A preparation method of a heterogeneous structure DP980 steel based on concave-convex roll forming

Abstract

The application discloses a preparation method of a heterogeneous structure DP980 steel based on concave-convex rolling, and belongs to the field of advanced steel materials. The application adds Nb and Ti elements to carry out solid solution strengthening, precipitation strengthening and fine-grain strengthening in the DP980 steel, then designs a unique rolling process to roll the DP980 steel plate with uneven thickness, and then carries out heat treatment to make the DP980 steel plate appear heterogeneous structure to improve yield strength. The specific steps are as follows: S1, adding 0.02-0.05% Nb and 0.01-0.03% Ti on the basis of the traditional DP980 composition when smelting the ingot; S2, adopting the concave-convex roller to roll the billet during rolling; and S3, carrying out a critical annealing process of 790-810 DEG C for 2-5 minutes for the rolled steel plate. The application can make the DP980 steel obtain the heterogeneous structure with uneven structure, and the ferrite grain size in the high deformation area is smaller. The DP980 steel prepared by the process has the yield strength of 720-800 MPa, and the yield strength is improved by about 15% compared with the existing DP980 steel.

Description

Technical Field

[0001] This invention relates to the field of advanced steel materials technology, and more specifically, to a method for preparing heterogeneous DP980 steel based on convex-concave roll forming. Background Technology

[0002] DP980 steel is an advanced high-strength steel that combines high strength, good formability, and lightweight properties. It is often used in the automotive manufacturing field, such as A-pillars, B-pillars, and anti-collision beams. However, its relatively low yield strength limits the application areas of DP980 steel.

[0003] Adding alloying elements and designing heterogeneous structures have become the main approaches to improving the yield strength of materials. Currently, the preparation of heterogeneous structures mainly relies on surface machining, chemical deposition, and other methods. However, these methods increase the cost of materials, and solid solution strengthening, grain refinement strengthening, and precipitation strengthening are limited by the chemical composition of the material. Due to the chemical composition factors of traditional DP980 steel, it is difficult to effectively carry out solid solution strengthening, grain refinement strengthening, and precipitation strengthening.

[0004] In view of the above situation, the present invention provides a method for preparing heterogeneous DP980 steel based on concave-convex roll rolling. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, the present invention provides a method for preparing heterogeneous DP980 steel based on concave-convex roll rolling, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing heterogeneous DP980 steel based on convex-concave roll forming, comprising the following steps:

[0007] S1. Smelting: Steel ingots are obtained by proportioning, smelting, and casting according to the following formula: C: 0.08-0.12wt.%, Mn: 1-2.5wt.%, Si: 0.2-0.5wt.%, Al: 0.3-0.7wt.%, Nb: 0.02-0.05wt.%, Ti: 0.08-0.12wt.%, S: 0.001-0.015wt.%, P: 0.001-0.015wt.%, with the balance being iron and unavoidable impurities.

[0008] S2. Forging: Heat the steel ingot to 1200-1250℃, hold for 2-3 hours, and forge it into a steel billet;

[0009] S3. Hot rolling: The steel billet is heated to 1100-1150℃, held for 2-3 hours, and then hot rolled in 6-7 passes at a rolling temperature range of 800-1000℃ with a total reduction rate of 40-50%. It is then cooled to room temperature.

[0010] S4. Cold rolling: The hot-rolled plate is cold-rolled using concave and convex rolls (crest / trough height difference Δh=0.2mm, wavelength 300μm) for 6-7 passes, with a total reduction rate of 50-90%, to obtain the cold-rolled plate.

[0011] S5. Critical Annealing: The rolled steel plate is subjected to a critical annealing process at 790℃-810℃ for 2-5 minutes to obtain high yield heterogeneous cold-rolled DP980 steel.

[0012] Preferably, the microalloying content added in step 1 is Nb: 0.02-0.05 wt.%, Ti: 0.08-0.12 wt.

[0013] When the Nb content is less than 0.02 wt.%, the solid solution dragging effect is not obvious, it cannot effectively refine the grains, and it cannot form enough precipitates, resulting in insufficient precipitation strengthening effect. When it is greater than 0.05 wt.%, the precipitation strengthening effect is not significant, which will lead to waste of Nb and reduce the toughness of the material. When the Ti content is less than 0.01 wt.%, the precipitates cannot effectively pin the interface; when it is greater than 0.03 wt.%, the precipitation strengthening effect is not significant, which will lead to waste of Nb and reduce the toughness of the material.

