A high-strength steel plate for low-temperature engineering structure with 1300 MPa grade and a manufacturing method thereof

Abstract

This invention discloses a high-efficiency, low-cost 1300MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method, belonging to the field of low-carbon microalloyed steel production technology. It employs optimized steelmaking, heating, rolling, pre-straightening, relaxation, controlled cooling, hot straightening, and tempering processes. The controlled cooling utilizes an ultra-fast cooling + laminar flow cooling mode, and the steel plate contains Ce in its chemical composition. This ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1300MPa grade hot-rolled steel plates with a thickness of 10-50mm using online cooling processes.

Description

Technical Field

[0001] This invention belongs to the field of low-carbon microalloyed steel production technology, and specifically relates to a low-cost and high-efficiency rolled high-strength steel plate with a thickness of 10-50mm and a yield strength of 1300MPa for low-temperature engineering structures and its manufacturing method. Background Technology

[0002] Currently, 1300MPa grade structural steel is the world's highest yield strength structural steel suitable for engineering machinery, widely used in offshore cranes, concrete pump trucks, jack-up offshore platform legs, and coal mine hydraulic supports. To ensure high strength and toughness, ultra-high strength steel is currently produced by adding microalloying elements such as Nb, V, Cr, Mo, and Ni, as well as carbides, nitrides, or highly hardenable elements, and employing an offline quenching + low-temperature tempering heat treatment process. However, this increases product cost and delivery time. Therefore, there is an urgent need to optimize its production process. Currently, domestic and international steel mills use an online cooling + offline tempering process to produce 1300MPa grade high-strength steel with a thickness of 10-50mm. This process achieves a high-density dislocation tempered martensite + precipitated carbides microstructure through microstructural control, ensuring strength. However, due to limitations of the online cooling process, the uniformity of the microstructure along the thickness of the steel plate and the impact of the quenched martensite on deformation coordination result in significantly lower low-temperature impact toughness, plate shape, yield strength ratio, and flaw detection pass rate compared to the offline quenching production process. This limits the market promotion of low-cost 10-50mm thick 1300MPa grade low-temperature high-strength steel. Therefore, improving and resolving the problems arising in the manufacturing process of 10-50mm thick 1300MPa grade low-temperature high-strength steel plates, and reducing manufacturing costs, are critical issues that urgently need to be addressed in the mass production of 1300MPa grade thick plates for engineering structures.

[0003] Compared with existing technologies:

[0004] To date, there has been very little research, both domestically and internationally, on efficient and low-cost production methods for low-temperature engineering structural steel with a thickness of 10–50 mm and a strength of 1300 MPa. Prior to this invention, the master's thesis "Evolution of Microstructure and Properties and Strengthening Mechanism of Q890D Engineering Machinery Steel" (Northeastern University, 2021.6) primarily produced high-strength steel through offline quenching and tempering, leading to a significant increase in production cycle and cost, and failing to meet the requirements for low-temperature impact toughness at -40℃ and 1300MPa grade. While the offline quenching and tempering process disclosed in the above literature can address the low-temperature toughness problem of steel plates at -20℃, it is not suitable for controlling and solving the problem of using TMCP + tempering to replace the offline quenching and tempering process to produce steel plates with a yield strength of 1300MPa, excellent low-temperature toughness at -40℃, and a thickness of 10-50mm, with an efficient and low-cost production method. Using the technical solution provided by this invention, the above shortcomings can be effectively overcome, solving the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1300MPa grade low-temperature structural steel plates with a thickness of 10-50mm using online cooling processes for continuously cast billets with a thickness of 250mm or less. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a high-efficiency, low-cost 1300MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method. This is an E-grade micro-alloyed high-strength steel plate with excellent low-temperature toughness. The method adopts optimized steelmaking, heating, rolling, pre-straightening, controlled cooling, hot straightening, and tempering processes. The controlled cooling adopts an ultra-fast cooling + laminar flow cooling mode, and the chemical composition of the steel plate contains Ce. This method ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1300MPa grade hot-rolled steel plates with a thickness of 10-50mm using online cooling processes.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] This invention provides a high-efficiency, low-cost 1300MPa grade high-strength steel plate for low-temperature engineering structures. The chemical composition of the steel plate by weight percentage is: C 0.15%~0.20%, Si 0.25%~0.35%, Mn 1.20%~1.35%, P≤0.02%, S≤0.010%, Cr 0.45%~0.55%, Ni 0.2%~0.3%, Al 0.015%~0.035%, V 0.1%~0.2%, B 0.0015%~0.0020%, N 0.02%~0.03%, Ti 0.01%~0.02%, Mo 0.40%~0.60%, Ce 0.05%~0.10%, with the balance being Fe and unavoidable impurities.

[0008] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0009] Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.15%–0.20%.

[0010] Mn: Mn strengthens steel through solid solution treatment, compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to finer low-temperature transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, hindering the improvement of low-temperature toughness and compromising the uniformity of the cross-sectional microstructure. Therefore, the Mn content range in this invention is designed to be 1.2%–1.35%.

