Method for improving performance of TRIP type bainite steel by microstructure regulation

Abstract

The application relates to the technical field of advanced steel material, and particularly discloses a method for improving the performance of TRIP type bainite steel by microstructure regulation, smelting a steel billet; hot rolling initial structure regulation: the microstructure is regulated to be granular bainite structure, the average grain size is 2.5-4.0 mu m, the proportion of large-angle grain boundaries is greater than or equal to 88% through EBSD analysis and statistics, and the width-length ratio is greater than 0.7 and less than 1.0; cold rolling structure regulation: the prepared hot rolling coil is subjected to an acid pickling process, and the surface reflectivity is greater than or equal to 70%; annealing microstructure regulation: bainite structure is obtained, the proportion of large-angle grain boundaries of the bainite structure is greater than or equal to 86% through EBSD analysis and statistics, and there is grain boundary austenite structure with a volume fraction greater than or equal to 18%; the microstructure design of the application improves the performance stability, and in particular, the stability of the product of strength and plasticity is significantly improved.

Description

Technical Field

[0001] This invention relates to the field of advanced steel materials technology, specifically to a method for improving the performance of TRIP-type bainitic steel through microstructure control. Background Technology

[0002] Automotive crash protection components (including crash beams, door crash bars, etc.) are core components of the vehicle's passive safety. They need to absorb energy through deformation during a collision while ensuring the integrity of the passenger compartment. Currently, the high-strength steels used in automotive crash protection components mainly include hot-formed steel, duplex steel, martensitic steel, and aluminum alloys. The main problems include:

[0003] 1. Hot-formed steel: Extremely high strength, with tensile strength exceeding 1500MPa. It can be used to manufacture front / rear bumpers, A / B pillar reinforcements, door anti-collision bars, etc. through hot forming process, and has low springback. However, it needs to be heated to about 950℃, and the surface is very easy to oxidize and form an oxide scale, usually requiring the addition of an Al-Si coating; in addition, the material has poor plasticity, and when the strength reaches 1500MPa, the elongation after fracture is only about 6%.

[0004] 2. Dual-phase steel: Possessing high strength, high elongation, and a high work hardening index, it can be used as front bumper beams and bumper bars. Currently, the highest strength achieved by domestically produced industrially manufactured steel is 1470 MPa, but its strength-ductility product is <14 GPa·%, which is a comprehensive indicator of a material's energy absorption capacity. It evaluates whether a material absorbs as much energy as possible during a high-speed collision to reduce the impact force transmitted to the occupants, thereby ensuring their safety performance.

[0005] 3. Martensitic steel: It has a high yield strength ratio and extremely high strength, with a tensile strength of up to 1800MPa, but a low elongation. It is suitable for roll forming and is suitable for use as simple cross-section parts such as bumpers and door sill reinforcement plates.

[0006] 4. Aluminum alloy materials: Their core advantages are significant weight reduction, good energy absorption, and corrosion resistance, making them suitable for use as front / rear bumper beams, energy-absorbing boxes, etc. However, compared to high-strength steel, they have the following drawbacks:

[0007] (1) High carbon emissions. Primary aluminum production is a typical high-energy-consuming industry, and its greenhouse gas emissions per unit mass are much higher than those of steel. Data shows that the carbon emissions from producing 1 kg of aluminum are about 13.9-15.5 kg CO2, while the carbon emissions from producing 1 kg of advanced high-strength steel (AHSS) are only 2.3-2.7 kg CO2.

[0008] (2) High material and manufacturing costs. The price of aluminum alloy raw materials is much higher than that of steel, and the forming and connecting processes are more complex, resulting in a significant increase in the cost of parts.

[0009] (3) High maintenance costs and easy to scrap. Aluminum alloy has poor plasticity. After collision and deformation, it is difficult to repair with sheet metal like steel. It often has to be replaced as a whole, resulting in extremely high maintenance costs.

[0010] (4) The connection and welding are difficult. The connection between aluminum alloy and steel (such as welding) is difficult. Riveting and gluing are commonly used, and the process requirements are high. The welding quality is unstable and problems such as rough welds are easy to occur.

