JP2002103048A - Method for spot welding of high strength steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stable method for welding applicable to a mass- production site, by which an excellent cruciform tensile strength of the weld zone is obtained at a spot welding of high strength steel plates. SOLUTION: The method for spot welding of the high strength steel plates is characterized by the fact that an electrode is separated from the steel plates after an elapse of holding time after welding after energizing for welding is terminated and the holding time after welding is within the value given by the relation. 300-500t+250t2<=HT<=560-900t+500t2, where t stands for the plate thickness (mm), HT stands for the holding time after welding (ms). Further, the method for spot welding of the high strength steel plates is characterized by the fact that an energizing is continued even after the energizing for welding is terminated to adjust the temperature drop rate during the time of cooling of the spot-welded zone. The post-energizing current is within the range of >=70% and <=90% of the energizing current for welding and the post- energizing time is within the range of >=60% and <=100% of the energizing time for welding.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spot welding method for improving the tensile strength of a weld in spot welding of a high strength steel sheet.

[0002]

2. Description of the Related Art In recent years, there has been an increasing need to use high-strength steel sheets for automobile bodies in order to take measures for vehicle safety, fuel efficiency and CO ₂ emission reduction. Spot welding is mainly used in assembling automobile bodies, and spot welding is also mainly used in welding high-strength steel plates. Tensile shear strength (tensile strength when a tensile load is applied in the shear direction of the joint) and cross tensile strength (tensile strength when a tensile load is applied in the peel direction of the joint) ) Is important.

[0003] Regarding the tensile shear strength of a spot welded portion of a high-strength steel sheet, the tensile shear strength tends to increase as the tensile strength of the steel sheet increases. However, the cross tensile strength of the joint increases only slightly with an increase in the tensile strength of the steel sheet, and may be reduced depending on the component. For example, if a high-strength steel plate having a tensile strength of 590 MPa is used instead of a mild steel plate having a tensile strength of 290 MPa, the tensile shear strength of the spot welded joint is almost 2
Although it is twice as large, the cross tensile strength hardly increases, and when a high strength steel sheet having a carbon equivalent Ceq value represented by the following formula (3) exceeding 0.20 is used, rather, when mild steel is used. May show lower values. Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (3) (where C, Si, Mn, P, and S indicate the contents (% by mass) of carbon, silicon, manganese, phosphorus, and sulfur in the steel, respectively. )

Generally, as the tensile strength of a steel sheet increases, the value of Ceq becomes higher. Therefore, as the steel sheet becomes stronger, the cross tensile strength becomes lower than that of mild steel. The reason why the cross tensile strength decreases as the carbon equivalent Ceq increases is that the nugget (welding) increases as the carbon equivalent increases.
This is considered to be because the hardness of the heat-affected zone and the heat-affected zone increases, the toughness decreases, and fracture easily occurs. In particular, in the case of the cross tensile strength, the stress concentration around the nugget is severe, so this effect is remarkable, and when the hardness of the welded portion and the heat affected zone show a high value, the cross tensile strength is high. Indicates a low value.

[0005] In general, when the diameter of the nugget is large, the fracture occurs around the nugget. However, when the Ceq is high, a crack partially enters the nugget and breaks. Therefore, the hardness of the inside of the nugget and the heat-affected zone greatly affects the joint strength.

In spot welding, the electrode is usually kept in contact with the steel plate for a certain period of time even after the completion of welding.
In particular, in welding high-strength steel sheets, the electrode holding time after energization is often set to be relatively long due to concerns about springback. On the other hand, since the spot welding electrode is water-cooled, heat is rapidly taken away from the welding portion toward the electrode during electrode holding after the completion of welding, and the welding portion is rapidly cooled. It is considered that the hardness of the steel increases.

On pages 34 to 38 of "Welding Techniques", March 1982, in order to improve the U-shaped tensile strength at the spot welded portion of a high-strength steel plate, the electrode holding time after the completion of energization is reduced. A method of reducing the hardness of the weld by shortening it is disclosed. However, if the electrode holding time is extremely short, the pressing force by the electrode is eliminated before the nugget portion solidifies, so that the molten metal jumps out of the nugget portion and a defect occurs in the nugget.

