890mpa grade low-cost super-thick plate for mine machinery having excellent low-temperature toughness and manufacturing method

Abstract

The present application relates to a kind of 890MPa grade low-cost super-thick plate for mine machinery with excellent low-temperature toughness and manufacturing method, steel plate C:0.12%~0.15%, Si:0.25%~0.35%, Mn:1.35%~1.45%, P≤0.02%, S≤0.010%, Cr:0.4%~0.45%, Al:0.015%~0.035%, Nb:0.05%~0.07%, V:0.06%~0.10%, B:0.0015%~0.0020%, Cu:0.45%~0.55%, Ti:0.015%~0.025%, N:0.01%~0.02%, Mo:0.5%~0.6%, Ni:0.65%~0.75%, the balance is Fe and impurity;Solve the problem such as high production cost, finished steel plate toughness, flaw detection qualified rate is low and plate shape unevenness exceeds standard.

Description

Technical Field

[0001] This invention relates to the field of steel production technology for mining machinery, and in particular to a low-cost extra-thick plate with excellent low-temperature toughness of 890MPa for mining machinery and its manufacturing method. Background Technology

[0002] The development of high-strength steel plates began in the mid-20th century, and their strength grades and comprehensive performance have continuously improved with advancements in steel production technology and market demand. In recent years, with the continuous increase in the depth and intensity of coal mining, the complexity and challenges of the working environment for coal mining machinery have also increased significantly. As a key structural material, the performance of high-strength steel plates directly determines the safety, reliability, and service life of coal mining machinery. Medium-thickness high-strength steel plates, due to their excellent comprehensive mechanical properties and processing performance, have been widely used in the production of key components such as hydraulic supports, scraper conveyors, and frames. Among them, 890MPa grade high-strength steel plates possess high strength and good toughness, meeting the stringent requirements of heavy loads, impacts, and harsh working conditions.

[0003] To meet the technical requirements of 890MPa grade extra-thick plates, traditional production processes mostly 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 and above) combined with offline quenching and tempering processes to produce thicker 890MPa 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 and heavy plate manufacturers are limited by the billet compression ratio, hindering the manufacturing of extra-thick plates. Meanwhile, 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 low-temperature toughness. Furthermore, the control over the plate shape is also limited; currently, steel companies can only use leveling equipment for correction, but plate shape leveling is difficult and inefficient. Regarding ensuring low-temperature toughness, domestic steel companies generally increase alloy content and employ offline quenching processes to guarantee the low-temperature toughness of 890MPa grade steel plates. However, these technologies remain insufficient for extra-thick plates rolled with low compression ratios.

[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) 890MPa grade low-temperature mining machinery steel plates with excellent low-temperature toughness and high flatness, solve the problems existing in the 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 mining machinery with thicknesses of 80–120 mm, yield strengths of 890 MPa, and excellent low-temperature toughness. Prior to this invention, Chinese patent application number 202211619343.7 disclosed a "Production Method for Controlling the Plate Shape of A514GrQ Extra-Thick Plates for Offshore Platforms," ​​which used ingot technology to produce extra-thick plates with thicknesses of 180–210 mm. However, its production cost was high, and the low-temperature toughness of the steel plate was not clearly defined. Summary of the Invention

[0007] This invention overcomes the problems and shortcomings of existing technologies and provides a low-cost extra-thick plate with excellent low-temperature toughness (890MPa grade) for mining machinery and its manufacturing method. When rolling low-temperature structural steel plates with a thickness of 80-120mm and a yield strength of 890MPa 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, and slow cooling.

[0008] To achieve the above objectives, the present invention employs the following technical solution: This is a low-cost, extra-thick steel plate for mining machinery with excellent low-temperature toughness and a grade of 890MPa. The chemical composition of the steel plate, by weight percentage, is: C: 0.12%–0.15%, Si: 0.25%–0.35%, Mn: 1.35%–1.45%, P≤0.02%, S≤0.010%, Cr: 0.4%–0.45%, Al: 0.015%–0.035%, Nb: 0.05%–0.07%, V: 0.06%–0. 10%, B: 0.0015%~0.0020%, Cu: 0.45%~0.55%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.5%~0.6%, Ni: 0.65%~0.75%, 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 ≥890MPa, a tensile strength of 980~1050MPa, and an elongation ≥18%; a transverse Charpy impact energy at -60℃ ≥135J, a yield strength ratio ≤0.90, a Z-direction elongation ≥30%, and a straightness ≤5mm / 2m; and a flaw detection pass rate ≥98%.

