Low-temperature toughness optimization method for medium and thick plate by controlling multi-stage phase transformation in a short process

Abstract

The present invention belongs to the technical field of steel material processing and preparation, and discloses a method for optimizing low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process. The problem of core microstructure refinement of medium and thick plates under low compression ratio is solved through the synergistic effect of two heatings, two ultra-fast coolings, one-stage rolling and post-rolling cooling. After the initial heating and heat preservation, ultra-fast cooling is performed to the surface room temperature, and secondary cooling is performed after returning to red; secondary heating and heat preservation is performed to regulate the original austenite grain size, and multi-pass uniform rolling is performed; laminar cooling is performed after rolling to a red-returning temperature of 310°C to 330°C, and a polygonal ferrite and granular bainite dual-phase structure is obtained after air cooling. The steel plate produced by the method of the present invention has a core impact energy of not less than 200J and not less than 100J at -80°C and -120°C, respectively, and a yield strength of ≥540MPa. It has both high strength and good low-temperature toughness, is suitable for the field of marine engineering, and has good industrial application value.

Description

Technical Field

[0001] The present invention relates to the technical field of steel material processing and preparation, and in particular to a method for optimizing low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process. Background Art

[0002] In recent years, with the rapid growth in demand for exploration in extreme regions such as the deep sea and polar regions, marine engineering equipment has gradually expanded to deeper (>1000m) and lower temperatures (annual average temperature <-30°C) extreme environments. The service conditions of medium / heavy thickness ship plate steel (≥30mm), a key structural material for ships, have expanded from conventional waters to the harsh conditions of deep-sea high pressure coupled with polar ultra-low temperatures. In this context, the market has placed higher demands on the strength and toughness matching of ship plate steel, especially its low-temperature toughness. Refining the microstructure through controlled rolling and controlled cooling technology (TMCP) is one of the effective means of improving the strength of steel plates while maintaining good toughness. The principle is mainly to refine the austenite grains through controlled rolling, and the cumulative processing strain is combined with subsequent controlled cooling to regulate and refine the phase transformation microstructure.

[0003] However, the current traditional TMCP technology improves the low-temperature toughness of the material by refining the austenite grain size, but it still faces challenges in the production of steel plates with a thickness of more than 30 mm. First, due to the low rolling reduction ratio in the core of the thick plate (usually the rolling reduction ratio is ≤4:1), the deformation penetration is insufficient, the austenite grains are difficult to fully refine, and the core structure coarsening problem is prominent, which seriously affects the low-temperature impact performance. Secondly, under the existing cooling process, there is a defect of large difference in cooling rate between the surface and the core of the steel plate, which can easily lead to uneven final structure, such as bainite enrichment on the surface and ferrite coarsening in the center. In addition, traditional processes often use multiple rolling passes and complex cooling paths to obtain fine-grained structure, resulting in high energy consumption, low production efficiency, and low adaptability to the needs of industrial large-scale production. In the existing technology, the industry mostly adopts a two-stage controlled rolling process in the austenite recrystallization zone and the non-recrystallization zone to control the microstructure. Patent CN116200682B discloses a method for manufacturing high-strength and toughness low-temperature marine steel plates. Through controlled rolling and controlled cooling combined with low-temperature tempering, a ferrite and granular bainite structure is achieved. The impact energy at -80°C is ≥70J, but the volume content of ferrite is ≤10%, and the carbon content of the applicable steel grade is above 0.085%. Patent CN104372257A discloses a method for improving the strength and toughness of medium and thick plate by utilizing residual heat from reheating. This method utilizes the TMCP process combined with a stack cooling treatment to achieve a mixed structure of acicular ferrite and granular bainite. However, this cooling process does not involve two ultra-rapid cooling techniques, and the reheating temperature control range is relatively wide (250-400°C), which can easily lead to insufficient structural stability. Therefore, in summary, there is an urgent need for a short-process, highly controllable low-temperature toughening method for medium and thick plate that can achieve ultra-fine core microstructure and high uniformity at low compression ratios, while significantly improving ultra-low temperature impact performance. Summary of the Invention

