KR102491219B1 - Resistance spot welding member and manufacturing method thereof - Google Patents

Abstract

By improving the structure of the nugget end, a resistance spot welded member excellent in delayed fracture resistance and a manufacturing method thereof are provided. The present invention is provided with two or more steel plates and a spot weld formed between the steel plates, the tensile strength of at least one of the steel plates is 980 MPa or more, and in the steel plate, X = [C] + [Si] / 40 + When X of the steel sheet having the largest coefficient X represented by [Mn]/200 is X _max , and Y of the steel sheet having the smallest coefficient Y represented by Y = [P] + 3 × [S] is Y _min , the Vickers hardness H _n (HV) of the nugget end of the spot weld is equal to or less than H _ob (HV) represented by H _ob = (800 × X _max + 300)/(0.7 + 20 × Y _min ), and the welding heat of the spot weld A resistance spot welded member in which the Vickers hardness H _min (HV) of the softest part of the affected part satisfies 0.4 × H _n ≤ H _min ≤ 0.9 × H _n .

Description

Resistance spot welding member and manufacturing method thereof

The present invention relates to a resistance spot welded member and a manufacturing method thereof.

[0002] Efforts are being made to increase the strength of steel sheets used and to reduce the sheet thickness in order to achieve both the reduction in weight of automobile bodies and the improvement of crash safety for improving fuel efficiency of automobiles. However, with the increase in tensile strength of automotive steel sheets to a level of 980 MPa or more, there has been concern about a decrease in delayed fracture resistance of welded parts.

To explain in detail, resistance spot welding is a welding method mainly used in automobile production processes, but a welded part of this resistance spot welding tends to undergo martensite transformation when the molten portion is rapidly cooled, resulting in a hard structure. In addition, tensile residual stress is generated in the welded portion due to thermal contraction during the cooling process. In addition, there are cases where hydrogen enters the weld metal during welding from a plating layer on the surface of the steel sheet, oil or moisture on the surface of the steel sheet, or hydrogen enters the welded portion from a working environment (for example, in an acidic environment). Therefore, the welded part of resistance spot welding may be in a very unfavorable state from the viewpoint of delayed fracture resistance characteristics.

Conventionally, since the strength of the steel sheet was not so high, the stress concentration in the welded portion was relatively small, and delayed failure was not a problem. However, in a high-strength steel sheet having a tensile strength of 980 MPa or more, since it contains a large amount of hardenable elements such as carbon, the nugget and its vicinity become very hard, and delayed fracture easily occurs.

In addition, a resistance spot welded portion having such a very hard nugget is highly sensitive to P and S, which are elements that decrease the grain boundary strength of the nugget structure. For this reason, in resistance spot welding members made of steel sheets containing a large amount of these elements, the strength of the nugget is lowered, and delayed fracture is more likely to occur. The problem of delayed fracture of this resistance spot welded member may occur even in plywood assembly with mild steel as long as it is a plate assembly made of high-strength steel plate. In addition, in the high-strength steel sheet, since press forming is difficult, gaps are likely to occur between the steel sheets when the high-strength steel sheets are stacked. For this reason, in resistance spot welding in which this gap is forcibly closed with a pair of opposing electrodes, tensile stress resulting from the gap between the steel plates is added to the end of the nugget, further causing delayed fracture. It is thought to be in an easy state.

As a method for preventing delayed fracture of such a welded portion, Patent Document 1 discloses that the structure and hardness of a welded portion are controlled by increasing the pressing force immediately after welding current (main current) and reducing the current to prevent delayed fracture. A technique for preventing this has been disclosed.

Further, Patent Literature 2 discloses a technique for preventing delayed fracture by controlling the structure and hardness of the welded part by increasing the pressing force immediately after welding current supply (main supply power supply) and supplying electricity after cooling time without electricity supply. .

Japanese Unexamined Patent Publication No. 2015-93282 Publication WO 2014/171495

However, these techniques require long-term post-energization in order to obtain the effect of preventing delayed fracture. For this reason, when the input heat amount is excessive, the strength of the heat-affected zone is reduced due to excessive softening, and fracture from the heat-affected zone may occur at low stress. In addition, these techniques do not consider the influence of P and S, which are elements that cause a decrease in the strength of the nugget, at all.

In addition, the problem that delayed fracture occurs due to hydrogen entering the weld metal having a high hydrogen embrittlement susceptibility during such welding is not only in the case of resistance spot welding of high-strength steel sheets for automobiles, but also in resistance spot welding of other steel sheets. exist the same

The present invention has been made in view of such circumstances, and its object is to provide a resistance spot welded member excellent in delayed fracture resistance and a manufacturing method thereof by improving the structure of the nugget end (nugget end).

