Preparation method of TWIP steel with multi-gradient structure

Abstract

The invention discloses a preparation method of TWIP steel with a multi-gradient structure, and belongs to the technical field of steel rolling, and the preparation method comprises the following steps: taking a hot-rolled TWIP steel plate as a raw material, pre-annealing, and then water-cooling to obtain a parent TWIP steel plate; and the TWIP steel is placed on an asynchronous rolling mill to be subjected to multiple times of asynchronous rolling, annealing treatment is conducted in the rolling process, and the TWIP steel with the gradient grain structure and the TWIP steel with the multi-gradient structure are obtained in sequence. According to the method, the double advantages that the TWIP steel has a multi-deformation mechanism and gradient shear stress is generated through an asymmetrical rolling process are fully played, the TWIP steel of the multi-gradient structure with gradient dislocation density, gradient twin crystal spacing and gradient grain size distribution is prepared, and synergistic improvement of strength and plasticity is achieved; while the yield strength is remarkably improved, good ductility is kept, and the increasingly stringent requirements of modern automobiles for light weight, energy conservation, safety and the like can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of steel rolling, and in particular relates to a method for preparing TWIP steel with a multiple gradient structure. Background Art

[0002] In view of the dual technical challenges of energy conservation and emission reduction and collision safety faced by the current automotive industry, it is urgent to develop advanced high-strength steel materials with breakthrough mechanical properties. Twin-induced plasticity (TWIP) steel exhibits excellent mechanical properties due to its unique deformation twinning mechanism and continuous strain hardening characteristics. Its strength-plasticity product can reach more than 50,000 MPa·%, showing significant advantages in the field of lightweight body. However, this material system has the inherent defect of low initial yield strength (usually 200-400 MPa), which seriously restricts its bearing capacity and application in engineering. Although traditional grain refinement strengthening methods can improve yield strength, they often lead to significant deterioration of elongation. This inverted relationship between strength and plasticity has become the main bottleneck restricting the engineering application of high-performance TWIP steel. Therefore, the development of new microstructure control technology to significantly improve the yield strength of the material while maintaining its excellent plasticity has become a key scientific issue that needs to be urgently solved in the current research and development of high-strength and tough steel. Summary of the invention

[0003] In order to solve the above problems, the present invention provides a method for preparing TWIP steel with a multi-gradient structure.

[0004] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A method for preparing a multi-gradient structure TWIP steel comprises the following steps: S1: selecting a hot-rolled TWIP steel plate as a raw material, wherein the thickness of the hot-rolled TWIP steel plate is 1-5 mm; removing an oxide layer on the surface of the hot-rolled TWIP steel plate; S2: Pre-annealing is performed in a protective gas, the pre-annealing temperature is 700-900°C, and the time is 15-30 minutes; S3: Pre-annealing and water cooling to obtain the parent TWIP steel plate; S4: placing the parent TWIP steel plate on an asynchronous rolling mill for asynchronous rolling, wherein the roller speed ratio of the upper and lower rollers of the asynchronous rolling mill is 1-1.3, and performing annealing treatment at 750-850° C. and keeping warm for 10-15 minutes after 2-5 passes during the rolling process; obtaining a TWIP steel plate with a gradient grain structure; S5: The TWIP steel plate is continuously subjected to asynchronous rolling, and an annealing treatment is performed at 500-600°C for 10-15 min after 2-5 passes during the rolling process to obtain a multi-gradient structure TWIP steel.

[0005] Furthermore, the composition of the hot-rolled TWIP steel plate by mass percentage includes 15-30% of Mn, 0.3-0.6% of C, 0-3% of Al, 0-3% of Si, and the balance of Fe.

[0006] Furthermore, in step S1, the surface oxide layer is removed by sandblasting or pickling.

[0007] Furthermore, in step S2, the protective gas is Ar gas.

[0008] Furthermore, in step S3, after pre-annealing, it is water-cooled to room temperature.