[0011] Si (Si) plays a role in deoxidation in steelmaking and improving the strength of the matrix. Increasing the Si content can purify ferrite and reduce the pearlite content, which is beneficial for reducing the Bauschinger effect in the matrix material; however, excessive Si will reduce the toughness of the heat-affected zone in the weld. Therefore, the Si content is set at 0.25% to 0.35% in this invention.

[0012] Ti and N: In addition to forming fine TiN particles to refine austenite grains, nitrogen in steel also readily forms BN with boron, affecting the yield of free boron and reducing the hardenability of the steel plate. Therefore, the bonding ability of TiN is greater than that of BN. Thus, the N content range selected in this invention is 0.012% to 0.03%, and the Ti content is 0.01% to 0.02%. The remaining Ti combines with C to form TiC, which plays a role in dispersion strengthening.

[0013] V (V): A strong solid nitrogen element, existing in the form of VN in continuously cast billets. Fine VN particles effectively inhibit austenite grain growth during reheating of the continuously cast billet. During the rolling stage, it can precipitate in the high-temperature austenite region, providing phase deformation nuclei for acicular ferrite; and during slow cooling, it forms nano-precipitates, significantly improving the strength of the steel and enhancing the impact toughness of the weld heat-affected zone. When the V content exceeds a certain value, the V particles coarsen, increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention selects a V content range of 0.1% to 0.2%.

[0014] Al: Commonly used as a deoxidizer in steel, it can also refine the microstructure if it forms AlN. When the Al content exceeds 0.035%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, deoxidation will be insufficient, and easily oxidized elements such as Ti will form oxides. Therefore, the lower limit for Al content is set at 0.015%.

[0015] Cr: It is a major element that can effectively improve hardenability, inhibit ferrite formation and promote bainite formation. It plays an important role in controlling phase transformation structure, promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, and improves the strength, plasticity and toughness of steel plate. The Cr content range selected in this invention is 0.45% to 0.55%.

[0016] Mo can significantly improve hardenability, increase strength, promote microstructure transformation at medium and low temperatures, and optimize the microstructure and properties of steel plates and heat-affected zones at welds. However, excessively high molybdenum content will increase production costs. Therefore, this invention controls the Mo content to be between 0.40% and 0.60%.

[0017] Ni: Nickel exhibits a similar phase transformation behavior to Mo in steel, which can lower the phase transformation temperature of steel, improve the microstructure, refine the grains, and increase the strength of steel while maintaining good plasticity and toughness. However, excessively high Ni content will increase production costs. Therefore, this invention controls the Ni content to 0.2%–0.3%.

[0018] B: A relatively effective element for significantly improving the hardenability of steel. It readily segregates at grain boundaries, preventing carbon precipitation. Even trace amounts of boron can have a significant effect. However, excessive boron content can easily form boron carbonitrides, reducing toughness and causing hot brittleness. The reasonable range is 0.0015% to 0.0020%.

[0019] Ce: Rare earth sulfur and oxides can easily become the nuclei of heterogeneous formation, refining the grains; at the same time, an appropriate amount of rare earth can refine the size of inclusions and improve their morphology; small rare earth inclusions pinned to the grain boundaries hinder austenite growth. In this invention, the rare earth Ce content is controlled at 0.05% to 0.10%.

[0020] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, due to considerations of smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.010%, respectively.

[0021] The steel plate of the present invention has a thickness of 10 to 50 mm, and is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of less than 250 mm, with water as the cooling medium.

[0022] The steel plate described in this invention is an E-grade microalloyed high-strength steel plate with excellent low-temperature toughness.

[0023] The objective of this invention is achieved through the following technical solution:

[0024] This invention provides a high-efficiency, low-cost manufacturing method for 1300MPa grade high-strength steel plates used in low-temperature engineering structures, comprising: steel smelting → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling and pre-straightening → relaxation, cooling and straightening → tempering; specifically including the following steps:

[0025] 1) Steel smelting to continuous casting: Smelting according to the following composition, with the following chemical composition by weight percentage: C 0.15%~0.20%, Si 0.25%~0.35%, Mn 1.20%~1.35%, P≤0.02%, S≤0.010%, Cr 0.45%~0.55%, Ni 0.2%~0.3%, Al 0.015%~0.035%, V 0.1%~0.2%, B 0.0015%~0.0020%, N 0.02%~0.03%, Ti 0.01%~0.02%, Mo 0.40%~0.60%, Ce 0.05%~0.10%, with the balance being Fe and unavoidable impurities. The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. After RH vacuum degassing, Al is added according to Als requirements; Ti-Fe is added for micro-titanium treatment, followed by B-Fe alloy; this ensures the iron is added to the molten steel and avoids floating on the slag layer. Slab continuous casting is then performed with a superheat of 8–15°C and a casting speed of 0.8–1.1 m / min (reducing superheat and casting speed improves macroscopic segregation in the slab, reduces the spacing of secondary dendrite arms in the solidification structure, and helps reduce slab segregation and internal structural defects). Electromagnetic stirring is used in the secondary cooling zone during continuous casting, with alternating forward and reverse stirring. Forward stirring time is 30–40 s, and reverse stirring time is 20–30 s, with a current of 800–100 kW. 00A, with a frequency of 50-100Hz, allows molten steel to be continuously cast into a billet, with an isometric crystal ratio of not less than 90%. Simultaneously, heavy pressure is applied at the end of solidification, with a billet reduction of 15-20mm (this optimizes the electromagnetic stirring process during continuous casting, significantly increasing the isometric crystal ratio of the billet; the heavy pressure also helps reduce billet segregation, internal structural defects, and promotes core grain breakage, ensuring the strength and toughness of subsequent steel plates). After leaving the production line, the billet undergoes slow cooling in a pit. The billet is then hot-charged into the slow cooling pit, with a slow cooling temperature (entering the slow cooling pit) greater than or equal to 630℃. When the billet temperature is greater than or equal to 630℃, the slow cooling pit is not heated, and the slow cooling time is not less than 72 hours (this promotes the diffusion of elements such as Mn, Cr, C, B, and Ce, mitigating their impact on microstructure and properties due to component segregation).

[0026] 2) Billet Heating: Before rolling, the billet is descaled for 1-2 minutes using high-pressure water at a pressure of 15-20 MPa. The billet is then fed into a walking beam furnace for heating, passing through a preheating section, a heating section, and a soaking section before exiting the furnace. The preheating section temperature range is 850-1100℃ (to promote the rapid and complete dissolution of Ti and Cr carbides and Ce sulfides into the matrix and facilitate their diffusion), the heating section temperature range is 1200-1230℃, and the soaking section temperature range is 1130-1150℃. The total time spent in the furnace during heating and soaking is controlled at 3.5-4.5 hours. The temperature of the billet in the furnace is adjusted according to the temperature of the heating section. The opening of the lower burner is controlled to maintain an air-fuel ratio of 1:1.8 to 1:2.2, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15℃. (The heating section provides high-temperature heating to ensure temperature uniformity in all parts of the billet, further promoting the diffusion of elements such as Mn, Cr, C, B, and Ce, reducing their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface; combined with the billet composition, by reducing the heating temperature of the billet soaking section, energy consumption is reduced, and the rapid precipitation of Ce sulfides at 1130-1150℃ has a modifying effect on high-strength steel, refining austenite grains and inhibiting the coarsening of the original austenite grains.)

[0027] 3) Controlled rolling and pre-straightening: The billet is rolled in two stages: the first stage is recrystallization rolling (rough rolling), with an initial rolling temperature ≥1130℃ and a final rolling temperature range of 1000~1050℃. The reduction rate of the first two passes in the rough rolling stage is not less than 25% per pass. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than or equal to four (utilizing the high-temperature deformation-induced effect to promote the precipitation of Ce sulfides and VN phases within the austenite grains, providing nucleation sites for acicular ferrite, thereby promoting the formation of intragranular acicular ferrite and improving strength and toughness; it also induces TiN precipitation and inhibits BN precipitation). Simultaneously, it promotes dynamic recrystallization of austenite grains, refining the grains. The thickness of the intermediate billet is 3 to 4 times the thickness of the finished product. During the intermediate billet's warming process, descaling water is sprayed onto the billet for 1 to 1.5 minutes at a pressure of 20 to 25 MPa (this inhibits austenite grain growth, generates a temperature gradient on the inner and outer surfaces of the billet, promotes the penetration of rolling deformation into the center of the thickness, refines the grains at half the thickness, improves the core structure of thick steel plates, reduces structural stress caused by uneven structure, and accelerates the temperature drop of the intermediate billet, reducing the warming time). The second stage is also recrystallization rolling (finishing rolling). The initial rolling temperature range for finishing is 910–960℃, and the final rolling temperature range is 830–890℃. The reduction rate in each pass is 30% or higher for the first two passes, and the rolled length of the steel plate is controlled to be within 25m. Finishing rolling has no more than five passes. (By controlling the final rolling temperature in the roughing stage, the intermediate billet is kept within the austenite recrystallization temperature range during warming and finishing rolling, ensuring microstructure uniformity. Simultaneously, controlling the reduction rate in the finishing stage ensures the density of dislocations, vacancies, and deformation bands within the austenite body, promoting the precipitation of Ce sulfides and VN, providing more nucleation sites. Furthermore, the addition of Ce expands the austenite phase transformation.) The region is adjusted to shift the CCT (Continuous Cooling Transformation) curve downwards. This promotes the bainitic and martensitic phase transformations during cooling, refines the grain structure, ensures the strength and toughness of the steel plate, and controls the length of the rolled steel plate to avoid excessively rapid temperature drop during later rolling, which could affect the uniformity of the steel plate's microstructure after controlled cooling. The rolled steel plate is then pre-straightened at high temperature with a straightening force between 1500KN and 2000KN, a bending roll amount of 2–2.5 mm, and an inclination value of 2–3 mm (to promote full release of internal stress; setting appropriate bending roll amount and inclination value ensures good flatness of the straightened plate and uniform water immersion temperature).

