JP2022047845A - Bonded joint, automobile component, and method for manufacturing bonded joint - Google Patents

Abstract

To provide a bonded joint having a high bond strength, and an automobile component, and to provide a method for manufacturing a bonded joint having a high bond strength.SOLUTION: A bonded joint includes a first member that has a shaft part and is made of steel, a second member that has a bond surface larger than a cross section of the shaft part and is made of steel, and a bonded part obtained by friction pressure welding of a first end of the first member and the bond surface, in which in a cross section of the bonded joint passing through a shaft line of the shaft part, an average value Have of highest hardness on both ends of the bonded part obtained by continuously measuring hardness at a position with a depth of 0.1 mm from the bonded surface in the bonded part and its periphery is 761.6×C3-1897.2×C2+1848.8×C+168.3 or more.SELECTED DRAWING: Figure 2

Description

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

Friction welding is known as one of the methods for manufacturing a joint by joining members. Friction welding is a technique for solid-phase welding members by applying pressure while rubbing the members. Friction between members is performed, for example, by forming the pressure contact surface of one member into a rotationally symmetric shape (for example, a circular or polygonal shape) and rotating the member at high speed.

One of the features of friction welding is that the members do not melt or stir at the joint. Welding is a joining method in which a joint is formed by melting and resolidifying a member. Friction stir welding is a joining method in which a press-fit member is rotated at high speed to stir the contact portions between the members to form a joint portion. Therefore, the members are mixed at the joint portion of the joint joint formed by welding or friction stir welding. On the other hand, in the joint formed by friction welding, the region where the members are mixed does not exist or is extremely small. By observing the cross section of the joint portion of the joint formed by friction welding, it is possible to determine how the members are separated through the friction welding surface. For example, Patent Document 1 and Patent Document 2 and the like disclose a method of friction welding.

Another feature of friction welding is that it can be applied to the manufacture of joints made of different materials. For example, it is possible to join by abutting the end faces of steel and aluminum round bars, rotating one of them under appropriate conditions, and applying pressure. In addition, by utilizing friction welding, it is possible to manufacture joints made of different materials in stacked plates. For example, in a joint joint consisting of a laminated upper plate and a lower plate and a joining member that penetrates the upper plate and is friction-welded to the lower plate, the lower plate is used by using the upper plate and the head of the joining member. By sandwiching the above plate, the materials of the upper plate and the lower plate can be made different.

Welding (particularly spot welding) is most often used as a means of joining stacked plates (particularly metal plates). For example, Patent Document 3 discloses a spot welding method for a high-strength steel sheet. However, it is difficult to apply welding, which is a joining means for causing a mixture of molten materials in a welded portion, to join dissimilar materials. Friction welding has an advantage over spot welding, for example, in that it improves the degree of freedom in material selection.

Japanese Unexamined Patent Publication No. 2006-297398 Japanese Unexamined Patent Publication No. 2004-141933 International Publication No. 2014/171495

The present inventors have attempted to apply friction welding to the bonding of various materials. When a round steel bar or a polygonal column is friction-welded to a high-strength steel plate, the present inventors tend to crack the joint when an external force is applied to the joint, and the strength of the joint decreases. I found out that In the prior art, there are few examples of applying friction welding to high-strength steel materials, and no reduction in bonding strength has been investigated.

In view of the above circumstances, it is an object of the present invention to provide a joint joint having high joint strength and an automobile part. Further, it is an object of the present invention to provide a method for manufacturing a joint joint having a 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) The joint according to one aspect of the present invention includes a first steel member having a shaft portion, a second steel member having a joint surface larger than the cross section of the shaft portion, and the first member. A joint including a first end portion of a member 1 and a joint portion formed by friction welding the joint surface, and the joint is formed in a cross section of the joint joint passing through the axis of the shaft portion. The average value Have of the maximum hardness at both ends of the joint, which is obtained by continuously measuring the hardness at a depth of 0.1 mm from the surface over the joint and its periphery, satisfies Equation 1.
Have ≤ 761.6 x C ^3-1897.2 x C ² + 1848.8 x C + 148.3 ... (Equation 1)
Here, the symbol "C" in the formula 1 is the amount of carbon in the unit mass% of the second member.
(2) In the joint according to (1) above, in the region where the second member is within 0.1 mm in depth from the friction welding interface, which is the interface between the first member and the second member. , May have tempered martensite.
(3) In the joint according to the above (1) or (2), the carbon content of the second member may be 0.20% by mass or more.
(4) In the joint according to any one of (1) to (3) above, the friction welding interface, which is the interface between the first member and the second member, is formed on the second member. The maximum depth of the friction welding interface of the second member with respect to the joint surface, which is intruded and passes through the axis and is measured in the cross section of the joint, may be 0.3 mm or more.
(5) In the joint according to 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 according to any one of (1) to (5) above, the area of the joint surface may be 3.0 times or more the cross-sectional area of the shaft portion.
(7) In the joint according to any one of (1) to (6) above, the cross section of the shaft portion perpendicular to the axis has a rotationally symmetric shape, and the diameter of the circumscribed circle of the cross section is 2. It may be 0.0 to 10.0 mm.
(8) In the joint according to (7) above, the cross section of the shaft portion perpendicular to the axis may be a circle or a regular polygon.
(9) In the joint according to any one of (1) to (8) above, the first member has a head arranged at its second end, and the joint is A third member provided between the head and the second member is further provided, and the shaft portion of the first member penetrates the third member and the third member. May be sandwiched between the head of the first member and the joint surface of the second member.
(10) In the joint according to (9) above, the third member may be a steel plate.
(11) Automobile parts according to another aspect of the present invention The joint has the joint according to any one of (1) to (10) above.
(12) In the method for manufacturing a joint joint according to another aspect of the present invention, the first end portion of the first steel member having a shaft portion is made of steel having a joint surface larger than the cross section of the shaft portion. The step of forming a joint portion formed by friction-welding the first end portion of the shaft portion and the joint surface by pressing the joint portion against the second member made of the product while rotating, and the above-mentioned using an electrode. The first member and the second member are sandwiched in the axial direction of the shaft portion of the first member, and the joint portion is burnt back by energizing the current value as I. The current value I is within a range that satisfies the following equation 2.

