JP7485946B2 - Joint, automobile part, and method for manufacturing joint - Google Patents

Description

The present invention relates to a joint, an automobile part, and a method for manufacturing a joint.

Friction welding is known as one method for joining components to produce a welded joint. Friction welding is a technique for joining components together in a solid state by applying pressure while rubbing them together. The friction between the components is achieved, for example, by making the weld surface of one of the components rotationally symmetrical (e.g., circular or polygonal) and rotating it at high speed.

One of the features of friction welding is that the materials are not melted or stirred at the joint. Welding is a joining method in which a joint is formed by melting and resolidifying the materials. Friction stir welding is a joining method in which a press-in member is rotated at high speed to stir the contact area between the materials, forming a joint. Therefore, the materials are mixed at the joint of a joint formed by welding or friction stir welding. On the other hand, in a joint formed by friction welding, the area where the materials are mixed is either nonexistent or extremely small. When observing the cross section of the joint of a joint formed by friction welding, it is possible to distinguish how the materials are separated via the friction welding surface. For example, Patent Document 1 and Patent Document 2 disclose methods of friction welding.

Another feature of friction welding is that it can be used to manufacture joints of dissimilar materials. For example, the end faces of steel and aluminum round bars can be butted together and one side can be rotated and pressed under appropriate conditions to join them. Friction welding can also be used to manufacture joints of dissimilar materials between stacked plates. For example, in a joint consisting of stacked upper and lower plates and a joining member that penetrates the upper plate and is friction-welded to the lower plate, the upper plate and the head of the joining member can be used to clamp the lower plate, making the upper and lower plates made of different materials.

The most commonly used method for joining stacked plates (particularly metal plates) is welding (particularly spot welding). For example, Patent Document 3 discloses a method for spot welding high-strength steel plates. However, it is difficult to apply welding, which is a joining method that causes the molten materials to mix at the weld, to joining dissimilar materials. Friction welding has an advantage over spot welding in that it improves the freedom of material selection, for example.

JP 2006-297398 A JP 2004-141933 A International Publication No. 2014/171495

The inventors have attempted to apply friction welding to the joining of various materials. They have discovered that when a steel round bar or polygonal column is friction-welded to a high-strength steel plate, cracks are likely to occur at the joint when an external force is applied to the joint, reducing the strength of the joint. In the prior art, there are almost no examples of applying friction welding to high-strength steel materials, and no consideration has been given to the reduction in joint strength.

In view of the above, the present invention aims to provide a joint having high joint strength and an automobile part. Furthermore, the present invention aims to provide a method for manufacturing a joint having high joint strength.

ここで、式３、式４、及び式５によって定義されるＩｍｉｎ、Ｉｍａｘ１、及びＩｍａｘ２の単位はｋＡであり、式３、式４、及び式５において、記号「Ｄ_１」は、前記軸部の前記断面の外接円の、単位ｍｍでの直径であり、記号「Ｄ_２」は、前記軸部の中空部の前記断面の外接円の、単位ｍｍでの直径であり、前記軸部が中実である場合はＤ_２＝０とされ、記号「ｔ」は、単位秒での通電時間であり、記号「Ｆ」は、単位ｋＮでの加圧力であり、式６において、記号「Ｃ」「Ｓｉ」「Ｍｎ」及び「Ｃｒ」は、それぞれ、前記第２の部材の単位質量％でのＣ含有量、Ｓｉ含有量、Ｍｎ含有量、及びＣｒ含有量である。
（１３）上記（１２）に記載の接合継手の製造方法は、さらに、前記接合部の形成の前に、前記第２の部材に第３の部材を重ねる工程と、前記第３の部材に前記第１の部材の前記軸部を貫通させる工程と、を備え、前記第１の部材が、その第２の端部に頭部を有してもよい。 The gist of the present invention is as follows.
(1) A bonded joint according to one aspect of the present invention is a bonded joint comprising a first steel member having a shaft, a second steel member having a bonding surface larger than a cross section of the shaft, and a bonded joint formed by frictionally welding a first end of the first member and the bonding surface, wherein an average value Have of maximum hardness at both ends of the bonded joint obtained by continuously measuring hardness at a position 0.1 mm deep from the bonding surface in a cross section of the bonded joint passing through the axis of the shaft, over the bonded joint and its periphery satisfies Equation 1.
Have≦761.6×C ³ −1897.2×C ² +1848.8×C+168.3……(Formula 1)
Here, the symbol "C" in the formula 1 is the carbon content in unit mass % of the second member.
(2) In the welded joint described in (1) above, the second member may have tempered martensite in a region within a depth of 0.1 mm from a friction welded interface, which is an interface between the first member and the second member.
(3) In the welded joint described in (1) or (2) above, the carbon content of the second member may be 0.20 mass % or more.
(4) In the bonded joint described in any one of (1) to (3) above, a friction welded interface, which is an interface between the first member and the second member, may penetrate into the second member, and a maximum depth of the friction welded interface relative to the bonding surface of the second member, measured in the cross section of the bonded joint passing through the axis, may be 0.3 mm or more.
(5) In the joint described in any one of (1) to (4) above, the second member may be a steel plate having a plate thickness of 0.8 to 3.0 mm.
(6) In the joint described in any one of (1) to (5) above, an area of the joining surface may be 3.0 times or more the cross-sectional area of the shaft portion.
(7) In the joint described in any one of (1) to (6) above, a cross section of the shaft portion perpendicular to the axis may have a rotationally symmetric shape, and a diameter of a circumscribing circle of the cross section may be 2.0 to 10.0 mm.
(8) In the joint described in (7) above, a cross section of the shaft portion perpendicular to the axis may be a circle or a regular polygon.
(9) In the joint joint described in any one of (1) to (8) above, the first member has a head disposed at a second end thereof, and the joint joint further includes a third member provided between the head and the second member, the shaft portion of the first member penetrating the third member, and the third member being clamped between the head of the first member and the joint surface of the second member.
(10) In the joint described in (9) above, the third member may be a steel plate.
(11) An automobile part according to another aspect of the present invention, comprising the bonded joint according to any one of (1) to (10) above.
(12) A method for manufacturing a joined joint according to another aspect of the present invention includes the steps of: pressing a first end of a first steel member having a shank against a second steel member having a joining surface larger than a cross section of the shank while rotating the first end of the shank, thereby frictionally welding the first end of the shank and the joining surface; and tempering the joint by using electrodes to hold the first member and the second member in the axial direction of the shank of the first member and passing a current of I through the electrodes, wherein the current I is within a range that satisfies the following formula 2:

Here, the units of Imin, Imax1, and Imax2 defined by Equation 3, Equation 4, and Equation 5 are kA, and in Equation 3, Equation 4, and Equation 5, the symbol "D ₁ " is the diameter, in mm, of the circumscribing circle of the cross section of the shaft portion, the symbol "D ₂ " is the diameter, in mm, of the circumscribing circle of the cross section of the hollow portion of the shaft portion, and if the shaft portion is solid, D ₂ = 0, the symbol "t" is the current flow time, in seconds, and the symbol "F" is the pressing force, in kN, and in Equation 6, the symbols "C", "Si", "Mn", and "Cr" are the C content, Si content, Mn content, and Cr content, in mass %, of the second member, respectively.
(13) The method for manufacturing a joint described in (12) above may further include a step of overlapping a third member on the second member before forming the joint, and a step of passing the shaft portion of the first member through the third member, and the first member may have a head at its second end.

According to the present invention, it is possible to provide a joint having high joining strength, and an automobile part. Furthermore, according to the present invention, it is possible to provide a manufacturing method for a joint having high joining strength.

1 is a photograph of a two-phase region heated portion of a welded joint after a fracture test. 3 is a cross-sectional view including the axis of the shaft portion of the joint according to the embodiment. FIG. FIG. 1 is a schematic diagram of hardness distribution curves obtained by continuously measuring the hardness at a position 0.1 mm deep from the joint surface of a welded joint according to this embodiment and a welded joint that was not subjected to post-treatment after friction welding of steel materials. 11 is a cross-sectional view including the axis of the shaft portion of a joint according to the present embodiment, the joint including a third member. FIG. FIG. 2 is a schematic diagram of an example of a method for electric tempering of a joint according to the present embodiment. 10 is a schematic diagram illustrating a method for determining a diameter _D1 of a shaft portion 111 having a tapered shape. FIG.

The inventors conducted fracture tests on various joints obtained by friction welding a first steel member 11 having a shaft portion 111 and a second steel member 12 having a joint surface 121 larger than the cross section of the shaft portion 111. The inventors then investigated the fractured portions in detail and discovered the following:

First, it was discovered that fractures tend to occur near the outer periphery of the joint 14 and at locations that correspond to the two-phase region heated portion of the second member 12. The two-phase region heated portion is a portion that is heated to a temperature range (two-phase region) between Ac1 and Ac3 by frictional heat during friction welding and then cooled. The metal structure of the two-phase region heated portion is a mixture of hard phases (e.g., martensite, bainite, and tempered martensite) and soft phases (e.g., ferrite).

Next, the inventors closely observed the two-phase region heated area after the fracture test and confirmed the presence of numerous voids between the hard and soft phases. Figure 1 shows a photograph of the two-phase region heated area after the fracture test. It was presumed that these numerous voids acted as the starting points of fracture. It was also presumed that the reason the voids were generated was because the difference in hardness between the soft and hard phases was large, causing strain to concentrate at the interface between them.

Based on the above findings, the inventors attempted to reduce the hardness difference and alleviate strain concentration by electrically tempering the two-phase region heated portion. However, when the amount of current flow was small, no improvement in the joint strength was observed. On the other hand, when the amount of current flow was large, the first member 11 softened and buckled during electrically heating.

Electric current tempering is sometimes performed in spot welding. However, in electric current tempering of spot welds, it is possible to predict the current conditions to achieve the temperature required for tempering by considering the actual current value and heat generation value during spot welding. On the other hand, such prediction of current conditions is not possible in electric current tempering of friction welded joints. In addition, since buckling of the weld does not occur in electric current tempering of spot welds, no consideration has been given to avoiding buckling in the prior art. Therefore, it has been difficult to simply convert electric current tempering of spot welds to electric current tempering of friction welded joints.

The inventors then conducted further investigations. As a result, they discovered that by optimizing the current conditions while comprehensively taking into account the components of the second member 12, the size of the first member 11, the current application time, and the pressure, it is possible to appropriately soften the joint 14 and improve the joint strength.

A bonded joint 1 according to one embodiment of the present invention completed based on the above findings comprises, for example, as shown in FIG. 2 , a first steel member 11 having a shaft portion 111, a second steel member 12 having a joint surface 121 larger than a cross section of the shaft portion 111, and a bonded joint 14 formed by frictionally welding a first end of the first member 11 to the joint surface 121, and the average value Have of the maximum hardness at both ends of the bonded joint 14 obtained by continuously measuring the hardness of the bonded joint 14 and its periphery at a position X at a depth of 0.1 mm from the joint surface 121 in a cross section of the bonded joint 1 passing through the axis Y of the shaft portion 111 satisfies Equation 1.
Have≦761.6×C ³ −1897.2×C ² +1848.8×C+168.3……(Formula 1)
Here, the symbol "C" in formula 1 is the carbon content in unit mass % of the second member 12. Hereinafter, the welded joint 1 according to this embodiment will be described in detail.

