JP6473192B2 - Spheroidal graphite cast iron and method for producing the same - Google Patents

Description

  The present invention relates to spheroidal graphite cast iron and a method for producing the same.

  Spheroidal graphite cast iron has excellent strength and contains graphite, so it has superior machinability, damping capacity and wear resistance compared to steel, and is widely used in various applications such as automobile parts, machine parts, civil engineering parts, etc. Has reached the point.

  Conventional high strength spheroidal graphite cast iron has a limit of about 800 to 900 MPa in tensile strength in the as-cast state, and a limit of about 900 to 1000 MPa in tensile strength even after normalization.

For example, in JP2008-229640A (Patent Document 1), C: 3.2 to 4.2 mass%, silicon: 1.9 to 4.5 mass%, manganese: 0.5 mass% Hereinafter, spheroidal graphite cast iron obtained by pouring and solidifying a molten spherical graphite containing phosphorus: 0.08% by mass or less, sulfur: 0.03% by mass or less, and 0.02-0.1% by mass of magnesium into a sand mold and the temperature is cooled in a sand mold until it reaches the a ₁ transformation point, tensile strength of the material obtained by the method of performing the predetermined cooling with disassembling the mold after the a ₁ transformation point reached (MPa) 463, Vickers strong An example of (HV) 161 is described.

  Also, for example, in the test in “Technical Contact 1999 Summary” (Fatigue characteristics of high-strength graphite cast iron) (Non-Patent Document 1), the tensile strength when normalizing cast iron of FCD700 is 843 MPa, The fatigue strength is 305 MPa, and the hardness is HRC27 (about 265 in terms of HB).

  In addition, “New version of cast iron material” (published by Japan Foundry Engineering Society 2012) (Non-Patent Document 2) states that “relation between pearlite amount and mechanical properties of spheroidal graphite cast iron”. However, the tensile strength is up to 900 MPa and no further strength is shown. This graph is shown in FIG.

Japanese Patent Laid-Open No. 2002-317219 (Patent Document 2) describes C: 3.20 to 4.00% by mass ratio, Si: 2.00 to 3.20%, and Mn: 0.3 to 2. 50%, P: 0.035% or less, S: 0.12% or less, Cu: 0.30 to 3.50%, Mg: 0.02 to 0.08%, rare earth element: 0.005 to 0. 30%, balance Fe, and cooling of the molten metal is promoted by any of the in-mold method, cooling metal, gas suction from the mold, cooling gas injection into the mold, or a combination thereof at the time of casting. A cast iron product having a metal structure including a petal-like structure in which ferrite and pearlite are intricately disclosed is disclosed as having a strength of 900 to 1000 N / mm ² as cast. In the in-mold method used therefor, Si is less effective if it is less than 0.01%, and if it exceeds 0.4%, graphite grows, so 0.01-0.4%. However, a tensile strength greater than 1000 N / mm ² is not obtained with this method.

Further, in the in austempered cast iron in JIS G 5503, tensile although strength is 900~1400N / mm ² and the normalized, heated austempering the austenite region after casting, and quenched in a salt bath After isothermal transformation by holding for a predetermined time, air cooling and a very complicated process are required. In addition, austempered spheroidal graphite cast iron is extremely poor in machinability, so its use is limited, and expensive heat treatment is a cause of cost increase.

  Then, the solution of the weight reduction by further high intensity | strength is an important subject. As a countermeasure, Mo or Ni, V or Nb alone or in combination is proposed. For example, JP-A-1997-087797 (Patent Document 3) discloses spheroidal graphite cast iron containing Ni: 1.0 to 4.0% and Mo: 0.1 to 2.0%. However, the use of expensive alloy elements increases the cost, and is not practical because it becomes a base structure with inferior machinability in which bainite is mixed.

JP 2008-229640 A JP 2002-317219 A JP 1997-087797 A

1999 Issue Technical Meeting Summary of 1999 Presentation (Fatigue Properties of High-Strength Graphite Cast Iron) Japan Casting Engineering Society 2012 issue New material of cast iron

  As described above, the secondary inoculation of the in-mold method, the promotion of cooling, and the normalizing may be used alone or in combination of two types, and thereby a spheroidal graphite cast iron product having a tensile strength of 1080 MPa or more. It was difficult to cast.

