阅读说明：本技术 钢轴部件 (Steel shaft component ) 是由 西原基成 多比良裕章 末安遥子 于 2020-04-09 设计创作，主要内容包括：提供具有优异疲劳强度的钢轴部件。钢轴部件的化学组成为C：0.40～0.60％、Si：0.05～1.00％、Mn：1.00～2.00％、P：0.030％以下、S：0.005～0.100％、Cr：0.10～0.50％、V：0.10～0.30％、Al：0.005～0.050％、N：0.0050～0.0200％、Ti：0～0.050％、以及余量：Fe和杂质,轴部的表面的维氏硬度Hs为620HV以上,R/2位置处的维氏硬度Hb满足式(1),维氏硬度为620HV以上的硬化层深度Hr(mm)满足式(2)。R/2位置处的显微组织包含铁素体和珠光体。在硬化层内,圆当量直径超过100nm的含V析出物为10个/276μm～(2)以下。Hs/2.3≤Hb≤350(1),0.05≤Hr/R≤0.40(2)。(Provided is a steel shaft member having excellent fatigue strength. The chemical composition of the steel shaft component is C: 0.40-0.60%, Si: 0.05 to 1.00%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.005-0.100%, Cr: 0.10-0.50%, V: 0.10 to 0.30%, Al: 0.005-0.050%, N: 0.0050 to 0.0200%, Ti: 0-0.050%, and the balance: fe and impurities, Vickers hardness of surface of shaft portionHs is 620HV or more, Vickers hardness Hb at the R/2 position satisfies formula (1), and depth Hr (mm) of the hardened layer having a Vickers hardness of 620HV or more satisfies formula (2). The microstructure at the R/2 position contains ferrite and pearlite. In the hardened layer, the number of V-containing precipitates having a circle-equivalent diameter of more than 100nm is 10/276 μm 2 The following. Hs/2.3 is not less than Hb not less than 350(1), and Hr/R is not less than 0.05 and not more than 0.40 (2).)

1. A steel shaft member having 1 or more shaft portions,

the shaft portion has a circular cross section perpendicular to the axial direction, and the surface layer has a hardened layer having a Vickers hardness of 620HV or more,

the chemical composition of the steel shaft part is calculated by mass percent

C：0.40～0.60％、

Si：0.05～1.00％、

Mn：1.00～2.00％、

P: less than 0.030%,

S：0.005～0.100％、

Cr：0.10～0.50％、

V：0.10～0.30％、

Al：0.005～0.050％、

N：0.0050～0.0200％、

Ti: 0 to 0.050%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

the surface of the shaft portion has a Vickers hardness Hs of 620HV or more,

in a cross section perpendicular to the axial direction of the shaft portion, a Vickers hardness Hb at a R/2 position corresponding to a center position of a radius R of the shaft portion satisfies formula (1),

the microstructure at the R/2 position comprises ferrite and pearlite,

the depth Hr (mm) of the hardened layer having a Vickers hardness of 620HV or more satisfies formula (2),

in the hardened layer of the cross section perpendicular to the axial direction of the shaft portion, the number of V-containing precipitates containing V and having a circle-equivalent diameter of more than 100nm is 10/276 [ mu ] m²In the following, the following description is given,

Hs/2.3≤Hb≤350 (1)

0.05≤Hr/R≤0.40 (2)

here, R of formula (2) is a radius (mm) of the shaft portion.

2. The steel shaft component according to claim 1,

the chemical composition contains, in mass%, Ti: 0.005-0.050%.

3. The steel shaft component according to claim 1 or 2,

the steel shaft part is a crankshaft or a camshaft.

Technical Field

The present invention relates to a steel shaft member, and more particularly, to a steel shaft member including 1 or more shaft portions, such as a crankshaft and a camshaft.

Background

A steel shaft member represented by a crankshaft includes 1 or more shaft portions. When the steel shaft member is a crankshaft, the crankpin and the crankshaft journal correspond to a shaft portion. Such steel shaft members are used as machine parts of industrial machines, construction machines, and transportation machines such as automobiles.

The steel shaft member is manufactured by the following steps. An intermediate product is produced by hot forging a steel material as a material for a steel shaft member. The produced intermediate product is subjected to thermal refining as necessary. The hot forged non-heat-treated intermediate product or the heat-treated intermediate product is machined into a component shape by cutting, punching, or the like. The machined intermediate product is subjected to a surface hardening heat treatment such as induction hardening. After the case hardening heat treatment, the intermediate product is subjected to finish machining by grinding, thereby manufacturing a steel shaft member.

Excellent fatigue strength is required for steel shaft members used for the above applications. For example, japanese patent laid-open nos. 2013-7098 (patent document 1), 2010-270346 (patent document 2), and 2004-137237 (patent document 3) propose techniques for improving the fatigue strength of a machine component to be subjected to high-frequency quenching after hot forging.

Patent document 1 proposes a steel material for machine parts to be subjected to induction hardening after hot forging. The steel for hot forging disclosed in patent document 1 contains, in mass%, C: more than 0.30% and less than 0.60%, Si: 0.10 to 0.90%, Mn: 0.50-2.0%, P: 0.080% or less, S: 0.010-0.10% of Al: more than 0.005% and not more than 0.10%, Cr: 0.01 to 1.0%, Ti: 0.001% or more and less than 0.040%, Ca: 0.0003-0.0040%, Te: 0.0003% or more and less than 0.0040%, N: 0.0030-0.020%, O: 0.0050% or less, and the balance Fe and impurities, wherein the steel for hot forging satisfies Ca/Te >1.00, and the circle-equivalent diameter of sulfide-based inclusions is 20 μm or less.

Patent document 2 proposes a steel material having excellent bending fatigue strength for machine parts to be subjected to induction hardening after hot forging. The non-heat-treated steel for hot forging disclosed in patent document 2 has the following composition: contains, in mass%, C: 0.25 to 0.50%, Si: 0.05 to 1.00%, Mn: 0.60-1.80%, P is less than or equal to 0.030%, S is less than or equal to 0.060%, Cr: 0.50% or less, Mo: 0.03% or less, V: 0.050 to 0.250%, Ti: 0.005-0.020%, Al: 0.050% or less, N: 0.008 to 0.015% of the composition satisfying formula (1), and the balance being Fe and unavoidable impurities, wherein a ferrite volume fraction (F%) and a ferrite average particle diameter (mum) in a microstructure of a hot forged raw material and a V content (%) in steel satisfy formula (2). Here, the formula (1) is 3.10. ltoreq. 2.7 XMn +4.6 XCr + V. ltoreq.5.60, and the formula (2) is 0.04. ltoreq. ferrite volume fraction (F%). times.V/ferrite average particle diameter (μm) is 0.18.

Patent document 3 proposes a steel material having excellent fatigue strength for machine parts to be subjected to induction hardening after hot forging. The high-strength high-workability steel for induction hardening disclosed in patent document 3 contains, in mass%, C: 0.5 to 0.7%, Si: 0.5 to 1.0%, Mn: 0.5 to 1.0%, Cr: 0.4% or less, S: 0.035% of the following, V: 0.01 to 0.15%, Al: more than 0.015% and less than 0.050%, N: more than 0.010% and less than 0.025%, and the balance of Fe and inevitable impurities, and is used by subjecting a part of the component to induction hardening after forging.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-7098

Patent document 2: japanese patent application laid-open No. 2010-270346

Patent document 3: japanese laid-open patent application No. 2004-137237

Disclosure of Invention

Problems to be solved by the invention

As in the above patent documents 1 to 3, conventionally, the fatigue strength of a steel shaft member, which is a final product after hot forging and induction hardening, has been improved by adjusting the chemical composition and structure of a steel material to be a steel shaft member material. However, even when the steel material is used, the steel shaft member as a final product may not sufficiently obtain high fatigue strength.

