JP5402711B2 - Steel product having carbonitriding layer and method for producing the same - Google Patents

Description

  The present invention relates to a steel product having a carbonitriding layer and a method for producing the steel product. More specifically, the present invention relates to a steel product having a carbonitriding layer such as a power transmission component that is required to have excellent surface fatigue strength, in particular, high limit strength against pitching, and a method for manufacturing the same.

  Conventionally, power transmission parts such as gears used as transmissions for automobiles are formed by molding alloy steel for machine structural use specified in JIS G 4053 (2008) into a predetermined shape by processing such as forging and cutting. Thus, it is manufactured by carburizing and quenching or carbonitriding and quenching, followed by further tempering.

  In recent years, demands for improving the fuel efficiency of automobiles have become increasingly severe. Under these circumstances, in order to reduce the weight of the vehicle body, which directly leads to improved fuel efficiency, the above parts are also required to be further reduced in size and strength, and the limit strength against pitching, which is a type of surface pressure fatigue ( Hereinafter, the emphasis is on improving “pitting strength” and wear resistance.

  The alloy steels for machine structural use that contain about 0.2% by mass of carbon and are used as materials for carburized parts and carbonitrided parts include manganese steel typified by SMn420 and manganese chromium steel typified by SMnC420. There are chrome steel typified by SCr420 and chrome molybdenum steel typified by SCM420, which are properly used according to the use level and the strength level of the product (hereinafter also referred to as “member”).

  Among the above, SCM420 contains molybdenum in the range of 0.15 to 0.25%, and keeps the hardenability of the surface layer higher at the time of quenching after carburizing or carbonitriding than other steel types. Therefore, it is widely used. However, in recent years, the price of molybdenum has risen remarkably, and there is a great social need for steel and a processing method that can provide high strength even if the Mo content is minimized.

  “Carbonitriding” is a heat treatment in which nitrogen is contained in the surface layer of a member, and “gas carbonitriding” in which ammonia gas is mixed in a carburizing atmosphere and carburizing and nitriding is known.

  Nitrogen introduced by nitriding is said to have an effect of increasing the so-called “temper softening resistance”, and is expected to increase the pitching strength as compared with a gas carburized and quenched. However, since nitrogen has a strong affinity with Cr contained in the steel, it is easy to generate CrN in the nitrogenous layer, and the amount of solid solution Cr that increases the hardenability of the nitrogenous layer is reduced. For this reason, there existed a problem that it was easy to produce incomplete hardening structure in the surface layer of a member.

  Thus, techniques for solving the above problems in carbonitriding are disclosed in, for example, Patent Documents 1 to 3.

  In Patent Document 1, in mass%, C: 0.15 to 0.25%, Si: 0.40 to 0.9%, Mn: 0.05 to 0.7%, Cr: 1.25 to 2 0.5%, Mo: 0.35 to 1%, Al: 0.02 to 0.06%, and N: 0.007 to 0.015%, and if necessary, Cu, Ni, Nb, It consists of steel containing one or more elements of Ti, B, S, Ca, Zr, Sb, Pb and Bi, with the balance being substantially Fe, and the amount of [Si + Mn + Mo] is 1.0-2. 20%, having a hardened layer that has been subjected to carbonitriding or carbonitriding followed by quenching and tempering, the C amount [Cs] from the surface to 0.1 mm is 0.7% or more, and the N amount [Ns] is 0 0.6-2.0%, and [R value = 1.11 × [Cs] + 1.25 × [Ns] + 1.89 × Si + 1.22 × Mn + 0.67 × M O + 3.94] discloses an “carbonitriding component” characterized in that the R value obtained is 7.5 or more.

  In Patent Document 2, by mass%, C: 0.10 to 0.30%, Si: 1.5% or less (not including 0%), Mn: 1.5% or less (not including 0%) , Cr: 0.5 to 2.5%, Mo: 0.5% or less (not including 0%), respectively, and if necessary, Ni, B, Al, Nb, Ti, V, N , S, Ca, Mg, Pb and Bi containing one or more elements, the balance: a steel part made of iron and unavoidable impurities is formed into a predetermined shape and then carburized or carbonitrided. In addition, the depth of the surface abnormal layer is 5 μm or less, the C concentration [Cm] at the position of 50 μm depth from the tooth surface is 0.4 to 1.5%, and at the position of 25 μm depth from the tooth surface. The ratio [Cs] / [Cm] of the C concentration [Cs] and the C concentration [Cm] is less than 1.0. "Excellent gear part conformability" is disclosed to symptoms.

  Patent Document 3 contains, in mass%, C: 0.1 to 0.5%, Si: more than 0.35% to 1.5%, Mn: 2% or less, Cr: 3% or less, and is necessary In addition, from 6 types of Ni: 4% or less, Mo: 2% or less, V: 1% or less, Nb: 0.5% or less, Ti: 0.5% or less, Al: 0.1% or less Si-Mn charcoal containing carbon nitride with a mass ratio content of Si + Mn of 50% or more obtained by carbonitriding and quenching a steel part containing one or more selected types and the balance Fe and inevitable impurities A “high-hardness steel part excellent in pitting resistance and wear resistance” is disclosed, in which a nitride is deposited at a rate of 1% or more within 0.2 mm of the outermost surface of the carbonitrided layer.

JP 2001-73072 A JP 2008-121075 A JP 2002-339039 A

  In the carbonitriding component disclosed in the above-mentioned Patent Document 1, the decrease in the hardenability of the carbonitriding layer due to the reduction of the solid solution Cr in the nitrogenizing layer by CrN generated in the nitrogenating layer during carbonitriding is supplemented. Therefore, it is characterized by containing Mo by 0.35% or more. However, the technique proposed in Patent Document 1 does not necessarily meet the recent needs to reduce the Mo content as much as possible from the viewpoint that the price of rare metal elements in recent years, particularly Mo, has risen remarkably. .

  The gear part disclosed in Patent Document 2 is worn by optimizing the carbonitriding conditions using a vacuum carburizing furnace and lowering the carbon concentration at the 25 μm position from the tooth surface to the carbon concentration at the 50 μm position from the tooth surface. It promotes and, as a result, improves the compatibility of the sliding surfaces. However, no particular consideration is given to the conformability when quenching by gas carbonitriding.

  The high-hardness steel part disclosed in Patent Document 3 is characterized in that Si—Mn carbonitride having a mass ratio content of Si + Mn of 50% or more is precipitated in the carbonitriding layer. However, in the case of a steel type having a high Cr content and a high affinity with nitrogen, the amount of nitrogen in the nitriding process is increased compared to a steel type having a low Cr content as well as the generation of CrN. Therefore, even if quenching after carbonitriding, the amount of austenite remaining without transformation increases. For this reason, when the Cr content is large, not only a sufficient surface hardness cannot be obtained, but also the coarse CrN generated in the vicinity of the grain boundary tends to roughen the surface of the sliding surface and obtain a high pitching strength. There was a problem that was not possible.

  As described above, the carbonitriding techniques proposed so far have been insufficient to efficiently provide a carbonitriding member with low cost and excellent pitching strength.

  Therefore, in recent years, the present invention has reduced the content of Mo, which is an expensive alloy element whose price has risen remarkably, and has reduced wear resistance, and has excellent wear resistance and large pitching while being cheaper than conventional steel. It aims at providing the steel product which has a carbonitriding layer which can ensure intensity | strength. It is also an object of the present invention to provide a production method capable of efficiently obtaining a steel product having the carbonitriding layer.

