Resistance spot welding method for ultrahigh-strength hot forming steel above 1800 Mpa grade

Abstract

The invention discloses a resistance spot welding method for ultrahigh-strength hot forming steel above 1800 Mpa grade. The method comprises a soft standard welding step and a low-current tempering step. Wherein in the soft standard welding step, multiple soft standard pulses are adopted for welding steel to be welded; and a low-current tempering step: welding the steel welded in the soft standard welding step by adopting low-current tempering pulse current. The method has the advantages that by adopting the multi-pulse soft standard welding and controlling the low-current tempering pulse method, the nugget diameter of a welding spot is remarkably increased, the hardness of the welding spot combined hot extension line is reduced, and the stress concentration at the position is reduced.

Description

Technical Field

[0001] The invention relates to the field of production welding of ultra-high strength hot-formed steel for automobiles, and in particular to a resistance spot welding method for ultra-high strength hot-formed steel of 1800Mpa or above. Background Art

[0002] With the continuous development of automobile lightweight technology, hot-formed parts above 1800MPa have been widely used in automobile bodies, such as A-pillars, B-pillars, C-pillars, door frame reinforcement beams, front cross beams, left and right longitudinal beams, door bumpers and other safety parts.

[0003] Resistance spot welding has been widely used in the automotive industry due to its advantages of high production efficiency and easy automation. It is also the main welding method for hot-formed parts. During the welding process, the material at the welding point of the hot-formed steel parts undergoes a thermal cycle of heating-pressurization-solidification-cooling during the welding process. Compared with the non-welding area, the organization and composition of each area of ​​the weld are uneven, the hardness is distributed in a gradient, and there is a large welding internal stress in the weld core and its surroundings. During service, the weld is very likely to break and fall off, especially in the event of a collision, it cannot effectively absorb energy, which poses a great safety hazard.

[0004] In order to improve the spot welding performance of 1800MPa grade hot-formed steel, the overall heat treatment method of the parts after welding is usually adopted in production and research. However, the overall heat treatment will seriously affect the production rhythm, increase the time cost and process flow, and affect the production of steel. For example, the patent publication number CN112247332A provides a resistance spot welding method for ultra-high strength hot-formed steel plates. Based on this method, ultra-high strength hot-formed steel plates with a carbon content of ≥0.33wt% and a tensile strength of ≥1800MPa are welded. Four consecutive pulses do not require a cooling section, which improves efficiency and can improve CTS strength. However, the above method has no intermediate cooling between spot welding pulses, the electrode is continuously heated and pressurized, the electrode life is greatly reduced, it is not easy to control welding spatter, and the weld spot is rapidly cooled to generate hardenable martensite under the still high fourth welding current, and the collision safety performance is difficult to guarantee. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a resistance spot welding method for ultra-high strength hot-formed steel above 1800Mpa level. By adopting a multi-pulse soft specification welding method and controlling a low current tempering pulse method, the diameter of the weld nugget is significantly increased, the hardness of the weld combined with the hot extension line is reduced, the stress concentration here is reduced, the tensile shear and cross-tensile properties of the joint are greatly improved, the fracture position of the destructive test is located in the heat-affected zone, the parts have a combination of high strength and plasticity, have a large energy absorption effect when a collision occurs, and the collision safety performance is greatly improved.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a resistance spot welding method for ultra-high strength hot-formed steel of 1800Mpa or above, the method comprising a soft specification welding step and a low current tempering step; wherein

[0007] Soft specification welding steps: Use multiple soft specification pulses to weld the steel to be welded;

[0008] Low current tempering step: The steel after the soft specification welding step is welded using a low current tempering pulse current.

[0009] In the soft specification welding step, the welding current during each soft specification welding is less than the critical spatter current I0.

[0010] Before welding the steel in the soft specification welding step, a single pulse spatter current test is performed on the steel to obtain the corresponding critical spatter current I0.

[0011] An intermediate cooling time is set between two adjacent soft specification welding steps and after the last soft specification is completed.

[0012] In the soft specification welding step, the welding current of each soft specification welding is calculated by the critical spatter current I0 and the welding current of each welding is less than the critical spatter current I0.

