Method for reducing LME cracks of GPa-grade galvanized high-strength steel

Abstract

The invention provides a method for reducing LME cracks of GPa-grade galvanized high-strength steel, which comprises the following steps: step 1, selecting a GPa-grade galvanized high-strength steel plate, cleaning a to-be-welded surface with acetone or alcohol, removing impurities, and processing the to-be-welded surface into a spot welding sample; secondly, an electrode is selected, spot welding parameters are set, and the sample is welded through a multi-section spot welding process; and 3, after welding is finished, the LME crack density and the maximum crack depth of the sample are detected. The purpose of inhibiting the LME cracks can be achieved, and the method has important significance on application and popularization 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 reducing LME cracks in GPa-grade galvanized high-strength steel. Background Art

[0002] With the rapid development of the automobile industry and the increasing innovation of production technology, automobile lightweighting has become an important goal of research and development and manufacturing. High-strength steel plates have a higher specific strength. While reducing the weight of the car, they can also improve the safety performance of the car. They have become the main material in automobile production and manufacturing at this stage. In order to improve corrosion resistance, the surface of high-strength steel for automobiles is often plated with a zinc layer, which causes liquid metal brittle cracks (LME) to form in the galvanized high-strength steel during the resistance spot welding process. LME is a phenomenon in which two metals with different melting points melt under the action of heat and contact the high-melting point metal. Under the action of force, the liquid metal penetrates along the grain boundary of the high-melting point metal, forming cracks and reducing plasticity. Galvanized high-strength steel is prone to LME cracks during the resistance spot welding process, which reduces the performance of the spot welding joint.

[0003] When galvanized high-strength steel is resistance spot welded, as the heat input increases, the surface galvanized layer melts, and at the same time, under the action of higher electrode pressure, the molten liquid zinc penetrates into the base material along the grain boundary, and LME cracks will appear in the spot weld joint. The density of LME cracks is usually 3-6 per cm, and the crack depth is usually 5-10μm. The presence of LME cracks causes the shear performance of the spot weld joint to be unable to exceed 20kN.

[0004] Patent document with publication number CN114686651A discloses a zinc-coated steel with reduced liquid metal embrittlement (LME) sensitivity, which reduces the LME sensitivity of spot welded joints by annealing the steel substrate in an oxygen-containing atmosphere to form an oxide-containing layer on the surface of the steel substrate. However, this method has high requirements on steel production equipment.

[0005] Patent document with publication number CN111451625A discloses a method for improving the quality of resistance spot welding of coated ultra-high strength steel. By adding a high-conductivity and thermal-conductivity gasket between the high-strength steel plate and the welding electrode, the surface cracks of the coated high-strength steel plate are improved during spot welding, thereby achieving the purpose of improving the mechanical properties of the weld. However, this method has a high spot welding cost and cumbersome steps.

[0006] Patent document with publication number CN116657033A discloses a high-strength steel composite galvanized sheet resistant to spot welding LME cracks and its preparation method, which prevents liquid metal from penetrating into the parent material along the grain boundary during welding by adding a low-carbon steel composite layer between the high-strength steel body and the galvanized layer, thereby alleviating the spot welding LME crack problem and improving the resistance spot welding performance of the high-strength steel body. However, this method has a high production cost. Summary of the invention

[0007] According to the technical problems raised above, a method for reducing LME cracks in GPa-grade galvanized high-strength steel is provided. The invention mainly achieves the purpose of suppressing LME cracks by optimizing the spot welding process, which is of great significance to the promotion and application of galvanized high-strength steel.

[0008] The technical means adopted by the present invention are as follows:

[0009] A method for reducing LME cracks in GPa-grade galvanized high-strength steel comprises the following steps:

[0010] Step 1: Select a GPa-grade galvanized high-strength steel plate, clean the surface to be welded with acetone or alcohol, remove impurities, and process it into a spot welding specimen;

[0011] Step 2: Select electrodes, set spot welding parameters, and weld the sample using a multi-stage spot welding process;

[0012] Step 3: After welding, test the LME crack density and maximum crack depth of the sample.

[0013] Furthermore, in step 2, the multi-stage spot welding process includes a first spot welding process and a second spot welding process. The first spot welding process is performed by using spot welding parameters of low current, long time and low electrode pressure. After a first interval time, the second spot welding process is used to weld the sample. The second spot welding process is performed by using spot welding parameters of multi-stage pulse current, short time and high electrode pressure.

