Method for lifting GPa-grade high-strength galvanized steel spot welding window

Abstract

The invention discloses a method for lifting a GPa-grade high-strength galvanized steel spot welding window, and belongs to the field of high-strength steel welding processes. The method comprises the following steps that firstly, high-strength galvanized steel is selected to machine a spot welding test plate, and an electrode is selected; and secondly, a two-stage spot welding process is adopted, spot welding parameters are set, and welding is conducted. According to the two-stage spot welding process, comprehensive heat input is controlled through two pulses, so that the growth speed of the nugget diameter is restrained, the limiting current when splashing occurs is improved, and the purpose of lifting a spot welding window is achieved.

Description

Technical Field

[0001] The invention belongs to the field of high-strength steel welding technology, is suitable for galvanized high-strength steel welding, and specifically relates to a method for improving a GPa-level high-strength galvanized steel spot welding window. Background Art

[0002] As people's requirements for automobile safety performance increase and environmental awareness increases, automobile manufacturers use a large amount of high-strength steel in production to reduce CO2 emissions and improve vehicle safety. Galvanized high-strength steel has excellent corrosion resistance and its use in car body manufacturing is also increasing. Resistance spot welding is the most widely used welding method in automobile assembly. Generally speaking, automobile manufacturers' requirements for GPa-grade high-strength steel spot welding performance are mainly reflected in two aspects: one is the spot welding process window, which requires the spot welding process window to be as wide as possible to meet the production rhythm requirements, and the other is the mechanical properties of the spot welding joint to meet the collision safety requirements. The right boundary of the spot welding process window refers to the limiting current when spatter occurs. The spot welding process window of GPa-grade galvanized high-strength steel is usually lower than 1.0kA. The reason is that the alloy content is high and spatter is very easy to occur, resulting in a smaller limiting current at the right boundary of the spot welding process window.

[0003] CN117206654A "A method for determining the weldable current range for resistance spot welding of galvanized ultra-high strength steel" discloses a method for determining the weldable current range for resistance spot welding of galvanized ultra-high strength steel. This method can only be used to determine the current range and cannot expand the spot welding window.

[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. This method can improve the LME cracks of the spot weld joints, but cannot expand the spot welding window.

[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 base material along the grain boundaries during spot welding, thereby reducing the spot welding LME sensitivity of the high-strength steel composite galvanized sheet. This method can alleviate spot welding LME cracks but cannot expand the spot welding window. Summary of the invention

[0006] In order to solve the problem that the welding performance of GPa-grade high-strength steel is reduced due to its high alloy element content and large carbon equivalent, especially after surface galvanizing, and its spot welding window width usually does not exceed 1.0kA, the present invention provides a method for improving the spot welding window of GPa-grade high-strength galvanized steel, and adopts a variable pressure two-stage spot welding process to increase the limiting current of the right boundary of the spot welding process window, so as to achieve the purpose of improving the spot welding window of GPa-grade high-strength galvanized steel, which is of great significance to the promotion and application of galvanized high-strength steel.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] A method for improving a GPa-level high-strength galvanized steel spot welding window comprises the following steps:

[0009] Step 1: Select high-strength galvanized steel for spot welding test plate and select electrodes;

[0010] Step 2: Use a 2-stage spot welding process, set spot welding parameters and perform welding; wherein, the first stage spot welding process is as follows: welding pressure 4.2-5.0kN, slow rise current 3.8-4.8kA, welding time 80-150ms, welding current 4.5-5.5kA, cooling time 120-180ms; the second stage spot welding process is as follows: welding pressure 5.5-6.0kN, slow rise current 6.0-6.5kA, welding time 200-280ms, welding current 6.5-12.0kA, cooling time 0-20ms.

[0011] Preferably, the first spot welding process is: welding pressure 4.5-5.0kN, slow rise current 3.8-4.8kA, welding time 90-120ms, welding current 4.8-5.2kA, cooling time 120-180ms; the second spot welding process is: welding pressure 5.5-5.8kN, slow rise current 6.0-6.5kA, welding time 240-280ms, welding current 7.5-10.0kA, cooling time 0-20ms.

