JP7267430B2 - Steel plate preparation method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an extra-thick steel plate and a method for producing the same, and more particularly to an extra-thick steel plate of 690 MPa grade and a method for producing the same.

With the implementation of the national offshore development strategy and the gradual expansion of the exploration of oil and gas resources from land to deep water and polar directions, the requirements for the performance and structural safety of offshore platforms are becoming higher. The steel materials required for the manufacture of offshore platforms are evolving towards high strength and high toughness, and the demand for plate for high strength and high toughness offshore platforms with a yield strength grade of 690 MPa is increasing. The mechanical properties of the core of 690 MPa grade extra heavy steel for conventional marine engineering are difficult to improve. In order to uniformly improve the overall performance of the steel sheet, a large number of elements such as Ni, Mo, Cr, and Cu are usually added, and the total amount of these elements is 4% or more, so the alloy cost is high. . In addition, the production method requires a large number of steps such as quenching, making the production difficult. In recent years, the development of high-strength grade extra-thick steel plates has been widely concerned to meet the improvement of marine engineering construction.

Chinese Invention Patent No. 201510125485.1 discloses a low yield ratio, high toughness and high strength extra thick steel plate with excellent low temperature impact toughness and a method for producing the same. Since 3.6 to 5.5% of Ni is contained in the chemical composition of the high-strength extra-thick steel plate, the cost is high.

A Chinese invention patent with patent number 201610026446.0 discloses a high-strength steel plate for marine engineering and a method for producing the same, but despite using Nb and V micromyerization methods, such as Mn Since the hardenability is high and the content is not high, the maximum thickness of the steel sheet cannot exceed 100 mm.

In view of the current state of material development, the performance of high-performance marine engineering heavy steel plates still needs to be improved.

Object of the Invention: In order to overcome the shortcomings of the prior art, the present invention provides a 690MPa grade with excellent core mechanical properties, which can meet the demand for high-performance extra-thick steel plates in harsh use environments such as marine engineering. provide extra heavy steel plates.
Another object of the present invention is to provide a method for producing the above 690 MPa grade extra heavy steel plate.

Technical solution: In the 690MPa grade extra heavy steel plate of the present invention, the mass percentage of the chemical composition is C: 0.04-0.08%, Mn: 5.2-6.0%, Si: 0.1 ~0.4%, Mo: 0.1-0.5%, Ni: 0.2-0.6%, Cr: 0.2-0.6%, Ti: 0.01-0.05%, S: ≤ 0.005%, P: ≤ 0.010%, the balance containing Fe and impurities.

Further, the steel plate has a thickness of 80 to 150 mm.

Moreover, the microstructure of the steel sheet comprises martensite and austenite, in which the volume fraction of austenite is 4-10%.

Furthermore, the mechanical properties of the core of the steel sheet are such that the yield strength is 690 MPa or more, the tensile strength is 770 MPa or more, the elongation after breaking is 14% or more, and the Charpy pendulum impact test of the V-shaped test piece at -60 ° C. is absorbed. energy is 80J or more.

Further, the steel sheet has a cross-sectional shrinkage of 50% or more due to elongation in the thickness direction.

The method for producing 690MPa grade extra heavy steel plate according to the present invention adopts a technical solution including the following steps.
(1) After iron water desulfurization treatment, converter smelting is performed to reduce the S and P contents in the steel water to S≦0.005% and P≦0.010%.
(2) After completing the alloying of the required mass fractions of C, Mn, Si, Mo, Ni and Ti elements by LF refining, perform RH treatment to make the degree of vacuum ≦4 mbar and the working time ≧20 min.
(3) The slab is obtained by casting, and the ratio of the thickness of the slab to the thickness of the steel plate is 4 or more.
(4) The temperature of the slab is heated to 1060-1140° C., and the soaking time is 40-90 minutes.
(5) Roll the heated slab to a rolling start temperature ≤ 1020°C, a rolling end temperature ≥ 930°C, and a transit deformation amount ≥ 10%.
(6) The steel sheet after rolling is immediately water-cooled, and the temperature at which the surface of the steel sheet after cooling returns to red is ≤360°C, and the average cooling rate is 1 to 5°C/s.
(7) Perform quenching heat treatment, reheat the steel sheet to 780 ~ 830 ℃, soaking time is 5 ~ 15 minutes, water cooling until the temperature of the steel sheet surface returns to red is ≤ 110 ℃, average cooling rate is 2 ~8°C/s.
(8) Perform tempering heat treatment, heat the quenched steel sheet to 610 to 640° C., soak for 40 to 70 minutes, and air-cool the tempered steel sheet to room temperature.

