Steel sheet and method for producing the same

Abstract

This application provides a method for producing steel plates, which involves heating, rolling, cooling, and heat treatment of continuously cast billets. The hot zone temperature of the continuously cast billets is T2+60~T1-260℃; the initial rolling temperature in the recrystallization zone is T2~T2-40℃, and the final rolling temperature is T3+50~T3+80℃; the initial rolling temperature in the non-recrystallization zone is T3-80~T3-100℃, and the final rolling temperature is T4+40~T4+60℃; the initial water cooling temperature is >T4+20℃, and the final cooling temperature is T5+30~T5+60℃. The steel plate is quenched and tempered sequentially. When the target thickness t of the steel plate is 20≤t<40mm, the quenching temperature is 910℃~920℃, and the furnace time is 1.6~2.0min / mm; the tempering temperature is 550~600℃, and the furnace time is 2.0~3.0min / mm. When the target thickness t of the steel plate is 40≤t≤60mm, the quenching temperature is 920~930℃, and the furnace time is 1.6~2.0min / mm; the tempering temperature is 500~550℃, and the furnace time is 1.8~2.5min / mm.

Description

Technical Field

[0001] This application relates to the field of iron and steel smelting technology, and in particular to a steel plate and a method for producing the same. Background Technology

[0002] Traditional quenched and tempered steels typically only guarantee impact energy at -20°C or -40°C, which is insufficient to meet the safety reserve requirements for winter operation of hydropower stations in Northeast and Northwest my country and high-altitude frigid regions (where temperatures can drop below -50°C). Under these ultra-low temperature conditions, conventional steels often experience brittle cleavage fracture. Furthermore, due to the thickness effect of cooling rate, coarse upper bainite or Widmanstätten structure easily forms in the core, resulting in significantly lower low-temperature toughness in the core compared to the surface, failing to meet the requirements for full-section safety assessment. In addition, these steels usually have a high nickel content, leading to high costs and making them uneconomical for large-scale use in large-scale hydropower projects. Summary of the Invention

[0003] The purpose of this application is to provide a method for producing steel plates that solves the problems of complex processes and insufficient toughness at ultra-low temperatures in the prior art by controlling the rolling and heat treatment processes.

[0004] To achieve one of the aforementioned objectives, one embodiment of this application provides a method for producing steel plates, which involves obtaining a continuously cast billet through heating, rolling, cooling, and heat treatment processes.

[0005] During the heating process, the continuously cast billet is placed in the heating furnace, and the temperature of the soaking zone is T2+60℃~T1-260℃; In the rolling process, the heated continuous casting billet is rolled in the recrystallization zone and the non-recrystallization zone to obtain steel plate; the initial rolling temperature of the recrystallization zone is T2℃~T2-40℃, and the final rolling temperature is T3+50℃~T3+80℃; the initial rolling temperature of the non-recrystallization zone is T3-80℃~T3-100℃, and the final rolling temperature is T4+40℃~T4+60℃. During the cooling process, the rolled steel plate is water-cooled, with an initial cooling temperature of >T4+20℃ and a final cooling temperature of T5+30℃~T5+60℃. In the heat treatment process, the steel plate, after being cooled to room temperature by water, is quenched and tempered successively. When the target thickness t of the steel plate satisfies 20mm≤t<40mm, the quenching temperature is 910℃~920℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 550℃~600℃, and the furnace time is 2.0min / mm~3.0min / mm. When the target thickness t of the steel plate meets the requirement of 40mm≤t≤60mm, the quenching temperature is 920℃~930℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 500℃~550℃, and the furnace time is 1.8min / mm~2.5min / mm. T1=1536-415.5[C]-12.3[Si]-6.8[Mn]-124.5[P]-183.9[S]-4.3[Ni]-1.4[Cr]-4.1[Al],

[0006] T3 = 887 + 464[C] + (6445[Nb] - 644) )+(732[V]-230 )+890[Ti]+363[Al]-357[Si], T4=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo], T5=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo], Among them, [C], [Si], [Mn], [P], [S], [Ni], [Cr], [Al], [Nb], [V], [Ti], [Cu], and [Mo] are 100 times the mass percentage of each element in the steel plate, and [NbC] is 100 times the mass percentage of NbC in the steel plate.

[0007] In one embodiment of this application, in the cooling process, after water cooling is completed, the steel plate is taken off the production line and sent into an insulation box. The temperature of the steel plate when it is taken off the production line is 380℃~420℃. It is stacked in the insulation box at a temperature above 350℃ for a stacking time of not less than 1.2h / mm. The mm in the unit is the unit of the target thickness of the steel plate.

[0008] In one embodiment of this application, during the cooling process, the ratio of water volume below the steel plate to water volume above the steel plate is 1.1 to 1.3, the water pressure is >0.2MPa, and the forward speed of the steel plate is 0.8m / s to 1.2m / s.

[0009] In one embodiment of this application, during the quenching heat treatment, the steel plate is rapidly transported to the water inlet of the cooling device after exiting the furnace, with a temperature drop ≤30℃, a high-pressure section water pressure of 0.7MPa~0.8MPa, and a water volume of 4200m³. 3 / h~5400m 3 / h, the ratio of water flow below the steel plate to water flow above it is 1.2~1.3; the water pressure in the low-pressure section is 0.4~0.5MPa, and the total water flow is 6000m³. 3 / h~7000m 3 / h, the ratio of water flow below the steel plate to water flow above the steel plate is 1.2~1.3.

