CN112195402B

CN112195402B - Precipitation-strengthened high-strength and high-toughness medium manganese steel plate and preparation method thereof

Info

Publication number: CN112195402B
Application number: CN202011041199.4A
Authority: CN
Inventors: 邹英; 韩赟; 朱国森; 滕华湘; 邱木生; 阳锋; 刘华赛; 谢春乾; 姜英花; 李飞; 章军
Original assignee: Shougang Group Co Ltd
Current assignee: Shougang Group Co Ltd
Priority date: 2020-09-28
Filing date: 2020-09-28
Publication date: 2022-08-05
Anticipated expiration: 2040-09-28
Also published as: CN112195402A

Abstract

The invention discloses a precipitation strengthening type high-strength and high-toughness medium manganese steel plate which comprises the following chemical components in percentage by mass: c: 0.04-0.10%, Si: 0.2-0.6%, Mn: 4.0% -6.0%, Al: 0.4% -1.0%, Nb: 0.02% -0.08%, Cu: 1.0% -2.5%, Ni: 1.5% -2.5%, B: 0.001-0.004%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, and the balance is Fe and inevitable impurities. The invention also discloses a preparation method: smelting and forging the chemical components to obtain a steel billet; rolling the billet to obtain a hot rolled plate; sequentially annealing and tempering the hot rolled plate to obtain the hot rolled plate; the annealing is carried out at the temperature of 610-650 ℃ for 1-2 h; the tempering is carried out at the temperature of 450-550 ℃ for 1.5-2.5 h. The steel plate has high strength and strong impact toughness.

Description

Precipitation-strengthened high-strength and high-toughness medium manganese steel plate and preparation method thereof

Technical Field

The invention relates to the technical field of steel preparation, in particular to a precipitation-strengthened high-strength and high-toughness medium manganese steel plate and a preparation method thereof.

Background

The medium plate is a key structural material of large engineering equipment such as engineering machinery, pressure vessels, ocean platforms and the like, and plays a significant role in the fields of basic manufacturing and engineering equipment. At present, China makes great progress in the development of medium plate varieties, but high-quality medium plates used in certain special fields, such as steel for engineering machinery with high formability of over 960MPa and steel for low-cost high-strength naval vessel pressure-resistant shells, still have defects in the research and development aspects. In particular, China has not made a breakthrough in the development of high-quality medium plates integrating strength and plastic toughness. In recent years, medium manganese steels having excellent strong plasticity have received much attention. The medium manganese steel contains a certain amount of retained austenite, can form a Transformation-induced plasticity (TRIP) effect in the deformation process, and can greatly improve the tensile strength and the low-temperature toughness while improving the plasticity. However, as a metastable phase, austenite stabilization strongly depends on element enrichment such as C and Mn, and a medium manganese steel generally requires a long-time annealing treatment in consideration of a low diffusion rate of Mn atoms. With C, Mn element distribution and dislocation recovery, a large amount of ferrite soft phase structure is generated in the annealing process, and the yield strength is obviously reduced. The yield strength can be improved to a certain extent by introducing Cu-rich equal precipitated particles into the medium manganese steel subjected to simple tempering treatment, but the effect is not obvious. This is because low temperature tempering can produce significant strengthening effect, but the content of retained austenite is low, and the plastic toughening effect is limited; while high-temperature tempering can improve the content of residual austenite and further improve the ductility and toughness, the growth, coarsening and reduction of the precipitation density of the Cu-rich phase can greatly reduce the strengthening effect. Although the intermediate tempering treatment can reconcile the contradiction to a certain degree, obviously, the strengthening effect of the Cu-rich phase and the toughening effect of the residual austenite cannot be optimally utilized.

Therefore, how to prepare the medium manganese steel with high strength, strong impact toughness and good plasticity becomes a technical problem to be solved urgently.

