Preheat-free weldable steel plates and their production methods, preheat-free welding methods

Abstract

This application discloses a preheat-free weldable steel plate and its production method, as well as a preheat-free welding method. The production method includes sequential processes of billet preparation, rolling, cooling, quenching, and tempering. In the billet preparation process, the continuously cast billet is placed in a heat-insulating pit for heat preservation at a temperature of 630~670℃ for a holding time t≥d. 2 / 40D, where d is the thickness of the continuously cast billet and D is the hydrogen diffusion coefficient in the continuously cast billet; in the quenching process, the steel plate is heated and held at a temperature of (A) c3 -30)℃~(A c3 -10℃, furnace time was (1.5a+20)min~(1.5a+30)min, followed by rapid water cooling to room temperature, with a cooling rate of (V B +15)℃ / s~(V B +20)℃ / s; In the tempering process, the heating temperature is (Ms-235)℃~(Ms-210)℃, and the furnace time is (5a+10)min~(5a+30)min.

Description

Technical Field

[0001] This application belongs to the technical field of steel alloy materials, and relates to a preheat-free weldable steel plate and its production method, as well as a preheat-free welding method for steel plates. Background Technology

[0002] Energy development and construction, especially the extraction of oil, gas, and mineral resources, are inseparable from large-scale engineering machinery and equipment. With the growth of energy demand, development areas have expanded from ordinary temperate regions to extreme low-temperature environments such as the Arctic and Antarctic, and high-altitude permafrost zones. Energy development operations in these areas place stringent demands on the performance of engineering machinery and equipment.

[0003] Due to transportation limitations of large equipment and the manufacturing requirements of complex structures, welding is often unavoidable during on-site splicing and assembly. Welding operations face significant challenges in extremely cold environments. To ensure the low-temperature toughness of welded joints, complex preheating and post-weld heat treatment processes are often required. These processes are not only difficult to implement and energy-intensive in extremely cold outdoor conditions, but also struggle to guarantee process stability. Summary of the Invention

[0004] The purpose of this application is to provide a preheat-free weldable steel plate, its production method, and a preheat-free welding method.

[0005] To achieve the above-mentioned application objectives, one embodiment of this application provides a method for producing steel plates, including sequential processes of billet preparation, rolling, cooling, quenching, and tempering.

[0006] In the billet preparation process, a continuous casting billet is prepared and then placed in a heat-insulating pit for heat preservation. The temperature in the heat-insulating pit is 630~670℃, and the heat preservation time is t≥d. 2 / 40D, in seconds; where d is the thickness of the continuously cast billet in mm; and D is the hydrogen diffusion coefficient in the continuously cast billet in mm. 2 / s;

[0007] In the rolling process, the continuously cast billet is heated and rolled into a steel plate;

[0008] In the cooling process, the steel plate is cooled;

[0009] In the quenching process, the cooled steel plate is placed in a heating furnace for heating and then held at a temperature of (A). c3 -30)℃~(A c3 -10)℃, furnace time was (1.5a+20)min~(1.5a+30)min, followed by rapid water cooling to room temperature, with a cooling rate of (V B +15) ℃ / s~(V B +20)℃ / s;

[0010] In the tempering process, the steel plate that has undergone the quenching process is sent into a heating furnace for heating and then held at a temperature of (Ms-235)℃~(Ms-210)℃, and the time in the furnace is (5a+10)min~(5a+30)min.

[0011] Among them, the austenitization completion temperature A c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu;

[0012] Critical cooling rate V for bainite formation B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600;

[0013] Martensitic transformation onset temperature Ms = 561-474°C - 33Mn-17Cr-17Ni-21Mo;

[0014] 'a' represents the thickness of the steel plate, in mm.

[0015] In some embodiments, the chemical composition of the continuously cast billet, by mass percentage, includes: C 0.20~0.23%, Si 0.28~0.36%, Mn 0.90~1.10%, Cr 0.35~0.42%, Ti 0.012~0.019%, Alt 0.021~0.049%, B 0.0016~0.0021%, Zr 0.012~0.019%, Ca 0.0022~0.0052%, N 20~45ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, with the remainder being Fe and unavoidable impurities.

[0016] In some embodiments, the chemical composition of the continuously cast billet, by mass percentage, satisfies the following: carbon equivalent (CEV) of 0.42-0.52%, Mn equivalent (∑Mn) of 1.20-1.35%, and hardenability index (J) ≥ 2.1.

[0017] CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15;

[0018] ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb);

[0019] J = (1.24C - 0.95C) 2)×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1).

[0020] In some embodiments, during the quenching process, water cooling is performed on a quenching machine with a water-to-water ratio of 0.75 to 0.85, and the running speed v of the steel plate in the quenching machine is 34 / a to (34 / a+0.4) m / s.

[0021] In some embodiments, during the quenching process, the quenching machine is turned on 20-30 seconds before the steel plate enters the quenching machine.

[0022] In some embodiments, during the quenching process, the steel plate passes through a high-pressure section and a low-pressure section sequentially during water cooling. The water pressure in the high-pressure section is 0.79~0.81MPa, and the water volume is 6100~6400m³. 3 / h; the water pressure in the low-pressure section is 0.39~0.41MPa, and the water volume is 5600~5900m³. 3 / h.

[0023] In some embodiments, during the rolling process, the continuously cast billet is heated in a heating furnace to a soaking temperature of (T). Nb(C,N) +30)℃~(T Nb(C,N) +60)℃, the soaking time is 0.1d~0.5dmin, and the total time in the furnace is 1.1d~1.5dmin;

[0024] Among them, T Nb(C,N) =6770 / (2.26 lg((C+12N / 14)×Nb))-273.

[0025] In some embodiments, in the rolling process, the continuously cast billet is heated and then subjected to rough rolling and finish rolling in sequence to form the steel plate, wherein the initial rolling temperature of the rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃;

[0026] Among them, T Nb(C,N) =6770 / (2.26 lg((C+12N / 14)×Nb))-273;

[0027] recrystallization temperature T nr=887+464C+6445Nb-644 +890Ti+363Alt-357Si;

[0028] A r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, A r3 This is the temperature at which ferrite precipitates from austenite during the cooling process.

[0029] In some embodiments, the cooling rate in the cooling process is (V B +5) ℃ / s~(V B +10)℃ / s.

[0030] To achieve the above-mentioned application objectives, one embodiment of this application provides a steel plate, which is prepared using the steel plate production method described above.

[0031] In some embodiments, the thickness 'a' of the steel plate is 6~30mm, the yield strength is ≥1100MPa, the tensile strength is ≥1400MPa, the hardness is 420~460HBW, the elongation is ≥12%, and the impact energy AKV at -40℃ is ≥50J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a, and there are no cracks on the surface of the steel plate; the strength-ductility product of the steel plate is ≥16.5GPa·%, and the residual stress is ≤150MPa.

[0032] In some embodiments, the steel plate is welded without preheating during the cold crack welding test, and there are no cracks on the surface, root, or cross-section of the steel plate.

[0033] The steel plate was subjected to gas-protected flux-cored wire arc welding without preheating, and the welded joint showed no delayed cracking.

