1000 MPa grade hot-rolled dual-phase steel sheet with high hole-expanding property and method for producing the same

Abstract

The present invention discloses a 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property, which contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.07 to 0.15%, Si: 0.1 to 0.8%, Mn: 1.5 to 2.2%, Al: 0.02 to 0.1%, Ti: 0.05 to 0.18%, Nb ≤ 0.06%, B ≤ 0.003%, and at least one of 0.2% ≤ Cr ≤ 1.5% and 0.05% ≤ Mo ≤ 0.5%; provided that the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053%. The 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property can be used as automobile chassis and structural parts, and meets the technical requirements of flanging, pressing of complex automobile parts, and automobile weight reduction.

Description

Technical Field

[0001] The present invention relates to duplex steel and a method for manufacturing the same, and particularly to a hot-rolled duplex steel sheet with high hole expansion property and a method for manufacturing the same.

Background Art

[0002] In recent years, with the rapid development of the automobile industry, the market and user requirements for automobile lightweighting have become increasingly high. Lightweighting has become a trend in the development of the automobile industry, and the proportion of high-strength steel sheets in automobile structural parts has also been increasing.

[0003] To improve strength, many current vehicle models use 80 kg-class steel sheets to produce automobile chassis parts. However, the strength of generally available duplex steel usually does not reach the level of 1000 MPa, and in the disclosed patented technical solutions, the tensile strength of duplex steel is mostly at the level of 800 MPa.

[0004] Therefore, in order to meet the requirements of lightweighting, the inventors of the present invention hope to further improve the strength of duplex steel and obtain a new 1000 MPa high hole expansion property duplex steel with higher strength and ultra-high hole expansion rate in line with the inevitable trend of the future development of duplex steel.

[0005] According to research, there are several 1000 MPa-class high hole expansion property duplex steels in the prior art. For example, in the Chinese patent document with the publication number CN106119702A, the publication date of November 16, 2016, and the title of "980 MPa-class hot-rolled high-strength high hole expansion property steel and a method for manufacturing the same", a 980 MPa-class hot-rolled high-strength high hole expansion property steel and a method for manufacturing the same are disclosed. The design of its chemical composition features low-carbon V-Ti microalloying design and contains V element. This technical solution has a relatively high cost and does not consider the use of B element.

[0006] Also, for example, in a Chinese patent document with a publication number of CN114107797A, a publication date of March 1, 2022, and a title of "980MPa Grade Bainite Precipitation Strengthened High Hole Expansion Steel and Its Manufacturing Method", a 980Mpa grade bainite precipitation strengthened high hole expansion steel and its manufacturing method are disclosed. The design of its chemical composition features low-carbon V-Ti microalloying design, and the inclusion of V makes the cost relatively high. At the same time, its microstructure is bainite-ferrite.

[0007] Furthermore, for example, in a Chinese patent document with a publication number of CN113122769A, a publication date of July 16, 2021, and a title of "Low-Silicon Low-Carbon Equivalent GPa Grade Duplex Steel Sheet / Strip and Its Manufacturing Method", a low-silicon low-carbon equivalent GPa grade duplex steel sheet / strip and its manufacturing method are disclosed. The design of its chemical composition has a low carbon content, its microstructure contains ferrite, and the hot rolling coiling temperature is high.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Generally speaking, it can be seen that the elongation, hole expansion rate, and strength of materials generally show an inverse correlation with each other. Therefore, in order to obtain a 1000MPa grade high hole expansion hot-rolled duplex steel sheet with high strength, high hole expansion rate, and high elongation, it is necessary to properly match alloy elements, the law of phase transformation, and the microstructure in the design.

Means for Solving the Problems

[0009] One object of the present invention is to provide a 1000MPa grade high hole expansion hot-rolled duplex steel sheet. The 1000MPa grade high hole expansion hot-rolled duplex steel sheet adopts a reasonable chemical composition design, can obtain good comprehensive mechanical properties, has high strength and high elongation, and at the same time has the characteristic of high hole expansion rate. It can be used as automotive body structure parts and automotive chassis parts, and can also be used in other application fields where high strength and lightweight are required, and has good application prospects.

[0010] In order to achieve the above object, the present invention contains Fe and inevitable impurity elements, and further contains the following chemical elements in the following mass percentage contents: C: 0.07 to 0.15%, Si: 0.1 to 0.8%, Mn: 1.5 to 2.2%, Al: 0.02 to 0.1%, Ti: 0.05 to 0.18%, Nb ≤ 0.06%, B ≤ 0.003%, and at least one of 0.2% ≤ Cr ≤ 1.5% and 0.05% ≤ Mo ≤ 0.5%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053%, and provide a hot-rolled duplex steel sheet with high hole expansion property of 1000 MPa grade.

[0011] Furthermore, in the hot-rolled duplex steel sheet with high hole expansion property of 1000 MPa grade according to the present invention, the mass percentage contents of its respective chemical elements are: C: 0.07 to 0.15%, Si: 0.1 to 0.8%, Mn: 1.5 to 2.2%, Al: 0.02 to 0.1%, Ti: 0.05 to 0.18%, Nb ≤ 0.06%, B ≤ 0.003%, and at least one of 0.2% ≤ Cr ≤ 1.5% and 0.05% ≤ Mo ≤ 0.5%; the balance is Fe and inevitable impurity elements; preferably, 0.015% ≤ Nb ≤ 0.06%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053%.

[0012] In the hot-rolled duplex steel sheet with high hole expansion property of 1000 MPa grade according to the present invention, the design principle of each chemical element is as follows.

