JP2010059451A - Welded joint and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded joint which is small in nugget diameter and is excellent in strength in a peeling direction when high-tension steel sheets having ≥440 MPa tensile strength are subjected to lap resistance welding, and to provide a manufacturing method the welded joint. <P>SOLUTION: When the welded joint is manufactured by applying a spot-welding to a lapped member formed by lapping a plurality of high tension steel sheets of ≥440 MPa the tensile strength, a welding metal for welded joint has chemical composition composed, by mass%, of C×P≤0.0025%, ≤0.015% P, ≤0.01% S, and the spot-welding part is subjected to heat treatment to satisfy 300≤T×(log<SB>10</SB>(t)+1)≤1,000, [wherein, T is the heat-treatment temperature (100°C≤T≤400°C) and t is a heat-treatment time (holding time in the heat-treatment time; min)]. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a welded joint and a manufacturing method thereof, and more particularly, to a welded joint manufactured by resistance welding mainly used in assembling an automobile and having excellent peel strength and a manufacturing method thereof.

In recent years, in the automotive industry field, the use of high-tensile steel plates having a tensile strength of, for example, 440 MPa or more has been expanded in order to reduce the weight of the vehicle body and improve the collision safety.
In spot welding, which is mainly used in the assembly of car bodies, it is required to secure a nugget diameter according to the plate thickness. From the current value at which the standard nugget diameter is obtained to the current value at which dust (melting and scattering phenomenon of the base material) occurs The appropriate current range represented by the range is an important indicator. Further, the spot welded joint is required to have both high shear strength and peel direction strength.

  In addition, the spot welded portion has a fracture path that is less than the fracture in the nugget where the fracture path in the tensile test passes through the nugget (melted portion) and the interface fracture in which this fracture path passes through the interface between the nugget and the base material. It is desirable to provide a quality that results in a fracture of the base material that passes outside the nugget.

  However, the strength in the peel direction of the spot welded joint of the high-tensile steel plate having a tensile strength of 440 MPa or more is weaker than the shear strength, and the strength is significantly reduced when the interface breaks. For this reason, in order to expand the application of a high-tensile steel sheet having a tensile strength of 440 MPa or more to the body of an automobile, an improvement in peel direction strength is required.

In general, it is considered effective to increase the nugget diameter in order to improve the strength in the peeling direction of the spot welded joint. Patent Documents 1 to 5 also propose improvement methods by post-energization after the end of welding energization.
JP 2002-103048 A JP 2002-103054 A Hiratsuka, K. et al., “Study on selection of tempering conditions in spot welding of weathering steel”, Journal of the Japan Welding Society Vol. Nobuyuki Yamauchi et al., “Spot Weldability of High Tensile Steel”, Sumitomo Metals, Vol. 33 (1981) no. 4, pages 109-120 Bessho Kiyoshi et al., “Spot Welding of High Tensile Steel Sheet”, Sumitomo Metals, Vol. 26 (1974) No. 2, pages 38-48

  However, the former means for enlarging the nugget diameter has a limit because dust tends to be generated in the case of a high-tensile steel plate. In addition, the latter means for performing subsequent energization is not practical because the construction time increases.

  An object of the present invention is to provide a welded joint having a small nugget diameter and excellent peel strength in a lap resistance welding of a high-tensile steel sheet having a tensile strength of 440 MPa or more, and a method for producing the same. In particular, an object of the present invention is to provide a welded joint that is excellent in strength in the peeling direction even when an overlapping surface is present in the center portion of the thickness of the welded joint, and a method for manufacturing the welded joint.

  As disclosed in Patent Documents 4 and 5, as a cause of a decrease in peel direction strength of a spot welded joint of a high-tensile steel plate, a decrease in toughness due to an increase in weld metal hardness accompanying an increase in C content is known. It is also known that P and S segregation in the high-tensile steel plate is also the cause. The segregation of P and S is more susceptible to increase as the C content increases. That is, in order to improve the strength in the peeling direction of spot welded joints, it is required to reduce the C content and the P content, but C is an essential element for increasing the strength of steel and is indispensable. I can't.

  Therefore, as a result of further investigations, the present inventors have determined that the peel direction of a spot welded joint manufactured by spot welding on an overlapping member formed by overlapping a plurality of high-tensile steel sheets having a tensile strength of 440 MPa or more. In order to improve the strength, it has been found that the P and S contents of the weld metal of the welded joint should be reduced, and that the spot welds should be heat-treated under specific conditions. Completed the invention.