(First member 11)
The first member 11 made of steel has a shaft portion 111 to be joined to the second member 12. Any shape can be applied to the shaft portion 111 as long as it has a shape that enables it to be friction-welded to the second member 12. For convenience, of both ends of the first member 11, the end on the side to be joined to the joining surface 121 of the second member 12 will be referred to as the first end, and the other end will be referred to as the second end.

(Second member 12)
The second member 12 has a joint surface 121 to which the first member 11 is joined that is larger in size than the cross section of the shaft portion 111 of the first member 11 (more specifically, a cross section perpendicular to the axis Y at a location near the first end of the shaft portion 111 where deformation due to friction welding is small). In other words, when the joint surface 121 is viewed in plan, it encompasses the cross section of the shaft portion 111. As long as this requirement is satisfied, any shape can be applied to the second member 12. Note that the term "join surface 121" means a surface to be used for joining, and is a different concept from the friction welding interface 15 (a surface where two members are joined) described later.

(Joint 14)
The joint 14 is a portion formed by friction welding an end of the first member 11 and a joint surface 121 of the second member 12. In this embodiment, the joint 14 is defined as a region where thermal effects and deformation occur due to the effects of friction welding. In FIG. 2, the outer edge of the joint 14 is shown by a dashed line. However, unlike a welded portion formed by welding, in the joint 14 formed by friction welding, the first member 11 and the second member 12 are not mixed together, and the interface between them can be clearly confirmed. In this embodiment, the interface between the first member 11 and the second member 12 is referred to as a friction welding interface 15.

(Average value Have of maximum hardness at both ends of joint 14)
In a normal welded joint obtained by friction welding steel members, quench hardening occurs in the welded portion 14 due to frictional heat. On the other hand, in the welded joint 1 according to this embodiment, the weld strength is increased by mitigating the quench hardening of the welded portion 14. Therefore, in the welded joint 1 according to this embodiment, the average value Have of the maximum hardness at both ends of the welded portion 14 satisfies formula 1. The average value Have of the maximum hardness is obtained by continuously measuring the hardness of the welded portion and its periphery at a position X at a depth of 0.1 mm from the welded surface 121 in a cross section of the welded joint 1 passing through the axis Y of the shaft portion 111.
Have≦761.6×C ³ −1897.2×C ² +1848.8×C+168.3……(Formula 1)
Here, the symbol “C” in formula 1 is the carbon content in unit mass % of the second member 12 .

Position X, which is 0.1 mm deep from the joint surface 121, is the position indicated by the straight line marked with the symbol "X" in FIG. 2. The joint surface 121 is significantly deformed at the joint 14. However, position X, which is 0.1 mm deep from the joint surface 121, is identified based on the joint surface 121 that is not deformed. That is, by extending the joint surface 121 that is not deformed, the joint surface 121 before friction welding is performed is estimated, and position X, which is 0.1 mm deep from the joint surface 121, is identified based on this. Usually, the friction welding interface 15 is embedded in the second member 12. Therefore, in the vicinity of the axis Y, the first member 11 may be located at position X, which is 0.1 mm deep from the joint surface 121.

By continuously measuring the hardness at position X, 0.1 mm deep from the joint surface 121, throughout the joint and its periphery, it is possible to obtain a hardness distribution curve along position X, 0.1 mm deep from the joint surface 121, as shown in FIG. 3, for example. The maximum hardness at both ends of the joint 14 is the maximum hardness value on the left side of axis Y and the maximum hardness value on the right side of axis Y in the hardness distribution curve.

The point where the hardness is maximum on the hardness distribution curve is usually near the intersection of position X, 0.1 mm deep from the joint surface 121, and the friction welded interface 15. This is the point where significant quench hardening occurs due to the frictional heat generated during friction welding. The dashed hardness distribution curve shown in Figure 3 is an example of a hardness distribution curve for a welded joint in which no special post-processing was performed after the steel materials were friction welded. In the hardness distribution curve for such a welded joint, significant peaks exist on both sides of the axis Y due to quench hardening.

On the other hand, in the bonded joint 1 according to this embodiment, quench hardening is mitigated. Therefore, the height of the hardness peak in the hardness distribution curve is smaller. The solid line hardness distribution curve shown in FIG. 3 is an example of the hardness distribution curve of the bonded joint 1 according to this embodiment. In the hardness distribution curve of the bonded joint 1 according to this embodiment, the height of the hardness peak is lower than the dashed line on at least one side of the axis Y.

In order to quantitatively define the average value Have of the maximum hardness at both ends of the joint 14 in the joint joint 1 according to this embodiment, the inventors focused on the carbon content of the second member 12. The hardness of the second member 12 after quenching is determined according to the carbon content of the second member 12. Therefore, the upper limit value of the average value Have of the maximum hardness at both ends of the joint 14 is set to a function of the carbon content of the second member 12, "761.6 x ^C3 - 1897.2 x ^C2 + 1848.8 x C + 168.3". When the average value Have of the maximum hardness at both ends of the joint 14 satisfies formula 1, the joint joint 1 has good joint strength. As long as the average value Have of the maximum hardness at both ends of the joint 14 satisfies formula 1, it is permitted that the maximum hardness at one end of the joint 14 exceeds "761.6 x C ³ - 1897.2 x C ² + 1848.8 x C + 168.3". The means for setting the average value Have of the maximum hardness at both ends of the joint 14 within the above range is not particularly limited, but for example, current tempering according to the conditions described below is suitable. The upper limit value of the average value Have is preferably 95% or less, 90% or less, or 85% or less of "761.6 x C 3 - 1897.2 x C 2 + 1848.8 x C + 168.3". The lower limit value of the average value Have is not particularly limited, but may be specified as, for example, 60% or more, 70% or more, 80% or more, or 90% or more of the base material hardness of the second member 12. The base material hardness of the second member 12 refers to the hardness of a region of the second member 12 that is not affected by heat or deformed by friction welding.

