Wear-resistant component

Abstract

To provide a highly durable wear-resistant component.SOLUTION: A crawler link 7, which is a wear-resistant component, comprises: a high-hardness part 7A that is composed of steel having a specific component composition, has a hardness of 57-60 HRC, and has a martensite structure; and a low-hardness part 7B that is a region other than the high-hardness part 7A, has a lower hardness than the high-hardness part 7A, and has a martensite structure. The high-hardness part 7A includes: a parent phase composed of martensite; and carbides 92 dispersed in a lath of the parent phase. The area ratio of the carbides 92 in the cross section of the high-hardness part 7A is at least 9%.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to wear-resistant parts.

Background Art

[0002] In addition to crawler parts such as crawler links, crawler bushes, crawler pins, sprocket teeth, lower rollers, upper rollers, and shoe plates, which are components that make up the undercarriage of crawler-type work machines such as hydraulic excavators and bulldozers, wear-resistant parts including earth and sand wear-resistant parts used in environments where they come into contact with hard objects such as earth and sand, ore, and concrete, such as ripper points and teeth of crushers, are required to have improved durability by achieving both wear resistance and toughness.

[0003] In order to achieve both wear resistance and toughness, techniques for forming a hardened layer in an appropriate region in wear-resistant parts are known (see, for example, Patent Document 1). In addition, steel having an appropriate component composition is adopted as the material, and M 23 C 6 Techniques for achieving both wear resistance and toughness by avoiding the formation of carbides have been proposed (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] The crawler parts disclosed in Patent Document 2 can be said to be wear-resistant parts capable of achieving both wear resistance and toughness. However, with the popularization of automatic driving and unmanned driving of work machines, the operation of work machines emphasizes efficiency, and work machines may be operated under more severe conditions such as high-speed driving. As a result, further improvement in durability is required for wear-resistant parts including earth and sand wear-resistant parts.

[0006] One of the objectives of the present disclosure is to provide a wear-resistant part with excellent durability.

Means for Solving the Problems

[0007] The wear-resistant part according to the first aspect of the present disclosure is made of steel containing C (carbon) of 0.41% by mass or more and 0.44% by mass or less, Si (silicon) of 0.2% by mass or more and 0.6% by mass or less, Mn (manganese) of 0.2% by mass or more and 1.5% by mass or less, S (sulfur) of 0.0005% by mass or more and 0.0050% by mass or less, Ni (nickel) of 0.6% by mass or more and 2.0% by mass or less, Cr (chromium) of 0.7% by mass or more and 1.5% by mass or less, Mo (molybdenum) of 0.1% by mass or more and 0.6% by mass or less, Nb (niobium) of 0.02% by mass or more and 0.04% by mass or less, Ti (titanium) of 0.015% by mass or more and 0.03% by mass or less, B (boron) of 0.0005% by mass or more and 0.0030% by mass or less, and N (nitrogen) of 20 ppm by mass or more and 120 ppm by mass or less, with the balance being iron and impurities. This wear-resistant part has a Rockwell hardness of 57 HRC or more and 60 HRC or less, and includes a high-hardness part having a martensite structure and a low-hardness part in a region other than the high-hardness part, which has a lower hardness than the high-hardness part and has a martensite structure. The high-hardness part includes a matrix phase composed of martensite and carbides dispersed in the laths of the matrix phase. The area ratio of the carbides in the cross-section of the high-hardness part is 9% or more.

[0008] The wear-resistant part according to the second aspect of the present disclosure contains C of 0.41% by mass or more and 0.44% by mass or less, Si of 0.2% by mass or more and 0.6% by mass or less, Mn of 0.2% by mass or more and 1.5% by mass or less, S of 0.0005% by mass or more and 0.0050% by mass or less, Ni of 0.6% by mass or more and 2.0% by mass or less, Cr of 0.7% by mass or more and 1.5% by mass or less, Mo of 0.1% by mass or more and 0.6% by mass or less, Nb of 0.02% by mass or more and 0.04% by mass or less, Ti of 0.015% by mass or more and 0.03% by mass or less, B of 0.0005% by mass or more and 0.0030% by mass or less, and N of 20 ppm by mass or more and 120 ppm by mass or less. Further, it contains at least one selected from the group consisting of V (vanadium) of 0.05% by mass or more and 0.20% by mass or less, Zr (zirconium) of 0.01% by mass or more and 0.15% by mass or less, Co (cobalt) of 0.1% by mass or more and 2.0% by mass or less, Cu (copper) of 0.05% by mass or more and 0.40% by mass or less, and Al (aluminum) of 0.005% by mass or more and 0.050% by mass or less, and the balance is steel composed of iron and impurities. This wear-resistant part has a hardness of 57 HRC or more and 60 HRC or less, and includes a high-hardness part having a martensite structure and a low-hardness part that is a region other than the high-hardness part, has a lower hardness than the high-hardness part, and has a martensite structure. The high-hardness part includes a matrix phase composed of martensite and carbides dispersed in the laths of the matrix phase. The area ratio of the carbides in the cross section of the high-hardness part is 9% or more.

Advantages of the Invention

[0009] According to the above wear-resistant part, a wear-resistant part excellent in durability can be provided.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0011] [Overview of the Embodiment] The wear-resistant part according to the first aspect of the present disclosure is made of steel containing 0.41% by mass or more and 0.44% by mass or less of C, 0.2% by mass or more and 0.6% by mass or less of Si, 0.2% by mass or more and 1.5% by mass or less of Mn, 0.0005% by mass or more and 0.0050% by mass or less of S, 0.6% by mass or more and 2.0% by mass or less of Ni, 0.7% by mass or more and 1.5% by mass or less of Cr, 0.1% by mass or more and 0.6% by mass or less of Mo, 0.02% by mass or more and 0.04% by mass or less of Nb, 0.015% by mass or more and 0.03% by mass or less of Ti, 0.0005% by mass or more and 0.0030% by mass or less of B, and 20 ppm by mass or more and 120 ppm by mass or less of N, with the balance being iron and impurities. This wear-resistant part has a hardness of 57 HRC or more and 60 HRC or less, and includes a high-hardness part having a martensite structure and a low-hardness part in a region other than the high-hardness part, which has a lower hardness than the high-hardness part and has a martensite structure. The high-hardness part includes a matrix phase composed of martensite and carbides dispersed in the laths of the matrix phase. The area ratio of the carbides in the cross-section of the high-hardness part is 9% or more.