[0034] To achieve the aforementioned objectives, one embodiment of this application provides a preheat-free welding method for steel plates. The steel plate is subjected to gas-shielded flux-cored wire arc welding without preheating. The ambient temperature is ≥15℃. For the root pass, low-hydrogen welding materials of 50kg grade or below are used, with a diffused hydrogen content ≤5mL / 100g. The welding current is controlled at ≤200A, welding voltage at ≤26V, and welding heat input at ≤18kJ / cm. For the fill and cover passes, low-hydrogen welding materials of 80kg grade or below are used, with a diffused hydrogen content ≤5mL / 100g. The welding current is controlled at ≤260A, welding voltage at ≤31V, and welding heat input at ≤25kJ / cm. The interpass temperature is 200~250℃.

[0035] Compared with the prior art, the beneficial effects of this application are as follows:

[0036] (1) In the production method of the steel plate, after the continuous casting billet is removed from the production line, it is placed in the heat preservation pit to achieve slow cooling and deep hydrogen diffusion treatment, thereby reducing the hydrogen content. Combined with the quenching and tempering process, residual stress is eliminated, thereby improving the welding performance of the steel plate, so that the steel plate can be welded without preheating and has excellent welding performance.

[0037] (2) When the cold crack welding test is carried out on the steel plate prepared by the above production method according to GB 4675.1-84 "Weldability Test Method for Y-groove Welding Crack", the steel plate is welded without preheating and there are no cracks on the surface, root and cross section of the steel plate; the steel plate is gas shielded flux-cored wire arc welded without preheating and there are no delayed cracks in the welded joint. Attached Figure Description

[0038] Figure 1 This is a metallographic diagram of the steel plate according to an embodiment of this application;

[0039] Figure 2 These are photographs of weld crack tests on the steel plate with a beveled Y-groove, as described in this application.

[0040] Figure 3 This is a metallographic diagram of the steel plate used in the comparative example of this application;

[0041] Figure 4 This is a photograph of a weld crack test on a steel plate with a beveled Y-groove, which is the comparative example of this application. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0043] One embodiment of this application provides a preheat-free weldable steel plate and a method for producing the same.

[0044] Regarding the chemical composition, the steel plate comprises, by mass percentage: C 0.20~0.23%, Si 0.28~0.36%, Mn 0.90~1.10%, Cr 0.35~0.42%, Ti 0.012~0.019%, Alt 0.021~0.049%, B 0.0016~0.0021%, Zr 0.012~0.019%, Ca 0.0022~0.0052%, N 20~45ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, with the remainder being Fe and unavoidable impurities.

[0045] Preferably, the chemical composition of the steel plate, by mass percentage, satisfies the following: carbon equivalent (CEV) of 0.42~0.52%, Mn equivalent (∑Mn) of 1.20~1.35%, and hardenability index (J) ≥ 2.1;

[0046] CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15;

[0047] ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb);

[0048] J = (1.24C - 0.95C) 2 )×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1).

[0049] It is understood that the calculation formulas are only applicable to the numerical values ​​of the corresponding indicators, and the units of each indicator will be explained separately. The same applies to the formulas mentioned later. For example, the units of CEV and ∑Mn here are both percentages.

[0050] In the calculation formula, the symbols of each element represent the mass percentage of the corresponding element. That is, C, Mn, Cr, Mo, V, Cu, Ni, W, Si, Alt, and Nb in the calculation formula represent the mass percentages of C, Mn, Cr, Mo, V, Cu, Ni, W, Si, Alt, and Nb, respectively.

[0051] For example, if the mass percentage of carbon in the steel plate is 0.20%, then the C in the calculation formula represents the mass percentage of 0.20.

[0052] Of course, if the chemical composition of the steel plate does not include a certain element, the mass percentage of that element in the calculation formula shall be calculated as 0.

[0053] The element symbols in the calculation formulas below will follow the same pattern, and will not be repeated hereafter.

[0054] The functions of each chemical component and the details of content control are explained below.

[0055] Carbon (C) is the most economical strengthening element in steel, exhibiting significant solid solution strengthening. It substantially improves the strength and hardness of steel by pinning dislocation movement. The higher the C content, the more pronounced the solid solution strengthening effect. However, excessive C addition will form coarse cementite, leading to a sharp decrease in toughness and weldability. Therefore, in this application, the C content is limited to 0.20~0.23%, and the carbon equivalent (CEV) is limited to 0.42~0.52%.

[0056] Si: Silicon plays a solid solution strengthening role in steel, and can also inhibit carbide decomposition and improve tempering stability. Si combines with impurity elements such as P and S in steel to form high-melting-point oxides, thus acting as nucleation cores to refine grains in heterogeneous formation. If too much Si is added, Si will form a low-melting-point FeS-SiO2 composite phase with S, increasing the tendency for hot cracking in the weld. Therefore, the Si content is limited to 0.28~0.36% in this application.

[0057] Mn: Manganese plays a solid solution strengthening role in steel, which can improve hardenability, enhance wear resistance, and lower the austenite decomposition temperature. However, if too much Mn is added, the grains in the hardened austenite zone (HAZ) will coarsen during high-temperature holding, and upon cooling, coarse ferrite or pearlite will form, thereby reducing impact toughness and increasing the tendency for cold cracking in welds. Therefore, in this application, the Mn content is limited to 0.90~1.10%, and the Mn equivalent ∑Mn is limited to 1.20~1.35%.

[0058] Cr: In steel, chromium can form stable carbides with carbon, pinning grain boundaries and inhibiting austenite grain growth, thus refining the matrix structure and improving the strength and toughness of the steel. However, if too much Cr is added, the hard martensite structure easily forms in the weld zone (HAZ) during cooling, leading to increased brittleness of the weld joint and delayed cracking. Therefore, the Cr content is limited to 0.35~0.42% in this application.

[0059] Ti combines with C and N to form carbonitride particles, which pin austenite grain boundaries during steel heating processes such as rolling or welding thermal cycles, inhibiting grain coarsening at high temperatures and reducing the risk of cracking. Furthermore, Ti can react with N in steel to form stable TiN precipitates, thereby reducing the reaction between B and N and providing some protection for B. However, excessive Ti addition can lead to the formation of coarse carbonitrides, which become crack initiation sites. Therefore, in this application, the Ti content is limited to 0.012~0.019%.

[0060] Al: Aluminum is a deoxidizing element in steel, which can significantly reduce the oxygen content in molten steel. Al can combine with nitrogen to form AlN particles, which can inhibit austenite grain growth, thus refining the base metal grains and inhibiting weld grain coarsening. However, excessive Al addition will form coarse Al2O3 inclusions, which can become crack initiation sites. Therefore, in this application, the Al content is limited to 0.021~0.049%.

[0061] B: Boron can segregate at austenite grain boundaries, lower grain boundary energy, inhibit ferrite nucleation, and significantly improve the hardenability of steel. However, excessive B addition can lead to the precipitation of a large amount of boron nitrides, which in turn promotes ferrite nucleation. Therefore, the B content is limited to 0.0016~0.0021% in this application.

