Resistance spot welding method for aluminum-silicon coated hot-formed steel having tensile strength of 1,700 mpa or above

Abstract

Disclosed in the present invention is a resistance spot welding method for aluminum-silicon coated hot-formed steel having a tensile strength of 1,700 MPa or above, comprising: when the thickness T of an alloy layer, very close to a substrate, in a coating layer of aluminum-silicon coated hot-formed steel is less than 5 μm, using single-pulse welding; when the thickness T of the alloy layer, very close to the substrate, in the coating layer of the aluminum-silicon coated hot-formed steel is greater than or equal to 5 μm, using multi-pulse welding, and, as the welding process progresses, the current of each welding pulse gradually increasing, wherein the number of welding pulses, n, and the thickness T of the alloy layer, very close to the substrate, in the coating layer of the aluminum-silicon coated hot-formed steel satisfy: when 5 μm ≤ T < 10 μm, n = 2-3; when 10 μm ≤ T < 15 μm, n = 3-4; when 15 μm ≤ T < 20 μm, n = 4-6; when 20 μm ≤T < 25 μm, n = 5-7; when 25 μm ≤ T < 30 μm, n = 6-9; and when 30 μm ≤ T, n = 7-10.

Description

Resistance spot welding method for hot-formed aluminum-silicon coated steel with tensile strength of 1700MPa or higher Technical Field

[0001] This invention relates to a welding process, and more particularly to a resistance spot welding method for coated hot-formed steel. Background Technology

[0002] Hot-formed steel, as an advanced high-strength steel, is used extensively in the automotive industry. In order to improve the corrosion resistance of hot-formed steel, the application of aluminum-silicon coated hot-formed steel is also increasing.

[0003] Hot-formed steel has a high carbon equivalent and high electrical resistance, resulting in rapid heat generation during spot welding and a tendency to cause welding spatter and an excessively narrow weldable area. Therefore, some existing technologies have attempted to control the spot welding of hot-formed steel, for example:

[0004] Chinese patent document CN116117294A, published on May 16, 2023, entitled "Resistance Spot Welding Process for 2000MPa Grade Hot-Formed Steel with Aluminum-Silicon Coating", discloses a method for improving the CTS strength of weld points of 2000MPa aluminum-silicon coated hot-formed steel using a traditional three-pulse method, but the method does not disclose the effect on the width of the weldable section.

[0005] Chinese patent document CN112247332A, published on January 2, 2021, entitled "A Resistance Spot Welding Method for Ultra-High Strength Hot-Formed Steel Plate", discloses a method for improving the CTS strength of hot-formed steel welds by using preheating and a three-step pulse method. However, this method is applicable to uncoated hot-formed steel.

[0006] However, in hot-formed aluminum-silicon coated steel, an alloy layer containing Al, Si, and Fe exists in the coating close to the substrate. This alloy layer has poor conductivity, severely affecting spot welding performance and making it more prone to spattering and electrode explosion during spot welding. This results in an excessively narrow weldable area, failing to meet welding production requirements. Furthermore, the thickness of this alloy layer increases with the heating temperature and holding time of the hot stamping process; the thicker the alloy layer, the worse the spot welding performance. Existing methods cannot solve the spot welding performance problem of aluminum-silicon coated hot-formed steel. Summary of the Invention

[0007] One of the objectives of this invention is to provide a resistance spot welding method for hot-formed steel with aluminum-silicon coating and a tensile strength of 1700MPa or higher. This method can better suppress spot welding problems such as spatter and electrode explosion, thereby increasing the width of its weldable range and meeting the requirements of actual welding production.

[0008] Because hot-formed steel has a high carbon equivalent and high electrical resistance, it generates heat quickly during spot welding, easily leading to welding spatter and an excessively narrow weldable area. Hot-formed steel with an aluminum-silicon coating is even more prone to these problems. This is because the coating of hot-formed aluminum-silicon steel is a multi-layered structure, containing an alloy layer (also known as an "interdiffusion layer," see CN 101583486 B) containing Al, Si, and Fe, which increases in thickness with the heating temperature and holding time of the hot stamping process. This alloy layer itself has poor conductivity, severely affecting resistance spot welding performance. Furthermore, the coating of hot-formed aluminum-silicon steel is prone to severe surface oxidation, increased porosity, excessively thick coating, and poor overall conductivity caused by the hot stamping process, thus failing to meet welding production requirements (generally requiring a weldable area width greater than 1000 Å).

