JP7375809B2 - Method for manufacturing plated black core malleable cast iron member and plated black core malleable cast iron member - Google Patents

Description

The present disclosure relates to a method for manufacturing a plated black-core malleable cast iron member, and a plated black-core malleable cast iron member, particularly a pipe joint, manufactured by the manufacturing method.

Cast iron is classified into flake graphite cast iron, spheroidal graphite cast iron, malleable cast iron, etc. depending on the form of carbon present. Malleable cast iron is further classified into white core malleable cast iron, black core malleable cast iron, pearlite malleable cast iron, etc. Black-core malleable cast iron, which is the object of the present invention, is also called malleable cast iron, and has a form in which graphite is dispersed in a matrix of ferrite. In the manufacturing process of black core malleable cast iron, carbon after casting and cooling exists in the form of cementite, which is a compound with iron. Thereafter, by heating and holding the casting at a temperature of 720° C. or higher, cementite is decomposed and graphite is precipitated. In this specification, the process of precipitating graphite by heat treatment is hereinafter referred to as "graphitization."

Black-core malleable cast iron has superior mechanical strength compared to flaky graphite cast iron, and since the matrix is ferrite, it also has superior toughness. For this reason, black-core malleable cast iron is widely used as a material constituting members such as automobile parts and pipe joints that require mechanical strength. The surface of pipe joints made of black-core malleable cast iron is often hot-dip galvanized for corrosion protection. The hot-dip galvanized layer has excellent durability and can be plated at relatively low cost, so it is suitable as a means for preventing corrosion of pipe joints.

In the prior art, oxides such as iron and silicon are likely to be generated on the surface of a member made of black-core malleable cast iron (hereinafter referred to as "black-core malleable cast iron member") during the graphitization process. If you try to form a plating layer on a surface where these oxides have been generated, there will likely be a situation where there is no plating film locally and the material surface is exposed (hereinafter sometimes referred to as "unplated"). . Therefore, in order to form a plating layer with good adhesion on a black-core malleable cast iron member, a black-core malleable cast iron member having a surface where oxide formation is suppressed as much as possible is prepared, and a plating layer is applied to the surface of the black-core malleable cast iron member. need to be formed.

In order to produce black-core malleable cast iron parts with less oxides on the surface, various methods for removing oxides generated on the surface have been studied. For example, Patent Document 1 describes a method of removing oxides by immersing a black-core malleable cast iron member in an acidic solution. This method is sometimes called "pickling." Further, for example, Patent Document 2 describes a method of removing oxides formed on the surface of a black-core malleable cast iron member by long-term shot blasting.

Japanese Patent Application Publication No. 2014-19878 Japanese Unexamined Patent Publication No. 58-151463 International Publication No. 2013/146520

In the pickling process described in Patent Document 1, the acidic solution itself and the gas generated by the reaction with the black-core malleable cast iron are harmful to the human body and must be handled with care, and the acidic solution after use is There are issues such as the large impact on the environment when disposing of gas or exhausting the generated gas outdoors. Furthermore, the method disclosed in Patent Document 2 has a problem in that it is difficult to form a hot-dip plating layer well on the surface.

The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to manufacture a black core malleable cast iron member in which a hot-dip plating layer is satisfactorily formed on the surface without pickling.

Aspect 1 of the present invention is
A method for producing a plated black-core malleable cast iron member, wherein a plating layer is formed on the surface of the black-core malleable cast iron member,
a step of graphitizing in a non-oxidizing and decarburizing atmosphere;
A step of performing particle projection treatment on the surface of the black-core malleable cast iron member after graphitization so that silicon oxide remains on the surface;
immersing the black-core malleable cast iron member after the particle projection treatment in flux for 3.0 minutes or more;
The method for producing a plated black-core malleable cast iron member includes the step of hot-dipping the black-core malleable cast iron member after being immersed in the flux.

Aspect 2 of the present invention is
In Embodiment 1, the non-oxidizing and decarburizing atmosphere is an atmosphere in which the oxygen partial pressure is 10 times or less the equilibrium oxygen partial pressure of the following chemical formula 1, and higher than the equilibrium oxygen partial pressure of the following chemical formula 2. This is a method of manufacturing the plated black-core malleable cast iron member.

Aspect 3 of the present invention is the method for manufacturing a plated black-core malleable cast iron member according to aspect 1 or 2, wherein the particle projection treatment is any one of shot blasting, shot peening, sandblasting, and air blasting. .

Aspect 4 of the present invention is a method for producing a plated black-core malleable cast iron member according to any one of Aspects 1 to 3, wherein the particle projection treatment is performed for 3.0 minutes or more and 20 minutes or less. It is.

Aspect 5 of the present invention is any one of Aspects 1 to 4, which further includes a step of preheating the black core malleable cast iron member at a temperature of 275° C. or more and 425° C. or less before the graphitizing step. This is a method for manufacturing a plated black-core malleable cast iron member as described in .

Aspect 6 of the present invention is that the graphitization step includes first graphitization heating at a temperature exceeding 900°C, a starting temperature of 720°C or more and 800°C or less, and a completion temperature of 680°C or more. The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 5, including second graphitization at a temperature of 780° C. or lower.

Aspect 7 of the present invention is a method for manufacturing a plated black-core malleable cast iron member according to aspect 6, in which at least the first graphitization of the graphitization step is performed in a non-oxidizing and decarburizing atmosphere. It is.

Aspect 8 of the present invention is the plating formation according to any one of Aspects 1 to 7, wherein the non-oxidizing and decarburizing atmosphere contains a metamorphosed gas generated by combustion of a mixed gas of combustion gas and air. This is a method for manufacturing a black core malleable cast iron member.

Aspect 9 of the present invention is the plated black-core malleable cast iron according to any one of aspects 1 to 8, further comprising the step of heating the black-core malleable cast iron member to 90° C. or higher after taking it out from the flux. This is a method for manufacturing a member.

Aspect 10 of the present invention is the method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 9, wherein the flux is an aqueous solution containing a weakly acidic chloride.

Aspect 11 of the present invention is the method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 10, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.

A twelfth aspect of the present invention is a method for manufacturing the plated black-core malleable cast iron member according to any one of aspects 1 to 11, wherein the hot-dip plating step includes a hot-dip galvanizing step.

A thirteenth aspect of the present invention is the method for producing a plated black core malleable cast iron member according to any one of aspects 1 to 12, wherein the black core malleable cast iron member is a pipe joint.

Aspect 14 of the present invention is
A plated black core malleable cast iron member in which a plating layer is formed on the surface of the black core malleable cast iron member,
The plating layer is a hot-dip galvanized layer,
The plated black-core malleable cast iron member has a work-altered region on the cast iron surface of the black-core malleable cast iron member, and the hot-dip galvanized layer contains silicon oxide.

Aspect 15 of the present invention is the plated black-core malleable cast iron member according to aspect 14, which is a pipe joint.

According to the method for manufacturing a plated black-core malleable cast iron member according to the embodiment of the present invention, the surface can be adjusted to be suitable for forming a plating layer by using the graphitization process that is essential for manufacturing. In addition, the pickling step, which was considered indispensable in the conventional formation of a plating layer, can be omitted. In addition, plating defects can be reliably prevented by light particle projection treatment and flux treatment. As a result, the production of black-core malleable cast iron members having a plating layer can reduce the burden on the environment, and can also reduce costs compared to conventional methods.

FIG. 1A is an example of a backscattered electron image of a cross section near the surface of a black core malleable cast iron member after graphitization and before particle projection treatment in an example. FIG. 1B is an elemental mapping image of silicon in the same region as FIG. 1A. FIG. 1C is an elemental mapping image of oxygen in the same region as FIG. 1A. FIG. 2 is an example of a backscattered electron image of the surface of a black core malleable cast iron member after graphitization and before particle projection treatment in an example. FIG. 3 is an example of a backscattered electron image of a cross section near the surface of a black core malleable cast iron member after shot blasting as a particle projection process and before immersion in flux in an example. FIG. 4 is an example of a backscattered electron image of the surface of a black-core malleable cast iron member after shot blasting as a particle projection process and before immersion in flux in an example. FIG. 5 is an example of a backscattered electron image of a cross section including the entire thickness of the plating layer of the black core malleable cast iron member after hot dipping in the example. FIG. 6 is an example of a backscattered electron image showing the vicinity of the boundary between the cast iron surface and the plating layer of the black core malleable cast iron member after hot dipping in the example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments for carrying out the present invention will be described in detail below with reference to figures and tables. Note that the embodiments described here are merely examples, and the embodiments for carrying out the present invention are not limited to the embodiments described here. Furthermore, the mechanism described here is only a hypothesis established by the inventors as one that can rationally explain the currently known facts, and does not limit the technical scope of the present invention. Please note that this is not a thing.

Plating formation In the manufacturing process of black-core malleable cast iron parts, when plating is applied to black-core malleable cast iron parts such as pipe fittings without pickling, the quality of the plating depends on the surface of the black-core malleable cast iron parts. Depends on the condition. Furthermore, the shape of the black core malleable cast iron member and the plating conditions also affect the quality of the plating. For example, when manufacturing a pipe fitting as a black-core malleable cast iron member and the immersion time in a plating bath is relatively short, minute defects with a diameter of several mm or less may be formed on the inner surface of the pipe fitting. Plating tends to occur easily. Even if a pipe joint in which minute unplating occurs is immersed in a plating bath again, a plating layer will not be formed on the unplated portion, making it difficult to repair the unplated area.

Furthermore, the present inventors have discovered that the quality of plating depends on the immersion state of the black-core malleable cast iron member when the black-core malleable cast iron member is immersed in the plating bath, and specifically, that poor plating is caused by poor plating. This is also due to the fact that when black-core malleable cast iron parts such as pipe fittings are immersed in a plating bath, the pipe fittings float above the plating bath (hereinafter, this phenomenon is sometimes referred to as "pot floating"). I paid attention. When the above-described pot lifting occurs, there are problems such as plating defects such as the thickness of the plating layer becoming non-uniform and the formation of pinholes due to air bubbles in the plating layer.

