A twin-induced plasticity steel and a method of manufacturing the same

Abstract

The application provides a kind of twinning induced plasticity steel and its preparation method, and relates to the technical field of high strength and toughness steel iron material preparation, the twinning induced plasticity steel has sandwich composite structure, the sandwich composite structure includes first ultrafine grain area, second ultrafine grain area and the coarse grain area between the first ultrafine grain area and the second ultrafine grain area. Utilize the characteristics of high strength, high passivation ability of ultrafine grain structure to strengthen TWIP steel and retain corrosion resistance; At the same time, the soft coarse grain structure of the core area is retained, the core coarse grain area is used to coordinate deformation, and the stress concentration of the processing area in the stretching process is eliminated, the strength is improved while the loss of plasticity is minimized, and good strength and plasticity matching is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-toughness steel material preparation, and particularly relates to a high-strength and high-plasticity corrosion-resistant twinning-induced plasticity steel and a preparation method thereof. BACKGROUND

[0002] Twinning-induced plasticity (TWIP) steel has outstanding work hardening capacity, excellent uniform elongation and high tensile strength, and has broad application prospects in many fields, and has become a research hotspot that people widely pay attention to in recent years. At present, TWIP steel can be mainly divided into Fe-Mn-C system, Fe-Mn-Al-Si system and Fe-Mn-Cr-C-N system according to its chemical composition. These TWIP steels have excellent mechanical properties, but the Fe-Mn-C system and the Fe-Mn-Al-Si system do not have corrosion resistance. The Fe-Mn-Cr-C-N system TWIP steel mainly stabilizes austenite with Mn elements, adds Cr elements to endow the material with corrosion resistance and increase the solubility of N elements, and the N elements are dissolved in the steel in the form of interstitial atoms, which can not only effectively stabilize austenite, but also play the role of solid solution strengthening and improve corrosion resistance, so this new type of TWIP steel not only has excellent plasticity, but also has significantly enhanced corrosion resistance, and is an advanced steel with excellent performance.

[0003] For the Fe-Mn-Cr-C-N system TWIP steel, although the tensile strength can reach more than 1 GPa, the yield strength is mostly lower than 500 MPa, and the low yield strength seriously limits its application range. Therefore, how to improve the yield strength under the premise of ensuring plasticity and corrosion resistance is a key problem for further research and application of the Fe-Mn-Cr-C-N system TWIP steel.

[0004] In the prior art, a heterogeneous structure of surface nanocrystalline-core coarse grain is prepared by cold deformation method such as ultrasonic surface rolling treatment, which significantly improves the strength of the TWIP steel, and the plasticity is not obviously reduced. However, for the Fe-Mn-Cr-C-N system TWIP steel, the high-density dislocations introduced by ultrasonic surface rolling treatment will cause the stability of the passivation film to decrease, and the corrosion resistance will be significantly deteriorated, so it is not applicable; there is also a method of carrying out hot dip aluminum plating on the TWIP steel after cold deformation to improve the corrosion resistance of the TWIP steel on the basis of ensuring high strength and plasticity, but this process needs to be strictly treated for surface oil and rust removal and complex plating aid treatment, which increases the process cost and reduces the production efficiency, especially the Fe-Mn-Cr-C-N system TWIP steel has a wide sensitization interval, and the heat action will cause the second phase to precipitate, so that the strength is reduced and the corrosion resistance is difficult to guarantee. Therefore, it is necessary to provide a TWIP steel with high strength and plasticity and corrosion resistance. SUMMARY

[0005] Therefore, the application provides a twin-induced plasticity steel and a preparation method thereof to solve the problem that the existing twin-induced plasticity steel cannot have high strength plasticity and corrosion resistance.

[0006] To solve the above problems, the application provides a twin-induced plasticity steel having a sandwich composite structure, which comprises a first ultra-fine grain region, a second ultra-fine grain region and a coarse grain region between the first and second ultra-fine grain regions.

[0007] Further, the microstructure of the first and second ultra-fine grain regions is equiaxed austenite; and / or

[0008] The average grain size of the first and second ultra-fine grain regions is 0.1-0.9 microns; and / or

[0009] The average grain width of the coarse grain region is 20-60 microns.

[0010] Further, the thickness ratio of the first ultra-fine grain region, the coarse grain region and the second ultra-fine grain region is 3:1:3-1:5:1.

[0011] In another aspect, the application also provides a preparation method of the twin-induced plasticity steel, comprising the following steps: first performing underwater friction stir processing on the upper surface of a base material, and then performing secondary underwater friction stir processing on the lower surface of the base material to obtain the twin-induced plasticity steel.

[0012] The process of the first friction stir processing and the second friction stir processing is consistent, and there is an unprocessed region between the first friction stir processing and the second friction stir processing; after the first friction stir processing and the second friction stir processing, the first ultra-fine grain region and the second ultra-fine grain region are formed on the upper surface and the lower surface of the twin-induced plasticity steel base material, respectively, and the positions of the first ultra-fine grain region and the second ultra-fine grain region correspond.

[0013] Further, the tool used for the first friction stir processing and the second friction stir processing is a stirring tool, and the stirring tool only comprises a shoulder;

[0014] Preferably, the shoulder is a flat shoulder, and the diameter of the shoulder is 5-15 mm.

[0015] Further, the material of the stirring tool is one of tungsten alloy and hard alloy.

[0016] Further, the rotating speed of the stirring tool is 180-400 rpm; and / or

[0017] The advancing speed is 20-100 mm / min; and / or

[0018] The pressing amount is 1 / 6-3 / 7 of the thickness of the parent material of the twin-induced plasticity steel; and / or

[0019] The included angle between the stirring tool and the vertical direction is 0-3°; and / or

[0020] The pressure maintaining time is 2-10 seconds.

[0021] Further, flowing water is used to cool the processing area during the processing of the stirring tool; wherein the water temperature is 15-25 DEG C, the diameter of the water outlet is 5-15 mm, and the water flow rate is 2-5 L / min.

[0022] Further, the parent material comprises the following components in percentage by weight:

[0023] C: 0.03-0.05%, Mo: 2.0-3.0%, Mn: 15.0-16.0%, Cr: 18.0-19.0%, N: 0.5-0.7%, and the balance of Fe.

