CN114480811A

CN114480811A - High-strength-ductility medium manganese steel with gradient structure and preparation method thereof

Info

Publication number: CN114480811A
Application number: CN202210135100.XA
Authority: CN
Inventors: 刘涛; 张喜亮; 王艳辉; 石银冬; 周骞; 刘宏基; 邢振国
Original assignee: Hebei University of Engineering
Current assignee: Hebei University of Engineering
Priority date: 2022-02-14
Filing date: 2022-02-14
Publication date: 2022-05-13
Anticipated expiration: 2042-02-14
Also published as: CN114480811B

Abstract

The invention relates to a preparation method of high-strength-ductility medium manganese steel with a gradient structure, which comprises the following steps: smelting, forging, hot rolling and two-phase region annealing according to predetermined medium manganese steel alloy components to obtain an annealed plate blank, processing the plate blank into a rod-shaped sample, performing torsion treatment at room temperature, immediately performing two-phase region annealing again after the torsion treatment, and air-cooling to room temperature; the twisting treatment is to process the plate blank annealed by the two-phase region into a bar-shaped sample, and then twist the bar-shaped sample at the speed of 70-120 DEG/min to 90-180 deg. According to the invention, the hot rolling and the two-phase zone annealing are carried out, and then the twisting treatment and the two-phase zone annealing are carried out, so that the ferrite and the austenite in the medium manganese steel finished product present a microstructure with gradient distribution from the surface to the inside, and compared with the traditional medium manganese steel with a homogeneous structure and the same alloy components, the yield strength is greatly improved, and the product of strength and elongation can be improved by more than 60%.

Description

High-strength-ductility medium manganese steel with gradient structure and preparation method thereof

Technical Field

The invention relates to the technical field of advanced high-strength steel plate production, in particular to high-strength medium manganese steel with a gradient structure and a preparation method thereof.

Background

The medium manganese steel (with the manganese content of 4-13%) is taken as a typical representative of the third-generation advanced high-strength steel, has attracted wide attention of scholars and industries at home and abroad, and has great application prospect in the fields of automobile steel, mining machinery, steel for ocean platforms (marine steel) and the like. With the gradual increase of national energy conservation and emission reduction, low carbon and environmental protection, the advanced high-strength steel with high strength and high plasticity becomes the first choice of the steel used in the fields. The medium manganese steel generally consists of austenite and ferrite two-phase structures, and the high-strength high-plasticity excellent performance (the product of strength and elongation can reach 30 GPa.) is obtained by the transformation induced plasticity (TRIP) effect in the deformation process. However, the yield strength of the current medium manganese steel is low, generally between 500-650MPa, and it is difficult to meet the current demand of the above fields for high quality steel, for example, the yield strength of high-end marine steel must reach 690MPa or higher. For another example, in order to ensure safety of passengers and light weight of automobiles, development of automobile steel sheets with higher yield strength is a goal pursued by current automobile steels. At present, the main methods for improving the yield strength of the medium manganese steel comprise precipitation strengthening or adding a martensite and other strong phase structures in a matrix. However, the above technique will cause the plasticity of the steel to be greatly reduced, and further the product of strength and elongation thereof is insufficient, and it is difficult to adapt to the actual service environment. Therefore, how to further improve the strength of medium manganese steel without significantly reducing its plasticity has become a great challenge in this field.

Disclosure of Invention

Technical problem to be solved

In view of the above disadvantages and deficiencies of the prior art, the present invention provides a method for preparing high-strength-ductility medium manganese steel with a gradient structure, which utilizes a micro-alloying technology in combination with torsional shear deformation (shear strain generation) and a critical annealing process to obtain a novel microstructure characteristic that austenite and ferrite are distributed in a gradient manner in a matrix in the conventional medium manganese steel, so that the medium manganese steel obtains high strength and high ductility, the yield strength of the prepared medium manganese steel is above 740MPa and the product of strength and ductility is above 45 GPa%, thereby solving the technical problem that the yield strength of the medium manganese steel is high but the product of strength and ductility is insufficient due to precipitation strengthening or increasing of a strong phase structure such as martensite at present.

(II) technical scheme

In order to achieve the purpose, the invention adopts the main technical scheme that:

a preparation method of high-strength-ductility medium manganese steel with a gradient structure comprises the following steps:

smelting, forging, hot rolling and two-phase region annealing according to predetermined medium manganese steel alloy components to obtain an annealed plate blank, processing the plate blank into a rod-shaped sample, performing torsion treatment at room temperature, immediately performing two-phase region annealing again after the torsion treatment, and air-cooling to room temperature; the twisting treatment is to process the plate blank annealed in the two-phase region into a rod-shaped sample, and then twist the rod-shaped sample at the speed of 70-120 DEG/min to 90-180 deg.

