JPH04141545A - High temperature low thermal expansion cast iron - Google Patents

Abstract

PURPOSE:To provide cast iron with excellent damping capacity and mechanical characteristics and to stabilize its castability by specifying the content of Ni and Co and their relationship in a steel. CONSTITUTION:The compsn. of cast iron is formed of a one constituted of, by weight, 1.5 to 3.0% C, 0.5 to 2.0% Si, <=1.0% Mn, 22 to 30% Ni, 3 to 20% Co and the balance Fe with inevitable impurities. Furthermore, the content of Ni and Co lies in the range of 31.5<=Ni+0.75Co<=37. After casting, the cast iron is heated to about 900 to 1150 deg.C and is held for desired time. If required, this cast iron is furthermore incorporated with either or both of Mg and Ca by 0.02 to 0.1% in all. By the above compsn., a high temp. low thermal expansion member having a complicated shape can stably be provided with good castability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a cast iron material with an extremely low coefficient of thermal expansion, particularly a material that maintains its properties relatively even at temperatures above room temperature and 250°C.

[Prior Art] Conventionally, materials that are incorporated into devices or machines as members have sometimes been required to have a functionally extremely small coefficient of thermal expansion. For example, precision machine parts, molds, wrapping plates, etc. must expand with changes in external temperature to a small extent, or the delicate finish may be distorted or the size of the product may vary, resulting in quality problems. It may have a negative impact on reliability.

For this purpose, low thermal expansion materials have been developed for specific members, and a wide variety of materials have already been provided in practice.

Common iron alloys usually have a thermal expansion coefficient of lθ~18X10
-6/''C, attempts have been made to significantly lower this value by adding various alloying elements.
Invar was developed as the most prominent material.

Invar is a copper system with C (1.10 or less) and N1 of 35 to 3
This material contains 7% of Cr, Mo, and Co, and the average coefficient of thermal expansion at room temperature to 100℃ is 1.88X 10-67" as forged, and 0 after quenching at 1830℃, G4X 10-8. A low thermal expansion coefficient of 1.02X10-8/'C after quenching and tempering at 1830°C and 2.01X10-'/'C even when furnace-cooled from 830°C has been reported.

Furthermore, super invar (F e
-32N I-5G o) 0. IX10-
6/'C indicates a value close to O. (The above is from Toru Gyudon, "New Formed Material - Low Thermal Expansion Casting Material": Casting, Forging and Heat Treatment, January 1989 issue, pages 21-28) However, Invar's low coefficient of thermal expansion is caused by spontaneous bulk magnetostriction due to ferromagnetism below the magnetic transformation point. It is explained that (contraction) overlaps with the original thermal expansion and suppresses the apparent expansion.
Even if it is fine from room temperature to the magnetic transformation point, this persistence becomes a problem when the temperature is raised to 200° C. or higher.

Since then, many low thermal expansion materials with an increased C content compared to Invar have been proposed. For example, in JP-A-1-306540, C083 to 2.0% (0.
73), Si 0.3-20%, Ni 28-3B%
, Co 2.0-7.0%, Mg or Ca 0.1% ductile austenitic cast iron is disclosed, and the thermal expansion coefficient up to 200°C is 1.077X 10-6/'C. . Also, Japanese Patent Application Laid-Open No. 64-21037 No. 0
Even C: o, e~1.0%, SI: 0.3~1
．． 0%, l'Ji: 2B-32%, CO: 8.0-18
．． 0% (N i + CO) is 40.0 to 46.0
In this example, C: OJ is 2 to 0.98, and the average coefficient of thermal expansion α between 30 and 500°C of five samples within the above component range is 6. .6~? , 4X10-"/"C. Further, JP-A-1-283342 discloses C: 3.0 or less, N125.0 to 40.0, co:
Specifying a wide content range of C: 8.0 to 12.0 (however, in the examples, measurement results are shown in a relatively narrow range of C: 2.00 to 1.70, and C: from room temperature to 200 ° C.
The average coefficient of thermal expansion α is 5.2 to 4. IX 10-6
/ ``It has become clear that C&K will fit.

[Problems to be Solved by the Invention] Low thermal expansion coefficient materials starting with Invar have achieved various developments mainly through adjustment of additive elements.

In terms of components, Fe-Pt system, Fe-Pd system, Z
Although there are a wide variety of materials such as r-Nb-Fe type and Cr-Fe-Mn type, research has focused on materials based on Fe-N1-Co as iron-based materials in practical use.

