Composite coating, coating method, steel plate with coating, and spot-welded joint

Abstract

Disclosed in the present invention is a composite coating for steel. The composite coating comprises a first coating and a second coating which are sequentially arranged in the thickness direction of the coating, wherein the first coating is in direct contact with a steel substrate and comprises Mo and / or Cr, and the second coating is a zinc-based coating. Further disclosed in the present invention are a coating method for the composite coating, a steel plate comprising the coating, and a spot-welded joint comprising the steel plate. The present invention can greatly inhibit the phenomenon of liquid metal embrittlement caused by welding during a spot welding process, reduce cracks generated due to the phenomenon, and ensure the performance stability and functional stability of a steel plate, especially a galvanized high-strength steel plate.

Description

Composite coating and coating method, steel plate with the coating, and spot welding joint Technical Field

[0001] The present invention relates to the field of coating, in particular to a composite coating for steel and a method for coating the composite coating, a steel plate having the composite coating, and a spot welding joint comprising the steel plate. Background Art

[0002] The research and development and application of lightweight automotive materials are crucial for energy conservation, emission reduction, safety, and cost reduction. They are crucial for conserving global energy, natural resources, and protecting the environment, and have become a leading trend in automotive material development. High-strength materials, such as advanced high-strength steel (AHSS), have demonstrated significant potential for achieving energy savings by reducing vehicle weight.

[0003] With the advancement of lightweight vehicles, the automotive industry is increasingly demanding high-strength steel. At the same time, to improve the corrosion resistance of high-strength steel, galvanizing processes such as hot-dip galvanizing and electroplating are widely adopted. The application of galvanized high-strength steel in the automotive field inevitably requires the use of joining technology. However, the subsequent thermal processing of galvanized high-strength steel, such as hot stamping and resistance spot welding, is prone to liquid metal embrittlement (LME), which has become a major obstacle to the application of galvanized high-strength steel in the automotive field.

[0004] The LME phenomenon is characterized by the fact that when a base metal, under externally applied stress or internal stress caused by constraints, thermal expansion, or phase transformation, comes into contact with liquid metal, the liquid metal infiltrates along the base metal's grain boundaries, forming cracks in severe cases and thus reducing the base metal's plasticity. The three necessary conditions for LME crack formation are contact between the base metal and the liquid metal, stress, and a suitable temperature range, which are sensitive to LME. High-strength galvanized steel sheets meet all three of these conditions simultaneously during resistance spot welding, making the LME problem particularly severe in spot welds of high-strength galvanized steel sheets. The industry generally believes that second- and third-generation advanced high-strength steels (AHSS) are most sensitive to spot weld LME. These steels are generally characterized by the presence of retained austenite, high strength, and relatively high carbon, silicon, and manganese contents.

[0005] The LME phenomenon occurs during the resistance spot welding of typical galvanized high-strength steels, such as DP, TRIP, TWIP, and QP medium-manganese steels. This phenomenon undoubtedly has a significant negative impact on the safety of galvanized automotive sheet during application. Therefore, it is imperative to investigate the formation mechanism and countermeasures of LME cracking during the spot welding of galvanized high-strength steel. While reasonable spot welding process adjustments and parent material manipulation can mitigate LME cracking in galvanized high-strength steel to a certain extent, neither approach has fundamentally resolved the LME problem. Chinese Patent Authorization No. CN108015401B discloses a method for suppressing spot welding LME while ensuring the performance of the weld joint by adjusting the spot welding process parameters. However, this method cannot fundamentally solve the LME problem. Chinese Patent Publication No. CN109385515A discloses a method for suppressing spot welding LME by decarburizing the surface of a steel plate to form a multi-layer steel structure. However, this method has the problems of difficulty in controlling the thickness and uniformity of the decarburization layer and unstable surface properties of the parent material. Chinese Patent Publication No. CN110892087A discloses a method for suppressing spot welding LME by improving the surface composition distribution of the steel plate by increasing the dew point to form an inner oxide layer. However, this method has the problems of difficulty in controlling the thickness and uniformity of the inner oxide layer and the uniformity of the surface composition distribution of the steel plate. Chinese patents CN110573335A, CN111263829A, CN111263830A, CN111279006A, CN111279007A, and CN111356783A address spot welding LME by nickel plating or pre-nickel plating, but nickel is relatively expensive and the cost is high. Chinese patent CN105849304A optimizes the plating layer to form an Fe-Zn alloy layer with a thickness of [(3.4×t) / 6] microns to control LME, but the thickness of the Fe-Zn alloy layer is difficult to accurately control. Summary of the Invention

[0006] The present invention provides a composite coating for steel to solve the above problems.

[0007] An embodiment of the present invention discloses a composite coating for steel, comprising a first coating and a second coating sequentially arranged along the coating thickness direction, wherein the first coating is in direct contact with a steel substrate, the first coating comprises Mo and / or Cr, and the second coating is a zinc-based coating.

[0008] This technical solution effectively suppresses the liquid metal embrittlement that occurs during resistance spot welding, ensuring stable steel plate performance. Furthermore, both Mo and Cr are readily available. By applying a second coating between the steel substrate and the zinc-based coating, the coating's uniformity and thickness can be precisely controlled by controlling the coating method, making it easy to operate.

[0009] Optionally, the first coating is a molybdenum-chromium alloy, and the first coating comprises, by weight percentage, Cr ≥ 2%, Mo ≥ 2%, and Cr + Mo ≥ 15%, with the remainder being unavoidable impurity elements. In some embodiments, the first coating comprises, by weight percentage, Cr ≥ 2%, Mo ≥ 2%, and Cr + Mo ≥ 15%, with the remainder being Fe and unavoidable impurity elements.

[0010] Optionally, the first coating is a molybdenum-chromium alloy, and the first coating includes, by weight percentage: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, 0 ≤ B + Be + K + Rb ≤ 10%, and the remainder is unavoidable impurity elements. In some embodiments, the first coating is a molybdenum-chromium alloy, and the first coating includes: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, 0 ≤ B + Be + K + Rb ≤ 10%, and the remainder is Fe and unavoidable impurity elements.

