Steel joint

Abstract

To provide a steel material joint body that is capable of effectively improving the bonding strength between steel materials, and capable of improving also wear resistance of an outer peripheral surface in the vicinity of a joint part.SOLUTION: In a steel material joint body 1, a plurality of steel materials 10, 20 are joined to each other, and a carbon concentration of a joint interface 30 between the steel materials 10, 20 is 0.20 mass% to 2.10 mass%. Further, the steel material joint body 1 includes concentration gradients in which the carbon concentration decreases when alienating from the joint interface 30.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] \] Conventionally, there has been a problem of developing a technique that can easily and efficiently join hot-rolled steel materials in an actual factory and obtain a high joint strength that does not hinder subsequent rolling processes. Thus, a hot joining method for steel materials has been disclosed in which a carbonaceous substance is applied or scattered on the joint surface, the hot-rolled steel materials are overlapped or butted, and then heated and pressure-welded in a reducing atmosphere (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, the joint strength of the steel material joints obtained by the technique described in Patent Document 1 cannot be said to have been effectively improved. Therefore, an object of the present invention is to provide a steel material joint that can effectively improve the joint strength between steel materials and also improve the wear resistance of the outer peripheral surface near the joint portion.

Means for Solving the Problems

[0005] The steel material joint according to the present invention is a steel material joint in which a plurality of steel materials are joined, and the carbon concentration at the joint interface between the steel materials is 0.20 mass% or more and 2.10 mass% or less, and it is characterized by having a concentration gradient layer in which the carbon concentration decreases as the distance from the joint interface increases.

[0006] In addition, the steel material joint according to the present invention is a steel material joint in which a plurality of steel materials, which are medium carbon steels, are joined to each other. The carbon concentration at the joint interface between the steel materials is 0.50 mass% or more and 2.10 mass% or less, and it has a concentration gradient layer in which the carbon concentration decreases as the distance from the joint interface increases.

Advantages of the Invention

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

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

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

[0010] FIG. 1 is a conceptual diagram for explaining the steel material joint according to the present embodiment. As shown in Fig. 1, the steel material joint 1 according to this embodiment has a plurality of steel materials 10 and 20 joined together. Further, the carbon concentration (carbon concentration A shown in Fig. 1) at the joint interface 30 (the hatched portion in Fig. 1) where a plurality of steel materials 10 and 20 (hereinafter also referred to as "materials") are joined together is 0.20 mass% or more and 2.10 mass% or less. Furthermore, it has concentration gradient layers 15 and 25 in which the carbon concentration decreases as the distance from the joint interface 30 increases.

[0011] Since the carbon concentration at the joint interface 30 of the steel material joint 1 according to this embodiment is 0.20 mass% or more and 2.10 mass% or less, 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 portion 31 (within the range where the concentration gradient layers 15 and 25 are formed) can also be improved. Specifically, since the carbon concentration at the joint interface 30 is 2.10 mass% or less, the crystallization of the solidification structure at the joint interface 30 is suppressed. Therefore, at the joint interface 30, the crystallization of a hard and brittle solidification structure can be suppressed, so that the joint strength between the steel materials 10 and 20 can be effectively improved. In addition, since the carbon concentration at the joint interface 30 is 0.20 mass% or more, the hardness of the joint interface 30 can be increased. Therefore, the wear resistance of the outer peripheral surface in the vicinity of the joint portion 31 can also be improved.

[0012] Preferably, the carbon concentration is 0.20 mass% or more and 0.90 mass% or less. By setting the carbon concentration at the joint interface 30 to 0.20 mass% or more and 0.90 mass% or less, the joint strength between the steel materials 10 and 20 can be more effectively improved. Specifically, by setting the carbon concentration at the joint interface 30 to 0.90 mass% or less, the precipitation of cementite at the austenite grain boundary is suppressed. Therefore, at the joint interface 30, the precipitation of cementite at the hard and brittle austenite grain boundary can be suppressed, so that the joint strength between the steel materials 10 and 20 can be more effectively improved.

[0013] In the steel material joined body 1 according to the present embodiment, the carbon concentration at the joining interface 30 can be measured by cutting the steel material joined body 1 along the joining interface 30, polishing the cross section, and using an element distribution measuring device such as an electron probe microanalyzer (EPMA) or energy dispersive X-ray spectroscopy (EDX). In the present invention, the numerical value of the carbon concentration at the joining interface 30 is calculated by measuring five arbitrary positions on the polished cross section and taking the average value thereof.

