JP2008308740A - Steel member for soft nitriding - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel member for soft nitriding not only having excellent bending straightening properties but also capable of imparting high fatigue strength to a soft-nitrided component subjected to bending straightening treatment in a finish stage after soft nitriding treatment. <P>SOLUTION: The steel member for soft nitriding has a chemical composition comprising 0.35 to 0.45% C, 0.10 to 0.35% Si, 1.2 to 1.8% Mn, ≤0.12% S, 0.001 to 0.02% Ti and 0.008 to 0.025% N, and the balance Fe with impurities, and in which, in the impurities, ≤0.08% P, ≤0.01% Al, ≤0.20% Cr, <0.01% V and Ti/N<1.0 are satisfied, and its yield ratio before soft nitriding treatment is ≥0.6. One or more elements selected from the two groups of: (a) Mo≤0.3%, and (b) one or more selected from Pb≤0.3%, Ca≤0.01% and Bi≤0.3% can be incorporated therein. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a steel material for soft nitriding, and in particular, a soft material suitable for use as a material for a part used for soft nitriding, such as a crankshaft which is an engine part of an automobile or construction machine. It relates to nitriding steel. More specifically, the present invention relates to a steel material for soft nitriding that is suitable for use as a material for soft nitriding parts that require bending correction, and that allows the parts to have high fatigue strength after bending correction.

  Parts such as automobiles and construction machinery that require high fatigue strength and wear resistance, such as crankshafts, are molded into a predetermined shape by forging or machining, and then subjected to induction hardening or softening for surface hardening. It is often produced by nitriding.

  Among the above, the soft nitriding treatment is characterized by less heat treatment distortion than the induction hardening treatment.

  For this reason, in particular, parts such as a crankshaft are often subjected to soft nitriding, but even in the case of soft nitriding, no distortion can be eliminated.

  Therefore, the nitrocarburized parts are subjected to a bending correction process in the finishing process after the nitrocarburizing process, thereby removing the strain generated by the nitrocarburizing process in addition to the strain existing before the nitrocarburizing process. .

  However, when the above bending correction treatment is performed, it is difficult to avoid that the fatigue strength of the nitrocarburized part is significantly reduced. This is because the surface nitriding layer formed by the soft nitriding process is cracked by the bending correction process.

  Therefore, the industry has an excellent bend straightening property that the surface nitrided layer does not crack until a large amount of bending displacement is obtained when the bend straightening process is performed in the finishing process after the soft nitriding treatment. There has been a demand for a soft nitriding steel material that can provide a high fatigue strength to a part subjected to nitriding treatment.

  Therefore, in order to meet the above-described demand, for example, Patent Documents 1 to 4 propose various techniques relating to soft nitriding. Patent Document 5 proposes a technique related to nitriding.

  Specifically, Patent Document 1 includes “C: 0.5% or less, Cr: 0.5% or less, Al: 0.2% or less, and V: 0.3% or less. (B) Nb + Ta: 0.3% or less, (c) Si, Mn, Cu, Mo, W, Co: 1 each .5% or less, (d) S: 0.15% or less, Pb + Te: 0.4% or less, Se: 0.3% or less, 1 type or 2 types or more, 4 groups of elements 1 or 2 or more combinations Steel, the balance being Fe and inevitable impurities are processed to give the desired shape of the machine part, and heat treatment is performed to precipitate at least part of the contained V, followed by soft nitriding treatment A method for manufacturing a machine part "is disclosed.

  Patent Document 2 states that “in weight percent, C: 0.4 to 0.7%, Si: 1.0% or less, Mn: 0.6 to 2.0%, Cr: 0.2% or more and ( C content + 0.1)% or less, Al: 0.05% or less, and if necessary, (a) P: 0.015% or less, (b) S: 0.15% or less, Pb : 0.3% or less, Bi: 0.3% or less, Se: 0.1% or less, and Ca: at least one selected from 0.0005 to 0.010%, at least one of two element groups A low-alloy nitrocarburized steel containing an element selected from one element group, the balance being substantially made of Fe, and having high strength and high toughness after nitrocarburizing treatment is disclosed.

