JPS63114936A - Low thermal expansion cast iron and its production - Google Patents

Abstract

PURPOSE:To reduce the coefficient of thermal expansion and also to improve tensile strength and damping capacity, by adding specific amounts of Sb and As to a cast iron having a specific composition containing large amounts of Ni so as to spheroidize graphite. CONSTITUTION:A molten metal consisting of, by weight, 1.0-2.7% C, 0.8-3.0% Si, 30-40% Ni, <=1.0% Mo, <=3.0% Cr, <=0.5% Ti, 0.002-0.02% Sb and/or As, and the balance Fe is prepared. After Mg and/or rare earth elements are added to the above molten metal so that residual quantity is <=0.2%, inoculation in which additive quantity is regulated to >=0.05% by Si equivalent is applied once or more by using an inoculant in which Fe-Si is used as a base alloy. In this way, a low thermal expansion CV graphite cast iron free from chunky graphite in the structure, excellent in mechanical properties, and particularly reduced in thickness sensitivity can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a low thermal expansion cast iron suitable for use in ultra-precision machining machine tool members, etc., and a method for producing the same.

[Conventional technology]

Traditionally, flake graphite cast iron has been used for machine tool beds. The physical and mechanical properties of this material are RT~
The thermal expansion coefficient at 100℃ is about 10-12'X10-',
The tensile strength is about 10 to 20 kgf/mm2. In recent years, requirements for machining accuracy using machine tools have become dramatically stricter, and along with this, the materials constituting the machine tools are also required to be low thermal expansion materials with little dimensional change due to temperature.

The thermal expansion coefficient from RT to 100°C is 5 to meet these requirements.
．． As cast iron materials of the order of 0 to 6.0 x 10-', minover cast iron has existed for a long time as a flaky graphite-based material, and Niresist cast iron (D-5) as a spheroidal graphite-based material.

[Problem that the invention seeks to solve]

However, although flaky graphite-based materials have a high damping capacity, which is another property required for machine tool parts, they have a tensile strength of 20k.
Low gf/ff1m2 or less, 20kgf/mm”
It is difficult to apply this method to the above required functional components.

On the other hand, the tensile strength of spheroidal graphite cast iron material is 40 kgf/m.
Although it shows a high value of m2 or more, the damping capacity often required for machine tool parts is low and may not be able to meet the requirements.

When graphite is spheroidized using Mg-based and/or RE-based spheroidizing agents, which are commonly used for cast iron containing a large amount of Ni, eutectic graphite (chunky graphite) or partial spheroidization occurs. Defective graphite is often generated, which can significantly deteriorate mechanical properties.6Furthermore, depending on the presence or absence of molten metal treatment after spheroidization treatment and the quality of that method, thermal expansion due to the generation of carbides in thin-walled parts of parts can occur. In thick-walled parts, problems such as the deterioration of mechanical properties due to the generation of coarse graphite occur in thick-walled parts.

The purpose of the present invention is to first contain chunky graphite which has a low coefficient of thermal expansion and a tensile strength greater than that of flaky graphite-based low thermal expansion cast iron, and which has an additional function of damping capacity greater than that of spheroidal graphite-based low thermal expansion cast iron. The present invention provides a low thermal expansion cast iron without CV graphite and a method for stably producing cast iron at a high quality level.

Another object of the present invention is to provide a spheroidal graphite low thermal expansion cast iron that has an extremely low coefficient of thermal expansion, does not contain chunky graphite, and has small variations in graphite shape due to changes in wall thickness, and a method for stably producing the cast iron at a high quality level. It is.

[Means for solving problems]

The chemical composition of the present invention is C1, O ~ 2.7%, S
i0.8-3.0%, N130-40%, ~1n1.0
% or less, Cr 3.0% or less, Ti 0.5% or less.

The total amount of one or both of S b and A s is 0.00
After adding Mg and/or rare earth elements (RE) to the molten metal so that the residual amount is 0.2% or less, the balance is iron and unavoidable elements, and then the Fe-8i-based alloy is The total added amount of the inoculant is 0.05% in terms of Si equivalent.
A method for manufacturing low thermal expansion cast iron characterized by performing inoculation one or more times as described above, and a Cv that does not contain chunky graphite and has a graphite structure with a spheroidization rate of 30% or more and less than 70%. These are low thermal expansion cast iron of graphite, and low thermal expansion cast iron of spheroidal graphite that does not contain chunky graphite and has a graphite structure with a spheroidization rate of 70% or more.