[0028] 4) Relaxation, Cooling, and Hot Straightening: After rolling, the steel plate undergoes relaxation and temperature control for 30–45 seconds (this is to control the immersion temperature of the steel plate in water, ensuring a ferrite content of 10%–20% before immersion, guaranteeing the soft phase ratio and deformation coordination of the post-rolled steel plate microstructure, ensuring the yield strength ratio, and also increasing the proportion of large-angle grain boundaries and the density of precipitated phases to ensure strength and toughness). The initial cooling temperature of the steel plate is 720–730℃, the roller speed in the controlled cooling zone is 1.5–2 m / s, and the acceleration is 0.003–0.005 m / s².2 During the steel plate cooling process, the water flow ratio between the upper and lower manifolds is 1:1.8 to 1:2.3. A UFC (Ultra-Fast Cooling) + ACC (Laminar Flow) cooling mode is adopted. The cooling rate in the UFC zone is controlled at 35-50℃ / s, with a final cooling temperature range of 400-450℃. The cooling rate in the ACC zone is controlled at 10-15℃ / s, with a final cooling temperature range of 200-300℃. (UFC uses a fast cooling rate to promote rapid cooling of the steel plate by quickly breaking through the film boiling zone, followed by a slow cooling rate to promote a gradual decrease in temperature from the surface to the core, thus cooling to 200-300℃. This ensures sufficient hardening of the quenched steel plate while reducing stress levels during quenching, thus maintaining the plate shape.) After the steel plate exits controlled cooling, the side spray and air purging are activated. The side spray pressure and water flow rate are 5-10 MPa and 25-40 m³ / s, respectively. 3 / h, the air purging pressure is 5-10MPa (through calculation and experiment, the fastest precipitation temperature range of VN is 730-750℃. Controlling the cooling temperature ensures the density of VN nanoprecipitates, which can significantly improve the mechanical properties of steel through precipitation strengthening. In addition, the addition of B, Cr, and Mo can improve the hardenability of steel, increase the stability of austenite, and increase the driving force of phase transformation. At the same time, the addition of Ce promotes the gradual increase of bainite transformation, the size of islands in the microstructure gradually becomes finer, and the morphology gradually changes from large blocks to long strips distributed along the grain boundaries. Meanwhile, the rolling length (within 25 meters), the speed of the cooling roller, and the matching of appropriate acceleration are controlled to ensure the temperature uniformity of the steel plate along its length. At the same time, the water ratio of the upper and lower manifolds is optimized to ensure the shape of the steel plate after quenching. Side spraying and air purging are used to reduce the residual water in the steel plate after quenching, ensuring that the shape of the steel plate is controlled and the uniformity of the steel plate performance is improved). After controlled cooling, the steel plate is hot straightened.

[0029] 5) Tempering: The steel plates after controlled cooling and hot straightening are tempered at a temperature of 300~450℃ and a furnace time of 3~5 min / mm. After being taken out of the furnace, they are air-cooled to obtain the finished steel plate (the tempering temperature is controlled to ensure that the width of the martensite laths is 200-300nm, and the proportions of precipitates and large-angle grain boundaries are above 0.5% and 60% respectively, so as to ensure strength and toughness).

[0030] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 0.015%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.020%, and the C content is controlled to be between 0.15% and 0.20% at the end of converter smelting; argon gas is blown for more than 30 minutes when tapping the steel (argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition); then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

[0031] Furthermore, before rolling, the billet after exiting the furnace is descaled for 1-2 minutes using high-pressure water, with the descaling machine pressure being 15-20 MPa.

[0032] Furthermore, the number of UFC zone cooling manifolds in operation is 2 to 4, with a single manifold water flow rate of 300 to 400 m³. 3 / h; The number of ACC zone cooling manifolds in operation is 2-3, and the water flow rate per manifold is 150-200m³. 3 / h.

[0033] Furthermore, in step 4), the steel plate after controlled cooling is straightened in 3 passes with a straightening force between 2000KN and 3000KN. The position of the inlet roller is -0.8mm to -1.3mm, and the position of the outlet roller is -1.1mm to -2.0mm (the straightening process is optimized by using multiple passes of small bending to ensure that the cumulative strain at 35% to 45% of the distance from the steel plate surface exceeds the elastic limit strain, thus promoting the further and full release of internal stress).

[0034] A highly efficient and low-cost production method for 1300MPa grade microalloyed steel with a thickness of 10-50mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, pre-straightening, relaxation, cooling, hot straightening, and tempering processes, and employing an ultra-fast cooling + laminar flow cooling mode, and considering the presence of Ce in the steel plate's chemical composition, the method ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the online quenching process for producing 1300MPa grade hot-rolled steel plates with a thickness of 10-50mm. Furthermore, it eliminates the need for subsequent offline quenching and straightening treatments to meet the technical requirements of the plate.

