CN113528946B - 1200 MPa-grade reinforced forming complex phase steel and preparation method thereof - Google Patents

1200 MPa-grade reinforced forming complex phase steel and preparation method thereof Download PDF

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CN113528946B
CN113528946B CN202110680950.3A CN202110680950A CN113528946B CN 113528946 B CN113528946 B CN 113528946B CN 202110680950 A CN202110680950 A CN 202110680950A CN 113528946 B CN113528946 B CN 113528946B
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complex phase
phase steel
forming
cooling
mass fraction
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CN113528946A (en
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刘华赛
朱国森
韩赟
李翔宇
邱木生
阳锋
邹英
姜英花
白雪
滕华湘
李飞
章军
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Shougang Group Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

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Abstract

The invention provides 1200 MPa-grade reinforced forming complex phase steel and a preparation method thereof, belonging to the technical field of production and manufacturing of complex phase steel, wherein the reinforced forming complex phase steel comprises the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.4-0.7%, nb:0.02-0.04%, strengthening elements, and the balance of Fe and inevitable impurities; the strengthening element is at least one of the following elements: ti and Cu, wherein the mass fraction of the Ti is 0.01-0.04%, and the mass fraction of the Cu is 0.03-0.08%. The reinforced forming complex phase steel provided by the invention has the yield strength of 951-1148MPa, the tensile strength of 1201-1344MPa, the elongation A80 after fracture of 8-13%, the uniform elongation Ag of 6-10% and the hole expansion rate of 40-59% during room temperature stretching, and can effectively avoid cracking caused by too low hole expansion rate in the flanging and hole expansion processes. The reinforced forming complex phase steel obtained by the invention has good hole expanding performance and reinforced forming performance, and is particularly suitable for automobile parts with flanging hole expanding design and certain reinforced forming capability.

Description

1200 MPa-grade reinforced forming complex phase steel and preparation method thereof
Technical Field
The invention belongs to the technical field of production and manufacturing of super-strong steel, and particularly relates to 1200 MPa-grade reinforced forming complex phase steel and a preparation method thereof.
Background
The complex phase steel is a high strength steel with a multi-phase metallographic structure, and is widely applied to automobile structural members due to high strength so as to reduce thickness and weight. However, as the strength is increased, the deformability of the material is also significantly reduced, which limits the design use of the part with the forming requirements. In addition to the insufficient forming capability, the materials which are required to be selected in the design of the structural member have certain flange flanging and hole expanding capability, and the further application of the ultrahigh-strength steel is limited due to the insufficient capability. For example, a chinese patent with publication number CN201811621827.9 discloses a cold-rolled complex phase steel with tensile strength of 1200MPa grade and a preparation method thereof: the complex phase steel comprises the following chemical components in percentage by mass, 0.10-0.15% of C, si:0.1-0.5%, mn 1.5-2.6%, cr 0.4-0.7%, mo 0.2-0.5%, nb 0.02-0.05%, ti:0.02-0.05%, P is less than or equal to 0.02%, S is less than or equal to 0.015%, and the balance is Fe and inevitable impurities; the cold-rolled complex phase steel has high strength, but the lower limit of the elongation of the material is only 5%, and the elongation in the embodiment reaches 7.5%, so that the requirement of complex forming of complex parts cannot be met.
Disclosure of Invention
In order to solve the technical problems, the invention provides 1200 MPa-grade reinforced forming complex phase steel and a preparation method thereof, and the steel has good drawing and hole expanding performances on the premise of high strength so as to meet the complex forming requirements of complex parts.
On one hand, the invention provides 1200 MPa-grade reinforced forming complex phase steel which comprises the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.4-0.7%, nb:0.02-0.04%, strengthening elements, and the balance of Fe and inevitable impurities;
the strengthening element is at least one of the following elements: ti and Cu, wherein the mass fraction of the Ti is 0.01-0.04%, and the mass fraction of the Cu is 0.03-0.08%.
Further, the mass fraction of C is 0.21-0.23%, the mass fraction of Si is 1.1-1.4%, the mass fraction of Mn is 2.2-2.5%, the mass fraction of Cr is 0.5-0.7%, the mass fraction of Nb is 0.02-0.04%, the mass fraction of Ti is 0.02-0.04%, and the mass fraction of Cu is 0.04-0.06%.
Further, the metallographic structure of the complex phase steel comprises, by volume fraction, 3-6% of residual austenite, 10-20% of ferrite, 35-50% of bainitic ferrite and 30-50% of tempered martensite.