The method for measuring the average value Have of the maximum hardness at both ends of the joint 14 is as follows.
First, the welded joint 1 is cut at a cross section that substantially includes the axis Y of the shaft portion 111 of the first member 11, and the cross section is appropriately polished. Note that it is permissible for this cross section to deviate slightly from the axis Y.
Next, position X is identified at a depth of 0.1 mm from the joint surface 121. Since the joint surface 121 is significantly deformed at the joint 14, the joint surface 121 before friction welding is performed is estimated by extending the joint surface 121 that is not deformed, and a line X (see FIG. 2) that is parallel to the joint surface 121 and spaced 0.1 mm apart is identified. If the joint surface 121 is a curved surface, the line X is also made to be a curve that matches the curve.
Then, the Vickers hardness is measured continuously over the joint and its periphery along the line X. The load for measuring the Vickers hardness is 200 g, and the measurement interval for the Vickers hardness is 0.2 mm.
The hardness measurement results are plotted on a graph with the horizontal axis representing the distance from the axis Y and the vertical axis representing the hardness to create a hardness distribution curve. Then, the maximum hardness value on the left side of the axis Y and the maximum hardness value on the right side of the axis Y are identified, and these values are averaged to calculate the average value Have of the maximum hardnesses at both ends of the joint 14.

The above describes the basic aspects of the joint 1 according to this embodiment. Below, we will explain more preferred aspects of the joint 1 according to this embodiment.

The metal structure of the welded joint 1 according to this embodiment is not particularly limited. As long as the quench hardening of the welded portion 14 is suppressed as described above, the welded joint 1 according to this embodiment can ensure excellent joint strength regardless of its metal structure. On the other hand, for example, the second member 12 may have tempered martensite in a region within 0.1 mm deep from the friction welded interface 15, which is the interface between the first member 11 and the second member 12. In other words, the metal structure of part or all of the region within 0.1 mm deep from the friction welded interface 15 in the second member 12 may be tempered martensite.

In a welded joint in which no special post-processing is performed after the steel materials are friction welded, tempered martensite does not exist in the region within 0.1 mm depth from the friction weld interface 15. This is because the region is quenched by the frictional heat during friction welding. Even if the steel material contains tempered martensite, this tempered martensite becomes fresh martensite (untempered martensite) due to quenching during friction welding. On the other hand, by making the metal structure of part or all of the region within 0.1 mm depth from the friction weld interface 15 in the second member 12 into tempered martensite, it is possible to further suppress fracture occurring in the second member 12 and further increase the joining strength of the welded joint 1.

The metal structure of the joint 14 is preferably a tempered structure, but it is not necessary for parts other than the joint 14 to be tempered. For example, the first member 11, the second member 12, and part or all of the third member 13 described below may have an untempered structure (e.g., fresh martensite).

The first member 11 and the second member 12 are made of steel. Their chemical components are not particularly limited. On the other hand, the carbon content of one or both of the first member 11 and the second member 12 may be, for example, 0.20 mass% or more, 0.22 mass% or more, 0.25 mass% or more, or 0.30 mass% or more. This allows for more stable friction welding with the high-strength second member 12. In addition, in friction welding of steel materials in which the carbon content in the second member 12 is 0.20 mass% or more, a decrease in joint strength is a problem. However, in the joint joint 1 according to this embodiment, the quench hardening of the joint 14 is suppressed so that the average value Have of the maximum hardness at both ends of the joint 14 satisfies formula 1, so that the joint strength is ensured even if the carbon content of the second member 12 is 0.20 mass% or more.

The mechanical properties of the first member 11 and the second member 12 are not particularly limited. On the other hand, the higher the strength of the first member 11 and the second member 12, the better because the characteristics of the part having the joint 1 can be improved. In addition, in the joint joint 1 according to this embodiment, quench hardening at the joint 14 is suppressed, so that the joint strength can be ensured even if the strength of the first member 11 and the second member 12 is high. Therefore, for example, the tensile strength of one or both of the first member 11 and the second member 12 may be 590 MPa or more.

In the bonded joint 1 according to this embodiment, the shape of the bonded portion 14 is not particularly limited. As shown in FIG. 2, the first member 11 and the second member 12 may be deformed during friction welding, resulting in the formation of burrs. The bonded portion 14 may have such burrs. Alternatively, the burrs may be mechanically removed from the bonded portion 14.

In addition, it is preferable that the first member 11 penetrates into the second member 12 at the joint 14. In other words, the friction welded interface 15, which is the interface between the first member 11 and the second member 12, may penetrate into the second member 12. In this case, the maximum depth of the friction welded interface 15 relative to the joint surface 121 of the second member 12, measured in a cross section of the welded joint 1 passing through the axis Y, may be 0.1 mm or more, 0.3 mm or more, or 0.5 mm or more. This suppresses poor welding at the friction welded interface 15, and further improves the joint strength of the welded joint 1.

The shapes of the first member 11 and the second member 12 are not particularly limited, but the following are preferred examples of the shapes of the first member 11 and the second member 12.

The second member 12 is preferably a steel plate having a thickness of, for example, 0.8 to 3.0 mm. Steel plates of such thickness are very suitable as materials for automobile parts. The thickness of the second member 12 may be 1.0 mm or more, 1.2 mm or more, or 1.5 mm or more. The thickness of the second member 12 may be 2.8 mm or less, 2.5 mm or less, or 2.0 mm or less. On the other hand, it is not prevented that the second member 12 is a round bar, a square bar, or the like.

The area of the joint surface 121 of the second member 12 may be 3.0 times or more, 4.0 times or more, or 5.0 times or more the cross-sectional area of the shaft portion 111 of the first member 11. This allows for flexible design of the shape of the joint joint 1. If the cross-sectional area of the shaft portion 111 of the first member 11 is not constant along the axis Y direction (for example, if the shaft portion 111 has a tapered shape), the cross-sectional area of the shaft portion 111 of the first member 11 means the area of the region formed by the intersection line between the outer circumferential surface of the shaft portion 111 and the joint surface 121 of the second member 12. If the joint portion 14 has burrs, the outer circumferential surface of the shaft portion 111 means its extended surface.

