A method for improving the rolling rhythm of steel plates with a yield strength of 550 MPa

Abstract

The present invention discloses a method for improving the rolling rhythm of a steel plate with a yield strength of 550 MPa, belonging to the field of metal processing. The chemical composition of the steel plate by weight percentage includes: C 0.13% - 0.17%, Si 0.15% - 0.35%, Mn 1.45% - 1.65%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.03% - 0.05%, Ti 0.015% - 0.0250%, Al 0.015% - 0.035%, V 0.02% - 0.05%, B 0.0004% - 0.0006%, N 0.01% - 0.02%, and the balance is Fe and inevitable impurities. The method includes molten steel smelting → continuous casting → slab heating → rolling → straightening (hot straightening) → air cooling to room temperature. The rolling rhythm of the steel plate of the present invention is significantly improved, and the rolling time of each steel plate is reduced by 60 - 100 seconds on average, which has a significant effect on improving production efficiency and reducing production costs.

Description

Technical Field

[0001] The present invention belongs to the field of metal processing, and particularly relates to a method for improving the rolling rhythm of steel plates with a thickness specification of 16 - 50 mm and a yield strength of 550 MPa grade. Background Art

[0002] Facing the severe market situation, user requirements and manufacturing cost pressure in the steel industry, it is particularly important to improve the rolling rhythm while ensuring product quality, so as to maximize the production capacity of the production line. The improvement of the medium plate rolling rhythm is to increase the hourly rolling pieces, that is, the output per unit time of the rolling line, under the condition of ensuring production and equipment safety, and ultimately to achieve high-efficiency production with the highest efficiency and the lowest consumption while ensuring product quality. In particular, medium plate varieties represented by the 550 MPa grade are the main types of high-strength steels. Typical varieties include low-alloy Q550C / D / E, and X80M pipeline steel, etc. These steels are mainly based on carbon manganese steel, and at the same time, micro-alloying elements such as Nb, V, Ti, Mo and other carbon and nitride forming elements are added to the steel to improve the strength and toughness of the steel plate through solid solution strengthening, precipitation strengthening and fine grain strengthening. According to statistics, medium plates of this strength grade account for 10 - 15% of the annual output. Therefore, for the production of medium plate products, especially 550 MPa grade medium plate varieties in the future, it is urgent to further optimize the production process, further reduce energy consumption, alloy cost, compress manufacturing cost, improve production efficiency, and ultimately rapidly enhance the market competitiveness of products.

[0003] Currently, for 550 MPa grade steel plates with a thickness of 16 - 50 mm, in order to reduce alloy cost, a composition design system of medium carbon, medium Mn and addition of Nb, V, Ti, Mo alloys is usually adopted, and at the same time, the process paths of rolling and controlled cooling are optimized to reduce the performance fluctuations caused by the reduction of alloy cost. Generally speaking, TMCP (Thermo-Mechanical Control Process) is the general term for the technology of implementing air cooling or controlled cooling and accelerated cooling on the basis of controlling the rolling temperature and reduction in the rolling process. Since the TMCP process can produce high-strength and high-toughness steel without adding too many alloying elements and without complex subsequent heat treatment, it is considered a green reduction process that saves alloys and has become an indispensable technology for producing medium plates. However, the steel plates after alloy reduction are usually produced by two-stage or even three-stage controlled rolling TMCP processes. At the same time, in order to ensure the cumulative reduction rate in the finishing rolling stage, the waiting time of the intermediate billet is long, and the final rolling temperature is all at 850 °C or even lower. The lower temperature increases the deformation resistance of the rolled piece, thus increasing the load of the rolling mill and increasing the difficulty of shape control. Moreover, low-temperature rolling requires a long intermediate waiting time, which affects the rolling rhythm and production efficiency, resulting in an increase in the production cost of economic 550 MPa grade steel plates.

[0004] It can be seen that how to improve the rolling rhythm of steel plates with a thickness specification of 16 - 50 mm and a yield strength of 550 MPa, and reduce their manufacturing costs, are the key problems that need to be solved urgently in the mass production process of economical 550 MPa medium and heavy plates.

