Low-temperature tough structural steel plate and production method thereof

Abstract

The invention discloses a low-temperature toughness structural steel plate and a production method thereof, and belongs to the technical field of structural steel, the low-temperature toughness structural steel plate comprises the following chemical components by mass percentage: 0.130-0.145% of C, 0.25-0.4% of Si, 1.45-1.55% of Mn, less than or equal to 0.018% of P, less than or equal to 0.003% of S, 0.018-0.028% of Nb, 0.015-0.035% of Als, less than or equal to 0.43% of CEV, and the balance of Fe. According to the mechanical properties of the steel plate, the yield Rel is larger than or equal to 400 MPa, the tensile strength Rm is larger than or equal to 540 MPa, the elongation A is larger than or equal to 28%, the longitudinal impact toughness at the temperature of-60 DEG C is larger than or equal to 100 J, the transverse impact toughness at the temperature of-60 DEG C is larger than or equal to 80 J, the CEV is smaller than or equal to 0.40%, and the steel plate has excellent obdurability, low-temperature impact performance and welding performance and can meet the use requirements of various light and thin foundation steel structural parts at the temperature of-60 DEG C.

Description

Technical Field

[0001] This invention belongs to the field of structural steel technology, and relates to a low-temperature toughness structural steel plate and its production method, specifically to a 355MPa grade low-temperature toughness structural steel plate and its production method. Background Technology

[0002] With the accelerated development of polar resources, polar shipbuilding, and infrastructure construction in high-altitude and cold regions, the demand for thin-gauge low-temperature structural steel has increased significantly. While traditional medium-thick plates (>16mm) can achieve stable low-temperature toughness through normalizing, thin-gauge steel plates face challenges such as controlling microstructure uniformity, imbalance between strength and toughness, and sensitivity to cold cracking during welding. Current solutions often rely on adding expensive alloying elements (e.g., Ni ≥ 0.3%), leading to a cost increase of over 30%. Furthermore, existing conventional TMCP processes have stringent requirements for controlled cooling precision in thin-gauge applications, making it difficult to balance plate shape and performance stability.

[0003] How to optimize the composition system and innovate the rolling process to ensure the low-temperature toughness of thin plates while also taking into account strength, shape control, weldability, and production cost has become a key issue that urgently needs to be solved in this field.

[0004] Chinese patent CN 115233108 A discloses an economical Q355NF steel plate for wind turbine towers and its production method. It employs a conventional C-Mn series composition design, combined with Nb-Ti microalloying, and utilizes controlled cooling, normalizing treatment followed by ACC accelerated cooling to produce Q355NF steel plates with a thickness of 16-40mm and good low-temperature impact resistance. The drawback is that the production process requires ACC accelerated cooling after normalizing to ensure strength. Currently, most domestic manufacturers lack this capability, requiring additional investment in facilities and equipment. Furthermore, the immediate re-cooling after normalizing poses certain risks to the uniformity of the plate's performance and shape control.

[0005] Chinese patent CN 119753521 A discloses an S355NL steel and its preparation method. This patent uses Nb-V-Mo alloying elements for control, and adds rare earth elements Ce or Y. By adding trace amounts of high-purity rare earth elements, it achieves inclusion modification and uniform refinement of pearlite lamellar spacing. The synergistic effect gives the steel excellent low-temperature impact resistance at -50℃. The manufacturing process employs casting and forging, including a series of steps such as ingot homogenization annealing followed by forging and heating treatment. The disadvantages are that rare earth elements are expensive, the molten steel tends to be viscous during casting, requiring a special nozzle to avoid clogging, and the forging process has lower production efficiency compared to continuous casting and rolling.

[0006] Chinese patents CN 117144101 A, CN 119615005 A, and CN 110042314 A all involve manufacturing methods for steel plates with similar low-temperature grades. They all use C-Mn series compositions, combined with one or more of Nb-V-Ti series, and some even add the precious metal Ni. The cost is relatively high. In terms of process, they mostly use ACC cooling combined with subsequent heat treatment for strength control. The impact performance is not related to below -50℃. Summary of the Invention

[0007] To solve the above-mentioned technical problems, the present invention provides a low-temperature toughness structural steel plate and its production method. The yield strength of the low-temperature toughness structural steel plate is 355MPa, and it has excellent strength and toughness, low-temperature impact performance and weldability, which can meet the usage requirements of various light and thin basic steel structural components at -60℃.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] This invention provides a low-temperature toughness structural steel plate, which comprises the following chemical composition by mass percentage: C: 0.130~0.145%, Si: 0.25~0.4%, Mn: 1.45~1.55%, P: ≤0.018%, S: ≤0.003%, Nb: 0.018~0.028%, Als: 0.015~0.035%, CEV ≤0.43%, with the remainder being Fe and unavoidable impurities.

