CN113416890A

CN113416890A - High-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate and preparation method thereof

Info

Publication number: CN113416890A
Application number: CN202110555917.8A
Authority: CN
Inventors: 胡智评; 刘仁东; 林利; 徐鑫; 蒋睿婷
Original assignee: Angang Steel Co Ltd
Current assignee: Angang Steel Co Ltd
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2021-09-21
Anticipated expiration: 2041-05-21
Also published as: CN113416890B

Abstract

The invention provides a high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate and a preparation method thereof, wherein the steel plate comprises the following components in percentage by mass: c: 0.15% -0.18%, Mn: 1.8-2.5%, Cr: 0.3% -0.8%, Si: 0.5% -1.5%, Al: 0.05-0.6%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, Ti: less than or equal to 0.03 percent, and the balance of Fe and inevitable impurities. The preparation method comprises smelting, hot rolling, acid washing, cold rolling, continuous annealing and finishing; the method can be used for producing the DH cold rolling continuous annealing steel plate with the tensile strength of over 980MPa, the yield strength of 700-850 MPa, the elongation of 16-18 percent and the hole expansion value of over 50 percent; and a DH cold rolling continuous annealing steel plate with tensile strength of more than 980MPa, yield strength of 650-850 MPa, elongation of 20-22% and hole expansion value of 40-50%.

Description

High-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate and preparation method thereof

Technical Field

The invention belongs to the field of metal materials, particularly relates to the field of automobile steel manufacturing, and particularly relates to a 980MPa grade dual-phase steel cold-rolled continuous annealing steel plate with high hole expansion and high plasticity and a preparation method thereof.

Background

In the competition between cold-formed and hot-formed steels, the extremely poor formability has been the key to limiting the development of cold-formed steels. When the strength reaches over 980MPa, the lower ductility causes that a more complex structural part is difficult to produce by a cold stamping mode, and the rolling mode greatly improves the manufacturing cost of the material. This makes many vehicle enterprises can only choose the hot forming part that the cost is higher to replace, abandons the use of high strength cold forming spare part. For cold rolled automotive steel sheets, the incompatibility of strength and plasticity is indeed a technical bottleneck that restricts the development of the material. Taking the cold-rolled dual-phase steel widely applied in the market as an example, the DP590 can be used for most body structural members, link members and reinforcing members except for panels and some extremely complex structural members, and the DP590 is closely inseparable from the good stamping performance predicted by the elongation of about 30 percent of the DP 590; the DP780 plasticity is about 17% generally, compared with DP590 in application, the proportion of the competent structural parts is greatly reduced, and the method is only applied to complex parts with deformation such as B columns, longitudinal beams, side beams and the like; the steel plasticity of DP980 steel is generally 10-12%, and compared with DP780 steel, the steel has few and few applications on a vehicle body structural part and is only used for reinforcing parts such as doorsills, bumpers and the like. Meanwhile, because the hardness difference between ferrite and martensite in the dual-phase steel is large, the yield strength of the DP980 steel is often low, and the low yield ratio leads to poor hole expansion performance, so that the dual-phase steel cannot be used as a high-flanging part. In the current material design, researchers have proposed personalized DP980 steel designs, such as high counterbored DP980 and high plasticity DP980 or DH980 (plasticized DP steel), for the performance requirements of different parts. However, 980MPa components with limited markets are customized in a plurality of varieties and individualities, which undoubtedly increases the manufacturing cost of steel mills. Therefore, the 980MPa high-strength steel with high hole expansion and high plasticity is developed, and parts meeting the requirements of high flanging and high drawing are important for industrialized low-cost operation of 980 MPa-level products.

Patent document CN 103290202B discloses a production method of 1000 MPa-level high-strength steel yield ratio and hole expansibility, which mainly comprises the following chemical components: c: 0.15% -0.25%, Si: 1.0% -2.0%, Mn: 1.5 to 2.5 percent of the total weight of the alloy, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, and the balance of Fe and inevitable impurities. The 1000MPa high-strength steel is obtained, the yield ratio is 0.7-0.9, the elongation is more than 10%, and the hole expansion rate is more than 50%. The patent realizes the force performance index of high hole expansion through the regulation and control organization of the process, but the control process is difficult to be suitable for large-scale industrial production, and the problem of compromise of yield and plasticity is still not solved.

