High-strength steel plate for low-temperature engineering structure with 890mpa grade and manufacturing method thereof

Abstract

This invention discloses a high-efficiency, low-cost 890MPa 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. By optimizing the steelmaking, heating, rolling, pre-straightening, relaxation, controlled cooling, and hot straightening processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-30mm thick 890MPa grade hot-rolled steel plates using online cooling processes are ultimately solved.

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 10-30mm and a yield strength of 890MPa for low-temperature engineering structures and its manufacturing method. Background Technology

[0002] In recent years, with the continuous advancement of steel materials, welding materials manufacturing, and welding technology, the strength levels of structural steel have been continuously increasing. 890MPa-grade high-strength steel is widely used in marine cranes, marine liquid tanks, self-elevating offshore platform legs, and coal mine hydraulic supports, especially in the marine engineering field where it has become an important steel grade. This type of steel is mainly based on carbon-manganese steel, with the addition of microalloying elements such as Nb, V, Cr, Mo, and Ni, as well as carbon and nitride or highly hardenable elements, to improve the strength and toughness of the steel plate through solid solution strengthening, precipitation strengthening, and grain refinement strengthening. Statistics show that the annual market demand for medium and heavy plates of this strength level is approximately 20,000 to 50,000 tons. Therefore, research on developing low-cost steel plate manufacturing technologies is of great significance. Currently, 890MPa-grade high-strength steel is generally produced using an offline quenching and tempering process, leading to increased manufacturing costs and product delivery time. With the upgrading of online controlled cooling equipment both domestically and internationally, the online cooling water volume and model control accuracy have been significantly improved, making online cooling of high-strength steel a possibility. In recent years, steel mills both domestically and internationally have adopted online cooling combined with offline tempering processes to produce high-strength steel with a thickness of 10-30mm. However, due to the influence of a series of processes such as composition design, steelmaking, heating, and rolling, and because of the two-stage rolling process (recrystallization and non-recrystallization zones), the longitudinally deformed austenite grains in the rolled steel plate microstructure tend to form martensite laths that penetrate the original austenite grains under subsequent high cooling rates. Simultaneously, during the cooling process, the temperature inhomogeneity along the thickness direction of the steel plate increases significantly, leading to greater internal stress. This results in products with significantly lower low-temperature impact toughness, plate shape, and flaw detection pass rate compared to the offline quenching + tempering production process. These factors limit the efficient and low-cost production and market promotion of 890MPa-grade low-temperature high-strength steel with a thickness of 10-30mm. Therefore, improving the manufacturing process of 890MPa grade high-strength steel plates with thicknesses of 10-30mm and resolving issues such as performance, plate shape, and flaw detection pass rate caused by online cooling are key problems that urgently need to be solved in the mass production of 890MPa grade high-strength steel medium and thick plates.

[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 10-30mm thick, 890MPa grade low-temperature engineering structural steel. Prior to this invention, the journal article "Influence of Tempering Temperature on the Microstructure and Properties of Q690D Grade High-Strength Structural Steel" (Heat Treatment Technology, 2014.8) mainly enhanced the toughness of steel plates through offline tempering heat treatment, but this offline tempering heat treatment significantly increased the production cycle and cost. The master's thesis "Evolution of Microstructure and Properties and Strengthening Mechanism of Q890D Engineering Machinery Steel" (Northeastern University, 2021.6) mainly produced high-strength steel through offline quenching and tempering treatment, which also significantly increased the production cycle and cost and failed to meet the -40℃ low-temperature impact toughness index. The above-disclosed literature on the influence of tempering temperature on the microstructure and properties of Q690D and Q890D high-strength structural steel can solve the low-temperature toughness problem of steel plates at -20℃, but it is not suitable for controlling and solving the problem of producing steel plates with a yield strength of 890MPa and excellent low-temperature toughness at -40℃ with a thickness of 10-30mm using TMCP instead of offline heat treatment. The technical solution provided by this invention can effectively overcome the above shortcomings and solve the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-30mm thick 890MPa grade low-temperature structural steel plates using online cooling processes on continuously cast billets with a thickness of less than 250mm. 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 890MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method. This plate is an E-grade microalloyed high-strength steel plate with excellent low-temperature toughness. The method employs optimized steelmaking, heating, rolling, pre-straightening, relaxation, controlled cooling, and hot straightening processes, ultimately solving the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-30mm thick 890MPa grade hot-rolled steel plates using online cooling processes.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a high-efficiency, low-cost 890MPa grade high-strength steel plate for low-temperature engineering structures. Its chemical composition, by weight percentage, is as follows: C 0.06%–0.10%, Si 0.1%–0.15%, Mn 1.40%–1.50%, P≤0.02%, S≤0.010%, Nb 0.02%–0.04%, Cr 0.4%–0.6%, Ni 0.35%–0.45%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0015%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.25%–0.35%, 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] 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 0.06% to 0.10%.