[0010] The metallographic structure of the low-temperature toughness structural steel plate is ferrite and pearlite, wherein the volume fraction of pearlite is 25~30% and the grain size grade is 9.5~10.5.

[0011] The thickness of the low-temperature toughness structural steel plate is 8~16mm; the yield strength of the low-temperature toughness structural steel plate is 355MPa grade, its yield strength Rel≥390MPa, tensile strength Rm≥530MPa, elongation A≥26%; longitudinal impact toughness at -60℃≥100J, and transverse impact toughness at -60℃≥80J.

[0012] The present invention also provides a method for producing the low-temperature toughness structural steel plate, the method comprising the following steps: steelmaking, continuous casting, slow cooling of the billet, heating, rolling, and heat treatment.

[0013] The steelmaking process specifically includes hot metal pretreatment → converter smelting → LF refining → continuous casting.

[0014] In the molten iron pretreatment step, the molten iron after KR pretreatment meets the following requirements: As≤0.016% and Sn≤0.012%.

[0015] In the converter smelting process, pretreated molten iron and scrap steel are used as raw materials. Top and bottom blowing control is adopted, with the final P ≤ 0.015%. The final temperature of converter smelting is 1590-1650℃ when tapping. 500±50kg of lime is added during tapping to perform slag washing process. The argon blowing pressure in the ladle is 1.4~1.75Mpa, and the argon blowing flow rate is 300-1200L / min.

[0016] In the LF refining step, lime and fluorite are added to form slag for desulfurization. The amount of lime is 500kg~1500kg and the amount of fluorite is no more than 150kg. Alloy is added according to the sampling analysis results. The outlet temperature is controlled at 1610~1645℃.

[0017] RH vacuum treatment is an optional step. Whether to perform RH vacuum treatment depends on the hydrogen and oxygen content of the LF refining process and the flaw detection control requirements. In the RH vacuum treatment step, the vacuum degree is ≤133pa, the vacuum time is ≥15min, the limit time is ≥10min, and the steel tapping temperature is controlled at 1560~1580℃. Calcium wire is fed for 100~150m in the RH or LF process, and the soft blowing argon time is ≥10min.

[0018] In the continuous casting step, medium carbon protective slag ZT-03-1 is used for protective casting; argon gas is used for the entire casting process, the tundish temperature is controlled at 1525~1545℃, and the casting speed is controlled at 1.05±0.05m / min.

[0019] In the slow cooling step of the billet, the billet is cooled in the sink for 48 hours after being removed from the casting line, and the sink temperature is ≥750℃.

[0020] The slab after slow cooling of this invention has a center segregation C≤1.5, an intermediate crack ≤0.5, and no other defects.

[0021] In the heating step, the temperature of the billet preheating section is controlled at 700~900℃, the temperature of the first heating section is 1100~1150℃, the temperature of the second heating section is 1200~1240℃, the temperature of the soaking section is 1200~1250℃, and the furnace time is (1.1~1.3)×h minutes, where h is the thickness of the continuously cast billet in mm.

[0022] In the rolling process, the roughing rolling start temperature is ≥1000℃, and the reduction rate of the last three roughing rolling passes is ≥12%; the thickness to be heated is (4~5.5)×H, where H is the finished plate thickness in mm; the finishing rolling start temperature is ≤980℃, the finishing rolling temperature is 800~880℃, and air cooling is performed after rolling.

[0023] The heat treatment process includes normalizing, which involves controlling the holding temperature to 870-890℃, holding the plate in the furnace for (2-4.5) × H minutes, where H is the thickness of the finished plate in mm, and then air cooling after removing it from the furnace.

