Method for regulating and controlling structure of cold-rolled medium manganese steel

Abstract

The invention belongs to the technical field of metal material preparation, and relates to a structure regulation and control method for cold-rolled medium manganese steel, which comprises the following steps: a cold-rolled raw material is a medium manganese steel billet of which the microstructure mainly consists of film austenite and ferrite; the microstructure of the cold-rolled medium manganese steel billet is mainly composed of a manganese-rich martensite area and a manganese-poor ferrite area. And carrying out austenite-ferrite double-phase region annealing on the cold-rolled medium-manganese steel billet. The microstructure of the medium manganese steel billet subjected to cold rolling treatment is mainly composed of thin-film austenite and ferrite, a manganese-rich martensite area and a manganese-poor ferrite area can be formed in the cold rolling process, the obtained finished product is high in cold rolling reduction rate, the austenite has a blocky shape and a thin-film shape, and a space is provided for regulating and controlling the mechanical property of the medium manganese steel; the problems that in the prior art, cold rolling machining is difficult, and the austenite morphology of the steel is difficult to regulate and control are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material preparation, and particularly to a method for controlling the microstructure of cold-rolled medium manganese steel. Background Art

[0002] Advanced high-strength steels have developed to the third generation. Medium manganese steel belongs to the third generation of advanced high-strength steels and has high strength and plasticity. According to different processing technologies, medium manganese steel can be divided into cold-rolled medium manganese steel and hot-rolled medium manganese steel. The cold-rolling process can obtain higher metal dimensional accuracy, better surface quality and thickness uniformity, and can meet the needs of high-precision products. The traditional processing technology of cold-rolled medium manganese steel is to obtain a cold-rolled medium manganese steel sheet through stress relief annealing and multi-pass cold rolling, and then through reverse transformation annealing, an equiaxed austenite and ferrite duplex structure is obtained.

[0003] On the one hand, as an important austenite stabilizing element, Mn element partitions from ferrite to austenite during annealing. The increase in its content helps to increase the volume fraction of metastable austenite in medium manganese steel. However, the increase in Mn content can significantly increase the hardness and work hardening ability of medium manganese steel, increasing the processing difficulty and putting forward higher requirements for the cold-rolling process. On the other hand, the morphology of austenite has a significant impact on the stability of austenite and the mechanical behavior of medium manganese steel. However, the existing thermo-mechanical treatment process for cold-rolled medium manganese steel is difficult to control the austenite morphology in steel, limiting the performance range of cold-rolled medium manganese steel. Summary of the Invention

[0004] In view of the above analysis, aiming at the deficiencies in the prior art, the present invention aims to provide a method for controlling the microstructure of cold-rolled medium manganese steel to solve at least one of the problems existing in the prior art, such as the difficulty in controlling the austenite morphology in steel and the difficulty in cold-rolling processing.

[0005] The object of the present invention is mainly achieved through the following technical solutions:

[0006] The present invention discloses a method for controlling the microstructure of cold-rolled medium manganese steel, including:

[0007] The raw material for cold rolling is a medium manganese steel billet whose microstructure is mainly composed of thin-film austenite and ferrite;

[0008] The microstructure of the medium manganese steel billet after cold rolling is mainly composed of a manganese-rich martensite region and a manganese-poor ferrite region;

[0009] The cold-rolled medium manganese steel billet is subjected to austenite-ferrite duplex zone annealing.

[0010] Preferably, the method for controlling the microstructure of the cold-rolled medium manganese steel further includes: performing austenite-ferrite duplex zone pre-annealing on the medium manganese steel billet before cold rolling.

[0011] Preferably, a hot-rolled raw material blank is used to obtain a hot-rolled medium manganese steel blank;

[0012] The hot-rolled medium manganese steel blank is subjected to austenite-ferrite dual-phase region pre-annealing, and the process conditions are controlled such that the microstructure of the medium manganese steel blank after austenite-ferrite dual-phase region pre-annealing mainly consists of thin-film austenite and ferrite.

[0013] Preferably, the annealing process parameters of the austenite-ferrite dual-phase region pre-annealing and the austenite-ferrite dual-phase region annealing include: annealing temperature, annealing heating rate, annealing holding time, and annealing cooling rate.

[0014] Preferably, the annealing temperature of the austenite-ferrite dual-phase region pre-annealing and the austenite-ferrite dual-phase region annealing is 600°C to 800°C.

[0015] Preferably, the method for controlling the microstructure of cold-rolled medium manganese steel includes:

[0016] Step (1), preparing raw materials according to the designed composition, and obtaining a medium manganese steel blank through smelting and casting;

[0017] Step (2), heating, holding, and then forging the medium manganese steel blank to prepare a medium manganese steel forging blank;

[0018] Step (3), heating and holding the medium manganese steel forging blank and then hot-rolling to obtain a hot-rolled sheet;

[0019] Step (4), performing the first annealing on the hot-rolled sheet in the austenite-ferrite dual-phase region, and air-cooling to room temperature to obtain the first annealed sheet;

[0020] Step (5), performing the first cold rolling on the first annealed sheet obtained in step (4) to obtain the original cold-rolled sheet;

[0021] Step (6), performing the second annealing on the original cold-rolled sheet in the austenite-ferrite dual-phase region, and air-cooling to room temperature to obtain the second annealed sheet with the volume fraction of thin-film austenite in all austenite being greater than 10%.

[0022] Preferably, the reduction ratio of cold rolling in step (5) is 10% to 80%.