The present invention is a method for producing a welded joint having a welded portion by performing resistance welding on an overlapping member formed by superimposing a plurality of steel plates having a tensile strength of 440 MPa or more, wherein the weld metal of the welded joint is: By mass%, C × P ≦ 0.0025, P: 0.015% or less, and S: 0.01% or less. A method for producing a welded joint, characterized in that heat treatment is performed at a heat treatment temperature and a heat treatment time satisfying the formula (1).
300 ≦ T · (log ₁₀ (t) +1) ≦ 1000 (1)
In the formula (1), T represents a heat treatment temperature (100 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes).

In the method for manufacturing a welded joint according to the present invention, it is desirable that the heat treatment satisfies the following formula (2).
400 ≦ T · (log ₁₀ (t) +1) ≦ 750 (2)
In the formula (2), T represents a heat treatment temperature (150 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes). In this case, the heat treatment temperature T is desirably 200 ° C. or higher and 400 ° C. or lower.

Furthermore, it is desirable that the heat treatment satisfies the following formula (3).
450 ≦ T · (log ₁₀ (t) +1) ≦ 700 (3)
In the formula (3), T represents a heat treatment temperature (200 ° C. ≦ T ≦ 300 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes).

  In these welded joint manufacturing methods according to the present invention, for example, it is preferable that the overlapping member is applied when the overlapping surface of a plurality of steel plates is present at the center in the thickness direction. However, the central portion in the thickness direction refers to a region where the amount of deviation between the overlapping surface and the center line in the thickness direction is within 10% of the total thickness of the overlapping members.

In the method for manufacturing a welded joint according to the present invention, it is desirable that the plurality of steel plates are two steel plates.
From another viewpoint, the present invention is a welded joint manufactured by the above-described manufacturing method according to the present invention.

  By this invention, the peeling direction intensity | strength of the welded joint obtained by performing resistance welding to the lamination | stacking member which consists of a steel plate whose tensile strength is 440 Mpa or more can be improved. In particular, it is possible to improve the peeling direction strength of the welded joint of the plate assembly in which the central portion of the thickness of the overlapping member becomes the overlapping surface.

  Therefore, according to the present invention, it is possible to improve the strength in the peeling direction of the welded joint with a small nugget diameter, that is, a welded joint of a high-tensile steel plate having a tensile strength of 440 MPa or more, which is excellent in the strength in the peeling direction, Can be provided.

  This makes it possible to expand the application of a high-tensile steel plate having a tensile strength of 440 MPa or more to the vehicle body of an automobile.

  The best mode for carrying out a welded joint and a method for producing the same according to the present invention will be described below in detail with reference to the accompanying drawings. The present invention is widely applied to spot welding joints manufactured by resistance welding such as spot welding, one-side spot welding, series spot welding, and direct spot welding. In the following description, spot welding widely used in the field of automobiles is taken as an example.

  In the present embodiment, a spot welded joint having a spot welded portion is manufactured by spot welding a superposed member formed by superposing a plurality of high strength steel plates having a tensile strength of 440 MPa or more.

At this time, the spot welded portion is heated, and the spot welded joint is manufactured by subjecting the welded portion to heat treatment at a heat treatment temperature and a heat treatment time satisfying the following formula (1).
300 ≦ T · (log ₁₀ (t) +1) ≦ 1000 (1)
However, T in Formula (1) shows heat processing temperature (100 degreeC or more and 400 degrees C or less), and t shows heat processing time (holding time in heat processing temperature; minute).

  Since the weld after completion of normal spot welding is air-cooled to room temperature, it is necessary to raise the temperature of the spot weld. In the heat treatment for maintaining the temperature in the cooling process after welding, it is difficult to aim at a predetermined temperature because the cooling rate of spot welding is very fast. Further, such heat treatment must be carried out by energization heating with a spot welding electrode, but this is not preferable because it increases the construction time and deteriorates the continuous spotting property.

