JP2022079181A - Steel for nitriding and nitriding treatment component - Google Patents

Abstract

To provide a steel for nitriding and a nitriding treatment component which are excellent in machinability even in hot-working, and secure surface fatigue strength and rotation bending fatigue strength after nitriding treatment.SOLUTION: Mn is effective so as to increase hardness before nitriding in association with increase in C content and aggregate machinability, and enhance a bainite structure fraction of a base material so as to increase core part age hardening capability before and after nitriding, and Cr and V are effective so as to increase hardness of a surface layer part after nitriding, and V is effective so as to improve core part age hardening capability. The nitriding steel contains, by mass%, 0.01-0.11% C, 0.01-0.30% Si, 1.20-2.50% Mn, 0.025% or less P, 0.100% or less S, 0.20-0.90% Cr, 0.05-0.50% V, 0.050% or less Al, 0.0250% or less N, and the balance Fe with impurities.SELECTED DRAWING: None

Description

The present invention relates to nitriding steel and gas nitriding steel parts.

Steel parts used in automobiles and various industrial machines are subjected to surface hardening heat treatment such as carburizing and quenching, induction hardening, and nitriding in order to improve mechanical properties such as fatigue strength, surface fatigue strength, and seizure resistance. Often given.

In the carburizing and quenching treatment, after C (carbon) is infiltrated and diffused into the surface layer of the steel, the steel is heated to the austenite region of Ac3 or more, and then cooled to harden the surface layer by martensitic transformation, so that the surface layer is hardened for a relatively short time. It is a heat treatment that can obtain a deep hardened layer and a high surface hardness. However, since the entire component undergoes phase transformation, there is a problem that the dimensional change of the component after heat treatment is large.

Induction hardening is a heat treatment in which only the surface layer of steel is rapidly heated to the austenite region and quenched, and the quenching strain is smaller than that of carburizing and quenching. However, unlike induction hardening, induction hardening is not a heat treatment method for increasing the C (carbon) concentration of the surface layer, and is not suitable for parts that require high surface fatigue strength and bending fatigue strength.

In the nitriding treatment and the soft nitriding treatment, the steel is heated in a ferrite region of A1 point or less, and the surface layer is hardened by solid solution strengthening by N (nitrogen) infiltrated and diffused into the surface layer and particle dispersion strengthening of the nitride. The heat treatment strain can be reduced without the above. Further, by performing induction hardening after nitriding or soft nitriding to deeply form a martensite structure having a high N concentration on the surface layer, surface fatigue strength and bending fatigue strength can be increased while suppressing distortion of parts. Therefore, the nitriding treatment and the soft nitriding treatment are often used for parts having high dimensional accuracy and large-sized parts, and are applied to, for example, gears used for transmission parts of automobiles and crank shafts used for engines. However, in nitriding and soft nitriding, in order to increase the surface hardness, the steel material before nitriding treatment (hereinafter, simply referred to as “base material”), which is the base material, may contain a nitride-forming element such as Cr or V. In order to secure the hardness of the steel core portion which is a non-nitriding layer, it is necessary to contain an alloy element to increase the fatigue strength. Therefore, the nitriding steel has a relatively hard base material as compared with the carburizing and quenching steel, and poor machinability may be a problem.

Therefore, in order to achieve both machinability before nitriding and fatigue strength after nitriding, for example, the soft nitriding steels shown in Patent Documents 1 to 3 have been proposed.
Patent Document 1 discloses a method for manufacturing a soft nitriding steel and a soft nitriding processed part that secures machinability by containing Mo and V in a certain range.
Patent Document 2 proposes a steel for soft nitriding, which contains Mo, V, and Ti in a certain range and has a bainite structure having an area ratio of more than 50%.
Patent Document 3 describes that hot rolling remains or heat is obtained by obtaining the precipitation effect and fatigue strength by precipitating Cu in ferrite and by reducing the content of C and N in steel to suppress pearlite. A steel for soft nitriding having excellent cold forging property, which has a hardness of 150 HV or less as it is forged, has been proposed.

Japanese Unexamined Patent Publication No. 2006-193827 International Publication No. 2016/152167 Japanese Unexamined Patent Publication No. 2002-69571.

The soft nitriding steel disclosed in Patent Document 1 requires the addition of Mo and B in order to ensure hardenability, and there is room for improvement in alloy cost and manufacturability.
The soft nitriding steel disclosed in Patent Document 2 has a hardness of 233 to 363 HV before the nitriding treatment, and cannot be said to be excellent in machinability.
Although the hardness of the soft nitriding steel disclosed in Patent Document 3 before nitriding is low, it is necessary to contain 0.5% by mass or more of Cu in the base material, and it is a technique premised on cold forging. In the parts machined with hot forging, the hardness of the core after nitriding is low, and it is difficult to obtain high fatigue strength.

The present invention solves the above-mentioned problems, is excellent in machinability in hot working (hot rolling, hot forging, etc.), and secures surface fatigue strength and rotational bending fatigue strength after nitriding treatment. It is an object of the present invention to provide steel for nitriding and nitriding parts having excellent characteristics.

In order to solve the above-mentioned problems, the present inventors independently change various alloy components, and each element has the hardness and structure morphology of the base material before nitriding (before nitriding treatment), and after nitriding (after nitriding treatment). Various effects on the hardness of the surface layer portion and the core portion of the diffusion layer were investigated, and the findings of (a) to (f) were obtained.

(A) The lower the surface hardness after hot working (hot rolling or hot forging, etc.) (before nitriding), the better the machinability, and the hardness of the surface layer and core of the diffusion layer after nitriding. The higher the value, the higher the surface fatigue strength and the bending fatigue strength.

(B) The hardness before nitriding increases remarkably as the C content increases. Therefore, from the viewpoint of machinability, it is preferable to reduce the amount of C in the base metal.