[0011] In some embodiments, the Cr content in the first coating layer is ≥5%. In some embodiments, the Cr content in the first coating layer is ≥10%. In some embodiments, the Cr content in the first coating layer is ≥15%. In some embodiments, the Cr content in the first coating layer is 2-100%, such as 5-100%, 10-100%, 15-100%, 5-25%, 10-25%, 15-25%, or 15-22%.

[0012] In some embodiments, the content of Mo in the first coating layer is 2-100%, such as 2-50%, 2-8%, 2-6% or 2-5%.

[0013] In some embodiments, the content of B is 0.01 to 0.10%, such as 0.03 to 0.07%.

[0014] In some embodiments, the content of Rb is 0.5-1.5%, such as 0.8-1.2%.

[0015] In some embodiments, the Be content is 0.5-1.5%, such as 0.8-1.2%.

[0016] In some embodiments, the content of Rb is 0.5-1.5%, such as 0.8-1.2%.

[0017] In some embodiments, the content of B+Be+K+Rb is 0≤B+Be+K+Rb≤5%, preferably 0≤B+Be+K+Rb≤2.5%.

[0018] In some embodiments, the first coating layer contains: Mo: 2-5%, Cr: 15-20%, Ni: 8-13%, C: 0.05-0.12%, Si: 0.5-1.2%, P: ≤0.020%, S: ≤0.005%, and the balance is Fe and unavoidable impurities.

[0019] In some embodiments, the first coating consists of Mo or Cr.

[0020] In some embodiments, the composition of the first coating is that of 316 stainless steel.

[0021] In some embodiments, the first coating layer contains 13-17% Cr and 2-5% Mo, with the balance being Fe and unavoidable impurities.

[0022] In some embodiments, the first coating layer contains: 13-17% Cr, 2-5% Mo, 0.01-0.10% B and 0.5ε1.5% Rb, with the balance being Fe and unavoidable impurities.

[0023] In some embodiments, the first coating layer contains: 13-17% Cr, 2-5% Mo, 0.5-1.5% Be, and 0.5-1.5% Rb, with the balance being Fe and unavoidable impurities.

[0024] Optionally, the second coating is pure Zn.

[0025] Optionally, the element composition of the second coating is: Zn≥90%, 0.5≤Al≤3%, 0.1≤Mg≤5%, and the rest are Cr, Mo, Fe and unavoidable impurity elements.

[0026] Optionally, at room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<130μΩ.cm, the resistivity ρ2 of the second coating satisfies 5μΩ.cm<ρ2<10μΩ.cm, and 1<ρ1 / ρ2<25. In some embodiments, at room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<100μΩ.cm. In some embodiments, at room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<80μΩ.cm. In some embodiments, at room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<20μΩ.cm. In some embodiments, at room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<15μΩ.cm. In some embodiments, at room temperature, the resistivity ρ2 of the second coating satisfies 5μΩ.cm<ρ2<7μΩ.cm.

[0027] According to another specific embodiment of the present invention, an embodiment of the present invention discloses a method for coating a composite coating on a steel substrate, comprising the following steps:

[0028] Applying the first coating: applying the first coating on the steel substrate by physical vapor deposition or electroplating;

[0029] Applying a second coating: applying a second coating on the first coating by physical vapor deposition, electroplating or hot-dip coating to form a composite coating consisting of the first coating and the second coating.

[0030] Optionally, coating the first coating layer includes: using a magnetron sputtering plating process to deposit a first coating layer on the surface of the steel substrate, wherein the vacuum degree is 0.5-0.65 Pa, the process gas is argon, the gas flow rate is 350-450 sccm, the magnetron sputtering current is 6.5-7.5 A, the distance between the target material and the steel plate is 50-100 mm, and the sputtering time is 10-30 minutes.

[0031] Optionally, coating the second coating layer includes: depositing the second coating layer on the surface of the first coating layer using an induction evaporation spray deposition process to form a composite coating consisting of the first coating layer and the second coating layer, wherein the vacuum degree is 3×10 -4 Pa or less, the crucible temperature is greater than 700 ° C, the injection speed is 200 ~ 500 m / min, the vapor temperature is 580 ~ 780 ° C, and the injection time is 10 ~ 30 minutes.

[0032] Optionally, before applying the first coating, the process further includes pre-treating the surface of the steel substrate, preheating the pre-treated steel substrate to 150-300°C, and plasma cleaning the pre-heated steel substrate. Optionally, the pre-treatment includes performing alkaline cleaning, brushing, electrolytic cleaning, rinsing, or a combination thereof on the surface of the steel substrate, and drying. Optionally, the plasma cleaning includes plasma cleaning the steel substrate in a vacuum chamber, wherein the vacuum degree is 3×10 -1 Below Pa, the cleaning time is 5 to 30 minutes, and the cleaning working gas is argon.

[0033] Optionally, the following steps are included:

[0034] Pretreatment: Pretreatment of the steel substrate, wherein the pretreatment includes alkaline cleaning, brushing, electrolytic cleaning, rinsing, or a combination thereof, and drying the surface of the steel substrate;

[0035] Preheating: Place the cleaned steel substrate in a vacuum chamber for preheating at a temperature of 150-300°C;

[0036] Plasma cleaning: The steel substrate is plasma cleaned in a vacuum chamber, where the vacuum degree is 1×10-1 Pa, the cleaning time is 10 min, and the cleaning working gas is argon;

[0037] Coating the first coating layer: using a magnetron sputtering process to deposit the first coating layer on the surface of the steel substrate, wherein the vacuum degree is 0.5-0.65 Pa, the process gas is argon, the gas flow rate is 350-450 sccm, the magnetron sputtering current is 6.5-7.5 A, the distance between the target and the steel plate is 50-100 mm, and the sputtering time is 10-30 minutes;

[0038] Coating the second coating: The second coating is deposited on the surface of the first coating by using an induction evaporation spray deposition process to form a composite coating consisting of the first coating and the second coating, wherein the vacuum degree is 1×10 -4 Pa, crucible temperature is greater than 700 ° C, injection speed is 200 ~ 500 m / min, steam temperature is 580 ~ 780 ° C, injection time is 10 ~ 30 minutes.

[0039] This method can achieve good coating and facilitate the preparation of composite coatings with excellent thickness and performance.

