Method for improving the performance of a resistance spot welded joint of a heavy scale hot-formed steel

Abstract

The application discloses a method for improving the performance of a thick iron oxide skin hot forming steel resistance spot welding joint, and comprises the following steps: (1) determining the size of a preheating current I p and a preheating time t p ;(2) determining a pulse time t w by using a three-pulse welding; (3) determining a critical spatter current I0, and then determining the current value I t of a tempering pulse and a tempering time t t ;(4) determining the cooling interval time t c1 between a preheating pulse and a welding pulse, the interval time t c2 between welding pulses, and the cooling interval time t c3 between a welding pulse and a tempering pulse; (5) determining a welding pressure P; (6) determining a pressure maintaining time t h ;(7) adopting a medium-frequency direct-current spot welding machine to implement resistance spot welding. The application has the advantages of inhibiting spatter, widening a welding process window, improving the performance of a joint and the like.

Description

Technical Field

[0001] This invention relates to a method for improving the performance of resistance spot welded joints of hot-formed steel with thick iron oxide scale, mainly applied to the welding of automotive A-pillars, B-pillars, C-pillars, center consoles, etc., and belongs to the field of welding technology. Background Technology

[0002] Hot-formed steel is a crucial material that simultaneously meets the requirements for automotive lightweighting and safety, and it has been increasingly applied in automobile manufacturing. Currently, some hot-formed steels are in mass production, but some hot-formed steels with thick iron oxide scale exhibit surface spatter during spot welding at OEMs, which is completely unacceptable. Furthermore, during the spot welding process, the iron oxide scale on the steel plate surface enters the weld nugget, causing numerous defects within the nugget and severely degrading the mechanical properties of the joint. This renders such hot-formed steel unsuitable for automotive production. Therefore, developing a resistance spot welding process suitable for hot-formed steel with thick iron oxide scale is essential. Summary of the Invention

[0003] This invention proposes a novel welding method consisting of a preheating pulse, a three-pulse welding process, and a tempering pulse. The preheating pulse utilizes an ultra-high preheating current and a short preheating time, while extending the cooling time between the three welding pulses and the tempering pulse. This method yields weld nuggets without significant welding defects and significantly improves joint performance.

[0004] The technical solution of the present invention is as follows:

[0005] A method for improving the resistance spot weld performance of hot-formed steel with thick iron oxide scale, wherein the thickness of the hot-formed steel with thick iron oxide scale is in the range of 1 mm ≤ plate thickness ≤ 2.5 mm, and the thickness of the iron oxide scale is ≥ 5 μm, the method comprising the following steps:

[0006] (1) Determine the preheating current I based on the thickness of the iron oxide scale. p The size is determined by the plate thickness, and the preheating time t is determined accordingly. p Size;

[0007] (2) Using three-pulse welding, the pulse time t is determined according to the plate thickness. w ;

[0008] (3) The critical spatter current I0 is determined by traditional single-pulse welding of hot-formed steel, and then the current value I of the tempering pulse is determined based on the critical spatter current I0. t Determine the tempering time t t ;

[0009] (4) Determine the cooling interval t between the preheating pulse and the welding pulse based on the plate thickness. c1 Welding pulse interval time t c2The cooling interval t between welding pulse and tempering pulse c3 ;

[0010] (5) Determine the welding pressure P based on the tensile strength of the steel plate;

[0011] (6) Based on the cooling interval t between the welding pulse and the tempering pulse c3 Determine the holding time t h .

[0012] (7) Use a medium-frequency DC spot welding machine to perform resistance spot welding according to the parameters determined above and in the sequence of pre-pressure, preheating, welding, tempering and holding pressure.

[0013] Furthermore, in step (1), the preheating current I p The size range is 20kA≤I p ≤27kA; preheating time t p The size range is generally 10ms ≤ t p ≤20ms. Preferably, when 5μm ≤ iron oxide scale thickness ≤10μm, the preheating current I... p The size range is 20kA≤I p ≤23kA; when 10μm < iron oxide scale thickness ≤20μm, the preheating current I p The size range is 23kA < I p ≤25kA; when the iron oxide scale thickness is >20μm, the preheating current I p The size range is 25kA < I p ≤27kA. When 1mm ≤ plate thickness ≤ 1.5mm, the preheating time t p The size range is generally 10ms ≤ t p ≤15ms; when 1.5mm < plate thickness ≤ 2.5mm, preheating time t p The size range is generally 15ms < t p ≤20ms.