Next, the reason for limiting the numerical value of the chemical composition of the low thermal expansion cast iron of the present invention will be explained.

C: The limit for graphite generation in the structure is about 0.8%, but if it is less than 1.0%, the amount of black in the structure decreases significantly, resulting in poor castability and workability.

If it exceeds 2.7%, a large amount of Quiche graphite will crystallize, especially in thick-walled castings, resulting in a decrease in tensile strength and also making casting defects more likely to occur.

Si: If it is less than 0.8%, the original graphitization promoting function will not be exhibited, and the amount of graphite in the structure will decrease significantly, and in some cases, there may be a risk of generation of carbides.

If it exceeds 3.0%, it becomes difficult to suppress the coefficient of thermal expansion to a low level and the toughness decreases. Depending on the amount of C, the generation of quiche graphite is promoted.

Ni: If other chemical compositions include less than 30% or more than 40% of Ni in the scope of this claim, the coefficient of thermal expansion from RT to 100°C will be 9X10-'' or more, which is consistent with the purpose of the present invention. I won't be able to do it.

Mn: Mn is an element necessary to clean molten cast iron and prevent casting defects, but if it exceeds 1%, the coefficient of thermal expansion becomes 9X10-' or more, which does not meet the purpose.

Cr: Cr improves corrosion resistance, but if it exceeds 3%, it becomes extremely difficult to crystallize graphite, and in thin-walled castings, carbides are often crystallized and the coefficient of thermal expansion increases.

Furthermore, hardness increases and machinability decreases.

Tj: Tj is an effective element for stably converting the shape of graphite into Cv, but as the content increases, the coefficient of thermal expansion increases, and when it exceeds 1%, it inhibits the crystallization of graphite and Decreases machinability. Note that Tj is not added to spheroidal graphite cast iron in principle.

Sb, As: Features of the present invention are usually avoided as harmful elements. The purpose is to contain one or more of Sb and As.

When high Ni-based molten cast iron is treated with Mg and/or rare earth elements, it is extremely easy to crystallize eutectic graphite (chunky graphite), and the final structure is eutectic graphite + CV graphite or eutectic graphite spheroidal graphite. Therefore, the mechanical properties are significantly deteriorated.

The addition of S b and As has a remarkable effect in suppressing the crystallization of this eutectic graphite, and does not increase the coefficient of thermal expansion. The lower limit of the content is 0.002% for the total amount of one or both of Sb and As, and if it is less than this, the effect will be weak.

The upper limit of the total amount is 0.02%, and if the content exceeds this, flaky graphite will crystallize in the structure even if treated with Mg and/or rare earth elements, resulting in a stable CV-shaped graphite structure or spheroidal graphite structure. It becomes difficult to obtain.

Mg and/or rare earth element (RE): Mg and/or RE are included to make the graphite shape of low thermal expansion cast iron into a Cv shape with a spheroidization rate of 30 or more and less than 70% or a spherical shape with a spheroidization rate of 70 or more. . In order to obtain low thermal expansion cast iron of CV graphite, the total amount of Mg and/or RE is usually 0.008 to 0.
．． 03% content. Furthermore, 0.05 to 0.3% Ti
By including it, a more stable CV-shaped graphite structure can be obtained.

To obtain low thermal expansion cast iron of spheroidal graphite, Mg and or R are usually used.
The total amount of E is 0.03 to 0.3%. However, when RE exceeds 0.2%, carbides tend to crystallize in thin-walled castings even if molten metal graphitization promotion treatment by inoculation is performed, resulting in an increase in the coefficient of thermal expansion and a decrease in machinability.

Next, the effect of inoculation, which is a manufacturing method, will be explained. 1st
The table shows the chemical composition of the molten metal before inoculation of the spheroidal graphite low thermal expansion cast iron according to the present invention. Table 2 shows the results of investigating the effect of inoculation by varying the wall thickness of the casting. - Fe-8i (75) was added at 0.2% Si equivalent for both secondary inoculations, and Fe-3i (73) -Ca(2)-Al(3
) alloy added in an amount of 0.1% in terms of Si equivalent.

Table 1 (wt%) Table 2 Note: () indicates carbide generation The inventors experimentally found that carbides were generated by inoculating 81 equivalents of 0.05% into the poured metal flow in 5 mm thick castings. confirmed to disappear. Note that although the results for spheroidal graphite low thermal expansion cast iron are illustrated here, CV
It has been confirmed that the increase in the number of graphite grains due to inoculation and the stability of the spheroidization rate with differences in wall thickness show similar trends in graphite low thermal expansion cast iron.

