A method for producing a super-thick steel plate based on forging rolling combination and a super-thick steel plate

Abstract

The application relates to the technical field of steel metallurgy, and provides a method for producing an extra-thick steel plate based on forging and rolling and the extra-thick steel plate, which comprises the following steps: performing forging treatment on a special-component blank, sequentially adopting large-pressing-down forging and small-pressing-down flattening to obtain an intermediate blank; slowly cooling the intermediate blank to room temperature and performing surface fold cleaning; re-heating the cleaned intermediate blank, and performing rolling treatment in an austenite non-recrystallization temperature interval; and performing water cooling and stack cooling treatment on the rolled extra-thick steel plate. The forging and rolling combined process effectively eliminates heart defects and refines grains, cancels the normalizing process, and reduces the production cost while guaranteeing the internal quality and low-temperature toughness.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a method for producing extra-thick steel plates based on a combination of forging and rolling, and the extra-thick steel plates themselves. Background Technology

[0002] In fields such as heavy equipment manufacturing, marine engineering, and super high-rise buildings, there are stringent performance requirements for extra-thick steel plates exceeding 100mm in thickness. These plates need not only excellent low-temperature impact toughness but also extremely high internal density to meet ultrasonic testing standards. Currently, the production of extra-thick steel plates mainly employs full rolling or full forging processes. However, the full rolling process is limited by mill capacity and deformation penetration depth. When the compression ratio is insufficient, the core deformation of the steel plate is inadequate, making it difficult to effectively eliminate inherent metallurgical defects such as porosity and segregation in continuously cast billets, resulting in unstable core flaw detection pass rates. While the full forging process can press together core defects under immense pressure, its production efficiency is low, and the finished product has poor dimensional accuracy and surface quality, leading to high subsequent processing costs. Furthermore, to ensure the mechanical properties of extra-thick steel plates, traditional processes typically require normalizing heat treatment after rolling or forging, which not only increases energy consumption and production costs but also significantly extends delivery cycles. Therefore, how to balance the internal quality, surface accuracy, and production efficiency of extra-thick steel plates, while eliminating the normalizing process to reduce costs, is a pressing technical problem to be solved in this field. Summary of the Invention

[0003] To address the problems of existing technologies that make it difficult to balance the internal quality and surface precision of extra-thick steel plates, and that rely on normalizing processes leading to high production costs and long cycles, this application proposes a method and an extra-thick steel plate based on a combination of forging and rolling. This method achieves the goal of refining grains through microstructure control while ensuring the density of the steel plate core and the surface quality, eliminating the normalizing process, significantly improving low-temperature impact toughness, and shortening the delivery cycle.

[0004] To achieve the above objectives, this application adopts the following technical solution: A method for producing extra-thick steel plates based on a combination of forging and rolling, wherein the chemical composition of the extra-thick steel plate, by mass percentage, includes: C: 0.08-0.12%, Mn: 1.4-1.6%, Si: 0.20-0.40%, Cr: 0.15-0.35%, Nb: 0.03-0.05%, V: 0.02-0.04%, Alt: 0.015-0.040%, P≤0.02%, S≤0.01%, with the balance being Fe and unavoidable impurities; the method includes: forging a billet by sequentially using high-reduction forging and low-reduction leveling to obtain an intermediate billet; slowly cooling the intermediate billet to room temperature and cleaning the surface creases; reheating the cleaned intermediate billet and rolling it within the austenite non-recrystallization temperature range; and water-cooling and stacking cooling of the rolled extra-thick steel plate.

[0005] The above scheme utilizes a specific chemical composition design combined with a "forging + rolling" composite process. The large reduction deformation of forging effectively compresses the metallurgical defects in the core, while rolling ensures dimensional accuracy and surface quality. The cold connection method, which involves slow cooling of the intermediate billet to room temperature and cleaning of surface creases, avoids the forging creases from being pressed in and forming cracks during subsequent rolling. By rolling in the austenite non-recrystallization temperature range and combining it with water-cooled stacking, the strain accumulation distortion energy is used to refine the final grains, thereby obtaining excellent comprehensive mechanical properties without the need for normalizing.

[0006] The forging process includes: using 2-3 passes of high-reduction forging, with a single-pass reduction rate of 10%-25%; using 2-3 passes of low-reduction leveling, with a single-pass reduction rate of 1%-15%; the thickness of the intermediate billet is 1.2-2.5 times the finished thickness of the extra-thick steel plate.

[0007] By limiting the number of passes and reduction rates for large and small reductions, a balance was ensured between effective healing of core defects and preliminary surface quality finishing. Controlling the thickness ratio of intermediate billets not only guaranteed the compression ratio requirements for subsequent rolling but also avoided excessive forging loads, thus optimizing the stability of the process flow.