[0012] The wear-resistant part according to the second aspect of the present disclosure contains C of 0.41% by mass or more and 0.44% by mass or less, Si of 0.2% by mass or more and 0.6% by mass or less, Mn of 0.2% by mass or more and 1.5% by mass or less, S of 0.0005% by mass or more and 0.0050% by mass or less, Ni of 0.6% by mass or more and 2.0% by mass or less, Cr of 0.7% by mass or more and 1.5% by mass or less, Mo of 0.1% by mass or more and 0.6% by mass or less, Nb of 0.02% by mass or more and 0.04% by mass or less, Ti of 0.015% by mass or more and 0.03% by mass or less, B of 0.0005% by mass or more and 0.0030% by mass or less, and N of 20 ppm by mass or more and 120 ppm by mass or less. Further, it contains at least one selected from the group consisting of V (vanadium) of 0.05% by mass or more and 0.20% by mass or less, Zr (zirconium) of 0.01% by mass or more and 0.15% by mass or less, Co (cobalt) of 0.1% by mass or more and 2.0% by mass or less, Cu (copper) of 0.05% by mass or more and 0.40% by mass or less, and Al (aluminum) of 0.005% by mass or more and 0.050% by mass or less, and the balance is steel composed of iron and impurities. This wear-resistant part has a hardness of 57 HRC or more and 60 HRC or less, and includes a high-hardness part having a martensite structure and a low-hardness part which is a region other than the high-hardness part, has a lower hardness than the high-hardness part, and has a martensite structure. The high-hardness part includes a matrix phase composed of martensite and carbides dispersed in the laths of the matrix phase. The area ratio of the carbides in the cross section of the high-hardness part is 9% or more.

[0013] First, the reasons for limiting the component composition of the steel constituting the wear-resistant part of the present disclosure to the above ranges will be described.

[0014] Carbon (C): 0.41% by mass or more and 0.44% by mass or less Carbon is an element that has a great influence on the hardness of steel. When the carbon content is less than 0.41% by mass, it becomes difficult to obtain sufficient hardness, for example, a hardness of 57 HRC or more, by quenching and tempering. On the other hand, when the carbon content exceeds 0.44% by mass, the reduction value in the tensile test decreases and the toughness decreases. Therefore, the carbon content needs to be within the above range. Further, from the viewpoint of easily ensuring sufficient hardness, the carbon content is preferably 0.42% by mass or more.

[0015] Silicon (Si): 0.2% by mass or more and 0.6% by mass or less In addition to effects such as improving the hardenability of steel, strengthening the matrix phase of steel, and improving the resistance to temper softening, silicon is an element having a deoxidizing effect in the steelmaking process. When the silicon content is 0.2% by mass or less, the above effects cannot be sufficiently obtained. On the other hand, when the silicon content exceeds 0.6% by mass, the drawing value tends to decrease. Therefore, the silicon content needs to be within the above range.

[0016] Manganese (Mn): 0.2% by mass or more and 1.5% by mass or less Manganese is effective in improving the hardenability of steel and is an element having a deoxidizing effect in the steelmaking process. When the manganese content is 0.2% by mass or less, the above effects cannot be sufficiently obtained. On the other hand, when the manganese content exceeds 1.5% by mass, the hardness before hardening by quenching increases and the workability tends to decrease. Therefore, the manganese content needs to be within the above range. Further, from the viewpoint of ensuring sufficient hardenability of steel, the manganese content is preferably 0.40% by mass or more. Also, when importance is attached to workability, the manganese content is preferably 0.9% by mass or less, and more preferably 0.8% by mass or less.

[0017] Sulfur (S): 0.0005% by mass or more and 0.0050% by mass or less Sulfur is an element that improves the machinability of steel. Also, sulfur is an element that is mixed in even if not intentionally added in the steelmaking process. When the sulfur content is less than 0.0005% by mass, the machinability decreases and the manufacturing cost of steel increases. On the other hand, according to the study by the inventors, in the component composition of the above steel of the present application, the sulfur content greatly affects the drawing value. And when the sulfur content exceeds 0.0050% by mass, the drawing value decreases and it becomes difficult to obtain sufficient toughness. Therefore, the sulfur content needs to be within the above range. Also, by setting the sulfur content to 0.0040% by mass or less, the toughness can be further improved.

[0018] Nickel (Ni): 0.6% by mass or more and 2.0% by mass or less Nickel is an element effective in improving the toughness of the matrix phase of steel. If the nickel content is less than 0.6% by mass, this effect cannot be fully exerted. On the other hand, when the nickel content exceeds 2.0% by mass, nickel has a stronger tendency to segregate in steel. As a result, the problem that the mechanical properties of the steel vary may occur. Therefore, the nickel content needs to be within the above range. Also, when the nickel content exceeds 1.5% by mass, while the improvement in toughness becomes moderate, the manufacturing cost of the steel increases. From such a perspective, it is preferable that the nickel content is 1.5% by mass or less. On the other hand, in steel having a hardness of 57 HRC or more, in order to fully exert the effect of improving the toughness of the matrix phase of the steel, the nickel content is preferably 1.0% by mass or more.

[0019] Chromium (Cr): 0.7% by mass or more and 1.5% by mass or less Chromium improves the hardenability of steel and enhances the tempering softening resistance. In particular, by the combined addition with molybdenum, niobium, vanadium, etc., the tempering softening resistance of the steel can be significantly enhanced. If the chromium content is less than 0.7% by mass, such an effect cannot be fully exerted. Also, when the chromium content exceeds 1.5% by mass, while the improvement in tempering softening resistance becomes moderate, the manufacturing cost of the steel increases. Therefore, the chromium content needs to be within the above range.

