A method for controlling the structure of a cold-rolled medium manganese steel

Abstract

The present application belongs to the technical field of metal material preparation, and relates to a microstructure regulation method of cold-rolled medium manganese steel, comprising: the raw material of cold-rolling treatment is medium manganese steel billet, the microstructure of which is mainly composed of thin film austenite and ferrite; the microstructure of the medium manganese steel billet after cold-rolling is mainly composed of manganese-rich martensite region and manganese-poor ferrite region; austenite-ferrite dual-phase zone annealing is performed on the medium manganese steel billet after cold-rolling. The microstructure of the medium manganese steel billet after cold-rolling treatment is mainly composed of thin film austenite and ferrite, which can form manganese-rich martensite region and manganese-poor ferrite region in the cold-rolling process, and obtain finished products with high cold-rolling reduction rate, blocky and thin film austenite, which provides space for the mechanical property regulation of medium manganese steel; the problems of cold-rolling processing difficulty and the difficulty of regulating the austenite morphology of steel in the prior art are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal material preparation, and particularly relates to a microstructure regulation method of cold-rolled medium manganese steel. BACKGROUND

[0002] Advanced high strength steel has developed to the third generation, and medium manganese steel belongs to the third generation of advanced high strength steel and has high strength and plasticity. According to different processing technologies, the 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 size precision, better surface quality and thickness uniformity, and can meet the needs of high-precision products. The traditional processing technology of the cold-rolled medium manganese steel is to obtain the medium manganese steel cold-rolled plate through stress relief annealing and multi-pass cold rolling, and then obtain the equiaxed austenite and ferrite dual-phase structure through reverse variable annealing.

[0003] On the one hand, as an important austenite stabilizing element, Mn element is partitioned from ferrite to austenite in the annealing process, and the increase of the content of Mn element helps to increase the volume fraction of the metastable austenite in the medium manganese steel, but the increase of the content of Mn element can significantly increase the hardness and work hardening capacity of the medium manganese steel, and increase the processing difficulty, which puts forward higher requirements for the cold-rolling process. On the other hand, the morphology of the austenite has a significant influence on the stability of the austenite and the mechanical behavior of the medium manganese steel. However, the existing hot mechanical processing technology of the cold-rolled medium manganese steel is difficult to regulate the morphology of the austenite in the steel, which limits the performance range of the cold-rolled medium manganese steel. SUMMARY

[0004] In view of the above analysis, in view of the deficiencies in the prior art, the present application aims to provide a microstructure regulation method of cold-rolled medium manganese steel, which at least solves one of the problems in the prior art, such as the difficulty in regulating the morphology of the austenite in the steel and the difficulty in cold-rolling.

[0005] The main purpose of the present application is achieved by the following technical solutions:

[0006] The present application discloses a microstructure regulation method of cold-rolled medium manganese steel, comprising:

[0007] The raw material for cold-rolling treatment is a medium manganese steel billet, the microstructure of which mainly consists of thin film austenite and ferrite;

[0008] After cold-rolling, the microstructure of the medium manganese steel billet mainly consists 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 dual-phase zone annealing.

[0010] Preferably, the microstructure regulation method of the cold-rolled medium manganese steel further comprises: pre-annealing the medium manganese steel billet in the austenite-ferrite dual-phase zone before cold-rolling.

[0011] Preferably, the hot-rolled raw blank obtains a hot-rolled medium-manganese steel blank;

[0012] The hot-rolled medium-manganese steel blank is subjected to an austenite-ferrite two-phase zone preannealing, and the process conditions are controlled so that the microstructure of the medium-manganese steel blank after the austenite-ferrite two-phase zone preannealing mainly consists of thin-film austenite and ferrite.

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

[0014] Preferably, the annealing temperature of the austenite-ferrite two-phase zone preannealing and the austenite-ferrite two-phase zone annealing is 600-800℃.

[0015] Preferably, the microstructure regulation method of the cold-rolled medium-manganese steel comprises:

[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), preparing a medium-manganese steel forged blank by heating, holding and post-forging the medium-manganese steel blank;

[0018] Step (3), obtaining a hot-rolled plate by heating and holding the medium-manganese steel forged blank and then hot-rolling;

[0019] Step (4), obtaining a first annealing plate by first annealing the hot-rolled plate in the austenite-ferrite two-phase zone and then air cooling to room temperature;

[0020] Step (5), obtaining an original cold-rolled plate by first cold-rolling the first annealing plate obtained in step (4);

[0021] Step (6), obtaining a second annealing plate with thin-film austenite accounting for more than 10% of the total volume fraction of austenite by second annealing the original cold-rolled plate in the austenite-ferrite two-phase zone and then air cooling to room temperature.