[0009] Furthermore, the annealing treatments in steps S4 and S5 are both carried out in a protective gas, the heating rate is 10°C / min, and water cooling is carried out after the heat preservation ends. Among them, the protective gas is Ar gas, and a single-phase austenite structure is obtained after water cooling.

[0010] Furthermore, in step S4, the reduction per pass in the rolling process is 0.1-0.2 mm.

[0011] Furthermore, in steps S4 and S5, the plate is flipped and the feeding direction is alternately changed after each pass of rolling. When the plate thickness is close to the target value, the roll speed difference between the upper and lower rolls is gradually reduced until the roll speed ratio of the upper and lower rolls is 1:1.

[0012] Compared with the prior art, the technical progress achieved by the present invention is as follows: The present invention uses a hot-rolled TWIP steel plate as the raw material, water-cools it to room temperature after pre-annealing, then places it on an asynchronous rolling mill for asynchronous rolling, and finally undergoes an annealing treatment to obtain a multi-gradient structure TWIP steel. The present invention gives full play to the dual advantages of TWIP steel having multiple deformation mechanisms and the gradient shear stress generated by the asynchronous rolling process, prepares a multi-gradient structure TWIP steel with a gradient dislocation density, gradient twin spacing, and gradient grain size distribution, and realizes the simultaneous improvement of strength and plasticity; while significantly increasing the yield strength, it maintains an elongation similar to that of coarse grains, and can meet the increasingly stringent requirements of modern automobiles for lightweight, energy conservation, and safety. Brief Description of the Drawings

[0013] The drawings are used to provide a further understanding of the present invention, and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention, and do not constitute a limitation to the present invention.

[0014] In the drawings: Figure 1 is a schematic diagram of asynchronous rolling of a multi-gradient structure TWIP steel provided by an embodiment of the present invention; Figure 2 is a process flow chart for preparing a multi-gradient structure TWIP steel provided by an embodiment of the present invention; Figure 3 It is the morphology of the bent sheet during the asynchronous rolling process in the embodiment of the present invention; Figure 4 It is the morphology of the flattened sheet after rolling in the embodiment of the present invention; Figure 5 It is the EBSD characterization result diagram of the grain size in the edge and center regions of the GS1 sheet in the embodiment of the present invention; Figure 6 It is the statistical result diagram of the grain size in the edge region of the GS1 sheet in the embodiment of the present invention; Figure 7 It is the statistical result diagram of the grain size in the center region of the GS1 sheet in the embodiment of the present invention; Figure 8 It is the XRD characterization result diagram of the dislocation density in the edge and center regions of the GS1 sheet in the embodiment of the present invention; Figure 9 It is the TEM characterization result diagram of the twin spacing and lamellar thickness in the edge and center regions of the GS1 sheet in the embodiment of the present invention; Figure 10 It is the engineering stress-strain curve diagram of comparing GS1, GS0.6, and GS0.4 with the comparative example HS2 in the embodiment of the present invention; Figure 11 It is the statistical chart of the elongation and yield strength of comparing GS1, GS0.6, and GS0.4 with the comparative example HS2 and the approximate composition TWIP steel in the embodiment of the present invention; In the figure: 1 - upper working roll; 2 - lower working roll; 3 - TWIP steel sheet. Specific embodiments

[0015] The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be repeated in some embodiments. The embodiments of the present invention will be described below with reference to the accompanying drawings.

[0016] A preparation method of a multi-gradient structure TWIP steel provided by the present invention includes the following steps: S1: Select a hot-rolled TWIP steel sheet as the raw material, and the thickness of the hot-rolled TWIP steel sheet is 1 - 5 mm; remove the surface oxide layer of the hot-rolled TWIP steel sheet by sandblasting or pickling; The composition of the selected hot-rolled TWIP steel sheet by mass percentage includes 15 - 30% of Mn, 0.3 - 0.6% of C, 0 - 3% of Al, 0 - 3% of Si, and the balance of Fe.