[0010] A method for manufacturing low-cost extra-thick plates for mining machinery with excellent low-temperature toughness (890MPa grade) includes steelmaking, LF refining, RH refining, continuous casting, billet heating, controlled rolling, controlled cooling, hot straightening, slow cooling, and heat treatment processes. The specific processes are controlled as follows: 1) Continuous casting: The superheating temperature for continuous casting is 8–12℃, and the casting speed is 0.8–1.1 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 40–50 s, and the reverse stirring time is 35–45 s. The current is 800–1000 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 500–700 L / min from the first to the fourth section, ensuring the surface and core temperatures of the solidified billet are within acceptable limits. The temperature difference between the surface and core of the continuously cast billet is greater than 300℃, and it is simultaneously subjected to light pressure with a reduction of 4-8mm. The total cooling water flow from the 5th to the 8th stage is 800-1200L / min to ensure that the temperature difference between the solidification end surface and the core of the billet is greater than 500℃. At the same time, it is subjected to heavy pressure with a reduction of 15-20mm. After the billet is removed from the production line, it is placed in the pit for heat preservation, homogenization and dehydrogenation treatment. The temperature of the billet in the pit is between 650-800℃ and the holding time is not less than 72h. Then it is slowly cooled to room temperature at a cooling rate of 20-30℃ / hour to ensure that the H content in the billet is not greater than 2ppm. 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 850-950℃, the heating section temperature is 1230-1250℃, and the soaking section temperature is 1270-1280℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 6.0-7.5h, of which the total furnace time for the heating and soaking sections is 5.5-7.0h. The calorific value of the gas is controlled at 2000-2500J / kg, and the air-fuel ratio in the furnace is controlled at 1:1.7-1:2.3. At the same time, the 2-4 sets of burners in the upper part of the soaking section are closed. The temperature difference between the upper and lower surfaces of the billet is within the range of 5-10℃ to ensure that the original austenite grain size is 30-50μm. 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; a three-stage rolling process is adopted. The first stage is austenitic deformation recrystallization rolling. The final rolling temperature of the first stage is 1010-1050℃, and the number of rolling passes is ≤3. The single-pass reduction rate of the last two passes is not less than 12%. In the last two passes of the first stage, descaling water is sprayed into the mill for each pass, the descaling time is 8-12s, and the descaling pressure is 15-20MPa. The second stage is low-temperature austenitic deformation rolling. The starting rolling temperature of the second stage is 970-1000℃, and the finishing rolling temperature of the second stage is 930-950℃. The number of rolling passes is ≤4. The single-pass reduction rate of the first two passes is not less than 15%, and each pass is allowed to warm up for 15-20 seconds before the start of the first two passes. After the first two stages of rolling are completed, the thickness of the intermediate billet is 1.5 to 2.5 times that of the finished steel plate. The intermediate billet is then allowed to heat up and sprayed with descaling water for 1 to 2 passes. The descaling time is 0.5 to 1 minute and the descaling machine pressure is 10 to 15 MPa to ensure that the temperature difference between the surface and the core is above 200°C. The third stage involves rolling in the austenite and ferrite two-phase region, with an initial rolling temperature of 890–910°C and a final rolling temperature of 830–850°C. The single-pass reduction rate for the first two passes in the third stage is >20%. The bite and rolling speeds are 0.9–1.2 m / s, and the ratio of upper and lower roll speeds is controlled at 95%–98%. During rolling, the cooling water volume of the roller table from the mill to the pre-straightening section is controlled at 100–150 m³ / s. 3 / h; the final pass in the third stage uses a small reduction rate, ranging from 0.5% to 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 for the steel plate is 750–770℃, and the final cooling temperature is 520–550℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -300–-500mm, and the lower manifold opening correction value is -700–-900mm. 7–13 water sets are opened on both the upper and lower manifolds, with a single manifold opening water flow rate of 230–260m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.0 to 1:2.4, and the cooling rate is controlled at 20 to 30℃ / s; 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 15-20 MPa and the water flow rate is 120-180 m³ / h. 3 / h, the air purging pressure is 10~15MPa; 5) Heat treatment: The quenching-critical quenching-tempering process, i.e., QLT quenching, is adopted. The initial quenching temperature is 880-900℃, and the furnace time is 1.4-2.2 min / mm; the critical quenching temperature is 770-820℃, and the furnace time is 1.3-2.1 min / mm; the tempering temperature is 450-500℃, and the furnace time is 3-5 min / mm; after exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate.

