Low-cost high-performance Sb-Nb-Ti microalloyed steel for ocean platform and preparation method of low-cost high-performance Sb-Nb-Ti microalloyed steel

Abstract

The invention relates to low-cost high-performance Sb-Nb-Ti microalloyed steel for an ocean platform and a preparation method of the low-cost high-performance Sb-Nb-Ti microalloyed steel, and belongs to the field of weathering resistant steel design and manufacturing, the steel comprises the following chemical components in percentage by weight (wt.%): 0.06 to 0.08 percent of C, 0.25 to 0.35 percent of Si, 1.45 to 1.55 percent of Mn, 0.20 to 0.30 percent of Cu, 0.75 to 0.85 percent of Ni, 0.01 to 0.03 percent of Ti, 0.01 to 0.03 percent of Nb, 0.01 to 0.05 percent of Al, 0.15 to 0.25 percent of Sb and the balance of Fe and inevitable impurities, the preparation method sequentially comprises the steps of converter smelting, external refining, slab casting, billet heating, controlled rolling, controlled cooling and air cooling. The rolling process comprises the following steps: sequentially rolling a heated steel billet in an austenite recrystallization region, rolling an austenite non-recrystallization region and rolling an austenite and ferrite two-phase region; according to the steel plate, the chemical components are simple, the Cr element is not added, the contents of Cu and Ni are controlled at a low level, a simple TMCP process is adopted to replace a complex TMCP + tempering or TMCP + hardening and tempering process, it is guaranteed that the steel plate has good obdurability matching, low-temperature toughness and welding performance, meanwhile, the production cost of the steel plate is reduced, and the production efficiency is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of weathering steel design and manufacturing, and relates to a low-cost high-performance Sb-Nb-Ti micro-alloyed steel for offshore platforms and a preparation method thereof. BACKGROUND

[0002] With the development of global marine resources to deep water and polar regions, offshore platforms need to adapt to extreme environments of-60℃ or even lower, and higher requirements are put forward for the low-temperature toughness, strength, corrosion resistance and fatigue resistance of steel. At present, high-strength offshore platform steels are gradually developing towards ultra-high strength of 550 and 620 MPa. Offshore platform steels not only have to face low temperature, high humidity, chloride salt corrosion, microbial corrosion and bear the effects of sea wind, sea waves and ocean currents, but also have to bear natural disasters such as typhoons, icebergs and earthquakes. Therefore, offshore platform steels must have the characteristics of high strength, high toughness, low yield ratio, corrosion resistance, easy welding and good cold working performance. However, when the yield strength of offshore platform steel is ≥550 MPa, it is necessary to simultaneously consider the excellent comprehensive performance of high toughness (≥120 J) and low yield ratio (≤0.80) at-80℃, which puts forward extremely high requirements and severe challenges for the comprehensive synergistic control of the chemical composition design, controlled rolling and controlled cooling process and production cost of the steel.

[0003] At present, for offshore platform steels with a yield strength of ≥550 MPa, Cr, Ni, Cu or Mo alloying elements are mainly added, and a TMCP+tempering or TMCP+quenching and tempering process is adopted to obtain a tempered sorbite structure, so as to simultaneously consider the high strength and toughness, low yield ratio, easy weldability and corrosion resistance of the steel plate.

[0004] The patent application with the publication number CN119194243A discloses a S420KT-50 heavy steel plate for offshore oil and gas platforms with a maximum thickness of not more than 120 mm and excellent low-temperature toughness, and a manufacturing method thereof. The steel plate has a yield strength of ≥420 MPa, a tensile strength of 500-690 MPa, an elongation of ≥19%, an impact energy at-50℃ of ≥150 J, and a yield ratio of ≤0.89. However, the strength grade is low, the low-temperature impact toughness only meets-50℃, the yield ratio is high, and the corrosion resistance problem is not solved, which cannot meet the needs of offshore platform construction.

[0005] The patent application with the publication number CN116043105A discloses a FH460 offshore platform steel with excellent corrosion resistance and a preparation method thereof. The steel plate has a yield strength of 515-541 MPa, a tensile strength of 570-590 MPa, an elongation of ≥29%, an impact energy at-60℃ of ≥300 J, a yield ratio of ≤0.91, and a corrosion rate of ≤0.013 mm / a. However, the strength grade is low, the yield ratio is too high, it is difficult to meet the requirements of deep-sea offshore platform construction, and the Mo content is 0.11%-0.12%.

