Gigapascal grade quenched steel with excellent spot welding capabilities and methods for manufacturing such materials.

Abstract

Invention details;

Description

A kind of GIP steel with excellent spot welding performance and its manufacturing method

[0001] This application claims priority from the Chinese invention patent application No. 202310033276.9 filed on January 10, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present invention relates to a high-strength steel and a manufacturing method thereof, in particular to a GPA steel with a coating and a manufacturing method thereof. Background Art

[0003] At present, with the advancement of automobile lightweighting and the increasing requirements for automobile corrosion resistance, the demand for coated ultra-high-strength steel is increasing.

[0004] On the other hand, resistance spot welding has become the most important connection method in automotive welding production due to its advantages such as fast welding speed and low cost.

[0005] While many zinc-coated ultra-high-strength steel joints exhibit good quasi-static mechanical properties during spot welding, surface cracking, as shown in Figure 1, often develops on the steel plate surface and its surroundings where the electrodes come into contact. Suppressing the formation of these surface cracks is extremely difficult, and they have become a significant barrier and bottleneck to the automotive application of zinc-coated GPA steel. Meanwhile, the automotive industry has identified surface cracking as a key indicator for evaluating the spot welding performance of zinc-coated high-strength steel.

[0006] There are some technical solutions in the prior art to control welding cracks:

[0007] For example, the Chinese patent document with publication number CN108015401A, publication date May 11, 2018, and titled "Resistance spot welding method for galvanized high-strength steel with good joint performance" discloses a method for suppressing spot weld surface cracks while ensuring that the performance of the weld joint is not reduced through innovation in the spot welding process.

[0008] For example, the Chinese patent document with publication number CN109385515A, published on February 26, 2019, and titled "Multi-layer steel and method for reducing liquid metal embrittlement", discloses a method for suppressing the occurrence of spot welding cracks in high-strength steel. The method suppresses the occurrence of spot welding cracks by first decarburizing the high-strength steel and then galvanizing it, so that the thickness of the decarburized layer is between 10 and 50 microns.

[0009] Another example is the Chinese patent document CN110892087A, published on March 17, 2020, and titled "Zinc-Coated Steel Sheet with High Resistance Spot Weldability." It discloses a technical solution for reducing spot weld cracking by increasing the dew point to form an inner oxide layer on the steel sheet, thereby improving the surface composition distribution. However, this technical solution suffers from the difficulty in controlling the thickness and uniformity of the inner oxide layer and the uniformity of the surface composition distribution of the steel sheet.

[0010] Summary of the Invention

[0011] One of the purposes of the present invention is to provide a coated GIP steel with excellent spot welding performance. The coated GIP steel has very excellent quality and performance. While meeting the user's requirements for the performance of the coated GIP steel and the mechanical properties of the spot welded joints, it also has low spot weld crack sensitivity and has good prospects for promotion and application. In the present invention, GIP steel refers to steel with a tensile strength of ≥1000MPa.

[0012] To achieve the above object, the present invention provides a coated GPA steel with excellent spot welding performance, comprising a steel substrate and a zinc-containing coating on the surface of the steel substrate, wherein the steel substrate contains Fe and unavoidable impurity elements, and further contains the following chemical elements in the following mass percentages:

[0013] C: 0.18~0.22%; Si: 0.80~1.20%; Mn: 1.50~2.50%; Cr: 2.00~4.00%; Al: 0.40~0.90%.

[0014] Accordingly, the present invention also provides a coated GPA steel with excellent spot welding performance, which comprises a steel substrate and a zinc-containing coating on the surface of the steel substrate, wherein the mass percentage of each chemical element of the steel substrate is:

[0015] C: 0.18-0.22%; Si: 0.80-1.20%; Mn: 1.50-2.50%; Cr: 2.00-4.00%; Al: 0.40-0.90%; the remainder is Fe and inevitable impurity elements.

[0016] Furthermore, in some embodiments of the present invention, the chemical elements of the steel substrate may further include at least one of the following elements (in mass percentage):

[0017] 0<Nb≤0.03%;

[0018] 0<Ti≤0.03%;

[0019] 0<V≤0.03%.

[0020] Furthermore, the mass percentage of each chemical element of the steel substrate satisfies at least one of the following:

[0021] 0<Nb≤0.01%;

[0022] 0<Ti≤0.01%;

[0023] 0<V≤0.01%.

[0024] Furthermore, the mass percentage of each chemical element of the steel substrate satisfies at least one of the following:

[0025] C: 0.20~0.22%;

[0026] Si: 0.80~1.00%;

[0027] Mn: 2.00~2.50%;

[0028] Cr: 2.00~3.00%.

[0029] Furthermore, in the coated GIP steel of the present invention, the mass percentage of the inevitable impurity elements in the steel substrate satisfies: P≤0.01%, S≤0.01%, and N≤0.003%.

