Low-temperature toughness optimization method for short-process multi-stage phase change control of medium-thickness plate

Abstract

The invention belongs to the technical field of steel and iron material processing and preparation, and discloses a low-temperature toughness optimization method for short-process multi-stage phase change control of a medium-thickness plate. Through the synergistic effect of two times of heating, two times of ultrafast cooling, one-stage rolling and cooling after rolling, the problem of refining the core structure of the medium-thickness plate under the low compression ratio is solved. After primary heating and heat preservation, ultrafast cooling is conducted to the surface room temperature, and secondary cooling is conducted after re-reddening; carrying out secondary heating and heat preservation to regulate and control the grain size of the original austenite, and carrying out multi-pass uniform rolling; and after rolling, laminar cooling is performed to reach the self-tempering temperature of 310-330 DEG C, and a polygonal ferrite and granular bainite double-phase structure is obtained after air cooling. The core impact energy of the steel plate produced through the method is not smaller than 200 J and not smaller than 100 J at the temperature of-80 DEG C and-120 DEG C respectively, the yield strength is larger than or equal to 540 MPa, and the steel plate has high strength and good low-temperature toughness, is suitable for the field of ocean 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 particularly relates to a method for optimizing low-temperature toughness by multi-stage phase transformation control in a short process of medium and heavy plates. Background Art

[0002] In recent years, with the rapid growth of the exploration needs in extreme regions such as the deep sea and polar regions, marine engineering equipment has gradually extended to more extreme environments with greater depth (>1000m) and lower temperature (average annual temperature < -30°C). As the medium / large thickness ship plate steel (≥30mm), which is a key structural material for ships, its service conditions have expanded from conventional sea areas to the harsh working conditions of the coupled action of deep-sea high pressure and polar ultra-low temperature. Under this background, the market has put forward higher requirements for the strength-toughness matching of ship plate steel, especially its low-temperature toughness. Refining the microstructure by thermo-mechanical control process (TMCP) is one of the effective means to improve the strength of steel plates while ensuring good toughness. Its principle is mainly to refine austenite grains by controlled rolling, and combine the accumulated processing strain with subsequent controlled cooling to regulate and refine the phase transformation microstructure.

[0003] However, the current traditional TMCP technology still faces challenges in the production of steel plates with a thickness of more than 30mm by refining the austenite grain size to improve the low-temperature toughness of materials. First, due to the low rolling reduction ratio in the core of thick plates (usually the rolling reduction ratio ≤ 4:1), insufficient deformation penetration occurs, making it difficult to fully refine austenite grains, and the problem of coarsening of the core microstructure is prominent, seriously affecting the low-temperature impact performance. Second, there is a defect of a large difference in the cooling rate between the surface and the core of the steel plate under the existing cooling process, which easily leads to non-uniform final microstructure, such as the enrichment of bainite on the surface and the coarsening of ferrite in the core. In addition, the traditional process often uses multi-pass rolling and complex cooling paths to obtain fine-grained microstructures, resulting in high energy consumption, low production efficiency, and low adaptability to the requirements of industrial large-scale production. In the prior art, the industry mostly adopts a two-stage controlled rolling process in the austenite recrystallization zone and the non-recrystallization zone to regulate the microstructure. The patent CN116200682B discloses a manufacturing method of a high-strength and tough low-temperature marine steel plate, which obtains ferrite and granular bainite microstructures through controlled rolling and controlled cooling combined with low-temperature tempering, and the impact energy at -80°C ≥ 70J, but the volume content of ferrite ≤ 10% and the carbon content of the applicable steel grades is above 0.085%. The patent CN104372257A discloses a method for improving the strength and toughness of medium and heavy plates by using post-red heat, which adopts a TMCP process combined with stacking cooling treatment to obtain a mixed microstructure of acicular ferrite + granular bainite, but its cooling process does not involve two ultra-fast cooling technologies, and the control range of the post-red heat temperature is relatively wide (250~400°C), which easily leads to insufficient tissue stability. Therefore, in summary, there is an urgent need for a short-process and highly controllable low-temperature toughening method for medium and heavy plates to achieve ultra-fine refinement and high uniformity of the core microstructure under low reduction ratio conditions, and significantly improve the extremely low-temperature impact performance. Summary of the Invention