[0008] The single-pass reduction rate of the high-reduction forging is 13%-20%, the single-pass reduction rate of the low-reduction leveling is 2%-10%, and the thickness of the intermediate billet is 1.5-2.0 times the finished thickness of the extra-thick steel plate.

[0009] High reduction can generate sufficient triaxial compressive stress to completely eliminate core porosity, while low reduction can effectively flatten the surface without introducing new folding defects. The thickness ratio of the intermediate billet is most conducive to temperature uniformity and microstructure refinement in the subsequent rolling process.

[0010] The reheating temperature is 850-950℃; the initial rolling temperature of the rolling process is 740-820℃, and the final rolling temperature is 730-800℃.

[0011] A thermodynamic regime for ultra-low temperature heating and fine rolling in the non-recrystallization zone was established. This temperature window can effectively suppress the growth of the original austenite grains in the intermediate billet, while ensuring that the rolling process is in the non-recrystallization zone, so as to accumulate sufficient deformation energy for subsequent phase transformation to refine the ferrite grains.

[0012] In one embodiment, the reheating temperature is 880-940℃, and the holding time is ≥0.12min / mm; the initial rolling temperature is 760-800℃, and the final rolling temperature is 750-790℃.

[0013] The thermal parameters were further optimized, and the high-temperature residence time was shortened to the maximum extent while ensuring uniform heat transmission, thereby reducing the risk of oxidation burn-off and grain coarsening. The precise control of the initial rolling and final rolling temperature ranges enabled the strain-induced precipitation and grain refinement effects to achieve the best match, significantly improving the low-temperature toughness of the steel plate.

[0014] As one implementation, the surface crease removal includes: using a flame to remove creases from the surface of the intermediate billet; the water cooling temperature is 500-700℃; and the stacking cooling time is ≥24 hours.

[0015] The specific methods for cold connection and the post-rolling controlled cooling system were clarified. Flame cleaning completely removed the surface defect sources that could lead to rolling cracks. The control of the red-heat temperature and the stacking cooling time aimed to promote the uniform transformation of the bainite / ferrite structure, release residual stress, and prevent the generation of hydrogen-induced cracks.

[0016] In one implementation, the reddening temperature is 550-650℃, and the stack cooling time is ≥48 hours.

[0017] More stringent cooling parameters were provided, and at this reddening temperature, the phase transformation products were finer and more dispersed. Combined with sufficient stacking cooling time, the uniformity of microstructure and properties in the cross-sectional direction of the extra-thick steel plate was ensured, further improving the flaw detection pass rate and impact energy stability.

[0018] In one embodiment, the finished thickness of the extra-thick steel plate is 100-300mm; the billet is prepared by converter smelting and vacuum treatment, with a vacuum time of ≥20 minutes; the billet is a continuously cast billet with a thickness of 300-500mm, and the slow cooling time of the billet is ≥72 hours.

[0019] The applicable product specifications and raw material preparation requirements were defined. For the characteristics of extra-thick plates of 100-300mm, the gas content was reduced by vacuum treatment and the internal stress and hydrogen embrittlement sensitivity were eliminated by slow cooling of the continuous casting billet, laying the foundation for purity and homogenization for the subsequent forging and rolling composite process.

[0020] As one implementation method, before the forging process, the billet is heated to 1200-1250℃ and held for ≥0.12min / mm; during the forging process, when the temperature of the billet is lower than 900℃, it is returned to the furnace for reheating.

[0021] This ensures that the billet has sufficient plasticity at high temperatures to withstand large compression deformation, while a strict remelting mechanism avoids the risk of work hardening or cracking due to excessive temperature drop, thus guaranteeing the safety and effectiveness of the forging process.

[0022] In addition, this application also provides an extra-thick steel plate, which is prepared by the method described above.

[0023] The aforementioned extra-thick steel plates, due to the combination of forging and rolling and a specific heat treatment process, have a dense internal structure and fine grains, possessing both excellent core flaw detection performance and low-temperature impact toughness, and can meet high-standard usage requirements without normalizing treatment. Beneficial effects

[0024] 1. This application adopts a dual-pressure forging system of "high-pressure forging + low-pressure leveling". The strong triaxial compressive stress generated by high pressure effectively compresses the porosity and segregation defects in the core of the continuously cast billet, fundamentally solving the problem of unstable core quality in the whole rolling process. At the same time, the surface is repaired by low pressure leveling, which provides good billet conditions for subsequent rolling, and achieves synergistic improvement of internal quality and surface precision.

[0025] 2. This application innovatively designs a cold-state connection process of "post-forging slow cooling cleaning + reheating rolling", which completely removes the surface creases generated during forging and avoids them being pressed into the matrix to form crack defects during rolling. It overcomes the technical bias that traditional direct forging and rolling connection easily leads to surface quality deterioration and significantly improves the yield and flaw detection pass rate of extra-thick steel plates.

