Preparation method of large-span lamellar equiaxed austenite grain size heterogeneous stainless steel

Abstract

The present invention relates to an austenitic stainless steel with large-span lamellar equiaxed austenite grain size heterogeneity and a preparation method thereof. The present invention first performs isothermal treatment on the stainless steel after smelting, forging and hot rolling to obtain stainless steel with large grain size; then performs rolling deformation treatment to introduce a linear temperature-induced martensite nucleation particle shear band; then performs low-temperature deep-cold treatment on the pre-deformed stainless steel to generate a banded martensite lath group in the large-grained austenite; then performs heating treatment on the stainless steel to reversely transform the banded martensite lath group into austenite grains, and finally obtains austenitic stainless steel with large-span lamellar grain size heterogeneity in which large-sized austenite grains are distributed with banded small-grained equiaxed austenite, which is a single-phase equiaxed austenite with heterogeneous grain size, and the grain size is distributed in a large span, and has the characteristics of large grains enveloping small grains, and at the same time, the small-grained austenite is distributed in the large-grained austenite in a regular linear shape.

Description

Technical Field

[0001] The invention belongs to the technical field of stainless steel heterogeneous material production, and particularly relates to a large-span lamellar equiaxed austenite grain size heterogeneous austenitic stainless steel and a preparation method thereof. Background Art

[0002] Austenitic stainless steel is widely used in various industrial fields due to its high plasticity, easy forming, corrosion resistance and other performance characteristics. However, when austenitic stainless steel is supplied with coarse grain structure during production, due to the coarse grain size (50-150μm), it has a low yield strength of 130-200MPa, which seriously limits its application in high-strength components. Therefore, improving the yield strength of austenitic stainless steel has become a problem that needs to be solved in its development.

[0003] Grain refinement can effectively improve the yield strength of austenitic stainless steel. The method of grain refinement of austenitic stainless steel is mainly to perform large plastic deformation on the stainless steel plate to produce deformation-induced martensite, and then perform annealing to obtain small-grain austenite to achieve grain refinement. However, this method has high requirements on equipment and high energy consumption, and due to large plastic deformation, it has great limitations on the production of thick steel plates. Studies have shown that temperature-induced martensite can be introduced into the austenitic structure to improve the yield strength of stainless steel. The disadvantage of this method is that it introduces the magnetic phase martensite, which makes it impossible to apply to service environments with magnetic requirements. With the rapid development of the economy, the demand for high-strength and corrosion-resistant components in various industrial fields has continued to increase. In this regard, the development of high-yield strength, non-magnetic austenitic stainless steel is an important research direction. Summary of the invention

[0004] In view of the problems of high equipment requirements and large losses when improving the yield strength of austenitic stainless steel by existing grain size refinement means, and the problem that the introduction of temperature-induced martensite strengthening phase will increase the magnetism of the steel itself, the present invention innovatively proposes austenitic stainless steel with large-span grain size heterogeneity, whose organizational characteristics are that large-sized equiaxed austenite grains wrap small-sized equiaxed austenite grains, and the small grains are distributed in the large grains in strips. The small grains distributed in strips inside the large grains are used to improve the yield strength of the stainless steel, and the back stress strengthening effect is introduced by utilizing the heterogeneous characteristics between large and small grains, thereby improving the mechanical properties of the steel and realizing high-strength plastic design. In addition, the magnetic phase martensite is not introduced into the austenitic structure, which ensures that the austenitic stainless steel is non-magnetic. At the same time, the production process of the designed organization has low equipment requirements, low losses, and is easy to implement. The present invention firstly performs high-temperature isothermal treatment on hot-rolled stainless steel to obtain an equiaxed austenite structure with uniform composition; then uses a cold rolling mill to perform rolling pre-deformation treatment on the stainless steel to introduce a temperature-induced martensite nucleation particle shear band; then uses a low-temperature environment to perform deep-cryogenic treatment on the deformed stainless steel to introduce temperature-induced martensite; finally, uses an annealing process to reverse the temperature-induced martensite in the austenite structure into an equiaxed austenite with a fine grain size, thereby obtaining austenitic stainless steel with a large-span lamellar grain size heterogeneity.