Examples of the present invention will be described in more detail below.

[Example 1] Return shoulder containing Ni, raw material mainly composed of metal Nl
kg was melted in a high-frequency electric furnace, and a Cr alloy, a Tj alloy, and other alloys for adjusting the composition were added thereto to form a base melt.

The following five experiments were conducted by sequentially using this source hot water for 60 kg each.

First, the molten metal processing method and mold will be explained.

1. Molten metal treatment (a) Cv treatment Fe-5i (45) Mg (4,5)-RE (1,5
) The so-called sandwich method was used, in which 0.45% of the molten metal was set at the bottom of a ladle and the molten metal was poured from above.

(b) Addition of sb and As For 180 kg of residual hot water from which Samples 1 and 2 described below were collected,
0.02% of sb and 0.005% of As were added by a phosphorizer in the furnace.

(c) Inoculation The primary inoculation is Fe-
3i(75) alloy was added in an Si equivalent amount of 0.3% of the amount of the molten metal and stirred.

Secondary inoculation was Fe-5L(73)-Ca(2)-Al(
3) A method was adopted in which alloy grains were added to the flow of molten metal in an amount of 0.1% of the amount of molten metal poured into the mold.

In addition, for molten metal that is not subjected to inoculation treatment, S is inoculated with Fe-5i (75) alloy together with the CV treated alloy in advance.
I set the amount commensurate with the L increase.

2. By pouring the same molten metal into Y block molds with mold wall thicknesses of 25 mm and 75 mm, we investigated the effects of the presence or absence of addition of sb and As, and the presence or absence and method of inoculation treatment on wall thickness sensitivity.

Next, the method and purpose of experiments on five types of CV cast iron will be explained.

a. Experiment 1 SB and As were not added to the starting hot water, graphite was subjected to Cv roasting in a ladle, and the hot water was poured without primary or secondary inoculation.

b. Experiment 2 In contrast to Experiment 1, the effects of primary and secondary inoculation were investigated.

C6 Experiment 3 SB and As were added to the starting water, CV treatment of graphite was performed, and the water was poured without subsequent or secondary inoculation. In comparison with Experiment 1, the effect of preventing chunky graphite by adding sb and As and its influence on mechanical properties was investigated.

d. Experiment 4 In contrast to Experiment 3, only primary inoculation was performed and hot water was poured to examine the effect of primary inoculation.

e. Experiment 5 A secondary inoculation was also performed in Experiment 4, and the effect of the secondary inoculation was investigated by pouring hot water.

The analysis results for each sample were as shown in Table 3, including iron and inevitable impurities. Note that the sample number corresponds to the above experiment number.

Table 4 shows the thermal expansion coefficient (20 to 100°C), mechanical properties, and graphite structure.

Table 4 Thickness is mm [Example, 2] Return scrap containing Ni, raw material mainly consisting of metallic Ni is 300 mm
kg was melted in a high-frequency electric furnace, and a Cr alloy and other alloys for adjusting the composition were added thereto to form a base melt.

The following five experiments were conducted by sequentially using this source hot water for 60 kg each.

First, the molten metal processing method and mold will be explained.

1. Molten metal treatment (a) Spheroidization treatment Fe-5i (45) Mg (4,5)-RE (1,5
) The so-called Sanderuch method was used, in which 0.85% of the molten metal was set at the bottom of a ladle and the molten metal was poured from above.

(b) Addition of sb 0.025% of Sb was added to 180 kg of residual hot water from which Samples 6 and 7 described below were taken using a phosphorizer in a furnace.

(c) Inoculation The primary and secondary inoculations of the molten metal in the ladle that had undergone the spheroidization process were carried out in the same manner as in [Example 1].

For molten metal that is not subjected to inoculation treatment, S is inoculated with Fe-5i (75) alloy together with the spheroidized alloy in advance.
I set the amount commensurate with the increase.

2. By pouring the same molten metal into Y-block molds with mold wall thicknesses of 25 mm and 75 mm, we investigated the effects of the presence or absence of Sb addition and the presence or absence of inoculation treatment on wall thickness sensitivity.

