IMPACT AND WEAR RESISTANT COMPONENT, AND METHOD FOR PRODUCING THE SAME

- KOMATSU LTD.

A ripper shank as the impact and wear resistant component is made of a steel of a specific component composition which has a hardness of HRC 53 or more and HRC 57 or less. The steel includes a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN. The steel does not include a M23C6 carbide.

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Description
TECHNICAL FIELD

The present invention relates to a component (impact and wear resistant component) that is subjected to repeated impact and wears by contact with earth and sand, such as a ground engaging tool (hereinafter, GET) component used in construction or mining equipment, and to a method for producing the same.

This application claims priority based on Japanese Patent Application No. 2018-243881 filed on Dec. 27, 2018, the entire contents of which are incorporated herein by reference.

BACKGROUND ART

A ripper device is a rear attachment of a work vehicle such as a bulldozer, and is used to scrape up earth, sand, and bedrock. Ripping work can be performed as the work machine is advanced with a ripper point attached to the distal end of the ripper shank being penetrated into the ground. While the ripper shank is a strength member of the ripper device, it is an impact and wear resistant component that suffers wear and deformation. Although SCrB steel, JIS SNCM431H steel, etc. have conventionally been used as the steel material constituting the ripper shank, a material having even better durability is desired.

To improve the durability of an impact and wear resistant component, it is necessary to impart high wear resistance and high proof stress (strength) to the material constituting the component. Simply increasing the strength of a component, however, leads to reduction in toughness of the material constituting the component. The surface of the component may crack or the component may break, giving rise to the need for replacement of the component. As such, in order to improve the durability of the impact and wear resistant component, it is necessary to maintain ductility (toughness) at a high level while achieving high proof stress (strength) of the material.

As a steel material constituting a component of construction equipment, a high-toughness and wear-resistant steel having excellent durability has been proposed (see, for example, Japanese Patent Application Laid-Open No. S61-166954 (Patent Literature 1)). Further, as a steel for a tracked undercarriage component, a steel containing about 0.4 mass % carbon and various alloy elements added therein has been proposed (see, for example, Japanese Translation of PCT International Publication No. 2014/185337 (Patent Literature 2)).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. S61-166954

Patent Literature 2: Japanese Translation of PCT International Publication No. 2014/185337

SUMMARY OF INVENTION Technical Problem

When an impact and wear resistant component, particularly a GET component, is produced using the steel disclosed in Patent Literature 1 or 2, the resultant component will have a high strength. Further, a steel having an improved 0.2% proof stress will be able to, for example, suppress deformation (plastic flow) of the contact surface with the ripper point in the ripper shank. However, when the steel material disclosed in Patent Literature 1 is used to produce a large ripper shank having a wall thickness of 100 mm and a mass of about 1 ton, for example, the component will suffer a decrease in strength (insufficient hardenability) at the center in its wall thickness. Further, a component produced using the steel disclosed in Patent Literature 2 through a common production process tends to exhibit a small reduction of area in a tensile test. According to the investigations conducted by the present inventors, the smaller reduction of area in the tensile test leads to lower resistance to breakage. That is, further improvement in durability is desired for the impact and wear resistant component produced through a common production process using the steel disclosed in Patent Literature 2.

One of the objects of the present invention is to provide an impact and wear resistant component excellent in durability and a method for producing the same.

Solution to Problem

An impact and wear resistant component according to the present invention is made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities, and having a hardness of HRC 53 or more and HRC 57 or less. The steel includes a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN. The steel does not include a carbide represented as M23C6 (where M represents the metallic elements constituting the steel).

A method for producing an impact and wear resistant component according to the present invention includes the steps of: preparing a steel material made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities; hot forging or hot rolling the steel material to obtain a formed body; performing normalizing treatment on an entirety of the formed body by cooling the formed body from a temperature not lower than 945° C. and not higher than 1000° C. to a temperature not higher than a temperature corresponding to the Ms point of the steel; and performing quench hardening treatment on the formed body having undergone the normalizing treatment and, thereafter, adjusting a hardness of the formed body to be HRC 53 or more and HRC 57 or less by heating the formed body to a temperature not lower than 150° C. and not higher than 250° C.

Effects of the Invention

According to the impact and wear resistant component and its producing method described above, it is possible to provide an impact and wear resistant component excellent in durability and a method for producing the same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing the structure of a ripper device including a ripper shank and a ripper point;

FIG. 2 is a schematic perspective view showing the state of connection between the ripper shank and the ripper point;

FIG. 3 is a schematic cross-sectional view showing the structure of the ripper shank;

FIG. 4 is a flowchart schematically illustrating the steps of producing a ripper shank;

FIG. 5 shows optical micrographs of a microstructure of the steel;

FIG. 6 shows SEM photographs of nonmetallic particles;

FIG. 7 shows observation results using an optical microscope and SEM, and elemental mapping results;

FIG. 8 shows a result of identification of a product present at a grain boundary; and

FIG. 9 shows a relationship between heating temperature and reduction of area.

