A 460mpa grade fracture toughness steel plate and a production method thereof

Abstract

This application provides a 460MPa grade crack-arresting toughness steel plate and its production method. The production method includes heating a continuously cast billet to 1150~1250℃ and holding it for (0.3~0.8)t0min, where t0 is the thickness of the continuously cast billet in mm; rough rolling the heated billet at 1100~1200℃, with at least two rolling passes having a single-pass reduction ≥30mm, and a total rough rolling reduction ≥40%; finishing rolling with an initial rolling temperature of 1050~1150℃ and a final rolling temperature of 900~950℃, with a waiting time of 10~30s between each pass, to obtain a steel plate with a target thickness t≥70mm; wherein, the steel plate obtained after rolling has uniformly distributed equiaxed austenite grains with an average grain size ≤20μm. By controlling the waiting time between each pass of finishing rolling, the austenite has sufficient time to recrystallize, forming fine and uniform equiaxed austenite.

Description

Technical Field

[0001] This application relates to the field of steel plate production technology, and in particular to a 460MPa grade crack-arresting toughness steel plate and its production method. Background Technology

[0002] Currently, the demand for ultra-wide (≥3500mm) and extra-thick (≥70mm) steel plates is increasing in ultra-large ships and marine engineering, especially in ultra-large container ships with a carrying capacity exceeding 24,000 TEU. The stress level and stress concentration of hatch coamings and strong decks have increased dramatically, creating an urgent need for wide and thick steel plates with high crack arrest toughness.

[0003] When faced with ultra-wide and extra-thick specifications, none of them could solve the following common bottlenecks: (1) Coarse and uneven austenite grains: In conventional hot rolling and heat treatment processes, especially inside large-size steel plates, the austenitizing heating temperature is high and the holding time is long, which easily leads to coarse austenite grains. If not properly controlled during the subsequent cooling process, coarse structures such as Widmanstätten and bainite will be formed, affecting toughness.

[0004] (2) The strengthening mechanism is simple and the toughness improvement is limited: The traditional quenching and tempering process mainly relies on martensitic phase transformation strengthening and precipitation phase strengthening. However, as the strength level increases, the toughness often decreases significantly, making it difficult to meet the requirements of ultra-high strength and ultra-high toughness at the same time.

[0005] (3) Insufficient crack arrest toughness: Crack arrest toughness is a key indicator for evaluating the energy resistance of steel plates during crack propagation. For large structural components, it is crucial to effectively prevent the initial crack from propagating to catastrophic fracture. Existing technologies struggle to achieve sufficiently high crack arrest toughness in ultra-wide and ultra-thick steel plates, especially under low-temperature or impact loads.

[0006] (4) Significant size effect: During the heating, rolling and cooling process of ultra-wide and extra-thick steel plates, the temperature and stress state of different parts inside the plates are very different, which makes it difficult to guarantee the uniformity of the structure and properties, and local performance defects may occur.

[0007] (5) The preparation process is complex and costly: In order to compensate for the above shortcomings, multiple heating, special alloying or complex rolling processes may be required, resulting in long production cycles, high energy consumption and increased costs. Summary of the Invention

[0008] The purpose of this application is to provide a production method for 460MPa grade crack-arresting toughness steel plates, which solves the problems of coarse microstructure, large difference between core and surface microstructure, and insufficient toughness and crack-arresting toughness in the existing technology of ultra-wide and ultra-thick steel plates.

[0009] To achieve one of the aforementioned objectives, one embodiment of this application provides a method for producing a 460MPa grade crack-arresting toughness steel plate, which is obtained by heating, rolling, and cooling a continuously cast billet. In the heating process, the continuously cast billet is heated to 1150~1250℃ and held for (0.3~0.8)t0min, where t0 is the thickness of the continuously cast billet in mm; In the rolling process, the heated continuous casting billet is rough rolled at 1100~1200℃, and the number of rolling passes with a single pass reduction of ≥30mm is not less than 2, and the total reduction rate of rough rolling is ≥40%; the starting rolling temperature of finish rolling is 1050~1150℃, the finishing rolling temperature is 900~950℃, and there is a waiting time of 10~30s between each pass, and a steel plate with a target thickness t≥70mm is obtained. The steel plate obtained after rolling has uniformly distributed equiaxed austenitic grains inside, with an average grain size of ≤20μm.

[0010] As a further improvement of one embodiment of this application, in the rolling process, the reduction rate of the first pass of the finishing rolling stage is ≥15%, the reduction rate of the last pass is <8%, the reduction rate of the intermediate passes is 8~15%, and the rolling temperature of the intermediate passes is 900~1000℃.

[0011] As a further improvement to one embodiment of this application, the cooling process employs any one of the following three schemes: Option 1: Air cool the rolled steel plate to 680~720℃, then cool it down to 300~350℃ at a cooling rate of ≥15℃ / s, and then air cool it to room temperature. Option 2: Cool the rolled steel plate to 750~780℃ at a cooling rate of ≥20℃ / s, then air cool to 680~720℃, then cool to 300~350℃ at a cooling rate of ≥15℃ / s, and finally air cool to room temperature. Option 3: Cool the rolled steel plate to 250-300℃ at a cooling rate of 25-40℃ / s, and then air cool it to room temperature.