The inventor of the present invention adjusts the hardness of the spot welded part by improving the structure of the nugget end, which is the starting point of delayed fracture, so that even in the case of a plate assembly including a steel plate having a tensile strength of 980 MPa or more, spot welding excellent in delayed fracture resistance I thought it was possible to provide a member.

Therefore, the elements P and S, which cause the hardness of the nugget end and the decrease in the grain boundary strength, which are the factors of delayed fracture, were investigated, and the following knowledge was obtained.

Examples of alloying elements in the steel sheet that affect the hardness of the nugget include C, Mn, and Si. As the content of these elements in the steel sheet increases, the delayed fracture susceptibility of the resistance spot welded member increases. However, by reducing the hardness of the nugget end according to the content of these elements, it becomes possible to improve the delayed fracture resistance of the resistance spot welded member of the plate assembly including the above steel plate.

However, when the steel sheet contains a large amount of P and S, the concentration of P and S at the end of the nugget increases, and delayed fracture is more likely to occur. Further, the nugget is a structure formed by melting a plurality of steel sheets to be welded. Therefore, even if the high-strength steel sheet does not contain much P and S, for example, when the content of P and S is high in other steel sheets included in the plate assembly, delayed fracture easily occurs. Therefore, it is necessary to appropriately control the hardness of the nugget end in consideration of not only the contents of C, Mn, and Si, but also the contents of P and S in the steel sheet, thereby making it possible to manufacture a resistance spot welded member having excellent delayed fracture resistance. do.

As a method for controlling the hardness of the nugget end, in the resistance spot welding process, after the main energization process for forming the nugget is completed, it is an effective means to apply a secondary energization process for the purpose of tempering. However, when the post-energization step for tempering the nugget end takes a long time, the effect of tempering by post-energization extends to the heat-affected zone outside the nugget (hereinafter sometimes referred to as a weld heat-affected zone) and excessive softening. It may cause a decrease in strength. Therefore, controlling the hardness of the nugget end without excessively softening the heat-affected zone by passing a high current equal to or higher than the current value in the main energization step after the main energization step and supplying the amount of heat necessary for tempering to the nugget end in a short time it becomes necessary

This invention was made based on the above knowledge, and the summary is as follows.

[1] A resistance spot welding member having two or more steel plates and a spot welded portion formed between the steel plates, wherein at least one of the two or more steel plates has a tensile strength of 980 MPa or more, and in the two or more steel plates, When X of the steel sheet having the largest coefficient X expressed by the formula (1) is X _max , and Y of the steel sheet having the smallest coefficient Y expressed by the following formula (2) is Y _min , the nugget end of the spot welded portion is The Vickers hardness H _n (HV) of is less than or equal to H _ob (HV) represented by the following formula (3), and the Vickers hardness H _min (HV) of the softest zone in the heat-affected zone of the spot welded part satisfies the following formula (4) , resistance spot welding member.

X = [C] + [Si]/40 + [Mn]/200 (1)

Y = [P] + 3 × [S] (2)

H _ob = (800 × X _max + 300)/(0.7 + 20 × Y _min ) (3)

0.4 × H _n ≤ H _min ≤ 0.9 × H _n (4)

In the formulas (1) and (2), [C], [Si], [Mn], [P], and [S] represent the content (% by mass) of each element. However, when it does not contain, it is set to 0.

[2] A method for manufacturing a resistance spot welded member, in which two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more are overlapped, sandwiched by a pair of welding electrodes, and then energized while pressing to form a nugget. process, and a cooling process of cooling the nugget by pressing and holding the steel sheet with the welding electrode for a cooling time C _t (ms) represented by the following formulas (5) and (6) after the main energization process, and after the cooling process , a method for manufacturing a resistance spot welded member having a post-energization step of conducting current at a current value I _p (kA) satisfying the following formula (7).

C _t ≥ 160 × t ² (t ≤ 1.6) (5)

_Ct ≥ 256 × t (t > 1.6) (6)

0.8 × I _min ≤ I _p < 1.5 × I _max (7)

In the above formulas (5), (6) and (7),

t: Average plate thickness of steel plates to be joined (mm)

I _max : maximum current value in the main energization process (kA)

I _min : Minimum current value (kA) in the main energization process

to be.

[3] After the post-energization process, cooling and energization in which cooling is performed under conditions satisfying the following equation (8), and re-energization in which re-energization is performed under conditions satisfying the following equation (9) after the cooling and energization The method for manufacturing a resistance spot welded member according to [2], which includes a repetitive energization step performed n times.

