Normalized Q370R low-temperature storage tank steel plate with thickness of less than 40mm and preparation method thereof

Abstract

The invention discloses a steel plate for a normalized Q370R low-temperature storage tank with the thickness smaller than 40 mm and a preparation method of the steel plate. The steel plate for the normalized Q370R low-temperature storage tank comprises, by weight, 0.16%-0.18% of C, 0.20%-0.30% of Si, 1.50%-1.65% of Mn, smaller than or equal to 0.015% of P, smaller than or equal to 0.005% of S, 0.040%-0.050% of Nb, 0.040%-0.050% of V, 0.010%-0.020% of Ti, 0.05%-0.15% of Cr, 0.020%-0.035% of Als and the balance Fe and inevitable impurities. The steel plate has the advantages of being high in strength and toughness, easy to weld and easy to form, simple in production process, low in cost and high in steel plate performance stability. The method can be used for manufacturing various low-temperature storage tank spherical tanks such as liquid carbon dioxide storage tanks.

Description

Technical Field

[0001] The present invention relates to the production technology of steel for containers, in particular to a normalized Q370R steel plate for low-temperature storage tanks with a thickness of less than 40 mm and a preparation method thereof. Background Art

[0002] Q370R is a grade in the national standard GB / T713.2 - 2023 Steel plates and strips for pressure equipment. As a classic steel grade, Q370R is widely used and in great demand in the boiler and pressure vessel industry. Q370R requires delivery in the normalized state. Practical production experience shows that after normalization, the yield strength and tensile strength of the steel plate decrease significantly, and it is very easy to have a situation where the strength is too low or even unqualified, which affects the qualification rate of the steel products and causes losses to production enterprises. Therefore, it is particularly important to improve the performance stability of Q370R steel plates.

[0003] Chinese Patent CN101713044A discloses "A Q370R steel for pressure vessels and its preparation method", and the chemical composition by weight percentage is: C: 0.14 - 0.18%, Si: 0.25 - 0.50%, Mn: 1.45 - 1.60%, P≤0.020%, S≤0.015%, Nd: 0.28 - 0.35%, and the rest is Fe and inevitable impurities. The Q370R steel plate meeting the GB713 - 2008 standard is produced by the method of TMCP + post-normalization cooling. The disadvantages are that, firstly, a large amount of Nd element is added to the composition, which greatly increases the smelting and continuous casting difficulty and at the same time increases the cost, making it difficult to produce stably. Secondly, the control method of rapid cooling after normalization puts forward requirements for the production equipment of steel mills, increases the production difficulty, and is not applicable to all steel mills.

[0004] Chinese Patent CN105603304A discloses "A thick Q370R steel plate for pressure vessels with good HIC and SSCC resistance and a manufacturing method", and the composition by mass percentage is: C 0.12 - 0.17%, Si 0.15 - 0.40%, Mn 1.4 - 1.65%, Cu 0.1 - 0.3%, Mo 0.05 - 0.08%, Nb 0.015 - 0.040%, V 0.015 - 0.05%, P<0.012%, S<0.002%, and the rest is Fe and inevitable impurities. The Q370R steel plate with a thickness of 10 - 60 mm and good performance, especially with good HIC and SSCC resistance, is produced by the method of TMCP + rapid cooling after normalization. The disadvantages are that, firstly, precious metals such as Mo and Cu are added to the composition, increasing the production cost. Secondly, the control method of rapid cooling after normalization puts forward requirements for the production equipment of steel mills, increases the production difficulty, and is not applicable to all steel mills.

