1180 mpa-grade steel plate having high baking hardening value and manufacturing method therefor

Abstract

Disclosed in the present invention is a 1180 MPa-grade steel plate having a high baking hardening value. The steel plate comprises Fe and inevitable impurities and further comprises the following chemical elements in percentage by mass: C: 0.17-0.27%, Si: 0.5-1.7%, Mn: 2.3-3.0%, Al: 0.02-0.90%, and 0 < B ≤ 0.003%, wherein Al + Si ≤ 2.00%. The microstructure of the 1180 MPa-grade steel plate has at least one of martensite, tempered martensite, and partitioned martensite having an area proportion of not less than 50%, and the dislocation density in the martensite, the tempered martensite, and the partitioned martensite is not less than 0.8×1015 / m2. Further disclosed in the present invention is a method for manufacturing the steel plate, comprising the steps of: smelting and continuous casting; hot rolling; pickling and cold rolling; and continuous annealing.

Description

1180mpa 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 Nb, Ti, Mo carbide forming elements. 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, and the existence of the bake hardening characteristic improves the strength of the outer plate to a certain extent, creating 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 Nb, Ti, Mo precious alloy elements, it will help to further expand the high-strength lightweight space of automobile body materials.

[0004] For example: CN107995931A, published on May 4, 2018, entitled "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 then cooled to 650℃ at a speed of 2-10℃ / s. The high-strength steel sheet 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 1180 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 1180 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: 0.17 to 0.27%;

[0008] Si: 0.5 to 1.7%;

[0009] Mn: 2.3 to 3.0%;

[0010] Al: 0.02 to 0.90%;

[0011] 0 < B ≤ 0.003%;

[0012] The microstructure of the 1180 MPa grade steel sheet has at least one of martensite, tempered martensite and partitioned martensite in an area ratio of not less than 50%, and a dislocation density in the martensite, tempered martensite and partitioned martensite of not less than 0.8 x 10 15 / m 2 .

[0013] In some embodiments, the total area ratio of the martensite, tempered martensite and partitioned martensite in the microstructure of the 1180 MPa grade steel sheet is 50 to 80%, such as 54 to 74%.

[0014] In some embodiments, the dislocation density in the martensite, tempered martensite and partitioned martensite is 0.80 x 10 15 / m 2 - 1.00 x 10 15 / m 2 .

[0015] In the present application, the martensite generated by the TRIP effect and the martensite, tempered martensite or partitioned martensite present in the structure can ensure a decrease in a hardness range between soft and hard phases after baking while ensuring a high strength of the steel sheet, improve the uniformity of the structure, and thus enhance the bending properties of the steel sheet.

[0016] 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 residual austenite in the pre-deformation stage 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 0.8×10 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, and carbide precipitation in subsequent baking treatment, which provides sufficient particles for pinning of dense dislocations.

[0017] Further, in the 1180MPa grade steel plate described in the present application, the mass percentage of each chemical element is:

[0018] C: 0.17-0.27%;

[0019] Si: 0.5-1.7%;

[0020] Mn: 2.3-3.0%;

[0021] Al: 0.02-0.90%;

[0022] 0 < B ≤ 0.003%;

[0023] The balance is Fe and inevitable impurities.

[0024] In the technical solution described in the present application, the design principles of each chemical element are as follows:

[0025] C: In the 1180MPa 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 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 1180MPa grade steel plate with high baking hardening value described in the present application, the mass percentage of C element can be controlled between 0.17-0.27%, such as between 0.17-0.2% or between 0.25-0.27%.

[0026] Si: In the 1180 MPa 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 many carbides are precipitated, which is not conducive to the formation and stabilization of residual austenite. Therefore, in the 1180 MPa 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 between 0.5-1% or between 1.4-1.7%.

[0027] Mn: In the 1180 MPa grade steel plate with high bake hardening value described in the present application, Mn 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 precipitation of banded structure of ferrite and pearlite at low cooling rate, which is not conducive to improving the elongation of the steel. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Mn element can be controlled between 2.3-3.0%.