  Accordingly, an object of the present invention is to provide a spheroidal graphite cast iron having an even higher strength than the spheroidal graphite cast iron, that is, a super high strength spheroidal graphite cast iron having a tensile strength of 1080 MPa or more, and a casting method thereof.

  The present invention as a means for solving the above problems is the selection of the chemical structure of spheroidal graphite cast iron, review of the addition amounts of Si and P, Cu and Sn, and the implementation and addition of secondary inoculation of Fe-Si. Normalization is carried out as a treatment of, specifically, in mass%, C: 3.0 to 4.0%, Si: 1.5 to 2.5%, Mn: 1.0 % Or less, P: 0.04% or less, S: 0.03% or less, Mg: 0.02-0.1%, Cu: 1.8-3.3%, Sn: 0.01-0.05 In a molten metal having a chemical composition composed of the remaining Fe and inevitable impurities, Fe-Si is inoculated with 0.05 to 0.15 mass% in terms of Si equivalent, casted, and released at room temperature. A method for producing spheroidal graphite cast iron, characterized by performing austenitization by reheating and subsequent cooling.

  Further, in the above method for producing spheroidal graphite cast iron, the austenitizing by reheating and the subsequent cooling are performed by normalization.

  In the method for producing spheroidal graphite cast iron, the cooling after the reheating is performed by forced air cooling.

Further, in terms of mass%, C: 3.0 to 4.0%, Si: 1.5 to 2.5%, Mn: 1.0% or less, P: 0.04% or less, S: 0.03% Hereinafter, Mg: 0.02 to 0.1%, Cu: 1.8 to 3.3%, Sn: 0.01 to 0.05%, and a chemical composition consisting of the remainder Fe and inevitable impurities. And a spheroidal graphite cast iron having a tensile strength of 1080 MPa or more.

  According to the present invention, an increase in the number of graphite grains could be promoted by performing secondary inoculation such as an in-mold method during pouring. However, although the ferrite formation is promoted and the strength is lowered as it is, the ferrite is reduced by normalization, and thereby a denser and more stubborn pearlite structure can be obtained. As the cooling rate increased, ferrite decreased, the pearlite structure increased, and high strength spheroidal graphite cast iron having a tensile strength exceeding 1080 MPa to 1300 MPa could be obtained.

  However, the method has the composition of the appropriate component range of Cu, Sn, Si, P, the step of casting by secondary inoculation of Fe-Si, mold release, air cooling after austenization by reheating after normal temperature High strength spheroidal graphite cast iron having a tensile strength of 1080 MPa or more could be obtained by an extremely simple method of an innovative combination of normalizing processes.

It is a photograph of this invention metal structure. It is a related figure about the mechanical property regarding the material of cast iron.

  The base structure of normal spheroidal graphite cast iron is a mixed type of ferrite and pearlite, and when it is cooled from austenite, various transformations occur depending on the cooling rate, and the pearlite transformation, which is one of the transformations, is at a slow cooling rate. It is a metastable co-folding transformation that appears in the surface, it tends to be pearlite when the cooling rate is high in the co-folding transformation, and the influence of additive elements on the as-cast ferritization of Mg-treated spheroidal graphite cast iron is moderate. Is known.

  Hereinafter, the present invention will be described in detail. The present invention is mass%, C: 3.0-4.0%, Si: 1.5-2.5%, Mn: 1.0% or less, P: 0.04% or less, S: 0.03 %, Mg: 0.02 to 0.1%, Cu: 1.8 to 3.3%, Sn: 0.01 to 0.05%, and the chemical composition comprising the balance Fe and inevitable impurities , Fe-Si is 0.05 to 0.15% by mass in terms of Si equivalent, cast after secondary inoculation, mold release, normal temperature, austenite by reheating and subsequent cooling This is a method for producing graphite cast iron. The content of Si and P is regulated, and Fe-Si is 0.05 to 0.15 mass% in Si equivalent at the time of pouring into a molten metal having a high chemical content of Cu and a small amount of Sn, and secondary inoculation is performed. Thus, further increasing the number of graphite grains is a major feature of the present invention. Although the method of secondary inoculation is not particularly limited, an in-mold method or a pouring flow inoculation can be used.

