Steel for nitrocarburizing use, steel product for nitrocarburizing use and crankshaft   

This application is a formal application of the U.S. Provisional Application No. 61/129,155 filed on Jun. 6, 2008.

TECHNICAL FIELD

The present invention relates to a steel for nitrocarburizing use, a steel product for nitrocarburizing use, and a crankshaft produced by using the said steel product for nitrocarburizing use. More specifically, the present invention relates to: a steel for nitrocarburizing use, which is used as a raw material for a machine component such as a crankshaft and/or a connecting rod for automobiles, industrial machines, construction machines and so on which has high fatigue strength and excellent straightenability, and which is produced by subjecting the said steel to, for example, hot forging into an intended shape, followed by normalizing and nitrocarburizing; a steel product for nitrocarburizing use, which is produced by working the said steel for nitrocarburizing use into an intended shape by various methods; and a crank shaft produced by using the said steel product for nitrocarburizing use.

BACKGROUND ART

Conventionally, a crankshaft, a connecting rod, and the like for automobiles, industrial machines and construction machines have been produced by subjecting a steel to, for example, hot forging into an intended shape, followed by heat treatment of quenching and tempering for obtaining a fine microstructure, and then a nitrocarburizing treatment to mainly enhance fatigue strength.

Generally, the above-mentioned “nitrocarburizing treatment” is a technique of simultaneously allowing the invasion and diffusion of N and C in the temperature range of 500 to 600° C. in order to harden the surface of a steel. The said “nitrocarburizing treatment” is particularly advantageous in improving fatigue strength, as compared with a “nitriding treatment” whose purpose is mainly improving wear resistance, and accordingly, the nitrocarburizing treatment has been spread rapidly.

Subjecting a steel to a nitrocarburizing treatment may cause strain, with the result that a dimensional precision of a machine component may deteriorate. After nitrocarburizing treatment, a straightening treatment for bending is frequently performed on a machine component, in particular, to such as a crankshaft and/or a connecting rod. Therefore, an excellent straightenability is also demanded for a machine component in which a nitrocarburizing treatment is performed (hereinafter, the said machine component is referred to as a “nitrocarburized component”).

On the other hand, in recent years, a demand for reducing the cost of heat treatment has been increasing. Accordingly, the use of a steel which can be used in a hot forged condition; in other words, the use of a non-heat treated steel has been increasing.

However, generally, a microstructure of a non-heat treated steel product is a coarse ferrite-pearlite (a composite microstructure of pearlite and ferrite), that is to say, a soft ferrite phase exists in the non-heat treated steel product. Accordingly, in many cases, it may be difficult to ensure high fatigue strength, if a nitrocarburized component is made of the non-heat treated steel product.

Therefore, it has been proposed an approach of producing a non-heat treated steel having a fine microstructure by containing various alloy elements, and enhancing fatigue strength of a nitrocarburized component by increasing the hardness of a soft ferrite phase. However, merely increasing the content of an alloy element may increase the hardness of the outer layer of the steel product subjected to nitrocarburizing treatment. As the result, a degradation of the said straightenability is unavoidable.

Moreover, in recent years, a demand for reducing the weight of an automobile has been increasing particularly in the aspect of reducing the amount of CO2 in the exhaust gas. Accordingly, there is an increasing demand for a crankshaft having a still higher fatigue strength than ever. However, the conventional non-heat treated steel has failed to sufficiently satisfy the said demand.

In view of these circumstances, there is an increasing demand for a steel for nitrocarburizing use which is capable of suppressing the heat treatment cost of quenching and tempering, and also having both the “high fatigue strength” and the “excellent straightenability” described in the following and have not been provided in a conventional non-heat treated steel.

“High fatigue strength”: fatigue strength of 460 MPa or more by the Ono type rotating bending fatigue test using a notched fatigue test specimen, which is suitable for evaluating fatigue strength of a machine component having a complicated shape such as a crankshaft, at room temperature in the atmosphere.

“Excellent straightenability”: there is no crack on the surface of a machine component until a large bending displacement is applied, or the crack length is sufficiently short, even if there is a crack. Concretely, there is no crack, or a crack length of 0.1 mm or less is observed by a bending test at room temperature in the atmosphere, using a test piece of 20 mm in diameter, which is described later.

Therefore, the Patent Documents 1 to 4 propose various steels for nitrocarburizing use in order to satisfy the aforementioned demand. Moreover, the Patent Documents 5 to 7 propose non-heat treated steels suitable for an engine component such as a crank.

Specifically, the Patent Document 1 discloses “a low-alloy steel for nitrocarburizing use which is capable of obtaining high strength and high toughness after nitrocarburizing treatment, which contains by weight %, C: 0.4 to 0.7%, Si: not more than 1.0%, Mn: 0.8 to 2.0%, Cr: not more than 0.2%, Al: not more than 0.05%, Ti+V: not less than 0.02% to not more than the content of [C/6]%, and which further contains, according to need;

[a] P: not more than 0.015%

and/or

[b] at least one element selected from S: not more than 0.15%, Pb: not more than 0.3%, Bi: not more than 0.3%, Se: not more than 0.1% and Ca: 0.0005 to 0.0100%; with the balance being substantially Fe, and N among the impurities of 0.007% or less being allowed”.

