CN117535583A

CN117535583A - 850 MPa-level high-elongation quenched and tempered steel plate and production method thereof

Info

Publication number: CN117535583A
Application number: CN202210921328.1A
Authority: CN
Inventors: 唐骜; 刘刚; 王巍; 杨阿娜; 宋凤明
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-08-02
Filing date: 2022-08-02
Publication date: 2024-02-09

Abstract

A850 MPa-level high-elongation quenched and tempered steel plate and a production method thereof. The weight percentages of the components are as follows: c:0.10 to 0.2 percent, si:0.05 to 0.3 percent of Mn:0.80 to 1.60 percent, cr:0.30 to 0.70 percent, mo:0.2 to 0.5 percent, B:0.0005 to 0.003 percent of Al:0.02 to 0.06 percent, ca: 0.001-0.004%, N is less than or equal to 0.005%, P is less than or equal to 0.020%, S is less than or equal to 0.005%, O is less than or equal to 0.004%, and one or more of Nb, ti and V are less than or equal to 0.015%, and the balance is Fe and unavoidable impurities; after hot rolling into a substrate, heating to Ac ₃ Preserving heat for 5-40 min at 80-130 ℃ and cooling to room temperature at a speed of more than or equal to 100 ℃/s; then heating to 500-600 ℃, preserving heat for 5-15 min, and finally air-cooling to room temperature.

Description

850 MPa-level high-elongation quenched and tempered steel plate and production method thereof

Technical Field

The present disclosure belongs to the field of ferrous metallurgy, and in particular relates to a 850 MPa-level high-elongation quenched and tempered steel plate and a production method thereof.

Background

Engineering machinery is an important component of the equipment industry. With the development of large-scale, high-end and light-weight engineering machinery industry, the strength level of steel is continuously improved, and the strength level is generally increased from original 500-600 MPa to 800MPa and 900MPa. Meanwhile, because the service environment of the high-strength steel for engineering machinery is harsh, the steel plate has strict requirements on the strength, bending, welding and other performances and plate shape characteristics of the steel plate besides high strength.

Currently, 800-900 MPa-level quenched and tempered high-strength steel is widely applied in engineering machinery industry, but has high strength, generally poor ductility and formability, and is difficult to be used for producing components with complex shapes. From the component, the steel grade often contains microalloy elements of refined grains such as Nb, V, ti and the like, and the production process adopts the quenching heating temperature of the intersection to inhibit the growth of the grains and improve the strength of the steel grade. Chinese patent No. CN110195193a discloses a method for producing 800 MPa-grade quenched and tempered steel plate, by adding Nb:0.010% -0.035%, V:0.025% -0.055%, ti: micro-alloy elements such as 0.007% -0.014% and the like are used for obtaining a uniform and fine lower bainite+lath martensitic structure, and the average grain size is below 20 mu m, but the elongation is only 15%. There is a need to develop a quenched and tempered steel plate having both high strength and high elongation.

Disclosure of Invention

The invention aims to provide a 850 MPa-grade high-elongation quenched and tempered steel plate and a production method thereof, wherein the content of elements for refining grains such as Nb, ti, V and the like is controlled at a lower level, and the quenching heating temperature is properly increased, so that the structure is coarsened, the plastic deformation capacity of martensite is improved, and the 850 MPa-grade high-strength quenched and tempered steel plate with the yield strength of the elongation more than or equal to 20% is obtained.

In order to achieve the above purpose, the present disclosure adopts the following technical scheme:

a850 MPa-grade quenched and tempered steel plate comprises the following chemical components in percentage by weight (unless specifically stated otherwise, the component contents referred to in the specification refer to weight percent): c:0.10 to 0.2 percent, si:0.05 to 0.3 percent of Mn:0.80 to 1.60 percent, cr:0.30 to 0.70 percent, mo:0.2 to 0.50 percent, B:0.0005 to 0.003 percent of Al:0.02 to 0.06 percent, ca: 0.001-0.004%, N less than or equal to 0.005%, P less than or equal to 0.020%, S less than or equal to 0.005%, O less than or equal to 0.004%, one or more than one of Nb less than or equal to 0.015%, ti less than or equal to 0.015%, V less than or equal to 0.015%, and the balance of Fe and unavoidable impurities.