Disclosure of Invention

The invention aims to provide a precipitation strengthening type high-strength and high-toughness medium manganese steel plate and a preparation method thereof, wherein the precipitation strengthening type high-strength and high-toughness medium manganese steel plate has high strength, namely has yield strength of more than 960MPa and tensile strength of more than 1000 MPa; strong impact toughness-with a room temperature impact energy of more than 120J; good plasticity-with elongation after fracture greater than 25%, product of strength and elongation exceeding 25 GPa%.

In order to achieve the above object, the present invention provides a precipitation-strengthened high-toughness medium manganese steel sheet, which comprises the following chemical components in mass fraction: c: 0.04-0.10%, Si: 0.2-0.6%, Mn: 4.0-6.0%, Al: 0.4 to 1.0%, Nb: 0.02 to 0.08%, Cu: 1.0-2.5%, Ni: 1.5-2.5%, B: 0.001-0.004%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, and the balance is Fe and inevitable impurities.

Further, the internal microstructure of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate comprises: ultra-fine grained ferrite, retained austenite, tempered martensite and nano precipitated phases.

Furthermore, the internal microstructure of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate comprises the following components in percentage by volume: 45 to 70 percent of ultra-fine grained ferrite, 20 to 40 percent of residual austenite, 5 to 20 percent of tempered martensite and less than 1 percent of nano precipitated phase.

Furthermore, the thickness of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate is 6-20 mm.

The invention also provides a preparation method of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate, which comprises the following steps:

smelting and forging the chemical components of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate to obtain a steel billet;

rolling the billet to obtain a hot rolled plate;

sequentially annealing and tempering the hot rolled plate to obtain a precipitation-strengthened high-strength and high-toughness medium manganese steel plate; wherein the annealing temperature is 610-650 ℃, and the soaking time of the annealing is 1-2 h; the tempering temperature is 450-550 ℃, and the tempering soaking time is 1.5-2.5 h.

Further, the smelting temperature is 1620-1640 ℃.

Further, the rolling the billet to obtain a hot-rolled plate includes:

heating the steel billet to 1150-1250 ℃, and preserving heat for 2-3 h to obtain a preheated steel billet;

rolling the preheated billet to obtain a rolled steel plate, wherein the initial rolling temperature is 1050-1100 ℃, the final rolling temperature is more than 950 ℃, and the accumulated reduction rate is 80-94%;

and quenching the rolled steel plate, and then cooling to room temperature to obtain a hot rolled plate.

Further, the quenching the rolled steel plate and then cooling to room temperature to obtain a hot rolled plate comprises:

and quenching the rolled steel plate, and then cooling to room temperature at an average cooling speed of more than or equal to 50 ℃/s to obtain a hot rolled plate.

Furthermore, the rolling passes are 5 to 11.

Further, the method for sequentially annealing and tempering the hot rolled plate to obtain the precipitation strengthening type high-strength and high-toughness medium manganese steel plate comprises the following steps:

heating the hot rolled plate from room temperature to 610-650 ℃ at a heating speed of 1-5 ℃/s, preserving heat for 1-2 h, and then air-cooling to room temperature to obtain an annealed steel plate;

heating the annealed steel plate from room temperature to 450-550 ℃ at a heating speed of 1-5 ℃/s, preserving the heat for 1.5-2.5 h, and then air-cooling to room temperature to obtain the precipitation-strengthened high-strength and high-toughness medium manganese steel plate.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

the precipitation-strengthened high-strength and high-toughness medium manganese steel plate prepared finally has yield strength larger than 960MPa, tensile strength larger than 1000MPa, elongation after fracture larger than 25%, product of strength and elongation larger than 25GPa%, and room-temperature impact energy larger than 120J. Compared with microalloyed medium plate in the same level, the steel plate has high hardenability, so the uniformity of the structure and the mechanical property in the plate thickness direction is good, and the introduction of the residual austenite obviously improves the elongation after fracture.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.