[0020] Molybdenum (Mo): 0.1% by mass or more and 0.6% by mass or less Molybdenum improves the hardenability of steel and enhances the tempering softening resistance. Also, molybdenum has a function of improving the high-temperature temper brittleness. If the molybdenum content is less than 0.1% by mass, these effects cannot be fully exerted. On the other hand, when the molybdenum content exceeds 0.6% by mass, the above effects saturate. Therefore, the molybdenum content needs to be within the above range.

[0021] Niobium (Nb): 0.02% by mass or more and 0.04% by mass or less Niobium has the effect of refining the crystal grains of steel and is effective in improving the strength and toughness of steel. In order to ensure such an effect, the niobium content needs to be 0.02% by mass or more. On the other hand, when the niobium content exceeds 0.04% by mass, problems such as precipitation of coarse eutectic NbC and reduction of the carbon amount in the matrix phase due to the formation of a large amount of NbC occur, leading to a decrease in strength and toughness. Also, when the niobium content exceeds 0.04% by mass, the manufacturing cost of steel also increases. Therefore, the niobium content needs to be within the above range.

[0022] Titanium (Ti): 0.015% by mass or more and 0.03% by mass or less Titanium effectively acts on improving the strength and toughness of steel by forming fine carbonitrides and refining the crystal grains. When the titanium content is less than 0.015% by mass, such an effect is small. On the other hand, when the titanium content exceeds 0.03% by mass, there is a risk that the strength and toughness of the steel will deteriorate instead. Therefore, the titanium content needs to be within the above range.

[0023] Boron (B): 0.0005% by mass or more and 0.0030% by mass or less Boron is an element that significantly improves the hardenability of steel. By adding boron, the addition amount of other elements added for the purpose of improving hardenability can be reduced, and the manufacturing cost of steel can be reduced. Also, boron has a strong tendency to segregate more than phosphorus (P) and sulfur at the boundaries of prior austenite crystal grains, and in particular, it discharges sulfur from the grain boundaries and improves the grain boundary strength. When the boron content is less than 0.0005% by mass, such an effect is not fully exerted. On the other hand, when the boron content exceeds 0.0030% by mass, the added boron combines with nitrogen to form BN (boron nitride), deteriorating the strength and toughness of the steel. Therefore, the boron content needs to be within the above range.

[0024] Nitrogen (N): 20 ppm by mass or more and 120 ppm by mass or less If the nitrogen content is too high, excessive nitrides may be formed, which may deteriorate the toughness of the steel. Therefore, the nitrogen content needs to be 120 mass ppm or less. On the other hand, if the nitrogen content is less than 20 mass ppm, the manufacturing cost of the steel increases. Therefore, the nitrogen content needs to be within the above range. From the perspective of emphasizing the toughness of the steel, the nitrogen content is preferably 100 mass ppm or less.

[0025] Vanadium (V): 0.05 mass% or more and 0.20 mass% or less Vanadium is not an essential element. However, vanadium forms fine carbides and contributes to the refinement of crystal grains. If the vanadium content is less than 0.05 mass%, such an effect cannot be obtained sufficiently. On the other hand, if the vanadium content exceeds 0.20 mass%, the above effect saturates. Also, since vanadium is a relatively expensive element, it is preferable to keep the addition amount to the minimum required. Therefore, when adding vanadium, the addition amount is appropriately within the above range. Vanadium is not added to the steel constituting the wear-resistant parts of the first aspect of the present disclosure, but is added to the steel constituting the wear-resistant parts of the second aspect.

[0026] Zirconium (Zr): 0.01 mass% or more and 0.15 mass% or less Zirconium is not an essential element, but has the effect of further improving the toughness of the steel by spheroidizing and dispersing carbides in the steel. If the zirconium content is less than 0.01 mass%, the effect cannot be obtained sufficiently. On the other hand, if the zirconium content exceeds 0.15 mass%, the toughness of the steel may deteriorate instead. Therefore, when adding zirconium, the addition amount is appropriately within the above range. Zirconium is not added to the steel constituting the wear-resistant parts of the first aspect of the present disclosure, but is added to the steel constituting the wear-resistant parts of the second aspect.

[0027] Cobalt (Co): 0.1 mass% or more and 2.0 mass% or less Cobalt is not an essential element, but it increases the solubility of carbide-forming elements such as chromium and molybdenum in the matrix phase and improves the tempering softening resistance of steel. Therefore, by adding cobalt, the refinement of carbides and the increase in the tempering temperature can be achieved, thereby improving the toughness of the steel. If the cobalt content is less than 0.1% by mass, such an effect cannot be sufficiently obtained. On the other hand, since cobalt is an expensive element, adding a large amount increases the manufacturing cost of steel. When the cobalt content exceeds 2.0% by mass, such problems become prominent. Therefore, when adding cobalt, it is appropriate that the addition amount is within the above range. Cobalt is not added to the steel constituting the wear-resistant parts of the first aspect of the present disclosure, but is added to the steel constituting the wear-resistant parts of the second aspect.

[0028] Copper (Cu): 0.05% by mass or more and 0.40% by mass or less Copper is not an essential element, but it has the effect of improving the hardenability of steel. If the copper content is less than 0.05% by mass, such an effect cannot be sufficiently obtained. On the other hand, when the copper content exceeds 0.40% by mass, the hot ductility of the steel decreases, promoting the occurrence of cracks, scratches, etc. during continuous casting and hot rolling. As a result, the ease of manufacturing the steel may be impaired. Therefore, when adding copper, it is appropriate that the addition amount is within the above range. From the viewpoint of emphasizing the hot ductility of the steel, the copper content is preferably 0.30% by mass or less. Copper is not added to the steel constituting the wear-resistant parts of the first aspect of the present disclosure, but is added to the steel constituting the wear-resistant parts of the second aspect.