Beneficial effect: The steel sheet of the present invention uses manganese as the main alloying element, and reduces the addition amount of expensive elements such as nickel during the production of extra thick steel sheet, thereby reducing the alloying cost. By using a specific manufacturing process, the produced extra-thick steel plate has excellent core mechanical properties of high strength, high plasticity, high toughness, and anti-lamellar tearing performance, making it suitable for harsh uses such as marine engineering. It can meet the demand for high-performance extra-thick steel plates in the environment.

1 is a transmission electron micrograph of the core composition of the steel sheet in Example 1. FIG.

In the following, the chemical composition, manufacturing process, configuration and performance of the present invention will be described in accordance with examples.

In the chemical composition design of the 690 MPa grade extra heavy steel plate of the present invention, C can significantly increase the structural strength through interstitial solid solution strengthening, and is an important strengthening element and an important austenite stabilizing element. In order to ensure low temperature impact toughness and weldability, its addition should be controlled to a low level. Mn can greatly improve the stability of austenite at the same time as increasing the strength of the structure through substitution and solid solution strengthening. Appropriate addition of C and Mn can significantly improve hardenability, lower the phase transition temperature of supercooled austenite, and obtain a high-strength martensitic structure. On the other hand, in the martensite tempering process, the addition of C and Mn lowered the temperature required to form a certain amount of reverse-transformed austenite. Since C and Mn are rich in reverse-transformed austenite, they can maintain a stable structure even at low temperatures, making them important structures for improving the plasticity and toughness of the present invention. If the tempering temperature of the present invention is lower than 600°C, especially around 550°C, grain boundary segregation of elements such as Mn and P tends to occur, which may reduce toughness. The inventor fully studied the mechanism of action of the C and Mn elements in the present invention, and found that "low carbon manganese" with 0.04 to 0.08% C and 5.2 to 6.0% Mn A compositional design was decided.

Si is a deoxidizing element in the steelmaking process, and an appropriate amount of Si can suppress segregation of Mn and P and improve toughness. Solid solution strengthening is also possible with Si, but if the content exceeds 0.3%, the toughness may be significantly reduced. In the present invention, Si is controlled to 0.1 to 0.4%.

Mo can increase the strength of martensite after tempering, and within a specific content range, can also weaken grain boundary segregation of Mn to improve toughness. In the present invention, since the Mo content is controlled to 0.1 to 0.5%, Mo does not significantly increase the cost while fulfilling the role of Mo.

Ni stabilizes the austenite phase, improves hardenability, and lowers the ductility-brittle transition temperature. It is an element effective in improving low-temperature toughness and also helps improve weldability. However, Ni is expensive, and the present invention controls the Ni content to 0.2 to 0.6%, and fully exhibits the beneficial effects of the Ni element without significantly increasing the cost. .

Cr can produce a distinct solid-solution strengthening effect, which is beneficial for increasing strength and can improve corrosion resistance. However, in the case of adding a large number of Mn elements in the present invention, if the Cr content is too high, carbides of Cr and Mn are likely to be formed at the grain boundaries during tempering, which reduces the grain boundary barriers to crack growth and increases plasticity. and may reduce toughness. In the present invention, the Cr content range is controlled within an appropriate range of 0.2 to 0.6%.

In the present invention, a small amount of Ti is added, which can prevent grain boundary migration at high temperature through a fine and dispersed second phase precipitation morphology, thereby refining grains and improving mechanical properties. Improving. The amount added is controlled within the range of 0.01 to 0.05%.

It is necessary to strictly control the contents of P and S. In the present invention, when a medium content of Mn element is added, S easily forms Mn and MnS, which may reduce plasticity. P tends to segregate at grain boundaries, lowering crack growth resistance at grain boundaries and reducing toughness. The present invention requires S≤0.005% and P≤0.010%.

The remainder of the present invention is Fe, but the normal manufacturing process unavoidably introduces impurities from raw materials or the surrounding environment. Since these impurities are obvious to those skilled in the art, their names and contents are not specifically described herein.