The Patent Document 2 discloses “a non-heat treated forging steel for nitrocarburizing use, which has a steel composition by weight ratio, C: 0.30 to 0.60%, Si: 0.05 to 1.50%, Mn: 0.20 to 2.00%, P: not more than 0.02%, S: not more than 0.04%, Cr: not more than 0.30%, Al: not more than 0.005%, and N: 0.01 to 0.02%, and which further contains, according to need, at least one element selected from the group consisting of P: 0.02 to 0.07%, S: 0.04 to 0.10%, Ca: 0.0003 to 0.003% and Pb: 0.01 to 0.20%, with the balance being Fe and inevitable impurities, wherein a content of V as an impurity element is not more than 0.01%”.

The Patent Document 3 discloses a “steel for a machine component use, which contains by weight %, a content ratio of Fe: not less than 90%, C: 0.35 to 0.5%, Si: 0.01 to 0.3%, Mn: 0.6 to 1.8%, Cu: 0.01 to 0.5%, Ni: 0.01 to 0.5%, Cr: 0.01 to 0.5%, Al: 0.001 to 0.01% and N: 0.005 to 0.025%, and which further contains, according to need, one element or two or more elements selected form Pb: not more than 0.30%, S: not more than 0.20%, Ca: not more than 0.01%, Bi: not more than 0.30%, Ti: not more than 0.02%, Zr: not more than 0.02% and Mg: not more than 0.01%”.

The term “machine component” refers to a “machine component subjected to a surface hardening treatment by nitrocarburizing, wherein a Vickers hardness of the surface layer part in the standard position corresponding to a depth of 50 μm from a member surface of the said machine component is from 340 to 460 HV; a Vickers hardness of an inner region having a substantially constant hardness, where the influence of nitrocarburizing is not affected, is from 190 to 260 HV; and an effective case depth from the member surface, where the Vickers hardness is set to 270 HV, is adjusted to not less than 0.3 mm”.

The Patent Document 4 discloses a “steel for machine structural use having excellent seizure resistance and fatigue strength, and produced by performing nitrocarburizing treatment on a steel, which contains by weight ratio, C: 0.22 to 0.55%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: not more than 0.025%, S: 0.04 to 0.08%, Ni: not more than 0.15%, Cr: not more than 0.15%, Mo: not more than 0.05%, Cu: not more than 0.15%, Al: 0.002 to 0.018%, Ti: not more than 0.0030%, V: 0.01 to 0.06%, Nb: not more than 0.0030%, B: not more than 0.0005% and N: 0.0080 to 0.0200%, and which further contains, according to need, one element or two or more elements selected from Ca: 0.0010 to 0.0120%, Pb: 0.04 to 0.40%, Bi: 0.05 to 0.50%, Te: 0.05 to 0.35% and Se: 0.05 to 0.35%, and satisfies the condition of Ni+Mo+Cu≦0.25%, Ti+Nb+B≦0.0040%, with the balance being Fe and impurities, wherein the steel surface has a compound layer of not less than 12 μm on the average”.

The Patent Document 5 discloses a “non-heat treated steel for hot forging use having excellent wear resistance, which contains by weight unit,

C: 0.40 to 0.70%, Si: not more than 0.50%, Mn: 0.90 to 1.80%, Cr: 0.05 to 1.00%, s-Al: 0.010 to 0.045% and N: 0.005 to 0.025%, and which further contains, according to need, one element or two or more elements selected from Pb: not more than 0.030%, S: not more than 0.20%, Te: not more than 0.030%, Ca: not more than 0.01% and Bi: not more than 0.30%, with the balance being Fe and impurities, wherein the microstructure after hot-forging is ferrite+pearlite, and the area ratio of pro-eutectoid ferrite is not more than 10%”.

The Patent Document 6 discloses a “non-heat treated steel for hot forging use having excellent wear resistance, which contains by mass %, C: 0.3 to 0.8%, Mn: 0.3 to 2.0% and Si: 0.5 to 2.5%, and which further contains, according to need, one or more elements of one or more groups selected from [a] to [d]:

[a] one or more elements selected from the group consisting of V: not more than 0.4%, Nb: not more than 0.15% and Ti: not more than 0.15%;

[b] Cr: not more than 1.5%;

[c] Al: not more than 0.04%; and

[d] one or more elements selected from the group consisting of S: not more than 0.12%, Pb: not more than 0.3%, Zr: not more than 0.2%, Ca: not more than 0.01%, Te: not more than 0.1% and Bi: not more than 0.1%, wherein: the F value defined by F=Si+(Mn/3.5)+3V+2.5Nb+2.5Ti is not less than 1.0; and the number of oxide type inclusion with a mean grain size is not less than 20 μm per 300 mm2 of detected area is ten or less in an arbitrary vertical section”.

