AÇO DE ULTRA-ALTA RESISTÊNCIA REVESTIDO COM EXCELENTE DESEMPENHO DE SOLDAGEM POR PONTOS E MÉTODO DE PRODUÇÃO PARA O MESMO

Abstract

Disclosed in the present invention is coated ultrahigh-strength steel with excellent spot welding performance, comprising a steel substrate and a zinc-containing coating on the surface of the steel substrate. The steel substrate contains Fe and inevitable impurity elements. The steel substrate further contains the following chemical elements in percentage by mass: 0.10-0.22% of C, 0.30-0.79% of Si, 1.00-2.50% of Mn, 0.50-5.00% of Cr, and 0.03-1.0% of Al. Correspondingly, further disclosed in the present invention is a manufacturing method for the coated ultrahigh-strength steel. The coated ultrahigh-strength steel of the present invention can be effectively prepared by using the manufacturing method, and when a final finished product of the coated ultrahigh-strength steel is welded by means of a spot welding process, if a joint surface crack is generated, the maximum depth of the joint surface crack is smaller than 5% of the plate thickness.

Description

[0001] The present invention relates to a high-strength steel and a method of producing it, in particular, to an ultra-high-strength steel having a coating and a method of producing it. BACKGROUND

[0002] With the advancement of the light vehicle manufacturing process, as well as the automotive industry's demand for improved vehicle corrosion resistance, many automakers have become increasingly demanding regarding ultra-high-strength coated steel. For cold-stamped steel, the coating is primarily a zinc-containing coating, such as hot-dip galvanized pure zinc coating, hot-dip galvanized iron, and electrogalvanized pure zinc. Resistance spot welding has become the most important connection method in automotive welding production due to its advantages of fast welding speed and low cost, etc.The joints of many ultra-high-strength steels, having a zinc-containing coating, often exhibit good quasi-static mechanical properties during spot welding, but a type of welding crack appears on the surface of the steel sheet in contact with the electrodes and in the surrounding area, and this type of welding crack is called a spot welding crack. Figure 1 schematically shows the spot welding cracks of ultra-high-strength steel having a zinc coating. However, it is very difficult to eliminate the formation of these cracks. Therefore, this type of crack has become a significant obstacle and a bottleneck for the application of ultra-high-strength steel. Petition 870250079878, dated 05 / 09 / 2025, page 9 / 52 2 / 32 coating containing zinc in the automotive field.

[0003] Some technical solutions are known in the prior art for controlling welding cracks, but many shortcomings still exist.

[0004] For example, Chinese patent document with publication number CN108015401A, published on May 11, 2018 and entitled Resistance spot welding method of galvanized high-strength steel with good joint performance, shows a method for inhibiting surface cracks in spot welding, ensuring that the weld joint performance is not reduced by innovation in the spot welding process.

[0005] Another example is the Chinese patent document with publication number CN109385515A, published on February 26, 2019 and entitled Multilayer steel and method for reducing liquid metal embrittlement, which shows a method for eliminating crack generation in spot welding of high-strength steel, which inhibits the occurrence of cracks in spot welding first by decarburization and then by galvanizing the high-strength steel, so as to control the thickness of the decarburized layer between 10 and 50 microns.

[0006] Another example is the Chinese patent document with publication number CN110892087A, published on March 17, 2020, entitled Zinc-coated steel plate with high resistance spot weldability, which shows a technical solution for forming an inner oxide layer on the steel plate by increasing the dew point, so as to improve the distribution of surface components, thereby reducing the occurrence of cracks in spot welding. However, the technical solution has the problem of being difficult to control the thickness and uniformity of the inner oxide layer and the uniformity of the distribution of the surface components of the steel plate.

[0007] Based on this, it is expected to provide an ultra-high steel Petition 870250079878, dated 05 / 09 / 2025, page 10 / 52 3 / 32 resistance having a coating that not only possesses ultra-high resistance but also has excellent spot welding performance. SUMMARY

[0008] One of the objects of the present invention is to provide an ultra-high strength steel having a coating with excellent spot welding performance. It satisfies the performance requirements of high strength coated steel and the mechanical properties of spot welded joints, while exhibiting low susceptibility to cracking in spot welding.

[0009] To achieve the aforementioned purpose, the present invention provides an ultra-high-strength steel having a coating with excellent spot welding performance, comprising a steel substrate and a coating containing zinc on the surface of the steel substrate, wherein the steel substrate comprises Fe and unavoidable impurity elements, and the steel substrate further comprises the following chemical elements in mass percentage: C: 0.10~0.22%; Si: 0.30~0.79%; Mn: 1.00~2.50%; Cr: 0.50~5.00%; Al: 0.03~1.0%.

[0010] Furthermore, in ultra-high strength steel having a coating of the present invention, the steel substrate comprises each chemical element in a mass percentage as follows: C: 0.10~0.22%; Si: 0.30~0.79%; Mn: 1.00~2.50%; Cr: 0.50~5.00%; Al: 0.03~1.0%; with a remainder of Fe and unavoidable impurity elements.

[0011] Furthermore, in ultra-high strength steel having a coating of the present invention, wherein the mass percentage of each chemical element of the steel substrate satisfies at least one of the following: C: 0.15~0.20%; Petition 870250079878, dated 05 / 09 / 2025, page 11 / 52 4 / 32 Si: 0.50~0.79%; Mn: 1.00~2.00%; Cr: 1.00~3.00%.

[0012] The present invention adopts a composition pattern based primarily on carbon, silicon, manganese, and chromium, and is complemented by the process proposed in the present invention. It takes full advantage of the role of the elements carbon, silicon, manganese, and chromium in the phase transformation of the material, thus providing the unification of high mechanical properties and high spot welding performance in the ultra-high-strength steel of the present invention, and finally obtaining an ultra-high-strength steel product having a coating with excellent spot welding performance.

[0013] The chemical elements in ultra-high strength steel having a coating of the present invention are determined according to the following principles:

[0014] C: In the ultra-high-strength steel having a coating with low sensitivity to spot welding cracks of the present invention, the solubility of carbon in austenite is much greater than in ferrite, which can prolong the gestation period before the austenite transition and reduce the Ms temperature. The higher the mass percentage of carbon in the steel, the greater the fraction of residual austenite and the greater the degree of carbon enrichment in the residual austenite during partitioning, which contributes to increasing the stability of the residual austenite, producing the TRIP effect and improving the ductility of the material. Furthermore, carbon is also the most basic solid solution strengthening element in steel.However, for the component system of the present invention, when the carbon content in the steel is very high, a large number of bonds tend to form after rapid cooling (i.e., rapid cooling to 200-300°C at a cooling rate of 50-500°C / s, which is mentioned further on in the...). Petition 870250079878, dated 05 / 09 / 2025, page 12 / 52 5 / 32 of the present invention), which increases the sensitivity to cracking of the sheet. Therefore, the mass percentage of element C in the present invention is controlled at 0.10~0.22%.

[0015] It is clear that the mass percentage of element C can be controlled particularly to 0.15~0.20%.

[0016] Si: In the ultra-high-strength steel having a coating with excellent spot welding performance of the present invention, the solubility of silicon in carbides is very low. Si can strongly inhibit cementite formation during partition treatment, promote carbon enrichment in residual austenite, and improve residual austenite stability. However, an excessively high mass percentage of silicon will reduce the plasticity of the steel at high temperatures and will form a stable oxide on the surface of the steel sheet, reducing the wettability of the sheet. In particular, the inventors found in this case that silicon is an element that significantly increases the tendency for cracking on the surface of the weld joint. Therefore, the mass percentage of the element Si in the present invention is controlled to 0.30~0.79%.