[0028] Al: In the 1180 MPa 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 martensitic 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 number of oxide inclusions are easily formed, which leads to difficulties in continuous casting. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Al element can be controlled between 0.02-0.90%, such as between 0.05-0.90% or between 0.30-0.90%.

[0029] B: In the 1180 MPa 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 the element B is too high, it will cause the strength to rise too fast, resulting in a decrease in elongation. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of the element B can be controlled between 0 < B ≤ 0.003%, such as 0.0005 ≤ B ≤ 0.003% or 0.0015 ≤ B ≤ 0.003%.

[0030] Further, in the 1180 MPa grade steel plate described in the present application, the mass percentage content of each element also satisfies Al + Si ≤ 2.00%.

[0031] In the chemical composition design of the present application, while controlling the content of a single element, the mass percentage of the elements Al and Si also satisfies Al + Si ≤ 2.00%. 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 the elements Al and Si satisfies 1.3 ≤ Al + Si ≤ 1.8% or 1.4 ≤ Al + Si ≤ 1.75%.

[0032] Further, in the 1180 MPa grade steel plate described in the present application, it also contains at least one of the following chemical elements:

[0033] 0 < Nb ≤ 0.1 wt%;

[0034] 0 < V ≤ 0.1 wt%;

[0035] 0 < Ti ≤ 0.1 wt%;

[0036] 0 < N ≤ 0.01 wt%.

[0037] In the present application, the 1180 MPa grade steel plate described in the present application can also contain a small amount of Nb, V, Ti and N, thereby improving the performance of the steel. Among them:

[0038] Nb, V, Ti: In the 1180 MPa grade steel plate with high bake hardening value described in the present application, the Nb, V, Ti elements can refine the structure and increase the proportion of solute C elements on the grain boundary. At the same time, after hot rolling, the steel can precipitate (Nb, V, Ti) C type carbides, which can play a role in pinning dislocations during heat treatment, thereby increasing the bake hardening value. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of Nb element can be controlled between 0 < Nb ≤ 0.1 wt%, the mass percentage content of V element can be controlled between 0 < V ≤ 0.1 wt%, and the mass percentage content of Ti element can be controlled between 0 < Ti ≤ 0.1 wt%. In some embodiments, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, when contained, the mass percentage of Nb element is 0.01 ≤ Nb ≤ 0.1 wt% or 0.02 ≤ Nb ≤ 0.1 wt%. In some embodiments, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, when contained, the mass percentage of V element is 0.01 ≤ V ≤ 0.1 wt%. In some embodiments, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, when contained, the mass percentage of Ti element is 0.01 ≤ Ti ≤ 0.1 wt% or 0.03 ≤ Ti ≤ 0.1 wt%.

[0039] N: In the 1180 MPa 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 steel, which can play a certain precipitation strengthening effect during hot rolling stage. However, when the mass percentage content of N in steel is too high, it will affect the cold forming ability of material elongation and r value, and also easily cause continuous casting crack. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of N element can be controlled between 0 < N ≤ 0.01 wt%. In some embodiments, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, when contained, the mass percentage of N element is 0.001 ≤ N ≤ 0.010 wt% or 0.004 ≤ N ≤ 0.010 wt%.

[0040] Further, in the inevitable impurities of the 1180 MPa grade steel plate described in the present application: P ≤ 0.01%, S ≤ 0.005%.

[0041] It should be noted that in the technical solutions described in the present application, P, S and N are all inevitable impurity elements in steel. Among them:

[0042] P: In the 1180 MPa grade steel plate with high bake hardening value described in the present application, although the P element can play a solid solution strengthening role, inhibit carbide formation, and be beneficial to improve the stability of residual austenite, if the mass percentage content of P in the steel is too high, the P element will inhibit the precipitation of carbides, affect the bake hardening effect, and the segregation of P element at the grain boundary will also increase the brittleness of the steel. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of P element can be controlled as P≤0.01%, such as 0.007≤P≤0.01%.