[0011] It is evident that, as a core component of vehicle passive safety, automotive crash barriers place extremely stringent comprehensive performance requirements on the high-strength steel used. They not only require ultra-high strength but also high collision energy absorption capacity. Furthermore, ensuring the material's performance stability is crucial to further enhance its safety and reliability. However, currently available technologies have not addressed the performance stability improvement of bainitic steel used in automotive crash barriers, particularly the improvement of its strength-ductility product. Therefore, a method is needed to design to improve the performance stability of TRIP-type bainitic steel through microstructure control, thereby resolving the technical challenges in improving the performance stability of ultra-high-strength bainitic steel used in automotive crash barriers in existing technologies. Summary of the Invention

[0012] To address the problems existing in the prior art, the purpose of this invention is to provide a method for improving the performance of TRIP-type bainitic steel by microstructure control.

[0013] The technical solution adopted by this invention to solve its technical problem is: a method for improving the performance of TRIP-type bainitic steel by microstructure control, comprising the following steps:

[0014] S1, Smelting steel billets;

[0015] S2. Initial microstructure control during hot rolling: The microstructure is controlled to granular bainite microstructure, which is formed based on the original austenite grains, resulting in an average grain size of 2.5-4.0 μm. Through EBSD analysis and statistics, the proportion of large-angle grain boundaries is ≥88%, and 0.7 < width-to-length ratio <1.0.

[0016] S3. Cold Rolling Structure Control: After the prepared hot-rolled coil undergoes a pickling process, the surface reflectivity is ≥70%; then it is cold-rolled to 1.0-2.0mm, with a cumulative cold rolling reduction rate of 60-70%.

[0017] S4. Annealing microstructure control: Bainite structure is obtained. Through EBSD analysis and statistics, the proportion of large-angle grain boundaries in the bainite structure is ≥86%, and there is austenite structure with a volume fraction of ≥18%. The mass percentage of C content is 1.1%-1.3%.

[0018] Specifically, in step S1, the chemical composition of the steel billet by mass percentage is: C: 0.22%-0.25%, Si: 1.0%-1.3%, Mn: 2.2%-2.5%, Cr: 0.65%-0.85%, Mo: 0.35%-0.45%, Nb: 0.035%-0.055%, Ni: 0.6%-0.8%, while controlling impurity elements P≤0.005%, S≤0.0025%, N≤0.003%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities.

[0019] Specifically, the hot rolling process in step S2 includes the following steps:

[0020] S21. Heating process: The heat soaking temperature is 1220±10℃, the heat soaking section holding time is about 30min, and the total furnace time is 220±15min.

[0021] S22. Rolling process: In the rolling process, the exit temperature of the roughing mill is 1135±15℃, the finishing mill temperature is 920±15℃, and the thickness of the hot-rolled strip is 2.5-5.5mm. The pass allocation and reduction rate of each pass in the roughing and finishing milling processes are calculated by the roughing setting model of the secondary system.

[0022] S23. Cooling Process: A two-stage cooling mode of "Section I ultra-fast cooling + Section II sparse cooling" is adopted. Immediately after final rolling, Section I ultra-fast cooling is adopted with a cooling rate >120℃ and a final cooling temperature of 640~680℃. Then, Section II sparse cooling is adopted and a three-stage U-shaped cooling is performed. The cooling temperature of the head 30-55m and tail 25-35m of the strip is 485±15℃, and the temperature of the middle part of the strip is 440-470℃, with a cooling rate of about 15-20℃ / s.

[0023] S24. Coiling and annealing process: After adopting the II-stage sparse cooling process, the strip steel is coiled to obtain a hot-rolled coil, which is immediately placed in a resistance furnace. The resistance furnace temperature is set to 450±15℃, the annealing time is 2h, and then the coil is cooled to room temperature with the furnace.

[0024] Specifically, in step S3, the initial microstructure of hot rolling is effectively broken up and introduced during the cold rolling process, wherein the grain size of ≥85% by volume is ≤1.2μm; and the grain size of ≤15% by volume is 1.2-2.0μm.

[0025] Specifically, the annealing process in step S4 includes the following steps:

[0026] S41. Continuous annealing process: The chilled strip steel is rapidly heated to 840-870℃ at a heating rate of 80-120℃ / s and held for 30-60s; after annealing, it is rapidly cooled to the over-aging temperature of 405±10℃ at a cooling rate of >70℃ / s, and the over-aging isothermal time is 60-120s; then the strip steel is cooled to room temperature at a cooling rate of 20℃ / s.

[0027] S42. Leveling process: The actual elongation rate during the leveling process is controlled between 1.0% and 1.4%.