[0026] 3. This application utilizes the thermodynamic control of ultra-low temperature heating and precision rolling in the non-recrystallized austenite region to significantly refine the final phase transformation structure by leveraging strain accumulation distortion energy. Combined with ultra-fast cooling and stacking cooling processes after rolling, the normalizing process is successfully eliminated. While ensuring excellent low-temperature impact toughness, energy consumption and production costs are significantly reduced, delivery cycle is shortened, and production efficiency and market competitiveness are improved. Detailed Implementation

[0027] Example 1

[0028] In this embodiment, the deformation regime of the forging process is further defined. Specifically, the forging process includes: 2-3 passes of high-reduction forging, with a single-pass reduction rate of 10%-25%; 2-3 passes of low-reduction leveling, with a single-pass reduction rate of 1%-15%; and the thickness of the intermediate billet is 1.2-2.5 times the finished thickness of the extra-thick steel plate. The core design of this deformation regime is to solve the contradiction between the core density and surface quality of the extra-thick steel plate by combining functionally separated passes. The main function of high-reduction forging is to transfer sufficient triaxial compressive stress to the core of the billet to weld the inherent porosity and shrinkage defects inside the continuously cast billet; the main function of low-reduction leveling is to correct the surface ripples and dimensional deviations generated during forging, providing a billet with good geometric accuracy for subsequent rolling; and the setting of the intermediate billet thickness ratio is to ensure that the subsequent rolling process has a sufficient compression ratio to achieve grain refinement, while avoiding mill overload or uncontrolled final rolling temperature due to excessive billet thickness.

[0029] The single-pass reduction rate in high-reduction forging is controlled within the range of 10%-25% based on a comprehensive consideration of the material's high-temperature plastic behavior and deformation penetration depth. If the single-pass reduction rate is less than 10%, the deformation zone is often limited to the surface layer of the billet, making it difficult to effectively penetrate to the core area of ​​the extra-thick plate, resulting in insufficient healing of metallurgical defects in the core. Conversely, if the single-pass reduction rate exceeds 25%, although the deformation penetration depth increases, it is very easy to exceed the plastic processing limit of the material at high temperatures, inducing surface cracking or severe folding defects, which in turn damages the yield. As a more preferred implementation, the single-pass reduction rate in high-reduction forging can be further optimized to 13%-20%. Within this preferred range, it can ensure that the deformation zone penetrates deep into the center of the billet to achieve densification, while maintaining good surface integrity, making it particularly suitable for the production of extra-thick steel plates with a thickness of 100mm or more.

[0030] Regarding the single-pass reduction rate for minor reduction and leveling, limiting it to 1%-15% is to clearly distinguish its functional boundary from the main deformation passes. Minor reduction passes should not bear the primary task of thinning; otherwise, additional shear strain can be introduced into the already densified surface layer of the billet, or even press surface oxide scale or micro-creases into the matrix, forming new crack initiation points. Therefore, the minor reduction rate is typically maintained at a low level, preferably 2%-10%. Within this range, it is sufficient to eliminate anvil marks and unevenness left by forging, without adversely affecting the internal structure of the billet, achieving coordinated control of surface and internal quality.

[0031] Regarding the ratio of intermediate billet thickness to finished product thickness, a range of 1.2-2.5 times constitutes a safe window for the connection between forging and rolling composite processes. When this ratio is less than 1.2, it means that the deformation amount left for subsequent rolling is too small, making it difficult to accumulate sufficient distortion energy through rolling in the non-recrystallization zone to refine the grains, which may lead to insufficient toughness in the final product. When this ratio is greater than 2.5, although the rolling compression ratio is sufficient, it will significantly increase the number of rolling passes and energy consumption, and the excessive rolling time may lead to excessive temperature drop in the steel plate, deviating from the preset thermodynamic regime. Preferably, the thickness of the intermediate billet is 1.5-2.0 times the thickness of the finished extra-thick steel plate. This preferred range achieves the best balance between ensuring rolling refinement effect and production efficiency.

[0032] To illustrate the practical application of the above parameters more intuitively, the following example demonstrates the production process of two different specifications of extra-thick steel plates. When producing an extra-thick steel plate with a thickness of 150mm, two large-reduction forging passes can be used, with single-pass reductions of 80mm and 70mm respectively, corresponding to reduction rates of approximately 17.78% and 18.92%, both falling within the preferred range of 13%-20%. Subsequently, two small-reduction leveling passes are used, with single-pass reductions of 30mm and 10mm respectively, corresponding to reduction rates of approximately 10% and 3.70%, also within the preferred range of 2%-10%. The final intermediate billet thickness is 260mm, which is approximately 1.73 times the finished product thickness of 150mm, falling within the preferred range of 1.5-2.0 times. Similarly, in the production of extra-thick steel plates with a thickness of 250mm, two large-reduction forging passes are used, with single-pass reductions of 90mm and 70mm, corresponding to reduction rates of approximately 15.52% and 14.29%, respectively; two small-reduction leveling passes are used, with single-pass reductions of 30mm and 10mm, corresponding to reduction rates of approximately 7.14% and 2.56%, respectively; the final intermediate billet thickness is 380mm, which is approximately 1.52 times the finished product thickness of 250mm. Within the above-defined parameter range, both medium and extra-thickness specifications can stably achieve the dual goals of core densification and surface leveling, providing ideal billet conditions for subsequent rolling processes. Example 2