FIG. 1 is a schematic diagram of the rolling and heat treatment process of a precipitation-strengthened high-toughness medium manganese steel plate;

FIG. 2 is a morphology diagram of the retained austenite of the precipitation-strengthened high-toughness medium manganese steel plate prepared in example 2 of the present invention: (a) TEM bright field image; (b) TEM dark field image; (c) a selected area electron diffraction pattern;

FIG. 3 is a morphology chart of a Cu-rich precipitated phase on a ferrite substrate of the precipitation-strengthened high-toughness medium manganese steel plate prepared in example 2 of the invention;

FIG. 4 is a room temperature tensile stress-strain curve diagram of the precipitation-strengthened high-toughness medium manganese steel plate prepared in example 2 of the present invention.

Detailed Description

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are for the purpose of illustrating the invention and are not to be construed as limiting the invention.

Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be obtained by an existing method.

The technical scheme provided by the embodiment of the invention is to provide a precipitation-strengthened high-strength and high-toughness medium manganese steel plate, and the general idea is as follows:

in order to achieve the above object, an embodiment of the present invention provides a precipitation-strengthened high-toughness medium manganese steel sheet, which comprises the following chemical components in mass fraction: c: 0.04-0.10%, Si: 0.2-0.6%, Mn: 4.0-6.0%, Al: 0.4 to 1.0%, Nb: 0.02 to 0.08%, Cu: 1.0-2.5%, Ni: 1.5-2.5%, B: 0.001-0.004%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, and the balance is Fe and inevitable impurities.

The precipitation strengthening type high-strength and high-toughness medium manganese steel plate with the chemical components is formed by optimizing the composition elements, and is based on the following principle:

the control principle in the chemical composition design of the invention is as follows:

c: 0.04-0.10%, wherein C is a strong austenite stabilizing element and can be enriched to austenite in the annealing process of the medium manganese steel two-phase region, so that the residual austenite content at room temperature is increased; meanwhile, C is a solid solution strengthening element and can improve the hardness and strength of a matrix structure. If the content of C exceeds 0.10 percent, the impact toughness and the welding performance of the medium manganese steel are seriously damaged; if the C content is less than 0.04%, it is not favorable for stabilizing the retained austenite.

Si: 0.2 to 0.6 percent of the total weight of the steel, and Si is a solid solution strengthening element, so that the strength of the medium manganese steel matrix structure can be improved. Meanwhile, Si is insoluble in cementite, so that the decomposition of residual austenite in the tempering process can be effectively inhibited, and the method is very important for retaining austenite at room temperature. If the Si content exceeds 0.6%, the impact toughness is reduced to a certain extent; if the Si content is less than 0.2%, the decomposition of the residual austenite during tempering cannot be effectively suppressed.

Mn: 4 to 6 percent of Mn is an important alloy element in the medium manganese steel and is also a solid solution strengthening element, which is beneficial to ensuring the strength of the medium manganese steel matrix structure. Meanwhile, Mn is an important retained austenite stabilizing element in low-carbon medium-manganese steel and is important for retaining the retained austenite. If the Mn content exceeds 6%, the medium manganese steel matrix has high structural strength and is unfavorable for impact toughness; if the Mn content is less than 4%, it is not favorable to obtain sufficient content and stability of the retained austenite.

Al: 0.4 to 1.0 percent of Al, and can effectively inhibit the decomposition of residual austenite and the precipitation of carbide. If the Al content exceeds 1 percent, the continuous casting nozzle is easy to block; if the Al content is less than 0.4%, it is not preferable to improve the tempering stability of the retained austenite.

Nb: 0.02% -0.08%, and Nb is added into the steel to refine austenite grains of the manganese steel in hot rolling and improve the strength by forming carbide. If the Nb content exceeds 0.08%, the effective C content in the steel is excessively consumed, which is not favorable for stabilizing austenite; if the Nb content is less than 0.02%, the regulating and controlling effects on the microstructure and the mechanical properties are obviously weakened.

