A method for time-series control of high-toughness, low-carbon steel thick plates with multiple strength levels without composition adjustment

Abstract

This invention belongs to the field of metal processing technology and discloses a method for time-series control of multiple strength levels in thick high-toughness, low-carbon steel plates without composition adjustment. After initial heating and holding, the plate is rough-rolled and ultra-rapidly cooled to room temperature; a second heating and holding process controls the original austenite grain size, followed by air cooling to A4 grade. c1 After precision rolling at ~745℃, the steel is then ultra-rapidly cooled to a reddening temperature of 300℃~350℃, followed by air cooling to obtain ferrite / bainite multiphase microstructures with different proportions. Subsequently, it undergoes tempering treatment at 580℃-620℃ for no more than 3 hours. By precisely controlling the tempering time, the yield strength can be continuously adjusted from 510MPa to 620MPa. Under the process conditions of this invention, the core impact energy of steel plates of various strength grades is not less than 200J at -80℃, and the elongation is not less than 19%. It possesses both controllable strength and stable, good ductility and toughness, making it valuable for industrial applications.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a method for time-series control of multiple strength levels of high-toughness, low-carbon steel thick plates without composition adjustment. Background Technology

[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 the alloy composition (such as adding microalloying elements like Nb and V) to achieve high strength, but this leads to problems such as soaring costs, decreased weldability, and deterioration of ductility and toughness. Especially for the multi-level requirements within a wide strength range of 510MPa to 620MPa, existing processes require repeated adjustments to the composition design and corresponding differentiated heat treatment regimes, significantly restricting production flexibility and industrialization efficiency.

[0003] In recent years, industry professionals have attempted to optimize microstructure and improve performance through controlled rolling and cooling (TMCP) processes, but two major challenges remain. First, there is the problem of a narrow microstructure control window. Conventional TMCP-obtained ferrite / bainite multiphase microstructures struggle to simultaneously achieve high strength and ductility / toughness. While strength increases, ductility and toughness often decrease. For example, CN112126758B discloses a toughening control method for ultra-thick steel plates. This method effectively improves the low-temperature toughness of the steel plate through two controlled rolling and cooling processes, but its strength is only maintained at the 450 MPa level. It cannot achieve coordinated ductility and toughness while simultaneously controlling the production of steel plates with different strength levels without compositional adjustments. Second, the heat treatment process following TMCP is complex and prone to uncontrollable performance degradation. For instance, CN115747661B discloses a steel plate resistant to tempering softening at 550℃~600℃. Due to the tendency for coarsening of precipitates or microstructure degradation during the post-rolling tempering process, its production method can only guarantee a strength reduction of ≤20 MPa after tempering, and cannot further increase strength.

[0004] Therefore, there is an urgent need to develop a flexible manufacturing process that can achieve a wide range of strengths under the same composition conditions through a thermomechanical-aging coupled process chain. This process would break through the limitations of traditional composition dependence, and could cover multiple strength levels (such as Q510 / Q550 / Q620) using only a single steel composition. Furthermore, the process adjustment would be simple, significantly shortening the production cycle and reducing production line configuration costs. This would provide a solid 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 time-series control of 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 method solves the problem of adjustable multiple strength levels under the condition of stable plasticity and toughness of low-carbon steel of the same composition.

[0006] To achieve the above objectives, the technical solution adopted by this invention is as follows: A method for time-series control of multiple strength levels of high-toughness low-carbon steel thick plates without composition adjustment, for low-carbon steel with an alloy content not exceeding 4wt%, specifically including the following steps:

[0007] S1. Heat the continuously cast steel billet to 1150℃-1250℃ and hold it for 1h~4h. Perform multiple rough rolling at a temperature not lower than 1050℃. Then use an ultra-fast cooling device to cool it to room temperature to obtain a rough rolled steel plate.

[0008] S2. Heat the rough-rolled steel plate to 930℃-970℃ and hold for 2-4 hours, then air cool until its surface temperature is A. c1 After reaching ~745℃, multiple passes of finishing rolling are performed, with the final rolling temperature not lower than 700℃. Then, an ultra-fast cooling device is used to cool it to the red-hot temperature of 300℃~350℃, and then it is air-cooled to room temperature to obtain the finished rolled steel plate.