[0005] In order to achieve the above object, the present invention is achieved by the following means:

[0006] The first aspect of the present invention provides a method for preparing stainless steel with large-span lamellar equiaxed austenite grain size heterogeneity, comprising the following steps:

[0007] (1) subjecting stainless steel of size d to isothermal treatment at a temperature of 950 to 1140° C. for a time of 20 to 40*d minutes; the unit of d is mm;

[0008] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the temperature of the rolling pre-deformation treatment is -50 to 100° C., and the deformation amount of the rolling pre-deformation treatment is 3 to 8%;

[0009] (3) After the rolling pre-deformation treatment is completed, a cryogenic treatment is performed at -80 to -196°C for 2 to 30 minutes, and then the temperature is raised to room temperature;

[0010] (4) using a heat treatment furnace to perform annealing treatment on the stainless steel after deep cryogenic treatment, wherein the annealing temperature is 760 to 860° C. and the annealing time is 5 to 20 minutes.

[0011] Preferably, the stainless steel in step (1) is selected from stainless steel plate and / or stainless steel rebar; most preferably, the stainless steel is selected from 321 stainless steel plate and / or 321 stainless steel rebar; when the stainless steel is selected from stainless steel plate, a person skilled in the art will understand that the dimension d in step (1) is the thickness of the stainless steel plate; when the stainless steel is selected from stainless steel rebar, a person skilled in the art will understand that the dimension d in step (1) is the diameter of the stainless steel rebar.

[0012] Preferably, the temperature of the isothermal treatment in step (1) is 1050° C., and the time of the isothermal treatment is 30*d minutes.

[0013] Preferably, the temperature of the rolling pre-deformation treatment in step (2) is 0-50° C., and the deformation amount of the rolling pre-deformation treatment is 5-6%.

[0014] Preferably, the temperature of the cryogenic treatment in step (3) is -115 to -196°C, and the time of the cryogenic treatment is 5 to 10 minutes.

[0015] Preferably, the temperature of the annealing treatment in step (4) is 780-810° C., and the annealing treatment time is 8*d minutes.

[0016] The second aspect of the present invention provides a stainless steel with large-span lamellar equiaxed austenite grain size heterogeneity prepared according to the above preparation method.

[0017] Grain size refinement can effectively improve the yield strength of austenitic stainless steel, but this method has high equipment requirements and large losses, and has not been widely used. In this regard, how to improve the yield strength of austenitic stainless steel based on grain refinement theory is an important scientific research direction. Based on the theory of fine grain strengthening, the present invention proposes to introduce fine equiaxed austenite grains into coarse-grained austenite to improve the yield strength of austenitic stainless steel. At the same time, lamellar heterogeneous structures have excellent mechanical properties. In this regard, the present invention proposes to introduce lamellar fine equiaxed austenite grains to divide the parent austenite phase, obtain a heterogeneous structure of lamellar parent austenite-lamellar small-grain austenite, introduce the back stress strengthening effect of the heterogeneous structure, and improve the mechanical properties of the material. The invention is based on the following theories: (1) the introduction of small-sized equiaxed austenite grains has higher strength; (2) the grain boundaries of small-grained austenite are effective barriers to dislocation movement, so the occupation of small grains within large austenite grains effectively blocks the movement of dislocations, further improving the yield strength of austenitic stainless steel; (3) the heterogeneous structure of lamellar parent phase austenite-lamellar small-grained austenite will introduce a back stress strengthening effect, further improving the strength and plasticity of austenitic stainless steel.

[0018] The above scheme requires the completion of two phases of tasks: (1) introducing lamellar martensite lath groups into austenitic stainless steel; (2) causing the lamellar martensite lath groups to reversely transform into fine equiaxed austenite. The present invention has well completed the above two phases of tasks and prepared an austenitic stainless steel with large-span lamellar equiaxed austenite grain size heterogeneity.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) The present invention proposes a heterogeneous structure design in which large austenite grains enclose lamellar equiaxed annealed small austenite grains, and large and small grains are alternately distributed. This solves the problem that the high solution treatment temperature of 321 austenitic stainless steel leads to coarse grains and low yield strength, and provides a new idea for the high-performance design of metastable austenitic stainless steel.

[0021] (2) The present invention innovatively proposes a coupling process of rolling pre-deformation-low-temperature deep cooling-critical annealing, through which the equiaxed grains of austenitic stainless steel can be regulated and controlled, and a heterogeneous structure with alternating distribution of large and small grain lamellae can be constructed, which provides a new direction for material organization structure design and a new means for the design of high-performance austenitic stainless steel.