An object of the present application is to provide a steel shaft component having excellent fatigue strength.

Means for solving the problems

The steel shaft member of the present application is provided with 1 or more shaft portions,

the shaft portion has a circular cross section perpendicular to the axial direction and a surface layer having a cured layer with a Vickers hardness of 620HV or more,

the chemical composition of the steel shaft member is in mass%

C：0.40～0.60％、

Si：0.05～1.00％、

Mn：1.00～2.00％、

P: less than 0.030%,

S：0.005～0.100％、

Cr：0.10～0.50％、

V：0.10～0.30％、

Al：0.005～0.050％、

N：0.0050～0.0200％、

Ti: 0 to 0.050%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

the surface of the shaft portion has a Vickers hardness Hs of 620HV or more,

in a cross section perpendicular to the axial direction of the shaft portion, a Vickers hardness Hb at a R/2 position corresponding to a center position of a radius R of the shaft portion satisfies formula (1),

the microstructure at the aforementioned R/2 position contains ferrite and pearlite,

the depth Hr (mm) of the hardened layer having a Vickers hardness of 620HV or more satisfies the formula (2),

in the hardened layer of the cross section perpendicular to the axial direction of the shaft portion, the number of V-containing precipitates containing V and having a circle-equivalent diameter of more than 100nm is 10/276 μm²The following.

Hs/2.3≤Hb≤350 (1)

0.05≤Hr/R≤0.40 (2)

Here, R in formula (2) is the radius (mm) of the shaft portion.

ADVANTAGEOUS EFFECTS OF INVENTION

The steel shaft member of the present application has excellent fatigue strength.

Drawings

Fig. 1 is a schematic diagram showing a relationship between a strength distribution in a depth direction (radial direction) from a surface of a steel shaft member and a stress distribution applied to the steel shaft member in use.

Fig. 2 is a view showing a main part of a crankshaft as an example of the steel shaft member of the present embodiment.

Fig. 3 is a sectional view of a crank pin corresponding to the shaft portion in fig. 2.

FIG. 4 is a side view of a sample for a rotary bending fatigue test in an example.

Detailed Description

The present inventors have conducted investigations and studies on the fatigue strength of steel shaft members, more specifically, the rotational bending fatigue strength. Conventionally, as described in patent documents 1 to 3, for a steel shaft member to be subjected to hot forging and induction hardening, improvement of fatigue strength of the steel shaft member has been achieved by adjusting the chemical composition and structure of a steel material to be a steel shaft member material. However, the steel shaft member as a final product may not sufficiently obtain fatigue strength. Therefore, the present inventors focused on a steel shaft member as a final product and examined the fatigue strength of the steel shaft member.

As described above, conventionally: when a hardened layer is formed on the surface layer of the steel shaft member by induction hardening, the fatigue strength of the steel shaft member increases as the hardness of the hardened layer increases. The reason for this is as follows. In the past, it was thought that: the main cause of the reduction in fatigue strength is cracks generated on the surface of the steel shaft member. Therefore, conventionally: by increasing the hardness of the hardened layer as much as possible, the occurrence of cracks on the surface can be suppressed, and the fatigue strength can be improved.

However, the inventors of the present invention have investigated and found that: when the hardness of the hardened layer is increased, cracks may occur not on the surface of the steel shaft member but in the interior of the steel shaft member, more specifically, in the core portion (base material portion) near the hardened layer of the steel shaft member, and the fatigue strength may be reduced.

Therefore, the present inventors investigated the relationship between the radial strength distribution of the steel shaft member and the stress distribution applied to each position in the radial direction of the steel shaft member when the steel shaft member is used. In the steel shaft member, when a load is applied in a direction (radial direction of the steel shaft member) perpendicular to a longitudinal direction (axial direction) of the steel shaft member, bending fatigue is applied to the steel shaft member. Therefore, the present inventors investigated the stress distribution applied in the radial direction in the stress concentration region of the steel shaft member when a load is applied in the radial direction of the steel shaft member.

Fig. 1 is a schematic diagram showing a relationship between a strength distribution in a depth direction (radial direction) from a surface of a steel shaft member and a stress distribution applied to the steel shaft member in use. Solid lines a1 and a2 of fig. 1 are stress distributions applied in use. The solid line B1 in fig. 1 is the radial strength distribution of the steel shaft member.

Referring to fig. 1, the stress distribution (a1 and a2) applied to the steel shaft member in use is highest at the surface and continuously decreases with depth (radially). On the other hand, the radial strength distribution (B1) of the steel shaft member is high at the hardened layer region B11, becomes low at the core region B12 inside of the hardened layer, and the strength decreases abruptly in a discontinuous manner as it goes from the hardened layer region B11 to the core region B12. This is because the hardened layer region is a martensite structure, while the core region is a ferrite and pearlite structure.

As described above, the distribution of stress applied to the steel shaft member in use is continuous, whereas the distribution of hardness of the steel shaft member is discontinuous. As a result, when the stress distribution applied to the steel shaft member during use increases from a1 to a2, as shown by a region 100 in fig. 1, the stress distribution a2 exceeds the strength of the core region B12 in the core region B12 in the vicinity of the hardened layer region B11, and cracks are likely to occur therein.

In view of the above, the present inventors considered that: the fatigue strength can be further improved by increasing the hardness of the cured layer and reducing the difference between the hardness of the cured layer and the hardness of the core. Therefore, the present inventors have further studied the relationship between the hardness of the hardened layer and the hardness of the core. As a result, they found that: if the chemical composition of the steel shaft member is set to be C: 0.40-0.60%, Si: 0.05 to 1.00%, Mn: 1.00-2.00%, P: 0.030% or less, S: 0.005-0.100%, Cr: 0.10-0.50%, V: 0.10 to 0.30%, Al: 0.005-0.050%, N: 0.0050 to 0.0200%, Ti: 0-0.050%, and the balance: in the chemical composition of Fe and impurities, when the vickers hardness Hs of the surface of the shaft portion (in other words, the hardened layer) is set to 620HV or more, and further, if the vickers hardness Hb at the center position of the radius R of the shaft portion of the steel shaft member (hereinafter, referred to as R/2 position) corresponds to the core hardness, the vickers hardness Hb at the R/2 position is adjusted so as to satisfy the formula (1), there is a possibility that excellent fatigue strength is obtained in the steel shaft member.

Hs/2.3≤Hb≤350 (1)

When the formula (1) is satisfied, the difference between the hardness of the hardened layer and the hardness of the core portion is sufficiently reduced in the steel shaft member having the above chemical composition. Therefore, a decrease in fatigue strength due to internal cracking can be suppressed.