[0024] The 355MPa grade low-temperature toughness structural steel plate provided by this invention has a simple composition design, limiting the content range of only one microalloying element, Nb, further reducing production costs. The main composition design is described below:

[0025] Carbon (C) is a fundamental element in carbon steel, significantly improving its strength and hardness, and increasing its hardenability. However, it negatively impacts plasticity, toughness, and weldability. This invention aims to balance strength with good low-temperature toughness. Higher carbon content can ensure strength, but it leads to a surge in the pearlite volume fraction of the final product, resulting in a loss of low-temperature toughness. Therefore, this invention designs the C content to be controlled between 0.130% and 0.145%.

[0026] In steelmaking, silicon (Si) acts as a reducing agent and deoxidizer. It does not form carbides in steel and increases the strength of the solid solution. However, it also reduces the toughness and plasticity of steel to some extent. Therefore, considering the impact and low-temperature toughness effects, the Si content should not be too high. This invention designs the Si content to be controlled at 0.25~0.4%.

[0027] Mn can deoxidize and desulfurize, and form a solid solution with iron in steel, directly or indirectly improving the strength of ferrite and pearlite in steel without sacrificing toughness. This invention designs the Mn content to be controlled at 1.45~1.55%.

[0028] P and S are generally considered harmful elements in steel, causing brittleness, segregation, and inclusions, which negatively impact the steel's mechanical properties. Therefore, the lower the levels, the better. This invention designs P to be ≤0.018% and S to be ≤0.003%.

[0029] Among the three major microalloying elements, nitrogen (Nb) exhibits the most outstanding precipitation strengthening effect. It can form stable carbides and nitrides with C, N, and O. Simultaneously, Nb refines the grain structure of steel and strengthens it by dissolving into ferrite, thus significantly improving the strength and toughness of the steel. In this invention, the Nb content is designed to be controlled between 0.018% and 0.028%.

[0030] The steel plate of this invention is based on the national standard GB / T 1591-2018. Through reasonable composition design and rolling and heat treatment processes, the optimal microstructure is obtained, giving the steel plate excellent comprehensive performance. When flaw detection is required, RH furnace treatment can be used to achieve high steel purity, meeting the Class I flaw detection standard according to GB / T 2970-2016. The mechanical properties meet the following requirements: yield strength Rel ≥ 400 MPa, tensile strength Rm ≥ 540 MPa, elongation A ≥ 28%, conventional impact toughness at -60℃ ≥ 80 J, CEV ≤ 0.40%, and good weldability, which can meet the needs of various high-altitude and cold-weather infrastructure fields for thin-walled materials.

[0031] The method for producing low-temperature toughness structural steel plates provided by this invention preferably involves controlling the material heating process, the deformation amount in the final pass, and the final rolling temperature during the rolling process to achieve dynamic recrystallization nucleation of the microstructure, resulting in fine grains and laying the foundation for the steel plate's excellent impact performance. Normalizing heat treatment can further refine the grains and homogenize the microstructure, ultimately yielding a combination of strength and toughness in thin-gauge 355MPa grade target steel plates.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The low-temperature toughness structural steel plate provided by this invention has good strength, low-temperature toughness, and weldability, which can meet customers' needs for low-temperature thin-walled structural components.

[0034] Compared with the Q355NF steel plate and its production method for economical wind turbine towers disclosed in Chinese Patent CN 115233108 A, it only uses the microalloying element Nb and the content is lower. After rolling and heat treatment, no ACC cooling means are required. In particular, no additional cooling equipment is required after normalizing heat treatment, which further reduces the cost. The target microstructure ratio is obtained through its own composition and simple process path, and high performance conditions are also achieved.

[0035] Compared with the thin-gauge, low-cost normalized rolled steel plate Q355NE disclosed in Chinese Patent CN 117144101 A, the C and Mn contents are different, resulting in a microstructure volume fraction that matches the target performance. It has a higher elongation, which is about 7-10% higher, and also has lower low-temperature transverse and longitudinal impact toughness at -60℃. In terms of rolling process, this invention does not require ACC cooling in the normalized rolled state. Under the premise of meeting performance requirements, it effectively avoids the risk of performance differences and unstable plate shape control of the same plate under thin-gauge ACC cooling, and further saves energy, shortens working time, and reduces production costs.