[0006] The invention patent application with publication number CN119392118A discloses a high-strength and high-toughness super-thick rack steel EQ70 and a production method thereof. The yield strength of the steel plate is greater than or equal to 690 MPa, the tensile strength is 810-890 MPa, the longitudinal and transverse Charpy impact energy at 40 ℃ is greater than or equal to 120 J, and the thickness of the steel plate is 1 / 4, 1 / 2 40 ℃, but it fails to solve the problem of corrosion resistance, and the production method is complex, requiring quenching + tempering process, and Mo (0.50%-0.60%) is added, the price of Mo alloy is relatively high, and the cost is relatively high.

[0007] The production and preparation methods of these offshore engineering steel plates generally have the problems of adding many valuable alloy elements, long production process, complex production method, high manufacturing cost, and low cost, low yield ratio, high strength and toughness, and good weather resistance. Therefore, how to combine new chemical composition design with reasonable controlled rolling and controlled cooling process to prepare high-performance offshore platform steel with excellent comprehensive performance and low cost is an urgent direction to be developed and broken through. SUMMARY

[0008] Therefore, in order to solve the problems of adding many valuable alloy elements, long production process, high manufacturing cost, and low cost, low yield ratio, high strength and toughness, and good weather resistance of the existing production and preparation methods of offshore engineering steel plates, the present application provides a low-cost high-performance Sb-Nb-Ti micro-alloyed offshore platform steel and a preparation method thereof. By combining reasonable micro-alloying design with optimized controlled rolling and controlled cooling process, a high-performance offshore platform steel with low cost, low yield ratio, high strength and toughness, good weather resistance, and easy welding is developed.

[0009] To achieve the above-mentioned purpose, the present application provides the following technical solutions: A preparation method of a low-cost high-performance Sb-Nb-Ti micro-alloyed offshore platform steel, comprising the following steps: S1, smelting process: S11, vacuum induction smelting is used to obtain molten steel with the composition of the present application, and after refining, the steel ingot is cast, wherein the chemical composition of the steel billet is as follows: C: 0.06-0.08%, Si: 0.25-0.35%, Mn: 1.45-1.55%, Cu: 0.20-0.30%, Ni: 0.75-0.85%, Ti: 0.01-0.03%, Nb: 0.01-0.03%, Al: 0.01-0.05%, Sb: 0.15-0.25%, and the balance is Fe and inevitable impurities.

[0010] S12. The refined steel ingot is shaped and hot-forged into a steel billet with a cross section of 150 mm × 150 mm. S2, Rolling process: S21. After the continuous casting billet is opened, it is put into the heating furnace for billet heating. The specific process parameters for billet heating are: heating furnace temperature 1200~1250℃, holding time 2~3 h, and furnace exit temperature 1150~1200℃. S22. The heated steel billet is rolled in the austenite recrystallization zone at an initial rolling temperature of 1100~1150℃ and a final rolling temperature of 980~1030℃, for a total of 2~4 passes, with a total reduction rate > 50%. S23. The steel plate rolled in the austenite recrystallization zone is rolled in the austenite non-recrystallization zone. The initial rolling temperature is 930~980℃, the final rolling temperature is 830~880℃, and the rolling is performed in 3~5 passes with a total reduction rate > 50%. S24. The steel plate rolled in the non-recrystallized austenite region is rolled in the austenite + ferrite two-phase region. The initial rolling temperature is 800~830℃, the final rolling temperature is 750~780℃, and it is rolled in one pass with a total reduction rate ≥14%. S3, Cooling Process: After final rolling, the steel plate is cooled to M at a cooling rate of 15~25 ℃ / s. s Set the temperature below 50-80℃ and air cool to room temperature.

[0011] Furthermore, the steel prepared by the above method exhibits the following performance indicators: resistance to marine atmospheric corrosion rate < 0.08 mm / a, and mechanical properties: yield strength Rp 0.2 ≥600 MPa, tensile strength R m ≥800 MPa, yield strength ratio Rp 0.2 / R m ≤0.80, elongation after fracture A ≥21%, -80℃ / impact energy KV2≥140J.