The present inventors further investigated the relationship between the depth of the hardened layer and the fatigue strength. As a result, they found that: if the hardened layer is too deep, the fatigue strength is rather lowered. The reason for this is considered as follows. When the hardened layer has an appropriate depth, compressive residual stress is borne in the surface layer of the steel shaft member in the axial direction. At this time, it can be considered that: the occurrence of cracks at the surface of the steel shaft member due to the compressive residual stress is suppressed. On the other hand, when the hardened layer becomes too deep, the residual stress in compression becomes low or the residual stress in tension is borne in the surface layer of the steel shaft member. The results are believed to be: cracks are likely to occur on the surface of the steel shaft member, and the fatigue strength is reduced.

Accordingly, the present inventors examined an appropriate range of the ratio (Hr/R) of the depth Hr of the hardened layer to the radius R of the shaft portion of the steel shaft member, the ratio being 620HV or more in terms of vickers hardness. The results show that: in the steel shaft member having the above chemical composition and satisfying the formula (1), if the formula (2) is further satisfied, excellent fatigue strength can be obtained.

0.05≤Hr/R≤0.40 (2)

Here, R in formula (2) is the radius (mm) of the shaft portion.

However, even if the steel shaft member of the above chemical composition satisfies the formula (1) and the formula (2), the fatigue strength may be low. Therefore, the present inventors have further investigated the cause of the reduction in fatigue strength. As a result, the present inventors have obtained the following findings. In the case of a steel shaft member having the above chemical composition, a plurality of precipitates containing V (hereinafter also referred to as V-containing precipitates) are present in the hardened layer. Here, the V-containing precipitates mean precipitates having a V content of 10% by mass or more. The V-containing precipitates are, for example, V carbides, V nitrides, V carbonitrides, and the like. The V-containing precipitates enhance the hardness of the hardened layer by precipitation strengthening. However, when a plurality of V-containing precipitates having a circle-equivalent diameter of more than 100nm (hereinafter, also referred to as coarse V-containing precipitates) are present in the hardened layer, cracks are likely to occur from the coarse V-containing precipitates as starting points due to a load carried in the radial direction of the steel shaft member. Therefore, the fatigue strength may be reduced. When the steel shaft member has the above chemical composition, the above formulas (1) and (2) are satisfied, and the number of coarse V-containing precipitates in the hardened layer is 10/276. mu.m²Hereinafter, excellent fatigue strength can be obtained.

The steel shaft member of the present embodiment completed based on the above-described findings has the following configuration.

[1] A steel shaft member having 1 or more shaft portions,

the shaft portion has a circular cross section perpendicular to the axial direction and a surface layer having a cured layer with a Vickers hardness of 620HV or more,

the chemical composition of the steel shaft member is in mass%

C：0.40～0.60％、

Si：0.05～1.00％、

Mn：1.00～2.00％、

P: less than 0.030%,

S：0.005～0.100％、

Cr：0.10～0.50％、

V：0.10～0.30％、

Al：0.005～0.050％、

N：0.0050～0.0200％、

Ti: 0 to 0.050%, and

and the balance: fe and impurities in the iron-based alloy, and the impurities,

the surface of the shaft portion has a Vickers hardness Hs of 620HV or more,

in a cross section perpendicular to the axial direction of the shaft portion, a Vickers hardness Hb at a R/2 position corresponding to a center position of a radius R of the shaft portion satisfies formula (1),

the microstructure at the aforementioned R/2 position contains ferrite and pearlite,

the depth Hr (mm) of the hardened layer having a Vickers hardness of 620HV or more satisfies the formula (2),

in the hardened layer of the cross section perpendicular to the axial direction of the shaft portion, the number of V-containing precipitates containing V and having a circle-equivalent diameter of more than 100nm is 10/276 μm²The following.

Hs/2.3≤Hb≤350 (1)

0.05≤Hr/R≤0.40 (2)

Here, R in formula (2) is the radius (mm) of the shaft portion.

[2] The steel shaft member according to [1], wherein,

the chemical composition contains, in mass%, Ti: 0.005-0.050%.

[3] The steel shaft member according to [1] or [2], wherein,

the steel shaft part is a crankshaft or a camshaft.

The steel shaft member of the present embodiment will be described in detail below.

[ constitution of Steel shaft Member ]

The steel shaft member of the present embodiment includes 1 or more shaft portions. The shaft portion has a circular cross section perpendicular to the longitudinal direction (axial direction) of the steel shaft member, and has a hardened layer on the surface layer. In the present specification, the term "hardened layer" refers to a region having a Vickers hardness of 620HV or more. The hardened layer is formed on the surface layer of the steel shaft member by performing induction hardening as described later.

Fig. 2 is a view showing a main part of a crankshaft 1 as an example of the steel shaft member of the present embodiment. The crankshaft 1 includes a crank pin 2 corresponding to a shaft portion and a crank journal 3. The crankshaft 1 shown in fig. 2 further includes a crank arm 4 and a counterweight 6. The crank arm 4 is disposed between the crank pin 2 and the crank journal 3, and connects the crank pin 2 and the crank journal 3. The counterweight 6 is connected to the crank arm 4. The crank pin 2 includes a fillet 5 at a connection portion with the crank arm 4. Similarly, the crank journal 3 includes a rounded portion 5 at a connecting portion with the crank arm 4. The round portion 5 may be omitted.

Fig. 2 shows a structure of a crankshaft as an example of a steel shaft member. However, the steel shaft member is not limited to the crankshaft. The steel shaft component may be, for example, a camshaft. The steel shaft member may have 1 shaft portion, or may have a plurality of shaft portions (crank pin 2 and crank journal 3) as in the crankshaft 1 shown in fig. 2.

Fig. 3 is a sectional view at a face including the central axis of the crank pin 2 in fig. 2. Referring to fig. 3, a hardened layer 20 is formed on the surface of the crank pin 2 corresponding to the shaft. As described above, the vickers hardness of the hardened layer 20 is 620HV or more. In other words, the lowest value of the Vickers hardness in the hardened layer 20 is 620 HV. In the present specification, a portion of the crankpin 2 corresponding to the shaft portion that is more inside than the hardened layer 20 (in other words, a portion having a vickers hardness of less than 620 HV) is referred to as a core 21.

[ chemical composition ]

The chemical composition of the steel shaft member of the present embodiment contains the following elements. In the present specification, the term "% of an element" means "by mass" unless otherwise specified.

C：0.40～0.60％

Carbon (C) increases the hardness of the core portion and the hardened layer of the shaft portion of the steel shaft member, and increases the fatigue strength of the steel shaft member. If the C content is less than 0.40%, the above-described effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the C content exceeds 0.60%, the hardness of the steel shaft member may be excessively high and the fatigue strength of the steel shaft member may be rather lowered even if the content of other elements is within the range of the present embodiment. Further, the machinability of the steel material to be used as the steel shaft member material is lowered. Therefore, the C content is 0.40 to 0.60%. The lower limit of the C content is preferably 0.43%, more preferably 0.44%, even more preferably 0.45%, and even more preferably 0.46%. The upper limit of the C content is preferably 0.59%, more preferably 0.56%, even more preferably 0.54%, even more preferably 0.52%, and even more preferably 0.50%.

Si：0.05～1.00％

Silicon (Si) is solid-dissolved in ferrite to strengthen the ferrite. Therefore, the hardness of the core of the steel shaft member is increased. If the Si content is less than 0.05%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 1.00%, the steel is decarburized during hot forging even if the contents of other elements are within the ranges of the present embodiment. In this case, the cutting loss becomes large in the steel material (intermediate product) after hot forging. Therefore, the Si content is 0.05 to 1.00%. The lower limit of the Si content is preferably 0.10%, more preferably 0.20%, even more preferably 0.30%, and even more preferably 0.35%. The upper limit of the Si content is preferably 0.90%, more preferably 0.80%, even more preferably 0.75%, and even more preferably 0.70%.

