Automotive structural steel with yield strength ≥ 1000 MPa and method for manufacturing the same

Abstract

The present invention relates to a steel for automobile structures with a yield strength ≥ 1000 MPa and a manufacturing method thereof. Its composition is in weight percentage: C 0.15 - 0.23%; Si 0.12 - 0.5%; Mn 1.6 - 2.4%; B 0.001 - 0.004%; Al 0.01 - 0.04%; Cr 0.05 - 0.5%; Mo 0.15 - 0.5%; P ≤ 0.015%; S ≤ 0.005%; the balance contains Fe and other inevitable impurities, and satisfies the relationship of the characteristic value Bs of the formation rate of lower bainite = 0.6 - 1.4, Bs = (Mo + B * 100 - Mn / 5) / C; the lower bainite in its microstructure is ≥ 95%. The steel for automobile structures has a yield strength ≥ 1000 MPa, a tensile strength ≥ 1180 MPa, an elongation rate ≥ 7%, and a 180° bending performance d ≤ 3.5T. The steel for automobile structures obtained by the present invention not only has ultra-high strength but also has excellent forming performance, especially bending forming performance, and is particularly useful for the manufacture of automobile chassis system parts.

Description

Technical Field

[0001] Technical Field The present invention relates to steel for automobiles, and particularly to automotive structural steel having a yield strength ≥ 1000 MPa and a method for manufacturing the same. Background Art

[0002] Background Art In accordance with the "green and safe" development concept of new-generation automobiles, the strength required for automotive structural components has become increasingly higher. This allows the thickness of automotive structural components to be substantially reduced, enabling a substantial reduction in the weight of the entire vehicle. This not only achieves the green development goal of "carbon dioxide emission reduction", but also improves the maneuverability of the entire vehicle, shortens the braking distance, and enhances the safety of automobiles. Summary of the Invention Problems to be Solved by the Invention

[0003] For automotive structural components at the current stage, the hot-rolled or pickled steel plates and strip steels used therein do not have high strength, and generally have a tensile strength of about 800 MPa, so there is an urgent need to further improve the strength of the steel materials used. Of course, it is possible to further improve the strength of steel plates and strip steels by cold-rolling hot-rolled or pickled steel plates and then re-annealing them. However, on the one hand, there are still many automotive structural components that require the use of hot-rolled materials or pickled materials with a relatively large plate thickness; on the other hand, the manufacturing process of cold rolling followed by re-annealing increases the manufacturing cost and carbon emission of steel plates and strip steels. Therefore, the present invention is an inventive design that mainly focuses on improving the strength of hot-rolled materials or pickled materials for automobiles.

[0004] In conventional hot-rolled and pickled steel plates and strip steels, there are two measures for improving the strength of materials: 1. By introducing a large amount of martensite or retained (metastable) austenite into the steel sheet structure, a transformation from retained (metastable) austenite to martensite is induced in the material due to deformation during forming.

[0005] For example, Chinese patent applications CN200610025065.7 and CN201210461655.X both disclose hot-rolled high-strength steel with a martensitic matrix and a method for manufacturing it, achieving tensile strengths of 1150 MPa, and even over 1400 MPa. While the strength of steel sheets and strips can be significantly improved by introducing a large amount of martensite, martensite has low plasticity and toughness. Therefore, even if the overall average plasticity and toughness of the material can be improved by tempering or introducing other soft phases (e.g., ferrite), in localized areas, particularly at the interface between the martensite phase and other phases, the strength / hardness difference between the martensite phase and the surrounding phase is too large. As a result, local plasticity and toughness are inferior, the bending performance and hole-expanding / flaming performance of steel sheets and strips are not high, and cracking is likely to occur at the interface between the martensite phase and the surrounding phase. Furthermore, in order to obtain a sufficient amount of martensite or retained austenite, high levels of carbon, manganese, and silicon are often added to steel sheets and strips, but this degrades the weldability, surface paintability, and surface color of the steel sheets.

[0006] 2. A means of inducing the large-scale precipitation of microalloy carbides or carbonitrides to improve the strength of hot-rolled or pickled steel sheets or strips.

[0007] For example, Chinese patent application CN201610268167.5 describes a method for obtaining hot-rolled steel sheets and strips with a tensile strength of 1180 MPa or higher by precipitation of microalloy carbides or carbonitrides. However, this method still has two problems: on the one hand, the large-scale precipitation of microalloy carbides or carbonitrides means that large amounts of microalloy elements such as Ti, V, and Nb must be added to the steel, and the cost of these alloying elements is very high, so adding them in large quantities increases the manufacturing cost of large-scale production of the product; on the other hand, the large-scale precipitation of microalloy carbides or carbonitrides is unfavorable to the bending performance of the steel sheet, and when the material is bent, cracks are likely to occur at the points where microalloy carbides or carbonitrides are concentrated, or at the interface between the precipitates and the matrix. Furthermore, the above elements also form coarse, sharp nitrides (e.g., TiN) together with nitrogen, which are prone to causing bending cracks in steel sheets and strips. [Means for solving the problem]

[0008] Content of the invention The object of the present invention is to provide an automotive structural steel having a yield strength of ≥1000 MPa and a method for manufacturing the same, wherein the automotive structural steel has a tensile strength of ≥1180 MPa while also having good bending flanging performance, and the automotive structural steel has a yield strength of ≥1000 MPa, a tensile strength of ≥1180 MPa, an elongation of ≥7%, and a 180° bending performance d of ≤3.5T, making it particularly useful for automotive chassis structural components.