0 ≤ I _nc ≤ I _max (8)

0.8 × I _min ≤ I _nr < 1.5 × I _max (9)

In the above formulas (8) and (9),

I _nc : Current value at the nth cooling pass (kA)

I _nr : Current value at the nth re-energization (kA)

I _max : maximum current value in the main energization process (kA)

I _min : Minimum current value (kA) in the main energization process

n : a natural number greater than or equal to 1

to be.

According to the present invention, even in the case of performing resistance spot welding of high-strength steel sheet, it is possible to improve the structure of the nugget end while preventing the strength decrease due to softening of the heat-affected zone, and to obtain a resistance spot welded member having excellent delayed fracture resistance. .

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram for demonstrating a spot welding part.
2 is a cross-sectional view schematically showing an example of resistance spot welding.
Fig. 3(a) to Fig. 3(c) are views showing resistance spot welding test pieces, Fig. 3(a) is a plan view, and Figs. 3(b) and 3(c) are side views.
Fig. 4 is a cross-sectional view illustrating a nugget end of the present invention.
Fig. 5 (a) is a cross-sectional view illustrating the softest zone of the heat-affected zone according to the present invention, and Fig. 5 (b) is a graph showing the Vickers hardness at the nugget end and the heat-affected zone.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described. In addition, this invention is not limited to the following embodiment.

The resistance spot welding member of the present invention has two or more steel plates and a spot welded portion. Two or more steel plates and spot welds are described in order.

steel plate

The two or more steel plates include steel plates having a tensile strength of 980 MPa or more (sometimes referred to as "high-strength steel plates"). If a steel sheet having a tensile strength of 980 MPa or more is used, delayed fracture of the spot welded portion tends to become a problem. However, as will be described later, in the resistance spot welded member of the present invention, since the hardness of the spot welded part is adjusted, the delayed fracture resistance of the spot welded part is improved even when a high-strength steel sheet is used. In addition, since the problem of delayed fracture time tends to occur as the amount of steel sheets having a tensile strength of 980 MPa or more is increased (the number of sheets used), the effect of the present invention is that the tensile strength of at least half of the two or more steel sheets is 980 MPa. In the case of an abnormality, it appears more remarkably.

In addition, the upper limit is not particularly limited as long as the number of steel sheets is 2 or more, but considering the ease of welding and the like, the number of steel sheets is preferably 4 or less.

The component composition of the above two or more steel sheets is not particularly limited, but, in terms of mass%, C: 0.6% or less, Si: 3.0% or less, Mn: 20.0% or less, P: 1.0% or less, S: 0.8% or less, Al: 3.0 A component composition containing % or less and the balance being Fe and unavoidable impurities is preferable.

All or part of the said two or more steel plates may be plated and have a plated layer on the surface. As the plating, Zn-based plating and Al-based plating are exemplified. Examples of Zn-based plating include hot-dip galvanizing (GI), Zn-Ni-based plating, and Zn-Al-based plating. Moreover, as Al-type plating, Al-Si-type plating (For example, Al-Si-type plating containing 10-20 mass % of Si) etc. can be illustrated. The hot-dip plating layer may be an alloyed hot-dip plating layer. As an alloyed hot-dip galvanized layer, an alloyed hot-dip galvanized (GA) layer is mentioned, for example.

The sheet thickness of the above two or more steel sheets is not particularly limited, but is preferably within a range of 0.5 mm or more and 2.0 mm or less, for example. A steel sheet having a sheet thickness within this range can be suitably used as a member for automobiles.

The two or more steel plates may be the same or different, and may be steel plates of the same type and shape, or may be steel plates of different types or shapes.

Since the two or more steel plates are steel plates constituting the resistance spot welding member, they are in an overlapping state. The following spot weld is formed between the overlapping steel plates.

spot weld

As shown in FIG. 1 , the spot welded portion 12 is formed between two or more steel plates 15 and is composed of a central nugget 13 and a welding heat affected zone 14 outside the nugget. The boundary between the nugget 13 and the heat-affected zone 14 can be determined visually by performing corrosion using an aqueous solution of picric acid in the plate thickness section of the spot welded part.

In two or more steel plates, X of the steel plate in which the coefficient X represented by the following formula (1) is the largest is set to X _max , and Y of the steel plate in which the coefficient Y represented by the following formula (2) is the smallest is set to Y _min , the Vickers hardness H _n (HV) of the nugget end of the spot weld is equal to or less than H _ob (HV) expressed by the following formula (3).

X = [C] + [Si]/40 + [Mn]/200 (1)

Y = [P] + 3 × [S] (2)

H _ob = (800 × X _max + 300)/(0.7 + 20 × Y _min ) (3)

In Formula (1) and Formula (2), [C], [Si], [Mn], [P], and [S] are content (mass %) of each element. However, when it does not contain, it is set to 0.