Furthermore, in the metallographic structure of the complex phase steel, the volume fraction of bainitic ferrite and tempered martensite with the size less than 5 mu m is more than 60 percent.
Further, the thickness of the complex phase steel is 0.4-2.5mm.
On the other hand, the invention also provides a preparation method of the 1200 MPa-grade reinforced forming complex phase steel, which comprises the following steps,
obtaining a plate blank; the slab comprises the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.4-0.7%, nb:0.02-0.04%, alloying elements, and the balance of Fe and inevitable impurities; the alloy element is at least one of the following elements: ti:0.01-0.04%, cu:0.03-0.08 percent.
Heating, rough rolling, finish rolling and coiling the plate blank to obtain a hot rolled plate; in the heating process, the heating temperature is 1180-1230 ℃, and the heat preservation time is 1-2 hours; the outlet temperature of the rough rolling is 1000-1100 ℃; the finish rolling finishing temperature is 850-900 ℃; the coiling temperature is 550-600 ℃;
and carrying out softening annealing, pickling, cold rolling, continuous annealing and flattening on the hot rolled plate to obtain the complex phase steel.
Furthermore, in the softening annealing, a hood-type annealing furnace is adopted for softening annealing, the annealing temperature is 600-650 ℃, and the time is 1-4h.
Furthermore, the cold rolling adopts 5-pass rolling, and the total cold rolling reduction rate is 45-55%, wherein the cold rolling reduction rate of the 1 st pass accounts for 20-30% of the total cold rolling reduction rate.
Further, the continuous annealing comprises heating, first heat preservation, first cooling, second heat preservation and second cooling, wherein in the heating, the heating rate is 5-10 ℃/s, and the heating temperature is 800-900 ℃; the heat preservation time is 250-350s; in the first cooling, the cooling rate is 20-40 ℃/s, and the cooling finishing temperature is 370-470 ℃; the second heat preservation time is 500-600s; in the second cooling, the cooling rate is 10-20 ℃/s, and the cooling finishing temperature is 15-35 ℃.
Further, the flat elongation is 0.2-0.5%.
One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:
the invention provides 1200 MPa-grade reinforced forming complex phase steel and a preparation method thereof, the complex phase steel enables a material to obtain a bainite ferrite structure by adding a Cr element, so that the material has good hole expansion performance; the Nb and Ti elements are added to refine the crystal grains, so that the material has high yield strength and good hole expansion performance; by adding Cu element, on one hand, a nano precipitate can be formed to play a role in strengthening, and on the other hand, the nano precipitate can also inhibit the growth of TiC and TiN, so that the influence of the reduction of the enhanced forming performance caused by the growth of TiC and TiN is weakened; the reinforced forming complex phase steel provided by the invention has the yield strength of 951-1148MPa, the tensile strength of 1201-1344MPa, the elongation A80 after fracture of 8-13%, the uniform elongation Ag of 6-10%, good forming performance and the hole expansion rate of 40-59% during room temperature stretching, and can effectively avoid cracking caused by too low hole expansion rate in the flanging and hole expansion processes. The reinforced forming complex phase steel obtained by the invention has good hole expanding performance and forming performance, and is particularly suitable for automobile parts with flanging hole expanding design and certain forming capability.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the description below are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
FIG. 1 shows a microstructure of a reinforced formed complex phase steel according to an embodiment of the present invention, taken by scanning electron microscopy;
fig. 2 is a photograph of the microstructure of the reinforced formed complex phase steel according to the embodiment of the present invention by electron back scattering diffraction.
Detailed Description
The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are illustrative of the invention and are not to be construed as limiting the invention.
Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.
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.
It is noted that, in this document, relational terms such as "first" and "second," and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
In order to solve the technical problems, the embodiment of the invention provides the following general ideas:
on the one hand, the embodiment of the invention provides 1200 MPa-grade reinforced forming complex phase steel.
The reinforced forming complex phase steel comprises the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.4-0.7%, nb:0.02-0.04%, strengthening elements, and the balance of Fe and inevitable impurities;
the strengthening element is at least one of the following elements: ti and Cu, wherein the mass fraction of the Ti is 0.01-0.04%, and the mass fraction of the Cu is 0.03-0.08%.