  Patent Document 3 states that “in weight%, C: 0.4 to 0.7%, Si: 1.0% or less, Mn: 0.8 to 2.0%, Cr: 0.2% or less, Al : 0.05% or less, Ti + V: 0.02% or more and (C content / 6)% or less, and if necessary, (a) P: 0.015% or less, (b) S: 2 of at least one selected from 0.15% or less, Pb: 0.3% or less, Bi: 0.3% or less, Se: 0.1% or less, and Ca: 0.0005 to 0.010% A low-alloy nitrocarburized steel containing an element selected from at least one element group of the element group, the balance being substantially made of Fe, and having high strength and high toughness after nitrocarburizing treatment is disclosed.

  In Patent Document 4, “C: 0.35 to 0.65 wt%, Si: 0.35% to 2.00 wt%, Mn: 0.80 to 2.50 wt%, Cr: 0.20 wt% % Or less and Al: 0.035% by weight or less, and if necessary, (a) Ni: 3.0% by weight or less, Cu: 1.0% by weight or less and Mo: 0.5% by weight or less 1 or more types selected from the group consisting of: (b) Pb: 0.03 to 0.35 wt%, Ca: 0.0010 to 0.0100 wt%, and S: 0.04 to 0.13 wt% Soft nitriding comprising at least one element selected from the group consisting of at least one element selected from the group consisting of one or more elements selected from the group (c) B: 0.0080% by weight or less, with the balance being Fe and inevitable impurities Steel "is disclosed.

  Patent Document 5 states that “in weight percent, the Fe content is 90% or more, C: 0.35 to 0.5%, Si: 0.01 to 0.3%, and Mn: 0.6-1.8%, Cu: 0.01-0.5%, Ni: 0.01-0.5%, Cr: 0.01-0.5%, Al: 0.001-0. 01%, N: 0.005 to 0.025% component elements are contained, and if necessary, Pb: 0.30% or less, S: 0.20% or less, Ca: 0.01% Hereinafter, Bi: 0.30% or less, Ti: 0.02% or less, Zr: 0.02% or less, Mg: 0.01% or less, and one or more of the component elements are contained. In addition, the carbon equivalent and the chromium equivalent are adjusted to a specific range, the Vickers hardness at a position of 50 μm from the surface of the member is 340 to 460 HV, The effective hardened layer depth from the surface of the member in which the Vickers hardness of the inner layer portion showing substantially constant hardness that does not resonate is 190 to 260 HV, and the Vickers hardness is 270 HV is adjusted to 0.3 mm or more. Machine parts ".

JP 59-67365 A JP-A 64-4457 JP-A 64-25949 JP-A-4-83849 JP 2004-162161 A

  In the case of the technique disclosed in Patent Document 1 described above, V is added as an essential constituent element. For this reason, it cannot necessarily be said that sufficient bending straightness can be secured.

  The technique disclosed in Patent Document 2 contains a large amount of C and Cr. For this reason, it cannot necessarily be said that sufficient bending straightness can be secured.

  The technique disclosed in Patent Document 3 contains a large amount of C. For this reason, it cannot necessarily be said that sufficient bending straightness can be secured.

  In the case of the technique disclosed in Patent Document 4, an excessive amount of Si of 0.35 to 2.00% by weight is included. For this reason, it cannot necessarily be said that sufficient bending straightness can be secured.

  In the case of the technique disclosed in Patent Document 5, Ti is not necessarily contained. Therefore, it cannot be said that it is sufficient in terms of suppressing coarsening of crystal grains, and is not sufficiently excellent in fatigue strength.

  Therefore, the object of the present invention is to have an excellent bend straightening property that when the bend straightening process is performed in the finishing process after the soft nitriding process, the surface nitrided layer does not crack until a large bending displacement amount is reached. It is another object of the present invention to provide a steel for nitrocarburizing that can provide a high fatigue strength to a nitrocarburized part that has undergone bending correction in the finishing process after nitrocarburizing.

