(19)
(11)EP 3 348 664 A1

(12)EUROPEAN PATENT APPLICATION
published in accordance with Art. 153(4) EPC

(43)Date of publication:
18.07.2018 Bulletin 2018/29

(21)Application number: 16844470.1

(22)Date of filing:  08.09.2016
(51)International Patent Classification (IPC): 
C23C 8/26(2006.01)
C21D 9/30(2006.01)
C22C 38/60(2006.01)
C21D 9/00(2006.01)
C22C 38/00(2006.01)
(86)International application number:
PCT/JP2016/076529
(87)International publication number:
WO 2017/043609 (16.03.2017 Gazette  2017/11)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
MA MD

(30)Priority: 08.09.2015 JP 2015176498

(71)Applicant: Nippon Steel & Sumitomo Metal Corporation
Tokyo 100-8071 (JP)

(72)Inventors:
  • UMEHARA, Takahide
    Tokyo 100-8071 (JP)
  • YUYA, Masato
    Tokyo 100-8071 (JP)
  • DAITO, Yoshihiro
    Tokyo 100-8071 (JP)

(74)Representative: Vossius & Partner Patentanwälte Rechtsanwälte mbB 
Siebertstrasse 3
81675 München
81675 München (DE)

  


(54)NITRIDED STEEL COMPONENT AND MANUFACTURING METHOD THEREOF


(57) A nitrided steel part excellent in bending straightening ability and bending fatigue characteristic enabling reduction of size and decrease of weight of parts or enabling demand for high load capacities to be met, using as a material a steel material containing, by mass%, C: 0.2 to 0.6%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less, S: 0.003 to 0.05%, Cr: 0.05 to 0.5%, Al: 0.01 to 0.05%, and N: 0.003 to 0.025%, and having a balance of Fe and impurities, having formed on the steel surface a compound layer of a thickness 3 µm or less comprising iron, nitrogen, and carbon and a hardened layer formed below the compound layer, and having an effective hardened layer depth of 160 to 410 µm.




Description

TECHNICAL FIELD



[0001] The present invention relates to a nitrided steel part, more particularly a crankshaft or other nitrided steel part excellent in bending straightening ability and bending fatigue characteristic, and a method of production of the same.

BACKGROUND ART



[0002] Steel parts used in automobiles and various industrial machinery etc. are improved in fatigue strength, wear resistance, seizing resistance, and other mechanical properties by carburizing hardening, high-frequency hardening, nitriding, soft nitriding, and other surface hardening heat treatment.

[0003] Nitriding and soft nitriding are performed in the ferrite region of the A1 point or less. During treatment, there is no phase transformation, so it is possible to reduce the heat treatment strain. For this reason, nitriding and soft nitriding are often used for parts requiring high dimensional precision and large sized parts. For example, they are applied to the gears used for transmission parts in automobiles and the crankshafts used for engines.

[0004] Nitriding is a method of treatment diffusing nitrogen into the surface of a steel material. For the medium used for the nitriding, there are a gas, salt bath, plasma, etc. For the transmission parts of an automobile, gas nitriding is mainly being used since it is excellent in productivity. Due to gas nitriding, the surface of the steel material is formed with a compound layer of a thickness of 10 µm or more. Furthermore, the surface layer of a steel material at the lower side of the compound layer is formed with a nitrogen diffused layer forming a hardened layer. The compound layer is mainly comprised of Fe2-3N and Fe4N. The hardness of the compound layer is extremely high compared with the steel of the base material. For this reason, the compound layer improves the wear resistance and pitting resistance of a steel part in the initial stage of use.

[0005] However, a compound layer is low in toughness and low in deformability, so sometimes the compound layer and the base layer peel apart at their interface during use and the strength of the part falls. For this reason, it is difficult to use a gas nitrided part as a part subjected to impact stress and large bending stress.

[0006] Therefore, for use as a part subjected to impact stress and large bending stress, reduction of the thickness of the compound layer and, furthermore, elimination of the compound layer are sought. In this regard, it is known that the thickness of the compound layer can be controlled by the treatment temperature of the nitriding and the nitriding potential KN found from the NH3 partial pressure and H2 partial pressure by the following formula:



[0007] If lowering the nitriding potential KN, it is also possible to make the compound layer thinner and even eliminate the compound layer. However, if lowering the nitriding potential KN, it becomes hard for nitrogen to diffuse into the steel. In this case, the hardness of the hardened layer becomes lower and the depth becomes shallower. As a result, the nitrided part falls in fatigue strength, wear resistance, and seizing resistance. To deal with such a drop in performance, there is the method of mechanically polishing or shot blasting etc. the nitride part after gas nitriding to remove the compound layer. However, with this method, the production costs become higher.

