Multiphase steel plate / steel strip with low silicon content and low carbon content equivalent to GPA, and its manufacturing method.

Abstract

A low-silicon, low-carbon multiphase steel plate / strip equivalent to GPa, and a method for manufacturing the same, are disclosed. The steel plate / steel strip comprises the following components in weight percentage: 0.03-0.07% C, 0.1-0.5% Si, 1.7-2.0% Mn, P=0.02%, S=0.01%, N=0.01%, 0.01-0.05% Al, 0.4-0.7% Cr, 0.001-0.005% B, and 0.07-0.15% Ti, and also comprises one or both of 0.15-0.4% Mo or 0.02-0.08% Nb, the remainder being Fe and other unavoidable impurities; and, at the same time, the steel plate / steel strip satisfies: available B* content = 0.001, available B* content = B - [Ti - 3.4N - 1.2(C - Nb / 7.8)] / 22, available CE < 0.58, and CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15.The steel plate has a tensile strength of ≥980 MPa and a rupture strength of ≥780 MPa, and the hole expansion rate satisfies the following: if the original hole is a drilled hole, the hole expansion rate is >50%; and if the original hole is a deburred hole, the hole expansion rate is >60%. The steel plate is mainly used in the preparation of vehicle chassis and suspension system parts.

Description

1 / 23 “LOW SILICON AND LOW CARBON EQUIVALENT MULTIPHASE STEEL PLATE / STRIP EQUIVALENT TO GPA, AND ITS MANUFACTURING METHOD” Technical Field

[0001] The present disclosure pertains to the field of metallic materials, in particular, it relates to a multiphase steel plate / steel strip with low carbon content equivalent GPa and a method of manufacturing the same, which is mainly used in the manufacture of products for automotive chassis and suspension systems. Antecedent Technique

[0002] The lightweight nature of automobiles can directly reduce emissions and decrease fuel consumption, which is the goal of the automotive industry today. An important measure of lightweight automobiles is the use of high and ultra-high strength steel sheets instead of low strength steel sheets. Currently, the lightweight concept is also applied to automotive chassis and suspension systems, and automotive chassis materials are also required to adopt high-strength steel to achieve lightweight with increasingly stringent environmental and market demands.

[0003] However, in addition to the higher strength of the steel plate, it is also necessary for the structural parts of the automobile chassis and suspension system that the steel plates have good hole expansion performance, surface coating performance, and welding performance. Therefore, multiphase steel with ferrite, bainite, and carbide precipitation as the main structure has become the commonly used steel class for automobile chassis and suspension system parts due to its high strength and good hole expansion performance. However, the strength of common multiphase steels on the market Petition 870260051343, dated 05 / 28 / 2026, page 5 / 65 2 / 23 current strength generally cannot reach the GPa level. Most multiphase steels have a tensile strength of 600-700MPa and a tensile strength of 700-900MPa in published patents. Since a certain amount of ferrite and bainite is required in the multiphase steel structure to provide high hole expansion performance (the strength of these two structures is lower than that of martensite), it is difficult to further increase the strength of multiphase steel to the GPa level.

[0004] Currently, there are two common methods for improving the tensile strength of multiphase steel to the GPa level (i.e., tensile strength >980MPa). One method is to introduce a large amount of carbon, silicon, and manganese, especially silicon, into the steel to alter the structure of the multiphase steel, thus introducing retained martensite or austenite to increase strength. Another method is to add large amounts of other alloying elements to increase strength. However, introducing a large amount of silicon will deteriorate the surface quality of the steel plate, and introducing a large amount of other alloying elements will greatly increase the cost of the steel plate. In addition, these two methods considerably increase the carbon equivalent level of the steel plate.However, compared to body parts, the structure of automotive chassis components is complex, requiring various welding processes such as argon-shielded welding, laser welding, spot welding, etc. This necessitates higher requirements for the carbon equivalent level of the steel. There is a technical contradiction between the development of GPa-grade multiphase steel for chassis and the cost-effective control of the carbon equivalent level of the steel sheet, a challenge not addressed in published patents.

[0005] For example, CN101400816A causes the steel to reach the Petition 870260051343, dated 05 / 28 / 2026, page 6 / 65 3 / 23 GPa strength level by adding a large amount of precious alloying elements, such as Ni and Cu, etc. But this method increases not only the cost of the steel alloy, but also the carbon equivalent level of the steel. In addition, more than 0.50% silicon is added in most examples of this patent application.

