JP7758940B2 - steel parts - Google Patents

Description

The present invention relates to steel members.

Some steel components used in machinery, automobiles, and other parts require high rigidity. For example, in automobile internal combustion engines and transmissions, improving the rigidity of moving parts is desirable to reduce vibration and noise. To improve automobile fuel efficiency by making steel components smaller and lighter while maintaining or improving their rigidity, a material with a higher elastic modulus than steel (hereinafter referred to as a "high elastic modulus material") is required.

Examples of such high-elastic modulus materials include cemented carbides and cermets used in cutting tools. Cermets are composite materials in which hard compounds with a higher elastic modulus than steel, such as _TiB2 and NbC, are dispersed in metal particles, and many cermets have been proposed. However, these materials are very expensive compared to ordinary steel and are difficult to process, making plastic processing and cutting difficult. Therefore, replacing the entire steel component with such high-elastic modulus materials would significantly impair economic efficiency in terms of both material cost and processing cost.

Partial material replacement is one way to improve the rigidity of steel components without significantly increasing costs. In other words, by replacing only the areas of the steel component that require rigidity with a high-elasticity material, rather than the entire component, the rigidity of the steel component can be improved without significantly increasing costs. Furthermore, by replacing the outer periphery with a high-elasticity material, it is possible to efficiently improve bending rigidity and torsional rigidity.

For example, Patent Document 1 below describes a technology for providing a rotating shaft for a rotating body that is supported by bearings, where at least the section between the bearing and the rotating body is made of a high-elasticity material with a high Young's modulus, such as cemented carbide or cermet, thereby improving rigidity. Patent Document 1 also discloses that in the rotating shaft for a rotating body, at least the surface layer between the bearing and the rotating body is made of a composite material in which hard particles of at least one type of material selected from boride, carbide, and nitride are dispersed in a matrix metal.

Patent Document 2 also discloses a surface-hardened gear characterized in that the surface roughness of the gear surface in the load transfer direction satisfies 0.2 μm≦Rpk+0.5Rk≦0.8 μm.

Japanese Patent Application Laid-Open No. 2005-90587 Japanese Patent Application Laid-Open No. 2006-225741

As a result of the inventors' investigations, it was found that when a cermet or the like is coated on a sliding portion with another member (for example, the pin portion of a crankshaft), attention must be paid to the surface properties. For example, it was found that if hard compounds protrude from the surface, they act like abrasive grains in a grinding stone, causing significant wear and damage to the other member.
Although the above-mentioned Patent Document 1 discloses that the surface layer is made of a composite material in which hard particles are dispersed in a matrix metal, it does not take such issues into consideration and does not control the roughness.

Patent Document 2 discloses that if protruding "mountains" or, conversely, "valleys" of several microns exist within the contact area, stress concentration occurs around them, resulting in a decrease in pitting strength, and therefore, in order to improve pitting fatigue resistance, it is necessary to make the height of the roughness protrusions within the contact area approximately uniform. However, Patent Document 2 does not consider replacing the material with a high-elasticity material, and the rigidity of the component is insufficient. Furthermore, it does not consider wear and damage to other components caused by high-elasticity materials.
Furthermore, as a result of investigations by the present inventors, it was found that if the surface of the surface layer formed of cemented carbide or cermet becomes excessively smooth, it becomes difficult for lubricant to be supplied to the sliding surface, which may result in the stick-slip phenomenon or seizure. In other words, it was found that smoothing the surface as in Patent Document 2 may not be desirable depending on the target part.

In light of the above issues, the present invention aims to provide a steel member that has high rigidity, reduces wear and damage to other sliding components, and can also reduce seizure on the sliding surfaces.

The inventors investigated a method for suppressing wear and damage to other sliding components and suppressing seizure on the sliding surface of a steel component having a substrate and a surface layer made of a composite material containing a high-elasticity compound. As a result, they discovered that by controlling the height of the protruding peaks and the depth of the protruding valleys as the surface roughness in the sliding direction of the surface of the surface layer, it is possible to suppress wear and damage to other components while also suppressing seizure on the sliding surface.

The present invention was made based on the above findings. The gist of the present invention is as follows:

[1] A steel member comprising a substrate portion made of steel and a surface layer portion covering at least a portion of the substrate portion, wherein the surface layer portion is made of a composite material containing at least one compound of boride, carbide, and nitride, accounting for 10% or more of a cross section of the surface layer portion in terms of area percent, with the remainder being a binder, wherein the compound has a longitudinal elastic modulus of 300 GPa or more and a Vickers hardness of 1200 HV or more, an average particle diameter of 0.1 μm or more and 150 μm or less, an average thickness of the surface layer portion is 0.5 mm or more, and a surface roughness load curve in the sliding direction of the surface of the surface layer portion has a protruding peak height Rpk of 0.50 μm or less and a protruding valley depth Rvk of 0.10 μm or more.
[2] The steel member according to [1], wherein the substrate portion contains, as a chemical composition, in mass%, C: 0.10 to 0.55%, Si: 0.05 to 1.50%, Mn: 0.20 to 2.00%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0250%, P: 0.001 to 0.150%, S: 0.005 to 0.150%, with the balance being Fe and impurities.
[3] The base material further contains, as the chemical composition, one or two selected from, in mass%, Cr: 0.10 to 5.00% and Mo: 0.05 to 1.00%, in place of a portion of the remaining Fe. The steel member according to [2].
[4] The base material further contains, as the chemical composition, one or two selected from, by mass%, V: 0.05 to 0.50% and Ti: 0.05 to 0.30%, in place of a portion of the remaining Fe. The steel member according to [2] or [3].
[5] A steel member according to any one of [1] to [4], wherein the binder of the composite material is an iron-based alloy having a C content of 0.1 to 1.0 mass %, and the compound is one or more of NbC, TiC, VC, WC, SiC, Cr ₃ C ₂ , Mo ₂ C, ZrC, TiB ₂ , W ₂ B ₅ , Mo ₂ B ₅ , TiN, VN, NbN, and ZrN.
[ 6 ] The steel member according to any one of [1] to [ 5 ], wherein the surface layer portion has an average thickness of 0.5 to 30.0 mm.
[ 7 ] The steel member according to any one of [1] to [ 6 ], wherein an area ratio of the surface layer portion in a cross section passing through both the surface layer portion and the base material portion is 10.0 to 50.0%.

