ULTRA HIGH STRENGTH LOW COST LOW CARBON MULTIPHASE STEEL PLATE / STEEL STRIP AND METHOD OF MANUFACTURING THE SAME

Abstract

LOW-COST, LOW-CARBON ULTRA-HIGH-STRENGTH MULTIPHASE STEEL PLATE / STEEL STRIP AND METHOD OF MANUFACTURING THEREOF. The present invention relates to a low-cost, ultra-high-strength, multiphase steel plate / steel strip and a method of manufacturing the same. The steel plate / steel strip comprises the following components in weight percent: 0.03-0.07% C; 0.1-0.5% Si; 1.3-1.9% Mn, less than or equal to 0.02% P, less than or equal to 0.01% S; 0.01-0.05% Al; 0.2-0.5% Cr, 0.07-0.14% Ti, less than 0.03% (Ni+Nb+Mo+V), and a balance of Fe and unavoidable impurities; and Mn+1.5Cr+5(Ti+Al+CU)+10(Mo+Mi)+20(Nb+V)<3.0; Mn+2Cr+4Ti+4Nb+4V+4Mo-Si / 3+2C=3.0.The steel plate has a tensile strength of ≥780 MPa, a yield strength of ≥680 MPa, an elongation of ≥15%, and a yield-to-tensile ratio of ≥0.9; the steel plate has a hole expansion rate of ≥85% if an original hole is a stamped hole, and a hole expansion rate of ≥115% if an original hole is a countersunk hole; the steel plate has a bending property satisfying the condition in which, in a 180° bending test, rupture does not occur when a bending diameter is equal to 0.5, plate thickness (d=0.5a); the steel plate is mainly used for manufacturing automotive suspension and chassis system parts.

Description

1 / 28 LOW COST, LOW CARBON ULTRA HIGH STRENGTH MULTIPHASE STEEL PLATE / STRIP AND METHOD OF ITS MANUFACTURE FIELD OF THE INVENTION

[0001] The present invention relates to the field of metallic materials, in particular to a low-cost, low-carbon, ultra-high-strength, multiphase steel plate / strip and its manufacturing method, which is mainly used in the manufacture of parts and component products for automotive chassis and suspension systems. BACKGROUND OF THE INVENTION

[0002] The lightweight nature of automobiles can directly reduce emissions and fuel consumption, which is the goal of current automobile manufacturing industries. An important measure of lightweight automobiles is the use of ultra-high-strength and high-strength steel plates instead of low-strength steel plates. Currently, the lightweight concept is also applied in automotive chassis and suspension systems, and automotive chassis materials must also adopt high-strength steel to achieve lightweight with increasingly stringent environmental requirements and market demands.

[0003] However, in addition to the higher strength of the steel plate, it is also required for the structural parts of the automobile chassis and suspension system that the steel plates have a high tensile strength-to-yield ratio (i.e., the ratio of yield strength to tensile strength, hereinafter referred to as yield ratio), good elongation, good bore expansion performance, and good bending performance. Unlike some body parts that need to absorb collision energy or prevent rigid collision damage to pedestrians through certain plastic deformations during collisions, it is often required for systems Petition 870260055051, dated 08 / 06 / 2026, page 5 / 77 2 / 28 of the chassis and suspension components require the material to have no plastic deformation, so the material's yield ratio needs to be extremely high. Furthermore, it is well known that the fatigue limit of a material is almost always proportional to its yield strength. Thus, for the same level of tensile strength, a material with higher yield strength means it has a higher fatigue limit. Therefore, for chassis and suspension system components, one of the goals pursued is to have higher yield strength or a higher yield ratio at the same level of tensile strength.

[0004] Furthermore, automotive chassis and suspension system parts often have a complex structure (as shown in Figure 1) and procedures such as flanging, hole expansion, bending, stretching and the like coexist, so the requirement for material forming performance is extremely high. It is required that not only the steel have a high elongation, a higher hole expansion ratio, but also that the steel plate have excellent bending performance.However, it is known in the art that the strength of steel plate is mutually restrictive for three performance indicators, namely yield ratio, elongation and hole expansion ratio, because it requires phase components with high strength and hardness in the steel (such as martensite and bainite) to ensure strength, softer phase components (such as ferrite) to ensure plasticity and elongation, and sufficient uniform structure to improve bending performance, and also requires the difference in hardness between the soft and hard phases to be as small as possible to ensure high yield ratio and high hole expansion performance.

[0005] In addition to the stringent performance requirements mentioned above, the preparation of parts of the chassis and systems Petition 870260055051, dated 08 / 06 / 2026, page 6 / 77 3 / 28 Automotive suspension also requires materials with good surface quality, covering capacity, and normal and uniform color, as well as low equivalent levels of carbon to ensure the weldability of the parts, so the composition is commonly designated as comprising low carbon (carbon content less than 0.1%).Under this composition system, to obtain high-strength steel with a high yield ratio, high elongation, high hole expansion and high bending properties, and to meet the manufacturing requirements of automotive chassis and suspension system parts having a complicated shape, high-quality performance and high serviceability, a multiphase steel with bainite as the main body and ferrite as the supplement is normally selected and is obtained by precipitation strengthening by adding a large amount of alloying elements, especially niobium, molybdenum, vanadium, nickel, aluminum and other precious alloying elements, thereby improving the strength of ferrite and ensuring the plasticity of ferrite, thus improving the yield ratio and guaranteeing that the steel has a high elongation and hole expansion ratio.