[0014] Preferably, the thickness of the forged steel billet in step 2 is between 8-10 mm, in order to facilitate subsequent forging.

[0015] Preferably, the hot rolling temperature in step 3 is 1100-1150℃. A higher temperature facilitates the precipitation of the precipitated phase and enhances the precipitation strengthening effect.

[0016] Preferably, in step 4, the peak / trough height difference Δh of the concave-convex roll is 0.2 mm, and the wavelength is 300 μm. The height difference between the peak and trough directly affects the local reduction difference during rolling. When Δh = 0.2 mm, the actual reduction in the peak area is about 15-20% higher than the nominal value, and the reduction in the trough area is 10-15% lower, forming a significant dislocation density gradient. Furthermore, the ferrite grain size in DP980 steel is generally 3-5 μm, and the martensite lath width is generally 0.2-0.5 μm. A wavelength of 300 μm ensures that a complete hard / soft phase alternation distribution is formed within each waveform cycle (about 60-100 grain widths), avoiding disruption of the microstructure continuity. Finally, through the accumulation of uneven strain energy through deformation inhomogeneity, a driving force is provided for recrystallization in the subsequent critical heat treatment process, thereby forming a microstructure with a wavy grain size distribution.

[0017] Preferably, in step 4, during the rolling process, the particles precipitated during forging and hot rolling are squeezed from the high deformation zone to the low deformation zone. During this process, the precipitated particles can further break the grains in the high deformation zone, and at the same time, the strength of the low deformation zone is improved because the precipitated phases accumulate in the low deformation zone.

[0018] Preferably, the critical annealing is performed in the critical region of 790℃-810℃ (Ac1-Ac3 range) for 2-5 minutes. 790℃-810℃ is in the two-phase region, which ensures that DP980 forms a two-phase structure of martensite and ferrite after cooling. Holding for less than 2 minutes cannot be used to further refine the grains by dynamic recrystallization, while holding for more than 5 minutes will result in coarse grains and the formation of precipitates to further improve the strength.

[0019] The technical effects and advantages of this invention are as follows:

[0020] This invention achieves a breakthrough in the performance of DP980 steel through multi-process collaborative innovation. By introducing Nb and Ti microalloying elements into the composition design, combined with a unique cold rolling process (using concave and convex rolls) and critical annealing technology, the deformation energy storage of precipitated phases drives the dynamic recrystallization of ferrite, forming a wavy fine-grained structure and precipitating Nb and Ti phases. Through the multi-scale synergistic effect of fine-grain strengthening, precipitation strengthening and solid solution strengthening, the yield strength of the material is increased to over 700 MPa, and the tensile strength reaches 980 MPa while maintaining an elongation of 18-22%, successfully improving the yield strength of DP980. Attached Figure Description

[0021] Figure 1 This is a flowchart of the preparation process of the present invention.

[0022] Figure 2 The microstructure of DP980 steel after traditional processing.

[0023] Figure 3 The microstructure of DP980 steel prepared according to the present invention. Detailed Implementation

[0024] Case 1:

[0025] A novel method for preparing high-yield heterostructure cold-rolled DP980 steel, the specific steps of which include:

[0026] S1. Preparation: Steel ingots are prepared by proportioning, smelting and casting according to the following mass percentages: C: 0.08%, Mn: 1%, Si: 0.2%, Al: 0.3%, Nb: 0.02%, Ti: 0.08%, S: 0.015%, P: 0.015%, with the balance being iron and unavoidable impurities.

[0027] S2. Forging: Heat the steel ingot to 1200℃, hold for 2 hours, and forge it into a steel billet with a size of 100mm*10mm.

[0028] S3. Hot rolling: The steel billet is heated to 1100℃, held for 2 hours, and then hot rolled in 6-7 passes at a rolling temperature range of 800-1000℃ to a thickness of 6mm. It is then cooled to room temperature to obtain a 6mm thick hot-rolled plate.

[0029] S4. Cold rolling: The hot-rolled plate is cold-rolled using concave and convex rolls (crest / trough height difference Δh=0.2mm, wavelength 300mm) for 6-7 passes to roll to 2mm thickness, resulting in a 2mm thick cold-rolled plate.

