A heat treatment method for preparing a medium-manganese steel with different morphology residual austenite

Abstract

The application discloses a heat treatment method for preparing medium-manganese steel with residual austenite of different morphologies and belongs to the technical field of heat treatment of metal materials. First, a hot-rolled medium-manganese steel plate is subjected to cold rolling deformation, high-density dislocations and deformation bands are introduced through large reduction, a strong hardening structure mainly composed of deformed martensite is obtained, and sufficient deformation energy storage is provided for subsequent austenite reverse phase transformation. Then, the two-phase region is kept warm and synchronous warm rolling is performed to form precursor structure with obvious layered orientation characteristics. After that, short-time salt bath critical annealing treatment is performed, and the complex phase structure composed of residual austenite of different morphologies is obtained through air cooling. Through the regulation of the critical annealing temperature, the volume fraction, grain size, morphological characteristics and thermodynamic stability distribution of the residual austenite can be effectively changed, the flexible design of the proportion of austenite of different stabilities is realized, and a feasible heat treatment path is provided for the synergistic optimization of the strength, plasticity and work hardening behavior of the medium-manganese steel.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and in particular to a heat treatment method for preparing medium manganese steel with different morphologies of retained austenite based on a combination of warm rolling precursor and critical annealing process. Background Technology

[0002] Advanced high-strength steels for automobiles can be effectively applied to the rapid development of lightweighting in the automotive industry, addressing issues such as energy conservation, environmental protection, and safety. Medium-manganese steel, as a typical representative of third-generation advanced high-strength steels, combines high strength with high plasticity, meeting the requirements of lightweighting and safety in automobiles. Traditionally, medium-manganese steel is prepared using hot rolling + critical annealing or cold rolling + critical annealing processes. In cold rolling, deformed martensite forms the initial microstructure, and during critical annealing, austenite reverse transformation occurs, forming a submicron-sized austenite-ferrite dual-phase microstructure. The transformation-induced plasticity (TRIP) effect of metastable retained austenite is utilized to enhance the steel's strength and plasticity. However, the retained austenite obtained by traditional processes exhibits relatively small differences in morphology, size, and stability, and the TRIP effect often concentrates within a small strain range, limiting its sustained improvement in plasticity. To fully leverage the strengthening and toughening potential of the TRIP effect, it is urgent to develop a heat treatment method that can regulate the differentiated stability of retained austenite, allowing it to undergo martensitic phase transformation in stages over a wider strain range. This invention addresses the aforementioned technical problems by proposing a method for controlling the residual austenite in manganese steel based on warm rolling precursors combined with different critical annealing temperatures. Summary of the Invention

[0003] The purpose of this invention is to provide a heat treatment method for preparing medium-manganese steel with different morphologies of retained austenite based on a warm rolling-annealing process. Through a three-step process of cold rolling, two-phase region holding + two-phase region warm rolling, and critical annealing at different temperatures, a residual austenite microstructure with fine grains, adjustable content, and gradient stability distribution is obtained, thereby improving the persistence of the TRIP effect and the overall mechanical properties of the material.

[0004] This invention provides a method for preparing manganese steel with retained austenite of different morphologies based on a warm rolling-annealing process, comprising the following steps:

[0005] Step 1: Cold rolling deformation process

[0006] Hot-rolled medium-manganese steel sheets are cold-rolled at room temperature with a reduction of 40% to 60%, and then rolled in multiple passes to obtain cold-rolled steel sheets.

[0007] Step 2: Two-phase region heat preservation + two-phase region warm rolling process

[0008] The cold-rolled steel sheet obtained in step 1 is heated to the two-phase region temperature between Accm and Ac3, i.e., 650~670℃, held for 0.5h~1.5h and then warm-rolled with a reduction of 50%~70%. It is then air-cooled to room temperature and rolled in multiple passes to obtain a warm-rolled steel sheet containing a certain amount of residual austenite.

[0009] Step 3: Critical Zone Annealing Process

[0010] The warm-rolled steel sheet obtained in step 2 was subjected to critical annealing at different temperatures in a salt bath furnace, and then air-cooled to room temperature to obtain annealed test steel.

[0011] Preferably, the chemical composition of the hot-rolled manganese steel in step 1 is as follows (mass percentage): C 0.05~0.15%, Mn 7~11%, Al ≤ 0.1%, with the balance being Fe.

[0012] Preferably, the thickness of the hot-rolled steel plate in step 1 is 5mm, and the thickness of the cold-rolled steel plate in step 1 is 2.5mm.

[0013] Preferably, the cold rolling reduction in step 1 is 50%.

