Twip steel with high yield strength and high ductility and method for producing the same

Abstract

The present application belongs to the technical field of advanced steel material, and particularly relates to a TWIP steel with high yield strength and high plasticity and a preparation method thereof. According to the present application, the chemical composition is as follows in percentage by weight: C: 0.6-0.9%; Mn: 17.0-25.0%; Si: 0.3-0.8%; Al: 0.5-1.5%; P: 0.06-0.20%; B: 0.02-0.08%; V: 0.05-0.50%; Ti: 0.01-0.15%; Nb: 0.01-0.15%; S: 0.005%; total amount of other impurity elements: 0.008%; and the rest is Fe. The thick and large workpiece produced by the present application has optimal performance reaching a high level, wherein the yield strength is 648 MPa, the tensile strength is 1244 MPa, and the elongation after fracture is 65.1%, which is obviously superior to the existing TWIP steel.

Description

Technical Field

[0001] This invention belongs to the field of advanced steel materials, and particularly relates to a TWIP steel with high yield strength and high plasticity and its preparation method. Background Technology

[0002] TWIP steel is a single-phase austenitic steel. Due to its moderate stacking fault energy, it generates a large number of deformation twins during plastic deformation, exhibiting a significant strain hardening effect. This delays the localization of deformation, resulting in extremely high plasticity and strength. Its representative properties are: yield strength 250 MPa, tensile strength 600 MPa, and elongation after fracture 80%. The relatively low yield strength limits its application in support, reinforcement, and seismic-resistant structures (such as soft rock support anchors, bridge tie rods, seismic reinforcement, and heavy-duty bearings), where high strength, stiffness, and plastic deformation are required. Furthermore, although TWIP steel has strong energy absorption capabilities and can dissipate the work done by external impacts or static loads through large plastic deformation, its low yield strength results in a continuously rising, steeply sloped stress-strain curve with a trapezoidal lower envelope area. This significantly deviates from the ideal energy absorption area formed by tensile strength and elongation after fracture, leading to low energy absorption efficiency. Therefore, improving the yield strength of TWIP steel and enhancing its service behavior and performance has become an important research hotspot in this field.

[0003] Because TWIP steel contains 20-30 wt.% Mn, its austenitic structure is extremely stable and will not undergo phase transformation under rapid cooling or large strain conditions. Therefore, its strength cannot be improved by phase transformation heat treatment. It can only be strengthened by non-phase transformation methods such as solution treatment, precipitation, grain refinement, or strain. The main characteristics of various strengthening methods are as follows: (1) Precipitation enhancement Typically, alloying with elements such as V, Ti, and Nb is first performed, followed by solution treatment and aging to form a large number of dispersed fine carbide precipitates (such as VC, TiC, and NbC) on the austenitic matrix. These precipitates can hinder dislocation slip, thereby improving the yield strength and tensile strength of TWIP steel. However, there are significant drawbacks: the precipitates significantly inhibit the generation and proliferation of deformation twins in TWIP steel, which is extremely detrimental to plasticity; at the same time, the production cost is high due to the need to add precious metals and the requirement for long-term aging treatment.

[0004] (2) Refine grain strengthening

[0005] This method can be achieved through two approaches: one is to add trace amounts of elements such as V, Ti, and Nb, utilizing their heterogeneous nucleation effect and their inhibition of grain boundary migration to refine the grains; the other is to use deformation processing followed by recrystallization annealing to allow the deformed structure formed by plastic deformation to recrystallize and grow into fine equiaxed grains. Both approaches can obtain fine grains of different sizes to improve the strength of TWIP steel, but the fine grain structure strongly inhibits twinning deformation, which is extremely detrimental to its plasticity.

