Plasticized enhanced trip dual phase stainless steel processing method

Abstract

This invention provides a method for plasticizing and strengthening TRIP duplex stainless steel, relating to the field of steel material manufacturing technology. The method includes: S1, smelting according to the chemical composition of duplex steel to produce TRIP duplex stainless steel billets; S2, obtaining an initial microstructure with uniform ferrite and austenite distribution; S3, subjecting the initial microstructure to cyclic deformation pretreatment to create a multi-scale structure within the two phases; and S4, performing mechanical property analysis on the pre-deformed TRIP duplex stainless steel. This invention reduces process difficulty by performing cyclic pre-deformation pretreatment on TRIP duplex stainless steel. By preparing a multi-scale structure within the two phases through cyclic pre-deformation, it effectively regulates the strain distribution during tensile deformation, promotes the early, uniform, and stable occurrence of martensitic phase transformation during deformation, and increases the saturation value and transformation amount of martensitic phase transformation. This achieves simultaneous improvement in the strength and plasticity of TRIP duplex stainless steel, effectively enhancing its comprehensive mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of steel material manufacturing technology, specifically to a method for treating TRIP duplex stainless steel with plasticizing and strengthening properties. Background Technology

[0002] Duplex stainless steel possesses both high strength and good ductility. Economical TRIP duplex stainless steels, which utilize Mn and N to replace Ni, further enhance their strength and ductility by inducing a deformation-induced martensitic transformation to trigger the TRIP effect. However, the core of this performance enhancement lies in precisely controlling the phase transformation kinetics of the austenitic phase, ensuring that the martensitic transformation occurs uniformly and continuously throughout the deformation process. Pre-deformation is a common process for improving the strength of duplex stainless steel, but it presents a contradiction between strength and ductility. Both low-temperature cycling and quasi-static pre-straining lead to premature and excessive transformation of metastable austenite into martensite, prematurely depleting the TRIP effect potential. This results in a sharp drop in ductility during subsequent deformation due to a lack of sustained phase transformation strengthening. This defect limits the application of traditional pre-deformation processes in high-performance TRIP duplex stainless steels.

[0003] Pre-deformation, as a key pretreatment process before the service of engineering components, is an effective method to improve the strength of stainless steel. Zongchi Wang of Tianjin University used a low-temperature cyclic plastic strengthening method to strengthen 304 austenitic stainless steel. Through low-temperature cyclic pre-deformation, its yield strength at room temperature increased by 97.2%-227.7%, and its tensile strength increased by 36.5%-70.8%, but the elongation decreased significantly. JT Lloyd of the DEVCOM Army Research Laboratory increased the dynamic strength of TRIP steel by three times by applying a quasi-static pre-strain to 10%, similarly with a slight decrease in elongation. Fahri R Zulfi of Bandung Polytechnic University applied pre-strain of 14%-52% to 304 stainless steel, also resulting in varying degrees of strength improvement, but the elongation was reduced by half. The different pre-deformation methods mentioned above all effectively improved the strength of the experimental steel, but the elongation decreased to varying degrees. Furthermore, excessive pre-deformation would limit the application of duplex stainless steel in subsequent large deformation processes. Therefore, the balance between the strength and ductility of duplex stainless steel in the pre-deformation process is a key issue that needs to be addressed.

[0004] CN103890214A discloses a duplex stainless steel that utilizes the TRIP effect through alloying design to achieve high formability and corrosion resistance. However, it lacks the ability to control the microstructure and phase transformation kinetics of the two phases through a mild deformation process. Pre-deformation strengthening processes often lead to a decrease in plasticity, making it difficult to achieve a good balance between strength and plasticity. Therefore, it is necessary to propose a plasticizing and strengthening TRIP duplex stainless steel treatment method. Summary of the Invention

[0005] To address the shortcomings of the existing technology, the present invention aims to provide a method for plasticizing and strengthening TRIP duplex stainless steel. By performing cyclic pre-deformation pretreatment on TRIP duplex stainless steel, not only is the process difficulty reduced, but the defect of a significant decrease in elongation of TRIP duplex stainless steel caused by conventional tensile pre-deformation is also effectively overcome. Through cyclic pre-deformation, a multi-scale structure is prepared in the two phases of TRIP duplex stainless steel, effectively adjusting the strain distribution during tensile deformation and promoting... Martensite occurs earlier, more uniformly, and more stably during deformation, increasing the saturation value and transformation amount of martensitic phase transformation, thereby achieving a simultaneous improvement in the strength and plasticity of TRIP duplex stainless steel and effectively enhancing its comprehensive mechanical properties.

