CN117127099B

CN117127099B - 1300MPa ultra-high strength plastic cold-rolled Mn-TRIP steel and preparation method thereof

Info

Publication number: CN117127099B
Application number: CN202310476344.9A
Authority: CN
Inventors: 胡智评; 顾兴利; 刘仁东; 林利; 郭金宇; 李侠
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2023-04-28
Filing date: 2023-04-28
Publication date: 2024-04-16
Anticipated expiration: 2043-04-28
Also published as: CN117127099A

Abstract

The invention provides 1300MPa ultra-high strength plastic cold-rolled Mn-TRIP steel and a preparation method thereof, wherein the steel comprises the following ：C：0.25％～0.35％,Si：0.15％～1.45％,Mn：6.5％～10.5％,Al：3.0％～5.0％,Cr：0.2％～1.0％,Mo：0.2％～1.0％,Ti：0.01％～0.03％,Nb：0.02％～0.04％,V：0.15％～0.25％,P≤0.015％,S≤0.005％ weight percent of Fe and unavoidable impurities as the rest. The preparation method comprises smelting, continuous casting, heating, hot rolling, hood annealing, pickling, cold rolling, rapid heat treatment and finishing; the Mn-TRIP steel produced by the invention has the yield strength of 850-1100 MPa, the tensile strength of 1300-1500 MPa, the elongation after fracture of 30-35%, the reaming ratio of more than or equal to 55% and the strength-plastic product of more than 40GPa%.

Description

1300MPa ultra-high strength plastic cold-rolled Mn-TRIP steel and preparation method thereof

Technical Field

The invention belongs to the field of metal materials, and particularly relates to 1300MPa ultra-high strength plastic cold-rolled Mn-TRIP steel and a preparation method thereof.

Background

The green and intelligent development trend of steel is important in the manufacturing industry of automobile materials closely combined with civil life, and the high-strength and thinning development is a key core of automobile material research and development along with the energy-saving and safe production concept. Scientists Krupitzer and Heimbuch in the united states were first proposing the concept of third generation automotive steels, and the automotive/steel alliance was under the support of DOE and NSF to initiate development of third generation automotive steels for three years with integration of strength and plasticity and cost intermediate to those of first and second generation automotive steels at 10 months of 2007. In the same period, china, korea and the like also start the research and development work of high-strength and high-plastic steel for improving the strength and plastic product. So far, the technical ideas of developing third-generation automobile steel in various countries are still under active exploration. In many third-generation automobile steel researches, medium manganese steel (Mn-TRIP steel) has gained a great deal of attention due to the low cost advantage of no or a small amount of noble alloy elements added and the relatively good comprehensive mechanical properties. Based on Mn-TRIP steel, a plurality of scholars such as DeCooman professor gradually put forward the research direction of the medium manganese steel with Al instead of Si, the full austenitizing temperature can be improved by inhibiting the addition of Al of carbide, the process window of a critical region is enlarged, and meanwhile, the product quality problem caused by the excessively high Si content can be solved. The concept of manganese TRIP steel in the mn—al system has been proposed in this regard.

Chinese patent CN104651734a discloses a "1000 MPa-level high-strength high-plasticity aluminum-containing medium manganese steel and preparation method thereof", the cold-rolled steel sheet comprises the following components in percentage by weight: c:0.05 to 0.20 percent, mn:7.0 to 11.0 percent, al:1.50 to 3.50 percent, 0.20 to 0.40 percent of Si, 0.02 to 0.60 percent of Cr, less than or equal to 0.50 percent of Cu, less than or equal to 0.40 percent of Mo, less than or equal to 0.10 percent of Nb, less than or equal to 0.01 percent of N, and the balance of Fe and unavoidable impurities. The cold-rolled steel plate is added with a large amount of micro-alloy elements such as Cu, cr, mo and the like, so that the alloy cost of the medium manganese steel is greatly increased; in the selection of the heat treatment process, the two processes adopt a cover annealing process and a continuous annealing process, and obvious cost-saving improvement does not exist.

Chinese patent CN103695765A discloses a "high strength high plasticity cold rolled medium manganese steel and its preparation method", the cold rolled steel sheet comprises the following components in percentage by weight: c:0.15 to 0.20 percent, mn:8.0 to 11.0 percent, al:3.0 to 4.0 percent, nb:0 to 0.04 percent of Fe and unavoidable impurities. The cold rolled steel has the strength of 1000MPa and the elongation after fracture of more than 60 percent, and has good mechanical properties. The improvement of the strength and plasticity of the cold-rolled steel sheet is mainly concentrated in the aspect of plasticity, and the higher C, mn content ratio can not further improve the strength, thereby meeting the development concept of high-strength and thinning automobile steel. Meanwhile, the selected process is also a production method of the traditional medium-manganese steel, and obvious cost saving and innovation are avoided.

