JP5336972B2 - Nitriding steel and nitride parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel for nitriding in which machining before nitriding is facilitated, and further, even if the content of Mo as an expensive element is limited, by mass, to &le;0.50%, which has high bending fatigue strength and pitching resistance after nitriding. <P>SOLUTION: The steel for nitriding has a composition comprising 0.05 to 0.09% C, 0.10 to 0.35% Si, 1.0 to 2.0% Mn, 0.005 to 0.050% S, 1.0 to 2.0% Cr, 0.10 to 0.50% Mo, 0.010 to 0.10% Al and 0.05 to 0.40% V, and in which the C content, the Mo and the V content satisfy the inequality of [äMo/(2&times;95.94)}+(V/50.9415)&ge;C/12], and the balance Fe with impurities, and, in the impurities, the contents of P, N, Ti and O satisfy &le;0.030%P, &le;0.008%N, &le;0.005% Ti and &le;0.0030% O, respectively. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a nitriding steel and a nitrided component (hereinafter referred to as “nitrided component”). More specifically, the present invention relates to a nitriding steel that is easy to cut before nitriding, has high bending fatigue strength after nitriding, has excellent pitting resistance, and is suitable for use as a material for nitriding parts such as an automobile ring gear, and the nitriding part thereof.

  Parts used for automobile transmissions and the like are usually subjected to surface hardening treatment such as carburizing quenching, induction quenching and nitriding from the viewpoint of improving bending fatigue strength and pitching resistance.

Among the above, “carburizing and quenching” is a process in which low carbon steel is generally used, C is intruded and diffused in a high temperature austenite region of Ac ₃ point or higher, and then quenched. Although there is an advantage that a high surface hardness and a deep hardened layer depth can be obtained, there is a problem that the heat treatment deformation becomes large because the treatment involves transformation. Therefore, when high part accuracy is required, finishing such as grinding and honing after carburizing and quenching is required. In addition, a so-called “carburized abnormal layer” such as a grain boundary oxide layer or an incompletely hardened layer formed on the surface layer becomes a fracture starting point such as bending fatigue, and there is a problem that the fatigue strength is lowered.

“Induction hardening” is a process of quenching by rapid heating and cooling in a high temperature austenite region of Ac ₃ or higher. Although there is an advantage that the adjustment of the hardened layer depth is relatively easy, it is not a surface hardening treatment that penetrates and diffuses C like carburizing, so the required surface hardness, hardened layer depth and core hardness are reduced. In order to obtain it, it is common to use medium carbon steel having a higher C content than carburizing steel. However, since the medium carbon steel has a higher material hardness than the low carbon steel, there is a problem that machinability is lowered. There is also a problem that a high-frequency heating coil needs to be produced for each component.

“Nitriding” is a process for obtaining a high surface hardness and an appropriate hardened layer depth by invading and diffusing N at a temperature of 450 to 550 ° C. below the Ac ₁ point. Since the treatment temperature is lower than that of carburizing and induction hardening, there is an advantage in that heat treatment deformation is small.

Among the “nitriding”, “soft nitriding” is a process for obtaining high surface hardness by invading and diffusing N and C at a temperature of about 500 to 600 ° C. below the Ac ₁ point, with only small heat treatment deformation. In addition, since the processing time is as short as several hours compared with the case where only N enters and diffuses, the processing is suitable for mass production. However, the conventional nitriding steel has problems as shown in the following <1> and <2>.

  <1> Poor machinability before nitriding. That is, nitriding is a treatment that does not perform quenching treatment from a high-temperature austenite region, so that strengthening that involves martensitic transformation cannot be utilized. Therefore, it is necessary to increase the hardness before nitriding in order to ensure the desired strength of the nitrided part, and if the hardness is increased by containing a large amount of alloy elements, cutting becomes difficult. End up.

  <2> Low fatigue strength after nitriding. For example, aluminum chromium molybdenum steel (SACM645) specified in JIS G 4202 (2005), which is a typical nitriding steel, obtains high surface hardness because Cr, Al, etc. generate nitride near the surface. Although the hardened layer is shallow, high fatigue strength cannot be ensured.

  On the other hand, for example, Patent Documents 1 to 4 propose steels using age hardening.

  That is, in Patent Document 1, by mass ratio, C: 0.05 to 0.20%, Si: 0.05 to 0.50%, Mn: 1.00 to 2.50%, Cr: 0.30 to 1 50%, Mo: 0.35 to 1.00%, V: 0.05 to 0.50%, Al: 0.005 to 0.070%, N: 0.005 to 0.020%, O: 0.0025% or less, and if necessary, (a) S: 0.035 to 0.12%, Pb: 0.02 to 0.30%, and (b) Ni: 2.0% Hereinafter, “precipitation hardening type nitriding steel” is characterized in that B: 0.0005% to 0.0050% of two groups of elements are contained in combination of one or more, and the balance is composed of Fe and impurity elements. It is disclosed.