[0006] (3) Strain strengthening

[0007] By utilizing the prominent strain hardening characteristics of TWIP steel, plastic processing at different temperatures and with varying deformation amounts (such as forging, rolling, and extrusion) can create deformed grains, high-density dislocations, and complex defect configurations, hindering twinning and dislocation slip, thereby significantly improving the strength of TWIP steel. The biggest drawback of this method is that the TWIP effect is no longer the primary deformation mechanism, resulting in a significant decrease in plasticity while substantially increasing strength. Furthermore, plastic processing must be performed at low or room temperature, making it suitable only for processing thin and fine materials. Due to the high resistance to deformation at low temperatures, it is difficult to use for processing thick and large components.

[0008] The common problem with the three strengthening methods mentioned above is that they cause significant loss of plasticity while strengthening the material, and they also have obvious drawbacks in terms of economy and product adaptability. Therefore, considering the plasticity and energy absorption properties of the material, solid solution strengthening may be the most suitable choice.

[0009] Solid solution strengthening refers to the method of strengthening materials by alloy atoms entering the parent phase solid solution through interstitial or substitutional solid solutions, causing lattice distortion. Appropriate control of solute content can significantly improve material strength and hardness while maintaining high plasticity and toughness, thus achieving better comprehensive mechanical properties. Studies have shown that adding the solid solution element Cu to TWIP steel increases stacking fault energy, which is beneficial for improving yield strength and elongation. Adding Ni to Cu alloying further reduces work hardening and hot cracking tendency, significantly improving the properties of TWIP steel, achieving yield strength, tensile strength, and elongation (A50) of 300 MPa, 735 MPa, and 83.3%, respectively. Among all substitutional solid solution elements, P has the strongest solid solution strengthening effect, and its multivalent electrons can strongly reduce stacking fault energy, significantly promoting twinning deformation. Effective control of P segregation at grain boundaries and full utilization of its solid solution strengthening effect can improve the plasticity of TWIP steel. Experiments have confirmed that adding 0.06-0.20 wt.% P to TWIP steel can increase its yield strength to 425 MPa, tensile strength to 785 MPa, and elongation (A50) to 80%. These results indicate that while solid solution strengthening can improve the strength and toughness of TWIP steel to some extent, the overall effect is still not ideal. The inherent problems of low yield strength and tensile strength have not been effectively solved, and it still lags significantly behind traditional alloy steel materials. Summary of the Invention

[0010] To overcome the shortcomings of the prior art, this invention provides a TWIP steel with high yield strength and high plasticity, and a method for its preparation. The thick workpieces produced by this invention achieve optimal performance at a high level, with a yield strength of 648 MPa, tensile strength of 1244 MPa, and elongation after fracture of 65.1%, significantly superior to existing TWIP steels.

[0011] To achieve one of the above objectives, the present invention adopts the following technical solution: A TWIP steel with high yield strength and high ductility, the TWIP steel is Fe-Mn-Al-P based, and its chemical composition by weight percentage is: C: 0.6%. 0.9%; Mn: 17.0% 25.0%; Si: 0.3% 0.8%; Al: 0.5% 1.5%; P: 0.06% A: 0.20%; B: 0.02% 0.08%; V: 0.05% 0.20%; Ti: 0.10% 0.15%; Nb: 0.10% 0.15%; S: 0.005%; Total amount of other impurity elements: 0.008%; the remainder was Fe.

[0012] Preferably, the TWIP steel has the following chemical composition by weight percentage: C: 0.8%; Mn: 22.0%; Si: 0.4%; Al: 1.2%; P: 0.12%; B: 0.05%; V: 0.5%; Ti: 0.02%; Nb: 0.02%; S: 0.005%; Total amount of other impurity elements: 0.008%; the remainder was Fe.

[0013] To achieve the second objective mentioned above, the present invention provides a method for preparing TWIP steel with high yield strength and high ductility, comprising the following steps: S1. Smelting and Casting: Materials are batched according to the alloy composition, and primary smelting, ladle refining, vacuum degassing, and casting are carried out sequentially in a 30-ton electric arc furnace to produce 750 alloy. 7500kg steel ingot; S2. Forging: Forging steel ingots into bars or plates.