[0014] Preferably, the furnace heating rate in step 2 is 5~10 ℃ / min;

[0015] Preferably, the temperature of the two-phase region in step 2 is 660 ℃, and the heat preservation time is 1 h.

[0016] Preferably, the warm rolling reduction in step 2 is 60%, and the thickness of the warm rolled steel plate is 1 mm.

[0017] Preferably, the critical zone annealing temperature in step 3 is 630℃, 660℃, 690℃, or 730℃, and the critical zone annealing time is 3 min.

[0018] Preferably, the air cooling rate in step 3 is 5~10 ℃ / s.

[0019] A method for preparing medium-manganese steel with different morphologies of retained austenite based on a warm rolling-annealing process. The microstructure of the medium-manganese steel contains two morphologies of retained austenite: lamellar and equiaxed. The volume fraction of retained austenite is 6.59% to 28.60%. With the increase of critical annealing temperature, the proportion of equiaxed austenite increases and the proportion of lamellar austenite decreases.

[0020] The annealed test steel described in step 3 was prepared into standard tensile specimens with a gauge length of 25 mm in accordance with GB / T 228-2010.

[0021] The beneficial effects of this invention:

[0022] Compared with existing technologies, this invention achieves heterogeneous retained austenite structures in medium-manganese steel by combining warm-rolled precursors with critical annealing at different temperatures. The two morphologies of retained austenite differ in grain size, interface characteristics, and elemental distribution, resulting in a gradient distribution of mechanical stability. At a critical annealing temperature of 660℃, the retained austenite volume fraction reaches a peak of 28.60%, with equiaxed and lamellar retained austenite coexisting. The difference in stability between these two types allows the TRIP effect to be applied segmentally over a wider strain range, which helps delay necking and enhances work hardening capacity, thereby significantly improving the strength-ductility balance of medium-manganese steel. This method provides a new approach to designing for gradient stability of retained austenite through morphology control.

[0023] Furthermore, this invention clarifies the regulation law of annealing temperature on the stability and mechanical properties of retained austenite: at low-temperature annealing (630 ℃), the stability of retained austenite is too strong, and the TRIP effect is difficult to be fully activated; at high-temperature annealing (≥690 ℃), the stability is insufficient, and the retained austenite undergoes a large transformation in the early stage of deformation; only when the annealing temperature is 660 ℃ is the optimal match between content and stability achieved. The medium-manganese steel prepared by the method of this invention has a tensile strength of 1279~1802 MPa, an elongation after fracture of 8.5%~29.2%, and a strength-ductility product of up to 37.04 GPa·%, with comprehensive mechanical properties superior to those of the traditional cold rolling + critical annealing process. Attached Figure Description

[0024] Figure 1 : Process flow diagram of this invention;

[0025] Figure 2 SEM microstructure of medium manganese steel after cold rolling pretreatment in Examples 1-4;

[0026] Figure 3 Examples 1-4 share the same XRD patterns of medium manganese steel after cold rolling pretreatment;

[0027] Figure 4 SEM microstructure of medium manganese steel after heat preservation and warm rolling in Examples 1-4;

[0028] Figure 5 XRD patterns of medium manganese steel after heat preservation and warm rolling in Examples 1-4;

[0029] Figure 6 SEM images of microstructures at different critical annealing temperatures (a: 630℃, b: 660℃, c: 690℃, d: 720℃).

[0030] Figure 7 XRD patterns at different critical annealing temperatures;

[0031] Figure 8 Stress-strain curves of engineering materials at different critical annealing temperatures. Detailed Implementation

[0032] Example 1

[0033] A method for preparing manganese steel with retained austenite of different morphologies based on a warm rolling-annealing process includes the following steps:

[0034] Step 1: Cold rolling deformation process

[0035] Using hot-rolled medium-manganese steel sheet with a mass percentage of C: 0.099%, Mn: 8.93%, Al ≤ 0.1%, and the balance Fe as the initial material, multiple cold rolling passes were performed at room temperature with a cold rolling reduction of 50% to produce cold-rolled steel sheet. The resulting cold-rolled microstructure and XRD pattern are shown in [reference needed]. Figure 2 and Figure 3 .

[0036] Step 2: Two-phase region heat preservation + two-phase region warm rolling process

[0037] The cold-rolled steel sheet obtained in step 1 was heated to the two-phase region temperature of 660 ℃ and held for 1 h, followed by simultaneous warm rolling with a reduction of 60%. It was then air-cooled to room temperature to obtain a warm-rolled steel sheet containing a certain amount of retained austenite. The microstructure and XRD pattern of the obtained warm-rolled sheet are shown in [reference needed]. Figure 4 and Figure 5 .

