High strength low carbon martensite high hole expandability steel and its manufacturing method

Abstract

A low-carbon martensitic high hole-expandability steel having a tensile strength of 980 MPa or more and a chemical composition, in weight percentages, of C 0.03 to 0.10%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P≦0.02%, S≦0.003%, Al 0.02 to 0.08%, N≦0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O≦0.0030%, and the balance being Fe and other unavoidable impurities, and a method for manufacturing the same. The high hole-expandability steel according to the present invention has a yield strength of ≥ 800 MPa, a tensile strength of ≥ 980 MPa, and a transverse elongation A 50 ≥ 8%, hole expansion ratio ≥ 30%, and has passed the cold bending performance test (d ≤ 4a, 180°), making it suitable for use in passenger car chassis parts that require high strength and thin wall thickness, such as control arms and subframes.

Description

[Technical field]

[0001] The present invention relates to the field of high strength steels, and in particular to high strength low carbon martensitic high hole expandability steels and methods for producing the same. [Background technology]

[0002] With the development of the national economy, the number of automobiles produced has increased significantly, and the amount of plate materials used has also continued to increase. In China's domestic automobile industry, the original design of many vehicle model parts, such as automobile chassis parts, torsion beams, sedan subframes, wheel spokes and rims, front and rear axle assemblies, body structural parts, seats, clutches, seat belts, truck box panels, protective nets, car beams, etc., requires the use of hot-rolled plates and pickled plates. Among them, chassis steel accounts for 24-34% of the total amount of steel used in automobiles.

[0003] Lightweighting of passenger cars is not only a trend in the automotive industry, but also a requirement of regulations. Regulations stipulate fuel efficiency, but this is essentially a requirement for lighter body construction, which translates into higher strength, thinner walls, and lighter materials. Higher strength and lighter weight are inevitable requirements for future new car models, which will inevitably lead to higher steel usage levels and changes in chassis structure: for example, the increasing complexity of parts will lead to higher requirements for material performance and surface, as well as advances in forming technologies such as hydroforming, hot stamping, and laser welding, which will result in higher material strength, stamping, flanging, rebound, fatigue, and other performance.

[0004] Compared with overseas, the development of high-strength high-hole-expandability steel in China not only has a relatively low strength level, but also has poor performance stability. For example, most of the high-hole-expandability steels used by domestic auto parts manufacturers in China are high-hardness steels with a tensile strength of 600 MPa or less, and the competition for high-hole-expandability steels with a tensile strength of 440 MPa or less is intensifying. At present, high-hole-expandability steels with a tensile strength of 780 MPa are gradually being mass-produced, but the requirements for elongation and hole expansion ratio, which are two important indicators of forming, are also increasing. Meanwhile, high-hole-expandability steels with a tensile strength of 980 MPa or more are still in the research, development and certification stage and have not yet reached the stage of mass production. High-strength high-hole-expandability steels with higher strength levels, such as 1180 MPa or more, have not yet been developed by manufacturers. However, 980 MPa high-hole-expandability steels with higher strength and higher hole expansion ratios will inevitably become the direction of development in the future. In order to meet the potential needs of future users, there is a demand for the development of high hole expandability steel with good hole expandability of 980 MPa or more.

[0005] There are few documents on high hole expandability steels at the 980 MPa level, and the 1180 MPa level of high hole expandability steels is rather a blank slate. Most of the existing patent documents on high hole expandability steels at the 780 MPa level or less. There are few documents on high hole expandability steels at 980 MPa or more. Chinese patent application CN106119702A discloses a 980 MPa level hot rolled high hole expandability steel having a fine structure of granular bainite and a small amount of martensite, and a low carbon V-Ti fine alloying design with small amounts of Nb and Cr added as the main feature of its component design. It is significantly different from the present invention in terms of components, process, structure, etc. Summary of the Invention [Problem to be solved by the invention]

[0006] According to the literature, the elongation of a material is usually inversely proportional to its hole expansion ratio, i.e., the higher the elongation, the lower the hole expansion ratio, and conversely, the lower the elongation, the higher the hole expansion ratio. It is therefore very difficult to obtain a high hole-expandability steel that has high elongation and high hole expandability and at the same time high strength. Also, with the same or similar strengthening mechanism, the higher the strength of the material, the lower the hole expansion ratio.

[0007] To obtain steel materials with excellent plasticity and hole expansion / flanging properties, it is necessary to strike a better balance between the two. Of course, the hole expansion ratio of a material is closely related to many factors, among which the homogeneity of the structure, the control level of inclusions and segregation, the type of structure, and the measurement of the hole expansion ratio are the most important factors. In general, a single and homogeneous structure is favorable for improving the hole expansion ratio, while a two-phase or multi-phase structure is unfavorable for improving the hole expansion ratio. [Means for solving the problem]

[0008] The object of the present invention is to provide a low carbon martensite high hole expandability steel having a tensile strength of 980 MPa or more and a method for producing the same, and the high hole expandability steel has a yield strength of 800 MPa or more, a tensile strength of 980 MPa or more, a transverse elongation A 50 ≧8%, hole expansion ratio ≧30%, preferably ≧50%, and can be used for automobile chassis parts that require high strength and thin wall thickness, such as control arms and subframes. In some embodiments, the high hole-expandability steel has a yield strength of ≧900 MPa, a tensile strength of ≧1180 MPa, a lateral elongation A 50 ≧10%, and hole expansion ratio ≧30%. In some embodiments, the high hole expandability steel of the present invention passes the cold bending performance (d≦4a, 180°) test.

[0009] To achieve the above objectives, the technical solution of the present invention: According to the component design of the present invention, the C content is designed to be low in order to ensure excellent weldability, good hole expandability and impact toughness of the obtained martensite structure when used by the user; the Si content is designed to be high in order to obtain more retained austenite in accordance with the process and thereby improve the plasticity of the material; and at the same time, the Si content is designed to be high in order to contribute to lowering the non-recrystallization temperature of the steel and allow the steel to complete the dynamic recrystallization process within a wider rolling finish temperature range, thereby refining the size of the austenite crystal grains and the final martensite crystal grains, and improving the plasticity and hole expansion ratio.

[0010] Specifically, the low carbon martensitic high hole expandability steel according to the present invention having a tensile strength of 980 MPa or more has a chemical composition, in weight percentages, of C 0.03 to 0.10%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P≦0.02%, S≦0.003%, Al 0.02 to 0.08%, N≦0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O≦0.0030%, and the balance being Fe and other unavoidable impurities.

[0011] The alloy further contains one or more of the following elements: Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, and Ni≦0.5%.

[0012] In some embodiments, the low carbon martensitic high hole expandability steel according to the present invention has a tensile strength of 980 MPa or more and a chemical composition, in weight percentage, of C 0.03 to 0.10%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P≦0.02%, S≦0.003%, Al 0.02 to 0.08%, N≦0.004%, Mo 0.1 to 0.5%, Ti 0.01-0.05%, O≦0.0030%, Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5%, the balance being Fe and other inevitable impurities; preferably, the low carbon martensitic high hole expandability steel contains at least one of Cr, B, Ca, Nb, V, Cu and Ni. In some preferred embodiments, the low carbon martensitic high hole expandability steel contains at least Ni, preferably the Ni content is 0.1-0.5%, more preferably 0.1-0.3%. In some preferred embodiments, the low carbon martensitic high hole expandability steel contains at least Cr and / or B, preferably the Cr content is 0.1-0.5%, preferably 0.2-0.4%, and preferably the B content is 0.0005-0.002%.

[0013] In some embodiments, the preferred Cr content is 0.2 to 0.4%; the preferred B content is 0.0005 to 0.0015%; the preferred Ca content is ≦0.002%; the preferred Nb and V contents are each ≦0.03%; and the preferred Cu and Ni contents are each ≦0.3%.

