Method for solving interface failure of resistance spot-welded joint of 980MPa-grade galvanized complex-phase steel plate

Abstract

The invention relates to a method for solving interface failure of a resistance spot-welded joint of a 980MPa-grade galvanized complex-phase steel plate. The method comprises the following steps: determining preheating current and preheating time according to the plate thickness of a welding sample; determining the number of welding pulses and the single-pulse welding time according to the plate thickness of the welding sample; according to the plate thickness of the welding sample, the cooling interval time of preheating pulse and welding pulse and the welding pulse interval time are determined; determining welding pressure according to the steel plate thickness of the welding sample; and determining the dwell time according to the plate thickness of the welding sample. The method has the advantages that the pulse form with ultra-large preheating current and small preheating time is used, the content of a zinc layer in a weld nugget is greatly reduced, then the multi-pulse interval time is determined according to the plate thickness of a welding sample, the cooling interval time between welding pulses is long, and the welding precision is high. The method aims at providing sufficient time to enable nugget structures to tend to be relatively stable after a previous welding pulse is finished, and a next pulse is applied when the temperature of a welding spot is close to the normal temperature state, so that the nugget is further increased.

Description

Technical Field

[0001] The invention relates to a method for solving interface failure of resistance spot welding joints of 980MPa-grade galvanized complex phase steel plates. Background Art

[0002] In recent years, as the automotive industry has become increasingly stringent in terms of lightweight, safety, energy conservation and environmental protection, the proportion of GPa-grade high-strength steel used in automobile bodies has continued to increase. Among them, galvanized complex phase steel CP980+Z is widely used in important automotive structural parts due to its high yield strength ratio and good mechanical properties. However, during the resistance spot welding process of galvanized complex phase steel CP980+Z, the joint often suffers from interface failure in the shear tensile test at the lower limit of the window, resulting in reduced mechanical properties of the joint, and the fatigue test and energy absorption of the weld point do not meet the standards. This is completely unacceptable to the OEM and urgently needs to be improved.

[0003] In the prior art, the application number is 2021105938672, and the patent name is "A method and device for resistance spot welding of galvanized sheet metal". A stainless steel sheet is added between the galvanized steel sheet base material and the electrode. Since the electrode and the galvanized steel sheet base material are separated by a stainless steel sheet and are not in direct contact, the zinc atoms cannot adhere to the electrode to form an alloy during welding, and the original electrical and thermal conductivity of the electrode is maintained. Although this invention can improve the problem of adhesion of the electrode when the zinc layer of the galvanized sheet melts to a certain extent, thereby improving the quality of the joint, the addition of stainless steel material between the electrode and the galvanized sheet base material greatly reduces the cooling rate, resulting in a high soft phase content and low strength of the structure formed by the molten core, which is not suitable for ultra-high-strength steel. Summary of the invention

[0004] The purpose of the present invention is to provide a method for solving the interface failure of 980MPa grade galvanized complex phase steel plate resistance spot welding joints. The welding process using ultra-large current short-time pulse form and long cooling time not only changes the failure mode of the joint and thus improves the mechanical properties, but is also suitable for resistance spot welding of ultra-high strength galvanized plates.

[0005] To achieve the above object, the present invention is implemented through the following technical solutions:

[0006] A method for solving interface failure of resistance spot welding joints of 980MPa grade galvanized duplex steel plates, comprising:

[0007] S1. Cut the galvanized complex phase steel sample into square welding specimens, remove the oil stains on the surface of the welding specimens, and preload the specimens for 300-400ms;

[0008] S2. Determine the preheating current I according to the plate thickness of the welding sample p With preheating time t p size;

[0009] S3, determining the number of welding pulses and the single-pulse welding time according to the plate thickness of the welding sample;

[0010] S4, determining the preheating pulse and the welding pulse cooling interval time tc1, and the welding pulse interval time tc2 according to the plate thickness of the welding sample;

[0011] S5. Determine the welding pressure P according to the steel plate thickness of the welding sample;

[0012] S6. Determine the holding time th according to the plate thickness of the welding sample.

[0013] In S2, the preheating current Ip range is: 20kA≤Ip≤25kA, and the preheating time is: 10ms≤tp≤20ms.