[0022] (3) The yield strength of austenitic stainless steel is effectively improved without changing the magnetic properties of austenitic stainless steel or reducing its plasticity; at the same time, each process in the process designed by the present invention is a common process in the field, and has the advantages of simple operation, low cost, and continuous production. DETAILED DESCRIPTION

[0023] 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.

[0024] Example 1

[0025] A large-span lamellar equiaxed austenite grain size heterogeneous austenitic stainless steel, the preparation method of which comprises the following steps:

[0026] (1) subjecting a stainless steel hot-rolled plate having a thickness of 3 mm to isothermal treatment at a temperature of 1050° C. for 90 minutes;

[0027] (2) performing rolling pre-deformation treatment on the stainless steel plate after isothermal treatment, wherein the rolling pre-deformation temperature is 50° C. and the rolling pre-deformation amount is 6%;

[0028] (3) After the rolling pre-deformation is completed, the experimental steel is subjected to a cryogenic treatment at -196°C for 15 minutes, and then the temperature is raised to room temperature;

[0029] (4) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace. The annealing temperature is 790° C. and the annealing time is 24 minutes.

[0030] Example 2

[0031] A large-span lamellar equiaxed austenite grain size heterogeneous austenitic stainless steel, the preparation method of which comprises the following steps:

[0032] (1) subjecting a stainless steel hot-rolled plate having a thickness of 3 mm to isothermal treatment at a temperature of 1050° C. for 90 minutes;

[0033] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the rolling pre-deformation temperature is 0° C. and the rolling pre-deformation amount is 4%;

[0034] (3) After the rolling pre-deformation is completed, the experimental steel is subjected to a cryogenic treatment at -115°C for 30 minutes, and then the temperature is raised to room temperature;

[0035] (4) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace. The annealing temperature is 790° C. and the annealing time is 24 minutes.

[0036] Example 3

[0037] A large-span lamellar equiaxed austenite grain size heterogeneous austenitic stainless steel, the preparation method of which comprises the following steps:

[0038] (1) subjecting a stainless steel hot-rolled steel bar having a diameter of 3 mm to isothermal treatment at a temperature of 950° C. for 120 minutes;

[0039] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the rolling pre-deformation temperature is -50°C and the rolling pre-deformation amount is 3%;

[0040] (3) After the rolling pre-deformation is completed, the experimental steel is subjected to a cryogenic treatment at -150°C for 6 minutes, and then the temperature is raised to room temperature;

[0041] (4) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace. The annealing temperature is 760° C. and the annealing time is 60 minutes.

[0042] Example 4

[0043] A large-span lamellar equiaxed austenite grain size heterogeneous austenitic stainless steel, the preparation method of which comprises the following steps:

[0044] (1) subjecting a stainless steel hot-rolled steel bar having a diameter of 3 mm to isothermal treatment at a temperature of 1140° C. for 60 minutes;

[0045] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the rolling pre-deformation temperature is 100° C. and the deformation amount of the rolling pre-deformation is 8%;

[0046] (3) After the rolling pre-deformation is completed, the experimental steel is subjected to a cryogenic treatment at -80°C for 90 minutes, and then the temperature is raised to room temperature;

[0047] (4) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace. The annealing temperature is 860° C. and the annealing time is 15 minutes.

[0048] Comparative Example 1

[0049] A 321 stainless steel, the preparation method of which comprises the following steps:

[0050] (1) subjecting a stainless steel hot-rolled plate having a thickness of 3 mm to isothermal treatment at a temperature of 1050° C. for 90 minutes;

[0051] (2) After the isothermal treatment, the experimental steel was cryogenically treated at -196°C for 15 minutes, and then heated to room temperature;

[0052] (3) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace. The annealing temperature is 790° C. and the annealing time is 24 minutes.

[0053] Comparative Example 2

[0054] A 321 stainless steel, the preparation method of which comprises the following steps:

[0055] (1) subjecting a stainless steel hot-rolled plate having a thickness of 3 mm to isothermal treatment at a temperature of 1050° C. for 90 minutes;

[0056] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the rolling pre-deformation temperature is 50° C. and the rolling pre-deformation amount is 6%;

[0057] (3) The stainless steel after the rolling pre-deformation treatment is annealed in a heat treatment furnace. The annealing temperature is 790° C. and the annealing time is 24 minutes.

