High-strength steel plate for low-temperature engineering structure with 690mpa grade and low cost and manufacturing method thereof

Abstract

The application discloses a kind of high-efficiency low-cost 690MPa 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. By using optimized steelmaking, heating, rolling, controlled cooling and straightening processes, the problems of low toughness, high yield ratio, low flaw detection qualification rate and unevenness of plate shape in the production of 690MPa grade hot-rolled steel plates with a thickness of 10-40mm by online cooling process are finally solved.

Description

Technical Field

[0001] This invention belongs to the field of low-carbon microalloyed steel production technology, and specifically relates to a high-efficiency, low-cost 690MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method. Background Technology

[0002] Faced with the severe market situation, customer demands, and manufacturing cost pressures in the steel industry, it is crucial to improve production line efficiency, reduce process bottlenecks, and maximize production capacity while ensuring product quality. In particular, medium and heavy plates, represented by the 690MPa grade, are a major type of high-strength steel. Typical varieties include Q690. This type of steel is mainly based on carbon-manganese steel, with the addition of microalloying elements such as Nb, V, Ti, Mo, and Cr, which are carbide and nitride-forming elements. These elements enhance the steel's strength and toughness through solid solution strengthening, precipitation strengthening, and grain refinement. Statistics show that medium and heavy plates of this strength grade account for 5-10% of annual production. Currently, 690MPa high-strength steel generally adopts a controlled rolling and cooling combined with offline quenching and tempering process. However, this method has drawbacks: high alloy costs, and from the perspective of actual production and process control, the heat treatment process after steel plate rolling undoubtedly increases production costs. Therefore, the production of medium and heavy plates, especially high-strength steel medium and heavy plates represented by the 690MPa grade, urgently needs to further optimize the production process, reduce energy consumption, compress manufacturing costs, and improve production efficiency on the current basis, so as to ultimately achieve a rapid improvement in the market competitiveness of the products.

[0003] Currently, research on online quenching processes for high-strength steel plates with a thickness of 40mm and below is gradually increasing. The online thermomechanical control process (TMCP) involves direct quenching of the steel plate on the main rolling line, which not only saves energy but also shortens the delivery cycle of finished steel plates. It is a green process for short-process, reduced-volume production of quenched and tempered steel. With the continuous advancement of post-rolling water cooling equipment and plate shape control capabilities, such as the introduction of ultra-fast cooling, pre-straightening machines, and warm straightening machines, hardware support has been provided for the promotion of this process. Major medium and heavy plate mills have successively carried out research and application of the process based on new equipment, solving problems such as performance consistency. However, the low-temperature impact toughness, plate shape, yield strength ratio, and flaw detection pass rate of the products are significantly lower than those of the offline quenching production process, which is one of the common problems faced by the industry. Many steel mills have adopted methods such as increasing the tempering temperature to improve product toughness and offline straightening to ensure plate shape, but the effect is not obvious and also increases production costs. Therefore, in response to the aforementioned problems with 690MPa high-strength steel plates, there is an urgent need to develop 690MPa grade steel plates that require less production equipment, have lower costs, higher strength and toughness, and lower yield strength ratio, in order to meet the urgent demand of the rapidly developing manufacturing industry for high-strength steel with excellent performance and low cost.

[0004] Currently, with the optimization of alloy costs and steelmaking processes for 690MPa grade steel plates, the optimized alloys and processes require the use of TMCP (Transformed Metallized Processing) technology for production, with the austenitic uncrystallized zone rolling process commonly employed in the finishing rolling stage. Steel plates produced using this process exhibit high strength but low impact toughness. This is because the austenite grains in the rolled steel plate are primarily flattened. During subsequent direct cooling, this morphology easily forms martensite laths penetrating the original austenite grains, with a relatively uniform orientation, which is detrimental to preventing crack propagation. Therefore, the toughness is low, failing to reach the level of offline quenching, and the internal stress of the steel plate is high, making it difficult to guarantee the yield strength ratio and plate shape. This limits the efficient and low-cost production and promotion of low-cost 10-40mm thick 690MPa grade low-temperature engineering structural steel. Therefore, how to improve and solve the problems arising in the manufacturing process of 10-40mm thick 690MPa grade low-temperature engineering structural steel plates and reduce its manufacturing costs is a key issue that urgently needs to be addressed in the mass production of 690MPa grade engineering structural medium and thick plates.