[0006] According to Yang Yu, Chang Wang, Lin Wang, et al., Research on red iron scale defects in Si-containing steel based on high-temperature oxidation characteristics [J], Rolling Steel, 2016, 33(2): 10-15; and Research on the effect of the element Si on the interface microstructure of oxide scale formation in the heating well [J], Rolling Steel, 2016, 33(5): 6-10, when the silicon content in steel is high, defects such as red scales (red rust, tiger winkle defect, etc.) can be formed and thus the surface quality of the steel is reduced. Among them, automotive steel containing 0.5% silicon was found with an evenly spaced strip of iron scale on the surface of the steel strip, and defects such as red rust and tiger winkle defect accounted for about 30% of the steel strip surface. This surface condition cannot be used to prepare automotive parts products that are extremely demanding in surface appearance and color.In the only inventive example where the silicon content can meet the requirements of the automotive steel products disclosed in this patent application, the carbon equivalent is as high as 0.73 or more, much less the high alloy cost caused by the addition of high Cu and high Ni. Therefore, the products involved in this patent application cannot be used to manufacture low-cost, low-carbon equivalent GPa-grade multiphase steel products for automotive chassis that are urgently needed in the market.

[0007] Similarly, CN201710022118.8 and CN201180067938.X designed a multiphase steel product that Petition 870260051343, dated 05 / 28 / 2026, page 7 / 65 4 / 23 reaches the GPa level. Although no precious Ni and Cu alloying element is added in these two patent applications, the silicon content is above 0.5%, and the carbon equivalent level is high, so it cannot be used to prepare automotive parts products that have extremely stringent requirements on surface appearance and color and carbon equivalent, and will not be repeated here.

[0008] CN201380022062.6 discloses a GPa grade multiphase steel product that does not contain tabulated surface defects (i.e., red scale surface defects mentioned in this disclosure) with a low silicon composition design. However, according to the carbon equivalent formula CE=C+Mn / 6+(Cr+Mo+V) / 5+(Si+Ni+Cu) / 15 published by the American Society for Metals, the carbon equivalent of the product in this patent application is greater than 0.60, and the hole expansion ratio performance of the product is not evaluated in this patent application.

[0009] Therefore, existing technology cannot resolve the contradiction between the tensile strength of GPa grade and the low silicon carbon equivalent (i.e., surface quality and weldability) of multiphase steel products for automotive chassis. It is a problem in the current steel industry and an urgent need in the current automotive industry to obtain a GPa grade multiphase steel plate / strip with a combination of GPa grade strength, high hole expansion, and high weldability to meet the production and manufacturing requirements of automotive chassis structural components. Summary

[0010] An objective of the present disclosure is to provide a multiphase steel plate / strip with low silicon and low carbon content equivalent to GPa and a manufacturing method whereby the steel plate has a tensile strength of >980 MPa, a stress Petition 870260051343, dated 05 / 28 / 2026, page 8 / 65 5 / 23 of a rupture strength of >780MPa, a hole expansion rate that satisfies: if the initial hole is a drilled hole, the hole expansion rate is >50%; if the initial hole is a deburred hole, the hole expansion rate is >60%. The multiphase steel plate / strip with low silicon content and low carbon content equivalent to GPa is suitable for the manufacture of automotive chassis and suspension system parts.

[0011] To achieve the above objective, the technical solution of the present disclosure is as follows:

[0012] A multiphase steel plate / strip with low silicon and low carbon content equivalent to GPa comprises the following chemical elements in weight percent: C: 0.03-0.07%, Si: 0.1-0.5%, Mn: 1.7-2.0%, P < 0.02%, S < 0.01%, N < 0.01%, Al: 0.01-0.05%, Cr: 0.4-0.7%, B: 0.001-0.005%, Ti: 0.07-0.15%, and also comprises Mo: 0.15-0.4%, and / or Nb: 0.02-0.08%, and a remainder of Fe and unavoidable impurities; at the same time, it is necessary to satisfy:

[0013] an effective B* content > 0.001, the effective B* content = B - [Ti3.4N - 1.2(C - Nb / 7.8)] / 22;

[0014] EC< 0.58, EC=C+Mn / 6+(Cr+Mo+V) / 5+(Si+Ni+Cu) / 15.

[0015] Preferably, the C content is 0.045-0.06%, as a percentage by weight.

[0016] Preferably, the Si content is 0.15-0.27%, as a percentage by weight.

[0017] Preferably, the B content is 0.002-0.004%, as a percentage by weight.

[0018] The microstructure of the steel plate / steel strip of the present disclosure contains ferrite and lower bainite, as well as a small amount of carbide precipitation phase, other inclusion phase and / or martensite trace phase, wherein the ferrite content is <20%, and the ferrite + lower bainite content is >95%, by volume percentage. Petition 870260051343, dated 05 / 28 / 2026, page 9 / 65 6 / 23

[0019] Preferably, the microstructure of the steel plate / steel strip also contains TiN particles, and a single particle has a longer side length of <8 μm or an area of ​​<50 μm2.

[0020] Preferably, the average diameter of the ferrite grains is <6 μm, or an ASTM ferrite grain size classification >11.8.

