345mpa grade super thick plate for engineering equipment with excellent low temperature toughness and high flatness and manufacturing method thereof

Abstract

The present application relates to the technical field of engineering structure steel production, and particularly relates to a 345MPa grade super-thick plate with excellent low-temperature toughness and high flatness for engineering equipment and a manufacturing method. The chemical composition of the steel plate is as follows: C: 0.14%-0.19%, Si: 0.25%-0.35%, Mn: 1.40%-1.55%, P: 0.020% or less, S: 0.010% or less, Al: 0.015%-0.035%, Nb: 0.02%-0.04%, B: 0.0015%-0.0020%, Ti: 0.01%-0.02%, and the balance of Fe and impurities. The 360mm or less thickness continuous casting billet is rolled by a plate reciprocating mill, and the thickness of the finished steel plate is 80-130mm. Through optimization of the processes of steelmaking, heating, rolling, controlled cooling, straightening and slow cooling, the problems of high production cost, low toughness of the finished steel plate, low flaw detection qualified rate and out-of-standard plate flatness are solved.

Description

Technical Field

[0001] This invention relates to the field of steel production technology for engineering equipment, and in particular to an extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and a 345MPa grade, and its manufacturing method. Background Technology

[0002] With the development of marine engineering, heavy engineering machinery, shipbuilding, and defense industries, downstream manufacturing enterprises are increasingly demanding high-quality extra-thick slabs with a strength of 345MPa. The traditional production process for extra-thick slabs involves rolling large ingots or composite welded billets, 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 foreign steel companies generally use extra-thick continuously cast slabs (450mm or thicker) to produce plates with a thickness of 80mm or more through rolling. 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, conventional medium-thick plate manufacturers are limited by the slab 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.

[0003] To address these issues, steel companies both domestically and internationally have widely adopted 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. They cannot guarantee the low-temperature toughness of the finished product, and their 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.

[0004] 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-thickness steel plates (thickness 80-130mm) for cryogenic engineering equipment with excellent low-temperature toughness and high flatness, solve the problems existing in the production process, reduce manufacturing costs, and achieve mass production.

[0005] To date, there has been little research, both domestically and internationally, on how to achieve low-cost manufacturing of extra-thick plates for engineering equipment with thicknesses ranging from 80 to 130 mm, yield strengths of 345 MPa, and excellent low-temperature toughness. Prior to this invention, the journal article "Composite Rolling Process of Low-Alloy High-Strength Structural Steel Q345 Extra-Thick Plate" (Special Steel, Vol. 35, No. 4, August 2014) mainly employed a composite rolling process to produce extra-thick plates, leading to a significant increase in manufacturing costs, and the low-temperature toughness of the rolled steel plates remained unclear. Chinese patent application No. 201010572979.1 discloses an "Extra-Thick Steel Q345R-Z35 for HIC-Resistant Pressure Vessels and its Production Method," which uses a die-casting process to produce 84 mm thick extra-thick plates, but its production cost is high, and the low-temperature toughness of the finished steel plates only meets the 0℃ requirement. 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 345 MPa-level extra-thick steel plates. Summary of the Invention

[0006] This invention overcomes the problems and shortcomings of existing technologies and provides an extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and a yield strength of 345MPa, as well as a manufacturing method. When rolling low-temperature structural steel plates with a thickness of 80-130mm and a yield strength of 345MPa using continuous casting billets with a cross-section of less than 360mm, the invention solves the problems of high production costs, low toughness of finished steel plates, low flaw detection pass rate, and excessive flatness of plate shape by optimizing processes such as steelmaking, heating, rolling, controlled cooling, straightening, and slow cooling.

[0007] To achieve the above objectives, the present invention employs the following technical solution: This is an extra-thick steel plate for engineering equipment with excellent low-temperature toughness and high flatness, rated for 345MPa. The chemical composition of the steel plate, by weight percentage, is: C: 0.14%~0.19%, Si: 0.25%~0.35%, Mn: 1.40%~1.55%, P≤0.020%, S≤0.010%, Al: 0.015%~0.035%, Nb: 0.02%~0.04%, B: 0.0015%~0.0020%, Ti: 0.01%~0.02%, with the balance being Fe and unavoidable impurities. It is rolled from continuously cast billets with a thickness of less than 360mm using a medium-thickness reciprocating rolling mill, resulting in a finished steel plate thickness of 80~130mm.