  In Patent Document 2, C: 0.11 to 0.60 mass%, Si: 0.03 to 3.0 mass%, Mn: 0.01 to 2.5 mass%, Mo: 0.3 to 4.0 % By mass, V: 0.05 to 0.5% by mass, Cr: 0.1 to 3.0% by mass, further if necessary, (a) Al: 0.001 to 0.3% by mass, N: 0.005 to 0.025 mass%, (b) Nb: 0.5 mass% or less, Ti: 0.5 mass% or less, Zr: 0.5 mass% or less, (c) Cu: 1.0 mass% Hereinafter, Ni: 1.0 mass% or less, and (d) S: 0.01-0.20 mass%, Ca: 0.003-0.010 mass%, Pb: 0.3 mass% or less, Bi: It contains 4 groups of elements of 0.3 mass% or less in combination of 1 or 2 and the balance is composed of Fe and inevitable impurities. Between each component, [4C + Mn + 0.7Cr + .6Mo−0.2V ≧ 2.5], [C ≧ Mo / 16 + V / 5.7] and [V + 0.15Mo ≧ 0.4] are satisfied, and rolling, forging, or solution After the aging treatment, cooling is performed at an average cooling rate of 0.05 to 10 ° C./sec between a temperature of 800 ° C. and a temperature of 300 ° C., and before the aging treatment, the area ratio of the bainite structure is 50% or more and the hardness Is 40HRC or less, and “aging hardened steel” is disclosed, characterized in that the hardness is higher by 7 HRC or more than the hardness before aging treatment by aging treatment.

  In Patent Document 3, C: 0.05 to 0.15 mass%, Si: 0.50 mass% or less, Mn: 1.00 mass% or less, Cr: 1.00 to 2.00 mass%, Mo: 0 .90 to 1.50 mass%, Al: 0.010 to 0.100 mass%, N: 0.0070 to 0.0200 mass%, Ni: 1.00 mass% or less, V: 0.10 It contains one or more of 0.30% by mass, Ti: 0.10% by mass or less, Nb: 0.030% by mass or less, and further, if necessary, S: 0.005 to 0.100% by mass. %, Pb: 0.03 to 0.35 mass%, Ca: 0.0010 to 0.0100 mass%, Te: 0.001 to 0.100 mass%, Zr: 0.01 to 0.20 mass% It is a precipitation hardening steel containing one or more kinds, the balance being Fe and inevitable impurities, and having a temperature of 50 Disclosed is a “nitriding steel” characterized by having a property that the Vickers hardness (HV) of the core is 30 or more higher than that before the gas nitriding treatment by gas nitriding treatment at ˜600 ° C. Has been.

  In Patent Document 4, in mass%, C: 0.04 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.50 to 1.50%, Mo: 0.80 to 1.%. 50%, V: 0.10 to 0.30%, Cr: 0 to 0.50%, Nb: 0 to 0.05%, Al: 0 to 0.050%, if necessary, (A) Ti: 0.015-0.100%, B: 0.0005-0.0030%, (b) S: 0.005-0.100%, Ca: 0.001-0.01%, Te: 0.001 to 0.100% of two groups of elements are contained in combination of one or more, the balance is made of Fe and impurities, N in the impurities is 0.0070% or less, O (oxygen) is A “soft nitriding steel” characterized by being 0.0030% or less is disclosed.

JP-A-4-154936 JP 2006-37177 A Japanese Patent Laid-Open No. 11-124653 JP 2006-328457 A

  The steel for nitriding proposed in the above-mentioned Patent Document 1 has a high Vickers hardness (HV hardness) of 276 or more before nitriding as shown in the examples. For this reason, it is hard to say that machinability is good.

  When nitriding is applied as the aging treatment of the age-hardened steel proposed in Patent Document 2, since the C content is high, the hardness before nitriding is high and the machinability is good. I want.

  Since the steels proposed in Patent Document 3 and Patent Document 4 both have a high Mo content, the cost increases.

  The present invention has been made in view of the above-mentioned present situation, cutting before nitriding is easy, and even if the content of Mo, which is an expensive element, is limited to 0.5% by mass%, An object of the present invention is to provide a nitriding steel having high bending fatigue strength after nitriding and having excellent pitting resistance and a nitrided part thereof.

  The present inventors have made various studies in order to solve the above-described problems. As a result, the following findings (a) to (e) were obtained.

  (A) In order to increase the age hardening amount in the nitriding treatment, a bainite structure is preferable.

  (B) In order to improve the machinability, it is necessary to keep the hardness before age hardening, that is, before nitriding, low. For this reason, the C content and the N content are reduced as much as possible.

  (C) The strength that is reduced by lowering the C content is compensated by increasing the Mn content and the Cr content.