[0021] The manufacturing method for multiphase steel plate / steel strip according to the present disclosure, with low silicon content and low carbon content equivalent to GPa, comprises the following steps:

[0022] 1) Casting, continuous casting

[0023] wherein the above chemical composition is melted and cast into a plate by continuous casting, wherein a cooling rate of the plate is >5 °C / s during continuous casting;

[0024] 2) Hot plate transfer, lamination

[0025] wherein the plate enters the furnace at a temperature not lower than 700 °C, and the plate is heated to a heating temperature of 1100-1250 °C; wherein each reduction rate for the first two hot rolling passes is >55%, and a final rolling temperature of finished rolling is 850-950 °C;

[0026] 3) Cooling after lamination, winding

[0027] in which water cooling is carried out after rolling, and the rolling temperature is 550-630 °C;

[0028] 4) Pickling.

[0029] In addition, after step 3) of pickling, the method also includes the hot-dip annealing process to obtain the finished hot-rolled hot-galvanized steel plate.

[0030] Preferably, the thickness of the steel plate / steel strip is 0.7 to 4.0 mm.

[0031] The steel component according to the present disclosure is designed as follows: Petition 870260051343, dated 05 / 28 / 2026, page 10 / 65 7 / 23

[0032] Carbon (C): Carbon directly affects the strength, weldability, and formability of steel plates / steel 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 0.03%, the strength of the steel plate / steel strip cannot meet the objective requirements; if the carbon content is greater than 0.07%, it is easy to cause an excessively high carbon equivalent and deteriorate the weldability of the steel plates. Thus, the carbon content with a current development is controlled to be from 0.03 to 0.07%.

[0033] Silicon (Si): Silicon has a certain solid solution strengthening effect. The higher the Si content, the more conducive it is to improving the strength of the steel plate / steel strip. But when the silicon content is higher than 0.5%, it is prone to generating serious hot-rolled iron oxide scale on the surface of the hot-rolled steel plate / steel strip, which not only worsens the surface quality of the steel plate / steel strip, which is not conducive to the production of hot-dip galvanized steel plate / steel strip, but also impairs the platability of the steel plate / steel strip. Therefore, the silicon content according to the present disclosure is controlled to be from 0.1 to 0.5%.

[0034] Manganese (Mn): Because manganese can effectively increase 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 disclosure. However, when the manganese content exceeds 2.0%, the steel plate / steel strip develops structural weakness, which impairs the hole expansion performance; when the manganese content is less than 1.70%, the strength of the steel plate / steel strip is insufficient. Therefore, the manganese content in the present disclosure is controlled to be between 1.7% and 2.0%.

[0035] Aluminum (Al): Aluminum is added as the main Petition 870260051343, dated 05 / 28 / 2026, page 11 / 65 8 / 23 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 performance of the steel plate / steel strip in welding. Therefore, the aluminum content according to the present disclosure is controlled from 0.01 to 0.05%.

[0036] Chromium (Cr): Chromium is conducive to the expansion of the bainite phase region, ensuring that the bainite structure can be obtained upon cooling of the steel plate / steel strip after rolling, which is conducive to improving the strength and expansion ratio of the hole. However, when the amount added exceeds 0.7%, the increase in strength is no longer significant and is not conducive to the weldability of the steel plate / steel strip. However, when the content is less than 0.4%, the expansion of the bainite phase region is no longer significant. Therefore, the chromium and molybdenum content according to the present disclosure is controlled to be from 0.4 to 0.7%.

[0037] Titanium, niobium, and molybdenum (Ti, Nb, Mo): Titanium, niobium, and molybdenum are the main alloying elements in the multiphase steel of the present disclosure, which increases the strength of the multiphase steel by second-phase strengthening after the formation of a fine hard metal. The Nb element in all three has a stronger carbide-forming capacity. If the addition of microalloying elements is insufficient, the strength of the steel plate cannot meet the design requirements. In addition, the TiN element will also form TiN particles with the N element in the steel, and the oversized TiN will negatively affect the hole expansion. The Ti element also forms titanium boron with the B element in the steel, reducing the effective boron content in the steel. When the microalloying content is low, the strength of the steel plate / steel strip is insufficient. Furthermore, the particle size of Petition 870260051343, dated 05 / 28 / 2026, page 12 / 65 9 / 23 TiN must be controlled to ensure that a single particle has a longest side length of < 8 μm or an area of ​​< 50 μm2, thus avoiding coarse TiN particles that damage the expansion hole of the steel plate.

[0038] Boron (B): Boron is conducive to the expansion of the bainite phase region, ensuring that the steel plate / steel strip can obtain a bainite structure during cooling after rolling, and the strength and hardness of the steel are significantly improved. However, too much B will lead to an excessive martensitic structure in the steel plate, resulting in a decrease in the elongation and expansion ratio of the steel holes. Furthermore, the B element in steel that is truly conducive to the expansion of the bainite phase region is an effective B element that does not combine with Ti, N, and other elements to form a boride, and the influence of the effective B element is calculated according to the following formula:

[0039] B*=B-[Ti-3.4N-1.2(C-Nb / 7.8)] / 22 > 0.001.