[0005] Compared with existing technologies:

[0006] 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 10–40 mm and a strength of 690 MPa. Prior to this invention, the journal article "The Influence of Tempering Temperature on the Microstructure and Properties of Q690D High-Strength Structural Steel" (Heat Treatment Technology, 2014.8) mainly enhanced the toughness of the steel plate by using offline tempering heat treatment. However, due to the use of offline tempering heat treatment, its production cycle and cost increased significantly, and it did not meet the -60℃ service requirement. The influence of tempering temperature on the microstructure and properties of Q690D high-strength structural steel disclosed in the above literature can solve the problem of steel plate toughness, but it is not suitable for controlling and solving the problem of using TMCP to replace offline heat treatment process to produce steel plates with a yield strength of 690 MPa, excellent low-temperature toughness, and a thickness of 10–40 mm with an efficient and low-cost production method. 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 690MPa grade low temperature structural steel plates with a thickness of 10-40mm using the online cooling process of continuous casting billets with a thickness of 250mm or less. Summary of the Invention

[0007] The purpose of this invention is to overcome the aforementioned technical problems and deficiencies, and to provide a high-efficiency, low-cost 690MPa grade high-strength steel plate for low-temperature engineering structures and its manufacturing method. This plate is an F-grade microalloyed high-strength steel plate with excellent low-temperature toughness. The method adopts optimized steelmaking, heating, rolling, controlled cooling and straightening processes, and ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate and plate unevenness in the production of 10-40mm thick 690MPa grade hot-rolled steel plates by online cooling processes.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] This invention provides a high-efficiency, low-cost 690MPa grade high-strength steel plate for low-temperature engineering structures. The chemical composition of the steel plate, by weight percentage, is: C 0.04%–0.08%, Si 0.25%–0.35%, Mn 1.60%–1.70%, P≤0.02%, S≤0.010%, Nb 0.02%–0.04%, Cr 0.3%–0.5%, Ni 0.25%–0.35%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0012%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.08%–0.12%, with the balance being Fe and unavoidable impurities.

[0010] The roles of the main elements in the chemical composition of the steel plate of this invention are as follows:

[0011] Carbon (C): The most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. However, increasing the C content has a negative impact on the plasticity, toughness, and weldability of steel. Therefore, this invention sets the C content range to 0.04% to 0.08%.

[0012] Mn: Mn strengthens steel through solid solution treatment, while compensating for the strength loss caused by the reduction in carbon content. Furthermore, it lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to finer low-temperature transformation products, thus improving toughness. However, increasing the Mn content exacerbates center segregation in continuously cast billets, hindering the improvement of low-temperature toughness and compromising the uniformity of the cross-sectional microstructure. Therefore, the Mn content range in this invention is designed to be 1.6%–1.70%.

[0013] 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.

[0014] 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.

[0015] 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%.

[0016] 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%.

[0017] Cr: It is a major element that can effectively improve hardenability, inhibit ferrite formation and promote bainite formation. It plays an important role in controlling phase transformation structure, promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium and low temperature range, and improves the strength, plasticity and toughness of steel plate. The Cr content range selected in this invention is 0.3% to 0.5%.

[0018] 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.

[0019] 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 below 0.12%.

[0020] B: A highly 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 noticeable effect. However, excessive boron content can easily form boron carbonitrides, reducing toughness and causing hot brittleness. The reasonable range is 0.0012–0.0020%.

[0021] 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.

[0022] The steel plate of the present invention has a thickness of 10 to 40 mm, and is produced on a medium-thick plate reciprocating rolling mill using a billet with a thickness of less than 250 mm, with water as the cooling medium.

[0023] The steel plate described in this invention is an F-grade microalloyed high-strength steel plate with excellent low-temperature toughness.

[0024] The 690MPa grade high-strength steel plate for low-temperature engineering structures described in this invention is a hot-rolled steel plate.

