CN112095046A

CN112095046A - Ultrahigh-strength cold-rolled DH1180 steel and preparation method thereof

Info

Publication number: CN112095046A
Application number: CN202010929364.3A
Authority: CN
Inventors: 张瑞坤; 刘仁东; 林利; 张南; 徐鑫; 李文竹; 周敬; 李潇彤; 苏洪英; 郝志强
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2020-09-07
Filing date: 2020-09-07
Publication date: 2020-12-18
Anticipated expiration: 2040-09-07
Also published as: CN112095046B

Abstract

The invention discloses ultrahigh-strength cold-rolled DH1180 steel and a preparation method thereof. The steel contains C: 0.18-0.25%, Mn: 1.8% -2.8%, Si: 0.5% -1.4%, Al: 0.02% -1.4%, Cr: 0.03-0.60%, Mo: 0.04-0.4%, Ti: 0.002% -0.1%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, V is less than or equal to 0.05%, Nb is less than or equal to 0.1%, and Si + Al: 0.8% -2.0%, Mo + Ti: 0.10 to 0.50 percent, and the balance of iron and inevitable impurities. Heating the casting blank at 1200-1300 ℃, rolling at 1050-1150 ℃, rolling at 880 ℃ or higher and rolling at 550-650 ℃; the cold rolling reduction rate is 40-70%; the annealing temperature is 790-910 ℃, the annealing time is 30-600 s, the slow cooling outlet temperature is 680-750 ℃, the rapid cooling rate is more than 30 ℃/s, the overaging temperature is 350-450 ℃, and the overaging time is 30-3600 s; the steel sheet is excellent in formability and hydrogen-induced delayed fracture resistance.

Description

Ultrahigh-strength cold-rolled DH1180 steel and preparation method thereof

Technical Field

The invention belongs to the technical field of cold-rolled steel, and relates to ultrahigh-strength cold-rolled DH1180 steel with excellent hydrogen-induced delayed fracture resistance and a preparation method thereof.

Background

In the automobile industry, higher requirements on light weight of automobile bodies, emission limitation and safety standards are provided, and in order to better serve users, the automobile industry has more and more demands on parts with high formability. Conventional dual phase steels have difficulty meeting the requirements of complex cup punching with high drawability, and TRIP steels have limited their widespread use due to the high alloy content which brings with it expensive production costs. DH steel is the acronym for Dual Phase Steels with Improved Steel construction and was first mentioned in the 2016 German society for automotive industry, VDA 239-. The DH steel has good forming performance due to the introduction of a certain amount of residual austenite, can overcome the defects of DP steel and TRIP steel in the application process, and further has remarkable advantages in the application market of future steel.

Patent document CN 103667893B discloses a high-strength steel with yield ratio less than or equal to 0.5 and resistance to delayed fracture and a production method thereof, wherein the main chemical components are C: 0.11-0.25%, Mn: 15.5% -17.5%, Mo: 0.030-0.050%, Co: 0.005% -0.02%, B: 0.005% -0.009%, N: 0.009% -0.011%, La + Ce + Nd: 0.011-0.025%, and the balance of Fe and inevitable impurities; the yield strength of the product is 350-400 MPa, the tensile strength is above 800MPa, the elongation is more than or equal to 58%, the yield ratio is less than or equal to 0.5, the strain hardening index is more than or equal to 0.460, the product is TWIP steel, the microstructure is austenite and twin crystal substructure or austenite + martensite blank nucleus, the strength is 800MPa grade, the invention has low strength grade and poor forming capability.

Patent document CN 108359901A discloses 1200 MPa-grade low-carbon hot-rolled dual-phase steel and a preparation method thereof, wherein the main chemical components C are 0.16-0.21 wt%, Si is 0.2-1.0 wt%, Mn is 1.4-2.0 wt%, Al is 0.03-0.06 wt%, Nb is 0.03-0.06 wt%, P is less than or equal to 0.020 wt%, S is less than or equal to 0.005 wt%, and the balance is Fe and inevitable impurities; the invention is a hot-rolled dual-phase steel product, and has poor delayed fracture resistance.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to provide the ultrahigh-strength cold-rolled DH1180 steel and the preparation method thereof, which can meet the production conditions of the traditional production line, control the alloy cost and provide a technical scheme for automobile manufacturers and steel companies.