Mn：1.00～2.00％

Manganese (Mn) increases the hardness of the steel shaft member. If the Mn content is less than 1.00%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 2.00%, bainite is easily formed in the microstructure even if the content of other elements is within the range of the present embodiment. In this case, the machinability of the steel material is lowered. Therefore, the Mn content is 1.00 to 2.00%. The lower limit of the Mn content is preferably 1.05%, more preferably 1.10%, even more preferably 1.12%, and even more preferably 1.15%. The upper limit of the Mn content is preferably 1.90%, more preferably 1.80%, even more preferably 1.70%, and even more preferably 1.60%.

P: less than 0.030%

Phosphorus (P) is an impurity that is inevitably contained. In other words, the P content exceeds 0%. P segregates to grain boundaries, and decreases the fatigue strength of the steel shaft member. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%, and still more preferably 0.018%. The P content is preferably as low as possible. However, an extreme reduction in P content greatly increases manufacturing costs. Therefore, the lower limit of the P content is preferably 0.001%, more preferably 0.002%, and still more preferably 0.003% in view of industrial production.

S：0.005～0.100％

Sulfur (S) forms sulfides such as MnS, and improves the machinability of the steel material. If the S content is less than 0.005%, the above-described effects cannot be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the S content exceeds 0.100%, the hot workability of the steel material is lowered even if the contents of other elements are within the ranges of the present embodiment. Therefore, the S content is 0.005 to 0.100%. The lower limit of the S content is preferably 0.008%, more preferably 0.010%, even more preferably 0.012%, and even more preferably 0.014%. The upper limit of the S content is preferably 0.090%, more preferably 0.080%, still more preferably 0.075%, and yet more preferably 0.070%.

Cr：0.10～0.50％

Chromium (Cr) increases the hardness of the steel shaft component. If the Cr content is less than 0.10%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 0.50%, bainite is formed in the microstructure even if the content of other elements is within the range of the present embodiment, and the machinability of the steel material is lowered. Therefore, the Cr content is 0.10 to 0.50%. The lower limit of the Cr content is preferably 0.11%, more preferably 0.12%, and still more preferably 0.13%. The upper limit of the Cr content is preferably 0.40%, more preferably 0.35%, further preferably 0.30%, further preferably 0.29%, further preferably 0.28%, further preferably 0.25%, further preferably 0.20%.

V：0.10～0.30％

Vanadium (V) generates fine carbides, and increases the hardness of the core of the steel shaft member. As a result, the fatigue strength of the steel shaft member is improved. If the V content is less than 0.10%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.30%, coarse V carbonitrides are formed even if the content of other elements is within the range of the present embodiment, and the fatigue strength of the steel shaft member is reduced. Therefore, the V content is 0.10 to 0.30%. The lower limit of the V content is preferably 0.11%, more preferably 0.12%, and still more preferably 0.13%. The upper limit of the V content is preferably 0.25%, more preferably 0.20%, even more preferably 0.18%, and even more preferably 0.15%.

Al：0.005～0.050％

Aluminum (Al) deoxidizes steel. Al also generates nitrides, and prevents the coarsening of crystal grains. Therefore, significant reduction in hardness and toughness of the steel shaft member is suppressed. If the Al content is less than 0.005%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the content of Al is too high, Al is excessively generated even if the content of other elements is within the range of the present embodiment₂O₃Is an inclusion. Al (Al)₂O₃The inclusions lower the machinability of the steel material. Therefore, the Al content is 0.005-0.050%. The lower limit of the Al content is preferably 0.007%, andmore preferably 0.010%, and still more preferably 0.012%. The upper limit of the Al content is preferably 0.045%, more preferably 0.042%, still more preferably 0.040%, and still more preferably 0.039%. The Al content referred to herein means the content (mass%) of acid-soluble Al (sol.al).

N：0.0050～0.0200％

Nitrogen (N) generates nitrides and carbonitrides. The nitride and carbonitride suppress coarsening of crystal grains. This suppresses a decrease in the hardness of the steel material, and improves the fatigue strength of the steel shaft member. If the N content is less than 0.0050%, the above-described effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the N content exceeds 0.0200%, defects such as voids are likely to occur in the steel material even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.0050 to 0.0200%. The lower limit of the N content is preferably 0.0060%, more preferably 0.0070%, and still more preferably 0.0080%. The upper limit of the N content is preferably 0.0180%, more preferably 0.0170%, still more preferably 0.0160%, and still more preferably 0.0150%.

The balance of the chemical composition of the steel shaft member of the present embodiment is Fe and impurities. The impurities mentioned here mean: elements mixed from ores and scraps utilized as raw materials of steel materials constituting the steel shaft member, the environment of the manufacturing process, and the like.

The chemical composition of the steel shaft member of the present embodiment may further contain Ti instead of a part of Fe.

Ti：0～0.050％

Titanium (Ti) is an optional element, and may not be contained. In other words, the Ti content may be 0%. When it is contained, in other words, when the Ti content exceeds 0%, carbides and the like are generated from Ti, and coarsening of crystal grains at the time of hot forging is suppressed. If Ti is contained even in a small amount, the above-described effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse Ti nitrides are generated even if the contents of other elements are within the range of the present embodiment, and the fatigue strength of the steel shaft member is lowered. Therefore, the Ti content is 0 to 0.050%. The lower limit of the Ti content is preferably 0.001%, more preferably 0.003%, further preferably 0.005%, further preferably 0.008%, further preferably 0.010%, further preferably 0.011%. The upper limit of the Ti content is preferably 0.040%, more preferably 0.035%, even more preferably 0.030%, and even more preferably 0.025%.

[ microstructure of Steel shaft Member ]

In the steel shaft component, the microstructure of the core portion contains ferrite and pearlite. In the present embodiment, the R/2 position of the shaft portion corresponds to the core portion. Here, as shown in fig. 3, the R/2 position is a center position of a radius R in a cross section (cross section) perpendicular to the axial direction of the shaft portion, and is a center position of a line segment connecting a surface of the cross section of the shaft portion and a center C. In the present embodiment, the microstructure of the core portion of the steel shaft member is defined by the microstructure of the R/2 position of the shaft portion. The microstructure at the R/2 position of the shaft portion of the present embodiment includes ferrite and pearlite.

More specifically, the microstructure (matrix excluding precipitates and inclusions) at the R/2 position of the shaft portion includes ferrite and pearlite, and other phases (bainite, martensite) are substantially absent. The ferrite of "ferrite and pearlite" refers to proeutectoid ferrite. In the present specification, the microstructure including ferrite and pearlite means that: in the microstructure at the R/2 position of the shaft portion, the total area ratio of ferrite and pearlite is 95.0% or more.

