Stainless steel with hardened and annealed austenite lamellae alternately distributed and preparation method thereof

Abstract

The present invention relates to a 321 austenitic stainless steel having an interlaced organizational structure of lamellar hardened austenite and annealed austenite lamellae and a preparation method thereof. The present invention sequentially performs solid solution treatment, rolling deformation treatment, deep cryogenic treatment, and rapid heating treatment on a hot-rolled deformed steel plate, and finally obtains a 321 austenitic stainless steel having an interlaced organizational structure of lamellar hardened austenite and annealed austenite lamellae. The organizational characteristics thereof are that lamellar hardened austenite and annealed austenite lamellae are interlaced, and small hardened austenite grains are distributed in a strip-like manner in the parent phase equiaxed austenite grains, and the parent phase austenite grains are divided. In this structure, the lamellar hardened austenite as a strengthening phase can effectively improve the yield strength of coarse-grained austenitic stainless steel, and the lamellar annealed austenite can provide good plasticity, and the synergistic effect of the two realizes the high strength and plasticity of austenitic stainless steel.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel heterogeneous material production, and relates to a stainless steel with hardened and annealed austenite lamellae alternately distributed and a preparation method thereof, and in particular to a 321 austenitic stainless steel with a lamellar hardened austenite and annealed austenite lamellae alternately distributed organizational structure and a preparation method thereof. Background Art

[0002] Type 321 austenitic stainless steel is a typical representative of austenitic stainless steel and is widely used in various industrial fields such as rail transportation, petrochemicals, automobiles, and national defense. However, the supply structure of 321 austenitic stainless steel is a coarse-grained annealed structure, which gives it a lower yield strength. With the rapid development of my country's economy, the demand for high-strength components in various industrial fields continues to increase. 321 austenitic stainless steel with low yield strength is difficult to apply to high-strength components, which restricts the development of 321 austenitic stainless steel. Therefore, how to improve the yield strength of 321 austenitic stainless steel has become a problem that needs to be solved in the development of this steel grade.

[0003] At present, the main methods to improve austenitic stainless steel include fine grain strengthening, solid solution strengthening, deformation strengthening and martensitic phase transformation strengthening. Fine grain strengthening has high equipment requirements and large losses; solid solution strengthening has high cost and poor strength improvement effect; deformation strengthening introduces a large number of defects into the material, seriously reducing its plasticity and corrosion resistance; martensitic phase transformation strengthening changes the non-magnetic nature of austenite and seriously reduces the plasticity of austenitic stainless steel. The above methods all have some shortcomings when improving 321 austenitic stainless steel. It can be seen that it is imperative to improve the shortcomings of existing methods or develop new technologies to improve the yield strength of 321 austenitic stainless steel.

[0004] The austenite structure in austenitic stainless steel has high thermal stability, and it is difficult to introduce martensite strengthening phase into austenite grains by traditional process methods. To address this problem, the existing technology has innovatively proposed a pre-deformation-deep freezing coupling process based on the research results of martensite phase transformation, which successfully introduced martensite strengthening phase into austenitic stainless steel. Through this process, the yield strength of austenitic stainless steel is effectively improved, but the plasticity of austenitic stainless steel is seriously reduced.

[0005] Heat treatment is a key process to improve material properties. In recent years, the rapid heating process has become a new member of the heat treatment process and a focus of research by researchers due to its special treatment method and excellent influence on the microstructure and properties. Therefore, how to apply the rapid heating process to improve the yield strength of austenitic stainless steel is a potential important research direction. Summary of the invention

