Production method for high-fracture-toughness steel plate

Abstract

A production method for a high-fracture-toughness steel plate. The method comprises: steelmaking, wherein molten steel comprises 0.03-0.07% of C, 0.15-0.25% of Si, 1.43-1.55% of Mn, less than or equal to 0.015% of P, less than or equal to 0.003% of S, 0.05-0.25% of Cr, less than or equal to 0.23% of Ni, 0.13-0.25% of Cu, less than or equal to 0.15% of Mo, 0.02-0.04% of Nb, 0.01-0.02% of Ti and 0.02-0.05% of Alt, and has a reinforcement equivalent Req of 0.43-0.57%; continuous casting, wherein the central segregation grade of a continuously cast product is controlled to be higher than class B, grade 1.0; heating, wherein the temperature of a soaking zone is 1130-1220ºC, and the residence time is 320-570 min; rolling, wherein the final rolling temperature is 725-860ºC; and cooling, wherein the cooling speed is 3-26ºC / s, and the temperature at which a steel plate is taken from water is 210-520ºC.

Description

Production method of high fracture toughness steel plate

[0001] This application is based on the Chinese patent application with application number CN202410156193.3 and application date of February 4, 2024, and claims the priority of the Chinese patent application. The entire content of the above patent application is hereby introduced into this application as a reference. Technical Field

[0002] The present application belongs to the technical field of material preparation, and specifically relates to a method for producing a high fracture toughness steel plate. Background Art

[0003] Fracture toughness characterizes a material's ability to resist brittle fracture caused by existing defects and is a key indicator for evaluating the safety of steel. The CTOD test uses full-thickness specimens to simulate real-world loading methods and speeds. The characteristic value of CTOD (Crack Tip Opening Displacement) is used to characterize the fracture toughness of steel. CTOD is the resistance value exhibited by a material when a crack or crack-like defect in a specimen or component no longer fractures rapidly with increasing load, i.e., when unstable fracture occurs.

[0004] With the development of resources like oil and natural gas, and the widespread use of wind power, these energy sources are increasingly being exploited in deep-sea and polar environments. Consequently, demand for steel in offshore engineering has increased significantly, placing increasing demands on the quality and performance of steel plates. While the large-scale steel structures of deep-sea and polar platforms demand a certain level of strength, the safety requirements of these environments also place a higher premium on the steel plates, requiring them to exhibit enhanced toughness, particularly fracture toughness. Consequently, there is a need to develop steel plates with high fracture toughness for use in deep-sea and polar environments.

[0005] Summary of the Invention

[0006] The purpose of this application is to provide a method for producing a high fracture toughness steel plate.

[0007] To achieve one of the above-mentioned objectives, one embodiment of the present invention provides a method for producing a high fracture toughness steel plate, comprising the following steps performed in sequence:

[0008] (1) Molten steel smelting

[0009] The chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.07%, Si 0.15-0.25%, Mn 1.43-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.25%, Ni≤0.23%, Cu 0.13-0.25%, Mo≤0.15%, Nb 0.02-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, wherein the strengthening equivalent Req is 0.43-0.57%, Req=2.5C+Mn / 5+Cr / 7+Cu / 6+Mo / 4+Nb / 6;

[0010] (2) Continuous casting

[0011] Continuously casting the molten steel obtained in the molten steel smelting process into a continuous casting billet, controlling the center segregation level of the continuous casting billet to be better than Class B 1.0, the continuous casting billet to have no center porosity, and the crack level to be better than Class 1.0;

[0012] (3) Heating

[0013] The continuous casting slab is heated with the soaking section temperature at 1130-1220°C and the furnace time at 320-570 min;

[0014] (4) Two-stage rolling

[0015] The continuous casting slab is first rolled in a recrystallization zone to obtain an intermediate slab, and then rolled in a non-recrystallization zone to produce a steel plate. The reduction in at least two passes of the recrystallization zone rolling stage is ≥30 mm, and the thickness of the intermediate slab is T ≥170 mm or T ≥3t, where t is the thickness of the steel plate in mm. The finishing rolling temperature of the non-recrystallization zone rolling is 725-860°C.

[0016] (5) Controlled cooling

[0017] The steel plate is water-cooled, the cooling rate of the steel plate is controlled to be 3-26°C / s, and the outlet water temperature of the steel plate is 210-520°C.

[0018] As a further improvement of one embodiment, in the continuous casting process, the liquid level fluctuation amplitude of the crystallizer is controlled within the range of ±2mm, the water inlet temperature of the crystallizer is 28-35°C, the inlet and outlet water temperature difference is 4-10°C, the narrow surface water volume is 550±50L / min, the wide surface water volume is 3800±200L / min, the continuous casting speed is 0.6±0.05m / min, and the total reduction at the end of solidification is 5-6mm.

[0019] As a further improvement of one embodiment, in the two-stage rolling process, when 60mm≤t<80mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥30mm; when 80mm≤t<90mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥35mm; when 90mm≤t≤100mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥38mm.

[0020] As a further improvement of one embodiment, the molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence;

[0021] In the converter smelting step, the total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the carbon content in the molten steel at the end of the converter smelting is 0.02-0.06%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of the aluminum ingot added is (0.69+14[0])kg / t;

[0022] In the LF refining step, low-carbon ferrochrome and FeNb are added to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffuse deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Before leaving the station, sampling and testing are carried out and the alloy composition is adjusted. After the chemical composition of the molten steel reaches the target range, it is kept for at least 5 minutes before tapping;

[0023] In the RH vacuum refining step, 0.5±0.05 kg / t of ferrotitanium is added, and then the molten steel is vacuum degassed and inclusions are removed, the vacuum degree is controlled to be less than 2 mBar, and the degassing time is ≥10 min; then 140±5 m of silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment, the feeding speed of the silicon calcium wire is 1.5±0.5 m / s, the soft stirring time is greater than 12 min, and the slag surface is controlled to fluctuate but the molten steel is not exposed.

[0024] As a further improvement of one embodiment, in the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.06%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.15%, Cu 0.15-0.25%, Nb 0.02-0.03%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.43-0.53%;

[0025] In the converter smelting step, 1.8±0.3kg / t of copper blocks are loaded into a converter together with scrap steel and molten iron for smelting, the amount of ferrosilicon added is 1.5±0.3kg / t, the amount of manganese metal added is 12±2kg / t, and the amount of lime added is 3±1kg / t;

[0026] In the LF refining step, the amount of low-carbon ferrochrome added is 1.0±0.5 kg / t, and the amount of FeNb added is 0.3±0.1 kg / t;

[0027] In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (960-295[C]-77[Mn]-0.002t 2 -0.24t)±10℃;

[0028] In the controlled cooling process, the cooling rate of the steel plate is controlled to be (28-0.24t)±1°C / s, and the outlet water temperature of the steel plate is controlled to be (453-1.87t)±20°C.

[0029] As a further improvement of one embodiment, when t≤60mm, the structure of the steel plate is polygonal ferrite + low-carbon bainite, and the proportion of low-carbon bainite is 10-45%;

[0030] When t>60mm, the structure of the steel plate is quasi-polygonal ferrite + low carbon bainite + degenerate pearlite, with the proportion of quasi-polygonal ferrite being 5-25%, the proportion of low carbon bainite being 70-85%, and the proportion of degenerate pearlite being 3-13%;

[0031] The yield strength of the steel plate is ≥355MPa, the tensile strength is 470~630MPa, the Z-direction tensile strength is ≥450MPa, and the CTOD value at -10℃ is ≥1.6mm; the steel plate is welded at a heat input of 7~50kJ / cm, the low-temperature impact energy of the heat-affected zone of the weld joint at -40℃ is ≥230J, and the CTOD value of the coarse-grained zone and the critical zone at -10℃ is ≥1.2mm.

[0032] As a further improvement of one embodiment, in the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.04-0.07%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.10-0.20%, Ni 0.08-0.15%, Cu 0.20-0.25%, Nb 0.03-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.45-0.55%;

[0033] In the converter smelting step, 1.8±0.3kg / t of copper blocks and 0.85±0.1kg / t of nickel plates are loaded into a converter together with scrap steel and molten iron for smelting. The carbon content of the molten steel at the end of the converter smelting is 0.02-0.05%, the amount of ferrosilicon added is 1.7±0.1kg / t, the amount of metallic manganese added is 15±1kg / t, and the amount of lime added is 4.0±0.5kg / t.