[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 the continuously cast billet, 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 1.4%–1.5%.

[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 0.1% to 0.15% 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 0.015% to 0.025%, 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: The main 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 0.4% to 0.6%.

[0016] Nitrogen (Nb) is a commonly used element in modern microalloyed pipeline steel, exhibiting excellent grain refinement and precipitation strengthening effects; it also delays austenite recrystallization. However, excessive Nb increases production costs and complicates continuous casting process control. This invention selects an Nb content range of 0.02% to 0.04%, combined with appropriate heating and rolling processes, to achieve good toughness.

[0017] 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 0.25% and 0.35%.

[0018] 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 be between 0.35% and 0.45%.

[0019] 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 0.0015–0.0020%.

[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 0.015%, respectively.

[0021] The steel plate of the present invention has a thickness of 10-30mm, and is produced on a medium-thick plate reciprocating rolling mill using a continuous casting billet with a thickness of less than 250mm, 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] This invention provides a high-efficiency, low-cost method for manufacturing high-strength steel plates for low-temperature engineering structures with a strength of 890MPa, comprising: 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; specifically including the following steps:

[0024] 1) Steel smelting to continuous casting: Smelting according to the following composition, the chemical composition by weight percentage is C 0.06%~0.10%, Si 0.1%~0.15%, Mn 1.40%~1.50%, P≤0.02%, S≤0.010%, Nb 0.02%~0.04%, Cr 0.4%~0.6%, Ni 0.35%~0.45%, Al 0.015%~0.035%, V 0.1%~0.2%, Nb 0.02%~0.04%, B 0.0015%~0.0020%, N 0.015%~0.025%, Ti 0.01%~0.02%, Mo 0.25%~0.35%, with the balance being Fe and unavoidable impurities. 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 0.7–0.9 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 20–30 s, reverse stirring time is 5–10 s, current is 800–1000 A, and frequency is 20–50 Hz. The molten steel is continuously cast to obtain a continuously cast billet, and the isometric crystal ratio of the billet is not less than 80%. At the same time, strong cooling is adopted in the fan-shaped section, with a total cooling water flow of 600-800 L / min in the first half and 800-1000 L / min in the second half. At the end of solidification, the billet is subjected to heavy pressure, and the reduction of the billet is 15-20 mm. (By optimizing the electromagnetic stirring process in the continuous casting stage, the isometric crystal ratio of the billet is greatly improved. Strong cooling is adopted to ensure the temperature gradient in the thickness direction of the billet. At the same time, the heavy pressure can help reduce the segregation of the billet, reduce internal structural defects, and promote the core grain breakage, which can ensure the strength and toughness of the subsequent steel plate.) After the billet is removed from the line, it is slowly cooled in the pit at a temperature not lower than 800℃ for a time not less than 72 hours. (This promotes the diffusion of elements such as Mn, Cr, C, and B, and reduces their impact on the microstructure and properties due to component segregation.)

[0025] 2) Billet Heating: The billet is 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 has a temperature range of 950–1150℃ (to promote the rapid and complete dissolution and diffusion of Ti and Cr carbides into the matrix), the heating section has a temperature range of 1230–1260℃, and the soaking section has a temperature range of 1200–1220℃. The total time spent in the furnace during the heating and soaking sections is controlled to be 3–4 hours, 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, and B, 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, reducing the heating temperature of the soaking section reduces energy consumption and also suppresses the impact of coarsening of the original austenite grains on performance.)