(C) In order to increase the age hardening ability of the core portion before and after the nitriding treatment, it is necessary to increase the bainite structure fraction of the base material before nitriding. Increasing the Mn content is effective in increasing the bainite structure fraction.

(D) The hardness of the surface layer of the diffusion layer after nitriding increases as the content of Cr and V increases. In particular, V is also effective in improving the age hardening ability of the core portion.

(E) The hardness of the surface layer of the diffusion layer after nitriding decreases as the C and Si contents increase.

(F) As the amounts of C and Mo in the base metal increase, the volume fraction of the ε phase in the compound layer after nitriding increases, and the surface fatigue strength and bending fatigue strength decrease.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1]
By mass%,
C: 0.01 to 0.11%,
Si: 0.01-0.30%,
Mn: 1.20 to 2.50%,
P: 0.025% or less,
S: 0.100% or less,
Cr: 0.20 to 0.90%,
V: 0.05 to 0.50%,
Al: 0.050% or less, and N: 0.0250% or less,
A steel for nitriding, characterized in that the balance is Fe and impurities.
[2]
The nitriding steel according to [1], wherein the Vickers hardness of the surface is 220 HV or less.
[3]
In addition, by mass%,
Mo: 0.50% or less,
Cu: 0.45% or less,
Ni: 0.50% or less,
W: 0.50% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.200% or less,
Ti: 0.250% or less, and B: 0.0100 or less%,
The nitriding steel according to [1] or [2], which comprises one or more of the above.
[4]
In addition, by mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Te: 0.100% or less,
Pb: 0.090% or less,
Sn: 0.100% or less,
The nitriding according to any one of [1] to [3], which contains one or more of Sb: 0.0100% or less and REM: 0.0100% or less. Steel for use.
[5]
In the cross section perpendicular to the main axis direction, the component of the core portion, which is a similar figure region that shares the center of gravity with the cross section and has a similarity ratio of 1/4 of the cross section, is mass%.
C: 0.01 to 0.11%,
Si: 0.01-0.30%,
Mn: 1.20 to 2.50%,
P: 0.025% or less,
S: 0.100% or less,
Cr: 0.20 to 0.90%,
V: 0.05 to 0.50%,
It contains Al: 0.050% or less and N: 0.0250% or less, and consists of the balance Fe and impurities.
In the cross section, the N concentration at a depth of 50 μm from the surface is higher than the N concentration in the core portion.
Further, the nitriding-treated component is characterized in that the Vickers hardness at the 50 μm depth position is 700 HV or more and the Vickers hardness at the core portion is 220 HV or more.
[6]
Further, the component of the core portion is by mass%,
Mo: 0.50% or less,
Cu: 0.45% or less,
Ni: 0.50% or less,
W: 0.50% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.200% or less,
Ti: 0.250% or less, and B: 0.0100 or less%,
The nitrided component according to [5], which contains one or more of the above.
[7]
Further, the component of the core portion is by mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Te: 0.100% or less,
Pb: 0.090% or less,
Sn: 0.100% or less,
The nitrided component according to [5] or [6], which contains one or more of Sb: 0.0100% or less and REM: 0.0100% or less.

According to the present invention, it is possible to obtain a nitriding steel having good machinability because the hardness before nitriding is low after hot working, and further, by nitriding, high surface fatigue strength and bending fatigue can be obtained. It is possible to obtain a nitrided component having both strength. Therefore, for example, it is suitable for use as a material for internal gears for automobiles, which have a high degree of processing difficulty.

FIG. 1 is a diagram showing an example of a small roller for a roller pitching test. The unit of dimensions in the figure is mm. FIG. 2 is a diagram showing an example of a large roller for a roller pitching test. The unit of dimensions in the figure is mm. FIG. 3 is a diagram showing an example of a cylindrical test piece for a rotary bending fatigue test. The unit of dimensions in the figure is mm.

The present invention will be described by exemplifying a nitriding steel and a nitriding processed component according to an embodiment of the present invention. The nitriding-treated component (which is a nitriding-treated component and may be simply referred to as a “part” below) according to the present embodiment is a core portion (hereinafter, simply “core portion”) which is a central portion of a steel material to be a component. It has a compound layer located on the surface layer of the steel material and a nitrogen diffusion layer adjacent to the compound layer (hereinafter, may be simply referred to as a “diffusion layer”).
Here, the core portion is a portion that is not invaded by nitrogen due to the nitriding treatment, and for convenience, the cross section and the center of the figure are taken in a cross section perpendicular to the main axis direction (for example, the central axis in the longitudinal direction) of the nitriding processed component. In common, it refers to a region of similar shape of the cross section (1/16 in area ratio) where the similarity ratio of the cross section is 1/4. That is, in the cross section of the component after nitriding, the position at a depth of 1/2 of the thickness (plate thickness if plate-shaped, diameter if rod-shaped) in the direction perpendicular to the surface (depth direction) from the surface (hereinafter). , "It may be called" 1/2 position of thickness (diameter) "), and the width of 1/4 of the thickness (diameter) (one-sided swing width from the center (or center line) is ± thickness." Refers to the area of 1/8) of (diameter). For example, in the case of a columnar part having a diameter D, a circular region having a diameter D / 4 from the center of the cross section (center of the circle) is the core portion in the cross section perpendicular to the axial direction.

[Ingredients of base material]
The components of the base material will be described. Normally, the core of the nitrided component has the same composition as the base material, so unless otherwise specified, the core component of the base material and the core component of the component are the same. The "%" of the content of each component element in steel and the concentration of the element on the surface of the component means "mass%" unless otherwise specified.