Patent document CN103469112A discloses a high formability cold-rolled dual-phase strip steel and a manufacturing method thereof, wherein the main chemical components of the strip steel are as follows: c: 0.06% -0.095%, Si: less than or equal to 0.4 percent, Mn: 2.05% -2.35%, Nb: 0-0.04%, P is less than or equal to 0.015%, S is less than or equal to 0.003%, Ti: 0.01-0.05%, Al: 0.015-0.05 percent, a certain content of Cr, Mo, Ni and other elements, and the balance of Fe and inevitable impurities. The steel is 1000MPa grade steel, the elongation is 14% -16%, the yield strength is 580-800 MPa, and the hole expansion rate is more than 30%. However, the steel is added with noble elements such as Nb, Cr, Mo, Ni and the like in a certain content, so that the alloy cost is greatly increased.

Disclosure of Invention

The invention aims to overcome the problems and the defects and provides a 980 MPa-grade cold-rolled continuous annealing steel plate with high hole expansion and high plasticity and a preparation method thereof.

The purpose of the invention is realized as follows:

a980 MPa-grade cold-rolled continuous annealing steel plate with high hole expansion and high plasticity comprises the following components in percentage by weight: c: 0.15% -0.18%, Mn: 1.8-2.5%, Cr: 0.3% -0.8%, Si: 0.5% -1.5%, Al: 0.05-0.6%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, Ti: less than or equal to 0.03 percent, and the balance of Fe and inevitable impurities.

Furthermore, Mn and Cr in the steel plate are more than or equal to 2.3% and less than or equal to 2.8%, and Si and Al in the steel plate are more than or equal to 1.0% and less than or equal to 1.5%.

Further, the steel sheet has Nb: not more than 0.03%, V not more than 0.05%, Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent, less than or equal to 0.005 percent of Ca and less than or equal to 0.005 percent of B; preferably, Nb and V are more than or equal to 0.01 percent and less than or equal to 0.05 percent, and Ni, Mo and Cu are more than or equal to 0.3 percent and less than or equal to 0.6 percent in the steel plate.

The microstructure of the steel plate of the invention is two types:

the first one is: the microstructure of the continuous annealing steel plate comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite; in volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 8 to 12 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 700-850 MPa, the elongation of 16-18% and the hole expansion value of over 50%.

The second method is as follows: the microstructure of the continuous annealing steel plate comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite; in volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 12 to 15 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 650-850 MPa, the elongation of 20-22% and the hole expansion value of 40-50%.

The invention has the following design reasons:

c: c is one of important alloy elements in the invention and plays a role in strengthening the steel plate. Secondly, the addition of C atoms promotes austenite nucleation in the critical region. In addition, the final structure content of the retained austenite with a certain content is indispensable in the invention, and the addition of the element C is indispensable, and related documents report that the content of C in the retained austenite needs to be more than 1.2% so as to keep the phase stability of the retained austenite at room temperature. For the 980MPa steel, the structure composition and the mechanical property of the steel plate are influenced by the excessively low or high C addition. If the content of C is less than 0.15%, the formation of residual austenite at room temperature with enough content cannot be ensured, and the plasticity of the experimental steel is influenced; if the content of C is more than 0.18%, the Ms point of the steel plate in the critical zone under the isothermal condition is reduced, so that the optimized quenching temperature point in actual production is reduced, and the manufacturing difficulty and the production cost are increased.