The microstructure of the steel shaft member was observed by the following method. A microstructure observation sample was collected from the R/2 position (see FIG. 3) of the shaft portion of the steel shaft member. The size of the sample is not particularly limited as long as a visual field size described below can be secured in the observation plane. Among the surfaces of the sample collected, the surface corresponding to the cross section of the shaft portion perpendicular to the axial direction was taken as the observation surface. After the observation surface was mirror-polished, the observation surface was etched with 3% nitroethanol (Nital etchant). Any 5 fields of view on the etched observation surface were observed by an optical microscope at 500 magnifications to generate photographic images. The size of each field was set to 200. mu. m.times.200. mu.m. In each visual field, for ferritePhases such as body, pearlite, bainite have different contrast ratios. Therefore, each phase is determined according to the contrast. Ferrite and pearlite were determined in each field. The total area (. mu.m) of ferrite in all the visual fields was determined²) The total area (. mu.m) of pearlite in all the visual fields was determined²). The total area ratio (%) of ferrite and pearlite was determined from the sum of the total area of ferrite and pearlite in all the visual fields and the total area of all the visual fields. When the total area ratio of ferrite and pearlite is 95.0% or more, it is judged that the microstructure includes ferrite and pearlite.

[ conditions A to D for the shaft part ]

The steel shaft member of the present embodiment further satisfies all of conditions a to D in the shaft portion.

Condition a: the Vickers hardness Hs of the surface of the shaft portion is 620HV or more.

Condition B: in a cross section perpendicular to the axial direction of the shaft portion, the Vickers hardness Hb at a R/2 position corresponding to a center position of the radius R of the shaft portion satisfies formula (1).

Hs/2.3≤Hb≤350 (1)

Condition C: the depth Hr of the hardened layer having a Vickers hardness of 620HV or more satisfies formula (2).

0.05≤Hr/R≤0.40 (2)

Condition D: in the hardened layer of the cross section perpendicular to the axial direction of the shaft portion, the number of coarse V-containing precipitates was 10/276 μm²The following. Here, the coarse V-containing precipitates mean: v content is 10% by mass or more of precipitates, and the equivalent circle diameter exceeds 100 nm.

The following describes conditions a to D.

[ regarding condition A ]

The surface of the shaft portion of the steel shaft member (in other words, the surface of the hardened layer) has a Vickers hardness Hs of 620HV or more. Here, the vickers hardness Hs of the surface of the shaft portion was measured by the following method.

A vickers hardness test was performed on any 3 points of the surface of the hardened layer of the shaft portion of the steel shaft member in accordance with JIS Z2244 (2009). The test force was set to 1.96N. The arithmetic mean of the obtained Vickers hardnesses was defined as the Vickers hardness Hs (HV) of the surface of the shaft portion.

If the Vickers hardness Hs of the surface of the shaft portion is less than 620HV, cracks are likely to occur from the surface of the shaft portion due to stress applied to the steel shaft member when the steel shaft member is used. In this case, the steel shaft member cannot obtain sufficient fatigue strength. Therefore, the Vickers hardness Hs of the surface of the shaft portion is 620HV or more.

[ concerning condition B ]

In the steel shaft member, the Vickers hardness Hb at the R/2 position of the shaft portion also satisfies formula (1).

Hs/2.3≤Hb≤350 (1)

Here, "Hs" in formula (1) is substituted for the value of vickers hardness Hs of the shaft portion surface.

The Vickers hardness Hb at the R/2 position of the shaft portion corresponds to the Vickers hardness of the core portion. If the Vickers hardness Hb of the core portion is too low relative to the Vickers hardness Hs of the shaft portion surface, the difference between the hardness of the core portion and the hardness of the cured layer becomes too large. At this time, even if the vickers hardness Hs of the surface of the shaft portion is sufficiently high, cracks are likely to occur in the core portion in the vicinity of the hardened layer as described above, and the fatigue strength of the steel shaft member is lowered. If the Vickers hardness Hb at the R/2 position is Hs/2.3 or more, the difference between the hardness of the core and the hardness of the cured layer is sufficiently small. Therefore, the occurrence of cracks in the core portion in the vicinity of the hardened layer can be suppressed, and the fatigue strength of the steel shaft member can be improved.

The upper limit of the Vickers hardness Hb at the R/2 position is 350 HV. When the Vickers hardness Hb at the R/2 position exceeds 350HV, the machinability of the steel material is lowered, and the productivity is remarkably lowered. Further, when the Vickers hardness Hb at the R/2 position exceeds 350HV, the induction hardening is excessively performed and excessively hardened. Therefore, tensile residual stress is likely to occur in the surface layer of the steel shaft member. At this time, the fatigue strength of the steel shaft member is reduced. Therefore, the upper limit of the Vickers hardness Hb at the R/2 position is 350 HV. The lower limit of the Vickers hardness Hb at the R/2 position is preferably Hs/2.23, more preferably Hs/2.2, and still more preferably Hs/2.1. The upper limit of the Vickers hardness Hb at the R/2 position is preferably 345HV, more preferably 343HV, and still more preferably 340 HV.

The Vickers hardness Hb at the R/2 position was measured by the following method. In a cross section perpendicular to the axial direction of the shaft portion of the steel shaft member, a vickers hardness test was performed according to JIS Z2244 (2009) for any 3 points of the R/2 position of the shaft portion. The test force was set to 98N. The arithmetic mean of the resulting Vickers hardnesses was defined as the Vickers hardness Hb (HV) at the R/2 position.

[ regarding condition C ]

In the steel shaft member, the depth Hr (mm) of the hardened layer having a Vickers hardness of 620HV or more also satisfies the formula (2).

0.05≤Hr/R≤0.40 (2)

Here, R in formula (2) is substituted for the radius (mm) of the shaft portion.

As described above, in the present specification, a region having a vickers hardness of 620HV or more in the surface layer of the shaft portion is defined as a hardened layer. The depth Hr of the hardened layer was measured by the following method.

In a cross section perpendicular to the axial direction of the shaft portion, vickers hardness was measured at a pitch of 0.1mm in the depth direction (radial direction) from the surface. The vickers hardness was measured in accordance with JIS Z2244 (2009). The test force was set to 1.96N. From the obtained vickers hardness, a vickers hardness distribution in the depth direction (radial direction) is plotted. In the obtained Vickers hardness distribution, the depth at which the Vickers hardness becomes 620HV or more is defined as the depth of the hardened layer. In a cross section perpendicular to the axial direction of the shaft portion, the vickers hardness distribution in the depth direction is obtained from arbitrary 3 portions of the surface, and the arithmetic average of the hardened layer depths at the respective positions (3 portions) is defined as a hardened layer depth hr (mm). The ratio (Hr/R) of the depth Hr of the cured layer to the radius R (mm) of the shaft portion was obtained.

If Hr/R is less than 0.05, the depth of the hardened layer of the shaft part of the steel shaft member is insufficient. At this time, the fatigue strength of the steel shaft member is reduced. On the other hand, if Hr/R exceeds 0.40, the hardened layer is formed too deeply. In this case, tensile residual stress is likely to occur on the surface of the steel shaft member, and as a result, the fatigue strength of the steel shaft member is reduced. If Hr/R satisfies formula (2), the balance between the core hardness and the hardness of the hardened layer satisfies an appropriate relationship on the premise that conditions a, B and D are satisfied, and as a result, the fatigue strength of the steel shaft member is improved. The lower limit of Hr/R is preferably 0.08, more preferably 0.10, and still more preferably 0.15. The upper limit of Hr/R is preferably 0.38, more preferably 0.35, and still more preferably 0.32.

