690mpa grade extra-thick plate for engineering structure having excellent low-temperature toughness and method for manufacturing the same

Abstract

The present application relates to a kind of 690MPa grade engineering structure with excellent low temperature toughness super-thick plate, steel plate chemical composition is C:0.12~0.15%, Si:0.10~0.20%, Mn:1.25~1.35%, P≤0.02%, S≤0.010%, Cr:0.30~0.38%, Al:0.015~0.035%, Nb:0.03~0.05%, V:0.05~0.07%, B:0.0015~0.0020%, Cu:0.25~0.30%, Ti:0.015~0.025%, N:0.01~0.02%, Mo:0.40~0.50%, Ni:0.75~0.85%, the balance is Fe and impurity.The present application solves the problems of high production cost, low toughness of finished steel plate, low flaw detection qualified rate and out-of-tolerance plate shape flatness.

Description

Technical Field

[0001] This invention relates to the field of steel production technology for engineering structures, and in particular to an extra-thick plate for engineering structures with excellent low-temperature toughness (690MPa grade) and its manufacturing method. Background Technology

[0002] Extra-thick plates have a wide range of applications, serving as a core and critical basic material for major equipment and a vital guarantee for the national economy. They are primarily used in construction, bridge building, and machinery manufacturing. In construction, extra-thick plates are commonly used to manufacture steel structural components for high-rise buildings (such as steel beams, columns, and plates). In bridge construction, they are frequently used to build long-span bridges, highway bridges, and high-speed railway bridges. In machinery manufacturing, they are commonly used to manufacture large machinery, engineering equipment, ships, and offshore drilling platforms. Among these, 690MPa grade extra-thick plates, due to their high strength, high toughness, and reliable weldability, meet the requirements of complex service environments and are used in large quantities.

[0003] To meet the technical requirements of 690MPa grade extra-thick plates, traditional production processes typically employ large ingots or composite welded billets for rolling, resulting in low production efficiency, low yield, high cost, and high energy consumption. In recent years, with the development of continuous casting technology, domestic and international steel companies generally use extra-thick continuously cast slabs (450mm or thicker) combined with offline quenching and tempering processes to produce thick 690MPa grade plates. However, the large cross-sectional thickness of extra-thick continuously cast slabs, the long solidification time, and poor internal cooling conditions often lead to internal defects such as center segregation, porosity, and shrinkage cavities, as well as surface defects such as transverse cracks, severely affecting the performance indicators of steel products (such as low-temperature toughness). Furthermore, the use of offline quenching and tempering processes also significantly increases production costs. In addition, conventional medium-thick plate manufacturers are limited by the billet compression ratio, hindering the manufacturing of extra-thick plates. Simultaneously, downstream users are increasingly demanding strict requirements for the shape of extra-thick plates, typically requiring a transverse unevenness of no more than 5mm / 2m.

[0004] To address the aforementioned issues, domestic and international steel companies generally employ technologies such as dynamic light reduction and electromagnetic stirring to improve billet quality. The primary aim is to increase the equiaxed grain region and compensate for volume shrinkage during solidification within the billet. However, when rolling extra-thick plates from continuously cast billets with cross-sections below 360mm, the improvement effect of these technologies is limited, failing to guarantee the low-temperature toughness of the finished product. Furthermore, the control over the plate shape of extra-thick plates is also limited. Currently, steel companies can only use leveling equipment for correction, but plate shape leveling is difficult and inefficient. To ensure the low-temperature toughness of products, domestic steel companies generally increase the alloy content and employ offline quenching processes to guarantee the low-temperature toughness of 690MPa grade steel plates.

[0005] Therefore, it is evident that the urgent problem to be solved is how to utilize continuously cast billets with cross sections below 360mm to roll extra-thick (80-120mm) 690MPa grade low-temperature engineering structural steel plates with excellent low-temperature toughness and high flatness, solve the problems existing in their production and manufacturing process, reduce manufacturing process costs, and achieve mass production.

[0006] To date, there has been little research, both domestically and internationally, on how to achieve low-cost manufacturing of extra-thick plates for engineering structures with thicknesses of 80–120 mm, yield strengths of 690 MPa, and excellent low-temperature toughness. Prior to this invention, Chinese patent application No. 202211619343.7 disclosed a "Production Method for Controlling the Plate Shape of A514GrQ Extra-Thick Plates for Offshore Platforms," ​​which uses ingot technology to produce extra-thick plates with thicknesses of 180–210 mm. However, its production cost is high, and the low-temperature toughness of the steel plate is not clearly defined. A doctoral dissertation, "Optimization of Microstructure and Properties and Toughness Mechanism of 690 MPa Grade Extra-Thick Gear Steel Plates for Marine Engineering" (October 2022, Northeastern University, Wang Qinghai), uses a die-casting process to produce 178 mm thick 690 MPa grade extra-thick plates, but it also suffers from high production costs. It is evident that the above-mentioned publications all suffer from high manufacturing costs, complex processes, and an inability to meet the -40℃ low-temperature service requirements when manufacturing 690 MPa grade extra-thick steel plates. Summary of the Invention

[0007] This invention overcomes the problems and shortcomings of existing technologies and provides a 690MPa grade extra-thick plate for engineering structures with excellent low-temperature toughness and its manufacturing method. When rolling low-temperature structural steel plates with a thickness of 80-120mm and a yield strength of 690MPa using continuous casting billets with a cross section of less than 360mm, the invention solves the problems of high production cost, low toughness of finished steel plates, low flaw detection pass rate, and excessive plate flatness by optimizing processes such as steelmaking, heating, rolling, controlled cooling, straightening, slow cooling, and heat treatment.