[0062] Zirconium (Zr) has a strong affinity for carbon and nitrogen (N), forming stable carbonitrides. These compounds are finely dispersed in steel, acting as grain boundary pinning agents. During welding, they refine the columnar grains in the weld, reducing the formation of coarse structures and inhibiting cold cracking. Zr can lower the activity energy of free carbon and nitrogen, suppressing grain boundary weakening caused by C and nitrogen diffusion during welding and inhibiting cold cracking. Zr can also combine with sulfur to form high-melting-point ZrS, preventing the formation of low-melting-point eutectic phases at grain boundaries and inhibiting welding hot cracking. However, excessive Zr addition will form coarse ZrC and ZrN particles or form intermetallic compounds with Fe, increasing brittleness. Therefore, the Zr content in this application is limited to 0.012~0.019%.

[0063] Ca: Calcium has a strong affinity for O and S (thus protecting Zr and La and improving their yield), and can form high-melting-point calcium aluminates, calcium silicates, calcium sulfides, and other complex inclusions. Ca can reduce free oxygen and oxide inclusions in steel, reducing the oxidizing properties of molten steel; it can also reduce sulfide segregation at grain boundaries, improving the hot working properties and resistance to hot cracking of steel; and it can modify inclusions, transforming them into spherical or ellipsoidal calcium aluminates and calcium silicates, reducing the sharpness of inclusions and interfacial stress concentration. However, excessive Ca addition will form high-melting-point calcium aluminate inclusions, which are prone to agglomeration in the weld and promote the formation of brittle phases under welding thermal cycling, increasing the risk of cold cracking. Therefore, the Ca content in this application is limited to 0.0022~0.0052%.

[0064] Nitrogen (N) can form high-hardness nitrides with Nb, Ti, and Zr, acting as dispersed second-phase particles that hinder dislocation movement and grain growth, thereby improving resistance to abrasive wear and weldability. However, excessive N content can easily lead to porosity, HAZ embrittlement, and uneven microstructure during welding. Therefore, the N content is controlled at 20–45 ppm in this application.

[0065] H: Hydrogen is a harmful element in steel, easily accumulating at grain boundaries and defects to form diffusible hydrogen, which, together with stress, causes hydrogen-induced cracking. Therefore, the H content in this application is controlled to ≤1.5ppm.

[0066] Phosphorus (P) is a harmful element in steel, easily segregating at grain boundaries and increasing susceptibility to cold cracking during welding. During welding, localized high temperatures can exacerbate P segregation, forming low-melting-point phases and initiating hot cracking. Therefore, the P content in this application is controlled to ≤0.008%.

[0067] Sulfur is a harmful element in steel. During welding, sulfur tends to segregate at the solidification front of the weld metal, forming a low-melting-point eutectic liquid film that hinders intergranular bonding and increases the tendency for cold cracking in the weld. Therefore, the sulfur content is controlled to ≤0.0012% in this application.

[0068] Based on this, the steel plate of this application, in terms of chemical composition, reduces the sensitivity to welding cracks by fixing free N with Ti; it lowers grain boundary energy by adding trace amounts of B, inhibiting premature ferrite precipitation and regulating phase transformation kinetics; and it generates nanoscale Zr(C,N) particles through Zr microalloying, pinning grain boundaries and subgrain boundaries, inhibiting austenite grain coarsening, avoiding the formation of coarse martensite during welding, and maintaining the toughness of the weld joint. Thus, the chemical composition design scheme of this application can achieve a high strength and high toughness balance in the steel plate, enabling welding without preheating and exhibiting excellent weldability.

[0069] The metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the proportions of lath martensite and acicular martensite is 90-94%, the proportion of ferrite is 3-5%, and the proportion of retained austenite is 3-7%.

[0070] Here, the metallographic structure of the steel plate can be obtained by performing microstructure testing according to GB / T 13298-2015 "Metallic Microstructure Examination Method".

[0071] According to YB / T 4676-2018 "Analysis of Precipitated Phases in Steel by Transmission Electron Microscopy", nanoscale carbides are distributed on the matrix of the steel plate.

[0072] Measurements show that the steel plate has a thickness (a) of 6-30 mm, a yield strength ≥1150 MPa, a tensile strength ≥1400 MPa, a hardness of 420-460 HBW, an elongation ≥12%, and an impact energy (AKV) ≥50 J at -40℃. When cold-bent 180° with a bending mandrel diameter (D) of 3a, the steel plate surface shows no cracks.

[0073] The measurements of yield strength, tensile strength, and elongation mentioned above can be performed according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test method at room temperature". The measurements of hardness mentioned above can be performed according to GB / T231.1-2018 "Metallic materials, Brinell hardness testing—Part 1: Test method". The measurements of impact energy at -40℃ mentioned above can be performed according to GB / T 229-2020 "Metallic materials, Charpy pendulum impact test method". The measurements of cold bending performance mentioned above can be performed according to GB / T 232-2024 "Metallic materials, bending test method".

[0074] The steel plate has a strength-ductility product ≥ 16.5 GPa·%, and a residual stress ≤ 150 MPa.

[0075] The above measurements of the strength-ductility product can be performed according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Test method at room temperature". The above measurements of residual stress can be performed according to GB / T 31310-2014 "Metallic materials, determination of residual stress—drill strain method".

[0076] The steel plate was subjected to a cold crack welding test in accordance with GB 4675.1-84 "Weldability Test - Test Method for Cracks in Y-groove Welding". Welding was performed without preheating, and no cracks were found on the surface, root, or cross-section of the steel plate.

[0077] Gas-shielded flux-cored wire arc welding was performed on the steel plate without preheating. The welded joint showed no delayed cracks and exhibited excellent strength and toughness.

[0078] This application also provides a preheat-free welding method for the above-mentioned steel plate. In this welding method, the steel plate is subjected to gas-shielded flux-cored wire arc welding without preheating. Specifically, the ambient temperature is ≥15℃; for the root pass welding, low-hydrogen welding materials of 50kg grade or below are used, with a diffused hydrogen content ≤5mL / 100g; the welding current is controlled to ≤200A, the welding voltage ≤26V, and the welding heat input ≤18kJ / cm; for the fill and cover passes welding, low-hydrogen welding materials of 80kg grade or below are used, with a diffused hydrogen content ≤5mL / 100g; the welding current is controlled to ≤260A, the welding voltage ≤31V, and the welding heat input ≤25kJ / cm; the interpass temperature is 200~250℃.

[0079] One embodiment of this application also provides a method for producing steel plates.

[0080] The aforementioned steel plate was produced using this production method.

[0081] The steel plate production method includes, in sequence, a billet preparation process, a rolling process, a cooling process, a quenching process, and a tempering process.

[0082] In the billet preparation process, a continuous casting billet is prepared and then placed in a heat-insulating pit for heat preservation. The temperature in the heat-insulating pit is 630~670℃, and the heat preservation time is t≥d. 2 / 40D, in seconds. Where d is the thickness of the continuously cast billet, in mm; D is the hydrogen diffusion coefficient in the continuously cast billet, in mm. 2 / s. By keeping the continuously cast billet in a heat-insulating pit after it comes off the production line, and by controlling the temperature and holding time of the heat-insulating pit, the hydrogen diffusion coefficient can be increased by tens of thousands of times compared to room temperature. This can significantly shorten the hydrogen diffusion time and ensure that H in the core of the continuously cast billet diffuses out fully.

