Method for inhibiting Luders deformation of high-toughness ultra-fine grain austenitic stainless steel

Abstract

The invention relates to the field of manufacturing of high-quality ultra-high-strength austenitic stainless steel, and discloses a method for inhibiting Luders deformation of high-toughness ultra-fine grain austenitic stainless steel, the method is suitable for metastable austenitic stainless steel, and comprises the following steps: (1) carrying out multi-pass cold rolling on a hot rolled plate subjected to solution treatment, martensite and untransformed austenite in a proper proportion are obtained; and (2) carrying out short-time isothermal annealing on the cold-rolled sheet to obtain a heterogeneous structure consisting of ultra-fine grain reverted austenite and recrystallized coarse grain austenite. The production process is simple, the manufacturing cost is low, based on the reasonable isomerization design of the microstructure, the remarkable Luders deformation in the homogeneous ultra-fine grain austenitic stainless steel can be completely eliminated, and the high strength and high toughness of the material can be guaranteed in cooperation with multiple strengthening and toughening mechanisms. The finished product prepared through the method is the high-quality high-toughness ultra-fine grain austenitic stainless steel, the yield strength is larger than 800 MPa, the tensile strength is larger than 1000 MPa, the total ductility is larger than 40%, and Luders deformation is completely restrained.

Description

Technical Field

[0001] The present invention belongs to the field of manufacturing high-quality ultra-high-strength austenitic stainless steel, and in particular relates to a method for suppressing the Luders deformation of high-strength and tough ultrafine-grained austenitic stainless steel. Background Art

[0002] The research and development of advanced high-strength steel is accelerating in the direction of "high strength, high toughness, lightweight, long life, and high cost performance" to support green and low-carbon transformation and high-quality development. As an important member of the steel material family, austenitic stainless steel has many advantages such as non-magneticity, excellent corrosion resistance, weldability, formability, good work hardening ability and high-temperature oxidation resistance. It is widely used in people's livelihood, national defense, chemical industry, medical care, marine engineering, aerospace and other fields. However, the yield strength of austenitic stainless steel is generally low (about 150-300MPa), which seriously limits its application in high-strength structural fields such as anti-collision, load-bearing, and pressure resistance. Among the many strengthening and toughening methods, fine grain strengthening is undoubtedly the first choice for the preparation of high-strength and tough austenitic stainless steel. For metastable austenitic stainless steel, the coupling effect of cold rolling / deep cold rolling to induce martensitic transformation and reverse transformation annealing can be used to achieve ultrafine or even nanoscale austenite grains. Patents (CN106435131B and CN104451082B) have successfully refined austenitic stainless steel grains to below 100nm using this principle, increasing the material's yield strength to over 1GPa. However, excessively small grains significantly deteriorate the material's plasticity.

[0003] By regulating the grain size, the comprehensive mechanical properties can be optimized, thereby obtaining austenitic stainless steel with both high strength and high toughness, which has been published in patents (CN108531817B, CN110241364B). It should be noted that the high strength of the material at this time comes from fine grain strengthening, while the good plasticity is related to the Lüders deformation (the Lüders strain accounts for more than 50% of the total elongation). The Lüders deformation will not only seriously affect the forming performance of the material and the surface quality of the parts, but will also cause the pressure vessel subjected to high-pressure impact load to crack and cause safety accidents. At the same time, the appearance of the Lüders band will also increase the risk of hydrogen-induced cracking of the material. In industry, the Lüders band is often eliminated by applying a pre-deformation equivalent to the Lüders strain. This method is not suitable for eliminating the ultra-large Lüders strain in ultrafine-grained austenitic stainless steel, because the larger pre-strain will introduce too many defects or induce the transformation of part of the austenite to martensite, which not only overdrafts the deformation capacity of the material but may also cause galvanic corrosion and thus reduce the corrosion resistance. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for suppressing the Lüders deformation of high-strength and toughness ultrafine-grained austenitic stainless steel. By controlling the structure of the rolling process (not pursuing complete martensite but a cold-rolled structure of martensite + an appropriate amount of untransformed austenite) combined with the optimization of the subsequent annealing process (short-time isothermal annealing), a rational heterogeneous structure design of the microstructure is achieved, thereby completely suppressing the Lüders deformation while ensuring the high strength and toughness of the ultrafine-grained steel.