[0035] The beneficial effects of this invention are:

[0036] 1. After deep desulfurization pretreatment of KR molten iron, the slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as center segregation and excessive inclusions caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed improves macroscopic segregation in the continuously cast billet, decreasing the spacing between secondary dendrite arms in the solidification structure, which helps reduce billet segregation and internal structural defects. Optimizing the electromagnetic stirring process during continuous casting significantly increases the equiaxed crystal ratio of the billet. Simultaneously, applying heavy pressure helps reduce billet segregation and internal structural defects, while promoting core grain breakage, ensuring the strength and toughness of the subsequent steel plate. Furthermore, slow cooling of the billet after casting, with controlled cooling temperature and time, promotes the diffusion of Mn, Cr, C, B, and Ce elements, mitigating their impact on microstructure and properties due to component segregation.

[0037] 2. The composition of this invention is reasonable, and the amount of alloy added is low. Through the V-Cr-NB-Mo-Ce composite design, the alloy cost and high-temperature deformation resistance in the roughing and finishing stages are greatly reduced, which is conducive to increasing the reduction per pass and ensuring the comprehensive performance of the super steel plate. The effect of boron on the improvement of austenite stability is related to the amount of boron dissolved in the steel. In order to increase the amount of boron dissolved in the steel, Ti, which has a stronger bonding force with N than B, should be added. The amount of vanadium in the steel has almost no effect on the amount of boron dissolved in the steel, ensuring that boron exists in a free form. At the same time, the high-temperature deformation-induced effect is used to promote the precipitation of the VN phase in the austenite grains, providing nucleation sites for acicular ferrite, thereby promoting the formation of intragranular acicular ferrite and improving strength and toughness.

[0038] 3. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure that Cr and Ti carbides and nitrides dissolve rapidly and fully in the matrix and diffuse sufficiently. At the same time, the heating zone provides high-temperature heating to ensure temperature uniformity in all parts of the billet, further promoting the diffusion of elements Mn, Cr, C, B, and Ce, reducing their impact on microstructure and properties due to component segregation, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, the heating temperature of the soaking zone is reduced to decrease energy consumption. At the same time, the rapid precipitation of Ce sulfides at 1130-1150℃ has a modifying effect on high-strength steel, refining austenite grains and inhibiting the coarsening of the original austenite grains.

[0039] 4. A two-stage controlled rolling process is adopted. By controlling the final rolling temperature of the roughing stage, the intermediate billet is kept in the austenite recrystallization temperature range during the warming process, ensuring the uniformity of the microstructure. At the same time, the rolling reduction rate and the thickness of the intermediate billet are limited in both stages. The high-temperature deformation-induced effect is used to promote the precipitation of Ce sulfides and VN, providing more nucleation sites. Meanwhile, the addition of Ce expands the austenite phase transformation region and shifts the CCT curve downward. This process promotes the transformation of bainite and martensite during cooling, refines the grain structure, and ensures the strength and toughness of the steel plate. It also induces TiN precipitation and inhibits BN precipitation. Spraying descaling water inhibits austenite grain growth and creates a temperature gradient on the inner and outer surfaces of the billet, promoting the penetration of rolling deformation towards the thickness center and refining the grains at half the thickness. This improves the core structure of thick steel plates and reduces structural stress caused by uneven microstructure. Simultaneously, controlling the length of the rolled steel plate prevents excessively rapid temperature drop during later rolling stages, which could affect the uniformity of the steel plate's microstructure after controlled cooling. Pre-straightening promotes the full release of internal stress in the steel plate, and setting appropriate bending and tilting values ​​ensures good straightness of the straightened plate and uniform water immersion temperature.

[0040] 5. Through calculation and experimentation, the fastest precipitation temperature range for VN is 730–750℃. By controlling the initial cooling temperature, the ferrite proportion before the steel plate enters the water is maintained at 10%–20%, ensuring the proportion of soft phases and deformation coordination in the post-rolled steel plate microstructure, and guaranteeing the yield strength ratio. This also increases the proportion of large-angle grain boundaries and the density of precipitated phases, ensuring strength and toughness. Furthermore, the addition of B, Cr, and Mo can improve the hardenability of the steel, increase austenite stability, and enhance the phase transformation driving force. Simultaneously, the addition of Ce promotes a gradual increase in bainite transformation, resulting in a gradual refinement of the island-like structures in the microstructure, and a gradual transformation of the morphology from large blocks to elongated strips distributed along grain boundaries. Simultaneously, controlling the rolling length, increasing the speed of the controlled cooling rollers, and matching appropriate acceleration ensures temperature uniformity along the length of the steel plate. Additionally, optimizing the water ratio in the upper and lower manifolds controls the cooling rate in different temperature ranges. UFC employs a rapid cooling rate to quickly break through the film boiling zone of the steel plate for rapid cooling, followed by a slow cooling rate to gradually reduce the surface temperature to the core temperature, cooling to 200-300℃. This ensures sufficient hardening of the quenched steel plate while reducing the stress level during quenching, thus maintaining the plate shape after quenching. Side spraying and air purging are used to reduce residual water in the steel plate after quenching, ensuring controlled plate shape and improving the uniformity of steel plate performance. The straightening process is optimized by using multi-pass small bending straightening to ensure that the cumulative strain at 35%-45% of the steel plate surface exceeds the elastic limit strain, promoting further full release of internal stress. The tempering temperature is controlled to ensure the width of martensite laths, the proportion of precipitates and large-angle grain boundaries, thus ensuring strength and toughness.