[0012] As a further improvement to one embodiment of this application, the microstructure of the steel plate cooled using Scheme 1 consists of fine acicular ferrite, quasi-polygonal ferrite, and granular bainite / martensite-austenite, with a large-angle grain boundary fraction ≥70%; the microstructure of the steel plate cooled using Scheme 2 consists of fine-grained ferrite, fine acicular ferrite, and granular bainite, with a large-angle grain boundary fraction of 55~70%; and the microstructure of the steel plate cooled using Scheme 3 consists of bainite, fine acicular ferrite, and martensite, with a large-angle grain boundary fraction of 35~55%.

[0013] As a further improvement to one embodiment of this application, in the surface microstructure of the steel plate after cooling according to Scheme 1, the volume percentage of fine acicular ferrite is 82-90%, the volume percentage of quasi-polygonal ferrite is 5-8%, the volume percentage of granular bainite / martensite-austenite is 2-10%, and the large-angle grain boundary fraction is 80-85%; in the core microstructure, the volume percentage of fine acicular ferrite is 70-80%, the volume percentage of quasi-polygonal ferrite is 10-20%, the volume percentage of granular bainite / martensite-austenite is 5-10%, and the large-angle grain boundary fraction is 70-80%. In the surface microstructure of the steel plate cooled using Scheme 2, the volume percentage of fine-grained ferrite is 30-40%, the volume percentage of fine acicular ferrite is 40-50%, the volume percentage of granular bainite is 10-20%, and the large-angle grain boundary fraction is 60-70%. In the core microstructure, the volume percentage of fine-grained ferrite is 35-45%, the volume percentage of fine acicular ferrite is 45-55%, the volume percentage of granular bainite is 5-10%, and the large-angle grain boundary fraction is 55-65%. In the surface microstructure of the steel plate cooled using Scheme 3, the volume percentage of bainite is 55-65%, the volume percentage of fine acicular ferrite is 20-30%, the volume percentage of martensite is 10-15%, and the large-angle grain boundary fraction is 40-50%. In the core microstructure, the volume percentage of bainite is 60-68%, the volume percentage of fine acicular ferrite is 25-33%, the volume percentage of martensite is 2-7%, and the large-angle grain boundary fraction is 35-42%.

[0014] As a further improvement to one embodiment of this application, after the cooling process, a tempering process is also included, which corresponds to the three schemes in the cooling process respectively: In Scheme 1, the tempering temperature of the tempering process after cooling is controlled at 500~530℃; in Scheme 2, the tempering temperature of the tempering process after cooling is controlled at 520~560℃; and in Scheme 3, the tempering temperature of the tempering process after cooling is controlled at 540~580℃.

[0015] As a further improvement of one embodiment of this application, the holding time in the tempering process is 1.5t~2.5t. After the tempering holding is completed, the plate is cooled to room temperature at a cooling rate of 0.5~2℃ / s. Here, t is the target thickness of the steel plate in mm, and the holding time is in min.

[0016] As a further improvement of one embodiment of this application, the chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0017] An embodiment of this application also provides a steel plate produced using the aforementioned method for producing 460MPa grade crack-arresting toughness steel plates. The chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0018] As a further improvement of one embodiment of this application, the chemical composition of the steel plate, by mass fraction, includes: C: 0.06~0.10%, Si: 0.1~0.5%, Mn: 1.5~1.7%, P≤0.010%, S≤0.003%, Ni: 0.7~1.0%, Cr: 0.6~1.0%, Mo: 0.25~0.50%, Nb: 0.025~0.045%, Ti: 0.01~0.015%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0019] As a further improvement of one embodiment of this application, the difference in yield strength between the surface and core of the steel plate is ≤30MPa, and the impact energy of the core of the steel plate at -60℃ is ≤15% compared with the surface.

[0020] As a further improvement of one embodiment of this application, the steel plate has a yield strength ≥460MPa, a tensile strength of 570~720MPa, an elongation after fracture ≥17%, and a crack arrest toughness Kca value ≥8000 N / mm at -10℃. 3 / 2 .

[0021] One or more technical solutions provided in this application have at least the following technical effects or advantages: In the production method of 460MPa grade crack-arresting toughness steel plate provided in this application, the waiting time after each rolling pass during the finishing rolling is controlled to allow the previously deformed austenite to undergo sufficient recrystallization, forming a large number of new, fine, dislocation-free or low-dislocation-density equiaxed austenite grains. The cycle of "rolling-recrystallization-rolling-recrystallization" is repeated, so that the austenite grains inside the steel plate are gradually refined and tend to be uniformly distributed, resulting in smaller differences in the internal and external microstructures and strength of the ultra-thick steel plate. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] This application provides a method for producing a 460MPa grade crack-arresting toughness steel plate, which is obtained by heating, rolling, and cooling a continuously cast billet. In the heating process, the continuous casting billet is heated to 1150~1250℃ and held for (0.3~0.8)t0min, where t0 is the thickness of the continuous casting billet in mm; In the rolling process, the heated continuous casting billet is rough rolled at 1100~1200℃, and the number of rolling passes with a single pass reduction of ≥30mm is not less than 2, and the total reduction rate of rough rolling is ≥40%; the starting rolling temperature of finish rolling is 1050~1150℃, the finishing rolling temperature is 900~950℃, and there is a waiting time of 10~30s between each pass, and a steel plate with a target thickness t≥70mm is obtained. The steel plate obtained after rolling has uniformly distributed equiaxed austenitic grains inside, with an average grain size of ≤20μm.