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

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

[0024] Preferably, the chemical composition of the cold-rolled medium manganese steel is as follows in terms of 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 of Fe and inevitable impurities.

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

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

[0027] (1) The microstructure of the cold-rolled medium manganese steel billet prepared by the present application mainly consists of Mn-rich thin film austenite and Mn-poor ferrite. The non-uniform distribution of Mn elements is retained after mechanical deformation, so that the microstructure consisting of ferrite matrix, thin film austenite and bulk austenite can be generated after cold rolling and annealing. The initial microstructure of the cold-rolled medium manganese steel billet selected by the prior art is martensite, and a high 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 mainly consists of equiaxed austenite (bulk austenite) and ferrite. The thin film austenite prepared by the present application has a higher volume fraction, which is beneficial to the stability of the medium manganese steel.

[0028] (2) By pre-annealing the medium manganese steel billet in the austenite-ferrite two-phase region after hot rolling, the medium manganese steel billet with Mn-rich austenite region and Mn-poor ferrite region is obtained. The austenite mainly exists in the form of thin film austenite, which is converted into Mn-rich martensite region in the subsequent cold rolling. This is beneficial to obtaining a higher proportion of thin film austenite by annealing in the austenite-ferrite two-phase region again after cold rolling, and further improving the stability of the medium manganese steel.

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

[0030] (4) By using the two-step annealing and cold rolling process, the reduction rate of the finished cold-rolled medium manganese steel can be controlled, and the problem of difficult cold rolling of traditional medium manganese steel is overcome. BRIEF DESCRIPTION OF DRAWINGS

[0031] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0032] Figure 1aA secondary electron image of the microstructure of the sample obtained in Example 1 of the present invention after Step 6;

[0033] Figure 1b An electron backscatter diffraction technique (EBSD) map of the microstructure image of the sample obtained in Example 1 of the present invention after Step 6;

[0034] Figure 2a A secondary electron image of the microstructure of the sample obtained in Example 2 of the present invention after Step 6;

[0035] Figure 2b An electron backscatter diffraction technique (EBSD) map of the microstructure image of the sample obtained in Example 2 of the present invention after Step 6;

[0036] Figure 3a A secondary electron image of the microstructure of the sample obtained in Example 3 of the present invention after Step 6;

[0037] Figure 3b An electron backscatter diffraction technique (EBSD) map of the microstructure image of the sample obtained in Example 3 of the present invention after Step 6;

[0038] Figure 4 A secondary electron image of the microstructure of the sample obtained in Example 4 of the present invention after Step 6;

[0039] Figure 5a A secondary electron image of the microstructure of the sample obtained in Example 5 of the present invention after Step 6;

[0040] Figure 5b An electron backscatter diffraction technique (EBSD) map of the microstructure image of the sample obtained in Example 5 of the present invention after Step 6. DETAILED DESCRIPTION

[0041] In order to make the technical problems solved by the present invention, technical solutions and beneficial effects more clearly understood, the present invention will be further described in detail below in combination with the drawings and examples. It should be understood that the specific examples 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 size change of the material before and after processing in the metal forming process, and is usually used to describe the deformation of the material in plastic deformation.

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

[0045] Specifically, its chemical composition is as follows in terms of 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.

[0046] Specifically, thin film austenite accounts for 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% of the volume fraction of all austenite.

[0047] It should be noted that the applicant found that the austenite grains in traditional cold-rolled medium manganese steel are mostly blocky austenite, rather than thin film austenite which has higher stability, and thin film austenite helps to eliminate inhomogeneous deformation phenomena such as Luders band and improve the mechanical properties of steel.

[0048] Preferably, thin film austenite accounts for 25%~90% of the volume fraction of all austenite.

[0049] Further preferably, thin film austenite accounts for 30%~50% of the volume fraction of all austenite.

