JPS5917184B2 - As-cast pearlite terrestrial graphite cast iron - 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 pearlite and ferrite (hereinafter simply referred to as pearlite base).

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

In this case, to obtain pearlitic terrestrial graphite cast iron,
The method used was to increase or decrease the amount of Mn depending on the target hardness and product wall thickness, but since Mn is an element that promotes pearlite formation and at the same time promotes the crystallization (precipitation) of cementite, it is necessary to contain more Mn than necessary. Then, due to the above-mentioned undesirable structure, there were problems such as locally increasing hardness, making the steel brittle, or significantly reducing machinability.

Therefore, when manufacturing pearlitic terrestrial graphite cast iron to be used in the as-cast state, crystals of the above-mentioned undesirable structure (
In order to prevent precipitation, it has generally been done by increasing the Si content (2.5 to 3.0).

Pearlite terrestrial graphite cast iron manufactured by conventional methods is susceptible to the cooling rate within the casting mold due to its high Mn and Si contents, and even within the same product, pearlite is uniform. It could not become an organization.

That is, even if the product is the same, if the wall thickness is thin and the cooling rate is fast, it will become completely pearlite and have high hardness.

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

Furthermore, due to the influence of various variables such as the spheroidization treatment method between each heat, the difference in inoculation effect, the difference in the cooling time in the mold from the end of pouring of each framework to the mold release, and the difference due to the fading phenomenon of inoculation effect, When mass-producing the same product, there are usually large variations in structure and hardness from product to product.

This is because Si, which is an element that promotes ferrite formation, and Mn, which is an element that promotes pearlite formation, are contained in large quantities at the same time, elements that have opposite effects.

In view of the above points, the present invention is a pearlite-promoting element that suppresses the amount of Si, which is a ferrite-promoting element, and also suppresses carbide (cementite)-forming elements such as Mn, Cr, and V, and does not have a tendency to form carbides. The inclusion of Sn provides as-cast pearlite terrestrial graphite cast iron with more uniform structure, hardness, and properties.

Crystallization of cementite structure appearing in spheroidal graphite cast iron
If this can be suppressed, it is possible to lower the Si content, which is an element that promotes ferrite formation, which not only promotes pearlite formation, but also reduces variations in structure, hardness, and properties caused by high Si content. can be expected to decrease.

Pearlite stabilizing elements normally contained in spheroidal graphite cast iron include Mn, Cr, V, Sn, Mo, Cu, etc., but these elements segregate at the eutectic cell boundaries and cause crystallization of the cementite structure. The element that promotes production is Mn.

They are Cr, V, and M6.

Therefore, these elements can be mixed into crystals with the above-mentioned unfavorable structure (
The amount of Si can be reduced by suppressing the amount of Sn to below the precipitation limit and instead containing an appropriate amount of Sn in accordance with the target hardness.

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

Example: In a low frequency furnace, 40% of spheroidal graphite cast iron return waste and Sorel metal (
Product name) 20 parts and steel scrap 40 parts were blended and melted, and after raising the temperature and adjusting the composition, a spheroidization treatment was performed with Fe-8Fe-8i-%Mg) at the time of tapping.

Further, the material was inoculated with 0.6% Fe-8i and cast into a sand mold having a cavity in which plates of different thicknesses were arranged radially.

In this case, commercially available high quality steel scrap was used.

The chemical components at this time are shown in Table 1, the mechanical properties at different plate thicknesses are shown in Table 2, and the microscopic structures at plate thicknesses of 10 mm and 70 mm are shown in FIGS. 1 and 2, respectively.

As is clear from Table 2, a small amount of cementite is observed when the plate thickness is 5 mm, but when the plate thickness is 10 mm or more, the structure is a mixture of pearlite and ferrite, and the plate thickness is 1107I.
The difference in Brinell hardness between L and 70 mm is 24, and the tensile strength is 7.2 to 9/-, and both exhibit good elongation at plate thicknesses of 10 m7IL or more.

In this case, the test was not conducted on the plate having a thickness of 5 mm because it was difficult to process the tensile test piece.

Also, as can be seen from the analysis results in Table 1, the large amount of Mn contained in ordinary pearlite terrestrial graphite cast iron is 0.16.
%, the content is very low.

The content of Cr, Cu, Mo, V, etc., which is likely to segregate at the eutectic cell boundary, is kept low, and the total content of Cr, Cu, Mo, V, etc. is reduced to 0.1042%.
By doing so, crystallization (precipitation) of cementite structure could be prevented.

Due to these factors, it shows good elongation even at a plate thickness of 10 mm.

In the present invention, the limit content for crystallizing (precipitating) individual carbides of each component that is unavoidably contained is Cr,
V is 0.10%, and Mo is 0.25%.

Cn itself is not an element that promotes carbide precipitation, but Cn
If the total amount of r, V, Mo, and Cu exceeds a factor of 0.50, carbide precipitation will still be significant even if the content of each element is below the limit, so this amount should not be exceeded.