[0025] This invention provides a high-efficiency, low-cost method for manufacturing 690MPa grade high-strength steel plates for low-temperature engineering structures, comprising: steel smelting → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet heating → controlled rolling → cooling and hot straightening; specifically including the following steps:

[0026] 1) Steel smelting to continuous casting: Smelting according to the following composition, the chemical composition by weight percentage is C 0.04%~0.08%, Si 0.2%~0.35%, Mn 1.60%~1.70%, P≤0.02%, S≤0.010%, Cr 0.3%~0.5%, Ni 0.25%~0.35%, Al 0.015%~0.035%, Nb 0.02%~0.04%, V 0.1%~0.2%, B 0.0012%~0.0020%, N 0.015%~0.025%, Ti 0.01%~0.02%, Mo 0.08%~0.12%, 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 8–13°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 10–15 s, reverse stirring time is 3–6 s, current is 500–800 A, and frequency is 10–20 Hz, ensuring the molten steel... The continuous casting process yields a billet with an equiaxed crystal ratio of no less than 60%. In the fan-shaped section, strong cooling is employed, with a total cooling water flow of 800–1000 L / min in the first half and 1000–1200 L / min in the second half. Simultaneously, light reduction is applied at the end of solidification, with a reduction of 8–15 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 reduction helps reduce billet segregation and internal structural defects, while also promoting core grain breakage, thus guaranteeing the strength and toughness of the subsequent steel plate). After casting, the billet undergoes slow cooling in a pit at a temperature no lower than 600℃ for no less than 48 hours (this promotes the diffusion of elements such as Mn, Cr, and C, mitigating their impact on microstructure and properties due to component segregation).

[0027] 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 1220–1250℃, and the soaking section has a temperature range of 1200–1210℃. 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 inhibits the impact of coarsening of the original austenite grains on performance.)

[0028] 3) Controlled rolling: The billet is rolled in two stages: the first stage is recrystallization rolling (rough rolling), with an initial rolling temperature ≥1100℃ and a final rolling temperature range of 980~1030℃. The reduction rate in the first two passes of the rough rolling stage is no less than 25% per pass. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than four (this utilizes the high-temperature deformation-induced effect to promote the precipitation of the 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; simultaneously…) It also induces TiN precipitation, inhibits BN precipitation, and promotes dynamic recrystallization of austenite grains, refining the grains. The thickness of the intermediate billet is 2.5 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 0.5 to 1.5 minutes and a descaling machine pressure of 20 to 25 MPa (to inhibit austenite grain growth, while simultaneously generating a temperature gradient on the inner and outer surfaces of the billet, promoting the penetration of rolling deformation into the thickness center, refining the grains at 1 / 2 thickness, which is beneficial for improving the thickness of thick steel plates). The first stage involves improving the core structure, reducing structural stress caused by uneven microstructure, and accelerating the temperature drop of the intermediate billet, thus reducing the waiting time. The second stage is recrystallization rolling (finish rolling). The initial rolling temperature range for finish rolling is 930–960℃, and the final rolling temperature range is 850–870℃. The reduction regime for each pass is as follows: the first two passes ensure a reduction rate of over 25%, and the final pass uses a reduction rate of less than 5%. The rolling length of the steel plate is controlled within 25m, and the finish rolling has no more than five passes. (By controlling the final rolling temperature of the roughing stage, the waiting time of the intermediate billet is ensured.) During the finishing rolling process, the temperature is within the austenite recrystallization temperature range to ensure microstructure uniformity. Simultaneously, controlling the reduction rate during the finishing rolling stage ensures the density of dislocations, vacancies, and deformation bands within the austenite body, promoting VN precipitation, providing more nucleation sites, facilitating ferrite phase transformation, refining the ferrite microstructure, and ensuring the strength and toughness of the steel plate. A small reduction rate is used in the final pass to flatten the steel 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 microstructure uniformity of the steel plate after controlled cooling.

[0029] 4) Cooling and straightening: The steel plate is cooled at an initial cooling temperature of 770–830℃. The roller speed in the controlled cooling zone is 1.5–2.0 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature range is 450–500℃, and the cooling rate is controlled at 5–10℃ / s. During the cooling process of the steel plate, the water flow ratio of the upper and lower manifolds is 1:1.6–1:2.1. 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 730~760℃. By controlling the starting cooling temperature, the density of VN nano-precipitation is guaranteed, which can significantly improve the mechanical properties of steel through precipitation strengthening. In addition, the addition of B and Cr 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 bainite. Based on this, a high final cooling temperature and slow cooling rate are adopted to avoid excessive internal stress in the steel plate due to excessive cooling rate, resulting in poor plate shape; at the same time, the rolling length is controlled (within 25m), the speed of the controlled cooling roller is increased, and a suitable acceleration is matched to ensure the temperature uniformity of the steel plate along the length direction. At the same time, the water ratio of the upper and lower manifolds is optimized to ensure the plate shape of the steel plate after quenching. Side spraying and air purging are used to reduce the residual water in the steel plate after quenching, ensuring that the plate shape is controlled and the uniformity of steel plate performance is improved). Hot straightening is carried out at a temperature below 400℃, followed by air cooling to room temperature.