[0012] Furthermore, the metallographic structure of the steel prepared by the above preparation method is 25-40% fine and uniform ferrite and 60-75% lath bainite.

[0013] The beneficial effects of this invention are as follows: 1. The low-cost, high-performance Sb-Nb-Ti microalloyed steel for marine platforms disclosed in this invention employs a low-C, low-Cu+low-Ni alloying and Sb+Ti+Nb microalloying composition design system. This reduces alloy costs while ensuring the steel plate possesses high strength, corrosion resistance, and excellent low-temperature toughness. The addition of small amounts of Cu, Ni, and Sb improves the steel plate's resistance to marine atmospheric corrosion, avoiding the reaction of Cr with Cl in the marine atmosphere that occurs in traditional Cu, Ni, and Cr alloying schemes. -The reaction can cause the rust layer to be locally acidified, resulting in reduced corrosion resistance. The chemical composition is simple, no Cr element is added, the Cu and Ni contents are controlled at a low level, and a simple TMCP process is used to achieve good strength and toughness matching of the steel plate; that is, fine polygonal ferrite is obtained by rolling in the austenite + ferrite two-phase region, and the proportion of the ferrite is controlled, and then the austenite is subjected to bainite phase transformation by controlling the cooling, and finally a polygonal ferrite + lath bainite structure is formed. During the deformation process, the ferrite as a soft phase can blunt the crack tip and prevent the rapid propagation of the crack, and can also absorb a large amount of energy; in addition to the high strength of the lath bainite as a hard phase, the interlaced bainite laths can effectively deflect the crack propagation direction, further improving the toughness of the steel plate. Therefore, the polygonal ferrite + lath bainite structure obtained by a simple TMCP process can replace the tempered sorbite structure obtained by a complex TMCP + tempering or TMCP + quenching and tempering process. While ensuring that the steel plate has good strength and toughness matching, low-temperature toughness and welding performance, the production cost of the steel plate is reduced, the production efficiency is improved, and the steel plate is suitable for large-scale production and can be widely used in the construction of offshore platforms in shallow water, deep sea, tropical and polar regions.

[0014] 2. In the controlled rolling and controlled cooling process of the disclosed low-cost high-performance Sb-Nb-Ti micro-alloyed offshore platform steel preparation method, by large reduction rolling in the austenite recrystallization zone, the solute drag effect of Sb on the austenite grain boundary is fully utilized to ensure that fine and uniform austenite recrystallization grains are obtained; by slow large reduction rolling in the austenite unrecrystallization zone, the equiaxed fine austenite grains are elongated, the grain boundary area is increased, and a large number of dislocations are formed inside the grains, providing nucleation sites for ferrite phase transformation during two-phase zone rolling and bainite phase transformation during controlled cooling; by small strain rolling in the two-phase zone, the proportion of ferrite and austenite is controlled, and the dislocation density in the ferrite grains is increased during rolling to promote the precipitation of (Ti, Nb) C in the ferrite; in the controlled cooling process, by using a higher cooling rate and a suitable final cooling temperature, the bainite phase transformation of the supercooled austenite is induced, the refining effect of Sb on the bainite structure is further utilized, and the segregation of Sb at the ferrite grain boundary is avoided, and finally a steel plate with a structure of fine and uniform ferrite + lath bainite is obtained; the strength of the steel plate is improved by utilizing fine-grain strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening; the low-temperature toughness of the steel plate is ensured by refining the grains, controlling the proportion of ferrite and lath bainite, and controlling the segregation of Sb element; in summary, by the controlled rolling and controlled cooling process of austenite recrystallization zone rolling, austenite unrecrystallization zone rolling, austenite + ferrite two-phase zone rolling and controlled cooling, a high-performance offshore platform steel with a yield strength higher than 600 MPa is developed.