[0017] Naturally, the mass percentage of the element Si can be controlled particularly to 0.5~0.79%.

[0018] Mn: In the ultra-high-strength steel having a coating with excellent spot welding performance of the present invention, manganese can expand the austenitic phase zone, reduce the Ac3, Ms, and Mf points, improve austenite stability and hardenability of the steel, and reduce the critical transition rate, which is conducive to the preservation of residual austenite at room temperature. At the same time, manganese can also play a solid solution strengthening effect in the steel. However, when the mass percentage of manganese in the steel is too high, it will exacerbate the grain growth tendency, reduce plasticity, and Petition 870250079878, dated 05 / 09 / 2025, page 13 / 52 6 / 32 steel toughness and deteriorate corrosion resistance, especially, will increase manganese enrichment in the surface layer of the base metal under the coating, increase susceptibility to cracking on the weld joint surface and deteriorate welding performance. However, when the Mn content in the steel is too low, a ferrite-pearlite band structure will be formed at low cooling rates due to segregation. Therefore, the mass percentage of the element Mn in the present invention is controlled to 1.00~2.50% by weight.

[0019] Naturally, the mass percentage of the element Mn can be controlled particularly to 1.00~2.00%.

[0020] Cr: In the ultra-high-strength steel having a coating with excellent spot welding performance of the present invention, chromium increases the strength and hardness of the steel without reducing its plasticity and toughness. Above all, chromium can solve the problem of reduced hardenability of the steel, caused by the decreased addition of manganese, which is intended to mitigate manganese enrichment under the coating. Chromium can increase the hardenability of the steel and exert a secondary hardening effect, allowing an increase in the hardness and wear resistance of carbon steel without causing brittleness. The element chromium can expand the γ phase region, improve hardenability and heat resistance, reduce the temperature range for the existence of the δ phase region at high temperatures, promote the progression of the δ^γ transformation, and inhibit the precipitation of δ ferrite at high temperatures.Furthermore, the inventors also found that, with increasing chromium content, the tendency for surface cracks to occur at the welding points of high-strength steel decreases. However, the chromium content in carbon steel should also not be excessively high. Therefore, the mass percentage of chromium in the present invention is controlled at 0.50500% by weight. Petition 870250079878, dated 05 / 09 / 2025, page 14 / 52 7 / 32

[0021] Naturally, the mass percentage of the element Cr can be controlled particularly to 1.00 to 3.00%.

[0022] Al: In the ultra-high-strength steel having a coating with excellent spot welding performance of the present invention, when aluminum exists in a solid solution state, it can increase the stacking fault energy, inhibit cementite precipitation and the γ-to-martensite transition, and improve austenite stability. Furthermore, aluminum forms fine, diffusely distributed insoluble mass points with carbon and nitrogen that can refine the grains, but the reinforcing effect of aluminum is weaker than that of silicon, and its ability to stabilize austenite is also weaker than that of silicon. In addition, when the mass percentage of aluminum in the steel is too high, it is easy to form a large number of oxide inclusions, which is not conducive to continuous casting for steelmaking. Therefore, the mass percentage of the element Al is controlled to 0.03~1.00%.

[0023] Furthermore, in ultra-high-strength steel having a coating of the present invention, the mass percentage of the chemical element of the steel substrate satisfies at least one of the following items: 0 <Mo<0,03%; 0 <Nb<0,03%; 0 <Ti<0,03%; 0 <V<0,03%; e 0 <B<0,001%, por exemplo, 0<B<0,001%.

[0024] Molybdenum, niobium, titanium, vanadium, and boron can, in addition, improve the properties of ultra-high-strength steel having a coating of the present invention. Among these elements, molybdenum can improve the hardenability of the steel and adjust its strength. However, an excessively high mass percentage of Petition 870250079878, dated 05 / 09 / 2025, page 15 / 52 8 / 32 molybdenum will lead to an increase in the resistance to cold rolling deformation of the steel. Niobium, titanium, and vanadium can form fine carbides with carbon, thus promoting microstructure refinement. However, the formation of these fine carbides is detrimental to carbon enrichment in residual austenite and to the stabilization of residual austenite. Boron can significantly improve the hardenability of steel. Boron is easily segregated at grain boundaries, which fills grain boundary defects and reduces grain boundary energy. It increases the difficulty of nucleation of new phases at austenite grain boundaries, where ferrite nucleation originally occurs, and increases austenite stability, thus improving hardenability. But more boron is not always better.When grain boundary defects are filled, if there is still more non-equilibrium boron segregation, boron phase precipitation will form at the grain boundaries, increasing the grain boundary energy. At the same time, the boron phase will be used as the nucleus of a new phase, which will increase the nucleation rate, reduce austenite stability, and reduce hardenability. A high degree of boron phase precipitation will make the steel brittle, which will negatively impact the steel's mechanical properties.

[0025] Furthermore, the addition of the elements mentioned above will increase the cost of materials. Considering the balance between material service performance and cost control, at least one of the elements mentioned above may optionally be added, with its content controlled within the range specified above.

[0026] In the present invention, when the steel substrate contains Mo, the mass percentage of Mo can be from 0.001% to 0.03%. When the steel substrate contains Nb, the mass percentage of Nb can be from 0.001% to 0.03%. When the steel substrate contains Ti, Petition 870250079878, dated 05 / 09 / 2025, page 16 / 52 9 / 32 The mass percentage of Ti can be from 0.001% to 0.03%. When the steel substrate contains V, the mass percentage of V can be from 0.001% to 0.03%. When the steel substrate contains B, the mass percentage of B can be from 0.0001% to 0.001%, for example, 0.0001% to 0.0009%.

[0027] Furthermore, in ultra-high strength steel having a coating of the present invention, the mass percentage of unavoidable impurity elements in the steel substrate satisfies: P<0.01%, S<0.01% and N<0.003%.

[0028] In the technical solution mentioned above, the elements P, S and N are the impurity elements in ultra-high-strength steel having a coating of the present invention and, if technical conditions permit, in order to obtain steel with better performance and better quality, the content of impurity elements in ultra-high-strength steel having a coating should be reduced as much as possible.

[0029] Among them, although P can play a role in strengthening the solid solution, inhibiting carbide formation, and being conducive to improving the stability of residual austenite, an excessively high mass percentage of P will weaken grain boundaries, increase material brittleness, and deteriorate welding performance. That is, the positive effect of the element P is weaker than its negative effect. Therefore, the mass percentage of P is preferably controlled to P<0.01%.

[0030] The element S in steel easily forms a low-melting-point eutectic at the grain boundaries, and the plasticity of the material will be significantly impaired when its mass percentage is too high. Therefore, the mass percentage of the element S is controlled to S<0.01%.

[0031] When the mass percentage of N is too high, this causes difficulties in steel production and continuous casting, as well as Petition 870250079878, dated 05 / 09 / 2025, page 17 / 52 10 / 32 can be detrimental to inclusion control. Therefore, it is preferable to control the nitrogen mass percentage to N<0.003% by weight.

[0032] Furthermore, in ultra-high strength steel having a coating of the present invention, the microstructure of the steel substrate is ferrite + martensite + residual austenite.

[0033] Furthermore, in ultra-high strength steel having a coating of the present invention, the volume fraction of ferrite is 25% ~ 45%, and / or the volume fraction of martensite is 45% ~ 65%.

