Plated ultra-high-strength steel with excellent spot welding properties and method for manufacturing the same

Abstract

The present invention discloses a plated ultra-high-strength steel with excellent spot-welding properties, comprising a steel base and a zinc-containing plating on the surface of the steel base, wherein the steel base contains Fe and unavoidable impurity elements, and further contains each chemical element in the following mass percentages: C: 0.10-0.22%, Si: 0.30-0.79%, Mn: 1.00-2.50%, Cr: 0.50-5.00%, and Al: 0.03-1.0%. Accordingly, the present invention further discloses a method for manufacturing the above-mentioned plated ultra-high-strength steel, and by utilizing this manufacturing method, the above-mentioned plated ultra-high-strength steel of the present invention can be effectively manufactured, and the final finished product, when welding in the spot-welding process, has a maximum depth of welding point surface cracking of less than 5% of the plate thickness if welding point surface cracking occurs.

Description

Technical Field

[0001] The present invention relates to high-tensile steel and a method for manufacturing the same, and particularly to plated ultra-high-tensile steel and a method for manufacturing the same.

Background Art

[0002] As the weight reduction of automobiles progresses and corrosion resistance of vehicles is required in the automobile industry, the demand for plated ultra-high-tensile steel in many automobile manufacturers is increasing. In the case of cold-rolled steel, the plating is mainly zinc-containing plating, for example, plating such as hot-dip pure zinc plating, hot-dip zinc-iron alloy plating, and electro-galvanized pure zinc plating. Resistance spot welding has become the most important joining method in the welding process of automobiles because of its advantages such as high welding speed and low cost. Many zinc-containing plated ultra-high-tensile steels show excellent quasi-static mechanical properties at the joints during spot welding, but welding cracks occur on the surface of the steel sheet in contact with the electrode and its peripheral area, and this welding crack is called a spot welding crack. FIG. 1 is a schematic diagram schematically showing the spot welding crack of zinc-plated ultra-high-tensile steel. However, since it is very difficult to suppress the occurrence of this crack, this crack has become a major obstacle and bottleneck in the application of zinc-containing plated ultra-high-tensile steel in the automobile field.

[0003] In the existing technology, although several embodiments for suppressing welding cracks are known, there are still many drawbacks.

[0004] For example, a Chinese patent document with the disclosure number CN108015401A, the disclosure date of May 11, 2018, and the disclosure name of "Resistance spot welding method for zinc-plated high-tensile steel having excellent joint properties" discloses a method for suppressing surface cracks of spot welding without degrading the joint properties of the welded point through innovation of the spot welding process.

[0005] Furthermore, for example, a Chinese patent document with disclosure number CN109385515A, disclosure date February 26, 2019, and disclosure title "Method for reducing multilayer steel and liquid metal embrittlement" discloses a method for suppressing the occurrence of spot weld cracks in high-tensile steel. This method involves first decarburizing the high-tensile steel, and then zinc plating it to suppress the occurrence of spot weld cracks and control the thickness of the decarburized layer to 10-50 μm.

[0006] Furthermore, for example, a Chinese patent document with disclosure number CN110892087A, disclosure date March 17, 2020, and disclosure title "Galvanized steel sheet with high resistance spot weldability" discloses an embodiment that suppresses the occurrence of spot welding cracks by raising the dew point to form an internal oxide layer on the steel sheet and improving the component distribution on the surface. However, this embodiment has the problem that it is difficult to control the thickness and uniformity of the internal oxide layer and the uniformity of the component distribution on the surface of the steel sheet.

[0007] Therefore, it is desirable to provide plated ultra-high-strength steel that has extremely high strength and excellent spot-welding properties. [Overview of the project] [Problems that the invention aims to solve]

[0008] The present invention aims to provide a plated ultra-high-strength steel with excellent spot-welding properties that satisfies the requirements for the properties of plated high-strength steel and the mechanical properties of spot-welded joints, while simultaneously having low susceptibility to spot-weld cracking.

[0009] To achieve the above objective, the present invention provides a plated ultra-high-strength steel with excellent spot-welding properties, comprising a steel substrate and a zinc-containing plating on the surface of the steel substrate, wherein the steel substrate contains Fe and unavoidable impurity elements, and the steel substrate further contains the following chemical elements in the following mass percentages. Specifically, it contains C: 0.10-0.22%, Si: 0.30-0.79%, Mn: 1.00-2.50%, Cr: 0.50-5.00%, and Al: 0.03-1.0%.

[0010] Furthermore, in the plated ultra-high-strength steel according to the present invention, the mass percentage content of each chemical element in the steel substrate is: The composition is 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 the remainder being Fe and unavoidable impurity elements.