[0006] In view of the problem that the plasticity is seriously reduced and the magnetic properties change after the temperature-induced martensite is introduced into austenitic stainless steel, the present invention designs a 321 austenitic stainless steel with a lamellar hardened austenite and annealed austenite lamellar distribution structure. The present invention introduces lamellar hardened austenite small grains into large grain size austenite grains, and uses the lamellar structure to divide the parent phase austenite to form a heterogeneous structure with soft and hard lamellar alternations. The lamellar annealed austenite is designed to provide good plasticity and work hardening properties, and the lamellar hardened austenite is designed to provide high strength; at the same time, the back stress strengthening effect of the heterogeneous structure is introduced to improve the performance of the steel. The production process is as follows: first, the heat treatment furnace is used to carry out solid solution treatment on the heat-deformed steel plate to obtain coarse-grained austenite with uniform composition and equiaxed shape; then, through two-stage rolling deformation treatment, linear defects such as slip bands are introduced into the austenite grains; then, the steel plate is subjected to deep cryogenic treatment in a low-temperature environment, so that the parallel linear defects trigger the martensitic phase transformation and obtain the martensite-austenite lamellar structure; finally, the steel plate is heat-treated by a rapid heating process, so that the temperature-induced martensite lath group distributed in stripes is reversely transformed into fine austenite in the strip-hardened state by shear mechanism. Finally, 321 austenitic stainless steel with an interlaced organizational structure of lamellar hardened austenite and annealed austenite lamellar is obtained.

[0007] In order to achieve the above object, the present invention is achieved by the following means:

[0008] The first aspect of the present invention provides a method for preparing stainless steel having a lamellar hardened austenite and annealed austenite lamellae distributed alternately, comprising the following steps:

[0009] (1) using a heat treatment furnace to perform a solution treatment on a heat-deformed stainless steel plate (thickness d, unit: mm), wherein the temperature of the solution treatment is 960-1145° C., and the time of the solution treatment is 10-20*d minutes;

[0010] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 25 to 100° C., the deformation amount is 1 to 2%, and the temperature of the second stage variable temperature rolling deformation treatment is -50 to 0° C., and the deformation amount is 2 to 6%;

[0011] (3) After the variable temperature rolling deformation treatment is completed, the deformed steel plate is subjected to a deep cryogenic treatment in a low temperature environment, wherein the temperature of the deep cryogenic treatment is -100 to -196°C, the time of the deep cryogenic treatment is 15 to 30 minutes, and then the temperature is raised to room temperature;

[0012] (4) Then, the stainless steel after the cryogenic treatment is heated to 750-800° C. at a heating rate of 40-100° C. / min for rapid heating treatment, wherein the time of the rapid heating treatment is 2-4 minutes;

[0013] (5) After the rapid heating treatment, the steel plate is quenched to room temperature.

[0014] Preferably, the stainless steel plate in step (1) is a 321 austenitic stainless steel plate.

[0015] Preferably, the solution treatment time in step (1) is 14 to 16*d minutes.

[0016] Preferably, the temperature of the solution treatment in step (1) is 990-1040°C.

[0017] Preferably, in step (2), the temperature of the first stage variable temperature rolling deformation treatment is 50-70°C, and the deformation amount is 2%; the temperature of the second stage variable temperature rolling deformation treatment is -30--20°C, and the deformation amount is 4%.

[0018] Preferably, the temperature of the cryogenic treatment in step (3) is -120 to -150°C, and the time of the cryogenic treatment is 25*d minutes.

[0019] Preferably, the temperature of the rapid heating treatment in step (4) is 785° C., and the time of the rapid heating treatment is 3*d minutes.

[0020] Preferably, the heating rate in step (4) is 100°C / min.

[0021] The second aspect of the present invention provides stainless steel having a microstructure with lamellar hardened austenite and annealed austenite lamellae distributed alternately, which is prepared according to the above preparation method.