[0028] S43. Uniform tempering process: The annealed strip is placed in an annealing softening furnace for tempering heat treatment. The tempering temperature is set to 430±15℃ and the tempering time is 60-120min.

[0029] Specifically, the strip steel is tempered and heat-treated to obtain TRIP-type bainitic steel.

[0030] Specifically, the TRIP-type bainitic steel has a strength-ductility product of 26±2.5 GPa·%, tensile strength is stably controlled at 1550±25 MPa, yield strength is stably controlled at 1125±25 MPa, yield strength ratio is stably controlled at 0.73±0.018, and elongation is >15%.

[0031] The present invention has the following beneficial effects:

[0032] The method for improving the performance of TRIP-type bainitic steel by microstructure control designed in this invention has the advantages of high strength-ductility product and high performance stability. Its strength-ductility product is 26±2.5GPa·%, tensile strength is stably controlled at 1550±25MPa, yield strength is stably controlled at 1125±25MPa, yield strength ratio is stably controlled at 0.73±0.018, and elongation is >15%.

[0033] This invention presents a method for improving the performance of TRIP-type bainitic steel through microstructure control. The initial hot-rolled microstructure is controlled to form a granular bainitic structure, which is formed based on the original austenite grains with an average grain size of 2.5-4.0 μm. During tempering, it gradually tends towards a polygonal structure, with large-angle grain boundaries accounting for ≥88%, and a width-to-length ratio of 0.7 < 1.0. This is beneficial for forming a highly stable austenitic structure with a high proportion of large-angle grain boundaries during cold rolling and annealing.

[0034] This invention presents a method for improving the performance of TRIP-type bainitic steel through microstructure control. During cold rolling, the initial hot-rolled microstructure is effectively broken down, introducing defects such as high-density dislocations and distortion energy. Specifically, grains with a volume fraction ≥85% have a size ≤1.2 μm, and grains with a volume fraction ≤15% have a size of 1.2~2.0 μm. This facilitates microstructure control through annealing, resulting in a large-angle grain boundary ratio ≥86% and the presence of a grain boundary austenite structure with a volume fraction ≥18% and a carbon content of 1.1-1.3%. During deformation, with increasing applied stress, the TRIP effect can occur continuously and progressively, providing sustained work hardening capability. This microstructure design improves the stability of its performance, particularly significantly enhancing the stability of the strength-ductility product. Attached Figure Description

[0035] Figure 1 This is a microstructure diagram of the initial microstructure control of hot rolling in the example.

[0036] Figure 2 This is a microstructure diagram of the cold-rolled structure control in the embodiment.

[0037] Figure 3 This is a microscopic scanning image of the TRIP-type bainitic steel prepared in the example.

[0038] Figure 4 This is a micro-region morphology image of the interior of the grains of TRIP-type bainitic steel prepared by field emission electron probe microanalysis in the example.

[0039] Figure 5 This is a micro-area C element distribution map under field emission electron probe microanalysis for the preparation of TRIP-type bainitic steel in the example (and...). Figure 4 correspond). Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0041] like Figures 1-5 As shown, Examples 1-9 of this invention employ a method to improve the performance of TRIP-type bainitic steel through microstructure control, specifically including the following steps:

[0042] Step 1: Smelting steel billets

[0043] The chemical composition of the steel billets used in Examples 1-9 of this invention, by mass percentage, is as follows: C: 0.22%-0.25%, Si: 1.0%-1.3%, Mn: 2.2%-2.5%, Cr: 0.65%-0.85%, Mo: 0.35%-0.45%, Nb: 0.035%-0.055%, Ni: 0.6%-0.8%, while controlling the impurity elements P≤0.005%, S≤0.0025%, N≤0.003%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities.

[0044] The chemical composition of the steel billets in specific embodiments 1-9 is shown in Table 1.

[0045] Table 1. Chemical composition and corresponding mass percentage of steel billets in Examples 1-9

[0046]

[0047] Step 2: Initial microstructure control during hot rolling

[0048] The specific process parameters for controlling the initial microstructure of hot rolling are shown in Table 2. The microstructure is controlled to be granular bainite, which is formed based on the original austenite grains. The average grain size is 2.5-4.0 μm, and it gradually tends to a polygonal structure during tempering. Through EBSD analysis and statistics, the proportion of large-angle grain boundaries is ≥88%, and 0.7 < width-to-length ratio <1.0.

[0049] (1) Heating process: The heat spread temperature is 1220±10℃, the heat spread section is kept for about 30 minutes, and the total time in the furnace is 220±15 minutes.