In one embodiment of the 850MPa grade quenched and tempered steel plate of the present disclosure, the weight percentage of the chemical components satisfies 4Nb+3Ti+2V.ltoreq.12N. Therefore, excessive carbide of Nb, ti and V can be prevented from obstructing grain growth, and the invention effect is better realized.

In one embodiment of the 850 MPa-grade quenched and tempered steel plate disclosed by the invention, one or two of Nb less than or equal to 0.015%, ti less than or equal to 0.015% and V less than or equal to 0.015% are contained in the chemical components of the steel plate, and one is preferable.

The composition design principle of the 850 MPa-level quenched and tempered steel plate is as follows:

c:0.1% of C can ensure the strength of the quenched steel plate; however, a higher C content results in an increase in the overall C equivalent, and cracking is likely to occur during welding. Accordingly, the C content of the present disclosure ranges from 0.10 to 0.2%.

Si: si of more than 0.05 percent can play a better deoxidizing role, and meanwhile, the precipitation of carbide is inhibited in the tempering process, so that the toughness of the steel is improved; however, si exceeding 0.3% tends to produce red scale. Thus, the silicon content of the present disclosure ranges from 0.05 to 0.3%.

Mn: mn element is more than 0.8%, so that the hardenability of the steel can be improved; however, when the Mn content exceeds 1.6%, inclusions such as segregation and MnS are liable to occur, and the toughness of the martensitic high-strength steel is deteriorated. Accordingly, the Mn content of the present disclosure ranges from 0.80 to 1.60%.

Cr: cr element is more than 0.2%, so that the hardenability of the steel can be improved, a full martensitic structure is formed during quenching, cr forms Cr carbide during tempering, and the tempering softening resistance is realized; however, cr content exceeding 0.70% causes a large spark during welding, which affects the welding quality. Accordingly, the Cr content of the present disclosure ranges from 0.30 to 0.70%.

Mo: the Mo element of more than 0.2 percent can improve the hardenability of the steel, and is beneficial to forming a full martensitic structure during quenching; mo reacts with C to form carbide particles during high-temperature tempering, and has the functions of resisting high-temperature tempering softening and softening a welded joint; however, too high a Mo content results in an increase in carbon equivalent, deteriorating welding performance, while Mo is a noble metal, and increases costs. Accordingly, the Mo content of the present disclosure ranges from 0.2 to 0.5%.

Nb, ti and V: these three elements can react with N element preferentially to play a role of fixing N, so as to play a role of protecting B element. However, excessive Nb, ti and V elements react with C in the steel to form nano-sized carbides, preventing grain growth during quenching heating. Therefore, the alloy contains more than one of Nb less than or equal to 0.015%, ti less than or equal to 0.015% and V less than or equal to 0.015%. Meanwhile, according to the chemical proportion of Nb, ti and V respectively reacting with N, 3Nb+4Ti+2V is required to be less than or equal to 12N, so that no or little residual elements are ensured after the three elements react with N preferentially, and excessive carbide which prevents grains from growing is avoided to be formed by the reaction of the three elements with C.

B: the trace B can improve the hardenability of the steel and improve the strength of the steel; however, B exceeding 0.0030% tends to segregate to form a carbon-boron compound, severely deteriorating the toughness of the steel. Accordingly, the boron content of the present disclosure ranges from 0.0005 to 0.0030%.

Al: al is used as deoxidizer, and more than 0.02% of Al is added into the steel to refine grains and improve impact toughness; however, an Al content exceeding 0.06% tends to cause oxide inclusion defects of Al. Accordingly, the Al content of the present disclosure ranges from 0.02 to 0.06%.

Ca: the Ca element exceeding 0.001 percent can play a role of a purifying agent in the steel smelting process, and improve the toughness of the steel; however, ca content exceeding 0.004% tends to form a compound of Ca having a large size, which in turn deteriorates toughness. Therefore, the Ca content of the present disclosure ranges from 0.001 to 0.004%.

N: the present disclosure requires strict control of the range of N element, and an N content exceeding 0.005% tends to cause formation of coarse precipitate particles, deteriorating toughness. Therefore, the N content of the present disclosure is less than or equal to 0.005%.

P, S and O: p, S and O are used as impurity elements to influence the plasticity and toughness of steel, and the control range of the steel is less than or equal to 0.020 percent of P, less than or equal to 0.0050 percent of S and less than or equal to 0.0040 percent of O.