[0009] S3. According to the requirements of different strength levels, the rolled steel plate is heated to 580℃-620℃ and then tempered for no more than 3 hours. Then it is water-cooled to room temperature to obtain finished steel plates with different strength levels.

[0010] The strength grade of the finished steel plate increases with the extension of the tempering time, and the ductility and toughness corresponding to each strength grade remain stable.

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

[0012] Furthermore, the reduction rate of the finishing mill is not less than 46%.

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

[0014] Furthermore, 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%.

[0015] Furthermore, the cooling rate of the surface of the finished steel plate is 30℃ / s~50℃ / s; the volume fraction of ferrite in the finished steel plate is 40%~60%, and the volume fraction of bainite is 40%~60%.

[0016] Furthermore, the average austenite grain size of the finished rolled steel plate is 22μm~28μm.

[0017] Furthermore, the chemical composition of the continuously cast 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 unavoidable elements.

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

[0019] Furthermore, the finished steel plate has an elongation of not less than 19%, and exhibits ductile fracture at temperatures of -80℃ and above, with an impact energy of not less than 200J.

[0020] The beneficial effects of this invention are as follows: Under the premise of zero composition adjustment, the method of this invention achieves flexible manufacturing with a wide range of strength through a thermomechanical-aging coupled process chain. Industrial verification has shown that a single composition steel grade can cover three strength levels: Q510, Q550, and Q620. This significantly shortens the production cycle and reduces production line adaptation costs, demonstrating promising application prospects. Its beneficial effects are specifically reflected in the following three aspects:

[0021] (1) Microstructure genetic control: During the rough rolling stage, the microstructure mainly composed of lath bainite is obtained by rolling temperature and precise cooling control. The lath with high density of dislocations is used in conjunction with the subsequent lower austenitization temperature to lay the foundation for fine grain strengthening microstructure of finished steel plate.

[0022] (2) Multi-stage phase transformation control: The high finishing rolling temperature combined with precise cooling rate and reddening temperature control regulates the ferrite + bainite multiphase structure with the potential for multi-level strength regulation, and the appropriate proportion also provides space for stabilizing plasticity and toughness during subsequent tempering.

[0023] (3) Precipitation kinetics control: By extending the tempering and holding time after rolling in a directional manner, the response of nanoscale precipitates is stimulated, and under the premise of stabilizing the ductility and toughness fluctuation of steel plates of different strength levels, the strength level is continuously increased from 510MPa to 620MPa in a gradient manner. Attached Figure Description

[0024] Figure 1 The images show the microstructure of embodiments and comparative examples of the present invention; (a) is Example 1; (b) is Example 2; (c) is Example 3; (d) is Example 4; (e) is Comparative Example 1.

[0025] Figure 2 The tensile property curves are for embodiments and comparative examples of the present invention;

[0026] Figure 3 The diagram shows the impact toughness of the embodiments and comparative examples of the present invention. Detailed Implementation

[0027] The following specific examples illustrate the embodiments of the present invention. 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 embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. The accompanying drawings are for illustrative purposes only, representing schematic diagrams rather than actual objects, and should not be construed as limiting the present invention. To better illustrate the embodiments of the present invention, it is understood that some well-known structures or steps and their descriptions may be omitted in the drawings.

[0028] A method for time-series control of multiple strength levels in high-toughness low-carbon steel thick plates without composition adjustment, applicable to low-carbon steel with an alloy content not exceeding 4wt%, specifically includes the following steps:

[0029] S1. The continuously cast steel billet is heated to 1150℃-1250℃ and held for 1-4 hours to homogenize it. Then, it is subjected to multi-pass rough rolling at a temperature not lower than 1050℃, with a reduction rate of 28%-35% to close internal defects such as pores in the continuously cast billet. After rough rolling, it is cooled to room temperature using an ultra-fast cooling device. The relatively high starting cooling temperature, such as 950℃-1050℃, and the large amount of deformation energy storage can work together with the subsequent cooling rate of not less than 50℃ / s on the surface of the steel plate to form a rough-rolled steel plate with a high-density dislocation lath structure as the matrix.