[0040] The upper limit of impurity elements in steel is controlled at P <0.02%, S <0.01%, N <0.01%, and the purer the steel, the better the effect.

[0041] The microstructure of the steel plate / steel strip of the present disclosure is a lower ferrite + bainite microstructure, wherein the ferrite content is <20%. The lower ferrite + bainite content is >95%. If the ferrite structure is greater than 20%, the steel plate / steel strip will not be able to provide the required strength; if the lower ferrite + bainite content is less than 95%, the performance of the steel plate / steel strip in hole expansion will not meet the requirements. The microstructure of the steel plate / steel strip of the present disclosure may also contain a small amount of carbide precipitation phase (e.g., 5% or less), a martensite phase (e.g., 0.5% or less), or an infinitesimal phase of another inclusion. Petition 870260051343, dated 05 / 28 / 2026, page 13 / 65 10 / 23 (0.01% or less, occasionally found in the visible field). The other inclusion phase can be common inclusions in steels such as MnS, TiN, and AlN.

[0042] In the microstructure of the steel plate / steel strip according to the present disclosure, the average diameter of the ferrite grain is <6 μm, or the ASTM grain size classification is >11.8. If the average grain diameter is not less than 6 μm or the grain size classification is not greater than 11.8, the steel plate / steel strip will not be able to provide the required strength.

[0043] In addition, the measurement ratio between the above alloying elements and the carbon elements must also meet the following carbon equivalent calculation formula: CE=C+Mn / 6+(Cr+Mo+V) / 5+(Si+Ni+Cu) / 15 < 0.58 to ensure that the multiphase steel has a low carbon equivalent content and good weldability.

[0044] In the manufacturing method according to the present disclosure:

[0045] The cooling rate of the slab during continuous casting will affect the grain size in the final structure of the steel slab / steel strip, the size of the inclusions formed in the liquid phase, and the proportion of columnar crystals in the slab structure. If the cooling rate is less than 5°C / s, on the one hand, the thickness or proportion of columnar crystals in the slab will be greater than the design requirements, so it is easy to form a banded structure in the finished product structure, which affects the bending properties of the steel slab / steel strip; On the other hand, decreasing the cooling rate of the slab during continuous casting will result in the grain size in the final structure not meeting the design requirements and will lead to coarse inclusions (typically as TiN) generated in the liquid phase in the steel, which is Petition 870260051343, dated 05 / 28 / 2026, page 14 / 65 11 / 23 adverse to bore expansion and bending performance.

[0046] The minimum temperature before the plate enters the heating furnace will affect the final product properties. When the minimum temperature before the plate enters the heating furnace is below 700°C, titanium carbide will precipitate on the plate in large quantities, and in the subsequent reheating process, the titanium carbide that has precipitated on the plate cannot be completely re-dissolved, resulting in less titanium dioxide and titanium carbide in the matrix after hot rolling and insufficient product strength. When the final rolling temperature of the finishing roll is below 850°C, ferrite will precipitate before the finishing roll, resulting in a low bainite content in the final structure, so the steel plate / steel strip cannot reach the defined strength.However, considering the heating temperature of the plate, 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 passes of hot rolling 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. Additionally, the high reduction rate in step 2) must be combined with the high cooling rate of the plate during continuous casting in step 1).If the cooling rate of the continuous casting does not reach 5°C / s or more, the inclusions (mainly TiN) generated in the liquid phase in the slab will be very large; at this point, if a large reduction rate of >55% is used in step 2), this will lead to the cracking of the thick TiN, as shown in Figure 1, thus becoming the source of cracks within the steel slab / steel strip and resulting in... Petition 870260051343, dated 05 / 28 / 2026, page 15 / 65 12 / 23 deterioration of the hole expansion property 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 plate are small in size, as shown in Figure 2, and will not break up at the large hot rolling reduction rate in step 2), so it does not impair the hole expansion property of the steel plate / steel strip.

[0047] The coiling temperature is one of the most critical process parameters for achieving high strength and high hole expansion rate. When the coiling temperature exceeds 630°C, due to strong precipitation and thickening of alloy carbides, it has a negative effect on the hole expansion rate of the steel sheet. On the other hand, when the coiling temperature is below 550°C, carbide precipitation will be seriously inhibited, resulting in the steel sheet strength not meeting the established requirements. Therefore, the coiling temperature in the present disclosure is limited to 550-630°C.

[0048] After testing, the performance of the ultra-high strength hot-rolled steel plate / strip according to the present disclosure meets the following indices:

[0049] Mechanical properties at room temperature: tensile strength >980MPa, preferably >1000MPa; yield strength >780MPa, preferably >800MPa;

[0050] Hole expansion rate performance: if the original hole is drilled, the hole expansion rate is greater than 50%, preferably >55%; if the original hole is countersunk, the hole expansion rate is greater than 60%, preferably >65%.