[0009] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows. The automotive structural steel according to the present invention is bainite steel, and without introducing martensite or introducing and precipitation large amounts of carbides, the microstructure of the steel sheet or strip is controlled to be substantially pure lower bainite (the area ratio of lower bainite in the microstructure is ≥95%), thereby achieving a tensile strength of ≥1180 MPa while also possessing good bending flanging performance.

[0010] In steel plates and strips, to achieve a yield strength of ≥1000 MPa and a tensile strength of ≥1180 MPa, it is necessary to increase the proportion of lower bainite and improve the strength or hardness of the lower bainite. However, due to limitations in the hot rolling production process and equipment (e.g., short laminar flow cooling roller conveyors, fast rolling production speeds, etc.), the time available for lower bainite transformation is relatively short. Therefore, in the design of the present invention, it is necessary to consider accelerating the lower bainite transformation rate, shortening the lower bainite formation time, and expanding the bainite phase region in order to form as much lower bainite as possible as quickly as possible.

[0011] Specifically, the automotive structural steel according to the present invention, having a yield strength of ≥1000 MPa, contains each chemical element in the following weight percentages: C: 0.15~0.23%; Si: 0.12~0.5%; Mn: 1.6~2.4%; B: 0.001~0.004%; Al: 0.01~0.04%; Cr; 0.05~0.5%; Mo: 0.15~0.5%; P: ≤ 0.015%; S: ≤ 0.005%; contains, The remainder contains Fe and other unavoidable impurities, and satisfies the characteristic values ​​of the lower bainite formation rate Bs = 0.6 ~ 1.4 and the relationship Bs = (Mo + B * 100 - Mn / 5) / C; The area ratio of lower bainite in the microstructure of the aforementioned automotive structural steel is ≥ 95%.

[0012] Furthermore, the automotive structural steel according to the present invention may further contain at least one of Ti, V, and Nb, and the weight percentage of the elements must satisfy Ti+Nb+V≦0.03%, preferably Ti+Nb+V≦0.006%.

[0013] Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of lower bainite is ≥ 95%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.2%; Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of lower bainite is ≥ 95%, the area ratio of ferrite + carbonitride precipitate + granular bainite is ≤ 5%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.01%.

[0014] The aforementioned automotive structural steel has a yield strength of ≥1000 MPa, a tensile strength of ≥1180 MPa, an elongation of ≥7%, and a 180° bending performance d of ≤3.5T, preferably d ≤3T.

[0015] In the compositional design of automotive structural steel according to the present invention: The carbon (C) element primarily controls the microstructure phase transition in steel, the hardness / strength of the lower bainite, the time required for lower bainite formation, the martensite formation temperature (Ms), and the large-scale precipitation of microalloy carbides or carbonitrides, thereby influencing the material's mechanical properties. If the carbon content in steel is <0.15%, the steel's strength will not meet the target requirements, or the Ms will be too high, causing martensite to form. In this case, although the strength is high, the bending performance will be significantly degraded. On the other hand, if the carbon content in steel is >0.23%, it tends to lead to excessively high strength or excessive carbides, resulting in a decrease in the plasticity and bending performance of the steel sheet. Therefore, in this invention, the carbon content is controlled to 0.15-0.23%, for example, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, 0.21%, 0.22%, and preferably 0.18-0.21%.

[0016] Si:Si has a certain solid solution strengthening effect, but it affects the surface quality of the steel sheet. When the Si element content in the steel is <0.12%, it becomes difficult to obtain a sufficient strengthening effect. On the other hand, when the Si element content in the steel is >0.5%, oxide scale and tiger stripe-like color differences are easily formed on the surface of the steel sheet after pickling, which is unfavorable for the surface quality of automotive steel sheets. Therefore, in this invention, the Si content is controlled to 0.12-0.5%, for example, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, and 0.45%.

[0017] Mn: The element Mn affects the hardenability of steel sheets and influences the formation of martensite and lower bainite. The higher the Mn content, the lower the Ms point. However, Mn acts as a drag on the diffusion of C atoms, prolonging the time required for lower bainite formation. Therefore, the Mn content in the steel should not be too high; otherwise, pure lower bainite cannot be obtained. However, if the Mn content is too low, the Ms point temperature rises, which is also unfavorable for the formation of a pure lower bainite structure. Accordingly, in this invention, the Mn content is controlled to 1.6-2.4%, for example, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, and 2.3%, and preferably 1.8-2.2%.

[0018] B: The element B can increase the hardness of the lower bainite and, in synergy with the element Mo, can shorten the lower bainite formation time. However, if the B content is too high, brittle borides are more likely to form, affecting the plasticity and bending performance of the steel sheet. Therefore, in this invention, the B content is controlled to 0.001 to 0.004%, for example, 0.0015%, 0.002%, 0.0025%, 0.003%, and 0.0035%.

