CN111979490A

CN111979490A - High-ductility and high-formability cold-rolled DH590 steel and production method thereof

Info

Publication number: CN111979490A
Application number: CN202010928566.6A
Authority: CN
Inventors: 林利; 张瑞坤; 刘仁东; 冉茂宇; 李春林; 梁笑; 李萧彤; 苏洪英; 胡秀梅; 李毅
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: CN111979490B

Abstract

The invention discloses a high-ductility and high-formability cold-rolled DH590 steel and a production method thereof. The steel contains C: 0.08-0.14%, Mn: 1.2% -2.2%, Si: 0.35-0.75%, Al: 0.02-0.50%, Cr: 0.03% -0.45%, Mo: 0.02 to 0.25 percent of the total weight of the alloy, less than or equal to 0.01 percent of P, less than or equal to 0.01 percent of S, less than or equal to 0.03 percent of Nb, and the balance of iron and inevitable impurities. The heating temperature of a casting blank is 1200-1300 ℃, the initial rolling temperature is 1100-1180 ℃, the final rolling temperature is more than or equal to 910 ℃, and the coiling temperature is 550-700 ℃; the cold rolling reduction rate is 50-90%; the continuous annealing temperature is 750-850 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-730 ℃, the rapid cooling rate is more than 20 ℃/s, the rapid cooling outlet temperature is 300-420 ℃, the overaging temperature is 300-420 ℃, and the overaging time is 30-3600 s; the finishing elongation is 0.3-0.6%. The finished steel sheet has high ductility and high formability.

Description

High-ductility and high-formability cold-rolled DH590 steel and production method thereof

Technical Field

The invention belongs to the technical field of cold-rolled steel, and relates to high-ductility and high-formability cold-rolled DH590 steel and a production 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 109402355 a discloses a 600MPa grade hot-rolled TRIP type dual phase steel and a preparation method thereof, which mainly comprises the following chemical components:c: 0.12% -0.14%, Si: 0.3% -0.4%, Mn: 1.5% -1.7%, Al: 0.35-0.45 percent of the total weight of the product, less than or equal to 0.014 percent of P, and the production process mainly adopts a hot rolling process, the product of the patent has poor plasticity, A₈₀The elongation after fracture is only 24 percent, and the cold rolled product is not involved.

Patent document CN 103827332B discloses a TRIP type martensitic dual phase steel and an ultra-high strength steel workpiece using the same, which mainly comprises the following chemical components: c: 0.1% -0.7%, Si: 0.5% -2.5%, Mn: 0.5-3.0%, Cr: 0.5-2.0%, Mo is less than or equal to 0.5%, Al: 0.04 to 2.5 percent, the production process mainly adopts a cold rolling continuous annealing process, the strength level is 1500MPa, the product has poor extension and hole expansion performance, and the performance requirements of complex stamping parts are difficult to meet.

Patent document CN 107747039 a discloses a cold-rolled dual-phase steel with high hole expansion performance and a preparation method thereof, wherein the cold-rolled dual-phase steel comprises the following main chemical components: c: 0.10% -0.50%, Si: 0.1-0.5%, Mn: 1.60% -2.00%, V: 0.01-0.03%, Al: 0.02-0.06 percent, less than or equal to 0.020 percent of P, less than or equal to 0.015 percent of S and less than or equal to 0.006 percent of N, the production process is a cold rolling continuous annealing system, and the product is low-yield-ratio dual-phase steel and is limited in forming and ductility.

Disclosure of Invention

Aiming at the problems in the prior art, the invention aims to develop the cold-rolled DH590 steel with high ductility and high formability and the production method thereof through proper smelting components, continuous casting, hot rolling, acid pickling and cold rolling and continuous annealing processes, and provides a technical scheme for vast automobile manufacturers and steel companies.