[0050] The specific components of the cold-rolled medium manganese steel have the following effects:

[0051] C: is the main factor affecting the strength of steel plate, can improve the stability of austenite, and can combine with elements such as titanium (Ti), niobium (Nb), vanadium (V) and molybdenum (Mo) to form carbides to improve the performance of steel. According to the strength grade and plasticity demand of the steel of the present application, the content of C is accurately controlled in the range of 0.1%~0.3%.

[0052] Mn: as a key austenite stabilizing element, increasing the content of manganese helps to increase the content and stability of metastable austenite in medium manganese steel. However, too high manganese content may cause macrosegregation in the material, affecting smelting and processing. Therefore, the Mn content in the present application is controlled in the range of 3%~10% to balance the strength and plasticity demand, while ensuring the processing performance.

[0053] Si and Al: The addition of these two elements aims to suppress the precipitation of cementite, thereby increasing the content and stability of metastable austenite. However, excessive addition of Si and Al can adversely affect the surface quality of medium manganese steel, so the Si and Al content is precisely controlled within the range of 0-1%.

[0054] Nb, Mo, V, Ti, Cu, Cr: These micro-alloying elements improve the strength of medium manganese steel through mechanisms such as precipitation strengthening, solid solution strengthening, and fine-grain strengthening. However, excessive addition can damage the plasticity of the steel. Therefore, according to the strength and plasticity requirements of the steel according to the present application, the addition of these elements is controlled within the range of Nb 0-0.2%, Mo 0-3.0%, V 0-1.0%, Ti 0-0.5%, Cu 0-5.0%, and Cr 0-5.0%.

[0055] Ni: The addition of nickel helps to further improve the stability of austenite, but 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 residual austenite content increases, the product of tensile strength and elongation of medium manganese steel increases; the dual-phase structure of ferrite and austenite generally has better toughness than single-phase structure; ferrite can absorb impact energy, while austenite can prevent rapid crack propagation. The volume fraction of austenite and ferrite within the above range is beneficial to maintaining good plasticity and toughness.

[0058] Applicants have found that when the cold-rolled medium manganese steel blank has a thin film of manganese-rich martensite region and a manganese-poor deformed ferrite region, the manganese-rich region is more prone to nucleation and growth of austenite, thus forming thin film austenite during annealing; the microstructure of the medium manganese steel cold-rolled plate obtained by stress relief annealing and cold rolling in the prior art is mainly composed of equiaxed austenite (massive austenite) and ferrite; increasing the volume fraction of thin film austenite in austenite can effectively regulate the stability of austenite in medium manganese steel and its dislocation behavior during deformation.

[0059] Further, the present application discloses a microstructure regulation method of cold-rolled medium manganese steel, comprising:

[0060] The raw material for the cold rolling treatment is a medium manganese steel blank whose microstructure mainly consists of thin film austenite and ferrite;

[0061] The microstructure of the medium manganese steel blank after the cold rolling mainly consists of a manganese-rich martensite region and a manganese-poor ferrite region;

[0062] The medium manganese steel blank after the cold rolling is subjected to austenite-ferrite dual-phase zone annealing.

[0063] In the implementation, the thin film austenite in the microstructure of the medium manganese steel blank after the cold rolling treatment is mainly converted into martensite, forming a manganese-rich martensite region and a manganese-poor ferrite region;

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

[0065] The applicant has found that the formation of the thin film austenite in the finished cold-rolled medium manganese steel is mainly due to the fact that the microstructure of the raw material for the cold rolling treatment mainly consists of a manganese-poor ferrite matrix and a manganese-rich austenite. After the cold rolling, the microstructure consists of martensite and ferrite, and the non-uniform distribution of manganese is retained therein. The thin film manganese-rich martensite region is more likely to nucleate and grow into austenite during the annealing process, and thus can generate a thin film austenite structure. By changing the cold rolling reduction rate and the annealing process, the proportion of the thin film austenite can be controlled. Compared with the prior art, the microstructure of the medium manganese steel blank for the cold rolling treatment of the present application mainly consists of thin film austenite and ferrite, and thus a manganese-rich martensite region and a manganese-poor ferrite region can be formed during the cold rolling process. The prior art selects a medium manganese steel blank for the cold rolling treatment whose raw material has a lower proportion of thin film austenite, and thus it is difficult to form a manganese-rich martensite region and a manganese-poor ferrite region, and further obtain a higher proportion of thin film austenite 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 (massive austenite) and ferrite. The cold-rolled medium manganese steel prepared by the present application can obtain a certain proportion of thin film austenite, which expands the space for the microstructure control of the medium manganese steel.