[0040] Optionally, the composite coating is coated on at least one side of the steel substrate.

[0041] According to another specific embodiment of the present invention, an embodiment of the present invention discloses a steel plate comprising a steel substrate coated with a composite coating, wherein the chemical composition of the steel substrate comprises, by mass percentage, 0.1% ≤ C ≤ 0.3%, 1.0% ≤ Mn ≤ 3.0%, 0.1% ≤ Si ≤ 2.0%, and Al ≤ 1.0%. In some embodiments, the C content is 0.13-0.25%. In some embodiments, the Al content is 0.01-0.05%. In some embodiments, the Si content is 1.3-2.0%. In some embodiments, the Mn content is 1.8-2.6%.

[0042] The steel substrate with such a composition can form an excellent bond with the composite coating, reduce the possibility of LME phenomenon in the final prepared steel plate due to spot welding, and realize the safe application of the steel plate.

[0043] Optionally, the steel substrate further comprises one or more of B ≤ 0.005%, Cr ≤ 1.5%, Mo ≤ 0.6%, Ni ≤ 1.2%, Ti ≤ 0.6%, Nb ≤ 0.6%, and V ≤ 0.004%, with the remainder being iron and unavoidable impurity elements. In some embodiments, the steel substrate further comprises 0.010-0.030% Ti.

[0044] In some embodiments, the element composition of the steel substrate is, by mass percentage, 0.1%≤C≤0.3%, 1.0%≤Mn≤3.0%, 0.1%≤Si≤2.0%, 0.01%≤Al≤1.0%, 0.010%≤Ti≤0.6%, P≤0.015%, S≤0.003%, and the balance is Fe and unavoidable impurities.

[0045] In some embodiments, the element composition of the steel substrate is, by mass percentage, 0.13%≤C≤0.25%, 1.8%≤Mn≤2.6%, 1.3%≤Si≤2.0%, 0.01%≤Al≤0.05%, 0.010%≤Ti≤0.030%, P≤0.015%, S≤0.003%, and the balance is Fe and unavoidable impurities.

[0046] Optionally, the tensile strength of the steel substrate is ≥780 MPa, and the elongation at break is ≥10%.

[0047] According to another specific embodiment of the present invention, an embodiment of the present invention discloses a spot welding joint, which is formed by welding multiple layers of metal plates by resistance spot welding, and is characterized in that at least one layer of the metal plate is a steel plate coated with a composite coating, and when no welding spatter occurs, there is no LME crack on the steel plate with a length exceeding 3% of the steel plate thickness; when welding spatter occurs, the number of LME cracks on the steel plate with a length greater than 6% of the steel plate thickness does not exceed 3.

[0048] Optionally, the steel plate has no LME cracks located at the metal plate bonding surface or surface LME cracks located outside the electrode indentation area.

[0049] In some embodiments, the present invention provides a steel plate formed by resistance spot welding two or more steel plates coated with the composite coating described herein according to any embodiment of the present invention, comprising two or more steel plates coated with the composite coating and a spot weld joint between the two or more steel plates. Preferably, in the absence of weld spatter, the steel plate has no LME cracks with a length exceeding 3% of the steel plate thickness; in the presence of weld spatter, the number of LME cracks with a length exceeding 6% of the steel plate thickness on the steel plate does not exceed 3; preferably, the steel plate has no LME cracks located at the metal plate bonding surface or surface LME cracks located outside the electrode indentation area.

[0050] Herein, exemplary spot welding parameters include: electrode pressure 3.0-5.0 kN, number of pulses 2-4, duration of each pulse 120-160 ms, cooling time between pulses 30-50 ms, total welding time 400-500 ms, and dwell time 150-250 ms. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] FIG1 is a schematic structural diagram of a steel plate coated with a composite coating according to an embodiment of the present invention;

[0052] FIG2 is a schematic diagram showing an electrode indentation area and the outside of the electrode indentation area according to an embodiment of the present invention. DETAILED DESCRIPTION

[0053] The following is an explanation of the embodiments of the present invention by specific specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Although the description of the present invention will be introduced in conjunction with the preferred embodiment, this does not mean that the features of this invention are limited to this embodiment. On the contrary, the purpose of introducing the invention in conjunction with the embodiment is to cover other options or modifications that may be extended based on the claims of the present invention. In order to provide a deep understanding of the present invention, the following description will contain many specific details. The present invention can also be implemented without using these details. In addition, in order to avoid confusion or blurring the focus of the present invention, some specific details will be omitted in the description. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0054] It should be noted that in this specification, similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0055] The terms “first”, “second”, etc. are only used for distinguishing descriptions and should not be understood as indicating or implying relative importance.

[0056] In the description of this embodiment, it should be noted that, unless otherwise specified or limited, the terms "disposed," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this embodiment based on specific circumstances.

[0057] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0058] As shown in Figure 1, the composite coating includes a first coating 3 and a second coating 2 sequentially disposed along the coating thickness direction A. The first coating 3 is in direct contact with the steel substrate 4 and includes Mo and / or Cr. The second coating 2 is a zinc-based coating. The zinc-based coating is a coating whose main element is Zn.

[0059] In fact, the main manifestation of the LME problem in spot welding of high-strength galvanized steel sheets is that the molten Zn penetrates along the grain boundaries of the base material and produces cracks under the action of tensile stress. LME cracks will cause brittle fracture of the weld joint during the tensile process, and the strength of the weld joint will be reduced, especially for galvanized high-strength steel weld joints. The inventors noticed that the mixing enthalpies of Mo and Cr with Zn are all positive numbers, and Mo, Cr and Zn atoms will repel each other. Therefore, another coating layer including Mo and / or Cr is provided between the zinc-based coating and the steel substrate, so that the first coating layer is like a shield, which prevents the liquid Zn generated in the second coating layer from contacting the steel substrate during the spot welding process and penetrating along the grain boundaries of the substrate, thereby suppressing the LME of spot welding of high-strength galvanized steel sheets. At the same time, Mo and Cr are relatively easy to obtain elements and are suitable for large-scale application in industrial production. In addition, the bonding between the first coating layer and the second coating layer, and between the first coating layer and the steel substrate is stable.