[0023] A cold-rolled medium manganese steel obtained by the method for controlling the microstructure of cold-rolled medium manganese steel as described above, wherein the mass content of Mn in the cold-rolled medium manganese steel is 3% to 10%; the microstructure of the cold-rolled medium manganese steel mainly consists of two phases of ferrite and austenite, and the volume fraction of thin-film austenite in all austenite is 10% to 90%.

[0024] Preferably, the chemical composition of the cold-rolled medium manganese steel is as follows by mass percentage: C 0.1% - 0.3%, Mn 3% - 10%, Si 0 - 1%, Al 0 - 1%, Nb 0 - 0.2%, Mo 0 - 3.0%, V 0 - 1.0%, Ti 0 - 0.5%, Ni 0 - 5.0%, Cu 0 - 5.0%, Cr 0 - 5.0%, and the balance is Fe and unavoidable impurities.

[0025] Preferably, the volume fraction of thin-film austenite in the cold-rolled medium manganese steel accounts for 25% - 90% of all austenite.

[0026] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0027] (1) The microstructure of the medium manganese steel billet treated by cold rolling in the present invention is mainly composed of Mn-rich thin-film austenite and Mn-poor ferrite. The non-uniform distribution of Mn elements is retained after mechanical deformation, so that a microstructure composed of a ferrite matrix, thin-film austenite, and massive austenite can be formed after cold rolling and annealing. The initial microstructure of the medium manganese steel billet raw material selected by the prior art for cold rolling treatment is martensite, and a higher proportion of thin-film austenite is obtained in the subsequent heat treatment process. Therefore, the microstructure of the cold-rolled medium manganese steel prepared by the prior art is mainly composed of equiaxed austenite (massive austenite) and ferrite. The volume ratio of thin-film austenite in the cold-rolled medium manganese steel prepared by the present invention after cold rolling is higher, which is beneficial for the medium manganese steel to obtain higher stability.

[0028] (2) By subjecting the hot-rolled medium manganese steel billet to austenite-ferrite dual-phase region pre-annealing in the present invention, not only a medium manganese steel billet with austenite regions rich in Mn and ferrite regions poor in Mn is obtained, but also the austenite mainly exists as thin-film austenite. The thin-film austenite is transformed into Mn-rich martensite regions during subsequent cold rolling, which is beneficial for obtaining a higher proportion of thin-film austenite after austenite-ferrite dual-phase region annealing again after cold rolling, and further enables the medium manganese steel to obtain higher stability.

[0029] (3) By controlling the cold rolling reduction rate and annealing process in the present invention, the proportion of thin-film austenite in the cold-rolled medium manganese steel finished product and the mechanical behavior of the cold-rolled medium manganese steel finished product can be regulated.

[0030] (4) By adopting a two-step annealing and cold rolling process in the present invention, the reduction rate of the cold-rolled medium manganese steel finished product can be controlled, overcoming the problem of difficult cold rolling of traditional medium manganese steel. Description of the Drawings

[0031] The drawings are only used for the purpose of showing specific embodiments, and are not considered to be a limitation to the present invention. Throughout the drawings, the same reference signs denote the same components.

[0032] Figure 1aSecondary electron image of the microstructure of the sample obtained in Step 6 in Embodiment 1 of the present invention;

[0033] Figure 1b Electron backscatter diffraction (EBSD) quality distribution map of the microstructure image of the sample obtained in Step 6 in Embodiment 1 of the present invention;

[0034] Figure 2a Secondary electron image of the microstructure of the sample obtained in Step 6 in Embodiment 2 of the present invention;

[0035] Figure 2b Electron backscatter diffraction (EBSD) quality distribution map of the microstructure image of the sample obtained in Step 6 in Embodiment 2 of the present invention;

[0036] Figure 3a Secondary electron image of the microstructure of the sample obtained in Step 6 in Embodiment 3 of the present invention;

[0037] Figure 3b Electron backscatter diffraction (EBSD) quality distribution map of the microstructure image of the sample obtained in Step 6 in Embodiment 3 of the present invention;

[0038] Figure 4 Secondary electron image of the microstructure of the sample obtained in Step 6 in Embodiment 4 of the present invention;

[0039] Figure 5a Secondary electron image of the microstructure of the sample obtained in Step 6 in Embodiment 5 of the present invention;

[0040] Figure 5b Electron backscatter diffraction (EBSD) quality distribution map of the microstructure image of the sample obtained in Step 6 in Embodiment 5 of the present invention. Detailed implementation manners

[0041] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and 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.

[0042] Regarding technical terms

[0043] The reduction ratio (also known as the deformation ratio or compression ratio) refers to the degree of dimensional change of the material before and after processing during metal forming, and is usually used to describe the deformation of the material during plastic deformation.

[0044] The present invention relates to a cold-rolled medium manganese steel, and the mass content of Mn in the cold-rolled medium manganese steel is 3% to 10%; the microstructure of the cold-rolled medium manganese steel is mainly composed of two phases of ferrite and austenite, wherein the volume fraction of thin-film austenite in all austenite is 10% to 90%.

[0045] Specifically, its chemical composition by mass percentage is as follows: C 0.1% to 0.3%, Mn 3% to 10%, Si 0 to 1%, Al 0 to 1%, Nb 0 to 0.2%, Mo 0 to 3.0%, V 0 to 1.0%, Ti 0 to 0.5%, Ni 0 to 5.0%, Cu 0 to 5.0%, Cr 0 to 5.0%, and the balance is Fe and unavoidable impurities.