[0083] Here, the hydrogen diffusion coefficient D can be measured in accordance with the method disclosed in ISO 17081:2014.

[0084] Measurements showed that the hydrogen diffusion coefficient D was 0.01 mm at temperatures ranging from 630 to 670°C. 2 / s.

[0085] In the rolling process, the continuously cast billet is heated and rolled into a steel plate.

[0086] In the cooling process, the steel plate is cooled.

[0087] In the quenching process, the cooled steel plate is placed in a heating furnace for heating and then held at a temperature of (A). c3 -30)℃~(A c3 -10)℃, furnace time was (1.5a+20)min~(1.5a+30)min, followed by rapid water cooling to room temperature, with a cooling rate of (V B +15) ℃ / s~(V B +20)℃ / s. The quenching process, by controlling the heating temperature, retains some undissolved ferrite, which refines the austenite grains and reduces quenching stress. As the grain boundary area increases, the steel plate's resistance to crack propagation is improved. Rapid cooling is achieved by controlling the cooling rate, ensuring that the supercooled austenite directly crosses the bainite transformation zone, inhibiting the formation of non-martensite structures, especially coarse MA islands, to obtain a martensite + retained austenite structure, thereby improving toughness.

[0088] In the tempering process, the steel plate that has undergone the quenching process is placed in a heating furnace and heated and held at a temperature of (Ms-235)℃ to (Ms-210)℃ for a time of (5a+10)min to (5a+30)min. Low-temperature tempering precipitates fine carbides on the martensitic framework, reduces lattice distortion, effectively eliminates quenching stress, reduces the brittleness of the steel plate, improves impact resistance, and prevents failure due to stress concentration during use.

[0089] Among them, the austenitization completion temperature A c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu;

[0090] Critical cooling rate V for bainite formation B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600;

[0091] Martensitic transformation onset temperature Ms = 561-474°C - 33Mn-17Cr-17Ni-21Mo;

[0092] 'a' represents the thickness of the steel plate, in mm.

[0093] Thus, the steel plate production method involves slowly cooling the continuously cast billet by placing it in a heat-insulating pit after it comes off the production line, and then performing deep hydrogen diffusion treatment to reduce the hydrogen content. Combined with quenching and tempering processes, residual stress is eliminated, thereby improving the weldability of the steel plate. This allows the steel plate to be welded without preheating, and the weldability is excellent.

[0094] In addition, rapid cooling during steel plate quenching can form fine carbides on the steel plate matrix. These fine carbides do not grow after low-temperature tempering, thus forming nanoscale carbides on the steel plate matrix.

[0095] Further combining the aforementioned chemical composition design scheme, Ti microalloying is used to suppress austenite grain growth and promote deformation-induced precipitation, achieving the effect of grain refinement. Combined with the two-phase quenching process, the ratio of soft and hard phases of martensite, ferrite, and retained austenite is controlled, enabling the steel plate to possess both high strength and excellent plasticity and toughness. Deep hydrogen diffusion treatment is carried out through a slow cooling process to reduce hydrogen content. Combined with quenching and tempering processes, residual stress is eliminated, and the pinning effect of Zr carbonitride composite particles is used to further suppress grain coarsening in the weld heat-affected zone, thereby improving the weldability of the steel plate. This allows the steel plate to be welded without preheating, and the weldability is excellent.

[0096] In the quenching process, water cooling is performed on a quenching machine. Multiple nozzles impact the upper and lower surfaces of the steel plate with water to achieve rapid cooling. At least some nozzles are positioned correspondingly on the upper and lower surfaces of the steel plate, and the positional deviation of the water jets from these nozzles on the upper and lower surfaces of the steel plate is ≤1mm, so that corresponding positions on the upper and lower surfaces of the steel plate undergo simultaneous phase transformation, thereby reducing internal stress.

[0097] In some implementations, the water-to-water ratio of the quenching machine is 0.75 to 0.85, and the running speed v of the steel plate in the quenching machine is 34 / a to (34 / a+0.4) m / s. This allows for control of the cooling rate, thereby ensuring the plate shape.

[0098] In some embodiments, during the quenching process, the quenching machine is turned on 20-30 seconds before the steel plate enters it. This avoids uneven stress on the steel plate surface caused by unstable water flow rate when the quenching machine is turned on.

[0099] In some embodiments, during the quenching process, the steel plate passes through a high-pressure section and a low-pressure section sequentially during water cooling. The water pressure in the high-pressure section is 0.79~0.81MPa, and the water volume is 6100~6400m³. 3 / h; the water pressure in the low-pressure section is 0.39~0.41MPa, and the water volume is 5600~5900m³. 3 / h. The high-pressure section uses a large volume of water and high water pressure to rapidly cool the steel plate to the phase transformation point, while the low-pressure section controls the temperature recovery.

[0100] In some embodiments, during the rolling process, the continuously cast billet is heated in a heating furnace to a soaking temperature of (T). Nb(C,N) +30)℃~(T Nb(C,N) The temperature is +60℃, the soaking time is 0.1d~0.5dmin, and the total time in the furnace is 1.1d~1.5dmin. In other words, after the continuously cast billet is fed into the heating furnace, it is first heated to the soaking temperature, and then held at that temperature for 0.1d~0.5dmin. Thus, by controlling the total time in the furnace and the soaking time, the time to reach the soaking temperature can be controlled, which means the heating rate can be controlled.

[0101] Among them, T Nb(C,N) is the solid solution temperature of Nb carbonitrides.

[0102] T Nb(C,N) =6770 / (2.26 lg((C+12 / 14N)×Nb))-273.

[0103] By controlling the homogenization temperature, homogenization time, and total furnace time before rolling, it is possible to ensure that the carbonitrides of Nb are fully dissolved, avoid undissolved particles from affecting toughness, and prevent excessively high temperatures from causing coarsening of austenite grains and increased oxidation and burn-off of the continuously cast billet.

[0104] In the rolling process, the continuously cast billet is heated and then subjected to rough rolling and finish rolling in sequence to form a steel plate. The starting rolling temperature for rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃.

[0105] Among them, recrystallization temperature T nr =887+464C+6445Nb-644 +890Ti+363Alt-357Si.

[0106] A r3 This is the temperature at which ferrite precipitates from austenite during the cooling process.

[0107] A r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo.

[0108] By rolling in the non-recrystallized austenite region during the rough rolling stage, the austenite recrystallization is suppressed by strain energy, and the strain-induced precipitation of Nb carbonitrides is promoted, thus refining the grains. During the finish rolling stage, the austenite recrystallization region or critical region is rolled. By controlling the rolling temperature, the microstructure before phase transformation can be optimized, ensuring that a fine-grained ferrite + bainite composite microstructure is obtained after phase transformation.

[0109] In the cooling process, the cooling rate is (V B +5) ℃ / s~(V B +10)℃ / s.

[0110] Among them, V B V is the critical cooling rate for bainite formation. B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600.