[0038] Step 3: Critical Zone Annealing Process

[0039] The warm-rolled steel sheet obtained in step 2 was subjected to critical zone annealing in a salt bath furnace at a temperature of 630 °C for 3 min. After annealing, the sheet was removed and air-cooled to room temperature to obtain the annealed test steel.

[0040] The volume fraction of retained austenite in the microstructure obtained in this embodiment is 25.03%, and the microstructure is mainly composed of fine lamellar structures (see...). Figure 6 a and Figure 7 Tensile testing revealed a tensile strength of 1279.29 MPa, an elongation after fracture of 24.3%, and a strength-ductility product of 31.11 GPa·%. Under this process, the residual austenite exhibited excessive stability, and the TRIP effect was not fully activated (see...). Figure 8 ).

[0041] Example 2

[0042] A method for preparing manganese steel with retained austenite of different morphologies based on a warm rolling-annealing process includes the following steps:

[0043] Step 1: Cold rolling deformation process

[0044] Using hot-rolled medium-manganese steel sheet with the same composition as in Example 1, multiple cold rolling passes were performed at room temperature with a cold rolling reduction of 50% to produce cold-rolled steel sheet. The resulting cold-rolled microstructure and XRD pattern are shown in [reference needed]. Figure 2 and Figure 3 .

[0045] Step 2: Two-phase region heat preservation + two-phase region warm rolling process

[0046] The cold-rolled steel sheet obtained in step 1 was heated to a two-phase region temperature of 660 °C and held for 1 h, followed by warm rolling with a reduction of 60%. It was then air-cooled to room temperature to obtain a warm-rolled steel sheet containing a certain amount of retained austenite. The microstructure and XRD pattern of the obtained warm-rolled sheet are shown in [reference needed]. Figure 4 and Figure 5 .

[0047] Step 3: Critical Zone Annealing Process

[0048] The warm-rolled steel sheet obtained in step 2 was subjected to critical zone annealing in a salt bath furnace. The annealing temperature was changed to 660 ℃ and the annealing time was 3 min. The sheet was then removed and air-cooled to room temperature to obtain the annealed test steel.

[0049] In this embodiment, the residual austenite volume fraction reached a peak of 28.60%, and the microstructure was still mainly lamellar, accompanied by some small blocky austenite (see...). Figure 6 b and Figure 7 Tensile testing revealed a tensile strength of 1267.23 MPa, an elongation after fracture of 29.2%, and a strength-ductility product of 37.04 GPa·%. Its engineering stress-strain curve exhibited a long, uniform elongation phase and continuously increasing rheological stress, representing the optimal process for this invention (see [link to invention]). Figure 8 ).

[0050] Example 3

[0051] A method for preparing manganese steel with retained austenite of different morphologies based on a warm rolling-annealing process includes the following steps:

[0052] Step 1: Cold rolling deformation process

[0053] Using hot-rolled medium-manganese steel sheet with the same composition as in Example 1, multiple cold rolling passes were performed at room temperature with a cold rolling reduction of 50% to produce cold-rolled steel sheet. The resulting cold-rolled microstructure and XRD pattern are shown in [reference needed]. Figure 2 and Figure 3 .

[0054] Step 2: Two-phase region heat preservation + two-phase region warm rolling process

[0055] The cold-rolled steel sheet obtained in step 1 was heated to a two-phase region temperature of 660 °C and held for 1 h, followed by warm rolling with a reduction of 60%. It was then air-cooled to room temperature to obtain a warm-rolled steel sheet containing a certain amount of retained austenite. The microstructure and XRD pattern of the obtained warm-rolled sheet are shown in [reference needed]. Figure 4 and Figure 5 .

[0056] Step 3: Critical Zone Annealing Process

[0057] The warm-rolled steel sheet obtained in step 2 was subjected to critical zone annealing in a salt bath furnace. The annealing temperature was changed to 690 ℃ and the annealing time was 3 min. The sheet was then removed and air-cooled to room temperature to obtain the annealed test steel.

[0058] In this embodiment, the volume fraction of retained austenite in the microstructure drops sharply to 14.99%, the austenite grains coarsen, and some transform into blocky structures (see...). Figure 6 c and Figure 7 The tensile strength increased significantly to 1508.44 MPa, while the elongation after fracture decreased to approximately 19.1%, and the strength-ductility product decreased significantly. The retained austenite exhibited insufficient stability, undergoing significant transformation in the early stages of deformation (see...). Figure 8 ).