[0046] Specifically, the volume fraction of thin-film austenite in all austenite is 10%, 12%, 15%, 18%, 20%, 25%, 28%, 30%, 33%, 38%, 40%, 42%, 44%, 47%, 50%, 51%, 54%, 56%, 60%, 64%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 86%, 88% or 90%.

[0047] It should be noted that the applicant's research found that: the austenite grains in traditional cold-rolled medium manganese steel are mostly massive austenite, rather than thin-film austenite with higher stability, and thin-film austenite is relatively helpful for eliminating non-uniform deformation phenomena such as Lüders bands compared to massive austenite, and improving the mechanical properties of steel.

[0048] Preferably, the volume fraction of thin-film austenite in all austenite is 25% to 90%.

[0049] More preferably, the volume fraction of thin-film austenite in all austenite is 30% to 50%.

[0050] The specific components of the cold-rolled medium manganese steel function as follows:

[0051] C: It is the main factor affecting the strength of the steel plate, can improve the stability of austenite, and can form carbides by combining with elements such as titanium (Ti), niobium (Nb), vanadium (V), molybdenum (Mo), etc., to improve the properties of the steel. According to the strength grade and plastic toughness requirements of the steel type of the present invention, the C content is precisely controlled within the range of 0.1% to 0.3%.

[0052] Mn: As a key austenite stabilizing element, increasing the manganese content helps to increase the content and stability of metastable austenite in medium manganese steel. However, too high a manganese content may cause macroscopic segregation of the material, affecting smelting and processing. Therefore, in the present invention, the Mn content is controlled at 3% to 10% to balance the strength and plasticity requirements and ensure the processing performance at the same time.

[0053] Si and Al: The addition of these two elements aims to inhibit the precipitation of cementite, thereby increasing the content and stability of metastable austenite. However, excessive addition of Si and Al may have an adverse effect on the surface quality of medium manganese steel. Therefore, the contents of Si and Al are precisely controlled within the range of 0-1%.

[0054] Nb, Mo, V, Ti, Cu, Cr: These microalloying elements enhance the strength of medium manganese steel through mechanisms such as precipitation strengthening, solid solution strengthening, and grain refinement strengthening. However, excessive addition may impair the plasticity of the steel. Therefore, according to the strength, plasticity, and toughness requirements of the steel grades of the present invention, the addition amounts of these elements are controlled as follows: Nb 0-0.2%, Mo 0-3.0%, V 0-1.0%, Ti 0-0.5%, Cu 0-5.0%, Cr 0-5.0%.

[0055] Ni: The addition of nickel helps to further improve the stability of austenite. However, due to its high cost, to ensure the best balance between economy and performance, Ni should be controlled within the range of 0-5.0%.

[0056] Specifically, the volume fraction of austenite is 8%-35%, and the corresponding volume fraction of ferrite is 65%-92%; the volume fraction of austenite can be 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34% or 35%.

[0057] It should be noted that the presence of austenite helps to improve the plasticity of medium manganese steel. When the content of retained austenite increases, the product of the tensile strength and elongation (the strength-plasticity product) of medium manganese steel will increase; the duplex structure of ferrite and austenite generally has better toughness than a single-phase structure; ferrite can absorb impact energy, while austenite can prevent the rapid propagation of cracks. The volume fractions of austenite and ferrite within the above ranges are beneficial to maintaining good plasticity and toughness.

[0058] The applicant's research found that: when the medium manganese steel billet treated by cold rolling has a thin-film-like manganese-rich martensite region and a manganese-poor deformed ferrite region, the manganese-rich region is more likely to nucleate and grow austenite, so thin-film austenite can be formed during the annealing process; while the microstructure of the medium manganese steel cold-rolled sheet obtained by stress relief annealing and cold rolling in the prior art mainly consists of equiaxed austenite (blocky austenite) and ferrite; increasing the volume ratio of thin-film austenite in austenite can effectively regulate the austenite stability in medium manganese steel and the dislocation behavior during its deformation process.

[0059] Furthermore, the present invention discloses a method for regulating the microstructure of cold-rolled medium manganese steel, including:

[0060] The raw material for cold rolling is a medium manganese steel billet whose microstructure mainly consists of thin-film austenite and ferrite.

[0061] After cold rolling, the microstructure of the medium manganese steel billet mainly consists of manganese-rich martensite regions and manganese-poor ferrite regions.

[0062] The cold-rolled medium manganese steel billet is subjected to austenite-ferrite dual-phase zone annealing.

[0063] During implementation, the thin-film austenite in the microstructure of the cold-rolled medium manganese steel billet is mainly transformed into martensite, forming manganese-rich martensite regions and manganese-poor ferrite regions.

[0064] After austenite-ferrite dual-phase zone annealing, the manganese-rich martensite regions in the medium manganese steel billet are more likely to form thin-film austenite than the manganese-poor ferrite regions, obtaining a higher proportion of thin-film austenite.