[0030] In the above-mentioned technical solution of the present invention, a component design mainly based on carbon, silicon, manganese and chromium is adopted, which fully utilizes the role of carbon, silicon, manganese and chromium elements in the phase change of the material, thereby realizing the unity of high mechanical properties and high spot welding performance of the GIP steel described in the present invention, and ultimately realizing a coated steel plate product with excellent spot welding performance.

[0031] In the coated GIP steel of the present invention, the design principles of the chemical elements are as follows:

[0032] C: In the coated GIP steel with excellent spot welding performance described in the present invention, the solubility of carbon in austenite is much higher than its solubility in ferrite, which can extend the incubation period before austenite transformation and lower the Ms temperature. The higher the mass percentage of carbon, the greater the fraction of retained austenite, and the higher the carbon enrichment in the retained austenite during partitioning, which helps enhance the stability of retained austenite, produce the TRIP effect, and improve the material's ductility. Furthermore, carbon is also the most essential solid solution strengthening element in steel. However, the inventors of the present invention have discovered that excessively high mass percentages of carbon can reduce the weldability of the steel, particularly significantly increasing the tendency of surface cracking in welded joints. Furthermore, when the mass percentage of carbon in the steel exceeds 0.22%, more twins are likely to form after quenching, increasing the plate's crack sensitivity. Based on this, in the coated GIP steel with excellent spot welding performance described in the present invention, the mass percentage of carbon is controlled to 0.18-0.22%, for example, 0.19%, 0.20%, 0.21%, or within a range consisting of any two of the aforementioned percentages.

[0033] Of course, in some implementations, in order to obtain better implementation effects, the mass percentage of the C element may be further preferably controlled to be between 0.2% and 0.22%.

[0034] Si: In the coated GIP steel with excellent spot welding performance described in the present invention, the solubility of silicon in carbide is extremely small, which strongly inhibits the formation of cementite during the partitioning treatment, promotes the enrichment of carbon in the retained austenite, and improves the stability of the retained austenite. However, too high a silicon mass percentage will reduce the high-temperature plasticity of the steel, form stable oxides on the surface of the steel plate, and reduce the wettability of the steel plate. In particular, the inventors of the present invention have found that silicon is an element that significantly increases the tendency of cracks on the surface of the weld. Based on this, in the coated GIP steel with excellent spot welding performance described in the present invention, the silicon mass percentage is controlled to be 0.80-1.20%, for example, 0.85%, 0.90%, 0.95%, 1.00%, 1.05%, 1.10%, 1.15%, or within the range composed of any two of the above percentage contents.

[0035] Of course, in some embodiments, the mass percentage of Si element may be further preferably controlled to be between 0.80 and 1.00%.

[0036] Mn: In the coated GIP steel with excellent spot welding performance described in the present invention, manganese can expand the austenite phase region, reduce Ac3, Ms and Mf points, improve austenite stability and the hardenability of steel, reduce the critical transformation rate, and is conducive to the preservation of retained austenite to room temperature. At the same time, manganese can also play a solid solution strengthening effect in steel. However, if the mass percentage of manganese is too high, it will aggravate the grain coarsening trend, reduce the plasticity and toughness of steel, deteriorate corrosion resistance, and especially increase the enrichment of manganese in the shallow surface layer of the parent material under the coating, increase the surface crack sensitivity of the weld of the joint, and deteriorate welding performance. However, if the mass percentage of manganese is too low, ferrite and pearlite banded structure will be produced under low cooling rate due to segregation. Based on this, in the coated GIP steel with excellent spot welding performance described in the present invention, the mass percentage of manganese is controlled at 1.50-2.50%, for example, 1.60%, 1.70%, 1.80%, 1.90%, 2.00%, 2.10%, 2.20%, 2.30%, 2.40%, or within the range of any two of the above percentage contents.

[0037] Of course, in the embodiment, the mass percentage of the Mn element may be further preferably controlled to be between 2.00 and 2.50%.

[0038] Cr: In the coated GIP steel with excellent spot welding performance described in the present invention, chromium increases the strength and hardness of the steel while slightly reducing the plasticity and toughness. More importantly, chromium can reduce the manganese enrichment under the coating and reduce the problem of reduced hardenability of the steel caused by manganese addition. Chromium can increase the hardenability of steel and has a secondary hardening effect. It can improve the hardenability and wear resistance of carbon steel without making the steel brittle. Chromium is an element that expands the austenite γ phase region and can improve the hardenability and heat strength, reduce the temperature range of high temperature in the δ phase region, promote the δ→γ phase transformation, and inhibit the precipitation of high-temperature δ ferrite. In addition, the inventors of the present invention also found that the tendency of high-strength steel to develop surface cracks in welds decreases with increasing chromium content, but the chromium content in carbon steel cannot be too high. Therefore, in the coated GIP steel with excellent spot welding performance described in the present invention, the mass percentage of chromium is controlled at 2.00-4.00%, for example, 2.20%, 2.50%, 2.70%, 3.00%, 3.20%, 3.50%, 3.70%, or within the range composed of any two of the above percentage contents.

