CN109402355B - A kind of 600MPa grade hot-rolled TRIP type dual-phase steel and preparation method thereof - Google Patents

Abstract

The invention provides a 600MPa-grade hot-rolled TRIP type dual-phase steel and a preparation method thereof. The microstructure is retained austenite, martensite, and ferrite, and the surface layer structure is 4-6% retained austenite. , 5-15% martensite, the rest are ferrite, and there are rod-like carbides in the ferrite; the main structure of the steel is: 2-5% retained austenite, 25-40% martensite, the rest is ferrite, and no carbides are precipitated in the ferrite. The specified plastic elongation strength of the steel of the present invention is 333-351MPa, the tensile strength is 602-614MPa, and the elongation after fracture (80mm gauge length) is 23-25%. The steel of the invention can be applied to complex automobile structural parts and chassis parts, and has good economical efficiency and application prospect.

Description

A kind of 600MPa grade hot-rolled TRIP type dual-phase steel and preparation method thereof

technical field

The invention belongs to the technical field of advanced high-strength steel for hot-rolled automobiles, and particularly relates to a 600MPa-grade hot-rolled TRIP type dual-phase steel and a preparation method thereof.

Background technique

At present, hot-rolled advanced high-strength steels for automobiles, such as dual-phase (DP) steels and transformation-induced plasticity (TRIP) steels, have been used in the preparation of automobile structural parts. In the actual production process, we found that the traditional dual-phase steel is difficult to form on many parts with high ductility, and it is difficult to meet the requirements of complex stamping structural parts for automobile design. However, when the transformation-induced plasticity steel is used, the elongation has a certain margin, which causes waste, and the higher the strength grade, the worse the weldability, but the retained austenite in the TRIP steel can absorb energy when the external force acts, causing The phase transition induces a plastic transition, that is, the TRIP effect, which increases the safety factor.

Therefore, the development of a dual-phase steel with high strength, high ductility and TRIP effect can not only meet the requirements of complex stamping structural parts, but also have good economical applicability and safety.

SUMMARY OF THE INVENTION

In view of this, the present invention provides a 600MPa grade hot-rolled TRIP type dual-phase steel and a preparation method thereof, which have the characteristics of high strength, high elongation and TRIP effect by controlling the components of each phase in the structure.

A first aspect of the present invention provides a 600MPa-grade hot-rolled TRIP-type dual-phase steel, comprising a steel body and a surface layer covering the steel body. The microstructure of the 600MPa-grade hot-rolled TRIP-type dual-phase steel is composed of ferrite, The composition of three phases, namely, retained austenite and retained austenite; in terms of volume percentage, the surface structure is: 4-6% retained austenite, 5-15% martensite, and the rest is iron with rod-like carbide precipitation Element body; the main structure of the steel is: 2-5% retained austenite, 25-40% martensite, and the rest is ferrite without carbide precipitation.

Preferably, in the thickness range of 0-22 μm from the surface of the 600MPa hot-rolled TRIP type dual-phase steel, the average number of rod-shaped carbides per 1000 square microns is 23-27; in the thickness range of 22-43 μm from the surface The average number of rod-like carbides per 1000 square micrometers is 8-10; no carbides are precipitated from 43 μm from the surface to half the thickness of the steel strip. The carbide precipitation within the thickness of the surface layer shows a gradually decreasing trend, and the distribution trend of the precipitated carbide directly affects the steel properties and the formation of retained austenite.

Preferably, the surface layer refers to a thickness range of 43 μm from the surface of the steel plate.

Preferably, the 600MPa grade hot-rolled TRIP type dual-phase steel includes chemical composition: in mass percentage, C0.12-0.14%, Si 0.3-0.4%, Mn 1.5-1.7%, Al 0.35-0.45%, P≤ 0.014%, the rest is Fe and inevitable impurities.

Preferably, the retained austenite is in the form of a thin film. The film-like retained austenite can increase the elongation of the steel.