However, it has become possible to reduce the coefficient of thermal expansion of austenitic steels with low 0%, such as Invar, to almost 0 at room temperature if desired. Because it is caused by magnetostriction, for members used at temperatures slightly higher than room temperature, 200°C to 250°C, the values at room temperature cannot be used as is, and it is necessary to use a coefficient of thermal expansion that increases rapidly as the temperature rises. You won't get any more. For this reason, it may be difficult to use as a component of machines or devices.

Moreover, as a common element of low carbon austenitic steel, castability is extremely poor, and despite the high melting temperature, the fluidity of the molten metal is poor, and even today with improved casting technology, parts with complex shapes cannot be cast soundly. It's difficult.

Furthermore, low vibration damping properties, which is a common element of low carbon steels, have a negative effect on the functionality of devices to which they are applied. As the use of low thermal expansion materials has expanded from standard measurement equipment to electronic equipment (e.g. IC boards, thermostat elements) and low-temperature equipment (LPG tanks: superconducting systems), even if low thermal expansion can be achieved, vibration damping properties are low. Therefore, it is not uncommon for the functionality of the device to be reduced.

Next, as is well known, austenitic cast iron with a relatively high carbon content has excellent vibration damping properties and greatly improves machinability. If necessary, graphite can be spheroidized to provide strength and toughness.

However, even with the high carbon materials disclosed to date, it is obvious from the carbon content that they still fall short of conventional ordinary cast iron (ductile cast iron) in terms of castability and vibration damping properties. It is undeniable that it has a large difference in low thermal expansion coefficient compared to low carbon steels. Conventional prior art is on the right track in this regard, but there are problems with experimental technology that lacks the precision of rigorous and reliable screening and pushes it to the ultimate conditions, and experimental processing that controls the scope. This may be due to the lack of skill.

In order to solve the above-mentioned problems, the present invention aims to provide a high-temperature, low-thermal-expansion cast iron that has excellent vibration-damping properties and mechanical properties, has good castability, and can be stably supplied even in large quantities of high-temperature, low-thermal-expansion members of complex shapes. purpose.

However, the high temperature referred to here is limited to a range of 200 to 250°C.

[Means for Solving the Problems] The high temperature and low thermal expansion cast iron according to the present invention has C: 1.
5-3.0, S f: 0.5-2.0, Mn: 1.
0 or less, N1: 22-30, Co: 3-20, balance includes iron and unavoidable impurities, and the content of N+ and CO is basically within the range of 31.5≦N I+0.75G o≦37. The above problem was solved as follows. In addition, the requirements for heating to a temperature of 900 to 1150°C after casting and holding it for a desired period of time are added to this, and to the above requirements, Mg
or Ca or both together 0.02 to 0.1
A case was shown in which an embodiment adding the third requirement including weight % may also be effective.

[Operations] The main significance of the present invention is that it specifies the highest conditions by obtaining more precise and systematic experimental values than those of the prior art.

Unlike steel, cast iron has many impurities, which inevitably leads to large variations in data. Even if an attempt is made to keep the components constant, the components will fluctuate due to loss or incorporation due to reactions with refractories or gases during melting. Furthermore, when casting into a mold, the casting structure differs depending on the mold material used, and especially when molding sand is used, since it is a natural material, the variation caused by the mold is large.

On the other hand, when measuring the coefficient of thermal expansion, it is usually 20 to 4 (1wn
However, if the thermal expansion coefficient is on the order of 10-6, the expansion margin is on the order of 0.01 microns at 1°C, and the accuracy of the measuring equipment and the length of the sample may be affected. The surface roughness of the area to be measured and the strain of the sample due to the measurement load are also major causes of variation.

Due to the combination of the various fluctuation factors mentioned above, it is difficult to determine which alloy composition has a truly low coefficient of thermal expansion. Therefore, a rapid melting method using a high-frequency induction electric furnace was used for melting the cast iron, a compound mold was used for casting, and an average coefficient of thermal expansion of 50 to 200°C or 250°C was used for measuring the coefficient of thermal expansion.