The present invention provides a steel member that has high rigidity, reduces wear and damage to other sliding components, and further reduces seizure on the sliding surface.

FIG. 1 is a diagram showing an outline of a block-on-ring test. 10 is a cross-sectional photograph of a surface layer of a steel member according to the present embodiment, made of a composite material, when there is no corrosion. 2B is a cross-sectional photograph of the surface layer portion of the steel member according to the present embodiment, which is made of a composite material, and which is the same as that shown in FIG. 2A, when the surface layer portion is corroded with nital. 1 is a cross-sectional photograph showing an example of a valley formed by partial dropping of a compound. FIG. 2 is a diagram showing an example of a roughness curve in the sliding direction of the surface of the steel member according to the present embodiment (the surface of the surface layer portion made of a composite material). FIG. 2 is an explanatory diagram for explaining the shape of a three-point bending test piece.

Preferred embodiments of the present invention will be described in detail below.
The following embodiments do not limit the present invention. Furthermore, the components of the following embodiments may include those that are easily replaceable by those skilled in the art, or may include those that are substantially identical. Furthermore, the various aspects included in the following embodiments may be combined in any way within the scope of obviousness to those skilled in the art.

(Regarding steel members)
A steel member according to one embodiment of the present invention (steel member according to the present embodiment) includes a base portion made of steel and a surface layer portion covering at least a portion of the base portion.
Each of these is explained further below.

<About the base material>
First, the substrate portion constituting the steel member will be described.
The substrate portion of the steel member according to this embodiment is made of steel.
The substrate portion of the steel member according to this embodiment may be entirely covered with a surface layer portion described in detail below, or may be partially covered with a surface layer portion described in detail below, with the substrate portion partially exposed on the surface. In other words, the substrate portion of the steel member according to this embodiment may have a region covered with a surface layer portion described in detail below and a region not covered with such a surface layer portion.
For example, in a gear shaft using the steel member according to this embodiment, a portion of the base material may be exposed, but if high rigidity is not required in such exposed portion, the base material may be exposed in a gear shaft using the steel member according to this embodiment.
The shape and outer diameter of the substrate are not limited, but when application to parts such as connecting rods and gear shafts is envisaged, the substrate may be rod-shaped with a circular or rectangular cross section and an outer diameter (the long diameter of the cross section in the case of a rectangle) of 10 to 50 mm, for example. Alternatively, when application to the pin portion of a crankshaft is envisaged, the substrate may be circular with an outer diameter of 30 to 120 mm, for example.

<Chemical composition of the base material>
The chemical composition of the substrate can be determined depending on the required mechanical properties, but it is preferable to use the chemical composition shown below.
That is, the substrate portion of the steel member according to this embodiment (sometimes referred to as the substrate portion according to this embodiment) preferably contains, in mass%, C: 0.10 to 0.55%, Si: 0.05 to 1.50%, Mn: 0.20 to 2.00%, Al: 0.005 to 0.100%, N: 0.0010 to 0.0250%, P: 0.001 to 0.150%, and S: 0.005 to 0.150%, with the balance consisting of Fe and impurities. Furthermore, the substrate portion according to this embodiment may, if necessary, further contain, in mass%, one or two of Cr: 0.10 to 5.00%, Mo: 0.05 to 1.00%, and/or V: 0.05 to 0.50%, and Ti: 0.05 to 0.30%, in place of a portion of the remaining Fe.

In the following explanation, the reasons for limiting the content of each chemical component in the preferred chemical composition of the substrate portion of the steel member according to this embodiment will be explained. The percentages (%) of each element shown below all refer to mass % unless otherwise specified.

[C: 0.10-0.55%]
Carbon (C) is an important element that significantly affects the strength of steel members. If the C content is less than 0.10%, sufficient strength may not be obtained. On the other hand, if the C content exceeds 0.55%, forgeability and machinability during part processing deteriorate. Therefore, in the substrate according to this embodiment, the C content is preferably 0.10 to 0.55%. In order to further improve the strength of the substrate according to this embodiment, the C content is more preferably 0.18% or more, and even more preferably 0.35% or more. Furthermore, in order to more reliably maintain forgeability and machinability during part processing, in the substrate according to this embodiment, the C content is more preferably 0.50% or less, and even more preferably 0.45% or less.

[Si: 0.05-1.50%]
Silicon (Si) is a useful element that increases the strength of steel members, improves temper softening resistance, and suppresses softening due to temperature increases. If the Si content is less than 0.05%, the above effects cannot be achieved. On the other hand, if the Si content exceeds 1.50%, the above effects saturate, and effects commensurate with the content cannot be expected. Therefore, in the substrate portion according to this embodiment, the Si content is preferably 0.05 to 1.50%. In order to more reliably achieve the above effects, in the substrate portion according to this embodiment, the Si content is more preferably 0.15% or more, and even more preferably 0.50% or more. Furthermore, in the substrate portion according to this embodiment, the Si content is more preferably 1.20% or less, and even more preferably 0.70% or less.