[0017] S2: Perform pre-annealing in an Ar protective gas, the pre-annealing temperature is 700 °C, and the time is 30 min; S3: Water-cool the material to room temperature after pre-annealing to obtain the parent TWIP steel plate. The purpose is to eliminate the residual stress of hot rolling and homogenize the microstructure.

[0018] S4: Place the parent TWIP steel plate on an asynchronous rolling mill with separately adjustable upper and lower roll speeds for asynchronous rolling. It should be noted that the thickness of the plate corresponds to the roll diameter of the rolling mill. When the target plate is thinner, a smaller roll diameter is selected.

[0019] The rolling process adopts an asynchronous rolling process with multiple passes and small reduction ratios. During the rolling process, the roll speed ratio of the upper and lower rolls of the asynchronous rolling mill is 1 - 1.3. The roll speed difference between the upper and lower rolls ensures that a gradient shear strain is generated during the rolling process of the TWIP steel plate, and the deformation area is mainly concentrated on the surface of the plate. Considering that the plastic deformation of TWIP steel is jointly dominated by the slip and twinning mechanisms, the surface of the TWIP steel plate has a higher dislocation density and a higher twin volume.

[0020] During the rolling process, a progressive deformation is carried out with a single-pass reduction of 0.1 - 0.2 mm. This process design has multiple advantages: firstly, the lower deformation per pass can significantly reduce the rolling force requirement and simultaneously reduce the probability of surface defects; secondly, this precisely controlled progressive deformation method can ensure that the plate maintains good shape quality.

[0021] During the asynchronous rolling process, an annealing treatment at 750 - 850 °C for 10 - 15 minutes is carried out after every 2 - 5 passes; to obtain a TWIP steel plate with a gradient grain structure.

[0022] The annealing treatment is preferably carried out in a protective gas, with a heating rate of 10 °C / min, and water cooling is carried out after the holding is completed. In this temperature range, due to the high concentrated stress and dislocation density accumulated on the surface layer of the TWIP steel plate, fine grain structures can be formed on the surface layer of the plate through the recrystallization process. This process design can effectively prepare a TWIP steel plate with a gradient grain structure, that is, the surface layer is a fine grain area while the core part remains relatively coarse grain structure.

[0023] S5: Continue to perform asynchronous rolling on the TWIP steel plate, and carry out an annealing treatment at 500 - 600 °C for 10 - 15 minutes after every 2 - 5 passes during the rolling process to obtain a TWIP steel with a multi-gradient structure.

[0024] Similarly, the annealing treatment is preferably carried out in a protective atmosphere, with a heating rate of 10 °C / min, and water cooling is carried out after the holding is completed. This annealing temperature can ensure that while reducing work hardening, the formed twin structure is retained, and by controlling the matching of the deformation and recovery processes, a TWIP steel with a multi-gradient structure of gradient dislocation density, gradient twin spacing, and gradient grain size is finally obtained.

[0025] In addition, in order to obtain a sheet with good flatness, the following process control measures are taken during asynchronous rolling: First, for the problem that the rolled sheet is prone to bending, a strategy of flipping the steel sheet after each rolling pass and alternately changing the feeding direction is adopted; Second, when the sheet thickness approaches the target value, the roll speed difference between the upper and lower rolls is gradually reduced until the roll speed ratio of the upper and lower rolls is 1:1. While maintaining the effect of asynchronous rolling, the surface flatness of the material is significantly improved. This composite process design not only ensures the formation of a gradient structure but also effectively controls the flatness of the sheet.

[0026] Through the above preparation process, a multi-gradient structure TWIP steel sheet with a flat surface can be obtained.

[0027] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and the accompanying drawings.

