A high-strength steel plate for low-temperature engineering structure with 1100mpa grade and a manufacturing method thereof

Abstract

The application discloses a kind of high-efficiency low-cost 1100MPa-grade high-strength steel plate for low-temperature engineering structure and a manufacturing method thereof, and belongs to the technical field of low-carbon microalloy steel production.The optimized steelmaking, heating, rolling, pre-straightening, relaxation, controlled cooling, hot straightening and tempering process is used, the super-fast cooling+laminar cooling mode is used for controlled cooling, and finally the toughness of 1100MPa-grade hot-rolled steel plate with a thickness of 10-50mm produced by online cooling process is low, the yield ratio is high, the flaw detection qualified rate is low, and the plate shape unevenness problem is 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 10-50mm and a yield strength of 1100MPa for low-temperature engineering structures and its manufacturing method. Background Technology

[0002] In recent years, with the development of materials engineering technology, the strength grades of traditional industrial steels have been continuously improving. Represented by hydropower projects and engineering machinery, structural and equipment steels are developing towards the 1100MPa level. Offshore cranes and hydropower station pressure steel pipes have advantages such as high strength, good seepage prevention, and convenient on-site construction, and are widely used in the construction of large and medium-sized hydropower stations, pumped storage power stations, and marine engineering equipment. This type of steel is mainly based on carbon-manganese steel, with the addition of microalloying elements such as Nb, V, Cr, Mo, Cu, and Ni, as well as carbon and nitride compounds or highly hardenable elements, to ensure the microstructure and properties of the steel plate. Currently, this type of steel is mainly produced using an offline quenching and tempering process, leading to a significant increase in manufacturing costs and delivery time. Under the development goal of pursuing low-carbon and green production, reducing the manufacturing cost of 1100MPa-level high-strength steel with a thickness of less than 50mm has become a key objective for steel mills striving for high efficiency and low cost. Based on this, most steel mills both domestically and internationally have combined online controlled cooling equipment upgrades and rolling model optimizations to produce this grade of high-strength steel using an online cooling + offline tempering process. However, the low-temperature impact toughness, shape, yield strength ratio, and flaw detection pass rate of the product are significantly lower than those of the original offline quenching production process. Furthermore, to ensure low-temperature toughness, this steel grade often employs a two-stage rolling process. During the online rapid cooling process, the deformed austenite easily generates martensite, causing stress concentration, affecting crack propagation, and resulting in fluctuations in low-temperature toughness. The shape also cannot be guaranteed, limiting the development of green, low-cost production and promotion of 1100MPa-grade high-strength steel with a thickness of 10–50mm. Therefore, how to improve and solve the problems arising in the manufacturing process of 1100MPa-grade high-strength steel with a thickness of 10–50mm, reduce its manufacturing process costs, and improve product delivery cycle are key issues that urgently need to be addressed in the mass production of 1100MPa-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 an efficient and low-cost production method for low-temperature engineering structural steel with a thickness of 10–50 mm and a strength of 1100 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) mainly produced high-strength steel through offline quenching and tempering, which significantly increased the production cycle and cost, and could not meet the requirements for impact toughness at -40℃ and the 1100 MPa level. The influence of tempering temperature on the microstructure and properties of Q1100D high-strength structural steel disclosed in the above literature can solve the problem of low-temperature toughness 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 1100 MPa and excellent low-temperature toughness at -40℃ with a thickness of 10–50 mm by using TMCP + tempering instead of offline quenching and tempering. The technical solution provided by this invention can effectively overcome the above-mentioned shortcomings and solve the problems of low toughness, high yield strength ratio, low flaw detection pass rate and plate unevenness in the production of 1100MPa grade low temperature structural steel plates with a thickness of 10-50mm using online cooling process of continuous casting 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 1100MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method. This is an E-grade microalloyed 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, which ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1100MPa 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 1100MPa grade high-strength steel plate for low-temperature engineering structures. The chemical composition of the steel plate, by weight percentage, is: C 0.12%–0.17%, Si 0.25%–0.35%, Mn 1.10%–1.30%, P≤0.02%, S≤0.010%, Cr 0.5%–0.65%, 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 0.45%–0.65%, 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 0.12% to 0.17%.