The above pot floating tends to occur particularly when the immersion time in the flux is relatively short or when the black core malleable cast iron member is relatively light in weight. This problem also tends to occur when the shape of the pipe joint is complicated, as shown in the examples described later. One of the specific reasons why the pot floats above occurs is that immediately after the black-core malleable cast iron member is immersed in the plating bath, the flux attached to the surface of the black-core malleable cast iron member is rapidly heated in the plating bath. As a result, some kind of chemical reaction occurs and gas is generated on the surface of the black-core malleable cast iron member, and this gas may remain inside the pipe joint as bubbles. However, even if these bubbles are not generated or bubbles are released to the outside of the pipe joint, floating of the pot may occur.

The present inventors solved these problems, and in particular suppressed pot floating that was not caused by air bubbles, and created a plated black-core malleable cast iron member that suppressed plating defects such as non-plating, which was conventionally done. In order to obtain this product without pickling, we conducted extensive research into a method for producing plated black-core malleable cast iron parts. As a result, the graphitization essential for the production of plated black-core malleable cast iron parts is carried out in a specific atmosphere, and the pickling process is carried out by performing light particle blasting and immersion in flux under specific immersion conditions. Even if this is not done, a black-core malleable cast iron member having a surface suitable for forming a plating layer can be obtained, and floating in the pot is sufficiently suppressed during immersion in a plating bath, and a plating layer can be formed well during plating formation. I discovered that. Details are described below.
In this specification, a black-core malleable cast iron member on which a plating layer is formed is referred to as a "plated black-core malleable cast iron member." Further, the cast iron portion of a plated black-core malleable cast iron member that is in contact with the plating layer is sometimes referred to as the "cast iron surface."

<Alloy composition>
The main material constituting the black core malleable cast iron member in the present invention is black core malleable cast iron. The proportion of elements contained in black core malleable cast iron is carbon: 2.0% by mass or more and 3.4% by mass or less, silicon from 0.5% by mass or more and 2.0% by mass or less, and the balance is iron and Preferably, it contains unavoidable impurities. When the carbon content is 2.0% by mass or more, the fluidity of the molten metal is good, making casting work easier and reducing the rate of defects caused by the flow of the molten metal. When the carbon content is 3.4% by mass or less, precipitation of graphite during casting and the subsequent cooling process can be prevented. When the silicon content is 0.5% by mass or more, the effect of promoting graphitization due to silicon can be obtained, and graphitization can be completed in a short time. When the silicon content is 2.0% by mass or less, precipitation of graphite during casting and the subsequent cooling process can be prevented.

The black-core malleable cast iron in the embodiment of the present invention further contains a total of 0.005% by mass or more and 0.020% by mass or less of one or two elements selected from the element group consisting of bismuth and aluminum. is more preferable. If the total content of bismuth and aluminum is 0.005% by mass or more, precipitation of graphite during casting and the subsequent cooling process can be prevented. When the total content of bismuth and aluminum is 0.020% by mass or less, graphitization is not significantly inhibited. In addition to these elements, the black-core malleable cast iron in embodiments of the present invention may contain 0.5% by mass or less of manganese.

<Preheating>
In a preferred embodiment of the present invention, the black-core malleable cast iron member before graphitization is preferably preheated at a temperature of 275°C or higher and 425°C or lower. In the present invention, "preheating" refers to heat treatment in a low temperature range performed on a cast black-core malleable cast iron member prior to graphitization. By preheating, the graphite after graphitization exists dispersedly at the grain boundaries of ferrite, and the grain size of ferrite can be made finer than that of conventional black-core malleable cast iron. Moreover, the time required for graphitization can also be shortened. The effect of such preheating becomes more pronounced when the black-core malleable cast iron member contains one or two elements selected from the group of elements consisting of bismuth and aluminum.

Since the present invention relates to the forming of a plating layer, the alloy composition of the black core malleable cast iron member in the method for producing a plated black core malleable cast iron member according to the present invention is not limited to a specific alloy composition. do not have. The alloy composition in the present invention may be any alloy composition as long as it does not significantly deviate from the range of the above-mentioned alloy composition generally considered as the alloy composition of black core malleable cast iron. Similarly, the crystal grain size of ferrite after graphitization is not particularly limited in the present invention. Therefore, the above preheating is not an essential step in the present invention, and it is naturally permissible in the present invention to form a plating layer on the surface of black-core malleable cast iron that has been graphitized without preheating. be done.

<Graphitization temperature and holding time>
In the method for manufacturing a plated black-core malleable cast iron member according to the present invention, the black-core malleable cast iron member after casting, preferably the black-core malleable cast iron member after the above-mentioned preheating, is heated to a temperature of, for example, 720° C. or higher. A heat treatment called graphitization is performed to heat and hold the material. Graphitization is a process specific to the method of manufacturing black-core malleable cast iron. In the graphitization process, cementite is decomposed and graphite is precipitated by heating the black-core malleable cast iron member to a temperature exceeding 720°C, which corresponds to the A1 transformation point, and at the same time, by cooling the austenite matrix. It can be transformed into ferrite to impart toughness to black-core malleable cast iron members. Graphitization is preferably divided into first graphitization, which is performed first, and second graphitization, which is performed after the first graphitization.

The first graphitization is preferably a step in which cementite in austenite is decomposed and graphite is precipitated in a temperature range exceeding 900°C. In the first graphitization, carbon separated by decomposition of cementite contributes to the production of graphite. The temperature at which the first graphitization is performed is more preferably 920°C or higher and 980°C or lower. The holding time required for the first graphitization varies depending on the size of the black-core malleable cast iron member to be graphitized. When the above preheating is performed, the holding time for the first graphitization is preferably 30 minutes or more and 3 hours or less, more preferably 2 hours or less.

The second graphitization is preferably a step of transforming austenite into ferrite and decomposing cementite in ferrite and/or pearlite to precipitate graphite at a temperature lower than the temperature at which the first graphitization is performed. . The second graphitization is preferably performed while gradually lowering the temperature from the second graphitization start temperature to the second graphitization completion temperature. As a result, graphite can be precipitated while gradually lowering the solid solubility of carbon in austenite, so that the transformation from austenite to ferrite progresses reliably.

The second graphitization start temperature is preferably 720°C or higher and 800°C or lower. The second graphitization completion temperature is preferably 680°C or higher and 780°C or lower, more preferably 720°C or lower and lower than the second graphitization start temperature. The time required from the start to the completion of the second graphitization also varies depending on the size of the black-core malleable cast iron member to be graphitized. When the above preheating is performed, the time for second graphitization is preferably 30 minutes or more and 3 hours or less, more preferably 2 hours or less. When transitioning from the first graphitization to the second graphitization, it is preferable to lower the temperature from the temperature of the first graphitization to the starting temperature of the second graphitization. In the embodiment of the present invention, it is not necessary to lower the temperature from the first graphitization temperature to a temperature lower than the second graphitization start temperature, such as room temperature, and then increase the temperature to the second graphitization start temperature. do not. There is no particular restriction on the time required for temperature reduction during transition from first graphitization to second graphitization.

<Non-oxidizing atmosphere>
In the method for manufacturing a plated black-core malleable cast iron member according to an embodiment of the present invention, graphitization of the black-core malleable cast iron member is performed in a non-oxidizing and decarburizing atmosphere. In the present invention, "non-oxidizing atmosphere" means only a reducing atmosphere in the strict sense, that is, an atmosphere having an oxygen partial pressure lower than the equilibrium oxygen partial pressure of chemical formula 1 described later at the graphitization temperature. Rather, it refers to an atmosphere in which the iron contained in a black-core malleable cast iron member does not react with the gas constituting the atmosphere and generate iron oxides to the extent that they interfere with the formation of a plating layer. In other words, the "non-oxidizing atmosphere" in the present invention is a broad concept that includes an atmosphere in which a thick oxide layer that prevents the formation of a plating layer is not generated. Specifically, the term "non-oxidizing atmosphere" means that the oxygen partial pressure in the graphitization atmosphere is preferably 10 times or less the equilibrium oxygen partial pressure of Chemical Formula 1, which will be detailed below. Therefore, according to a preferred embodiment, the equilibrium oxygen partial pressure of chemical formula 1 at the temperature at which graphitization is performed is determined, and the oxygen partial pressure in the graphitization atmosphere is 10 times or less the equilibrium oxygen partial pressure of chemical formula 1. In addition, even if graphitization is performed in a state equal to or lower than the determined equilibrium oxygen partial pressure, this also falls under the non-oxidizing atmosphere in the present invention. The oxygen partial pressure in the graphitization atmosphere is more preferably 6 times or less, even more preferably 3 times or less, and even more preferably less than or equal to the equilibrium oxygen partial pressure of Chemical Formula 1.

A chemical formula representing a typical iron oxidation reaction is shown in Chemical Formula 1.

Here, Fe(s) represents solid iron, O ₂ (g) represents gaseous oxygen, and FeO(s) represents solid ferrous oxide (wustite). Several reactions other than Chemical Formula 1 are known for iron oxidation reactions, but the oxidation reaction with the lowest standard Gibbs energy at the graphitization temperature is the reaction of Chemical Formula 1. Therefore, in an atmosphere in which the oxidation reaction of iron expressed by Chemical Formula 1 is difficult to proceed, the oxidation reactions of iron expressed by other chemical formulas are also difficult to proceed.

To perform graphitization in a non-oxidizing atmosphere, find the equilibrium oxygen partial pressure of chemical formula 1 at the temperature at which graphitization is performed, and make sure that the oxygen partial pressure of the atmosphere is 10 times the equilibrium oxygen partial pressure of chemical formula 1, as described above. It is preferable that it is below. Particularly preferably, the oxygen partial pressure of the atmosphere is equal to or lower than the determined equilibrium oxygen partial pressure. Then, the reaction of Chemical Formula 1 will maintain chemical equilibrium or proceed from right to left, and the formation of iron oxides will be more fully inhibited. The value of the equilibrium oxygen partial pressure of Chemical Formula 1 at the temperature of graphitization can be calculated using the literature value of the standard Gibbs energy of Chemical Formula 1. Table 1 shows an example of calculating the equilibrium oxygen partial pressure of Chemical Formula 1 in the first graphitization (980° C.) and the second graphitization (760° C.). In this calculation, M. W. M.W. Chase, "NIST-JANAF Thermochemical Tables" (USA), 4th edition, American Institute of Physics, August 1, 1998. Reference was made to the standard Gibbs energy values listed in .

