Method for manufacturing steel material joined body

Abstract

To provide a method for manufacturing a steel material joined body which can effectively improve joint strength of steel materials, and can also improve wear resistance of an outer peripheral surface in the vicinity of a joint part.SOLUTION: A method for manufacturing a steel material joined body includes: arranging a carbonaceous material on at least one joint surface of steel materials to be joined; heating the steel materials to be joined through the carbonaceous material at the highest arrival temperature of 1,150°C or higher and 1,300°C or lower so that the joint surfaces of the steel materials are mutually overlapped; thereby generating a liquid phase at an interface between the steel material and the carbonaceous substance and diffusing carbon from the joint surface of the steel material to the inside; and extinguishing the liquid phase and joining the steel materials.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] In the past, with the objective of developing a technology that can easily and efficiently join hot steel materials in an actual factory and can obtain a high level of joining strength that does not interfere with the subsequent rolling process, a method for hot joining of 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 (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 members obtained by the technique described in Patent Document 1 effectively improve the joining strength between the steel members. Therefore, an object of the present invention is to provide a method for manufacturing a joined steel member that can effectively improve the joining strength between the steel members and also improve the wear resistance of the outer peripheral surface in the vicinity of the joint. [Means for solving the problem]

[0005] The manufacturing method of the steel member according to the present invention is characterized in that a carbonaceous material is placed on at least one of the joining surfaces of steel materials to be joined, the joining surfaces of the steel materials to be joined are overlapped with each other via the carbonaceous material, and heated to a maximum temperature of 1150°C to 1300°C, inclusive, whereby a liquid phase is generated at the interface between the steel materials and the carbonaceous material, carbon diffuses from the joining surfaces of the steel materials to the inside, the liquid phase disappears, and the steel materials are joined to each other.

[0006] In the method for producing a joined steel body according to the present invention, the carbon concentration at the joining interface is preferably 0.20 mass % or more and 0.90 mass % or less. Effect of the Invention

[0007] According to the present invention, it is possible to provide a manufacturing method for 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 in the vicinity of the joint. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a conceptual diagram for explaining a steel joint according to an embodiment of the present invention. [Diagram 2] FIG. 1 is a phase diagram of an iron-cementite system for explaining the effects of the present invention. [Diagram 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 joint is confirmed. [Diagram 5] 5A and 5B are metallographic images of the test specimen (steel joint) produced in Example 3 at locations A, B, and C on the L cross section shown in FIG. 4(b). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 the present embodiment. In the steel joint 1 according to the present embodiment, as shown in Fig. 1, a plurality of steel materials 10, 20 are joined together. The carbon concentration (carbon concentration A shown in Fig. 1) of a joint interface 30 (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 joint interface 30 has concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joint interface 30.

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

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

[0013] The carbon concentration at the joint interface 30 in the joined steel member 1 according to this embodiment can be measured by cutting the joined steel member 1 along the joint interface 30, polishing the cross section, and measuring the carbon concentration 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 joint interface 30 is calculated by averaging measurements taken at any five points on the polished cross section.

[0014] Moreover, the joined steel body 1 according to this embodiment has the concentration gradient layers 15, 25 in which the carbon concentration decreases with increasing distance from the joint interface 30. Therefore, since the carbon concentration is high at the joint interface 30 (see carbon concentration A shown in FIG. 1), the joint strength between the steel materials 10, 20 can be effectively improved. Furthermore, since the carbon concentration (carbon concentration B in FIG. 1) on the opposite side of the joint interface 30 in the concentration gradient layers 15, 25 (the side of the steel materials (materials 10, 20) before joining) is lower than that at the joint interface 30, the materials 10, 20 can exhibit "elongation." Therefore, the joined steel body 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, the term "continuously decreasing" refers to the carbon concentration decreasing proportionally from the joint interface 30 toward the multiple 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 joint interface 30 are also improved.

[0016] The carbon concentration of the concentration gradient layers 15, 25 in the steel joint 1 according to this embodiment can be measured by cutting the steel joint 1 along the joint interface 30, then further cutting it in a direction away from the cut surface of the joint interface 30 (towards the materials 10, 20), and then polishing the cross section cut in the direction away. The carbon concentration can be measured 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 straight line from the carbon concentration at the bonded interface 30 to the carbon concentration at the materials 10, 20 in each direction from the bonded interface 30 to the materials 10, 20 on a polished cross section in the separating direction using an element distribution measuring device as described above, and creating a graph as shown in FIG. 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] In addition, although the joined steel member 1 according to this embodiment does not depend on the structural form of the joint interface 30, it is preferable that the joint interface 30 is composed of pearlite. This pearlite can be obtained by air-cooling or slow-cooling the joined steel member 1 in an austenite state. When the joined interface 30 is composed of pearlite, the tensile strength and bending strength are increased, so that the joint strength of the joined interface 30 can be improved more effectively. The structural form of the joined interface 30 can be confirmed by cutting the joined steel member 1 along the joined interface 30, polishing the cut cross section, and then performing nital etching on the polished cross section, and then observing the resultant cross section with an optical microscope.