[0030] S2. Heat the rough-rolled steel plate to 930℃-970℃ and hold for 2-4 hours, then air cool until its surface temperature is A. c1 After reaching ~745℃, multiple passes of finishing rolling are performed, with the final rolling temperature not lower than 700℃. Subsequently, an ultra-fast cooling device is used to cool it to the red-hot temperature of 300℃~350℃, and then it is air-cooled to room temperature to obtain the finished rolled steel plate.

[0031] In traditional two-stage rolling processes, finishing rolling is performed directly after rough rolling and air-cooled to a certain temperature. Therefore, the original austenite grains are formed at relatively high temperatures during the rough rolling holding period, resulting in coarser grains. In this invention, however, after rough rolling, the steel is cooled to room temperature without an intermediate billet process. Finish rolling involves reheating the steel plate to 930℃-970℃ for austenitization, which is more conducive to obtaining finer original austenite grains. Furthermore, considering the phase transformation characteristics of the steel plate, this heating temperature and longer holding time fully utilize the microstructure formed after the ultra-rapid cooling of the rough rolling, allowing for high-density nucleation of austenite in the lath matrix without excessive growth. This ensures a more uniform distribution of original austenite grains in the subsequently finished rolled steel plate, with an average size in the range of 22μm to 28μm.

[0032] Compared to traditional temperatures above 800℃, it tends to be more A c3 Compared to the finishing rolling temperature, this invention lowers the finishing rolling start temperature to A. c1 The temperature range is ~745℃. On the one hand, a lower deformation temperature significantly increases the density of deformation bands within the austenite grains, thereby effectively enhancing the phase transformation driving force of dynamic phase transformation ferrite and leading to increased nucleation density. On the other hand, a lower deformation temperature also reduces the diffusion capacity of atoms, resulting in smaller ferrite grain sizes generated by phase transformation or dynamic phase transformation. Temperature control in this step is crucial for the microstructure preparation during subsequent tempering, as confirmed by experimental results. When the finishing rolling temperature is too low, the strength of the finished steel plate deteriorates, while when the finishing rolling temperature is too high, the ductility and toughness are adversely affected. Furthermore, with the gradual development of roll performance in recent years, its adaptability to lower rolling temperatures has been increasing. For the thick plate in this invention, during cooling, the core temperature of the steel plate is higher than the surface temperature. The red-hot phenomenon of the steel plate during rolling helps to reduce deformation resistance. When the final rolling temperature is controlled within a range not lower than 700℃, it matches well with the current mill load performance.

[0033] Meanwhile, to form finished rolled steel plates with optimized strength and ductility-toughness matching potential, post-finishing cooling control is also crucial. By using an initial cooling temperature, a cooling rate of 30℃ / s to 50℃ / s, and a reddening temperature of 300℃ to 350℃, the grain size, phase composition, morphology, and distribution in the microstructure can be further refined. For example, experimental results show that, under constant conditions, excessively high cooling rates and excessively low reddening temperatures will reduce the ferrite content in the microstructure, affecting ductility and toughness. Conversely, they will reduce the bainite content, failing to meet the requirements of the corresponding strength level.

[0034] S3. After heating the rolled steel plate to 580℃-620℃, temper it. 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. The strength level of the finished steel plate increases with the extension of the tempering holding time, and the ductility and toughness of each strength level steel plate remain stable.

[0035] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, examples, and comparative examples. To reduce experimental errors, the chemical composition of the continuously cast 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 continuously cast steel billet is subjected to the following steps:

[0038] S1. A 130mm thick continuously cast steel billet was heated to 1200℃ and held for 4 hours, followed by multi-pass rough rolling at 1150℃ with a reduction of approximately 28%. After rough rolling, it was cooled to room temperature using an ultra-fast cooling device to obtain a rough-rolled steel plate. The cooling rate of the steel plate surface was approximately 50℃ / s. Furthermore, metallographic observation results showed that the lath bainite content in the microstructure of the rough-rolled steel plate was approximately 95%.