[0013] C: In the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, the C content greatly determines the tensile strength level of the steel sheet. C contributes to solid solution strengthening and forms sufficient precipitation strengthening phases in combination with Nb, Ti, etc., ensuring the strength of the steel. However, if the mass percentage content of C is high, carbide particles coarsen, and excessive martensite and retained austenite are likely to form, which is disadvantageous to the hole expansion performance. Therefore, it is necessary to note that the C content in the steel should not be too high. Therefore, on the premise of ensuring the strength of the steel grade, in order to achieve both high hole expansion rate and good formability and weldability, in the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, the mass percentage content of the C element is controlled to be 0.07 - 0.15%.

[0014] Si: In the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, the Si element plays a role in solid solution strengthening to improve the strength of the steel sheet. By adding Si, the work hardening rate and the uniform elongation rate and total elongation rate at a predetermined strength can be improved, contributing to the improvement of the elongation rate of the steel sheet. Furthermore, Si can also prevent the precipitation of carbides and reduce the appearance of the pearlite phase. However, it should be noted that when the steel contains silicon, surface defects of fayalite (2FeO - SiO2) oxide scale are likely to form on the surface of the steel sheet, which has an adverse effect on the surface quality. Therefore, in the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, the mass percentage content of the Si element is controlled to be 0.1 - 0.8%.

[0015] Mn: In the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, the Mn element is a solid solution strengthening element. If the mass percentage content of the Mn element in the steel is low, it will lead to insufficient strength of the steel material. However, if the mass percentage content of the Mn element is too high, it will lead to a decrease in the plasticity of the steel sheet. Furthermore, Mn delays the pearlite transformation, improves the hardenability of the steel, lowers the bainite transformation temperature, refines the substructure of the steel's microstructure, and ensures the acquisition of the lath substructure, bringing good formability on the premise of ensuring high tensile strength of the steel material. Therefore, considering the influence of the Mn element content on the steel material performance, in the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, the mass percentage content of the Mn element is controlled to be 1.5 - 2.2%.

[0016] Al: In the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, Al is a deoxidizing element in the steel. It can reduce the oxide inclusions in the steel, purify the steel quality, and contribute to the improvement of the formability of the steel sheet. However, it should be noted that if the mass percentage content of the Al element in the steel is too high, oxidation will occur, further affecting continuous casting production. Therefore, considering the influence of the Al element content on the steel sheet performance, in the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, the mass percentage content of the Al element is controlled to be 0.02 - 0.1%.

[0017] Ti: In the 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to the present invention, Ti is one of the important grain refinement strengthening and precipitation strengthening elements. The Ti element can increase the recrystallization temperature of the steel material and refine the grain size of the crystal grains during the hot rolling process. At the same time, the combination of Ti element and C element has an excellent strengthening effect. However, if the content of Ti element in the steel is too high, it is easy to form TiN with a large size, which is disadvantageous to the impact toughness of the steel. Therefore, it is necessary to pay attention that the content of Ti element in the steel should not be too high. Therefore, in order to exert the beneficial effects of the Ti element, in the 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to the present invention, the mass percentage content of the Ti element is controlled to be 0.05 - 0.18%.

[0018] Nb: In the 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to the present invention, Nb is one of the important precipitation strengthening and grain refinement strengthening elements. However, when the mass percentage of Nb exceeds 0.06%, the strengthening effect of Nb reaches saturation and the cost increases. Therefore, in order to exert the beneficial effects of the Nb element and suppress the production cost, in the present invention, the mass percentage content of the Nb element is controlled to be Nb ≤ 0.06%.

[0019] B: In the 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to the present invention, B contributes to the expansion of the bainite phase region and can ensure the acquisition of bainite structure in the steel sheet during cooling after rolling, which is extremely effective in improving the strength and hardness of the steel material. However, excessive B element leads to the appearance of excessive massive martensite structure in the steel sheet and the decrease of the hole expansion rate and elongation rate of the steel material. Therefore, it is necessary to pay attention that the content of B element in the steel should not be too high. Therefore, in the present invention, the mass percentage content of the B element is controlled to be B ≤ 0.003%.

[0020] Correspondingly, in the 1000 MPa grade hot-rolled multiphase steel sheet designed according to the present invention, in addition to the above elements, Cr element and / or Mo element may be added to the steel. However, the Cr and Mo elements may be used alone or in combination.

[0021] Cr: In the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, Cr is an element that suppresses the formation of pearlite and contributes to the formation of bainite structure, and can improve the strength and hole expansion rate of the steel material. According to the research of the present inventors, when the mass percentage content of Cr element in the steel is less than 0.15%, the influence on the transformation curve becomes insignificant. However, when the mass percentage content of Cr element in the steel is too high, it not only causes an increase in the cost of the alloy, but also easily generates a large amount of martensite structure. Therefore, in the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, when added, the mass percentage content of Cr element is controlled to be 0.2 - 1.5%.

[0022] Mo: In the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, the Mo element can not only suppress the formation of pearlite, but also contribute to the formation of bainite structure and a small amount of martensite-austenite islands. Moreover, since the Mo element can promote the bainite microstructure transformation at high temperature, it enables the coiling of the steel material at high temperature, and such a high coiling temperature provides sufficient precipitation kinetics to cause significant precipitation strengthening. In the present invention, the Mo element also plays a very important role in the composite precipitation process with Nb and Ti, and can reduce the possibility of the particle size of the precipitation particles becoming coarsened. However, if the content of Mo element in the steel is too high, it not only causes an increase in the cost of the alloy, but also easily generates a large amount of martensite and austenite, which is disadvantageous to the performance of the steel material. Therefore, it is necessary to pay attention that the content of Mo element in the steel should not be too high. Therefore, in order to exert the beneficial effect of the Mo element, in the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention, when added, the mass percentage content of Mo element is controlled to be 0.05 - 0.5%.