[0005] Comparison with the prior art:

[0006] So far, there has been little research at home and abroad on methods to improve the rolling rhythm of steel plates with a thickness specification of 16 - 50 mm and a yield strength of 550 MPa. Before this invention, the patent with publication number CN 115446114 A disclosed a method to improve the rolling efficiency of A36 grade marine medium and heavy plates. This method uses a non - TMCP rolling process and controls the billet heating system to make the final rolling temperature of the steel plate fall within a pre - set target range; however, its alloy composition is relatively high, and the production thickness is mainly below 16 mm, without covering the production process for thickness above 16 mm, and its strength level is 355 MPa. The journal paper "Analysis of the reasons for unqualified elongation of low - alloy high - strength steel Q550D" (Shanxi Metallurgy, 2010.1) mainly enhances the toughness of the steel plate by means of improving the purity of molten steel, inclusion modification treatment, optimizing rolling and controlled cooling processes, and using temper heat treatment, etc. However, due to its use of off - line temper heat treatment, the production cycle and cost increase significantly, and the improvement effect on rolling efficiency is not clear.

[0007] Although the steel plate shape control methods disclosed in the above patent documents improve the rolling efficiency of some thickness specifications and 355 MPa grade steel plates or the improvement methods for the strength and toughness of 550 MPa grade steel plates, they are not suitable for improving the rolling efficiency of economical 550 MPa grade steel plates with a thickness of 16 mm and above. Using the technical solution provided by the present invention can effectively overcome the above - mentioned deficiencies, solve the problem of low rolling efficiency of economical medium and heavy plates with a yield strength of 550 MPa and a rolling thickness of 16 - 50 mm by using continuous casting billets with a thickness of 250 mm and below and optimizing processes such as heating and rolling. Summary of the Invention

[0008] The purpose of the present invention is to overcome the above - mentioned existing technical problems and deficiencies, and provide a method to improve the rolling rhythm of economical steel plates with a thickness specification of 16 - 50 mm and a yield strength of 550 MPa, which can meet the performance requirements of the steel plates without subsequent long - term controlled rolling, and solve problems such as the reduction of alloy composition of such steel plates, and in order to ensure the requirements of strength and toughness, it is necessary to control the rolling temperature during the rolling process, reduce the final rolling temperature, which affects the rolling efficiency of the steel plate and the first - pass rate of the plate shape.

[0009] The present invention provides a method for improving the rolling rhythm of a steel plate with a yield strength of 550 MPa. The chemical composition of the steel plate, by weight percentage, includes: C 0.13% - 0.17%, Si 0.15% - 0.35%, Mn 1.45% - 1.65%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.03% - 0.05%, Ti 0.015% - 0.0250%, Al 0.015% - 0.035%, V 0.02% - 0.05%, B 0.0004% - 0.0006%, N 0.01% - 0.02%, with the balance being Fe and unavoidable impurities; the thickness of the steel plate is 16 - 50 mm, and continuous casting billets with a thickness of less than 250 mm are used for production on a medium and heavy plate reciprocating rolling mill.

[0010] The object of the present invention is achieved through the following technical solutions: A method for improving the rolling rhythm of a steel plate with a yield strength of 550 MPa in the present invention is used to improve the rolling efficiency of an economical medium and heavy plate with a yield strength of 550 MPa and a thickness of 16 - 50 mm, including molten steel smelting → continuous casting → slab heating → rolling → straightening (hot straightening) → air cooling to room temperature, specifically including the following steps:

[0011] 1) Steelmaking and continuous casting: Smelt according to the following composition, and its chemical composition by weight percentage is C 0.13% - 0.17%, Si 0.15% - 0.35%, Mn 1.45% - 1.65%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.03% - 0.05%, Ti 0.015% - 0.0250%, Al 0.015% - 0.035%, V 0.02% - 0.05%, B 0.0004% - 0.0006%, N 0.01% - 0.02%, with the balance being Fe and unavoidable impurities. The raw materials are pretreated by KR hot metal, controlling the content of S to be less than 0.015%, and after slag skimming, they enter the converter; in the converter smelting, the double slag method is used to remove P, controlling the content of P ≤ 0.020%, and the content of C at the end of converter smelting is controlled at 0.13 - 0.17%; subsequently, slab continuous casting is carried out, the continuous casting drawing speed is 1.5 - 2.5 m / min, controlling the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage to be 250 - 300 A, and the secondary cooling specific water volume is 2.3 L / kg - 2.8 L / kg (reducing the average carbon segregation index, suppressing segregation, and at the same time limiting the secondary cooling intensity to suppress the deterioration trend of the central crack of the casting blank), applying heavy reduction at the horizontal segment, that is, the solidification end, with the reduction amount of the continuous casting billet being 12 - 15 mm (reducing the central porosity level and segregation of the casting blank), and the cast billet after being taken offline is stacked for more than 48 hours (reducing the aggregation of residual H, suppressing the generation of microcracks inside the steel billet, and ensuring the toughness of the steel plate).