  In order to investigate the influence of the heat treatment temperature and the heat treatment time on the strength of the spot weld when the heat treatment is performed on the spot weld, the present inventors have the weld metal in mass%, C: 0.17%, P: A tensile strength of 780 MPa having the composition shown in Table 1 (the numerical value is mass%, and Fe and inevitable impurities other than those shown in the table) so as to contain a chemical component of 0.009% and S: 0.001%. And two high-tensile steel plates 0a and 0b having a plate thickness of 1.8 mm are overlapped to assemble an L-shaped tensile test piece 0 having the shape shown in FIG. Spot welding is performed under the welding conditions shown (electrode, applied pressure, energization time, welding current, and hold time), and after this spot weld 0c is cooled to room temperature, a predetermined heat treatment temperature (room temperature to 400 ° C.) and a predetermined heat treatment time (0-30 Was heated held at minutes), it was investigated peel direction strength (L-shaped joint strength) by conducting a tensile test in an L-shape specimens 0. In the following, component% represents mass%.

  The results are summarized in Table 3 and graphically shown in FIGS.

  FIG. 2 is a graph showing the relationship between the heat treatment temperature and the L-shaped joint strength, and FIG. 3 is a graph showing the relationship between the heat treatment time and the heat treatment temperature and the fracture mode. In addition, the L-shaped joint strength in the graph of FIG. 2 shows the highest strength obtained at each heat treatment temperature. Moreover, the graph of FIG. 3 has shown the L-shaped joint intensity | strength in each heat processing conditions, and the base material, nugget, and interface which were described in each plot in the figure are base material fracture | ruptures, and it is a nugget internal fracture | rupture. , Indicating that it is an interfacial fracture. Moreover, the size of the plot in the figure indicates the magnitude of the intensity, and the larger the plot, the higher the intensity.

  As shown in Table 3, the strength in the peeling direction can be improved by performing the heat treatment of the present embodiment. Furthermore, as shown in the graph of FIG. 2, the influence of the heat treatment appears from the heat treatment temperature of about 100 ° C. As the temperature is higher, the strength in the peeling direction of the spot weld is improved even if the heat treatment time is shorter. However, as shown in Table 3, in the temperature range where the heat treatment temperature is higher than 200 ° C., if the heat treatment time is too long, the spot It can be seen that the increase in the peel direction strength of the welded portion is small. That is, it can be seen that there is an appropriate heat treatment time with respect to the heat treatment temperature in order to reliably increase the strength in the peeling direction of the spot weld. If the heat treatment temperature exceeds 400 ° C., the appropriate treatment time is narrow and impractical.

Furthermore, the items listed below can be understood from the results shown in Table 3 and the graphs of FIGS.
(A) By performing the heat treatment at a heat treatment temperature of 100 ° C. or higher and 400 ° C. or lower for an appropriate time, the strength in the peeling direction of the joint portion is significantly improved.

(B) The strength in the peeling direction is improved by performing a heat treatment of 300 ≦ T · (log ₁₀ (t) +1) ≦ 1000.
(C) If T · (log ₁₀ (t) +1) is less than 300, the peeling direction strength is small, and even if it exceeds 1000, the effect of improving the peeling direction strength is small compared to the energy input to the heat treatment. To become a.

(D) In particular, by performing a heat treatment of 400 ≦ T · (log ₁₀ (t) +1) ≦ 750, the base material breaks and the strength in the peeling direction is greatly improved.
As shown in Table 3, the strength in the peeling direction is improved by performing heat treatment at a heat treatment temperature and a heat treatment time satisfying the above-described formula (1): 300 ≦ T · (log ₁₀ (t) +1) ≦ 1000. In the formula (1), T represents a heat treatment temperature (100 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes).

Furthermore, when the heat treatment is performed at the heat treatment temperature and the heat treatment time satisfying the above-described formula (2): 400 ≦ T · (log ₁₀ (t) +1) ≦ 750, the base material breaks, and the strength in the peeling direction is greatly improved. . In the formula (2), T represents a heat treatment temperature (150 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes). In this case, the heat treatment temperature T is desirably 200 ° C. or higher and 400 ° C. or lower.

Furthermore, it is desirable that the heat treatment satisfies the following formula (3).
450 ≦ T · (log ₁₀ (t) +1) ≦ 700 (3)
In the formula (3), T represents a heat treatment temperature (200 ° C. ≦ T ≦ 300 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes).

  The strength in the peel direction is mainly affected by the toughness of the weld metal. In high-tensile steel sheets with a tensile strength of 440 MPa or more, the C content is large due to the high strength of the base metal, and the spot welds have a very fast cooling rate. It tends to be a martensitic structure with low toughness. The heat treatment temperes the spot welded portion and improves the toughness of the weld metal, so that it is assumed that the strength in the peeling direction is improved.