  In order to solve the above-mentioned problems, the present inventors increased the Si content based on manganese chromium steel represented by SMnC420 defined in JIS G 4053 (2008), and changed the Mn amount. Carbonitriding experiments were conducted under various conditions using chrome-based case-hardened steel typified by steel and SCr420, and the relationship between the pitching strength of the carbonitrided member and the microstructure of the surface hardened layer was investigated.

  As a result, the following findings (a) to (f) were obtained with respect to a method of expressing excellent pitching strength by carbonitriding and quenching.

(A) In the carbonitrided layer, CrN is generated in a component system having a high Cr content, and SiMnN ₂ is generated in a component system having a high Si and Mn content. When this phenomenon is arranged by a parameter [(Si + Mn) / Cr] using the contents of Si, Mn and Cr among the steel components, only SiMnN ₂ is found in steel materials having [(Si + Mn) / Cr] of 2 or more. On the other hand, in a steel material having [(Si + Mn) / Cr] smaller than 2, CrN is generated. When these alloy nitrides are generated, the amount of alloy elements dissolved in the matrix is reduced in any case, so that an incompletely quenched structure is likely to be generated in the surface layer after quenching. As a result, the surface hardness becomes low and the surface is easily worn.

(B) A wear test was performed using a small test piece that was subjected to carbonitriding and quenching so that the hardness of the surface layer became the same level, and then the surface roughness of each sliding portion was measured. As a result, even when the surface hardness was an equivalent level, the surface roughness of the steel material in which MnSiN ₂ was dispersed in the surface layer was smaller than that of the SCr420-based steel material in which CrN was dispersed in the surface layer.

  (C) Depending on the type of alloy nitride produced in the carbonitriding layer, the degree of decrease in surface roughness after sliding varies, so the alloy nitride produced in the carbonitriding layer also affects the pitting resistance. Is expected to affect.

(D) From the above (c), carbonitriding and quenching was performed using steel materials in which the content of the alloy element was changed in order to change the type of alloy nitride generated in the carbonitrided layer, and the surface hardness was equivalent. The pitching strength was investigated according to the level. As a result, when only MnSiN ₂ was generated without generating CrN on the surface layer, that is, when [(Si + Mn) / Cr] was 2 or more, when CrN was generated, that is, [(Si + Mn) / Cr] was 2 A higher pitching strength was obtained compared to the smaller case.

(E) Even when the above [(Si + Mn) / Cr] was 2 or more and only MnSiN ₂ was produced, a higher pitching strength was obtained when the surface austenite content was 30% or more and 40% or less.

  (F) Furthermore, the hardenability of the carbonitriding layer is increased by containing a trace amount of Mo of 0.10% or less, or the quenching speed is increased by changing the cooling conditions even if Mo is not added. Even if the change in the amount of austenite is small, a remarkably high pitching strength was obtained.

Since MnSiN ₂ and CrN have the crystal structures and lattice constants shown in Table 1, each alloy nitride can be identified by photographing an electron diffraction pattern and analyzing it.

本発明は、上記の知見に基づいて完成されたものであり、その要旨は、下記（１）〜（３）に示す鋼製品および（４）に示す鋼製品の製造方法にある。

This invention is completed based on said knowledge, The summary exists in the manufacturing method of the steel products shown to following (1)-(3), and (4).

(1) Steel product having a carbonitriding layer, and the steel material of the material is mass%, C: 0.10 to 0.35%, Si: 0.40 to 1.00%, Mn: 0.60 -1.50%, Cr: 0.40-0.80%, Al: 0.01-0.05%, S: 0.05% or less and N: 0.0020-0.0300%, Fn represented by the following formula (1) is 2 or more, the remainder has a chemical composition composed of Fe and impurities, and the dispersed alloy nitride is in a region from the carbonitriding layer surface to a depth of 50 μm. A steel product comprising only MnSiN ₂ and having an austenite amount on the surface of the carbonitriding layer of 30% to 40% by volume.
fn = (Si + Mn) / Cr (1)
However, the element symbol in the formula (1) represents the content in mass% in the steel material of the material of the element.

  (2) The steel product as described in (1) above, wherein the steel material of the dough contains Mo: 0.10% or less in mass% instead of part of Fe.

  (3) The material of the steel material is one containing at least one of Ti: 0.10% or less and Nb: 0.080% or less in mass% instead of part of Fe. The steel product according to (1) or (2) above.

(4) The above-mentioned (1), wherein the following steps <1> to <4> are sequentially performed on a steel material having the chemical composition of the steel material of the fabric according to any one of (1) to (3) above. The manufacturing method of the steel product in any one of (3).
<1> Carburization is performed in a temperature range of 900 to 950 ° C.
<2> Carbonitriding is performed in a temperature range of 800 to 900 ° C. with a nitrogen potential of 0.2 to 0.6%.
<3> Perform quenching.
<4> Tempering in a temperature range of 150 ° C. to 250 ° C.

  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.

Since MnSiN ₂ has the crystal structure and lattice constant shown in Table 1, it can be identified by photographing an electron diffraction pattern and analyzing it.

  The steel product of the present invention has excellent wear resistance and high pitching strength. For this reason, in order to realize the weight reduction of the vehicle body directly linked to the improvement of fuel consumption, it can be used for power transmission parts such as gears for automobile transmissions that are required to be further reduced in size and strength. In addition, the steel product of the present invention can be produced by the method of the present invention, and is made of an inexpensive steel with a low content of Mo, which is an expensive alloy element, or Mo is not added. Further, the manufacturing cost can be reduced as compared with the conventional power transmission component.

Steel 3 used in Example and materials, is a diagram showing a photograph observed with a transmission electron microscope MnSiN ₂ at the position of depth 20μm from the surface of the sample that remains oil quenching after carbonitriding by extraction replica method . Incidentally, it spots black in the figure are MnSiN _2. It is a figure which illustrates typically an example of the "quenching" process after carbonitriding process, "carbonitriding" process, and carbonitriding in this invention. In this figure, the “quenching” step is illustrated as “oil quenching”. “Cp” and “Np” in the figure represent a carbon potential and a nitrogen potential, respectively. It is a figure which shows the shape of the small roller test piece used for the roller pitching test (two-cylinder rolling fatigue test) of an Example. The unit of dimension is “mm”. It is a figure which shows the shape of the block test piece used for the block on-ring abrasion test of an Example. The unit of dimension is “mm”. It is a figure which shows the shape of the test piece for chip collection used for the nitrogen concentration measurement of an Example. The unit of dimension is mm. It is a figure which illustrates typically the conditions of the "carburizing" process performed in the Example, the "carbonitriding" process, the "quenching" process after carbonitriding, and the "tempering" process after quenching. “Cp” and “Np” in the figure represent a carbon potential and a nitrogen potential, respectively. In the figure, the cooling in the atmosphere is expressed as “AC”. It is a figure which illustrates typically the wear part which generate | occur | produced in the contact surface of the method of the block on-ring abrasion test implemented in the Example, and a block test piece.

  In the present invention, the reason why the chemical composition, microstructure and manufacturing conditions of the steel material as defined above are defined as described above will be described in detail below. In addition, “%” of the content of each component element means “mass%”.

