Method for lifting 1200 MPa-grade high-strength galvanized steel spot welding window

Abstract

The invention discloses a method for lifting a spot welding window of 1200 MPa-grade high-strength galvanized steel, 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 three-section spot welding process is adopted, spot welding parameters are set, and welding is conducted. According to the three-section spot welding process, comprehensive heat input is controlled through three pulses, then the point welding head nugget diameter and the plastic ring growth speed are controlled, the limiting current generating splashing is increased, and finally a spot welding window is lifted.

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 1200MPa-grade high-strength galvanized steel spot welding window. Background Art

[0002] With the rapid development of automobile lightweight technology, its use in car bodies is increasing. 1200MPa-level galvanized high-strength steel has also attracted the attention of automobile manufacturers. However, with the increase in strength, the content of alloy elements in the steel is getting higher and higher, and its welding performance is reduced accordingly. Especially for galvanized high-strength steel, the volatilization of zinc during welding can easily contaminate the electrode, causing the electrode to stick to the steel plate and splash, resulting in a narrow spot welding process window and affecting the production rhythm. This aspect has become a hot topic for automobile manufacturers.

[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 alloy element content of 1200MPa grade galvanized high-strength steel is relatively high and the carbon equivalent is also increased accordingly, and when the ordinary medium-frequency spot welding method is used for welding, the spot welding window width does not exceed 1.0kA, which affects the promotion and application of the product. The present invention provides a method for improving the spot welding window of 1200MPa grade high-strength galvanized steel. A three-stage spot welding process is used to achieve the purpose of improving the spot welding window of 1200MPa grade high-strength galvanized steel, which is of great significance to the promotion and application of galvanized steel.

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

[0008] A method for improving a 1200MPa grade 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 the 3-stage spot welding process, set the spot welding parameters and perform welding; welding pressure 3.5-4.5kN, wherein the first stage spot welding process is as follows: slow rise current 2.8-3.8kA, welding time 100-180ms, welding current 3.5-4.8kA, cooling time 60-100ms; the second stage spot welding process is as follows: slow rise current 4.5-5.2kA, welding time 300-420ms, welding current 5.0-11.0kA, cooling time 20-40ms, the third stage spot welding process is as follows: slow rise current 4.5-5.5kA, welding time 100-200ms, welding current 3.0-5.0kA, cooling time 0-20ms.

[0011] Preferably, the welding pressure is 4.0-4.5kN, and the first spot welding process is: slow rising current 3.0-3.5kA, welding time 120-160ms, welding current 3.5-4.0kA, and cooling time 70-80ms; the second spot welding process is: slow rising current 4.5-5.0kA, welding time 350-400ms, welding current 6.0-10.0kA, and cooling time 25-40ms; the third spot welding process is: slow rising current 4.5-5.5kA, welding time 120-180ms, welding current 4.0-5.0kA, and 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: 0.105-0.135% C, 0.25-0.35% Si, 1.6-2.0% Mn, 0.15-0.25% Mo, 0.35-0.45% Cr, 0.01-0.02% Ti, 0.030-0.050% Nb, 0.008-0.020% P+S, and the balance is Fe and other inevitable impurities.

[0013] Furthermore, the high-strength galvanized steel is of 1200MPa grade, and has the following mechanical properties: yield strength ≥900MPa, tensile strength ≥1200MPa, and elongation after fracture ≥10%.

[0014] Furthermore, the thickness of the high-strength galvanized steel is 1.2-1.6 mm, and the thickness of the zinc layer is 80-100 g / m 3 .

[0015] Furthermore, the electrode is a copper-based electrode, preferably a copper-based conical or spherical electrode, with an end face diameter of 6.0-7.0 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 6.0kA 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 three-stage spot welding process of the present invention controls the comprehensive heat input through three pulses, thereby controlling the diameter of the spot welding joint nugget and the growth rate of the plastic ring, increasing the limiting current for spattering, and ultimately improving the spot welding window. The first stage of the spot welding process of this method is mainly to preheat the plate, and initially form 2.0t by controlling the heat input. 1 / 2 -3.0t 1 / 2 (t is the plate thickness) of the nugget diameter, the second spot welding process is to expand the nugget diameter and increase the growth rate of the plastic ring to form a 4.25t 1 / 2 (t is the plate thickness) and the growth rate of the molten core diameter is lower than the growth rate of the plastic ring. The third spot welding process is to heat treat the molten core and control the temperature to improve the microstructure of each area of ​​the spot welding joint and enhance the mechanical properties of the spot welding joint. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the microstructure of 1200MPa grade galvanized high-strength 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 embodiment, the 1200MPa grade galvanized high-strength 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: 0.128% C, 0.29% Si, 1.93% Mn, 0.22% Mo, 0.42% Cr, 0.018% Ti, 0.045% Nb, 0.018% P+S, and the balance is Fe and other inevitable impurities; the yield strength of the high-strength galvanized steel sheet is 935MPa, the tensile strength is 1250MPa, the elongation after fracture is 11%, and the thickness of the zinc layer is 86g / m 3 The thickness of the galvanized steel sheet is 1.2 mm, and the microstructure is as follows Figure 1 As shown, it is composed of martensite and ferrite.