[0005] Chinese Patent CN102605245A discloses "A Ni-free normalized Q370R pressure vessel steel and its preparation method". The chemical composition by weight percentage is: C: 0.15 - 0.18%, Si: 0.25 - 0.50%, Mn: 1.40 - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.02 - 0.03%, V 0.07 - 0.08%, Ti 0.006 - 0.010%, and the balance is Fe and unavoidable impurities. The Q370R steel plate meeting the GB713-2008 standard is produced by the method of TMCP + air cooling after normalizing. The disadvantage is that the national standard version has been updated from GB713-2008 to GB / T713-2023, and this invention is no longer applicable to the production of Q370R. Summary of the Invention

[0006] In view of the deficiencies of the above patent, the present invention provides a normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40mm and its preparation method. The performance of the normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40mm meets the following requirements: yield strength ≥ 370MPa, tensile strength 530 - 630MPa, elongation after fracture ≥ 20%, transverse impact energy at -20°C ≥ 47J. It has the characteristics of easy welding and easy forming, and the production process is simple, the cost is low, and the steel plate performance has high stability. It can be used to manufacture various cryogenic storage tanks such as liquid carbon dioxide storage tanks.

[0007] The present invention is mainly realized through the following technical solutions.

[0008] In the first aspect of the present invention, a normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40mm is provided. Its chemical composition by mass percentage is: C: 0.16 - 0.18%, Si: 0.20 - 0.30%, Mn: 1.50 - 1.65%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.040 - 0.050%, V: 0.040 - 0.050%, Ti: 0.010 - 0.020%, Cr: 0.05 - 0.15%, Als: 0.020 - 0.035%, and the balance is Fe and unavoidable impurities.

[0009] In some embodiments, the chemical composition of the normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40mm by mass percentage is: C: 0.165 - 0.173%, Si: 0.24 - 0.27%, Mn: 1.56 - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.042 - 0.048%, V: 0.040 - 0.042%, Ti: 0.013 - 0.016%, Cr: 0.08 - 0.11%, Als: 0.020 - 0.022%, and the balance is Fe and unavoidable impurities.

[0010] In some embodiments, the properties of the normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40 mm meet the following requirements: yield strength ≥ 370 MPa, tensile strength 530 - 630 MPa, elongation after fracture ≥ 20%, and transverse impact energy at -20°C ≥ 47 J.

[0011] In some embodiments, the properties of the normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40 mm meet the following requirements: yield strength ≥ 400 MPa, tensile strength 550 - 630 MPa, elongation after fracture ≥ 25%, and transverse impact energy at -20°C ≥ 130 J.

[0012] On the other hand, the present invention provides a method for preparing a normalized Q370R steel plate for cryogenic storage tanks with a thickness of less than 40 mm, which includes the following processes: hot metal pretreatment, converter smelting, LF secondary refining, RH vacuum treatment, continuous casting, slow cooling, heating, rolling and cooling, and normalizing heat treatment.

[0013] In some embodiments, the method includes the following processes:

[0014] Hot metal pretreatment: After desulfurization and dephosphorization of hot metal by the KR stirring method, ensure that S ≤ 0.005% and P ≤ 0.015% in the hot metal;

[0015] Converter smelting: In the converter smelting stage, use hot metal pretreated by desulfurization and high-quality scrap steel as raw materials, with the hot metal temperature ≥ 1260°C. Reduce the phosphorus and carbon contents through the efficient top and bottom combined blowing technology to ensure that P ≤ 0.010% and S ≤ 0.004% in the molten steel;

[0016] LF secondary refining: Precisely control the composition of the molten steel in the secondary refining stage, carry out deoxidation alloying, and further reduce the non-metallic inclusions and harmful impurities in the molten steel, with S ≤ 0.004%, to obtain clean molten steel;

[0017] RH vacuum treatment: Adopt this treatment mode to greatly reduce the contents of hydrogen, oxygen, and nitrogen gases in the molten steel and reduce the adverse effects of harmful gases on the steel quality; the vacuum degree is less than 90 Pa, the vacuum time is maintained for more than 20 minutes, the pure degassing time is greater than 15 minutes, and the soft blowing time is greater than 15 minutes to ensure that the hydrogen content of the molten steel in the off-position is ≤ 1.6 ppm, the oxygen content is ≤ 30 ppm, and the nitrogen content is ≤ 50 ppm;

