NON-TEMPERED STEEL SOFT NITRIDED COMPONENT

Abstract

 Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ' nitride of more than 20µm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C:0.30 to 0.45%, Si:0.1 to 0.5%, Mn:0.6 to 1.0%, Ti:0.005 to 0.1%, N:0.010 to 0.030%, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.
The non-heat treated steel may contain one or more elements of 0.001 to 0.1% of Nb, 0.01 to 1.0% of Mo:, 0.01 to 1.0% of Cu, 0.01 to 1.0% of Ni and 0.001 to 0.005%of B, 0.01 to 0.1°/of S and 0.0001 to 0.005% of Ca.
This invention provides machinery parts, based on this non-heat treated steel for soft-nitriding, which has further improved high fatigue strength and excellent bending straightening property after soft-nitrided even omitting thermal refining.

Description

[Technical Field]

[0001] The present invention relates to soft-nitrided machinery parts made of non-heat treated steel and, more specifically, relates to soft-nitrided machinery parts such as a crankshaft and a connecting rod for automobiles, industrial machinery, construction machinery and the like.

[Background Art]

[0002] Conventionally, machinery parts such as a crankshaft and a connecting rod for automobiles, industrial machinery, construction machinery and the like are manufactured by performing thermal refining (quenching, tempering, normalizing, annealing) after hot working by means of hot forging or the like. The thermal refining leads to a homogenous and fine microstructure. After the thermal refining, soft-nitriding is performed mainly for the purpose of enhancing fatigue resistance.

[0003] The soft-nitriding causes distortion, which impairs the dimensional accuracy of the parts, therefore, bending straightening is frequently performed after the soft-nitriding. Therefore, the parts after soft nitriding require an excellent bending straightening property in addition to high fatigue resistance.

[0004] The above-mentioned "excellent bending straightening property" means that the parts never crack up on the surface even to an extent of large bending displacement.

[0005] In the manufacture of machinery parts, it has been asked to omit the thermal refining in order to reduce manufacturing cost and save energy, and this requirement has been increasing in recent years.

[0006] However, if the thermal refining is omitted, a nonuniform microstructure formed at the time of hot working is apt to remain, and large grains that are coarsened during the heating of the steel materials before starting the hot working remain in the steel products. These result in the deterioration of the mechanical properties of the product. This is the reason why normalizing after the hot working is performed. If the normalizing is not performed after hot working, the grains remain as coarsened, or the resulting microstructure is not uniform with a mixture of partially hot-deformed microstructure. Therefore, those materials in which normalizing has been omitted cannot attain the desired fatigue strength even if soft-nitrided.

[0007] Although the parts after soft-nitriding are required to be excellent in bending straightening property as described above, omission of the thermal refining frequently results in a remarkably inferior bending straightening property of the parts after soft-nitriding due to the above-mentioned coarse-grained microstructure and/or nonuniform microstructure.

[0008] Therefore, it is desirable to develop machinery parts that can have both high fatigue resistance and excellent bending straightening property after soft-nitriding, even if the thermal refining is omitted for the purpose of cost reduction and saving energy.

[0009] Regarding the thermal refining, "normalizing", that is a typical thermal refining, will be explained as a representative case in the following. Some methods have been proposed so far in order to obtain a non-heat treated steel for soft nitriding that can form parts with high fatigue strength and excellent "bending straightening property" after nitriding even if the thermal refining is omitted. These are roughly classified into two methods described below.

[0010] 

(1) A method of avoiding and reducing the grain coarsening in the steel during hot forging, while keeping the steel in the same mixed microstructure of ferrite and pearlite as a heat treated steel (e.g., refer to the following Patent Documents 1 to 4).

[0011] 

(2) A method of making the microstructure into a bainite (e.g., refer to the following Patent Documents 5 to 9).

[0012] The following Patent Document 1 discloses "a nitrided steel, having contents of alloy elements of, by mass%, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, and the balance Fe and inevitable impurities, in which the microstructure after hot working is a ferrite-pearlite microstructure, the ferrite area fraction is 30% or more, the ferrite grain size number is not less than No. 5, and has the average dimension of pearlite is 50 µm or less". It is described that this steel is excellent in fatigue strength and bending straightening property after nitriding even if normalizing is omitted.

[0013] The following Patent Document 2 discloses "nitrided parts obtained by nitriding a steel, the steel comprising, by mass%, C: 0.15 to 0.40%, Si: not more than 0.50%, Mn: 0.20 to 1.50%, Cr: 0.05 to 0.50%, and the balance Fe and inevitable impurities, and the steel having a mixed microstructure of ferrite and pearlite as hot forged, in which the average dimension of grain of the ferrite is 50 µm or less, the average dimension of grain of the pearlite is 50 µm or less, the average hardening depth by the nitriding is 0.3 mm or more, and the fluctuation of the hardening depth is 0.1 mm or less". It is described that these parts are excellent in fatigue strength and bending straightening property after nitrided even if the normalizing after hot forging is omitted.

[0014] The following Patent Document 3 discloses "a steel product for soft nitriding, having a chemical composition comprising, by mass%, C: 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.3 to 1.0%, P: 0.05% or less, S: 0.005 to 0.10%, Cr: 0.3% or less, Al: 0.08% or less, Ti: 0.03% or less, N: 0.008 to 0.020%, Ca: 0.005% or less, Pb: 0.30% or less, Cu: 0.30% or less, Ni: 0.30% or less, Mo: 0.30% or less, V: 0.20% or less, Nb: 0.05% or less, with satisfaction of an inequality of 221C(%) + 99.5Mn(%) + 52.5Cr(%) - 304Ti(%) + 577N(%) + 25 ≥ 150, and the balance Fe and inevitable impurities, and the steel having a mixed microstructure of ferrite and pearlite whose ferrite fraction is 10% or more".