[0014] Furthermore, the microstructure of the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention is martensite. In some embodiments, the microstructure is martensite or tempered martensite. Preferably, the content of retained austenite in the microstructure of the low carbon martensitic high hole expandability steel is ≦5% by volume. In some embodiments, the content of austenite is 0.5 to 5%.

[0015] Furthermore, the low carbon martensitic high hole expandability steel of the present invention has a tensile strength of 980 MPa or more, a yield strength of ≧800 MPa, preferably ≧900 MPa, a tensile strength of ≧980 MPa, preferably ≧1180 MPa, and a transverse elongation A 50 ≧8%, preferably ≧10%, and the hole expansion ratio is ≧30%, preferably ≧50%.

[0016] Preferably, the low carbon martensitic high hole expandability steel of the present invention having a tensile strength of 980 MPa or more has a -40°C impact toughness of ≥ 60 J, preferably ≥ 70 J. In some embodiments, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a -40°C impact toughness of ≥ 140 J, preferably ≥ 150 J, more preferably ≥ 160 J.

[0017] Preferably, the low carbon martensitic high hole expandability steel according to the present invention having a tensile strength of 980 MPa or more passes a cold bending performance test (d≦4a, 180°).

[0018] In some embodiments, the present invention provides an ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more and a chemical composition, in weight percentages, of 0.03-0.06% C, 0.5-2.0% Si, 1.0-2.0% Mn, P≦0.02%, S≦0.003%, 0.02-0.08%, N≦0.004%, 0.1-0.5% Mo, 0.01-0.05% Ti, O≦0.0030%, and the balance being Fe and other unavoidable impurities.

[0019] Furthermore, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more further contains one or more elements selected from the group consisting of Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, and Ni≦0.5%.

[0020] In some embodiments, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a chemical composition, in weight percentages, of C 0.03 to 0.06%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P ≦ 0.02%, S ≦ 0.003%, Al 0.02 to 0.08%, N ≦ 0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O ≦ 0.0030%, Cr ≦ 0.5%, B ≦ 0.002%, Ca ≦ 0.005%, Nb ≦ 0.06%, V ≦ 0.05%, Cu ≦ 0.5%, Ni ≦ 0.5%, and the balance being Fe and other unavoidable impurities. Preferably, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more contains at least Cr and / or B, and preferably, the Cr content is 0.1 to 0.5%, preferably 0.2 to 0.4%, and preferably, the B content is 0.0005 to 0.002%.

[0021] In some embodiments, the preferred Cr content is 0.2 to 0.4%; the preferred B content is 0.0005 to 0.0015%; the preferred Ca content is ≦0.002%; the preferred Nb and V contents are each ≦0.03%; and the preferred Cu and Ni contents are each ≦0.3%.

[0022] In a preferred embodiment, the microstructure of the ultra-low carbon martensite high hole expandability steel having a tensile strength of 980 MPa or more is martensite or tempered martensite. In some embodiments, the microstructure of the ultra-low carbon martensite high hole expandability steel having a tensile strength of 980 MPa or more further contains a small amount of retained austenite. Preferably, the content of retained austenite in the microstructure of the ultra-low carbon martensite high hole expandability steel is ≦5% by volume. In some embodiments, the content of austenite is 0.5 to 5%.

[0023] In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a yield strength of ≧800 MPa, preferably ≧820 MPa, a tensile strength of ≧980 MPa, preferably ≧1000 MPa, and a transverse elongation A 50 ≧8%, preferably ≧10%, hole expansion ratio ≧50%, preferably ≧55%, and passes the cold bending performance test (d≦4a, 180°). In a preferred embodiment, the ultra-low carbon martensitic high hole-expandability steel having a tensile strength of 980 MPa or more has a -40°C impact toughness ≧140 J, preferably ≧150 J, more preferably ≧160 J. In a preferred embodiment, the ultra-low carbon martensitic high hole-expandability steel having a tensile strength of 980 MPa or more has a yield strength of 800 to 890 MPa, a tensile strength of 980 to 1150 MPa, a transverse elongation A 50 The microstructure of the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more is preferably martensite and retained austenite, and the volume percentage of the retained austenite in the microstructure is ≦5% as described above.

[0024] In some embodiments, the low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention is a high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more, and its chemical composition, in weight percentages, is 0.06 to 0.10% C, 0.8 to 2.0% Si, 1.5 to 2.0% Mn, P≦0.02%, S≦0.003%, 0.02 to 0.08%, N≦0.004%, 0.1 to 0.5% Mo, 0.01 to 0.05% Ti, O≦0.0030%, and the balance is Fe and other unavoidable impurities.

[0025] Furthermore, the high plasticity, high hole expandability steel having a tensile strength of 1180 MPa or more further contains one or more elements selected from the group consisting of Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, and Ni≦0.5%.

[0026] In some embodiments, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more has a chemical composition, in weight percentages, of C 0.06 to 0.10%, Si 0.8 to 2.0%, Mn 1.5 to 2.0%, P ≦ 0.02%, S ≦ 0.003%, Al 0.02 to 0.08%, N ≦ 0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O ≦ 0.0030%, Cr ≦ 0.5%, B ≦ 0.002%, Ca ≦ 0.005%, Nb ≦ 0.06%, V ≦ 0.05%, Cu ≦ 0.5%, Ni ≦ 0.5%, and the balance being Fe and other unavoidable impurities. In some embodiments, the high-plasticity, high-hole-expandability steel having a tensile strength of 1180 MPa or more preferably contains at least Ni, and preferably the Ni content is 0.1 to 0.3%. The high-plasticity, high-hole-expandability steel having a tensile strength of 1180 MPa or more preferably contains at least Cr and / or B, and preferably the Cr content is 0.1 to 0.5%, preferably 0.2 to 0.4%, and preferably the B content is 0.0005 to 0.002%.

[0027] In some embodiments, the preferred Cr content is 0.2-0.4%; the preferred Cu and Ni contents are each ≦0.3%; the preferred Nb and V contents are each ≦0.03%; the preferred B content is 0.0005-0.0015%; and the preferred Ca content is ≦0.002%.

[0028] In a preferred embodiment, the microstructure of the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite. In some embodiments, the microstructure of the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more further contains a small amount of retained austenite. Preferably, the content of retained austenite in the microstructure is ≦5% by volume. In some embodiments, the content of austenite is 2 to 5%.

[0029] In a preferred embodiment, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength of ≧900 MPa, preferably ≧930 MPa, more preferably ≧950 MPa, a tensile strength of ≧1180 MPa, preferably ≧1200 MPa, more preferably ≧1220 MPa, and a lateral elongation A 50 ≧10%, and the hole expansion ratio is ≧30%, preferably ≧35%. In a preferred embodiment, the ultra-low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more has a −40° C. impact toughness of ≧60 J, preferably ≧70 J, more preferably ≧80 J. Preferably, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more passes a cold bending performance test (d≦4a, 180°).

[0030] In a preferred embodiment, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength of 900 to 1000 MPa, a tensile strength of 1200 to 1280 MPa, a lateral elongation of 10 to 13%, a hole expansion ratio of 30 to 50%, and a -40 ° C. impact toughness of 60 to 100 J. Preferably, the microstructure of the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite and retained austenite, with the volume percentage of the retained austenite in the microstructure being ≦ 5% as described above. Preferably, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more has passed a cold bending performance test (d ≦ 4a, 180 °).

[0031] In another preferred embodiment, the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more has a yield strength of 940 to 1000 MPa, a tensile strength of 1210 to 1300 MPa, a lateral elongation of 10 to 13%, a hole expansion ratio of 30 to 50%, a -40 ° C. impact toughness of 80 to 110 J, and has passed a cold bending performance test (d ≦ 4a, 180 °). Preferably, the microstructure of the high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more is tempered martensite and retained austenite, with the volume percentage of the retained austenite in the microstructure being ≦ 5% as described above.

