Component-adjustment-free time sequence regulation and control high-toughness low-carbon steel thick plate multi-strength-grade method

Abstract

The invention belongs to the technical field of metal material processing, and discloses a component-adjustment-free time sequence regulation and control high-toughness low-carbon steel thick plate multi-strength-grade method. After primary heating and heat preservation, performing rough rolling and ultrafast cooling to room temperature; the method comprises the following steps of: carrying out secondary heating and heat preservation to regulate and control the original austenite grain size, carrying out air cooling to Ac1-745 DEG C, then carrying out finish rolling, then carrying out ultrafast cooling to a self-tempering temperature of 300-350 DEG C, carrying out air cooling to obtain ferrite / bainite complex phase structures with different proportions, then carrying out tempering treatment at 580-620 DEG C for not more than 3 hours, and accurately controlling the tempering time to realize continuous and adjustable yield strength of 510-620 MPa. Under the technological conditions, the core impact energy of steel plates of all strength levels at-80 DEG C is not smaller than 200 J, the ductility is not lower than 19%, and the steel plates have controllable strength, stable and good plasticity and toughness performance and have good industrial application value.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material processing, and in particular to a method for sequentially controlling multiple strength levels of a high-toughness low-carbon steel thick plate without requiring component adjustment. Background Art

[0002] As an important engineering structural material, the synergistic improvement of strength, ductility, and toughness in low-carbon microalloyed steel has always been a key issue in the fields of metallurgy and materials. Traditional technical approaches often rely on adjusting alloy composition (such as adding microalloying elements such as Nb and V) to achieve high strength, but this leads to cost increases, reduced weldability, and deterioration in ductility and toughness. In particular, for multi-level strength requirements across a wide strength range of 510MPa to 620MPa, existing processes require repeated adjustments to composition design and the implementation of differentiated heat treatment regimes, significantly restricting production flexibility and industrial efficiency.

[0003] In recent years, industry experts have attempted to optimize microstructure and improve performance through the controlled rolling and controlled cooling (TMCP) process, but two major challenges remain. The first is a narrow microstructure control window. Conventional TMCP produces a ferrite / bainite duplex structure that struggles to achieve both high strength and ductility. While strength increases, ductility and toughness often decrease. For example, CN112126758B discloses a method for toughening and controlling ultra-thick steel plates. While this method effectively improves low-temperature toughness through two controlled rolling and controlled cooling processes, its strength remains at only 450 MPa. This method fails to achieve a balance between ductility and toughness, allowing for the production of steel plates with varying strength levels without requiring compositional adjustments. Second, the post-TMCP heat treatment process is complex and prone to uncontrollable performance degradation. For example, CN115747661B discloses a steel plate resistant to tempering softening at 550°C to 600°C. Because the post-rolling tempering process can easily induce precipitate coarsening or microstructural degradation, this production method only guarantees a post-tempering strength reduction of ≤20 MPa, preventing further strength gains.

[0004] Therefore, it is urgent to develop a flexible manufacturing process that can achieve wide-range strength through a thermo-mechanical-aging coupled process chain under the same composition conditions, break through the limitations of traditional composition dependence, and cover multiple strength levels (such as Q510 / Q550 / Q620, etc.) using only a single-component steel grade with simple process adjustments, greatly shortening the production cycle and production line configuration costs, and providing good underlying material support for the design of major equipment such as bridges and ships. Summary of the Invention

[0005] In view of this, the present invention provides a method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without composition adjustment. By precisely designing the hot working route and utilizing multiple strengthening mechanisms, the problem of adjustable multiple strength levels under the condition of stable plasticity and toughness of low-carbon steel with the same composition is solved.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without composition adjustment, which is used for low-carbon steel with an alloy content of no more than 4wt%, and specifically comprises the following steps:

[0007] S1. Heating the continuous casting steel slab to 1150-1250°C and holding the temperature for 1-4 hours, performing multiple rough rolling at a temperature not lower than 1050°C, and then cooling the slab to room temperature using an ultra-fast cooling device to obtain a rough-rolled steel plate;

[0008] S2. Heat the rough rolled steel plate to 930℃-970℃ and keep it warm for 2h~4h, then air cool it to the surface temperature of A. c1 After the temperature reaches 745℃, multiple passes of finishing rolling are carried out, and the final rolling temperature of the finishing rolling is not less than 700℃. Then, an ultra-fast cooling device is used to cool it to the red-return temperature of 300℃~350℃, and then air-cooled to room temperature to obtain the finished rolled steel plate;

[0009] S3. According to the requirements of different strength levels, the finished rolled steel plate is heated to 580°C-620°C, and then a tempering time of no more than 3 hours is set accordingly, and then the plate is water-cooled to room temperature to obtain finished steel plates with different strength levels;

[0010] The strength level of the finished steel plate increases with the extension of the tempering time, and the plasticity and toughness corresponding to each strength level can be maintained stable.