[0026] 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 ≥1120℃ and a final rolling temperature range of 980~1030℃. 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 VN phase 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, inhibits BN precipitation, and promotes austenite...). The first stage involves 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, two passes of descaling water are sprayed, with a descaling time of 1 to 1.5 minutes and a descaling machine pressure of 20 to 25 MPa (this inhibits austenite grain growth, simultaneously generates a temperature gradient on the inner and outer surfaces of the billet, promotes the penetration of rolling deformation into the thickness center, refines the grains at half the thickness, which is beneficial for improving the core structure of thick steel plates, reducing structural stress caused by structural inhomogeneity, and accelerating the temperature drop of the intermediate billet while reducing the warming time). The second stage is also recrystallization rolling (finish rolling). The initial rolling temperature range is 920–950℃, and the final rolling temperature range is 860–890℃. The reduction rate for each pass is 30% or higher for the first two passes, and less than or equal to 5% for the final pass. The rolling length of the steel plate is controlled to be within 25m, and the finishing rolling process 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 uniform microstructure. Simultaneously, controlling the reduction rate in the finishing stage ensures the density of dislocations, vacancies, and deformation bands within the austenite body, promoting VN precipitation, providing more nucleation sites, and promoting ferrite formation.) Phase transformation occurs, refining the ferrite structure and ensuring the strength and toughness of the steel plate. A low reduction rate is used in the final rolling pass to flatten the plate shape, reduce internal stress, and control the length of the rolled steel plate to avoid excessively rapid temperature drop in later rolling stages, which could affect the uniformity of the steel plate's microstructure after controlled cooling. The rolled steel plate undergoes high-temperature pre-straightening with a straightening force between 1000KN and 1200KN, a bending roll amount of 1–1.5mm, and an inclination value of 3–3.5mm (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).

[0027] 4) Relaxation, Cooling, and Hot Straightening: After rolling, the steel plate undergoes relaxation and temperature control for 30–40 seconds (to ensure the initial plate shape upon immersion in water, while controlling the immersion temperature to ensure a ferrite content of 10%–20% before immersion, ensuring the soft phase ratio and deformation compatibility of the post-rolled steel plate microstructure, and ensuring the yield strength ratio). The initial cooling temperature is 710–730℃, and the roller speed in the controlled cooling zone is 2–2.5 m / s, with an acceleration of 0.003–0.005 m / s². 2The final cooling temperature range is 400–420℃, and the cooling rate is controlled at 15–25℃ / s. During the cooling process of the steel plate, the water ratio between the upper and lower manifolds is 1:1.8–1:2.3. After the steel plate exits controlled cooling, the side spray and air purging are activated, with the side spray pressure and water flow rate being 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 710-740℃; controlling the starting cooling temperature ensures the density of VN nano-precipitation, 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, increase the phase transformation driving force, and promote the transformation of acicular ferrite, bainite, and martensite. Based on this, a high final cooling temperature and a low cooling rate are adopted to avoid excessively rapid cooling. This can lead to excessive internal stress in the steel plate, resulting in poor plate shape. Simultaneously, controlling the rolling length (within 25m), increasing the speed of the controlled cooling rollers, and matching appropriate acceleration are crucial to ensure temperature uniformity along the length of the steel plate. Optimizing the water ratio in the upper and lower manifolds is also essential to maintain the plate shape after quenching. Side spraying and air purging are used to reduce residual water after quenching, ensuring controlled plate shape and improved uniformity of steel plate performance. After controlled cooling, the steel plate undergoes hot straightening at temperatures below 300℃, followed by air cooling to room temperature.

[0028] 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 0.06% and 0.10% 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 40 minutes.

[0029] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0030] Furthermore, in the continuous casting process, the first half is sections 1 to 4, and the second half is sections 5 to 8.

[0031] Furthermore, the thickness of the cast billet is less than 250 mm.

[0032] Furthermore, 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.

[0033] Furthermore, in step 4), the number of controlled cooling manifold groups opened is 3 to 6, and the water flow rate of a single manifold is 150 to 200 m³. 3 / h.