[0014] Further, the steel material joined body 1 according to the present embodiment has concentration gradient layers 15 and 25 in which the carbon concentration decreases as the distance from the joining interface 30 increases. Therefore, at the joining interface 30, since the carbon concentration is high (see the carbon concentration A shown in FIG. 1), the joining strength between the steel materials 10 and 20 can be effectively improved. Further, since the carbon concentration (carbon concentration B in FIG. 1) on the side opposite to the joining interface 30 (the side of the steel materials (materials 10 and 20) before joining) in the concentration gradient layers 15 and 25 is lower than that at the joining interface 30, the materials 10 and 20 sides can exhibit "elongation". Therefore, the steel material joined body 1 according to the present embodiment can be suitably used for applications where "elongation" is required on the materials 10 and 20 sides.

[0015] As shown in FIG. 1, it is preferable that the carbon concentration in the concentration gradient layers 15 and 25 continuously decreases as the distance from the joining interface 30 increases. "Continuously decreasing" as used in the present invention means that, as shown in FIG. 1, the numerical value of the carbon concentration proportionally decreases from the joining interface 30 toward the plurality of steel materials (materials) 10 and 20 sides. Since the concentration gradient layers 15 and 25 have such a decreasing tendency, in addition to the above-described effects, the ductility and toughness at the joining interface 30 are also improved.

[0016] In the steel material joined body 1 according to the present embodiment, the carbon concentration in the concentration gradient layers 15 and 25 can be measured by cutting the steel material joined body 1 along the joining interface 30, then cutting further in the direction away from the surface of the cut joining interface 30 (the direction of the materials 10 and 20 sides), and then polishing the cross section cut in the separating direction, and using an element distribution measuring device such as an electron probe microanalyzer (EPMA) or energy dispersive X-ray spectroscopy (EDX). In the present invention, the measurement of the decreasing tendency of the concentration gradient layers 15 and 25 is carried out by measuring the carbon concentrations at any five locations (a total of ten locations) on a straight line from the carbon concentration at the bonding interface 30 to the carbon concentrations of the materials 10 and 20 in each direction from the polished cross-section in the separating direction with respect to the bonding interface 30 using the above-described element distribution measuring apparatus. By creating a graph as shown in FIG. 1 with the distance from the bonding interface 30 on the horizontal axis and the carbon concentration on the vertical axis and plotting the carbon concentration against the distance, it can be confirmed.

[0017] Further, although the steel material joint 1 according to the present embodiment is independent of the tissue form of the bonding interface 30, it is preferable that the bonding interface 30 is composed of pearlite. This pearlite can be obtained by air-cooling or slow-cooling the steel material joint 1 in the austenite state. Since the bonding interface 30 is composed of pearlite, the tensile strength and the bending strength are increased, so that the bonding strength of the bonding interface 30 can be more effectively improved. The tissue form of the bonding interface 30 can be confirmed with an optical microscope in a state where the steel material joint 1 is cut along the bonding interface 30, the cut cross-section is polished, and then nital corrosion is applied to the polished cross-section.

[0018] It is preferable that the bonding interface 30 does not include cementite at the austenite grain boundary. If grain boundary cementite exists, there is a possibility that cracks are likely to occur at the starting point in tension and bending. Note that the above-mentioned "not including" does not mean that there is no grain boundary cementite at all, but means that the abundance ratio of the grain boundary cementite at the bonding interface 30 is less than 10%. Here, the confirmation of the abundance ratio of the grain boundary cementite at the bonding interface 30 is carried out using a point counting method conforming to JIS G0555 on the cross-section after applying nital corrosion to the cross-section.

[0019] Incidentally, in the steel material joint 1 according to the present embodiment, the materials of the steel materials (materials) 10 and 20 to be joined are not particularly limited as long as they are arbitrary steel materials and metals that can be integrated with each other. Further, the alloying elements other than carbon in the joint interface 30 and the plurality of steel materials (materials) 10 and 20 before joining are not particularly limited. For example, as defined in JIS G 4051, generally, it contains Si: 1.50 mass% or less and Mn: 1.00 mass% or less, and has a composition consisting of the balance Fe and inevitable impurities. The shapes of the plurality of steel materials (materials) 10 and 20 before joining are not particularly limited as long as they each have a joint surface and these joint surfaces can be overlapped with each other and integrated with each other. The steel materials (materials) 10 and 20 can adopt, for example, a cylindrical shape, a prismatic shape, a screw shape, a concavo-convex shape, etc.