[0014] The preferred method for forging the bar stock is as follows: (1) The steel ingot is surface-machined, and the machined steel ingot is homogenized and then forged according to a forging ratio of 4. 8. Upset the steel ingot, then forge it according to the ratio 9. 16. Unidirectional elongation along the ingot axis, forging 70 120mm round blank or 60mm side length 100mm square billet; then 4 10 Forging ratio is used to forge billets.

[0015] (2) Solution treatment or stress relief annealing: According to the performance requirements, the forging billet is subjected to solution treatment or stress relief annealing.

[0016] Preferably, the forging method for the plate is as follows: heating the steel ingot to 1150°C 1170℃, homogenization holding time 3 hours, then elongated along the axial direction of the steel ingot, and forged in several stages to a length of 1000 mm. 4000mm wide, 500mm deep 2000mm, 30mm thick 200mm slab, forging ratio of 4 10. According to performance requirements, the forging billet shall be subjected to solution treatment or stress-relieving annealing.

[0017] Preferably, the alloy composition includes scrap steel, elemental materials, and intermediate alloys, wherein the elemental materials include C, Mn, Si, Al, and Fe, and the intermediate alloys include P-Fe, B-Fe, Ti-Fe, and Nb-Fe.

[0018] Preferably, the scrap steel and elemental Fe (industrial pure iron) in the alloy composition are added during the initial smelting, while the elemental C, Mn, Al, Si and intermediate alloys are added during the ladle refining stage.

[0019] Preferably, after the steel ingot is poured, it is covered and kept warm with insulating material (dry sand) for ≥24 hours.

[0020] Preferably, the machining allowance for surface turning is 2. 5mm; the homogenization treatment temperature is 1130~1170℃, and the time is ≥3; the length of the forging billet is 1000mm. 6000mm.

[0021] Preferably, the solution treatment temperature is 1050°C. 1150℃, heat preservation time is 2.0 seconds. 4.0h, air cooling; stress-relief annealing temperature is 350-500℃, holding time is 3.0h. 5.0h, then cool to room temperature.

[0022] The advantages of this invention are: (1) This invention addresses the inherent problems of low yield strength and difficulty in balancing strength and plasticity in existing TWIP steels using existing strengthening methods. Based on high-C, medium-Mn type TWIP steel, it adopts a design scheme with P as the main strengthening element and B, Ti, and Nb as auxiliary alloying elements. By leveraging the outstanding solid solution strengthening effect of P, combined with the auxiliary strengthening effects of B, Ti, and Nb, stacking fault energy and deformation resistance are appropriately reduced, while dislocation slip resistance is increased. Ultimately, this achieves the goal of both improving the yield strength of TWIP steel and promoting twinning deformation while ensuring its plasticity. In addition, through the combined effects of elements such as B, V, Ti, and Nb, grain boundary embrittlement caused by the segregation of C and P atoms to the grain boundaries can be effectively suppressed, fully utilizing the solid solution strengthening efficiency of C and P. Finally, a new type of TWIP steel with significantly improved yield strength, tensile strength, and plasticity is obtained, laying a solid foundation for its application in a wider range of industrial fields. Thick workpieces produced using the technology of this invention achieve a high level of optimal performance, with a yield strength of 648 MPa, tensile strength of 1244 MPa, and elongation after fracture of 65.1%, which are significantly better than existing TWIP steel.

[0023] (2) This invention uses solid solution strengthening as the main strengthening mechanism, supplemented by strain strengthening caused by different plastic processing techniques, processing temperatures, and deformation amounts, to achieve the goal of significantly improving the yield strength, tensile strength, and yield ratio of TWIP steel while minimizing the decrease in plasticity. Compared with the ultra-high Mn type TWIP steel reported in the literature, this invention appropriately reduces the Mn content to reduce stacking fault energy and deformation resistance, creating favorable conditions for the occurrence of twinning deformation; at the same time, it utilizes the hindering effect of C on dislocations and the solid solution strengthening effect of P to further improve the yield strength of TWIP steel.