[0006] This invention provides a method for treating TRIP duplex stainless steel with plasticizing and strengthening properties, the steps of which include: S1. Based on the chemical composition of duplex steel, TRIP duplex stainless steel billets are produced by casting.

[0007] S2. The duplex stainless steel is forged at 1150℃ to obtain a forged duplex stainless steel billet with a forging ratio >4. The stainless steel billet is heated to 1050℃ and held for 30 minutes, then water-cooled to room temperature for solution treatment to obtain the initial structure of TRIP duplex stainless steel with uniform distribution of ferrite and austenite phases.

[0008] S3. The initial microstructure of TRIP duplex stainless steel is subjected to cyclic deformation pretreatment. By adjusting the strain rate, strain amplitude and number of cycles of cyclic loading, a multi-scale structure is formed within the two phases of TRIP duplex stainless steel.

[0009] Adjusting the strain distribution between the two phases during tensile deformation, thereby regulating... Martensitic phase transformation kinetics.

[0010] S4. Mechanical property analysis: The pre-deformed TRIP duplex stainless steel was subjected to tensile testing at different strain rates until fracture failure to verify the mechanical properties of the pre-deformed TRIP duplex stainless steel.

[0011] Preferably, the chemical composition of the TRIP duplex stainless steel in step S1, by mass percentage, is: C: 0.025%, Si: 0.51%, Mn: 5.06%, Cr: 19.94%, Ni: 0.32%, N: 0.25%, with the remainder being Fe.

[0012] Preferably, after solution treatment and water cooling in step S2, the volume fraction of the austenite phase accounts for 55% to 65% of the initial microstructure volume fraction.

[0013] Preferably, the strain rate range of the symmetrical strain cyclic loading in step S3 is 1×10⁻⁶. -3 s-1 ~ 5×10 -3 s -1 The strain amplitude is 0.5%~0.8%, and the cycle number is 50 cycles.

[0014] Preferably, after the cyclic pre-deformation treatment in step S3, a spatial dislocation structure is formed in the ferrite phase, and a multi-scale structure including slip bands, dislocations, stacking faults and ε-martensite is formed in the austenite phase.

[0015] Preferably, the strain rate range for stretching in step S4 is 1×10⁻⁶. -3 s -1 ~ 5×10 -3 s -1 Until the pre-deformed TRIP duplex stainless steel fractures and fails.

[0016] Preferably, the TRIP duplex stainless steel after pre-deformation in step S4 has a tensile strength ≥1136 MPa and an elongation increased to 0.408 at a strain amplitude of 0.8%.

[0017] Compared with the prior art, the beneficial effects of the present invention are reflected in: (1) The method for treating TRIP duplex stainless steel with plasticization and reinforcement of the present invention prepares a multi-scale structure in the two phases of TRIP duplex stainless steel through cyclic pre-deformation, which can effectively adjust the strain distribution in tensile deformation and promote the plasticization and reinforcement of TRIP duplex stainless steel. The martensitic transformation occurs earlier, more uniformly, and more stably during deformation, which not only increases the saturation value and transformation amount of the martensitic transformation, but also enables the TRIP effect to continue to play its role to a greater extent and for a longer period of time, thereby improving the comprehensive mechanical properties of TRIP duplex stainless steel.

[0018] (2) The method for processing TRIP duplex stainless steel with plasticization and reinforcement of the present invention performs cyclic pre-deformation pre-treatment on TRIP duplex stainless steel. Compared with traditional low temperature cyclic pre-deformation or conventional tensile pre-deformation, it not only reduces the process difficulty, but also effectively solves the problem of the decrease in elongation of TRIP duplex stainless steel caused by tensile pre-deformation. Attached Figure Description

