Steel plate and production method thereof

Abstract

The invention provides a steel plate and a production method thereof. The production method comprises the steps that the rough rolling temperature is controlled to be 900-1100 DEG C, the thickness of a to-be-warmed blank is larger than or equal to 1.5 t, the finish rolling starting temperature is controlled to be smaller than or equal to 870 DEG C, the finish rolling temperature is T1-20-T1 + 20 DEG C, the final cooling temperature is 660-700 DEG C, and T1 is equal to 836-0.64 * t; the target thickness t of the steel plate is smaller than or equal to 100 mm, the normalizing temperature is 880-900 DEG C, the in-furnace time is 1.8-2.0 t min, the steel plate enters water to be cooled after being discharged from the furnace, the self-tempering temperature is T2-20-T2 + 20 DEG C, and T2 is equal to 752-2.66 * t + 0.007 * t2; or, the target thickness t of the steel plate is smaller than or equal to 50 mm, the normalizing temperature ranges from 850 DEG C to 870 DEG C, the in-furnace time ranges from 1.6 t min to 1.7 t min, and the steel plate is air-cooled after being discharged out of the furnace; t is the target thickness unit of the steel plate as mm. By adopting the controlled rolling and normalizing process, the obdurability attenuation value of the steel plate caused by the process chain of forming, heat treatment and PWHT is reduced.

Description

Technical Field

[0001] This application relates to the technical field of steel smelting, and particularly to a steel plate and a production method thereof. Background Art

[0002] The current trend in oil and gas exploration and development is as follows: ① The exploration maturity of onshore and shallow water resources continues to improve; ② The deep water area has rich reserves but a low proven rate (the global average proven rate < 30%), and it is gradually becoming the core replacement area for increasing oil and gas reserves and production. In the deep water development environment, the equipment is large-scale and the service conditions are harsh, which puts higher requirements on the performance of steel materials.

[0003] Specifically, it is manifested in the following aspects: ① In terms of forming and processing, key components such as the cargo holds of large liquefied petroleum gas / liquid ammonia carriers, fuel tanks, and pressure vessel heads need to withstand cold forming plastic deformation of ≥ 5%. Thick specification steel plates (t ≥ 50 mm) are limited by the forming equipment capacity and need to adopt high-temperature large deformation processes, resulting in the deterioration of the original properties of the materials, and subsequent heat treatment for restoring properties must be carried out; ② The restoration of properties depends on secondary heat treatment that matches the original steel plate production process; ③ Welded components need to undergo multiple rounds of local / overall post-weld heat treatment (PWHT) to eliminate residual stress, stabilize the structural dimensional tolerance, and improve the microstructure morphology of the heat affected zone of the weld, ensuring the safe operation of the equipment throughout its life cycle. The above process chain (forming → heat treatment for restoring properties → PWHT) leads to a significant attenuation of the strength and toughness indexes of the steel plate. Therefore, developing a new type of low-temperature steel with both adaptability to large deformation cold forming or high-temperature forming and tolerance to multi-stage heat treatment has become a technical bottleneck that urgently needs to be broken through in the field of deep water oil and gas equipment manufacturing. Summary of the Invention

[0004] The purpose of this application is to provide a production method of a steel plate, which solves the problem that the process chain of forming → heat treatment for restoring properties → PWHT in the prior art leads to a significant attenuation of the strength and toughness indexes of the steel plate through the cooperation of rolling and normalizing heat treatment.