[0111] The cooling process controls the cooling rate to form a partial acicular ferrite + lath bainite structure, thereby obtaining a good precursor structure and ensuring improved toughness and weldability after quenching.

[0112] In some embodiments, the cooling process involves water cooling of the steel plate, i.e., water cooling is used to cool the steel plate. The water immersion temperature of the steel plate is ≥ (A). r3-20)℃, the final cooling temperature of the steel plate is (Bs-10)℃~(Bs-30)℃, where Bs is the bainite transformation start temperature, Bs=630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W.

[0113] In one embodiment, the billet-making process involves continuous casting of molten steel to prepare a continuously cast billet. The superheat of the molten steel during continuous casting is 8-20°C. Light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.2-1.4 mm / m. The casting speed v satisfies: 200×L / F-0.05≤v≤200×L / F+0.05, where L is the perimeter of the continuously cast billet obtained in the continuous casting process (in mm) and F is the cross-sectional area of ​​the continuously cast billet (in mm²). 2 Thus, by controlling the casting of molten steel with low superheat, the reduction rate, and the casting speed, the quality of the continuously cast billet can be improved, and the center segregation and center porosity of the continuously cast billet can be controlled.

[0114] It is understandable that the chemical composition of the continuously cast billet is roughly the same as that of the molten steel and the final steel plate product, all of which conform to the chemical composition of the steel plate mentioned above, and will not be repeated here.

[0115] The thickness of the continuously cast billet is 220~320mm.

[0116] Measurements showed that the center segregation level of the continuously cast billet was ≤C1.5 or ≤B1.0, with no Class A segregation. Class B segregation was discontinuous linear segregation, and Class C segregation was dotted segregation. The center porosity level was ≤0.5, and the levels of Class A, B, and C inclusions were all ≤0.5. The levels of Class D and Ds inclusions were all ≤1.

[0117] The above measurements of center segregation level, center porosity level, and inclusion level of continuously cast billets can be based on YB / T 4003. The 2016 "Low-Magnification Microstructure Defect Rating Chart for Continuously Cast Steel Slabs" was developed.

[0118] In the preparation of continuously cast billets, molten steel is poured into a continuous casting machine to form the billet. Specifically, the molten steel is first distributed from the tundish to the crystallizer, where it solidifies to form a billet shell. The shell still contains molten steel. After exiting the crystallizer, it is cooled in the secondary cooling zone, which uses air mist cooling. The specific water volume in the secondary cooling zone is 0.5~0.7L / kg. In this way, rapid cooling through a strong cooling mode can prevent inclusions from agglomerating during the solidification process of the molten steel, thereby reducing the inclusion level.

[0119] The steel plate production method also includes a steel smelting process. The molten steel obtained from the steel smelting process is used for continuous casting to prepare continuously cast billets.

[0120] In one embodiment, the steelmaking process includes sequential steps of hot metal pre-desulfurization, converter smelting, LF refining, and RH refining.

[0121] In the converter smelting step, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.8~1.2 Nm³. 3 / (min·t), lime and dolomite are used for slag making, the slag basicity is controlled at 3.0~3.5, and the P content in the molten steel at the end of the converter is less than 0.005% by mass percentage.

[0122] In the LF refining step, heating and alloying are performed first, followed by slag formation. The slag basicity is controlled at 4.5-5.0, and the sulfur content in the molten steel is controlled to be less than 0.0010% by mass. After alloying, 100-300 m / furnace of aluminum wire is fed in and stirred for 3-5 minutes, controlling the oxygen content in the molten steel to be less than 0.0040% by mass. The steel is then tapped at a temperature of 1650-1670℃. Thus, by controlling the slag basicity to 4.5-5.0, deep desulfurization can be achieved, and by controlling the timing and amount of aluminum wire feeding, deep deoxidation can be achieved.

[0123] Among them, the LF refining process uses an LF furnace to refine molten steel, with a steel loading of 180t per furnace.

[0124] In the RH refining step, after the molten steel is allowed to stand under vacuum, calcium wire is fed in after the vacuum is broken to control the Ca, O, and S content in the molten steel to meet the following condition: 2.0 ≤ (Ca - 3.5O) / S ≤ 2.5. Then, the mixture is stirred for 2-3 minutes. Zirconium-iron alloy is added to control the Zr content in the molten steel to 0.012-0.019%, and the mixture is gently stirred with bottom-blown argon gas for 5-8 minutes, with an argon flow rate ≤ 1 Nm³. 3 / h, then stir for 15~20min.

[0125] Thus, by controlling the content of calcium wire fed into the molten steel, Ca in the steel preferentially reacts with O to form CaO, and Ca completely combines with S, avoiding the formation of low-melting-point FeS. Static stirring after feeding the calcium wire promotes the aggregation and growth of CaO and CaS inclusions, facilitating subsequent flotation and removal. By adding zirconium-iron alloy, Zr preferentially reacts with O to form high-melting-point ZrO2, further reducing active oxygen and achieving deep purification of the molten steel in conjunction with rare earth elements. Furthermore, Zr can react with C and N in the molten steel to form compounds such as ZrN and ZrC, which act as heterogeneous nucleation sites, inhibiting grain growth and improving the steel's strength, toughness, and weldability. Weak stirring with bottom-blowing argon after adding Zr promotes uniform dissolution of Zr, followed by prolonged static stirring, allowing inclusions to float fully, thereby reducing the size and number of inclusions in the finished steel.

[0126] The zirconium-iron alloy contains 30-50% Zr by mass.

[0127] In some embodiments, in the pre-desulfurization step of the molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the S content in the molten iron after desulfurization to be less than 0.0012% by mass percentage; wherein, the amount of magnesium powder injected is 0.35~0.45 kg per ton of molten iron.

[0128] In some embodiments, during the LF refining step, the heating and alloying process is carried out at a rate of 3~4 Nm for the first 10 minutes. 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.6~1.2Nm 3 A bottom-blown argon flow rate of / h is used. Thus, in the early stages of refining, a higher stirring intensity can accelerate the deoxidation reaction and the collision and aggregation of inclusions; while in the later stages of refining, a lower stirring intensity can be used to avoid slag entrapment caused by excessive agitation of molten steel, while promoting the flotation and removal of inclusions.

[0129] In some embodiments, the RH refining step, specifically the vacuum settling of molten steel, includes: setting the molten steel under a vacuum of ≤30 Pa for 25-30 minutes to control the O content in the molten steel to be 0.001-0.003% by mass. This allows dissolved O, N, and H in the molten steel to escape due to the reduced partial pressure under high vacuum.

[0130] According to GB 4675.1-84 "Test Method for Welding Cracks in Y-groove Welding", cold crack welding tests were conducted on steel plates prepared by the above production method. Welding was carried out without preheating, and no cracks were found on the surface, root, or cross-section of the steel plate.