 DESCRIPTION OF EMBODIMENT Outline of Embodiment

An impact and wear resistant component of the present application is made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities, and having a hardness of HRC 53 or more and HRC 57 or less. The steel includes a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN. The steel does not include a carbide represented as M23C6 (where M represents the metallic elements constituting the steel).

In the impact and wear resistant component described above, the steel may further contain at least one species selected from the group consisting of not less than 0.05 mass % and not more than 0.20 mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr, and not less than 0.1 mass % and not more than 2.0 mass % Co.

Firstly, a description will be made about the reasons for limiting the component composition of the steel constituting the impact and wear resistant component of the present application to the above-described ranges.

Carbon (C): Not Less than 0.41 Mass % and Not More than 0.44 Mass %

Carbon is an element that greatly affects the hardness of the steel. If the carbon content is less than 0.41 mass %, it will be difficult to obtain a hardness of HRC 53 or more in a portion having a wall thickness of about 100 mm, for example, with quenching and tempering. On the other hand, the carbon content exceeding 0.44 mass % will decrease the reduction of area and reduce the breakage resistance. The carbon content is thus necessary to be within the above-described range. From the standpoint of readily securing a sufficient hardness, the carbon content is preferably 0.42 mass % or more.

Silicon (Si): Not Less Than 0.2 Mass % and Not More Than 0.5 Mass %

Silicon is an element that has the effects of improving the hardenability of the steel, enhancing the matrix of the steel, and improving the resistance to temper softening, and also has a deoxidizing effect in the steelmaking process. If the silicon content is 0.2 mass % or less, the above effects cannot be obtained sufficiently. If the silicon content exceeds 0.5 mass %, however, the reduction of area tends to decrease. The silicon content is thus necessary to be within the above-described range.

Manganese (Mn): Not Less Than 0.2 Mass % and Not More Than 1.5 Mass %

Manganese is an element effective in improving the hardenability of the steel, and also having a deoxidizing effect in the steelmaking process. If the manganese content is 0.2 mass % or less, the above effects cannot be obtained sufficiently. If the manganese content exceeds 1.5 mass %, however, the hardness before quench hardening will increase, leading to degradation in workability. From the standpoint of securing sufficient hardenability of the steel, the manganese content is preferably 0.4 mass % or more. Further, focusing on the workability, the manganese content is preferably 0.9 mass % or less, and more preferably 0.8 mass % or less.

Sulfur (S): Not Less Than 0.0005 Mass % and Not More Than 0.0050 Mass %

Sulfur is an element that improves the machinability of the steel. Sulfur is also an element that is mixed during the steelmaking process even if not added intentionally. If the sulfur content is less than 0.0005 mass %, the machinability will decrease, and the production cost of the steel will increase. On the other hand, according to the investigations of the present inventors, in the component composition of the steel of the present application, the sulfur content greatly affects the reduction of area. If the sulfur content exceeds 0.0050 mass %, the reduction of area will decrease, making it difficult to obtain sufficient breakage resistance. The sulfur content is thus necessary to be within the above-described range. The sulfur content of 0.0040 mass % or less can further improve the breakage resistance.

Nickel (Ni): Not Less Than 0.6 Mass % and Not More Than 2.0 Mass %

Nickel is an effective element in improving the toughness of the matrix of the steel. If the nickel content is less than 0.6 mass %, such an effect cannot be exerted sufficiently. If the nickel content exceeds 2.0 mass %, however, nickel becomes more likely to segregate in the steel. This may cause variation in the mechanical properties of the steel. The nickel content is thus necessary to be within the above-described range. Further, with the nickel content exceeding 1.5 mass %, the improvement in toughness will become moderate, and the production cost of the steel will increase. From these standpoints, the nickel content is preferably 1.5 mass % or less. On the other hand, in the case of a steel having a hardness of HRC 53 or more, in order to sufficiently exert the effect of improving the toughness of the matrix of the steel, the nickel content is preferably 1.0 mass % or more.

Chromium (Cr): Not Less Than 0.7 Mass % and Not More Than 1.5 Mass %

Chromium improves the hardenability of the steel and also enhances the resistance to temper softening. In particular, chromium being added in combination with molybdenum, niobium, vanadium, and the like considerably enhances the resistance to temper softening of the steel. If the chromium content is less than 0.7 mass %, the above effects cannot be exerted sufficiently. If the chromium content exceeds 1.5 mass %, however, the improvement of the resistance to temper softening will become moderate, and the production cost of the steel will increase. The chromium content is thus necessary to be within the above-described range.

Molybdenum (Mo): Not Less Than 0.1 Mass % and Not More Than 0.6 Mass %

Molybdenum improves the hardenability of the steel and enhances the resistance to temper softening. Molybdenum also has the function of improving the high temperature tempering brittleness. If the molybdenum content is less than 0.1 mass %, the above effects cannot be exerted sufficiently. If the molybdenum content exceeds 0.6 mass %, however, the above effects will be saturated. The molybdenum content is thus necessary to be within the above-described range.