[0015] It is described in the following Patent Document 3 that without normalizing nitrided parts excellent in fatigue strength and bending straightening property can be obtained, when the fatigue strength, which is expressed in the form of a regression equation of alloy chemistries, is not less than a specified magnitude and the microstructure is composed of ferrite and pearlite with a ferrite fraction of 10% or more.

[0016] The following Patent Document 4 discloses "a steel for nitriding, having a chemical composition comprising, by weight%, C: 0.30 to 0.43%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.60%, P: 0.08% or less, S: 0.10% or less, sol. Al: 0.010% or less, Ti: 0.013% or less, Ca: 0.0030% or less, Pb: 0.20% or less, N: 0.010 to 0.030%, and the balance Fe and impurities, the content of Cr in the impurities being restricted to 0.10% or less and the content of V in the impurities being restricted to 0.01% or less".

[0017] It is described in the following Patent Document 4 that nitriding without prior normalizing can obtain a product excellent in fatigue strength and bending straightening property by making the hardness gradient in a nitrided layer moderate.

[0018] The following Patent Document 5 discloses "a steel for structural use with a high fatigue strength that have a chemical composition comprising C: 0.1 to 0.35%, Si: 0.05 to 0.35%, Mn: 0.6 to 1.50%, P: 0.01% or less, S: 0.015% or less, Cr: 1.1 to 2.0%, Mo: 0.5 to 1.0%, V: 0.03 to 0.13%, B: 0.0005 to 0.0030%, Ti: 0.01 to 0.04%, Al: 0.01 to 0.04%, and the balance Fe and inevitable impurities".

[0019] It is described in the following Patent Document 5 that Cr is effective for improving the hardenability and nitriding-hardening property and that V is effective for enhancing the fatigue strength by refining carbide precipitates. Cr and V improve the fatigue strength based on the precipitation hardening because the improved nitriding-hardening property by Cr is based on precipitation of the Cr nitrides. However, since a steel product once manufactured is reheated and cooled in order to obtain the bainite microstructure, this steel is contained in the category of heat-treated steel.

[0020] The following Patent Document 6 discloses "a non-heat treated steel for soft-nitriding, having a chemical composition comprising, by mass%, C: 0.1 to less than 0.3%, Si: 0.01 to 1.0%, Mn: 1.5 to 3.0%, Cr: 0.01 to 0.5%, Mo: 0.1 to 1.0%, acid-soluble Al: 0.01 to 0.045%, N: 0.005 to 0.025%, and the balance Fe and inevitable impurities", and the like.

[0021] It is described in the following Patent Document 6 that a steel having a bainite microstructure, which is obtained by air-cooling from a hot working temperature, has an excellent toughness and bending straightening property after it is subjected to soft-nitriding. The concentration of C is restricted to less than 0.3% so that the machinability is not impaired by an excessively increased hardness of bainite, and the concentration of Mn is prescribed to 1.5% or more for ensuring the hardenability of steel for generating the bainite. Further, the hardness of the nitrided layer is enhanced by precipitation hardening of Cr nitrides by adding 0.01 to 0.05% of Cr.
Namely, the reason for the improved bending straightening property due to the bainite microstructure is in the fact that a bainite microstructure has a higher toughness than that of a ferrite-pearlite microstructure when these two microstructures have the same hardness. As described above in the following Patent Document 6, the concentration of C is restricted to less than 0.3% so that the hardness of bainite is not excessively increased. However, a carbon concentration of less than 0.3% might adversely affect the wear resistance, which is also a very important factor for machinery parts such as a crankshaft and a connecting rod.

[0022] The following Patent Document 7 discloses "a steel for soft nitriding, having a chemical composition comprising, by weight %, C: 0.05 to 0.30%, Si: 1.20% or less, Mn: 0.60 to 1.30%, Cr: 0.70 to 1.50%, Al: 0.10% or less, N: 0.006 to 0.020%, V: 0.05 to 0.20%, Mo: 0 to 1.00%, B: 0 to 0.0050%, S: 0 to 0.060%, Pb: 0 to 0.20%, Ca: 0 to 0.010%, with satisfaction of an inequality of 0.60≤C+0.1Si+0.2Mn+0.25Cr+1.65V≤1.35 or an inequality of 0.60≤C+0.1Si+0.2Mn+0.25Cr+1.65V+0.55Mo+8B≤1.35, and the balance Fe and inevitable impurities, with a core part hardness of 200 to 300 Hv and a fully bainitic microstructure of a mixed microstructure of "ferrite + bainite" whose ferrite fraction is less than 80%, these being obtained by cooling after hot rolling or after hot forging, without post heat treatment".

[0023] The invention of the following Patent Document 7 adopts the idea of improving the fatigue strength by using precipitation hardening by Cr and V, similar to the following Patent Document 5. However, it might negatively affect the wear resistance because the concentration of C is restricted to less than 0.3%, similar to the following Patent Document 6.

[0024] The following Patent Document 8 discloses "a steel for soft nitriding, having a chemical composition comprising, by weight %, C: 0.15 to 0.40%, Si: 1.20% or less, Mn: 0.60 to 1.80%, C: 0.20 to 2.00%, Al: 0.02 to 0.10%, N: 0.006 to 0.020%, V: 0.05 to 0.20%, and the balance Fe and inevitable impurities, with satisfaction of inequalities of 0.60≤C + O.lSi + 0.2Mn + 0.25Cr + 1.65V ≤ 1.35 and 0.25Cr + 2V ≤ 0.85, with a core part hardness of 200 to 300 Hv and a mixed microstructure of "ferrite + pearlite" or a mixed microstructure "ferrite + pearlite (+ bainite)" whose bainite fraction is less than 20%, without heat treatment, by being cooled after hot rolling or after hot forging, and the steel having high surface hardness, large hardening depth, and low thermal distortion after further soft-nitrided".

