Method for joining steel member and method for manufacturing steel product

Abstract

To provide a steel member joining method for more efficiently and more easily joining steel members: and to provide a steel product manufacturing method using the joining method.SOLUTION: A method for joining a steel member comprises: subjecting a face to be joined of the steel member to a carburization treatment at a temperature of 500°C or higher and lower than 850°C; then cooling the steel member; and joining the steel member to a joining member. A method for manufacturing a steel product by joining a steel member and a joining member comprises: subjecting a face to be joined of the steel member to a carburization treatment at a temperature of 500°C or higher and lower than 850°C; then cooling the steel member; and joining the steel member to the joining member.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a method for joining steel members and a method for manufacturing a steel product. [Background technology]

[0002] A method for joining steel members to joining members has been used in which a material with a lower melting point than both members is applied to the joining area, but this method can be time-consuming depending on the shape of each member, and can also easily cause poor adhesion and peeling at the interface.

[0003] Patent Document 1 discloses a method in which the joining surfaces of one or both steel members are carburized, and then the joining surfaces of both steel members are directly joined by liquid phase diffusion bonding. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2008-279479 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the method described in Patent Document 1, the carburizing treatment of the joining surfaces of the steel members is performed at a high temperature of 850 to 990°C, so that the carbon concentration on the steel member surfaces may not increase sufficiently, and it may be necessary to set the heating temperature during joining higher. In addition, since both the carburizing temperature and the heating temperature during joining are high, the working time may be long, and the working efficiency may be reduced.

[0006] The present disclosure has been made in consideration of such problems, and aims to provide a method for joining steel members to join steel members more efficiently and more easily, and a method for manufacturing a steel product using the joining method. [Means for solving the problem]

[0007] One embodiment for achieving the above object is a method for joining steel members, which comprises carburizing the surfaces to be joined of the steel members at a temperature of 500° C. or higher and lower than 850° C., cooling the surfaces, and joining the steel members to a joining member. The cooling method is preferably slow cooling or air cooling. Furthermore, the thickness from the surface of the joining surface of the steel member, where the carbon concentration in the joining surface is increased by the carburization treatment compared to before the carburization treatment, may be 0.02 mm or more and 0.20 mm or less. The carbon concentration in the thickness from the surface of the bonded surface may be 1.00 mass % or more.

[0008] One aspect for achieving the above object is a method for manufacturing a steel product by joining a steel member and a joining member, the method comprising the steps of: subjecting a joining surface of the steel member to a carburizing treatment at a temperature of 500°C or higher and lower than 850°C; thereafter, cooling the steel member; and joining the joining member.

[0009] In the joining method and manufacturing method according to the present disclosure, the carburizing temperature of the joining surfaces of the steel members is between 500° C. and 850° C., which is lower than the usual carburizing temperature. This makes it possible to more efficiently increase the carbon concentration only in the surface portions that will become the joining surfaces of the steel members during joining, and to further lower the melting temperature of the surface, which allows the heating temperature during joining to be lowered and shortens the working time. Effect of the Invention

[0010] According to the present disclosure, it is possible to provide a method for joining steel members more efficiently and more simply, and a method for manufacturing a steel product using the joining method. [Brief description of the drawings]

[0011] [Figure 1] 4 is a diagram for explaining the relationship between the melting temperature of each portion in the thickness direction of a steel member after carburizing treatment and the heating temperature during joining in this embodiment. FIG. [Diagram 2] 1A and 1B are graphs for explaining the difference between a general carburizing process and the carburizing process of this embodiment, in which (a) is a graph showing the relationship between temperature and time during carburizing process and cooling, and (b) is a graph showing the relationship between the distance from the joined surface after carburizing process and the carbon concentration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] As described above, when joining steel members together or a steel member to another joining member, a material having a lower melting point than those members (e.g., solder) is applied to the joining points of each member and dried, and then both members are joined by heating and cooling, etc. However, it is necessary to use a material other than the steel members and the joining members for joining, and furthermore, the number of steps is required for applying and drying the low melting point material, which can be burdensome in terms of cost and work efficiency.In addition, since different materials are used for both members, the low melting point material may not penetrate to the joining part if the wetting is poor, or if both members are dissimilar metals, the difference in thermal expansion coefficients may cause stress to be applied to the interface during use, causing the interface to peel off.

[0013] Therefore, in the method described in Patent Document 1, the joining surfaces of the steel members are carburized to increase the carbon concentration of the joining surfaces, thereby lowering the melting temperature of the surface portions, and the joining surfaces of both steel members are directly joined by liquid phase diffusion bonding.