[0011] The steel smelting process is as follows: smelting is carried out according to the set composition. The raw materials are pretreated by KR hot metal to control the S content ≤0.010%. After slag removal, the raw materials enter the converter. 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% to 0.15% at the end of the converter smelting. Argon gas is blown for more than 30 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 30 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 450-500℃, the straightening force is controlled at 4000-5500kN, the position of the inlet roller is -1.5--2.0mm, the position of the outlet roller is -3.2--4.0mm, 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 650-800°C, the number of stacked plates is not less than 12, and the slow cooling time is more than 72 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, so as to avoid 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 the segregation of the billet and internal structural defects. By optimizing the electromagnetic stirring process in the continuous casting stage, the equiaxed crystal ratio of the continuous casting billet is greatly improved. Strong cooling is adopted in the fan-shaped section and the cooling water volume of different sections is controlled to ensure the temperature gradient in the thickness direction of the billet. At the same time, light pressure or heavy pressure is applied in different sections, which helps to reduce the segregation of the billet and ensures that the strain penetrates to 1 / 4 of the billet thickness (the position improved by light pressure) and the core (the position improved by heavy pressure), significantly improving the defects in different positions of the billet cross section, and providing a guarantee for the strength and toughness of the subsequent steel plate. After the billet is removed from the production line, it is kept warm to homogenize and dehydrogenate, and the temperature and time are controlled to promote the diffusion of elements such as Mn, C, B, H, V and N, reduce the impact of component segregation on microstructure and properties, and ensure that the H content is not greater than 2ppm.

[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 per pass, thereby 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 of Ti, Nb and V carbonitrides in the matrix and their full diffusion. At the same time, the heating and soaking sections provide high-temperature heating conditions, and the calorific value of the gas is controlled to ensure the temperature uniformity of each part of the billet and to ensure that the partial solidification of Ti, Nb and V carbonitrides in the matrix. At the same time, it 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℃ 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. This ensures the uniformity of transverse and longitudinal metal flow on the surface of the steel plate. Controlling the total furnace time is to effectively promote the full diffusion of elements such as C, Mn, V, Cr and N, while also ensuring that the original austenite grain size is 30-50 μm. When the austenite size is large, there are fewer austenite grain boundaries intersecting with the high Ar3 region, and the ferrite nucleation density is low. Ferrite grains are difficult to grow together and eventually cannot form a ferrite / pearlite banded structure, which can improve the toughness of the steel plate.

[0018] (4) A three-stage controlled rolling process was adopted, which controlled the reduction and rolling temperature of each rolling stage, optimized the thickness of the intermediate billet, promoted the temperature range for static recrystallization of austenite during the waiting process, ensured the uniformity of the microstructure, and ensured the dispersed precipitation of Nb, V, Cr and Ti carbides or nitrides. The last two passes of the first stage used sprayed descaling water to create a temperature gradient between the inner and outer surfaces of the billet, promoted the penetration of rolling deformation into the thickness center, refined the grains at 1 / 2 thickness, and helped improve the core microstructure of thick steel plates. The first two passes of the second stage used a large reduction rate, and used the high-temperature deformation-induced effect to promote the further precipitation of Nb, V and Ti, Cr carbonitrides in the austenite grains to improve strength and toughness. At the same time, the waiting process before the start of the first two passes promoted the dynamic recrystallization of austenite grains to refine the grains. Intermediate billet cooling process is employed to suppress austenite grain growth and simultaneously generate a temperature gradient on the inner and outer surfaces of the billet. This promotes the penetration of rolling deformation into the thickness center, refines the grains at half the thickness, and improves the core microstructure of thick steel plates. It also reduces microstructural stress caused by microstructural inhomogeneity and accelerates the temperature drop of the intermediate billet, reducing the waiting time. By controlling the reduction rate in the third stage and the reduction rate in the finishing rolling stage, the dislocation density, vacancies, and deformation bands within the austenite body are ensured, promoting the precipitation of Nb, Ti, and V carbonitrides, providing more nucleation sites, facilitating ferrite phase transformation, and refining the ferrite microstructure. A small reduction rate is used in the final finishing pass (leveling pass) to level the plate shape, reduce internal stress, and prevent water retention in the plate after controlled cooling due to breakage and warping, which would affect the uniformity of microstructure and properties.

[0019] (5) During the UFC cooling process, the final cooling temperature and cooling rate are controlled, and the head and tail of the steel plate are shielded. The purpose is to effectively regulate the temperature uniformity at different positions of the head, middle and tail of the steel plate to ensure the strength, toughness and shape of the steel plate. The introduction of side spraying and air blowing is conducive to the control of the steel plate shape, which can improve the uniformity of the steel plate performance and reduce the probability of shape problems such as buckling head and tail. A high-temperature hot straightening machine is used and appropriate roll gap and tilting values ​​are set. At the same time, multiple passes of small bending amount straightening are adopted to ensure that the cumulative strain of 15% to 20% near the surface of the steel plate exceeds the elastic limit strain, promote the further full release of internal stress, and ensure good flatness of the straightened plate.