Preferably, the specified plastic elongation strength of the 600 MPa-grade hot-rolled TRIP type dual-phase steel is 333-351 MPa, the tensile strength is 602-614 MPa, and the elongation after fracture under the gauge length of 80 mm is 23-25%.

The second aspect of the present invention provides the preparation method of the above-mentioned 600MPa grade hot-rolled TRIP type dual-phase steel, the steps comprising:

S1. The molten iron is smelted and cast to obtain the casting billet that meets the requirements of chemical composition, and the billet is heated and kept warm under the protection of inert gas;

S2, control the hot rolling of the heat-preserved casting billet to obtain a hot-rolled steel strip;

S3. Control the cooling and coiling of the hot-rolled steel strip to obtain the product.

The invention forms the TRIP type dual-phase steel with the above-mentioned specific microstructure through the technological process of heat preservation, hot rolling, cooling and coiling, and especially the specific coiling process after cooling is beneficial to the formation of film-like retained austenite.

Preferably, in step S1, the heat preservation is to heat the billet at 1150-1250° C. for 1.5-2.5 hours, under 99.99% high-purity nitrogen protection. Insulation keeps the steel temperature uniform.

Preferably, in step S2, the controlled hot rolling includes 2-3 passes of rough rolling and 3-5 passes of finishing rolling, the total reduction of the rough rolling is 82-84%, and the finishing rolling is 7- The speed of 8m/s is uniformly rolled, and the total reduction of finishing rolling is 72-82%; the thickness of the hot-rolled steel strip is 4.3-4.7mm. Both the heat preservation and hot rolling process affect the distribution trend of the precipitated carbides in the steel.

Preferably, in step S3, the control cooling step includes: cooling the steel strip obtained in step S2 to 690-710°C by water at an ultra-fast cooling rate of 40-55°C/s, then air-cooling for 20-30s, and then cooling at 25- The cooling rate of 35°C/s is water-cooled to 250-285°C; the cooled steel strip is coiled, and then pile-cooled to room temperature. The cooling process affects the formation of the three-phase composite phase of the steel surface and the main body. Too high temperature at the end of water cooling before air cooling will reduce the volume fraction of ferrite in the microstructure and reduce the elongation of the steel plate; too low end temperature of water cooling will cause too much volume fraction of ferrite and reduce the strength of the steel. If the end temperature of water cooling before coiling is too low, a large amount of martensite will be formed in the structure, which will greatly reduce the elongation of the steel plate; if the end temperature of water cooling is too high, the formation of martensite will be insufficient and the strength of the steel plate will be reduced.

Compared with the prior art, the present invention has the following advantages: the specified plastic elongation strength of the 600MPa-grade hot-rolled TRIP type dual-phase steel provided by the present invention can reach 333-351MPa, the tensile strength can reach 602-614MPa, and the elongation after fracture can reach 333-351MPa. (80mm gauge length) can reach 23-25%, which can be widely used in complex automobile structural parts and chassis parts. It has the advantages of forming, and the TRIP effect can also occur when the external force acts, which improves the safety factor, and has good economic applicability and application prospects.

Description of drawings

Accompanying drawing 1 is the carbide morphology photograph and energy spectrum in Example 3 of the present invention.

Detailed ways

In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and in detail below with reference to the embodiments, but the protection scope of the present invention is not limited to the following specific embodiments.

Unless otherwise defined, all technical terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments, and are not intended to limit the protection scope of the present invention.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or can be prepared by existing methods.