[0034] Furthermore, in step 4), the straightening force of the hot straightening is between 2000KN and 3000KN, the position of the inlet roller is -0.8mm to -1.5mm, and the position of the outlet roller is -2.1mm to -3.0mm (to optimize the straightening process and promote the further full release of internal stress).

[0035] A highly efficient and low-cost production method for 890MPa grade microalloyed steel with a thickness of 10-30mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, pre-straightening, relaxation, cooling, and hot straightening processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-30mm thick 890MPa grade hot-rolled steel plates were finally solved. Furthermore, the technical requirements of the plate can be met without subsequent offline tempering and straightening treatments.

[0036] The beneficial effects of this invention are:

[0037] 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 in the billet caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed can improve macroscopic segregation in the continuously cast billet, decrease the spacing of secondary dendrite arms in the solidification structure of the billet, and help reduce billet segregation and internal structural defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed grain ratio of the continuously cast billet is significantly improved. Strong cooling is used to ensure the temperature gradient in the thickness direction of the billet. At the same time, heavy pressure is applied to reduce billet segregation and internal structural defects, while promoting core grain breakage, which ensures the strength and toughness of the subsequent steel plate. In addition, the billet is slowly cooled in the pit after it leaves the production line, and the slow cooling temperature and time are limited, which helps to promote the diffusion of Mn, Cr, C and B elements and reduce their impact on microstructure and properties due to component segregation.

[0038] 2. The composition of this invention is reasonable, and the amount of alloy added is low. Through the V-Cr-NB-Mo composite design, the alloy cost and high-temperature deformation resistance in the roughing and finishing rolling 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 content of boron dissolved in the steel, Ti, which has a stronger bonding force with N than B, should be added. The content of V in the steel has almost no effect on the content of boron dissolved in the steel, ensuring that B exists in a free form. At the same time, the high-temperature deformation-induced effect is used to promote the precipitation of 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.

[0039] 3. Billet Heating: The temperatures and times of the billet in the preheating, heating, and soaking zones are limited to ensure that Nb, Cr, and Ti carbides and nitrides dissolve rapidly and fully in the matrix and diffuse sufficiently. Simultaneously, the heating zone provides high-temperature heating to ensure temperature uniformity across the billet, further promoting the diffusion of Mn, Cr, C, and B, mitigating 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, reducing the soaking zone heating temperature decreases energy consumption and also suppresses the impact of primary austenite grain coarsening on performance.

[0040] 4. A two-stage controlled rolling process is adopted. By controlling the final rolling temperature of the roughing stage, the intermediate billet is kept within the austenite recrystallization temperature range during the warming process, ensuring microstructure uniformity. Simultaneously, the rolling reduction rate and intermediate billet thickness are limited in both stages. Utilizing the high-temperature deformation-induced effect, the precipitation of the VN phase within the austenite grains is promoted, 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. Descaling water is sprayed to inhibit austenite grain growth and also descaling the inner and outer surfaces of the billet. The surface generates a temperature gradient, promoting the penetration of rolling deformation towards the thickness center, refining the grains at half the thickness, which is beneficial for improving the core structure of thick steel plates and reducing structural stress caused by uneven structure. The final pass uses a small reduction rate to flatten the plate shape, reduce internal stress, and control the length of the rolled plate to avoid excessively rapid temperature drop in the later rolling stages, which would affect the uniformity of the steel plate structure after controlled cooling. By incorporating pre-straightening, the internal stress of the steel plate is fully released, and appropriate bending roll amount and tilting value are set to ensure good straightness of the straightened plate and uniform temperature of the steel plate when entering the water.