[0004] In view of this, the present invention provides a method for optimizing the low-temperature toughness by multi-stage phase transformation control in a short process for medium-thick plates, so as to solve the problems of cumbersome rolling processes, large fluctuations in post-rolling properties, and contradictions between strength and low-temperature toughness optimization during the production of low-carbon microalloy medium-thick steel plates with a carbon content of 0.03wt% - 0.05wt%.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for optimizing the low-temperature toughness by multi-stage phase transformation control in a short process for medium-thick plates, which is used for low-carbon microalloy steel with a carbon content of 0.03wt% - 0.05wt%, specifically includes the following steps:

[0006] S1. The steel billet is initially heated to the austenite recrystallization temperature range of 1150°C - 1200°C, and after holding for 1h - 2h, it is initially cooled to room temperature on the surface of the steel billet through an ultra-fast cooling device. After the steel billet turns red again, it is secondarily cooled to room temperature through the ultra-fast cooling device to obtain a homogenized steel billet;

[0007] S2. The homogenized steel billet is reheated to 815°C - 845°C, and after holding for 2h - 4h, one-stage rolling is carried out. The starting rolling temperature is 780°C - 830°C, and the total rolling reduction ratio is 3.6:1 - 4:1;

[0008] S3. The steel plate obtained by one-stage rolling enters a laminar flow cooling device, and is cooled by laminar flow until the surface temperature of the steel plate turns red to 310°C - 330°C, and then 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%, and the average effective grain size is 2.2μm - 3.2μm, and the standard deviation is ≤4.0μm.

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

[0010] The duration of the steel billet turning red in step S1 is ≤60s.

[0011] The original austenite grain size formed after the secondary heating of the homogenized steel billet is 17μm - 22μm.

[0012] The one-stage rolling includes 5 - 7 passes of rolling, and the reduction per pass is the same; the total rolling reduction ratio is 3.6:1 - 3.8:1.

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

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

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

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

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

[0018] Advantages of the present invention: According to the above technical solution, a method for optimizing the low-temperature toughness by multi-stage phase transformation control in a short process of medium and heavy plates provided by the present invention refines the grain size of the final microstructure under a lower reduction ratio through precise regulation of the parameters of two heating processes, two ultra-fast cooling processes, one-stage rolling, and post-rolling cooling processes, forming a duplex structure mainly composed of ferrite and supplemented by bainite, and significantly optimizing the low-temperature toughness on the premise of ensuring strength. This method has been successfully applied to the industrial production of 35 mm thick shipbuilding steel. The impact energy of the core at -120 °C is stably ≥100 J. And compared with the traditional two-stage process of rough rolling and finish rolling, the one-stage rolling process of the present invention has a short process flow, low energy consumption, and better uniformity of the microstructure of the final steel plate, and has good industrial application value and economic benefits. Description of the Drawings

[0019] By reading the detailed description of the preferred embodiments below, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention, wherein:

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

[0021] Figure 2 is the statistical chart of the original austenite grain size distribution of the core structure of the steel plate after secondary heating in Example 1 of the present invention;

[0022] Figure 3It is a schematic diagram of the typical metallographic structure at the 1 / 2 thickness of the steel plate in Embodiment 1 of the present invention;

[0023] Figure 4 It is a schematic diagram of the specimen at the core of the steel plate in Embodiment 1 of the present invention after impact at -120°C; (a) is the macroscopic SEM image, and (b) is the enlarged view of the crack initiation area;

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

[0025] Figure 6 It is a schematic diagram of the specimen at the core of the steel plate in Comparative Example 1 after impact at -120°C; (a) is the macroscopic SEM image, and (b) is the enlarged view of the crack initiation area;

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

[0027] Figure 8 It is a schematic diagram of the specimen at the core of the steel plate in Comparative Example 2 after impact at -120°C; (a) is the macroscopic SEM image, and (b) is the enlarged view of the crack initiation area. Detailed implementation manners

[0028] The following illustrates the implementation manners 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 content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. Among them, the drawings are only for illustrative purposes, showing only schematic diagrams rather than actual diagrams, and should not be construed as a limitation to the present invention; for better illustration of the embodiments of the present invention, it is understandable that some well-known structures or steps and their descriptions in the drawings may be omitted for those skilled in the art.