[0033] This embodiment provides a basic method for producing extra-thick steel plates based on a combination of forging and rolling. This method, through specific composition design and the synergy of forging and rolling composite processes, constructs a complete technical framework from billet to finished product, aiming to solve the problems of difficulty in simultaneously achieving internal quality and surface precision in extra-thick steel plates, as well as the high cost resulting from reliance on normalizing processes. Specifically, the method includes the following steps: Step S100: Prepare a billet with a specific chemical composition. The chemical composition of the extra-thick steel plate, by mass percentage, includes: C: 0.08-0.12%, Mn: 1.4-1.6%, Si: 0.20-0.40%, Cr: 0.15-0.35%, Nb: 0.03-0.05%, V: 0.02-0.04%, Alt: 0.015-0.040%, P≤0.02%, S≤0.01%, with the balance being Fe and unavoidable impurities. This composition system is the material basis for achieving normalizing-free production of high-performance extra-thick steel plates in this application. Specifically, C and Mn, as basic strengthening elements, ensure the hardenability and lower strength limit of the steel plate; the addition of Cr further improves hardenability and corrosion resistance; Nb and V, as microalloying elements, can form fine carbonitrides through strain-induced precipitation during subsequent thermomechanical control processes, effectively pinning grain boundaries to inhibit austenite grain growth and providing nucleation sites for ferrite phase transformation, which is key to achieving grain refinement; Alt is mainly used for deoxidation and nitrogen fixation, purifying the steel and refining the grains; strict control of P and S content is to ensure the impact toughness of the steel plate at low temperatures. This low-alloy, high-strength composition design allows the steel plate to obtain excellent comprehensive mechanical properties through deformation heat treatment alone, without the need for normalizing heat treatment.

[0034] Step S200 involves forging the billet, sequentially employing high-reduction forging and low-reduction leveling to obtain an intermediate billet. This step establishes the functional positioning of the forging process within the overall process, focusing on the dual tasks of addressing metallurgical defects in the core of the continuously cast billet and correcting its geometric dimensions. Specifically, high-reduction forging utilizes the immense pressure and triaxial compressive stress of the forging machine to allow deformation to penetrate deeply into the core of the billet, forcibly welding together the inherent porosity, shrinkage cavities, and segregation bands within the continuously cast billet, fundamentally improving the internal density of the steel plate—something that conventional rolling processes cannot achieve due to insufficient compression ratios. The subsequent low-reduction leveling primarily eliminates surface ripples and dimensional deviations generated during the high-reduction process, providing a well-shaped billet for subsequent rolling. Through this functionally separated pass combination, the advantages of forging for core treatment are leveraged while avoiding the shortcomings of poor surface quality and low efficiency inherent in the entire forging process, achieving a preliminary synergy between internal quality and surface precision.

[0035] Step S300 involves slowly cooling the intermediate billet to room temperature and cleaning the surface creases. This is a key cold-state connection step that distinguishes this application from traditional direct forging and rolling processes or hot-feeding and hot-charging processes. Specifically, during the forging process, due to friction and uneven deformation between the anvil and the billet surface, creases or micro-cracks inevitably occur on the surface of the intermediate billet. If the high-temperature intermediate billet is directly fed into the rolling mill, these surface defects are easily pressed into the matrix under rolling pressure, forming crack sources that are difficult to repair, seriously damaging the finished product's flaw detection pass rate. Therefore, this application specifically arranges for the intermediate billet to be slowly cooled to room temperature. On the one hand, this homogenizes the temperature inside and outside the billet, eliminating thermal stress; on the other hand, it facilitates thorough cleaning of surface creases at room temperature, such as by using flame cleaning to remove the defect layer. Although this cold-state connection increases the process and time costs, it plays an irreplaceable role in ensuring the yield of extra-thick steel plates, especially products with high flaw detection requirements, reflecting a quality-centric process design logic.

[0036] Step S400 involves reheating the cleaned intermediate billet and rolling it within the austenite non-recrystallization temperature range. This step is the core thermodynamic control step for achieving the normalizing-free process. Specifically, the purpose of reheating is not to coarsen the grains, but to bring the billet to a suitable plastic state for rolling, while strictly controlling the heating temperature to avoid excessive growth of the original austenite grains. Subsequent rolling within the austenite non-recrystallization temperature range means that the austenite grains will not undergo dynamic or static recrystallization during the rolling deformation process; instead, they will be elongated and broken, accumulating a large amount of deformation energy and crystal defects. These distortions will become high-energy sites for ferrite nucleation during the subsequent cooling phase transformation, significantly increasing the nucleation rate and thus obtaining a fine and uniform room-temperature microstructure. This strategy of refining grains by replacing normalizing recrystallization with deformation accumulation not only saves energy consumption but also shortens the production cycle, representing the core inventiveness of the technical solution in this application.