[0024] Compared with the prior art, the present application has at least the following beneficial effects:

[0025] 1. The present application provides a twin-induced plasticity steel having a sandwich composite structure, which comprises a coarse-grained region in the core and ultra-fine-grained regions on both sides of the coarse-grained region. The grain size of the ultra-fine-grained region is greatly reduced compared with the grain size of the parent material, which can effectively improve the yield strength of the TWIP steel; at the same time, the grains in the ultra-fine-grained region have anisotropy, no obvious texture, uniform structure and low defect density, which can effectively improve the corrosion resistance of the TWIP steel.

[0026] In addition, the deformation of the coarse-grained structure in the core of the sandwich composite structure is coordinated, and the stress concentration in the processing area during the stretching process is eliminated, which is beneficial to obtain a good strength-plasticity match, i.e. the plasticity of the TWIP steel will not be greatly reduced while the yield strength is improved. Specifically, during the stretching process, the deformation of the coarse-grained region in the core occurs first, the load is transferred from the coarse-grained region in the core to the ultra-fine-grained region, the ultra-fine-grained structure has high strength, and the coarse-grained structure can provide greater plastic deformation, the mutual coordination of the coarse-grained and ultra-fine-grained structures makes the load continuously transferred, which is beneficial to the dispersion of stress and prevents failure caused by local stress concentration; before yielding, the "hard-soft-hard" (i.e. ultra-fine-grained-coarse-grained-ultra-fine-grained) structure can bear greater tensile stress; at the same time, the strain gradient generated by the mutual coordination of the coarse-grained and ultra-fine-grained structures also produces a geometrically necessary dislocation gradient, which provides additional work hardening capacity. This special work hardening is inherent to the gradient sandwich composite structure and does not exist in uniform materials, which helps to improve the strength and delay necking. During the stretching process, the strain is transferred between the ultra-fine-grained region and the coarse-grained region in the core, which avoids local deformation and effectively suppresses the generation of micro-cracks, thereby improving the elongation of the gradient sandwich composite structure.

[0027] 2. The first ultra-fine grain zone, core coarse grain zone, second ultra-fine grain zone thickness ratio of the present application is set to 3:1:3-1:5:1, which facilitates the controllable preparation of a structure with good strength and plasticity matching. Too high a proportion of the ultra-fine grain zone will result in too high a yield strength, but very low plasticity, which is due to the fact that a too high proportion of the ultra-fine grain processing zone greatly reduces the ability of the core coarse grain zone to coordinate deformation, which easily leads to local stress concentration and brittle fracture; a too high proportion of the core coarse grain zone will weaken the strengthening effect of the ultra-fine grain processing zone and cannot effectively improve the yield strength of the TWIP steel.

[0028] 3. Further, the present application also provides a preparation method of a twinning-induced plasticity steel, which comprises sequentially performing stir friction processing on the upper surface and the lower surface of a base material under water to obtain a sandwich composite structure with a core coarse grain zone and two ultra-fine grain zones on both sides, thereby avoiding the problem of reduced corrosion resistance caused by traditional severe plastic deformation methods and effectively improving the strength of the material. By adding water cooling (i.e., performing the processing under water), the residence time and cooling rate of the high-temperature zone in the stir friction processing are effectively controlled, the phase transformation in the processing is inhibited, and finally ultra-fine equiaxed austenite grains are obtained, thereby effectively maintaining the corrosion resistance of the TWIP steel.

[0029] 4. By reducing the heat input in the stir friction processing, the peak temperature of the stir friction processing is reduced, the ultra-fine grain structure obtained can simultaneously ensure the strength and passivation ability of the gradient sandwich structure, and finally an excellent TWIP steel with high strength, plasticity and corrosion resistance is prepared. The low-heat-input stir friction processing is closely related to the stirring tool, and the present application uses a needle-free flat shoulder stirring head, which can eliminate forming defects such as holes and tunnels caused by the stirring needle and improve the formability of the processing zone; the stirring tool is made of superhard and wear-resistant materials such as tungsten alloy and hard alloy, which can also avoid the weakening effect of tool wear on the corrosion resistance of the processing zone. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. The drawings in the following description are only exemplary, and for those skilled in the art, other drawings can also be derived from the provided drawings without creative labor.

[0031] Figure 1 It is a schematic diagram of underwater double-sided stir friction processing of Example 1;

[0032] Figure 2 It is the cross-sectional macro-morphology of the gradient sandwich composite structure obtained in Example 1;

[0033] Figure 3Electron backscatter diffraction typical images of the TWIP steel base material, the core coarse grain zone, the primary processing zone microstructure and the secondary processing zone microstructure in the sandwich composite structure of Example 1;

[0034] Figure 4 Typical tensile stress-strain curves of the TWIP steel base material, the primary processing zone microstructure and the gradient sandwich composite structure in Example 1;

[0035] Figure 5 Polarization curves of the TWIP steel base material and the primary processing zone microstructure in Example 1 in 3.5wt.% NaCl solution. DETAILED DESCRIPTION

[0036] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. The drawings in the following description are only exemplary, and for those skilled in the art, other drawings can be derived from the provided drawings without creative labor.

[0037] The present application provides a twin-induced plasticity steel, which has a sandwich composite structure, the sandwich composite structure comprising a first ultra-fine grain zone, a second ultra-fine grain zone and a coarse grain zone between the first ultra-fine grain zone and the second ultra-fine grain zone, i.e. the core of the sandwich composite structure is the coarse grain zone, and the two sides are ultra-fine grain structures. The TWIP steel is strengthened by using the characteristics of high strength and high passivation ability of the ultra-fine grain structure and the corrosion resistance is retained; at the same time, the soft coarse grain structure of the core region is retained, the deformation of the core coarse grain zone structure is coordinated, the stress concentration of the processing zone in the stretching process is eliminated, the loss of plasticity is minimized while the strength is improved, and good strength-plasticity matching is achieved.

[0038] In some embodiments, the microstructure of the first ultra-fine grain zone and the second ultra-fine grain zone is equiaxed austenite; and / or

[0039] The average grain size of the first ultra-fine grain zone and the second ultra-fine grain zone is 0.1-0.9 microns; and / or

[0040] The average grain width of the coarse grain zone is 20-60 microns. Since the coarse grain zone is subjected to the action of the tip force, the grains are compressed and deformed, so the grain size of the coarse grain zone is expressed by the grain width.

[0041] The equiaxed austenite refers to that the austenite is composed of equiaxed polygonal grains, there are twins in the grains, and the grain boundaries are straight. The ultra-fine grain zone has a significantly reduced grain size compared to the base material, which can effectively improve the yield strength, and the grains of the ultra-fine grain zone have anisotropy, there is no obvious texture, the microstructure is uniform and the defect density is low, which can effectively improve the corrosion resistance of the twin-induced plasticity steel.