Wherein, before the twisting treatment, the working procedures of smelting, forging, hot rolling, two-phase region annealing and the like can be carried out according to the traditional/conventional preparation process of the medium manganese steel.

In the present invention, as a preferred embodiment of the present invention, the total angle of twist is 90 to 180 °. The torsion angle is too small, so that the shear stress or strain of the steel bar matrix is insufficient, the austenite phase variable is small (the martensite content is insufficient), and an effective gradient structure cannot be generated; on the contrary, the torsion angle is too large, the shear strain is too large, a large number of defects (particularly surface regions) are formed in the matrix, such as crack initiation and the like, and the material performance is greatly reduced.

According to a preferred embodiment of the present invention, the two-phase annealing conditions before the twisting process are as follows: keeping the temperature of the plate blank at 650-700 ℃ for 1-2h, and then cooling the plate blank to room temperature in air; the two-phase region annealing conditions after the twisting treatment are as follows: the slab is kept at 650-700 ℃ for 5-20min and then cooled to room temperature by air.

According to a preferred embodiment of the present invention, the preparation method comprises the steps of:

s1, smelting, forging and hot rolling: preparing smelting raw materials according to the alloy components of the medium manganese steel, carrying out vacuum smelting to prepare a steel ingot, heating the steel ingot to 1200 ℃, preserving heat for 1.5-3h, forging the steel ingot into a plate blank with the thickness of 30-50mm, heating the plate blank to 1200 ℃, preserving heat for 1-2h, then rolling the plate blank into a plate blank with the thickness of 5-15mm through multiple passes, wherein the final rolling temperature is not lower than 900 ℃, and obtaining a hot rolled plate blank;

s2, primary two-phase region annealing: annealing the hot-rolled plate blank at 650-700 ℃ for 1-2h, and then air-cooling to room temperature to obtain a hot-rolled steel plate;

s3, room temperature torsion and secondary two-phase region annealing: processing the hot rolled steel plate into a bar-shaped test sample, twisting the bar-shaped test sample at the speed of 70-120 DEG/min for 90-180 DEG, then keeping the temperature at 650-700 ℃ for 5-20min, and then cooling the bar-shaped test sample to room temperature in air.

The medium manganese steel needs homogenization treatment before forging or hot rolling, and the conventional temperature of the homogenization treatment is 1200 ℃, so as to ensure that the steel billet does not crack in the forging and hot rolling processes of the medium manganese steel. The homogenization treatment temperature is generally 1200 ℃, the prior austenite grain size is too coarse when the temperature is too high, the strength is insufficient, and the homogenization degree is not enough when the temperature is too low. The temperature range of the two-phase region is directly related to the alloy composition of the medium manganese steel. The time of primary two-phase region annealing and heat preservation is 60-120 min; if the temperature is too high, the grain size may be large, recrystallization may be too sufficient, and the strength may be insufficient.

The secondary two-phase region annealing heat preservation time is only 5-20min (shorter than the primary annealing heat preservation time), because the hot-rolled and annealed medium manganese steel has a large number of dislocation and other defects in a matrix after being twisted, the power of medium manganese steel recrystallization and austenite reverse phase transformation is sufficient in the secondary annealing process, and the process can be completed in a short time, so the secondary annealing time is not suitable to be overlong. According to the experimental process, the strength of the medium manganese steel is reduced and the plasticity is not improved after the secondary annealing time exceeds 20 min.

According to the preferred embodiment of the invention, the alloy components of the medium manganese steel are as follows by mass percent: c: 0.15-0.3%, Mn 6-10%, Al 1.5-3%, Ni 1.5-3%, Ce: 0.04-0.08%, and the balance of Fe.

In the alloy components of the medium manganese steel, a certain content of C can ensure that the content of austenite in the steel is increased and the stability of the austenite is improved, and the fault energy of the medium manganese steel can be adjusted; increasing Al and Ni with mass fraction of 1-3% can generate NiAl precipitation so as to increase precipitation strengthening effect; meanwhile, the content ratio of austenite can be increased by a proper amount of Ni. In addition, the addition of Al and Ni can regulate the stacking fault energy of the medium manganese steel and improve the stability of austenite. Rare earth Ce is added for purifying the matrix, so that the grain size of the medium manganese steel is refined, and the stability of austenite is improved. The high austenite stability is beneficial to improving the martensite gradient rate of the round bar from the surface to the inside in the twisting process. In the secondary annealing process, the reverse transformation of austenite from the surface to the inside of the round bar and the recrystallization power of ferrite are obviously different, so that an obvious austenite and ferrite phase distribution gradient is generated.