The Patent Document 7 discloses a “non-heat treated steel for hot forging use, which contains by mass %, C: 0.30 to 0.80%, Si: 0.1 to 2.5%, Mn: 0.30 to 2.0%, Al: 0.001 to 0.06%, N: 0.005 to 0.10%, P: not more than 0.30%, S: not more than 0.12%, Cr: not more than 1.0%, Cu: not more than 0.3% and Ni: not more than 0.3%, and which further contains, according to need, one or more elements selected from Pb: not more than 0.3%, Zr: not more than 0.2%, Ca: not more than 0.010%, Te: not more than 0.10% and Bi: not more than 0.1%, with the balance being Fe and inevitable impurities, wherein:

the steel satisfies the relations:

Si+3.4Mn+19.5P−13.4S+2.7Cr≧3.5;

C+1.1Mn−1.9Si+1.5Cu+1.8Ni+0.6Cr≦2.6; and

the steel has tensile strength from 600 to 900 N/mm2”.

Patent Document 1: Japanese Unexamined Patent Publication No. 64-25949

Patent Document 2: Japanese Unexamined Patent Publication No. 8-170146

Patent Document 3: Japanese Unexamined Patent Publication No. 2004-162161

Patent Document 4: Japanese Unexamined Patent Publication No. 7-18379

Patent Document 5: Japanese Unexamined Patent Publication No. 2000-265242

Patent Document 6: Japanese Unexamined Patent Publication No. 2000-328193

Patent Document 7: Japanese Unexamined Patent Publication No. 9-310152

In order to ensure both the high fatigue strength and the straightenability, the technique described in the above-mentioned Patent Document 1 is directed to ensure the toughness of a surface hardened layer after the nitrocarburizing treatment and also to improve the strength of the core portion by minimizing the contents of Cr, Al, V and Ti which may deteriorate the straightenability; and the addition of Mn to compensate for strength deterioration will minimize the contents of the above-mentioned elements. However, the “straightenability” in the said Patent Document 1 is evaluated based on a measure of bending flexure which can be obtained when a large crack has occurred in the bending test using a test piece with a thickness as small as 5 mm. Accordingly, the steel proposed in the Patent Document 1 has failed to ensure the “straightenability”, in the case where both the “high fatigue strength” and the “excellent straightenability” as described above are required.

Without performing a thermal refining treatment, by limiting the contents of N, Cr and V, the technique disclosed in the Patent Document 2 allows for ensuring both the fatigue strength and the straightenability substantially equivalent to those of a steel which will be performed a thermal refining treatment. However, the “fatigue strength” in the Patent Document 2 is evaluated by using a smooth fatigue test specimen. Accordingly, the steel proposed in the said Patent Document 2 has failed to particularly ensure the “fatigue strength” under such a circumstance that the weight reduction in automobiles has been required in the aspect of reducing the amount of CO2 in the exhaust gas, and a demand for a crankshaft having a still higher fatigue strength than ever has been increasing, and both the “high fatigue strength” and the “excellent straightenability” as described above are required.

In order to ensure both the fatigue strength and the straightenability, the technique disclosed in the Patent Document 3 is directed to controlling a hardness distribution after the nitrocarburizing treatment by controlling the contents of C, Mn, Si, Cr, Cu and Ni. However, even with use of the steel proposed in the said Patent Document 3, both the “high fatigue strength” and the “excellent straightenability” as described above could not be ensured.

The steel disclosed in the Patent Document 4 is directed to increasing the thickness of a compound layer to be formed after the nitrocarburizing treatment by reducing the contents of Ni, Cu, Mo and V so as to ensure both the seizure resistance and the fatigue strength. However, the steel proposed in the Patent Document 4 does not take into account the straightenability after the nitrocarburizing treatment. Accordingly, the said steel disclosed in the Patent Document 4 has failed to particularly ensure the “straightenability”, in the case where both the “high fatigue strength” and the “excellent straightenability” as described above are required.

The steel disclosed in the Patent Document 5 is a non-heat treated steel, having high wear resistance in a hot forged condition, without performing a surface hardening treatment such as high frequency induction hardening, nitrocarburizing and so on. Accordingly, even if the steel proposed in the said Patent Document 5 is subjected to nitrocarburizing, the intended fatigue strength could not be obtained, not to mention the intended straightenability.

The technique disclosed in the Patent Document 6 is directed to providing a non-heat treated steel for hot forging use having excellent wear resistance, whose microstructure comprises ferrite-pearlite, by dissolving Si and Mn in the ferrite, and also precipitating fine carbonitrides such as V, Nb, Ti and so on in the said ferrite. The steel proposed in the Patent document 6 does not take into account the nitrocarburizing. Accordingly, if the steel proposed in the Patent Document 6 is subjected to nitrocarburizing treatment, not only the “excellent straightenability” as described above but also the said intended fatigue strength could not be obtained.

