WO2020240961A1 - 抵抗スポット溶接部および抵抗スポット溶接方法、並びに抵抗スポット溶接継手および抵抗スポット溶接継手の製造方法 - Google Patents
抵抗スポット溶接部および抵抗スポット溶接方法、並びに抵抗スポット溶接継手および抵抗スポット溶接継手の製造方法 Download PDFInfo
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- WO2020240961A1 WO2020240961A1 PCT/JP2020/009011 JP2020009011W WO2020240961A1 WO 2020240961 A1 WO2020240961 A1 WO 2020240961A1 JP 2020009011 W JP2020009011 W JP 2020009011W WO 2020240961 A1 WO2020240961 A1 WO 2020240961A1
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- nugget
- resistance spot
- region
- spot welded
- hardness
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/10—Spot welding; Stitch welding
- B23K11/11—Spot welding
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K11/00—Resistance welding; Severing by resistance heating
- B23K11/24—Electric supply or control circuits therefor
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
Definitions
- the present invention relates to a resistance spot welded portion and a resistance spot welded method, and a method for manufacturing a resistance spot welded joint and a resistance spot welded joint.
- HITEN high-strength steel sheets
- resistance spot welding is mainly used for joining members, for example, structural members of automobiles having high-strength steel plates. Welded joints joined by resistance spot welding are required to have strength (tensile strength) that does not break even during collision deformation in order to ensure collision safety as described above.
- the joint strength of the welded part is the shear tensile strength (TSS: Tensile shear strength) which is the tensile strength in the shear direction of the joint and the cross tensile strength (CTS:) which is the tensile strength in the peeling direction of the joint. It is evaluated by Cross tension (strength).
- TSS shear tensile strength
- CTS cross tensile strength
- the TSS of the resistance spot weld tends to increase with the tensile strength of the base metal, but the CTS of the resistance spot weld may decrease when the tensile strength of the base metal is 780 MPa or more.
- the CTS decreases, the fracture morphology changes from a plug fracture that ductilely fractures in the base metal or HAZ (heat-affected zone) around the resistance spot weld to an interface fracture or a partial plug fracture that brittlely fractures in the nugget. Transition.
- the cause of the decrease in CTS is that brittle fracture occurs due to hardening of the nugget end portion after quenching.
- Patent Document 1 describes that the nugget (melt-solidified zone) and the heat-affected zone in the base material of a specific steel type have a tempered martensite structure or a tempered bainite structure.
- Patent Document 2 describes that the maximum temperature at the interface between the nugget and the corona bond when the post-energization process is performed is specified.
- Patent Document 3 describes that the hardness on the outside of the nugget and the structure inside the nugget are specified.
- Patent Document 4 describes that tempering is performed at a high current value.
- Patent Document 1 only defines the component composition of the base material, and does not consider the welding conditions for the nugget and the heat-affected zone to obtain the above structure, specifically, the temperature range of post-energization. ..
- Patent Document 2 makes it possible to reduce segregation by energizing after raising the temperature to a high temperature in a short time, thereby improving the joint strength.
- a cooling time is set so that the temperature at the end of the nugget does not fall below the Ms point. For this reason, the structure is limited to those that do not undergo martensitic transformation in the cooling process after the main energization, and tempered martensite cannot be obtained. As a result, the toughness of the nugget end does not improve.
- Patent Document 3 stipulates that the structure inside the nugget is an equiaxed martensite structure and that a softening region having a hardness lower than that of the base material exists outside the nugget in order to achieve both TSS and CTS. In addition, as post-energization, a high current about twice that of the main energization step is applied in a short time (0.1 seconds or less). However, since the tissue in the obtained nugget is a martensite structure, sufficient toughness cannot be obtained. That is, Patent Document 3 does not consider that the hardness is appropriately controlled by baking at a high temperature.
- Patent Document 4 since tempering is performed at a current value higher than that of the main energization, there is a concern that the end of the nugget exceeds the melting point and melts. If the nugget end melts, it becomes martensite after cooling, and strength cannot be secured.
- a resistance spot welding method with only single energization there is also a resistance spot welding method with only single energization.
- a high-strength steel sheet having a tensile strength of 780 MPa or more and containing 1.5 to 10.0% by mass of Mn as a component composition of the steel sheet (hereinafter, this steel sheet is referred to as a medium Mn steel sheet) has a resistance spot of only this single energization.
- the austenite structure contained in the medium Mn steel sheet becomes a martensite structure when the molten portion formed by energization melts and hardens. As a result, the tissue becomes hard and brittle, so that there is a problem that the CTS is low.
- the present invention improves the joint strength by improving the toughness of the nugget end of the resistance spot weld even in the high-strength steel plate having a tensile strength of 780 MPa or more, particularly a medium Mn steel plate. It is an object of the present invention to provide a resistance spot welded portion and a resistance spot welded method capable of forming, and a method for manufacturing a resistance spot welded joint and a resistance spot welded joint.
- a mechanism for lowering CTS and a method for improving CTS in resistance spot welding of a plate frame including a high-strength steel plate having a tensile strength of 780 MPa or more have been intensively studied.
- the CTS decreases as the strength of the steel sheet increases.
- the fracture mode changes from a plug fracture that ductilely fractures at the base metal or HAZ around the resistance spot weld to an interfacial fracture or a partial plug fracture that brittlely fractures in the nugget.
- the cause of the interfacial fracture is the embrittlement of the nugget end due to the formation of a hardened structure by quenching after the nugget is formed. As a result, the nugget end is cracked and the interface is broken. Therefore, in order not to cause this brittle fracture, it is necessary to make the nugget end a tough structure.
- the present inventors change the fracture surface from a ductile fracture surface to a brittle fracture surface due to the progress of tempering in the range where the above-mentioned brittle fracture occurs, and the fracture surface is brittle due to this. It was found that it causes brittleness. That is, it was clarified that the toughness of the nugget end can be improved by tempering the nugget end at a temperature higher or lower than the temperature range of the temper embrittlement region.
- main energization is performed by heating to a temperature range above the melting point for nugget formation, and then cooling is performed to quench the temperature from the austenite structure to the martensite structure through solidification of the melting part.
- the process is carried out, and then the nugget end is rebaked in an appropriate temperature range and then energized.
- the metal structure at the end of the nugget becomes a structure having a tempered martensite structure as the main phase.
- the nugget end when the hardness Hv in a specific region of the nugget end (nugget tip region described later) satisfies a predetermined relational expression with respect to the hardness Hmw of the martensite structure of the entire nugget, the nugget end becomes It has a tempered martensite structure with high toughness. As a result, it was found that the effect of avoiding the interface breakage of the resistance spot welded portion can be obtained. Further, in the above-mentioned tempered resistance spot welded portion having a high CTS, the hardness Hh in a specific region of HAZ (strong HAZ region described later) is also the hardness Hh of the martensite structure of the superposed steel sheet. It was found that the predetermined relational expression was satisfied.