[0011] Furthermore, in the plated ultra-high-strength steel according to the present invention, the mass percentage content of each chemical element in the steel substrate is as follows: C: 0.15~0.20%, Si: 0.50~0.79%, Mn: 1.00~2.00%, The Cr: satisfies at least one of the following conditions: 1.00 to 3.00%.

[0012] This invention employs a compositional design primarily composed of carbon, silicon, manganese, and chromium, and, combined with the process according to the present invention, fully utilizes the effects of carbon, silicon, manganese, and chromium elements in the phase transformation of the material. By achieving both the high mechanical properties and high spot welding properties of the ultra-high-strength steel according to the present invention, a plated steel sheet product with excellent spot welding properties is obtained.

[0013] In the plated ultra-high-strength steel according to the present invention, the design principles for each chemical element are as follows.

[0014] C: In the plated ultra-high-strength steel with low spot-weld crack susceptibility according to the present invention, carbon has a much higher solubility in austenite than in ferrite, which extends the induction period before austenite transformation and can lower the Ms temperature. The higher the mass percentage content of carbon in the steel, the greater the proportion of retained austenite, and the higher the degree of carbon enrichment of the retained austenite during partitioning, which increases the stability of the retained austenite, generates a TRIP effect, and improves the ductility of the material. Carbon is also the most basic solid solution strengthening element in steel. However, in the component system of the present invention, if the carbon content in the steel is too high, many twins are likely to occur after quenching (i.e., when rapidly cooled to 200-300°C at a cooling rate of 50-500°C / s as described in the present invention), increasing the crack susceptibility of the plate material. Based on this, the present invention controls the mass percentage content of element C to 0.10-0.22%.

[0015] Of course, more preferably, the mass percentage content of element C is controlled to 0.15-0.20%.

[0016] Si: In the plated ultra-high-strength steel with excellent spot welding properties according to the present invention, silicon has extremely low solubility in carbides, strongly suppresses cementite formation during the partitioning process, promotes carbon enrichment of retained austenite, and enhances the stability of retained austenite. However, if the mass percentage content of silicon is too high, it reduces the high-temperature plasticity of the steel, forms stable oxides on the surface of the steel sheet, and reduces the wettability of the steel sheet. In particular, the inventors have discovered that silicon is an element that significantly increases the tendency for cracking at the welding point surface. Based on this, the present invention controls the mass percentage content of silicon to 0.30 to 0.79%.

[0017] Of course, more preferably, the mass percentage content of Si element is controlled to 0.5 to 0.79%.

[0018] Mn: In the plated ultra-high tensile steel with excellent spot welding characteristics according to the present invention, manganese expands the austenite phase region, Ac3, M s Point, M f By reducing the critical point, improving austenite stability and steel hardenability, and lowering the critical transformation rate, manganese helps retain austenite at room temperature, while also providing a solid solution strengthening effect in steel. However, if the mass percentage content of manganese is too high, it accelerates the tendency for grain coarsening, reduces the plasticity and toughness of the steel, worsens corrosion resistance, and leads to manganese enrichment, especially in the shallow surface layer of the base material under plating, increasing the susceptibility of joints to surface cracking at weld points and degrading welding properties. However, if the mass percentage content of manganese is too low, segregation causes the formation of ferrite and pearlite bands at low cooling rates. Based on this, the present invention controls the manganese mass percentage content to 1.00 to 2.50 wt%.

[0019] Of course, more preferably, the mass percentage content of Mn element is controlled to 1.00 to 2.00%.

[0020] Cr: In the plated ultra-high-strength steel with excellent spot welding properties according to the present invention, chromium improves the strength and hardness of the steel without significantly reducing plasticity and toughness. More importantly, chromium can solve the problem of reduced hardenability of steel due to reduced manganese addition to reduce manganese enrichment under plating. Chromium enhances the hardenability of steel, has a secondary hardening effect, does not make the steel brittle, and can improve the hardness and wear resistance of carbon steel. The chromium element expands the γ phase region, increases hardenability and hot strength, reduces the high-temperature range of the δ phase region, promotes the progression of δ→γ phase transformation, and suppresses the precipitation of high-temperature δ ferrite. Furthermore, the inventors have discovered that the tendency for surface cracking at weld points in high-tensile steel decreases with increasing chromium content, and that the chromium content in carbon steel should not be too high. Therefore, the present invention controls the mass percentage chromium content to 0.50~5.00 wt%.

[0021] Of course, more preferably, the mass percentage content of the Cr element is controlled to be 1.00 to 3.00%.