  (D) In order to increase the age-hardening amount, Mo and V are combined and contained, and then the balance with the C content is optimized. That is, the age hardening amount is increased by precipitating fine precipitates of Mo and V by nitriding.

  (E) Further, by limiting the Ti content and the N content, the formation of hard inclusions (TiN) that adversely affect the bending fatigue strength and pitting resistance is suppressed.

  The present invention has been completed on the basis of the above findings, and the gist thereof resides in the nitriding steel shown in (1) below and the nitrided part shown in (2).

(1) By mass%, C: 0.05 to 0.09%, Si: 0.10 to 0.35%, Mn: 1.0 to 2.0%, S: 0.005 to 0.050% , Cr: 1.0 to 2.0%, Mo: 0.10 to 0.50%, Al: 0.010 to 0.10%, V: 0.05 to 0.40%, and C , Mo and V satisfy the following formula (1), the balance consists of Fe and impurities, and P, N, Ti and O in the impurities are each P: 0.030% or less, N: 0 0.008% or less, Ti: 0.005% or less, and O: 0.0030% or less.
{Mo / (2 × 95.94)} + (V / 50.9415) ≧ C / 12 (1)
However, the element symbol in the formula (1) represents the content in mass% of the element.

  (2) It has the chemical composition described in (1) above, the surface hardness is 700 to 900 in terms of Vickers hardness, the core hardness is 230 or more in terms of Vickers hardness, and the effective hardened layer depth is 0.20 mm or more. A nitrided part characterized by the above.

  In the present invention, “nitriding” includes not only the process of invading and diffusing only N but also the process of invading and diffusing N and C, that is, “soft nitriding”.

  The “impurities” in the remaining “Fe and impurities” refers to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  Further, “surface hardness” refers to any 10 points at a depth of 0.03 mm from the surface of the test piece in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003). The Vickers hardness (hereinafter referred to as “HV hardness”) is an arithmetic average value of values measured by a Vickers hardness tester with a test force of 0.98 N.

  The “effective hardened layer depth” was determined using a distribution diagram of HV hardness (that is, a transition curve of HV hardness) when the test force was 1.96 N and measured at a predetermined interval from the test piece surface. The distance from the surface to the position where the HV hardness is 420.

  The nitriding steel of the present invention is easy to cut before nitriding, and the nitriding component made from this nitriding steel has a Mo content of 0.5% by mass, which is an expensive element. In spite of the following, it has high bending fatigue strength and excellent pitting resistance. Therefore, the nitriding steel of the present invention is suitable for use as a material for nitriding parts such as automobile ring gears that require high bending fatigue strength and a long pitching life.

It is a figure which shows the rough shape as cut out from the steel bar of the Ono-type rotary bending fatigue test piece with a notch used in the Example. It is a figure which shows the rough shape as cut out from the bar steel of the roller pitching small roller test piece used in the Example. It is a figure which shows the rough shape as cut out from the steel bar of the roller pitching large roller test piece used in the Example. In FIG. 3, (a) is a front view when a coarse roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line. In an Example, it is a figure which shows the heat pattern of the soft nitriding given to the test piece shown in FIG. 1 and FIG. 2 which uses steel 1-12 as a raw material. In an Example, it is a figure which shows the heat pattern of the carburizing quenching-tempering given to the test piece shown in FIG.1 and FIG.2 which uses the steel 13 as a raw material. In an Example, it is a figure which shows the heat pattern of the carburizing quenching-tempering given to the test piece shown in FIG. 3 which uses the steel 13 as a raw material. It is a figure which shows the finishing shape of the Ono type | formula rotation bending fatigue test piece with a notch used in the Example. It is a figure which shows the finishing shape of the roller pitching small roller test piece used in the Example. It is a figure which shows the finishing shape of the roller pitching large roller test piece used in the Example. In FIG. 9, (a) is a front view when a roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition of steel C: 0.05 to 0.09%
C is an essential element for securing the strength of the nitrided part, and a content of 0.05% or more is necessary. However, if the C content increases and exceeds 0.09%, the hardness before nitriding increases and the machinability deteriorates. Therefore, the C content is set to 0.05 to 0.09%. When the machinability is more important, the C content is preferably 0.05 to 0.08%.

Si: 0.10 to 0.35%
Si has a deoxidizing action. In order to obtain this effect, a Si content of 0.10% or more is necessary. However, when the Si content increases and exceeds 0.35%, the hardness before nitriding increases and the machinability decreases. Therefore, the content of Si is set to 0.10 to 0.35%. The Si content is preferably 0.10 to 0.25%.

Mn: 1.0-2.0%
Mn has the effect of ensuring the bending fatigue strength and pitting resistance of nitrided parts, and the deoxidizing action. In order to obtain these effects, a content of 1.0% or more is necessary. However, if the Mn content increases and exceeds 2.0%, the hardness before nitriding becomes too high, and the machinability deteriorates. Therefore, the Mn content is set to 1.0 to 2.0%. In addition, it is preferable to make Mn content into 1.0 to 1.5% when machinability is regarded as important.