[0004] In view of this, the present invention provides a low-temperature toughness optimization method for medium and thick plates with short-process multi-stage phase transformation control, which is used to solve the problems of complicated rolling process, large fluctuations in post-rolling performance and contradiction between strength and low-temperature toughness optimization in the production process of low-carbon microalloy medium and thick steel plates with a carbon content of 0.03wt%~0.05wt%.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a method for optimizing the low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process, which is used for low-carbon microalloyed steel with a carbon content of 0.03wt% to 0.05wt%, specifically comprising the following steps:

[0006] S1. The steel billet is initially heated to the austenite recrystallization temperature range of 1150°C to 1200°C, kept at this temperature for 1 to 2 hours, and then cooled to the room temperature of the steel billet surface by an ultra-fast cooling device. After the steel billet returns to red, it is secondarily cooled to the room temperature by an ultra-fast cooling device to obtain a homogenized steel billet.

[0007] S2. reheating the homogenized steel billet to 815° C. to 845° C., holding the temperature for 2 h to 4 h, and then performing one-stage rolling with a starting rolling temperature of 780° C. to 830° C. and a total rolling reduction ratio of 3.6:1 to 4:1;

[0008] S3. The steel plate obtained by one-stage rolling enters a laminar cooling device, is laminar-cooled until the surface temperature of the steel plate returns to 310°C~330°C, and is air-cooled to room temperature; the final steel plate thickness is ≥30mm, and its core structure includes ferrite with a volume fraction of 85%~95% and bainite with a volume fraction of 5%~15%, an average effective grain size of 2.2μm~3.2μm, and a standard deviation of ≤4.0μm.

[0009] The primary cooling rate is 100°C / s to 200°C / s; the secondary cooling rate is 40°C / s to 70°C / s.

[0010] In step S1, the duration of the steel billet returning to red is ≤60s.

[0011] The size of the original austenite grains formed after the homogenized steel billet is secondary heated is 17 μm to 22 μm.

[0012] The one-stage rolling includes 5 to 7 rolling passes, with the same reduction in each pass; the total rolling reduction ratio is 3.6:1 to 3.8:1.

[0013] The cooling rate of the laminar cooling is 10°C / s to 20°C / s, and the cooling start temperature is 750°C to 780°C.

[0014] The ferrite is polygonal ferrite, the bainite is granular bainite, and nano-scale granular MA islands exist at some grain boundaries of the core structure.

[0015] The chemical composition of the low-carbon microalloyed steel also includes: Mn: 1.55wt%~2.05wt%, Si: 0.15wt%~0.30wt%, Ni: 0.50wt%~1.95wt%, Cu: 0.10wt%~0.60wt%, Nb: 0.01wt%~0.05wt%, Ti: 0.01wt%~0.05wt%, Mo: 0.05wt%~0.50wt%, S<0.008wt%, P<0.008wt%, Fe and other inevitable elements.

[0016] When the temperature of the final steel plate is -80°C, the impact energy of the core is ≥200J; when the temperature of the final steel plate is -120°C, the impact energy of the core is ≥100J.

[0017] The yield strength of the final steel plate is ≥540 MPa, the yield strength ratio is ≤0.85, and the elongation is ≥18%.

[0018] Beneficial effects of the present invention: According to the above technical solution, the present invention provides a method for optimizing low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process. By precisely controlling the parameters of the two-stage heating, two-stage ultra-fast cooling, one-stage rolling, and post-rolling cooling processes, the grain size of the final microstructure is refined under low compression ratio conditions, forming a two-phase structure with ferrite as the main phase and bainite as the auxiliary phase, significantly optimizing low-temperature toughness while ensuring strength. This method has been successfully applied to the industrial production of 35mm thick shipbuilding steel. The impact energy of the core at -120°C is stable at ≥100J. Compared with the traditional two-stage process of rough rolling and finishing rolling, the one-stage rolling process of the present invention has a short process flow, low energy consumption, and the microstructure of the final steel plate is more uniform, with good industrial application value and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Various other advantages and benefits will become apparent to those skilled in the art by reading the detailed description of the preferred embodiment below. The accompanying drawings are only for the purpose of illustrating the preferred embodiment and are not to be considered as limiting the present invention, wherein:

[0020] Figure 1 This is a distribution diagram of the original austenite grains in the core structure of the steel plate after secondary heating in Example 1 of the present invention;