[0039] Of course, in some embodiments, the mass percentage of Cr element may be further preferably controlled to be between 2.00 and 3.00%.

[0040] Al: In the coated GIP steel with excellent spot welding performance described in the present invention, when aluminum exists in a solid solution state, it can increase the stacking fault energy, inhibit the precipitation of cementite and the transformation from γ to martensite, and improve the stability of austenite. In addition, aluminum forms fine and dispersed insoluble particles with carbon and nitrogen, which can refine the grains, but the strengthening effect of aluminum is weaker than that of silicon, and its ability to stabilize austenite is also weaker than that of silicon. In addition, if the mass percentage of aluminum is too high, a large amount of oxide inclusions will easily form, which is not conducive to steelmaking and continuous casting. Therefore, in the coated GIP steel with excellent spot welding performance described in the present invention, the mass percentage of aluminum is controlled to be between 0.40% and 0.90%, for example, 0.50%, 0.60%, 0.70%, 0.80%, or within the range composed of any two of the above percentage contents.

[0041] Nb, Ti, and V: These elements can further enhance the performance of the high-strength steel described herein. Niobium, titanium, and vanadium can form fine carbides with carbon, promoting microstructure refinement. However, the formation of these fine carbides hinders carbon enrichment in retained austenite and its stabilization. Furthermore, the addition of these elements increases material cost. Considering both material performance and cost control, the technical solution described herein preferably includes at least one of these elements.

[0042] In some embodiments, the chemical elements of the coated GIP steel of the present invention contain one or more elements in the following mass percentages:

[0043] 0.001%≤Nb≤0.03%;

[0044] 0.001%≤Ti≤0.03%;

[0045] 0.001%≤V≤0.03%.

[0046] P, S, and N elements are all impurity elements in the coated GIP steel described in the present invention. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the coated GIP steel should be reduced as much as possible.

[0047] While phosphorus (P) can provide solid solution strengthening, inhibit carbide formation, and improve the stability of retained austenite, excessive P content can weaken grain boundaries, increase material brittleness, and deteriorate weldability. In other words, the positive effects of P outweigh its negative effects. Therefore, the P content is preferably controlled to 0.01% by weight. In some embodiments, the P content in the steel substrate satisfies the requirement of 0 < P ≤ 0.01%, for example, 0.001 ≤ P ≤ 0.01%.

[0048] However, a high N mass percentage can cause difficulties in steelmaking and continuous casting, and is not conducive to inclusion control. Therefore, the N mass percentage is preferably controlled to N≤0.003%. In some embodiments, the N mass percentage in the steel substrate satisfies 0<N≤0.003%, for example, 0.001%≤N≤0.003%.

[0049] Accordingly, the S element in steel tends to form a low-melting eutectic at grain boundaries. Excessive S content significantly deteriorates the material's plasticity. Therefore, the S content is controlled to be S ≤ 0.01%. In some embodiments, the S content in the steel substrate satisfies the following conditions: 0 < S ≤ 0.01%, for example, 0.001 ≤ S ≤ 0.01%.

[0050] Furthermore, in the coated GPA steel described in the present invention, the microstructure of the steel substrate is ferrite+martensite+retained austenite.

[0051] Furthermore, in the coated GIP steel described in the present invention, the volume proportion of ferrite is 20% to 35%, for example, 25%, 30%; and / or the volume proportion of martensite is 55% to 70%, for example, 60%, 65%.

[0052] Furthermore, in the coated GIP steel described in the present invention, in the ferrite, the volume proportion of grains with a size of less than 10 μm is ≥85%, for example, 85% to 96%, and the volume proportion of grains with a size of less than 5 μm is ≥55%, for example, 55% to 71%.

[0053] Furthermore, in the coated GIP steel of the present invention, the average grain size of the retained austenite is ≤2 μm, such as 0.3-2 μm; and / or the average C content in the retained austenite is ≥1.0 wt%, such as 1.0 wt%-1.3 wt%.

[0054] Furthermore, in the coated GIP steel of the present invention, if surface cracks are generated at the weld spots during spot welding, the maximum length of the surface cracks at the weld spots is less than 5% of the plate thickness.

[0055] Furthermore, the resistivity of the coated GPa steel of the present invention is 4.5×10 -7 ~7.0×10 -7 Ωm, for example 5.0×10 -7 Ωm、5.5×10 -7 Ωm、6.0×10 -7 Ωm、6.5×10 -7 Ωm, or within the range consisting of any two of the above values.

[0056] In some embodiments, the yield strength of the coated GPa steel described in the present invention is 850MPa to 1000MPa, such as 880MPa, 900MPa, 920MPa, 950MPa, 980MPa, or within the range of any two of the above values.

[0057] In some embodiments, the tensile strength of the coated GPa steel described in the present invention is 1180MPa to 1300MPa, for example, 1200MPa, 1230MPa, 1250MPa, 1280MPa, or within the range of any two of the above values.

