JP4500708B2 - Non-tempered steel nitrocarburized parts - Google Patents

Description

  The present invention relates to a machine part obtained by nitrocarburizing non-heat treated steel. Specifically, the present invention relates to mechanical parts subjected to soft nitriding treatment such as crankshafts and connecting rods of automobiles, industrial machines and construction machines.

  Conventionally, mechanical parts such as crankshafts and connecting rods of automobiles, industrial machines and construction machines are subjected to tempering treatment (quenching, tempering, normalization) after hot working by a method such as hot forging. (Annealing). The tempering process results in homogenization and refinement of the tissue. After the tempering treatment, soft nitriding treatment is performed mainly for the purpose of increasing the fatigue strength.

  Strain is generated by the soft nitriding treatment. Since the distortion impairs the dimensional accuracy of the part, bending correction is often performed after the soft nitriding treatment. Accordingly, the parts after the soft nitriding treatment are required to have high bending strength as well as high bending strength.

  The above “excellent bend straightening” means that the surface of the part does not crack until a large amount of bending displacement is reached.

  In the manufacture of mechanical parts, it is desired to omit the tempering process in order to reduce the manufacturing cost and save energy, and in recent years, the demand has been particularly strong.

  However, if the tempering treatment is omitted, the inhomogeneous structure generated during hot working tends to remain, and the crystal grains grown and coarsened during heating of the raw material before the start of hot working remain in the product. And the mechanical properties of the product deteriorate. Therefore, this problem is usually solved by performing a normalizing process after hot working. When the normalizing treatment is not performed after the hot working, the crystal grains remain coarsened or become an inhomogeneous structure in which the hot deformed structure partially remains. Therefore, in a material in which the normalizing process is omitted, a desired fatigue strength cannot be obtained even if a soft nitriding process is performed.

  In addition, as described above, the parts after soft nitriding are required to have excellent bend straightening properties. However, when the tempering treatment is omitted, the above-described coarse crystal grain structure and / or non-refining properties are not required. Due to the uniform structure, the bend straightening of the parts after nitrocarburizing is often extremely inferior.

  Therefore, even when the tempering treatment is omitted for the purpose of cost reduction and energy saving, it is desired to develop a mechanical component having high fatigue strength and excellent bend straightening after nitrocarburizing treatment.

  Hereinafter, “normalization” will be described as a representative example in the tempering process. Regarding the method of obtaining non-tempered steel for soft nitriding that can be a part with high fatigue strength and excellent “bend straightening” after nitriding even when normalizing treatment is omitted, I have a suggestion. They are roughly divided into the following two.

  (1) A method of avoiding the coarsening of the structure by hot forging as much as possible while keeping the microstructure of the steel in ferrite and pearlite like the tempered steel (see, for example, Patent Documents 1 to 4).

  (2) A method in which the microstructure of steel is made bainite (see, for example, Patent Documents 5 to 9).

  Patent Document 1 states that “alloy element content is mass%, C: 0.15 to 0.40%, Si ≦ 0.50%, Mn: 0.20 to 1.50%, Cr: 0.00. It consists of 05-0.50%, the balance Fe and inevitable impurities, and the structure after hot working is substantially a ferrite-pearlite structure, the ferrite area ratio is 30% or more, and the ferrite grain size number is 5 or more And a nitrided steel characterized in that the average dimension of pearlite is 50 μm or less ”. This steel is described as being excellent in fatigue strength and bend straightening after nitriding even if the normalizing treatment is omitted.

  Patent Document 2 states that “it is a nitriding part formed by nitriding steel, and the steel is mass% as an alloy component, C: 0.15 to 0.40%, Si: 0.50% or less. , Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, the balance is Fe and inevitable impurities, and the steel remains hot-worked with ferrite and pearlite. The average size of the ferrite crystal grains is 50 μm or less, the average size of the pearlite crystal grains is 50 μm or less, and the average hardening depth by the nitriding treatment is 0.3 mm or more. There is also disclosed a nitriding component characterized in that the variation of the curing depth is within 0.1 mm. And it is described that even if this part is nitrided by omitting the normalizing process after hot forging, it is excellent in fatigue strength and bending straightness.

  Patent Document 3 states that “in weight%, C: 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.3 to 1.0%, P: 0.05%. Hereinafter, S: 0.005 to 0.10%, Cr: 0.3% or less, Al: 0.08% or less, Ti: 0.03% or less, N: 0.008 to 0.020%, Ca: 0.005% or less, Pb: 0.30% or less, Cu: 0.30% or less, Ni: 0.30% or less, Mo: 0.30% or less, V: 0.20% or less, Nb: 0.00. 05% or less and 221C (%) + 99.5 Mn (%) + 52.5 Cr (%) − 304 Ti (%) + 577 N (%) + 25 ≧ 150, with the balance being the chemical composition of Fe and inevitable impurities, Soft nitriding treatment characterized in that the structure is composed of ferrite and pearlite and the ferrite fraction is 10% or more. Use steel "and the like have been disclosed.

  In this Patent Document 3, if fatigue strength is expressed as a regression equation of contained elements, the factor is not less than a specific size, the structure is composed of ferrite and pearlite, and the ferrite fraction is not less than 10%. Further, it is described that a nitriding part excellent in fatigue strength and bend straightening can be obtained even if the normalizing treatment is omitted.

