Method for inhibiting LME cracks of galvanized high-strength steel

Abstract

The invention relates to the technical field of high-strength steel welding processes, in particular to a method for inhibiting LME cracks of galvanized high-strength steel, which comprises the following steps of: 1, galvanizing a high-strength steel plate, and cleaning a to-be-welded surface of the galvanized high-strength steel; and 2, the two galvanized high-strength steel plates are in lap joint and then placed between spot welding electrodes, and spot welding process parameters are set for welding. Components of the plating solution are optimized, a layer of compound is generated on the surface of the high-strength steel by means of the Zn element, the Al element, the Ni element and the Cu element, so that liquid metal is prevented from diffusing into the high-strength steel, spot welding LME cracks of the galvanized high-strength steel are effectively improved in combination with a spot welding process, and an obtained spot-welded joint is excellent in mechanical property and good in corrosion resistance. And the method has important significance on popularization and application of the galvanized steel strong steel.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel welding technology, and in particular to a method for inhibiting LME cracks in galvanized high-strength steel. Background Art

[0002] With the continuous enhancement of environmental awareness in various countries, automobile lightweight technology has been rapidly developed, resulting in an increasing amount of advanced high-strength steel in automobile manufacturing, especially galvanized high-strength steel, a product with excellent corrosion resistance, which is favored by automobile manufacturers. However, the welding technology of galvanized high-strength steel restricts its development. This is because during the hot working process of galvanized high-strength steel, liquid zinc inevitably enters the high-strength steel matrix, resulting in liquid metal embrittlement (LME), which has a negative effect on the safety performance of parts. LME is a phenomenon in which the liquid metal infiltrates along the grain boundary of the parent material after the parent material contacts with the liquid metal under the action of external stress or internal stress, forming cracks and causing plasticity reduction. The necessary conditions for its occurrence include the following parts: LME-sensitive parent material, liquid metal, stress, and temperature. High-strength steel has a high content of alloy elements and large structural stress, and LME cracks are very likely to occur during welding.

[0003] CN105269137A "Medium-frequency spot welding of dissimilar materials of aluminum alloy and galvanized high-strength steel" optimizes the process parameters of resistance spot welding to control the thickness of intermetallic compounds at the aluminum alloy / galvanized high-strength steel interface to below 5.0μm. This method is suitable for connecting steel and aluminum dissimilar materials, and the shear force of the spot welding joint is only 5.4kN.

[0004] CN114686651A "Zinc-coated steel with reduced liquid metal embrittlement (LME) sensitivity" achieves the effect of reducing the LME sensitivity of spot welds by annealing the steel substrate in an oxygen-containing atmosphere to form an oxide-containing layer on the surface of the steel substrate. The production cost of the steel in this method is relatively high.

[0005] CN116657033A "High-strength steel composite galvanized sheet resistant to spot welding LME cracks and its preparation method" compositely rolls a low-carbon steel composite layer on the surface of a high-strength steel body to prevent liquid metal from penetrating into the parent material along the grain boundary during spot welding, thereby reducing the spot welding LME sensitivity of the high-strength steel composite galvanized sheet and alleviating the spot welding LME crack problem. This method has high requirements for steel production equipment and is costly. Summary of the invention

[0006] In order to overcome the defects of the prior art, the present invention provides a method for inhibiting LME cracks in galvanized high-strength steel.

[0007] High-strength steel needs to add a large amount of alloy elements to improve its strength. In order to improve its corrosion resistance, it is necessary to galvanize the surface of high-strength steel. Galvanized high-strength steel is subjected to high temperature and pressure during spot welding, and LME cracks will appear in the spot welding joints. The present invention optimizes the zinc liquid composition and combines the spot welding process to achieve the purpose of inhibiting LME cracks, which is of great significance to the promotion and application of galvanized steel.

[0008] In order to achieve the above object, the technical solution of the present invention is as follows:

[0009] The present invention provides a method for inhibiting LME cracks of galvanized high-strength steel, comprising the following steps:

[0010] Step 1: galvanize the high-strength steel plate and clean the galvanized high-strength steel surface to be welded for later use;

[0011] The plating solution comprises the following components by mass fraction: 10%-15% Al, 2%-5% Si, 5%-10% Ni, 2%-5% Cu, and the remainder is Zn and other inevitable impurities;

[0012] Step 2: overlap two galvanized high-strength steel plates, then place them between spot welding electrodes, set spot welding process parameters for welding;

[0013] Among them, the spot welding process parameters are: the welding pressure of the first stage spot welding is 3.0-4.5kN, the slow rise current is 3.0-3.5kA, the welding time is 60-120ms, the welding current is 4.0-5.0kA, and the cooling time is 100-200ms; the welding pressure of the second stage spot welding is 4.5-6.0kN, the slow rise current is 5.0-7.0kA, the welding time is 160-260ms, the welding current is 7.5-9.5kA, and the cooling time is 0-80ms.

[0014] In the above technical solution, further, in step 1, the galvanizing temperature is 470-520°C, and the coating thickness is 60-90g / m 3 .

[0015] In the above technical solution, further, in step 1, acetone or alcohol is used for cleaning.