[0034] Furthermore, in ultra-high strength steel having a coating of the present invention, in ferrite, the volume of grains having a size of 10 μm or less represents >85%, and the volume of grains having a size of 5 μm or less represents >55%. In some embodiments, in ferrite, the volume fraction of grains having a size of 10 μm or less is 85-96%. In some embodiments, in ferrite, the volume fraction of grains having a size of 5 μm or less is 55-75%.

[0035] Furthermore, in ultra-high strength steel having a coating of the present invention, the residual austenite has an average grain size < 2 μm, and / or the residual austenite has an average carbon content >1.0%. In some embodiments, the average carbon content in the residual austenite is between 1.0%~1.30%, for example, 1.0%~1.28%. Additionally, in ultra-high strength steel having a coating of the present invention, when spot welding is adopted for welding, if cracks are generated on the surface of the weld joint, the maximum depth of the cracks on the surface of the weld joint is less than 5% of the plate thickness.

[0036] Furthermore, in ultra-high strength steel having a coating of the present invention, when spot welding is adopted for welding, if cracks are generated on the surface of the weld joint, the maximum depth of the cracks on the surface of the joint is Petition 870250079878, dated 05 / 09 / 2025, page 18 / 52 11 / 32 weld is less than 5% of the sheet thickness.

[0037] Furthermore, in ultra-high strength steel having a coating of the present invention, its mechanical properties satisfy: a yield strength of 600 MPa~850 MPa, a tensile strength of 980 MPa~1,150 MPa, a uniform elongation of not less than 13%, an elongation at the rupture point of not less than 15%.

[0038] Furthermore, in ultra-high strength steel having a coating of the present invention, its mechanical properties satisfy: a yield strength of >750 MPa; a tensile strength of >1,000 MPa; a uniform elongation of >14.5%; an elongation at the rupture point of >22%.

[0039] Furthermore, in ultra-high strength steel having a coating of the present invention, the coating is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating, or an aluminum-zinc coating. Among these, the pure zinc coating may be a hot-dip galvanized coating or an electrogalvanized coating.

[0040] Consequently, another object of the present invention is to provide a method for producing ultra-high strength steel having a coating, which is simple to produce and can provide high strength steel with significantly improved spot welding performance, especially resistance to cracking on the surface of the weld joint, while maintaining the same mechanical properties.

[0041] To achieve the purpose presented above, the present invention provides a method for producing ultra-high-strength steel having a coating, comprising the steps of: (1) continuous melting and casting into steel billets; (2) heating; (3) hot rolling: in which the thickness of the oxide scale in Petition 870250079878, dated 05 / 09 / 2025, page 19 / 52 12 / 32 The surface of the steel strip, after hot rolling, is controlled to <4μm, and the mass percentage of FeO + Fe3O4 in the oxide scale on the surface of the steel strip, after hot rolling, is <50% by weight; (4) pickling, or pickling + cold rolling; (5) Continuous annealing: wherein the steel is annealed at 800~920°C, then slowly cooled to 700~770°C at a cooling rate of 3~10°C / s; then rapidly cooled to 200~300°C at a cooling rate of 50~500°C / s; then reheated to 360~460°C, held for 50~600 s; and finally cooled to room temperature; and (6) electrodeposition of a coating containing zinc.

[0042] The production method of the present invention can obtain an ultra-high strength steel having a coating with low sensitivity to cracking in spot welding. The ultra-high strength steel, having a coating produced by this production method, can be welded using the conventional spot welding process of automobile manufacturers, and the maximum crack depth on the surface of the weld joint is less than 5% of the sheet thickness, which has a very low sensitivity to cracking by spot welding.

[0043] In step (2), the thickness of the oxide scale on the surface of the steel strip, after hot rolling, is controlled to <4μm, and the content of (FeO+ Fe3O4) in the oxide scale on the surface of the steel strip, after hot rolling, is <50% by weight, which is conducive to the implementation of the subsequent steps and has an important influence on the properties of the steel sheet obtained after continuous annealing. This is because, in the technical solution of the present invention, FeO and Fe3O4 are more difficult to pickle than Fe2O3. Controlling the thickness of the oxide scale on the surface of the steel strip, after hot rolling, and the content of (FeO+Fe3O4) in the oxide scale in Petition 870250079878, dated 05 / 09 / 2025, page 20 / 52 13 / 32 The surface of the steel strip, after hot rolling, prepared according to the present invention, to be <50% by weight, can effectively improve the pickling effect and obtain a pickled sheet surface that can be used for direct continuous annealing. Because the pickled sheet can be directly subjected to continuous annealing, the deformation rate of the hot-rolled microstructure is small, and the microstructure of the steel sheet is dominated by pearlite and ferrite. Therefore, under the same continuous annealing conditions, the material's strength can be reduced, thus providing a more uniform microstructure and achieving excellent ductility.

[0044] The present invention has led to an optimized design for the continuous annealing process. A homogenized structure of austenite or austenite + ferrite can be formed by controlling the annealing temperature from 800 to 920°C. Then, the ferrite content in the structure can be further adjusted by slowly cooling the steel to 700-770°C at a cooling rate of 3-10°C / s to obtain a specific ferrite content, thus improving the material's plasticity. After this, it is cooled to 200-300°C (i.e., between Ms (the initial temperature of the martensite transition) and Mf (the final temperature of the martensite transition)) at a rate of 50-500°C / s. At this point, a portion of the austenite is transformed into martensite, which can ensure the high strength of the steel.Next, it is reheated to 360-460°C and held for 50-600 seconds, which can split the carbon into martensite and austenite to form a certain amount of carbon-rich residual austenite, which can be stably maintained at room temperature. Due to the TRIP effect, the work hardening capacity and plasticity of the steel can be significantly improved, and it is possible to obtain a high-strength steel sheet with excellent ductility. Petition 870250079878, dated 05 / 09 / 2025, page 21 / 52 14 / 32

[0045] Furthermore, in view of the inventors' understanding of the influence of carbon, silicon, manganese, and chromium on cracks in the surface of the weld joint, especially the understanding that it is easy to enrich silicon and manganese in the surface layer of the steel sheet to significantly increase the sensitivity to cracks in the surface of the weld joint, the present invention not only limits the carbon, silicon, manganese, and chromium content of the steel in the composition formulation, but also reduces the content of the elements carbon, silicon, and manganese, compared to the same strength grade of steel, and adds the element chromium to increase the hardenability of the steel, so as to ensure that the zinc-coated sheet has low sensitivity to cracks in the surface of the weld joint and that the maximum value of the crack depth in the surface of the weld joint is less than 5% of the sheet thickness.

[0046] It should be noted that when the zinc-containing coating is electrodeposited in step (6), the zinc-containing coating may be produced by, but is not limited to, hot-dip galvanizing, electrogalvanizing and vacuum evaporation technology.

[0047] As the ultra-high strength steel, having a coating designed in the present invention, adopts a carbon, silicon, manganese and chromium design, as well as ferrite grain refinement, during the continuous annealing process, the nucleation point of the reverse phase transition of austenite increases and the grain size can be further refined, the average grain size of the residual austenite, which can be stably maintained at room temperature, is < 2 μm and the average carbon content in the residual austenite is > 1.0%.

[0048] Furthermore, in the method of producing ultra-high strength steel having a coating of the present invention, in step (2), a plate is heated to 1,200~1,300°C.