[0019] Figure 1 Image of the initial microstructure of the TRIP duplex stainless steel prepared according to an embodiment of the present invention; Figure 2 This is a comparison of the mechanical property curves of TRIP duplex stainless steel before and after 50 cycles at a strain amplitude of 0.8% in Example 1 of the present invention. Figure 3 This refers to the multi-scale structure prepared in the two phases of TRIP duplex stainless steel after 50 cycles at a strain amplitude of 0.8% in Example 1 of the present invention. Figure 4This is a typical TEM image of the TRIP duplex stainless steel in Example 1 of the present invention after 50 cycles at a strain amplitude of 0.8% and subsequent tensile failure. Figure 5 The images show the EBSD microstructures of TRIP duplex stainless steel in Example 1 of this invention after direct tensile failure and after pre-deformation and re-tensile failure with a strain amplitude of 0.8%. Figure 6 The images show SEM images of the fracture surfaces of TRIP duplex stainless steel in Example 1 of this invention after direct tensile failure and after pre-deformation and re-tensile failure with a strain amplitude of 0.8%. Figure 7 This is a comparison of the mechanical property curves of TRIP duplex stainless steel before and after 50 cycles at a strain amplitude of 0.5% in Example 2 of the present invention. Figure 8 The images show the EBSD microstructures of TRIP duplex stainless steel in Example 2 of this invention after direct tensile failure and after pre-deformation and re-tensile failure with a strain amplitude of 0.5%. Figure 9 The images show SEM images of the fracture surfaces of TRIP duplex stainless steel in Embodiment 2 of the present invention after direct tensile failure and after failure after pre-deformation with a strain amplitude of 0.5% followed by tensile failure. Figure 10 This is a comparison of the mechanical property curves of TRIP duplex stainless steel before and after 50 cycles at a strain amplitude of 0.6% in Example 3 of the present invention. Figure 11 This is a comparison of the mechanical property curves of TRIP duplex stainless steel before and after 50 cycles at a strain amplitude of 0.7% in Example 4 of the present invention. Figure 12 This is a comparison of the mechanical property curves of the TRIP duplex stainless steel of the present invention after pretreatment at various strain amplitudes. Detailed Implementation

[0020] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0021] The present invention provides a method for treating TRIP duplex stainless steel with plasticizing and strengthening properties, such as... Figures 1-12 As shown, the steps include: S1. Based on the chemical composition of duplex steel, TRIP duplex stainless steel billets are produced by casting.

[0022] The chemical composition of TRIP duplex stainless steel by mass percentage is: C: 0.025%, Si: 0.51%, Mn: 5.06%, Cr: 19.94%, Ni: 0.32%, N: 0.25%, with the remainder being Fe.

[0023] S2. The duplex stainless steel is forged at 1150℃ to obtain a forged duplex stainless steel billet with a forging ratio >4. The stainless steel billet is heated to 1050℃ and held for 30 minutes, then water-cooled to room temperature for solution treatment to obtain the initial structure of TRIP duplex stainless steel with uniform distribution of ferrite and austenite phases.

[0024] After solution treatment and water cooling, the volume fraction of the austenite phase accounts for 55% to 65% of the initial microstructure volume fraction.

[0025] S3. The initial microstructure of TRIP duplex stainless steel is subjected to cyclic deformation pretreatment. By adjusting the strain rate, strain amplitude and number of cycles of cyclic loading, a multi-scale structure is formed within the two phases of TRIP duplex stainless steel.

[0026] The strain rate range for symmetrical strain cyclic loading is 1×10⁻⁶. -3 s -1 ~ 5×10 -3 s -1 The strain amplitude is 0.5%~0.8%, and the cycle number is 50 cycles.

[0027] Adjusting the strain distribution between the two phases during tensile deformation, thereby regulating... Martensitic phase transformation kinetics.

[0028] After cyclic pre-deformation treatment, a spatial dislocation structure is formed in the ferrite phase, and a multi-scale nucleation point network including slip bands, dislocations, stacking faults and ε-martensite is formed in the austenite phase.

[0029] S4. Mechanical property analysis: The pre-deformed TRIP duplex stainless steel was subjected to tensile testing at different strain rates until fracture failure to verify the mechanical properties of the pre-deformed TRIP duplex stainless steel.

[0030] The strain rate range for tension is 1×10⁻⁶. -3 s -1 ~ 5×10 -3 s -1 Until the pre-deformed TRIP duplex stainless steel fractures and fails, the pre-deformed TRIP duplex stainless steel has a tensile strength ≥1136 MPa and an elongation increased to 0.408 at a strain amplitude of 0.8%.

[0031] The following describes the method for treating TRIP duplex stainless steel with plasticization and reinforcement according to the present invention in further detail with reference to embodiments: TRIP duplex stainless steel was subjected to cyclic deformation pretreatment at room temperature with a strain amplitude of 0.8% and a symmetrical strain cyclic loading method at a strain rate of 0.002 s⁻¹. -1The cycle length was 50 cycles. Then, the pre-deformed TRIP duplex stainless steel was subjected to monotonic loading at a strain rate of 0.002 s⁻¹. -1 The obtained mechanical property curves are as follows: Figure 1 and Figure 2 As shown, after cyclic pre-deformation treatment, the yield strength of TRIP duplex stainless steel increased from 482 MPa to 609 MPa, the tensile strength increased from 954 MPa to 1136 MPa, and the elongation increased from 0.35 to 0.408.