[0051] In some embodiments, the ultra-high strength hot-rolled steel plate / strip according to the present disclosure has a tensile strength of 980-1100MPa, a limit Petition 870260051343, dated 05 / 28 / 2026, page 16 / 65 13 / 23 resistance of 780-900MPa; hole expansion rate performance: if the original hole is a drilled hole, the hole expansion rate is 55% to 70%; if the original hole is a countersunk hole, the hole expansion rate is 65-80%.

[0052] The design of components with low silicon and low carbon equivalent is adopted in the present disclosure to meet the surface quality and weldability requirements of multiphase steel for automobile chassis. First, the silicon content is designed to be Si: 0.1-0.5%, preferably 0.1-0.4%, more preferably the silicon content Si: 0.15-0.27%. Second, the carbon equivalent satisfies CE=C+Mn / 6+(Cr+Mo+V) / 5+(Si+Ni+Cu) / 15 <0.7 (carbon equivalent formula suggested by the American Society for Metals), preferably < 0.58.

[0053] Under the premise of designing with low silicon and low carbon equivalent content, in order to ensure that the steel sheet reaches the GPa strength level, in addition to adding a certain amount of alloying elements such as Mn and Cr to the steel, the distribution of the microalloying elements B, Ti, and Nb is further optimized. Although it is known that the trace element B can greatly improve the strength and hardness of the steel sheet, there is no clear research on how much element B is added to multiphase steel products. In fact, the element B added to the steel will react with various alloying elements, and the most active reaction is the production of BN with the element N in the steel, but the formation of BN will greatly impair the manufacturing and performance of the final steel product. Therefore, some element Ti will be added to the steel containing B to prevent the reaction between N and B, preferentially forming TiN.However, the remaining Ti in the steel is also a strong boride-forming element, which will react with element B to form titanium boride. On the other hand, the... Petition 870260051343, dated 05 / 28 / 2026, page 17 / 65 14 / 23 element Ti will also form TiC with effective element C. Therefore, the effective boron content in steel depends on the content of Ti and N elements, on the one hand, and is also affected by the effective carbon element, on the other hand, and the latter is also affected by the content of strong carbide-forming elements and even bainite. Thus, the content of effective element B in steel is affected by a combination of very complex factors. With regard to effective element B (represented by B*) in steel, after exhaustively considering all factors, it is proposed in the present disclosure that to ensure that the steel sheet reaches the GPa strength level, the effective boron element must satisfy B*=B-[Ti-3.4N-1.2(C-Nb / 7.8)] / 22>0.001.

[0054] Small, highly uniform structure and small inclusions can be achieved through process optimization to obtain excellent hole expansion performance. In one aspect, a high cooling rate design is adopted in continuous casting. On the one hand, the proportion of columnar crystals in the slab is reduced to increase the proportion of fine equiaxed crystals, and on the other hand, the size of the inclusions generated in the liquid phase (represented by TiN) is reduced. In another aspect, in the first and second hot rolling passes, a high-reduction rolling process design is adopted to obtain a fine structure while further destroying the columnar crystals and achieving a combination of high strength and high hole expansion rate.

[0055] The ultra-high strength hot-rolled steel plate / strip manufactured by the present disclosure has a combination of low silicon content, low carbon equivalent, high GPa grade strength and high hole expansion performance. The ultra-high strength hot-rolled steel plate / strip is hot-dip galvanized to obtain a hot-rolled galvanized steel plate. Petition 870260051343, dated 05 / 28 / 2026, page 18 / 65 15 / 23 hot finished. Ultra-high strength hot-rolled steel plate products, steel strip products and hot-dip galvanized steel plate products can be used for the manufacture of automobile chassis and suspension system parts, achieving the “lightweight” of automobiles. Description of the Drawings

[0056] Figure 1 shows the size of TiN particles when the continuous casting cooling rate reaches 5 °C / s or more and their morphology after hot rolling at a large reduction (photograph of the structure in the hot rolling state).

[0057] Figure 2 shows the size of TiN particles when the cooling rate of the continuous casting is less than 5°C / se and its morphology after hot rolling in great reduction (photograph of the structure in the hot rolling state).

[0058] FIGURE 3 is a photo of hot-rolled red iron scale defects (tiger winkle) on the surface of the steel strip when the Si element exceeds 0.5% (FIGURE 3 shows that the Si content is 0.55%, Comparative Example L).

[0059] FIGURE 4 is a photo of the surface of the steel strip when the element Si is less than 0.5% (FIGURE 4 shows that the Si content is 0.25%, Example C).

[0060] FIGURE 5 shows that in the microstructure of the steel plate / steel strip of the present disclosure, the ferrite + bainite content is less than >95%. Detailed Description

[0061] The present revelation will be further described with reference to the following examples and figures.

