Steel material joined body

Abstract

To provide a steel material joined body capable of effectively improving joint strength of steel materials and improving abrasion resistance of an outer peripheral surface in the vicinity of a joint part.SOLUTION: In order to solve the above problem, a steel material joined body is obtained by joining a plurality of steel materials. A carbon concentration of a joint interface of the steel materials is equal to or greater than 0.20 mass% and equal to or less than 2.10 mass%. The steel material joined body has a concentration inclination layer in which a carbon concentration decreases as separating from the joint interface. The joint interface is formed by perlite.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel joint. [Background technology]

[0002] To date, a hot joining method for steel materials has been disclosed in which a carbonaceous substance is applied or sprayed onto the joining surfaces, hot steel materials are overlapped or butted together, and then heated and pressure-welded in a reducing atmosphere, with the aim of developing a technology that can join hot steel materials simply and efficiently in an actual factory and that can obtain a high level of joining strength that does not interfere with the subsequent rolling process (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-7970 Summary of the Invention [Problem to be solved by the invention]

[0004] However, it cannot be said that the joined steel material obtained by the technique described in Patent Document 1 effectively improves the joining strength between the steel materials. Therefore, an object of the present invention is to provide a joined steel material that can effectively improve the joining strength between the steel materials and also improve the wear resistance of the outer peripheral surface near the joint. [Means for solving the problem]

[0005] The steel joined body according to the present invention is a steel joined body in which a plurality of steel materials are joined together, characterized in that the carbon concentration at the joining interface between the steel materials is 0.20 mass% or more and 2.10 mass% or less, the joining interface has a concentration gradient layer in which the carbon concentration decreases with increasing distance from the joining interface, and the joining interface is composed of pearlite.

[0006] In the joined steel member according to the present invention, it is preferable that the carbon concentration of the concentration gradient layer continuously decreases with increasing distance from the joining interface. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a joined steel material body that can effectively improve the joining strength between steel materials and also improve the wear resistance of the outer peripheral surface near the joint. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a conceptual diagram for explaining a steel joint according to an embodiment of the present invention. FIG. [Figure 2] FIG. 1 is a phase diagram of an iron-cementite system for explaining the effect of the present invention. [Figure 3] FIG. 1 is a conceptual diagram for explaining the effect of the present invention. [Figure 4] FIG. 2 is a conceptual diagram illustrating a location where the metal structure of a steel welded body is confirmed. [Figure 5] 5A and 5B are metallographic images of the test specimen (welded steel member) produced in Example 3 at points A, B, and C on the L cross section shown in FIG. 4B. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0010] FIG. 1 is a conceptual diagram for explaining a steel member joint according to this embodiment. As shown in Fig. 1, the joined steel product 1 according to this embodiment is formed by joining a plurality of steel materials 10, 20 together. The carbon concentration (carbon concentration A shown in Fig. 1) at a joining interface 30 (the shaded area in Fig. 1) where the plurality of steel materials 10, 20 (hereinafter also referred to as "materials") are joined together is 0.20 mass% or more and 2.10 mass% or less. Furthermore, the joined steel product 1 has concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joining interface 30.

[0011] In the steel material joint 1 according to this embodiment, the carbon concentration at the joint interface 30 is 0.20 mass% or more and 2.10 mass% or less, and therefore the joint strength between the steel materials 10 and 20 can be effectively improved, and the wear resistance of the outer peripheral surface in the vicinity of the joint 31 (within the range in which the concentration gradient layers 15 and 25 are formed) can also be improved. Specifically, since the carbon concentration at the bonded interface 30 is 2.10 mass% or less, crystallization of the solidification structure at the bonded interface 30 is suppressed. Therefore, since it is possible to suppress crystallization of a hard and brittle solidification structure at the bonded interface 30, it is possible to effectively improve the bond strength between the steel materials 10 and 20. Furthermore, since the carbon concentration at the bonded interface 30 is 0.20 mass% or more, it is possible to increase the hardness of the bonded interface 30. Therefore, it is possible to improve the wear resistance of the outer peripheral surface of the bonded area 31.

[0012] The carbon concentration is preferably 0.20 mass% or more and 0.90 mass% or less. By setting the carbon concentration of the bonding interface 30 to 0.20 mass% or more and 0.90 mass% or less, the bonding strength between the steel materials 10, 20 can be more effectively improved. Specifically, precipitation of cementite at the austenite grain boundaries is suppressed by setting the carbon concentration at the bonded interface 30 to 0.90 mass% or less. Therefore, since precipitation of cementite at the hard and brittle austenite grain boundaries at the bonded interface 30 can be suppressed, the bond strength between the steel materials 10 and 20 can be more effectively improved.