[0012] 2) Slab heating: The slab (with a thickness of less than 250 mm) is fed into a walking beam reheating furnace for heating. The slab is heated to 1130 - 1180 °C. The total heating time in the soaking zone and the heating zone is 2.2 - 3.8 hours, and the total residence time in the furnace is 3.5 - 5 hours. Adjust the opening degrees of the upper and lower burners in the soaking zone, control the air-fuel ratio at 1:1.7 - 1:2.3, and ensure that the temperature difference between the upper and lower surfaces of the billet is within 15 °C. (Combined with the billet composition, by reducing the billet heating temperature, energy consumption is reduced, while ensuring that its final rolling temperature is within the austenite temperature range. In addition, ensure the residence time in the soaking zone and the heating zone, and at the same time regulate the air-fuel ratio in the soaking zone to shorten the temperature difference between the upper and lower surfaces and the core of the continuous casting billet, improve the uniformity of the transverse and longitudinal metal flow on the steel plate surface, and also effectively inhibit the excessive growth of austenite grains, ensuring the steel plate properties and subsequent rolling shape), and at the same time leave 3 - 6 empty positions at the furnace head in the soaking zone of the reheating furnace (to prevent the low temperature at the furnace head from affecting the temperature uniformity of the billet).

[0013] 3) Rolling: Two-stage rolling is adopted. The first stage is recrystallization rolling (rough rolling). The reduction ratio of each of the first three passes in rough rolling is greater than 20%, and descaling water is sprayed on the rolling mill in the first three passes. The time for each pass is 0.2 - 0.5 min, the pressure is 10 - 15 MPa, and the thickness of the intermediate billet is 1.5 - 2 times the finished product thickness. The second stage is non-recrystallization rolling (finish rolling). The starting rolling temperature range is 950 - 980 °C, and the final rolling temperature range is 860 - 900 °C. The finish rolling is no more than four passes, and the reduction ratio of the last pass in rolling is controlled below 5% (In the rough rolling stage, in the first three passes of rolling, the rolling mill capacity is exerted as much as possible, large reduction ratios are adopted to increase the cumulative deformation amount in the steel plate, accelerate the occurrence of static recrystallization during the short-time temperature holding process of the intermediate billet. At the same time, through a flexible and variable high-pressure water descaling process, the tissue distribution from the steel plate surface to the core is made uniform, improving the strength and toughness of the steel plate; reducing the thickness of the intermediate billet during temperature holding, accelerating the temperature drop of the intermediate billet, and reducing the subsequent temperature holding time. In the later stage of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, a small reduction ratio is used in the last pass of rolling to flatten the shape of the steel plate and reduce the internal stress in the steel plate); In the finish rolling stage, when the rolling length of this pass reaches 75%, the pass table in the TCS jumps to the next pass for positioning preparation in advance (receiving the set roll gap from the secondary level in advance and performing roll gap distribution; after the signal of the steel in each pass disappears, positioning of equipment unrelated to rolling is allowed), and at the same time, the speed of the steel transfer roller table is increased from 1.2 - 2.5 m / s to 3 - 4 m / s. After rolling, the steel is ejected, and the ejection speed is 5 - 6.5 m / s. Subsequently, a combined controlled cooling mode of ultra-fast cooling + laminar cooling is adopted. The starting cooling temperature range of ultra-fast cooling is 820 - 850 °C, the final cooling temperature range is 580 - 620 °C, the cooling speed is 35 - 40 °C / s, and then it enters laminar cooling. The final cooling temperature range is 450 - 480 °C, the cooling speed is 5 - 10 °C / s, and the side spray pressure and water volume are 2 - 5 MPa and 50 - 70 m 3 / h. (Control the starting cooling temperature to ensure that the structure of the steel plate is austenitic when it enters the water. During the cooling process, adopt different segmented cooling rates. A large cooling rate ensures the transformation of the core structure, and then a small cooling rate is adopted to ensure the influence of the core temperature rise on the surface temperature. By controlling the cooling rate and the final cooling temperature, the formation of bainite and martensite phases on the steel plate surface and the amount of bainite phase in the core structure are inhibited, avoiding the influence of bainite or martensite phase transformation on the surface on the strength and toughness of the steel plate. Through side spraying, it is beneficial to control the shape of the steel plate, improve the uniformity of the steel plate performance, and reduce the probability of shape problems such as head buckling and tail buckling.)