[0008] The finished steel plate has a yield strength of ≥345MPa in the transverse tensile test, a tensile strength of 490~570MPa, and an elongation of ≥22%; a transverse Charpy impact energy of ≥120J at -40℃ and a yield strength ratio of ≤0.88; a Z-direction elongation of ≥35% and a straightness of ≤5mm / 2m; and a flaw detection pass rate of ≥99%.

[0009] The manufacturing method of extra-thick plates for engineering equipment with excellent low-temperature toughness and high flatness (345MPa grade) includes steel smelting, LF refining, RH refining, continuous casting, billet heating, controlled rolling, controlled cooling, hot straightening, and slow cooling processes. The specific processes are controlled as follows: 1) Continuous casting: The superheat of continuous casting is 10-16℃, and the casting speed is 0.8-1.0m / min; the fan-shaped section of continuous casting adopts a strong cooling process, with a total cooling water volume of 800-1200L / min for the first to fourth sections and 600-800L / min for the fifth to eighth sections. The temperature difference between the surface and core of the solidified billet is >500℃, and heavy pressure is used for reduction of 15-20mm; after the billet leaves the production line, it is hot-charged into a slow cooling pit. When the billet temperature is above 700℃, the slow cooling pit is not heated, and the slow cooling time is not less than 48h; the H content in the billet is guaranteed to be no more than 5ppm. 2) Billet heating: Billets with a thickness ≤360mm are fed into a walking beam furnace for heating. The preheating section temperature is 600~900℃, the heating section temperature is 1220~1250℃, and the soaking section temperature is 1190~1210℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 5.5~7.5h, of which the total furnace time for the heating and soaking sections is 4.5~6.0h. The air-fuel ratio in the furnace is controlled at 1:1.7~1:2.2 to ensure that the temperature difference between the upper and lower surfaces of the billet is ≤10℃. 3) Controlled rolling: Before rolling, use high-pressure water to descale the cast billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 15-20 MPa; Rolling is done in two stages: The first stage is the recrystallization zone rolling, i.e., rough rolling, with a rough rolling start temperature ≥1150℃ and a rough rolling finish temperature of 1020-1050℃. The single-pass reduction rate of the last two passes and the last pass in the rough rolling stage is not less than 15%. Simultaneously, descaling water is sprayed in the last two passes and the last pass in the rough rolling stage, with a descaling time of 10-30 seconds and a descaling machine pressure of 15-20 MPa; The thickness of the intermediate billet is 2.0-3.0 times the thickness of the finished product; During the intermediate billet's warming process, 1-2 passes of descaling water are sprayed to remove scale. The scaling time is 1–1.5 min, the descaling machine pressure is 20–25 MPa, and the temperature difference between the surface and core of the intermediate billet is ≥50℃. During the intermediate billet's warming process, pre-straightening is performed with a straightening force of 1000–1500 kN, a roll bending amount of 1.0–2.0 mm, and a tilting value of 2.0–3.0 mm. The second stage is the rolling of the non-recrystallized zone, i.e., finishing rolling. The starting temperature of finishing rolling is 930–960℃, the bite and rolling speed of finishing rolling is 1.0–1.2 m / s, and the finishing rolling temperature is 870–890℃. The single-pass reduction rate of the first two passes in the finishing rolling stage is ≥20%, the reduction rate of the last pass is <5%, and the finishing rolling does not exceed five passes. 4) Controlled Cooling: After rolling, a UFC ultra-fast cooling system is used for cooling. The UFC ultra-fast cooling process employs a fully automatic controlled cooling mode. The initial cooling temperature of the steel plate is 770–810℃, and the final cooling temperature is 620–650℃. 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 -1000–-1200mm. 8–12 sets of water are opened in both the upper and lower manifolds, with a single manifold opening water flow of 200–230m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2 to 1:2.5; the roller speed of the steel plate conveyor is 0.5 to 0.8 m / s, and the acceleration is 0.003 to 0.005 m / s². 2 .

[0010] The steelmaking process is as follows: the raw materials are pretreated with KR hot metal to control the S content to be less than 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 to be ≤0.020%; the C content is controlled to be 0.14% to 0.19% at the end of the converter smelting, and argon gas is blown for more than 30 minutes when tapping the steel.

