CN111979489A

CN111979489A - 780 MPa-grade high-plasticity cold-rolled DH steel and preparation method thereof

Info

Publication number: CN111979489A
Application number: CN202010928563.2A
Authority: CN
Inventors: 张瑞坤; 刘仁东; 林利; 徐鑫; 苏洪英; 李岩; 张南; 李春林; 梁笑; 吴萌
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2020-09-07
Filing date: 2020-09-07
Publication date: 2020-11-24
Anticipated expiration: 2040-09-07
Also published as: CN111979489B

Abstract

The invention discloses 780MPa grade high plasticity cold rolling DH steel and a preparation method thereof. The steel contains C: 0.10-0.18%, Mn: 1.5% -2.5%, Si: 0.4-0.8%, Al: 0.02-0.7%, Cr: 0.02-0.50%, P is less than or equal to 0.01%, S is less than or equal to 0.01%, Nb is less than or equal to 0.1%, Ti is less than or equal to 0.1%, and Si + Al: 0.5 to 1.5 percent, and the balance of iron and inevitable impurities. The initial rolling temperature is 1050-1150 ℃, the final rolling temperature is more than or equal to 900 ℃, and the coiling temperature is 550-700 ℃; the cold rolling reduction rate is 40-80 percent; the continuous annealing preheating temperature is 200-600 ℃, the annealing temperature is 760-880 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-760 ℃, the rapid cooling rate is more than 20 ℃/s, the overaging temperature is 300-420 ℃, and the overaging time is 30-3600 s; the finishing elongation is 0.3-0.7%. The product is used for parts with high requirements on formability in the automobile industry.

Description

780 MPa-grade high-plasticity cold-rolled DH steel and preparation method thereof

Technical Field

The invention belongs to the technical field of cold-rolled steel, and relates to DH780 cold-rolled automobile steel with high ductility and high formability 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 106119716 a discloses a plasticity-enhanced cold-rolled hot-dip galvanized dual-phase steel and a production method thereof, and the main chemical components of the steel are as follows: c: 0.12% -0.18%, Si: 0.3% -0.6%, Mn: 1.3% -2.3%, Al: 0.4-0.9 percent, less than or equal to 0.01 percent of P, less than or equal to 0.01 percent of S, and the production process mainly adopts cold rolling hot galvanizing treatment, and the finished product is a hot galvanizing product, and the patent does not relate to cold rolling continuous annealing steel plates.

Patent document CN 109554616 a discloses a 700MPa grade hot-rolled TRIP assisted dual-phase steel and a preparation method thereof, which mainly comprises the following chemical components: c: 0.14% -0.16%, Si: 0.42-0.65%, Mn: 1.6-1.7%, Al: 0.5 to 0.8 percent, less than or equal to 0.014 percent of P and less than or equal to 0.003 percent of S, the production process mainly adopts hot rolling, the strength grade is 700MPa, the strength grade of the patent can not reach the DH780 performance requirement, and the product is a hot rolled product.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to develop an economical cold-rolled DH780 steel and a manufacturing method thereof, which can meet the production conditions of the traditional production line and control the alloy cost, and the cold-rolled DH780 product manufactured by the invention has excellent ductility and formability, thereby providing a technical scheme for vast automobile manufacturers and steel companies.

The specific technical scheme is as follows:

a780 MPa grade high plasticity cold rolled DH steel, the chemical composition in the steel contains by the mass percent: c: 0.10-0.18%, Mn: 1.5% -2.5%, Si: 0.4-0.8%, Al: 0.02-0.7%, Cr: 0.02-0.50%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, Nb is less than or equal to 0.1%, Ti is less than or equal to 0.1%, and Si + Al: 0.5 to 1.5 percent, the balance being Fe and inevitable impurities, sampling the steel along the direction vertical to the rolling direction (transverse direction), wherein the yield strength is 450 to 550MPa, the tensile strength is 780 to 880MPa, and A₈₀The elongation after fracture is more than or equal to 21 percent, and the requirements of high strength, high extensibility and high formability of automobile parts are met.

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 meanwhile, the sufficient carbon element is beneficial to stabilizing austenite, so that necessary conditions are provided for a certain amount of stable residual austenite existing in the steel at normal temperature, and the forming performance of the steel is further improved. 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, in the invention, the content of the C element is controlled as C: 0.10 to 0.18 percent.