In “Iron and Steel,” Vol. 68, No. 9, pp. 1444 to 1451, after welding current is completed, current is again applied after a certain period of time (tempering), and the nugget portion and the heat-affected zone are exposed. A method of annealing to reduce the hardness and improve the cross tensile strength of the spot welded part. The same page of “Welding Technique” also discloses that post-heating after welding is effective in recovering the fracture shape and strength in a peeling test of a welded portion. However, this method was considered to be impractical in mass-production factories due to the narrow range of appropriate conditions.

[0009]

In the spot welding of high-strength steel sheets, the prior arts aimed at improving the cross tensile strength of the welded portions are all applied to mass production sites as described above. Does not have sufficient stability.

In order to ensure the quality of the weld, it is necessary to keep the cooling conditions after welding constant. The proper operating conditions for keeping the cooling conditions after welding constant vary depending on the type and thickness of the steel sheet. Therefore, it is necessary to find out the optimal operating conditions for each type and thickness of the steel plate to be spot-welded, and the production management is complicated.

[0011] The present invention relates to a method for spot welding of high-strength steel sheets, which provides a welding method with excellent cross-tensile strength at a welded portion, and provides a stable welding method applicable to mass production sites. It is a second object of the present invention to provide a welding method capable of securing the quality of a welded portion even when welding steel sheets of different steel types and thicknesses.

[0012]

That is, the gist of the present invention is as follows. (1) In the spot welding method for a high-strength steel sheet, the electrode is separated from the steel sheet after a lapse of a holding time after welding after the completion of welding, and the holding time after welding is within a range of the following formula (1). Spot welding method for high strength steel sheet. ^{300-500t + 250t 2 ≦ HT ≦ 560-900t} + 500t 2 (1) except, t is thickness (mm), HT retention time after welding (m
s) (2) In the spot welding method for a high-strength steel sheet, the post-energization is continued even after the end of the welding energization, and the rate of temperature decrease during cooling of the spot-welded portion is adjusted. . (3) The temperature drop rate of the spot weld is adjusted by setting the post-energization current to a range of 70% to 90% of the welding energization current, and setting the post-energization time to a range of 60% to 100% of the welding energization time. (2) characterized in that:
The spot welding method for a high-strength steel sheet according to item 1. (4) The high-strength steel sheet according to any one of (1) to (3), wherein the high-strength steel sheet has a component range of the following formula (2) and a tensile strength of 420 MPa or more and 1200 MPa or less. Spot welding method for high strength steel sheets. 0.20 ≦ C + Si / 30 + Mn / 20 + 2P + 4S ≦ 0.60 (2) where C, Si, Mn, P and S are the contents (% by mass) of carbon, silicon, manganese, phosphorus and sulfur in the steel, respectively. is there.

[0013]

FIG. 1 is a diagram for explaining a spot welding method used in the present invention. As shown in FIG. 1, two high-strength steel sheets 1 to be welded are overlapped,
Electric current is applied while applying pressure with the copper electrode 3 to form a molten portion between the two high-strength steel sheets. After the energization is completed, the molten portion is solidified to form the nugget 2 and spot-welded. At this time, FIG.
As shown by, the control device 4 can control the conduction current and the conduction time.

The above (1) of the present invention will be described. 2. Description of the Related Art A method for reducing the hardness of a welded portion by shortening a post-weld holding time after completion of energization to improve the quality of a spot welded portion of a high-strength steel plate has been conventionally known. The above (1)
In the present invention, the post-weld holding time after the completion of the energization is not fixed, but the post-weld holding time is adjusted according to the thickness of the steel plate to be welded. This makes it possible to always perform treatment at the most appropriate timing. That is, there is no trouble that the molten metal jumps out of the nugget part because the electrode holding time is too short, and there is no trouble that the hardness of the welded part increases because the electrode holding time is too long.

In spot welding, the cooling rate of the weld decreases as the plate thickness increases. Therefore, it is desirable to set the holding time as a function of the sheet thickness. As a result of experiments, the present inventors have found that, at each plate thickness, when the holding time after welding HT is shorter than the lower limit of the formula (1), the molten metal is scattered, and the holding time after welding is longer than the upper limit of the formula (1). In this case, it was found that the cooling rate of the weld became too fast, the hardness of the nugget and the heat affected zone increased, and the cross tensile strength decreased.

In the above (2) of the present invention, the post-energization is continued even after the end of the welding energization, and the post-energization is performed at a current value lower than the welding energizing current, thereby lowering the cooling rate of the welded portion after the welding is completed. Therefore, even in the case of spot welding of a high-strength steel plate, the quality of a welded portion can be improved.