A first aspect of the present invention provides a 600MPa-grade hot-rolled TRIP type dual-phase steel, comprising a steel body and a surface layer covering the steel body, and the thickness of the surface layer is 43 μm. The microstructure of the 600MPa-grade hot-rolled TRIP type dual-phase steel is composed of three phases: ferrite, martensite, and film-like retained austenite; in terms of volume percentage, the surface structure is: 4-6% Film-like retained austenite, 5-15% martensite, the rest is ferrite, and rod-like carbides are present in the ferrite; the main structure of the steel is: 2-5% film-like residual Austenite, 25-40% martensite, the remainder ferrite, and no carbide precipitation in the ferrite. Specifically, within the thickness range of 0-22 μm from the surface, the average number of rod-shaped carbides per 1000 square microns is 23-27; within the thickness range of 22-43 μm from the surface, the average number of rod-shaped carbides per 1000 square microns is within the range of 22-43 μm. The number of objects is 8-10; no carbides are precipitated at 43 μm from the surface to half the thickness of the steel strip. The chemical composition of TRIP type dual-phase steel includes: by mass percentage, C 0.12-0.14%, Si 0.3-0.4%, Mn 1.5-1.7%, Al 0.35-0.45%, P≤0.014%, the rest are Fe and unavoidable of impurities.

The specified plastic elongation strength of the above-mentioned 600 MPa grade hot-rolled TRIP type dual-phase steel is 333-351 MPa, the tensile strength is 602-614 MPa, and the elongation after fracture under the gauge length of 80 mm is 23-25%.

The second aspect of the present invention provides the preparation method of the above-mentioned 600MPa grade hot-rolled TRIP type dual-phase steel, the steps comprising:

S1. The molten iron is smelted and cast to obtain the cast billet that meets the chemical composition requirements, and the cast billet is heated to 1150-1250 ℃ under the protection of 99.99% high-purity nitrogen and kept for 1.5-2.5h;

S2. Controlling hot rolling of the heat-preserved slab, the controlled hot rolling includes 2-3 passes of rough rolling and 3-5 passes of finishing rolling, and the total reduction of the rough rolling is 82-84%. The finishing rolling is uniformly rolled at a speed of 7-8m/s, and the total reduction of finishing rolling is 75-82% to obtain a hot-rolled steel strip with a thickness of 4.3-4.7mm;

S3. Cool the hot-rolled steel strip to 690-720°C at a cooling rate of 40-55°C/s, then air-cool it for 20-30s, and then cool it to 250-280°C at a cooling rate of 35-35°C/s; The cooled steel strip is coiled, and then pile-cooled to room temperature to obtain the product.

The 600MPa-grade hot-rolled TRIP type dual-phase steel of the present application and the preparation method thereof will be described in detail below with reference to five specific embodiments.

Example 1

The present embodiment provides a preparation method of 600MPa grade hot-rolled TRIP type dual-phase steel, the steps comprising:

The casting billet was heated to 1200 ℃ under the protection of 99.99% high-purity nitrogen for 2 hours, and protected with nitrogen with a purity of 99.99% during heating. The chemical composition of the cast slab is as follows: in terms of mass percentage, C: 0.124%, Si: 0.39%, Mn: 1.67%, Alt: 0.42%, P: 0.014%, and the rest are Fe and inevitable impurities.

The hot-rolled slab after heating and heat preservation is started at 1180°C, and the hot-rolling process is controlled as follows: 3 passes of rough rolling, the total rough rolling reduction is 83%, and 4 passes of finishing rolling are carried out. The speed of 7.5m/s was uniformly rolled, the entry temperature of the final pass of finishing rolling was 880°C, and the total reduction of finishing rolling was 79%, and a hot-rolled steel strip with a thickness of 4.5mm was obtained;

The obtained hot-rolled steel strip is controlled to be cooled, and the microstructure of the hot-rolled steel strip is controlled to be a three-phase structure of ferrite, martensite and retained austenite. The specific control cooling process is as follows: the steel strip is water-cooled to 700°C at a cooling rate of 50°C/s, then air-cooled for 25s, and then water-cooled to 275°C at a cooling rate of 30°C/s; Coiled and then pile-cooled to room temperature to obtain the product.