In the present invention, the effects of the respective elements are as follows:
c: the C element is used as an important solid solution element and an austenite forming element in the steel, so that the strength grade of the material can reach 1200MPa, 3-6% of residual austenite in a finished product can be ensured, and the forming capability of the complex phase steel is enhanced. However, since too high C content causes the retained austenite to be too stable and the effect of enhancing the formability to be limited, the C content is set to 0.21 to 0.24% in the present invention.
Si: si is also a solid solution strengthening element, and can inhibit precipitation of carbide and discharge C element into austenite, thereby improving stability of austenite and being helpful for improving deformability of materials. When the Si content is too low, the yield strength of the strip is low and the retained austenite stability is insufficient, and the material has a low uniform elongation and a low post-fracture elongation, so that the Si content is set to 1.0 to 1.4% in the present invention.
Mn: mn is also a solid solution strengthening element and an austenite stabilizing element, and the addition of Mn can prevent the transformation of austenite to pearlite in the cooling process and improve the hardenability of the material. When the content of Mn is too low, the stability of the retained austenite is poor, and when the content of Mn is too high, a serious edge crack problem occurs during hot rolling, thereby deteriorating the workability of the material, so that the content of Mn element is set to 2.1-2.6%.
P and S: p and S are used as impurity elements, wherein the P is in solid solution in ferrite and can deteriorate the toughness of the material, so that the lower the content of the P element, the better; the S element interacts with the Mn element to form MnS, which affects the hole-expanding performance and corrosion resistance of the material, so that the lower the S element content, the better, the upper limit of the P content is set to 0.01% and the upper limit of the S content is set to 0.005% in the present invention.
Cr: the main function of the Cr element is to delay the transformation of bainite, so that a bainitic ferrite structure can be obtained in the final finished product, and the structure can ensure that the material obtains higher hole expansion performance. However, too high Cr greatly reduces the uniform deformability of the material, so that the content of Cr is 0.4-0.7%.
Nb: the Nb element and the C element can form NbC precipitation, and can prevent ferrite grains from growing to refine the grains and improve the strength, if no fine-grain strengthening exists, the size of the ferrite in the structure is too large, and even if the subsequent precipitation strengthening of Ti and Cu occurs, the size of the ferrite is still insufficient to enable the material to have higher yield strength, so that the requirement of high hole expansion cannot be met, and the Nb element needs to be added. However, since the cost of Nb is relatively high and the material cost is excessively high due to the addition of excess Nb, the content of Nb is set to 0.02 to 0.04%.
Ti: the action of Ti element is very similar to that of Nb element, ti element can form TiC and TiN precipitation with C and N element, the yield strength of the material can be obviously improved, but the precipitation temperature of Ti is higher, meanwhile, ti element can prevent ferrite grains from growing up so as to refine the grains and improve the strength, but excessive precipitation and growth of TiC and TiN can reduce the action of Ti and reduce the reinforced forming performance of the material, therefore, the content of Ti element is set to be 0.02-0.04%.
Cu: the Cu element has good aging strengthening capability, ferrite can be effectively strengthened through the Cu nanometer precipitate, heat preservation is needed after the strip steel is annealed and cooled to a certain temperature, and the aging strengthening effect can be effectively generated. In addition, the Cu content is set to 0.03-0.08% because the Cu diffusion precipitation can inhibit the formation of coarse TiC and TiN, and plays a role in composite strengthening, but too high Cu increases the brittleness of steel, and causes too high strength of hot-rolled strip steel and serious edge crack during cold-rolling.
As an embodiment of the present invention, the mass fraction of C is 0.21-0.23%, the mass fraction of Si is 1.1-1.4%, the mass fraction of Mn is 2.2-2.5%, the mass fraction of Cr is 0.5-0.7%, the mass fraction of Nb is 0.02-0.04%, the mass fraction of Ti is 0.02-0.04%, and the mass fraction of Cu is 0.04-0.06%.
Further, the mass fraction of Ti is 0.02-0.04%, and the mass fraction of Cu is 0.04-0.06%.
As one mode of the embodiment of the invention, the metallographic structure of the reinforced forming complex phase steel comprises 3-6% of residual austenite, 10-20% of ferrite, 35-50% of bainitic ferrite and 30-50% of tempered martensite in percentage by volume.
In the invention, the main function of the retained austenite is to enhance the forming performance of the material, when the proportion of the retained austenite is less than 3 percent, the content is too low, even if the transformation induced plasticity effect occurs during deformation, the uniform elongation of the material cannot be obviously improved, and the material cannot obtain enhanced forming capability; and when the content of the residual austenite is more than 6%, transformation induced plasticity effect occurs during deformation and the residual austenite undergoes martensitic transformation, resulting in a reduction in hole-expanding capacity.