[0043] S: In the 1180 MPa grade steel plate with high bake hardening value described in the present application, the S element will combine with the Mn element to form MnS inclusions, which is not conducive to improving the cold forming ability of the steel. Therefore, in the 1180 MPa grade steel plate with high bake hardening value described in the present application, the mass percentage content of S element can be controlled as S≤0.005%, such as S≤0.004%, S≤0.003%, 0.002≤S≤0.003% or 0.002≤S≤0.005%.

[0044] Further, in the 1180 MPa grade steel plate described in the present application, the microstructure further contains 10-35% of ferrite in area ratio, such as 10-31% or 33-35%.

[0045] In the present application, the 10-35% ferrite content can enhance the coordinated deformation ability during the processing of the steel plate and improve the forming performance of the steel.

[0046] Further, in the 1180 MPa grade steel plate described in the present application, the microstructure further contains 10-17% of residual austenite in area ratio.

[0047] In the present application, the 10-17% residual austenite in area ratio plays a TRIP effect during deformation to realize the strengthening and plasticizing of the steel plate and further improve the elongation level.

[0048] Further, in the 1180 MPa grade steel plate described in the present application, the microstructure is 10-35% of ferrite in area ratio + 10-17% of residual austenite in area ratio + the balance of at least one of martensite, tempered martensite and partitioned martensite.

[0049] Further, in the 1180 MPa grade steel plate described in the present application, the performance satisfies at least one of the following:

[0050] The bake hardening value in the strain range of 2-15% is not less than 120 MPa;

[0051] The VDA maximum bending angle of the steel plate after bake hardening is not less than 114°.

[0052] Tensile strength ≥ 1180 MPa, uniform elongation ≥ 12%, and elongation at break ≥ 18%.

[0053] In some embodiments, the 1180 MPa grade steel sheet described in the present application has a bake hardening value in the range of 2-15% strain ≥ 125 MPa, such as 120-300 MPa or 125-291 MPa.

[0054] In some embodiments, the pre-strain amount can be in the range of 5-15%.

[0055] In some embodiments, the 1180 MPa grade steel sheet described in the present application has a VDA maximum bending angle of the steel sheet after bake hardening in the range of 114°-132°.

[0056] In some embodiments, the 1180 MPa grade steel sheet described in the present application has a tensile strength in the range of 1180-1280 MPa or 1195-1273 MPa.

[0057] In some embodiments, the 1180 MPa grade steel sheet described in the present application has a uniform elongation in the range of 12%-20% or 12%-18%.

[0058] In some embodiments, the 1180 MPa grade steel sheet described in the present application has an elongation at break in the range of 18%-26% or 18%-25.3%.

[0059] Another object of the present application is to provide a manufacturing method of a 1180 MPa grade steel sheet having a high bake hardening value, by which a 1180 MPa grade steel sheet having a high bake hardening value with excellent performance can be obtained.

[0060] To achieve the above object, the present application provides a manufacturing method of a 1180 MPa grade steel sheet having a high bake hardening value, which comprises the steps of:

[0061] (1) smelting and continuous casting;

[0062] (2) hot rolling;

[0063] (3) pickling and cold rolling;

[0064] (4) continuous annealing: controlling the annealing temperature to be in the range of 800-920℃, the holding time to be in the range of 30-200s, then slowly cooling at a cooling rate not higher than 10℃ / s to (Ac1±50)℃, then cooling at a rate not lower than 50℃ / s to a temperature range of (Ms-10)~Mf, 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 final temperature of martensitic transformation.