[0008] PLT 1 proposes the method of dealing with such a problem by controlling the atmosphere of the gas nitriding by a nitriding parameter KN=(NH3 partial pressure)/[(H2 partial pressure)1/2] different from the nitriding potential and reducing the variation in depth of the hardened layer.

[0009] PLT 2 proposes a gas nitriding method enabling formation of a hardened layer (nitrided layer) without forming a compound layer. The method of PLT 2 first removes the oxide film of a part by fluoride treatment then nitrides the part. A non-nitriding material is necessary as a fixture for placing the treated part in a treatment furnace.

[0010] However, the nitriding parameter proposed in PLT 1 may be useful for control of the depth of the hardened layer, but does not improve the functions of a part.

[0011] As proposed in PLT 2, in the case of the method of preparing a non-nitriding fixture and first performing fluoride treatment, the problems arise of the selection of the fixture and the increase in the number of work steps.

CITATION LIST


PATENT LITERATURE



[0012] 

PLT 1: Japanese Patent Publication No. 2006-28588A

PLT 2: Japanese Patent Publication No. 2007-31759A


SUMMARY OF INVENTION


TECHNICAL PROBLEM



[0013] An object of the present invention is to provide a nitrided steel part excellent in bending straightening ability and bending fatigue characteristic solving the two simultaneously difficult to solve problems of reduction of the thickness of a low toughness and low deformability compound layer and increase of the depth of the hardened layer and able to answer the demands for reduction of the size and decrease of the weight of a part or a higher load capacity and to provide a nitriding method of the same.

SOLUTION TO PROBLEM



[0014] The inventors studied the method of making the compound layer formed on the surface of the steel material by nitriding thinner and obtaining a deep hardened layer. Furthermore, they simultaneously studied methods of keeping the nitrogen from forming a gas and creating voids near the surface of a steel material at the time of nitriding (in particular, at the time of treatment by a high KN value). In addition, they investigated the relationship between the nitriding conditions and the bending straightening ability and bending fatigue characteristic. As a result, the inventors obtained the following findings (a) to (d):

(a) Regarding KN value in gas nitriding



[0015] In general, the KN value is defined by the following formula using the NH3 partial pressure and the H2 partial pressure in the atmosphere in the furnace performing the gas nitriding (below, referred to as the "nitriding atmosphere" or simply the "atmosphere").



[0016] The KN value can be controlled by the gas flow rates. However, a certain time is required after setting the gas flow rates until the nitriding atmosphere reaches the equilibrium state. For this reason, the KN value changes with each instant even before the KN value reaches the equilibrium state. Further, even if changing the KN value in the middle of the gas nitriding, the KN value fluctuates until reaching the equilibrium state.

[0017] The above such fluctuation of the KN value has an effect on the compound layer, surface hardness, and depth of the hardened layer. For this reason, not only the target value of the KN value, but also the range of variation of the KN value during gas nitriding have to be controlled to within a predetermined range.

(b) Regarding realization of both suppression of formation of compound layer and securing surface hardness and depth of hardened layer



[0018] In the various experiments conducted by the inventors, the thickness of the compound layer, voids in the compound layer, surface hardness, and depth of the hardened layer were related to the bending straightening ability and bending fatigue characteristic of the nitrided part. If the compound layer is thick and, further, there are many voids in the compound layer, cracks easily form starting from the compound layer and the bending straightening ability and bending fatigue strength fall.

[0019] Further, the lower the surface hardness and the shallower the depth of the hardened layer, the more cracks and fractures occur starting from the diffused layer and the more the bending fatigue strength falls. Furthermore, if the surface hardness is too high, the bending straightening ability deteriorates. That is, the inventors discovered that if the compound layer is thin, there are few voids in the compound layer, and the surface hardness is in a certain range, and as the depth of the hardened layer increases, the bending straightening ability and the bending fatigue characteristic become better.

[0020] From the above, to achieve both a bending straightening ability and bending fatigue characteristic, it is important to prevent the formation of a compound layer as much as possible, to control the surface hardness to a certain range, and increase the depth of the hardened layer.