[0037] Step S500 involves water cooling and stacking cooling of the rolled extra-thick steel plate. As the final control step in the process flow, this step determines the final microstructure and residual stress state of the steel plate. Specifically, post-rolling water cooling aims to suppress excessive precipitation of proeutectoid ferrite through rapid cooling, promote the formation of high-strength and high-toughness microstructures such as bainite or acicular ferrite, and simultaneously fix the refining effect accumulated during the rolling stage. The subsequent stacking cooling provides a slow cooling environment for the steel plate, promoting the outward diffusion of hydrogen atoms to prevent hydrogen-induced cracking, and making the temperature of the steel plate section more uniform, releasing the thermal and structural stresses caused by uneven cooling, and stabilizing the dimensions and properties of the steel plate. Through the organic combination of water cooling and stacking cooling, the extra-thick steel plate can still maintain excellent low-temperature impact toughness and internal flaw detection quality under normalizing-free conditions. Example 3

[0038] Based on Example 2, this example further specifies the rolling thermodynamic regime. Specifically, the reheating temperature is 850-950℃; the initial rolling temperature is 740-820℃, and the final rolling temperature is 730-800℃. The core of this temperature regime design lies in establishing a synergistic mechanism between "ultra-low temperature heating" and "precision rolling in the non-recrystallization zone," which is a key thermodynamic window for achieving normalization-free production of extra-thick steel plates and obtaining excellent low-temperature toughness. Unlike traditional controlled rolling and cooling processes that typically use high-temperature heating above 1000℃, this application strictly controls the reheating temperature within the lower range of 850-950℃. The microscopic mechanism is that this temperature range is located between the Ac1 and Ac3 critical points of the steel or slightly above Ac3 but much lower than the conventional austenitizing temperature. This ensures that the intermediate billet completes the full austenitizing transformation, eliminates the inheritance of forging structure, and effectively inhibits the excessive growth of the original austenite grains, providing fine initial grain conditions for subsequent rolling. If the heating temperature is below 850℃, incomplete austenitization may occur, resulting in uneven distribution of residual ferrite or carbides and impairing the final properties. If the temperature is above 950℃, the original austenite grains will coarsen rapidly and will be difficult to refine completely even with subsequent large deformation, leading to a deterioration in the toughness of the steel plate that cannot be recovered by subsequent heat treatment.

[0039] Regarding the rolling temperature window, the setting of an initial rolling temperature of 740-820℃ and a final rolling temperature of 730-800℃ aims to ensure that the entire finishing rolling process remains within the austenite non-recrystallization temperature range. Within this temperature range, plastic deformation prevents the elongated austenite grains from undergoing dynamic or static recrystallization recovery, instead leading to the continuous accumulation of high-density dislocations, deformation bands, and crystal defects. This stored deformation energy constitutes high-energy sites for ferrite nucleation during subsequent cooling phase transformation, significantly increasing the nucleation rate and resulting in a fine and uniform room-temperature microstructure. This strategy of refining grains by replacing normalizing recrystallization with strain accumulation is the physical basis for eliminating the normalizing process in this application. If the initial rolling temperature is higher than 820℃, some passes may enter the recrystallization zone, causing the release of stored deformation energy and weakening the grain refinement effect; if the final rolling temperature is lower than 730℃, rolling may enter the two-phase region, inducing mixed-grain structures or generating excessive residual stress, increasing the risk of cracking.

[0040] As a preferred implementation, the reheating temperature is preferably 880-940℃, and the holding time is ≥0.12 min / mm; the initial rolling temperature is preferably 760-800℃, and the final rolling temperature is preferably 750-790℃. Within this preferred range, the narrowing of the heating temperature further reduces the probability of grain coarsening while ensuring sufficient austenitization driving force; while the limitation of holding time ≥0.12 min / mm is designed for the characteristics of large cross-section and slow thermal conductivity of extra-thick steel plates, ensuring that the temperature difference between the core and surface of the billet is controlled within the allowable range, avoiding gradient differences in microstructure and properties caused by uneven heat penetration. For example, when producing extra-thick steel plates with a thickness of 150mm, the intermediate billet can be reheated to 911℃, with the holding time coefficient controlled at approximately 0.14min / mm. After exiting the furnace, high-pressure water descaling and pre-mill temperature control ensure that the initial rolling temperature is precisely stabilized at 795℃, and the final rolling temperature is controlled at 789℃. When producing extra-thick steel plates with a thickness of 250mm, the reheating temperature is set at 892℃, with the holding time coefficient also maintained above 0.14min / mm. The initial rolling temperature is controlled at 794℃, and the final rolling temperature is controlled at 788℃. These two typical production cases demonstrate that, under the optimized thermodynamic regime, the steel plate can achieve excellent toughness indicators of 157J and 173J respectively in impact absorption energy at -20℃ without normalizing, and the flaw detection pass rate consistently meets the national standard Class I requirements. Example 4