[0029] A method for optimizing the low-temperature toughness by short-process multi-stage phase transformation control of medium and heavy plates specifically includes the following steps:

[0030] The billet of low-carbon microalloyed steel with a carbon content of 0.03wt% - 0.05wt% is heated to the austenite recrystallization temperature range of 1150°C - 1200°C and held for 1 - 2 hours to ensure complete homogenization of austenite. Subsequently, an ultra-fast cooling device is used for primary cooling at a cooling rate of 100°C / s - 200°C / s to rapidly cool the surface of the billet to room temperature. During this process, the return red time is controlled ≤ 60 seconds, and then secondary cooling is carried out at a cooling rate of 40°C / s - 70°C / s to room temperature to further prepare the microstructure for secondary heating austenitization, and finally a homogenized billet is obtained. Compared with conventional cooling, laminar cooling, and accelerated cooling technologies, the ultra-fast cooling has a high cooling rate and better cooling uniformity. After solution treatment, using ultra-fast cooling can rapidly cool the steel plate, pass through the austenite phase transformation range in a short time, obtain finer microstructures and inhibit precipitation, providing a higher density of nucleation sites for subsequent phase transformations, so that the steel plate forms finer austenite grains at the same subsequent heating temperature.

[0031] The homogenized billet is reheated to 815°C - 845°C and held for 2 - 4 hours to precisely control the original austenite grain size within 17μm - 22μm. The relatively low heating temperature above the critical region for complete austenite transformation and the relatively long holding time can make the austenitization of the steel plate more complete while inhibiting grain growth, thereby obtaining a finer original austenite grain size. Subsequently, one-stage rolling is carried out in the temperature range of 780°C - 830°C, with a total reduction ratio of 3.6:1 - 4:1 (preferably 3.6:1 - 3.8:1), completed in 5 - 7 passes with the same reduction per pass. This process combines the tissue genetic characteristics with multi-pass uniform deformation to accumulate dislocation density, accelerates the phase transformation driving force while inhibiting dynamic recrystallization, retains the deformation energy storage, and provides a driving force for ferrite phase transformation.

[0032] The rolled steel plate enters the laminar cooling device and is cooled at a cooling rate of 10°C / s to 20°C / s to a surface recrystallization temperature of 310°C to 330°C. The starting cooling temperature is 750°C to 780°C, and then it is air-cooled to room temperature. The precise control of the final cooling temperature enables the bainite phase transformation rate to match the ferrite growth rate, forming a duplex structure with polygonal ferrite (85% - 95%) as the matrix and granular bainite (5% - 15%) dispersed at the grain boundaries. Granular M-A islands at the nanoscale are also distributed at the grain boundaries to further hinder crack propagation. Finally, the average effective grain size at the center of the steel plate is 2.2μm to 3.2μm, and the size distribution is uniform with a standard deviation ≤ 4.0μm. To ensure the applicability of the process, 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. The mechanical properties of the final steel plate are as follows: good low-temperature toughness, that is, the impact energy at the center at -80°C ≥ 200J, and the impact energy at the center at -120°C ≥ 100J; at the same time, it has a good match between strength and plasticity, that is, the yield strength ≥ 540MPa, the yield ratio ≤ 0.85, and the elongation ≥ 18%, improving the comprehensive performance of the steel plate and being beneficial to extending the service life.

[0033] The following further details the specific implementation manners of the present invention in conjunction with the accompanying drawings, examples, and comparative examples. Among them, to reduce the experimental error, the chemical compositions of the steel billets in the examples and comparative examples are: C: 0.05wt%, Mn: 1.85wt%, Si: 0.20wt%, Ni: 0.96wt%, Cu: 0.25wt%, Nb: 0.025wt%, Ti: 0.02wt%, Mo: 0.20wt%, S < 0.008wt%, P < 0.008wt%, and the balance is Fe and other inevitable elements.

[0034] Example 1

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

[0036] 1. Primary heating and cooling: The steel billet is heated to 1150°C and held for 2 hours, and then rapidly cooled to room temperature at a primary cooling rate of 100°C / s by the ultra-rapid cooling device. After the recrystallization time of about 10 seconds, it is cooled to room temperature at a secondary cooling rate of 40°C / s.