[0009] The thicker the alloy layer in close contact with the substrate in the above-mentioned coating, the worse its conductivity. At the same time, the thickness of the alloy layer can indirectly indicate whether the coating has problems such as severe surface oxidation, increased porosity defects, and overall excessive thickness of the coating caused by the hot stamping process, resulting in poor overall conductivity of the coating, and the degree of these problems. In other words, the thicker the alloy layer, the more serious the above-mentioned coating problems are, and the worse the overall conductivity of the coating is.

[0010] Based on the above characteristics of aluminum-silicon coated hot-formed steel, this invention uses the thickness of the alloy layer in the coating that is in close contact with the substrate to determine the spot welding performance of aluminum-silicon coated hot-formed steel, and formulates corresponding spot welding schemes and countermeasures to improve the weldability of the material.

[0011] To achieve the above objectives, based on the above design concept, this invention provides a resistance spot welding method for hot-formed steel with an aluminum-silicon coating and a tensile strength of 1700 MPa or higher, comprising:

[0012] When the thickness T of the alloy layer adjacent to the substrate in the coating of aluminum-silicon coated hot-formed steel is less than 5 μm, single-pulse welding is used;

[0013] When the thickness T of the alloy layer adjacent to the substrate in the aluminum-silicon coated hot-formed steel coating is ≥ 5 μm, multi-pulse welding is used, and the current of each welding pulse gradually increases as the welding process progresses. The number of welding pulses n and the thickness T of the alloy layer adjacent to the substrate in the aluminum-silicon coated hot-formed steel coating satisfy the following:

[0014] When 5μm≤T<10μm, n=2~3;

[0015] When 10μm≤T<15μm, n=3~4;

[0016] When 15μm≤T<20μm, n=4~6;

[0017] When 20μm≤T<25μm, n=5~7;

[0018] When 25μm≤T<30μm, n=6~9;

[0019] When 30μm≤T, n=7~10.

[0020] In this invention, the number of welding pulses n is determined by the thickness T of the alloy layer in the coating that is in close contact with the substrate after the aluminum-silicon coated hot-formed steel has completed hot forming. The thicker the alloy layer T in the coating of the aluminum-silicon coated hot-formed steel that is in close contact with the substrate, the worse the conductivity of the coating and the greater the resistance.

[0021] When the alloy layer thickness T is less than 5 μm, the coating condition is good, and conventional single-pulse spot welding process can be used. Therefore, the pulse number n is set to 1.

[0022] However, as the alloy layer thickness T increases, the coating resistance also increases. Using a large heat input in the initial welding stage can easily lead to excessively rapid heat generation, resulting in premature welding spatter. Therefore, it is necessary to apply heat input gradually, first slowly melting the coating and the less conductive alloy layer, and then applying a relatively larger heat input to form a weld nugget. Since the heat in resistance spot welding comes from resistance heat (Q = I...),... 2 When R is constant, the magnitude of resistance heat depends on the welding current and welding time. Therefore, in this technology, when T≥5μm, a stepped multi-pulse welding scheme is adopted, that is, "as the welding process progresses, the current of each welding pulse gradually increases".

[0023] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, when the welded joint has two or more layers of aluminum-silicon coated hot-formed steel, and the different layers of aluminum-silicon coated hot-formed steel have different alloy layer thicknesses T, the maximum value of T is taken.

[0024] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, for aluminum-silicon coated hot-formed steel with a determined plate thickness, the time WT1 of the first welding pulse is inversely related to the number of welding pulses n.

[0025] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the duration of each welding pulse gradually increases as the welding process progresses.

[0026] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the time WT of each welding pulse is... a Satisfy the following formula:

[0027] In the formula, n is the number of welding pulses, ms represents the unit parameter "millisecond", t represents the plate thickness of aluminum-silicon coated hot-formed steel in mm, and a represents the a-th pulse in n welding pulses, a = 1, 2, ... n.

[0028] The time for each welding pulse is WT. a (1≤a≤n, representing the a-th pulse out of n pulses) Parameters in milliseconds (ms).