[Mn: 0.20-2.00%]
Manganese (Mn) is an element that improves the hardenability of steel material, thereby increasing the strength of the steel member, while suppressing red shortness and improving hot ductility. If the Mn content is less than 0.20%, the above effects cannot be achieved. On the other hand, if the Mn content exceeds 2.00%, the above effects saturate, and no effects commensurate with the content can be expected. Therefore, in the substrate portion according to this embodiment, the Mn content is preferably 0.20 to 2.00%. In order to more reliably achieve the above effects, in the substrate portion according to this embodiment, the Mn content is more preferably 0.60% or more, and even more preferably 1.00% or more. Furthermore, in the substrate portion according to this embodiment, the Mn content is more preferably 1.80% or less, and even more preferably 1.50% or less.

[Al: 0.005-0.100%]
Aluminum (Al) is an element that has a deoxidizing effect and also has the effect of suppressing coarsening of austenite grains by combining with N to form AlN during heat treatment. When the Al content is less than 0.005%, the above effect is not exerted. On the other hand, when the Al content exceeds 0.100%, the above effect is saturated. Therefore, in the substrate portion according to this embodiment, the Al content is preferably 0.005 to 0.100%. In order to more reliably exert the above effect, in the substrate portion according to this embodiment, the Al content is more preferably 0.015% or more, and even more preferably 0.030% or more. Furthermore, in the substrate portion according to this embodiment, the Al content is more preferably 0.080% or less, and even more preferably 0.050% or less.

[N:0.0010-0.0250%]
Nitrogen (N) is an element that combines with Al to form AlN, thereby suppressing the coarsening of austenite grains during heat treatment. When the N content is less than 0.0010%, the above effect cannot be fully achieved. On the other hand, when the N content exceeds 0.0250%, the above effect saturates. Therefore, in the substrate portion according to this embodiment, the N content is preferably 0.0010 to 0.0250%. In order to more reliably achieve the above effect, in the substrate portion according to this embodiment, the N content is more preferably 0.0030% or more, and even more preferably 0.0100% or more. Furthermore, in the substrate portion according to this embodiment, the N content is more preferably 0.0200% or less, and even more preferably 0.0150% or less.

[P: 0.001-0.150%]
Phosphorus (P) is an element typically contained as an impurity. Since P segregates at grain boundaries and reduces grain boundary strength, it is desirable to keep the P content as low as possible. However, although P can be reduced during the steelmaking process, reducing the P content to less than 0.001% significantly increases manufacturing costs. Furthermore, reducing the P content to less than 0.001% does not significantly improve grain boundary strength. Furthermore, steel for fracture split connecting rods may intentionally contain a large amount of P to obtain a brittle fracture surface during the splitting process. Therefore, the P content of the substrate according to this embodiment is preferably 0.001% or more. To more reliably obtain a brittle fracture surface, the P content is more preferably 0.050% or more, and even more preferably 0.080% or more.
On the other hand, if the P content exceeds 0.150%, the above effect saturates. Therefore, in the substrate according to this embodiment, the P content is preferably 0.150% or less. The P content is more preferably 0.120% or less, and even more preferably 0.100% or less.

[S: 0.005-0.150%]
Sulfur (S) is an element that improves the machinability of steel members. To obtain this effect, the S content is preferably 0.005% or more. To more reliably improve machinability, the S content is more preferably 0.040% or more, and even more preferably 0.060% or more.
On the other hand, if the S content is too high, S that is not fixed by Mn is generated as FeS at grain boundaries, resulting in a decrease in hot ductility. Therefore, in the base material according to this embodiment, the S content is preferably 0.150% or less. The S content is more preferably 0.120% or less, and even more preferably 0.100% or less.

The chemical composition of the substrate basically contains the above elements with the remainder being iron (Fe) and impurities, where impurities refer to components that are mixed in from ores or scrap used as raw materials for steel, or from the manufacturing process environment, etc., and are not components that are intentionally contained in the steel material.
On the other hand, for the purpose of improving mechanical properties, etc., one or more of Cr, Mo, V, and Ti shown below may be further contained in place of a portion of Fe. However, since it is not essential to contain these elements, the content may be 0%, or they may be contained as impurities in a content below the range described below.

[Cr:0.10-5.00%]
Chromium (Cr) is a useful element that improves the hardenability of steel and simultaneously improves the elastic modulus of steel members. Therefore, the substrate according to this embodiment may contain a predetermined amount of Cr in place of a portion of the remaining Fe. If the Cr content is less than 0.10%, the above effects may not be achieved. On the other hand, if the Cr content exceeds 5.00%, the forgeability and machinability during part processing may be reduced. Therefore, when Cr is contained in the substrate according to this embodiment, the Cr content is preferably 0.10 to 5.00%. In order to more reliably improve the hardenability and elastic modulus of the steel material in the substrate according to this embodiment, the Cr content is more preferably 0.90% or more. Furthermore, in order to suppress a reduction in forgeability and machinability during part processing, the Cr content is more preferably 3.00% or less in the substrate according to this embodiment.

[Mo: 0.05-1.00%]
Molybdenum (Mo) is a useful element for increasing the strength and hardenability of steel. Therefore, the substrate according to this embodiment may contain a predetermined amount of Mo in place of a portion of the remaining Fe. If the Mo content is less than 0.05%, the above effects may not be achieved. On the other hand, if the Mo content exceeds 1.00%, forgeability and machinability during part processing may be reduced. Therefore, when Mo is contained in the substrate according to this embodiment, the Mo content is preferably 0.05 to 1.00%. In order to more reliably improve the strength and hardenability of the steel material in the substrate according to this embodiment, the Mo content is more preferably 0.15% or more. Furthermore, in order to suppress a reduction in forgeability and machinability during part processing, the Mo content is more preferably 0.60% or less in the substrate according to this embodiment.