- the present invention has been made based on the above findings, and has the following gist.
- a resistance spot welded portion of a welded member obtained by superimposing two or more steel plates and performing resistance spot welding. At least one of the steel sheets has a component composition of mass%. C: 0.05-0.6%, Si: 0.1-3.5%, A high-strength steel sheet satisfying the range of Mn: 1.5 to 10.0% and P: 0.1% or less. Two points on the boundary of the nugget that intersect the overlapping surfaces of the steel sheets are defined as the first end and the second end. Let D (mm) be the length of the line segment X connecting the first end and the second end.
- the positions on the line segment X from the first end and the second end toward the center of the nugget are designated as points O and P, and from the first end to the point O and from the second end.
- the region in the nugget where each distance L (mm) to the point P satisfies the following formula (1) is defined as the nugget tip region, one or more of the nugget tip regions corresponding to the overlapping surfaces
- the metallographic structure of the nugget tip region has tempered martensite as the main phase.
- the hardness Hv of the nugget tip region satisfies the following formula (4) with respect to the hardness Hmw of martensite of the entire nugget calculated by the following formulas (2) and (3).
- Cw (% by mass): C content per volume from each steel sheet in the nugget
- Ci (mass%): C content of each stacked steel sheet
- Vi (mm 2 ): The molten area of each steel sheet in the region surrounded by the nugget boundary and each line X in the thickness direction cross section passing through the center of the nugget.
- n The number of stacked steel plates.
- M D / 20 ... (5) 0 ⁇ T ⁇ D / 10 ... (6)
- Hmh 884 x Ch x (1-0.3 x Ch 2 ) +294 ... (7) Hh ⁇ Hmh-25 ...
- Ch (mass%): C content of the steel plate on the upper side with respect to the overlapped surface, or C content of the steel plate on the lower side with respect to the overlapped surface.
- two points on the boundary of the nugget located in the middle of the gap and intersecting the straight line Y parallel to the surface of the steel plate are the first end portion and the said. It is the second end.
- the resistance spot welded portion according to [1] wherein the proportion of carbides in the tempered martensite exceeds 20% in terms of area ratio.
- the resistance spot welded portion according to [2] wherein the carbide has an average crystal grain size of 300 nm or less.
- the toughness is improved and the joint strength is increased by defining the metal structure and hardness of the nugget end portion and the hardness of the HAZ around the nugget end portion in the resistance spot welded portion of the high-strength steel plate. Can be improved.
- FIG. 1 (A) and 1 (B) are cross-sectional views illustrating a resistance spot welded portion according to an embodiment of the present invention.
- FIG. 2 is a cross-sectional view illustrating a case where there is a plate gap in the resistance spot welded portion shown in FIG. 1 (A).
- 3 (A) and 3 (B) are cross-sectional views illustrating a resistance spot welded portion in another embodiment of the present invention.
- FIG. 4 is a cross-sectional view illustrating a state of resistance spot welding according to an embodiment of the present invention.
- FIG. 1 (A) to 3 (B) show cross-sectional views in the plate thickness direction for explaining an example of the resistance spot welded portion obtained in the present invention.
- FIG. 1 (A) shows the entire resistance spot welded portion obtained by superimposing and welding two steel plates
- FIG. 1 (B) shows a partially enlarged portion of the resistance spot welded portion shown in FIG. 1 (A).
- FIG. 2 shows an example in which a plate gap exists on the overlapping surface of the steel plates in the resistance spot welded portion shown in FIG. 1 (A).
- FIG. 3A shows the entire resistance spot welded portion obtained by superimposing and welding three steel plates
- FIG. 3B shows a partially enlarged portion of the resistance spot welded portion shown in FIG. 3A. The figure is shown.
- the present invention is a resistance spot welded portion of a welded member in which two or more steel plates are superposed and joined by resistance spot welding.
- the steel sheets to be stacked include at least one high-strength steel sheet described later.
- the steel plate 1 arranged on the lower side and the steel plate 2 arranged on the upper side are overlapped with each other.
- the lower steel plate 1 and / or the upper steel plate 2 is a high-strength steel plate.
- the resistance spot welded portion (hereinafter referred to as a welded portion) of the present invention has a nugget 3 and a heat affected zone (HAZ) 6.
- HZ heat affected zone
- the cross-sectional shape in the thickness direction of the welded portion formed in a circular shape passing through the center of the nugget 3 is elliptical.
- the welded portion in the plate assembly of the two steel plates will be described with reference to FIGS. 1 (A), 1 (B) and 2.
- the welded portion has a tip region 31 having a highly tough metal structure in the nugget 3 and a strong HAZ region 61 having a predetermined hardness in the HAZ 6.
- two points on the boundary of the elliptical nugget 3 intersecting the overlapping surfaces 7 of the stacked steel plates 1 and 2 are the first end 8a and the second end. Let it be part 8b.
- the straight line connecting the first end portion 8a and the second end portion 8b is referred to as a line segment X, and the length of the line segment X is D (mm).
- the position on the line segment X from the first end 8a toward the center of the nugget 3 is defined as the point O, and the distance from the first end 8a to the point O is L (mm).
- nugget tip region 31 A region within the nugget 3 in which these distances L satisfy the following formula (1) is referred to as a nugget tip region 31 (hereinafter, may be referred to as a “tip region”). As shown in FIG. 1 (A), the tip region 31 exists at both ends of the nugget 3.
- the tip region 31 can be defined for each of the overlapping surfaces 7 of the steel sheets. That is, in the case of a welded member in which three or more steel plates are overlapped and resistance spot welded, the welded member has two or more overlapped surfaces, and a nugget tip region can be defined for each overlapped surface. For example, as shown in FIG. 3A described later, when three steel plates are assembled, there are two overlapping surfaces, and a nugget tip region exists for each overlapping surface.
- the tip region 31 is a region where the distance L from the first end portion 8a to the point O and the distance L from the second end portion 8b to the point P satisfy the equation (1).
- the distance L does not satisfy the condition of the formula (1) (that is, 0 ⁇ L ⁇ 0.25 ⁇ D)
- the metal structure of the present invention described later is provided in the region of the nugget end that affects the joint strength. There will be no.
- the distance L is preferably 0 ⁇ L ⁇ 0.20 ⁇ D.
- the metal structure of the central portion of the nugget (in the example shown in FIG. 1A, it refers to a region other than the tip region 31 in the nugget 3) does not affect the joint strength. No particular metal structure is specified.
- the metal structure of the tip region 31 at both ends of the nugget has a tempered martensite structure as the main phase.
- the main phase means that the tempered martensite structure has an area ratio of 60% or more with respect to the entire metal structure in the nugget 3.