(A) Chemical composition of the steel material C: 0.10 to 0.35%
C is the most important element for determining the strength of the steel material, and in order to ensure the strength of the dough, that is, the strength of the core portion that is not hardened by quenching after carbonitriding, it is necessary to contain 0.10% or more. is there. On the other hand, when the content exceeds 0.35%, the toughness of the core part is lowered or the machinability is lowered. Therefore, the content of C is set to 0.10 to 0.35%. The C content is preferably 0.15% or more and 0.30% or less.

Si: 0.40 to 1.00%
Si is an element that has an effect of suppressing transformation of austenite to pearlite and contributes to an increase in core strength as a solid solution strengthening element. Si also has the effect of suppressing cementite precipitation and increasing temper softening resistance when tempering martensite. Further, Si has an action of combining Mn and invading nitrogen during carbonitriding to produce MnSiN ₂ and reducing the surface roughness during sliding to improve the pitching strength. These effects are obtained when the Si content is 0.40% or more. However, when the Si content increases, the carburization rate and ductility decrease. In particular, when the Si content exceeds 1.00%, the hot workability deteriorates and the carburization rate significantly decreases. . Therefore, the Si content is set to 0.40 to 1.00%. The Si content is desirably 0.50% or more and 0.90% or less.

Mn: 0.60 to 1.50%
Mn is an austenite stabilizing element that lowers the activity of C in austenite and promotes carburization. Mn also forms MnS together with S and has an effect of improving machinability. Mn combines with Si and invading nitrogen during carbonitriding to produce MnSiN ₂ , and has the effect of reducing the surface roughness during sliding and improving the pitching strength. In order to obtain these effects, a Mn content of 0.60% or more is necessary. However, even if Mn is contained in an amount exceeding 1.50%, the effect is saturated and the cost increases, and the size of MnSiN ₂ produced during carbonitriding becomes too large, and the pitching strength may even decrease. Therefore, the content of Mn is set to 0.60 to 1.50%. The Mn content is preferably 0.70% or more and 1.40% or less.

Cr: 0.40 to 0.80%
Cr has a large affinity with carbon and nitrogen, and has an effect of promoting carbonitriding by lowering the activities of C and N in austenite during carbonitriding. Cr also has the effect of increasing the hardenability of the core and increasing the hardness of the core after quenching. These effects are obtained when the Cr content is 0.40% or more. However, when the Cr content increases, coarse CrN is generated in the grain boundaries and grains, and Cr near the grain boundaries is deficient. As a result, an incompletely hardened structure is easily generated in the surface layer of the member, and the sliding is caused. The surface roughness inside increases and the pitching strength deteriorates. In particular, when the Cr content exceeds 0.8%, CrN is likely to be generated in the surface layer of the member, and the pitching strength and wear resistance are significantly reduced. Therefore, the Cr content is set to 0.40 to 0.80%. The Cr content is desirably 0.50% or more and 0.70% or less.

Al: 0.01 to 0.05%
Al has the effect of forming fine nitrides by bonding with N in the steel and making the crystal grains finer. However, the effect is poor when the Al content is less than 0.01%. On the other hand, if the content of Al becomes excessive, especially exceeding 0.05%, AlN becomes coarse and the above-mentioned crystal grain refining effect is lowered. Therefore, the Al content is set to 0.01 to 0.05%.

S: 0.05% or less S is an element contained as an impurity. Moreover, it is an element which forms MnS with Mn and improves machinability. In order to obtain this effect, the S content is desirably 0.01% or more. On the other hand, when the content of S becomes excessive, especially when it exceeds 0.05%, the hot ductility is lowered and cracking is likely to occur during forging. Therefore, the S content is 0.05% or less. The S content is preferably 0.03% or less.

N: 0.0020 to 0.0300%
N combines with Al, Nb, Ti, and C in steel to form fine nitrides and / or carbonitrides, and has the effect of refining crystal grains. In order to obtain this effect, the N content is set to 0.0020% or more. On the other hand, when the N content is excessive, not only the above effects are saturated, but also nitrides and / or carbonitrides become coarse, which may cause a decrease in hot forgeability. Therefore, the N content is set to 0.0020 to 0.0300%. The N content is preferably 0.0100% or more and 0.0250% or less.

fn: 2 or more Si, Mn, and Cr are elements that affect the alloy nitride formed in the carbonitrided layer, each content is in an appropriate range, and in the above formula (1) The expressed fn, that is, [(Si + Mn) / Cr] needs to be 2 or more.

When fn represented by the above formula (1) is 2 or more, the alloy nitride generated in the surface layer by carbonitriding is only SiMnN ₂ , whereas when fn is less than ₂ , in addition to SiMnN ₂ CrN is generated or, in some cases, only CrN is generated.

When the surface hardness is the same level, the surface roughness of the sliding portion is smaller and the “familiarity” is better when only MnSiN ₂ is dispersed in the surface layer than when CrN is dispersed in the surface layer. Therefore, it is possible to suppress an excessive increase in the surface pressure in a part of the sliding portion, and thus high pitching strength can be obtained. Therefore, fn needs to be 2 or more.

Note that fn is preferably 2.3 or more. fn may be 6.25 with Si and Mn contents of 1.00% and 1.50% and Cr content of 0.40%, respectively, but if it is too high, its effect will be In order to saturate, 4.0 or less is desirable.
One of the chemical compositions of the steel material of the steel product according to the present invention is that the balance is composed of Fe and impurities in addition to the above elements.

  Another chemical composition of the steel material of the steel product dough according to the present invention contains one or more elements selected from Mo, Ti and Nb instead of a part of Fe.

  Hereinafter, the effect of Mo, Ti, and Nb, which are optional elements, and the reason for limiting the content will be described.

Mo: 0.10% or less Mo has an effect of increasing the surface hardness and improving the pitching strength by suppressing the formation of an abnormal layer due to incompletely quenched structure and / or grain boundary oxidation in the surface layer of the member. . In order to obtain these effects, Mo may be contained. However, if the Mo content exceeds 0.10%, not only the material cost increases, but also the machinability deteriorates remarkably. Therefore, the amount of Mo in the case of inclusion is set to 0.10% or less. In addition, when Mo is included, the Mo amount is desirably in the range of 0.02% to 0.08%.

  Both Ti and Nb have the effect of refining crystal grains to increase bending fatigue strength and pitching strength. For this reason, when it is desired to obtain the above effect, these elements may be contained. Hereinafter, Ti and Nb will be described.

Ti: 0.10% or less Ti combines with C and N to form carbides, nitrides, carbonitrides, and refines crystal grains to improve bending fatigue strength and pitching strength. In order to obtain the above effect, Ti may be contained. However, if the Ti content increases and exceeds 0.10%, coarse Ti nitrides are likely to be generated, and the bending fatigue strength is lowered. Therefore, when Ti is included, the amount of Ti is set to 0.10% or less. In addition, when Ti is included, the amount of Ti is desirably 0.06% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the characteristics of Ti, the amount of Ti when contained is preferably 0.01% or more, and more preferably 0.02% or more.

Nb: 0.080% or less Nb combines with C and N to form carbides, nitrides, carbonitrides, and has the effect of refining crystal grains to improve bending fatigue strength and pitching strength. In order to obtain the effect, Nb may be contained. However, if the Nb content increases and exceeds 0.080%, coarse Nb carbonitrides are likely to be generated, and the bending fatigue strength is lowered. Therefore, the amount of Nb in the case of inclusion is set to 0.080% or less. When Nb is included, the Nb content is desirably 0.05% or less.