In the production method of the present invention, the S and P contents are reduced to S ≤ 0.005% and P ≤ 0.010% by performing converter smelting after iron water desulfurization treatment, and a sufficiently high degree of vacuum ( The content of gas impurity elements is reduced by RH treatment with a vacuum degree ≤ 4 mbar) and a sufficiently long vacuum time (treatment time ≥ 20 min), but the addition of C, Mn, Si, Mo, Ni, Ti and other alloys is completed by LF refining, so a high-purity refining effect can be obtained.

Slab casting can be continuous casting or die casting + forging to obtain slabs of various sizes. When the thickness of the slab is ≦320 mm, the production efficiency is high because it is obtained by continuous casting. Thicker slabs (>320 mm) are obtained by die casting + forging. In order to achieve the mechanical properties of the core required in the present invention, a sufficient rolling total deformation amount is required as a necessary condition. In the present invention, since the ratio of the thickness of the slab to the thickness of the steel sheet must be 4 or more, it is possible to ensure that the total rolling deformation amount is 75% or more. If the steel plate thickness is 80 mm, the required slab thickness is 320 mm or more, and if the steel plate thickness is 150 mm, the required slab thickness is 600 mm or more. The resulting slab acquires the required structure and properties through rolling and heat treatment processes.

Within the composition range of the present invention, the Ac3 temperature of the steel is 770°C or less. When the slab is heated to 1060-1140° C., a high temperature austenitic structure is formed and at the same time the alloying elements such as C, Mn are homogenized by diffusion. During soaking when the temperature of the core of the slab is close to the surface temperature and the heat retention continues, the austenite is homogenized throughout the slab, and the soaking time of 40-90min can ensure the uniform diffusion of the elements. At temperatures below 1140° C., Ti second phase particles can play a role in hindering grain growth. However, if the temperature is less than 1060° C., the diffusion of the elements slows down and the austenite homogenization efficiency may decrease.

In the present invention, the heated slab is recrystallized at 930° C. or higher and rolled to refine grains. When the rolling start temperature ≤ 1020°C, it is possible to prevent the crystal grain growth rate after recrystallization from becoming too fast. If the passing deformation amount is 10% or more, the austenite can have sufficient accumulated strain energy after deformation to ensure the refinement effect of recrystallization.

After rolling the steel sheet, it is necessary to immediately water-cool the steel sheet after rolling in order to avoid excessive growth of recrystallized grains refined after rolling deformation. Martensite transformation may also occur during water cooling. By adding a sufficient amount of Mn element in the present invention, the critical cooling rate for martensitic conversion is less than 1° C./s, but a martensitic structure can be obtained even at low cooling rates. When the thickness of the steel plate is large, the cooling of the core is usually much slower than the cooling of the surface, but the compositional design of the present invention allows the martensite transformation to occur even in the core of the steel plate with a thickness of 80 to 150 mm. can be guaranteed. However, if the cooling rate is too fast, the thermal stress of the steel sheet tends to be too high, and cracks may even occur in the steel sheet. ing. The temperature at which the surface of the steel plate cooled after rolling returns to red is selected below 360°C, which can prevent obvious elemental segregation during cooling and suppress the precipitation of coarse carbides. In addition, it is lower than the martensite transformation start temperature in the composition of the present invention. The post-rolling cooling process selected in the present invention can provide a precursor structure suitable for subsequent heat treatment processes.

In the present invention, a steel sheet is subjected to quenching and tempering heat treatment. The quenching temperature of 780-830°C is higher than Ac3, and the austenitic structure is obtained by soaking. The steel sheet is water-cooled after rolling, which avoids the segregation of elements and the formation of coarse carbides during cooling, thus greatly shortening the homogenization time of the elements in the austenite. The soaking time for quenching heating selected in the present invention is 5 to 15 minutes, and it is possible to effectively refine the grain size while ensuring homogenization of austenite, which is beneficial for improving the mechanical properties of the steel plate. is. The selection of the cooling rate for quenching is controlled at 2-8° C./s for the same reason as the selection of the cooling rate after rolling. However, the temperature at which the surface of the quenched steel sheet returns to red must be 110°C or lower, which is lower than the martensite transformation end temperature under the composition of the present invention, and the entire steel sheet can be quenched with high strength. Obtaining a martensitic structure can be guaranteed.