Niobium (Nb): Not Less Than 0.02 Mass % and Not More Than 0.03 Mass %

Niobium is effective in improving the strength and toughness of the steel. In particular, niobium is a highly effective element in improving the toughness because it makes the crystal grains of the steel extremely fine when added in combination with chromium and molybdenum. To secure such effects, the niobium content should be 0.02 mass % or more. If the niobium content exceeds 0.03 mass %, however, the crystallization of coarse eutectic NbC and the formation of a large amount of NbC cause a decrease in the amount of carbon in the matrix, leading to degradation in strength and toughness of the steel. Further, the niobium content exceeding 0.03 mass % will increase the production cost of the steel. The niobium content is thus necessary to be within the above-described range.

Titanium (Ti): Not Less Than 0.01 Mass % and Not More Than 0.04 Mass %

Titanium is effective in improving the toughness of the steel. Further, the addition of Ti can form Ti(C,N) and refine the crystal grains of the steel. If the titanium content is less than 0.01 mass %, such effects are small. If the titanium content exceeds 0.04 mass %, however, the toughness of the steel may rather deteriorate. The titanium content is thus necessary to be within the above-described range.

Boron (B): Not Less Than 0.0005 Mass % and Not More Than 0.0030 Mass %

Boron is an element that considerably improves the hardenability of the steel. The addition of boron can decrease the addition amounts of the other elements added for the purpose of improving the hardenability, and can reduce the production cost of the steel. As compared to phosphorus (P) and sulfur, boron is more likely to segregate in the prior austenite grain boundary, and it particularly expels sulfur from the grain boundary, thereby improving the grain boundary strength. If the boron content is 0.0005 mass % or less, the above effects cannot be exerted sufficiently. The boron content exceeding 0.0030 mass %, however, may decrease the toughness of the steel. The boron content is thus necessary to be within the above-described range.

Nitrogen (N): Not Less Than 20 Mass ppm and Not More Than 60 Mass ppm

Nitrogen may deteriorate the toughness of the steel, except the case where nitrogen together with carbon forms carbonitrides with Ti or Nb to refine the crystal grains. The nitrogen content is thus necessary to be 60 mass ppm or less. The nitrogen content of less than 20 mass ppm, however, will increase the production cost of the steel. The nitrogen content is thus necessary to be within the above-described range.

Vanadium (V): Not Less Than 0.05 Mass % and Not More Than 0.20 Mass %

Vanadium is not an indispensable element. Vanadium, however, forms fine carbides, contributing to the refinement of crystal grains. If the vanadium content is less than 0.05 mass %, the above effect cannot be obtained sufficiently. If the vanadium content exceeds 0.20 mass %, however, the above effect will be saturated. Vanadium is a relatively expensive element, so it is preferably added in a minimum required amount. Thus, in the case of adding vanadium, the addition amount within the above-described range is appropriate.

Zirconium (Zr): Not Less Than 0.01 Mass % and Not More Than 0.15 Mass %

Zirconium is not an indispensable element, but it has the effect of further improving the toughness of the steel by making carbides in the form of fine spherical particles dispersed in the steel. If the zirconium content is less than 0.01 mass %, its effect cannot be obtained sufficiently. If the zirconium content exceeds 0.15 mass %, however, the toughness of the steel may rather deteriorate. Thus, in the case of adding zirconium, the addition amount within the above-described range is appropriate.

Cobalt (Co): Not Less Than 0.1 Mass % and Not More Than 2.0 Mass %

Cobalt is not an indispensable element, but it increases the solid solubility of chromium, molybdenum, and other carbide-forming elements to the matrix, and also improves the resistance to temper softening of the steel. The addition of cobalt thus achieves finer carbides and a higher tempering temperature, thereby improving the strength and toughness of the steel. If the cobalt content is less than 0.1 mass %, the above effects cannot be obtained sufficiently. On the other hand, because of its expensiveness, cobalt added in a large amount will increase the production cost of the steel. These problems become prominent with a cobalt content exceeding 2.0 mass %. Thus, in the case of adding cobalt, the addition amount within the above-described range is appropriate.

Unavoidable Impurities

Besides the components intentionally added during the production process, elements other than those described above may be mixed into the steel as unavoidable impurities. Phosphorus (P) as an unavoidable impurity is preferably contained in an amount of 0.010 mass % or less. Copper (Cu) as an unavoidable impurity is contained in an amount of preferably 0.1 mass % or less and more preferably 0.05 mass % or less. Aluminum (Al) as an unavoidable impurity is contained in an amount of preferably 0.04 mass % or less.