- the tempered martensite structure is less than 60%, it is considered that the tempering has not progressed or the metal structure contains a large amount of martensite structure that has appeared due to the tempering temperature being too high. As the martensite structure of the remaining structure, which will be described later, increases, the nugget end becomes a hard and brittle structure, resulting in brittle fracture, and thus the joint strength becomes low. Therefore, the tempered martensite structure is 60% or more. It is preferably 80% or more. More preferably, it is 90% or more. In the present invention, since it is desirable to have a large amount of tempered martensite structure having toughness at the nugget end, an upper limit of the tempered martensite structure is not particularly set. The tempered martensite structure is preferably 100% or less.
- the metal structure in the tip region 31 may contain a martensite structure as a structure other than the tempered martensite structure (hereinafter, may be referred to as “residual structure”).
- a martensite structure as a structure other than the tempered martensite structure (hereinafter, may be referred to as “residual structure”).
- residual structure a martensite structure that can exist if the tempering temperature is too high. Since the martensite structure is a particularly brittle structure, there is a concern that it will greatly affect the decrease in joint strength. Therefore, the martensite structure is preferably reduced as much as possible, preferably less than 40%.
- Carbides are deposited in the tempered martensite structure. It was found that when tempering was performed so that the hardness Hv of the tip region 31 secured the hardness desired in the present invention, the proportion of carbides in the tempered martensite structure exceeded 20% in terms of area ratio. .. As the tempering progresses, the carbides become coarser and the distance between adjacent carbides becomes narrower. Also, as tempering progresses, carbides increase. Therefore, by setting the ratio of carbides in the tempered martensite structure to more than 20% in terms of area ratio together with the above-mentioned hardness index, it can be determined that the temperature is not in the tempered embrittlement region more appropriately. That is, it can be determined that the tempering temperature is more appropriate.
- the ratio of carbides in the tempered martensite structure is preferably more than 20% in terms of area ratio. It is more preferably 23% or more, and further preferably 40% or more. It is preferably 85% or less, more preferably 75% or less, and further preferably 50% or less.
- Carbides appear by discharging supersaturated C by tempering. Therefore, it is shown that tempering is progressing depending on the proportion of carbides.
- the ratio of carbides having an average crystal grain size (hereinafter, sometimes referred to as an average particle size) of 300 nm or less is specified. This is because if the average particle size of the carbide exceeds 300 nm, the tempering temperature may rise to the temperature range of the embrittlement range due to the growth of grains.
- the structure of the nugget tip region and the carbides in the tempered martensite structure can be measured by the method described in Examples described later.
- the hardness Hv of the tip region 31 satisfies the following formula (4) with respect to the hardness Hmw of the martensite of the entire nugget 3 calculated by the following formula (2) and the following formula (3). ..
- Cw (% by mass): C content per volume from each steel sheet in the nugget
- Ci mass%: C content of each stacked steel sheet
- Vi (mm 2 ): The molten area of each steel sheet in the region surrounded by the nugget boundary and each line X in the thickness direction cross section passing through the center of the nugget.
- n The number of stacked steel plates.
- tempering proceeds, and the fracture surface changes from a ductile fracture surface to a brittle fracture surface, resulting in embrittlement. I found it to wake up. That is, it was found that when tempering progresses too much and causes embrittlement, the hardness of the nugget end may decrease as compared with proper tempering.
- the present inventors performed SEM observation of the fracture surface of the joint in this embrittlement region. As a result, it was found that the entire nugget had a large number of grain boundary fracture surfaces, and the end of the nugget also occupied a large number of grain boundary fracture surfaces, indicating that the nugget was a brittle fracture surface.
- a ductile fracture surface including dimples can be seen around the nugget end. From this, it was found that the embrittled region contained a large number of grain boundaries even on the fracture surface. That is, it was found that in this embrittlement region, P segregates at the grain boundaries, resulting in a brittle fractured form and a decrease in joint strength. Therefore, in the present invention, the tempered state is determined from both the metallographic structure and hardness of the nugget end.
- the end portion of the nugget 3 is lower or higher than the embrittlement region and has a melting point. Since it is in a state of being tempered in the following temperature range, it can be seen that a good joint is obtained. Further, the smaller the value of the hardness Hv of the tip region 31, the more the tempering progresses. As the tempering progresses, the toughness of the nugget end is improved and the cracks proceed to the outside of the nugget, so that the cracks do not grow inside the nugget and the plug breaks. From this, it is considered that the joint strength is improved. Considering tempering to obtain a better joint, the hardness Hv of the tip region 31 is preferably (Hmw-55) or less.
- the lower limit of the hardness Hv of the tip region 31 is not particularly specified. In order to properly obtain the metal structure of the present invention, it is considered that there is a limit to the decrease in hardness of the tip region 31 due to tempering. Therefore, the hardness Hv of the tip region 31 is set to (Hmv-700) or more. Is preferable.
- a part of the nugget end having tempered martensite as the main phase may rise to the austenite region by an appropriate post-energization temperature, and the reverse-transformed tissue may become a martensite structure after the post-energization is completed.
- the nugget end portion is configured as described above, and HAZ6 has a configuration described below. That is, the hardness Hh in a specific region (strong HAZ region 61 described later) in HAZ6 satisfies the following relational expression (formula (8)) with respect to the hardness Hm of the martensite structure of each steel sheet. As a result, it can be confirmed that the tempering has not progressed too much even in the region in HAZ which may affect the joint strength, so that the effect of the present invention can be appropriately obtained.
- the strong HAZ region 61 in the steel plate 2 on the upper side and / or the steel plate 1 on the lower side with respect to the overlapping surface 7 has the following configuration.
- the intersection of the boundary between the straight line Z parallel to the overlapping surface 7 (or the line segment X described above) and the nugget 3 is defined as a point q
- the position in HAZ 6 on the straight line Z is defined as a point r.
- HAZ6 in which the distance M (mm) between the straight line Z and the overlapping surface 7 in the plate thickness direction satisfies the following formula (5), and each distance T (mm) from the point q to the point r satisfies the following formula (6).
- the inner region is defined as the strong HAZ region 61.
- the line segment X (overlapping surface 7) and the straight line Z are lines perpendicular to the thickness direction of the steel sheet.
- a strong HAZ region 61 exists on the upper steel plate 2 and the lower steel plate 1 on the outside of the tip region 31, respectively.
- the strong HAZ region 61 may exist only in the upper steel plate 2 or only in the lower steel plate 1.
- the hardness Hh in the strong HAZ region 61 satisfies the following formula (8) with respect to the hardness Hm of the martensite of the steel sheet calculated by the following formula (7), respectively.
- M D / 20 ... (5) 0 ⁇ T ⁇ D / 10 ... (6)
- Hmh 884 x Ch x (1-0.3 x Ch 2 ) +294 ... (7) Hh ⁇ Hmh-25 ... (8)
- the hardness Hh of the strong HAZ region 61 does not satisfy the above formula (8), that is, when it is (Hmh-25) or more, the tempering of the strong HAZ region 61 proceeds too much and rises to the melting point. There may be. As a result, the structure of the strong HAZ region 61 becomes martensite and the toughness is lowered, so that the joint strength may be lowered. Therefore, from the viewpoint of performing appropriate tempering to HAZ6, the hardness Hh of the strong HAZ region 61 is set to less than (Hmh-25). It is preferably (Hmh-40) or less.