[0131] The steel plate was subjected to gas-shielded flux-cored wire arc welding without preheating. The ambient temperature was ≥15℃. For the root pass, low-hydrogen welding materials of 50kg grade or below were used, with a diffused H content of ≤5mL / 100g. The welding current was controlled at ≤200A, welding voltage at ≤26V, and welding heat input at ≤18kJ / cm. For the fill and cover passes, low-hydrogen welding materials of 80kg grade or below were used, with a diffused H content of ≤5mL / 100g. The welding current was controlled at ≤260A, welding voltage at ≤31V, and welding heat input at ≤25kJ / cm. The interpass temperature was 200~250℃. There were no cracks on the surface, root, or cross-section of the weld joint, and the weld joint exhibited excellent strength and toughness.

[0132] The following describes specific embodiments and comparative examples to further illustrate the beneficial effects of this application.

[0133] Example

[0134] The production process of steel plates includes the following steps in sequence: steel smelting, billet preparation, rolling, cooling, quenching, and tempering.

[0135] The steelmaking process includes the sequential steps of hot metal pre-desulfurization, converter smelting, LF refining, and RH refining.

[0136] In the pre-desulfurization step of molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the sulfur content in the molten iron after desulfurization to be 0.0009%; among them, the amount of magnesium powder injected is 0.5 kg per ton of molten iron.

[0137] In the converter smelting process, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.8 Nm³. 3 / (min·t), lime and dolomite are used for slag making, the slag basicity is controlled at 3.2, and the P content in the molten steel at the end of the converter is 0.0012%.

[0138] In the LF refining process, an LF furnace is used to refine the molten steel, with each furnace charging 180 tons of steel. After the ladle arrives at the station, it undergoes heating and alloying, followed by slag formation. The slag basicity is controlled at 4.8, and the sulfur content in the molten steel is controlled at 0.0008%. After alloying, 265m / furnace of aluminum wire is fed in and stirred statically for 4.7 minutes, controlling the oxygen content in the molten steel to 0.0030%. The steel is then tapped at a temperature of 1666℃. During the heating and alloying process, the first 10 minutes are carried out at a rate of 3.5 Nm... 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.8 Nm³ / h. 3 Bottom-blown argon gas at a flow rate of / h.

[0139] In the RH refining step, molten steel is allowed to stand under a high vacuum of 29 Pa for 25 minutes to control the O content in the molten steel to 0.0017%. Then, the vacuum is broken and calcium wire is fed into the molten steel to adjust the Ca content to 0.0073%, followed by static stirring for 22 minutes. Zirconium-iron alloy is added to adjust the Zr content in the molten steel to 0.015%, and the steel is gently stirred with bottom-blown argon gas for 7 minutes at a flow rate of 0.8 Nm³. 3 / h, then stir statically for 18min.

[0140] In this zirconium-iron alloy, Zr accounts for 40% of the total mass.

[0141] After the above steps, the chemical composition of the molten steel obtained from the steelmaking process, by mass percentage, includes: C 0.21%, Si 0.33%, Mn 0.98%, Cr 0.38%, Ti 0.016%, Alt 0.033%, B 0.0018%, Zr 0.015%, Ca 0.0035%, N 33ppm, O 9ppm, H 1.2ppm, P 0.008%, S 0.0009%, carbon equivalent CEV=0.45%, Mn equivalent ∑Mn=1.24%, and hardenability index J=2.2.

[0142] In the billet-making process, the molten steel obtained from smelting is continuously cast in a continuous casting machine to prepare a continuously cast billet with a thickness of 220 mm. The billet is then placed in a heat-preserving pit for heat preservation. The chemical composition of the continuously cast billet is roughly the same as that of the molten steel obtained from the smelting process.

[0143] During continuous casting, the superheat of the molten steel is 11℃, the casting speed v is 1.0m / min, and after exiting the crystallizer, it is cooled in the secondary cooling zone using air mist cooling. The specific water volume in the secondary cooling zone is 0.6L / kg. At the end of the continuous casting, a light reduction is applied, with a reduction rate of 1.3mm / m.

[0144] Measurements showed that the center segregation level of the continuously cast billet was C1.0, with no Class A segregation, and the center porosity level was 0.5. Inclusions of Class A, B, and C were all at level 0, and inclusions of Class D and Ds were all at level 0.5.

[0145] After the continuously cast billet comes off the continuous casting machine, it is immediately sent to the heat preservation pit for heat preservation. The temperature in the heat preservation pit is 660℃ and the heat preservation time is 39h.

[0146] In the rolling process, the continuously cast billet is first fed into a heating furnace and heated to 1180℃ and held for 45 minutes. The total time the continuously cast billet is in the furnace is 268 minutes. Then, the continuously cast billet is first rough rolled to an intermediate billet with a thickness of 65mm. The initial rolling temperature of the rough rolling is 1055℃. Then, it is finished rolled into a steel plate with a thickness of 16mm. The initial rolling temperature of the finish rolling is 882℃ and the final rolling temperature is 833℃.

[0147] During the cooling process, the rolled steel plate immediately enters the ultra-fast cooling system and is rapidly cooled by water cooling. The water temperature of the steel plate is 795℃, the cooling rate is 9℃ / s, and the final cooling temperature of the steel plate is 540℃.

[0148] In the quenching process, the cooled steel plate is placed in a heating furnace and heated to 850℃ for 50 minutes. Afterward, it is rapidly water-cooled to room temperature in a quenching machine. The quenching machine is started 25 seconds before the steel plate enters it. The water-to-top ratio in the quenching machine is 0.80. The steel plate's running speed (v) in the quenching machine is 2.2 m / s. The steel plate passes through a high-pressure section and a low-pressure section. The water temperature is 20℃. The positional deviation of the water jets from the nozzles in the high-pressure section on the upper and lower surfaces of the steel plate is 0.6 mm. The water pressure in the high-pressure section is 0.80 MPa, and the water volume is 6300 m³ / h. 3 / h; the water pressure in the low-pressure section is 0.40MPa, and the water volume is 5800m³ / h. 3 / h, the water cooling rate is 18℃ / s.

[0149] In the tempering process, the steel plate that has undergone quenching is sent into a heating furnace for heating and then held at a temperature of 200℃ for 100 minutes.

[0150] Samples of the finished steel plates were taken and tested, and the results were as follows:

[0151] (1) such as Figure 1 As shown, the metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the area ratios of lath martensite and acicular martensite in the metallographic sampling surface is approximately 92%, ferrite accounts for approximately 5%, and retained austenite accounts for approximately 3%.

[0152] (2) The yield strength is 1130MPa, the tensile strength is 1460MPa, the hardness is 438HBW, the elongation is 15%, and the impact energy at -40℃ is AKV 68J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a and there are no cracks on the surface.

[0153] (3) The strength-ductility product of the steel plate is 21.9 GPa·%, and the residual stress is 122 MPa.

[0154] (4) Perform cold crack welding tests on the steel plate according to GB 4675.1-84 "Weldability Test - Test Method for Cracks in Y-groove Welding", see [link to relevant documentation]. Figure 2 Welding was performed without preheating, and no cracks were found on the surface, root, or cross-section of the steel plate.