[0008] To achieve the above objectives, the present invention employs the following technical solution: A type of extra-thick steel plate with excellent low-temperature toughness and a strength of 690 MPa for engineering structures. The chemical composition of the steel plate, by weight percentage, is: C: 0.12%–0.15%, Si: 0.10%–0.20%, Mn: 1.25%–1.35%, P≤0.02%, S≤0.010%, Cr: 0.30%–0.38%, Al: 0.015%–0.035%, Nb: 0.03%–0.05%, V: 0.05%–0.07%. %, B: 0.0015%~0.0020%, Cu: 0.25%~0.30%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.40%~0.50%, Ni: 0.75%~0.85%, with the balance being Fe and unavoidable impurities; continuously cast billets with a thickness of less than 360mm are rolled on a medium-thick plate reciprocating rolling mill, and the thickness of the finished steel plate after rolling is 80~120mm.

[0009] The finished steel plate has a transverse tensile yield strength ≥690MPa, a tensile strength of 800~850MPa, and an elongation ≥20%; a transverse Charpy impact energy at -60℃ ≥150J, a yield strength ratio ≤0.89, a Z-direction elongation ≥35%, and a straightness ≤5mm / 2m; and a flaw detection pass rate ≥98%.

[0010] A method for manufacturing an extra-thick plate for engineering structures with excellent low-temperature toughness (690MPa grade) includes steel smelting, LF refining, RH refining, continuous casting, billet heating, controlled rolling, controlled cooling, hot straightening, and slow cooling processes, specifically controlling the following processes: 1) Continuous casting: The superheating temperature for continuous casting is 12–15℃, and the billet pulling speed is 0.9–1.2 m / min. Electromagnetic stirring is used in the secondary cooling zone of continuous casting, with alternating forward and reverse stirring. The forward stirring time is 30–45 s, and the reverse stirring time is 30–40 s. The current is 600–800 A, and the frequency is 30–50 Hz. The isometric crystal ratio in the continuously cast billet is not less than 90%. The fan-shaped section of continuous casting adopts a strong cooling process, with a total cooling water flow of 600–800 L / min from section 1 to section 4, ensuring… The temperature difference between the surface and core of the continuously cast billet at the solidification end is >200℃, and it is simultaneously subjected to light pressure with a reduction of 3-5mm; the total cooling water flow rate from the 5th to the 8th stage is 1000-1500L / min, ensuring that the temperature difference between the surface and core of the continuously cast billet at the solidification end is >500℃, and it is simultaneously subjected to heavy pressure with a reduction of 10-15mm; after the billet is removed from the production line, it is subjected to heat preservation, homogenization and dehydrogenation treatment in an electrically heated bell furnace, with a heat preservation temperature of 650-750℃ and a heat preservation time of ≥48h, ensuring that the H content in the billet is ≤5ppm; 2) Billet heating: Billets with a thickness of less than 360mm are fed into a walking beam furnace for heating. The preheating section temperature is 750-900℃, the heating section temperature is 1230-1250℃, and the soaking section temperature is 1210-1225℃. The total furnace time for the preheating, heating, and soaking sections is 6.0-8.0h, of which the total furnace time for the heating and soaking sections is 5.5-7.0h. The air-fuel ratio in the furnace is controlled at 1:1.7-1:2.1, and the 2-4 sets of burners in the upper part of the soaking section are closed to ensure that the temperature difference between the upper and lower surfaces of the billet is within the range of 5-10℃. 3) Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1.5–2.5 minutes, with a descaling machine pressure of 20–25 MPa; rolling is carried out in two stages: the first stage is rolling in the austenite recrystallization zone, i.e., rough rolling, with a finishing rolling start temperature ≥1100℃ and a rough rolling finish temperature of 980–1010℃; the single-pass reduction rate of the last three passes in the rough rolling stage is ≥15%, and each of the last three passes is allowed to warm up for 15–20 seconds before rolling; each of the last three passes in the rough rolling stage is sprayed with descaling water, with a descaling time of 10–30 seconds and a descaling machine pressure of 15–25 MPa. 0 MPa; the thickness of the intermediate billet is 1.5 to 2.5 times the thickness of the finished product; during the intermediate billet warming process, spray descaling water for 2 to 3 passes, descaling time is 2 to 3 minutes, descaling machine pressure is 20 to 25 MPa, ensuring the temperature difference between the surface and the core is ≥200℃; the second stage is the rolling of the austenite non-recrystallization zone, i.e., finishing rolling, the finishing rolling start temperature is 945 to 960℃, the bite and rolling speed in the finishing rolling stage is 0.9 to 1.2 m / s, and the ratio of upper and lower roll speeds is 95% to 98%; during finishing rolling, the cooling water volume of the roller table from the mill to the pre-straightening zone is controlled at 150 to 200 m³ / s. 3 / h, the finishing rolling temperature is 860~880℃; the single-pass reduction rate of the first two passes in the finishing rolling stage is above 20%, the finishing rolling is no more than five passes, and the final finishing rolling pass adopts a small reduction rate of 0.5%~1%; 4) Controlled Cooling: The UFC ultra-fast cooling system is used. During UFC cooling, a fully automatic controlled cooling mode is employed. The initial cooling temperature of the steel plate is 760–790℃, and the final cooling temperature is 500–580℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -400–-600mm, and the lower manifold opening correction value is -800–-1000mm. 8–12 sets of water are opened on both the upper and lower manifolds, with an opening water flow rate of 250–280 m³ / h. 3 / h; the water ratio between the upper and lower manifolds is 1:2.1 to 1:2.4; the roller speed of the steel plate conveyor in the controlled cooling zone is 0.3 to 0.5 m / s, and the acceleration is 0.002 to 0.005 m / s. 2 After the steel plate is cooled, the side spray and air purging are turned on. The side spray pressure is 10-15 MPa and the water flow is 100-150 m3 / h. The air purging pressure is 8-12 MPa. 5) Heat treatment: The QLT quenching process of quenching-critical quenching-tempering is adopted. The initial quenching temperature is 890~910℃, and the furnace time is 1.5~2min / mm; the critical quenching temperature is 770~820℃, and the furnace time is 1.5~2min / mm; the tempering temperature is 250~300℃, and the furnace time is 3~5min / mm; after exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate.