[0005] In order to achieve one of the above-mentioned invention purposes, an embodiment of this application provides a production method of a steel plate, including converter smelting, LF refining, RH refining, continuous casting, heating, rolling, and normalizing heat treatment carried out in sequence. Among them, In the rolling process, the rough rolling temperature is 900 - 1100 °C, the thickness of the waiting-for-temperature billet ≥ 1.5t, the starting rolling temperature of finish rolling ≤ 870 °C, the finishing rolling temperature is T1 - 20 to T1 + 20 °C, the final cooling temperature is 660 - 700 °C, and T1 = 836 - 0.64 × t; The normalizing heat treatment process is as follows: When the target thickness t of the steel plate is ≤ 100 mm, the normalizing temperature is 880 - 900 °C, the time in the furnace is 1.8t - 2.0t min. After the steel plate is taken out of the furnace, it is cooled in water, and the return red temperature is T2 - 20 to T2 + 20 °C, where T2 = 752 - 2.66×t + 0.007×t 2 ; Alternatively, the normalizing heat treatment process is as follows: When the target thickness t of the steel plate is ≤ 50 mm, the normalizing temperature is 850 - 870 °C, the time in the furnace is 1.6t - 1.7t min, and after the steel plate is taken out of the furnace, it is air-cooled; where t is the target thickness of the steel plate, with the unit of mm.

[0006] In one embodiment of the present application, in the heating process, the soaking section temperature is 1100 - 1150 °C, and the time in the furnace is ≥ st + 80 min, where st is the thickness of the slab obtained in the continuous casting process, with the unit of mm.

[0007] In one embodiment of the present application, in the converter process, alloys and slag materials are added in the order of ferrosilicon, ferromanganese, aluminum lumps, and lime during converter tapping. Among them, the aluminum addition amount is 0.65 + 15[O] kg / t, where [O] is 100 times the oxygen content measured before tapping.

[0008] In one embodiment of the present application, in the LF refining process, lime is added, and the lime addition amount is 0.9 + 40[O] kg / t, where [O] is 100 times the oxygen content measured before converter tapping.

[0009] In one embodiment of the present application, after the LF refining process is completed, a calcium wire of 92 + 845[Al] m is fed, and the wire feeding speed is 1.0 - 2.0 m / s, where [Al] is 100 times the aluminum content measured after the LF refining process is completed.

[0010] In one embodiment of the present application, after the calcium wire feeding is completed, the soft stirring time is ≥ 8 min, the soft stirring time in the RH refining process is ≥ 12 min, and adding Al is strictly prohibited in the RH refining process.

[0011] The present application also provides a steel plate obtained by the production method of the steel plate as described above. The chemical composition of the steel plate in mass percentage includes: C: 0.16 - 0.19%, Si: 0.30 - 0.40%, Mn: 1.35 - 1.45%, P ≤ 0.015%, S ≤ 0.005%, Ni: 0.15 - 0.25%, Nb: 0.010 - 0.020%, V: 0.020 - 0.030%, Ti: 0.010 - 0.020%, Alt: 0.02 - 0.05%, and the balance is iron and inevitable impurities; The microstructure of the steel plate is ferrite and pearlite. In the normalizing heat treatment process, for the steel plate water-cooled after normalizing, the pearlite is diffusely distributed; for the steel plate air-cooled after normalizing, the banded structure rating of the pearlite is ≤ 2 levels.

[0012] In one embodiment of the present application, the yield strength of the steel plate is ≥ 355 MPa, the tensile strength is 540 - 620 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51 °C is ≥ 100 J, and the Z-direction tensile area reduction rate is ≥ 65%.

[0013] In one embodiment of the present application, after the steel plate undergoes large deformation cold forming or hot forming, normalizing to restore performance, and post-weld heat treatment, the yield strength is ≥ 300 MPa, the tensile strength is 485 - 540 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51 °C is ≥ 100 J, and the Z-direction tensile area reduction rate is ≥ 65%.

[0014] In one embodiment of the present application, the strain of the large deformation cold forming process is 5 - 10%; the temperature of the hot forming process is 920 ± 10 °C, the holding time is 1.6t ± 0.2t min, the heating rate is ≤ 50 °C / h, and the cooling is natural cooling in the air; the process of normalizing to restore performance is the same as the normalizing heat treatment process of the steel plate; for the post-weld heat treatment process, the temperature is 635 ± 15 °C, the holding time is 360 min, the heating and cooling rates are ≤ 50 °C / h, and it undergoes 4 cycles.

