Cold-rolled plasticity-enhanced complex phase steel with tensile strength above 1370 MPa and preparation method thereof

Abstract

The present invention relates to a cold-rolled plastic reinforced complex phase steel with a tensile strength of more than 1370 MPa and a preparation method thereof, wherein the chemical composition of the steel is C: 0.21% to 0.24%, Si: 1.2% to 1.6%, Mn: 2.3% to 2.7%, P≤0.01%, S≤0.08%, Al: 0.1% to 0.12%, Cr: 0.3% to 0.6%, Mo: 0.22% to 0.50%, Ti: 0.03% to 0.06%, Nb: 0.03% to 0.06%, B: 0.0003% to 0.0005%, and Cr+Mo: 0.60% to 0.95%, Ti+Nb: 0.07% to 0.11%, and the rest is Fe and impurities. The preparation process includes continuous casting, hot rolling, curling, pickling, cold rolling, continuous annealing and leveling processes. By matching the composition and process, the complex phase steel has higher drawing and hole expansion properties.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile steel manufacturing, and in particular to a cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa for automobiles and a preparation method thereof. Background Art

[0002] For the automotive industry, reducing the weight of the car body is an important means to achieve carbon dioxide emission reduction, and the use of high-strength steel to manufacture body structural parts can effectively reduce the weight of the car body, such as dual-phase (DP) steel, complex phase (CP) steel, etc. Compared with dual-phase steel of the same strength level, complex phase steel is a ferrite or bainite matrix with martensite, residual austenite, and pearlite structure, so it has better flanging and hole expansion performance and bending performance, which can greatly reduce the risk of sheet forming cracking. Cold-rolled CP steel is mainly used to produce parts with high requirements for flanging performance, such as bumpers, door sills, seat rails, etc. At present, the research on high-plasticity complex phase steel is mainly focused on the tensile strength level below 1200MPa, and there is less research on high-plasticity complex phase steel with higher strength levels. The manufacture of higher-strength steel plates helps to reduce the thickness of parts. Therefore, it is urgent to develop high-strength and formable complex phase steels to meet the lightweight needs of the automotive manufacturing industry and complete product upgrades and iterations.

[0003] The Chinese invention patent with the authorization announcement number CN113528946B discloses "a 1200MPa grade reinforced forming complex phase steel and its preparation method". The main chemical composition of the complex phase steel is: C: 0.21-0.24%, Si: 1.0-1.4%, Mn: 2.1-2.6%, P≤0.01%, S≤0.005%, Cr: 0.4-0.7%, Nb: 0.02-0.04%, strengthening elements, and the rest are Fe and unavoidable impurities. The performance of the steel plate can reach yield strength: 951-1148MPa, tensile strength: 1201-1344MPa, elongation after fracture: A80 is 8-13%, and hole expansion rate: 40-59%. Although the complex phase steel has a high hole expansion rate and elongation performance, it still cannot meet the requirements for structural parts with higher tensile strength requirements.

[0004] The Chinese patent application with publication number CN114574760A discloses "a 1500MPa grade high formability alloyed hot-dip galvanized dual-phase steel plate and its preparation method". The main chemical components of the steel plate are: C: 0.21%~0.24%, Si: 0.71%~0.8%, Mn: 2.66%~2.85%, Alt: 0.91%~1.1%, Cr: 0.71%~1.1%, Mo: 0.21%~0.35%, Nb: 0.041%~0.085%, Ti: 0.051%~0.095%, B: 0.003~0.005%, P: 0.035~0.07%, S≤0.004%, N≤0.0025%, and Si+Al: 1.62%~1.9%, Cr+Mo: 0.92%~1.45%, and the rest are Fe and unavoidable impurities. Its product performance is: tensile strength ≥1510MPa, yield strength ≥805MPa, elongation after break ≥8%. It forms a large number of fine and dispersed nanoscale second phase particles through organizational regulation to make the strength reach 1500MPa level, but it does not mention the flanging and hole expansion performance. Summary of the invention

[0005] The present invention provides a cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa and a preparation method. Through the matching of ingenious component design and process design, the complex phase steel has higher drawing performance and hole expansion performance, completing the upgrade and iteration of products by steel enterprises and meeting the higher requirements for steel plate performance in the manufacture of complex-shaped parts in the automotive industry.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa, wherein the chemical composition of the steel is, by mass percentage, C: 0.21%-0.24%, Si: 1.2%-1.6%, Mn: 2.3%-2.7%, P≤0.01%, S≤0.08%, Al: 0.1%-0.12%, Cr: 0.3%-0.6%, Mo: 0.22%-0.50%, Ti: 0.03%-0.06%, Nb: 0.03%-0.06%, B: 0.0003%-0.0005%, Cr+Mo: 0.60%-0.95%, Ti+Nb: 0.07%-0.11%, and the rest is Fe and unavoidable impurity elements.