[0013] The carbon concentration at the bonded interface 30 in the joined steel member 1 according to this embodiment can be measured by cutting the joined steel member 1 along the bonded interface 30, polishing the cross section, and measuring using an element distribution measuring device such as an electron probe microanalyzer (EPMA) or an energy dispersive X-ray analyzer (EDX). In the present invention, the carbon concentration at the bonded interface 30 is calculated by measuring any five points on the polished cross section and averaging the measured values.

[0014] The joined steel member 1 according to this embodiment also has concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joint interface 30. Therefore, the carbon concentration is high at the joint interface 30 (see carbon concentration A in FIG. 1 ), which effectively improves the joint strength between the steel members 10, 20. Furthermore, the carbon concentration (carbon concentration B in FIG. 1 ) on the side of the concentration gradient layers 15, 25 opposite the joint interface 30 (the side of the steel members (materials 10, 20) before joining) is lower than that at the joint interface 30, which allows the materials 10, 20 to exhibit "elongation." Therefore, the joined steel member 1 according to this embodiment can be suitably used in applications where "elongation" is required on the materials 10, 20 side.

[0015] As shown in FIG. 1, the concentration gradient layers 15 and 25 preferably have a carbon concentration that decreases continuously with increasing distance from the bonded interface 30 . In the present invention, "continuously decreasing" means that the carbon concentration decreases proportionally from the bonded interface 30 toward the plurality of steel materials (raw materials) 10, 20, as shown in FIG. Since the concentration gradient layers 15, 25 have such a decreasing tendency, in addition to the above-mentioned effects, the ductility and toughness at the bonding interface 30 are also improved.

[0016] The carbon concentration of the concentration gradient layers 15, 25 in the steel joined body 1 according to this embodiment can be measured by cutting the steel joined body 1 along the joining interface 30, then further cutting it in a direction away from the surface of the cut joining interface 30 (towards the raw materials 10, 20), polishing the cross section cut in the direction away, and using an element distribution measuring device such as an electron probe microanalyzer (EPMA) or an energy dispersive X-ray analyzer (EDX). In the present invention, the decreasing tendency of the concentration gradient layers 15, 25 can be measured by measuring the carbon concentration at any five points (10 points in total) on a line from the carbon concentration at the bonded interface 30 to the carbon concentration at the material 10, 20 in each direction from the bonded interface 30 to the material 10, 20 on a polished cross section in the separating direction using the element distribution measuring device as described above, and creating a graph as shown in Figure 1 in which the carbon concentration is plotted against the distance, with the distance from the bonded interface 30 on the horizontal axis and the carbon concentration on the vertical axis.

[0017] Furthermore, although the joined steel member 1 according to this embodiment does not depend on the microstructural form of the joint interface 30, it is preferable that the joint interface 30 be composed of pearlite. This pearlite can be obtained by air-cooling or slow-cooling the joined steel member 1 in an austenitic state. When the joined interface 30 is composed of pearlite, the tensile strength and bending strength are increased, and therefore the joint strength of the joined interface 30 can be more effectively improved. The microstructural form of the joint interface 30 can be confirmed by cutting the joined steel member 1 along the joint interface 30, polishing the cut cross section, and then etching the polished cross section with nital using an optical microscope.

[0018] It is preferable that the bonded interface 30 does not have cementite at the austenite grain boundaries. If grain boundary cementite is present, it may become the starting point for cracks to occur during tension and bending. The above-mentioned "not present" does not mean that there is no grain boundary cementite at all, but means that the abundance ratio of grain boundary cementite at the bonded interface 30 is less than 10%. Here, the abundance ratio of grain boundary cementite at the bonded interface 30 is confirmed by subjecting the cross section to nital etching and then using a point counting method in accordance with JIS G0555 on the cross section.