[0019] Al: Aluminum element is added to steel only as a deoxidizing element, which can remove oxygen element from steel to ensure the performance and quality of steel. However, an excessively high content of Al element will cause cost surge and greatly increase the difficulty of continuous casting production. Therefore, in the present invention, the content of Al is controlled to 0.01 to 0.04%, for example, 0.015%, 0.02%, 0.025%, 0.03%, 0.035%.

[0020] Cr: Cr is mainly used to facilitate the obtainment of lower bainite in steel by expanding the bainite phase region. At the same time, the strength of the steel sheet can be further improved through solid solution strengthening. However, Cr and C can form carbides, and an excessively high Cr content is disadvantageous to the plasticity and bending performance of the steel sheet. Therefore, in the present invention, the content of Cr is controlled to 0.05 to 0.50%, for example, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, preferably 0.15 to 0.35%.

[0021] Mo: Mo can facilitate the obtainment of lower bainite in steel by expanding the bainite phase region, and can also shorten the formation time of lower bainite through a synergistic effect with B element. In addition, Mo can further improve the strength of the steel sheet through solid solution strengthening, or through precipitation by forming carbides or carbonitrides. However, an excessively high Mo content will cause coarsening of carbides in steel, which is disadvantageous to the bending performance of the steel sheet. Therefore, in the present invention, the content of Mo is controlled to 0.15 to 0.50%, for example, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, preferably 0.15 to 0.38%.

[0022] Among the above elements, Mo, B and Mn all affect the formation rate (or formation time) of lower bainite. In order to ensure that the area fraction of lower bainite is ≧95%, it is further necessary to control the weight percentages of Mo, B, Mn and C to satisfy Bs=(Mo+B*100-Mn / 5) / C and 0.6≦Bs≦1.4, wherein Bs is a characteristic value representing the formation rate of lower bainite. When Mo and B elements are added in combination, the bainite transformation C curve can be greatly shifted to the left, so these two elements contribute positively to accelerating the formation rate of lower bainite; in addition, the formation of lower bainite is also affected by the diffusion rate of carbon element. Generally, the faster the diffusion rate of carbon atoms, the faster the formation rate of lower bainite. Since Mn element has a dragging effect on the diffusion of carbon atoms, it contributes negatively to accelerating the formation rate of lower bainite. Similarly, the diffusion rate of carbon atoms is positively correlated with the formation rate of bainite, so the above formula needs to be divided by the content of carbon element. If Bs<0.6, the formation rate of lower bainite is too low and the formation time is too long, so that it cannot be effectively ensured that ≧95% of lower bainite is obtained within the controlled cooling time. If Bs>1.4, the formation rate of lower bainite is sufficient, but the deterioration and decomposition of lower bainite and the precipitation of carbides are also positively correlated with the diffusion rate of atoms such as carbon atoms. Therefore, an excessively high Bs value means that lower bainite is prone to deteriorate to form granular bainite, and the precipitation rate of carbonitrides is also too fast. As a result, in steel plates and strip steels, carbides are prone to excessive formation, agglomeration or coarsening, which ultimately leads to deterioration of bending performance. In some embodiments, Bs is 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or within a range between any two of the foregoing values.

[0023] Ti, Nb, and V: While Ti, Nb, and V can be added to steel as optional alloying elements, their addition is not recommended. Ti, Nb, and V can precipitate in large quantities as second-phase precipitates, forming microalloy carbides or carbonitrides, which can further improve the strength of steel sheets. However, if too many or too large microalloy carbides or carbonitrides precipitate, it can conversely degrade the bending performance. Furthermore, because nitrogen is inevitably present in steel, these alloying elements, along with N, form coarse, sharp nitrides (e.g., TiN), further degrading the bending performance of steel sheets and strips. In addition, the addition of these alloying elements increases the cost of the material, so a comprehensive consideration of performance and cost control is necessary. Therefore, in the present invention, the sum of the mass percentages of Nb, Ti, and V is controlled to Ti + Nb + V ≤ 0.03%, for example, 0.001 to 0.03%, and preferably to Ti + Nb + V ≤ 0.006%, for example, 0.001 to 0.006%.

[0024] In some embodiments, the unavoidable impurities include N in a mass percentage of ≤0.005.

[0025] The area ratio of lower bainite in the microstructure of the automotive structural steel according to the present invention is ≥95%. For example, ≥96%, ≥97%, ≥98%, and ≥99%.

[0026] Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of ferrite + carbonitride precipitate + granular bainite is ≤5%, for example, ≤4%, ≤3%, ≤2%, or ≤1%.

[0027] Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤0.2%, for example, ≤0.1%, ≤0.05%, ≤0.02%, or ≤0.01%.

[0028] Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of phases other than lower bainite, ferrite, carbonitride precipitates, and granular bainite is ≤0.2%, for example, ≤0.1%, ≤0.05%, ≤0.02%, and ≤0.01%. In some embodiments, in the microstructure of the automotive structural steel according to the present invention, the phases other than lower bainite, ferrite, carbonitride precipitates, and granular bainite are one or more selected from the group consisting of retained austenite, martensite, tempered martensite, and upper bainite.

