980 mpa-grade steel plate having high bake hardening value and manufacturing method therefor

Abstract

Disclosed in the present invention is a 980 MPa-grade steel plate having a high bake hardening value, comprising Fe and inevitable impurities, and further comprising the following chemical elements in percentage by mass: 0.15-0.25% of C, 0.5-1.7% of Si, 1.50-2.50% of Mn, 0.03-0.80% of Al, and greater than 0% and less than 0.001% of B. The 980 MPa-grade steel plate does not contain Nb, V, Ti, Cr, and Mo elements. The microstructure of the 980 MPa-grade steel plate has: (1) retained austenite; and (2) at least one of martensite, tempered martensite, and partitioned martensite, wherein the total area proportion of the retained austenite and the at least one of martensite, tempered martensite, and partitioned martensite is not less than 50%, and the dislocation density in the martensite, tempered martensite, and partitioned martensite is not less than 0.7×1015 / m2.

Description

980mpa grade steel sheet having high bake hardening value and method for manufacturing the same TECHNICAL FIELD

[0001] The present invention relates to a steel sheet and a method for manufacturing the same, and more particularly, to a bake hardening steel sheet and a method for manufacturing the same. BACKGROUND

[0002] Paint baking after part forming is an important link in the automobile manufacturing process. For automobile steel, the strength after paint baking increases, and the part has higher strength than the steel sheet itself, which is called the so-called bake hardening characteristic. The presence of a certain dislocation and solid solution atom in the steel sheet is a necessary condition for obtaining the bake hardening characteristic, and generally needs to add carbide forming elements of Nb, Ti and Mo. The strength of the automobile outer plate steel is low, and in order to obtain the deep drawing performance, the carbon content needs to be controlled to be low. The existence of the bake hardening characteristic improves the strength of the outer plate to a certain extent, and creates a good foundation for lightweight.

[0003] At present, the steel for automobile body structure is mainly high-strength steel and ultra-high-strength steel. Due to the more complex composition and structure, the bake hardening characteristic is less researched and utilized. If the composition and phase change characteristics of high-strength steel are utilized to develop products with higher bake hardening characteristics, and without adding valuable alloying elements such as Nb, Ti and Mo, it will help to further expand the high-strength lightweight space of automobile body materials.

[0004] For example, the Chinese patent document with the publication number CN107995931A, the publication date of May 4, 2018, and the name of "High-strength thin steel sheet excellent in drawability and bake hardenability and method for manufacturing the same" discloses a high-strength steel sheet excellent in bake hardenability, the composition of which is as follows in terms of weight percentage: C: 0.0005-0.003%, Si: 0-0.5% (except 0%), Mn: 0-1.2% (except 0), P: 0.005-0.12%, S: <0.008%, N: <0.005%, acid-soluble Al: 0-0.1% (except 0%), Ti: 0.01-0.04%, and the balance of Fe and other inevitable impurities. The steel sheet has a bake hardening value of 4 MPa or more. The manufacturing method is as follows: after hot rolling of the steel billet, the steel sheet is coiled at 450-750℃, cold-rolled at a reduction ratio of 75-85% and a final roll reduction ratio of 5-15%, heated to 830-880℃ at a speed of 0-7℃ / s and kept for 30-80s, and then cooled to 650℃ at a speed of 2-10℃ / s. The high-strength steel sheet of the patent application can be used for manufacturing automobile outer plate parts, and the strength thereof is about 200MPa. SUMMARY

[0005] An object of the present application is to provide a 980 MPa grade steel sheet having a high bake hardening value, which has excellent cold formability, bake hardening characteristics and bending properties, can fully utilize a paint baking process in a part manufacturing process, further improve a strength level of a part, and improve a collision energy absorption performance.