[0016] In the above technical solution, further, in step 2, the spot welding electrode is a chromium-zirconium-copper conical electrode with an end face diameter of 6.0 mm.

[0017] In the above technical solution, further, the chemical composition of the high-strength steel is as follows in weight percentage: 0.115%-0.135% C, 0.3%-0.4% Si, 2.2%-2.4% Mn, 0.08%-0.12% Mo, 0.57%-0.63% Cr, 0.025%-0.035% Ti, 0.020%-0.030% Nb, 0.004%-0.020% P+S, and the balance is Fe and other inevitable impurities.

[0018] In the above technical solution, further, the yield strength of the high-strength steel is ≥980MPa, the tensile strength is ≥1180MPa, and the elongation after fracture is ≥10%.

[0019] The present invention adopts two pulses for spot welding. In the first spot welding process, a compound layer composed of two or more elements of Al, Ni, Cu, Zn, Fe and O is generated on the surface of galvanized high-strength steel, and the density of the compound layer is used to hinder the liquid metal from penetrating into the interior of the high-strength steel. A smaller welding pressure is used in this stage to prevent a large number of cracks from occurring in the compound layer, and a longer cooling time is used to improve the stability of the compound layer. A larger welding pressure is used in the second spot welding process to increase the current density of the welding point, and a larger welding time and welding current are combined to allow the molten core to obtain sufficient energy to grow, thereby improving the welding mechanical properties.

[0020] The beneficial effects of the present invention are:

[0021] The present invention optimizes the composition of the plating solution, and utilizes the Zn element and the Al, Ni, and Cu elements to generate a layer of compounds on the surface of the high-strength steel, thereby hindering the diffusion of liquid metal into the interior of the high-strength steel. In addition, the spot welding process is combined to effectively improve the LME cracks of the galvanized high-strength steel spot welding. The obtained spot welding joint has excellent mechanical properties, which is of great significance to the promotion and application of galvanized high-strength steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The microstructure of high-strength steel plate; DETAILED DESCRIPTION

[0023] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0024] In the following embodiment, the thickness of the high-strength steel is 1.2 mm, and the chemical composition is as follows by weight percentage: 0.12% C, 0.34% Si, 2.35% Mn, 0.10% Mo, 0.59% Cr, 0.027% Ti, 0.028% Nb, 0.016% P+S, and the balance is Fe and other inevitable impurities; the mechanical properties are a yield strength of 1030 MPa, a tensile strength of 1256 MPa, and an elongation after fracture of 11%.

[0025] Example 1

[0026] Step 1: Select the high-strength steel plate with the above composition for galvanizing. The plating solution includes the following components by mass fraction: 13.2% Al, 3.8% Si, 6.9% Ni, 2.6% Cu, the balance is Zn and other inevitable impurities, the galvanizing temperature is 485℃, and the coating thickness is 68g / m 3 , Use acetone or alcohol to clean the galvanized high-strength steel surface to be welded, remove oil and other impurities and set aside;

[0027] Step 2: overlap two galvanized high-strength steel plates, and then place them between spot welding electrodes. The electrodes are chromium-zirconium-copper conical electrodes with an end face diameter of 6.0 mm. Set the spot welding process parameters and perform welding.

[0028] Among them: the first spot welding process is as follows: welding pressure is 3.5kN, slow rising current is 3.0kA, welding time is 80ms, welding current is 4.5kA, cooling time is 120ms;

[0029] The second spot welding process is as follows: welding pressure is 5.5kN, slow-rise current is 5.8kA, welding time is 220ms, welding current is 8.5kA, and cooling time is 20ms.

[0030] After inlaying, grinding and polishing, the LME crack density and maximum crack depth of the spot welding joint were tested. The test showed that the LME crack density was 2.8 lines / cm, the maximum crack depth was 0.12mm, and the shear force of the spot welding joint was 17.25kN.

[0031] Example 2

[0032] Step 1: Select the high-strength steel plate with the above composition for galvanizing. The plating solution includes the following components by mass fraction: 11.8% Al, 2.3% Si, 9.2% Ni, 4.6% Cu, and the remainder is Zn and other inevitable impurities. The galvanizing temperature is 495°C and the coating thickness is 80g / m 3 , Use acetone or alcohol to clean the galvanized high-strength steel surface to be welded, remove oil and other impurities and set aside;

[0033] Step 2: overlap two galvanized high-strength steel plates, and then place them between spot welding electrodes. The electrodes are chromium-zirconium-copper conical electrodes with an end face diameter of 6.0 mm. Set spot welding parameters and perform welding.

[0034] Among them: the first stage spot welding process is as follows: welding pressure is 4.2kN, slow rising current is 3.5kA, welding time is 70ms, welding current is 4.8kA, cooling time is 180ms;

[0035] The second spot welding process is as follows: welding pressure is 5.8kN, slow-rise current is 5.3kA, welding time is 180ms, welding current is 9.5kA, and cooling time is 40ms.

[0036] After inlaying, grinding and polishing, the LME crack density and maximum crack depth of the spot welding joint were tested. The test showed that the LME crack density was 1.6 lines / cm, the maximum crack depth was 0.08mm, and the shear force of the spot welding joint was 16.32kN.

