Method for reducing brittle cracks of liquid metal during welding

Abstract

The invention provides a method for reducing brittle cracks of liquid metal during welding, which comprises the following steps of: 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; 2, selecting a spherical electrode; thirdly, welding is conducted through a multi-section type spot welding technology; and 4, after welding is finished, the LME crack density and the maximum crack depth of the sample are detected. According to the method, stress concentration in the spot welding process can be reduced, the number and depth of LME cracks are reduced, and the shearing performance of a spot-welded joint is improved.

Description

Technical Field

[0001] The invention relates to the technical field of high-strength steel welding technology, and in particular to a method for reducing brittle cracks of liquid metal during welding. Background Art

[0002] With the continuous innovation of automobile production technology, automobile lightweighting has become an important goal of research and development and manufacturing. In this context, advanced high-strength steel has become the main material in automobile manufacturing, especially GPa-grade high-strength steel, a high-strength-to-weight ratio product, which is favored by automobile manufacturers. In order to improve the corrosion resistance, the surface of GPa-grade high-strength steel for automobiles is often plated with a zinc layer, but the presence of the zinc layer often reduces the performance of high-strength steel after welding. This is because the coating will inevitably enter the high-strength steel matrix under the heat and stress of the welding process, resulting in liquid metal embrittlement (LME) phenomenon, which reduces the mechanical properties of the welded joint during use. LME is a phenomenon in which two metals with different melting points melt under the action of heat and the low-melting-point metal contacts with 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 causing plasticity reduction. Galvanized high-strength steel is very prone to LME cracks during resistance spot welding.

[0003] When galvanized high-strength steel is subjected to resistance spot welding, the base metal softens locally as the resistance heat continues to increase. The softened position is sunken under the action of welding pressure, and the electrode head sinks. At this time, a higher stress is generated at the edge of the electrode head in contact with the base metal. Traditional resistance spot welding uses round flat electrodes, such as Figure 2 As shown. During the welding process, the edge of this electrode in contact with the galvanized high-strength steel has a large deformation degree and high stress, and LME cracks are often generated. 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.

[0004] CN115106633A "A resistance spot welding method for hot-dip galvanized hot-formed steel" uses spot welding pre-pressure and two welding pulses to suppress the formation of liquid metal embrittlement (LME) cracks. This method has high requirements for spot welding equipment.

[0005] CN108015401A "Resistance spot welding method for galvanized high-strength steel with good joint performance" uses three welding pulses to suppress the generation of liquid metal embrittlement (LME) cracks. This method has high requirements for spot welding equipment.

[0006] 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 has high requirements for steel production equipment. Summary of the invention

[0007] According to the technical problems raised above, a method for reducing brittle cracks of liquid metal during welding is provided. The present invention mainly achieves the purpose of inhibiting and reducing LME cracks by optimizing the geometric dimensions of spot welding joints and combining spot welding processes, which is of great significance to the promotion and application of galvanized steel and strong steel.

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

[0009] A spot welding joint for reducing brittle cracks of liquid metal during welding comprises the following steps:

[0010] Step 1: Select 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 a spherical electrode;

[0012] Step 3: Set the spot welding parameters and weld the sample through a multi-stage spot welding process;

[0013] Step 4: After welding, test the LME crack density and maximum crack depth of the sample.

[0014] Furthermore, the spherical electrode is an electrode with a spherical head, and the curvature of the spherical shape is 90°-100°.

[0015] Furthermore, in step three, a three-stage spot welding process is used to weld the sample.

[0016] Furthermore, the three-stage spot welding process includes a first spot welding process, a second spot welding process and a third spot welding process, and the first spot welding process, the second spot welding process and the third spot welding process are performed at intervals.

[0017] Furthermore, the welding pressure of the first spot welding process is 3.0-4.5 kN, the slow-rise current is 3.0-3.5 kA, the welding time is 60-120 ms, the welding current is 4.0-5.0 kA, and the cooling time is 100-200 ms.