[0058] In some embodiments, the uniform elongation of the coated GPa steel of the present invention is not less than 11%, such as 12%, 13%, 14%, 15%, 16%, or within the range of any two of the above values.

[0059] In some embodiments, the elongation at break of the coated GIP steel of the present invention is not less than 15%, preferably not less than 20%, for example, 21%, 22%, 23%, 24%, 25%, or within the range of any two of the above values.

[0060] Furthermore, in the coated GIP steel described in the present invention, its mechanical properties meet the following requirements: its yield strength is 850MPa~1000MPa, its tensile strength is 1180MPa~1300MPa, its uniform elongation is not less than 11%, and its elongation at break is not less than 15%.

[0061] The resistivity and mechanical properties of the coated GPa steel described in the present invention refer to the resistivity and mechanical properties of the steel substrate of the coated GPa steel.

[0062] The present application has no particular restrictions on the coating, and any known zinc-containing coating can be used in the present invention. Furthermore, the coating of the coated GPA steel described in the present invention is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating (ZM), or an aluminum-zinc coating (AM). These coatings all meet industry, regional, or national standards.

[0063] The pure zinc coating can be a hot-dip pure zinc coating (GI) or an electroplated pure zinc coating (EG). The zinc-iron alloy coating can be a hot-dip galvanized iron alloy coating (GA).

[0064] Correspondingly, another object of the present invention is to provide a method for manufacturing coated GIP steel with excellent spot welding performance. The manufacturing method is simple to produce, and the resulting high-strength steel has significantly improved spot welding performance, especially the ability to resist surface cracks in welds, under the same mechanical properties.

[0065] In order to achieve the above object, the present invention proposes a method for manufacturing a coated GIP steel with excellent spot welding performance, which specifically comprises the steps of:

[0066] (1) Smelting and thin plate continuous casting;

[0067] (2) heating;

[0068] (3) Hot rolling: the thickness of the oxide scale on the surface of the steel strip after hot rolling is controlled to be ≤6 μm, and the mass percentage of FeO+Fe3O4 in the oxide scale on the surface of the steel strip after hot rolling is ≤55 wt%;

[0069] (4) pickling, or pickling + cold rolling;

[0070] (5) Continuous annealing: anneal at 800-920°C, then slowly cool to 680-750°C at a cooling rate of 3-10°C / s; then quickly cool to 180-280°C at a cooling rate of 50-100°C / s; then heat to 350-450°C, keep warm for 60-300s; finally cool to room temperature;

[0071] (6) Coating with zinc coating.

[0072] In the technical solution described in the present invention, the manufacturing method utilizes a thin slab continuous casting process in conjunction with a pickling or pickling rolling process, followed by continuous annealing and coating, to produce coated GIP steel with excellent spot welding properties. The coated GIP steel produced by this manufacturing method can be welded using conventional spot welding processes used in car factories, and the maximum length of surface cracks at the welds is less than 5% of the plate thickness. It exhibits very low spot weld crack sensitivity and excellent quality.

[0073] In the above step (1) of the present invention, since thin slab continuous casting is used, the rough rolling process can be omitted, the hot rolling deformation is reduced, and the steel plate performance in the subsequent steps (4) and (5) is guaranteed. In addition, since thin slab continuous casting is used in step (1), it can fully utilize the heat of the slab and reduce the energy consumption required for heating, thereby obtaining a more uniform ferrite or ferrite + pearlite structure, which is conducive to maintaining a certain amount of fine-grained ferrite in the microstructure of the finished substrate in step (6), thereby improving the uniformity of the structure.

[0074] In step (3), controlling the thickness of the oxide scale on the surface of the hot-rolled steel strip to be ≤6 μm and the content of (FeO+Fe3O4) in the oxide scale on the surface of the hot-rolled steel strip to be ≤55 wt% is beneficial to the subsequent step (4) and has an important influence on the performance of the steel plate obtained after continuous annealing. This is because: in the technical solution described in the present invention, FeO and Fe3O4 are more difficult to pickle than Fe2O3, and controlling the thickness of the oxide scale on the surface of the hot-rolled steel strip prepared by the present invention and the content of (FeO+Fe3O4) in the oxide scale on the surface of the hot-rolled steel strip to be ≤55 wt% can effectively improve the pickling effect and obtain a pickled plate surface that can be used for direct continuous annealing. Since the pickled plate can be directly annealed continuously, the deformation of the hot-rolled structure is small, and the steel plate structure is mainly pearlite and ferrite. Therefore, under the same continuous annealing conditions, the material strength can be reduced, the structure can be made more uniform, and excellent ductility can be obtained.