[0041] Based on Example 1, this example further specifies the cold connection and post-rolling controlled cooling process. Specifically, surface crease removal includes: flame cleaning of creases on the surface of the intermediate billet; a water-cooled red-hot temperature of 500-700℃; and a stacking cooling time of ≥24 hours. This process combination is the core feature that distinguishes this application from the traditional "hot delivery and hot charging" short-process technology. Its design aims to resolve the contradiction between surface quality inheritance and the risk of internal hydrogen-induced cracking faced by extra-thick steel plates in forging and rolling composite production. Although hot delivery and hot charging have advantages from an energy-saving perspective, for extra-thick plates, if the surface creases caused by the anvil pressing down during forging are not completely removed, they are easily pressed into the matrix during high-temperature rolling, forming crack sources that are difficult to repair through subsequent heat treatment, resulting in failure of ultrasonic testing. Therefore, this application specifically sets up a process of slow cooling to room temperature and flame cleaning. Taking advantage of the clear visibility of the interface between defects and the matrix at room temperature, the creases and affected layers are completely removed by high-temperature flame melting and cutting, fundamentally cutting off the path of surface defects to internal inheritance. Although this step increases the production cycle and energy consumption, it plays an irreplaceable role in ensuring the yield of extra-thick steel plates, especially products with high flaw detection levels.

[0042] The control of the water-cooled reheating temperature, limiting it to the range of 500-700℃, is based on considerations of the phase transformation kinetics and microstructure properties of extra-thick steel plates. The reheating temperature essentially reflects the dynamic balance between the heat conduction from the core of the steel plate to the surface and the surface cooling rate. If the reheating temperature is higher than 700℃, it indicates insufficient cooling intensity or excessively high final rolling temperature, which may lead to a large amount of proeutectoid ferrite precipitation along austenite grain boundaries or grain coarsening, impairing low-temperature toughness. If the reheating temperature is lower than 500℃, the cooling is too intense, easily forming a hard and brittle martensite structure on the surface of the steel plate or generating excessive thermal stress, increasing the risk of cracking. As a more preferred implementation, the reheating temperature can be further optimized to 550-650℃. Within this preferred range, the steel plate cross-section can obtain a uniform microstructure dominated by bainite or acicular ferrite, ensuring sufficient strength while achieving excellent low-temperature impact toughness, thus achieving the best match between strength and toughness.

[0043] Regarding the stacking cooling time, setting it to ≥24 hours is a necessary constraint on the hydrogen diffusion behavior and stress relaxation characteristics of extra-thick steel plates. Due to their large cross-sectional dimensions, extra-thick steel plates have extremely long paths for hydrogen atoms to diffuse from the core to the surface, and the forging and rolling composite process introduces a complex residual stress field. If the stacking cooling time is too short, the hydrogen content in the core will still be above the critical value, and coupled with the superposition of microstructure transformation stress, hydrogen-induced delayed cracks or white spot defects are easily induced. Preferably, the stacking cooling time is ≥48 hours. The extended stacking cooling time provides sufficient thermal activation time for the full escape of hydrogen atoms, while also making the temperature difference between the inside and outside of the steel plate more uniform, effectively releasing quenching stress and microstructure stress, and ensuring the dimensional stability and safety of the steel plate during subsequent cutting and processing.

[0044] To illustrate the practical application of the above parameters more intuitively, the following examples demonstrate the production processes of two different specifications of extra-thick steel plates. When producing 150mm thick extra-thick steel plates, after the forged intermediate billet is slowly cooled to room temperature, a flame gun is used to thoroughly clean the surface creases, ensuring no visible defects remain. After rolling, ultra-fast water cooling is employed, with the re-heating temperature strictly controlled at 612℃, followed by 52 hours of stacking cooling. Test results show that the average impact absorption energy of this specification of steel plate at -20℃ reaches 157J, and the ultrasonic flaw detection of the entire plate meets the national standard level one requirement. Similarly, when producing 250mm thick extra-thick steel plates, a rigorous flame cleaning process is also performed, the water cooling re-heating temperature is controlled at 582℃, and the stacking cooling time is extended to 60 hours to accommodate the thicker cross-section. The final product has an average impact absorption energy of 173J at -20℃, and the flaw detection pass rate also consistently meets the standard. These two typical cases fully demonstrate that, within the range of cold connection and controlled cooling parameters defined in this application, even at the cost of sacrificing some thermal efficiency, a significant improvement in the internal quality and overall performance of extra-thick steel plates can be achieved, proving the necessity and effectiveness of this technical route in the production of high-quality extra-thick plates. Example 5