Here, the unit of Imin, Imax1, and Imax2 defined by the formulas 3, 4, and 5 is kA, and in the formulas 3, 4, and 5, the symbol "D ₁ " is the shaft portion. Is the diameter of the circumscribed circle of the cross section in mm, and the symbol “D ₂ ” is the diameter of the circumscribed circle of the cross section of the hollow portion of the shaft portion in mm. If it is true, D ₂ = 0, the symbol "t" is the energization time in unit seconds, the symbol "F" is the pressing force in the unit kN, and the symbol "C" in Equation 6 “Si”, “Mn” and “Cr” are C content, Si content, Mn content and Cr content in unit mass% of the second member, respectively.
(13) The method for manufacturing a joint according to the above (12) further comprises a step of superimposing the third member on the second member and the step of superimposing the third member on the third member before forming the joint portion. The first member may have a head at a second end thereof, comprising a step of penetrating the shaft portion of the first member.

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

It is a photograph of the two-phase region heating part in the joint after the rupture test. It is sectional drawing which includes the axis of the shaft part of the joint joint which concerns on this embodiment. Hardness distribution obtained by continuously measuring the hardness of the joint joint according to the present embodiment and the joint joint that has not been post-treated after friction welding of steel materials at a depth of 0.1 mm from the joint surface. It is a schematic diagram of a curve. It is sectional drawing which includes the axis of the shaft part of the joint joint which concerns on this embodiment which includes the 3rd member. It is a schematic diagram of an example of the electric current tempering method of the joint joint which concerns on this embodiment. It is a schematic diagram explaining the method of specifying the diameter D1 of the shaft portion ₁₁₁ having a tapered shape.

The present inventors 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. Fracture tests were carried out on the various joints made. Then, the present inventors investigated the fractured portion in detail and found the following matters.

First, it was discovered that the fracture tends to occur near the outer periphery of the joint portion 14 and at a portion corresponding to the two-phase region heating portion of the second member 12. The two-phase region heating portion is a portion that is heated to a temperature region (two-phase region) of Ac 1 point or more and Ac 3 points or less by friction heat during friction welding, and then cooled. The metal structure of the two-phase region heating portion is a mixture of a hard phase (for example, martensite, bainite, tempered martensite, etc.) and a soft phase (for example, ferrite, etc.).

Next, when the present inventors observed the two-phase region heating portion after the fracture test in detail, it was confirmed that a large number of voids were present between the hard phase and the soft phase. FIG. 1 shows a photograph of the two-phase region heating portion after the fracture test. It was presumed that many of these voids acted as the starting point of destruction. Further, it was presumed that the reason why the void was generated was that the hardness difference between the soft phase and the hard phase was large and the strain was concentrated on these interfaces.

Based on the above findings, the present inventors attempted to reduce the above-mentioned hardness difference and alleviate the strain concentration by energizing and tempering the two-phase region heating portion. However, when the amount of energization was small, no improvement in bonding strength was observed. On the other hand, when the amount of energization was large, the first member 11 softened, and buckling of the first member 11 occurred during energization heating.

In addition, energization tempering may be carried out in spot welding. However, in the energization tempering of the spot welded portion, the current condition for achieving the temperature required for tempering can be predicted by considering the actual value of the current value and the calorific value at the time of spot welding. On the other hand, such current condition prediction cannot be made in the current current tempering of the friction welding joint. Further, since buckling of the welded portion does not occur in the current current tempering of the spot welded portion, the avoidance of buckling has not been studied in the prior art. Therefore, it has been difficult to simply divert the energization tempering of the spot welded portion to the energization tempering of the friction welding joint.

The present inventors conducted further studies there. As a result, the present inventors have joined by optimizing the energization conditions while comprehensively considering the components of the second member 12, the size of the first member 11, the energization time, the pressing force, and the like. It was found that the joint strength can be improved by appropriately softening the portion 14.