- the lower limit of the hardness Hh of the strong HAZ region 61 is not particularly specified. Since it is considered that there is a limit for reducing the hardness by tempering, the hardness Hh of the strong HAZ region 61 is preferably (Hmh-700) or more.
- FIG. 2 shows an example of a welded portion having a plate gap G.
- a straight line equidistant from two straight lines formed by the surface of the steel plate on the side where the steel plates 1 and 2 face each other that is, located in the middle of the plate gap G in the plate thickness direction, and the steel plates 1 and 2
- the two points on the boundary of the nugget 3 intersecting the straight line Y) parallel to the surface of the steel plate are defined as the first end portion 8a and the second end portion 8b.
- the region in the nugget 3 where the distance L from the first end 8a to the point O and the distance L from the second end 8b to the point P satisfy the equation (1) is the tip region 31. ..
- the straight line Y may be regarded as an "overlapping surface", and the strong HAZ region 61 may be defined in the same manner as described above.
- the number of overlapping surfaces 7 is two or more, and each overlapping surface 7 has a tip region 31 and a strong HAZ region 61.
- the tip region 31 corresponding to the overlapping surfaces has the above-mentioned metal structure and hardness Hv
- the strong HAZ region 61 has the above-mentioned hardness. If it has Hh, the effect of the present invention can be obtained in the same manner.
- D 1 be the length of.
- the region in the nugget 3 where the distance L 1 satisfies the above equation (1) is the tip region 31.
- the intersection of the boundary between the straight line Z 1 parallel to the overlapping surface 7 and the nugget 3 is set as a point q 1
- the straight line Z 1 the point r 1 the position in HAZ6 above.
- the distance M 1 (mm) in the plate thickness direction between the straight line Z 1 and the first overlapping surface 7a satisfies the above formula (5)
- the distance T 1 (mm) between the point q 1 and the point r 1 is the above formula ( The region in HAZ6 that satisfies 6) becomes strong HAZ61.
- the second lap surface 7b and two points of the first end portion 8a 2 on the boundary of the nugget 3, the second end portion 8b 2 and to a line connecting the first end portion 8a 2 of the second end portion 8b 2 Let the length of the minute X 2 be D 2 .
- the distance between the point O 2 on the line X 2 and the first end portion 8a 2, and the point P 2 on the line segment X 2 the distance between the second end 8b 2, and each of L 2.
- the region in the nugget 3 where the distance L 2 satisfies the above equation (1) is the tip region 31.
- the intersection of the boundary between the straight line Z 2 parallel to the second overlapping surface 7b and the nugget 3 is set as a point q 2 .
- the position in HAZ 6 on the straight line Z 2 is defined as point r 2 .
- the distance M 2 (mm) between the straight line Z 2 and the second overlapping surface 7b in the plate thickness direction satisfies the above formula (5), and the distance T 2 (mm) between the point q 2 and the point r 2 is the above formula ( The region satisfying 6) is the strong HAZ61.
- the tip region 31 and the strong HAZ region 61 corresponding to either one of the first overlapping surface 7a and the second overlapping surface 7b correspond to both the first overlapping surface 7a and the second overlapping surface 7b.
- the hardness of the nugget tip region and the hardness of the heat-affected zone (HAZ) can be measured by the method described in Examples described later.
- the high-strength steel sheet used in the present invention will be described.
- at least one of the steel sheets to be overlapped is a high-strength steel sheet having the following composition. That is, the component composition of the high-strength steel sheet may satisfy C, Si, Mn, and P in the ranges shown below. If this range is satisfied, the resistance spot welding method according to the embodiment of the present invention can be effectively applied.
- “mass%" in the component composition is simply described as "%".
- C 0.05-0.6% C is an element that contributes to the strengthening of steel. If the C content is less than 0.05%, the strength of the steel becomes low, and it is extremely difficult to produce a steel sheet having a tensile strength of 780 MPa or more. On the other hand, when the C content exceeds 0.6%, the strength of the steel sheet increases, but the amount of hard martensite becomes excessive and microvoids increase. Further, the nugget and its surrounding heat-affected zone (HAZ) are excessively hardened and embrittled, so that it is difficult to improve the cross tensile strength (CTS). Therefore, the C content is set to 0.05 to 0.6%. The C content is more preferably 0.1% or more, and more preferably 0.3% or less.
- the Si content 0.1-3.5%
- the Si content is set to 0.1 to 3.5%.
- the Si content is more preferably 0.2% or more, and more preferably 2.0% or less.
- Mn 1.5 to 10.0%
- the Mn content is set to 1.5% or more and 10.0% or less.
- the Mn content is more preferably 2.0% or more, and more preferably 8.0% or less.
- P 0.1% or less
- P is an unavoidable impurity, but if the P content exceeds 0.1%, strong segregation appears at the nugget end of the weld, making it difficult to improve the joint strength. Therefore, the P content is set to 0.1% or less. More preferably, the P content is 0.05% or less, and more preferably, the P content is 0.02% or less.
- one or more elements selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al, and Ca may be added. ..
- Cu, Ni, and Mo are elements that can contribute to improving the strength of steel.
- Cr is an element whose strength can be improved by improving hardenability.
- Nb and V are elements that can reinforce steel by controlling the structure by precipitation hardening.
- Ti and B are elements that can improve hardenability and strengthen steel.
- Al is an element capable of controlling the structure for austenite granulation.
- Ca is an element that can contribute to improving the workability of steel.
- one or more elements selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al, and Ca if necessary. May be added. If these elements are excessively contained, toughness deterioration and cracks may occur. Therefore, when these elements are added, a total content of 5% or less is acceptable.
- composition of components other than these is Fe and unavoidable impurities.
- the tensile strength of the high-strength steel sheet having the above-mentioned component composition is preferably 780 MPa or more.
- CTS may decrease, especially when the tensile strength of the base metal is 780 MPa or more.
- a high-strength steel plate having a tensile strength of 780 MPa or more becomes a structure having toughness by tempering the metal structure such as the tip region of the nugget into martensite. Can prevent brittle fracture.
- the welded portion can suppress the decrease in CTS.
- the above effect can be obtained even with a high-strength steel sheet having a tensile strength of less than 780 MPa.
- the galvanized steel sheet refers to a steel sheet having a plating layer containing zinc as a main component.
- the zinc-based plating layer shall include all known zinc plating layers.
- the zinc-based plating layer includes a hot-dip galvanizing layer, an electrogalvanizing layer, a Zn—Al plating layer, a Zn—Ni layer, and the like.
- the steel plate to be stacked a plurality of steel plates of the same type may be stacked, or a plurality of steel plates of different types may be stacked.
- a surface-treated steel sheet having a plating layer and a steel sheet not having a plating layer may be overlapped.
- the thickness of each steel plate is the same or different.