[0018] Furthermore, the second-stage spot welding process is as follows: welding pressure is 4.5-6.0 kN, slow-rise current is 5.0-7.0 kA, welding time is 160-260 ms, welding current is 7.5-9.5 kA, and cooling time is 0-80 ms.

[0019] Furthermore, the welding pressure of the third stage spot welding process is 3.0-4.5 kN, the slow-rise current is 3.0-3.5 kA, the welding time is 60-120 ms, the welding current is 4.0-5.0 kA, and the cooling time is 100-200 ms.

[0020] Furthermore, in step three, during the welding process, the cooling water flow rate is 20 to 30 L / min.

[0021] Furthermore, in the step one, the thickness of the GPa-grade galvanized high-strength steel plate is 1.2 to 2.0 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%.

[0022] 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, finish-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-finished 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 80-100 g / m 3 .

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The present invention reduces stress concentration during spot welding by improving the geometric shape of the electrode head, reduces the number and depth of LME cracks, and improves the shear performance of the spot welding joint.

[0025] 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.

[0026] 3. The present invention redesigns the geometric shape of the spot welding joint, adopts a spherical electrode, R = 90-100°, and when the electrode head with this geometric shape is used for resistance spot welding, the deformation degree of the parent material at the contact edge of the electrode head and the parent material can be reduced, and the stress concentration can be reduced, thereby reducing the number and depth of LME cracks in the spot welding joint. At the same time, it is combined with a three-stage spot welding process, in which the first stage of the welding process adopts a smaller welding pressure to reduce the current density of the welding point, and the purpose of combining with a shorter welding time and welding current is to preheat the sample and reduce the thermal shock of the instantaneous temperature rise during the second stage of welding on the sample, so as to reduce stress concentration and cause LME cracks. The second stage of the spot welding process adopts a larger welding pressure to increase the current density of the welding point, and with a larger welding time and welding current, the molten core obtains enough energy to grow, thereby improving the welding mechanical properties. The third stage of the welding process adopts a smaller welding pressure to reduce the current density of the welding point, and the purpose of combining with a shorter welding time and welding current is to prevent stress concentration caused by rapid cooling of the sample and reduce the generation of LME cracks. The setting of the slow current in the three-stage welding process is to reduce the thermal shock caused by the instantaneous temperature rise and reduce the LME cracks caused by stress concentration.

[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 Schematic diagram of the structure of a round flat head electrode.

[0031] Figure 3 Schematic diagram of the structure of the hemispherical electrode of the present invention. DETAILED DESCRIPTION

[0032] 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.

[0033] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions 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.

[0034] 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.

[0035] 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.

[0036] Example 1

[0037] When galvanized high-strength steel is subjected to resistance spot welding, the base metal softens locally as the resistance heat continues to increase. The softened position is sunken under the action of welding pressure, and the electrode head sinks. At this time, a higher stress is generated at the edge of the electrode head in contact with the base metal. Traditional resistance spot welding uses round flat electrodes, such as Figure 2As shown. During the welding process, the edge of this electrode in contact with the galvanized high-strength steel has a large degree of deformation and high stress, and LME cracks are often generated. 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 deficiencies of the prior art, the present invention provides a method for reducing brittle cracks of liquid metal during welding, which 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. By improving the geometric shape of the electrode head, the stress concentration during the spot welding process is reduced, the number and depth of LME cracks are reduced, and the shear performance of the spot welding joint is improved.

[0038] In the present invention, the thickness of the GPa-grade galvanized high-strength steel is 1.2 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 950 MPa, tensile strength 1195 MPa, and elongation after fracture ≥ 14%. 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 hot-dip galvanized, heat treated and smoothed 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%, and the remainder is Zn and inevitable impurities. The thickness of the zinc layer is 90 g / m 3 .

[0039] like Figure 1 As shown, the present invention provides a method for reducing brittle cracks of liquid metal during welding, and the specific steps are as follows:

[0040] 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.

[0041] Step 2: Select a spherical electrode.

[0042] Step 3: Set the three-stage welding current and weld the sample.