[0032] Figure 3 As shown, the multi-scale structure of the two phases of TRIP duplex stainless steel after 50 cycles at a strain amplitude of 0.8% is illustrated. Spatial dislocation structures are observed in the ferrite, which softens the ferrite and allows it to absorb a large amount of strain in the initial stages of subsequent tensile deformation. This prevents excessive strain absorption by the austenite in the TRIP duplex stainless steel, thus avoiding significant deformation. Martensitic transformation, which consumes excessive austenite, makes phase transformation difficult in the later stages of deformation, thus preventing the initiation of the TRIP effect. Numerous slip bands, dislocations, stacking faults, and ε-martensite appear within the austenite; these structures... The martensitic phase transformation provides numerous nucleation sites, offering sufficient driving force for the transformation. This effectively alters the characteristic of TRIP duplex stainless steel, which undergoes sufficient nucleation followed by rapid phase transformation during monotonic loading, allowing the phase transformation to occur earlier and effectively increasing the phase transformation saturation value.

[0033] Figure 4 As shown, this is a typical TEM image of TRIP duplex stainless steel after 50 cycles at a strain amplitude of 0.8% and subsequent tensile failure. The dislocation structure within the ferrite grains disappears, replaced by densely interwoven dislocation lines. Dislocations multiply and accumulate significantly, resulting in an increased dislocation density compared to the pre-deformation stage. After cyclic pre-deformation, the ferrite phase of the TRIP duplex stainless steel already possesses a certain dislocation density. During subsequent tensile deformation, dislocations multiply more rapidly, causing the ferrite phase to harden quickly. During tensile deformation, the strain distribution gradually shifts towards austenite, allowing the austenite phase to accumulate sufficient plastic strain in the later stages of tensile deformation. This induces deformation-induced martensitic transformation, enabling the TRIP duplex stainless steel to retain its transformation capability in the later stages of tensile deformation and promoting the continued effectiveness of the TRIP effect. A large number of dislocations appear in the austenite phase. Martensite enhances the hardening rate of TRIP duplex stainless steel while reducing strain concentration, increasing strength, and delaying necking.

[0034] Figure 5The image shows EBSD images of the microstructure of TRIP duplex stainless steel after direct tensile failure and after pre-deformation with a strain amplitude of 0.8% followed by tensile failure. After cyclic pre-deformation treatment, the martensite transformation amount of TRIP duplex stainless steel is greater than that in the untreated direct tensile failure state, ensuring that the TRIP effect can be exerted to a greater extent.

[0035] Figure 6 The image shows SEM images of the fracture surfaces of TRIP duplex stainless steel after direct tensile failure and after cyclic pre-deformation followed by tensile failure. After cyclic pre-deformation, the fracture surface of TRIP duplex stainless steel consists entirely of cleavage and quasi-cleavage planes; while untreated TRIP duplex stainless steel, after direct tensile failure, still exhibits a small number of dimples. The appearance of dimples is mainly due to the presence of retained austenite, demonstrating that the martensite transformation amount is further enhanced and the TRIP effect is more significant after cyclic pre-deformation.

[0036] Example 2 The prepared TRIP duplex stainless steel was subjected to cyclic deformation pretreatment at a strain amplitude of 0.5% and a symmetrical strain cyclic loading method with a strain rate of 0.002 s⁻¹. -1 The cycle length was 50 cycles. Then, the pre-deformed TRIP duplex stainless steel was subjected to monotonic loading at a strain rate of 0.002 s⁻¹. -1 The obtained mechanical property curves are as follows: Figure 7 As shown, after cyclic pre-deformation treatment, the yield strength of TRIP duplex stainless steel increased from 482 MPa to 533 MPa, the tensile strength increased from 954 MPa to 1027 MPa, and the elongation increased from 0.35 to 0.378.

[0037] Figure 8 The image shows EBSD images of the microstructure of TRIP duplex stainless steel after direct tensile failure and after pre-deformation with a strain amplitude of 0.5% followed by tensile failure. It also shows that after cyclic pre-deformation treatment, the martensite transformation of TRIP duplex stainless steel is greater than that in the untreated direct tensile failure state, ensuring that the TRIP effect plays a greater role.

[0038] Figure 9 The images show SEM images of the fracture surfaces of TRIP duplex stainless steel after direct tensile failure and after cyclic pre-deformation followed by tensile failure. After cyclic pre-deformation, the fracture surface of TRIP duplex stainless steel consists of cleavage planes and quasi-cleavage planes, and also contains a small number of dimples; while the untreated TRIP duplex stainless steel, after direct tensile failure, still exhibits a large number of dimples. This also indicates that the martensitic transformation is further enhanced and the TRIP effect is stronger after cyclic pre-deformation treatment.