[0014] Furthermore, the first spot welding process performs welding by using a one-stage first current.

[0015] Furthermore, in the first spot welding process, the electrode pressure is 3.2kN to 4.5kN, the first current is 3.0kA to 5.5kA, the welding time is 100ms to 500ms, and the cooling time is 100 to 300ms; during the welding process, the cooling water flow rate is 20 to 30L / min.

[0016] Furthermore, the second spot welding process uses a three-stage current for welding, and the three-stage current includes a first current, a second current and a third current. After welding with the first current, a second interval time is passed, and welding is performed with the second current. Then, a third interval time is passed, and welding is performed with the third current.

[0017] Furthermore, the first current, the second current and the third current are all greater than the welding current of the first spot welding process, and the electrode pressure of the second spot welding process is greater than the electrode pressure of the first spot welding process.

[0018] Furthermore, the first current, the second current and the third current are the same, all of which are 5.2-8.0 kA; the electrode pressure is 4.0-5.6 kN, the pulse time of each section is 70-180 ms, and the cooling time between two adjacent current sections is the same, all of which are 40 ms; during the welding process, the cooling water flow rate is 20-30 L / min.

[0019] Furthermore, in step 2, the electrode is a chromium-zirconium-copper conical electrode with an end face radius of 6.0 mm or 8.0 mm.

[0020] Furthermore, in the step one, the thickness of the GPa-grade galvanized high-strength steel plate is 1.2 to 1.9 mm, and the chemical composition by weight percentage is: C: 0.098%, Si: 0.28, Mn: 2.55, P: 0.008, S: 0.008, Cr: 0.52, Nb: 0.015, Ti: 0.028, the remainder is Fe and other inevitable impurities, the microstructure is martensite and bainite, and the mechanical properties are yield strength of 950 MPa, tensile strength of 1195 MPa, and elongation after fracture ≥14%.

[0021] Furthermore, the GPa-grade galvanized high-strength steel sheet is prepared by the following process: the high-strength steel substrate slab is heated, rough rolled, fine rolled, cooled, coiled, cold rolled and continuously annealed to obtain an annealed substrate, and the annealed substrate is subjected to hot-dip galvanizing, heat treatment and skin-finishing processes to prepare a GPa-grade galvanized high-strength steel sheet; the zinc liquid composition in the galvanizing process is Al: 7.1%, Si: 5.2%, Mg: 2.8%, and the remainder is Zn and unavoidable impurities, and the zinc layer thickness is 80-100g / m 3 .

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] 1. The present invention achieves the purpose of suppressing LME cracks by optimizing the spot welding process, which is of great significance to the promotion and application of galvanized steel and strong steel.

[0024] 2. The present invention can effectively improve the LME crack problem of galvanized high-strength steel spot welding, and the obtained spot welding joint has excellent mechanical properties, which has great promotion and application value in the automobile manufacturing industry.

[0025] 3. The present invention adopts two-stage current for welding, adjusts the electrode pressure, pulse current and pulse time in each stage, reduces the content of liquid zinc penetrating into the base material during the welding process, reduces stress concentration, reduces the number and depth of LME cracks, and improves the shear performance of spot welding joints.

[0026] 4. The present invention adopts two-stage welding, wherein the first stage selects spot welding parameters of low current, long time and low electrode pressure for welding. This process melts the zinc layer with a relatively small current density, evaporates part of the molten liquid zinc for a long time, removes part of the molten liquid zinc, and causes the galvanized layer to react with the steel plate to alloy, thereby reducing the liquid zinc that penetrates into the base material, and the low electrode pressure reduces the stress state on the base material surface. The second stage adopts three-stage pulse current, short time and high electrode pressure welding parameters to form a target size nugget. This two-stage welding process makes it difficult to meet the temperature, stress and liquid metal conditions required for the generation of liquid metal brittleness during resistance spot welding, ultimately suppressing liquid metal brittle cracks and improving the mechanical properties of spot welded joints.

[0027] Based on the above reasons, the present invention can be widely promoted in the fields of galvanized high-strength steel welding and the like. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0029] Figure 1 Flow chart of the method of the present invention.