[0041] 6. A high-efficiency, low-cost production method for 1300MPa grade microalloyed steel with a thickness of 10-50mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, cooling, straightening, and tempering processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate flatness in the production of 1300MPa grade hot-rolled steel plates with a thickness of 10-50mm using online quenching processes were finally solved. Furthermore, no subsequent offline quenching and straightening treatment is required to meet the technical requirements of the plate. The mechanical properties of the produced plate are as follows: transverse tensile yield strength ≥1300MPa, tensile strength between 1450-1600MPa, elongation ≥10%, transverse Charpy impact energy at -40℃ ≥60J, yield strength ratio ≤0.9, flaw detection pass rate ≥98%, and flatness below 4mm / 2m. Detailed Implementation

[0042] The following examples are used to illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0043] This invention provides a high-efficiency, low-cost 1300MPa grade high-strength steel plate for low-temperature engineering structures. Its chemical composition, by weight percentage, is: C 0.15%–0.20%, Si 0.25%–0.35%, Mn 1.20%–1.35%, P≤0.02%, S≤0.010%, Cr 0.45%–0.55%, Ni 0.2%–0.3%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0015%–0.0020%, N 0.02%–0.03%, Ti 0.01%–0.02%, Mo 0.40%–0.60%, Ce 0.05% to 0.10%, with the balance being Fe and unavoidable impurities; the steel plate has a thickness of 10-50 mm and is produced on a medium-thick plate reciprocating rolling mill using continuously cast billets with a thickness of less than 250 mm.

[0044] The aforementioned high-efficiency, low-cost manufacturing method for 1300MPa grade high-strength steel plates used in cryogenic engineering structures includes steel smelting → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling and pre-straightening → relaxation, cooling and hot straightening → tempering; specifically, it includes the following steps:

[0045] 1) Steelmaking to continuous casting: The steel is smelted according to the above composition. The molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. After RH vacuum degassing, Al is added according to Als requirements; after Ti-Fe micro-titanium treatment, B-Fe alloy is added; this is ensured to prevent it from floating on the slag layer. Slab continuous casting is then carried out, with a superheat of 8–15℃ and a casting speed of 0.8–1.1 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage, with alternating forward and reverse stirring. The forward stirring time is 30-40s, the reverse stirring time is 20-30s, the current is 800-1000A, and the frequency is 50-100Hz. The molten steel is continuously cast to obtain a continuously cast billet, and the isometric crystal ratio of the billet is not less than 90%. At the end of solidification, the billet is subjected to heavy pressure, and the reduction of the billet is 15-20mm. After the billet is removed from the line, it is slowly cooled in a pit. After the billet is removed from the line, it is hot-charged into the slow cooling pit. When the billet temperature is greater than or equal to 630℃, the slow cooling pit is not heated, and the slow cooling time is not less than 72h.

[0046] 2) Billet Heating: The billet (thickness below 250mm) is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section, and soaking section in sequence before exiting the furnace. The temperature range of the preheating section is 850-1100℃, the temperature range of the heating section is 1200-1230℃, and the temperature range of the soaking section is 1130-1150℃. The total time in the furnace during the heating and soaking sections is controlled at 3.5-4.5 hours. The opening degree of the upper and lower burners in the furnace is adjusted to control the air-fuel ratio at 1:1.8-1:2.2, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15℃.

[0047] 3) Controlled rolling and pre-straightening: Before rolling, the billet is descaled with high-pressure water for 1-2 minutes after exiting the furnace, with a descaling machine pressure of 15-20 MPa; the billet is rolled in two stages: the first stage is recrystallization rolling (rough rolling), with a rough rolling start temperature ≥1130℃ and a rough rolling finish temperature range of 1000-1050℃. The reduction rate of the first two passes in the rough rolling stage is not less than 25% per pass. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than or equal to four, and the thickness of the intermediate billet is 3-4 times the thickness of the finished product; during the intermediate billet's warming process, descaling water is sprayed from the rolling mill to remove scale. The time is 1-1.5 minutes, and the descaling machine pressure is 20-25 MPa; the second stage is also recrystallization rolling (finish rolling), with the initial rolling temperature range of 910-960℃ and the final rolling temperature range of 830-890℃. The reduction rate for the first two passes is guaranteed to be above 30%, and the rolling length of the steel plate is controlled within 25m. The finishing rolling is no more than five passes. The rolled steel plate is then pre-straightened at high temperature with a straightening force between 1500KN and 2000KN, a bending amount of 2-2.5mm, and an inclination value of 2-3mm.