The shape of the shaft portion 111 of the first member 11 is not particularly limited, but for example, it is preferable that the cross section of the shaft portion 111 perpendicular to the axis Y (hereinafter, sometimes abbreviated as "cross section of the shaft portion 111") is rotationally symmetric. Specifically, it is preferable that the cross section of the shaft portion 111 is a circle or a regular polygon. The size of the shaft portion 111 is not particularly limited, but for example, it is preferable that the diameter of the circumscribing circle of the cross section of the shaft portion 111 (when the cross section of the shaft portion 111 is a circle, the diameter of the cross section) is within the range of 2.0 to 10.0 mm. The diameter of the circumscribing circle of the shaft portion 111 may be constant along the axis Y or may vary. For example, the shaft portion 111 may have a tapered shape in which the thickness decreases toward the tip. In addition, the shaft portion 111 may have a hollow structure (for example, a cylindrical structure, etc.) or a solid structure. When the shaft portion 111 has a hollow structure, it is preferable that the inner shape of the cross section of the shaft portion 111 is rotationally symmetric, for example, a circle or a regular polygon.

The first member 11 may have a head 112 at the second end, the head 112 having a cross-sectional diameter larger than that of the shaft 111. The head 112 has a function of fixing the third member 13 described later. In this case, the first member 11 may be rivet-shaped. The diameter of the circumscribing circle of the head 112 is preferably 1.5 times or more the diameter of the circumscribing circle of the shaft 111. In addition, the length of the shaft 111 (the total length of the first member 11 minus the thickness of the head 112) is preferably 1.5 times or more the thickness of the third member 13. On the other hand, the joint 1 may not include the third member 13, and the first member 11 may be a rod-shaped member consisting of only the shaft 111.

When the first member 11 has a head 112, the joint 1 may further include a third member 13 provided between the head 112 of the first member 11 and the second member 12. For example, as shown in FIG. 4, the first member 11 may be shaped to have a shaft 111 and a head 112 like a rivet, and the shaft 111 may be inserted through the third member 13, and the third member 13 may be sandwiched between the head 112 and the joint surface 121 of the second member 12. In this case, the second member 12 and the third member 13 are preferably tightly joined using the first member 11. In addition, in the joint 1 having such a configuration, the first member 11 and the third member 13 are preferably made of different materials to which welding cannot be applied. The third member 13 is, for example, a steel plate, a light metal plate, a resin plate, etc. The light metal plate is, for example, an aluminum plate. If the first member 11 is a high-strength steel material, the first member 11 can penetrate the third member 13 even if the third member 13 is a high-strength steel plate. There is no prohibition on the joint 1 having further members.

The third member 13 may already have a hole through which the shaft portion 111 is inserted. The hole may be provided in the third member 13 before the shaft portion 111 is inserted, or may be formed by pressing the shaft portion 111 rotating at high speed against the third member 13. The shape of the hole in the third member 13 is not particularly limited. In order to fix the third member 13 using the head portion 112, it is preferable to make the diameter of the through hole smaller than the diameter of the head portion 112.

One or more of the first member 11, the second member 12, and the third member 13 may have a surface treatment layer. The surface treatment layer may be, for example, a plating layer, a coating, or a chemical conversion coating. The surface treatment layers of the first member 11 and the second member 12 are removed during friction welding, so they do not interfere with the formation of the joint 14. In addition, the third member 13 is mechanically joined to the second member 12 via the first member 11, but the second member 12 and the third member 13 may be further joined by a means other than the first member 11 (for example, welding). That is, the joint joint 1 according to this embodiment having the third member 13 may further include an additional joint (for example, a welded part) that joins the second member 12 and the third member 13.

Next, an automobile part according to another aspect of the present invention will be described below. The automobile part according to this embodiment has the joint 1 according to this embodiment. As a result, the automobile part according to this embodiment has high joint strength.

Next, an example of a method for manufacturing a joint according to another aspect of the present invention will be described below. The method for manufacturing the joint according to the present embodiment described above is not particularly limited, but the method described below makes it possible to manufacture the joint in an appropriate manner.

式２において、記号Ｉｍｉｎは式３によって定義される値であり、記号Ｉｍｉｎは式４によって定義される値であり、記号Ｉｍｉｎは式５によって定義される値であり、これら値の単位はｋＡである。
式３、式４、及び式５において、記号「Ｄ_１」は、軸部の断面の外接円の、単位ｍｍでの直径である。記号「Ｄ_２」は、軸部の中空部の断面の外接円の、単位ｍｍでの直径である。ここで、軸部が中実である場合はＤ_２＝０とされる。記号「ｔ」は、単位秒での通電時間である。記号「Ｆ」は、単位ｋＮでの加圧力である。記号「Ａ」は、式６によって定義される値である。
式６において、記号「Ｃ」「Ｓｉ」「Ｍｎ」及び「Ｃｒ」は、それぞれ、第２の部材の単位質量％でのＣ含有量、Ｓｉ含有量、Ｍｎ含有量、及びＣｒ含有量である。 The manufacturing method of the joint according to the present embodiment is as follows:
(S1) a step of pressing a first end of a first steel member 11 having a shaft portion 111 against a second steel member 12 having a joint surface 121 larger than a cross section of the shaft portion 111 while rotating the first steel member 11 against the joint surface 121 to form a joint 14 formed by friction welding the first end of the shaft portion 111 and the joint surface 121;
(S2) a process of using an electrode E to clamp the first member 11 and the second member 12 in the axis Y direction of the shaft portion 111 of the first member 11, and passing a current of a value I through the electrodes E to temper the joint portion 14;
Here, the current value I is set to be within a range that satisfies the following formula 2.