The specific technical scheme is as follows:

the ultrahigh-strength cold-rolled DH1180 steel comprises the following chemical components in percentage by mass: c: 0.18-0.25%, Mn: 1.8% -2.8%, Si: 0.5% -1.4%, Al: 0.02% -1.4%, Cr: 0.03-0.60%, Mo: 0.04-0.4%, Ti: 0.002% -0.1%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, V is less than or equal to 0.05%, Nb is less than or equal to 0.1%, and Si + Al: 0.8% -2.0%, Mo + Ti: 0.10 to 0.50 percent, and the balance of Fe and inevitable impurities.

The reason for the alloy design of the present invention is as follows:

c: the carbon element guarantees the strength requirement of the steel through solid solution strengthening, and sufficient carbon element is beneficial to stabilizing austenite, thereby improving the forming performance of the steel. The content of the element C is too low, so that the mechanical property of the steel in the invention can not be obtained; too high a content can embrittle the steel, with the risk of delayed fracture. Therefore, the content of the C element is controlled to be 0.18-0.25 percent in the invention.

Mn: manganese is an austenite stabilizing element in steel, can expand an austenite phase region, reduce the critical quenching speed of the steel, and can refine grains, thereby being beneficial to solid solution strengthening to improve the strength. The content of Mn element is too low, the super-cooled austenite is not stable enough, and the plasticity, the toughness and other processing performances of the steel plate are reduced; the excessively high content of the Mn element causes deterioration in the weldability of the steel sheet, and increases in the production cost, which is not favorable for industrial production. Therefore, the content of the Mn element is controlled to be 1.8-2.8 percent in the invention.

Si: the silicon element has a certain solid solution strengthening effect in ferrite, so that the steel has enough strength, and meanwhile, the Si can inhibit the decomposition of residual austenite and the precipitation of carbide, thereby reducing the inclusion in the steel. The Si element content is too low to play a role in strengthening; too high content of Si element may degrade the surface quality and weldability of the steel sheet. Therefore, the content of the Si element is controlled to be 0.5 to 1.4 percent in the invention.

Al: the aluminum element contributes to deoxidation of the molten steel. It is also possible to suppress decomposition of residual austenite and precipitation of carbide, and to accelerate bainite transformation to improve the ability to coordinate transformation. Too high content of Al element not only increases production cost, but also causes difficulties in continuous casting production, etc. Therefore, the content of the Al element is controlled within the range of 0.02 to 1.4 percent in the invention. In addition, the invention also controls the ratio of Si + Al: 0.8 to 2.0 percent, and the main purpose is to play the synergistic action of Si and Al so as to improve the toughness and plasticity of the steel.

Cr: the chromium element can increase the hardenability of the steel to ensure the strength of the steel and stabilize the retained austenite, the hardenability of the steel is influenced by too low content of the Cr, and the production cost is increased by too high content of the Cr. Therefore, the content of Cr element is controlled within 0.03-0.60%.

Mo: the molybdenum element is a strengthening element in the steel, is beneficial to stabilizing the residual austenite, has an obvious effect of improving the hardenability of the steel, and is matched with Ti to use, so that the high strength and the high toughness are both considered, and the delayed fracture resistance of the steel can be improved and the good comprehensive mechanical property can be kept. The invention controls the content range of the Mo element at 0.04-0.4%.

Ti: the addition of a proper amount of Ti element can refine the grain size and obviously improve the toughness of the material, and the cooperation of the Ti element and Mo can realize both high strength and high toughness, thereby realizing the improvement of the delayed fracture resistance of the steel and simultaneously keeping good comprehensive mechanical property; the invention controls the content range of Ti element at 0.002% -0.1%. In addition, the present invention also controls Mo + Ti: 0.10 to 0.50 percent, and the main purpose is to play the synergistic action of Mo and Ti, thereby improving the delayed fracture resistance and the comprehensive mechanical property of the steel.