Cu: 1.0% -2.5%, adding Cu into the medium manganese steel, and obviously improving the yield strength by utilizing a Cu-rich precipitated phase; in addition, Cu can also improve hardenability. If the Cu content exceeds 2.5 percent, the alloy cost is increased, and excessive precipitation of a Cu-rich phase is not beneficial to improving the impact toughness; if the Cu content is less than 1%, the precipitation strengthening effect is obviously reduced, and the yield strength of more than 960MPa cannot be ensured.

Ni: 1.5 to 2.5 percent, and the Ni is added into the steel, so that the hot brittleness phenomenon of the Cu-containing steel can be avoided; in addition, Ni and Al are combined to form a NiAl precipitated phase with a B2 structure, and the NiAl precipitated phase and the Cu-rich phase can form composite precipitation with a core-shell structure, so that coarsening of the nano Cu-rich phase is inhibited. If the Ni content exceeds 2.5 percent, the alloy cost is obviously increased; if the Ni content is less than 1.5%, a sufficient amount of NiAl precipitates cannot be formed.

B: 0.001% -0.004%, the hardenability can be improved, and if the content of B exceeds 0.004%, the alloy cost is obviously improved; if the B content is less than 0.001%, the effect of improving hardenability is not obvious.

P is less than or equal to 0.005 percent, and P can improve the strength of the steel plate properly, but is easy to be segregated in grain boundaries to deteriorate plasticity, so the content of P cannot exceed 0.005 percent.

S is less than or equal to 0.003 percent, S is easily combined with Mn to form coarse MnS inclusions, the forming performance of punching processing and the like of the steel plate is deteriorated, and the S content is required to be controlled to be less than 0.003 percent in order to avoid cost increase caused by excessive S removal.

Preferably, the internal microstructure of the precipitation strengthening type high-toughness medium manganese steel plate comprises: ultra-fine grained ferrite, retained austenite, tempered martensite and nano precipitated phases.

The retained austenite and tempered martensite: the hot-rolled martensite structure is annealed for full austenite reverse transformation, and after cooling, a large amount of room-temperature retained austenite with moderate stability can be obtained, and a certain amount of tempered martensite transformed from unstable reverse austenite is generated.

The nanometer precipitated phase: tempering treatment is carried out for 1.5 h-2.5 h at 450 ℃ -550 ℃, so as to form a high-density nano Cu-rich precipitated phase on the tempered martensite matrix.

The plasticity and toughness of the medium manganese steel are obviously improved by the TRIP effect of a large amount of residual austenite with moderate stability, the yield strength of the high-density nano Cu-rich precipitated phase is greatly improved by the interaction with dislocation, and the balance of high yield strength, high plasticity and excellent toughness of the medium manganese steel is realized.

Preferably, the internal microstructure of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate comprises the following components in percentage by volume: 45 to 70 percent of ultra-fine grained ferrite, 20 to 40 percent of residual austenite, 5 to 20 percent of tempered martensite and less than 1 percent of nano precipitated phase.

The reason why the volume fraction of the ferrite is controlled to be 45-70% in the invention is as follows: ferrite is formed by diffusion of alloying elements such as the martensite phase C, Mn into the reversed austenite during the two-phase annealing. Ferrite is a matrix structure of the medium manganese steel, has low hardness and strength, and is essential for ensuring good plasticity and toughness. When the medium manganese steel is annealed, only ferrite and reverse transformation austenite exist in the two-phase region, so the content and the stability of the reverse transformation austenite in the two-phase region can be adjusted to a certain degree by adjusting the proportion of the ferrite in the medium manganese steel. If the ferrite content exceeds 70%, the tensile strength of more than 1000MPa cannot be ensured; if the content of the martensite is less than 45%, the content of reverse transformation austenite in the two-phase region is too high, the stability is greatly reduced, a large amount of martensite is generated in the cooling process, and the plasticity and the toughness are obviously reduced.