[0066] Preferably, the microstructure control method of the cold-rolled medium manganese steel further comprises: subjecting the medium manganese steel blank to austenite-ferrite dual-phase zone pre-annealing before the cold rolling.

[0067] Specifically, the microstructure control method of the cold-rolled medium manganese steel comprises:

[0068] The hot-rolled raw material blank obtains a hot-rolled medium manganese steel blank;

[0069] The hot-rolled medium-manganese steel blank is pre-annealed in an austenite-ferrite two-phase zone, and process conditions are controlled so that the microstructure of the pre-annealed medium-manganese steel blank mainly consists of thin-film austenite and ferrite.

[0070] In implementation, after the hot-rolled medium-manganese steel blank is pre-annealed in an austenite-ferrite two-phase zone, the microstructure of the medium-manganese steel blank mainly consists of thin-film austenite and ferrite, and the austenite and ferrite have different manganese contents, thus forming a manganese-rich austenite region and a manganese-poor ferrite region.

[0071] Applicants have found that in the hot-rolled medium-manganese steel, thin-film austenite can be formed in the ferrite matrix through two-phase zone annealing; this thin-film austenite is usually distributed at the original martensite lath boundaries. This is because austenite tends to nucleate at interfaces, and the special orientation relationship and morphology of the lath martensite make the austenite nucleated between the lath boundaries tend to form thin-film austenite morphology by growing. Thus, pre-annealing the hot-rolled medium-manganese steel blank in an austenite-ferrite two-phase zone is conducive to obtaining a high proportion of thin-film austenite in the medium-manganese steel blank used for cold rolling, and further conducive to obtaining a microstructure mainly consisting of thin-film austenite and ferrite in the medium-manganese steel blank after cold rolling and re-annealing in an austenite-ferrite two-phase zone.

[0072] It should be noted that compared with another common form of austenite in medium-manganese steel, i.e. equiaxed austenite (blocky austenite), thin-film austenite has higher stability due to its special morphology. In addition, compared with medium-manganese steel containing thin-film austenite, medium-manganese steel containing blocky austenite is more likely to form Lueders bands during stretching, which adversely affects the hydrogen embrittlement and fatigue properties of the medium-manganese steel.

[0073] Compared with the prior art, the present application pre-anneals the hot-rolled medium-manganese steel blank in an austenite-ferrite two-phase zone, not only obtaining a medium-manganese steel blank having a manganese-rich austenite region and a manganese-poor ferrite region, but also having austenite mainly in the form of thin-film austenite, which is converted into a manganese-rich martensite region during subsequent cold rolling, which is conducive to obtaining a higher proportion of thin-film austenite by re-annealing in an austenite-ferrite two-phase zone after cold rolling, thus expanding the space for designing the microstructure and properties of medium-manganese steel.

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

[0075] Specifically, the annealing temperature of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing is 600-800℃, for example, it can be 600℃, 605℃, 640℃, 650℃, 698℃, 700℃, 712℃, 725℃, 745℃, 765℃, 798℃ or 800℃.

[0076] It should be noted that the annealing temperature has a significant impact on the microstructure of the medium manganese steel: on the one hand, at a lower annealing temperature (such as 570℃), the microstructure may contain part of the deformed structure, and the recrystallization is not complete; as the temperature increases, the recrystallization degree is higher, the microstructure gradually tends to be equiaxed, the grain size increases, and the equiaxed austenite (massive austenite) tends to be generated, and the proportion of thin film austenite decreases; at a higher annealing temperature (such as 800℃ and above), martensite structure may appear, and the volume fraction of austenite decreases.

[0077] Preferably, the annealing temperature of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing is 600-680℃.

[0078] Further preferably, the annealing temperature of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing before and after cold rolling is 630-660℃.

[0079] It should be noted that the annealing temperature of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing can be the same or different.

[0080] Specifically, the heating rate of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing ranges from 1℃ / s to 300℃ / s, for example, it can be 1℃ / s, 5℃ / s, 40℃ / s, 50℃ / s, 98℃ / s, 100℃ / s, 112℃ / s, 125℃ / s, 145℃ / s, 165℃ / s, 198℃ / s, 200℃ / s, 212℃ / s, 225℃ / s, 232℃ / s, 250℃ / s, 298℃ / s or 300℃ / s.