Mn: the steel is infinitely solid-dissolved in austenite to play a role in solid-solution strengthening. Mn is an important element for expanding an austenite phase region, reduces the critical quenching speed of the experimental steel and delays the transformation from austenite to pearlite; meanwhile, the Ms point (martensite start temperature) in the experimental steel can be reduced, the austenite is stabilized, and the proper phase stability of the retained austenite is ensured. Too low a Mn content is insufficient to stabilize a sufficient content of austenite in the critical region and reduces the phase stability of the retained austenite at room temperature, resulting in poor work hardening behavior of the experimental steel. Most importantly, the addition of sufficient Mn element improves the hardenability of the steel plate and ensures the transformation quantity of martensite in a quenching state. However, excessive Mn addition causes serious Mn segregation in the continuous casting process, and meanwhile, the slab continuous casting is easy to generate a hot cracking phenomenon; furthermore, the addition of high Mn will also cause an increase in carbon equivalent in the subsequent welding stage, thereby deteriorating the welding performance. Therefore, the Mn content in the steel of the invention is controlled to be 1.8-2.5%.

Cr: cr is added into the steel as a Mn element supplement element, and when the content of Mn is lower, a proper Cr element can be added to stabilize austenite and improve the hardenability of the steel plate; meanwhile, the addition of Cr can improve the oxidation resistance of the steel plate to a certain extent and improve the internal oxidation state of the steel plate.

Mn + Cr: both Mn and Cr are elements that increase hardenability, ensuring the martensite content obtained during the quenching or rapid cooling phase, whereas too high hardenability leads to limited formation of retained austenite, which is not conducive to higher plasticity. Therefore, the Mn + Cr is more than or equal to 2.3 percent and less than or equal to 2.8 percent in the invention.

Si: si itself promotes ferrite formation and strengthens the ferrite matrix. In the present invention, Si mainly acts to suppress the precipitation of cementite at the overaging stage. However, adding too much Si reduces the surface quality of the steel. Therefore, the content of the Si element is controlled to be 0.5 to 1.5 percent in the invention.

Al: al is a deoxidizer in the steel-making process in the traditional process, and meanwhile, N in steel can be combined with Al to form AlN and refine grains. However, the main purpose of adding more Al in the invention is to accelerate the transformation kinetic process from austenite to ferrite in the cooling process, simultaneously inhibit the precipitation of cementite together with Si, simultaneously improve the austenitizing temperature and facilitate the selection of a better process window. Too little Al content has limited influence on austenitizing temperature, and simultaneously, the precipitation speed of ferrite is slowed down during cooling; and too high Al content can cause the water gap to be blocked in the continuous casting process, and the production efficiency is influenced. Therefore, the content of the Al element is controlled within the range of 0.05-0.6 percent in the invention.

Si + Al: the main function of Si and Al in the invention is to inhibit carbide precipitation in the overaging stage, however, too high Si addition will cause the surface quality of the steel plate to be reduced, and too high Al addition will increase the smelting difficulty, so that Si + Al is more than or equal to 1.0% and less than or equal to 1.5%.

Ti: ti can capture free N atoms in the steel and plays a role in fixing N. Meanwhile, TiN can be precipitated in the solidification process to play a role in pinning a crystal boundary, and the Ti (C, N) is precipitated in the hot rolling stage to play a role in pinning a prior austenite crystal boundary and refining the prior austenite crystal grain. Meanwhile, a small amount of Ti is precipitated in the continuous annealing stage to play a role in strengthening ferrite and bainite, but excessive Ti precipitation occupies C atoms required for remaining austenite to remain.

Nb: in the invention, the appropriate addition of Nb can promote the strain-induced precipitation behavior in the hot rolling recrystallization rolling stage, promote the recrystallization of prior austenite grains and play a role in refining the grains.

V: in the invention, the V element is properly added to strengthen the precipitation strengthening effect in the coiling stage, inhibit the dislocation self-recovery phenomenon in the cold rolling process, improve the retention of deformation energy storage and promote the recrystallization behavior in the continuous annealing stage; meanwhile, VC is precipitated in ferrite in the continuous annealing isothermal stage, and plays a role in precipitation strengthening.