[V:0.05-0.50%]
Vanadium (V) is an element that forms vanadium carbide and/or vanadium carbonitride in steel to increase the strength of the steel and prevent coarsening of austenite grains during heat treatment. Furthermore, the formation of vanadium carbide and/or vanadium carbonitride in steel facilitates the formation of brittle fracture surfaces during the splitting process of a fracture split connecting rod when the steel member according to this embodiment is used to manufacture the fracture split connecting rod. Therefore, the substrate according to this embodiment may contain a predetermined amount of V in place of part of the remaining Fe. If the V content is less than 0.05%, the above-described effect may not be achieved. On the other hand, if the V content exceeds 0.50%, not only will the steel production cost increase, but effects commensurate with the content cannot be expected. Therefore, when V is contained in the substrate according to this embodiment, the V content is preferably 0.05 to 0.50%. To more reliably obtain the above-described effect in the substrate according to this embodiment, the V content is more preferably 0.15% or more. In addition, in the substrate according to this embodiment, in order to more reliably achieve an effect commensurate with the content while suppressing costs, the V content is more preferably 0.35% or less.

[Ti: 0.05-0.30%]
Titanium (Ti) is an element that forms titanium carbide and/or titanium carbonitride in steel to increase the strength of the steel and prevent coarsening of austenite grains during heat treatment. Furthermore, by incorporating Ti in combination with V, Ti and V composite carbides are more likely to form in the steel, making it easier to obtain brittle fracture surfaces during the splitting process of a fracture split-type connecting rod. Therefore, the substrate according to this embodiment may contain a predetermined amount of Ti in place of part of the remaining Fe. If the Ti content is less than 0.05%, the above-described effect may not be achieved. On the other hand, if the Ti content exceeds 0.30%, not only will the steel production cost increase, but the effect commensurate with the content cannot be expected. Therefore, when Ti is incorporated into the substrate according to this embodiment, the Ti content is preferably 0.05 to 0.30%. To more reliably obtain the above-described effect, the Ti content of the substrate according to this embodiment is more preferably 0.10% or more. In addition, in the substrate according to this embodiment, in order to more reliably achieve an effect commensurate with the content while suppressing costs, the Ti content is more preferably 0.20% or less.

As described above, the chemical composition of the substrate preferably contains C, Si, Mn, Al, N, P, and S, with the balance being iron (Fe) and impurities, or contains C, Si, Mn, Al, N, P, and S, and one or more of Cr, Mo, V, and Ti, with the balance being iron (Fe) and impurities.

<About the surface layer>
Next, the configuration of the surface layer portion of the steel member according to this embodiment (sometimes referred to as the surface layer portion according to this embodiment) will be described in detail.
As shown in FIGS. 2A and 2B , the surface layer 1 according to this embodiment is made of a composite material containing at least one compound 11 selected from borides, carbides, and nitrides, occupying 10% or more of the cross section of the surface layer in terms of area percent, with the remainder being a binder 12, and the compound 11 has a longitudinal elastic modulus of 300 GPa or more and a Vickers hardness of 1200 HV or more.
The average thickness of the surface layer portion according to this embodiment is 0.5 mm or more, and in the surface roughness load curve in the sliding direction of the surface of the surface layer portion, the protruding peak height Rpk is 0.50 μm or less and the protruding valley depth Rvk is 0.10 μm or more.

<<Compounds in the surface layer>>
The composite material constituting the surface layer of the steel member according to this embodiment contains at least one compound selected from the group consisting of borides, carbides, and nitrides.

[Modulus of Young's Modulus of Compound]
The longitudinal modulus of steel is approximately 200 GPa. Therefore, if the longitudinal modulus of the compound (high-modulus compound) contained for the purpose of improving rigidity (longitudinal modulus of elasticity) is less than 300 GPa, a sufficient effect of improving the modulus of elasticity cannot be obtained. Therefore, in the surface layer portion according to this embodiment, the longitudinal modulus of the compound contained in the composite material constituting the surface layer portion is set to 300 GPa or more. The longitudinal modulus of the compound is preferably 400 GPa or more, more preferably 500 GPa or more. On the other hand, the larger the longitudinal modulus of the compound, the better, and its upper limit is not particularly specified. However, when tensile stress is applied to the surface layer portion provided in the steel member according to this embodiment, there is a concern that the difference in elastic deformation between the metal portion serving as the binder and the compound may cause peeling at the interface between the binder (base material) and the high-modulus compound, or that large stress concentration may occur around the high-modulus compound. Therefore, from the viewpoint of suppressing such interfacial peeling and stress concentration, the longitudinal modulus of the compound is preferably 900 GPa or less, more preferably 700 GPa or less.

Examples of the compound having a longitudinal modulus of 300 GPa or more include NbC, TiC, VC, WC, SiC, _Cr3C2 , _Mo2C , ZrC, _TiB2 , _W2B5 , _Mo2B5 , TiN, VN, NbN, ZrN _, etc. In terms of specific Young _'s modulus (Young's modulus per specific gravity of 1), the compound _is preferably TiC or _TiB2 .

The longitudinal modulus of elasticity of compounds contained in composite materials can be measured as follows. That is, for compounds for which test specimens on the order of millimeters or larger can be prepared, the longitudinal modulus can be measured using a tensile test as specified in JIS Z2241:2011, or the resonance method or ultrasonic pulse method as specified in JIS Z2280:1993. Furthermore, for compounds for which it is difficult to prepare test specimens on the order of millimeters or larger, the longitudinal modulus can be measured using the nanoindentation method as specified in ISO 14577. With the nanoindentation method, measurements are taken at five or more points around the center of the compound with a load of 5 mN. This is also performed on 20 or more randomly selected compounds, and the average value is used as the longitudinal modulus of the compound.