[0019] In the steel joint 1 according to this embodiment, the materials of the steel materials (raw materials) 10, 20 to be joined are not particularly limited as long as they are any steel material and can be integrated with each other. Furthermore, the alloying elements other than carbon in the joint interface 30 and the multiple steel materials (raw materials) 10, 20 before joining are not particularly limited. For example, as specified in JIS G 4051, the steel materials (raw materials) 10, 20 have a composition containing approximately 1.50 mass% or less of Si, 1.00 mass% or less of Mn, and the balance being Fe and unavoidable impurities. The shapes of the multiple steel materials (raw materials) 10, 20 before joining are not particularly limited as long as they each have a joining surface and can be integrated with each other by overlapping these joining surfaces. The steel materials (raw materials) 10, 20 can have, for example, a cylindrical shape, a prismatic shape, a threaded shape, a concave-convex shape, or the like.

[0020] As shown in Fig. 1, another steel joined body 1A according to this embodiment is formed by joining a plurality of steel materials 10, 20 made of medium carbon steel together. The carbon concentration (carbon concentration A shown in Fig. 1) at a joining interface 30 (the shaded area in Fig. 1) where the plurality of steel materials 10, 20 made of medium carbon steel are joined together is 0.50 mass% or more and 2.10 mass% or less. Furthermore, the joined body has concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joining interface 30. That is, the joined steel member 1A has a different range of carbon concentration from the joined steel member 1.

[0021] In this way, when medium carbon steel is used as the multiple steel materials (raw materials) before joining, setting the carbon concentration to 0.50 mass% or more and 2.10 mass% or less is advantageous in achieving the object of the present invention. Specifically, since the carbon concentration at the joining interface 30 is 2.10 mass% or less, crystallization of the solidification structure at the joining interface 30 is suppressed. Therefore, since crystallization of a hard and brittle solidification structure can be suppressed at the joining interface 30, the joining strength between the steel materials 10 and 20 can be effectively improved. Furthermore, since the carbon concentration at the bonded interface 30 is 0.50 mass % or more, it is possible to increase the hardness of the bonded interface 30. Therefore, it is possible to improve the wear resistance of the peripheral surface of the bonded portion vicinity 31.

[0022] Furthermore, the carbon concentration in the joined steel body 1A according to this embodiment is preferably 0.50 mass% or more and 0.90 mass% or less. By setting the carbon concentration at the joining interface 30 to 0.50 mass% or more and 0.90 mass% or less, the joining strength between the steel materials 10 and 20 can be more effectively improved. Specifically, precipitation of cementite at the austenite grain boundaries is suppressed by setting the carbon concentration at the bonded interface 30 to 0.90 mass% or less. Therefore, since precipitation of cementite at the hard and brittle austenite grain boundaries at the bonded interface 30 can be suppressed, the bond strength between the steel materials 10 and 20 can be more effectively improved.

[0023] Another joined steel member 1A according to this embodiment has concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joint interface 30. Therefore, the carbon concentration is high at the joint interface 30 (see carbon concentration A in FIG. 1 ), which effectively improves the joint strength between the steel members 10, 20. Furthermore, the carbon concentration (carbon concentration B in FIG. 1 ) on the side of the concentration gradient layers 15, 25 opposite the joint interface 30 (the side of the steel members (materials 10, 20) before joining) is lower than that at the joint interface 30, which allows the materials 10, 20 to exhibit "elongation." Therefore, the joined steel member 1A according to this embodiment can be suitably used in applications where "elongation" is required on the materials 10, 20 side.

[0024] Similarly, in the other joined steel member 1A according to this embodiment, the concentration gradient layers 15, 25 preferably have a carbon concentration that decreases continuously with increasing distance from the joint interface 30, as shown in FIG. Since the concentration gradient layers 15, 25 have such a decreasing tendency, in addition to the above-mentioned effects, the ductility and toughness at the bonding interface 30 are also improved. Incidentally, the carbon concentrations of the bonded interface 30 and the concentration gradient layers 15 and 25 can be measured under the same conditions and by the same method as those for the bonded steel member 1 according to the present embodiment described above.

[0025] In the present invention, medium carbon steel refers to a steel material having a carbon concentration of 0.30 mass% or more and 0.50 mass% or less. For reference, low carbon steel refers to a steel material having a carbon concentration of less than 0.30 mass%, and high carbon steel refers to a steel material having a carbon concentration of more than 0.50 mass%. The metal structure of the joining interface 30 of the joined steel material 1A according to this embodiment is the same as that of the joined steel material 1 according to the present embodiment described above, and therefore a description thereof will be omitted here.