[0024] (3) In terms of auxiliary alloying, the present invention adds appropriate amounts of elements such as B, Ti and Nb: by utilizing the strong surface activity of B, it preferentially accumulates on the grain surface, thereby inhibiting the segregation of P at the grain boundary and reducing the embrittlement effect of P on the grain boundary; by utilizing the strong carbide formation tendency of Ti and Nb, the solid C effect is achieved, avoiding the diffusion of C and P to the grain boundary, and ensuring that the solid solution strengthening effect of C and P is fully utilized.

[0025] (4) Through high-temperature plastic deformation processing, a deformed structure with high dislocation density and complex configuration (including dislocation intersection, climb and network) can be obtained, which reduces the dislocation mobility when the material is stretched; then through solid solution or stress relief annealing, the stress generated during plastic processing is reduced and a small amount of fine grains are formed. Although the tensile strength of the material is slightly reduced, the plasticity is significantly improved, and finally the strengthening effect of significantly improved yield strength and tensile strength while maintaining good plasticity is achieved.

[0026] (5) The novel TWIP steel prepared by this invention has the following significant characteristics compared with the existing TWIP steel: First, the metallographic structure shows single-phase austenitic equiaxed crystals with adjustable size and a small amount of annealed twins, with the annealed twin integral number being 15%-25%; Second, the typical tensile mechanical properties are as follows: conventional properties are yield strength 436MPa, tensile strength 888MPa, and elongation after fracture 40%, and the optimal properties can reach yield strength 648MPa, tensile strength 1244MPa, and elongation after fracture 65.1%. Attached Figure Description

[0027] Figure 1 The diagram shows the tensile mechanical properties of TWIP steels with different P contents according to the present invention.

[0028] Figure 2 The image shows the metallographic structure of TWIP steel with a P content of 0.039 wt.% according to this invention.

[0029] Figure 3 The image shows the metallographic structure of TWIP steel with a P content of 0.068 wt.% according to this invention.

[0030] Figure 4 The image shows the metallographic structure of TWIP steel with a P content of 0.105 wt.% according to this invention.

[0031] Figure 5 The image shows the metallographic structure of TWIP steel with a P content of 0.110 wt.% according to this invention.

[0032] Figure 6 This is Embodiment 1 of the present invention. Tensile mechanical properties diagram of 100×6000mm tie rod forging blank.

[0033] Figure 7 The tensile mechanical properties diagram of the 1700×3300×150mm thick plate of this invention is shown.

[0034] Figure 8 The tensile stress-strain curves of the 63×76×2130mm TWIP steel billet of this invention are shown. Detailed Implementation

[0035] Example 1

[0036] This embodiment provides a high yield strength TWIP steel for bridge tie rods and its preparation method, with a tie rod diameter of 100mm. The specific implementation steps are as follows: S1. Smelting: According to alloy composition (wt.%): C: 0.9; Mn: 25.0; Si: 0.8; Al: 0.5; P: 0.10; B: 0.03; V: 0.20; Ti: 0.10; Nb: 0.10; S: 0.005; Total amount of other impurity elements: 0.008; the remainder is Fe. Elemental materials C, Mn, Al, Si, Fe, and intermediate alloys such as P-Fe, B-Fe, Ti-Fe, and Nb-Fe are weighed and added to a 30-ton electric arc furnace for primary refining, ladle refining, and vacuum degassing, and finally cast into several steel ingots of approximately 750 kg each.

[0037] S2. High-temperature forging: The steel ingot is heated to 1150℃, homogenized and held at that temperature for 3 hours, and then forged in two stages into a square billet with a cross-section of 200mm and a length of about 1000mm. The first stage involves elongating along the axial direction of the steel ingot and removing the riser, with a forging ratio of 4; the second stage involves upsetting, with a forging ratio of 9.

[0038] S3, Medium-temperature forging: The square billet formed by high-temperature forging is reheated to 900℃, held for 3 hours, and then drawn and cut into pieces in one pass. 120 6000mm or 100 A round blank of approximately 6000mm.