The technique disclosed in the Patent Document 7 is directed to providing a non-heat treated steel for hot forging use which has, in a non-heat treated condition, high proof strength and fatigue property substantially equivalent to those of a steel containing V or the like, and which has excellent machinability, by controlling the chemical compositions of the steel to enhance the proof strength. However, the steel proposed in the Patent document 7 also does not take into account the nitrocarburizing. And consequently, if the steel proposed in the Patent Document 7 is subjected to nitrocarburizing treatment, not only the said “excellent straightenability” but also the said intended fatigue strength could not be obtained.

Accordingly, it is an objective of the present invention to provide a steel for nitrocarburizing use, which has the high fatigue strength and the excellent straightenability after the nitrocarburizing treatment, and is used suitably as a raw material for a machine component, such as a crankshaft and/or a connecting rod for automobiles, industrial machines, construction machines and so on, without performing the expensive heat treatment of quenching and tempering; a steel product for nitrocarburizing use, which is produced by working the said steel for nitrocarburizing use into an intended shape by various methods; and a crankshaft produced by using the said steel product for nitrocarburizing use.

The specific fatigue property and straightenability after the nitrocarburizing treatment, which are attained by the present invention, are as follows.

Fatigue property: fatigue strength of 460 MPa or more by the Ono type rotating bending fatigue test, using a notched fatigue test specimen shown in FIG. 1, at room temperature in the atmosphere.

Straightenability: there is no crack, or the crack length is 0.1 mm or less, in the case where a load is applied to a test specimen of 20 mm in diameter in a state that a strain gauge is attached to the longitudinal middle portion of the said test specimen, with two fulcrums being spaced away from each other by the distance of 70 mm, until a reading of the strain gauge reaches 17000μ (corresponding to 1.7% of bending strain) by a 3-point bending method at room temperature in the atmosphere.

In order to accomplish the above objective, the present inventors first investigated how to omit the heat treatment of quenching and tempering.

(a) By reducing the precipitation of soft ferrite included in the microstructure as much as possible and forming a fine microstructure, the high fatigue strength can be ensured for a steel product which is not subjected to the heat treatment of quenching and tempering.

(b) However, a large amount of soft ferrite may precipitate in the case of a steel in a hot forged condition, that is to say, a non-heat treated steel, and a fine microstructure may not be obtained.

(c) By using normalizing in place of the heat treatment of quenching and tempering, it is possible to reduce the heat treating cost to approximately one-half of that required for quenching and tempering. In this case, as compared with non-heat treated steel, soft ferrite precipitation is somewhat suppressed, and a fine microstructure is formed to a certain degree. However, the degree of refinement of the ferrite phase is not sufficient.

Accordingly, the present inventors investigated microstructures in detail by melting various kinds of steels. By evaluating the fatigue strength and the straightenability after the nitrocarburizing treatment, the present inventors also investigated the influences of microstructures on the fatigue strength and the straightenability. As a result of the said investigation, the following findings (d) to (h) were obtained.

(d) Simply increasing the content of C, which is an indispensable element for forming carbides, fails to sufficiently suppress the generation of pro-eutectoid ferrite at the time of cooling the steel from an austenite temperature region during normalizing. However, increasing the content of Mn as an austenite-forming element as well as increasing the C content, and limiting the content of Cr as a ferrite-forming element suppresses the generation of pro-eutectoid ferrite.

(e) By controlling the contents of C, Mn and Cr so that the value of “fn1”, which is expressed by the formula (1), becomes not less than 1.90, the ratio of ferrite is suppressed to not more than 10%;

fn1=1.25C+Mn−0.1Cr  (1).

(f) Furthermore, in the case where the grain diameter of austenite (hereinafter, referred to as “prior-austenite”) in normalizing is coarse, the ferrite-pearlite to be formed after cooling may become coarse, and therefore, the high fatigue strength can not be ensured. However, in the case where the prior-austenite grain diameter is small, fine ferrite-pearlite is obtained, and the fatigue strength and the straightenability are significantly improved.

(g) A coarsening of the prior-austenite grain diameter is suppressed by processing a steel into an intended shape by hot forging and so on, which is followed by normalizing, and by utilizing a pinning effect of nitrides of Ti and/or Al during the said normalizing. In particular, at a normalizing temperature region between 800 and 900° C., the prior-austenite grain diameter is sufficiently reduced to approximately 5 to 45 μm.

(h) Containing, by mass %, N: more than 0.007% to not more than 0.030%, and containing one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, wherein the contents of N, Al and Ti satisfy a requirement that the value of “fn2”, which is expressed by the formula (2) is more than 0, obtains the said pinning effect by nitrides of Ti and/or Al in the above-mentioned item (g);

fn2=N−0.45Al−( 1/22)Ti  (2).