[0014] Furthermore, in step (2), the pulse time t w The size range is 70ms≤t w ≤170ms. Preferably, when 1.0mm ≤ plate thickness ≤ 1.3mm, the pulse time t w The size range is generally 70ms≤t w ≤90ms; when 1.3mm < plate thickness ≤ 1.9mm, pulse time t w The size range is generally 90ms < t w ≤120ms; when 1.9mm < plate thickness ≤ 2.5mm, pulse time t w The size range is generally 120ms < t w ≤170ms.

[0015] Furthermore, in step (3), the current value I of the tempering pulse t The formula for calculation is: I t = (0.6~0.8)I0, the tempering time is in the range of 120ms≤t t ≤170ms.

[0016] Furthermore, in step (4), the cooling interval t between the preheating pulse and the welding pulse... c The size range is 20ms≤t c1 ≤40ms, welding pulse interval t c2 The size range is 20ms≤t c2 ≤40ms, cooling interval t between welding pulse and tempering pulse c3 80ms≤t c3 ≤120ms.

[0017] Furthermore, in step (5), the welding pressure P is in the range of 5kN≤P≤6kN.

[0018] Furthermore, in step (6), the holding time t h The formula for calculating t is: h =(2~4)t c3 .

[0019] Furthermore, in step (7), the pre-compression time is 100-500ms.

[0020] This invention is applicable to resistance spot welding of hot-formed steel with a tensile strength of 1500 MPa or higher and a thick iron oxide scale. The hot-formed steel can be PHS2000, PHS1500 hot-formed steel, etc.

[0021] Furthermore, the thick iron oxide scale hot-formed steel is replaced with coated hot-formed steel, wherein the coated hot-formed steel is formed by plating a coating on the surface of the thick iron oxide scale hot-formed steel, and the coating thickness is 20-50 μm. The coated hot-formed steel can be aluminum-silicon coated hot-formed steel, etc.

[0022] Furthermore, for hot-formed steel with a coating (20μm ≤ coating thickness ≤ 50μm), the preheating current I... p The size range is 25kA≤I p ≤27kA.

[0023] The purpose of this invention, which uses a pulse welding method with a large preheating current and a short preheating time, is to remove the iron oxide scale on the surface of hot-formed steel plates, resulting in a purer weld nugget structure and improved joint quality.

[0024] The purpose of using three-pulse welding in this invention is to gradually increase the diameter of the weld nugget and to avoid spatter caused by prolonged continuous welding.

[0025] The purpose of using tempering pulses in this invention is to slow down the cooling rate of the welded joint, thereby reducing residual stress at the joint and improving joint performance.

[0026] The present invention has a long cooling interval between the welding pulse and the tempering pulse. The purpose is to make the weld nugget structure relatively stable after the three welding pulses are completed, so that the residual iron oxide scale and other impurities are completely squeezed to the outside of the weld nugget during the solidification process, thereby improving the joint performance and weldability.

[0027] Beneficial effects:

[0028] This invention relates to a method for resistance spot welding of hot-formed steel with thick iron oxide scale. This method utilizes a pulsed pattern of high preheating current and short preheating time to disperse the iron oxide scale on the surface of the hot-formed steel, significantly reducing the iron oxide scale content in the weld nugget. The three-pulse welding method can solve the problem of spatter on the steel plate surface and also allows for a further increase in weld nugget size. The increased cooling time after three-pulse welding is equivalent to supercooling aging; the tempering pulse reduces the cooling rate of the joint after welding, decreasing residual stress at the joint. Extending the holding pressure time reduces shrinkage cavities within the weld nugget, improving joint performance. This method has advantages such as suppressing spatter, widening the welding process window, and improving joint performance. This welding process can accelerate the application of hot-formed steel with thick iron oxide scale in the automotive manufacturing industry.

[0029] Compared with existing technologies, this method has the following significant advantages:

[0030] (1) It is applicable not only to ordinary hot-formed steel sheets, but also to hot-formed steel sheets with coatings.

[0031] (2) It can effectively suppress surface splashing (surface splashing is unacceptable to OEMs).

[0032] (3) The internal structure of the weld nugget is uniform and there are no obvious welding defects (no obvious shrinkage cavities and cracks).

[0033] (4) No grinding or pickling is required to remove the surface iron oxide scale, which improves production efficiency and reduces costs.

[0034] (5) The welding process window is significantly widened (the window is around 2kA, while the traditional process window is usually less than 1kA).

[0035] (6) The joint performance is significantly improved (tensile shear strength is increased by more than 60%, and cross tensile strength is increased by more than 50%).