[0030] Figure 2 It is a schematic diagram of the two-stage welding process of the present invention. DETAILED DESCRIPTION

[0031] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0032] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0034] Unless otherwise specifically stated, the relative arrangement, numerical expressions and numerical values ​​of the parts and steps set forth in these embodiments do not limit the scope of the present invention. Meanwhile, it should be clear that, for ease of description, the sizes of the various parts shown in the accompanying drawings are not drawn according to the actual proportional relationship. The technology, method and equipment known to those of ordinary skill in the relevant art may not be discussed in detail, but in appropriate cases, the technology, method and equipment should be considered as a part of the specification. In all examples shown and discussed here, any specific value should be interpreted as being merely exemplary, rather than as a limitation. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters represent similar items in the following drawings, and therefore, once a certain item is defined in an accompanying drawing, it does not need to be further discussed in subsequent drawings.

[0035] Example 1

[0036] When galvanized high-strength steel is resistance spot welded, as the heat input increases, the surface galvanized layer melts, and at the same time, under the action of higher electrode pressure, the molten liquid zinc penetrates into the parent material along the grain boundary, and LME cracks will appear in the spot welding joint. The density of LME cracks is usually 3-6 / cm, and the crack depth is usually 5-10μm. The presence of LME cracks causes the shear performance of the spot welding joint to be unable to exceed 20kN. In view of the shortcomings of the prior art, the present invention provides a method for reducing LME cracks in GPa-grade galvanized high-strength steel, that is, a resistance spot welding method for galvanized high-strength steel, and also a method for improving the performance of galvanized high-strength steel spot welding joints. The present invention achieves the purpose of suppressing LME cracks by optimizing the spot welding process, which is of great significance to the promotion and application of galvanized steel. The present invention can effectively improve the problem of LME cracks in spot welding of galvanized high-strength steel, and the obtained spot welding joint has excellent mechanical properties, which has great promotion and application value in the automobile manufacturing industry.

[0037] In the present invention, the thickness of the GPa-grade galvanized high-strength steel sheet is 1.2 mm, the chemical composition is as follows by weight percentage: C: 0.098%, Si: 0.28, Mn: 2.55, P: 0.008, S: 0.008, Cr: 0.52, Nb: 0.015, Ti: 0.028, the balance is Fe and other inevitable impurities, the microstructure is martensite and bainite, the mechanical properties are yield strength 950 MPa, tensile strength 1195 MPa, and elongation after fracture ≥ 14%. The high-strength steel substrate slab is heated, rough rolled, finished rolled, cooled, coiled, cold rolled and continuously annealed to obtain an annealed substrate, and the annealed substrate is hot-dip galvanized, heat treated and skin-passed to obtain a galvanized high-strength steel sheet. The zinc liquid composition in the galvanizing process is Al: 7.1%, Si: 5.2%, Mg: 2.8%, the balance is Zn and unavoidable impurities, and the zinc layer thickness is 90g / m 3 .

[0038] like Figure 1 As shown, the present invention provides a method for reducing LME cracks in GPa-grade galvanized high-strength steel, and the specific steps are as follows:

[0039] Step 1: Select the GPa-grade galvanized high-strength steel plate with the above composition, clean the surface to be welded with acetone or alcohol, remove impurities such as oil, and process it into a spot welding specimen.

[0040] Step 2: Select a chromium-zirconium-copper conical electrode with an end face radius of 6.0 mm, set spot welding parameters and perform welding, wherein the sample is welded by a multi-stage spot welding process.

[0041] Step 3: After welding, detect the LME crack density and maximum crack depth.

[0042] Preferably, in step 2, the multi-stage spot welding process includes a first spot welding process and a second spot welding process. The first spot welding process is performed by using spot welding parameters of low current, long time and low electrode pressure. After a first interval time, the second spot welding process is used to weld the sample. The second spot welding process uses spot welding parameters of multi-stage pulse current, short time and high electrode pressure for welding.

[0043] Specifically, Figure 2As shown, the present invention adopts a two-stage welding. The first stage selects spot welding parameters of low current, long time and low electrode pressure for welding. This process melts the zinc layer with a relatively small current density, evaporates part of the molten liquid zinc for a long time, removes part of the molten liquid zinc, and causes the galvanized layer to react with the steel plate to alloy, thereby reducing the liquid zinc that penetrates into the base material. The low electrode pressure reduces the stress state on the surface of the base material. The second stage adopts three-stage pulse current, short time and high electrode pressure welding parameters to form a target size nugget. The two-stage welding process of the present invention makes it difficult to meet the temperature, stress and liquid metal conditions required for the generation of liquid metal brittleness in the resistance spot welding process, and ultimately suppresses the liquid metal brittle cracks and improves the mechanical properties of the spot welding joint.