S: 0.005 to 0.050%
S combines with Mn to form MnS and has the effect of improving machinability. However, if the S content is less than 0.005%, the above effect is difficult to obtain. On the other hand, if the S content exceeds 0.050%, coarse MnS is formed, and hot forgeability and bending fatigue strength are reduced. Therefore, the content of S is set to 0.005 to 0.050%. When the hot forgeability and bending fatigue strength are more important, the S content is preferably 0.005 to 0.030%.

Cr: 1.0-2.0%
Cr has the effects of increasing the surface hardness and core hardness in nitriding, deepening the hardened layer, and ensuring the bending fatigue strength and pitting resistance of the part. However, if the Cr content is less than 1.0%, the above effect cannot be obtained. On the other hand, when the Cr content increases and exceeds 2.0%, the hardness before nitriding increases and the machinability decreases. Therefore, the Cr content is set to 1.0 to 2.0%. In addition, when the machinability is more important, the Cr content is preferably 1.0 to 1.5%.

Mo: 0.10 to 0.50%
Mo combines with C in the steel at the nitriding temperature to form carbides, and has the effect of improving the core hardness after nitriding. However, if the Mo content is less than 0.10%, the desired core hardness cannot be obtained. On the other hand, if the Mo content exceeds 0.50%, the hardness before nitriding increases and the machinability decreases. Therefore, the Mo content is set to 0.10 to 0.50%. When the machinability is important, the Mo content is preferably 0.10 to 0.40%.

Al: 0.010 to 0.10%
Al has a deoxidizing action. Moreover, it combines with N that penetrates and diffuses from the surface during nitriding to form AlN, and has the effect of improving the surface hardness. In order to obtain these effects, it is necessary to contain 0.010% or more of Al. However, if the Al content increases and exceeds 0.10%, not only does the hard Al ₂ O ₃ form and the machinability decreases, but the hardened layer in nitriding becomes shallow and bending fatigue strength and There arises a problem that the pitting resistance is lowered. Therefore, the content of Al is set to 0.010 to 0.10%. In addition, the minimum with preferable Al content is 0.020%, and a preferable upper limit is 0.070%.

V: 0.05 to 0.40%
V, like Mo, combines with C in steel at the nitriding temperature to form carbides, and has the effect of improving the core hardness after nitriding. It also has the effect of improving the surface hardness by forming a nitride or carbonitride by combining with N or C that penetrates and diffuses from the surface during nitriding. In order to obtain these effects, it is necessary to contain 0.05% or more of V. However, if the content of V increases and exceeds 0.40%, the hardness before nitriding becomes too high and the machinability deteriorates, and V is contained in the matrix by hot forging and subsequent normalization. Will not be dissolved, and the above effect will be saturated. Therefore, the content of V is set to 0.05 to 0.40%. A preferable V content is 0.10 to 0.40%.

{Mo / (2 × 95.94)} + (V / 50.9415) value: (C / 12) or more Before nitriding, C is mainly present in cementite. When C in this steel is nitrided, it combines with Mo and V to form carbides and contributes to age hardening. That is, at the nitriding temperature, Mo precipitates mainly as Mo ₂ C and V mainly as VC, which contributes to age hardening. And the atomic weights of C, Mo and V are 12, 95.94 and 50.9415, respectively,
{Mo / (2 × 95.94)} + (V / 50.9415) ≧ C / 12 (1)
When the above formula is satisfied, Mo and V are excessively contained with respect to C, so that a sufficient age hardening amount can be obtained at the nitriding temperature. Therefore, the contents of C, Mo and V are limited to the already described range, and further, the above expression (1) is satisfied.

  In the present invention, the contents of P, N, Ti and O in the impurities are P: 0.030% or less, N: 0.008% or less, Ti: 0.005% or less, and O: 0, respectively. It is necessary to limit it to 0030% or less.

  This will be described below.

P: 0.030% or less P is an impurity contained in the steel and segregates at the grain boundaries to embrittle the steel. In particular, when the content exceeds 0.030%, the degree of embrittlement May become noticeable. Therefore, in the present invention, the content of P in the impurities is set to 0.030% or less. In addition, it is preferable that content of P in an impurity shall be 0.020% or less.

N: 0.008% or less N in steel is easy to form carbonitride by combining with elements such as C and V, and when carbonitride such as VCN is formed before nitriding, the hardness becomes high, In the present invention, N is an undesirable element because machinability is reduced. In addition, since this carbonitride has a high solid solution temperature, it becomes difficult for V to be dissolved in the matrix by hot forging or heating in the subsequent normalization, and when the N content in the steel is high, age hardening at the nitriding temperature. The effect of improving the hardness due to is not sufficiently obtained. Therefore, in the present invention, the content of N in the impurities is set to 0.008% or less. In addition, the preferable content of N in the impurities is 0.006% or less.