[0036] (7) The failure mode of the joint is the pull-out of the weld nugget (interface failure often occurs when welding with low current in the welding window). Attached Figure Description

[0037] Figure 1 This is a welding timing diagram for the present invention. Detailed Implementation

[0038] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0039] Example 1

[0040] The welding material is a 1.4mm thick PHS2000 hot-formed steel bare plate with a surface iron oxide scale thickness of 6.3μm. The pre-pressing time is 300ms, and a medium-frequency DC spot welding machine is used.

[0041] (1) Determine the preheating current I based on the thickness of the iron oxide scale. p =21kA, preheating time t is determined based on plate thickness. p =14ms.

[0042] (2) Using three-pulse welding, determine the single-pulse welding time t based on the plate thickness. w =100ms;

[0043] (3) The critical spatter current I0 = 8.8 kA was determined using traditional single-pulse welding. t =0.68I0, select the tempering current I t =6kA; tempering time t t =150ms.

[0044] (4) Determine the cooling interval t between the preheating pulse and the welding pulse based on the plate thickness. c1 =20ms, welding pulse interval time t c2 =20ms, cooling interval t between welding pulse and tempering pulse c3 =90ms.

[0045] (5) Select welding pressure P = 6kN based on the strength of the steel plate.

[0046] (6) Holding time t h =2.78t c3 Select t h =250ms.

[0047] (7) Perform resistance spot welding according to the parameters determined above and in the sequence of pre-pressing, preheating, welding, tempering and holding pressure.

[0048] Based on the above welding parameters, the resistance spot welding process window of the 1.4mm PHS2000 is 2.1kA, which is much greater than the standard requirement of more than 1kA; the shear tensile strength of the joint is 21kN, the cross tensile strength is 5.4kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0049] Comparative Example 1

[0050] Using the welding materials of the conventional welding example 1, the process was as follows: welding pressure of 3.6 kN, single-pulse welding, welding time of 200 ms, and holding time of 40 ms. The welding window was 0.5 kA (not meeting the standard requirement of greater than 1 kA), the shear tensile strength of the joint was 13 kN, and the cross tensile strength was 3.5 kN. Therefore, the process of Example 1 represents a significant improvement over the process of Comparative Example 1, with a 1.6 kA increase in the welding window, a 62% improvement in the shear tensile strength of the joint, and a 54% improvement in the cross tensile strength.

[0051] Example 2

[0052] The welding material is a 1.0mm thick PHS1500 hot-formed steel bare plate with a surface iron oxide scale of 12.3μm. The pre-pressing time is 300ms, and a medium-frequency DC spot welding machine is used.

[0053] (1) Determine the preheating current I based on the thickness of the iron oxide scale. p =24kA, preheating time t is determined based on plate thickness. p =10ms.

[0054] (2) Using three-pulse welding, determine the single-pulse welding time t based on the plate thickness. w =70ms;

[0055] (3) The critical spatter current I0 = 8.2 kA was determined using traditional single-pulse welding. t =0.61I0, select the tempering current I t =5kA; tempering time t t =120ms.

[0056] (4) Determine the cooling interval t between the preheating pulse and the welding pulse based on the plate thickness. c1 =20ms, welding pulse interval time t c2 =20ms, cooling interval t between welding pulse and tempering pulse c3 =80ms.

[0057] (5) Select welding pressure P = 5kN based on the strength of the steel plate.

[0058] (6) Holding time t h =2.5t c3 Select t h=200ms.

[0059] (7) Perform resistance spot welding according to the parameters determined above and in the sequence of pre-pressing, preheating, welding, tempering and holding pressure.

[0060] Based on the above welding parameters, the resistance spot welding process window of the 1.0mm PHS1500 is 1.9kA, which is much greater than the standard requirement of more than 1kA; the shear tensile strength of the joint is 16kN, the cross tensile strength of the joint is 4.5kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0061] Comparative Example 2

[0062] Using the welding materials of the conventional welding example 2, the process was as follows: welding pressure of 2.6 kN, single-pulse welding, welding time of 150 ms, and holding time of 40 ms. The welding window was 0.7 kA (not meeting the standard requirement of greater than 1 kA), the shear tensile strength of the joint was 9.8 kN, and the cross tensile strength was 2.9 kN. Therefore, the process of example 2 represents a significant improvement over the process of comparison example 2, with a 1.2 kA increase in the welding window, a 63% improvement in the shear tensile strength of the joint, and a 55% improvement in the cross tensile strength.

[0063] Example 3

[0064] The welding material is 2.5mm thick Al-10%Si coated PHS1500 hot-formed steel with a coating thickness of 30μm. The iron oxide scale thickness before coating is 2μm. The pre-pressing time is 300ms. A medium-frequency DC spot welding machine is used.

[0065] (1) Determine the preheating current I based on the thickness of the iron oxide scale. p =26kA, preheating time t is determined based on plate thickness. p =20ms.