In Equation 2, the symbol Imin is a value defined by Equation 3, the symbol Imin is a value defined by Equation 4, and the symbol Imin is a value defined by Equation 5, and these values are in kA.
In Equation 3, Equation 4, and Equation 5, the symbol "D ₁ " is the diameter, in mm, of the circumscribing circle of the cross section of the shaft portion. The symbol "D ₂ " is the diameter, in mm, of the circumscribing circle of the cross section of the hollow portion of the shaft portion. Here, when the shaft portion is solid, D ₂ =0. The symbol "t" is the current application time, in seconds. The symbol "F" is the applied pressure, in kN. The symbol "A" is a value defined by Equation 6.
In formula 6, the symbols "C", "Si", "Mn" and "Cr" respectively represent the C content, Si content, Mn content and Cr content in unit mass % of the second component.

(S1)
In the manufacturing method of the joint according to the present embodiment, first, a first end of a first steel member 11 having a shaft portion 111 is pressed against a second steel member 12 having a joint surface 121 larger than the cross section of the shaft portion 111 while rotating, thereby forming a joint 14 formed by frictionally welding the first end of the shaft portion 111 and the joint surface 121. The conditions of the friction welding are not particularly limited, and may be appropriately selected according to the shapes of the first member 11 and the second member 12. For example, when friction welding is performed by utilizing the rotation of the shaft portion 111 having a diameter of less than 30 mm, the rotation speed of the shaft portion 111 may be set to 1000 to 8000 rpm, and the pressure when pressing the shaft portion 111 against the second member 12 may be set to 5 kN or more.

When friction welding is performed using the rotation of the shaft portion 111, it is preferable that the shape of the tip of the shaft portion 111 before friction welding is, for example, a cone shape, a pyramid shape, or a partial sphere shape. It is preferable that the apex of the tip of the shaft portion 111 is on the axis Y of the shaft portion 111. Furthermore, if the height of the cone or pyramid or the height of the partial sphere is too large, it may not be possible to completely crush them during welding, so the height is preferably 2.0 mm or less.

(S2)
In the manufacturing method of the joint joint according to the present embodiment, as shown in Fig. 5, the first member 11 and the second member 12 are clamped in the axial direction Y of the shaft portion 111 of the first member 11 using an electrode E, and a current value I is passed through the electrode E to temper the joint 14. This electric tempering reduces the quench hardening of the joint 14. The type of the electrode E is not particularly limited, but a copper electrode is preferable, for example. The device for performing electric tempering is not particularly limited, but for example, a resistance spot welding machine may be used as the device for electric tempering.

In electric tempering, the current value I must be equal to or greater than Imin, which is calculated using formulas 3 and 6. The current value I may be constant or may be changed during electric tempering. If the current value I is not constant, the average current value during electric tempering is regarded as the current value I. The symbols in formulas 3 and 6 have the same meanings as described above.

It should be noted that "D ₁ ² -D ₂ ² " included in Equation 3 etc. is an index of the cross-sectional area of shaft portion 111. As described above, shaft portion 111 may have a hollow structure (e.g., a cylindrical structure, etc.) or a solid structure. Considering the case where shaft portion 111 has a hollow structure, Equation 3 includes the size of the hollow portion of shaft portion 111 as a variable. When shaft portion 111 has a solid structure, 0 is substituted for diameter D ₂ of the circumscribing circle of the hollow portion, and Imin is calculated based only on circumscribing circle D ₁ of shaft portion 111.

As described above, the shaft portion 111 of the first member 11 may have a tapered shape. In this case, _D1 and _D2 may be values measured on a surface estimated to coincide with the joining surface 121 of the second member 12. Such a surface can be specified by taking into consideration the amount of pressing P of the first member 11. Therefore, the circumscribing circle of the shaft portion 111 and the circumscribing circle of the hollow portion in a cut surface obtained by cutting the shaft portion 111 of the first member 11 along the axis Y by a distance P from the tip of the shaft portion 111 and perpendicular to the axis Y may be defined as _D1 and _D2 , respectively.

FIG. 6 illustrates a method for measuring _D1 of the shaft portion 111 having a tapered shape. For convenience of explanation, the taper angle of the shaft portion 111 illustrated in FIG. 6 is made larger than the actual angle. The horizontal dashed line illustrated in FIG. 6 indicates a surface that is P away from the tip of the shaft portion 111 along the axis Y and perpendicular to the axis Y. When the shaft portion 111 illustrated in FIG. 6 is joined to the second member 12 with the amount of pushing in set to P, the joining surface 121 of the second member 12 is P away from the tip of the shaft portion 111 along the axis Y and coincides with a surface perpendicular to the axis Y. Although a hollow portion is not illustrated in FIG. 6, even when the shaft portion 111 has a hollow portion, _D2 may be measured in the same manner as _D1 . During friction welding, the amount of pushing in P is controlled to a predetermined value by a normal method, and the lower limit value Imin of the current value may be determined based on _D1 and _D2 measured based on the amount of pushing in P. Imax1 and Imax2, which will be described later, may be determined in the same manner as Imin.

In addition, "A" calculated by formula 6 is an index related to the electrical resistance of the second member 12. The larger A is, the greater the increase in hardness due to quenching.

Imin is a value related to the heat treatment conditions of joint 14. The larger the index "D ₁ ² -D ₂ ² " of the cross-sectional area of shaft portion 111, the larger Imin becomes. This is because the larger the cross-sectional area of joint 14 formed by shaft portion 111, the larger the current value required to ensure current density becomes. On the other hand, since the heat input is proportional to the product of the current value and the current flow time, the longer the current flow time t, the smaller Imin becomes. Also, the larger the electrical resistance index A, the smaller Imin becomes.