[0011] The steelmaking process is as follows: the raw materials are pretreated with KR hot metal to control the S content ≤0.010%, and then enter the converter after slag removal; the converter smelting adopts the double slag method to remove P, and controls the P content ≤0.020%; the C content is controlled at 0.12%~0.15% at the end of the converter smelting; and argon gas is blown for more than 40 minutes when tapping the steel.

[0012] The RH refining process is as follows: after the vacuum degree reaches the set value during RH vacuum degassing, it is maintained for more than 35 minutes; after the degassing treatment is completed, Al is added according to the Al content requirements of the finished product, Ti-Fe is added for micro-titanium treatment, and then B-Fe alloy is added.

[0013] The hot straightening process is as follows: the steel plate is subjected to three high-temperature hot straightening passes, the straightening temperature is controlled at 450-550℃, the straightening force is controlled at 3500-5000kN, the position of the inlet roller is -1.5--2.1mm, the position of the outlet roller is -3.5--4.2mm, and the tilt value is set to 8-12mm.

[0014] The slow cooling process is as follows: hot plates are used to cover and stack the hot-straightened steel plates. The stacking temperature of the hot plates is 600-800°C, the number of stacked plates is not less than 10, and the slow cooling time is more than 48 hours.

[0015] Compared with the prior art, the beneficial effects of the present invention are: (1) After deep desulfurization of KR hot metal pretreatment, the slag is removed cleanly. The converter adopts the double slag method to remove P, so that the P and S content of the billet is controlled at a low level. The argon blowing time is controlled and the RH vacuum degassing time is maintained, thereby avoiding defects such as central segregation and excessive inclusions in the billet caused by high Mn, Cr and C content, which is conducive to improving the plasticity and toughness of the steel plate. By reducing the superheat of continuous casting and the continuous casting speed, the macro segregation of the continuous casting billet is improved, and the spacing of secondary dendrite arms in the solidification structure of the continuous casting billet is reduced, which helps to reduce billet segregation and internal structural defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuously cast billet is significantly improved. Strong cooling is employed in the sector section, and the cooling water volume in different sections is controlled to ensure a temperature gradient along the billet thickness. Simultaneously, light or heavy pressure is applied to different sections to help reduce billet segregation and ensure strain penetration to the 1 / 4 (improved under light pressure) and 1 / 2 (improved under heavy pressure) portions of the billet thickness. This significantly improves defects at different locations on the billet cross-section, ensuring the strength and toughness of the subsequent steel plate. After casting, the billet is subjected to heat-holding homogenization and dehydrogenation treatment in an electrically heated bell-type furnace, with controlled temperature and time. This promotes the diffusion of elements such as Mn, C, B, H, V, and N, mitigating the impact of component segregation on microstructure and properties, and ensuring that the H content does not exceed 5 ppm.

[0016] (2) The composition design of this invention is reasonable and the amount of alloy added is low. The conventional C-Mn-Nb-V-Cr-Mo-Ni microalloying composition design system is adopted, and continuous casting billets with a cross section of less than 360mm are used for rolling, which greatly reduces the alloy cost and the high temperature deformation resistance in the roughing and finishing stages, which is conducive to increasing the reduction in each pass and ultimately ensuring the comprehensive performance of the extra-thick steel plate. The effect of B on the improvement of austenite stability is related to the amount of B dissolved in the steel. In order to increase the B content in boron-containing steel, this invention adds Ti, which has a stronger bonding force with C than B, to ensure that B exists in a free form; at the same time, the high temperature deformation induction effect is used to promote the precipitation of carbonitrides of Nb, V and Ti, providing more nucleation sites, promoting the occurrence of ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate.

[0017] (3) The temperature and time of the billet in the preheating, heating and soaking sections are limited to promote the partial solidification and diffusion of carbonitrides of Ti, Nb and V into the matrix. At the same time, the heating section provides high-temperature heating conditions to ensure the temperature uniformity of each part of the billet and to ensure that the partial solidification of carbonitrides of Ti, Nb and V into the matrix. It also further promotes the full diffusion of elements such as Mn, Cr, C, B, V, Nb and Mo, and reduces the impact of component segregation on the microstructure and properties. The air-fuel ratio is controlled and some nozzles in the upper part of the soaking section are closed to ensure that the temperature difference of the upper surface of the billet is 5-10°C higher than that of the lower surface. This avoids the lower surface of the billet being affected by the water beam at the bottom of the furnace, which would cause the temperature to be too low and affect the uniformity of rolling deformation of the upper and lower surfaces, so as to ensure the uniformity of transverse and longitudinal metal flow on the surface of the steel plate. The total time in the furnace is controlled to effectively promote the full diffusion of elements such as C, Mn, V, Cr and N, while also inhibiting the excessive growth of the original austenite grains and improving the toughness of the steel plate.

[0018] (4) A two-stage controlled rolling process is adopted. In the last three passes of the roughing stage, a large reduction rate is used to promote the precipitation of carbonitrides of Nb, V and Ti in the austenite grains by utilizing the high-temperature deformation-induced effect, thereby improving strength and toughness. The preheating before the last three passes promotes the dynamic recrystallization of austenite grains to refine the grains. The descaling water sprayed in the last three passes creates a temperature gradient on the inner and outer surfaces of the billet, promoting the penetration of rolling deformation into the thickness center and refining the grains at 1 / 2 thickness, which is beneficial to improving the core structure of thick steel plates. The intermediate billet cooling process is adopted to inhibit the growth of austenite grains and at the same time create a temperature gradient on the inner and outer surfaces of the billet, promoting the penetration of rolling deformation into the thickness center and refining the grains at 1 / 2 thickness, which is beneficial to improving the core structure of thick steel plates and reducing the structural stress caused by uneven structure. At the same time, the intermediate billet temperature drops faster and the preheating time is reduced. By controlling the reduction speed and reduction rate during the finishing rolling stage, as well as the ratio range of the upper and lower roll speeds and the cooling water in the roller table from the mill to the pre-straightening section, a large amount of cooling water is prevented from flowing on the lower surface of the steel plate, reducing uncontrolled temperature drops on the lower surface and thus preventing the "head-cutting" phenomenon. Simultaneously, the density of dislocations, vacancies, and the number of deformation bands within the austenite body are ensured, promoting the precipitation of carbonitrides of Nb, Ti, and V, providing more nucleation sites, promoting ferrite phase transformation, and refining the ferrite microstructure. The final finishing rolling pass (leveling pass) uses a small reduction rate to level the steel plate shape, reduce internal stress, and prevent water retention in the steel plate after controlled cooling due to breakage and head-cutting phenomena, which would affect the uniformity of the microstructure and properties.