[0037] 2. Reheating and one - pass rolling: In this embodiment, in order to inhibit grain growth and coarsening, a reheating temperature closer to the complete transformation point of austenite, 815 °C, was selected. After holding for 2.5 hours, one - pass rolling was directly carried out. Among them, the rolling start temperature was 780 °C, the number of rolling passes was 7 (the reduction per pass was the same). For more favorable industrial application, the total reduction ratio was 3.75:1, and the thickness of the finally formed steel plate was 35 mm.

[0038] 3. laminar cooling + air cooling: The post - rolling cooling process adopted the combination of laminar cooling and air cooling processes. Among them, the cooling start temperature was 750 °C, the laminar cooling rate was 20 °C / s, and after laminar cooling to the surface recrystallization temperature of 330 °C, it was air - cooled to room temperature.

[0039] Example 2

[0040] 1. Primary heating and cooling: Heat to 1200 °C and hold for 2 hours. The ultra - fast cooling device ultra - fast cools to the room temperature on the surface of the steel billet at a primary cooling rate of 200 °C / s. After the recrystallization time of 20 seconds, it is cooled to room temperature at a secondary cooling rate of 70 °C / s. Due to the relatively fast cooling rate of the primary cooling, the core part has no time to cool. Therefore, after recrystallization, a higher secondary cooling rate is controlled. On the one hand, it inhibits the recovery of the surface structure of the steel plate due to recrystallization, and on the other hand, it quickly cools the core part to avoid affecting the original austenite grain size after secondary heating.

[0041] 2. Reheating and one - pass rolling: Considering the relatively fine original structure formed under the two high - cooling - rate processes in the previous process, in order to improve production efficiency, the reheating temperature can be appropriately increased to 845 °C. To avoid grain coarsening, the holding time is shortened to 2 hours, and then one - pass rolling is directly carried out. Among them, the rolling start temperature is set at 830 °C, the number of rolling passes is 7 (the reduction per pass is the same). For more favorable industrial application, the total reduction ratio is 3.75:1, and the thickness of the finally formed steel plate is 35 mm.

[0042] 3. laminar cooling + air cooling: The post - rolling cooling process adopted the combination of laminar cooling and air cooling processes. Among them, the cooling start temperature was 780 °C, the laminar cooling rate was 10 °C / s, and after laminar cooling to the surface recrystallization temperature of 310 °C, it was air - cooled to room temperature. To ensure that the final structure after cooling has an appropriate ferrite content and bainite content, the control of the recrystallization temperature in this step is extremely important. If it is too low, it is easy to cause changes in the morphology of ferrite and bainite and an increase in the bainite content, affecting the low - temperature toughness, while if it is too high, it will promote ferrite transformation and inhibit the formation of bainite, reducing the service strength of the steel plate.

[0043] Statistics on the structure after secondary heating of Example 1 and Example 2 found that the original austenite grain size of Example 2 was 21.97 μm, while that of Example 1 was slightly smaller. As Figure 1As shown, it is the microstructure of the prior austenite grains after secondary heating in Example 1. Its average grain size is 18.57 μm, and the statistics of the grain size distribution are as follows Figure 2 As shown, it can be seen that the prior austenite grain size is small and the distribution is relatively uniform. The proportion of grains larger than 30 μm is relatively low. Such prior austenite microstructure is beneficial to obtaining a finer final microstructure during subsequent cooling, thereby improving the low-temperature toughness of the steel plate. The main reasons for the smaller prior austenite grain size in the two examples are as follows: (1) At a relatively low secondary heating temperature, the driving force is small, and the growth of austenite will be restricted. Moreover, the precipitates remaining in the matrix have a strong pinning effect on the grain boundaries, which can effectively inhibit the growth of austenite grains; (2) Enough transformation energy is stored in the two ultra-fast cooling processes in the previous process, providing a higher density of nucleation sites for secondary austenitization. In addition, compared with recrystallized austenite, deformed austenite has more deformation stored energy, which can effectively enhance the transformation driving force. Therefore, controlling the total reduction ratio of controlled rolling in the range of 3.6 - 4:1, especially in the range of 3.6:1 - 3.8:1, and then through the control of the lower cooling rate and appropriate return red temperature during subsequent laminar cooling, it is easier for the steel plate to undergo ferrite transformation and the microstructure after transformation is finer, ultimately optimizing the low-temperature toughness.