[0015] Additional advantages, objects, and features of the application will be apparent to those skilled in the art upon examination of the following specification. It is intended that the application not be limited by the disclosed BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to make the objects, technical solutions and advantages of the present application clearer, the preferred embodiments of the present application will be described in detail below with reference to the drawings, in which: Figure 1 OM microstructure of the Sb-Nb-Ti micro-alloyed offshore platform steel of the present application; Figure 2 SEM impact fracture of the Sb-Nb-Ti micro-alloyed offshore platform steel of the present application; Figure 3 Processing map of the Sb-Nb-Ti micro-alloyed offshore platform steel of the present application; Figure 4 Dynamic continuous cooling transformation curve of the Sb-Nb-Ti micro-alloyed offshore platform steel of the present application; Figure 5 SEM microstructure of the lath bainite of the Sb-Nb-Ti micro-alloyed offshore platform steel of the present application. DETAILED DESCRIPTION

[0017] The present application is described below by way of specific examples. Other advantages and benefits of the present application will be apparent to those skilled in the art from the disclosure herein. The present application can be implemented or applied in other different embodiments, and the details in the present specification can be modified or changed based on different views and applications without departing from the spirit of the present application. It should be noted that the figures provided in the following examples only schematically illustrate the basic concept of the present application, and the following examples and features in the examples can be combined with each other without conflict.

[0018] The role and ratio of each element in the low-cost high-performance Sb-Nb-Ti micro-alloyed offshore platform steel are as follows: C: an important strengthening element in steel, which ensures the strength and moderate plasticity and toughness of the steel. However, when the carbon content is too high, it is not conducive to the welding performance and corrosion resistance, and it will also deteriorate the plasticity and toughness. Therefore, the C content of the present application is 0.06-0.08%.

[0019] Si: mainly through solid solution strengthening to improve the strength of steel, while in the smelting process can remove part of the oxygen and improve the corrosion resistance, but, too high silicon content will deteriorate the toughness and welding performance of offshore platform steel, and easy to form surface quality defects. Therefore, the Si content of the present application is 0.25~0.35%.

[0020] Mn: can significantly improve the hardenability of steel, expand the austenite phase region, significantly refine ferrite grains by reducing the phase change starting temperature, also has a certain solid solution strengthening effect. However, when the Mn content is too high, its segregation tendency in the billet increases, and brittle fracture is easy to occur, in addition, it will increase the sensitivity of welding hot crack, and the weld area is easy to produce crack. Therefore, the Mn content of the present application is 1.45~1.55%.

[0021] Cu: austenite stabilizing element, can improve the hardenability of steel; Cu can significantly improve the corrosion resistance of steel, and also can improve the strength of steel through solid solution and precipitation strengthening. However, when the Cu content is too high, the billet is easy to produce surface defects; Cu is easy to segregate at the grain boundary at high temperature, which reduces the hot plasticity of steel, causes crack during hot rolling or forging, increases the sensitivity of welding hot crack, and affects the weld quality. Therefore, the Cu content of the present application is 0.20~0.30%.

[0022] Ni: through solid solution strengthening to improve the strength of offshore platform steel, and can improve the hardenability of steel. In addition, Ni can effectively improve the plasticity, toughness and corrosion resistance of steel. However, Ni is a precious alloy element, and its content is too high, which will significantly increase the production cost and increase the risk of solidification crack of weld. Therefore, the Ni content of the present application is 0.75~0.85%.

[0023] Ti: strong carbide forming element, adding trace titanium element in steel can easily form fine and stable Ti-containing second phase, effectively inhibit the growth of austenite grains during soaking process; TiN particles can prevent grain coarsening and improve welding performance during welding process. Therefore, the Ti content of the present application is 0.01~0.03%.

[0024] Nb: appropriate Nb and C, N form second phase, which can refine austenite grains and improve toughness during rolling, at the same time, Nb improves the unrecrystallization temperature by precipitate pinning and solute drag to promote the refinement of austenite and phase change organization and improve the strength; in addition, Nb can optimize the welding performance and improve the seawater corrosion resistance. However, too high Nb content will increase the production cost and easily deteriorate the hot working performance. Therefore, the Nb content of the present application is 0.01~0.03%.

[0025] Al: the main deoxidizing element in steel, trace amount of Al can reduce the inclusion content. Al can refine the austenite grain, and increase the yield ratio of steel. But too high Al content will deteriorate the hot workability of steel, and make the steel prone to cracking during hot rolling. Therefore, the Al content of the present application is 0.01~0.05%.