P: the P element is a harmful element in steel, seriously reduces the plasticity and the deformability of the steel, and the lower the content, the better the content. In the invention, the content of the P element is controlled to be less than or equal to 0.03 percent in consideration of the cost.

S: the S element is a harmful element in steel, seriously affects the formability of steel, and the lower the content, the better the formability. In consideration of cost, the content of the S element is controlled to be less than or equal to 0.03 percent.

V: the microalloying element vanadium mainly exists in a VC form, the comprehensive performance of the material is improved through fine grain strengthening and dispersion strengthening, when the annealing temperature is increased to a two-phase zone, the VC is low in dissolving temperature, so that the VC is fully dissolved in a matrix, and simultaneously solid-dissolved C atoms are enriched into austenite to improve the stability of the material; during the annealing process, VC in the ferrite is separated out again, thereby producing obvious precipitation strengthening. Therefore, in the present invention, the V element may be added in an amount of not more than 0.05% as appropriate depending on the actual situation, or the V micro-alloying element may not be added in order to control the production cost.

Nb: the microalloying element Nb is used for improving the comprehensive performance of the material through fine grain strengthening, and Nb of not more than 0.1 percent can be added according to actual conditions, and the Nb element can not be added in order to control the production cost. The invention controls the Nb content to be less than or equal to 0.1 percent.

The invention also provides a preparation method of the ultrahigh-strength cold-rolled DH1180 steel, which is characterized by comprising the following steps of: converter smelting, medium and thin slab continuous casting, hot rolling, acid pickling and cold rolling and continuous annealing. The preparation process comprises the following specific steps:

smelting in a converter: smelting by a converter to obtain molten steel meeting the following component requirements in percentage by mass, C: 0.18-0.25%, Mn: 1.8% -2.8%, Si: 0.5% -1.4%, Al: 0.02% -1.4%, Cr: 0.03-0.60%, Mo: 0.04-0.4%, Ti: 0.002% -0.1%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, V is less than or equal to 0.05%, Nb is less than or equal to 0.1%, and Si + Al: 0.8% -2.0%, Mo + Ti: 0.10 to 0.50 percent, and the balance of Fe and inevitable impurities.

Hot rolling: the charging temperature of the casting blank is 550-700 ℃, the heating temperature is 1200-1300 ℃, the initial rolling temperature is 1050-1150 ℃, the final rolling temperature is 880 ℃ or more, and the coiling temperature is 550-650 ℃. The thickness of the hot-rolled plate is 2.0-4.0 mm, and the microstructure of the hot-rolled steel plate comprises 20-50% of ferrite, 30-60% of pearlite, 5-20% of bainite and 1-5% of cementite by volume percentage, and the total is 100%.

Acid pickling and cold rolling: the iron scale on the surface of the steel coil is removed by acid liquor before cold rolling, and the cold rolling reduction rate is 40-70%. The rolling reduction is too high, so that the deformation resistance is too high, and the rolling is difficult to reach the target thickness; the reduction ratio is too low, resulting in a decrease in the elongation of the cold-rolled steel sheet.

And (3) continuous annealing: controlling the annealing temperature of a critical zone to be 790-910 ℃, the annealing time to be 30-600 s, the slow cooling outlet temperature to be 680-750 ℃, the rapid cooling rate to be more than 30 ℃/s, the overaging temperature to be 350-450 ℃, and the overaging time to be 30-3600 s; the polishing elongation in the polishing process is controlled within the range of 0.3-1.2%. The microstructure of the cold-rolled continuous annealing product comprises 10-30% of ferrite, 40-70% of martensite, 4-12% of residual austenite and 5-20% of bainite according to volume percentage.

The annealing temperature of the critical zone is 770-910 ℃, and if the annealing temperature is too high, the ductility of the steel is reduced due to complete austenitizing and insufficient ferrite proportion; if the annealing temperature is too low, the proportion of soft phase ferrite in the final material is too high, which may significantly reduce the strength of the material. The annealing time is 30-600 s, if the annealing time is too long, the crystal grains of the steel plate are coarse, the annealing time is too short, and the steel plate does not finish the annealing and recrystallization processes quickly, so that the elongation of the steel plate is reduced.