[0044] Preferably, the first spot welding process is welded by a one-stage first current. In the first spot welding process, the electrode pressure is 3.2kN to 4.5kN, the first current is 3.0kA to 5.5kA, the welding time is 100ms to 500ms, and the cooling time is 100 to 300ms; during the welding process, the cooling water flow rate is 20 to 30L / min. The combination of low electrode pressure, low current and long welding time in the first spot welding process can preheat the sample, reduce the thermal shock caused by the instantaneous temperature rise of high electrode pressure and high current during the second spot welding, and lead to higher stress concentration. At the same time, the first short spot welding process also helps to remove Zn from the surface of the sample, reducing the generation of LME cracks from the basic conditions for the generation of liquid metal.

[0045] Preferably, the second spot welding process uses a three-stage current for welding, and the three-stage current includes a first current, a second current and a third current. After welding with the first current, a second interval time is passed, and welding is performed with the second current. Then, a third interval time is passed, and welding is performed with the third current.

[0046] Preferably, the first current, the second current and the third current are all greater than the welding current of the first spot welding process, and the electrode pressure of the second spot welding process is greater than the electrode pressure of the first spot welding process. Among them, the first current, the second current and the third current are the same, all 5.2-8.0kA; the electrode pressure is 4.0-5.6kN, the pulse time of each section is 70-180ms, and the cooling time between two adjacent current sections is the same, all 40ms; during welding, the cooling water flow rate is 20-30L / min. The second spot welding process uses a larger welding pressure to increase the current density of the weld, and with a larger welding time and welding current, the molten core obtains enough energy to grow, thereby improving the welding mechanical properties.

[0047] Example 2

[0048] In this embodiment, a method for reducing LME cracks in GPa-grade galvanized high-strength steel includes the following steps:

[0049] Step 1: Select a GPa-grade galvanized high-strength steel plate with a predetermined composition, clean the surface to be welded with acetone or alcohol, remove impurities such as oil, and process it into a spot welding specimen.

[0050] Step 2: Select a chromium-zirconium-copper conical electrode with an end radius of 6.0 mm, set the spot welding parameters and perform welding; cooling water flow: 26 L / min, the first stage spot welding process is as follows: electrode pressure 3.2 kN, welding current 3.0 kA, welding time 800 ms, cooling time 300 ms. The second stage spot welding process is as follows: three-stage welding current, welding pressure 5.6 kN, welding current 5.2 kA, each pulse time 180 ms, cooling time 40 ms.

[0051] Step 3: Shear performance test: The maximum shear force of the spot welding joint is 22.48kN. LME crack detection: The crack density is 2.3 / cm and the maximum crack depth is 2.0μm.

[0052] Example 3

[0053] In this embodiment, a method for reducing LME cracks in GPa-grade galvanized high-strength steel includes the following steps:

[0054] Step 1: Select a GPa-grade galvanized high-strength steel plate with a predetermined composition, clean the surface to be welded with acetone or alcohol, remove impurities such as oil, and process it into a spot welding specimen.

[0055] Step 2: Select a chromium-zirconium-copper conical electrode with an end radius of 6.0 mm, set the spot welding parameters and perform welding; cooling water flow: 26 L / min, the first stage of spot welding process is as follows: electrode pressure 3.5 kN, welding current 3.5 kA, welding time 500 ms, cooling time 300 ms. The second stage of spot welding process is as follows: three-stage welding current, welding pressure 5.6 kN, welding current 5.2 kA, each pulse time 180 ms, cooling time 40 ms.

[0056] Step 3: Shear performance test: The maximum shear force of the spot welding joint is 20.88kN. LME crack detection: The crack density is 2.5 / cm and the maximum crack depth is 2.3μm.

[0057] Comparative Example 1

[0058] In this comparative example 1, the method is:

[0059] Step 1: Select a GPa-grade galvanized high-strength steel plate with a predetermined composition, clean the surface to be welded with acetone or alcohol, remove impurities such as oil, and process it into a spot welding specimen.