[0020] As shown in FIG. 1, in another steel material joint 1A according to the present embodiment, a plurality of steel materials 10 and 20 made of medium carbon steel are joined to each other. Further, the carbon concentration (carbon concentration A shown in FIG. 1) of the joint interface 30 (hatched portion in FIG. 1) where a plurality of steel materials 10 and 20 made of medium carbon steel are joined is 0.50 mass% or more and 2.10 mass% or less. Furthermore, it has concentration gradient layers 15 and 25 in which the carbon concentration decreases as it moves away from the joint interface 30. That is, the steel material joint 1A has a different carbon concentration range compared to the steel material joint 1.

[0021] Thus, when medium carbon steel is used as the plurality of steel materials (materials) before joining, setting the carbon concentration to 0.50 mass% or more and 2.10 mass% or less is advantageous for achieving the object of the present invention. Specifically, since the carbon concentration of the joint interface 30 is 2.10 mass% or less, the crystallization of the solidification structure at the joint interface 30 is suppressed. Therefore, at the joint interface 30, the crystallization of a hard and brittle solidification structure can be suppressed, so that the joint strength between the steel materials 10 and 20 can be effectively improved. In addition, since the carbon concentration of the joint interface 30 is 0.50 mass% or more, the hardness of the joint interface 30 can be increased. Therefore, the wear resistance of the circumferential surface of the vicinity 31 of the joint portion can be improved.

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

[0023] In addition, another steel material joint 1A according to the present embodiment has concentration gradient layers 15 and 25 in which the carbon concentration decreases as the distance from the joint interface 30 increases. Therefore, at the joint interface 30, since the carbon concentration is high (see the carbon concentration A shown in FIG. 1), the joint strength between the steel materials 10 and 20 can be effectively improved. Also, since the carbon concentration (carbon concentration B in FIG. 1) on the side of the steel materials (materials 10 and 20) before joining on the opposite sides of the joint interface 30 in the concentration gradient layers 15 and 25 is lower than that of the joint interface 30, the "elongation" can be exhibited on the side of the materials 10 and 20. Therefore, it can be suitably used for applications where "elongation" is required on the side of the steel material joint 1A, materials 10, and 20 according to the present embodiment.

[0024] Similarly, in another steel material joint 1A according to the present embodiment, the concentration gradient layers 15 and 25 preferably decrease continuously in carbon concentration as the distance from the joint interface 30 increases, as shown in FIG. 1. Since the concentration gradient layers 15 and 25 have such a decreasing tendency, in addition to the above-described effects, the ductility and toughness at the joint interface 30 are also improved. Incidentally, for the measurement of the carbon concentration of these joint interfaces 30 and concentration gradient layers 15 and 25, the same conditions and methods as those of the steel material joint 1 according to the above-described present embodiment can be adopted.

[0025] The medium carbon steel referred to in the present invention means a steel material having a carbon concentration of 0.30 mass% or more and 0.50 mass% or less. For reference, the low carbon steel means a steel material having a carbon concentration of less than 0.30 mass%, and the high carbon steel means a steel material having a carbon concentration exceeding 0.50 mass%. Further, since the metal structure of the bonding interface 30 of the other steel material bonding body 1A according to the present embodiment is the same as that of the steel material bonding body 1 according to the present embodiment described above, the description thereof is omitted here.

[0026] Hereinafter, in the steel material bonding bodies 1 and 1A according to the present embodiment, the mechanism for increasing the bonding force between the steel materials will be described with reference to the drawings. FIG. 2 is an iron-cementite phase diagram for explaining the effects of the present invention. FIG. 3 is a conceptual diagram for explaining the effects of the present invention, and more specifically, is a conceptual diagram showing the reaction occurring at the bonding interface. As a manufacturing method of the steel material bonding bodies 1 and 1A according to the present embodiment, carbon powder (carbonaceous substance) is disposed on at least one of the bonding surfaces of the steel materials to be bonded, and the steel materials in which the bonding surfaces of the steel materials to be bonded are overlapped via this carbonaceous substance are heated at a maximum temperature of 1150° C. or higher and 1500° C. or lower (preferably 1150° C. or higher and 1300° C. or lower) in a predetermined atmosphere (for example, in an air atmosphere) (see FIG. 3(a)).