The 600MPa-grade hot-rolled TRIP type dual-phase steel prepared by the above preparation method has a microstructure composed of three phases: ferrite, martensite and thin-film retained austenite. The percentage of each phase (volume percentage, %) of the surface structure is: 4.8% retained austenite, 8.6% martensite, the rest are ferrite, and there are rod-like carbides in the ferrite. The percentage of each phase of the main structure of the steel except the surface structure is: 2.5% retained austenite, 31% martensite, the rest is ferrite, no carbide precipitation; within the thickness range of 0-22μm from the surface , the average number of rod-shaped carbides per 1000 square microns is 23; in the thickness range of 22 μm-343 μm from the surface, the average number of rod-shaped carbides per 1000 square microns is 10; 43 μm from the surface - the entire thickness There is no carbide precipitation at one-half of the structure; the retained austenite in the structure is all film-like.

The specified plastic elongation strength of the hot-rolled dual-phase steel obtained in this example was determined to be 351 MPa, the tensile strength was 614 MPa, and the elongation after fracture (80 mm gauge length) was 23.5%.

Example 2

This embodiment provides a 600MPa-grade hot-rolled TRIP-type dual-phase steel, which adopts basically the same preparation steps as in Embodiment 1, except that the chemical composition of the cast slab is: in terms of mass percentage, C: 0.131%, Si : 0.36%, Mn: 1.55%, Alt: 0.45%, P: 0.012%, and the rest are Fe and inevitable impurities.

The obtained 600MPa hot-rolled TRIP type dual-phase steel microstructure is composed of three phases: ferrite, martensite and thin-film retained austenite. The percentage of each phase (volume percentage, %) of the surface structure is: 5.2% retained austenite, 11% martensite, the rest are ferrite, and there are rod-like carbides in the ferrite. The percentage of each phase of the main structure of the steel except the surface structure is: 3.5% retained austenite, 38% martensite, the rest is ferrite, no carbide precipitation; within the thickness range of 0-22μm from the surface , the average number of rod-shaped carbides per 1000 square microns is 26; in the thickness range of 22 μm-343 μm from the surface, the average number of rod-shaped carbides per 1000 square microns is 8; 43 μm from the surface - the entire thickness There is no carbide precipitation at one-half of the structure; the retained austenite in the structure is all film-like.

The specified plastic elongation strength of the hot-rolled dual-phase steel obtained in this example was measured to be 337 MPa, the tensile strength was 613 MPa, and the elongation after fracture (80 mm gauge length) was 24.5%.

Example 3

This embodiment provides a 600MPa-grade hot-rolled TRIP-type dual-phase steel, which adopts basically the same preparation steps as in Embodiment 1, except that the chemical composition mass percentages of the cast slab are C: 0.136%, Si: 0.34%, Mn : 1.61%, Alt: 0.37%, P: 0.014%, and the rest are Fe and inevitable impurities.

The obtained 600MPa hot-rolled TRIP type dual-phase steel microstructure is composed of three phases: ferrite, martensite and thin-film retained austenite. The percentage of each phase (volume percentage, %) of the surface structure is: 4.4% retained austenite, 9% martensite, the rest are ferrite, and there are rod carbides in the ferrite. The percentage of each phase of the main structure of the steel except the surface structure is: 2.5% retained austenite, 29% martensite, and the rest are ferrite, no carbide precipitation; within the thickness range of 0-22μm from the surface , the average number of rod-shaped carbides per 1000 square microns is 27; in the thickness range of 22 μm-343 μm from the surface, the average number of rod-shaped carbides per 1000 square microns is 10; 43 μm from the surface - the entire thickness There is no rod-like carbide precipitation at one-half of the structure; the retained austenite in the structure is all film-like.

The morphology photo and energy spectrum of the carbide in this example are shown in FIG. 1 , and the specific data corresponding to the energy spectrum in FIG. 1 is shown in Table 1.

Carbide energy spectrum in the embodiment 3 of table 1

The specified plastic elongation strength of the hot-rolled dual-phase steel obtained in this example was determined to be 333 MPa, the tensile strength was 602 MPa, and the elongation after fracture (80 mm gauge length) was 24%.