The impact and wear resistant component of the present application is made of a steel having the above-described appropriate component composition. Further, in the impact and wear resistant component of the present application, the steel constituting the impact and wear resistant component does not include a carbide represented as M23C6 (where M represents the metallic elements constituting the steel, mainly at least one of Cr and Mo; hereinafter, referred to as “M23C6 carbide”).

According to the investigations conducted by the present inventors, in the case of adopting a steel having the above-described appropriate component composition as the steel constituting an impact and wear resistant component, when the component is produced with a common production process, M23C6 carbides are generated at the grain boundaries of the steel. With the M23C6 carbides generated, the Cr and Mo contents decrease in the region around the M23C6 carbides. The hardenability in the region thus decreases, and a bainite structure is formed. That the steel contains not only a martensite structure, but also brittle M23C6 carbides at the grain boundaries as well as brittle bainite structure near the grain boundaries attributable thereto, results in a smaller reduction of area in the tensile test of the steel. A lower reduction of area of the steel leads to a reduced breakage resistance of the impact and wear resistant component made of the steel.

As a result of investigating the way of improving the durability of the impact and wear resistant components, the present inventors have obtained findings that adopting a steel having the above-described appropriate component composition and eliminating the M23C6 carbides from the steel structure can obtain an impact and wear resistant component improved in breakage resistance and excellent in durability. In the impact and wear resistant component of the present application, the steel having the above-described appropriate component composition is adopted as the steel constituting the impact and wear resistant component, and no M23C6 carbides are included in the steel structure. The impact and wear resistant component of the present application is thus an impact and wear resistant component excellent in durability.

In the present application, the state where the steel includes no M23C6 carbides means a state where M23C6 carbides are not found when the cross section of the impact and wear resistant component is observed using a field-emission scanning electron microscope (FE-SEM) and an area of 80 μm2 including the grain boundary of the steel is examined for 10 or more fields of view. The M23C6 carbide can be identified, when a possible product of M23C6 carbide is found for example in the above-described manner, by detecting the product in a bright-field image of a scanning transmission electron microscope (STEM) and then confirming the selected area diffraction (SAD) pattern of the product.

In the impact and wear resistant component described above, the matrix may have a grain size number of 5 or more and 8 or less. With this configuration, excellent toughness can readily be imparted to the impact and wear resistant component.

In the impact and wear resistant component described above, the martensite phase constituting the matrix may be a low temperature-tempered martensite phase. With this configuration, excellent toughness can readily be imparted to the impact and wear resistant component.

As used herein, the low temperature-tempered martensite phase means a phase made up of a structure (obtained through low temperature tempering) which is obtained when a steel that has been quenched is tempered at a temperature not lower than 150° C. and not higher than 250° C. The phase being the low temperature-tempered martensite phase can be confirmed through investigation of the hardness, carbide precipitation state, etc. of the phase.

A method for producing an impact and wear resistant component of the present application includes the steps of: preparing a steel material made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities; hot forging or hot rolling the steel material to obtain a formed body; performing normalizing treatment on an entirety of the formed body by cooling the formed body from a temperature not lower than 945° C. and not higher than 1000° C. to a temperature not higher than a temperature corresponding to the Ms point of the steel; and performing quench hardening treatment on the formed body having undergone the normalizing treatment and, thereafter, adjusting a hardness of the formed body to be HRC 53 or more and HRC 57 or less by heating the formed body to a temperature not lower than 150° C. and not higher than 250° C.

In the impact and wear resistant component producing method described above, the steel may further contain at least one species selected from the group consisting of not less than 0.05 mass % and not more than 0.20 mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr, and not less than 0.1 mass % and not more than 2.0 mass % Co.

In the impact and wear resistant component producing method of the present application, after a steel material made of the steel having the above-described appropriate component composition is prepared, the steel material is hot forged or hot rolled to obtain a formed body. In the cooling process following the hot forging or hot rolling, M23C6 carbides are generated at the grain boundaries of the steel. Thereafter, in the impact and wear resistant component producing method of the present application, normalizing treatment is performed on the entirety of the formed body in which the formed body is cooled from a temperature not lower than 945° C. and not higher than 1000° C. to a temperature not higher than the temperature corresponding to the Ms point of the steel. With the normalizing treatment of heating to a temperature range of not lower than 945° C. and then cooling being performed, the M23C6 carbides previously generated dissolve into the matrix of the steel and disappear. Thereafter, quench hardening treatment is performed and then the formed body is heated to a temperature not lower than 150° C. and not higher than 250° C. to adjust the hardness of the steel to be HRC 53 or more and HRC 57 or less. In this manner, it is readily possible to produce the impact and wear resistant component of the present application that is made of the steel including no M23C6 carbides.

Specific Example of Embodiment

An embodiment of the impact and wear resistant component of the present invention will be described below with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.

Firstly, referring to FIGS. 1 to 3, a ripper shank as an impact and wear resistant component in the present embodiment will be described. FIG. 1 is a schematic view showing the structure of a ripper device including a ripper shank and a ripper point. FIG. 2 is an exploded perspective view of the ripper shank and the ripper point. FIG. 3 is a schematic cross-sectional view showing the structure of the ripper shank.