[0060] The first coating layer may use pure Mo or pure Cr. Preferably, the first coating layer is a molybdenum-chromium alloy, and the first coating layer includes, by weight percentage: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, with the remainder being unavoidable impurity elements. In some embodiments, the first coating layer includes: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, with the remainder being Fe and unavoidable impurity elements. In some embodiments, the Cr content in the first coating layer is ≥ 5%. In some embodiments, the Cr content in the first coating layer is ≥ 10%. In some embodiments, the Cr content in the first coating layer is ≥ 15%. In some embodiments, the Cr content in the first coating layer is 2 to 100%, such as 5 to 100%, 10 to 100%, 15 to 100%, 5 to 25%, 10 to 25%, 15 to 25% or 15 to 22%. In some embodiments, the content of Mo in the first coating is 2-100%, such as 2-50%, 2-8%, 2-6%, or 2-5%. In some embodiments, the first coating contains: Mo: 2-5%, Cr: 15-20%, Ni: 8-13%, C: 0.05-0.12%, Si: 0.5-1.2%, P: ≤0.020%, S: ≤0.005%, and the balance is Fe and unavoidable impurities. In some embodiments, the first coating contains: 13-17% Cr and 2-5% Mo, and the balance is Fe and unavoidable impurities.

[0061] Compared to using pure Mo or pure Cr, using the above composition combination can reduce production costs and can directly use stainless steel for coating, which is convenient and effective. Therefore, in some embodiments, the composition of the first coating is the composition of 316 stainless steel.

[0062] Furthermore, the first coating is a molybdenum-chromium alloy and includes, by weight percentage, the following: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, 0 ≤ B + Be + K + Rb ≤ 10%, with the remainder being unavoidable impurities. In addition to Mo and Cr, the mixing enthalpies of B, Be, K, and Rb with Zn are all positive, mutually repelling Zn, further enhancing the first coating's "shield" effect and significantly suppressing the LME phenomenon. Furthermore, B, Be, K, and Rb are readily available. The first coating is a molybdenum-chromium alloy and includes the following: Cr ≥ 2%, Mo ≥ 2% and Cr + Mo ≥ 15%, 0 ≤ B + Be + K + Rb ≤ 10%, with the remainder being Fe and unavoidable impurities. In some embodiments, the B content is 0.01 to 0.10%, such as 0.03 to 0.07%. In some embodiments, the Rb content is 0.5 to 1.5%, such as 0.8 to 1.2%. In some embodiments, the Be content is 0.5-1.5%, such as 0.8-1.2%. In some embodiments, the Rb content is 0.5-1.5%, such as 0.8-1.2%. In some embodiments, the B+Be+K+Rb content is 0≤B+Be+K+Rb≤5%, preferably 0≤B+Be+K+Rb≤2.5%. In some embodiments, the first coating contains: 13-17% Cr, 2-5% Mo, 0.01-0.10% B and 0.5-1.5% Rb, with the balance being Fe and unavoidable impurities. In some embodiments, the first coating contains: 13-17% Cr, 2-5% Mo, 0.5-1.5% Be and 0.5-1.5% Rb, with the balance being Fe and unavoidable impurities.

[0063] In a specific embodiment of the present invention, the second coating layer is pure Zn.

[0064] Preferably, the element composition of the second coating is: Zn ≥ 90%, 0.5 ≤ Al ≤ 3%, 0.1 ≤ Mg ≤ 5%, and the remainder is Cr, Mo, Fe, and unavoidable impurity elements. With this composition combination, the second coating is more effectively bonded to the first coating.

[0065] In a specific embodiment of the present invention, at room temperature, specifically 18-26°C, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<130μΩ.cm, the resistivity ρ2 of the second coating satisfies 5μΩ.cm<ρ2<10μΩ.cm, and 1<ρ1 / ρ2<25. The resistivity measurement method is as follows: According to the resistivity formula Using a low-resistance meter, the resistance R of a sample with a length of l is measured, and the resistivity is calculated based on the formula, where W is the sample width and t is the thickness. The occurrence of LME in spot welds requires that the joint be subjected to tensile stress during the welding process and be within the appropriate temperature range; both are essential. The heat input during spot welding comes from resistive heat, and the actual heat input determines when the joint is within the aforementioned suitable temperature range. The resistivity of the coating is a key factor influencing resistive heat. The inventors discovered that when the resistivity of the first coating is 5μΩ.cm < ρ1 < 130μΩ.cm, the resistivity of the second coating is 5μΩ.cm < ρ2 < 10μΩ.cm, and 1 < ρ1 / ρ2 < 25, the spot weld joint can be reduced from being subjected to tensile stress and being within the appropriate temperature range, thereby better avoiding the occurrence of LME in spot welds. By designing the components of the first and second coatings, the resistivity can be achieved within the above range. Preferably, 13 < ρ1 / ρ2 < 25. By combining the first and second coatings with a certain composition, the LME problem is improved. More preferably, 19<ρ1 / ρ2<25. Within this range, the LME phenomenon can be more effectively suppressed regardless of whether there is splashing or not.

[0066] Preferably, the thickness of the first coating is 0.5-5 μm. If the first coating is too thin, the repelling force against Zn is insufficient and cannot provide good protection; if the first coating is too thick, the cost is too high and the adhesion of the second coating is also affected.

[0067] Preferably, the thickness of the second coating layer is 2.5-15 μm. If the second coating layer is too thin, the corrosion resistance is poor; if the second coating layer is too thick, the cost is increased.

[0068] When the thickness of the first coating layer and the second coating layer is within the above-mentioned range, good bonding between the first coating layer and the second coating layer can be ensured, and their respective functions can be fully exerted.

[0069] In another specific embodiment of the present invention, the method of coating a composite coating on a steel substrate may include: first coating a first coating on the steel substrate by physical vapor deposition or electroplating, and then coating a second coating on the first coating by physical vapor deposition, electroplating or hot-dip coating, so that a composite coating consisting of the first coating and the second coating can be formed. Optionally, coating the first coating includes: using a magnetron sputtering process to deposit the first coating on the surface of the steel substrate, wherein the vacuum degree is 0.5 to 0.65 Pa, the process gas is argon, the gas flow rate is 350 to 450 sccm, the magnetron sputtering current is 6.5 to 7.5 A, the distance between the target and the steel plate is 50 to 100 mm, and the sputtering time is 10 to 30 minutes. Optionally, coating the second coating includes: using an induction evaporation spray deposition process to deposit the second coating on the surface of the first coating, to form a composite coating consisting of the first coating and the second coating, wherein the vacuum degree is 3×10 -4 Pa or less, the crucible temperature is greater than 700 ° C, the injection speed is 200 ~ 500 m / min, the vapor temperature is 580 ~ 780 ° C, and the injection time is 10 ~ 30 minutes.