[0010] In one embodiment of this application, in the rolling process, the first pass reduction in the recrystallization zone is ≤20mm, and the single pass reduction for at least three subsequent passes is >26mm.

[0011] In one embodiment of this application, during the rolling process, the intermediate billet obtained after the recrystallization zone rolling is 2.5t~4.0t in mm, and the intermediate billet swings back and forth on the roller table to wait for heating.

[0012] In one embodiment of this application, the total heating time in the heating process is 460 min to 550 min, wherein the soaking time is ≥ 60 min.

[0013] In one embodiment of this application, during the heating process, the air-fuel ratio in the soaking zone of the heating furnace is ≤1.0, the furnace pressure is in a slightly positive pressure state of 10Pa~20Pa, and the furnace atmosphere is reducing.

[0014] In one embodiment of this application, descaling is further included after the heating process. After the continuously cast billet is heated and removed from the heating furnace, a first descaling is performed with a water pressure of 5 MPa to 18 MPa and a nozzle angle of 15°. A second coarse descaling is performed at intervals of 0.5s to 1.0s, with a descaling pressure of 15MPa to 18MPa and a nozzle angle of 15°. .

[0015] In one embodiment of this application, the continuously cast billet is obtained through hot metal desulfurization, converter refining, LF refining, RH vacuum treatment, and continuous casting processes, wherein... In the hot metal desulfurization process, the sulfur content of the hot metal after desulfurization is <0.005%, the hot metal temperature is maintained >1300℃, and slag is removed after desulfurization; In the converter refining process, nickel plates and ferromolybdenum alloys are added to the converter along with the scrap steel. Ferrosilicon, metallic manganese, aluminum blocks, and lime are added during tapping. The amount of aluminum blocks added is based on the oxygen content of the molten steel: 2.3 kg / ton for 0-500 ppm oxygen content, 2.4 kg / ton for 501-600 ppm oxygen content, 2.5 kg / ton for 601-700 ppm oxygen content, 2.6 kg / ton for 701-800 ppm oxygen content, and 2.7 kg / ton for above 800 ppm oxygen content. Argon gas is blown throughout the tapping process, with the pressure controlled at 0.5 MPa~0.6 MPa, adjusted to 0.4 MPa~0.5 MPa when three-quarters of the steel has been tapped. In the LF refining process, after the molten steel arrives at the station, a steel sample is taken after 4 to 5 minutes of power-on. The alloy composition is adjusted based on the temperature measurement and analysis results. Pure calcium wire is fed 2 to 3 minutes before tapping, with a feeding rate of 120m / heat to 150m / heat and a wire feeding speed of 1.4m / s to 1.6m / s. The tapping temperature is maintained at 1601℃ to 1621℃. In the RH vacuum treatment process, the net circulation time after alloying shall not be less than 10 minutes, the soft stirring shall not be less than 12 minutes, the vacuum degree ≤ 2mbar time shall not be less than 15 minutes, and the hydrogen shall be determined before the molten steel breaks the vacuum to ensure that H ≤ 1.5ppm; In the continuous casting process, a full protective casting process is adopted, using a long nozzle and argon seal, tundish covering agent, and submerged nozzle. The tundish covering agent adopts a double-layer insulation structure of alkaline covering agent and carbonized rice husk. The argon blowing flow rate of the long nozzle is 150L / min ~ 200L / min, the argon blowing flow rate of the stopper rod and submerged nozzle is 3L / min ~ 6L / min, and the slab drawing speed is 0.55m / min ~ 0.65m / min.

[0016] This application also provides a steel plate produced by the aforementioned steel plate production method, the chemical composition of which, by weight percentage, includes: C: 0.07%~0.09%, Si: 0.15%~0.22%, Mn: 1.30%~1.40%, P<0.012%, S<0.003%, Cr: 0.20%~0.28%, Ni: 0.12%~0.20%, Mo: 0.10%~0.17%, Nb: 0.02%~0.03%, V: 0.032%~0.042%, Ti: 0.008%~0.016%, Al: 0.045%~0.070%.

[0017] In one embodiment of this application, the microstructure changes in a gradient from the surface of the steel plate to the core.

[0018] In one embodiment of this application, the surface of the steel plate is tempered lath bainite, the microstructure at the 1 / 4 thickness position is a mixture of tempered lath bainite and acicular ferrite, and the microstructure at the 1 / 2 thickness position is a mixture of acicular ferrite and fine-grained ferrite.

[0019] In one embodiment of this application, the steel plate has a yield strength > 490 MPa, a tensile strength > 610 MPa, an impact energy of > 200 J at -80°C at the 1 / 4 and 1 / 2 thickness positions, and an impact energy of > 200 J at -80°C after 5% strain aging of the surface.

[0020] One or more technical solutions provided in this application have at least the following technical effects or advantages: In the steel plate production method provided in this application, by controlling the rolling process in conjunction with quenching and tempering, a gradient microstructure of "surface tempered lath bainite - 1 / 4 thickness tempered lath bainite + acicular ferrite - core acicular ferrite + fine-grained ferrite" can be constructed in the thickness direction of the steel plate. This achieves excellent full-thickness impact toughness and surface strain-aged impact toughness (>200J) of 600MPa grade hydropower steel in an environment of -80℃, meeting the stringent requirements for material safety in hydropower projects in extremely cold regions. Attached Figure Description

[0021] Figure 1 The metallographic structures of the steel plate in Embodiment 1 of this application are those at the surface, at 1 / 4 of the thickness, and at 1 / 2 of the thickness.