[0030] Furthermore, in step 1), the raw material is pretreated with KR molten iron to control the S content to be below 0.015%, and then enters the converter after slag removal; during converter smelting, the P content is controlled to be ≤0.020%, and the C content is controlled to be between 0.04% and 0.08% at the end of converter smelting; argon gas is blown for 25 to 30 minutes when tapping the steel (argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition); then LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

[0031] Furthermore, during the converter smelting process, a double-slag method is used for phosphorus removal.

[0032] Furthermore, in the continuous casting process, the first half is sections 1 to 4, and the second half is sections 5 to 8.

[0033] Furthermore, the thickness of the cast billet is less than 250 mm.

[0034] 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 15-25 MPa.

[0035] 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.

[0036] Furthermore, in step 4), the straightening force of the hot straightening is between 2500KN and 3000KN, the position of the inlet roller is -0.8mm to -1.8mm, and the position of the outlet roller is -2.3mm to -3.0mm (to optimize the straightening process and promote the further full release of internal stress).

[0037] A highly efficient and low-cost production method for 690MPa grade microalloyed steel with a thickness of 10-40mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, cooling, and straightening processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate unevenness in the production of 10-40mm thick 690MPa grade hot-rolled steel plates were finally solved. Furthermore, the technical requirements of the plates can be met without subsequent offline tempering and straightening treatments.

[0038] The beneficial effects of this invention are:

[0039] 1. After deep desulfurization pretreatment of KR molten iron, slag is thoroughly removed. The converter employs a double-slag method for P removal, resulting in lower P and S content in the billet. Controlling the argon blowing time and maintaining the RH vacuum degassing time overcomes defects such as center segregation and excessive inclusions caused by high Mn, Cr, and C content, thus improving the plasticity and toughness of the steel plate. Reducing superheat and continuous casting speed improves macroscopic segregation in the continuously cast billet, decreasing the spacing between secondary dendrite arms in the solidification structure, which helps reduce billet segregation and internal structural defects. Optimizing the electromagnetic stirring process during continuous casting significantly increases the equiaxed grain ratio of the billet. Strong cooling ensures a temperature gradient along the billet thickness, while applying light pressure helps reduce billet segregation and internal structural defects, while also promoting core grain breakage, ensuring the strength and toughness of the subsequent steel plate. Simultaneously, slow cooling of the billet after casting, with controlled cooling temperature and time, promotes the diffusion of Mn, Cr, and C elements, mitigating their impact on microstructure and properties due to component segregation.

[0040] 2. The composition of this invention is reasonable, and the amount of alloy added is low. By using a V-Cr-NB 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 B on the improvement of austenite stability is related to the amount of B dissolved in the steel. To increase the amount of dissolved B in boron-containing steel, Ti, which has a stronger bonding force with N than B, should be added. The V content in the steel has almost no effect on the amount of dissolved B, 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.

[0041] 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.

[0042] 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 the uniformity of the microstructure. At the same time, the reduction rate and thickness of the intermediate billet are limited in both stages of rolling. The high-temperature deformation-induced effect is used 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 and inhibits BN precipitation. Descaling water is sprayed to inhibit austenite grain growth and generate a temperature gradient on the inner and outer surfaces of the billet, promoting the penetration of rolling deformation into the thickness center, refining the grains at 1 / 2 thickness, which is beneficial to improving the core microstructure of thick steel plates and reducing the microstructure inhomogeneity that causes structural stress. The final pass uses a small reduction rate to flatten the shape of the steel plate, reduce the internal stress of the steel plate, and control the length of the rolled steel plate to avoid excessively rapid temperature drop in the later rolling stages, which would affect the microstructure uniformity of the steel plate after controlled cooling.