[0023] Furthermore, in this technical solution designed according to the present invention, the inventors need to note that while controlling the mass percentage content of a single chemical element in the matrix plate, the mass percentage contents of N, Ti, and Nb in the steel plate are further controlled to satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053%. Here, N is an impurity element in the steel plate.

[0024] In the present invention, the inventors adopt a design of high Ti and high Nb, which mainly exhibits the following three grain refinement effects and one precipitation strengthening effect: (1) During the slab heating process, the precipitates of Nb and Ti can prevent the growth of prior austenite grains; (2) During the hot rolling process, (Nb,Ti)(C,N) contributes to the increase in the recrystallization temperature and further refines austenite grains; (3) The already precipitated (Nb,Ti)(C,N) or (Nb,Ti)(Mo,Cr)(C,N) contributes to the refinement of transformed bainite and a small amount of martensite grains; (4) During the laminar flow cooling process, the nano-scale precipitates of (Nb,Ti)(C,N) or (Nb,Ti)(Mo,Cr)(C,N) can achieve a strong precipitation strengthening effect.

[0025] Therefore, in the present invention, in addition to matching the design of the C element with the Ti and Nb contents to ensure sufficient precipitation of Ti and Nb compounds, it is also necessary to control the mass percentage contents of N, Ti, and Nb by a reasonable combination of Nb and Ti to satisfy the relationship of "0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053%", and optimize the manufacturing process accordingly to obtain a 1000 MPa grade high hole expansion hot rolled duplex steel plate with bainite grain size ≤ 6 μm.

[0026] Furthermore, in the 1000 MPa grade high hole expansion hot rolled duplex steel plate according to the present invention, among the inevitable impurity elements, P ≤ 0.02%, S ≤ 0.005%, and N ≤ 0.005%.

[0027] In the above technical solution, P, S, and N are all impurity elements in the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention. Technically, as long as it is permitted, in order to obtain steel materials with better performance and superior quality, the content of impurity elements in the steel sheet should be reduced as much as possible.

[0028] Therefore, in the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, the content of P element is controlled to P≤0.02%, the content of S element is controlled to S≤0.005%, and the content of N element is controlled to N≤0.005%.

[0029] Furthermore, in the 1000 MPa grade hot-rolled multi-phase steel sheet with high hole expansion property according to the present invention, the contents of Cr and Mo in the steel satisfy any one of the following: (1) When 0.2%≤Cr≤0.7%, the mass percentage content of Mo satisfies 0.2%≤Mo≤0.35%; (2) When 0.7%<Cr≤1.0%, the mass percentage content of Mo satisfies 0.05%≤Mo<0.2%; (3) When 1.0%<Cr≤1.5%, Mo is not contained; (4) When 0.35%<Mo≤0.5%, Cr is not contained.

[0030] In the above technical solution of the present invention, in terms of the design of chemical composition, the Cr and Mo elements may be used alone or in combination. The addition of an appropriate amount of Cr element or Mo element is to ensure that in the hot-rolling coiling process, by obtaining a bainite structure and a small amount of martensite-austenite islands, it does not contain pearlite or massive martensite that affects the hole expansion performance.

[0031] Therefore, the inventors designed a manufacturing process to further match it, obtaining a 1000 MPa grade hot-rolled duplex steel sheet with excellent overall mechanical properties. To meet the requirements of users and the market, the contents of Cr and Mo in the steel were further optimized, and during practical application, it is preferable to add according to any one of the composition ratios of Cr and Mo designed in the above (1) to (4).

[0032] Furthermore, in the 1000 MPa grade hot-rolled duplex steel sheet according to the present invention, the matrix of its microstructure is mainly bainite, and the matrix has nano-scale precipitates.

[0033] In the present invention, the matrix of the microstructure of such a 1000 MPa grade hot-rolled duplex steel sheet designed by the present invention has nano-scale precipitates. These micro-alloy nano-scale precipitates include TiC and (Ti,Nb)C, and their specific precipitation sizes are controlled in the range of 3 to 20 nm.

[0034] Furthermore, in the 1000 MPa grade hot-rolled duplex steel sheet according to the present invention, its bainite volume fraction is ≧85%.

[0035] Furthermore, in the 1000 MPa grade hot-rolled duplex steel sheet according to the present invention, the matrix of its microstructure further contains martensite and / or retained austenite. Furthermore, in the 1000 MPa grade hot-rolled duplex steel sheet according to the present invention, the total volume fraction of the above-mentioned martensite and retained austenite is ≦15%.

[0036] Furthermore, in the 1000 MPa grade hot-rolled duplex steel sheet according to the present invention, the size of the above-mentioned nano-scale precipitates is 3 to 20 nm.

[0037] Furthermore, in the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention, the size of the bainite crystal grains is ≦6 μm, and the size of the martensite and / or retained austenite crystal grains is ≦3 μm.

[0038] In some embodiments, the microstructure of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention includes martensite with a volume fraction of 85 to 96% and a total volume fraction of martensite and retained austenite of 4 to 15%. However, the size of the bainite crystal grains is 3.2 to 5 μm, and the size of the martensite crystal grains is 0.5 to 3 μm.