[0022] Figure 2 The metallographic structures of the steel plate in Embodiment 2 of this application are at the surface, at 1 / 4 of the thickness, and at 1 / 2 of the thickness.

[0023] Figure 3 The metallographic structures of the steel plate in Embodiment 3 of this application are those at the surface, at 1 / 4 of the thickness, and at 1 / 2 of the thickness. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This application provides a method for producing steel plates, which involves heating, rolling, cooling, and heat treatment of continuously cast billets. During the heating process, the continuously cast billet is placed in the heating furnace, and the temperature of the soaking zone is T2+60℃~T1-260℃; In the rolling process, the heated continuous casting billet is rolled in the recrystallization zone and the non-recrystallization zone to obtain steel plate; the initial rolling temperature of the recrystallization zone is T2℃~T2-40℃, and the final rolling temperature is T3+50℃~T3+80℃; the initial rolling temperature of the non-recrystallization zone is T3-80℃~T3-100℃, and the final rolling temperature is T4+40℃~T4+60℃. During the cooling process, the rolled steel plate is water-cooled, with an initial cooling temperature of >T4+20℃ and a final cooling temperature of T5+30℃~T5+60℃. In the heat treatment process, the steel plate, after being cooled to room temperature by water, is quenched and tempered successively. When the target thickness t of the steel plate satisfies 20mm≤t<40mm, the quenching temperature is 910℃~920℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 550℃~600℃, and the furnace time is 2.0min / mm~3.0min / mm. When the target thickness t of the steel plate meets the requirement of 40mm≤t≤60mm, the quenching temperature is 920℃~930℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 500℃~550℃, and the furnace time is 1.8min / mm~2.5min / mm. T1=1536-415.5[C]-12.3[Si]-6.8[Mn]-124.5[P]-183.9[S]-4.3[Ni]-1.4[Cr]-4.1[Al],

[0026] T3 = 887 + 464[C] + (6445[Nb] - 644) )+(732[V]-230 )+890[Ti]+363[Al]-357[Si], T4=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo], T5=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo], Where [C], [Si], [Mn], [P], [S], [Ni], [Cr], [Al], [Nb], [V], [Ti], [Cu], and [Mo] are 100 times the mass percentage of each element in the steel plate, and [NbC] is 100 times the mass percentage of NbC in the steel plate. [NbC] is calculated as the product of 100 times the mass percentage of Nb and 100 times the mass percentage of C in the steel plate.

[0027] In this application, during the heating process, the continuously cast billet is heated to above the solution temperature of Nb to ensure complete Nb solution. Simultaneously, the maximum temperature in the soaking zone is controlled at 260°C below the melting point of the continuously cast billet to prevent the coarsening and dissolution of TiN particles and the growth of austenite grains. Rolling in the recrystallization zone breaks up the austenite, resulting in fine and uniform grains. The initial rolling temperature in the non-recrystallization zone is 80°C to 100°C below the recrystallization temperature, ensuring that the intermediate billet is not within the recrystallization temperature range. The lower final rolling temperature further refines the microstructure, resulting in a high-strength and high-toughness steel plate.

[0028] After rolling, the steel plate is quickly immersed in water to avoid temperature drop and cool it above the bainite transformation temperature. This prevents the steel plate from entering the bainite phase region and forming coarse granular bainite. The final water cooling temperature of T5+30℃~T5+60℃ can obtain a fine ferrite structure, which can prevent the formation of coarse austenite during the heat treatment heating process, thus affecting the structure after subsequent cooling.

[0029] During the quenching process, the surface of the steel plate cools rapidly with a large degree of supercooling, causing austenite to transform into lath bainite. The core cools relatively slowly, at which point the pre-existing carbonitrides in the steel come into play, inducing austenite to preferentially nucleate and transform into interlocking acicular ferrite within the grains, rather than coarse upper bainite. From the surface to the core, the microstructure of the steel plate transforms from lath bainite to ferrite, forming a gradient microstructure along the thickness direction of the steel plate.

[0030] In some embodiments of this application, during the cooling process, after water cooling is completed, the steel plate is taken off the production line and sent into an insulation box. The temperature of the steel plate when it is taken off the production line is 380°C to 420°C. The steel plate is stacked in the insulation box at a temperature above 350°C for a stacking time of not less than 1.2 hours per mm. The mm in the unit is the unit of the target thickness of the steel plate.

[0031] After water cooling, the steel plate temperature is still relatively high, so it is transported to a cooling bed for air cooling. Once the steel plate cools to 380℃~420℃, it is placed in an insulated box and stacked for slow cooling at above 350℃. On the one hand, this temperature range is located in the ferrite / bainite phase region, where hydrogen solubility is low and diffusion coefficient is high, allowing for sufficient diffusion to remove hydrogen atoms from the steel and prevent white spots. On the other hand, it promotes the early precipitation of carbonitrides, preventing them from coarsening during subsequent rapid heating, and providing favorable microstructure inheritance conditions for grain refinement and acicular ferrite nucleation during the final quenching and tempering treatment.

[0032] During slow cooling of the stack, other steel plates at 400℃~420℃ need to be placed above and below the steel plate to utilize the residual heat of the steel plates for long-term slow cooling.

[0033] In some embodiments of this application, during the cooling process, the ratio of water volume below the steel plate to water volume above the steel plate is 1.1 to 1.3, the water pressure is >0.2 MPa, and the forward speed of the steel plate is 0.8 m / s to 1.2 m / s. The steel plate is rapidly cooled to 30°C to 60°C above the bainite transformation temperature to obtain a fine ferrite structure.