[0039] S2. The rough-rolled steel plate is heated to 950℃ and held for 4 hours. After air cooling to a surface temperature of 745℃, it is subjected to multiple passes of finish rolling with a reduction of about 46% and a final rolling temperature of 730℃. Then, it is cooled to a reddening temperature of 330℃ using an ultra-fast cooling device with a surface cooling rate of about 50℃ / s. Finally, it is air-cooled to room temperature to obtain a 50mm finish-rolled steel plate with an average austenite grain size of about 26μm.

[0040] S3. In this embodiment, the finished steel plate is a precision rolled steel plate and has not undergone tempering treatment.

[0041] Example 2

[0042] Compared with Example 1, step S3 in Example 2 involves heating the rolled steel plate to 600°C, tempering and holding it for 1 hour, and then water cooling to obtain the finished steel plate. Other process parameters remain unchanged.

[0043] Example 3

[0044] Compared with Example 1, step S3 in Example 3 involves heating the rolled steel plate to 600°C, tempering and holding it for 2 hours, and then water cooling to obtain the finished steel plate. Other process parameters remain unchanged.

[0045] Example 4

[0046] Compared with Example 1, step S3 in Example 3 involves heating the rolled steel plate to 600°C, tempering and holding it for 3 hours, and then water cooling 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 cooling process was used in steps S1 and S2 of Comparative Example 1, namely: the continuously cast steel billet was air-cooled to a surface temperature of 745°C after rough rolling and then directly subjected to finish rolling, while other process parameters remained unchanged.

[0049] Comparative Example 2

[0050] Compared with Comparative Example 1, in Comparative Example 2, step S3 involves heating the finished rolled steel plate to 600°C, tempering and holding it for 1 hour, and then water cooling to obtain the finished steel plate. 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 continuously cast steel billet directly undergoes step S3 after rough rolling, and other process parameters remain unchanged.

[0053] Comparative Example 4

[0054] Compared with Comparative Example 3, in Comparative Example 4, step S3 involves heating the rough-rolled steel plate to 600°C, tempering and holding it for 1 hour, and then water cooling to obtain the finished steel plate. 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 continuously cast steel billet is not rough rolled, and steps S2 and S3 are performed directly, while other process parameters remain unchanged.

[0057] Comparative Example 6

[0058] Compared with Comparative Example 5, in Comparative Example 6, step S3 involves heating the finished rolled steel plate to 600°C, tempering and holding it for 1 hour, and then water cooling to obtain the finished steel plate. Other process parameters remain unchanged.

[0059] Combination Figure 1 , Figure 2 , Figure 3 The tissue performance data of the embodiments and comparative examples in Tables 1 and 2 provide a detailed explanation of the technical effects of the process of the present invention. For example... Figure 1As shown, although the size, morphology and content of ferrite and bainite in the microstructure of the various embodiments 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. The large blocky MA islands are significantly reduced and softened, and the dotted precipitation is significantly increased, but the overall microstructure is not significantly coarsened.

[0060] Figure 2 , Figure 3 The mechanical data in Tables 1 and 2 show that the strength of Examples 1 and 2 reaches 510 MPa, the strength of Example 3 reaches 550 MPa, and the strength of Example 4 reaches 620 MPa. Further comparison of the plasticity properties of Examples 1 to 4 reveals that the elongation of the steel plates at each strength level is above 19%, with fluctuations not exceeding 2.6%. Moreover, comparison of the toughness properties of each example shows that the steel plates at each strength level exhibit ductile fracture at -80°C, with impact energy exceeding 200 J, greatly satisfying the comprehensive safe service requirements under room temperature and various low-temperature environments.

[0061] Table 1 shows the experimental data on the tensile properties of different steel plates.

[0062]

[0063] Table 2 shows the experimental data on impact toughness of different steel plates.