[0155] (5) The steel plate is subjected to gas-shielded flux-cored wire arc welding without preheating. The ambient temperature is ≥15℃. For the root pass welding, low-hydrogen welding materials of grade 50kg or below are used, with a diffused H content of 4mL / 100g. The welding current is ≤200A, the welding voltage is ≤26V, and the welding heat input is ≤18kJ / cm. For the fill and cover passes welding, low-hydrogen welding materials of grade 80kg or below are used, with a diffused H content of 4mL / 100g. The welding current is ≤260A, the welding voltage is ≤31V, and the welding heat input is ≤25kJ / cm. The interpass temperature is 200~250℃. The welded joint has no delayed cracks, the tensile strength of the welded joint is 865MPa, the impact energy at -20℃ is AKV=38J, and the steel plate is bent at 90° with a bending mandrel diameter D=6a. There are no cracks on the surface of the steel plate.

[0156] Comparative Example

[0157] The production process of steel plates includes the following steps in sequence: steel smelting, billet preparation, rolling, cooling, quenching, and tempering.

[0158] The steelmaking process includes the sequential steps of hot metal pre-desulfurization, converter smelting, LF refining, and RH refining.

[0159] In the pre-desulfurization step of molten iron, magnesium powder, lime powder and fluorite powder are mixed and then injected into the molten iron to control the sulfur content in the molten iron after desulfurization to be 0.0010%; among them, the amount of magnesium powder injected is 0.39 kg per ton of molten iron.

[0160] In the converter smelting process, the molten pool in the converter is subjected to top and bottom blowing, with oxygen blown from the top and argon blown from the bottom. The flow rate of the bottom-blown argon is 0.9 Nm³. 3 / (min·t), lime and dolomite are used for slag making, the slag basicity is controlled at 3.3, and the P content in the molten steel at the end of the converter is 0.0011%.

[0161] In the LF refining process, an LF furnace is used to refine the molten steel, with each furnace charging 180 tons of steel. After the ladle arrives at the station, it undergoes heating and alloying, followed by slag formation. The slag basicity is controlled at 4.8, and the sulfur content in the molten steel is controlled at 0.0008%. After alloying, 265m / furnace of aluminum wire is fed in and stirred for 4.6 minutes, controlling the oxygen content in the molten steel to 0.0030%. The steel is then tapped at a temperature of 1668℃. During the heating and alloying process, the first 10 minutes are carried out at a rate of 3.6 Nm... 3 Argon gas was bottom-blown at a flow rate of / h, followed by 0.8 Nm³ / h. 3 Bottom-blown argon gas at a flow rate of / h.

[0162] In the RH refining step, the molten steel is allowed to stand under a high vacuum of 28 Pa for 24 minutes to control the O content in the molten steel to 0.0018%. Then, the vacuum is broken and calcium wire is fed in to adjust the Ca content in the molten steel to 0.0072%. After that, it is stirred for 20 minutes.

[0163] After the above steps, the chemical composition of the molten steel obtained from the steelmaking process, by mass percentage, includes: C 0.21%, Si 0.37%, Mn 0.90%, Cr 0.32%, Ti 0.014%, Alt 0.035%, B 0.0018%, Ca 0.0032%, N 36ppm, O 10ppm, H 2.2ppm, P 0.008%, S 0.0010%, carbon equivalent CEV=0.46%, Mn equivalent ∑Mn=1.31%, and hardenability index J=2.33.

[0164] In the billet preparation process, the molten steel obtained from smelting is continuously cast in a continuous casting machine to prepare a continuously cast billet with a thickness of 220 mm. The chemical composition of the continuously cast billet is roughly the same as that of the molten steel obtained from the steel smelting process.

[0165] During continuous casting, the molten steel is superheated to 10℃, and the casting speed v is 1.0 m / min. After exiting the crystallizer, it is cooled in the secondary cooling zone using air mist cooling, with a specific water volume of 0.6 L / kg. Light reduction is applied at the end of the continuous casting process, with a reduction rate of 1.3 mm / m.

[0166] Measurements showed that the center segregation level of the continuously cast billet was C1.0, with no Class A segregation, and the center porosity level was 0.5. Inclusions of Class A, B, and C were all at level 0, and inclusions of Class D and Ds were all at level 0.5.

[0167] In the rolling process, the continuously cast billet is first fed into a heating furnace and heated to 1180℃ and held for 46 minutes. The total time the continuously cast billet is in the furnace is 266 minutes. Then, the continuously cast billet is first rough rolled to an intermediate billet with a thickness of 65mm. The initial rolling temperature of the rough rolling is 1050℃. Then, it is finished rolled into a steel plate with a thickness of 16mm. The initial rolling temperature of the finish rolling is 880℃ and the final rolling temperature is 830℃.

[0168] During the cooling process, the rolled steel plate immediately enters the ultra-fast cooling system and is rapidly cooled by water cooling. The water temperature of the steel plate is 780℃, the cooling rate is 10℃ / s, and the final cooling temperature of the steel plate is 500℃.

[0169] In the quenching process, the cooled steel plate is placed in a heating furnace and heated to 850℃ for 50 minutes. Afterward, it is rapidly water-cooled to room temperature in a quenching machine. The quenching machine is started 25 seconds before the steel plate enters it. The water-to-top ratio in the quenching machine is 0.80. The steel plate's running speed (v) in the quenching machine is 2.2 m / s. The steel plate passes through a high-pressure section and a low-pressure section. The water temperature is 20℃. The positional deviation of the water jets from the nozzles in the high-pressure section on the upper and lower surfaces of the steel plate is 0.6 mm. The water pressure in the high-pressure section is 0.80 MPa, and the water volume is 6300 m³ / h. 3 / h; the water pressure in the low-pressure section is 0.40MPa, and the water volume is 5800m³ / h. 3 / h, the cooling rate of water cooling is 17℃ / s.

[0170] In the tempering process, the steel plate that has undergone quenching is sent into a heating furnace for heating and then held at a temperature of 200℃ for 100 minutes.

[0171] Samples of the finished steel plates were taken and tested, and the results were as follows:

[0172] (1) such as Figure 3 As shown, the metallographic structure of the steel plate is a composite structure of lath martensite, acicular martensite, ferrite, and retained austenite. The sum of the area ratios of lath martensite and acicular martensite in the metallographic sampling surface is approximately 93%, ferrite accounts for approximately 3%, and retained austenite accounts for approximately 4%.

[0173] (2) The yield strength is 1126MPa, the tensile strength is 1430MPa, the hardness is 438HBW, the elongation is 12%, and the impact energy at -40℃ is AKV 55J; the steel plate is cold-bent 180° with a bending mandrel diameter D=3a and there are no cracks on the surface.

[0174] (3) The strength-ductility product of the steel plate is 17.6 GPa·%, and the residual stress is 180 MPa.

[0175] (4) Perform cold crack welding tests on the steel plate according to GB 4675.1-84 "Weldability Test - Test Method for Cracks in Y-groove Welding", see [link to relevant documentation]. Figure 4 If welding is performed without preheating, cracks will appear on the cross-section of the steel plate.