[0034] In some embodiments of this application, during the rolling process, the reduction in the first pass of the recrystallization zone is ≤20mm, and the reduction in each of the following at least three passes is >26mm. Rolling in the recrystallization zone, with at least three passes using large reductions after the first pass, utilizes the recrystallization capacity of the recrystallization zone. After the austenite grains are flattened and elongated, rapid recrystallization occurs, producing new, finer grains, thus gradually refining the grain size. The first pass uses a smaller reduction to avoid slippage of the steel sheet and ensure proper roll gripping.

[0035] In some embodiments of this application, during the rolling process, the intermediate billet obtained after the recrystallization zone rolling is 2.5t~4.0t in mm, and the intermediate billet swings back and forth on the roller table to wait for heating.

[0036] After the recrystallization zone rolling is completed, the steel plate is still above the recrystallization temperature and swings back and forth on the roller table to wait for the temperature to drop to 80℃~100℃ below the recrystallization temperature. This ensures that the temperature of all parts of the steel plate is not within the recrystallization temperature range before non-recrystallization rolling begins. The number of rolling passes in the non-recrystallization zone rolling is ≤7.

[0037] To prevent the formation of iron oxide scale, intermittent fine descaling is performed during the rolling process. Descaling is performed in the first pass, but not in the second and third passes. Subsequently, the surface condition of the steel plate is observed and descaling is performed multiple times, with an interval of no less than 3 minutes between two descaling operations.

[0038] In some embodiments of this application, during quenching heat treatment, the steel plate is rapidly transported to the water inlet of the cooling device after exiting the furnace, with a temperature drop of <30°C. The water pressure in the high-pressure section is 0.7MPa~0.8MPa, and the water volume is 4200m³. 3 / h~5400m 3 / h, the ratio of water flow below the steel plate to water flow above it is 1.2~1.3; the water pressure in the low-pressure section is 0.4~0.5MPa, and the total water flow is 6000m³. 3 / h~7000m 3 / h, the ratio of water flow below the steel plate to water flow above the steel plate is 1.2~1.3.

[0039] The high-pressure section uses a smaller volume of water to prevent the steel plate surface from over-quenching and hardening, while providing surface strain aging properties. The steel plate oscillates back and forth during cooling for 10-30 minutes, with the outlet water temperature below 100℃. To ensure effective quenching, all water collection pipes are fully opened when the steel plate enters the water and closed sequentially as the tail of the steel plate leaves its cooling zone.

[0040] In some embodiments of this application, the total heating time in the heating process is 460 min to 550 min, of which the heat soaking period is ≥ 60 min, in order to ensure the uniformity of heating.

[0041] In some embodiments of this application, in order to avoid the formation of surface iron oxide scale during the heating process, the air-fuel ratio in the soaking zone of the heating furnace is ≤1.0, the furnace pressure is in a slightly positive pressure state of 10Pa~20Pa, and the furnace atmosphere is reducing.

[0042] In some embodiments of this application, descaling is included after the heating process. After the continuously cast billet is heated and removed from the heating furnace, a first descaling is performed with a water pressure of 5 MPa to 18 MPa and a nozzle angle of 15°. A second coarse descaling is performed at intervals of 0.5s to 1.0s, with a descaling pressure of 15MPa to 18MPa and a nozzle angle of 15°. .

[0043] Utilizing the physical thermal effect, the iron oxide scale is loosened in the first step, reducing its bonding force with the slab substrate. In the second step, the temperature difference between the cooling water and the steel plate surface generates steam explosion, removing the stubborn iron oxide scale.

[0044] Descaling after heating results in a better surface quality of the continuously cast billet and prevents iron oxide scale from being pressed into the steel plate surface during rolling, which would affect the strain aging performance of the steel plate.

[0045] In some embodiments of this application, the continuously cast billet is obtained through hot metal desulfurization, converter refining, LF refining, RH vacuum treatment, and continuous casting processes, wherein... In the hot metal desulfurization process, the sulfur content of the hot metal after desulfurization is <0.005%, the hot metal temperature is maintained >1300℃, and the slag is removed after desulfurization.

[0046] In the converter refining process, nickel plates and ferromolybdenum alloys are added to the converter along with the scrap steel. When tapping the steel, ferrosilicon, metallic manganese, aluminum blocks, and lime are added. The amount of aluminum blocks added is based on the oxygen content of the molten steel: 2.3 kg / ton for oxygen content of 0-500 ppm, 2.4 kg / ton for oxygen content of 501-600 ppm, 2.5 kg / ton for oxygen content of 601-700 ppm, 2.6 kg / ton for oxygen content of 701-800 ppm, and 2.7 kg / ton for oxygen content above 800 ppm. Argon gas is blown throughout the tapping process, with the pressure controlled at 0.5 MPa~0.6 MPa. When three-quarters of the steel has been tapped, the pressure is adjusted to 0.4 MPa~0.5 MPa.

[0047] Nickel plates and ferromolybdenum alloys are added according to the target Ni and Mo content, at amounts of 1 kg / t and 3 kg / t respectively. The nickel plates contain ≥95% nickel, with the remainder being unavoidable impurities. The ferromolybdenum alloy contains 55-70% molybdenum and 25-40% iron, with the remainder being unavoidable impurities. Alloys and lime are added during tapping for alloying and slag formation. Aluminum blocks are added for deoxidation, based on the oxygen content in the molten steel. During tapping alloying, a higher argon gas pressure is used to promote alloy melting and dispersion, while a lower pressure is used for soft stirring to maintain uniformity and stability of the molten steel.