Disclosure of Invention

The invention aims to overcome the problems and the defects and provide the 1300MPa ultra-high strength cold-rolled Mn-TRIP steel with the yield strength of 850-1100 MPa, the tensile strength of 1300-1500 MPa, the elongation after fracture of more than 30-35%, the hole expansion rate of more than or equal to 55% and the strength-plastic product of more than 40GPa percent and the preparation method thereof.

The invention aims at realizing the following steps:

A1300 MPa ultra-high strength cold-rolled Mn-TRIP steel comprises the following ：C：0.25％～0.35％,Si：0.15％～1.45％,Mn：6.5％～10.5％,Al：3.0％～5.0％,Cr：0.2％～1.0％,Mo：0.2％～1.0％,Ti：0.01～0.03％,Nb：0.02～0.04％,V：0.15％～0.25％,P≤0.015％,S≤0.005％ parts by weight of Fe and unavoidable impurities as the balance.

The Mn-TRIP steel has the yield strength of 850-1100 MPa, the tensile strength of 1300-1500 MPa, the elongation after fracture of 30-35%, the hole expansion rate of more than or equal to 55% and the strength-plastic product of more than 40GPa%.

The Mn-TRIP steel microstructure is a critical zone ferrite, epitaxial ferrite, residual austenite, bainite and martensite structure; each microstructure comprises the following components in percentage by volume: 10-30% of ferrite in the critical area, 8-15% of epitaxial ferrite, 35-45% of retained austenite and the balance of bainite and martensite structure.

Preferably, the average grain size of ferrite is between 0.5 and 1.5 μm, and the average grain size of retained austenite is between 0.1 and 0.3 μm.

Preferably, the mass percentage of C in the residual austenite is 1.3-1.6%.

The reason for designing the components of the invention is as follows:

C: c is an essential element for obtaining the expected strength of high-strength steel. In addition, in the invention, C mainly affects the content and stability of the residual austenite, thereby affecting the plasticity of the steel plate; the content of C is too low to reach the expected strength, the content and stability of austenite are reduced, and the plasticity is reduced; the high C content deteriorates the weldability of the steel sheet, and the high austenite stability deteriorates the TRIP effect, and further deteriorates both strength and plasticity.

Si: the Si element has the main function of inhibiting carbide precipitation in the high-strength steel, and is beneficial to the strength of the steel plate. In the present invention, si is mainly considered to expand the two-phase interval of the steel sheet, promote ferrite formation, and contribute to improvement of ferrite hardness.

Mn: mn is an essential element for obtaining the intended strength of high-strength steel. In addition, in the invention, mn mainly considers the content and stability of the residual austenite, thereby affecting the plasticity of the steel plate; meanwhile, mn element ensures the transformation quantity of martensite in the annealing rapid cooling stage, and prevents excessive ferrite or other phases from forming in the stage.

Al: the Al element conventionally acts as a deoxidizer in the steel. However, the effect of the Al content in the invention mainly considers that the temperature interval of the two-phase region is enlarged, the surface quality of the steel plate is improved, and the concentration of C in austenite is promoted to reach the standard.

Cr: cr is an element for improving the strength of high-strength steel. The Cr in the invention has the main effects of improving the surface quality of the steel plate, reducing edge cracks caused by too high addition of Mn and ensuring the performance uniformity of the steel plate from edge to middle to edge.

Mo: mo is an element that increases strength in high-strength steel. The Mo in the invention has the effects of improving the surface quality of the steel plate, improving the stability of austenite and further ensuring the strength and the plasticity.

V: the V element acts on precipitation strengthening in high-strength steel. The main effect of V in the invention is that the V is precipitated in the hot rolling stage, retained in ferrite in the rapid heat treatment stage, and the V is retained in martensite to improve the toughness of the steel plate.

Nb: nb element acts as precipitation strengthening in high-strength steel. In the invention, nb mainly considers strain-induced precipitation in the hot rolling stage, so that original austenite grains are refined, the size of the original austenite grains is ensured to be less than 10 mu m, and the final strength and plasticity are ensured.

Ti: nb element acts as precipitation strengthening in high-strength steel. In the invention, ti is mainly precipitated in a continuous casting solidification stage, so that original austenite grains are refined, the size of the original austenite grains is ensured to be less than 10 mu m, and the final strength and plasticity are ensured.