The rapid melting method, which takes 1/10 to 1/20 of the normal melting time, has little variation in composition from the blended components, and by keeping the mold temperature constant, compounding molds prevent component segregation and fluctuations in the cast structure of the cast product. few. However, the structure cast with a compound mold is a rapidly quenched structure in which carbon becomes a carbide, so 11
It was placed in a heat treatment furnace at 00°C for 2 hours and then rapidly cooled to form a structure of graphite and austenite. When measuring the coefficient of thermal expansion, the temperature is gradually increased from room temperature while maintaining a constant temperature increase rate, but avoiding temperatures near room temperature where there are many fluctuations, and measuring the expansion range from 50°C to 200°C or 250°C. By measuring the surface roughness of the location where the length of the sample is measured,
This reduces the influence of strain caused by measurement load on thermal expansion coefficient fluctuations.

As a result of these small fluctuations, the accuracy of the data increased and a very clear graph of the coefficient of thermal expansion was obtained.

As a result, there are slight deviations in chemical composition from sample numbers 1 to 30, but the intention is to increase N1% sequentially from 22 to 32, and for each N1%.
%, Co was distributed at appropriate intervals over a range of 0-18%, and the average thermal expansion coefficient α (X 10-8/°C) between 50 and 200°C was measured for the combination of both (
Other ingredients are almost the same).

Now, observing the whole, α is 2.5X 10-6/”C
Table 1 is a summary of the following groups with A1 and above groups as B and written in the notes column.

(The following is a blank space) The next task is to plot the correlation between the variation in Co% for each N1% and the average coefficient of thermal expansion α. In other words, the curve 22 in Figure 1 is 00 for sample numbers 1 to 3.
The relationship between % and α is connected by a curved line, and a remarkable minimum point is formed for each N1% group. Inequality % formula % that is satisfied by picking out only the minimum points for each curve and N1% and Co% Now, for the sample in Table 1, l'tN% + 0.75Co
Table 2 shows the results of calculating each percentage.

(The following is a blank space) The results of Table 2 on the second back All the samples in rank A are included within the range where the above inequality holds true and are completely consistent. However, the converse is not true, and there are some samples that do not fall into A rank even if they are included in the inequality. For example, sample number 1.3.21.2
For 2.26-29, the value of N1% + 0.75% CO is 31
．． It is in the range of 5-37.

This means that N1 for this type of austenitic cast iron.
There are strict critical values at the lowest and highest values, which suggests that components around these values do not meet the requirements, so N1% was limited to 22 to 30 after checking consistency with A rank.

In addition, to briefly mention other component elements, C: Adding carbon lowers the melting point of the alloy and improves castability. Furthermore, crystallization of graphite in the structure improves machinability and vibration damping properties.

If it is less than 1.50%, the melting point will be high and crystallization of graphite in the structure will be significantly reduced, and the advantages of good castability, cutting workability, and vibration damping properties will be lost. If it exceeds 3.0%, casting defects tend to occur, graphite becomes larger, and material strength decreases.

Sl: Lowers the melting point of the alloy, improving castability.

Furthermore, since it helps graphitize the carbon, machinability is improved, and as a result, vibration damping performance is also improved.

When it is less than 0.5%, castability deteriorates, and when it exceeds 2.0%, the coefficient of thermal expansion increases.

Mn: Mn is useful for improving material strength, but it tends to segregate and increases the coefficient of thermal expansion, so it is limited to 1.0% or less.

Co: Co is determined by itself from the above inequality, but it is assumed to be 3% to 20% (when Nl is 22% and the inequality is 37%).

Adding heat treatment as a second requirement after specifying the above chemical components also leads to good results.

The purpose of heating is to decompose carbides that remain in the cast structure and are harmful to the coefficient of thermal expansion, and to diffuse nickel, cobalt, silicon, and manganese that are segregated in the cast structure into a uniform alloy phase. If this heat treatment is not performed, the coefficient of thermal expansion will increase and the variation will increase. If the temperature is 900° C. or lower, there is no effect, and if the temperature is 1150° C. or higher, the deformation becomes large, which is not preferable. The higher the temperature, the shorter the heating time.

Next, depending on the embodiment, priority may be given to improving tougher mechanical properties even at the expense of some low thermal expansion.

Mg or Ca: Spheroidizes graphite and improves the strength of cast iron. However, the combination of one or two of these is 0.
If it is less than 0.02%, the graphite will not become spheroidized, so there will be no significant improvement in strength.If it is more than 0.1%, the coefficient of thermal expansion will increase, so it is limited to 0.1% or less. Add strength only when required, and do not normally add it.

The third requirement of adding MgtCa is rather a negative factor when focusing on low thermal expansion.