[0048] 4) Relaxation, cooling, and hot straightening: After rolling, the steel plate undergoes relaxation and temperature control for 30–45 seconds (ensuring a ferrite content of 10%–20% before immersion in water). The initial cooling temperature is 720–730℃, and the roller speed in the controlled cooling zone is 1.5–2 m / s with an acceleration of 0.003–0.005 m / s². 2 During the steel plate cooling process, the water flow ratio (water ratio) between the upper and lower manifolds is 1:1.8 to 1:2.3. A UFC (Ultra-Fast Cooling) + ACC (Laminar Flow) cooling mode is adopted. The number of UFC zone-controlled cooling manifolds open is 2 to 4, and the water flow rate per manifold is 300 to 400 m³. 3 The cooling rate is controlled at 35~50℃ / s, with a final cooling temperature range of 400~450℃. The number of ACC zone cooling manifolds open is 2~3, and the water flow rate per manifold is 150~200m³. 3 The cooling rate is controlled at 10~15℃ / s, with a final cooling temperature range of 200~300℃. After the steel plate exits controlled cooling, the side spray and air purging are activated, with side spray pressure and water flow rate of 5~10MPa and 25~40m³ / h respectively.3 / h, the air blowing pressure is 5~10MPa, the steel plate is straightened in 3 stages after controlled cooling, the straightening force is between 2000KN~3000KN, the position of the inlet roller is -0.8mm~-1.3mm, and the position of the outlet roller is -1.1mm~-2.0mm;

[0049] 5) Tempering: The steel plate after controlled cooling and hot straightening is tempered at a temperature of 300~450℃ and in the furnace for 3~5 min / mm. After being taken out of the furnace, it is air-cooled to room temperature to obtain the finished steel plate.

[0050] Examples 1-6

[0051] Table 1 shows the chemical composition of the steel plate in the examples; Table 2 shows the smelting process of the steel plate in the examples; Table 3 shows the heating process of the billet of the steel plate in the examples; Table 4 shows the rolling and pre-straightening process of the steel plate in the examples; Table 5 shows the cooling process of the steel in the examples; Table 6 shows the hot straightening and tempering process of the steel in the examples; Table 7 shows the dimensions, properties and flatness of the steel plate in the examples.

[0052] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0053]

[0054] Note: Impurity elements in steel: P≤0.02%, S≤0.010%.

[0055] Table 2. Smelting process of steel in the examples

[0056]

[0057] Table 3 Heating regime of steel billets in the examples

[0058]

[0059] Table 4 Rolling and pre-straightening processes of steel in the examples

[0060]

[0061] Table 5 Cooling process of steel in the examples

[0062]

[0063] Table 6 Hot straightening and tempering process of steel plates in the examples

[0064]

[0065] Table 7 Dimensions, properties, and flatness of the steel plates in the examples

[0066]

[0067] Therefore, compared with existing technologies, the efficient and low-cost production method of 1300MPa grade microalloyed steel with a thickness of 10-50mm in this invention optimizes the steelmaking, heating, rolling, pre-straightening, relaxation, cooling, hot straightening, and tempering processes. Furthermore, the steel plate contains Ce in its chemical composition. This method ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1300MPa grade hot-rolled steel plates with a thickness of 10-50mm produced by online quenching processes. Simultaneously, it eliminates the need for subsequent offline quenching and straightening treatments to meet the technical requirements of the plate.

[0068] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high-efficiency, low-cost, 1300MPa-grade high-strength steel plate for cryogenic engineering structures, characterized in that, The weight percentage of the chemical composition of the steel plate includes: C 0.15%~0.20%, Si 0.25%~0.35%, Mn 1.20%~1.35%, P≤0.02%, S≤0.010%, Cr 0.45%~0.55%, Ni 0.2%~0.3%, Al 0.015%~0.035%, V 0.1%~0.2%, B 0.0015%~0.0020%, N 0.02%~0.03%, Ti 0.01%~0.02%, Mo 0.40%~0.60%, Ce 0.05%~0.10%, with the balance being Fe and unavoidable impurities.

2. The high-efficiency, low-cost 1300MPa grade low-temperature engineering structure high-strength steel plate according to claim 1, characterized in that, The thickness of the steel plate is 10-50 mm.

3. The high-efficiency, low-cost 1300MPa grade low-temperature engineering structure high-strength steel plate according to claim 1, characterized in that, The steel plate has a transverse tensile yield strength ≥1300MPa, a tensile strength of 1450~1600MPa, an elongation ≥10%, a transverse Charpy impact energy of -40℃ ≥60J, a yield strength ratio ≤0.9, a flaw detection pass rate ≥98%, and a flatness of less than 4mm / 2m.