[0034] In the LF refining step, the amount of low-carbon ferrochrome added is 2.1±0.5 kg / t, and the amount of FeNb added is 0.5±0.05 kg / t;

[0035] In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (854-295[C]-77[Mn]-0.04t 2 +4.21t)±10℃;

[0036] In the controlled cooling process, the cooling rate of the steel plate is controlled to be (23+0.002t 2 -0.33t)±1℃ / s, the outlet water temperature of the steel plate is (0.0023t 3 -0.48t 2 +28.3t)±20℃.

[0037] As a further improvement of one embodiment, the structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, and the proportion of low-carbon bainite is 35-80%;

[0038] The yield strength of the steel plate is ≥420MPa, the tensile strength is 500~640MPa, the Z-direction tensile strength is ≥475MPa, and the CTOD value at -10℃ is ≥1.3mm; the steel plate is welded at a heat input of 7~50kJ / cm, the low-temperature impact energy of the heat-affected zone of the weld joint at -40℃ is ≥150J, and the CTOD value of the coarse-grained zone and the critical zone at -10℃ is ≥1.0mm.

[0039] As a further improvement of one embodiment, in the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.06%, Si 0.15-0.25%, Mn 1.43-1.53%, P≤0.015%, S≤0.003%, Cr 0.15-0.25%, Ni 0.13-0.23%, Cu 0.13-0.22%, Mo 0.07-0.15%, Nb 0.025-0.035%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.47-0.57%;

[0040] In the converter smelting step, 1.4±0.3kg / t of copper blocks, 1.4±0.3kg / t of nickel plates, and 1.4±0.3kg / t of ferromolybdenum are loaded into a converter together with scrap steel and molten iron for smelting. The carbon content of the molten steel at the end of the converter smelting is 0.02-0.06%, the amount of ferrosilicon added is 2.5±0.3kg / t, the amount of metallic manganese added is 12±2kg / t, and the amount of lime added is 3±1kg / t.

[0041] In the LF refining step, the amount of low-carbon ferrochrome added is 2.7±0.5 kg / t, and the amount of FeNb added is 0.4±0.1 kg / t;

[0042] In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (955-295[C]-77[Mn]-82[Mo]-0.002t 2 -0.24t)±10℃;

[0043] In the controlled cooling process, the cooling rate of the steel plate is controlled to be (19-0.13t)±1℃ / s, and the outlet water temperature of the steel plate is (560+0.02t 2 -4.5t)±20℃.

[0044] As a further improvement of one embodiment, the structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, the low-carbon bainite accounts for 65-90%, and the average grain size of the quasi-polygonal ferrite is 4-9 μm;

[0045] The steel plate must have a yield strength of 460 MPa or greater, a tensile strength of 520-675 MPa, a Z-axis tensile strength of 495 MPa or greater, and a CTOD value of 1.1 mm or greater at -10°C. The steel plates must be welded at a heat input of 7-50 kJ / cm2. The low-temperature impact energy of the heat-affected zone (HAZ) at -40°C must be 150 J or greater, and the CTOD value of the coarse-grained and critical zones at -10°C must be 0.8 mm or greater.

[0046] Compared with the prior art, the beneficial effects of the present application are: (1) by optimizing the design of the chemical composition scheme, the low C+Mn-Cr-Ni-Cu-Mo composite alloying+Nb-Ti-Al composite microalloying composition system is adopted, which not only has an excellent strengthening effect on the finally prepared steel plate, controls the central segregation of the steel plate, reduces non-metallic inclusions, and makes the steel plate have excellent fracture toughness, lamellar tearing resistance, low-temperature impact toughness and welding performance, but also, the combination of Ni and Cu can change the structure of the copper-rich phase in the oxide layer, inhibit the penetration of Cu into the matrix, and avoid the risk of copper embrittlement; and it also controls the cost and avoids the problems of great production difficulty caused by overly strict content control requirements;

[0047] (2) By optimizing the chemical composition design scheme and combining it with the control of the production process, not only the aging treatment, quenching heat treatment and tempering heat treatment in the existing technology are eliminated, but the TMCP short process can be used to complete the preparation of high fracture toughness steel plates without the need for stacking and slow cooling. The rolling and cooling processes are also simplified, the rolling efficiency and rolling stability are improved, and the stable execution of the rolling process is facilitated. Moreover, the final prepared steel plate has high fracture toughness, low yield strength ratio, excellent low-temperature toughness, welding performance and lamellar tearing resistance, good safety, can withstand large deformation cold forming, and is resistant to high temperatures. It has good application prospects in the field of marine engineering steel such as offshore platforms and offshore wind power.

[0048] As used herein, the term "comprise" and variations of the term, such as "comprises," "comprised," "comprising," "including," and "containing" do not exclude other features, components, elements, or steps unless the context clearly requires otherwise. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] FIG1 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 1 of the present invention;

[0050] FIG2 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 4 of the present invention;

[0051] FIG3 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 5 of the present invention;

[0052] FIG4 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 8 of the present invention;

[0053] FIG5 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 9 of the present invention;

[0054] FIG6 is a metallographic structure diagram of a steel plate at 1 / 2 thickness according to Example 12 of the present invention. DETAILED DESCRIPTION

[0055] The technical solution of the present invention is further described below in conjunction with specific implementation methods, but the scope of protection required is not limited to the description.

[0056] One embodiment of the present invention provides a production method of a high fracture toughness steel plate and a high fracture toughness steel plate prepared by the production method.

[0057] The various steps of the production method are introduced in detail below.

[0058] (1) Molten steel smelting

[0059] The chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.07%, Si 0.15-0.25%, Mn 1.43-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.25%, Ni≤0.23%, Cu 0.13-0.25%, Mo≤0.15%, Nb 0.02-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities. Among them, the strengthening equivalent Req is 0.43-0.57%.

[0060] In the chemical composition of the present invention:

[0061] C is the most basic strengthening element in steel, but if the C content is too high, the fracture toughness of the heat-affected zone of the steel will deteriorate. In the present invention, the C content is limited to 0.03-0.07%.

[0062] Si is a strengthening and deoxidizing element in steel, but too much Si will reduce the low-temperature toughness of the steel. In the present invention, the Si content is limited to 0.15-0.25%. Without affecting the low-temperature toughness, the deoxidation effect can be guaranteed and the oxide inclusions in the steel can be reduced.

[0063] Mn is a solid solution strengthening and grain refining strengthening element that can improve the strength and hardenability of steel. However, Mn is an element that is easily segregated and easily forms inclusions. Excessive Mn content can affect the fracture toughness, lamellar tear resistance, and low-temperature impact toughness of the steel plate. In the present invention, the Mn content is limited to 1.43-1.55%, which not only ensures the strength of the steel plate but also reduces segregation, thereby preventing the deterioration of the fracture toughness, lamellar tear resistance, and low-temperature impact toughness of the steel plate caused by MnS inclusions.

[0064] Cr can reduce the critical cooling rate of steel, improve hardenability, and improve the uniformity of microstructure and properties in the thickness direction of thick steel plates. However, excessive Cr is detrimental to the plasticity and toughness of the steel plates. In the present invention, the Cr content is limited to 0.05% to 0.25%, which can achieve a reasonable match between strength and toughness.

[0065] Ni is an effective element for improving fracture toughness and low-temperature toughness, but the alloy cost is relatively high. The present invention limits Ni to ≤ 0.23%, which is beneficial to controlling costs.

[0066] Cu can improve the strength, toughness and high temperature resistance of the steel plate to ensure the heat treatment performance of the steel plate after welding. In the present invention, the Cu content is limited to 0.13-0.25%.

[0067] Mo is a strong hardenability element and a fine grain element, which can improve strength and toughness, but has a high cost. In the present invention, Mo is limited to 0.15%.

[0068] Nb has a fine grain strengthening effect, but too much Nb will induce bainite formation during welding, which is not conducive to the low-temperature toughness of the heat-affected zone of the weld joint. In the present invention, the Nb content is limited to 0.02-0.04%, which can ensure the fine grain strengthening effect while avoiding adverse effects on the low-temperature toughness of the heat-affected zone of the weld joint.