[0030] S5. Critical Annealing: The rolled steel plate is subjected to a critical annealing process at 790℃ for 2 minutes to obtain high yield heterogeneous cold-rolled DP980 steel.

[0031] 10mm × 10mm heat-treated samples were prepared by wire cutting. The samples were progressively ground down to 2000# using different grades of sandpaper, followed by polishing and etching. The DP980 steel samples were then characterized using an optical metallographic microscope. The characterization results are as follows: Figure 2 As shown. The results show that: (1) Compared with the current traditional cold rolling process for DP980 steel, the microstructure of martensite and ferrite is obtained ( Figure 1 The microstructure produced by the process of this invention exhibits a wavy, fine-grained structure. Tensile property tests were then conducted on both conventional and experimental samples processed according to this invention. The results showed that this method can achieve a yield strength of 750 MPa and an elongation of 15% for DP980 steel. Therefore, this invention is a method for improving the yield strength of DP980 steel.

[0032] Case 2:

[0033] A novel method for preparing high-yield heterostructure cold-rolled DP980 steel, the specific steps of which include:

[0034] S1. Preparation: Steel ingots are prepared by proportioning, smelting and casting according to the following mass percentages: C: 0.12%, Mn: 2.5%, Si: 0.5%, Al: 0.7%, Nb: 0.05%, Ti: 0.12%, S: 0.015%, P: 0.015%, with the balance being iron and unavoidable impurities.

[0035] S2. Forging: Heat the steel ingot to 1250℃, hold for 3 hours, and forge it into a steel billet with dimensions of 100mm*9mm;

[0036] S3. Hot rolling: The steel billet is heated to 1100-1150℃, held for 3 hours, and then hot rolled in 6-7 passes at a rolling temperature range of 800-1000℃ to a thickness of 4.5mm. It is then cooled to room temperature to obtain a 4.5mm thick hot-rolled plate.

[0037] S4. Cold rolling: The hot-rolled plate is cold-rolled using concave and convex rolls (crest / trough height difference Δh=0.2mm, wavelength 300mm) for 6-7 passes to obtain a 1.2mm cold-rolled plate.

[0038] S5. Critical Annealing: The rolled steel plate is subjected to a critical annealing process at 810℃ for 5 minutes to obtain high yield heterogeneous cold-rolled DP980 steel.

[0039] Tensile property tests were conducted on these two samples, and the results showed that this method can achieve a yield strength of 736 MPa and an elongation of 12% for DP980 steel. Therefore, the process of this invention is a method that can improve the yield strength of DP980 steel.

[0040] Case 3:

[0041] A novel method for preparing high-yield heterostructure cold-rolled DP980 steel, the specific steps of which include:

[0042] S1. Preparation: Steel ingots are prepared by proportioning, smelting and casting according to the following mass percentages: C: 0.1%, Mn: 2%, Si: 0.3%, Al: 0.5%, Nb: 0.03%, Ti: 0.1%, S: 0.01%, P: 0.01%, with the balance being iron and unavoidable impurities.

[0043] S2. Forging: Heat the steel ingot to 1200-1250℃, hold for 3 hours, and forge it into a steel billet with a size of 100mm*8mm;

[0044] S3. Hot rolling: The steel billet is heated to 1125℃, held for 2 hours, and then hot rolled in 6-7 passes at a rolling temperature range of 800-1000℃ to a thickness of 5mm. It is then cooled to room temperature to obtain a 5mm thick hot-rolled plate.

[0045] S4. Cold rolling: The hot-rolled plate is cold-rolled using convex and concave rolls (crest / valley height difference Δh=0.2mm, wavelength 300mm) for 6-7 passes to obtain a 1.5mm thick cold-rolled plate.

[0046] S5. Critical Annealing: The rolled steel plate is subjected to a critical annealing process at 800℃ for 3 minutes to obtain high yield heterogeneous cold-rolled DP980 steel.