[0019] During UFC cooling, controlling the reheat temperature and the head and tail shielding parameters of the cooled steel plate effectively regulates the temperature uniformity at different positions (head, middle, and tail) to ensure the steel plate's strength, toughness, and shape. Side spraying and air purging further facilitate shape control, improve performance uniformity, and reduce the likelihood of misshapen sections. Controlling the final cooling temperature and implementing head and tail shielding during UFC cooling also effectively regulates temperature uniformity at different positions (head, middle, and tail) to ensure the steel plate's toughness and shape. A high-temperature hot straightener is used, with appropriate roll gap and tilt values ​​set. Multiple passes with small bending amounts are employed to ensure that 20%–30% of the cumulative strain near the steel plate surface exceeds the elastic limit strain, promoting further release of internal stress and ensuring good flatness of the straightened plate.

[0020] (5) High-temperature stacking method is adopted to reduce banded structure and allow C, Mn and H elements to diffuse fully; the hot plate self-tempering effect is used to improve the plasticity and toughness of the steel plate and improve the flaw detection pass rate. The QLT process is adopted for the rolled steel plate. Its core is two-phase quenching. After holding in the temperature range where austenite and ferrite coexist, quenching is performed to form a lamellar structure. This structure can refine the grains, increase the proportion of large-angle grain boundaries, and promote the formation of reverse-transformed austenite, thereby improving toughness; by controlling the tempering temperature, the width of martensite laths, the proportion of precipitates and large-angle grain boundaries are ensured to reach 55% to 70% to ensure strength and toughness.

[0021] (6) Through composition and process design, a low-cost production method was obtained for producing extra-thick plates for engineering structures with a thickness of 80-120mm, a yield strength of 690MPa, and excellent low-temperature toughness. By optimizing the steelmaking, heating, rolling, cooling, straightening, slow cooling, and heat treatment processes, the problems of low toughness, low flaw detection pass rate, and excessive plate flatness that existed when using continuous casting billets with a cross-section of less than 360mm to produce low-cost low-temperature structural steel plates with a thickness of 80-120mm and a yield strength of 690MPa were finally solved. The technical indicators of the prepared plates are as follows: transverse tensile yield strength ≥690MPa, tensile strength between 800-850MPa, elongation ≥20%, transverse Charpy impact energy at -60℃ ≥150J, yield strength ratio ≤0.89, Z-direction elongation ≥35%, straightness below 5mm / 2m, and flaw detection pass rate ≥98%. Detailed Implementation

[0022] The present invention discloses an extra-thick steel plate for engineering structures with excellent low-temperature toughness and a strength of 690 MPa. The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.12%–0.15%, Si: 0.10%–0.20%, Mn: 1.25%–1.35%, P≤0.02%, S≤0.010%, Cr: 0.30%–0.38%, Al: 0.015%–0.035%, Nb: 0.03%–0.05%, V: 0.05%–0.05%. 0.07%, B: 0.0015%~0.0020%, Cu: 0.25%~0.30%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.40%~0.50%, Ni: 0.75%~0.85%, with the balance being Fe and unavoidable impurities; continuously cast billets with a thickness of less than 360mm are rolled on a medium-thick plate reciprocating rolling mill, and the thickness of the finished steel plate after rolling is 80~120mm.

[0023] The roles of the main elements in the chemical composition of the steel plate described in this invention are as follows: C: The most economical and basic strengthening element in steel, it significantly improves 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%–0.15%.

[0024] Mn (metallurgical nitric acid) enhances the strength of steel through solid solution strengthening, while compensating for the strength loss caused by the reduction in carbon (C) content. Furthermore, Mn lowers the γ-α phase transformation temperature, thereby refining ferrite grains and contributing to the production of finer low-temperature phase transformation products, thus improving the toughness of the steel plate. However, increasing the Mn content exacerbates segregation at the center of the continuously cast billet, which is detrimental to improving the low-temperature toughness of the steel plate and also fails to guarantee the uniformity of the cross-sectional microstructure. Therefore, this invention sets the Mn content range to 1.25%–1.35%.

[0025] 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 content of pearlite, which is beneficial to reducing the Bauschinger effect in the matrix material. However, excessive Si will reduce the toughness of the heat-affected zone of the base metal. Therefore, this invention sets the Si content range to 0.10% to 0.20%.

[0026] Niobium in steel mainly improves strength and toughness by refining grains and precipitation strengthening, while also improving weldability, corrosion resistance and other properties. In this invention, the Nb content is set to a range of 0.03% to 0.05%.

[0027] V (V) is a strong solid element, existing in the form of VC / VN in continuously cast billets. Fine V particles can 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 nucleation sites for tough phases such as acicular ferrite. 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. However, when the V content exceeds a certain value, the V particles coarsen, thereby increasing the stress concentration level at the particle interface and the matrix. Therefore, this invention sets the V content range to 0.05%–0.07%.

[0028] Al: It is usually used as a deoxidizer in steel, and if it forms AlN, it can also refine the microstructure. When the Al content exceeds 0.035%, excessive alumina inclusions will reduce the cleanliness of the steel. If the Al content is too low, it will lead to insufficient deoxidation, and easily oxidized elements such as Ti will form oxides; therefore, this invention sets the Al content range to 0.015% to 0.035%.