[0005] The technical solution to achieve the purpose of the present invention is: a method for suppressing the Lüders deformation of high-strength and tough ultrafine-grained austenitic stainless steel, the method is suitable for -20℃≤M d30 ≤50℃ and 10mJ / m 2 ≤SFE≤25mJ / m 2 The method comprises the following steps:

[0006] Step (1): Homogenization: homogenization to obtain an initial solid solution structure with an equiaxed, uniform austenite single-phase structure;

[0007] Step (2): cold rolling deformation: cold rolling to obtain a dual-phase cold-rolled structure consisting of martensite + deformed austenite with a martensite content of 70% to 85%;

[0008] Step (3): Isomerization annealing: Annealing to obtain an isomerized annealed structure in which recrystallized coarse-grained austenite is embedded in an ultrafine-grained austenite matrix, wherein the average grain size of the ultrafine-grained austenite is 200-300 nm, accounting for about 70%-80%; the average grain size of the coarse-grained austenite is 2-4 μm.

[0009] Furthermore, the composition of metastable austenitic stainless steel is determined according to the following formula:

[0010] M d30 (℃)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb-1.42(Gs-8)

[0011]

[0012] Among them, the content of each element is mass percentage, and Gs is the grain size.

[0013] Furthermore, the homogenization process in step (1) is specifically as follows:

[0014] A metastable austenitic stainless steel hot-rolled slab with a thickness of 3-10 mm is solution treated at 1050-1100° C., kept at this temperature for 0.5-2 hours, and then water-cooled to room temperature.

[0015] Furthermore, the cold rolling deformation in step (2) is specifically as follows: the steel plate after homogenization treatment in step (1) is pickled, and then subjected to multiple cold rolling deformations, with a total reduction rate of 60-90%, and a final cold rolled plate thickness of 0.6-2 mm; M d30 The cold rolling temperature of metastable austenitic stainless steel above room temperature is room temperature or liquid nitrogen low temperature rolling, M d30 The cold rolling temperature of metastable austenitic stainless steel below room temperature is liquid nitrogen low-temperature rolling.

[0016] Furthermore, step (3): isomerization annealing is specifically as follows: the cold-rolled sheet in step (2) is subjected to short-time isothermal annealing at an annealing temperature of 700-800° C. for a holding time of 100-300 seconds, followed by water cooling after annealing.

[0017] Furthermore, the annealing in step (3) is performed in a tubular annealing furnace, and the holding time includes a heating process and an isothermal process.

[0018] A high-strength and tough ultrafine-grained austenitic stainless steel plate is prepared by the above method, has a yield strength greater than 800 MPa, a tensile strength greater than 1000 MPa, a total elongation greater than 40%, and has no Lüders strain.

[0019] Compared with the prior art, the present invention has the following significant advantages:

[0020] (1) The method of the present invention achieves precise control of the isomerization of the metastable austenitic stainless steel microstructure by controlling cold rolling and short-time annealing, utilizing the difference in driving force between the reverse phase transformation of cold-rolled martensite and the recrystallization of deformed austenite. Ultimately, while ensuring the high strength and high toughness of the ultrafine-grained austenitic stainless steel plate, its Lüders deformation behavior is completely eliminated.

[0021] (2) The process of the present invention is simple, has no special requirements for equipment and technology, and has low production cost.

[0022] (3) The method of the present invention is suitable for the industrial production of high-quality, high-strength and high-toughness austenitic stainless steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the OM diagram of the initial solid solution structure of the experimental steel in Example 1 of the present invention.

[0024] Figure 2 This is the cold-rolled microstructure XRD of the experimental steel in Examples 1 and 2 of the present invention.

[0025] Figure 3 These are SEM images of the finished product structures of the experimental steels in Examples 1 and 2 of the present invention, where (a) is Example 1 and (b) is Example 2.