[0022] Due to the coarse grains of coarse-grained equiaxed austenite, dislocations in its structure are easy to multiply and start during deformation, which makes austenitic stainless steel have a lower yield strength. The introduction of small lamellar hardened austenite grains into coarse-grained austenite first increases the effective obstacles to the movement of dislocations in the coarse-grained austenite, blocks the dislocation movement, and can effectively improve the yield strength of the coarse-grained austenite. At the same time, after the introduction of lamellar hardened austenite, the parent phase coarse-grained austenite is divided. Under the action of the segmentation effect, the grains of the parent phase coarse-grained austenite are refined, and lamellar annealed austenite is obtained. The above two types of lamellar austenite constitute a new structure of austenitic stainless steel, which has heterogeneous characteristics and is a new type of heterogeneous structure in austenitic stainless steel.

[0023] During the deformation process of the above-mentioned heterogeneous structure, the annealed austenite is refined in size, which improves its own yield strength; at the same time, the lamellar hardened austenite has a higher hardness, which can also improve the yield strength of austenitic stainless steel; finally, since the heterogeneous structure has a back stress strengthening effect, it can also improve the yield strength of austenitic stainless steel.

[0024] The above three effects work together to effectively improve the yield strength of coarse-grained austenitic stainless steel and maintain its good plasticity.

[0025] The above scheme requires the completion of two stages of tasks: (1) introducing lamellar temperature-induced martensite lath groups into austenite grains; (2) using a rapid heating process to reverse the phase transformation of the lamellar martensite lath groups to lamellar hardened austenite, and the reverse phase transformation mechanism is a shear mechanism. The above two steps are coupled to achieve the preparation of 321 austenitic stainless steel with an interlaced microstructure of lamellar hardened austenite and annealed austenite lamellae.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) This invention proposes for the first time a heterogeneous microstructure design of 321 austenitic stainless steel with alternating distribution of lamellar hardened austenite and annealed austenite lamellae from the grain scale. This design achieves the regulation of martensitic phase transformation and reverse phase transformation process, providing a new idea for the microstructure design of metastable austenitic stainless steel.

[0028] (2) Compared with the existing technology for improving the yield strength of austenitic stainless steel, the present invention effectively improves the yield strength of austenitic stainless steel without seriously reducing the plasticity and magnetic properties of austenitic stainless steel, thus providing a new direction for the high-performance design of austenitic stainless steel.

[0029] (3) The present invention proposes a new process for preparing heterogeneous structures. The new process is the coupling effect of traditional processes and current new technologies. It has low equipment requirements, low energy consumption, and uniform tissue properties. At the same time, the production process is simple and can be mass-produced. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0031] Example 1

[0032] A 321 austenitic stainless steel having a lamellar hardened austenite and an annealed austenite lamellar structure alternately distributed, wherein the preparation method comprises the following steps:

[0033] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 1040°C for solution treatment for 70 min.

[0034] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 70°C, the deformation amount is 2%, and the temperature of the second stage variable temperature rolling deformation treatment is -30°C, and the deformation amount is 4%;

[0035] (3) placing the stainless steel plate after the variable temperature rolling deformation treatment in a low temperature environment of -150°C for 125 minutes;

[0036] (4) using rapid heating means to heat the deep-cooled steel plate to 785°C at a heating rate of 100°C / min for a rapid heating treatment time of 15 minutes;

[0037] (5) Quench to room temperature.

[0038] Example 2

[0039] A 321 austenitic stainless steel having a lamellar hardened austenite and an annealed austenite lamellar structure alternately distributed, wherein the preparation method comprises the following steps:

[0040] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 990 °C for solution treatment for 80 min.

[0041] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 50°C, the deformation amount is 2%, and the temperature of the second stage variable temperature rolling deformation treatment is -20°C, and the deformation amount is 4%;

[0042] (3) placing the stainless steel plate after the variable temperature rolling deformation treatment in a low temperature environment of -120°C for 125 minutes;

[0043] (4) using rapid heating means to heat the steel plate after deep cooling to 785°C at a heating rate of 100°C / min for a rapid heating treatment time of 20 minutes;

[0044] (5) Quench to room temperature.