[Vickers hardness of compound]
In addition, in the surface layer according to this embodiment, the compound has a Vickers hardness of 1200 HV or more.
In the steel member according to this embodiment, the height of the protruding peaks and the depth of the protruding valleys on the surface are controlled by polishing the surface layer made of a composite material containing the compound. In particular, the depth of the protruding valleys is controlled by partially removing the compound by polishing, thereby creating appropriate depressions (valleys).
As a result of investigations by the present inventors, it was found that if the Vickers hardness of the compound is 1200 HV or more, the compound can be partially removed in an appropriate size by polishing.
If the Vickers hardness of the compound is less than 1200 HV, the compound is polished smoothly, and Rvk cannot be set within the preferred range.
FIG. 3 is a cross-sectional photograph showing an example of a valley 21 formed by partial dropping of the compound 11.

The Vickers hardness of a compound can be measured using the Vickers hardness test specified in JIS Z2244:2009. Measurements are taken at the center of the compound under a load of 10 gf. This is done for 20 or more randomly selected compounds, and the average value is taken as the Vickers hardness of the compound.

[Compound content]
Furthermore, in the cross section of the surface layer portion of the steel member according to this embodiment, if the proportion of the compound relative to the total area of the cross section is less than 10 area %, a sufficient effect of improving the elastic modulus is not exhibited. Therefore, in the cross section of the surface layer portion of the steel member according to this embodiment, the proportion of the compound (high elastic modulus compound) is set to 10 area % or more. The proportion of the high elastic modulus compound in the cross section of the surface layer portion is preferably 30 area % or more, and more preferably 50 area % or more.
On the other hand, the proportion of the high elastic modulus compound in the cross section of the surface layer portion is the better, and there is no particular upper limit. However, if the proportion of the metal part, which is the binder, in the surface layer portion is reduced, the toughness of the surface layer portion may be reduced. Therefore, the proportion of the compound (high elastic modulus compound) in the cross section of the surface layer portion may be 70 area % or less.

The area ratio of the compound in the cross section of the surface layer can be measured as follows. First, an optical microscope digital photograph is taken of an area of 50,000 μm2 or ^more of the cross section of the surface layer. Next, if the compound and other binders can be clearly distinguished in the optical microscope digital photograph, the area ratio of the high modulus compound is measured using the point counting method specified in JIS G0555:2003. If the high modulus compound and other binders cannot be clearly distinguished, the cross section of the surface layer can be corroded so that the metal portion (the binder) can be distinguished from the high modulus compound, and then a digital optical microscope photograph can be taken. For example, if the binder is an iron-based alloy, nital can be used as the etchant. For example, Figure 2B is a photograph of the cross section of Figure 2A, where the binder is an iron-based alloy, after nital corrosion. It can be seen that by performing corrosion, compound 11 and binder 12 can be more clearly distinguished, as shown in Figure 2B.

[Average particle size of compound]
The surface layer is also required to be strong against repeated tensile and bending forces.
If the average particle size of the compound exceeds 150 μm, the high elastic modulus compound may become a crack initiation source, which may result in a decrease in fatigue strength. Therefore, when the fatigue strength of the steel member is also to be increased, it is preferable that the average particle size of the compound contained in the composite material of the surface layer portion is 150 μm or less. The average particle size of the compound is more preferably 100 μm or less, and even more preferably 45 μm or less. On the other hand, since a smaller average particle size of the compound reduces the toughness of the surface layer portion, it is preferable that the average particle size is 0.1 μm or more.
The average particle size is preferably changed within the above range depending on the method for forming the high elastic modulus material into a composite material. For example, when high-velocity flame spraying is used as the method for forming the composite material, the average particle size of the compound is preferably 15 to 45 μm. When powder plasma welding is used as the method for forming the composite material, the average particle size of the compound is preferably 75 to 150 μm. When laser cladding is used as the method for forming the composite material, the average particle size of the compound is preferably 45 to 150 μm.

The average particle size of a compound can be determined by observing the cross section of the surface layer using an optical microscope. Specifically, the long and short diameters of the particles are measured, and their average value is taken as the particle size. This is done for 20 or more randomly selected particles, and the average value is taken as the average particle size.

<About the binder in the surface layer>
In the composite material constituting the surface layer portion of the steel member according to this embodiment, the remainder other than the compound (high elastic modulus compound) as described above is substantially a binder, i.e., the compound is present in the binder that serves as the base material (matrix).

[Chemical composition of binder]
The binder for the surface layer portion according to this embodiment is not limited as long as it is a metal material, and examples thereof include an iron-based alloy having a C content of 0.10 to 1.00 mass %, stainless steel, Ni, Ni-based alloy, Cr, Cr-based alloy, Co, or Co-based alloy.
When the binder is an iron-based alloy, if the C content is less than 0.10%, there is a possibility that the strength of the surface layer portion will not be sufficient. On the other hand, if the C content of the iron-based alloy exceeds 1.00%, the ductility and toughness will decrease, and, combined with the presence of compounds, the surface layer portion will likely become brittle. There are no particular restrictions on the chemical components of the iron-based alloy other than C, and it is sufficient that the alloy appropriately contains various elements useful for achieving the desired properties.