In some embodiments of the present disclosure, 850 MPa-grade temper steel plates according to the present disclosure are composed of the above elements, fe, and unavoidable impurities. In another embodiment of the present disclosure, a 850 MPa-grade quenched and tempered steel sheet according to the present disclosure contains the above elements, fe, and unavoidable impurities.

In one embodiment of the 850 MPa-grade quenched and tempered steel plate of the present disclosure, the yield strength of the steel plate is equal to or more than 850MPa, and the tensile strength is equal to or more than 900MPa. The yield strength and tensile strength may be determined by methods commonly used in the art, for example according to national standard GB/T228.1-2010 tensile test Standard for metallic materials.

In one embodiment of the 850 MPa-grade quenched and tempered steel sheet of the present disclosure, the elongation of the steel sheet is 18% or more, preferably 20% or more, more preferably 21% or more. The elongation can be measured by a method commonly used in the art, for example, according to national standard GB/T228.1-2010 tensile test Standard for metallic materials.

In one embodiment of the 850 MPa-grade quenched and tempered steel sheet of the present disclosure, the microstructure of the steel sheet is high temperature tempered martensite. Wherein, the original austenite grain size is 30-60 mu m, the length of the martensite lath is 15-60 mu m, and the width of the martensite lath is 0.5-5 mu m.

One embodiment of the present disclosure relates to a method for producing 850 MPa-grade quenched and tempered steel plate, comprising the steps of:

1) Smelting and casting

Typically, a converter or an electric furnace is used for steelmaking and refining, and casting is performed to form a casting blank;

2) Heating

Typically, the casting blank is heated in a furnace at 1150-1270 ℃ and heat preservation is started after the core of the casting blank reaches the furnace temperature, and the heat preservation time is more than 1.5h.

3) Rolling

Typically, rolling the casting blank obtained in step 2) to a target thickness by single-stand reciprocating rolling or multi-stand hot continuous rolling, wherein the reduction rate of the last pass of rolling is more than 15%; the final rolling temperature is 820-920 ℃.

4) Cooling

Typically, the rolled sheet obtained in step 3) is coiled at a cooling rate of 60 ℃ or more to a temperature in the range of 480 to 620 ℃ and then air-cooled to room temperature.

5) Quenching heat treatment

Typically, the millboard obtained in step 4) is heated to (Ac) ₃ And (3) after the core of the steel plate reaches the furnace temperature, preserving heat for 5-40 min, and rapidly cooling to room temperature at a cooling speed of more than or equal to 100 ℃/s.

6) Tempering heat treatment

Typically, the tempering temperature is 500-600 ℃, and the steel plate core is kept for 5-15 min after reaching the furnace temperature.

In the production method of the present disclosure:

in the smelting and casting of step (1), the steel typically comprises the following chemical components in percentage by weight: c:0.10 to 0.2 percent, si:0.05 to 0.3 percent of Mn:0.80 to 1.60 percent, cr:0.30 to 0.70 percent, mo:0.2 to 0.50 percent, B:0.0005 to 0.003 percent of Al:0.02 to 0.06 percent, ca: 0.001-0.004%, N less than or equal to 0.005%, P less than or equal to 0.020%, S less than or equal to 0.005%, O less than or equal to 0.004%, further, one or more than one of Nb less than or equal to 0.015%, ti less than or equal to 0.015%, V less than or equal to 0.015%, and the balance of Fe and unavoidable impurities. Further, it is preferable that 4Nb+3Ti+2V.ltoreq.12N in weight percentage of the chemical components.

In the casting blank heating process in the step (2), the heating temperature is controlled to be higher than 1150 ℃, and the core heat preservation time is controlled to be higher than 1.5h, so that the alloy elements can be ensured to be fully dissolved in solid; when the heating temperature exceeds 1270 ℃, austenite grains grow excessively, so that the inter-crystal binding force is weakened, and cracks are easy to generate during rolling; in addition, the heating temperature exceeds 1270 ℃ is easy to cause decarburization of the surface of the steel billet, and the mechanical property of the finished product is influenced.

In the rolling in the step (3), austenite grains of a metallographic structure of the steel plate can be refined by setting the rolling reduction rate of the last pass of rolling to be more than 15 percent and the final rolling temperature to be 820-920 ℃.