[0025] The steel of the following Patent Document 8 is supposed to have an improved wear resistance because the concentration of C is 0.15 to 0.40%. However, the invention adopts the idea of improving the fatigue strength by precipitation hardening by Cr and V, similar to the invention of the following Patent Document 7.

[0026] The following Patent Document 9 discloses "non-heat treated nitrided forged parts having a chemical composition comprising C: 0.15 to 0.35%, Mn: 1.00 to 3.00%, Cr: 0 to 0.15%, V: 0 to 0.02%, Cu: 0.50 to 1.50%, Ni: not less than 0.4 times the content of Cu, and the sum of B, N and Ti is described by Bsol and Bsol: 0.0010 to 0.0030%, where Bsol is defined by an equation of Bsol = B - (11/14){N - (14/48)Ti}, and the balance Fe and inevitable impurities".

[0027]  It is described in the following Patent Document 9 that, for the steel for nitriding, a ferrite-oiented microstructure is the most favorable, and a single-phase microstructure of martensite or bainite is more desirable than a ferrite + pearlite mixed microstructure if the ferrite-oriented microstructure is difficult to obtain.
The idea of using precipitation hardening by Cu is alternatively adapted although the precipitation hardening by Cr and V is avoided. The concentration of Mn must be 1.0% or more for ensuring a bainite single-phase microstructure, which means that a non-heat treated steel of bainite single phase is recommended.
On the other hand, in relation to the soft-nitriding condition, a soft-nitriding method for shortening the time for forming a compound layer (refer to the following Patent Document 10), a soft-nitriding method for enhancing the corrosion resistance of a compound layer (refer to the following Patent Document 11), and a soft-nitriding method for enhancing the dent resistance (refer to the following Patent Document 12) are disclosed, while no soft-nitriding method for improving the fatigue strength or bending straightening property has been examined.

[0028] The following Patent Document 12, for example, discloses a soft-nitriding method for enhancing the wear resistance and the dent resistance of the machinery parts by further performing, after soft-nitriding, a heat treatment which comprises a reheating to an austenite temperature range followed by a rapid cooling, thereby turning a diffusion layer and a base metal into martensitic structure, and then performing tempering again. However, with respect to the soft-nitriding itself, it is only described that gas soft-nitriding is performed at a standard temperature range (570 to 580°C).

[0029] Patent Document 1: Japanese Patent Unexamined Publication No. H9-291339
Patent Document 2: Japanese Patent Unexamined Publication No. H9-324258
Patent Document 3: Japanese Patent Unexamined Publication No. H 9-324241
Patent Document 4: Japanese Patent Unexamined Publication No. H10-46287
Patent Document 5: Japanese Patent Unexamined Publication No. H5-65592
Patent Document 6: Japanese Patent Unexamined Publication No. 2000-309846
Patent Document 7: Japanese Patent Unexamined Publication No. H7-157842
Patent Document 8: Japanese Patent Unexamined Publication No. H 8-176733
Patent Document 9: Japanese Patent Unexamined Publication No. 2000-160287
Patent Document 10: Japanese Patent Unexamined Publication No. 2003-253420
Patent Document 11: Japanese Patent Unexamined Publication No. 2002-302756
Patent Document 12: Japanese Patent Unexamined Publication No. H11-269631

[Disclosure of the Invention]

[Problems to be solved by the Invention]

[0030] As described above, it is known that a non-heat treated steel for soft nitriding that forms parts, excellent in fatigue strength and bending straitening property after soft-nitriding, is obtained by exploiting a bainite microstructure. However, enhancing the fatigue strength by precipitation hardening by adding alloy elements causes deterioration of the bending straightening property. Namely, the problem of achieving the combination of high fatigue strength and excellent bending straightening property has not been yet solved.

[0031] In order to respond to the recent demand of a further increased strength of parts, non-heat treated steels for soft-nitrided parts that are excellent in bending straightening property with higher fatigue strength than ever before are requested. However, the above-mentioned conventional technique of "precipitation hardening and microstructural control into bainite" cannot necessarily meet such requirements.

[0032] From the viewpoint of such circumstances, some of the present inventors have completed a prior invention and filed an application in the Application Number of PCT/JP2004/012372. This prior invention is a non-heat treated steel for soft-nitriding, which can form parts having high fatigue strength and excellent bending straightening property even if it is soft-nitrided without thermal refining. This prior invention is related to "a non-heat treated steel for soft-nitriding, comprising, by mass %, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.1%, N: 0.015 to 0.030%, and the balance Fe and impurities, the steel having a mixed microstructure of bainite and ferrite or a mixed microstructure of bainite, ferrite and pearlite with the bainite fraction in the mixed microstructure of 5 to 90%". This steel may further contain one or more element(s) selected from Nb: 0.003 to 0.1%, Mo: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, B: 0.001 to 0.005%, S: 0.01 to 0.1%, and Ca: 0.0001 to 0.005%.

[0033] It is the objective of the present invention to provide machinery parts, based on this non-heat treated steel for soft-nitriding, which have further improved high fatigue strength and excellent bending straightening property after soft-nitrided even omitting thermal refining.

[0034] The present inventors have continuously investigated these issues after the above application. As a result, it has been found that the fatigue strength and bending straightening property of the parts after soft-nitriding can be further improved by adjusting the cooling rate in the soft-nitriding, and the following knowledge was obtained as a result of further examinations.

[0035] 

(a) Among the precipitates observed in the area of a diffusion layer after soft-nitriding, iron nitrides are mainly precipitated in the cooling process of the soft-nitriding, and the preparation behavior strongly depends on the cooling condition.

[0036] 

(b) Main iron nitrides formed in the diffusion layer are rod-shaped γ'-Fe₄N and disk-shaped α"-Fe₁₆N₂.