[0012] Furthermore, the high-strength galvanized steel is obtained by galvanizing the surface of high-strength steel, and the chemical composition of the high-strength steel is as follows by weight percentage: 0.125-0.145% C, 0.35-0.55% Si, 1.9-2.2% Mn, 0.10-0.14% Mo, 0.65-0.80% Cr, 0.03-0.05% Ti, 0.020-0.030% Nb, 0.008-0.020% P+S, and the remainder is Fe and other inevitable impurities.

[0013] Furthermore, the high-strength galvanized steel is of 1180MPa grade, and has the following mechanical properties: yield strength ≥1000MPa, tensile strength ≥1180MPa, and elongation after fracture ≥12%.

[0014] Furthermore, the thickness of the high-strength galvanized steel is 1.0-1.6 mm, and the thickness of the zinc layer is 120-150 g / m 3 .

[0015] Further, the electrode is a copper-based electrode, preferably a copper-based conical or spherical electrode, with an end face diameter of 6.0-8.0 mm. Preferably, the electrode is a copper-based spherical electrode, with an end face diameter of 6.0-6.8 mm.

[0016] Furthermore, after spot welding, the spot welding window width is measured. The method for measuring the spot welding window is to fix the process parameters except the welding current (I2) in the second stage of the spot welding process, and measure the spot welding window by changing I2, as follows: I2 takes 7.0kA as the initial current, and gradually increases I2 by 0.2kA until spatter occurs. After spatter occurs, I2 is gradually reduced by 0.1kA until no spatter occurs when three samples are welded continuously at a certain current. The welding current at this time is the limiting current Imax, and then all spot welding samples are cut into metallographic samples by wire cutting. After inlaying-grinding-polishing-corrosion, the core diameter is measured under an optical microscope. When the core diameter reaches 4.25t for the first time, 1 / 2 (t is the plate thickness), the corresponding current is the assumed minimum current Ip, and the welding current is increased or decreased by 0.1kA near Ip. Three metallographic specimens are welded at each current and the nugget diameter is measured. When the nugget diameters of all three specimens at a certain current are greater than or equal to 4.25t 1 / 2 (t is the plate thickness), the corresponding current is Imin, and the spot welding process window is from Imin to Imax, and its width is the difference between the two.

[0017] Beneficial effects of the present invention:

[0018] The two-stage spot welding process of the present invention controls the comprehensive heat input through two pulses, thereby suppressing the growth rate of the molten core diameter, thereby increasing the limiting current when spatter occurs, and achieving the purpose of improving the spot welding window. The first stage spot welding process achieves the effect of preheating the plate, and forms a molten core of about 2.5-3.5mm by controlling the spot welding process. The second stage spot welding process increases the diameter of the molten core, thereby obtaining excellent mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the microstructure of GPa grade high-strength galvanized steel. DETAILED DESCRIPTION

[0020] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0021] In the following embodiments, the GPa-grade high-strength galvanized steel is obtained by galvanizing the surface of the high-strength steel. The chemical composition of the high-strength steel is as follows by weight percentage: 0.131% C, 0.38% Si, 2.02% Mn, 0.12% Mo, 0.69% Cr, 0.035% Ti, 0.022% Nb, 0.012% P+S, and the balance is Fe and other inevitable impurities; the yield strength of the high-strength galvanized steel sheet is 1050 MPa, the tensile strength is 1230 MPa, the elongation after fracture is 13%, the thickness of the galvanized steel sheet is 1.2 mm, and the thickness of the zinc layer is 130 g / m 3 , microstructure such as Figure 1 As shown, it is mainly composed of martensite and ferrite.

[0022] A method for improving the GPa-level high-strength galvanized steel spot welding window, the specific steps are as follows:

[0023] Step 1: Select the galvanized high-strength steel with the above composition for spot welding test plate and select the electrode.

[0024] Step 2: Set spot welding parameters and perform welding.

[0025] Step 3: Determine the spot welding window width.

[0026] Example 1

[0027] Step 1: Select the high-strength steel plate with the above composition and process it into a 25×120 mm test plate to be welded. The electrode is a copper-based spherical electrode with an end face diameter of 6.2 mm.

[0028] Step 2: Set the spot welding parameters and perform welding. The fixed process parameters are as follows: The first spot welding process is welding pressure 4.6kN, slow rise current 4.0kA, welding time 100ms, welding current 4.9kA, cooling time 160ms. The second spot welding process is welding pressure 5.5kN, slow rise current 6.2kA, welding time 280ms, welding current 8.5kA, cooling time 10ms.