[0018] Continuous casting: During continuous casting, dynamic soft reduction is adopted, and the soft reduction positions are in segments 8, 9, and 10, with a total reduction of 7.5 mm; electromagnetic stirring is adopted during continuous casting, the electromagnetic stirring positions are at the outlet of segment 3 and the inlet of segment 4, the electromagnetic stirring frequency is 5 Hz, and the current is 350 A; the water flow on the wide side of the mold is 4500 L / min, the water flow on the narrow side is 370 L / min, the inlet water temperature of the mold is 36 ± 2 °C, the inlet water temperature of the mold is 38 ± 1 °C, the secondary cooling water temperature is 22 - 25 °C, and the water quality index should meet the process requirements; protective casting is adopted, the argon pressure for sealing the long nozzle is greater than 0.3 MPa, and the flow rate is 130 - 160 L / min; the argon pressure for sealing the submerged nozzle in the tundish is greater than 0.2 MPa, and the flow rate is 15 - 20 L / min; the superheat during continuous casting is 20 - 30 °C, and the tundish liquid level during ladle change is not less than 30 tons; constant casting speed is adopted, and the casting speed is stable at 1.0 m / min; the straightening temperature of the slab is controlled at 950 - 1000 °C, and the temperature difference along the width direction of the slab shall not exceed 50 °C; finally, a continuous casting slab with a thickness of 250 mm is produced, and the central segregation of the slab in the macrostructure is controlled below class C 3.0 level. After the slab is taken off the production line, it is stacked and cooled or pit-cooled for more than 36 hours;

[0019] Heating: The slab is heated in a walking beam reheating furnace, and the furnace atmosphere is strictly controlled to ensure the heating temperature and heating time of the slab. The heating temperature is 1210 °C - 1250 °C; the total time in the furnace is greater than 270 min, among which the heating section time is greater than 120 min, and the soaking section time is greater than 30 min to ensure the full solution of alloying elements and the uniform temperature of the slab;

[0020] Rolling and cooling: Two-stage controlled rolling is adopted for rolling, usually referred to as the rough rolling stage and the finish rolling stage; rough rolling is carried out on a 3800 mm roughing mill, the starting rolling temperature is above 1170 °C, and the relative reduction per pass is controlled at least two passes above 15%; the set torque during the first stage of rolling is 2280 kN·m; during finish rolling, the deformation of each pass is strictly controlled, the starting rolling temperature of finish rolling is ≤ 930 °C, and the final rolling temperature is ≤ 830 °C; the set torque during the second stage of rolling is 2320 kN·m; after the steel plate is rolled, laminar cooling is carried out, the water temperature is 17 - 20 °C, the final cooling temperature is 650 - 700 °C, the head shielding is 0 - 2.0 m, the tail shielding is 0 - 2.5 m, the edge shielding is 0 - 2.0 m, and the overall temperature difference after the steel plate turns red is controlled ≤ 50 °C; the steel plate is stacked and slowly cooled for 12 hours before sampling and inspection;

[0021] Normalizing heat treatment: The steel plate after shot blasting enters the normalizing furnace, the normalizing temperature is 850 °C, and the holding time is t × 1.5 min / mm + 30 minutes, where t is the thickness of the steel plate in mm.

[0022] In some embodiments, the method obtains a fine and uniform ferrite + pearlite structure.

[0023] The beneficial effects of the present invention are:

[0024] According to GB / T 713.2-2023, the present invention adopts a medium-carbon-niobium-vanadium-titanium-chromium alloying composition design, and uses a thermo-mechanical rolling + normalizing heat treatment technology to produce steel plates for low-temperature storage tanks of Q370R with a thickness of less than 40 mm. The yield strength of the steel plate is ≥370 MPa, the tensile strength is 530-660 MPa, the elongation after fracture is ≥20%, the transverse impact energy at -20°C is ≥47 J, and it has the characteristics of easy welding and easy forming. Moreover, the production process is simple, the cost is low, and the performance stability of the steel plate is high. Description of the Drawings

[0025] Figure 1 It is a macrostructure photograph of the continuous casting billet corresponding to the steel plate of the embodiment of the present invention.