[0006] For example, the publication of a Chinese patent CN109055657A describes a high-strength, ultra-low-carbon multiphase steel plate that has a yield strength of > 690 MPa and a yield ratio of 0.89-0.92 by adding a large amount of niobium, molybdenum, nickel, and other precious metal elements, but does not consider the hole expansion performance of the steel plate. Chinese patent publication CN109055657A also describes a high-strength hot-rolled steel plate that has a tensile strength greater than 780 MPa and a hole expansion ratio (the original hole is drilled) of 40% or greater by adding a large amount of niobium, molybdenum, nickel, and other precious metal elements. Petition 870260055051, dated 08 / 06 / 2026, page 7 / 77 4 / 28

[0007] However, with increasingly fierce internal competition in the steel and automotive industries, this type of steel with the addition of a large amount of alloying elements, especially precious alloying elements such as molybdenum, vanadium, and niobium, etc., has greatly increased the pressure on the entire automotive industry. Therefore, there is an urgent market need for a steel plate having excellent service and comprehensive forming performance with little or no addition of precious alloying elements (i.e., low cost), and having high strength, high yield ratio, high elongation, high hole expansion ratio, and high bending performance.

[0008] However, since precious alloying elements such as molybdenum, vanadium, and niobium, etc., are not added, it is difficult for low-cost multiphase steels to achieve high strength levels (such as a tensile strength of 800 MPa). For example, Chinese patent publication CN10366794A describes a multiphase steel free of niobium, vanadium, nickel, molybdenum, and other precious metal microalloy elements and its manufacturing method. Although the multiphase steel described in the patent application has a low alloy cost and better bore expansion performance, its strength is lower. The steel only has a yield strength no greater than 530 MPa and a tensile strength no greater than 725 MPa in the working examples. Therefore, it can only be designated for the manufacture of automobile wheels, and cannot meet the requirements of automotive parts and chassis for the strength of multiphase steel product parts.

[0009] To solve the problem of insufficient strength in low-cost multiphase steel, one of the main methods is to add a large amount of relatively inexpensive alloying elements, such as manganese, chromium, and titanium, instead of precious alloying elements. Petition 870260055051, dated 08 / 06 / 2026, page 8 / 77 5 / 28 such as molybdenum, vanadium, and niobium, etc., and improve the strength of multiphase steel through phase transformation strengthening, solid solution strengthening, and second-phase strengthening. For example, Chinese patent publication CN102732790A describes an ultra-low carbon bainite steel plate having a tensile strength greater than 770 MPa and its manufacturing method. Although the steel plate does not comprise niobium, molybdenum, vanadium, and other precious metal microalloy elements, it still comprises 3.0% - 4.5% manganese, still leading to a high cost. And the bore expansion and bending properties of multiphase steel are not considered in this patent application. For example, Chinese patent publication CN101285156A describes a bainite steel produced using continuous thin sheet casting and rolling technology.Although a high-strength bainite steel without niobium, molybdenum, vanadium, and other precious metal microalloy elements, possessing high strength, high yield ratio, high elongation, and certain bending performance, is designed and manufactured using continuous thin sheet casting and rolling technology under patent, due to the addition of large amounts of chromium, magnesium, titanium, and other elements, the production and manufacturing cost of its products remains high, and the product's bending performance is average.The hole expansion performance is not yet considered; in fact, manganese and chromium elements are very prone to segregation in steel plates, resulting in differences between local composition and phase composition, leading to deterioration of the hole expansion ratio and bending performance of the steel plate. Meanwhile, manganese is prone to causing coarse columnar crystal formation in the plate during continuous casting, and this coarse columnar crystal will have a genetic impact on the hot-rolled and cold-rolled structure of the subsequent steel plate, which in turn affects the uniformity of the steel plate structure and adversely affects the... Petition 870260055051, dated 08 / 06 / 2026, page 9 / 77 6 / 28 bore expansion performance and bending performance.

[0010] Another method of preparing high-strength, low-cost multiphase steel is to increase the strength of the multiphase steel by adding a large amount of silicon through the solid solution strengthening effect. For example, Chinese patent publication CN1756853A describes a high-strength hot-rolled multiphase steel comprising ferrite, bainite / martensite, and a second phase composition, and its manufacturing method. A large amount of silicon (up to 1.5%) is added in this patent to ensure the ultra-high strength of the multiphase steel, thereby reducing the amount of other alloying elements.However, an excessively high content of the element silicon will lead to serious red iron oxide incrustation on the surface of the hot-rolled steel plate / strip, which will directly cause serious color differences on the surface of the finished steel plate / strip, and directly affect its surface quality and aesthetics as an automotive structural part.According to known techniques: Yang Yu, Chang Wang, Lin Wang, et al., Research in defects of red iron scale on Si-containing steel based on high temperature oxidation characteristics [J], Rolling Steel, 2016, 33(2): 10-15; Yang Yu, Chang Wang, Lin Wang and others, Research on defects of red iron scale on Si-containing steel based on high temperature oxidation characteristics [J], Rolling Steel, 2016, 33 (2): 10-15; Chang Wang, Tang Lu, Lin Wang, et al., Research on the effect of Si element on the interface microstructure of oxide scale forming in the heating furnace [J], Rolling Steel, 2016, 33(5): 6-10, when the silicon content in the steel is high, defects such as red scale can be formed and thus the surface quality of the steel is reduced. Automotive steel containing 0.5% silicon has been found to have an evenly spaced strip of iron incrustation on the surface of the steel strip, and a. Petition 870260055051, dated 08 / 06 / 2026, p. 10 / 77 The proportion of red iron fouling defects on the surface of the steel strip is approximately 30%. Therefore, an ultra-high strength multiphase steel, designed and manufactured using a high silicon element solution, is not suitable for the automotive industry.

[0011] Therefore, for such low-carbon multiphase steels with tensile strength up to 800 MPa (hereinafter referred to as high-strength low-carbon multiphase steels) used in the manufacture of automotive chassis and suspension system parts, existing technology cannot solve the contradiction between their low-cost design and high comprehensive service performance. Even if the existing low-cost design is adopted, it is impossible to provide excellent strength, yield ratio, bore expansion, and bending properties simultaneously. Thus, the long-standing challenge in the steel industry is how to obtain low-cost, high-strength, high-bore expansion multiphase steel plates / strips to meet the production and manufacturing requirements of automotive parts. SUMMARY

[0012] An object of the present invention is to provide an ultra-high strength, low-cost multiphase steel plate / steel strip and its manufacturing method, wherein the steel plate has a tensile strength >780 MPa, a yield strength >680 MPa, an elongation >15°, a yield ratio >0.9, a hole expansion ratio that satisfies: if the initial hole is a drilled hole, the hole expansion ratio is >85%; if the initial hole is a countersunk hole, the hole expansion ratio is >115%, and a bending performance that meets that no cracking occurs when the diameter of the bending angle of the bending test is equal to the plate thickness of 0.5 (d = 0.5a). The ultra-high strength, low-cost multiphase steel plate / steel strip is mainly used in the manufacture of parts Petition 870260055051, dated 08 / 06 / 2026, page 11 / 77 8 / 28 of the automotive chassis and suspension system.