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

[0082] Specifically, the cooling rate of the austenite-ferrite dual-phase zone pre-annealing and the austenite-ferrite dual-phase zone annealing before and after cold rolling is 5℃ / min-1200℃ / min, for example, it can be 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 12℃ / min, 13℃ / min, 14℃ / min, 15℃ / min, 16℃ / min, 17℃ / min, 18℃ / min, 19℃ / min or 20℃ / 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 sample after cooling. However, too slow cooling rate may lead to the precipitation of cementite during cooling, and too fast cooling rate may result in high residual stress.

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

[0085] It should be noted that, with the extension of annealing time, the volume fraction of austenite increases until it reaches the equilibrium austenite volume fraction. After that, the austenite volume fraction no longer increases, but the grain size of the sample increases, which reduces the stability of austenite and deteriorates the mechanical properties. The growth rate of austenite increases with the increase of annealing temperature. Therefore, it is necessary to select appropriate annealing time according to the annealing temperature.

[0086] Specifically, the microstructure regulation method of the medium-manganese steel, comprising:

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

[0088] Step (2), preparing medium-manganese steel forgings by heating, holding and forging the medium-manganese steel billets;

[0089] Step (3), obtaining hot-rolled plates by heating and holding the medium-manganese steel forgings and then hot rolling;

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

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

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

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

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

[0095] It should also be noted that when the medium manganese steel is heated at 1180℃-1250℃, which belongs to the austenite zone, the billet in step (2) and the forging billet in step (3) are preferably heated for 60-120min, because: when the medium manganese steel is heated in the austenite zone, it is necessary to ensure that the carbides inside the billet are fully dissolved, and the austenite grains are uniformized; If the holding time is too short, it may lead to incomplete austenitization, affecting the subsequent processing performance.

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

[0097] In addition, in the austenite zone, with the extension of holding time, the austenite grains will gradually grow; appropriate holding time can control the grain growth rate, avoid excessively coarse grains, and thus ensure the mechanical properties of the material. The austenite grain growth kinetics of medium manganese steel shows that excessive holding time will lead to grain coarsening, reducing the toughness and strength of the material;

[0098] In addition, medium manganese steel contains a certain amount of alloying elements (such as manganese), which will affect the austenitizing speed and grain growth behavior. Appropriate holding time can ensure the full diffusion and uniform distribution of alloying elements, thereby obtaining ideal organization and performance.

[0099] Specifically, the final rolling temperature of step (3) is not less than 900℃.

[0100] It should be noted that the final rolling temperature should not be too low in order to ensure that the steel has sufficient plasticity and deformation ability during rolling, while avoiding rolling difficulties and uneven organization and performance caused by too low temperature; at the same time, the final rolling temperature has an important influence on the microstructure and performance of the steel. Higher final rolling temperature may lead to grain growth, thereby reducing the strength and toughness of the material; while 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 the medium manganese steel in step (3) belongs to the heating in the austenite region, and the finish rolling temperature should be no less than 900 DEG C.

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

[0103] It should be noted that if the reduction rate of the cold rolling is too low, it can be difficult to meet the final thickness requirement, and if the reduction rate is too high, it can cause the plate to crack. The applicant found that increasing the reduction rate can cause more deformation defects in the cold-rolled sample, thereby causing the recrystallization behavior to occur more easily during the subsequent annealing process, so that the grain size of the annealed sample is reduced and the proportion of blocky austenite is increased. Reasonably selecting the reduction rate can not only optimize the process flow, but also control the microstructure and mechanical properties of the material.

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

[0105] Preferably, the cold rolling in step (5) to step (6) can be repeated multiple times within the operating temperature range to ensure that the reduction rate of each cold rolling is 10% to 80%.

[0106] In implementation, the residual austenite film austenite after annealing in step (4); during the cold rolling in step (5), the metastable austenite (film austenite) of the annealed plate is converted into martensite, but the non-uniform distribution of Mn is retained in the structure of the cold-rolled plate, forming a rich-manganese martensite region and a poor-manganese ferrite region.

[0107] After step (6), the original rich-manganese martensite region is more likely to form metastable austenite (film austenite), thereby forming a dual-phase structure mainly composed of poor-manganese ferrite and rich-manganese film austenite.