[0041] 5. Through calculation and experimentation, the fastest precipitation temperature range for VN is 710–740℃. Controlling the initial cooling temperature ensures a ferrite content of 10%–20% before the steel plate is immersed in water, guaranteeing the proportion of soft phases, VN nano-precipitation density, and deformation compatibility in the post-rolled steel plate microstructure, thus ensuring the yield strength ratio and mechanical properties of the steel. Furthermore, the addition of B, Cr, and Mo can improve the hardenability of the steel, increase austenite stability, enhance the phase transformation driving force, and promote the transformation of acicular ferrite and bainite. Based on this, a high final cooling temperature and slow cooling rate are adopted to avoid… Excessive cooling rate caused excessive internal stress in the steel plate, resulting in poor plate shape. To address this, the rolling length was controlled, the speed of the controlled cooling rollers was increased, and appropriate acceleration was matched to ensure temperature uniformity along the length of the steel plate. The water ratio between the upper and lower manifolds was optimized to ensure the plate shape after quenching. Side spraying and air blowing were used to reduce residual water in the steel plate after quenching, ensuring that the plate shape was controlled and the uniformity of the steel plate performance was improved. Hot straightening was adopted, and the positions and pressures of the inlet and outlet rollers were set to ensure that the straightened steel plate was straight and had a good shape.

[0042] 6. A high-efficiency, low-cost production method for 890MPa grade microalloyed steel with a thickness of 10-30mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, pre-straightening, relaxation, cooling, and hot straightening processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate flatness in the production of 10-30mm thick 890MPa grade hot-rolled steel plates using online cooling processes were finally solved. Furthermore, no subsequent offline tempering 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 ≥890MPa, tensile strength between 1050-1160MPa, elongation ≥12%, transverse Charpy impact energy at -40℃ ≥80J, yield strength ratio ≤0.85, flaw detection pass rate ≥98%, and flatness below 5mm / 2m. Detailed Implementation

[0043] 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.

[0044] This invention provides a high-efficiency, low-cost 890MPa grade high-strength steel plate for low-temperature engineering structures. Its chemical composition, by weight percentage, is as follows: C 0.06%–0.10%, Si 0.1%–0.15%, Mn 1.40%–1.50%, P≤0.02%, S≤0.010%, Nb 0.02%–0.04%, Cr 0.4%–0.6%, Ni 0.35%–0.45%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0015%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.25%–0.35%, with the balance being Fe and unavoidable impurities. The steel plate has a thickness of 10-30mm and is produced on a medium-thick plate reciprocating rolling mill using continuously cast billets with a thickness of less than 250mm.

[0045] The aforementioned high-efficiency, low-cost manufacturing method for 890MPa grade high-strength steel plates used in low-temperature 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; specifically, it includes the following steps:

[0046] 1) Steelmaking to continuous casting: Smelting according to the above composition. Raw materials are pretreated with KR hot metal to control the S content below 0.015%, and after slag removal, they enter the converter. In the converter smelting, the double slag method is used to remove P, controlling the P content ≤0.020%. At the end of the converter smelting, the C content is controlled between 0.06% and 0.10%. Argon gas is blown for more than 30 minutes during tapping (argon blowing and killing 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 40 minutes. After the degassing treatment, Al is added according to the Al requirements. After adding Ti-Fe for micro-titanium treatment, B-Fe alloy is added. Ensure that it is added to the molten steel and avoids floating on the slag layer. Then, slab continuous casting is carried out, and the superheating temperature of continuous casting is 8-15℃. Continuous casting billet is then drawn. The speed is 0.7-0.9 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 20-30 s, the reverse stirring time is 5-10 s, the current is 800-1000 A, and the frequency is 20-50 Hz. This allows the molten steel to be continuously cast into a billet, and the isometric crystal ratio of the billet is not less than 80%. At the same time, strong cooling is used in the fan-shaped section. The total cooling water volume of the first half is 600-800 L / min, and the total cooling water volume of the second half is 800-1000 L / min. At the same time, heavy pressure is applied at the end of solidification, and the billet is reduced by 15-20 mm. After the billet is removed from the line, it is slowly cooled in the pit. The slow cooling temperature is not lower than 800℃, and the slow cooling time is not less than 72 h.

[0047] 2) Billet heating: The billet (thickness less than 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 950-1150℃, the temperature range of the heating section is 1230-1260℃, and the temperature range of the soaking section is 1200-1220℃. The total time in the furnace for the heating and soaking sections is controlled at 3-4 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃.