[0026] Figure 2 It is an optical metallographic structure diagram (500×) of the near-surface of the steel plate of Example 3 of the present invention.

[0027] Figure 3 It is a macro-morphology diagram of the impact fracture of the steel plate of Example 3 of the present invention.

[0028] Figure 4 It is a high-magnification morphology diagram of the impact fracture of the steel plate of Example 3 of the present invention.

[0029] Figure 5 It is a cold bending specimen of the steel plates of Examples 1-3 of the present invention. Detailed Embodiments

[0030] The present invention will be described in more detail below with reference to the embodiments and the drawings. These embodiments are only descriptions of the best embodiments of the present invention and do not limit the scope of the present invention in any way.

[0031] The chemical composition of the steel plate of the present invention is by weight percentage: C: 0.16-0.18%, Si: 0.20-0.30%, Mn: 1.50-1.65%, P≤0.015%, S≤0.005%, Nb: 0.040-0.050%, V: 0.040-0.050%, Ti: 0.010-0.020%, Cr: 0.05-0.15%, Als: 0.020-0.035%, and the balance is Fe and unavoidable impurities. In addition, the present invention also provides a manufacturing method of the steel plate. The main steps and process parameters are as follows:

[0032] S1 Molten iron pretreatment: After the molten iron is desulfurized and dephosphorized by the KR stirring method, ensure that S≤0.005% and P≤0.015% in the molten iron;

[0033] S2 Converter Smelting: In the converter smelting stage, hot metal pretreated by desulfurization and high-quality scrap steel are used as raw materials. The temperature of the hot metal is ≥1260°C. The top-bottom combined blowing technology is used to reduce the phosphorus and carbon content, ensuring that the P content in the molten steel is ≤0.010% and the S content is ≤0.004%.

[0034] S3 LF Ladle Furnace Refining: In the ladle furnace refining stage, the composition of the molten steel is precisely controlled, deoxidized and alloyed, and the non-metallic inclusions and harmful impurities in the molten steel are further reduced. The S content is ≤0.004% to obtain clean molten steel.

[0035] S4 RH Vacuum Treatment: This treatment mode is adopted to greatly reduce the contents of hydrogen, oxygen and nitrogen gases in the molten steel and minimize the adverse effects of harmful gases on the steel quality. The vacuum degree is less than 90 Pa, the vacuum time is maintained for more than 20 minutes, the pure degassing time is more than 15 minutes, and the soft blowing time is more than 15 minutes, ensuring that the hydrogen content of the off-position molten steel is ≤1.6 ppm, the oxygen content is ≤30 ppm, and the nitrogen content is ≤50 ppm.

[0036] S5 Continuous Casting: Dynamic soft reduction is adopted during continuous casting. The soft reduction positions are segments 8, 9, and 10, and the total reduction amount is 7.5 mm. Electromagnetic stirring is adopted during continuous casting. The electromagnetic stirring positions are at the outlet of segment 3 and the inlet of segment 4. The electromagnetic stirring frequency is 5 Hz and the current is 350 A. The water flow on the wide side of the mold is 4500 L / min, and the water flow on the narrow side is 370 L / min. The inlet water temperature of the mold is 36±2°C, and the inlet water temperature of the mold is 38±1°C. The secondary cooling water temperature is 22 - 25°C, and the water quality index should meet the process requirements. Protective casting is adopted. The argon pressure for sealing the long nozzle is greater than 0.3 MPa, and the flow rate is 130 - 160 L / min. The argon pressure for sealing the submerged nozzle in the tundish is greater than 0.2 MPa, and the flow rate is 15 - 20 L / min. The superheat during continuous casting is 20 - 30°C. The tundish liquid level during ladle change is not less than 30 tons. Constant casting speed is adopted, and the casting speed is stable at 1.0 m / min. The straightening temperature of the slab is controlled between 950 - 1000°C, and the temperature difference along the width direction of the slab shall not exceed 50°C. Finally, a continuous casting slab with a thickness of 250 mm is produced. The central segregation of the slab in the macrostructure is controlled below class C 3.0. After the slab is taken off the production line, it is stacked and cooled or pit-cooled for more than 36 hours.