[0006] To achieve the above object, the present application provides a 980 MPa grade steel sheet having a high bake hardening value, which contains Fe and inevitable impurities, and further contains the following chemical elements in mass %:

[0007] C: 0.15 to 0.25%;

[0008] Si: 0.5 to 1.7%;

[0009] Mn: 1.50 to 2.50%;

[0010] Al: 0.03 to 0.80%;

[0011] 0 < B ≤ 0.001%;

[0012] The 980 MPa grade steel sheet does not contain Nb, V, Ti, Cr and Mo elements;

[0013] The microstructure of the 980 MPa grade steel sheet has (1) residual austenite; and (2) at least one of martensite, tempered martensite and partitioned martensite, wherein the total area ratio of the residual austenite and the at least one of the martensite, the tempered martensite and the partitioned martensite is not less than 50%, and the dislocation density of the martensite, the tempered martensite and the partitioned martensite is not less than 0.7 x 10 15 / m 2 .

[0014] In some embodiments, in the 980 MPa grade steel sheet according to the present application, the total area ratio of the residual austenite and the at least one of the martensite, the tempered martensite and the partitioned martensite is 50 to 70%.

[0015] In some embodiments, the dislocation density of the martensite, the tempered martensite and the partitioned martensite is 0.7 x 10 15 / m 2 - 1.3 x 10 15 / m 2 .

[0016] In the present application, the martensite generated by the TRIP effect and the martensite, the tempered martensite or the partitioned martensite present in the structure can ensure a decrease in a hardness difference between soft and hard phases after baking while ensuring a high strength of the steel sheet, improve a structure uniformity, and thus enhance the bending properties of the steel sheet.

[0017] In the present application, a large number of movable dislocations are generated in ferrite due to martensitic transformation in the quenching process, and the transformation of the pre-deformation stage residual austenite increases the dislocation density in the martensite, and the dislocation density increases with the increase of the pre-deformation amount, so that the dislocation density in the martensite, the tempered martensite and the partitioned martensite is not less than 0.7×10 15 / m 2 , and the baking hardening performance can be improved. These carbon supersaturated martensites occur carbon redistribution, carbon atom cluster or cox gas cluster formation, carbide precipitation in subsequent baking treatment, which provides sufficient particles for the pinning of dense dislocations.

[0018] Further, in the 980MPa grade steel plate described in the present application, the mass percentage of each chemical element is:

[0019] C: 0.15-0.25%;

[0020] Si: 0.5-1.7%;

[0021] Mn: 1.50-2.50%;

[0022] Al: 0.03-0.80%;

[0023] 0≤B≤0.001%;

[0024] The balance is Fe and inevitable impurities.

[0025] In the technical solution described in the present application, low-cost C, Si, Mn, Al, B composition design is adopted, and no valuable elements such as Nb, V, Ti, Cr, Mo are added. Specifically, the design principles of each chemical element are as follows:

[0026] C: In the 980MPa grade steel plate with high baking hardening value described in the present application, C element is an austenite forming and stabilizing element, and can prolong the incubation temperature before austenite transformation. In the quenching-partitioning process, by using the transfer of C element from martensite to residual austenite, the stability of residual austenite can be improved, which helps the residual austenite to play TRIP effect to improve the strength and elongation of the material. At the same time, sufficient C element is more likely to precipitate and form carbide in the paint baking process after the deformation of the steel plate, which can pin the movable dislocations generated by pre-deformation and improve the baking hardening value. However, when the content of C element is too high, the welding performance of the steel will be poor, and the crack sensitivity after quenching will increase. Therefore, in the 980MPa grade steel plate with high baking hardening value described in the present application, the mass percentage of C element can be controlled between 0.15-0.25%, such as 0.18-0.25%, 0.15-0.22%, 0.18-0.22%.

[0027] Si: In the 980MPa grade steel plate with high bake hardening value described in the present application, the solubility of Si element in carbide is extremely small, which can inhibit the formation of cementite. Si element can also promote the enrichment of carbon in residual austenite, improve the stability of residual austenite, and improve the elongation of the steel. When the mass percentage content of Si element is too high, it is difficult to form carbide, which is not conducive to pinning dislocations to improve the bake hardening value; when the mass percentage content of Si element is too low, too much carbide precipitates, which is not conducive to the formation and stabilization of residual austenite. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Si element can be controlled between 0.5-1.7%, such as 0.8-1.7%, 0.8-1.4%, 1.1-1.7%.