As measures for improving the quality of spot welds of high-strength steel sheets, reduction of the electrode holding time after welding and post-heating tempering have been conventionally performed. In shortening the electrode holding time after welding, a very small change in the steel type, plate thickness, and electrode condition may cause a situation in which the molten metal jumps out of the nugget and a defect occurs in the nugget. Further, the post-heating tempering treatment has a narrow range of appropriate conditions, and cannot be applied as a practical method in a mass-production factory. On the other hand, in the present invention (2), the quality is improved based on slow cooling by post-energization after the completion of welding, and even if the steel type, plate thickness, and electrode condition change, it is possible to reduce the electrode holding time. It is characterized in that no trouble occurs, the range of appropriate conditions is wider than in the post-heating tempering process, and the quality improvement effect can be stably obtained.

In the above (3) of the present invention, the post-energization current of the invention of the above (2) is set to a range of 70% to 90% of the welding energization current, and the post-energization time is set to 6% of the welding energization time.
The temperature falling rate is adjusted by setting the range of 0% to 100%. If the current is less than 70% and the time is less than 60%, the cooling rate in the temperature range of 1000 ° C. or more in the nugget portion and the heat-affected zone is too fast, and if the current exceeds 90%, the welded portion is melted again and the time is 100%. This is because, if it exceeds, the work efficiency will be poor.

Generally, in the nugget part and the heat-affected zone, S
Although segregation such as i, Mn, P, and S is observed, when Ceq is high, the segregation becomes remarkable, and as a result, the cross tensile strength decreases. However, the cooling rate is reduced to 100
By increasing the holding time at 0 ° C. or higher, diffusion occurs, and this segregation is alleviated, and the cross tensile strength is improved.

The welding welding current and welding welding time are adjusted according to the type of steel to be welded and the thickness so as to optimize the shape of the nugget generated by spot welding, particularly the nugget diameter. If the post-energization current is determined according to the welding energization current and the post-energization time is determined according to the welding energization time as in the invention of the above (3), the welding energization current and the welding energization time are determined by the steel type. Therefore, the cooling control by post-energization after welding is also optimized for the steel type and the sheet thickness.

In the above invention (4), the grounds for determining the numerical value range are as follows. When Ceq in the formula (2) is 0.
When it is less than 20, hardness does not matter, and it is 0.60.
This is because, if it exceeds, even the method of the present invention cannot improve cross tensile strength. Similarly, the tensile strength is 4
When it is less than 20 MPa, hardness does not matter, and 1
If it exceeds 200 MPa, the method of the present invention cannot improve the cross tensile strength.

The high-strength steel sheet used in the present invention is not particularly limited as long as the steel sheet has a tensile strength of 420 MPa or more and 1200 MPa or less. Any type of steel sheet, such as a two-phase structure type (a structure containing martensite in ferrite or a structure containing bainite in ferrite) or a work-induced transformation type (a structure containing residual austenite in ferrite), may be used. . About the thickness, the thickness of a steel sheet generally used in automobiles, for example, 0.4 mm to
It may be about 4.0 mm. The method for producing the steel sheet may be a hot rolling method or a cold rolling method.

Spot welding conditions for forming a nugget,
That is, the electrode shape, electrode pressing force, welding current, and welding time may be in accordance with general spot welding conditions.

[0024]

(Example 1) Spot welding of a high-strength steel sheet was performed by using the invention of the above (1). As a material to be welded, a high-strength steel plate (symbol: 780T) of a work-induced transformation composite structure having a thickness of 1.2 mm and a tensile strength of 812 MPa, and a mild steel plate having the same thickness and a tensile strength of 362 MPa as a comparative material (Symbol: SPHC) was used. CFR type was used as the spot welding electrode, the tip diameter was 5.5 mm, the tip curvature diameter was 40 mm, and the material was chromium copper. A cross tensile test piece was prepared based on a tensile test method (JIS Z3137) for spot welded joints.

Since the plate thickness is 1.2 mm, the lower limit of HT in equation (1) is calculated to be 60 ms, and the upper limit is calculated to be 200 ms.