[0015] One or more technical solutions provided by the present application have at least the following technical effects or advantages: In the production method of the steel plate provided by the present application, by controlling the refinement of the original austenite grains during rolling, a uniform initial tissue basis is established, and the generation of banded structures is inhibited; by controlling the normalizing heat treatment, the grains are refined, the pearlite is diffusely distributed, the banded structures are eliminated, and the internal stress is reduced, ensuring the dimensional appearance stability of subsequent processing (cutting, welding, etc.). The "controlled rolling + normalizing" process can meet the requirements of the process chain of large deformation cold forming / hot forming → heat treatment to restore performance → PWHT, and has good application prospects in the fields of steel for ships, offshore engineering, pressure vessels, etc. Description of the Drawings

[0016] Figure 1 It is the metallographic structure diagram at the 1 / 2 thickness of the steel plate in Example 1.

[0017] Figure 2 It is the metallographic structure diagram at the 1 / 2 thickness of the steel plate in Example 2.

[0018] Figure 3 It is the metallographic structure diagram at the 1 / 4 thickness of the steel plate in Example 6.

[0019] Figure 4 It is the metallographic structure diagram at the 1 / 2 thickness of the steel plate in Example 6. Specific Embodiments

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] The embodiments of the present application provide a production method of steel plates, including converter smelting, LF refining, RH refining, continuous casting, heating, rolling, and normalizing heat treatment carried out in sequence. Among them, In the rolling process, the rough rolling temperature is 900°C to 1100°C, the thickness of the billet waiting for temperature is ≥ 1.5t, the starting rolling temperature of finish rolling is ≤ 870°C, the finishing rolling temperature is T1 - 20 to T1 + 20°C, the finishing cooling temperature is 660 to 700°C, and T1 = 836 - 0.64×t; The normalizing heat treatment process is as follows: when the target thickness t of the steel plate ≤ 100mm, the normalizing temperature is 880 to 900°C, the time in the furnace is 1.8t to 2.0t min, the steel plate is cooled by water after leaving the furnace, and the return red temperature is T2 - 20 to T2 + 20°C, where T2 = 752 - 2.66×t + 0.007×t 2 ; Or, the normalizing heat treatment process is as follows: when the target thickness t of the steel plate ≤ 50mm, the normalizing temperature is 850 to 870°C, the time in the furnace is 1.6t to 1.7t min, and the steel plate is air-cooled after leaving the furnace; Wherein, t is the target thickness of the steel plate, with the unit of mm.

[0022] It should be noted here that in the formulas with the target thickness t of the steel plate in this article, the numerical values of the target thickness of the steel plate are taken when the unit is mm. The thickness of the billet waiting for temperature refers to the thickness after rolling the continuous casting billet to a certain thickness, and then starting finish rolling after the temperature reaches the starting rolling temperature of finish rolling, and this certain thickness is the thickness of the billet waiting for temperature.

[0023] Rolling in the austenite recrystallization zone in the rough rolling stage can fully recrystallize and refine the original austenite grains, laying a uniform initial tissue foundation. Rolling in the non-recrystallization zone in the finish rolling stage flattens the austenite grains through deformation induction, increases the deformation energy storage, forms a highly refined as-rolled structure, effectively inhibits the generation of banded structure, creates an ideal precursor structure for subsequent heat treatment, and improves the low-temperature toughness.

[0024] Precisely control the finish rolling temperature according to the target thickness of the steel plate to ensure that the steel plate has sufficient deformation energy storage and avoid phase transformation at this temperature. After rolling, a multi-functional intermittent cooling system is used for water cooling, controlling the cooling rate at 6 - 10 °C / s. After cooling to 660 - 700 °C, air cooling is carried out. The relatively fast cooling rate makes the grains in the steel plate refined.