[0020] (6) High-temperature stacking reduces banded structure and allows for full diffusion of C, Mn, and H elements; the self-tempering effect of hot plates improves the plasticity and toughness of the steel plate and increases the pass rate of flaw detection. The QLT process is used on the rolled steel plate, the core of which is two-phase quenching. After holding at a temperature range where austenite and ferrite coexist, lamellar structure is formed by quenching. This structure can refine grains, increase the proportion of large-angle grain boundaries, and promote the formation of reverse-transformed austenite, thereby improving toughness. The tempering temperature is controlled to ensure that the width of martensite laths, the proportion of precipitates, and the proportion of large-angle grain boundaries reach 60% to 75% to ensure strength and toughness.

[0021] (7) Through composition and process design, a low-cost production method for rolling extra-thick plates for mining machinery with a thickness of 80-120mm, a yield strength of 890MPa, and excellent low-temperature toughness was obtained. By optimizing the steelmaking, heating, rolling, cooling, straightening, and slow cooling 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 roll low-temperature structural steel plates with a thickness of 80-120mm and a yield strength of 890MPa were finally solved. The technical indicators of the prepared plates are as follows: transverse tensile yield strength ≥890MPa, tensile strength between 980 and 1050MPa, elongation ≥18%, transverse Charpy impact energy at -60℃ ≥135J, yield strength ratio ≤0.90, Z-direction elongation ≥30%, straightness below 5mm / 2m, and flaw detection pass rate ≥98%. Detailed Implementation

[0022] The present invention relates to a low-cost, extra-thick steel plate for mining machinery with excellent low-temperature toughness and a strength of 890 MPa. The steel plate's chemical composition, by weight percentage, is as follows: C: 0.12%–0.15%, Si: 0.25%–0.35%, Mn: 1.35%–1.45%, P≤0.02%, S≤0.010%, Cr: 0.4%–0.45%, Al: 0.015%–0.035%, Nb: 0.05%–0.07%, V: 0.06%. ~0.10%, B: 0.0015%~0.0020%, Cu: 0.45%~0.55%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.5%~0.6%, Ni: 0.65%~0.75%, 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 (methyl monoxide) 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 1.35%–1.45%.

[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 for reducing the Bauschinger effect in the matrix material. However, excessive Si will reduce the toughness of the heat-affected zone of the base metal during welding. Therefore, this invention sets the Si content range to 0.25% to 0.35%.

[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 range is set to 0.05% to 0.07%.

[0027] V (V) is a strong solid element containing carbon and nitrogen, 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 0.06%–0.10%.

[0028] Al: It is usually used as a deoxidizer in steel. If it forms AlN, it can also refine the microstructure. When the Al content exceeds 0.035%, the excessive alumina inclusions will reduce the cleanliness of the steel, while if the Al content is too low, it will lead to insufficient deoxidation, and easily oxidized elements such as Ti will form oxides. Therefore, the present 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, thereby improving the strength, plasticity and toughness of steel plates. In this invention, the Cr content range is set to 0.4% to 0.45%.

[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 0.45% to 0.55%.

[0031] 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 sets the Mo content range to 0.50% to 0.60%.

[0032] Ni: Nickel exhibits a similar phase transformation behavior to Mo in steel, which can lower the phase transformation temperature of steel, improve the microstructure, refine the grains, and increase the strength of steel while maintaining good plasticity and toughness. However, excessively high Ni content will increase production costs. Therefore, this invention controls the Ni content within the range of 0.65% to 0.75%.

[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 of 0.01% to 0.02% and a Ti content 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 manufacturing method of the 890MPa grade low-cost extra-thick plate for mining machinery with excellent low-temperature toughness described in this invention comprises the following process route: steel smelting → ladle refining + RH refining (B alloying) → continuous casting → billet heating → controlled rolling and controlled cooling → hot straightening and slow cooling → heat treatment. The specific control process is as follows: 1) Steelmaking and 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 30 minutes during tapping. Argon blowing and killing before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition.

[0037] After LF refining, RH refining is carried out. During RH vacuum degassing, the set vacuum degree is reached and maintained for more than 30 minutes. After the degassing treatment is completed, sufficient Al is added according to the Al content requirements of the finished product. Sufficient Ti-Fe is added for micro-titanium treatment, and then B-Fe alloy is added. It is ensured that the alloy is added directly to the molten steel to avoid floating on the slag layer.