[0039] Furthermore, in the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention, its performance is as follows: yield strength ≧750 MPa, tensile strength 950 to 1150 MPa, elongation at 50 mm A50 ≧12%, punching hole expansion rate ≧45%, and reaming hole expansion rate ≧65%.

[0040] In some embodiments, the yield strength of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is ≧780 MPa. In some embodiments, the yield strength of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is ≧800 MPa. In some embodiments, the yield strength of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is 750 to 960 MPa.

[0041] In some embodiments, the tensile strength of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is 980 to 1150 MPa.

[0042] In some embodiments, the punching hole expansion rate of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is ≧50%. In some embodiments, the punching hole expansion rate of the 1000 MPa-class hot-rolled dual-phase steel sheet with high hole expansion property according to the present invention is ≧55%.

[0043] In some embodiments, the reaming hole expansion rate of the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention is ≧70%. In some embodiments, the reaming hole expansion rate of the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention is ≧75%.

[0044] Correspondingly, another object of the present invention is to provide a manufacturing method of the 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property according to the present invention. The 1000 MPa grade hot-rolled duplex steel sheet obtained by adopting this manufacturing method has high strength and high elongation rate, and at the same time has the characteristic of high hole expansion rate, and has good application prospects.

[0045] In order to achieve the above object, the manufacturing method of the above 1000 MPa grade hot-rolled duplex steel sheet with high hole expansion property provided by the present invention includes the following steps: (1) Smelting and casting; (2) Hot rolling: Heat the slab to 1200 - 1300 °C and keep it warm; then perform rolling, provided that the rough rolling outlet temperature is 1000 - 1080 °C and the finish rolling end temperature is 840 - 950 °C; (3) Perform two-stage laminar flow cooling to water-cool the steel sheet to the coiling temperature: provided that the average cooling rate of the first stage is ≧100 °C / s, the average cooling rate of the second stage is ≧3 °C / s, the intermediate point temperature between the first stage cooling and the second stage cooling is the bainite transformation temperature Bs ± 30 °C, control the threading speed to 7 - 12 m / s, and control the cooling time from the intermediate point temperature to the coiling temperature to be ≧4.5 s, preferably ≧6 s, more preferably ≧6.5 s. Here, Bs = 844 - 597×C + 127×C 2 - 92×Mn + 8×Mn 2 - 32×Cr + 2.2×Cr 2 - 42×Mo, in which for each chemical element, the numerical value before the percentage symbol of the mass percentage content of the corresponding chemical element is substituted; (4) Coiling: Control the coiling temperature to 430 - 600 °C, and after coiling, cool it to room temperature at a cooling rate of ≦0.1 °C / s; (5) Pickling.

[0046] In the above technical solution of the present invention, in step (2), in the case of Ti-containing steel, the heating temperature of the slab is particularly important for performance. In the continuous casting process, Ti precipitates in large quantities as (Ti, Nb)(C, N) precipitates with large sizes. However, the main purpose of setting the heating temperature to ≥1200°C is to dissolve alloying elements such as Ti as much as possible during the heating process of the slab to ensure nano-scale precipitation of microalloys such as Ti in the subsequent hot rolling coiling process. However, when the heating temperature exceeds 1300°C, the crystal grains tend to coarsen, which is disadvantageous to the toughness of the steel plate. Therefore, in the hot rolling process of the present invention, preferably, the heating temperature is controlled at 1200 - 1300°C.

[0047] Furthermore, the control of the rough rolling temperature in the hot rolling process has a great influence on microalloys such as Ti. At a low rough rolling temperature and in the finish rolling process, Ti precipitates as carbides and carbonitrides of Ti. The precipitates in this process are large in size and are disadvantageous to the improvement of the final strength. However, the precipitated (Nb, Ti)(C, N) contributes to the refinement of austenite crystal grains. Therefore, in the hot rolling process of the present invention, the rough rolling exit temperature is controlled at 1000 - 1080°C. In some embodiments, the rough rolling exit temperature is controlled at 1050 - 1080°C. In some embodiments, the finish rolling end temperature is controlled at 880 - 950°C.

[0048] Furthermore, although the addition of Cr and / or Mo elements in the steel effectively suppressed the formation of ferrite and pearlite, massive secondary martensite and retained austenite were still likely to form, which had a significant impact on the volume fraction of bainite transformation during the hot rolling laminar cooling process in step (3). Therefore, in the present invention, in order to obtain suitable bainite transformation and small-sized martensite-austenite islands, it is necessary to control the laminar cooling time, cooling rate, and threading speed in step (3). Specifically, the cooling time from the midpoint temperature to the coiling temperature is controlled to be ≥4.5 s, preferably ≥6.5 s; the average cooling rate in the first stage before the midpoint temperature is ≥100 °C / s, the average cooling rate after the midpoint temperature is ≥3 °C / s, and the threading speed is controlled to be 7 - 12 m / s.

[0049] In the present invention, it is also necessary to control bainite transformation and the precipitation of microalloys. However, if the coiling temperature is too high, the ferrite content will increase, the sizes of secondary martensite and retained austenite will become larger, which is disadvantageous for improving the hole expansion rate. But it should be noted that if the coiling temperature is too low, primary martensite structure may appear, which may lead to a decrease in the elongation rate of the steel. Therefore, in the present invention, by controlling the coiling temperature to 430 - 600 °C, the problem of matching the elongation rate and the hole expansion rate can be solved. Of course, in order to obtain better implementation effects, the coiling temperature can be further controlled to 430 - 580 °C.