[0029] The time WT for each welding pulse can be seen from the above formula. a The duration WT of each welding pulse is determined by the number of welding pulses n, the thickness t of the aluminum-silicon coated hot-formed steel, and a. a It increases gradually with the increase of a. Furthermore, for each defined welding pulse, WT... a It increases with the increase of plate thickness t.

[0030] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, for a 2-layer plate welded joint, t is taken as the smaller value of the plate thickness of the 2-layer plate; for a welded joint with more than 2 layers of plates, t is taken as the average value of the thickness of all steel plates.

[0031] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, for aluminum-silicon coated hot-formed steel with a determined plate thickness, the welding current I1 of the first pulse is inversely related to the number of welding pulses n.

[0032] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, as the welding process progresses, the magnitude of the increase in current for each welding pulse also gradually increases.

[0033] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the current I of each welding pulse... a Satisfy the following formula:

[0034] When the welding pulse number n = 1, I a =I b ;

[0035] When the number of welding pulses n>1 and a=1

[0036] When the number of welding pulses n>1 and a>1

[0037] Where I b To set the welding current, 'a' represents the 'a'-th pulse out of n welding pulses, where a = 1, 2, ..., n, and I1 represents the current of the first welding pulse. a This represents the welding current for each welding pulse.

[0038] As can be seen from the above formula, the welding current I for each welding pulse a The value is determined by the number of pulses n and a. Furthermore, it can be seen that when the plate thickness t is constant, the larger n is, the smaller the welding current I1 of the first pulse, and the smaller the welding current I of each welding pulse. a It increases with increasing a, and the magnitude of the increase increases with increasing a, but decreases with increasing n. It should be noted that I... b This refers to the welding current when n = 1 (i.e., T < 5 μm), where WT1 = t * 150 ms. When n > 1, it can be determined according to I... b The value is determined by setting the current I for each welding pulse according to the formula above. a . When I b When the value of I changes a You need to reset the settings according to the formulas above.

[0039] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the welding current I is set... b The range is 3kA to 13kA.

[0040] In some implementations, I b It is related to the actual welding pressure, electrode end face diameter, and plate thickness t. The larger the welding pressure, electrode end face diameter, and t, the greater I... b The larger. Those skilled in the art can, based on actual needs and the above principles, [details about I]. b The range is 3kA to 13kA.

[0041] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, a medium-frequency DC spot welding machine is used for welding.

[0042] In other embodiments, an AC spot welding machine can also be used for welding, and the controller of the AC or DC spot welding machine has a maximum welding pulse output of not less than 10.

[0043] It should be noted that when using an AC spot welding machine, the duration of each welding pulse (WT) is... a The parameter is cyc, where 1 cyc = 20 ms (50 Hz AC).

[0044] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the welding pressure used is 2kN to 8kN.

[0045] The welding pressure used is related to the thickness of the joint plate and the strength of the steel; the greater the plate thickness and strength, the greater the welding pressure. In some further embodiments, the welding pressure is 3kN to 6kN.

[0046] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the end face shape of the electrode used for welding is spherical, and the radius of the sphere is 40mm to 100mm.

[0047] In some further embodiments, the radius of the sphere is 40–60 mm.

[0048] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the electrode end face diameter used for welding is 5mm to 10mm. The electrode end face diameter used for welding is related to the plate thickness of the joint; the greater the plate thickness, the larger the end face diameter. In some further embodiments, the electrode end face diameter is 6mm to 8mm.

[0049] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the holding time after the welding energization is completed is 50ms to 500ms.

[0050] In some further embodiments, the holding time after the welding energization ends is 60ms to 250ms.

[0051] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention, the thickness of the aluminum-silicon coated hot-formed steel plate is 0.8mm to 3mm.

[0052] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention, the single-sided coating thickness of the aluminum-silicon coated hot-formed steel before hot forming is 10 g / m. 2 ~120g / m 2 .

[0053] Furthermore, in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the substrate of the aluminum-silicon coated hot-formed steel has a yield strength of 1100-1500 MPa, a tensile strength of 1700-2150 MPa, and an elongation after fracture ≥5% (50 gauge length) after hot forming.