[0021] Figure 2 This is a statistical diagram of the size distribution of the original austenite grains in the core of the steel plate after secondary heating in Example 1 of the present invention;

[0022] Figure 3This is a schematic diagram of a typical metallographic structure at 1 / 2 thickness of a steel plate according to Example 1 of the present invention;

[0023] Figure 4 Schematic diagram of the core sample of the steel plate of Example 1 of the present invention after impact at -120°C; (a) is a macro SEM image, and (b) is an enlarged image of the crack initiation zone;

[0024] Figure 5 This is the original austenite grain distribution diagram of the core structure of the steel plate after secondary heating in Comparative Example 1 of the present invention;

[0025] Figure 6 Schematic diagram of the core sample of the steel plate of Comparative Example 1 after impact at -120°C; (a) is a macro SEM image, and (b) is an enlarged image of the crack initiation area;

[0026] Figure 7 This is the original austenite grain distribution diagram of the core structure of the steel plate of Comparative Example 2 of the present invention after secondary heating;

[0027] Figure 8 Schematic diagram of the core sample of the steel plate of comparative example 2 after impact at -120°C; (a) is a macro SEM image, and (b) is an enlarged image of the crack initiation area. DETAILED DESCRIPTION

[0028] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. Among them, the accompanying drawings are only for illustrative purposes and only represent schematic diagrams, not physical drawings, and cannot be understood as limiting the present invention. In order to better illustrate the embodiments of the present invention, it is understandable to those skilled in the art that some well-known structures or steps in the accompanying drawings and their descriptions may be omitted.

[0029] A method for optimizing low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process specifically comprises the following steps:

[0030] A low-carbon microalloyed steel billet with a carbon content of 0.03wt% to 0.05wt% is heated to the austenite recrystallization temperature range of 1150°C to 1200°C and held for 1-2 hours to ensure complete austenite homogenization. An ultra-rapid cooling system is then used for primary cooling at a cooling rate of 100°C / s to 200°C / s, rapidly cooling the billet surface to room temperature. During this process, the return to red heat time is controlled to ≤60 seconds. A secondary cooling process is then carried out at a cooling rate of 40°C / s to 70°C / s to room temperature, further preparing the microstructure for the secondary austenitization heating, ultimately resulting in a homogenized billet. Compared to conventional cooling, laminar cooling, and accelerated cooling techniques, ultra-rapid cooling offers higher cooling rates and better cooling uniformity. After solutionizing, ultra-rapid cooling allows the steel plate to cool rapidly, passing through the austenite transformation zone in a shorter time, resulting in a finer microstructure and inhibiting precipitation. This provides a higher density of nucleation sites for subsequent transformations, allowing the steel plate to form finer austenite grains at the same subsequent heating temperature.

[0031] The homogenized steel slab is reheated to 815°C–845°C and held for 2–4 hours to precisely control the prior austenite grain size to 17–22 μm. The lower heating temperature above the critical region for complete austenite transformation and the relatively long holding time achieve more complete austenitization of the steel plate while suppressing grain growth, resulting in a finer prior austenite grain size. Subsequently, rolling is performed in a single-stage process within the 780°C–830°C temperature range, with a total reduction ratio of 3.6:1–4:1 (preferably 3.6:1–3.8:1), completed in 5–7 passes with equal reductions per pass. This process leverages the inherited microstructure characteristics combined with the accumulated dislocation density from multi-pass uniform deformation to accelerate the phase transformation dynamics while suppressing dynamic recrystallization, preserving deformation energy and providing the driving force for the ferrite phase transformation.