[0029] Cr: It can effectively improve hardenability and is the main element that inhibits the formation of ferrite and promotes the formation of bainite. It plays an important role in controlling the phase transformation structure and promotes the formation of polygonal ferrite, pearlite and acicular ferrite with a large number of dislocations in the grain in the medium temperature and low temperature range, thus improving the strength, plasticity and toughness of steel plates. In this invention, the Cr content range is set to 0.30% to 0.38%.

[0030] Cu: It can improve the strength of steel through solid solution strengthening and also improve corrosion resistance. However, excessive Cu can cause hot brittleness and is detrimental to toughness. Therefore, the present invention sets the Cu content range to 0.25% to 0.30%.

[0031] Mo can significantly improve hardenability, increase strength, promote the transformation of microstructure 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, the present invention sets the Mo content range to 0.40% to 0.50%.

[0032] Ni: Nickel exhibits 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 sets the Ni content range to 0.75%–0.85%.

[0033] 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. Since TiN has a better bonding ability than BN, this invention selects an N content range of 0.01% to 0.02% and a Ti content range of 0.015% to 0.025%. The remaining Ti combines with C to form TiC, which plays a role in dispersion strengthening.

[0034] 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 significant effect. However, excessive boron content can easily form boron carbonitrides, reducing toughness and causing hot brittleness. Therefore, this invention sets the B content range to 0.0015%–0.0020%.

[0035] 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 limit of P and S content at 0.020% and 0.010%, respectively.

[0036] The present invention discloses a method for manufacturing an extra-thick plate for engineering structures with excellent low-temperature toughness (690MPa grade). The process route is as follows: steel smelting → ladle refining + RH refining → B alloying → continuous casting → slab reheating → controlled rolling and cooling → hot straightening and slow cooling → heat treatment (quenching + tempering). The specific control process is as follows: 1) Steelmaking to continuous casting process: Smelting is carried out according to the set composition. The raw materials are pretreated with KR hot metal to control the S content below 0.010%, and then enter the converter after slag removal. The converter smelting adopts the double slag method for P removal, controlling the P content ≤0.020%. The final carbon content of the converter smelting is controlled at 0.12%~0.15%. Argon gas is blown for more than 40 minutes during tapping. Argon blowing and killing before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition.

[0037] After LF refining, the steel undergoes RH vacuum degassing, maintaining the vacuum level at the set value for at least 35 minutes. Following degassing, sufficient Al is added according to the required Al content of the finished product, along with sufficient Ti-Fe for micro-titanium treatment. Then, B-Fe alloy is added, ensuring it is added directly to the molten steel to avoid floating on the slag layer. Slab continuous casting is then performed at a superheat of 12–15°C and a casting speed of 0.9–1.2 m / min. Reducing the superheat and casting speed improves macroscopic segregation in the continuously cast slab, decreases the spacing between secondary dendrite arms in the solidification structure, and helps reduce slab segregation and internal structural defects.

[0038] Electromagnetic stirring is applied in the secondary cooling zone of continuous casting, using alternating forward and reverse stirring. The forward stirring time is 30–45 s, and the reverse stirring time is 30–40 s, with a current of 600–800 A and a frequency of 30–50 Hz. The molten steel is then continuously cast to obtain a continuously cast billet, with an isometric crystal ratio of not less than 90%. The sector section of continuous casting employs a strong cooling process. The total cooling water flow rate for sections 1 to 4 is 600–800 L / min to ensure a large temperature gradient between the surface and core of the billet at the solidification end (>200℃), while light reduction of 3–5 mm is applied. The total cooling water flow rate for sections 5 to 8 is 1000–1500 L / min to ensure a large temperature gradient between the surface and core of the billet at the solidification end (>500℃), while heavy reduction of 10–15 mm is applied. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuously cast billet is significantly improved. By using strong cooling in the fan-shaped section and controlling the cooling water volume in different sections, the temperature gradient in the thickness direction of the billet is guaranteed. By applying light or heavy pressure in different sections, it helps to reduce billet segregation and ensure that strain penetrates to 1 / 4 of the billet thickness (the location improved under light pressure) and 1 / 2 of the core (the location improved under heavy pressure). This significantly improves defects in different locations of the billet cross-section and provides a guarantee for the strength and toughness of the subsequent steel plate.

[0039] After casting, the billets are subjected to heat treatment, homogenization, and dehydrogenation in an electrically heated bell-type furnace. The heat treatment temperature is 650–750℃, and the holding time is no less than 48 hours. The purpose of heat treatment, homogenization, and dehydrogenation of the continuously cast billets, with controlled temperature and time, is to promote the diffusion of elements such as Mn, C, B, H, V, and N, reduce the impact of component segregation on the microstructure and properties, and ensure that the H content in the billets does not exceed 5 ppm.

[0040] 2) Billet Heating Process: Billets with a thickness of 360mm or less are fed into a walking beam furnace for heating. The preheating section temperature range is 750–900℃, the purpose of which is to promote the partial dissolution and full diffusion of Ti, Nb, and V carbonitrides in the matrix. The heating section temperature range is 1230–1250℃, and the soaking section temperature range is 1210–1225℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 6.0–8.0 hours, of which the total furnace time for the heating and soaking sections is 5.5–7.0 hours. By adjusting the opening degree of the upper and lower burners in the furnace, the air-fuel ratio is controlled at 1:1.7–1:2.1. At the same time, the 2–4 sets of burners in the upper part of the soaking section are closed to ensure that the temperature difference between the upper and lower surfaces of the billet is between 5–10℃. The heating section provides high-temperature heating conditions to ensure temperature uniformity across all parts of the billet, guaranteeing the partial dissolution of Ti, Nb, and V carbonitrides in the matrix. Simultaneously, it further promotes the full diffusion of elements such as Mn, Cr, C, B, V, Nb, and Mo, mitigating the impact of component segregation on microstructure and properties. By controlling the air-fuel ratio and closing some nozzles in the upper part of the soaking section, the temperature difference between the upper surface and lower surface of the billet is maintained at 5–10°C higher. This prevents the lower surface of the billet from being too cold due to the influence of the water beam at the furnace bottom, which would affect the uniformity of rolling deformation between the upper and lower surfaces, thus ensuring uniform transverse and longitudinal metal flow on the steel plate surface. Controlling the total furnace time effectively promotes the full diffusion of elements such as C, Mn, V, Cr, and N, while inhibiting excessive growth of the original austenite grains and improving the toughness of the steel plate.