[0028] Mn: In the 980MPa grade steel plate with high bake hardening value described in the present application, Mn element can improve the hardenability and austenite stability of the steel, reduce the critical cooling rate, and also play a solid solution strengthening effect to improve the strength of the steel. When the mass percentage content of Mn element is too high, the grain will be coarsened, which will reduce the plasticity of the steel; when the mass percentage content of Mn element is too low, it is easy to cause the banded structure of ferrite and pearlite to precipitate at low cooling rate, which is not conducive to improving the elongation of the steel. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Mn element can be controlled between 1.50-2.50%, such as 1.5-2.3%, 1.8-2.3%, 1.5-2.1%.

[0029] Al: In the 980MPa grade steel plate with high bake hardening value described in the present application, when Al element exists in solid solution state, it can increase the stacking fault, inhibit the precipitation of cementite and martensite transformation, and improve the stability of austenite, thereby improving the elongation of the steel; Al element can also form fine and dispersed insoluble particles with C and N, which can refine the grain and prevent dislocation movement, thereby improving the strength and bake hardening value of the steel. However, when the mass percentage content of Mn element is too high, a large amount of oxide inclusions will be formed, which will lead to continuous casting difficulties. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Al element can be controlled between 0.03-0.80%, such as 0.2-0.8%, 0.2-0.7%, 0.4-0.8%.

[0030] B: In the 980MPa grade steel plate with high bake hardening value described in the present application, the element B can improve the hardenability of the steel, delay the transformation of austenite to ferrite, and increase the strength of the steel. However, when the mass percentage content of B is too high, the strength will increase too fast, resulting in a decrease in elongation. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of B can be controlled to 0 < B ≤ 0.001%, such as 0.0002 ≤ B ≤ 0.0009%, 0.0002 ≤ B ≤ 0.0006%.

[0031] Further, in the 980MPa grade steel plate described in the present application, the mass percentage content of each element also satisfies Al + Si ≤ 1.85%.

[0032] In the chemical composition design of the present application, while controlling the content of a single element, the mass percentage of Al and Si also satisfies Al + Si ≤ 1.85%. When the content of Al + Si is too high, it is easy to cause internal oxidation and deteriorate the surface quality of the steel plate. In some embodiments, the mass percentage of Al and Si satisfies Al + Si ≤ 1.73%, such as 1.3 ≤ Al + Si ≤ 1.73%.

[0033] Further, in the 980MPa grade steel plate described in the present application, among the unavoidable impurities: P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.008%.

[0034] It should be noted that in the technical solution described in the present application, P, S and N are all unavoidable impurity elements in steel.

[0035] P: In the 980MPa grade steel plate with high bake hardening value described in the present application, although the element P can play a role in solid solution strengthening, inhibit the formation of carbides, and be beneficial to improve the stability of residual austenite, if the mass percentage content of P in the steel is too high, the element P will inhibit the precipitation of carbides, affect the bake hardening effect, and its segregation at the grain boundary will also increase the brittleness of the steel. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of P can be controlled to P ≤ 0.015%, such as P ≤ 0.015%, 0.007 ≤ P ≤ 0.015% or 0.007 ≤ P ≤ 0.009%.

[0036] S: In the 980MPa grade steel plate with high bake hardening value described in the present application, the element S will combine with the element Mn to form MnS inclusions, which is not conducive to improving the cold forming ability of the steel. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of S can be controlled to S ≤ 0.005%, such as S ≤ 0.004%, S ≤ 0.003%, S ≤ 0.002%.

[0037] N: In the 980MPa grade steel plate with high bake hardening value described in the present application, N element is easy to combine with Ti to form TiN in the steel, which can precipitate in the hot rolling stage to play a certain precipitation strengthening effect. However, when the mass percentage content of N in the steel is too high, it will affect the cold forming ability of the material elongation and r value, and also easily cause continuous casting crack. Therefore, in the 980MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of N element can be controlled as N≤0.008%, such as 0.003≤N≤0.008% or 0.004≤N≤0.008%.

[0038] Further, in the 980MPa grade steel plate described in the present application, the area ratio of the residual austenite is 12-20%.