The joint joint 1 according to one aspect of the present invention completed based on the above findings is larger than the cross section of the first steel member 11 having the shaft portion 111 and the shaft portion 111, for example, as shown in FIG. The shaft portion 111 includes a second member 12 made of steel having a joint surface 121, a first end portion of the first member 11, and a joint portion 14 formed by friction welding the joint surface 121. Both ends of the joint 14 obtained by continuously measuring the hardness of the position X at a depth of 0.1 mm from the joint surface 121 over the joint 14 and its periphery in the cross section of the joint 1 passing through the axis Y. Have, the average value of the maximum hardness of, satisfies Equation 1.
Have ≤ 761.6 x C ^3-1897.2 x C ² + 1848.8 x C + 148.3 ... (Equation 1)
Here, the symbol "C" in the formula 1 is the amount of carbon in the unit mass% of the second member 12. Hereinafter, the joint joint 1 according to the present embodiment will be described in detail.

(First member 11)
The first steel member 11 has a shaft portion 111 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 friction welding with the second member 12. For convenience, of both ends of the first member 11, the end on the side joined to the joining surface 121 of the second member 12 is referred to as the first end, and the other end is referred to as the second end. It is called.

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

(Joint portion 14)
The joint portion 14 is a portion formed by friction welding the end portion of the first member 11 and the joint surface 121 of the second member 12. In the present embodiment, the joint portion 14 is defined as a region where thermal influence or deformation occurs due to the influence of friction welding. FIG. 2 schematically shows the outer edge of the joint portion 14 with a broken line. However, unlike the welded portion formed by welding, in the joint portion 14 formed by friction welding, the first member 11 and the second member 12 are not mixed, and their interfaces are clearly defined. You can check. In the present embodiment, the interface between the first member 11 and the second member 12 is referred to as a friction welding interface 15.

(Mean value of maximum hardness at both ends of the joint portion 14 Have)
In a normal joint joint obtained by friction welding a steel member, the joint portion 14 is hardened by quenching due to frictional heat. On the other hand, in the joint joint 1 according to the present embodiment, the joint strength is increased by relaxing the quench hardening of the joint portion 14. Therefore, in the joint joint 1 according to the present embodiment, the average value Have of the maximum hardness at both ends of the joint portion 14 satisfies the formula 1. The average value Have of the maximum hardness is a continuous hardness measurement of a position X at a depth of 0.1 mm from the joint surface 121 over the joint and its periphery in the cross section of the joint 1 passing through the axis Y of the shaft portion 111. Obtained by doing.
Have ≤ 761.6 x C ^3-1897.2 x C ² + 1848.8 x C + 148.3 ... (Equation 1)
Here, the symbol "C" in the formula 1 is the amount of carbon in the unit mass% of the second member 12.

The position X at a depth of 0.1 mm from the joint surface 121 is a position indicated by a straight line designated by the reference numeral “X” in FIG. The joint surface 121 is significantly deformed at the joint portion 14. However, the position X at a depth of 0.1 mm from the joint surface 121 is specified based on the joint surface 121 that has not been deformed. That is, by extending the joint surface 121 that has not been deformed, the joint surface 121 before friction welding is estimated, and based on this, the position X at a depth of 0.1 mm is specified from the joint surface 121. .. Normally, the friction welding interface 15 bites into the inside of the second member 12. Therefore, in the vicinity of the axis Y, the first member 11 may be arranged at a position X having a depth of 0.1 mm from the joint surface 121.

By continuously measuring the hardness of the position X at a depth of 0.1 mm from the joint surface 121 over the joint portion and its periphery, for example, the position at a depth of 0.1 mm from the joint surface 121 as shown in FIG. A hardness distribution curve along X can be obtained. The maximum hardness at both ends of the joint portion 14 is the maximum value of the hardness on the left side of the axis Y and the maximum value of the hardness on the right side of the axis Y in the hardness distribution curve.

The location where the hardness is maximized on the hardness distribution curve is usually near the intersection of the position X at a depth of 0.1 mm from the joint surface 121 and the friction welding interface 15. This location is where significant quench hardening occurs due to frictional heat during friction welding. The broken line hardness distribution curve shown in FIG. 3 is an example of the hardness distribution curve of a joint joint that has not undergone any special post-treatment after friction welding of steel materials. In such a hardness distribution curve of a joint, there are remarkable peaks on both sides of the axis Y due to quench hardening.

On the other hand, in the joint joint 1 according to the present embodiment, quench hardening is alleviated. Therefore, the height of the hardness peak in the hardness distribution curve becomes small. The solid line hardness distribution curve shown in FIG. 3 is an example of the hardness distribution curve of the joint 1 according to the present embodiment. In the hardness distribution curve of the joint 1 according to the present embodiment, the height of the hardness peak is lower than the broken 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 portion 14 in the joint joint 1 according to the present embodiment, the present inventors have focused on the carbon content of the second member 12. The hardness of the second member 12 after quenching is determined according to the amount of carbon in the second member 12. Therefore, the upper limit of the average value Have of the maximum hardness at both ends of the joint portion 14 is set to the function of the carbon content of the second member 12 “761.6 × C ^3-1897.2 × C ² + 1848.8 × C + 168. 3 ". When the average value Have of the maximum hardness at both ends of the joint portion 14 satisfies the 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 portion 14 satisfies Equation 1, the maximum hardness of one end portion of the joint portion 14 is "761.6 x C ^3-1897.2 x C ² +1848". It is permissible to exceed "0.8 x C + 168.3". The means for keeping the average value Have of the maximum hardness at both ends of the joint portion 14 within the above range is not particularly limited, but for example, electric tempering according to the conditions described later is preferable. The upper limit of the average value Have is preferably 95% or less, 90% or less, or 85% or less of "761.6 x C3-1897.2 x C2 + 1848.8 x C + 168.3". The lower limit of the average value Have is not particularly limited, but may be defined as, for example, 60% or more, 70% or more, 80% or more, or 90% or more of the hardness of the base material of the second member 12. The hardness of the base material of the second member 12 is the hardness of the region of the second member 12 where the thermal influence and deformation of the friction welding are not generated.