[0014] 4) Hot straightening: Conduct one more straightening during hot straightening. The inlet roll gap is -1.2 mm to -2.5 mm, and the outlet roll gap is -2.9 mm to -3.8 mm. The straightening force is between 2800 KN and 3500 KN (by setting appropriate roll gaps and straightening forces, ensure that the straightened steel plate is flat and has a good shape), and then air-cool to room temperature.

[0015] 5) Air-cool to room temperature.

[0016] By adopting the above composition and heating, rolling and cooling process plans, the deficiencies existing in the prior art are overcome, and problems such as the need for deep controlled rolling during the rolling process of such steel plates due to low alloy composition, which affect the rolling efficiency of the steel plate and the first-pass rate of the shape, are solved. And the performance of the finally obtained steel plate is the same as that produced by the existing TMCP process. The flatness of the steel plate is below 5 mm / 2 m. The yield strength of the transverse tensile of the steel plate is ≥550 MPa, the tensile strength is between 670 and 840 MPa, the elongation is ≥17%, and the transverse Charpy impact energy at -20 °C is ≥34 J. It meets the technical requirements of users. It is estimated that after optimizing the composition, process, caster withdrawal speed and casting speed, the rolling rhythm of the economic medium and heavy plates with a yield strength of 550 MPa and a thickness of 16 - 50 mm is significantly improved, and the rolling time of each steel plate is reduced by 60 - 100 seconds on average, which has a significant effect on improving production efficiency and reducing production costs.

[0017] The beneficial effects of the present invention:

[0018] 1. By controlling the casting speed, the current intensity of electromagnetic stirring in the secondary cooling zone and the specific water volume of secondary cooling, the average index of carbon segregation can be effectively reduced, and the tendency of segregation and the deterioration of center cracks in the continuous casting billet can be inhibited; applying heavy reduction at the end of solidification can effectively reduce the center porosity level and segregation of the continuous casting billet. At the same time, stack the billets after they are taken off the production line to reduce the accumulation of residual H and inhibit the generation of microcracks inside the billets;

[0019] 2. The components designed in the present invention are reasonable, and the alloy addition amount is low. By increasing the C content, reducing the Mn content, and adding a small amount of micro-alloying elements Nb, Ti, and V, the alloy cost is greatly reduced. By reducing the billet heating temperature, the energy consumption is reduced. At the same time, it is ensured that the carbides and nitrides of Nb and V are quickly and fully dissolved in the matrix, and the finish rolling temperature is also ensured to be in the austenite temperature range. In addition, the residence time in the soaking section and heating section of the furnace is ensured, and the air-fuel ratio in the soaking section is regulated to shorten the temperature difference between the upper and lower surfaces and the core of the continuous casting billet. In addition, a vacant position is left at the furnace head of the soaking section of the heating furnace to ensure the temperature uniformity of the steel billet, and finally the lateral and longitudinal metal flow uniformity of the steel plate surface is ensured, and the excessive growth of austenite grains is effectively inhibited, ensuring the performance of the steel plate and the subsequent rolling shape.