[0066] (2) Using three-pulse welding, determine the single-pulse welding time t based on the plate thickness. w =170ms;

[0067] (3) Using traditional single-pulse welding, the critical spatter current I0 = 11kA was determined. t =0.73I0, select the tempering current I t =8kA; tempering time t t =170ms.

[0068] (4) Determine the cooling interval t between the preheating pulse and the welding pulse based on the plate thickness. c1 =40ms, welding pulse interval time t c2 =40ms, cooling interval t between welding pulse and tempering pulse c3 =120ms.

[0069] (5) Select welding pressure P = 6kN based on the strength of the steel plate.

[0070] (6) Holding time t h =3.33t c3 Select t h =400ms.

[0071] (7) Perform resistance spot welding according to the parameters determined above and in the sequence of pre-pressing, preheating, welding, tempering and holding pressure.

[0072] Based on the above welding parameters, the resistance spot welding process window for 2.5mm thick PHS1500 is 1.8kA, which is much greater than the standard requirement of more than 1kA; the shear tensile strength of the joint is 35.2kN, the cross tensile strength of the joint is 15.2kN, the failure mode is pull-out of the weld nugget, and there is no spatter on the surface.

[0073] Comparative Example 3

[0074] Using the welding materials of Example 3, the welding process was as follows: welding pressure of 4 kN, three-pulse welding, single-pulse welding time of 130 ms, cooling time of 40 ms, and holding time of 180 ms. The welding window was 0.8 kA (not meeting the standard requirement of greater than 1 kA). The shear tensile strength of the joint was 21.3 kN, and the cross tensile strength was 10 kN. Therefore, Example 3 shows a significant improvement over Example 3, with a 1 kA increase in the welding window, a 65% improvement in the shear tensile strength of the joint, and a 52% improvement in the cross tensile strength.

Claims

1. A method for improving the properties of a resistance spot welded joint of a heavy scaled hot-formed steel, the heavy scaled hot-formed steel having a sheet thickness in the range of 1 mm < sheet thickness < 2.5 mm, the scale having a thickness of > 5 μm, characterized in that, The method comprises the following steps: (1) The magnitude of the preheating current I is determined in accordance with the thickness of the oxide scale p The magnitude of the preheating time t is determined in accordance with the thickness of the plate p ; (2) Using three-pulse welding, a pulse time t is determined according to the plate thickness w ; (3) using traditional single pulse welding hot forming steel to determine the critical spatter current I0, and then determining the current value I of the tempering pulse according to the critical spatter current I0 t , determining the tempering time t t ; (4) The preheating pulse and the welding pulse cooling interval time t is determined according to the plate thickness c1 The welding pulse interval time t c2 The welding pulse and the tempering pulse cooling interval time t c3 ; (5) determining the welding pressure P according to the tensile strength of the steel plate; (6) according to the cooling interval time t between the welding pulse and the tempering pulse c3 determining the dwell time t h ; (7) using a medium frequency direct current spot welding machine to implement resistance spot welding according to the determined parameters and the time sequence of pre-pressing, pre-heating, welding, tempering, and pressure maintaining; In step (1), the preheating current I p ranges in magnitude from 20 kA ≤ I p ≤ 27 kA; the preheating time t p ranges in magnitude from 10 ms ≤ t p ≤ 20 ms; In step (4), the preheat pulse is separated in time from the weld pulse cooling interval t c ranging in size from 20 ms ≤ t c1 ≤ 40 ms, the weld pulse interval t c2 ranging in size from 20 ms ≤ t c2 ≤ 40 ms, and the weld pulse to temper pulse cooling interval t c3 ranging from 80 ms ≤ t c3 ≤ 120 ms.

2. The method of claim 1, wherein, In step (2), the pulse time t w ranges in size from 70 ms ≤ t w ≤ 170 ms.

3. The method of claim 1, wherein, In step (3), the current value I of the tempering pulse t The calculation formula is: I t = (0.6 ~ 0.8) I0, and the tempering time ranges from 120 ms ≤ t t ≤ 170 ms.

4. The method of claim 1, wherein, In step (5), the welding pressure P ranges from 5kN to 6kN.

5. The method of claim 1, wherein, In step (6), the pressure holding time t h The calculation formula is: t h = (2-4) t c3 .

6. The method of claim 1, wherein, In step (7), the pre-pressing time is 100-500ms.

7. The method of claim 1, wherein, The thick-oxide hot forming steel is replaced by a coated hot forming steel, which is plated with a coating on the surface of the thick-oxide hot forming steel.

8. The method of claim 7, wherein, The thickness of the coating is 20-50μm.

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