[0049] Furthermore, in the ultra-high steel production method Petition 870250079878, dated 05 / 09 / 2025, page 22 / 52 15 / 32 resistance with coating of the present invention, in step (3), the final lamination temperature is controlled at 860~930°C, and the winding temperature is controlled at 450~600°C.

[0050] Furthermore, in the method of producing ultra-high strength steel having a coating of the present invention, in step (4), when pickling + cold rolling is adopted, the cold rolling strain rate is controlled at 40%~60%.

[0051] Furthermore, in the method of producing ultra-high strength steel having a coating of the present invention, the parameters of the annealing process of step (5) satisfy at least one of the following items: an annealing temperature of 820-870°C; Slow cooling to 700-730°C at a cooling rate of 3-10°C / s; rapid cooling to 250~300C; Reheating to 400-430°C, after rapid cooling, maintained for 180-300 s; and the volumetric hydrogen content in the reducing atmosphere, in the continuous annealing furnace, is controlled at 10-15%.

[0052] Ultra-high strength steel having a coating of the present invention and the method of production thereof have the following advantages and beneficial effects:

[0053] The ultra-high strength coated steel of the present invention is based on carbon, silicon, manganese and chromium in its composition design. Without adding expensive alloying elements, by optimizing the carbon, silicon, manganese and chromium contents, an ultra-high strength cold-rolled coated steel with excellent spot welding performance is obtained.

[0054] The ultra-high strength steel having a coating described in the present invention has excellent quality and performance, Petition 870250079878, dated 05 / 09 / 2025, page 23 / 52 16 / 32 that meets user requirements for the performance of high-strength steel having a coating and the mechanical properties of spot-welded joints, and also has low sensitivity to spot welding cracking.

[0055] The ultra-high strength steel having a coating prepared by this technical solution of the present invention comprises a steel substrate and a coating containing zinc on the surface of the steel substrate, and its mechanical properties satisfy: a yield strength of 600 MPa~850 MPa; a tensile strength of 980 MPa-1,150 MPa; a uniform elongation of not less than 13%; and an elongation at the rupture point of not less than 15%. Furthermore, when the spot welding process is actually used for welding, if cracks are generated on the surface of the weld joint, the maximum depth of the cracks on the surface of the weld joint is less than 5% of the plate thickness.

[0056] The production process of the production method developed by the present invention is simple, and the high-strength steel obtained significantly improved resistance to spot welding cracks, under the same mechanical properties, and will have a good prospect of application in the production of structural safety parts for downstream users. DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 schematically shows spot welding cracks in ultra-high strength steel having a zinc-containing coating. DETAILED DESCRIPTION

[0058] Ultra-high strength steel, having a coating of Examples 1-28 of the present invention, and the comparative steel of Comparative Examples 1-4 were prepared by the following steps: (1) continuous melting and casting in steel billets were carried out Petition 870250079878, dated 05 / 09 / 2025, page 24 / 52 17 / 32 according to the chemical compositions and relationships as shown in Table 1; (2) heating: in which a plate was heated to 1200~1300OC. (3) hot rolling: wherein the thickness of the oxide scale on the surface of the strip steel, after hot rolling, was controlled to <4μm, the content of (FeO+ Fe3O4) in the oxide scale on the surface of the strip steel, after hot rolling, was <50% by weight, the final rolling temperature was controlled at 860~930°C and the winding temperature was controlled at 450~600°C; (4) Pickling or Pickling + Cold Rolling: When pickling + cold rolling was adopted, the cold rolling strain rate was controlled to 40%~60%, and when the steel was galvanized with zinc or zinc alloy, directly after pickling, the cold rolling strain rate in Table 2-2 was 0; (5) Continuous annealing: wherein the hydrogen content by volume in the reducing atmosphere in the continuous annealing furnace was controlled to be 10~15%; wherein the steel was annealed at 800~920°C, preferably with the annealing temperature controlled between 820 and 870°C, then slowly cooled to 700~770°C, at a cooling rate of 3~10°C / s to obtain a certain percentage of ferrite, wherein the final cooling temperature of the slow cooling was preferably controlled at 700~730°C; then rapidly cooled to 200~300°C with a cooling rate of 50~500°C / s, in order to convert part of the austenite into martensite, wherein the final cooling temperature of the rapid cooling was preferably controlled at 250~300°C; Then, it was reheated to 360-460°C after rapid cooling, preferably to 400-430°C, held for 50-600 seconds, preferably 180-300 seconds, and finally cooled to room temperature;and (6) electrodeposition of a zinc or zinc alloy coating on; Petition 870250079878, dated 05 / 09 / 2025, page 25 / 52 18 / 32 that the coating can be specifically selected with a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating.

[0059] It should be noted that, in the present invention, the chemical composition and related process parameters adopted for the ultra-high strength steel having a coating of Examples 1-28 met all the control requirements designed in the present invention. Similarly, there were parameters in the chemical composition and related process parameters adopted for the comparative steel of Comparative Examples 1-4 that did not meet the requirements set forth in the present invention.

[0060] The coatings on the steels in Examples 1-28 and in the Comparative Examples were all hot-dip pure zinc coatings.