[0029] Aluminum (Al): 0.005% by mass or more and 0.050% by mass or less Aluminum is not an essential element but is an element effective as a deoxidizer. If the aluminum content is less than 0.005% by mass, such an effect cannot be sufficiently obtained. On the other hand, if the aluminum content exceeds 0.050% by mass, coarse nitrides may be formed, which may cause a reduction in drawing. Therefore, when adding aluminum, it is appropriate to set the addition amount within the above range. From the viewpoint of emphasizing the effect as a deoxidizer, the aluminum content is preferably 0.010% by mass or more. From the viewpoint of more reliably suppressing the formation of coarse nitrides, the aluminum content is preferably 0.040% by mass or less.

[0030] Impurities In addition to the components intentionally added in the manufacturing process, various components may be contained in the steel as impurities. For example, phosphorus (P) as an impurity is preferably 0.012% by mass or less. The total amount of impurities is preferably 0.050% by mass or less.

[0031] According to the study by the present inventors, by increasing the drawing value in the tensile test while maintaining sufficient hardness in the high-hardness part of the wear-resistant part, it is possible to achieve an improvement in durability by achieving both wear resistance and toughness of the wear-resistant part including the earth-and-sand wear-resistant part. Such an improvement in durability can be achieved by adopting a steel having an appropriate component composition as the steel constituting the wear-resistant part and setting the amount of carbides dispersed in the laths of martensite constituting the matrix phase within an appropriate range in the structure of the steel constituting the high-hardness part. More specifically, by setting the area ratio of the above carbides in the cross-section of the high-hardness part to 9% or more, it is possible to increase the drawing value while ensuring sufficient hardness in the high-hardness part.

[0032] In the quenching treatment of steel carried out to form a high-hardness part, the carbon contained in the steel in the form of carbide dissolves in the steel when the steel is heated to a temperature range above the A1 transformation point. From this state, when the steel is rapidly cooled to a temperature range below the MS point, the steel transforms from austenite to martensite with a large amount of carbon dissolved, and high hardness is obtained. In such a quenching treatment, it is common to set the cooling rate as large as possible within a range where quenching cracks do not occur. On the other hand, the inventors of the present invention deliberately set the cooling rate in the quenching treatment small within a range where sufficient quenching can be achieved, and increase the reduction value in the tensile test by generating a large amount of carbide in the laths of martensite constituting the parent phase, and found that it is possible to achieve both high wear resistance and toughness of wear-resistant parts at a high level.

[0033] The wear-resistant part of the present disclosure is made of steel having the above-mentioned appropriate component composition. Also, the hardness in the high-hardness part is set to 57 HRC or more. And the area ratio of carbide in the cross-section of the high-hardness part is set to 9% or more. As a result, according to the wear-resistant part of the present disclosure, it is possible to achieve an improvement in durability due to both wear resistance and toughness.

[0034] Here, the carbide dispersed in the laths of martensite is iron carbide and is mainly composed of cementite (Fe 3 C). From the viewpoint of imparting sufficient hardness to the high-hardness part, the area ratio of the carbide in the cross-section of the high-hardness part is preferably 15% or less, more preferably 14% or less. Also, from the viewpoint of facilitating an increase in the reduction value in the tensile test, the area ratio of the carbide in the cross-section of the high-hardness part is preferably 10% or more. Also, the size of the carbide dispersed in the laths of martensite is, for example, 500 nm or less, and may be 300 nm or less.

[0035] In the above wear-resistant part, the high-hardness part and the low-hardness part are represented by M 23 C 6 where M is a metal element constituting the steel carbide (hereinafter, "M 23 C 6It is preferably free of (referred to as "carbide"). This can further improve the toughness of the wear-resistant component. Here, M is a metal element constituting the steel, and is mainly at least one of Cr and Mo. In the present application, the high-hardness part and the low-hardness part are M 23 C 6 The state of not containing carbide means that when observing the cross-sections of the high-hardness part and the low-hardness part with an FE-SEM (field emission scanning electron microscope) and investigating 10 or more fields of a region of 80 μm including the grain boundaries of the steel 2 no M 23 C 6 carbide is found. M 23 C 6 carbide can be identified, for example, by detecting the product in the bright-field image of a STEM (scanning transmission electron microscope) and then confirming the SAD (selected area diffraction) pattern of the product when a product that may be M 23 C 6 carbide is found by the above method.

[0036] In the above wear-resistant component, the reduction value in the tensile test of the high-hardness part may be 40% or more. This can endow the wear-resistant component with better toughness.

[0037] In the above wear-resistant component, the crystal grain size number of the steel constituting the high-hardness part may be 5 or more and 12 or less. The crystal grain size number of the steel constituting the low-hardness part may be 5 or more and 9 or less. By doing so, it becomes easy to impart toughness to the wear-resistant component. The crystal grain size number of the steel constituting the high-hardness part is preferably 9 or more and 12 or less. The crystal grain size number of the steel constituting the low-hardness part is preferably 7 or more and 9 or less.

[0038] In the above wear-resistant component, the martensite structure of the steel constituting the high-hardness part may be a low-temperature tempered martensite structure. The martensite structure of the steel constituting the low-hardness part may be a high-temperature tempered martensite structure (sorbite structure). By doing so, it becomes easy to impart toughness to the wear-resistant component.

[0039] In the present application, the low-temperature tempering martensite structure means a structure obtained by tempering the quenched steel at a temperature of 150°C or higher and 250°C or lower (obtained by low-temperature tempering). The high-temperature tempering martensite phase means a structure obtained by tempering the quenched steel at a temperature of 500°C or higher (obtained by high-temperature tempering). Whether it is a low-temperature tempering martensite structure or a high-temperature tempering martensite structure can be confirmed by investigating the hardness of the structure, the precipitation state of carbides, etc.

[0040] [Specific Examples of Embodiments] Next, an example of a specific embodiment of the wear-resistant component of the present disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals and their descriptions will not be repeated.