[0050] Correspondingly, after coiling, cooling to room temperature at a cooling rate of ≤0.1 °C / s can not only promote further bainite transformation, but also contribute to the tempering of martensite and the further precipitation of microalloys, effectively improving the strength, hole expansion rate, and elongation rate of the steel.

[0051] Furthermore, in the present invention, the pickling process is not particularly limited. However, in some embodiments, when actually performing pickling, specifically, the elongation rate of pickling tension leveling is controlled to be 0.2 - 2%, the pickling speed is controlled to be 60 - 150 m / min, the temperature of the final pickling tank in the pickling process is controlled to be 80 - 90 °C, the iron ion concentration is controlled to be 30 - 40 g / L, and it should be noted that a finished steel plate can be obtained after pickling.

[0052] Furthermore, in the manufacturing method according to the present invention, in step (2), the total rolling reduction rate is controlled to be ≥ 80%, and the total finishing rolling reduction rate is controlled to be ≥ 50%. Preferably, the total rolling reduction rate is 90% - 95%, and the total finishing rolling reduction rate is 85% - 90%. More preferably, the obtained finished steel plate has a thickness of 5 mm or less.

[0053] Furthermore, in the manufacturing method according to the present invention, in step (2), the heat preservation time is 1 - 3 hours.

[0054] Furthermore, in the manufacturing method according to the present invention, in step (3), the cooling time from the intermediate point temperature to the coiling temperature is controlled to be ≥ 8 s.

[0055] Furthermore, in the manufacturing method according to the present invention, in step (3), the average cooling rate in the first stage is 100 - 160 °C / s, preferably 120 - 160 °C / s.

[0056] Furthermore, in the manufacturing method according to the present invention, in step (3), the average cooling rate in the second stage is 3 - 25 °C / s, preferably 3 - 22 °C / s.

[0057] Furthermore, in the manufacturing method according to the present invention, in step (4), the coiling temperature is controlled to be 430 - 580 °C. In some embodiments, in step (4), the coiling temperature is controlled to be 430 - 550 °C.

[0058] The 1000 MPa grade high hole-expanding hot-rolled duplex steel plate and its manufacturing method according to the present invention have the following advantages and beneficial effects compared with the prior art: In the present invention, an economical and reasonable chemical composition design is adopted, and in combination with an existing hot continuous rolling production line, it is possible to produce a novel hot-rolled dual-phase steel sheet with ultra-high strength and a hole expansion ratio of 1000 MPa grade and high hole expansion property.

[0059] The 1000 MPa grade hot-rolled dual-phase steel sheet with high hole expansion property manufactured by the present invention has characteristics such as a high hole expansion ratio, high strength, and high formability. It has a yield strength ≧ 750 MPa, a tensile strength of 950 - 1150 MPa, an elongation rate A50 ≧ 12%, a punching hole expansion rate ≧ 45%, and a reaming hole expansion rate ≧ 65%. Therefore, it can be used as automotive body structure parts and automotive chassis parts, and can also be used in other application fields where high strength and weight reduction are required, and has good application prospects.

Brief Description of the Drawings

[0060]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0061] Hereinafter, based on specific examples and drawings, the 1000 MPa grade hot-rolled dual-phase steel sheet with high hole expansion property and its manufacturing method according to the present invention will be further interpreted and described. However, such interpretation and description do not unduly limit the technical solution of the present invention.

Examples

[0062] Examples 1 - 13 and Comparative Examples 1 - 11 The mass percentage ratios of each chemical element of the 1000 MPa grade hot-rolled dual-phase steel sheet with high hole expansion property according to Examples 1 - 13 and the comparative steel sheets according to Comparative Examples 1 - 11 are shown in Table 1-1.

[0063]

Table 1-1

[0064] The matching of each chemical element in the steel plates according to Examples 1 to 13 and Comparative Examples 1 to 11 is shown in Table 1-2.

[0065]

Table 1-2

[0066] Remarks: In Table 1-2 above, Bs = 844 - 597×C + 127×C 2 - 92×Mn + 8×Mn 2 - 32×Cr + 2.2×Cr 2 - 42×Mo. For each chemical element in the calculation formula, the numerical value before the percentage symbol of the mass percentage content is substituted; TiCeq = (Ti - 3.43N + 0.52Nb) / 4. For N, Ti, and Nb in the formula, the corresponding mass percentage contents of each chemical element are substituted.

[0067] The 1000 MPa grade high hole expansion hot-rolled duplex steel plates according to Examples 1 to 13 of the present invention and the comparative steel plates according to Comparative Examples 1 to 11 were all prepared by the following steps: (1) Smelting and casting were carried out according to the chemical compositions shown in Table 1-1 and Table 1-2.

[0068] (2) Hot rolling: The slab obtained through smelting and continuous casting was heated to 1200 - 1300 °C and held for 1 - 3 h; then rolling was carried out, provided that the rough rolling exit temperature was 1000 - 1080 °C, the finish rolling end temperature was 840 - 950 °C, the total rolling reduction rate was ≥80%, and the finish rolling total reduction rate was ≥50%.

[0069] (3) After finish rolling, two-stage laminar cooling was carried out to cool the steel plate to the coiling temperature with water: However, the average cooling rate in the first stage is ≥100 °C / s, the average cooling rate in the second stage is ≥3 °C / s, the intermediate point temperature between the first stage cooling and the second stage cooling is the bainite transformation temperature Bs±30 °C, the threading speed is controlled at 7-12 m / s, and the cooling time from the intermediate point temperature to the coiling temperature is controlled to be ≥4.5 s. Preferably, the cooling time from the intermediate point temperature to the coiling temperature may be controlled to be ≥6 s.

