JP2005074467A - Spot welding method and spot-welded steel sheet member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spot welding method capable of stably spot-welding three or more superimposed steel sheets mutually without causing welding defects even by using a movable spot welding machine. <P>SOLUTION: Among three or more steel sheets 1-3 to be superimposed mutually, the strongest one 2 is positioned between the other two steel sheets 1, 3. In such superimposed condition, the three steel sheets are held between a pair of electrode tips 12, 14, and then, the parts so held in-between are spot-welded to each other. Thus, the steel sheets 1, 3 having a relatively low strength or rigidity are situated on the outer side in the thickness direction so as to be brought into contact with the pair of electrode tips 12, 14, while the steel sheet having the highest strength or rigidity is situated on the inner side in the thickness direction. Consequently, the three or more superimposed steel sheets 1-3 are suitably spot-welded with stable quality free from welding defects even if a movable spot welding machine is used. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a spot welding method and a spot-welded steel plate member, and more particularly to a technique for locally spot welding in a state where three or more steel plates are overlapped with each other.

  In recent years, there has been a demand for further increasing the strength of members in which steel plates are welded to each other without increasing the weight thereof. For example, with the stricter standards for improving safety at the time of automobile collision, it is required to increase the rigidity of the body (vehicle body) constituting the automobile and the structural parts constituting the automobile.

  On the other hand, research to improve the material strength of the steel sheet has been advanced, and a high-tensile material (high-tensile steel) having a tensile strength of 340 MPa or more and a steel sheet having a tensile strength of 980 MPa or more prepared by a hot press method have been proposed. It is being applied to applications such as shock absorbing members. In addition, a steel plate part having a sufficient strength by increasing the thickness of the steel plate or welding a reinforcing plate in a portion where strength is required by design has been proposed and is being applied to applications such as an impact absorbing member. For the welding, continuous welding such as arc welding, plasma welding, laser welding, and mash welding using a welding wire can be suitably used, but spot welding is the most excellent in terms of work efficiency. In particular, when welding three or more steel sheets in a stacked state, spot welding is optimal.

  For example, Patent Document 1 discloses a nugget that is a region in which three thin steel plates are overlapped with each other and sandwiched between a pair of electrode tips and a current is passed through the pair of electrode tips to melt and solidify between the steel plates. And a technique of locally spot welding the three thin plates to each other.

Japanese Patent Laid-Open No. 10-249537

  By the way, since the steel plate parts welded in the conventional spot welding method are actually often combined with press-formed parts, a gap is usually generated between them. As the strength increases and the plate thickness increases, the gaps are less likely to collapse during welding, and the repulsive force of the steel plate parts may increase, leading to the occurrence of weld defects such as weld cracks and blowholes. there were. In particular, when a movable spot welder provided on a robot arm is used, or when three or more sheets are stacked as described above, two or more gaps are formed and the total plate thickness is larger than two sheets stacked. Therefore, the above-mentioned welding defect is more likely to occur. For this reason, when the strength is increased and the increase in the thickness of the steel sheet is large as in the case of a reinforcing member of an automobile, the above-described disadvantage tends to become remarkable.

  On the other hand, it is conceivable to use a spot welder that increases the pressure of the pair of electrode tips or changes the front and back so as to crush the gap during spot welding. However, there is a disadvantage that the apparatus becomes large and expensive and the welding operability and workability are lowered.

  The present invention has been made against the background of the above circumstances, and the object is to stably stack three or more steel plates even when using a movable spot welder without generating welding defects. It is an object of the present invention to provide a spot welding method capable of spot welding with each other.

  By the way, high-tensile materials contain a large amount of chemical components such as additive elements Si, Mn, Cr, etc., so that they have high electrical resistance and are more likely to generate heat than other steel plates, and hot pressed materials also have a surface oxide layer. Since the surface resistance is high and it is easier to generate heat than other steel plates, when performing spot welding on three or more steel plates stacked on each other, in order to form a good nugget, the outer side in contact with the electrode tip of the spot welder In general, high-tensile materials and hot-pressed materials are arranged. However, when such a general spot welding method is actually applied in the production of steel plate parts, spot welding is not always successful, and the welding quality may become unstable.