[0054] Further, in the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention, the chemical element mass percentage content of the aluminum-silicon coated hot-formed steel substrate is as follows: C: 0.28-0.40%, Si≤0.80%, Mn: 1.0-2.5%, B≤0.005%, Cr≤0.50%, Ti≤0.10%, Mo≤0.40%, Al: 0.01-0.06%, with the balance being Fe and unavoidable impurity elements. In some embodiments, the Mn content in the aluminum-silicon coated hot-formed steel substrate is 1.5-2.2%. In some embodiments, the B content in the aluminum-silicon coated hot-formed steel substrate is 0.001-0.005%. In some embodiments, the Cr content in the aluminum-silicon coated hot-formed steel substrate is 0.20-0.40%. In some embodiments, the Ti content in the aluminum-silicon coated hot-formed steel substrate is 0.03-0.06%. In some embodiments, the Mo content in the substrate of the aluminum-silicon coated hot-formed steel is 0.15 to 0.40%.

[0055] It should be understood that in the resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention, the steel plates to be welded only need to include aluminum-silicon coated hot-formed steel with a tensile strength of 1700 MPa or higher as described herein; that is, the steel plates to be welded must at least include aluminum-silicon coated hot-formed steel with a tensile strength of 1700 MPa or higher. The two or more steel plates to be welded can both be aluminum-silicon coated hot-formed steel with a tensile strength of 1700 MPa or higher, or they can be aluminum-silicon coated hot-formed steel with a tensile strength of 1700 MPa or higher combined with other steel plates, such as various mild steels, high-strength steels, and ultra-high-strength steels. Exemplary uncoated steels may have the following chemical element mass percentages: C: 0.0001–0.20%, Si: 0.15–2.70%, Mn ≤ 0.50%, B ≤ 0.30%, Cr: 0.01–2.0%, Ti ≤ 0.08%, Mo ≤ 0.05%, Al ≤ 0.001%, Nb ≤ 0.70%, with the balance being Fe and unavoidable impurity elements. Such steels may have a yield strength of 120–1180 MPa, a tensile strength of 250–1380 MPa, and an elongation of 5.5–40%.

[0056] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention further includes the step of determining the thickness T of the alloy layer in close contact with the substrate in the aluminum-silicon coated hot-formed steel. In some embodiments, the alloy layer thickness T is detected by the following method: a cross-sectional metallographic image of the hot-formed steel plate is taken, ground and polished, etched with a 4% nitric acid alcohol solution, and then observed and measured under a metallographic microscope.

[0057] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention further includes the step of determining the thickness t of the steel plate to be welded.

[0058] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel of the present invention further includes testing to obtain a set welding current I. b The steps, in which I b The lower limit is that the diameter of the generated melt nucleus is 4t. 1 / 2 The welding current at the weld point is capped at the welding current when welding spatter occurs.

[0059] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention further includes the step of determining the number of welding pulses n based on T.

[0060] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention further includes determining the welding pulse time WT of the a-th pulse based on the plate thickness t and the number of welding pulses n. a The steps.

[0061] In some embodiments, the resistance spot welding method for aluminum-silicon coated hot-formed steel according to the present invention further includes, according to I b The current I for each welding pulse is determined by the number of pulses n. a The steps.

[0062] Compared with existing technologies, the resistance spot welding method for hot-formed steel with aluminum-silicon coating and a tensile strength of 1700MPa or higher described in this invention has the following advantages and beneficial effects:

[0063] The resistance spot welding method for aluminum-silicon coated hot-formed steel described in this invention can better suppress spot welding problems such as spatter and electrode explosion, thereby increasing the width of its weldable range and meeting the requirements of actual welding production. Attached Figure Description

[0064] Figure 1 shows a schematic diagram of the current for the stepped multi-pulse welding described in this invention. Detailed Implementation

[0065] The resistance spot welding method for hot-formed aluminum-silicon coated steel with a tensile strength of 1700MPa or higher described in this invention will be further explained and illustrated below with reference to specific embodiments. However, this explanation and illustration do not constitute an improper limitation on the technical solution of this invention.

[0066] Figure 1 shows a schematic diagram of the current for the stepped multi-pulse welding described in this invention.