- the thickness of the steel sheet is preferably 0.4 mm to 2.2 mm because it targets a general steel sheet for automobiles.
- FIG. 4 shows, as an example, a schematic view illustrating a state in which resistance spot welding is performed on two steel plates.
- the steel plate 1 arranged on the lower side and the steel plate 2 arranged on the upper side are overlapped.
- one or more of the steel sheets to be stacked are high-strength steel sheets having the above-mentioned component composition.
- the lower steel plate 1 and / or the upper steel plate 2 is a high-strength steel plate.
- the pair of upper and lower electrodes 4 and 5 sandwich the lower steel plate 1 and the upper steel plate 2 that are overlapped with each other, and energize while pressurizing.
- the steel plates 1 and 2 are sandwiched between the electrode 4 (lower electrode) arranged on the lower side and the electrode 5 (upper electrode) arranged on the upper side.
- the above-mentioned energization is an energization step described below.
- a nugget 3 having a required size is formed to obtain a welded member having the resistance spot welded portion described above.
- the process of energizing the lower steel plate 1 and the upper steel plate 2 by using the lower electrode 4 and the upper electrode 5 is controlled as follows.
- a main energization step of energizing with a current value I w (kA) is performed.
- a cooling process for cooling with a cooling time t c (ms) represented by the following formula (13) is given.
- the step tempering by the following equation (14) shows the current value I t (kA), during the energization time t p (ms) shown in the following equation (15), to energize. 400 ⁇ t c ...
- the main energization step is an energization step for forming a nugget 3 by melting the superposed portions of the superposed steel plates (in the example shown in FIG. 4, the lower steel plate 1 and the upper steel plate 2).
- the current value I w (kA) is applied to generate a welded portion.
- the energization conditions and pressurization conditions for forming the nugget 3 in the main energization step are not particularly limited. Welding conditions that have been used conventionally can be adopted.
- the energization conditions for the main energization are preferably such that the energization time t w is 120 to 400 ms and the current value I w is 4 to 8 kA.
- the pressurization condition is preferably 2.0 to 4.0 kN.
- the lower limit of the current value is preferably a current value that can secure a nugget diameter of 3 ⁇ t (t: plate thickness) (mm) or more, and the upper limit of the current value is the occurrence of scattering in order to obtain a stable nugget diameter. It is preferable that the current value is not accompanied by.
- a cooling process is provided between the main energization process and the tempering process described later.
- the structure of the tip region 31 is cooled to a temperature at which martensitic transformation occurs.
- cooling is performed with a cooling time t c (ms) represented by the above formula (13). If the cooling time t c is less than 400 ms, the nugget end cannot be cooled to a temperature at which martensitic transformation occurs. As a result, the retained austenite structure that could not be transformed into martensite in the tip region 31 becomes one or two types of martensite structure and retained austenite structure by re-energization and recooling in the tempering step described later.
- the cooling time t c is set to 400 ms or more.
- the cooling time t c is set. It is preferably 600 ms or more. More preferably, it is 800 ms or more. Even more preferably, it is 1000 ms or more.
- the upper limit of the cooling time t c is not particularly specified.
- the cooling time t c (ms) is preferably 8000 ms or less in order to shorten the workability. It is more preferably 4000 ms or less, further preferably 2000 ms or less, and even more preferably 1000 ms or less.
- the tempering step is a post-heat treatment step for tempering the tip region 31 of the nugget 3 formed in the main energization step to improve toughness.
- the tempering step is performed in an appropriate temperature range.
- the structure at the end of the nugget after welding will not be tempered, and a large amount of brittle martensite structure will remain. Further, when tempering is performed in the embrittlement region, impurities such as P remain at the grain boundaries. From these facts, the tip region 31 becomes an embrittled metal structure. As a result, the joint strength is low.
- the joint strength is improved. For this reason, it is necessary to control the temperature of the tempering process so that it becomes an appropriate temperature. Therefore, in the present invention, it is important to control the welding conditions in the tempering process as follows.
- the tempering step a current value shown in Equation (14) mentioned above I t (kA), during the energization time t p (ms) shown in equation (15) described above, to energize.
- the current value I t of the current tempering process exceeds (0.95 ⁇ I w) kA, since the current value of the energization tempering too high, the martensitic transformation tissue again in the cooling process, remelting or austenite The temperature rises to the region and eventually becomes a martensitic structure. As a result, the tip region 31 becomes a brittle structure, and the joint strength is not improved. Therefore, the current value I t and (0.95 ⁇ I w) kA or less.
- the current value I t is preferably set to (0.9 ⁇ I w) kA or less. More preferably, it is (0.8 ⁇ I w ) kA or less.
- the lower limit of the current value I t is not specifically defined. However, since the tip region 31 is tempered so that the metal structure as described above, the current value I t is preferably set to (0.4 ⁇ I w) kA or more. It is more preferably (0.5 ⁇ I w ) kA or more. It is even more preferable that it is (0.6 ⁇ I w ) kA or less.
- Is less than the energization time t p of the tempering process is 400 ms, it is impossible to martensitic structure tempered martensite structure of the tip region 31 generated in the cooling process. As a result, the tempered martensite structure cannot be produced in an area ratio of 60% or more in the tempering step. Further, the tip region 31 does not have the hardness Hv described above.
- the strong HAZ region 61 in order for the strong HAZ region 61 to have the above-mentioned hardness Hh, it is necessary to temper the HAZ at a temperature higher than the embrittlement region, but if the temperature becomes too high, the melting point will be exceeded and the martensite structure will be formed. It becomes a metal structure in which many appear. Therefore, the tempering process must not rise above an appropriate temperature that can be tempered. That is, it is not desirable to raise the current value too much. The longer the tempering process is, the more the tempering is promoted, but if it is too long, the temperature may rise above an appropriate temperature.
- the energization time t p is the least 400 ms.
- Energization time t p is more preferably not less than 600 ms, even more preferably at least 800 ms.
- the upper limit of the conduction time t p is not particularly defined. If a short time of for workability improvement, is preferably not more than energization time t p is 3000 ms, more preferably to less 2000 ms, more preferably to less 1500 ms, to be lower than or equal to 1000ms Is even more preferable.
- the average particle size of the carbides generated in the tempering step in the tip region 31 is 300 nm or less. It was found that in the above-mentioned target steel sheet of the present invention, an embrittlement region exists depending on the tempering temperature, and the joint strength does not improve in a temperature region exceeding the melting point. Therefore, it suffices to be able to bake in a temperature range below the melting point and below the embrittlement range, or in a temperature range below the melting point and above the embrittlement range. In the present invention, tempered martensite can be obtained and toughness can be maintained in either temperature range, so that the joint strength can be improved.
- the energization conditions for example, pressing force, electrodes, holding time, etc.
- the energization conditions are not particularly limited because they change depending on the plate assembly and plate thickness of the steel plate, and the number of times of energization is not limited.
- a pair of upper and lower electrodes may be provided, and a portion to be welded by the pair of electrodes may be sandwiched between them to pressurize and energize.