[0039] In the present application, 12-20% of residual austenite can play a TRIP effect in the deformation process to realize the strengthening and plasticizing of the steel plate, and further improve the elongation level.

[0040] Further, in the 980MPa grade steel plate described in the present application, the microstructure further contains 30-50% of ferrite in area ratio.

[0041] In the present application, the content of 30-50% of ferrite can enhance the coordinated deformation ability in the processing of the steel plate and improve the forming performance of the steel.

[0042] Further, in the 980MPa grade steel plate described in the present application, the microstructure is 30-50% of ferrite in area ratio + 12-20% of residual austenite in area ratio + at least one of the remaining amount of martensite, tempered martensite and partitioning martensite.

[0043] In some embodiments, in the 980MPa grade steel plate described in the present application, the total area ratio of martensite, tempered martensite and partitioning martensite is 30-55%.

[0044] Further, in the 980MPa grade steel plate described in the present application, the performance satisfies at least one of the following:

[0045] The bake hardening value in the strain range of 2-20% is not less than 110MPa;

[0046] The VDA maximum bending angle of the steel plate after bake hardening is not less than 108°;

[0047] The tensile strength is ≥980MPa, the uniform elongation is ≥15%, and the elongation at break is ≥21%.

[0048] In some embodiments, the 980 MPa grade steel sheet according to the present application has a bake hardening value of 110-260 MPa or 112-258 MPa in the strain range of 2-20%.

[0049] In some embodiments, the pre-strain amount is 5-20%.

[0050] In some embodiments, the 980 MPa grade steel sheet according to the present application has a VDA maximum bending angle of 108-140°, such as 108-136°, after bake hardening of the steel sheet.

[0051] In some embodiments, the 980 MPa grade steel sheet according to the present application has a tensile strength of ≥1000 MPa. In some embodiments, the 980 MPa grade steel sheet according to the present application has a tensile strength of 980-1060 MPa or 983-1055 MPa.

[0052] In some embodiments, the 980 MPa grade steel sheet according to the present application has an uniform elongation of 15-25%, such as 15-22%.

[0053] In some embodiments, the 980 MPa grade steel sheet according to the present application has an elongation at break of 21-30%.

[0054] Another object of the present application is to provide a method for manufacturing a 980 MPa grade steel sheet having a high bake hardening value, by which a 980 MPa grade steel sheet having a high bake hardening value with excellent performance can be obtained.

[0055] To achieve the above object, the present application provides a method for manufacturing a 980 MPa grade steel sheet having a high bake hardening value, which comprises the steps of:

[0056] (1) smelting and continuous casting;

[0057] (2) hot rolling;

[0058] (3) pickling and cold rolling;

[0059] (4) continuous annealing: controlling the annealing temperature to be 780-930℃, the holding time to be 30-240s, then slowly cooling at a cooling rate of not higher than 10℃ / s to (Ac1±50)℃, then cooling at a rate of not lower than 75℃ / s to (Ms-20)~(Mf+5)℃, then heating to 350-460℃, holding for 20-600s, and finally cooling to room temperature. Wherein, Ac1 represents the temperature at which the steel starts to transform into austenite when heated, Ms represents the starting temperature of martensitic transformation, and Mf represents the ending temperature of martensitic transformation.

[0060] In the present invention, by controlling the annealing temperature at 780-930℃ and holding for 30-240s, a complete austenite or a proportion controllable austenite + ferrite two-phase structure can be obtained, which is the primary condition for obtaining carbon supersaturated martensite and residual austenite in the fast cooling stage. This is because: the holding time is short when annealing in the austenite single-phase region, and the holding time is long when annealing in the austenite + ferrite two-phase region. Slow cooling at a cooling rate of not higher than 10℃ / s to (Ac1±50)℃ is used for further adjusting the ferrite content and improving the material coordination deformation ability. Cooling at a speed of not less than 75℃ / s can obtain sufficient martensite and obtain high dislocation density. By adjusting the fast cooling end temperature between (Ms-20)~(Mf+5)℃, the martensite transformation amount and the residual austenite content can be controlled. Preferably, the annealing temperature is 780-850℃, 860-880℃ or 780-880℃. Preferably, the annealing holding time is 30-200s, 60-240s or 60-180s. Preferably, the slow cooling rate is 4-10℃ / s. Preferably, the Ac1 temperature is 700-726℃. Preferably, the slow cooling temperature is (Ac1±46)℃, such as 680-750℃. Preferably, the fast cooling rate is 75-500℃ / s. Preferably, the Ms temperature is 344-391℃ or 344-359℃. Preferably, the Mf temperature is 229-280℃ or 229-243℃. Preferably, the fast cooling temperature is (Ms-50)~(Mf+5)℃, such as 235-305℃ or 235-290℃. Preferably, the reheating temperature is 350-420℃. Preferably, the reheating holding time is 30-600s, 20-500s or 30-500s.