Next, chemical components of the weld metal of the weld joint will be described.
In general, the strength characteristics of the spot weld joint 2 are mainly affected by the chemical components of the weld metal of the spot weld 4.

  FIG. 4 is an explanatory diagram showing the influence of the plate assembly on the strength in the peeling direction of the spot welded joints 1 and 2, and FIG. 4A is a three-layer spot welded portion of three high-tensile steel plates 1 a to 1 c. 4 shows a spot welded joint 1 having a two-layer spot welded portion 4 made of two high-tensile steel plates 2a and 2b. In FIG. 4A and FIG. 4B, the solid lines shown inside the spot welds 3 and 4 indicate the solidified segregation parts of P and S.

  The thickness of the spot welded joint 1 having a three-layer spot welded portion 3 made of three high-tensile steel plates 1a to 1c shown in FIG. When there is an overlapping surface of each of the high-tensile steel plates 1a to 1c at a position different from the central portion of the direction, as shown in the figure, a crack growth line along the overlapping surface indicated by a white arrow; The solidification segregation lines of P and S indicated by solid lines in the center of the plate thickness do not match.

  In contrast, a spot welded joint 2 having a two-layer spot welded portion 4 made of two high-tensile steel plates 2a and 2b shown in FIG. 4B, that is, an overlapping member of two high-tensile steel plates 2a and 2b. When there is an overlapping surface of the high-tensile steel plates 2a and 2b in the center portion in the thickness direction, a crack propagation line along the overlapping surface indicated by a white arrow and a solid line in the thickness center portion The solidification segregation lines of P and S coincide.

That is, as shown in FIG. 4B, when there is an overlapping surface at the center of the plate thickness, segregation of P and S of the weld metal has a strong effect on the strength in the peeling direction.
By the way, when spot-welding high-tensile steel plates made of the same steel type, the chemical component of the weld metal is equal to the chemical component of the base material. On the other hand, when welding high-tensile steel plates made of different steel types, the chemical components of the weld metal are mixed with the chemical components of the respective high-tensile steel plates. In this embodiment, based on the premise that the chemical components of each high-strength steel plate are uniformly stirred in the weld metal, as an example, welding is performed when high-tensile steel plates A and B having different chemical components are spot-welded. The chemical components of the metal are described below.

Chemical composition of high-tensile steel plate A, respectively melt volume C _A, together with the V _A, the chemical components of the high-tensile steel plate B, C _B melt volume _{respectively,} when V _B, the chemical components of the weld metal, (C the _{a V a + C B V B} ) / (V a + V B).

  In the present embodiment, in order to suppress the influence of solidification segregation of P and S in the spot welded portion, the weld metal of the welded joint is C × P ≦ 0.0025, P: 0.015% or less, S: 0.00. It has a chemical component that is 010% or less. The reason for this will be described below.

  C is an element effective for increasing the hardness and strength of the weld metal. However, if the C content is too high, the hardness of the weld metal increases too much, so that the nugget becomes brittle and the strength in the peeling direction decreases. It is considered that the toughness of the weld metal is improved by the heat treatment of the present embodiment, and the strength in the peeling direction is improved. Generally, since the amount of C contained in the high-tensile steel sheet for automobiles is 0.35% or less, the C content is desirably 0.35% or less in the present embodiment.

  On the other hand, P is considered to segregate at the boundary of dendrites during solidification, thereby reducing the toughness of the weld metal. Therefore, when the P content increases, the strength in the peeling direction of the spot welded joint 2 decreases. Such a decrease in peel direction strength due to the segregation of P may not be compensated only by the heat treatment of the present embodiment. Therefore, in this embodiment, the P content is suppressed to 0.015% or less. It is desirable to suppress the segregation of P itself.