  Conventionally, the characteristics of steel for soft nitriding are only two: fatigue strength with soft nitriding as it is, and bend straightening expressed by the amount of bending displacement or bending strain until the surface nitrided layer cracks. It was evaluated. This is because even if the fatigue strength after soft nitriding is high, if the surface nitriding layer is cracked by slight bending correction processing, the substantial fatigue strength of the parts subjected to bending correction is inferior. .

However, as a result of various studies to solve the above-described problems, the present inventor
(A) Even if the surface nitrided layer does not crack during bending correction, the fatigue strength may decrease after bending correction.
I noticed the fact.

Therefore, after further examination,
(B) There are two types of fatigue strength after bending correction: when the surface nitrided layer cracks generated during bending correction are determined as a factor and when residual stress during bending correction is determined as a factor. Specifically, the fatigue crack may progress from the compression correction side where cracks do not occur during bending correction, rather than from the tension correction side where cracks occur during bending correction.
I realized that.

  The fatigue crack may progress from the compression correction side because the residual stress after bending correction is the compression residual stress on the tension correction side, whereas the tensile residual stress on the compression correction side. It became clear that it was because.

  (C) Therefore, in order to ensure good fatigue strength after bending correction, it is not sufficient to ensure the fatigue strength of steel materials and to prevent cracking of the surface nitrided layer during bending correction. It is necessary to reduce the absolute value of the generated tensile residual stress.

  As a result of further studies, the following findings (d) were obtained.

  (D) In order to ensure good fatigue strength after bending correction, the chemical composition of the steel material is set to a specific range, and mechanical properties before nitrocarburizing treatment, in particular, the yield ratio (relative to tensile strength) before nitrocarburizing treatment. A value expressed by “yield stress / tensile strength” as a yield stress ratio (hereinafter also referred to as “YR”) may be controlled to be 0.6 or more.

  This invention is completed based on said knowledge, The summary exists in the steel material for soft nitriding shown to the following (1)-(3).

  (1) By mass%, C: 0.35 to 0.45%, Si: 0.10 to 0.35%, Mn: 1.2 to 1.8%, S: 0.12% or less, Ti: 0.001-0.02%, N: 0.008-0.025% is contained, the balance consists of Fe and impurities, P in impurities: 0.08% or less, Al: 0.01% or less, Cr: 0.20% or less, V: less than 0.01%, and a chemical composition satisfying Ti (%) / N (%) <1.0, and the yield ratio before soft nitriding is 0 A steel material for soft nitriding characterized by being 6 or more.

  (2) The steel material for soft nitriding as described in (1) above, which contains Mo: 0.3% or less in mass% instead of part of Fe.

  (3) Instead of a part of Fe, by mass%, Pb: not more than 0.3%, Ca: not more than 0.01% and Bi: not more than 0.3%, containing one or more The steel material for soft nitriding as described in (1) or (2) above.

  Hereinafter, the inventions related to the steel materials for soft nitriding (1) to (3) are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The steel material for nitrocarburizing of the present invention has only an excellent bending straightening property that, when the bending straightening treatment is performed in the finishing process after the soft nitriding treatment, the surface nitrided layer does not crack until reaching a large bending displacement amount. Instead, a high fatigue strength can be provided to the nitrocarburized part subjected to the bending correction process in the finishing process after the nitrocarburizing process. Therefore, it is suitable for use as a material for soft nitriding parts.

  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 C: 0.35 to 0.45%
C is an effective element for ensuring necessary fatigue strength, and in order to obtain high fatigue strength, it is necessary to make the content 0.35% or more. However, when the content of C is excessive, the fatigue strength of the steel material itself is decreased, and since YR is decreased and the tensile residual stress after bending is increased, the fatigue strength after bending is decreased, In particular, if it exceeds 0.45%, the YR is remarkably reduced, and the fatigue strength after bending correction is greatly reduced. Therefore, the content of C is set to 0.35 to 0.45%. In addition, the more desirable range of C content is 0.35 to 0.43%.

Si: 0.10 to 0.35%
Si is an element necessary for deoxidation at the time of melting, and in order to obtain such an effect, the content needs to be at least 0.10%. However, a large amount of Si causes a decrease in bending straightness, and particularly when the Si content exceeds 0.35%, the bending straightness is significantly reduced. Therefore, the content of Si is set to 0.10 to 0.35%. In addition, the more desirable range of Si content is 0.15-0.35%.