[0026] Hereinafter, the mechanism by which the joining strength between steel materials is increased in the joined steel materials 1, 1A according to this embodiment will be described with reference to the drawings. Fig. 2 is a phase diagram of an iron-cementite system for explaining the effects of the present invention. Fig. 3 is a conceptual diagram for explaining the effects of the present invention, specifically a conceptual diagram showing the reaction occurring at the joining interface. A method for producing the joined steel products 1 and 1A according to this embodiment involves placing carbon powder (carbonaceous material) on at least one joining surface of the steel materials to be joined, and heating the steel materials, with the joining surfaces of the steel materials to be joined via the carbonaceous material, in a predetermined atmosphere (for example, in the air atmosphere) at a maximum temperature of 1150°C to 1500°C (preferably 1150°C to 1300°C) (see Fig. 3(a)).

[0027] When the temperature of the joining surface of steel materials reaches, for example, 1250°C, a liquid phase L with a carbon concentration of 3.5 mass% is generated at the interface between the steel material and the carbonaceous material (see the area indicated by square in Figure 2). This liquid phase L increases until the carbonaceous material disappears (see Figure 3(b)).

[0028] At 1250°C, the carbon concentration at the interface between the "austenite γ" region and the "austenite γ + liquid L" region (see the area indicated by the circle in Figure 2) is 1.6 mass% (see Figure 2). Carbon diffusion is extremely fast at 1250°C, and when maintained at this temperature, carbon diffuses rapidly from the joining surface of the steel into the internal austenite γ. As a result, at the interface between the liquid L and austenite γ, the austenite γ side steals carbon from the liquid L side in an attempt to maintain its carbon concentration at 1.6 mass%. Meanwhile, the liquid L decreases in an attempt to maintain its carbon concentration at 3.5 mass% (see Figure 3(c)). Eventually, the liquid L disappears, and the joining of the steels is completed (see Figure 3(d)).

[0029] Note that the carbon concentration at the joining interface may be high immediately after the liquid phase L disappears. To suppress the precipitation of grain boundary cementite, the carbon concentration at the joining interface in the joined steel material 1 must be reduced to 0.20 mass% or more and 0.90 mass% or less. The carbon concentration at the joining interface in the joined steel material 1A must be reduced to 0.50 mass% or more and 0.90 mass% or less. The reduction in the carbon concentration at the joining interface can be controlled by extending the heating time at the maximum temperature.

[0030] The carbonaceous material referred to here is not particularly limited in material or shape as long as it is placed on at least one of the joining surfaces of the steel materials to be joined and can integrate the joining surfaces. For example, the carbonaceous material can be a powder of graphite particles (carbon powder) with an average particle size of 1 μm. [Example]

[0031] In Example 1, two steel materials (low-carbon steel) having a carbon concentration of 0.045 mass% and a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 were prepared. These two steel materials were then joined together to produce a steel joined body as shown by reference numeral 1 in FIG. 1 as a test specimen. Carbon powder (carbonaceous material) was placed on the joining surface of each of the two steel materials, and the joining surfaces of the steel materials to be joined via the carbonaceous material were placed on top of each other. Next, high-frequency induction heating was performed in an air atmosphere at a maximum temperature of 1250°C, followed by slow cooling. The mass of the carbon powder placed on the joining surface of each of the two steel materials was adjusted so that a reaction could occur at the joining interface as shown in FIGS. 2 and 3 . The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 0.20 mass% by controlling the heating maintenance time at the maximum temperature.

[0032] In Example 1, the test specimens (steel joints) prepared were subjected to the following "verification of joint strength and abrasion resistance." (Confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength and abrasion resistance) The fabricated specimens (steel joints) were examined for the presence or absence of a concentration gradient layer (a concentration gradient layer in which the carbon concentration decreases with increasing distance from the joint interface), the main microstructure of the joint interface, joint strength (high, low), and wear resistance (high, low). The presence or absence of a concentration gradient layer was determined by plotting the carbon concentration against the distance as shown in Figure 1 and determining whether or not there was a decreasing trend in carbon concentration. The main microstructure (metallographic structure) of the joint interface was confirmed by observing selected locations of the joint interface of the fabricated steel joints under an optical microscope after etching with nital. Figure 4 shows an illustration of the locations where the metallographic structure was examined. The steel joint shown in Figure 4(b) was obtained by cutting the steel joint at an L-section as shown in Figure 4(a). Metallographic images of locations A, B, and C on this L-section were processed to confirm the presence or absence of metallographic structures (e.g., pearlite) in each image. The joint strength was evaluated by measuring tensile strength. The tensile strength was tested using JIS No. 9A (G.L 100 mm). Abrasion resistance was evaluated using an abrasion tester, adjusting the speed and final load in a dry environment, and using a grinding wheel made of cubic silicon nitride with grit number 400 as the mating material, based on the specific wear rate of the outer periphery of the joint. Table 1 below shows the results of the above confirmation items. [Example]