[0011] 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 25 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.

[0012] The hot straightening process is as follows: the cooled steel plate is hot straightened at high temperature, the straightening force is 2000-3000kN, the bending amount of the roller is set to 2-2.5mm, and the tilting value is set to 3-5mm.

[0013] The slow cooling process is as follows: the hot-straightened steel plates are stacked and slow-cooled at a temperature of 300-400°C, with a minimum of 8 stacked plates and a slow cooling time of ≥10 hours.

[0014] 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 for desulfurization, 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 macroscopic 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. The strong cooling process is used to ensure the temperature gradient in the thickness direction of the billet. At the same time, it is subjected to heavy pressure, which helps to reduce the segregation of the billet and ensures that the strain penetrates to the core of the billet, thereby significantly improving the central defects of the billet and providing a guarantee for the strength and toughness of the subsequent steel plate. By stacking and slow cooling the billets after they come off the production line, and by limiting the slow cooling temperature and time, it helps to promote the diffusion of elements such as Mn, C, B, and H, reduce the impact of component segregation on the microstructure and properties, and ensure that the H content is no more than 5 ppm.

[0015] (2) The composition design of this invention is reasonable and the amount of alloy added is low. The conventional C-Mn-Nb microalloying composition design system is adopted, and the continuous casting billet with a cross section of less than 360mm is 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 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-induced effect is used to promote the precipitation of NbC and TiC phases, 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.

[0016] (3) The temperature and time of the billet in the preheating, heating, and soaking zones are limited to promote the partial dissolution of Ti and Nb carbides in the matrix and their full diffusion. At the same time, the heating zone provides high-temperature heating conditions to ensure the temperature uniformity of all parts of the billet and to ensure that the partial dissolution of Ti and Nb carbides in the matrix. It also further promotes the diffusion of elements such as Mn, Cr, C, B, and Nb, reducing the impact of component segregation on the microstructure and properties, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, by reducing the heating temperature of the billet in the soaking zone, energy consumption is reduced, and the impact of coarsening of the original austenite grains on the properties is also suppressed.

[0017] (4) A two-stage controlled rolling process is adopted. In the last two passes of the roughing stage, a large reduction rate is used to promote the precipitation of NbC and TiC phases in the austenite grains by utilizing the high-temperature deformation-induced effect, thereby improving strength and toughness. At the same time, it promotes the dynamic recrystallization of austenite grains to refine the grains. In the last two passes of the roughing stage, water spraying is used to descale the billet, creating a temperature gradient between the inner and outer surfaces of the billet. This promotes the penetration of rolling deformation into the thickness center, refines the grains at 1 / 2 thickness, and is beneficial to improving the core structure of thick steel plates. An intermediate billet cooling process is adopted to inhibit austenite grain growth. By controlling the reduction rate in the finishing stage, the dislocation density, vacancies, and deformation bands in the austenite body are ensured, promoting the precipitation of NbC and TiC phases, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite structure, and ensuring the strength and toughness of the steel plate. The final pass of the finishing mill uses a low reduction rate to flatten the steel plate shape and reduce internal stress. Simultaneously, the length of the rolled steel plate is controlled to prevent excessively rapid temperature drop in later rolling stages from affecting the uniformity of the microstructure after controlled cooling. During UFC cooling, parameters such as cooling rate and final cooling temperature are controlled, and head and tail shielding is employed to effectively regulate the temperature uniformity at different positions of the steel plate (head, middle, and tail) to ensure the steel plate's toughness and shape. The rolled steel plate undergoes high-temperature hot straightening to promote full release of internal stress. Appropriate bending roll amounts and tilting values ​​are set to ensure good straightness of the straightened plate. The hot-straightened steel plates are then stacked for slow cooling, reducing banded microstructure and promoting the full diffusion of C, Mn, and H elements, ensuring the steel plate's plasticity and toughness.