[0020] 3. In the rough rolling stage, in the first three rolling passes, the rolling mill capacity is exerted as much as possible, a large reduction ratio is adopted to increase the cumulative deformation amount in the steel plate, and the static recrystallization during the short-time temperature holding process of the intermediate billet is accelerated. At the same time, through a flexible high-pressure water descaling process, the tissue distribution from the surface to the core of the steel plate is made uniform, improving the strength and toughness of the steel plate; reducing the thickness of the intermediate billet during temperature holding, accelerating the temperature drop of the intermediate billet, and reducing the subsequent temperature holding time. In the later stage of rolling, due to the temperature drop of the steel plate and the increase in deformation resistance, a small reduction ratio is adopted in the last pass to flatten the shape of the steel plate and reduce the internal stress of the steel plate; in addition, through the rolling model setting, when the rolling length of this pass reaches 75%, the pass table in the TCS jumps to the next pass in advance for positioning preparation to receive the set roll gap from the secondary level in advance and perform roll gap distribution. After the signal of the steel in each pass disappears, the equipment unrelated to rolling is allowed to perform positioning; increasing the throwing distance between the rotating steel roller table speed and the throwing speed when the steel is discharged, significantly improving the rolling rhythm; controlling the starting cooling temperature to ensure that the structure of the steel plate is austenite when it enters the water. During the cooling process, different cooling rates are adopted. A large cooling rate ensures the transformation of the core structure, and then a small cooling rate is adopted to ensure the influence of the core turning red on the surface temperature. By controlling the cooling rate and the turning red temperature, the formation of bainite and martensite phases on the surface of the steel plate and the number of bainite phases in the core structure are inhibited, avoiding the influence on the strength and toughness of the steel plate due to the bainite or martensite phase transformation on the surface. Through the side spray input, it is beneficial to the control of the steel plate shape, improving the performance uniformity of the steel plate and reducing the probability of shape problems such as buckling at the head and tail.

[0021] 4. Thermal straightening is adopted, and the positions and pressures of the inlet and outlet rollers are set to ensure that the straightened steel plate is straight and has a good shape.

[0022] 5. By adopting the above heating, rolling and cooling process plans, problems such as the need for in-depth control during the rolling process due to low alloy composition of such steel plates, which affect the rolling efficiency of the steel plates and the first-pass rate of the plate shape, are solved. Moreover, the properties of the finally obtained steel plates are the same as those produced by the existing TMCP process. The flatness of the steel plates is below 5 mm / 2 m, saving the cost of subsequent cold straightening processes. The yield strength of the transverse tension of the steel plates is ≥550 MPa, the tensile strength is between 670 - 840 MPa, the elongation is ≥17%, and the transverse Charpy impact energy at -20°C is ≥34 J, meeting the technical requirements of users. It is estimated that after optimizing the composition, process, steel transfer speed and steel throwing speed, the rolling rhythm of the economic medium and heavy plates with a yield strength of 550 MPa and a thickness of 16 - 50 mm is significantly improved, and the rolling time of each steel plate is reduced by 60 - 100 seconds on average, which has a significant effect on improving production efficiency and reducing production costs. Detailed implementation manners

[0023] The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0024] Among them, Table 1 shows the chemical composition of the steel in the examples, Table 2 shows the smelting process system of the steel in the examples; Table 3 shows the heating system of the continuous casting billets of the steel in the examples and the high-pressure water descaling process before rolling the continuous casting billets; Table 4 shows the rolling method of the steel in the examples; Table 5 shows the cooling and steel straightening processes of the steel in the examples; Table 6 shows the dimensions, properties and flatness of the steel in the examples

[0025] Table 1 Chemical composition (wt, %) of the examples of the present invention

[0026]

[0027] Note: In the steel, the impurity elements P ≤ 0.02%, S ≤ 0.015%.

[0028] Table 2 Smelting process system of the steel in the examples

[0029]

[0030] Table 3 Heating system of the continuous casting billets of the steel in the examples and high-pressure water descaling process before rolling the continuous casting billets

[0031]

[0032] Table 4 Rolling method of the steel in the examples

[0033]

[0034]

[0035] Table 5 Cooling and steel straightening processes of the steel in the examples

[0036]

[0037] Table 6 Dimensions, Properties and Flatness of Steel in Examples

[0038]

[0039] It can be seen that, compared with the prior art, a method for improving the rolling rhythm of steel plates with a yield strength of 550 MPa according to the present invention. By using the technical solution provided by the present invention, problems such as low alloy composition of such steel plates, the need to control rolling during the rolling process, which affect the rolling efficiency of the steel plates and the first-pass rate of the plate shape, are solved. It is estimated that after the composition and process are optimized, for economic medium and heavy plates with a thickness of 16 - 50 mm and a yield strength of 550 MPa, the rolling time of each steel plate is reduced by 60 - 100 seconds, and the flatness of the steel plate is below 5 mm / 2m, which has a significant effect on improving production efficiency and reducing production costs.