In the electric tempering, the current value I needs to be equal to or less than Imax1 calculated by the above-mentioned formula 4 and formula 6, and equal to or less than Imax2 calculated by formula 5. In other words, the smaller of Imax1 and Imax2 is the upper limit of the current value I. The symbols in the formulas have the same meanings as described above.
Imax1 is a value related to the heat treatment conditions of the joint 14. If the current value during electric tempering exceeds Imax1, the joint 14 may be excessively tempered, reducing the strength of the joint, or the joint 14 may be quenched.
Imax2 is a value related to the prevention of buckling of the first member 11. If the current value during current tempering exceeds Imax2, the first member 11 may buckle, making it impossible to manufacture a joint. The reason why Imax2 increases as the cross-sectional area index "D ₁ ² -D ₂ ² " of the shaft portion 111 increases, and Imax2 decreases as the current flow time t increases, is the same as that of Imin. The reason why Imax2 decreases as the pressure force F (kN) increases is that the greater the pressure force F, the more likely buckling occurs, so the softening of the first member 11 due to current flow must be suppressed. In addition, if buckling can be tolerated as a dimensional accuracy of the joint, there may be a range in which a current value exceeding Imax2 is applicable.
The current application time t (s) is preferably 0.4 s or more in order to perform stable tempering, and is preferably 2.0 s or less because if the current application time t (s) is too long, the base material portion other than the joint portion may be tempered.

The method for manufacturing a bonded joint may further include a step of overlapping the third member 13 on the second member 12 and a step of passing the shaft portion 111 of the first member 11 through the third member 13 before forming the friction welded surface. The first member 11 may have a head portion 112 having a cross-sectional diameter larger than that of the shaft portion 111. This makes it possible to manufacture a bonded joint 1 having a configuration in which the third member 13 is sandwiched between the head portion 112 of the first member 11 and the bonding surface 121 of the second member 12.

The means for penetrating the shaft portion 111 through the third member 13 is not particularly limited. For example, if the hardness of the third member 13 is significantly softer than that of the shaft portion 111, the shaft portion 111 can be pressed against the third member 13 while rotating at high speed, thereby easily penetrating the third member 13. For example, the above-mentioned penetrating means can be used when the first member 11 is a high-strength steel material and the third member 13 is a light metal or a thin mild steel plate. The above-mentioned penetrating means is preferable in terms of the efficiency of the joining work, since it allows the penetration and friction welding to be performed continuously.

On the other hand, a through hole may be provided in advance in the third member 13, and the shaft portion 111 may be inserted through the through hole. The diameter of the through hole is preferably larger than the diameter of the shaft portion 111. However, if the material and the number of rotations of the shaft portion 111 are appropriately selected, it is possible to pass the shaft portion 111 through the third member 13 even if the diameter of the through hole is smaller than the diameter of the shaft portion 111. On the other hand, in order to fix the third member 13 using the head portion 112, it is necessary to make the diameter of the through hole smaller than the diameter of the head portion 112.

The above-mentioned manufacturing method for the joint is merely one example of a manufacturing method for the joint 1 according to the present embodiment. In the above-mentioned manufacturing method, the softening of the joint 14 is achieved by optimizing the current flow conditions, but other methods may also be used.

The effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are merely one example of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this one example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

Various types of joints were produced by a manufacturing method including a process of pressing a first end of a first steel member having a shaft against a second steel member having a joint surface larger than the cross section of the shaft while rotating the first end of the shaft to frictionally weld the first end of the shaft to the joint surface, and a process of using electrodes to hold the first member and the second member in the axial direction of the shaft of the first member and pass a current of I through the electrodes to temper the joint.

The test conditions were as follows. The type of the first member was the same for all the welded joints. In the table, values outside the range of the invention are underlined.
Type of first member: Steel material with a carbon content of 0.22% by mass and a tensile strength of 1000 MPa class Shape of first member: Round bar with a conical tip having a height of 0.8 mm (diameter as shown in Table 2)
● Type of second member: Any of high-strength steel materials a to c having the components listed in Table 1 ● Shape of second member: Steel plate with a joining surface area of ²⁵⁰⁰ cm2 or more Other conditions were as listed in Table 1 or Table 2. Note that no current tempering was performed on some of the joined joints, and these current conditions are indicated as "-" in Table 2. The balance of the components listed in Table 1 was iron and impurities.

The various bonded joints thus obtained were evaluated. Specifically, the average maximum hardness Have of both ends of the bonded joint and the strength of the bonded joint were measured, and the presence or absence of buckling was also confirmed.
The average value Have of the maximum hardness at both ends of the joint was measured by the method described above. The strength of the joint was defined as the maximum load measured when the end of the second member was fixed and the first member was pulled in the direction perpendicular to the joint surface until the first member broke.
Regarding the judgment of the presence or absence of buckling, those that were clearly bent by visual inspection, and those that, when a joint was made under the same conditions as those described in the table and a cross-section was observed and the diameter of the shaft of the first member was found to have increased from the original diameter, were judged to have buckling. However, although it is preferable that buckling does not occur, buckling is permissible as long as it does not impair the strength of the joint.
These evaluation results are shown in Table 3. Values outside the range of the invention are underlined. For reference, the value obtained by substituting the carbon content C of the second member into "761.6×C ³ - 1897.2×C ² + 1848.8×C + 168.3" is listed in Table 3 as the effective upper limit hardness.

No electric tempering was performed on the welded joint of No. 1. Therefore, in the welded joint of No. 1, Have did not satisfy Formula 1, and sufficient joint strength was not obtained.
The bonded joints Nos. 2 to 5 were manufactured under the same manufacturing conditions as No. 1, except for the current value and current flow time. Among these, the bonded joints Nos. 3 and 6, which were manufactured under conditions where the current value was too large compared to the current flow time, suffered from buckling and re-quenching of the round bar. The bonded joint No. 4, which was manufactured under conditions where the current value was too small compared to the current flow time, had excessive Have and could not increase the joint strength. On the other hand, the bonded joints Nos. 2 and 5, which were manufactured under conditions where the current value was within the appropriate range relative to the current flow time, satisfied the Have formula 1, and the joint strength was dramatically increased compared to the bonded joint No. 1.