That is to say, it is necessary not only to contain the proper amounts of N, Al and Ti, but also to satisfy the requirement that the value of “fn2” is more than 0, in order to contain a sufficient amount of N which is able to form sufficient nitrides of Al and/or Ti which can ensure the pinning effect.

As mentioned above, if “normalizing” is adopted as a heat treatment after hot forging, it can reduce the heat treating cost to approximately one-half of the one required for “quenching and tempering”.

The present invention has been accomplished on the basis of the above-described findings. The main points of the present invention are steels for nitrocarburizing use shown in the following (1) to (3), steel products for nitrocarburizing use shown in the following (4) to (6), and crankshafts shown in the following (7) to (9).

(1) A steel for nitrocarburizing use, which comprises by mass percent, C: more than 0.45% to not more than 0.60%, Si: less than 0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more than 0.007% to not more than 0.030%, and which further contains one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, “fn1” expressed by the following formula (1) is not less than 1.90, and “fn2” expressed by the following formula (2) is more than 0:

fn1=1.25C+Mn−0.1Cr  (1),

fn2=N−0.45Al−( 1/22)Ti  (2);

In the formulas (1) and (2), each element symbol represents the content by mass percent of the element concerned.

(2) The steel for nitrocarburizing use according to the above (1), which further contains by mass percent, Ca: not more than 0.005%.

(3) The steel for nitrocarburizing use according to the above (1) or (2), which further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%.

(4) A steel product for nitrocarburizing use, which has a chemical composition by mass percent, C: more than 0.45% to not more than 0.60%, Si: less than 0.50%, Mn: more than 1.30% to not more than 1.70%, P: not more than 0.05%, S: 0.02 to 0.10%, Cr: not more than 0.30% and N: more than 0.007% to not more than 0.030%, and which further contains one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, “fn1” expressed by the following formula (1) is not less than 1.90, and “fn2” expressed by the following formula (2) is more than 0; and has a microstructure which comprise ferrite-pearlite phase with a ratio of the ferrite of 10% or less:

fn1=1.25C+Mn−0.1Cr  (1),

fn2=N−0.45Al−( 1/22)Ti  (2);

In the formulas (1) and (2), each element symbol represents the content by mass percent of the element concerned.

(5) The steel product for nitrocarburizing use according to the above (4), whose chemical composition further contains by mass percent, Ca: not more than 0.005%.

(6) The steel product for nitrocarburizing use according to the above (4) or (5), whose chemical composition further contains by mass percent, one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2%.

(7) A crankshaft manufactured by using the steel product for nitrocarburizing use according to the above (4).

(8) A crankshaft manufactured by using the steel product for nitrocarburizing use according to the above (5).

(9) A crankshaft manufactured by using the steel product for nitrocarburizing use according to the above (6).

The above-mentioned “ferrite-pearlite phase” represents a composite microstructure of ferrite and pearlite. The ferrite, which is 10% or less by the “ratio” in the microstructure, represents the ferrite which forms the said “ferrite-pearlite phase” together with pearlite, and does not embrace the one which forms pearlite with cementite.

The above-mentioned inventions (1) to (3) related to the steels for nitrocarburizing use, the inventions (4) to (6) related to the steel products for nitrocarburizing use and the inventions (7) to (9) related to the crankshafts are referred to as “the present invention (1)” to “the present invention (9)”, respectively, or collectively referred to as “the present invention”.

The steels for nitrocarburizing use of the present invention have high fatigue strength and excellent straightenability after the nitrocarburizing treatment, without performing the expensive heat treatment of quenching and tempering. Accordingly, the said steels for nitrocarburizing use of the present invention are suitable as raw materials for nitrocarburized components such as crankshafts and/or connecting rods for automobiles, industrial machines, construction machines and so on.

FIG. 1 shows the shape of a notched fatigue test specimen used in the Ono type rotating bending fatigue test at room temperature in the atmosphere in the examples.

In the following, all of the requirements of the present invention are described in detail. The symbol “%” for the content of each chemical composition represents “% by mass”.

(A) Chemical Composition

C: more than 0.45% to not more than 0.60%

C is bonded to Fe and precipitates as cementite, and consequently forms lamellar pearlite and reduces the ratio of soft ferrite. Thus, C has an effect of enhancing fatigue strength. In order to obtain such effects, a content of C more than 0.45% is necessary. However, an excessive C content, in particular, a content of C exceeding 0.60% may increase the hardness of a steel product and thereby causes deterioration of machinability. Therefore, the content of C is set to more than 0.45% to not more than 0.60%. The lower limit of the C content is preferably 0.48%.