[0032] After rolling, the steel plate enters a laminar flow cooling system and is cooled at a rate of 10°C / s to 20°C / s to a surface red-hot temperature of 310°C to 330°C. The initial cooling temperature is 750°C to 780°C, followed by air cooling to room temperature. Precise control of the final cooling temperature aligns the bainite transformation rate with the ferrite growth rate, forming a dual-phase structure with polygonal ferrite (85% to 95%) as the matrix and granular bainite (5% to 15%) dispersed at the grain boundaries. Nanoscale granular MA islands are also distributed at the grain boundaries to further hinder crack propagation. The final average effective grain size in the core of the steel plate is 2.2μm to 3.2μm, with a uniform size distribution and a standard deviation of ≤4.0μm. To ensure process suitability, the chemical composition of the low-carbon microalloyed steel also includes: Mn: 1.55wt%-2.05wt%, Si: 0.15wt%-0.30wt%, Ni: 0.50wt%-1.95wt%, Cu: 0.10wt%-0.60wt%, Nb: 0.01wt%-0.05wt%, Ti: 0.01wt%-0.05wt%, Mo: 0.05wt%-0.50wt%, S <0.008wt%, P <0.008wt%, Fe, and other unavoidable elements. The resulting steel exhibits excellent low-temperature toughness (core impact energy ≥200J at -80°C and ≥100J at -120°C). Furthermore, it exhibits a well-balanced combination of strength and ductility, with yield strength ≥540MPa, yield strength ratio ≤0.85, and elongation ≥18%. This improves the overall performance of the steel and contributes to a longer service life.

[0033] The following describes the specific embodiments of the present invention in further detail with reference to the accompanying drawings, examples, and comparative examples. To reduce experimental errors, the chemical composition of the steel billets in the examples and comparative examples is as follows: C: 0.05 wt%, Mn: 1.85 wt%, Si: 0.20 wt%, Ni: 0.96 wt%, Cu: 0.25 wt%, Nb: 0.025 wt%, Ti: 0.02 wt%, Mo: 0.20 wt%, S < 0.008 wt%, P < 0.008 wt%, with the remainder being Fe and other unavoidable elements.

[0034] Example 1

[0035] The low carbon microalloyed steel billet is subjected to the following steps:

[0036] 1. Primary heating and cooling: The billet is heated to 1150℃ and kept at this temperature for 2 hours. The ultra-fast cooling device is used to ultra-fast cool the billet to room temperature at a primary cooling rate of 100℃ / s. After a return to red temperature of about 10 seconds, the billet is cooled to room temperature at a secondary cooling rate of 40℃ / s.

[0037] 2. Secondary heating and one-stage rolling: In this embodiment, in order to suppress grain growth and coarsening, a secondary heating temperature of 815°C, which is closer to the complete transformation point of austenite, was selected and kept warm for 2.5 hours before one-stage rolling was carried out directly. The starting rolling temperature was 780°C, and the number of rolling passes was 7 (with the same reduction in each pass). In order to be more conducive to industrial applications, the total compression ratio was 3.75:1, and the final thickness of the formed steel plate was 35 mm.

[0038] 3. Laminar cooling + air cooling: The post-rolling cooling process adopts a combination of laminar cooling and air cooling. The starting cooling temperature is 750°C, the laminar cooling rate is 20°C / s, and the laminar cooling is performed until the surface reddening temperature reaches 330°C, followed by air cooling to room temperature.

[0039] Example 2

[0040] 1. Primary Heating and Cooling: Heat to 1200°C and hold for 2 hours. Ultra-rapid cooling is performed using an ultra-fast cooling device at a primary cooling rate of 200°C / s to room temperature on the billet surface. After a 20-second return to red heat, the billet is cooled to room temperature at a secondary cooling rate of 70°C / s. Because the primary cooling rate is so high that the core has insufficient time to cool, a higher secondary cooling rate is used after red heat. This not only prevents the steel plate surface from recovering due to red heat, but also rapidly cools the core to avoid affecting the original austenite grain size after secondary heating.

[0041] 2. Secondary heating and one-stage rolling: Considering the relatively fine original structure formed by the two high cooling rates in the previous process, in order to improve production efficiency, the secondary heating temperature can be appropriately increased to 845°C. To avoid grain coarsening, the holding time is shortened to 2 hours, and then one-stage rolling is directly carried out. The starting rolling temperature is set to 830°C, and the number of rolling passes is 7 (the reduction amount in each pass is the same). In order to be more conducive to industrial application, the total reduction ratio is 3.75:1, and the final formed steel plate thickness is 35mm.