[0032] In the composition design of the high hole expandability steel according to the present invention: Carbon is a basic element in steel and is also one of the important elements for the present invention. Carbon expands the austenite phase region and stabilizes austenite. Carbon, as an interstitial atom in steel, plays a very important role in improving the strength of steel, and has the greatest effect on the yield strength and tensile strength of steel. In the present invention, since the structure to be obtained is low carbon or ultra-low carbon martensite, in order to obtain a high-strength steel with a tensile strength of 980 MPa, the carbon content must be ensured to be 0.03% or more; otherwise, if the carbon content is below 0.03%, the tensile strength cannot reach 980 MPa even if the steel is completely quenched to room temperature. However, a carbon content exceeding 0.10% is also not acceptable, and if the carbon content is too high, the strength of the low carbon martensite formed is too high, resulting in low elongation and low hole expansion ratio. Therefore, the carbon content should be controlled between 0.03 and 0.10%. In some embodiments, the preferred range of C is between 0.04 and 0.055%. In some other embodiments, the preferred range of C is 0.07 to 0.09%.

[0033] Silicon is a basic element in steel and is also one of the important elements for the present invention. Increasing the Si content can not only improve the solid solution strengthening effect, but more importantly, it can also play the following two roles. One is to significantly lower the non-recrystallization temperature of the steel, thereby completing dynamic recrystallization in the steel within a very low temperature range. Then, in the actual rolling process, rolling can be performed within a relatively wide rolling end temperature range, for example, within a rolling end temperature range such as 800 to 900 ° C., and the anisotropy of the structure can be significantly improved, thereby reducing the anisotropy of the final martensite structure, which contributes to improving strength and plasticity and also contributes to obtaining a good hole expansion ratio; another important role of Si is to inhibit the precipitation of cementite, and under appropriate rolling process conditions, especially when obtaining a structure mainly composed of martensite, a predetermined amount of retained austenite can be retained, which contributes to improving elongation. As is well known, the elongation of martensite is usually the lowest at the same strength level, but retaining a predetermined amount of stable retained austenite is one of the important means to improve the elongation of martensite. This role of Si generally begins to be expressed when its content is 0.5% or more, but too much Si content is also unacceptable, otherwise the rolling force load during the actual rolling process will be excessive, which is disadvantageous to stable product production. Therefore, in steel, the Si content is usually controlled between 0.5 and 2.0%, with a preferred range being between 0.8 and 1.4%. In some embodiments, the Si content is controlled between 1.0 and 1.4%.

[0034] Manganese is the most basic element in steel and is one of the most important elements for the present invention. As is well known, Mn is an important element that expands the austenite phase region, reduces the critical quenching rate of steel, stabilizes austenite, refines crystal grains, and delays the transformation from austenite to pearlite. In the present invention, in order to ensure the strength of the steel plate and stabilize the retained austenite, the Mn content is usually controlled to 1.0% or more, but the Mn content is usually not allowed to exceed 2.0%, otherwise, segregation of Mn is likely to occur during steelmaking and hot cracks are likely to occur during continuous slab casting. Therefore, in steel, the Mn content is usually controlled to 1.0-2.0%, with a preferred range being 1.4-1.8%. In some embodiments, the Mn content is controlled between 1.6-1.9%.

[0035] Phosphorus is an impurity element in steel. P tends to be concentrated in grain boundaries, and if the P content in steel is high (≧0.1%), it forms Fe2P and precipitates around grains, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better. In general, it is preferable to control it to within 0.02%, and the steelmaking cost will not increase.

[0036] Sulfur is an impurity element in steel. S in steel usually combines with Mn to form MnS inclusions. Especially when the contents of S and Mn are both high, a lot of MnS is formed in the steel. MnS itself has some plasticity, and in the subsequent rolling process, MnS deforms along the rolling direction, which not only reduces the transverse plasticity of the steel sheet, but also increases the anisotropy of the structure, which is disadvantageous to the hole expandability. Therefore, the lower the S content in steel, the better. Considering that the Mn content in the present invention must be at a high level, the S content must be strictly controlled in order to reduce the MnS content. The S content must be controlled within 0.003%, and the preferred range is 0.0015% or less.

[0037] In steel, aluminum mainly plays the role of deoxidization and nitrogen fixation. Assuming the presence of strong carbide forming elements such as Ti, Nb, and V, Al mainly plays the role of deoxidization and grain refinement. In the present invention, Al is a general deoxidizing element and grain refinement element, and its content may be controlled to usually 0.02 to 0.08%. If the Al content is less than 0.02%, it cannot contribute to grain refinement, and similarly, if the Al content is 0.08% or more, its grain refinement effect is saturated. Therefore, in steel, the Al content may be controlled usually between 0.02 and 0.08%, with the preferred range being between 0.02 and 0.05%.

[0038] In the present invention, nitrogen belongs to the impurity element, and the lower its content, the better. However, nitrogen is an unavoidable element in the steelmaking process. Although its content is small, when it is combined with strong carbide forming elements such as Ti, the formed TiN particles have a very negative effect on the performance of the steel, especially the hole expandability. In addition, since TiN has a square shape, there is a large stress concentration between its sharp corners and the substrate, and during the hole expansion deformation process, cracks are likely to occur due to the stress concentration between TiN and the substrate, which greatly reduces the hole expandability. On the premise that the nitrogen content is controlled as much as possible, the lower the content of strong carbide forming elements such as Ti, the better. In the present invention, the adverse effects of TiN are reduced as much as possible by adding a trace amount of Ti to fix nitrogen. Therefore, the nitrogen content should be controlled to 0.004% or less, and the preferred range is 0.003% or less.

[0039] Titanium is one of the important elements for the present invention. Ti plays two main roles in the present invention: one is to combine with the impurity element N in the steel to form TiN, which plays a part of the role of "nitrogen fixation"; the other is to form a certain number of fine TiN dispersed in the subsequent welding process of the material, suppress the size of austenite crystal grains, refine the structure, and improve low-temperature toughness. Therefore, in steel, the Ti content range is controlled to 0.01-0.05%, and the preferred range is 0.01-0.03%.

[0040] Molybdenum is one of the important elements for the present invention. Adding molybdenum to steel can significantly delay the transformation of ferrite and pearlite. This role of molybdenum is advantageous for adjusting various processes in the actual rolling process. For example, at the end of rolling, stepwise cooling may be performed, or air cooling followed by water cooling may be performed. In the present invention, even if the process of air cooling followed by water cooling or the process of direct water cooling after rolling is adopted, the addition of molybdenum can ensure that structures such as ferrite and pearlite are not formed during the air cooling process, and dynamic recovery of austenite deformed during the air cooling process occurs, contributing to the improvement of the homogeneity of the structure; molybdenum has strong resistance to welding softening. The main purpose of the present invention is to obtain a structure of a single low-carbon martensite and a small amount of retained austenite, but since low-carbon martensite is prone to softening phenomenon after welding, the degree of welding softening can be effectively reduced by adding a certain amount of molybdenum. Therefore, the molybdenum content should be controlled between 0.1 and 0.5%, with the preferred range being 0.15 to 0.35%.

[0041] Chromium is one of the elements that can be added in the present invention. The addition of a small amount of chromium is not for improving the hardenability of steel, but for combining with the B phase to contribute to the formation of an acicular ferrite structure in the weld heat affected zone after welding, thereby significantly improving the low-temperature toughness of the weld heat affected zone. Since the final application part according to the present invention is a chassis product for passenger cars, the low-temperature toughness of the weld heat affected zone is an important indicator. In addition to ensuring that the strength of the weld heat affected zone does not decrease too much, the low-temperature toughness of the weld heat affected zone must also meet certain requirements. In addition, chromium itself has a certain degree of weld softening resistance. Therefore, in steel, the amount of chromium added is usually ≦0.5%, and the preferred range is 0.2-0.4%.