[C: 0.01 to 0.11%]
C (carbon) is an important element for ensuring the hardness of the core of the nitrided component. In order to obtain this effect, the C content should be 0.01% or more. On the other hand, when the C content is high, the curing ability of the diffusion layer after the nitriding treatment becomes small, and the ε-phase volume ratio in the compound layer tends to be high, so that the surface fatigue strength and the bending fatigue strength decrease. do. In addition, the hardness of the base metal after hot working becomes too high, so that the machinability of the base metal is greatly reduced. Further, the hardness of the surface layer portion of the component after nitriding decreases as the C content increases. From these facts, it is preferable that the C content is low, and the upper limit thereof is preferably 0.11%.
More preferably, the C content may be 0.02% or more, 0.03% or more, or 0.04% or more. Similarly, the C content may be 0.10% or less, 0.09% or less, 0.08% or less, 0.07% or less, or 0.06% or less.

[Si: 0.01 to 0.30%]
Si has a deoxidizing effect. In order to obtain this effect, it is preferable to contain 0.01% or more of Si. On the other hand, if the Si content is high, the curability of the diffusion layer is lowered, and the hardness of the base metal after hot working is too high, so that the machinability of the base metal is greatly reduced. The Si content should be 0.30% or less.
More preferably, the Si content may be 0.02% or more, 0.03% or more, 0.04% or more, 0.05% or more, 0.06% or more, or 0.07% or more. Similarly, the Si content is 0.29% or less, 0.28% or less, 0.27% or less, 0.26% or less, 0.25% or less, 0.24% or less, 0.23% or less, 0.22% or less, 0.21% or less, 0.20% or less, 0.19% or less, 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less good.

[Mn: 1.20 to 2.50%]
Mn is an element that increases the hardness of the core by strengthening the solid solution. It is also an element that enhances hardenability and enhances the bainite structure fraction after hot working of the base metal. In addition, Mn is an element that forms fine nitrides (Mn3N2) in the compound layer and the diffusion layer by nitriding treatment to increase surface fatigue strength and bending fatigue strength. In order to obtain these effects, the Mn content should be 1.20% or more. On the other hand, if the Mn content is high, the hardness of the base metal after hot working becomes too high, and the machinability of the base metal is greatly reduced. Therefore, it is preferably 2.50% or less.
More preferably, the Mn content is 1.25% or more, 1.30% or more, 1.35% or more, 1.40% or more, 1.45% or more, 1.50% or more, 1.55% or more. It may be 1.60% or more, 1.65% or more, 1.70% or more, or 1.75% or more. Similarly, the Mn content is 2.45% or less, 2.40% or less, 2.35% or less, 2.30% or less, 2.25% or less, 2.20% or less, 2.15% or less, It may be 2.10% or less, or 2.05% or less.

[P: 0.025% or less]
Since P is an impurity and may cause grain boundary segregation to embrittle parts and reduce surface fatigue strength and bending fatigue strength, the P content is preferably 0.025% or less. It is preferable that the P content is as low as possible. The preferred upper limit of the P content is 0.018%, 0.015%, 0.013%, or 0.010%. The P content may be preferably 0%, but the P content is 0.001% or more, 0.005% or more, or 0.008% in consideration of the economic efficiency of refining because it leads to an increase in the cost of removing P. The above may be applied.

[S: 0.100% or less]
S is an element that combines with Mn to form MnS and improves machinability. However, when the S content is high, coarse MnS is likely to be generated, and the surface fatigue strength and the bending fatigue strength are greatly reduced. Therefore, the S content is preferably 0.100%. The preferred upper limit of the S content is 0.080%, 0.060%, or 0.040%. The S content may be 0%, but the S content may be 0.001% or more, 0.002%% or more, or 0.005% or more in consideration of the economic efficiency of refining because it leads to an increase in the cost of removing S. May be.

[Cr: 0.20 to 0.90%]
Cr is an element that forms fine nitrides (CrN) in the compound layer and the diffusion layer by the nitriding treatment and enhances the surface fatigue strength and the bending fatigue strength. In order to obtain these effects, the Cr content is preferably 0.20% or more. On the other hand, when the Cr content is high, not only the effect of improving the surface fatigue strength and the bending fatigue strength is saturated, but also the hardness of the base material after hot working becomes too high, so that the machinability of the base material becomes too high. However, the Si content should be 0.90% or less.
More preferably, the Cr content may be 0.25% or more, 0.30% or more, 0.35% or more, 0.40% or more, 0.45% or more, or 0.50% or more. Similarly, the Cr content may be 0.85% or less, 0.80% or less, 0.75% or less, 0.70% or less, 0.65% or less, or 0.60% or less.

[V: 0.05 to 0.50%]
V is an element that increases the surface fatigue strength and bending fatigue strength of parts by forming fine nitrides (VN) in the compound layer and diffusion layer by nitriding treatment and forming carbides (VC) in the core. be. In order to obtain these effects, the V content is preferably 0.05% or more. On the other hand, when the V content is high, not only the effect of improving the surface fatigue strength and the bending fatigue strength is saturated, but also the hardness of the base material after hot working becomes too high, so that the machinability of the base material becomes too high. However, the V content should be 0.50% or less.
More preferably, the V content is 0.06% or more, 0.08% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.18% or more, or 0.20% or more. May be. Similarly, the V content may be 0.48% or less, 0.45% or less, 0.43% or less, 0.40% or less, 0.38% or less, or 0.35% or less.

[Al: 0.050% or less]
Al is a deoxidizing element, but it does not have to be contained in particular. However, Al combines with N to form AlN, and the pinning action of the austenite grains has the effect of refining the structure of the base metal before the nitriding treatment and reducing the variation in the mechanical properties of the nitriding treated parts. In order to obtain this effect, the Al content is preferably 0.001% or more, more preferably 0.002% or more, 0.003% or more, 0.004% or more, or 0.005%. It is good to do the above.
On the other hand, Al tends to form hard oxide-based inclusions, and if the Al content is high, the bending fatigue strength is significantly reduced, and the desired bending fatigue strength cannot be obtained even if other requirements are satisfied. Since it may occur, the Al content should be 0.050% or less. The preferable upper limit of the Al content is preferably 0.040%, 0.030%, or 0.020% in order to prevent a decrease in bending fatigue strength.