Since the hardness of a spot welded part is affected by C, Si, and Mn, it is necessary to consider C, Si, and Mn. In the present invention, the influence of C, Si, and Mn is considered by X = [C] + [Si]/40 + [Mn]/200 (Formula (1) above). Further, P and S are elements that cause a decrease in the strength of the nugget structure. For this reason, in the present invention for improving delayed fracture, it is necessary to consider the influence of P or S. So, in this invention, the influence of S or P is considered by Y=[P]+3x[S] (the said (2) formula). Coefficients related to each element in X and Y are determined by considering the magnitude of the influence of each element.

The Vickers hardness H _n (HV) of the nugget end of the spot welded part is equal to or less than H _ob (HV) expressed by the above formula (3). When the Vickers hardness at the end of the nugget exceeds H _ob (HV) represented by formula (3), delayed fracture is likely to occur.

In the case where the effect of improving the delayed fracture resistance is intended to be exhibited more remarkably, the Vickers hardness H _n (HV) of the nugget end is set to H _ob2 (HV) or less represented by the following formula (10), or (0.95 × H _ob ) or less is more preferable.

H _ob2 = (800 × X _max + 300)/(0.7 + 20 × Y _max ) (10)

Here, in the formula (10), Y _max : Y of the steel sheet in which the coefficient Y represented by the formula (2) is the largest.

Also, the Vickers hardness H _n of the nugget end is preferably (0.4 × H _ob ) or more. When the Vickers hardness H _n of the nugget end is smaller than (0.4 × H _ob ), the strength of the nugget itself decreases, and joint performance other than delayed fracture resistance, such as joint strength, may decrease.

Here, the nugget end is described using FIG. 4 . 4 also shows an enlarged view of the nugget end 16 in the spot welded portion 12 . As shown in FIG. 4 , the nugget end 16 is a plate thickness cross section passing through the center of the nugget 13 of the resistance spot welded member, from the boundary between the nugget 13 and the heat-affected zone 14 to the nugget ( 13) means a position of 50 μm toward the center. Further, the nugget end 16 extends 50 μm toward the center of the nugget 13 from two intersections between the boundary line between the steel plates 15 and the nugget 13 in the plate thickness cross section passing through the center of the nugget 13. , and the one with the smaller Vickers hardness value is defined as the Vickers hardness H _n of the nugget end. In addition, when the value of the Vickers hardness is the same, the value is defined as the Vickers hardness H _n of the nugget end.

In the present invention, as long as the Vickers hardness H _n (HV) is within a certain range centered at a position of 50 µm from the boundary toward the center of the nugget 13, the above-described effect can be obtained similarly. The range within this constant range is a region of 40 to 60 µm from the boundary toward the center of the nugget. Therefore, it is assumed that the "nugget end" includes a region of 40 to 60 µm from the boundary toward the center of the nugget.

When three or more steel plates are welded, a plurality of nugget ends are formed, but it is only necessary that at least one nugget end of a plate assembly including steel plates having a tensile strength of 980 MPa or more satisfies Expression (3).

In the present invention, the Vickers hardness H _min (HV) of the softest part of the heat-affected zone of the spot weld zone satisfies the following formula (4).

0.4 × H _n ≤ H _min ≤ 0.9 × H _n (4)

When the Vickers hardness H _min (HV) of the softest zone of the weld heat-affected zone is less than (0.4 × H _n ) (HV), excessive softening of the weld heat-affected zone tends to cause a decrease in strength. In addition, when the Vickers hardness H _min (HV) of the softest zone of the weld heat-affected zone exceeds (0.9 × H _n ) (HV), high stress is concentrated locally at the end of the nugget, and delayed fracture tends to occur. In addition, when it is desired to exhibit the effect of suppressing delayed fracture more remarkably without causing a decrease in strength, the Vickers hardness H _min of the softest zone of the weld heat-affected zone is preferably (0.5 × H _n ) or more, and (0.8 × H _n ) or less is preferable.

Here, the softest zone of the welding heat-affected zone of the spot weld zone will be described using FIGS. 5(a) and 5(b). Fig. 5(a) is a sectional view in the plate thickness direction passing through the center of the nugget 13, and Fig. 5(b) shows the distance from the boundary (mm) and the Vickers hardness ( HV) is a graph that represents

As shown in FIG. 5(a) and FIG. 5(b), the softest part of the welding heat-affected zone 14 of the spot welded part 12 means the following position. 3 mm outside the nugget 13 from the boundary between the nugget 13 and the weld heat-affected zone 14 on a straight line passing through the center of the nugget 13 and the aforementioned nugget end 16 (not shown) Regarding the area, it means the part (position) showing the softest hardness when measured at intervals of 150 μm from the boundary. In addition, hardness can be measured by the method described in the Examples described later.