The method for measuring the average value Have of the maximum hardness at both ends of the joint portion 14 is as follows.
First, the joint 1 is cut with a cross section substantially including the axis Y of the shaft portion 111 of the first member 11, and the cross section is appropriately polished. It is permissible that this cross section deviates slightly from the axis Y.
Next, the position X having a depth of 0.1 mm from the joint surface 121 is specified. Since the joint surface 121 is significantly deformed at the joint portion 14, the joint surface 121 before the friction welding is estimated by extending the joint surface 121 that has not been deformed, and is parallel to this. And the line X (see FIG. 2) separated by 0.1 mm is specified. When the joint surface 121 is a curved surface, the line X is also a curved line corresponding to the curved surface.
Then, along the line X, the Vickers hardness is continuously measured over the joint and its periphery. The measurement load of Vickers hardness is 200 g, and the measurement interval of Vickers hardness is 0.2 mm.
The obtained hardness measurement result is plotted on a graph with the distance from the axis Y as the horizontal axis and the hardness as the vertical axis to create a hardness distribution curve. Then, the maximum value of the hardness on the left side of the axis Y and the maximum value of the hardness on the right side of the axis Y are specified, and by averaging these values, the average value of the maximum hardness at both ends of the joint portion 14 is obtained. Calculate Have.

The basic aspect of the joint joint 1 according to the present embodiment has been described above. Hereinafter, a more preferable embodiment of the joint joint 1 according to the present embodiment will be described.

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

In a joint that has not been subjected to special post-treatment after friction welding of steel materials, tempered martensite does not exist in the region within 0.1 mm in depth from the friction welding 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) by quenching during friction welding. On the other hand, by tempering a part or all of the metal structure of the region within 0.1 mm in depth from the friction welding interface 15 in the second member 12 as tempered martensite, the fracture generated from the second member 12 is further further destroyed. It can be suppressed and the joint strength of the joint joint 1 can be further increased.

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

The first member 11 and the second member 12 are made of steel. These 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 is, for example, 0.20% by mass or more, 0.22% by mass or more, 0.25% by mass or more, or 0.30% by mass. It may be the above. As a result, friction welding with the second member 12, which has high strength, can be performed more stably. In the friction welding of steel materials having a carbon content of 0.20% by mass or more in the second member 12, a decrease in joint strength becomes a problem, but in the joint joint 1 according to the present embodiment, the joint portion is used. Since quench hardening of the joint portion 14 is suppressed so that the average value Have of the maximum hardness at both ends of 14 satisfies Equation 1, even if the carbon content of the second member 12 is 0.20% by mass or more. Bonding strength is ensured.

The mechanical properties of the first member 11 and the second member 12 are also not particularly limited. On the other hand, the higher the strength of the first member 11 and the second member 12, the higher the characteristics of the component having the joint joint 1, which is preferable. Further, in the joint joint 1 according to the present embodiment, since the quench hardening at the joint portion 14 is suppressed, the joint strength is ensured even when the strength of the first member 11 and the second member 12 is high. can do. Therefore, for example, the tensile strength of one or both of the first member 11 and the second member 12 may be set to 590 MPa or more.

In the joint 1 according to the present embodiment, the shape of the joint 14 is not particularly limited. As shown in FIG. 2, during friction welding, the first member 11 and the second member 12 may be deformed to form burrs. The joint portion 14 may have such a burr. On the other hand, the burr may be mechanically removed from the joint portion 14.

Further, in the joint portion 14, it is preferable that the first member 11 enters the inside of the second member 12. In other words, the friction welding interface 15 which is the interface between the first member 11 and the second member 12 may enter the second member 12. In this case, the maximum depth of the friction welding interface 15 with respect to the joint surface 121 of the second member 12 measured in the cross section of the joint 1 passing through the axis Y is 0.1 mm or more, 0.3 mm or more, or 0.5 mm. It may be the above. As a result, poor joining at the friction welding interface 15 is suppressed, and the joining strength of the joining joint 1 is further improved.

The shapes of the first member 11 and the second member 12 are not particularly limited, but suitable examples of the shapes of the first member 11 and the second member 12 are shown below.