[0045] In this embodiment, specific limitations are made on the billet preparation and pre-forging heating regime to improve the integrity of the process chain and ensure the smooth implementation of subsequent forging and rolling composite processes. Specifically, the finished thickness of the extra-thick steel plate is 100-300mm; the billet is prepared by converter smelting and vacuum treatment, with a vacuum time of ≥20 minutes; the billet is a continuously cast billet with a thickness of 300-500mm, and the slow cooling time of the billet is ≥72 hours. The core design of this raw material preparation regime is to control metallurgical defects and internal stress from the source, laying the material foundation for the high yield of extra-thick steel plates. For the thickness range of 100-300mm, the steel plate has a huge cross-section and is extremely sensitive to internal purity and density. Any slight excess of gas content or residual stress may be amplified into fatal defects in subsequent processing or use. Therefore, using a continuously cast billet with a thickness of 300-500mm as raw material ensures that the subsequent forging and rolling processes have a sufficient total compression ratio to break the as-cast structure, while also taking into account the economy and feasibility of the steelmaking continuous casting process.

[0046] The limitation of a vacuum treatment time of ≥20 minutes stems from a microscopic mechanism that significantly reduces the hydrogen and nitrogen content, as well as the level of non-metallic inclusions, in the molten steel through deep degassing and inclusion flotation removal. For extra-thick steel plates, hydrogen is the primary cause of white spots and hydrogen-induced delayed cracking, while nitrogen easily leads to aging embrittlement, impairing low-temperature toughness. Only when the vacuum treatment time reaches ≥20 minutes can the molten steel be ensured to fully boil and degas under low pressure, reducing the hydrogen content below the safe threshold. Simultaneously, it promotes the aggregation, growth, and flotation separation of micro-inclusions, thereby significantly improving the ultrasonic flaw detection pass rate and low-temperature impact toughness stability of the steel plate. In actual production, for example, when producing extra-thick steel plates of 150mm or 250mm specifications, the vacuum treatment time can be controlled at 20 minutes and 21 minutes respectively to ensure the consistency of purity between different batches of raw materials.

[0047] The requirement of a slow cooling time of ≥72 hours for continuously cast billets aims to eliminate the enormous thermal and structural stresses generated during solidification and cooling, and to further promote the diffusion and escape of hydrogen atoms. If continuously cast billets are directly heated in a furnace or forged after exiting the furnace, the residual internal thermal stress and hydrogen embrittlement sensitivity can easily induce internal cracks or even fractures during high-pressure forging deformation. By using a slow cooling pit for at least 72 hours, the internal and external temperatures of the billet tend to balance, stress is fully relaxed, and the hydrogen content is further reduced, thereby significantly improving the forging plasticity and processing safety of the billet. As a preferred implementation, the slow cooling time can be extended to 75-80 hours to obtain even better stress relief and hydrogen diffusion, which is particularly important when producing ultra-thick steel plates with a thickness of 250mm or more.

[0048] In addition, before forging, the billet is heated to 1200-1250℃ and held for ≥0.12 min / mm. During forging, if the billet temperature drops below 900℃, it is reheated in the furnace. This heating and temperature control discipline is a key prerequisite for ensuring the effectiveness and safety of the forging process. Setting the heating temperature in the 1200-1250℃ range is to ensure that the continuously cast billet is fully austenitized and obtains uniform and coarse initial grains, giving the material excellent high-temperature plasticity to withstand subsequent large reduction deformation. If the temperature is too low, the core of the billet may not be fully heated or undissolved carbides may exist, resulting in increased deformation resistance and uneven microstructure. If the temperature is too high, it is easy to cause overheating and burning of grain boundaries or severe oxidation and burning. The requirement of a holding time ≥0.12 min / mm is to ensure that the temperature difference between the core and the surface is controlled within the allowable range, given the slow thermal conductivity of extra-thick continuously cast billets, thus avoiding the risk of forging cracks caused by uneven heating. For example, in actual operation, the heating temperature can be precisely controlled between 1228-1231℃, and the heat preservation coefficient should be strictly implemented to ensure that the billet is in the optimal forging temperature window. Example 6

[0049] This embodiment provides an extra-thick steel plate, prepared using the method described in any one of Embodiments 1 to 5 above. Specifically, due to the enormous cross-sectional dimensions of the extra-thick steel plate, there is a significant gradient in the cooling rate from the surface to the core, inevitably resulting in a non-uniform distribution of microstructure and mechanical properties along the thickness direction. If the product is limited solely by conventional chemical composition or mechanical property indicators at a specific location, the scope of protection can easily become ambiguous or be easily circumvented due to differences in sampling locations or batch fluctuations. In contrast, the core innovation of this application lies in fundamentally altering the forming mechanism and microstructure evolution path of the extra-thick steel plate through a specific forging-rolling composite process and thermodynamic control system.