  In order to investigate the influence of the C content and P content of the weld metal on the strength of the spot weld, the present inventors have the composition shown in Table 4 (Fe and unavoidable impurities other than those shown in the table), The welding shown in Table 5 is applied to an L-shaped tensile test piece 0 having the shape shown in FIG. 1, which is an overlapping member in which two high-tensile steel plates 0a and 0b having a thickness of 1.4 mm and a tensile strength of 800 MPa are overlapped. Spot welding is performed under the conditions (electrode, applied pressure, energization time, welding current, and hold time), and the spot welded part 0c of the welded joint as-welded and the heat-treated temperature of 250 ° C. after cooling the spot welded part 0c to room temperature A tensile test was performed on the spot welded portion 0c of the welded joint obtained by maintaining the heat treatment time for 20 minutes, and the strength in the peeling direction was compared and investigated. The results are summarized in the graph of FIG.

  FIG. 5 is a graph showing the fracture form of a welded joint, organized according to the C content and P content of the weld metal of this welded joint. In the graph of FIG. 5, a black circle mark indicates that the base material is broken, and a white circle mark indicates that the interface is broken.

  From the graph shown in FIG. 5, when two high-tensile steel plates having the same thickness are stacked, the influence of the C content and the P content on whether the base material fracture or the interface fracture is large, If (C content% × P content%) is 0.0025 or less, the base material breaks and it can have sufficient breaking strength by heat treatment, whereas (C content% × P content%) is When it exceeds 0.0025, interface fracture occurs, and it can be seen that the effect of improving the fracture strength by heat treatment is reduced.

  By the heat treatment of the present embodiment, the fracture mode is changed from the interface fracture (break in the nugget) to the base metal fracture, whereby the strength in the peeling direction of the welded joint is increased. For this reason, in the area | region (C content% x P content%) is 0.0025 or less, the improvement effect of the peeling direction strength of the welded joint by heat processing will be acquired.

  As described above, the strength in the peeling direction is easily affected by P segregation in the weld metal as the C content of the weld metal increases. Therefore, the P content is C × P ≦ 0.0025 according to the C content. To do.

  S segregates at the boundary of the dendrite during solidification and reduces the toughness of the weld metal. Therefore, when the S content increases, the strength in the peeling direction of the spot welded joint decreases. Therefore, in this Embodiment, S content is suppressed to 0.010% or less. According to the latest steelmaking technology, it is possible to desulfurize to a sufficiently low level, and there is also a balance with the P content. Therefore, the S content is more preferably 0.003% or less.

  Elements of the weld metal other than these elements do not need to be specified because they do not have a special effect in the present invention. What is necessary is just to determine suitably according to the chemical component of the base material which can obtain the tensile strength of 440 Mpa or more.

The inventors further conducted a test for investigating the relationship between the position in the thickness direction of the overlapping surface and the strength in the peeling direction.
Two types of steel plates of Type-A (tensile strength 780 MPa) and Type-B (tensile strength 1500 MPa) were used as test materials. Type-A and B are chemical components shown in Tables 1 and 6, respectively. Chemical components other than those shown in Tables 1 and 6 are Fe and inevitable impurities. The former is a steel plate having a chemical component within the range recited in claim 1, and the latter is a steel plate having a chemical component outside the range recited in claim 1.

  FIG. 6 is an explanatory view showing the influence of the plate assembly on the peel direction strength of the spot welded joint, and shows a two-layer spot welded joint when the plate thickness is different.

  In Type-A, the thickness of the upper and lower steel plates 5, 6 is changed by using two steel plates 5, 6 having a thickness of 1.2-1.8mm, and in Type-B, the thickness of 1.6-2.6mm. The joint 7 was prepared, and the peel direction strength as welded and the peel direction strength when heat treatment (temperature 250 ° C., time 20 minutes) was performed were investigated. Here, as shown in FIG. 6, the difference between the overlapping surface and the center line in the plate thickness direction is defined as the “deviation amount”.

  FIG. 7 is a graph showing the results of arranging the peel direction strengths of the joints 7 having different thicknesses of the upper and lower steel plates 5 and 6 of Type-B in terms of the deviation amount (%) with respect to the total plate thickness. The rhombus marks and the circle marks in the graph of FIG. 7 indicate the strength in the peel direction when heat treatment is performed while welding. In addition, the base material, nugget, and interface described in each plot in the figure indicate that the base material is fractured, nugget internal fracture, and interface fracture. In the Type-B steel plate, it can be seen that the effect of the post heat treatment is small when the shift amount is small. This is because the segregation of P and S of the weld metal 8 has a strong influence on the strength in the peeling direction when the overlapping surface exists at the center portion of the plate thickness as described above.