[0065] The applicant's research found that the formation of thin-film austenite in the cold-rolled medium manganese steel products is mainly due to the fact that the microstructure of the raw material for cold rolling mainly consists of manganese-poor ferrite matrix and manganese-rich austenite. After cold rolling, the microstructure is composed of martensite and ferrite, and the non-uniform distribution of manganese is retained therein. The thin-film manganese-rich martensite regions are more likely to nucleate and grow austenite during annealing, thus a thin-film austenite structure can be formed. By changing the cold rolling reduction rate and annealing process, the proportion of thin-film austenite therein can be regulated. Compared with the prior art, the microstructure of the medium manganese steel billet treated by cold rolling in the present invention mainly consists of thin-film austenite and ferrite, so that manganese-rich martensite regions and manganese-poor ferrite regions can be formed during cold rolling; the proportion of thin-film austenite in the raw material of the medium manganese steel billet selected for cold rolling treatment in the prior art is less, so it is difficult to form manganese-rich martensite regions and manganese-poor ferrite regions, and thus a higher proportion of thin-film austenite cannot be obtained during the subsequent heat treatment process. Therefore, the microstructure of the cold-rolled medium manganese steel prepared by the prior art mainly consists of equiaxed austenite (blocky austenite) and ferrite. The cold-rolled medium manganese steel prepared in the present invention after cold rolling can obtain a certain proportion of thin-film austenite, expanding the space for microstructure regulation of medium manganese steel.

[0066] Preferably, the method for microstructure regulation of the cold-rolled medium manganese steel further includes: performing austenite-ferrite dual-phase zone pre-annealing on the medium manganese steel billet before cold rolling.

[0067] Specifically, the method for microstructure regulation of the cold-rolled medium manganese steel includes:

[0068] Obtaining a hot-rolled medium manganese steel billet by hot-rolling a raw material embryo.

[0069] The hot-rolled medium manganese steel billet is subjected to austenite-ferrite dual-phase region pre-annealing, and the process conditions are controlled so that the microstructure of the medium manganese steel billet after pre-annealing mainly consists of thin-film austenite and ferrite.

[0070] During implementation, after the hot-rolled medium manganese steel billet is subjected to austenite-ferrite dual-phase region pre-annealing, the microstructure of the medium manganese steel billet mainly consists of thin-film austenite and ferrite. The manganese element contents of austenite and ferrite are different, so that a manganese-rich austenite region and a manganese-poor ferrite region can be formed.

[0071] The applicant's research finds that in the hot-rolled medium manganese steel, through dual-phase region annealing, thin-film austenite can be formed in the ferrite matrix; this kind of thin-film austenite is usually distributed at the original martensite lath boundaries. This is because austenite tends to nucleate at the interfaces, and the special orientation relationship and morphology of lath martensite make the austenite nucleated between the lath boundaries easy to grow into the thin-film austenite morphology. Therefore, subjecting the hot-rolled medium manganese steel billet to austenite-ferrite dual-phase region pre-annealing is beneficial for the raw material of the medium manganese steel billet used for cold rolling to obtain a high proportion of thin-film austenite, and further beneficial for the medium manganese steel billet after cold rolling and then subjected to austenite-ferrite dual-phase region annealing treatment to obtain a microstructure mainly composed of thin-film austenite and ferrite.

[0072] It should be noted that compared with another common form of austenite in medium manganese steel - equiaxed austenite (blocky austenite), thin-film austenite usually has higher stability due to its special morphology. In addition, compared with the medium manganese steel structure containing thin-film austenite, the medium manganese steel containing blocky austenite is more likely to form Lüder's bands during the tensile process, which has an adverse effect on the hydrogen embrittlement, fatigue and other properties of the medium manganese steel.

[0073] Compared with the prior art, the present invention subjects the hot-rolled medium manganese steel billet to austenite-ferrite dual-phase region pre-annealing, not only obtaining a medium manganese steel billet with a manganese-rich austenite region and a manganese-poor ferrite region, but also the austenite mainly exists in the form of thin-film austenite, and the thin-film austenite transforms into a manganese-rich martensite region during subsequent cold rolling, which is beneficial for obtaining a higher proportion of thin-film austenite after cold rolling and then performing austenite-ferrite dual-phase region annealing again, expanding the space for the microstructure and property design of medium manganese steel.

[0074] Specifically, the annealing process parameters of austenite-ferrite dual-phase region pre-annealing and austenite-ferrite dual-phase region annealing include: annealing temperature, annealing heating rate, annealing holding time and annealing cooling rate.

[0075] Specifically, the annealing temperatures for austenite-ferrite duplex zone pre-annealing and austenite-ferrite duplex zone annealing are 600°C to 800°C. For example, they can be 600°C, 605°C, 640°C, 650°C, 698°C, 700°C, 712°C, 725°C, 745°C, 765°C, 798°C or 800°C.

[0076] It should be noted that the annealing temperature has a significant impact on the microstructure of medium manganese steel. On the one hand, at a relatively low annealing temperature (such as 570°C), the microstructure may contain some deformed structures and the recrystallization is incomplete. As the temperature increases, the degree of recrystallization is higher, the microstructure gradually tends to equiaxed, the grain size increases, and it tends to generate equiaxed austenite (blocky austenite), and the proportion of thin-film austenite decreases. At a relatively high annealing temperature (such as 800°C and above), martensite structure may appear and the austenite volume fraction decreases.

[0077] Preferably, the annealing temperatures for austenite-ferrite duplex zone pre-annealing and austenite-ferrite duplex zone annealing are 600°C to 680°C.

[0078] More preferably, the annealing temperatures for austenite-ferrite duplex zone pre-annealing and austenite-ferrite duplex zone annealing before and after cold rolling are 630°C to 660°C.

[0079] It should be noted that the annealing temperatures for austenite-ferrite duplex zone pre-annealing and austenite-ferrite duplex zone annealing can be the same or different.