Mn: 1.2 to 1.8%
Mn is an element effective for deoxidation of steel, like Si. Mn also has an effect of improving hardenability and an effect of improving fatigue strength by increasing the amount of dissolved nitrogen during soft nitriding. In order to obtain the effect described above, the Mn content needs to be 1.2% or more. However, when the content of Mn exceeds 1.8%, the bending straightness deteriorates. Therefore, the Mn content is set to 1.2 to 1.8%. In addition, the more desirable range of Mn content is 1.3 to 1.7%.

S: 0.12% or less S has an effect of improving machinability. However, when the S content increases, fatigue strength and bend straightening properties decrease. In particular, when the S content exceeds 0.12%, the fatigue strength and bend straightening properties decrease significantly. Therefore, the content of S is set to 0.12% or less. In order to obtain the machinability improving effect, the S content is preferably 0.04% or more. For this reason, the more desirable range of S content is 0.04-0.12%, and if it is 0.04-0.07%, it is still more desirable.

Ti: 0.001 to 0.02%
Ti is an element that suppresses coarsening of crystal grains and refines the crystal grains. In order to obtain this effect, the Ti content needs to be 0.001% or more. On the other hand, when the Ti content is increased and exceeds 0.02%, the bending straightness deteriorates. Therefore, the content of Ti is set to 0.001 to 0.02%. A more desirable range of Ti content is 0.005 to 0.02%.

  The content of Ti within the above range must satisfy the condition of Ti (%) / N (%) <1.0 as will be described later.

N: 0.008 to 0.025%
N is an element effective for forming nitrides and refining crystal grains. However, if the content is less than 0.008%, the above effect cannot be sufficiently expected. On the other hand, even if N is contained in excess of 0.025%, the above effects are saturated and bending straightness is lowered. Therefore, the N content is set to 0.008 to 0.025%. In addition, the more desirable range of N content is 0.015-0.022%.

  The N content within the above range must satisfy the condition of Ti (%) / N (%) <1.0 as described below.

Ti (%) / N (%) <1.0
When the value of Ti (%) / N (%), which is the ratio of the content of Ti and N, is 1.0 or more, large nitrides or carbonitrides are formed, which becomes the starting point of fracture and fatigue strength. Leading to a decline. Therefore, Ti (%) / N (%) <1.0. In addition, the lower limit of the value of Ti (%) / N (%) is 0. 0 in the case of 0.01% which is the lower limit of the Ti content and 0.025% which is the upper limit of the N content. 04.

  In the steel for soft nitriding according to the present invention, the contents of P, Al, Cr and V in the impurities are respectively P: 0.08% or less, Al: 0.01% or less, Cr: 0.20% And V: limited to less than 0.01%.

  This will be described below.

P: 0.08% or less P is an impurity, which reduces fatigue strength and bending straightness. In particular, when the content exceeds 0.08%, the fatigue strength and the bending straightness are significantly lowered. Therefore, the content of P in the impurities is set to 0.08% or less. A more desirable range of the P content in the impurities is 0.03% or less.

Al: 0.01% or less Al is an impurity and reduces the bending straightness. In particular, when the content exceeds 0.01%, the bending straightness is significantly lowered. Therefore, the Al content in the impurities is set to 0.01% or less.

Cr: 0.20% or less Cr is an impurity, and if its content exceeds 0.20%, the bending straightness is remarkably lowered, and the fatigue strength after bending straightening is lowered. Therefore, in the present invention, the content of Cr in the impurities is set to 0.20% or less. In addition, the more desirable range of Cr content in impurities is 0.10% or less.

V: Less than 0.01% V is an impurity, and when the content is 0.01% or more, the bending straightness is lowered, YR is lowered, and the fatigue strength after bending straightening is lowered. Therefore, the content of V in the impurities is less than 0.01%.

  For the above reason, the steel material for soft nitriding according to the present invention (1) contains C, Si, Mn, S, Ti, N in the above-mentioned range, and the balance consists of Fe and impurities, P in the impurities, It was specified that Al, Cr, and V were in the above-described ranges and that Ti (%) / N (%) <1.0 was satisfied.