[0024] By keeping the continuously cast billet at a high temperature for a sufficient period of time, the chemical composition and temperature distribution inside the billet are made highly uniform, creating favorable conditions for the subsequent rolling process.

[0025] The roughing stage employs a large reduction to break down the coarse solidification structure of the continuously cast billet, providing a refined original grain structure for subsequent austenite cyclic recrystallization rolling.

[0026] In the finishing rolling process, namely, austenite cyclic recrystallization rolling (ACR), this application does not immediately proceed with the next rolling pass after each pass, but instead allows for a 10-30s interval, enabling the previously deformed austenite to undergo sufficient recrystallization, forming a large number of new, fine, dislocation-free or low-dislocation-density equiaxed austenite grains. This repeated "rolling-recrystallization-rolling-recrystallization" cycle gradually refines the austenite grains inside the steel plate and makes them tend to be evenly distributed, resulting in smaller differences in the internal and external microstructure and strength of the ultra-thick steel plate.

[0027] In some embodiments of this application, during the rolling process, the reduction rate of the first pass in the finishing rolling stage is ≥15%, the reduction rate of the last pass is <8%, and the reduction rate of the intermediate passes is 8~15%, with the rolling temperature of the intermediate passes being 900~1000℃. The first pass uses a relatively large reduction rate, ensuring that subsequent rolling processes achieve both a certain reduction effect and sufficient austenite recrystallization between passes. The smaller reduction in the final rolling stage ensures a straight steel plate shape, providing a good foundation for subsequent cooling and shape control.

[0028] In some embodiments of this application, the cooling process employs any one of the following three methods: Option 1: The rolled steel plate is air-cooled to 680~720℃, then cooled to 300~350℃ at a cooling rate of ≥15℃ / s, and finally air-cooled to room temperature. After air cooling, the rolled steel plate undergoes relaxation treatment to homogenize the carbon concentration in the supercooled austenite and stabilize the retained austenite. Rapid cooling is then performed to obtain a microstructure dominated by fine acicular ferrite, with dispersed grain orientation and high large-angle grain boundary density, which significantly improves low-temperature crack arrest toughness. This option is suitable for service scenarios with extremely high requirements for low-temperature crack arrest toughness (Kca). This option has a longer relaxation time, allows for complete carbide precipitation, has lower sensitivity to cooling rate, and exhibits optimal uniformity in the thickness direction.

[0029] Option 2: Cool the rolled steel plate to 750~780℃ at a cooling rate of ≥20℃ / s, then air-cool to 680~720℃, followed by a cooling rate of ≥15℃ / s to 300~350℃, and finally air-cool to room temperature. A short relaxation period between the two rapid cooling cycles allows for the release of latent heat of phase transformation and homogenization of carbon elements. The subsequent rapid cooling achieves a good balance between high strength and high toughness.

[0030] Option 3: Cool the rolled steel plate to 250-300℃ at a cooling rate of 25-40℃ / s, then air-cool to room temperature. Direct and rapid cooling results in abundant fine and dispersed precipitates, which can improve the strength of the steel plate.

[0031] This scheme has no relaxation, involves strong cooling throughout, and has a high martensite / bainite ratio, making it most sensitive to cooling rate gradients. However, because the austenite cyclic recrystallization rolling process ensures that the original austenite grains are highly uniform in the thickness direction, the strength difference is still controllable even if the phase transformation products on the surface and in the core are different.

[0032] In some embodiments of this application, the microstructure of the steel plate cooled using Scheme 1 consists of fine acicular ferrite (AF), quasi-polygonal ferrite (QF), and granular bainite / martensite-austenite (M / A), with a large-angle grain boundary fraction ≥70%; the microstructure of the steel plate cooled using Scheme 2 consists of fine-grained ferrite (GF), fine acicular ferrite, and granular bainite (GB), with a large-angle grain boundary fraction of 55-70%; the microstructure of the steel plate cooled using Scheme 3 consists of bainite (GB / LB), fine acicular ferrite, and martensite (M), with a large-angle grain boundary fraction of 35-55%. Bainite (GB / LB) refers to both granular bainite (GB) and lath bainite (LB).

[0033] Furthermore, in the surface microstructure of the steel plate cooled according to Scheme 1, the volume percentage of fine acicular ferrite is 82-90%, the volume percentage of quasi-polygonal ferrite is 5-8%, the volume percentage of granular bainite / martensite-austenite is 2-10%, and the large-angle grain boundary fraction is 80-85%; in the core microstructure, the volume percentage of fine acicular ferrite is 70-80%, the volume percentage of quasi-polygonal ferrite is 10-20%, the volume percentage of granular bainite / martensite-austenite is 5-10%, and the large-angle grain boundary fraction is 70-80%.