[0008] Furthermore, the metallographic structure of the finished steel plate includes intercritical ferrite, epitaxial ferrite, bainitic ferrite, martensite and residual austenite; and by volume percentage, intercritical ferrite: 3% to 6%, epitaxial ferrite: 4% to 9%, bainitic ferrite: 50% to 65%, martensite: 22% to 40%, and the rest is residual austenite; among which: the width of the bainitic ferrite lath is less than 5μm, and the width of the martensite lath is less than 5μm; fine precipitates including carbides, nitrides and carbonitrides are also dispersed in the matrix, and the diameter of the fine precipitates is 40 to 60nm.

[0009] Furthermore, the yield strength of the finished steel plate is 1020-1250 MPa, the tensile strength is ≥1370 MPa, the elongation is ≥8%, and the hole expansion rate is ≥35%.

[0010] A method for preparing cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa, comprising continuous casting, hot rolling, curling, pickling, cold rolling, continuous annealing and leveling processes; wherein the following processes are controlled:

[0011] 1) Hot rolling: the heating temperature of the slab is 1230℃~1250℃, and the temperature is kept for 1~2h; the starting rolling temperature is 1050℃~1130℃, and the final rolling temperature is 890℃~930℃; the steel plate is cooled to 600℃~650℃ by laminar flow after rolling; the coiling temperature is 550℃~650℃, and the temperature is kept for more than 2h, and then air-cooled to room temperature;

[0012] 2) Cold rolling; the cold rolling reduction rate is 50% to 65%;

[0013] 3) Continuous annealing; including heating, heat preservation, slow cooling, first stage rapid cooling, over-aging heat preservation and second stage cooling process; the strip is heated to 820℃~870℃ at a speed of 5~10℃ / s, kept for 150s~220s, slowly cooled to 770℃~800℃, the slow cooling rate is 1.5~4℃ / s, the first stage rapid cooling is cooled to 350℃~380℃, the rapid cooling rate is>25℃ / s, over-aging heat preservation is 350s~600s, and finally the second stage cooling is cooled to room temperature at 20~25℃ / s;

[0014] 4) Flatness; the elongation when flat is 0.2% to 0.4%.

[0015] Furthermore, the thickness of the hot-rolled steel plate is 2.5 to 3.2 mm, and the thickness of the finished steel plate is 0.8 to 2.0 mm.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1) By selecting and proportioning alloy elements, especially controlling the content of "Si+Al" and "Cr+Mo", the hardenability of steel is improved, the critical cooling rate of steel is reduced, and a certain amount of bainite and martensite structures are obtained; at the same time, a certain amount of oriented epitaxial ferrite is introduced to coordinate the deformation of the structure, and a certain amount of residual austenite is introduced to improve the plasticity of the steel;

[0018] 2) By reasonably controlling the hot rolling finishing temperature and the cold hard plate annealing temperature, annealing time and cooling rate, the organizational structure and phase ratio of the steel are controlled, and the strength of the steel plate is improved by means of solid solution strengthening, precipitation strengthening, fine grain strengthening, phase transformation strengthening, etc.;

[0019] 3) The production method of the present invention can meet the production requirements by utilizing the existing high-strength steel production line of the steel plant, thereby reducing production input and lowering production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 1 is a metallographic structure diagram of the finished steel plate in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The invention relates to a 1370MPa plasticity enhanced complex phase steel and a preparation method thereof; the chemical components (by mass percentage) in the steel are C: 0.21%-0.24%, Si: 1.2%-1.6%, Mn: 2.3%-2.7%, P≤0.01%, S≤0.08%, Al: 0.1%-0.12%, Cr: 0.3%-0.6%, Mo: 0.22%-0.50%, Ti: 0.03%-0.06%, Nb: 0.03%-0.06%, B: 0.0003%-0.0005%, Cr+Mo: 0.60%-0.95%, Ti+Nb: 0.07%-0.11%, and the rest are Fe and inevitable impurity elements.