[0037] Example 3

[0038] Step 1: Select the high-strength steel plate with the above composition for galvanizing. The plating solution includes the following components by mass fraction: 14.1% Al, 2.8% Si, 6.2% Ni, 3.1% Cu, and the balance is Zn and other inevitable impurities. The galvanizing temperature is 520℃ and the coating thickness is 85g / m 3 , Use acetone or alcohol to clean the galvanized high-strength steel surface to be welded, remove oil and other impurities and set aside;

[0039] Step 2: overlap two galvanized high-strength steel plates, and then place them between spot welding electrodes. The electrodes are chromium-zirconium-copper conical electrodes with an end face diameter of 6.0 mm. Set spot welding parameters and perform welding.

[0040] Among them: the first stage spot welding process is as follows: welding pressure is 4.8kN, slow rising current is 3.2kA, welding time is 80ms, welding current is 4.5kA, cooling time is 140ms;

[0041] The second spot welding process is as follows: welding pressure is 5.5kN, slow-rise current is 6.5kA, welding time is 200ms, welding current is 8.0kA, and cooling time is 20ms.

[0042] After inlaying, grinding and polishing, the LME crack density and maximum crack depth of the spot welding joint were tested. The test showed that the LME crack density was 1.7 lines / cm, the maximum crack depth was 0.07mm, and the shear force of the spot welding joint was 18.35kN.

[0043] Comparative Example 1

[0044] Step 1: Select the high-strength steel plate with the above composition for galvanizing. The plating solution includes the following components by mass fraction: 8.2% Al, 5.6% Si, 4.2% Ni, 4.2% Cu, and the remainder is Zn and other inevitable impurities. The galvanizing temperature is 485°C and the coating thickness is 68g / m 3 , Use acetone or alcohol to clean the galvanized high-strength steel surface to be welded, remove oil and other impurities and set aside;

[0045] Step 2: overlap two galvanized high-strength steel plates, and then place them between spot welding electrodes. The electrodes are chromium-zirconium-copper conical electrodes with an end face diameter of 6.0 mm. Set the spot welding process parameters and perform welding.

[0046] Among them: the first spot welding process is as follows: welding pressure is 3.5kN, slow rising current is 3.0kA, welding time is 80ms, welding current is 4.5kA, cooling time is 120ms;

[0047] The second spot welding process is as follows: welding pressure is 5.5kN, slow-rise current is 5.8kA, welding time is 220ms, welding current is 8.5kA, and cooling time is 20ms.

[0048] After inlaying, grinding and polishing, the LME crack density and maximum crack depth of the spot welding joint were tested. The test showed that the LME crack density was 8.6 lines / cm, the maximum crack depth was 0.35mm, and the shear force of the spot welding joint was 8.54kN.

[0049] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for inhibiting LME cracks in galvanized high-strength steel, characterized in that: The following steps are involved: Step 1: galvanize the high-strength steel plate and clean the galvanized high-strength steel surface to be welded for later use; The plating solution comprises the following components by mass fraction: 10%-15% Al, 2%-5% Si, 5%-10% Ni, 2%-5% Cu, and the remainder is Zn and other inevitable impurities; Step 2: overlap two galvanized high-strength steel plates, then place them between spot welding electrodes, set spot welding process parameters for welding; Among them, the spot welding process parameters are: the welding pressure of the first stage spot welding is 3.0-4.5kN, the slow rise current is 3.0-3.5kA, the welding time is 60-120ms, the welding current is 4.0-5.0kA, and the cooling time is 100-200ms; the welding pressure of the second stage spot welding is 4.5-6.0kN, the slow rise current is 5.0-7.0kA, the welding time is 160-260ms, the welding current is 7.5-9.5kA, and the cooling time is 0-80ms.

2. The method for inhibiting LME cracks in galvanized high-strength steel according to claim 1, characterized in that: In step 1, the galvanizing temperature is 470-520℃ and the coating thickness is 60-90g / m 3 .

3. The method for inhibiting LME cracks in galvanized high-strength steel according to claim 1, characterized in that: In step one, use acetone or alcohol for cleaning.

4. The method for inhibiting LME cracks in galvanized high-strength steel according to claim 1, characterized in that: In step 2, the spot welding electrode is a chromium-zirconium-copper conical electrode with an end face diameter of 6.0 mm.

5. The method for inhibiting LME cracks in galvanized high-strength steel according to claim 1, characterized in that: The chemical composition of the high-strength steel is as follows by weight percentage: 0.115%-0.135% C, 0.3%-0.4% Si, 2.2%-2.4% Mn, 0.08%-0.12% Mo, 0.57%-0.63% Cr, 0.025%-0.035% Ti, 0.020%-0.030% Nb, 0.004%-0.020% P+S, and the remainder is Fe and other inevitable impurities.

6. The method for inhibiting LME cracks in galvanized high-strength steel according to claim 1, characterized in that: The high-strength steel has a yield strength of ≥980MPa, a tensile strength of ≥1180MPa, and an elongation after fracture of ≥10%.

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