[0029] Preferably, in the microstructure of the automotive structural steel according to the present invention, the area ratio of lower bainite is ≥ 95%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.2%; preferably, the area ratio of lower bainite is ≥ 95%, the area ratio of ferrite + carbonitride precipitate + granular bainite is ≤ 5%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.01%.

[0030] In some embodiments, the yield strength of the automotive structural steel according to the present invention is ≥1000 MPa, for example, ≥1020 MPa, ≥1050 MPa, ≥1100 MPa, and ≥1150 MPa.

[0031] In some embodiments, the tensile strength of the automotive structural steel according to the present invention is ≥1180 MPa, for example, ≥1200 MPa, ≥1250 MPa, ≥1300 MPa, and ≥1320 MPa.

[0032] In some embodiments, the elongation of the automotive structural steel according to the present invention is ≥7%, for example, ≥7.5%, ≥8%, ≥8.5%, ≥9%, ≥9.5%, ≥10%, and ≥10.5%.

[0033] In some embodiments, the 180° bending performance of the automotive structural steel according to the present invention is d ≤ 3.5T, for example, d ≤ 3T and d ≤ 2.5T.

[0034] The present invention provides a method for manufacturing automotive structural steel with a yield strength of ≥1000 MPa, which includes the following steps: 1) Smelting, continuous casting The billets are smelted according to the above chemical composition and cast by continuous casting, with a billet cooling rate of ≥ 5K / s during continuous casting. 2) Hot rolling The billet is heated, and the heating temperature at the center point in the width direction of the billet is 1150-1220°C; The total rolling reduction ratio is ≥98%, provided that the reduction ratios for both the first and second passes are ≥60%; the finish rolling outlet temperature is 920-980°C; the billet rolling speed is controlled so that the elapsed time tp when any part of the rolled steel sheet or strip is transported from the finish rolling outlet temperature measurement point to the winding temperature measurement point is ≥(5 / Bs)+4 seconds; 3) Laminar cooling, winding After rolling, the steel strip is rapidly cooled to ≤530°C at a cooling rate of ≥150°C / s, and then further cooled to the coiling temperature at a cooling rate of ≥10°C / s, where the coiling temperature is (Ms + 10°C) ~ 400°C; where the martensitic transformation temperature of the steel strip, Ms = 498.9 - 333.3*(C) - 33.3*(Mn) - 27.8*(Cr) - 16.7*(Ni) - 11.1*(Si + Mo + W), is in °C; For steel plates and strips with Bs < 0.9, after winding, the steel coil is held in the winding machine for a time of ≥ (10 / Bs) + 5 seconds before being unwound. 4) Stacked cooling.

[0035] In some embodiments, in step 1), the billet cooling rate during continuous casting is ≥5K / s, for example, ≥6K / s, ≥7K / s, ≥8K / s, ≥9K / s, ≥10K / s, ≥11K / s, ≥12K / s, ≥13K / s, ≥14K / s, ≥15K / s, ≥16K / s.

[0036] In some embodiments, in step 2), the heating temperature at the center point in the width direction of the billet is within the range of 1150°C, 1160°C, 1170°C, 1180°C, 1190°C, 1200°C, 1210°C, 1220°C, or any two of the above values.

[0037] In some embodiments, in step 2), the reduction ratio of the first pass is ≥60%, for example ≥65%, ≥70%, ≥75%, and the reduction ratio of the second pass is ≥60%, for example ≥65%, ≥70%, ≥75%.

[0038] In some embodiments, in step 2), the finish roll outlet temperature is within the range of 920°C, 930°C, 940°C, 950°C, 960°C, 970°C, 980°C, or any two of the above values.

[0039] Furthermore, the method for manufacturing automotive structural steel having a yield strength of ≥1000 MPa according to the present invention may further include a step 5) of pickling hot-rolled steel sheets or strips to produce pickled sheets.

[0040] Preferably, after rolling in step 3), the steel strip is rapidly cooled to ≤530°C at a cooling rate of ≥180°C / s.

[0041] In some embodiments, the cooling rate used to cool the strip to ≤530°C after rolling in step 3) is within the range of 150°C / s, 160°C / s, 170°C / s, 180°C / s, 190°C / s, 200°C / s, or any two of the above values.

[0042] In some embodiments, the cooling rate of the second stage cooling in step 3) is within the range of 10°C / s, 12°C / s, 14°C / s, 16°C / s, 18°C / s, 20°C / s, or any two of the above values.

[0043] Preferably, for steel plates or strips with Bs ≥ 0.9, after winding, the steel coil is left in the winding machine for a time of ≥ (10 / Bs) + 5 seconds before being unwound.

[0044] Preferably, the stacking method for stacking and cooling in step 4) is to stack the steel coils in a heat-insulating bit at an ambient temperature of ≥280°C after unwinding and keep them warm for 2 to 6 hours; preferably, the steel coils are stacked in a flat position on one side.