Ti: 0.005% or less Ti has a high affinity with N, and is easily combined with N in steel to produce TiN which is a hard nitride. When the Ti content exceeds 0.005%, the generated coarse TiN reduces bending fatigue strength and pitting resistance. Therefore, in the present invention, the content of Ti in the impurities is set to 0.005% or less. In addition, the preferable content of Ti in the impurities is 0.003% or less.

O: 0.0030% or less O forms oxide-based inclusions that cause fatigue failure starting from inclusions, and lowers fatigue strength. In particular, when the O content exceeds 0.0030%, the fatigue strength is significantly reduced. Therefore, in the present invention, the content of O in the impurities is set to 0.0030% or less. In addition, the preferable content of O in the impurities is 0.0020% or less.

  As already mentioned, “impurities” refer to those mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

(B) Surface hardness of nitrided parts A nitrided part, that is, a part subjected to nitridation, has low surface fatigue, bending fatigue strength, pitting resistance, and wear resistance. If the HV hardness is 700 or more, the nitrided part can be provided with a desired strength. On the other hand, when the surface hardness increases, especially when the HV hardness exceeds 900, when the nitrided part is a gear, the aggression against the mating gear increases. Therefore, the nitrided part has a surface hardness of 700 to 900 in terms of HV hardness. In addition, the preferable minimum of surface hardness is 720 in HV hardness, and a preferable upper limit is 800 in HV hardness.

(C) Core part hardness of nitrided parts If the core part hardness of nitrided parts is low, plastic deformation occurs inside when a load is applied, and pitching occurs due to cracks generated inside. In order to suppress internal plastic deformation in the nitrided part, a core hardness of 230 or more in HV hardness is required. Therefore, the core hardness of the nitrided part of the present invention is 230 or more in terms of HV hardness. A preferable lower limit of the core hardness is HV hardness of 250.

  The upper limit of the core hardness is not particularly required, but the upper limit of the core hardness that can be reached when the nitriding steel of the present invention is nitrided without quenching is about 350 HV hardness. .

(D) Effective Hardened Layer Depth of Nitrided Parts If the effective hardened layer depth of the nitrided parts is shallow, it will cause destruction of the internal starting point and reduce bending fatigue strength and pitting resistance. In order to suppress destruction of the internal starting point, the effective hardened layer depth needs to be 0.20 mm or more. Therefore, the effective hardened layer depth of the nitrided part of the present invention is set to 0.20 mm or more. A preferred lower limit of the effective hardened layer depth is 0.25 mm.

  The upper limit of the effective hardened layer depth does not need to be specified in particular, but in order to increase the effective hardened layer depth, it is necessary to lengthen the nitriding treatment time, resulting in an increase in cost. Therefore, the effective hardened layer depth is preferably 0.50 mm or less, and more preferably 0.45 mm or less.

(E) Manufacturing method of nitrided part The nitrided part of the present invention is processed and heat-treated using steel having the chemical composition described in the above section (A) under the following conditions, for example, to perform nitriding treatment. Can be manufactured.

(E-1) Hot forging After cutting a steel slab or steel bar having the chemical composition described in (A) above, the steel is heated to 1150 to 1270 ° C. and hot forged into a rough shape.

(E-2) Normalization The structure of a steel material as hot forged has large crystal grains and may cause a decrease in bending fatigue strength. Therefore, it is preferable to perform the normalizing process at a temperature of 850 to 970 ° C. to make smaller crystal grains.

  When cooling after normalization is performed, slow cooling such as standing cooling or furnace cooling, carbonitrides such as VCN are precipitated in the cooling process and the hardness increases, and the machinability may decrease. Accordingly, in cooling after normalization, it is preferable to suppress precipitation of VCN and the like in the cooling process by taking appropriate measures such as air cooling.

  In order to suppress the precipitation of VCN and the like in the cooling process and maintain the machinability, the lower limit of the cooling rate is preferably 0.5 ° C./s, and the upper limit is 5 ° C./s. It is preferable.

(E-3) Cutting After the normalized rough product is cut with a lathe or the like, for example, in the case of a ring gear, it is processed with a processing machine such as a broaching machine or a gear shaper.

(E-4) Nitriding treatment The nitriding treatment method for obtaining the nitrided part of the present invention is not particularly defined, and gas nitriding treatment, salt bath nitriding treatment, ion nitriding treatment, etc. can be used. The treatment temperature of the nitriding treatment is preferably 500 to 650 ° C. If the soft-nitriding, for example a combination of RX gas in addition to NH _3, NH ₃ and RX gas 1: processing may be performed in one of an atmosphere.

  Although the treatment time varies depending on the treatment temperature, the desired surface hardness, core hardness and effective hardened layer depth can be obtained in 9 hours when the soft nitriding treatment is performed at 560 ° C.