[0011] Furthermore, the reduction rate of the rough rolling is 28% to 35%.

[0012] Furthermore, the reduction ratio of the finish rolling is not less than 46%.

[0013] Furthermore, the total reduction rate of the finished steel plate is 62% to 92%, and the thickness is 30 mm to 120 mm.

[0014] Furthermore, the cooling rate of the surface of the steel plate after rough rolling is not less than 50°C / s; and the volume fraction of lath bainite in the rough-rolled steel plate is not less than 95%.

[0015] Furthermore, the cooling rate of the surface of the steel plate after finish rolling is 30°C / s to 50°C / s; the volume fraction of ferrite in the finish-rolled steel plate is 40% to 60%, and the volume fraction of bainite is 40% to 60%.

[0016] Furthermore, the average grain size of the original austenite of the finish-rolled steel plate is 22 μm to 28 μm.

[0017] Furthermore, the chemical composition of the continuous casting steel billet includes: C: 0.05wt%~0.08wt%, Mn: 1.2wt%~1.9wt%, Si: 0.18wt%~0.20wt%, Ni: 0.5wt%~1.0wt%, Cr: 0.23wt%~0.25wt%, Cu: 0.15wt%~0.20wt%, Mo: 0.05wt%~0.08wt%, Nb+V+Ti<0.10wt%, Fe and other inevitable elements.

[0018] Furthermore, the range of the different strength levels is 510MPa~620MPa.

[0019] Furthermore, the elongation of the finished steel plate is not less than 19%, and the fracture is ductile at temperatures of -80°C and above, and the impact energy is not less than 200J.

[0020] Beneficial effects of the present invention: Under the premise of zero composition adjustment, the method of the present invention realizes flexible manufacturing of wide-range strength through a thermomechanical-aging coupled process chain. Industrial verification shows that a single-component steel grade can cover three strength grades, Q510 / Q550 / Q620, significantly shortening the production cycle and reducing the production line adaptation cost. The method has good application prospects and its beneficial effects are specifically reflected in the following three aspects:

[0021] (1) Microstructure genetic control: In the rough rolling stage, the microstructure dominated by lath bainite is obtained by precisely controlling the rolling temperature and cooling. The lath with high dislocation density is used in conjunction with the subsequent lower austenitization temperature to lay the foundation for fine-grained strengthening of the finished steel plate;

[0022] (2) Multi-level phase transformation control: The higher finishing temperature combined with precise cooling rate and red-returning temperature control regulates the ferrite + bainite multiphase structure with the potential for multi-strength level control, and the appropriate proportion composition also provides space for stable plastic toughness during subsequent tempering.

[0023] (3) Regulation of precipitation kinetics: By directional extension of the post-rolling tempering holding time, the nanoscale precipitate response is stimulated, and a continuous gradient increase in strength level from 510 MPa to 620 MPa is achieved while stabilizing the plasticity and toughness fluctuations of steel plates with different strength levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The microstructure diagrams of the embodiments of the present invention and the comparative examples are shown in FIG. 1 ; (a) is embodiment 1; (b) is embodiment 2; (c) is embodiment 3; (d) is embodiment 4; and (e) is comparative example 1.

[0025] Figure 2 Graph showing the tensile properties of the embodiments of the present invention and the comparative examples;

[0026] Figure 3 Graph showing the impact toughness of the embodiments of the present invention and the comparative example. DETAILED DESCRIPTION

[0027] 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.

[0028] A method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without composition adjustment is provided for low-carbon steel with an alloy content not exceeding 4 wt%, specifically comprising the following steps:

[0029] S1. The continuous cast steel slab is heated to 1150-1250°C and held for 1-4 hours for homogenization. It is then subjected to multiple rough rolling passes at a temperature of no less than 1050°C, with a reduction of 28%-35% to close defects such as internal pores in the slab. After rough rolling, the slab is cooled to room temperature using an ultra-fast cooling device. The high initial cooling temperature, such as 950-1050°C, and the large amount of deformation energy storage facilitate the subsequent cooling rate of the steel plate surface of no less than 50°C / s, thereby forming a rough-rolled steel plate with a high-density dislocation lath structure.