The specific technical scheme is as follows:

a high-ductility and high-formability cold-rolled DH590 steel is characterized in that the steel comprises the following chemical components (by mass percent): c: 0.08-0.14%, Mn: 1.2% -2.2%, Si: 0.35-0.75%, Al: 0.02-0.50%, Cr: 0.03% -0.45%, Mo: 0.02-0.25%, 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 0.03%, and Si + Al: 0.5 to 1.2 percent, the balance being Fe and inevitable impurities, sampling the steel along the direction vertical to the rolling direction (transverse direction), and the yield strength of the steel is 350 to 430MPaTensile strength of 590-700 MPa, A₈₀The elongation after fracture is 30-35 percent, and the hole expansion rate is not lower than 50 percent; meets the requirements of high ductility and high formability of automobiles.

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 free carbon can play a good stabilizing effect on austenite, so that the forming performance of the steel is 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, the content of the C element is controlled to be 0.08-0.14 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 1.2-2.2 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 0.35 to 0.75 percent in the invention.

Al: the aluminum element contributes to deoxidation of molten steel and can also suppress decomposition of residual austenite and precipitation of carbide. 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 0.50 percent in the invention. In addition, the invention also controls the ratio of Si + Al: 0.5 to 1.2 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 the Cr element is controlled to be 0.03-0.45 percent in the invention.

Mo: the molybdenum element is a strengthening element in the steel, is beneficial to stabilizing the retained austenite, has obvious effect of improving the hardenability of the steel, and controls the content range of the Mo element to be 0.02-0.25 percent.

P: phosphorus is a harmful element in steel, seriously reduces the plasticity and the deformability of the steel, and the lower the content, the better. 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: sulfur is a harmful element in steel, seriously affects the formability of steel, and the lower the content, the better. 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 0.03 percent can be added according to actual conditions.

The invention also provides a production method of the cold-rolled DH590 steel with high ductility and high formability, 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: 0.08-0.14%, Mn: 1.2% -2.2%, Si: 0.35-0.75%, Al: 0.02-0.50%, Cr: 0.03% -0.45%, Mo: 0.02-0.25%, 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 0.03%, Si + Al: 0.5 to 1.2 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 1100-1180 ℃, the final rolling temperature is above 910 ℃, 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 40-80% of ferrite, 10-40% 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 50-85%. 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 750-850 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-730 ℃, the rapid cooling rate is more than 20 ℃/s, the rapid cooling outlet temperature is 300-420 ℃, 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 0.3-0.6%. The microstructure of the cold-rolled continuous annealing product is 40-80% of ferrite, 10-50% of martensite, 3-12% of residual austenite and 3-10% of bainite structure by volume percentage, and the total is 100%.

The annealing temperature of the critical zone is 750-850 ℃, 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 grains of the steel plate are coarse, the annealing time is too short, and the steel plate does not finish annealing and recrystallization processes quickly, so that the ductility of the steel plate is reduced.

The overaging temperature is 300-400 ℃, and in order to ensure that a certain proportion of stable residual austenite is obtained at room temperature, the overaging temperature of the product 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.

By the method, the yield strength of the sample in the transverse direction is 350-430 MPa, the tensile strength is 590-700 MPa, and A₈₀The elongation after fracture is 30-35%, the thickness is 0.8-1.6 mm, and the hole expansion rate is not less than 50%.

Has the advantages that:

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

(1) the chemical components of the steel mainly take C, Mn, Al and Si as main elements, and a proper amount of Cr, Mo and Nb 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 rolling, acid pickling cold rolling and continuous annealing, can realize the industrial production of DH590 cold rolled products on the traditional cold rolled dual-phase steel production line, and has the advantages of low cost, no need of adding new production equipment and stable production process.

(3) The cold-rolled DH590 product produced by the invention introduces residual austenite and bainite on the basis of the traditional dual-phase steel, realizes that the ductility of the material is obviously improved in the forming process of parts under the coupling action of phase transformation induced plasticity (TRIP) effect and the coordinated deformation of martensite/lower bainite mixed structure, and simultaneously shows better hole expansion performance. The finished steel plate produced by the invention has the structure which consists of 40-80% of ferrite, 10-50% of martensite, 3-12% of residual austenite and 2-10% of bainite according to volume percentage.

(4) The transverse sampling of the cold-rolled DH590 steel strip produced by the invention can reach the yield strength of 350-430 MPa, the tensile strength of 590-700 MPa and A₈₀The elongation after fracture is 30-35 percent, and the hole expansion rate is not lower than 50 percent; meets the requirements of high ductility and high formability of automobiles.