[0041] Taking the case where the wear-resistant component of the present disclosure is adopted as a track link included in a crawler-type traveling device as an example, an example of an embodiment of the wear-resistant component of the present disclosure will be described.

[0042] First, with reference to FIGS. 1 to 3, the track link as a wear-resistant component in the present embodiment will be described. FIG. 1 is a schematic diagram showing the structure of a crawler-type traveling device. FIG. 2 is a schematic perspective view showing a part of the structure of the crawler. FIG. 3 is a schematic cross-sectional view showing the structure of the crawler.

[0043] Referring to FIG. 1, the crawler-type traveling device 1 in the present embodiment is a traveling device of a working machine such as a bulldozer or a hydraulic excavator. The crawler-type traveling device 1 includes a crawler 2, a track frame 3, an idler 4, a sprocket 5, a plurality of (here, seven) lower rollers 10, and a plurality of (here, two) upper rollers 11.

[0044] The crawler belt 2 includes a plurality of crawler links 7 connected in an annular (endless) shape, and tread plates 6 fixed to each crawler link 7. Referring to FIG. 2, a pair of crawler links 7 are fixed to each tread plate 6. The crawler links 7 form two rows extending along the circumferential direction of the crawler belt 2.

[0045] Referring to FIG. 1, an idler 4, a plurality of (here, seven) lower rollers 10, and a plurality of (here, two) upper rollers 11 are rotatably attached to the track frame 3 about their respective axes. The sprocket 5 is disposed on the side opposite to the end of the track frame 3 where the idler 4 is attached when viewed from the center of the track frame 3. The sprocket 5 is connected to a power source such as an engine and is driven by the power source to rotate about its axis. A plurality of sprocket teeth 51, which are protrusions protruding radially outward, are disposed on the outer peripheral surface of the sprocket 5. Each sprocket tooth 51 meshes with the crawler belt 2. Thereby, the rotation of the sprocket 5 is transmitted to the crawler belt 2. As a result, the crawler belt 2 is driven by the rotation of the sprocket 5 and rotates in the circumferential direction.

[0046] The idler 4 is attached to an end of the track frame 3 (the end opposite to the side where the sprocket 5 is disposed). Further, a plurality of lower rollers 10 are attached to the region of the track frame 3 sandwiched between the sprocket 5 and the idler 4 on the ground contact side, and a plurality of upper rollers 11 are attached to the side opposite to the ground contact side. The idler 4, the lower rollers 10, and the upper rollers 11 are in contact with the inner peripheral surface of the crawler belt 2 on their outer peripheral surfaces. As a result, the crawler belt 2 driven by the rotation of the sprocket 5 rotates in the circumferential direction while being guided by the idler 4, the sprocket 5, the lower rollers 10, and the upper rollers 11.

[0047] Next, the details of the structure of the crawler belt 2 will be described. Referring to FIG. 2, adjacent crawler links 7 in the circumferential direction of the crawler belt 2 are arranged such that a part of them overlaps when viewed from a direction perpendicular to the rotation surface of the crawler belt 2, and are connected by a crawler pin 23 and a crawler bushing 21.

[0048] More specifically, referring to FIGS. 2 and 3, each crawler link 7 is formed with a bush hole 75A and a crawler pin hole 75B that penetrate in a direction perpendicular to the rotating surface of the crawler 2. The bush hole 75A is formed at one end of the crawler link 7 in the longitudinal direction. The crawler pin hole 75B is formed at the other end of the crawler link 7 in the longitudinal direction. Further, a tread surface 77 is formed on the side of the crawler link 7 opposite to the side where the tread plate 6 is attached.

[0049] A pair of crawler links 7 fixed to each tread plate 6 are arranged such that the two bush holes 75A overlap and the crawler pin holes 75B overlap when viewed from a direction perpendicular to the rotating surface of the crawler 2. Also, adjacent crawler links 7 in the circumferential direction of the crawler 2 are arranged such that the crawler pin hole 75B and the bush hole 75A overlap when viewed from a direction perpendicular to the rotating surface of the crawler 2. And as shown in FIGS. 2 and 3, the crawler pin 23 is arranged to penetrate through the crawler pin hole 75B of the crawler link 7 constituting one row, the bush hole 75A of the crawler link 7 constituting one row, the bush hole 75A of the crawler link 7 constituting the other row, and the crawler pin hole 75B of the crawler link 7 constituting the other row. The crawler pin 23 is press-fitted into the crawler pin hole 75B and is fixed to the crawler link 7 by caulking both ends thereof at the boss portion 76 of the crawler link 7.

[0050] Referring to FIGS. 2 and 3, the crawler bush 21 has a hollow cylindrical shape having a through hole in a region including the central axis. The crawler bush 21 is fixed by being fitted into the bush hole 75A of the crawler link 7. The crawler pin 23 is arranged to penetrate through the through hole of the crawler bush 21. An annular dust seal 22 is arranged between both longitudinal ends of the crawler bush 21 and the crawler link 7.

[0051] Furthermore, referring to FIG. 3, in a region including the central axis of the crawler pin 23, a lubricant holding hole 23A is formed which extends in the axial direction and holds a lubricant such as lubricating oil. Also, in the crawler pin 23, a lubricant supply passage 23B is formed which extends in the radial direction and connects the outer peripheral surface and the lubricant holding hole 23A. Further, the lubricant holding hole 23A has an opening on one end face side of the crawler pin 23, and a plug 23C is fitted into the opening. A lubricant such as lubricating oil is supplied into the lubricant holding hole 23A from the opening of the lubricant holding hole 23A, and then, by fitting the plug 23C into the opening, it is held in the lubricant holding hole 23A. Then, the lubricant in the lubricant holding hole 23A is supplied between the outer peripheral surface of the crawler pin 23 and the inner peripheral surface of the crawler bush 21 via the lubricant supply passage 23B. As a result, the friction between the outer peripheral surface of the crawler pin 23 and the inner peripheral surface of the crawler bush 21 is reduced, and wear of the outer peripheral surface of the crawler pin 23 and the inner peripheral surface of the crawler bush 21 is suppressed. In such a crawler traveling device 1, wear of the tread surface 77 of the crawler link 7 may determine the life or the component replacement cycle of the crawler traveling device 1.