[0041] 3) Control the rolling process: Before rolling, use high-pressure water to descale the billet after it comes out of the furnace for 1.5 to 2.5 minutes, with the descaling machine pressure at 20 to 25 MPa.

[0042] The rolling process is divided into two stages: the first stage is the austenite recrystallization zone rolling (rough rolling), with a rough rolling start temperature of ≥1100℃ and a rough rolling finish temperature of 980~1010℃. The single-pass reduction rate of the last three passes in the rough rolling stage is not less than 15%, and each of the last three passes is allowed to wait for 15~20s before the start of rolling. At the same time, each of the last three passes is sprayed with descaling water, with a descaling time of 10~30s and a descaling machine pressure of 15~20MPa. The last three passes of the roughing stage all use a large reduction rate to utilize the high-temperature deformation-induced effect to promote the precipitation of Nb, V, and Ti carbonitrides within the austenite grains, thereby improving strength and toughness. The preheating before the start of the last three passes is intended to promote dynamic recrystallization of the austenite grains to refine the grains. The last three passes all use descaling water spraying to create a temperature gradient between 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, and thus improving the core microstructure of thick steel plates.

[0043] The thickness of the intermediate billet is 1.5 to 2.5 times the thickness of the finished product. During the intermediate billet's warming process, it is sprayed with descaling water for 2 to 3 passes, with a descaling time of 2 to 3 minutes and a descaling machine pressure of 20 to 25 MPa, ensuring that the temperature difference between the surface and the core is above 200°C. The purpose of spraying the intermediate billet with descaling water is to inhibit austenite grain growth, while simultaneously creating 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 half the thickness, improving the core microstructure of thick steel plates, reducing the structural stress caused by uneven microstructure, and accelerating the temperature drop of the intermediate billet, thus reducing the warming time.

[0044] The second stage is the non-recrystallization zone rolling (finishing rolling). The initial rolling temperature is 945–960℃, and the bite and rolling speeds in the finishing rolling stage are 0.9–1.2 m / s. The ratio of upper and lower roll speeds is 95%–98%. During rolling, the cooling water in the roller table from the mill to the pre-straightening zone is reduced to 150–200 m³. 3 / h; To avoid excessive cooling water flow on the lower surface of the steel plate, reduce uncontrolled temperature drop on the lower surface of the steel plate, and thus prevent the "butt-off" phenomenon. The finishing rolling temperature range is 860~880℃. The first two passes of the finishing rolling stage ensure a reduction rate of more than 20% per pass. The finishing rolling stage has no more than five passes. The last pass of the finishing rolling stage is a leveling pass with a small reduction rate of 0.5%~1%. By controlling the reduction speed and the reduction rate in the finishing rolling stage, the dislocation density, vacancies and deformation bands in the austenite are ensured, promoting the precipitation of carbonitrides of Nb, Ti and V, providing more nucleation sites, promoting ferrite phase transformation, and refining the ferrite structure. The purpose of using a small reduction rate in the last pass is to level the shape of the steel plate, reduce the internal stress of the steel plate, and avoid water retention in the steel plate due to broken waves and "butt-off" after controlled cooling, which would affect the uniformity of the structure and properties.

[0045] UFC (Ultra-Fast Cooling System) is used for cooling. The UFC cooling process employs a fully automatic controlled cooling mode. The initial cooling temperature for the steel plate is 760–790℃, and the final cooling temperature is 500–580℃. The steel plate is shielded at both ends before operation. The upper manifold opening correction value is -400–-600mm, and the lower manifold opening correction value is -800–-1000mm. 8–12 sets of water are opened on both the upper and lower manifolds, with an opening water flow rate of 250–280m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.1 to 1:2.4; the roller speed of the steel plate conveyor in the controlled cooling zone is 0.3 to 0.5 m / s, and the acceleration is 0.002 to 0.005 m / s. 2 After the steel plate is cooled under controlled conditions, the side spray and air purging are activated. The side spray pressure is 10-15 MPa, and the water flow rate is 100-150 m³ / h. 3 / h, with a wind purging pressure of 8-12MPa. During the UFC cooling process, the final cooling temperature is controlled, and the steel plate head and tail shielding is implemented to effectively regulate the temperature uniformity at different positions of the steel plate head, middle, and tail, so as to ensure the strength, toughness, and shape of the steel plate. The implementation of side spraying and wind purging is beneficial to the control of the steel plate shape, which can improve the uniformity of steel plate performance and reduce the probability of head and tail shape problems.

[0046] The cooled steel plate undergoes three passes of high-temperature hot straightening. The straightening temperature is controlled between 450 and 550°C, the straightening force is between 3500 and 5000 kN, the inlet roller position is -1.5 to -2.1 mm, the outlet roller position is -3.5 to -4.2 mm, and the tilt value is set to 8 to 12 mm. Utilizing high-temperature hot straightening and setting appropriate roller gaps and tilt values, multiple passes of small bending are employed to ensure that 20% to 30% of the cumulative strain near the steel plate surface exceeds the elastic limit strain. This promotes further and full release of internal stress in the steel plate, ensuring good straightness of the straightened plate.

[0047] 4) Stacking and Slow Cooling Process: Hot plates are used to cover and stack the hot-straightened steel plates. The stacking temperature of the hot plates is 600-800°C, and the number of stacked plates is no less than 10. The slow cooling time is more than 48 hours. The high-temperature stacking method reduces the banded structure, allows C, Mn and H elements to diffuse fully, and at the same time utilizes the self-tempering effect of the hot plates to improve the plasticity and toughness of the steel plates and increase the pass rate of flaw detection.