The welding current and welding time shown in Table 1 are optimized under the condition that the nugget diameter becomes 5.5 mm. The time from the end of energization to the separation of the electrode is the post-welding hold time H
T, as shown in Table 1, the holding time after welding HT
Under the conditions 1 and 2 (retention time after welding: 60 m, 100 ms) in which the 780T is within the range of the formula (1), the cross tensile strength of the 780T welded joint was at the same level as that of the SPHC welded joint. On the other hand, under conditions 3 and 4 (retention time after welding: 300 ms and 500 ms) in which the retention time HT after welding is outside the upper limit of the equation (1), the cross tensile strength of the 780T welded joint is SPHC welded joint (conditions 7 and 8). It showed a lower value. Further, under the conditions 5 and 6 (retention time after welding: 0 ms, 20 ms) in which the post-weld retention time HT is below the lower limit of the equation (1), the molten portion is scattered from between the steel plates and defects are generated in the nugget. 780
The cross tensile strength of the T welded joint is SPHC welded joint (condition 7
And 8) lower values. As described above, by adjusting the post-weld holding time HT based on the equation (1), 78
The cross tensile strength of the 0T welded joint can be made the same level as that of the SPHC welded joint. Similar experiments were performed with other steel types or plate thicknesses changed, but the results were similar.

[0027]

[Table 1]

Example 2 Spot welding of a high-strength steel sheet was performed using the invention of the above (3). As a material to be welded,
Work-induced transformation composite structure high-strength steel sheet (symbol: 780T) having a thickness of 1.2 mm and tensile strength: 812 MPa, and a mild steel sheet having the same thickness and a tensile strength of 362 MPa (symbol: SPHC) as a comparison material Was used. The same spot welding electrode as in Example 1 was used.

The welding current and welding time are optimized under the condition that the same nugget diameter can be obtained. At the time of spot welding, as shown in Table 2, a constant current was passed for a fixed time after welding. Table 2 shows the results of measuring the cross tensile strength.
Under conditions 1 and 2, the cross tensile strength of the 780T welded joint was S
It was at the same level as the PHC welded joint (condition 8). Condition 3
-6, the cross tensile strength of the 780T welded joint is SPHC
The value was lower than that of the welded joint (condition 8). That is, as described in the above (3) of the present invention, the post-current is set to 70% of the welding current.
It has been found that good cross tensile strength can be obtained by setting the post-energization time to a range of 60% to 100% of the welding energization time to 90% or less.

The invention of the above (3) was carried out when conditions different from those in Table 2 were adopted for the steel type and the plate thickness. Since the welding current and welding time are optimized for the steel type and the sheet thickness, and the post-energization current and post-energization time are adjusted based on the welding current and welding time, the cross tensile strength is set to SPH.
It could be maintained at the same level as the C welded joint (condition 8).

[0031]

[Table 2]

[0032]

According to the present invention, in a spot welding method for a high-strength steel sheet, the electrode is separated from the steel sheet after the elapse of a holding time after welding determined according to the sheet thickness, thereby stably maintaining good welding quality. Can be.

According to the present invention, in a spot welding method for a high-strength steel sheet, the post-energization is continued even after the end of the welding energization, and the temperature drop rate during cooling of the spot weld is adjusted to stabilize the welding quality. Can be kept good.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining a spot welding method according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 High-strength steel plate 2 Nugget 3 Copper electrode 4 Control device

Claims (4)

[Claims] 1. A spot welding method for a high-strength steel sheet, wherein the electrode is separated from the steel sheet after a lapse of a post-weld holding time after the end of welding, and the post-weld holding time is within a range of the following equation (1). Spot welding method for high strength steel sheet. ^{300-500t + 250t 2 ≦ HT ≦ 560-900t} + 500t 2 (1) except, t is thickness (mm), HT retention time after welding (m
s) 2. A spot welding method for a high-strength steel sheet, characterized in that the post-energization is continued even after the end of the welding and the temperature drop rate during cooling of the spot-welded portion is adjusted. . 3. The post-energization current is in the range of 70% to 90% of the welding energization current, and the post-energization time is set to 6% of the welding energization time.
3. The spot welding method for a high-strength steel sheet according to claim 2, wherein the temperature drop rate of the spot weld is adjusted by setting the range of 0% to 100%. 4. The high-strength steel sheet has a component range represented by the following formula (2), and has a tensile strength of 420 MPa or more and 1200 MPa.
The spot welding method for a high-strength steel sheet according to any one of claims 1 to 3, wherein a is equal to or less than a. 0.20 ≦ C + Si / 30 + Mn / 20 + 2P + 4S ≦ 0.60 (2) where C, Si, Mn, P, and S are the contents (% by mass) of carbon, silicon, manganese, phosphorus, and sulfur in the steel, respectively. is there.