  The inventors of the present invention have made various studies in order to achieve such an object. As a result, when spot welding is performed in a state where three or more steel plates including a high-strength material are overlapped with each other, the conventional general spot welding method described above is used. On the other hand, it has been found that spot weldability is improved when a steel plate located on the outer side has a relatively low material strength or a low plate strength having a thin thickness. In other words, the higher the steel sheet coefficient value represented by the product of the hardness (Hv) and the plate thickness (mm) of the steel sheet, the better the spot weldability by positioning it on the inner side in the thickness direction so as not to contact the electrode tip. Found the fact that it will be improved. That is, according to the analysis of the present inventors, when there is no gap between the steel plates when spot welding three or more stacked steel plates, the conventional general spot welding method is suitable. When manufacturing steel plate parts, most of them are pre-pressed on each steel plate, so when spot welding is performed with a gap formed between the stacked steel plates, it is rather located outside. It is preferable to use a steel plate having a relatively low material strength or a low plate strength having a relatively small thickness. The present invention has been made based on such findings.

  That is, the gist of the invention according to claim 1 is a spot welding method in which three or more steel plates are locally welded in a state of being overlapped with each other, and (a) the maximum of the three or more steel plates A stacking process in which high-strength steel sheets are stacked on each other so as to be positioned between other steel sheets; and (b) three or more steel sheets stacked in the stacking process are sandwiched between a pair of electrode chips, and the pair of electrode chips And a welding step of spot welding the sandwiched portions to each other.

  Further, the gist of the invention according to claim 2 is that the maximum strength steel plate among the three or more steel plates stacked on each other has a steel plate coefficient value represented by the product of the steel plate hardness and the plate thickness. It is the largest steel plate.

  Further, the gist of the invention according to claim 3 is that the maximum strength steel plate among the three or more steel plates stacked on each other is a steel plate that has been strengthened in the process of being formed by a hot press method. It is.

  The gist of the invention according to claim 4 is that the maximum strength steel plate among the three or more steel plates stacked on each other has a Vickers hardness of 350 Hv or more.

  Further, the gist of the invention according to claim 5 is that the maximum strength steel plate among the three or more steel plates stacked on each other has an iron-zinc alloy layer on the surface.

  A gist of the invention according to claim 6 is a steel plate member spot-welded by any one of the above-described invention methods.

  Further, the gist of the invention according to claim 7 is that the steel plate member constitutes a part of a vehicle body or a component thereof.

  According to the spot welding method according to claim 1, the three or more steel plates stacked on each other so that the steel plate having the maximum strength among the three or more steel plates stacked on each other is located between the other steel plates. Since the portions sandwiched between the pair of electrode tips and the portions sandwiched between the pair of electrode tips are spot-welded to each other, the outer side in the thickness direction so that the relatively low-strength steel plate can be brought into contact with the pair of electrode tips. The high strength steel sheet is positioned in the thickness direction, so that the steel sheets stacked in three or more sheets can be used with a movable spot welder without causing welding defects. It is preferably spot welded.

  According to the invention of claim 2, the maximum strength steel plate among the three or more steel plates stacked on each other has the maximum steel plate coefficient value K represented by the product of the steel plate hardness and the plate thickness. Therefore, by using this steel plate coefficient value K, there is an advantage that the steel plate having the maximum strength can be easily selected or specified without requiring strength measurement.

  According to the invention of claim 3, the maximum strength steel plate among the three or more steel plates stacked on each other is a steel plate that has been strengthened in the process of being formed by a hot press method. Therefore, by using a steel plate member that has been strengthened at the same time as hot pressing, the consumption of the mold and the like is suitably suppressed and the processing cost is reduced compared to the case of pressing a steel plate that has been strengthened. Is done. Moreover, since the high tensile strength material has a high electrical resistance with respect to general steel because it contains many steel plate chemical components such as additive elements Si, Mn, Cr, etc. When the high-tensile material is used as a steel sheet, heat generation due to the welding current is concentrated on the high-tensile material, and it is difficult to sufficiently form nuggets on the general steel layered on the top and bottom of the high-tensile material, which may cause poor welding. On the other hand, when a steel plate with high strength in the process of being formed by the hot press method is used as the maximum strength steel plate among the three or more steel plates as described above, Since there is not much difference, a good nugget of sufficient size is formed even in the general steel.

  According to the invention of claim 4, the maximum strength steel plate among the three or more steel plates stacked on each other has a Vickers hardness of 350 Hv or more. A highly rigid steel plate member is obtained.

  Further, according to the invention according to claim 5, the maximum strength steel plate located on the inner side among the three or more steel plates stacked on each other has an iron-zinc alloy layer on the surface. Oxidation scale is not generated during heat treatment for heat treatment to obtain a strong steel plate, and spot welding can be performed without performing a surface cleaning process such as shot blasting after forming.

  Further, according to the invention according to claim 6, since the steel plate member is spot-welded by any one of the above-described inventive methods, the steel plate member having stable welding quality and high rigidity or high corrosion resistance can be obtained.