Ferrite may provide the deformability of the material, but too high a ferrite content would seriously reduce the hole-expanding capability of the material.
The bainitic ferrite is one of the bainitic, and can enable supersaturated austenite stabilizing elements to diffuse into the residual austenite in the isothermal process after rapid cooling, so that the residual austenite has better stability, and the uniform deformation capability of the material is improved. Meanwhile, the strength of the bainitic ferrite is between the ferrite and the martensite, so that the strength can be uniformly changed, and the requirement that the material has higher hole expansion rate is met.
Tempered martensite can provide the strength of the material, if the content is too low, the tensile strength of the material cannot reach 1200MPa, and if the content of martensite is too high, the porosity of the material is reduced due to insufficient bainite ferrite content.
As an implementation mode of the embodiment of the invention, in the metallographic structure of the reinforced forming complex phase steel, the volume fraction of bainitic ferrite and tempered martensite, both of which have the size of less than 5 mu m, is more than 60%.
The proportion of the bainitic ferrite and the tempered martensite in the size can affect the hole-expanding performance of the material, and if the volume fraction of the bainitic ferrite and the tempered martensite with the sizes less than 5 mu m is less than 60%, the content of the large-size ferrite bainite and the tempered martensite is too high, so that stress concentration is easily generated at the interface during the deformation process of the strip steel, and microcracks are generated prematurely, so that the forming capability and the hole-expanding capability are low.
As an implementation mode of the embodiment of the invention, the thickness of the reinforced forming complex phase steel is 0.4-2.5mm.
In a second aspect, the embodiment of the invention also provides a preparation method of the 1200 MPa-grade reinforced forming complex phase steel, which comprises the following steps,
s1, obtaining a plate blank; the slab comprises the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.4-0.7%, nb:0.02-0.04%, alloy elements, and the balance of Fe and inevitable impurities; the alloy element is at least one of the following elements: ti:0.01-0.04%, cu:0.03-0.08 percent.
S2, heating, rough rolling, finish rolling and coiling the plate blank to obtain a hot rolled plate; in the heating process, the heating temperature is 1180-1230 ℃, and the heat preservation time is 1-2h; the temperature of the rough rolling outlet is 1000-1100 ℃; the finish rolling finishing temperature is 850-900 ℃; the coiling temperature is 550-600 ℃;
the finish rolling finishing temperature is in a full austenite region, the coiling temperature is in a bainite transformation region, the obtained hot rolled plate has a ferrite and bainite structure, and a more uniform structure can be obtained during subsequent annealing, so that the alloy distribution in the structure is more uniform; meanwhile, a more refined structure can be obtained, and the yield strength of the material is improved.
The heating temperature is too high, on one hand, the grain size in the structure is large, the mechanical property of the finished product is influenced, and on the other hand, too much energy is consumed for production, and the production efficiency is influenced. The heating temperature is too low, the heat preservation time is short, the temperature deviation exists between the core part of the casting blank and the surface, and the rolling deformation is uneven due to the temperature gradient existing in the rolling process; too low a finish rolling finishing temperature results in higher residual stress and a large reduction in austenite length.
The volume proportion of the tempered martensite can be reduced due to the excessively high coiling temperature, and the volume proportion of the tempered martensite can be improved due to the excessively low coiling temperature, so that the hole expansion performance of the material is reduced.
In the present invention, the finish rolling temperature is preferably 870 to 890 ℃ and the coiling temperature is preferably 560 to 580 ℃.
And S3, carrying out softening annealing, pickling, cold rolling, continuous annealing and flattening on the hot rolled plate to obtain the complex phase steel.
In the softening annealing, a hood-type annealing furnace is adopted for softening annealing, the annealing temperature is 600-650 ℃, and the time is 1-4h.
The softening annealing is carried out at the temperature, so that the grain size of the strip steel is not grown, the residual stress in the strip steel can be eliminated, the strength of the strip steel is reduced, and the strip breakage in the subsequent cold rolling process is avoided. The softening annealing temperature is too high, the time is too long, the grain size grows up, and the performance of the finished product is lower; the softening annealing temperature is too low, the time is too short, the softening is insufficient, the subsequent cold rolling force is too large, and the edge crack is easy to occur.
