JPH08193239A - Steel material with high strength and high coefficient of thermal expansion and its production - Google Patents

Abstract

PURPOSE: To produce a steel material with high strength and high coefficient of thermal expansion by heat-treating a high carbon steel having a specific composition containing Al, Si, etc., under specific temp. conditions. CONSTITUTION: A steel stock, which has a composition consisting of by weight, 0.40-2.0% C, 0.50-3.0% Si, <2.50% Mn, <0.50% Cr, <1.5% Al, and the balance Fe and satisfying (Al+Si)=0.6 to 4.5% or further containing one or >=2 kinds among specific amounts of B, Ni, Mo, Cu, and N and further, as machinability improving elements, one or >=2 kinds among specific small amounts of S, Te, Pb, Ca, and Bi, is heated to a temp. not lower than the Ac, point, cooled down to a temp. between the Ms point and 550 deg.C at a cooling rate higher than the critical cooling rate for pearlite formation, isothermally held at that temp., and cooled down to room temp. By this method, the steel material, having a mixed structure consisting of 30-70vol.% of austenite with a thermal expansion coefficient higher than that of ferrite and the balance bainite or bainite and martensite and also having >343Mpa yield strength and >13×10<-6> / deg.C thermal expansion coefficient, can be obtained.

Description

Detailed Description of the Invention

[0001]

[Industrial application] The present invention has a yield strength of 343 MPa (3 MPa).
The present invention relates to a high strength and high thermal expansion coefficient steel material having a thermal expansion coefficient of 5 kgf / mm ² ) or more and a room temperature to 373 K (a thermal expansion coefficient at 100 ° C.3 of 13 × 10 ⁻⁶ / ° C. or more) and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, cases where steel and non-ferrous metal materials are commonly used have been increasing. In particular, in order to reduce the weight of various mechanical devices, the use of non-ferrous metal materials such as aluminum or aluminum alloys instead of steel is increasing. Since these lightweight metal materials have relatively low strength, they are used in combination with high-strength steel to achieve both mechanical strength and weight reduction.

Since these non-ferrous alloys generally have a larger coefficient of thermal expansion than steel, technical troubles due to the difference in coefficient of thermal expansion are observed. However, since coefficient of thermal expansion is a physical property peculiar to a material, It was impossible to adjust or change.

However, there are two kinds of steels, one having a body-centered cubic crystal structure called ferrite and the other having a face-centered cubic crystal structure called austenite, the former at room temperature and the latter at high temperature. It is in a stable phase. Since austenite has a higher coefficient of thermal expansion than ferrite, austenitic phase steel (eg austenitic stainless steel) has a coefficient of thermal expansion that is about 1.5 times higher than that of normal ferritic phase steel. If the austenite phase can be mixed in the ferritic steel, the coefficient of thermal expansion can be freely changed between the values of both phases.

As an example of a new technology based on such an idea,
Japanese Unexamined Patent Publication No. 61-252912 discloses a technique for preventing seizure of a metal bearing by suppressing a temperature change of a metal clearance of a bearing portion by a combination of a crankshaft and an aluminum alloy cylinder block. As a means, a proposal is disclosed that a crankshaft made of spheroidal graphite cast iron is austempered so that the amount of retained austenite becomes 30 to 50%.

On the other hand, although the purpose is different from the adjustment of the coefficient of thermal expansion, in JP-B-58-42246, C: 0.4-
A method for producing high strength steel strips with a retained austenite content of 30 to 60% by a process of austenitizing 0.85%, Si: 1.4 to 2.5%, Mn: 0.3 to 1.0% steel and holding it at a constant temperature of 380 to 480 ° C is provided. It is disclosed.

The method disclosed in Japanese Patent Publication No. 58-42246 discloses a bainite structure of 65 to 85% and a retained austenite structure in order to improve the ductility of a high-strength heat-treated steel strip used for small parts for precision machinery. This is a manufacturing method of forming a composite structure, and its purpose is completely different from the method for increasing the thermal expansion coefficient, which is the object of the present invention.