[0070] Optionally, before applying the first coating, the process further includes pre-treating the surface of the steel substrate, preheating the pre-treated steel substrate to 150-300°C, and plasma cleaning the pre-heated steel substrate. Optionally, the pre-treatment includes performing alkaline cleaning, brushing, electrolytic cleaning, rinsing, or a combination thereof on the surface of the steel substrate, and drying. Optionally, the plasma cleaning includes plasma cleaning the steel substrate in a vacuum chamber, wherein the vacuum degree is 3×10 -1 Below Pa, the cleaning time is 5 to 30 minutes, and the cleaning working gas is argon.

[0071] Specifically, the specific method of coating the first coating layer and the second coating layer is:

[0072] Pretreatment: Pretreatment of the steel substrate, wherein the pretreatment includes alkaline cleaning, brushing, electrolytic cleaning, rinsing, or a combination thereof, and drying the surface of the steel substrate to remove grease and solid particles on the surface of the steel substrate to facilitate subsequent coating;

[0073] Preheating: Place the cleaned steel substrate in a vacuum chamber for preheating at a temperature of 150-300°C. Within this preheating temperature range, the adhesion of the coating can be further improved during the subsequent coating process.

[0074] Plasma cleaning: The steel substrate is plasma cleaned in a vacuum chamber, where the vacuum degree is 1×10 -1 Pa, the cleaning time is 10 min, and the cleaning working gas is argon;

[0075] Coating the first coating layer: using a magnetron sputtering process to deposit the first coating layer on the surface of the steel substrate, wherein the vacuum degree is 0.5-0.65 Pa, the process gas is argon, the gas flow rate is 350-450 sccm, the magnetron sputtering current is 6.5-7.5 A, the distance between the target and the steel plate is 50-100 mm, and the sputtering time is 10-30 minutes;

[0076] Coating the second coating: The second coating is deposited on the surface of the first coating by using an induction evaporation spray deposition process to form a composite coating consisting of the first coating and the second coating, wherein the vacuum degree is 1×10 -4 Pa, crucible temperature is greater than 700 ° C, injection speed is 200 ~ 500 m / min, steam temperature is 580 ~ 780 ° C, injection time is 10 ~ 30 minutes.

[0077] The coating thickness can be controlled more accurately by adjusting the deposition time, crucible temperature, flow rate, etc.

[0078] Through the above method and parameter design, the first coating layer can have a thickness of 0.5-5μm, and the resistivity ρ1 satisfies 5μΩ.cm<ρ1<130μΩ.cm, and the second coating layer can have a thickness of 2.5-15μm, and the resistivity ρ2 satisfies 5μΩ.cm<ρ2<10μΩ.cm, and 1<ρ1 / ρ2<25.

[0079] Furthermore, the composite coating is coated on at least one side of the steel substrate. As shown in FIG1 , the composite coating is coated on both sides of the steel substrate 4 to form a double-sided coated steel plate. In specific production, one or both sides can be coated according to actual product needs.

[0080] Another embodiment of the present invention discloses a steel plate comprising a steel substrate coated with a composite coating. The chemical composition of the steel substrate coated with the composite coating, measured by mass percentage, includes: 0.1% ≤ C ≤ 0.3%, 1.0% ≤ Mn ≤ 3.0%, 0.1% ≤ Si ≤ 2.0%, and Al ≤ 1.0%. Using a substrate with this composition as a target for coating can effectively prevent the formation and propagation of LME cracks.

[0081] Preferably, the steel substrate further comprises one or more components of B≤0.005%, Cr≤1.5%, Mo≤0.6%, Ni≤1.2%, Ti≤0.6%, Nb≤0.6%, and V≤0.004%, with the remainder being iron and unavoidable impurity elements, resulting in better mechanical properties, corrosion resistance, etc.

[0082] Specifically, the steel substrate has a tensile strength of ≥780 MPa and an elongation at break of ≥10%. More specifically, 780 MPa≤tensile strength≤1180 MPa, and 10%≤elongation at break≤25%.

[0083] In a specific embodiment of the present invention, a spot welding joint is disclosed, which is formed by resistance spot welding of multiple metal plates, and at least one layer of the metal plate is a steel plate coated with a composite coating. When no welding spatter occurs, there are no LME cracks on the steel plate with a length exceeding 3% of the thickness of the steel plate; when welding spatter occurs, the number of LME cracks on the steel plate with a length greater than 6% of the thickness of the steel plate does not exceed 3. As shown in Figure 2, when spot welding is performed, the steel plate includes an electrode indentation area a and an outer side b of the electrode indentation area. Furthermore, there are no LME cracks on the steel plate located at the joint surface of the metal plates or surface LME cracks located at the outer side b of the electrode indentation area. In other words, the steel plate coated with a composite coating effectively alleviates the LME problem of spot welding joints.

[0084] Furthermore, except for the steel plate coated with the composite coating, the other metal plates are steel plates or aluminum plates, or the multi-layer metal plates are all steel plates coated with the composite coating.

[0085] The above technical solutions and technical effects of the invention will be further explained and illustrated below in conjunction with more specific embodiments.

[0086] Example 1-2

[0087] Examples 1 and 2 are steel sheets with composite coatings applied to both surfaces of a high-strength steel substrate. The high-strength steel substrates used in Examples 1 and 2 are identical, 1.6 mm thick, and their compositions are shown in Table 1. The annealing process parameters used in the manufacturing of the high-strength steel substrates are shown in Table 2 below, and the mechanical properties of the substrates after annealing (tested according to ASTM E8 / E8M-08) are shown in Table 3.