[0080] Specifically, the heating rate range for austenite-ferrite duplex zone pre-annealing and austenite-ferrite duplex zone annealing is 1°C / s to 300°C / s. For example, they can be 1°C / s, 5°C / s, 40°C / s, 50°C / s, 98°C / s, 100°C / s, 112°C / s, 125°C / s, 145°C / s, 165°C / s, 198°C / s, 200°C / s, 212°C / s, 225°C / s, 232°C / s, 250°C / s, 298°C / s or 300°C / s.

[0081] It should be noted that due to the long annealing time, the heating rate has little effect on the austenite volume fraction in the microstructure. And the increase in the heating rate may lead to an increase in the nucleation density of the sample and a decrease in the grain size.

[0082] Specifically, the cooling rate for pre-annealing in the austenite-ferrite dual-phase region and annealing in the austenite-ferrite dual-phase region before and after cold rolling is 5°C / min to 1200°C / min. For example, it can be 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min, 16°C / min, 17°C / min, 18°C / min, 19°C / min or 20°C / min.

[0083] It should be noted that due to the long annealing time and high austenite stability, the cooling rate has little effect on the microstructure of the cooled sample. However, too slow a cooling rate may cause cementite precipitation during cooling, and too fast a cooling rate may result in high residual stress.

[0084] Specifically, the holding time for pre-annealing in the austenite-ferrite dual-phase region and annealing in the austenite-ferrite dual-phase region before and after cold rolling is 1 h to 10 h. For example, it can be 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h.

[0085] It should be noted that with the extension of the annealing time, the austenite volume fraction increases until it reaches the equilibrium austenite volume fraction. After that, continuing to extend the annealing time, the austenite volume fraction no longer increases, while the grain size of the sample increases, resulting in a decrease in austenite stability and deterioration of mechanical properties. As the annealing temperature increases, the austenite growth rate accelerates. Therefore, it is necessary to select an appropriate annealing time according to the annealing temperature.

[0086] Specifically, the method for controlling the microstructure of the cold-rolled medium manganese steel includes:

[0087] Step (1), preparing raw materials according to the designed composition, and obtaining a medium manganese steel billet through smelting and casting;

[0088] Step (2), heating, holding, and then forging the medium manganese steel billet to prepare a medium manganese steel forging billet;

[0089] Step (3), heating and holding the medium manganese steel forging billet and then hot rolling to obtain a hot rolled plate;

[0090] Step (4), performing the first annealing on the hot rolled plate in the austenite-ferrite dual-phase region and air cooling to room temperature to obtain the first annealed plate;

[0091] Step (5), performing the first cold rolling on the first annealed plate obtained in step (4) to obtain the original cold rolled plate;

[0092] Step (6), performing the second annealing on the original cold rolled plate in the austenite-ferrite dual-phase region and air cooling to room temperature to obtain the second annealed plate with the volume fraction of thin-film austenite greater than 10% of the total austenite volume.

[0093] It should be noted that step (1) can adopt the existing material balance method, use the raw materials for manganese steel smelting in the existing technology, prepare the raw materials according to the designed composition, and obtain steel billets through smelting and casting.

[0094] It should be noted that the solidus temperature of medium manganese steel is around 1300°C - 1350°C. Therefore, its maximum heating temperature should not exceed 1200°C. The steel billets in step (2) and the forging billets in step (3) are preferably heated to 1180°C - 1250°C, which can ensure that the steel billets have good plasticity and low deformation resistance, while avoiding overheating and overburning.

[0095] At the same time, it should be noted that when medium manganese steel is heated in the range of 1180°C - 1250°C, which is in the austenite region, the holding time for heating the steel billets in step (2) and the forging billets in step (3) is preferably 60 min - 120 min. The reasons are as follows: When medium manganese steel is heated in the austenite region, it is necessary to ensure that the carbides inside the steel billets are fully dissolved and the austenite grains are homogenized; too short a holding time may lead to incomplete austenitization and affect the subsequent processing performance.

[0096] In addition, during the heating process of medium manganese steel billets, after the surface temperature reaches the set value, the internal temperature still needs time to conduct and homogenize; the holding time ensures that the internal temperature of the steel billets is uniform, avoiding stress concentration and uneven deformation caused by too large a temperature gradient.

[0097] In addition, in the austenite region, as the holding time prolongs, the austenite grains will gradually grow; an appropriate holding time can control the grain growth rate and avoid overly large grains, thus ensuring the mechanical properties of the material. The research on the austenite grain growth kinetics of medium manganese steel shows that too long a holding time will lead to grain coarsening and reduce the toughness and strength of the material.

[0098] In addition, medium manganese steel contains a certain amount of alloying elements (such as manganese), and these elements will affect the austenitization rate and grain growth behavior. An appropriate holding time can ensure the full diffusion and uniform distribution of alloying elements, thus obtaining ideal microstructure and properties.

[0099] Specifically, the final rolling temperature in step (3) is not lower than 900°C.

[0100] It should be noted that the final rolling temperature should not be too low to ensure that the steel has sufficient plasticity and deformation ability during rolling, while avoiding rolling difficulties and uneven microstructure and properties caused by too low a temperature; at the same time, the final rolling temperature has an important impact on the microstructure and properties of the steel. A relatively high final rolling temperature may lead to grain growth, thus reducing the strength and toughness of the material; while an appropriate final rolling temperature can refine the grains and improve the comprehensive performance of the material.

[0101] It should be noted that the hot rolling of medium manganese steel in step (3) belongs to heating in the austenite region, and the finish rolling temperature should not be lower than 900 °C.