[0042] In some embodiments, the thickness ratio of the first ultra-fine grain zone, the coarse grain zone and the second ultra-fine grain zone is 3:1:3-1:5:1. In this way, a better strength and plasticity match can be obtained. If the proportion of the fine grain zone is too high (for example, the ratio is 4:1:4), the yield strength will be too high, but the elongation will be too low, and even brittle fracture will occur during the stretching process without reaching the yield stage. If the proportion of the coarse grain zone is too high (for example, the ratio is 1:6:1), the strengthening effect of the ultra-fine grain will be weakened, and the yield strength of the gradient sandwich composite structure cannot be effectively improved.

[0043] In another aspect, the present application also provides a preparation method of the twin-induced plasticity steel described in any one of the above, comprising the following steps: first performing a first friction stir processing on the upper surface of the base material under water, and then performing a second friction stir processing on the lower surface of the base material under water to obtain the twin-induced plasticity steel described above.

[0044] The process of the first friction stir processing and the second friction stir processing is consistent, and there is an unprocessed zone between the first friction stir processing and the second friction stir processing. The unprocessed zone is subjected to the action of the top end force during processing, resulting in compression deformation of the grain, and the grain size is also reduced compared with the base material. After the first friction stir processing and the second friction stir processing, a first ultra-fine grain zone (i.e., a first processed zone) and a second ultra-fine grain zone (i.e., a second processed zone) are formed on the upper surface and the lower surface of the twin-induced plasticity steel base material, respectively, and the positions of the first ultra-fine grain zone and the second ultra-fine grain zone correspond.

[0045] In the above method, the process of the first friction stir processing and the second friction stir processing is consistent. If it is not consistent, the thickness and the structure of the two processed zones will not be consistent, and serious non-uniform deformation will easily occur during the stretching deformation process, resulting in a significant reduction in plasticity. The unprocessed zone between the first ultra-fine grain zone and the second ultra-fine grain zone is a coarse grain zone, forming a "ultra-fine grain-coarse grain-ultra-fine grain" gradient sandwich composite structure with the first ultra-fine grain zone and the second ultra-fine grain zone. At the same time, by adding water cooling (i.e., processing under water), the residence time and the cooling rate of the high-temperature zone during the friction stir processing are effectively controlled, the phase change during processing is inhibited, and finally ultra-fine equiaxed austenite grains are obtained, effectively maintaining the corrosion resistance of the TWIP steel. If the processing is performed in air, the heat input will be too high, the structure of the processed zone will be coarsened, and then the austenite phase will change, damaging the strength and corrosion resistance of the TWIP steel.

[0046] In some embodiments, the tool used for the first friction stir processing and the second friction stir processing is a stirring tool, and the stirring tool only comprises a shoulder;

[0047] Preferably, the shoulder is a flat shoulder; the shoulder diameter is 5-15 mm. A shoulder diameter less than 5 mm will result in a too narrow processing area, weakening the strengthening effect of the processing area and reducing the material preparation efficiency; a shoulder diameter greater than 15 mm will result in a too large spindle torque, affecting the stability of the equipment during processing.

[0048] In some embodiments, the material of the stirring tool is one of tungsten alloy and hard alloy.

[0049] In some embodiments, the rotation speed of the stirring tool is 180-400 rpm; and / or

[0050] The travel speed is 20-100 mm / min; and / or

[0051] The pressing amount is 1 / 6-3 / 7 of the thickness of the twin-induced plasticity steel base material; and / or

[0052] The angle between the axial direction of the stirring tool and the vertical direction is 0-3°; and / or

[0053] The pressure maintaining time is 2-10 seconds. When the rotation speed of the stirring tool is greater than 400 rpm and the travel speed is less than 20 mm / min, the heat input per unit length of the processing area is too large, causing local overheating of the processing area, thereby affecting the corrosion resistance by converting austenite into martensite or causing surface collapse and other forming defects; when the rotation speed of the stirring tool is less than 180 rpm and the travel speed is greater than 100 mm / min, the friction heat is small, and the heat input per unit length of the processing area is too low, so that the material cannot flow sufficiently, resulting in tunnel-type defects or grooves on the surface; the selection of the pressing amount is closely related to the ratio of coarse and fine grain zones; the pressure maintaining time during the friction stir processing refers to the residence time of the stirring tool after reaching the preset pressing amount, if the residence time is too short, the workpiece has not yet reached a thermoplastic state, and the processing area is prone to tunnel and hole defects due to insufficient material filling, if the residence time is too long, the processed material is overheated, which is prone to composition segregation, affecting the processing quality.

[0054] In some embodiments, flowing water is used to cool the processing area during the processing of the stirring tool; wherein the water temperature is 15-25°C, the outlet diameter is 5-15 mm, and the water flow rate is 2-5 L / min. By adjusting the water temperature, the cooling of the processing area and the processing tool is avoided, which will result in too high heat input, causing the processing area to coarsen, thereby damaging the strength and corrosion resistance.

[0055] In some embodiments, the twin-induced plasticity steel base material includes the following components in terms of weight percentage:

[0056] C: 0.03-0.05%, Mo: 2.0-3.0%, Mn: 15.0-16.0%, Cr: 18.0-19.0%, N: 0.5-0.7%, balance Fe.

[0057] Before processing, the surface of the TWIP steel base material (workpiece) is polished and cleaned, and oil stains and oxide scales are removed. The surface is washed with alcohol and dried to prevent surface impurities from reducing the formability of the processing area. The friction stir processing is carried out in a water tank, and the bottom surface of the water tank serves as the processing platform. The workpiece is placed on the processing platform. Before friction stir processing, tap water is injected into the water tank, and the water level is 5-20 mm higher than the surface of the workpiece. During processing, if the water level is 5 mm lower than the surface of the workpiece, the processed position will be in a state without cooling water due to the rotation of the stirring tool and the heat generated. If the water level is 20 mm higher than the surface of the workpiece, the water flow in the water tank will be too slow, affecting the cooling efficiency.

[0058] After the first processing (first friction stir processing) is completed, the flash of the first processing area is polished flat, and then the workpiece is flipped and placed in the water tank. A pad strip with the same length and width as the first processing area is placed in the first processing area, and the thickness of the pad strip is consistent with the thickness reduction of the workpiece surface during the first processing, to ensure that the first processing area does not deform during the second processing. The same fixture as the first processing is used to fix the workpiece, and a centering tool is used to calibrate the placement position of the workpiece to ensure that the center of the second processing area is in the same plumb surface as the center of the first processing area. Two starting positions are used for the second processing (second friction stir processing), one of which is defined as the same direction double-sided friction stir processing when it is the same as the starting position of the first processing, and the other is defined as the opposite direction double-sided friction stir processing when it is the same as the end position of the first processing. In this application, the same direction double-sided friction stir processing is used.