[0048] In the LF refining process, after the molten steel arrives at the station, a steel sample is taken after 4 to 5 minutes of power-on. The alloy composition is adjusted based on the temperature measurement and analysis results. Pure calcium wire is fed 2 to 3 minutes before tapping, with a feeding amount of 120m / heat to 150m / heat and a wire feeding speed of 1.4m / s to 1.6m / s. The tapping temperature is maintained at 1601℃ to 1621℃.

[0049] In the RH vacuum treatment process, the net circulation time after alloying shall not be less than 10 minutes, the soft stirring shall not be less than 12 minutes, the vacuum degree ≤ 2mbar time shall not be less than 15 minutes, and the hydrogen content shall be determined before the molten steel breaks the vacuum to ensure that H ≤ 1.5ppm.

[0050] In the continuous casting process, a full protective casting process is adopted, using a long nozzle and argon seal, tundish covering agent, and submerged nozzle. The tundish covering agent adopts a double-layer insulation structure of alkaline covering agent and carbonized rice husk. The argon blowing flow rate of the long nozzle is 150L / min~200L / min, the argon blowing flow rate of the stopper rod and submerged nozzle is 3L / min~6L / min, and the slab drawing speed is 0.55m / min~0.65m / min.

[0051] This application embodiment also provides a steel plate produced by the aforementioned steel plate production method, characterized in that its chemical composition, by weight percentage, includes: C: 0.07%~0.09%, Si: 0.15%~0.22%, Mn: 1.30%~1.40%, P<0.012%, S<0.003%, Cr: 0.20%~0.28%, Ni: 0.12%~0.20%, Mo: 0.10%~0.17%, Nb: 0.02%~0.03%, V: 0.032%~0.042%, Ti: 0.008%~0.016%, Al: 0.045%~0.070%.

[0052] The following is an explanation of the mechanisms of action of each element: C: Carbon is the foundation for ensuring strength. This invention strictly controls the carbon content to an ultra-low carbon level of less than 0.09%, aiming to reduce the ductile-brittle transition temperature from the fundamental matrix and significantly improve the sensitivity to welding cracks, so as to adapt to extremely cold working conditions of -80℃.

[0053] Silicon (Si): As the main deoxidizing element in steel, silicon acts as a deoxidizer and provides solid solution strengthening, and its content needs to be controlled within a low range. Low silicon content helps reduce red iron oxide scale on the surface, improves surface quality, and reduces the adverse effects on low-temperature toughness. Excessive silicon content will increase the ductile-brittle transition temperature. Therefore, the silicon content range of this invention is 0.15%~0.22%.

[0054] Mn: Manganese is a common strengthening element in steel. While improving strength, it can stabilize austenite, expand the austenite phase region, lower the phase transformation temperature, and refine the grains. However, excessive manganese content can easily lead to segregation and inclusions, impairing the steel's properties. This invention sets the manganese content at 1.30%~1.40%, a range that balances strength with the risk of central segregation.

[0055] P: Phosphorus is the main impurity that causes low-temperature brittleness and must be controlled at extremely low levels to prevent grain boundary segregation.

[0056] S: Sulfur is also a major impurity element in steel. It easily forms long MnS inclusions, which need to be controlled at extremely low levels to ensure transverse impact toughness.

[0057] Cr: Chromium is a key element for improving hardenability, promoting bainite transformation, and improving the performance of steel.

[0058] Ni: Nickel is a key element for improving toughness. Nickel can promote cross-slip of screw dislocations, effectively reducing the ductile-brittle transition temperature without significantly increasing costs. This invention breaks away from the traditional approach of relying on high nickel content (>0.5%) for low-temperature steel. It adds only trace amounts of nickel to help improve the toughness of the matrix, ensuring low-temperature performance through microstructure regulation rather than simple solid solution strengthening, thereby significantly reducing costs.

[0059] Mo: Molybdenum is a key element to ensure a balance between strength and toughness. It can improve tempering resistance, suppress tempering brittleness, and promote the precipitation of fine carbonitrides.

[0060] Niobium (Nb) is an effective element for refining grain size. Niobium can increase the recrystallization temperature through the solute drag effect, ensuring austenite grain refinement and cumulative deformation of the non-recrystallized region at high temperatures. During cooling, it can increase ferrite nucleation sites, significantly refining the grain size.

[0061] Vanadium (V) is an effective element for precipitation strengthening. During tempering, fine V (C, N) particles precipitate, compensating for the strength loss caused by low carbon content.

[0062] Ti: Titanium is an effective element for refining grains. On the one hand, it forms TiN pins austenite grain boundaries, preventing coarsening upon heating; on the other hand, Ti carbonitrides act as nucleation sites for acicular ferrite (AF), which is crucial for core toughness.

[0063] Al: Aluminum is an effective element for deoxidation and grain refinement; a small amount can simultaneously improve toughness and strength. This invention uses a higher-than-conventional acid-soluble aluminum content. The addition of high aluminum improves the hardenability of the steel, and combined with the pinning effect of carbonitrides formed by trace amounts of titanium, it avoids the formation of coarse ferrite / pearlite during the slow cooling stage of the core, promotes the formation of fine needle-like structures, and thus improves the toughness of the core.

[0064] In some embodiments of this application, the microstructure changes in a gradient from the surface of the steel plate to the core. Specifically, the surface of the steel plate is tempered lath bainite, the microstructure at the 1 / 4 thickness position is a mixture of tempered lath bainite and acicular ferrite, and the microstructure at the 1 / 2 thickness position is a mixture of acicular ferrite and fine-grained ferrite.