(c) Whether or not these iron nitrides precipitate in the diffusion layer gives a strong influence on the fatigue strength and bending straightening property of soft-nitrided machinery parts. Particularly, precipitation of γ'-Fe₄N, that is a rod-shaped γ' nitride, causes serious deterioration of fatigue strength.

(d) Dissolving the nitrogen in ferrite grains in the diffusion layer leads the parts after soft-nitriding to an excellent fatigue strength and bending strengthening property. In other words, the parts are strengthened by means of introducing the nitrogen into ferrite grains as supersaturated solid solution with nitrogen as much as possible, avoiding iron nitrides formation.

[0037] The present invention has been completed based on the above-mentioned knowledge.

[0038] The present invention involves the following soft-nitrided parts made of non-heat treated steel.

[0039] 

(1) Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ' nitride of more than 20µm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C:0.30 to 0.45%, Si:0.1 to 0.5%, Mn:0.6 to 1.0%, Ti:0.005 to 0.1%, N:0.010 to 0.030%, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.

[0040] (2) Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ' nitride of more than 20µm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C:0.30 to 0.45%, Si:0.1 to 0.5%, Mn:0.6 to 1.0%, Ti:0.005 to 0.1%, N:0.010 to 0.030%, and either or both of one or more elements selected from the first group mentioned below and one or more elements selected from the second group mentioned below, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.
The first group: 0.001 to 0.1% of Nb, 0.01 to 1.0% of Mo, 0.01 to 1.0% of Cu, 0.01 to 1.0% of Ni and 0.001 to 0.005%of B.
The second group: 0.01 to 0.1%of S and 0.0001 to 0.005% of Ca.
In this context, "the diffusion layer" means a region that recognizes a diffusion of nitrogen and/or carbon near the surface of nitrided parts except the topmost compound layer, as defined by JIS (Japan Industrial Standard) G0562. A " γ' nitride" means a γ'-Fe₄N, as stated above.

[Effect of the Invention]

[0041]  According to the present invention, high-strength soft-nitrided parts excellent in fatigue strength and bending straightening property can be obtained from a non-heat treated steel. Accordingly, the cost manufacturing parts can be reduced.

[Best Mode for Carrying out the Invention]

[0042] Each essential requirement of the present invention will be described. In the following description, "%" represents "% by mass", unless otherwise specified.

(A) Chemical Composition

[0043] 

C: 0.30 to 0.45%

C is an essential element for obtaining a mixed microstructure of "bainite + ferrite" or "bainite + ferrite + pearlite". In order to stabilize austenite and ensure the wear resistance of the material, a content of 0.30% or more is needed. On the other hand, if the content exceeds 0.45%, the hardenability excessively increases, easily causing harmful martensite. Therefore, the proper range of the C content is 0.30 to 0.45%.

[0044] 

Si: 0.1 to 0.5%

Si is added in a steel making process as a deoxidizer. A content of 0.1% or more is needed since it is also effective for solid-solution strengthening of ferrite. On the other hand, a Si content exceeding 0.5% causes an increase in hot deformation resistance of steel or a deterioration of the toughness or machinability. Therefore, the proper range of the Si content is 0.1 to 0.5%.

[0045] 

Mn: 0.6 to 1.0%

Mn is added in steel making process as deoxidizer, similar to Si. It is also an essential element for stabilizing austenite in order to obtain the mixed microstructure of "bainite + ferrite" or the mixed microstructure of "bainite + ferrite + pearlite". Further, Mn combines with S in steel to form MnS, which effectively improves the machinability of the steel.

[0046] In the above mixed microstructure, the bainite fraction must be 5% or more. In order to ensure the hardenability in which bainite of this fraction is formed, a content of Mn of 0.6% or more is needed. On the other hand, if the content of Mn exceeds 1.0%, the hardenability excessively increases, easily causing harmful martensite. Therefore, the proper range of the Mn content is 0.6 to 1.0%.

[0047] 

Ti: 0.005 to 0.1%

Ti is an essential element for forming pinning particles for suppressing the grain-coarsening during hot working. The pinning particles include nitrides, carbides and carbonitrides of Ti, and a content of 0.005% or more is needed for forming the pinning particles at a sufficient distribution density. On the other hand, the content of Ti must be restricted to 0.1% or less in order to prevent the complete consumption of N in the steel which forms Fe nitrides and contributes to an increase in base metal strength. For the above reason, the proper range of the Ti content is 0.005 to 0.1% and, more desirably, 0.01 to 0.05%.

[0048] 

N: 0.010 to 0.030%

N is added for the purposes, of stabilizing austenite in order to obtain the mixed microstructure of "bainite + ferrite" or the mixed microstructure of "bainite + ferrite + pearlite", of forming the pinning particles that suppress the grain-coarsening, and also of providing dissolved nitrogen that strengthens the base metal through the solid-solution strengthening. Considering the amount consumed as pinning particles, a content of 0.010% or more is needed. On the other hand, if the content of N exceeds 0.030%, bubble defects are generated in an ingot, which may impair the material quality. Therefore, the proper range of the N content is 0.010 to 0.030%, desirably 0.015 to 0.030%, and more desirably, 0.015 to 0.025%.

[0049] One of the non-heat treated steels for soft nitriding that are used as the steel materials of the soft-nitrided parts of the present invention contains the balance Fe and impurities in addition to the above-mentioned elements.

[0050] Another one of non-heat treated steels for soft nitriding which are used as the steel materials of the soft-nitrided parts of the present invention contains, in addition to the above-mentioned elements, either or both of one or more elements selected from the above-mentioned first group and one or more element selected from the above-mentioned second group, and the balance Fe and impurities.

[0051] The elements, Nb, Mo, Cu, Ni and B, belonging to the first group have the common effect of enhancing the strength of the steels of the present invention. The respective effects and the reasons for limiting the contents are as follows.