[0025] In the normalizing heat treatment process, when the first normalizing heat treatment is adopted (the target thickness t of the steel plate ≤ 100 mm), water cooling is carried out after furnace discharging, controlling the water cooling rate at 6 - 10 °C / s. It can refine the grains, make the pearlite disperse, eliminate the banded structure, improve the strength and toughness, and enable the steel plate to withstand long-term post-weld heat treatment; controlling the red-return temperature during cooling can keep the tissue type of the steel plate unchanged, reduce the internal stress, and ensure the dimensional appearance stability of subsequent processing (cutting, welding, etc.).

[0026] When the second normalizing heat treatment is adopted (the target thickness t of the steel plate ≤ 50 mm), air cooling is carried out after furnace discharging. On the one hand, it can reduce the dependence on heat treatment equipment (cooling device, straightening machine); on the other hand, it simplifies the downstream manufacturing process. When performing heat treatment for restoring performance after large deformation, water cooling process is not required, significantly reducing the production difficulty of manufacturing enterprises.

[0027] Furthermore, in the converter process, ferro-silicon, ferromanganese, aluminum block, and lime are added in sequence during converter tapping. Among them, the aluminum addition amount is 0.65 + 15[O] kg / t, where [O] is one hundred times the oxygen content measured before tapping. For example, if the O content is 0.0544%, then [O] = 0.0544, which can also be understood as taking the value of the O content without %.

[0028] The unit kg / t of the aluminum addition amount and the lime addition amount in the following text means: the weight of aluminum / lime added per ton of molten steel, with the unit of kg. In the converter process, the lime addition amount is 0.995 - 1.005 kg / t.

[0029] Ferro-silicon is added first during converter tapping for deoxidation to avoid oxidation of the alloy by oxygen in the molten steel. At the same time, the aluminum addition amount is accurately calculated according to the oxygen content, effectively inhibiting the generation of high-melting-point Al2O3 inclusions and saving resources.

[0030] Ferro-silicon, ferromanganese, aluminum block, and lime are added in the early stage of tapping. For example, ferro-silicon, ferromanganese, aluminum block, and lime can be added in sequence starting from 1 / 8 - 1 / 6 of tapping.

[0031] In an embodiment of the present application, in the LF refining process, lime is added, and the lime addition amount is 0.9 + 40[O] kg / t, where [O] is one hundred times the oxygen content measured before converter tapping.

[0032] In the LF refining process, the addition amount of lime is precisely controlled by the measured oxygen content, which is beneficial to adsorb inclusions in the molten steel and purify the molten steel.

[0033] Furthermore, after the LF refining process is completed, a calcium wire 92 + 845[Al]m is fed, and the wire feeding speed is 1.0 - 2.0 m / s, where [Al] is a hundred times the aluminum content measured after the LF refining process.

[0034] The feeding amount of the calcium wire is dynamically adjusted according to the Al content to avoid overfeeding and save resources. Controlling the wire feeding speed can ensure that the calcium wire is completely dissolved and completely consumed during the floating process of calcium bubbles, improving the calcium absorption rate, enabling inclusions in the molten steel to be fully modified and float up and removed, ensuring the crystallinity and castability of the molten steel, and laying a foundation for the low-temperature toughness of the finished steel plate.

[0035] Furthermore, after feeding the calcium wire, the soft stirring time is ≥ 8 min, and the soft stirring time in the RH refining process is ≥ 12 min, and adding Al is strictly prohibited in the RH refining process. Through multiple soft stirrings, inclusions in the molten steel have enough time to float up, improving the purity of the molten steel.

[0036] For the remaining alloying elements, such as Ni, they are added in the converter smelting stage, Nb and V are added after the ladle arrives in the LF refining process, and Ti is added after adding lime.