Nb + V: nb and V are used as microalloy supplementary elements on the basis of Ti addition in the invention, and effectively play roles in refining original austenite grains and precipitation strengthening, but too high Nb and V addition causes too high strength of a hot rolled plate and is not beneficial to cold rolling. Therefore, Nb + V is more than or equal to 0.01 percent and less than or equal to 0.05 percent.

P: the P element is a harmful element in steel, and the lower the content, the better. In the invention, the content of the P element is controlled to be less than or equal to 0.02 percent in consideration of cost.

S: the S element is a harmful element in steel, and the lower the content, the better. In the invention, the content of the S element is controlled to be less than or equal to 0.005 percent in consideration of cost.

Ni, Mo and Cu: mo itself is a solid solution strengthening element, and plays a role in strengthening the steel plate. In the invention, Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite in the cooling stage and promote the formation of martensite; ni improves the corrosion resistance of the steel plate to a certain extent. The Cu element itself is dissolved in austenite to improve the strength of the steel sheet. In the continuous annealing stage, the simple substance Cu is precipitated in austenite to play a certain precipitation strength role. The addition of Cu has a certain effect of improving the corrosion resistance of the steel plate. Mo, Ni and Cu are all more noble alloys, and the total addition content is controlled to be lower than 0.6 percent by considering the alloy cost problem and the synergistic effect of each element.

Ca: the inclusion morphology can be controlled by adding a proper amount of Ca, thereby improving the quality of the casting blank steel plate.

B: the addition of B in the invention can supplement the hardenability of the steel plate and ensure the formation of martensite in the rapid cooling stage in the continuous annealing galvanization process. Too much B is added to increase the brittleness of the steel sheet and deteriorate the workability of the steel sheet.

The second technical scheme of the invention provides a DH cold rolling continuous annealing steel plate with high hole expansion and high plasticity of 980MPa grade and a preparation method thereof, comprising smelting, casting, hot rolling, acid washing, cold rolling, continuous annealing and finishing;

smelting:

and smelting by a converter to obtain the alloy components within the range.

Hot rolling:

heating at 1240-1270 ℃ for isothermal over 2 hours; the initial rolling temperature is 950-1100 ℃, and the final rolling temperature is above 850 ℃; the coiling temperature is 400-500 ℃; the thickness of the hot rolled coil is 3.2-3.6 mm;

the heating temperature is 1240-1270 ℃, the temperature is kept constant for more than 2 hours, the alloy elements are ensured to be fully dissolved in solution, when the Ti element is added, the precipitation of Ti (C, N) or TiN is ensured, and the precipitation and coarsening of Ti caused by overhigh temperature are prevented.

The initial rolling temperature is 950-1100 ℃, and the final rolling temperature is above 850 ℃; the recrystallization behavior of the prior austenite is controlled to a certain degree in the stage, and the grain size of the prior austenite is controlled to be more than 20 mu m. Because the final structure of the steel plate of the invention is mainly tempered martensite, larger original austenite grains promote the uniform distribution of tempered martensite lath bundles, prevent local C enrichment, cause the formation of island-shaped martensite and deteriorate the hole-expanding performance.

The coiling temperature is between 400 and 500 ℃: firstly, preventing the coil collapse phenomenon caused by overhigh coiling temperature; at the same time, more importantly, the serious phenomenon of internal oxidation caused by high Si content is prevented, and the Fe on the subsurface layer of the steel plate is inhibited₂SiO₄，SiO₂And MnO₂The formation of complex internal oxides affects the surface quality of cold-rolled steel sheets. The thickness of the hot rolled coil is 3.2-3.6 mm.

Acid washing:

and removing the scale generated on the surface of the hot rolling, and ensuring the surface quality of the cold-rolled steel plate.

Cold rolling:

the thickness of the cold-rolled plate is controlled to be 52-60%, and the cold-rolling reduction rate is prevented from being too high, so that the deformation resistance is too large, and the cold-rolled plate is difficult to roll to the target thickness.

The microstructure after cold rolling comprises ferrite, pearlite and bainite, wherein the microstructure of the steel plate after cold rolling comprises the following components in percentage by volume: ferrite: 25% -50%, pearlite: 18-30% of bainite: 23 to 30 percent.