Si: less than 0.50%

Si is contained as an impurity in a steel. Si may increase the hardness of a surface layer and deteriorate the straightenability. A content of Si not less than 0.50% may significantly deteriorate the straightenability. Furthermore, the content of Si not less than 0.50% may increase the ratio of ferrite in the microstructure and thereby deteriorates fatigue strength. In view of this, it is necessary to set the content of Si to less than 0.50%. As far as the content of Si is less than 0.50%, Si has substantially no effect on the ratio of ferrite in the microstructure. However, in order to suppress the increase in the ratio of ferrite stably and surely, the content of Si is preferably set to not more than 0.45%.

Mn: more than 1.30% to not more than 1.70%

Mn has an effect of ensuring high fatigue strength by lowering the ratio of ferrite in the microstructure, as an austenite-forming element, and by increasing the hardness of a base material, as a solid-solution strengthening element. In order to obtain such effect, it is necessary to contain more than 1.30% of Mn. However, an excessive Mn content, in particular, a content of Mn exceeding 1.70% may increase the hardness of a steel product and thereby causes deterioration of the straightenability and machinability. Therefore, the content of Mn is set to more than 1.30% to not more than 1.70%. The lower limit of the Mn content is preferably 1.35% and the upper limit thereof is preferably 1.65%.

P: not more than 0.05%

P is an impurity element including in a steel and may promote the occurrence of intergranular brittle fracture by segregating on the grain boundaries. In particular, a content of P exceeding 0.05% may significantly increase the occurrence of the said intergranular brittle fracture. In view of this, the content of P is set to not more than 0.05%. The content of P is preferably not more than 0.045%.

S: 0.02 to 0.10%

S is an effective element for improving the machinability of steel. In order to obtain this effect, it is necessary to contain not less than 0.02% of S. However, an excessive S content may cause deterioration of hot workability and fatigue strength. In particular, a content of S exceeding 0.10% may significantly deteriorate the said hot workability and fatigue strength. Therefore, the content of S is set to 0.02 to 0.10%. The upper limit of the S content is preferably 0.07%.

Cr: not more than 0.30%

Cr is contained as an impurity in a steel, and may increase the hardness of a surface layer after nitrocarburizing treatment and deteriorate the straightenability. Furthermore, as a ferrite-forming element, Cr may increase the ratio of ferrite in the microstructure and thereby deteriorates fatigue strength. In particular, a content of Cr exceeding 0.30% may significantly deteriorate the said straightenability and fatigue strength. In view of this, the content of Cr is set to not more than 0.30%. It is desirable that the Cr content be reduced as low as possible.

N: more than 0.007% to not more than 0.030%

N is bonded to Al and/or Ti, and forms fine nitrides of Al and/or Ti, thereby contributing to refine the prior-austenite grain diameter and has an effect of improving fatigue strength and the straightenability. In order to obtain this effect, it is necessary to contain more than 0.007% of N. However, in industry, it is difficult to add more N than 0.030% as a content thereof. And moreover, in such a case, for instance, the gas bubbles may generate in the ingot, which may impair the quality of the steel. Therefore, the content of N is set to more than 0.007% to not more than 0.030%. The lower limit of the N content is preferably 0.010% and the upper limit thereof is preferably 0.025%.

One or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more

Al and Ti precipitate as nitrides in s steel, and the prior-austenite diameter can be refined by the pinning effect of these nitrides. As the result, fatigue strength and the straightenability improve. In order to obtain such effects, it is necessary to contain one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more.

However, an excessive Al content may cause precipitation of nitrides during nitrocarburizing treatment, thereby increasing the hardness of a surface layer. As a result, the straightenability may be impaired. In particular, a content of Al exceeding 0.10% may significantly deteriorate the said straightenability. Furthermore, a content of Ti exceeding 0.035% may cause coarsening of the carbonitrides of Ti. As a result, not only the straightenability may be deteriorated, but also the effect of refining the prior-austenite diameter may not be obtained, thereby deteriorating fatigue strength.

Therefore, in the present invention, one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035% are contained, with Al+Ti being 0.015% or more.

In the present invention, the upper limit of the [Al+Ti] content may be 0.135% with the content of Al being 0.10% and the content of Ti being 0.035%.

In the present invention, Al and Ti may not be necessarily contained in combination, that is to say, either one of Al and Ti may be exclusively contained in the content of not less than 0.015%.

In the present invention, at least as far as Al is contained, the lower limit of the Al content is preferably 0.015% and the upper limit thereof is preferably 0.070%.

In the present invention, it is most preferable to contain Al+Ti of not less than 0.015% in combination, on the condition of Al: more than 0.010% to not more than 0.10% and Ti more than 0.005% to not more than 0.035%.