[0052] 

Nb: 0.001 to 0.1%

Nb is an element that can be used to form the pinning particles for suppressing the grain-coarsening during hot working. Nb is precipitated as fine carbonitrides during cooling after the end of hot working, effectively enhancing the strength of the base metal. In order to obtain such an effect, a content of 0.001% or more is needed. On the other hand, if the content exceeds 0.1%, not only the effect is saturated, but also coarse undissolved carbonitrides tend to form in steel making process, which deteriorates the quality of the steel product. Therefore, when Nb is added, the content is preferably set to 0.001 to 0.1%. The content is desirably 0.003 to 0.1%, more desirably, 0.005 to 0.1%, and most desirably to 0.005 to 0.05%.

[0053] 

Mo: 0.01 to 1.0%

Mo is an element that enhances the hardenability and strength of steel, and is also effective for improving the toughness. Addition of Mo facilitates obtaining the mixed microstructure of "bainite + ferrite" or the mixed microstructure of "bainite + ferrite + pearlite". In order to obtain such effects, a content of 0.01% or more is needed. On the other hand, if the content of Mo exceeds 1.0%, the formation of martensite is promoted because of the excessive hardenability, resulting in deterioration of the bending straightening property or toughness after soft-nitriding. Therefore, when Mo is added, the content is preferably set to 0.01 to 1.0%. A more desirable content is 0.05 to 0.6%.

[0054] 

Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%

Cu can be added for solid solution strengthening and for increasing the bainite fraction through the austenite stabilization. For these effects, Cu is included by a content of 0.01% or more.

[0055] Although Cu and Ni do not form carbonitrides effective for precipitation strengthening Cu itself can contribute to precipitation strengthening by precipitating in ferrites during aging. When a general soft-nitriding treatment with a temperature of about 580°C and treatment time of about several hours is regarded as an aging treatment, a content of Cu of 1.0% or more is needed for causing sufficient age hardening by Cu. However, in the soft-nitrided parts of the present invention, it is not necessary to expect the age hardening effect of Cu at the time of soft-nitriding. Further, since the melting point of Cu is as low as 1085°C, Cu remains in a liquid phase in the course of solidification in the steel making process for an extended period of time, and thus segregates to the grain boundary, inducing hot cracking. In order to eliminate this undesirable effect, the upper limit of the Cu content is set to 1.0% in the steel of the present invention. When Cu is added in a large quantity, Ni is desirably added along with Cu, in order to prevent the above-mentioned bad effect.

[0056] Ni is an austenite-stabilizing element, similar to Cu, and effective for ensuring solid solution strengthening and a desirable bainite fraction. Therefore, Ni is desirably included in a content of 0.01% or more. On the other hand, since the inclusion of a content exceeding 1.0% only results in saturation of the effect and incurs unnecessary high material cost, the upper limit is set to 1.0%. In the addition of Ni together with Cu, the Ni content is desirably set to not less than the half of the Cu content in order to ensure the effect of preventing the hot cracking.

[0057] 

B: 0.001 to 0.005%

B enhances the hardenability of steel and promotes the formation of the mixed microstructure of "bainite + ferrite" or the mixed microstructure of "bainite + ferrite + pearlite". The effect appears clearly when the B content is 0.001% or more. On the other hand, if the content of B exceeds 0.005%, the toughness of the steel is impaired. Therefore, when B is added, the content is preferably set to 0.001 to 0.005%.

[0058] The elements, S and Ca, of the second group improve the machinability of non-heat treated steels for soft-nitriding used as steel materials of the soft-nitrided parts of the present invention. The reason for limiting the respective contents is as follows.

[0059] 

S: 0.01 to 0.1%, Ca: 0.0001 to 0.005%

Both S and Ca are elements that improve the machinability of steel products. Since their addition enables further improvement in machinability, either or both of them are added if necessary. However, since an excessive addition causes segregation defects in steel billets or deterioration of the hot workability, the S content is favorably 0.01 to 0.1%, and the Ca content is favorably 0.0001 to 0.005%. A more favorable lower limit of Ca is 0.001%.

[0060] Elements other than those described above are not intentionally added because they are regarded as impurities in the non-heat treated steels for soft-nitriding, which is used for the soft-nitrided parts of the present invention. However, the allowable quantities of the impurities will be described below in order to prevent an unnecessary cost increase in steel making process.

[0061] P is preferably set to 0.05% or less since it segregates to the grain boundary to cause intercrystalline embrittlement cracking.

[0062] Al is generally added in steel making process as deoxidizer. Al remains as alumina particles in steel and/or is combined with N to form AlN. The alumina is an oxide-based inclusion with high hardness, which shortens the tool life used for machining. The AlN remarkably increases the hardness of a surface layer in soft-nitriding by their precipitation in the vicinity of the surface or promoting the growth of the compound layer at the surface, which results in the deteriorating bending straightening property. Further, the AlN cannot be expected to have the function as the pinning particles, since they dissolve at a hot working temperature, and is hardly useful for the grain-refinement. Therefore, a lower content of Al is more preferable. However, since the excessive removal of the Al from the steel causes an increased cost in the deoxidation process, the Al content is preferably set at 0.05% or less, which never disturbs the bending straightening property of the soft-nitrided parts of the present invention.

[0063] Neither Cr nor V is added to the steels of the present invention. These elements are impurities, and smaller contents are more preferable. Because Cr and V remarkably increase the hardness of the near-surface layer of the steel by precipitation of nitrides, which impairs the bending straightening property, as described above. From the view point of the purity of steel materials and the refining cost in a steel manufacturing, a Cr content of up to 0.15% is allowable as impurities and a V content of up to 0.02%is allowable as impurities, respectively. Cr is more desirably set to 0.1% or less.