It should be noted that the alloy components of the medium manganese steel are not particularly limited, as long as the austenite content in the hot-rolled annealed steel billet is greater than 40% and the austenite has higher stability, and the medium manganese steel with the high-strength and high-plasticity gradient structure can be obtained through the treatment of hot rolling, primary two-phase region annealing, twisting and secondary two-phase region annealing. The above-mentioned medium manganese steel alloy composition can satisfy the requirements of "the austenite content in the annealed steel billet matrix is greater than 40% and the austenite has high stability".

The invention mainly produces austenite and ferrite two-phase structure by annealing after hot rolling billet, then uses torsion to make round bar produce torsion deformation, because of gradient shear strain, the steel bar has larger gradient from surface to inner shear strain, so that different parts of the steel bar produce martensite phase transformation with different degrees, thereby producing defects of different dislocation density, etc. In the process of annealing the two-phase region again after twisting, the reverse austenite transformation power of the round bar is different from the surface to the inside due to the fact that the round bar has different contents of martensite and defect density (austenite nucleation and growth difference), in addition, the recrystallization power is also different in the ferrite annealing process, the microstructure characteristic of gradient distribution of austenite and ferrite from the surface to the inside is finally formed, the continuous TRIP effect and the strain gradient strengthening effect are synergistic in the deformation process, and compared with the manganese steel prepared by the traditional preparation method and having the same components, the yield strength of the medium manganese steel is remarkably improved, and meanwhile, the plasticity is synchronously and greatly improved.

The invention provides high-strength-ductility medium manganese steel with a gradient structure, which is prepared by adopting the scheme.

The yield strength of the medium manganese steel with the gradient structure prepared by the invention is more than 740MPa, the tensile strength is more than 1110MPa, and the elongation is as follows: 38-47%, the product of strength and elongation is more than 42GPa, and the highest product of strength and elongation reaches 52 GPa.

(III) advantageous effects

The invention has the beneficial effects that:

the gradient structure high-strength-ductility medium manganese steel has the following characteristics: (1) the preparation method is simple and feasible, the required equipment is conventional equipment, the process is relatively simple, the period is short, and the preparation method of the manganese steel in the gradient structure is suitable for most of the traditional manganese steel and most of steel types with TRIP effect. The invention can realize the microstructure with gradient structure distribution in the steel product and obviously improve the strength and the plasticity by the process steps of hot rolling, annealing in a two-phase region, and then twisting treatment and annealing in the two-phase region. (2) The prepared medium manganese steel presents a microstructure with gradient distribution from the surface to the inside of ferrite and austenite, the continuous TRIP effect and the strain gradient strengthening effect have synergistic effect in the deformation process, and compared with the traditional medium manganese steel with a homogeneous structure and the same alloy components, the yield strength of the medium manganese steel is greatly improved, the product of strength and ductility can be simultaneously improved by more than 60%, and the synchronous and large improvement of the strength and the plasticity is realized. (3) The mechanical property of the manganese TRIP steel in the gradient structure meets the requirements that the tensile strength is more than 1100MPa, the yield strength is more than 740MPa, the elongation is 38-47%, and the maximum product of strength and elongation can reach 52 GPa.

Drawings

FIG. 1 is an EBSD phase diagram of different parts of manganese steel with gradient structure and high product of strength and elongation in example 1 of the present invention.

FIG. 2 is a graph showing the hardness distribution of a high-strength-product medium manganese steel having a gradient structure from the center to the surface in example 1 of the present invention.

FIG. 3 is the engineering stress-strain curves of the high product of strength and elongation medium manganese steel with gradient structure of example 1 of the present invention and the comparative example.

FIG. 4 is the engineering stress-strain curve of the high product of strength and elongation medium manganese steel with gradient structure of example 2 of the present invention and the comparative example.

FIG. 5 is the engineering stress-strain curve of the high product of strength and elongation medium manganese steel with gradient structure of example 3 of the present invention and the comparative example.

Detailed Description

For the purpose of better explaining the present invention and to facilitate understanding, the present invention will be described in detail by way of specific embodiments with reference to the accompanying drawings.