[0043] Step 4: After welding, detect the LME crack density and maximum crack depth.

[0044] Preferably, the spherical electrode is an electrode with a spherical head, and the curvature of the spherical shape is 90°-100°.

[0045] Preferably, in step 3, the sample is welded using a three-stage spot welding process, which includes a first spot welding process, a second spot welding process and a third spot welding process, wherein the first spot welding process, the second spot welding process and the third spot welding process are performed at intervals.

[0046] Preferably, the welding pressure of the first section spot welding process 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 second section spot welding process is as follows: welding pressure 4.5-6.0kN, slow rise current 5.0-7.0kA, welding time 160-260ms, welding current 7.5-9.5kA, cooling time 0-80ms. The welding pressure of the third section spot welding process 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.

[0047] The first welding process uses a smaller welding pressure to reduce the current density of the weld. The purpose of the shorter welding time and welding current is to preheat the sample and reduce the thermal shock caused by the instantaneous temperature rise during the second welding to reduce the stress concentration that leads to LME cracks. The second spot welding process uses a larger welding pressure to increase the current density of the weld. With a longer welding time and welding current, the molten nugget obtains enough energy to grow, thereby improving the welding mechanical properties. The third welding process uses a smaller welding pressure to reduce the current density of the weld. The purpose of the shorter welding time and welding current is to prevent stress concentration caused by rapid cooling of the sample and reduce the occurrence of LME cracks. The setting of the slow current in the three welding processes is to reduce the thermal shock caused by the instantaneous temperature rise and reduce the LME cracks caused by stress concentration.

[0048] Preferably, in step three, during the welding process, the cooling water flow rate is 20 to 30 L / min.

[0049] The present invention redesigns the geometric shape of the spot welding joint, adopts a spherical electrode, R = 90°-100°, and the radius of the spherical electrode in this angle range is approximately equivalent to the contact area between the 6.0mm round flat electrode and the sample during welding. Figure 3As shown, when the electrode head with this geometric shape is used for resistance spot welding, the deformation degree of the base material at the contact edge of the electrode head and the base material can be reduced, the stress concentration can be reduced, and the number and depth of LME cracks in the spot welding joint can be reduced. At the same time, in conjunction with the two-stage spot welding process, the first stage mainly functions to preheat the galvanized high-strength steel plate, initially form a molten core, reduce the thermal shock under high heat input during the second stage welding process, and reduce stress concentration. The second stage mainly functions to grow the galvanized high-strength steel molten core and expand the cross-sectional area of ​​the molten core, thereby improving the shear performance of the spot welding joint and improving the performance of the galvanized high-strength steel spot welding joint.

[0050] The method of reducing liquid metal brittle cracks during welding can effectively improve the LME crack problem of galvanized high-strength steel spot welding, and the obtained spot welding joint has excellent mechanical properties and has great promotion and application value in the automobile manufacturing industry.

[0051] Example 2

[0052] In this embodiment, a method for reducing brittle cracks of liquid metal during welding includes the following steps:

[0053] 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.

[0054] Step 2: Select a chromium-zirconium-copper hemispherical electrode, R=90°.

[0055] Step 3: Set the spot welding parameters and perform welding; Cooling water flow rate: 26L / min, the first spot welding process is as follows: welding pressure 3.5kN, slow rise current 3.0kA, welding time 90ms, welding current 4.5kA, cooling time 180ms. The second spot welding process is as follows: welding pressure 5.2kN, slow rise current 5.5kA, welding time 220ms, welding current 8.2kA, cooling time 40ms. The third spot welding process is as follows: welding pressure 3.5kN, slow rise current 3.0kA, welding time 90ms, welding current 4.5kA, cooling time 180ms.

[0056] Step 4: Shear performance test: The maximum shear force of the spot welding joint is 23.55kN. LME crack detection: The crack density is 2.0 lines / cm and the maximum crack depth is 1.9μm.

[0057] Example 3

[0058] In this embodiment, a method for reducing brittle cracks of liquid metal during welding includes the following steps:

[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 hemispherical electrode, R=100°.