  Further, according to the invention according to claim 7, since the steel plate member constitutes a part of a vehicle body or a component thereof, the welding quality is stable, and high rigidity or high corrosion resistance is achieved. A car body or its components can be obtained.

  Here, the strength of the steel sheet indicates the degree of strength of the steel sheet and correlates with the rigidity indicating the ability to withstand bending and twisting, that is, the ability to hardly deform. It can also be called a steel plate. The strength of the steel sheet should be originally expressed by the tensile strength T (MPa) of the steel sheet, but is proportional to the Vickers hardness Hv in the cross section at the thickness center of the steel sheet (T = 3.1 × Hv). As such, it can be expressed using the Vickers hardness Hv. Further, the strength of the steel plate is proportional to the plate thickness. Therefore, the strength of the steel sheet is preferably indicated by a steel sheet coefficient value K represented by the product (= Hv × t) of the hardness Hv and the thickness t (mm) of the steel sheet. Can be used instead of other strengths, but other values may be used.

  Method for increasing the strength of a steel plate having a maximum steel plate coefficient value K, that is, a method for increasing the strength to obtain a steel plate having the maximum strength among the three or more steel plates stacked for spot welding. In addition to quenching by hot pressing, a rapid cooling method such as induction quenching is preferably used. Further, the strength can be increased by increasing the hardness of the steel sheet by nitride coating or carburization.

  High-tensile steel made high-tensile steel by containing many steel plate chemical components such as additive elements Si, Mn, Cr, etc., and hot-pressed in the austenite region heated to a temperature of Ac3 point or higher A hot press material that has been quenched by being rapidly cooled to a temperature below the Ms point in a press mold and causing martensitic transformation corresponds to the high-strength steel sheet having a Vickers hardness of 350 or more. . The strength of a high-strength steel plate is determined by the carbon equivalent calculated from the contents of carbon, manganese, chromium, and boron contained therein, and 1180 MPa class, 1470 MPa class, and the like are known. As a special case such as the above hot press, a part of the steel plate part having a low strength may be used by providing a locally slow cooling part by partially applying a heat insulating material during quenching. . This is a steel sheet in which the carbon equivalent is partially lowered by the tailored blank method, or the hardness after quenching is partially reduced by slowing the cooling rate to 150 Hv (Vickers hardness). Spot welding may be performed with the low strength portion of such a steel plate as the outside, or spot welding may be performed with the high strength portion as the inside.

  The iron-zinc alloy layer is generated on the surface of a steel plate after the steel plate having an alloyed hot-dip galvanized coating or electrogalvanized coating is quenched by hot pressing. The alloyed hot dip galvanized film is formed by a so-called hot dip plating method. The electrogalvanized film and the plated film formed by the so-called hot dipping method are zinc-based, zinc-iron-based, zinc-aluminum-based, zinc-nickel-based, zinc-cobalt-based, and the like. As a steel material having the iron-zinc alloy layer on the surface, for example, an alloyed hot-dip galvanized steel sheet (GA) and a hot-dip galvanized steel sheet (GI) are known. The iron-zinc alloy layer is generally an iron-zinc solid solution phase, an iron-zinc intermetallic compound phase, or the like, but they may be constituted singly or in a mixed state.

  In order to clamp the three or more steel plates between a pair of electrode tips during spot welding, an electrode tip clamping actuator such as a pneumatic cylinder, a hydraulic cylinder, and an electric motor is provided in the spot welding apparatus. The spot welding apparatus may be either a stationary type (stationary type) or a movable type (portable type or portable type). Also, either C type gun type or X type gun type may be used. As the material of the electrode tip, a copper alloy such as chromium copper, alumina-dispersed copper, and zirconium copper is preferably used. However, any material having low electrical resistance and high wear resistance may be used to pass a large current. .

  The method of the present invention includes, for example, a car body of an automobile, or a frame, a panel, a side member, which is a part of a component thereof, by spot welding in a state where three or more pressed parts formed from steel plates are stacked on each other. It is applied to manufacture steel parts such as doors, reinforcements, pillars such as center pillars and front pillars, and door impact beams. The steel parts constituting a part of the automobile body or its components obtained by applying the method of the present invention are stable in welding quality, and are highly rigid or highly corrosion resistant automobile bodies or their components. can get. Hereinafter, experimental examples performed by the present inventors and the evaluation results thereof will be described below.