  On the other hand, in order to reliably obtain the above-described effect of improving the characteristics of Nb, the amount of Nb when contained is preferably 0.010% or more, and more preferably 0.020% or more.

  In addition, said Ti and Nb can be contained only in any 1 type of them, or 2 types of composites. The total content of these elements may be 0.180%, but is preferably 0.150% or less.

  In addition, about the impurity in the chemical composition of the steel material of the base material of the steel product according to the present invention, it is particularly desirable to limit the P content to 0.05% or less, and it is more desirable to limit it to 0.03% or less. .

(B) Microstructure In the steel product of the present invention, in the region from the carbonitriding layer surface to a depth of 50 μm, the alloy nitride to be dispersed is only MnSiN ₂ , and the austenite amount on the carbonitriding layer surface is 30 by volume. % Or more and 40% or less. The “alloy nitride” does not include “iron-containing nitride”.

  Hereinafter, the microstructure will be described in detail.

<Dispersion of MnSiN ₂ >
In the steel material subjected to carbonitriding, particles of MnSiN ₂ and / or CrN, which are alloy nitrides, are precipitated and dispersed in the surface layer portion that becomes a hardened layer by quenching after carbonitriding, that is, the carbonitriding layer. Even if these alloy nitrides are quenched after carbonitriding and are further tempered after the quenching, they do not change. For this reason, it is expected that the surface hardness of the carbonitrided member is increased to improve the wear resistance and the pitching strength.

  However, precipitation of the alloy nitride is accompanied by a decrease in the amount of the solid solution alloy element in the member surface layer, so that the hardenability of the surface layer portion may be lowered. Therefore, in order to use alloy nitride, it is necessary to select alloy nitride particles that can improve the intended strength.

In the case of the steel material having the chemical composition described in the section (A), as already described, MnSiN ₂ is generated on the surface layer by carbonitriding, and when the surface hardness is an equivalent level, only MnSiN ₂ is present on the surface layer. In the case of dispersion, the surface roughness of the sliding part becomes smaller and the “fitability” becomes better than in the case where CrN is dispersed on the surface layer, so that an excessive increase in surface pressure occurs in a part of the sliding part. Occurrence is suppressed, and thus high pitching strength is obtained.

Only MnSiN ₂ needs to be dispersed in the region from the surface of the carbonitriding layer to a depth of 50 μm in order to reduce the surface roughness of the sliding portion and improve the “fitability”. For this reason, in the steel product of the present invention, only MnSiN ₂ was dispersed in the region from the carbonitrided layer surface to a depth of 50 μm.

Even if only MnSiN ₂ is dispersed in a region exceeding the depth of 50 μm from the surface of the carbonitrided layer, the surface roughness of the sliding portion is reduced and the “familiarity” is improved. However, it takes a long time for nitriding to disperse MnSiN ₂ from the surface of the carbonitriding layer to a deep region, resulting in a decrease in productivity and an increase in energy cost.

The MnSiN ₂ dispersed in the region from the carbonitrided layer surface to a depth of 50 μm preferably has an equivalent circle diameter of several tens to several hundreds of nm, particularly 50 to 300 nm. The MnSiN ₂ can be confirmed by, for example, preparing a thin film sample with an extraction replica and observing it with a transmission electron microscope (hereinafter referred to as “TEM”) to confirm its size. Further, the MnSiN ₂ determined by taking an electron beam diffraction pattern from a region containing, by obtaining the crystal structure and lattice constant analyzes the diffraction pattern can be identified as the MnSiN _2.

1, as an example of MnSiN ₂ dispersed in the region to a depth of 50μm from the carbonitrided layer surface, showing a case of using the steel 3 shown in the examples below. FIG. 1 is a photograph obtained by TEM observation of a sample collected by the extraction replica method, and shows MnSiN ₂ existing at a depth of 20 μm from the surface of the sample as it is oil-quenched after carbonitriding. Incidentally, it spots black in the figure are MnSiN _2.

<Austenite>
In the steel material having the chemical composition described in the section (A), the alloy nitride to be dispersed is only MnSiN ₂ in the region from the surface of the carbonitriding layer to a depth of 50 μm, and the amount of austenite on the surface of the carbonitriding layer is small. When the volume ratio is 30% or more and 40% or less, higher pitching strength can be obtained.

  This reason can be interpreted as follows. That is, in the case of carbonitriding and quenching, carbon and nitrogen are dissolved in the surface layer and the Ms point is lowered, so that the amount of austenite tends to be larger than in the case of carburizing and quenching. This nitrogen-containing austenite is similar to a so-called ausfoam in which austenite undergoes “bainite transformation” while being “processed” by repeated increases in surface temperature and high surface pressure due to frictional heat. As a result, it is possible to obtain an effect of increasing the hardness while adapting the surface so as to suppress the piece contact caused by the contact with the mating surface.

  However, when the amount of austenite is small, so-called “familiarity” is considered to be caused by plastic deformation of the martensitic structure of the surface layer or plastic deformation of the incompletely quenched structure. Furthermore, since the surface temperature rises due to frictional heat in the surface layer, it is considered that the surface layer with advanced “familiarity” causes structural changes such as recovery of the martensite structure and tempering of the incompletely quenched structure. And it is thought that it becomes easy to become a starting point of a pitching damage since hardness falls partially in the site | part in which such a structure | tissue change arose. In addition, when the amount of austenite is large, there is austenite that cannot be sufficiently transformed by temperature rise due to frictional heat or repeated surface pressure. Therefore, it is considered that pitching damage occurs starting from the interface between austenite and the transformed structure. For this reason, it can be interpreted that there is an optimum value for the amount of austenite generated by carbonitriding and quenching.

  In the case of a steel material having the chemical composition described in the item (A), the microstructure described in the item (B) is obtained, for example, by performing a heat treatment under the conditions described in the following item (C). be able to.

(C) Manufacturing conditions The heat treatment as the manufacturing conditions of the present invention is:
<1> Carburization is performed in a temperature range of 900 to 950 ° C.
<2> Carbonitriding is performed in a temperature range of 800 to 900 ° C. with a nitrogen potential of 0.2 to 0.6%.
<3> Perform quenching.
<4> Tempering in a temperature range of 150 ° C. to 250 ° C.
The steps <1> to <4> are sequentially performed.

  That is, the heat treatment as a production condition of the present invention is a “carburizing” step of maintaining a carburizing atmosphere at 900 to 950 ° C., and subsequently to the carburizing, the temperature is lowered to 800 to 900 ° C. to maintain the carburizing atmosphere. For example, the “carbonitriding” process in which the atmosphere is mixed with ammonia gas and the like and has a nitriding property, and the nitrogen potential is maintained in an atmosphere of 0.2 to 0.6%, and the “quenching” process after carbonitriding And a “tempering” step in a temperature range of more than 150 ° C. and not more than 250 ° C.