[0045] Preferably, the thickness of the steel strip after rolling is ≤4 mm. In the present invention, a method for manufacturing automotive structural steel with a yield strength of 1000 MPa or more: In step 1) above, the billet cooling rate during continuous casting affects the grain size in the final structure of the steel sheet or strip. The grain size ultimately affects the formation rate and time of lower bainite. The smaller the grain size, the faster the lower bainite is formed, and the shorter the required formation time. If the cooling rate is <5K / s, the grains of the billet structure become coarser, which is not only unfavorable for obtaining a fine crystalline structure during subsequent rolling, but also makes it easier for central segregation and zonal structures to form in the finished product. Both central segregation and zonal structures easily form martensite, so the proportion of martensite in the final steel sheet or strip increases significantly, degrading the bending performance of the steel sheet or strip.

[0046] In step 2) above, if the heating temperature is too high, the grain size in the steel tends to coarseen, slowing down the final lower bainite formation rate, resulting in insufficient lower bainite content in the steel sheet or strip. If the heating temperature is too low, the degree of austenitization of the billet is insufficient, which also results in insufficient lower bainite content in the steel sheet or strip. On the other hand, if the finish rolling completion temperature is <920°C, ferrite precipitates before finish rolling, resulting in a low lower bainite content in the final structure. However, considering the billet heating temperature, the finish rolling completion temperature should be controlled to ≤980°C. In order to ensure that the rolled steel sheet or strip has a relatively small grain structure, the overall reduction ratio of the rolling process should be ≥98%, and the reduction ratio for each pass (first and second passes) should be ≥60%. If the reduction ratio is insufficient, it will not be possible to obtain a fine and homogeneous structure, and the final lower bainite formation time will be too long, making it impossible to obtain ≥95% lower bainite. Similarly, in order to complete the lower bainite transformation before unwinding, the rolled steel sheet or strip is tp min The billet rolling speed must be controlled so that ≥ (5 / Bs) + 4 seconds, however tp minThis represents the minimum elapsed time when any point of a rolled steel sheet or strip is transported from the finish rolling outlet temperature measurement point to the winding temperature measurement point. The specific time may vary depending on the billet size and line equipment, but in any case, it is necessary to ensure that as much lower bainite transformation as possible occurs before the steel sheet or strip enters the winding machine, so this time has a high correlation with the characteristic value Bs of the bainite transformation rate. If the time is insufficient, it will be impossible to obtain ≥95% lower bainite.

[0047] In step 3) above, in order to avoid the ferrite and pearlite phase regions, the first stage of cooling must be performed at a cooling rate of ≥150°C / s, preferably ≥180°C / s, to cool to ≤530°C. If the cooling temperature is too high, the formation of upper bainite, granular bainite, and consequently structures such as pearlite and ferrite will be promoted, making it impossible to achieve a lower bainite content of ≥95%. After cooling to ≤530°C, the second stage of cooling is reduced to a cooling rate of ≥10°C / s to cool to the winding temperature. However, the cooling rate should not be too low in order to avoid the formation of granular bainite and upper bainite. Similarly, in order to ensure the formation of ≥95% lower bainite, the winding temperature must be controlled to (Ms + 10°C) ~ 400°C. If the winding temperature is too low, it will lead to the formation of martensite, but if the winding temperature is too high, it will lead to the precipitation of granular bainite, upper bainite, and carbides or carbonitrides. In some embodiments, the winding temperature is (Ms + 20°C) to 400°C. At the same time, for steel sheets and strips with Bs < 0.9, in order to ensure sufficient formation of lower bainite and avoid the formation of martensite or retained austenite, the steel coil needs to be left in the winder for a time of ≥ (10 / Bs) + 5 seconds before being unwound.

[0048] Preferably, for steel plates or strips with Bs≧0.9, after winding, the steel coil may be unwound and cooled in stages without being left in the winding machine, or the steel coil may be left in the winding machine for a time of ≧(10 / Bs)+5 seconds before unwinding; preferably, in order to ensure sufficient formation of lower bainite and further avoid the formation of martensite or retained austenite, the steel coil may be left in the winding machine for a time of ≧(10 / Bs)+5 seconds before unwinding.

[0049] Preferably, the thickness of the rolled steel strip is ≤4 mm. If the steel strip is too thick, the homogeneity of the structure in the thickness direction is poor, which is unfavorable for obtaining ≥95% lower bainite.

[0050] In step 4) above, in order to further avoid the formation of martensite, the steel coil may be unwound and then stacked in a heat-insulating bit at an ambient temperature of ≥280°C in a flat position (vertically) for 2 to 6 hours. However, if the stacking temperature is too high or the time is too long during heat insulation, it will not only induce the precipitation of carbides or carbonitrides, but the lower bainite will decompose to form granular bainite, which is unfavorable for bending performance. In some embodiments, the steel coil is stacked in a heat-insulating bit at an ambient temperature of 280 to 340°C for 2 to 6 hours.

[0051] Compared to the prior art, the advantages and beneficial effects of the present invention are as follows: The present invention aims to obtain steel sheets that combine high strength and high bending performance by forming substantially pure, high-strength lower bainite from steel sheets and strips.