Continuous annealing:

heating at an isothermal temperature of 810-870 ℃, keeping the isothermal time at 60-240 s, slowly cooling at a temperature of 650-680 ℃, and controlling the slow cooling speed at 0.5-5 ℃/s; after slow cooling, cooling to 245-280 ℃ at a cooling speed of more than 30 ℃/s, then heating to an overaging temperature of 360-430 ℃ at a heating speed of more than 10 ℃/s, keeping the isothermal time at 280-410 s, and then cooling to room temperature;

aiming at the alloy design, the invention adopts high-proportion austenitizing and slow cooling to match, and adjusts the contents of the oriented epizoic ferrite obtained in the critical zone ferrite and slow cooling stages, so that the content of the critical zone ferrite is less than or equal to 10 percent, and the content of the oriented epizoic ferrite is 10 to 20 percent.

After slow cooling, cooling to 245-280 ℃ at a cooling speed of more than 30 ℃/s, then heating to an overaging temperature of 360-430 ℃ at a heating speed of more than 10 ℃/s, keeping the isothermal time at 280-410 s, and then cooling to room temperature; the slow cooling speed is more than 30 ℃/s to prevent the generation of excessive epitopic ferrite; cooling to 245-280 ℃, and aiming at ensuring that 40% of martensite is obtained at the stage, so as to ensure the strength of the steel plate; then the heating speed is more than 10 ℃/s, aiming at ensuring the isothermal time of the subsequent adjustment stage; the overaging temperature is 360-430 ℃, and the purpose is that 10-20% of bainite phase needs to be introduced to promote the diffusion of C and the retention of residual austenite. The C diffusion of C atoms into austenite is inhibited at the excessively low overaging temperature, so that the content and stability of residual austenite are reduced, and the plasticity of the steel plate is reduced; an excessively high overaging temperature will result in a decrease in the martensite temper resistance and precipitation of cementite deteriorating the steel sheet properties. The isothermal time is controlled to be 280-410 s, and the purpose is to promote the C-rich behavior of austenite in the stage, improve the phase stability of residual austenite at room temperature, and simultaneously prevent the martensite from tempering and softening due to overlong time and deteriorating the performance of the steel plate.

Finishing: then, the steel plate enters a finishing machine to carry out plate shape adjustment, and the finishing elongation is controlled to be 0.1-0.4%.

The microstructure of the continuous annealing steel plate obtained by the preparation method comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite; the microstructures were as follows by volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 8 to 12 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 700-850 MPa, the elongation of 16-18% and the hole expansion value of 50%.

Further, the method comprises the following steps of; carrying out structure adjustment before continuous annealing, carrying out austenitizing quenching treatment on a cold-rolled sheet, wherein the steel sheet structure is ferrite and bainite structures, and the microstructure is calculated according to volume fraction: 20-50% of ferrite and 50-80% of martensite, and aims to provide more nucleation points of austenite in the continuous annealing isothermal stage, improve the proportion of retained austenite so as to meet the mechanical properties of high strength and high plasticity, wherein the optimized retained austenite content can reach 12-15%, and the TRIP can improve the plasticity to more than 20%. Then carrying out continuous annealing process; the obtained continuous annealing steel plate structure comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite, and the microstructure is calculated according to volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 12 to 15 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 650-850 MPa, the elongation of 20-22% and the hole expansion value of 40-50%.

The invention has the beneficial effects that:

(1) based on reasonable alloy design and innovative process design, the tensile strength is over 980MPa, the yield strength is 700-850 MPa, the elongation is 16% -18%, and the hole expansion value is over 50%; and the tensile strength is more than 980MPa, the yield strength is 650-850 MPa, the elongation is 20-22%, and the hole expansion value is 40-50%.

(2) The invention integrates the whole process flow, comprehensively considers a plurality of factors such as components, carbon equivalent, surface quality, process stability and the like, and gives full play to the subjective motility of regulating and controlling the tissue performance on the basis of controlling the cost.