[0018] (5) Through composition and process design, a low-cost production method was obtained for producing extra-thick plates for engineering equipment with a thickness of 80-130mm, a yield strength of 345MPa, and excellent low-temperature toughness. 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 produce low-cost low-temperature structural steel plates with a thickness of 80-130mm and a yield strength of 345MPa were finally solved. At the same time, no subsequent offline quenching and straightening treatment was required to meet the technical requirements of the plate. The technical indicators of the prepared plate are as follows: transverse tensile yield strength ≥345MPa, tensile strength between 490-570MPa, elongation ≥22%, transverse Charpy impact energy at -40℃ ≥120J, yield strength ratio ≤0.88, Z-direction elongation ≥35%, straightness below 5mm / 2m, and flaw detection pass rate ≥99%. Detailed Implementation

[0019] The extra-thick steel plate for engineering equipment with excellent low-temperature toughness, high flatness, and a strength of 345MPa, as described in this invention, has the following chemical composition by weight percentage: C: 0.14%–0.19%, Si: 0.25%–0.35%, Mn: 1.40%–1.55%, P≤0.02%, S≤0.010%, Al: 0.015%–0.035%, Nb: 0.02%–0.04%, B: 0.0015%–0.0020%, Ti: 0.01%–0.02%, with the balance being Fe and unavoidable impurities. It is rolled from continuously cast billets with a thickness of less than 360mm using a medium-thickness reciprocating rolling mill, resulting in a finished steel plate thickness of 80–130mm.

[0020] 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 0.14%–0.19%.

[0021] 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 1.40%–1.55%.

[0022] 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%.

[0023] 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 0.02% to 0.04%.

[0024] Ti: In steel, Ti combines with C to form TiC, which plays a role in dispersion strengthening. In this invention, the Ti content is set in the range of 0.01% to 0.02%.

[0025] 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%.

[0026] 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%.

[0027] P and S are unavoidable impurity elements in steel, and their content should be as low as possible. However, considering smelting costs and processes, their content cannot be infinitely low. Therefore, this invention sets the upper limits for P and S content at 0.020% and 0.010%, respectively.

[0028] The manufacturing method of the 345MPa grade extra-thick plate for engineering equipment with excellent low-temperature toughness and high flatness, as described in this invention, comprises the following process route: steel smelting → ladle refining + RH refining → B alloying → continuous casting → slab reheating → controlled rolling and cooling → hot straightening and slow cooling. 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 0.14%~0.19%. Argon gas is blown for more than 30 minutes before tapping. Argon blowing and calming before continuous casting can promote the removal of inclusions in the molten steel and improve the uniformity of the steel composition.

[0029] After LF refining, the steel undergoes RH vacuum degassing, maintaining the vacuum level for at least 25 minutes after reaching the set value. Following degassing, sufficient Al is added according to the finished product Als requirements, along with sufficient Ti-Fe for micro-titanium treatment. Then, a 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 10–16°C and a casting speed of 0.8–1.0 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 solidified structure, and helps reduce slab segregation and internal structural defects.

[0030] The continuous casting sector employs a forced cooling process. The total cooling water flow rate for sections 1-4 is 800–1200 L / min, and for sections 5-8 it is 600–800 L / min. This ensures a large temperature gradient between the surface and core of the continuously cast billet at the solidification end, exceeding 500°C. Heavy compression is applied, with a reduction of 15–20 mm. Forced cooling maintains the temperature gradient along the billet's thickness, while the heavy compression helps reduce billet segregation and ensures strain penetration to the billet's core. This significantly improves central defects in the billet, guaranteeing the strength and toughness of the subsequent steel plate.

[0031] After the billet comes off the production line, it is hot-charged into a slow cooling pit. When the billet temperature is above 700℃, the slow cooling pit is not heated, and the slow cooling time is not less than 48 hours. Slow cooling promotes the diffusion of elements such as Mn, C, B, and H, reduces the impact of component segregation on the microstructure and properties, and ensures that the H content in the billet is not greater than 5ppm.

[0032] 2) Billet Heating: The billet (thickness below 360mm) is fed into a walking beam furnace for heating. The preheating section temperature range is 600–900℃, the purpose of which is to promote the partial dissolution of Ti and Nb carbides in the matrix and to facilitate sufficient diffusion. The heating section temperature range is 1220–1250℃, and the soaking section temperature range is 1190–1210℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 5.5–7.5 hours, with the total furnace time for the heating and soaking sections being 4.5–6.0 hours. By adjusting the opening degree of the upper and lower burners in the walking beam furnace, the air-fuel ratio in the furnace is controlled at 1:1.7–1:2.2 to ensure that the temperature difference between the upper and lower surfaces of the billet is within 10℃.