  Patent Document 4 states that “in weight%, C: 0.30 to 0.43%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.60%, P: 0.08%. Hereinafter, S: 0.10% or less, sol.Al: 0.010% or less, Ti: 0.013% or less, Ca: 0.0030% or less, Pb: 0.20% or less, and N: 0.010 Nitride steel characterized by containing 0.030%, the balance being Fe and impurities, Cr in impurities being 0.10% or less, and V being 0.01% or less " .

  This Patent Document 4 describes that even if the nitriding treatment is omitted without carrying out the normalizing treatment, a product excellent in fatigue strength and bending straightness can be obtained by smoothing the hardness gradient in the nitrided layer. ing.

  In Patent Document 5, “C: 0.1 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.6 to 1.50%, P: 0.01% or less, S: 0.015% or less, Cr: 1.1-2.0%, Mo: 0.5-1.0%, V: 0.03-0.13%, B: 0.0005-0.0030%, "High fatigue strength structural steel characterized by comprising Ti: 0.01 to 0.04%, Al: 0.01 to 0.04%, balance: Fe and inevitable impurities" and the like are disclosed.

  In Patent Document 5, Cr is effective in improving hardenability and nitriding hardenability, and V is effective in increasing the fatigue strength by refining the precipitated carbide. Here, since the nitriding hardenability by Cr is due to the precipitation of Cr nitride, the improvement in fatigue strength here is based on the precipitation strengthening by Cr and V. However, in Patent Document 5, the steel material once manufactured is heated and cooled again to form a bainite structure, and this steel is included in the category of tempered steel.

  Patent Document 6 states that “mass%, C: less than 0.1 to 0.3%, Si: 0.01 to 1.0%, Mn: 1.5 to 3.0%, Cr: 0.01 -0.5%, Mo: 0.1-1.0%, acid-soluble Al: 0.01-0.045%, N: 0.005-0.025%, the remainder Fe and inevitable impurities Non-tempered steel for soft nitriding characterized by comprising “and the like”.

  According to Patent Document 6, steel having a bainite structure obtained by air cooling from a hot working temperature is excellent in toughness and has excellent bend straightening after soft nitriding treatment. Here, the C concentration is less than 0.3% in order to prevent the bainite from becoming too hard and impairing the machinability, and Mn to ensure the hardenability of the steel for generating bainite. The concentration is specified as 1.5% or more. Moreover, 0.01 to 0.05% of Cr is added, and the hardness of the nitride layer is increased by precipitation strengthening with Cr nitride. That is, in Patent Document 6, the reason why the bend straightening is improved by the bainite structure is that bainite has higher toughness at the same hardness than the ferrite pearlite structure, as described above. The C concentration is less than 0.3% so that the hardness of bainite does not become too hard. However, if the C concentration is less than 0.3%, there is a concern about insufficient wear resistance. Wear resistance is also a very important factor for machine parts such as crankshafts and connecting rods.

  Patent Document 7 states that “in weight percent, C: 0.05 to 0.30%, Si: 1.20% or less, Mn: 0.60 to 1.30%, Cr: 0.70 to 1.50. %, Al: 0.10% or less, N: 0.006 to 0.020%, V: 0.05 to 0.20%, Mo: 0 to 1.00%, B: 0 to 0.0050%, S: 0 to 0.060%, Pb: 0 to 0.20%, Ca: 0 to 0.010%, and 0.60 ≦ C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V ≦ 1.35, or 0 .60 ≦ C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V + 0.55Mo + 8B ≦ 1.35, having a steel composition consisting of the balance Fe and inevitable impurities, cooled after hot rolling or hot forging and without heat treatment , The core hardness is Hv200-300, the structure is bainite or Fe Soft nitriding steel for site fraction is characterized in that a mixed microstructure of "ferrite + bainite" of less than 80% "is disclosed.

  The invention of Patent Document 7 also adopts the idea of improving fatigue strength by utilizing precipitation strengthening by Cr and V, as in Patent Document 5. However, as in Patent Document 6, since the C concentration is defined to be less than 0.3%, the concern regarding wear resistance cannot be wiped out.

  Patent Document 8 states that “in weight percent, C: 0.15 to 0.40%, Si: 1.20% or less, Mn: 0.60 to 1.80%, Cr: 0.20 to 2.00. %, Al: 0.02 to 0.10%, N: 0.006 to 0.020%, V: 0.05 to 0.20%, and the balance Fe and unavoidable impurities, and 0.60 ≦ C + 0.1Si + 0.2Mn + 0.25Cr + 1.65V ≦ 1.35 and steel having conditions of 0.25Cr + 2V ≦ 0.85, and after cooling by hot rolling or hot forging, without heat treatment The core has a hardness of Hv200 to 300 and the structure is a mixed structure of “ferrite + pearlite” or “ferrite + pearlite (+ bainite) with a bainite fraction of less than 20%”, and is subjected to soft nitriding treatment High surface hardness and deep curing depth It discloses a soft nitriding steel ", characterized by further comprising a low heat treatment distortion characteristics.

  Since the steel of Patent Document 8 has a C concentration of 0.15 to 0.40%, it is expected that the wear resistance is improved. However, the idea of improving the fatigue strength using precipitation strengthening by Cr and V as well as the invention of Patent Document 7 is adopted for this steel.