(B) Microstructure of steel materials for the soft-nitrided parts according to the present invention

[0064] The non-heat treated steels for soft-nitriding for the soft-nitrided parts according to the present invention has a mixed microstructure of bainite and ferrite or a mixed microstructure of bainite, ferrite and pearlite. The bainite fraction in these mixed microstructures is 5 to 90%.

[0065] As described before, a martensite microstructure can be avoided by exploiting a bainite transformation, and leads to a finer microstructure than that composed of coarse pearlite colonies. The bainite microstructure is composed of bamboo leaf-like ferrites (bainitic ferrites) as shown in Fig. 1 and carbides. The bainitic ferrites, which are dispersed within the former austenite grain, are smaller than the pro-eutectoid ferrites (polygonal ferrites) developed from the former austenite grain boundaries. Namely, this bainite has "a microstructure that has relatively fine ferrites (bainitic ferrites) dispersed in a pearlite colony although the shape is bamboo leaf-like", and the pearlite colony with these bainitic ferrites dispersed therein has a relatively irregular lamellar microstructure, compared with a perfect pearlite colony free from these bainitic ferrites.

[0066] Fig. 2 is a SEM image of a former austenite grain in which the bainitic ferrites are dispersed. As is apparent from this photograph, a ferrite/cementite lamellar in the pearlite colony does not have a well-ordered microstructure, in which irregularities are observed in many positions. This lamellar microstructure has lower strength than the one in which the former austenite grain is entirely transformed to pearlite, but it has a better crack propagation resistance than the coarse pearlite colony, because bending of crack path and/or plastic deformation at a crack tip occur in the bainitic ferrite.

[0067] In other words, the bainite fraction of not less than 5% by area fraction can lead to maintaining the high crack propagation resistance due to a mixed microstructure with bainite although it might slightly be coarse-grained. Although the bainite fraction of 100% can be permitted, it might not be obtained in reality because a mixed microstructure with martensite must develop when the bainite fraction exceeds 90%. Since martensite deteriorates the bending strengthening property and impairs the machinability, a mixed microstructure with an excessive martensite is not preferable. Therefore, the bainite fraction in the mixed microstructure is 5 to 90%. A more desirable bainite fraction is 10 to 80%. The microstructure other than bainite is essentially ferrite or ferrite and pearlite.

(C) Method for manufacturing the steel materials for the soft-nitrided parts according to the present invention

[0068] The microstructure of non-heat treated steels for soft-nitriding that are used for the soft-nitrided parts according to the present invention can be obtained, for example, by the following method.

[0069] The steel materials for hot forging that have a defined chemical composition are prepared as any one of the followings: a billet obtained by blooming and rolling an ingot, a billet obtained by blooming and rolling a continuous cast material, and bar steels obtained by hot rolling these billets. The heating temperature of the steel materials for hot forging is set to 1100 to 1250°C. Regarding the cooling after hot forging, an air-cooling in the atmosphere is performed or forced air-cooling using a fan is performed. The steel materials may be cooled rapidly up to the vicinity of an eutectoid transformation temperature and then slowly cooled in the temperature range of 700 to 500°C, or may be immediately cooled to about 500 to 300°C just after hot forging, and held at this temperature, in order to promote the bainite transformation. The adjustment of the cooling rate can be performed by making continuous cooling transformation diagrams (CTT curve) in advance, determining the cooling rate range passing the bainite transformation area, and adjusting the cooling rate at the production to the pre-determined cooling rate range.

(D) Microstructure of diffusion layer in the soft-nitrided parts according to the present invention

[0070] A rod-shaped γ' nitride in ferrite grains of the diffusion layer of the soft-nitrided parts of the present invention is of 20µm or less in the longitudinal axis.

[0071] As described above, if supersaturated nitrogen dissolved in the diffusion layer precipitates as a γ' nitride, or the dissolved nitrogen is further reduced by extensive growth of the precipitated γ' nitride, the strength of the ferrite grain decreases, causing a reduction in fatigue strength. Further, since the γ' nitride has a rod-shaped shape and grows so as to extend from the ferrite grain boundary into the ferrite grain interior, the γ' nitride is distributed in a state where it transversely crosses the inside of the ferrite grain when extensively grown. The strength is reduced in the vicinity of the γ' nitride because of the remarkably reduced dissolved nitrogen concentration therein. Therefore, if the long rod-shaped γ' nitride is dispersed transversely in the ferrite grain cracks penetrating in the ferrite grain easily propagates along the γ' nitride/ferrite interface, deteriorating the crack propagation resistance. Namely, the precipitation of the γ' nitride facilitates a fatigue rupture since the crack propagation resistance is locally decreased in the vicinity of the γ' nitride in addition to the reduction of its own average strength of the ferrite grain by the precipitation of γ' nitride. Therefore, the precipitation of γ' nitride and its growth are to be suppressed.

[0072] The reason for restricting the longitudinal size of the rod-shaped γ' nitride to 20µm or less is explained as follows. The ferrite grains in non-heat treated steels that are steel materials for the soft-nitrided parts of the present invention have grain sizes of about 10 to 50µm. Therefore, the length of each rod-shaped γ' nitride must be controlled to not more than the half of the ferrite grain size in order to avoid the coalescence of the rod-shaped γ' nitrides that grow from the opposite ferrite grain boundaries into the interior, otherwise the coalescent γ' nitrides behave as if one huge rod-shaped γ' nitride. Therefore, the longitudinal size of each rod-shaped γ' nitride inside the ferrite grain at the diffusion layer is restricted to 20µm or less, desirably to 10µm or less, and more desirably to 5µm or less.