[0045] Example 3

[0046] A 321 austenitic stainless steel having a lamellar hardened austenite and an annealed austenite lamellar structure alternately distributed, wherein the preparation method comprises the following steps:

[0047] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 960 °C for solution treatment for 100 min;

[0048] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 100°C, the deformation amount is 1%, and the temperature of the second stage variable temperature rolling deformation treatment is -50°C, and the deformation amount is 6%;

[0049] (3) placing the stainless steel plate after variable temperature rolling deformation treatment in a low temperature environment of -100°C for 150 minutes;

[0050] (4) using rapid heating means to heat the steel plate after deep cooling to 750°C at a heating rate of 40°C / min for a rapid heating treatment, and the rapid heating treatment time is 20 minutes;

[0051] (5) Quench to room temperature.

[0052] Example 4

[0053] A 321 austenitic stainless steel having a lamellar hardened austenite and an annealed austenite lamellar structure alternately distributed, wherein the preparation method comprises the following steps:

[0054] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 1145 °C for solution treatment for 100 min.

[0055] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, the temperature of the first stage variable temperature rolling deformation treatment is 100°C, the deformation amount is 2%, and the temperature of the second stage variable temperature rolling deformation treatment is 0°C, and the deformation amount is 6%;

[0056] (3) placing the stainless steel plate after variable temperature rolling deformation treatment in a low temperature environment of -196°C for 75 minutes;

[0057] (4) using rapid heating means to heat the steel plate after deep cooling to 800°C at a heating rate of 40°C / min for a rapid heating treatment time of 10 minutes;

[0058] (5) Quench to room temperature.

[0059] Comparative Example 1

[0060] A 321 stainless steel, the preparation method of which comprises the following steps:

[0061] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 1040 °C for solution treatment for 70 min;

[0062] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 70°C, the deformation amount is 2%, and the temperature of the second stage variable temperature rolling deformation treatment is -30°C, and the deformation amount is 4%;

[0063] (3) placing the stainless steel plate after the variable temperature rolling deformation treatment in a low temperature environment of -150°C for 125 minutes;

[0064] (4) Warm up to room temperature.

[0065] Comparative Example 2

[0066] A 321 stainless steel, the preparation method of which comprises the following steps:

[0067] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 1040 °C for solution treatment for 70 min;

[0068] (2) The solution treated stainless steel plate was placed in a low temperature environment of -150°C for 125 min;

[0069] (3) The steel plate after deep cooling was rapidly heated to 785°C at a heating rate of 100°C / min, and the rapid heating time was 15 minutes.

[0070] Comparative Example 3

[0071] A 321 stainless steel, the preparation method of which comprises the following steps:

[0072] (1) A 5 mm thick hot-rolled 321 austenitic stainless steel plate was heated to 1040 °C for solution treatment for 70 min;

[0073] (2) using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment on the stainless steel plate after solid solution treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 70°C, the deformation amount is 2%, and the temperature of the second stage variable temperature rolling deformation treatment is -30°C, and the deformation amount is 4%;

[0074] (3) The steel plate after variable temperature rolling deformation treatment is rapidly heated to 785°C at a heating rate of 100°C / min, and the rapid heating treatment time is 15 minutes.

[0075] Verification Example 1

[0076] Commercial 321 stainless steel plates, stainless steels prepared in Examples 1-4 and Comparative Examples 1-3 were respectively taken, and their organizational structure (whether a lamellar hardened austenite and annealed austenite lamellar structure with alternating distribution) yield strength and elongation were tested using conventional technical methods in the field. The specific test results are shown in Table 1 below.

[0077] Table 1 Test results of commercial 321 stainless steel, examples 1-4 and comparative examples 1-3 stainless steel

[0078]

[0079] By comparing the performance of the stainless steel of Examples 1-4 with commercial stainless steel plates, it is found that a microstructure in which lamellar hardened austenite and annealed austenite lamellae are distributed alternately can significantly improve the yield strength of the stainless steel without seriously damaging the plasticity (≥45%).