[0033] High-temperature heating is provided by the heating section to ensure temperature uniformity in all parts of the billet and to completely dissolve Ti and Nb carbides in the matrix. At the same time, it further promotes the diffusion of elements such as Mn, Cr, C, B, and Nb, reducing the impact of compositional segregation on microstructure and properties, and improving the uniformity of transverse and longitudinal metal flow on the steel plate surface. Combined with the billet composition, the energy consumption is reduced by lowering the heating temperature of the billet soaking section, while also suppressing the impact of coarsening of the original austenite grains on properties.

[0034] 3) Controlled rolling: Before rolling, use high-pressure water to descale the billet after it comes out of the furnace for 1 to 2 minutes, with the descaling machine pressure being 15 to 20 MPa.

[0035] The rolling process is divided into two stages: the first stage is roughing in the recrystallization zone, with an initial rolling temperature ≥1150℃ and a final rolling temperature of 1020~1050℃. The single-pass reduction rate for the last two and final passes in the roughing stage is not less than 15%. Simultaneously, descaling water is sprayed onto the mill for the last two and final passes, with a descaling time of 10~30s and a descaling mill pressure of 15~20MPa. The high reduction rate in the last two passes of the roughing stage aims to utilize the high-temperature deformation-induced effect to promote the precipitation of NbC and TiC phases within the austenite grains, improving strength and toughness; it also promotes dynamic recrystallization of the austenite grains, thereby refining the grain size. The use of descaling water on the mill in the last two passes of the roughing stage aims to create a temperature gradient between the inner and outer surfaces of the billet, promoting the penetration of rolling deformation towards the center of thickness, refining the grain size at half the thickness, and improving the core microstructure of thick steel plates.

[0036] The thickness of the intermediate billet is 2.0 to 3.0 times that of the finished product. During the intermediate billet's warming process, 1 to 2 passes of descaling water are sprayed, with a descaling time of 1 to 1.5 minutes and a descaling machine pressure of 20 to 25 MPa. This ensures that the temperature difference between the surface and the core is above 50°C, inhibiting austenite grain growth. At the same time, a temperature gradient is generated 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, which is beneficial for improving the core structure of thick steel plates, reducing the structural stress caused by uneven structure, and also accelerating the temperature drop of the intermediate billet, reducing the warming time.

[0037] Pre-straightening is performed during the intermediate billet's warming process. The straightening force is between 1000 and 1500 kN, the bending amount is set to 1.0 to 2.0 mm, and the tilting value is set to 2.0 to 3.0 mm. Pre-straightening ensures the billet's shape during the warming process and promotes the full release of internal stress. By setting appropriate straightening force, bending amount, and tilting value, good flatness of the straightened plate is ensured, which is beneficial for shape control during the finishing rolling stage.

[0038] The initial rolling temperature for the second stage, the non-recrystallization zone rolling (finish rolling), is 930–960℃. The bite and rolling speed in the finish rolling stage is 1.0–1.2 m / s, and the final rolling temperature is 870–890℃. In the first two passes of the finish rolling stage, the single-pass reduction rate is guaranteed to be above 20%, and the reduction rate in the last pass is less than 5%. The finish rolling stage consists of no more than five passes. By controlling the reduction speed and the reduction rate in the finish rolling stage, the dislocation density, vacancies, and deformation bands in the austenite body are ensured, promoting the precipitation of NbC and TiC phases, providing more nucleation sites, promoting ferrite phase transformation, refining the ferrite microstructure, and ensuring the strength and toughness of the steel plate. The final pass of the finish rolling uses a small reduction rate to flatten the plate shape, reduce internal stress, and control the length of the rolled steel plate to avoid excessively rapid temperature drop in the later rolling stages, which would affect the uniformity of the steel plate microstructure after controlled cooling.