Furthermore, in order to uniformly transform the matrix structure into pearlite by incorporating Sn, unless the content of other pearlite-promoting elements is kept low, the synergistic effect of these elements will cause variations in the matrix structure.

For example, Cu has almost no effect of promoting pearlite formation up to 0.20%, but when the content exceeds 0.25%, its effect rapidly increases.

Therefore, if 0.050% Sn is added to a Cu content of 0.10% and 0.30%, the latter will promote pearlite formation, have higher hardness, and have higher tensile strength. It will also be expensive.

This is due to the synergistic effect of the pearlitization promoting effects of Cu and Sn, and is due to the addition of Sn.

In order to obtain a uniform pearlite structure with hardness with little variation between heats, it is necessary to keep the Cu content at 0.20% or less.

The effect of Mn on accelerating the precipitation of carbide products (precipitation) varies depending on the plate thickness, but in thin walls where the cooling rate is fast, the effect becomes noticeable at 0.30% or more, so it is necessary to reduce the amount below this value. .

If Mn is 0.1 to 0.2, which can be achieved in normal operations, then Si may be in the range of 2.0 or less, and the amount of Sn required to turn the base structure into pearlite depends on the product thickness and target. Although there are some differences depending on the hardness, if the Sn content is 0.025% or less, the pearlite formation promotion effect decreases and a sufficient pearlite structure cannot be obtained even in a thin wall part.

The upper limit of 0.2 is the amount necessary to form a complete pearlite structure in the thick part, and there is no need to contain more than that.

Table 3 shows the chemical composition of conventional pearlitic terrestrial graphite cast iron.
Table 4 shows its mechanical properties.

This comparative example clearly shows how superior the pearlitic terrestrial graphite cast iron according to the invention is.

Comparative example: 40 pieces of spheroidal graphite cast iron returned to a low frequency furnace, Sorel metal (
After blending 20% (trade name) and 40% steel scrap, melting and raising the temperature, and adjusting the composition, a spheroidizing treatment was performed with Fe-Fe-8i-5%Mg).

This molten metal was inoculated with Fe-8i at a rate of 0.6, and a test piece similar to that in the example was cast.

Commercially available general steel scrap was used as the steel scrap in this case.

The chemical components and mechanical properties at each plate thickness are shown in Tables 3 and 4, respectively, and the microstructures at plate thicknesses of 10 mm and 70 mrn are shown in FIGS. 3 and 4, respectively.

In the case of this example, as is clear from Tables 3 and 4 and Figures 3 and 4, the plate thickness is 5 mm, 10v271+ and 3
At 0 mm, cementite crystallizes (precipitates) and has high hardness, so the elongation is extremely small.

When the plate thickness is 5 mm, most of the base structure is cementite, and it is clear that there is a marked tendency to white iron.

Conversely, when the plate thickness is 70mm, the amount of ferrite is higher than that of pearlite, the tensile strength is lower, and the elongation is increased.

The difference in Brinell hardness between plate thicknesses of 10 mrn and 70 mm is 67, and the difference in tensile strength is 23 kg/ma.
It is clear that there are large variations in both cases compared to the examples of the present invention.

In this case, the total amount of Cr, Mo, V, and Sn is 0.18
5%, which is within the numerical limit set forth in claim 3, but the Cr content exceeds the carbide main formation limit value of 0.10%, and the Mn content is also extremely high at 0.58%. expensive.

Because the content of these elements is high, carbides mainly form in the thin-walled parts as mentioned above, while the amount of Si is high at 2.66%, so the amount of pearlite in the thick-walled parts decreases, resulting in a decrease in hardness. It shows.

As is clear from the above explanation, Mn has a strong tendency to form carbides.
By suppressing , Cr, Mo, and V to appropriate amounts, and further reducing the Si content and containing an appropriate amount of Sn as a pearlitization promoting element according to the target hardness, it has a higher This makes it possible to obtain extremely useful pearlitic terrestrial graphite cast iron with little variation in structure and mechanical properties.

[Brief explanation of the drawing]

Figures 1 and 2 show plate thicknesses of 1101n and 7 according to the present invention.
0mm microscopic structure Figures 3 and 4 are for conventional plate thickness 10mm.
It is a photograph showing microscopic structures of mm and 70 mm.

Claims (1)

[Claims] IC3-4.2%, S i 1.3-2.2%, S
n0.025-0.20%, Mg 0.02-0.0
As-cast pearlite terrestrial graphite cast iron, characterized in that the balance is 8% Fe and impurities such as Mn, Cr, V, and Mo. 2. The as-cast pearlite terrestrial graphite cast iron according to claim 1, wherein the Mn content is 0.30% or less. 3 Cr<0.10%, Cu<0.20%, V<
The as-cast pearlite terrestrial graphite cast iron according to claim 1 or 2, wherein 0.10%Mo<0.25%. 4. The as-cast pearlite terrestrial graphite cast iron according to any one of claims 1 to 3, wherein the sum of Cr, Cu, V, and Mo is 0.50% or less.