  In Patent Document 9, "C: 0.15 to 0.35%, Mn: 1.00 to 3.00%, Cr: 0 to 0.15%, V: 0 to 0.02%, Cu: 0 .50 to 1.50%, Ni: Cu content of 0.4 times or more, B, N and Ti content is Bsol = B- (11/14) {N- (14/48) Non-tempered nitriding forged parts characterized in that Bsol defined by Ti} is 0.0010 to 0.0030%, and the balance is made of Fe and inevitable impurity elements.

According to Patent Document 9, “the nitriding steel has a ferrite main structure, or if this is difficult, a martensite or bainite single phase structure is preferable to a ferrite + pearlite structure”. Here, precipitation strengthening by Cr and V is avoided, but the idea is to use precipitation strengthening by Cu instead. Further, in order to obtain a bainite single-phase structure, the Mn concentration must be 1.0% or more, and the bainite single-phase non-tempered steel is intended.
On the other hand, regarding contrivance of soft nitriding treatment conditions, conventionally, a soft nitriding treatment method (see Patent Document 10) that shortens the time for forming a compound layer, and a soft nitriding treatment method that improves the corrosion resistance of a compound layer (see Patent Document 11). ) And a soft nitriding method for improving resistance to dents (see Patent Document 12), but a soft nitriding method for improving fatigue strength and bend straightening has not been studied.

  For example, in Patent Document 12, after the soft nitriding treatment, the diffusion layer and the base material base are martensite by a quenching treatment that is heated to an austenite temperature range and rapidly cooled, and then a heat treatment is performed to re-temper the machined parts. A soft nitriding method for improving the wear resistance and resistance to dents is disclosed. However, the soft nitriding treatment itself is only performed by performing the gas soft nitriding treatment at a standard temperature (570 to 580 ° C.).

Japanese Patent Laid-Open No. 9-291339 JP-A-9-324258 Japanese Patent Laid-Open No. 9-324241 Japanese Patent Laid-Open No. 10-46287 JP-A-5-65592 JP 2000-309846 A JP-A-7-157842 JP-A-8-176733 JP 2000-160287 A JP 2003-253420 A JP 2002-302756 A JP-A-11-269631

  As described above, by utilizing a bainite structure, a method for obtaining a non-tempered steel for nitrocarburizing treatment that becomes a component excellent in fatigue strength and bend straightening after nitrocarburizing treatment is already known. However, increasing the fatigue strength by precipitation strengthening with an additive alloy element, on the other hand, decreases the bending straightness. That is, the problem of achieving both high fatigue strength and excellent bend straightening has not yet been solved.

  In addition, in order to meet the recent demand for higher strength of parts, there is a need for non-tempered steel for nitrocarburized parts that has higher fatigue strength than ever and is also excellent in bend straightening. ing. However, the conventional technique of “precipitation precipitation and bainite formation of the structure” described above cannot always meet such a request.

  In view of these circumstances, some of the inventors have previously found non-adjusted parts that have high fatigue strength and excellent bend straightening properties even when soft nitriding is performed without the tempering treatment. An invention aimed at providing high quality nitrocarburizing steel was completed and filed as PCT / JP2004 / 012372. The gist of the present invention is “mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0 0.005 to 0.1% and N: 0.015 to 0.030%, with the balance being Fe and impurities, and having a mixed structure consisting of bainite and ferrite or a mixed structure consisting of bainite, ferrite and pearlite The tempered steel for soft nitriding is characterized by having a bainite fraction in the mixed structure of 5 to 90%. In addition, this steel has Nb: 0.003-0.1%, Mo: 0.01-1.0%, Cu: 0.01-1.0%, Ni: 0.01-1.0%, One or more of B: 0.001 to 0.005%, S: 0.01 to 0.1%, and Ca: 0.0001 to 0.005% may be contained.

  The object of the present invention is based on the steel for non-tempered nitrocarburizing treatment, even when soft nitriding is performed in a state where the tempering treatment is omitted, further improved high fatigue strength and excellent bending It is to provide a machine part having a straightening property.

  The inventors have continued research after the application of PCT / JP2004 / 012372. As a result, we noticed that by improving the cooling rate during soft nitriding, the fatigue strength and bend straightening of the parts after soft nitriding could be further improved. It was.

  (A) Among the precipitates observed in the diffusion layer region after the soft nitriding treatment, iron nitride is generated exclusively during the cooling process of the soft nitriding treatment, and the generation behavior strongly depends on the cooling conditions.

(B) The main iron nitrides generated in the diffusion layer are rod-like γ′-Fe 4 N and disk-like α ″ -Fe 16 N 2.
(C) The presence or absence of these iron nitrides in the diffusion layer greatly affects the fatigue strength and bend straightening properties of the nitrocarburized machine parts. In particular, the precipitation of γ′-Fe 4 N, which is a rod-like γ ′ nitride, greatly reduces the fatigue strength.
(D) adjusting the presence form of nitrogen in the ferrite grains in the diffusion layer, that is, as much as possible as saturated solid solution nitrogen without precipitation of nitrogen introduced during soft nitriding as iron nitride, By strengthening, the parts after the soft nitriding treatment can have excellent fatigue strength and bend correction.

  The present invention has been completed based on the above findings.

  The gist of the present invention resides in the following non-tempered steel nitrocarburized parts.

  (1) By mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1% N: 0.010 to 0.030%, the balance being Fe and impurities, and having a mixed structure composed of bainite and ferrite or a mixed structure composed of bainite, ferrite and pearlite, and bainite in the mixed structure A nitrocarburized component obtained by nitrocarburizing a non-tempered steel having a fraction of 5 to 90%, and a longitudinal size of γ ′ nitride existing in ferrite grains of the diffusion layer A soft nitriding component having a thickness of 20 μm or less.