[0022] Al: In the plated ultra-high strength steel excellent in spot welding characteristics according to the present invention, when aluminum exists in a solid solution state, it can increase the stacking defect energy, suppress the precipitation of cementite and the transformation from γ to martensite, and enhance the stability of austenite. Further, aluminum can combine with carbon and nitrogen to form fine insoluble particles distributed dispersedly, thereby refining the crystal grains. However, the strengthening effect of aluminum is weaker than that of silicon, and the ability to stabilize austenite is also weaker than that of silicon. Furthermore, if the mass percentage content of aluminum is too high, it is easy to form a large amount of oxide inclusions, which is disadvantageous for the continuous casting of steelmaking. Therefore, the present invention controls the mass percentage content of aluminum to be 0.03 to 1.00 wt%.

[0023] Furthermore, in the plated ultra-high strength steel according to the present invention, the chemical elements of the steel base include the following items, that is, 0 < Mo ≤ 0.03%, 0 < Nb ≤ 0.03%, 0 < Ti ≤ 0.03%, 0 < V ≤ 0.03%, 0 < B ≤ 0.001%, for example, it further includes at least one item of 0 < B < 0.001%.

[0024] Molybdenum, niobium, titanium, vanadium, and boron can further improve the properties of the plated ultra-high strength steel according to the present invention. Among them, molybdenum can increase the hardenability of steel and adjust the strength of steel. However, if the mass percentage content of molybdenum is too high, the cold rolling deformation resistance of steel will increase. Niobium, titanium, and vanadium elements combine with carbon to form fine carbides, promoting the refinement of the structure. However, the formation of these fine carbides is disadvantageous for the carbon enrichment of retained austenite and the stabilization of retained austenite. Boron can significantly increase the hardenability of steel. Boron tends to segregate at grain boundaries, fill the defects at grain boundaries, and lower the grain boundary energy, thereby increasing the difficulty of new phase nucleation at austenite grain boundaries, which are originally ferrite nucleation sites, enhancing austenite stability, and improving hardenability. However, it is not the case that the more the content, the better. When more boron segregates non-equilibrium after the defects at grain boundaries are filled, "boron phase" precipitates at grain boundaries, increasing the grain boundary energy. At the same time, the "boron phase" becomes the nucleus of the new phase, increasing the nucleation rate, lowering austenite stability, and reducing hardenability. In addition, the precipitation of a large amount of "boron phase" makes the steel brittle and has an adverse effect on the mechanical properties of the steel.

[0025] Furthermore, the addition of the above elements increases the cost of the material. Therefore, considering the use properties and cost management of the material comprehensively, at least one of the above elements is selectively added and its content is controlled within the above range.

[0026] In the present invention, when the steel substrate contains Mo, the mass percentage content of Mo is 0.001 - 0.03%. When the steel substrate contains Nb, the mass percentage content of Nb is 0.001 - 0.03%. When the steel substrate contains Ti, the mass percentage content of Ti is 0.001 - 0.03%. When the steel substrate contains V, the mass percentage content of V is 0.001 - 0.03%. When the steel substrate contains B, the mass percentage content of B is 0.0001 - 0.001%, for example, 0.0001 - 0.0009%.

[0027] Furthermore, in the plated ultra-high-strength steel according to the present invention, the mass percentage content of unavoidable impurity elements in the steel substrate satisfies P ≤ 0.01%, S ≤ 0.01%, and N ≤ 0.003%.

[0028] In the above embodiments, P, S, and N are all impurity elements in the plated ultra-high-strength steel according to the present invention, and to the extent that it is technically permissible, the content of impurity elements in the plated ultra-high-strength steel should be reduced as much as possible in order to obtain steel materials with better properties and quality.

[0029] Of these, P has a solid solution strengthening effect, suppressing carbide formation and leading to improved retained austenite stability. However, if the mass percentage content of P is too high, it weakens grain boundaries, increases material brittleness, and deteriorates welding properties. In other words, since the positive effect of element P is weaker than its negative effect, it is preferable to control the mass percentage content of P to P ≤ 0.01%.

[0030] In steel, sulfur (S) elements tend to form low-melting-point eutectic crystals at grain boundaries. If the mass percentage content of S is too high, it significantly deteriorates the plasticity of the material. Therefore, the mass percentage content of S is controlled to S ≤ 0.01%.

[0031] If the mass percentage content of nitrogen is too high, it will cause difficulties in steelmaking and continuous casting and will be unfavorable for inclusion control. Therefore, it is preferable to control the mass percentage content of nitrogen to N ≤ 0.003 wt%.

[0032] Furthermore, in the plated ultra-high-strength steel according to the present invention, the microstructure of the steel substrate is ferrite + martensite + retained austenite.

[0033] Furthermore, in the plated ultra-high-strength steel according to the present invention, the volume ratio of ferrite is 25% to 45%, and / or the volume ratio of martensite is 45% to 65%.