[0059] Example 4

[0060] A method for preparing manganese steel with retained austenite of different morphologies based on a warm rolling-annealing process includes the following steps:

[0061] Step 1: Cold rolling deformation process

[0062] Using hot-rolled medium-manganese steel sheet with the same composition as in Example 1, multiple cold rolling passes were performed at room temperature with a cold rolling reduction of 50% to produce cold-rolled steel sheet. The resulting cold-rolled microstructure and XRD pattern are shown in [reference needed]. Figure 2 and Figure 3 .

[0063] Step 2: Two-phase region heat preservation + two-phase region warm rolling process

[0064] The cold-rolled steel sheet obtained in step 1 was heated to a two-phase region temperature of 660 °C and held for 1 h, followed by warm rolling with a reduction of 60%. It was then air-cooled to room temperature to obtain a warm-rolled steel sheet containing a certain amount of retained austenite. The microstructure and XRD pattern of the obtained warm-rolled sheet are shown in [reference needed]. Figure 4 and Figure 5 .

[0065] Step 3: Critical Zone Annealing Process

[0066] The warm-rolled steel sheet obtained in step 2 was subjected to critical zone annealing in a salt bath furnace. The annealing temperature was changed to 720 ℃ and the annealing time was 3 min. The sheet was then removed and air-cooled to room temperature to obtain the annealed test steel.

[0067] In this embodiment, the volume fraction of retained austenite in the microstructure was further reduced to 6.59%, and the microstructure was mainly composed of martensite and ferrite (see...). Figure 6 d and Figure 7 Tensile testing revealed a tensile strength as high as 1802.21 MPa, but the elongation after fracture was only 8.5%, the strength-ductility product was approximately 15.38 GPa·%, the TRIP effect was almost nonexistent, and the material exhibited hard and brittle characteristics (see...). Figure 8 ).

[0068] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the scope of protection defined by the claims submitted herein.

Claims

1. A method for preparing manganese steel with retained austenite of different morphologies, characterized in that, Includes the following steps: Step 1: Cold rolling deformation process Hot-rolled medium-manganese steel sheets are cold-rolled at room temperature with a reduction of 40% to 60%, and then rolled in multiple passes to obtain cold-rolled steel sheets. Step 2: Two-phase region insulation + two-phase region warm rolling process The cold-rolled steel sheet obtained in step 1 is heated to the two-phase region temperature between Accm and Ac3, i.e., 650~670℃, held for 0.5h~1.5h and then warm-rolled with a reduction of 50%~70%. It is then air-cooled to room temperature and rolled in multiple passes to obtain a warm-rolled steel sheet containing retained austenite. Step 3: Critical Zone Annealing Process The warm-rolled steel sheet obtained in step 2 was subjected to critical annealing at different temperatures in a salt bath furnace, and then air-cooled to room temperature to obtain annealed test steel.

2. The method for preparing manganese steel with retained austenite of different morphologies according to claim 1, characterized in that: The chemical composition of the hot-rolled manganese steel in step 1 is as follows (mass percentage): C 0.05~0.15%, Mn 7~11%, Al ≤ 0.1%, with the balance being Fe.

3. The method for preparing manganese steel with retained austenite of different morphologies according to claim 1, characterized in that: The thickness of the hot-rolled steel plate mentioned in step 1 is 5mm, and the thickness of the cold-rolled steel plate mentioned in step 1 is 2.5mm.

4. The method for preparing manganese steel with different morphologies of retained austenite based on warm rolling-annealing process according to claim 1, characterized in that: The cold rolling reduction in step 1 is 50%.

5. The method for preparing manganese steel with retained austenite of different morphologies according to claim 1, characterized in that: The furnace heating rate mentioned in step 2 is 5~10 ℃ / min.

6. A method for preparing manganese steel with retained austenite of different morphologies according to claim 1, characterized in that: The temperature of the two-phase region in step 2 is 660 ℃, and the heat preservation time is 1 h.

7. The method for preparing manganese steel with retained austenite of different morphologies according to claim 1, characterized in that: In step 2, the warm rolling reduction is 60%, and the thickness of the warm rolled steel plate is 1 mm.

8. The method for preparing manganese steel with different morphologies of retained austenite according to claim 1, characterized in that: The critical zone annealing temperatures in step 3 are 630℃, 660℃, 690℃, and 730℃, and the critical zone annealing time is 3 min.

9. The method for preparing manganese steel with different morphologies of retained austenite according to claim 1, characterized in that: The air cooling rate mentioned in step 3 is 5~10 ℃ / s.

10. The medium-manganese steel prepared by the method according to any one of claims 1-9, characterized in that: The microstructure of the medium manganese steel contains two types of retained austenite: lamellar and equiaxed. The volume fraction of retained austenite is 6.59% to 28.60%, and the proportion of equiaxed austenite increases while the proportion of lamellar austenite decreases with increasing critical annealing temperature.

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