<Average thickness of the surface layer in cross section>
In order to improve the rigidity of the steel member, the surface layer needs to cover at least a part of the base material. In order to efficiently improve the bending rigidity and torsional rigidity, it is preferable that the surface layer covers the outer periphery of the base material.
In the steel member according to this embodiment, the average thickness (in the direction perpendicular to the surface) of the surface layer portion at the position where the surface layer portion is formed on the surface of the base material is important in order to obtain a sufficient rigidity improvement effect. If the average thickness of the surface layer portion is thinner than 0.5 mm (less than 0.5 mm), a sufficient rigidity improvement effect cannot be obtained. Therefore, the average thickness of the surface layer portion is set to 0.5 mm or more. The average thickness of the surface layer portion is preferably 1.0 mm or more, and more preferably 3.0 mm or more.
On the other hand, if the average thickness of the surface layer portion exceeds 30.0 mm, the proportion of expensive composite material increases, resulting in a significant increase in cost. Therefore, the average thickness of the surface layer portion is preferably 30.0 mm or less. The average thickness of the surface layer portion is more preferably 20.0 mm or less, and even more preferably 10.0 mm or less.

<<Area ratio of the surface layer in the cross section>>
To obtain a sufficient rigidity improvement effect, the area ratio of the surface layer portion in a cross section passing through both the surface layer portion and the base material portion is preferably 10.0% or more relative to the total area of the cross section. The area ratio of the surface layer portion in the cross section is more preferably 20.0% or more. On the other hand, the larger the area ratio of the surface layer portion in the cross section, the better, and there is no particular upper limit. However, from the viewpoint of reducing the material cost and processing cost of the steel member, the area ratio of the surface layer portion in the cross section is preferably 50.0% or less, and more preferably 40.0% or less.

The area ratio and average thickness of the surface layer portion in the cross section of a steel member can be measured by performing nital etching on the cross section of a sample, observing the entire cross section under an optical microscope, and determining that the portion containing high elastic modulus compound particles is the surface layer portion. For example, if the surface layer portion is approximately plate-shaped in the cross section, the average thickness of the surface layer portion can be calculated by dividing the area of the surface layer portion in the cross section of the sample by the length of the line segments that form the surface of the surface layer portion. For example, if the surface layer portion is approximately ring-shaped in the cross section, 20 or more half-lines can be drawn radially from the center of the ring, and the average length of the line segments that pass through the surface layer portion can be calculated as the average thickness of the surface layer portion.
The area ratio of the surface layer portion in the cross section can be determined from the average thickness of the surface layer portion in the total thickness.

<Surface roughness of the surface layer in the sliding direction>
The surface of the surface layer portion according to this embodiment (i.e., the surface of the steel member according to this embodiment) has a protruding peak height Rpk of 0.50 μm or less and a protruding valley depth Rvk of 0.10 μm or more in a surface roughness load curve in the sliding direction.
The steel member according to this embodiment is intended for use in a member having a sliding portion with another member. If hard compounds protrude from the surface, they act like abrasive grains in a grinding wheel, causing significant wear and damage to the other member. If the protruding peak height Rpk in the surface roughness load curve in the sliding direction exceeds 0.50 μm, the wear and damage of the sliding counterpart member will be significantly increased.
Therefore, the protruding peak height Rpk is set to 0.50 μm or less.
On the other hand, if the surface of the surface layer is excessively smooth, it becomes difficult for the lubricant to be supplied to the sliding surface, resulting in stick-slip and seizure. In the surface roughness load curve in the sliding direction, if the protruding valley depth Rvk is 0.10 μm or more, the valleys become oil reservoirs, thereby suppressing stick-slip and seizure. If the protruding valley depth Rvk is less than 0.10 μm, sufficient effect cannot be obtained.
Here, the sliding direction refers to the direction in which the steel member comes into contact with and slides against another member, and refers to the direction of movement or rotation of the steel member. However, it may also refer to the direction of movement or rotation of a part of the steel member by focusing on that part. For example, in the case of a pin portion of a crankshaft, it is the circumferential direction, and in the case of a skirt portion of a piston, it is the axial direction.

The protruding peak height Rpk and the protruding valley depth Rvk are measured by the following method.
A roughness curve is measured by the method specified in JIS B0601:2001, and Rpk and Rvk are calculated from a load curve obtained based on this roughness curve in accordance with JIS B0671-2:2002.
Generally, Ra (arithmetic mean roughness) and Rz (maximum height roughness) are often used as indicators of surface roughness, but these indicators cannot evaluate the roughness of the surface layer portion according to this embodiment.

(Regarding the manufacturing method of steel members)
The method for manufacturing a steel member according to this embodiment will be described in detail below.
As described above, the steel member according to this embodiment is manufactured, for example, by manufacturing a steel material by integrating steel having a chemical composition desired as the substrate with a composite material containing a high-elasticity compound, then machining the steel material as needed to form a part, quenching and tempering the steel material as needed, and then performing shot peening or wet blasting. Furthermore, such a steel member is manufactured by machining a steel material having a chemical composition desired as the substrate with a part, integrating the processed steel material with a composite material containing high-elasticity compound particles, quenching and tempering the steel material as needed, and then performing shot peening or wet blasting.

An example of a method for manufacturing a steel member according to this embodiment will be described.
First, a steel substrate with a desired chemical composition and a composite material containing a specific compound (high-elasticity compound) are prepared, and the steel is formed into a desired shape. The composite material containing the compound is then integrated with the substrate using powder plasma welding, high-velocity flame spraying, laser cladding, or other techniques to produce a steel material.

The resulting steel material is then machined as needed to create the part shape. Furthermore, to further adjust the mechanical properties of the component, quenching and tempering are performed as needed, and shot peening and wet blasting are performed to achieve the desired compressive residual stress value.