[0036] Compared with Chinese patents CN 119753521 A, CN 119615005 A, and CN 110042314 A, the composition design does not include precious metals such as Ni, Mo, Cu, or rare earth elements, nor does it include V or Ti, two other microalloying elements, giving it a competitive advantage in cost control. The continuous casting and rolling process is convenient and stable; after online hot rolling, it can be directly sheared and heat-treated online after air cooling, eliminating the need for secondary online cutting, resulting in significant logistical advantages. Furthermore, none of the above mentions isothermal impact performance below -50℃, and the low-temperature performance below -60℃ is still unclear. Attached Figure Description

[0037] Figure 1 Metallographic images of the steel plate matrix produced in Example 1;

[0038] Figure 2 This is a metallographic image of the steel plate produced in Example 1 at 1 / 2 thickness.

[0039] Figure 3 Metallographic images of the steel plate matrix produced in Example 2;

[0040] Figure 4 Metallographic images of the steel plate matrix produced in Comparative Example 3;

[0041] Figure 5 Metallographic images of the steel plate matrix produced in Comparative Example 4;

[0042] Figure 6 The image shows the metallographic structure of the steel plate matrix produced in Comparative Example 6. Detailed Implementation

[0043] This invention provides a thin-gauge 355MPa grade low-temperature toughness structural steel plate, comprising the following chemical composition by mass percentage: C: 0.135~0.145%, Si: 0.25~0.4%, Mn: 1.45~1.55%, P: ≤0.018%, S: ≤0.003%, Nb: 0.018~0.025%, Als: 0.015~0.035%, CEV ≤0.43%, with the remainder being Fe and unavoidable impurities.

[0044] The production method of the thin-gauge 355MPa grade low-temperature toughness structural steel plate includes the following steps: hot metal pretreatment, converter smelting, LF refining, RH vacuum treatment, continuous casting, slow cooling of the billet, heating, rolling, and normalizing heat treatment.

[0045] In the molten iron pretreatment step, the molten iron after KR pretreatment meets the following requirements: As≤0.016% and Sn≤0.012%.

[0046] In the converter smelting process, pretreated molten iron and scrap steel are used as raw materials. Top and bottom blowing control is adopted, with the final P ≤ 0.015%. The final temperature of converter smelting is 1590-1650℃ when tapping. 500±50kg of lime is added during tapping to perform slag washing process. The argon blowing pressure in the ladle is 1.4~1.75Mpa, and the flow rate is adjusted to 300-1200L / min depending on the condition of the steel slag.

[0047] In the LF refining step, lime and fluorite are added for slag formation and desulfurization. The amount of lime is 500kg~1500kg and the amount of fluorite is no more than 150kg. Alloy is added according to the sampling analysis results. The outlet temperature is controlled at 1610~1645℃.

[0048] RH vacuum treatment is an optional step, depending on the hydrogen and oxygen content and flaw detection control requirements of the LF refining process. The vacuum degree is ≤133pa, the vacuum time is ≥15min, the limit time is ≥10min, the calcium wire is fed for 100~150m in the RH or LF process, the soft blowing argon time is ≥10min, and the steel tapping temperature is controlled at 1560~1580℃ when the RH vacuum treatment step is performed.

[0049] In the continuous casting step, medium carbon protective slag ZT-03-1 is used, and argon gas is used for casting throughout the process. The tundish temperature is controlled at 1525~1545℃, and the casting speed is controlled at 1.05±0.05m / min. The continuous casting yields a continuously cast billet with a thickness of 223mm.

[0050] The billet is cooled in the sink for 48 hours after being removed from the casting line, with a sink temperature ≥750℃, to eliminate stress, prevent cracks, and prevent hydrogen-induced defects.

[0051] The low-magnification analysis results of the billet material obtained by the above smelting and continuous casting meet the following requirements: central segregation C≤1.5, intermediate crack ≤0.5, and no other defects.

[0052] In the heating step, the temperature of the billet preheating section is controlled at 700~900℃, the temperature of the first heating section is 1100~1150℃, the temperature of the second heating section is 1200~1240℃, the temperature of the soaking section is 1200~1250℃, and the furnace time is (1.1~1.3)×h minutes, where h is the thickness of the continuously cast billet in mm. This is to prevent coarse original grains, and the temperature difference between the upper and lower surfaces of the billet is ≤20℃.

[0053] In the rolling process, the roughing rolling start temperature is ≥1000℃, the reduction rate of the last three roughing rolling passes is ≥12%, the thickness to be heated is (4~5.5)×H, where H is the finished plate thickness in mm, the finishing rolling start temperature is ≤980℃, the finishing rolling temperature is 800~880℃, and the cooling method after rolling is air cooling.

[0054] The heat treatment process includes normalizing.