(2) By mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1% And N: 0.010 to 0.030%, containing one or both of one or more elements selected from the following first element group and one or more elements selected from the second element group And the balance is Fe and impurities, and has a mixed structure composed of bainite and ferrite or a mixed structure composed of bainite, ferrite and pearlite, and the bainite fraction in the mixed structure is 5 to 90%. A soft nitriding component obtained by soft nitriding a non-tempered steel, wherein the longitudinal size of γ ′ nitride existing in the ferrite grains of the diffusion layer is 20 μm or less Processing parts.
First element group:
Nb: 0.001 to 0.1%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%, and B: 0.001 to 0.005%
Second element group:
S: 0.01 to 0.1%, and Ca: 0.0001 to 0.005%
The “diffusion layer” here is defined by JIS G0562, and is a layer in which diffusion of nitrogen, carbon, etc. is recognized, excluding the compound layer in the surface layer of the soft-nitrided component. It is. Further, “γ ′ nitride” refers to the above-mentioned γ′-Fe 4 N.

  According to the present invention, a high-strength nitrocarburized part excellent in fatigue strength and bending straightness can be obtained using non-heat treated steel as a raw material. Therefore, the part manufacturing cost can be reduced.

  Hereinafter, each requirement of the present invention will be described. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition C: 0.30 to 0.45%
C is an essential element for obtaining a mixed structure of “bainite + ferrite” or “bainite + ferrite + pearlite”. A content of 0.30% or more is necessary for stabilizing austenite and ensuring wear resistance of the material. On the other hand, if it exceeds 0.45%, the hardenability is excessively increased and harmful martensite is easily generated. Therefore, the appropriate range of C content is 0.30 to 0.45%.

Si: 0.1 to 0.5%
Si is added as a deoxidizer in the steel making process, but it is also effective for strengthening the solid solution of ferrite, so a content of 0.1% or more is necessary. On the other hand, if the Si content exceeds 0.5%, the hot deformation resistance of steel is increased, and the toughness and machinability are deteriorated. Therefore, the appropriate range of Si content is 0.1 to 0.5%.

Mn: 0.6 to 1.0%
Mn is added in the steel making process as a deoxidizing agent similarly to Si. Further, it is an essential element for stabilizing austenite to obtain a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”. Furthermore, Mn combines with S in steel to form MnS, which is effective in improving machinability.

  In the above mixed structure, the bainite fraction must be 5% or more. And in order to ensure the hardenability which produces | generates this fraction of bainite, content of 0.6% or more of Mn is required. On the other hand, if the Mn content exceeds 1.0%, the hardenability is excessively increased and harmful martensite is easily generated. Therefore, the appropriate range of the Mn content is 0.6 to 1.0%.

Ti: 0.005 to 0.1%
Ti is an essential element for forming pinning particles for suppressing coarsening of crystal grains during hot working. Pinning particles include Ti nitrides, carbides, and carbonitrides, and a content of 0.005% or more is required to generate pinning particles having a sufficient distribution density. On the other hand, in order to prevent the consumption of N in the steel, which contributes to the increase in the strength of the base metal by making Fe nitride, it is necessary to suppress the Ti content to 0.1% or less. For the above reasons, the proper range of Ti content is 0.005 to 0.1%. More desirable is 0.01 to 0.05%.

N: 0.010 to 0.030%
N stabilizes austenite to obtain a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”, so as to constitute pinning particles for suppressing grain coarsening and as solid solution nitrogen It is added to contribute to solid solution strengthening and increase the strength of the base material. Here, considering the amount consumed as the pinning particles, the content of 0.010% or more is necessary. On the other hand, if N exceeds 0.030%, bubble defects may be generated in the ingot and the material may be damaged. Therefore, the appropriate range of N content is 0.010 to 0.030%. Desirable is 0.015 to 0.030%, and more desirable is 0.015 to 0.025%.

  One of the non-tempered steels for nitrocarburizing treatment used as a material for the nitrocarburized component of the present invention is steel composed of Fe and impurities in the balance in addition to the elements described above.

  Another non-tempered steel for nitrocarburizing treatment used as a material for the nitrocarburized component of the present invention includes one or more elements selected from the first element group, in addition to the elements described above, and This steel contains one or both of one or more elements selected from the second element group, and the balance is Fe and impurities.

  Elements belonging to the first group, that is, Nb, Mo, Cu, Ni and B have a common effect of increasing the strength of the steel of the present invention. The reasons for limiting the respective effects and contents are as follows.

Nb: 0.001 to 0.1%
Nb is an element that can be used to form pinning particles for suppressing crystal grain coarsening during hot working. In addition, during the cooling after the hot working is finished, it is precipitated as fine carbonitride, which is effective in increasing the strength of the base material. In order to obtain such an effect, a content of 0.001% or more is necessary. On the other hand, even if the content exceeds 0.1%, the effect is saturated, and a coarse undissolved carbonitride is formed at the time of steel making, which may deteriorate the quality of the steel slab. Therefore, when Nb is added, the content is preferably 0.001 to 0.1%. Desirable is 0.003 to 0.1%, more desirable is 0.005 to 0.1%, and most desirable is 0.005 to 0.05%.