[0029] Step 3: Determine the spot welding window width. According to the above method for measuring the spot welding window, Ip is 8.0kA, Imin is 8.2kA, Imax is 9.8kA, the spot welding window width is 1.6kA, and the maximum shear force of the spot welding joint is 21.45kN.

[0030] Example 2

[0031] Step 1: Select the high-strength steel plate with the above composition and process it into a 25×120 mm test plate to be welded. The electrode is a copper-based spherical electrode with an end face diameter of 6.8 mm.

[0032] Step 2: Set the spot welding parameters and perform welding. The fixed process parameters are as follows: The first spot welding process is welding pressure 5.0kN, slow rise current 4.8kA, welding time 120ms, welding current 5.2kA, cooling time 140ms. The second spot welding process is welding pressure 5.8kN, slow rise current 6.4kA, welding time 240ms, welding current 9.5kA, cooling time 0ms.

[0033] Step 3: Determine the spot welding window width. According to the above method for measuring the spot welding window, Ip is 7.8kA, Imin is 7.9kA, Imax is 9.6kA, the spot welding window width is 1.7kA, and the maximum shear force of the spot welding joint is 20.75kN.

[0034] Comparative Example 1

[0035] Step 1: Select the high-strength steel plate with the above composition and process it into a 25×120 mm test plate to be welded. The electrode is a copper-based spherical electrode with an end face diameter of 6.5 mm.

[0036] Step 2: Set the spot welding parameters and perform welding. The fixed process parameters are as follows: The first spot welding process is welding pressure 3.5kN, slow rise current 3.0kA, welding time 70ms, welding current 4.4kA, cooling time 150ms. The second spot welding process is welding pressure 3.5kN, slow rise current 7.2kA, welding time 300ms, welding current 8.5kA, cooling time 0ms.

[0037] Step 3: Determine the spot welding window width. According to the above method for measuring the spot welding window, Ip is 7.0kA, Imin is 7.2kA, Imax is 8.0kA, the spot welding window width is 0.8kA, and the maximum shear force of the spot welding joint is 15.24kN.

Claims

1. A method for improving GPa-level high-strength galvanized steel spot welding window, characterized in that: The steps include: Step 1: Select high-strength galvanized steel for spot welding test plate and select electrodes; Step 2: Use a 2-stage spot welding process, set spot welding parameters and perform welding; wherein, the first stage spot welding process is as follows: welding pressure 4.2-5.0kN, slow rise current 3.8-4.8kA, welding time 80-150ms, welding current 4.5-5.5kA, cooling time 120-180ms; the second stage spot welding process is as follows: welding pressure 5.5-6.0kN, slow rise current 6.0-6.5kA, welding time 200-280ms, welding current 6.5-12.0kA, cooling time 0-20ms.

2. The method according to claim 1, characterized in that: The first spot welding process is: welding pressure 4.5-5.0kN, slow rise current 3.8-4.8kA, welding time 90-120ms, welding current 4.8-5.2kA, cooling time 120-180ms; the second spot welding process is: welding pressure 5.5-5.8kN, slow rise current 6.0-6.5kA, welding time 240-280ms, welding current 7.5-10.0kA, cooling time 0-20ms.

3. The method according to claim 1, characterized in that: The high-strength galvanized steel is obtained by galvanizing the surface of high-strength steel. The chemical composition of the high-strength steel is as follows by weight percentage: 0.125-0.145% C, 0.35-0.55% Si, 1.9-2.2% Mn, 0.10-0.14% Mo, 0.65-0.80% Cr, 0.03-0.05% Ti, 0.020-0.030% Nb, 0.008-0.020% P+S, and the remainder is Fe and other inevitable impurities.

4. The method according to claim 1, characterized in that: The mechanical properties of the high-strength galvanized steel are: yield strength ≥1000MPa, tensile strength ≥1180MPa, and elongation after fracture ≥12%.

5. The method according to claim 1, characterized in that: The thickness of the high-strength galvanized steel is 1.0-1.6 mm, and the thickness of the zinc layer is 120-150 g / m 3 .

6. The method according to claim 1, characterized in that: The electrode is a copper-based electrode.

7. The method according to claim 6, characterized in that: The electrode is a copper-based conical or spherical electrode with an end face diameter of 6.0-8.0 mm.

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