Next, the manufacturing method of the resistance spot welded member of this invention is demonstrated. The manufacturing method of this invention has a main energization process, a cooling process, and a post energization process. The production method of the present invention is described using FIGS. 1 and 2 .

The main energization step is a step in which two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more are overlapped, held together by a pair of welding electrodes, and energized while pressurizing to form a nugget.

As shown in Fig. 2, a steel plate disposed on the lower side (hereinafter referred to as lower plate 1) and a steel plate disposed on the upper side (hereinafter referred to as upper steel plate 2) are overlapped. At this time, at least one of the lower plate 1 and the upper plate 2 is a steel plate having a tensile strength of 980 MPa or more. Next, a pair of welding electrodes, that is, an electrode arranged on the lower side (hereinafter referred to as lower electrode 4) and an electrode disposed on the upper side (hereinafter referred to as upper electrode 5) are overlapped with a steel plate (lower plate). (1) and the upper steel plate (2) are clamped and energized while pressurizing. The configuration for applying pressure by the lower electrode 4 and the upper electrode 5 and controlling the pressing force is not particularly limited, and conventionally known devices such as air cylinders and servomotors can be used. In addition, there is no particular limitation on the configuration of supplying current at the time of power supply and controlling the current value, and conventionally known devices can be used. In addition, the present invention can be applied to either direct current or alternating current. In the case of alternating current, "current" means "effective current". Also, the shape of the tip of the lower electrode 4 or the upper electrode 5 is not particularly limited, and for example, DR type (dome radius type) and R type (radius type) described in JIS C 9304: 1999 ), D type (dome type), and the like. Moreover, the tip diameter (tip diameter) of an electrode is 4 mm - 16 mm, for example.

The main energization step performed as described above is a step of melting the steel sheet to form a nugget. The energization conditions and pressurization conditions for forming the nugget are not particularly limited, and conventionally used welding conditions can be employed. For example, the current value is 1.0 kA or more and 15.0 kA or less, and the pressing force is 2.0 kN or more and 7.0 kN or less. Moreover, the energization time is not particularly limited either, and is, for example, 100 ms or more and 1000 ms or less. In addition, a "nugget" is a portion that is melted and solidified at a spot welded portion in lap resistance welding.

After the main energization step, a cooling step is performed in which the nugget is cooled by holding the steel sheet while being pressed with the welding electrode for a cooling time C _t (ms) represented by the following expressions (5) and (6).

C _t ≥ 160 × t ² (t ≤ 1.6) (5)

_Ct ≥ 256 × t (t > 1.6) (6)

In Formula (5) and Formula (6), t: It is the average plate|board thickness (mm) of the to-be-joined steel plate.

A cooling process is a process necessary in order to obtain the effect of tempering by the post-energization process mentioned later. If the cooling time C _t (ms) does not satisfy the equations (5) and (6), the nugget end is heated by subsequent energization without being sufficiently cooled. In that case, the effect of tempering cannot be obtained, and the hardness of the nugget end cannot be reduced. The cooling time C _t (ms) depends on the plate thickness of the steel plates, and when joining steel plates having different plate thicknesses, it is taken as the average value of the plate thicknesses of the respective steel plates. Further, in the case where the end of the nugget is to be cooled more sufficiently and the effect of tempering in subsequent energization is more remarkably exerted, in the case of t ≤ 1.6, C _t ≥ 200 × t ² , in the case of t > 1.6, It is preferable to set C _t ≥ 320 × t.

Although the upper limit of the cooling time C _t (ms) is not particularly specified in the present invention, it is preferable to set it as C _t < 800 × t. When C _t is (800 × t) or more, the total time of the welding process itself becomes long and productivity decreases.

From the above, in the production method of the present invention, the cooling step is performed after passing through the process of determining the conditions for the cooling time based on the formulas (5) and (6). In addition, the average plate thickness of the steel plates to be joined means the average of the plate thicknesses of all steel plates to be welded.

After the cooling process, a post-energizing process is performed. The subsequent energization step is a step of conducting current at a current value I _p (kA) that satisfies the following formula (7).

0.8 × I _min ≤ I _p < 1.5 × I _max (7)

(7) Formula WHEREIN: I _max : The maximum current value (kA) in a main power supply process, I _min : It is the minimum current value (kA) in a main power supply process.