(3) The steel plate is a 980MPa grade cold rolling and annealing product which is rare in the current market and has the functions of hole expansion and ductility.

(4) The steel plate is not limited by the traditional process thinking of DP, QP and TRIP products, creatively provides the composite structure composition of the ferrite in the critical area, the oriented periphytic ferrite, the tempered martensite, the residual austenite and the bainite, and realizes the optimized index of the mechanical property.

Drawings

FIG. 1 is a gold phase diagram of a microstructure according to example 1 of the present invention.

Detailed Description

The present invention is further illustrated by the following examples.

According to the embodiment of the invention, smelting, hot rolling, acid washing, cold rolling, continuous annealing and finishing are carried out according to the component proportion of the technical scheme.

(1) Hot rolling:

(2) cold rolling:

the cold-rolled structure comprises ferrite, pearlite and bainite, wherein the microstructure of the cold-rolled steel plate comprises the following components in percentage by volume: ferrite: 25% -50%, pearlite: 18-30% of bainite: 23 to 30 percent.

(3) Continuous annealing:

heating at an isothermal temperature of 810-870 ℃, keeping the isothermal time at 60-240 s, slowly cooling at a temperature of 650-680 ℃ or above, and controlling the slow cooling speed at 0.5-5 ℃/s; after slow cooling, cooling to 245-280 ℃ at a cooling speed of more than 30 ℃/s, then heating to an overaging temperature of 360-430 ℃ at a heating speed of more than 10 ℃/s, keeping the isothermal time at 280-410 s, and then cooling to room temperature;

(4) finishing: the steel plate enters a finishing machine to carry out plate shape adjustment, and the finishing elongation is controlled to be 0.1-0.4%.

The microstructure of the continuous annealing steel plate obtained by the preparation method comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite; the microstructure is calculated by volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 8 to 12 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 700-850 MPa, the elongation of 16-18% and the hole expansion value of 50%.

Further, the method comprises the following steps of; performing structure adjustment before continuous annealing, and performing austenitizing quenching treatment on a cold-rolled sheet to enable the microstructure of the quenched steel sheet to be ferrite and bainite, wherein the microstructure is calculated according to volume fraction: 20 to 50 percent of ferrite and 50 to 80 percent of martensite; then, carrying out the continuous annealing process in the step (3); the microstructure of the obtained continuous annealing steel plate comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite, and the microstructure is calculated according to volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 12 to 15 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 650-850 MPa, the elongation of 20-22% and the hole expansion value of 40-50%.

Table 1 the compositions of the steels of the examples of the present invention are shown in table 1. The main process parameters of hot rolling of the steel of the embodiment of the invention are shown in Table 2. The main process parameters of the continuous annealing of the steel of the embodiment of the invention are shown in Table 3. The microstructure of the steels of the examples of the invention is shown in Table 4. The mechanical properties of the steels of the examples of the invention are shown in Table 5.

TABLE 2 Hot Rolling Main Process parameters of steels of examples of the present invention

TABLE 3 main process parameters for continuous annealing of steel of examples of the present invention

TABLE 4 microstructure of inventive example steels

TABLE 5 mechanical Properties of steels according to examples of the invention

Examples	Rp0.2/MPa	Rm/MPa	A80/％	λ/％
					1	754	1035	16.6	53.6
2	724	1054	17.5	54.5
					3	827	1058	16.8	53.8
4	756	1035	17.9	52.3
					5	784	1026	16.2	51.4
6	793	1034	17.5	51.9
					7	709	1019	16.5	52.2
8	736	1026	17.1	55.3
					9	755	1033	16.5	52.7
10	748	1051	16.8	51.5
					11	682	1076	20.6	43.5
12	699	1084	21.2	41.9
					13	668	1088	21.8	47.3
14	683	1036	20.8	42.5
					15	675	1052	21.1	41.5

As can be seen from the above, the tensile strength of the steel plate is over 980MPa, the yield strength is 700-850 MPa, the elongation is over 16% -18%, and the hole expansion value is over 50%; and two innovative DH cold rolling continuous annealing steel plates with tensile strength of more than 980MPa, yield strength of 650-850 MPa, elongation of more than 20-22% and hole expansion value of more than 40-50%.