[0065] The steel plate has a yield strength >490MPa, tensile strength >610MPa, impact energy at 1 / 4 and 1 / 2 of the thickness at -80℃ >200J, and impact energy at 5% strain aging of the surface at -80℃ >200J.

[0066] This application employs a low-carbon, low-sulfur, low-phosphorus, and micro-alloyed composition system, combined with the aforementioned rolling, cooling, and heat treatment, to obtain a gradient microstructure. This results in an impact energy exceeding 200J at -80℃, far surpassing the standard of conventional hydropower steel at -40℃, significantly improving the safety of hydropower stations in extreme climates. Furthermore, it exhibits strong resistance to strain aging; even after 5% strain aging, the impact energy at -80℃ remains >200J, indicating that the steel plate retains extremely high toughness reserves after being rolled into tubes, ensuring the safety of pumped-storage hydropower stations in extremely cold regions.

[0067] The technical solution of this application will be further described below with reference to some specific embodiments.

[0068] Example 1 A 320mm thick continuously cast billet was obtained through steelmaking and continuous casting. Its chemical composition is: C: 0.07%, Si: 0.16%, Mn: 1.35%, P: 0.010%, S: 0.001%, Cr: 0.23%, Ni: 0.15%, Mo: 0.14%, Nb: 0.025%, V: 0.036%, Ti: 0.010%, Al: 0.055%.

[0069] The slab is heated to 1200℃, and the total time in the furnace is 500 minutes, including a soaking time of 70 minutes.

[0070] After exiting the furnace, the initial rolling temperature is 1050℃, the final rolling temperature is 995℃, the reduction in the first pass is 18mm, and the reduction in the second, third and fourth passes is 30, 28 and 27mm respectively.

[0071] The thickness is 80mm, the initial rolling temperature is 840℃, the final rolling temperature is 810℃, and there are a total of 7 rolling passes in the second stage.

[0072] The initial cooling temperature after rolling is 780℃, and the final cooling temperature is 700℃.

[0073] The steel plates are rolled off the production line at 400℃, and then stacked in an insulated box at 400℃ for 34 hours to cool.

[0074] The heat treatment quenching temperature was 910℃, the furnace time was 56 minutes, the water immersion temperature of the steel plate after exiting the furnace was 885℃, and the final cooling temperature was <100℃. The tempering temperature was 600℃, the furnace time was 84 minutes, and the steel plate was naturally cooled to room temperature after exiting the furnace, finally obtaining a 28mm thick finished steel plate.

[0075] Sampling and testing revealed that the surface layer consisted entirely of tempered lath bainite. At the 1 / 4 position, the composition was 38.4% tempered lath bainite + 61.6% acicular ferrite; at the 1 / 2 position, it was 46.3% acicular ferrite + 53.7% fine-grained ferrite. Metallographic images are shown below. Figure 1 As shown. The steel plate has a yield strength of 520 MPa, a tensile strength of 652 MPa, and impact energy at -80℃ at 1 / 4 of the thickness position is 301 J, 323 J, and 298 J, respectively. The impact energy at -80℃ at 1 / 2 of the thickness position is 285 J, 294 J, and 302 J, respectively. After 5% strain aging on the surface, the impact energy at -80℃ is 235 J, 221 J, and 257 J.

[0076] Example 2 A 320mm thick continuously cast billet was obtained through steelmaking and continuous casting. Its chemical composition is as follows: C: 0.08%, Si: 0.17%, Mn: 1.33%, P: 0.011%, S: 0.002%, Cr: 0.25%, Ni: 0.13%, Mo: 0.16%, Nb: 0.023%, V: 0.033%, Ti: 0.009%, Al: 0.057%.

[0077] The slab is heated to 1200℃ and the total time in the furnace is 520 minutes, including a soaking time of 75 minutes.

[0078] After exiting the furnace, the initial rolling temperature is 1070℃, the final rolling temperature is 1000℃, the reduction in the first pass is 20mm, and the reduction in the second, third and fourth passes is 30, 31 and 30mm respectively.

[0079] The thickness is 120mm, the initial rolling temperature is 840℃, the final rolling temperature is 800℃, and there are a total of 6 rolling passes in the second stage.

[0080] The initial cooling temperature after rolling is 785℃, and the final cooling temperature is 690℃.

[0081] After rolling, the steel plates are placed in an insulated box at 400°C and stacked for 50 hours to cool.

[0082] The heat treatment quenching temperature is 920℃, the furnace time is 80 minutes, the water temperature of the steel plate after exiting the furnace is 900℃, and the final cooling temperature is <100℃.

[0083] The tempering temperature is 500℃, the furnace time is 100 minutes, and after being taken out of the furnace, it is naturally cooled to room temperature to finally obtain a 40mm thick finished steel plate.

[0084] Sampling and testing revealed that the surface layer consisted entirely of tempered lath bainite. At the 1 / 4 position, the composition was 30.5% tempered lath bainite + 69.5% acicular ferrite; at the 1 / 2 position, it was 33.2% acicular ferrite + 66.8% fine-grained ferrite. Metallographic images are shown below. Figure 2 As shown. The steel plate has a yield strength of 510 MPa, a tensile strength of 640 MPa, and impact energy at -80℃ at 1 / 4 of the thickness position is 288 J, 275 J, and 280 J, respectively. The impact energy at -80℃ at 1 / 2 of the thickness position is 215 J, 243 J, and 224 J, respectively. After 5% strain aging of the surface, the impact energy at -80℃ is 220 J, 211 J, and 233 J.