The post-energizing process is a process of reheating the end of the nugget and reducing the hardness of the end of the nugget by tempering. When the current value I _p (kA) in the post-energizing step is less than (0.8 × I _min ), the input heat amount is insufficient and the hardness of the nugget end cannot be reduced. In addition, when the current value I _p (kA) in the post-energization step is more than (1.5 × I _max ), the amount of heat input becomes excessive and exceeds the temperature range in which the effect of tempering the nugget end is obtained. Therefore, the hardness of the nugget end cannot reduce Further, in the case where the effect of reducing the hardness of the nugget end and improving the delayed fracture characteristics is desired to occur more remarkably, the current value I _p (kA) in the post-energizing step is 0.95 × I _min ≤ I _p <1.2 × I _max is preferably satisfied. Further, the lower limit of the current value I _p (kA) in the post-energizing step is more preferably set to (1.0 × I _min ) or more. From the above, in the manufacturing method of the present invention, after going through the process of determining the conditions for the current value _Ip based on the formula (7), the post-energizing step is performed.

If the conduction time of the post-energization step is less than 20 ms, the nugget may not be sufficiently heated and the effect of tempering may not be obtained. If the conduction time of the post-energization step exceeds 200 ms, the amount of heat input is sufficient and the hardness of the nugget end is reduced, but the strength of the heat-affected zone is reduced due to excessive softening, and fracture occurs from the heat-affected zone at low stress there is In addition, when the current value in the post-energizing step is high and the input heat amount is excessive, the nugget melts again, and the effect of tempering cannot be obtained, causing problems such as scattering. Therefore, the conduction time of the post-energization step is preferably within the range of 20 to 200 ms. More preferably, it is 20-100 ms.

Moreover, you may perform a repetitive energization process after the said post-energization process. The repeated energization step is a cooling energization in which cooling is performed after the post-energization step and energization is performed under conditions satisfying the following equation (8), and a re-energization in which re-energization is performed under conditions satisfying the following equation (9) after the cooling and energization, n It is a process that is carried out once in a while.

0 ≤ I _nc ≤ I _max (8)

0.8 × I _min ≤ I _nr < 1.5 × I _max (9)

In the formulas (8) and (9), I _nc : current value (kA) in the n-th cooling pass, I _nr : current value (kA) in the n-th re-energization, n: 1 or more is a natural number I _max and I _min are the same as in Formula (7).

By performing the repeated energization process, the effect of reducing the hardness by tempering the nugget end can be exhibited more remarkably, and the delayed fracture resistance of the welded member can be further improved.

In the repeated energization process, by repeating cooling and heating, the end of the nugget can be kept in an appropriate temperature range for a long time, and the effect of tempering can be further exhibited. Further, since the cooling current is a process of cooling the end of the nugget, it may be performed with no current applied. In addition, when the current value I _nc (kA) in the cooling pass is greater than I _max , the effect of cooling the nugget end is not obtained, and an appropriate temperature range cannot be maintained in subsequent re-energization. Further, in the case where the effect of cooling the end of the nugget in the cooling pass and maintaining it in an appropriate temperature range in the subsequent re-energization is desired to be exhibited more remarkably, the current value I _nc (kA) in the cooling pass is 0 ≤ I _nc It is desirable to satisfy ≤ 0.5 × I _max .

In addition, when the current value I _nr (kA) during re-energization is less than (0.8 × I _min ), the nugget end cannot be sufficiently reheated and an appropriate temperature range cannot be maintained. In addition, when the current value I _nr (kA) during re-energization is more than (1.5 × I _max ), the input heat amount is excessive and the nugget melts again, so that the effect of tempering cannot be obtained. Further, in the case where the effect of maintaining the nugget end in an appropriate temperature range during re-energization is intended to be exhibited more remarkably, the current value I _nr (kA) during re-energization is 0.95 × I _min ≤ I _nr < 1.2 × I _max It is desirable to satisfy In addition, it is preferable to set the energization time in the re-energization step within the range of 20 ms to 200 ms. More preferably, they are 20 ms - 100 ms.

In the present invention, the upper limit of the energization time of the cooling energization in the repeated energization step is not specified, but it is preferably (800 × t) or less. If it exceeds (800 × t), the total time of the welding process itself may be increased, and productivity may decrease.

On the other hand, the lower limit of the energization time of the cooling energization is not particularly specified, but it is preferably 20 ms or more. If it is less than 20 ms, the effect of cooling the end of the nugget cannot be obtained, and an appropriate temperature range may not be maintained during subsequent re-energization.

In the repetitive energization process, the above-described cooling energization and re-energization are repeated n times. In the case of carrying out a plurality of times, the conditions for each time may be the same or different as long as they are within the above range. However, the welding process itself is prolonged by repeating the cooling pass-through process and the re-energization process, resulting in a decrease in the work efficiency of resistance spot welding. For this reason, the repetition number n of the cooling pass-through step and the re-energization step is preferably 3 or less.