P: the P element is a precipitate phase formed with C in high-strength steel, and the performance of the steel plate is deteriorated, and the P element needs to be strictly controlled at 0.005%.

S: the S element is a precipitate phase formed with C in high-strength steel, and the performance of the steel plate is deteriorated, and is required to be strictly controlled at 0.005%.

The second technical scheme of the invention is to provide a preparation method of 1300MPa ultra-high strength plastic cold-rolled Mn-TRIP steel, which comprises smelting, continuous casting, heating, hot rolling, cover annealing, pickling, cold rolling, rapid heat treatment and finishing;

(1) Continuous casting: the casting blank pulling speed is less than or equal to 1.0m/min, so that the cracking and the steel leakage of the casting blank are prevented, and the warning is performed; the temperature of the tundish is 1500-1600 ℃; the thickness of the casting blank is 220-280 mm, the rolling reduction of the hot rolling is ensured, and the grain refinement effect is further enhanced.

(2) Heating: the heating temperature is 1180-1280 ℃, the precipitation behavior of Ti atoms is ensured, the formation and the precipitation of TiN or Ti (C, N) are ensured, and the effect of pinning the prior austenite grain boundary and refining the prior austenite grain is achieved.

(3) And (3) hot rolling:

The initial rolling temperature is 1060-1150 ℃, the rolling temperature of a recrystallization region is ensured, nb strain induction precipitation is promoted, the dynamic recrystallization behavior of the prior austenite grains in the hot rolling stage is regulated and controlled, the grains are refined, and then the prior austenite grains after final rolling are refined. The final rolling temperature is 850-950 ℃, the dislocation state of the steel plate before laminar cooling is ensured, the pre-eutectoid transformation of supercooled austenite is prevented in advance, and the grain size of original austenite after hot rolling final rolling is ensured to be below 10 mu m. The coiling temperature is 450-490 ℃, the internal oxidation and the grain boundary oxidation of the steel plate are prevented, the surface quality of the steel plate is ensured, and the thickness of the hot rolled coil is 2.2-4.0 mm.

(3) Hood annealing: placing the coiled steel plate in a hood-type annealing furnace for 24-120 hours, wherein the annealing temperature is 620-750 ℃, the steel plate tissue is annealed to ferrite and austenite without martensite or bainite, and the microstructure comprises the following components in percentage by volume: the ferrite content is 30% -50% and the austenite content is 50% -70%.

(4) Acid washing: and (5) removing the oxidized iron scales generated on the hot-rolled surface and ensuring the surface quality of the cold-rolled steel plate.

(5) Cold rolling: the cold rolling reduction is 40% -58%, so that the rolling reduction of more than 40% of cold rolling is ensured, and tissue fibrosis in cold rolling configuration is promoted; meanwhile, the cold rolling reduction is prevented from being too high, so that deformation resistance is too high, and the rolling is difficult to achieve the target thickness.

(6) And (3) rapid heat treatment:

① The temperature of the steel plate is raised to 500-600 ℃ at a speed of 10-15 ℃/s, and austenite transformation nucleation does not exist during the temperature.

② Heating the steel plate to 850-950 ℃ at a speed of more than 150 ℃/s, and keeping the isothermal time at 5-15 s; the rapid temperature rise prevents excessive transformation of austenite at the stage, when the temperature is raised to 850-950 ℃, the austenite rapidly transforms to form nuclei, the isothermal time ensures that austenite grains grow greatly in the temperature range, and preferably, the reversed austenite grain size in a microstructure in the steel plate is 0.3-0.5 mu m, so that the performance of the steel plate is ensured.

③ The slow cooling temperature is 750-800 ℃, and the slow cooling speed is controlled to be 3-5 ℃/s; the austenite content is ensured, excessive epitaxial ferrite is prevented from forming, the epitaxial ferrite content is controlled to be 8-15%, and the performance of the steel plate is ensured.

④ Cooling to 80-150 ℃ at a cooling rate of more than 30 ℃/s, and preventing further formation of epitaxial ferrite at a high cooling rate, so as to ensure the performance of the steel plate; and prevents the formation of martensite at too low a temperature and bainite or other phases at too high a temperature.

⑤ And then heating to 200-340 ℃, wherein the isothermal time is 10-20 min, so that the diffusion of C with too low temperature is prevented from being inhibited, and the bainite transformation is prevented from being caused when the temperature is too high.

⑥ And then cooling to room temperature at a cooling rate of more than 5 ℃ per second to prevent excessive bainite formation at a low cooling rate.