  The chemical composition of the spheroidal graphite cast iron of the present invention will be described in detail. The constitution of C: 3.0 to 4.0% by mass, Mn: 1.0% by mass or less, and S: 0.03% by mass or less is a general composition of spheroidal graphite cast iron. Regarding the chemical composition, an important component of the present invention is the chemical components of Si, P, Cu, and Sn. In addition to the importance of the amount of these single chemical components, their mutual relationship is important. The effects of these chemical components will be described in detail below.

  The critical amount of Cu varies depending on the researcher and is said to be 1.5 to 2.2%. If the Cu content exceeds that, it segregates at the austenite grain boundaries and crystallizes irregular graphite to inhibit spheroidization. In addition, it has been pointed out that addition of 2% or more also increases the tendency to chill. On the other hand, the present inventor is Japanese Patent No. 3723706, and by adding a small amount of Sn to high Cu added exceeding the critical amount, the graphite shape and the base structure are improved, there is no chill, tensile strength and elongation are improved. It has been disclosed that it is greatly improved and has been put into practical use. In the present invention, about Cu, this component range is almost applied and the content is set to 1.8 to 3.3% by mass. The present inventors have clarified that the action effect of Cu is distributed around the precipitated graphite, suppresses the growth (enlargement of diameter) of graphite, and creates spherical graphite with high sphericity. Yes.

  The upper limit of Si was regulated to 2.5% by mass and the upper limit of P was regulated to 0.04% by mass. When the blending amount was larger than that, the step of air cooling after austenitization, that is, normalizing This is because both the hardness and strength of spheroidal graphite cast iron are reduced (described in Table 1).

  The action of Sn has the effect of reducing the pearlite interlayer distance when Cu is lower than 1.8 mass%. Furthermore, when Cu is 1.8% by mass or higher Cu as in the present invention, there is an effect of concentrating in the vicinity of graphite to suppress the collapse of the graphite and to increase the spheroidization. Due to the presence of Sn, a thin film of Cu—Sn is formed on the surface of the graphite grains accompanying with the pro-eutectoid Cu, and this inhibits the precipitation of C atoms to the graphite grains, thereby suppressing the decomposition of pearlite and reducing the graphite shape. I was able to prevent it. If the action is less than 0.01% by mass, the effect cannot be obtained, and if it is more than 0.05% by mass, the graphite shape collapses conversely.

  Comprehensively explaining the effect of the composition of the chemical composition, the number of graphite particles increases due to the effect of creating a highly sphericity spherical graphite of a special material by combining a large amount of Cu and a small amount of Sn. Collapse is suppressed. As a result of the restriction of the upper limit values of Si and P suppressing the strength reduction, it is possible to increase the strength.

The method for producing the spheroidal graphite cast iron is not limited to this method, but the sandwich method using a graphite spheroidizing agent for Fe-Si-Mg-RE alloy by adding Cu and Sn to the molten metal using steel scrap and return material. Spheroidizing treatment, etc., temporarily inoculate 0.2% by mass of Fe-Si with Si equivalent to form a molten metal having the above composition, and 0.1% by mass of Fe-Si with Si equivalent at the time of casting (in-mold) ) Secondary inoculation by the method of pouring and pouring, etc., and sand casting. After the cast iron has completed the A ₁ transformation, it performs type shakeout, to room temperature by cooling. Thereafter, normalization, that is, austenitization by reheating and subsequent cooling are performed. In addition, it is good also as forced air cooling using a fan etc. instead of cooling by as-casting.