[0018] It is preferable that the bond interface 30 does not have cementite at the austenite grain boundaries. If grain boundary cementite is present, it may become a starting point for cracks to easily occur during tension and bending. The above term "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 bond interface 30 is less than 10%. Here, the abundance ratio of grain boundary cementite at the bond 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] Incidentally, in the steel joint 1 according to this embodiment, the material of the steel materials (materials) 10 and 20 to be joined is not particularly limited as long as it is any steel material and can be integrated with each other. In addition, the alloy elements other than carbon of the joint interface 30 and the multiple steel materials (materials) 10 and 20 before joining are not particularly limited, and for example, as specified in JIS G4051, it has a composition that contains approximately Si: 1.50 mass% or less, Mn: 1.00 mass% or less, and the balance is Fe and unavoidable impurities. The shape of the multiple steel materials (materials) 10 and 20 before joining is not particularly limited as long as they each have a joint surface and can be integrated with each other by overlapping these joint surfaces. For example, the steel materials (materials) 10 and 20 can adopt a cylindrical shape, a rectangular column shape, a screw shape, a concave-convex shape, etc.

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

[0021] In this way, when medium carbon steel is used as the multiple steel materials (raw materials) before joining, it is advantageous to achieve the object of the present invention by setting the carbon concentration to 0.50 mass% or more and 2.10 mass% or less. In detail, since the carbon concentration of 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. Moreover, 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 31.

[0022] The carbon concentration in the other steel joint 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 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, precipitation of cementite at the austenite grain boundaries is suppressed by setting the carbon concentration at the joint interface 30 to 0.90 mass% or less. Therefore, since precipitation of cementite at the hard and brittle austenite grain boundaries at the joint interface 30 can be suppressed, the joint strength between the steel materials 10 and 20 can be more effectively improved.

[0023] Another joined steel member 1A according to this embodiment has a concentration gradient layer 15, 25 in which the carbon concentration decreases with increasing distance from the joint interface 30. Therefore, since the carbon concentration is high at the joint interface 30 (see carbon concentration A in FIG. 1), the joint strength between the steel members 10, 20 can be effectively improved. Furthermore, since the carbon concentration (carbon concentration B in FIG. 1) of the steel members (materials 10, 20) before joining on the side opposite the joint interface 30 in the concentration gradient layer 15, 25) is lower than that at the joint interface 30, the materials 10, 20 can 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 steel joint 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 joint interface 30 are also improved. Incidentally, the carbon concentrations of the joint interface 30 and the concentration gradient layers 15 and 25 can be measured under the same conditions and by the same method as those of the jointed steel member 1 according to the present embodiment described above.

[0025] In the present invention, the 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, the low carbon steel refers to a steel material having a carbon concentration of less than 0.30 mass%, and the high carbon steel refers to a steel material having a carbon concentration of more than 0.50 mass%. The metal structure of the joint interface 30 of the other steel joint 1A according to the present embodiment is the same as that of the steel joint 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 effect of the present invention. Fig. 3 is a conceptual diagram for explaining the effect of the present invention, more specifically, a conceptual diagram showing a reaction occurring at a joint interface. In a manufacturing method of the joined steel bodies 1 and 1A according to this embodiment, carbon powder (carbonaceous material) is placed on at least one of the joint surfaces of the steel materials to be joined, and the steel materials in which the joint surfaces of the steel materials to be joined via the carbonaceous material are overlapped are heated 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 joint surface between 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 the 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 phase L" region (see the part marked with a 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 at a high rate from the joining surface of the steel into the internal austenite γ phase. As a result, at the interface between the liquid phase L and the austenite γ phase, the austenite γ phase side tries to maintain its carbon concentration at 1.6 mass%, stealing carbon from the liquid phase L side. Meanwhile, the liquid phase L tries to maintain its carbon concentration at 3.5 mass%, so the liquid phase L decreases (see Figure 3(c)). Finally, the liquid phase L disappears and the joining of the steel materials is completed (see Figure 3(d)).

[0029] Incidentally, the carbon concentration at the joining interface may be high immediately after the liquid phase L disappears. In order to suppress the above-mentioned precipitation of grain boundary cementite, the carbon concentration at the joining interface of the joined steel body 1 must be reduced to 0.20 mass% or more and 0.90 mass% or less. In the joined steel body 1A, the carbon concentration 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 maintenance time at the maximum temperature.