[0042] Boron is one of the elements that can be added in the present invention. The main role of boron in steel is to be unevenly distributed in the prior austenite grain boundaries and to suppress the formation of proeutectoid ferrite; the addition of boron to steel can also greatly improve the hardenability of the steel. However, in the present invention, the main purpose of adding a trace amount of boron element is not to improve the hardenability, but to combine with the chromium phase to improve the structure of the weld heat affected zone and obtain an acicular ferrite structure with excellent low-temperature toughness. The boron element added to steel is usually controlled to 0.002% or less, and the preferred range is between 0.0005 and 0.0015%.

[0043] Calcium is an element that can be added to the present invention. Calcium improves the morphology of sulfides such as MnS, converts long-striped sulfides such as MnS into spherical CaS, and contributes to improving the morphology of inclusions, thereby reducing the adverse effect of long-striped sulfides on hole expandability. However, if too much calcium is added, the number of calcium oxides increases, which is detrimental to hole expandability. Therefore, in steel, the amount of calcium added is usually ≦0.005%, and the preferred range is ≦0.002%.

[0044] Oxygen is an inevitable element in the steelmaking process, and in the present invention, the O content in steel can usually reach 30 ppm or less after deoxidation, and there is no obvious adverse effect on the performance of the steel sheet. Therefore, it is sufficient to control the O content in steel to within 30 ppm.

[0045] Niobium is one of the elements that can be added in the present invention. Niobium, like titanium, is a strong carbide-forming element in steel. By adding niobium to steel, the non-recrystallization temperature of steel can be significantly increased, and deformed austenite with higher dislocation density can be obtained in the finish rolling stage, and the final transformation structure can be refined in the subsequent transformation process. However, the amount of niobium added should not be too much. On the other hand, if the amount of niobium added exceeds 0.06%, it is easy to form relatively coarse niobium carbonitride in the structure, consuming part of the carbon atoms and reducing the precipitation strengthening effect of carbides. In addition, if the niobium content is high, anisotropy is easily generated in the austenite structure in the hot rolled state, which is carried over to the final structure in the subsequent cooling transformation process, which is disadvantageous to hole expansion properties. Therefore, in steel, the niobium content is usually controlled to ≦0.06%, and the preferred range is ≦0.03%.

[0046] Vanadium is an element that can be added in the present invention. Vanadium, like titanium and niobium, is a strong carbide forming element. However, vanadium carbides have low solid solution temperature and precipitation temperature, and are usually all dissolved in austenite at the finish rolling stage. When the temperature drops and transformation begins, vanadium begins to form in ferrite. In the present invention, the main purpose of adding vanadium is to improve the softening resistance of the weld heat affected zone together with molybdenum. In terms of the weld softening resistance effect, the effects of molybdenum and vanadium are the strongest, but when molybdenum is contained, vanadium may be selectively added. Therefore, in steel, the vanadium addition amount is usually ≦0.05%, and the preferred range is ≦0.03%.

[0047] Copper is one of the elements that can be added in the present invention. The addition of copper to steel can improve the corrosion resistance of the steel, and when added together with P, the corrosion resistance is even better; when the amount of Cu added exceeds 1%, it can form an ε-Cu precipitate phase under certain conditions, and can achieve a strong precipitation strengthening effect. However, the addition of Cu is likely to cause the "Cu embrittlement" phenomenon during the rolling process, and in order to fully utilize the corrosion resistance improvement effect of Cu without significantly causing the "Cu embrittlement" phenomenon in a certain application environment, the Cu element content is usually controlled within 0.5%, with the preferred range being within 0.3%.

[0048] Nickel is one of the elements that can be added in the present invention. The addition of nickel to steel gives it a certain degree of corrosion resistance, but the corrosion resistance effect is weaker than that of copper. The addition of nickel to steel does not affect the tensile performance of the steel very much, but it can refine the structure and precipitation phase of the steel and greatly improve the low-temperature toughness of the steel; in addition, the addition of a small amount of nickel to steel with the addition of copper element can suppress the occurrence of "Cu embrittlement". The addition of a large amount of nickel has no obvious adverse effect on the performance of the steel itself. The simultaneous addition of copper and nickel not only improves the corrosion resistance, but also refines the structure and precipitation phase of the steel, and greatly improves the low-temperature toughness. However, since both copper and nickel are relatively expensive alloying elements, in order to minimize the cost of the alloy, the amount of nickel added is usually ≦0.5%, and the preferred range is ≦0.3%.

[0049] The method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C and a heat retention time of 1-2 hours; 3) Hot rolling The main purpose is to refine the austenite grains, the rolling start temperature is 950-1100°C, 3-5 passes are performed under high rolling pressure at 950°C or more, and the cumulative deformation amount is ≧50%; optionally, the intermediate billet temperature is 900-950°C, and then the final rolling is 3-7 passes, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-950°C; 4) Cooling First, air-cool for 0-10 seconds to achieve dynamic recovery and dynamic recrystallization, then water-cool the steel strip at a cooling rate of ≥30°C / s to a temperature below the Ms point (between room temperature and the Ms point), coil it, and then cool it to room temperature (preferably at a cooling rate of ≤20°C / h); alternatively, air-cool for 0-10 seconds, then water-cool the steel strip directly to room temperature at a cooling rate of ≥30°C / s, and coil it; alternatively, air-cool for 0-10 seconds, then water-cool the steel strip at a cooling rate of ≥30°C / s to a temperature below the Ms point, which is the martensitic transformation start point, coil it, and then slowly cool it to room temperature (preferably at a cooling rate of ≤20°C / h); 5) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 100 m / min, and the pickling temperature is controlled in the range of 75 to 85°C to reduce the elongation loss of the steel strip. Elongation in tension leveling process is controlled to ≦2%, then rinsed, the strip surface is dried and oiled.

[0050] Preferably, after pickling in step 5), the strip steel is rinsed at a temperature range of 35-50°C to ensure the surface quality, and the surface is dried at a temperature range of 120-140°C and oiled.

[0051] In some embodiments, between steps 4) and 5), step 4-1): annealing by bell annealing, with a heating rate of ≧20°C / h, a bell annealing temperature of 100-300°C, and a bell annealing time of 12-48h; cooling the steel sheet to ≦100°C at a cooling rate of ≦50°C / h, and tapping.

[0052] In some embodiments, a method for producing a low carbon martensitic high hole expandability steel having a tensile strength of 980 MPa or more according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Heat the billet or ingot to 1100-1200℃ and keep it at that temperature for 1-2 hours. Reheat with; 3) Hot rolling For the main purpose of refining austenite grains, the rolling start temperature is 950-1100°C, 3-5 passes are performed under high pressure at 950°C or higher, and the cumulative deformation amount is ≧50%, preferably ≧60%; next, the intermediate billet temperature is 920-950°C, and then the final rolling is performed 3-5 passes, and the cumulative deformation amount is ≧70%, preferably ≧85%; the rolling end temperature is 800-920°C; 4) Cooling First, air-cool for 0-10 seconds to achieve dynamic recovery and dynamic recrystallization, then water-cool the strip steel at a cooling rate of ≧50°C / s, preferably 50-85°C / s, to a temperature below the Ms point (between room temperature and the Ms point), coil it, and then cool it to room temperature (preferably at a cooling rate of ≦20°C / h); 5) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 100 m / min, and the pickling temperature is controlled in the range of 75 to 85°C to reduce the elongation loss of the steel strip. Elongation in tension leveling process is controlled to ≦2%, then rinsed, the strip surface is dried and oiled.