Referring to FIG. 1, the ripper device 1 of the present embodiment is, for example, a ripper device attached to a bulldozer. The ripper device 1 is attached to the rear (opposite the side on which a blade (soil removal plate) is disposed) of the vehicle body of the bulldozer. The ripper device 1 includes an arm 31, a lift cylinder 32, a tilt cylinder 33, a ripper support member 34, a ripper shank 10, and a ripper point 20.

The arm 31 has a rod shape. The arm 31 has one end connected to a bracket (not shown) mounted on the vehicle body of the bulldozer, and the other end connected to the ripper support member 34. The ripper support member 34 is pivotably connected to the other end of the arm 31.

The lift cylinder 32 and the tilt cylinder 33 have their one ends connected to the bracket (not shown) mounted on the vehicle body of the bulldozer. The lift cylinder 32 and the tilt cylinder 33 have their other ends connected to the ripper support member 34. The lift cylinder 32 and the tilt cylinder 33 are hydraulic cylinders that can be extended and contracted in the longitudinal direction. The ripper support member 34 is pivotably connected to the other ends of the lift cylinder 32 and the tilt cylinder 33. Of the ripper support member 34, the region connected to the lift cylinder 32 is located between the region connected to the arm 31 and the region connected to the tilt cylinder 33.

Referring to FIGS. 1 and 2, the ripper shank 10 is made of steel. The ripper shank 10 includes a distal end 15 as one end and a proximal end 14 as the other end in the longitudinal direction. The region including the distal end of the ripper shank 10 is bent toward the side approaching the vehicle body of the bulldozer. The region of the ripper shank 10 between its distal end 15 and proximal end 14 is supported by the ripper support member 34. The ripper point 20 is attached to the distal end 15 of the ripper shank 10. Of the ripper support member 34, the region connected to the arm 31 is positioned closer to the ripper point 20 as compared to the region connected to the tilt cylinder 33 and the region connected to the lift cylinder 32.

In the ripper device 1, the extension and contraction of the lift cylinder 32 cause the ripper shank 10 to move up and down. The extension and contraction of the tilt cylinder 33 cause the ripper shank 10 to tilt. With the ripper shank 10 in a lowered state and tilted to cause the ripper point 20 to penetrate the ground the vehicle body of the bulldozer is advanced, whereby earth, sand, and bedrock are scraped up.

Referring to FIGS. 1 to 3, the ripper shank 10 has a through hole, a ripper shank through hole 11, formed therein. The ripper point 20 has a through hole, a ripper point through hole 25, formed therein. In the state where the ripper point 20 is attached to the ripper shank 10, the ripper point through hole 25 and the ripper shank through hole 11 form a continuous through hole. A pin 51 inserted into the continuous through hole secures the ripper point 20 to the ripper shank 10.

Referring to FIG. 3, the ripper point 20 has a recess 22 formed to recess from its proximal end 23 side toward its distal end 21 side. The ripper shank 10 includes a body portion 12 including its proximal end 14 and an insert portion 13 including its distal end 15 on the side to be inserted into the recess 22. The recess 22 formed in the ripper point 20 has its bottom region 22A not in contact with the distal end 15 of the ripper shank 10. There is a space 29 between the bottom region 22A of the recess 22 and the distal end 15.

In the ripper device 1 in the present embodiment, the ripper shank 10 as the impact and wear resistant component is made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities, and having a hardness of HRC 53 or more and HRC 57 or less. The steel includes a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN. The steel does not include a carbide represented as M23C6 (where M represents the metallic elements constituting the steel). The amount of the residual austenite included in the matrix is 10 vol % or less, for example, and preferably 5 vol % or less.

The steel constituting the ripper shank 10 may further contain at least one species selected from the group consisting of not less than 0.05 mass % and not more than 0.20 mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr, and not less than 0.1 mass % and not more than 2.0 mass % Co.

The ripper shank 10 as the impact and wear resistant component of the present embodiment adopts the steel having the above-described appropriate component composition as the material, and the steel structure does not include M23C6 carbides. Accordingly, the ripper shank 10 as the impact and wear resistant component of the present embodiment is an impact and wear resistant component excellent in durability.

In the ripper shank 10, the matrix of the steel constituting the ripper shank 10 preferably has the grain size number (ASTM) of 5 or more and 8 or less. This facilitates imparting excellent toughness to the ripper shank 10.

In the ripper shank 10, the martensite phase constituting the matrix of the steel is preferably a low temperature-tempered martensite phase. This facilitates imparting excellent toughness to the ripper shank 10.

An exemplary method of producing a ripper shank 10 as the impact and wear resistant component of the present embodiment will now be described with reference to FIG. 4. In the method of producing the ripper shank 10 in the present embodiment, firstly, a steel material preparing step is performed as a step S10. In the step S10, a steel material made of the steel having the above-described appropriate component composition is prepared.

