JPS5917185B2 - Spheroidal graphite cast iron and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to spheroidal graphite cast iron whose matrix structure is a mixed structure of fine pearlite and ferrite (hereinafter referred to as fine pearlite base), and a method for producing the same.

Spheroidal graphite cast iron, whose matrix structure is pearlite, has been manufactured and used for a long time because it can be easily obtained in an as-cast state.

In this case, in order to obtain pearlitic terrestrial graphite cast iron, the amount of Mn was increased or decreased depending on the desired hardness and product wall thickness. Since it is an element that promotes precipitation, when the Mn content increases, the amount of pearlite in the base structure increases, and carbide crystals (precipitation) also occur, locally increasing hardness, becoming brittle, and reducing machinability. There were problems such as extreme deterioration.

Therefore, in order to prevent the crystallization (precipitation) of carbides when manufacturing pearlitic terrestrial graphite cast iron used in the as-cast state, it is generally necessary to increase the Mn content and at the same time suppress the precipitation of carbide products (precipitation). The amount of Si, which is an element, is also increased (2.5~
3.0%).

Pearlitic terrestrial graphite cast iron produced by such conventional methods is susceptible to the cooling rate in the casting mold and other variable factors due to its high Mn and Si contents, making it difficult to produce the same product. It was not possible to form a uniform pearlite structure even within the grain.

That is, even within the same product, where the wall thickness is thin and the cooling rate is fast, the material becomes completely pearlite and has high hardness.

On the other hand, in areas where the cooling rate is slow, such as in thick-walled parts, a pearlite texture containing a large amount of ferrite is formed, and the hardness is low and there is a strong tendency to become soft.

Furthermore, there are various variable factors such as differences in the spheroidization processing conditions and inoculation effects between each heat, differences in the cooling time in the mold from the end of pouring for each framework to the mold release, and differences in inoculation effects due to fading phenomenon.

When mass-producing the same product, there is usually large variation in the structural hardness of each product, and the material itself lacks reliability.

This is because these materials simultaneously contain a large amount of elements that have opposite effects to Si, which is an element that promotes ferrite formation, and Mn, which is an element that promotes pearlite formation.

Pearlitic terrestrial graphite cast iron, which has traditionally been produced as-cast, has low manufacturing costs but has large variations in structure and mechanical properties, so heat treatment is required for castings that require high reliability. Improvements in material quality have been made.

The heat treatment method in this case was to heat and maintain the casting in the austenite region at 900° C. or higher to uniformly austenite each part of the casting, and then cool it at an appropriate cooling rate.

The reason why it is necessary to completely austate at a high temperature of 900° C. or higher is that when conventional pearlitic terrestrial graphite cast iron is in an as-cast state, the amount of pearlite varies depending on each part of the casting and each casting.

High-temperature heating of 900° C. or higher not only increases heat treatment cost but also causes distortion during heat treatment, which is not at all preferable.

In addition, the conventional method for making the base structure into a fine pearlite ground corresponds to rapid cooling from point A in the iron-carbon equilibrium phase diagram shown in Figure 1, and the austite at this time contains more than the amount of eutectoid carbon. Therefore, during rapid cooling, fine pro-eutectoid cementite is crystallized (precipitated) near the grain boundaries on the AC'IrL line, or supersaturated carbon is contained in pearlite, which causes a decrease in toughness.

In view of the above points, the present invention heats homogeneous pearlitic terrestrial graphite cast iron obtained by as-casting to a temperature range of 750 to 850°C where α phase and γ phase coexist, and at that temperature, α phase and γ phase coexist. The present invention aims to provide fine pearlite terrestrial graphite cast iron with excellent toughness and uniform pearlite fineness by holding it for an appropriate time without reaching an equilibrium state and rapidly cooling it from that temperature, and a method for producing the same. be.

According to the present invention, since the maximum heating temperature is significantly lower than that of conventional methods, not only is the heat treatment cost reduced, but there is no risk of heat treatment distortion, and the resulting structure can be cooled from point B in Figure 1. This corresponds to a uniform fine pearlite base that does not contain supersaturated carbon or pro-eutectoid cementite, and can be made to be highly tough.

In order to obtain a uniform fine pearlite base in each part of the casting and each cast article through relatively simple heat treatment, it is necessary to have a uniform pearlite structure in the as-cast state, which is not possible in the conventional method. This is difficult to achieve with spheroidal graphite cast iron having a high content of Si and Mn.

Therefore, in order to obtain a uniform pearlite structure in the as-cast state, which is the first condition of the present invention, the content of Mn, Cr, V, Mo, etc., which are pearlitization stable elements and carbide forming elements, is It is characterized in that the content of Si, which is an element that promotes ferrite formation, is kept below the limit of mustard, and the content of Si, which is an element that promotes ferrite formation, is as low as possible, and an appropriate amount of Sn, which is an element that promotes pearlite formation that does not have a tendency to form carbides, is contained.