[0038] During continuous casting, the superheat is controlled at 8–12℃, and the casting speed is 0.8–1.1 m / min. By reducing the superheat and casting speed, macroscopic segregation in the continuously cast billet is improved, and the spacing of secondary dendrite arms in the solidification structure of the billet is reduced, which helps to reduce billet segregation and internal structural defects. 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 40–50 s, and the reverse stirring time is 35–45 s. The current is 800–1000 A, and the frequency is 30–50 Hz. The molten steel is then continuously cast to obtain a continuously cast billet, and the isometric crystal ratio in the billet is not less than 90%. The continuous casting sector adopts a strong cooling process. The total cooling water volume of the first to fourth sections is 500-700 L / min to ensure a small gradient temperature field with a temperature difference of >300℃ between the surface and core of the solidified billet. At the same time, light pressure is applied with a reduction of 4-8 mm. The total cooling water volume of the fifth to eighth sections is 800-1200 L / min to ensure a large gradient temperature field with a temperature difference of >500℃ between the surface and core of the solidified billet. At the same time, heavy pressure is applied with a reduction of 15-20 mm. 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 used in the sector section, and the cooling water volume of different sections is controlled to ensure the temperature gradient in the thickness direction of the billet. Light or heavy pressure is applied to different sections to help reduce billet segregation and ensure that strain penetrates to 1 / 4 of the billet thickness (improved under light pressure) and the core (improved under heavy pressure), thereby significantly improving defects in different positions of the billet cross-section and providing a guarantee for the strength and toughness of the subsequent steel plate.

[0039] After being removed from the production line, the cast billets undergo heat preservation, homogenization, and dehydrogenation treatment in a pit. The initial heat preservation temperature is between 650 and 800℃, and the holding time is no less than 72 hours. Subsequently, the billets are slowly cooled to room temperature at a rate of 20 to 30℃ / hour. This heat preservation, homogenization, and dehydrogenation treatment, along with controlled stacking temperature, time, and cooling rate, aims 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, prevent corner cracks in the billets, and ensure that the H content in the billets does not exceed 2 ppm.

[0040] 2) Billet Heating Process: Billets with a thickness of less than 360mm are fed into a walking beam furnace for heating. The preheating section temperature range is 850–950℃, aiming to promote the partial dissolution and 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 1270–1280℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 6.0–7.5 hours, with the heating and soaking sections taking 5.5–7.0 hours. The calorific value of the gas is controlled at 2000–2500 J / kg. The air-fuel ratio in the furnace is controlled at 1:1.7–1:2.3 by adjusting the opening degree of the upper and lower burners. Simultaneously, 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℃. The heating and soaking sections provide high-temperature heating conditions while controlling the calorific value of the gas. This aims to ensure temperature uniformity across all parts of the billet, guarantee that the carbonitrides of Ti, Nb, and V are partially dissolved in the matrix, and further promote the full diffusion of elements such as Mn, Cr, C, B, V, Nb, and Mo, thus mitigating the impact of component segregation on the microstructure and properties. Controlling the air-fuel ratio and closing some nozzles in the upper part of the soaking section aims to ensure that the temperature difference between the upper surface and the lower surface of the billet is 5–10°C higher. This prevents the lower surface of the billet from being affected by the water beam at the bottom of the furnace, which could lead to a lower temperature and affect the uniformity of rolling deformation between the upper and lower surfaces. Ultimately, this ensures uniform transverse and longitudinal metal flow on the steel plate surface. Controlling the total furnace time is to effectively promote the full diffusion of elements such as C, Mn, V, Cr, and N, while also ensuring that the original austenite grain size is within the range of 30–50 μm. When the austenite size is large, there are fewer austenite grain boundaries that intersect with the high Ar3 region, resulting in a lower ferrite nucleation density. Ferrite grains are difficult to grow together and ultimately cannot form a ferrite / pearlite banded structure, thus failing to improve the toughness of the steel plate.

[0041] 3) Control the rolling process: Before rolling, use high-pressure water to descale the cast billet after it exits the furnace for 1.5–2.5 minutes, with a descaling machine pressure of 20–25 MPa; adopt a three-stage rolling process: The first stage is austenitic deformation recrystallization rolling, with a final rolling temperature of 1010-1050℃ and ≤3 rolling passes, of which the single-pass reduction rate of the last two passes is not less than 12%; the last two passes of the first stage are sprayed with descaling water in each pass, with a descaling time of 8-12s and a descaling machine pressure of 15-20MPa.

[0042] The second stage is low-temperature austenitic deformation rolling, with an initial rolling temperature of 970-1000℃ and a final rolling temperature of 930-950℃. The number of rolling passes is ≤4, with the single-pass reduction rate of the first two passes not less than 15%, and each pass is allowed to warm up for 15-20 seconds before the start of the first two passes.