[0085] Example 3 A 320mm thick continuously cast billet was obtained through steelmaking and continuous casting. Its chemical composition is: C: 0.09%, Si: 0.16%, Mn: 1.34%, P: 0.010%, S: 0.001%, Cr: 0.26%, Ni: 0.19%, Mo: 0.17%, Nb: 0.026%, V: 0.034%, Ti: 0.010%, Al: 0.060%.

[0086] The slab is heated to 1200℃ and the total time in the furnace is 530 minutes, including a soaking time of 75 minutes.

[0087] After exiting the furnace, the initial rolling temperature is 1085℃, the final rolling temperature is 1010℃, the reduction in the first pass is 20mm, and the reduction in the second, third and fourth passes is 30mm, 28mm and 27mm respectively.

[0088] The thickness is 150mm, the initial rolling temperature is 850℃, the final rolling temperature is 800℃, and there are a total of 7 rolling passes in the second stage.

[0089] The initial cooling temperature after rolling is 775℃, and the final cooling temperature is 690℃.

[0090] After rolling, the steel plates are placed in an insulated box at an temperature of 390℃ and stacked for 60 hours to cool.

[0091] The heat treatment quenching temperature is 920℃, the furnace time is 90 minutes, the water temperature of the steel plate after exiting the furnace is 900℃, and the final cooling temperature is <100℃.

[0092] The tempering temperature is 500℃, the furnace time is 100 minutes, and after being taken out of the furnace, it is naturally cooled to room temperature to finally obtain a 50mm thick finished steel plate.

[0093] Sampling and testing revealed that the surface layer consisted entirely of tempered lath bainite. At the 1 / 4 position, the composition was 26.2% tempered lath bainite + 73.8% acicular ferrite, while at the 1 / 2 position, it was 31.1% acicular ferrite + 68.9% fine-grained ferrite. Metallographic images are shown below. Figure 3 As shown. The steel plate has a yield strength of 505 MPa, a tensile strength of 621 MPa, and impact energy at -80℃ at 1 / 4 of the thickness position is 260 J, 275 J, and 263 J, respectively. The impact energy at -80℃ at 1 / 2 of the thickness position is 210 J, 202 J, and 212 J, respectively. After 5% strain aging of the surface, the impact energy at -80℃ is 224 J, 218 J, and 207 J.

[0094] Tensile tests were performed according to GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Tests at room temperature" on an Instron 5585H 250 kN electronic universal testing machine. Impact tests were performed according to GB / T 229-2020 "Metallic materials, Charpy impact test" on an Instron IMP450 J instrumented pendulum impact testing machine.

[0095] 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.

[0096] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for producing steel plates, characterized in that, It is obtained by heating, rolling, cooling, and heat treatment of continuously cast billets, wherein... During the heating process, the continuously cast billet is placed in the heating furnace, and the temperature of the soaking zone is T2+60℃~T1-260℃; In the rolling process, the heated continuous casting billet is rolled in the recrystallization zone and the non-recrystallization zone to obtain steel plate; the initial rolling temperature of the recrystallization zone is T2℃~T2-40℃, and the final rolling temperature is T3+50℃~T3+80℃; the initial rolling temperature of the non-recrystallization zone is T3-80℃~T3-100℃, and the final rolling temperature is T4+40℃~T4+60℃. During the cooling process, the rolled steel plate is water-cooled, with an initial cooling temperature of >T4+20℃ and a final cooling temperature of T5+30℃~T5+60℃. In the heat treatment process, the steel plate, after being cooled to room temperature by water, is quenched and tempered successively. When the target thickness t of the steel plate satisfies 20mm≤t<40mm, the quenching temperature is 910℃~920℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 550℃~600℃, and the furnace time is 2.0min / mm~3.0min / mm. When the target thickness t of the steel plate meets the requirement of 40mm≤t≤60mm, the quenching temperature is 920℃~930℃, and the furnace time is 1.6min / mm~2.0min / mm; the tempering temperature is 500℃~550℃, and the furnace time is 1.8min / mm~2.5min / mm. T1=1536-415.5[C]-12.3[Si]-6.8[Mn]-124.5[P]-183.9[S]-4.3[Ni]-1.4[Cr]-4.1[Al], T3=887+464[C]+(6445[Nb]-644 )+(732[V]-230 )+890[Ti]+363[Al]-357[Si], T4=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo], T5=830-270[C]-90[Mn]-37[Ni]-70[Cr]-83[Mo], Among them, [C], [Si], [Mn], [P], [S], [Ni], [Cr], [Al], [Nb], [V], [Ti], [Cu], and [Mo] are 100 times the mass percentage of each element in the steel plate, and [NbC] is 100 times the mass percentage of NbC in the steel plate.

2. The method for producing steel plates according to claim 1, characterized in that, In the cooling process, after water cooling is completed, the steel plate is taken off the production line and sent to the heat preservation box. The temperature of the steel plate when it is taken off the production line is 380℃~420℃. It is stacked in the heat preservation box at a temperature above 350℃ for a stacking time of not less than 1.2h / mm. The mm in the unit is the unit of the target thickness of the steel plate.

3. The method for producing steel plates according to claim 2, characterized in that, During the cooling process, the ratio of water volume below the steel plate to water volume above the steel plate is 1.1 to 1.3, the water pressure is >0.2MPa, and the forward speed of the steel plate is 0.8m / s to 1.2m / s.