(E) Means for obtaining the diffusion layer of the soft-nitrided parts according to the present invention

[0073] Gas soft-nitriding, salt-bath soft-nitriding (Tufftride treatment), ion nitriding and the like can be adapted for the soft-nitriding. Each method can homogenously form a compound layer (nitride layer) about 20µm thick on the surface of a product and a diffusion layer just underneath. If any soft-nitriding is adapted, it is necessary to control the longitudinal size of the γ' nitrides to 20µm or less by suppressing the precipitation and growth of γ' nitrides. The cooling process from the holding temperature of soft-nitriding to room temperature needs to be adjusted. The cooling process from the holding temperature of soft-nitriding will be illustrated by an example of a gas soft-nitriding.

[0074] In order to obtain machinery parts by gas soft-nitriding, for example, treatment of several tens of minutes intervals to several hours is performed at a holding temperature of 550 to 620°C in a 1:1 mixed atmosphere of RX gas and ammonia gas. If the holding temperature is excessively low, a sufficient hardening effect cannot be obtained because of slow intrusion and diffusion of nitrogen into the material steel, as well as slow growth of the compound layer on the surface. On the other hand, if the holding temperature is excessively high, the dimensional change (distortion) of parts in the cooling process becomes a problem. Therefore, the holding temperature is preferably set to 550 to 620°C. A more desirable holding temperature is within the range between 580 and 600°C. The thickness of the compound layer on the surface and the quantity of nitrogen that diffusively intrudes into the steel are determined according to the holding time (treatment time) at the holding temperature. From the viewpoint of industrial production efficiency and ensuring the desired effect of improving the fatigue strength, the holding time is preferably set at 30 minutes to 3 hours, more preferably set at 1 to 2 hours.

[0075] During the isothermal holding of soft nitriding, the nitrogen that diffusively intrudes forms only the compound layer with Fe on the surface, and then it dissolves in Fe matrix in the diffusion layer, without causing precipitation of Fe nitrides. If the cooling rate is decreased in the cooling process after isothermal holding, the nitrogen in the diffusion layer cannot be dissolved in the matrix, resulting in precipitation and growth of the γ' nitrides. If the cooling rate is increased enough, the nitrogen in the diffusion layer is kept in supersaturation in the matrix, resulting in the suppression of precipitation and growth of the γ' nitrides. However, even an increased cooling rate is applied, the precipitation of an α" nitride, that is a metastable phase, occurs when the parts are held at 100 to 200°C for a long time after cooling. And it may cause the transformation of the a" nitride to γ' nitride when the parts are held at that temperature for a longer period of time after cooling. The precipitation of such nitrides causes deterioration of fatigue strength, particularly, the γ' nitride has the salient effect of deteriorating the fatigue resistance.

[0076]  In the present invention, in order to suppress the precipitation of γ' nitrides, it is effective to increase the cooling rate to 1.0°C /sec or more in the temperature range from the holding temperature to 200°C, at which temperature the precipitation of γ' nitrides does not occur. The cooling rate is more desirably set to 1.5°C /sec or more. Further, when oil quenching is adopted, it is effective to ensure a high fatigue strength by being careful that the parts are not to be exposed in the temperature range between 100 and 200°C for a long time (more than 30 minutes) during the cooling process after soft-nitriding. For this, the followings are effective: one is to use a big oil tank having a large heat capacity enough to ensure sufficient heat extraction of treated parts, with an oil temperature set to 100°C or lower, the other is to reduce the number of parts to be treated at once.

[0077] In general, oil cooling (oil quenching) that has a smaller cooling rate than water cooling is used for cooling from the holding temperature to room temperature in the gas soft-nitriding in order to reduce the thermal distortion due to quenching. Further, the cooling condition is adjusted by changing the oil temperature, or using various heat treatment oils differed in properties. In the industrial process of gas soft-nitriding, parts to be nitrided are not directly oil-quenched into the oil tank from the treatment atmosphere of RX gas and ammonia gas, but is first transferred from a heating furnace filled with the treatment atmosphere to a space filled with an inert gas, and then oil-quenched into the oil tank.

[0078] In such a cooling process in the gas soft-nitriding, for example when the parts are nitrided at 580°C in the treatment atmosphere, and then moved to another space, and successively oil-quenched in an oil tank held at 100°C, precipitations of γ' nitrides and α" nitrides are often observed in the diffusion layer. The reason for this is described as follows.

[0079] 

1) In the stage where the parts ready to be cooled are transferred to another space prior to oil-quenching, the temperature of the parts is reduced from 580°C to about 400°C. Since the cooling rate at this time is small, the precipitation of γ' nitrides occurs.

[0080] 

2) Immersing the parts in an oil tank of not lower than 100°C for an excessive amount of time causes the precipitation of metastable α" nitrides and further causes the precipitation of the γ' nitrides due to the transformation of α" nitrides as mentioned above.

[0081] Accordingly, in case of such a cooling process, the cooling rate in the stage where the parts are carried to another space prior to oil-quenching must be increased, as well as the long-time immersion of the nitrided parts in a high-temperature oil tank must be avoided.

[Example 1]

[0082] The present invention will be described in more detail by a working example.

[0083] 180kg of steel, having a chemical composition shown in Table 1, was melted in a vacuum and formed to an ingot. The steel ingot was heated to 1200°C and hot-forged into round bars, in diameter of 50mm, while keeping the temperature not lower than 1000°C. An air cooling in the atmosphere was performed after hot forging. Test pieces were machined from the round bars for microstructure observation for a bending test using a stepped round bar, and a plane bending fatigue test.

[0084] 

[0085] Part of the test pieces for microstructure observation was sectioned, and the as-hot forged microstructure was observed with an optical microscope with the magnification of 200 to measure a bainite fraction by area. An area defined to be bainite comprised bamboo leaf-like bainitic ferrites that were surrounded by a continuous closed curve, and the bainite fraction was calculated from the percentage of the bainite area to the whole visual field area. The bainite fraction of each steel specimen is also shown in Table 1. The steel of No. 7 had a microstructure of ferrite and pearlite, and no bainite was observed. The steel of No. 9 had a bainite fraction exceeding 90%, in which martensite was formed. The remaining part of the test piece for microstructure observation was soft-nitrided together with the test piece for plane bending fatigue test, and the microstructure of the diffusion layer was observed by scanning electron microscopy (SEM) to examine the size of γ' nitrides. The size of γ' nitrides is defined by a longitudinal size of the longest one of the γ' nitrides observed in 10 sheets of photographs that were taken with the magnification of 1000.