Mg and ca remaining after solidification are thought to cause microscopic segregation and become a factor inhibiting α reduction, so even with the best performance of conventional ductile austenitic cast iron, α < 2 × 10−'/ I can't find any reports of breaking the 'C wall.

In the present invention, only in this embodiment where toughness is particularly required, the solution to the problem is to combine all three requirements mentioned above.

[Example 1] As described above, we were able to identify a desirable range of ingredients through experiments aimed at precision and accuracy, but it was not possible to follow the above experiments when implementing the invention, and we used ordinary sand casting with ordinary melting. We have to go back to known means.

The present invention will now be implemented and the merits of improvement will be evaluated by comparison with the conventional techniques already cited.

Using electrolytic nickel, electrolytic cobalt, Quebec pig iron, electrolytic iron, and ferrosilicon (75% 81) as raw materials, the raw materials were precisely weighed by changing the blending ratio, and 10 kg of the blended raw materials were mixed into 55%
A of JIS-G5122 made with COQ type using silica sand, put in KVA's high frequency induction electric furnace and melted in the atmosphere.
The test piece was cast in a No. 1 test piece mold. Then 1100°C
Samples for measuring thermal expansion coefficients of 5IΦ x 20mmL were cut from test pieces that had been placed in a furnace for 2 hours and then water-cooled, a test piece that had been cooled in a furnace for comparison, and an as-cast test piece. 2
The thermal expansion coefficient α was measured at 50° C. in the same manner as in the previous section.

The measurement results are shown in Table 3. First, when observed with a microscope,
Samples A, B, and C all had an austenitic structure with no cementite and flaky graphite precipitated therein.

On the other hand, the sample is another example of the present invention, in which graphite was spheroidized by Mg treatment, heated to the austenite region after casting, held, and then rapidly or gradually cooled.

All comparison materials were taken directly from publicly available documents that boast outstanding results. That is,
Comparative example a is from JP-A-1-308540/Table 1,
Comparative example JP-A-64-21037/Table 29 Table 3
Therefore, Comparative Example C is disclosed in JP-A-1-283342, Table 1.
2 From Table 2, the numerical values announced by the applicants themselves have been taken directly and used for comparison.

(Left below) Table 4 shows how the mechanical properties of sample B (flake graphite) and sample D (spheroidal graphite) change in mechanical properties due to water cooling heat treatment.
In D, the tensile strength increases by about 10%, while the thermal expansion coefficient also decreases by about 15%, illustrating the effect of heat treatment.

Table 4 (Note) *I B is an invention alloy with no Mg added *2
D is an invented alloy containing Mg [Effects of the Invention] The effects of the present invention are clear from the Examples and Comparative Examples in Table 3. It shows a value close to that of a certain low carbon-based low thermal expansion material (Comparative Example a), and in particular, in heat-treated water-cooled products, it reaches a level where results are almost comparable to, or even superior to, Comparative Example a. Furthermore, compared to Comparative Example C, which is a high carbon type material and is considered to have a comparative advantage in vibration damping properties, castability, etc., a significant difference in the average coefficient of thermal expansion is observed.

FIG. 2 shows a comparison between the prior art and the present invention with respect to the relationship between N1 and 00%.

In the figure, ■ indicates the present invention, and ■ indicates the publication of Japanese Patent Application Laid-open No. 84-21037 (
Comparative example b), ■ is published in JP-A-1-306540 (Comparative example a),
) is shown. Of course, it is acknowledged that there are some prior art techniques that partially overlap with the present invention, but if the unique negative effects of the C content are taken into account, there is no doubt that the present invention is advantageous. .

[Brief explanation of the drawing]

FIG. 1 is a diagram plotting experimental results for specifying the present invention, and FIG. 2 is a diagram showing the scope of the present invention and three different prior arts. Figure Figure Figure N i ”/.

Claims (3)

[Claims] (1) C: 1.5-3.0, Si: 0.5-2 in weight%
．． 0, Mn: 1.0 or less, Ni: 22-30, Co: 3
~20, including the balance iron and unavoidable impurities, and the content of Ni and Co is 31.5≦Ni+0.75Co≦37
A high temperature and low thermal expansion cast iron characterized by being within the range of . (2) The high-temperature, low-thermal-expansion cast iron according to claim 1, which is heated to a temperature of 900 to 1150°C and held for a desired time after casting. (3) The high-temperature, low-thermal-expansion cast iron according to claim 2, characterized in that it contains 0.02 to 0.1% by weight of either Mg or Ca or both in total.