[0069] Ti is an important nitrogen-fixing and deoxidizing element, but excessive Ti can easily form large-particle Ti carbonitrides in the core of the steel plate, thereby affecting the fracture toughness, core impact toughness and lamellar tearing resistance of the steel plate. In the present invention, the Ti content is limited to 0.01-0.02%.

[0070] Al is a deoxidizing element, but excessive Al tends to increase Al2O3 inclusions in the steel, thereby affecting the low-temperature toughness of the steel. In the present invention, the Al content is limited to 0.02-0.05%.

[0071] P, S, N, and O are unavoidable impurity elements. In the present invention, P is limited to 0.015%, S is limited to 0.003%, N is limited to 0.005%, and O is limited to 0.002%.

[0072] Strengthening equivalent Req = 2.5C + Mn / 5 + Cr / 7 + Cu / 6 + Mo / 4 + Nb / 6. By limiting the elemental relationship and content of C, Mn, Cr, Cu, Mo, and Nb, the proportions of solid solution strengthening, grain refinement strengthening, precipitation strengthening, and phase transformation strengthening can be comprehensively controlled, thereby giving the steel plate sufficient strength.

[0073] In summary, through the optimization design of the chemical composition design scheme, the low C + Mn-Cr-Ni-Cu-Mo composite alloying + Nb-Ti-Al composite microalloying composition system is adopted, which not only can have an excellent strengthening effect on the finally prepared steel plate, control the central segregation of the steel plate, reduce non-metallic inclusions, and make the steel plate have excellent fracture toughness, lamellar tearing resistance, low-temperature impact toughness and welding performance. In addition, the combination of Ni and Cu can change the structure of the copper-rich phase in the oxide layer, inhibit the penetration of Cu into the matrix, and avoid the risk of copper embrittlement; it also controls costs and avoids problems such as high production difficulty caused by overly strict content control requirements.

[0074] It is understandable that the chemical composition of the molten steel obtained in the molten steel smelting process, the chemical composition of the continuous casting billet obtained in the subsequent continuous casting process, and the chemical composition of the high fracture toughness steel plate finally produced by the production method are all consistent.

[0075] Preferably, the molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence.

[0076] a. Converter smelting steps

[0077] The steelmaking raw materials are fed into the converter to be mixed into molten steel and then deoxidized and alloyed.

[0078] The total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the C content in the molten steel at the end of the converter smelting is 0.02-0.06%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of aluminum ingot added is (0.69+14[O])kg / t.

[0079] By controlling the converter smelting steps, not only can harmful residual elements be controlled to improve the toughness of the final steel plate, but also efficient smelting and safe production can be achieved.

[0080] b. LF refining step

[0081] The molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment, temperature control, and inclusion control.

[0082] Specifically, low-carbon ferrochrome and FeNb are added to the LF refining furnace to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffusely deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Samples are taken for testing and alloy composition adjustment before leaving the station. After the chemical composition of the molten steel reaches the target range, it is maintained for at least 5 minutes before steel is tapped. In other words, it is prohibited to add smelting raw materials and auxiliary materials at least 5 minutes before the end of refining.

[0083] In this way, desulfurization, deoxidation and inclusion adsorption can be fully achieved, thereby improving the fracture toughness, low-temperature toughness and lamellar tearing resistance of the steel plate.

[0084] c. RH vacuum refining step

[0085] The RH cycle degassing equipment is used to vacuum degas and remove inclusions from the molten steel after LF refining, and then silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment.

[0086] Specifically, 0.5±0.05kg / t of ferrotitanium is added to the molten steel, and then the molten steel is vacuum degassed and inclusions are removed, the vacuum degree is controlled to be less than 2mBar, and the degassing time is ≥10min; then 140±5m of silicon-calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment, the feeding speed of the silicon-calcium wire is 1.5±0.5m / s, the soft stirring time is greater than 12min, and the slag surface fluctuation is controlled without exposing the molten steel, so that the inclusions in the molten steel are fully denatured and floated, thereby improving the fracture toughness, low-temperature toughness and lamellar tearing resistance of the steel plate.

[0087] (2) Continuous casting

[0088] The molten steel obtained from the molten steel smelting process is continuously cast into a continuous casting billet with a thickness of 320 mm. The center segregation level of the continuous casting billet is controlled to be better than Class B level 1.0, the continuous casting billet has no center porosity, and the crack level is better than Level 1.0, so that the center segregation level of the finally prepared steel plate can be better than Class B level 0.5.

[0089] Specifically, during continuous casting, the liquid level fluctuation amplitude of the crystallizer is controlled within the range of ±2mm to avoid surface longitudinal cracks and slag inclusions in the continuous casting billet, and thus avoid surface cracks and warping defects in the finished steel plate; the water inlet temperature of the crystallizer is 28-35°C, the inlet and outlet water temperature difference is 4-10°C, the narrow side water volume is 550±50L / min, the wide side water volume is 3800±200L / min, the continuous casting speed is 0.6±0.05m / min, and the total reduction at the end of solidification is 5-6mm. This facilitates the control of internal quality such as central segregation and looseness of the continuous casting billet, and thus controls the central segregation of the steel plate, thereby improving the fracture toughness and low-temperature impact toughness of the core of the steel plate.

[0090] The chemical composition of the continuous casting billet cast from molten steel is the same as that of the molten steel obtained by smelting, and will not be described in detail here.

[0091] (3) Heating

[0092] The continuous casting billet is heated with the soaking section temperature at 1130-1220°C and the furnace time at 320-570 min. This not only allows the alloy elements to be fully dissolved and the grains to not grow excessively, but also facilitates production scheduling. It can be scheduled for production together with high-alloy steel that requires high-temperature heating, and can also be scheduled for production together with high-performance steel that requires low-temperature heating.

[0093] (4) Two-stage rolling

[0094] The continuously cast slab is first rolled through a recrystallization zone to produce an intermediate billet, which is then rolled through a non-recrystallization zone to produce steel plate. During the recrystallization zone rolling stage, at least two passes have a reduction of 30 mm or greater. The intermediate billet has a thickness of T ≥ 170 mm or T ≥ 3t, where t is the thickness of the steel plate in mm. This ensures a reasonable distribution of reductions between the two stages. For a given total reduction, this allows for sufficient recrystallization and crushing of the original austenite grains, improving the low-temperature toughness and weldability of the steel plate. Furthermore, it controls the oxide scale thickness during rolling, ensuring excellent surface quality of the steel plate.

[0095] Furthermore, the final rolling temperature of the non-recrystallization zone rolling is 725-860° C., so as to fully refine the rolled structure and improve the low-temperature toughness, welding performance and large deformation cold formability of the steel plate.

[0096] Preferably, when 60mm≤t<80mm, the reduction in at least two passes during the recrystallization zone rolling stage is ≥30mm; when 80mm≤t<90mm, the reduction in at least two passes during the recrystallization zone rolling stage is ≥35mm; and when 90mm≤t≤100mm, the reduction in at least two passes during the recrystallization zone rolling stage is ≥38mm. By distributing and controlling the reduction in each pass during the recrystallization zone rolling stage based on the target thickness of the final steel plate, deformation can be fully penetrated into the core of the steel plate, fully recrystallizing the original austenite grains and refining the microstructure, thereby improving the low-temperature toughness and large-deformability cold formability of the steel plate.

[0097] (5) Controlled cooling

[0098] The steel plates are water-cooled, specifically using a multifunctional intermittent cooling system, with a controlled cooling rate of 3-26°C / s and a water outlet temperature of 210-520°C. By controlling the cooling method, cooling rate, and water outlet temperature, the steel plate structure can be precisely controlled, resulting in high fracture toughness, low yield strength ratio, excellent low-temperature toughness, weldability, and lamellar tearing resistance. The steel plates can also withstand large deformation cold forming and high-temperature resistance.

[0099] After testing, the central segregation grade of the obtained steel plate is better than Class B 0.5 grade, so the CTOD value of the heat-affected zone of the steel plate after welding will not deteriorate due to central segregation; by sampling the obtained steel plate and conducting mechanical property testing, the yield strength ratio of the obtained steel plate is ≤0.85, the -60°C low-temperature impact energy at different positions in the thickness direction is ≥250J, the Z-direction tensile section shrinkage rate is ≥65%, and the CTOD value at -10°C is ≥1.1mm.