[0065] In the present application, by controlling the annealing temperature at 800-930 °C and holding for 30-200 s, a fully austenitic structure or a structure with controllable proportions of austenite + ferrite two-phase structure can be obtained, which is the primary condition for obtaining carbon supersaturated martensite and residual austenite in the rapid 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 rate of not higher than 10 °C / s to (Ac1±50) °C is used for further adjusting the ferrite content and improving the material's coordinated deformation ability. Cooling at a rate of not lower than 50 °C / s can obtain sufficient martensite and high dislocation density. By adjusting the rapid cooling end temperature between (Ms-10) and Mf, the amount of martensite transformation and the content of residual austenite can be controlled. Preferably, the annealing temperature is 800-850 °C or 855-890 °C. Preferably, the annealing holding time is 30-50 s or 55-185 s. Preferably, the slow cooling rate is 3-10 °C / s, such as 4-10 °C / s. Preferably, the Ac1 temperature is 700-720 °C, such as 700-716 °C. Preferably, the slow cooling temperature is (Ac1±36) °C, such as 680-720 °C. Preferably, the rapid cooling rate is 50-1000 °C / s, such as 100-1000 °C / s. Preferably, the Ms temperature is 315-348 °C, such as 315-322 °C or 338-348 °C. Preferably, the Mf temperature is 197-233 °C, such as 222-233 °C or 197-204 °C. Preferably, the rapid cooling temperature ((Ms-10)~Mf) is 200-310 °C, such as 200-290 °C. Preferably, the reheating temperature is 370-430 °C. Preferably, the reheating holding time is 30-550 s. In the present application, during 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 presence of high-carbon residual austenite and high-dislocation-density martensite, the further increase in dislocation density and the formation of sufficient pinning points during the pre-deformation and baking of the steel plate are ensured, and the bake hardening value is improved.

[0066] Further, in step (2) of the manufacturing method described in the present application, the slab is heated to 1180-1280 °C and held for 60-240 min, and then rolled. Preferably, the slab is heated to 1260-1280 °C. Preferably, the holding time is 120-160 min or 200-240 min.

[0067] In the present application, the slab heating temperature is controlled between 1180-1280℃, which can reduce the slab dendrite coarsening and surface decarburization caused by too high heating temperature, avoid the hot rolling deformation resistance rising caused by too low heating temperature, thereby ensuring the smooth production of hot rolling. The holding time is controlled between 60-240min, which can ensure the internal temperature and structure uniformity of the slab, and avoid the structure coarsening caused by too long time.

[0068] Further, in the step (2) of the manufacturing method, the finish rolling temperature is controlled to be 880-950℃.

[0069] In the present application, by controlling the finish rolling temperature to be 880-950℃, the rolling can be carried out in the austenite single phase region, thereby improving the rollability; at the same time, the too large rolling force caused by too low temperature is avoided, and the production stability is ensured.

[0070] Further, in the step (2) of the manufacturing method, the coiling temperature is controlled to be 400-650℃, and the coiling is carried out in the range of not lower than the coiling temperature and not higher than 100℃ above the coiling temperature for 2-720min. Preferably, the coiling temperature is 400-600℃ or 400-580℃. Preferably, the coiling is carried out in the range of 20-50℃ above the coiling temperature, such as 450-670℃.

[0071] In the present application, by controlling the coiling temperature and the holding temperature and time after coiling, the steel coil with suitable strength and good surface quality can be ensured. When the coiling temperature is too low, the hot coil strength is too high, and the low temperature long time cover annealing treatment can be carried out to reduce the hot rolling strength. When the coiling temperature is high, the Si and Mn elements in the steel are easy to form internal oxidation layer, which is enriched between the iron oxide scale and the steel plate substrate, causing the pickling to be difficult to remove completely. The high temperature short time continuous annealing treatment can be carried out to improve the removal effect of the steel plate surface oxide scale and internal oxidation layer. Based on this, in the manufacturing method, the coiling temperature can be controlled to be 400-650℃, and the coiling is carried out in the range of not lower than the coiling temperature and not higher than 100℃ above the coiling temperature for 2-720min.

[0072] Further, in the step (4) of the manufacturing method, the hot galvanizing process is further carried out after the process of reheating to 350-460℃ and holding for 20-600s, to obtain the corresponding hot galvanizing product.