[0008]

Here, a general object of the present invention is to provide a steel material for machine structure having a yield strength of 343 MPa (35 kgf / mm ² ) or more, and at room temperature to 373 K (100 ° C.). To provide a high thermal expansion coefficient steel material having a thermal expansion coefficient of 13 × 10 ⁻⁶ / K or more and a manufacturing method thereof.

Therefore, the present inventor has made various studies in order to achieve the above object, and firstly, the prior art was reviewed and organized as follows. Austenitic stainless steel (JIS-G4303, etc.) JP-B-58-42246, JP-A-1-108842.

(1) Austenitic stainless steels: There are two types of steel, one having a body-centered cubic crystal structure called a ferrite phase and the other having a face-centered cubic crystal structure called an austenite phase. In the steel material, the former is stable at room temperature and the latter is stable at high temperature. However, there are alloy steels in which the austenite phase is stable even at room temperature by containing a large amount of Cr and Ni appropriately, and there are alloy steels generally referred to as austenitic stainless steels, which are widely used.

Since the austenitic phase has a higher coefficient of thermal expansion than the ferritic phase, the coefficient of thermal expansion of austenitic stainless steel is approximately the same as that of general ferritic steels.
1.5 times bigger. Therefore, if austenitic stainless steel is used, the coefficient of thermal expansion can be made higher than that of ordinary steel for machine structural use. However, austenitic steel cannot be used as a mechanical structural steel with a high coefficient of thermal expansion due to the following three drawbacks: Low yield strength: Mechanical structural steels considering strength often yield strength. Is required to be 343 MPa (35 kgf / mm ² ) or more [This means JIS-G3113 (steel rod for reinforced concrete), G4051 (carbon steel steel for machine structure), G4102 to 41
09 (various alloy steel materials), etc. However, the yield strength (proof stress) of austenitic steel materials is around 196 MPa (20 kgf / mm ² ) and it is necessary to consider strength. As a result, it lacks strength.

Expensive: Austenitic steel contains a large amount of expensive Ni and Cr in order to stabilize the austenitic phase and is very expensive, so it is difficult to use as a steel for machine structural use.

The coefficient of thermal expansion decreases at low temperatures: For machine structural steel, -213K, so that it can be used even in cold regions.
It is necessary to maintain a stable and large coefficient of thermal expansion even after cooling to (-60 ° C), but the austenitic phase of austenitic stainless steel is often unstable and cannot be put to practical use. Since it is necessary to add a larger amount of Ni, it becomes extremely expensive.

(2) Method of Japanese Patent Publication No. 58-42246: C:
Steel of 0.4 to 0.85%, Si: 1.4 to 2.5%, Mn: 0.3 to 1.0% has low hardenability and has a cooling rate Ac of ₃ points or more from the critical cooling rate of the TTT diagram to 380 to 480 ° C. It is possible to rapidly cool to the temperature range of 1 mm because it is a steel strip with a plate thickness of 1 mm as in this method, and it must be a small part as much as when targeting mechanical parts. It is difficult to achieve the above-mentioned object of the present invention with such a heat treatment method with steel having such a composition.

(3) JP-A-1-108342: C: 0.5-
Since it is steel with 2.5%, Si: 1.0 to 4.0%, and Mn: 0.6% or less, it has low hardenability, so it must be a small part as much as a machine part that undergoes heat treatment. Even if it is manufactured, it is difficult to realize a heat treatment method therefor. Further, since the present invention is a proposal relating to "iron-based castings", hot-rolled steel products are not taught by this.

Therefore, a more specific object of the present invention is a steel material for mechanical structure which is inexpensive and has a yield strength of 343 MPa (35 kgf / mm ² ) or more, which can be used as a hot-rolled material for mechanical structure, and , The coefficient of thermal expansion from room temperature to 373K (100 ℃) is 13 × 10 ^-6
(EN) A high thermal expansion coefficient steel material having a temperature of / ° C or higher and a method for producing the same.