[0100] Furthermore, after the steel plate is subjected to 5% pre-deformation treatment, or 5% pre-deformation + aging at (250±10)°C for 1h, the low-temperature impact toughness of the steel plate does not decrease, that is, after the steel plate is subjected to large deformation cold forming, or large deformation cold forming + long-term aging, the low-temperature impact toughness does not decrease.

[0101] The steel plates are welded at a heat input of 7 to 50 kJ / cm, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥0.8 mm; after welding, the steel plates are heat treated at 590±10°C for no more than 6 hours, and the strength, low-temperature impact toughness, CTOD fracture toughness, and Z-direction tensile properties are not reduced.

[0102] That is to say, through the optimization design of the chemical composition design scheme and the control of the production process, not only the aging treatment, quenching heat treatment and tempering heat treatment in the existing technology are eliminated, but the TMCP short process can be used to complete the preparation of high fracture toughness steel plates without stacking and slow cooling. It also simplifies the rolling and cooling processes, improves the rolling efficiency and rolling stability, and facilitates the stable execution of the rolling process; and the final prepared steel plate has high fracture toughness, low yield strength ratio, excellent low-temperature toughness, welding performance and lamellar tearing resistance, good safety, can withstand large deformation cold forming, and is resistant to high temperatures. It has good application prospects in marine engineering steel fields such as offshore platforms and offshore wind power.

[0103] Three preferred embodiments of the present invention are provided below, each providing a method for producing a high fracture toughness steel plate and a high fracture toughness steel plate produced by the production method, to further illustrate the technical solution of the present invention.

[0104] First embodiment

[0105] (1) Molten steel smelting

[0106] The chemical composition of the molten steel obtained by smelting in this embodiment preferably includes, by mass percentage, the following: C 0.03-0.06%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.15%, Cu 0.15-0.25%, Nb 0.02-0.03%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities. The strengthening equivalent Req is 0.43-0.53%.

[0107] Preferably, the molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence.

[0108] a. Converter smelting steps

[0109] 1.8±0.3kg / t copper ingots are loaded into a converter along with scrap steel and molten iron for smelting, and the resulting molten steel is deoxidized and alloyed. In other words, the raw materials for steelmaking include copper ingots, scrap steel, and molten iron.

[0110] The total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the C content in the molten steel at the end of the converter smelting is 0.02-0.06%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of aluminum ingot added is (0.69+14[O])kg / t, the amount of ferrosilicon added is 1.5±0.3kg / t, the amount of metallic manganese added is 12±2kg / t, and the amount of lime added is 3±1kg / t.

[0111] b. LF refining step

[0112] The molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment, temperature control, and inclusion control.

[0113] Specifically, 1.0±0.5kg / t of low-carbon ferrochrome and 0.3±0.1kg / t of FeNb are added to the LF refining furnace to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffusely deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Samples are taken for testing and alloy composition adjustment before leaving the station. After the chemical composition of the molten steel reaches the target range, it is maintained for at least 5 minutes before steel is tapped. In other words, it is prohibited to add smelting raw materials and auxiliary materials at least 5 minutes before the end of refining.

[0114] c. RH vacuum refining step

[0115] The RH cycle degassing equipment is used to vacuum degas and remove inclusions from the molten steel after LF refining, and then silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment.

[0116] Specifically, 0.5±0.05kg / t of ferrotitanium is added to the molten steel, and then the molten steel is vacuum degassed and inclusions are removed, the vacuum degree is controlled to be less than 2mBar, and the degassing time is ≥10min; then 140±5m of silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment, the feeding speed of the silicon calcium wire is 1.5±0.5m / s, the soft stirring time is greater than 12min, and the slag surface fluctuation is controlled but the molten steel is not exposed.

[0117] (2) Continuous casting

[0118] The molten steel obtained from the molten steel smelting process is continuously cast into a continuous casting billet with a thickness of 320 mm. The central carbon segregation level of the continuous casting billet is controlled to be better than Class B level 1.0, the continuous casting billet has no central porosity, and the crack level is better than Level 1.0, so that the central segregation level of the finally prepared steel plate can be better than Class B level 0.5.

[0119] Specifically, during continuous casting, the liquid level fluctuation amplitude of the crystallizer is controlled within the range of ±2mm, the water inlet temperature of the crystallizer is 28-35°C, the inlet and outlet water temperature difference is 4-10°C, the narrow surface water volume is 550±50L / min, the wide surface water volume is 3800±200L / min, the continuous casting speed is 0.6±0.05m / min, and the total reduction at the end of solidification is 5-6mm.

[0120] The chemical composition of the continuous casting billet cast from molten steel is the same as that of the molten steel obtained by smelting, and will not be described in detail here.

[0121] (3) Heating

[0122] The continuous casting billet is heated, the soaking section temperature is 1130-1220°C, and the furnace time is 320-570 minutes.

[0123] (4) Two-stage rolling

[0124] The continuous casting billet is first rolled in a recrystallization zone to obtain an intermediate billet, and then rolled in a non-recrystallization zone to form a steel plate. The reduction of at least two passes in the recrystallization zone rolling stage is ≥30 mm, and the thickness of the intermediate billet is T ≥170 mm or T ≥3t, where t is the thickness of the steel plate in mm.

[0125] Furthermore, the final rolling temperature of the non-recrystallization zone rolling is (960-295[C]-77[Mn]-0.002t 2 -0.24t)±10℃.

[0126] Wherein, t is the thickness of the steel plate, in mm; [C] represents the mass percentage of C in the continuous casting billet, [Mn] represents the mass percentage of Mn in the continuous casting billet, and so on.

[0127] That is, the final rolling temperature for non-recrystallization rolling is calculated based on the actual mass percentages of C and Mn in the continuous casting slab. In practice, the continuous casting slab can be sampled before the heating process to test the chemical composition of the continuous casting slab, including the actual mass percentage of C [C] and the actual mass percentage of Mn [Mn]. The final rolling temperature for non-recrystallization rolling is then calculated using the aforementioned formula, and the temperature control scheme for the two-stage rolling process is then determined.

[0128] Thus, this preferred embodiment determines the final rolling temperature for non-recrystallization rolling based on the actual chemical composition content of the continuous casting slab, thereby establishing a matching relationship between the actual chemical composition content and the final rolling temperature for non-recrystallization rolling. This makes the temperature control range in the two-stage rolling process more reasonable, further controls the microstructure of the steel plate, and reasonably matches the strength and toughness of the steel plate. Of course, using the above formula to calculate the final rolling temperature for non-recrystallization rolling is only a preferred embodiment of the present invention. The method for determining the final rolling temperature for non-recrystallization rolling in the present invention is not limited to this. For example, in alternative embodiments, it can also be obtained based on experience or other methods.

[0129] Preferably, when 60mm≤t<80mm, the reduction in at least two passes in the rolling stage of the recrystallization zone is ≥30mm; when 80mm≤t<90mm, the reduction in at least two passes in the rolling stage of the recrystallization zone is ≥35mm; when 90mm≤t≤100mm, the reduction in at least two passes in the rolling stage of the recrystallization zone is ≥38mm.

[0130] (5) Controlled cooling

[0131] The steel plate is water-cooled. Specifically, a multifunctional intermittent cooling system can be used for water cooling, and the cooling rate of the steel plate is controlled to be (28-0.24t)±1°C / s, and the outlet water temperature of the steel plate is (453-1.87t)±20°C, where t is the thickness of the steel plate in mm.

[0132] The chemical composition of the obtained steel plate is the same as that of the molten steel obtained by smelting, and will not be repeated here.

[0133] Thus, this preferred embodiment determines the cooling rate and outlet water temperature during water cooling based on the target thickness of the steel plate to be produced, thereby establishing a matching relationship between the steel plate thickness, cooling rate, and outlet water temperature. This allows for more reasonable control of the cooling process during the cooling step, further controlling the microstructure of the steel plate and properly matching the strength and toughness of the steel plate. Of course, using the above formula to calculate the cooling rate and outlet water temperature during water cooling is only a preferred embodiment of the present invention. The method for determining the cooling rate and outlet water temperature during water cooling in the present invention is not limited to this. For example, in alternative embodiments, they can also be obtained based on experience or other methods.

[0134] After testing, the central segregation grade of the obtained steel plate is better than Class B 0.5.