Next, the method and purpose of experiments on five types of spheroidal graphite cast iron will be explained.

a. Experiment 6 After Sb was not added to the starting hot water and graphite was spheroidized in a ladle, the hot water was poured without subsequent or secondary inoculation.

b. Experiment 7 Regarding Experiment 5, the effects of primary and secondary inoculation were investigated.

C0 Experiment 8 After adding sb to the original hot water and performing graphite spheroidization treatment, the hot water was poured without subsequent or secondary inoculation. In comparison with Experiment 6, we examined the effect of adding Sb on preventing chunky graphite and improving the spheroidization rate, and the effect thereof on mechanical properties.

d. Experiment 9 In Experiment 8, only the primary inoculation was added and water was poured to examine the effect of the primary inoculation.

b. Experiment 10 In contrast to Experiment 9, secondary inoculation was further performed and hot water was poured to examine the effect of secondary inoculation.

The analysis results for each sample were as shown in Table 5, including iron and inevitable impurities. Note that the sample number corresponds to the above experiment number.

Table 5 (wt%) Table 6 shows the thermal expansion coefficient (20 to 100°C) 1 mechanical properties and graphite structure.

Table 6 Wall thickness is mm [Effect of the invention] As is clear from Table 4, in Cv graphite cast iron, s
In Samples 1 and 2 without the addition of sb and As, chunky graphite crystallizes and the mechanical properties are low, whereas in Samples 3, 4 and 5 with the addition of sb and As, chunky graphite crystallizes and the mechanical properties are low. No crystallization was observed and the mechanical properties were high.

Next, looking at the effect of inoculation, the mechanical properties of the inoculated samples improved regardless of the presence or absence of sb and As.
Thickness sensitivity is low. This tendency is observed in samples 4 and 5.
As is clear from the comparison, this effect becomes more noticeable after secondary vaccination. Note that the coefficient of thermal expansion does not change depending on whether or not sb and As are added and whether or not there is inoculation.

As mentioned above, by including sb and As and by performing subsequent or secondary inoculation.

It does not contain chunky graphite in its structure and has excellent mechanical properties.
In particular, low thermal expansion CV graphite cast iron with low wall thickness sensitivity could be obtained.

As is clear from Table 6, in spheroidal graphite cast iron, sb
Chunky graphite was crystallized in both samples 6 and 7 without addition of Sb, and the mechanical properties were low, whereas no chunky graphite crystallization was observed in samples 8, 9, and 10 with addition of Sb. Moreover, the spheroidization rate of graphite is increased and the mechanical properties are high.

Next, looking at the effect of inoculation, regardless of whether sb was added or not, the spheroidization rate and mechanical properties of the inoculated samples improved, and the wall thickness sensitivity became lower. This tendency was observed in sample 9.
As is clear from comparing sample 10 and sample 10, it becomes even more noticeable after secondary inoculation.

Note that no change in the coefficient of thermal expansion is observed in Samples 6 to 10.

As mentioned above, by containing sb and performing secondary or secondary inoculation, the structure does not contain chunky graphite, has a high spheroidization rate, has excellent mechanical properties, and is particularly effective against wall thickness sensitivity. We were able to obtain low thermal expansion spheroidal graphite cast iron.

Claims (1)

[Claims] 1. Chemical composition is C1.0-2.7% by weight, Si0
．． 8 to 3.0%, Ni 30 to 40%, Mn 1.0% or less, Cr 3.0% or less, Ti 0.5% or less, Mg and/or rare earth elements (RE) 0.2% or less, one type of Sb, As, or A low thermal expansion cast iron in which the total amount of the two types is 0.002 to 0.02%, and the balance is iron and unavoidable elements. 2. The low thermal expansion cast iron according to claim 1, wherein the graphite structure has a spheroidization rate of 30% or more and less than 70%. 3. The low thermal expansion cast iron according to claim 1, wherein the graphite structure has a spheroidization rate of 70% or more. 4. Chemical composition is C1.0-2.7% by weight, Si0
．． 8-3.0%, Ni 30-40%, Mn 1.0% or less, Cr 3.0% or less, Ti 0.5% or less, the total amount of one or both of Sb and As is 0.002-0.02%, The remainder consists of iron and unavoidable elements, and the residual amount in the molten metal is 0.
Mg and/or rare earth elements (RE) to be 2% or less
A method for producing low thermal expansion cast iron, which comprises adding inoculation at least once with an inoculant based on Fe-Si so that the total amount added becomes 0.05% or more in terms of Si equivalent.