[N: 0.0250% or less]
N (nitrogen) does not have to be contained in particular. However, N combines with Mn, Cr, Al, and V to form Mn3N2, CrN, AlN, and VN, and in particular, Al and V have a fine structure of the base material before the nitriding treatment due to the pinning action of the austenite grains. It has the effect of reducing variations in the mechanical properties of nitrided parts. From the viewpoint of obtaining this effect, the N content is 0.0010% or more, 0.0015% or more, 0.0020% or more, 0.0025% or more, 0.0030% or more, 0.0035% or more, or 0. It may be 0040% or more.
On the other hand, when the N content is high, coarse AlN and VN are likely to be formed, so that the bending fatigue strength is significantly reduced, and the desired surface fatigue strength and bending fatigue strength can be obtained even if other requirements are satisfied. The N content should be 0.0250% or less because it may not be possible. Preferably, the N content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

The rest of the above elements are basically composed of Fe and impurities. Impurities include components contained in raw materials, components mixed in during the manufacturing process, and elements that are not intentionally contained, and are the characteristics of the nitriding steel and nitriding parts according to the present embodiment. It is acceptable as long as it does not impair.

As a component that can be further contained in the base material, the following elements can be contained in place of a part of Fe. However, the parts according to the present embodiment can solve the problem without containing the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[Mo: 0.50% or less]
Mo is an element that increases the hardness of the core by strengthening the solid solution. It is also an element that enhances hardenability and enhances the bainite structure fraction after hot working of the base metal. In addition, Mn forms fine nitrides (Mо ₂ N) in the compound layer and diffusion layer by nitriding treatment, and forms carbides (Mo ₂ C) in the core to form surface fatigue strength and bending fatigue. It is an element that increases strength. In order to obtain these effects, the Mo content may be 0.01% or more. On the other hand, when the Mo content is high, the ε-phase volume ratio in the compound layer after nitriding tends to be high, and the surface fatigue strength and bending fatigue strength are lowered. Therefore, the Mo content should be 0.50% or less. Further, since the hardness of the base metal after hot working becomes too high, the machinability of the base metal is greatly reduced.
When Mo is contained, the Mo content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Mo content may be 0.45% or less, 0.40% or less, or 0.30% or less.

[Cu: 0.45% or less]
Cu is an element that increases the hardness of the core by strengthening the solid solution. In order to surely obtain this effect, the Cu content may be 0.01% or more. On the other hand, if the Cu content is high, the hardness of the base metal after hot working becomes too high, and the machinability of the base metal is greatly reduced. Therefore, the Cu content should be 0.45% or less.
When Cu is contained, the Cu content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Cu content may be 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[Ni: 0.50% or less]
Ni is an element that increases the hardness of the core by strengthening the solid solution. In order to surely obtain this effect, the Ni content may be 0.01% or more. On the other hand, if the Ni content is high, the hardness of the base metal after hot working becomes too high, and the machinability of the base metal is greatly reduced. Therefore, the Ni content should be 0.50% or less.
When Ni is contained, the Ni content may be preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Ni content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[W: 0.50% or less]
W improves the core hardness and the surface hardness by strengthening the solid solution and precipitating carbides (WC and W2C). In order to surely obtain the action of W, the W content should be 0.01% or more. On the other hand, if W is high, the hardness of the base metal after hot working becomes too high, and the machinability of the base metal is greatly reduced. Therefore, the W content should be 0.50% or less.
When W is contained, the W content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the W content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[Bi: 0.50% or less]
Bi has the effect of reducing cutting resistance and prolonging the life of the tool. In order to surely obtain this effect, the Bi content should be 0.01% or more. On the other hand, if the Bi content is high, cracks and scratches are likely to occur during hot working, so the Bi content should be 0.50% or less.
When Bi is contained, the Bi content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Bi content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[Co: 0.50% or less]
Co is an element that increases the hardness of the core by strengthening the solid solution. In order to surely obtain this effect, the Co content should be 0.01% or more. On the other hand, if the Co content is high, the hardness of the base metal after hot working becomes too high, and the machinability of the base metal is greatly reduced. Therefore, the Co content should be 0.50% or less. ..
When Co is contained, the Co content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Co content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[Nb: 0.200% or less]
Nb can be combined with N that has penetrated into the surface layer of steel during nitriding and C of the matrix to form fine nitrides (NbN) and carbides (NbC), and can improve surface hardness and core hardness. can. In order to surely obtain this effect, the Nb content should be 0.010% or more. On the other hand, if the Nb content is high, coarse nitrides and carbonitrides are likely to be produced, so the Nb content should be 0.200% or less.
When Nb is contained, the Nb content is preferably 0.015% or more, 0.020% or more, 0.025% or more, or 0.030% or more. Similarly, the Nb content may be 0.175% or less, 0.150% or less, 0.125% or less, or 0.100% or less.

[Ti: 0.250% or less]
Ti combines with N that has penetrated into the surface layer of the base metal and C in the base metal during nitriding to form fine nitrides (TiN) and carbides (TiC), and improves surface hardness and core hardness. be able to. In order to surely obtain this effect, the Ti content should be 0.005% or more. On the other hand, if the Ti content is high, coarse nitrides and carbides are likely to be generated. Therefore, the Ti content should be 0.250% or less.
When Ti is contained, the Ti content is preferably 0.007% or more, 0.010% or more, 0.015% or more, or 0.020% or more. Similarly, the Ti content may be 0.200% or less, 0.150% or less, or 0.100% or less.

[B: 0.0100% or less]
The solid solution B has the effect of suppressing the grain boundary segregation of P and improving the toughness. In addition, BN that binds to N and precipitates improves machinability. In order to surely obtain these effects, the B content is preferably 0.0005% (5 ppm) or more. On the other hand, if the B content is high, segregation of a large amount of BN is promoted, which may lead to cracking of the steel material. Therefore, the B content should be 0.0100% or less.
When B is contained, the B content is preferably 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. Similarly, the B content may be 0.0080% or less, 0.0070% or less, or 0.0060% or less.