In the steel for soft nitriding according to the present invention, if necessary, instead of a part of Fe in the present invention (1),
First group: Mo: 0.3% or less,
Second group: Pb: 0.3% or less, Ca: 0.01% or less and Bi: 0.3% or less
One or more elements of at least one group of the above may be contained.

  That is, in order to obtain even more excellent characteristics, one or more elements of at least one element of the first group and the second group are part of Fe in the steel for soft nitriding of the present invention (1). Instead, it may be contained.

  Hereinafter, the above elements will be described.

First group: Mo: 0.3% or less Mo is an element that contributes to improvement of fatigue strength and toughness by increasing hardenability, and may be added and contained as necessary. However, even if Mo exceeding 0.3% is contained, the above effect is saturated, so the cost is increased and the economic efficiency is impaired. Therefore, the content of Mo when added is set to 0.3% or less.

  In order to reliably obtain the effect of improving the hardenability of Mo described above, the Mo content is preferably 0.05% or more. For this reason, the more desirable Mo content in the case of adding is 0.05 to 0.3%. In addition, when adding, the more desirable content of Mo is 0.05 to 0.15%.

Second group: Pb: 0.3% or less, Ca: 0.01% or less, and Bi: 0.3% or less One or more of Pb, Ca, and Bi all have machinability. Has the effect of improving. For this reason, when it is desired to obtain better machinability, it may be contained in the following range.

Pb: 0.3% or less Pb has an effect of improving machinability, and may be contained for further machinability improvement. However, if the Pb content exceeds 0.3%, inclusions increase and the fatigue strength is significantly reduced. Therefore, the content of Pb when added is set to 0.3% or less.

  In order to reliably obtain the effect of improving the machinability of Pb described above, the Pb content is preferably 0.05% or more. For this reason, the more desirable Pb content in the case of adding is 0.05 to 0.3%. In addition, the more desirable content of Pb in the case of adding is 0.1 to 0.3%.

Ca: 0.01% or less Ca has an effect of improving machinability, and therefore may be contained for further machinability improvement. However, if the Ca content exceeds 0.01%, the inclusion of large inclusions is unavoidable and causes a reduction in fatigue strength. Therefore, when Ca is added, the content of Ca is set to 0.01% or less.

  In order to reliably obtain the above-described Ca machinability improving effect, the Ca content is preferably set to 0.0003% or more. For this reason, the more desirable Ca content in the case of adding is 0.0003 to 0.01%. In addition, when adding, the more desirable content of Ca is 0.0005 to 0.01%.

Bi: 0.3% or less Bi also has an action of improving the machinability, so it may be contained for further improving machinability. However, if the Bi content exceeds 0.3%, fatigue characteristics and bend straightening properties are significantly reduced. Therefore, the Bi content when added is set to 0.3% or less.

  In order to reliably obtain the effect of improving the machinability described above, it is preferable that the Bi content is 0.05% or more. For this reason, the more desirable Bi content in the case of adding is 0.05 to 0.3%. When Bi is added, the more desirable Bi content is 0.1 to 0.3%.

  Said Pb, Ca, and Bi can contain only 1 type, or 2 or more types of composites.

  For the reasons described above, the chemical composition of the steel for soft nitriding according to the present invention (2) is replaced with a part of Fe of the steel for soft nitriding according to the present invention (1), and Mo: 0.3% or less is contained. It was stipulated that

  Further, the chemical composition of the soft nitriding steel material according to the present invention (3) is replaced with a part of Fe in the soft nitriding steel material of the present invention (1) or the present invention (2), and Pb: 0.3% or less , Ca: 0.01% or less and Bi: 0.3% or less are defined as containing one or more.

(B) Yield ratio before nitrocarburizing treatment Even if the chemical composition of the steel material for nitrocarburizing is as described in the above section (A), if YR before nitrocarburizing treatment is less than 0.6, after bending correction The fatigue strength of the steel is greatly reduced. Therefore, the YR before soft nitriding is set to 0.6 or more. In order to secure a YR exceeding 0.9, a large amount of alloy elements must be contained, and the surface hardness becomes too high, so that the surface nitrided layer tends to crack during bending correction. Therefore, it is preferable that YR before soft nitriding is 0.9 or less.