[0052] In the crawler traveling device 1 according to the present embodiment, the crawler link 7, which is a crawler component, is made of steel containing 0.41% by mass or more and 0.44% by mass or less of C, 0.2% by mass or more and 0.6% by mass or less of Si, 0.2% by mass or more and 1.5% by mass or less of Mn, 0.0005% by mass or more and 0.0050% by mass or less of S, 0.6% by mass or more and 2.0% by mass or less of Ni, 0.7% by mass or more and 1.5% by mass or less of Cr, 0.1% by mass or more and 0.6% by mass or less of Mo, 0.02% by mass or more and 0.04% by mass or less of Nb, 0.015% by mass or more and 0.03% by mass or less of Ti, 0.0005% by mass or more and 0.0030% by mass or less of B, and 20 ppm by mass or more and 120 ppm by mass or less of N, with the balance being iron and impurities. The region including the tread surface 77 of the crawler link 7 is a high-hardness portion 7A having a hardness of 57 HRC or more and 60 HRC or less. The high-hardness portion 7A is, for example, a hardened layer formed by partially quenching and tempering treatment. The partial quenching and tempering treatment can be carried out, for example, by high-frequency quenching. The region other than the high-hardness portion 7A of the crawler link 7 is a low-hardness portion 7B having a lower hardness than the high-hardness portion 7A. The hardness of the low-hardness portion 7B is, for example, 30 HRC or more and 45 HRC or less.

[0053] The high-hardness portion 7A has a martensite structure. The martensite structure constituting the high-hardness portion 7A is a low-temperature tempered martensite structure. The low-hardness portion 7B has a martensite structure. The martensite structure constituting the low-hardness portion 7B is a high-temperature tempered martensite structure. The high-hardness portion 7A includes a parent phase and carbides dispersed in the laths of martensite constituting the parent phase. The area ratio of the carbides in the cross-section of the high-hardness portion 7A is 9% or more.

[0054] The steel constituting the crawler link 7 may further contain at least one selected from the group consisting of 0.05% by mass or more and 0.20% by mass or less of V, 0.01% by mass or more and 0.15% by mass or less of Zr, 0.1% by mass or more and 2.0% by mass or less of Co, 0.05% by mass or more and 0.40% by mass or less of Cu, and 0.005% by mass or more and 0.050% by mass or less of Al.

[0055] The crawler link 7, which is a wear-resistant part of this embodiment, is made of steel having the above appropriate component composition as a material, and the area ratio of carbides in the cross section of the high-hardness part 7A is set to 9% or more. As a result, the crawler link 7 of this embodiment is a wear-resistant part in which the durability is improved by achieving both wear resistance and toughness.

[0056] In the crawler link 7, the high-hardness part 7A and the low-hardness part 7B are represented by M 23 C 6 and M is a metal element constituting the steel and preferably does not contain carbides (M 23 C 6 carbides). Thereby, the toughness of the crawler link 7 can be further improved.

[0057] In the crawler link 7, the reduction value in the tensile test of the high-hardness part 7A is preferably 40% or more. Thereby, it becomes possible to impart more excellent toughness to the crawler link 7.

[0058] In the crawler link 7, the crystal grain size number of the steel constituting the high-hardness part 7A may be 5 or more and 12 or less. The crystal grain size number of the steel constituting the low-hardness part 7B may be 5 or more and 9 or less. Thereby, it becomes easy to impart toughness to the crawler link 7.

[0059] Next, an example of the manufacturing method of the crawler link 7, which is a wear-resistant part of this embodiment, will be described with reference to FIG. 4. In the manufacturing method of the crawler link 7 in this embodiment, first, a steel material preparation step is carried out as step (S10). In this step (S10), a steel material made of steel having the above appropriate component composition is prepared.

[0060] Next, a hot forging process is carried out as step (S20). In this step (S20), hot forging is performed on the steel material prepared in step (S10). As a result, a formed body having the schematic shape of the crawler link 7 is obtained. The hot forging is carried out, for example, when the steel material prepared in step (S10) is heated to 1200 °C or higher, for example, 1250 °C. During the cooling process after hot forging, M 23 C 6 carbides are formed at the grain boundaries of the steel.

[0061] Next, a quenching and tempering process is carried out as step (S30). In this step (S30), a quenching and tempering treatment is performed on the formed body obtained in step (S20). Specifically, first, after the formed body is heated to a temperature range of 945 °C or higher and 1050 °C or lower, it is cooled from this temperature range to a temperature below the M S point of the steel. In this way, the entire formed body is quenched. Next, after the quenched formed body is heated to a temperature range of 570 °C or higher and 620 °C or lower, it is cooled to room temperature. In this way, the entire formed body is subjected to high-temperature tempering treatment. As a result, the structure of the steel constituting the formed body becomes a high-temperature tempered martensite structure throughout the entire area. Here, by setting the heating temperature for the quenching treatment to a temperature range of 945 °C or higher and 1050 °C or lower, which is higher than the general heating temperature, the M 23 C 6 carbides dissolve and disappear in the matrix of the steel.

[0062] Next, a high-hardness part forming process is carried out as step (S40). In this step (S40), first, the region where the high-hardness part 7A is to be formed in the formed body tempered in step (S30) is heated to a temperature range of 900 °C or higher, and then cooled from this temperature range to a temperature below the M S point of the steel. In this way, the high-hardness part 7A is formed in the formed body. The partial heating of the formed body can be carried out, for example, by induction heating. The M SCooling to a temperature below the point can be carried out, for example, by cooling using water or an aqueous solution of water-soluble oil as a cooling medium. The cooling is preferably continued until the surface temperature of the molded body reaches a temperature of 100°C or lower. The cooling is continued, for example, until the surface temperature of the molded body reaches a temperature of 50°C or higher and 100°C or lower.