When it is desired to suppress the formation of brittle compounds, or using fluorine gas as a pretreatment for nitriding treatment with NH _3, it is preferable to use a mixed gas of NH ₃ and H ₂ to nitriding treatment.

  Hereinafter, the present invention will be described in more detail by way of examples treated with gas soft nitriding, but the present invention is not limited to these examples.

  Steels 1 to 13 having the chemical composition shown in Table 1 were melted in a vacuum melting furnace or converter to produce an ingot or slab.

  Specifically, about the steels 1-12, after melting with a 180 kg vacuum melting furnace, it ingot-formed and produced the ingot.

  Steel 13 was melted by a 70-ton converter and then continuously cast to produce a slab.

  Steels 1 to 5 in Table 1 are steels of the present invention examples whose chemical compositions are within the range specified by the present invention, while Steels 6 to 13 are from the conditions specified by the chemical composition of the present invention. It is steel of the comparative example which has come off.

  Among the steels of the above comparative examples, steel 13 is steel corresponding to SCM420H defined in JIS G 4052 (2008).

  The ingots of the above steels 1 to 12 were subjected to a solution treatment that was held at 1250 ° C. for 5 hours and homogenized, and then heated to 1200 ° C. to perform hot forging, and the diameters were 25 mm and 35 mm, respectively. Steel bars each having a length of 1000 mm were produced.

  The steel 13 slab was heated at 1250 ° C. for 3 hours to be rolled into a steel slab, then heated to 1200 ° C. to perform hot forging, and the diameters were 25 mm and 35 mm, respectively. Steel bars having a length of 140 mm and a length of 1000 mm were prepared.

  Among the above steel bars, steels 1 to 12 having a diameter of 25 mm and a diameter of 35 mm were subjected to “normalization” in which they were kept at 950 ° C. for 1 hour and then air-cooled.

  On the other hand, the steel bar having a diameter of 25 mm and a diameter of 35 mm was subjected to “normalization” in which the steel 13 was held at 900 ° C. for 1 hour and then allowed to cool. Further, the steel 13 having a diameter of 140 mm was subjected to “normalization” in which it was kept at 900 ° C. for 4 hours and then allowed to cool. The steel 13 was allowed to cool, not air cooled after being held at 900 ° C., because the steel 13 did not precipitate carbide even when allowed to cool, and conversely, when air cooled, there were many bainite structures. This is because the hardness increases and the machinability decreases.

  Various test pieces were collected from each of the steel bars 1 to 13 normalized as described above.

  Specifically, first, a steel bar having a diameter of 25 mm is cut so-called “crossing”, that is, perpendicularly to the axial direction (length direction), and embedded in the resin so that the cut surface becomes the test surface. The sample was polished so that the cut surface had a mirror finish, and used as a Vickers hardness test piece and a microstructure observation sample after normalization.

  Further, a turning test piece having a diameter of 30 mm and a length of 300 mm was collected from a steel bar having a diameter of 35 mm.

  Further, from the central part of a steel bar having a diameter of 25 mm, a coarse notched Ono-type rotary bending fatigue test piece shown in FIG. 1 was cut out in parallel to the axial direction. Similarly, from the central part of the steel bar having a diameter of 35 mm in the axial direction. A coarse roller pitching small roller test piece shown in FIG. 2 was cut out in parallel.

  Further, a coarse roller pitching large roller test piece shown in FIG. 3 was cut out in parallel with the axial direction from the center of a steel bar having a diameter of 140 mm. In FIG. 3, (a) is a front view when a rough roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line.

  1 to 3 are all “mm”, and the finishing symbols “▽”, “▽▽” and “▽▽▽” in the figures are JIS B 0601 (1982) is a “triangle symbol” indicating the surface roughness in Table 1.

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Among the test pieces prepared as described above, the heat pattern shown in FIG. 4 is used for the Ono-type rotary bending fatigue test piece with a rough shape of steel 1 to 12 and the coarse roller pitching small roller test piece. “Gas soft nitriding” was performed. Note that “120 ° C. oil cooling” indicates that the oil was cooled in oil at an oil temperature of 120 ° C.

  On the other hand, the ono-type rotating bending fatigue test piece with a rough notch and a coarse roller pitching small roller test piece of steel 13 were subjected to “carburization quenching and tempering” by the heat pattern shown in FIG. Note that “Cp” in FIG. 5 represents a carbon potential. In addition, “120 ° C. oil quenching” indicates that it was put into an oil having an oil temperature of 120 ° C. and quenching, and “AC” represents air cooling.

  Furthermore, the carburizing quenching-tempering by the heat pattern shown in FIG. In FIG. 6, as in FIG. 5, “Cp” indicates carbon potential, and “50 ° C. oil quenching” indicates that quenching was performed by putting in oil with an oil temperature of 50 ° C. "Represents air cooling.