[0080] By comparing and analyzing the results of Examples 1-4 and Comparative Examples 1-3, the following conclusions can be drawn:

[0081] (1) By comparing the examples with the comparative examples, it can be seen that the construction of the heterogeneous structure can effectively improve the yield strength of commercial 321 stainless steel.

[0082] (2) No rapid heating process was set in Comparative Example 1; no pre-deformation process was set in Comparative Example 2; and no cryogenic process was set in Comparative Example 3. Comparative Example 1 achieved the construction of a martensite-austenite heterostructure, but failed to achieve the construction of a hardened austenite-annealed austenite heterostructure, while Comparative Examples 2 and 3 also failed to obtain a hardened austenite-annealed austenite heterostructure. The above analysis reveals that the pre-deformation process, cryogenic process, and rapid heating process are all essential conditions of the present invention, and the absence of any process will not allow 321 stainless steel to obtain the best performance.

[0083] The above specific implementation method part specifically introduces the analytical method involved in the present invention. It should be noted that the above introduction is only to help those skilled in the art better understand the method and ideas of the present invention, rather than limiting the relevant content. Without departing from the principle of the present invention, those skilled in the art may also make appropriate adjustments or modifications to the present invention, and the above adjustments and modifications shall also fall within the scope of protection of the present invention.

Claims

1. A method for preparing stainless steel having a lamellar hardened austenite and annealed austenite lamellae distributed alternately, characterized in that: The steps include: (1) performing a solution treatment on the heat-deformed stainless steel plate in a heat treatment furnace, wherein the temperature of the solution treatment is 990-1040° C., and the time of the solution treatment is 14-16*d minutes; wherein d is the thickness of the stainless steel plate, in mm; (2) Using a four-roll cold rolling mill to perform two-stage variable temperature rolling deformation treatment, wherein the temperature of the first stage variable temperature rolling deformation treatment is 25~100℃, the deformation amount is 1~2%, and the temperature of the second stage variable temperature rolling deformation treatment is -50~0℃, and the deformation amount is 2~6%; (3) After the variable temperature rolling deformation treatment is completed, a low temperature deep cryogenic treatment is performed, wherein the temperature of the low temperature deep cryogenic treatment is -100~-196°C, the time of the low temperature deep cryogenic treatment is 15~30*d minutes, and then the temperature is raised to room temperature; (4) then heating the temperature to 750-800°C at a heating rate of 40-100°C / min for rapid heating treatment, wherein the time for the rapid heating treatment is 2-4*d minutes; (5) After the rapid heating treatment, the stainless steel plate is quenched to room temperature.

2. The preparation method according to claim 1, characterized in that: The stainless steel plate in step (1) is a 321 austenitic stainless steel plate.

3. The preparation method according to claim 1, characterized in that: The temperature of the solution treatment in step (1) is 990-1040° C., and the time of the solution treatment is 14-16 minutes.

4. The preparation method according to claim 1, characterized in that: In step (2), the temperature of the first stage variable temperature rolling deformation treatment is 50~70°C, and the deformation amount is 2%; the temperature of the second stage variable temperature rolling deformation treatment is -30~-20°C, and the deformation amount is 4%.

5. The preparation method according to claim 1, characterized in that: The temperature of the low temperature deep cryogenic treatment in step (3) is -120~-150°C, and the time of the low temperature deep cryogenic treatment is 25*d minutes.

6. The preparation method according to claim 1, characterized in that: The temperature of the rapid heating treatment in step (4) is 785° C., and the time of the rapid heating treatment is 3*d minutes.

7. The preparation method according to claim 1, characterized in that: The heating rate in step (5) is 100°C / min.

8. A stainless steel having a structure in which lamellar hardened austenite and annealed austenite lamellae are distributed alternately, prepared by the preparation method according to any one of claims 1 to 7.

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