[0061] Table 1 listed the mass percentage of each chemical element of the ultra-high strength steel having a coating from Examples 1-28, and of the comparative steel from Comparative Examples 1-4. Table 1. (Percentage by weight, the remainder being Fe and other unavoidable impurities, except P, S and N) Ex. 4 1.9 0 2.2 0 1.0 0 0.01 0 / 0.00 5 0.02 3 0.000 6 0.00 6 0.00 3 0.003 0 Ex. 3 0.1 1 0.7 5 2.3 0 1.2 0 0.0 5 / / 0.00 5 0.03 0 0.000 3 0.00 8 0.00 4 0.000 7 Ex. 4 0.1 5 0.5 5 1.9 0 1.8 0 0.3 5 / 0.01 0 0.01 5 0.02 0 0.000 1 0.00 6 0.00 2 0.000 8 Ex. 0.1 0.5 1.6 1.9 0.4 0.01 0.00 / 0.02 0.000 0.00 0.00 0.000 Petition 870250079878, dated 05 / 09 / 2025, page 26 / 52 19 / 32 5 6 0 0 0 0 5 5 0 7 7 3 9 Ex. 6 0,1 4 0,4 6 2,0 5 2,3 5 0,4 8 0,01 0 0,01 0 0,02 5 / 0,000 5 0,00 5 0,00 3 0,000 3 Ex. 7 0,1 2 0,3 7 2,2 0 2,6 0 0,7 0 0,00 5 0,01 5 / 0,02 7 0,000 5 0,00 5 0,00 3 0,000 5 Ex. 8 0,2 0 0,6 7 1,3 5 1,4 5 0,1 3 / 0,02 3 0,01 0 0,00 5 0,000 2 0,00 7 0,00 4 0,000 1 Ex. 9 0,2 0 0,4 0 1,3 0 4,0 0 0,6 6 / / / / / 0,00 8 0,00 2 0,001 3 Ex. 10 0,2 1 0,3 1 1,1 0 2,7 0 0,9 5 0,01 5 / 0,02 7 0,00 5 0,000 8 0,00 8 0,00 3 0,001 7 Ex. 11 0,1 5 0,6 8 2,1 0 0,5 0 0,1 4 0,02 3 0,01 0 / 0,00 5 0,000 5 0,00 8 0,00 3 0,001 9 Ex. 12 0,1 9 0,4 7 1,5 0 3,0 0 0,5 3 0,01 0 0,02 5 0,01 0 / 0,000 8 0,00 7 0,00 3 0,002 5 Ex. 13 0,1 0 0,3 0 2,5 0 3,0 0 0,8 0 0,00 1 0,01 5 0,03 0 / 0,00 4 0,00 4 0,002 7 Ex. 14 0,2 2 0,7 6 1,0 5 1,0 5 0,0 4 0,01 5 0,02 7 0,00 5 / / 0,00 6 0,00 4 0,002 9 Ex. 15 0,1 3 0,7 4 2,3 0 1,4 0 0,0 9 0,02 7 0,00 5 0,01 5 / 0,000 4 0,00 7 0,00 3 0,000 2 Ex. 16 0,1 7 0,4 5 1,6 5 2,1 0 0,5 0 0,00 5 / 0,02 0 0,01 5 0,001 0 0,00 5 0,00 2 0,000 8 Ex.17 0.1 0 0.7 9 2.5 0 1.1 0 0.0 4 0.03 0 0.00 1 / 0.01 5 0.000 3 0.00 6 0.00 4 0.000 4 Ex. 18 0.1 9 0.3 8 1.3 5 2.4 0 0.6 5 / 0.00 5 0.02 3 0.01 0 0.000 9 0.00 4 0.00 2 0.001 4 Ex. 19 0.1 8 0.6 0 1.6 5 1.7 5 0.2 8 0.00 5 0.02 0 0.01 5 / / 0.00 8 0.00 4 0.001 8 Ex. 20 0.1 3 0.6 5 2.1 0 1.5 0 0.2 0 0.01 0 0.01 0 0.01 0 0.02 5 0.000 2 0.00 7 0.00 4 0.002 8 Ex. 0.1 0.3 2.4 2.9 0.9 / 0.01 0.03 0.00 0.000 0.00 0.00 0.002. Petition 870250079878, dated 05 / 09 / 2025, page 27 / 52 20 / 32 21 0 2 5 5 8 5 0 1 7 7 3 2 Ex. 22 0,2 1 0,3 3 1,1 0 5,0 0 0,7 9 / 0,03 0 0,00 1 0,01 5 0,001 0 0,00 5 0,00 2 0,002 4 Ex. 23 0,1 2 0,3 9 2,2 5 2,6 5 0,6 3 / / / / / 0,00 6 0,00 3 0,001 6 Ex. 24 0,1 8 0,5 4 1,7 0 2,5 0 0,4 0 0,01 5 0,02 0 / 0,01 0 0,000 7 0,00 6 0,00 3 0,001 0 Ex. 25 0,1 6 0,5 3 1,8 5 2,0 5 0,3 3 / 0,01 5 0,02 0 0,01 0 0,000 4 0,00 4 0,00 3 0,001 6 Ex. 26 0,2 0 0,6 9 1,2 0 1,3 0 0,1 0 0,02 5 0,01 0 / 0,01 0 0,000 9 0,00 7 0,00 3 0,002 4 Ex. 27 0,1 7 0,6 1 1,9 0 2,0 0 0,2 7 0,02 0 0,01 5 0,00 5 / 0,000 6 0,00 5 0,00 3 0,002 8 Ex. 28 0,2 2 0,7 9 1,0 0 1,0 0 0,0 3 0,03 0 / / 0,00 5 0,001 0 0,00 8 0,00 2 0,003 0 CEx . 1 0,2 4 1,1 0 2,3 0 / 0,2 8 0,01 0 0,01 0 0,01 0 0,02 5 0,000 3 0,00 7 0,00 4 0,003 5 CEx . 2 0,2 0 0,3 0 1,3 0 / 0,1 3 / 0,01 0 0,01 5 0,02 0 / 0,00 6 0,00 2 0,001 4 CEx . 3 0,1 6 0,5 0 1,7 0 / 0,0 4 0,00 5 / 0,02 0 0,01 5 0,000 1 0,00 5 0,00 2 0,000 3 CEx . 4 0,1 0 0,7 0 1,9 0 / 0,7 9 / 0,00 5 0,02 3 0,01 0 0,001 0 0,00 4 0,00 2 0,002 6

[0062] Table 2-1 and Table 2-2 listed the specific process parameters for the ultra-high strength steel having a coating from Examples 1-28, and the comparative steel from Comparative Examples 1-4.

[0063] In Table 2-1, the mass percentage of FeO and Fe3O4 in the oxide scale on the surface of the steel strip after hot rolling, based on the mass of the oxide scale (content of (FeO and Fe3O4)), was determined by X-ray diffraction (XRD).

[0064] Table 2-1 Petition 870250079878, dated 05 / 09 / 2025, page 28 / 52 21 / 32 No. Heating temperature of the plate (°C) Thickness of the oxide scale of hot-rolled coil (μm) Content of (FeO+ Fe3Ü4) (%) Final temperature of hot rolling (T) Winding temperature of hot-rolled coil (°C) Ex. 1 1,205 2.7 35.8 910 465 Ex. 2 1,235 2.5 49.5 903 496 Ex. 3 1,215 2.2 25.6 930 530 Ex. 4 1,240 3.5 42.3 865 475 Ex. 5 1,210 4.0 21.7 860 450 Ex. 6 1,230 3.7 33.5 920 600 Ex. 7 1,300 2.5 37.8 870 550 Ex. 8 1,200 3.3 27.5 910 500 Ex. 9 1,215 2.1 42.3 900 590 Ex. 10 1,245 3.9 45.9 880 510 Ex. 11 1,225 2.8 17.3 890 520 Ex. 12 1,220 1.5 23.8 885 580 Ex. 13 1,290 2.6 35.7 875 575 Ex. 14 1,240 2.8 46.8 889 565 Ex. 15 1,235 3.4 42.2 895 555 Ex. 16 1,240 2.9 36.8 897 525 Ex. 17 1,225 3.7 27.3 875 535 Ex. 18 1,265 3.2 29.5 882 490 Ex. 19 1,210 1.8 26.4 890 515 Ex. 20 1,205 2.4 23.8 895 535 Ex. 21 1,200 1.1 23.6 863 480 Ex. 22 1,210 2.7 35.4 878 470 Ex. 23 1,240 1.8 47.9 899 510 Ex. 24 1,235 1.2 35.2 915 520 Ex. 25 1,225 2.6 28.3 920 550 Ex. 26 1.290 3.3 22.5 908 545. Petition 870250079878, dated 05 / 09 / 2025, page 29 / 52 22 / 32 Ex. 27 1,230 3.5 24.3 915 565 Ex. 28 1,210 3.1 27.8 920 585 CEx. 1 1,220 2.8 33.6 910 550 CEx. 2 1,240 3.5 38.7 905 530 CEx. 3 1,215 1.6 41.5 915 560 CEx. 4 1,230 2.2 29.1 890 540