- a pressurizing control device and a welding current control device capable of arbitrarily controlling the pressurizing force and the welding current during welding, respectively.
- the pressurizing mechanism for example, air cylinder, servomotor, etc.
- current control mechanism for example, AC, DC, etc.
- type for example, stationary type, robot gun, etc.
- the type of power supply is not particularly limited.
- the shape of the electrode is also not particularly limited. Examples of the type of the tip of the electrode include DR type (dome radius type), R type (radius type), and D type (dome type) described in JIS C 9304: 1999.
- the present invention is a resistance spot welded joint having the above-mentioned resistance spot welded portion.
- the resistance spot welded joint of the present invention is, for example, a joint in which two or more stacked steel plates are joined by a molten portion defined by the above-mentioned metal structure and hardness and a resistant spot welded portion having HAZ.
- the steel plate, welding conditions, metal structure of the welded portion, etc. are the same as those described above, and are therefore omitted.
- the present invention is a method for manufacturing a resistance spot welded joint by using the above-mentioned resistance spot welding method.
- a plate set in which two or more steel plates are stacked is sandwiched by a pair of electrodes, and resistance spot welding is performed while applying pressure under the above-mentioned welding conditions.
- the tip region 31 has the above-mentioned metal structure and hardness.
- the strong HAZ region obtains the hardness Hh described above.
- the hardness Hh of the strong HAZ region 61 it is possible to determine the possibility that the nugget end portion and the HAZ are at the temperature of the embrittlement region. As a result, the toughness of the welded portion can be improved, and the CTS can be further improved. That is, both TSS and CTS can be compatible.
- the joint strength of the obtained welded joint can also be improved. Therefore, even when the plate assembly contains a medium Mn steel plate (high-strength steel plate) having the above-mentioned steel plate component, the effect of further improving the joint strength (particularly CTS) can be obtained.
- a plate assembly in which two steel plates (lower steel plate 1 and upper steel plate 2) are superposed is spot-welded by a resistance welding machine which is a servomotor pressure type and has a DC power supply attached to a C gun.
- a resistance spot welded joint was produced by forming a nugget 3 having a required size.
- test piece high-strength steel plates (steel plates A to H) having a plate thickness of 0.8 mm and a plate thickness of 1.2 mm from 780 MPa class to 1180 MPa class were used.
- the size of the test piece was 150 mm on the long side and 50 mm on the short side.
- steel sheets A to H those having the following component compositions were used.
- % representing the component composition of the steel sheet means “mass%" unless otherwise specified.
- the plate thicknesses of the plate sets a to h and the board set j were all the same 1.2 mm.
- the plate set i three medium-Mn steel plates A of the same type were laminated, and the plate thickness was 0.8 mm each.
- the structure and hardness of the nugget end, the hardness of the heat-affected zone (HAZ), and CTS were evaluated by the methods described below.
- the sample used for observing the tissue at the nugget end was obtained as follows.
- the prepared resistance spot welded joint was cut into a test piece, and the test piece was ultrasonically cleaned, then resin-filled, the cross section was polished, and etching was performed using a nital solution to obtain a sample.
- SEM was used for observing the tissue, and the observation was performed at a magnification of 1000 to 100,000.
- Hardness was measured by a Vickers hardness tester by the method specified in JISZ2244.
- Tables 3-1 and 3-2 show the structure of the nugget tip region in the resistance spot welded joint after welding, and the hardness of the nugget tip region, the entire nugget, and the heat-affected zone, respectively.