[0026] Sb: the solid solubility in ferrite and austenite is 2.6% and 10.3% respectively, a small amount of Sb can be dissolved in the steel matrix and produce strong solid solution strengthening effect; Sb can inhibit recrystallized grain growth in austenite through solute drag effect, and also can inhibit the bainite lath coarsening during bainite transformation, which is beneficial to the refinement of the steel structure; Sb can significantly improve the corrosion resistance of steel in marine atmosphere, when Sb2O3 is formed in the rust layer, the alkalization effect produced by Sb2O3 on one hand promotes the transformation of γ-FeOOH to Fe3O4, promotes the formation of protective rust layer, and inhibits the electrochemical corrosion process, on the other hand, it consumes a large amount of H + , inhibits local acidification; Sb 3+ has similar reduction potential with Cu + in acidic chloride medium, which makes Sb 3+ accelerate the re-deposition reaction of Cu + , and promote the formation of dense and thick Cu, Sb-rich protective rust layer. But Sb is a grain boundary segregation element, and unreasonable controlled rolling and controlled cooling process will lead to Sb segregation at ferrite grain boundaries, resulting in embrittlement of steel, and too high Sb content will also deteriorate the hot deformation performance of steel. Therefore, the Sb content of the present application is controlled at 0.15~0.25%.

[0027] The preparation method of the low-cost high-performance Sb-Nb-Ti micro-alloyed marine platform steel comprises the following steps: S1, smelting process: S11, using vacuum induction smelting to obtain molten steel with the composition of the present application, and casting the molten steel into ingots after refining, specifically: Referring to the chemical composition of the steel material of each embodiment and the comparative example shown in the following Table 1, a vacuum induction furnace is used to smelt 150 kg of molten steel with the chemical composition shown in Table 1, and the molten steel is cast into ingots after refining; Table 1: Melting chemical composition of the steel of the embodiments and the comparative steel (wt. %)

[0028] S12, the ingots after refining are broken down and hot forged into steel billets with a cross section of 150 mm x 150 mm; S2, rolling process: S21, after the continuous casting billet is broken down, it is loaded into a heating furnace for steel billet heating, and the steel billet heating process parameters of each embodiment and the comparative example are shown in the following Table 2; Table 2: Heating process parameters of the steel of the embodiments of the present application and the comparative steel

[0029] S22, the heated billet is rolled in the austenite recrystallization zone, and the austenite recrystallization zone rolling process parameters of each embodiment and the comparative example are shown in Table 3 below; Table 3: Austenite recrystallization zone rolling process parameters of the steel of the embodiments of the present application and the comparative steel

[0030] S23, the steel plate after austenite recrystallization zone rolling is rolled in the austenite non-recrystallization zone, and the austenite non-recrystallization zone rolling process parameters of each embodiment and the comparative example are shown in Table 4 below; Table 4: Austenite non-recrystallization zone rolling process parameters of the steel of the embodiments of the present application and the comparative steel

[0031] S24, the steel plate after austenite non-recrystallization zone rolling is rolled in the austenite + ferrite two-phase zone, and the austenite + ferrite two-phase zone rolling process parameters of each embodiment and the comparative example are shown in Table 5 below; Table 5: Two-phase zone rolling process parameters of the steel of the embodiments of the present application and the comparative steel

[0032] S3, cooling process: after the end of the finish rolling of the steel plate, the cooling rate is 15-25 ℃ / s to cool to 50-80 ℃ below the M s point, and air cooling to room temperature. The controlled cooling process parameters of each embodiment and the comparative example are shown in Table 6 below; Table 6: Controlled cooling process parameters of the steel of the embodiments of the present application and the comparative steel

[0033] According to GB / T19746 2005 Standard for Corrosion Salt Solution Circumferential Immersion Test of Metals and Alloys, circumferential immersion corrosion tests are carried out on the steel plate products rolled for offshore platforms of each embodiment and the comparative example to simulate the corrosion rate in the marine atmospheric environment. The average corrosion rate formula is shown according to ASTM G1-03 (2011): (1) Wherein in formula (1), CR is the average corrosion rate (mm / a); is the mass change of the sample before and after corrosion (g); S is the total surface area of the sample (cm 2 ); p is the density of the sample (g / cm 3 ); t is the corrosion time (h).

[0034] The corrosion rate and mechanical properties of the rolled offshore platform steel plate products of the embodiments are shown in Table 7 below.