In the above, the case of welding two sheets of steel was mainly described, but the manufacturing method of the present invention is similarly applicable also to the case of welding three or more steel sheets.

In addition, resistance spot welding is performed in a state where the electrode is always cooled by water.

Example

Hereinafter, the present invention will be described using examples. In addition, this invention is not limited to the following example.

Resistance spot welding was performed in each condition of the two-sheet plate assembly and the three-sheet plate assembly. Resistance spot welding was performed at room temperature, and the electrode was always cooled in water. Both the lower electrode 4 and the upper electrode 5 were made of chrome copper DR type electrodes with a tip diameter of 6 mm and a radius of curvature of 40 mm. Further, the pressing force was controlled by driving the lower electrode 4 and the upper electrode 5 with a servo motor, and a single-phase alternating current having a frequency of 50 Hz was supplied at the time of energization. In the case of a two-plate assembly, the lower plate 1 and the upper plate 2 were used, and in the case of a three-plate assembly, the lower plate 9, the middle plate 10, and the upper plate 11 were used. As plate assembly, it was set as the following (A), (B), and (C).

(A): The lower plate 1 and the upper plate 2 have a tensile strength of 1470 MPa, and plating treatment in which X represented by the formula (1) is 0.24 and Y represented by the formula (2) is 0.020 (hot-dip galvanized) (GI), the coating weight was 50 g/m 2 per side) and a steel sheet having a sheet thickness of 1.4 mm was used.

(B): The lower plate 1 has a tensile strength of 1470 MPa, has a plating treatment in which X expressed by the formula (1) is 0.24 and Y expressed by the formula (2) is 0.020 (hot-dip galvanizing (GI), the amount of coating is 50 g/m 2 per side)), a steel plate with a plate thickness of 1.6 mm, and an upper steel plate 2, a plating having a tensile strength of 1180 MPa, X represented by the formula (1) being 0.20, and Y represented by the formula (2) being 0.024. A steel plate with a plate thickness of 2.0 mm without treatment was used.

(C): As the lower plate 9 and the middle steel plate 10, the tensile strength is 1470 MPa, and the plate thickness without plating is 1.4, where X expressed by the formula (1) is 0.22 and Y expressed by the formula (2) is 0.024. As the steel plate and the upper steel plate 11 of mm, the tensile strength is 270 MPa, and the plating treatment in which X expressed by the formula (1) is 0.027 and Y expressed by the formula (2) is 0.044 (hot-dip galvanized (GA), the coating amount is 45 g/m 2 per side) and a steel sheet with a sheet thickness of 0.7 mm were used.

In the case of (A), X _max = 0.24, Y _min = 0.020, in the case of (B), X _max = 0.24, Y _min = 0.020, and in the case of (C), X _max = 0.22, Y _min = 0.024. The tensile strength is the tensile strength obtained by preparing a JIS No. 5 tensile test piece in a direction parallel to the rolling direction from each steel plate and conducting a tensile test in accordance with the provisions of JIS Z 2241:2011.

Resistance spot welding, in the case of a two-plate assembly, as shown in Figs. 3(a) and 3(b), 2 of the steel plate (length in the longitudinal direction: 150 mm, length in the width direction: 50 mm) Between the sheets, 1.6 mm thick and 50 mm square spacers 6 are temporarily welded on both sides, and the center of the plate assembly in which two steel plates are overlapped is welded under the conditions described above and in Table 1-1, welding A seam was made. Fig. 3(a) to Fig. 3(c) are a plan view (Fig. 3(a)) and a side view (Fig. 3(b), Fig. 3(c)) showing a resistance spot welding test piece, and Fig. 3(a) 3(c), reference numeral 7 denotes a welding point, and reference numeral 8 denotes a temporary welding point. In addition, in the case of a three-plate assembly, as shown in FIG. 3(c), the lower plate 9, the middle plate 10, and the upper plate 11 are overlapped, and the lower plate 9 and the middle plate ( 10), welding was performed with spacers 6 sandwiched between both sides. In addition, the dimensions of the steel plate and the spacer are the same as in the case of the above two-plate assembly.

The obtained weld joint was left still in the air at normal temperature (20°C), and the presence or absence of delayed fracture was examined after 24 hours had elapsed. A result is shown in Table 1-2. Regarding delayed fracture, the symbol "○" was given to the case where delayed fracture did not occur after standing still for 24 hours, and the symbol "x" was given to the case where destruction occurred. Regarding the determination of delayed fracture, when peeling of the nugget (a phenomenon in which the nugget peels into two parts at the joint interface) was visually observed after welding, delayed fracture was determined to have occurred.