When preparing the composite material to be used for integration, it is preferable to appropriately adjust the content (area %) of the high elastic modulus compound to be contained, taking into consideration the integration method to be used, so that the area percentage of the high elastic modulus compound in the cross section of the surface layer of the manufactured steel member is the desired level.

The steel member thus obtained is polished at least in the portion where it is expected to slide against another member, to control the surface roughness (height of the protruding peaks and depth of the protruding valleys). The polishing is carried out in the sliding direction.
There are no particular limitations on the polishing conditions, but it is preferable to use, for example, a CBN grindstone with a grit size of #60 for polishing.

The sliding direction refers to the direction in which a steel member comes into contact with and slides against another member, and refers to the direction of movement or rotation of the steel member. However, it may also refer to the direction of movement or rotation of a portion of the steel member. For example, in the case of the pin portion of a crankshaft, it would be the circumferential direction, and in the case of the skirt portion of a piston, it would be the axial direction.

Next, the steel member of the present invention will be described in detail using examples and comparative examples. The examples shown below are merely examples of the steel member of the present invention, and the steel member of the present invention is not limited to the examples shown below.

Steel having the chemical composition shown in the "substrate portion" in Table 1 was vacuum melted and then cast using a mold to produce a steel billet.
A blank space in Table 1 means that the corresponding element content is 0% in significant figures (numbers to the least significant digit) specified in the embodiment. In other words, when the corresponding element content is rounded to the nearest significant digit (numbers to the least significant digit) specified in the embodiment, it means that it is 0%. For example, the Cr content described in this embodiment is specified to two decimal places. Therefore, for steel grade code A in Table 1, when the measured Cr content is rounded to three decimal places, it means that it is 0%. The contents of Mo, V, and Ti described in this embodiment are also specified to two decimal places. Similarly, when the measured contents are rounded to three decimal places, it means that it is 0%. Rounding means that if the digit (fraction) below the least significant digit specified is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

The resulting steel billet was heated to 1250°C and then hot forged into a round bar with an outer diameter of 25 mm. From this round bar, square bars with a width of 14.0 mm, heights of 3.0 to 8.5 mm, and length of 150 mm were machined to obtain square bars (substrate portions) Nos. 1 to 11. However, during machining, the heights of the square bars were set to 8.0 mm, 3.0 mm, and 8.5 mm for Example No. 1, Example No. 4, and Comparative Example No. 10, respectively, in order to vary the average thickness of the surface layer portion made of the composite material. For the other examples, the height was set to 5.0 mm.
Furthermore, in order to measure the bending rigidity when there is no surface layer made of a composite material, a three-point bending test piece, which will be described later, was separately prepared from the round bar.

The square timbers (substrate portions) No. 1 to No. 11 obtained by the above machining were subjected to the following processing.
First, a mixture was prepared by mixing powder of the compound shown as "contained compound" in the "surface layer portion" of Table 2 with powder of JIS S55C (carbon steel for mechanical structures, C content: 0.54 mass%), JIS SUS410, or Ni-based alloy ("Inconel 625" manufactured by Special Metals Corporation). The compounds used here were commercially available products, the details of which are as follows:
NbC: average particle diameter 36 μm
TiC: average particle diameter 52 μm
TiN: average particle diameter 69 μm
TiB ₂ : Average particle size 124 μm
VB ₂ : Average particle diameter 76 μm
WSi ₂ : Average particle size 55 μm

Next, the mixture was plasma-welded to the upper and lower surfaces of the square bar so that surface layers made of the composite material were formed on both surfaces with the same average thickness, and the height of the square bar including the surface layers was approximately 10.0 mm. For the plasma-welding, Ar gas was used as the shielding gas, and the welding conditions were a welding speed of 5 mm/s, a welding current of 200 A, and a powder supply rate of 20 g/min, with the conditions adjusted appropriately depending on the powder combination.
The powder plasma welded square bars were then machined to prepare three-point bending test specimens measuring 5.0 mm wide, 9.0 mm high, and 120 mm long, and block test specimens measuring 6.35 mm wide, 9.0 mm high, and 15.75 mm long, so that the test specimen center, height, and longitudinal directions remained unchanged. The top and bottom surfaces of each test specimen were finished by surface grinding. Test specimens 1 to 10 were ground using a CBN grinding wheel with a grit size of #60, and test specimen No. 11 was ground using a CBN grinding wheel with a grit size of #20.

Next, the three-point bending test specimen was cut so that the cross section passing through the center of the specimen and perpendicular to the longitudinal direction of the specimen was the observation surface, and then embedded in resin, polished, and etched with nital. A 45 ^mm2 cross section was observed in 10 fields of view using an optical microscope (Nikon Corporation ECLIPSE L150). In each field of view, the portion containing the compound particles was determined to be the surface layer, and the average thickness of the upper surface layer and the lower surface layer were measured. The average of the average thicknesses of the upper surface layer and the lower surface layer was further calculated to determine the average thickness of the surface layer.
The results are shown in Table 2.

Furthermore, the cross section of the surface layer portion of the three-point bending test piece was observed at 200 magnifications using an optical microscope (Nikon Corporation ECLIPSE L150), and the average particle size of the compound particles was determined within a 50,000 μm ^2- minute field of view by a cutting method in accordance with JIS G0551:2013.
The longitudinal elastic modulus of the compound was measured by nanoindentation. Specifically, measurements were performed at five or more points at the center of the compound under a load of 5 mN. This was performed on 20 randomly selected compounds, and the average value was taken as the longitudinal elastic modulus of the compound.
The Vickers hardness of the compound was measured by a Vickers hardness test. Specifically, the measurement was performed at the center of the compound under a load of 10 gf. This test was performed on 20 randomly selected compounds, and the average value was taken as the Vickers hardness of the compound.
Finally, the cross section of the surface layer portion of the three-point bending test piece was observed at 200 magnifications using an optical microscope (Nikon Corporation ECLIPSE L150), and the area ratio of compound particles that appeared white in the optical microscope photograph within a 50,000 μm ^2- minute field of view was calculated using the point counting method specified in JIS G0555:2003.
The results are shown in Table 2.