[0017]

The gist of the present invention is as follows. (1)% by mass, C: 0.40 to 2.0%, Si: 0.50 to 3.0%,
Mn: less than 2.50%, Cr: less than 0.50%, Al: less than 1.5%, Al
+ Si: 0.6 to 4.5%, and if necessary, one or more of B: 0.01% or less, Ni: 3% or less, Mo: 2% or less, Cu: 2% or less, and N: 0.03% or less. And / or S: 0.02 to 0.2%, Te: 0.001 to 0.1%,
Pb: 0.001 to 0.5%, Ca: 0.001 to 0.02%, and Bi:
It has a steel composition that contains 0.001 to 0.5% of one or more machinability improving elements, and the balance Fe and unavoidable impurity elements. The austenite content of 30% or more and 70% or less and the balance are It is composed of bainite or a composite structure of bainite and martensite and has a yield strength of 343 MPa (3 MPa).
5 kgf / mm ² ) or more, and the coefficient of thermal expansion at room temperature to 100 ° C is 13
It is a high-strength, high-thermal-expansion-strength steel material with a temperature of × 10 ^-6 / ° C or higher.

(2) C: 0.4 to 2%, Si: 0.5 by mass%
~ 3.0%, Mn: less than 2.5%, Cr: less than 0.5%, Al: 1.5
%, Al + Si: 0.6-4.5%, and if necessary,
B: 0.01% or less, Ni: 3% or less, Mo: 2% or less, Cu: 2
% Or less, and N: 0.03% or less and one or more types are contained, and / or S: 0.02 to 0.2%, Te: 0.00
1 to 0.1%, Pb: 0.001 to 0.5%, Ca: 0.001 to 0.02
%, And Bi: 0.001 to 0.5% of one or more machinability improving elements, and a steel having a steel composition consisting of the balance Fe and unavoidable impurity elements, after heating to Ac ₁ point or more. , At a cooling rate higher than the critical cooling rate for producing pearlite to a temperature range from the Ms point to 550 ° C. or lower, and keeping the temperature constant at that temperature range, and then cooling to room temperature, the volume ratio is 30%. % To 70% austenite and the balance bainite or bainite and martensite, and the yield strength is 343 MPa (35 kgf
/ mm ² ) or more, and the coefficient of thermal expansion at room temperature to 100 ° C is 13 × 10
It is a method for producing steel materials with high strength and high coefficient of thermal expansion that are ^-6 / ° C or higher.

[0019]

Next, the operation of the present invention will be described. In the present invention, it is intended to obtain a mixed structure of bainite and austenite mainly by a high temperature transformation treatment known as austempering treatment as disclosed in Japanese Patent Publication No. 58-42246. In the steel of the component system of, the amount of retained austenite obtained is small and unstable, and at low temperatures [−213K (−60 ° C)], the retained austenite transforms to martensite and the amount of retained austenite decreases further. Therefore, it has been found that it is not possible to meet the demand as a stable structural steel having a high coefficient of thermal expansion.

Therefore, in the present invention, in order to increase the amount of residual austempered by austempering and to obtain stable retained austenite that is unlikely to martensite even when exposed to low temperatures, the combined addition of Si and Al and the addition of Cr are performed. The amount is reduced.

In order to generate a large amount of stable retained austenite during austempering, it is extremely important that pearlite transformation does not occur during cooling during austempering. In order to secure the above, the alloy elements Mn, Ni, Mo, Cu, B and N that delay the pearlite transformation are utilized together with Al and Si.

On the other hand, as Cr, which is most often used as a hardenability improving means, when the content thereof is large, the retained austenite produced by the austempering treatment becomes unstable at a low temperature and easily becomes martensite, so its utilization is limited. To do.