  In the chemical composition of the present invention, Fe-Si is added at a Si equivalent by 0.05 to 0.15 mass% by in-mold method or the like, and further increase in the number of graphite grains is promoted. . However, the increase in the number of graphite grains promotes the ferritization of the base structure, and as it is, the strength is lowered. However, normalization increases the cooling rate of cast iron and promotes pearlite formation. A dense and robust pearlite structure can be obtained by combining Sn, Si, and P with the chemical component Cu of the cast iron at the top. The improvement in the strength of pearlite due to the cooling rate by normalizing is far superior to the reduction in strength due to the promotion of ferritization by secondary inoculation of Fe-Si.

Disassembling the mold is an operation of breaking the mold and removing the sand adhering to the casting after casting, and is also called an open frame. Note that the mold release includes disassembly of the entire mold and partial disassembly of the mold. The timing of the general type disassembling, it is usual cast spheroidal graphite cast iron is the give away type after completing the A ₁ transformation. Because the temperature difference in contact with the atmosphere occurs in the spheroidal graphite cast iron member due to the timing of mold release, the base structure is likely to vary, it is difficult to stabilize the quality, and strict adjustment of the timing of mold release is required. That's why.

  In the present invention, the secondary chemical inoculation of Fe-Si with the above-described chemical composition of spheroidal graphite cast iron is performed, and after casting, sand mold cooling, mold release, normalizing after normal temperature, Can be obtained by reducing the ferrite and increasing the pearlite by performing austenite formation by reheating and then cooling to accelerate the pearlite formation. The cooling after austenitizing is preferably forced air cooling by fan air cooling or the like. This is because by forced air cooling, the cooling rate is increased and the strength of the spheroidal graphite cast iron is further increased.

  In the chemical composition of the present invention, Fe-Si is added at a Si equivalent by 0.05 to 0.15 mass% by in-mold method or the like, and further increase in the number of graphite grains is promoted. . Fe-Si Si inoculation amount is less effective if it is less than 0.05%, and if it exceeds 0.15%, the effect is saturated, and the inoculant is likely to remain undissolved, resulting in inclusion defects. Therefore, the Si equivalent was set to 0.05 to 0.15 mass%. However, the increase in the number of graphite grains promotes the ferritization of the base structure, and as it is, the strength decreases, but the ferrite decreases by normalization, and a more compact and robust pearlite structure is obtained. The strength improvement of pearlite formation by normalization is far superior to the strength reduction by ferrite formation by secondary inoculation.

  A comparative experiment between the spheroidal graphite cast iron of the present invention and the spheroidal graphite cast iron of the prior art component or manufacturing method was performed using a test piece. The manufacturing method of a test material is created based on JIS G5502, and the shape of the test material is Y type B. Examples and comparative examples are shown in Table 1. The chemical composition, secondary inoculation, and cooling treatment were performed according to the conditions shown in Table 1.

  According to the conditions of the present invention 1 in Table 1, a 1000 kg capacity high frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2% of Si-equivalent Fe-Si was inoculated. Table 1 shows the results obtained by sand casting cast iron subjected to secondary inoculation by 0.1% in-mold method with Fe-Si at Si equivalent at the time of casting, and normalizing at 950 ° C. by a conventional method. To A photograph of the metal structure is shown in FIG. Tensile strength exceeds 1200 MPa, 0.2% proof stress is 885 MPa, ultrahigh strength, and elongation is 3%. What is overwhelmingly stronger than Comparative Example 3 is the difference in chemical components of Si, P, Cu, and Sn and the effect of in-mold inoculation.

  In accordance with the conditions of Invention 2 in Table 1, a 1000 kg capacity high frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2% of Si-equivalent Fe-Si was inoculated. The results of casting cast iron, which was subjected to secondary inoculation by 0.1% in-mold method with Fe-Si at the Si equivalent at the time of casting, and normalized at 860 ° C. by a conventional method are shown in Table 1 of the present invention 2. I write. Even when the normalizing temperature is 860 ° C., the strength is extremely high. It is a dense and robust pearlite structure with reduced ferrite.