Comparative Example 1

The chemical composition of the dual-phase steel billet provided in this comparative example is the same as that of Example 3, and the preparation steps are basically the same as those of Example 3. The difference is that the hot rolling adopts 2 passes of rough rolling + 5 passes of finishing rolling, The total reduction is 60%, the total reduction of finishing rolling is 77.5%, the entry temperature of the final pass of finishing rolling is 860 ℃, air-cooled to 670 ℃ after finishing rolling, and then water-cooled to 450 ℃, coiled, and pile-cooled to room temperature.

All the structures of the dual-phase steel provided in this comparative example are composed of 55% ferrite and 45% bainite. The specified plastic elongation strength of the obtained hot-rolled dual-phase steel is 449MPa, and the tensile strength is 592MPa, the elongation after fracture (80mm gauge length) is 20%.

Combining the examples of the present invention with the comparative chemical composition preparation process and mechanical properties, it can be concluded that all the examples of the present invention have better performance, and the obtained 600MPa-grade hot-rolled TRIP type dual-phase steel has a specified plastic elongation strength of 333-351MPa, and a resistance to The tensile strength is 602-614MPa, and the elongation after break (80mm gauge length) is 23-25%. The structure is a three-phase structure of ferrite, martensite and retained austenite, and the surface ferrite structure contains rod-like carbides. Although the chemical composition of the comparative example is the same as that of the example, the microstructure control is different. The obtained microstructure is ferrite and bainite. The carbon expulsion in the ferrite is insufficient, the retained austenite is not formed, and the lower specified plasticity is not obtained. Elongation strength, higher tensile strength and higher elongation cannot form the TRIP effect.

Example 4

The present embodiment provides a preparation method of 600MPa grade hot-rolled TRIP type dual-phase steel, the steps comprising:

The slab was heated to 1180°C for 2.5h, and protected with nitrogen with a purity of 99.99% during heating. The chemical composition of the cast slab is as follows: in terms of mass percentage, C: 0.124%, Si: 0.39%, Mn: 1.67%, Alt: 0.42%, P: 0.014%, and the rest are Fe and inevitable impurities.

The hot-rolled slab after heating and heat preservation is started at 1150°C, and the hot-rolling process is controlled as follows: 3 passes of rough rolling, the total rough rolling reduction is 83%, and 4 passes of finishing rolling are carried out. The speed of 7m/s was uniformly rolled, the entry temperature of the final pass of finishing rolling was 850℃, the total reduction of finishing rolling was 76%, and a hot-rolled steel strip with a thickness of 4.7mm was obtained;

The obtained hot-rolled steel strip is controlled to be cooled, and the microstructure of the hot-rolled steel strip is controlled to be a three-phase structure of ferrite, martensite and retained austenite. The specific control cooling process is: cooling to 690°C at a cooling rate of 52°C/s, then air cooling for 20s, and then cooling to 265°C at a cooling rate of 25°C/s; coiling the cooled steel strip, and then stack cooling The product was obtained at room temperature. The properties of the obtained product are basically the same as those of Example 1.

Example 5

The present embodiment provides a preparation method of 600MPa grade hot-rolled TRIP type dual-phase steel, the steps comprising:

The slab was heated to 1250°C for 1.5h, and protected with nitrogen with a purity of 99.99% during heating. The chemical composition of the cast slab is as follows: in terms of mass percentage, C: 0.124%, Si: 0.39%, Mn: 1.67%, Alt: 0.42%, P: 0.014%, and the rest are Fe and inevitable impurities.