[0061] In the present invention, in the subsequent reheating process, partitioning of carbon from the supersaturated martensite into the residual austenite and stabilization of the residual austenite occur, so that a high content of residual austenite can be retained in the structure after final cooling to room temperature, ensuring the strength and elongation of the steel plate. At the same time, due to the existence of high-carbon-content residual austenite and high-dislocation-density martensite, the further increase of dislocation density and the formation of sufficient pinning points during the pre-deformation and baking of the steel plate are ensured, and the baking hardening value is improved.

[0062] Further, in the step (2) of the manufacturing method, the slab is heated to 1180-1280℃, and held for 60-240 minutes, and then rolled. Preferably, the slab is heated to 1230-1280℃ or 1180-1230℃. Preferably, the holding time is 70-240 minutes, 70-160 minutes or 190-240 minutes. In the present application, the slab is heated to a temperature of 1180-1280℃, which can reduce the slab dendrite coarsening and surface decarburization caused by excessively high heating temperature, and avoid the hot rolling deformation resistance increase caused by excessively low heating temperature, thereby ensuring the smoothness of hot rolling production. The holding time is controlled to 60-240 minutes, which can ensure the uniformity of the internal temperature and structure of the slab, and avoid the structure coarsening caused by excessively long time.

[0063] Further, in the step (2) of the manufacturing method, the finishing temperature is controlled to 880-950℃, such as 890-950℃, 880-920℃ or 940-950℃.

[0064] In the present application, by controlling the finishing temperature to 880-950℃, the slab can be rolled in the austenite single-phase region, thereby improving the rollability; at the same time, the excessively high rolling force caused by excessively low temperature is avoided, and the production stability is ensured.

[0065] Further, in the step (2) of the manufacturing method, the coiling temperature is controlled to 400-550℃, and the coiled product is held for 2-300 minutes at a temperature not lower than the coiling temperature and not higher than 100℃ above the coiling temperature. Preferably, the coiled product is held for 0-90℃ above the coiling temperature, such as 450-640℃.

[0066] In the present application, by controlling the coiling temperature to 400-550℃, the steel coil can have appropriate strength and good surface quality. When the coiling temperature is excessively low, the hot coil strength is high, and the subsequent pickling and cold rolling are difficult; when the coiling temperature is excessively high, the Si and Mn elements in the steel are easily oxidized, and the oxide layer is enriched between the iron oxide scale and the steel plate substrate, which makes it difficult to remove the acid.

[0067] In the present application, by controlling the holding temperature and time after coiling, the hot rolled coil can be uniformly softened without obvious Si and Mn internal oxidation layer, i.e. high-temperature short-time or low-temperature long-time holding treatment, which realizes the martensite tempering or near-isothermal bainite phase change.

[0068] Further, in the step (4) of the manufacturing method, the hot galvanizing process is further performed after the process of reheating to 350-460℃ and holding for 20-600 seconds, so as to obtain the corresponding hot galvanizing product.

[0069] Further, in the step (4) of the manufacturing method, after the cooling to room temperature process, a galvanizing process can also be performed to obtain a corresponding galvanized product.

[0070] The 980MPa grade steel plate with high bake hardening value and the manufacturing method thereof have the following advantages and beneficial effects compared to the prior art:

[0071] The 980MPa grade steel plate with high bake hardening value has a simple and low-cost component system, and through component and process design, a microstructure with a high residual austenite content is obtained. The TRIP effect of the residual austenite is utilized to obtain a tensile strength of ≥980MPa, and the steel plate has excellent cold forming performance with a uniform elongation of ≥15% and a fracture elongation of ≥21%.