[0013] To achieve the above objective, the technical solution of the present invention is as follows.

[0014] A low-cost, ultra-high-strength, multiphase steel plate / strip comprises the following chemical elements in weight percentages: C: 0.03-0.07%; Si: 0.1-0.5%; Mn: 1.3-1.9%, P <0.02%, S < 0.0%; Al: 0.01-0.05%; Cr: 0.2-0.5%, and also includes Ti: 0.07-0.14%, (Ni+Nb+Mo+V) <0.03%, and a balance of Fe and unavoidable impurities; at the same time it is required to satisfy: [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Mi)+20(Nb+V)]<3.0; (Mn+2Cr+4Ti+4NB+4V+4Mo-Si / 3+2C)<3.0

[0015] Preferably, the steel plate / steel strip comprises the chemical element C: 0.04-0.06%, by weight percentage.

[0016] Preferably, the steel plate / steel strip comprises the chemical element Si: 0.1-0.27% by weight.

[0017] Preferably, the steel plate / steel strip comprises the chemical element Mn: 1.45-1.75% by weight.

[0018] Preferably, the steel plate / steel strip comprises the chemical element Cr: 0.35-0.50% by weight.

[0019] Preferably, the steel plate / steel strip does not comprise V, Mo, and Ni are present, and the Nb content is <0.03%.

[0020] The microstructure of the steel plate / steel strip of the present invention comprises ferrite, lower bainite, and also contains carbide precipitation phase, inclusion phase and / or martensite trace phase, wherein the ferrite content is <70%, and the lower bainite ferrite content is >90%.

[0021] Preferably, in the sheet casting structure of steel plate / steel strip, the proportion of columnar crystals is <10%, or the thickness of the columnar crystal region is <40 mm.

[0022] Preferably, in the microstructure of the steel plate / steel strip Petition 870260055051, dated 08 / 06 / 2026, page 12 / 77 9 / 28 finished, the average diameter of ferrite grains is <6 pm, or an ASTM grain size classification of >11.8.

[0023] Preferably, the microstructure of the finished steel plate / steel strip contains TiN particles, and a single joint has a longer side length of <10 pm or an area of ​​<50 pm2. Preferably, the length of the longer side of a single TiN particle is <8 pm.

[0024] According to the above solution, the steel plate / steel strip has a tensile strength of >780 MPa, a yield strength of >680 MPa, and a hole expansion ratio performance index: if the original hole is a drilled hole, the hole expansion ratio is >85%; if the original hole is a countersunk hole, the hole expansion ratio is >115%; 180° bending is qualified at d=0.5a.

[0025] Preferably, according to the above solution, the steel plate / steel strip has a tensile strength of >800 MPa, a yield strength of >710 MPa, and a hole expansion ratio performance index: if the original hole is a drilled hole, the hole expansion ratio is >95%; if the original hole is a countersunk hole, the hole expansion ratio is >120%; 180° bending is qualified at d=0.5a.

[0026] Preferably, the steel plate / steel strip according to the above solution has a yield ratio of >0.9.

[0027] The steel composition according to the present invention is designated as follows:

[0028] Carbon (C): Carbon directly affects the strength, weldability, formability, and continuous casting manufacturing capacity of thin steel plates / strips. The higher the carbon content, the more likely it is to increase the strength of the steel plate. If the carbon content is less than Petition 870260055051, dated 08 / 06 / 2026, page 13 / 77 10 / 28 If the carbon content is greater than 0.03%, the strength of the steel plate / steel strip cannot meet the target requirements; if the carbon content is greater than 0.07%, it is easy to cause excessively high strength of the steel plate / steel strip, resulting in an unacceptable hole expansion ratio. Thus, the carbon content according to the present invention is controlled to be between 0.03% and 0.07%.

[0029] Silicon (Si): Silicon has a certain solid solution strengthening effect. The higher the Si content, the more conducive it is to improving the yield strength of the steel plate / steel strip. At the same time, silicon also has the effect of inhibiting carbide precipitation, and can form a bainite structure without carbide precipitates by adding silicon elements. But when the silicon content is greater than 0.5%, it is prone to generating serious red iron oxide fouling on the surface of the hot-rolled steel plate / steel strip, which not only worsens the surface quality of the steel plate / steel strip, but also damages the forming ability of the steel plate / steel strip, which is not conducive to the production of hot-dip galvanized steel plate / steel strip. Therefore, the silicon content according to the present invention is controlled to be from 0.1 to 0.5%.

[0030] Manganese (Mn): Since manganese can effectively improve the strength of steel plate / steel strip, and its cost is relatively low compared to other alloying elements, manganese is used as the main added element in the present invention. However, when the manganese content is greater than 1.90%, not only will the proportion or thickness of columnar crystals in the sheet casting structure increase significantly, which will have a serious adverse effect on controlling the uniformity of the finished product structure in the future, but the increase in Mn content will also lead to an increase in the martensite content in the finished structure. Petition 870260055051, dated 08 / 06 / 2026, page 14 / 77 11 / 28 final, which damaged the hole expansion performance; when the manganese content is less than 1.40%, the strength of the steel plate / steel strip is insufficient. Therefore, the manganese content according to the present invention is controlled to be from 1.30 to 1.90%.