[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 1.1%–1.3%.

[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 present invention selects an N content range of 0.02% to 0.03% and a Ti content of 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 plates. The Cr content range selected in this invention is 0.5% to 0.65%.

[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 0.45% and 0.65%.

[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] 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.

[0020] 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.

[0021] The steel plate described in this invention is an E-grade microalloyed high-strength steel plate with excellent low-temperature toughness.

[0022] This invention provides a high-efficiency, low-cost manufacturing method for 1100MPa 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 hot straightening → tempering; specifically including the following steps:

[0023] 1) Steel smelting to continuous casting: Smelting according to the following composition, the chemical composition by weight percentage is C 0.12%~0.17%, Si 0.25%~0.35%, Mn 1.10%~1.30%, P≤0.02%, S≤0.010%, Cr 0.5%~0.65%, 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 0.45%~0.65%, 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 0.7–1.0 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, reverse stirring time is 20–30 s, current is 800–1000 A, and frequency is 50–100 Hz, ensuring the molten steel achieves the desired properties after continuous casting. The continuously cast billet has an equiaxed crystal ratio of no less than 90%. In the fan-shaped section, strong cooling is employed, with a total cooling water flow of 500–700 L / min in the first half and 800–1000 L / min in the second half. Simultaneously, heavy pressure is applied at the end of solidification, with a reduction of 15–20 mm (this optimizes the electromagnetic stirring process during continuous casting, significantly increasing the equiaxed crystal ratio; strong cooling ensures a temperature gradient along the billet thickness; and light pressure helps reduce billet segregation and internal structural defects, while also promoting core grain breakage, ensuring the strength and toughness of subsequent steel plates). After finishing, the billet undergoes slow cooling in a pit. The billet is hot-charged into the slow cooling pit after finishing. When the billet temperature exceeds 600℃, the slow cooling pit is not heated, and the slow cooling time is no less than 72 hours (this promotes the diffusion of elements such as Mn, Cr, C, and B, mitigating their impact on microstructure and properties due to component segregation).

[0024] 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 temperature range is 850–1100℃ (to promote the rapid and complete dissolution and diffusion of Ti and Cr carbides into the matrix), the heating section temperature range is 1250–1270℃, and the soaking section temperature range is 1210–1220℃. The total time spent in the furnace during heating and soaking is controlled at 3.5–4.5 hours. The opening degree of the upper and lower burners in the furnace is adjusted. The air-fuel ratio is controlled between 1:1.8 and 1:2.2 to ensure that the temperature difference between the upper and lower surfaces of the billet is within 15℃ (the heating section provides high-temperature heating and controls the air-fuel ratio 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, by reducing the heating temperature of the billet soaking section, energy consumption is reduced, and the impact of coarsening of the original austenite grains on performance is also suppressed).

[0025] 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 1000~1065℃. 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, thus reducing the warming time). The second stage is recrystallization rolling (finishing rolling). The initial rolling temperature range for finishing is 930–970℃, and the final rolling temperature range is 860–900℃. 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 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 VN precipitation, providing more nucleation sites, and promoting…) Ferrite phase transformation occurs, refining the ferrite microstructure 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 microstructure after controlled cooling. The rolled steel plate undergoes high-temperature pre-straightening with a straightening force between 1000KN and 1500KN, 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).

[0026] 4) Relaxation, Cooling, and Hot Straightening: After rolling, the steel plate undergoes relaxation and temperature control for 25–35 seconds (this is to control the immersion temperature of the steel plate in water, ensuring a ferrite content of 10%–15% before immersion, guaranteeing the proportion of soft phases and deformation coordination in the post-rolling 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. 2During 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 350~400℃. The cooling rate in the ACC zone is controlled at 10~15℃ / s, with a final cooling temperature range of 150~250℃. (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 150~250℃. 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 5~10MPa and 25~40m³, 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 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 and martensite. At the same time, controlling the rolling length (within 25 meters), the speed of the cooling roller, and matching the appropriate acceleration ensures the temperature uniformity of the steel plate along its length. At the same time, optimizing the water ratio of the upper and lower manifolds ensures the shape of the steel plate after quenching. By side spraying and air purging, the residual water in the steel plate after quenching is reduced, ensuring that the shape of the steel plate is controlled and improving the uniformity of the steel plate performance). After controlled cooling, the steel plate is straightened.