[0048] 5) Quenching-Critical Quenching-Tempering (QLT) Process: The QLT quenching process is applied to the steel plates after slow cooling in the stack. The initial quenching temperature is 890–910℃, with a furnace time of 1.5–2 min / mm; the critical quenching temperature is 770–820℃, with a furnace time of 1.5–2 min / mm; the tempering temperature is 250–300℃, with a furnace time of 3–5 min / mm. After removal from the furnace, the plates are air-cooled to obtain the finished steel plate. The core of the QLT process lies in two-phase quenching. By holding the plates at a temperature range where austenite and ferrite coexist, a lamellar structure is formed during quenching. This refines the grains, increases the proportion of large-angle grain boundaries, and promotes the formation of reverse-transformed austenite, thereby improving toughness. By controlling the tempering temperature, the martensite lath width, precipitated phase, and large-angle grain boundary proportion are ensured to reach 55%–70%, guaranteeing both strength and toughness.

[0049] This invention, employing the aforementioned components and processes, ultimately yields a low-cost production method for extra-thick engineering structural plates with a thickness of 80-120mm, a yield strength of 690MPa, and excellent low-temperature toughness. By optimizing the steelmaking, heating, rolling, cooling, straightening, slow cooling, and heat treatment processes, it solves the problems of low toughness, low flaw detection pass rate, and excessive plate flatness encountered when using continuously cast billets with a cross-section of less than 360mm to produce low-cost low-temperature structural steel plates with a thickness of 80-120mm and a yield strength of 690MPa. The technical specifications of the prepared plates are: transverse tensile yield strength ≥690MPa, tensile strength between 800-850MPa, elongation ≥20%, transverse Charpy impact energy at -60℃ ≥150J, yield strength ratio ≤0.89, Z-direction elongation ≥35%, straightness below 5mm / 2m, and flaw detection pass rate ≥98%.

[0050] To more intuitively illustrate the present invention, the embodiments of the present invention will be further described in conjunction with the examples. The following examples are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention, including simple variations or equivalent substitutions, are all within the scope of protection of the present invention.

[0051]

Example

[0052] Table 1 - Chemical composition of steel (wt, %) 1 0.13 0.19 1.28 0.37 0.016 0.05 0.019 0.015 0.04 0.26 0.0016 0.77 0.47 2 0.15 0.15 1.33 0.35 0.021 0.06 0.033 0.017 0.03 0.28 0.0018 0.82 0.42 3 0.14 0.18 1.25 0.32 0.019 0.07 0.029 0.013 0.04 0.25 0.0019 0.83 0.41 4 0.13 0.14 1.27 0.37 0.018 0.05 0.025 0.012 0.05 0.30 0.0018 0.77 0.49 5 0.12 0.13 1.34 0.33 0.023 0.06 0.030 0.018 0.03 0.29 0.0020 0.79 0.48 6 0.15 0.17 1.29 0.32 0.017 0.07 0.031 0.019 0.05 0.28 0.0019 0.84 0.50 Note: Impurity elements in steel: P≤0.02%, S≤0.010%.

[0053] Table 2 - Steel smelting and continuous casting process parameters

[0054] Table 3 - Billet Heating Process Parameters

[0055] Table 4 - Process parameters for rough rolling and intermediate billet treatment of steel plates

[0056] Table 5 - Finishing process parameters for steel plates

[0057] Table 6 - Cooling process parameters for steel plates

[0058] Table 7 - Straightening, Slow Cooling and Heat Treatment Process Parameters for Steel Plates

[0059] Table 8 - Dimensions and Performance Indicators of Finished Steel Plates

[0060] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A 690MPa grade extra-thick plate for engineering structures with excellent low-temperature toughness, characterized in that, The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.12%–0.15%, Si: 0.10%–0.20%, Mn: 1.25%–1.35%, P≤0.02%, S≤0.010%, Cr: 0.30%–0.38%, Al: 0.015%–0.035%, Nb: 0.03%–0.05%, V: 0.05%–0.07%, B: 0.0015%– 0.0020%, Cu: 0.25%~0.30%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.40%~0.50%, Ni: 0.75%~0.85%, with the balance being Fe and unavoidable impurities; continuously cast billets with a thickness of less than 360mm are rolled on a medium-thick plate reciprocating rolling mill, and the thickness of the finished steel plate after rolling is 80~120mm.

2. The extra-thick plate for engineering structures with excellent low-temperature toughness (690MPa grade) according to claim 1, characterized in that, The finished steel plate has a transverse tensile yield strength ≥690MPa, a tensile strength of 800~850MPa, and an elongation ≥20%; a transverse Charpy impact energy at -60℃ ≥150J, a yield strength ratio ≤0.89, a Z-direction elongation ≥35%, and a straightness ≤5mm / 2m; and a flaw detection pass rate ≥98%.