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 1.5-2.5 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 0.4-0.8%.

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 transformation of bainite to improve deformability. 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 0.7 percent in the invention. In addition, the invention also controls the ratio of Si + Al: 0.5 to 1.5 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 the range of 0.02 to 0.50 percent in the invention.

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.

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.

Ti: the grain size can be refined by adding a small amount of Ti element, the toughness of the material can be obviously improved, not more than 0.1 percent of Ti can be added according to actual conditions, and Ti micro-alloying elements can not be added in order to control the production cost. The invention controls the Ti content to be less than or equal to 0.1 percent.

The invention also provides a preparation method of the 780MPa grade high plasticity cold-rolled DH steel, which is characterized by comprising the following steps: converter smelting, 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: 0.10-0.18%, Mn: 1.5% -2.5%, Si: 0.4-0.8%, Al: 0.02-0.7%, Cr: 0.02 to 0.50 percent of Fe, less than or equal to 0.03 percent of P, less than or equal to 0.03 percent of S, less than or equal to 0.1 percent of Nb, less than or equal to 0.1 percent of Ti, and the balance of Fe and inevitable impurities.

Hot rolling: the charging temperature of the casting blank is 500-700 ℃, the heating temperature is 1180-1300 ℃, the initial rolling temperature is 1050-1150 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 550-700 ℃. The thickness of the hot rolled steel plate is 2-6 mm, and the hot rolled microstructure of the steel plate comprises 30-70% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite according to volume percentage.

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 40-80%. 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: the annealing temperature is 760-880 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-760 ℃, the rapid cooling rate is more than 20 ℃/s, the overaging temperature is 300-420 ℃, and the overaging time is 30-3600 s; the polishing elongation in the polishing process is controlled within the range of 0.3-0.7%. The microstructure of the cold-rolled continuous annealing product is 30-60% of ferrite, 30-60% of martensite, 3-12% of residual austenite and 3-15% of bainite structure by volume percentage, and the total is 100%.

The annealing temperature of the critical zone is 760-880 ℃, 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 10-600 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 plasticity of the steel plate is reduced.

The overaging temperature is 300-420 ℃, and in order to ensure that a certain proportion of stable residual austenite is obtained at room temperature, the overaging temperature is improved compared with that of the traditional dual-phase steel, but the overaging temperature is not too high, otherwise, the target structure and performance are difficult to obtain; the overaging time is controlled to be 30-3600 s.

The yield strength of the finished cold-rolled continuous-annealed DH steel sheet/steel strip obtained by the method is 450-550 MPa, the tensile strength is 780-880 MPa, and A is₈₀The elongation after fracture is more than or equal to 21 percent, and the thickness is 0.8-1.6 mm.

Has the advantages that:

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

(1) the chemical components of the steel material mainly comprise C, Mn, Al and Si, and a proper amount of Cr, Nb, Ti and other elements are selectively added on the basis, so that the original cost is low. Simultaneously, the synergistic effect of Si and Al is exerted, thereby improving the toughness and the plasticity of the steel.

(2) The invention adopts the production process of converter smelting, slab continuous casting, hot continuous rolling, acid pickling cold rolling and continuous annealing, can realize the industrial production of DH780 cold rolled products on the traditional cold rolled dual-phase steel production line, and has the advantages of low production cost, no need of adding new production equipment and stable production process.

(3) The cold-rolled DH steel plate produced by the invention introduces the residual austenite and the bainite on the basis of the traditional cold-rolled dual-phase steel, realizes that the ductility of the material is obviously improved in the forming process of parts under the coupling action of the phase transformation induced plasticity (TRIP) effect and the coordinated deformation of the martensite/lower bainite mixed structure, and simultaneously shows better collision energy-absorbing effect.

(4) The yield strength of the cold-rolled DH steel sheet strip produced by the invention sampled along the transverse direction is 450-550 MPa, the tensile strength is 780-880 MPa, A₈₀The elongation after fracture is more than or equal to 21 percent, and the thickness of the finished product is 0.8-1.6 mm.