  The carburizing ability and nitriding ability of the atmosphere are defined as carbon potential and nitrogen potential, respectively. That is, it is represented by the carbon concentration and the nitrogen concentration on the surface of the processing member when equilibrium is reached with the atmosphere at a specific atmospheric temperature. The carbon concentration profile and the nitrogen concentration profile in the depth direction from the surface of the processing member are determined by the carbon potential, the nitrogen potential, the processing temperature, and the processing time. However, in the present invention, the average concentration of nitrogen from the outermost surface of the processing member to the position of 50 μm when reaching an equilibrium with the atmosphere at a specific atmosphere temperature as in the examples described later is “nitrogen potential”. I will say. This is a chemical analysis of the cutting powder when the outer peripheral curved surface portion of a rod-shaped sample having a diameter of 26 mm and a length of 100 mm as a processing member is cut from the outermost surface toward the center along the radial direction by a depth of 50 μm. This is because this was defined as “surface nitrogen concentration”.

  FIG. 2 schematically shows an example of the “carburizing” step, “carbonitriding” step, and “quenching” step after carbonitriding in the present invention. In this figure, the “quenching” step is illustrated as “oil quenching”. “Cp” and “Np” in the figure represent a carbon potential and a nitrogen potential, respectively.

  Note that the carbon potential is not necessarily maintained in the state shown in FIG. 2, that is, it is not necessary to maintain a constant state in both the carburizing and carbonitriding processes. From the viewpoint of target surface carbon concentration, required carburization depth, and efficient operation, it may be appropriately changed.

  For example, by setting the carbon potential in the carburizing process higher than the target surface carbon concentration of the carbonitriding member and lowering the carbon potential to the target surface carbon concentration when moving to the next carbonitriding process, And the total processing time of carbonitriding can be shortened.

In the “carburizing” process, which is the process of <1>, for example, an endothermic gas (this gas is a mixed gas of CO, H ₂ and N ₂ , which is modified by mixing hydrocarbon gas such as butane and propane with air) “Gas carburizing”, in which carburizing is performed using an atmosphere to which a so-called “enriched gas” such as butane or propane is added, is usually applied. The processing temperature in this “carburizing” step, that is, the temperature maintained in the carburizing atmosphere is set to 900 to 950 ° C. This is because if the temperature is higher than 950 ° C., the crystal grains are likely to be coarsened and the strength after quenching is likely to be lowered. If the temperature is lower than 900 ° C., a sufficient hardened layer depth can be obtained. This is because it becomes difficult. The time for maintaining the temperature depends on the desired depth of the hardened layer, but may be, for example, about 2 to 15 hours. The above carbon potential can be controlled exclusively by the amount of enriched gas added.

  The “carbonitriding” step which is the step <2> following the “carburizing” step as the step <1> is a carbonitriding atmosphere at a temperature of 800 to 900 ° C. and a nitrogen potential of 0.2 to 0.6%. To do.

This is carbonitriding with a nitrogen potential of 0.2% or more at a temperature of 800 to 900 ° C., which is about 50 ° C. higher than the conventional general “carbonitriding” step, and the solubility of nitrogen in austenite is reduced. This is because only MnSiN ₂ having a preferred circle equivalent diameter of 50 to 300 nm can be precipitated and dispersed as the alloy nitride. In addition, by performing carbonitriding with a nitrogen potential of 0.2% or more, austenite is stabilized, and the above-mentioned amount of austenite is easily obtained. If nitrogen potential is less than 0.2%, with a preferred size of a circle equivalent diameter 50 to 300 nm, not only can not be precipitated and dispersed MnSiN ₂ is an alloy nitride, other than austenite and martensite non A fully quenched structure may occur. However, if the nitrogen potential is too high, especially exceeding 0.6%, the above MnSiN ₂ is likely to be coarsened, and the equivalent circle diameter exceeds 300 nm, resulting in destruction starting from MnSiN _2. It may become easy and pitching intensity may fall. For this reason, the nitrogen potential in the said temperature range shall be 0.6% or less.

  The above-mentioned “carbonitriding” step, which is the step of <2>, is performed by, for example, reducing the furnace temperature to 800 to 900 ° C., which is the temperature for carbonitriding, in the gas atmosphere of the carburizing step, May be added. The nitrogen potential at this time can be controlled by the amount of ammonia gas added. In addition, what is necessary is just to set the time hold | maintained in the said carbonitriding atmosphere to several hours, for example, 1-2 hours.

  The “quenching” step, which is the step <3> after carbonitriding, may be oil quenching as illustrated in FIG.

  In the carbonitriding process, nitrogen dissolves in the austenite, so the austenite is stabilized, and even when quenched by oil quenching, austenite that does not transform into martensite, that is, retained austenite is easily generated. Become. This retained austenite increases in hardness by transforming like an ausfoam while exhibiting a conforming effect in contact with the mating surface due to a rise in surface temperature due to frictional heat during sliding and / or repeated high surface pressure. Expected to be effective. As a result, it is considered that the wear resistance is improved and the pitching strength is greatly improved.

  For this reason, the tempering temperature in the step <4> after quenching in the step <3> is not the temperature exceeding 250 ° C. at which the austenite in which nitrogen is dissolved is isothermally bainite transformed, but a temperature of 150 to 250 ° C. Range.

The 50 to 300 nm MnSiN ₂ existing before tempering is not changed by this tempering. For this reason, the “familiarity” is improved, so that an excessive increase in the surface pressure is suppressed in a part of the sliding portion, and thus high pitching strength is obtained.

  For the above reasons, the steel product manufacturing method of the present invention sequentially performs the steps <1> to <4>.

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

  Steels 1 to 15 having the chemical compositions shown in Table 2 were melted in a 50 kg vacuum melting furnace to produce ingots.

  The above steels 1 to 6 are all steels whose chemical compositions are within the range defined by the present invention, while the steels 7 to 15 are steels deviating from the conditions defined by the present invention.

  Among the steels having the above chemical composition within the range defined in the present invention, Steel 1 is a steel in which the amount of Mn is reduced and the amount of Si is increased based on SMnC420 described in JIS G 4053 (2008). . Steel 2 is a steel containing Nb by increasing the amount of C and Si based on SMnC420, and Steel 3 is a steel containing Ti by increasing the amount of Si based on SMnC420. Steels 4 to 6 are steels based on SMnC420, in which the amount of C and Mn is reduced and the amount of Si is increased and Mo is contained, among which steel 5 further includes Nb, and steel 6 further includes It is a steel containing Nb and Ti.

  Among the steels whose chemical compositions deviate from the range defined in the present invention, Steel 7 is a steel in which the amount of Mn is decreased and the amount of Si and Cr is increased based on SMnC420. Steel 8 is steel in which the amount of Si is reduced with respect to steel 1, and steel 9 is steel in which the amounts of C, Si, and Mn are reduced with respect to steel 1. Steel 10 is steel in which the amount of Si and Mn is reduced with respect to steel 1. Steel 11 and steel 12 are steels in which the amount of Si and the amount of Mn are increased with respect to steel 1, respectively. Steel 13 is steel in which the amount of Cr is reduced with respect to steel 1. Further, steel 14 and steel 15 are steels corresponding to SCr420 and SCM420 described in the above JIS, respectively.

  In any of the steels 1 to 15, the content of Ni as an impurity was 0.02%, and the content of Cu was 0.02%.

このようにして得たインゴットを、１２５０℃に加熱した後、仕上げ温度が１０００℃となるように熱間鍛造して、直径３５ｍｍの丸棒とした。なお、熱間鍛造終了後は、大気中で放冷した。

The ingot thus obtained was heated to 1250 ° C. and then hot forged to a finishing temperature of 1000 ° C. to obtain a round bar having a diameter of 35 mm. In addition, after completion | finish of hot forging, it stood to cool in air | atmosphere.