As an implementation mode of the embodiment of the invention, the cold rolling adopts 5-pass rolling, and the total reduction rate of the cold rolling is 45-55%, wherein the reduction rate of the cold rolling of the 1 st pass accounts for 20-30% of the total reduction rate of the cold rolling.
As an implementation manner of the embodiment of the present invention, the continuous annealing includes heating, first heat preservation, first cooling, second heat preservation, and second cooling, in the heating, the heating rate is 5-10 ℃/s, and the heating temperature is 800-900 ℃; the heat preservation time is 250-350s; in the first cooling, the cooling rate is 20-40 ℃/s, and the cooling finishing temperature is 370-470 ℃; the second heat preservation time is 500-600s; in the second cooling, the cooling rate is 10-20 ℃/s, and the cooling finishing temperature is 15-35 ℃.
The heating temperature is 800-900 ℃ in the most austenite or full austenite region, the martensite content obtained after the heating temperature is too high and then the cooling is carried out to the bainite temperature is too high, the forming capability of the material is influenced, and the heating temperature is preferably 830-850 ℃. The heating rate is too fast, which causes temperature uprush, increases the control difficulty, and the heating rate is too low, which causes the increase of the furnace time, the growth of austenite grains and the reduction of the hole expansion rate. The heating temperature is too high, austenite grains grow large, the performance is low, and the heating temperature is too low, so that the retained austenite of the dog cannot be obtained.
In the heating process, the heat preservation time is too long, the tissue is uniform, but the growth of austenite is easy to occur, and the mechanical property is influenced.
If the cooling finishing temperature in the first cooling is lower than 370 ℃, which means that the second heat preservation temperature is too low, the bainitic ferrite in the structure has low content, the martensite content is high, the stability of the retained austenite is poor, and deformation induced phase transformation occurs in subsequent flattening, so that the uniform elongation of the material is reduced, and the hole expansion rate is reduced; if the cooling end temperature in the first cooling is higher than 470 ℃, the bainitic ferrite content is high, and the martensite content is low, so that the tensile strength of the material cannot reach 1200MPa, and good uniform elongation of the material cannot be obtained, it is necessary to set the cooling end temperature in the first cooling to 370 to 470 ℃, preferably 370 to 400 ℃.
In the first cooling, the cooling rate is controlled to obtain a structure in which martensite and bainite are mixed, the martensite is more in the structure with the excessively high cooling speed, and the production efficiency is influenced by the slow cooling speed. The second heat preservation time is used for promoting the balance of elements in the structure, the decomposition of martensite and the diffusion of carbon in bainite into austenite, and the martensite and the bainite are decomposed and the strength is reduced when the second heat preservation time is too long; the second heat preservation time is too short, elements are not completely diffused, the elongation is low, and the forming performance is poor.
In the second cooling, the function of controlling the cooling rate is to control the comprehensive mechanical properties of the product.
As an implementation of the embodiments of the present invention, the flat elongation is 0.2-0.5%.
Leveling is a means for obtaining a suitable roughness on the surface of the strip, and too low or too high a leveling elongation causes too low or too high a surface roughness, and in addition too high a leveling elongation causes martensite transformation of the retained austenite in the structure, resulting in a significant reduction in uniform elongation of the strip, so that the leveling elongation is controlled to be 0.2 to 0.5%, preferably 0.3 to 0.4%.
The 1200 MPa-grade reinforced forming complex phase steel and the preparation method thereof according to the invention will be described in detail with reference to examples, comparative examples and experimental data.
Examples 1 to 23
Embodiments 1 to 23 provide a reinforced forming complex phase steel, and the preparation process includes: smelting → continuous casting → hot rolling → softening annealing → acid pickling cold rolling → continuous annealing → leveling. The method comprises the following specific steps:
1. qualified molten steel is smelted through a converter furnace and continuously cast to obtain a plate blank, the chemical components of the plate blank are shown in table 1, and the balance is Fe and inevitable impurities. The slab components of examples 1 to 4 were component 1, the slab components of examples 5 to 7 were component 2, the slab components of examples 8 to 11 were component 3, the slab components of examples 12 to 15 were component 4, the slab components of examples 16 to 18 were component 5, the slab components of examples 19 to 21 were component 6, and the slab components of examples 22 to 23 were component 7.
2. Heating, rough rolling, finish rolling and coiling the plate blank obtained in the step 1 to obtain a hot rolled plate; the specific process control for this step is shown in table 2.