[0031] Aluminum (Al): Aluminum is added as the main deoxidizer in the steelmaking process. But when the aluminum content is less than 0.01%, the deoxidation effect is insufficient; when the aluminum content exceeds 0.05%, the viscosity of the molten steel is affected, which can cause nodules in the water nozzle and damage the weld performance of the steel plate / steel strip. Therefore, the aluminum content according to the present invention is controlled to be from 0.01 to 0.05%.

[0032] Chromium (Cr): Chromium is conducive to expanding the bainite phase region, ensuring that the bainite structure can be obtained by cooling the steel plate / steel strip after rolling, which is conducive to improving strength and hole expansion ratio. However, when the amount added exceeds 0.5%, the increase in strength is no longer significant, and it is not conducive to the weldability of the steel plate / steel strip, resulting in a significant increase in the proportion or thickness of crystals in the sheet casting structure, and also leads to the enrichment of chromium elements on the surface of the steel plate, thus affecting the uniformity of the final product structure. However, when the content is less than 0.2%, the expansion of the bainite phase region is not significant. Therefore, the chromium content according to the present invention is controlled to be from 0.2 to 0.5%.

[0033] Titanium (Ti): Titanium is the main alloying element in the multiphase steel of the present invention, which improves the strength of the multiphase steel by solid solution strengthening and second-phase strengthening after the formation of a fine carbide. When the microalloy content is Petition 870260055051, dated 08 / 06 / 2026, page 15 / 77 12 / 28 less than 0.07%, the strength of the steel plate / steel strip is insufficient; when the microalloy content is greater than 0.14%, it increases the cost on the one hand, and it is easy to form central carbide segregation on the other hand, which is not conducive to hole expansion performance.

[0034] Niobium, vanadium, molybdenum, and nickel (Nb, V, No, Ni): the addition of niobium, vanadium, molybdenum, and nickel can also produce solid solution strengthening and second-phase strengthening, thus increasing the strength of multiphase steels. Furthermore, the inclusion of these precious metal microalloy elements can also achieve a grain refinement effect, which is beneficial for the hole expansion ratio of multiphase steel. However, compared to titanium, niobium, vanadium, molybdenum, and nickel alloys are extremely expensive. Although these microalloy elements can be added as an additional element to steel, it is not recommended to add niobium, vanadium, molybdenum, and nickel considering the low cost, i.e., Nb+V+Mo+Ni < 0.03%.

[0035] The upper limit of impurity elements in steel is controlled at P<0.02%, S<0.01%, and the purer the steel, the better the effect.

[0036] The microstructure of the steel plate / steel strip of the present invention is a ferrite + lower bainite microstructure, wherein the ferrite content is <70%. The ferrite + lower bainite content is >90%. If the ferrite structure is greater than 70%, the steel plate / steel strip will not be able to provide the required strength; if the ferrite + lower bainite content is less than 90%, the hole expansion performance of the steel plate / steel strip will not meet the requirements. The microstructure of the steel plate / steel strip of the present invention may also contain a carbide precipitation phase, an inclusion phase and / or a martensite trace phase, wherein the content of Petition 870260055051, dated 08 / 06 / 2026, page 16 / 77 13 / 28 The carbide precipitation phase is typically 5% or less, the inclusion phase content is typically 0.01% or less (occasionally found in the visible field), and the martensite phase content is typically 0.5% or less. The inclusions may be common inclusions in steels such as MnS, TiN, and AlN.

[0037] In the microstructure of steel plate / steel strip according to the present invention, the average diameter of the ferrite grain is <6 µm, or the ASTM grain size classification is >12.3. If the average grain diameter is not less than 6 µm or the grain size classification is not greater than 12.3, the steel plate / steel strip will not be able to provide the required strength.

[0038] The microstructure of the steel plate / steel strip of the present invention contains TiN particles, of which the length of the longer side of a single particle is <10 pm. If the length of the longer side of a single particle is not less than 10 pm, it will cause the steel plate / steel strip to fail to provide the required hole expansion performance.

[0039] Furthermore, based on the percentage of each element, the measurement ratio between the alloying elements above and the carbon elements must also meet the following formula: (1) [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]<3.0, to ensure that the steel plate / steel strip has a low alloy cost; (2) (Mn+2Cr+4Ti+4Nb+4V+4Mo-0.5Si+5C)<3.0, to ensure that a certain amount of carbide particles can be precipitated in the microstructure of the steel plate / steel strip, without reducing the hole expansion ratio caused by excessive, very coarse precipitation or too much carbide, thus ensuring that the steel plate / steel strip has high strength and hole expansion properties. [00 40] The manufacturing method of steel plate / steel strip Petition 870260055051, dated 08 / 06 / 2026, page 17 / 77 The 14 / 28 ultra-high resistance, low-cost, and ultra-low carbon multiphase circuit, according to the present invention, comprises the following steps; 1) Fusion, continuous casting in which the chemical composition is melted into a casting plate by continuous casting, wherein a cooling rate of the plate is >5°C / s during continuous casting; 2) Hot rolling, cooling after rolling, in which the sheet enters the furnace at a temperature of not less than 700°C, and the casting sheet is heated to a heating temperature of 1100-1250°C; wherein each reduction rate for the first and second hot rolling pass is >55%, and the final rolling temperature is 850-950°C; 3) Cooling after rolling, cooling in which water cooling is carried out after rolling, and the cooling temperature is 550-630°C; 4) Pickling.

[0041] In addition, after pickling step 4), the method still comprises the hot-dip galvanizing annealing process to obtain the finished hot-dip galvanized, hot-rolled steel plate.

[0042] In step 1) above, the cooling rate of the sheet during continuous casting will affect the grain size in the final structure of the steel plate / steel strip, the size of the inclusions formed in the liquid phase, and the proportion of columnar crystals in the sheet structure. If the cooling rate is less than 5°C / s, on the one hand, the thickness or proportion of columnar crystals in the sheet will be greater than the design requirements, so it is easy to form a banded structure in the structure of the subsequent finished product, which affects the bending properties of the steel plate / steel strip; on the other hand, decreasing the cooling rate of the sheet during continuous casting will result in Petition 870260055051, dated 08 / 06 / 2026, page 18 / 77 15 / 28 that the grain size in the final structure cannot meet the design requirement, and will lead to coarse inclusion size (typically such as TiN) generated in the liquid phase in the steel, which is adverse to hole expansion and bending performance.