In the manufacture of joints Nos. 7 to 12, the diameter of the round bar (corresponding to the diameter of the shaft of the first member) and the current value were different, but the other conditions were the same. In joint No. 7, which was manufactured under conditions where the current value was too large compared to the diameter of the round bar, buckling and re-quenching occurred. In joints Nos. 9 and 11, which were manufactured under conditions where the current value was too large compared to the diameter of the round bar, re-quenching occurred. These joints did not have high joint strength. On the other hand, joints Nos. 8, 10, and 12, which were manufactured under conditions where the relationship between the diameter of the round bar and the current value was within the appropriate range, had high joint strength, with Have satisfying formula 1.

The applied pressure was different in the manufacture of joints No. 13 and 14, but the other conditions were the same. In joint No. 14, which was manufactured under conditions where the current value was too high compared to the applied pressure, buckling occurred in the round bar. On the other hand, in joint No. 13, which was manufactured under conditions where the relationship between the applied pressure and the current value was within the appropriate range, Have satisfied formula 1 and had high joint strength. However, although buckling occurred in joint No. 14, the joint strength met the pass/fail criteria. Therefore, joint No. 14 was also treated as an example of the invention.

No electric tempering was performed on the welded joints of Nos. 15 and 17. Therefore, in the welded joints of Nos. 15 and 17, Have did not satisfy Formula 1, and sufficient joint strength was not obtained.
The bonded joints of Nos. 16 and 18 had the same composition of the second member as the bonded joints of Nos. 15 and 17, respectively. The bonded joints of Nos. 16 and 18 were subjected to electric tempering under appropriate conditions, so that Have satisfied Formula 1 and the bond strength was dramatically increased compared to the bonded joints of Nos. 15 and 17, respectively.

Reference Signs List 1: Welded joint 11: First member 111: Shank 112: Head 12: Second member 121: Welding surface 13: Third member 14: Welded portion 15: Friction welded interface X: Position Y at a depth of 0.1 mm from the welded surface: Axis E: Electrode

Claims (13)

a first member made of steel and having a shaft portion;
a second member made of steel having a joining surface larger than a cross section of the shaft portion;
a joint formed by frictionally welding a first end portion of the first member and the joint surface;
A joint joint comprising:
A bonded joint, in which an average value Have of maximum hardness at both ends of the bonded joint, obtained by continuously measuring hardness over the bonded joint and its periphery at a position 0.1 mm deep from the bonded surface in a cross section of the bonded joint passing through the axis of the shaft portion, satisfies Formula 1.
Have≦761.6×C ³ −1897.2×C ² +1848.8×C+168.3……(Formula 1)
Here, the symbol "C" in the formula 1 is the carbon content in unit mass % of the second member. The welded joint according to claim 1, characterized in that the second member has tempered martensite in a region within a depth of 0.1 mm from a friction welded interface, which is an interface between the first member and the second member. 3. The welded joint according to claim 1, wherein the carbon content of the second member is 0.20 mass % or more. a friction welded interface between the first member and the second member penetrates into the second member,
The welded joint according to any one of claims 1 to 3, characterized in that a maximum depth of the friction welded interface with respect to the welded surface of the second member, measured in the cross section of the welded joint passing through the axis, is 0.3 mm or more. The joint according to any one of claims 1 to 4, characterized in that the second member is a steel plate having a thickness of 0.8 to 3.0 mm. 6. The joint according to claim 1, wherein the area of the joining surface is 3.0 times or more the cross-sectional area of the shaft portion. A cross section of the shaft portion perpendicular to the axis line has a rotationally symmetric shape,
7. The joint according to claim 1, wherein the diameter of the circumscribing circle of the cross section is 2.0 to 10.0 mm. 8. The joint according to claim 7, wherein a cross section of the shaft portion perpendicular to the axis is a circle or a regular polygon. the first member having a head disposed at a second end thereof;
The joint further includes a third member provided between the head and the second member,
The shaft portion of the first member passes through the third member,
The joint joint according to any one of claims 1 to 8, characterized in that the third member is clamped between the head of the first member and the joint surface of the second member. The bonded joint of claim 9, wherein the third member is a steel plate. An automobile part having a joint according to any one of claims 1 to 10. a step of pressing a first end of a first steel member having a shaft portion against a second steel member having a joint surface larger than a cross section of the shaft portion while rotating the first steel member, thereby forming a joint by frictionally welding the first end of the shaft portion and the joint surface;
a step of using electrodes to clamp the first member and the second member in an axial direction of the shaft portion of the first member, and passing a current of a value I through the electrodes to temper the joint;
Equipped with
A method for manufacturing a joint, in which the current value I is within a range that satisfies the following formula 2.

where Imin, Imax1, and Imax2 defined by Equation 3, Equation 4, and Equation 5 have units of kA;
In Equation 3, Equation 4, and Equation 5, the symbol "D ₁ " is the diameter, in mm, of the circumscribing circle of the cross section of the shaft portion, the symbol "D ₂ " is the diameter, in mm, of the circumscribing circle of the cross section of the hollow portion of the shaft portion, and when the shaft portion is solid, D ₂ = 0, the symbol "t" is the current application time, in seconds, and the symbol "F" is the applied pressure, in kN.
In formula 6, the symbols "C", "Si", "Mn" and "Cr" respectively represent the C content, Si content, Mn content and Cr content in unit mass % of the second member. The method for manufacturing the bonded joint further comprises, before forming the bonded portion,
overlaying a third member on the second member;
a step of passing the shaft portion of the first member through the third member;
Equipped with
The method of claim 12, wherein the first member has a head at a second end thereof.

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