Drawings

FIG. 1 is a 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 mechanical properties and key parameters of the example steels;

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

Examples	C	Mn	Si	Al	Cr	Mo	P	S	Nb
										1	0.12	1.82	0.62	0.46	0.15	0.15	0.006	0.003	-
2	0.13	1.66	0.54	0.31	0.24	0.02	0.002	0.004	-
										3	0.14	1.57	0.38	0.47	0.03	0.21	0.001	0.003	-
4	0.12	1.64	0.42	0.23	0.32	0.03	0.003	0.002	-
										5	0.11	1.53	0.65	0.36	0.04	0.12	0.002	0.001	0.013
6	0.10	1.62	0.55	0.42	0.22	0.05	0.005	0.002	0.018
										7	0.14	1.52	0.43	0.27	0.06	0.08	0.002	0.003	0.027
8	0.13	1.68	0.70	0.02	0.40	0.04	0.003	0.002	-
										9	0.14	1.54	0.69	0.05	0.31	0.09	0.002	0.004	-
10	0.11	1.97	0.42	0.45	0.23	0.11	0.008	0.006	-
										11	0.09	2.10	0.76	0.23	0.44	0.05	0.002	0.004	0.021
12	0.13	1.63	0.54	0.08	0.36	0.20	0.003	0.002	0.012
										13	0.08	2.20	0.45	0.40	0.13	0.23	0.005	0.003	0.010

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 mechanical properties and Key parameters of the steels of the examples

The embodiment shows that the transverse sampling of the cold-rolled DH590 product with high extensibility and high formability is prepared by adopting the component design, rolling and continuous annealing process, and the yield strength is 350-430 MPa, the tensile strength is 590-700 MPa, and A is₈₀The elongation after fracture is 30-35 percent, the hole expansion rate is more than or equal to 50 percent, and the requirements of high extensibility and high formability of the automobile steel are met.

Claims

1. A high-ductility and high-formability cold-rolled DH590 steel is characterized in that the steel comprises the following chemical components in percentage by mass: c: 0.08-0.14%, Mn: 1.2% -2.2%, Si: 0.35-0.75%, Al: 0.02-0.50%, Cr: 0.03% -0.45%, Mo: 0.02-0.25%, 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 0.03%, and Si + Al: 0.5 to 1.2 percent of iron and the balance of inevitable impurities.

2. The DH590 steel with high ductility and high formability according to claim 1, wherein the microstructure of the finished steel plate comprises, by volume percentage, 40-80% ferrite, 10-50% martensite, 3-12% retained austenite, and 3-10% bainite.

3. The DH590 steel sheet for cold rolling with high ductility and formability according to claim 1 or 2, wherein the finished steel sheet is sampled perpendicular to the rolling direction and has a yield strength of 350-430 MPa, a tensile strength of 590-700 MPa, A₈₀The elongation after fracture is 30-35 percent, and the hole expansion rate is not lower than 50 percent.

4. A method of producing a high ductility, high formability cold rolled DH590 steel according to 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 the casting blank is 550-700 ℃, the heating temperature is 1200-1300 ℃, the initial rolling temperature is 1100-1180 ℃, the final rolling temperature is above 910 ℃, and the coiling temperature is 550-700 ℃; the thickness of the hot-rolled steel plate is 2-6 mm;

acid pickling and cold rolling: the cold rolling reduction rate is 50-85%;

and (3) continuous annealing: the annealing temperature is 750-850 ℃, the annealing time is 10-600 s, the slow cooling outlet temperature is 660-730 ℃, the rapid cooling rate is more than 20 ℃/s, the rapid cooling outlet temperature is 300-420 ℃, 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 0.3-0.6%.

5. Method for producing a high ductility, high formability cold-rolled DH590 steel according to claim 4, characterized in that: the microstructure of the hot-rolled steel plate comprises 40-80% of ferrite, 10-40% of pearlite, 5-20% of bainite and 1-5% of cementite by volume percentage.