- the measurement positions of the hardness of the "nugget tip region" shown in Tables 3-1 and 3-2 are the boundary between the nugget 3 having an elliptical cross-sectional shape in the plate thickness direction and the steel plate.
- the position was 0.02 mm away from the point P in the HAZ6 direction. Indentations were made at these two points, and their values were measured.
- the measurement of carbides in the tempered martensite structure of the nugget tip region was performed as follows. For the observation of the cross section of the welded portion, a thin tital etching was performed on the cross section sample for observation, and an image was taken at 30,000 times using an SEM (Scanning Electron Microscope). The size of the carbide was measured using image processing software based on SEM photographs. For the ratio of carbides (area ratio), the ratio of carbides (area ratio) was calculated using image binarization software. As described above, the ratio (area ratio) of carbides was calculated for those having an average particle size of carbides of 300 nm or less.
- the hardness measurement positions of the "strong HAZ region 61" shown in Tables 3-1 and 3-2 were determined as shown in FIG. 1 (A).
- an indentation was made at a position 0.3 mm away from the point q toward the HAZ6 side, and the value was measured.
- the same region as above was set as the strong HAZ region, and indentations were made at a position 0.3 mm away from the point q toward the HAZ6 side, and the value was measured.
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Abstract
Description
[1] 2枚以上の鋼板を重ね合わせて抵抗スポット溶接した溶接部材の抵抗スポット溶接部であって、
前記鋼板のうち少なくとも1枚の鋼板は、成分組成が、質量%で、
C:0.05~0.6%、
Si:0.1~3.5%、
Mn:1.5~10.0%、および
P:0.1%以下
の範囲を満足する高強度鋼板であり、
前記鋼板の重ね面と交わるナゲットの境界上の二点を第1端部および第2端部とし、
前記第1端部および前記第2端部を結ぶ線分Xの長さをD(mm)とし、
前記第1端部および前記第2端部から前記ナゲットの中心方向に向けた線分X上の位置を点Oおよび点Pとし、前記第1端部から点Oまでおよび前記第2端部から点Pまでの各距離L(mm)が下記式(1)を満たす、前記ナゲット内の領域をナゲット先端領域とするとき、前記重ね面に対応する前記ナゲット先端領域のうち1つ以上で、
前記ナゲット先端領域の金属組織が、焼き戻しマルテンサイトを主相とし、
前記ナゲット先端領域の硬さHvが、下記式(2)および下記式(3)で算出される前記ナゲット全体のマルテンサイトの硬さHmwに対して、下記式(4)を満たし、
前記重ね面に対して上側および/または下側の鋼板では、
前記重ね面に平行な直線Zと前記ナゲットの境界の交点を点qとし、直線Z上で熱影響部内の位置を点rとし、
直線Zと前記重ね面の板厚方向の距離M(mm)が下記式(5)を満たし、かつ、点qから点rまでの各距離T(mm)が下記式(6)を満たす、前記熱影響部内の領域を強HAZ領域とするとき、前記重ね面に対応する前記強HAZ領域のうち1つ以上で、
前記強HAZ領域における硬さHhが、下記式(7)で算出される鋼板のマルテンサイトの硬さHmhに対して、下記式(8)を満たす、抵抗スポット溶接部。
Cw(質量%):ナゲット内における各鋼板からの体積当たりのC含有量、
Ci(質量%):重ね合わせた各鋼板のC含有量、
Vi(mm2):ナゲットの中心を通る板厚方向断面において、ナゲットの境界と各線分Xにより囲まれた領域における各鋼板の溶融面積、
n:重ね合わせた鋼板の数、とする。
M=D/20 ・・・・・・・(5)
0<T≦D/10・・・・・・・(6)
Hmh=884×Ch×(1-0.3×Ch2)+294・・・(7)
Hh<Hmh-25・・・・・(8)
ここで、式(7)において、Ch(質量%):重ね面に対して上側の鋼板のC含有量、あるいは、重ね面に対して下側の鋼板のC含有量、とする。
ただし、前記重ね面において前記鋼板間の隙間がある場合には、前記隙間の中間に位置し前記鋼板表面に平行な直線Yと交わる前記ナゲットの境界上の二点を前記第1端部および前記第2端部とする。
[2] 前記焼き戻しマルテンサイト中の炭化物の割合は、面積率で20%超えである、[1]に記載の抵抗スポット溶接部。
[3] 前記炭化物は、平均結晶粒径が300nm以下である、[2]に記載の抵抗スポット溶接部。
[4] 前記高強度鋼板は、引張強度が780MPa以上である、[1]~[3]のいずれか1つに記載の抵抗スポット溶接部。
[5] [1]~[4]のいずれか1つに記載の抵抗スポット溶接部を生成する抵抗スポット溶接方法であって、
主通電工程として、電流値Iw(kA)で通電し、溶接部を生成し、
その後、下記式(13)に示す冷却時間tc(ms)で冷却し、
その後、焼き戻し工程として、下記式(14)に示す電流値It(kA)で、下記式(15)に示す通電時間tp(ms)の間、通電を行う、抵抗スポット溶接方法。
400≦tc ・・・(13)
It≦0.95×Iw ・・・(14)
400≦tp ・・・(15)
[6] [1]~[4]のいずれか1つに記載の抵抗スポット溶接部を有する、抵抗スポット溶接継手。
[7] [5]に記載の抵抗スポット溶接方法を用いて抵抗スポット溶接継手を製造する、抵抗スポット溶接継手の製造方法。
上記のように、第1端部8aから点Oまでの距離L、および第2端部8bから点Pまでの距離Lが、式(1)を満たす領域が先端領域31である。距離Lが式(1)の条件(すなわち、0<L≦0.25×D)を満たさない場合、継手強度に影響を及ぼすナゲット端部の領域に、後述する本発明の金属組織を有さないことになる。継手強度をより向上させる観点から、距離Lは、0<L≦0.20×Dとすることが好ましい。
Cw(質量%):ナゲット内における各鋼板からの体積当たりのC含有量、
Ci(質量%):重ね合わせた各鋼板のC含有量、
Vi(mm2):ナゲットの中心を通る板厚方向断面において、ナゲットの境界と各線分Xにより囲まれた領域における各鋼板の溶融面積、
n:重ね合わせた鋼板の数、とする。
M=D/20 ・・・・・・・(5)
0<T≦D/10・・・・・・・(6)
Hmh=884×Ch×(1-0.3×Ch2)+294・・・(7)
Hh<Hmh-25・・・・・(8)
ここで、式(7)において、Ch(質量%):重ね面に対して上側の鋼板のC含有量、あるいは、重ね面に対して下側の鋼板のC含有量、とする。
Cは鋼の強化に寄与する元素である。C含有量が0.05%未満では、鋼の強度が低くなり、引張強度780MPa以上の鋼板を製作することは極めて困難である。一方、C含有量が0.6%を超えると、鋼板の強度は高くなるものの、硬質なマルテンサイト量が過大となり、マイクロボイドが増加する。更にナゲットとその周辺の熱影響部(HAZ)が過度に硬化し、脆化も進むため、十字引張強度(CTS)を向上させることは困難である。そのため、C含有量は0.05~0.6%とする。C含有量は、より好ましくは0.1%以上であり、より好ましくは0.3%以下である。
Si含有量が0.1%以上であると、鋼の強化に有効に作用する。一方、Si含有量が3.5%を超えると、鋼は強化されるものの、靱性に悪影響を与えることがある。そのため、Si含有量は0.1~3.5%とする。Si含有量は、より好ましくは0.2%以上であり、より好ましくは2.0%以下である。
Mn含有量が1.5%未満であると、本発明のように長時間の冷却を与えずとも、高い継手強度を得ることができる。一方、Mn含有量が10.0%を超えると、溶接部の脆化あるいは脆化に伴う割れが顕著に現れるため、継手強度を向上させることは困難である。そのため、Mn含有量は1.5%以上10.0%以下とする。Mn含有量は、より好ましくは2.0%以上であり、より好ましくは8.0%以下である。