[0044] The microstructure of both groups of examples is mainly composed of polygonal ferrite, and at the same time contains a small amount of granular bainite to improve the service strength. Due to the higher return red temperature in Example 1, the content of polygonal ferrite increases accordingly. The specific metallographic structure is as follows Figure 3 As shown. The final microstructure of the steel plate obtained in Example 1 includes approximately 91% polygonal ferrite and 7% granular bainite in the core part. There are also granular M-A islands at some grain boundary positions. Through EBSD grain boundary statistics, it is found that the average effective grain size of the core microstructure of the steel plate in Example 1 is 2.92 μm, and the standard deviation is 3.32 μm. It can be seen that the final microstructure inherits the microstructure characteristics of the previous process, maintaining a small grain size and a relatively uniform size distribution. Further mechanical property tests show that the cores of the steel plates in both 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 in Example 1 can reach 119.6 J, and the impact energy of the steel plate in Example 2 can also reach 107.7 J. Among them, the macroscopic and microscopic morphologies of the impact fracture of Example 1 at -120 °C are as follows Figure 4 As shown, it is a typical ductile fracture. To ensure the reliability of the data, more than three samples are taken for each experiment for repeated experiments and the average value is obtained.

[0045] Through the TMCP technology, various microstructures can be regulated in steel, such as acicular ferrite, polygonal ferrite, granular bainite, lath bainite, and martensite, etc. The content and morphological distribution of different microstructures directly affect the mechanical properties of the steel plate. For the low-carbon microalloyed steel with a carbon content of 0.03wt% - 0.05wt% applicable to the present invention, due to its low carbon content, good welding performance can be ensured during service. However, as an important alloy strengthening element, the decrease in the content of carbon also means limited increase in strength. How to improve the toughness while ensuring the strength of the steel plate through microstructure regulation is the main problem to be solved in the present invention. The present invention utilizes the tissue inheritance characteristics to regulate a microstructure with an average effective grain size of 2.2μm - 3.2μm in the core of the steel plate by refining the thermomechanical control process parameters, which includes polygonal ferrite with a volume fraction of 85% - 95% and granular bainite with a volume fraction of 5% - 15%, and there are also granular M-A islands at some grain boundaries. According to the classical Hall-Petch relationship, grain refinement can improve the strength and toughness of materials simultaneously. Therefore, regulating the grain size range between 2.2μm and 3.2μm is beneficial to improving the comprehensive mechanical properties of the steel plate. Research has found that the behavior of crack propagation is closely related to the distribution of dislocations. Compared with the diffusional ferrite phase transformation, bainite with a lower transformation temperature often has a higher dislocation density, which can promote the increase in strength, but at the same time, it is 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% of polygonal ferrite structure is regulated as the matrix structure in the core of the steel plate to improve the low-temperature toughness of the steel plate. At the same time, 5 - 15% of granular bainite structure is regulated and distributed on the polygonal ferrite matrix to ensure a sufficient strength level; while the smaller granular M-A islands can effectively blunt 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, after the initial heating and holding, it was directly water-cooled to room temperature without including the secondary ultra-fast cooling process, and other parameters remained unchanged. The test results showed that since the original microstructure before the secondary heating was coarser than the secondary ultra-fast cooling microstructure, according to the tissue inheritance characteristics, the original austenite grain size in Comparative Example 1 increased to 29.4μm after the secondary heating, as Figure 5 shown. Due to the larger original austenite grain size, the final cooled microstructure was coarser. The statistical results showed that the average effective grain size of the steel plate in this comparative example increased to 4.1μm, and its impact energy at -120°C also decreased to 19.8J, as Figure 6 shown, and its fracture surface presented obvious cleavage characteristics.

[0048] Comparative Example 2

[0049] Compared with Example 1, the temperature of the secondary heating in 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 after secondary heating in Comparative Example 2 increased to 31.5 μm. As Figure 7 shown, its final average effective grain size increased to 4.4 μm, and the impact energy at -120 °C also dropped sharply to 5.3 J. As Figure 8 shown, its fracture surface also showed obvious cleavage characteristics.