[0070] (4) Coiling: The water-cooled steel plate was coiled, and the coiling temperature was controlled at 430-600 °C. Preferably, it may also be controlled at 430-580 °C. After coiling, it was cooled to room temperature at a cooling rate of ≤0.1 °C / s.

[0071] (5) Pickling: The pickling tension leveling elongation rate was controlled at 0.2-2%, the pickling speed was controlled at 60-150 m / min, the temperature of the final pickling tank in the pickling process was controlled at 80-90 °C, the iron ion concentration was controlled at 30-40 g / L, and a steel plate with a thickness of ≤5 mm was obtained after pickling.

[0072] In the present invention, the chemical composition design and related processes of the 1000 MPa grade high hole expansion hot rolled dual phase steel plates according to Examples 1-13 all met the requirements of the standards designed by the present invention. On the other hand, for the comparative steel plates according to Comparative Examples 1-11, although the adopted processes were carried out according to the above processes (1)-(5), there were parameters in the chemical composition design and / or related processes of the comparative steel plates according to Comparative Examples 1-11 that did not meet the design requirements of the present invention.

[0073] The specific process parameters of the 1000 MPa grade high hole expansion hot rolled dual phase steel plates according to Examples 1-13 and the comparative steel plates according to Comparative Examples 1-11 are shown in Table 2-1 and Table 2-2.

[0074]

Table 2-1

[0075]

Table 2-2

[0076] In the present invention, the inventors respectively sampled from the 1000 MPa grade hot-rolled multiphase steel sheets with high hole expansion property according to Examples 1 to 13 of the finished products obtained through the above-mentioned processes and steps and the comparative steel sheets according to Comparative Examples 1 to 11, observed and analyzed the microstructures of the steel sheets according to each example and comparative example, and the results obtained from the observation and analysis are shown in Table 3 below.

[0077] The results of the observation and analysis of the microstructures of the 1000 MPa grade hot-rolled multiphase steel sheets with high hole expansion property according to Examples 1 to 13 and the comparative steel sheets according to Comparative Examples 1 to 11 are shown in Table 3.

[0078]

Table 3

[0079] As can be seen from the observation, in the present invention, for the 1000 MPa grade hot-rolled multiphase steel sheets with high hole expansion property according to Examples 1 to 13 prepared, the microstructure matrixes are all bainite + a small amount of martensite and retained austenite, and the volume fraction of bainite is 85 - 96%, the volume fraction of martensite and / or retained austenite is 4 - 15%, the size of bainite crystal grains is 3.2 - 5 μm, and the size of martensite crystal grains is 0.5 - 3 μm.

[0080] It should be noted that when actually prepared, in the matrix of the 1000 MPa grade hot-rolled multiphase steel sheets with high hole expansion property according to Examples 1 to 13, there are also nano-scale precipitates containing TiC and (Ti,Nb)C, and the diameters of these nano-scale precipitates are 3 - 20 nm.

[0081] Correspondingly, after the analysis of the microstructure was completed, samples were respectively taken from the finished products of Examples 1 to 13 of the 1000 MPa grade high hole-expanding hot-rolled dual-phase steel sheets and the comparative steel sheets of Comparative Examples 1 to 11 obtained through the above-mentioned processes and steps, and the mechanical properties of the steel sheets according to each example and comparative example were measured. The measurement results of the related mechanical properties are shown in Table 4 below.

[0082] The measuring means for the related properties were as follows: (1) Measurement of tensile properties: JIS 5# tensile test pieces were taken along the longitudinal direction, and a tensile test was carried out using the standard of GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" to obtain the yield strength, tensile strength and elongation of the steel sheets according to each example and comparative example.

[0083] (2) Hole-expanding test: The hole-expanding rate was measured by a hole-expanding test in which a sample with a central hole was pushed into a die using a punch die until the hole edge of the plate was constricted or a through crack occurred to expand the central hole of the sample. Since the manufacturing method of the original hole in the center of the sample has a great influence on the measurement results of the hole-expanding rate, the original holes in the centers of the samples were respectively manufactured by punching and reaming, and the subsequent test and measurement methods were carried out in accordance with the measurement method of the hole-expanding rate specified in the ISO / DIS 16630 standard. The measurement results are shown in Table 4.

[0084] The measurement results of the mechanical properties of the 1000 MPa grade high hole-expanding hot-rolled dual-phase steel sheets according to Examples 1 to 13 and the comparative steel sheets according to Comparative Examples 1 to 11 are shown in Table 4.

[0085]

Table 4

[0086] As shown in Table 4, compared with the comparative steel sheets according to Comparative Examples 1 to 11, the 1000 MPa grade high hole-expanding hot-rolled dual-phase steel sheets according to Examples 1 to 13 of the present application had better comprehensive mechanical properties.

[0087] In the present invention, Examples 1 to 13 adopted reasonable Cr and Mo ratios shown in Table 1-1 and Table 1-2, added Ti and Nb to improve the precipitation strengthening effect during the tempering process, and also satisfied the hot rolling processes shown in Table 2-1 and Table 2-2. Therefore, the 1000 MPa grade high hole expansion hot rolled duplex steel sheets according to Examples 1 to 13 of the finally obtained finished products obtained the microstructures shown in Table 3.