Explanation of Experimental Sample Each sample No. used in the experiment 1 to No. 5 is a three-ply steel plate part in which a steel piece (30 mm width × 100 mm length) is spot-welded under the following conditions. 6 and no. Reference numeral 7 is a steel plate part of four test pieces which are spot-welded. Each of these sample Nos. 1 to No. The hardness, thickness t, and steel plate coefficient value K of three or more steel plates used in 7 are those shown in FIG. Each sample No. 1 to No. Reference numeral 7 denotes a combination of the steel plates 1 to 3 or the steel plates 1 to 4 shown in FIG. In each sample, the maximum strength steel sheet having a steel sheet hardness of 350 Hv or more, that is, the steel sheet with the maximum steel sheet coefficient value K, has a steel chemical composition of 0.21% C (carbon) and 0.2% Si. (Silicon), 1.2% Mn (manganese), 0.007% P (phosphorus), 0.002% S (sulfur) hardened steel sheet alloyed hot dip galvanized layer (plating) Within 10 seconds after the heat treatment of holding a 900 ° C.-5 minute test piece of a steel sheet having an adhesion amount of 60 g / m ² ) at a temperature of Ac 3 point or higher in a heating furnace, the temperature inside the Ms point or less This is a hot-pressed steel sheet that has been quenched by quenching at the same time as pressing with a pressurizing force of 5 MPa using a flat-pressed metal mold cooled with water. The maximum strength steel sheet is sample No. 1 to No. 3, Sample No. 6 is the steel plate 2 and the sample No. 4 and no. 5 is the steel plate 3 and the sample No. 7 is a steel plate 1.

Welding machine / DR type spot welding machine / Electrode tip: made of chrome copper, tip original diameter is 16 mmφ, tip diameter is 6 mmφ, and tip curvature radius is 40 mmR.
・ Pressure: 3920N (400kgf)
・ Welding current: 9000A
・ Energization conditions: 10 cycles of preliminary energization, 40 cycles of main energization, 1 cycle of holding (60 Hz)
・ Cooling water flow: 3L / min

As shown in FIG. 2, in order to simulate the same gap conditions and current conditions as the actual product, three steel plate pieces 1, 2, 3, Alternatively, four steel plate pieces 1, 2, 3, 4 are sequentially stacked and positioned between the pair of electrode chips 12 and 14 (stacking step), and then the pair of electrode chips as shown in FIG. 3 or FIG. The above-mentioned energization conditions are set in advance between the pair of electrode tips 12 and 14 by closing the gap S between the steel plates by pressurizing (clamping) between 12 and 14 and bringing them into contact with each other. Then, energization is performed (welding process), and then the clamping pressure by the pair of electrode tips 12 and 14 is released.

Evaluation criteria Each sample No. 1 to No. For the spot welding state of No. 7, the cross section where the nugget of the spot welding appears is observed using an optical microscope, and if the nugget is well formed, the circle mark is formed around the boundary of the steel plate. When a blow hole having a diameter of 20% or more of the diameter of the nugget is present, a Δ mark is attached to the result column of FIG.

Evaluation Results As shown in FIG. 1, the hardness is 350 Hv or more among the three steel plate pieces 1, 2, 3, or the four steel plate pieces 1, 2, 3, and 4 that are overlapped with each other. Sample No. to which the method of the present invention is applied, in which a steel plate having a certain maximum strength is located between other steel plates and is located inside the thickness direction. 1, no. 2, No. 3, no. No. 6 shows that the nugget is well formed, whereas the sample No. 6 as a comparative example in which the steel plate with the maximum strength is located outside. 4, no. In No. 7, blow holes and cracks were observed in the nugget, and the welding quality was unstable. Sample No. 4 and sample no. 5 is sample No. 5 above. 1 and sample no. It is indicated that the nugget evaluation effect is larger as the steel plate coefficient value K is larger because the evaluation of the nugget is x and 2 levels lower than 3. Further, in the overlapping process, the state shown in FIG. 1, no. 2, no. 3, the deformation and distortion of the outer steel plates 1 and 3 are small as shown in FIG. 4, as shown in FIG. 4, the deformation and distortion of the outer steel plate 1 on the opposite side to the steel plate having the maximum strength is increased. As a result, the nugget formation becomes unstable or difficult to form.

  Therefore, according to the present embodiment, the steel plate 2 having the maximum strength among the three or more steel plates 1 to 3 or 1 to 4 stacked on each other is positioned between the other steel plates 1 and 3 or 1 and 4. Since the portions sandwiched between the pair of electrode tips 12 and 14 are overlapped with each other and the portions sandwiched between the pair of electrode tips 12 and 14 are spot-welded to each other, relatively low strength or low plate Since the rigid steel plates 1 and 3 or 1 and 4 are positioned on the outer side in the thickness direction so as to be in contact with the pair of electrode tips 12 and 14, the maximum strength or the maximum rigidity steel plate is positioned on the inner side in the thickness direction. Three or more steel plates 1 to 3 or 1 to 4 are spot-welded suitably with stable quality even if a movable spot welder is used without generating welding defects.