[0027] 5) Tempering: The steel plates after controlled cooling and hot straightening are tempered at a temperature of 590~650℃ and in the furnace for 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 the width of the martensite laths, the proportion of precipitates and large-angle grain boundaries, so as to ensure strength and toughness).

[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 0.12% and 0.17% at the end of converter smelting; argon gas is blown for more than 30 minutes when tapping the steel (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.

[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 UFC zone cooling manifold groups opened is 2 to 4, and the water flow rate per manifold is 250 to 350 m³. 3 / h, the number of ACC zone cooling manifolds open is 2 to 4, and the water flow rate per manifold is 150 to 200 m³ / h. 3 / h.

[0034] Furthermore, in step 4), the steel plate after controlled cooling is straightened in 3 passes with a straightening force between 1500KN and 2500KN. The position of the inlet roller is -0.8mm to -1.5mm, and the position of the outlet roller is -1.3mm to -2.2mm (the straightening process is optimized by using multiple passes with small reductions to ensure that the cumulative strain at 30% to 40% of the distance from the steel plate surface exceeds the elastic limit strain, thus promoting the further and full release of internal stress).

[0035] A highly efficient and low-cost production method for 1100MPa 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 adopting an ultra-fast cooling + laminar flow cooling mode, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1100MPa grade hot-rolled steel plates with a thickness of 10-50mm using online quenching processes were finally solved. At the same time, the technical requirements of the plate can be met without subsequent offline quenching and straightening treatment.

[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. By using a V-Cr-NB-Mo-Ni composite design, the cost of the alloy and the resistance to high-temperature deformation during roughing and finishing rolling are greatly reduced, which is beneficial 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. To increase the content of dissolved boron in boron-containing steel, Ti, which has a stronger bonding force with nitrogen than boron, should be added. The content of vanadium in the steel has almost no effect on the content of dissolved boron, 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.

[0039] 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 into 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, 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, the heating temperature of the soaking zone is reduced to decrease energy consumption and also to suppress the impact of coarsening of the original austenite grains 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 730–750℃. Controlling the initial cooling temperature ensures a ferrite proportion of 10%–15% before the steel plate enters the water, guaranteeing the proportion of soft phases and deformation coordination in the post-rolling steel plate microstructure, and maintaining the yield strength ratio. It 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 improves the hardenability of the steel, increases austenite stability, enhances the phase transformation driving force, and promotes the transformation of acicular ferrite and martensite. 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. Optimizing the water ratio in the upper and lower manifolds controls the cooling rate in different temperature ranges. The UFC employs a rapid cooling rate to promote rapid film breaking through the steel plate. Rapid cooling is applied in the boiling zone, followed by slow cooling to gradually reduce the surface temperature to the core temperature. Cooling to 150-250℃ ensures sufficient hardening of the quenched steel plate while minimizing stress levels during quenching, thus maintaining 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 performance. The straightening process is optimized with multiple passes of small bending to ensure that the cumulative strain at 30%-40% of the plate surface exceeds the elastic limit strain, further promoting the release of internal stress. The tempering temperature is controlled to maintain the martensite lath width, precipitate proportion, and large-angle grain boundary ratio, ensuring strength and toughness.

[0042] 6. A high-efficiency, low-cost production method for 1100MPa 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, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate flatness in the production of 1100MPa grade hot-rolled steel plates with a thickness of 10-50mm using online quenching processes were ultimately 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 ≥1100MPa, tensile strength between 1300-1400MPa, elongation ≥10%, transverse Charpy impact energy at -40℃ ≥85J, yield strength ratio ≤0.88, flaw detection pass rate ≥98%, and flatness below 4mm / 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 1100MPa grade high-strength steel plate for low-temperature engineering structures. Its chemical composition, by weight percentage, is: C 0.12%–0.17%, Si 0.25%–0.35%, Mn 1.10%–1.30%, P≤0.02%, S≤0.010%, Cr 0.5%–0.65%, 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 0.45%–0.65%, with the balance being Fe and unavoidable impurities. The steel plate has a thickness of 10-50mm and is produced using continuously cast billets with a thickness of less than 250mm on a medium-thickness reciprocating rolling mill.