  Each of the above-mentioned "gas soft nitriding" or "carburization quenching-tempering" test pieces is finished, and the Ono rotary bending fatigue test piece with a notch shown in FIG. 7, the roller pitching small roller test piece shown in FIG. The roller pitching large roller test piece shown in FIG. 9 was produced. In FIG. 9, (a) is a front view when a roller pitching large roller test piece is halved by a center line, and (b) is a cross-sectional view at the center line.

  The units of the dimensions of the above-mentioned test pieces shown in FIGS. 7 to 9 are all “mm”, and the finish symbols “▽” and “▽▽▽” in the above figures are the same as those in FIGS. Similarly, each is a “triangular symbol” indicating the surface roughness in the explanatory table 1 of JIS B 0601 (1982).

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Furthermore, “˜” is a “waveform symbol”, which means that it is a dough, that is, it remains on the aforementioned “gas soft nitriding” or “carburizing and tempering” surface.

  Tests shown in the following << 1 >> to << 6 >> were performed using the test pieces prepared as described above.

<< 1 >> Vickers hardness test after normalization Five points in total, one point at the center of the Vickers hardness test piece after normalization and four R / 2 parts ("R" represents the radius of the steel bar), In accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003), the test force was measured with a Vickers hardness tester at 9.8 N, and the average value of 5 points was measured after Say it.

<< 2 >> Microstructure observation after normalization The microstructure observation sample after normalization was corroded with nital and the R / 2 part was observed with an optical microscope at a magnification of 400 times. As a result, the microstructure was either bainite, a two-phase mixed structure composed of ferrite and bainite, or a three-phase mixed structure composed of ferrite, pearlite, and bainite.

<3> Turning test Using a turning test piece,
・ Tool: Carbide tool (Material type: ST20E),
・ Peripheral speed: 200 m / min,
・ Feeding: 0.4mm / rev,
・ Incision: 1.5mm,
・ Lubricant: None (dry)
A turning test was conducted under the conditions of: In addition, the cutting resistance at the time of turning was measured, and when the cutting resistance was less than 1300 N, it was evaluated that the machinability was good. In addition, the chip when turning is also evaluated, and when the chip is divided into small pieces and there is no problem such as wrapping around the material under test, the chip is treated with good chip treatment. The case where the trouble of winding occurred was defined as “poor chip disposal”.

<< 4 >> Measurement of surface hardness, core hardness and effective hardened layer depth after "gas soft nitriding" and "carburizing quenching-tempering" Finish after "gas soft nitriding" or "carburizing quenching-tempering" Using a processed roller pitching small roller test piece before the test, cross the part with a diameter of 26 mm, bury it in the resin so that the cut surface becomes the test surface, and then polish so that the surface has a mirror finish The surface hardness, core hardness, and effective hardened layer depth were examined using a Vickers hardness tester.

  Specifically, in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003), HV hardness at any 10 points at a depth of 0.03 mm from the surface of the test piece. Was measured with a Vickers hardness tester with a test force of 0.98 N, and the value was arithmetically averaged to obtain “surface hardness”.

  In addition, using the same embedded sample, as in the above case, the HV hardness at any 10 points at a depth of 2 mm from the surface of the mirror-finished test piece was set to a test force of 1.96 N and Vickers hardness. It was measured with a thickness tester, and the value was arithmetically averaged to obtain “core hardness”.

  Further, using the same embedded sample, in the same manner as in the above case, in the direction from the surface of the mirror-finished test piece to the center, the test force is set to 1.96 N with a Vickers hardness tester, and the HV hardness is set at a predetermined interval. The thickness was measured and a distribution chart of HV hardness was created. The distance from the surface to the position where the HV hardness is 420 was defined as the “effective hardened layer depth”.

"5" using the Ono-type rotary bending fatigue test finishing the Ono-type rotary bending fatigue test pieces were conducted rotary bending fatigue test Ono-type by the test under the following conditions, the maximum intensity repetition is not broken at 10 ⁷ times The “rotary bending fatigue strength” was evaluated. Bending fatigue strength is excellent when it has a rotational bending fatigue strength equal to or higher than that of test No. 13 "carburized and tempered" using steel 13 corresponding to SCM420H defined in JIS G 4052 (2008). It was.

・ Temperature: Room temperature,
・ Atmosphere: In air
-Number of rotations: 3000 rpm.

<6> Roller pitching test Using the finished roller pitching small roller test piece and roller pitching large roller test piece, a roller pitching test is carried out under the following test conditions, and no pitching with a major axis of 1 mm or more occurs. Lifespan was measured. The above test was performed 3 times, and the average life of 3 times was defined as “pitching life”. The maximum number of repetitions evaluated was 10 ⁷ times. When the steel 13 corresponding to SCM420H defined in JIS G 4052 (2008) has a pitching life equal to or greater than that of the test number 13 "carburized and tempered", it has a long pitching life. .