  In addition, when the chemical composition satisfies the condition of the item (A), in order to set YR before soft nitriding to 0.6 or more, for example, normalization is performed at 820 to 870 ° C., and then 0.5 What is necessary is just to cool to room temperature with the cooling rate of -1.0 degreeC / sec. By carrying out the treatment as described above, fine crystal grains can be obtained, so that a large YR of 0.6 or more can be provided before the soft nitriding treatment.

Note that soft nitriding treatment conditions for obtaining high surface hardness by invading and diffusing N and C in a temperature range of about 500 to 600 ° C. below Ac ₁ point are not particularly required. Bath soft nitriding, plasma soft nitriding, or the like may be used as appropriate. For example, in the case of gas soft nitriding, “NH ₃ ”, which is a mixing ratio of NH ₃ gas (ammonia gas) and RX gas (“RX gas” is a trademark of endothermic metamorphic gas), as is normally performed. / RX "is 1, that is, treatment is performed at 600 ° C. for about 2 hours in an atmosphere of NH ₃ gas: RX gas = 1: 1, and then cooled in oil at 100 ° C.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  A slab obtained by subjecting steels 1 to 16 having the chemical composition shown in Table 1 to 70-ton converter melting-continuous casting was subjected to block rolling to obtain a square member having a side of 180 mm.

  A round bar having a diameter of 50 mm was manufactured by hot forging with heating at 1200 ° C. and finishing at 1000 ° C. using a square material having a side of 180 mm as a raw material.

  Next, a round bar having a diameter of 50 mm was normalized for 2 hours at the temperature shown in Table 2. After normalization, it was cooled to room temperature at the cooling rate shown in Table 2.

  No. 4 tensile test piece described in JIS Z 2201 (1998) from R / 2 part (where “R” represents the radius of the round bar) of each round bar having a diameter of 50 mm obtained as described above. (However, the length of the parallel part: 30 mm, the diameter: 7 mm, the shoulder R: 15 mm) was sampled, and the tensile test was performed at room temperature with a gauge distance of 25 mm. The yield stress (YS) and the tensile strength (TS ) Was measured.

Further, a plane bending fatigue test piece having the shape shown in FIG. 1 was taken from the central portion of each round bar having a diameter of 50 mm, and 2 at 600 ° C. in an atmosphere of NH ₃ gas: RX gas = 1: 1. Treated for hours and then cooled in oil at 100 ° C. In addition, the unit of the dimension in the plane bending fatigue test piece shown in the said FIG. 1 is all "mm".

  Next, for some of the plane bending fatigue test pieces subjected to the soft nitriding treatment, a strain gauge of 0.3 mm was adhered to the notch bottom, and strain was applied until the strain gauge was disconnected, and the strain gauge was disconnected. The bend straightness was evaluated based on the amount of strain at the time, that is, the amount of bend straightening.

  In addition, the remaining plane bending fatigue test piece subjected to the soft nitriding treatment has a 0.3 mm strain gauge bonded to the notch bottom, and the output of the strain gauge is 15000 με (corresponding to 1.5% in bending correction strain). Bending correction strain was applied until.

  Using each of the above-mentioned test piece with 15,000 με bending correction strain and the test piece without soft nitriding treatment with no bending correction strain at room temperature and in air atmosphere, the condition of both swings at a frequency of 20 Hz A plane bending fatigue test was conducted to investigate the fatigue strength (σw) before and after bending correction.

  The target values for each test are as follows: bending correctable strain amount is 20000 με or more, fatigue strength after bending correction is 500 MPa or more, “before and after bending correction represented by“ fatigue strength after bending correction / fatigue strength before bending correction ”. The fatigue strength change rate was set to 0.70 or more.

  Table 3 summarizes the above test results.