Value of fn1: not less than 1.90

Even if the contents of C, Mn and Cr fall within the aforementioned ranges, in other words, satisfy the requirements that C: more than 0.45% to not less than 0.60%, Mn: more than 1.30% to not more than 1.70%, and Cr: not more than 0.30%, when the value of “fn1” expressed by the formula (1) is less than 1.90, the ratio of ferrite in the ferrite-pearlite phase exceeds 10%, and therefore, the intended high fatigue strength cannot be obtained:

fn1=1.25C+Mn−0.1Cr  (1);

Therefore, the value of “fn1” expressed by the formula (1) is set to not less than 1.90. The value of “fn1” is preferably not less than 1.93. The upper limit of the value of “fn1” may be close to 2.45, wherein the contents of C and Mn are respectively 0.60% and 1.70%, which are the upper limits thereof, and the Cr content is close to 0.

Value of fn2: more than 0

Even if the contents of N, Al and Ti fall within the aforementioned ranges, in other words, satisfy the requirement that N: more than 0.007% to not more than 0.030%, and further satisfy the requirements that one or two elements selected from Al: more than 0.010% to not more than 0.10% and Ti: more than 0.005% to not more than 0.035%, with Al+Ti being 0.015% or more, when the value of “fn2” expressed by the formula (2) is not more than 0, the said pinning effect by nitrides of Al and/or Ti cannot be obtained, and the prior-austenite grain diameter may coarsen, and therefore the intended high fatigue strength and excellent straightenability can not be obtained:

fn2=N−0.45Al−( 1/22)Ti  (2);

Therefore, the value of “fn2” expressed by the formula (2) is set to more than 0. The value of “fn2” is preferably not less than 0.0005. The upper limit of the value of “fn2” may be 0.0293, wherein the content of N is 0.030%, which is the upper limit thereof, A1 is not contained, and the content of Ti is 0.015%.

In the present invention, it is necessary to suppress the content of V among the impurities in the following manner.

V: not more than 0.010%

V may increase the hardness of a surface layer and degrade the straightenability. In particular, a content of V exceeding 0.010% may significantly degrade the straightenability. Moreover, a content of V more than 0.010% may increase the ratio of ferrite in the microstructure, thereby deteriorating fatigue strength. In view of this, the content of V among the impurities is not more than 0.010%.

From the reasons mentioned above, the steel for nitrocarburizing use according to the present invention (1) is defined as the one containing C, Si, Mn, P, S, Cr, N, Al and Ti within the aforementioned ranges, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, the “fn1” expressed by the said formula (1) is not less than 1.90, and the “fn2” expressed by the said formula (2) is more than 0.

The chemical composition of the steel product for nitrocarburizing use according to the present invention (4) is also defined as the one containing C, Si, Mn, P, S, Cr, N, Al and Ti within the aforementioned ranges, with the balance being Fe and impurities, wherein V among the impurities is not more than 0.010%, the “fn1” expressed by the said formula (1) is not less than 1.90, and the “fn2” expressed by the said formula (2) is more than 0.

The steel for nitrocarburizing use of the present invention may further contain Ca, according to need. The chemical composition of the steel product for nitrocarburizing use of the present invention also may further contain Ca, according to need.

In the following, Ca as an optional element will be explained.

Ca: not more than 0.005%

Ca, if added, has the effect of improving the machinability of steel. In order to obtain this effect, Ca may be added. However, an excessive Ca content may cause deterioration of hot workability and fatigue strength. In particular, a content of Ca exceeding 0.005% may significantly deteriorate the said hot workability and fatigue strength. Therefore, the content of Ca is set to not more than 0.005%.

On the other hand, in order to obtain the effect of improving machinability by Ca sufficiently, the lower limit of the Ca content is preferably set to not less than 0.0005%.

From the above-mentioned reason, the steel for nitrocarburizing use according to the present invention (2) is defined as the one which further contains Ca: not more than 0.005% in addition to the steel for nitrocarburizing use according to the present invention (1).

The chemical composition of the steel product for nitrocarburizing use according to the present invention (5) is also defined as the one which further contains Ca: not more than 0.005% in addition to the chemical composition of the steel product for nitrocarburizing use according to the present invention (4).

The steel for nitrocarburizing use of the present invention may further contain Cu and/or Ni, according to need. The chemical composition of the steel product for nitrocarburizing use of the present invention also may further contain Cu and/or Ni, according to need.

In the following, Cu and Ni as optional elements will be explained.

Cu: not more than 0.3%

Cu, if added, has the effect of ensuring the further higher fatigue strength by increasing the hardness of a base material as a solid-solution strengthening element. In order to obtain this effect, Cu may be added. However an excessive Cu content may increase the hardness of a steel product and thereby causes deterioration of machinability. In particular, a content of Cu exceeding 0.3% may significantly deteriorate machinability. Therefore, the content of Cu set to not more than 0.3%.

In order to obtain the effect of enhancing the fatigue strength by Cu sufficiently, the lower limit of the Cu content is preferably set to not less than 0.05%.

Ni: not more than 0.2%

Ni, if added, also has the effect of ensuring the further higher fatigue strength by increasing the hardness of a base material as a solid-solution strengthening element. In order to obtain this effect, Ni may be added. However an excessive Ni content may increase the hardness of a steel product and thereby causes deterioration of machinability. In particular, a content of Ni exceeding 0.2% may significantly deteriorate machinability. Therefore, the content of Ni set to not more than 0.2%.