[0108] Specifically, the volume ratio of ferrite, austenite and film austenite in the microstructure of the cold-rolled medium manganese steel can be statistically and identified by the area of each region of ferrite, austenite and film austenite by means of the prior art.

[0109] In order to better illustrate the present application, the following embodiments are further provided:

[0110] Embodiment 1

[0111] The present embodiment discloses a microstructure control method of a cold-rolled medium manganese steel, the control method comprising:

[0112] Step 1, smelting molten steel with set composition and casting to obtain billets, the composition 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, heating the billets to 1200℃ and keeping for 2h, and obtaining forged billets through forging;

[0114] Step 3, keeping the forged billets at 1200℃ for 2h, and obtaining hot-rolled plates with a thickness of about 4.2mm through multi-pass hot rolling, and the final rolling temperature of the hot rolling is not lower than 900℃;

[0115] Step 4, pre-annealing the hot-rolled plates in the austenite-ferrite two-phase region at 630℃ for 5h and air cooling to room temperature;

[0116] Step 5, cold rolling the pre-annealed hot-rolled plates to a thickness of about 2.37mm with a reduction of about 44%;

[0117] Step 6, annealing the obtained cold-rolled plates in the austenite-ferrite two-phase region at 630℃ for 2h and air cooling to room temperature.

[0118] The temperature rising rate is 50℃ / s and the temperature falling rate is 10℃ / min in the two austenite-ferrite two-phase region annealing processes of Step 4 and Step 6.

[0119] The embodiment discloses a cold-rolled medium-manganese steel prepared by the above method, which contains Figures 1a-1b It is shown that the red grains are austenite, and the statistical results show that the volume fraction of the austenite is about 20%; the thin film austenite accounts for 30% of the volume fraction of all the austenite.

[0120] Embodiment 2

[0121] The embodiment discloses a microstructure regulation method of a cold-rolled medium-manganese steel,

[0122] Step 1, smelting molten steel with set composition and casting to obtain billets, the composition 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, heating the billets to 1200℃ and keeping for 2h, and obtaining forged billets through forging;

[0124] Step 3, keeping the forged billets at 1200℃ for 2h, and obtaining hot-rolled plates with a thickness of about 4.2mm through multi-pass hot rolling, and the final rolling temperature of the hot rolling is not lower than 900℃;

[0125] Step 4, pre-annealing the hot-rolled plates in the austenite-ferrite two-phase region at 650℃ for 5h and air cooling to room temperature;

[0126] Step 5, the pre-annealed hot-rolled plate is cold-rolled to a thickness of about 2.37 mm, with a reduction of about 44%;

[0127] Step 6, the obtained cold-rolled plate is annealed in the austenite-ferrite two-phase region at 630℃ for 1h and air-cooled to room temperature.

[0128] The heating rate of the austenite-ferrite two-phase region annealing process in steps 4 and 6 is 50℃ / s, and the cooling rate is 10℃ / min.

[0129] The present embodiment discloses a cold-rolled medium-manganese steel prepared by the above method, which comprises Figures 2a-2b It is shown that the red grains are austenite, and the statistical results show that the volume fraction of the austenite is about 18%; the thin film austenite accounts for more than 50% of the volume fraction of all austenite.

[0130] Embodiment 3

[0131] The present embodiment discloses a microstructure regulation method of a cold-rolled medium-manganese steel,

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

[0133] Step 2, heat the billet to 1200℃ and keep for 2h, and then obtain a forged billet by forging;

[0134] Step 3, keep the forged billet at 1200℃ for 2h, and then obtain a hot-rolled plate with a thickness of about 4.2mm by multi-pass hot rolling, and the final rolling temperature of the hot rolling is not lower than 900℃;

[0135] Step 4, perform austenite-ferrite two-phase region pre-annealing treatment on the hot-rolled plate at 660℃ for 5h and air-cool to room temperature;

[0136] Step 5, cold-roll the pre-annealed hot-rolled plate to a thickness of about 2.37 mm, with a reduction of about 44%;

[0137] Step 6, anneal the obtained cold-rolled plate in the austenite-ferrite two-phase region at 630℃ for 5h and air-cool to room temperature.

[0138] The heating rate of the austenite-ferrite two-phase region annealing process in steps 4 and 6 is 50℃ / s, and the cooling rate is 10℃ / min.