[0027] When the temperature of the bonding surface between the steel materials reaches, for example, 1250° C., a liquid phase L having a carbon concentration of 3.5 mass% is generated at the interface between the steel material and the carbonaceous substance (see the portion indicated by □ in FIG. 2). This liquid phase L increases until the carbonaceous substance disappears (see FIG. 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) with a carbon concentration of 0.045 mass% were prepared in the shape of a cylinder (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)) as shown in FIG. 1. Then, as a test piece, these two steel materials were joined together to produce a steel material joined body as shown by reference numeral 1 in FIG. 1. At this time, carbon powder (carbonaceous substance) was placed on the bonding surface of each of the two steel materials, and the bonding surfaces of the steel materials to be joined through this carbonaceous substance were overlapped. Next, high-frequency induction heating was performed at a maximum temperature of 1250° C. in an air atmosphere, and then slowly cooled. Here, the mass of the carbon powder placed on the bonding surface of each of the two steel materials was adjusted to be a mass at which a reaction at the bonding interface as shown in FIGS. 2 and 3 could occur. Also, by controlling the heating maintenance time at the maximum temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to be 0.20 mass%.

[0032] In Example 1, for the produced test piece (steel material joined body), the following "confirmation of bonding strength and wear resistance" was carried out. (Confirmation of the presence or absence of a concentration gradient layer, the main tissue structure of the bonding interface, bonding strength, and wear resistance) For the fabricated test specimens (steel joints), the presence or absence of a concentration gradient layer (a concentration gradient layer where the carbon concentration decreases as the distance from the joint interface increases), the main microstructure of the joint interface, the joint strength (〇 high, △ low), and the wear resistance (〇 high, △ low) were confirmed. The presence or absence of the concentration gradient layer was determined by creating a graph as shown in Fig. 1, which plots the carbon concentration against the above-mentioned distance, and checking whether there is a decreasing trend in the carbon concentration. The confirmation of the main microstructure (metallic structure) of the joint interface was carried out by observing a predetermined location of the joint interface of the fabricated steel joint under a nitriding corrosion state using an optical microscope. Fig. 4 shows a diagram explaining the locations where the metallic structure is confirmed. Note that the steel joint shown in Fig. 4(b) is in a state where the steel joint is cut in an L-section as shown in Fig. 4(a). By image processing the metallic structure images at each of the locations A, B, and C on this L-section, the presence or absence of the metallic structure (pearlite, etc.) in each image was confirmed. The joint strength was measured and evaluated as the tensile strength. This tensile strength test was carried out using JIS 9 A (G.L 100 mm). Also, the wear resistance was evaluated by adjusting the speed and the final load in a dry environment using a wear testing machine, using a grinding wheel made of cubic silicon nitride with a particle size number of 400 as the counter material, and based on the specific wear rate of the outer peripheral surface of the joint. Table 1 shown below presents the results of the above confirmations.

Example

[0033] In Example 2, two steel materials (medium carbon steel) with a carbon concentration of 0.450 mass% were prepared in a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joint surface)) as shown in Fig. 1. Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the joint interface after high-frequency induction heating was adjusted to 0.50 mass%. Other than this, test specimens (steel joints) were fabricated under the same conditions and methods as in Example 1.

[0034] In Example 2, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was performed. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the explanations regarding these conditions and methods are omitted. Table 1 shown below presents the confirmation results of Example 2 together with those of Example 1.

Example

[0035] In Example 3, two steel materials (medium carbon steel) with a carbon concentration of 0.450 mass% were prepared in the shape of a cylinder (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)) as shown in Fig. 1. Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to be 0.90 mass%. Other than this, a test piece (steel material bonded body) was produced under the same conditions and methods as in Example 1.

[0036] In Example 3, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was performed. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the explanations regarding these conditions and methods are omitted. Table 1 shown below presents the confirmation results of Example 3 together with those of Example 1. For reference, Fig. 5 shows the metal microstructure images at locations A, B, and C on the L cross-section shown in Fig. 4(b) of the test piece (steel material bonded body) produced in Example 3.