Mo: 0.01 to 1.0%
Mo is an element that enhances the hardenability of steel, contributes to high strength, and is also effective in improving toughness. Further, when Mo is added, a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite” is easily obtained. In order to obtain such an effect, a content of 0.01% or more is necessary. On the other hand, if the Mo content exceeds 1.0%, the hardenability is excessively increased, so that the formation of martensite is promoted and the bend straightening and toughness after the soft nitriding treatment are deteriorated. Therefore, when adding Mo, the content is preferably 0.01 to 1.0%. A more desirable content is 0.05 to 0.6%.

Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%
When adding Cu, the increase in the bainite fraction by the solid solution strengthening and austenite stabilization is expected. Therefore, Cu contains 0.01% or more.

  Cu and Ni do not have the effect of precipitation strengthening due to carbonitride formation, but Cu can age precipitate in ferrite and contribute to precipitation strengthening. However, when the temperature of a general soft nitriding treatment (about 580 ° C.) and the treatment time (about several hours) are substituted for an aging treatment, the Cu content is set to 1 in order to cause sufficient Cu precipitation. 0.0% or more is necessary. However, in the soft nitriding part of the present invention, it is not necessary to expect the age hardening effect of Cu during the soft nitriding process. Further, since the melting point of Cu is as low as 1085 ° C., the time remaining as a liquid phase in the solidification process in the steel making process is long, and therefore segregates at the grain boundaries of the steel to induce hot cracking. In order to eliminate this harmful effect, the upper limit of the Cu content is set to 1.0% in the steel of the present invention. In addition, when adding a lot of Cu, it is desirable to add Ni in order to prevent the above-described adverse effects.

  Ni, like Cu, is an austenite stabilizing element and is effective in securing solid solution strengthening and a desirable bainite fraction. Therefore, it is preferable to contain Ni by 0.01% or more. On the other hand, even if the content exceeds 1.0%, the effect is saturated and the material cost only increases, so the upper limit was made 1.0%. In addition, when using together with Cu, in order to ensure the effect which prevents the said hot crack, it is desirable to contain Ni more than 1/2 of Cu content.

B: 0.001 to 0.005%
B increases the hardenability of the steel and promotes the formation of a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”. The effect is clearly manifested at a content of 0.001% or more. On the other hand, if the B content exceeds 0.005%, the toughness of the steel is impaired. Therefore, when adding B, the content is good to be 0.001 to 0.005%.

  The elements of the second group are S and Ca, and these improve the machinability of the non-tempered steel for nitrocarburizing treatment used as a material for the nitrocarburized component of the present invention. The reasons for limiting the respective contents are as follows.

S: 0.01 to 0.1%, Ca: 0.0001 to 0.005%
S and Ca are both elements that improve the machinability of the steel material. If added, the machinability is further improved. Therefore, either one or two of them are added as necessary. However, if added excessively, segregation defects are generated in the steel slab or hot workability is deteriorated, so the S content range is 0.01 to 0.1%, and the Ca content range. 0.0001 to 0.005% is appropriate. A desirable lower limit of Ca is 0.001%.

  Except for the elements described above, they are impurities in the non-refined steel for soft nitriding used as the material for the soft nitriding component of the present invention, so that they are not added intentionally. However, the allowable amount of impurities will be described below in order not to cause an unreasonable cost increase in the steel making process.

  P segregates at the grain boundary and promotes grain boundary embrittlement cracking, so 0.05% or less is preferable.

  Al is usually added as a deoxidizer during melting. Al remains in the steel as alumina particles or binds to N to form AlN. Alumina is an oxide-based inclusion with high hardness, and shortens the life of tools used for cutting. AlN precipitates in the vicinity of the surface during soft nitriding treatment or promotes the growth of the surface compound layer to remarkably increase the surface layer hardness and deteriorate the bending straightness. In addition, since AlN is dissolved at the hot working temperature, it cannot be expected to function as pinning particles, and is hardly useful for refinement of crystal grains. Therefore, it is better that the Al content is low. However, minimizing the lower limit of the Al content causes a restriction in the deoxidation process and leads to an increase in cost. Therefore, it is preferable that the lower limit of the Al content is 0.05% or less that does not hinder the bending straightening property of the steel of the present invention.

  Cr and V are not added to the steel of the present invention. These are impurities, and the smaller the content, the better. The reason for this is that, as already described, Cr and V precipitate nitrides and remarkably increase the hardness of the near-surface layer of the steel, thereby impairing the bending straightness. Considering that the effects of the present invention are not impaired and that the refining cost and the purity of the material in the slab manufacturing method other than the blast furnace-converter method are taken into account, Cr is up to 0.15%, and V is 0.02%. Up to is allowed as an impurity. Note that Cr is more preferably 0.1% or less.

(B) Structure of the material of the nitrocarburized part of the present invention The structure of the non-tempered steel for soft nitriding used as the material of the nitrocarburized part of the present invention is a mixed structure of bainite and ferrite, or bainite and ferrite. It is a mixed structure of pearlite. And the bainite fraction in these mixed structures is 5 to 90%.

  As described above, the use of the bainite transformation can avoid the formation of martensite and can obtain a finer structure than a coarse pearlite colony. This bainite structure is made of bamboo leaf-shaped ferrite (bainitic ferrite) as shown in FIG. 1 and carbide. This bainitic ferrite is dispersed inside the prior austenite grains and is smaller than the proeutectoid ferrite (polygonal ferrite) developed from the prior austenite grain boundaries. In other words, this bainite is “a structure in which a relatively fine ferrite (bainitic ferrite) is dispersed in a pearlite colony, and the shape is a bamboo leaf shape”, and the pearlite colony in which this bainitic ferrite is dispersed. Compared with the case of this complete pearlite colony not containing bainitic ferrite, the lamellar structure is disordered.