  Moreover, according to the present embodiment, the maximum strength steel plate 2 among the three or more steel plates 1 to 3 or 1 to 4 stacked on each other is a steel plate represented by the product of the steel plate hardness Hv and the plate thickness t. Since the steel sheet has the maximum coefficient value K (= Hv × t), the use of the steel sheet coefficient value K has an advantage that the steel sheet having the maximum strength can be easily selected or specified without requiring strength measurement.

  In addition, according to the present embodiment, the maximum strength steel plate 2 among the three or more steel plates 1 to 3 or 1 to 4 that are stacked on each other was strengthened in the process of being formed by the hot press method. Because it is a steel plate, by using a steel plate member that has been strengthened at the same time as hot pressing, wear of the mold and the like is suitably suppressed as compared to when the steel plate after strengthening is pressed. Cost is reduced. Further, since the high-tensile material contains a large amount of chemical components such as additive elements Si, Mn, Cr, etc., it has a high electric resistance with respect to the steel plates 1 and 3 made of general steel. When the high-tensile material is used as the steel plate 2 having the maximum strength, the heat generation due to the welding current is concentrated on the high-tensile material, and it is difficult to sufficiently form the nugget on the general steel layered on the top and bottom of the high-tensile material. However, there is a risk that welding failure may occur, as in this embodiment, a steel plate that has been strengthened in the process of being formed by the hot press method as the maximum strength steel plate 2 of the three or more steel plates. When used, since there is not much difference in electrical resistance from the steel plates 1 and 3 made of general steel, there is an advantage that a good nugget having a sufficient size is formed in the steel plates 1 and 3.

  In addition, according to the present embodiment, the maximum strength steel plate 2 among the three or more steel plates 1 to 3 or 1 to 4 that are stacked on each other has a Vickers hardness of 350 Hv or more. A steel plate member with high rigidity is obtained after welding.

  Further, since the steel plate member obtained by the spot welding method of this embodiment has a stable welding quality and a steel plate member having high rigidity or high corrosion resistance can be obtained, it constitutes a part of a vehicle body or a component thereof. By being applied to the manufacture of a steel plate member, a car body or a component thereof of an automobile having stable welding quality and high rigidity or high corrosion resistance can be obtained.

  As mentioned above, although the Example of this invention was described in detail based on drawing, these are one Embodiment to the last, This invention is implemented in the aspect which added the various change and improvement based on the knowledge of those skilled in the art. be able to.

It is a graph which shows the evaluation result of the steel plate component manufactured according to the spot welding method of one Example of this invention in contrast with the evaluation result of the steel plate component manufactured according to the conventional spot welding method. It is a figure explaining the spot welding method of one Example of this invention, Comprising: It is a figure which shows the overlap process. It is a figure explaining the spot welding method of one Example of this invention, Comprising: It is a figure which shows the welding process. It is a figure explaining the welding process of the conventional spot welding method, Comprising: It is a figure which shows the welding process.

Explanation of symbols

1: Steel plate 2: Steel plate 3: Steel plate 4: Steel plate 10: Spacer 12, 14: Electrode tip

Claims (7)

It is a spot welding method in which three or more steel plates are locally welded in a state where they are overlapped with each other,
An overlapping step of stacking the maximum strength steel plates among the three or more steel plates so as to be positioned between other steel plates;
A spot welding method comprising: a welding step of sandwiching three or more steel plates stacked by the stacking step between a pair of electrode tips, and spot welding the portions sandwiched between the pair of electrode tips. . The spot welding method according to claim 1, wherein the steel plate having the maximum strength among the three or more steel plates is a steel plate having a maximum steel plate coefficient value represented by a product of steel plate hardness and plate thickness. 3. The spot welding method according to claim 1, wherein a steel plate having the maximum strength among the three or more steel plates is a steel plate that has been strengthened in a process of being formed by a hot press method. The spot welding method according to any one of claims 1 to 3, wherein the steel plate having the maximum strength among the three or more steel plates has a Vickers hardness of 350 Hv or more. The spot welding method according to any one of claims 1 to 4, wherein the steel plate having the maximum strength among the three or more steel plates has an iron-zinc alloy layer on a surface thereof. A steel plate member spot-welded by the method according to claim 1. The steel plate member according to claim 6, wherein the steel plate member constitutes a part of a vehicle body or a component thereof.

Images