As described above, the present invention addresses the above technical problems by appropriately adjusting the composition of the hot-rolled steel for machine structural use and by subjecting the steel to an appropriate heat treatment. Trying to solve a problem,
Here, the reason for limiting the steel composition and manufacturing conditions in the present invention as described above will be explained. In this specification, unless otherwise specified, "%" indicating the steel composition is "mass%".

(1) C: 0.40 to 2.0%, preferably 0.80 to
The 1.50% C content is selected according to the required properties of the steel material, but if the addition amount is 0.4% or less, the residual austenite content is 30%.
Since it is less than%, the lower limit is set to 0.4%. Further, if the C content exceeds 2%, C cannot be completely dissolved in austenite during heat treatment, so the upper limit was made 2%.

In order to stably achieve the target amount of retained austenite of 30% or more, the amount of C is preferably 0.80% or more,
In addition, the amount of C is 1.50 for stable production without cracking during hot rolling when producing as hot rolled steel.
% Or less is preferable.

(2) Si: 0.50 to 3.0%, preferably 1.0 to
2.5% Si has a function as a deoxidizing agent as well as a function as a deoxidizing agent so as to be contained in ordinary steel, and an effect of improving mechanical properties by solid solution strengthening and hardenability improvement. It has a very important function and effect in increasing the amount and stability of retained austenite generated in such heat treatment.

When the Si amount is 0.5% or less, the amount of retained austenite does not exceed 30%. Further, if the Si content exceeds 3%, the melting point of steel lowers and cracking frequently occurs during hot rolling. Therefore, the lower limit of the amount of Si was 0.50% and the upper limit was 3.0%.

The required amount of Si has a composite action with Al, which will be described later, and the required amount is determined in consideration of the Al content.
If it is 1.0% or more, a stable amount of retained austenite can be obtained regardless of the amount of Al, and if the amount of Si is 2.5% or less, the risk of forging cracking is reduced even in hot forging at a high temperature, so 1.0-2.5% Was the preferred range.

(3) Mn: less than 2.50%, preferably 1.0-2.
0% Mn is an essential alloying element for improving the hardenability of steel and heat treating mechanical parts to form a mixed structure of bainite and retained austenite (which may contain some martensite). Hardenability is insufficient with C and Si alone, and in many cases,
Since pearlite transformation is accompanied during heat treatment and it interferes with the subsequent formation of bainite and retained austenite,
For general use, the content of 1% or more is preferable. The amount added is 2.
If it exceeds 5%, cracking tends to occur during hot rolling,
If it exceeds 2.0%, the heat treatment time required for producing retained austenite becomes long, so 1.0 to 2.0% was made the preferred range.

(4) Cr: less than 0.5%, preferably 0.10% or less Cr is generally the alloy element most often used as a hardenability improving means, but in the present invention, the Cr content is 0.50%.
If it is contained in the above amount, the amount of retained austenite generated by the austempering treatment becomes extremely small and it becomes unstable, so that it becomes a mixed structure of bainite and martensite. Since it becomes more unstable and easily becomes martensite at low temperature, 0.10% or less is preferable for the retained austenite to be stable even at -60 ° C.

(5) Al: less than 1.5%, preferably 0.05 to 1.
0% Al is an element effective in improving the amount and stability of retained austenite. Al is generally added to the steel as a deoxidizing agent, but with such an amount of content, there is little effective effect to generate a stable retained austenite of 30% or more, which is the object of the present invention, and the retained austenite is stable. It is desirable that the content be 0.05% or more for the purpose of conversion. On the other hand, if a large amount of Al is added, the hot workability deteriorates, so the Al content is less than 1.5%, preferably 1.0% or less.