[0022] The 1370MPa grade plasticity enhanced complex phase steel of the present invention has the following properties for the finished steel plate: yield strength of 1020-1250MPa, tensile strength ≥1370MPa, elongation ≥8%, and hole expansion rate ≥35%.

[0023] The metallographic structure of the finished steel plate prepared by the present invention includes intercritical ferrite (3% to 6%), epitaxial ferrite (4% to 9%), bainitic ferrite (50% to 65%), martensite (22% to 40%) and residual austenite (remainder). Among them: the width of the bainitic ferrite lath is less than 5μm, the width of the martensite lath is less than 5μm, and fine precipitates including carbides, nitrides and carbonitrides are dispersed in the matrix of the steel plate, and the diameter of the fine precipitates is 40 to 60nm.

[0024] In the 1370MPa plasticity enhanced complex phase steel described in the present invention, the role and content of each element are selected as follows:

[0025] C: Carbon atoms are interstitial atoms in steel, which ensure the strength of steel through solid solution strengthening. Increasing carbon content can improve the hardenability of steel and promote the formation of hard phase structures such as bainite and martensite. Increasing the carbon content in austenite can prevent supercooled austenite from undergoing phase change during cooling, ensuring that the steel has 4% to 7% residual austenite at room temperature, thereby improving the plasticity of the steel. However, too high carbon content will affect the welding performance of steel, so the present invention controls the C content to 0.21% to 0.24%.

[0026] Si: It can dissolve in ferrite and improve the strength of ferrite. Si does not form carbides and can effectively inhibit the precipitation of carbides. However, excessive addition of Si can cause Si oxides to form on the surface of the steel plate, affecting the surface quality of the steel plate. Therefore, the present invention controls the Si content to 1.2% to 1.6%.

[0027] Mn: solid dissolves in ferrite and austenite, and can expand the austenite phase region. It can reduce the martensite transformation temperature and phase transformation speed in steel, thereby improving the hardenability of steel and increasing the residual austenite content. If the manganese content is too low, the stability of the retained austenite will be poor and the strength of the steel plate will be low; if the manganese content is too high, Mn segregation will occur, making the formability of the steel poor. Therefore, the present invention controls the Mn content to 2.3% to 2.7%.

[0028] P: dissolves in ferrite and can improve the strength and hardness of steel. However, P will segregate in steel and have adverse effects on welding. Generally speaking, P is a harmful element and should be strictly controlled. Therefore, the present invention controls the upper limit of P content to 0.01%.

[0029] S: is a harmful element in steel. It forms MnS inclusions with Mn, which is also the starting point for crack initiation, thereby reducing the flanging performance of the steel plate. Therefore, the present invention controls the upper limit of the S content to 0.08%.

[0030] Al: It can form high melting point Al2O3 with O in steel, reducing oxide inclusions in steel; it can also combine with N in steel to form AlN, hindering austenite grain boundary migration and preventing austenite grain coarsening. However, too high Al will increase the viscosity of molten steel and block the continuous casting nozzle, so the present invention controls the Al content to 0.1% to 0.12%.

[0031] Cr: can effectively improve the hardenability of steel, so that the steel obtains sufficient martensite, thereby ensuring the strength of the steel; at the same time, the Cr element is a ferrite zone expansion element, and excessive Cr elements will reduce the two-phase zone, so the present invention controls the Cr content to 0.3% to 0.6%.

[0032] Mo: can effectively improve the hardenability of steel, and can effectively delay the pearlite transformation, obtain more bainite and martensite structures, and ensure the high strength of steel. However, the cost of Mo is relatively high, so the present invention controls the Mo content to 0.22% to 0.50%, and controls Cr+Mo to 0.60% to 0.95%.

[0033] Ti: It can precipitate TiC and TiN with C and N elements in steel, refine grains, and significantly improve the yield strength of steel. However, excessive precipitation of titanium compounds will reduce the formability of steel, so the present invention controls the Ti content to 0.03% to 0.06%.