[0073] Further, in the step (4) of the manufacturing method, the electro-galvanizing process can be further carried out after the process of cooling to room temperature, to obtain the corresponding electro-galvanizing product.

[0074] The 1180MPa 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:

[0075] The 1180MPa grade steel plate with high bake hardening value has a microstructure with a higher content of residual austenite by component and process design, and a tensile strength of ≥1180MPa is obtained by using the TRIP effect of residual austenite, while the uniform elongation is ≥12%, the fracture elongation is ≥17%, and the cold forming performance of the steel plate is excellent.

[0076] The 1180MPa grade steel plate with high bake hardening value has a higher carbon content and a high dislocation density of martensite, tempered martensite and / or partitioned martensite, which can fully utilize the steel plate in part manufacturing and paint baking; at the same time, the 1180MPa grade steel plate can obtain a bake hardening value of 120-300MPa in a larger strain range, further improving the strength of the part and the collision energy absorption effect.

[0077] The content of each type of martensite in the 1180MPa grade steel plate with high bake hardening value is not less than 50%, which can reduce the hardness difference of each phase in the structure after part forming and paint baking, thereby improving the uniformity of the structure and obtaining excellent bending performance, and the VDA maximum bending angle of 1.2mm plate thickness is not less than 114°. The excellent bending performance after baking can improve the ability of the part to resist local deformation. BRIEF DESCRIPTION OF DRAWINGS

[0078] Figure 1 shows the SEM microstructure of the 1180MPa grade steel plate of Example 2 of the present application.

[0079] Figure 2 schematically shows the change of bake hardening value of Example 2 of the present application in the strain range of 2-15%. DETAILED DESCRIPTION

[0080] The 1180MPa grade steel plate with high bake hardening value and the manufacturing method thereof will be further explained and described below in combination with specific examples and the accompanying drawings of the specification, but the explanation and description do not constitute an improper limitation on the technical solutions of the present application.

[0081] Examples 1-25 and Comparative Examples 1-2

[0082] Table 1 lists the mass percentages of each chemical element in the 1180MPa grade steel plate with high bake hardening value of Examples 1-25 and the comparative steel plate of Comparative Examples 1-2.

[0083] Table 1. (wt%, the balance is Fe and other unavoidable impurities except P, S and N)

[0084] The 1180 MPa grade steel sheets having high bake hardening values of Examples 1-25 and the comparative steel sheets of Comparative Examples 1-2 according to the present application were produced by the following steps.

[0085] (1) Smelting and continuous casting;

[0086] (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;

[0087] (3) Pickling and cold rolling; pickling and cold rolling after uncoiling the hot rolled coil;

[0088] (4) Continuous annealing; controlling the annealing temperature to be 800-920°C, holding for 30-200 s, then slowly cooling to (Ac1±50) °C at a cooling rate of not higher than 10°C / s, then cooling to the (Ms-10) ~ Mf temperature range at a rate of not lower than 50°C / s, then heating to 350-460°C, holding for 20-600 s, and finally cooling to room temperature.

[0089] In some embodiments, after the heating to 350-460°C and holding for 20-600 s in step (4), a hot-dip galvanizing process can be further performed to obtain a hot-dip galvanized product.

[0090] In other embodiments, after the cooling to room temperature in step (4), an electro-galvanizing process can be further performed.

[0091] It should be noted that the compositions and processes of Examples 1-25 according to the present application are all in accordance with the requirements of the present application, while the compositions and process parameters of Comparative Examples 1-2 are all not in accordance with the present application.

[0092] Table 2-1, Table 2-2 and Table 2-3 list the specific process parameters of the 1180 MPa grade steel sheets having high bake hardening values of Examples 1-25 and the comparative steel sheets of Comparative Examples 1-2 according to the present application.

[0093] Table 2-1.

[0094] Table 2-2.

[0095] Table 2-3. Note: "CR" in the table means no plating on the surface; "GI" means hot-dip galvanized plating; "EG" means electro-galvanized plating; and "GA" means hot-dip galvannealing plating.