The reason why the volume fraction of the retained austenite is controlled to be 20-40% in the invention is as follows: retained austenite is an important constituent phase in medium manganese steel to regulate plasticity and toughness. In the present invention, the content of the retained austenite is at least 20% or more in order to ensure that the deformation process has enough retained austenite to generate TRIP effect to improve plasticity. However, when the content of the retained austenite exceeds 40%, the average C, Mn enrichment therein is significantly reduced, resulting in insufficient stability, easy transformation into martensite at the early stage of the subsequent cooling process and tensile deformation, and rather failure to provide a sustained TRIP effect.

The reason why the volume fraction of the tempered martensite is controlled to be 5-20% in the invention is as follows: martensite is formed in the quench cooling stage after the two-phase zone annealing. In the invention, if the martensite content exceeds 20 percent, the stability of the reverse transformation austenite in the two-phase region is seriously insufficient, and obvious phase transformation occurs in the cooling process, which is not beneficial to the plasticity and toughness of the medium manganese steel; if the martensite content is less than 5%, it is not possible to ensure that a sufficient amount of the nano Cu-rich phase is precipitated in the martensite during tempering, and thus the yield strength cannot be further increased.

The reason why the nano precipitated phase is controlled to be less than 1 percent is as follows: the formation of the nano precipitated phase needs a certain time, and for the alloy composition of the invention, if the nano Cu-rich phase with volume fraction of more than 1% is precipitated, the nano Cu-rich phase is inevitably grown and coarsened seriously, and the precipitation strengthening effect is greatly reduced.

Preferably, the thickness of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate is 6-20 mm. For medium manganese steel plates for structures, a thickness of more than 6mm is necessary; however, in the manganese steel and the preparation method thereof according to the present invention, if the thickness of the steel sheet exceeds 20mm, the rate of temperature rise during annealing and tempering is reduced, dislocation and various defects are recovered remarkably, and nucleation and precipitation of the Cu-rich phase are not facilitated.

rolling the billet to obtain a hot rolled plate;

The preparation method of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate provided by the invention is based on the following principle:

the Cu-containing medium manganese steel hot rolled plate is subjected to two-phase zone annealing-tempering heat treatment, the contradiction between Cu-rich phase precipitation and austenite reverse phase transformation existing in a single tempering process is coordinated, and the synergy of Cu-rich phase precipitation strengthening and austenite plastic toughening is fully utilized to realize the unification of high strength, high plasticity and excellent toughness of the medium manganese steel. The design principle of the two-stage annealing-tempering heat treatment process is as follows:

the transformation temperature points Ac1 and Ac3 of the precipitation-strengthened medium manganese steel sheet are respectively above 560 ℃ and below 790 ℃, preferably the reverse transformation temperature range of austenite is 600-650 ℃, and sufficient retained austenite can be obtained by annealing treatment in the temperature range. Especially, when the annealing temperature is higher, a large amount of residual austenite can obviously improve the ductility and toughness of the medium manganese steel. However, the Cu-rich phase precipitated in this temperature range has a large size and a low number density, and the strengthening effect on the ferrite matrix is extremely limited. If the Cu precipitation is optimized by greatly lowering the annealing temperature, the content of the retained austenite cannot be secured. The invention innovatively designs a two-stage heat treatment process of two-phase region annealing-tempering: firstly, annealing at 610-650 ℃ in a two-phase region for 1-2 h, performing full austenite reverse phase transformation, cooling to obtain a large amount of room-temperature retained austenite, and generating a certain amount of secondary martensite converted from unstable reverse austenite; then tempering treatment is carried out for 1.5 h-2.5 h at 450 ℃ -550 ℃, so as to form a high-density nano Cu-rich precipitated phase on the secondary martensite matrix. The novel two-phase region annealing-tempering heat treatment process can optimize reverse transformation of austenite and precipitation of a Cu-rich phase in separate heat treatment steps. The plasticity and the toughness of the medium manganese steel are obviously improved by a large number of residual austenite TRIP effects with moderate stability, the yield strength of the high-density nano Cu-rich precipitated phase is greatly improved through the interaction with dislocation, and the balance of high yield strength, high plasticity and excellent toughness of the medium manganese steel is realized.