[0039] After rolling, UFC (Ultra-Fast Cooling System) is used for cooling. The UFC cooling process employs fully automatic controlled cooling. The initial cooling temperature of the steel plate is 770–810℃, and the final cooling temperature is 620–650℃. The steel plate is shielded at both ends. The upper manifold opening correction value is -300 to -500mm, and the lower manifold opening correction value is -1000 to -1200mm. 8–12 sets of water are opened on both the upper and lower manifolds, with a single manifold opening water flow of 200–230m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2 to 1:2.5. The roller speed of the steel plate conveyor is 0.5 to 0.8 m / s, and the acceleration is 0.003 to 0.005 m / s². 2 During the UFC cooling process, parameters such as cooling rate and final cooling temperature are controlled, and methods such as shielding the head and tail of the steel plate are used to effectively regulate the temperature uniformity at different positions of the head, middle, and tail of the steel plate, so as to ensure the toughness and shape of the steel plate.

[0040] The rolled steel plate is subjected to high-temperature hot straightening with a straightening force between 2000 and 3000 kN, a bending amount of 2 to 2.5 mm, and an inclination value of 3 to 5 mm. High-temperature hot straightening promotes the full release of internal stress, and by setting appropriate bending amounts and inclination values, good flatness of the straightened plate is ensured.

[0041] 4) Stacking and slow cooling: After hot straightening, the steel plates are stacked and slow cooled at a temperature of 300-400°C. The number of stacked plates is no less than 8, and the slow cooling time is more than 10 hours. By adopting the stacking and slow cooling process, the banded structure is reduced and the C, Mn and H elements are fully diffused, thus ensuring the plasticity and toughness of the steel plate.

[0042] This invention, employing the aforementioned components and processes, ultimately yields a low-cost method for producing extra-thick plates for engineering equipment with a thickness of 80–130 mm, a yield strength of 345 MPa, and excellent low-temperature toughness. By optimizing processes such as steelmaking, heating, rolling, cooling, straightening, and slow stacking cooling, it solves the problems of low toughness, low flaw detection pass rate, and excessive plate flatness that are easily encountered when using continuous casting billets with a cross-section of less than 360 mm to produce low-temperature structural steel plates with a thickness of 80–130 mm and a yield strength of 345 MPa. The method eliminates the need for subsequent offline quenching and straightening treatments, thus meeting the technical requirements of the plate material.

[0043] The technical specifications of the steel plate manufactured according to the method of the present invention are as follows: transverse tensile yield strength ≥345MPa, tensile strength between 490 and 570MPa, elongation ≥22%; transverse Charpy impact energy at -40℃ ≥120J, yield strength ratio ≤0.88, Z-direction elongation ≥35%, straightness below 5mm / 2m; flaw detection pass rate ≥99%.

[0044] To more intuitively illustrate the present invention, the embodiments are 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.

[0045]

Example

[0046] Table 1 - Chemical composition of steel (wt, %)

[0047] Note: Impurity elements in steel: P≤0.02%, S≤0.010%.

[0048] Table 2 - Steel smelting process parameters

[0049] Table 3 - Billet Heating Process Parameters

[0050] Table 4 - Roughing rolling process parameters and intermediate billet treatment process parameters

[0051] Table 5 - Finishing Rolling Process Parameters

[0052] Table 6 - Cooling, straightening and slow cooling process parameters of steel plates

[0053] Table 7 - Dimensions and Performance Indicators of Finished Steel Plates

[0054] 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. An extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa strength, characterized in that... The chemical composition of the steel plate, by weight percentage, is: C: 0.14%–0.19%, Si: 0.25%–0.35%, Mn: 1.40%–1.55%, P≤0.020%, S≤0.010%, Al: 0.015%–0.035%, Nb: 0.02%–0.04%, B: 0.0015%–0.0020%, Ti: 0.01%–0.02%, with the balance being Fe and unavoidable impurities. The steel plate is rolled from a continuous casting billet with a thickness of less than 360 mm using a medium-thickness reciprocating rolling mill, resulting in a finished steel plate thickness of 80–130 mm.

2. The extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa grade as described in claim 1, characterized in that, The finished steel plate has a yield strength of ≥345MPa in the transverse tensile test, a tensile strength of 490~570MPa, and an elongation of ≥22%; a transverse Charpy impact energy of ≥120J at -40℃ and a yield strength ratio of ≤0.88; a Z-direction elongation of ≥35% and a straightness of ≤5mm / 2m; and a flaw detection pass rate of ≥99%.