[0064]

[0065] However, in Comparative Example 1, although the steel plate strength reached 510 MPa, its toughness was far lower than that of the embodiments of the present invention, and it experienced significant brittle fracture at -60°C, greatly reducing its overall service safety in low-temperature environments. Meanwhile, tensile strength test results showed that in Comparative Example 2, after tempering at 600°C for 1 hour, its yield strength decreased from 532 MPa to 496 MPa, indicating that it was not possible to control multiple strength levels through tempering.

[0066] Furthermore, in this application, steps S1 and S2 are used as an integrated control process. Their synergistic effect is the foundation for subsequent tempering to control strength and stabilize ductility and toughness. A single roughing or finishing rolling process cannot produce a steel plate with the potential for multi-level strength control. Mechanical experimental results also show that in Comparative Example 3, when only roughing was performed, the finished steel plate had a lath structure due to the lack of subsequent microstructure control. It had a high yield strength of approximately 750 MPa, but an elongation of only about 15%. In Comparative Example 4, although the ductility improved after tempering at 600℃ for 1 hour, the strength also decreased significantly, making multi-level strength control impossible. When only finishing rolling is performed, on the one hand, due to the lack of homogenization treatment of the billet, the yield strength fluctuation of the final finished steel plate is extremely large, exceeding 20%, which does not meet the requirements of industrial production for the pass rate; on the other hand, due to the lack of pre-processing preparation, the comprehensive mechanical properties of the final finished steel plate deteriorate significantly. The experimental results show that the yield strength of Comparative 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 either; in Comparative Example 6, the tempering further coarsens the microstructure, causing its strength to drop to about 400 MPa, and it also does not have the possibility of multi-strength level control.

[0067] In summary, the process of this invention can cover three strength levels (Q510 / Q550 / Q620) using a single steel composition without reducing ductility and toughness under the same production line layout, significantly reducing production cycle and cost, and showing 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, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions 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 time-series control of multiple yield strength levels in high-toughness, low-carbon steel thick plates without composition adjustment, characterized in that, For low-carbon steel with an alloy content not exceeding 4 wt%, the specific steps include: S1. Heat the continuously cast steel billet to 1150℃-1250℃ and hold it for 1h~4h. Perform multiple rough rolling at a temperature not lower than 1050℃. Then use an ultra-fast cooling device to cool it to room temperature to obtain a rough rolled steel plate. S2. Heat the rough-rolled steel plate to 930℃-970℃ and hold for 2-4 hours, then air cool until its surface temperature is A. c1 After reaching ~745℃, the steel is subjected to multiple passes of finishing rolling, with a final rolling temperature of not less than 700℃. It is then cooled to a red-hot temperature of 300℃~350℃ using an ultra-fast cooling device, and finally air-cooled to room temperature to obtain a finished rolled steel plate. The finished rolled steel plate contains 40%~60% ferrite and 40%~60% bainite by volume. S3. According to the requirements of different yield strength levels, the rolled steel plate is heated to 580℃-620℃ and then tempered for no more than 3 hours. Then it is water-cooled to room temperature to obtain finished steel plates with different yield strength levels. The yield strength grade of the finished steel plate increases with the extension of the tempering time.

2. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The reduction rate of the rough rolling is 28% to 35%.

3. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The reduction rate of the finishing mill is not less than 46%.

4. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The total reduction rate of the finished steel plate is 62%~92%, and the thickness is 30mm~120mm.

5. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The cooling rate of the surface of the rough-rolled steel plate is not less than 50℃ / s; the volume fraction of lath bainite in the rough-rolled steel plate is not less than 95%.

6. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The cooling rate of the surface of the finished steel plate is 30℃ / s to 50℃ / s.

7. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The average austenite grain size of the finished rolled steel plate is 22μm~28μm.

8. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment as described in claim 1, characterized in that, The chemical composition of the continuously cast 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 unavoidable elements.

9. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment according to claim 1, characterized in that, The range of the different yield strength levels is 510MPa~620MPa.

10. The method for time-series control of multiple yield strength levels of high-toughness low-carbon steel thick plates without composition adjustment according to claim 1, characterized in that, The finished steel plate has an elongation of not less than 19%, and exhibits ductile fracture at temperatures of -80℃ and above, with an impact energy of not less than 200J.

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