[0034] Furthermore, in the plated ultra-high-strength steel according to the present invention, in the ferrite, the volume ratio of crystal grains with a size of 10 μm or less is ≥ 85%, and the volume ratio of crystal grains with a size of 5 μm or less is ≥ 55%. In some embodiments, in the ferrite, the volume ratio of crystal grains with a size of 10 μm or less is 85-96%. In some embodiments, in the ferrite, the volume ratio of crystal grains with a size of 5 μm or less is 55-75%.

[0035] Furthermore, in the plated ultra-high-strength steel according to the present invention, the average grain size of the retained austenite is ≤2 μm, and / or the average carbon content in the retained austenite is ≥1.0%. In some embodiments, the average carbon content in the retained austenite is 1.0 to 1.30%, for example, 1.0 to 1.28%.

[0036] Furthermore, in the plated ultra-high-strength steel according to the present invention, when welding occurs in a spot welding process, the maximum depth of the welding point surface crack is less than 5% of the plate thickness.

[0037] Furthermore, in the plated ultra-high-strength steel according to the present invention, the mechanical properties of the plated ultra-high-strength steel satisfy the following: yield strength of 600 MPa to 850 MPa, tensile strength of 980 MPa to 1150 MPa, uniform elongation of 13% or more, and elongation at break of 15% or more.

[0038] Furthermore, in the plated ultra-high-strength steel according to the present invention, the mechanical properties of the plated ultra-high-strength steel satisfy the following: yield strength ≥ 750 MPa, tensile strength ≥ 1000 MPa, uniform elongation ≥ 14.5%, and elongation at break ≥ 22%.

[0039] Furthermore, in the plated ultra-high-strength steel according to the present invention, the plating is pure zinc plating, zinc-iron alloy plating, zinc-aluminum-magnesium alloy plating, or aluminum-zinc alloy plating. Of these, the pure zinc plating may be a hot-dip galvanized layer or an electro-galvanized layer.

[0040] Accordingly, the present invention further aims to provide a method for manufacturing plated ultra-high-strength steel. This manufacturing method is simple to produce, and the resulting high-strength steel exhibits significantly improved spot welding properties, particularly resistance to cracking at the weld point, under equivalent mechanical property conditions.

[0041] To achieve the above objective, the present invention presents a method for manufacturing plated ultra-high-strength steel, specifically the following steps, namely: (1) Steelmaking and continuous casting, (2) Heating and, (3) Hot rolling in which the scale thickness on the surface of the strip steel after hot rolling is controlled to be ≤4 μm and the mass percentage content of FeO + Fe3O4 in the scale on the surface of the strip steel after hot rolling is controlled to be ≤50 wt%, (4) Pickling, or pickling + cold rolling, (5) Continuous annealing, which involves annealing at 800-920°C, then slowly cooling to 700-770°C at a cooling rate of 3-10°C / s, then rapidly cooling to 200-300°C at a cooling rate of 50-500°C / s, then heating again to 360-460°C, holding for 50-600 seconds, and finally cooling to room temperature. (6) Including the implementation of zinc-containing plating.

[0042] By utilizing the manufacturing method of the present invention, plated ultra-high-strength steel with low spot-weld crack susceptibility can be obtained. Plated ultra-high-strength steel produced by this manufacturing method can be welded in the typical spot-welding processes of automobile manufacturers, and its maximum depth of surface cracks at the weld point is less than 5% of the plate thickness, exhibiting very low spot-weld crack susceptibility.

[0043] In step (2), controlling the scale thickness on the surface of the hot-rolled steel strip to ≤4 μm and the (FeO + Fe3O4) content in the scale on the surface of the hot-rolled steel strip to ≤50 wt% is advantageous for subsequent processes and has a significant impact on the properties of the steel sheet obtained after continuous annealing. This is because, in the embodiments of the present invention, FeO and Fe3O4 are more difficult to pickle than Fe2O3, but by controlling the scale thickness on the surface of the hot-rolled steel strip and the (FeO + Fe3O4) content in the scale on the surface of the hot-rolled steel strip produced by the present invention to ≤50 wt%, the pickling effect can be effectively enhanced, and a pickled sheet surface that can be used directly for continuous annealing can be obtained. Since the pickled sheet can be continuously annealed as is, the amount of structural deformation during hot rolling is small, and the steel sheet structure mainly consists of pearlite and ferrite. Therefore, by reducing the material strength and making the structure more uniform under the same continuous annealing conditions, excellent ductility can be obtained.