[0043] After the first two stages of rolling, the thickness of the intermediate billet is 1.5 to 2.5 times the thickness of the finished steel plate. After the second stage of rolling, the intermediate billet is allowed to heat up and is sprayed with descaling water for 1 to 2 passes. The descaling time is 0.5 to 1 minute, and the descaling machine pressure is 10 to 15 MPa. This ensures that the temperature difference between the surface and the core is above 200°C, which inhibits the growth of austenite grains. At the same time, a temperature gradient is generated on the inner and outer surfaces of the billet, which promotes the penetration of rolling deformation into the thickness center, refines the core grains, and helps improve the uniformity of the microstructure, reduce the microstructural stress caused by microstructural inhomogeneity, and reduce the fluctuation of the properties of the steel plate at various locations.

[0044] The third stage involves rolling in the austenite and ferrite two-phase region, with an initial rolling temperature of 890–910°C and a final rolling temperature of 830–850°C. The single-pass reduction rate for the first two passes in the third stage is >20%. The bite and rolling speeds are 0.9–1.2 m / s, and the ratio of upper and lower roll speeds is controlled at 95%–98%. During rolling, the cooling water level in the roller table from the mill to the pre-straightening section is reduced to 100–150 m³. 3 / h, to avoid a large amount of cooling water flowing on the lower surface of the steel plate, reducing uncontrolled temperature drop on the lower surface of the steel plate, and thus preventing the buckling phenomenon. The third stage final pass is a leveling pass, using a small reduction rate, which is 0.5% to 1%.

[0045] A three-stage controlled rolling process is adopted, with separate control of the reduction and rolling temperature for each stage. The intermediate billet thickness is optimized to promote the static recrystallization temperature range of austenite during the warming process, ensuring uniform microstructure and dispersed precipitation of Nb, V, Cr, and Ti carbides or nitrides. In the last two passes of the first stage, descaling water is used in the spray mill to create a temperature gradient between the inner and outer surfaces of the billet, promoting the penetration of rolling deformation towards the thickness center and refining the grains at the 1 / 2 thickness, which is beneficial for improving the core microstructure of thick steel plates. In the first two passes of the second stage, a large reduction rate is used, utilizing the high-temperature deformation-induced effect to further promote the precipitation of Nb, V, Ti, and Cr carbonitrides within the austenite grains, improving strength and toughness. Simultaneously, the warming process before the start of rolling in the first two passes of the second stage promotes dynamic recrystallization of austenite grains, refining the grain size. Controlling the reduction rate and reduction ratio in the third stage ensures the density of dislocations, vacancies, and deformation bands within the austenite body, promotes the precipitation of carbonitrides of Nb, Ti, and V, provides more nucleation sites, promotes ferrite phase transformation, and refines the ferrite microstructure. The final pass in the third stage is a leveling pass with a small reduction rate. The purpose 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 curling heads after controlled cooling, which would affect the uniformity of microstructure and properties.

[0046] 4) Cooling process control: UFC (Ultra-Fast Cooling System) is used for cooling. The UFC cooling process employs a fully automatic controlled cooling mode. The initial cooling temperature of the steel plate is 750–770℃, and the final cooling temperature is 520–550℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -300–-500mm, and the lower manifold opening correction value is -700–-900mm. 7–13 sets of water are opened on both the upper and lower manifolds, with a single manifold opening water flow of 230–260m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.0 to 1:2.4, and the cooling rate is controlled at 20 to 30℃ / s; 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 15–20 MPa and the water flow rate is 120–180 m³ / h. 3 The air purging pressure is 10-15 MPa per hour. During UFC cooling, controlling the final cooling temperature and cooling rate, and employing head and tail shielding of the steel plate, aims to effectively regulate the temperature uniformity at different positions (head, middle, and tail) of the steel plate, ensuring its strength, toughness, and shape. Side spraying and air purging are beneficial for controlling the steel plate shape, improving the uniformity of steel plate performance, and reducing the likelihood of head and tail shape issues.

[0047] The steel plate undergoes three passes of high-temperature hot straightening, with the straightening temperature controlled between 450 and 500℃, the straightening force between 4000KN and 5500KN, the inlet roller position at -1.5 to -2.0 mm, the outlet roller position at -3.2 to -4.0 mm, and the tilt value set at 8 to 12 mm. Using a high-temperature hot straightening machine, with appropriate roller gaps and tilt values, multiple passes of small bending are employed to ensure that the cumulative strain near the steel plate surface exceeds the elastic limit strain by 15% to 20%, promoting further and full release of internal stress and ensuring good straightness of the straightened plate.

[0048] 5) Stacking and slow cooling process: Cooled steel plates are covered and stacked using hot plates at a temperature of 650–800°C. At least 12 plates are stacked, and the slow cooling time is over 72 hours. High-temperature stacking reduces banded microstructure, allowing for full diffusion of C, Mn, and H elements. Simultaneously, the self-tempering effect of the hot plates further improves the steel plate's ductility and toughness, and increases the pass rate of flaw detection.