Preferably, the annealing and tempering are sequentially carried out on the hot rolled plate to obtain the precipitation strengthening type high-strength and high-toughness medium manganese steel plate, and the method comprises the following steps:

The heating rate is selected to be 1 ℃/s-5 ℃/s, because: for the steel plate with the thickness of the invention, the heating rate exceeds 5 ℃/s, the equipment capability needs to be improved, and the energy consumption needs to be increased; if the heating rate is lower than 1 ℃/s, the energy storage and dislocation recovery of the steel plate are obvious, and the large precipitation of the Cu-rich phase is not facilitated.

Preferably, the smelting temperature is 1620 ℃ to 1640 ℃. If the smelting temperature is lower than 1620 ℃, the dissolution and homogenization of various alloy elements are not facilitated; above 1640 deg.C, the energy consumption increases.

In the embodiment, a high-frequency vacuum induction furnace is adopted for smelting to obtain ingots, and each 100Kg of ingots are forged into a billet with the cross-sectional dimension of 100mm multiplied by 100 mm;

preferably, the multi-pass rolling of the billet to obtain the hot-rolled plate comprises:

The rolling temperature during rolling is 1050-1100 ℃ for the following reasons: if the initial rolling temperature is higher than 1100 ℃, the hot rolling crystal grain size is easy to be large; if the initial rolling temperature is lower than 1050 ℃, the load of the hot rolling mill is increased, and the control of the final rolling temperature is not facilitated.

The reason that the finishing temperature is more than 950 ℃ is as follows: the final rolling temperature is too low, the deformation resistance of the hot rolled plate is increased, and edge cracking is easy to occur. .

The reason why the cumulative reduction ratio is controlled to be 80-94%: if the accumulated reduction rate is less than 80%, the target thickness cannot be achieved, and the crystal grains are easy to be uneven; if the cumulative reduction is higher than 94%, the steel plate has excessive deformation resistance, which increases the load of the rolling mill and is easy to crack

The reason why the average cooling speed is more than or equal to 50 ℃/s is as follows: if the cooling speed of the steel plate is too low, a fine and uniform quenched martensite structure cannot be obtained, and defects such as a large amount of dislocation and the like are not kept, so that austenite nucleation and Cu-rich phase precipitation in the subsequent annealing stage are influenced.

More preferably, the rolling passes are between 5 and 11. If the rolling pass is higher than 11 passes, the pass reduction is insufficient, and the recrystallization of austenite is not facilitated; if the pass is less than 5, the pass reduction is too large, the load of the rolling mill is increased, and the control of the plate shape is not facilitated.

According to the precipitation strengthening type high-strength and high-toughness medium manganese steel plate and the preparation method thereof, provided by the invention, low-carbon medium manganese is adopted in chemical components, a proper amount of alloy elements are added, and a two-stage heat treatment process of two-phase region annealing-tempering is innovatively designed in the preparation method; the finally prepared precipitation-strengthened high-strength and high-toughness medium manganese steel plate has yield strength larger than 960MPa, tensile strength larger than 1000MPa, elongation after fracture larger than 25%, product of strength and elongation exceeding 25GPa%, and room-temperature impact energy larger than 120J. Compared with microalloyed medium plate of the same grade, the steel plate has high hardenability, so the uniformity of the structure and the mechanical property in the plate thickness direction is good, and the introduction of the residual austenite obviously improves the elongation after fracture.

The precipitation strengthening type high strength and toughness medium manganese steel plate and the preparation method thereof will be described in detail below with reference to examples, comparative examples and experimental data.