In the cooling process of the step (4), the hot rolled steel is cooled to 480-620 ℃ at a cooling speed of more than or equal to 60 ℃/s and coiled to obtain a bainite structure, so that the carbide Cheng Misan is distributed, and the dissolving time of the carbide in the subsequent quenching and heating process is shortened.

In the quenching heat treatment process of the step (5), the quenching temperature is typically Ac ₃ ++ (80-130) DEG C, wherein Ac ₃ For the austenite transformation end temperature, ac can be found according to the empirical formula ₃ =955-350C-25mn+51si+106nb+100ti+68al-11 Cr-33Ni-16cu+67mo. In the quenching heat treatment, the grains can be fully grown by quenching at a higher heating temperature.

Typically, in the quenching heat treatment process, the soaking time of the steel plate is 5-20 min after the core of the steel plate reaches the heating temperature; and after soaking, rapidly cooling to room temperature at a cooling speed of more than or equal to 100 ℃/s to obtain a full martensitic structure.

In the quenching heat treatment process, the heating temperature is lower than Ac ₃ Alloy elements and carbides are insufficiently dissolved at the temperature of +80 ℃ for soaking time of less than 5min, the growth driving force of austenite grains is small, the growth is not obvious, and the effect of improving the plastic deformation capacity of martensite cannot be achieved; heating at a temperature greater than Ac ₃ The soaking time is more than 40min at the temperature of +130 ℃, and the treatment cost is high. Therefore, by controlling the quenching heating temperature and the quenching soaking time within a certain range in the present disclosure, it is possible toEnsures that coarser austenite grains are obtained, thereby coarsening the martensitic structure after quenching and improving the elongation of the steel.

And (6) tempering heat treatment process: the tempering temperature of the steel of the component system exceeds 500 ℃, the steel plate is kept for more than 5 minutes after the core part of the steel plate reaches the furnace temperature, the internal stress of the steel plate can be effectively removed, meanwhile, the dislocation in martensite is recovered to improve the plasticity of the steel, and the alloys Mo and Cr react with C at the temperature to form tiny alloy carbide, so that the yield strength of the steel is improved; tempering temperature exceeding 600 ℃ or long heat preservation time, coarsening alloy carbide, and deteriorating toughness and strength of steel; the optimal matching of the strength and the plasticity can be realized by adjusting the tempering temperature and the tempering time.

In addition, in the step (5) and the step (6), by controlling the heat treatment time and the cooling speed, the rapid heat treatment can be realized, and the economy and the time cost are saved.

In summary, the beneficial effects of the present disclosure at least include:

(1) The method strictly limits the content of Nb, ti and V elements, properly increases the quenching heating temperature, coarsens the structure and improves the plastic deformation capacity of the steel; (2) The thermal histories of quenching and tempering are reasonably controlled to further improve the elongation percentage, and meanwhile, the high strength is ensured, so that the high-strength steel with the yield strength of more than or equal to 850MPa and the elongation percentage of more than or equal to 20% is obtained.

Drawings

Fig. 1 a metallographic structure image after heat treatment of example 4 of the present disclosure.

Fig. 2 a metallographic image of example 6 of the present disclosure after heat treatment.

Detailed Description

The present disclosure is further illustrated below with reference to examples.

The production process flow of the ultra-high strength steel comprises the following steps: converter or electric furnace steelmaking, external refining, continuous casting, heating, rolling, cooling and rapid heat treatment.

Specifically, the manufacturing method of 850 MPa-level high-elongation quenched and tempered steel of examples 1 to 8 of the present disclosure comprises the steps of:

1) Smelting and casting: smelting by using a 500kg vacuum electric furnace, wherein the compositions are shown in table 1, and pouring the smelted molten steel into a casting blank with the thickness of 120 mm.

2) Heating: and heating the casting blank in an electric furnace. The heating temperature is 1150-1270 ℃, and the heat preservation is started after the core of the casting blank reaches the furnace temperature, and the heat preservation time is more than 1.5h.

3) Rolling: and rolling the casting blank into a steel plate with the target thickness of 8mm by adopting multiple passes, wherein the final rolling temperature is 820-920 ℃, and the reduction rate of the last pass is set to 17%.