[0014] However, in normal carburizing treatments such as the method described in Patent Document 1, carbon is added for the purpose of hardening the surface portion (surface layer), so the carburizing treatment is carried out at a high temperature (e.g., 850-950°C) of 850°C or higher and 990°C or lower, and then quenched. In this way, in normal carburizing treatments, as shown by symbol A in Fig. 2(b) (where symbol A represents a general carburizing treatment), the carburizing treatment is carried out so that the carbon concentration (mass%) is high in the surface portion of the component and gradually decreases from there toward the inside. On the other hand, in the method for joining steel members of the present disclosure (hereinafter also referred to as the present joining method), carburization is performed at a temperature lower than that of normal carburization, that is, at a temperature of 500°C or higher and lower than 850°C, and then cooled, so that the surface of the steel member has a higher carbon concentration than that of general carburization process A, as shown by symbol B in FIG. 2(b) (where symbol B represents the carburization process in the present disclosure). Furthermore, in the carburization process B of the present disclosure, only the thickness portion that is required to be melted during joining has a high carbon concentration, and from there toward the inside, the carbon concentration decreases at a steeper rate than that of general carburization process A. Therefore, when joining the steel member and the joining member, it is possible to melt only the surface portion required for joining, and it becomes possible to join the steel members more efficiently and more easily. FIG. 2 is a graph for explaining the difference between a general carburizing process and the carburizing process of this embodiment, where (a) is a graph showing the relationship between temperature and time during carburizing process and cooling, and (b) is a graph showing the relationship between the distance from the joined surface after carburizing process and the carbon concentration.

[0015] Hereinafter, the present embodiment will be described in detail with reference to the drawings. However, the present disclosure is not limited to the following embodiment. In addition, the following description and drawings are appropriately simplified for clarity of explanation.

[0016] In this joining method, the surfaces to be joined of the steel members are carburized at a temperature of 500°C or higher but lower than 850°C, then cooled and joined to the joining member. As the carburizing method, a conventionally known method can be appropriately used, for example, a vacuum carburizing method can be applied, but the carburizing temperature is set to 500°C or higher and less than 850°C, which is lower than the normal carburizing temperature. If the carburizing temperature is 500°C or higher, the carburizing process can be completed in a reasonable time. From the same viewpoint, the carburizing temperature is preferably 550°C or higher, more preferably 600°C or higher. Also, if the carburizing temperature is less than 850°C, only the surface portion that is required to be melted (melted) during joining can be selectively carburized (the carbon concentration can be increased). As a result, not only can the joining of the steel member and the joining member be made efficient, but the unnecessary internal portion is not carburized, so that the occurrence of distortion due to stress caused by that portion can be prevented. From the same viewpoint, the carburizing temperature is preferably 840°C or lower, more preferably 830°C or lower, and even more preferably 800°C or lower.

[0017] The shape and specific composition of the steel member are not particularly limited, and conventionally known steel members can be used as appropriate. For example, when S25C (steel with a carbon concentration of 0.25% by mass) is used as the steel member and the carburizing treatment of the present disclosure is performed, the carbon concentration can be made 1.00% by mass or more only in the surface portion of the steel (specifically, for example, a portion 0.20 mm thick from the surface). In addition, this carburizing treatment can lower the melting temperature of the surface portion of the steel by 150°C or more compared to the melting temperature of the other portions.

[0018] FIG. 1 is a diagram for explaining the relationship between the melting temperature of each part in the thickness direction of the steel member after carburization in this embodiment and the heating temperature during joining. In FIG. 1, the symbol X indicates the heating temperature during joining of the steel member and the joining member, and the symbol Y indicates the melting temperature of each part of the steel member. In the carburized steel member 1, the carbon concentration increases toward the surface of the steel member in the carburized layer 2, so that the melting temperature decreases as shown in FIG. 1. Here, the symbol X1 indicates a part that does not melt when heated during joining because the melting temperature is higher than the heating temperature when heating to join the steel member and the joining member. On the other hand, the symbol X2 indicates a part that melts when heated during joining because the melting temperature is lower than the heating temperature when heating to join the steel member and the joining member. Furthermore, as shown in FIG. 2(b), in the carburizing process B of the present disclosure, the carbon concentration can be made higher only in the surface portion of the steel member compared to the general carburizing process A which uses a higher carburizing process temperature. As a result, the heating temperature (symbol X) when joining the two members can be set lower, improving work efficiency.

[0019] In this manner, in the carburizing treatment of the present disclosure, only the surface portions of the steel members that are required to be dissolved during joining can be selectively carburized, so that the amount of gas (e.g., acetylene gas) used in the carburizing treatment can be reduced. Also, the carburizing treatment time (e.g., gas injection time) can be shortened, which ultimately shortens the work time required to join the steel members and the joining members.

[0020] The thickness from the surface of the joining surface of the steel member where the carbon concentration is increased by the carburizing treatment compared to before the carburizing treatment (thickness of the carburized layer 2) can be set appropriately, but can be 0.02 mm or more and 0.20 mm or less. This thick portion can be melted (fused) by heating during joining to join the joining member. The carburizing treatment in the present disclosure can carburize only the thickness required for joining, and the thickness of the carburized layer 2 can be made thinner than in normal carburizing treatment which is performed at a higher temperature. The carbon concentration in the thickness from the surface of the joining surface can be, for example, 1.00 mass% or more.