[0026] Figure 41 is the engineering stress-engineering strain curve of the finished steel in Examples 1 to 4 of the present invention.

[0027] Figure 5 Comparison of the mechanical behaviors of ultrafine-grained austenitic stainless steel (comparative example) and heterogeneous ultrafine-grained steel (Example 3). DETAILED DESCRIPTION

[0028] The present invention is further described in detail below with reference to the accompanying drawings.

[0029] The method of suppressing Lüders deformation of high-strength and tough ultrafine-grained austenitic stainless steel described in the present invention can completely eliminate Lüders deformation by optimizing the heterogeneous microstructure of the microstructure while ensuring the high strength and toughness of the ultrafine-grained austenitic stainless steel, thereby facilitating the industrial production and application of high-quality ultrafine-grained austenitic stainless steel. This method is applicable to metastable austenitic stainless steel, and the material should have the following characteristics: -20℃≤M d30 ≤50℃,10mJ / m 2 ≤SFE≤25mJ / m 2 .

[0030] Example 1

[0031] The present invention discloses a method for suppressing Lüders deformation of high-strength and ultrafine-grained austenitic stainless steel. The material used in the current embodiment is 304 austenitic stainless steel. d30 At about 30°C, SFE is about 18mJ / m 2 .

[0032] The specific steps are as follows:

[0033] Step 1: Homogenization: 8mm thick 304 austenitic stainless steel hot-rolled slabs were solution treated at 1080°C for 1.5 hours and then cooled to room temperature to obtain an initial solution structure with uniform composition and structure.

[0034] Step 2, cold rolling: The steel plate homogenized in step (1) is pickled to remove surface scale and then subjected to multiple passes of room temperature cold rolling, with a single pass reduction of 0.5 to 1 mm and a total reduction of 75%. The final cold-rolled plate thickness is 2 mm. The cold-rolled structure consists of 76% martensite and 24% untransformed austenite.

[0035] Step 3, isomerization annealing: The cold-rolled sheet in step (2) is subjected to short-term isothermal annealing in a tubular annealing furnace at 800°C for 150 seconds, followed by water cooling after annealing. The finished product structure is a bimodal isomerization structure consisting of an ultrafine-grained austenite matrix (from martensite reverse transformation) + recrystallized coarse-grained austenite (derived from the recrystallization of deformed austenite). Among them, the average grain size of the ultrafine-grained austenite is 250nm, accounting for about 75%; the average grain size of the coarse-grained austenite is 2.5μm, accounting for about 25%.

[0036] The mechanical properties test results show that the finished heterogeneous ultrafine grain steel (finished product organization SEM picture see Figure 3 ) not only completely eliminates the unusually significant Lüders deformation in ultrafine-grained steel but also maintains high strength and toughness, with a yield strength of 820 MPa, a tensile strength of 1082 MPa, and a total elongation of 43% (the engineering stress-engineering strain curve of the finished steel is shown in Figure 4 ).

[0037] Example 2

[0038] The present invention discloses a method for suppressing Lüders deformation of high-strength and ultrafine-grained austenitic stainless steel. The material used in the current embodiment is 301 austenitic stainless steel. d30 At about 30°C, the SFE is about 12mJ / m 2 .

[0039] The specific steps are as follows:

[0040] Step 1: Homogenization: A 5 mm thick 301 austenitic stainless steel hot-rolled slab was solution treated at 1050°C for 1 hour and then cooled to room temperature to obtain an initial solution structure with uniform composition and structure.

[0041] Step 2, cold rolling: The steel plate homogenized in step (1) is pickled to remove surface scale and then subjected to multiple passes of room temperature cold rolling, with a single pass reduction of 0.5 to 1 mm and a total reduction of 60%. The final cold-rolled plate thickness is 2 mm. The cold-rolled structure consists of 85% martensite and 15% untransformed austenite.