[Ca: 0.0100% or less]
Ca has a function of refining MnS and improving surface fatigue strength. In order to surely obtain this effect of Ca, the Ca content should be 0.0010% or more. On the other hand, even if the Ca content is high, the effect is saturated and the economic efficiency is impaired. Therefore, the Ca content should be 0.0100% or less.
When Ca is contained, the Ca content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Ca content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[Mg: 0.0100% or less]
Mg has a function of refining MnS and improving surface fatigue strength. In order to surely obtain the action of Mg, the Mg content should be 0.0010% or more. On the other hand, if the Mg content is high, the effect is saturated and the economy is impaired. Therefore, the Mg content should be 0.0100% or less.
When Mg is contained, the Mg content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Mg content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[Te: 0.100% or less]
Te has a function of refining MnS and improving surface fatigue strength. In order to surely obtain the action of Te, the Te content should be 0.010% or more. On the other hand, if the Te content is high, the effect is saturated and the economic efficiency is impaired. Therefore, the Te content should be 0.0100% or less.
When Te is contained, the Te content is preferably 0.020% or more, 0.030% or more, or 0.040% or more. Similarly, the Te content may be 0.090% or less, 0.080% or less, or 0.070% or less.

[Pb: 0.090% or less]
Pb has the effect of lowering the cutting resistance and prolonging the life of the tool. However, if the content of Pb is increased, the effect is saturated and the economic efficiency is impaired. Therefore, the Pb content is preferably 0.090% or less.
When Pb is contained, the Pb content is preferably 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[Sn: 0.100% or less]
Sn has the effect of reducing cutting resistance and prolonging the life of the tool. In order to surely obtain this effect, the Sn content may be 0.010% or more. On the other hand, if the Sn content is high, the effect is saturated and the economic efficiency is impaired. Therefore, the Sn content should be 0.100% or less.
When Sn is contained, the Sn content is preferably 0.020% or more, 0.030% or more, or 0.040% or more. Similarly, the Sn content may be 0.090% or less, 0.080% or less, or 0.070% or less.

[Sb: 0.0100% or less]
Sb has the effect of reducing cutting resistance and prolonging the life of the tool. In order to surely obtain this effect, the Sb content may be 0.0010% or more. On the other hand, if the Sb content is high, the effect is saturated and the economic efficiency is impaired. Therefore, the Sb content should be 0.0100% or less.
When Sb is contained, the Sb content may be preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Sb content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[REM: 0.0100% or less]
REM (rare earth element) refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and REM content means the total content of these 17 elements. When lanthanides are used as REMs, industrially, REMs are added in the form of mischmetal.
REM has a function of refining MnS and improving surface fatigue strength. In order to surely obtain the action of REM, the REM content should be 0.0010% or more. On the other hand, if the REM content is high, the effect is saturated and the economy is impaired. Therefore, the REM content should be 0.0100% or less.
When REM is contained, the REM content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the REM content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[Hardness]
The nitriding steel according to the present invention is melted, hot-worked (hot rolling or hot forging, etc.), then cut into a predetermined part shape such as a gear, and then nitrided to be nitrided. It becomes a processing part. In one embodiment of the present invention, the hardness after hot working (hereinafter referred to as “pre-nitriding hardness”) and machinability, and the hardness in the surface layer portion of the diffusion layer after nitriding treatment (hereinafter referred to as “surface layer hardness”). It has been found that there is a certain correlation between the hardness of the core portion (hereinafter referred to as "core hardness") and the surface fatigue strength and the bending fatigue strength. Therefore, the pre-nitriding hardness, which is an index of machinability of nitriding steel, and the surface layer hardness and core hardness, which are one of the indexes of fatigue strength of nitrided parts, will be described below. The hardness in the present invention refers to Vickers hardness (HV) according to JIS Z 2244: 2009 "Vickers hardness test-test method".

[Hardness after hot working of steel for nitriding (hardness before nitriding)]
The pre-nitriding hardness of the nitriding steel according to the present invention is the test force obtained by mirror-polishing the cross section of the base metal after hot working by cutting it perpendicular to the working direction (rolling or forging direction). Refers to the average value of Vickers hardness at any 10 points excluding the region up to 1 mm from the surface measured at 1.96 N.
This pre-nitriding hardness affects the machinability of parts, and it is empirically understood that the machinability is good when the pre-nitriding hardness is 220 HV or less. On the other hand, if the hardness before nitriding exceeds 220 HV, the machinability of the component becomes poor. Therefore, the hardness of the nitriding steel after hot working (before nitriding) was set to 220 HV or less. The hardness after hot working (before nitriding) is preferably 210 HV, more preferably 200 HV or less.

[Hardness in the surface layer of the diffusion layer after nitriding treatment (hardness of the surface layer)]
The surface layer hardness is the position of 0.05 mm (50 μm) from the surface at a depth of 0.05 mm (50 μm) from the surface by mirror-polishing the cross section that appears by cutting the part after nitriding (nitriding part) perpendicular to the main axis direction or the longitudinal direction. It refers to the average value of Vickers hardness calculated by measuring any 10 points (positions perpendicular to the surface) with a test force of 1.96 N.
This surface layer hardness affects the surface fatigue strength and bending fatigue strength of the component. When the surface layer hardness is 700 HV or more, the surface fatigue strength and the bending fatigue strength are good. On the other hand, when the hardness of the surface layer portion is less than 700 HV, the surface fatigue strength and the bending fatigue strength of the component are low. Therefore, the hardness of the surface layer of the nitrided component is preferably 700 HV or more. The preferable lower limit of the surface hardness is 720 HV, and more preferably 740 HV.