[0037] S6 Heating: The slab is heated in a walking beam reheating furnace, and the furnace atmosphere is strictly controlled to ensure the heating temperature and heating time of the slab. The heating temperature is 1210°C - 1250°C. The total time in the furnace is greater than 270 min, including the heating section time greater than 120 min and the soaking section time greater than 30 min, ensuring the full solution of alloying elements and uniform slab temperature.

[0038] S7 Rolling and Cooling: Two-stage controlled rolling is adopted in rolling, usually referred to as the rough rolling stage and the finish rolling stage; rough rolling is carried out on a 3800mm rough rolling mill, the starting rolling temperature is above 1170°C, and the relative reduction per pass is controlled at least two passes above 15%; the set torque during the first stage of rolling is 2280 kN·m; during finish rolling, the deformation of each pass is strictly controlled, the starting rolling temperature of finish rolling ≤ 930°C, and the finishing rolling temperature ≤ 830°C; the set torque during the second stage of rolling is 2320 kN·m; after the steel plate is rolled, laminar cooling is carried out, the starting cooling temperature is 750 - 800°C, the water temperature is 17 - 20°C, the cooling rate is 7 - 10°C / s, the final cooling temperature is 650 - 700°C, the head shielding is 0 - 2.0m, the tail shielding is 0 - 2.5m, the edge shielding is 0 - 2.0m, and the overall temperature difference after the steel plate returns to red is controlled ≤ 50°C; the steel plate is stacked and slowly cooled for 12 hours and then sampled for inspection;

[0039] S8 Normalizing Heat Treatment: The steel plate after shot blasting enters the normalizing furnace, the normalizing temperature is 850°C, and the holding time is t×1.5 min / mm + 30 minutes, where t is the thickness of the steel plate in mm; a fine and uniform ferrite + pearlite structure is obtained.

[0040] The present invention will be described in detail below in conjunction with actual embodiments.

[0041] Table 1 lists the chemical compositions of the embodiments, and Table 2 lists the rolling process parameters of the embodiments.

[0042] Table 1 Chemical Compositions (wt%) of Embodiments of the Present Invention

[0043] Embodiment C Si Mn P S Nb V Ti Cr Als 1 0.165 0.24 1.56 0.012 0.003 0.042 0.041 0.013 0.08 0.021 2 0.171 0.24 1.57 0.013 0.002 0.045 0.040 0.015 0.09 0.021 3 0.170 0.26 1.57 0.012 0.004 0.046 0.042 0.015 0.10 0.020 4 0.173 0.25 1.58 0.011 0.002 0.047 0.040 0.013 0.10 0.022 5 0.170 0.27 1.58 0.011 0.002 0.045 0.042 0.016 0.11 0.022 6 0.172 0.25 1.60 0.012 0.003 0.048 0.041 0.013 0.08 0.022

[0044] Table 2 Process Parameters of Embodiments of the Present Invention

[0045]

[0046] The mechanical properties, low-temperature impact properties, and cold bending properties of the steel plates of the embodiments of the present invention are inspected, and the results are shown in Table 3.