[0033] In Example 2, two steel materials (medium carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.450 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 0.50 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0034] In Example 2, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as in Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Example 2 together with the confirmation results of Example 1. [Example]

[0035] In Example 3, two steel materials (medium carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.450 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 0.90 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0036] In Example 3, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as in Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Example 3 together with the confirmation results of Example 1. For reference, FIG. 5 shows metallographic images of the specimen (welded steel member) produced in Example 3 at points A, B, and C on the L cross section shown in FIG. 4(b). [Example]

[0037] In Example 4, two steel materials (medium carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.450 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 1.30 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0038] In Example 4, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as in Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Example 4 together with the confirmation results of Example 1. [Example]

[0039] In Example 5, two steel materials (medium carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.450 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 1.70 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0040] In Example 5, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as in Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Example 5 together with the confirmation results of Example 1. [Example]

[0041] In Example 6, two steel materials (medium carbon steel) with a carbon concentration of 0.450 mass% and a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 2.10 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0042] In Example 6, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Example 6 together with the confirmation results of Example 1. Comparative Example

[0043] [Comparative Example 1] In Comparative Example 1, two steel materials (low carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.045 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 0.10 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0044] In Comparative Example 1, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Comparative Example 1 together with the confirmation results of Example 1.

[0045] Comparative Example 2 In Comparative Example 2, two steel materials (medium carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joining surface)) as shown in FIG. 1 and a carbon concentration of 0.450 mass% were prepared. The carbon concentration at the joining interface after high-frequency induction heating was adjusted to 2.30 mass% by controlling the heating maintenance time at the maximum temperature. Except for this, test specimens (joined steel materials) were produced under the same conditions and by the same method as in Example 1.

[0046] In Comparative Example 2, similarly to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, the same conditions and methods as Example 1 were used in this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 below shows the confirmation results of Comparative Example 2 together with the confirmation results of Example 1.

[0047] [Table 1]

[0048] (Results and Discussion) As can be seen from the results in Table 1, when the carbon concentration at the bonded interface of the test specimens (steel bonded bodies) was 0.20 mass% or more and 2.10 mass% or less (Examples 1 to 6), it was confirmed that the bond strength and wear resistance were high. On the other hand, in Comparative Example 1, the carbon concentration at the bonded interface was low, so it was confirmed that the wear resistance was reduced. Furthermore, in Comparative Example 2, crystallization of a solidification structure was confirmed as the structure at the bonded interface, so it was confirmed that the bond strength was reduced.

[0049] Furthermore, in the joined steel materials of Examples 4 to 6, in addition to pearlite, cementite (including cementite at austenite grain boundaries) was confirmed as the main structure of the joint interface. On the other hand, in the joined steel materials of Examples 1 to 3, pearlite was confirmed as the main structure of the joint interface, but cementite was not. Therefore, it is believed that the joined steel materials of Examples 1 to 3, in which hard and brittle cementite was not confirmed (when the carbon concentration at the joint interface was 0.20 mass% or more and 0.90 mass% or less), have higher joint strength and wear resistance. Furthermore, based on these confirmed results, when medium carbon steel is used as the steel material to be joined, it is believed that a carbon concentration at the joint interface of 0.50 mass% or more and 0.90 mass% or less is more advantageous in achieving the object of the present invention. [Explanation of symbols]

[0050] 1, 1A steel joint 10 Steel (material) 15 Concentration gradient layer 20 Steel 25 Concentration gradient layer 30 Joint interface 31 Joint A Carbon concentration (joint interface) B Carbon concentration (opposite side of the concentration gradient layer from the bonding interface) X length (steel) φ Diameter (steel material) γ austenite phase L liquid phase

Claims

1. A steel material joint in which a plurality of steel materials are joined together, The carbon concentration at the joining interface between the steel materials is 0.20 mass% or more and 2.10 mass% or less, a concentration gradient layer in which the carbon concentration decreases with increasing distance from the bonding interface; The joining interface of the steel members is made of pearlite.

2. 2. The joined steel member according to claim 1, wherein the carbon concentration in the concentration gradient layer decreases continuously with increasing distance from the joint interface.

Images