[0037] In an embodiment of the present application, in the heating process, the soaking section temperature is 1100 - 1150 °C, and the time in the furnace is ≥ st + 80 min, where st is the thickness of the slab obtained in the continuous casting process, with the unit of mm. When calculating the time in the furnace, the numerical value of the slab thickness is taken.

[0038] Heating in this temperature range can effectively inhibit the growth of austenite grains, which is beneficial to improving the low-temperature toughness of the steel plate; calculating the time in the furnace through the thickness of the slab can ensure that the slab temperature is fully homogenized, resulting in good slab shape during the rolling process.

[0039] The present application also provides a steel plate obtained by the production method of the foregoing steel plate. The chemical composition of the steel plate in mass percentage includes: C: 0.16 - 0.19%, Si: 0.30 - 0.40%, Mn: 1.35 - 1.45%, P ≤ 0.015%, S ≤ 0.005%, Ni: 0.15 - 0.25%, Nb: 0.010 - 0.020%, V: 0.020 - 0.030%, Ti: 0.010 - 0.020%, Alt: 0.02 - 0.05%, and the balance is iron and unavoidable impurities; The structure of the steel plate is ferrite and pearlite. In the normalizing heat treatment process, for the steel plate water-cooled after normalizing, its pearlite is diffusely distributed; for the steel plate air-cooled after normalizing, the banded structure rating of pearlite is ≤ 2 levels.

[0040] Using C, Si, Mn, Ni as the matrix and combining with a multi - micro - alloying composition system of Nb, V, Ti, Al, compared with the traditional composition system adding Cu / Mo alloy and strictly restricting the P / S content, the alloy cost and smelting difficulty are reduced.

[0041] The functions and selection of the amounts of each chemical component in the steel plate described in the present invention are described as follows: Carbon (C): It is a key strengthening element. When the C content is low, the steel plate cannot withstand long - time post - weld heat treatment. When the C content exceeds 0.19%, the low - temperature toughness of the steel plate becomes poor, the surface hardness is high, which is not conducive to large - deformation cold forming. In the present invention, the C content is controlled at 0.16 - 0.19%. Combining with the overall design of other elements and processes, while ensuring the strengthening effect, the low - temperature toughness and cold formability are improved.

[0042] Silicon (Si): It is a deoxidizing and solution - strengthening element, which can increase the carbon activity so that it does not dissolve in carbides, thereby inhibiting carbide coarsening and reducing the strength reduction during long - time post - weld heat treatment. However, when the Si content is high, the low - temperature toughness of the steel plate will decrease. In the present invention, the Si content is controlled at 0.30 - 0.40%. Combining with the overall design of other elements and processes, on the premise of not affecting the low - temperature toughness, the deoxidation effect is ensured, the oxide inclusions in the steel are reduced, and its strengthening effect is fully exerted.

[0043] Manganese (Mn): It is a solution - strengthening and grain - refining element. At the same time, it is an element that is prone to segregation and easy to form inclusions, which will affect the low - temperature impact toughness of the steel plate core. In the present invention, the Mn content is controlled at 1.35 - 1.45%. On the one hand, it can ensure the strength of the steel plate, and on the other hand, it can reduce segregation and avoid the deterioration of the low - temperature impact toughness of the steel plate core caused by MnS inclusions.

[0044] Nickel (Ni): It can make the edge of cementite smooth and disperse, making dislocations easier to slip under low - temperature conditions. It is an effective element to improve the low - temperature toughness of the steel plate, but the alloy cost is relatively high. In the present invention, the Ni content is controlled at 0.15 - 0.25%, which can not only ensure the low - temperature toughness of the steel plate but also reasonably control the alloy cost.

[0045] Niobium (Nb): It can reduce the overheating sensitivity and temper brittleness of the steel, and reduce the strength reduction during long - time post - weld heat treatment. However, when the Nb content is too high, the strengthening trend weakens. In the present invention, the Nb content is controlled at 0.01 - 0.02% to ensure the strengthening and toughening effect.