4) And (3) cooling: and (3) carrying out on-line laminar cooling on the rolled piece, and cooling to 480-620 ℃ at a cooling speed of more than or equal to 60 ℃ per second to obtain a lower bainite structure.

5) The quenching heat treatment process comprises the following steps: heating the hot rolled substrate to Ac ₃ And (3) preserving heat for 5-40 min at the temperature of 80-130 ℃ and after the temperature of the core part of the steel plate reaches the temperature, and rapidly cooling to room temperature at a cooling speed of more than or equal to 100 ℃/s.

6) Tempering heat treatment process: heating the quenched steel plate to 500-600 ℃, and preserving heat for 5-15 mm. The specific process parameters are shown in table 2.

Further, specific process parameters of comparative examples 1 to 4 are shown in Table 2, in which comparative examples 1 and 3 were not subjected to quenching heat treatment. Comparative example 1 relates to CN104513937a "a high strength steel with yield strength of 800MPa grade" disclosed in 2015 and its production "; comparative example 2 relates to CN104532158A "a quenched and tempered high-strength steel with yield strength of 800MPa grade" disclosed in 2015 and a production method thereof; comparative example 3 relates to a "low-cost, high-toughness and excellent weldability 800 MPa-grade quenched and tempered steel plate of CN110195193a published in 2019 and a method for producing the same"; comparative example 4 relates to CN 104532157A' a quenched and tempered high-strength steel with yield strength of 900-1000 MPa grade disclosed in 2015 and a production method thereof.

Mechanical properties of the examples were tested according to national standard GB/T228.1-2010 "tensile test Standard for Metal materials", and the quenched and tempered steel sheets of each example and comparative example were subjected to longitudinal tensile and longitudinal impact tests, and the properties of the templates of each example are shown in Table 3.

The optical microscope structure photographs of examples 4 and 6 are shown in fig. 1 and 2, respectively, and it can be seen from the photographs that the metallographic structure of the finished steel plate is uniform equiaxial tempered martensite, and the structure is coarse, the original austenite grain size measured by the intercept method is 30-60 μm, the length of the martensite lath is 15-60 μm, and the width is 0.5-5 μm.

From a comparison of table 1, the present disclosure relates to steel grades with significant differences in composition from the comparison patent. Comparative examples 1 to 4 all add relatively more Nb, ti and V elements in composition, but this patent adds only one or two of them, and the addition amount is low.

Various mechanical property indexes are obtained, see table 3. From the comparison of mechanical properties, the steel disclosed by the invention has excellent plasticity, the elongation is more than 20%, and the elongation is obviously higher than that of the comparative steel 1-4 with the same strength level. This is because the present disclosure reduces the content of fine-grained elements Nb, ti, and V, and adopts a higher quenching temperature to coarsen the grains, thereby improving the deformability of tempered martensite.

TABLE 2

TABLE 3 Table 3

The above results show that the examples of the present disclosure have a specific element composition, particularly, have contents of specific Nb, ti and V elements, and perform specific quenching and tempering processes, with respect to the comparative examples related to the prior art, whereby the obtained steel sheet has an elongation increased to 20% or more while securing high strength.

The 850 MPa-grade quenched and tempered steel plate can be used in various fields requiring the use of high-strength steel, in particular to the use of a steel plate for engineering machinery requiring high strength and high elongation performance.

Claims

1. A850 MPa-grade quenched and tempered steel plate is characterized by comprising the following chemical components in percentage by weight: c:0.10 to 0.2 percent, si:0.05 to 0.3 percent of Mn:0.80 to 1.60 percent, cr:0.30 to 0.70 percent, mo:0.2 to 0.50 percent, B:0.0005 to 0.003 percent of Al:0.02 to 0.06 percent, ca: 0.001-0.004%, N less than or equal to 0.002-0.005%, P less than or equal to 0.020%, S less than or equal to 0.005%, O less than or equal to 0.004%, one or more of Nb less than or equal to 0.015%, ti less than or equal to 0.015%, V less than or equal to 0.015%, and the balance of Fe and unavoidable impurities.