[0135] When t≤60mm, the structure of the steel plate is polygonal ferrite + low-carbon bainite, and the proportion of low-carbon bainite is 10-45%;

[0136] When t>60mm, the structure of the steel plate is quasi-polygonal ferrite + low carbon bainite + degenerate pearlite, the proportion of quasi-polygonal ferrite is 5-25%, the proportion of low carbon bainite is 70-85%, and the proportion of degenerate pearlite is 3-13%.

[0137] The yield strength of the steel plate is ≥355MPa, the tensile strength is 470~630MPa, the Z-direction tensile strength is ≥450MPa, the yield strength ratio is ≤0.85, the -60℃ low-temperature impact energy at different positions in the thickness direction of the steel plate is ≥250J, the Z-direction tensile section shrinkage is ≥65%, and the CTOD value at -10℃ is ≥1.6mm.

[0138] After the steel plate is subjected to 5% pre-deformation treatment, or 5% pre-deformation + aging at (250±10)℃ for 1h, the low-temperature impact toughness of the steel plate does not decrease, that is, after the steel plate is subjected to large deformation cold forming, or large deformation cold forming + long-term aging, the low-temperature impact toughness does not decrease.

[0139] The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥230 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥1.2 mm.

[0140] After welding, the steel plate is heat treated at 590±10℃ for no more than 6h, and the strength, low-temperature impact toughness, CTOD fracture toughness and Z-direction tensile properties are not reduced.

[0141] Second embodiment

[0142] (1) Molten steel smelting

[0143] The chemical composition of the molten steel obtained by smelting in this embodiment preferably includes, by mass percentage, C 0.04-0.07%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.10-0.20%, Ni 0.08-0.15%, Cu 0.20-0.25%, Nb 0.03-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities. The strengthening equivalent Req is 0.45-0.55%.

[0144] Preferably, the molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence.

[0145] a. Converter smelting steps

[0146] 1.8±0.3kg / t of copper ingots and 0.85±0.1kg / t of nickel plates are loaded into a converter along with scrap steel and molten iron for smelting. The resulting molten steel is then deoxidized and alloyed. In other words, the steelmaking raw materials include copper ingots, nickel plates, scrap steel, and molten iron.

[0147] The total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the C content in the molten steel at the end of the converter smelting is 0.02-0.05%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of aluminum ingot added is (0.69+14[O])kg / t, the amount of ferrosilicon added is 1.7±0.1kg / t, the amount of metallic manganese added is 15±1kg / t, and the amount of lime added is 4.0±0.5kg / t.

[0148] b. LF refining step

[0149] The molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment, temperature control, and inclusion control.

[0150] Specifically, 2.1±0.5kg / t of low-carbon ferrochrome and 0.5±0.05kg / t of FeNb are added to the LF refining furnace to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffuse deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Samples are taken for testing and alloy composition adjustment before leaving the station. After the chemical composition of the molten steel reaches the target range, it is maintained for at least 5 minutes before steel is tapped. In other words, it is prohibited to add smelting raw materials and auxiliary materials at least 5 minutes before the end of refining.

[0151] c. RH vacuum refining step

[0152] This step is the same as that in the first embodiment and will not be described again here.

[0153] (2) Continuous casting

[0154] This process is the same as that of the first embodiment and will not be described again.

[0155] (3) Heating

[0156] This process is the same as that of the first embodiment and will not be described again.

[0157] (4) Two-stage rolling

[0158] The only difference between this process and the first embodiment is that:

[0159] The final rolling temperature of the non-recrystallization zone rolling is (854-295[C]-77[Mn]-0.04t 2+4.21t)±10℃.

[0160] Wherein, t is the thickness of the steel plate, in mm; [C] represents the mass percentage of C in the continuous casting billet, [Mn] represents the mass percentage of Mn in the continuous casting billet, and so on.

[0161] Except for the above differences, this process is no different from the first embodiment and will not be described again.

[0162] (5) Controlled cooling

[0163] The steel plate is water-cooled. Specifically, a multifunctional intermittent cooling system can be used for water cooling, and the cooling rate of the steel plate is controlled to be (23+0.002t 2 -0.33t)±1℃ / s, the outlet water temperature of the steel plate is (0.0023t 3 -0.48t 2 +28.3t)±20℃, where t is the thickness of the steel plate in mm.

[0164] The chemical composition of the obtained steel plate is the same as that of the molten steel obtained by smelting, and will not be repeated here.

[0165] Thus, this preferred embodiment determines the cooling rate and outlet water temperature during water cooling based on the target thickness of the steel plate to be produced, thereby establishing a matching relationship between the steel plate thickness, cooling rate, and outlet water temperature. This allows for more reasonable control of the cooling process during the cooling step, further controlling the microstructure of the steel plate and properly matching the strength and toughness of the steel plate. Of course, using the above formula to calculate the cooling rate and outlet water temperature during water cooling is only a preferred embodiment of the present invention. The method for determining the cooling rate and outlet water temperature during water cooling in the present invention is not limited to this. For example, in alternative embodiments, they can also be obtained based on experience or other methods.

[0166] After testing, the central segregation grade of the obtained steel plate is better than Class B 0.5.

[0167] The structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, and the proportion of low-carbon bainite is 35% to 80%.

[0168] The yield strength of the steel plate is ≥420MPa, the tensile strength is 500-640MPa, the Z-direction tensile strength is ≥475MPa, the yield strength ratio is ≤0.85, the -60°C low-temperature impact energy at different positions in the thickness direction of the steel plate is ≥250J, the Z-direction tensile section shrinkage is ≥65%, and the CTOD value at -10°C is ≥1.3mm.

[0169] After the steel plate is subjected to 5% pre-deformation treatment, or 5% pre-deformation + aging at (250±10)℃ for 1h, the low-temperature impact toughness of the steel plate does not decrease, that is, after the steel plate is subjected to large deformation cold forming, or large deformation cold forming + long-term aging, the low-temperature impact toughness does not decrease.

[0170] The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥1.0 mm.

[0171] After welding, the steel plate is heat treated at 590±10℃ for no more than 6h, and the strength, low-temperature impact toughness, CTOD fracture toughness and Z-direction tensile properties are not reduced.

[0172] Third embodiment

[0173] (1) Molten steel smelting

[0174] The chemical composition of the molten steel obtained by smelting in this embodiment preferably includes, by mass percentage, C 0.03-0.06%, Si 0.15-0.25%, Mn 1.43-1.53%, P≤0.015%, S≤0.003%, Cr 0.15-0.25%, Ni 0.13-0.23%, Cu 0.13-0.22%, Mo 0.07-0.15%, Nb 0.025-0.035%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities. The strengthening equivalent Req is 0.47-0.57%.

[0175] Preferably, the molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence.

[0176] a. Converter smelting steps

[0177] 1.4±0.3kg / t of copper ingots, 1.4±0.3kg / t of nickel plates, and 1.4±0.3kg / t of ferromolybdenum are charged into a converter along with scrap steel and molten iron for smelting. The resulting molten steel is then deoxidized and alloyed. In other words, the steelmaking raw materials include copper ingots, nickel plates, ferromolybdenum, scrap steel, and molten iron.

[0178] The total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the C content in the molten steel at the end of the converter smelting is 0.02-0.06%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of aluminum ingot added is (0.69+14[O])kg / t, the amount of ferrosilicon added is 2.5±0.3kg / t, the amount of metallic manganese added is 12±2kg / t, and the amount of lime added is 3±1kg / t.

[0179] b. LF refining step

[0180] The molten steel after converter smelting is sent to the LF refining furnace for chemical composition adjustment, temperature control, and inclusion control.

[0181] Specifically, 2.7±0.5kg / t of low-carbon ferrochrome and 0.4±0.1kg / t of FeNb are added to the LF refining furnace to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffuse deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Samples are taken for testing and alloy composition adjustment before leaving the station. After the chemical composition of the molten steel reaches the target range, it is maintained for at least 5 minutes before steel is tapped. In other words, it is prohibited to add smelting raw materials and auxiliary materials at least 5 minutes before the end of refining.

[0182] c. RH vacuum refining step

[0183] This step is the same as that in the first embodiment and will not be described again here.

[0184] (2) Continuous casting

[0185] This process is the same as that of the first embodiment and will not be described again.

[0186] (3) Heating

[0187] This process is the same as that of the first embodiment and will not be described again.