・ Slip rate: 40%
・ Surface pressure: 2000 MPa,
-Number of rotations of small roller test piece: 1000 rpm,
Lubrication: A lubricating oil for automatic transmission with an oil temperature of 100 ° C. was sprayed at a rate of 2 liters / min to the contact portion between the roller pitching small roller test piece and the roller pitching large roller test piece.

However, the above-mentioned “slip rate” is a value calculated by the following formula, where “V1” is the tangential speed of the roller pitching small roller test piece surface and “V2” is the tangential speed of the roller pitching large roller test piece surface. Point to.
{(V2-V1) / V1} × 100.

  Table 2 shows the test results investigated using test specimens collected after “normalizing”, and Table 3 shows tests using test specimens that were finished after “gas soft nitriding” or “carburization quenching-tempering”. Each test result is shown together.

  “B”, “F”, and “P” in the “Microstructure” column of Table 2 mean bainite, ferrite, and pearlite, respectively. In addition, “◯” in the “Chip Disposability” column indicates that the chips were divided into small pieces and no defects such as wrapping around the material to be tested were generated, and “Chip Disposability was good”.

  From Table 2 and Table 3, in the case of test numbers 1 to 5 using steels 1 to 5 that satisfy the conditions specified in the present invention as materials, machinability before soft nitriding is good, and JIS G 4052 (2008) It has a rotating bending fatigue strength exceeding 470 MPa of test number 13 “carburized and tempered” using steel 13 corresponding to SCM420H specified in JIS No. 13 and a pitching life equal to or greater than test number 13, and soft nitriding It is clear that it has high bending fatigue strength later and is excellent in pitting resistance.

  On the other hand, in the case of test numbers 6 to 12 of comparative examples that deviate from the conditions specified in the present invention, the machinability is reduced, or the rotational bending fatigue strength and the pitching life are the same as those of the steel 13. It is inferior to the case of the test number 13 that was.

Specifically, in the case of test number 6, since steel 6 does not satisfy the formula (1), the core hardness is low, and the rotational bending fatigue strength and the pitching life are 450 MPa and 9.35 × 10 ⁵ times, respectively. Therefore, it is inferior to the case of test number 13 using steel 13.

  In the case of test number 7, the C content of steel 7 is greater than the value specified in the present invention, and the HV hardness after normalization is high. For this reason, cutting resistance is 1353N and is inferior to machinability.

In the case of test number 8, since the Mn content of steel 8 is less than the value specified in the present invention, the rotational bending fatigue strength and the pitching life are 450 MPa and 1.25 × 10 ⁶ times, respectively. It is inferior to the case of test number 13 used.

In the case of test number 9, since the Cr content of steel 9 is less than the value specified in the present invention, the rotational bending fatigue strength and the pitching life are 440 MPa and 1.15 × 10 ⁶ times, respectively. It is inferior to the case of test number 13 used.

In the case of test number 10, since the Ti and N contents of steel 10 are larger than the values specified in the present invention, the pitching life is 2.03 × 10 ⁶ times, and in the case of test number 13 using steel 13 Is inferior to

In the case of test number 11, since the contents of Mo and V in steel 11 are less than the values specified in the present invention and do not satisfy formula (1), the rotational bending fatigue strength and the pitching life are 420 MPa and 8.91, respectively. × 10 ⁵ times, which is inferior to the case of test number 13 using steel 13.

  In the case of test number 12, the N content of steel 12 is greater than the value specified in the present invention, and the HV hardness after normalization is high. For this reason, cutting resistance is 1373N and is inferior to machinability.

  The nitriding steel of the present invention is easy to cut before nitriding, and the nitriding component made from this nitriding steel has a Mo content of 0.5% by mass, which is an expensive element. In spite of the following, it has high bending fatigue strength and excellent pitting resistance. Therefore, the nitriding steel of the present invention is suitable for use as a material for nitriding parts such as automobile ring gears that require high bending fatigue strength and a long pitching life.

Claims (2)

In mass%, C: 0.05 to 0.09%, Si: 0.10 to 0.35%, Mn: 1.0 to 2.0%, S: 0.005 to 0.050%, Cr: 1.0 to 2.0%, Mo: 0.10 to 0.50%, Al: 0.010 to 0.10%, V: 0.05 to 0.40%, C, Mo and The content of V satisfies the following formula (1), the balance is composed of Fe and impurities, and P, N, Ti and O in the impurities are respectively P: 0.030% or less, N: 0.008% Hereinafter, Ti: 0.005% or less and O: 0.0030% or less.
{Mo / (2 × 95.94)} + (V / 50.9415) ≧ C / 12 (1)
However, the element symbol in the formula (1) represents the content in mass% of the element.   It has the chemical composition according to claim 1, has a surface hardness of 700 to 900 in terms of Vickers hardness, a core hardness of 230 or more in terms of Vickers hardness, and an effective hardened layer depth of 0.20 mm or more. A featured nitrided part.

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