Background
Currently, hot rolled advanced high strength steels for automobiles, such as Dual Phase (DP) steels, transformation induced plasticity (TRIP) steels, have been applied to the preparation of automotive structural members. In the actual production process, the traditional dual-phase steel is difficult to form on a plurality of high-ductility parts and difficult to meet the requirement of complicated stamping structural parts for automobile design. The elongation of the transformation induced plasticity steel is a certain margin, waste is caused, the higher the strength grade is, the poorer the weldability is, but the residual austenite in the TRIP steel can absorb energy when acting as an external force, the transformation of the transformation induced plasticity is generated, namely the TRIP effect is generated, and the safety coefficient is improved.
Therefore, the dual-phase steel with high strength, high ductility and TRIP effect is developed, not only can meet the requirements of complex stamping structural parts, but also has better economic applicability and safety.
Disclosure of Invention
In view of the above, the invention provides 700MPa grade hot-rolled TRIP assisted dual-phase steel and a preparation method thereof, and the steel has the characteristics of high strength, high elongation and TRIP effect through the control of each phase component in the structure.
The invention provides 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel, which comprises a steel main body and a surface layer covering the steel main body, wherein the microstructure of the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel consists of three phases of ferrite, martensite and retained austenite; the superficial tissues are as follows by volume percentage: 3-6% of residual austenite, 16-25% of martensite and the balance of ferrite precipitated by point carbides; the main structure of the steel is as follows: 2-5% of retained austenite, 26-40% of martensite and the balance of ferrite without carbide precipitation.
Preferably, the number of the point-like carbides in the range of 0 to 44 μm in thickness from the surface of the 700MPa class hot-rolled TRIP assist type dual-phase steel is 15 to 19 on average per 1000 μm square; at a distance of 44 μm from the surface to half the thickness of the strip, no carbides of any shape are precipitated. In the surface layer thickness range of 44 mu m from the surface of the steel plate, the precipitation of carbides from the surface to the inside is in a descending trend, and the distribution trend of the precipitated carbides directly influences the steel performance and the formation of residual austenite.
Preferably, the surface layer is a thickness range of 44 μm from the surface of the steel sheet.
Preferably, the 700MPa grade hot-rolled TRIP assisted dual-phase steel comprises the following chemical components: by mass percentage, 0.14 to 0.16 percent of C, 0.42 to 0.65 percent of Si, 1.6 to 1.7 percent of Mn, 0.5 to 0.8 percent of Al, less than or equal to 0.014 percent of P, less than or equal to 0.003 percent of S, and the balance of Fe and inevitable impurities.
Preferably, the retained austenite is in the form of a thin film. The thin film of retained austenite can increase the elongation of the steel.
Preferably, the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel has the specified plastic elongation strength of 384-406MPa, the tensile strength of 692-707MPa and the elongation after fracture at the gauge length of 80mm of 22.5-24.5%.
The invention provides a preparation method of the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel, which comprises the following steps:
s1, smelting and casting molten iron to obtain a casting blank meeting the chemical component requirement, and preserving the heat of the casting blank under the protection of inert gas;
s2, controlling hot rolling of the heat-preserved casting blank to obtain a hot rolled steel strip;
and S3, controlling and cooling the hot rolled steel strip, and coiling to obtain the product.
The TRIP type dual-phase steel with the specific microstructure is formed through the process flows of heat preservation, hot rolling, cooling and coiling, and particularly the special coiling process after cooling is beneficial to forming of film-shaped residual austenite.
Preferably, in step S1, the heat preservation is to heat the casting blank to 1150-1250 ℃ under 99.99% high-purity nitrogen and preserve the heat for 1.5-2.5 h. The temperature is kept to be uniform.
Preferably, in step S2, the controlled hot rolling includes 2-3 passes of rough rolling and 3-5 passes of finish rolling, the total reduction of the rough rolling is 82-84%, the steel sheet is fed into a coil box for heat preservation after each pass of the rough rolling, the finish rolling is carried out at a constant speed of 7-8m/S, and the total reduction of the finish rolling is 84-85%; the thickness of the hot rolled steel strip is 2.8-3.2 mm. The distribution trend of precipitated carbide in steel is influenced in the heat preservation and hot rolling processes.