[0049] 6) Heat Treatment Process: The quench-critical quench-temper (QLT quenching) process is adopted. The initial quenching temperature is 880–900℃, with a furnace time of 1.4–2.2 min / mm; the critical quenching temperature is 770–820℃, with a furnace time of 1.3–2.1 min / mm. Subsequently, the steel plate undergoes tempering treatment at a tempering temperature of 450–500℃, with a furnace time of 3–5 min / mm. After exiting the furnace, it is air-cooled to obtain the finished steel plate. The core of the QLT quenching process lies in two-phase quenching. It forms a lamellar structure by holding the steel at a temperature range where austenite and ferrite coexist. 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 and reducing the yield strength ratio. By controlling the tempering temperature, the hard phase martensite laths are gradually decomposed and their width is controlled during the tempering process, reducing the internal dislocation density, gradually restoring the lattice distortion, and causing the surrounding soft phase ferrite to undergo slight coordinated deformation, ensuring that the proportion of large-angle grain boundaries reaches 60% to 75%, ultimately guaranteeing strength and toughness.

[0050] This invention, employing the aforementioned components and processes, ultimately yields a low-cost production method for extra-thick plates used in mining machinery, characterized by a thickness of 80–120 mm, a yield strength of 890 MPa, and excellent low-temperature toughness. By optimizing the steelmaking, heating, rolling, cooling, straightening, stacking, and heat treatment processes, the invention 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 360 mm to produce low-cost low-temperature structural steel plates with a thickness of 80–120 mm and a yield strength of 890 MPa. The technical specifications of the prepared plates are as follows: transverse tensile yield strength ≥ 890 MPa, tensile strength between 980 and 1050 MPa, elongation ≥ 18%, transverse Charpy impact energy at -60℃ ≥ 135 J, yield strength ratio ≤ 0.90, Z-direction elongation ≥ 30%, straightness below 5 mm / 2 m, and flaw detection pass rate ≥ 98%.

[0051] 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.

[0052]

Example

[0053] Table 1 - Chemical composition of steel (wt, %)

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

[0055] Table 2 - Steel smelting and continuous casting process parameters

[0056] Table 3 - Billet Heating Process Parameters

[0057] Table 4 - Rolling process parameters for the first and second stages

[0058] Table 5 - Intermediate Billet Treatment and Third-Stage Rolling Process Parameters

[0059] Table 6 - Cooling and Straightening Process Parameters for Steel Plates

[0060] Table 7 - Slow Cooling and Heat Treatment Process Parameters for Steel Plates

[0061] Table 8 - Dimensions and Performance Indicators of Finished Steel Plates

[0062] 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 low-cost, extra-thick plate for mining machinery with excellent low-temperature toughness and a strength of 890MPa, characterized in that... The chemical composition of the steel plate, by weight percentage, is as follows: C: 0.12%–0.15%, Si: 0.25%–0.35%, Mn: 1.35%–1.45%, P≤0.02%, S≤0.010%, Cr: 0.4%–0.45%, Al: 0.015%–0.035%, Nb: 0.05%–0.07%, V: 0.06%–0.10%, B: 0.0015%– 0.0020%, Cu: 0.45%~0.55%, Ti: 0.015%~0.025%, N: 0.01%~0.02%, Mo: 0.5%~0.6%, Ni: 0.65%~0.75%, 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 low-cost extra-thick plate for mining machinery with excellent low-temperature toughness of 890MPa as described in claim 1, characterized in that, The finished steel plate has a transverse tensile yield strength ≥890MPa, a tensile strength of 980~1050MPa, and an elongation ≥18%; a transverse Charpy impact energy at -60℃ ≥135J, a yield strength ratio ≤0.90, a Z-direction elongation ≥30%, 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 (890 MPa grade) as described in claim 1 or 2, characterized in that, This includes the processes of steel smelting, LF refining, RH refining, continuous casting, billet heating, controlled rolling, controlled cooling, hot straightening, slow cooling, and heat treatment. The specific processes controlled are as follows: 1) Continuous casting: The superheating temperature for continuous casting is 8–12℃, and the casting speed is 0.8–1.1 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 40–50 s, and the reverse stirring time is 35–45 s. The current is 800–1000 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 500–700 L / min from the first to the fourth section, ensuring the surface and core temperatures of the solidified billet are within acceptable limits. The temperature difference between the surface and core of the continuously cast billet is greater than 300℃, and it is simultaneously subjected to light pressure with a reduction of 4-8mm. The total cooling water flow from the 5th to the 8th stage is 800-1200L / min to ensure that the temperature difference between the solidification end surface and the core of the billet is greater than 500℃. At the same time, it is subjected to heavy pressure with a reduction of 15-20mm. After the billet is removed from the production line, it is placed in the pit for heat preservation, homogenization and dehydrogenation treatment. The temperature of the billet in the pit is between 650-800℃ and the holding time is not less than 72h. Then it is slowly cooled to room temperature at a cooling rate of 20-30℃ / hour to ensure that the H content in the billet is not greater than 2ppm. 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 850-950℃, the heating section temperature is 1230-1250℃, and the soaking section temperature is 1270-1280℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 6.0-7.5h, of which the total furnace time for the heating and soaking sections is 5.5-7.0h. The calorific value of the gas is controlled at 2000-2500J / kg, and the air-fuel ratio in the furnace is controlled at 1:1.7-1:2.