Next, a hot working step is performed as a step S20. In the step S20, the steel material prepared in the step S10 is subjected to hot forging or hot rolling and forming processing. With this, a formed body having an approximate shape of the ripper shank 10 is obtained. Hot forging or hot rolling is performed by, for example, heating the steel material prepared in the step S10 to a temperature not lower than 1200° C., such as 1250° C. In the cooling process following the hot forging or hot rolling, M23C6 carbides are formed at the grain boundaries of the steel.

Next, a normalizing step is performed as a step S30. In the step S30, the formed body obtained in the step S20 is subjected to normalizing treatment. Specifically, the formed body is firstly heated to a temperature range of not lower than 945° C. and not higher than 1000° C., and then cooled from the temperature range to a temperature not higher than the temperature corresponding to the Ms point of the steel. In this manner, the entirety of the formed body is normalized. Performing the normalizing treatment of heating to the temperature range of 945° C. or higher and 1000° C. or lower and then cooling causes the M23C6 carbides generated in the step S20 to dissolve into the matrix of the steel and disappear.

Next, a hardening treatment step is performed as a step S40. In the step S40, the formed body having undergone the normalizing treatment in the step S30 is firstly heated to a temperature range of 840° C. or higher and 920° C. or lower, for example, and then cooled from the temperature range to a temperature not higher than the Ms point of the steel. In this manner, the entirety of the formed body is quench hardened. The cooling to the temperature not higher than the Ms point of the steel can be performed, for example, by water cooling or oil cooling adopting water or oil as a cooling medium. The water cooling or oil cooling is continued until, for example, the surface temperature of the formed body becomes a temperature not lower than 50° C. and not higher than 100° C. Thereafter, the formed body is heated to a temperature range of not lower than 150° C. and not higher than 250° C. and then cooled to a room temperature (low temperature tempering). With this, the hardness of the steel constituting the formed body is adjusted to a range of HRC 53 or more and HRC 57 or less.

Next, a finishing step is performed as a step S50 as required. In the step S50, the formed body obtained through the steps S10 to S40 is subjected to any necessary finishing or other treatment. The ripper shank 10 in the present embodiment can be produced through the above-described process. The obtained ripper shank 10 is combined with a separately prepared ripper point 20, to obtain a ripper device 1.

According to the method for producing the ripper shank 10 of the present embodiment, the M23C6 carbides, generated along the grain boundaries of the steel during hot forging or hot rolling and forming the steel material made of the steel having the above-described appropriate component composition, are made to disappear by the normalizing treatment in the step S30, before the hardening treatment in the step S40. In this manner, the ripper shank 10 as the impact and wear resistant component excellent in durability can be produced.

Examples

Samples corresponding to the impact and wear resistant component of the present application were prepared using four types of steel materials, including one made of a steel having the above-described appropriate component composition, and experiments for evaluating their properties were conducted. The experimental procedures were as follows.

Table 1 shows chemical compositions of the steels used in the experiments. The values in Table 1 are in mass %. The steel material A has a component composition corresponding to the steel constituting the impact and wear resistant component of the present invention (Inventive Example). The steel materials B, C, and D have component compositions falling outside the scope of the present invention (Comparative Examples). The steel materials B, C, and D correspond to SCrB430H, JIS standard SNCM431H, and the steel disclosed in the aforementioned Patent Literature 1, respectively.

TABLE 1 C Si Mn P S Ni Cr Mo Nb Ti Al B N Fe A 0.43 0.30 0.40 0.008 0.004 1.29 0.99 0.48 0.03 0.02 0.033 0.0024 0.0035 Bal. B 0.30 0.23 0.93 0.021 0.015 0.05 1.09 0.03 not not 0.030 0.0017 not Bal. measured measured measured C 0.34 0.17 0.68 0.017 0.007 1.62 0.73 0.18 not not 0.028 not not Bal. measured measured measured measured D 0.41 0.30 0.47 0.010 0.007 0.03 0.96 0.50 0.03 0.02 0.044 0.0022 0.0051 Bal.

(Experiments on Mechanical Properties)

The steel materials in Table 1 were used to prepare samples through a process similar to the steps S10 to S40 in the above embodiment. From the obtained samples, tensile test specimens and Charpy impact test specimens (2 mm U-notch) were produced, and a tensile test, an impact test, and a Rockwell hardness measurement were conducted.

For the steel material A (Inventive Example) alone, the amount of residual austenite was measured using an X ray. The test results are shown in Table 2.