[0030] S2. Heat the rough rolled steel plate to 930℃-970℃ and keep it warm for 2h~4h, then air cool it to the surface temperature of A. c1 After reaching 745℃, multiple finishing rolling is carried out, and the final rolling temperature of the finishing rolling is not lower than 700℃. Subsequently, an ultra-fast cooling device is used to cool it to the red-return temperature of 300℃~350℃, and then air-cooled to room temperature to obtain the finished rolled steel plate.

[0031] In the traditional two-stage rolling process, since finishing rolling is carried out directly after rough rolling and air cooling to a certain temperature, its original austenite grains are formed at a higher temperature during the rough rolling and heat preservation, and are therefore usually relatively coarse. In the present invention, after the rough rolling is completed, the steel plate is cooled to room temperature, and there is no intermediate billet process. Finishing rolling is carried out by reheating the steel plate to 930℃-970℃ and then austenitizing, which is more conducive to obtaining fine original austenite grains. At the same time, combined with the phase transformation law of the steel plate, this heating temperature and the longer holding time can make full use of the structure formed after the ultra-fast cooling of the rough rolling, so that the austenite nucleates at a high density on the lath structure matrix without excessive growth, ensuring that the original austenite grains of the subsequent finished rolled steel plate are more evenly distributed, with an average size within the range of 22μm~28μm.

[0032] Compared with the traditional 800℃ and above, it is closer to A c3 Compared with the finishing rolling temperature of the present invention, the finishing rolling start temperature is lowered to A c1 ~745℃ range. On the one hand, a lower deformation temperature will significantly increase the density of deformation bands within the austenite crystal, thereby effectively increasing the phase transformation driving force of the dynamic phase transformation ferrite and increasing the nucleation density; on the other hand, a lower deformation temperature will also reduce the diffusion ability of atoms, resulting in a smaller ferrite grain size generated by phase transformation or dynamic phase transformation. The temperature control in this step is crucial for the microstructure preparation in the subsequent tempering process, and the experimental results also confirm this. When the finishing temperature is too low, the strength of the finished steel plate will deteriorate, while when the finishing temperature is too high, the plastic toughness will be adversely affected. In addition, with the gradual development of roller performance in recent years, its adaptability to lower rolling temperatures has also been increasing. For the thick plate in the present invention, the core temperature of the steel plate is higher than the surface temperature during cooling. The reddening phenomenon of the steel plate during rolling is beneficial to reducing deformation resistance. When the final rolling temperature is controlled within the range of not less than 700℃, it is well matched with the current rolling mill load performance.

[0033] At the same time, post-finishing cooling control is also crucial to achieving the desired match between finished steel plate strength and ductility. By adjusting the initial cooling temperature, a cooling rate of 30°C / s to 50°C / s, and a return-to-red temperature of 300°C to 350°C, the grain size, phase composition, morphology, and distribution in the microstructure can be further fine-tuned. Experimental results show that, when other conditions remain constant, excessively high cooling rates and excessively low return-to-red temperatures will reduce the ferrite content in the microstructure, impacting ductility and toughness. Conversely, excessively low cooling rates will reduce the bainite content in the microstructure, making it impossible to meet the corresponding strength level requirements.

[0034] S3. The finished rolled steel plate is heated to 580°C-620°C and then tempered. According to the requirements of different strength levels, the tempering holding time is set to no more than 3 hours to ensure that the precipitation strengthening effect is greater than the dislocation recovery softening effect. After water cooling, finished steel plates with different strength levels can be obtained, and the strength level of the finished steel plate increases with the extension of the tempering holding time. The corresponding plasticity and toughness properties of the steel plates of each strength level remain stable.

[0035] 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 continuous casting steel billets used in the examples and comparative examples is as follows: C: 0.05wt%, Mn: 1.6wt%, Si: 0.18wt%, Cr: 0.23wt%, Nb: 0.04wt%, V: 0.03wt%, Ti: 0.02wt%, Mo: 0.06wt%, Ni: 0.9wt%, Cu: 0.20wt%, Fe, and other unavoidable elements.

[0036] Example 1

[0037] The above continuous casting billet is subjected to the following steps:

[0038] S1. A 130mm thick continuous cast steel slab was heated to 1200°C and held for 4 hours. It was then rough-rolled in multiple passes at 1150°C with a reduction of approximately 28%. After rough rolling, the slab was cooled to room temperature using an ultra-rapid cooling device to produce a rough-rolled steel plate. The cooling rate on the steel plate surface was approximately 50°C / s. Metallographic observations revealed that the lath bainite content in the rough-rolled steel plate microstructure was approximately 95%.