[0028] In this specific embodiment, the preparation process of the multi-gradient structure TWIP steel is as Figure 2 shown. First, a hot-rolled TWIP steel sheet is selected as the initial TWIP steel. Calculated by mass percentage, the initial TWIP steel consists of the following components: the content of manganese (Mn) is 15.68%, the content of carbon (C) is 0.59%, the content of aluminum (Al) is 1.19%, the content of silicon (Si) is 0.047%, and the rest is iron (Fe).

[0029] The thickness of the initial TWIP steel sheet is 2 mm. First, sandblasting is carried out to remove the surface oxide layer, and then pre-annealing is carried out at 700 °C for 30 min under argon protection, and then water-cooled to room temperature. The hot-rolled residual stress is eliminated and the structure is homogenized. The average grain size of the parent TWIP steel sheet obtained after annealing is 9 μm.

[0030] As Figure 1 shown, the parent TWIP steel sheet is placed on an asynchronous rolling mill with separately adjustable upper and lower roll speeds for asynchronous rolling. Figure 3 and Figure 4 are the sheet shapes during and after rolling respectively. During asynchronous rolling, the sheet shows an arc-shaped bend. After approaching the target thickness, the roll speed difference is reduced, and finally a good sheet shape is obtained.

[0031] The working roll diameter of the asynchronous rolling mill is 20 mm. The roll speed of the upper working roll 1 is adjusted to 1 m / min, and the roll speed of the lower working roll 2 is adjusted to 1.3 m / min to establish a roll speed ratio of 1:1.3. The roll speed difference between the upper and lower rolls generates a gradient shear strain on the surface of the TWIP steel sheet, resulting in a high dislocation density and twinning volume on the surface of the TWIP steel. While the core still maintains a relatively low degree of deformation and still retains a coarse-grained structure.

[0032] During the rolling process, progressive deformation is carried out with a single-pass reduction of 0.1 mm. This process design has multiple advantages: Firstly, the lower deformation per pass can significantly reduce the rolling force requirement and at the same time decrease the probability of surface defects; Secondly, this precisely controlled progressive deformation method can ensure that the sheet metal maintains good shape quality.

[0033] During the asynchronous rolling process, an annealing treatment process is carried out after 5 passes of rolling. The annealing treatment is carried out in an Ar gas atmosphere; the heating rate is 10 °C / min, heated to 800 °C and held for 15 min, and after the holding is completed, water cooling is carried out. After water cooling, a single-phase austenite phase structure is obtained. At this temperature, due to the accumulation of high concentrated stress and dislocation density on the surface layer of the TWIP steel sheet, fine grain structures can be formed on the surface layer of the sheet through the recrystallization process. This process design can effectively prepare TWIP steel sheets with a gradient grain structure, that is, the surface layer is a fine grain area while the core remains relatively coarse grain structure.

[0034] The obtained TWIP steel sheet with a gradient grain structure is further subjected to asynchronous rolling, and an annealing treatment process at 550 °C for 15 min is carried out after an interval of 3 - 5 passes. The annealing treatment is carried out in an Ar gas atmosphere, the heating rate is 10 °C / min, and water cooling is carried out after the holding is completed. This annealing temperature can ensure that while reducing work hardening, the formed twin structure is retained. By regulating the matching of the deformation and recovery processes, a multi-gradient structure TWIP steel with gradient dislocation density, gradient twin spacing, and gradient grain size is finally obtained.

[0035] During the rolling process, due to the relatively large rotational speed of the lower working roll, greater deformation will occur below the parent material, which will in turn cause the parent material to bend upward. To obtain sheets with good shape, for the problem that the rolled sheets are prone to bending, a strategy of flipping the steel sheet after each pass of rolling and alternately changing the feeding direction is adopted. When the sheet thickness approaches the target value, the reduction amount is no longer increased, and the roll speed difference between the upper and lower rolls is gradually reduced until the roll speed ratio of the upper and lower rolls is 1:1. By gradually reducing the roll speed difference, while maintaining the effect of asynchronous rolling, the surface flatness of the material is significantly improved. This composite process design not only ensures the formation of a gradient structure but also effectively controls the flatness of the sheet.