[0188] (4) Two-stage rolling

[0189] The only difference between this process and the first embodiment is that:

[0190] The final rolling temperature of the non-recrystallization zone rolling is (955-295[C]-77[Mn]-82[Mo]-0.002t 2 -0.24t)±10℃.

[0191] Wherein, t is the thickness of the steel plate in mm, [C] represents the mass percentage of C in the continuous casting billet, [Mn] represents the mass percentage of Mn in the continuous casting billet, and [Mo] represents the mass percentage of Mo in the continuous casting billet, and so on.

[0192] Except for the above differences, this process is no different from the first embodiment and will not be described again.

[0193] (5) Controlled cooling

[0194] The steel plate is water-cooled. Specifically, a multifunctional intermittent cooling system can be used for water cooling, and the cooling rate of the steel plate is controlled to be (19-0.13t) ± 1℃ / s, and the outlet water temperature of the steel plate is (560+0.02t 2 -4.5t)±20℃, where t is the thickness of the steel plate in mm.

[0195] The chemical composition of the obtained steel plate is the same as that of the molten steel obtained by smelting, and will not be repeated here.

[0196] Thus, this preferred embodiment determines the cooling rate and outlet water temperature during water cooling based on the target thickness of the steel plate to be produced, thereby establishing a matching relationship between the steel plate thickness, cooling rate, and outlet water temperature. This allows for more reasonable control of the cooling process during the cooling step, further controlling the microstructure of the steel plate and properly matching the strength and toughness of the steel plate. Of course, using the above formula to calculate the cooling rate and outlet water temperature during water cooling is only a preferred embodiment of the present invention. The method for determining the cooling rate and outlet water temperature during water cooling in the present invention is not limited to this. For example, in alternative embodiments, they can also be obtained based on experience or other methods.

[0197] After testing, the central segregation grade of the obtained steel plate is better than Class B 0.5.

[0198] The structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, the proportion of low-carbon bainite is 65-90%, and the average grain size of the quasi-polygonal ferrite is 4-9 μm.

[0199] The yield strength of the steel plate is ≥460MPa, the tensile strength is 520~675MPa, the Z-direction tensile strength is ≥495MPa, the yield strength ratio is ≤0.85, the -60℃ low-temperature impact energy at different positions in the thickness direction of the steel plate is ≥250J, the Z-direction tensile section shrinkage is ≥65%, and the CTOD value at -10℃ is ≥1.1mm.

[0200] After the steel plate is subjected to 5% pre-deformation treatment, or 5% pre-deformation + aging at (250±10)℃ for 1h, the low-temperature impact toughness of the steel plate does not decrease, that is, after the steel plate is subjected to large deformation cold forming, or large deformation cold forming + long-term aging, the low-temperature impact toughness does not decrease.

[0201] The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥0.8 mm.

[0202] After welding, the steel plate is heat treated at 590±10℃ for no more than 6h, and the strength, low-temperature impact toughness, CTOD fracture toughness and Z-direction tensile properties are not reduced.

[0203] To further clarify the objectives, technical solutions, and advantages of one embodiment of the present invention, the present embodiment will be further described below with reference to Examples 1 to 12 according to one embodiment of the present invention. Obviously, Examples 1 to 12 described herein are only a portion of the embodiments of the present invention, not all of them. Other embodiments based on the aforementioned embodiments do not depart from the technical spirit of the present invention.

[0204] Specifically, Examples 1 to 12 all provide a high fracture toughness steel plate, the chemical composition of which is shown in Table 1, with the remainder being Fe and unavoidable impurities.

[0205] Table 1

[0206] The production methods of the high fracture toughness steel plates of Examples 1 to 12 are specifically as follows.

[0207] (1) Molten steel smelting

[0208] The chemical composition of the molten steel obtained by smelting is shown in Table 1.

[0209] a. Converter smelting steps

[0210] The smelting raw materials are loaded into a converter for smelting, and the resulting molten steel is deoxidized and alloyed. The steelmaking raw materials include scrap steel, molten iron, and at least one of copper blocks, nickel plates, and ferromolybdenum. The total charge of the converter is 200±2t, and the amount of smelting raw materials added is shown in Table 2; the target alkalinity of the final slag is 4.0, the C, P, and S contents in the molten steel at the end of the converter smelting are shown in Table 2, and the tapping temperature is 1640±20°C. When 1 / 6 of the steel is tapped, alloys and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingots-lime, wherein the amounts of aluminum ingots, ferrosilicon, metallic manganese, and lime added are shown in Table 2, respectively.

[0211] Table 2

[0212] b. LF refining step

[0213] Molten steel from the converter is fed into the LF refining furnace for chemical composition adjustment, temperature control, and inclusion control. Low-carbon ferrochrome and FeNb are added to the LF refining furnace to adjust the steel's alloy composition. The amounts of low-carbon ferrochrome and FeNb added are shown in Table 3. Calcium carbide is then added for diffusion deoxidation of the steel. Deoxidation continues after the slag turns white. The deoxidation time is shown in Table 3. Before leaving the station, samples are collected for testing and alloy composition adjustment. Once the steel's chemical composition reaches the target range, it is maintained for at least 5 minutes before tapping.

[0214] c. RH vacuum refining step

[0215] The RH cycle degassing equipment is used to vacuum degas and remove inclusions from the molten steel after LF refining, and then silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment.

[0216] Specifically, 0.5±0.05 kg / t of ferrotitanium was added to the molten steel, and then the molten steel was vacuum degassed and inclusions were removed, the vacuum degree was controlled to be less than 2 mBar, and the degassing time was ≥10 min; then 140±5 m of calcium silicon wire was fed into the molten steel to denature the inclusions and perform soft stirring treatment, the feeding speed of the calcium silicon wire was 1.5±0.5 m / s, and the soft stirring time was shown in Table 3, and the slag surface fluctuation was controlled but the molten steel was not exposed.

[0217] Table 3

[0218] (2) Continuous casting

[0219] The molten steel obtained from the steelmaking process was continuously cast into continuous casting billets with a thickness of 320 mm. The chemical composition of the continuous casting billets is shown in Table 1. During continuous casting, the liquid level fluctuation amplitude of the crystallizer was controlled within the range of ±2 mm. The crystallizer water inlet temperature was 28-35°C, with an inlet and outlet water temperature difference of 4-10°C. The water flow rate on the narrow side was 550±50 L / min, the water flow on the wide side was 3800±200 L / min, the continuous casting speed was 0.6±0.05 m / min, and the total reduction at the end of solidification was 5-6 mm.

[0220] The continuous casting slabs of Examples 1 to 12 were sampled and tested and cold acid etched according to GB / T 226-2015, with the sample direction being the transverse direction. Macrostructure defect rating of the continuous casting slabs was performed according to YB / T 4003-2016.

[0221] After testing, the center segregation level of the continuous casting billets of Examples 1 to 12 is better than Class B level 1.0, the continuous casting billets have no center porosity, and the crack level is better than level 1.0.

[0222] (3) Heating

[0223] The continuous casting slab is heated, and the soaking section temperature and the time in the furnace are shown in Table 4.

[0224] (4) Two-stage rolling

[0225] The continuous casting billet is first rolled in the recrystallization zone to obtain an intermediate billet, and then rolled in the non-recrystallization zone to form a steel plate. When 60mm≤t<80mm, the reduction in at least two passes in the recrystallization zone rolling stage is ≥30mm; when 80mm≤t<90mm, the reduction in at least two passes in the recrystallization zone rolling stage is ≥35mm; when 90mm≤t≤100mm, the reduction in at least two passes in the recrystallization zone rolling stage is ≥38mm.

[0226] The thickness T of the intermediate billet, the thickness t of the steel plate, and the final rolling temperature of the non-recrystallization zone rolling are shown in Table 4.

[0227] (5) Controlled cooling

[0228] The steel plate is water-cooled using a multifunctional intermittent cooling system. The cooling rate and outlet water temperature of the steel plate are shown in Table 4.