(6) Al + Si: 0.6 to 4.5%, preferably 1.
05-3.5% It was found that the combined addition of Si and Al is more effective than the single addition, because it is a stable retained austenite that does not easily form martensite even when the steel material or steel parts after austempering is exposed to low temperature. did. The effect of such complex addition appears when Al + Si exceeds 0.6%, but especially 1.05
Since the action and effect are remarkable at ˜3.5%, 1.05 to 3.5% is set as the preferable range. Further, when Al + Si exceeds 4.5%, the hot workability is extremely deteriorated, so the upper limit was made 4.5%.

(7) B: 0.01% or less, preferably 0.001 to
0.005%, Ni: 3.0% or less, Mo: 2.0% or less, Cu: 2.0
% Or less and N: 0.03% or less, one kind or two or more kinds. All of these alloy elements are elements added for improving hardenability. The heat treatment in the present invention is performed by converting austenite
Since it maintains a constant temperature in the temperature range of points to 550 ° C and produces a structure containing 30 to 70% of retained austenite, it forms pearlite when the austenite is cooled to the relevant temperature range. Since the amount decreases, it is necessary to make the critical cooling rate of the target steel material or steel part smaller than the critical cooling rate for producing pearlite. For this purpose, it is only necessary to add a hardenability improving element that has been conventionally applied, but in the present invention, the stability of retained austenite as found in Cr is impaired. must not. At the same time, the retained austenite stabilizes while it transforms from austenite to bainite in the temperature range of Ms point to 550 ° C while it is kept at a constant temperature.However, when the temperature is kept longer, the retained austenite becomes ferrite and carbide in that temperature range. Since it is thermally decomposed, it is counterproductive to lengthen the time for the hardenability-improving element to transform from austenite to bainite at a constant temperature. That is, the alloying element to be used in the present invention has a remarkable hardenability-improving effect in the sense that it delays the pearlite transformation, and at the same time, transformation during transformation from austenite to bainite at constant temperature. It must also have the adverse effect of not increasing the time, that is to say that the increase in bainite hardenability is small.

The above five elements were selected to be suitable for these purposes. However, even if the contents of these elements are too large, the transformation time at the time of transformation from austenite to bainite at a constant temperature becomes too long, so the upper limits of the contents were limited to the respective values.

(8) S: 0.02 to 0.2%, Te: 0.001 to 0.1
%, Pb: 0.001 to 0.5%, and Ca: 0.001 to 0.02%,
Bi: One or more machinability-improving elements selected from the group consisting of 0.001 to 0.5% In mechanical structural steels, some kind of machining is performed in the process of machining parts. Machinability is always a problem in that case, and its improvement can be an essential issue in some cases.

Therefore, it is important in the present invention whether or not the known improvement in machinability is possible. As a result of investigation and investigation, it was found that these known machinability improving means had no adverse effect on the effect of the present invention in the steel of the present invention.

(9) Heating at Ac ₁ point or higher, preferably Ac ₃ point or higher or Ac _m point or higher: First, in order to heat treat steel or steel parts to mix an austenitic structure of 30 to 70% by volume, The first necessary heat treatment condition is that the steel has an austenitic structure.

In this case, it is desirable to heat the mixture to the Ac ₃ point or more or the Ac _m point or more to completely austenite the whole, but depending on the case, Ac ₁ point to Ac ₃ point or Ac _m point.
It is also possible to heat to an intermediate temperature range of points.

(10) Cooling at a cooling rate higher than the critical cooling rate for producing pearlite: After austenitizing and heating, in order to transform austenite into a mixed structure of bainite and retained austenite, prior to the transformation, austenite It is necessary to cool at a cooling rate higher than the critical cooling rate so that pearlite is not generated during the cooling process. This is because if pearlite is generated, the amount of untransformed austenite is reduced, and thus the amount of retained austenite that can be generated in the subsequent isothermal transformation is reduced.