[0013] The welding electrode used in the welding process is a curved electrode, the pre-pressure time is TP, and the set time threshold range and electrode pressure range are maintained.

[0014] The steel is welded using a low current tempering pulse current, with the welding current maintained at 3-4KA for a set time range.

[0015] The soft standard pulse welding step includes at least three soft standard pulse current weldings.

[0016] During each soft standard pulse current welding process, the welding current is maintained at (0.85-0.95)I 0, and the welding time T1 = (100-200) ms.

[0017] During the welding process, the welding electrode pressure ranges from 4kN to 8kN, and the holding time is 200-300ms.

[0018] The advantages of the present invention are: by adopting a method of multi-pulse soft specification welding and controlling a low current tempering pulse, the diameter of the weld nugget is significantly increased, the hardness of the weld joint at the hot extension line is reduced, the stress concentration there is reduced, the tensile shear and cross-tensile properties of the joint are greatly improved, the fracture position of the destructive test is located in the heat-affected zone, the parts have a combination of high strength and plasticity, have a large energy absorption effect when a collision occurs, and the collision safety performance is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The following is a brief description of the contents expressed in the drawings of the present invention and the symbols in the drawings:

[0020] Figure 1 A schematic diagram of welding current, electrode pressure and their corresponding time during the spot welding method of the present invention;

[0021] Figure 2 It is a timing principle diagram corresponding to soft standard pulse current welding and low current tempering welding in the spot welding process of the present invention;

[0022] Figure 3 It is a schematic diagram of microhardness dot punching of the present invention;

[0023] Figure 4 The microhardness distribution diagram of the weld nugget of Example 1 of the present invention and Comparative Example 1 is shown.

[0024] The marks in the above figures are: F is the electrode pressure, Tp is the pre-pressure time, I1, I2, I3, I4 are the welding currents, T, T1, T2, T3, T4 are the welding times, Tc1, Tc2, Tc3 are the cooling times, T5 is the holding time, and T6 is the waiting time for the next welding cycle. DETAILED DESCRIPTION

[0025] The specific implementation of the present invention will be further explained in detail below by describing the optimal embodiment with reference to the accompanying drawings.

[0026] The present invention relates to a resistance spot welding method for ultra-high strength hot-formed steel for automobiles. The method adopts multi-pulse soft specification welding and controls low current tempering pulses, and is applied to spot welding connections between hot-formed parts above 1800MPa level, which can significantly increase the diameter of the spot welding nugget, reduce stress concentration at the welding point, and improve the strength and plasticity of the spot welding joint of the hot-formed parts. It belongs to the field of production and welding of ultra-high strength parts for automobiles.

[0027] The present invention provides a resistance spot welding method for ultra-high strength hot-formed steel of 1800MPa or above, which is suitable for steel of 1800MPa or above. Other materials have good welding performance and do not need to adopt this method. The welding of steel and steel plates is achieved by adopting multi-pulse soft specification welding and controlling low current tempering pulses, thereby meeting the requirements of spot welding, increasing the diameter of the spot welding nugget, reducing stress concentration at the welding point, and improving the strength and plasticity of the spot welding joints of hot-formed parts. It belongs to the field of production and welding of ultra-high strength parts for automobiles. The specific steps of the spot welding method include:

[0028] Step 1: Perform a single pulse spatter current test on the 1800MPa grade or above steel plate to be spot welded to obtain a critical spatter current I0;

[0029] Step 2: Use soft standard pulse 1, welding current I1 = (0.85-0.95) I0, welding time T1 = (100-200) ms, intermediate cooling time Tc1 = (20-60) ms; soft standard pulse refers to a small current and a long time of welding heat input, which is one of the conventional welding methods in the prior art.