In order to obtain the effect of enhancing the fatigue strength by Ni sufficiently, the lower limit of the Ni content is preferably set to not less than 0.05%.

From the reasons mentioned above, the steel for nitrocarburizing use according to the present invention (3) is defined as the one which further contains one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2% in addition to the steel for nitrocarburizing use according to the present invention (1) or (2).

The chemical composition of the steel product for nitrocarburizing use according to the present invention (6) is also defined as the one which further contains one or two elements selected from Cu: not more than 0.3% and Ni: not more than 0.2% in addition to the chemical composition of the steel product for nitrocarburizing use according to the present invention (4) or (5).

(B) Microstructure

A coarsening of the prior-austenite grain diameter can be suppressed by processing the steel, which satisfies the chemical composition requirements described in the item (A), into the intended shape by hot forging, followed by normalizing. The process forms a microstructure being ferrite-pearlite phase with a ratio of the ferrite of 10% or less, without coarsening the microstructure of the steel product. And thereby the intended high fatigue strength can be obtained.

In view of the above, the microstructure of the steel products for nitrocarburizing use according to the present inventions (4) to (6) is defined as the one being ferrite-pearlite phase with a ratio of the ferrite of 10% or less.

As already described above, the “ferrite-pearlite phase” represents a composite microstructure of ferrite and pearlite. The ferrite of 10% or less by the “ratio” in the microstructure represents the ferrite which forms the said “ferrite-pearlite phase” together with pearlite, and does not encompass the one which forms pearlite with cementite. In order to obtain the high fatigue strength, it is desirable that the ferrite ratio in the microstructure be reduced as much as possible. However, the ferrite ratio of approximately 1% is the lower limit in the industrial scale production by normalizing.

From the reasons mentioned above, each of the crankshafts according to the present inventions (7) to (9) is defined as the one which is respectively manufactured by using the steel product for nitrocarburizing use according to the present inventions (4) to (6).

At the heating temperature of 800 to 900° C. on the said “normalizing” treatment, a microstructure of a fine ferrite-pearlite phase whose ferrite ratio is 10% or less can be ensured. Therefore, in that case, fatigue strength and the straightenability can be extremely improved.

The heat treating cost for “normalizing” is reduced to approximately one-half of that required for “quenching and tempering”.

A nitrocarburized component such as a crankshaft and/or a connecting rod for automobiles, industrial machines, and construction machines, as well as the crankshafts according to the present inventions (7) to (9), can be manufactured by using the steel product for nitrocarburizing use which is obtained by working the steel for nitrocarburizing use according to the present invention into the intended shape in several methods.

Specifically, for instance, the steel product for nitrocarburizing use of the present invention can be obtained by: subjecting a cast bloom or a steel ingot of the steel for nitrocarburizing use of the present invention, or a semi-finished steel product manufactured from the said cast bloom or steel ingot to hot working into the intended shape, followed by normalizing. Then, the said nitrocarburized component can be obtained by machining the steel product for nitrocarburizing use into a predetermined shape, followed by nitrocarburizing.

In the aforementioned process, the hot working condition for the intended shape is not particularly defined. However, it is preferable to set the heating temperature before hot working for the intended shape to 1100 to 1300° C. Moreover, the hot working is preferably terminated at 1100 to 900° C., and after the hot working, cooling is preferably performed in the atmosphere.

Also, the normalizing condition is not necessarily defined. However, it is preferable to set the heating temperature for austenitization to 800 to 900° C.

Moreover, the condition for nitrocarburizing treatment is not necessarily defined. An ordinary method such as gas nitrocarburizing, salt bath nitrocarburizing and plasma nitrocarburizing may be conveniently used. Any of the above methods can form stably and homogeneously an approximately 20 μm thick compound layer and a diffusion layer immediately beneath the said compound layer on the surface of a nitrocarburized component.

In order to obtain the intended nitrocarburized component by the gas nitrocarburizing method, for example, the treatment may be performed in an atmosphere of 570° C., which contains a mixture of endothermic gas (RX gas) and ammonia gas at a ratio of 1:1, for approximately 3 hours, followed by cooling in an oil of 100° C.

In the following, the present invention is described in detail by referring to examples.

The steels 1 to 18 having the chemical compositions shown in Table 1 were melted by use of a vacuum melting furnace with a volume of 180 kg and made into ingots.

The steels 1 to 6, 17 and 18 shown in Table 1 are steels whose chemical compositions fall within the range regulated by the present invention. On the other hand, the steels 7 to 16 are steels of comparative examples whose chemical compositions are out of the range regulated by the present invention.

TABLE 1 Chemical composition (% by mass) Balance: Fe and impurities Steel C Si Mn P

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