[0075] Accordingly, in step (5), the inventors optimized the design of the continuous annealing process. By controlling the annealing temperature to 800-920°C, a homogenized austenite structure or austenite + ferrite structure can be formed; then the material is slowly cooled to 680-750°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; then the material is cooled to 180-280°C (i.e., between M and C) at a cooling rate of 50-100°C / s. s With M f Temperature), at this time, part of the austenite is transformed into martensite, ensuring that the steel has a high strength; then heated to 350 ~ 450 ℃ and kept warm for 60 ~ 300s, so that carbon is partitioned in martensite and austenite, forming a certain amount of carbon-rich retained austenite, which is stable to room temperature. Due to the TRIP effect, the work hardening ability and formability of the steel can be significantly improved, and a high-strength steel plate with excellent ductility can be obtained.

[0076] In addition, in view of the inventors' understanding of the effects of carbon, silicon, manganese and chromium on weld surface cracks, especially the understanding that silicon and manganese are easily enriched in the shallow surface layer of the steel plate and significantly increase the sensitivity of weld surface cracks, the contents of carbon, silicon, manganese and chromium are strictly limited when designing the steel composition. At the same time, the contents of carbon, silicon and manganese are reduced compared with steels of the same strength level, and chromium is added to increase the hardenability of the steel. This ensures that the designed proportions of martensite and austenite and the carbon distribution are achieved in step (5), while ensuring that the zinc-coated plate passing step (6) has low weld surface crack sensitivity, that is, the maximum length of the weld surface crack is less than 5% of the plate thickness.

[0077] Because the GIP steel of the present invention utilizes carbon, silicon, manganese, and chromium in its design and refines the ferrite grains, during continuous annealing, the number of nucleation sites for reverse austenite transformation increases while further refining the grain size. The average grain size of the retained austenite stably maintained at room temperature can be ≤2μm, and the average carbon content in the retained austenite is ≥1.0wt%.

[0078] It should be noted that, when the zinc-containing coating is plated in step (6), the zinc-containing coating can be produced by, but is not limited to, hot-dip, electroplating, or vacuum plating techniques.

[0079] Furthermore, in the method for manufacturing coated GIP steel of the present invention, in step (1), the thickness of the slab at the outlet end of the thin strip continuous casting is controlled to be 55 to 65 mm.

[0080] Furthermore, in the method for manufacturing coated GPA steel of the present invention, in step (1), the drawing speed of the thin strip continuous casting is controlled to be 2 to 5 m / min.

[0081] Furthermore, in the method for manufacturing coated GPA steel of the present invention, in step (2), the slab is heated to 1200-1250°C.

[0082] Furthermore, in the method for manufacturing coated GIP steel of the present invention, in step (3), the finishing rolling temperature is controlled to be 850-900°C, and the coiling temperature is controlled to be 500-600°C.

[0083] Furthermore, in the method for manufacturing coated GIP steel of the present invention, in step (4), when pickling + cold rolling is adopted, the cold rolling deformation is controlled to be 50% to 60%.

[0084] Furthermore, in the method for manufacturing coated GIP steel of the present invention, in step (5), the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled to be 10-15%.

[0085] Furthermore, in the method for manufacturing coated GIP steel of the present invention, the annealing process parameters of step (5) satisfy at least one of the following:

[0086] Annealing temperature is 820~870℃;

[0087] Slowly cool to 680-720°C at a cooling rate of 3-10°C / s;

[0088] Cool rapidly to 200-250°C at a rate of 50-100°C / s;

[0089] After rapid cooling, heat again to 400-430°C and keep warm for 150-250 seconds.

[0090] Compared with the prior art, the coated GIP steel with excellent spot welding performance and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0091] The coated GIP steel with excellent spot welding performance described in the present invention is based on carbon, silicon, manganese and chromium, without adding any expensive alloy elements. By optimizing the ratio of carbon, silicon, manganese and chromium, the coated GIP steel with excellent spot welding performance is obtained.

[0092] The manufacturing method of the present invention has a simple production process. The obtained GIP steel has significantly improved resistance to weld surface cracks under the same mechanical property conditions, and will have good application prospects in the production of safety structural parts for downstream users. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] Figure 1 shows the spot weld cracks in the zinc-coated ultra-high strength steel. DETAILED DESCRIPTION

[0094] The following will further explain and illustrate the coated GIP steel with excellent spot welding performance and the manufacturing method thereof described in the present invention in conjunction with specific embodiments. However, such explanation and illustration do not constitute an improper limitation to the technical solution of the present invention.

[0095] Examples 1-35 and Comparative Examples 1-4

[0096] The coated GIP steels of Examples 1-35 of the present invention and the comparative steels of Comparative Examples 1-4 were prepared by the following steps:

[0097] (1) Smelting and thin strip continuous casting are carried out according to the chemical composition ratio designed in Table 1, wherein the slab thickness at the outlet end of the thin strip continuous casting is controlled to be 55-65 mm, and the pulling speed of the thin strip continuous casting is controlled to be 2-5 m / min to obtain the corresponding slab.

[0098] (2) Heating: Heat the slab to 1200-1250°C.

[0099] (3) Hot rolling: The thickness of the oxide scale on the surface of the steel strip after hot rolling is controlled to be ≤6μm, and the (FeO+Fe3O4) in the oxide scale on the surface of the steel strip after hot rolling is ≤55wt%. The finishing temperature is controlled to be 850-900℃, and the coiling temperature is controlled to be 500-600℃.