[0062] The steels with different components after casting shown in Table 1 were heated and hot-rolled according to the process shown in Table 2 to obtain sheets of Petition 870260051343, dated 05 / 28 / 2026, page 19 / 65 16 / 23 steel with a thickness less than 4 mm. The yield strength, tensile strength, and elongation were measured for tensile specimens with reference lengths of 50 mm and 5 mm along the longitudinal direction, and the hole expansion ratio and 180° bending performance were measured in the central 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 cracks or perforated cracks appeared on the edge of the hole. Since the method of preparing the original hole in the center of the specimen has a great influence on the results of the hole expansion ratio test, the original holes in the center of the specimens were prepared by punching and enlarging, respectively, and subsequent tests were performed.in accordance with the hole expansion rate test method specified in ISO / DIS 16630.

[0063] In Table 1, Examples AI are the steels of the present disclosure. Comparative Examples JM are comparative steels, in which the carbon or manganese content or other alloying elements exceed the scope of the composition of the present disclosure. Comparative Examples O and P use the component and process according to the published patent application. Comparative Example O is an example of CN201380022062.6, in which the alloying ingredient is different from that of the present disclosure and the carbon equivalent is higher than that of the present disclosure; Comparative Example P is an example of CN201180067938.X, in which the alloying ingredient is also different from that of the present disclosure and the carbon equivalent is higher than that of the present disclosure.

[0064] Table 2 shows the different manufacturing processes of Petition 870260051343, dated 05 / 28 / 2026, page 20 / 65 17 / 23 various steel grades in Table 1, which are also divided into two categories of Examples and Comparative Examples, wherein the processes of Comparative Example O and Comparative Example P are the processes disclosed in the corresponding patent applications. But Comparative Example O is a cold-rolled product that does not involve a hot rolling process, and its product performance is the product performance after cold rolling and annealing. Some parameters in Comparative Example P are not mentioned, and other parameters are partially different from those of the present disclosure. Table 3 lists the value of the tested mechanical property of the Examples and Comparative Examples mentioned above.

[0065] It can be observed that when the content of C, Mn, Ti, Nb, B or B* deviates from the scope of the present disclosure, for example, when the content of Mn, Ti, and Nb, or B*, is relatively low, as in Comparative Examples K, L, and N, leading to a steel plate strength lower than the design requirements; and when the content of C or B is higher than the composition range of the present disclosure, as in Comparative Examples J and M, leading to the production of a large amount of martensite in the structure, which deteriorates the hole expansion performance of the material, not meeting the purpose of the present disclosure.

[0066] When the Si content is greater than the scope of the present disclosure, as in Comparative Example L, severe red iron scale (tiger winkle) defects appear on the surface of the steel plate after hot rolling and pickling, as shown in FIGURE 3; and when the Si content is within the scope of the present disclosure, the surface of the steel plate is of normal color after hot rolling and pickling, as in Example C, as shown in FIGURE 4.

[0067] When the temperature of the plate entering the oven is too Petition 870260051343, dated 05 / 28 / 2026, page 21 / 65 18 / 23 low, as in Comparative Example A-2, the strength does not meet the design standards of the present disclosure; when the coiling temperature is too high, as in Comparative Example D-2, a large amount of coarse carbide particles is generated in the steel plate after coiling, which deteriorates the elongation and hole expansion performance. When the reduction ratio 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 fully refined to achieve structure uniformity, which leads to deterioration of the elongation and hole expansion performance of the steel plate, as in Comparative Example B-2.When the cooling rate of continuous casting is insufficient, but a large reduction rate is sought in hot rolling, the coarse TiN particles in the steel are broken and a potential source of cracking is formed, which greatly deteriorates the elongation and hole expansion performance of the steel material, such as Comparative Example C-2.