[0229] Table 4

[0230] The steel plates of Examples 1 to 12 were sampled and subjected to metallographic structure testing, center segregation testing, mechanical property testing, and welding performance testing according to the same testing methods. The specific testing methods and test results are as follows:

[0231] (1) Metallographic structure

[0232] A 15×15 cm sample was taken from the head of each steel plate, and a metallographic sample was made along the rolling direction. After mechanical polishing and nitric acid etching, the sample was placed under a metallographic microscope for microstructure observation. The average grain size of the quasi-polygonal ferrite in Examples 9 to 12 was measured according to GB / T 6394-2017. It was found that the microstructures of the steel plates of Examples 1 to 12, the proportion of each microstructure phase, and the grain size were shown in Table 5, respectively. The metallographic microstructure images of the steel plates of Examples 1, 4, 5, 8, 9, and 12 are shown in Figures 1 to 6, respectively. In the three preferred embodiments, the metallographic microstructure images of the steel plates with the minimum thickness and the maximum thickness are respectively selected for display, which can characterize the metallographic microstructure of the steel plates of each preferred embodiment. The other embodiments are omitted.

[0233] Table 5

[0234] (2) Central segregation

[0235] The steel plates of Examples 1 to 12 were sampled, tested, and cold-acid-etched with reference to GB / T 226-2015, with the specimens in the transverse direction. Macrostructural defect rating was performed in accordance with YB / T 4003-2016.

[0236] The central segregation grades of the steel plates of Examples 1 to 12 were all measured to be Class A, Grade 1.0.

[0237] (3) Mechanical properties

[0238] The tensile properties of the steel plates of the above embodiments were tested with reference to the GB / T 228.1-2021 standard, and the Z-direction tensile properties of the steel plates of the above embodiments were tested with reference to the GB / T 5313-2010 standard. The test results are shown in Table 6.

[0239] Referring to the standard ISO 12135:2021, a three-point bending test was used to test the crack tip opening displacement (CTOD) characteristic value of the steel plate at -10°C. The test results are shown in Table 6.

[0240] Among them, the Z-direction tensile properties of steel plates with a thickness of 16 mm and below are usually better, and the specimens are difficult to process, so there is no need to test the Z-direction tensile properties; the CTOD characteristic value and welding performance of thin-gauge steel plates are usually better, so the CTOD characteristic value test and welding performance test are only performed on thick steel plates.

[0241] Table 6

[0242] (4) Impact performance

[0243] The steel plates of each embodiment were subjected to a -60°C impact performance test with reference to GB / T 229-2020. The -60°C low-temperature impact energy near the surface and at 1 / 2 thickness of the steel plates is shown in Table 7.

[0244] The steel plates of each embodiment were strain-aged according to GB / T 4160-2004. The steel plates of each embodiment were subjected to 5% pre-deformation, 5% pre-deformation, and aging at 250±10°C for 1 h. Afterwards, a -60°C impact performance test was performed. The -60°C low-temperature impact energy of the steel plates near the surface and at 1 / 2 thickness was obtained, as shown in Table 7.

[0245] Table 7

[0246] Tables 6 and 7 show that the steel plates have a yield ratio of ≤0.85, low-temperature impact energy at -60°C at various locations along the thickness of the plates is ≥250 J, the Z-axis tensile reduction of area is ≥65%, and the CTOD value at -10°C is ≥1.1 mm. Furthermore, the low-temperature impact toughness of the steel plates does not decrease after a 5% pre-deformation treatment or a 5% pre-deformation followed by aging at (250±10)°C for 1 hour. In other words, the low-temperature impact toughness of the steel plates does not decrease after large-deformation cold forming or large-deformation cold forming followed by long-term aging.

[0247] Furthermore, the steel plates of Examples 4, 8, and 12 were respectively subjected to gas shielded arc welding at a heat input of 7 kJ / cm and submerged arc welding at a heat input of 50 kJ / cm, and then performance tests were performed, as shown below.

[0248] (1) The impact performance test of the welded joints cooled to room temperature after welding was carried out in accordance with GB / T 2650-2008. The -40°C low-temperature impact performance test results of the heat-affected zone of the welded joint near the surface and 1 / 2 thickness are shown in Table 8. The -40°C low-temperature impact energy kV2 of the impact notch positions at the fusion line FL, 2 mm outside the fusion line (i.e., FL+2), and 5 mm outside the fusion line (i.e., FL+5) were tested. Three sampling test results are shown for each test position.

[0249] Table 8

[0250] (2) Referring to the standard ISO 15653:2018, a three-point bending test was used to detect the crack tip opening displacement CTOD characteristic value of the welded joint at -10°C. The notch positions were located in the coarse-grained zone (i.e., GCHAZ) and the critical zone (i.e., SCHAZ / ICHAZ junction zone) of the weld heat affected zone. Three sampling test results were shown at each test position. The test results are shown in Table 9.

[0251] Table 9

[0252] It can be seen from Tables 8 and 9 that when the steel plates are welded at a heat input of 7 to 50 kJ / cm, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥0.8 mm.

[0253] The welded steel plate was further subjected to post-weld heat treatment at a temperature of 590°C for 6 hours, followed by a performance test, as detailed below.

[0254] (1) Mechanical properties

[0255] The tensile properties of the steel plates after post-weld heat treatment were tested with reference to the GB / T 228.1-2021 standard, and the Z-direction tensile properties of the steel plates after post-weld heat treatment were tested with reference to the GB / T5313-2010 standard. The test results are shown in Table 10.

[0256] Referring to the standard ISO 12135:2021, the three-point bending test was used to test the crack tip opening displacement (CTOD) characteristic value of the post-weld heat treated steel plate at -10°C. The test results are shown in Table 10.

[0257] Table 10

[0258] (2) Impact performance

[0259] The -60℃ impact performance test of the post-weld heat treated steel plate was carried out with reference to GB / T 229-2020. The -60℃ low-temperature impact energy near the surface and 1 / 2 thickness of the steel plate is shown in Table 11.

[0260] The strain aging treatment was performed on the post-weld heat treated steel plates according to GB / T 4160-2004, with 5% pre-deformation, 5% pre-deformation, and aging at 250±10℃ for 1h. Afterwards, the impact performance test was performed at -60℃. The results are shown in Table 11.

[0261] Table 11

[0262] (3) Impact performance tests were carried out on welded joints after post-weld heat treatment according to GB / T 2650-2008. The -40°C low-temperature impact performance test results of the heat-affected zone near the surface and 1 / 2 thickness of the welded joint are shown in Table 12. The -40°C low-temperature impact energy kV2 of the impact notch positions at the fusion line FL, 2 mm outside the fusion line (i.e., FL+2), and 5 mm outside the fusion line (i.e., FL+5) were tested. Three sampling test results are shown for each test position.

[0263] Table 12

[0264] (4) Referring to the standard ISO 15653:2018, a three-point bending test was used to detect the crack tip opening displacement CTOD characteristic value at -10°C of the welded joint of the steel plate after post-weld heat treatment. The notch positions were located in the coarse-grained zone (i.e., GCHAZ) and the critical zone (i.e., SCHAZ / ICHAZ junction zone) of the welding heat affected zone. Three sampling test results were shown at each test position. The test results are shown in Table 13.

[0265] Table 13

[0266] It can be seen from Tables 10 to 13 that after welding, the strength, low-temperature impact toughness, CTOD fracture toughness and Z-direction tensile properties of the steel plate are not reduced when the heat treatment is performed at 590±10℃ for no more than 6h.

[0267] It should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each implementation method can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0268] The detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for producing a steel plate, characterized in that: The process includes the following steps in sequence: (1) Molten steel smelting The chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.07%, Si 0.15-0.25%, Mn 1.43-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.25%, Ni≤0.23%, Cu 0.13-0.25%, Mo≤0.15%, Nb 0.02-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, wherein the strengthening equivalent Req is 0.43-0.57%, Req=2.5C+Mn / 5+Cr / 7+Cu / 6+Mo / 4+Nb / 6; (2) Continuous casting Continuously casting the molten steel obtained in the molten steel smelting process into a continuous casting billet, controlling the center segregation level of the continuous casting billet to be better than Class B 1.0, the continuous casting billet to have no center porosity, and the crack level to be better than Class 1.0; (3) Heating The continuous casting slab is heated with the soaking section temperature at 1130-1220°C and the furnace time at 320-570 min; (4) Two-stage rolling The continuous casting slab is first rolled in a recrystallization zone to obtain an intermediate slab, and then rolled in a non-recrystallization zone to produce a steel plate. The reduction in at least two passes of the recrystallization zone rolling stage is ≥30 mm, and the thickness of the intermediate slab is T ≥170 mm or T ≥3t, where t is the thickness of the steel plate in mm. The finishing rolling temperature of the non-recrystallization zone rolling is 725-860°C. (5) Controlled cooling The steel plate is water-cooled, the cooling rate of the steel plate is controlled to be 3-26°C / s, and the outlet water temperature of the steel plate is 210-520°C.