(11) Cooling to a temperature range above the Ms point and below 550 ° C., maintaining a constant temperature in the temperature range, and then cooling to room temperature:
The reason why the quenching temperature from austenite is above the Ms point and below 550 ° C is to transform the bainite structure. The reason above the Ms point is to prevent the martensitic transformation from occurring during this cooling process, and below 550 ° C is to cause the bainite transformation, and at temperatures above this, the temperature range in which the pearlite transformation occurs occurs.

To maintain the temperature in this temperature range, the austenite is maintained in this temperature range to form bainite,
This is for stabilizing the untransformed austenite and converting it to retained austenite. When the amount of bainite produced is too small, the untransformed austenite becomes insufficiently stabilized and cannot exist stably as retained austenite, so it is necessary to maintain a constant temperature for a sufficient amount of time for transformation production of bainite. Since this required time varies depending on the alloy component content, it is necessary to determine the appropriate time depending on the steel type component.

(12) A composite structure having a volume ratio of austenite of 30% or more and 70% or less and the balance of bainite or the balance of bainite and martensite: thermal expansion coefficient of 13 × 10 ⁻⁶ / ° C. at room temperature to 100 ° C. To achieve the above, volume ratio is 30%
The above austenite must be present in the residual. On the other hand, when the amount of retained austenite exceeds 70% (volume), it is necessary to use 343 MPa (35 kgf / mm ² ) for machine structural steel.
It becomes difficult to give the above yield strength.

(13) The coefficient of thermal expansion at room temperature to 100 ° C. is 13 × 10 ^-6
/ ° C or higher: When steel and aluminum and non-ferrous metal materials such as aluminum or aluminum alloys are used in common because weight reduction of various mechanical devices is required, these light weight metal materials have a coefficient of thermal expansion larger than that of steel, and various differences arise due to the mismatch. However, there is a technical limit to the reduction of the coefficient of thermal expansion of non-ferrous metal materials, and it is not completely the same as that of steel so far. Therefore, the coefficient of thermal expansion of steel is set to 13 × 10
^By setting the temperature to ^-6 / ° C or higher, the difference in thermal expansion between the two metals can be eliminated.

(14) Yield strength of 343 MPa (35 kgf / mm ² ) or more:
In many cases, mechanical structural steels considering strength require a yield strength of at least 343 MPa (35 kgf / mm ² ) [This means that JIS-G 3113 (steel rod for reinforced concrete), G4051 (machine Structural carbon steels), G 4102-4109 (various alloy steels), etc.]. Therefore, the yield strength of the present invention, which is intended for use in machine structural steel, is 343.
Must be at least MPa (35 kgf / mm ² ). Next, the function and effect of the present invention will be described more specifically with reference to Examples.

[0045]

【Example】

(Example 1) Each steel shown in Table 1 was melted in a 100 kg induction heating melting furnace, and the obtained ingot was sufficiently soaked and then hot forged to forge a square bar having a cross section of 50 mm x 50 mm. did.

Steel types Nos. 1, 7, 11, 13, and 14 are all out of the composition range of the steel of the present invention. In addition, Ac
₁ , Ac ₃ , and Ms points are 1008K (Ac ₁ point), 1073K (Ac ₃ points), and 553K, respectively, for steel type No. 15.
(Ms point).

Table 2 shows combinations of manufacturing conditions. Manufacturing conditions 1, 2, and 3 are all methods of the present invention. Manufacturing conditions 4 and 5 are such that the quenching temperature in the salt bath in the austempering treatment is out of the scope of the present invention.

Manufacturing condition 6 is a comparison with manufacturing condition 1 in which a sub-zero treatment (deep cooling treatment) was carried out by cooling and holding at 213K (-60 ° C) for 2 hours in consideration of the use environment in cold regions. Is.

The amount of retained austenite was obtained from the integrated value of the X-ray diffraction intensity from the ferrite phase and the austenite phase. The coefficient of thermal expansion was obtained from the coefficient of linear expansion between room temperature and 373K (100 ° C). Table 3 collectively shows the results of the examples.