[0043] In step 2) above, the minimum temperature before the sheet enters the heating furnace will affect the final product properties. When the minimum temperature before the sheet enters the heating furnace is less than 700°C, titanium carbide will precipitate on the sheet in large quantities, and in the subsequent reheating process, the titanium carbide that has precipitated on the sheet cannot be completely redissolved, resulting in less solid solution of titanium and titanium carbide in the die after hot rolling and insufficient product strength. When the final rolling temperature of the finishing rolling is less than 850°C, ferrite will precipitate before finishing rolling, resulting in a low bainite content in the final structure, so the steel plate / steel strip cannot achieve the determined strength.However, considering the plate heating temperature, the final rolling temperature of the finishing roll should not exceed 950°C. Furthermore, in step 2) above, to ensure that the steel plate / steel strip has a fine and highly uniform structure, each reduction rate for the first and second hot rolling passes is >55%; when the reduction rate is insufficient, it is impossible to obtain a fine and uniform structure, resulting in insufficient strength of the steel plate / steel strip, and the bending performance cannot meet the design requirements. Not only that, the high reduction rate in step 2) above must be combined with the high cooling rate of the plate during continuous casting in step 1). If the continuous casting cooling rate cannot reach 5°C / s or more, inclusions (mainly...) Petition 870260055051, dated 08 / 06 / 2026, page 19 / 77 16 / 28 The amount of TiN inclusions generated in the liquid phase in the sheet will be very large; at this point, if a large reduction rate of >55% is used in step 2), it will lead to coarse TiN cracking, as shown in Figure 1, thus becoming the source of cracks within the steel plate / steel strip and resulting in the deterioration of the hole expansion and bending properties of the steel plate / steel strip. However, if the continuous casting cooling rate can reach 5°C / s or more, the inclusions (mainly TiN) generated in the liquid phase in the sheet are small in size, as shown in Figure 2, and will not break at the large hot rolling reduction rate in step 2), so it does not adversely affect the hole expansion and bending properties of the steel plate / steel strip.

[0044] In step 3) above, the cooling temperature is one of the most critical process parameters for achieving a high strength-to-high hole expansion ratio. When the cooling temperature is higher than 640°C, due to strong precipitation and thickening of alloy carbides, it has a negative effect on the hole expansion ratio of the steel plate. On the other hand, when the cooling temperature is lower than 550°C, carbide precipitation will be seriously inhibited, resulting in the steel plate strength not meeting the specified requirements. Therefore, the cooling temperature in the present invention is limited to 550-630°C.

[0045] After verification, the performance of the ultra-high-strength hot-rolled steel plate / strip according to the present invention satisfies the following indices: 1. Mechanical properties at room temperature: A tensile strength of >780 MPa, preferably >800 MPa; a yield strength of >680 MPa, preferably >710 MPa; a yield ratio of >0.9. In some embodiments, the Petition 870260055051, dated 08 / 06 / 2026, page 20 / 77 17 / 28 feed resistance is 780=900 MPa and yield resistance is 680-830 MPa. 2. Elongation at rupture: A50 >15% or A5 >19% 3. Bore expansion ratio performance: If the original hole is a drilled hole, the hole expansion ratio is greater than 85%, preferably >95%, and up to 100% or more; in some embodiments, the hole expansion ratio is 86% to 110%; If the original hole is a countersunk hole, the hole expansion ratio is greater than 115%, preferably >120%, and up to 130% or more; in some embodiments, the hole expansion ratio is 117% to 140%. 4. Bending performance: Cold bending of 180° is qualified at d=0.5a.

[0046] The beneficial effects of the present invention are: 1. The present invention adopts a low-cost composition design to reduce the addition of alloying elements, especially precious metal alloying elements, and it is required that [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)] <3.0, and (Nb+V+Mo+Ni) <0.03 to ensure extremely low alloy costs for multiphase steel. Among the common alloying elements of multiphase steel, niobium and vanadium have the highest cost, followed by molybdenum and nickel. Considering the classification, the unit price of niobium and vanadium can reach 20 times the unit cost of manganese; considering the classification, the unit price of molybdenum and nickel can reach 10 times the unit cost of manganese. Therefore, this type of element is basically not used in the present invention. Other alloying elements, such as titanium, aluminum, manganese, chromium, etc., are selected to minimize the amount of addition for the optimized composition design to achieve the lowest Petition 870260055051, dated 08 / 06 / 2026, p. 21 / 77 18 / 28 total cost. 2. Due to the market demand for low cost, basically no precious alloying elements are added in the present invention, which easily causes insufficient material strength, such as in CN103667948A. To increase the material strength, fine-grained reinforcement combined with Ti element reinforcement is mainly adopted in the present invention, and a combination of high strength, high elongation, high hole expansion ratio, and excellent bending performance is obtained by optimizing the ratio of precipitation reinforcement and solid solution reinforcement and providing a highly uniform microstructure and sufficiently small inclusions. A certain amount of manganese, chromium, titanium, especially chromium and titanium, are still added in the present invention. On the one hand, reinforcement is obtained by solid solution of these elements in the matrix, i.e., solid solution reinforcement; on the other hand, reinforcement is obtained by forming carbide precipitation, i.e., second-phase reinforcement.For these two strengthening mechanisms, the increase in strength from solid solution strengthening is weaker than that from second-phase strengthening, but carbide precipitation will damage the hole expansion performance of multiphase steel. Therefore, it is necessary to find a balance between the two strengthening mechanisms, that is, the following ratio is required between the addition of alloying elements and the carbon content: (Mn+2Cr+4Ti+4Nb+4V+4Mo-Si / 3+2C) <3.0. The formula of the present invention represents the contribution of each alloying element in the material to the second-phase precipitation effect. Among them, the abilities of the three types of elements Mn, Cr, Ti (or Nb, Mo, V) to form the carbide precipitation phase increase gradually, thus the gradient coefficient design is adopted; and the element Si has the effect of inhibiting carbide precipitation, thus it is designated as a negative coefficient in the formula. Therefore, the higher the value of... Petition 870260055051, dated 08 / 06 / 2026, p. 22 / 77 19 / 28 of the formula above, the greater the contribution of the alloying elements added to the material to the overall precipitation reinforcement. It is observed in the present invention that when the value of the formula above is greater than 3.0, the hole expansion performance of the material will be gradually reduced. 3. A small, highly uniform structure and small inclusion size can be achieved through design and process composition, in order to obtain excellent bending performance. In the design, lower Mn and Cr content is adopted firstly to avoid the appearance of large columnar grains in the steel sheet after continuous casting, so as to minimize adverse effects of columnar grains in subsequent production to obtain a highly uniform structure. Secondly, a high cooling rate design is adopted in continuous casting. On the one hand, the proportion of columnar crystals in the sheet is continuously reduced to increase the proportion of fine equiaxed crystals, and on the other hand, the size of inclusions generated in the liquid phase (represented by TiN) is reduced.Finally, in the first and second hot rolling passes, a high-reduction rolling process design is adopted to obtain a fine structure while still destroying the columnar crystals, and to achieve a combination of high strength, high bending, and high elongation.