[0044] This invention optimizes the continuous annealing process. By performing annealing while controlling the annealing temperature to 800-920°C, a uniform austenite or austenite-ferrite structure can be formed. Subsequently, the material is slowly cooled to 700-770°C at a cooling rate of 3-10°C / s to further adjust the ferrite content in the structure and improve the plasticity of the material by obtaining a constant ratio of ferrite. After that, the material is cooled again to 200-300°C (i.e., M) at a cooling rate of 50-500°C / s. s (Initiation temperature of martensitic transformation) and M f The material is cooled to the temperature between the end of the martensitic transformation. At this time, a portion of the austenite transforms into martensite, allowing the steel to gain higher strength. Subsequently, it is heated again to 360-460°C and held for 50-600 seconds, causing carbon to partition between martensite and austenite, forming a certain amount of carbon-enriched retained austenite. This is then stably maintained at room temperature, and due to the TRIP effect, the work hardening ability and formability of the steel are significantly improved, resulting in a high-tensile steel sheet with excellent ductility.

[0045] Furthermore, based on the understanding that carbon, silicon, manganese, and chromium have an effect on surface cracking at weld points, particularly that silicon and manganese enrich the shallow surface layer of steel sheets, significantly increasing susceptibility to surface cracking at weld points, the inventors controlled the content of carbon, silicon, manganese, and chromium during the steel composition design. Compared to steel types of the same strength level, they reduced the content of carbon, silicon, and manganese and added chromium to improve the hardenability of the steel, thereby achieving the designed ratio of martensite to austenite and carbon partitioning in the continuous annealing process. At the same time, they ensured that zinc-containing plated sheets have low susceptibility to surface cracking at weld points and that the maximum depth of surface cracking at weld points is less than 5% of the sheet thickness.

[0046] In step (6), zinc-containing plating can be manufactured using, but is not limited to, hot-dip plating, electroplating, or vacuum plating techniques.

[0047] The plated ultra-high-strength steel according to the present invention employs a carbon, silicon, manganese, and chromium design and refinement of ferrite crystal grains. As a result, in the continuous annealing process, the nucleation sites of austenite reverse phase transformation increase, and the size is further refined, resulting in an average crystal grain size of retained austenite that is ≤2 μm and maintains stably at room temperature, with an average carbon content of ≥1.0% in the retained austenite.

[0048] Furthermore, in the method for manufacturing plated ultra-high-strength steel according to the present invention, in step (2), the slab is heated to 1200 to 1300°C.

[0049] Furthermore, in the method for manufacturing plated ultra-high-strength steel according to the present invention, in step (3), the final rolling temperature is controlled to 860 to 930°C and the winding temperature to 450 to 600°C.

[0050] Furthermore, in the method for manufacturing plated ultra-high-strength steel according to the present invention, in step (4), when pickling + cold rolling is employed, the amount of cold rolling deformation is controlled to 40% to 60%.

[0051] Furthermore, in the method for manufacturing plated ultra-high-strength steel according to the present invention, the parameters of the annealing process in step (5) are the following items, namely, The annealing temperature is 820-870°C. Slowly cool to 700-730°C at a cooling rate of 3-10°C / s. Rapid cooling to 250-300°C. After rapid cooling, reheat to 400-430°C and hold for 180-300 seconds. At least one of the following conditions is met: the volume content of hydrogen gas in the reducing atmosphere inside the continuous annealing furnace is controlled to 10-15%.

[0052] The plated ultra-high-strength steel and its manufacturing method according to the present invention have the following advantages and superiorities.

[0053] The plated ultra-high-strength steel according to the present invention is based on carbon, silicon, manganese, and chromium in its compositional design. By optimizing the blending ratio of carbon, silicon, manganese, and chromium without adding expensive alloying elements, a plated ultra-high-strength cold-rolled steel with excellent spot welding properties is obtained.

[0054] The plated ultra-high-strength steel according to the present invention has excellent quality and properties, meeting customer requirements for the properties of plated high-strength steel and the mechanical properties of spot-welded joints, while also having low susceptibility to spot-weld cracking.

[0055] The plated ultra-high-strength steel manufactured according to this embodiment of the present invention includes a steel substrate and a zinc-containing plating on the surface of the steel substrate, and its mechanical properties satisfy the following requirements: a yield strength of 600 MPa to 850 MPa, a tensile strength of 980 MPa to 1150 MPa, a uniform elongation of 13% or more, and a fracture elongation of 15% or more. Furthermore, when welding in the spot welding process, if surface cracking occurs at the weld point, the maximum depth of the surface cracking at the weld point is less than 5% of the plate thickness.

[0056] The manufacturing method designed in this invention simplifies the production process, and the resulting high-tensile steel exhibits significantly improved resistance to spot weld cracking under equivalent mechanical property conditions, offering excellent potential for application in the production of safety structural components by downstream users. [Brief explanation of the drawing]

[0057] [Figure 1] This is a schematic diagram illustrating spot weld cracking in galvanized ultra-high-strength steel. [Modes for carrying out the invention]

[0058] The following interpretation and explanation will be based on specific examples and further describes the plated ultra-high-strength steel with excellent spot-welding properties and the method for manufacturing the same according to the present invention, but this interpretation and explanation will not inappropriately limit the embodiments of the present invention.