(5) The finished cold-rolled DH780 steel plate of the invention has the structure according to the volume percentage: 30 to 60 percent of ferrite, 30 to 60 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite structure.

Drawings

FIG. 1 is a metallographic 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	P	S	Nb	Ti
										1	0.16	2.12	0.75	0.05	0.02	0.006	0.002	-	-
2	0.14	1.88	0.55	0.25	0.25	0.003	0.001	0.015	-
										3	0.15	2.06	0.68	0.65	0.04	0.008	0.003	-	0.024
4	0.17	1.62	0.44	0.64	0.38	0.003	0.002	-	0.025
										5	0.16	1.77	0.45	0.58	0.06	0.006	0.002	-	-
6	0.13	2.08	0.56	0.03	0.47	0.005	0.001	0.018	-
										7	0.18	1.85	0.45	0.62	0.15	0.002	0.003	-	0.025
8	0.15	1.56	0.62	0.02	0.34	0.004	0.003	-	-
										9	0.13	2.04	0.57	0.64	0.36	0.003	0.004	-	-
10	0.12	2.31	0.77	0.03	0.44	0.006	0.001	-	0.071
										11	0.16	1.85	0.54	0.51	0.16	0.004	0.002	-	-
12	0.10	2.26	0.76	0.04	0.43	0.008	0.004	0.065	-
										13	0.11	2.43	0.72	0.32	0.25	0.002	0.005	0.018	0.056
14	0.15	2.15	0.50	0.70	0.20	0.004	0.002	0.016	-

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

As can be seen from the above examples, the cold-rolled continuous annealing steel plate prepared by adopting the component design, rolling and continuous annealing process of the invention has the yield strength of 450-550 MPa, the tensile strength of 780-880 MPa and A₈₀The elongation after fracture is 21-26%, the thickness of the product is 0.8-1.6 mm, the requirements of high strength and high plasticity of the automobile steel are met, and the steel can be used for parts and parts with higher requirements on formability in the automobile industry.

Claims

1. A780 MPa grade high plasticity cold rolling DH steel, characterized by that, the chemical composition in the steel is according to the mass percent: c: 0.10-0.18%, Mn: 1.5% -2.5%, Si: 0.4-0.8%, Al: 0.02-0.7%, Cr: 0.02-0.50%, P is less than or equal to 0.03%, S is less than or equal to 0.03%, Nb is less than or equal to 0.1%, Ti is less than or equal to 0.1%, and Si + Al: 0.5 to 1.5 percent, and the balance of iron and inevitable impurities.

2. The 780MPa grade high plasticity cold rolled DH steel of claim 1, wherein the microstructure of the finished steel plate after cold rolling is as follows according to volume percentage: 30 to 60 percent of ferrite, 30 to 60 percent of martensite, 3 to 12 percent of residual austenite and 3 to 15 percent of bainite.

3. The 780MPa grade high plasticity cold rolled DH steel according to the claim 1 or 2, characterized in that the steel plate samples along the direction vertical to the rolling direction, the yield strength is 450-550 MPa, the tensile strength is 780-880 MPa, A₈₀The elongation after fracture is more than or equal to 21 percent.

4. A method for preparing 780MPa grade high plasticity cold rolled DH steel according to claim 1, 2 or 3, the steel sheet is produced by the following process: 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 500-700 ℃, the heating temperature is 1180-1300 ℃, the initial rolling temperature is 1050-1150 ℃, the final rolling temperature is above 900 ℃, and the coiling temperature is 550-700 ℃; the thickness of the hot rolled steel plate is 2-6 mm;

acid pickling and cold rolling: removing iron scale on the surface of the steel plate before cold rolling, wherein the cold rolling reduction rate is 40-80%;

and (3) continuous annealing: the annealing temperature is 760-880 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-760 ℃, the rapid cooling rate is more than 20 ℃/s, the overaging temperature is 300-420 ℃, and the overaging time is 30-3600 s; the polishing elongation in the polishing process is 0.3-0.7%.

5. The method for preparing 780MPa grade high plasticity cold rolled DH steel according to claim 4, characterized in that: the microstructure of the hot-rolled steel plate comprises 30-70% of ferrite, 20-50% of pearlite, 5-20% of bainite and 1-5% of cementite by volume percentage.