[0053] In some embodiments, a method for producing a high plasticity, high hole expandability steel having a tensile strength of 1180 MPa or more according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C and a heat retention time of 1-2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, 3-5 passes are performed at 950°C or higher under high pressure, and the cumulative deformation amount is ≧50%, preferably ≧60%; next, 3-7 passes are performed, and the cumulative deformation amount is ≧70%, preferably ≧85%; the rolling end temperature is 800-950°C; 4) Cooling First, air-cool for 0-10 seconds, then water-cool the steel strip to room temperature at a cooling rate of ≧30°C / s, preferably 30-65°C / s, and then coil it; 5) Annealing Annealing by bell annealing, with a heating rate of ≧20°C / h, preferably 20-40°C / h, a bell annealing temperature of 100-300°C, and a bell annealing time of 12-48h; cooling the steel sheet to below 100°C at a cooling rate of ≦50°C / h, preferably 15-50°C / h, and tapping; 6) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 90 m / min, and the pickling temperature is controlled in the range of 75 to 85°C. Elongation in tension leveling process The content is controlled to be ≦1.5%, rinsed at a temperature range of 35-50°C, and the surface is dried at a temperature range of 120-140°C and oiled.

[0054] Preferably, after pickling in step 6), the steel is rinsed at a temperature range of 35 to 50° C., and the surface is dried at a temperature range of 120 to 140° C., and oil is applied.

[0055] In some other embodiments, a method for producing a high plasticity high hole expandability steel having a tensile strength of 1180 MPa or more according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, 3-5 passes are performed under high pressure at 950°C or higher, and the cumulative deformation amount is ≧50%, preferably ≧60%; next, the temperature of the intermediate billet is 900-950°C, and then 3-7 passes are performed, and the cumulative deformation amount is ≧70%, preferably ≧85%; the rolling end temperature is 800-900°C; 4) Cooling First, air-cool for 0 to 10 seconds, then water-cool the steel sheet at a cooling rate of ≧30°C / s, preferably 30 to 70°C / s, to a certain temperature below the Ms point, which is the martensitic transformation start point, and then coil it up, and then slowly cool it to room temperature (preferably at a cooling rate of ≦20°C / h); 5) Annealing Annealing by bell annealing, the heating rate is ≧20°C / h, preferably 20-50°C / h, the bell annealing temperature is 100-300°C, and the bell annealing time is 12-48h; cooling the steel sheet to ≦100°C at a cooling rate of ≦50°C / h, preferably 20-50°C / h, and tapping; 6) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 90 m / min, and the pickling temperature is controlled in the range of 75 to 85°C to reduce the elongation loss of the steel strip. Elongation in tension leveling process is controlled to ≦1.5%, then rinsed, the strip surface is dried and oiled.

[0056] Preferably, after pickling in step 6), the steel is rinsed at a temperature range of 35 to 50° C., and the surface is dried at a temperature range of 120 to 140° C., and oil is applied.

[0057] The innovations of the present invention are as follows: According to the composition design of the present invention, the C content is designed to be low in order to ensure excellent weldability, good hole expandability and impact toughness of the obtained martensite structure during use by the user. When the tensile strength is ≧1180 MPa, the lower the carbon content, the better in satisfying the tensile strength ≧1180 MPa. In accordance with the process, the Si content is designed to be high in order to obtain more retained austenite and improve the plasticity of the material; at the same time, the Si content is designed to be high in order to contribute to lowering the non-recrystallization temperature of the steel and to make the steel complete the dynamic recrystallization process within a wider final rolling temperature range, thereby refining the size of the austenite crystal grains and the final martensite crystal grains, and improving the plasticity and hole expansion ratio. In addition, in the process of bell-shaped annealing, the purpose of improving the homogeneity of the structure and improving the plasticity and hole expansion ratio can be achieved by removing part of the quenching stress.

[0058] In the composition design, a low carbon martensite design concept is adopted and more silicon is added to suppress and reduce the formation of cementite, while the non-recrystallization temperature is lowered and rolling and air cooling after rolling are performed in a relatively wide rolling end temperature range, resulting in fine crystal grains and homogeneous, equiaxed prior austenite grains, and finally a homogeneous structure of martensite and retained austenite. The retained austenite provides the steel plate with high plasticity and cold bending performance, the martensite with high strength, and the homogeneous, fine structure with higher hole expandability and low temperature toughness.

[0059] In the design of the rolling process, the rolling process should be completed as fast as possible in the rough rolling and finish rolling stages. After the rolling is completed, the steel is first air-cooled for a specified time. The main purpose of air-cooling is to design the steel with a large amount of manganese and molybdenum, as manganese is an element that stabilizes austenite, and molybdenum significantly delays the transformation of ferrite and pearlite. Therefore, in the process of air-cooling for a specified time, the rolled deformed austenite undergoes dynamic recrystallization and relaxation without transformation, that is, without forming a ferrite structure. When the deformed austenite undergoes dynamic recrystallization, it can form austenite with a homogeneous structure and pseudo-equiaxed structure, and after relaxation, the dislocations inside the austenite grains are greatly reduced, and the combination of the two can obtain a martensite with a homogeneous structure in the subsequent water-cooled quenching process. In order to obtain a martensite structure, the water-cooling rate should exceed the critical cooling rate of low-carbon martensite. In the present invention, the water-cooling rate of the strip steel must be 30°C / s or more so that martensite can be obtained in all composition designs.

[0060] Since the microstructure according to the present invention is low carbon or ultra-low carbon martensite, after the rolling is completed, the strip steel may be cooled to below Ms, which is the martensite transformation start point, at a cooling rate exceeding the critical cooling rate. The content of the retained austenite at room temperature also differs depending on the cooling stop temperature. Usually, there is an optimal quenching / cooling stop temperature range, which varies depending on the alloy composition, but is generally between 150 and 350°C. In order to obtain high strength steel with good plasticity and hole expansion ratio, it is necessary to quench the strip steel to a certain temperature range below the Ms point, but according to theoretical calculations and actual tests, if the strip steel is quenched to ≦400°C, a structure with excellent overall performance can be obtained. In addition, when the quenching temperature is ≧400°C, the amount of retained austenite is large, but a bainite structure is generated in the structure, and the strength requirement of 980 MPa or more cannot be met. For the above reasons, it is necessary to control the coiling temperature to ≦400°C. Based on such innovative components and process design concepts, the present invention makes it possible to obtain an ultra-low carbon martensitic high hole expandability steel at the 980 MPa level that has excellent properties such as strength, plasticity, toughness, cold bending, and hole expandability.

[0061] In some embodiments, since the microstructure according to the present invention is low-carbon tempered martensite, after rolling, the strip steel is cooled to room temperature at a cooling rate exceeding the critical cooling rate. In the subsequent bell-type annealing process, the bell-type annealing temperature and time are controlled within a predetermined range. In this manner, an ultra-high strength hole expandability steel having uniform properties such as strength, plasticity, and hole expandability can be obtained.

[0062] In the bell-type annealing process, the steel coil is first heated to 100-300°C at a rate of ≧20°C / s, and then kept at that temperature for a long period of time, 12-48 hours, to homogenize the temperature throughout the steel coil and contribute to stabilizing the structure and performance. The lower the temperature is, the longer the time is; conversely, the higher the temperature is, the shorter the time is. Finally, the steel coil is cooled to below 100°C at a cooling rate of ≦50°C / s, and then the steel is tapped from the bell-type annealing furnace and allowed to cool naturally.

[0063] Generally, the bell-shaped annealing temperature and the bell-shaped annealing time are inversely proportional to each other. The lower the bell-shaped annealing temperature, the longer the bell-shaped annealing time; conversely, the higher the bell-shaped annealing temperature, the shorter the bell-shaped annealing time. If the bell-shaped annealing temperature is less than 100°C, the strength will be high, the hole expansion ratio will be low, and the hole expansion ratio of 30% or more will not be reached; if the bell-shaped annealing temperature is more than 300°C, it will be difficult to meet the requirement of strength ≧ 1180MPa. Therefore, the bell-shaped annealing temperature is between 100 and 300°C. It is because of the adoption of the high silicon composition design concept that in the low-temperature bell-shaped annealing process, silicon can effectively inhibit the formation of cementite in the steel, and also contribute to the diffusion of carbon atoms from martensite to retained austenite, further improving the stability of retained austenite, and giving the strip steel higher elongation and better forming performance than high-strength steel of the same strength level.