[0088] Table 1: Composition of high-strength steel substrates in Examples 1 and 2

[0089] (wt%, the balance is Fe and other inevitable impurities except P and S)

[0090] Table 2: Annealing process of high-strength steel substrate in Example 1 and Example 2

[0091] Table 3: Mechanical properties of substrates in Example 1 and Example 2

[0092] First, the first coating layer is applied on the steel substrate. The target material of the first coating layer is 316 stainless steel, and its composition is shown in Table 4 below:

[0093] Table 4: 316 stainless steel composition

[0094] (wt%, the balance is Fe and other inevitable impurities except P and S)

[0095] The annealed high-strength steel substrate is pre-plated with 316 stainless steel using a PVD vacuum coating method. The specific steps of the PVD vacuum coating method are as follows:

[0096] Pre-treating the substrate, wherein the pre-treatment includes performing one or a combination of alkaline cleaning, brushing, electrolytic cleaning, rinsing, and drying on the surface of the substrate to remove grease and solid particles on the surface of the substrate;

[0097] The cleaned substrate is placed in a vacuum chamber for preheating at a temperature of 150 to 300°C;

[0098] After preheating, the steel plate was plasma cleaned in a vacuum chamber with a vacuum degree of 1×10 -1 Pa cleaning time is 10min; the cleaning working gas is argon;

[0099] The first coating layer is deposited on the substrate surface using a magnetron sputtering process. The specific process parameters are as follows:

[0100] Vacuum degree: 0.65Pa;

[0101] Process gas: argon, flow rate: 400 sccm;

[0102] Magnetron sputtering current: 7.0A;

[0103] Distance between target and steel plate: 50mm;

[0104] Sputtering time: 10 minutes;

[0105] Target material: 316 stainless steel.

[0106] The thickness of the first coating layer is controlled by adjusting the sputtering current and the sputtering time. The thickness of the first coating layer is 3 μm.

[0107] Subsequently, the second coating layer is deposited on the surface of the first coating layer using an induction evaporation spray deposition process. The second coating layer is a pure Zn coating layer. The specific process parameters are as follows:

[0108] Vacuum degree: 1×10 -4 Pa;

[0109] Crucible temperature: 700℃;

[0110] Injection speed: 200m / min;

[0111] Steam temperature: 680℃;

[0112] Spraying time: 10 minutes.

[0113] The thickness of the second coating layer was controlled by adjusting the crucible temperature, spraying speed, spraying time, etc. The thickness of the second coating layer was 10 μm.

[0114] Through the above process, the steel plates coated with the composite coating according to Example 1 and Example 2 were formed.

[0115] At room temperature, use a low resistance meter to measure the resistance R of a sample with a length of 1, and then use the resistivity formula The resistivity was calculated, where W is the sample width and t is the thickness. The resistivity of the first coating was 80 μΩ·cm, and the resistivity of the second coating was 5.2 μΩ·cm.

[0116] Subsequently, the two steel plates of Example 1 were spot welded using the spot welding process parameters shown in Table 5, and the two steel plates of Example 2 were spot welded using the spot welding process parameters shown in Table 5 to obtain spot welded joints. The obtained spot welded joints were removed from the surface coating of the joints with dilute hydrochloric acid, and the distribution and direction of the spot welding cracks were observed under a microscope. The cross-section through the center of the weld nugget and the most spot welding cracks that could be cut were selected as the metallographic section of the joint. Wire cutting was used for sampling, and the cross-section contained all the welding feature areas of the spot welding joint. The surface of the intercepted sample was rinsed to prevent foreign matter from interfering with the test results. The rinsed sample was dried; the dried sample was mounted, ground and polished, and observed under a metallographic microscope. The LME crack conditions in the joint were recorded in Table 6.

[0117] Table 5: Spot welding parameters

[0118] Table 6: LME cracks in spot welded joints of Example 1 and Example 2

[0119] Comparative Example 1-2

[0120] Comparative Examples 1 and 2 are steel plates without a composite coating on the surface of the high-strength steel substrate.

[0121] The chemical composition of the steel substrates used in Comparative Examples 1 and 2 was the same as listed in Table 1, and the mechanical properties were as shown in Table 3 for Examples 1 and 2, respectively. The plate thickness was 1.6 mm. The annealing process parameters during the manufacture of the high-strength steel substrate in Comparative Example 1 were the same as those for Example 1 listed in Table 2, and the annealing process parameters during the manufacture of the high-strength steel substrate in Comparative Example 2 were the same as those for Example 2 listed in Table 2.

[0122] For the annealed high-strength steel substrate, a pure Zn coating is deposited on the substrate surface using an induction evaporation spray deposition process. The specific process parameters are as follows:

[0123] Vacuum degree: 1×10 -4 Pa;

[0124] Crucible temperature: 700℃;

[0125] Injection speed: 200m / min;

[0126] Steam temperature: 680℃;

[0127] Spraying time: 10 minutes.

[0128] The thickness of the second coating layer was controlled by adjusting the crucible temperature, spraying speed, spraying time, etc. The thickness of the second coating layer was 10 μm.

[0129] Through the above process, steel sheets coated with only the Zn coating layer of Comparative Example 1 and Comparative Example 2 were formed.

[0130] At room temperature, use a low resistance meter to measure the resistance R of a sample with a length of l, and then use the resistivity formula The resistivity was calculated as follows: where W is the width of the sample and t is the thickness. The resistivity of the Zn coating was measured to be 5.2 μΩ·cm.

[0131] Subsequently, the two steel plates of Comparative Example 1 were spot welded using the spot welding process parameters shown in Table 5, and the two steel plates of Comparative Example 2 were spot welded using the spot welding process parameters shown in Table 5 to obtain spot welded joints. The obtained spot welded joints were removed from the surface coating of the joints 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 where the most spot welding cracks could be cut was selected as the metallographic section of the joint. Wire cutting was used for sampling, and the cross-section contained all welding feature areas of the spot welding joint. The surface of the intercepted sample was rinsed to prevent foreign matter from interfering with the test results. The rinsed sample was dried; the dried sample was mounted, ground and polished, and observed under a metallographic microscope. The LME crack conditions in the joint were recorded in Table 7.