[0034] In the surface microstructure of the steel plate cooled using Scheme 2, the volume percentage of fine-grained ferrite is 30-40%, the volume percentage of fine acicular ferrite is 40-50%, the volume percentage of granular bainite is 10-20%, and the large-angle grain boundary fraction is 60-70%. In the core microstructure, the volume percentage of fine-grained ferrite is 35-45%, the volume percentage of fine acicular ferrite is 45-55%, the volume percentage of granular bainite is 5-10%, and the large-angle grain boundary fraction is 55-65%.

[0035] In the surface microstructure of the steel plate cooled using Scheme 3, the volume percentage of bainite is 55-65%, the volume percentage of fine acicular ferrite is 20-30%, the volume percentage of martensite is 10-15%, and the large-angle grain boundary fraction is 40-50%. In the core microstructure, the volume percentage of bainite is 60-68%, the volume percentage of fine acicular ferrite is 25-33%, the volume percentage of martensite is 2-7%, and the large-angle grain boundary fraction is 35-42%.

[0036] In some embodiments of this application, after the cooling process, a tempering process is also included to eliminate the internal stress generated during the cooling process, regulate the size and distribution of the precipitated phase, improve the uniformity of the microstructure, and further enhance the low-temperature toughness and crack arrest toughness while maintaining the strength.

[0037] The tempering process corresponds to one of the three schemes in the cooling process: In Option 1, the tempering temperature of the tempering process after cooling is controlled at 500~530℃. This temperature range allows the dislocation structure in the fine acicular ferrite matrix to recover moderately, while promoting the dispersion precipitation of microalloyed carbonitrides such as NbC and TiN within the grains and on the dislocation lines, resulting in a significant precipitation strengthening effect without causing obvious coarsening of the acicular ferrite structure.

[0038] In Scheme 2, the tempering temperature after cooling is controlled at 520~560℃. Since the final microstructure is mainly fine-grained ferrite and fine acicular ferrite, appropriately increasing the tempering temperature helps to eliminate the micro-strain concentration generated during the rapid cooling-relaxation-rapid cooling process, making the ferrite grain boundaries tend to be straight, reducing local stress concentration, and at the same time promoting the transformation of carbides from thin film to granular, thus improving toughness.

[0039] In Scheme 3, the tempering temperature of the tempering process after cooling is controlled at 540~580℃. Because this scheme has a high proportion of bainite and the quenching stress generated by direct rapid cooling to a lower temperature is the greatest, a higher tempering temperature is required to fully release the internal stress and promote the spheroidization of carbides in bainite, thereby effectively improving plasticity and toughness while maintaining high strength.

[0040] In some embodiments of this application, the holding time in the tempering process is 1.5t~2.5t. After the tempering holding is completed, the plate is cooled to room temperature at a cooling rate of 0.5~2℃ / s. Here, t is the target thickness of the steel plate in mm, and the holding time is in min.

[0041] After tempering and heat preservation, the steel plate is cooled to room temperature by air cooling or stack cooling. This is to avoid rapid cooling that introduces new thermal stress and to ensure the stable existence of the tempered precipitates.

[0042] In some embodiments of this application, the chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0043] The following explains the role and dosage of each chemical component in the steel plate described in this invention: C: The basic strength of the material is guaranteed while ensuring weldability; therefore, C: 0.04~0.12%.

[0044] Si: Si is one of the main deoxidizing elements in the steelmaking process and is also a solid solution strengthening element. However, if the Si content is too high, it is easy to form ferrous silicate Fe2SiO4, which is difficult to remove, during the heating process of steel billet and steel plate. This makes it difficult to peel off the iron oxide scale on the surface of the steel plate and affects the surface quality of the steel plate. Therefore, the Si content is controlled at 0.10~0.50%.

[0045] Mn, Cr, and Mo significantly improve the hardenability of steel, ensuring that the pearlite transformation can be suppressed during cooling in the core of steel plates with a thickness of ≥70mm, thus obtaining acicular ferrite and bainite structures. Therefore, the steel of this invention contains Mn: 1.0~2.0%; Cr: 0.50~1.5%; and Mo: 0.20~0.80%.

[0046] Ni: Solid solution strengthening and a significant reduction in the ductile-brittle transition temperature make it a key element for ensuring high toughness at -10℃ and lower. This solution, based on process strengthening, controls the Ni content to 0.50~1.50%.

[0047] Nb and Ti: Composite microalloying. Ti forms high-temperature stable TiN particles in molten steel, effectively pinning austenite grain boundaries and preventing grain coarsening during heating and high-temperature rolling. Nb strongly inhibits recrystallization during rolling in the non-recrystallized austenite region, increasing the total grain boundary area of ​​the deformed austenite and providing more nucleation sites for quenching phase transformation, thus being the core component for obtaining a fine final microstructure. This scheme controls Nb at 0.02~0.08% and Ti at 0.01~0.05%.