[0047] Table 3 Mechanical Properties of Steel Plates of Embodiments of the Present Invention

[0048]

[0049] The present invention produces a normalized Q370R steel plate for low-temperature storage tanks with a thickness of less than 40 mm by combining reasonable chemical components with a specific process flow. The yield strength is ≥370 MPa, the tensile strength is 530 - 630 MPa, the elongation after fracture is ≥20%, and the transverse impact energy at -20°C is ≥47 J. Preferably, it meets the following requirements: the yield strength is ≥400 MPa, the tensile strength is 550 - 630 MPa, the elongation after fracture is ≥25%, and the transverse impact energy at -20°C is ≥130 J. It has the characteristics of easy welding and easy forming, and the production process is simple, the cost is low, and the performance stability of the steel plate is high. It can be used to manufacture various low-temperature storage tanks such as liquid carbon dioxide storage tanks.

[0050] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A normalized Q370R steel plate for low-temperature storage tanks with a thickness of less than 40 mm, characterized in that, The chemical composition of the normalized Q370R low-temperature storage tank steel plate with a thickness of less than 40 mm is as follows by mass percentage: C: 0.16 - 0.18%, Si: 0.20 - 0.30%, Mn: 1.50 - 1.65%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.040 - 0.050%, V: 0.040 - 0.050%, Ti: 0.010 - 0.020%, Cr: 0.05 - 0.15%, Als: 0.020 - 0.035%, and the balance is Fe and inevitable impurities.

2. The normalized Q370R steel plate with a thickness of less than 40 mm for low-temperature storage tanks according to claim 1, characterized in that, The chemical composition of the normalized Q370R low-temperature storage tank steel plate with a thickness of less than 40 mm is as follows by mass percentage: C: 0.165 - 0.173%, Si: 0.24 - 0.27%, Mn: 1.56 - 1.60%, P ≤ 0.015%, S ≤ 0.005%, Nb: 0.042 - 0.048%, V: 0.040 - 0.042%, Ti: 0.013 - 0.016%, Cr: 0.08 - 0.11%, Als: 0.020 - 0.022%, and the balance is Fe and inevitable impurities.

3. The normalized Q370R steel plate for low-temperature storage tanks with a thickness of less than 40 mm according to claim 1 or 2, characterized in that, The properties of the normalized Q370R low-temperature storage tank steel plate with a thickness of less than 40 mm meet the following requirements: yield strength ≥ 370 MPa, tensile strength 530 - 630 MPa, elongation after fracture ≥ 20%, and -20°C transverse impact energy ≥ 47 J.

4. The normalized Q370R steel plate with a thickness of less than 40 mm for low-temperature storage tanks according to claim 3, characterized in that, The properties of the normalized Q370R low-temperature storage tank steel plate with a thickness of less than 40 mm meet the following requirements: yield strength ≥ 400 MPa, tensile strength 550 - 630 MPa, elongation after fracture ≥ 25%, and -20°C transverse impact energy ≥ 130 J.

5. The preparation method of the normalized Q370R low-temperature storage tank steel plate with a thickness of less than 40 mm according to any one of claims 1-4, characterized in that, The method includes the following processes: Hot metal pretreatment, converter smelting, LF secondary refining, RH vacuum treatment, continuous casting, slow cooling, heating, rolling and cooling, normalizing heat treatment.