[0065] Note: In Table 2-1 above, (FeO+Fe3O4) content refers to the mass percentage of FeO and Fe3O4 in the steel scale on the surface of the strip steel after hot rolling, based on the steel scale table. Table 2-2 No. Cold rolling reduction rate (%) Re-annealing temperature (C) Slow cooling rate (C / s) Initial cooling temperature of slow cooling (°C) Final cooling temperature of slow cooling (C) Rapid cooling rate (C / s) Reheating temperature (C) Reheating retention time (s) Ex. 1 0 830 10 700 200 450 360 600 Ex. 2 0 820 3 730 240 410 460 50 Ex. 3 40 870 7 750 210 460 370 500 Ex. 4 60 835 5 770 250 420 450 80 Ex. 5 45 860 4 720 290 50 380 400 Ex. 6 55 855 9 715 220 350 440 140 Ex. 7 50 840 6 705 230 100 390 350 Ex. 8 52 800 8 725 300 300 430 180 Ex. 9 0 810 9 735 280 490 400 300 Ex. 10 0 920 6 755 270 500 420 220 Ex. 11 43 910 3 765 205 390 410 260 Ex. 12 58 880 5 745 295 380 405 295 Ex. 13 0 900 7 704 215 470 415 240 Petition 870250079878, dated 05 / 09 / 2025, page 30 / 52 23 / 32 Ex. 14 0 910 4 716 255 430 428 300 Ex. 15 44 845 8 728 285 480 425 300 Ex. 16 56 855 10 720 245 440 418 230 Ex. 17 0 843 8 715 275 445 423 225 Ex. 18 0 852 6 755 235 485 413 250 Ex. 19 48 838 7 770 265 475 408 290 Ex. 20 57 857 5 745 225 460 403 300 Ex. 21 49 864 9 735 253 150 398 340 Ex. 22 54 915 3 765 218 250 393 380 Ex. 23 58 905 4 754 257 200 387 420 Ex. 24 50 918 6 749 263 350 383 460 Ex. 25 0 903 7 736 277 435 378 500 Ex. 26 0 896 9 728 283 495 363 590 Ex. 27 46 858 10 714 297 425 368 570 Ex. 28 44 846 5 718 293 415 372 540 CEx. 1 35 830 7 720 255 445 480 180 CEx. 2 0 810 5 715 265 585 400 300 CEx. 3 55 870 4 705 270 475 430 300 CEx. 4 0 835 9 735 280 460 401 180

[0066] Note: In Table 2-2 above, a cold rolling strain rate of 0 indicates that no cold rolling was applied, and only pickling was performed.

[0067] Before the electrodeposition process described in step (6), the inventor collected samples of the ultra-high strength steel substrate, having a coating of Examples 1-28, obtained after the continuous annealing process of step (5) and of the comparative steel substrate of Comparative Examples 1-4, and performed observations of the substrate microstructure of each example and comparative example. The results of the observation of the substrate microstructure of each example and comparative example were listed in Table 3 below.

[0068] Table 3 listed the result of the observation of Petition 870250079878, dated 05 / 09 / 2025, page 31 / 52 24 / 32 microstructure of the ultra-high strength steel substrate, having a coating in Examples 1-28, and the comparative steel substrate in Comparative Examples 1-4.

[0069] The methods of observation and determination of the microstructure were as follows:

[0070] The volume fractions of ferrite and martensite were determined by metallographic analysis.

[0071] Grain size and volume fractions of ferrite were determined by metallographic analysis.

[0072] The average grain size of residual austenite was determined by metallographic analysis.

[0073] The average carbon content in residual austenite was determined using an electron probe Casting microanalysis (EPMA). Table 3 No. Ferrite fraction (%) Martensite fraction (%) Volume fractions of ferrite having a grain size of < 10 μm (%) Volume fractions of ferrite having a grain size of < 5 μm (%) Average grain size of residual austenite (qm) Carbon content in residual austenite (%) Ex. 1 25.25 64.67 95.78 73.87 1.1 1.14 Ex. 2 26.31 63.34 93.64 72.69 1.3 1.21 Ex. 3 26.18 63.58 94.56 72.58 1.5 1.23 Ex. 4 25.78 63.79 94.37 73.24 1.9 1.15 Ex. 5 43.11 45.65 85.23 57.43 0.8 1.02 Ex. 6 41.25 47.21 85.98 56.89 0.4 1.04 Ex. 7 44.59 45.32 86.76 57.65 0.3 1.03 Ex. 8 42.76 46.45 87.33 57.12 0.6 1.05 Petition 870250079878, dated 05 / 09 / 2025, page 32 / 52 25 / 32 Ex. 9 28,45 59,87 91,22 66,34 0,9 1,26 Ex. 10 30,22 58,64 92,56 65,67 0,7 1,22 Ex. 11 28,43 60,65 92,43 65,87 1,0 1,20 Ex. 12 30,56 59,25 92,48 66,18 1,2 1,18 Ex. 13 28,34 61,25 94,52 69,75 1,4 1,28 Ex. 14 28,11 62,11 94,67 69,24 1,6 1,27 Ex. 15 27,78 61,56 94,74 69,38 1,8 1,23 Ex. 16 27,56 61,23 94,83 68,54 2,0 1,26 Ex. 17 32,97 56,32 92,78 67,89 1,7 1,21 Ex. 18 34,23 55,68 92,43 67,78 1,8 1,18 Ex. 19 34,54 55,34 92,65 67,56 1,4 1,16 Ex. 20 34,87 54,98 92,47 66,87 1,3 1,17 Ex. 21 40,88 48,33 88,76 59,87 1,5 1,08 Ex. 22 39,68 49,11 87,69 59,63 1,3 1,09 Ex. 23 39,12 48,76 88,45 58,45 1,7 1,06 Ex. 24 40,85 48,34 87,35 58,93 1,9 1,05 Ex. 25 37,66 51,89 90,55 63,77 0,5 1,09 Ex. 26 36,64 52,45 90,63 62,95 0,8 1,08 Ex. 27 37,25 52,23 90,12 62,87 0,6 1,09 Ex. 28 38,27 51,69 90,67 63,43 0,7 1,11 CEx. 1 40,67 48,34 83,75 59,87 1,2 1,15 CEx. 2 22,87 66,76 90,23 58,92 2,5 1,19 CEx. 3 41,73 46,23 89,34 58,34 1,3 0,93 CEx. 4 42,55 47,12 88,94 49,45 1,1 1,17

[0074] Note: The residual austenite volume fraction not listed in Table 3 was the remainder, excluding ferrite and martensite.

[0075] As can be seen in Table 3 above of the present invention, the microstructure of the steel substrate elaborated by Examples 128 of the present invention was: ferrite + martensite + residual austenite. Furthermore, the microstructure of the substrate of each example satisfied the following indices: the volume fraction of ferrite was from 25% to 45% and the volume fraction of martensite was from 45% to 65%; wherein the fraction Petition 870250079878, dated 05 / 09 / 2025, page 33 / 52 26 / 32 volumetric fraction of ferrite grains, with a size of 10 μm or less, represented >85% and the volumetric fraction of ferrite grains, with a size of 5 μm or less, represented >55%, the average size of residual austenite grains was <2 μm and the average carbon content in residual austenite was >1.0%.