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

[0031] In Example 1, two steel materials (low carbon steel) having a cylindrical shape (length X: 150 mm, diameter φ: 15 mm (diameter of the joint surface)) as shown in FIG. 1 and a carbon concentration of 0.045 mass% were prepared. Then, as a test specimen, these two steel materials were joined together to produce a steel joint as shown by reference numeral 1 in FIG. 1. At this time, carbon powder (carbonaceous material) was placed on the joint surface of each of the two steel materials, and the joint surfaces of the steel materials to be joined via the carbonaceous material were overlapped. Next, high-frequency induction heating was performed at a maximum temperature of 1250°C in an air atmosphere, and then the steel materials were slowly cooled. Here, the mass of the carbon powder placed on each joint surface of the two steel materials was adjusted so that a reaction at the joint interface as shown in FIG. 2 and FIG. 3 could occur. In addition, the carbon concentration at the joint 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) thus produced were subjected to the "verification of joint strength and abrasion resistance" described below. (Check the presence or absence of a concentration gradient layer, the main structure of the bonding interface, bonding strength and abrasion resistance) The prepared test specimens (steel joints) were checked 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 structure 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 creating a graph as shown in Figure 1, which plots the carbon concentration against the above-mentioned distance, and then checking whether there was a tendency for the carbon concentration to decrease. The main structure (metal structure) of the joint interface was confirmed by observing a predetermined portion of the joint interface of the prepared steel joints under nital corrosion with an optical microscope. Figure 4 shows a diagram explaining the portion where the metal structure was confirmed. The steel joint shown in Figure 4(b) is in a state in which the steel joint is cut at the L cross section as shown in Figure 4(a), and the metal structure images at the A, B, and C points on this L cross section were processed to confirm the presence or absence of the metal structure (pearlite, etc.) in each image. The joint strength was measured and evaluated as tensile strength. The tensile strength was tested using JIS No. 9A (G.L 100mm). Abrasion resistance was evaluated using an abrasion tester, adjusting the speed and final load in a dry environment, and using a grindstone made of cubic silicon nitride with grain size 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. EXAMPLES

[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 of 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, a test specimen (jointed steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, the same conditions and methods as Example 1 were adopted in performing this confirmation. Therefore, a description of these conditions and methods will be omitted. Table 1 shown below shows the confirmation results of Example 2 together with the confirmation results of Example 1. EXAMPLES

[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 of 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, a test specimen (joined steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown below shows the confirmation results of Example 3 together with the confirmation results of Example 1. For reference, FIG. 5 shows metal structure images of the test specimen (steel joint) produced in Example 3 at points A, B, and C on the L cross section shown in FIG. 4(b). EXAMPLES

[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 of 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, a test specimen (jointed steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown below shows the confirmation results of Example 4 together with the confirmation results of Example 1. EXAMPLES

[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 of 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, a test specimen (jointed steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown below shows the confirmation results of Example 5 together with the confirmation results of Example 1. EXAMPLES

[0041] In Example 6, 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 of 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, a test specimen (jointed steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was carried out. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown 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 of 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, a test specimen (joined steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown 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 of 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, a test specimen (joined steel material) was 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 microstructure of the bonding interface, bonding strength, and abrasion resistance" was performed. Here, in carrying out this confirmation, the same conditions and methods as in Example 1 were adopted. Therefore, a description of these conditions and methods will be omitted. Table 1 shown 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 joint interface of the test specimens (steel joints) was 0.20 mass% or more and 2.10 mass% or less (Examples 1 to 6), it was confirmed that the joint strength and wear resistance were high. On the other hand, it was confirmed that the wear resistance was reduced in Comparative Example 1 because the carbon concentration at the joint interface was low. Also, in Comparative Example 2, it was confirmed that the crystallization of the solidification structure was confirmed as the structure at the joint interface, and therefore the joint strength was reduced.

[0049] In addition, in the steel joints 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 steel joints of Examples 1 to 3, pearlite was confirmed as the main structure of the joint interface, and cementite was not confirmed. Therefore, it is considered that the joint strength and wear resistance are higher in the steel joints of Examples 1 to 3 in which hard and brittle cementite was not confirmed (when the carbon concentration of the joint interface is 0.20 mass% or more and 0.90 mass% or less). Furthermore, from this confirmation result, it is considered that when medium carbon steel is used as the steel to be joined, it is more advantageous to achieve the object of the present invention when the carbon concentration of the joint interface is 0.50 mass% or more and 0.90 mass% or less. [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 carbonaceous material is placed on at least one of the joining surfaces of the steel materials to be joined; The joining surfaces of the steel materials to be joined are overlapped with each other via the carbonaceous material and heated at a maximum temperature of 1150°C or more and 1300°C or less, whereby a liquid phase is generated at the interface between the steel material and the carbonaceous material, and carbon diffuses from the joining surfaces of the steel materials to the inside, The method for manufacturing a steel member in which the liquid phase disappears and the steel members are joined to each other.

2. 2. The method for producing a joined steel member according to claim 1, wherein a carbon concentration at a joining interface is 0.20 mass% or more and 0.90 mass% or less.

3. 3. The method for producing a steel joint according to claim 1, wherein the carbonaceous material is a powder of graphite particles.

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