[0034] Nb: It can form NbC precipitation with C element in steel, effectively preventing ferrite grain growth, thereby refining the grains; niobium dispersed in ferrite can improve the yield strength of steel, so that the steel plate has higher hole expansion performance. However, excessive addition of Nb element will increase the production cost of steel, so the present invention controls the Nb content to 0.03% to 0.06%, and controls Ti+Nb: 0.07% to 0.11%.

[0035] B: can strongly increase the hardenability of steel, reduce the cooling rate when phase change occurs, and reduce P segregation. Therefore, the content of B element is controlled within 0.0003% to 0.0005% in the present invention.

[0036] The method for preparing a cold-rolled complex phase steel with plasticity enhanced tensile strength of more than 1370 MPa described in the present invention includes a series of processes such as continuous casting, hot rolling, curling, pickling, cold rolling, continuous annealing, and leveling. The following processes are specifically controlled:

[0037] 1) Continuous casting: molten steel with a set chemical composition is smelted in a converter and then continuously cast to obtain slabs;

[0038] 2) Hot rolling;

[0039] ① The heating temperature is controlled at 1230℃~1250℃ and kept at this temperature for 1~2h. During the heating period, the precipitation of Ti(C,N) is ensured to pin the original austenite grain boundary, prevent the migration of austenite grain boundary, and play a role in refining austenite grains. The appropriate holding time can make the alloy elements completely dissolved and prevent the precipitation and coarsening of TiN, which will lead to the reduction of grain boundary strength.

[0040] ② The starting rolling temperature is 1050℃~1130℃, and the final rolling temperature is 890℃~930℃; the starting rolling temperature is in the austenite region to ensure austenite recrystallization; the steel plate after rolling is laminar cooled to 600℃~650℃, and the thickness of the hot rolled plate is 2.5~3.2mm.

[0041] 3) Coiling: The coiling temperature is 550℃~650℃, and the temperature is kept for more than 2 hours, followed by air cooling to room temperature; the coiling temperature is determined according to the phase transformation temperature of the components, and the steel plate structure obtained after coiling is ferrite, bainite, and martensite. Too high a coiling temperature will cause defects such as poor surface quality and coil collapse; too low a coiling temperature will lead to excessive strength of the hot-rolled plate, which will bring difficulties to subsequent cold rolling.

[0042] 4) Pickling: used to remove the iron oxide scale generated on the surface of hot-rolled steel plates.

[0043] 5) Cold rolling; the cold rolling reduction rate is 50% to 65%; the reduction rate is guaranteed to be above 50% in order to allow the steel plate to have sufficient deformation energy storage, which is conducive to recrystallization during annealing; at the same time, the reduction rate is controlled below 65%, because if the deformation is too large, edge cracks or even strip breaks are likely to occur during the rolling process.

[0044] 6) Continuous annealing; including heating, insulation, slow cooling, first stage rapid cooling, over-aging insulation, second stage cooling and other processes.

[0045] The steel plate is heated to 820℃~870℃ at a speed of 5~10℃ / s, kept warm for 150s~220s, slowly cooled to 770℃~800℃, with a slow cooling rate of 1.5~4℃ / s. In the first stage, it is cooled to 350℃~380℃, with a fast cooling rate greater than 25℃ / s. After aging and keeping warm for 350s~600s, it is cooled to room temperature at 20~25℃ / s in the second stage.

[0046] The reasons for selecting the continuous annealing process parameters are as follows:

[0047] ① The steel plate is heated at a heating rate of 5℃ / s to 10℃ / s. If the heating rate is too low, the steel plate will stay in the heating furnace for too long, which will not only cause the austenite grains to grow and affect the steel strength, but also reduce production efficiency. If the heating rate is too high, it will cause temperature overshoot, which will increase the difficulty of equipment control.

[0048] ② Heat to 820℃~870℃; this temperature is in the two-phase region. If the heating temperature is too low, the total content of bainite and martensite will be insufficient, resulting in insufficient tensile strength of the product; if the heating temperature is too high, more martensite will be obtained during the secondary cooling process, which will reduce the formability of the product.

[0049] ③ Keep warm for 150s to 220s. If the holding time is too short, the alloy elements cannot be evenly distributed. If the holding time is too long, the austenite grains will gradually grow. Both too short and too long holding times will affect the final performance of the product.