[0039] S2. The rough-rolled steel plate is heated to 950°C and kept warm for 4 hours. After air cooling to a surface temperature of 745°C, multiple passes of finish rolling are performed with a reduction of about 46% and a final rolling temperature of 730°C. Subsequently, an ultra-fast cooling device is used to cool the steel plate to a red-return temperature of 330°C with a surface cooling rate of about 50°C / s. The steel plate is then air cooled to room temperature to obtain a 50 mm finish-rolled steel plate with an average grain size of the original austenite of about 26 μm.

[0040] S3. The finished steel plate in this embodiment is a finish-rolled steel plate and has not been tempered.

[0041] Example 2

[0042] Compared with Example 1, step S3 in Example 2 is to heat the finished rolled steel plate to 600° C., temper and keep it for 1 hour, and then water-cool it to obtain the finished steel plate. Other process parameters remain unchanged.

[0043] Example 3

[0044] Compared with Example 1, step S3 in Example 3 is to heat the finished rolled steel plate to 600° C., temper and keep it for 2 hours, and then water-cool it to obtain the finished steel plate. Other process parameters remain unchanged.

[0045] Example 4

[0046] Compared with Example 1, step S3 in Example 3 is to heat the finished rolled steel plate to 600° C., temper and keep it for 3 hours, and then water-cool it to obtain the finished steel plate. Other process parameters remain unchanged.

[0047] Comparative Example 1

[0048] Compared with Example 1, the conventional TMCP two-stage controlled rolling and controlled cooling process was adopted in steps S1 and S2 of Comparative Example 1, that is, the continuous casting billet was air-cooled to a surface temperature of 745°C after rough rolling and then directly subjected to finish rolling, and other process parameters remained unchanged.

[0049] Comparative Example 2

[0050] Compared with Comparative Example 1, in step S3 of Comparative Example 2, the finished rolled steel plate is heated to 600° C., tempered and kept warm for 1 hour, and then water-cooled to obtain the finished steel plate, and other process parameters remain unchanged.

[0051] Comparative Example 3

[0052] Compared with Example 1, Comparative Example 3 does not include step S2, that is, the continuous casting steel billet is directly subjected to step S3 after rough rolling, and other process parameters remain unchanged.

[0053] Comparative Example 4

[0054] Compared with Comparative Example 3, in step S3 of Comparative Example 4, the rough-rolled steel plate is heated to 600° C., tempered and kept warm for 1 hour, and then water-cooled to obtain the finished steel plate, and other process parameters remain unchanged.

[0055] Comparative Example 5

[0056] Compared with Example 1, Comparative Example 3 does not include step S1, that is, the continuous casting billet is not subjected to rough rolling, and steps S2 and S3 are directly performed, and other process parameters remain unchanged.

[0057] Comparative Example 6

[0058] Compared with Comparative Example 5, in step S3 of Comparative Example 6, the finished steel plate is heated to 600° C., tempered and kept warm for 1 hour, and then water-cooled to obtain the finished steel plate, and other process parameters remain unchanged.

[0059] Combine Figure 1 、 Figure 2 、 Figure 3 The organizational performance data of the embodiments and comparative examples in Tables 1 and 2 illustrate the technical effects of the process of the present invention in detail. Figure 1As shown in the figure, although the size, morphology and content of ferrite and bainite in the microstructure of each embodiment of the process of the present invention are slightly different, the volume fraction of each phase is in the range of 40% to 60%, which is significantly different from the microstructure of the conventional rolling process in Comparative Example 1, in which the large MA islands are significantly reduced and softened, and the point precipitation is significantly increased, but the overall structure is not obviously coarsened.

[0060] Figure 2 、 Figure 3 The mechanical data in Tables 1 and 2 show that the strength of Examples 1 and 2 can reach 510 MPa, the strength of Example 3 can reach 550 MPa, and the strength of Example 4 can reach 620 MPa. Further comparison of the plastic properties of Examples 1 to 4 reveals that the elongation of the steel plates at each strength level is above 19%, with fluctuations of no more than 2.6%. Furthermore, a comparison of the toughness properties of the various examples reveals that the steel plates at each strength level all fracture ductilely at -80°C, with impact energies exceeding 200 J, effectively meeting the comprehensive safety requirements for service at room temperature and various low-temperature environments.

[0061] Table 1 is the experimental data of tensile properties of different steel plates

[0062]

[0063] Table 2 is the impact toughness test data of different steel plates

[0064]

[0065] However, in Comparative Example 1, while the steel plate achieved a strength of 510 MPa, its toughness was far lower than that of the various examples of the present invention, with significant brittle fracture occurring at -60°C, significantly reducing its overall service safety in low-temperature environments. Furthermore, tensile strength test results showed that in Comparative Example 2, after tempering at 600°C for 1 hour, the yield strength decreased from 532 MPa to 496 MPa, implying that tempering could not achieve multiple strength levels.