TABLE 2 0.2% Proof Tensile Reduction Impact Residual Stress Strength Elongation of Area Value Hardness γ Amount (MPa) (MPa) (%) (%) (J/cm2) (HRC) (vol %) A 1592 2131 14 44 62 56 2.3 B 1417 1655 13 47 65 48 not measured C 1414 1752 15 44 60 50 not measured D 1599 1935 13 43 59 53 not measured

Referring to Table 2, when comparing the Inventive Example with the Comparative Examples, the Inventive Example has achieved high values for the 0.2% proof stress, tensile strength, and impact value, while maintaining the reduction of area comparable to those of the Comparative Examples. Further, for the steel material A as the Inventive Example, as compared to the steel material D, the tensile strength has improved considerably despite their comparable 0.2% proof stress. The above demonstrates that the impact and wear resistant component of the present application is excellent in durability.

(Experiment on Steel Structure)

The steel material A in Table 1 (the steel material corresponding to the example of the present invention) was used to prepare a sample of a ripper shank in a similar procedure as in the above embodiment. A test specimen was taken from the sample. The surface of the obtained test specimen was polished and then etched with a nitric acid alcohol solution, and a microstructure was observed using an optical microscope. FIG. 5 shows optical micrographs showing the microstructure of the steel.

Referring to FIG. 5, it can be seen from the microstructure of the steel that the matrix includes a low temperature-tempered martensite phase. In the impact and wear resistant component of the present application, the presence of some residual austenite (of 10 vol % or less) is acceptable. For the sample obtained in a similar manner, the amount of residual austenite was measured using an X ray, and it was found that the residual austenite of 1 vol % to 3 vol % was present. The above demonstrates that the matrix of the steel includes the martensite phase and the residual austenite phase.

FIG. 6 shows photographs indicating the results of analysis by energy dispersive X-ray spectroscopy (EDX) of products that were found through observation of the steel structure with SEM. As shown in FIG. 6, it is confirmed that nonmetallic particles having a size of about 1 μm to about 20 μm (first nonmetallic particles including at least one species selected from the group consisting of MnS, TiCN, and NbCN) are dispersed in the matrix of the steel.

(Experiments on Carbides Formed at Grain Boundaries)

The steel material A (the steel material corresponding to the example of the present invention) in Table 1 was used to prepare a test specimen (as quenched; sample A) by performing the process of the above embodiment up to the step S20 (with the forging temperature of 1250° C.), not performing the step S30, and performing quenching treatment in the step S40 after heating the material to 870° C. A test specimen (as quenched; sample B) was also prepared, by similarly performing the process up to the step S20, performing normalizing treatment in the step S30 by heating the material to 970° C., and further performing quenching treatment in the step S40 after heating the material to 870° C. For the samples A and B, the microstructures were observed with an optical microscope and SEM, and for products present along the grain boundaries, elemental mapping was conducted with EDX. The experimental results are shown in FIG. 7.

Referring to FIG. 7, it can be seen that carbides of Mo and Cr are present along the grain boundaries in the sample A for which the step S30 was omitted, and that a bainite structure is formed around the carbides. The formation of the bainite structure is conceivably attributable to the local decrease in the amount of alloy elements because of the formation of the above carbides, and the resultant reduction in hardenability. In contrast, in the sample B corresponding to the impact and wear resistant component of the present invention for which normalizing treatment was conducted in the step S30 with the heating temperature of 970° C., no carbides as described above were found. The above experimental results show that although the above-described carbides formed during the hot working process remain with the quenching temperature of 870° C., the carbides dissolve and disappear with the normalizing temperature of 970° C.

An example of the identification of carbides present in the sample A is shown in FIG. 8, in which a carbide was detected in a bright-field image of STEM and then the selected area diffraction (SAD) pattern of the carbide was confirmed. As shown in FIG. 8, it can be seen that the carbide is a M23C6 carbide. That is to say, it has been confirmed that in the method of producing an impact and wear resistant component of the present application, the M23C6 carbides formed during the hot working process disappear by the heating during the normalizing conducted in the step S30.

(Experiment on Relationship between Heating Temperature and Reduction of Area)

The steel material A in Table 1 was used to prepare test specimens which were quench hardened by rapid cooling from various temperatures and then tempered at high temperature. The test specimens were subjected to a tensile test. At this time, the heating temperature upon quenching was varied to investigate the effect of the heating temperature on the reduction of area in the tensile test. The experimental results are shown in FIG. 9.

Referring to FIG. 9, it can be seen that the reduction of area clearly increases with the heating temperature of 945° C. or higher. This temperature range of not lower than 945° C. agrees with the temperature range in which M23C6 carbides cease to be seen in the experiment on the carbides formed at the grain boundaries. This indicates that the M23C6 carbides generated at the grain boundaries of the steel can be eliminated by the heating to the temperature range of not lower than 945° C., whereby the reduction of area is improved.

While the ripper shank was described as an example of the impact and wear resistant component of the present application in the above embodiment, the impact and wear resistant component of the present application is applicable to a variety of impact and wear resistant components made of a steel having a hardness of HRC 53 or more and HRC 57 or less, such as bucket teeth, bucket adapters, bucket shrouds, ripper points, protectors, cutting edges, end bits, crusher teeth, sprocket teeth, springs, shoe plates, shoe bolts, and the like.