2. The method for producing a steel plate according to claim 1, wherein: During the continuous casting process, the liquid level fluctuation amplitude of the crystallizer is controlled within the range of ±2 mm, the water inlet temperature of the crystallizer is 28-35°C, the inlet and outlet water temperature difference is 4-10°C, the water volume on the narrow side is 550±50 L / min, the water volume on the wide side is 3800±200 L / min, the continuous casting speed is 0.6±0.05 m / min, and the total reduction at the end of solidification is 5-6 mm.

3. The method for producing a steel plate according to claim 1, wherein: In the two-stage rolling process, when 60mm≤t<80mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥30mm; when 80mm≤t<90mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥35mm; when 90mm≤t≤100mm, the reduction of at least two passes in the recrystallization zone rolling stage is ≥38mm.

4. The method for producing a steel plate according to claim 1, wherein: The molten steel smelting process includes converter smelting, LF refining and RH vacuum refining steps performed in sequence; In the converter smelting step, the total charge of the converter is 200±2t, the target basicity of the final slag is 4.0, the carbon content in the molten steel at the end of the converter smelting is 0.02-0.06%, P≤0.012%, S≤0.008%, the tapping temperature is 1640±20°C, and when 1 / 6 of the steel is tapped, the alloy and slag are added in the order of ferrosilicon-metallic manganese-aluminum ingot-lime, wherein the amount of the aluminum ingot added is (0.69+14[0])kg / t; In the LF refining step, low-carbon ferrochrome and FeNb are added to adjust the alloy composition of the molten steel, and then calcium carbide is added to diffuse deoxidize the molten steel. After the slag turns white, deoxidation is continued for more than 15 minutes. Before leaving the station, sampling and testing are carried out and the alloy composition is adjusted. After the chemical composition of the molten steel reaches the target range, it is kept for at least 5 minutes before tapping; In the RH vacuum refining step, 0.5±0.05 kg / t of ferrotitanium is added, and then the molten steel is vacuum degassed and inclusions are removed, the vacuum degree is controlled to be less than 2 mBar, and the degassing time is ≥10 min; then 140±5 m of silicon calcium wire is fed into the molten steel to denature the inclusions and perform soft stirring treatment, the feeding speed of the silicon calcium wire is 1.5±0.5 m / s, the soft stirring time is greater than 12 min, and the slag surface is controlled to fluctuate but the molten steel is not exposed.

5. The method for producing a steel plate according to claim 4, wherein: In the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.06%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.05-0.15%, Cu 0.15-0.25%, Nb 0.02-0.03%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.43-0.53%; In the converter smelting step, 1.8±0.3kg / t of copper blocks are placed in a converter together with scrap steel and molten iron for smelting. The addition amount of nitrate is 1.5±0.3kg / t, the addition amount of metallic manganese is 12±2kg / t, and the addition amount of lime is 3±1kg / t; In the LF refining step, the amount of low-carbon ferrochrome added is 1.0±0.5 kg / t, and the amount of FeNb added is 0.3±0.1 kg / t; In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (960-295[C]-77[Mn]-0.002t 2 -0.24t)±10℃; In the controlled cooling process, the cooling rate of the steel plate is controlled to be (28-0.24t)±1°C / s, and the outlet water temperature of the steel plate is controlled to be (453-1.87t)±20°C.

6. The method for producing a steel plate according to claim 5, wherein: When t≤60mm, the structure of the steel plate is polygonal ferrite + low-carbon bainite, and the proportion of low-carbon bainite is 10-45%; When t>60mm, the structure of the steel plate is quasi-polygonal ferrite + low carbon bainite + degenerate pearlite, with the proportion of quasi-polygonal ferrite being 5-25%, the proportion of low carbon bainite being 70-85%, and the proportion of degenerate pearlite being 3-13%; The yield strength of the steel plate is ≥355MPa, the tensile strength is 470-630MPa, the Z-direction tensile strength is ≥450MPa, and the CTOD value at -10℃ is ≥1.6mm; The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥230 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥1.2 mm.

7. The method for producing a steel plate according to claim 4, wherein: In the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.04-0.07%, Si 0.15-0.25%, Mn 1.45-1.55%, P≤0.015%, S≤0.003%, Cr 0.10-0.20%, Ni 0.08-0.15%, Cu 0.20-0.25%, Nb 0.03-0.04%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.45-0.55%; In the converter smelting step, 1.8±0.3kg / t of copper blocks and 0.85±0.1kg / t of nickel plates are loaded into a converter together with scrap steel and molten iron for smelting. The carbon content of the molten steel at the end of the converter smelting is 0.02-0.05%, the amount of ferrosilicon added is 1.7±0.1kg / t, the amount of metallic manganese added is 15±1kg / t, and the amount of lime added is 4.0±0.5kg / t. In the LF refining step, the amount of low-carbon ferrochrome added is 2.1±0.5 kg / t, and the amount of FeNb added is 0.5±0.05 kg / t; In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (854-295[C]-77[Mn]-0.04t 2 +4.21t)±10℃; In the controlled cooling process, the cooling rate of the steel plate is controlled to be (23+0.002t 2 -0.33t)±1℃ / s, the outlet water temperature of the steel plate is (0.0023t 3 -0.48t 2 +28.3t)±20℃.

8. The method for producing a steel plate according to claim 7, wherein: The structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, with the proportion of low-carbon bainite being 35-80%; The yield strength of the steel plate is ≥420MPa, the tensile strength is 500-640MPa, the Z-direction tensile strength is ≥475MPa, and the CTOD value at -10℃ is ≥1.3mm; The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥1.0 mm.

9. The method for producing a steel plate according to claim 4, wherein: In the molten steel smelting process, the chemical composition of the molten steel obtained by smelting includes, by mass percentage, C 0.03-0.06%, Si 0.15-0.25%, Mn 1.43-1.53%, P≤0.015%, S≤0.003%, Cr 0.15-0.25%, Ni 0.13-0.23%, Cu 0.13-0.22%, Mo 0.07-0.15%, Nb 0.025-0.035%, Ti 0.01-0.02%, Alt 0.02-0.05%, N≤0.005%, O≤0.002%, and the balance is Fe and unavoidable impurities, and the strengthening equivalent Req is 0.47-0.57%; In the converter smelting step, 1.4±0.3kg / t of copper blocks, 1.4±0.3kg / t of nickel plates, and 1.4±0.3kg / t of ferromolybdenum are loaded into a converter together with scrap steel and molten iron for smelting. The carbon content of the molten steel at the end of the converter smelting is 0.02-0.06%, the amount of ferrosilicon added is 2.5±0.3kg / t, the amount of metallic manganese added is 12±2kg / t, and the amount of lime added is 3±1kg / t. In the LF refining step, the amount of low-carbon ferrochrome added is 2.7±0.5 kg / t, and the amount of FeNb added is 0.4±0.1 kg / t; In the two-stage rolling process, the final rolling temperature of the non-recrystallization zone rolling is (955-295[C]-77[Mn]-82[Mo]-0.002t 2 -0.24t)±10℃; In the controlled cooling process, the cooling rate of the steel plate is controlled to be (19-0.13t)±1℃ / s, and the outlet water temperature of the steel plate is (560+0.02t 2 -4.5t)±20℃.

10. The method for producing a steel plate according to claim 9, wherein: The structure of the steel plate is quasi-polygonal ferrite + low-carbon bainite, with the proportion of low-carbon bainite being 65-90%, and the average grain size of the quasi-polygonal ferrite being 4-9 μm; The yield strength of the steel plate is ≥460MPa, the tensile strength is 520-675MPa, the Z-direction tensile strength is ≥495MPa, and the CTOD value at -10℃ is ≥1.1mm; The steel plates are welded at a heat input of 7 to 50 kJ / cm2, the low-temperature impact energy of the heat-affected zone of the weld joint at -40°C is ≥150 J, and the CTOD value of the coarse-grained zone and the critical zone at -10°C is ≥0.8 mm.

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