[0072] The 980MPa grade steel plate with high bake hardening value has a high carbon content and high dislocation density of martensite, tempered martensite, and partitioned martensite, which can fully utilize the pre-deformation and baking treatment of the steel plate during part manufacturing and paint baking, further enhance the dislocation density, precipitate a large number of pinning dislocation particles, and obtain a bake hardening value of not less than 110MPa in a larger strain range, thereby further improving the strength of the part and the collision energy absorption effect.

[0073] The content of residual austenite and various types of martensite in the 980MPa grade steel plate with high bake hardening value is not less than 50%, which can reduce the hardness difference between each phase in the microstructure after part forming and paint baking, improve the uniformity of the microstructure, and obtain excellent bending performance with a VDA maximum bending angle of not less than 108° for a 1.2mm thick plate. The excellent bending performance after baking can improve the ability of the part to resist local deformation. BRIEF DESCRIPTION OF DRAWINGS

[0074] Figure 1 shows the SEM microstructure of the 980MPa grade steel plate of Example 11 of the present application.

[0075] Figure 2 schematically shows the change in bake hardening value of Example 11 of the present application in the strain range of 2-20%. DETAILED DESCRIPTION

[0076] The 980MPa grade steel plate with high bake hardening value and the manufacturing method thereof will be further explained and described below in conjunction with specific examples and the accompanying drawings, but the explanation and description do not constitute an improper limitation on the technical solutions of the present application.

[0077] Examples 1-25 and Comparative Examples 1-2

[0078] Table 1 lists the mass percentages of each chemical element in the 980 MPa grade steel sheets having high bake hardening values of Examples 1-25 and the comparative steel sheets of Comparative Examples 1-2.

[0079] Table 1. (wt%, balance being Fe and unavoidable impurities other than P, S, N)

[0080] The 980 MPa grade steel sheets having high bake hardening values of Examples 1-25 and the comparative steel sheets of Comparative Example 1-2 described in the present application are each prepared using the following steps:

[0081] (1) Smelting and continuous casting;

[0082] (2) Hot rolling: heating the slab to 1180-1280°C, holding for 60-240 min, controlling the finish rolling temperature to be 880-950°C, and controlling the coiling temperature to be 400-550°C, and holding for 2-300 min in the range of not lower than the coiling temperature and not higher than the coiling temperature + 100°C after coiling;

[0083] (3) Pickling and cold rolling: pickling and cold rolling after uncoiling the hot-rolled coil;

[0084] (4) Continuous annealing: annealing at 780-930°C for 30-240 s, then slowly cooling to (Ac1± 50) °C at a cooling rate of not higher than 10°C / s, then cooling to (Ms-20)-(Mf+5) °C at a rate of not lower than 75°C / s, and then heating to 350-460°C for 20-600 s.

[0085] In some embodiments, after the heating to 350-460°C for 20-600 s in step (4), a hot-dip galvanizing process can be further performed to obtain a hot-dip galvanized product.

[0086] In other embodiments, after the cooling to room temperature in step (4), an electro-galvanizing process can be further performed.

[0087] It should be noted that the components and processes of Examples 1-25 of the present application all meet the requirements of the present application, while the components and process parameters of Comparative Example 1-2 all do not meet the requirements of the present application.

[0088] Table 2-1, Table 2-2 and Table 2-3 list the specific process parameters of the 980 MPa grade steel sheets having high bake hardening values of Examples 1-25 and the comparative steel sheets of Comparative Example 1-2.

[0089] Table 2-1.

[0090] Table 2-2.

[0091] Table 2-3.

[0092] Note: "CR" in the table means no plating layer on the surface; "GI" means hot-dip galvanized plating layer; "EG" means electro-galvanized plating layer; and "GA" means hot-dip galvannealed plating layer.

[0093] In order to verify the implementation effect of the present application, the 980MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel of Comparative Example 1-2 were sampled, and microstructure observation and dislocation density measurement were performed by using scanning electron microscope and X-ray diffraction, and the observation results are listed in Table 3.