[0030] Step 3: using the soft standard pulse 2, welding current I2 = (0.85-0.95) I0, welding time T2 = (100-200) ms, intermediate cooling time Tc2 = (20-60) ms;

[0031] In the three soft specification pulse currents, the welding current, duration, and cooling time of each time can be different, but they all need to be within the set value range, that is, the welding current, duration, etc. between soft specification pulse 1 and soft specification pulse 2 can be the same or take different values ​​within the value range. The soft specifications of each time are not necessarily the same. When they are processed in the same way, the main consideration is to make it easier to operate when writing programs. There are no special requirements. The purpose of doing this is to effectively and stably grow the weld nugget. Step 4: Use soft specification pulse 3, welding current I 3 = (0.85-0.95) I0, welding time T3 = (100-200) ms, intermediate cooling time Tc3 = (20-60) ms; Steps 2, 3, and 4 use three soft specification pulse current welding as an example. In fact, it can be 4-5 times or even more. However, in actual application, it is necessary to consider the production rhythm. Generally, more pulses will not be used, and the number of welding times will generally be counted. The welding parameters used in the soft specification are very important for welding. It has been verified by experiments that if the range is exceeded, the welding effect cannot be achieved.

[0032] Step 5: After the soft standard pulse spot welding in step 4 is completed and cooling is completed, the steel after the spot welding in step 4 is continued to be welded using low current tempering pulse 4, wherein the current I4 = 3-4kA and the time T4 = (80-160)ms during low current tempering pulse welding. After the welding in step 5 is completed, all the steps of spot welding are completed. The reason for setting the welding current and duration numbers is that when the welding current is very low, the heating temperature is not enough, and when the welding current is very high, the weld core temperature is high. The heat affected zone structure generated during rapid cooling is still a brittle structure, which does not have the effect of tempering. The duration mainly depends on the production rhythm and the matching with the welding current.

[0033] like Figure 1 , 2As shown, it is a timing diagram of a welding process. After the electric welding is turned on, the spot welding machine first performs pre-stressing, and the pre-stressing time is Tp. Before the end of the pre-stressing time, the electrode voltage reaches F. After the end of the pre-stressing time, the first soft standard pulse current welding is started, and its duration is T1 and the welding current is I1. After the end of time T1, the second soft standard welding is started after the cooling time Tc1. After the welding with a duration of T2 and a welding current of I2, the second soft standard welding is started after the cooling time Tc2. The third soft standard welding duration is T3 and the welding current is I3. After the end of T3, it is cooled for a duration of Tc3, and then enters the low current tempering welding process after the cooling time reaches Tc3. In this process, the welding duration is T4 and the welding current is I4. After the end of the duration T4, the spot welding machine electrode pressure is maintained at F and the duration is T5. After T5, the electrode pressure gradually decreases until the next spot welding is started, and the electrode pressure F gradually decreases to 0 until the next welding is started. The duration is T6, and T6 is the waiting time for the next welding cycle.

[0034] like Figure 3 , 4 , which is a schematic diagram of microhardness points corresponding to steel obtained by using the present method and the spot welding method in the prior art and a microhardness distribution diagram of the weld core. It can be seen from the figure that the diameter of the weld obtained by using the present method is larger than that of the prior art. At the same time, the hardness value of the weld softening zone is reduced, and the hardness value of the molten zone where the weld core bonding line extends into the position is reduced, thereby improving the toughness of the spot welding joint.

[0035] The electrode used in the welding process is a φ6-8mm (end face diameter 6-8mm) arc electrode, with a pre-pressing time of 40ms, an electrode pressure range of 4kN-8kN, and a holding time of 200-300ms. The electrode end face size limit is because the hot-formed steel itself has a large resistance and more alloy elements. When using an electrode with a smaller end face, the current density is concentrated, which is more likely to cause spatter, the welding process window is small, and the weld core diameter cannot meet the requirements. Therefore, the conventional choice is a diameter of φ6mm. As the electrode wears, the electrode is allowed to wear to 8mm. Correspondingly, when the degree of electrode wear is large, the required welding current increases. At the same time, studies have found that the larger the electrode diameter, the larger the welding process window, but the welding current is often limited by the welding machine controller. Therefore, the electrode end face diameter is limited to φ6-8mm.

[0036] When spot welding is performed on steel by the technical solution in the above-mentioned embodiment of the present invention, when spot welding ultra-high strength hot-formed steel of 1800Mpa and above is performed, a method of multi-pulse soft specification welding and controlling low current tempering pulses is adopted, so that the weld nugget gradually grows without splashing, thereby significantly increasing the diameter of the weld nugget; at the same time, a low current tempering pulse is adopted to reduce the cooling rate of the weld, and the hardness of the weld combined with the hot extension line is reduced, thereby reducing the stress concentration there, and the tensile shear and cross-tensile properties of the joint are greatly improved. The fracture position of the destructive test is located in the heat-affected zone, and the components have a combination of high strength and plasticity, have a large energy absorption effect when a collision occurs, and the collision safety performance is greatly improved.