  FIG. 2 is an SEM image of prior austenite grains in which bainitic ferrite is dispersed. As is clear from this figure, the arrangement of the ferrite / cementite lamellar structure in the pearlite colony is not an orderly lamellar structure, but disorder is observed in various places. Although this structure has lower strength than that of a prior austenite grain that has undergone pearlite transformation as a whole, the crack progresses due to crack bending and plastic deformation at the crack tip in bainitic ferrite. In terms of resistance, it is superior to coarse pearlite colonies.

  That is, by mixing the bainite structure, the crack growth resistance can be kept high even if the crystal grain structure becomes somewhat coarse. For that purpose, it is necessary to contain 5% or more of bainite by area ratio. Here, the entire structure may be bainite, but in a structure where the bainite fraction exceeds 90%, it is practically inevitable that martensite is mixed. Martensite deteriorates the bending straightening property and also deteriorates the machinability, so that the mixture is not preferable. Therefore, the bainite fraction in the mixed structure is set to 5 to 90%. A more desirable bainite fraction is 10 to 80%. The structure other than bainite is substantially ferrite or ferrite and pearlite.

(C) Production method of nitrocarburized component material of the present invention The structure of non-tempered steel for nitrocarburized treatment used as the material of the nitrocarburized component of the present invention can be obtained by, for example, the following method .

  The material for hot forging may be either a billet obtained by performing ingot rolling on an ingot, a billet obtained by performing ingot rolling on a continuous cast material, or a bar steel obtained by hot rolling these, but a material having a specified chemical composition range. Prepare. The heating temperature of these hot forging materials is 1100 to 1250 ° C. Cooling after hot forging should be in the air or by forced air cooling using a fan. Also, for example, it may be cooled rapidly to the vicinity of the eutectoid transformation temperature, and the range of 700 to 500 ° C. may be slowly cooled, or after hot forging, immediately cooled to about 500 to 300 ° C. You may hold | maintain and may promote a bainite transformation. The cooling rate may be adjusted by preparing a continuous cooling transformation diagram (CCT curve diagram) in advance, obtaining a cooling rate range that passes through the bainite transformation region, and adjusting the obtained cooling rate range.

(D) Structure of the diffusion layer in the nitrocarburized part of the present invention In the diffusion layer of the nitrocarburized part of the present invention, the longitudinal size of the rod-like γ 'nitride existing in the ferrite grains is 20 µm or less. .

  As described above, when supersaturated nitrogen dissolved in the diffusion layer precipitates as γ ′ nitride, or when the precipitated γ ′ nitride grows greatly and solid solution nitrogen further decreases, the ferrite grains The strength is reduced, leading to a reduction in fatigue strength. Furthermore, since γ 'nitride has a rod-like form and grows so as to extend from the ferrite grain boundary into the ferrite grain, when γ' nitride grows long and large, it is in a state where it crosses the ferrite grain. Distributed by. In the vicinity of the γ 'nitride, the strength is also reduced because the concentration of dissolved nitrogen is remarkably reduced. Therefore, if the elongated γ' nitride is dispersed so as to cross the ferrite grain, it penetrates into the ferrite grain. The cracks that have been generated are more likely to propagate through the γ ′ nitride / ferrite interface, and the crack propagation resistance is reduced. In other words, the precipitation of γ ′ nitride reduces the average strength of the ferrite grains themselves, and the crack growth resistance locally decreases in the vicinity of the γ ′ nitride. It becomes easier. Therefore, precipitation of γ ′ nitride and its growth are suppressed.

  The reason why the length in the longitudinal direction of the rod-like γ ′ nitride is specified to be 20 μm or less is as follows. The grain size of the ferrite grain of the non-tempered steel used as the material of the soft nitriding part of the present invention is approximately 10 to 50 μm. Therefore, a rod-like γ 'nitride that grows from the ferrite grain boundary toward the inside of the grain is connected in the ferrite grain, and it is as if one coarse γ' nitride crosses the ferrite grain. In order to avoid this, it is necessary to suppress the length of the rod-like γ ′ nitride to ½ or less of the ferrite grain size. From this, the rod-like γ ′ nitride present in the ferrite grains of the diffusion layer The size in the longitudinal direction was defined as 20 μm or less. Desirably, it is 10 μm or less, and more desirably 5 μm or less.

(E) Means for Obtaining Diffusion Layer of Soft Nitrided Component of the Present Invention For the soft nitriding treatment, gas soft nitriding treatment, salt bath soft nitriding treatment (tuftride treatment), ion nitriding or the like can be used. In any method, a compound layer (nitride layer) having a thickness of about 20 μm and a diffusion layer immediately below the compound layer can be uniformly formed on the surface of the product. However, regardless of which soft nitriding treatment is used, it is necessary to suppress the precipitation and growth of γ ′ nitride and to reduce the size of γ ′ nitride in the longitudinal direction to 20 μm or less. For that purpose, it is necessary to devise in the cooling process from the soaking temperature of soft nitriding to room temperature. Hereinafter, the cooling process from the soaking temperature of soft nitriding will be described by taking gas soft nitriding as an example.