(7) And (3) finishing: then, the steel plate enters a finishing machine to carry out plate shape adjustment, the finishing elongation is controlled to be 0.1-0.6%, and the yield strength is regulated and controlled to be 850-1100 MPa.

The invention has the beneficial effects that:

(1) Compared with the traditional high Al medium manganese steel phase, the alloy proportion of the ultra-high-strength plastic cold-rolled Mn-TRIP steel plate is reduced, a small amount of noble alloy elements such as Cr, mo and the like are added, the process stability is good, the structure is uniform, and the production safety is high.

(2) After cold rolling, rapid temperature raising and two-phase zone annealing treatment, more dislocation generated after cold rolling is reserved in the two-phase zone annealing process, the recrystallization behavior of coarse ferrite is promoted, more nucleation positions of reverse austenite are provided, and a double-peak ferrite and reverse austenite structure is obtained.

(3) The rapid temperature raising and two-phase zone process improves the annealing temperature of the two-phase zone, promotes the enrichment of C, mn elements, enhances the phase stability of reversed austenite, and provides a tissue foundation for improving the work hardening behavior of experimental steel and enhancing plasticization in the subsequent deformation process.

Drawings

FIG. 1 is a SEM image of the microstructure of example 1 of the present invention.

Detailed Description

The invention is further illustrated by the following examples.

According to the component proportions of the technical scheme, smelting, heating, hot rolling, hood annealing, pickling, cold rolling, rapid heat treatment and finishing are carried out. The composition of the steel of the example of the invention is shown in Table 1. The main technological parameters of continuous casting and hot rolling of the steel of the embodiment of the invention are shown in Table 2. The main process parameters of the rapid heat treatment of the steel of the embodiment of the invention are shown in Table 3, and the properties of the steel of the embodiment of the invention are shown in Table 4. The structure of the steel of the example of the invention is shown in Table 5.

TABLE 1 composition (wt%) of the inventive example steel

Examples	C	Mn	Cr	Mo	Si	Al	Ti	Nb	V	P	S
												1	0.27	8.56	0.94	0.52	1.43	3.52	0.015	0.035	0.18	0.011	0.003
2	0.35	8.47	0.46	0.24	0.17	4.69	0.028	0.037	0.19	0.012	0.005
												3	0.28	9.53	0.52	0.51	1.32	3.59	0.014	0.04	0.22	0.009	0.003
4	0.29	10.28	0.4	0.94	1.39	4.31	0.012	0.021	0.24	0.015	0.004
												5	0.32	7.65	0.51	0.46	1.28	4.82	0.023	0.027	0.23	0.005	0.003
6	0.33	8.94	0.94	0.51	0.65	3.52	0.028	0.024	0.17	0.014	0.005
												7	0.32	6.82	0.46	0.94	0.52	4.05	0.019	0.032	0.19	0.013	0.004
8	0.34	6.56	0.52	0.46	1.39	3.32	0.05	0.037	0.22	0.01	0.005
												9	0.25	8.23	0.94	0.58	1.28	3.64	0.016	0.025	0.24	0.009	0.003
10	0.27	8.92	0.46	0.52	0.65	3.65	0.025	0.025	0.25	0.013	0.005
												11	0.26	9.47	0.52	0.94	0.52	4.5	0.027	0.024	0.15	0.005	0.003
12	0.28	9.82	0.4	0.46	0.58	4.52	0.024	0.024	0.23	0.009	0.004
												13	0.32	9.56	0.76	0.52	0.52	4.78	0.023	0.032	0.19	0.008	0.005
14	0.34	9.02	0.41	0.4	0.56	4.39	0.018	0.037	0.18	0.012	0.003
												15	0.34	9.62	0.55	0.37	0.68	3.62	0.018	0.025	0.24	0.012	0.005

TABLE 2 main process parameters for continuous casting and hot rolling of the inventive example steel

TABLE 3 main process parameters for rapid heat treatment of example steels according to the invention

TABLE 4 Properties of the inventive example Steel

Examples	Rp0.2/MPa	Rm/MPa	A80/％	PSE/Gpa％	λ/％
						1	1082	1356	32.4	43.9	58.6
2	1024	1478	33.4	49.4	55.9
						3	896	1396	30.8	42.9	62.3
4	878	1378	34.9	48.1	62.3
						5	956	1345	34.5	46.4	61.4
6	1042	1367	34.2	46.8	61.9
						7	989	1325	33.5	43.1	61.4
8	971	1487	31.7	47.1	61.9
						9	963	1492	35	52.2	62.2
10	874	1411	34.2	48.3	65.3
						11	856	1423	30.2	42.9	62.7
12	1097	1476	33.1	48.9	61.9
						13	888	1405	34.8	48.9	62.2
14	1056	1385	30.9	42.8	65.3
						15	1004	1462	31.5	46.1	62.7