  Next, the round bar having a diameter of 35 mm was heated to 925 ° C. and held for 120 minutes, and then subjected to a normalizing treatment in which it was allowed to cool in the atmosphere to obtain a mixed structure of ferrite and pearlite.

  For any steel, from the central part of each round bar with a diameter of 35 mm, parallel to the forging direction (forging axis), a small shape for a roller pitching test (two-cylinder rolling fatigue test) having the shape shown in FIG. A roller specimen was cut out.

  For the steel 1, steel 2 and steel 14, a block test piece having the shape shown in FIG. 4 was also cut out from the center of each of the normalized round bars having a diameter of 35 mm in parallel with the forging direction (forging axis). For the steel 15, a test piece for cutting chips having the shape shown in FIG. The units of dimensions in the test pieces shown in FIGS. 3 to 5 are all “mm”.

  The test piece for cutting chips collected from Steel 15 is cut out, and a small roller test piece for roller pitching test collected from Steels 1 to 14, and a block test taken from Steel 1, Steel 2 and Steel 14 are used. As shown in FIGS. 3 and 4, the pieces were subjected to carbonitriding quenching or carburizing quenching after grinding the surfaces in contact with the large roller test piece and the ring test piece, and then tempered.

  Carbonitriding and quenching includes three steps of carburizing, carbonitriding, and oil quenching under heat treatment conditions as schematically shown in FIG.

  The “oil quenching” was performed in 120 ° C. oil as shown in FIG. However, as described later, some of them were quenched in oil at 60 ° C. instead.

  In the carburizing step, the temperature was 930 ° C., the holding time was 180 minutes, and the carbon potential was constant at 0.6%.

  In the subsequent carbonitriding process, the temperature is set to 820 ° C., the holding time is set to 90 minutes, the carbon potential is set to 0.6% as in the carburizing process, and the flow rate of ammonia gas introduced into the furnace is changed to change the nitrogen potential. Changed to

  In the carbonitriding quenching, in addition to the small roller test piece and the block test piece for the roller pitching test, the chip collection sample taken from the steel 15 is treated in the same manner. Used for analysis.

  About the small roller test piece for the roller pitching test extract | collected from the steel 1, in order to evaluate the influence which the amount of austenite which exists after hardening and tempering has on the pitching strength, the carbonitriding process was performed with various nitrogen potentials.

  Block test specimens taken from steel 1, steel 2 and steel 14 and small roller specimens for roller pitching test taken from steels 2 to 14 are subjected to carbonitriding under conditions where the nitrogen potential is 0.4%. did.

  In addition, about the small roller test piece for the roller pitching test extract | collected from the steel 1 and the steel 15, the comparative test which gave the carburizing quenching instead of the carbonitriding quenching was also performed. In this case, the conditions of temperature and time are the same as in FIG. 6, the carbon potential in the carburizing process is constant at 0.8%, and the carbon potential is set to 0. 0 during the holding at 820 ° C. corresponding to the carbonitriding process. The condition was 8%, and ammonia gas was not allowed to flow into the furnace.

  Nitrogen potential was measured using a specimen for collecting chips, which was oil-quenched after carbonitriding. Under any condition, the analysis of the nitrogen potential by collecting chips was performed on steel 15 corresponding to JIS SCM420. The method is based on the melted antipyretic conductivity method in which a curved portion of a rod-shaped sample having a diameter of 26 mm and a length of 100 mm shown in FIG. The analysis was performed with an analyzer Leco TC-136, and the nitrogen concentration determined by this analysis was defined as “nitrogen potential”. As a comparative test, in the case of carburizing and quenching instead of carbonitriding, ammonia gas was not flowed into the furnace during holding at 820 ° C. corresponding to the carbonitriding process. No analytical investigation was conducted.

  In the tempering step, the holding temperature was set to 160 ° C. and the holding time was set to 120 minutes, and then the product was taken out from the furnace and allowed to cool in the atmosphere. In FIG. 6, the cooling in the atmosphere is expressed as “AC”.

  A block-on-ring wear test was performed using the block test pieces of Steel 1, Steel 2 and Steel 14 produced as described above, and the surface roughness before and after the block-on-ring wear test was measured.

  The ring test piece used for the block-on-ring wear test is machined with SCM822 defined in JIS G 4053 (2008), temperature is 930 ° C., holding time is 180 minutes, and carbon potential is 0.8%. After carburizing with gas under the above conditions, oil quenching was carried out, followed by tempering at 180 ° C. for 120 minutes and allowing to cool in the air, and then grinding the surface layer by 50 μm was used.

  About said ring test piece, the surface roughness was measured. That is, using a contact type surface roughness meter with a tip diameter of 2 μm, the sliding surface of the test piece was scanned 3 mm in the axial direction to measure the surface roughness. In each ring test piece, the arithmetic average roughness Ra specified by JIS B 0601 (2001) was 0.4 μm or less, and the maximum height roughness Rz was 0.7 μm or less.

  Before performing the block-on-ring wear test, the block test piece was prepared by using a contact type surface roughness meter having the same stylus tip diameter of 2 μm as used in the case of the ring test piece. The surface roughness was measured by scanning the stylus with a measurement length of 2 mm in the width direction.

  Next, with a test load of 1000 N, a sliding speed of 0.1 m / s, a lubricating oil of automatic transmission oil, a lubricating oil temperature of 90 ° C., and a lubricating oil amount of 100 ml in the test machine, the total sliding distance is 2000 m. A block-on-ring wear test was conducted. After the test, the surface roughness in the vicinity of the central portion of the block test piece wear portion was measured by scanning the stylus in the width direction of the test piece using the same surface roughness meter as before the test.

  In addition, in FIG. 7, the schematic diagram of the abrasion part which generate | occur | produced in the contact surface of the method of a block on ring abrasion test and a block test piece is shown.

  Furthermore, various accuracy investigations were performed using the block test pieces after the block-on-ring wear test.

  First, the austenite amount in the volume ratio in the carbonitriding layer surface was measured using the non-contact part among the test surfaces of the block test piece after the test.

  For the measurement, X-ray diffraction was used, and the diffraction peak intensities of the three faces of the ferrite phase (110), (200), (211) and the diffraction peaks of the three faces of the austenite phases (111), (200), (220). The strength was measured, and the value multiplied by the correction coefficient was averaged for each of the ferrite phase and the austenite phase, and the amount of austenite at the volume ratio on the carbonitriding layer surface was evaluated based on the strength ratio.

  Next, the block test piece after the test was cut at a position of 5 mm from the end, and a 3 mm × 10 mm cross section was embedded in the resin as an observation surface, and then the observation surface was mirror-polished and Vickers hardness (hereinafter referred to as “HV”). Hardness ") was measured. That is, the hardness at the 30 μm depth and the 50 μm depth position from the surface of the mirror-polished test piece was measured in accordance with JIS Z 2244 (2009) at 5 points each with a test force of 2.94 N, and arithmetic The average value was evaluated as the surface hardness of the test piece.