In addition, Rpk and Rvk were measured on the top and bottom surfaces of the block test pieces using a surface texture measuring instrument (SV-C3200, manufactured by Mitutoyo Corporation).

Next, a three-point bending test was performed to evaluate the bending rigidity of the above three-point bending test specimen.

Specifically, as shown in Figure 5, a three-point bending test was conducted using the three-point bending test specimen, with two lower supports 100 mm apart in the longitudinal direction of the specimen and one upper support located in the longitudinal center of the specimen, and a downward displacement applied to the upper support. The upper support was lowered at a speed of 0.5 mm/s, and the bending rigidity of the three-point bending test specimen was calculated from the load applied to the upper support and the deflection at the center of the specimen. Furthermore, the bending rigidity ratio was calculated, normalized by the bending rigidity of a three-point bending test specimen without a surface layer made of composite material. A bending rigidity ratio of 1.10 or greater was deemed to have excellent bending rigidity and passed the test.

Furthermore, a block-on-ring test was carried out on the block test piece under the following conditions in the manner shown in FIG. 1 to measure the presence or absence of seizure and the amount of wear of the ring test piece.
The test conditions were as follows:
The ring test piece was made of pure aluminum (JIS A1070) with an outer diameter of 37.32 mm. The rotation speed of the ring test piece was 1000 rpm, the pressing load was 50 N, the test time was 1 hour, the lubricant was engine oil (Mobil 1 0W-30 manufactured by ExxonMobil), and the oil temperature was 60°C.
If the amount of wear on one side of the ring test piece was 50 μm or less, it was judged to be acceptable.

As can be seen from Tables 1 and 2, in addition to high rigidity, Nos. 1 to 6 did not experience seizure in the block-on-ring test and the amount of wear of the pure aluminum ring test pieces was small. However, Nos. 4 and 5 had an extremely high bending rigidity ratio, which prevented seizure and also reduced the amount of wear of the pure aluminum ring test pieces, but the surface area ratio was large, resulting in high material costs.
In contrast, Nos. 7 to 11 had a low bending rigidity ratio, or seizure occurred, or the amount of wear of the pure aluminum ring test piece was large.
Specifically, in No. 7, the compound contained in the surface layer had a low modulus of longitudinal elasticity, and therefore sufficient bending rigidity could not be obtained.
In No. 8, the proportion of compound particles contained in the surface layer was low, and therefore sufficient bending rigidity was not obtained.
In No. 9, the compound particles contained in the surface layer had a low Vickers hardness, so the surface was polished smoothly, resulting in a low Rvk, and seizure occurred in the block-on-ring test.
In No. 10, the average thickness of the surface layer portion was small (and the area ratio of the surface layer portion was also low), so sufficient bending rigidity was not obtained.
In No. 11, the grinding stone used in the surface grinding process was rough and the Rpk of the surface layer was high, so the mating material was significantly worn in the block-on-ring test.

1 surface layer portion 11 compound 12 binder 21 valley portion

Claims (7)

a base portion made of steel;
a surface layer portion covering at least a portion of the base material portion;
Equipped with
the surface layer portion is made of a composite material containing at least one compound selected from the group consisting of boride, carbide, and nitride, occupying 10% or more of a cross section of the surface layer portion in terms of area percent, and the remainder being a binder;
the compound has a longitudinal elastic modulus of 300 GPa or more and a Vickers hardness of 1200 HV or more, and an average particle size of 0.1 μm or more and 150 μm or less;
The average thickness of the surface layer portion is 0.5 mm or more,
In a surface roughness load curve in the sliding direction of the surface of the surface layer portion, the protruding peak height Rpk is 0.50 μm or less and the protruding valley depth Rvk is 0.10 μm or more.
steel parts. The substrate portion has a chemical composition, in mass%, of
C: 0.10-0.55%,
Si: 0.05-1.50%,
Mn: 0.20-2.00%,
Al: 0.005-0.100%,
N: 0.0010-0.0250%,
P: 0.001-0.150%,
S: 0.005-0.150%,
and the balance being Fe and impurities.
The steel member according to claim 1 . The base material has the chemical composition, in mass %, replacing a part of the remaining Fe,
Cr: 0.10-5.00%,
Mo: 0.05-1.00%,
Further containing one or two selected from
The steel member according to claim 2. The base material has the chemical composition, in mass %, replacing a part of the remaining Fe,
V: 0.05-0.50%,
Ti: 0.05-0.30%,
Further containing one or two selected from
The steel member according to claim 2 or 3. The binder of the composite material is an iron-based alloy having a C content of 0.1 to 1.0 mass %, and
The compound is one or more of NbC, TiC, VC, WC, _SiC , _Cr3C2 , _{Mo2C , ZrC} , _TiB2 , _W2B5 , _Mo2B5 , TiN, VN, NbN, and ZrN;
The steel member according to any one of claims 1 to 4. The average thickness of the surface layer portion is 0.5 to 30.0 mm.
The steel member according to any one of claims 1 to 5 . an area ratio of the surface layer portion in a cross section passing through both the surface layer portion and the base material portion is 10.0 to 50.0%;
The steel member according to any one of claims 1 to 6 .