[0132] Table 7: LME cracks in spot welds of Comparative Example 1 and Comparative Example 2

[0133] Comparative Examples 1 and 2 share the same substrates as Examples 1 and 2, but differ in coating. Examples 1 and 2 utilize the composite coating technology described in this invention, while Comparative Examples 1 and 2 utilize conventional hot-dip galvanizing. Under identical welding conditions, the results in Tables 6 and 7 demonstrate that the present invention effectively suppresses LME cracks in spot welds, both in terms of quantity and length.

[0134] Examples 3-17 and Comparative Examples ac

[0135] Examples 3-17 and Comparative Examples ac are steel plates in which a composite coating is coated on one surface of a high-strength steel substrate.

[0136] On an annealed steel plate having the composition shown in Table 1, the basic mechanical properties shown in Example 1 in Table 3, and a thickness of 1.6 mm, a first coating layer was applied by physical vapor deposition and electroplating, and a second coating layer was applied by physical vapor deposition, electroplating, and hot-dip coating.

[0137] At room temperature, use a low resistance meter to measure the resistance R of a sample with a length of 1, and then use the resistivity formula The resistivity of each coating is calculated, where W is the width of the sample and t is the thickness. The specific coating conditions are shown in Table 8:

[0138] Table 8: Specific details of coatings in Examples 3-17 and Comparative Examples ac

[0139] For Examples 3-17 and Comparative Examples ac, two steel plates were taken from each Example and spot welded using the spot welding process parameters shown in Table 5. The surface coating of the obtained spot welded sample joints was removed using dilute hydrochloric acid, and the distribution and direction of the spot weld cracks were observed under a microscope. The cross-section passing through the center of the weld nugget and capable of cutting the most spot weld cracks was selected as the metallographic section of the joint. Wire cutting was used for sampling, and the cross-section included all welding feature areas of the spot welded joint. The surface of the cut sample was rinsed to prevent foreign matter from interfering with the test results. The rinsed sample was dried. The dried sample was mounted, ground and polished, and observed under a metallographic microscope. The LME crack conditions in the joint were recorded in Table 9.

[0140] Table 9: LME cracks in joints of Examples 3-17 and Comparative Example ac

[0141] In Examples 3-17, when no weld spatter occurred, no LME cracks exceeding 3% of the thickness of the high-strength steel plate coated with the composite coating of the present invention occurred. When weld spatter occurred, no more than three LME cracks exceeding 6% of the thickness of the high-strength steel plate coated with the composite coating of the present invention occurred. Furthermore, no LME cracks were located at the metal plate bonding surface or on the surface of the high-strength steel plate coated with the composite coating of the present invention. In Examples 15 and 16, based on Example 8, the elements B, Rb, Be, and K were added to the first coating layer, respectively, while the second coating layer remained unchanged. The results in Table 9 show that the LME crack lengths in Examples 15 and 16 were significantly reduced compared to those in Example 8. Especially in the presence of spatter, the addition of B, Rb, and Be to the first coating layer effectively suppresses the LME phenomenon compared to other types of coatings.

[0142] Compared to Comparative Examples 1-2, which lack a first coating, Comparative Examples ac exhibited reduced maximum crack length in the presence of spatter and a lower ratio of maximum crack length to steel plate thickness, mitigating the impact of spatter on steel plate performance. Furthermore, Comparative Example a, compared to Example 5, included relatively low levels of Cr and Mo in the first coating, with the Cr+Mo content being less than 15%, while the second coating remained unchanged. Comparative Examples b and c, based on Example 15, added over 10% B, Rb, and Be to the first coating, while leaving the second coating unchanged. Consequently, Examples 5 and 15 exhibited greater LME suppression than Comparative Examples ac. Compared to Examples 5 and 15, Comparative Examples ac exhibited greater LME cracking severity. In the absence of weld spatter, all Comparative Examples ac exhibited LME cracks exceeding 3% of the high-strength steel plate thickness. In the presence of weld spatter, all exhibited at least three LME cracks exceeding 6% of the high-strength steel plate thickness. These cracks were located at the metal plate bonding surface or on the surface of the electrode indentation region (b).

[0143] Although the present invention has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is provided as a further detailed description of the present invention in conjunction with specific embodiments thereof, and that the specific implementation of the present invention is not limited to these descriptions. Those skilled in the art may make various changes in form and details, including simple deductions or substitutions, without departing from the spirit and scope of the present invention.

Claims

1. A composite coating for steel, characterized in that: The composite coating includes a first coating and a second coating sequentially arranged along the coating thickness direction. The first coating is in direct contact with the steel substrate. The first coating includes Mo and / or Cr. The second coating is a zinc-based coating.

2. The composite coating according to claim 1, wherein: In terms of weight percentage, the first coating is a molybdenum-chromium alloy, and the first coating includes: Cr≥2%, Mo≥2% and Cr+Mo≥15%, and the rest are unavoidable impurity elements; or in terms of weight percentage, the first coating is a molybdenum-chromium alloy, and the first coating includes: Cr≥2%, Mo≥2% and Cr+Mo≥15%, 0≤B+Be+K+Rb≤10%, and the rest are unavoidable impurity elements.

3. The composite coating according to claim 2, wherein: In terms of weight percentage, the first coating is a molybdenum-chromium alloy, and the first coating includes: Cr≥2%, Mo≥2% and Cr+Mo≥15%, and the rest is Fe and unavoidable impurity elements; or in terms of weight percentage, the first coating is a molybdenum-chromium alloy, and the first coating includes: Cr≥2%, Mo≥2% and Cr+Mo≥15%, 0≤B+Be+K+Rb≤10%, and the rest is Fe and unavoidable impurity elements.

4. The composite coating according to claim 2, wherein: In the first plating layer: The Cr content is ≥10% or ≥15%, or the Cr content in the first coating is 2-100%, such as 5-100%, 10-100%, 15-100%, 5-25%, 10-25%, 15-25% or 15-22%; The content of Mo is 2-100%, 2-50%, 2-8%, 2-6% or 2-5%; The content of B is 0-0.10%, 0.01-0.10% or 0.03-0.07%; The Rb content is 0-1.5%, 0.5-1.5% or 0.8-1.2%; The Be content is 0-1.5%, 0.5-1.5% or 0.8-1.2%; The Rb content is 0-1.5%, 0.5-1.5% or 0.8-1.2%; and / or The content of B+Be+K+Rb is 0≤B+Be+K+Rb≤5%, or 0≤B+Be+K+Rb≤2.5%.