[0102] Specifically, the reduction ratio during cold rolling in step (5) is 10% - 80%, for example, it can be 10%, 20%, 30%, 40%, 50%, 60%, 70% or 80%.

[0103] It should be noted that if the reduction ratio during cold rolling is too low, it may be difficult to meet the final thickness requirement, while if the reduction ratio is too high, it may cause the sheet to crack. The applicant's research found that increasing the reduction ratio will result in more deformation defects in the cold-rolled sample, thus making recrystallization behavior more likely to occur during the subsequent annealing process, reducing the grain size and increasing the proportion of massive austenite in the annealed sample. Reasonably selecting the reduction ratio can not only optimize the process flow but also regulate the microstructure and mechanical properties of the material.

[0104] Compared with the prior art, the present invention optimizes the process flow of cold-rolled medium manganese steel and can regulate the austenite morphology in medium manganese steel by controlling the reduction ratio during cold rolling.

[0105] Preferably, in steps (5) - (6), within the operable temperature range of cold rolling, it can be repeated multiple times according to the requirements of the rolled plate thickness, ensuring that the reduction ratio for each cold rolling is 10% - 80%.

[0106] During implementation, after annealing in step (4), the retained austenite in the annealed sample is thin-film austenite; during cold rolling in step (5), the metastable austenite (thin-film austenite) of the annealed plate transforms into martensite, but the non-uniform distribution of Mn is retained in the cold-rolled plate structure, forming manganese-rich martensite regions and manganese-poor ferrite regions;

[0107] After step (6), the original manganese-rich martensite regions are more likely to form metastable austenite (thin-film austenite), thus forming a duplex structure mainly composed of manganese-poor ferrite and manganese-rich thin-film austenite.

[0108] Specifically, the volume ratios of ferrite, austenite, and thin-film austenite phases in the microstructure of cold-rolled medium manganese steel can be used to statistically analyze and identify the areas of ferrite, austenite, and thin-film austenite phases respectively with the help of the prior art.

[0109] To better illustrate the present invention, the following embodiments are further provided:

[0110] Example 1

[0111] This example discloses a method for regulating the microstructure of cold-rolled medium manganese steel, and the regulation method includes:

[0112] Step 1: Smelt molten steel according to the set composition and cast to obtain a steel billet, the composition of which contains C 0.18%, Mn 4.95%, Si 0.4%, Mo 0.0021% by weight percentage, and the rest is Fe and other inevitable impurities;

[0113] Step 2: Heat the steel billet to 1200 °C, hold for 2 h, and obtain a forged billet through forging;

[0114] Step 3: Hold the forged billet at 1200 °C for 2 h, and obtain a hot-rolled sheet with a thickness of about 4.2 mm through multi-pass hot rolling, and the final rolling temperature of hot rolling is not lower than 900 °C;

[0115] Step 4: Perform austenite-ferrite dual-phase region pre-annealing treatment on the hot-rolled sheet at 630 °C for 5 h and air-cool to room temperature;

[0116] Step 5: Cold-roll the pre-annealed hot-rolled sheet to a thickness of about 2.37 mm, and the reduction ratio is about 44%;

[0117] Step 6: Perform austenite-ferrite dual-phase region annealing on the obtained cold-rolled sheet at 630 °C, isothermal for 2 h and air-cool to room temperature.

[0118] The heating and cooling rate parameters of the austenite-ferrite dual-phase region annealing process in Step 4 and Step 6 are the same: the heating rate is 50 °C / s; the cooling rate is 10 °C / min.

[0119] This example discloses a cold-rolled medium manganese steel, prepared by the above method, consisting of Figure 1a - Figure 1b Shown: The red grains are austenite, and statistics show that the volume fraction of austenite is about 20%; the thin-film austenite accounts for 30% of the volume fraction of all austenite.

[0120] Example 2

[0121] This example discloses a method for controlling the microstructure of cold-rolled medium manganese steel,

[0122] Step 1: Smelt molten steel according to the set composition and cast to obtain a steel billet, the composition of which contains C 0.18%, Mn 4.95%, Si 0.4%, Mo 0.0021% by weight percentage, and the rest is Fe and other inevitable impurities;

[0123] Step 2: Heat the steel billet to 1200 °C, hold for 2 h, and obtain a forged billet through forging;

[0124] Step 3: Hold the forged billet at 1200 °C for 2 h, and obtain a hot-rolled sheet with a thickness of about 4.2 mm through multi-pass hot rolling, and the final rolling temperature of hot rolling is not lower than 900 °C;

[0125] Step 4: Perform austenite-ferrite dual-phase region pre-annealing treatment on the hot-rolled sheet at 650 °C for 5 h and air-cool to room temperature;

[0126] Step 5: Cold-roll the pre-annealed hot-rolled sheet to a thickness of about 2.37 mm with a reduction ratio of about 44%.

[0127] Step 6: Anneal the obtained cold-rolled sheet in the austenite-ferrite dual-phase region at 630 °C, isotherm for 1 h and air-cool to room temperature.

[0128] The heating and cooling parameters of the two austenite-ferrite dual-phase region annealing processes in Step 4 and Step 6 are the same: the heating rate is 50 °C / s; the cooling rate is 10 °C / min.

[0129] This example discloses a cold-rolled medium manganese steel prepared by the above method, and it consists of Figure 2a - Figure 2b It shows that: the red grains are austenite, and statistics show that the volume fraction of austenite is about 18%; the thin-film austenite accounts for more than 50% of the volume of all austenite.