In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.

Claims

1. A high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate is characterized by comprising the following components in percentage by weight: c: 0.15% -0.18%, Mn: 1.8-2.5%, Cr: 0.3% -0.8%, Si: 0.5% -1.5%, Al: 0.05-0.6%, P is less than or equal to 0.02%, S is less than or equal to 0.005%, Ti: less than or equal to 0.03 percent, and the balance of Fe and inevitable impurities.

2. The high hole expansion high plasticity 980MPa grade cold rolling continuous annealing steel plate according to claim 1, characterized in that 2.3% to 2.8% of Mn + Cr and 1.0% to 1.5% of Si + Al in the steel plate.

3. The high hole expansion high plasticity 980MPa grade cold rolled continuous annealing steel plate according to claim 1, characterized in that the steel plate further comprises Nb: not more than 0.03%, V not more than 0.05%, Ni: less than or equal to 0.5 percent, Mo: less than or equal to 0.5 percent, Cu: less than or equal to 0.5 percent, less than or equal to 0.005 percent of Ca and less than or equal to 0.005 percent of B.

4. The high hole expansion high plasticity 980MPa grade cold rolling continuous annealing steel plate according to claim 3, characterized in that Nb + V is more than or equal to 0.01% and less than or equal to 0.05%, and Ni + Mo + Cu is more than or equal to 0.3% and less than or equal to 0.6%.

5. The high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate according to claim 1, wherein the microstructure of the continuous annealing steel plate comprises critical zone ferrite, oriented ferrite, tempered martensite, bainite and retained austenite; the microstructure is calculated by volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 8 to 12 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 700-850 MPa, the elongation of 16-18% and the hole expansion value of over 50%.

6. The high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate according to claim 1, wherein the continuous annealing steel plate structure comprises critical zone ferrite, oriented ferrite, tempered martensite, bainite and retained austenite; the microstructure is calculated by volume fraction: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 12 to 15 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 650-850 MPa, the elongation of 20-22% and the hole expansion value of 40-50%.

7. A method for preparing a high-hole-expansion high-plasticity 980MPa cold-rolled continuous-annealing steel plate as claimed in any one of claims 1 to 6, which comprises smelting, casting, hot rolling, acid washing, cold rolling, continuous annealing and finishing; the method is characterized in that:

(1) hot rolling:

(2) cold rolling:

the cold rolled structure comprises ferrite, pearlite and bainite; wherein the microstructure of the cold-rolled steel plate is as follows according to volume percentage: ferrite: 25% -50%, pearlite: 18-30% of bainite: 23% -30%;

(3) continuous annealing:

8. The preparation method of the high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous annealing steel plate according to claim 7 is characterized by comprising the following steps of:

9. The preparation method of the high-hole-expansion high-plasticity 980 MPa-grade cold-rolled continuous-annealed steel plate as claimed in claim 7, is characterized in that:

performing structure adjustment before continuous annealing, and performing austenitizing quenching treatment on a cold-rolled sheet to enable the steel sheet structure after quenching treatment to comprise ferrite and bainite structures, wherein the microstructure is calculated according to volume fraction: 20 to 50 percent of ferrite and 50 to 80 percent of martensite; then, carrying out the continuous annealing process in the step (3); the obtained continuous annealing steel plate structure comprises critical zone ferrite, oriented periphytic ferrite, tempered martensite, bainite and residual austenite, and the volume fractions are as follows: the ferrite in the critical area is less than or equal to 10 percent, the oriented periphytic ferrite is 10 to 20 percent, the tempered martensite is 40 to 50 percent, the bainite is 10 to 20 percent, and the residual austenite is 12 to 15 percent; the continuous annealing steel plate has the tensile strength of over 980MPa, the yield strength of 650-850 MPa, the elongation of 20-22% and the hole expansion value of 40-50%.