[0059] The application will be further described in detail through specific examples:

[0060] Example 1

[0061] This example provides a kind of twinning induced plasticity steel and its preparation method, underwater double-sided friction stir processing is carried out to 3 millimeter thick TWIP steel plate (base material), and the chemical composition of TWIP steel plate is as follows in weight percentage: C: 0.04%, Mo: 2.88%, Mn: 15.13%, Cr: 18.87%, N: 0.68%, balance Fe. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in 3.5wt.% NaCl solution is 0.356V sceThe yield strength of the base material is 523 MPa, the tensile strength is 917 MPa, and the total elongation is 59%. The stirring tool is a flat shoulder W-5Re stirring head with a diameter of 12 mm, the stirring tool rotation speed is 300 rpm, the travel speed is 20 mm / min, the shoulder is 3 / 8 TWIP steel plate thickness, i.e. 1.1 mm, the inclination angle is 3°; during processing, flowing water is used to cool the processing area, the water gun outlet diameter used for flowing water is 8 mm, the flow rate is 2.2 L / min, and the water temperature at the outlet is 18°C.

[0062] The main steps are:

[0063] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool, the stirring tool is stopped after moving for 1 second, then the stirring tool is withdrawn, and the first friction stir processing is completed, obtaining a first processing area (i.e. a first ultra-fine grain area);

[0064] Then the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing area using the stirring tool, taking the back of the starting position of the first processing layer as the starting position of the second processing, the processing parameters such as the moving path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the first friction stir processing, and a second processing area (i.e. a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing, a high-strength and high-plasticity, corrosion-resistant twinning-induced plasticity steel is obtained.

[0065] The processes of the two friction stir processings are as shown in Figure 1 The first and second processing areas both obtain ultra-fine scale structures, and the coarse grain area between the two processing areas is not affected by stirring, forming a "ultra-fine grain-coarse grain-ultra-fine grain" gradient sandwich composite structure together with the first and second processing areas. The cross-sectional morphology of the sandwich composite structure is as shown in Figure 2 The thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 3:2:3, as measured and calculated. The structures of the first and second processing areas are both equiaxed austenite grains, and the average grain size is 0.4 microns. The average grain width of the core coarse grain area is about 39 microns, as shown in Figure 3 The self-corrosion potential of the first processing area measured in a 3.5wt.% NaCl solution is 0.514 V sce , as shown in Figure 5 , which is significantly higher than that of the base material, i.e. the corrosion resistance is improved.

[0066] The sandwich composite structure zone in this embodiment was tested to have a yield strength of 1272 MPa, a tensile strength of 1368 MPa, an increase of 143% and 49% respectively compared to the base material, and a total elongation of 29%, a slight decrease compared to the base material. The sample from the primary processing zone (without the unprocessed zone sample) was taken for tensile property testing, and the yield strength of the primary processing zone structure was found to be 1434 MPa, the tensile strength was 1641 MPa, an increase of 174% and 81% respectively compared to the base material, but the total elongation was only 2%, the plasticity was severely reduced, see Figure 4 It can be seen that the ultra-fine grain zone formed after friction stir processing has high strength but low plasticity.

[0067] Comparative Example 1

[0068] This comparative example only friction stir processes the upper surface of the TWIP steel plate to obtain a processed TWIP steel. The TWIP steel plate, the stirring tool and the parameters during friction stir processing are all the same as in Example 1.

[0069] Therefore, the processed TWIP steel has a processing zone and an unprocessed zone. The thickness ratio of the processing zone and the unprocessed zone is 3:5. The microstructure of the processing zone is equiaxed austenite grains with an ultra-fine grain size, with an average grain size of 0.4 microns, and the rest of the TWIP steel plate (i.e. the unprocessed zone) is coarse-grained, with an average grain size of 60 microns.

[0070] The processed TWIP steel in this comparative example was tested in tension, and the tensile strength was only 837 MPa (lower than the base material of 917 MPa), because only one friction stir processing was performed, and the thickness ratio of the ultra-fine grain zone to the coarse grain zone was too large, resulting in severe non-uniform deformation during tension, with the tension sample sinking to one side of the primary processing zone, causing the single-layer processing zone composite structure to fail and break before reaching the yield stage.

[0071] Example 2

[0072] This example provides a twinning-induced plasticity steel and a method for preparing the same. A 2 mm thick TWIP steel plate (base material) was subjected to underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate, in terms of weight percentage, is: C: 0.04%, Mo: 2.19%, Mn: 15.81%, Cr: 18.36%, N: 0.66%, and the balance is Fe. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in a 3.5 wt.% NaCl solution is 0.368 V sce, the yield strength of the base material is 543 MPa, the tensile strength is 926 MPa, and the total elongation is 58%. The stirring tool adopts a flat shoulder W-25Re stirring head with a diameter of 10 mm, the stirring tool rotation speed is 300 rpm, the travel speed is 20 mm / min, the shoulder is 1 / 7TWIP steel plate thickness, that is, 0.3 mm, the inclination angle is 3°; during the processing, the processing area is cooled by flowing water, the water gun outlet diameter used for applying flowing water is 8 mm, the flow rate is 2.2 L / min, and the water temperature at the outlet is 18℃.

[0073] The main steps are:

[0074] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool, the stirring tool is stopped after moving for 1 second, then the stirring tool is withdrawn, and the first friction stir processing is completed, obtaining a first processing area (i.e. a first ultra-fine grain area);

[0075] Then, the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing area using the stirring tool, taking the back of the starting position of the first processing layer as the starting position of the second processing, the processing parameters such as the moving path, rotation speed, travel speed, and plunge depth of the stirring tool are the same as those of the first friction stir processing, and a second processing area (i.e. a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing, a high-strength and high-plasticity, corrosion-resistant twinning-induced plasticity steel is obtained.

[0076] According to the measurement and calculation, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 1:5:1. The microstructure of the first and second processing areas is equiaxed austenitic grains, and the average grain size is 0.3 microns. The average grain width of the core coarse grain area is about 47 microns. The self-corrosion potential of the first processing area measured in a 3.5wt.% NaCl solution is 0.596V sce , which is higher than the corrosion resistance of the base material. According to the test, the yield strength of the sandwich composite structure area in this embodiment is 998 MPa, the tensile strength is 1179 MPa, which is 84% and 27% higher than the base material respectively, and the total elongation is 45%, which is slightly lower than the base material.