To know whether the oxygen partial pressure of the atmosphere during graphitization is less than or equal to the equilibrium oxygen partial pressure of chemical formula 1 shown in Table 1, and how many times the equilibrium oxygen partial pressure of chemical formula 1 is, it is necessary to check the oxygen content of the atmosphere. You need to know the pressure. Examples of methods for measuring the oxygen partial pressure of the atmosphere include a method of directly measuring the oxygen partial pressure of the atmosphere using a zirconia oxygen concentration meter, a quadrupole mass spectrometer, or the like. However, when measuring extremely low oxygen partial pressures as shown in Table 1, these direct methods may not always provide sufficient measurement accuracy.

When using a metamorphosed gas as an atmospheric gas for graphitization, for example, as described in Patent Document 3, the partial pressure ratio of carbon monoxide and carbon dioxide or the partial pressure ratio of hydrogen and water vapor in the atmosphere is measured. , the partial pressure of oxygen in equilibrium with these gases can be determined indirectly by calculation. This calculation is based on the reaction in which carbon monoxide and oxygen react in a heat treatment furnace to produce carbon dioxide (2CO + O ₂ = 2CO ₂ ) or the reaction in which hydrogen and oxygen react to produce water vapor (2H ₂ + O ₂ = 2H ₂ O) is assumed to be established.

In an embodiment of the present invention, a known method capable of lowering the oxygen partial pressure can be used to make the graphitization atmosphere a non-oxidizing atmosphere. Specific methods include, for example, a method of maintaining the inside of the heat treatment furnace in a high vacuum, a method of filling the inside of the heat treatment furnace with non-oxidizing gas, but are not limited to these.

In a preferred embodiment of the invention, the non-oxidizing atmosphere includes a metamorphosed gas generated by burning a mixture of combustion gas and air. Since the converted gas can be produced relatively inexpensively, the production cost required for graphitization can be suppressed compared to the case where other non-oxidizing atmospheres are used. Combustion gases that can be used to generate converted gas include propane gas, butane gas, mixed gases thereof, liquefied petroleum gas, liquefied natural gas, and the like.

A gas generator can be used to generate the converted gas. When the mixing ratio of air mixed with combustion gas is increased, a complete combustion type gas containing many components of CO ₂ gas and N ₂ gas is generated. When the air mixture ratio is reduced, an incomplete combustion type gas containing a large amount of CO gas and _H2 gas is generated. A portion of the water vapor contained in the converted gas can be removed by a refrigeration dehydrator.

When a modified gas is used to form a non-oxidizing atmosphere, the oxygen partial pressure in the heat treatment furnace, which can be determined by any of the above methods, is calculated from the equilibrium oxygen partial pressure of chemical formula 1 shown in Table 1. If the temperature is quite high, either lower the mixing ratio of the air mixed with the combustion gas to increase the ratio of CO gas and _H2 gas, or lower the cooling temperature of the refrigeration dehydrator to lower the dew point of the converted gas. Oxygen partial pressure can be lowered by this method. Alternatively, both of these methods may be used.

In the embodiment of the present invention, as described later, graphitization is performed in the non-oxidizing and decarburizing atmosphere, that is, the graphitizing atmosphere is also a decarburizing atmosphere, but it is not a decarburizing atmosphere. It is not so important to make the graphitization atmosphere a non-oxidizing atmosphere. In other words, even if a slight oxide layer is formed on the surface of the black-core malleable cast iron member during graphitization, it does not need to be a major hindrance to the formation of the plating layer. Therefore, the "non-oxidizing atmosphere" in the present invention is a broad concept as described above.

In a preferred embodiment of the present invention, the second graphitization is performed in a reducing atmosphere, ie, an atmosphere in which the oxygen partial pressure is lower than the equilibrium oxygen partial pressure of Formula 1 above. Even if oxides are generated on the surface of the black-core malleable cast iron member during the first graphitization, the oxides that have been generated can be reduced by performing the second graphitization in a reducing atmosphere. The thickness can be reduced to a level that does not interfere with the formation of the plating layer.

<Decarburizing atmosphere>
In the method for manufacturing a plated black-core malleable cast iron member according to the present invention, the atmosphere for graphitizing the black-core malleable cast iron member is also a decarburizing atmosphere. In the present invention, a "decarburizing atmosphere" means that carbon contained in a black-core malleable cast iron member is oxidized by oxygen gas in the atmosphere to carbon monoxide, and the carbon monoxide gas is This refers to an atmosphere in which carbon removal progresses by separating from the surface to the outside. This chemical reaction can be represented by Chemical Formula 2 below.

Here, C(s) represents solid carbon, O ₂ (g) represents gaseous oxygen, and CO(g) represents gaseous carbon monoxide. In addition to chemical formula 2, carbon oxidation reactions include reactions in which carbon reacts with oxygen to produce carbon dioxide (C + O ₂ = CO ₂ ); The reaction of chemical formula 2, which has lower energy, proceeds preferentially.

To perform graphitization in a decarburizing atmosphere, find the equilibrium oxygen partial pressure of chemical formula 2 at the graphitization temperature, and perform graphitization in a state where the oxygen partial pressure of the graphitization atmosphere is higher than this equilibrium oxygen partial pressure. Just go. Then, the reaction of chemical formula 2 proceeds from left to right, and the carbon contained in the black core malleable cast iron reacts with oxygen and becomes carbon monoxide, which is released to the outside, and decarburization progresses. The value of the equilibrium oxygen partial pressure of Chemical Formula 2 at the graphitization temperature can be calculated using the literature value of standard Gibbs energy, as in the case of Chemical Formula 1 described above. Table 1 also shows an example of calculating the equilibrium oxygen partial pressure of Chemical Formula 2 in the first graphitization (980° C.) and the second graphitization (760° C.).

In order to know whether the oxygen partial pressure of the atmosphere during graphitization is higher than the equilibrium oxygen partial pressure of chemical formula 2 shown in Table 1, it is necessary to measure the oxygen partial pressure of the atmosphere. Since the method for measuring the oxygen partial pressure of the atmosphere has already been explained, the explanation will be omitted here. If the determined oxygen partial pressure of the atmosphere is higher than the equilibrium oxygen partial pressure of chemical formula 2 shown in Table 1, graphitization can be performed in the decarburizing atmosphere. When a modified gas is used in the atmosphere and the oxygen partial pressure in the heat treatment furnace is equal to or lower than the equilibrium oxygen partial pressure of chemical formula 2, for example, air mixing in the modified gas generator The oxygen partial pressure can be adjusted to be higher than the equilibrium oxygen partial pressure of Chemical Formula 2 by increasing the ratio or increasing the dew point of the converted gas. However, the method of adjusting the oxygen partial pressure is not limited to these.

In the embodiment of the present invention, since graphitization is performed in a decarburizing atmosphere, graphite is not generated on the surface of the black-core malleable cast iron member during the graphitization process. Therefore, according to the manufacturing method of the present invention, it is possible to manufacture a black-core malleable cast iron member in which almost no graphite is generated on the surface after graphitization but before the formation of a plating layer, and the surface has excellent adhesion. A plating layer can be formed.

In the embodiment of the present invention, both the first graphitization and the second graphitization may be performed in a non-oxidizing and decarburizing atmosphere, and even if this is not the case, at least Preferably, the first graphitization is performed in a non-oxidizing and decarburizing atmosphere. In the latter case, the second graphitization may be performed in an atmosphere that is not a decarburizing atmosphere. However, since the temperature of the second graphitization is lower than that of the first graphitization, the speed at which graphite is precipitated on the surface of the black-core malleable cast iron member is slower than that of the first graphitization. Therefore, the effects of the present invention can be obtained by performing at least the first graphitization in a decarburizing atmosphere.

As described above, the method for manufacturing a plated black-core malleable cast iron member according to the present invention includes a step of graphitizing in a non-oxidizing and decarburizing atmosphere. For example, to achieve a non-oxidizing and decarburizing atmosphere for the first graphitization (980°C), the oxygen partial pressure in the furnace is set to the equilibrium oxygen partial pressure of chemical formula 2 shown in Table 1. The oxygen partial pressure may be higher than 2.6×10 ⁻¹⁹ atm and 10 times or less than 3.4×10 ⁻¹⁶ atm, which is the equilibrium oxygen partial pressure of chemical formula 1 shown in Table 1.

As described above, according to the method for manufacturing a plated black-core malleable cast iron member according to the present invention, it is possible to prepare a surface suitable for forming a plated layer by using the graphitization process that is essential for manufacturing. can. In particular, since graphitization is carried out in a decarburizing atmosphere, there is almost no formation of graphite, which is one of the causes of unplating, on the surface of the black-core malleable cast iron member. Furthermore, since graphitization is performed in a non-oxidizing atmosphere, there is almost no oxide layer, and even if there is, it is extremely thin. Therefore, it is possible to obtain a surface of a black-core malleable cast iron member suitable for plating.

<Ferrite layer>
In a preferred embodiment of the present invention, the black-core malleable cast iron member after graphitization and before the particle blasting treatment described below has a ferrite layer on its surface with a thickness of more than 100 μm. The ferrite layer refers to a layered structure composed of ferrite, which is called α phase in the iron-carbon binary phase diagram and contains almost no carbon. In a preferred embodiment, decarburization progresses on the surface of the black-core malleable cast iron member, resulting in the formation of carbon-poor austenite, which, when cooled after graphitization is complete, becomes a ferrite layer with a thickness of more than 100 μm. When a ferrite layer is formed, graphite is not present not only on the surface of the black-core malleable cast iron member but also inside the vicinity of the surface layer. Therefore, it is possible to form a stronger plating layer with excellent adhesion, which is preferable.

White core malleable cast iron is decarburized in a decarburizing atmosphere, but black core malleable cast iron and pearlitic malleable cast iron are usually not graphitized in a decarburizing atmosphere. However, in the present invention, graphitization is performed in a decarburizing atmosphere in order to enable formation of a plating layer with excellent adhesion. As a result, even if a ferrite layer is formed on the surface of the black-core malleable cast iron member, as long as the ferrite layer is not very thick, it will have little effect on the mechanical properties.