[Hardness in the core after nitriding (core hardness)]
The core hardness means that the cross section exposed by cutting the part after nitriding (nitriding part) perpendicular to the main axis direction or the longitudinal direction is mirror-polished, and any 10 points in the core are tested. Refers to the average value of Vickers hardness measured and calculated at 96N.
The reason for using the core is as follows. It is difficult to strictly identify the boundary between the diffusion layer in which nitrogen has invaded by the nitriding treatment and the portion where nitrogen has not invaded, but nitriding is performed in the core portion at the position of 1/2 of the thickness (diameter). There is almost no effect of nitrogen intrusion by the treatment. Therefore, by measuring the hardness of the core portion of the component, the hardness of the core portion can be measured without being affected by the chemical composition due to nitriding. The core hardness affects the surface fatigue strength and bending fatigue strength of the part. When the core hardness is 220 HV or more, the surface fatigue strength and the bending fatigue strength are good. On the other hand, if the core hardness is less than 220 HV, plastic deformation occurs in the non-nitriding layer on the surface layer side, which has the same hardness as the core, cracks occur from the vicinity of the interface with the diffusion layer, or the diffusion layer becomes By releasing the generated compressive residual stress, the surface fatigue strength and bending fatigue strength of the component become low. Therefore, the hardness of the core of the nitriding component is preferably 220 HV or more. The preferable lower limit of the core hardness is 230 HV, and more preferably 240 HV. On the other hand, since it is not harder than the diffusion layer, the upper limit of the core hardness may be 700 HV.

[Manufacturing method of nitriding parts]
An aspect of a method for manufacturing a nitriding steel according to the present invention and a nitriding-treated component using the same will be described. The method for manufacturing a nitriding steel and a nitriding processed part according to the present invention is not limited to this aspect.
First, steel having the above-mentioned chemical composition is melted by a conventional method to produce steel materials such as ingots, slabs, and billets. Next, these steel materials can be manufactured by subjecting them to processing and heat treatment by, for example, the following methods.

[Hot working]
After cutting the steel material having the above-mentioned chemical composition into an appropriate size, it is heated and held at 1050 to 1250 ° C., and then hot-worked into a rough shape. The heating retention time is preferably 0.5 to 4.0 hours in order to reduce the heat equalization of the steel and reduce the surface oxide film. The hot working is mainly hot rolling or hot forging, but the hot working is not particularly limited to this working method, and the hot working may be performed according to the above-mentioned method.
For the purpose of equalizing the crystal grain size of the structure after hot working, normalizing according to JIS B 6911: 2010 "Normalizing and Annealing of Steel" is performed before cutting, which will be described later. May be.

[Cutting]
After cutting a rough product after hot working with a lathe or the like, for example, in the case of a gear, it is machined into a predetermined part shape by broaching or the like.

[Nitriding treatment]
Nitriding is applied to the parts that have been cut into a predetermined shape. The method for nitriding the nitrided component in this embodiment is not particularly limited, and well-known gas nitriding, gas nitrocarburizing, salt bath nitriding, plasma nitriding and the like can be applied. The gas used for the nitriding treatment may be only NH3, or may be a mixed gas containing N2 and H2 in addition to NH3. Further, a carburizing gas may be contained in these gases to carry out a soft nitriding treatment. Further, as a pretreatment or a posttreatment of the nitriding treatment, a chemical treatment such as film removal or an oxidation treatment may be carried out as long as the temperature does not exceed the nitriding treatment temperature.
However, in nitriding including carburizing gas (for example, gas nitriding, salt bath nitriding), the constituent phase of the compound layer formed on the surface of the component after nitriding tends to be a fragile ε single phase. Therefore, it is preferable to perform nitriding treatment in an atmosphere that does not contain carburizing gas.
In order to stably generate a compound layer and a diffusion layer and secure surface fatigue strength and bending fatigue strength, the temperature of the nitriding treatment may be 550 to 650 ° C. and the holding time of the nitriding treatment may be 1 to 10 hours. preferable.

[Induction hardening]
As a post-process of the nitriding treatment, induction hardening may be performed for the purpose of increasing surface fatigue strength and bending fatigue strength. By induction hardening, the surface layer portion becomes a hardened layer having a high N martensite structure in which the nitride forming element is solid-solved. Therefore, in the region where the tooth surface temperature rises (about 200 to 400 ° C.) due to contact friction in surface fatigue, the hardness does not easily decrease due to the formation of nitride clusters by alloys such as Cr and V, resulting in high surface fatigue strength. can get. In addition, by deepening the hardened layer, it is possible to suppress internal origin fracture due to bending fatigue. In order to obtain these effects, it is preferable to perform induction hardening at a depth of 100 μm or more from the surface of the component. Further, the heating temperature of the induction hardening treatment may be 1000 ° C. or higher and 1200 ° C. or lower, and the time required for raising the temperature from room temperature to the heating temperature may be 4 seconds or less. The time for holding the steel material in the temperature range of 1000 ° C. or higher and 1200 ° C. or lower is preferably 0.2 seconds or longer and 2 seconds or shorter.

Hereinafter, examples will be shown as an example of the present invention. The present invention is not limited to the following examples.
Steels a to ad having the chemical components shown in Table 1 were melted and melted in a 50 kg vacuum melting furnace and cast to produce an ingot. Further, among the components of steels a to 〇 shown in Table 1, the balance other than the components shown in Table 1 is Fe and impurities. In all steels, O (oxygen) was contained as an impurity at about 10 ppm.

The ingots of each steel shown in Table 1 were hot forged to obtain a round bar having a diameter of 40 mm. In hot forging, the ingot was held in a heating furnace at a temperature between 1100 ° C. and 1200 ° C. for 2 hours, forged into a round bar having a diameter of 40 mm, and after forging, it was allowed to cool in the air.
[Measurement of hardness before nitriding]
Each round bar after hot forging was cut vertically in the longitudinal direction at the central portion in the longitudinal direction, and then the cut surface was mirror-polished. Then, the Vickers hardness at any 10 points in the cross section was measured using a micro Vickers hardness tester (manufactured by Shimadzu Corporation; HMV-G31-FA) under the condition of a test force of 1.96 N. The average value of these 10 points was defined as the hardness before nitriding.