When the grain size, dislocation, phase composition, element content, texture and the like in the steel substrate are in continuous gradient distribution in the material, the yield strength-plasticity inversion relation of the metal material can be effectively overcome. At present, gradient structures are reported in copper, magnesium alloy, IF steel, TWIP steel, high-entropy alloy and the like, but are not found in medium manganese steel. Based on the essence that martensite is induced by medium manganese steel deformation and a large amount of dislocation is generated, the gradient distribution of shear strain is generated from the surface to the inside of a matrix by utilizing a twisting process, the gradient distribution characteristic of the density of the phase-change martensite and the dislocation is further realized, and the microstructure characteristic that reverse transformation austenite and ferrite are continuously and gradiently distributed from the surface to the inside of the matrix is successfully realized by matching with the subsequent two-phase zone annealing technology, so that the medium manganese steel with both high yield strength and high strength-elongation product is prepared.

The idea or design idea of the invention is as follows: the manganese steel with the TRIP effect realizes torsional deformation by utilizing a torsion mode, and due to the existence of gradient shear strain, the steel bar has larger gradient from the surface to the inside of the steel bar, so that different parts of the steel bar are subjected to martensite phase transformation with different degrees, and the defects of different dislocation densities and the like are generated. After twisting, two-phase annealing treatment is carried out again, the austenite reverse transformation dynamics of the martensite at different parts are different (austenite nucleation and growth difference), and in addition, because of different density of matrix defects, the ferrite recrystallization dynamics are also different, and finally the microstructure characteristics of austenite and ferrite phase gradient distribution from the front to the back are formed.

Example 1

The embodiment provides a gradient-structure high-strength-ductility medium manganese steel which comprises the following chemical components in percentage by weight: c: 0.2%, Mn 8%, Al 2%, Ni 2%, Ce: 0.05%, P < 0.008%, S < 0.008%, and the balance Fe. The preparation method of the gradient structure high-strength-ductility medium manganese steel comprises the following steps:

(1) smelting, forging and hot rolling: and (3) vacuum smelting according to alloy components to prepare a steel ingot, keeping the temperature of the steel ingot at 1200 ℃ for 2h, forging the steel ingot into a plate blank with the thickness of 40mm, and cooling the plate blank to room temperature in air. Then heating the plate blank to 1200 ℃, preserving the heat for 2 hours, rolling the plate blank into a plate blank with the thickness of 15mm for 3 times, wherein the final rolling temperature is not lower than 900 ℃;

(2) and (3) annealing the two-phase region: annealing the hot-rolled plate blank at 680 ℃ for 1h, and then air-cooling to room temperature;

(3) room temperature torsion + two-phase region annealing: the hot rolled steel plate is processed into a bar-shaped sample, twisted 180 degrees at the speed of 90 degrees/min, and then kept at 680 degrees for 10min and cooled to room temperature in air.

The microstructure (EBSD phase diagram) of the manganese steel in the gradient structure prepared in this example from the center to different portions of the surface is shown in fig. 1, where gray is austenite, black is ferrite, the austenite content in the center is 38.5%, the austenite content in the middle (around the center) is 64%, and the austenite content in the surface is 73.7%. The hardness of the manganese steel in the gradient structure is gradually increased from the center to the surface (as shown in figure 2), the engineering stress-strain curve of the manganese steel in the gradient structure is shown in figure 3, the tensile strength is 1119MPa, the yield strength is 771MPa, the elongation is 46.2%, and the product of strength and elongation is as high as 52 GPa%.

Comparative example

The alloy composition of the manganese steel of this comparative example was the same as that of examples 1 and 3, but the preparation method was different. Mainly, on the basis of the embodiments 1 and 3, the twisting and the subsequent two-phase region annealing treatment are not carried out, and the other steps including the steps of smelting, homogenizing treatment, forging, hot rolling and two-phase region annealing are the same as the embodiments 1 and 3. And finally, preparing the manganese steel in the homogeneous structure.

The engineering stress-strain curve of the manganese steel in the homogeneous structure of the comparative example is shown in FIG. 5, the tensile strength is 1249MPa, the yield strength is 545MPa, the total elongation is 23%, and the product of strength and elongation is 28.7 GPa. The yield strength and the product of strength and elongation of the alloy are obviously lower than those of the manganese steel in the gradient structure.

As can be seen by comparison, the yield strength of the medium manganese steel with the gradient structure in the example 1 is improved by 41.46%, the total elongation is improved by 100%, and the product of strength and elongation is improved by 81.1% compared with the medium manganese steel with the homogeneous structure in the comparative example.