6. The method according to claim 5, characterized in that, The method includes the following processes: Hot metal pretreatment: After desulfurization and dephosphorization of hot metal by the KR stirring method, ensure that S ≤ 0.005% and P ≤ 0.015% in the hot metal; Converter smelting: In the converter smelting stage, use hot metal after desulfurization pretreatment and high-quality scrap steel as raw materials, the hot metal temperature ≥ 1260°C, and reduce the phosphorus and carbon content through the efficient top-bottom combined blowing technology to ensure that P ≤ 0.010% and S ≤ 0.004% in the molten steel; LF secondary refining: Precisely control the composition of the molten steel in the secondary refining stage, carry out deoxidation alloying, further reduce the non-metallic inclusions and harmful impurities in the molten steel, S ≤ 0.004%, and obtain clean molten steel; RH vacuum treatment: Adopt this treatment mode to greatly reduce the content of hydrogen, oxygen, and nitrogen gases in the molten steel, reduce the adverse effects of harmful gases on the steel quality; the vacuum degree is less than 90 Pa, the vacuum time is maintained for more than 20 minutes, the pure degassing time is greater than 15 minutes, and the soft blowing time is greater than 15 minutes to ensure that the hydrogen content of the molten steel in the off-position is ≤ 1.6 ppm, the oxygen content is ≤ 30 ppm, and the nitrogen content is ≤ 50 ppm; Continuous casting: During continuous casting, dynamic soft reduction is adopted, and the soft reduction positions are sections 8, 9, and 10, with a total reduction of 7.5 mm; electromagnetic stirring is adopted during continuous casting, and the electromagnetic stirring positions are at the outlet of section 3 and the inlet of section 4, with an electromagnetic stirring frequency of 5 Hz and a current of 350 A; the water flow rate on the wide side of the mold is 4500 L / min, the water flow rate on the narrow side is 370 L / min, the inlet water temperature of the mold is 36 ± 2 °C, the inlet water temperature of the mold is 38 ± 1 °C, the secondary cooling water temperature is 22 - 25 °C, and the water quality index should meet the process requirements; protective casting is adopted, the argon pressure for sealing the long nozzle is greater than 0.3 MPa, and the flow rate is 130 - 160 L / min; the argon pressure for sealing the submerged nozzle in the tundish is greater than 0.2 MPa, and the flow rate is 15 - 20 L / min; the superheat during continuous casting is 20 - 30 °C, and the tundish liquid level during ladle change is not less than 30 tons; constant casting speed is adopted, and the casting speed is stable at 1.0 m / min; the straightening temperature of the slab is controlled at 950 - 1000 °C, and the temperature difference along the width direction of the slab shall not exceed 50 °C; finally, a continuous casting slab with a thickness of 250 mm is produced, and the central segregation of the slab in the macrostructure is controlled below grade C 3.

0. After the slab is taken offline, it is stacked and cooled or pit-cooled for more than 36 hours; Heating: The slab is heated in a walking beam reheating furnace, and the furnace atmosphere is strictly controlled to ensure the heating temperature and heating time of the slab. The heating temperature is 1210 °C - 1250 °C; the total time in the furnace is greater than 270 min, among which the heating section time is greater than 120 min, and the soaking section time is greater than 30 min to ensure the full solution of alloying elements and uniform slab temperature; Rolling and cooling: Two-stage controlled rolling is adopted for rolling, usually referred to as the rough rolling stage and the finish rolling stage; rough rolling is carried out on a 3800 mm roughing mill, the starting rolling temperature is above 1170 °C, and the relative reduction per pass is controlled at least two passes above 15%; the set torque during the first stage of rolling is 2280 kN·m; during finish rolling, the deformation of each pass is strictly controlled, the starting rolling temperature of finish rolling is ≤ 930 °C, and the final rolling temperature is ≤ 830 °C; the set torque during the second stage of rolling is 2320 kN·m; after the steel plate is rolled, laminar cooling is carried out, the water temperature is 17 - 20 °C, the final cooling temperature is 650 - 700 °C, the head shielding is 0 - 2.0 m, the tail shielding is 0 - 2.5 m, and the edge shielding is 0 - 2.0 m. The overall temperature difference after the steel plate turns red is controlled ≤ 50 °C; the steel plate is stacked and slowly cooled for 12 hours and then sampled for inspection; Normalizing heat treatment: The steel plate after shot blasting enters the normalizing furnace, the normalizing temperature is 850 °C, and the holding time is t × 1.5 min / mm + 30 minutes, where t is the thickness of the steel plate in mm.

7. The method according to claim 5 or 6, characterized in that, The method obtains a ferrite + pearlite structure with fine and uniform grains.

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