[0100] (4) Pickling, or pickling + cold rolling. When pickling + cold rolling is used, the cold rolling deformation is controlled to be 50% to 60%. When galvanizing or zinc alloy is directly plated after pickling, the cold rolling deformation in Table 2-2 is 0.

[0101] (5) Continuous annealing: Control the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace to 10-15%, anneal at 800-920°C, then slowly cool to 680-750°C at a cooling rate of 3-10°C / s; then quickly cool to 180-280°C at a cooling rate of 50-100°C / s; then heat to 350-450°C, keep warm for 60-300s; finally cool to room temperature.

[0102] (6) Coating with zinc or zinc alloy, wherein the coating can be specifically selected from pure zinc coating, zinc-iron alloy coating, zinc-aluminum-magnesium coating or aluminum-zinc coating.

[0103] It should be noted that the chemical composition designs and related manufacturing processes used in the coated GPA steels of Examples 1-35 of the present invention all meet the specification requirements of the present invention. Accordingly, the chemical composition designs and related manufacturing processes used in the comparative steels of Comparative Examples 1-4 all contain process parameters that do not meet the design requirements of the present invention.

[0104] The coatings of Examples 1-35 and Comparative Examples 1-4 are all hot-dip pure zinc.

[0105] Table 1 lists the mass percentages of the chemical elements in the steel plate substrates of the coated GIP steels of Examples 1-35 and the comparative steels of Comparative Examples 1-4.

[0106] Table 1. (wt%, the balance is Fe and other inevitable impurities except P, S, and N)

[0107] Accordingly, Table 2-1 and Table 2-2 list the specific process parameters of the coated GIP steel of Examples 1-35 and the comparative steel of Comparative Examples 1-4 in the above process steps (1) to (6).

[0108] Table 2-1.

[0109] Note: In the above Table 2-1, "(FeO+Fe3O4) ratio" means the mass percentage of FeO and Fe3O4 in the oxide scale on the surface of the strip after hot rolling.

[0110] Table 2-2.

[0111] It should be noted that before the plating process of step (6) above is carried out, in order to verify that the coated GIP steel of Examples 1-35 has very excellent performance, the inventors sampled the coated GIP steel substrates of Examples 1-35 obtained after the continuous annealing process of step (5) and the comparative steel substrates of Comparative Examples 1-4 respectively before the plating process was carried out, and the microstructure of the substrates of each embodiment and comparative example was observed, and the mechanical properties of the substrates of each embodiment and comparative example were tested. Among them, the microstructure observation results of the substrates of each embodiment and comparative example are listed in Table 3 below, and the mechanical property test results of the substrates of each embodiment and comparative example are listed in Table 4 below.

[0112] Table 3 lists the microstructure observation results of the coated GIP steel substrates of Examples 1-35 and the comparative steel substrates of Comparative Examples 1-4.

[0113] The methods for observing and measuring the microstructure are as follows:

[0114] The volume fractions of ferrite and martensite were determined by metallographic method;

[0115] The size and volume fraction of ferrite grains were determined by metallographic method;

[0116] The average grain size of retained austenite was determined by metallographic method;

[0117] The average carbon content in retained austenite was determined by Castaing electron probe microscopy (EPMA).

[0118] Table 3.

[0119] As can be seen from Table 3, the microstructures of the substrates of Examples 1-35 designed according to the present invention are all composed of ferrite + martensite + retained austenite. Furthermore, the microstructures of the substrates of each example meet the following specifications: the volume fraction of ferrite is 20% to 35%, and the volume fraction of martensite is 55% to 70%. The volume fraction of ferrite grains 10 μm or smaller is ≥ 85%, and the volume fraction 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 the retained austenite is ≥ 1.0 wt%.

[0120] Accordingly, when testing the mechanical properties of the substrates of each embodiment and comparative example, the relevant mechanical properties testing methods are as follows:

[0121] Tensile property test: The tensile test was carried out using GB / T228.1-2010 Tensile Tests of Metallic Materials Part 1: Room Temperature Test Method to test the yield strength, tensile strength, uniform elongation and elongation at break of the coated GIP steel substrates of Examples 1-35 and the comparative steel substrates of Comparative Examples 1-4.

[0122] Resistivity test: GB / T 351-2019 Metal Material Resistivity Measurement Method is used for testing.

[0123] Table 4 lists the mechanical properties and resistivity test results of the coated GPa steels of Examples 1-35 and the comparative steels of Comparative Examples 1-4.

[0124] Table 4.