  In accordance with the conditions of Invention 3 in Table 1, a 1000 kg capacity high frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2% of Si-equivalent Fe-Si was inoculated. The cast iron which was subjected to secondary inoculation by 0.1% in-mold method with Fe-Si at the equivalent of Si at the time of casting was cast into sand mold and normalized at 910 ° C. by a conventional method. I write. The fatigue strength is 420 MPa. It can be seen that when the Si content is decreased, the strength becomes higher.

  In accordance with the conditions of Invention 4 in Table 1, a 1000 kg capacity high frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2% of Si-equivalent Fe-Si was inoculated. Cast iron secondary inoculated with 0.1% in-mold method with Fe-Si at the time of casting at the time of casting was cast into sand mold, normalized by a conventional method, and the result of fan air cooling (forced air cooling) at 910 ° C. It describes in this invention 4 of Table 1. When the fan is air-cooled during normalization, the cooling rate is further increased and the strength of the spheroidal graphite cast iron is further increased. The difference in strength from the normalizing fan air cooling of the FCD700 material of Comparative Example 1 is remarkable.

  As a comparative example, the test results in normalization (fan air cooling) with high-strength spheroidal graphite cast iron shown in Non-Patent Document 1 (Technical Contact 1999 Summary of Presentations) are shown in Comparative Example 1 in Table 1. The tensile strength of the spheroidal graphite cast iron of Comparative Example 1 is 843 MPa.

  As another comparative example, in accordance with the conditions of Comparative Example 1 in Table 1, a 1000 kg capacity high-frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2 mass% of Fe-Si was inoculated with Si equivalent. The results of casting in JIS Y type B sand mold test pieces and normalizing at 930 ° C. are shown in Comparative Example 2 in Table 1. The spheroidal graphite cast iron of Comparative Example 2 has a chemical composition of 2.6 mass% Cu and 0.03 mass% Sn, and a tensile strength of 1040 MPa.

  As another comparative example, according to the conditions of Comparative Example 2 in Table 1, a 1000 kg capacity high-frequency induction furnace was used to make a molten metal equivalent to FCD700 using steel scraps and return materials. Cu and Sn were added to this molten metal, and a spheroidizing treatment was performed by a sandwich method using a graphite spheroidizing agent of a commercially available Fe-Si-Mg-RE alloy, and 0.2 mass% of Fe-Si was inoculated with Si equivalent. The results of casting in JIS Y type B sand mold test pieces and normalizing at 930 ° C. are shown in Comparative Example 3 of Table 1. The spheroidal graphite cast iron of Comparative Example 3 has a chemical composition of Cu 1.6 mass% and Sn 0.06 mass%, and a tensile strength of 985 MPa.

  According to the present invention as described above, an ultra-high-strength as-cast spheroidal graphite cast iron having a tensile strength of 1080 MPa or more can be obtained by a very simple method. For this reason, the range of use of conventional spheroidal graphite cast iron has been greatly expanded, and for example, it has become possible to manufacture even products that could only be manufactured with forged products. In addition, it can be used as a member that could only be manufactured with forged steel and cast steel such as various gears, pins, and plates, and can be used as an extremely large number of parts and members.

Claims (3)

In mass%, C: 3.0 to 4.0%, Si: 1.5 to 2.1 %, Mn: 1.0% or less, P: 0.04% or less, S: 0.03% or less, Mg: 0.02 to 0.1%, Cu: 1.8 to 3.3%, Sn: 0.01 to 0.05%, a molten metal having a chemical composition consisting of the remainder Fe and inevitable impurities, Spheroidal graphite cast iron characterized in that Fe-Si is casted by secondary inoculation at 0.05 to 0.15% by mass with Si equivalent, separated into molds, brought to room temperature, austenitized by reheating and then cooled. Manufacturing method.   2. The method for producing spheroidal graphite cast iron according to claim 1, wherein the austenitization by the reheating and the subsequent cooling are performed by normalization.   The method for producing spheroidal graphite cast iron according to claim 1 or 2, wherein the cooling after the reheating is performed by forced air cooling.