[ concerning condition D ]

In the steel shaft member, the number of coarse V-containing precipitates in the hardened layer of the cross section perpendicular to the axial direction of the shaft portion was 10/276. mu.m²The following. Here, the V-containing precipitates mean precipitates in which the V content in the precipitates is 10% by mass or more. In addition, coarse V-containing precipitates mean: v-containing precipitates having a circle-equivalent diameter of more than 100 nm.

Even if the steel shaft member having the above chemical composition satisfies the conditions A to C, the number density of coarse V-containing precipitates in the hardened layer is more than 10/276 [ mu ] m²In the hardened layer, the number of coarse V-containing precipitates which become crack origins with respect to the radial load is too large. At this time, the fatigue strength of the steel shaft member is reduced. On the other hand, if the number density of coarse V-containing precipitates in the hardened layer is 10/276. mu.m²Hereinafter, the number of coarse V-containing precipitates in the hardened layer is sufficiently small. Therefore, excellent fatigue strength can be obtained in the steel shaft member. The number density of coarse V-containing precipitates in the hardened layer is preferably 9 precipitates/276. mu.m²The number of them is more preferably 8/276. mu.m²The following.

The number density of coarse V-containing precipitates in the hardened layer was measured by the following method. The shaft portion of the steel shaft member was cut in a direction perpendicular to the longitudinal direction (axial direction) of the steel shaft member. In the cross section of the shaft portion, the hardened layer of the cross section of the shaft portion is determined according to the method of measuring the depth of the hardened layer described in the above condition C. The sample was taken from a position of the determined hardened layer at a depth of approximately 1/2 degrees from the surface (in other words, the central position in the depth direction of the hardened layer in the shaft section). The surface corresponding to a cross section perpendicular to the longitudinal direction (axial direction) of the steel shaft member among the sample surfaces was set as an observation surface. Using a Transmission Electron Microscope (TEM), any 10 of the observation surfaces were examined at a magnification of 30000 timesVisual field (area per 1 visual field is 27.6 μm²) And (6) carrying out observation. The thickness of the sample (thin film sample) was set to about 50 nm.

The precipitates and inclusions in each visual field (referred to as an observation plane) were quantitatively analyzed for the content of the elements contained in each of the precipitates and inclusions by energy dispersive X-ray spectroscopy (EDX), and the V-containing precipitates were identified. Specifically, among the precipitates and inclusions in the visual field, those having a V content of 10% by mass or more are defined as "V-containing precipitates".

The circle-equivalent diameter of each of the V-containing precipitates determined was determined. Here, the circle-equivalent diameter refers to a diameter (nm) when the area of the V-containing precipitates is converted into a circle. V-containing precipitates having a circle equivalent diameter of more than 100nm among V-containing precipitates in the entire visual field are defined as "coarse V-containing precipitates". The total number of coarse V-containing precipitates in all the visual fields was determined. From the total number of the coarse V-containing precipitates thus obtained, the number density (number/276. mu.m) of the coarse V-containing precipitates was determined²)。

As described above, the steel shaft member of the present embodiment has the above-described chemical composition, and the microstructure of the core portion of the shaft portion includes ferrite and pearlite and satisfies all of the conditions a to D, so that the occurrence of cracks in the core portion region near the conventionally unknown hardened layer can be suppressed, and excellent fatigue strength can be obtained.

[ method for producing Steel shaft Member ]

An example of the method for producing the steel shaft member will be described. The manufacturing method described below is an example, and the manufacturing method of the steel shaft member according to the present embodiment is not limited to this. In other words, the steel shaft member of the present embodiment having the above-described configuration can be manufactured, and is not limited to the manufacturing method described below. The manufacturing method described below is a manufacturing method suitable for manufacturing the steel shaft member of the present embodiment.

In the method of manufacturing the steel shaft member according to the present embodiment, first, the steel material for the steel shaft member having the chemical composition is prepared. As the steel material for the steel shaft member, a steel material manufactured by a third party can be used. The steel material for the steel shaft member may be produced by a manufacturer of the steel shaft member.

The steel material for the steel shaft member can be produced, for example, by the following method. Molten steel having the above chemical composition is produced by a known method. Molten steel is used to manufacture a billet (slab or ingot). Specifically, a slab is manufactured by a continuous casting method using molten steel. Alternatively, an ingot can be produced by an ingot casting method using molten steel.

The produced billet is hot worked to produce a steel material for a steel shaft member. The steel material for the steel shaft member is, for example, bar steel. In the hot working step, hot working is usually carried out 1 or more times. The hot working is performed a plurality of times, and the first hot working is, for example, roughing or hot forging, and the subsequent hot working is finish rolling using a continuous rolling mill. In a continuous rolling mill, a horizontal rolling mill having a pair of horizontal rolls and a vertical rolling mill having a pair of vertical rolls are alternately arranged in a row. The finish-rolled steel material for steel shaft members is cooled to room temperature. Through the above steps, the steel material for a steel shaft member of the present embodiment is manufactured. The heating temperature of the billet in the hot working is, for example, 950 to 1350 ℃.

The steel shaft member is produced using the steel material for steel shaft members. The manufacturing method of the steel shaft component comprises the following steps: a hot forging step, a machining step, a high-frequency quenching step, a tempering step, and a cutting step. Hereinafter, each step will be described.

[ Hot forging Process ]

In the hot forging step, the steel material for the steel shaft member is hot forged to manufacture an intermediate product having the approximate shape of the steel shaft member. Here, the rough shape is a shape close to the final shape of the steel shaft member. The heating temperature in the hot forging step is, for example, 950 to 1350 ℃. Here, the heating temperature T1 refers to the heating temperature (deg.c) in the heating furnace or soaking furnace before hot forging. When the heating temperature is 950 to 1350 ℃, the V-containing precipitates in the steel material for a steel shaft member are sufficiently dissolved in a solid state on the premise that other production conditions are satisfied. Therefore, in the steel shaft member after the induction hardening step, the number density of coarse V-containing precipitates in the hardened layer can be controlled to 10/276. mu.m²The following.

After the final rolling in the hot forging, the steel material is cooled. In this case, the average cooling rate CR of the steel material temperature is set to be not less than 12 ℃/min between 800 and 500 ℃. When the average cooling rate CR is less than 12 ℃/min, V-containing precipitates are formed and grown by precipitation at the interface between the steel material and the steel material at a temperature of 800 to 500 ℃. At this time, the V-containing precipitates formed in the hot forging become coarse in the subsequent induction hardening step. As a result, the number density of coarse V-containing precipitates in the hardened layer may exceed 10/276. mu.m². If the average cooling rate CR is less than 12 ℃/min, the hardness of the core also becomes low, and the Vickers hardness Hb at the R/2 position becomes less than the lower limit of formula (1). Therefore, the average cooling rate of the steel material at 800 to 500 ℃ is 12 ℃/min or more.

Here, the average cooling rate at a steel temperature of 800 to 500 ℃ is determined by the following method. The temperature of the steel material after the final rolling in the hot forging step was measured by a thermometer, and the time taken for the temperature of the steel material to decrease from 800 ℃ to 500 ℃ was determined. From the obtained time, the average cooling rate (DEG C/min) of the steel material temperature between 800 and 500 ℃ is obtained.

The upper limit of the average cooling rate CR is preferably 25 ℃/min, more preferably 20 ℃/min, and still more preferably 15 ℃/min.