[0077] Comparative Example 2

[0078] In this comparative example, the TWIP steel plate is subjected to underwater double-sided friction stir processing. By controlling the plunge depth of the stirring tool, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 1:6:1, and the rest of the conditions are the same as those of Example 2, that is, except that the plunge depth of the stirring tool is 1 / 8TWIP steel plate thickness (0.25 mm), the TWIP steel plate, the stirring tool, and the other friction stir processing parameters used in this comparative example are the same as those of Example 2.

[0079] The main steps are:

[0080] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool. After the stirring tool stops moving, it stays for 1 second, and then the stirring tool is withdrawn, and the first friction stir processing is completed, obtaining a first processing zone (i.e., a first ultra-fine grain zone).

[0081] Then, the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing zone using the stirring tool. The back of the starting position of the first processing layer is taken as the starting position of the second processing, and the processing parameters such as the moving path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the first friction stir processing. A second processing zone (i.e., a second ultra-fine grain zone) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing is completed, the processed TWIP steel is obtained.

[0082] It is measured that the thickness ratio of the first processing zone, the core coarse grain zone, and the second processing zone of the processed TWIP steel is 1:6:1. The microstructure of the first and second processing zones is equiaxed austenite grains, and the average grain size is 0.25 microns. The average grain width of the core coarse grain zone is about 63 microns.

[0083] It is tested that the yield strength of the sandwich composite structure zone in the comparative example is 546 MPa, and the tensile strength is 1011 MPa, which only has a small amount of improvement of 9%, and the yield strength has almost no change relative to the base material. This is because the thickness ratio of the processing zone is low, and the strengthening effect of the ultra-fine grain zone is weakened, which cannot enhance the yield strength. At the same time, because the grain size of the coarse grain zone is large, the distance of the dislocation pile-up stress field to the intragranular dislocation source is large, which increases the time of dislocation movement, thereby affecting the strengthening effect.

[0084] Example 3

[0085] This example provides a twinning-induced plasticity steel and a preparation method thereof. A 2.1 mm thick TWIP steel plate (base material) is subjected to underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate is, in terms of weight percentage: C: 0.04%, Mo: 2.19%, Mn: 15.81%, Cr: 18.36%, N: 0.66%, and the balance is Fe. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in a 3.5 wt.% NaCl solution is 0.368 V sceThe base material has a yield strength of 543 MPa, a tensile strength of 926 MPa, and a total elongation of 58%. The stirring tool is a flat shoulder W-25Re stirring head with a diameter of 10 mm, the stirring tool rotation speed is 300 rpm, the travel speed is 20 mm / min, the shoulder is 3 / 7 TWIP steel plate thickness, that is, 0.9 mm, and the inclination angle is 3°; during the processing, the processing area is cooled by flowing water, the water gun outlet diameter used for applying flowing water is 8 mm, the flow rate is 2.2 L / min, and the water temperature at the outlet is 18℃.

[0086] The main steps are:

[0087] After placing the TWIP steel plate in the water tank, the upper surface of the TWIP steel plate is first subjected to a first friction stir processing by using a stirring tool, the stirring tool is stopped after moving for 1 second, then the stirring tool is withdrawn, and the first friction stir processing is completed, thereby obtaining a first processing area (i.e., a first ultra-fine grain area);

[0088] Then, the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing area by using the stirring tool, the back of the starting position of the first processing layer is used as the starting position of the second processing, the processing parameters such as the moving path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the first friction stir processing, and a second processing area (i.e., a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing is completed, a high-strength and high-plasticity corrosion-resistant twinning-induced plasticity steel is obtained.

[0089] According to the measurement and calculation, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 3:1:3. The microstructure of the first and second processing areas is equiaxed austenitic grains, and the average grain size is 0.5 microns. The average grain width of the core coarse grain area is about 30 microns. The self-corrosion potential of the first processing area measured in a 3.5wt.% NaCl solution is 0.484V sce , which is improved compared with the corrosion resistance of the base material. According to the test, the yield strength of the sandwich composite structure area in the embodiment is 1384 MPa, the tensile strength is 1509 MPa, which is increased by 155% and 63% respectively compared with the base material, and the total elongation is 15%.

[0090] Comparative Example 3

[0091] In this comparative example, the TWIP steel plate is subjected to underwater double-sided friction stir processing. By controlling the pressing amount of the stirring tool, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 4:1:4, and the rest of the conditions are the same as those of Example 3, that is, except that the pressing amount of the stirring tool is 4 / 9 TWIP steel plate thickness (0.95 mm), the TWIP steel plate, the stirring tool, and the other friction stir processing parameters used in this comparative example are the same as those of Example 3.

[0092] The main steps are:

[0093] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool, the stirring tool is stopped after moving for 1 second, and then the stirring tool is withdrawn, and the first friction stir processing is completed, and a first processed area (i.e., a first ultra-fine grain area) is obtained.

[0094] Then, the workpiece is turned over, and a second friction stir processing is performed on the back of the first processed area using the stirring tool, the back of the starting position of the first processed layer is taken as the starting position of the second processing, the moving path, the rotating speed, the traveling speed, the pressing amount and other processing parameters of the stirring tool are the same as those of the first friction stir processing, and a second processed area (i.e., a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing is completed, the TWIP steel after processing is obtained.

[0095] It is measured that the thickness ratio of the first processed area, the core coarse grain area and the second processed area of the TWIP steel after processing is 4:1:4. The microstructure of the first and second processed areas is equiaxed austenite grains, and the average grain size is 1.1 microns. The average grain width of the core coarse grain area is about 15 microns.

[0096] It is tested that the yield strength of the sandwich composite structure area in the comparative example is 1437 MPa, the tensile strength is 1644 MPa, the relative base material is increased by 165% and 78% respectively, the total elongation is greatly reduced, and is almost 0. This is because the high proportion of the processed area will lead to high yield strength of the gradient sandwich composite structure, and the small grain width of the core coarse grain area will greatly reduce the coordinated deformation ability, easily produce large local stress concentration, lead to the formation of micro-cracks, greatly reduce the elongation, and even in the tensile process, brittle fracture occurs before the yield stage.