[0064] The advantageous effects of the present invention are: (1) By adopting a relatively economical composition design concept and an innovative cooling process route, a high hole expandability steel with excellent properties such as strength, plasticity, toughness, cold bending, and hole expandability at the 980 MPa level was obtained; (2) The steel coil or steel plate has excellent strength, plasticity and toughness compatibility, and also has good cold bending performance and hole expansion and flanging performance. Its yield strength is ≥ 800MPa, tensile strength is ≥ 980MPa, and good elongation (transverse A 50 It also has a bending toughness (d≦4a, 180°) and has passed the cold bending performance (d≦4a, 180°) test and hole expandability (hole expansion ratio ≧30%). It can be used in the manufacture of parts such as automobile chassis and subframes that require high strength, thinning, hole expansion, and flanging, and is expected to have a very wide range of applications. [Brief description of the drawings]

[0065] [Figure 1] FIG. 1 is a process flow chart of a method for producing an ultra-low carbon martensitic high hole expandability steel of 980 MPa level according to the present invention. [Diagram 2] FIG. 2 is a conceptual diagram of a rolling process in a manufacturing method of an ultra-low carbon martensite steel with high hole expandability at a strength of 980 MPa according to the present invention. [Diagram 3] FIG. 3 is a conceptual diagram of a cooling process in a manufacturing method of an ultra-low carbon martensitic steel with high hole expandability of 980 MPa level according to the present invention. [Figure 4] FIG. 4 is a process flow chart of a method for producing high plasticity, high hole expandability steels of 1180 MPa level according to Preparation Examples II and III of the present invention. [Diagram 5] FIG. 5 is a conceptual diagram of a rolling process in a manufacturing method of a high plasticity, high hole expandability steel of 1180 MPa level according to Preparation Example II of the present invention. [Figure 6] FIG. 6 is a conceptual diagram of a cooling process in a manufacturing method of a high plasticity, high hole expandability steel of 1180 MPa level according to Preparation Example II of the present invention. [Figure 7] FIG. 7 is a conceptual diagram of a bell-type annealing process in a manufacturing method of high plasticity and high hole expandability steel of 1180 MPa level according to Preparation Examples II and III of the present invention. [Figure 8] FIG. 8 is a typical metal structure photograph of Example 10 of the high hole expandability steel according to the present invention. [Figure 9] FIG. 9 is a typical metal structure photograph of Example 12 of the high hole expandability steel according to the present invention. [Figure 10] FIG. 10 is a typical metal structure photograph of Example 14 of the high hole expandability steel according to the present invention. [Figure 11] FIG. 11 is a typical metal structure photograph of Example 16 of the high hole expandability steel according to the present invention. [Figure 12] FIG. 12 is a conceptual diagram of a rolling process in a manufacturing method of a high plasticity, high hole expandability steel of 1180 MPa level according to Preparation Example III of the present invention. [Figure 13] FIG. 13 is a conceptual diagram of a cooling process in a manufacturing method of a high plasticity, high hole expandability steel of 1180 MPa level according to Preparation Example III of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0066] In each of the following examples, the tensile properties (yield strength, tensile strength, elongation) were measured according to the ISO 6892-2-2018 international standard, the hole expansion ratio was measured according to the ISO 16630-2017 international standard, the -40 ° C. impact toughness was measured according to the ISO 14556-2015 international standard, and the cold bend performance was measured according to the ISO 7438-2005 international standard.

[0067] Preparation example I Referring to FIG. 1 to FIG. 3, the method for producing ultra-low carbon martensitic high hole expandability steel of 980 MPa level according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100-1200°C and a heat retention time of 1-2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high pressure at 950°C or more, and the cumulative deformation amount is ≧50%; then, the intermediate billet temperature is 920-950°C, and finally, 3-5 passes are performed for rolling, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-920°C; 4) Cooling First, air-cool for 0-10 seconds to achieve dynamic recovery and dynamic recrystallization, then water-cool the strip steel at a cooling rate of ≥50°C / s to a temperature below the Ms point (between room temperature and the Ms point), coil it, and then cool it to room temperature (at a cooling rate of ≤20°C / h); 5) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 100 m / min, and the pickling temperature is controlled in the range of 75 to 85°C. Elongation in tension leveling process The content of the steel strip is controlled to be ≦2%, and the steel strip is rinsed at a temperature range of 35-50°C, and the surface of the steel strip is dried at a temperature range of 120-140°C and oiled.

[0068] The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 1, and the production process parameters of the example of the steel according to the present invention are shown in Tables 2 and 3, where the thickness of the steel billet in the rolling process is 120 mm; the mechanical properties of the steel plate according to the example of the present invention are shown in Table 4.

[0069] As can be seen from Table 4, the steel coils all have a yield strength of ≥ 800 MPa, a tensile strength of ≥ 980 MPa, an elongation generally between 8% and 13%, an impact energy is relatively stable, and the low-temperature impact energy at -40°C is stable at 140-180 J, the residual austenite content generally varies between 1.5% and 5% according to the coiling temperature, and the hole expansion ratio meets ≥ 50%.

[0070] As can be seen from the above examples, the 980 MPa high strength steel of the present invention has good strength, plasticity, toughness and hole expandability, and is particularly suitable for manufacturing parts such as control arms and other parts for automobile chassis that require high strength, thinning, hole expanding and flanging. It can also be applied to parts such as wheels that require hole flanging, and its wide range of applications is expected.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] [Table 4]

[0075] Preparation Example II Referring to FIG. 4 to FIG. 7, the manufacturing method of the high plasticity high hole expandability steel of 1180 MPa level according to the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Heat the billet or ingot to 1100-1200℃ and keep it at that temperature for 1-2 hours. Reheat with; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high rolling pressure at 950°C or higher, and the cumulative deformation amount is ≧50%; then, 3-7 passes are performed, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-950°C; 4) Cooling First, air-cool for 0-10 seconds, then water-cool the steel strip to room temperature at a cooling rate of ≥30°C / s, and then coil it; 5) Annealing Annealing by bell annealing, the heating rate is ≧20℃ / h, the bell annealing temperature is 100-300℃, and the bell annealing time is 12-48h; the steel plate is cooled to below 100℃ at a cooling rate of ≦50℃ / h, and then tapped; 6) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 90 m / min, and the pickling temperature is controlled in the range of 75 to 85°C. Elongation in tension leveling process The content is controlled to be ≦1.5%, rinsed at a temperature range of 35-50°C, and the surface is dried at a temperature range of 120-140°C and oiled.

[0076] The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 5, and the production process parameters of the example of the steel according to the present invention are shown in Tables 6 and 7, where the thickness of the steel billet in the rolling process is 120 mm; the mechanical properties of the steel plate according to the example of the present invention are shown in Table 8.

[0077] As can be seen from Table 8, the steel coils all have a yield strength of ≥ 900 MPa, a tensile strength of ≥ 1180 MPa, an elongation generally between 10-13%, a relatively stable impact energy, a stable impact energy at -40°C at 60-100 J, a residual austenite content that varies with the coiling temperature, and a hole expansion ratio of ≥ 30%.

[0078] As can be seen from the above examples, the 1180 MPa high hole expandability steel of the present invention has good strength, plasticity, toughness and hole expandability compatibility, and is particularly suitable for manufacturing parts such as control arms and other parts for automobile chassis that require high strength, thinning and hole expanding / flanging. It can also be applied to parts such as wheels that require hole flanging, and a wide range of applications are expected.