[0052] Conventional martensitic high-strength steels have a microstructure that is mainly martensite or tempered martensite, supplemented by small amounts of other microstructures such as ferrite, bainite, and carbide precipitates, but such martensitic high-strength steels have poor bending performance. In contrast, the substantially pure lower bainite high-strength steel obtained by the present invention has strength equivalent to that of martensitic high-strength steel, while having superior bending performance. Furthermore, according to the literature (Wear Resistance of Medium Carbon Steel with Different Microstructures - PMC (nih.gov)), the fracture toughness of steel sheets mainly composed of bainite microstructure is higher than that of martensitic steel sheets and tempered martensitic steel sheets.

[0053] On the other hand, conventional precipitation-strengthened ultra-high-strength hot-rolled steel sheets and strips all have many microalloy elements such as Nb, Ti, and V added to improve the strength of the steel sheets and strips through the large-scale precipitation of microalloy carbides or carbonitrides.

[0054] In contrast, the present invention avoids the formation of microalloy carbides or carbonitride precipitates by adding large amounts of expensive alloying elements, while also avoiding the adverse effects of large-scale precipitation of microalloy carbides or carbonitrides on bending performance. Therefore, the steel sheets and strips obtained by the present invention have superior bending performance at the same strength level.

[0055] The core problem that this invention aims to solve is how to achieve the formation of a substantially pure, ultra-high-strength or ultra-high-hardness lower bainite structure in hot-rolled or pickled steel sheets and strips. Hot rolling has a short production time and a fast pace, so the time for the temperature control and controlled cooling phases of laminar flow cooling is very short, and the accuracy of temperature control and controlled cooling is also low. However, because the temperature window for lower bainite formation is narrow, the formation temperature becomes low and the lower bainite formation rate becomes slow. As a result, the lower bainite formation temperature, formation rate, and hot-rolling production time are in conflict with each other, making the design and manufacturing process extremely difficult.

[0056] Therefore, in compositional design, on the one hand, the lower the formation temperature, the higher the strength or hardness of the lower bainite. Thus, it is necessary to extend the lower bainite transformation interval as much as possible so that the lower bainite is formed at the lowest possible transformation temperature. However, since the transformation of the lower bainite is related to the diffusion dynamics process of related atoms such as carbon atoms, and the lower the phase transition temperature, the slower the transformation of the lower bainite. Therefore, in compositional design, on the other hand, the mixing ratio of related elements that affect diffusion and rate is rationally designed, and the transformation rate of the lower bainite, i.e., the value of Bs mentioned in this invention, is optimized. This makes it possible to form as much lower bainite as possible in the short time of hot rolling production.

[0057] Of course, in addition to compositional design, the present invention also requires consideration of the formation interval and formation time in the manufacturing process, on the one hand, to form at the lowest possible temperature in order to improve the strength of the product, and on the other hand, to form as much as possible in order to improve bending performance. On the one hand, the invention designs a manufacturing process that can refine the crystal grains, such as increasing the billet cooling rate during continuous casting or increasing the reduction ratio during hot rolling, thereby accelerating atomic diffusion and lower bainite transformation through crystal grain refinement, and on the other hand, optimizing temperature control so that lower bainite is formed at the lowest possible temperature without generating other structures such as martensite or upper bainite. More importantly, the present invention pays particular attention to matching the hot rolling production time with the rate of lower bainite formation, and designs appropriate production times in each process step of hot rolling production so that ≥95% lower bainite is produced in the final structure.

[0058] As a result of the above composition and manufacturing process design, the automotive structural steel obtained by the present invention not only has ultra-high strength but also excellent formability, especially bendability. The automotive structural steel has a yield strength of ≥1000 MPa, a tensile strength of ≥1180 MPa, an elongation of ≥7%, and a 180° bending performance d of ≤3.5T, which can meet the future demand for weight reduction in automotive structures and is particularly useful for chassis system components. [Brief explanation of the drawing]

[0059] [Figure 1] Figure 1 is a microstructure photograph of steel according to Example D1 of the present invention. [Modes for carrying out the invention]

[0060] Specific Embodiments The present invention will be further described below based on examples and drawings.

[0061] Examples and Comparative Examples The automotive structural steels according to the examples and comparative examples of the present invention were all manufactured by the following process: 1) Smelting, continuous casting The billets were cast by continuous casting after smelting according to the composition shown in Table 1, and the billet cooling rate during continuous casting was ≥ 5 K / s. 2) Hot rolling The billet was heated, and the heating temperature at the center point in the billet's width direction was 1150-1220°C; The total rolling reduction ratio was ≥98%, however, the reduction ratios for both the first and second passes were ≥60%; the finish rolling outlet temperature was 920-980°C; the slab rolling speed was controlled so that the elapsed time tp when any part of the rolled steel sheet or strip was transported from the finish rolling outlet temperature measurement point to the winding temperature measurement point was ≥(5 / Bs)+4 seconds; the thickness of the rolled strip was ≤4mm; 3) Laminar cooling, winding After rolling, the steel strip was rapidly water-cooled to ≤530°C at a cooling rate of ≥150°C / s, and then further cooled to the coiling temperature at a cooling rate of ≥10°C / s, with the coiling temperature being (Ms + 10°C) ~ 400°C; where the martensitic transformation temperature of the steel strip, Ms = 498.9 - 333.3*(C) - 33.3*(Mn) - 27.8*(Cr) - 16.7*(Ni) - 11.1*(Si + Mo + W), is in °C; The additional residence time in the winding machine for each example and comparative example is shown in Table 2.