[0042] Step 3, isomerization annealing: The cold-rolled sheet in step (2) is subjected to short-term isothermal annealing in a tubular annealing furnace at a temperature of 700°C and a holding time of 200s, followed by water cooling after annealing. The finished product structure is a bimodal isomerized structure consisting of an ultrafine-grained austenite matrix (from martensite reverse transformation) + recrystallized coarse-grained austenite (derived from the recrystallization of deformed austenite). Among them, the average grain size of the ultrafine-grained austenite is 200nm, accounting for about 70%; the average grain size of the coarse-grained austenite is 3μm, accounting for about 30%.

[0043] The mechanical properties test results show that the finished heterogeneous ultrafine grain steel (finished product organization SEM picture see Figure 3 ) not only completely eliminates the unusually significant Lüders deformation in ultrafine-grained steel but also maintains high strength and toughness, with a yield strength of 800MPa, a tensile strength of 1064MPa, and a total elongation of 43% (the engineering stress-engineering strain curve of the finished steel is shown in Figure 4 ).

[0044] Example 3

[0045] The present invention discloses a method for preparing a high-strength and ultrafine-grained austenitic stainless steel to suppress Lüders deformation. The material used in the current embodiment is 304 austenitic stainless steel. d30 At about 30°C, SFE is about 18mJ / m 2 .

[0046] The specific steps are as follows:

[0047] Step 1: Homogenization: A 3 mm thick metastable austenitic stainless steel hot-rolled slab was solution treated at 1050°C for 0.5 h and then cooled to room temperature to obtain an initial solution structure with uniform composition and structure.

[0048] Step 2, cold rolling: The 304 austenitic stainless steel homogenized in step (1) is pickled to remove surface scale and then subjected to multiple passes of room temperature cold rolling, with a single pass reduction of 0.1 to 0.5 mm and a total reduction of 80%. The final cold-rolled sheet thickness is 0.6 mm. The cold-rolled structure consists of 80% martensite and 20% untransformed austenite.

[0049] Step 3, isomerization annealing: The cold-rolled sheet in step (2) is subjected to short-term isothermal annealing in a tubular annealing furnace at 720°C for 100 seconds, followed by water cooling after annealing. The finished product structure is a bimodal isomerization structure consisting of an ultrafine-grained austenite matrix (from martensite reverse transformation) + recrystallized coarse-grained austenite (derived from the recrystallization of deformed austenite). Among them, the average grain size of the ultrafine-grained austenite is 200nm, accounting for about 80%; the average grain size of the coarse-grained austenite is 4μm, accounting for about 20%.

[0050] The mechanical properties test results show that the finished heterogeneous ultrafine-grained steel not only completely eliminates the unusually significant Lüders deformation in ultrafine-grained steel but also maintains high strength and toughness. Its yield strength is 883MPa, tensile strength is 1100MPa, and total elongation is 45% (the engineering stress-engineering strain curve of the finished steel is shown in Figure 2). Figure 4 ).

[0051] Example 4

[0052] The present invention discloses a method for suppressing Lüders deformation of high-strength and ultrafine-grained austenitic stainless steel. The material used in the current embodiment is a 321 austenitic stainless steel. d30 At -20°C, the SFE is about 22 mJ / m 2 The specific steps are as follows:

[0053] Step 1: Homogenization: A 10 mm thick metastable austenitic stainless steel hot-rolled slab was solution treated at 1100°C for 2 hours and then cooled to room temperature to obtain an initial solution structure with uniform composition and structure.

[0054] Step 2, cold rolling: The steel plate homogenized in step (1) is pickled to remove surface scale and then subjected to multiple passes of liquid nitrogen deep cold rolling. The reduction per pass is controlled between 0.5 and 2 mm, the total reduction is 90%, and the final cold-rolled plate thickness is 1 mm. The cold-rolled structure consists of 73% martensite and 27% untransformed austenite.

[0055] Step 3, isomerization annealing: The cold-rolled sheet in step (2) is subjected to short-term isothermal annealing in a tubular annealing furnace at a temperature of 750°C for 300 seconds, followed by water cooling after annealing. The finished product structure is a bimodal isomerization structure consisting of an ultrafine-grained austenite matrix (from martensite reverse transformation) + recrystallized coarse-grained austenite (derived from the recrystallization of deformed austenite). Among them, the average grain size of the ultrafine-grained austenite is 300nm, accounting for about 75%; the average grain size of the coarse-grained austenite is 3.5μm, accounting for about 25%.