[0050] ④ Slow cooling to 770℃~800℃ is to obtain a certain amount of oriented epitaxial ferrite, which has a higher alloy content than the two-phase ferrite. As a soft phase, it can coordinate the deformation of the organization and improve the hole expansion performance.

[0051] ⑤ Complete the first stage of cooling at a cooling rate of not less than 25℃ / s; this cooling rate avoids the pearlite transformation and causes the supercooled austenite to undergo bainite transformation in the bainite transformation zone.

[0052] ⑥ Cool to 350℃~380℃ for overaging; this temperature is the bainite phase transformation temperature. If the overaging temperature is too low, the bainite content will be low, and the martensite content will be high in the subsequent cooling process. The low carbon content in the retained austenite will reduce its stability. In the subsequent flattening process, martensite phase transformation will occur, reducing the product elongation and hole expansion performance. If the overaging temperature is too high, more bainite will be generated, and less martensite will be generated in the subsequent cooling process, and the product tensile strength cannot reach 1370MPa.

[0053] ⑦ Keep warm for 350s to 600s; by controlling the over-aging time, a certain amount of bainite is obtained. The carbon atoms in the bainite diffuse into the martensite, which increases the stability of the supercooled austenite. In the subsequent cooling process, a certain amount of residual austenite is obtained to ensure the formability of the steel.

[0054] ⑧ The second stage of cooling is cooling to room temperature at 20℃ / s~25℃ / s; the purpose is to obtain a certain amount of martensite and retained austenite, as well as a certain amount of martensite to ensure the strength of the steel plate; a certain amount of retained austenite undergoes phase transformation during the deformation process, which can improve the plasticity of the steel plate.

[0055] 7) Flattening: The elongation during flattening is 0.2% to 0.4%, the purpose is to improve the yield strength of the steel plate. Flattening can produce a suitable roughness on the surface of the steel plate (too small or too large flattening elongation will change the surface roughness of the steel plate). In addition, too large flattening elongation will cause the residual austenite in the steel to undergo martensite transformation, reducing the plasticity of the steel.

[0056] In order to make the purpose, technical scheme and technical effect of the present invention clearer, the technical scheme in the embodiment of the present invention is now clearly and completely described. However, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. In combination with the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] Embodiment 1:

[0058] Table 1 lists the chemical composition of the steel of each embodiment, Table 2 lists the continuous casting and hot rolling process parameters of the steel of each embodiment, Table 3 lists the cold rolling and continuous annealing process parameters of the steel plate of each embodiment, and Table 4 lists the mechanical properties of the finished steel plate of each embodiment.

[0059] Figure 1 This is the metallographic structure diagram of the finished steel plate prepared in Example 1.

[0060] Table 1 Chemical composition of steel, wt%

[0061] Example C Si Mn P S Al Cr Mo Ti Nb B 1 0.213 1.34 2.63 0.008 0.06 0.118 0.53 0.33 0.044 0.053 0.0004 2 0.225 1.29 2.46 0.005 0.05 0.111 0.47 0.44 0.053 0.039 0.0003 3 0.231 1.32 2.31 0.005 0.05 0.102 0.33 0.29 0.053 0.050 0.0005 4 0.219 1.27 2.59 0.007 0.04 0.115 0.56 0.37 0.036 0.032 0.0004 5 0.236 1.40 2.33 0.006 0.05 0.107 0.33 0.41 0.043 0.037 0.0004 6 0.227 1.51 2.42 0.007 0.05 0.109 0.60 0.22 0.035 0.044 0.0003

[0062] Table 2 Continuous casting and hot rolling process parameters of steel

[0063] Example Heating temperature / ℃ Rolling temperature / ℃ Final rolling temperature / ℃ Curling temperature / ℃ Hot rolled plate thickness / mm 1 1233 1067 897 609 2.92 2 1245 1087 904 637 3.08 3 1247 1118 914 642 3.14 4 1230 1079 900 614 3.02 5 1250 1122 921 621 2.89 6 1242 1098 902 618 2.97

[0064] Table 3 Process parameters of cold rolling, continuous annealing and leveling of steel plates