[0039] S4. Stress-relieving annealing: Heat the round billet to 550℃, hold for 2 hours, and then air cool it after removing it from the furnace.

[0040] S5. Finishing: Turn the round blank to... 100 2000mm or 70 A 6000mm pull rod semi-finished product.

[0041] Appendix Figure 6 As shown 100 Tensile stress-strain curves obtained from sampling of a 2000mm tie rod. (From...) Figure 6 It can be seen that the yield strength of the tie rod reaches 618. 648MPa, tensile strength reaches 1114 1165 MPa, with an elongation after fracture of 40.7%. 45.2%.

[0042] Example 2

[0043] This embodiment provides a thick TWIP steel plate for a ring damper of a railway bridge and its preparation method. The specific implementation steps are the same as in Embodiment 1, with the only difference being: (1) The chemical composition (wt.%) of the material is as follows: C: 0.6; Mn: 22.0; Si: 0.4; Al: 0.6; P: 0.07; B: 0.03; V: 0.20; Ti: 0.05; Nb: 0.05; S: 0.005; Total amount of other impurity elements: 0.008; the remainder is Fe.

[0044] (2) Smelting is carried out in a 30-ton electric arc furnace. The smelting process includes primary smelting, ladle refining, vacuum degassing, etc., and finally cast into 4 steel ingots of about 7.5 tons each.

[0045] (3) High-temperature forging process: Because the plate produced in this embodiment is relatively large, reaching 1700×3300×150mm, the resistance to plastic deformation during forging is very high. Therefore, a large-tonnage press was used for multi-pass pressing during production. Heating temperature: 1170℃, holding time: 3h. During forging, the steel ingot is elongated along its axial direction; the first forging ratio... 5. Subsequent forging ratios are approximately 3 per forging pass. Between each two forging passes, the billet is heated to 1150℃ and held for a specified time. 1 hour.

[0046] (4) Properties of forged materials: Tensile tests were conducted on samples taken along the length of the thick plate, and the stress-strain curves were obtained as shown in the attached figure. Figure 7 As shown. Sampling yield strength 436MPa, tensile strength 888MPa, elongation after fracture 52% of them met the technical requirements for railway bridge dampers.

[0047] Example 3

[0048] This embodiment provides a TWIP steel square bar for a U-shaped damper used in construction and its preparation method. The specific implementation steps are the same as in Embodiment 1, with the only difference being: (1) The chemical composition (wt.%) of the material is as follows: C: 0.8; Mn: 20.0; Si: 0.4; Al: 1.2; P: 0.12; B: 0.05; V: 0.50; Ti: 0.02; Nb: 0.02; S: 0.005; Total amount of other impurity elements: 0.008; the remainder is Fe.

[0049] (2) High-temperature forging process: A 750kg rectangular steel ingot is heated to 1170℃, homogenized, and held at that temperature for 3 hours. Then, it is drawn along the axial direction of the ingot and forged in three stages into a square billet of approximately 80×90×2150mm. The first forging stage is... 10, Second and third forging ratio 9.

[0050] (3) Billet machining: The forged billet was machined into a 63×76×2130mm square bar using a plane milling machine. At the same time, tensile specimens were machined along the axial direction for tensile performance testing. The measured tensile stress-strain curves are shown in the attached figure. Figure 8 As shown.

[0051] (4) Properties of forged materials: (See attached document) Figure 8 This indicates the material's yield strength. 537MPa, tensile strength 1244MPa, elongation after fracture 65.1%, meeting the technical requirements for U-shaped dampers used in buildings.