In Test No. 1, the amount of residual austenite is small and the coefficient of thermal expansion is too small because the amount of C is insufficient. Test No. 11 was a sample that could not be put to practical use because the amount of Mn was too large and therefore hot working cracks occurred during hot forging. Test No. 13 had a small amount of retained austenite and a too small thermal expansion coefficient because the Cr content was too large. In Steel 14 and Steel 28 of Test No. 14 and Test No. 28, the amount of residual austenite was small and the coefficient of thermal expansion was too small because the amount of Si + Al was small.

In Test No. 31, since the amounts of Si and Al were too large, severe cracking occurred during hot forging of the steel ingot, and hot rolled steel could not be manufactured. Exam number 33 and
34 is steel 13 and Si that have too much Cr as in test numbers 13 and 14.
Steel 14 with a small amount of + Al was subjected to subzero treatment at -60 ° C after austempering. In any case, the stability of retained austenite formed in austemper was extremely unstable at low temperatures, and most of it was subzero. Retained austenite transforms into martensite. Therefore, the coefficient of thermal expansion also drops to an extremely small value.

Test No. 40 is one in which retained temperature in the austemper was too low and retained austenite was not formed, and martensite was formed. Test No. 41 is a test in which the pearlite transformation proceeded due to the too high isothermal holding temperature in the austemper and no retained austenite was generated.

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

[0056]

As described above, according to the present invention, a large amount of extremely stable retained austenite is generated by the heat treatment, which results in a large coefficient of thermal expansion and a high yield which cannot be realized by the conventional steel for machine structural use. Strength can be realized. According to the present invention, the yield strength is 343 MPa (35 kgf / mm ² )
Production and supply of high strength and high coefficient of thermal expansion steels with a coefficient of thermal expansion of 13 × 10 ^-6 / K or more at room temperature to 373K (100 ° C), and parts made of these steels It becomes industrially possible to bring about an extremely useful effect in industry.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/60

Claims (4)

[Claims] 1. In mass%, C: 0.40 to 2.0%, Si: 0.50 to 3.0%, Mn: less than 2.50%, Cr: less than 0.50%, Al: less than 1.5%, Al + Si: 0.6 to 4.5.
%, The balance Fe, and an unavoidable impurity element, and a composite structure consisting of austenite with a volume ratio of 30% or more and 70% or less and the balance bainite or bainite and martensite, and the yield strength is 343 MPa ( 35kgf / mm ² )
Above, and coefficient of thermal expansion from room temperature to 100 ℃ 13 × 10 ^-6 / ℃
The high strength and high coefficient of thermal expansion steel materials described above. 2. The steel composition further comprises B:
0.01% or less, Ni: 3.0% or less, Mo: 2.0% or less, Cu: 2.
1 selected from the group consisting of 0% or less and N: 0.03% or less
High strength according to claim 1, containing two or more species.
Steel with high coefficient of thermal expansion. 3. The steel composition further comprises, in mass%, S:
0.02 to 0.2%, Te: 0.001 to 0.1%, Pb: 0.001 to 0.5
%, Ca: 0.001 to 0.02%, and Bi: 0.001 to 0.5%, and one or more kinds of machinability improving elements selected from the group consisting of high strength and high thermal expansion according to claim 1 or 2. Rate steel material. 4. A steel having the steel composition according to any one of claims 1 to 3 is heated to an Ac ₁ point or higher, and then at a cooling rate higher than a critical cooling rate at which pearlite is generated, a Ms point or higher and 550 ° C. Cooling to the following temperature range, holding at a constant temperature in the temperature range, and then cooling to room temperature, austenite with a volume ratio of 30% or more and 70% or less and the balance is bainite or bainite and martensite composite A method for producing a high-strength / high-thermal-expansion steel material having a structure, a yield strength of 343 MPa (35 kgf / mm ² ) or more, and a thermal expansion coefficient of 13 × 10 ⁻⁶ / ° C or more at room temperature to 100 ° C.