If the Si content is high, the amount of pearlite as cast and after heat treatment will be easily affected by the cooling rate, so it must be kept as low as possible. If the Si content is below, the tendency of carbide formation increases in the thin wall portion, making it difficult to reduce the amount of Si.

Therefore, as mentioned above, it is necessary to control the content of the elements that are precipitated in the carbide product (precipitated).

The present invention will be explained in detail below based on examples.

Example 1 - Mix and melt 40 parts of spheroidal graphite cast iron returned scraps, 20 parts of Sorel Metal (trade name), and 40% of steel scraps in a low frequency furnace, raise the temperature,
After adjusting the components, a spheroidizing treatment was performed with Fe-8Fe-8i-%Mg) at the time of tapping.

Thereafter, the sample was inoculated with Fe-8i at a rate of 0.6, and plate-shaped test pieces having different wall thicknesses were cast into a sand mold having cavities arranged radially.

The steel scrap used was commercially available high quality steel scrap.

The chemical components at this time are shown in Table 1, and FIGS. 2 and 3 show the microscopic structure of the plate having a thickness of 10 m7ft and 70rIL7Il.

As shown in Figures 2 and 3, the plate thickness is 10mff1.7
The structure at 0 mm is a mixture of pearlite and ferrite, and the difference in hardness is 27 in terms of Brinell hardness, but in the case of normal spheroidal graphite cast iron with high Mn and Si content, this difference is about 60.

Furthermore, as can be seen from the analysis results in Table 1, Mn, which is contained in a large amount in ordinary pearlite terrestrial graphite cast iron, has a very low content of 0.17%.

Furthermore, Cr segregates at the eutectic cell boundaries and tends to crystallize (precipitate) carbides.

By setting the total amount of Mo, V, etc. to 0.046%, it was possible to prevent carbide crystallization (precipitation) even when the Si amount was 2.03%.

In this case, if the Si content is 2.2% or more and Mn is contained in an amount of 0.30%, the influence of the cooling rate of the casting becomes significant and the difference in hardness depending on the thickness of the casting increases, so it is necessary to keep the Si content below this value.

Further, the limit content for crystallizing (precipitating) individual carbides of each of the unavoidably contained components is Cr.

(2) is 0.10%, and Mo is 0.25%.

Cu itself is not an element that promotes carbide precipitation, but C
If the total amount of r, V, Mo, and Cu exceeds 0.50%, carbide precipitation will become significant even if the content of each element is below the limit, so do not exceed this amount. It won't happen.

Furthermore, in order to uniformly transform the base structure into pearlite by incorporating Sn, the content of other pearlite-promoting elements must be kept low, otherwise the synergistic effect will cause variations in the base structure.

For example, Cu has almost no effect of promoting pearlite formation up to 0.20%, but when it exceeds 0.25%,
Its effects are rapidly increasing.

Therefore, if 0.050% Sn is added to a material with a Cu content of 0.10% and a material with a Cu content of 0 and 30%, the latter will promote pearlite formation, have higher hardness, and have higher tensile strength. Become something.

This is due to the synergistic effect of the pearlitization promoting effects of Cu and Sn.In order to obtain a uniform pearlite structure with hardness with little variation between heats by adding Sn, the Cu content must be 0.20% or less. need to be kept.

Furthermore, the effect of Mn on promoting the precipitation of carbide products varies depending on the wall thickness of the casting, but in thin-walled parts where the cooling rate is fast, the effect is 0.
If it exceeds 30%, the effect becomes noticeable, so it is necessary to keep it below this range.

The amount of Sn required to obtain a uniform pearlite structure within such a component range varies somewhat depending on the thickness of the casting and the target hardness, but normally, if the Sn amount is less than 0.025%, the pearlite formation promotion effect decreases. A sufficient pearlite structure cannot be obtained even in thin-walled parts.

Further, the upper limit of 0.20% is the amount necessary to form a complete pearlite structure in the thick portion, and there is no need to contain more than that.

The thus obtained casting with a uniform pearlite structure was heated to 800°C, which is the coexistence temperature range of α and γ phases, and then
FIG. 4 shows the structure obtained by holding the sample for a certain period of time, taking it out of the furnace, and then rapidly cooling it by blast cooling (forced air cooling).

The pearlite structure in FIG. 4 is clearly finer and more uniformly dispersed than in FIGS. 2 and 3.

In addition to blast cooling, liquid quenching methods are available as rapid cooling methods.
Even in that case, the same effect can be obtained.

Spheroidal graphite cast iron in which pearlite is finely divided and uniformly dispersed has mechanical properties as shown in Table 2.

It can be seen that the spheroidal graphite cast iron according to the present invention has excellent elongation and impact properties, and is a material with high toughness.