[0063] At this time, the cooling rate in the above cooling is determined such that the area ratio of the above carbides (fine carbides (having a size of 500 nm or less) dispersed in the laths of martensite) in the cross-section of the high-hardness portion 7A is 9% or more. Specifically, the cooling rate can be determined as follows. When there are no plate-like carbides generated in the martensite constituting the parent phase and the area ratio of the fine carbides having a size of 500 nm or less is less than 9% as shown in, for example, FIG. 7 (in FIG. 7, the area ratio of the fine carbides is 3.3%), the area ratio of the fine carbides increases by reducing the cooling rate. By reducing the cooling rate, the area ratio of the fine carbides can be made 9% or more as shown in, for example, FIG. 5 (in FIG. 5, the area ratio of the fine carbides is 14.0%). When the cooling rate is further reduced, plate-like carbides (carbides having a size exceeding 500 nm) are generated at the boundary portion between adjacent laths as shown in, for example, FIG. 6, and the area ratio of the carbides having a size of 500 nm or less in the laths decreases to 5.0%. The generation of such plate-like carbides at the boundary portion of the laths and a small amount of carbides in the laths reduces the necking value in the tensile test of the high-hardness portion 7A. Therefore, it is preferable to achieve avoidance of the generation of plate-like carbides and optimization of the area ratio of the fine carbides in the laths (9% or more). Therefore, when the generation of plate-like carbides is confirmed and the amount of fine carbides in the laths is small (less than 9%), it is preferable to increase the cooling rate. By adjusting the cooling rate in this way, the area ratio of the fine carbides in the high-hardness portion 7A can be made 9% or more, for example, 9% or more and 15% or less, and the necking value in the tensile test of the high-hardness portion 7A can be made 40% or more.

[0064] Thus, by deliberately setting the cooling rate in the quenching process to be small within the range where sufficient quenching can be achieved, a large amount of carbide can be generated in the laths of martensite that constitute the matrix of the high-hardness portion 7A. Thereafter, after the high-hardness portion 7A is heated to a temperature range of 150°C or higher and 200°C or lower, it is cooled to room temperature (low-temperature tempering). As a result, the hardness of the steel constituting the high-hardness portion 7A is adjusted to a range of 57 HRC or higher and 60 HRC or lower.

[0065] Next, as step (S50), a finishing process is carried out as necessary. In this step (S50), necessary finishing processes and the like are carried out on the formed body obtained by carrying out steps (S10) to (S40). Through the above process, the crawler link 7 in the present embodiment can be manufactured.

[0066] According to the manufacturing method of the crawler link of the present embodiment, by appropriately setting the cooling rate in the quenching process of step (S40), the area ratio of carbide in the cross-section of the high-hardness portion 7A can be made 9% or more. Also, in step (S20), when hot forging and forming a steel material made of steel having the above appropriate component composition, M 23 C 6 carbide is made to disappear by raising the heating temperature in the quenching process of step (S30) to 945°C or higher. In this way, the crawler link 7, which is a wear-resistant part with excellent durability, can be manufactured.

Example

[0067] An experiment was conducted to prepare samples corresponding to the high-hardness portions of the wear-resistant parts of the present disclosure using steel materials A to D made of steel having the above appropriate component composition and evaluate their properties. The experimental procedure is as follows.

[0068] First, steel materials A to D made of steel having the above appropriate component composition were prepared. Also, for comparison, steel materials E and F having component compositions outside the range of the steel constituting the wear-resistant parts of the present disclosure were also prepared. The specific component compositions of the steel materials are shown in Table 1. In Table 1, the unit of each numerical value is mass%.

[0069] [Table 1] (Experiment on mechanical properties) Using the steel materials in Table 1, samples corresponding to the high-hardness parts were produced by the same process as the steps (S10) to (S40) in the above embodiment. Tensile test pieces and Charpy impact test pieces (JIS standard; 2 mm U-notch) were produced from the obtained samples, and tensile tests and impact tests were carried out. Also, the hardness of the samples obtained in the same way was measured. The experimental results are shown in Table 2. In Table 2, the display of "-" means that the corresponding experiment was not carried out.

[0070] [Table 2]

[0071] Referring to Table 2, although steel materials E and F, which are outside the range of the steel constituting the wear-resistant parts of the present disclosure, have large drawing values and impact values, their hardness, 0.2% proof stress, and tensile strength are insufficient. On the other hand, steel materials A, B, C, and D, which are within the range of the steel constituting the wear-resistant parts of the present disclosure, ensure a hardness of 57 HRC or more, a 0.2% proof stress of 1530 MPa or more (more preferably 1550 MPa or more), and a tensile strength of 2100 MPa or more (more preferably 2150 MPa or more), while ensuring a drawing value of 40% or more. From this, it is confirmed that the steel constituting the wear-resistant parts of the present disclosure is an appropriate steel from the viewpoint of achieving both wear resistance and toughness.

[0072] (Experiment on area ratio of carbides) Next, using the steel material D in Table 1 above, in the same process as the steps (S10) to (S40) in the above embodiment, a tensile test piece (a test piece corresponding to the high-hardness part) with the area ratio of carbides changed by changing the cooling rate in step (S40) was produced. Then, a tensile test was carried out using the test piece, and the relationship between the area ratio of carbides and the drawing value in the tensile test was investigated.