[0139] The present embodiment discloses a cold-rolled medium-manganese steel prepared by the above method, which comprises Figures 3a-3bDisplay: red grains are austenite, statistics show that the volume fraction of austenite is about 20%; thin film austenite accounts for about 40% of the volume fraction of all austenite.

[0140] Example 4

[0141] The embodiment discloses a microstructure regulation method of cold-rolled medium manganese steel, and is different from example 1 in that the reduction rate of the cold rolling step in step 5 is 20%, and the rest of the 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 the embodiment is about 18%; and the volume fraction of thin film austenite accounts for about 22% of all austenite.

[0142] Comparative Example 1

[0143] The comparative example discloses a microstructure regulation method of cold-rolled medium manganese steel, and compared with example 1, the hot-rolled plate is subjected to a stress relief annealing treatment at 450 DEG C for 2h and air cooling to room temperature:

[0144] Step 1, smelting molten steel according to the set composition and casting to obtain a billet, the composition contains C 0.18%, Mn 4.95%, Si 0.4%, Mo 0.0021% by weight, and the rest is Fe and other inevitable impurities;

[0145] Step 2, heating the billet to 1200 DEG C and keeping for 2h, and obtaining a forged billet through forging;

[0146] Step 3, keeping the forged billet at 1200 DEG C for 2h, and obtaining a hot-rolled plate with a thickness of about 4.2mm through multi-pass hot rolling, and the final rolling temperature of the hot rolling is not lower than 900 DEG C;

[0147] Step 4, subjecting the hot-rolled plate to a stress relief annealing treatment at 450 DEG C for 2h and air cooling to room temperature, no austenite is formed at this temperature, only the internal stress in the material is eliminated to avoid cracking in the subsequent cold rolling process;

[0148] Step 5, cold rolling the hot-rolled plate to a thickness of about 2.37mm, and the reduction rate is about 44%;

[0149] Step 6, isothermally annealing the cold-rolled plate at 630 DEG C for 5h and air cooling to room temperature;

[0150] The comparative example discloses a cold-rolled medium manganese steel, which is prepared by the above method, Figure 4 Display, the microstructure of the obtained sample is composed of ferrite and blocky austenite, and there is no thin film austenite.

[0151] Comparative Example 2

[0152] The comparative example discloses a microstructure regulation method of cold-rolled medium manganese steel, compared with example 1, the step 4 hot-rolled plate is tempered at 450 DEG C for 2h and air-cooled to room temperature, and is not annealed in the dual-phase region, no austenite is formed at this temperature, only internal stress in the material is eliminated:

[0153] Step 1, smelting molten steel according to the set composition and casting to obtain a billet, the composition contains C 0.18%, Mn 4.95%, Si 0.4%, Mo 0.0021%, the rest is Fe and other inevitable impurities;

[0154] Step 2, heating the billet to 1200 DEG C, keeping for 2h, and obtaining a forged billet through forging;

[0155] Step 3, keeping the forged billet at 1200 DEG C for 2h, and obtaining a hot-rolled plate with a thickness of about 4.2mm through multi-pass hot rolling, and the final rolling temperature of hot rolling is not less than 900 DEG C;

[0156] Step 4, tempering the hot-rolled plate at 450 DEG C for 2h and air-cooling to room temperature, no austenite is formed at this temperature, only internal stress in the material is eliminated, and cracking in the subsequent cold rolling process is avoided;

[0157] Step 5, cold rolling the tempered hot-rolled plate to a thickness of about 2.37mm, and the reduction rate is about 44%;

[0158] Step 6, isothermally annealing the cold-rolled plate at 660 DEG C for 1h, and air-cooling to room temperature;

[0159] The comparative example discloses a cold-rolled medium manganese steel prepared by the above method, Figures 5a-5b It is shown that the microstructure of the obtained sample is composed of ferrite and blocky austenite, and no thin film austenite.

[0160] As known from the above, the volume fraction of austenite in the cold-rolled medium manganese steel prepared in example 1 to example 4 is about 18% to 20%; the volume fraction of thin film austenite in all austenite is greater than 20%, and preferably, the volume fraction of thin film austenite is 30% to 50%; according to common sense, the cold-rolled medium manganese steel prepared in the application has better stability than the conventional cold-rolled medium manganese steel with blocky austenite-ferrite dual-phase structure.