[0062] 4) Stacked cooling In the stacked cooling method, after unwinding, the steel coils were stacked on a heating bit at an ambient temperature of ≥280°C with one side flat, and kept warm for 2 to 6 hours.

[0063] The compositions of the steels in the embodiments and comparative examples of the present invention are shown in Table 1, provided that the remainder consists of Fe and other unavoidable impurities. The process parameters of the steels in the embodiments and comparative examples of the present invention are shown in Table 2, with comparative example data that does not conform to the present invention being underlined.

[0064] In this invention, the methods for measuring yield strength, tensile strength, and elongation are those specified in GB / T228.1-2021, "Tensile Tests of Metallic Materials, Part 1: Room Temperature Test Methods." The 180° bending performance d / T was measured using the bending performance measurement method specified in GB / T232-2010, with a 180° bending test (bending diameter d=1a). The microstructure content was measured by etching the surface of a buffed metallographic sample with a 4% nitric acid-alcohol solution and then examining it with an SEM electron microscope.

[0065] Table 3 shows the performance and microstructure of steel sheets and strips corresponding to the steel in the embodiments and comparative examples of the present invention. Since all of the indicators in the embodiments satisfied the design of the present invention, hot-rolled steel sheets and strips were obtained with a yield strength of ≥1000 MPa, a tensile strength of ≥1180 MPa, an elongation of ≥7%, and a 180° bending performance d of ≤3.5T.

[0066] However, in Examples E1 and E2, due to their high Mn, Cr, and Mo content, the tensile strength of the product reached 1300 MPa, but the elongation at break was relatively low, resulting in a 180° bending performance of d=3.5T.

[0067] Examples D1 and H1 promoted granular bainite transformation and carbonitride precipitation by stacking and slow cooling after winding, but the bending performance did not reach the highest level, with a 180° bending performance of d=3.5T. Examples B1 and F1 also had a 180° bending performance of d=3.5T. However, in Example F1, although the strength was relatively high and the Bs value was relatively low, the rate of lower bainite formation was relatively slow, and martensite was easily formed after winding and unwinding and during stacking and cooling. Therefore, to ensure the formation of lower bainite, the material was left in the winding machine for an additional 33 seconds after winding, but ultimately, about 0.1% of martensite was still formed, leading to a decrease in bending performance. On the other hand, Example B1 also had a similar problem. In Example B1, the Bs value was ≥0.9, which was relatively appropriate, but because it was not left in the winding machine for an additional time after winding, ultimately, about 0.15% of martensite was still formed, leading to a decrease in bending performance. Examples A1, B2, C1, D2, G1, and I1 preferably achieved d≦3.0T in 180° bending performance, and the lower bainite content in the microstructure of the five examples other than Example D2 was >99%, which contributed to the improvement in their bending performance; however, Examples A1 and C1, due to their relatively low strength, achieved d≦2.5T in 180° bending performance.

[0068] Among the comparative examples, the main difference between Comparative Example H2 and Comparative Example H1 was that the winding temperature was too high. As a result, excessive carbonitride precipitation and granular bainite formation occurred in Comparative Example H2, leading to insufficient lower bainite content. Although the strength improved, the bending performance deteriorated significantly.

[0069] In comparative example C2, the winding temperature was too low, resulting in the formation of a virtually pure martensitic structure. Consequently, while the strength was significantly improved, both the plasticity and bending properties deteriorated considerably.

[0070] Compared to Example D1, Comparative Example J1 had a similar single-element content in its composition design, but its Bs value was too high. Therefore, even though the manufacturing process was made closer to that of Example D1, the excessively high Bs value still resulted in excessive carbonitride precipitation and granular bainite, leading to inferior bending performance.

[0071] Compared to Example G1, Comparative Example K1 had a similar content of single elements in its composition design, but K1's Bs value was too low. Furthermore, in the manufacturing process, it was only given a very short time for lower bainite formation and was not allowed to remain in the winding machine. As a result, the lower bainite content was insufficient, and a large amount of martensite and retained austenite was produced. Although the strength and elongation reached the standard, the bending performance was very poor.

[0072] Compared to Example G1, the main difference in Comparative Example G2 was that the tp time was insufficient, and no additional retention was allowed in the winding machine after winding. As a result, the Bs value of the composition of Example G was relatively low, the rate of lower bainite formation was slow, and due to the short tp time and the lack of additional retention after winding, the lower bainite formation time was ultimately insufficient. Consequently, Comparative Example G2 had an insufficient lower bainite content, and martensite and retained austenite were formed. Although the strength and elongation reached the standard, the bending performance was inferior.

[0073] As can be seen from Figure 1, which shows the microstructure according to Example D1 of the present invention, the area ratio of lower bainite was ≥95%, the area ratio of ferrite + carbide + granular bainite was <5%, and the area ratio of martensite + tempered martensite + upper bainite was ≤0.01%.

[0074] In summary, the automotive structural steel obtained by the present invention not only possesses ultra-high strength but also excellent formability, particularly bending performance, and can meet the future demand for weight reduction in automotive structures, making it particularly useful for chassis system components.