In addition, a welded joint was produced under the same conditions and the same shape separately from the delayed fracture test. The obtained weld seam was cut at the center of the nugget, etched with picric acid to clarify the boundary line of the molten portion, and then the Vickers hardness of the nugget end and the softest portion was measured under a load of 200 gf and a load holding time of 15 s. The hardness of the end of the nugget was determined as a hardness measurement value of a portion inside the nugget 50 μm from the boundary line of the molten portion. The hardness of the softest part was taken as the measured value of the softest part in the case of measuring a region 3 mm long from the nugget end to the outside of the nugget at intervals of 150 µm.

In addition, apart from the delayed fracture test, a cross tensile test was conducted on the welded joint produced under the same conditions. About the cross tension test, it was implemented with the test body of the shape based on JISZ 3137. Compared to the joint strength in the case of welding only in the main energization process without conducting post-energization, the symbol "x" is given for those whose strength has decreased by 10% or more, and the symbol "○" is given to those whose strength has not decreased by 10% or more. did

The results obtained in each evaluation are shown in Table 1-2. In the column of "determination" shown in Table 1-2, the symbol "○" is written when both "delayed failure" and "seam strength" were "○", and at least one of "delayed failure" and "seam strength" The symbol "x" was written where the dog was "x". As is clear from Table 1-2, in all of the invention examples, welded joints excellent in delayed fracture resistance without lowering joint strength were obtained, whereas in comparative examples, good welded joints were not obtained.

[Table 1-1]

[Table 1-2]

1 : Descent plate
2: upper plate
3 : Nugget
4: lower electrode
5: phase electrode
6 : spacer
7: welding point
8: Temporary welding point
9: descending plate
10: heavy steel plate
11: upper steel plate
12: spot welding
13 : Nugget
14: welding heat affected zone
15 : steel plate
16: nugget end

Claims (3)

A resistance spot welding member having two or more steel plates and a spot welded portion formed between the steel plates,
The tensile strength of at least one of the two or more steel plates is 980 MPa or more,
In the above two or more steel sheets, when X of the steel sheet having the largest coefficient X represented by the following formula (1) is X _max , and Y of the steel sheet having the smallest coefficient Y represented by the following formula (2) is Y _min E.g., the Vickers hardness H _n (HV) of the nugget end of the spot welded part is equal to or less than H _ob (HV) represented by the following formula (3),
The resistance spot welded member in which the Vickers hardness H _min (HV) of the softest part of the welding heat affected zone of the spot welded part satisfies the following formula (4).
X = [C] + [Si]/40 + [Mn]/200 (1)
Y = [P] + 3 × [S] (2)
H _ob = (800 × X _max + 300)/(0.7 + 20 × Y _min ) (3)
0.4 × H _n ≤ H _min ≤ 0.9 × H _n (4)
In the formulas (1) and (2), [C], [Si], [Mn], [P], and [S] represent the content (% by mass) of each element. However, when it does not contain, it is set to 0. As a method of manufacturing a resistance spot welded member,
A main energization step of overlapping two or more steel plates including at least one steel plate having a tensile strength of 980 MPa or more, holding them with a pair of welding electrodes and applying current while pressing to form a nugget;
A cooling step of cooling the nugget by pressing and holding the steel sheet with the welding electrode for a cooling time C _t (ms) represented by the following expressions (5) and (6) after the main power supply step;
A method of manufacturing a resistance spot welded member comprising a post-energization step of conducting current at a current value I _p (kA) that satisfies the following formula (7) after the cooling step.
C _t ≥ 160 × t ² (t ≤ 1.6) (5)
_Ct ≥ 256 × t (t > 1.6) (6)
0.95 × I _min ≤ I _p ＜ 1.5 × I _max and post-energization time: 20 ~ 200 ms (7)
In the above formulas (5), (6) and (7),
t: Average plate thickness of steel plates to be joined (mm)
I _max : maximum current value in the main energization process (kA)
I _min : minimum current value in the main power supply process (kA)
to be. According to claim 2,
After the post-energization process, repetitive energization in which cooling is performed and energization is performed under conditions satisfying the following equation (8), and re-energization is performed n times after the cooling and energization, re-energization under conditions satisfying the following equation (9) A manufacturing method of a resistance spot welded member having a process.
0 ≤ I _nc ≤ I _max (8)
0.8 × I _min ≤ I _nr < 1.5 × I _max (9)
In the above formulas (8) and (9),
I _nc : Current value at the nth cooling pass (kA)
I _nr : Current value at the nth re-energization (kA)
I _max : maximum current value in the main energization process (kA)
I _min : minimum current value in the main power supply process (kA)
n : a natural number greater than or equal to 1
to be.

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