[0097] Example 4

[0098] This embodiment provides a twinning induced plasticity steel and a preparation method thereof. A 3mm thick TWIP steel plate (base material) is subjected to underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate is 0.04% of C, 2.29% of Mo, 15.83% of Mn, 18.56% of Cr, 0.68% of N, and the balance of Fe in terms of weight percentage. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in a 3.5wt.% NaCl solution is 0.469V sceThe yield strength of the base material is 656 MPa, the tensile strength is 1238 MPa, and the total elongation is 47%. The stirring tool is a flat shoulder W-25Re stirring head with a diameter of 5 mm, the stirring tool rotation speed is 400 rpm, the travel speed is 100 mm / min, the shoulder is 1 / 5TWIP steel plate thickness, i.e. 0.6 mm, the inclination angle is 3°; during processing, flowing water is used to cool the processing area, the water gun outlet diameter used for flowing water is 11 mm, the flow rate is 3.4 L / min, and the water temperature at the outlet is 22°C.

[0099] The main steps are:

[0100] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool, the stirring tool is stopped after moving for 1 second, then the stirring tool is withdrawn, and the first friction stir processing is completed, obtaining a first processing area (i.e. a first ultra-fine grain area);

[0101] Then the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing area using a stirring tool, taking the back of the starting position of the first processing layer as the starting position of the second processing, the processing parameters such as the moving path, rotation speed, travel speed and pressing amount of the stirring tool are the same as those of the first friction stir processing, and a second processing area (i.e. a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing, a high-strength, high-plasticity, corrosion-resistant twinning-induced plasticity steel is obtained.

[0102] According to the measurement and calculation, the thickness ratio of the first processing area, the core coarse grain area and the second processing area is 1:3:1. The microstructure of the first and second processing areas is equiaxed austenite grains, and the average grain size is 0.6 microns. The average grain width of the core coarse grain area is about 27 microns. The self-corrosion potential of the first processing area measured in a 3.5wt.% NaCl solution is 0.463V sce According to the test, the yield strength of the sandwich composite structure area in the embodiment is 1521 MPa, the tensile strength is 1770 MPa, which is increased by 1532% and 43% compared with the base material, and the total elongation is 33%.

[0103] Comparative Example 4

[0104] In this comparative example, the TWIP steel plate is subjected to underwater double-sided friction stir processing. By controlling the pressing amount of the stirring tool, there is no unprocessed area between the first processing area and the second processing area. The rest of the conditions are the same as those of Example 4, i.e. except that the pressing amount of the stirring tool is 1 / 2TWIP steel plate thickness (1.5 mm), the TWIP steel plate, the stirring tool and the other friction stir processing parameters used in this comparative example are the same as those of Example 4.

[0105] The main steps are:

[0106] After placing the TWIP steel plate in the water tank, the upper surface of the TWIP steel plate is first subjected to a stirring friction process using a stirring tool. After the stirring tool stops moving, it is paused for 1 second, and then the stirring tool is pulled out. This completes the first stirring friction process and obtains the first processing zone (i.e., the first ultrafine grain zone).

[0107] The workpiece is then flipped over, and a secondary friction stir process is performed on the back of the primary processing zone using a stirring tool. The back of the primary processing layer is used as the starting position for the secondary processing. The movement path, rotation speed, travel speed, and pressure of the stirring tool are the same as those for the primary friction stir process. A secondary processing zone (i.e., the second ultrafine grain zone) is obtained on the upper surface of the TWIP steel plate. After the secondary friction stir process is completed, the treated TWIP steel is obtained.

[0108] The treated TWIP steel has no unprocessed area between the primary and secondary processed zones, making it impossible to form a "hard-soft-hard" gradient sandwich composite structure. The microstructure of both the primary and secondary processed zones consists of equiaxed austenite grains with an average grain size of 1.0 micrometers.

[0109] Tests showed that the tensile strength of the TWIP steel treated in this comparative example was 1348 MPa, and the total elongation was almost zero, indicating a significant reduction in plasticity compared to the parent material. Due to the absence of a coordinating deformation effect in the coarse-grained core region, the load accumulated in the ultrafine-grained processing zone during tensile testing, resulting in significant local stress concentration. Consequently, the TWIP steel, entirely composed of processed microstructure, underwent brittle fracture before reaching the yield stage.

[0110] Example 5

[0111] This embodiment provides a twinned induced ductile steel and its preparation method. A 4 mm thick TWIP steel plate (base material) is subjected to underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate, by weight percentage, is: C: 0.03%, Mo: 2.06%, Mn: 15.8%, Cr: 18.25%, N: 0.57%, with the balance being Fe. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained in a 3.5 wt.% NaCl solution is 0.442 V. sce The base material has a yield strength of 629 MPa, a tensile strength of 1147 MPa, and a total elongation of 52%. The mixing tool uses a W-25Re mixing head with a flat shoulder and a diameter of 14 mm. The mixing tool rotates at 180 rpm, travels at 20 mm / min, and the shoulder has a downward pressure of 2 / 5 TWIP steel plate thickness (1.6 mm) and an inclination angle of 3°. During processing, flowing water is used to cool the processing area. The water gun used to apply the flowing water has an outlet diameter of 6 mm, a flow rate of 4.4 L / min, and an outlet water temperature of 19°C.

[0112] The main steps are:

[0113] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to primary friction stir processing using a stirring tool. After the stirring tool stops moving, it stays for 1 second, and then the stirring tool is extracted. Thus, the primary friction stir processing is completed, and a primary processing area (i.e., a first ultra-fine grain area) is obtained.

[0114] Then, the workpiece is flipped over, and secondary friction stir processing is performed on the back of the primary processing area using the stirring tool. The back of the starting position of the primary processing layer is used as the starting position of the secondary processing. The processing parameters such as the moving path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the primary friction stir processing. A secondary processing area (i.e., a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the secondary friction stir processing is completed, a high-strength and high-plasticity, corrosion-resistant twinning-induced plasticity steel is obtained.

[0115] According to the measurement and calculation, the thickness ratio of the primary processing area, the core coarse grain area, and the secondary processing area is 2:1:2. The microstructure of the primary and secondary processing areas is equiaxed austenite grains, and the average grain size is 0.2 microns. The average grain width of the core coarse grain area is about 51 microns. The self-corrosion potential of the primary processing area measured in a 3.5wt.% NaCl solution is 0.4908V sce , which is significantly improved compared to the corrosion resistance of the base material. According to the test, the yield strength of the sandwich composite structure area in this embodiment is 1578 MPa, and the tensile strength is 1835 MPa, which is increased by 151% and 60% respectively compared to the base material, and the total elongation is 19%, which is reduced compared to the base material.