[0043] 5. Through calculation and experimentation, the fastest precipitation temperature range for VN is 730–760℃. By controlling the initial cooling temperature, the density of VN nanoprecipitates is ensured, which can significantly improve the mechanical properties of steel through precipitation strengthening. In addition, the addition of B and Cr can improve the hardenability of steel, increase the stability of austenite, increase the driving force of phase transformation, and promote the transformation of acicular ferrite and bainite. Based on this, a high final cooling temperature and a slow cooling rate are adopted to avoid excessive internal stress in the steel plate and poor plate shape due to excessive cooling rate. At the same time, the rolling length is controlled, the speed of the controlled cooling roller is increased, and a suitable acceleration is matched to ensure the temperature uniformity of the steel plate along its length. The water ratio of the upper and lower manifolds is optimized to ensure the plate shape of the steel plate after quenching. Side spraying and air blowing are used to reduce the residual water in the steel plate after quenching, ensuring that the plate shape is controlled and the uniformity of steel plate performance is improved. Hot straightening is adopted, and the position and pressure of the inlet and outlet rollers are set to ensure that the straightened steel plate is straight and has a good plate shape.

[0044] 6. A high-efficiency, low-cost production method for 690MPa grade microalloyed steel with a thickness of 10-40mm was obtained using the above-mentioned composition and process scheme. By optimizing the steelmaking, heating, rolling, cooling, and straightening processes, the problems of low toughness, high yield strength ratio, low flaw detection pass rate, and plate flatness in the production of 10-40mm thick 690MPa 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 ≥690MPa, tensile strength between 800-950MPa, elongation ≥16%, transverse Charpy impact energy at -60℃ ≥100J, yield strength ratio ≤0.85, flaw detection pass rate ≥98%, and straightness below 5mm / 2m; the microstructure of the steel plate is bainite + martensite, with bainite accounting for 80%-90% and martensite accounting for 10%-20%. Detailed Implementation

[0045] 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.

[0046] This invention provides a high-efficiency, low-cost 690MPa grade high-strength steel plate for low-temperature engineering structures. Its chemical composition, by weight percentage, is as follows: C 0.04%–0.08%, Si 0.2%–0.35%, Mn 1.60%–1.70%, P≤0.02%, S≤0.010%, Cr 0.3%–0.5%, Ni 0.25%–0.35%, Al 0.015%–0.035%, Nb 0.02%–0.04%, V 0.1%–0.2%, B 0.0012%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.08%–0.12%, with the balance being Fe and unavoidable impurities. The steel plate has a thickness of 10-40mm and is produced on a medium-thick plate reciprocating rolling mill using continuously cast billets with a thickness of less than 250mm.

[0047] The aforementioned high-efficiency, low-cost manufacturing method for 690MPa grade high-strength steel plates used in cryogenic engineering structures includes steel smelting → ladle refining (LF refining) + RH vacuum degassing → B alloying → continuous casting → billet reheating → controlled rolling → cooling and hot straightening; specifically, it includes the following steps:

[0048] 1) Steelmaking and Continuous Casting: Smelting is carried out 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 to ≤0.020%. At the end of converter smelting, the C content is controlled between 0.04% and 0.08%. Argon gas is blown for 25–30 minutes during tapping (argon blowing and calming before continuous casting promotes the removal of inclusions in the molten steel and improves the uniformity of the steel composition). Then, LF refining and RH vacuum degassing are performed, with RH vacuum maintained for at least 30 minutes. After degassing, Al is added according to Al requirements. After micro-titanium treatment with Ti-Fe, B-Fe alloy is added, ensuring it is added to the molten steel and avoids floating on the slag layer. Slab continuous casting is then performed, with a superheat of 8–13°C, followed by continuous casting of the billet. The speed is 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. The forward stirring time is 10-15 s, the reverse stirring time is 3-6 s, the current is 500-800 A, and the frequency is 10-20 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 60%. At the same time, strong cooling is used in the fan-shaped section. The total cooling water volume of the first to fourth sections is 800-1000 L / min, and the total cooling water volume of the fifth to eighth sections is 1000-1200 L / min. At the same time, light reduction is applied at the end of solidification, and the reduction of the billet is 8-15 mm. After the billet is removed from the line, it is slowly cooled in the pit. The slow cooling temperature is not lower than 600℃, and the slow cooling time is not lower than 48 h.