An intermediate product of the steel shaft member is manufactured through the hot forging process. In the present embodiment, after the hot forging step, quenching in which the entire intermediate product is heated and then rapidly cooled using a heat treatment furnace and tempering after quenching are not performed on the intermediate product. In other words, the heat treatment step after the hot forging step and before the induction hardening step is omitted.

[ machining Process ]

The intermediate product after the hot forging step is subjected to machining such as cutting, and the intermediate product is finished into the final shape of the steel shaft member. The machining may be performed by a known method.

[ high-frequency hardening step ]

The intermediate product after the machining step is subjected to induction hardening treatment. Specifically, at least the surface of the portion corresponding to the shaft portion of the intermediate product is subjected to induction hardening. In the high-frequency quenching, a high-frequency induction heating apparatus is used. The intermediate product is heated while relatively moving the high-frequency induction heating apparatus with respect to the intermediate product in the longitudinal direction (axial direction) of the intermediate product. A water cooling device is disposed on the outlet side of the high-frequency induction heating device. The intermediate product is rapidly quenched in a portion passing through the high-frequency induction heating apparatus by a water cooling apparatus.

In the induction hardening, the output power of the induction heating device is set to 20 to 60kW, and the frequency is set to 150 to 300 kHz. Further, the moving speed of the high-frequency induction heating device is set to be 4.0 to 8.0 mm/sec. If the moving speed of the high-frequency induction heating apparatus is less than 4.0 mm/sec, the shaft portion of the intermediate product is excessively heated. At this time, the hardened layer depth Hr becomes too deep, and the hardened layer depth Hr does not satisfy formula (2). As a result, the fatigue strength of the steel shaft member is reduced. Furthermore, V-containing precipitates are excessively formed in the surface layer, and the number of coarse V-containing precipitates in the surface layer exceeds 10/276. mu.m². On the other hand, if the moving speed of the high-frequency induction heating apparatus exceeds 8.0 mm/sec, the hardened layer depth Hr becomes too shallow. In this case, cracks are generated in the core region near the hardened layer of the steel shaft member, and the fatigue strength of the steel shaft member is reduced. If the moving speed of the high-frequency induction heating device is 4.0 to 8.0 mm/sec, a hardened layer with a proper depth is formed, and the depth Hr of the hardened layer satisfies the formula (2). As a result, a steel shaft member having excellent fatigue strength can be obtained.

[ tempering step ]

Tempering is performed on the intermediate product after the induction hardening step. The tempering temperature is 150-280 ℃. The lower limit of the tempering temperature is preferably 160 ℃ and preferably 170 ℃. The upper limit of the tempering temperature is preferably 270 ℃. The tempering temperature and time are, for example, 15 to 150 minutes.

[ cutting Process ]

A part of the surface of the intermediate product after the tempering step is cut to manufacture a steel shaft member as a final product.

The steel shaft member of the present embodiment is manufactured by the manufacturing process described above. The method for manufacturing the steel shaft member is not limited to the above method. The steel shaft member of the present embodiment may be produced by another production method as long as the steel shaft member having the above-described chemical composition, having a microstructure including ferrite and pearlite at the R/2 position of the shaft portion, and satisfying all of the conditions a to D can be produced.

Examples

Hereinafter, the effects of one embodiment of the steel shaft member of the present embodiment will be described in more detail by way of examples. The conditions in the examples are one example of conditions employed for confirming the feasibility and the effects of the present invention. Therefore, the steel shaft member of the present embodiment is not limited to the one condition example.

Steel materials for steel shaft members having chemical compositions shown in Table 1 were prepared.

[ Table 1]

TABLE 1

The content (mass%) of the corresponding element is shown in each element symbol column in table 1. In table 1, "-" indicates that the corresponding element was not detected (in other words, impurity level). Specifically, the V content of test No. 5 means: the third position after the decimal point is "0"% "when rounded. The Ti content of test No. 5 means: the fourth decimal place is rounded to 0%. The balance of the chemical composition of the steel material of each test number was Fe and impurities. The chemical composition of test No. 5 corresponds to 38MnS6 which is a German industrial standard widely used as a steel material for steel shaft members.

The steel materials for steel shaft members of the respective test numbers were hot forged to produce round bars corresponding to intermediate products. The heating temperature in the hot forging step is within the range of 1000 to 1200 ℃. The average cooling rate CR (DEG C/min) at a temperature of 800 to 500 ℃ of the steel after the final rolling is shown in Table 2. The diameter of the round bar produced was 60 mm.

The test material for the rotational bending fatigue test shown in fig. 4 was produced from a round bar material by machining. The test piece had a round bar shape, and the diameter was set to 12mm and the length to 120 mm. A notch is formed at the center of the test piece in the longitudinal direction. The notch angle was set to 60 °, and the notch depth was set to 1 mm. The diameter of the test piece at the bottom of the notch was set to 10mm, and the radius of the bottom of the notch was set to 0.5 mm. The longitudinal direction (axial direction) of the test material was parallel to the longitudinal direction (axial direction) of the round bar.

The obtained test material was subjected to high-frequency quenching. In the induction hardening, an induction heating apparatus is used. The output of the high-frequency induction heating apparatus was 40kW, the frequency was 220kHz, and the moving speed (mm/sec) of the high-frequency induction heating apparatus was as shown in Table 2. A water cooling device is disposed on the outlet side of the high-frequency induction heating device. The water cooling device cools the test material by water within 1 second after the test material passes through the high-frequency induction heating device. The hardness of the hardened layer and the depth Hr of the hardened layer are adjusted by adjusting the moving speed of the high-frequency induction heating apparatus. The test material after the high-frequency quenching was tempered. The tempering temperature is shown in Table 2. The tempering time was set to 90 minutes. The test piece simulating the steel shaft member was produced by the above-described production process.

[ Table 2]

The following test was performed on the test material produced through the above-described steps.

[ microscopic Structure Observation test ]

Samples for microstructure observation were taken from the R/2 position of the test material after tempering. The surface (observation surface) of the sample was mirror-polished, and then the observation surface was etched with 3% nitroethanol (Nital etchant). Any 5 fields of view on the etched observation surface were observed by an optical microscope at 500 magnifications to generate photographic images. The size of each field was set to 200. mu. m.times.200. mu.m. In each visual field, the contrast of each phase is different for each phase such as ferrite, pearlite, bainite. Therefore, each phase is determined according to the contrast. In each field, ferrite andpearlite was determined as the total area (μm) of ferrite in all visual fields²) The total area (. mu.m) of pearlite in all the visual fields was determined²). The ratio of the total area of ferrite in all the fields to the total area of pearlite in all the fields is defined as the total area ratio (%) of ferrite to pearlite. When the total area ratio of ferrite and pearlite is 95.0% or more, it is judged that the microstructure contains ferrite and pearlite. The judgment results are shown in the column "microstructure" in table 2. "F + P" indicates that the total area ratio of ferrite and pearlite in the microstructure is 95.0% or more, and the microstructure is a structure including ferrite and pearlite. "M + B" indicates that the microstructure is a structure containing martensite and bainite.

[ Vickers hardness Hs measurement test ]

A vickers hardness test was performed on any 3 points of the surface of the test piece after tempering in accordance with JIS Z2244 (2009). The test force was set to 1.96N. The arithmetic mean of the resulting Vickers hardnesses is defined as the Vickers hardness Hs (HV). The obtained Vickers hardness Hs is shown in Table 2 under the heading "Hs". Hs/2.3 is shown in Table 2 under "Hs/2.3".