The second member 12 is preferably a steel plate having a plate thickness of 0.8 to 3.0 mm, for example. A steel plate having such a thickness is very suitable as a material for automobile parts. The plate 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 plate 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 hindered 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 makes it possible to flexibly design the shape of the joint joint 1. When the cross-sectional area of the shaft portion 111 of the first member 11 is not constant along the axis Y direction (for example, when the shaft portion 111 has a tapered shape), the shaft portion 111 of the first member 11 The cross-sectional area means the area of the region formed by the line of intersection between the outer peripheral surface of the shaft portion 111 and the joint surface 121 of the second member 12. When the joint portion 14 has burrs, the outer peripheral surface of the shaft portion 111 means an extension surface thereof.

The shape of the shaft portion 111 of the first member 11 is not particularly limited, but for example, the cross section of the shaft portion 111 perpendicular to the axis Y (hereinafter, may be abbreviated as “cross section of the shaft portion 111”) is rotationally symmetric. The shape is preferable. 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, the diameter of the circumscribed 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 2.0 to 10.0 mm. It is preferably within the range. The diameter of the circumscribed circle of the shaft portion 111 may be constant or different along the axis Y. For example, the shaft portion 111 may have a tapered shape whose thickness decreases toward the tip thereof. Further, the shaft portion 111 may have a hollow structure (for example, a cylindrical structure or the like) or a solid structure. When the shaft portion 111 has a hollow structure, the internal shape of the cross section of the shaft portion 111 is preferably a rotationally symmetric shape, for example, a circle or a regular polygon.

The first member 11 may be provided with a head 112 having a cross-sectional diameter larger than that of the shaft portion 111 at the second end portion. The head 112 has a function of fixing a third member 13 described later. In this case, the first member 11 may have a rivet shape. The diameter of the circumscribed circle of the head 112 is preferably 1.5 times or more the diameter of the circumscribed circle of the shaft portion 111. Further, the length of the shaft portion 111 (the value obtained by subtracting the thickness of the head 112 from the length of the entire first member 11) is 1.5 times or more the thickness of the third member 13. Is preferable. On the other hand, the joint 1 may not include the third member 13, or the first member 11 may be a rod-shaped member having only the shaft portion 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. good. For example, as shown in FIG. 4, the first member 11 has a shape having a shaft portion 111 like a rivet and a head portion 112, and the shaft portion 111 is passed through the third member 13 to form a third member. 13 may be sandwiched by the joint surface 121 of the head 112 and the second member 12. In this case, the second member 12 and the third member 13 are preferably joined tightly by using the first member 11. Further, in the joint 1 having such a configuration, the first member 11 and the third member 13 can be made of different materials to which welding cannot be applied, which is preferable. The third member 13 is, for example, a steel plate, a light metal plate, a resin plate, or the like. The light metal plate is, for example, an aluminum plate. If the first member 11 is made of a high-strength steel material, the first member 11 can be penetrated through the third member 13 even if the third member 13 is a high-strength steel plate. It is not prevented that the joint 1 has yet another member.

The third member 13 may have a hole through which the shaft portion 111 is inserted in advance. The hole may be provided in the third member 13 in advance before the shaft portion 111 is inserted, or may be formed by pressing the shaft portion 111 that rotates at high speed against the third member 13. The shape of the hole of the third member 13 is not particularly limited. In order to fix the third member 13 using the head 112, the diameter of the through hole may be smaller than the diameter of the head 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-treated layer is, for example, a plating layer, a coating film, a chemical conversion-treated film, or the like. Since the surface treatment layer in the first member 11 and the second member 12 is removed during friction welding, it does not hinder the formation of the joint portion 14. Further, 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 are the first members. It may be further joined by means other than the member 11 (for example, welding). That is, the joint 1 according to the present embodiment having the third member 13 may further include an additional joint portion (for example, a welded portion) for joining 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 parts according to the present embodiment have the joint joint 1 according to the present embodiment. As a result, the automobile parts according to the present embodiment have 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 a joint according to the present embodiment described above is not particularly limited, but the means described below can be used to suitably manufacture the joint.

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 method for manufacturing a joint according to the present embodiment is as follows.
(S1) While rotating the first end portion of the steel first member 11 having the shaft portion 111 to the steel second member 12 having the joint surface 121 larger than the cross section of the shaft portion 111. A step of forming a joint portion 14 formed by friction welding the first end portion of the shaft portion 111 and the joint surface 121 by pressing the shaft portion 111.
(S2) Using the electrode E, the first member 11 and the second member 12 are sandwiched in the axis Y direction of the shaft portion 111 of the first member 11, and are joined by energizing with the current value as I. The process of burning back part 14 and
To prepare for. Here, the current value I is set within the range that satisfies the following equation 2.

In Equation 2, the symbol Imin is the value defined by Equation 3, the symbol Imin is the value defined by Equation 4, the symbol Imin is the value defined by Equation 5, and the unit of these values is kA. be.
In formulas 3, 4, and 5, the symbol "D ₁ " is the diameter of the circumscribed circle of the cross section of the shaft in mm. The symbol "D ₂ " is the diameter of the circumscribed circle of the cross section of the hollow portion of the shaft portion in mm. Here, when the shaft portion is solid, D ₂ = 0. The symbol "t" is the energization time in a unit second. The symbol "F" is a pressing force in the unit kN. The symbol "A" is a value defined by Equation 6.
In formula 6, the symbols "C", "Si", "Mn" and "Cr" are the C content, Si content, Mn content and Cr content in the unit mass% of the second member, respectively. ..