[0125] As can be seen from Table 4 above, the steel substrates of Examples 1-35 designed by the invention all have very excellent mechanical properties, and their mechanical properties all meet the design requirements of the present invention. The yield strength of the coated GPa steel substrates of Examples 1-35 is specifically between 855 and 995 MPa, the tensile strength is between 1180 and 1285 MPa, the uniform elongation is between 13.6 and 15.4%, and the elongation at break is between 21.1 and 23.9%. In addition, the resistivity of each embodiment of the present invention is between 4.5×10 -7 ~7.0×10 -7 Ωm. The yield strength of comparative example 1 is lower, and the uniform elongation of comparative examples 3 and 4 is lower. Although the mechanical properties of comparative example 2 meet the requirements of the present invention, its resistivity is lower than 4.5×10 -7 Ωm. And it can be seen that the resistivity of comparative examples 1-4 is lower than 4.5×10 -7 Ωm.

[0126] Accordingly, in order to verify that the finished products of Examples 1-35 with coated GIP steel prepared through the above steps (1)-(6) have excellent low spot welding crack sensitivity, the inventors took samples of the finally prepared finished products of Examples 1-35 with coated GIP steel and the comparative steel plates of Comparative Examples 1-4, and conducted welding tests using a spot welding process on the finished steel plate samples of each embodiment and comparative example. The relevant spot welding process parameters are shown in Table 5 below.

[0127] Table 5 lists the specific spot welding process parameters of the coated GIP steels of Examples 1-35 and the comparative steel plates of Comparative Examples 1-4.

[0128] Table 5.

[0129] During the actual spot welding process, one tensile shear (TSS) specimen, one cross tensile (CTS) specimen, and one metallographic specimen were welded at each welding current. The TSS and CTS specimens were tested using a tensile testing machine according to ISO 14273-2016 and ISO 14272-2016 standards to measure the bearing capacity of the spot-welded joints of the finished steel plate samples of each embodiment and comparative example. The results are shown in Table 6 below.

[0130] Accordingly, for the metallographic specimens obtained, the coating on the surface of the joint was first removed with dilute hydrochloric acid, and the distribution and direction of the spot welding cracks were observed under a microscope. The cross section passing through the center of the weld nugget and capable of cutting the most spot welding cracks was selected as the metallographic section of the joint. Wire cutting was used for sampling, and the cross section included all welding characteristic areas of the spot welding joint. The surface of the cut specimens was rinsed to prevent foreign matter from interfering with the test results. The rinsed samples were dried. The dried specimens were mounted, ground and polished, and measured under a metallographic microscope. The maximum crack length was recorded in Table 6.

[0131] Table 6 lists the mechanical properties of the welds and the crack test results of the welds of the coated GPa steels of Examples 1-35 and Comparative Examples 1-4 after spot welding.

[0132] Table 6.

[0133] Note: The two values ​​of the nominal weight of the coating in Table 6 refer to the coating weight on the front and back of the steel plate.

[0134] At the same time, it can be seen from Table 6 that for the finished coated GIP steel of Examples 1-35 of this case, when the welding current is less than the spatter current, the welded welds (referred to as non-spatter welds) have no spot welding cracks; when the welding current is greater than the spatter current, the longest crack of the welded welds (referred to as spatter welds) is also less than 5% of the plate thickness. Thus, it is explained that the coated GIP steel of each embodiment of Examples 1-35 designed by the technical solution of the present invention has excellent low spot welding crack sensitivity while ensuring the performance of the plate. For the comparative steel plates of Comparative Examples 1-4 designed in this case, regardless of whether the welding current is greater than the spatter current, the welded welds will have cracks, and the cracks of the spatter welds are more serious than those of the non-spatter welds, and the ratio of the maximum crack length to the plate thickness is much greater than 5%.

[0135] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0136] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

DEPCT6903 / 10 / 25681. Gigapascal grade steel with a coating that exhibits excellent spot welding performance, consisting of a steel substrate and a zinc coating on the surface of the steel substrate. The steel substrate contains Fe and unavoidable impurities, with the following chemical elements present as a percentage by mass: C: 0.18~0.22%, Si: 0.80~1.20%, Mn: 1.50~2.50%, Cr: 2.00~4.00%, Al: 0.40~0.90%.

2. Gigapascal steel with a coating according to Requisition 1, where the steel substrate contains each chemical element in the following percentages by mass: C: 0.18~0.220%, Si: 0.80~1.20%, Mn: 1.50~2.50%, Cr: 2.00~4.00%, Al: 0.40~0.90%, with the remainder being Fe and unavoidable impurities.