[0055] The normalizing process includes: controlling the holding temperature to 870~890℃, the furnace time to be 2~4.5×H minutes, where H is the thickness of the finished plate in mm, and air cooling after exiting the furnace.

[0056] The present invention will now be described in detail with reference to the embodiments.

[0057] The chemical composition and weight percentage of the steel plates in each embodiment and comparative example are shown in Table 1.

[0058] Table 1 Chemical composition of examples and comparative examples

[0059]

[0060] The production process parameters of the steel plates in each embodiment and comparative example are shown in Table 2. Among them, the components of comparative example 5 and comparative example 2 are the same, and the components of comparative example 7 and comparative example 6 are the same.

[0061] Table 2 Production process parameters for examples and comparative examples

[0062]

[0063] The mechanical property test results of the steel plates in each embodiment and comparative example are shown in Table 3.

[0064] Table 3. Mechanical property test results of the examples and comparative examples

[0065]

[0066] A series of temperature shock tests were conducted on the steel plates in Examples 1 and 2 and Comparative Examples 3 and 6. The results are shown in Tables 4 and 5. The results show that the steel plates provided in each embodiment of the present invention have good transverse and longitudinal low-temperature impact performance.

[0067] Table 4 Results of a series of temperature shock tests

[0068]

[0069] Table 5 Results of a series of temperature shock tests

[0070]

[0071] In Tables 4 and 5, T / 4 represents the point where the thickness is 1 / 4.

[0072] The metallographic structure of the steel plates in Examples 1 and 2 and Comparative Examples 3, 4 and 6 was analyzed, and their microstructure and grain size grade were determined. The volume fraction of pearlite was calculated. The test results are shown in Table 6.

[0073] Table 6 Metallographic Structure Detection

[0074]

[0075] As can be seen from the above, the mechanical properties of the structural steel plate provided by the present invention meet the following requirements: yield strength Rel≥400MPa, tensile strength Rm≥540MPa, elongation A≥28%, longitudinal impact toughness at -60℃≥100J, transverse impact toughness at -60℃≥80J, CEV≤0.40%, and have good welding performance, which can meet the needs of various high-altitude and cold infrastructure fields for thin-walled materials.

[0076] In Comparative Example 3, the C content (0.17%) was much higher than the 0.130~0.145% required by this invention, resulting in a pearlite volume fraction (24.1%) lower than the 25~30% required by this invention. Simultaneously, the Mn content (1.04%) was too low, failing to effectively strengthen ferrite and pearlite, and the P content (0.026%) exceeded the standard, increasing the risk of brittleness. Furthermore, the Nb content (0.011%) was too low, failing to fully utilize grain refinement and precipitation strengthening effects. The addition of redundant elements such as Ni, V, and Ti increased costs without improving performance. In terms of process, although the rolling and normalizing parameters basically meet the requirements, the compositional defects cause the yield strength (285~337 MPa) and tensile strength (449~499 MPa) to be substandard. The longitudinal impact toughness (64~76 J) and transverse impact toughness (9~24 J) at -60℃ are far lower than the requirements of the present invention, which are longitudinal impact toughness ≥100 J at -60℃ and transverse impact toughness ≥80 J at -60℃. In addition, the grain size (grade 9.0) is low.

[0077] In Comparative Example 4, the low Mn content (1.22%) and lack of Nb resulted in insufficient strength (yield strength 341~355 MPa, tensile strength 504~527 MPa) and poor low-temperature toughness (longitudinal impact toughness 52~68 J at -60℃). Nb is a key microalloying element in this invention; its absence resulted in insufficient grain refinement, with a grain size of only 9.0 grade, and the pearlite volume fraction (21.3%) was also below the required range (25~30%). Although process parameters such as the final rolling temperature (789℃) and normalizing temperature (880℃) were within reasonable ranges, the compositional design defects could not be compensated for by the process, and the final performance did not meet the requirements.

[0078] In Comparative Example 5, the composition was the same as in Example 2, but the normalizing temperature (900°C) was too high, exceeding the 870~890°C range required by this invention. The excessively high normalizing temperature resulted in coarse grains and uneven microstructure, further deteriorating strength and toughness. As a result, the yield strength (340~360 MPa) and tensile strength (502~515 MPa) still did not meet the requirements, and the longitudinal impact toughness (76~100 J) at -60°C fluctuated significantly with an average value below 100 J, indicating that defects in the process window can lead to performance fluctuations and failure.