[0079] 8 to 11 show typical metal structures of the steel sheets according to Examples 10#, 12#, 14# and 16#, respectively. As can be seen from the metal structure photographs, the structure is a single-phase low-carbon martensite with a certain amount of retained austenite, and exhibits higher elongation and hole expansion ratio at the same strength level.

[0080] [Table 5]

[0081] [Table 6]

[0082] [Table 7]

[0083] [Table 8]

[0084] Preparation example III Referring to Figures 4, 7, 12 and 13, the manufacturing method of the 1180 MPa level high plasticity high hole expandability steel of the present invention includes the following steps: 1) Smelting and casting Smelting in a converter or electric furnace according to the composition described, followed by secondary refining in a vacuum furnace, then casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high rolling pressure at 950°C or higher, and the cumulative deformation amount is ≧50%; then, the intermediate billet temperature is 900-950°C, and 3-7 passes are performed after that, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-900°C; 4) Cooling First, air-cool for 0-10 seconds, then water-cool the steel sheet at a cooling rate of ≥30°C / s to a temperature below the Ms point, roll it up, and then slowly cool it to room temperature (at a cooling rate of ≤20°C / h); 5) Annealing Anneal by bell annealing, the heating rate is ≧20℃ / h, the bell annealing temperature is 100~300℃, and the bell annealing time is 12~48h; the steel plate is cooled to ≦100℃ at a cooling rate of ≦50℃ / h and then tapped; 6) Pickling The pickling speed of the steel strip is adjusted in the range of 30 to 90 m / min, and the pickling temperature is controlled in the range of 75 to 85°C. Elongation in tension leveling processThe content of the steel strip is controlled to be ≦1.5%, and the steel strip is rinsed at a temperature range of 35-50°C, and the surface of the steel strip is dried at a temperature range of 120-140°C and oiled.

[0085] The composition of the example of the high hole expandability steel according to this preparation example is shown in Table 9, and the production process parameters of the example of the steel according to the present invention are shown in Tables 10 and 11, where the thickness of the steel billet in the rolling process is 120 mm; the mechanical properties of the steel plate according to the example of the present invention are shown in Table 12.

[0086] As can be seen from Table 12, the steel coils all have a yield strength of ≥ 900 MPa, a tensile strength of ≥ 1180 MPa, an elongation generally between 10-13%, a relatively stable impact energy, a stable impact energy at -40°C at 80-110 J, a residual austenite content that varies with the coiling temperature, and a hole expansion ratio of ≥ 30%.

[0087] As can be seen from the above examples, the 1180 MPa high strength steel of the present invention has good strength, plasticity, toughness and hole expandability, and is particularly suitable for manufacturing parts such as control arms and other parts for automobile chassis that require high strength, thinning, hole expanding and flanging. It can also be applied to parts such as wheels that require hole flanging, and its wide range of applications is expected.

[0088] [Table 9]

[0089] [Table 10]

[0090] [Table 11]

[0091] [Table 12]

Claims

1. A low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more, the chemical composition of which is, in weight percentage, C 0.03 to 0.10%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P ≦ 0.02%, S ≦ 0.003%, Al 0.02 to 0.08%, N ≦ 0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O ≦ 0.0030%, and the balance being Fe and other unavoidable impurities. The microstructure of the low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more is martensite or tempered martensite and retained austenite, provided that the volume percentage of the retained austenite in the microstructure is ≦5%, The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more has a yield strength of ≧800 MPa, a tensile strength of ≧980 MPa, a lateral elongation A 50 ≧8%, and a hole expansion ratio of ≧30%.

2. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 1, wherein the chemical composition is, in weight percentage, C 0.03 to 0.06%, Si 0.5 to 2.0%, Mn 1.0 to 2.0%, P ≦ 0.02%, S ≦ 0.003%, Al 0.02 to 0.08%, N ≦ 0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O ≦ 0.0030%, and the balance is Fe and other inevitable impurities.

3. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 1, wherein the chemical composition is, in weight percentage, C 0.06 to 0.10%, Si 0.8 to 2.0%, Mn 1.5 to 2.0%, P ≦ 0.02%, S ≦ 0.003%, Al 0.02 to 0.08%, N ≦ 0.004%, Mo 0.1 to 0.5%, Ti 0.01 to 0.05%, O ≦ 0.0030%, and the balance is Fe and other unavoidable impurities.

4. Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, Ni≦0.5%. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to any one of claims 1 to 3, further comprising one or more elements selected from the group consisting of Cr≦0.5%, B≦0.002%, Ca≦0.005%, Nb≦0.06%, V≦0.05%, Cu≦0.5%, and Ni≦0.5%.

5. A low-carbon martensitic high hole expandability hot-rolled steel plate having a tensile strength of 980 MPa or more as described in claim 4, wherein the chemical composition satisfies one or more of the following characteristics. The Cr content is 0.2-0.4%; The content of B is 0.0005 to 0.0015%; The Ca content is ≦0.002%; The Nb content is ≦0.03%; The content of V is ≦0.03%; The Cu content is ≦0.3%; The Ni content is ≦0.3%.

6. The C content is 0.04 to 0.055%, the Si content is 0.8 to 1.4%, the Mn content is 1.4 to 1.8%, the S content is controlled to 0.0015% or less, the Al content is 0.02 to 0.05%, the N content is controlled to 0.003% or less, the Ti content is 0.01 to 0.03%, and the Mo content is 0.15 to 0.35%. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 2 or 4, having one or more characteristics selected from the group consisting of.

7. The C content is 0.07 to 0.09%, the Si content is 1.0 to 1.4%, the Mn content is 1.6 to 1.9%, the S content is controlled to 0.0015% or less, the Al content is 0.02 to 0.05%, the N content is controlled to 0.003% or less, the Ti content is 0.01 to 0.03%, and the Mo content is 0.15 to 0.35%. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 3 or 4, having one or more characteristics selected from the group consisting of.

8. The low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 1, wherein the high hole expandability hot rolled steel sheet has a -40 ° C. impact toughness of ≧ 60 J.

9. The low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 2 or 6, wherein the high hole expandability hot rolled steel sheet has a hole expansion ratio of ≧50%.

10. The high hole-expandability hot-rolled steel plate is a low-carbon martensitic high hole-expandability hot-rolled steel plate having a tensile strength of 980 MPa or more as described in claim 9, wherein the high hole-expandability hot-rolled steel plate has a -40°C impact toughness of ≧140J.

11. The low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more has a yield strength of 800 to 890 MPa, a tensile strength of 980 to 1150 MPa, and a transverse elongation A ５０ The low-carbon martensite high hole expandability hot-rolled steel sheet has a diameter of 8 to 13%, a hole expansion ratio of 50 to 85%, a -40 ° C. impact toughness of 140 to 185 J, and a tensile strength of 980 MPa or more. The microstructure of the low-carbon martensite high hole expandability hot-rolled steel sheet is martensite and retained austenite, and the volume percentage of the retained austenite in the microstructure is ≦5%. The low-carbon martensite high hole expandability hot-rolled steel sheet according to claim 9,

12. The high hole expandability hot rolled steel sheet has a yield strength of ≧900 MPa, a tensile strength of ≧1180 MPa, and a transverse elongation A ５０ The low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 3 or 7, wherein the tensile strength is ≧ 10%.

13. The high hole-expandability hot-rolled steel plate having a tensile strength of 980 MPa or more as described in claim 12, wherein the high hole-expandability hot-rolled steel plate has a -40°C impact toughness of ≧60J.

14. The high hole expandability hot-rolled steel sheet has a yield strength of 900 to 1000 MPa, a tensile strength of 1200 to 1280 MPa, a lateral elongation of 10 to 13%, a hole expansion ratio of 30 to 50%, and a −40 ° C. impact toughness of 60 to 100 J. The low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 12.