Tempering heat treatment is performed on the steel plate after quenching. In the tempering process with a tempering temperature of 610 to 640 ° C. and a soaking time of 40 to 70 minutes, in addition to improving the strength and toughness consistency of martensite, reverse-transformed austenite with a volume fraction of 4 to 10% can be obtained. is mainly in the form of a film and is distributed between the martensite slats. During tempering, austenite stabilizing elements such as C and Mn are enriched in the austenite, which not only increases the stability of the austenite, but also during air cooling to room temperature after tempering and even at lower temperatures. Austenite can still maintain the stability of the crystal structure and no martensite transformation occurs. Air-cooling the steel plate after tempering can also reduce the thermal stress of the extra-thick steel plate and improve the quality of the steel plate.

In the present invention, after heat treatment, a martensite+austenite structure was obtained in the entire thickness direction of the steel sheet, particularly in the core portion of the steel sheet. During tensile deformation, the martensite matrix provides a yield strength of over 690 MPa, and the plasticity of martensite after tempering is also improved. Austenite acts as a soft phase to relieve local stress concentrations during the early and middle stages of deformation, whereas martensite occurs during the later stages of deformation and can play a strengthening role. Therefore, the presence of austenite plays an important role in retarding crack initiation and propagation and improving tensile strength and post-break elongation. During impact deformation, the presence of austenite improved impact toughness by impeding crack propagation and increasing crack propagation resistance. The austenite in the present invention has sufficient stability to exert beneficial effects on impact toughness even at -60°C. The beneficial effect of the austenitic structure in the present invention is closely related to its volume fraction and elemental enrichment, and the selection of process parameters of the manufacturing method, especially the tempering heat treatment, is most directly related to the austenitic structure. nature is determined.

Specifically, the mechanical properties of the core of the steel sheet of the present invention are a yield strength of 690 MPa or more, a tensile strength of 770 MPa or more, an elongation after breakage of 14% or more, and a V-shaped test piece at -60 ° C. The energy absorbed in the Charpy pendulum impact test is 80 J or more.

The definition of mechanical properties in the present invention follows standards GB/T228.1, GB/T229 and GB/T5313. No explanation.

In particular, the present invention achieves excellent core mechanical properties of the steel sheet, while the mechanical properties of other positions in the thickness of the steel sheet also reach the core mechanical properties. Since the present invention effectively controls the structure and performance of each position of the extra thick steel plate, the steel plate has a high cross-sectional shrinkage in the thickness direction, and the cross-sectional shrinkage due to elongation in the thickness direction is 50%. % and above, its anti-lamellar tear performance is very good.

The details of the steel sheet and the method for producing the steel sheet will be described below by way of specific examples.

Example 1: 80 mm thick and 0.06% C, 5.7% Mn, 0.22% Si, 0.35% Mo, 0.2% Ni, 0.31% A core with a chemical composition containing Cr, 0.02% Ti, S≤0.005%, P≤0.010%, the balance Fe and other unavoidable impurity elements (content expressed in mass percentage) 690 MPa grade extra heavy steel plate with excellent mechanical properties.

The manufacturing method of the steel plate is as follows.
After iron water desulfurization treatment, converter smelting is performed to reduce the S and P contents in the steel water to S≦0.005% and P≦0.010%. After completing the alloying of the required mass fraction of elements such as C, Mn, Si, Mo, Ni, Ti by LF refining, RH treatment is performed, the vacuum degree is 3 mbar, the treatment time is 23 minutes, and the reduce the content of gas impurity elements in A slab is obtained by continuous casting to obtain a slab with a thickness of 320 mm. The temperature of the slab is heated up to 1140° C. and the soaking time is 60 min. The heated slab is rolled, the rolling start temperature is 1005 ° C., the rolling end temperature is 952 ° C., and the pressing regulation of the rolling mill is 320 mm - 280 mm - 240 mm - 200 mm - 165 mm - 135 mm - 110 mm - 90 mm - 80 mm. ing. The steel sheet after rolling is immediately water-cooled, the temperature at which the surface of the steel sheet after cooling returns to red is 350°C, and the average cooling rate is 3.1°C/s. The steel plate is quenched and tempered. The quenching temperature is 810°C, the soaking time is 10 minutes, and the surface of the steel sheet is water-cooled to a temperature of 77°C at which the surface returns to red. The average cooling rate is 2.9°C/s, the tempering temperature is 626°C, and the soaking time is 55 minutes. Then, the tempered steel sheet is air-cooled to room temperature.