[0047] The ultra-high strength hot-rolled steel plate products, steel strip products, and hot-dip galvanized steel plate products manufactured by the present invention can be used for the manufacture of automotive chassis and suspension system parts, and at the same time have a combination of low cost, high strength, high bore expansion, and good bending performance, thus filling the gap for high-quality, low-cost steel chassis products urgently needed by the market. Petition 870260055051, dated 08 / 06 / 2026, page 23 / 77 20 / 28 of the automotive industry chain. DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 shows the Ti particle size when the continuous casting cooling rate reaches 5°C / s or more and its morphology after hot rolling at a large reduction (photograph of the structure in the hot rolling state).

[0049] Figure 2 shows the particle size of Ti when the cooling rate is less than 5°C / s and its morphology after hot rolling in great reduction (photograph of the structure in the hot rolling state).

[0050] Figure 3 shows that the microstructure of the steel plate / steel strip of the present invention contains ferrite and lower bainite, wherein the ferrite + lower bainite content is >90%. DETAILED DESCRIPTION

[0051] The present invention will be further described with reference to the following examples.

[0052] The steels with different compositions after melting shown in Table 1 were heated and hot-rolled according to the process shown in Table 2 to obtain steel plates with a thickness less than 4 mm. Yield strength, tensile strength, and elongation were measured by tensile specimens with calibration lengths of 50 mm and 5 mm along the longitudinal direction, and the hole expansion ratio and 180° bending performance were measured in the middle area of ​​the steel plate. The test data are shown in Table 2. Among them, the hole expansion ratio is measured by the hole expansion test. The specimen with a hole in the center was pressed into a concave die with a punch, so that the central hole of the specimen was enlarged until trunks and perforated cracks appeared at the edge of the hole. Since the method of preparing the original hole in the center of the specimen has a large Petition 870260055051, dated 08 / 06 / 2026, page 24 / 77 21 / 28 influence on the hole expansion ratio test results, the original holes in the center of the specimens were prepared by drilling and countersinking, respectively, and subsequent tests were performed according to the hole expansion ratio test method specified in ISO / DIS standard 16630. The 180° bending test was performed using the method for determining bending properties in GB / T232-2010 standard.

[0053] In Table 1, Examples AI are steels of the present invention. The carbon or manganese content and other alloying elements in Comparative Examples JN exceed the scope of the composition of the present invention. M in the table refers to the calculated value of Item [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)], R refers to the calculated value of Item (Mn+2Cr+4Ti+4Nb+4V+4Mo-Si / 3+2C) in the composition. Furthermore, Comparative Example O and Comparative Example P are examples described in CN101906567A and CN101285156A, respectively. It can be seen from the comparison that the values ​​of M and R of Comparative Example O and Comparative Example P exceed the range of the present invention, indicating that the cost of the alloy in these two comparative examples is higher than that of the present invention, and the optimized alloy ratio according to the formula designed in the present invention is not adopted.

[0054] Table 2 shows different manufacturing processes for various steel classifications from Table 1, which are also divided into two categories: Examples and Comparative Examples, where the processes in Comparative Example O and Comparative Example P are the manufacturing processes described in corresponding patent applications. Table 3 shows the values ​​detected for mechanical properties of the aforementioned examples and comparative examples, where the properties of Comparative Example O and Petition 870260055051, dated 08 / 06 / 2026, page 25 / 77 22 / 28 Comparative Example P refers to those described in the corresponding patent applications. It can be seen from the table that the properties of Comparative Example O and Comparative Example P are inferior to those of the examples in the present invention.

[0055] It can be seen that when C, Mn, Ti and other alloying components deviate from the scope of the present invention, for example, when the content of Mn and Ti is low, such as Examples K and M, the strength of the steel plate is less than the design requirements; and when the content of C, Ti or R value is greater than the compositional range of the present invention, such as comparative examples J, L, and N, the excessive content of C r Mn leads to the production of a large amount of martensite in the structure, which deteriorates the hole expansion and bending properties of the material, while if the content of Ti and the R value are too high, the carbides in the structure are thickened, and the hole expansion performance of the material is deteriorated, which is not in accordance with the purpose of the present invention.