[0060] Step 2: Select a chromium-zirconium-copper conical electrode with an end face radius of 6.0 mm, set spot welding parameters and perform welding; cooling water flow: 26 L / min, three-stage welding current, welding pressure 5.6 kN, welding current 5.2 kA, each pulse time 180 ms, cooling time 40 ms.

[0061] Step 3: Shear performance test: The maximum shear force of the spot welding joint is 17.58kN. LME crack detection: The crack density is 5.5 / cm and the maximum crack depth is 4.4μm.

[0062] From the above, it can be seen that the present invention adopts two-stage current for welding. By adjusting the electrode pressure, pulse current and pulse time in each stage, the content of liquid zinc penetrating into the base material during the welding process can be reduced, stress concentration can be reduced, the number and depth of LME cracks can be reduced, and the shear performance of the spot welding joint can be improved.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for reducing LME cracks in GPa-grade galvanized high-strength steel, characterized in that: The steps include: Step 1: Select a GPa-grade galvanized high-strength steel plate, clean the surface to be welded with acetone or alcohol, remove impurities, and process it into a spot welding specimen; Step 2: Select electrodes, set spot welding parameters, and weld the sample using a multi-stage spot welding process; Step 3: After welding, test the LME crack density and maximum crack depth of the sample.

2. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 1, characterized in that: In the step 2, the multi-stage spot welding process includes a first-stage spot welding process and a second-stage spot welding process. The first-stage spot welding process is performed by using spot welding parameters of low current, long time and low electrode pressure. After a first interval time, the second-stage spot welding process is used to weld the sample. The second-stage spot welding process is performed by using spot welding parameters of multi-stage pulse current, short time and high electrode pressure.

3. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 2, characterized in that: The first spot welding process performs welding by using a one-stage first current.

4. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 3, characterized in that: In the first spot welding process, the electrode pressure is 3.2kN~4.5kN, the first current is 3.0kA~5.5kA, the welding time is 100ms~500ms, and the cooling time is 100~300ms; during the welding process, the cooling water flow rate is 20~30L / min.

5. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 2, characterized in that: The second spot welding process uses a three-stage current for welding, and the three-stage current includes a first current, a second current and a third current. After welding with the first current, a second interval time is interval, and welding is performed with the second current. Then, a third interval time is interval, and welding is performed with the third current.

6. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 5, characterized in that: The first current, the second current and the third current are all greater than the welding current of the first spot welding process, and the electrode pressure of the second spot welding process is greater than the electrode pressure of the first spot welding process.

7. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 5 or 6, characterized in that: The first current, the second current and the third current are the same, all of which are 5.2-8.0 kA; the electrode pressure is 4.0-5.6 kN, the pulse time of each section is 70-180 ms, and the cooling time between two adjacent current sections is the same, all of which are 40 ms; during the welding process, the cooling water flow rate is 20-30 L / min.

8. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 1, characterized in that: In the step 2, the electrode is a chromium-zirconium-copper conical electrode with an end face radius of 6.0 mm or 8.0 mm.

9. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 1, characterized in that: In the step 1, the thickness of the GPa-grade galvanized high-strength steel plate is 1.2-1.9 mm, and the chemical composition is as follows by weight percentage: C: 0.098%, Si: 0.28, Mn: 2.55, P: 0.008, S: 0.008, Cr: 0.52, Nb: 0.015, Ti: 0.028, the remainder is Fe and other inevitable impurities, the microstructure is martensite and bainite, and the mechanical properties are yield strength of 950 MPa, tensile strength of 1195 MPa, and elongation after fracture ≥14%.

10. The method for reducing LME cracks in GPa-grade galvanized high-strength steel according to claim 9, characterized in that: The GPa-grade galvanized high-strength steel sheet is prepared by the following process: the high-strength steel substrate slab is subjected to heating, rough rolling, finish rolling, cooling, coiling, cold rolling and continuous annealing to obtain an annealed substrate, and the annealed substrate is subjected to hot-dip galvanizing, heat treatment and skin-finishing processes to prepare the GPa-grade galvanized high-strength steel sheet; the zinc liquid composition in the galvanizing process is Al: 7.1%, Si: 5.2%, Mg: 2.8%, and the balance is Zn and unavoidable impurities, and the zinc layer thickness is 80-100 g / m 3 .

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