3. The manufacturing method of the extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa 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 superheat of continuous casting is 10-16℃, and the casting speed is 0.8-1.0m / min; the fan-shaped section of continuous casting adopts a strong cooling process, with a total cooling water volume of 800-1200L / min for the first to fourth sections and 600-800L / min for the fifth to eighth sections. The temperature difference between the surface and core of the solidified billet is >500℃, and heavy pressure is used for reduction of 15-20mm; after the billet leaves the production line, it is hot-charged into a slow cooling pit. When the billet temperature is above 700℃, the slow cooling pit is not heated, and the slow cooling time is not less than 48h; the H content in the billet is guaranteed to be no more than 5ppm. 2) Billet heating: Billets with a thickness ≤360mm are fed into a walking beam furnace for heating. The preheating section temperature is 600~900℃, the heating section temperature is 1220~1250℃, and the soaking section temperature is 1190~1210℃. The total furnace time for the preheating, heating, and soaking sections is controlled at 5.5~7.5h, of which the total furnace time for the heating and soaking sections is 4.5~6.0h. The air-fuel ratio in the furnace is controlled at 1:1.7~1:2.2 to ensure that the temperature difference between the upper and lower surfaces of the billet is ≤10℃. 3) Controlled rolling: Before rolling, use high-pressure water to descale the cast billet after it exits the furnace for 1-2 minutes, with a descaling machine pressure of 15-20 MPa; Rolling is done in two stages: The first stage is recrystallization zone rolling, i.e., rough rolling, with a rough rolling start temperature ≥1150℃ and a rough rolling finish temperature of 1020-1050℃. The single-pass reduction rate of the last two passes and the last pass in the rough rolling stage is not less than 15%. Simultaneously, descaling water is sprayed into the last two passes and the last pass in the rough rolling stage, with a descaling time of 10-30 seconds and a descaling machine pressure of 15-20 MPa; the thickness of the intermediate billet is [missing information - likely a percentage] of the finished product thickness. 2.0–3.0 times; during the intermediate billet warming process, spray descaling water for 1–2 passes, with a descaling time of 1–1.5 min and a descaling machine pressure of 20–25 MPa, ensuring a temperature difference of ≥50℃ between the surface and core of the intermediate billet; during the intermediate billet warming process, perform pre-straightening with a straightening force of 1000–1500 kN, a roll bending amount of 1.0–2.0 mm, and a tilting value of 2.0–3.0 mm; the second stage is the non-recrystallization zone rolling, i.e., finishing rolling, with a finishing rolling start temperature of 930–960℃, a bite and rolling speed of 1.0–1.2 m / s, and a finishing rolling finish temperature of 870–890℃; the single-pass reduction rate of the first two passes in the finishing rolling stage is ≥20%, the reduction rate of the last pass is <5%, and the finishing rolling does not exceed five passes; 4) Controlled Cooling: After rolling, a UFC ultra-fast cooling system is used for cooling. The UFC ultra-fast cooling process employs a fully automatic controlled cooling mode. The initial cooling temperature of the steel plate is 770–810℃, and the final cooling temperature is 620–650℃. 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 -1000–-1200mm. 8–12 sets of water are opened in both the upper and lower manifolds, with a single manifold opening water flow of 200–230m³. 3 / h; the water ratio between the upper and lower manifolds is 1:2 to 1:2.5; the roller speed of the steel plate conveyor is 0.5 to 0.8 m / s, and the acceleration is 0.003 to 0.005 m / s². 2 .

4. The manufacturing method of the extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa grade as described in 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 to be less than 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 to be ≤0.020%; the C content is controlled to be 0.14% to 0.19% at the end of the converter smelting, and argon gas is blown for more than 30 minutes when tapping the steel.

5. The manufacturing method of the extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa 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 25 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 manufacturing method of the extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa grade according to claim 3, characterized in that, The hot straightening process is as follows: the cooled steel plate is hot straightened at high temperature, the straightening force is 2000-3000kN, the bending amount of the roller is set to 2-2.5mm, and the tilting value is set to 3-5mm.

7. The manufacturing method of the extra-thick plate for engineering equipment with excellent low-temperature toughness, high flatness, and 345MPa grade according to claim 3, characterized in that, The slow cooling process is as follows: the hot-straightened steel plates are stacked and slow-cooled at a temperature of 300-400°C, with a minimum of 8 stacked plates and a slow cooling time of ≥10 hours.

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