[0035] Table 7: Properties of the steel of the embodiments and the comparative steels

[0036] The rolling process and the cooling process are designed according to the thermal processing map (Fig. 1) and the dynamic continuous cooling transformation curve (Fig. 2) of the steel. As shown in Fig. 1, when the deformation temperature is high and the strain rate is low, the power dissipation value of the steel is large, which is beneficial to the recrystallization of austenite, so the low strain rate is used for the rolling in the austenite recrystallization zone to fully refine the austenite grains. As shown in Fig. 2, when the cooling rate is 15-25 ℃ / s, the austenite mainly undergoes bainite transformation, and the room temperature structure after the phase transformation is fine lath bainite, as shown in Fig. 3. Figure 3 Figure 4 Figure 3 Figure 4 Figure 5 Figure 1 Figure 2

[0037] ​​​​​​​Finally, it is to be explained that the above embodiments are only used to illustrate the technical solutions of the present application but not to limit the present application. Although the present application is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions, and all should be covered in the scope of the claims of the present application.

Claims

1. A low-cost, high-performance Sb-Nb-Ti microalloyed steel for marine platforms, characterized in that, The chemical composition of the steel by weight percentage is as follows: C: 0.06~0.08%, Si: 0.25~0.35%, Mn: 1.45~1.55%, Cu: 0.20~0.30%, Ni: 0.75~0.85%, Ti: 0.01~0.03%, Nb: 0.01~0.03%, Al: 0.01~0.05%, Sb: 0.15~0.25%, with the balance being Fe and unavoidable impurities. The metallographic structure is 25~40% polygonal ferrite and 60~75% lath bainite.

2. The low-cost, high-performance Sb-Nb-Ti microalloyed steel for marine platforms as described in claim 1, characterized in that, Mechanical properties: Yield strength Rp 0.2 ≥600 MPa, tensile strength R m ≥800 MPa, yield strength ratio Rp 0.2 / R m ≤0.80, elongation after fracture A ≥21%, -80℃ / impact energy KV2≥140J.

3. The low-cost, high-performance Sb-Nb-Ti microalloyed steel for marine platforms as described in claim 1, characterized in that, Resistance to marine atmospheric corrosion rate < 0.08 mm / a.

4. The method for preparing low-cost, high-performance Sb-Nb-Ti microalloyed marine platform steel as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Smelting process: The steel with the composition of this invention is obtained by vacuum induction smelting, refined and cast into steel ingots, and then hot forged into steel billets. S2, Rolling process: S21. After the continuous casting billet is opened, it is put into the heating furnace for billet heating. The specific process parameters for billet heating are: heating furnace temperature 1200~1250℃, holding time 2~3 h, and furnace exit temperature 1150~1200℃. S22. The heated steel billet is rolled in the austenite recrystallization zone at an initial rolling temperature of 1100~1150℃ and a final rolling temperature of 980~1030℃, for a total of 2~4 passes, with a total reduction rate > 50%. S23. The steel plate rolled in the austenite recrystallization zone is rolled in the austenite non-recrystallization zone. The initial rolling temperature is 930~980℃, the final rolling temperature is 830~880℃, and the rolling is performed in 3~5 passes with a total reduction rate > 50%. S24. The steel plate rolled in the non-recrystallized austenite region is rolled in the austenite + ferrite two-phase region. The initial rolling temperature is 800~830℃, the final rolling temperature is 750~780℃, and it is rolled in one pass with a total reduction rate ≥14%. S3, Cooling Process: After final rolling, the steel plate is cooled to M at a cooling rate of 15~25 ℃ / s. s Set the temperature below 50-80°C and air cool to room temperature.

5. The preparation method according to claim 4, characterized in that, In step S1, the refined steel ingot is shaped and hot-forged into a steel billet with a cross-section of 150 mm × 150 mm.

6. The preparation method according to claim 4, characterized in that, Step S2 rolling process is as follows: the heated steel billet is rolled in the austenite recrystallization zone, the austenite non-recrystallization zone, and the austenite + ferrite two-phase zone in sequence.

7. The preparation method according to claim 4, characterized in that, After the steel plate is air-cooled to room temperature following the final rolling in step S3, it is not subjected to tempering treatment.

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