[0036] After the above series of technological processes, a TWIP steel with a multi-gradient structure can finally be manufactured. In this embodiment, it is named according to the thickness of the multi-gradient structure steel after rolling. GS1 is the multi-gradient structure TWIP steel with a thickness of 1 mm, and HS2 is the initial TWIP steel with a thickness of 2 mm. It gradually transitions from the surface layer (with a strengthening surface having a smaller grain size, smaller twin lamellae, and a higher dislocation density) to the central region (with a structure having a larger grain size, larger twin lamellae, and a lower dislocation density). The surface layer serves as the strengthening layer, providing higher strength for the material, while the central structure can provide better plasticity for the material, thus achieving an excellent combination of strength and plasticity. In addition, the gradient structure will produce a strain gradient effect during the deformation process, thereby generating additional geometrically necessary dislocations, which cause additional strengthening and hardening effects. In the present invention, the yield strength of the multi-gradient structure TWIP steel is approximately 1300 MPa, and the elongation is 17%.

[0037] In the following specific embodiments, the preparation process steps of GS0.6, GS0.4, and GS1 are the same. The difference is that the reduction amount in the preparation of GS1 is 1 mm, and the reduction amounts of GS0.6 and GS0.4 are 1.4 mm and 1.6 mm respectively. GS1 is annealed at 800 °C once and 550 °C once for 15 min. GS0.6 is annealed at 800 °C once and 550 °C twice for 15 min, and GS0.4 is annealed at 800 °C once and 550 °C three times for 15 min.

[0038] The EBSD characterization results of the grain sizes in the edge and central regions of the GS1 sheet are as Figure 5 shown; the statistical results of the grain sizes in the edge and central regions of the GS1 sheet are as Figure 6 、 7 shown. The average grain size in the edge region is 4 μm, and the average grain size in the core region is 7 μm. The XRD characterization results of the dislocation densities in the edge and central regions of the GS1 sheet are as Figure 8 shown. The TEM characterization results of the twin spacing and lamella thickness in the edge and central regions of the GS1 sheet are as Figure 9 shown. The following Table 1 summarizes the grain sizes, dislocation densities, and twin spacings in the edge and central regions of the GS1 sheet in the embodiment.

[0039]

[0040] The average grain size at the edge is 4 μm, the dislocation density is 1.1×10 15 m -2 , the average twin spacing is 18 nm, the average grain size in the central region is 7 μm, and the dislocation density is 9×10 14 m -2, the average twin spacing is 20 nm. That is, the edge region has smaller grain size, higher dislocation density and smaller twin spacing compared with the central region, constituting a multi-gradient structure TWIP steel with gradient grain, gradient dislocation density and gradient twin spacing distribution.

[0041] Figure 10 and Figure 11 are the results of room temperature tensile tests on the obtained multi-gradient structure TWIP steel according to the national standard GB / T228.1-2021 Method B. It can be seen from Figure 11 that compared with Examples GS1, GS0.6 and GS0.4, the yield strength of Comparative Example HS2 is significantly improved. To further compare the contribution of gradient structuring to the synergistic improvement of strength and plasticity, Figure 11 also gives the results of the matching relationship between the yield strength and elongation of various TWIP steels with similar compositions. It can be seen from Figure 11 that the strength and plasticity effect of the multi-gradient structure TWIP steel is significantly better than that of other TWIP steels with similar compositions, and the multi-gradient structuring significantly improves the comprehensive mechanical properties of the TWIP steel.