[0176] (5) The steel plate is subjected to gas-shielded flux-cored wire arc welding without preheating. The ambient temperature is ≥15℃. For the root pass welding, low-hydrogen welding materials of grade 50kg or below are used, with a diffused H content of 4mL / 100g. The welding current is ≤200A, the welding voltage is ≤26V, and the welding heat input is ≤18kJ / cm. For the fill and cover passes welding, low-hydrogen welding materials of grade 80kg or below are used, with a diffused H content of 4mL / 100g. The welding current is ≤260A, the welding voltage is ≤31V, and the welding heat input is ≤25kJ / cm. The interpass temperature is 200~250℃. Delayed cracks occur in the welded joint. The tensile strength of the welded joint is 857MPa. The impact energy at -20℃ is AKV=22J. When the plate is bent 90° inside and outside with a bending mandrel diameter D=6a, the steel plate breaks.

[0177] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. 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.

[0178] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for producing steel plates, characterized in that, This includes the sequential processes of billet preparation, rolling, cooling, quenching, and tempering. In the billet preparation process, a continuous casting billet is prepared and then placed in a heat-insulating pit for heat preservation. The temperature in the heat-insulating pit is 630~670℃, and the heat preservation time is t≥d. 2 / 40D, in seconds; where d is the thickness of the continuously cast billet in mm; and D is the hydrogen diffusion coefficient in the continuously cast billet in mm. 2 / s; In the rolling process, the continuously cast billet is heated and rolled into a steel plate; In the cooling process, the steel plate is cooled; In the quenching process, the cooled steel plate is placed in a heating furnace for heating and then held at a temperature of (A). c3 -30)℃~(A c3 -10)℃, furnace time was (1.5a+20)min~(1.5a+30)min, followed by rapid water cooling to room temperature, with a cooling rate of (V B +15) ℃ / s~(V B +20)℃ / s; In the tempering process, the steel plate that has undergone the quenching process is sent into a heating furnace for heating and then held at a temperature of (Ms-235)℃~(Ms-210)℃, and the time in the furnace is (5a+10)min~(5a+30)min. The chemical composition of the continuously cast billet, by mass percentage, includes: C 0.20~0.23%, Si 0.28~0.36%, Mn 0.90~1.10%, Cr 0.35~0.42%, Ti 0.012~0.019%, Alt 0.021~0.049%, B 0.0016~0.0021%, Zr 0.012~0.019%, Ca 0.0022~0.0052%, N 20~45ppm, O≤15ppm, H≤1.5ppm, P≤0.008%, S≤0.0012%, with the remainder being Fe and unavoidable impurities, and satisfying the following conditions: carbon equivalent CEV is 0.42~0.52%, Mn equivalent ∑Mn is 1.20~1.35%, and hardenability index J≥2.1; CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Cu+Ni) / 15; ∑Mn=Mn+3.28Mo+0.64Cr+0.5W+0.37Ni+0.23Cu+0.03Si+0.1(Alt+V+Ti+Nb); J = (1.24C - 0.95C) 2 )×(0.7Si+1)×(3.33Mn+1)×(2.16Cr+1)×(3Mo+1)×(0.36Ni+1)×(0.37Cu+1)×(1.73V+1)×(1.55Alt+1) Austenitizing end temperature A c3 =912-250C-16Mn+48Si-2Cr-16Ni+95V+96Ti+210Alt-10Cu; Critical cooling rate V for bainite formation B =e (13.08-8.8C-1.07Mn-0.7Ni-0.57Cr-9.2Mo-366B) / 3600; Martensitic transformation onset temperature Ms = 561-474°C - 33Mn-17Cr-17Ni-21Mo; 'a' represents the thickness of the steel plate, in mm.

2. The method for producing steel plates according to claim 1, characterized in that, In the quenching process, water cooling is carried out on a quenching machine with a water-to-water ratio of 0.75 to 0.

85. The running speed v of the steel plate in the quenching machine is 34 / a to (34 / a+0.4) m / s.

3. The method for producing steel plates according to claim 2, characterized in that, In the quenching process, the quenching machine is turned on 20-30 seconds before the steel plate enters the quenching machine.

4. The method for producing steel plates according to claim 1, characterized in that, In the quenching process, the steel plate passes through a high-pressure section and a low-pressure section during water cooling. The water pressure in the high-pressure section is 0.79~0.81MPa, and the water volume is 6100~6400m³. 3 / h; the water pressure in the low-pressure section is 0.39~0.41MPa, and the water volume is 5600~5900m³. 3 / h.

5. The method for producing steel plates according to claim 1, characterized in that, In the rolling process, the continuously cast billet is heated in a heating furnace, and the soaking temperature is (T). Nb(C,N) +30)℃~(T Nb(C,N) +60)℃, the soaking time is 0.1d~0.5dmin, and the total time in the furnace is 1.1d~1.5dmin; Among them, T Nb(C,N) =6770 / (2.26 lg((C+12N / 14)×Nb))-273.

6. The method for producing steel plates according to claim 1, characterized in that, In the rolling process, the continuously cast billet is heated and then subjected to rough rolling and finish rolling in sequence to form the steel plate. The starting rolling temperature for rough rolling is (T). Nb(C,N) -130)℃~(T Nb(C,N) -50)℃, the initial rolling temperature for finishing rolling is (T nr -20)℃~(T nr +10)℃, final rolling temperature is (A) r3 +60)℃~(A r3 +100)℃; Among them, T Nb(C,N) =6770 / (2.26 lg((C+12N / 14)×Nb))-273; recrystallization temperature T nr =887+464C+6445Nb-644 +890Ti+363Alt-357Si; A r3 =910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, A r3 This is the temperature at which ferrite precipitates from austenite during the cooling process.

7. The method for producing steel plates according to claim 1, characterized in that, In the cooling process, the cooling rate is (V B +5) ℃ / s~(V B +10)℃ / s.

8. A steel plate, characterized in that, The steel plate is prepared by the steel plate production method described in any one of claims 1 to 7.

9. The steel plate according to claim 8, characterized in that, The steel plate has a thickness 'a' of 6~30mm, a yield strength ≥1100MPa, a tensile strength ≥1400MPa, a hardness of 420~460HBW, an elongation ≥12%, and an impact energy AKV ≥50J at -40℃; it can be cold-bent 180° with a bending mandrel diameter D=3a, and the steel plate surface is free of cracks; the steel plate has a strength-ductility product ≥16.5GPa·%, and a residual stress ≤150MPa.

10. The steel plate according to claim 8, characterized in that, When the steel plate was subjected to a cold crack welding test, it was welded without preheating, and no cracks appeared on the surface, root, or cross-section of the steel plate. The steel plate was subjected to gas-protected flux-cored wire arc welding without preheating, and the welded joint showed no delayed cracking.

11. A preheat-free welding method for steel plates as described in any one of claims 8 to 10, characterized in that, The steel plate is subjected to gas-shielded flux-cored wire arc welding without preheating. The ambient temperature is ≥15℃. For the root pass, low-hydrogen welding materials of 50kg grade or below are used, with a diffused H content of ≤5mL / 100g. The welding current is controlled at ≤200A, welding voltage at ≤26V, and welding heat input at ≤18kJ / cm. For the fill and cover passes, low-hydrogen welding materials of 80kg grade or below are used, with a diffused H content of ≤5mL / 100g. The welding current is controlled at ≤260A, welding voltage at ≤31V, and welding heat input at ≤25kJ / cm. The interpass temperature is 200~250℃.

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