[0037] The following examples and comparative examples are used to verify the technical advantages of spot welding in this solution:

[0038] Example 1: Steel plate strength: 1835 MPa, thickness 1.4 mm.

[0039] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 7.8 kA;

[0040] Step 2: Use the soft standard pulse 1, welding current I1 = 7kA, welding time T1 = 180ms, intermediate cooling time T c1 =20ms;

[0041] Step 3: Use the soft standard pulse 2, welding current I2 = 7kA, welding time T2 = 180ms, intermediate cooling time T c2 =20ms;

[0042] Step 4: Use the soft standard pulse 3, welding current I3 = 7kA, welding time T3 = 180ms, intermediate cooling time T c3 =20ms;

[0043] Step 5: Use the low current tempering pulse 4, current I4 = 3kA, time T4 = 100ms.

[0044] Example 2: Steel plate strength: 1835 MPa, thickness 1.8 mm.

[0045] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 8.8 kA;

[0046] Step 2: Use the soft standard pulse 1, welding current I1 = 7.9 kA, welding time T1 = 180 ms, intermediate cooling time T c1 =20ms;

[0047] Step 3: Use the soft standard pulse 2, welding current I2 = 7.9kA, welding time T2 = 180ms, intermediate cooling time T c2 =20ms;

[0048] Step 4: Use the soft standard pulse 3, welding current I3 = 7.9 kA, welding time T3 = 180 ms, intermediate cooling time T c3 =20ms;

[0049] Step 5: Use the low current tempering pulse 4, current I4 = 4kA, time T4 = 100ms.

[0050] Example 3: Steel plate strength: 1835 MPa, thicknesses are 1.4 mm and 1.8 mm respectively.

[0051] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 8.2 kA;

[0052] Step 2: Use the soft standard pulse 1, welding current I1 = 7.4kA, welding time T1 = 180ms, intermediate cooling time T c1 =20ms;

[0053] Step 3: Use the soft standard pulse 2, welding current I2 = 7.4kA, welding time T2 = 180ms, intermediate cooling time T c2 =20ms;

[0054] Step 4: Use the soft standard pulse 3, welding current I3 = 7.4kA, welding time T3 = 180ms, intermediate cooling time T c3 =20ms;

[0055] Step 5: Use the low current tempering pulse 4, current I4 = 3kA, time T4 = 100ms.

[0056] Comparative Example 1: Steel plate strength: 1835 MPa, thickness 1.4 mm.

[0057] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 7.8 kA;

[0058] Step 2: Use critical spatter current 7.8kA, welding time T = 540ms;

[0059] Comparative Example 2: Steel plate strength: 1835 MPa, thickness 1.4 mm.

[0060] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 7.8 kA;

[0061] Step 2: Use the soft standard pulse 1, welding current I1 = 7kA, welding time T1 = 180ms, intermediate cooling time T c1 =20ms;

[0062] Step 3: Use the soft standard pulse 2, welding current I2 = 7kA, welding time T2 = 180ms, intermediate cooling time T c2 =20ms;

[0063] Step 4: Use the soft standard pulse 3, welding current I3 = 7kA, welding time T3 = 180ms, intermediate cooling time T c3 =20ms;

[0064] Step 5: Use the low current tempering pulse 4, current I4 = 2kA, time T4 = 100ms.

[0065] Comparative Example 3: Steel plate strength: 1835MPa, thickness 1.4mm.

[0066] Step 1: Perform a single pulse spatter current test on the steel plate, and obtain a critical spatter current of 7.8 kA;

[0067] Step 2: Use the soft standard pulse 1, welding current I1 = 7kA, welding time T1 = 180ms, intermediate cooling time T c1 =20ms;

[0068] Step 3: Use the soft standard pulse 2, welding current I2 = 7kA, welding time T2 = 180ms, intermediate cooling time T c2 =20ms;

[0069] Step 4: Use the soft standard pulse 3, welding current I3 = 7kA, welding time T3 = 180ms, intermediate cooling time T c3 =20ms;

[0070] Step 5: Use the low current tempering pulse 4, current I4 = 5kA, time T4 = 100ms.