[0050] (2) Rolling process: In the rolling process, the exit temperature of the roughing mill is 1135±15℃, the finishing mill temperature is 920±15℃, and the thickness of the hot-rolled strip is 2.5-5.5mm. The pass allocation and reduction rate of each pass in the roughing and finishing milling processes are calculated by the roughing setting model of the secondary system.

[0051] (3) Cooling process: A two-stage cooling mode of "I-stage ultra-fast cooling + II-stage sparse cooling" is adopted. The I-stage ultra-fast cooling mode is adopted immediately after the final rolling, with a cooling rate >120℃ and the final cooling temperature of the I-stage is 640-680℃; then, the II-stage sparse cooling mode is adopted, and a three-stage U-shaped cooling is performed. The cooling temperature of the head (30~55)m and tail (25~35)m of the strip is 485±15℃, and the temperature of the middle part of the strip is 440-470℃, with a cooling rate of about 15-20℃ / s.

[0052] (4) Coiling and annealing process: After adopting the second-stage sparse cooling process, the strip steel is coiled to obtain a hot-rolled coil, which is immediately placed in a resistance furnace. The resistance furnace temperature is set to 450±15℃, the annealing time is 2h, and then the strip is cooled to room temperature with the furnace.

[0053] Table 2 Specific process parameters for hot rolling initial microstructure control in Examples 1-9

[0054]

[0055] Step 3: Microstructure Control in Cold Rolling

[0056] After the hot-rolled coil prepared above undergoes a pickling process, its surface reflectivity is ≥70%; then it is cold-rolled to 1.0-2.0 mm, with a cumulative cold rolling reduction of 60-70%. During the cold rolling process, the initial microstructure of the hot-rolled coil is effectively broken up and defects such as high-density dislocations and distortion energy are introduced, of which grains with a volume fraction ≥85% have a size ≤1.2 μm; and grains with a volume fraction ≤15% have a size of 1.2-2.0 μm.

[0057] Step 4: Annealing Microstructure Control

[0058] The specific process parameters for cold rolling and annealing microstructure control are shown in Table 3. Bainitic microstructure was obtained, and its large-angle grain boundary ratio was ≥86% through EBSD analysis and statistics. In addition, there is a grain boundary austenitic microstructure with a volume fraction of ≥18% and a carbon content of 1.1-1.3%. It has good thermomechanical stability. During tensile deformation, it can continuously and gradually undergo the TRIP effect with the increase of applied stress, which can provide continuous work hardening ability. It is the key to obtaining excellent and stable strength-plasticity matching of bainitic steel.

[0059] (1) Continuous annealing process: The above-mentioned cold-hardened strip steel is rapidly heated to 840-870℃ at a heating rate of 80-120℃ / s and held for 30-60s; after annealing, it is rapidly cooled to the over-aging temperature of 405±10℃ at a cooling rate of >70℃ / s, and the over-aging isothermal time is 60-120s; then the strip steel is cooled to room temperature at a cooling rate of 20℃ / s.

[0060] (2) Leveling process: The actual elongation rate during the leveling process is controlled between 1.0 and 1.4%.

[0061] (3) Uniform tempering process: The annealed strip steel is placed in an annealing softening furnace for tempering heat treatment. The tempering temperature is set to 430±15℃ and the tempering time is 60-120min.

[0062] Table 3 Specific process parameters for cold rolling and annealing microstructure control in Examples 1-9

[0063]

[0064] The sample preparation and mechanical property testing methods at room temperature were carried out in accordance with the national standard GB / T 228.1-2021 "Metallic materials, tensile testing—Part 1: Room temperature test method". The mechanical property test results of Examples 1-9 are shown in Table 4. Analysis of the mechanical property test results shows that the TRIP-type bainitic steel prepared in Examples 1-9 of this invention has the advantages of high strength-ductility product and high performance stability. Its strength-ductility product is 26±2.5 GPa·%, tensile strength is stably controlled at 1550±25 MPa, yield strength is stably controlled at 1125±25 MPa, yield ratio is stably controlled at 0.73±0.018, and elongation is >15%.

[0065] Table 4. Mechanical property test results of Examples 1-9

[0066]

[0067] This invention is not limited to the above-described embodiments. Anyone should know that any structural changes made under the guidance of this invention, and any technical solutions that are the same as or similar to this invention, fall within the protection scope of this invention.