[0088] As can be seen from reading Table 4, in this embodiment, the 1000 MPa grade high hole expansion hot rolled duplex steel sheets according to the designed Examples 1 to 13 had a yield strength of 755 to 953 MPa, a tensile strength of 982 to 1150 MPa, an elongation rate A50 of 12 to 18.5%, a punching hole expansion rate of 45 to 65%, and a reaming hole expansion rate of 65 to 81%.

[0089] Compared with Examples 1 to 5, Comparative Examples 1 to 2 also adopted the same type A steel grades but adopted different hot rolling coiling temperatures. However, Comparative Example 1 adopted a low coiling temperature of 380°C, but the martensite content in the final microstructure reached 80%, and finally, the tensile strength increased and the elongation rate decreased. On the other hand, Comparative Example 2 adopted a high coiling temperature of 620°C, but the ferrite content in its microstructure decreased, the bainite content was not low, and the tensile strength of the steel material became insufficient.

[0090] Compared with Examples 6 to 8, Comparative Examples 3 to 6 also adopted the same type B steel grades, but their processes did not meet the design requirements. However, in Comparative Example 3, due to the low intermediate point temperature, the cooling time from the intermediate point temperature to the coiling temperature was shortened during the laminar flow cooling process, the bainite transformation in the laminar flow cooling process became insufficient, the ratio of the transformation of supercooled austenite during the coiling process increased, the martensite content in the final microstructure increased, and the elongation rate and hole expansion rate of the obtained comparative steel materials were relatively low.

[0091] On the other hand, in Comparative Example 4, since the total finishing rolling reduction rate was as low as 20%, recrystallization was insufficient, the crystal grains became relatively coarse, and the elongation rate and hole expansion rate became relatively low. In Comparative Example 5, in the hot rolling process of step (2), the rough rolling exit temperature and the finishing rolling end temperature were low, and relatively coarse micro alloy particles had already precipitated during the rolling process, and their contribution to strength was not so great. Finally, the tensile strength of the steel material became insufficient. On the other hand, in Comparative Example 6, since the heating temperature adopted in the hot rolling process of step (2) was as low as 1120°C, the contents of Nb and Ti became insufficient. During the solution and continuous casting process, coarse (Ti,Nb)(C,N) particles did not completely dissolve, and their contribution to strength became small, and the tensile strength of the steel material also became insufficient.

[0092] Compared with Example 9, Comparative Example 7 also adopted the same type C steel grade. However, in Comparative Example 7, since the cooling rate after hot rolling coiling was too fast at 0.2°C / s, the proportion of supercooled austenite that underwent martensite transformation after coiling increased, and the proportion of bainite transformation decreased. Finally, the elongation rate and hole expansion rate of the steel material became low.

[0093] Different from the above Comparative Examples 1 to 7, in Comparative Examples 8 to 11, since the chemical composition did not meet the requirements of the present invention, finally, the performance of the steel material was inferior.

[0094] In Comparative Example 8, since the ratios of Cr and Mo in the steel were not reasonable, the proportion of bainite transformation decreased, and the proportions of martensite and retained austenite transformations increased. Since their contribution to strength was too great, conversely, the elongation rate and hole expansion rate decreased.

[0095] In Comparative Example 9, since the Ti content in the steel was low, the precipitation strengthening effect thereof became weak, the contribution to the strength of the steel material became small, and the strength of the finally obtained steel plate became insufficient.

[0096] In Comparative Example 10, since the N content in the steel was high, a large amount of Ti was consumed, and a large amount of massive TiN precipitated. The contribution of 5 - 20um TiN to strength became small, and the strength of the steel plate decreased.

[0097] In Comparative Example 11, since the C and Mn contents in the steel were low, the effects of solid solution strengthening and bainite transformation strengthening by them became weak, and finally, the strength of the steel sheet became low.

[0098] FIG. 1 is a microstructural photograph of a 1000 MPa class high hole-expanding hot-rolled dual-phase steel sheet according to Example 3.

[0099] As shown in FIG. 1, in the present embodiment, the microstructure of the 1000 MPa class high hole-expanding hot-rolled dual-phase steel sheet according to Example 3 is 92% bainite + 8% martensite and retained austenite. However, the size of the bainite crystal grains was 4.3 μm, and the sizes of the martensite and retained austenite crystal grains were 0.5 to 2 μm.

[0100] FIG. 2 is a microstructural photograph of a 1000 MPa class high hole-expanding hot-rolled dual-phase steel sheet according to Example 5.

[0101] As shown in FIG. 2, in the present embodiment, the microstructure of the 1000 MPa class high hole-expanding hot-rolled dual-phase steel sheet according to Example 5 is 85% bainite + 15% martensite and retained austenite. However, the size of the bainite crystal grains was 5.0 μm, and the sizes of the martensite and retained austenite crystal grains were 0.5 to 3 μm.

[0102] FIG. 3 is a microstructural photograph of a comparative steel material according to Comparative Example 6. As shown in FIG. 3, in the present embodiment, the microstructure of the comparative steel material according to Comparative Example 6 is 20% bainite + 80% martensite. However, the size of the bainite crystal grains was 3.0 μm, and the size of the martensite crystal grains was >3 μm.

[0103] In addition, the combination of each technical feature in the present application is not limited to the combination described in the claims of the present application or the combination described in the specific embodiments. As long as they do not conflict with each other, all the technical features described in the present application can be freely combined or joined in any form.