  Moreover, the deposit of the surface layer of a block test piece was also identified using the above-mentioned resin-embedded sample. That is, after measuring the HV hardness, the measurement surface was mirror-polished again and corroded with nital, and then a precipitate in a region from the test surface to a depth of 50 μm was collected as a replica sample using bromoethanol. Next, the collected replica sample was observed with a TEM, and the alloy nitride was identified from the obtained diffraction pattern.

  Table 3 shows the surface roughness before and after the test, the hardness of the surface layer, the amount of austenite, and the types of alloy nitrides observed on the surface layer from the surface of the carbonitriding layer to a depth of 50 μm. Note that “Rpk” and “Rk” in Table 3 refer to “projection peak height” and “level difference between core portions” defined in JIS B 0671-2 (2002), respectively.

表３から、（１）式で表されるｆｎ、つまり、〔（Ｓｉ＋Ｍｎ）／Ｃｒ〕が２以上である鋼１および鋼２の場合には、浸炭窒化層表面から５０μｍ深さの位置までの表層に生じる合金窒化物がＭｎＳｉＮ₂のみであるのに対して、ｆｎが２を下回る鋼１４の場合にはＣｒＮであるため、表層硬さおよびオーステナイト量はほぼ同程度であっても、ブロックオンリング摩耗試験前後の表面粗さの変化に違いが生じていることが明らかである。すなわち、鋼１および鋼２の場合には、試験後の粗さの各パラメータは試験前よりも小さくなっているのに対して、鋼１４の場合には、試験後の粗さの各パラメータは試験前よりも大きくなっている。

From Table 3, in the case of steel 1 and steel 2 in which fn represented by the formula (1), that is, [(Si + Mn) / Cr] is 2 or more, from the surface of the carbonitriding layer to a position at a depth of 50 μm. The alloy nitride produced in the surface layer is only MnSiN ₂ , whereas in the case of steel 14 with fn less than 2, it is CrN, so even if the surface layer hardness and the austenite amount are approximately the same, the block on It is clear that there is a difference in the change in surface roughness before and after the ring wear test. That is, in the case of Steel 1 and Steel 2, each parameter of roughness after the test is smaller than that before the test, whereas in the case of Steel 14, each parameter of roughness after the test is It is larger than before the test.

Next, paying attention to the type of alloy nitride, it was decided to compare the steel type in which MnSiN ₂ is precipitated after carbonitriding with the steel type in which CrN is precipitated in a roller pitching test.

  That is, the roller pitching test was implemented on the conditions shown in Table 4 using the small roller test piece of steel 1-15 produced as mentioned above, and pitching strength was investigated.

なお、ローラーピッチング試験に用いる大ローラー試験片には、ＪＩＳ Ｇ ４０５３（２００８）で規定されたＳＣＭ８２２を機械加工して、温度が９３０℃、保持時間が１８０分、炭素ポテンシャルが０．８％の条件でガス浸炭した後油焼入れし、次いで、１８０℃で１２０分焼戻したものを使用した。

The large roller test piece used in the roller pitching test is machined with SCM822 defined in JIS G 4053 (2008), and has a temperature of 930 ° C., a holding time of 180 minutes, and a carbon potential of 0.8%. After gas carburizing under the conditions, oil quenching and then tempering at 180 ° C. for 120 minutes were used.

In the roller pitching test, the test is performed under four conditions where the maximum surface pressure between the small roller test piece and the large roller test piece is 2050 MPa, 2400 MPa, 2600 MPa, and 2800 MPa, and the cumulative number of rotations under each condition is 2 When fatigue peeling did not occur even after reaching × 10 ⁷ times, it was determined that it was durable. As will be described later, steel 15 which is equivalent to JIS SCM420 is durable at a maximum surface pressure of 2050 MPa and not durable at a maximum surface pressure of 2400 MPa, 2600 MPa and 2800 MPa. Was determined to have high pitching strength.

  In addition, using an untested small roller test piece, the accuracy was investigated in the same manner as the block test piece.

  Specifically, first, the amount of austenite at the volume ratio in the “surface contacting the large roller test piece” which is the surface of the carbonitrided layer was evaluated by the method using X-ray diffraction described above. For some, cut at the center of a small roller test piece, embed a cross-section with a diameter of 26 mm in the resin as an observation surface, then mirror-polishing the observation surface, and measure the HV hardness by the method described above Went. That is, the hardness at the 30 μm depth and the 50 μm depth position from the surface of the mirror-polished test piece was measured in accordance with JIS Z 2244 (2009) at 5 points each with a test force of 2.94 N, and arithmetic The average value was evaluated as the surface hardness of the test piece.

  Moreover, the sample which measured said HV hardness was corroded with nital, and the deposit of the area | region from a test surface to a depth of 50 micrometers was extract | collected as a replica sample using brom ethanol after that. Next, the collected replica sample was observed with a TEM, and the alloy nitride was identified from the obtained diffraction pattern.

  Table 5 shows the results of the above investigations together with the results of Np measurement performed using steel 15 corresponding to Cp and JIS SCM420. In Table 5, except for the test symbol h, all the oil quenching was quenching in 120 ° C. oil shown in FIG. 6, while the test symbol h was quenching in 60 ° C. oil.

表５における試験記号ａは、ＪＩＳに記載のＳＣＭ４２０に相当する鋼１５を浸炭焼入れした場合の調査結果で、この試験記号ａをローラーピッチング試験評価の基準とした。すなわち、試験記号ａよりも高面圧の条件で繰返し数が２×１０⁷回以上耐久した場合、換言すれば、前述のとおり、最大面圧が２４００ＭＰａ以上での累積回転数が２×１０⁷回に至っても疲労はく離が生じない場合に、ピッチング強度に優れているものとした。

The test symbol a in Table 5 is a result of investigation when carburizing and quenching steel 15 corresponding to SCM420 described in JIS, and this test symbol a was used as a reference for evaluating the roller pitching test. That is, when the number of repetitions is 2 × 10 ⁷ times or more under conditions of higher surface pressure than the test symbol a, in other words, as described above, the cumulative number of rotations when the maximum surface pressure is 2400 MPa or more is 2 × 10 ^7. In the case where fatigue separation does not occur even after reaching the number of rotations, the pitching strength was excellent.

  Test symbols b to h are investigation results when the steel 1 having a chemical composition within the range defined by the present invention is processed under various heat treatment conditions. Among the above, the test symbols c to h are treatments targeting the surface carbon concentration, that is, Cp (carbon potential) of 0.60% in the same carburizing process, and Np (nitrogen potential) in the nitriding process. The amount of austenite is changed by various changes. In addition, the test symbol b was a treatment aimed at a surface carbon concentration of 0.80% in the carburizing process, and a carburizing atmosphere was used without flowing ammonia into the furnace in the nitriding process. For this reason, since Np during processing is 0 (zero), the above-described analysis investigation of “nitrogen potential” was not performed.

  In addition, as described above, Np in the nitriding step is obtained by turning 50 μm from the outermost periphery to the center of the curved surface portion of the steel 15 corresponding to JIS SCM420 processed at the same time and having a diameter of 26 mm and a length of 100 mm. The collected swarf was determined by analyzing with an analyzer Leco TC-136 based on the melting and antipyretic conductivity method in a helium gas atmosphere.

  As shown in the test symbols c to g in Table 5, the amount of austenite on the surface of the carbonitriding layer could be changed by changing Np while keeping Cp constant. Furthermore, it is clear that the pitching strength also changes as the amount of austenite on the carbonitrided layer surface changes.