[0042] 3. Laminar Cooling + Air Cooling: The post-rolling cooling process utilizes a combination of laminar and air cooling. The initial cooling temperature is 780°C, the laminar cooling rate is 10°C / s, and laminar cooling is performed to a surface red-hot temperature of 310°C, followed by air cooling to room temperature. Controlling the red-hot temperature during this step is crucial to ensure the final microstructure has appropriate ferrite and bainite contents after cooling. A too low red-hot temperature can easily alter the ferrite and bainite morphologies and increase the bainite content, affecting low-temperature toughness. A too high temperature can promote ferrite transformation, inhibit bainite formation, and reduce the service strength of the steel plate.

[0043] The statistical analysis of the microstructures after secondary heating of Example 1 and Example 2 revealed that the original austenite grain size of Example 2 was 21.97 μm, while that of Example 1 was slightly smaller. Figure 1The microstructure of the original austenite grains after secondary heating of Example 1 is shown in FIG. 1 , wherein the average grain size is 18.57 μm and the statistics of the grain size distribution are shown in FIG. Figure 2 As shown in the figure, it can be seen that the original austenite grain size is small and the distribution is relatively uniform. The proportion of grains larger than 30μm is relatively low. Such original austenite structure is conducive to obtaining a finer microstructure in the subsequent cooling process, thereby improving the low-temperature toughness of the steel plate. The main reasons for the smaller original austenite grain size after secondary heating in the two embodiments are: (1) at a lower secondary heating temperature, the driving force is small, the growth of austenite will be restricted, and the residual precipitates in the matrix have a strong pinning effect on the grain boundaries, which can effectively inhibit the growth of austenite grains; (2) the two ultra-fast cooling processes in the previous process store sufficient phase transformation energy, providing a higher density of nucleation sites for secondary austenitization. In addition, compared with recrystallized austenite, deformed austenite has more deformation storage energy and can effectively enhance the driving force of phase transformation. Therefore, the total rolling reduction ratio is controlled in the range of 3.6~4:1, especially in the range of 3.6:1~3.8:1. Then, through subsequent laminar cooling with a lower cooling rate and appropriate red-return temperature control, the steel plate is more likely to undergo ferrite phase transformation and the structure after phase transformation is finer, ultimately optimizing the low-temperature toughness.

[0044] The microstructures of both examples are mainly polygonal ferrite, and contain a small amount of granular bainite to improve the service strength. Since the red-hot temperature of Example 1 is higher, the content of polygonal ferrite increases accordingly. The specific metallographic structure is as follows: Figure 3 As shown. The final core structure of the steel plate obtained in Example 1 includes about 91% polygonal ferrite and 7% granular bainite, and granular MA islands also exist at some grain boundary positions. Through EBSD grain boundary statistics, it was found that the average effective grain size of the core structure of the steel plate of Example 1 is 2.92μm, and the standard deviation is 3.32μm. It can be seen that the final structure inherits the organizational characteristics of the previous process, maintains a smaller grain size and a more uniform size distribution. Further mechanical property tests show that the core of the steel plates of the two groups of examples have good comprehensive mechanical properties and excellent low-temperature toughness. The test results show that at -120°C, the impact energy of the steel plate of Example 1 can reach 119.6J, and the impact energy of the steel plate of Example 2 can also reach 107.7J. Among them, the macroscopic and microscopic morphologies of the impact fracture of Example 1 at -120°C are as follows Figure 4 To ensure the reliability of the data, the above tests were repeated on three or more samples for each experiment and the average value was obtained.

[0045] Through TMCP technology, a variety of microstructures can be regulated in steel, such as acicular ferrite, polygonal ferrite, granular bainite, lath bainite and martensite, etc. Different tissue contents and morphological distributions directly affect the mechanical properties of the steel plate. For the low-carbon microalloy steel with a carbon content of 0.03wt%~0.05wt% applicable to the present invention, due to its low carbon content, it can ensure good welding performance during service. However, as an important alloy strengthening element, the reduction of carbon content also means that the strength improvement is limited. How to ensure the strength of the steel plate while improving the toughness through tissue regulation is the main problem to be solved by the present invention. The present invention utilizes the genetic characteristics of the tissue to refine the controlled rolling and controlled cooling process parameters to regulate a microstructure with an average effective grain size of 2.2μm~3.2μm in the core of the steel plate and includes polygonal ferrite with a volume fraction of 85%~95% and granular bainite with a volume fraction of 5~15%. Granular MA islands also exist at some grain boundaries. According to the classic Hall-Petch relationship, grain refinement can simultaneously improve the strength and toughness of the material, so regulating the grain size in the range of 2.2μm~3.2μm is beneficial to improving the comprehensive mechanical properties of the steel plate. Studies have found that the behavior of crack propagation is closely related to the distribution of dislocations. Compared with the diffusion-type ferrite phase transformation, bainite with a lower transformation temperature often has a higher dislocation density, which can promote the increase in strength, but it is also more likely to reach the deformation limit and fracture. Therefore, in order to give full play to the role of ferrite in preventing crack propagation, 85~95% polygonal ferrite structure is regulated as the matrix structure of the core of the steel plate to improve the low-temperature toughness of the steel plate. At the same time, 5~15% granular bainite structure is regulated and distributed on the polygonal ferrite matrix to ensure sufficient strength level; and the smaller granular MA islands can effectively passivate the crack tip and improve the crack resistance of the steel plate.