[0045] The aforementioned high-efficiency, low-cost manufacturing method for 1100MPa 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 and cooling → hot straightening → tempering; specifically, it includes the following steps:

[0046] 1) Steelmaking to continuous casting: Smelting according to the above composition. Raw materials undergo KR hot metal pretreatment to control the S content below 0.015%, and after slag removal, they enter the converter. During converter smelting, a double-slag method is used to remove P, controlling the P content ≤0.020%. At the end of converter smelting, the C content is controlled between 0.12% and 0.17%, and argon gas is blown for at least 30 minutes during tapping. Next, LF refining and RH vacuum degassing are performed, with RH vacuum maintained for at least 40 minutes. After RH vacuum degassing, Al is added according to Al requirements. After Ti-Fe micro-titanium treatment, B-Fe alloy is added, ensuring it is added to the molten steel to avoid floating on the slag layer. Then, slab continuous casting is performed, with a superheat of 8–15℃ and a casting speed of 0.7–1.0 m / min. Secondary cooling is controlled during the continuous casting stage. Electromagnetic stirring is used in the zone, with alternating forward and reverse stirring. The forward stirring time is 30-40 seconds, and the reverse stirring time is 20-30 seconds. The current is 800-1000A, and the frequency is 50-100Hz. This allows the molten steel to be continuously cast into a billet, with an isometric crystal ratio of not less than 90%. In the fan-shaped section, strong cooling is used. The total cooling water volume for sections 1-4 is 500-700L / min, and the total cooling water volume for sections 5-8 is 800-1000L / min. At the end of solidification, heavy pressure is applied, and the billet is reduced by 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 600℃, the slow cooling pit is not heated, and the slow cooling time is not less than 72 hours.

[0047] 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 1250-1270℃, and the temperature range of the soaking section is 1210-1220℃. 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℃.

[0048] 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 1000-1065℃. 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, two passes of descaling water are sprayed, 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 930-970℃, and the final rolling temperature range is 860-900℃. 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 then pre-straightened at high temperature with a straightening force between 1000KN and 1500KN, a bending roll amount of 2-2.5mm, and a tilting value of 2-3mm.

[0049] 4) Relaxation, Cooling, and Hot Straightening: After rolling, the steel plate undergoes relaxation and temperature control for 25–35 seconds (ensuring a ferrite content of 10%–15% 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, with a single manifold water flow of 250 to 350 m³ / h. 3 The cooling rate is controlled at 35~50℃ / s, with a final cooling temperature range of 350~400℃. The number of ACC zone cooling manifolds in operation is 2~4, 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 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 / 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 1500KN~2500KN, the position of the inlet roller is -0.8mm~-1.5mm, and the position of the outlet roller is -1.3mm~-2.2mm.

[0050] 5) Tempering: The steel plates after controlled cooling and hot straightening are tempered at a temperature of 590~650℃ and in the furnace for 3~5 min / mm. After being taken out of the furnace, they are air-cooled to room temperature to obtain the finished steel plate.

[0051] Examples 1-6

[0052] 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 parameters of the steel plate in the examples; Table 5 shows the relaxation and cooling process of the steel plate in the examples; Table 6 shows the hot straightening and tempering process of the steel plate in the examples; Table 7 shows the dimensions, properties and flatness of the steel plate in the examples.

[0053] Table 1 Chemical composition (wt, %) of the steel plates in the examples

[0054]

[0055] Note: Impurity elements in steel: P≤0.02%, S≤0.010%.