[0048] Al: Al is a strong deoxidizing and grain-refining element in the steelmaking process. It effectively removes the O gas content in the molten steel and forms a stable AlN precipitate with N, which plays a role in pinning the growth of austenite grains during the heating process. However, adding too much Al will leave too much Al2O3 inclusion in the molten steel, which will agglomerate at the casting nozzle and cause accidents. Therefore, the Al content is controlled at 0.015~0.045%.

[0049] Ultra-low phosphorus (P) and sulfur (S): Reduce grain boundary segregation and harmful inclusions, which is fundamental to improving toughness, especially crack arrest toughness. P: ≤0.015%; S: ≤0.005%.

[0050] This invention, by controlling the chemical composition of the steel plate and the aforementioned production process, achieves homogenization of strain and microstructure in various regions of the steel plate through multiple deformation and phase transformation cycles during rolling, overcoming the performance inhomogeneity problem caused by the size effect in traditional processes. After rolling, uniform and fine equiaxed austenite grains (≤20μm) are formed inside the steel plate, laying the foundation for subsequent phase transformation to form fine ferrite, bainite, and other microstructures. Through alloy design (Mo, Nb, Cr) and the synergistic effect of fine grains and dispersed precipitates, yield strength, tensile strength, and fracture toughness are simultaneously and significantly improved. Furthermore, the fine grain structure and grain boundary strengthening mechanism effectively inhibit crack initiation and propagation, endowing the steel plate with excellent crack arrest properties, especially under low temperature or dynamic loading conditions.

[0051] An embodiment of this application also provides a steel plate produced using the aforementioned method for producing 460MPa grade crack-arresting toughness steel plates, i.e., the steel plate is a 460MPa grade crack-arresting toughness steel plate. The chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0052] Preferably, the chemical composition of the steel plate, by mass fraction, includes: C: 0.06~0.10%, Si: 0.1~0.5%, Mn: 1.5~1.7%, P≤0.010%, S≤0.003%, Ni: 0.7~1.0%, Cr: 0.6~1.0%, Mo: 0.25~0.50%, Nb: 0.025~0.045%, Ti: 0.01~0.015%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

[0053] The steel plate in this application has a thickness t ≥ 70 mm, a width ≥ 3500 mm, a yield strength difference between the surface and core of the steel plate ≤ 30 MPa, and an impact energy fluctuation of -60℃ in the core of the steel plate compared to the surface ≤ 15%.

[0054] The steel plate has a yield strength ≥460MPa, a tensile strength of 570~720MPa, an elongation after fracture ≥17%, and a crack arrest toughness (Kca) ≥8000 N / mm at -10℃. 3 / 2 .

[0055] The technical solution of this application will be further described below with reference to some specific embodiments.

[0056] Example 1 The chemical composition of the steel plate (wt.%) is as follows: C 0.08, Si 0.25, Mn 1.60, P 0.008, S 0.002, Ni 0.85, Cr 0.60, Mo 0.25, Nb 0.025, Ti 0.012, Als 0.030, with the remainder being Fe and unavoidable impurities.

[0057] Process flow: Heating: The 320mm thick continuous casting billet is heated to 1170℃ and held for 120 minutes to ensure that the chemical composition and temperature distribution inside the continuous casting billet are highly uniform.

[0058] Rough rolling: Rough rolling is carried out in the temperature range of 1100~1200 ℃. There are 2 passes with a reduction of ≥30mm per pass. The thickness after rough rolling is 190mm and the rough rolling reduction rate is 40.6%.

[0059] Finishing rolling: A total of 6 passes are rolled. The first pass starts at a rolling temperature of 1085℃ with a reduction rate of 16%. There is a 20-second warm-up period between passes. The second to fifth passes are rolled at a temperature range of 900-1000℃, with a reduction rate of 8%-15% for each pass. There is a 20-second warm-up period between each pass. The final rolling pass has a reduction rate of 7%, a final rolling temperature of 920℃, and a final rolling thickness of 90 mm.

[0060] Cooling (Option 1): After the final rolling is completed, the temperature is first air-cooled to 700 ℃ for relaxation treatment; then rapid cooling is performed at a cooling rate of 22 ℃ / s to 320 ℃, and then air-cooled to room temperature.

[0061] Tempering: The cooled steel plate is subjected to high-temperature tempering treatment at a tempering temperature of 520 ℃ and a holding time of 180 min. After tempering, it is air-cooled to room temperature at a cooling rate of 1 ℃ / s.

[0062] Tissue performance results:

[0063] Steel plate specifications: 90 mm thick, 3600 mm wide, 10 m long.

[0064] Example 2 Chemical composition of steel plate (wt.%): Same as in Example 1.

[0065] Process flow: Heating: Heat the 320 mm thick continuous casting billet to 1170 ℃ and hold for 120 min.

[0066] Rough rolling: Rough rolling is carried out in the temperature range of 1100~1200 ℃. There are 2 passes with a reduction of ≥30 mm per pass. The thickness after rough rolling is 190 mm and the rough rolling reduction rate is 40.6%.