The ultrahigh-strength cold-rolled DH1180 steel plate obtained by the method is sampled along the direction vertical to the rolling direction (transverse direction), the yield strength is 850-1050 MPa, the tensile strength is 1180-1300 MPa, and A is₈₀The elongation after fracture is more than or equal to 10 percent, and the minimum bending core radius of transverse 180-degree cold bending is less than or equal to 2.0 t; the thickness of the finished product is 1.0-1.6 mm; the product of the invention not only meets the requirements of high strength and high plasticity of the steel for automobiles, but also has excellent forming and hydrogen-induced delayed fracture resistance.

Has the advantages that:

compared with the prior art, the invention has the following beneficial effects:

(1) the steel material of the invention mainly takes C, Mn, Si, Al and Cr as main elements, and has lower original cost.

(2) The invention adopts the production process of converter smelting, slab continuous casting, acid pickling cold rolling and continuous annealing, can realize the industrial production of the automobile steel on the traditional production line, and has the advantages of low cost, no need of adding new production equipment and stable production process.

(3) The ultrahigh-strength cold-rolled DH1180 steel plate prepared by the method is added with residual austenite and bainite on the basis of the traditional cold-rolled dual-phase steel, and has the characteristics of high strength, high plasticity and high hole expanding performance under the coupling action of transformation induced plasticity (TRIP) effect and bainite coordinated deformation. The microstructure of the cold-rolled continuous annealing product comprises 10-30% of ferrite, 40-70% of martensite, 4-12% of residual austenite and 5-20% of bainite according to volume percentage.

(4) The ultrahigh-strength cold-rolled DH1180 steel plate prepared by the method is sampled along the transverse direction, the yield strength is 850-1050 MPa, the tensile strength is 1180-1300 MPa, and A₈₀The elongation after fracture is more than or equal to 10 percent, the minimum bending core radius of transverse 180-degree cold bending is less than or equal to 2.0t, and t is the thickness of a finished steel plate; the thickness of the finished product is 1.0-1.6 mm; the product of the invention not only meets the requirements of high strength and high plasticity of the steel for automobiles, but also has excellent forming and hydrogen-induced delayed fracture resistance.

Drawings

FIG. 1 is a typical microstructure of a steel sheet of example 1;

FIG. 2 is an engineering stress-strain curve of example 1.

Detailed Description

The following examples are intended to illustrate the invention in detail, and are intended to be a general description of the invention, and not to limit the invention.

The chemical compositions of the example steels are listed in table 1, the continuous casting and hot rolling process parameters of the example steels are listed in table 2, the cold rolling and continuous annealing process parameters of the example steels are listed in table 3, and the structures of the example steels are given in table 4; table 5 gives the overall properties of the steels of the examples;

table 1 chemical composition of the example steels, wt.%

Examples	C	Mn	Si	Al	Cr	Mo	P	S	V	Nb	Ti
												1	0.21	2.24	0.72	0.74	0.05	0.35	0.004	0.002	-	-	0.003
2	0.23	2.18	0.65	0.55	0.34	0.26	0.005	0.001	-	-	0.017
												3	0.20	2.55	0.88	0.85	0.36	0.09	0.008	0.002	0.032	-	0.011
4	0.22	2.42	1.23	0.36	0.06	0.18	0.003	0.003	-	0.022	0.005
												5	0.18	2.74	0.54	1.33	0.57	0.06	0.004	0.002	-	0.036	0.042
6	0.20	2.11	0.62	1.06	0.05	0.14	0.007	0.003	-	-	0.018
												7	0.25	1.85	0.55	0.52	0.35	0.07	0.004	0.001	-	-	0.031
8	0.23	2.32	1.32	0.64	0.24	0.08	0.002	0.003	0.021	-	0.036
												9	0.19	2.65	1.40	0.34	0.03	0.31	0.005	0.002	0.046	-	0.006
10	0.22	2.68	0.78	0.04	0.42	0.23	0.003	0.001	-	0.016	0.021