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

When friction welding is performed by utilizing the rotation of the shaft portion 111, the shape of the tip of the shaft portion 111 before the friction welding is preferably a conical shape, a pyramid shape, or a partial spherical shape, for example. It is preferable that the apex of the tip of the shaft portion 111 is on the axis Y of the shaft portion 111. Further, if the height of the cone or the pyramid or the height of the partial sphere is too large, the height may not be completely crushed by joining, so the height is preferably 2.0 mm or less.

(S2)
In the method for manufacturing a joint according to the present embodiment, as shown in FIG. 5, the first member 11 and the second member 12 are attached to the shaft portion 111 of the first member 11 by using the electrode E. The joint portion 14 is burnt back by sandwiching it in the Y direction of the axis of No. 1 and energizing it with the current value as I. This energization tempering reduces the quench hardening of the joint portion 14. The type of the electrode E is not particularly limited, but for example, a copper electrode is preferable. The device for performing energization tempering is not particularly limited, but for example, a resistance spot welder may be used as a device for energization tempering.

In energization tempering, the current value I needs to be equal to or greater than Imin calculated by the formulas 3 and 6. The current value I may be constant or may be changed during energization tempering. If the current value I is not constant, the average value of the current values during energization tempering is regarded as the current value I. The meanings of the symbols in the formulas 3 and 6 are as described above.

In addition, "D ₁ ² -D ₂ ² " included in the formula 3 and the like is an index of the cross-sectional area of the shaft portion 111. As described above, the shaft portion 111 may have a hollow structure (for example, a cylindrical structure or the like) or a solid structure. Considering the case where the shaft portion 111 has a hollow structure, the formula 3 includes the size of the hollow portion of the shaft portion 111 as a variable. When the shaft portion 111 has a solid structure, 0 is substituted for the diameter D ₂ of the circumscribed circle of the hollow portion, and Imin is calculated based only on the circumscribed circle D ₁ of the shaft portion 111.

Further, as described above, the shaft portion 111 of the first member 11 may have a tapered shape. In this case, D ₁ and D ₂ may be values measured on a surface presumed to coincide with the joint surface 121 of the second member 12. Such a surface can be specified in consideration of the pushing amount P of the first member 11. Therefore, the shaft portion 111 on the cut surface obtained by cutting the shaft portion 111 of the first member 11 on a plane that is separated from the tip of the shaft portion 111 by P along the axis Y and is perpendicular to the axis Y. The circumscribed circle of the hollow portion may be D ₁ and the circumscribed circle of the hollow portion may be D ₂ .

FIG. 6 illustrates a method of measuring D ₁ of the shaft portion 111 having a tapered shape. For convenience of explanation, the taper angle of the shaft portion 111 shown in FIG. 6 is made larger than the actual one. The alternate long and short dash line in the horizontal direction shown in FIG. 6 is separated from the tip of the shaft portion 111 by P along the axis Y and indicates a plane perpendicular to the axis Y. When the shaft portion 111 shown in FIG. 6 is joined to the second member 12 with a pushing amount of P, the joint surface 121 of the second member 12 is separated from the tip of the shaft portion 111 by P along the axis Y. It coincides with the plane perpendicular to the axis Y. Although the hollow portion is not shown in FIG. 6, even when the shaft portion 111 has the hollow portion, D ₂ may be measured in the same manner as D ₁ . At the time of friction welding, the push-in amount P is controlled to a predetermined value by a normal method, and the lower limit value Imin of the current value is determined based on D ₁ and D ₂ measured based on the push-in amount P. do it. Imax1 and Imax2, which will be described later, may be determined in the same manner as Imin.

Further, "A" calculated by the formula 6 is an index relating to the electric resistance of the second member 12. The larger the A, the greater the increase in hardness due to quenching.

Imin is a value related to the heat treatment conditions of the joint portion 14. The larger the index “D ₁ ² − D ₂ ² ” of the cross-sectional area of the shaft portion 111, the larger the Imin. This is because the wider the cross-sectional area of the joint portion 14 formed by the shaft portion 111, the higher the current value for ensuring the current density. On the other hand, since the amount of heat input is proportional to the product of the current value and the energization time, the larger the energization time t, the smaller the Imin. Further, the larger the index A of the electric resistance, the smaller the Imin.

Further, in the energization tempering, the current value I needs to be Imax1 or less calculated by the above formula 4 and the above formula 6 and Imax2 or less calculated by the above formula 5. In other words, the smaller of Imax1 and Imax2 is the upper limit of the current value I. The meanings of the symbols in the formula are as described above.
Imax1 is a value related to the heat treatment conditions of the joint portion 14. If the current value at the time of energization tempering exceeds Imax1, the joint portion 14 may be excessively tempered to reduce the strength of the joint portion or the joint portion 14 may be quenched.
Imax2 is a value related to buckling prevention of the first member 11. If the current value at the time of energization tempering exceeds Imax2, the first member 11 may buckle and the joint may not be manufactured. The reason why Imax2 becomes larger as the index “D ₁ ² − D ₂ ² ” of the cross-sectional area of the shaft portion 111 becomes larger and Imax2 becomes smaller as the energization time t becomes larger is the same as that of Imin. The reason why Imax2 becomes smaller as the pressing force F (kN) is larger is that buckling is more likely to occur as the pressing force F is larger, and it is necessary to suppress the softening of the first member 11 due to energization. Further, if buckling is acceptable as the dimensional accuracy of the joint portion, there may be an applicable range even if the current value exceeds Imax2.
The energization time t (s) is preferably 0.4 s or more in order to perform stable tempering. Further, if the energization time t (s) is too long, the base metal portion other than the joint portion may be tempered. Therefore, 2.0 s or less is preferable.