[0116] Comparative Example 5

[0117] In this comparative example, TWIP steel plates are subjected to underwater double-sided friction stir processing. The rotation speed of the stirring tool is higher, and the other conditions are the same as those of Example 5, i.e., except that the rotation speed of the stirring tool is increased to 420 revolutions per minute, the TWIP steel plates, stirring tools, and other friction stir processing parameters used in this comparative example are the same as those of Example 5.

[0118] The main steps are:

[0119] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to primary friction stir processing using a stirring tool. After the stirring tool stops moving, it stays for 1 second, and then the stirring tool is extracted. Thus, the primary friction stir processing is completed, and a primary processing area (i.e., a first ultra-fine grain area) is obtained.

[0120] The workpiece is flipped over and secondary friction stir processing is performed on the back of the primary processing zone using a stirring tool. The back of the starting position of the primary processing layer is used as the starting position of the secondary processing. The movement path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the primary friction stir processing. A secondary processing zone (i.e., a second ultra-fine grain zone) is obtained on the upper surface of the TWIP steel plate. After the secondary friction stir processing is completed, the processed TWIP steel is obtained.

[0121] The thickness ratio of the primary processing zone, the core coarse grain zone, and the secondary processing zone of the processed TWIP steel is 2:1:2. The microstructure of the primary and secondary processing zones is equiaxed austenite grains, and the average grain size is 1.4 microns. The average grain width of the core coarse grain zone is about 18 microns. There are obvious wear particles in the primary and secondary processing zones, and the average size of the wear particles is close to 0.3 microns. This is because the rotation speed of the stirring tool is increased, which increases the rheological stress of the stirring tool relative to its rotation direction, resulting in severe wear of the stirring tool, and the wear particles fall into the processing zone.

[0122] The yield strength of the processing zone (primary processing zone and secondary processing zone) in the comparative example is 1198 MPa, and the tensile strength is 1376 MPa, which is 90% and 20% higher than the base material, respectively. The total elongation is only 5%, and the plasticity of the base material is severely reduced. After the wear particles fall into the processing zone, microcracks are easily generated between the wear particles and the grains in the processing zone under the action of tensile stress, so the plasticity is severely reduced. The self-corrosion potential of the primary processing zone is -0.437 V sce in a 3.5wt.% NaCl solution, which is a significant decrease in corrosion resistance compared to the base material. Due to the difference in corrosion potential between the wear particles and the material in the processing zone, galvanic corrosion occurs, making the tool wear particles extremely sensitive to corrosion. At the same time, due to the excessively large grain size of the fine grain zone, the diffusion of Cr elements in the TWIP steel is not sufficient, which reduces the re-passivation ability of the TWIP steel and causes a decrease in corrosion resistance.

[0123] Example 6

[0124] This example provides a twinning-induced plasticity steel and a method for preparing the same. A 1.8 millimeter thick TWIP steel plate (base material) is subjected to underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate is 0.05% C, 2.59% Mo, 16% Mn, 18.86% Cr, 0.78% N, and the balance being Fe in terms of weight percentage. The microstructure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in a 3.5wt.% NaCl solution is 0.393 V sceThe yield strength of the base material is 612 MPa, the tensile strength is 1007 MPa, and the total elongation is 56%. The stirring tool is a flat shoulder tungsten cobalt alloy stirring head with a diameter of 12 mm, the stirring tool rotation speed is 180 rpm, the travel speed is 100 mm / min, the shoulder is 1 / 3TWIP steel plate thickness, that is, 0.6 mm, and the inclination angle is 3°; during processing, flowing water is used to cool the processing area, the water gun outlet diameter used for applying flowing water is 10 mm, the flow rate is 3.5 L / min, and the water temperature at the outlet is 21°C.

[0125] The main steps are:

[0126] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is subjected to a first friction stir processing using a stirring tool, the stirring tool is stopped after moving for 1 second, then the stirring tool is withdrawn, and the first friction stir processing is completed, obtaining a first processing area (i.e., a first ultra-fine grain area);

[0127] Then, the workpiece is turned over, and a second friction stir processing is performed on the back of the first processing area using the stirring tool, taking the back of the starting position of the first processing layer as the starting position of the second processing, and the processing parameters such as the moving path, rotation speed, travel speed, and pressing amount of the stirring tool are the same as those of the first friction stir processing, and a second processing area (i.e., a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing is completed, a high-strength and high-plasticity, corrosion-resistant twinning-induced plasticity steel is obtained.

[0128] According to the measurement and calculation, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 1:1:1. The microstructure of the first and second processing areas is equiaxed austenite grains, and the average grain size is 0.1 microns. The average grain width of the core coarse grain area is about 56 microns. The self-corrosion potential of the first processing area measured in a 3.5wt.% NaCl solution is 0.987V sce , which is improved compared with the corrosion resistance of the base material. According to the test, the yield strength of the sandwich composite structure area in the embodiment is 1468 MPa, the tensile strength is 1500 MPa, which is increased by 110% and 49% respectively compared with the base material, and the total elongation is 32%, which is reduced compared with the base material.

[0129] Comparative Example 6

[0130] In this comparative example, the TWIP steel plate is subjected to underwater double-sided friction stir processing, and the travel speed of the stirring tool is faster, and the other conditions are the same as those of Example 6, that is, except that the travel speed of the stirring tool is increased to 120 mm / min, the TWIP steel plate, the stirring tool, and the other friction stir processing parameters used in this comparative example are the same as those of Example 6.

[0131] The main steps are:

[0132] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is processed by using the stirring tool for one time of friction stir processing, the stirring tool stops moving and stays for 1 second, then the stirring tool is extracted, and one time of friction stir processing is completed, and the processed TWIP steel is obtained.

[0133] Due to the too high traveling speed of the stirring tool, the friction heat production is small, the heat input of the processing zone per unit length is too low, the material cannot flow sufficiently, so that the heat input is obviously insufficient after the processing starts, and the processing cannot continue; the surface of the processed area appears grooves, a large number of holes and obvious tunnel defects exist at the bottom, and the width of the processing zone is less than 8 mm, so that the preparation of the sandwich composite structure TWIP steel cannot be realized. At the same time, the stirring tool is severely deformed after processing, and obvious wear phenomenon exists.