In an embodiment of the present invention, when a ferrite layer is formed on the surface of the black core malleable cast iron member, a thin oxide layer of iron may be formed on the surface of the ferrite layer. Even if an oxide layer is formed, if it is thin, it can be removed through particle projection treatment and flux treatment, which are subsequent steps. Furthermore, the formation of a thin oxide layer is preferable because it prevents excessive decarburization of the surface of the black-core malleable cast iron member. The allowable thickness of the oxide layer that can be formed on the surface of the ferrite layer is preferably 20 μm or less, more preferably 10 μm or less.

<Silicon oxide>
In an embodiment of the present invention, silicon oxide is present on the surface of the black-core malleable cast iron member after graphitization is completed. As mentioned above, silicon is one of the elements constituting black core malleable cast iron. Silicon is an element that is more easily oxidized than iron and carbon. Therefore, even when graphitization is performed in a non-oxidizing atmosphere in the present invention, it is inevitable that the silicon contained in the black core malleable cast iron will be oxidized and silicon oxides will be produced. Silicon oxides produced during the graphitization process mainly exist on the surface of black-core malleable cast iron members. When the above-mentioned ferrite layer is formed on the surface of the black core malleable cast iron member, silicon oxide is present on the surface of the ferrite layer. As mentioned above, the generation of oxides on the surface of black-core malleable cast iron members causes non-plating. However, in the present invention, non-plating can be prevented by subjecting the silicon oxide present on the surface of the black-core malleable cast iron member to particle projection treatment, which will be described later.

<Particle projection processing>
In an embodiment of the present invention, after the above-mentioned graphitization and before immersion in flux, the surface of the black-core malleable cast iron member is subjected to particle projection treatment so that silicon oxide remains on the surface. The particle projection treatment in the embodiment of the present invention is a treatment to the extent that cracks and strain energy are introduced to the surface of the black core malleable cast iron member, and unlike the conventional technology, the oxide film on the surface of the member to be subjected to plating treatment is removed. It is not a highly destructive process that removes it. If high enough energy is applied to the surface of the component to remove an oxide film made of silicon oxide, etc., the formation rate of the plating layer (described later) will be higher than necessary, making it difficult to control the plating thickness. This is not preferable because it becomes difficult. Therefore, the particle projection treatment is performed so that silicon oxide remains on the surface of the black-core malleable cast iron. The area of the particle projection treatment on the surface of the black-core malleable cast iron member may not be the entire surface, but may be a part of the surface of the member.

The particle projection process is a process in which particles are sprayed at high speed onto the surface of a black-core malleable cast iron member, and is classified into mechanical type and pneumatic type. As a mechanical method, there is a method in which the centrifugal force of an impeller is used to project projecting particles (media) onto a member to be processed (workpiece). Specific examples of the mechanical processing include shot processing such as shot blasting and shot peening, and sand processing such as sandblasting. The pneumatic method includes a method of projecting projecting particles using compressed air (air blast). In order to perform a homogeneous particle projection process, it is preferable that the number of projection positions by the impeller or compressed air is two or more. The object to be processed (work) may move during processing, such as stirring or rotating, or may be fixed.

The material, particle size, hardness, etc. of the projected particles (media) do not matter. Examples of the material of the projection particles include steel, cast steel, stainless steel, alumina, ceramic, glass, and silica sand. The preferred projecting particles are steel balls, grit, and sand, in that order. Examples of the shape of the projected particles include a spherical shape, a short cylindrical cut wire obtained by simply cutting a metal wire, and a grit having acute corners. The preferable range of the particle diameter of the projected particles varies depending on the size and shape of the object to be treated, the material of the projected particles, and the like. For example, when using steel balls as the projection particles, the preferred particle size range is 5 to 10 mm. By using projection particles with a particle size of 5 mm or more, a sufficient impact can be applied to the surface of the object to be treated. Furthermore, by using projection particles with a particle size of 10 mm or less, it is possible to project even into the recesses of a workpiece having a complex shape, and at the same time, it is possible to prevent the plating layer from becoming too thick due to excessive impact. A more preferable range of particle size of the steel balls used for the projection particles is 6 to 8 mm.

As a specific example of the particle projection process, for example, a steel ball with a diameter of 6 mm to 8 mm is used as the projecting particles, and while stirring the black core malleable cast iron member, a large number of the above projecting particles are blasted through the black core using an impeller. One example is a method of striking the surface of a forged cast iron member.

In the embodiment of the present invention, the surface of the black-core malleable cast iron member is subjected to particle projection treatment so that silicon oxide remains on the surface. The core malleable cast iron member has silicon oxide on its surface. Since the purpose of the above treatment in the present invention is not to remove silicon oxide, silicon oxide remains on the surface of the black core malleable cast iron member after the above treatment. A large amount of the silicon oxide may remain. For example, as shown in the examples below, the area ratio of silicon oxide on the surface of the black-core malleable cast iron member after the particle projection treatment is the same as the silicon oxide present on the surface of the black-core malleable cast iron member before the particle projection treatment. For example, it may be 50% or more, further 70% or more, and even more still 90% or more with respect to the quantity. Whether or not silicon oxide remains on the surface of the black core malleable cast iron member after the particle projection treatment can be determined by taking an elemental mapping image of silicon and oxygen on the surface or cross section of the sample, as exemplified in the example below. This can be confirmed by, for example,

Further, the surface of the black-core malleable cast iron member after the particle projection treatment of the present invention has a work-altered region due to the particle projection treatment. That is, in a plated black-core malleable cast iron member obtained as a final product, the cast iron surface of the black-core malleable cast iron member has a process-altered region.

In the embodiment of the present invention, the projection time, projection speed, projection angle, projection amount, etc. are not particularly limited as long as silicon oxide remains on the surface after the treatment. It can be set appropriately depending on the size of the object to be processed (workpiece) (for example, when the object to be processed is a pipe joint, the nominal size is 1/8 to 8 inches). From the viewpoint of differentiation from conventional silicon oxide removal processing, the projection time is, for example, 20 minutes or less, preferably 10 minutes or less, and can be, for example, 3.0 minutes or more. The embodiment of the present invention differs from Patent Document 2 in that a light particle blasting process is performed as described above, in which shot blasting is performed for a long time of 30 to 40 minutes instead of pickling.

As mentioned above, the reason why pot floating and non-plating are suppressed by performing a light particle projection treatment is not fully understood yet, but it is thought to be as follows. Silicon oxide present on the surface of the black core malleable cast iron member is considered to be the cause of unplatedness and pot floating. By performing the above particle projection treatment, the ferrite layer present on the surface of the black-core malleable cast iron member is deformed and flattened, and cracks are generated in the silicon oxide. It is thought that this causes stress to be introduced into the ferrite layer and silicon oxide on the surface of the black core malleable cast iron member, promoting the reaction with the plating solution. Furthermore, the plating solution can easily reach the silicon oxide buried in the ferrite layer. It is thought that due to these effects, the silicon oxide is likely to be detached when immersed in a plating bath.

<Flux treatment>
The method for manufacturing a plated black-core malleable cast iron member according to the present invention includes graphitizing in a non-oxidizing and decarburizing atmosphere, not performing pickling treatment before plating treatment, and black-core malleable. In addition to subjecting the cast iron member to the above-mentioned light particle projection treatment, the cast iron member is also characterized by being immersed in flux for a predetermined period of time or more, as explained above.

As the flux used in the embodiment of the present invention, a known weakly acidic chloride aqueous solution suitable for flux can be used. Generally, flux has the effect of forming a thin film on the surface of the member to be plated, improving wettability with molten metal, and preventing rusting before hot-dip plating. It has the effect of making the thickness of the plating layer formed on the surface of the plated member uniform and improving the adhesion of the plating layer. For this reason, in hot-dip plating, the step of immersing the member to be plated in flux has become a step that cannot be omitted. In addition to the above-mentioned effects, the immersion of the black-core malleable cast iron member in the flux in the present invention has the unique effect of removing the thin oxide layer produced by graphitization.

In an embodiment of the present invention, the immersion in flux has the novel function of removing the oxide layer generated on the surface of the black-core malleable cast iron member during the casting and graphitization process, thereby eliminating the conventional The step of removing oxides by pickling can be omitted. Since the flux made of an aqueous chloride solution can be used repeatedly, there is no need to dispose of the acidic solution when pickling is performed. In addition, the chemical reaction between black-core malleable cast iron parts and the weakly acidic chloride aqueous solution used for flux is slower than the chemical reaction with the strong acid used in conventional pickling, and the Occurrence is also low. Therefore, according to the manufacturing method according to the present invention, the load on the environment can be significantly reduced compared to conventional manufacturing methods.

When the flux is a chloride aqueous solution, the concentration of chloride in the chloride aqueous solution is preferably 10% by mass or more and 50% by mass or less. When the concentration is 10% by mass or more, the effect of removing the oxide layer becomes significant. The effectiveness of removing the oxide layer does not change significantly when the concentration is increased beyond 50% by weight. When the concentration is 50% by mass or less, chloride consumed in preparing the flux can be saved. Further, the thickness of the flux formed is not too thick and is easy to dry. A more preferable concentration of the chloride aqueous solution is 20% by mass or more and 40% by mass or less.

In a preferred embodiment of the present invention, the chloride contained in the flux is one or more of zinc chloride, ammonium chloride, and potassium chloride. More preferably, the flux is an aqueous solution containing zinc chloride and ammonium chloride. The molar ratio of the content of zinc chloride and ammonium chloride in the flux is preferably 2 or more and 4 or less of ammonium chloride to 1 part of zinc chloride. Among these, if the molar ratio is 1 part zinc chloride to 3 parts ammonium chloride, that is, if the mass ratio is 46% zinc chloride to 54% ammonium chloride, it will be easier to dry. preferable.