15. A low carbon martensite high hole expandability hot rolled steel plate having a tensile strength of 980 MPa or more as described in claim 14, wherein the microstructure of the high hole expandability hot rolled steel plate is tempered martensite and retained austenite, and the volume percentage of the retained austenite in the microstructure is ≦5%.

16. A method for producing a low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to any one of claims 1 to 15, comprising the following steps. 1) Smelting and casting smelting in a converter or electric furnace according to the composition according to any one of claims 1 to 7, followed by secondary refining in a vacuum furnace, followed by casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high rolling at 950°C or more, and the cumulative deformation amount is ≧50%; then, 3-7 passes are performed in the final rolling, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-950°C; optionally, after 3-5 passes under high rolling, the temperature of the intermediate billet is 900-950°C, and then 3-7 passes are performed on the rolling; 4) Cooling First, air-cooling is performed for 0-10 seconds, then the steel strip is water-cooled at a cooling rate of ≧50° C. / s until the temperature is between room temperature and the Ms point, and then coiled, and then cooled to room temperature; alternatively, air-cooling is performed for 0-10 seconds, then the steel strip is water-cooled directly to room temperature at a cooling rate of ≧30° C. / s, and then coiled; alternatively, air-cooling is performed for 0-10 seconds, then the steel strip is water-cooled at a cooling rate of ≧30° C. / s until the temperature is below the Ms point, which is the martensitic transformation starting point, and then coiled, and then slowly cooled to room temperature; 5) Pickling The pickling speed of the strip is adjusted in the range of 30-100 m / min, the pickling temperature is controlled between 75-85°C, and the elongation in the tension leveling process is controlled to be ≦2%, and then rinsed, the strip surface is dried and oiled.

17. 17. The method for producing a low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16, wherein after the pickling in step 5), the steel sheet is rinsed in a temperature range of 35 to 50 ° C, and the surface is dried and oiled between 120 and 140 ° C. The method for producing a low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16.

18. Between steps 4) and 5), step 4-1): Annealing with bell-type annealing, with a heating rate of ≧20 ° C. / h, a bell-type annealing temperature of 100 to 300 ° C., and a bell-type annealing time of 12 to 48 h; The method for producing a low-carbon martensite high hole expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16 or 17, which includes cooling the steel sheet to ≦100 ° C. at a cooling rate of ≦50 ° C. / h and tapping.

19. The manufacturing method of a low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16 or 17, comprising the following steps. 1) Smelting and casting smelting in a converter or electric furnace according to the composition as defined in claim 2, 4 or 6, followed by secondary refining in a vacuum furnace, followed by casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high pressure at 950°C or more, and the cumulative deformation amount is ≧50%; then, the intermediate billet temperature is 920-950°C, and finally, 3-5 passes are performed for rolling, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-920°C; 4) Cooling First, air-cool for 0 to 10 seconds to achieve dynamic recovery and dynamic recrystallization, then water-cool the steel strip at a cooling rate of ≧50° C. / s to a temperature between room temperature and the Ms point, coil it, and then cool it to room temperature; 5) Pickling The pickling speed of the strip steel is adjusted in the range of 30-100 m / min, the pickling temperature is controlled between 75-85°C, and the elongation rate in the tension leveling process is controlled to be ≦2% so as to reduce the elongation loss of the strip steel, and then rinsed, the strip steel surface is dried and oiled.

20. A method for manufacturing a low-carbon martensitic high hole-expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more as described in claim 19, comprising one or more of the following features: In step 3), 3 to 5 passes are performed at 950° C. or higher under atmospheric pressure, and the cumulative deformation amount is ≧60%; In step 3), rolling is performed 3 to 5 passes, and the cumulative deformation amount is ≧85%; In step 4), the strip is water-cooled at a cooling rate of 50-85° C. / s.

21. The manufacturing method of a low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16 or 17, comprising the following steps. 1) Smelting and casting smelting in a converter or electric furnace according to the composition as defined in claim 3, 4 or 7, followed by secondary refining in a vacuum furnace, followed by casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high pressure at 950°C or higher, and the cumulative deformation amount is ≧50%; then, 3-7 passes are performed, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-950°C; 4) Cooling First, air-cool for 0-10 seconds, then water-cool the strip to room temperature at a cooling rate of ≧30° C. / s and coil it; 5) Annealing Annealing by bell annealing, the heating rate is ≧20° C. / h, the bell annealing temperature is 100-300° C., and the bell annealing time is 12-48 h; cooling the steel sheet to ≦100° C. at a cooling rate of ≦50° C. / h and tapping; 6) Pickling The pickling speed of the strip steel is adjusted in the range of 30-90 m / min, the pickling temperature is controlled between 75-85°C, the elongation in the tension leveling process is controlled to be ≦1.5%, rinsed in the temperature range of 35-50°C, and the surface is dried and oiled between 120-140°C.

22. A method for manufacturing a low-carbon martensitic high hole-expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more as described in claim 21, comprising one or more of the following features: In step 3), 3 to 5 passes are performed at 950° C. or higher under atmospheric pressure, and the cumulative deformation amount is ≧60%; In step 3), rolling is performed 3 to 7 passes, and the cumulative deformation amount is ≧85%; In step 4), the strip is water-cooled at a cooling rate of 30-65° C. / s; In step 5), bell-shaped annealing is carried out at a heating rate of 20-40° C. / h; In step 5), the steel plate is cooled to ≦100° C. at a cooling rate of 15-50° C. / h.

23. The manufacturing method of a low carbon martensite high hole expandability hot rolled steel sheet having a tensile strength of 980 MPa or more according to claim 16 or 17, comprising the following steps. 1) Smelting and casting smelting in a converter or electric furnace according to the composition as defined in claim 3, 4 or 7, followed by secondary refining in a vacuum furnace, followed by casting into billets or ingots; 2) Reheat the billet or ingot at a heating temperature of 1100 to 1200°C and a heat retention time of 1 to 2 hours; 3) Hot rolling The rolling start temperature is 950-1100°C, and 3-5 passes are performed under high rolling pressure at 950°C or more, and the cumulative deformation amount is ≧50%; then, the intermediate billet temperature is 900-950°C, and 3-7 passes are performed after that, and the cumulative deformation amount is ≧70%; the rolling end temperature is 800-900°C; 4) Cooling First, air-cooling is performed for 0 to 10 seconds, then the steel sheet is water-cooled at a cooling rate of ≧30° C. / s to a certain temperature below the Ms point, which is the martensite transformation start point, and then coiled, and then slowly cooled to room temperature; 5) Annealing Annealing by bell annealing, the heating rate is ≧20° C. / h, the bell annealing temperature is 100-300° C., and the bell annealing time is 12-48 h; cooling the steel sheet to ≦100° C. at a cooling rate of ≦50° C. / h and tapping; 6) Pickling The pickling speed of the steel strip is adjusted in the range of 30-90 m / min, the pickling temperature is controlled between 75-85°C, and the elongation rate in the tension leveling process is controlled to be ≦1.5% so as to reduce the elongation loss of the steel strip, and then rinsed, the steel strip surface is dried and oiled.

24. A method for producing a low-carbon martensitic high hole-expandability hot-rolled steel sheet having a tensile strength of 980 MPa or more as described in claim 23, comprising one or more of the following steps. In step 3), 3 to 5 passes are performed at 950° C. or higher under atmospheric pressure, and the cumulative deformation amount is ≧60%; In step 3), rolling is performed 3 to 7 passes, and the cumulative deformation amount is ≧85%; In step 4), the strip is water-cooled at a cooling rate of 30-70° C. / s; In step 5), bell-shaped annealing is carried out at a heating rate of 20-50° C. / h; In step 5), the steel plate is cooled to ≦100° C. at a cooling rate of 25-50° C. / h.

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