3. Gigapascal steel with a coating according to Requisition 1 or 2, where the chemical composition of the steel substrate contains at least one of the following elements. <Nbน้อยกว่าหรือเท่ากับ0.03%0<Tiน้อยกว่าหรือเท่ากับ0.03%0<Vน้อยกว่าหรือเท่ากับ0.03%4.Gigapascal grade steel with a coating conforming to claim 3, where the percentage by mass of each chemical element in the steel substrate conforms to at least one of the following list of elements: <Nbน้อยกว่าหรือเท่ากับ0.01%0<Tiน้อยกว่าหรือเท่ากับ0.01%0<Vน้อยกว่าหรือเท่ากับ0.01%5.เหล็กกล้าระดับจิกะปาสคาลที่มีส่วนเคลือบตามข้อถือสิทธิที่1หรือ2โดยที่ปริมาณเป็นเปอร์เซ็นต์โดยมวลของธาตุทางเคมีแต่ละชนิดในซับสเตรตเหล็กกล้าเป็นไปตามรายการธาตุต่อไปนี้อย่างน้อยหนึ่งรายการC:0.20~0.22%Si:0.80~1.00%Mn:2.00~2.50%Cr:2.00~3.00%6.เหล็กกล้าระดับจิกะปาสคาลที่มีส่วนเคลือบตามข้อถือสิทธิที่1หรือ2โดยที่เปอร์เซ็นต์โดยมวลของธาตุเจือปนที่หลีกเลี่ยงไม่ได้ในซับสเตรตเหล็กกล้าเป็นไปตาม:Pน้อยกว่าหรือเท่ากับ0.01%,Sน้อยกว่าหรือเท่ากับ0.01%,Nน้อยกว่าหรือเท่ากับ0.003%7.เหล็กกล้าระดับจิกะปาสคาลที่มีส่วนเคลือบตามข้อถือสิทธิที่1หรือ2โดยที่โครงสร้างระดับจุลภาคของซับสเตรตเหล็กกล้าเป็นเฟอร์ไรต์+มาร์เทนไซต์+ออสเทนไนต์ส่วนที่เหลือ8.

8. Gigapascal-grade steel with a coating according to Requisition 7 where the volume fraction of ferrite is 20%~35% and / or the volume fraction of martensite is 55%~70%.

9. Gigapascal-grade steel with a coating according to Requisition 7 where the volume of grains 10 µm or less in ferrite is greater than or equal to 85% and the volume of grains 5 µm or less is greater than or equal to 55%.

10. Gigapascal-grade steel with a coating according to Requisition 7 where the remaining austenite has an average grain size less than or equal to 2 µm and / or the remaining austenite has an average C content greater than or equal to 1.0%.

11. Gigapascal-grade steel with a coating according to Requisition 1 or 2 where, when spot welding is applied for welding, if cracks occur on the surface of the solder joint, the maximum length of the crack on the surface of the solder joint is less than 5% of the plate thickness. 12.

12. Gigapascal-grade steel with coatings according to Requisition 1 or 2, where the mechanical properties of such coatings are: yield strength 850 MPa ~ 1000 MPa, tensile strength 1180 MPa ~ 1300 MPa, uniform design elongation not less than 11%, and elongation at break not less than 15%.

13. Gigapascal-grade steel with coatings according to Requisition 1 or 2, where the electrical resistance of such coatings is 4.5 x 10⁻⁷ ~ 7.0 x 10⁻⁷ ohm-meters.

14. Gigapascal-grade steel with coatings according to Requisition 1 or 2, where the coating is pure zinc, zinc-iron alloy, zinc-aluminum-magnesium, or aluminum-zinc. 15.The process of producing gigapascal steel with coatings according to one of the claims 1-14, which includes the following steps: (1) continuous melting and casting of thin flat bars; (2) heating; (3) hot rolling, where the thickness of the oxide flake on the surface of the steel coil after hot rolling is controlled to be less than or equal to 6 micrometers and the percentage by mass of FeO+Fe3O4 in the oxide flake on the surface of the steel coil after hot rolling is less than or equal to 55% by weight; (4) acid cleaning or Acid cleaning + cold rolling (5) Continuous annealing in which the steel is annealed at 800~920°C, then slowly cooled to 680~750°C at a cooling rate of 3~10°C / sec, then rapidly cooled to 180~280°C at a cooling rate of 50~100°C / sec, then reheated to 350~450°C, held for 60~300 seconds and final cooled to room temperature. (6) Zinc coating plating. 16.

17. Production method according to claim 15 where in step (1) the thickness of the flat bar at the outlet of continuous thin flat bar casting is controlled at 55~65 mm.

18. Production method according to claim 15 where in step (1) the drawing speed of continuous thin flat bar casting is controlled at 2~5 m / min.

19. Production method according to claim 15 where in step (2) the flat bar is heated to 1200~1250 degrees Celsius.

20. Production method according to claim 15 where in step (2) (3) The final rolling temperature is controlled at 850~900°C, the coiling temperature is controlled at 500~600°C.

20. Production method according to claim 15 where in step (4) when acid cleaning + cold rolling is applied, the deformation rate in cold rolling is controlled at 50%~60%.

21. Production method according to claim 15 where in step (5) the volumetric amount of hydrogen in the reducing atmosphere in the continuous furnace is controlled at 10~15%. 22.The production method according to claim 15, where the parameters of the annealing process of step (5) are at least one of the following items; annealing temperature 820~870°C, slowly cooled to 680~720°C at a cooling rate of 3~10°C / s, quickly cooled to 200~250°C at a cooling rate of 50~100°C / s, reheated to 400~430°C after rapid cooling, which is held for 150~250 seconds;

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