[0046] Vanadium (V): It reduces the area of pearlite clusters, increases the number of cementite breakpoints and the degree of fragmentation in pearlite lamellae, improves the strength and toughness of the steel plate. At the same time, during the long-term post-weld heat treatment process, it plays a certain precipitation strengthening role. However, when the V content is too high, it is not conducive to weldability. In the present invention, the V content is controlled at 0.02 - 0.03%, which can ensure the strengthening and toughening effect.

[0047] Titanium (Ti): It is an element for nitrogen fixation and deoxidation, but it is easy to form large particles of Ti(C, N) in the center of the steel plate, which affects the low-temperature impact toughness at 1 / 2 of the thickness. In the present invention, the Ti content is controlled at 0.01 - 0.02%.

[0048] Aluminum (Al): It is an element for deoxidation and grain refinement. Excessive aluminum is likely to increase the Al2O3 inclusions in the steel, affecting the low-temperature toughness of the steel. In the present invention, Alt is controlled at 0.02 - 0.05%.

[0049] Phosphorus and sulfur (P, S): They are inevitable impurity elements in steel. P is prone to segregation in the center of the steel plate, and S is easy to combine with Mn to form MnS inclusions, affecting the low-temperature toughness at 1 / 2 of the steel plate thickness. During the production process, the contents of P and S should be reduced as much as possible, but removing P and S will increase the difficulty of the steelmaking process and production cost. In the present invention, P ≤ 0.015% and S ≤ 0.005% are controlled. Combining the overall chemical composition design and process control scheme, on the one hand, it effectively controls the center segregation grade and the content of non-metallic inclusions, ensuring that the steel plate has excellent low-temperature impact toughness; on the other hand, it avoids the problems of increased production difficulty and rising production cost caused by overly strict control requirements for P and S contents.

[0050] After adopting the above two normalizing heat treatments (water cooling after normalizing and air cooling after normalizing), the pearlite of the steel plate is dispersed or slightly banded, making the steel plate have strength and toughness and can withstand long-term post-weld heat treatment.

[0051] Furthermore, the yield strength of the steel plate is ≥ 355 MPa, the tensile strength is 540 - 620 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51°C is ≥ 100 J, and the Z-direction tensile area reduction rate is ≥ 65%. The steel plate is inspected by NB / T 47013.3 and is of grade I. Among them, the Z-direction is the thickness direction of the steel plate.

[0052] Furthermore, after the steel plate undergoes large-deformation cold forming or hot forming, normalizing to restore performance, and post-weld heat treatment, the yield strength is ≥ 300 MPa, the tensile strength is 485 - 540 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51°C is ≥ 100 J, and the Z-direction tensile area reduction rate is ≥ 65%.

[0053] Furthermore, the strain of the large deformation cold forming process is 5-10%; the temperature of the hot forming process is 920±10°C, the heat preservation time is 1.6t±0.2t min, the heating rate is ≤50°C / h, and the cooling is natural cooling in the air; the normalizing and performance recovery process is the same as the normalizing heat treatment process of the steel plate; the post-weld heat treatment process, the temperature is 635±15°C, the heat preservation time is 360 min, the heating and cooling rate is ≤50°C / h, and it goes through 4 cycles.

[0054] The technical solutions of the present application will be further described below in conjunction with some specific embodiments.