[0061] Step 3: Set the spot welding parameters and perform welding; Cooling water flow rate: 26L / min, the first spot welding process is as follows: welding pressure 4.0kN, slow rise current 3.2kA, welding time 90ms, welding current 5.0kA, cooling time 180ms. The second spot welding process is as follows: welding pressure 5.2kN, slow rise current 5.5kA, welding time 220ms, welding current 8.2kA, cooling time 40ms. The third spot welding process is as follows: welding pressure 4.0kN, slow rise current 3.2kA, welding time 90ms, welding current 5.0kA, cooling time 180ms.

[0062] Step 4: Shear performance test: The maximum shear force of the spot welding joint is 23.55kN. LME crack detection: The crack density is 1.8 / cm and the maximum crack depth is 2.2μm.

[0063] Comparative Example 1

[0064] In this comparative example 1, the method is:

[0065] 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.

[0066] Step 2: Select a chromium-zirconium-copper spherical electrode with an end face diameter of 6.0 mm.

[0067] Step 3: Set the spot welding parameters and perform welding; the spot welding process is as follows: welding pressure 5.2kN, slow rise current 5.5kA, welding time 220ms, welding current 8.2kA, cooling time 40ms.

[0068] Step 4: Shear performance test: The maximum shear force of the spot welding joint is 18.02kN. LME crack detection: The crack density is 4.2 / cm and the maximum crack depth is 5.4μm.

[0069] It can be seen from the above that the present invention achieves the purpose of suppressing LME cracks by optimizing the geometric dimensions of the spot welding joint and combining the three-stage spot welding process, which is of great significance to the promotion and application of galvanized steel and strong steel.

[0070] 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 brittle cracks in liquid metal during welding, characterized in that: The steps include: Step 1: Select 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 a spherical electrode; Step 3: Welding the sample by multi-stage spot welding process; Step 4: After welding, test the LME crack density and maximum crack depth of the sample.

2. The method for reducing brittle cracks of liquid metal during welding according to claim 1, characterized in that: The spherical electrode is an electrode with a spherical head, and the curvature of the spherical shape is 90°-100°.

3. The method for reducing brittle cracks of liquid metal during welding according to claim 1, characterized in that: In the step three, the sample is welded using a three-stage spot welding process.

4. The method for reducing brittle cracks of liquid metal during welding according to claim 3, characterized in that: The three-stage spot welding process includes a first-stage spot welding process, a second-stage spot welding process and a third-stage spot welding process, and the first-stage spot welding process, the second-stage spot welding process and the third-stage spot welding process are performed at intervals.

5. The method for reducing brittle cracks of liquid metal during welding according to claim 4, characterized in that: The welding pressure of the first stage spot welding process is 3.0-4.5 kN, the slow-rise current is 3.0-3.5 kA, the welding time is 60-120 ms, the welding current is 4.0-5.0 kA, and the cooling time is 100-200 ms.

6. The method for reducing brittle cracks of liquid metal during welding according to claim 4, characterized in that: The welding pressure of the second spot welding process is 4.5-6.0 kN, the slow-rise current is 5.0-7.0 kA, the welding time is 160-260 ms, the welding current is 7.5-9.5 kA, and the cooling time is 0-80 ms.

7. The method for reducing brittle cracks of liquid metal during welding according to claim 4, characterized in that: The welding pressure of the third stage spot welding process is 3.0-4.5 kN, the slow-rise current is 3.0-3.5 kA, the welding time is 60-120 ms, the welding current is 4.0-5.0 kA, and the cooling time is 100-200 ms.

8. The method for reducing brittle cracks of liquid metal during welding according to claim 1, characterized in that: In step three, during the welding process, the cooling water flow rate is 20 to 30 L / min.

9. The method for reducing brittle cracks of liquid metal during welding according to claim 1, characterized in that: In the step 1, the thickness of the GPa-grade galvanized high-strength steel plate is 1.2-2.0 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 brittle cracks of liquid metal during welding 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-passing 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 .

Images