[Machinability evaluation test]
After hot forging, each round bar is machined to a diameter of 20 mm and a length of 300 mm, and then a carbide tool (K10 grade, TiN coating), feed rate 0.20 mm / rev, peripheral speed 100 m / min, cutting. A cutting test was carried out under the condition of 0.9 mm, and their cutting resistance was measured.
As an evaluation index of machinability, steel satisfying the SCr420 standard of JIS G 4053: 2016 was machined into the same shape, and the above-mentioned cutting test was carried out. If the cutting resistance of the SCr420 steel is used as the reference cutting resistance and the ratio of the cutting resistance of each steel to the reference cutting resistance (specific cutting resistance) is 1.40 or less when this is set to 1.00, the machinability Was evaluated as good.

Subsequently, each round bar after hot forging was machined to produce a small roller for a roller pitching test for evaluating the surface fatigue strength shown in FIG. 1. Multiple small rollers were manufactured from one ingot for the roller pitching test, and at that time, more small rollers were manufactured than required for the roller pitching test, assuming that the hardness in the cross section was investigated. .. Further, using the same round bar as a material, a cylindrical test piece for evaluating the rotational bending fatigue strength shown in FIG. 3 was produced. Multiple cylindrical test pieces were also prepared from one ingot for the rotary bending fatigue test.

The collected test pieces were subjected to gas nitriding treatment, gas soft nitriding treatment, and plasma nitriding treatment. Table 2 shows the correspondence between each test piece and the nitriding treatment. In any of the nitriding treatments, the nitriding treatment temperature was 590 ° C., the nitriding treatment time was 5 hours, and the test piece after the nitriding treatment was oil-cooled with oil at 80 ° C.
For gas nitriding, test pieces are charged into a gas nitriding furnace, and NH ₃ , H ₂ , and N ₂ gases are introduced into the furnace. For gas nitriding tests, CO ₂ gas is added to these gases. Was introduced at a volume ratio of 3%.

The H ₂ partial pressure in the atmosphere was measured using a heat conduction type H ₂ sensor directly mounted on the gas nitride furnace body. The difference in thermal conductivity between the standard gas and the measured gas was converted into gas concentration and measured. _The H2 partial pressure was continuously measured during the gas nitriding process.

The NH ₃ partial pressure was measured using an infrared absorption type NH ₃ analyzer installed outside the furnace. The NH ₃ partial pressure was continuously measured during the gas nitriding process. Regarding test number 5, which is in an atmosphere of CO ₂ gas mixing, (NH ₄ ) ₂ CO ₃ was deposited in the infrared absorption type NH ₃ analyzer, and there was a risk that the equipment would break down. The NH ₃ partial pressure was measured every 10 minutes using a ₃ analyzer.

The nitriding potential KN of the gas nitriding process is defined by the following equation.
KN (atm- ^{1 / 2} ) = (NH ₃ partial pressure (atm)) / [(H ₂ partial pressure (atm)) ^3/2 ]

The NH ₃ flow rate and the N ₂ flow rate were controlled so that the nitriding potential KN calculated in the apparatus converged to the target value. The nitriding potential KN was recorded every 10 minutes, and the average value of KN measured during the processing time was calculated.
For plasma nitriding treatment, test pieces are charged into plasma nitriding equipment, H ₂ and N ₂ gases are introduced into the furnace, and the partial pressure ratio of H ₂ gas and N ₂ gas becomes constant at 3: 1. The gas flow rate was controlled.
A part of the nitriding test piece was induction hardened. In each treatment, the heating temperature was set to 1100 ° C, the time required to raise the temperature from room temperature to the heating temperature was set to 3 seconds, the time to hold the steel material at 1100 ° C was set to 1 second, and immediately after induction hardening, water at room temperature was used. Quenched. Then, tempering was performed at 170 ° C. for 1.5 hours.

[Measurement of surface hardness and core hardness]
Each small roller subjected to the nitriding treatment or the nitriding induction hardening treatment was cut to a length of 10 mm on a surface perpendicular to the center in the longitudinal direction, and then the cut surface was mirror-polished. After that, the Vickers hardness at any 10 points at a depth of 0.05 mm (50 μm) from the surface was adjusted to a test force of 1.96 N using a Micro Vickers hardness tester (manufactured by Shimadzu Corporation; HMV-G31-FA). Was measured. The average value of these 10 points was defined as the surface hardness.
Similarly, using a test piece whose surface hardness was measured, the Vickers hardness at any 10 points of the core was measured using the same device under the condition of a test force of 1.96 N, and the average value of 10 points was measured. Was defined as core hardness.

[Surface fatigue strength evaluation test]
The surface fatigue strength was evaluated by the following method using a roller pitching tester (manufactured by Komatsu Equipment Co., Ltd .; RP102). The small rollers for the roller pitching test were subjected to finishing of the grip portion for the purpose of removing heat treatment strain, and then subjected to each roller pitching test piece. The shape after finishing is shown in FIG.

As shown in FIG. 1, the small roller, which is a roller pitching test piece, includes a central test surface portion having a diameter of 26 mm and a width of 28 mm, and grip portions having a diameter of 22 mm provided on both side portions thereof. In the roller pitching test, the test surface portion was brought into contact with the large roller, and a predetermined surface pressure was applied before rotation.
The roller pitching test was performed under the conditions shown in Table 3 with a combination of the above-mentioned small roller for roller pitching test and a large roller for roller pitching test having the shape shown in FIG.