  In order to obtain mechanical parts by gas soft nitriding, for example, processing is performed for several tens of minutes to several hours at a soaking temperature of 550 to 620 ° C. in an atmosphere in which RX gas and ammonia gas are mixed 1: 1. If the soaking temperature is too low, the growth of the surface compound layer is slowed and the diffusion and penetration of nitrogen into the material steel is also slowed, so that a sufficient hardening action cannot be obtained. On the other hand, when the soaking temperature is too high, a dimensional change (distortion) of the component in the cooling process becomes a problem. Therefore, the soaking temperature is preferably 550 to 620 ° C, and the more desirable soaking temperature is in the range of 580 to 600 ° C. The holding time (treatment time) at the soaking temperature determines the thickness of the surface compound layer and the amount of nitrogen that diffuses and penetrates into the steel. From the viewpoint of obtaining a desired fatigue strength improvement and industrial production efficiency, the holding time is preferably 30 minutes to 3 hours, and more preferably 1 to 2 hours.

  In the soaking state of soft nitriding treatment, nitrogen that has diffused and penetrated only forms a compound layer on the surface of Fe, and precipitation of Fe nitride does not occur in the diffusion layer region, and nitrogen does not form Fe in the parent phase. It is in a solid solution state. Thereafter, in the cooling process after soaking, if the cooling rate is low, the nitrogen in the diffusion layer region cannot be completely dissolved in the parent phase, and γ ′ nitride precipitates and grows. On the other hand, when the cooling rate is increased, nitrogen in the diffusion layer region dissolves in the supersaturated state in the matrix phase, and precipitation and growth of γ ′ nitride is suppressed. However, even if the cooling rate is increased, if it is maintained at a temperature of 100 to 200 ° C. for a long time after cooling, a metastable phase α ″ nitride precipitates, or if it is maintained at that temperature for a long time. In some cases, the α ″ nitride may be transformed into γ ′ nitride. When such a nitride precipitates, the fatigue strength decreases. In particular, γ ′ nitride has a great effect of reducing fatigue strength.

  In the present invention, in order to suppress the precipitation of γ ′ nitride, it is effective to increase the cooling rate from the soaking temperature to 200 ° C. at which γ ′ nitride does not precipitate increases to 1.0 ° C./second or more. More desirably, the cooling rate should be 1.5 ° C./second or more. In addition, when using oil quenching, use an oil tank with a large heat capacity so that the oil temperature is 100 ° C. or less and sufficient heat removal from machine parts occurs, or the number of parts to be processed at one time It is also important to ensure that high fatigue strength is ensured that the parts are not kept in the temperature range of 100 to 200 ° C. for a long time (30 minutes or more) in the cooling process after the soft nitriding treatment by reducing the number of parts. .

  In general, in order to reduce quenching distortion, oil cooling (oil quenching) having a cooling rate lower than that of water cooling is used for cooling from a soaking temperature to room temperature in gas soft nitriding. In order to adjust the cooling conditions, various heat-treated oils having different properties or different oil temperatures are used. In addition, in the industrial gas soft nitriding treatment, for the reasons of operational efficiency and safety, the heating furnace filled with the treatment atmosphere is not directly quenched into the oil tank from the treatment atmosphere of RX gas and ammonia gas. In many cases, the treated product is once drawn into a space filled with another inert gas, and then oil-quenched there.

  In such a cooling process from the soaking temperature of the gas soft nitriding treatment, for example, when a treatment object is drawn from a treatment atmosphere of 580 ° C. to another space and subsequently oil-cooled in an oil tank maintained at 100 ° C., Precipitation of γ ′ nitride and α ″ nitride may be observed. This is due to the following reason.

  1) At the stage where the treated product is transported to another space before oil cooling, the temperature of the treated product decreases from 580 ° C. to about 400 ° C., and since the cooling rate at this time is small, γ ′ nitride is precipitated. Occur.

  2) While the treated product is immersed in an oil bath at 100 ° C. or higher for a long time, metastable phase α ″ nitride and further γ ′ nitride precipitate.

  Therefore, even when such a cooling process is taken, the cooling rate at the stage where the processed material is conveyed to another space before oil cooling is increased, and the processed material is prevented from being immersed in a hot oil bath for a long time. There must be.

  Hereinafter, the present invention will be described in detail by way of examples.

  After melting 180 kg of steel with chemical components shown in Table 1 in a vacuum melting furnace, the steel slab is heated to 1200 ° C. and hot forged so that the steel temperature does not fall below 1000 ° C. did. Cooling after hot forging was performed by cooling in the atmosphere. From this round bar, specimens for microstructural observation, stepped round bar bending specimens, and plane bending fatigue test specimens were collected.

  A part of the specimen for microstructural observation was cut, and the microstructure as hot forged was observed with an optical microscope having a magnification of 200 times, and the bainite fraction (area ratio) was measured. The area defined as bainite was calculated from the area ratio with respect to the total visual field area of the area where the area where the bamboo leaf-like bainitic ferrite exists was surrounded by a continuous closed curve. The bainite fraction of each steel type is shown in Table 1. No. In the steel No. 7, no bainite was observed in the structure of ferrite and pearlite. No. Steel No. 9 had a bainite fraction of over 90% and martensite was generated. The remainder of the specimen for microstructural observation is soft-nitrided together with the specimen for plane bending fatigue test, and the microstructure of the diffusion layer is observed with a scanning electron microscope (SEM). I investigated. The size of γ ′ nitride was the longest γ ′ nitride in the longitudinal direction observed when 10 fields of view were photographed at a magnification of 1000 times.