3. Softening and annealing the hot rolled plate obtained in the step 2 by using a hood-type annealing furnace, wherein the annealing temperature and the annealing time are shown in the table 2;
4. and (3) carrying out acid pickling and cold rolling on the hot rolled plate softened and annealed in the step (3) to obtain a cold rolled plate, which specifically comprises the following steps: the reduction and the total reduction of each pass of the cold rolling of 5 passes are shown in table 3;
5. continuously annealing the strip steel after the cold rolling in the step 4, wherein the continuous annealing sequentially comprises heating, first heat preservation, first cooling, second heat preservation and second cooling, and the specific process of the continuous annealing is shown in tables 3 and 4;
6. and (5) flattening the strip steel continuously annealed in the step (5), wherein the flattening elongation is shown in a table 4.
TABLE 1
Numbering C/％ Si/％ Mn/％ P/％ S/％ Nb/％ Cr/％ Mo/％ Ti/％ Cu/％ Remarks for note
Component 1 0.211 1.18 2.26 0.007 0.003 0.022 0.44 - 0.027 - Examples 1 to 4
Component 2 0.238 1.07 2.41 0.008 0.003 0.028 0.51 - 0.019 - Examples 5 to 7
Component 3 0.222 1.25 2.14 0.007 0.002 0.033 0.47 - 0.032 0.032 Examples 8 to 11
Component 4 0.217 1.33 2.34 0.008 0.002 0.025 0.59 - - 0.042 Examples 12 to 15
Component 5 0.228 1.05 2.44 0.006 0.003 0.035 0.54 - 0.035 - Examples 16 to 18
Component 6 0.21 1.38 2.56 0.009 0.004 0.031 0.66 - 0.027 0.055 Examples 19 to 21
Component 7 0.213 1.27 2.47 0.009 0.004 0.027 0.55 - 0.031 0.033 Examples 22 to 23
Comparative example 1 0.112 0.25 1.93 0.01 0.005 0.028 0.45 0.24 0.022 - -
Comparative example 2 0.12 0.27 2.17 0.013 0.006 0.029 0.52 0.23 0.031 - -
Comparative example 3 0.105 0.28 2.2 0.008 0.004 0.03 0.43 0.21 0.021 - -
Comparative example 4 0.099 0.32 2.16 0.008 0.005 0.031 0.46 0.26 0.033 - -
TABLE 2
Figure GDA0003760858490000081
TABLE 3
Figure GDA0003760858490000091
TABLE 4
Figure GDA0003760858490000101
Comparative examples 1 to 4
Comparative examples 1 to 4 provide a cold-rolled complex phase steel, the preparation process comprising: smelting → continuous casting → hot rolling → softening annealing → acid pickling and cold rolling → continuous annealing → leveling. The method comprises the following specific steps:
1. qualified molten steel is smelted through a converter furnace and continuously cast to obtain a plate blank, the chemical components of the plate blank are shown in Table 1, and the balance is Fe and inevitable impurities.
2. Heating, rough rolling, finish rolling and coiling the plate blank obtained in the step 1 to obtain a hot rolled plate; the specific process control for this step is shown in table 5.
3. Softening and annealing the hot rolled plate obtained in the step 2 by using a hood-type annealing furnace, wherein the annealing temperature and the annealing time are shown in table 5;
4. and (4) carrying out acid pickling and then cold rolling on the hot rolled plate after softening and annealing in the step (3) to obtain a cold rolled plate, which specifically comprises the following steps: the reduction ratios and the total reduction ratios of the respective passes of the cold rolling of 5 passes are shown in table 5;
5. continuously annealing the strip steel after the cold rolling in the step 4, wherein the continuous annealing sequentially comprises heating, first heat preservation, first cooling, second heat preservation and second cooling, and the specific process of the continuous annealing is shown in table 5;
6. the strip steel continuously annealed in the step 5 was flattened, and the flattening elongation was as shown in table 5.