Examples 1 to 6 and comparative examples 1 to 3 were prepared by melting the chemical components shown in table 1 at 1630 ℃ in the alloy composition ratios designed in table 1, and then forging the molten alloy into billets having a cross-sectional size of 100mm × 100 mm;

TABLE 1 Mass fractions of chemical components of examples and comparative examples

Group of	C％	Si％	Mn％	P％	S％	Al％	Nb％	Cu％	Ni％	B％
											Example 1	0.04	0.3	4.5	≤0.005	≤0.0003	0.5	0.04	1.4	1.6	0.003
Example 2	0.06	0.36	5.1	≤0.005	≤0.0003	0.56	0.06	1.6	2.1	0.0025
											Example 3	0.06	0.36	5.1	≤0.005	≤0.0003	0.56	0.06	1.6	2.1	0.0025
Example 4	0.08	0.50	5.0	≤0.005	≤0.0003	0.65	0.04	2.0	2.4	0.0025
											Example 5	0.08	0.50	5.0	≤0.005	≤0.0003	0.65	0.04	2.0	2.4	0.0025
Example 6	0.05	0.6	5.5	≤0.005	≤0.0003	0.8	0.04	2.2	2.5	0.0025
											Comparative example 1	0.06	0.36	5.1	≤0.005	≤0.0003	0.56	0.06	1.6	2.1	0.0025
Comparative example 2	0.06	0.36	5.1	≤0.005	≤0.0003	0.56	0.06	1.6	2.1	0.0025
											Comparative example 3	0.06	0.36	5.1	≤0.005	≤0.0003	0.56	0.06	1.6	2.1	0.0025

Step 2: rolling in a recrystallization zone:

(1) heating the steel slab to a slab heating temperature as shown in table 2;

(2) rolling for multiple times: the initial rolling temperature, the final rolling temperature, the pass number, and the cumulative reduction ratio were measured to obtain a hot rolled plate having a thickness as shown in Table 2.

(3) Cooling the hot-rolled plate to room temperature by water quenching at the cooling rate shown in table 2;

and step 3: two-stage heat treatment:

(1) and (3) annealing the two-phase region: heating the hot rolled plate to an annealing temperature, and then air-cooling the hot rolled plate to a room temperature to obtain an annealed steel plate; the annealing temperature, the rate of heating from room temperature to the annealing temperature, and the soaking time for annealing are shown in table 2;

(2) tempering: and heating the annealed steel plate to a tempering temperature, and then air-cooling to room temperature to obtain the precipitation-strengthened high-strength and high-toughness medium manganese steel plate, wherein the tempering temperature, the speed of heating from the room temperature to the tempering temperature, and the tempering time are shown in table 2.

To further illustrate the excellent mechanical properties of the steel sheets of the present invention and to further emphasize the novel two-stage heat treatment process required to achieve such mechanical properties, the following comparative examples 1-3 were designed, with comparative examples 1-3 employing a one-step heat treatment process.

TABLE 2 Process parameters for the examples and comparative examples

The properties of the final medium manganese steel sheets obtained from each group are shown in table 3.

TABLE 3 mechanical property chart of manganese steel plate

As can be seen from Table 3, the impact energy at room temperature of the medium manganese steel plate obtained in the comparative example 1 by adopting the one-step heat treatment process is relatively large and reaches 164J, but the yield strength is only 885MPa, and the tensile strength is only 960 MPa. Comparative examples 2-3 all adopt one-step heat treatment process, and the obtained medium manganese steel plate has yield strength of 1015MPa-1160MPa, tensile strength of 1040MPa-1180MPa, but small impact energy at room temperature of only 5J-29J, and small elongation and product of strength and elongation after fracture.

The precipitation-strengthened high-strength and high-toughness medium manganese steel plate finally prepared in the embodiments 1 to 6 of the invention has yield strength larger than 960MPa, tensile strength larger than 1000MPa, elongation after fracture larger than 25%, product of strength and elongation larger than 25GPa%, and room-temperature impact energy larger than 120J.

Fig. 2 is a morphology diagram of retained austenite of the precipitation-strengthened high-toughness medium manganese steel plate manufactured in example 2 of the present invention, and it can be seen from fig. 2 that a retained austenite entity exists, the retained austenite is in a slender slab shape, and is alternately distributed with lath ferrite, and the width of the lath of the retained austenite is less than 500 nm.