[0014] In S3, when the plate thickness of the welding sample is 0.8mm≤<1.2mm, double-pulse welding is used, and the single-pulse welding time is 140-160ms; when the plate thickness of the welding sample is 1.2mm≤<1.7mm, triple-pulse welding is used, and the single-pulse welding time is 120-140ms; when the plate thickness of the welding sample is 1.7mm≤≤2.0mm, quad-pulse welding is used, and the single-pulse welding time is 130-150ms.

[0015] In S4, when the plate thickness of the welding sample is 0.8mm≤<1.5mm, the welding pulse cooling interval time tc1=15~25ms, and the welding pulse cooling interval time tc2=450~550ms. When the plate thickness of the welding sample is 1.5mm≤≤2.0mm, the welding pulse cooling interval time tc1=35~45ms, and the welding pulse cooling interval time tc2=650~750ms.

[0016] In S5, when 0.8mm≤the plate thickness of the welding sample<1.2mm, the welding pressure P=2.5~3.0kN; when 1.2mm≤the plate thickness of the welding sample<1.5mm, the welding pressure P=3.0~3.5kN; when 1.5mm≤the plate thickness of the welding sample<1.7mm, the welding pressure P=3.5~4.0kN; when 1.7mm≤the plate thickness of the welding sample≤2.0mm, the welding pressure P=4.5~5.0kN.

[0017] In S6, when the plate thickness of the welding sample is 0.8 mm ≤ <1.5 mm, the holding time th is 150 to 200 ms; when the plate thickness of the welding sample is 1.5 mm ≤ <2.0 mm, the holding time th is 300 to 350 ms.

[0018] Then, a medium frequency DC spot welding machine is used to perform resistance spot welding according to the parameters determined above and in the sequence of pre-pressing, preheating, welding and pressure holding.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. Use a pulse form with ultra-large preheating current and short preheating time to push away the zinc layer on the surface of galvanized complex phase steel, so that the zinc content in the weld core is greatly reduced. Then determine the multi-pulse interval time according to the plate thickness of the welding sample. The cooling interval time between welding pulses is relatively long. The purpose is to have sufficient time after the previous welding pulse to make the weld core structure relatively stable. Apply the next pulse when the temperature of the welding point is close to room temperature to further increase the weld core and completely melt the two steel plates together, so as to achieve the purpose of improving joint performance and weldability, making the weld core structure composition purer, improving the joint quality, and ensuring that there will be no interface failure.

[0021] 2. Prolonging the holding time can reduce the shrinkage inside the weld core and improve the joint performance;

[0022] 3. The purpose of using multi-pulse welding is to gradually increase the diameter of the weld nugget, and it can avoid spatter caused by long-term continuous welding, broaden the welding process window, and improve the performance of the joint;

[0023] 4. This welding process can accelerate the wider application of 980MPa grade galvanized complex phase steel plates in the automotive manufacturing field. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a welding timing diagram. DETAILED DESCRIPTION

[0025] The present invention is described in detail below in conjunction with the accompanying drawings, but it should be noted that the implementation of the present invention is not limited to the following embodiments.

[0026] The following examples are implemented on the premise of the technical solution of the present invention, and provide detailed implementation methods and specific operation processes, but the protection scope of the present invention is not limited to the following examples. The methods used in the following examples are conventional methods unless otherwise specified.

[0027] [Example 1]

[0028] The welding material is 0.8mm thick galvanized composite steel CP980+Z, using a medium frequency DC spot welder, see Figure 1 , the content is as follows:

[0029] (1) Cut the 0.8 mm thick CP980+Z sample into 150 mm × 50 mm welding specimens, wipe the oil stains on the surface with alcohol, and the pre-pressing time is 300 ms.

[0030] (2) Determine the preheating current Ip = 20kA and the preheating time tp = 10ms according to the plate thickness of the welding sample.

[0031] (3) Use double pulse welding, and determine the single pulse welding time tw = 150ms according to the plate thickness of the welding sample;

[0032] (4) According to the plate thickness of the welding sample, the preheating pulse and the welding pulse cooling interval time tc1 = 20ms, and the welding pulse interval time tc2 = 500ms are determined.

[0033] (5) Select the welding pressure p = 2.5 kN according to the plate thickness of the welding sample.