[0068] Based on the foregoing, the present disclosure adopts a design with low silicon and low carbon equivalent content and optimizes the ratio of each element by reasonably designing the effective element B content range based on carbon-manganese steel. By further increasing the cooling rate in continuous casting, the hot rolling reduction rate, and the coiling temperature based on the conventional automotive steel production line, the present disclosure produces an ultra-high strength GPa grade hot-rolled steel plate / strip with a combination of high strength, high hole expansion performance, excellent surface quality, and weldability performance, which has a yield strength of not less than 780 MPa, a tensile strength of not less than 980 MPa, and a hole expansion rate greater than 50%. Petition 870260051343, dated 05 / 28 / 2026, page 22 / 65 19 / 23 (the original bore is drilled) or greater than 60% (the original bore is enlarged), to compensate for the urgent market demand from the automotive industry for chassis and suspension materials with a combination of ultra-high strength, high-performance bore expansion, and low-carbon equivalent. Petition 870260051343, dated 05 / 28 / 2026, page 23 / 65 20 / 23 Table 1 (unit: percentage) Aço No. C Si Mn P S N Al Cr Ti Mo Nb B CE B* Ex. A 0,059 0,30 1,88 0,012 0,002 0,004 0,02 0,53 0,081 0,32 0,05 0,0015 0,562 0,0013 Ex. B 0,040 0,45 1,98 0,013 0,004 0,003 0,01 0,66 0,113 0,18 0,02 0,0045 0,568 0,0019 Ex. C 0,069 0,25 1,8 0,015 0,003 0,005 0,02 0,44 0,141 0,37 0,02 0,0031 0,548 0,0011 Ex. D 0,054 0,36 1,75 0,014 0,001 0,005 0,04 0,58 0,090 0,22 0,06 0,0020 0,530 0,0012 Ex. E 0,048 0,15 1,85 0,010 0,001 0,004 0,01 0,69 0,104 0,28 0,04 0,0035 0,560 0,0017 Ex. F 0,033 0,42 1,94 0,008 0,005 0,002 0,03 0,42 0,075 0,25 0,07 0,0043 0,518 0,0025 Ex. G 0,037 0,33 1,73 0,011 0,001 0,007 0,02 0,47 0,121 0,39 0,07 0,0040 0,519 0,0011 Ex. H 0,065 0,18 1,77 0,009 0,006 0,006 0,04 0,50 0,131 0,35 0,03 0,0037 0,542 0,0020 Ex. I 0,051 0,20 1,83 0,016 0,007 0,006 0,01 0,62 0,097 0,16 0,03 0,0025 0,525 0,0016 Ex. Comp. J 0,075 0,23 1,75 0,012 0,002 0,005 0,03 0,44 0,092 0,19 0,05 0,0044 0,472 0,0020 Ex. Comp.K 0.062 0.47 1.56 0.015 0.001 0.005 0.04 0.42 0.084 0.35 0.05 0.002 0.497 0.0022 Ex. Comp. L 0.051 0.55 1.86 0.01 0.001 0.003 0.03 0.5 0.061 0.35 0.01 0.0022 0.568 0.0026 Ex. Comp. M 0.042 0.25 1.96 0.01 0.002 0.005 0.03 0.47 0.09 0.32 0.04 0.0071 0.543 0.0053 Ex. Comp. N 0.045 0.22 1.93 0.01 0.001 0.005 0.01 0.55 0.135 0.29 0.03 0.0039 0.549 0.0008 Ex. Comp. O 0.14 0.06 2.29 0.001 0.0012 0.001 0.292 0.54 0.029 0 0 0.0015 0.634 0.0080 Ex. Comp. P 0.16 0.86 2.05 Not revealed Not revealed 0.004 0.033 0.33 0.12 0 0 0.002 0.625 0.0059. Petition 870260051343, dated 05 / 28 / 2026, page 24 / 65 21 / 23 Table 2 Steel No. Continuous casting cooling rate °C / s Hot rolling furnace slab temperature °C Reheating temperature °C First pass reduction rate % Second pass reduction rate % Finished rolling temperature °C Coiling temperature °C Ex. A-1 12 720 1200 55 57 910 590 Ex. Comp. A-2 10 580 1200 56 57 890 600 Ex. B-1 15 800 1220 58 58 900 610 Ex. Comp. B-2 10 780 1220 45 40 880 615 Ex. C-1 10 770 1210 57 58 870 570 Ex. Comp. C-2 1 780 1210 57 57 920 575 Ex. D-1 13 750 1230 60 59 930 580 Ex. Comp. D-2 10 750 1200 58 58 860 640 Ex. E 11 790 1240 61 55 925 620 Ex. F 17 730 1190 65 55 875 605 Ex. G 20 810 1180 57 56 850 595 Ex. H 16 780 1250 58 59 940 560 Ex. I 14 740 1215 60 60 915 585 Ex. Comp. J 15 730 1200 58 57 890 600 Ex. Comp. K 14 720 1200 55 55 910 590 Ex. Comp. 13 760 1230 57 55 900 610 Petition 870260051343, dated 05 / 28 / 2026, page 25 / 65 22 / 23 Steel No. Continuous casting cooling rate °C / s Hot rolling furnace slab temperature °C Reheating temperature °C First pass reduction rate % Second pass reduction rate % Finished rolling temperature °C Coiling temperature °C Ex. Comp. M 10 720 1210 55 57 910 600 Ex. Comp. N 15 710 1210 60 55 900 610 Ex. Comp. O Not revealed Not revealed Not revealed Not revealed Not revealed Not revealed Not revealed Ex. Comp. P Not revealed Not revealed 1260 900 Not revealed Not revealed 500 Table 3 Steel No. Yield Strength, MPa Tensile Strength, MPa A50% Hole Expansion Ratio (Punched Hole), % Hole Expansion Ratio (Countersink Hole), % Ex. A-1 799 1011 11.3 63 76 Ex. Comp. A-2 723 942 14 79 92 Ex. B-1 827 1039 10.8 57 71 Ex. Comp. B-2 800 1015 11.1 43 55 Ex. C-1 884 1097 10.2 55 69 Ex. Comp. C-2 832 1054 7.7 37 48 Ex. D-1 785 992 12.1 66 79 Ex. Comp. D-2 811 1046 10.0 47 58 Ex. E 839 1025 10.6 61 76 Ex. F 802 1040 10.9 56 67 Ex. G 812 1012 10.6 59 70 Petition 870260051343, dated 05 / 28 / 2026, page 26 / 65 23 / 23 Ex. H 891 1066 10.1 Ex. I 765 1012 11.3 Ex. Comp. J 893 1087 9.5 Ex. Comp. K 711 922 15 Ex. Comp. L 696 877 16.5 Ex. Comp. M 902 1103 8.1 Ex. Comp. N 752 966 12.3 Ex. Comp. O 748 1094 6.1(A80) Ex. Comp. P 825 1203 9.6(A80) 55 60 44 73 80 39 68 Not revealed Not revealed 65 72 61 90 95 52 84 Not revealed Not revealed Petition 870260051343, dated 05 / 28 / 2026, page 27 / 65