[0067] As shown in Figure 1, in this invention, the number of welding pulses n is determined by the thickness T of the alloy layer in the coating that is in close contact with the substrate after the aluminum-silicon coated hot-formed steel has completed hot forming. In this paper, the method for detecting the alloy layer thickness T is as follows: a cross-sectional metallographic image of the hot-formed steel plate is taken, ground and polished, etched with a 4% nitric acid alcohol solution, and then observed and measured under a metallographic microscope.

[0068] The time for each welding pulse is WT. a (1≤a≤n, representing the a-th pulse out of n pulses) The number of pulses n, determined by T, the plate thickness t, and a are determined. When the plate thickness t is constant, the larger n is, the smaller the time WT1 of the first pulse, and the time WT of each welding pulse is... a It gradually increases as a increases;

[0069] The welding current I for each welding pulse a (1≤a≤n, representing the a-th pulse out of n pulses) The number of pulses n and a are determined by T. When the plate thickness t is constant, the larger n is, the smaller the welding current I1 of the first pulse, and the welding current I of each welding pulse... a It increases with the increase of a, and the magnitude of the increase increases with the increase of a, but decreases with the increase of n.

[0070] Examples 1-19

[0071] Examples 1-19 all employed a medium-frequency direct welding machine for welding 2150MPa grade aluminum-silicon coated hot-formed steel with different plate thicknesses and alloy layer thicknesses T. The objects of Examples 1-19 were all two-layer plate joints consisting of two 2150MPa grade aluminum-silicon coated hot-formed steel plates. When the aluminum-silicon coated hot-formed steel plates in the joint combination had different alloy layer thicknesses T, T was taken as the maximum value; when the steel plates in the joint combination had different plate thicknesses t, t was taken as the smaller value.

[0072] Table 1 lists the template information for the connectors in Examples 1-19.

[0073] Table 1.

[0074] Table 2 lists the substrate chemical composition ratios and properties of the 2150MPa grade aluminum-silicon coated hot-formed steel used in Examples 1-19.

[0075] Table 2.

[0076] Table 3 lists the process parameters used in Examples 1-19.

[0077] Table 3.

[0078] Based on the welding pressure, electrode parameters, and holding time of the above embodiments, according to the welding method of the present invention, the set welding current I for the joint combinations corresponding to Examples 1-19 when T < 5 μm is determined. a =I b Set the welding current I. b The lower limit was obtained through actual spot welding tests, and it was found to be a weld nugget diameter of 4t. 1 / 2 The welding current at the weld joint is capped at the welding current at which weld spatter occurs. (I of each embodiment) b The ranges are listed in Table 4. According to the welding method of the present invention, the number of pulses n and the welding time and welding current for each pulse are determined and listed in Table 4.

[0079] Tables 4-1, 4-2, and 4-3 list the I used in Examples 1-19 b The number of pulses n and the welding time and welding current for each pulse.

[0080] Table 4-1.

[0081] Table 4-2.

[0082] Table 4-3.

[0083] Based on the above parameter settings, spot welding was performed on the joints of Examples 1-19. According to the definition of the weldable range in this method, when n>1, the welding current I is set according to the setting suitable for multi-pulse welding at this time. b Actual range I b Define the solderable region at this point. The specific definition method is as follows: The lower limit of the solderable region is the region where, under these conditions, the diameter of the weld nugget generated is 4t. 1 / 2 The minimum I required for the solder joint b min', the upper limit of the weldable range is the minimum I that causes welding spatter under this condition. b max',I b max'-I b min' is the width of the weldable section at this point. Spot welding was performed on the joints of Examples 1-19, and the diameter of the resulting weld nugget was recorded as 4t. 1 / 2 The welding current of the weld joint is denoted as I. b min'; Record the welding current at which welding spatter occurs, denoted as I. b max'.

[0084] Table 5 lists the I of embodiments 1-19 of the present invention. b min' and I bmax' is used to calculate the width of the weldable section.

[0085] Table 5.

[0086] As can be seen from Table 5 above, the weldable range width of the joints in Examples 1-19 of this technology is greater than 1000A, which meets the welding production requirements.