[0034] (6) Holding time th = 150 ms.

[0035] Based on the above welding parameters, the resistance spot welding process window of 0.8mm thick galvanized complex phase steel CP980+Z is 2.2kA, which is much larger than the standard requirement of more than 1kA; the shear tensile performance of the joint is greater than 12kN, the cross tensile performance is greater than 4.1kN, and the failure mode is the pull-out of the molten core without splashing on the surface.

[0036] The conventional welding process is as follows: welding pressure is 2.6kN, single pulse welding, welding time is 230ms, and pressure holding time is 40ms. The welding window is 1.1kA (just meeting the standard requirement of greater than 1kA), the shear tensile performance of the joint is 10.5kN, and the cross tensile performance is 3.5kN. A small amount of interface failure occurred in the window lower limit shear and cross welding tensile tests. It can be concluded that this welding method is a great improvement over the traditional welding process.

[0037] [Example 2]

[0038] The welding material is 1.2mm thick galvanized complex phase steel CP980+Z, using a medium frequency DC spot welding machine, see Figure 1 , the content is as follows:

[0039] (1) Cut the 1.2 mm thick CP980+Z sample into 150 mm × 50 mm welding specimens, wipe the oil stains on the surface with alcohol, and the pre-pressing time is 300 ms.

[0040] (2) Determine the preheating current Ip = 22 kA and the preheating time tp = 15 ms based on the plate thickness of the welding sample.

[0041] (3) Using three-pulse welding, the single-pulse welding time tw = 130ms is determined according to the plate thickness of the welding sample;

[0042] (4) According to the plate thickness of the welding sample, the preheating pulse and the welding pulse cooling interval time tc1 = 20ms, and the welding pulse interval time tc2 = 500ms are determined.

[0043] (5) Select the welding pressure p = 3.2 kN according to the plate thickness of the welding sample.

[0044] (6) Holding time th = 180 ms.

[0045] The resistance spot welding process window of 1.2mm thick CP980+Z obtained based on the above welding parameters is 2.4kA, which is much larger than the standard requirement of more than 1kA; the shear tensile performance of the joint is greater than 20.3kN, and the cross tensile performance of the joint is greater than 8.5kN. The failure mode is the pull-out of the molten core, and there is no spatter on the surface.

[0046] The conventional welding process is as follows: welding pressure is 2.6kN, single pulse welding, welding time is 270ms, and pressure holding time is 40ms. The welding window is 0.9kA (does not meet the standard requirement of greater than 1kA), the shear tensile performance of the joint is 17.3kN, and the cross tensile performance is 6.7kN. Interface failure occurs in the window lower limit shear and cross welding tensile tests. It can be concluded that this welding method is a great improvement over the conventional welding process.

[0047] [Example 3]

[0048] The welding material is 2.0mm thick CP980+Z, using a medium frequency DC spot welder, see Figure 1 , the content is as follows:

[0049] (1) Cut the 2.0 mm thick CP980+Z sample into 150 mm × 50 mm welding specimens, wipe the oil stains on the surface with alcohol, and the pre-pressing time is 300 ms.

[0050] (2) Determine the preheating current Ip = 25kA and the preheating time tp = 20ms according to the plate thickness of the welding sample.

[0051] (3) Using four-pulse welding, the single-pulse welding time tw = 140ms is determined according to the plate thickness of the welding sample;

[0052] (4) According to the plate thickness of the welding sample, the preheating pulse and the welding pulse cooling interval time tc1 = 40ms, and the welding pulse interval time tc2 = 700ms are determined.

[0053] (5) Select the welding pressure p = 5.0 kN according to the plate thickness of the welding sample.

[0054] (6) Holding time th = 350 ms.

[0055] The resistance spot welding process window of 2.0mm thick CP980+Z obtained based on the above welding parameters is 1.7kA, which is much larger than the standard requirement of more than 1kA; the shear tensile performance of the joint is greater than 38.2kN, and the cross tensile performance of the joint is greater than 20.2kN. The failure mode is the pull-out of the molten core, and there is no spatter on the surface.