[0052] Appendix Table 1: Comparison of the mechanical properties of the P-reinforced TWIP steel of this invention with existing technologies

[0053] As can be seen from Appendix Table 1, the yield strength and tensile strength of the novel TWIP steel prepared by this invention are significantly higher than those of ordinary TWIP steel or TWIP steel strengthened by other methods, while the elongation after fracture is very similar, indicating that the P-strengthening method proposed in this invention is effective. Figure 1 It can be seen that as the phosphorus (P) content increases, the strength of TWIP steel increases while its ductility decreases. A moderate P content, such as 0.068 wt.% P, can achieve both high strength and high ductility. (See attached...) Figure 2-5It can be seen that phosphorus alloying has no significant effect on the microstructure of TWIP steel, which remains a single-phase austenite with annealing twins present within many austenite grains. Therefore, the method proposed in this invention for strengthening TWIP steel through phosphorus alloying effectively solves the problems of low yield strength and ineffective general strengthening methods in ordinary TWIP steel. This lays a crucial foundation for the application of TWIP steel in large-scale support structures, bridges, and earthquake-resistant buildings, providing an important material solution for improving the service performance of these engineering structures.

[0054] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A TWIP steel with high yield strength and high plasticity, characterized in that, This TWIP steel is an Fe-Mn-Al-P system steel, with a chemical composition of C: 0.6% by weight. 0.9%; Mn: 17.0% 25.0%; Si: 0.3% 0.8%; Al：0.5% 1.5%； P：0.06% 0.20%； B：0.02% 0.08%； V：0.05% 0.50%； If: 0.010% 0.15%; Nb ：0.01% 0.15%； S: 0.005%; Total amount of other impurity elements: 0.008%; the remainder was Fe.

2. The TWIP steel with high yield strength and high plasticity according to claim 1, characterized in that, The chemical composition of this TWIP steel, by weight percentage, is: C: 0.8%; Mn: 22.0%; Si: 0.4%; Al：1.2%； P：0.12%； B：0.05%； V：0.5%； Ti: 0.02%; Nb ：0.02%； S: 0.005%; Total amount of other impurity elements: 0.008%; the remainder was Fe.

3. A method for preparing TWIP steel with high yield strength and high plasticity as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Smelting and casting: According to the alloy composition, the raw materials are batched and then subjected to primary smelting, ladle refining, vacuum degassing and casting in an electric arc furnace to produce steel ingots. S2. Forging: Forging steel ingots into bars or plates.

4. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 3, characterized in that, The forging method of the bar is as follows: (1) The steel ingot is surface-machined, and the machined steel ingot is homogenized and then forged according to a forging ratio of 4.

8. Upset the steel ingot, then forge it according to the ratio 9.

16. The steel ingot is drawn unidirectionally along its axis and forged into a round or square billet, then... 10 Forging ratio is used to forge billets; (2) Solution treatment or stress relief annealing: The forging billet is subjected to solution treatment or stress relief annealing.

5. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 3, characterized in that, The forging method for the plate is as follows: heating the steel ingot to 1150°C. 1170℃, homogenization holding time After 3 hours, the steel ingot is drawn lengthwise along its axis and forged into slabs in several passes, with a forging ratio of 4.

10. Perform solution treatment or stress-relieving annealing on the forging billet.

6. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 3, characterized in that: The alloy composition includes scrap steel, elemental materials, and intermediate alloys. The elemental materials include C, Mn, Si, Al, and Fe, and the intermediate alloys include P-Fe, B-Fe, Ti-Fe, and Nb-Fe.

7. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 3, characterized in that: The scrap steel and elemental Fe in the alloy composition are added during the initial refining, while the elemental C, Mn, Al, Si, and intermediate alloys are added during the ladle refining stage.

8. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 3, characterized in that: After the steel ingot is poured, it is covered with insulation material and kept warm for ≥24 hours.

9. The method for preparing TWIP steel with high yield strength and high plasticity according to claim 4, characterized in that: The machining allowance for surface turning is 2. 5mm; the homogenization treatment temperature is 1130~1170℃, and the time is ≥3; the length of the forging billet is 1000mm. 6000mm.

10. A method for preparing TWIP steel with high yield strength and high plasticity according to claim 4 or 5, characterized in that: The solution treatment temperature is 1050°C. 1150℃, heat preservation time is 2.0 seconds. 4.0h, air cooling; stress-relief annealing temperature is 350-500℃, holding time is 3.0h. 5.0h, then cool to room temperature.

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