[0075] Table 1

[0076] Table 2

[0077] Table 3

Claims

1. Each chemical element is given in the following weight percentages: C: 0.15-0.23%; Si: 0.12-0.5%; Mn: 1.6-2.4%; B: 0.001-0.004%; Al: 0.01-0.04%; Cr; 0.05-0.5%; Mo: 0.15-0.5%; P:≦0.015%; S: ≤0.005%; contains, The remainder consists of Fe and other unavoidable impurities, and satisfies the characteristic values ​​of the lower bainite formation rate Bs = 0.6 to 1.4 and the relationship Bs = (Mo + B * 100 - Mn / 5) / C; A structural steel for automobiles having a yield strength of ≥1000 MPa, characterized in that the area ratio of lower bainite in its microstructure is ≥95%.

2. The automotive structural steel according to claim 1, characterized in that the automotive structural steel further contains at least one of Ti, V, and Nb, and Ti + Nb + V ≤ 0.03%.

3. The automotive structural steel according to Claim 2, characterized in that Ti + Nb + V ≤ 0.006%, having a yield strength of ≥ 1000 MPa.

4. The automotive structural steel according to claim 1, characterized in that the C content is 0.18 to 0.21 wt%, and having a yield strength of ≥ 1000 MPa.

5. The automotive structural steel according to claim 1, characterized in that the Mn content is 1.8 to 2.2 wt%, and having a yield strength of ≥ 1000 MPa.

6. The automotive structural steel according to claim 1, characterized in that the Cr content is 0.15 to 0.35 wt%, and having a yield strength of ≥ 1000 MPa.

7. The automotive structural steel according to claim 1, characterized in that the Mo content is 0.15 to 0.38 wt%, and having a yield strength of ≥ 1000 MPa.

8. The automotive structural steel according to claim 1, characterized in that, in the microstructure of the automotive structural steel, the area ratio of lower bainite is ≥ 95%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.2%.

9. The automotive structural steel according to claim 1, characterized in that, in the microstructure of the automotive structural steel, the area ratio of lower bainite is ≥ 95%, the area ratio of ferrite + carbonitride precipitate + granular bainite is ≤ 5%, and the area ratio of retained austenite + martensite + tempered martensite + upper bainite is ≤ 0.01%.

10. The aforementioned structural steel for automobiles is characterized in that it has a yield strength of ≥1000 MPa, a tensile strength of ≥1180 MPa, an elongation of ≥7%, and a 180° bending performance d of ≤3.5T, as described in claim 1.

11. The automotive structural steel according to Claim 1, characterized in that the 180° bending performance d is ≤ 3T, wherein the yield strength is ≥ 1000 MPa.

12. A method for producing automotive structural steel having a yield strength of ≥ 1000 MPa according to any one of claims 1 to 11, characterized by comprising the following steps. 1) Smelting, continuous casting The billet is cast by smelting according to any one of claims 1 to 7, and the billet is cast by continuous casting, wherein the billet cooling rate during continuous casting is ≥ 5 K / s; 2) Hot rolling The billet is heated to a temperature of 1150-1220°C; The total rolling reduction ratio is ≥98%, provided that the reduction ratios for both the first and second passes are ≥60%; the finish rolling outlet temperature is 920–980°C; 3) Laminar cooling and winding After rolling, the steel strip is cooled to ≤530°C at a cooling rate of ≥150°C / s, and then further cooled to the coiling temperature at a cooling rate of ≥10°C / s, where the coiling temperature is (Ms + 10°C) to 400°C; the elapsed time tp when any part of the rolled steel plate or strip is transported from the finish rolling outlet temperature measurement point to the coiling temperature measurement point is ≥(5 / Bs) + 4 seconds; however, The martensitic transformation temperature Ms of steel strip is given by 498.9 - 333.3 * (C) - 33.3 * (Mn) - 27.8 * (Cr) - 16.7 * (Ni) - 11.1 * (Si + Mo + W), and the unit is °C. For steel plates and strips with Bs < 0.9, after winding, the steel coil is held in the winding machine for a time of ≥ (10 / Bs) + 5 seconds before being unwound. 4) Stacked cooling.

13. A method for producing automotive structural steel having a yield strength of ≥ 1000 MPa, characterized by further comprising step 5) of pickling hot-rolled steel sheets or strip steel to produce pickled sheets.

14. A method for producing automotive structural steel having a yield strength of ≥1000 MPa, characterized in that, after rolling in step 3), the strip steel is rapidly cooled to ≤530°C at a cooling rate of ≥180°C / s, as described in claim 12.

15. A method for manufacturing automotive structural steel having a yield strength of ≥ 1000 MPa, as described in claim 12, characterized in that, after winding a steel plate or strip steel with Bs ≥ 0.9, the steel coil is left in a winding machine for a time of ≥ (10 / Bs) + 5 seconds before being unwound.

16. The method for manufacturing automotive structural steel having a yield strength of ≥ 1000 MPa, as described in claim 12, characterized in that, after unwinding, the steel coils are stacked in a heat-insulating bit at an ambient temperature ≥ 280°C and kept warm for 2 to 6 hours.

Images