[0086] The stepped round bar specimen for the bending test has a 10mm-wide stepped portion in the center part with a diameter larger than that at the both ends, in which the diameter of the center part is 15 mm, the diameter of the mother body part is 10 mm, and the stepped portion has a corner R having a curvature radius of 2 mm. After the test piece of stepped round bar was soft-nitrided, a strain gauge is attached to the cured corner of the stepped portion, and a bending straightening test was performed in a manner similar to a three-point bending. The bending straightening property was evaluated by a push-in stroke at which a disconnection of the strain gauge occurs in application of a load to the center portion of the bar. A satisfactory bending straightening property was defined the one which did not cause the disconnection of strain gauge up to a push-in stroke of 3 mm.

[0087] The test piece for the plane bending fatigue test has a shape of a cylindrical body 44mm in diameter having a tapered neck part (neck part diameter of 20 mm). After this test piece was soft-nitrided, a plane bending fatigue test was carried out by fixing the heat side of the test piece and applying repetitive load to the opposite end part.

[0088] Gas soft-nitriding in an atmosphere of RX gas: ammonia gas=1:1 was used for soft-nitriding. The isothermal holding temperature was 600°C, and the holding time was 2 hours. The test piece that finished isothermal holding was once carried to another chamber which was separated from a nitriding chamber by a shutter and filled with nitrogen atmosphere, and then put into an oil tank installed to a lower part of the chamber of nitrogen atmosphere and oil-quenched. At this time, the degree of precipitation and growth of γ' nitrides was changed by varying the time before the test piece was put into the oil tank after carried to the chamber of nitrogen atmosphere. The oil temperature of the oil tank was controlled to a predetermined temperature within the range between 80 and 150°C, and the holding time of the test piece in the oil tank after oil-quenching was set to a predetermined time within the range between 10 and 90 minutes. The cooling rate in the cooling process was separately measured by a Pt-Rh thermocouple spot-welded to the surface of the test piece for the plane bending fatigue test in a state where the inside of the furnace was entirely filled with the nitrogen atmosphere, since the atmosphere of actual gas soft-nitriding (RX gas: ammonia gas) gave serious damage to the thermocouple. In the measurement, the care was taken to measure the variation of the temperature and the cooling rate during cooling before the test piece is put into the oil tank after it is carried from the nitriding chamber to the other chamber.

[0089] The size of γ' nitrides, the fatigue strength and the bending straightening property of each steel specimen are shown in Table 2. The remarks illustrate the average cooling rate in the cooling process from the holding temperature in gas soft-nitriding and the holding time at not less than 100°C including the time kept in the oil tank after oil-quenching.

[0090] 

[0091] As is apparent from Table 2, the present invention provides a satisfactory bending straightening property (bending stroke of 3 mm or more) and a high fatigue strength of not less than 550Mpa, which is classified to the category of high strength for a normalizing-free type.

[0092] On the other hand, in a case that the size of γ' nitrides exceeds 20µm due to a low cooling rate in the gas soft-nitriding, the fatigue strength was consequently reduced. A typical microstructure of the diffusion layer where γ' nitrides are dispersed is shown in Fig. 3. Those shown by arrows in the picture are particularly coarse γ' nitrides.

[0093] If the chemical composition or the microstructure of the non-heat treated steels was deviated from the present invention, the fatigue strength was reduced or the bending straightening property was inferior even if the size of γ' nitrides was not larger than 20µm.

[Industrial Applicability]

[0094] According to the present invention, soft-nitrided steel parts excellent in fatigue strength and bending straightening property can be obtained by using non-heat treated steels. Therefore, the costs of manufacturing parts can be reduced.

[Brief Description of the Drawings]

[0095] 

[Fig. 1] A typical microstructure image of a mixed microstructure of "bainite + ferrite + pearlite" of non-heat treated steels for soft nitriding, which are used for steel materials of soft-nitrided parts according to the present invention;

[Fig. 2] A SEM image of a former austenite grain in which bainitic ferrites are dispersed; and

[Fig. 3] A microstructure image of a diffusion layer where coarse γ' nitrides (shown by arrows) are dispersed.

Claims

1. Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ' nitride of more than 20µm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C:0.30 to 0.45%, Si:0.1 to 0.5%, Mn:0.6 to 1.0%, Ti:0.005 to 0.1%, N:0.010 to 0.030%, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.

2. Soft-nitrided parts wherein a ferrite grain in the diffusion layer does not have a γ' nitride of more than 20µm in the longitudinal axis, which is made of a non-heat treated steel that has a chemical composition of, by mass %, C:0.30 to 0.45%, Si:0.1 to 0.5%, Mn:0.6 to 1.0%, Ti:0.005 to 0.1%, N:0.010 to 0.030%, , and either or both of one or more elements selected from the first group mentioned below and one or more elements selected from the second group mentioned below, and the balance Fe and impurities, and that has a mixed microstructure of bainite and ferrite whose bainite fraction is 5 to 90% or a mixed microstructure of bainite, ferrite and pearlite whose bainite fraction is 5 to 90%.
The first group: 0.001 to 0.1% of Nb, 0.01 to 1.0% of Mo:, 0.01 to 1.0% of Cu, 0.01 to 1.0% of Ni and 0.001 to 0.005% of B.
The second group: 0.01 to 0.1% of S and 0.0001 to 0.005% of Ca.

Drawing