The resulting steel plate structure contains martensite and austenite, with the austenite volume fraction being 6.5%. Figure 1 shows a transmission electron micrograph of the core structure of the steel sheet. It can be observed in the photograph that martensite and austenite are distributed at intervals, among which the slat-like structure with bright contrast is martensite. , the dark-contrast film-like structure is austenite. The core of the steel sheet has a yield strength of 758 MPa, a tensile strength of 842 MPa, an elongation after breaking of 16%, and an energy absorbed of 135 J in a Charpy pendulum impact test of a V-shaped specimen at -60°C. The cross-sectional shrinkage in the thickness direction of the steel sheet is 63%.

Example 2: 80 mm thick and 0.04% C, 5.2% Mn, 0.4% Si, 0.1% Mo, 0.6% Ni, 0.6% A core with a chemical composition containing Cr, 0.01% Ti, S≤0.005%, P≤0.010%, the balance Fe and other unavoidable impurity elements (content expressed in mass percentage) 690 MPa grade extra heavy steel plate with excellent mechanical properties.

The manufacturing method of the steel plate is as follows.
After iron water desulfurization treatment, converter smelting is performed to reduce the S and P contents in the steel water to S≦0.005% and P≦0.010%. After completing the alloying of the required mass fraction of elements such as C, Mn, Si, Mo, Ni, and Ti by LF refining, RH treatment is performed, the vacuum degree is 3 mbar, the treatment time is 20 min, and the reduce the content of gas impurity elements in A slab is obtained by continuous casting to obtain a slab with a thickness of 320 mm. The temperature of the slab is heated up to 1105°C and the soaking time is 40min. The heated slab is rolled, the rolling start temperature is 1001°C, the rolling end temperature is 930°C, and the press down regulation of the rolling mill is 320mm-280mm-240mm-200mm-165mm-135mm-110mm-90mm-80mm. ing. The steel sheet after rolling is immediately water-cooled, the temperature at which the surface of the steel sheet after cooling returns to red is 271°C, and the average cooling rate is 4.7°C/s. The steel plate is quenched and tempered. The quenching temperature is 830°C, the soaking time is 5 minutes, and the surface of the steel sheet is water-cooled to a temperature of 51°C at which the surface returns to red. The average cooling rate is 4.2°C/s, the tempering temperature is 640°C, and the soaking time is 40 minutes. Then, the tempered steel sheet is air-cooled to room temperature.

The resulting steel plate structure contains martensite and austenite, with the austenite volume fraction being 10%. The core of the steel sheet has a yield strength of 741 MPa, a tensile strength of 821 MPa, an elongation after fracture of 17.5%, and an energy absorbed of 165 J in a Charpy pendulum impact test of a V-shaped specimen at -60°C. The cross-sectional shrinkage in the thickness direction of the steel plate is 71%.

Example 3: 150 mm thick and 0.08% C, 6.0% Mn, 0.1% Si, 0.5% Mo, 0.5% Ni, 0.2% A core with a chemical composition containing Cr, 0.05% Ti, S≤0.005%, P≤0.010%, the balance Fe and other unavoidable impurity elements (content expressed in mass percentage) 690 MPa grade extra heavy steel plate with excellent mechanical properties.

The manufacturing method of the steel plate is as follows.
After iron water desulfurization treatment, converter smelting is performed to reduce the S and P contents in the steel water to S≦0.005% and P≦0.010%. After completing the alloying of the required mass fraction of elements such as C, Mn, Si, Mo, Ni, and Ti by LF refining, RH treatment is performed, the degree of vacuum is 3 mbar, the treatment time is 26 min, and the reduce the content of gas impurity elements in A slab with a thickness of 610 mm is obtained by forging after die casting. The temperature of the slab is heated up to 1060°C and the soaking time is 90min. The heated slab is rolled, the rolling start temperature is 1015°C, the rolling end temperature is 942°C, and the pressing range of the rolling mill is 610mm-540mm-470mm-400mm-340mm-290mm-245mm-215mm-190mm-170mm. -150 mm. The steel sheet after rolling is immediately water-cooled, the temperature at which the surface of the steel sheet after cooling returns to red is 327°C, and the average cooling rate is 1.5°C/s. The steel plate is quenched and tempered. The quenching temperature is 780°C, the soaking time is 15 minutes, and the surface of the steel sheet is water-cooled to 102°C, the temperature at which the surface returns to red, the average cooling rate is 1.2°C/s, the tempering temperature is 610°C, and the soaking time is 70 minutes. Then, the tempered steel sheet is air-cooled to room temperature.