  For example, for the test symbols b and c having an austenite amount of less than 30% on the surface of the carbonitriding layer and the test symbol g having an austenite amount exceeding 40%, the carburized material of steel 15 corresponding to JIS SCM420 of the test symbol a Pitching strength is low compared to In other words, as specified in the present invention, by setting the austenite amount on the surface of the carbonitriding layer to 30% or more and 40% or less, it is possible to obtain a higher pitching strength than the carburizing material of JIS SCM420 equivalent steel.

  In addition, the surface hardness of the test symbol e which was quenched in oil at 120 ° C. shown in FIG. 6 was 540 in HV hardness, whereas the test which was quenched in oil at 60 ° C. The surface hardness of symbol h was 710 in HV hardness, which was significantly higher. For this reason, test symbol h and test symbol e have substantially the same austenite amount on the surface of the carbonitrided layer, but the pitching strength of test symbol h having a higher surface hardness is much larger than that of test symbol e. It became high.

  Test symbols i to u are subjected to the same carbonitriding and tempering and tempering as the test symbol e having the highest pitching strength among the test symbols b to g.

  Among the above, the test symbols i to m all satisfy the conditions defined by the present invention in terms of chemical composition and microstructure. In these cases, the test symbols i to m are more than the carburized material of steel 15 corresponding to JIS SCM420. High pitching strength could be obtained. Among these, in particular, the test symbol k using the steel 4 containing Mo, the test symbol 1 using the steel 5 and the test symbol m using the steel 6 are on the surface of the carbonitriding layer as compared with the test symbol e. The austenite content of the steel is reduced by several percent, but the pitching strength is significantly higher than that of the carburized material of JIS SCM420 equivalent steel of test symbol a.

  The test symbols n to u are all out of the conditions defined in the present invention, and therefore the pitching strength is low.

Specifically, in the test symbol n, the Cr content of the steel 7 used exceeds the range specified in the present invention, and fn is less than 2. For this reason, not only MnSiN ₂ but also CrN exists as an alloy nitride generated in the surface layer from the carbonitriding layer surface to a depth of 50 μm, and the austenite content on the carbonitriding layer surface exceeds 40%. As a result, the pitching strength was equivalent to that of the carburized material of JIS SCM420 equivalent steel of test symbol a, and high strength could not be achieved.

In the test symbol o, the Si content of the steel 8 used is lower than the range defined in the present invention, and fn is less than 2. As a result, not only MnSiN ₂ but also CrN is present as an alloy nitride generated in the surface layer from the carbonitriding layer surface to a depth of 50 μm, and the austenite amount on the carbonitriding layer surface exceeds 40% or more. For this reason, the pitching strength is lower than that of the carburized material of JIS SCM420 equivalent steel of test symbol a, and it has not been possible to increase the strength.

In the test symbol p, the Mn content of the steel 9 used is lower than the range specified in the present invention, and fn is less than 2. As a result, not only MnSiN ₂ but also CrN is present as alloy nitride generated in the surface layer from the carbonitriding layer surface to a depth of 50 μm, and the austenite content on the carbonitriding layer surface exceeds 40% or more. For this reason, the pitching strength is lower than that of the carburized material of JIS SCM420 equivalent steel of test symbol a, and it has not been possible to increase the strength.

In the test symbol q, the content of each element of the steel 10 used is within the range defined in the present invention, but fn is less than 2 or less. As a result, not only MnSiN ₂ but also CrN is present as an alloy nitride generated in the surface layer from the carbonitriding layer surface to a depth of 50 μm, and the austenite content on the carbonitriding layer surface exceeds 40% or more. For this reason, the pitching strength is lower than that of the carburized material of JIS SCM420 equivalent steel of test symbol a, and it has not been possible to increase the strength.

In the test symbol r, since the steel 11 used has a high Si content of 1.20%, coarse MnSiN ₂ is generated in the carbonitriding process, and it is considered that pitting fracture has occurred from this point. It was not possible to increase the strength compared to the carburized material of SCM420 equivalent steel.

In the test symbol s, since the steel 12 used has a large Mn content of 1.70%, coarse MnSiN ₂ is generated on the surface layer from the carbonitrided layer surface to a depth of 50 μm, and pitching fracture is caused from this. It was considered that the strength could not be increased as compared with the carburized material of JIS SCM420 equivalent steel of test symbol a.

In the test symbol t, since the fn of the steel 13 used exceeded 2, the alloy nitride formed in the surface layer from the carbonitrided layer surface to a depth of 50 μm was MnSiN ₂ . However, since the Cr content is as low as 0.30%, the hardenability of the core material is low, the pitching damage is early, and the strength is higher than the carburized material of JIS SCM420 equivalent steel of test symbol a. I could not.

  The test symbol u is steel corresponding to the steel 14 used is JIS SCr420. This steel u is a steel whose chemical composition deviates from the conditions specified in the present invention, but was used for the test for comparison with the block-on-ring test. In the case of this test symbol u, CrN was recognized as an alloy nitride in the surface layer from the carbonitrided layer surface to a depth of 50 μm, and the austenite amount exceeded 40%. For this reason, the pitching strength could not be increased as compared with the carburized material of JIS SCM420 equivalent steel of test symbol a.

  The steel product of the present invention has excellent wear resistance and high pitching strength. For this reason, in order to realize the weight reduction of the vehicle body directly linked to the improvement of fuel consumption, it can be used for power transmission parts such as gears for automobile transmissions that are required to be further reduced in size and strength. In addition, the steel product of the present invention can be produced by the method of the present invention, and is made of an inexpensive steel with a low content of Mo, which is an expensive alloy element, or Mo is not added. Further, the manufacturing cost can be reduced as compared with the conventional power transmission component.

Claims (4)

A steel product having a carbonitriding layer,
Steel material of the material is mass%, C: 0.10 to 0.35%, Si: 0.40 to 1.00%, Mn: 0.60 to 1.50%, Cr: 0.40 to 0.00. 80%, Al: 0.01-0.05%, S: 0.05% or less and N: 0.0020-0.0300%,
Fn represented by the following formula (1) is 2 or more, and the balance has a chemical composition composed of Fe and impurities,
In the region from the carbonitriding layer surface to a depth of 50 μm, the dispersed alloy nitride is only MnSiN ₂ ,
The amount of austenite on the carbonitrided layer surface is 30% or more and 40% or less by volume ratio
Steel products characterized by that.
fn = (Si + Mn) / Cr (1)
However, the element symbol in the formula (1) represents the content in mass% in the steel material of the material of the element.   The steel product according to claim 1, wherein the steel material of the dough contains Mo: 0.10% or less in mass% instead of part of Fe.   The dough steel material is replaced with a part of Fe and contains at least one of Ti: 0.10% or less and Nb: 0.080% or less in mass%. Item 3. The steel product according to item 1 or 2. The steel material having the chemical composition of the steel material of the fabric according to any one of claims 1 to 3 is subjected to the following steps <1> to <4> in order. The manufacturing method of the steel product as described in 2.
<1> Carburization is performed in a temperature range of 900 to 950 ° C.
<2> Carbonitriding is performed in a temperature range of 800 to 900 ° C. with a nitrogen potential of 0.2 to 0.6%.
<3> Perform quenching.
<4> Tempering in a temperature range of 150 ° C. to 250 ° C.

Images