[0046] Comparative Example 1

[0047] Compared with Example 1, in Comparative Example 1, the initial heating and heat preservation were followed by direct water cooling to room temperature, without the secondary ultra-fast cooling process, and other parameters remained unchanged. The test results showed that since the original structure before the secondary heating was coarser than the secondary ultra-fast cooling structure, according to the genetic characteristics of the structure, the original austenite grain size of Comparative Example 1 increased to 29.4μm after the secondary heating, as shown in Figure 1. Figure 5 As shown in Figure 2, the microstructure after cooling is coarser due to the larger size of the original austenite grains. Statistical results show that the average effective grain size of the comparative steel plate increases to 4.1 μm, and its -120°C impact energy also decreases to 19.8 J. Figure 6 As shown, its fracture surface shows obvious cleavage characteristics.

[0048] Comparative Example 2

[0049] Compared with Example 1, the secondary heating temperature of Comparative Example 2 was increased to 920°C, and other parameters remained unchanged. The test results showed that the higher austenitizing temperature accelerated the growth and coarsening of grains. The original austenite grain size of Comparative Example 2 after secondary heating increased to 31.5μm. Figure 7 As shown in Figure 2, the final average effective grain size increases to 4.4 μm, and the impact energy at -120 °C also drops sharply to 5.3 J, as shown in Figure 2. Figure 8 As shown, its fracture surface also shows obvious cleavage characteristics.

[0050] Table 1 Schematic table of low temperature impact toughness of steel plates of embodiments of the present invention and comparative examples

[0051]

[0052] The specific mechanical property parameters of Examples 1 and 2 and the two comparative examples are shown in Table 1. When the cooling parameters of the primary heating and the secondary heating temperature exceed the limit range of the examples, the increase in the original austenite grain size will result in the inability to optimize the core structure of the steel plate during the subsequent cooling process. In Example 1, after secondary heating to 815°C and holding, the ferrite phase transformation range is expanded through the control of the rolling process and the red-hot temperature. During its nucleation and growth, the continuous consumption of the original austenite grains reduces the nucleation site of bainite, thereby inhibiting the formation of the bainite phase. Therefore, the steel plate is more likely to undergo ferrite phase transformation to form polygonal ferrite. The competitive nucleation and growth of a large amount of ferrite further limits the growth space of the grains, ultimately promoting the effective refinement of the grain size. In Comparative Examples 1 and 2, due to the increase in the original austenite grain size, under the same rolling and cooling conditions, the final microstructural composition changes, the grain size is significantly coarsened, and the low-temperature toughness is deteriorated. Furthermore, the study found that, based on Example 1, only the surface red-hot temperature in step S3 was changed; when the red-hot temperature was set to 250°C and other parameters remained unchanged, the ferrite content in the final microstructure of the steel plate dropped sharply and its morphology changed, with the matrix structure dominated by lath bainite. This led to a significant decrease in the steel plate's low-temperature toughness and brittle fracture at -120°C. However, when the red-hot temperature was set to 350°C and other parameters remained unchanged, the granular bainite content in the final microstructure of the steel plate was significantly reduced, with its content below 5%. Tensile test results showed that at this point, the strength of the steel plate dropped below 540 MPa, indicating that a moderately dispersed content of granular bainite is beneficial for improving the strength of the steel plate.