[0040] The above embodiments are only used to illustrate the technical concept and features of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and it cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for improving the rolling rhythm of a steel plate with a yield strength of 550 MPa, characterized in that, It includes the following steps: 1) Steelmaking and continuous casting: The raw materials are pretreated by KR hot metal, controlling the S content to be less than 0.015%, and after slag skimming, they enter the converter; in the converter smelting, the double slag method is used to remove P, controlling the P content ≤ 0.020%, and the C content at the end of the converter smelting is controlled at 0.13 - 0.17%; then continuous casting is carried out, the continuous casting drawing speed is 1.5 - 2.5 m / min, controlling the electromagnetic stirring current intensity in the secondary cooling zone of the continuous casting stage to be 250 - 300 A, the secondary cooling specific water volume is 2.3 L / kg - 2.8 L / kg, in the horizontal segment, that is, at the solidification end, heavy reduction is applied, the reduction amount of the continuous casting billet is 12 - 15 mm, and the cast billet after being taken off the line is stacked for more than 48 hours; 2) Billet heating: The billet is sent into a walking beam reheating furnace for heating, the billet is heated to 1130 - 1180 °C, the total heating time of the soaking section and the heating section is 2.2 - 3.8 hours, and the total time in the furnace is 3.5 - 5 hours; control the air-fuel ratio in the soaking section to be 1:1.7 - 1:2.3, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15 °C; at the same time, leave 3 - 6 empty positions at the furnace head of the soaking section of the heating furnace; 3) Rolling: Two-stage rolling is adopted. The reduction rate of each of the first three passes in rough rolling is greater than 20%, and descaling water is sprayed by the rolling mill in the first three passes. The time for each pass is 0.2 - 0.5 min, the pressure is 10 - 15 MPa. The thickness of the intermediate billet is 1.5 - 2 times the thickness of the finished steel plate. The finishing rolling starting temperature is 950 - 980 °C, the finishing rolling ending temperature is 860 - 900 °C. The finishing rolling has no more than four passes, and the reduction rate of the last pass in rolling is controlled below 5%. In the finishing rolling stage, when the rolling length of this pass reaches 75%, the pass table in TCS jumps to the next pass in advance for positioning preparation. At the same time, the speed of the steel turning roller table is increased to 3 - 4 m / s. After rolling, the steel is discharged, and the discharging speed is 5 - 6.5 m / s. Subsequently, a combined controlled cooling mode of ultra-fast cooling + laminar cooling is adopted. The starting temperature of ultra-fast cooling is 820 - 850 °C, the final cooling temperature is 580 - 620 °C, and the cooling speed is 35 - 40 °C / s. Then it enters laminar cooling, the final cooling temperature is 450 - 480 °C, and the cooling speed is 5 - 10 °C / s. The side spray pressure and water volume are 2 - 5 MPa and 50 - 70 m 3 / h respectively; 4) Hot straightening: The inlet roll gap is -1.2 mm - -2.5 mm, the outlet roll gap is -2.9 mm - -3.8 mm, and the straightening force is 2800 KN - 3500 KN; 5) Air-cool to room temperature.

2. The method according to claim 1, characterized in that, The chemical composition of the said steel plate by weight percentage includes: C 0.13% - 0.17%, Si 0.15% - 0.35%, Mn 1.45% - 1.65%, P ≤ 0.02%, S ≤ 0.015%, Nb 0.03% - 0.05%, Ti 0.015% - 0.0250%, Al 0.015% - 0.035%, V 0.02% - 0.05%, B 0.0004% - 0.0006%, N 0.01% - 0.02%, and the balance is Fe and unavoidable impurities.

3. The method according to claim 1, wherein In step 2), by adjusting the opening degrees of the upper and lower burners in the soaking section, the air-fuel ratio is controlled to be 1:1.8 - 1:2.

3.

4. The method according to claim 1, characterized in that, In step 4), the said hot straightening is one-pass straightening.

5. The method according to claim 1, wherein The thickness of the said steel plate is 16 - 50 mm.

6. The method according to claim 1, wherein The thickness of the said billet is less than 250 mm.

7. The method according to claim 1, wherein The said steel plate is obtained by rolling a billet on a medium and heavy plate reciprocating rolling mill.

8. The method according to claim 1, wherein The yield strength of the transverse tensile of the said steel plate ≥ 550 MPa, the tensile strength is 670 - 840 MPa, the elongation ≥ 17%, the transverse Charpy impact energy at -20 °C ≥ 34 J, and the flatness is below 5 mm / 2 m.

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