[0130] Example 3

[0131] This example discloses a method for controlling the microstructure of cold-rolled medium manganese steel,

[0132] Step 1: Smelt molten steel according to the set composition and cast to obtain a steel billet, whose composition contains 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo by weight percentage, and the rest is Fe and other inevitable impurities;

[0133] Step 2: Heat the steel billet to 1200 °C, hold for 2 h, and obtain a forged billet through forging;

[0134] Step 3: Hold the forged billet at 1200 °C for 2 h, and obtain a hot-rolled sheet with a thickness of about 4.2 mm through multi-pass hot rolling, and the final rolling temperature of hot rolling is not lower than 900 °C;

[0135] Step 4: Perform austenite-ferrite dual-phase region pre-annealing treatment on the hot-rolled sheet at 660 °C for 5 h and air-cool to room temperature;

[0136] Step 5: Cold-roll the pre-annealed hot-rolled sheet to a thickness of about 2.37 mm with a reduction ratio of about 44%.

[0137] Step 6: Anneal the obtained cold-rolled sheet in the austenite-ferrite dual-phase region at 630 °C, isotherm for 5 h and air-cool to room temperature.

[0138] The heating and cooling parameters of the two austenite-ferrite dual-phase region annealing processes in Step 4 and Step 6 are the same: the heating rate is 50 °C / s; the cooling rate is 10 °C / min.

[0139] This example discloses a cold-rolled medium manganese steel prepared by the above method, and it consists of Figure 3a - Figure 3bObservation: The red grains are austenite. Statistics show that the volume fraction of austenite is approximately 20%; the volume fraction of thin-film austenite in all austenite is approximately 40%.

[0140] Example 4

[0141] This example discloses a method for controlling the microstructure of cold-rolled medium manganese steel. The difference from Example 1 is that the reduction rate in the cold rolling step of Step 5 is 20%, and the remaining steps and components are the same as those in Example 1; the volume fraction of austenite in the cold-rolled medium manganese steel prepared by the method of this example is approximately 18%; the volume fraction of thin-film austenite in all austenite is approximately 22%.

[0142] Comparative Example 1

[0143] This comparative example discloses a method for controlling the microstructure of cold-rolled medium manganese steel. Compared with Example 1, the hot-rolled sheet is subjected to stress relief annealing at 450 °C for 2 h and air-cooled to room temperature:

[0144] Step 1: Molten steel is smelted according to the set composition and cast into a steel billet. Its composition contains 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo by weight percentage, and the rest is Fe and other inevitable impurities;

[0145] Step 2: The steel billet is heated to 1200 °C, held for 2 h, and forged to obtain a forged billet;

[0146] Step 3: The forged billet is held at 1200 °C for 2 h and hot-rolled through multiple passes to obtain a hot-rolled sheet with a thickness of approximately 4.2 mm. The final rolling temperature of hot rolling is not lower than 900 °C;

[0147] Step 4: The hot-rolled sheet is subjected to stress relief annealing at 450 °C for 2 h and air-cooled to room temperature. Austenite is not formed during annealing at this temperature, only the internal stress in the material is eliminated to avoid cracking during subsequent cold rolling;

[0148] Step 5: The hot-rolled sheet is cold-rolled to a thickness of approximately 2.37 mm, and the reduction rate is approximately 44%;

[0149] Step 6: The cold-rolled sheet is isothermally held at 630 °C for 5 h and air-cooled to room temperature;

[0150] This comparative example discloses a cold-rolled medium manganese steel prepared by the above method, Figure 4 Observation shows that the microstructure of the obtained sample consists of ferrite and massive austenite, and there is no thin-film austenite.

[0151] Comparative Example 2

[0152] This comparative example discloses a method for controlling the microstructure of cold-rolled medium manganese steel. Compared with Example 1, in step 4, the hot-rolled sheet is subjected to tempering treatment at 450 °C for 2 h and air-cooled to room temperature, without annealing in the duplex region. At this temperature, no austenite is formed during annealing, and only the internal stress in the material is eliminated:

[0153] Step 1: Smelt molten steel according to the set composition and cast to obtain a steel billet. Its composition contains 0.18% C, 4.95% Mn, 0.4% Si, 0.0021% Mo by weight percentage, and the rest is Fe and other inevitable impurities;

[0154] Step 2: Heat the steel billet to 1200 °C, hold for 2 h, and obtain a forged billet through forging;

[0155] Step 3: Hold the forged billet at 1200 °C for 2 h, and obtain a hot-rolled sheet with a thickness of about 4.2 mm through multi-pass hot rolling. The finishing rolling temperature of hot rolling is not lower than 900 °C;

[0156] Step 4: Carry out tempering treatment on the hot-rolled sheet at 450 °C for 2 h and air-cool it to room temperature. No austenite is formed during annealing at this temperature, and only the internal stress in the material is eliminated to avoid cracking during subsequent cold rolling;

[0157] Step 5: Cold-roll the tempered hot-rolled sheet to a thickness of about 2.37 mm, with a reduction ratio of about 44%;

[0158] Step 6: Isothermally hold the cold-rolled sheet at 660 °C for 1 h and air-cool it to room temperature;

[0159] This comparative example discloses a cold-rolled medium manganese steel prepared by the above method, Figure 5a - Figure 5b It shows that the microstructure of the obtained sample consists of ferrite and massive austenite, without thin-film austenite.