TABLE 2 continuous casting and Hot Rolling Process of the steels of the examples

Table 3 cold rolling annealing process of steel of examples

TABLE 4 Structure of the steels of the examples

TABLE 5 comprehensive Properties of the steels of the examples

Note: the delayed fracture resistance evaluation adopts U-shaped bending soaking evaluation, the 180-degree cold bending radius is 5mm, each group of 5 parallel samples are placed in 0.1mol/L HCl solution to be soaked for 14 days, and if no fracture occurs, the samples are judged to have no delayed fracture risk.

As can be seen from the above embodiments, the ultrahigh-strength cold-rolled DH1180 steel plate prepared by adopting the component design, rolling and continuous annealing process of the invention is sampled along the transverse direction, the yield strength is 850-1050 MPa, the tensile strength is 1180-1300 MPa, and A is₈₀The elongation after fracture is more than or equal to 10 percent, and the minimum bending core radius of transverse 180-degree cold bending is less than or equal to 2.0t (t is the thickness of a steel plate); the thickness of the finished product is 1.0-1.6 mm; the product of the invention not only meets the requirements of high strength and high plasticity of the steel for automobiles, but also has excellent forming and hydrogen-induced delayed fracture resistance.

Claims

1. The ultrahigh-strength cold-rolled DH1180 steel is characterized by comprising the following chemical components in percentage by mass: c: 0.18-0.25%, Mn: 1.8% -2.8%, Si: 0.5% -1.4%, Al: 0.02% -1.4%, Cr: 0.03-0.60%, Mo: 0.04-0.4%, Ti: 0.002% -0.1%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, V is less than or equal to 0.05%, Nb is less than or equal to 0.1%, and Si + Al: 0.8% -2.0%, Mo + Ti: 0.10 to 0.50 percent, and the balance of iron and inevitable impurities.

2. The ultra-high strength cold-rolled DH1180 steel according to claim 1, wherein the microstructure of the finished cold-rolled steel plate is 10-30% ferrite, 40-70% martensite, 4-12% retained austenite and 5-20% bainite by volume percentage.

3. The ultra-high strength cold-rolled DH1180 steel according to claim 1 or 2, wherein the thickness of the finished steel plate is 1.0-1.6 mm, the finished steel plate is sampled along the direction vertical to the rolling direction, the yield strength is 850-1050 MPa, the tensile strength is 1180-1300 MPa, A₈₀The elongation after fracture is more than or equal to 10 percent, and the minimum bending core radius of transverse 180-degree cold bending is less than or equal to 2.0t, wherein t is the thickness of a steel plate.

4. A method for producing the ultra-high strength cold rolled DH1180 steel of claim 1, 2 or 3, by a process comprising: converter smelting, slab continuous casting, hot rolling, acid pickling cold rolling and continuous annealing, and is characterized in that:

hot rolling: the charging temperature of a casting blank is 550-700 ℃, the heating temperature is 1200-1300 ℃, the initial rolling temperature is 1050-1150 ℃, the final rolling temperature is 880 ℃, and the coiling temperature is 550-650 ℃; the thickness of the hot rolled steel plate is 2.0-4.0 mm;

acid pickling and cold rolling: the cold rolling reduction rate is 40-70%;

and (3) continuous annealing: the annealing temperature is controlled to be 790-910 ℃, the annealing time is 30-600 s, the slow cooling outlet temperature is 680-750 ℃, the rapid cooling rate is more than 30 ℃/s, the overaging temperature is 350-450 ℃, and the overaging time is 30-3600 s; the polishing elongation in the polishing process is controlled within the range of 0.3-1.2%.

5. The method of making an ultra-high strength cold rolled DH1180 steel according to claim 4, wherein the method comprises: the microstructure of the hot-rolled steel plate comprises 20-50% of ferrite, 30-60% of pearlite, 5-20% of bainite and 1-5% of cementite by volume percentage.