The hot-rolled slab after heating and heat preservation is started at 1220°C, and the hot-rolling process is controlled as follows: 3 passes of rough rolling, the total rough rolling reduction is 83%, and 4 passes of finishing rolling are carried out. The speed of 8m/s is uniformly rolled, the entry temperature of the final pass of finishing rolling is 860 °C, and the total reduction of finishing rolling is 82%, and a hot-rolled steel strip with a thickness of 4.3 mm is obtained;

The obtained hot-rolled steel strip is controlled to be cooled, and the microstructure of the hot-rolled steel strip is controlled to be a three-phase structure of ferrite, martensite and retained austenite. The specific control cooling process is as follows: the steel strip is water-cooled to 710°C at a cooling rate of 45°C/s, then air-cooled for 20s, and then water-cooled to 275°C at a cooling rate of 28°C/s; Coiled and then pile-cooled to room temperature to obtain the product. The properties of the obtained product are basically the same as those of Example 1.

Although preferred embodiments of the present invention have been described, additional changes and modifications to these embodiments may occur to those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be construed to include the preferred embodiment and all changes and modifications that fall within the scope of the present invention.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, provided that these 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 these modifications and variations.

Claims (3)

1. A preparation method of 600 MPa-grade hot-rolled TRIP type dual-phase steel comprises the following steps:

s1, smelting and casting molten iron to obtain a casting blank meeting the chemical component requirement, heating the casting blank under the protection of inert gas and preserving heat; wherein, by mass percent, the casting blank comprises the following chemical components: 0.12 to 0.14 percent of C, 0.3 to 0.4 percent of Si, 1.5 to 1.7 percent of Mn, 0.35 to 0.45 percent of Al0.014 percent of P, and the balance of Fe and inevitable impurities; the heat preservation is to preserve the heat of the casting blank for 1.5 to 2.5 hours at 1150-1250 ℃ under the protection of 99.99 percent of high-purity nitrogen;

s2, controlling hot rolling of the heat-preserved casting blank to obtain a hot rolled steel strip; the controlled hot rolling comprises 2-3 times of rough rolling and 3-5 times of finish rolling, the total reduction of the rough rolling is 82-84%, the finish rolling is carried out at a constant speed of 9m/s, and the total reduction of the finish rolling is 75-82%; the thickness of the hot rolled steel strip is 4.3-4.7 mm;

s3, controlling and cooling the hot rolled steel strip, and coiling to obtain a product; the step of controlling cooling comprises: water-cooling the steel strip obtained in the step S2 to 690-720 ℃ at a cooling speed of 40-55 ℃/S, then air-cooling for 20-30S, and then water-cooling to 250-285 ℃ at a cooling speed of 25-35 ℃/S; coiling the cooled steel strip, and then stacking and cooling to room temperature;

the 600 MPa-grade hot-rolled TRIP-type dual-phase steel comprises a steel main body and a surface layer covering the steel main body, wherein the microstructure of the 600 MPa-grade hot-rolled TRIP-type dual-phase steel consists of three phases of ferrite, martensite and retained austenite; the superficial tissues are as follows by volume percentage: 4-6% of retained austenite, 5-15% of martensite, and the balance of ferrite with rod-shaped carbide separated out; the main structure of the steel is as follows: 2-5% of retained austenite, 25-40% of martensite and the balance of ferrite without carbide precipitation; the residual austenite is in a thin film shape; wherein the surface layer is a thickness range of 43 μm from the surface of the steel sheet.

2. The method of manufacturing hot-rolled TRIP type dual phase steel of 600MPa grade according to claim 1, characterized in that: the number of the rodlike carbides in the range of 0-22 μm thickness from the surface of the 600MPa grade hot-rolled TRIP type dual-phase steel is 23-27 on average per 1000 square micrometers; the number of rod-like carbides in the range of 22-43 μm thickness from the surface is 8-10 on average per 1000 square micrometers; at a distance of 43 μm from the surface to half the thickness of the strip, no carbides were precipitated.

3. The method of manufacturing hot-rolled TRIP type dual phase steel of 600MPa grade according to claim 1, characterized in that: the specified plastic elongation strength of the 600 MPa-grade hot-rolled TRIP-type dual-phase steel is 333-351MPa, the tensile strength is 602-614MPa, and the elongation after fracture at a gauge length of 80mm is 23-25%.

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