[0094] Table 3 lists the microstructure observation results of the 980MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel of Comparative Example 1-2 of the present application.

[0095] Table 3.

[0096] As can be seen from the above Table 3, in the microstructure of the 980MPa grade steel plates of Examples 1-25, the area proportion of ferrite is between 30-50%, the area proportion of residual austenite is between 12-20%, the total area proportion of residual austenite and at least one of martensite, tempered martensite and partitioned martensite is not less than 50%, and the dislocation density in the martensite, tempered martensite and partitioned martensite is not less than 0.7x10 15 / m 2 .

[0097] In addition, Figure 1 shows the SEM microstructure diagram of the 980MPa grade steel plate of Example 11 of the present application.

[0098] As can be seen from Figure 1, the microstructure of Example 11 of the present application is ferrite + residual austenite + martensite + partitioned martensite, wherein the proportion of ferrite is 47%, the proportion of residual austenite is 20%, the proportion of martensite is 7%, and the proportion of partitioned martensite is 26%.

[0099] The 980MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel of Comparative Example 1-2 were resampled, and various performance tests were performed thereon, and the test results are listed in Table 4. Among them, the various performance tests include:

[0100] Bake hardening value in 2-20% strain range: according to GB / T 24174-2022 standard, after pre-stretching the sample at different strain amounts of 2%-20%, the sample is baked at 170℃ for 20min, then the sample is stretched until fracture, the increase value of the yield strength of the baked sample relative to the yield strength of the original sample is the bake hardening value.

[0101] VDA maximum bending angle of the baked steel sheet with a sheet thickness of 1.2mm: according to VDA238-100 standard, a sample of 60*60mm is taken from the steel sheet, a punch with a radius of 0.4mm is used to measure the maximum bending angle of the sample.

[0102] Mechanical properties: according to GB / T 228-2010 standard, a tensile sample is taken from the steel sheet vertically to the rolling direction, tensile test is carried out at room temperature until fracture, and the tensile strength, uniform elongation and fracture elongation are obtained.

[0103] Table 4 lists the performance test results of the 980MPa grade steel sheet with high bake hardening value of the inventive examples 1-25 and the comparative steel of comparative examples 1-2.

[0104] Table 4.

[0105] As can be seen from the above table 4, the bake hardening value of the 980MPa grade steel sheet of the inventive examples 1-25 in the 2-20% pre-strain range is between 112-258MPa, the VDA maximum bending angle after baking is greater than or equal to 108°, the tensile strength is greater than or equal to 983MPa, the uniform elongation is greater than or equal to 15%, and the fracture elongation is greater than or equal to 21%.

[0106] Figure 2 schematically shows the change of the bake hardening value in the 2-20% strain range of the inventive example 11.

[0107] As shown in Figure 2, the bake hardening value of the inventive example 11 under the conditions of 2%, 5%, 8%, 10%, 15%, 20% pre-strain is 115MPa, 132MPa, 164MPa, 189MPa, 212MPa, 253MPa respectively.

[0108] It should be noted that the scope of protection of the present application is not limited to the embodiments given in the present application file, all prior art, including but not limited to prior patent documents, prior published documents, prior public uses, etc. that are not contradictory to the scheme of the present application can be included in the scope of protection of the present application. In addition, the combination of technical features in the present case is not limited to the combination of technical features as recited in the claims or as recited in the specific embodiments, all technical features recited in the present case can be freely combined or integrated in any manner, unless contradictory to each other.

[0109] It should also be noted that the above listed embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above embodiments, and similar changes or modifications that can be directly derived or easily conceived by those skilled in the art from the disclosure of the present application should all be included in the scope of protection of the present application.