[0071] The above examples 1-3 and comparative examples 1-3 are analyzed as follows:

[0072] According to the process of the embodiment and the comparative example, a welding joint is obtained, and the welding joint is subjected to shear tensile and cross tensile test detection, and the shear strength and cross tensile strength of the spot welding joint are obtained as shown in Table 1. From the data in Table 1, it can be seen that the shear strength and cross tensile strength of the spot welding joint obtained in the embodiment are greatly improved compared with the comparative example, the mechanical specimen of the welding joint of the embodiment undergoes plastic fracture, and the welding joint of the comparative example undergoes brittle fracture. Therefore, the spot welding method provided in this scheme adopts a method of multi-pulse soft specification welding and controlling low current tempering pulses when spot welding ultra-high strength hot-formed steel of 1800Mpa and above in the prior art, so that the weld nugget gradually grows without splashing, significantly increasing the diameter of the weld nugget; at the same time, a low current tempering pulse is used to reduce the cooling rate of the weld, and the hardness of the weld combined with the hot extension line is reduced, reducing the stress concentration here, and the tensile shear and cross tensile properties of the joint are greatly improved. The fracture position of the destructive test is located in the heat-affected zone, and the parts have a combination of high strength and plasticity, and have a large energy absorption effect when a collision occurs, and the collision safety performance is greatly improved.

[0073] Table 1 Mechanical properties of spot welded joints

[0074] Nugget diameter / mm Weld spatter / Yes / No Shear strength / kN Cross tensile strength / kN Example 1 6.22 No 23.56 5.32 Example 2 6.62 No 28.12 5.64 Example 3 6.14 No 22.56 5.21 Comparative Example 1 5.81 Yes 14.64 3.46 Comparative Example 2 6.17 No 16.52 3.85 Comparative Example 3 6.24 No 15.86 3.94

[0075] Obviously, the specific implementation of the present invention is not limited to the above-mentioned methods. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, they are all within the protection scope of the present invention.

Claims

1. A resistance spot welding method for ultra-high strength hot-formed steel of 1800Mpa or above, characterized in that: The method comprises a soft specification welding step and a low current tempering step; in Soft specification welding steps: Use multiple soft specification pulses to weld the steel to be welded; Low current tempering step: The steel after the soft specification welding step is welded using a low current tempering pulse current.

2. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in claim 1, characterized in that: In the soft specification welding step, the welding current during each soft specification welding is less than the critical spatter current I0.

3. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in claim 2, characterized in that: Before welding the steel in the soft specification welding step, a single pulse spatter current test is performed on the steel to obtain the corresponding critical spatter current I0.

4. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as described in any one of claims 1 to 3, characterized in that: An intermediate cooling time is set between two adjacent soft specification welding steps and after the last soft specification is completed.

5. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in any one of claims 1 to 3, characterized in that: In the soft specification welding step, the welding current of each soft specification welding is calculated by the critical spatter current I0 and the welding current of each welding is less than the critical spatter current I0.

6. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in claim 5, characterized in that: The welding electrode used in the welding process is a curved electrode, the pre-pressing time is TP, and the set time threshold range and electrode pressure range are maintained.

7. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in any one of claims 1 to 3, characterized in that: The steel is welded using a low current tempering pulse current, with the welding current maintained at 3-4KA for a set time range.

8. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in any one of claims 1 to 3, characterized in that: The soft standard pulse welding step includes at least three soft standard pulse current weldings.

9. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in claim 8, characterized in that: During each soft standard pulse current welding process, the welding current is maintained at (0.85-0.95)I0, and the welding time T1 = (100-200)ms.

10. A resistance spot welding method for ultra-high strength hot-formed steel of 1800 MPa or above as claimed in claim 6, characterized in that: During the welding process, the welding electrode pressure ranges from 4kN to 8kN, and the holding time is 200-300ms.

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