[0096] To verify the implementation effect of the present application, the 1180 MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel plates of Comparative Examples 1-2 were sampled, and microstructure observation and dislocation density measurement were performed by using a scanning electron microscope and X-ray diffraction, and the observation results are listed in Table 3.

[0097] Table 3 lists the microstructure observation results of the comparative steel of the 1180 MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel plates of Comparative Examples 1-2 of the present application.

[0098] Table 3.

[0099] As can be seen from the above Table 3, in the microstructure of the 1180 MPa grade steel plates of Examples 1-25, the area proportion of ferrite is between 10-35%, the area proportion of residual austenite is between 10-17%, the total area proportion of at least one of martensite, tempered martensite and partitioned martensite is not less than 50%, and the dislocation density of martensite, tempered martensite and partitioned martensite is not less than 0.8 x 1014 / m2. 15 2 .

[0100] Figure 1 shows the SEM microstructure of the 1180 MPa grade steel plate of Example 2 of the present application.

[0101] As can be seen from Figure 1, the microstructure of Example 2 of the present application is ferrite + residual austenite + martensite + partitioned martensite, wherein the proportion of ferrite is 28%, the proportion of residual austenite is 14%, the proportion of martensite is 18%, and the proportion of partitioned martensite is 40%.

[0102] The 1180 MPa grade steel plates with high bake hardening value of Examples 1-25 and the comparative steel of Comparative Examples 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:

[0103] Bake hardening value in the strain range of 2-15%: according to the standard GB / T 24174-2022, after pre-stretching the sample by different strain amounts of 2%-20%, the sample is baked at 170°C for 20 min, then the sample is stretched until fracture, and the increase value of the yield strength of the baked sample relative to the yield strength of the original state of the sample is the bake hardening value.

[0104] ​VDA maximum bending angle of the bake-hardening steel plate with a thickness of 1.2 mm: according to the VDA 238-100 standard, a 60*60 mm sample is taken from the steel plate, a punch with a radius of 0.4 mm is used to measure the maximum bending angle of the sample.

[0105] Mechanical properties: according to the GB / T 228-2010 standard, a tensile sample is taken from the steel plate perpendicular to the rolling direction, and tensile test is carried out at room temperature until fracture to obtain the tensile strength, uniform elongation and fracture elongation.

[0106] Table 4 lists the performance test results of the 1180 MPa grade steel plates with high bake-hardening values of the embodiments 1-25 of the present application and the comparative steels of the comparative examples 1-2.

[0107] Table 4.

[0108] As can be seen from the above Table 4, the bake-hardening values of the 1180 MPa grade steel plates 2-15% of the embodiments 1-25 of the present application are all higher than 120 MPa, the VDA maximum bending angles of the bake-hardening steel plates with a thickness of 1.2 mm are all higher than 114°, the tensile strengths are all greater than 1180 MPa, the uniform elongations are all greater than or equal to 12%, and the fracture elongations are all greater than or equal to 18%.

[0109] Figure 2 schematically shows the change of the bake-hardening values of the embodiment 2 of the present application in the range of 2-15% strain.

[0110] As shown in Figure 2, the bake-hardening values of the embodiment 2 of the present application under the conditions of 2%, 5%, 8%, 10% and 15% pre-strain are 122 MPa, 170 MPa, 212 MPa, 245 MPa and 287 MPa respectively, and the bake-hardening values gradually increase with the increase of the pre-strain, but are all in the range of 120-300 MPa.

[0111] It should be noted that the prior art part in the protection scope of the present application is not limited to the embodiments given in the present application file, all prior art not contradictory to the scheme of the present application, including but not limited to prior patent documents, prior published publications, prior public use, etc., can be included in the protection scope of the present application. In addition, the combination manner of the technical features in the present case is not limited to the combination manner or the combination manner recorded in the specific embodiments recorded in the claims of the present case, all the technical features recorded in the present case can be freely combined or combined in any manner, unless contradictory to each other.