Example 2 As-cast pearlite terrestrial graphite cast iron obtained from the same molten metal as in Example 1 was heated to 840°C, which is the coexistence temperature range of the α phase and γ phase, and held for 3 hours, and then oil quenched after furnace removal. The structure when this casting was tempered at 500°C is shown in Figure 5, and the mechanical properties are shown in Table 3.

The pearlite in the base structure is further refined compared to Fig. 4.

In this case, the quenching start temperature is higher than in Example 1, so
The base structure is mostly perlite.

Spheroidal graphite cast iron with such a structure has superior wear resistance and toughness compared to as-cast pearlite base having the same hardness.

Since the simple heat treatment as in Examples 1 and 2 does not involve a high temperature range as conventionally used, there is no strain generated during the heat treatment, and the heat treatment is energy-saving, and its economical effects are significant.

As is clear from the above explanation, the content of elements with a strong tendency to form carbides is suppressed to an appropriate amount or less, and at the same time, the Si content is
．． S is lowered to below 2 coefficient and S
By containing an appropriate amount of n depending on the target hardness, it is possible to obtain a casting with less variation in structure compared to ordinary pearlite terrestrial graphite cast iron, and furthermore, it is possible to obtain a casting with less variation in structure than normal pearlitic terrestrial graphite cast iron. A very useful fine pearlite terrestrial graphite cast iron with a more uniform fine pearlite base and excellent toughness can be obtained by a simple heat treatment of quenching from ~850°C or tempering the quenched product.

[Brief explanation of drawings]

Fig. 1 is an equilibrium state diagram, Figs. 2 and 3 are micrographs showing the structure of plates with thicknesses of 10 mm and 70 mm according to the present invention, Fig. 4 is a micrograph showing the structure when forced air cooling is performed, and Fig. 5
The figure is a micrograph showing the quenched and tempered structure.

Claims (1)

[Claims] IC3-4.2%, Si 1.3-2.2%, Sn0.
Spheroidal graphite cast iron having a fine pearlite structure consisting of impurities such as Fe and Mn, Cr, V, Mo, Cu, etc. as 0.025 to 0.20 and Mg 0.02 to 0.08. 2. Spheroidal graphite cast iron according to claim 1, having a Mn content of 0.30% or less. 3 Cu<0.20%, Cr<0.10%, V<0
．． 10. Spheroidal graphite cast iron according to claim 1 or 2, wherein Mo < 0.25%. 4. Spheroidal graphite cast iron according to any one of claims 1 to 3, wherein the sum of Cu, Cr, V, and Mo is 0.50% or less. 5C3-4.2%, Si 1.3-2.2%, Sn0.
025-0.20%, Mg 0.02-0.08%, balance Fe and impurities such as Mn, Cr, Cu, V, Mo, etc. Spheroidal graphite cast iron is heated to a coexistence temperature of α phase and γ phase of 750 to 850°C. A method for producing spheroidal graphite cast iron having a fine pearlite structure obtained by rapid cooling from a temperature range. 5. The method for producing spheroidal graphite cast iron according to claim 5, wherein the Mn content is 0.3 or less. 7 Cu<0.2%, Cr<0.01%, V<0
．． 10% and Mo <0.25%. The method for producing spheroidal graphite cast iron according to claim 5 or 6. 8. The method for producing spheroidal graphite cast iron according to any one of claims 5 to 1, wherein the sum of Cr, Cu, V, and Mo is 0.50% or less. 9. A method for producing spheroidal graphite cast iron according to any one of claims 5 to 8, characterized in that forced air cooling is performed from a temperature range of 9750 to 850°C where α and γ phases coexist. The method for producing spherical graphite according to any one of claims 5 to 8, characterized in that liquid quenching is carried out at a temperature range of 750 to 850°C, where the α phase and the γ phase coexist. 11C3-4.2%, Si1.3-2.2%, Sn0.
025~0.20%, Mg 0.02~0.08%, balance Fe and impurities such as Mn, Cr, Cu, V, Mo, etc. Spheroidal graphite cast iron is heated at 750~850°C with coexistence of α phase and γ phase. A method for producing spheroidal graphite cast iron having a fine pearlite structure, which comprises rapidly cooling from a temperature range and then tempering at 150 to 550°C. 12. The method for producing spheroidal graphite cast iron according to claim 11, wherein the Mn content is 0.30% or less. 13 CLI<0.20%, Cr<0.10%, Sn
<0.02, V<0.10%, Mn<0.
The method for producing spheroidal graphite cast iron according to claim 11 or 12, wherein the spheroidal graphite cast iron is set to 25%. 14. The method for producing spheroidal graphite cast iron according to any one of claims 11 to 13, wherein the total content of 14 Cr, Cu, V, and Mo is 0.50% or less.