The resulting steel plate structure contains martensite and austenite, with the volume fraction of austenite being 4%. The core of the steel sheet has a yield strength of 745 MPa, a tensile strength of 819 MPa, an elongation after fracture of 15%, and an energy absorbed of 106 J in a Charpy pendulum impact test of a V-shaped specimen at -60°C. The cross-sectional shrinkage in the thickness direction of the steel sheet is 57%.

Example 4: Four sets of parallel experiments were designed, the details of which are shown in Table 1 below. was the same.

As can be seen from Table 1, groups 1 and 2 have rolling start temperatures within the range of the present invention, but groups 3 and 4 have rolling start temperatures outside the range of the present invention, whereby the steel sheets prepared thereby break. Performance such as post-elongation, low-temperature impact energy, and cross-sectional shrinkage in the plate thickness direction are degraded.

Example 5: Three sets of parallel experiments were designed, the details of which are shown in Table 2 below. was the same as

As can be seen from Table 2, Group 1 is the average cooling rate of water cooling after quenching within the range of the present invention, but Groups 2 and 3 are average cooling rates outside the range of the present invention. Yield strength and low temperature impact energy performance are degraded. The low temperature impact energy performance of the Group 3 steel sheets is degraded and cracks due to thermal stress appear in the steel sheets.

(Appendix)
(Appendix 1)
The mass percentage of the chemical composition is C: 0.04 to 0.08%, Mn: 5.2 to 6.0%, Si: 0.1 to 0.4%, Mo: 0.1 to 0.5%, Ni: 0.2 to 0.6%, Cr: 0.2 to 0.6%, Ti: 0.01 to 0.05%, S: ≤0.005%, P: ≤0.010%, balance containing Fe and impurities,
A 690 MPa grade extra-thick steel plate characterized by:

(Appendix 2)
The steel plate has a thickness of 80 to 150 mm,
A 690 MPa grade extra-thick steel plate according to appendix 1, characterized in that:

(Appendix 3)
The microstructure of the steel sheet has martensite and austenite, in which the volume fraction of austenite is 4-10%.
A 690 MPa grade extra-thick steel plate according to appendix 1, characterized in that:

(Appendix 4)
wherein the austenite is in the form of a thin film and the austenite is distributed between the martensitic slats;
A 690 MPa grade extra-thick steel plate according to appendix 3, characterized in that:

(Appendix 5)
The mechanical properties of the core of the steel plate are yield strength of 690 MPa or more, tensile strength of 770 MPa or more, elongation after breakage of 14% or more, and energy absorbed in a Charpy pendulum impact test of a V-shaped test piece at -60 ° C. is greater than or equal to 80 J,
A 690 MPa grade extra-thick steel plate according to appendix 1, characterized in that:

(Appendix 6)
The cross-sectional shrinkage rate due to elongation in the thickness direction of the steel sheet is 50% or more.
A 690 MPa grade extra-thick steel plate according to appendix 1, characterized in that:

(Appendix 7)
A method for preparing a 690 MPa grade extra heavy steel plate according to any one of appendices 1 to 6, characterized in that it comprises the steps of:
(1) After the iron water desulfurization treatment, converter smelting is performed to reduce the S and P contents in the steel water to S ≤ 0.005% and P ≤ 0.010%,
(2) After completing the alloying of the required mass fractions of C, Mn, Si, Mo, Ni, and Ti elements by LF refining, perform RH treatment to make the degree of vacuum ≤ 4 mbar and the processing time ≥ 20 min,
(3) A slab is obtained by casting, and the ratio of the thickness of the slab to the thickness of the steel plate is 4 or more,
(4) heating the slab to a temperature of 1060-1140°C;
(5) Roll the heated slab to a rolling start temperature ≤ 1020°C, a rolling end temperature ≥ 930°C, and a passing deformation amount ≥ 10%;
(6) The steel sheet after rolling is immediately water-cooled, the temperature at which the surface of the steel sheet returns to red after cooling is ≦360° C., and the average cooling rate is 1 to 5° C./s,
(7) Perform quenching heat treatment, reheat the steel sheet to 780 ~ 830 ℃, soaking time is 5 ~ 15 minutes, water cooling until the temperature of the steel sheet surface returns to red is ≤ 110 ℃, average cooling rate is 2 ~8°C/s,
(8) Perform tempering heat treatment, heat the steel plate after quenching to 610 to 640 ° C., soak for 40 to 70 minutes, and air cool the steel plate after tempering to room temperature.
Method of preparation.

(Appendix 8)
The slab in step (3) adopts a continuous cast slab or a forged slab after die casting.
The preparation method according to appendix 6, characterized in that:

(Appendix 9)
If the thickness of the slab is 320 mm or less, adopt the continuous cast slab, if the thickness of the slab exceeds 320 mm, adopt the forged slab after die casting.
The preparation method according to appendix 7, characterized in that

(Appendix 10)
In the step (4), the soaking time of the slab is 40 to 90 minutes,
The preparation method according to appendix 6, characterized in that:

Claims (6)

The mass percentage of the chemical composition is C: 0.04 to 0.08%, Mn: 5.2 to 6.0%, Si: 0.1 to 0.4%, Mo: 0.1 to 0.5%, Ni: 0.2 to 0.6%, Cr: 0.2 to 0.6%, Ti: 0.01 to 0.05%, S: ≤0.005%, P: ≤0.010%, balance consists of Fe and impurities,
The microstructure of the core part of the steel sheet consists of martensite and austenite, the volume fraction of austenite is 4-10%, the austenite is in the form of a thin film, and the austenite is distributed between the martensite thin plates. ,
A method for preparing a steel plate, comprising:
(1) After the desulfurization treatment of the molten iron, converter smelting is performed to reduce the S and P contents in the molten iron to S ≤ 0.005% and P ≤ 0.010%,
(2) After completing the alloying of the required mass fractions of C, Mn, Si, Mo, Ni, and Ti elements by LF refining, perform RH treatment to make the degree of vacuum ≤ 4 mbar and the processing time ≥ 20 min,
(3) A slab is obtained by casting, and the ratio of the thickness of the slab to the thickness of the steel plate is 4 or more,
(4) heating the slab to a temperature of 1060-1140°C;
(5) Rolling the heated slab to a rolling start temperature ≤ 1020 ° C, a rolling end temperature ≥ 930 ° C, a passing deformation per pass ≥ 10%,
(6) The steel sheet after rolling is immediately water-cooled, the surface temperature of the steel sheet after cooling is ≦360° C., and the average cooling rate is 1 to 5° C./s,
(7) Perform quenching heat treatment, reheat the steel plate to 780-830°C, soak for 5-15 minutes, water-cool the steel plate surface temperature to ≤110°C, and average cooling rate is 2-8°C. /s,
(8) Perform tempering heat treatment, heat the steel plate after quenching to 610 to 640 ° C., soak for 40 to 70 minutes, and air cool the steel plate after tempering to room temperature.
A method for preparing a steel sheet, comprising the steps of : The steel plate has a thickness of 80 to 150 mm,
The method for preparing a steel sheet according to claim 1, characterized in that: The mechanical properties of the core of the steel sheet are that the yield strength is 690 MPa or more, the tensile strength is 770 MPa or more , and the energy absorbed in the Charpy pendulum impact test of the V-shaped test piece at -60 ° C. is 80 J or more.
The method for preparing a steel sheet according to claim 1, characterized in that: The slab in step (3) adopts a continuous cast slab or a forged slab after die casting.
The method for preparing a steel sheet according to claim 1 , characterized in that: If the thickness of the slab is 320 mm or less, adopt the continuous cast slab, if the thickness of the slab exceeds 320 mm, adopt the forged slab after die casting.
The method for preparing a steel sheet according to claim 1 , characterized in that: In the step (4), the soaking time of the slab is 40 to 90 minutes,
The method for preparing a steel sheet according to claim 1 , characterized in that:

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