  FIG. 8 is a graph showing the relationship between the joint strength increase rate and the deviation (%) relative to the total plate thickness when the plate thicknesses of the upper and lower steel plates 5 and 6 of Type-A are different. Here, the joint strength increase rate is the peel direction strength (%) after heat treatment with respect to the peel direction strength as-welded. In Type-A steel plates 5 and 6, it can be seen that the strength in the peeling direction is remarkably increased by post-heat treatment when the shift amount is small. It is desirable that the amount of deviation between the overlapping surface and the center line in the thickness direction is within 10% of the total thickness.

  That is, in the case of a joint in which two steel plates are combined so that there is an overlapping surface in the central portion of the plate thickness, the peeling direction is reduced by performing the heat treatment specified in the present invention, compared to the case where no heat treatment is performed. Hardness is greatly improved.

  Thus, according to the present embodiment, improving the strength in the peeling direction of the welded joint with a small nugget diameter, that is, a welded joint of a high-tensile steel plate having a tensile strength of 440 MPa or more excellent in the peeling direction strength, A manufacturing method thereof is provided.

  In the present embodiment, the weld metal of the spot welded portion subjected to spot welding in the overlapping member constituted by overlapping a plurality of high-tensile steel plates having a tensile strength of 440 MPa or more is mass%, and C × P ≦ 0. .0025, P: 0.015% or less, S: 0.01% or less, and the spot welded portion is heat-treated at a heat treatment temperature and a heat treatment time satisfying the above formula (1). The spot weld is tempered and the toughness of the welded structure is improved. Thereby, when the spot weld part of this spot welded joint is subjected to a tensile test, the strength in the peeling direction is improved.

  The heat treatment means in this embodiment is not limited to a specific means, and any means used for this kind of heat treatment such as heating in a furnace can be used. However, heat treatment by energization heating with a spot welding electrode is not preferable because of problems such as an increase in construction time and deterioration in continuous spotting performance.

  The high-tensile steel sheet having a tensile strength of 440 MPa or more, which is the subject of the present invention, is mainly for automobiles, but is not necessarily limited to automobiles, and is used for other applications that require a high strength of 440 MPa or more. Of course, it is also applicable. The chemical component of this high-tensile steel sheet can maintain a tensile strength of 440 MPa or more, and the weld metal is in mass%, C × P ≦ 0.0025, P: 0.015% or less, S: 0.01% or less. What is necessary is just to have a certain chemical component, and it is not limited to a specific chemical component.

  The thickness of the high-tensile steel plate need not be specified. In general, the plate thickness of a steel plate used in automobile parts and vehicle bodies is 0.4 mm or more and 4.0 mm or less, and the present invention has a sufficient effect in this range.

  There is no particular need to specify the type of high-tensile steel plate. For example, it can be applied to various known high strength steel plates having a tensile strength of 440 MPa or more, such as precipitation strengthened steel, DP steel, TRIP (work-induced transformation) steel, and steel plates for hot pressing. The high-tensile steel plate may be a cold-rolled steel plate or a hot-rolled steel plate. Further, the high-tensile steel plate may be a bare steel plate that is not plated or a plated steel plate.

  Further, spot welding is usually performed on two high-strength steel plates that are overlapped, but the present invention is also effective for three or more high-strength steel plates that are overlapped.

The present invention will be described more specifically with reference to examples.
In this embodiment, the L-shaped tensile test piece 0 in which two high-tensile steel plates are overlapped as shown in FIG. 1 and the L-shaped tensile member in which three high-tensile steel plates 9a to 9c are overlapped as shown in FIG. Using the test piece 9, the effect of heat treatment on the peel joint strength of the spot welds 0c and 9d was examined.

  Two types of high-strength steel plates Type-A (tensile strength: 780 MPa) and B (tensile strength: 1500 MPa) having a thickness of 1.8 mm were used for the high-tensile steel plates 0a, 0b, and 9a to 9c. High-tensile steel plates Type-A and B are chemical components shown in Tables 1 and 6, respectively. Chemical components other than those shown in Tables 1 and 6 are Fe and inevitable impurities.

  Spot welding is a combination of high-strength steel sheets of the same steel type, with the welding conditions (electrode, pressure, energization time, welding current, and hold time) shown in Table 7 for two-ply welding and three-ply welding. went. After the spot weld was cooled to room temperature, it was charged into a furnace and subjected to heat treatment at a predetermined heat treatment temperature (300 ° C.) and heat treatment time (5 minutes, 10 minutes). For comparison, a test piece without heat treatment was also produced.