[0073] Figs. 5, 6 and 7 are SEM (scanning electron microscope) photographs showing the microstructure of the steel constituting the tensile test piece. Referring to Figs. 5 to 7, it can be seen that the martensite structure constituting the matrix phase of the steel is in a state of being divided into a large number of laths 91, and a large number of fine carbides 92 having a size of 100 nm or less are dispersed in the laths 91. The area ratio of the carbide 92 was investigated by binarizing these photographs into black and white using image processing software and calculating the area ratio of the carbide 92 detected as white dots. In Fig. 5, the area ratio of the carbide 92 is 14.0% which is within the range required for the wear-resistant parts of the present disclosure, and the reduction value of the corresponding test piece was 44%. On the other hand, in Fig. 6, the area ratio of the carbide 92 is 5.0% which is outside the range required for the wear-resistant parts of the present disclosure, and the reduction value of the corresponding test piece was 29%. It is considered that the reduction value of the reduction decreased significantly because the area ratio of the carbide in the lath was small, a large amount of carbon was dissolved in the matrix phase, and in addition, plate-like carbides were precipitated at the lath boundaries, making the lath boundaries fragile. Further, in Fig. 7, the area ratio of the carbide 92 is 3.3% which is outside the range required for the wear-resistant parts of the present disclosure, and the reduction value of the corresponding test piece was 32%. This is considered to be because, as a result of the small amount of carbide precipitation, a large amount of carbon was dissolved in the matrix phase, reducing the reduction value.

[0074] Fig. 8 is a diagram showing the relationship between the area ratio of the carbide and the reduction. Fig. 8 illustrates the results of investigating the correspondence between the area ratio of the carbide and the reduction value as described above. Referring to Fig. 8, it is confirmed that there is a clear correlation between the area ratio of the carbide and the reduction value. When the area ratio of the carbide is 9% or more, the reduction value is 40% or more which is a preferable value, whereas when the area ratio of the carbide is less than 9%, the reduction value is less than 40%.

[0075] From the above experimental results, it is confirmed that according to the wear-resistant parts of the present disclosure, an improvement in durability can be achieved by achieving both wear resistance and toughness.

[0076] In the above-described embodiment, the crawler link has been described as an example of the wear-resistant component of the present disclosure. However, the wear-resistant component of the present disclosure includes various wear-resistant components having a high-hardness portion with a hardness of 57 HRC or more and 60 HRC or less in a part of the component, for example, a crawler bushing, a crawler pin, a sprocket tooth, a lower idler, an upper idler, and a track shoe as crawler components which are wear-resistant components subjected to an impact load. In addition, it can also be applied to a ripper point, a tooth of a crusher, a work machine pin, and the like.

[0077] It should be understood that the embodiments and examples disclosed this time are illustrative in all respects and not restrictive in any way. The scope of the present invention is defined not by the above description but by the claims, and it is intended that all modifications within the meaning and scope equivalent to the claims be included.

Description of Reference Numerals

[0078] 1 Crawler-type traveling device, 2 Crawler, 3 Truck frame, 4 Idler, 5 Sprocket, 6 Track shoe, 7 Crawler link, 7A High-hardness portion, 7B Low-hardness portion, 10 Lower idler, 11 Upper idler, 21 Crawler bushing, 22 Dust seal, 23 Crawler pin, 23A Lubricant holding hole, 23B Lubricant supply path, 23C Plug, 51 Sprocket tooth, 75A Bushing hole, 75B Crawler pin hole, 76 Boss portion, 77 Tread surface, 91 Lath, 92 Carbide.

Claims

1. 0.41% by mass or more and 0.44% by mass or less of C; 0.2% by mass or more and 0.6% by mass or less of Si; 0.2% by mass or more and 1.5% by mass or less of Mn; 0.0005% by mass or more and 0.0050% by mass or less of S; 0.6% by mass or more and 2.0% by mass or less of Ni; 0.7% by mass or more and 1.5% by mass or less of Cr; 0.1% by mass or more and 0.6% by mass or less of Mo; 0.02% by mass or more and 0.04% by mass or less of Nb; 0.015% by mass or more and 0.03% by mass or less of Ti; 0.0005% by mass or more and 0.0030% by mass or less of B; The steel contains 20 mass ppm or more and 120 mass ppm or less of N, and the remainder is iron and impurities, a high-hardness portion having a hardness of 57 HRC or more and 60 HRC or less and having a martensite structure; A low hardness portion is a region other than the high hardness portion, has a hardness lower than that of the high hardness portion, and has a martensite structure, The high hardness portion is A matrix composed of martensite; Carbides dispersed within the laths of the matrix; The area ratio of the carbides in a cross section of the high hardness portion is 9% or more.

2. 0.41% by mass or more and 0.44% by mass or less of C; 0.2% by mass or more and 0.6% by mass or less of Si; 0.2% by mass or more and 1.5% by mass or less of Mn; 0.0005% by mass or more and 0.0050% by mass or less of S; 0.6% by mass or more and 2.0% by mass or less of Ni; 0.7% by mass or more and 1.5% by mass or less of Cr; 0.1% by mass or more and 0.6% by mass or less of Mo; 0.02% by mass or more and 0.04% by mass or less of Nb; 0.015% by mass or more and 0.03% by mass or less of Ti; 0.0005% by mass or more and 0.0030% by mass or less of B; N in an amount of 20 ppm by mass or more and 120 ppm by mass or less; and at least one selected from the group consisting of 0.05% by mass or more and 0.20% by mass or less of V, 0.01% by mass or more and 0.15% by mass or less of Zr, 0.1% by mass or more and 2.0% by mass or less of Co, 0.05% by mass or more and 0.40% by mass or less of Cu, and 0.005% by mass or more and 0.050% by mass or less of Al, with the balance being iron and impurities; a high-hardness portion having a hardness of 57 HRC or more and 60 HRC or less and having a martensite structure; A low hardness portion is a region other than the high hardness portion, has a hardness lower than that of the high hardness portion, and has a martensite structure, The high hardness portion is A matrix composed of martensite; Carbides dispersed within the laths of the matrix; The area ratio of the carbides in a cross section of the high hardness portion is 9% or more.

3. The high hardness portion and the low hardness portion are M ２３ C ６ M is a metal element constituting the steel. ２３ C ６ 3. The wear-resistant part according to claim 1 or 2, which is free of carbides.

4. 3. The wear-resistant part according to claim 1, wherein the high hardness portion has a reduction in area of ​​40% or more in a tensile test.

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