[0048] 3) Controlled rolling and pre-straightening: Before rolling, use high-pressure water to descale the cast billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 15-20 MPa; rolling is carried out in two stages: the first stage is recrystallization rolling (rough rolling), with a rough rolling start temperature ≥1120℃ and a rough rolling finish temperature range of 980-1030℃. The reduction rate of the first two passes in the rough rolling stage is not less than 25% per pass. Using 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 waiting period of the intermediate billet, spray two passes of descaling water, with a descaling time of 1-1. The first stage is 5 minutes, with the descaling machine pressure at 20-25 MPa. The second stage is also recrystallization rolling (finish rolling). The initial rolling temperature range for finish rolling is 920-950℃, and the final rolling temperature range is 860-890℃. The reduction ratio for each pass is 30% or more for the first two passes, and less than or equal to 5% for the last pass. The rolling length of the steel plate is controlled within 25m, and there are no more than five finish rolling passes. The rolled steel plate is pre-straightened at high temperature with a straightening force between 1000KN and 1200KN, a bending amount of 1-1.5mm, and an inclination value of 3-3.5mm.

[0049] 4) Relaxation, Cooling, and Hot Straightening: After rolling, the steel plate undergoes relaxation and temperature control for 30–40 seconds (ensuring a ferrite content of 10%–20% before immersion in water). The initial cooling temperature is 710–730℃, and the roller speed in the controlled cooling zone is 2–2.5 m / s with an acceleration of 0.003–0.005 m / s². 2 The final cooling temperature range is 400–420℃, the number of controlled cooling manifold opening groups is 3–6, and the water flow rate per manifold is 150–200 m³. 3 The cooling rate is controlled at 15~25℃ / s per hour. During the steel plate cooling process, the water ratio between the upper and lower manifolds is 1:1.8 to 1:2.3. 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 purging pressure is 5~10MPa, hot straightening is performed, the straightening temperature is below 300℃, the straightening force is between 2000KN~3000KN, the inlet roller position is -0.8mm~-1.5mm, the outlet roller position is -2.1mm~-3.0mm, and then air-cooled to room temperature.

[0050] Examples 1-6

[0051] Table 1 shows the chemical composition of the steel plates in the examples; Table 2 shows the smelting process of the steel plates in the examples; Table 3 shows the heating process of the billets for the steel plates in the examples; Table 4 shows the rolling and pre-straightening process parameters of the steel plates in the examples; Table 5 shows the main process parameters for relaxation, controlled cooling, and hot straightening of the steel plates in the examples; Table 6 shows the dimensions, properties, and flatness of the steel plates in the examples.

[0052] Table 1 Chemical composition (wt, %) of the steel plates in the examples

[0053]

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

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

[0056]

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

[0058]

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

[0060]

[0061] Table 5. Main process parameters for relaxation, controlled cooling, and hot straightening of the steel plates in the examples.

[0062]

[0063] Table 6. Dimensions, properties, and flatness of the steel plates in the examples.

[0064]