TABLE 5
Figure GDA0003760858490000111
TABLE 6
Figure GDA0003760858490000112
Figure GDA0003760858490000121
TABLE 7
Serial number Rp0.2/MPa Rm/MPa A80/％ Ag/％ Hole expansion ratio/%
Example 1 953 1203 12.5 9 43
Example 2 962 1223 13 10 45
Example 3 987 1235 12 10 43
Example 4 951 1221 11 8 40
Example 5 1148 1344 8 6 59
Example 6 1096 1333 9.5 8 45
Example 7 1025 1299 10 9 51
Example 8 1056 1266 11 9 53
Example 9 1024 1254 10.5 8.5 51
Example 10 1001 1239 11 9 47
Example 11 997 1247 11 7 47
Example 12 1063 1256 10 8 56
Example 13 1059 1277 10.5 8 53
Example 14 1013 1233 11 9 55
Example 15 987 1243 12 9 47
Example 16 1056 1289 10 8 46
Example 17 1002 1311 9 7 42
Example 18 1067 1278 9 7 48
Example 19 959 1207 10 8 47
Example 20 951 1201 10 8 43
Example 21 977 1233 11 9 45
Example 22 981 1233 11 9 49
Example 23 962 1211 11 8 51
Comparative example 1 973 1234 7.5 4 40
Comparative example 2 981 1222 7 4.5 42
Comparative example 3 945 1228 7.5 5 41
Comparative example 4 924 1188 7.5 4.5 43
The complex phase steels prepared in examples 1 to 23 and comparative examples 1 to 4 were subjected to microstructure observation, the retained austenite distribution was measured by EBSD, as shown in table 6, and the mechanical properties and hole expansion properties were measured, as shown in table 7.
Wherein, the mechanical properties are determined by adopting a tensile testing machine according to part 1 of a GBT 228.1-2010 metal material tensile test: the test is carried out by a room temperature test method, and the size of the sample is a P6 sample in appendix B; the hole expanding rate is detected according to GBT 15825.4-2008 metal sheet reinforced forming performance and test method.
As can be seen from the data in table 5, the metallographic structures of the reinforced formed complex phase steels provided in examples 1 to 23 of the present invention were ferrite, bainitic ferrite, tempered martensite, and retained austenite. As can be seen from the data in Table 5, the complex phase steel provided in the embodiments 1 to 23 of the present invention has a yield strength of 951 to 1148MPa, a tensile strength of 1201 to 1344MPa, an elongation A80 after fracture of 8 to 13%, a uniform elongation Ag of 6 to 10%, and a hole expansion ratio of 40 to 59%. The ultrahigh-strength steel plate is suitable for producing automobile parts which have flanging and reaming designs and need certain forming capacity, such as side plates of seats, locally needs to have good uniform deformation capacity, and meanwhile, parts are provided with a large number of reaming flanges. The cold-rolled complex phase steel provided by the comparative examples 1 to 4 has the yield strength of 924 to 981MPa, the tensile strength of 1188 to 1234MPa, the elongation A80 after fracture of 7 to 7.5 percent, the uniform elongation Ag of 4 to 5 percent and the hole expansion rate of 40 to 43 percent, although the strength level is equivalent to that of the examples 1 to 23 of the invention, the uniform elongation and the elongation after fracture are low, only the strengthening and forming of a hole expansion flange can be met, the uniform deformation capability is poor, a large amount of punching cracks can occur in cold punching, and the forming performance or the hole expansion performance of the cold-rolled complex phase steel cannot meet the target requirement at the same time.
In fig. 1: the lath-shaped structure is bainitic ferrite, the block-shaped structure is tempered martensite and retained austenite, and the gray concave position is ferrite, so that as can be seen from fig. 1, the metallographic structure of the reinforced forming complex phase steel provided by the embodiment of the invention consists of ferrite, bainitic ferrite, tempered martensite and retained austenite.
In fig. 2, the black circles indicate retained austenite, which is actually blue, the lath-shaped structure is bainitic ferrite, the massive structure with larger size is tempered martensite, and the massive structure with smaller size is ferrite. As can be seen from fig. 2, the metallographic structure of the reinforced shaped complex phase steel provided by the example of the present invention is composed of ferrite, bainitic ferrite, tempered martensite, and retained austenite, which is consistent with the observation results of fig. 1.
The invention provides 1200 MPa-grade reinforced forming complex phase steel and a preparation method thereof, the complex phase steel enables a material to obtain a bainite ferrite structure by adding a Cr element, so that the material has good hole expansion performance; the grains are refined by adding Nb and Ti elements, so that the material has higher yield strength and good hole expansion performance; by adding Cu element, on one hand, a nano precipitate can be formed to play a role in strengthening, and on the other hand, the nano precipitate can also inhibit the growth of TiC and TiN, so that the influence of the reduction of the forming performance caused by the growth of TiC and TiN is weakened; the reinforced forming complex phase steel prepared by matching with the hot rolling and continuous annealing processes has the yield strength of 951-1148MPa, the tensile strength of 1201-1344MPa, the elongation A80 after fracture of 8-13%, the uniform elongation Ag of 6-10% and the hole expansion rate of 40-59% during room temperature stretching, and can effectively avoid cracking caused by too low hole expansion rate in the flanging and hole expansion processes. The reinforced forming complex phase steel obtained by the invention has good hole expansion performance and reinforced forming performance, and is particularly suitable for automobile parts with flanging hole expansion design and certain forming capability.
Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
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 the preferred embodiment and all changes and modifications that 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.

Claims (9)

1. The 1200 MPa-grade reinforced forming complex phase steel is characterized by comprising the following chemical components in percentage by mass: c:0.21-0.24%, si:1.0-1.4%, mn:2.1-2.6%, P is less than or equal to 0.01%, S is less than or equal to 0.005%, cr:0.51-0.7%, nb:0.02-0.04%, strengthening elements, and the balance of Fe and inevitable impurities;
the strengthening element is at least one of the following elements: ti and Cu, wherein the mass fraction of the Ti is 0.01-0.04%, and the mass fraction of the Cu is 0.03-0.08%;
the preparation method of the reinforced forming complex phase steel comprises the following steps:
obtaining a plate blank;
heating, rough rolling, finish rolling and coiling the plate blank to obtain a hot rolled plate;
softening annealing, pickling, cold rolling, continuous annealing and flattening are carried out on the hot rolled plate to obtain reinforced forming complex phase steel;
in the softening annealing, a hood-type annealing furnace is adopted for softening annealing, the annealing temperature is 600-650 ℃, and the time is 1-4h;
the room-temperature yield strength of the reinforced forming complex phase steel is 951-1148MPa, the tensile strength is 1201-1344MPa, the elongation A80 after fracture is 8-13%, the uniform elongation Ag is 6-10%, and the hole expansion rate is 40-59%.
2. The complex phase steel for reinforcing and forming at 1200MPa level according to claim 1, wherein the mass fraction of C is 0.21-0.23%, the mass fraction of Si is 1.1-1.4%, the mass fraction of Mn is 2.2-2.5%, the mass fraction of Nb is 0.02-0.04%, the mass fraction of Ti is 0.02-0.04%, and the mass fraction of Cu is 0.04-0.06%.
3. The complex phase steel for reinforcing and forming in the 1200MPa grade according to claim 1, characterized in that the metallographic structure of the complex phase steel for reinforcing and forming comprises, in volume fraction, 3-6% of retained austenite, 10-20% of ferrite, 35-50% of bainitic ferrite and 30-50% of tempered martensite.
4. The complex phase steel for reinforcing and forming in the 1200MPa grade according to claim 3, characterized in that the volume fraction of the metallographic structure of the complex phase steel for reinforcing and forming is more than 60%, wherein the volume fraction of the bainitic ferrite and the tempered martensite are both less than 5 μm.
5. The complex phase steel for reinforcing and forming at 1200MPa level as claimed in claim 1, wherein the thickness of the complex phase steel for reinforcing and forming is 0.4-2.5mm.
6. The 1200 MPa-grade reinforced forming complex phase steel as claimed in claim 1, wherein in the heating, the heating temperature is 1180-1230 ℃, and the heat preservation time is 1-2h; the temperature of the rough rolling outlet is 1000-1100 ℃; the finish rolling finishing temperature is 850-900 ℃; the coiling temperature is 550-600 ℃.
7. The complex phase steel for reinforcing and forming at the level of 1200MPa as claimed in claim 1, wherein the cold rolling adopts 5-pass rolling, and the total reduction rate of the cold rolling is 45-55%, wherein the reduction rate of the cold rolling of the 1 st pass accounts for 20-30% of the total reduction rate of the cold rolling.
8. The 1200 MPa-grade reinforced forming complex phase steel as claimed in claim 1, wherein the continuous annealing comprises heating, first heat preservation, first cooling, second heat preservation and second cooling, wherein in the heating, the heating rate is 5-10 ℃/s, and the heating temperature is 800-900 ℃; the heat preservation time is 250-350s; in the first cooling, the cooling rate is 20-40 ℃/s, and the cooling finishing temperature is 370-470 ℃; the second heat preservation time is 500-600s; in the second cooling, the cooling rate is 10-20 ℃/s, and the cooling finishing temperature is 15-35 ℃.
9. The complex phase steel for reinforcing and forming at the level of 1200MPa as claimed in claim 1, wherein the flat elongation is 0.2-0.5%.
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