[0056] When the temperature of the sheet entering the furnace is too low, such as Comparative Steel A-2, the strength does not meet the design standards of the present invention; if the cooling temperature is too low, such as Comparative Example D-2, carbide precipitation in the steel is inhibited, resulting in very low strength of the steel plate. When the reduction rate of the first two hot rolling passes is insufficient, the banded structure of the steel plate cannot be completely eliminated, and the grains cannot be completely refined to obtain structure uniformity, leading to deterioration of the bending performance of the steel plate for elongation, such as Comparative Example B-2. When the continuous casting cooling rate is insufficient, but a large reduction rate is pursued in Petition 870260055051, dated 08 / 06 / 2026, page 26 / 77 23 / 28 hot rolling, the coarse TiN particles in the steel are broken up and a potential crack source is formed, which greatly deteriorates the elongation, hole expansion performance and bending performance of the material, such as Comparative Example C-2.

[0057] Based on the above, the present invention greatly reduces alloy costs by controlling a reasonable composition range, limiting the content of alloying elements, and optimizing the ratio of each element based on carbon-manganese steel.By further controlling the cooling rate in continuous casting, the hot rolling reduction rate, and the cooling temperature based on the automotive steel production line, the present invention produces a low-cost, ultra-high-strength hot-rolled steel plate / strip with high strength, high hole expansion performance, and excellent bending performance, which has a yield strength of not less than 680 MPa, a tensile strength of not less than 780 MPa, and a hole expansion ratio of not less than 85% (the original hole is drilled) or not less than 115% (the original hole is countersunk), 180° bending d=0.5a, to meet the urgent market demand of the automotive industry for chassis and suspension materials with a combination of low cost, high strength, and high forming performance. Petition 870260055051, dated 08 / 06 / 2026, page 27 / 77 24 / 28 Table 1 (unit: percentage) Steel No. C Si Mn PS Al Cr Ti V Mo Ni Cu Nb MR Example A 0.058 0.30 1.55 0.012 0.002 0.02 0.43 0.090 0 0 0 0 0 2.75 2.79 Example B 0.040 0.48 1.79 0.013 0.004 0.01 0.38 0.110 0 0 0 0 0 2.96 2.91 Example C 0.068 0.25 1.32 0.015 0.003 0.02 0.40 0.130 0 0 0 0 0.01 2.87 2.73 Example D 0.053 0.36 1.85 0.014 Example E: 0.046 0.14 1.49 0.010 0.001 0.01 0.49 0.077 0 0 0 0 0 2.66 2.82 Example F: 0.035 0.43 1.37 0.008 0.005 0.03 0.33 0.101 0 0 0 0 0.02 2.92 2.440666667 Example G: 0.062 0.21 1.72 0.007 0.006 0.01 0.25 0.140 0 0 0 0 0 2.85 Example H 0.038 0.39 1.65 0.014 0.004 0.01 0.35 0.135 0 0 0 0 0 2.90 2.836 Example I 0.049 0.17 1.42 0.013 0.001 0.05 0.46 0.120 0 0 0 0 0 2.96 2.861333333 Example J 0.072 0.23 1.75 0.012 0.002 0.03 0.40 0.085 0 0 0 0 0 2.93 2.96 Example K 0.052 0.47 1.28 0.015 0.001 0.04 0.42 0.102 0 0 0 0 0.01 2.82 2.52 Comparative Example L 0.056 0.20 1.42 0.01 0.001 0.03 0.45 0.148 0 0 0 0 0 2.99 2.Example 96 Comparative M 0.042 0.29 1.59 0.01 0.002 0.03 0.47 0.067 0 0 0 0 0 2.78 2.79 Example Comparative N 0.049 0.16 1.56 0.01 0.001 0.01 0.48 0.132 0 0 0 0 0 2.99 3.09 Petition 870260055051, dated 08 / 06 / 2026, page 28 / 77 25 / 28 Comparative Example O 0.085 0.18 1.45 0.015 0.002 0.042 0.03 0.162 0 0.21 0.01 0 0.051 5.735 3.315 Comparative Example P 0.07 0.1 1.85 0.02 0.01 Not described 0.80 0.12 0 0 0 0 0 3.65 4.04 Table 2 Steel No. Continuous casting cooling rate °C / s Hot rolling furnace sheet temperature °C Reheating temperature °C First pass reduction rate % Second pass reduction rate % Finishing rolling temperature °C Cooling temperature °C Example A-1 10 720 1190 55 57 900 600 Example Com. A-2 10 620 1220 55 55 890 600 Example B-1 15 800 1210 58 56 890 615 Example Com. B-2 10 780 1220 45 40 880 610 Example C-1 12 810 1200 59 59 880 630 Example Com. C-2 1 780 1225 57 57 890 600 Example D-1 17 750 1230 65 60 870 605 . Example With D-2 10 850 1240 58 58 910 520 Petition 870260055051, dated 08 / 06 / 2026, page 29 / 77 26 / 28 Example E 13 830 1225 63 61 920 560 Example F 14 790 1220 62 62 930 575 Example G 16 850 1245 64 59 945 595 Example H 9 710 1160 56 56 855 580 Example I 11 770 1170 58 58 865 585 Example Com. M 15 730 1210 58 57 900 610 Example Com. N 15 720 1210 55 55 890 580 Example Com. J 15 760 1220 57 55 900 610 Example Com. K 10 720 1230 55 57 890 610 Example Com. L 10 710 1230 56 55 900 600 Example Com. O Not described Not described >1150 Not described Not described 900 525 Example Com. P Not described 1000°C 1180 Not described Not described 930 640 Petition 870260055051, dated 08 / 06 / 2026, page 30 / 77 27 / 28 Table 3 Steel No. Yield Strength, MPa Tensile Strength MPa A50% A5% Hole Expansion Ratio (Drilled Hole), % Hole Expansion Ratio (Countersunk Hole), % 180° Bending (d=0.5a) Example A-1 762 843 17.3 21.2 99 123 qualified Comparative Example A-2 652 743 16.2 19.5 101 126 qualified Example B-1 737 822 18.4 23.1 105 136 qualified Comparative Example B-2 725 823 15.8 19.6 85 115 cracked Example C-1 803 830 15.2 19.1 99 123 qualified Comparative Example C-2 681 811 11.1 14.3 70 83 cracked Example D-1 818 873 15.3 19.1 92 118 qualified Example Comparative D-2 636 731 12.7 16.5 112 139 qualified Example E 712 785 20.7 24.3 103 132 qualified Example F 724 805 18.5 25.0 108 139 qualified Example G 829 899 15.1 19.2 86 117 qualified Example H 749 815 16.3 20.7 89 120 qualified Example I 772 833 16.0 20.5 91 122 qualified Example Comparative J 853 916 7.0 10.0 72 85 cracked Comparative Example K 601 727 20.2 24.5 112 139 qualified Petition 870260055051, dated 08 / 06 / 2026, page 31 / 77 28 / 28 Comparative Example L 805 853 15.3 19.4 70 87 cracked Comparative Example M 636 731 24.7 29.6 112 139 qualified Comparative Example N 798 832 16.1 20.2 76 101 qualified Comparative Example O 698 803 18.0 Not described 79 Not described Not described Comparative Example P 755-800 780-885 Not described 17-23 Not described Not described 180° bending d=a Petition 870260055051, dated 08 / 06 / 2026, page 32 / 77