[0049] 2) Billet heating: The billet (thickness less than 250mm) is sent into the walking beam furnace for heating. The temperature range of the preheating section is 950-1150℃, the temperature range of the heating section is 1220-1250℃, and the temperature range of the soaking section is 1200-1210℃. 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℃.

[0050] 3) Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, with a descaling machine pressure of 15-25 MPa; rolling is carried out in two stages: the first stage is recrystallization rolling (rough rolling), with a rough rolling start temperature ≥1100℃ 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. Utilizing the pass locking function, the number of rolling passes in the rough rolling stage is less than four, and the thickness of the intermediate billet is 2.5 times the thickness of the finished product. ~4 times; during the intermediate billet warming process, spray descaling water twice, with a descaling time of 0.5~1.5min and a descaling machine pressure of 20~25MPa; the second stage is recrystallization rolling (finish rolling), with a finishing rolling temperature range of 930~960℃ and a finishing rolling temperature range of 850~870℃. The pass reduction regime is that the first two passes ensure a reduction rate of more than 25%, and the last pass adopts a reduction rate of less than 5%, and the steel plate rolling length is controlled within 25m, with no more than five finishing rolling passes;

[0051] 4) Cooling and straightening: The steel plate is cooled at an initial cooling temperature of 770–830℃. The roller speed in the controlled cooling zone is 1.5–2.0 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature range is 450–500℃, the number of controlled cooling manifolds open is 3–6, and the water flow rate per manifold is 150–200 m³. 3 The cooling rate is controlled at 5~10℃ / s. During the steel plate cooling process, the water ratio between the upper and lower manifolds is 1:1.6~1:2.1. After the steel plate exits controlled cooling, the side spray and air purging are activated, with side spray pressure and water volume of 5~10MPa and 25~40m³ / h, respectively. 3 / h, the air purging pressure is 5~10MPa, hot straightening is performed, the hot straightening temperature is below 400℃, the straightening force is between 2500KN~3000KN, the inlet roller position is -0.8mm~-1.8mm, the outlet roller position is -2.3mm~-3.0mm, and then air-cooled to room temperature.

[0052] Examples 1-6

[0053] Table 1 shows the chemical composition of the steel plate in the example; Table 2 shows the smelting process of the steel plate in the example; Table 3 shows the heating process of the billet of the steel plate in the example; Table 4 shows the rolling process parameters of the steel plate in the example; Table 5 shows the main cooling and straightening process parameters of the steel plate in the example; Table 6 shows the dimensions, properties and flatness of the steel in the example.

[0054] Table 1 Chemical composition (wt, %) of the steel plates in the examples

[0055]

[0056] Note: Impurity elements in steel: P≤0.02%, S≤0.01%.

[0057] Table 2. Smelting process of steel plates in the examples

[0058]

[0059] Table 3 Heating regime of steel plate billets in the examples

[0060]

[0061] Table 4 Rolling process parameters of steel plates in the examples

[0062]

[0063] Table 5. Main process parameters for cooling and straightening of steel plates in embodiments of the present invention.

[0064]

[0065] Table 6. Dimensions, properties, and flatness of the steel plates in the examples.

[0066]

[0067] Therefore, compared with existing technologies, the efficient and low-cost production method of 690MPa grade, 10-40mm thick microalloyed steel of this invention, through optimization of steelmaking, heating, rolling, cooling and straightening processes, ultimately solves the problems of low toughness, high yield strength ratio, low flaw detection pass rate and plate flatness in the production of 10-40mm thick 690MPa grade hot-rolled steel plates using online cooling processes. Simultaneously, it meets the technical requirements of the plate without subsequent offline tempering and straightening treatment. The mechanical properties of the produced plate are as follows: transverse tensile yield strength ≥690MPa, tensile strength between 800 and 950MPa, elongation ≥16%, transverse Charpy impact energy at -60℃ ≥100J, yield strength ratio ≤0.85, flaw detection pass rate ≥98%, and flatness below 5mm / 2m; the microstructure of the steel plate is bainite + martensite, with bainite accounting for 80%–90% and martensite accounting for 10%–20%.