When the flux is an aqueous solution containing zinc chloride and ammonium chloride, the temperature of the flux is preferably 60°C or higher and 95°C or lower. When the temperature is 60° C. or higher, the effect of removing the oxide layer becomes significant. When the temperature is below 95℃, boiling of the flux can be prevented, so immersion of black-core malleable cast iron parts in flux can be performed more safely, and the removal of the oxide layer can also be more stable. It can be carried out. When the temperature of the flux is 90° C. or higher, hydrolysis of ammonium chloride progresses, the concentration of the flux becomes stable, and the effect of removing the oxide layer increases, so it is more preferable.

The preferred time for immersing the black-core malleable cast iron member in flux is determined by the composition of the flux, concentration, temperature, degree of deterioration of the flux, size of the black-core malleable cast iron member, and the amount of time formed on the surface of the black-core malleable cast iron member. It depends on conditions such as the thickness of the oxide layer. The immersion time in the flux is 3.0 minutes or more, preferably 5.0 minutes or more, and preferably 60 minutes or less. It is preferable that the immersion time is 5.0 minutes or more because the effect of removing the oxide layer becomes significant. The effectiveness of removing the oxide layer does not change significantly even after soaking for more than 60 minutes. Therefore, when the immersion time is 60 minutes or less, excessive melting of the black-core malleable cast iron member can be prevented and the flux can be made to last for a long time. The immersion time in the flux is more preferably 10 minutes or more and 50 minutes or less, and even more preferably 15 minutes or more and 40 minutes or less. However, if the thickness of the oxide layer formed on the surface of the black core malleable cast iron member is very thick, it may be immersed in the flux for more than 60 minutes.

When black-core malleable cast iron parts are repeatedly immersed in flux, the flux turns green. This is presumed to be because iron dissolves in the flux and produces iron(II) chloride (ferrous chloride). If you continue to use it further, the flux will turn reddish brown. This is presumably because iron (II) chloride is oxidized to produce iron (III) chloride (ferric chloride). If it continues to be used, oxidation will progress further and iron (III) hydroxide will be produced and precipitated. If iron (III) hydroxide adheres to the surface of a black-core malleable cast iron member, it will cause unplating, so it is preferable to remove it from the flux by filtration. By controlling the concentration of flux within a desirable range while removing iron (III) hydroxide through filtration, the flux once prepared can be used for a long period of time.

The concentration of the flux can be controlled by known means such as analysis of the specific gravity of the flux, pH, or chemical components contained in the flux. For example, when using a chloride aqueous solution containing 3 parts of ammonium chloride to 1 part of zinc chloride in molar ratio as a flux, the solute should be By adjusting the amount of dissolved chloride, the concentration of the chloride aqueous solution can be adjusted to a preferable range of 10% by mass or more and 50% by mass or less. Furthermore, if the specific gravity measured at 90°C is adjusted to be 1.10 or more and 1.20 or less, the concentration of the chloride aqueous solution can be adjusted to a more preferable range of 20% by mass or more and 40% by mass or less. I can do it. If the concentration of the flux decreases due to continued use of the flux, the concentration of the flux can be managed so as not to deviate from the desired range by adding a solute so that the specific gravity of the flux falls within the above range. . The specific gravity of the flux can be measured using, for example, a floating scale. The preferred pH range of the flux used in the present invention is 3.0 or more and 6.0 or less.

<Hot-dip plating>
The method for manufacturing a plated black-core malleable cast iron member according to the present invention includes a step of hot-dipping the black-core malleable cast iron member taken out from the flux. A plating layer is formed on the surface of the black-core malleable cast iron member by hot-dip plating. According to the manufacturing method of the present invention, almost no graphite is generated on the surface before the plating layer is formed after graphitization, and by passing through the particle projection treatment and flux treatment, the plating layer with excellent adhesion is formed on the surface. can be formed. As the plating layer in the present invention, a metal or alloy plating layer can be used. Specifically, metals such as zinc, tin, aluminum, or alloys thereof can be used, but the plating layer is not limited to these. Preferably it is hot-dip galvanizing.

In a preferred embodiment of the present invention, the step of hot-dip plating includes the step of hot-dip galvanizing. Zinc is preferred because it has a strong ionization tendency and has a sacrificial anticorrosion effect. When the first plating applied is hot-dip galvanizing, a zinc layer (η (eta) layer) is generated on the outermost surface of the plated black-core malleable cast iron member, and the difference between the zinc layer and the black-core malleable cast iron member is An alloy layer of iron and zinc (one δ (delta) layer and one ζ (zeta) layer) is generated between the surface and the surface. These layers adhere firmly to each other, forming a plating layer with good adhesion as a whole.

In an embodiment of the present invention, by performing graphitization in a decarburizing atmosphere, a ferrite layer can be generated on the surface of a black-core malleable cast iron member after graphitization but before formation of a plating layer, as described above. . The same is true when this ferrite layer is formed; in this case, ferrite and zinc react to form an alloy layer. After applying hot-dip plating (for example, after forming a hot-dip galvanized layer), the ferrite layer may remain inside the plating layer, or the ferrite layer may disappear.

When the step of applying hot-dip plating includes the step of applying hot-dip galvanizing, the temperature of the galvanizing bath used for hot-dip galvanizing is preferably 450°C or higher and 550°C or lower. When the temperature is 450°C or higher, coagulation of zinc in the galvanizing bath can be prevented. When the temperature is 550° C. or lower, excessive reaction between the galvanized layer and the surface of the black-core malleable cast iron member can be prevented. A more preferable temperature of the galvanizing bath is 480°C or higher and 520°C or lower.

In a preferred embodiment of the present invention, when the step of hot-dip galvanizing includes the step of hot-dip galvanizing, the galvanizing bath used for hot-dip galvanizing may contain aluminum. When aluminum is molten in the galvanizing bath, the formation of a zinc oxide film on the surface of the plating bath is suppressed, and the liquid surface becomes clean. In addition, the formed plating layer also has increased gloss, and the aesthetic appearance is improved.

According to the method for manufacturing a plated black core malleable cast iron member according to the present invention, it is possible to form a plated layer by hot-dip plating without causing unplatedness even if pickling is omitted. The reason for this is not necessarily clear, but it is probably due to the following reasons. The first reason is that there are few substances on the surface of the black-core malleable cast iron member after graphitization and before hot-dip plating that cause non-plating. Since graphitization is carried out in a decarburizing atmosphere, there is almost no formation of graphite, which is one of the causes of non-plating. Furthermore, since graphitization is performed in a non-oxidizing atmosphere, there is almost no oxide layer, and even if there is, it is extremely thin.

Even if an oxide layer remains, it is thought that most of it will be removed when it is immersed in flux after the above-described particle projection treatment. If the immersion time in the flux is short, gas generated during hot-dip plating may adhere as bubbles to the surface of the member to be plated, resulting in the above-mentioned pot floating. The details of this cause are unknown, but it is speculated that if the immersion time in the flux is insufficient, substances that cause gas generation remain on the surface of the black-core malleable cast iron member. However, in the embodiment of the present invention, if the immersion time in the flux is made sufficiently long, floating of the pot hardly occurs.

The second reason is that the thin oxide layer formed on the surface of the black core malleable cast iron member is peeled off from the surface of the black core malleable cast iron member during the hot-dip plating process and rendered harmless. When the flux is an aqueous solution containing zinc chloride and ammonium chloride, iron oxide on the surface of the black-core malleable cast iron member may chemically react with ammonium chloride to produce a black product. This product is usually difficult to peel off and is one of the causes of non-plating. However, in an embodiment of the present invention, during hot-dip plating, a phenomenon is observed in which black products peel off from the surface of the black core malleable cast iron member and float to the surface of the plating bath. Therefore, in the embodiment of the present invention, even if the above-mentioned black product is generated, it will peel off during the hot-dip plating process, so no unplated material will occur even if pickling is omitted. It is assumed that.

<Heat treatment>
The plated black-core malleable cast iron member according to the present invention can be manufactured by immersing the black-core malleable cast iron member in flux and then subjecting it to hot-dip plating without performing heat treatment. One possible embodiment of the present invention may further include the step of heating the black core malleable cast iron member after removal from the flux and before hot dipping, as described below.

By preheating the black-core malleable cast iron member before applying hot-dip plating, the occurrence of unplated parts tends to be suppressed. The heating temperature of the black-core malleable cast iron member depends on the size and shape of the black-core malleable cast iron member. The heating temperature when heating the black-core malleable cast iron member is preferably 90° C. or higher. More preferably, the temperature is 100°C or higher and 250°C or lower. When the temperature is 100° C. or higher, the flux can be sufficiently dried, and detoxification due to the reaction between the flux and the oxide layer on the surface of the black-core malleable cast iron member is promoted. When the temperature is 250° C. or lower, the flux does not decompose due to temperature rise, and peeling of the flux and additional oxidation of the surface of the black-core malleable cast iron member can be prevented. A more preferable heating temperature is 150°C or higher and 200°C or lower.

When heating, a known heating means such as a heat treatment furnace can be used. For example, a black-core malleable cast iron member removed from flux is inserted into a heat treatment furnace that has been heated to a predetermined temperature, and when the temperature of the black-core malleable cast iron member reaches a desired temperature, it is removed from the heat treatment furnace. , hot-dip plating may be applied before the temperature of the black-core malleable cast iron member drops significantly. In this case, the temperature of the black-core malleable cast iron member does not need to be uniform throughout the black-core malleable cast iron member, and the temperature of at least a portion of the surface on which the flux film is formed is kept at a predetermined temperature. It is sufficient that the temperature has been reached. However, if a part of the surface to which hot-dip plating is to be applied has not reached a predetermined temperature, there is a risk that unplated parts will occur on the surface of that part. Therefore, it is preferable that the temperature of all surfaces to be subjected to hot-dip plating reaches the above-mentioned preferred temperature range.

The time required for heating depends on the size and shape of the black core malleable cast iron member. For example, when hot-dipping a large black-core malleable cast iron component, the temperature at the center of the component should be heated for a sufficient amount of time depending on the heat capacity of the component until the temperature at the center of the component reaches the desired temperature range. It is more preferable to keep it. By doing so, it is possible to prevent the temperature of the surface of the black-core malleable cast iron member from decreasing during the hot-dip plating process, thereby preventing the occurrence of non-plating.