[0053] In summary, the medium and thick plate steel prepared in the embodiment of the present invention, while ensuring the thickness, refines the grain size of the final microstructure by controlling the original austenite grain size and combining the rolling and cooling processes, forming a two-phase structure with polygonal ferrite as the main phase and granular bainite as the auxiliary phase, significantly optimizing the low-temperature toughness while ensuring the strength.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A method for optimizing low-temperature toughness of medium and thick plates by controlling multi-stage phase transformation in a short process, characterized in that: For low carbon microalloyed steel with a carbon content of 0.03wt% to 0.05wt%, the method specifically includes the following steps: S1. The steel billet is initially heated to the austenite recrystallization temperature range of 1150°C to 1200°C, kept at this temperature for 1 to 2 hours, and then cooled to the room temperature of the steel billet surface by an ultra-fast cooling device. After the steel billet returns to red, it is secondarily cooled to the room temperature by an ultra-fast cooling device to obtain a homogenized steel billet. S2. reheating the homogenized steel billet to 815° C. to 845° C., holding the temperature for 2 h to 4 h, and then performing one-stage rolling with a starting rolling temperature of 780° C. to 830° C. and a total rolling reduction ratio of 3.6:1 to 4:1; S3. The steel plate obtained by one-stage rolling enters a laminar cooling device, is laminar-cooled until the surface temperature of the steel plate returns to 310°C-330°C, and is air-cooled to room temperature; the final steel plate has a thickness of ≥30 mm, a core structure comprising 85%-95% by volume of ferrite and 5%-15% by volume of bainite, an average effective grain size of 2.2μm-3.2μm, and a standard deviation of ≤4.0μm; The ferrite is polygonal ferrite, the bainite is granular bainite, and nano-scale granular MA islands exist at some grain boundaries of the core structure.

2. The low-temperature toughness optimization method for medium and thick plate with short-process multi-stage phase transformation control according to claim 1 is characterized in that: The primary cooling rate is 100°C / s to 200°C / s; the secondary cooling rate is 40°C / s to 70°C / s.

3. The low-temperature toughness optimization method for medium and thick plate with short-process multi-stage phase transformation control according to claim 1 is characterized in that: In step S1, the duration of the steel billet returning to red is ≤60s.

4. The low-temperature toughness optimization method for medium and thick plate with short-process multi-stage phase transformation control according to claim 1 is characterized in that: The size of the original austenite grains formed after the homogenized steel billet is secondary heated is 17 μm to 22 μm.

5. The low-temperature toughness optimization method for medium and thick plate with short-process multi-stage phase transformation control according to claim 1 is characterized in that: The one-stage rolling includes 5 to 7 rolling passes, with the same reduction in each pass; the total rolling reduction ratio is 3.6:1 to 3.8:

1.

6. The low-temperature toughness optimization method for medium and thick plate with short-process multi-stage phase transformation control according to claim 1 is characterized in that: The cooling rate of the laminar cooling is 10°C / s to 20°C / s, and the cooling start temperature is 750°C to 780°C.

7. The method for optimizing low-temperature toughness of medium and thick plates by controlling short-process multi-stage phase transformation according to claim 1, characterized in that: The chemical composition of the low-carbon microalloyed steel also includes: Mn: 1.55wt%~2.05wt%, Si: 0.15wt%~0.30wt%, Ni: 0.50wt%~1.95wt%, Cu: 0.10wt%~0.60wt%, Nb: 0.01wt%~0.05wt%, Ti: 0.01wt%~0.05wt%, Mo: 0.05wt%~0.50wt%, S<0.008wt%, P<0.008wt%, Fe and other inevitable elements.

8. The method for optimizing low-temperature toughness of medium and thick plates by controlling short-process multi-stage phase transformation according to claim 1, characterized in that: When the temperature of the final steel plate is -80°C, the impact energy of the core is ≥200J; when the temperature of the final steel plate is -120°C, the impact energy of the core is ≥100J.

9. The method for optimizing low-temperature toughness of medium and thick plates by controlling short-process multi-stage phase transformation according to claim 1, characterized in that: The yield strength of the final steel plate is ≥540 MPa, the yield strength ratio is ≤0.85, and the elongation is ≥18%.

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