[ Vickers hardness Hb measurement test ]

The tempered test material was cut perpendicularly to the axial direction. A vickers hardness test was performed according to JIS Z2244 (2009) for any 3 points of the R/2 position in the cross section of the cut test piece. The test force was set to 98N. The arithmetic mean of the resulting Vickers hardnesses was defined as the Vickers hardness Hb (HV) at the R/2 position. The obtained vickers hardness Hb is shown in table 2 in the column "Hb". When vickers hardness Hb is not less than the lower limit of formula (1), it is expressed as "t (true)" in the column of "lower limit of formula (1)" in table 2. When the Vickers hardness Hb is less than the lower limit of formula (1), it is expressed as "F (false)" in the column of "lower limit of formula (1)" in Table 2. When vickers hardness Hb is not more than the upper limit of formula (1), it is expressed as "t (true)" in the column of "upper limit of formula (1)" in table 2. When the Vickers hardness Hb is higher than the upper limit of formula (1), it is expressed as "F (false)" in the column of "upper limit of formula (1)" in Table 2.

[ measurement test for depth Hr of hardened layer ]

The test material was cut perpendicularly to the axial direction. In the cross section of the cut test piece, vickers hardness was measured at a pitch of 0.1mm in the depth direction (radial direction) from the surface. The vickers hardness was measured in accordance with JIS Z2244 (2009) and the test force was set to 1.96N. From the obtained vickers hardness, a vickers hardness distribution in the depth direction (radial direction) was prepared. In the Vickers hardness distribution, the depth at which the Vickers hardness becomes 620HV or more is defined as the depth of a hardened layer (mm). In a cross section perpendicular to the axial direction of the shaft portion, the vickers hardness distribution in the depth direction is obtained from arbitrary 3 portions of the surface, and the arithmetic average of the hardened layer depths at the respective positions (3 portions) is defined as a hardened layer depth hr (mm). The ratio of the depth Hr of the cured layer to the radius R (6mm) of the test material (Hr/R) was determined. The Hr/R thus obtained is shown in Table 2 under the heading "Hr/R".

[ number density of coarse V-containing precipitates (number/276. mu.m)²) Determination test]

The cutting was performed in a direction perpendicular to the axial direction of the test material. In the cut surface, a hardened layer is determined from the result of the hardened layer depth Hr measurement test. Samples were taken from approximately 1/2 depth positions of the hardened layer determined. Among the sample surfaces, a surface corresponding to a cross section perpendicular to the axial direction of the sample material was used as an observation surface. Using TEM, any 10 fields of view (each 1 field of view having an area of 27.6 μm) were observed at a magnification of 30000 times²) And (6) carrying out observation. The precipitates and inclusions in each visual field (referred to as an observation plane) were quantitatively analyzed by EDX for the content of the elements contained in each precipitate and inclusion, and a substance having a V content of 10% by mass or more among the precipitates and inclusions in the visual field was identified as "V-containing precipitates". The thickness of the sample (film sample) was set to about 50 nm.

The circle-equivalent diameter of each of the V-containing precipitates determined was determined. The V-containing precipitates having a circle equivalent diameter of more than 100nm among the V-containing precipitates in all the visual fields were defined as "coarse V-containing precipitates", and the total number of the coarse V-containing precipitates in all the visual fields was determined. Determining the number density of coarse V-containing precipitates from the determined total number of coarse V-containing precipitates: (276 mu m in each²)。

[ rotational bending fatigue test ]

The test materials of the respective test numbers were used to carry out the rotary bending fatigue test in accordance with JIS Z2274 (1978). Specifically, the number of repetitions of the stress load was set to 3600rpm and 10⁷The maximum stress at which fracture did not occur after one cycle was set as the fatigue strength (MPa). The ratio of the fatigue strength of each test number to the fatigue strength of test number 5 (hereinafter referred to as fatigue strength ratio) was determined by the following formula, using the fatigue strength of the test material of test number 5 as a reference.

Fatigue strength ratio fatigue strength of each test No./fatigue strength of test No. 5

The fatigue strength ratio is determined by rounding off the third decimal place of the obtained numerical values. When the fatigue strength ratio is 1.10 or more, the fatigue strength is judged to be excellent.

The position of occurrence of a crack (fatigue fracture starting point position) in the test material that fractured after the rotational bending fatigue test was determined by observing the cross section with an SEM. In the column of "starting point of failure" in table 2, "surface" means the starting point at which fatigue failure was observed in the surface of the test material. "internal" means: in the test material, the starting point of fatigue failure was observed at a portion more inside than the hardened layer (in other words, the core portion).

[ test results ]

Referring to tables 1 and 2, the chemical compositions of test numbers 1 to 4 are suitable. Further, the production conditions are also appropriate. Therefore, the microstructure at the R/2 position includes ferrite and pearlite, and the Vickers hardness Hs of the surface is 620HV or more. Furthermore, the Vickers hardness Hb at the R/2 position satisfies formula (1), and the ratio (Hr/R) of the depth Hr of the cured layer to the radius R of the test material satisfies formula (2). Furthermore, the number density of coarse V-containing precipitates was 10 precipitates/276. mu.m²The following. As a result, the fatigue strength ratios of test nos. 1 to 4 were 1.10 times or more the fatigue strength of the reference test No. 5, and excellent fatigue strength was obtained.

On the other hand, in test No. 6, the C content and V content were too low. Therefore, in test No. 6, the surface Vickers hardness Hs was less than 620HV, the fatigue strength ratio was less than 1.10, and the fatigue strength was low.

Test No. 7 contained no V. Therefore, in test No. 7, the Vickers hardness Hb at the R/2 position does not satisfy formula (1). As a result, the fatigue strength ratio was less than 1.10, and the fatigue strength was low. In addition, the test material after the rotational bending fatigue test was observed, and as a result, cracks were generated in the core region near the hardened layer of the test material.

In test No. 8, the moving speed of the high-frequency induction heating apparatus was too slow. Thus, the test material north was over-quenched. As a result, Hr/R exceeds the upper limit of formula (2). Furthermore, the number density of coarse V-containing precipitates exceeds 10/276. mu.m². As a result, the Vickers hardness Hb at the R/2 position exceeded 350 HV. Further, the fatigue strength ratio is less than 1.10, and the fatigue strength is low.

In test No. 9, the moving speed of the high-frequency induction heating apparatus was too high. Therefore, Hr/R is less than the lower limit of formula (2). As a result, the fatigue strength ratio was less than 1.10, and the fatigue strength was low. In addition, the test material after the rotational bending fatigue test was observed, and as a result, cracks were generated in the core region near the hardened layer of the test material.

In test No. 10, the tempering temperature was too high. Therefore, the Vickers hardness Hs of the surface of the test material is less than 620 HV. As a result, the fatigue strength ratio was less than 1.10, and the fatigue strength was low.

In test No. 11, the average cooling rate CR was too low when the temperature of the steel material after hot forging was 800 to 500 ℃. Therefore, the number density of coarse V-containing precipitates exceeds 10/276. mu.m². Further, the Vickers hardness Hb at the R/2 position is less than the lower limit of formula (1). As a result, the fatigue strength ratio was less than 1.10, and the fatigue strength was low.

The embodiments of the present invention have been described above, but the above embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments may be appropriately modified and implemented without departing from the scope of the invention.