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

Claims

1. A high-efficiency, low-cost 690MPa 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 0.04%–0.08%, Si 0.25%–0.35%, Mn 1.60%–1.70%, P≤0.02%, S≤0.010%, Nb 0.02%–0.04%, Cr 0.3%–0.5%, Ni 0.25%–0.35%, Al 0.015%–0.035%, V 0.1%–0.2%, B 0.0012%–0.0020%, N 0.015%–0.025%, Ti 0.01%–0.02%, Mo 0.08%–0.12%, with the balance being Fe and unavoidable impurities.

2. The high-efficiency, low-cost 690MPa grade low-temperature engineering structure high-strength steel plate according to claim 1, characterized in that, The thickness of the steel plate is 10-40 mm; the microstructure of the steel plate is bainite + martensite, wherein the bainite accounts for 80%-90% and the martensite accounts for 10%-20%.

3. The high-efficiency, low-cost 690MPa 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 ≥690MPa, a tensile strength of 800~950MPa, an elongation ≥16%, a transverse Charpy impact energy of -60℃ ≥100J, a yield strength ratio ≤0.85, a flaw detection pass rate ≥98%, and a flatness of less than 5mm / 2m.

4. The method for manufacturing high-strength steel plate for low-temperature engineering structures with high efficiency and low cost as described in 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 8–13°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 lasts 10–15 seconds, and reverse stirring... The stirring time is 3-6 seconds, the current is 500-800A, and the frequency is 10-20Hz. The molten steel is continuously cast to obtain a billet, and the isometric crystal ratio of the billet is not less than 60%. At the same time, strong cooling is used in the fan-shaped section. The total cooling water volume of the first half is 800-1000L / min, and the total cooling water volume of the second half is 1000-1200L / min. At the same time, light reduction is applied at the end of solidification, and the reduction of the continuously cast billet is 8-15mm. After the billet is removed from the line, it is slowly cooled in the pit. The slow cooling temperature is not less than 600℃ and the slow cooling time is not less than 48h. 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 1220-1250℃, and the temperature of the soaking section is 1200-1210℃. 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℃. 3) Controlled rolling: The billet is rolled in two stages. The first stage is rough rolling, with an initial rolling temperature of ≥1100℃ and a final rolling temperature of 980~1030℃. 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 2.5 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 0.5~1.5min and a descaling machine pressure of 20~25MPa. The second stage is finish rolling, with an initial rolling temperature of 930~960℃ and a final rolling temperature of 850~870℃. The reduction rate of the first two passes is guaranteed to be above 25%, and the reduction rate of the last pass is less than 5%. The rolling length of the steel plate is controlled within 25m, and the finish rolling has no more than five passes. 4) Cooling and straightening: The steel plate is cooled at an initial cooling temperature of 770–830℃. The roller speed in the controlled cooling zone is 1.5–2.0 m / s, and the acceleration is 0.003–0.005 m / s². 2 The final cooling temperature is 450–500℃, and the cooling rate is controlled at 5–10℃ / s. During the cooling process of the steel plate, the water flow ratio of the upper and lower manifolds is 1:1.6–1:2.

1. 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 400℃, and then air-cooled to room temperature.

5. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 690MPa as described in claim 4, characterized in that, In step 1), the raw material is pretreated with KR molten iron to control the S content to be less than 0.015%, and then enters the converter after slag removal. During converter smelting, the P content is controlled to be ≤0.020%, and the C content is controlled to be 0.04% to 0.08% at the end of converter smelting. Argon gas is blown for 25 to 30 minutes when tapping the steel. Then, LF refining and RH vacuum degassing are carried out, and the RH vacuum is maintained for more than 30 minutes.

6. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 690MPa as described in 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 low-temperature engineering structures at 690MPa as described in claim 4, characterized in that, The thickness of the cast billet is less than 250mm; the steel plate is produced by casting the billet on a medium-thick plate reciprocating rolling mill, and the cooling medium is water.

8. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 690MPa as described in claim 4, characterized in that, In step 3), before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1-2 minutes, and the pressure of the descaling machine is 15-25 MPa.

9. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 690MPa as described in claim 4, characterized in that, 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.

10. The method for manufacturing high-strength steel plates for low-temperature engineering structures at 690MPa as described in claim 4, characterized in that, In step 4), the straightening force of the hot straightening is between 2500KN and 3000KN, the position of the inlet roller is -0.8mm to -1.8mm, and the position of the outlet roller is -2.3mm to -3.0mm.

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