[0066] Furthermore, in this application, steps S1 and S2 serve as an integrated control process. Their synergistic effect is the foundation for subsequent tempering to control strength and stabilize plasticity and toughness. Roughing or finishing rolling alone cannot produce a pre-cured steel plate with the potential for multi-strength level control. Mechanical test results also show that in Comparative Example 3, where only roughing rolling was performed, the lack of subsequent microstructure control resulted in a finished steel plate with a lath structure and a high yield strength of approximately 750 MPa, but an elongation of only approximately 15%. Furthermore, in Comparative Example 4, after tempering at 600°C for 1 hour, although plasticity improved, strength also decreased significantly, making multi-strength level control impossible. However, when only finishing rolling is performed, on the one hand, since the steel billet is not homogenized, the yield strength fluctuation rate of the final finished steel plate is extremely large, exceeding 20%, which does not meet the qualification rate requirements of industrial production; on the other hand, due to the lack of organizational preparation in the previous process, the comprehensive mechanical properties of the final finished steel plate are significantly deteriorated. The experimental results show that the yield strength of Example 5 is significantly lower than that of Example 1, and its qualified finished steel plate is only about 430 MPa, and the elongation is not high; in Comparative Example 6, since the tempering further coarsens the structure, its strength drops to about 400 MPa, and it also does not have the possibility of multi-strength level regulation.

[0067] In summary, the process of the present invention can cover three strength levels of Q510 / Q550 / Q620 using a single steel component under the same production line layout without reducing the plasticity and toughness properties, greatly reducing the production cycle and cost, and has good prospects for industrial application.

[0068] 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 sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without adjusting components, characterized in that: For low carbon steel with an alloy content not exceeding 4 wt%, the method comprises the following steps: S1. Heating the continuous casting steel slab to 1150-1250°C and holding the temperature for 1-4 hours, performing multiple rough rolling at a temperature not lower than 1050°C, and then cooling the slab to room temperature using an ultra-fast cooling device to obtain a rough-rolled steel plate; S2. Heat the rough rolled steel plate to 930℃-970℃ and keep it warm for 2h~4h, then air cool it to the surface temperature of A. c1 After the temperature reaches 745℃, multiple passes of finishing rolling are carried out, and the final rolling temperature of the finishing rolling is not less than 700℃. Then, an ultra-fast cooling device is used to cool it to the red-return temperature of 300℃~350℃, and then air-cooled to room temperature to obtain the finished rolled steel plate; S3. According to the requirements of different strength levels, the finished rolled steel plate is heated to 580°C-620°C, and then a tempering time of no more than 3 hours is set accordingly, and then the plate is water-cooled to room temperature to obtain finished steel plates with different strength levels; The strength level of the finished steel plate increases with the extension of the tempering time, and the plasticity and toughness corresponding to each strength level can be maintained stable.

2. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1 is characterized in that: The reduction rate of the rough rolling is 28% to 35%.

3. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1 is characterized in that: The reduction ratio of the finish rolling is not less than 46%.

4. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The finished steel plate has a total reduction rate of 62% to 92% and a thickness of 30 mm to 120 mm.

5. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The cooling rate of the surface of the rough-rolled steel plate is not less than 50° C. / s; and the volume fraction of lath bainite in the rough-rolled steel plate is not less than 95%.

6. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The cooling rate of the surface of the steel plate after finish rolling is 30°C / s to 50°C / s; the volume fraction of ferrite in the finish-rolled steel plate is 40% to 60%, and the volume fraction of bainite is 40% to 60%.

7. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The average grain size of the original austenite of the finish-rolled steel plate is 22 μm to 28 μm.

8. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The chemical composition of the continuous casting steel billet includes: C: 0.05wt%~0.08wt%, Mn: 1.2wt%~1.9wt%, Si: 0.18wt%~0.20wt%, Ni: 0.5wt%~1.0wt%, Cr: 0.23wt%~0.25wt%, Cu: 0.15wt%~0.20wt%, Mo: 0.05wt%~0.08wt%, Nb+V+Ti<0.10wt%, Fe and other inevitable elements.

9. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The range of the different strength levels is 510MPa~620MPa.

10. The method for sequentially controlling multiple strength levels of high-toughness low-carbon steel thick plates without component adjustment according to claim 1, characterized in that: The elongation of the finished steel plate is not less than 19%, and it fractures ductilely at temperatures of -80°C and above, and the impact energy is not less than 200J.

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