[0056] The mechanical properties test results show that the finished heterogeneous ultrafine-grained steel not only completely eliminates the unusually significant Lüders deformation in ultrafine-grained steel but also maintains high strength and toughness. Its yield strength is 828MPa, tensile strength is 1076MPa, and total elongation is 44% (the engineering stress-engineering strain curve of the finished steel is shown in Figure 4 ).

[0057] Comparative Example

[0058] The cold-rolled sample in Example 3 was isothermally annealed in a tubular annealing furnace at 650°C for 1 hour, and then water-cooled after annealing. The finished steel was a uniform ultrafine-grained austenite structure with an average grain size of about 200nm. The mechanical properties test results showed that the yield strength of the ultrafine-grained steel was 1030MPa, the tensile strength was 1057MPa, and the total elongation was 34%, of which the Lüdes strain was 22.3%, accounting for 65.6% of the total elongation. (Comparison of engineering stress-engineering strain curves of ultrafine-grained steel and heterogeneous ultrafine-grained steel can be seen in Figure 5 ).

Claims

1. A method for suppressing Lüders deformation of high-strength and tough ultrafine-grained austenitic stainless steel, characterized in that: The method is applicable to -20℃≤M d30 ≤50℃ and 10mJ / m 2 ≤SFE≤25mJ / m 2 The method comprises the following steps: Step (1): Homogenization: homogenization to obtain an initial solid solution structure with an equiaxed, uniform austenite single-phase structure; Step (2): cold rolling deformation: cold rolling to obtain a dual-phase cold-rolled structure consisting of martensite + deformed austenite with a martensite content of 70% to 85%; Step (3): Isomerization annealing: Annealing to obtain an isomerized annealed structure in which recrystallized coarse-grained austenite is embedded in an ultrafine-grained austenite matrix, wherein the average grain size of the ultrafine-grained austenite is 200-300 nm, accounting for about 70%-80%; the average grain size of the coarse-grained austenite is 2-4 μm.

2. The method according to claim 1, characterized in that The composition of metastable austenitic stainless steel is determined according to the following formula: M d30 (℃)=551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo -68Nb-1.42(Gs-8) Among them, the content of each element is mass percentage, and Gs is the grain size.

3. The method according to claim 1, characterized in that The homogenization process in step (1) is specifically as follows: A metastable austenitic stainless steel hot-rolled slab with a thickness of 3-10 mm is solution treated at 1050-1100° C., kept at this temperature for 0.5-2 hours, and then water-cooled to room temperature.

4. The method according to claim 1, wherein Step (2) cold rolling deformation is specifically as follows: pickling the steel plate after homogenization treatment in step (1), and then performing multiple cold rolling deformation, with a total reduction rate of 60-90%, and the final cold rolled plate thickness is 0.6-2 mm; M d30 The cold rolling temperature of metastable austenitic stainless steel above room temperature is room temperature or liquid nitrogen low temperature rolling, M d30 The cold rolling temperature of metastable austenitic stainless steel below room temperature is liquid nitrogen low-temperature rolling.

5. The method according to claim 1, wherein Step (3): isomerization annealing is specifically as follows: subjecting the cold-rolled sheet in step (2) to short-term isothermal annealing at an annealing temperature of 700-800° C. for a holding time of 100-300 seconds, followed by water cooling after annealing.

6. The method according to claim 5, characterized in that The annealing in step (3) is performed in a tubular annealing furnace, and the holding time includes a heating process and an isothermal process.

7. A high-strength and tough ultrafine-grained austenitic stainless steel plate, characterized in that: The composite material is prepared by the method according to any one of claims 1 to 6, has a yield strength greater than 800 MPa, a tensile strength greater than 1000 MPa, a total elongation greater than 40%, and has no Lüders strain.

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