[0059] Examples 1-28 and Comparative Examples 1-4 The plated ultra-high-strength steels of Examples 1 to 28 and the comparative steel materials of Comparative Examples 1 to 4 according to the present invention were all manufactured by the following process.

[0060] (1) Steelmaking and continuous casting are carried out according to the chemical composition ratios designed in Table 1 to form slabs.

[0061] (2) Heating: Heat the slab to 1200-1300°C. (3) Hot rolling: The scale thickness on the surface of the strip after hot rolling is controlled to be ≤4 μm, and the (FeO + Fe3O4) content in the scale on the surface of the strip after hot rolling is controlled to be ≤50 wt%, the final rolling temperature is controlled to be 860~930°C, and the winding temperature is controlled to be 450~600°C.

[0062] (4) Pickling, or pickling + cold rolling: When pickling + cold rolling is used, the amount of cold rolling deformation is controlled to 40% to 60%, and when zinc plating or zinc alloy plating is performed immediately after pickling, the amount of cold rolling deformation in Table 2-2 is 0.

[0063] (5) Continuous annealing: The volume content of hydrogen gas in the reducing atmosphere inside the continuous annealing furnace is controlled to 10-15%. Annealing is performed at 800-920°C, preferably controlling the annealing temperature to 820-870°C, and then slowly cooling to 700-770°C at a cooling rate of 3-10°C / s to obtain a certain proportion of ferrite, preferably controlling the final cooling temperature of the slow cooling to 700-730°C. Then, it is rapidly cooled again to 200-300°C at a cooling rate of 50-500°C / s to transform a portion of the austenite into martensite, preferably controlling the final cooling temperature of the rapid cooling to 250-300°C. After rapid cooling, it is heated again to 360-460°C, preferably heating to 400-430°C and holding for 50-600 seconds, preferably holding for 180-300 seconds. Finally, it is cooled to room temperature.

[0064] (6) The vehicle may be plated with zinc or a zinc alloy, and the plating may be specifically selected from pure zinc plating, zinc-iron alloy plating, zinc-aluminum-magnesium alloy plating, or aluminum-zinc alloy plating.

[0065] In this invention, the chemical composition designs and related manufacturing processes used in the plated ultra-high-strength steels of Examples 1 to 28 all satisfy the normative requirements designed in this invention. In contrast, the chemical composition designs and related manufacturing processes used in the comparative steels of Comparative Examples 1 to 4 contain process parameters that do not satisfy the design requirements of this invention.

[0066] The plating of the steel in Examples 1-28 and Comparative Examples 1-4 is all hot-dip pure zinc plating. Table 1 shows the mass percentage composition ratios of each chemical element in the steel substrates of the plated ultra-high-strength steels of Examples 1 to 28 and the comparative steel materials of Comparative Examples 1 to 4.

[0067] [Table 1-1]

[0068] [Table 1-2]

[0069] Tables 2-1 and 2-2 show the specific process parameters for the plated ultra-high-strength steels of Examples 1-28 and the comparative steels of Comparative Examples 1-4.

[0070] In Table 2-1, the method for measuring the mass percentage ratio of FeO and Fe3O4 in the scale on the surface of hot-rolled steel strip (i.e., the ratio of (FeO + Fe3O4)) is X-ray diffraction (XRD).

[0071] [Table 2-1-1]

[0072] [Table 2-1-2]

[0073] [Table 2-2-1]

[0074] [Table 2-2-2]

[0075] Before performing the plating process in step (6) described above, the inventors sampled the plated ultra-high-strength steel substrates of Examples 1 to 28 and the comparative steel substrates of Comparative Examples 1 to 4, obtained after performing the continuous annealing process in step (5), and observed the microstructure of the substrates of each example and comparative example. The results of the microstructure observation of the substrates of each example and comparative example are shown in Table 3 below.

[0076] Table 3 shows the microstructure observation results for plated ultra-high-strength steel substrates of Examples 1-28 and comparative steel substrates of Comparative Examples 1-4.

[0077] The methods for observing and measuring microstructure are as follows: The volume ratio of ferrite to martensite was measured using the metallurgical method.

[0078] The size and volume ratio of ferrite crystal grains were measured using the metallurgical method. The average grain size of retained austenite was measured using the metallurgical method.

[0079] The average carbon content in retained austenite was measured using a Castaing electron probe (EPMA).