It should be understood that the embodiment and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST

1: ripper device; 10: ripper shank; 11: ripper shank through hole; 12: body portion; 13: insert portion; 14: proximal end; 15: distal end; 20: ripper point; 21: distal end; 22: recess; 22A: bottom region; 23: proximal end; 25: ripper point through hole; 29: space; 31: arm; 32: lift cylinder; 33: tilt cylinder; 34: ripper support member; and 51: pin.

Claims

1. An impact and wear resistant component made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities, and having a hardness of HRC 53 or more and HRC 57 or less,

the steel including a matrix including a martensite phase and a residual austenite phase, and first nonmetallic particles dispersed in the matrix and including at least one species selected from the group consisting of MnS, TiCN, and NbCN,
the steel not including a carbide represented as M23C6 (where M represents the metallic elements constituting the steel).

2. The impact and wear resistant component according to claim 1, wherein the steel further contains at least one species selected from the group consisting of not less than 0.05 mass % and not more than 0.20 mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr, and not less than 0.1 mass % and not more than 2.0 mass % Co.

3. The impact and wear resistant component according to claim 1, wherein the matrix has a grain size number of 5 or more and 8 or less.

4. The impact and wear resistant component according to claim 1, wherein the martensite phase constituting the matrix is a low temperature-tempered martensite phase.

5. A method for producing an impact and wear resistant component, comprising the steps of:

preparing a steel material made of a steel containing not less than 0.41 mass % and not more than 0.44 mass % C, not less than 0.2 mass % and not more than 0.5 mass % Si, not less than 0.2 mass % and not more than 1.5 mass % Mn, not less than 0.0005 mass % and not more than 0.0050 mass % S, not less than 0.6 mass % and not more than 2.0 mass % Ni, not less than 0.7 mass % and not more than 1.5 mass % Cr, not less than 0.1 mass % and not more than 0.6 mass % Mo, not less than 0.02 mass % and not more than 0.03 mass % Nb, not less than 0.01 mass % and not more than 0.04 mass % Ti, not less than 0.0005 mass % and not more than 0.0030 mass % B, and not less than 20 mass ppm and not more than 60 mass ppm N, with the balance consisting of iron and unavoidable impurities;
hot forging or hot rolling the steel material to obtain a formed body;
performing normalizing treatment on an entirety of the formed body by cooling the formed body from a temperature not lower than 945° C. and not higher than 1000° C. to a temperature not higher than a temperature corresponding to the Ms point of the steel; and
performing quench hardening treatment on the formed body having undergone the normalizing treatment and, thereafter, adjusting a hardness of the formed body to be HRC 53 or more and HRC 57 or less by heating the formed body to a temperature not lower than 150° C. and not higher than 250° C.

6. The impact and wear resistant component producing method according to claim 5, wherein the steel further contains at least one species selected from the group consisting of not less than 0.05 mass % and not more than 0.20 mass % V, not less than 0.01 mass % and not more than 0.15 mass % Zr, and not less than 0.1 mass % and not more than 2.0 mass % Co.

7. The impact and wear resistant component according to claim 2, wherein the matrix has a grain size number of 5 or more and 8 or less.

8. The impact and wear resistant component according to claim 2, wherein the martensite phase constituting the matrix is a low temperature-tempered martensite phase.

9. The impact and wear resistant component according to claim 3, wherein the martensite phase constituting the matrix is a low temperature-tempered martensite phase.

10. The impact and wear resistant component according to claim 7, wherein the martensite phase constituting the matrix is a low temperature-tempered martensite phase.

Patent History
Publication number: 20220025475
Type: Application
Filed: Dec 26, 2019
Publication Date: Jan 27, 2022
Applicants: KOMATSU LTD. (Tokyo), NIPPON STEEL CORPORATION (Tokyo)
Inventors: Eiji AMADA (Tokyo), Kouji KITAMURA (Tokyo), Kazuo MAEDA (Tokyo), Naomi KOBAYASHI (Tokyo), Takashi NODA (Tokyo), Mamoru HATANO (Tokyo), Takafumi AMATA (Tokyo), Yutaka NEISHI (Tokyo), Kei MIYANISHI (Tokyo), Ryoji NISHIJIMA (Tokyo), Daisuke TAKIGUCHI (Tokyo)
Application Number: 17/311,886
Classifications
International Classification: C21D 9/00 (20060101); C21D 8/00 (20060101); C21D 6/00 (20060101); C21D 6/02 (20060101); C21D 1/18 (20060101); C22C 38/54 (20060101); C22C 38/50 (20060101); C22C 38/48 (20060101); C22C 38/46 (20060101); C22C 38/44 (20060101); C22C 38/04 (20060101); C22C 38/02 (20060101); C22C 38/00 (20060101); E02F 9/28 (20060101);