Example 2

The embodiment provides a gradient structure high-strength-ductility medium manganese steel, which comprises the following alloy components in percentage by weight: c: 0.2%, Mn 6%, Al 1.5%, Ni 1.5%, Ce: 0.05%, P < 0.008%, S < 0.008%, and the balance Fe. The preparation method of the gradient structure high-strength-ductility medium manganese steel comprises the following steps:

(1) smelting, forging and hot rolling: and (3) vacuum smelting according to alloy components to prepare a steel ingot, forging the steel ingot into a plate blank with the thickness of 50mm after the steel ingot is subjected to heat preservation at 1200 ℃ for 2 hours, and cooling in air to room temperature. Then heating the plate blank to 1200 ℃, preserving heat for 2 hours, rolling the plate blank into a plate blank with the thickness of 15mm for 4 times, wherein the final rolling temperature is not lower than 900 ℃;

(2) and (3) annealing the two-phase region: annealing the hot-rolled plate blank at 700 ℃ for 1h, and then air-cooling to room temperature;

(3) room temperature torsion + two-phase region annealing: the hot rolled steel plate is processed into a bar-shaped sample, twisted 180 degrees at the speed of 120 degrees/min, and then kept at 700 degrees for 10min and then cooled to room temperature by air.

The engineering stress-strain curve of the manganese steel in the gradient structure of the embodiment is shown in fig. 4, the tensile strength is 1141MPa, the yield strength is 752MPa, the elongation is 41.1%, and the product of strength and elongation is as high as 47 GPa.

As can be seen by comparison, the yield strength of the medium manganese steel with the gradient structure in the example 2 is improved by 38%, the total elongation is improved by 48.7%, and the product of strength and elongation is improved by 63.76% compared with the medium manganese steel with the homogeneous structure in the comparative example.

Example 3

This example provides a gradient structure high-product-strength medium manganese steel, the alloy composition of which is the same as that of example 1, and the preparation method is the same as that of example 1 in steps (1) to (2), except that step (3) is: room temperature torsion + two-phase region annealing: the hot-rolled annealed steel plate is processed into a bar-shaped sample, twisted by 180 degrees at the speed of 90 degrees/min, and then kept at 700 degrees for 5min and cooled to room temperature in air.

The engineering stress-strain curve of the manganese steel in the gradient structure of the embodiment is shown in fig. 5, the tensile strength is 1101MPa, the yield strength is 782MPa, the elongation is 38.8%, and the product of strength and elongation is as high as 43 GPa.

As can be seen by comparison, the yield strength of the medium manganese steel with the gradient structure in the embodiment 3 is improved by 43.48 percent, the total elongation is improved by 68.7 percent, and the product of strength and elongation is improved by 49.83 percent compared with the medium manganese steel with the homogeneous structure in the comparative example.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. The preparation method of the high-strength-product medium manganese steel with the gradient structure is characterized by comprising the following steps of: smelting, forging, hot rolling and two-phase region annealing according to predetermined medium manganese steel alloy components to obtain an annealed plate blank, processing the plate blank into a rod-shaped sample, performing torsion treatment at room temperature, immediately performing two-phase region annealing again after the torsion treatment, and air-cooling to room temperature; the twisting treatment is to process the plate blank annealed in the two-phase region into a rod-shaped sample, and then twist the rod-shaped sample at the speed of 70-120 DEG/min to 90-180 deg.

2. The method according to claim 1, wherein the two-phase region annealing conditions before the twisting treatment are: keeping the temperature of the plate blank at 650-700 ℃ for 1-3h, and then cooling the plate blank to room temperature in air; the two-phase region annealing conditions after the twisting treatment are as follows: the slab is kept at 650-700 ℃ for 5-20min and then cooled to room temperature by air.

3. The method of claim 1, comprising the steps of:

4. The method according to claim 1, wherein the rod specimen has an austenite content of > 40%.

5. The preparation method according to claim 1 or 4, wherein the medium manganese steel comprises the following alloy components in percentage by mass: c: 0.15-0.3%, Mn 6-10%, Al 1.5-3%, Ni 1.5-3%, Ce: 0.04-0.08%, and the balance of Fe.

6. High-product-strength-and-elongation medium manganese steel with a gradient structure, which is prepared by the preparation method of any one of claims 1 to 5.

7. The high-product-of-strength-and-elongation medium manganese steel with a gradient structure according to claim 6, characterized by a yield strength >740MPa, a tensile strength >1110MPa, an elongation: 38-47%, the product of strength and elongation is more than 42GPa, and the highest product of strength and elongation reaches 52 GPa.