[0055] Table 1 Chemical compositions of the steel plates in Examples 1-6

[0056] Table 2 Parameters of converter smelting, LF refining, and RH refining processes

[0057] Table 3 Parameters of heating, rolling, and normalizing heat treatment processes

[0058] The steel plates in Examples 1-6 were inspected for flaws in accordance with NB / T 47013.3, and all of Examples 1-6 could meet the Class I requirements. Sampling was carried out in accordance with the ASME SA-20 / SA-20M standard to detect the metallographic structure and mechanical properties. The specific test methods and test results are as follows: (1) Metallographic structure: A specimen of 15 cm×15 cm was taken, made into a metallographic sample along the rolling direction, mechanically polished and etched with nitric acid alcohol, and then placed under a metallographic microscope for tissue observation. The metallographic structures of Examples 1-6 were all ferrite + pearlite structures. The pearlite in Examples 1, 3, 5, and 6 was diffusely distributed, without obvious banded structure. The pearlite in Examples 2 and 4 showed slight banded distribution. According to GB / T 34474.1, the banded structure of Examples 2 and 4 was rated, and the rating results were Grade 1 and Grade 1.5 respectively. The metallographic structure pictures of the steel plates in Examples 1, 2, and 6 are respectively as Figures 1 to 4 shown, which can represent the metallographic structure of the steel plates in each embodiment, and the remaining examples are omitted.

[0059] (2) Mechanical properties: The tensile and impact properties of the above-mentioned example steel plates were tested with reference to ASTM A370 standard, and the Z-direction tensile properties of the above-mentioned example steel plates were tested with reference to ASTM A770 standard. The test results are shown in Table 4.

[0060] Table 4 Mechanical properties of Examples 1-6

[0061] Using a tensile testing machine and a box-type resistance furnace, the steel plates of Examples 1 to 4 were subjected to simulated large-deformation cold forming → recovery performance heat treatment → post-weld heat treatment, specifically: ① The pre-deformation tensile strain of the steel plates of Examples 1 to 4 was 8%, 7%, 9%, and 6% respectively; ② The process requirements for the recovery performance heat treatment were the same as those for the normalizing heat treatment process of the steel plates; ③ The simulated post-weld heat treatment temperature was 635 ± 15 °C, the holding time was 360 min, the heating and cooling rate was ≤ 50 °C / h, and 4 cycles were experienced. After the treatment, tensile, impact, and Z-direction tensile properties were tested, and the test results are shown in Table 5.

[0062] Using a box-type resistance furnace, the steel plates of Examples 5 to 6 were subjected to simulated hot forming → recovery performance heat treatment → post-weld heat treatment, specifically: ① The simulated hot forming temperature was 920 ± 10 °C, the holding time was 1.5t min, the heating rate was ≤ 50 °C / h, and the cooling was natural cooling in air; ② The process for the recovery performance heat treatment was the same as that for the normalizing heat treatment process of the steel plates; ③ The simulated post-weld heat treatment temperature was 635 ± 15 °C, the holding time was 360 min, the heating and cooling rate was ≤ 50 °C / h, and 4 cycles were experienced. After the treatment, tensile, impact, and Z-direction tensile properties were tested, and the test results are shown in Table 5.

[0063] Table 5 Mechanical properties of steel plates after large-deformation cold forming / hot forming → recovery performance heat treatment → post-weld heat treatment

[0064] It should be understood that although this specification is described according to the embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0065] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of this application, and they are not intended to limit the protection scope of this application. Any equivalent embodiments or changes made without departing from the technical spirit of this application should be included in the protection scope of this application.

Claims

1. A production method of a steel plate, characterized in that, It includes the following processes in sequence: converter smelting, LF refining, RH refining, continuous casting, heating, rolling, and normalizing heat treatment. Among them, In the rolling process, the rough rolling temperature is 900 - 1100 °C, the thickness of the billet waiting for temperature is ≥ 1.5t, the starting rolling temperature of finish rolling is ≤ 870 °C, the finishing rolling temperature is T1 - 20 to T1 + 20 °C, the final cooling temperature is 660 - 700 °C, and T1 = 836 - 0.64×t; The normalizing heat treatment process is as follows: When the target thickness t of the steel plate is ≤ 100 mm, the normalizing temperature is 880 - 900 °C, the time in the furnace is 1.8t - 2.0t min. After the steel plate is taken out of the furnace, it is cooled in water, and the return red temperature is T2 - 20 ~ T2 + 20 °C, where T2 = 752 - 2.66×t + 0.007×t 2 ; Alternatively, the normalizing heat treatment process is as follows: when the target thickness t of the steel plate ≤ 50 mm, the normalizing temperature is 850 - 870 °C, the time in the furnace is 1.6t - 1.7t min, and the steel plate is air-cooled after being taken out of the furnace; Among them, t is the target thickness of the steel plate, with the unit of mm.