[0080] [Table 3-1]

[0081] [Table 3-2]

[0082] According to Table 3 above, the microstructure of the steel sheet substrates in Examples 1 to 28 of the present invention is all ferrite + martensite + retained austenite. Furthermore, the microstructure of the substrates in each example satisfies the following criteria: the volume ratio of ferrite is 25% to 45%, the volume ratio of martensite is 45% to 65%, of which the volume ratio of ferrite grains 10 μm or smaller is ≥ 85%, the volume ratio of ferrite grains 5 μm or smaller is ≥ 55%, the average grain size of retained austenite is ≤ 2 μm, and the average carbon content in retained austenite is ≥ 1.0%.

[0083] Furthermore, the inventors detected the mechanical properties of the plated steel sheets of each example and comparative example, and the detection results are shown in Table 4 below. The relevant mechanical property detection methods for detecting the mechanical properties of the plated steel sheets of each example and comparative example are as follows.

[0084] Tensile properties test: Using GB / T228.1-2010 Tensile testing of metallic materials, Part 1: Room temperature test method, tensile tests were performed to obtain the yield strength, tensile strength, uniform elongation, and elongation at break of plated ultra-high-strength steels of Examples 1-28 and comparative steels of Comparative Examples 1-4.

[0085] [Table 4-1]

[0086] [Table 4-2]

[0087] According to Table 4 above, the steel sheets of Examples 1 to 28 designed in the present invention have excellent mechanical properties, with a yield strength of ≥600 MPa, a tensile strength of ≥980 MPa, a uniform elongation of ≥13%, and a fracture elongation of ≥15%.

[0088] Furthermore, in order to verify that the plated ultra-high-strength steel of Examples 1 to 28, which are the finished products, possesses extremely low spot weld crack susceptibility, the inventors sampled the plated ultra-high-strength steel of Examples 1 to 28 and the comparative steel plates of Comparative Examples 1 to 4, and performed welding tests using the spot welding process on each example and comparative example steel plate sample. See Table 5 below for the relevant spot welding process parameters.

[0089] Table 5 shows the specific spot welding process parameters for the plated ultra-high-strength steel of Examples 1-28 and the comparative steel sheets of Comparative Examples 1-4.

[0090] [Table 5-1]

[0091] [Table 5-2]

[0092] During spot welding, one tensile shear (TSS) specimen, one cross-tensile (CTS) specimen, and one metallurgical specimen were welded at each welding current. The load-bearing capacity of the joints was measured using a tensile testing machine according to ISO 14273-2016 and ISO 14272-2016 standards for TSS and CTS. See Table 6 for the results.

[0093] For the metallurgical specimens, the plating layer on the joint surface was first removed with dilute hydrochloric acid. The distribution and direction of surface cracks at the weld points were then observed under a microscope. The cross-section that cut through the center of the weld core and contained the most numerous surface cracks was selected as the metallurgical cross-section of the joint, and samples were taken by wire-cut electrical discharge machining. The cross-section included all the welding characteristic regions of the spot-welded joint. To prevent interference with the test results by foreign matter, the surface of the specimen was cleaned after cutting, and the cleaned sample was dried. After drying, the specimen was subjected to embedding and polishing treatments, and the maximum crack depth was measured using a metallurgical microscope, as shown in Table 6.

[0094] Table 6 shows the mechanical properties of the weld points and the results of the weld point cracking test after spot welding for the plated ultra-high-tensile steel of Examples 1 to 28 and Comparative Examples 1 to 4.

[0095] [Table 6-1]

[0096] [Table 6-2]

[0097] According to Table 6, in Examples 1 to 28 of this application, when the welding current is less than the sputtering current, there are no cracks on the surface of the welded points (abbreviated as sputter-free welded points). However, when the welding current is greater than the sputtering current, the longest crack on the surface of the welded points (abbreviated as sputtered welded points) is less than 5% of the plate thickness. In contrast, in Comparative Examples 1 to 4, regardless of whether the welding current is greater than the sputtering current, there are cracks on the surface of the welded points, the surface cracks at sputtered welded points are more severe than those at sputter-free welded points, and the ratio of the maximum crack length to the plate thickness is much greater than 5%. Thus, the plated ultra-high-strength steel of each example of this application guarantees plate material properties while simultaneously possessing excellent low susceptibility to weld point surface cracking.

[0098] Furthermore, the combination methods of each technical feature in this application are not limited to the combination methods described in the claims of this application or the combination methods described in the specific embodiments. All technical features described in this application can be freely combined or combined in any manner, as long as they do not contradict each other.

[0099] Furthermore, the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and similar changes or modifications can be obtained from or easily conceived by those skilled in the art from the content disclosed herein, and are within the scope of protection of the present invention.