3. A method for manufacturing an extra-thick plate for engineering structures with excellent low-temperature toughness (690 MPa grade) as described in claim 1 or 2, characterized in that, This includes steelmaking, LF refining, RH refining, continuous casting, billet heating, controlled rolling, controlled cooling, hot straightening, and slow cooling processes, specifically controlling the following processes: 1) Continuous casting: The superheating temperature for continuous casting is 12–15℃, and the billet pulling speed is 0.9–1.2 m / min. Electromagnetic stirring is used in the secondary cooling zone of continuous casting, with alternating forward and reverse stirring. The forward stirring time is 30–45 s, and the reverse stirring time is 30–40 s. The current is 600–800 A, and the frequency is 30–50 Hz. The isometric crystal ratio in the continuously cast billet is not less than 90%. The fan-shaped section of continuous casting adopts a strong cooling process, with a total cooling water flow of 600–800 L / min from section 1 to section 4, ensuring… The temperature difference between the surface and core of the continuously cast billet at the solidification end is >200℃, and it is simultaneously subjected to light pressure with a reduction of 3-5mm; the total cooling water flow rate from the 5th to the 8th stage is 1000-1500L / min, ensuring that the temperature difference between the surface and core of the continuously cast billet at the solidification end is >500℃, and it is simultaneously subjected to heavy pressure with a reduction of 10-15mm; after the billet is removed from the production line, it is subjected to heat preservation, homogenization and dehydrogenation treatment in an electrically heated bell furnace, with a heat preservation temperature of 650-750℃ and a heat preservation time of ≥48h, ensuring that the H content in the billet is ≤5ppm; 2) Billet heating: Billets with a thickness of less than 360mm are fed into a walking beam furnace for heating. The preheating section temperature is 750-900℃, the heating section temperature is 1230-1250℃, and the soaking section temperature is 1210-1225℃. The total furnace time for the preheating, heating, and soaking sections is 6.0-8.0h, of which the total furnace time for the heating and soaking sections is 5.5-7.0h. The air-fuel ratio in the furnace is controlled at 1:1.7-1:2.1, and the 2-4 sets of burners in the upper part of the soaking section are closed to ensure that the temperature difference between the upper and lower surfaces of the billet is within the range of 5-10℃. 3) Controlled rolling: Before rolling, the billet after exiting the furnace is descaled with high-pressure water for 1.5–2.5 minutes, with a descaling machine pressure of 20–25 MPa; rolling is carried out in two stages: the first stage is rolling in the austenite recrystallization zone, i.e., rough rolling, with a finishing rolling start temperature ≥1100℃ and a rough rolling finish temperature of 980–1010℃; the single-pass reduction rate of the last three passes in the rough rolling stage is ≥15%, and each of the last three passes is allowed to warm up for 15–20 seconds before rolling; each of the last three passes in the rough rolling stage is sprayed with water. Descaling water is used for 10–30 seconds, and the descaling machine pressure is 15–20 MPa. The thickness of the intermediate billet is 1.5–2.5 times the thickness of the finished product. During the intermediate billet warming process, 2–3 passes of descaling water are sprayed, with a descaling time of 2–3 minutes and a descaling machine pressure of 20–25 MPa, ensuring a temperature difference between the surface and the core of ≥200℃. The second stage is the finishing rolling in the non-recrystallized austenite zone. The starting temperature for finishing rolling is 945–960℃, and the bite and rolling speeds during the finishing rolling stage are 0.9–1.2 m / s, with the ratio of upper and lower roll speeds being 95%–98%. During finishing rolling, the cooling water volume of the roller table from the mill to the pre-straightening section is controlled at 150–200 m³ / s. 3 / h, the finishing rolling temperature is 860~880℃; the single-pass reduction rate of the first two passes in the finishing rolling stage is above 20%, the finishing rolling is no more than five passes, and the final finishing rolling pass adopts a small reduction rate of 0.5%~1%; 4) Controlled Cooling: The UFC ultra-fast cooling system is used. During UFC cooling, a fully automatic controlled cooling mode is employed. The initial cooling temperature of the steel plate is 760–790℃, and the final cooling temperature is 500–580℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -400–-600mm, and the lower manifold opening correction value is -800–-1000mm. 8–12 sets of water are opened on both the upper and lower manifolds, with an opening water flow rate of 250–280 m³ / h. 3 / h; the water ratio between the upper and lower manifolds is 1:2.1 to 1:2.4; the roller speed of the steel plate conveyor in the controlled cooling zone is 0.3 to 0.5 m / s, and the acceleration is 0.002 to 0.005 m / s. 2 After the steel plate is cooled, the side spray and air purging are turned on. The side spray pressure is 10-15 MPa and the water flow is 100-150 m3 / h. The air purging pressure is 8-12 MPa. 5) Heat treatment: The QLT quenching process of quenching-critical quenching-tempering is adopted. The initial quenching temperature is 890~910℃, and the furnace time is 1.5~2min / mm; the critical quenching temperature is 770~820℃, and the furnace time is 1.5~2min / mm; the tempering temperature is 250~300℃, and the furnace time is 3~5min / mm; after exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate.

4. The method for manufacturing a 690MPa grade extra-thick plate for engineering structures with excellent low-temperature toughness according to claim 3, characterized in that, The steelmaking process is as follows: the raw materials are pretreated with KR hot metal to control the S content ≤0.010%, and then enter the converter after slag removal; the converter smelting adopts the double slag method to remove P, and controls the P content ≤0.020%; the C content is controlled at 0.12%~0.15% at the end of the converter smelting; and argon gas is blown for more than 40 minutes when tapping the steel.

5. The method for manufacturing a 690MPa grade extra-thick plate for engineering structures with excellent low-temperature toughness according to claim 3, characterized in that, The RH refining process is as follows: after the vacuum degree reaches the set value during RH vacuum degassing, it is maintained for more than 35 minutes; after the degassing treatment is completed, Al is added according to the Al content requirements of the finished product, Ti-Fe is added for micro-titanium treatment, and then B-Fe alloy is added.

6. The method for manufacturing an extra-thick plate for engineering structures with excellent low-temperature toughness (690MPa grade) according to claim 3, characterized in that, The hot straightening process is as follows: the steel plate is subjected to three high-temperature hot straightening passes, the straightening temperature is controlled at 450-550℃, the straightening force is controlled at 3500-5000kN, the position of the inlet roller is -1.5--2.1mm, the position of the outlet roller is -3.5--4.2mm, and the tilt value is set to 8-12mm.

7. The method for manufacturing a 690MPa grade extra-thick plate for engineering structures with excellent low-temperature toughness according to claim 3, characterized in that, The slow cooling process is as follows: hot plates are used to cover and stack the hot-straightened steel plates. The stacking temperature of the hot plates is 600-800°C, the number of stacked plates is not less than 10, and the slow cooling time is more than 48 hours.

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