Pは不可避的不純物であるが、P含有量が0.1%を超えると、溶接部のナゲット端に強偏析が現れるため継手強度を向上させることは困難である。そのため、P含有量は0.1%以下とする。より好ましくは、P含有量は0.05%以下であり、より好ましくは、P含有量は0.02%以下である。
400≦tc ・・・(13)
It≦0.95×Iw ・・・(14)
400≦tp ・・・(15)
〔主通電工程〕
主通電工程とは、重ね合わせた鋼板(図4に示す例では、下の鋼板1と上の鋼板2)の重ね合わせ部を溶融してナゲット3を生成するための通電工程である。本発明では、電流値Iw(kA)で通電を行い、溶接部を生成する。
主通電工程と後述する焼き戻し工程の間に冷却過程を設ける。この冷却過程において、先端領域31の組織がマルテンサイト変態を生じる温度まで冷却を行う。本発明では、上記の式(13)に示す冷却時間tc(ms)で冷却する。冷却時間tcが400ms未満では、ナゲット端部をマルテンサイト変態が生じる温度まで冷却できない。その結果、先端領域31のマルテンサイト変態することができなかった残留オーステナイト組織は、後述する焼き戻し工程における再通電、再冷却によりマルテンサイト組織および残留オーステナイト組織の1種または2種となる。これらの組織は、靱性を有する焼き戻しマルテンサイト組織ではないため、硬い組織のままである。またこれらの組織は、焼き戻しマルテンサイト組織ではなく、靱性の無い組織であることから、先端領域31は脆化した組織となる。したがって、冷却時間tcは400ms以上とする。
焼き戻し工程とは、主通電工程で形成されたナゲット3における先端領域31を焼き戻し、靱性を向上させるための後熱処理工程である。本発明では、冷却過程でマルテンサイト組織となった先端領域31の組織を焼戻すために、適切な温度域で焼き戻し工程を行う。
[鋼板Aの成分組成]
C:0.20%、Si:0.6%、Mn:4.0%、P:0.01%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Bの成分組成]
C:0.10%、Si:0.2%、Mn:6.0%、P:0.01%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Cの成分組成]
C:0.10%、Si:1.1%、Mn:1.2%、P:0.01%、Ti:0.03%、B:0.002%、Cr:0.40%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Dの成分組成]
C:0.13%、Si:0.8%、Mn:1.2%、P:0.01%、Cu:0.50%、Ni:0.51%、Mo:0.19%、Al:0.03%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Eの成分組成]
C:0.58%、Si:0.25%、Mn:0.75%、P:0.03%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Fの成分組成]
C:0.30%、Si:3.5%、Mn:2.5%、P:0.01%、Nb:0.04%、V:0.03%、Ca:0.004%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Gの成分組成]
C:0.60%、Si:2.0%、Mn:1.5%、P:0.01%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
[鋼板Hの成分組成]
C:0.20%、Si:0.3%、Mn:1.5%、P:0.01%を含有し、残部がFeおよび不可避的不純物を含有する鋼板
表1に示すように、上記の鋼板A~鋼板Hより2枚以上の鋼板を選び、重ね合わせて各板組とした。板組a~板組hおよび板組jの板厚は、全て同じ1.2mmとした。板組iは、同一種類の中Mn鋼板Aを3枚重ね合わせ、板厚はそれぞれ0.8mmとした。
ナゲット端部(ナゲット先端領域)の組織の観察に用いたサンプルは、次のように得た。作製した抵抗スポット溶接継手を切断して試験片とし、試験片を超音波洗浄した後に樹脂埋めを行い、断面を研磨し、ナイタール溶液を用いてエッチングを行ってサンプルを得た。組織の観察にはSEMを用い、1000倍~100000倍で観察を行った。硬さはヴィッカース硬度計により、JISZ2244に規定の方法で測定した。表3-1および表3-2に、溶接後の抵抗スポット溶接継手におけるナゲット先端領域の組織、およびナゲット先端領域、ナゲット全体、熱影響部の硬さをそれぞれ示す。
熱影響部として、上記した強HAZ領域の硬さを測定した。「強HAZ領域61」の硬さの測定は、上記したナゲット端部(ナゲット先端領域)の組織の観察と同様の方法でサンプルを作製し、測定を行った。また、「強HAZ領域61」の硬さはヴィッカース硬度計により、JISZ2244に規定の方法で測定した。
CTSの評価は、作製した抵抗スポット溶接継手に対し、JISZ3137に規定の方法で十字引張試験を行い、CTS(十字引張力)を測定して行った。CTSの基準は、測定値がJIS A級(3.4kN)以上であったものに対して記号○を付し、JIS A級未満であったものに対して記号×を付した。なお、本実施例では、記号○の場合を良好(継手強度に優れる)と評価し、記号×の場合を劣ると評価する。表4に、溶接後の抵抗スポット溶接継手におけるCTSの評価結果を示す。
2 鋼板
3 ナゲット
31 ナゲット先端領域
4 下の電極
5 上の電極
6 熱影響部(HAZ)
61 強HAZ領域
7 重ね面
8a 第1端部
8b 第2端部
10 鋼板
Claims (7)
- 2枚以上の鋼板を重ね合わせて抵抗スポット溶接した溶接部材の抵抗スポット溶接部であって、
前記鋼板のうち少なくとも1枚の鋼板は、成分組成が、質量%で、
C:0.05~0.6%、
Si:0.1~3.5%、
Mn:1.5~10.0%、および
P:0.1%以下
の範囲を満足する高強度鋼板であり、
前記鋼板の重ね面と交わるナゲットの境界上の二点を第1端部および第2端部とし、
前記第1端部および前記第2端部を結ぶ線分Xの長さをD(mm)とし、
前記第1端部および前記第2端部から前記ナゲットの中心方向に向けた線分X上の位置を点Oおよび点Pとし、前記第1端部から点Oまでおよび前記第2端部から点Pまでの各距離L(mm)が下記式(1)を満たす、前記ナゲット内の領域をナゲット先端領域とするとき、前記重ね面に対応する前記ナゲット先端領域のうち1つ以上で、
前記ナゲット先端領域の金属組織が、焼き戻しマルテンサイトを主相とし、
前記ナゲット先端領域の硬さHvが、下記式(2)および下記式(3)で算出される前記ナゲット全体のマルテンサイトの硬さHmwに対して、下記式(4)を満たし、
前記重ね面に対して上側および/または下側の鋼板では、
前記重ね面に平行な直線Zと前記ナゲットの境界の交点を点qとし、直線Z上で熱影響部内の位置を点rとし、
直線Zと前記重ね面の板厚方向の距離M(mm)が下記式(5)を満たし、かつ、点qから点rまでの各距離T(mm)が下記式(6)を満たす、前記熱影響部内の領域を強HAZ領域とするとき、前記重ね面に対応する前記強HAZ領域のうち1つ以上で、
前記強HAZ領域における硬さHhが、下記式(7)で算出される鋼板のマルテンサイトの硬さHmhに対して、下記式(8)を満たす、抵抗スポット溶接部。
ここで、式(2)~式(3)において、
Cw(質量%):ナゲット内における各鋼板からの体積当たりのC含有量、
Ci(質量%):重ね合わせた各鋼板のC含有量、
Vi(mm2):ナゲットの中心を通る板厚方向断面において、ナゲットの境界と各線分Xにより囲まれた領域における各鋼板の溶融面積、
n:重ね合わせた鋼板の数、とする。
M=D/20 ・・・・・・・(5)
0<T≦D/10・・・・・・・(6)
Hmh=884×Ch×(1-0.3×Ch2)+294・・・(7)
Hh<Hmh-25・・・・・(8)
ここで、式(7)において、Ch(質量%):重ね面に対して上側の鋼板のC含有量、あるいは、重ね面に対して下側の鋼板のC含有量、とする。
ただし、前記重ね面において前記鋼板間の隙間がある場合には、前記隙間の中間に位置し前記鋼板表面に平行な直線Yと交わる前記ナゲットの境界上の二点を前記第1端部および前記第2端部とする。 - 前記焼き戻しマルテンサイト中の炭化物の割合は、面積率で20%超えである、請求項1に記載の抵抗スポット溶接部。
- 前記炭化物は、平均結晶粒径が300nm以下である、請求項2に記載の抵抗スポット溶接部。
- 前記高強度鋼板は、引張強度が780MPa以上である、請求項1~3のいずれか1項に記載の抵抗スポット溶接部。
- 請求項1~4のいずれか1項に記載の抵抗スポット溶接部を生成する抵抗スポット溶接方法であって、
主通電工程として、電流値Iw(kA)で通電し、溶接部を生成し、
その後、下記式(13)に示す冷却時間tc(ms)で冷却し、
その後、焼き戻し工程として、下記式(14)に示す電流値It(kA)で、下記式(15)に示す通電時間tp(ms)の間、通電を行う、抵抗スポット溶接方法。
400≦tc ・・・(13)
It≦0.95×Iw ・・・(14)
400≦tp ・・・(15) - 請求項1~4のいずれか1項に記載の抵抗スポット溶接部を有する、抵抗スポット溶接継手。
- 請求項5に記載の抵抗スポット溶接方法を用いて抵抗スポット溶接継手を製造する、抵抗スポット溶接継手の製造方法。
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