The above heating was carried out in order to change the iron oxide on the surface of the black-core malleable cast iron into a black product and sufficiently peel it off, as detailed below, with the aim of sufficiently suppressing the occurrence of unplated parts. It's okay. On the other hand, bubbles are likely to be generated on the surface of the black-core malleable cast iron member due to the above heating. When the bubbles are generated, depending on the shape of the black core malleable cast iron member, the pot is likely to float. Therefore, depending on the shape of the black core malleable cast iron member, it is also an embodiment of the present invention to not perform the heat treatment from the viewpoint of sufficiently suppressing the pot floating.

During hot-dip plating, the above phenomenon in which black products peel off from the surface of black-core malleable cast iron parts and float to the surface of the plating bath is caused by the black core being removed from the flux and before hot-dip plating. This tendency is particularly noticeable when the malleable cast iron member is further heated. The details of the reason are unknown, but it is probably that the surface temperature of the black-core malleable cast iron member heated to a preferred temperature range immediately after being immersed in the hot-dipping bath is lower than that of the black-core malleable cast iron member that was immersed without heating. It is presumed that this is related to the fact that it is higher than the case. In other words, when immersed without heating, when the flux on the surface of the black-core malleable cast iron comes into contact with the molten metal and decomposes, the decomposition products of the flux and the iron on the surface of the black-core malleable cast iron are mixed. The reaction rate is slow due to the low reaction temperature with the oxide. For this reason, all of the iron oxide cannot be converted into a black product, and it is thought that peeling is less likely to occur. On the other hand, when immersed in a hot-dip plating bath after heating, the reaction temperature is high and the reaction rate is fast, and the reaction between the flux decomposition products and iron oxide is completed in a short time, and the iron It is believed that the entire oxide changes to a black product that can be easily peeled off from the surface of the black-core malleable cast iron member.

In the plated black-core malleable cast iron member of the present invention, the hot-dip galvanized layer contains silicon oxide. As will be explained in detail in the examples below, it is presumed that this silicon oxide was taken into the hot-dip galvanized layer after being separated from the surface of the black-core malleable cast iron during the hot-dip galvanizing process. Since the silicon oxide contained in the hot-dip galvanized layer is separated from the surface of the black-core malleable cast iron member, it does not cause non-plating. Further, since the plated black-core malleable cast iron member of the present invention is subjected to particle projection treatment during the manufacturing process, the cast iron surface of the black-core malleable cast iron member has a process-altered region.

<Pipe fitting and its manufacturing method>
The plated black-core malleable cast iron member of the present invention includes a pipe joint. That is, the present invention may include a method for manufacturing a plated black-core malleable cast iron member, wherein the plated black-core malleable cast iron member is a pipe joint. The plated black-core malleable cast iron member according to the present invention has excellent adhesion of the plating layer formed on the surface, and can be suitably used for pipe fittings that require a high degree of corrosion resistance. When using the plated black-core malleable cast iron according to the present invention as a pipe fitting, after applying hot-dip plating, the male or female threads used to connect the fitting are machined to the ends of the pipe fitting. can be provided.

The plated black-core malleable cast iron members and pipe joints according to the present invention only need to have a hot-dip galvanized layer formed thereon, and further coated with a thermosetting resin or lining with a thermosetting resin on the hot-dip galvanized layer. , other layers may be applied by chemical conversion treatment, metal sputtering, thermal spraying, etc.

[Example 1]
A molten metal containing 3.1% by mass of carbon, 1.5% by mass of silicon, 0.4% by mass of manganese, and the balance iron and unavoidable impurities was prepared. Next, 700 kg of the prepared molten metal was poured into a ladle, 210 g (0.030% by mass) of bismuth was added, and after stirring, the molten metal was immediately poured into a mold and an elbow of the size shown in Table 3 was poured. A plurality of pipe fittings having the same shape were cast. The content of bismuth, which has a high vapor pressure, in the pipe joint was 0.020% by mass or less. After the cast pipe fittings were removed from the mold, they were lightly shot blasted to remove casting sand adhering to the surface. The maximum wall thickness of the obtained pipe joints was approximately 8 mm, and the mass per piece was approximately 900 g.

Next, the obtained pipe joint was preheated in an air atmosphere at a temperature of 275° C. or more and 425° C. or less, and then graphitized. Graphitization was performed in two stages: first graphitization, in which the casting was held at 980° C. for 90 minutes, and second graphitization, in which the temperature was lowered from 760° C. to 720° C. over 90 minutes. The temperature drop time from the end temperature of the first graphitization to the start temperature of the second graphitization was 90 minutes.

Graphitization was performed using a heat treatment furnace in which the atmosphere inside the furnace was controlled. Converter gas generated by an exothermic converter gas generator was supplied to the heat treatment furnace. The converted gas was generated by mixing combustion gas, which is a mixture of 30 vol% propane gas and 70 vol% butane gas, with air and combusting the mixture. The air mixing ratio in the mixed gas of combustion gas and air was between 95.4 vol% and 95.6 vol%.

The generated metamorphic gas was passed through a refrigeration dehydrator set at a temperature of 2° C. to remove some of the water vapor, and then supplied to a heat treatment furnace. The total pressure of the transformed gas supplied into the heat treatment furnace was atmospheric pressure. The gas in the heat treatment furnace during the first graphitization and the second graphitization was sampled from the outlet, and the gas concentration was measured using an infrared absorption type CO concentration meter and CO ₂ concentration meter, and the gas concentration was measured using a dew point meter. The dew point was measured. Table 2 shows the obtained volume percentages of CO and CO ₂ in the heat treatment furnace, the dew point, and estimated values of the oxygen partial pressure in the furnace determined by equilibrium calculation. Dew point corresponds to the amount of water contained in the gas. Note that the remainder of the gases not listed in Table 2 were hydrogen and nitrogen.

Comparing the estimated value of the in-furnace oxygen partial pressure shown in Table 2 with the equilibrium oxygen partial pressure shown in Table 1, the in-furnace oxygen partial pressure of the first graphitization is 3.4× which is the equilibrium oxygen partial pressure of chemical formula 1. While it was a value of 10 to the minus 16th power, which is the same as ^{10 −16} atm, it was several thousand times the equilibrium oxygen partial pressure of chemical formula 2, 2.6×10 ⁻¹⁹ atm. From this, it is presumed that the atmosphere of the first graphitization was non-oxidizing and had strong decarburization properties.

Next, looking at the second graphitization, the oxygen partial pressure in the furnace for the second graphitization is 10 times or less of the equilibrium oxygen partial pressure of chemical formula 1, 5.1×10 ^-21 atm, and the chemical formula 2 The value was on the order of 10 to the minus 20th power, which is higher than the equilibrium oxygen partial pressure of 2.8×10 ⁻²¹ atm. From this, it is presumed that the atmosphere of the second graphitization was non-oxidizing and decarburizing.

The surface color of the pipe fitting after graphitization was light gray. In the interior far from the surface, a typical structure of black-core malleable cast iron consisting of a ferrite matrix and lumpy graphite contained in the matrix was formed. A ferrite layer with a thickness of approximately 200 μm consisting of only a ferrite phase was formed near the surface of the pipe joint. A thin oxide layer with a thickness of about 20 μm was formed on the outermost surface of the ferrite layer.

As a comparative example, Experiment No. 3 in Table 3 was used. In Nos. 2 to 5, after the graphitization and before immersion in the flux bath, pickling was performed by immersion in an acidic solution of 10% hydrochloric acid for the time shown in Table 3.

Experiment No. in Table 3. In Nos. 6 and 7, shot blasting was performed under the following conditions as a particle projection treatment after the graphitization. On the other hand, experiment No. 3 in Table 3. In Nos. 1 to 5, flux immersion, which will be described later, was performed without shot blasting. Shot blasting was performed under the following conditions. In other words, using an apron-type shot blasting device, a workpiece (workpiece) is placed in the recess of a loop-shaped rubber apron, and the apron is rotated to change the orientation of the workpiece. A steel ball with a diameter of about 6 mm was projected toward the workpiece from a projecting means (rotating impeller). In one treatment, the amount of workpiece input was 400 kg, and the treatment time was 10 minutes.

Next, a flux raw material containing 46% by mass of zinc chloride and 54% by mass of ammonium chloride was dissolved in tap water, the concentration was adjusted so that the specific gravity at 50°C was 1.25, and the flux was heated to 90°C. I prepared a bath. The pipe fittings were then immersed in a flux bath for the times shown in Table 3. The pipe fitting taken out from the flux was inserted into the furnace chamber of a muffle furnace heated to 300° C. in an air atmosphere and heated for 5 minutes. It is estimated that the surface temperature of the pipe joint at this time was heated to 150°C or more and 200°C or less.

Thereafter, the pipe fitting was taken out of the muffle furnace, immediately immersed in a hot-dip galvanizing bath, and taken out after 1 minute, washed with water, dried, and cooled to produce a black-core malleable cast iron pipe fitting with a plating layer on the surface. The hot-dip galvanizing bath used contained 0.03% by mass of Al and the balance was Zn. The temperature of the hot-dip galvanizing bath was 500°C or higher and 520°C or lower in all cases. Then, the presence or absence of floating in the pot during immersion in the plating bath was investigated. The results are also listed in Table 3.

As shown in Table 3, Experiment No. 1 was neither pickled nor shot blasted. In No. 1, floating occurred in the plating bath. In addition, experiment No. 1 was subjected to pickling but not shot blasting. In samples 2 to 5, floating occurred in the plating bath. In addition, experiment No. Nos. 2 to 4 are comparative examples in which pipe joints of the same shape were used and the pickling time was varied, but even if the pickling time was increased, the pot floated. In these pipe fittings, as a result of the floating of the pot, non-plating was more likely to occur.

In contrast, experiment no. In Nos. 6 and 7, shot blasting was performed under the above conditions, and then the flux treatment and immersion in the plating bath were performed, so that no pot floating occurred in the plating bath. As a result, no non-plating occurred.

[Example 2]
Experiment No. in Table 3. 1 (no shot blasting) and Experiment No. 1 in Table 3. 6 (with shot blasting), using elbow-shaped pipe fittings with a nominal diameter of 3/4 inch, the immersion time in the hot-dip galvanizing bath was varied as shown in Table 4, and the number of unplated pieces was investigated. Ta. Note that the conditions were the same as in Example 1 except for the immersion time in the plating bath.