4. The method for manufacturing high-efficiency, low-cost 1300MPa grade low-temperature engineering structure high-strength steel plate according to any one of claims 1-3, characterized in that, Includes the following steps: 1) Steelmaking to Continuous Casting: Molten iron undergoes pretreatment, converter smelting, LF refining, RH vacuum degassing, and continuous casting. After RH vacuum degassing, Al is added according to Als requirements. After micro-titanium treatment with Ti-Fe, B-Fe alloy is added. Slab continuous casting is then performed with a superheat of 8–15°C and a casting speed of 0.8–1.1 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage, with alternating forward and reverse stirring. The stirring time is 30-40s for the first stirring and 20-30s for the second stirring. The current is 800-1000A and the frequency is 50-100Hz. The molten steel is continuously cast to obtain a continuously cast billet, and the isometric crystal ratio of the billet is not less than 90%. At the end of solidification, the billet is subjected to heavy pressure and the reduction is 15-20mm. After the billet is removed from the line, it is slowly cooled in a pit. The billet is hot-charged into the slow cooling pit after it is removed from the line. When the billet temperature is greater than or equal to 630℃, the slow cooling pit is not heated and the slow cooling time is not less than 72h. 2) Billet Heating: The billet is fed into a walking beam furnace for heating. The billet passes through the preheating section, heating section, and soaking section in sequence before exiting the furnace. The temperature of the preheating section is 850-1100℃, the temperature of the heating section is 1200-1230℃, and the temperature of the soaking section is 1130-1150℃. The total time in the furnace during the heating and soaking sections is controlled at 3.5-4.5 hours. The opening degree of the upper and lower burners in the furnace is adjusted to control the air-fuel ratio at 1:1.8-1:2.2, ensuring that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) Controlled rolling and pre-straightening: The billet is rolled in two stages: the first stage is rough rolling, with an initial rolling temperature ≥1130℃ and a final rolling temperature range of 1000~1050℃. The reduction rate in the first two passes of the rough rolling stage is no less than 25% per pass, and the number of rolling passes in the rough rolling stage is less than or equal to four. The thickness of the intermediate billet is 3~4 times the thickness of the finished product. During the intermediate billet's warming process, descaling water is sprayed onto the rolling mill for 1~1.5 minutes at a pressure of 2. The first stage is 0-25MPa; the second stage is finishing rolling, with an initial rolling temperature of 910-960℃ and a final rolling temperature of 830-890℃. The reduction rate for the first two passes is guaranteed to be above 30%, and the rolling length of the steel plate is controlled within 25m. The finishing rolling is not more than five passes. The rolled steel plate is pre-straightened at high temperature, with a straightening force between 1500KN and 2000KN, a bending amount of 2-2.5mm, and an inclination value of 2-3mm. 4) Relaxation, cooling, and hot straightening: After rolling, the steel plate is relaxed and allowed to heat up for 30–45 seconds. The initial cooling temperature is 720–730℃. The roller speed in the controlled cooling zone is 1.5–2 m / s, and the acceleration is 0.003–0.005 m / s². 2 During the steel plate cooling process, the water flow ratio between the upper and lower manifolds is 1:1.8 to 1:2.3, employing an ultra-fast cooling + laminar flow cooling mode. The cooling rate in the ultra-fast cooling zone is controlled at 35~50℃ / s, with a final cooling temperature of 350~400℃. The cooling rate in the laminar flow zone is controlled at 10~15℃ / s, with a final cooling temperature of 150~250℃. After the steel plate exits controlled cooling, the side spray and air purging are activated, with side spray pressure and water flow rate of 5~10MPa and 25~40m³ / s, respectively. 3 / h, the air blowing pressure is 5~10MPa, and the steel plate is hot straightened after controlled cooling; 5) Tempering: The steel plate after controlled cooling and hot straightening is tempered at a temperature of 300~450℃ and in the furnace for 3~5 min / mm. After being taken out of the furnace, it is air-cooled to obtain the finished steel plate.

5. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 1300MPa as described in claim 4, characterized in that, In step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.015%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.020%, and the C content is controlled to be 0.15% to 0.20% at the end of converter smelting. Argon gas is blown for more than 30 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

6. The method for manufacturing high-strength steel plates for 1300MPa-grade low-temperature engineering structures according to claim 4, characterized in that, During the converter smelting process, the double slag method is used for phosphorus removal.

7. The method for manufacturing high-strength steel plates for 1300MPa-grade low-temperature engineering structures according to claim 4, characterized in that, The thickness of the cast billet is less than 250mm; the steel plate is produced by casting the billet on a medium-thick plate reciprocating rolling mill, and the cooling medium is water.

8. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 1300MPa as described in claim 4, characterized in that, In step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, and the descaling machine pressure is 15-20 MPa.

9. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 1300MPa as described in claim 4, characterized in that, In step 4), the ferrite content of the steel plate before cooling is 10%–20%; the number of controlled cooling manifolds in the ultra-fast cooling zone is 2–4 groups, and the water flow rate of a single manifold is 300–400 m³. 3 / h; the number of controlled cooling manifolds in the laminar flow cooling zone is 2-3 groups, and the water flow rate per manifold is 150-200m³. 3 / h.

10. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 1300MPa as described in claim 4, characterized in that, In step 4), the steel plate after controlled cooling is straightened in 3 passes with a straightening force between 2000KN and 3000KN. The position of the inlet roller is -0.8mm to -1.3mm, and the position of the outlet roller is -1.1mm to -2.0mm.

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