[0022] A method for improving a 1200MPa grade 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 conical electrode with an end face diameter of 6.5 mm.

[0028] Step 2: Set the spot welding parameters and perform welding. The fixed process parameters are as follows: welding pressure 4.0kN, the first stage spot welding process is: slow rise current 3.5kA, welding time 120ms, welding current 4.0kA, cooling time 80ms. The second stage spot welding process is as follows: slow rise current 5.0kA, welding time 350ms, welding current 6.0kA, cooling time 40ms, the third stage spot welding process is as follows: slow rise current 4.5kA, welding time 120ms, welding current 5.0kA, cooling time 20ms.

[0029] Step 3: Determine the spot welding window width. According to the above method for measuring the spot welding window, Ip is 8.4kA, Imin is 8.5kA, Imax is 10.5kA, the spot welding window width is 2.0kA, and the maximum shear force of the spot welding joint is 22.15kN.

[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 conical electrode with an end face diameter of 6.5 mm.

[0032] Step 2: Set the spot welding parameters and perform welding. The fixed process parameters are as follows: welding pressure 4.5kN, the first stage spot welding process is as follows: slow rise current 3.0kA, welding time 160ms, welding current 3.5kA, cooling time 70ms; the second stage spot welding process is as follows: slow rise current 4.5kA, welding time 400ms, welding current 10.0kA, cooling time 25ms, the third stage spot welding process is as follows: slow rise current 5.5kA, welding time 180ms, welding current 4.0kA, 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 8.2kA, Imin is 8.1kA, Imax is 9.9kA, the spot welding window width is 1.8kA, and the maximum shear force of the spot welding joint is 23.24kN.

[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 conical 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: welding pressure 3.5kN, the first stage spot welding process is as follows: slow rise current 4.0kA, welding time 200ms, welding current 5.0kA, cooling time 70ms. The second stage spot welding process is as follows: slow rise current 3.5kA, welding time 120ms, welding current 5.0kA, cooling time 50ms, the third stage spot welding process is as follows: slow rise current 5.5kA, welding time 80ms, welding current 5.0kA, 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 7.2kA, Imin is 7.3kA, Imax is 7.8kA, the spot welding window width is 0.5kA, and the maximum shear force of the spot welding joint is 16.46kN.

Claims

1. A method for improving a 1200MPa grade 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 the 3-stage spot welding process, set the spot welding parameters and perform welding; welding pressure 3.5-4.5kN, wherein the first stage spot welding process is as follows: slow rise current 2.8-3.8kA, welding time 100-180ms, welding current 3.5-4.8kA, cooling time 60-100ms; the second stage spot welding process is as follows: slow rise current 4.5-5.2kA, welding time 300-420ms, welding current 5.0-11.0kA, cooling time 20-40ms, the third stage spot welding process is as follows: slow rise current 4.5-5.5kA, welding time 100-200ms, welding current 3.0-5.0kA, cooling time 0-20ms.

2. The method according to claim 1, characterized in that The welding pressure is 4.0-4.5kN, among which, the first spot welding process is: slow rise current 3.0-3.5kA, welding time 120-160ms, welding current 3.5-4.0kA, cooling time 70-80ms; the second spot welding process is: slow rise current 4.5-5.0kA, welding time 350-400ms, welding current 6.0-10.0kA, cooling time 25-40ms; the third spot welding process is: slow rise current 4.5-5.5kA, welding time 120-180ms, welding current 4.0-5.0kA, 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: 0.105-0.135% C, 0.25-0.35% Si, 1.6-2.0% Mn, 0.15-0.25% Mo, 0.35-0.45% Cr, 0.01-0.02% Ti, 0.030-0.050% Nb, 0.008-0.020% P+S, and the balance 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 ≥900MPa, tensile strength ≥1200MPa, and elongation after fracture ≥10%.

5. The method according to claim 1, characterized in that The thickness of the high-strength galvanized steel is 1.2-1.6 mm, and the thickness of the zinc layer is 80-100 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 6.0-7.0 mm.

Images