[0058] Comparative Example 3

[0059] A 321 stainless steel, the preparation method of which comprises the following steps:

[0060] (1) subjecting a stainless steel hot-rolled plate having a thickness of 3 mm to isothermal treatment at a temperature of 1050° C. for 90 minutes;

[0061] (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the rolling pre-deformation temperature is 50° C. and the rolling pre-deformation amount is 6%;

[0062] (3) After the rolling pre-deformation is completed, the experimental steel is subjected to a cryogenic treatment at -196°C for 15 minutes, and then the temperature is raised to room temperature;

[0063] Verification Example 1

[0064] Commercial 321 stainless steel, stainless steel prepared in Examples 1-4 and Comparative Examples 1-3 were respectively taken, and their organizational structure (whether a large-span lamellar equiaxed austenite grain size heterogeneous structure was obtained), yield strength and elongation were tested using conventional technical methods in the field. The specific test results are shown in Table 1 below.

[0065] Table 1 Test results of commercial 321 stainless steel, examples 1-4 and comparative examples 1-3 stainless steel

[0066]

[0067]

[0068] By comparing the performance of the stainless steels of Examples 1-4 with commercial stainless steels, it was found that the large-span lamellar equiaxed austenite grain size heterogeneous structure can significantly improve the yield strength of the stainless steel without seriously damaging the plasticity (≥47%).

[0069] By comparing and analyzing the results of Examples 1-4 and Comparative Examples 1-3, the following conclusions can be drawn:

[0070] (1) It can be seen from the comparative examples that the construction of lamellar equiaxed austenite grain size heterogeneous structure can effectively improve the yield strength of commercial 321 stainless steel.

[0071] (2) No pre-deformation process is set in Comparative Example 1; no cryogenic treatment process is set in Comparative Example 2; and no isothermal annealing heat treatment process is set in Comparative Example 3. Comparative Examples 1-2 have little effect on the existing structure of commercial stainless steel and do not achieve the construction of heterogeneous structure; although Comparative Example 3 introduces the strengthening phase martensite to improve the yield strength, it seriously reduces the plasticity of the experimental steel and introduces a magnetic phase into the material, destroying the non-magnetic characteristics. The above analysis reveals that pre-deformation, cryogenic treatment and isothermal annealing heat treatment are all essential conditions for this design. Without any of the processes, 321 stainless steel cannot achieve the best performance.

[0072] 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 with large-span lamellar equiaxed austenite grain size heterogeneity, characterized in that: The steps include: (1) subjecting stainless steel of size d to isothermal treatment at a temperature of 950-1140° C. and a time of (20-40)*d minutes; the unit of d is mm; the stainless steel is selected from 321 stainless steel plate and / or 321 stainless steel rebar; when the stainless steel is selected from stainless steel plate, d is the thickness of the stainless steel plate; when the stainless steel is selected from stainless steel rebar, d is the diameter of the stainless steel rebar; (2) performing rolling pre-deformation treatment on the stainless steel after isothermal treatment, wherein the temperature of the rolling pre-deformation treatment is 0-50° C., and the deformation amount of the rolling pre-deformation treatment is 3-8%; (3) After the rolling pre-deformation treatment is completed, a cryogenic treatment is performed in an environment of -80~-196°C, the cryogenic treatment time is (2~30)*d minutes, and then the temperature is raised to room temperature; (4) The stainless steel after the cryogenic treatment is annealed in a heat treatment furnace, wherein the annealing temperature is 760-860° C. and the annealing time is (5-20)*d minutes.

2. The preparation method according to claim 1, characterized in that: The temperature of the isothermal treatment in step (1) is 1050° C., and the time of the isothermal treatment is 30*d minutes.

3. The preparation method according to claim 1, characterized in that: The deformation amount of the rolling pre-deformation treatment in step (2) is 5-6%.

4. The preparation method according to claim 1, characterized in that: The temperature of the cryogenic treatment in step (3) is -115~-196°C, and the time of the cryogenic treatment is (5~10)*d minutes.

5. The preparation method according to claim 1, characterized in that: The temperature of the annealing treatment in step (4) is 780-810° C., and the annealing treatment time is 8*d minutes.

6. A stainless steel with large-span lamellar equiaxed austenite grain size heterogeneity is prepared according to the preparation method according to any one of claims 1 to 5.

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