[0134] Example 7

[0135] This embodiment provides a twinning induced plasticity steel and a preparation method thereof. A 2 mm thick TWIP steel plate (base material) is processed by underwater double-sided friction stir processing. The chemical composition of the TWIP steel plate is as follows in terms of weight percentage: C: 0.04%, Mo: 2.08%, Mn: 15.94%, Cr: 18.86%, N: 0.66%, and the balance is Fe. The structure of the TWIP steel plate is equiaxed austenite, and the self-corrosion potential obtained by testing in a 3.5 wt.% NaCl solution is 0.403 V sce , the yield strength of the base material is 551 MPa, the tensile strength is 928 MPa, and the total elongation is 57%. The stirring tool adopts a flat shoulder W-25Re stirring head with a diameter of 10 mm, the stirring tool rotating speed is 300 rpm, the traveling speed is 20 mm / min, the shoulder is 3 / 7 TWIP steel plate thickness, that is, 0.9 mm, and the inclination angle is 3°; during the processing, the processing zone is cooled by using flowing water, the water gun outlet diameter used for applying the flowing water is 8 mm, the flow rate is 2.2 L / min, and the water temperature at the outlet is 18°C.

[0136] The main steps are as follows:

[0137] After placing the TWIP steel plate in the water tank, first, the upper surface of the TWIP steel plate is processed by using the stirring tool for one time of friction stir processing, the stirring tool stops moving and stays for 1 second, then the stirring tool is extracted, and one time of friction stir processing is completed, and the processed TWIP steel is obtained.

[0138] The workpiece is flipped over, and a second friction stir processing is performed on the back of the first processing area using a stirring tool. The back of the starting position of the first processing layer is used as the starting position of the second processing. The processing parameters of the stirring tool, such as the moving path, rotation speed, travel speed, and pressing amount, are the same as those of the first friction stir processing. A second processing area (i.e., a second ultra-fine grain area) is obtained on the upper surface of the TWIP steel plate. After the second friction stir processing, a high-strength and high-plasticity and corrosion-resistant twin-induced plasticity steel is obtained.

[0139] According to measurement and calculation, the thickness ratio of the first processing area, the core coarse grain area, and the second processing area is 3:1:3. The microstructure of the first and second processing areas is equiaxed austenite grains, and the average grain size is 0.5 microns. The average grain width of the core coarse grain area is about 32 microns. The self-corrosion potential of the first processing area is 0.492V sce , which is higher than that of the base material. The yield strength of the sandwich composite structure area in the embodiment is 1400MPa, and the tensile strength is 1559MPa, which is 154% and 68% higher than that of the base material, respectively. The total elongation is 14%, which is lower than that of the base material.

[0140] Comparative Example 7

[0141] In this comparative example, underwater double-sided friction stir processing is performed on a TWIP steel plate. The stirring tool is made of metal ceramic, and the other conditions are the same as those of Example 7. That is, in addition to using a flat shoulder metal ceramic stirring tool with a diameter of 10mm, the TWIP steel plate, the size of the stirring tool, and the processing parameters of the friction stir processing in this comparative example are the same as those of Example 7.

[0142] The main steps are as follows:

[0143] After placing the TWIP steel plate in the water tank, first, a stirring tool is used to perform first friction stir processing on the upper surface of the TWIP steel plate. After the stirring tool stops moving, it stays for 1 second, and then the stirring tool is pulled out. The first friction stir processing is completed, and a processed TWIP steel is obtained.

[0144] During processing, the metal ceramic stirring tool is severely worn and cannot continue processing. The length of the processing area is less than 15mm. There are a large number of holes and obvious tunnels at the bottom of the processed area, and the width of the processing area is less than 5mm, which cannot realize the preparation of sandwich composite structure TWIP steel.

[0145] Those skilled in the art will readily understand that the advantageous technical features of the above-mentioned modes can be freely combined and superimposed without conflict.

[0146] The above merely describes preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application. The above merely describes preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing twinned ductile steel, characterized in that, Includes the following steps: The upper surface of the base material is first subjected to a stirring friction process underwater using a stirring tool, and then the lower surface of the base material is subjected to a second stirring friction process underwater to obtain twin-induced plastic steel. The twin-induced plastic steel has a sandwich composite structure, which includes a first ultrafine grain region, a second ultrafine grain region, and a coarse grain region located between the first ultrafine grain region and the second ultrafine grain region. The stirring tool has a rotation speed of 180-400 rpm, a travel speed of 20-100 mm / min, a pressure of 1 / 6-3 / 7 of the thickness of the twin-induced plastic steel base material, an angle of 0-3° between the stirring tool and the vertical direction, and a holding time of 2-10 seconds.

2. The method for preparing twinned induced ductile steel according to claim 1, characterized in that, The microstructure of both the first and second ultrafine-grained regions is equiaxed austenite; and / or The average grain size of both the first and second ultrafine grain regions is 0.1-0.9 micrometers; and / or The average grain width in the coarse-grained region is 20-60 micrometers.

3. The method for preparing twinned induced ductile steel according to claim 1, characterized in that, The thickness ratio of the first ultrafine crystal region, the coarse crystal region, and the second ultrafine crystal region is 3:1:3 to 1:5:

1.

4. The method for preparing twinned induced ductile steel according to any one of claims 1 to 3, characterized in that, The processes of the first and second friction stir processing are the same, and there is an unprocessed area between the first and second friction stir processing. After the first and second friction stir processing, the first ultrafine crystalline region and the second ultrafine crystalline region are formed on the upper and lower surfaces of the twin-induced plastic steel base material, respectively, and the positions of the first and second ultrafine crystalline regions correspond to each other.

5. The method for preparing twinned induced ductile steel according to claim 4, characterized in that, The tools used in the primary and secondary friction stir machining processes are stirring tools, which only include a shoulder.

6. The method for preparing twinned induced ductile steel according to claim 5, characterized in that, The shoulder is a flat shoulder; the shoulder diameter is 5-15 mm.

7. The method for preparing twinned induced ductile steel according to claim 5, characterized in that, The stirring tool is made of either tungsten alloy or cemented carbide.

8. The method for preparing twinned induced ductile steel according to claim 5, characterized in that, During the processing of the mixing tool, flowing water is used to cool the processing area; the water temperature is 15-25℃, the outlet diameter is 5-15 mm, and the water flow rate is 2-5 liters / minute.

9. The method for preparing twinned induced ductile steel according to any one of claims 4 to 8, characterized in that, The base material comprises the following components by weight percentage: C: 0.03-0.05%, Mo: 2.0-3.0%, Mn: 15.0-16.0%, Cr: 18.0-19.0%, N: 0.5-0.7%, balance Fe.

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