2. The production method of the steel plate according to claim 1, characterized in that In the heating process, the soaking section temperature is 1100 - 1150 °C, and the time in the furnace is ≥ st + 80 min, where st is the thickness of the slab obtained in the continuous casting process, with the unit of mm.

3. The production method of the steel plate according to claim 1, characterized in that, In the converter process, alloys and slag materials are added in the order of ferrosilicon, ferromanganese, aluminum lumps, and lime during converter tapping. Among them, the aluminum addition amount is 0.65 + 15[O] kg / t, where [O] is 100 times the oxygen content measured before tapping.

4. The production method of the steel plate according to claim 1, characterized in that, In the LF refining process, lime is added, and the lime addition amount is 0.9 + 40[O] kg / t, where [O] is 100 times the oxygen content measured before converter tapping.

5. The production method of the steel plate according to claim 4, characterized in that, After the LF refining process is completed, 92 + 845[Al] m of calcium wire is fed, and the wire feeding speed is 1.0 - 2.0 m / s, where [Al] is 100 times the aluminum content measured after the LF refining process is completed.

6. The production method of the steel plate according to claim 5, characterized in that, After feeding the calcium wire, the soft stirring time is ≥ 8 min, the soft stirring time in the RH refining process is ≥ 12 min, and adding Al is strictly prohibited in the RH refining process.

7. The steel plate obtained by the production method of the steel plate described in claim 1, characterized in that, The chemical composition of the steel plate includes, by mass percentage: C: 0.16 - 0.19%, Si: 0.30 - 0.40%, Mn: 1.35 - 1.45%, P ≤ 0.015%, S ≤ 0.005%, Ni: 0.15 - 0.25%, Nb: 0.010 - 0.020%, V: 0.020 - 0.030%, Ti: 0.010 - 0.020%, Alt: 0.02 - 0.05%, and the balance is iron and inevitable impurities; The microstructure of the steel plate is ferrite and pearlite. In the normalizing heat treatment process, for the steel plate water-cooled after normalizing, the pearlite is diffusely distributed; for the steel plate air-cooled after normalizing, the banding structure rating of the pearlite is ≤ 2 levels.

8. The steel plate according to claim 7, characterized in that, The yield strength of the steel plate is ≥ 355 MPa, the tensile strength is 540 - 620 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51 °C is ≥ 100 J, and the Z-direction tensile area reduction is ≥ 65%.

9. The steel plate according to claim 8, characterized in that, After the steel plate undergoes large-deformation cold forming or hot forming, normalization to restore performance, and post-weld heat treatment, the yield strength is ≥ 300 MPa, the tensile strength is 485 - 540 MPa, the elongation after fracture is ≥ 30%, the low-temperature impact energy at -51 °C is ≥ 100 J, and the Z-direction tensile area reduction is ≥ 65%.

10. The steel plate according to claim 9, characterized in that, The strain of the large deformation cold forming process is 5 - 10%; the temperature of the hot forming process is 920 ± 10 °C, the holding time is 1.6t ± 0.2t min, the heating rate ≤ 50 °C / h, and the cooling is natural cooling in air; the normalization for performance recovery process is the same as the normal heat treatment process of the steel plate; for the post-weld heat treatment process, the temperature is 635 ± 15 °C, the holding time is 360 min, the heating and cooling rates ≤ 50 °C / h, and it experiences 4 cycles.

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