[0087] Examples 20-31

[0088] Examples 20-31 all employ a medium-frequency DC welding machine. According to this welding method, combinations of 2-layer or 3-layer plates of 1800MPa grade aluminum-silicon coated hot-formed steel and other steel grades are used. In Examples 20-31, the 1800MPa grade aluminum-silicon coated hot-formed steel has different plate thicknesses and different alloy layer thicknesses T in close contact with the substrate. Other steel grades include mild steel, high-strength steel, and ultra-high-strength steel. When the aluminum-silicon coated hot-formed steel in the joint combination has different T values, the larger value of T is taken. When the steel plates in the joint combination have different thicknesses t, the smaller value of t is taken for 2-layer plate joints, and for joints with more than 2 layers, the average value of t is taken.

[0089] Tables 6-1 and 6-2 list the template information for the connectors in Examples 20-31.

[0090] Table 6-1.

[0091] Table 6-2.

[0092] Table 7 lists the chemical composition ratios and properties of the various steel grades used in Examples 20-31.

[0093] Table 7.

[0094] Table 8 lists the spot welding process parameters used in Examples 20-31.

[0095] Table 8.

[0096] Based on the welding pressure, electrode, and holding time used above, according to the welding method of the present invention, the set welding current I for the joint assembly corresponding to Examples 20-31 when T < 5 μm is determined. b and its range. Set the welding current I. b The lower limit was obtained through actual spot welding tests, and it was found to be a weld nugget diameter of 4t. 1 / 2 The welding current at the weld joint is capped at the welding current at which weld spatter occurs. (I of each embodiment) bThe ranges are listed in Table 9. According to the welding method of the present invention, the number of pulses n and the welding time and welding current for each pulse are determined and listed in Table 9.

[0097] Tables 9-1, 9-2, and 9-3 list the I used in Examples 20-31 b The number of pulses n and the welding time and welding current for each pulse.

[0098] Table 9-1.

[0099] Table 9-2.

[0100] Table 9-3.

[0101] Based on the above parameter settings, spot welding was performed on the joints of Examples 20-31. According to the definition of the weldable range in this method, when n>1, the appropriate I value for multi-pulse welding at this time is determined. b Actual range I b Define the solderable region at this point. The specific definition method is as follows: The lower limit of the solderable region is the region where, under these conditions, the diameter of the weld nugget generated is 4t. 1 / 2 Minimum I required for solder joints b min', the upper limit of the weldable range is the minimum I that causes welding spatter under this condition. b max',I b max'-I b min' is the width of the weldable section at this point. When spot welding the joints in Examples 20-30, the diameter of the generated weld nugget was recorded as 4t. 1 / 2 The welding current of the weld joint is denoted as I. b min'; Record the welding current at which welding spatter occurs, denoted as I. b max'.

[0102] Table 10 lists the embodiments 20-31 of the present invention. b min' and I b max' is used to calculate the width of the weldable section.

[0103] Table 10.

[0104] The results in Table 10 show that the weldable range width of the joints in Examples 20-31 using this technology is greater than 1000A, which meets the welding production requirements.

[0105] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0106] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A resistance spot welding method for hot-formed steel with an aluminum-silicon coating and a tensile strength of 1700 MPa or higher, characterized in that: When the thickness T of the alloy layer adjacent to the substrate in the coating of aluminum-silicon coated hot-formed steel is less than 5 μm, single-pulse welding is used. When the thickness T of the alloy layer adjacent to the substrate in the aluminum-silicon coated hot-formed steel coating is ≥ 5 μm, multi-pulse welding is used, and the current of each welding pulse gradually increases as the welding process progresses. The number of welding pulses n and the thickness T of the alloy layer adjacent to the substrate in the aluminum-silicon coated hot-formed steel coating satisfy the following: When 5μm≤T<10μm, n=2~3; When 10μm≤T<15μm, n=3~4; When 15μm≤T<20μm, n=4~6; When 20μm≤T<25μm, n=5~7; When 25μm≤T<30μm, n=6~9; When 30μm≤T, n=7~10.

2. The resistance spot welding method as described in claim 1, characterized in that, When the aluminum-silicon coated hot-formed steel in the welded joint assembly has multiple different alloy layer thicknesses T, T is taken as the maximum value among them.

3. The resistance spot welding method as described in claim 1, characterized in that, For hot-formed steel with aluminum-silicon coating and a fixed plate thickness, the time WT1 of the first welding pulse is inversely related to the number of welding pulses n.