  The stepped round bar bending test piece has a shape with a 10 mm wide step with a diameter larger than both ends at the center. The diameter of the center is 15 mm, the diameter of the main body is 10 mm, and the step is The corner R has a curvature of 2 mm. After the stepped round bar bending test piece was soft-nitrided, a strain gauge was attached to the curvature portion, and a bending correction test was performed in the manner of three-point bending. The bending straightness was evaluated by the indentation stroke until the strain gauge was disconnected when a load was applied to the center. When the indentation stroke was up to 3 mm and the strain gauge did not break, it was determined that the bending straightness was good.

  The test piece for the plane bending fatigue test is obtained by processing a tapered neck portion (neck portion diameter is 20 mm) on a cylindrical body having a diameter of 44 mm. After subjecting this test piece to nitrocarburizing treatment, a plane bending fatigue test was conducted by fixing the head side of the test piece and applying a load repeatedly to the opposite end.

  For soft nitriding, gas soft nitriding in an atmosphere of RX gas: ammonia gas = 1: 1 was used. The soaking temperature was 600 ° C., and the holding time at the soaking temperature was 2 hours. The test piece that has been soaked is separated from the soaking chamber by a shutter and is transported to a separate chamber adjusted to a nitrogen atmosphere, and then inserted into an oil tank installed at the bottom of this separate chamber. Oil cooled. At this time, the degree of precipitation and growth of γ ′ nitride was changed by changing the time from the transfer to a separate chamber to the insertion into the oil tank. The oil temperature in the oil tank was set to a predetermined temperature in the range of 80 to 150 ° C., and the time for which the test piece was held in the oil tank after oil cooling was set to a predetermined time in the range of 10 to 90 minutes. The cooling rate in the cooling process from the soaking temperature was separately measured with a Pt-Rh thermocouple spot-welded to the surface of the test piece for the plane bending fatigue test in a state where the entire furnace was in a nitrogen atmosphere. This is because damage to the thermocouple becomes a problem in an atmosphere (RX gas: ammonia gas) in an actual gas soft nitriding treatment. When measuring, pay attention to the ultimate temperature of the test piece and the cooling rate until it reaches the same temperature when changing the time from when the test piece is transferred from the soaking chamber to being inserted into the oil bath. Went.

  Table 2 summarizes the size of γ 'nitride, fatigue strength, and bending straightness for each steel type. In the remarks column, the cooling rate in the cooling step from the soaking temperature of the gas soft nitriding treatment and the time during which the temperature was kept at 100 ° C. or higher including the holding after oil cooling are shown.

  As is apparent from Table 2, in the present invention, the standard fatigue strength classified into the category of high strength as a normalization omitted type: high fatigue strength equal to or higher than 550 MPa and good bending correction (Bending stroke of 3 mm or more) is obtained.

  On the other hand, due to the low cooling rate in the gas soft nitriding treatment, the fatigue strength was uniformly reduced when the size of the γ ′ nitride exceeded 20 μm. FIG. 3 shows a typical structure of the diffusion layer when γ ′ nitride is dispersed. Here, what is indicated by an arrow in the drawing is particularly coarse γ ′ nitride.

  Further, if the chemical composition and microstructure of the non-heat treated steel that is the material deviates from the present invention, even if the size of the γ ′ nitride is 20 μm or less, the fatigue strength becomes low, or the bending The result was poor correction.

  As described above, according to the present invention, a high-strength nitrocarburized steel part having excellent fatigue strength and bend straightening properties can be obtained using non-tempered steel as a raw material. Therefore, the part manufacturing cost can be reduced.

2 is a representative structural photograph of a “bainite + ferrite + pearlite” mixed structure of non-tempered steel for soft nitriding used as a material for the soft nitriding component of the present invention. It is a SEM image of former austenite grains in which batinic ferrite is dispersed. This is the structure of the diffusion layer when coarse γ ′ nitride (indicated by an arrow) is dispersed.

Claims (2)

  In mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1% and N: Containing 0.010-0.030%, the balance being Fe and impurities, having a mixed structure consisting of bainite and ferrite or a mixed structure consisting of bainite, ferrite and pearlite, and the bainite fraction in the mixed structure is A nitrocarburized component obtained by nitrocarburizing non-tempered steel, characterized in that it is 5 to 90%, and the longitudinal size of γ ′ nitride existing in the ferrite grains of the diffusion layer is 20 μm A soft nitriding component characterized by the following: In mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1% and N: Containing 0.010 to 0.030%, containing one or both of one or more elements selected from the following first element group and one or more elements selected from the second element group, and the balance Is composed of Fe and impurities and has a mixed structure composed of bainite and ferrite or a mixed structure composed of bainite, ferrite and pearlite, and the bainite fraction in the mixed structure is 5 to 90%. A nitrocarburized component obtained by nitrocarburizing a carbon steel, wherein the γ ′ nitride present in the ferrite grains of the diffusion layer has a longitudinal size of 20 μm or less.
First element group:
Nb: 0.001 to 0.1%,
Mo: 0.01 to 1.0%,
Cu: 0.01 to 1.0%,
Ni: 0.01 to 1.0%, and B: 0.001 to 0.005%
Second element group:
S: 0.01 to 0.1%, and Ca: 0.0001 to 0.005%

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