5. The composite coating according to claim 1, wherein: Calculated by mass percentage, the first coating contains: Mo: 2-5%, Cr: 15-20%, Ni: 8-13%, C: 0.05-0.12%, Si: 0.5-1.2%, P: ≤0.020%, S: ≤0.005%, and the balance is Fe and unavoidable impurities; or The first coating layer is composed of Mo or Cr; or The composition of the first coating is the composition of 316 stainless steel; or The first coating contains: 13-17% Cr and 2-5% Mo, with the balance being Fe and unavoidable impurities; or The first coating contains: 13-17% Cr, 2-5% Mo, 0.01-0.10% B and 0.5-1.5% Rb, with the balance being Fe and unavoidable impurities; or The first coating layer contains: 13-17% Cr, 2-5% Mo, 0.5-1.5% Be and 0.5-1.5% Rb, with the remainder being Fe and unavoidable impurities.

6. The composite coating according to claim 1, wherein: The second coating is pure Zn, or the element composition of the second coating is: Zn≥90%, 0.5≤Al≤3%, 0.1≤Mg≤5%, and the rest are Cr, Mo, Fe and unavoidable impurity elements.

7. The composite coating according to claim 1, wherein: At room temperature, the resistivity ρ1 of the first coating satisfies 5μΩ.cm<ρ1<130μΩ.cm, the resistivity ρ2 of the second coating satisfies 5μΩ.cm<ρ2<10μΩ.cm, and 1<ρ1 / ρ2<25.

8. The composite coating according to claim 1, wherein: The thickness of the first coating layer is 0.5-5 μm, and / or the thickness of the second coating layer is 2.5-15 μm.

9. A steel plate comprising a steel substrate and the composite coating according to any one of claims 1 to 8 on either or both surfaces of the steel substrate.

10. The steel plate according to claim 9, wherein: The chemical composition of the steel substrate includes, by mass percentage, 0.1%≤C≤0.3%, 1.0%≤Mn≤3.0%, 0.1%≤Si≤2.0%, and Al≤1.0%; optionally, the steel substrate further includes, by mass percentage, one or more of B≤0.005%, Cr≤1.5%, Mo≤0.6%, Ni≤1.2%, Ti≤0.6%, Nb≤0.6%, and V≤0.004%, with the remainder being iron and unavoidable impurity elements; and / or The tensile strength of the steel substrate is ≥780 MPa, and the elongation at break is ≥10%.

11. A method for coating a composite coating according to any one of claims 1 to 8 on a steel substrate, characterized in that: The steps include: Applying a first coating: applying a first coating on the steel substrate by physical vapor deposition or electroplating; preferably, applying the first coating includes: depositing the first coating on the surface of the steel substrate by a magnetron sputtering process, wherein the vacuum degree is 0.5-0.65 Pa, the process gas is argon, the gas flow rate is 350-450 sccm, the magnetron sputtering current is 6.5-7.5 A, the distance between the target and the steel plate is 50-100 mm, and the sputtering time is 10-30 minutes; Coating the second coating layer: coating the second coating layer on the first coating layer by physical vapor deposition, electroplating or hot plating to form a composite coating layer consisting of the first coating layer and the second coating layer; preferably, coating the second coating layer includes: depositing the second coating layer on the surface of the first coating layer by an induction evaporation spray deposition process to form a composite coating layer consisting of the first coating layer and the second coating layer, wherein the vacuum degree is 3×10 -4 Pa or less, the crucible temperature is greater than 700 ° C, the injection speed is 200-500 m / min, the vapor temperature is 580-780 ° C, and the injection time is 10-30 minutes; Preferably, the composite coating is coated on at least one side of the steel substrate.

12. The method according to claim 11, wherein The steps include: Pretreatment: Pretreatment of the steel substrate, wherein the pretreatment includes alkaline cleaning, brushing, electrolytic cleaning, rinsing, or a combination thereof, and drying the surface of the steel substrate; Preheating: Place the cleaned steel substrate in a vacuum chamber for preheating at a temperature of 150-300°C; Plasma cleaning: The steel substrate is plasma cleaned in a vacuum chamber, where the vacuum degree is 1×10 -1 Pa, the cleaning time is 10 min, and the cleaning working gas is argon; Coating the first coating layer: using a magnetron sputtering process to deposit the first coating layer on the surface of the steel substrate, wherein the vacuum degree is 0.5-0.65 Pa, the process gas is argon, the gas flow rate is 350-450 sccm, the magnetron sputtering current is 6.5-7.5 A, the distance between the target and the steel plate is 50-100 mm, and the sputtering time is 10-30 minutes; Coating the second coating: depositing the second coating on the surface of the first coating by an induction evaporation spray deposition process to form a composite coating consisting of the first coating and the second coating, wherein the vacuum degree is 1×10-4Pa, the crucible temperature is greater than 700°C, the spray speed is 200-500m / min, the vapor temperature is 580-780°C, and the spray time is 10-30 minutes.

13. A spot welded joint formed by resistance spot welding of multiple metal plates, characterized in that: At least one metal plate is a steel plate coated with the composite coating according to any one of claims 1 to 8. When no welding spatter occurs, there is no LME crack on the steel plate with a length exceeding 3% of the thickness of the steel plate; when welding spatter occurs, the number of LME cracks on the steel plate with a length greater than 6% of the thickness of the steel plate does not exceed 3.

14. The spot welding joint according to claim 13, wherein: The steel plate has no LME cracks located at the metal plate bonding surface or surface LME cracks located outside the electrode indentation area.

15. A steel plate formed by resistance spot welding of the steel plates according to claim 9, comprising spot weld joints between two or more layers of the steel plates according to claim 9; preferably, when no welding spatter occurs, there are no LME cracks on the steel plate with a length exceeding 3% of the thickness of the steel plate, and when welding spatter occurs, the number of LME cracks on the steel plate with a length greater than 6% of the thickness of the steel plate does not exceed 3; preferably, there are no LME cracks on the steel plate located at the bonding surface of the metal plates or surface LME cracks located outside the electrode indentation area.

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