[0067] Finishing rolling: A total of 6 passes are rolled. The first pass starts at a rolling temperature of 1120℃ with a reduction rate of 16%. There is a 22s waiting period between passes. The 2nd to 5th passes are rolled at a temperature range of 900~1000℃, with a reduction rate of 8%~15% for each pass. There is a 22s waiting period between each pass. The final rolling pass has a reduction rate of 7%, a final rolling temperature of 923℃, and a final rolling thickness of 90 mm.

[0068] Cooling (Option 2): After the final rolling is completed, the temperature is first rapidly cooled to 750°C at a cooling rate of 25°C / s, and then air-cooled to 700°C for relaxation treatment; then it is rapidly cooled again to 330°C at a cooling rate of 20°C / s, and then air-cooled to room temperature.

[0069] Tempering: The cooled steel plate is subjected to high-temperature tempering treatment at a tempering temperature of 540 ℃ and a holding time of 180 min. After tempering, it is air-cooled to room temperature at a cooling rate of 1.5 ℃ / s.

[0070] Performance results:

[0071] Steel plate specifications: 90 mm thick, 3700 mm wide, and 9 m long.

[0072] Example 3 Chemical composition (wt.%): Same as in Example 1.

[0073] Heating: Heat the 320 mm thick continuous casting billet to 1170 ℃ and hold for 120 min.

[0074] Rough rolling: Rough rolling is carried out in the temperature range of 1100~1200 ℃. There are 2 passes with a reduction of ≥30 mm per pass. The thickness after rough rolling is 190 mm and the rough rolling reduction rate is 40.6%.

[0075] Finishing rolling: A total of 6 passes are rolled. The first pass starts at a temperature of 1100℃ with a reduction rate of 16%. There is an 18-second warm-up period between passes. The 2nd to 5th passes are rolled at a temperature range of 900 to 1000℃, with a reduction rate of 8% to 15% for each pass. There is an 18-second warm-up period between each pass. The final rolling pass has a reduction rate of 7%, a final rolling temperature of 934℃, and a final rolling thickness of 90 mm.

[0076] Cooling (Option 3): After the final rolling is completed, the surface is rapidly cooled to 280°C by continuous water jet cooling at a surface cooling rate of 35°C / s, and then air-cooled to room temperature.

[0077] Tempering: The cooled steel plate is subjected to high-temperature tempering treatment at a tempering temperature of 560 ℃ and a holding time of 200 min. After tempering, it is air-cooled to room temperature at a cooling rate of 0.8 ℃ / s.

[0078] Performance results:

[0079] Steel plate specifications: 90 mm thick, 3580 mm wide, 11 m long.

[0080] Example 4 The only difference from Example 1 is that there is no tempering process.

[0081] Performance results:

[0082] (4) Steel plate specifications: 90 mm thick, 3550 mm wide, and 10 m long.

[0083] Example 5 The only difference from Example 2 is that there is no tempering process.

[0084] Performance results:

[0085] Steel plate specifications: 90 mm thick, 3560 mm wide, 10 m long.

[0086] Comparative Example 1 Chemical composition of steel plate (wt.%): Same as in Example 1.

[0087] Process flow: Heating: Heat the 320 mm thick continuous casting billet to 1180 ℃ and hold for 120 min.

[0088] Conventional TMCP rolling: Traditional two-stage controlled rolling is employed. The first stage involves rough rolling at 1050–1000 °C; the second stage involves finish rolling at 860–820 °C, with a total reduction of approximately 65% ​​and a final rolling thickness of 90 mm. The final rolling temperature is controlled at 840 °C. ACR technology is not used; rolling is continuous between passes, and no inter-pass recrystallization or temperature-controlled recrystallization interval is provided.

[0089] Cooling: After final rolling, ACC (accelerated cooling) is used at a cooling rate of 12 ℃ / s to cool to 550 ℃, and then air-cooled to room temperature.

[0090] No tempering process is performed.

[0091] Performance results:

[0092] Steel plate specifications: 90 mm thick, 3580 mm wide, and 9.5 m long.

[0093] Comparative analysis of Comparative Example 1 and the Example: Comparative Example 1, using conventional TMCP process, under the same chemical composition, showed a yield strength (surface 476 MPa / core 424 MPa) that essentially reached the lower limit of 460 MPa, but the core yield strength was only 424 MPa, and the crack arrest toughness (Kca) at -10℃ was only 5200 N / mm. 3 / 2 It is far lower than the 8110~8500 N / mm of Examples 1~5. 3 / 2 The yield strength difference between the surface and the core reaches 52 MPa, which is far greater than the ≤30 MPa requirement of this invention. This indicates that without the ACR mechanism, even with the same chemical composition, insufficient strain accumulation in the core of a thick plate leads to limited grain refinement, significantly deteriorated crack arrest toughness, and the size effect results in significant differences in core and surface properties.