[0065] Example Cold rolling reduction rate / % Heating temperature / ℃ Insulation time / s Slow cooling temperature / ℃ Slow cooling rate / ℃ / s The first cooling speed / ℃ / s Over-aging temperature / ℃ Insulation time / s Second stage cooling speed / ℃ / s Flat elongation% 1 56 862 164 777 3.1 26 359 433 21.4 0.23 2 62 869 178 782 2.6 27 366 358 20.7 0.31 3 51 842 215 788 2.3 32 352 374 24.2 0.27 4 56 857 191 776 3.2 29 377 392 21.9 0.29 5 62 881 187 793 1.9 34 361 469 23.8 0.33 6 60 860 182 783 2.7 39 369 504 22.6 0.38

[0066] Table 4 Mechanical properties of finished steel plates

[0067] Example Yield strength / MPa Tensile strength / MPa <![CDATA[Elongation (A 50 ) / %]]> Hole expansion rate λ / % 1 1036 1389 8.5 38.5 2 1126 1385 8.7 43.4 3 1228 1403 8.1 36.4 4 1173 1382 8.5 45.2 5 1201 1395 8.4 35.3 6 1156 1377 9.3 42.7

[0068] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for preparing cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa, characterized in that: The chemical composition of the steel is calculated by mass percentage as follows: C: 0.21% ~ 0.24%, Si: 1.2% ~ 1.6%, Mn: 2.3% ~ 2.7%, P ≤ 0.01%, S ≤ 0.08%, Al: 0.1% ~ 0.12%, Cr: 0.3% ~ 0.6%, Mo: 0.22% ~ 0.50%, Ti: 0.03% ~ 0.06%, Nb: 0.03% ~ 0.06%, B: 0.0003% ~ 0.0005%, and Cr + Mo: 0.60% ~ 0.95%, Ti + Nb: 0.07% ~ 0.11%, and the rest is Fe and unavoidable impurity elements; The method for preparing the cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa comprises continuous casting, hot rolling, curling, pickling, cold rolling, continuous annealing and leveling processes; wherein the following processes are controlled: 1) Hot rolling: the heating temperature of the slab is 1230℃~1250℃, and the temperature is kept for 1~2h; the starting rolling temperature is 1050℃~1130℃, and the final rolling temperature is 890℃~930℃; the steel plate is cooled to 600℃~650℃ by laminar flow after rolling; the coiling temperature is 550℃~650℃, and the temperature is kept for more than 2h, and then air-cooled to room temperature; 2) Cold rolling; the cold rolling reduction rate is 50% to 65%; 3) Continuous annealing; including heating, heat preservation, slow cooling, first stage rapid cooling, over-aging heat preservation and second stage cooling process; the strip is heated to 820℃~870℃ at a speed of 5~10℃ / s, kept for 150s~220s, slowly cooled to 770℃~800℃, the slow cooling rate is 1.5~4℃ / s, the first stage rapid cooling is cooled to 350℃~380℃, the rapid cooling rate is>25℃ / s, over-aging heat preservation is 350s~600s, and finally the second stage cooling is cooled to room temperature at 20~25℃ / s; 4) Flatness; the elongation when flat is 0.2% to 0.4%.

2. The method for preparing a cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa according to claim 1, characterized in that: The metallographic structure of the finished steel plate includes intercritical ferrite, epitaxial ferrite, bainitic ferrite, martensite and residual austenite; and by volume percentage, intercritical ferrite: 3% to 6%, epitaxial ferrite: 4% to 9%, bainitic ferrite: 50% to 65%, martensite: 22% to 40%, and the rest is residual austenite; among which: the width of bainitic ferrite lath is less than 5μm, and the width of martensite lath is less than 5μm; fine precipitates including carbides, nitrides and carbonitrides are also dispersed in the matrix, and the diameter of the fine precipitates is 40 to 60nm.

3. The method for preparing a cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa according to claim 1, characterized in that: The yield strength of the finished steel plate is 1020~1250MPa, the tensile strength is ≥1370MPa, the elongation is ≥8%, and the hole expansion rate is ≥35%.

4. The method for preparing a cold-rolled plasticity-enhanced complex phase steel with a tensile strength of more than 1370 MPa according to claim 1, characterized in that: The thickness of the hot-rolled steel plate is 2.5 to 3.2 mm, and the thickness of the finished steel plate is 0.8 to 2.0 mm.

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