The appearance of the plating layer of the obtained pipe joint was visually observed to determine the presence or absence of so-called "non-plating" in which no galvanized layer was formed. Three pipe fittings were prepared for each condition, and the number of pipe fittings in which non-plating occurred among the three was determined. The results are shown in Table 4. In addition, in all the examples shown in Table 4, when the plating thickness was measured at 10 locations where plating was formed, it was 70 μm or more.

Table 4 shows that when mild shot blasting was performed, no plating occurred, but when shot blasting was not performed, no plating occurred even if the immersion time in the plating bath was prolonged. . Although not shown in Table 4, the immersion time to reach the target film thickness (for example, 70 μm) can be shorter when shot blasting is performed than when shot blasting is not performed. I understand. The reason for such a difference is that the surface is activated by shot blasting, and therefore plating is more likely to be formed than when shot blasting is not performed even with the same immersion time.

From the results of this example, it was found that according to the method of manufacturing a plated black-core malleable cast iron member according to the present invention, unplating could be suppressed even if the conventionally required pickling after graphitization was omitted. It can be seen that it is possible to form a good plated layer, preferably without any unplated areas. In this process, graphitization, which is essential for the production of plated black-core malleable cast iron parts, is carried out in a specific atmosphere, as well as a light particle blasting process and then immersion in flux under specific immersion conditions. This is thought to be because a black-core malleable cast iron member with a surface suitable for layer formation was obtained, and floatation in the pot was sufficiently suppressed during immersion in the plating bath, so that a plating layer was formed well during plating formation. It will be done.

[Example 3]
In order to investigate the effect of the particle projection treatment in the present invention, the metal structure near the surface of the sample after each step was observed using a scanning electron microscope.

<After graphitization>
FIG. 1A is an example of a backscattered electron image of a cross section near the surface of a black core malleable cast iron member after graphitization under the same conditions as in Example 1 and before particle projection treatment. The light gray phase shown in FIG. 1A is a matrix of ferrite. No lumpy graphite is seen in the matrix. This is considered to be because graphitization was performed in a decarburizing atmosphere, so that the chemical reaction shown in the above-mentioned chemical formula 2 progressed on the surface of the sample, and graphite disappeared. The ferrite layer made of such a ferrite matrix existed with a thickness of approximately 200 μm in the depth direction from near the surface shown in FIG. 1A.

In the region from the outermost surface of the sample shown in FIG. 1A to a depth of about 10 μm, a nearly spherical phase shown in dark gray was distributed in the ferrite matrix. The size of this near-spherical phase is approximately greater than 1 μm. In a region deeper than about 10 μm from the outermost surface of the sample, elongated phases shown in dark gray in the ferrite matrix and finely dispersed phases between the elongated phases were observed. The width of this elongated phase is finer than 1 μm, and the size of the finely dispersed phase is even smaller than that. Further, the thickness of the region where these phases were present was approximately 20 μm.

FIG. 1B is an elemental mapping image of silicon in the same region as FIG. 1A. Moreover, FIG. 1C is an elemental mapping image of oxygen in the same region as FIG. 1A. The positions where silicon and oxygen were distributed matched extremely well with the positions where the dark gray phase described above in FIG. 1A was distributed. Furthermore, according to an elemental mapping image of iron (not shown) in the same region as in FIG. 1A, iron was deficient in the areas where silicon and oxygen were enriched. From these facts, it is thought that the dark gray phase is not an iron oxide phase but a silicon oxide phase. Silicon is an element contained in black core malleable cast iron members. The elongated phase is considered to be a silicon oxide phase formed along the grain boundaries of ferrite. Moreover, the finely distributed phase is considered to be a silicon oxide phase formed within the crystal grains of ferrite.

FIG. 2 is an example of a backscattered electron image of the surface of a black-core malleable cast iron member after graphitization under the same conditions as in Example 1 and before particle projection treatment. According to an elemental mapping image (not shown) taken of the same area as in Figure 2, the light gray or dark gray area in Figure 2 is silicon oxide, and the white area and white particles are ferrite made of iron, a heavy element. It is thought that.

<After particle projection processing (shot blasting)>
FIG. 3 is an example of a backscattered electron image of a cross section near the surface of a black-core malleable cast iron member after shot blasting as a particle projection treatment was performed under the same conditions as in Example 1 and before immersion in flux. As in FIG. 1A above, the light gray phase is a ferrite matrix, and the dark gray phase is a silicon oxide phase. A flat structure was observed on the surface of the sample, and voids were observed underneath. Moreover, the nearly spherical silicon oxide phase observed in FIG. 1A was not observed. Furthermore, the thickness of the region where the silicon oxide phase is distributed is thinner than in FIG. 1A. It should be noted that the magnifications are different between FIG. 1A and FIG. 3.

FIG. 4 is an example of a backscattered electron image of the surface of a black core malleable cast iron member after shot blasting as a particle projection treatment under the same conditions as in Example 1 and before immersion in flux. The white particles of ferrite observed in FIG. 2 were hardly observed in FIG. 4, and instead, flat ferrite particles indicated by white areas were observed. In addition, cracks have occurred in the ferrite, and it is observed that many silicon oxide phases shown in dark gray are distributed in granular form in the cracked portions of the ferrite. Further, granular silicon oxide is also present on the surface of the flat portion of the ferrite.

From the observation of the photographs shown in FIGS. 1 to 4, it is clear that the ferrite and nearly spherical silicon oxide phases were distributed on the outermost surface of the black core malleable cast iron member after graphitization and before particle blasting treatment. It is thought that a portion of the area containing the area was removed by shot blasting or was crushed in the depth direction and plastically deformed. That is, the region near the surface shown in FIG. 3 is an example of the "process-affected region" in the present invention.

Further, from observation of the photographs shown in FIGS. 1 to 4, it is recognized that the silicon oxide phase is not completely removed by shot blasting, but much of it remains. In particular, it can be seen in FIG. 3 that many of the silicon oxide phases finely dispersed between the elongated phases seen in FIG. 1A remain in deep positions away from the surface. That is, in the embodiment of the present invention, as shown in the photograph exemplified above, the particle projection treatment is performed so that the silicon oxide remains on the surface. On the other hand, in the prior art, when the oxide phase or the process-affected region is removed by pickling, all silicon oxides are also removed. Therefore, the photographs shown in FIGS. 3 and 4, in which silicon oxide is present on the surface of black-core malleable cast iron parts, indicate that pickling has not been performed before immersion in flux as a pretreatment for hot-dip plating. This can be indirect evidence that there is no such thing.

<After hot-dip plating>
Figure 5 shows the backscattered electrons of a cross section including the full thickness of the plating layer of a black-core malleable cast iron member after hot-dip plating was applied under the same conditions as in Example 1, except that heating after immersion in flux was not performed. This is an example of a statue. The dark gray region from the bottom to the fourth in FIG. 5 is a cross section of the black-core malleable cast iron member, and the light gray region above it is a cross section of the hot-dip galvanized layer. The thickness of the hot-dip galvanized layer is approximately 70 μm. The boundary between these two regions is flat with no gap. According to an elemental mapping image of iron (not shown) in the same region as in FIG. was found to be almost non-existent. From this, the hot-dip galvanized layer has a region that is a solid solution of iron and zinc located near the boundary with the black core malleable cast iron surface, and a region that is a solid solution of iron and zinc that is located outside of that region where iron is slightly hardened. It is believed that there are at least two regions, one consisting of a dissolved phase.

A black phase is distributed in the region near the surface of the black-core malleable cast iron member in FIG. 5 and in the central region in the thickness direction of the hot-dip galvanized layer. According to an elemental mapping image of silicon and an elemental mapping of oxygen (not shown) in the same region as in FIG. 5, the positions where silicon and oxygen are distributed closely match the positions where the above-mentioned black phase is distributed in FIG. Ta. Furthermore, according to an elemental mapping image of zinc (not shown) in the same region as in FIG. 5, zinc was deficient in the areas where silicon and oxygen were concentrated. From these facts, it is considered that the black phase is not a zinc oxide phase but a silicon oxide phase.

Figure 6 shows the vicinity of the boundary between the cast iron surface and the hot-dip galvanized layer of a black-core malleable cast iron member after hot-dip plating was applied under the same conditions as in Example 1, except that no heating was performed after immersion in flux. This is an example of a backscattered electron image. The dark gray area at the bottom of FIG. 6 is a cross section of the black-core malleable cast iron member, and the light gray area above it is a cross section of the hot-dip galvanized layer. A black silicon oxide phase exists near the surface of the black-core malleable cast iron in contact with the hot-dip galvanized layer, and this is a structure resulting from the work-affected layer shown in FIG. 3 above. Further, in the hot-dip galvanized layer, a silicon oxide phase was present at a position slightly away from the boundary with the black core malleable cast iron surface in the upper part of FIG. On the other hand, almost no silicon oxide phase was present near the boundary with the black core malleable cast iron surface.

From observation of the photographs shown in FIGS. 5 and 6, the following can be inferred. Since the flux and plating bath do not contain compounds containing silicon, the black phase in FIGS. 5 and 6, which appears to be a phase of silicon oxide contained in the hot-dip galvanized layer, is the black core shown in FIG. 3. It is assumed that the silicon oxide present in the process-altered region of the malleable cast iron member was peeled off from the surface of the black-core malleable cast iron member during the hot-dip galvanizing process and incorporated into the hot-dip galvanized layer. .

During hot-dip galvanizing, the process-altered region has large residual stress and contains many voids, so it reacts violently with the molten zinc. During this reaction process, the oxide layer containing silicon oxide separates from the surface of the black-core malleable cast iron member and falls apart, remaining in a dispersed state in the hot-dip galvanized layer as shown in Figures 5 and 6. It is thought that this may be the case. Therefore, the presence of a dispersed silicon oxide phase in the hot-dip galvanized layer means that silicon oxide remains on the surface of the black-core malleable cast iron member after graphitization, which has not been pickled. This could be indirect evidence that a particle projection process such as shot blasting was performed in this way.