[0112] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all belong to the protection scope of the present application.

Claims

1. A 1180 MPa grade steel sheet having 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：0.17～0.27％； Si: 0.5-1.7%; Mn: 2.3-3.0%; Al：0.02～0.90％； 0＜B≤0.003％； The microstructure of the 1180 MPa grade steel plate has at least one of martensite, tempered martensite and partitioned martensite with an area ratio of not less than 50%, and a dislocation density of not less than 0.8×10 15 / m 2 ; Preferably, the total area percentage of martensite, tempered martensite and partitioned martensite is 50-80%. Preferably, the dislocation density in the martensite, the tempered martensite and the partitioned martensite is 0.80 x 10 15 / m 2 -1.00 x 10 15 / m 2 .

2. The 1180 MPa grade steel sheet according to claim 1, characterized by, It contains the following chemical elements in the following mass percentages: C：0.17～0.27％； Si: 0.5-1.7%; Mn: 2.3-3.0%; Al：0.02～0.90％； 0＜B≤0.003％； The balance is Fe and inevitable impurities.

3. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, The mass percentage of each element also satisfies Al+Si≤2.00%, preferably 1.3≤Al+Si≤1.8% or 1.4≤Al+Si≤1.75%.

4. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, It also contains at least one of the following chemical elements: 0 0 0 0 5. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, Among the inevitable impurities: P≤0.01%, S≤0.005%.

6. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, It also contains 10-35% of ferrite in the microstructure, preferably 10-31% or 33-35%.

7. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, It also contains 10-17% of residual austenite in the microstructure.

8. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, The microstructure is 10-35% of ferrite + 10-17% of residual austenite + the balance of at least one of martensite, tempered martensite and partitioned martensite.

9. The 1180 MPa grade steel sheet according to claim 1 or 2, characterized by, Its performance satisfies at least one of the following: The bake hardening value in the strain range of 2-15% is not less than 120 MPa, preferably 120-300 MPa; The VDA maximum bending angle of the steel plate after bake hardening is not less than 114°, preferably 114°-132°; Tensile strength≥1180 MPa, preferably 1180-1280 MPa; Uniform elongation≥12%, preferably 12%-20%; Breaking elongation≥18%, preferably 18%-26%.

10. The method of producing a 1180 MPa grade steel sheet according to any one of claims 1 to 9, characterized by, It includes the following steps: (1) Smelting and continuous casting; (2) Hot rolling; (3) Pickling and cold rolling; (4) Continuous annealing: control the annealing temperature to be 800-920℃, the holding time to be 30-200s, then slowly cool to (Ac1±50)℃ at a cooling rate not higher than 10℃ / s, then cool to the temperature range of (Ms-10)~Mf at a rate not lower than 50℃ / s, then heat to 350-460℃, hold for 20-600s, and finally cool to room temperature.

11. The production method according to claim 10, wherein In step (2), the slab is heated to 1180-1280℃ and held for 60-240min, and then rolled.

12. The production method according to claim 10, wherein In step (2), the finish rolling temperature is controlled to be 880-950℃.

13. The production method according to claim 10, wherein In step (2), the coiling temperature is controlled to be 400-650℃, and after coiling, the temperature is held in the range of not lower than the coiling temperature and not higher than 100℃ above the coiling temperature for 2-720min.

14. The production method according to claim 10, wherein In step (4), after the process of reheating to 350-460℃ and holding for 20-600s, a hot galvanizing process is further carried out; and / or, after the process of cooling to room temperature, a zinc electroplating process is further carried out.

15. The production method according to claim 10, wherein Step (4) has one or more of the following characteristics: The slow cooling rate is 3-10℃ / s; The Ac1 temperature is 700-720℃; The slow cooling temperature is 680-720℃; The rapid cooling rate is 50-1000℃ / s; The Ms temperature is 315-348℃; The Mf temperature is 197-233℃; The rapid cooling temperature ((Ms-10)~Mf) is 200-310℃.

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