[0076] In addition, the inventors also tested the mechanical properties of the coated steel sheets in each example and comparative example, and the test results are listed in Table 4 below. When the mechanical properties of the coated steel sheets of each example and comparative example were tested, the test methods for the relevant mechanical properties were as follows:

[0077] Tensile properties test: the tensile test was performed in accordance with standard GB / T228.1-2010, Metallic materials-Tensile testing-Part 1: Method of test at room temperature, to test the yield strength, tensile strength, uniform elongation and elongation at the breaking point of the ultra-high strength steel, having a coating obtained in Examples 1-28, and the comparative steel in Comparative Examples 1-4. Table 4 No. Yield Strength (MPa) Tensile Strength (MPa) Uniform Elongation (%) Elongation at Rupture Point (%) Ex. 1 755 1,080 15.6 23.1 Ex. 2 780 1,090 16.1 24.8 Ex. 3 765 1,085 15.9 23.1 Ex. 4 845 1,125 14.7 22.8 Ex. 5 795 1,105 15.2 23.2 Ex. 6 790 1,110 15.4 23.4 Ex. 7 825 1,115 15.1 23.9 Ex. 8 820 1,120 15.3 23.3 Petition 870250079878, dated 05 / 09 / 2025, page 34 / 52 27 / 32 Ex. 9 815 1,115 14.8 22.6 Ex. 10 835 1,135 14.6 22.4 Ex. 11 830 1,140 14.9 22.9 Ex. 12 820 1,110 15.2 23.3 Ex. 13 800 1,090 15.8 23.5 Ex. 14 785 1,045 16.2 24.8 Ex. 15 795 1,075 15.7 23.4 Ex. 16 790 1,090 15.5 23.8 Ex. 17 770 1,070 15.6 24.1 Ex. 18 650 980 13.2 15.0 Ex. 19 785 1,105 15.1 23.9 Ex. 20 780 1,110 15.3 23.5 Ex. 21 750 1,030 16.2 24.7 Ex. 22 600 1,050 14.1 18.2 Ex. 23 765 Ex. 28 800 1,075 15.9 24.4 CEx. 1 765 1,050 15.7 23.5 CEx. 2 780 970 15.4 25.4 CEx. 3 770 1,060 12.9 14.8 CEx. 4 870 1,165 15.8 24.3

[0078] As can be seen in Table 4 above, the steel plate in Examples 1-28 of the present invention exhibited many excellent mechanical properties: yield strength >600 MPa, tensile strength >980 MPa, uniform elongation >13%, and elongation at rupture >15%.

[0079] Furthermore, in order to verify that the final product of ultra-high strength steel, having a coating prepared in Examples 1-28, had a greater sensitivity to spot welding cracking Petition 870250079878, dated 05 / 09 / 2025, page 35 / 52 28 / 32 than excellent, the inventors collected samples respectively of the final product of the ultra-high-strength steel, with coating from Examples 1-28, and of the comparative steel sheet from Comparative Examples 1-4, and performed welding tests with a spot welding process on the final product of each example and comparative example. The relevant parameters of the spot welding process are listed in Table 5 below.

[0080] Table 5 lists the specific parameters of the spot welding process of the ultra-high strength coated steel of Examples 1-28, and of the comparative steel plate of Comparative Examples 1-4. Table 5 No. Steel plate thickness (mm) Electrode pressure (kN) Number of pulses Time for 1 pulse (ms) Cooling time (ms) Total welding time (ms) Retention time (ms) Ex. 1 1.0 2.6 1 230 0 230 250 Ex. 2 1.0 2.6 1 230 0 230 250 Ex. 3 1.2 2.6 1 270 0 270 250 Ex. 4 1.2 2.6 1 270 0 270 250 Ex. 5 1.8 3.6 3 130 40 470 250 Ex. 6 1.8 3.6 3 130 40 470 250 Ex. 7 1.4 3.6 3 120 20 400 250 Ex. 8 1.4 3.6 3 120 20 400 250 Ex. 9 1.5 3.6 3 120 20 400 250 Ex. 10 1.5 3.6 3 120 20 400 250 Ex. 11 1.3 3.6 3 120 20 400 250 Ex. 12 1.3 3.6 3 120 20 400 250 Ex. 13 2.0 4.0 4 120 40 600 250 Ex. 14 2.0 4.0 4 120 40 600 250 Petition 870250079878, dated 05 / 09 / 2025, page 36 / 52 29 / 32 Ex. 15 1,6 3,6 3 130 40 470 250 Ex. 16 1,6 3,6 3 130 40 470 250 Ex. 17 1,85 3,6 3 130 40 470 250 Ex. 18 1,85 3,6 3 130 40 470 250 Ex. 19 1,85 3,6 3 130 40 470 250 Ex. 20 1,85 3,6 3 130 40 470 250 Ex. 21 1,2 2,6 1 270 0 270 250 Ex. 22 1,2 2,6 1 270 0 270 250 Ex. 23 1,2 2,6 1 270 0 270 250 Ex. 24 1,2 2,6 1 270 0 270 250 Ex. 25 1,4 3,6 3 120 20 400 250 Ex. 26 1,4 3,6 3 120 20 400 250 Ex. 27 1,4 3,6 3 120 20 400 250 Ex. 28 1,4 3,6 3 120 20 400 250 CEx. 1 1,2 2,6 1 270 0 270 250 CEx. 2 1,5 3,6 3 120 20 400 250 CEx. 3 1,4 3,6 3 120 20 400 250 CEx. 4 1,6 3,6 3 130 40 470 250

[0081] In the spot welding process, all tensile shear (TSS) samples, the transverse tensile (CTS) sample, and the metallographic sample were welded under each of the welding currents. The load capacities of the TSS and CTS joints were measured by a tensile testing machine, according to ISO 14273-2016 and ISO 14272-2016 standards. The results are presented in Table 6 below.

[0082] For the metallographic samples, the coating on the surface of the joint was first removed with diluted hydrochloric acid, and the distribution and direction of the cracks on the surface of the weld joint were observed under a microscope. The cross-section passing through the center of the weld point, where most of the surface cracks could be cut, was selected as the metallographic section of the joint, and the Petition 870250079878, dated 05 / 09 / 2025, page 37 / 52 A 30 / 32 cross-section was sampled by wire cutting, where the cross-section included all characteristic welding areas of the spot weld joint. The surface of the intercepted sample was washed to avoid interference from foreign matter in the test results, and the washed sample was dried. The dried sample was mounted, ground, and polished, and measured by a metallographic microscope, and the maximum crack length was entered in Table 6.

[0083] Table 6 listed the mechanical properties of weld joints and the results of weld joint crack tests for ultra-high strength steel, having a coating in Examples 1-28 and Comparative Examples 1-4, after spot welding. Table 6 Nominal weight of coating (g / m2) Shear strength under tension (kN) Transverse tensile strength of the joint (kN) Without weld spatter Weld spatter Maximum crack length (pm) Maximum crack length / steel plate thickness (%) Maximum crack length (μm) Maximum crack length / steel plate thickness (%) Ex. 1 62 / 66 >10 >7.8 0 0 38.6 3.86 Ex. 2 67 / 63 >10 >7.8 0 0 39.3 3.93 Ex. 3 68 / 69 >12 >8.2 0 0 46.1 3.84 Ex. 4 74 / 76 >12 >8.2 0 0 47.4 3.95 Ex. 5 86 / 86 >21 >13.8 0 0 0 0 Ex. 6 72 / 73 >21 >13.8 0 0 0 0 Ex. 7 70 / 71 >15 >11 0 0 0 0 Ex. 8 68 / 68 >15 >11 0 0 0 0 Petition 870250079878, dated 05 / 09 / 2025, page 38 / 52 31 / 32 Ex. 9 65 / 67 >16,5 >11 0 0 35,3 2,35 Ex. 10 68 / 69 >16,5 >11 0 0 36,3 2,42 Ex. 11 65 / 67 >13,6 >10,5 0 0 29,8 2,29 Ex. 12 69 / 69 >13,6 >10,5 0 0 30,8 2,37 Ex. 13 71 / 72 >24 >13,8 0 0 66,4 3,32 Ex. 14 71 / 73 >24 >13,8 0 0 69,0 3,45 Ex. 15 72 / 74 >18 >11,8 0 0 54,1 3,38 Ex. 16 75 / 73 >18 >11,8 0 0 54,7 3,42 Ex. 17 73 / 70 >22 >13,8 0 0 26,8 1,45 Ex. 18 71 / 72 >22 >13,8 0 0 25,7 1,39 Ex. 19 70 / 69 >22 >13,8 0 0 24,4 1,32 Ex. 20 75 / 75 >22 >13,8 0 0 26,1 1,41 Ex. 21 76 / 77 >12 >8,2 0 0 4,4 0,37 Ex. 22 87 / 85 >12 >8,2 0 0 4,7 0,39 Ex. 23 66 / 67 >12 >8,2 0 0 5,4 0,45 Ex. 24 72 / 73 >12 >8,2 0 0 4,2 0,35 Ex. 25 74 / 74 >15 >11 0 0 34,4 2,46 Ex. 26 76 / 77 >15 >11 0 0 33,3 2,38 Ex. 27 73 / 72 >15 >11 0 0 34,0 2,43 Ex. 28 73 / 73 >15 >11 0 0 33,0 2,36 CEx. 1 64 / 65 >12 >8,2 42,4 3,53 231,4 19,28 CEx. 2 86 / 86 >16,5 >11 56,3 3,75 194,7 12,98 CEx. 3 75 / 75 >12 >8,2 52,3 4,36 213,5 17,79 CEx. 4 72 / 73 >18 >11,8 69,2 4,33 292,9 18,31