[0094] Comparative Example 2 Chemical composition of steel plate (wt.%): C 0.19, Si 0.32, Mn 1.95, P 0.015, S 0.004, Ni 1.60, Cr 1.50, Mo 0.60, Nb 0.040, Ti 0.06, Als 0.025, with the remainder being Fe and unavoidable impurities.

[0095] Process flow: Heating: Heat the 320 mm thick continuous casting billet to 1200 ℃ and hold for 130 min.

[0096] The rolling process is the same as in Example 1, and the cooling process is the same as in Example 2.

[0097] Tempering: Tempering temperature 540 ℃, holding time 180 min, air cooling to room temperature.

[0098] Performance results:

[0099] Steel plate specifications: 90 mm thick, 3590 mm wide, and 9.5 m long.

[0100] Comparative Example 2 has a higher carbon and alloy content. Although its yield strength and tensile strength are relatively high, its crack arrest toughness (Kca) at -10 ℃ is only 5200 N / mm. 3 / 2 The required ≥8000 N / mm² of this invention was not achieved. 3 / 2 The surface impact energy at -60 ℃ is only 65 J, far lower than that in Examples 1-5. This indicates that although high carbon and higher alloys can improve strength, they severely impair low-temperature toughness and crack arrest toughness, failing to meet the structural safety requirements of large structural components such as ultra-large container ships.

[0101] Comparative Example 3 Chemical composition of steel plate (wt.%): Same as in Example 1.

[0102] Process flow: Heating, rolling, and cooling are the same as in Example 2.

[0103] Tempering: Tempering temperature 620℃ (exceeding the range of 520~560℃ corresponding to Scheme 2 of the present invention), holding time 180min, air cooling to room temperature.

[0104] Performance results:

[0105] Steel plate specifications: 90 mm thick, 3550 mm wide, 10 m long.

[0106] The tempering temperature of Comparative Example 3 (620°C) is higher than the upper limit of the scope of this invention (560°C). Although the elongation is slightly increased, the yield strength is lower than 460 MPa (only 416 MPa in the core), and the crack arrest toughness Kca at -10°C drops to 5500 N / mm. 3 / 2 It is far lower than the ≥8000 N / mm of this invention. 3 / 2 The requirements are as follows: Excessive tempering at high temperatures leads to significant coarsening of the bainitic microstructure, with a large accumulation and spheroidization of carbides, weakening both precipitation strengthening and grain boundary strengthening effects. Furthermore, the large carbides become favorable sites for crack initiation, severely impairing crack arrest toughness. This indicates that the tempering temperature must be precisely controlled within the range specified in Scheme 2 (520~560 ℃). Excessively high tempering temperatures will destroy the microstructural advantages established by the ACR+ controlled cooling process, resulting in a significant decrease in crack arrest toughness.

[0107] Based on the combined experimental results of Comparative Examples 1-3, the following conclusions can be drawn: (1) ACR rolling is fundamental: Comparative Example 1 shows that even with the exact same chemical composition, without the ACR rolling process, the core strain accumulation of thick plates is insufficient, the grain refinement effect is limited, and the crack arrest toughness Kca is only 5200 N / mm. 3 / 2 The values ​​are significantly lower than the 8110~8500 N / mm in the embodiments of the present invention. 3 / 2 Furthermore, the yield strength difference from the surface to the core reaches 52 MPa, exceeding the ≤30 MPa range of this invention.

[0108] (2) Precise control of composition is the guarantee: Comparative Example 2 shows that when the carbon content and alloy content increase, although the strength is significantly improved, the crack arrest toughness and low temperature impact toughness are severely deteriorated.

[0109] (3) Precise matching of tempering temperature is a guarantee of performance: Comparative Example 3 shows that if the tempering temperature deviates from the matching range of the present invention (Scheme 2 corresponds to 520~560 ℃, the actual temperature is 620 ℃), it will lead to coarsening of the precipitate, yield strength below 460 MPa, and Kca dropping to 5500 N / mm.3 / 2 This verified the technical rationality of the matching relationship between tempering temperature and cooling scheme.

[0110] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0111] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for producing a 460MPa grade crack-arresting toughness steel plate, characterized in that, It is obtained by heating, rolling, and cooling a continuously cast billet. In the heating process, the continuous casting billet is heated to 1150~1250℃ and held for (0.3~0.8)t0min, where t0 is the thickness of the continuous casting billet in mm; In the rolling process, the heated continuous casting billet is rough rolled at 1100~1200℃, and the number of rolling passes with a single pass reduction of ≥30mm is not less than 2, and the total reduction rate of rough rolling is ≥40%; the starting rolling temperature of finish rolling is 1050~1150℃, the finishing rolling temperature is 900~950℃, and there is a waiting time of 10~30s between each pass, and a steel plate with a target thickness t≥70mm is obtained. The steel plate obtained after rolling has uniformly distributed equiaxed austenitic grains inside, with an average grain size of ≤20μm.

2. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 1, characterized in that, In the rolling process, the reduction rate of the first pass in the finishing rolling stage is ≥15%, the reduction rate of the last pass is <8%, the reduction rate of the intermediate passes is 8~15%, and the rolling temperature of the intermediate passes is 900~1000℃.

3. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 1, characterized in that, In the cooling process, any one of the following three schemes is adopted: Option 1: Air cool the rolled steel plate to 680~720℃, then cool it down to 300~350℃ at a cooling rate of ≥15℃ / s, and then air cool it to room temperature. Option 2: Cool the rolled steel plate to 750~780℃ at a cooling rate of ≥20℃ / s, then air cool to 680~720℃, then cool to 300~350℃ at a cooling rate of ≥15℃ / s, and finally air cool to room temperature. Option 3: Cool the rolled steel plate to 250-300℃ at a cooling rate of 25-40℃ / s, and then air cool it to room temperature.

4. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 3, characterized in that, The microstructure of the steel plate cooled using Scheme 1 consists of fine acicular ferrite, quasi-polygonal ferrite, and granular bainite / martensite-austenite, with a large-angle grain boundary fraction ≥70%. The microstructure of the steel plate cooled using Scheme 2 consists of fine-grained ferrite, fine acicular ferrite, and granular bainite, with a large-angle grain boundary fraction of 55~70%. The microstructure of the steel plate cooled using Scheme 3 consists of bainite, fine acicular ferrite, and martensite, with a large-angle grain boundary fraction of 35~55%.

5. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 4, characterized in that, In the surface microstructure of the steel plate cooled using Scheme 1, the volume percentage of fine acicular ferrite is 82-90%, the volume percentage of quasi-polygonal ferrite is 5-8%, the volume percentage of granular bainite / martensite-austenite is 2-10%, and the large-angle grain boundary fraction is 80-85%. In the core microstructure, the volume percentage of fine acicular ferrite is 70-80%, the volume percentage of quasi-polygonal ferrite is 10-20%, the volume percentage of granular bainite / martensite-austenite is 5-10%, and the large-angle grain boundary fraction is 70-80%. In the surface microstructure of the steel plate cooled using Scheme 2, the volume percentage of fine-grained ferrite is 30-40%, the volume percentage of fine acicular ferrite is 40-50%, the volume percentage of granular bainite is 10-20%, and the large-angle grain boundary fraction is 60-70%. In the core microstructure, the volume percentage of fine-grained ferrite is 35-45%, the volume percentage of fine acicular ferrite is 45-55%, the volume percentage of granular bainite is 5-10%, and the large-angle grain boundary fraction is 55-65%. In the surface microstructure of the steel plate cooled using Scheme 3, the volume percentage of bainite is 55-65%, the volume percentage of fine acicular ferrite is 20-30%, the volume percentage of martensite is 10-15%, and the large-angle grain boundary fraction is 40-50%. In the core microstructure, the volume percentage of bainite is 60-68%, the volume percentage of fine acicular ferrite is 25-33%, the volume percentage of martensite is 2-7%, and the large-angle grain boundary fraction is 35-42%.

6. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 3, characterized in that, Following the cooling process, a tempering process is also included, which corresponds to the three schemes in the cooling process: In Scheme 1, the tempering temperature of the tempering process after cooling is controlled at 500~530℃; in Scheme 2, the tempering temperature of the tempering process after cooling is controlled at 520~560℃; and in Scheme 3, the tempering temperature of the tempering process after cooling is controlled at 540~580℃.

7. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 6, characterized in that, The holding time in the tempering process is 1.5t~2.5t. After the tempering holding is completed, the steel plate is cooled to room temperature at a cooling rate of 0.5~2℃ / s. Here, t is the target thickness of the steel plate in mm, and the holding time is in min.

8. The method for producing 460MPa grade crack-arresting toughness steel plate according to claim 1, characterized in that, The chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

9. A steel plate produced by the production method of the 460MPa grade crack-arresting toughness steel plate according to claim 1, characterized in that, The chemical composition of the steel plate, by mass fraction, includes: C: 0.04~0.12%, Si: 0.1~0.5%, Mn: 1.0~2.0%, P≤0.015%, S≤0.005%, Ni: 0.5~1.5%, Cr: 0.5~1.5%, Mo: 0.2~0.8%, Nb: 0.02~0.08%, Ti: 0.01~0.05%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

10. The steel plate according to claim 9, characterized in that, The chemical composition of the steel plate, by mass fraction, includes: C: 0.06~0.10%, Si: 0.1~0.5%, Mn: 1.5~1.7%, P≤0.010%, S≤0.003%, Ni: 0.7~1.0%, Cr: 0.6~1.0%, Mo: 0.25~0.50%, Nb: 0.025~0.045%, Ti: 0.01~0.015%, Al: 0.015~0.045%, with the remainder being Fe and unavoidable impurities.

11. The steel plate according to claim 9, characterized in that, The difference in yield strength between the surface and core of the steel plate is ≤30MPa, and the impact energy of the core at -60℃ fluctuates by ≤15% compared to the surface.

12. The steel plate according to claim 9, characterized in that, The steel plate has yield strength ≥ 460 MPa, tensile strength of 570 ~ 720 MPa, elongation after fracture ≥ 17%, and Kca value of arrest toughness at -10℃ ≥ 8000 N / mm 3 / 2 .

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