Further, the method of manufacturing the joint is a step of superimposing the third member 13 on the second member 12 and penetrating the shaft portion 111 of the first member 11 through the third member 13 before forming the friction welding surface. It may be provided with a step of making it. The first member 11 may have a head portion 112 having a cross-sectional diameter larger than that of the shaft portion 111. Thereby, the joint joint 1 having a structure in which the third member 13 is sandwiched by the joint surface 121 of the head 112 of the first member 11 and the second member 12 can be manufactured.

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

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 rotation speed of the shaft portion 111 are appropriately selected, it is possible to allow the shaft portion 111 to penetrate 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 112, it is required that the diameter of the through hole is smaller than the diameter of the head 112.

The above-mentioned method for manufacturing a joint is only an example of the above-mentioned method for manufacturing a joint 1 according to the present embodiment. In the above-mentioned manufacturing method, the joint portion 14 is softened by optimizing the energization conditions, but the use of other methods is not hindered.

The effects of one embodiment of the present invention will be described more specifically by way of examples. However, the conditions in the examples are only one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.

By pressing the first end of the first steel member having the shaft portion against the second member made of steel having a joint surface larger than the cross section of the shaft portion while rotating, the second member of the shaft portion is formed. The first member and the second member are sandwiched in the axial direction of the shaft portion of the first member by using the step of forming a joint portion formed by friction welding the end portion of 1 and the joint surface and the electrode. Then, various joints were manufactured by a manufacturing method including a step of burning back the joint by energizing with the current value as I.

The test conditions were as follows. The type of the first member was the same for all joints. In the table, values outside the scope of the invention are underlined.
● Type of first member: Steel material with carbon content of 0.22% by mass and tensile strength of 1000 MPa class ● Shape of first member: Round bar with a conical shape with a tip of 0.8 mm in height (diameter is Table 2) As described in)
● Type of second member: Any of high-strength steel materials a to c having the components shown in Table 1. Shape of second member: Steel plate with a joint surface area of 2500 cm ² or more Other conditions are shown in Table. 1 or as described in Table 2. It should be noted that some of the joints were not tempered by energization, and these energization conditions are indicated by "-" in Table 2. The rest of the components listed in Table 1 were iron and impurities.

The various joints thus obtained were evaluated. Specifically, in these joints, the average value Have of the maximum hardness at both ends of the joint and the strength of the joint were measured, and the presence or absence of buckling was 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 set to the maximum load measured until the end of the second member was fixed, the first member was pulled in the direction perpendicular to the joint surface, and the first member was broken.
Regarding the judgment of the presence or absence of buckling, the diameter of the shaft part of the first member seen when the joint part is manufactured under the same conditions as those described in the table and the cross section is observed. Those that had increased from the original diameter were judged to have buckling. However, although it is preferable that buckling does not occur, buckling is permitted as long as the strength of the joint is not impaired by buckling.
The results of these evaluations are shown in Table 3. Values outside the scope of the invention are underlined. For reference, the value obtained by substituting the carbon content C of the second member into "761.6 x C ^3-1897.2 x C ² + 1848.8 x C + 148.3" is used as the effective upper limit hardness. As shown in Table 3.

The joint of No. 1 was not energized and tempered. Therefore, in the joint of No. 1, Have did not satisfy the formula 1 and sufficient joint strength could not be obtained.
The joints of Nos. 2 to 5 are those to which the same manufacturing conditions as those of No. 1 are applied except for the current value and the energization time. Of these, in the joints of Nos. 3 and 6 manufactured under the condition that the current value was too large with respect to the energization time, buckling and re-quenching of the round bar occurred. In the joint of No. 4 manufactured under the condition that the current value was too small with respect to the energization time, the Have was excessive and the joint strength could not be increased. On the other hand, the joints of No. 2 and 5 manufactured under the condition that the current value with respect to the energization time is within the appropriate range have Have satisfying the formula 1 and the joint strength is dramatically increased as compared with the joint of No. 1. Was being done.

In the manufacture of the joints of Nos. 7 to 12, the diameter of the round bar (corresponding to the diameter of the shaft portion of the first member) and the current value were different, but the other conditions were the same. Buckling and requenching occurred in the No. 7 joint manufactured under conditions where the current value was too large for the diameter of the round bar. Requenching occurred in the joints of numbers 9 and 11 manufactured under conditions where the current value was too large for the diameter of the round bar. These joints did not have high joint strength. On the other hand, in the joints of Nos. 8, 10, and 12 manufactured under the condition that the relationship between the diameter of the round bar and the current value is within an appropriate range, Have satisfied Equation 1 and had high joint strength.