Claims

1 / 4 CLAIMS 1. Ultra-high strength multiphase steel plate / strip, characterized in that it comprises the following chemical elements in weight percentages: C: 0.03-0.07%; Si: 0.1-0.5%; Mn: 1.3-1.9%, P <0.02%, S < 0.01%; Al: 0.01-0.05%; Cr: 0.2-0.5%; Ti: 0.07-0.14%, (Ni+Nb+Mo+V) < 0.03%, and a balance of Fe and unavoidable impurities; at the same time it is required to satisfy: [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Mi)+20(Nb+V)]<3.0; (Mn+2Cr+4Ti+4Nb+4V+4Mo-Si / 3+2C)<3.0; wherein the steel plate / steel strip has a tensile strength of >780 MPa, a yield strength of >680 MPa, an A50 elongation of >15%, and a hole expansion ratio performance index that satisfies: if the original hole is a drilled hole, the hole expansion ratio is >85%; if the original hole is a countersunk hole, the hole expansion ratio is >115%; and a bending performance that satisfies 180° bending is qualified at d=0.5a; wherein the hole expansion ratio is tested using the hole expansion ratio test method specified in ISO / DIS 16630, the 180° bending test is performed using the method for determining bending properties in GB / T232-2010.

2. Ultra-high strength multiphase steel plate / steel strip according to claim 1, characterized in that the C content is 0.04-0.06% by weight.

3. Ultra-high strength multiphase steel plate / steel strip according to claim 1, characterized in that the Si content is 0.1-0.27% by weight.

4. Ultra-high strength multiphase steel plate / steel strip according to claim 1, characterized in that the Mn content is 1.45-1.75% by weight.

5. Ultra-high strength multiphase steel plate / strip, according to claim 1, Petition 870260055051, dated 08 / 06 / 2026, page 33 / 77 2 / 4characterized in that the Cr content is 0.35-0.50% by weight.

6. Ultra-high strength multiphase steel plate / steel strip according to claim 1, characterized in that the chemical elements Nb+Mo+V<0.03% by weight.

7. Ultra-high strength multiphase steel plate / steel strip according to any one of claims 1 to 6, characterized in that the steel has a structure containing ferrite, lower bainite, and also containing carbide precipitation phase, inclusion phase and / or martensite trace phase, and in which the ferrite content is <70%, and the ferrite and lower bainite content is >90%.

8. Ultra-high strength multiphase steel plate / steel strip according to claim 7, characterized in that the microstructure of the steel plate / steel strip also comprises TiN particles,and a single particle has a longer side length of <10 pm or an area of ​​<50 pm2.

9. Ultra-high strength multiphase steel plate / steel strip according to claim 7, characterized in that the average diameter of ferrite grains is <6 pm, or an ASTM classification of ferrite grain size is >11.

8.

10. Ultra-high strength multiphase steel plate / steel strip according to any one of claims 1 to 9, characterized in that the steel plate / steel strip has a tensile strength of >800 MPa, a yield strength of >710 MPa, and a hole expansion ratio performance index that satisfies: if the original hole is a drilled hole, the hole expansion ratio is >95%; if the original hole is a countersunk hole, the hole expansion ratio is >120%; and a bending performance that satisfies 180° bending is qualified at d=0.5a. Petition 870260055051, dated 08 / 06 / 2026,Page 34 / 77 3 / 4 11. Ultra-high strength multiphase steel plate / strip according to claim 1, characterized in that the steel plate / strip has a yield ratio of >0.

9.

12. Manufacturing method for the ultra-high strength multiphase steel plate / strip as defined in any one of claims 1 to 11, characterized in that it comprises the following steps: 1) Reduction, reducing in continuous casting wherein the chemical elements, as defined in any one of claims 1 to 6, are melted into a casting plate by continuous casting, wherein a cooling rate of the plate is >5°C / s during continuous casting; 2) Enter hot, roll, cool after rolling and cool the sheet where the sheet enters the furnace at a temperature of not less than 700°C, and the sheet is heated to a heating temperature of 1100-1250°C; where each reduction rate for the first and second hot rolling pass is >55%, and a final rolling temperature is 850-950°C; and the cooling temperature is 550-630°C; and 3) Pickling.

13. Manufacturing method for ultra-high strength multiphase steel plate / steel strip according to claim 12, characterized in that, after step 3) of pickling, the method further comprises a hot-dip galvanizing annealing process to obtain the finished hot-rolled hot-dip galvanized steel plate.

14. Manufacturing method for ultra-high strength multiphase steel plate / steel strip according to claim 12, characterized in that, in step 1), a proportion of columnar crystals in the sheet casting structure is <10%, or a thickness of the columnar crystal region is <40 mm.

15. Manufacturing method for ultra-high strength multiphase steel plate / strip according to claim 12, characterized in that the thickness of the steel plate / strip is 0.7-4.0 mm. Petition 870260055051, dated 08 / 06 / 2026, p. 36 / 77

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