2. A850 MPa-grade quenched and tempered steel plate is characterized by comprising the following chemical components in percentage by weight: c:0.10 to 0.2 percent, si:0.05 to 0.3 percent of Mn:0.80 to 1.60 percent, cr:0.30 to 0.70 percent, mo:0.2 to 0.50 percent, B:0.0005 to 0.003 percent of Al:0.02 to 0.06 percent, ca: 0.001-0.004%, N less than or equal to 0.002-0.005%, P less than or equal to 0.020%, S less than or equal to 0.005%, O less than or equal to 0.004%, and one or more of Nb less than or equal to 0.015%, ti less than or equal to 0.015%, V less than or equal to 0.015%, fe and unavoidable impurities.

3. The 850 MPa-grade quenched and tempered steel plate as claimed in claim 1 or 2, which contains one of Nb 0.015% or less, ti 0.015% or less, and V0.015% or less.

4. The 850 MPa-grade quenched and tempered steel plate according to claim 1 or 2, wherein Nb, ti, V, N is further satisfied with 4nb+3ti+2v.ltoreq.12n by weight.

5. The 850 MPa-grade quenched and tempered steel plate according to claim 1 and 2, wherein the yield strength is not less than 850MPa and the tensile strength is not less than 900MPa.

6. The 850 MPa-grade quenched and tempered steel plate as claimed in claim 1 and 2, wherein the elongation is not less than 20%.

7. The 850 MPa-grade quenched and tempered steel plate according to claim 1 or 2, wherein the microstructure is high-temperature tempered martensite, the prior austenite grain size is 30 to 60 μm, the martensite lath length is 15 to 60 μm, and the martensite lath width is 0.5 to 3 μm.

8. The production method of the 850 MPa-level quenched and tempered steel plate is characterized by comprising the following steps of:

(1) Smelting and casting;

(2) Heating a casting blank;

(3) Rolling;

(4) Cooling;

(5) Quenching heat treatment, heating the rolled plate obtained in the step (4) to Ac ₃ +80～Ac ₃ 130 Heat preservation is carried out for 5-40 min after the core of the steel plate reaches the furnace temperature at the temperature of more than or equal to 100 ℃/s, and the steel plate is rapidly cooled to room temperature, wherein Ac ₃ Indicating the austenite transformation end temperature; and

(6) And (5) tempering heat treatment.

9. The production method of a 850MPa grade quenched and tempered steel plate according to claim 7, wherein in the step (1), the chemical components described in any one of claims 1 to 3 are used for smelting and casting.

10. The production method of 850 MPa-grade quenched and tempered steel plate according to claim 7 or 8, wherein in step (2), the cast slab obtained in step (1) is heated in a furnace at 1150-1270 ℃ and the heat preservation is started after the core of the cast slab reaches the furnace temperature for a period of time of >1.5 hours.

11. The production method of 850 MPa-grade quenched and tempered steel plate according to claim 7 or 8, wherein in the step (4), the rolled plate obtained in the step (4) is cooled to a temperature of 480 to 620 ℃ at a cooling rate of not less than 60 ℃/s, coiled, and then cooled to room temperature by air.

12. The production method of 850 MPa-grade quenched and tempered steel plate according to claim 7 or 8, wherein in the step (4), the rolling reduction of the last pass is >15%; the final rolling temperature is 820-920 ℃.

13. The production method of 850 MPa-grade quenched and tempered steel plate according to claim 7 or 8, wherein in the step (6), the tempering temperature is 500-600 ℃, the temperature of the steel plate core is kept for 5-15 min after reaching the furnace temperature, and the steel plate is air-cooled to room temperature.

14. The production method of 850 MPa-grade quenched and tempered steel plate according to claim 7 or 8, wherein the yield strength of the obtained steel plate is equal to or more than 850MPa, the tensile strength is equal to or more than 900MPa, and the elongation is equal to or more than 20%.

15. The production method of 850 MPa-grade quenched and tempered steel plate as claimed in claim 7 or 8, wherein the microstructure of the steel plate is high-temperature tempered martensite, the prior austenite grain size is 30-60 μm, the martensite lath length is 15-60 μm, and the martensite lath width is 0.5-3 μm.

16. A850 MPa-grade quenched and tempered steel plate has yield strength of not less than 850MPa, tensile strength of not less than 900MPa, elongation of not less than 20%, microstructure of high-temperature tempered martensite, original austenite grain size of 30-60 μm, length of martensite lath of 15-60 μm, and width of martensite lath of 0.5-3 μm.