[0068] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A method for improving the performance of TRIP-type bainitic steel through microstructure control, characterized in that, Includes the following steps: S1, Smelting steel billets; S2. Initial microstructure control during hot rolling: The microstructure is controlled to granular bainite microstructure, which is formed based on the original austenite grains, resulting in an average grain size of 2.5-4.0 μm. Through EBSD analysis and statistics, the proportion of large-angle grain boundaries is ≥88%, and 0.7 < width-to-length ratio <1.

0. S3. Cold-rolled microstructure control: After the prepared hot-rolled coil undergoes a pickling process, the surface reflectivity is ≥70%; Then it is cold rolled to 1.0-2.0mm, with a cumulative cold rolling reduction rate of 60-70%; S4. Annealing microstructure control: Bainite structure is obtained. Through EBSD analysis and statistics, the proportion of large-angle grain boundaries in the bainite structure is ≥86%, and there is austenite structure with a volume fraction of ≥18%. The mass percentage of C content is 1.1%-1.3%.

2. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 1, characterized in that, In step S1, the chemical composition of the steel billet by mass percentage is as follows: C: 0.22%-0.25%, Si: 1.0%-1.3%, Mn: 2.2%-2.5%, Cr: 0.65%-0.85%, Mo: 0.35%-0.45%, Nb: 0.035%-0.055%, Ni: 0.6%-0.8%, while controlling the impurity elements P≤0.005%, S≤0.0025%, N≤0.003%, O≤0.002%, H≤0.0003%, with the balance being Fe and unavoidable impurities.

3. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 1, characterized in that, The hot rolling process in step S2 includes the following steps: S21. Heating process: The heat soaking temperature is 1220±10℃, the heat soaking section holding time is about 30min, and the total furnace time is 220±15min. S22. Rolling process: In the rolling process, the exit temperature of the roughing mill is 1135±15℃, the finishing mill temperature is 920±15℃, and the thickness of the hot-rolled strip is 2.5-5.5mm. The pass allocation and reduction rate of each pass in the roughing and finishing milling processes are calculated by the roughing setting model of the secondary system. S23. Cooling process: A two-stage cooling mode of "Section I ultra-fast cooling + Section II sparse cooling" is adopted. Immediately after final rolling, Section I ultra-fast cooling is adopted with a cooling rate >120℃ and a final cooling temperature of 640~680℃. Then, Section II sparse cooling is adopted and a three-stage U-shaped cooling is performed. The cooling temperature of the head 30-55m and tail 25-35m of the strip is 485±15℃, and the temperature of the middle part of the strip is 440-470℃, with a cooling rate of about 15-20℃ / s. S24. Coiling and annealing process: After adopting the II-stage sparse cooling process, the strip steel is coiled to obtain a hot-rolled coil, which is immediately placed in a resistance furnace. The resistance furnace temperature is set to 450±15℃, the annealing time is 2h, and then the coil is cooled to room temperature with the furnace.

4. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 1, characterized in that, In step S3, the initial microstructure of hot rolling is effectively broken up and introduced during the cold rolling process, wherein the grain size of ≥85% volume fraction is ≤1.2μm; and the grain size of ≤15% volume fraction is 1.2-2.0μm.

5. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 1, characterized in that, The annealing process in step S4 includes the following steps: S41. Continuous annealing process: The chilled strip steel is rapidly heated to 840-870℃ at a heating rate of 80-120℃ / s and held for 30-60s; after annealing, it is rapidly cooled to the over-aging temperature of 405±10℃ at a cooling rate of >70℃ / s, and the over-aging isothermal time is 60-120s; then the strip steel is cooled to room temperature at a cooling rate of 20℃ / s. S42. Leveling process: The actual elongation rate during the leveling process is controlled between 1.0% and 1.4%. S43. Uniform tempering process: The annealed strip is placed in an annealing softening furnace for tempering heat treatment. The tempering temperature is set to 430±15℃ and the tempering time is 60-120min.

6. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 5, characterized in that, The strip steel is subjected to tempering heat treatment to obtain TRIP-type bainitic steel.

7. The method for improving the performance of TRIP-type bainitic steel by microstructure control according to claim 6, characterized in that, The TRIP-type bainitic steel has a strength-ductility product of 26±2.5 GPa·%, tensile strength is stably controlled at 1550±25 MPa, yield strength is stably controlled at 1125±25 MPa, yield strength ratio is stably controlled at 0.73±0.018, and elongation is >15%.

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