[0056] Table 2. Smelting process of steel plates in the examples

[0057]

[0058] Table 3 Heating regime of steel plate billet in the examples

[0059]

[0060] Table 4 Rolling and pre-straightening processes of steel plates in the examples

[0061]

[0062] Table 5 Relaxation and Cooling Processes of Steel Plates in Examples

[0063]

[0064] Table 6 Hot straightening and tempering process of steel in the examples

[0065]

[0066] Table 7 Dimensions, properties, and flatness of the steel plates in the examples

[0067]

[0068] Therefore, compared with existing technologies, the efficient and low-cost production method of 1100MPa 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. The cooling process employs an ultra-fast cooling + laminar flow cooling mode, ultimately solving the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 1100MPa grade hot-rolled steel plates with a thickness of 10-50mm using online quenching processes. Furthermore, it meets the technical requirements of the plate without the need for subsequent offline quenching and straightening treatments.

[0069] 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, 1100MPa-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 0.12%–0.17%, Si 0.25%–0.35%, Mn 1.10%–1.30%, P≤0.02%, S≤0.010%, Cr 0.5%–0.65%, 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 0.45%–0.65%, with the balance being Fe and unavoidable impurities.

2. The high-efficiency, low-cost 1100MPa 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 1100MPa 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 ≥1100MPa, a tensile strength of 1300~1400MPa, an elongation ≥10%, a transverse Charpy impact energy of -40℃ ≥85J, a yield strength ratio ≤0.88, a flaw detection pass rate ≥98%, and a straightness of less than 4mm / 2m.

4. A method for manufacturing a high-efficiency, low-cost 1100MPa 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 0.7–1.0 m / min. Electromagnetic stirring is used in the secondary cooling zone during the continuous casting stage, with alternating forward and reverse stirring. Forward stirring time is 30–40 s, and reverse stirring time is 20–30 s. The flow rate is 800-1000A and the frequency is 50-100Hz, so that the molten steel is continuously cast to obtain a continuously cast billet. The proportion of isometric crystals in the continuously cast billet is not less than 90%. At the same time, strong cooling is used in the fan-shaped section. The total cooling water flow rate of the first half is 500-700L / min, and the total cooling water flow rate of the second half is 800-1000L / min. 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. After the billet is removed from the line, it is hot-charged into the slow cooling pit. When the billet temperature is greater than 600℃, 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 range of the heating section is 1250-1270℃, and the temperature of the soaking section is 1210-1220℃. 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 1000~1065℃. The reduction rate of the first two passes in 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 to 4 times the thickness of the finished product. During the warming process of the intermediate billet, 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. a; The second stage is finishing rolling. The initial rolling temperature range for finishing rolling is 930-970℃, and the final rolling temperature is 860-900℃. 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 the finishing rolling is no more than five passes. The rolled steel plate is pre-straightened at high temperature with a straightening force between 1000KN and 1500KN, a bending roll amount of 2-2.5mm, and a tilting value of 2-3mm. 4) Relaxation, cooling, and hot straightening: After rolling, the steel plate is relaxed and allowed to heat for 25–35 seconds. 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, 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 590~650℃ 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 1100MPa-grade low-temperature engineering structures according to 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 0.12% to 0.17% 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.

6. The method for manufacturing high-strength steel plates for 1100MPa-grade cryogenic engineering structures according to claim 4, characterized in that, During the converter smelting process, the double slag method is used for phosphorus removal; during the continuous casting process, the first half is sections 1 to 4, and the second half is sections 5 to 8.

7. The method for manufacturing high-strength steel plates for 1100MPa-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 1100MPa-grade low-temperature engineering structures according to 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 1100MPa-grade cryogenic engineering structures according to claim 4, characterized in that, In step 4), the ferrite content of the steel plate before cooling is 10%–15%; 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 250–350 m³ / h. 3 / h; The number of controlled cooling manifold groups in the laminar flow cooling zone is 2 to 4, and the water flow rate per manifold is 150 to 200 m³ / h. 3 / h.

10. The method for manufacturing high-strength steel plates for 1100MPa-grade cryogenic engineering structures according to claim 4, characterized in that, In step 4), the steel plate after controlled cooling is straightened in 3 passes with a straightening force between 1500KN and 2500KN. The position of the inlet roller is -0.8mm to -1.5mm, and the position of the outlet roller is -1.3mm to -2.2mm.

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