[0047] Tensile property tests were conducted on these two samples, and the results showed that this method can achieve a yield strength of 782 MPa and an elongation of 13% for DP980 steel. Therefore, the process of this invention is a method that can improve the yield strength of DP980 steel.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a novel high-yield heterostructure cold-rolled DP980 steel, characterized in that, Includes the following steps: S1. Smelting: Steel ingots are obtained by proportioning, smelting, and casting according to the following formula: C: 0.08-0.12wt.%, Mn: 1-2.5wt.%, Si: 0.2-0.5wt.%, Al: 0.3-0.7wt.%, Nb: 0.02-0.05wt.%, Ti: 0.08-0.12wt.%, S: 0.001-0.015wt.%, P: 0.001-0.015wt.%, with the balance being iron and unavoidable impurities. S2. Forging: Heat the steel ingot to 1200-1250℃, hold for 2-3 hours, and forge it into a steel billet; S3. Hot rolling: The steel billet is heated to 1100-1150℃, held for 2-3 hours, and then hot rolled in 6-7 passes at a rolling temperature range of 800-1000℃ with a total reduction rate of 40-50%. It is then cooled to room temperature. S4. Cold rolling: The hot-rolled plate is cold-rolled using concave and convex rolls (crest / trough height difference Δh=0.2mm, wavelength 300μm) for 6-7 passes, with a total reduction rate of 50-90%, to obtain the cold-rolled plate. S5. Critical Annealing: The rolled steel plate is subjected to a critical annealing process at 790℃-810℃ for 2-5 minutes to obtain high yield heterogeneous cold-rolled DP980 steel.

2. A method for preparing a novel high-yield heterogeneous cold-rolled DP980 steel according to claim 1, characterized in that: In step 1, the microalloying content is Nb: 0.02-0.05 wt.%, Ti: 0.08-0.12 wt.%. When the Nb content is less than 0.02 wt.%, the solid solution dragging effect is not obvious, it cannot effectively refine the grains, and it cannot form enough precipitates, resulting in insufficient precipitation strengthening effect. When it is greater than 0.05 wt.%, the precipitation strengthening effect is not significant, which will lead to waste of Nb and reduce the toughness of the material. When the Ti content is less than 0.01 wt.%, the precipitates cannot effectively pin the interface; when it is greater than 0.03 wt.%, the precipitation strengthening effect is not significant, which will lead to waste of Nb and reduce the toughness of the material.

3. A method for preparing a novel high-yield heterostructure cold-rolled DP980 steel according to claim 1, characterized in that: The thickness of the forged steel billet in step 2 is between 8-10mm, which is to facilitate subsequent forging.

4. A method for preparing a novel high-yield heterostructure cold-rolled DP980 steel according to claim 1, characterized in that: In step 3, the hot rolling temperature is 1100-1150℃. The higher temperature facilitates the precipitation of the precipitated phase and enhances the precipitation strengthening effect.

5. A method for preparing a novel high-yield heterogeneous cold-rolled DP980 steel according to claim 1, characterized in that: In step 4, the peak / trough height difference of the concave and convex rolls is Δh = 0.2 mm, with a wavelength of 300 μm. This height difference directly affects the local reduction difference during rolling. When Δh = 0.2 mm, the actual reduction in the peak area is about 15-20% higher than the nominal value, while it is 10-15% lower in the trough area, forming a significant dislocation density gradient. Furthermore, the ferrite grain size in DP980 steel is generally 3-5 μm, and the martensite lath width is generally 0.2-0.5 μm. A wavelength of 300 μm ensures a complete alternating distribution of hard and soft phases within each waveform cycle (approximately 60-100 grain widths), avoiding disruption of the microstructure continuity. Ultimately, the uneven strain energy accumulated through deformation provides the driving force for recrystallization during subsequent critical heat treatment, resulting in a microstructure with a wavy grain size distribution.

6. A method for preparing a novel high-yield heterostructure cold-rolled DP980 steel according to claim 1, characterized in that: In step 4, during the rolling process, the particles precipitated during forging and hot rolling are squeezed from the high deformation zone to the low deformation zone. During this process, the precipitated particles can further break the grains in the high deformation zone, and at the same time, the strength of the low deformation zone is improved because the precipitated phases accumulate in the low deformation zone.

7. A method for preparing a novel high-yield heterogeneous cold-rolled DP980 steel according to claim 1, characterized in that: Critical annealing involves holding the material in the critical region (Ac1-Ac3 range) at 790℃-810℃ for 2-5 minutes. The 790℃-810℃ range is in the two-phase region, ensuring that DP980 forms a two-phase structure of martensite and ferrite after cooling. Holding the material for less than 2 minutes does not allow for further grain refinement through dynamic recrystallization, while holding it for more than 5 minutes will result in coarse grains and the formation of precipitates that further enhance strength.

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