[0079] In Comparative Example 6, the C content (0.17%) was too high, the Mn content (1.32%) was too low, and Nb was not added, resulting in insufficient strength (yield strength 379~392 MPa, close to but not fully reaching 390 MPa) and elongation (25~26%). Simultaneously, the normalizing temperature (850℃) was too low, failing to achieve sufficient microstructure homogenization and grain refinement, resulting in a low pearlite volume fraction (22.6%). Although the conventional impact toughness at -60℃ (107~121 J) met the standard, the transverse impact toughness (46~61 J) was far below 80 J, indicating an imbalance between toughness and strength.

[0080] In Comparative Example 7, the composition was the same as in Comparative Example 6 (C=0.17%, Mn=1.32%, no Nb). Although the normalizing temperature (880℃) met the requirements, the compositional defects could not be compensated for. As a result, the yield strength (367~369MPa) and tensile strength (519~528MPa) did not meet the standards, the elongation (24.5~26%) was barely acceptable, and the impact toughness at -60℃ (137~82~96 J) fluctuated greatly and sometimes did not meet 100J. The overall performance was unstable and could not meet the consistent requirements of this invention for strength, toughness and microstructure.

[0081] The above detailed description of a low-temperature toughness structural steel plate and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments may be listed within the defined scope. Therefore, variations and modifications that do not depart from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A low-temperature toughness structural steel plate, characterized in that, The low-temperature toughness structural steel plate comprises the following chemical composition by mass percentage: C: 0.130~0.145%, Si: 0.25~0.4%, Mn: 1.45~1.55%, P: ≤0.018%, S: ≤0.003%, Nb: 0.018~0.028%, Als: 0.015~0.035%, CEV ≤0.43%, with the remainder being Fe and unavoidable impurities.

2. The low-temperature toughness structural steel plate according to claim 1, characterized in that, The metallographic structure of the low-temperature toughness structural steel plate is ferrite and pearlite, wherein the volume fraction of pearlite is 25~30% and the grain size grade is 9.5~10.

5.

3. The low-temperature toughness structural steel plate according to claim 1, characterized in that, The thickness of the low-temperature toughness structural steel plate is 8~16mm; the yield strength of the low-temperature toughness structural steel plate is 355MPa grade, with a yield strength Rel≥390MPa, tensile strength Rm≥530MPa, elongation A≥26%; longitudinal impact toughness at -60℃≥100J, and transverse impact toughness at -60℃≥80J.

4. A method for producing a low-temperature toughness structural steel plate as described in any one of claims 1-3, characterized in that, The production method includes the following steps: steelmaking, continuous casting, slow cooling of the billet, heating, rolling, and heat treatment.

5. The production method according to claim 4, characterized in that, In the continuous casting process, argon gas is used for protection throughout the casting process, the tundish temperature is controlled at 1525~1545℃, and the casting speed is controlled at 1.05±0.05m / min.

6. The production method according to claim 4, characterized in that, In the slow cooling step of the billet, the billet is cooled in the sink for 48 hours after being removed from the casting line, and the sink temperature is ≥750℃.

7. The production method according to claim 4, characterized in that, After slow cooling, the center segregation of the billet is C≤1.5, the intermediate crack is ≤0.5, and there are no other defects.

8. The production method according to claim 4, characterized in that, In the heating step, the temperature of the billet preheating section is controlled at 700~900℃, the temperature of the first heating section is 1100~1150℃, the temperature of the second heating section is 1200~1240℃, the temperature of the soaking section is 1200~1250℃, and the furnace time is (1.1~1.3)×h minutes, where h is the thickness of the continuously cast billet in mm.

9. The production method according to claim 4, characterized in that, In the rolling process, the roughing rolling start temperature is ≥1000℃, and the reduction rate of the last three roughing rolling passes is ≥12%; the thickness to be heated is (4~5.5)×H, where H is the thickness of the finished plate in mm; the finishing rolling start temperature is ≤980℃, the finishing rolling temperature is 800~880℃, and air cooling is performed after rolling.

10. The production method according to claim 4, characterized in that, The heat treatment process includes normalizing, which involves controlling the holding temperature to 870-890℃, holding the plate in the furnace for (2-4.5) × H minutes, where H is the thickness of the finished plate in mm, and then air cooling after removing it from the furnace.

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