  The peel direction strength was determined for the L-shaped tensile test pieces 0 and 9 of the obtained spot welded joint. The results are summarized in Table 8.

  In the test piece made of Type-A high-strength steel plate, the chemical composition of the weld metal satisfies the range specified in the present invention, and by performing the heat treatment specified in the present invention, two sheets and three sheets are stacked. It can be seen that the peel joint strength increases in both cases.

  On the other hand, in the test piece made of Type-B high tensile strength steel plate, the chemical component of the weld metal does not satisfy the range defined by the present invention, and the central portion in the plate thickness direction is subjected to the heat treatment defined by the present invention. The peel joint strength increases in the case of three-sheet superimposition where the overlap surface exists at a position different from that of the peel joint, but in the case of two-sheet overlap where the overlap surface exists in the center in the thickness direction, the peel joint It can be seen that the effect of increasing the strength cannot be obtained.

It is explanatory drawing which shows the shape of the L-shaped tension test piece of 2 sheets overlap. It is a graph which shows the relationship between heat processing temperature and L-shaped joint strength. It is a graph which shows the relationship between heat processing time and heat processing temperature, and a fracture | rupture form. FIG. 4A is an explanatory diagram showing the influence of a plate assembly on the peel strength of a spot welded joint, and FIG. 4A shows a spot welded joint having a three-layer spot welded portion made of three high-tensile steel plates. (B) shows a spot welded joint having a two-layer spot welded portion made of two high-tensile steel plates. It is a graph which arrange | positions and shows the fracture | rupture form of a welded joint according to C content and P content of the weld metal of this welded joint. It is explanatory drawing which shows the influence of the board assembly which acts on the peeling direction intensity | strength of a spot welded joint, and shows the 2 sheet | seat spot welded joint in case plate | board thickness differs. It is the graph which arranged the peeling direction intensity | strength of the joint from which the upper and lower plate | board thickness of Type-B differs according to the deviation | shift amount (%) with respect to a total board thickness. It is a graph which shows the relationship of the deviation | shift amount (%) with respect to the joint intensity | strength increase rate and total plate | board thickness in Type-A. It is explanatory drawing which shows the shape of a 3 sheet L-shaped tension test piece.

Explanation of symbols

0 L-shaped tensile test pieces 0a, 0b High-tensile steel plate 0c Spot welded portion 1, 2 Spot welded joints 1a-1c, 2a, 2b High-tensile steel plate 3 Three-layer spot welded portion 4 Two-layer spot welded portion 5, 6 Steel plate 7 Two-lap joint 8 Weld metal 9 L-shaped tensile test pieces 9a to 9c High strength steel plate 9d Spot weld

Claims (5)

A method for manufacturing a welded joint having a welded portion by performing resistance welding on an overlapping member formed by superimposing a plurality of steel plates having a tensile strength of 440 MPa or more, wherein the weld metal of the welded joint is in mass%. , C × P ≦ 0.0025, P: 0.015% or less, S: 0.01% or less, and by increasing the temperature of the welded portion, the welded portion has the following formula (1 And a heat treatment temperature and a heat treatment time satisfying the above.
300 ≦ T · (log ₁₀ (t) +1) ≦ 1000 (1)
In the formula (1), T represents a heat treatment temperature (100 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes). The said heat processing is a manufacturing method of the welded joint described in Claim 1 which satisfies following formula (2).
400 ≦ T · (log ₁₀ (t) +1) ≦ 750 (2)
In the formula (2), T represents a heat treatment temperature (150 ° C. ≦ T ≦ 400 ° C.), and t represents a heat treatment time (heat treatment temperature holding time; minutes). 3. The method for manufacturing a welded joint according to claim 1, wherein the overlapping member has an overlapping surface of the plurality of steel plates at a central portion in a thickness direction thereof.
However, the central portion of the overlapping member in the thickness direction refers to a region where the amount of deviation between the overlapping surface and the center line in the thickness direction is within 10% of the total thickness of the overlapping members.   The method for manufacturing a welded joint according to any one of claims 1 to 3, wherein the plurality of steel plates are two steel plates.   A welded joint manufactured by the manufacturing method according to any one of claims 1 to 4.

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