Example

[0037] In Example 4, two steel materials (medium carbon steel) with a carbon concentration of 0.450 mass% were prepared in the shape of a cylinder (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)) as shown in Fig. 1. Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to be 1.30 mass%. Other than this, a test piece (steel material bonded body) was produced under the same conditions and methods as in Example 1.

[0038] In Example 4, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was carried out. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the explanations regarding these conditions and methods are omitted. Table 1 shown below presents 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) with a carbon concentration of 0.450 mass% were prepared in the shape of a cylinder as shown in Fig. 1 (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)). Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to 1.70 mass%. Except for this, the test specimens (bonded steel materials) were fabricated under the same conditions and methods as in Example 1.

[0040] In Example 5, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was carried out. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the explanations regarding these conditions and methods are omitted. Table 1 shown below presents 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% were prepared in the shape of a cylinder as shown in Fig. 1 (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)). Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to 2.10 mass%. Except for this, the test specimens (bonded steel materials) were fabricated under the same conditions and methods as in Example 1.

[0042] In Example 6, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was carried out. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the descriptions of these conditions and methods are omitted. Table 1 shown below presents 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) with a carbon concentration of 0.045 mass% were prepared in the shape of a cylinder (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)) as shown in Fig. 1. Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to 0.10 mass%. Other than this, the test specimens (bonded steel materials) were fabricated under the same conditions and methods as in Example 1.

[0044] In Comparative Example 1, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was carried out. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the descriptions of these conditions and methods are omitted. Table 1 shown below presents 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) with a carbon concentration of 0.450 mass% were prepared in the shape of a cylinder (length X: 150 mm, diameter φ: 15 mm (diameter of the bonding surface)) as shown in Fig. 1. Then, by controlling the heating maintenance time at the maximum reaching temperature, the carbon concentration at the bonding interface after high-frequency induction heating was adjusted to 2.30 mass%. Other than this, the test specimens (bonded steel materials) were fabricated under the same conditions and methods as in Example 1.

[0046] In Comparative Example 2, similar to Example 1, "confirmation of the presence or absence of a concentration gradient layer, the main microstructure of the bonding interface, bonding strength, and wear resistance" was carried out. Here, when conducting this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, the explanations regarding these conditions and methods are omitted. Table 1 shown below presents 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 bonding interface of the test piece (steel material joint) was 0.20 mass% or more and 2.10 mass% or less (Examples 1 to 6), it was confirmed that the bonding strength and wear resistance were high. On the other hand, in Comparative Example 1, since the carbon concentration at the bonding interface was low, it was confirmed that the wear resistance decreased. Also, in Comparative Example 2, since the crystallization of the solidification structure was confirmed as the structure of the bonding interface, it was confirmed that the bonding strength decreased.

[0049] Moreover, in the steel material joints of Examples 4 to 6, in addition to pearlite, cementite (including cementite at the austenite grain boundary) was also confirmed as the main structure of the bonding interface. On the other hand, in the steel material joints of Examples 1 to 3, pearlite was confirmed as the main structure of the bonding interface, and cementite was not confirmed. Therefore, it is considered that the bonding strength and wear resistance are higher in the steel material joints of Examples 1 to 3 (when the carbon concentration at the bonding interface is 0.20 mass% or more and 0.90 mass% or less) where hard and brittle cementite was not confirmed. Furthermore, from this confirmation result, when medium carbon steel is used for the steel material to be joined, it is considered that a carbon concentration at the bonding interface of 0.50 mass% or more and 0.90 mass% or less is more advantageous for achieving the object of the present invention. [Explanation of Reference Numerals]

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

Claims

1. A steel member joint body in which a plurality of steel members are joined together, wherein the carbon concentration at the joint interface between the steel members is 0.20 mass% or more and 0.90 mass% or less, has a concentration gradient layer in which the carbon concentration decreases as the distance from the joint interface increases, and the joint interface is composed of pearlite and does not have cementite at the austenite grain boundary. A steel member joint body.

2. A steel member joint body in which a plurality of medium carbon steel members are joined together, wherein the carbon concentration at the joint interface between the steel members is 0.50 mass% or more and 0.90 mass% or less, has a concentration gradient layer in which the carbon concentration decreases as the distance from the joint interface increases, and the joint interface is composed of pearlite and does not have cementite at the austenite grain boundary. A steel member joint body.

3. The steel member joint body according to claim 1 or 2, wherein the carbon concentration in the concentration gradient layer continuously decreases as the distance from the joint interface increases.

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