[0050] Thanks to the aforementioned unique manufacturing process, the extra-thick steel plate provided in this embodiment possesses a series of significant performance characteristics that distinguish it from traditional fully rolled or fully forged products. First, the steel plate has a dense core free of metallurgical defects. This is because a high-reduction forging process is employed during manufacturing, utilizing intense triaxial compressive stress to forcibly weld together the inherent porosity and shrinkage cavities within the continuously cast billet, eliminating the core quality risks caused by insufficient compression ratio in fully rolled processes. Second, the steel plate exhibits fine and uniform grains overall. This is attributed to the synergistic effect of ultra-low temperature heating and precision rolling in the austenite non-recrystallization zone, which inhibits the growth of original grains and accumulates high-density deformation energy, significantly increasing the phase deformation nucleation rate, thereby obtaining a fine-grained structure superior to that of conventional normalized states. Third, the steel plate possesses excellent low-temperature impact toughness and requires no normalizing heat treatment. Through precise matching of strain-induced refinement and controlled cooling phase transformation, the steel plate directly obtains a room-temperature microstructure combining strength and toughness after rolling, significantly reducing energy consumption and production cycle time, and avoiding the risk of grain coarsening that may occur during the normalizing process.

[0051] To verify the stability and reliability of the aforementioned performance characteristics, the following are two sets of measured data from typical product specifications as supporting evidence. When producing extra-thick steel plates with a finished thickness of 150mm, testing showed that the average longitudinal V-notch impact absorption energy at -20℃ reached 157J, far exceeding the 60J lower limit required by the national standard Q355D; simultaneously, the ultrasonic testing results for the entire plate met the AB11 standard, proving that the core density reached an extremely high level. When producing extra-thick steel plates with a finished thickness further increased to 250mm, although the cross-sectional thickness increased by 67%, the average longitudinal V-notch impact absorption energy at -20℃ still reached as high as 173J, and the ultrasonic testing results consistently met the GK11 standard. This counterintuitive performance fully demonstrates that the manufacturing process of this application can effectively overcome the technical bottleneck of the rapid deterioration of performance in extra-thick steel plates with increasing thickness, ensuring stable and excellent comprehensive mechanical properties and internal quality at different thickness specifications.

[0052] The above description is merely a specific implementation of this application, but the scope of protection of this application is not limited thereto.

Claims

1. A method for producing extra-thick steel plates based on a combination of forging and rolling, characterized in that, The chemical composition of the extra-thick steel plate, by mass percentage, includes: C: 0.08-0.12%, Mn: 1.4-1.6%, Si: 0.20-0.40%, Cr: 0.15-0.35%, Nb: 0.03-0.05%, V: 0.02-0.04%, Alt: 0.015-0.040%, P≤0.02%, S≤0.01%, with the balance being Fe and unavoidable impurities; The method includes: The billet is forged by sequentially applying high-reduction forging and low-reduction leveling to obtain an intermediate billet. The forging process includes: 2-3 passes of high-reduction forging, with a single-pass reduction rate of 13%-20%; and 2-3 passes of low-reduction leveling, with a single-pass reduction rate of 2%-10%. The thickness of the intermediate billet is 1.5-2.0 times the finished thickness of the extra-thick steel plate. The intermediate billet is slowly cooled to room temperature, and surface creases are removed. The cleaned intermediate billet is reheated to 850-950℃ and rolled in the austenite non-recrystallization temperature range; the initial rolling temperature is 740-820℃ and the final rolling temperature is 730-800℃. The rolled extra-thick steel plate is subjected to water cooling and stacking cooling treatment.

2. The method according to claim 1, characterized in that, The reheating temperature is 880-940℃, and the holding time is ≥0.12min / mm; The initial rolling temperature is 760-800℃, and the final rolling temperature is 750-790℃.

3. The method according to claim 1, characterized in that, The surface crease removal includes: using a flame to remove creases from the surface of the intermediate blank; The water-cooled temperature is 500-700℃. The stack cooling time for the stack cooling process is ≥24 hours.

4. The method according to claim 3, characterized in that, The reddening temperature is 550-650℃, and the stack cooling time is ≥48 hours.

5. The method according to claim 1, characterized in that, The finished thickness of the extra-thick steel plate is 100-300mm; The billet is prepared by converter smelting and vacuum treatment, with a vacuum time of ≥20 minutes; The billet is a continuously cast billet with a thickness of 300-500 mm, and the slow cooling time of the billet is ≥72 hours.

6. The method according to claim 1, characterized in that, Before the forging process, the billet is heated to 1200-1250℃ for 1.0-1.6 min / mm and held for ≥0.12 min / mm. During the forging process, when the temperature of the billet is below 900°C, it is returned to the furnace for reheating.

7. An extra-thick steel plate, characterized in that, It is prepared by the method described in any one of claims 1-6.