Preferably, in step S3, the controlling and cooling step includes: cooling the steel strip obtained in the step S2 to 665-; and coiling the cooled steel strip, and then stacking and cooling the steel strip to room temperature of 10-35 ℃. The formation of three-phase composite phases on the surface layer and the main body of the steel is influenced in the cooling process, and the volume fraction of ferrite in the structure is reduced and the elongation of the steel plate is reduced due to overhigh water cooling finishing temperature before air cooling; too low a water cooling finishing temperature may result in too much ferrite volume fraction, reducing the strength of the steel. The water cooling finishing temperature before coiling is too low, so that a large amount of martensite is formed in the structure, and the elongation of the steel plate is greatly reduced; when the temperature is too high after the water cooling, the martensite is insufficiently formed, and the strength of the steel plate is reduced.
Compared with the prior art, the invention has the following advantages: the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel provided by the invention has the specified plastic elongation strength of 384-406MPa, the tensile strength of 692-707MPa and the elongation (80mm gauge length) after fracture of 22.5-24.5%, can be widely applied to complex automobile structural members and chassis members, can solve the complex stamping problem in the stamping forming process, has certain forming advantages in the processes of bending, flanging and bulging, can generate the TRIP effect under the action of external force, improves the safety coefficient, and has better economic applicability and application prospect.
Detailed Description
In order to facilitate understanding of the present invention, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited to the following specific examples.
Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention.
Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.
The invention provides 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel, which comprises a steel main body and a surface layer covering the steel main body, wherein the thickness of the surface layer is 44 mu m. The microstructure of the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel consists of three phases of ferrite, martensite and film-shaped residual austenite; the surface layer comprises 3-6% of film-shaped retained austenite, 16-25% of martensite and the balance of ferrite in percentage by volume, and point-shaped carbides are arranged in the ferrite; the steel body comprises 2-5% of film-like retained austenite, 26-40% of martensite, and the balance of ferrite, and no carbide of any shape is precipitated in the ferrite. Specifically, the number of the point-like carbides in the range of 0 to 44 μm in thickness from the surface is 15 to 19 on average per 1000 square micrometers; at a distance of 44 μm from the surface to half the thickness of the strip, no carbides of any shape are precipitated. The chemical components of the TRIP type dual-phase steel comprise: by mass percentage, 0.14 to 0.16 percent of C, 0.42 to 0.65 percent of Si, 1.6 to 1.7 percent of Mn, 0.5 to 0.8 percent of Al, less than or equal to 0.014 percent of P, less than or equal to 0.003 percent of S, and the balance of Fe and inevitable impurities.
The specified plastic elongation strength of the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel is 384-406MPa, the tensile strength can reach 692-707MPa, and the elongation after fracture under the gauge length of 80mm can reach 22.5-24.5%.
The invention provides a preparation method of the 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel, which comprises the following steps:
s1, smelting and casting molten iron to obtain a casting blank meeting the requirements of chemical components, heating the casting blank to 1150-1250 ℃ under the protection of 99.99% high-purity nitrogen, and preserving heat for 1.5-2.5 h;
s2, controlling hot rolling of the heat-preserved casting blank, wherein 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 blank is fed into a coil box for heat preservation after each time of the rough rolling, the finish rolling is carried out at a constant speed of 7-8m/S, the total reduction of the finish rolling is 84-85%, and a hot rolled steel strip with the thickness of 2.8-3.2mm is obtained;
s3, water-cooling the hot-rolled steel strip to 680-700 ℃ at a cooling speed of 60-70 ℃/S, air-cooling for 20-30S, and water-cooling to 290-320 ℃ at a cooling speed of 25-35 ℃/S; and coiling the cooled steel strip, and then performing stack cooling to room temperature to obtain the product.
The 700MPa class hot rolled TRIP assisted dual phase steel of the present application and the method of manufacturing the same will be described in detail with reference to four specific examples.
Examples1
The embodiment provides a preparation method of 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel, which comprises the following steps:
heating the casting blank to 1200 ℃, preserving heat for 2h, and protecting by using nitrogen with the purity of 99.99% during heating. The casting blank comprises the following chemical components: and C, calculated by mass percent: 0.142%, Si: 0.65%, Mn: 1.68%, Al: 0.62%, P: 0.014%, S: 0.002%, and the balance of Fe and inevitable impurities.