Claims

1. A 980MPa grade steel plate with a high bake hardening value, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C：0.15～0.25％； Si: 0.5–1.7%; Mn: 1.50–2.50%; Al：0.03～0.80％； 0＜B≤0.001％； The 980MPa grade steel plate does not contain Nb, V, Ti, Cr and Mo elements; The microstructure of the 980MPa grade steel plate comprises (1) retained austenite; and (2) at least one of martensite, tempered martensite, and partitioned martensite, wherein the total area ratio of retained austenite and at least one of martensite, tempered martensite, and partitioned martensite is not less than 50%, and the dislocation density in the martensite, tempered martensite, and partitioned martensite is not less than 0.7×10⁻⁶. 15 / m 2 .

2. The 980MPa grade steel plate as described in claim 1, characterized in that, The total area of ​​retained austenite and at least one of martensite, tempered martensite, and partitioned martensite is 50-70%; and / or, the dislocation density in the martensite, tempered martensite, and partitioned martensite is 0.7 × 10⁻⁶. 15 / m 2 -1.3×10 15 / m 2 .

3. The 980MPa grade steel plate as described in claim 1, characterized in that, Its mass percentage of each chemical element is as follows: C：0.15～0.25％； Si: 0.5–1.7%; Mn: 1.50–2.50%; Al：0.03～0.80％； 0＜B≤0.001％； The balance is Fe and unavoidable impurities.

4. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, The mass percentage of each element also satisfies Al+Si≤1.85%, preferably 1.3≤Al+Si≤1.73%.

5. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, In unavoidable impurities: P ≤ 0.015%, S ≤ 0.005%, N ≤ 0.008%.

6. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, The area ratio of the retained austenite is 12-20%; and / or the total area ratio of martensite, tempered martensite and partitioned martensite is 30-55%.

7. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, Its microstructure also contains ferrite with an area ratio of 30-50%.

8. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, Its microstructure consists of 30-50% ferrite in area, 12-20% retained austenite in area, and the remainder of at least one of martensite, tempered martensite, or partitioned martensite.

9. The 980MPa grade steel plate as described in any one of claims 1-3, characterized in that, Its performance satisfies at least one of the following: The bake hardening value within the strain range of 2-20% is not less than 110 MPa, preferably 110-260 MPa; After baking and hardening, the maximum VDA bending angle of the steel plate shall not be less than 108°, preferably 108°-140°; Tensile strength ≥980MPa, preferably 980-1060MPa; Uniform elongation ≥15%, preferably 15-25%; The elongation at break is ≥21%, preferably 21-30%.

10. The method for manufacturing 980MPa grade steel plate according to any one of claims 1-9, characterized in that, Including the following steps: (1) Smelting and continuous casting; (2) Hot rolling; (3) Pickling and cold rolling; (4) Continuous annealing: control the annealing temperature to 780-930℃, hold for 30-240s, then slowly cool to (Ac1±50)℃ at a cooling rate not higher than 10℃ / s, then cool to (Ms-20)~(Mf+5)℃ at a rate not lower than 75℃ / s, then heat to 350-460℃, hold for 20-600s, and finally cool to room temperature.

11. The manufacturing method as described in claim 10, characterized in that, In step (2), the slab is heated to 1180-1280℃, held for 60-240 minutes, and then rolled.

12. The manufacturing method as described in claim 10, characterized in that, In step (2), the final rolling temperature is controlled to be 880-950℃.

13. The manufacturing method as described in claim 10, characterized in that, In step (2), the winding temperature is controlled at 400-550℃, and after winding, it is kept at a temperature not lower than the winding temperature and not higher than 100℃ above the winding temperature for 2-300 minutes.

14. The manufacturing method as described in claim 10, characterized in that, In step (4), a hot-dip galvanizing process is performed after reheating to 350-460°C and holding for 20-600 seconds; and / or, in step (4), an electro-galvanizing process is performed after cooling to room temperature.

15. The manufacturing method as described in claim 10, characterized in that, Step (4) has one or more of the following characteristics: Annealing holding time is 30-200 seconds; The slow cooling rate is 4-10℃ / s; The temperature of Ac1 is 700-726℃; The slow cooling temperature is 680-750℃; The rapid cooling rate is 75-500℃ / s; Ms temperature is 344-391℃; The Mf temperature is 229-280℃; The rapid cooling temperature is 235-305℃; The reheating temperature is 350-420℃; The reheating and heat preservation time is 30-600 seconds.

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