[0039] Example 3 The prepared TRIP duplex stainless steel was subjected to cyclic deformation pretreatment at a strain amplitude of 0.6% and a symmetrical strain cyclic loading method with a strain rate of 0.002 s⁻¹. -1 The cycle length was 50 cycles. Then, the pre-deformed TRIP duplex stainless steel was subjected to monotonic loading at a strain rate of 0.002 s⁻¹. -1 The obtained mechanical property curves are as follows: Figure 10 As shown, after cyclic pre-deformation treatment, the yield strength of TRIP duplex stainless steel increased from 482 MPa to 556 MPa, the tensile strength increased from 954 MPa to 1059 MPa, and the elongation increased from 0.35 to 0.386.

[0040] Example 4 The prepared TRIP duplex stainless steel was subjected to cyclic deformation pretreatment at a strain amplitude of 0.7% using symmetrical strain cyclic loading at a strain rate of 0.002 s⁻¹. -1 The cycle length was 50 cycles. Then, the pre-deformed TRIP duplex stainless steel was subjected to monotonic loading at a strain rate of 0.002 s⁻¹. -1 The obtained mechanical property curves are as follows: Figure 11 As shown, after cyclic pre-deformation treatment, the yield strength of TRIP duplex stainless steel increased from 482 MPa to 580 MPa, the tensile strength increased from 954 MPa to 1092 MPa, and the elongation increased from 0.35 to 0.391.

[0041] In summary, compared with untreated TRIP duplex stainless steel, Examples 1, 2, 3, and 4 show improved strength and plasticity, verifying that the plasticizing and strengthening TRIP duplex stainless steel treatment method of the present invention can effectively adjust the strain distribution of each phase, enabling the martensitic phase transformation to occur earlier and more uniformly and stably throughout the tensile deformation process, increasing the phase transformation saturation value, thereby allowing the phase transformation-induced plasticity effect to play a full role and promoting the engineering design and application of TRIP duplex stainless steel.

[0042] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for treating TRIP duplex stainless steel with plasticizing and strengthening properties, characterized in that: The steps include: S1. Based on the chemical composition of duplex steel, TRIP duplex stainless steel billets are produced by casting. S2. The duplex stainless steel is forged at 1150℃ to obtain a forged duplex stainless steel billet with a forging ratio >4. The stainless steel billet is heated to 1050℃ and held for 30 minutes. Then it is water-cooled to room temperature for solution treatment to obtain the initial structure of TRIP duplex stainless steel with uniform distribution of ferrite and austenite phases. S3. The initial microstructure of TRIP duplex stainless steel is subjected to cyclic deformation pretreatment. By adjusting the strain rate, strain amplitude and number of cycles of cyclic loading, a multi-scale structure is formed in the two phases of TRIP duplex stainless steel. Adjusting the strain distribution between the two phases during tensile deformation, thereby regulating... Martensitic phase transformation kinetics; S4. Mechanical property analysis: The pre-deformed TRIP duplex stainless steel was subjected to tensile testing at different strain rates until fracture failure to verify the mechanical properties of the pre-deformed TRIP duplex stainless steel.

2. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: The chemical composition of the TRIP duplex stainless steel in step S1, by mass percentage, is: C: 0.025%, Si: 0.51%, Mn: 5.06%, Cr: 19.94%, Ni: 0.32%, N: 0.25%, with the remainder being Fe.

3. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: After solution treatment in step S2, the volume fraction of the austenite phase accounts for 55% to 65% of the initial microstructure volume fraction.

4. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: The strain rate range for symmetrical strain cyclic loading in step S3 is 1×10⁻⁶. -3 s -1 ~ 5×10 -3 s -1 The strain amplitude is 0.5%~0.8%, and the cycle number is 50 cycles.

5. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: In step S3, after cyclic pre-deformation treatment, a spatial dislocation structure is formed in the ferrite phase, and a multi-scale structure including slip bands, dislocations, stacking faults and ε-martensite is formed in the austenite phase.

6. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: The strain rate range for tension in step S4 is 1×10⁻⁶. -3 s -1 ~ 5×10 -3 s -1 Until the pre-deformed TRIP duplex stainless steel fractures and fails.

7. The method for treating TRIP duplex stainless steel with plasticization and reinforcement according to claim 1, characterized in that: In step S4, the pre-deformed TRIP duplex stainless steel exhibits a tensile strength ≥1136 MPa and an elongation increased to 0.408 at a strain amplitude of 0.8%.

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