[0042] In summary, the present invention uses hot-rolled TWIP steel plate as raw material, cools it to room temperature by pre-annealing and then water-cooling, and then places it on an asynchronous rolling mill for asynchronous rolling. During the rolling process, it undergoes multiple annealing treatments to obtain a multi-gradient structure TWIP steel. It has the following advantages: (1) Prepare a multi-gradient structure TWIP steel with gradient grains, gradient dislocations and gradient twins, which has a good synergistic improvement effect of strength and plasticity.

[0043] (2) The multi-gradient structure TWIP steel is composed of an asynchronous rolling process and an annealing process, and has high process stability. This rolling process can be applied to industrial production.

[0044] The present invention gives full play to the dual advantages of TWIP steel having multiple deformation mechanisms and the gradient shear stress generated by the asynchronous rolling process, and prepares a multi-gradient structure TWIP steel with gradient dislocation density, gradient twin spacing and gradient grain size distribution. The gradient structuring combines high-strength fine grains and high-plasticity coarse grains in an orderly manner, successfully breaking through the technical bottleneck of the strength-plasticity mismatch in the traditional grain refinement process. The multi-gradient structure TWIP steel prepared by the present invention can achieve the synergistic improvement of strength and plasticity, significantly improve the yield strength while maintaining an elongation similar to that of coarse grains, and can meet the increasingly stringent requirements of modern automobiles for lightweight, energy conservation and safety.

[0045] Finally, it should be noted that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing TWIP steel with a multi-gradient structure, characterized in that: The following steps are involved: S1: selecting a hot-rolled TWIP steel plate as a raw material, wherein the thickness of the hot-rolled TWIP steel plate is 1-5 mm; removing an oxide layer on the surface of the hot-rolled TWIP steel plate; S2: Pre-annealing is performed in a protective gas, the pre-annealing temperature is 700-900°C, and the time is 15-30 minutes; S3: Pre-annealing and water cooling to obtain the parent TWIP steel plate; S4: placing the parent TWIP steel plate on an asynchronous rolling mill for asynchronous rolling, wherein the roller speed ratio of the upper and lower rollers of the asynchronous rolling mill is 1-1.3, and performing annealing treatment at 750-850° C. and keeping warm for 10-15 minutes after 2-5 passes during the rolling process; obtaining a TWIP steel plate with a gradient grain structure; S5: The TWIP steel plate is continuously subjected to asynchronous rolling, and an annealing treatment is performed at 500-600°C for 10-15 min after 2-5 passes during the rolling process to obtain a multi-gradient structure TWIP steel.

2. The method for preparing a multi-gradient structure TWIP steel according to claim 1, characterized in that: The composition of the hot-rolled TWIP steel plate includes, by mass percentage, 15-30% Mn, 0.3-0.6% C, 0-3% Al, 0-3% Si and the balance Fe.

3. The method for preparing a multi-gradient structure TWIP steel according to claim 2, characterized in that: In step S1, the surface oxide layer is removed by sandblasting or pickling.

4. The method for preparing a multi-gradient structure TWIP steel according to claim 2, characterized in that: In step S2, the protective gas is Ar gas.

5. The method for preparing a multi-gradient structure TWIP steel according to claim 4, characterized in that: In step S3, the pre-annealing is followed by water cooling to room temperature.

6. The method for preparing a multi-gradient structure TWIP steel according to claim 2, characterized in that: The annealing treatments in steps S4 and S5 are both carried out in a protective gas, with a heating rate of 10° C. / min, and water cooling is performed after the insulation is completed.

7. The method for preparing a TWIP steel with a multi-gradient structure according to claim 6, characterized in that: In step S4, the reduction amount of a single pass during the rolling process is 0.1-0.2 mm.

8. The method for preparing a TWIP steel with a multi-gradient structure according to claim 7, characterized in that: In steps S4 and S5, the plate is turned over after each rolling pass and the feeding direction is changed alternately. When the plate thickness approaches the target value, the speed difference between the upper and lower rollers is gradually reduced until the speed ratio of the upper and lower rollers is 1:1.

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