Fig. 3 is a morphology chart of the Cu-rich precipitated phase on the ferrite matrix of the precipitation-strengthened high-toughness medium manganese steel plate prepared in example 2 of the present invention, and it can be seen from fig. 3 that the Cu-rich precipitated phase on the ferrite matrix is distributed in a high density.

FIG. 4 is a room temperature tensile stress-strain curve diagram of the precipitation-strengthened high-toughness medium manganese steel plate prepared in example 2 of the present invention, and it can be seen from FIG. 4 that the yield strength of the steel plate is greater than 960MPa, the tensile strength is greater than 1000MPa, and the elongation after fracture exceeds 25%.

In conclusion, the precipitation strengthening type high-strength and high-toughness medium manganese steel plate and the preparation method thereof provided by the invention have the advantages that low-carbon medium manganese is adopted in chemical components, a proper amount of alloy elements are added, and a two-stage heat treatment process of two-phase region annealing-tempering is innovatively designed in the preparation method; the finally prepared precipitation-strengthened high-strength and high-toughness medium manganese steel plate has yield strength larger than 960MPa, tensile strength larger than 1000MPa, elongation after fracture larger than 25%, product of strength and elongation exceeding 25GPa%, and room-temperature impact energy larger than 120J. Compared with microalloyed medium plate in the same level, the steel plate has high hardenability, so the uniformity of the structure and the mechanical property in the plate thickness direction is good, and the introduction of the residual austenite obviously improves the elongation after fracture.

Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims

1. The precipitation-strengthened high-strength and high-toughness medium manganese steel plate is characterized by comprising the following chemical components in percentage by mass: c: 0.04-0.10%, Si: 0.2-0.6%, Mn: 4.0-6.0%, Al: 0.4% -1.0%, Nb: 0.02% -0.08%, Cu: 1.0% -2.5%, Ni: 1.5% -2.5%, B: 0.001-0.004%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, and the balance is Fe and inevitable impurities, wherein the internal microstructure of the precipitation-strengthened high-strength and toughness medium manganese steel plate comprises the following components in percentage by volume: 45-70% of ultrafine grained ferrite, 20-40% of slender lath-shaped residual austenite, 5-20% of tempered martensite and less than 1% of nano-scale Cu-rich precipitated phase, wherein the thickness of the precipitated and strengthened high-strength and toughness medium manganese steel plate is 6-20 mm, the yield strength of the strengthened high-strength and toughness medium manganese steel plate is greater than 960MPa, the tensile strength is greater than 1000MPa, the elongation after fracture is greater than 25%, the product of strength and elongation exceeds 25GPa%, and the room-temperature impact power is greater than 120J, and the preparation method of the high-strength and toughness medium manganese steel plate comprises the following steps:

smelting and forging the chemical components to obtain a steel billet;

rolling the steel billet to obtain a rolled steel plate;

quenching the rolled steel plate, and then cooling to room temperature to obtain a hot rolled plate;

2. The precipitation-strengthened high-toughness medium-manganese steel sheet according to claim 1, wherein said billet obtained by melting and forging said chemical components comprises:

the chemical components of the precipitation strengthening type high-strength and high-toughness medium manganese steel plate are smelted and forged at the smelting temperature of 1620-1640 ℃, so that a billet is obtained.

3. The precipitation-strengthened high-toughness medium-manganese steel sheet according to claim 1, wherein said step of rolling said billet to obtain a rolled steel sheet comprises:

and rolling the preheated billet to obtain a rolled steel plate, wherein the initial rolling temperature is 1050-1100 ℃, the final rolling temperature is more than 950 ℃, and the cumulative reduction rate is 80-94%.

4. The precipitation-strengthened high-toughness medium-manganese steel plate according to claim 1, wherein the hot-rolled steel plate obtained by quenching the rolled steel plate and then cooling the quenched steel plate to room temperature comprises:

5. The precipitation strengthening type high-strength and high-toughness medium manganese steel plate as claimed in claim 1, wherein the rolling passes are 5 to 11.