[0160] As can be seen from the above, the volume fraction of austenite in the cold-rolled medium manganese steel prepared in Examples 1 - 4 of the present invention is about 18% - 20%; the volume fraction of thin-film austenite in all austenite is > 20%, and preferably the volume fraction of thin-film austenite is 30% - 50%; According to common sense, it is inferred that the cold-rolled medium manganese steel prepared by the present invention has better stability than the conventional massive austenite-ferrite duplex structure in cold-rolled medium manganese steel.

[0161] By comparing Example 1, Comparative Example 1 and Comparative Example 2, it can be seen that the present invention pre-anneals the medium manganese steel billet after hot rolling in the austenite-ferrite duplex region, so that the microstructure of the annealed medium manganese steel billet mainly consists of thin-film austenite and ferrite. Furthermore, the microstructure of the cold-rolled billet has a manganese-rich martensite region and a manganese-poor ferrite region, and a higher proportion of thin-film austenite is obtained during subsequent heat treatment, making the volume fraction of thin-film austenite in the cold-rolled medium manganese steel greater than 20% of the total volume of austenite, and preferably 30% - 50%.

[0162] Comparing Comparative Example 1 and Example 4, it can be seen that when the rolling reduction during cold rolling is between 10% and 80%, the volume fraction of thin-film austenite in medium manganese cold-rolled steel is greater than 20% of the total volume of austenite. Preferably, it is between 30% and 50%.

[0163] As mentioned above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for controlling the microstructure of medium manganese cold-rolled steel, characterized in that, Comprising: The raw material for cold rolling is a medium manganese steel billet whose microstructure is mainly composed of thin-film austenite and ferrite; After cold rolling, the microstructure of the medium manganese steel billet is mainly composed of manganese-rich martensite regions and manganese-poor ferrite regions; The cold-rolled medium manganese steel billet is subjected to austenite-ferrite dual-phase region annealing.

2. The method for controlling the microstructure of medium manganese cold-rolled steel according to claim 1, characterized in that The method for controlling the microstructure of the cold-rolled medium manganese steel further includes: performing austenite-ferrite dual-phase region pre-annealing on the medium manganese steel billet before cold rolling.

3. The method for microstructure control of medium manganese cold-rolled steel according to claim 2, characterized in that, The hot-rolled raw material blank is used to obtain a hot-rolled medium manganese steel billet; The hot-rolled medium manganese steel billet is subjected to austenite-ferrite dual-phase region pre-annealing, and the process conditions are controlled such that the microstructure of the medium manganese steel billet after austenite-ferrite dual-phase region pre-annealing is mainly composed of thin-film austenite and ferrite.

4. The microstructure control method of the medium manganese cold-rolled steel according to claim 3, characterized in that, The annealing process parameters of the austenite-ferrite dual-phase region pre-annealing and the austenite-ferrite dual-phase region annealing include: annealing temperature, annealing heating rate, annealing holding time, and annealing cooling rate.

5. The method for controlling the microstructure of the medium manganese cold-rolled steel according to claim 4, characterized in that, The annealing temperature of the austenite-ferrite dual-phase region pre-annealing and the austenite-ferrite dual-phase region annealing is 600°C to 800°C.

6. The method for microstructure control of medium manganese cold-rolled steel according to any one of claims 1-5, characterized in that The method for controlling the microstructure of the cold-rolled medium manganese steel includes: Step (1), preparing raw materials according to the designed composition, and obtaining a medium manganese steel billet through smelting and casting; Step (2), heating, holding, and then forging the medium manganese steel billet to prepare a medium manganese steel forging billet; Step (3), heating and holding the medium manganese steel forging billet and then hot rolling to obtain a hot-rolled plate; Step (4), performing the first annealing on the hot-rolled plate in the austenite-ferrite dual-phase region and air-cooling to room temperature to obtain the first annealed plate; Step (5), performing the first cold rolling on the first annealed plate obtained in step (4) to obtain the original cold-rolled plate; Step (6), performing the second annealing on the original cold-rolled plate in the austenite-ferrite dual-phase region and air-cooling to room temperature to obtain the second annealed plate with the volume fraction of thin-film austenite in the total volume of austenite being greater than 10%.

7. The method for controlling the microstructure of medium manganese cold-rolled steel according to claim 6, characterized in that, The reduction ratio of cold rolling in step (5) is 10% to 80%.

8. A cold-rolled medium manganese steel, characterized in that, Obtained by the method for controlling the microstructure of cold-rolled medium manganese steel according to any one of claims 1-7, the mass content of Mn in the cold-rolled medium manganese steel is 3% to 10%; the microstructure of the cold-rolled medium manganese steel is mainly composed of two phases of ferrite and austenite, and the volume fraction of thin-film austenite in all austenite is 10% to 90%.

9. The cold-rolled medium manganese steel according to claim 8, characterized in that, The chemical composition of the cold-rolled medium manganese steel is as follows by mass percentage: C 0.1% to 0.3%, Mn 3% to 10%, Si 0 to 1%, Al 0 to 1%, Nb 0 to 0.2%, Mo 0 to 3.0%, V 0 to 1.0%, Ti 0 to 0.5%, Ni 0 to 5.0%, Cu 0 to 5.0%, Cr 0 to 5.0%, and the balance is Fe and inevitable impurities.

10. The cold-rolled medium manganese steel according to claim 8 or 9, characterized in that, The volume fraction of thin-film austenite in all austenite of the cold-rolled medium manganese steel is 25% to 90%.

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