And (3) starting hot rolling the heated and heat-preserved casting blank at 1180 ℃, and controlling the structure of the hot rolled steel strip to be a ferrite, martensite and a small amount of film-shaped residual austenite three-phase structure in the processes of hot rolling and cooling. The control hot rolling process comprises the following steps: carrying out 3-pass rough rolling, wherein the total reduction of the rough rolling is 83%, and feeding the steel plate into a plate rolling box for heat preservation after each pass; and performing 4 times of finish rolling, wherein the finish rolling is performed at a constant speed of 7m/s, and the total reduction of the finish rolling is 85%. Cooling the finish rolled steel strip by using an 'ultra-fast cooling + air cooling + fast cooling' control process: firstly, cooling the finish-rolled steel strip to 690 ℃ at an ultra-fast cooling speed of 70 ℃/s, then air-cooling for 30s, and then cooling to 290 ℃ at a cooling speed of 30 ℃/s; the cooled steel strip was coiled and then cooled in a heap to room temperature to obtain a hot rolled steel strip of 3.0mm thickness.
The 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel prepared by the preparation method has a microstructure consisting of three phases of ferrite, martensite and film-shaped residual austenite. The percentage of each phase (volume percentage,%) of the surface layer tissue is as follows: 4.8% of retained austenite, 21% of martensite and the balance ferrite in which point carbides are present. The main body structure of the steel except the surface structure comprises the following phases in percentage: 3.5 percent of film-shaped retained austenite, 37 percent of martensite and the balance of ferrite, and no carbide is precipitated; the number of point-like carbides in the range of 0-44 μm thickness from the surface is 18 on average per 1000 square micrometers; at 44 μm to half of the entire thickness from the surface, no carbides of any shape precipitate; the retained austenite in the structure is thin film.
The hot-rolled dual-phase steel obtained in this example was measured to have a predetermined plastic elongation of 384MPa, a tensile strength of 692MPa, and an elongation after fracture (80mm gauge length) of 22.5%.
Example 2, example 3 and example 4 all provide a preparation method of 700MPa grade hot rolled TRIP assisted dual phase steel, the preparation method is basically the same as example 1, the difference is in the chemical composition of the casting blank and part of the preparation process parameters, specifically, examples 2 to 4 adopt the chemical composition of the casting blank as shown in table 1, the control requirements of the preparation process parameters are shown in table 2, and the other parts which are not mentioned are the same as the preparation process of example 1. The microstructure and mechanical properties of the products obtained in examples 2 to 4 are shown in Table 2.
TABLE 1 quality percentage of chemical composition of cast slab in each example (%)
TABLE 2 control requirements and mechanical Properties (80mm gauge length) of the examples
As can be seen from table 2, the TRIP-assisted dual-phase steel prepared by the preparation process provided by the present application has high strength and high elongation, the thin-film retained austenite formed in the steel can significantly improve the elongation, and examples 1 to 4 all have almost uniform microstructures, wherein a scanning electron micrograph of the surface structure of the steel obtained in example 2 is shown in fig. 1.
Comparative example 1
The chemical components of the dual-phase steel casting blank provided by the comparative example are the same as those of the dual-phase steel casting blank provided by the example 4, and the preparation steps are basically the same as those of the dual-phase steel casting blank provided by the example 4, except that: the difference lies in that: the hot rolling adopts 2-pass rough rolling and 5-pass finish rolling, the total reduction of the rough rolling is 60%, the total reduction of the finish rolling is 77.5%, the entry temperature of the last pass of the finish rolling is 860 ℃, air cooling is carried out to 670 ℃ after the finish rolling is finished, then water cooling is carried out to 450 ℃, coiling is carried out, and the stack cooling is carried out to the room temperature after the coiling. The dual-phase steel provided in this comparative example had a structure composed of two types of 53% ferrite and 47% bainite, and the resulting hot-rolled dual-phase steel had a predetermined plastic elongation of 432MPa, a tensile strength of 581MPa, and an elongation after fracture (80mm gauge length) of 20%.
The chemical component preparation process and the mechanical property can be obtained by combining the embodiment of the invention and the comparison: the performance of all embodiments of the invention is better, the specified plastic elongation strength of the obtained 700 MPa-grade hot-rolled TRIP auxiliary type dual-phase steel is 384-406MPa, the tensile strength can reach 692-707MPa, and the elongation after fracture under the gauge length of 80mm can reach 22.5-24.5%. The structure is a ferrite, martensite and retained austenite three-phase structure, and the surface layer ferrite structure contains point-like carbides. The chemical composition of the comparative example was the same as that of the example, but the structure control was different, and the obtained structure was ferrite and bainite, and the ferrite had insufficient carbon removal, no retained austenite was formed, a lower specific plastic elongation, a higher tensile strength and a higher elongation were not obtained, and the TRIP effect was not further developed.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such 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 such modifications and variations.