[0056] The traditional welding process is as follows: welding pressure is 4kN, four-pulse welding, single-pulse welding time is 120ms, cooling time is 40ms, and pressure holding time is 100ms. The welding window is 1.0kA (just meeting the standard requirement of more than 1kA), the shear tensile performance of the joint is 31.3kN, and the cross tensile performance is 16kN. In the welding shear test at the lower limit of the window, interface failure occurs. It can be concluded that this welding method is a great improvement over the traditional welding process.

[0057] The present invention uses a pulse form with an ultra-large preheating current and a short preheating time to push away the zinc layer on the surface of the galvanized complex phase steel, thereby significantly reducing the zinc layer content in the weld core, and then determining the multi-pulse interval time according to the plate thickness of the welding sample. The cooling interval time between the welding pulses is relatively long, so that there is sufficient time after the previous welding pulse to make the weld core structure relatively stable, and the next pulse is applied when the temperature of the welding point is close to room temperature, so that the weld core is further enlarged, and the two steel plates are completely melted together, thereby achieving the purpose of improving the joint performance and improving the weldability, making the weld core structure component purer, improving the joint quality, and ensuring that no interface failure occurs; extending the pressure holding time can reduce the shrinkage cavity inside the weld core, and improving the joint performance; the purpose of using multi-pulse welding is to gradually increase the weld core diameter, and can avoid spatter caused by long-term continuous welding, broaden the welding process window, and improve the joint performance; this welding process can accelerate the wider application of 980MPa-grade galvanized complex phase steel plates in the field of automobile manufacturing.

Claims

1. A method for solving the interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint, characterized in that: include: S1. Cut the galvanized complex phase steel sample into square welding specimens, remove the oil stains on the surface of the welding specimens, and preload the specimens for 300-400ms; S2. Determine the preheating current Ip and preheating time tp according to the plate thickness of the welding sample; S3, determining the number of welding pulses and the single-pulse welding time according to the plate thickness of the welding sample; S4, determining the preheating pulse and the welding pulse cooling interval time tc1, and the welding pulse interval time tc2 according to the plate thickness of the welding sample; S5. Determine the welding pressure P according to the steel plate thickness of the welding sample; S6. Determine the holding time th according to the plate thickness of the welding sample.

2. A method for solving interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint according to claim 1, characterized in that: In S2, the preheating current Ip range is: 20kA≤Ip≤25kA, and the preheating time is: 10ms≤tp≤20ms.

3. A method for solving interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint according to claim 1, characterized in that: In S3, when the plate thickness of the welding sample is 0.8mm≤<1.2mm, double-pulse welding is used, and the single-pulse welding time is 140-160ms; when the plate thickness of the welding sample is 1.2mm≤<1.7mm, triple-pulse welding is used, and the single-pulse welding time is 120-140ms; when the plate thickness of the welding sample is 1.7mm≤≤2.0mm, quad-pulse welding is used, and the single-pulse welding time is 130-150ms.

4. A method for solving interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint according to claim 1, characterized in that: In S4, when the plate thickness of the welding sample is 0.8mm≤<1.5mm, the welding pulse cooling interval time tc1=15~25ms, and the welding pulse cooling interval time tc2=450~550ms. When the plate thickness of the welding sample is 1.5mm≤≤2.0mm, the welding pulse cooling interval time tc1=35~45ms, and the welding pulse cooling interval time tc2=650~750ms.

5. A method for solving interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint according to claim 1, characterized in that: In S5, when 0.8mm≤the plate thickness of the welding sample<1.2mm, the welding pressure P=2.5~3.0kN; when 1.2mm≤the plate thickness of the welding sample<1.5mm, the welding pressure P=3.0~3.5kN; when 1.5mm≤the plate thickness of the welding sample<1.7mm, the welding pressure P=3.5~4.0kN; when 1.7mm≤the plate thickness of the welding sample≤2.0mm, the welding pressure P=4.5~5.0kN.

6. A method for solving interface failure of 980MPa grade galvanized composite steel plate resistance spot welding joint according to claim 1, characterized in that: In S6, when the plate thickness of the welding sample is 0.8 mm ≤ <1.5 mm, the holding time th is 150 to 200 ms; when the plate thickness of the welding sample is 1.5 mm ≤ <2.0 mm, the holding time th is 300 to 350 ms.

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