The unit of dimensions in FIGS. 1 and 2 is mm.
The large roller for roller pitching test uses steel that meets the SCM420 standard of JIS G 4053: 2016, and is a general manufacturing process, that is, "normalizing-> test piece processing-> eutectoid carburizing by gas carburizing furnace-> low-temperature tempering-> It was produced by the process of "polishing", and the Vickers hardness HV at a position of 0.05 mm (50 μm) from the surface was 740 to 760, and the depth of Vickers hardness Hv of 550 or more was 0. It was in the range of 8 to 1.0 mm.

Table 3 shows the test conditions for which the surface fatigue strength was evaluated. Table 3 shows the test conditions for which the surface fatigue strength was evaluated. The number of repetitions of the test cutoff was 2.0 × 107 times, which indicates the fatigue limit of general steel, and the maximum surface pressure reached 2 × ^{10 7 times} without pitching in the small roller test piece was set to the small roller. The fatigue limit of the test piece was set. In the roller pitching test, the surface pressure was measured in increments of 50 MPa, especially near the fatigue limit. That is, in the values of the pitching strength shown in Table 2, in the target test number, pitching did not occur in the small roller test piece tested under the same surface pressure, but the test was performed under a surface pressure 50 MPa higher than the same surface pressure. It is shown that pitching occurred in the small roller test piece that was subjected to.

The occurrence of pitching was detected by a vibration meter installed in the testing machine, and after the vibration was generated, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitching and the rotation speed were confirmed. In this embodiment, assuming application to gear parts, the target is that the surface pressure at the fatigue limit in the roller pitching test shown in Table 3 is 2000 MPa or more.

[Rotary bending fatigue strength evaluation test]
An Ono-type rotary bending fatigue test based on JIS Z 2274: 1978 "Rotating bending fatigue test method for metal materials" was carried out on a cylindrical test piece subjected to gas nitriding treatment. The number of rotations is 3000 rpm, the number of test cutoff repetitions is 1 × 10 ⁷ times, which indicates the fatigue limit of general steel, and the maximum stress reached 1 × 10 ⁷ times without fracture in the rotary bending fatigue test piece. The fatigue limit of the rotary bending fatigue test piece was set. In the rotary bending fatigue test, the stress was tested in increments of 10 MPa, especially near the fatigue limit. That is, the values of the rotational bending fatigue strength shown in Table 2 showed that the cylindrical test piece tested under the same stress did not break under the same stress, but the test was performed under a stress 10 MPa higher than the same stress. It is shown that the columnar test piece subjected to the above was broken.

In the parts of the present invention, assuming application to gear parts, the goal was to have a stress of 550 MPa or more in the fatigue limit in the Ono-type rotary bending fatigue test.

[Test results]
The results are shown in Table 2. In test numbers 1 to 19, since the steel component is within the range of the present invention, the hardness before nitriding is low and the machinability is excellent, and the surface layer hardness and core hardness after nitriding or induction hardening are high. The results obtained that the surface fatigue strength and the rotational bending fatigue strength were also high.

On the other hand, in test numbers 20 to 35, the steel component is out of the range of the present invention, the hardness before nitriding is high and the machinability is inferior, or the hardness of the surface layer after nitriding or induction hardening. The core hardness was low, and the desired surface fatigue strength and rotational bending fatigue strength did not reach the objects of the present invention.

The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.

The present invention can be widely used in industrial fields such as mechanical parts such as gears, transmissions and camshafts of transportation machines.

Claims (7)

By mass%,
C: 0.01 to 0.11%,
Si: 0.01-0.30%,
Mn: 1.20 to 2.50%,
P: 0.025% or less,
S: 0.100% or less,
Cr: 0.20 to 0.90%,
V: 0.05 to 0.50%,
Al: 0.050% or less, and N: 0.0250% or less,
A steel for nitriding, characterized in that the balance is Fe and impurities. The nitriding steel according to claim 1, wherein the Vickers hardness of the surface is 220 HV or less. In addition, by mass%,
Mo: 0.50% or less,
Cu: 0.45% or less,
Ni: 0.50% or less,
W: 0.50% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.200% or less,
Ti: 0.250% or less, and B: 0.0100% or less,
The nitriding steel according to claim 1 or 2, wherein the steel contains one or more of the two. In addition, by mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Te: 0.100% or less,
Pb: 0.090% or less,
Sn: 0.100% or less,
The nitriding steel according to any one of claims 1 to 3, which contains one or more of Sb: 0.0100% or less and REM: 0.0100% or less. .. In the cross section perpendicular to the main axis direction, the component of the core portion, which is a similar figure region that shares the center of gravity with the cross section and has a similarity ratio of 1/4 of the cross section, is mass%.
C: 0.01 to 0.11%,
Si: 0.01-0.30%,
Mn: 1.20 to 2.50%,
P: 0.025% or less,
S: 0.100% or less,
Cr: 0.20 to 0.90%,
V: 0.05 to 0.50%,
Al: 0.050% or less,
N: 0.0250% or less,
Containing, the balance consists of Fe and impurities,
In the cross section, the N concentration at a depth of 50 μm from the surface is higher than the N concentration in the core portion.
Further, the nitriding-treated component is characterized in that the Vickers hardness at the 50 μm depth position is 700 HV or more and the Vickers hardness at the core portion is 220 HV or more. Further, the component of the core portion is by mass%,
Mo: 0.50% or less,
Cu: 0.45% or less,
Ni: 0.50% or less,
W: 0.50% or less,
Bi: 0.50% or less,
Co: 0.50% or less,
Nb: 0.200% or less,
Ti: 0.250% or less, and B: 0.0100 or less%,
The nitrided component according to claim 5, wherein the nitriding component contains one or more of the two or more. Further, the component of the core portion is by mass%,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Te: 0.100% or less,
Pb: 0.090% or less,
Sn: 0.100% or less,
The nitrogen treatment component according to claim 5 or 6, wherein Sb: 0.0100% or less, and REM: 0.0100% or less, one or more of them are contained.

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