  From Table 3, the test numbers 1 to 11 relating to “examples of the present invention” satisfying the conditions specified in the present invention have a bend-correctable strain amount of 20000 με or more, a fatigue strength after bending correction of 500 MPa or more, and “after bending correction”. The rate of change in fatigue strength before and after bending correction represented by "Fatigue Strength of Bending / Fatigue Strength Before Bending Correction" has been achieved. It has high fatigue strength in addition to excellent bending straightening properties. It is clear.

  On the other hand, test numbers 12 to 14 use steel 5 whose chemical composition satisfies the conditions defined in the present invention, but the YR before soft nitriding is 0.59, which is low from the provisions of the present invention. It is off. For this reason, in any of the test numbers, the fatigue strength after bending correction and the fatigue strength change rate before and after bending correction have not reached the targets.

  Test Nos. 15 to 17 use steel 6 whose chemical composition satisfies the conditions defined in the present invention, but YR before soft nitriding is 0.58, which is not within the scope of the present invention. For this reason, in any of the test numbers, the fatigue strength after bending correction and the fatigue strength change rate before and after bending correction have not reached the targets.

  Test Nos. 18 to 20 use steel 7 whose chemical composition satisfies the conditions defined in the present invention, but YR before the soft nitriding treatment is low at 0.59 and is not within the scope of the present invention. For this reason, in any of the test numbers, the fatigue strength after bending correction and the fatigue strength change rate before and after bending correction have not reached the targets.

  In test number 21, the value of Ti (%) / N (%) of steel 12 is as high as 1.16, which is outside the conditions specified in the present invention. For this reason, coarse nitrides are generated, the fatigue strength after bending correction is low, and the target is not achieved.

  In test number 22, the Mn content of steel 12 is as low as 1.00%, which is out of the conditions defined in the present invention. For this reason, the amount of dissolved nitrogen at the time of soft nitriding is low, the fatigue strength after bending correction is low, and the target is not achieved.

  In test number 23, the Cr content of steel 13 is as high as 0.22%, which is out of the conditions defined in the present invention. For this reason, the bend straightening property is deteriorated, and the amount of strain that can be straightened and the fatigue strength after the straightening have not reached the target.

  In Test No. 24, the V content of Steel 15 is as high as 0.012%, and the YR before soft nitriding is as low as 0.54, which is outside the conditions specified in the present invention. For this reason, the bend straightening property is lowered, and the amount of strain that can be bend straightened, the fatigue strength after bending straightening, and the rate of change in fatigue strength before and after bending straightening have not reached the target.

  In test number 25, the C content of steel 16 is as high as 0.50%, and the YR before soft nitriding is as low as 0.56, which is outside the conditions defined in the present invention. For this reason, the bend straightening property is lowered, and the amount of strain that can be bend straightened, the fatigue strength after bending straightening, and the rate of change in fatigue strength before and after bending straightening have not reached the target.

  The steel material for soft nitriding of the present invention has an excellent bending straightening property that, when a bending straightening treatment is performed in the finishing process after the soft nitriding treatment, no cracks occur in the surface nitrided layer until a large amount of bending displacement is reached. In addition, high fatigue strength can be provided to the nitrocarburized parts that have been subjected to the bending correction process in the finishing process after the nitrocarburizing process. Therefore, it is suitable for use as a material for soft nitriding parts.

It is a figure which shows the shape of the plane bending fatigue test piece used in the Example.

Claims (3)

  In mass%, C: 0.35 to 0.45%, Si: 0.10 to 0.35%, Mn: 1.2 to 1.8%, S: 0.12% or less, Ti: 0.001 -0.02%, N: 0.008-0.025% is contained, the balance consists of Fe and impurities, P in impurities is 0.08% or less, Al: 0.01% or less, Cr: 0 .20% or less, V: less than 0.01% and a chemical composition satisfying Ti (%) / N (%) <1.0, and yield ratio before soft nitriding is 0.6 or more A steel material for soft nitriding characterized by   The steel material for soft nitriding according to claim 1, wherein the steel material contains Mo: 0.3% or less in mass% instead of part of Fe.   Instead of a part of Fe, it contains one or more of Pb: 0.3% or less, Ca: 0.01% or less, and Bi: 0.3% or less in mass%. The steel material for soft nitriding according to claim 1 or 2.

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