In the production of the joints of Nos. 13 and 14, the pressing force was different, but the other conditions were the same. In the joint of No. 14 manufactured under the condition that the current value is too large with respect to the applied pressure, buckling of the round bar occurred. On the other hand, in the joint of No. 13 manufactured under the condition that the relationship between the pressing force and the current value is within an appropriate range, Have satisfied Equation 1 and had high joint strength. However, although buckling occurred in the joint of No. 14, the strength of the joint satisfied the pass / fail criteria. Therefore, the joint of No. 14 is also treated as an example of the invention.

No energization tempering was performed on the joints of numbers 15 and 17. Therefore, in the joints of Nos. 15 and 17, Have did not satisfy the formula 1 and sufficient joint strength could not be obtained.
The joints of Nos. 16 and 18 had the same components of the second member as the joints of Nos. 15 and 17, respectively. Since the joints of Nos. 16 and 18 were tempered by energization under appropriate conditions, Have satisfied Equation 1 and the joint strength was dramatically increased as compared with the joints of Nos. 15 and 17, respectively. ..

1 Joint joint 11 First member 111 Shaft portion 112 Head 12 Head 12 Second member 121 Joint surface 13 Third member 14 Joint portion 15 Friction welding interface X Position 0.1 mm deep from the joint surface Y Axis E Electrode

Claims (13)

A first steel member with a shaft and
A second member made of steel having a joint surface larger than the cross section of the shaft portion,
A joint portion formed by friction welding the first end portion of the first member and the joint surface.
It is a joint joint provided with
The joint portion obtained by continuously measuring the hardness of the joint portion in the cross section of the joint portion passing through the axis of the shaft portion at a depth of 0.1 mm from the joint surface over the joint portion and its periphery. A joint joint in which the average value Have of the maximum hardness at both ends satisfies Equation 1.
Have ≤ 761.6 x C ^3-1897.2 x C ² + 1848.8 x C + 148.3 ... (Equation 1)
Here, the symbol "C" in the formula 1 is the amount of carbon in the unit mass% of the second member. Claim 1 is characterized in that the second member has tempered martensite in a region within 0.1 mm in depth from the friction welding interface, which is the interface between the first member and the second member. The joint described in. The joint according to claim 1 or 2, wherein the carbon content of the second member is 0.20% by mass or more. The friction welding interface, which is the interface between the first member and the second member, has entered the second member.
Claims 1 to 1, wherein the maximum depth of the friction welding interface of the second member with respect to the joint surface measured in the cross section of the joint passing through the axis is 0.3 mm or more. The joint according to any one of 3. The joint according to any one of claims 1 to 4, wherein the second member is a steel plate having a plate thickness of 0.8 to 3.0 mm. The joint according to any one of claims 1 to 5, wherein the area of the joint surface is 3.0 times or more the cross-sectional area of the shaft portion. The cross section of the shaft portion perpendicular to the axis has a rotationally symmetric shape.
The joint according to any one of claims 1 to 6, wherein the diameter of the circumscribed circle of the cross section is 2.0 to 10.0 mm. The joint according to claim 7, wherein the cross section of the shaft portion perpendicular to the axis is a circle or a regular polygon. The first member has a head disposed at its second end.
The joint further comprises a third member provided between the head and the second member.
The shaft portion of the first member penetrates the third member.
The third member according to any one of claims 1 to 8, wherein the third member is sandwiched between the head of the first member and the joint surface of the second member. Joined joint. The joint according to claim 9, wherein the third member is a steel plate. An automobile part having the joint according to any one of claims 1 to 10. The shaft portion is formed by pressing the first end portion of the steel first member having the shaft portion against the steel second member having a joint surface larger than the cross section of the shaft portion while rotating. A step of forming a joint portion formed by friction welding the first end portion of the above and the joint surface.
Using the electrodes, the first member and the second member are sandwiched in the axial direction of the shaft portion of the first member, and the joint portion is burnt back by energizing with a current value of I. Process and
Equipped with
A method for manufacturing a joint joint in which the current value I is within the range satisfying the following formula 2.

Here, the units of Imin, Imax1, and Imax2 defined by Equations 3, 4, and 5 are kA.
In formulas 3, 4, and 5, the symbol "D ₁ " is the diameter of the circumscribed circle of the cross section of the shaft portion in mm, and the symbol "D ₂ " is the hollow portion of the shaft portion. It is the diameter of the circumscribed circle of the cross section in the unit mm, and when the shaft portion is solid, D ₂ = 0, and the symbol "t" is the energization time in the unit second, and the symbol "t". "F" is the pressing force in the unit kN.
In the formula 6, the symbols "C", "Si", "Mn" and "Cr" are the C content, Si content, Mn content and Cr content in the unit mass% of the second member, respectively. be. The method of manufacturing the joint is further described before the formation of the joint.
The step of superimposing the third member on the second member and
A step of allowing the third member to penetrate the shaft portion of the first member, and
Equipped with
The method for manufacturing a joint according to claim 12, wherein the first member has a head at a second end thereof.

Images