[0084] It can be noted in Table 6 that, for Examples 1-28 in the present invention, when the welding current was less than the current when spatter occurred, there was no surface cracking in the weld joint (hereinafter referred to as no weld spatter); when the welding current was greater than the current when spatter occurred, the longest surface crack in the weld joint (hereinafter referred to as with weld spatter) was less than 5% of the plate thickness. In contrast, for the Petition 870250079878, dated 05 / 09 / 2025, p. 39 / 52 32 / 32 Comparative Examples 1-4, regardless of whether the welding current was greater than the current when spatter occurred, the surface of the weld joints exhibited cracks, and the surface cracks of the weld joints with weld spatter were more severe than those of the weld joints without weld spatter, and the ratio between the maximum crack length and the plate thickness was much greater than 5%. Thus, this showed that the ultra-high strength steel, having a coating for each example of the present invention, had excellent low sensitivity to cracking on the surface of the weld joint, ensuring the plate performance.

[0085] It should be noted that the combinations of the various technical features are not limited, in this case, to the combinations presented in the claims of this case or to the combinations presented in the specific examples. All the technical features presented in this case may be freely combined or associated in any way, unless a contradiction arises.

[0086] It should also be noted that the examples listed above are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and alterations or modifications made to it may be derived directly from the present invention or easily devised by those skilled in the art, all of which fall within the scope of protection of the present invention.

Claims

1. Ultra-high strength steel having a coating with excellent spot welding performance, characterized in that it comprises a steel substrate and a coating containing zinc on the surface of the steel substrate, wherein the steel substrate comprises Fe and unavoidable impurity elements, and the steel substrate further comprises the following chemical elements in mass percentage: C: 0.10~0.22%; Si: 0.30~0.79%; Mn: 1.00~2.50%; Cr: 0.50~5.00%; Al: 0.03~1.0%.

2. Ultra-high strength steel having a coating, according to claim 1, characterized in that the steel substrate comprises each of the following chemical elements in mass percentage: C: 0.10~0.22%; Si: 0.30~0.79%; Mn: 1.00~2.50%; Cr: 0.50~5.00%; Al: 0.03~1.0%; with a remainder of Fe and unavoidable impurity elements. 3.Ultra-high-strength steel having a coating, according to either claim 1 or 2, characterized in that the mass percentage of each chemical element of the steel substrate satisfies at least one of the following: C: 0.15~0.20%; Si: 0.50~0.79%; Mn: 1.00~2.00%; and Cr: 1.00~3.00%.

4. Ultra-high-strength steel having a coating, according to either claim 1 or 2, characterized in that the mass percentage of each chemical element of the steel substrate satisfies at least one of the following: 0 <Mo<0,03%; Petição 870250079878, de 05 / 09 / 2025, pág. 41 / 52 2 / 4 0<Nb<0,03%; 0<Ti<0,03%; 0<V<0,03%; e 0<B<0,001%.

5. Aço de ultra-alta resistência tendo um revestimento, de acordo com qualquer uma das reivindicações 1 ou 2, caracterizado pelo fato de que a porcentagem em massa dos elementos de impureza inevitáveis do substrato de aço satisfaz: P<0,01%, S<0,01%, N<0,003%. 6.Ultra-high strength steel having a coating, according to either of claims 1 or 2, characterized in that the microstructure of the steel substrate is ferrite + martensite + residual austenite.

7. Ultra-high strength steel having a coating, according to claim 6, characterized in that the volume fraction of ferrite is 25%~45%; and / or the volume fraction of martensite is 45%~65%.

8. Ultra-high strength steel having a coating, according to claim 6, characterized in that in the ferrite, the volume of grains having a size of 10 μm or less represents >85%, and the volume of grains having a size of 5 μm or less represents >55%.

9. Ultra-high strength steel having a coating, according to claim 6, characterized in that the residual austenite has an average grain size < 2 μm; and / or the residual austenite has an average carbon content >1.0%. 10.Ultra-high strength steel having a coating, according to either of claims 1 or 2, characterized in that, when spot welding is adopted for welding, if cracks are generated on the surface of the weld joint, the maximum depth of the cracks on the surface of the weld joint is less than 5% of the plate thickness.

11. Ultra-high strength steel having a coating, according to either of claims 1 or 2, characterized in that the mechanical properties of the ultra-high strength steel having a coating satisfy: a yield strength of 600 MPa~850 MPa, a tensile strength of 980 MPa~1,150 MPa, a uniform elongation of not less than 13% and an elongation at the rupture point of not less than 15%. 12.Ultra-high strength steel having a coating, according to any one of claims 1 or 2, characterized in that the coating is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating.

13. Method of production for ultra-high strength steel having a coating, as defined in any one of claims 1 to 12, characterized in that it comprises the steps of: (1) melting and continuous casting; (2) heating; (3) hot rolling: wherein the thickness of the oxide scale on the surface of the strip steel, after hot rolling, is controlled to <4μm, and the mass percentage of FeO+ Fe3O4 in the oxide scale on the surface of the strip steel, after hot rolling, is <50% by weight; (4) pickling or pickling + cold rolling; (5) Continuous annealing: in which the steel is annealed at 800~920°C, then slowly cooled to 700~770°C at a cooling rate of 3~10°C / s; then rapidly cooled to 200~300°C at a cooling rate of 50~500°C / s; then reheated to 360~460°C, held for 50~600 s; and finally cooled to room temperature; and Petition 870250079878, dated 05 / 09 / 2025, p. 43 / 52 4 / 4 (6) Electrodeposition of a coating containing zinc.

14. Production method according to claim 13, characterized in that the production method has one or more features selected from: wherein, in step (2), a plate is heated to 1200~1300°C; wherein, in step (3), the final rolling temperature is controlled at 860~930°C, the winding temperature is controlled at 450~600C; and wherein, in step (4), when pickling + cold rolling is adopted, the cold rolling strain rate is controlled at 40%~60%. 15.Production method according to claim 13, characterized in that the annealing process parameters of step (5) satisfy at least one of the following items: an annealing temperature of 820~870°C; slow cooling to 700~730°C at a cooling rate of 3~10°C / s; rapid cooling to 250~300°C; reheating to 400~430°C after rapid cooling, maintained for 180~300s; and the volumetric hydrogen content in the reducing atmosphere in the continuous annealing furnace is controlled to 10~15%.

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