Claims

1. In a plated ultra-high-strength steel with excellent spot-welding properties, comprising a steel substrate and a zinc-containing plating on the surface of the steel substrate, wherein the steel substrate contains Fe and unavoidable impurity elements, the steel substrate further contains the following chemical elements in the following mass percentages: Specifically, a plated ultra-high-strength steel containing C: 0.10–0.22%, Si: 0.30–0.79%, Mn: 1.00–2.50%, Cr: 0.50–5.00%, and Al: 0.03–1.0%.

2. The mass percentage content of each chemical element in the aforementioned steel substrate is: The plated ultra-high-strength steel according to claim 1, characterized in that the composition is 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 the remainder being Fe and unavoidable impurity elements.

3. The mass percentage content of each chemical element in the aforementioned steel substrate is as follows: C：0.15～0.20%、 Si: 0.30–0.79%, Mn: 1.00–2.00%, A plated ultra-high-strength steel according to claim 1 or 2, characterized in that it satisfies at least one of the following conditions: Cr: 1.00 to 3.00%.

4. The chemical elements of the aforementioned steel substrate are as follows: 0 < Mo ≤ 0.03%, 0 < Nb ≤ 0.03%, 0 < Ti ≤ 0.03%, 0＜V≦0.03%、 The plated ultra-high-strength steel according to claim 1 or 2, further comprising at least one clause of 0 < B ≤ 0.001%.

5. The plated ultra-high-strength steel according to claim 1 or 2, characterized in that the mass percentage content of unavoidable impurity elements in the steel substrate satisfies P ≤ 0.01%, S ≤ 0.01%, and N ≤ 0.003%.

6. The plated ultra-high-strength steel according to claim 1 or 2, characterized in that the microstructure of the steel substrate is ferrite + martensite + retained austenite.

7. The plated ultra-high-strength steel according to claim 6, characterized in that the volume ratio of ferrite is 25% to 45% and / or the volume ratio of martensite is 45% to 65%.

8. The plated ultra-high-strength steel according to claim 6, characterized in that, in the ferrite, the volume ratio of crystal grains with a size of 10 μm or less is ≥ 85%, and the volume ratio of crystal grains with a size of 5 μm or less is ≥ 55%.

9. The plated ultra-high-strength steel according to claim 6, characterized in that the average grain size of retained austenite is ≤2 μm and / or the average carbon content in retained austenite is ≥1.0%.

10. The plated ultra-high-strength steel according to claim 1 or 2, characterized in that when a crack occurs on the surface of the weld point during welding in a spot welding process, the maximum depth of the crack on the surface of the weld point is less than 5% of the plate thickness.

11. The plated ultra-high-strength steel according to claim 1 or 2, characterized in that the mechanical properties of the plated ultra-high-strength steel satisfy the following: yield strength of 600 MPa to 850 MPa, tensile strength of 980 MPa to 1150 MPa, uniform elongation of 13% or more, and elongation at break of 15% or more.

12. The plated ultra-high-strength steel according to claim 1 or 2, characterized in that the plating is pure zinc plating, zinc-iron alloy plating, zinc-aluminum-magnesium alloy plating, or aluminum-zinc alloy plating.

13. The following steps, namely, (1) Steelmaking and continuous casting, (2) Heating and, (3) The scale thickness on the surface of the strip after hot rolling is ≤4 μm, and the FeO+Fe in the scale on the surface of the strip after hot rolling. 3 O 4 Hot rolling that controls the mass percentage content to ≤50 wt%, (4) Pickling, or pickling + cold rolling, (5) Continuous annealing, which involves annealing at 800-920°C, then slowly cooling to 700-770°C at a cooling rate of 3-10°C / s, then rapidly cooling to 200-300°C at a cooling rate of 50-500°C / s, then heating again to 360-460°C, holding for 50-600 seconds, and finally cooling to room temperature. (6) A method for producing plated ultra-high-strength steel according to any one of claims 1 to 12, characterized by comprising the steps of (6) carrying out zinc-containing plating.

14. The following characteristics, namely, In step (2), the slab is heated to 1200-1300°C. In process (3), the final rolling temperature is controlled to 860-930°C and the winding temperature to 450-600°C. The manufacturing method according to claim 13, characterized in that, in step (4), when pickling + cold rolling is employed, the amount of cold rolling deformation is controlled to 40% to 60%.

15. The parameters for the annealing process in step (5) are the following items, namely: The annealing temperature is 820-870°C. Slowly cool to 700-730°C at a cooling rate of 3-10°C / s. Rapid cooling to 250-300°C. After rapid cooling, reheat to 400-430°C and hold for 180-300 seconds. The manufacturing method according to claim 13, characterized in that at least one of the following is satisfied: controlling the volume content of hydrogen gas in the reducing atmosphere in a continuous annealing furnace to 10 to 15%.

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