[0065] Therefore, compared with existing technologies, the efficient and low-cost production method of 890MPa grade, 10-30mm thick microalloyed steel of the present invention, through optimizing the steelmaking, heating, rolling, pre-straightening, relaxation, cooling, and straightening processes, ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-30mm thick 890MPa grade hot-rolled steel plates using online cooling processes. Furthermore, it eliminates the need for subsequent offline tempering and straightening treatments to meet the technical requirements of the plate. 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 according to 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 890MPa grade high-strength steel plate for cryogenic engineering structures, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C 0.06%–0.10%, Si 0.1%–0.15%, Mn 1.40%–1.50%, P≤0.02%, S≤0.010%, Nb 0.02%–0.04%, Cr 0.4%–0.6%, Ni 0.35%–0.45%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0015%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.25%–0.35%, with the balance being Fe and unavoidable impurities; The manufacturing method of the high-strength steel plate for low-temperature engineering structures with high efficiency and low cost of 890MPa 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 0.7–0.9 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage, with alternating forward and reverse stirring. Forward stirring lasts 20–30 seconds, and reverse stirring... The interval is 5-10 seconds, the current is 800-1000A, and the frequency is 20-50Hz, so that the molten steel is continuously cast to obtain a continuously cast billet, and the isometric crystal ratio of the continuously cast billet is not less than 80%. At the same time, strong cooling is used in the fan-shaped section, with a total cooling water volume of 600-800L / min in the first half and 800-1000L / min in the second half. At the same time, heavy pressure is applied at the end of solidification, and the continuously cast billet is reduced by 15-20mm. After the billet is removed from the line, it is slowly cooled in the pit, with a slow cooling temperature of not less than 800℃ and a slow cooling time of 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 950-1150℃, the temperature of the heating section is 1230-1260℃, and the temperature of the soaking section is 1200-1220℃. The total time in the furnace for heating and soaking is controlled to be 3-4 hours to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃. 3) Controlled rolling and pre-straightening: Rolling is performed in two stages: The first stage is rough rolling, with an initial rolling temperature ≥1120℃ and a final rolling temperature of 980~1030℃. The reduction rate in the first two passes of the rough rolling stage is not 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, two passes of descaling water are sprayed, with a descaling time of 1~1.5min and a descaling machine pressure of 20~25MPa. The second stage... The section is for finishing rolling, with an initial rolling temperature of 920-950℃ and a final rolling temperature of 860-890℃. The reduction rate per pass is guaranteed to be above 30% for the first two passes and less than or equal to 5% for the last pass. The rolling length of the steel plate is controlled within 25m, and the finishing rolling passes are no more than five. The rolled steel plate is pre-straightened at high temperature with a straightening force between 1000KN and 1200KN, a bending roll amount of 1-1.5mm, and an inclination value of 3-3.5mm. 4) Relaxation, cooling, and hot straightening: After rolling, the steel plate is relaxed and allowed to heat up for 30–40 seconds. The initial cooling temperature of the steel plate is 710–730℃. The roller speed in the controlled cooling zone is 2–2.5 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature is 400–420℃, and the cooling rate is controlled at 15–25℃ / s. During the cooling process of the steel plate, the water flow ratio of the upper and lower manifolds is 1:1.8–1:2.

3. After the steel plate exits controlled cooling, the side spray and air purging are activated, with side spray pressure and water flow rate of 5–10 MPa and 25–40 m³ / s, respectively. 3 / h, the air purging pressure is 5~10MPa, hot straightening is performed, the hot straightening temperature is below 300℃, and then air-cooled to room temperature.

2. The high-efficiency, low-cost 890MPa grade low-temperature engineering structure high-strength steel plate according to claim 1, characterized in that, The thickness of the steel plate is 10-30 mm.

3. The high strength steel plate for cryogenic engineering structure of 890 MPa grade with high efficiency and low cost according to claim 1, characterized in that, The steel plate has a transverse tensile yield strength ≥890MPa, a tensile strength of 1050~1160MPa, an elongation ≥12%, a transverse Charpy impact energy of -40℃ ≥80J, a yield strength ratio ≤0.85, a flaw detection pass rate ≥98%, and a straightness of less than 5mm / 2m.

4. The high strength steel plate for cryogenic engineering structure of 890 MPa grade with high efficiency and low cost according to claim 1, 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 0.06% to 0.10% 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 40 minutes.

5. The high strength steel plate for cryogenic engineering structure of 890 MPa grade with high efficiency and low cost according to claim 1, characterized in that, During the converter smelting process, the double-slag method is used for phosphorus removal; During the forced cooling process, the first half consists of stages 1 to 4, and the second half consists of stages 5 to 8.

6. The high strength steel plate for cryogenic engineering structure of 890 MPa grade with high efficiency and low cost according to claim 1, 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.

7. The high strength low cost 890 MPa grade high strength steel plate for cryogenic engineering structure according to claim 1, 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.

8. The high strength low cost 890 MPa grade high strength steel plate for cryogenic engineering structure according to claim 1, characterized in that, In step 4), the ferrite content of the steel plate before cooling is 10%–20%; the number of controlled cooling manifolds is 3–6, and the water flow rate per manifold is 150–200 m³. 3 / h.

9. The high-efficiency, low-cost 890MPa grade low-temperature engineering structure high-strength steel plate according to claim 1, characterized in that, In step 4), the straightening force of the hot straightening is between 2000KN and 3000KN, the position of the inlet roller is -0.8mm to -1.5mm, and the position of the outlet roller is -2.1mm to -3.0mm.

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