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

[0051]

[0052] The specific mechanical property parameters of Example 1 and Example 2 and two groups of comparative examples are shown in Table 1. When the cooling parameters of the primary heating change and the secondary heating temperature exceeds the limit range of the examples, the increase in the original austenite grain size causes the microstructure of the steel plate core to be unable to be optimized during the subsequent cooling process. In Example 1, after secondary heating to 815 °C and holding, by controlling the rolling process and the return red temperature, the phase transformation range of ferrite is expanded. During its nucleation and growth process, the continuous consumption of the original austenite grains reduces the nucleation sites 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, and the competitive nucleation and growth of a large amount of ferrite further limit the growth space of grains, ultimately promoting the effective refinement of grain size. In Comparative Example 1 and Comparative Example 2, due to the increase in the original austenite grain size, under the same rolling and cooling conditions, the final microstructure composition changes, the grain size significantly coarsens, and the low-temperature toughness deteriorates. In addition, the research also found that based on Example 1, only the surface return red temperature in step S3 was changed; when the return red temperature was set to 250 °C and other parameters remained unchanged, the ferrite content in the final microstructure of the steel plate decreased sharply and its morphology changed, and the matrix microstructure was mainly lath bainite, resulting in a significant reduction in the low-temperature toughness of the steel plate, and brittle fracture occurred at -120 °C. When the return red temperature was set to 350 °C and other parameters remained unchanged, the granular bainite in the final microstructure of the steel plate significantly decreased, and its content was less than 5%. The tensile test results showed that the strength of the steel plate dropped below 540 MPa at this time, indicating that an appropriate content of granular bainite distributed dispersedly is beneficial to the improvement of the strength of the steel plate.

[0053] In summary, while ensuring the thickness, the medium and heavy plate steel prepared in the examples of the present invention refines the grain size of the final microstructure by controlling the original austenite grain size in combination with the rolling and cooling processes, forming a duplex structure mainly composed of polygonal ferrite and supplemented by granular bainite, and significantly optimizing the low-temperature toughness on the premise of ensuring 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 them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and 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 by short-process multi-stage phase transformation control of medium-thick plates, characterized in that For a low-carbon microalloyed steel with a carbon content of 0.03 wt% to 0.05 wt%, the specific steps are as follows: S1. The steel billet is initially heated to the austenite recrystallization temperature range of 1150 °C to 1200 °C, held for 1 h to 2 h, then initially cooled to room temperature on the surface of the steel billet by an ultra-fast cooling device. After the steel billet turns red again, it is secondarily cooled to room temperature by the ultra-fast cooling device to obtain a homogenized steel billet; S2. The homogenized steel billet is reheated to 815 °C to 845 °C, held for 2 h to 4 h, and then subjected to one-stage rolling. The starting rolling temperature is 780 °C to 830 °C, and the total rolling reduction ratio is 3.6:1 to 4:1; S3. The steel plate obtained by one-stage rolling enters a laminar cooling device, and is laminarly cooled until the surface temperature of the steel plate turns red to 310 °C to 330 °C, and then air-cooled to room temperature. The final steel plate has a thickness of ≥30 mm, and its core structure includes ferrite with a volume fraction of 85% to 95% and bainite with a volume fraction of 5% to 15%. The average effective grain size is 2.2 μm to 3.2 μm, and the standard deviation is ≤4.0 μm.

2. The method for optimizing the low-temperature toughness by short-process multi-stage phase transformation control of medium-thick plates according to claim 1, wherein The initial 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 short-process multi-stage phase transformation control of medium-thick plates according to claim 1, characterized in that In step S1, the duration for which the steel billet turns red is ≤60 s.

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

5. The method for optimizing low-temperature toughness by short-process multi-stage phase transformation control of medium-thick plates according to claim 1, wherein The one-stage rolling includes 5 to 7 passes of rolling, with the same reduction per pass; the total rolling reduction ratio is 3.6:1 to 3.8:

1.

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

7. The method for optimizing low-temperature toughness by short-process multi-stage phase transformation control of medium-thick plates according to claim 1, characterized in that, The ferrite is polygonal ferrite, the bainite is granular bainite, and nano-scale granular M-A islands exist at some grain boundaries of the core structure.

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

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

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

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