4. The resistance spot welding method as described in claim 2, characterized in that, As the welding process progresses, the duration of each welding pulse gradually increases; preferably, the duration of each welding pulse is WT. a Satisfy the following formula: In the formula, ms represents milliseconds, n represents the number of welding pulses, t represents the thickness of the aluminum-silicon coated hot-formed steel plate in mm, and a represents the a-th pulse in n welding pulses, where a = 1, 2, ..., n.

5. The resistance spot welding method as described in claim 4, characterized in that, For welded joints with two layers of plates, t is taken as the smaller value; for welded joints with more than two layers of plates, t is taken as the average value of the thickness of all plates.

6. The resistance spot welding method as described in claim 1, characterized in that, For hot-formed steel with aluminum-silicon coating and a fixed plate thickness, the welding current I1 of the first pulse is inversely related to the number of welding pulses n.

7. The resistance spot welding method as described in claim 1, characterized in that, As the welding process progresses, the increase in current for each welding pulse gradually increases; preferably, the current I of each welding pulse... a Satisfy the following formula: Current n=1 time, I a =I b ; When n>1 and a=1 When n>1 and a>1 Where I b To set the welding current, 'a' represents the a-th pulse out of n welding pulses, where a = 1, 2, ..., n, and I1 represents the current of the first welding pulse.

8. The resistance spot welding method as described in claim 7, characterized in that, Set welding current I b The current is 3kA to 13kA; preferably, I b The lower limit is that the diameter of the generated melt nucleus is 4t. 1 / 2 The welding current at the weld point is capped at the welding current when welding spatter occurs.

9. The resistance spot welding method as described in claim 1, characterized in that, Welding was performed using a medium-frequency DC spot welding machine.

10. The resistance spot welding method as described in claim 1, characterized in that: The welding pressure used for welding is 2kN to 8kN; and / or, the end face shape of the electrode used for welding is spherical with a radius of 40mm to 100mm; and / or, the end face diameter of the electrode used for welding is 5mm to 10mm; and / or, the holding time after the welding is completed is 50ms to 500ms.

11. The resistance spot welding method as described in claim 1, characterized in that, The thickness of the aluminum-silicon coated hot-formed steel is 0.8 mm to 3 mm; and / or, the single-sided coating thickness of the aluminum-silicon coated hot-formed steel before hot forming is 10 g / m. 2 ~120g / m 2 .

12. The resistance spot welding method as described in claim 1, characterized in that, The chemical element mass percentage content of the substrate of the aluminum-silicon coated hot-formed steel is as follows: C: 0.28-0.40%, Si≤0.80%, Mn: 1.0-2.5%, B≤0.005%, Cr≤0.50%, Ti≤0.10%, Mo≤0.40%, Al: 0.01-0.06%, with the balance being Fe and unavoidable impurity elements; preferably, the content of Mn is 1.5-2.2%, the content of B is 0.001-0.005%, the content of Cr is 0.20-0.40%, the content of Ti is 0.03-0.06%, and the content of Mo is 0.15-0.40%.

13. The resistance spot welding method as described in claim 12, characterized in that, The substrate of the aluminum-silicon coated hot-formed steel has a yield strength of 1100-1500 MPa, a tensile strength of 1700-2150 MPa, and an elongation after fracture of ≥5% after hot forming.

14. The resistance spot welding method as described in claim 1, characterized in that, The resistance spot welding method includes the following steps: (1) The step of determining the thickness T of the alloy layer in close contact with the substrate in the hot-formed steel with aluminum-silicon coating; (2) Steps for determining the thickness t of the steel plate to be welded; (3) Determine the welding current I b The steps; where I b The lower limit is that the diameter of the generated melt nucleus is 4t. 1 / 2 The welding current at the weld joint is limited to the welding current at which welding spatter occurs. (4) Steps for determining the number of welding pulses n based on T; (5) Determine the welding pulse time WT of the a-th pulse based on the plate thickness t and the number of welding pulses n. a Steps; (6) According to I b The current I for each welding pulse is determined by the number of pulses n. a The steps; and (7) Welding steps according to the set parameters.

15. The resistance spot welding method as described in claim 14, characterized in that, The alloy layer thickness T was determined using the following method: a cross-sectional metallographic sample was taken from the hot-formed steel plate, ground and polished, etched with a 4% nitric acid alcohol solution, and then observed and measured under a metallographic microscope.

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