[0161] As known from comparative example 1, comparative example 1 and comparative example 2, by pre-annealing the hot-rolled medium manganese billet in the austenite-ferrite dual-phase region, the microstructure of the annealed medium manganese billet is mainly composed of thin film austenite and ferrite, and then the microstructure of the cold-rolled billet has a manganese-rich martensite region and a manganese-poor ferrite region, and a higher thin film austenite proportion is obtained in the subsequent heat treatment process, so that the thin film austenite accounts for more than 20% of the total volume fraction of austenite in the cold-rolled medium manganese steel, and preferably, 30% to 50%.

[0162] As can be seen from Comparative Example 1 and Example 4, the cold rolling reduction ratio between 10% and 80% makes the total volume fraction of the film austenite in the cold-rolled manganese steel greater than 20%, preferably 30% to 50%.

[0163] The above merely provides the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of the changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method of microstructure control of cold-rolled medium manganese steel, characterized by, The application relates to a cold-rolled medium-manganese steel and a microstructure regulation method thereof. Before cold rolling, austenite-ferrite dual-phase zone preannealing is carried out on the medium-manganese steel blank at 600-800 DEG C for 1-10 hours to obtain a medium-manganese steel blank whose microstructure mainly consists of thin-film austenite and ferrite; the cold-rolled medium-manganese steel has the following chemical component composition in percentage by mass: C 0.1-0.3%, Mn 3-4.95%, Si 0.4-1%, Al 0-1%, Nb 0-0.2%, Mo 0.0021-3.0%, V 0-1.0%, Ti 0-0.5%, Ni 0-5.0%, Cu 0-5.0%, Cr 0-5.0%, and the balance of Fe and inevitable impurities; The preannealed medium-manganese steel blank is subjected to cold rolling treatment, and the cold rolling reduction is 30-44% to obtain a medium-manganese steel blank whose microstructure mainly consists of a manganese-rich martensite region and a manganese-poor ferrite region; After the cold rolling, austenite-ferrite dual-phase zone annealing is carried out on the medium-manganese steel blank at 600-630 DEG C to finally obtain a cold-rolled medium-manganese steel whose thin-film austenite accounts for 25-90% of the total volume fraction of austenite; The austenite-ferrite dual-phase zone annealing has a heating rate of 1-50 DEG C / s, a cooling rate of 5-10 DEG C / min, and a holding time of 1-10 hours.

2. The microstructure controlling method of cold-rolled medium manganese steel according to claim 1, characterized by, The microstructure regulation method of the cold-rolled medium-manganese steel comprises the following steps: Step (1): preparing raw materials according to the designed components, smelting and casting to obtain a medium-manganese steel blank; Step (2): heating, holding and then forging the medium-manganese steel blank to prepare a medium-manganese steel forged blank; Step (3): heating and holding the medium-manganese steel forged blank and then hot rolling to obtain a hot-rolled plate; Step (4): carrying out first annealing on the hot-rolled plate in the austenite-ferrite dual-phase zone and then air cooling to room temperature to obtain a first annealed plate; Step (5): carrying out first cold rolling on the first annealed plate obtained in step (4) to obtain an original cold-rolled plate; Step (6): carrying out second annealing on the original cold-rolled plate in the austenite-ferrite dual-phase zone and then air cooling to room temperature to obtain a second annealed plate whose thin-film austenite accounts for 25-90% of the total volume fraction of austenite.

3. A cold-rolled medium manganese steel, characterized in that, The cold-rolled medium-manganese steel obtained by the microstructure regulation method of the cold-rolled medium-manganese steel in claim 1 or 2 has the following chemical component composition in percentage by mass: C 0.1-0.3%, Mn 3-4.95%, Si 0.4-1%, Al 0-1%, Nb 0-0.2%, Mo 0.0021-3.0%, V 0-1.0%, Ti 0-0.5%, Ni 0-5.0%, Cu 0-5.0%, Cr 0-5.0%, and the balance of Fe and inevitable impurities; the microstructure of the cold-rolled medium-manganese steel mainly consists of ferrite and austenite, and the thin-film austenite accounts for 25-90% of the total volume fraction of austenite.

4. The cold rolled medium manganese steel according to claim 3, characterized in that, The thin-film austenite accounts for 30-90% of the total volume fraction of austenite in the cold-rolled medium-manganese steel.

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