Further, as described above, the distribution of the silicon oxide phase is small near the boundary with the surface of the black core malleable cast iron. The reason for this is not clear, but it is probably that when a region of the hot-dip galvanized layer consisting of a solid solution of iron and zinc is formed, silicon oxide is not incorporated into the solid solution and is ejected into the molten zinc, and then the This is thought to be due to the fact that when the region with less zinc and more zinc solidifies, it solidifies while containing this discharged silicon oxide. The fact that there is almost no silicon oxide phase in the plating layer near the boundary with the surface of the black core malleable cast iron is thought to mean that unplating is prevented during the formation of the plating layer. .

態様３：
前記フラックスに浸漬させる黒心可鍛鋳鉄部材は、その表面にケイ素酸化物を有する態様１又は２に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様４：
前記粒子投射処理は、ショットブラスト、ショットピーニング、サンドブラスト、エアブラストのうちのいずれかである態様１～３のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様５：
前記粒子投射処理の実施時間は、３．０分以上、２０分以下である態様１～４のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様６：
前記黒鉛化を行う工程の前に、黒心可鍛鋳鉄部材を２７５℃以上、４２５℃以下の温度で予備加熱する工程を更に有する態様１～５のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様７：
前記黒鉛化を行う工程は、９００℃を超える温度で加熱する第１黒鉛化と、開始温度が７２０℃以上、８００℃以下であり、かつ完了温度が６８０℃以上、７８０℃以下である第２黒鉛化とを含む態様１～６のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様８：
前記黒鉛化を行う工程のうち、少なくとも第１黒鉛化を、非酸化性かつ脱炭性の雰囲気で行う態様７に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様９：
前記非酸化性かつ脱炭性の雰囲気は、燃焼ガスと空気との混合ガスの燃焼によって発生した変成ガスを含む態様１～８のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１０：
前記フラックスから取り出した後、黒心可鍛鋳鉄部材を加熱する工程において、前記黒心可鍛鋳鉄部材を１００℃以上、２５０℃以下に加熱する態様１～９のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１１：
前記フラックスが、弱酸性の塩化物を含有する水溶液である態様１～１０のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１２：
前記フラックスが、塩化亜鉛及び塩化アンモニウムを含有する水溶液である態様１～１１のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１３：
前記溶融めっきを施す工程が、溶融亜鉛めっきを施す工程を含む態様１～１２のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１４：
前記黒心可鍛鋳鉄部材が、管継手である態様１～１３のいずれか１項に記載のめっき形成黒心可鍛鋳鉄部材の製造方法。
態様１５：
黒心可鍛鋳鉄部材の表面にめっき層が形成されためっき形成黒心可鍛鋳鉄部材であって、
前記めっき層が溶融亜鉛めっき層であり、
前記黒心可鍛鋳鉄部材の鋳鉄表面に加工変質領域を有し、かつ
前記溶融亜鉛めっき層にケイ素酸化物が含まれるめっき形成黒心可鍛鋳鉄部材。
態様１６：
管継手である態様１５に記載のめっき形成黒心可鍛鋳鉄部材。The disclosure content of this specification includes the following aspects described in Japanese Patent Application No. 2019-053581, which is the basis for claiming priority.
Aspect 1:
A method for producing a plated black-core malleable cast iron member, wherein a plating layer is formed on the surface of the black-core malleable cast iron member,
a step of graphitizing in a non-oxidizing and decarburizing atmosphere;
a step of performing particle projection treatment on the surface of the black-core malleable cast iron member after graphitization;
immersing the black-core malleable cast iron member after the particle projection treatment in flux for 3.0 minutes or more;
heating the black core malleable cast iron member to 90°C or higher after removing it from the flux;
A method for producing a plated black-core malleable cast iron member, comprising the step of applying hot-dip plating to the heated black-core malleable cast iron member.
Aspect 2:
In Embodiment 1, the non-oxidizing and decarburizing atmosphere is an atmosphere in which the oxygen partial pressure is 10 times or less the equilibrium oxygen partial pressure of the following chemical formula 1, and higher than the equilibrium oxygen partial pressure of the following chemical formula 2. The method for manufacturing the plated black-core malleable cast iron member described above.

Aspect 3:
The method for manufacturing a plated black-core malleable cast iron member according to aspect 1 or 2, wherein the black-core malleable cast iron member immersed in the flux has silicon oxide on its surface.
Aspect 4:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 3, wherein the particle projection treatment is any one of shot blasting, shot peening, sandblasting, and air blasting.
Aspect 5:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 4, wherein the particle projection treatment is performed for a period of 3.0 minutes or more and 20 minutes or less.
Aspect 6:
The plated black core according to any one of aspects 1 to 5, further comprising a step of preheating the black core malleable cast iron member at a temperature of 275° C. or higher and 425° C. or lower before the graphitizing step. A method for manufacturing malleable cast iron parts.
Aspect 7:
The graphitization step includes a first graphitization step in which heating is performed at a temperature exceeding 900°C, and a second graphitization step in which the starting temperature is 720°C or more and 800°C or less, and the completion temperature is 680°C or more and 780°C or less. The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 6, including graphitization.
Aspect 8:
The method for manufacturing a plated black-core malleable cast iron member according to aspect 7, wherein at least the first graphitization of the graphitization step is performed in a non-oxidizing and decarburizing atmosphere.
Aspect 9:
In the plated black-core malleable cast iron member according to any one of aspects 1 to 8, the non-oxidizing and decarburizing atmosphere contains a metamorphosed gas generated by combustion of a mixed gas of combustion gas and air. Production method.
Aspect 10:
The plating according to any one of aspects 1 to 9, wherein in the step of heating the black core malleable cast iron member after taking it out from the flux, the black core malleable cast iron member is heated to 100 ° C. or higher and 250 ° C. or lower. A method of manufacturing a formed black core malleable cast iron member.
Aspect 11:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 10, wherein the flux is an aqueous solution containing a weakly acidic chloride.
Aspect 12:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 11, wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride.
Aspect 13:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 12, wherein the step of applying hot-dip plating includes the step of applying hot-dip galvanization.
Aspect 14:
The method for producing a plated black-core malleable cast iron member according to any one of aspects 1 to 13, wherein the black-core malleable cast iron member is a pipe joint.
Aspect 15:
A plated black core malleable cast iron member in which a plating layer is formed on the surface of the black core malleable cast iron member,
The plating layer is a hot-dip galvanized layer,
A plated black-core malleable cast iron member, wherein the cast iron surface of the black-core malleable cast iron member has a process-altered region, and the hot-dip galvanized layer contains silicon oxide.
Aspect 16:
The plated black core malleable cast iron member according to aspect 15, which is a pipe joint.

This application claims priority to the Japanese patent application, Japanese Patent Application No. 2019-053581, as the basic application. Japanese Patent Application No. 2019-053581 is incorporated herein by reference.

Claims (14)

A method for producing a plated black-core malleable cast iron member, wherein a plating layer is formed on the surface of the black-core malleable cast iron member,
a step of graphitizing in a non-oxidizing and decarburizing atmosphere;
A step of performing particle projection treatment on the surface of the black-core malleable cast iron member after graphitization so that silicon oxide remains on the surface;
immersing the black-core malleable cast iron member after the particle projection treatment in flux for 3.0 minutes or more;
a step of hot-dipping the black-core malleable cast iron member after immersion in the flux ;
The non-oxidizing and decarburizing atmosphere is an atmosphere in which the oxygen partial pressure is 10 times or less the equilibrium oxygen partial pressure of the following chemical formula 1 and higher than the equilibrium oxygen partial pressure of the following chemical formula 2. A method for manufacturing core malleable cast iron parts.

The method for manufacturing a plated black-core malleable cast iron member according to claim 1 , wherein the particle projection treatment is one of shot blasting, shot peening, sandblasting, and air blasting. The method for manufacturing a plated black-core malleable cast iron member according to claim 1 or 2, wherein the particle projection treatment is performed for a period of 3.0 minutes or more and 20 minutes or less. The plating-forming black according to any one of claims 1 to 3 , further comprising a step of preheating the black core malleable cast iron member at a temperature of 275° C. or more and 425° C. or less before the graphitizing step. A method for manufacturing core malleable cast iron parts. The graphitization step includes a first graphitization step in which heating is performed at a temperature exceeding 900°C, and a second graphitization step in which the starting temperature is 720°C or more and 800°C or less, and the completion temperature is 680°C or more and 780°C or less. 5. The method for producing a plated black-core malleable cast iron member according to any one of claims 1 to 4 , comprising graphitization. 6. The method for manufacturing a plated black-core malleable cast iron member according to claim 5 , wherein at least the first graphitization of the graphitization steps is performed in a non-oxidizing and decarburizing atmosphere. The plated black-core malleable cast iron member according to any one of claims 1 to 6 , wherein the non-oxidizing and decarburizing atmosphere contains a metamorphosed gas generated by combustion of a mixed gas of combustion gas and air. manufacturing method. The method for producing a plated black-core malleable cast iron member according to any one of claims 1 to 7 , further comprising the step of heating the black-core malleable cast iron member to 90° C. or higher after being removed from the flux. The method for producing a plated black-core malleable cast iron member according to any one of claims 1 to 8 , wherein the flux is an aqueous solution containing a weakly acidic chloride. The method for producing a plated black-core malleable cast iron member according to any one of claims 1 to 9 , wherein the flux is an aqueous solution containing zinc chloride and ammonium chloride. The method for manufacturing a plated black-core malleable cast iron member according to any one of claims 1 to 10 , wherein the step of applying hot-dip plating includes the step of applying hot-dip galvanization. The method for manufacturing a plated black-core malleable cast iron member according to any one of claims 1 to 11 , wherein the black-core malleable cast iron member is a pipe joint. A plated black core malleable cast iron member in which a plating layer is formed on the surface of the black core malleable cast iron member,
The plating layer is a hot-dip galvanized layer,
A plated black-core malleable cast iron member, wherein the cast iron surface of the black-core malleable cast iron member has a work-altered region formed by particle projection treatment , and the hot-dip galvanized layer contains silicon oxide. The plated black-core malleable cast iron member according to claim 13 , which is a pipe joint.

Images