3. At the same time, the 2-4 sets of burners in the upper part of the soaking section are closed. The temperature difference between the upper and lower surfaces of the billet is within the range of 5-10℃ to ensure that the original austenite grain size is 30-50μm. 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; a three-stage rolling process is adopted. The first stage is austenitic deformation recrystallization rolling. The final rolling temperature of the first stage is 1010-1050℃, and the number of rolling passes is ≤3. The single-pass reduction rate of the last two passes is not less than 12%. In the last two passes of the first stage, descaling water is sprayed into the mill for each pass, the descaling time is 8-12s, and the descaling pressure is 15-20MPa. The second stage is low-temperature austenitic deformation rolling. The starting rolling temperature of the second stage is 970-1000℃, and the finishing rolling temperature of the second stage is 930-950℃. The number of rolling passes is ≤4. The single-pass reduction rate of the first two passes is not less than 15%, and each pass is allowed to warm up for 15-20 seconds before the start of the first two passes. After the first two stages of rolling are completed, the thickness of the intermediate billet is 1.5 to 2.5 times that of the finished steel plate. The intermediate billet is then allowed to heat up and sprayed with descaling water for 1 to 2 passes. The descaling time is 0.5 to 1 minute and the descaling machine pressure is 10 to 15 MPa to ensure that the temperature difference between the surface and the core is above 200°C. The third stage involves rolling in the austenite and ferrite two-phase region, with an initial rolling temperature of 890–910°C and a final rolling temperature of 830–850°C. The single-pass reduction rate for the first two passes in the third stage is >20%. The bite and rolling speeds are 0.9–1.2 m / s, and the ratio of upper and lower roll speeds is controlled at 95%–98%. During rolling, the cooling water volume of the roller table from the mill to the pre-straightening section is controlled at 100–150 m³ / s. 3 / h; the final pass in the third stage uses a small reduction rate, ranging from 0.5% to 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 for the steel plate is 750–770℃, and the final cooling temperature is 520–550℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -300–-500mm, and the lower manifold opening correction value is -700–-900mm. 7–13 water sets are opened on both the upper and lower manifolds, with a single manifold opening water flow rate of 230–260m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2.0 to 1:2.4, and the cooling rate is controlled at 20 to 30℃ / s; 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 15-20 MPa and the water flow rate is 120-180 m³ / h. 3 / h, the air purging pressure is 10~15MPa; 5) Heat treatment: The quenching-critical quenching-tempering process, i.e., QLT quenching, is adopted. The initial quenching temperature is 880-900℃, and the furnace time is 1.4-2.2 min / mm; the critical quenching temperature is 770-820℃, and the furnace time is 1.3-2.1 min / mm; the tempering temperature is 450-500℃, and the furnace time is 3-5 min / mm; after exiting the furnace, the steel plate is air-cooled to obtain the finished steel plate.

4. The method for manufacturing a low-cost, extra-thick plate for mining machinery with excellent low-temperature toughness (890MPa grade) according to claim 3, characterized in that, The steel smelting process is as follows: smelting is carried out according to the set composition. The raw materials are pretreated by KR hot metal to control the S content ≤0.010%. After slag removal, the raw materials enter the converter. 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% to 0.15% at the end of the converter smelting. Argon gas is blown for more than 30 minutes when tapping the steel.

5. The method for manufacturing a low-cost, extra-thick plate with excellent low-temperature toughness for mining machinery of 890MPa grade 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 30 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 a low-cost, extra-thick plate for mining machinery with excellent low-temperature toughness (890MPa 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 450-500℃, the straightening force is controlled at 4000-5500kN, the position of the inlet roller is -1.5--2.0mm, the position of the outlet roller is -3.2--4.0mm, and the tilt value is set to 8-12mm.

7. The method for manufacturing a low-cost, extra-thick plate for mining machinery with excellent low-temperature toughness (890MPa grade) 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 650-800°C, the number of stacked plates is not less than 12, and the slow cooling time is more than 72 hours.

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