Claims

1 / 3 CLAIMS 1. Multiphase steel plate / steel strip with low silicon content and low carbon content equivalent to GPa, characterized in that it consists of the following chemical elements by weight percentage: C: 0.03-0.07%, Si: 0.1-0.5%, Mn: 1.7-2.0%, P<0.02%, S<0.01%, N<0.01%, Al: 0.01-0.05%, Cr: 0.4-0.7%, B: 0.001-0.005%, Ti: 0.07-0.15%, and also consisting of Mo: 0.15-0.4%, and / or Nb: 0.02-0.08%, and a remainder of Fe and other unavoidable impurities; At the same time, it satisfies: an effective B* content > 0.001, the effective content of: B* = B - [Ti - 3.4N - 1.2(C - Nb / 7.8)] / 22; CE < 0.58, CE = C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15; wherein the microstructure of the steel plate / steel strip contains ferrite and lower bainite, as well as a small amount of carbide precipitation phase, another inclusion phase and / or trace martensite phase, and wherein the ferrite content is < 20%, and the ferrite + lower bainite content is > 95%. 2.Steel plate / multiphase steel strip with low silicon content and low carbon content equivalent to GPa, according to claim 1, characterized in that the C content is 0.0450.06% by weight.

3. Steel plate / multiphase steel strip with low silicon content and low carbon content equivalent to GPa, according to claim 1, characterized in that the Si content is 0.150.27% by weight.

4. Steel plate / multiphase steel strip with low silicon content and low carbon content equivalent to GPa, according to claim 1, characterized in that the B content is 0.0020.004% by weight. 5.A multiphase steel plate / steel strip with low silicon content and low carbon content equivalent to GPa, according to claim 1, characterized in that the microstructure of the steel plate / steel strip also contains TiN particles, and a single particle has a longer side length of <8 μm or an area of ​​<50 μm².

6. A multiphase steel plate / steel strip with low silicon content and low carbon content equivalent to GPa, according to claim 1, characterized in that the average diameter of the ferrite grains is <6 μm, or an ASTM ferrite grain size classification is >11.

8. 7.Multiphase steel plate / steel strip with low silicon and low carbon content equivalent to GPa, according to claim 1, characterized in that the steel plate / steel strip has a tensile strength of >980MPa, a rupture strength of >780MPa; a hole expansion performance rate that satisfies that if the original hole is a drilled hole, the hole expansion rate is >50%; if the original hole is a deburred hole, the hole expansion rate is >60%. Petition 870260051343, dated 05 / 28 / 2026, page 29 / 65 3 / 3 8. Multiphase steel plate / steel strip with low silicon and low carbon content equivalent to GPa, according to claim 1, characterized in that the thickness after processing is from 0.7 to 4.0 mm. 9.Method for manufacturing a multiphase steel plate / steel strip with low silicon and low carbon equivalent GPa, as defined in any one of claims 1 to 8, characterized in that it comprises the following steps: (1) Melting, continuous casting wherein the chemical elements, as defined in any one of claims 1 to 4, are melted and cast into a plate by continuous casting, wherein a cooling rate of the plate is >5 °C / s during continuous casting; (2) Hot transfer of the plate, lamination wherein the plate enters the furnace at a temperature not lower than 700 °C, and the plate is heated to a heating temperature of 1100-1250 °C; wherein each reduction rate for the first two hot lamination passes is >55%, and a final lamination temperature of finished lamination is 850-950 °C; (3) Cooling after lamination, winding where cooling with water is carried out after lamination, and the winding temperature is 550-630 °C; (4) Pickling.

10. Method according to claim 9, characterized in that after the pickling step (3), the method further comprises the hot-dip galvanizing annealing process to obtain the finished hot-rolled hot-dip galvanized steel plate. Petition 870260051343, dated 05 / 28 / 2026, p. 30 / 65

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