TABLE 5 microstructure of the inventive example steel

As is clear from the above, the Mn-TRIP steel produced by the method has the yield strength of 850-1100 MPa, the tensile strength of 1300-1500 MPa, the elongation after fracture of 30-35%, the reaming ratio of more than or equal to 55% and the strength-plastic product of more than 40GPa%. The microstructure is a critical area ferrite, epitaxial ferrite, residual austenite, bainite and martensite structure; each microstructure comprises the following components in percentage by volume: 10-30% of ferrite in the critical area, 8-15% of epitaxial ferrite, 35-45% of retained austenite and the balance of bainite and martensite structure.

The present invention has been properly and fully described in the foregoing embodiments by way of example only, and not by way of limitation, and various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the invention, any modification, equivalent substitution, improvement, etc. should be included in the scope of the invention, and the scope of the invention is defined by the claims.

Claims

1. A 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel, characterized in that the steel has the following components by weight percentage: C: 0.25% to 0.35%, Si: 0.15% to 1.45%, Mn: 6.5% to 10.5%, Al: 3.0% to 5.0%, Cr: 0.2% to 1.0%, Mo: 0.2% to 1.0%, Ti: 0.01% to 0.03%, Nb: 0.02% to 0.04%, V: 0.15% to 0.25%, P≤0.015%, S≤0.005%, and the balance is Fe and unavoidable impurities;

The method for preparing a 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel comprises smelting, continuous casting, heating, hot rolling, bell annealing, pickling, cold rolling, rapid heat treatment, and skin-passing;

(1) Heating: Heating temperature 1180~1280℃;

(2) Hot rolling:

The starting rolling temperature is 1060~1150℃, the final rolling temperature is 850~950℃, and the coiling temperature is 450~490℃;

(3) Hood annealing: the annealing temperature of the steel plate after coiling is 620~750℃, and the annealing time is 24~120 hours;

(4) Cold rolling: The cold rolling reduction rate is 40%~58%;

(5) Rapid heat treatment:

① Raise the temperature of the steel plate to 500~600℃ at a rate of 10~15℃/s;

② Raise the temperature of the steel plate to 850~950℃ at a rate greater than 150℃/s, and keep the isothermal time at 5~15s;

③ The slow cooling temperature is 750~800℃, and the slow cooling rate is controlled at 3~5℃/s;

④ Cool to 80~150℃ at a cooling rate greater than 30℃/s;

⑤ Reheat to 200~340℃, isothermal time is 10~20min;

⑥ Then cool to room temperature at a cooling rate greater than 5℃/s;

(6) Finishing: The finishing elongation is controlled at 0.1%~0.6%.

2. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 1 is characterized in that the Mn-TRIP steel has a yield strength of 850-1100MPa, a tensile strength of 1300-1500MPa, an elongation after fracture of 30%-35%, a hole expansion rate of ≥55%, and a strength-plasticity product of greater than 40GPa%.

3. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 1 is characterized in that the microstructure of the Mn-TRIP steel is intercritical ferrite + epitaxial ferrite + retained austenite + bainite + martensite; the volume percentage of each microstructure is as follows: intercritical ferrite 10%~30%, epitaxial ferrite 8%~15%, retained austenite 35%~45%, and the remainder is bainite and martensite.

4. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 3, characterized in that the mass percentage of C in the retained austenite is 1.3% to 1.6%.

5. The 1300 MPa ultra-high strength and plasticity cold-rolled Mn-TRIP steel according to claim 3, characterized in that the average ferrite grain size is 0.5-1.5 μm, and the average retained austenite grain size is 0.1-0.3 μm.

6. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 1, characterized in that: the continuous casting: the billet pulling speed is ≤1.0 m/min, the tundish temperature is 1500-1600°C; the billet thickness is 220-280 mm.

7. The 1300MPa ultra-high strength and plasticity cold-rolled Mn-TRIP steel according to claim 1, characterized in that the original austenite grain size after hot rolling is less than 10 μm.

8. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 1, characterized in that after the bell annealing, the steel plate structure is ferrite and reversed austenite, the volume percentage of ferrite is 30%-50%, and the volume percentage of austenite is 50%-70%.

9. The 1300MPa ultra-high strength and plastic cold-rolled Mn-TRIP steel according to claim 1, characterized in that: after step ②, the reversed austenite grain size in the steel plate microstructure is 0.3-0.5μm.