[0021] The type, shape, specific composition, etc. of the joining member to be joined to the steel members are not particularly limited. For example, steel members having the same or different compositions may be used to join the steel members together, or a member completely different from the steel members (for example, a member containing a dissimilar metal) may be used. Furthermore, when joining steel members together, the carburizing treatment according to the present disclosure may be applied to only the surface of one of the steel members, or the carburizing treatment according to the present disclosure may be applied to the surfaces of both steel members.

[0022] In FIG. 2(a), A1 denotes quenching (quenching) using oil or water, B1 denotes air cooling, and B2 denotes slow cooling (furnace cooling). In this way, in the general carburizing process A, after the carburizing process is performed at a high temperature of 850 to 990°C, the steel member is rapidly cooled (quenching A1) to harden it. However, in the carburizing process B of the present disclosure, the carburizing process is performed at a low temperature of 500°C or more and less than 850°C to further increase the carbon concentration of the surface and improve the joining quality. Thereafter, it is preferable to cool the steel by air cooling B1 or slow cooling B2 (furnace cooling) so that the martensitic transformation does not occur. In this way, by cooling after the carburizing process by slow cooling or air cooling, the deformation of the steel member after the carburizing process can be reduced. In addition, from the viewpoint of maintaining the carbon concentration of the steel member surface, it is preferable to cool the carburizing process in a carburizing atmosphere.

[0023] The heating temperature when joining the carburized and cooled steel members to the joining member can be set appropriately. However, in the carburization process according to the present disclosure, the surface portion of the steel member can be selectively carburized (the carbon concentration can be increased), so that joining can be performed at a temperature lower than the heating temperature during normal joining. For example, the heating temperature during joining can be 1350°C or lower, 1300°C or lower, 1250°C or lower, 1200°C or lower, or 1150°C or lower. The cooling temperature during joining can also be set appropriately.

[0024] From the above, the present bonding method can include the following steps. A process in which the surfaces of the steel components to be joined are carburized at a temperature of 500°C or higher but lower than 850°C (carburizing process). A step of cooling the carburized steel component (for example, by slow cooling or air cooling) (cooling step). A process of joining the carburized and cooled steel member and the joining member by heating and cooling (joining process).

[0025] These steps can be performed in sequence. Furthermore, the present joining method can also include other steps used in conventionally known carburizing methods and joining methods, as long as the problem of the present disclosure can be solved.

[0026] The manufacturing method of a steel product according to the present disclosure (hereinafter also referred to as the present manufacturing method) is a method for manufacturing a steel product by joining a steel member and a joining member, in which the joining surface of the steel member is carburized at a temperature of 500°C or higher and lower than 850°C, cooled, and joined to the joining member. In this manner, in the present manufacturing method, the steel member and the joining member can be joined using the above-mentioned present joining method. Note that the present manufacturing method can be applied to any steel product having a structure in which a steel member is joined to another member.

[0027] In this way, the present joining method and manufacturing method can join steel members more efficiently and more simply. The present joining method and manufacturing method can be applied to a variety of applications, such as various parts that use steel members, vehicles such as automobiles, and aircraft. The present disclosure is not limited to the above-described embodiment, and can be modified as appropriate without departing from the spirit and scope of the present disclosure. [Explanation of symbols]

[0028] 1. Carburized steel components 2 Carburized layer X Heating temperature during bonding X1: Part that does not melt when heated during bonding X2: The part that melts when heated during joining Y Melting temperature of each part A General carburizing treatment A1 Hardening B. Carburizing treatment in this disclosure B1 Air Cooled B2 slow cooling

Claims

1. A method for joining steel members, comprising carburizing a joining surface of the steel member at a temperature of 500°C or higher and lower than 850°C, cooling the steel member, and joining the steel member to a joining member.

2. The method for joining steel members according to claim 1 , wherein the cooling method is slow cooling or air cooling.

3. 2. The method for joining steel members according to claim 1, wherein the carbon concentration in the joining surfaces of the steel members is increased by the carburizing treatment compared to before the carburizing treatment, and the thickness of the joining surfaces from the surfaces is 0.02 mm or more and 0.20 mm or less.

4. The method for joining steel members according to claim 3 , wherein a carbon concentration in a thickness direction from the surface of the joined surfaces is 1.00 mass % or more.

5. A method for manufacturing a steel product by joining a steel member and a joining member, comprising: The method for manufacturing a steel product includes subjecting the joining surfaces of the steel members to a carburizing treatment at a temperature of 500° C. or higher and lower than 850° C., cooling the steel members, and joining the joining members to the joining members.

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