[0104] Furthermore, it should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. The present invention is not limited to the above embodiments, and it is obvious that any similar changes or modifications that can be directly derived from or easily conceived by those skilled in the art from the disclosure of the present invention are included in the protection scope of the present invention.

Claims

1. Containing Fe and inevitable impurity elements, and further containing the following chemical elements in the following mass percentage contents: C: 0.07 - 0.15%, Si: 0.1 - 0.8%, Mn: 1.5 - 2.2%, Al: 0.02 - 0.1%, Ti: 0.05 - 0.18%, Nb ≤ 0.06%, B ≤ 0.003%, and at least one of 0.2% ≤ Cr ≤ 1.5%, 0.05% ≤ Mo ≤ 0.5%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053% A 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property, characterized by the above.

2. The mass percentage contents of its respective chemical elements are: C: 0.07 - 0.15%, Si: 0.1 - 0.8%, Mn: 1.5 - 2.2%, Al: 0.02 - 0.1%, Ti: 0.05 - 0.18%, Nb ≤ 0.06%, B ≤ 0.003%, and at least one of 0.2% ≤ Cr ≤ 1.5%, 0.05% ≤ Mo ≤ 0.5%; the balance is Fe and inevitable impurity elements; preferably, 0.015% ≤ Nb ≤ 0.06%; However, the mass percentage contents of N, Ti, and Nb further satisfy 0.01% ≤ (Ti - 3.43N + 0.52Nb) / 4 ≤ 0.053% A 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to Claim 1, characterized by the above.

3. Among the inevitable impurity elements, P ≤ 0.02%, S ≤ 0.005%, N ≤ 0.005%, a 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to Claim 1 or 2, characterized by the above.

4. The contents of Cr and Mo in the steel satisfy any one of the following: (1) When 0.2% ≤ Cr ≤ 0.7%, the mass percentage content of Mo satisfies 0.2% ≤ Mo ≤ 0.35%; (2) When 0.7% < Cr ≤ 1.0%, the mass percentage content of Mo satisfies 0.05% ≤ Mo < 0.2%; (3) When 1.0% < Cr ≤ 1.5%, Mo is not contained; (4) When 0.35% < Mo ≤ 0.5%, Cr is not contained A 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to Claim 1 or 2, characterized by the above.

5. The main body of the matrix of its microstructure is bainite, and the matrix has nano-scale precipitates, a 1000 MPa grade hot-rolled multiphase steel sheet with high hole expansion property according to Claim 1 or 2, characterized by the above.

6. The bainite volume fraction is ≥85%, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 5 is characterized in that.

7. The matrix of the microstructure further contains martensite and / or retained austenite, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 1 or 2 is characterized in that.

8. The size of the nano-scale precipitate is 3 to 20 nm, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 5 is characterized in that.

9. The size of the bainite crystal grains is ≤6 μm, and the size of the martensite and / or retained austenite crystal grains is ≤3 μm, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 7 is characterized in that.

10. The microstructure of the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade contains martensite with a volume fraction of 85 to 96% and a total volume fraction of martensite and retained austenite of 4 to 15%. However, the size of the bainite crystal grains is 3.2 to 5 μm, and the size of the martensite crystal grains is 0.5 to 3 μm, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 1 or 2 is characterized in that.

11. Its performance is characterized in that the yield strength is ≥750 MPa, the tensile strength is 950 to 1150 MPa, the elongation rate A50 is ≥12%, the punching hole expansion rate is ≥45%, and the reaming hole expansion rate is ≥65%, and the hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to claim 1 or 2.

12. The method for manufacturing a hot-rolled dual-phase steel sheet with high hole expansion property of 1000 MPa grade according to any one of claims 1 to 11, characterized by including the following steps. (1) Smelting and casting; (2) Hot rolling: Heat the slab to 1200 to 1300 °C and hold it; then perform rolling, provided that the rough rolling exit temperature is 1000 to 1080 °C and the finish rolling end temperature is 840 to 950 °C; (3) Perform two-stage laminar flow cooling so as to water-cool the steel sheet to the coiling temperature: However, the average cooling rate in the first stage is ≥ 100 °C / s, the average cooling rate in the second stage is ≥ 3 °C / s, the intermediate point temperature between the first stage cooling and the second stage cooling is the bainite transformation temperature Bs ± 30 °C, control the threading speed to 7 to 12 m / s, and control the cooling time from the intermediate point temperature to the coiling temperature to ≥ 4.5 s. Here, Bs = 844 - 597×C + 127×C ２ - 92×Mn + 8×Mn ２ - 32×Cr + 2.2×Cr ２ - 42×Mo. For each chemical element in the formula, the numerical value before the percentage symbol of the mass percentage content of the corresponding chemical element is substituted; (4) Coiling: Control the coiling temperature to 430 to 600 °C, and after coiling, cool it to room temperature at a cooling rate of ≤0.1 °C / s; (5) Pickling.

13. In step (2), the total rolling reduction ratio is controlled to be ≥80%, and the finish rolling total reduction ratio is controlled to be ≥50%; preferably, the total rolling reduction ratio is 90% to 95%, and the finish rolling total reduction ratio is 85% to 90%; more preferably, the steel plate of the obtained finished product has a thickness of 5 mm or less, and the manufacturing method according to claim 12 is characterized in that.

14. In step (3), the time for cooling from the intermediate point temperature to the coiling temperature is controlled to be ≥6 s, and the manufacturing method according to claim 12 is characterized in that.

15. In step (4), the coiling temperature is controlled to be 430 to 580°C, and the manufacturing method according to claim 12 is characterized in that.

Images