4. The method for producing steel plates according to claim 1, characterized in that, During quenching heat treatment, the steel plate is transported to the water inlet of the cooling device after being discharged from the furnace, the temperature drop is ≤30℃, the water pressure of the high-pressure section is 0.7MPa~0.8MPa, the water quantity is 4200m 3 / h~5400m 3 / h, the ratio of the water quantity below the steel plate to the water quantity above the steel plate is 1.2~1.3; the water pressure of the low-pressure section is 0.4~0.5MPa, the total water quantity is 6000m 3 / h~7000m 3 / h, the ratio of the water quantity below the steel plate to the water quantity above the steel plate is 1.2~1.

3.

5. The method for producing steel plates according to claim 1, characterized in that, In the rolling process, the first pass reduction in the recrystallization zone is ≤20mm, and the single pass reduction for at least the next 3 passes is >26mm.

6. The method for producing steel plates according to claim 5, characterized in that, In the rolling process, the intermediate billet obtained after the recrystallization zone rolling is 2.5t~4.0t thick, in mm, and the intermediate billet swings back and forth on the roller table to wait for heating.

7. The method for producing steel plates according to claim 1, characterized in that, In the heating process, the total heating time is 460 min to 550 min, of which the soaking time is ≥ 60 min.

8. The method for producing steel plates according to claim 7, characterized in that, During the heating process, the air-fuel ratio in the soaking zone of the heating furnace is ≤1.0, the furnace pressure is in a slightly positive pressure state, 10Pa~20Pa, and the furnace atmosphere is reducing.

9. The method for producing steel plates according to claim 1, characterized in that, The heating process is followed by descaling. After the continuously cast billet is heated and removed from the heating furnace, it undergoes a first descaling process with a water pressure of 5MPa~18MPa and a nozzle angle of 15°. A second coarse descaling is performed at intervals of 0.5s to 1.0s, with a descaling pressure of 15MPa to 18MPa and a nozzle angle of 15°. .

10. The method for producing steel plates according to claim 1, characterized in that, The continuously cast billet is obtained through hot metal desulfurization, converter refining, LF refining, RH vacuum treatment, and continuous casting processes. In the hot metal desulfurization process, the sulfur content of the hot metal after desulfurization is <0.005%, the hot metal temperature is maintained >1300℃, and slag is removed after desulfurization; In the converter refining process, nickel plates and ferromolybdenum alloys are added to the converter along with the scrap steel. Ferrosilicon, metallic manganese, aluminum blocks, and lime are added during tapping. The amount of aluminum blocks added is based on the oxygen content of the molten steel: 2.3 kg / ton for 0-500 ppm oxygen content, 2.4 kg / ton for 501-600 ppm oxygen content, 2.5 kg / ton for 601-700 ppm oxygen content, 2.6 kg / ton for 701-800 ppm oxygen content, and 2.7 kg / ton for above 800 ppm oxygen content. Argon gas is blown throughout the tapping process, with the pressure controlled at 0.5 MPa~0.6 MPa, adjusted to 0.4 MPa~0.5 MPa when three-quarters of the steel has been tapped. In the LF refining process, after the molten steel arrives at the station, a steel sample is taken after 4 to 5 minutes of power-on. The alloy composition is adjusted based on the temperature measurement and analysis results. Pure calcium wire is fed 2 to 3 minutes before tapping, with a feeding rate of 120m / heat to 150m / heat and a wire feeding speed of 1.4 to 1.6m / s. The tapping temperature is maintained at 1601℃ to 1621℃. In the RH vacuum treatment process, the net circulation time after alloying shall not be less than 10 minutes, the soft stirring shall not be less than 12 minutes, the vacuum degree ≤ 2mbar time shall not be less than 15 minutes, and the hydrogen shall be determined before the molten steel breaks the vacuum to ensure that H ≤ 1.5ppm; In the continuous casting process, a full protective casting process is adopted, using a long nozzle and argon seal, tundish covering agent, and submerged nozzle. The tundish covering agent adopts a double-layer insulation structure of alkaline covering agent and carbonized rice husk. The argon blowing flow rate of the long nozzle is 150L / min ~ 200L / min, the argon blowing flow rate of the stopper rod and submerged nozzle is 3L / min ~ 6L / min, and the slab drawing speed is 0.55m / min ~ 0.65m / min.

11. A steel plate produced by the method for producing steel plates according to claim 1, characterized in that, Its chemical composition, by weight percentage, includes: C: 0.07%~0.09%, Si: 0.15%~0.22%, Mn: 1.30%~1.40%, P<0.012%, S<0.003%, Cr: 0.20%~0.28%, Ni: 0.12%~0.20%, Mo: 0.10%~0.17%, Nb: 0.02%~0.03%, V: 0.032%~0.042%, Ti: 0.008%~0.016%, Al: 0.045%~0.070%.

12. The steel plate according to claim 11, characterized in that, The microstructure changes in a gradient from the surface of the steel plate towards the core.

13. The steel plate according to claim 12, characterized in that, The surface of the steel plate is tempered lath bainite, the microstructure at 1 / 4 of the thickness is a mixture of tempered lath bainite and acicular ferrite, and the microstructure at 1 / 2 of the thickness is a mixture of acicular ferrite and fine-grained ferrite.

14. The steel plate according to claim 13, characterized in that, The steel plate has a yield strength >490MPa, tensile strength >610MPa, impact energy at 1 / 4 and 1 / 2 of the thickness at -80℃ >200J, and impact energy at 5% strain aging of the surface at -80℃ >200J.

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