[0021] To finally suppress the formation of the compound layer and secure the depth of the hardened layer, it is efficient to form a compound layer once, then break down the formed compound layer and utilize it as a source of supply of nitrogen to the hardened layer. Specifically, in the first half of the gas nitriding, gas nitriding raising the nitriding potential (high KN value treatment) is performed to form the compound layer. Further, in the second half of the gas nitriding, gas nitriding lowered in nitriding potential than the high KN value treatment (low KN value treatment) is performed. As a result, the compound layer formed in the high KN value treatment is broken down into Fe and N. The N diffuses, thereby promoting the formation of a nitrogen diffused layer (hardened layer). Finally, at the nitrided part, it is possible to make the compound layer thinner, raise the surface hardness, and increase the depth of the hardened layer.

(c) Regarding suppression of formation of voids



[0022] When nitriding by the high KN value in the first half of the gas nitriding, sometimes a layer including voids (porous layer) is formed in the compound layer (FIG. 1A). In this case, even after the nitrides break down and the nitrogen diffused layer (hardened layer) is formed, voids remain as they are inside the nitrogen diffused layer. If voids remain inside the nitrogen diffused layer, the nitrided part falls in fatigue strength. If restricting the upper limit of the KN value when forming the compound layer in the high KN value treatment, it is possible to suppress the formation of the porous layer and voids (FIG. IB).

(d) Regarding relationship of components of steel material and compound layer and nitrogen diffused layer



[0023] If C is present in the steel material, the compound layer easily becomes thicker. Further, if Mn, Cr, and other nitride compound forming elements are present, the hardness of the nitrogen diffused layer and the depth of the diffused layer changes. The bending straightening ability is improved the thinner the thickness of the compound layer or the lower the surface hardness and the bending fatigue characteristic is improved the higher the surface hardness or the deeper the diffused layer, so it becomes necessary to set the optimal range of the steel material components.

[0024] The present invention was made based on the above discoveries and has as its gist the following:
  1. [1] A nitrided steel part comprising a steel material as a material, the steel material consisting of, by mass%, C: 0.2 to 0.6%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less, S: 0.003 to 0.05%, Cr: 0.05 to 0.5%, Al: 0.01 to 0.05%, N: 0.003 to 0.025% and a balance of Fe and impurities, the nitrided steel part comprising a compound layer of a thickness of 3 µm or less comprising iron, nitrogen, and carbon formed on the steel surface and a hardened layer formed under the compound layer, an effective hardened layer depth of the nitrided steel part being 160 to 410 µm.
  2. [2] The nitrided steel part of [1] wherein the steel material contains, in place of part of Fe, one or both of Mo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
  3. [3] The nitrided steel part of [1] or [2] wherein the steel material contains, in place of part of Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
  4. [4] The nitrided part of any one of [1] to [3] wherein the steel material contains, in place of part of Fe, Ti: 0.005 to less than 0.05%.
  5. [5] A method of nitriding comprising using as a material a steel material consisting of, by mass%, C: 0.2 to 0.6%, Si: 0.05 to 1.5%, Mn: 0.2 to 2.5%, P: 0.025% or less, S: 0.003 to 0.05%, Cr: 0.05 to 0.5%, Al: 0.01 to 0.05%, N: 0.003 to 0.025% and a balance of Fe and impurities and gas nitriding by heating the steel material in a gas atmosphere containing NH3, H2, and N2 to 550 to 620°C, and making the overall treatment time A 1.5 to 10 hours, the gas nitriding comprised of high KN value treatment having a treatment time of X hours and a low KN value treatment after the high KN value treatment having a treatment time of Y hours, the high KN value treatment having a nitriding potential KNX determined by formula (1) of 0.15 to 1.50 and having an average value KNXave of the nitriding potential KNX determined by formula (2) of 0.30 to 0.80, the low KN value treatment having a nitriding potential KNY determined by formula (3) of 0.02 to 0.25, having an average value KNYave of the nitriding potential KNY determined by formula (4) of 0.03 to 0.20 and having an average value KNave of the nitriding potential determined by formula (5) of 0.07 to 0.30:









    where, in formula (2) and formula (4), the subscript "i" is a number indicating the number of measurements for each constant time interval, X0 indicates the measurement interval (hours) of the nitriding potential KNX, Y0 indicates the measurement interval (hours) of the nitriding potential KNY, KNXi indicates the nitriding potential at the i-th measurement during the high KN value treatment, and KNYi indicates the nitriding potential at the i-th measurement during the low KN value treatment.
  6. [6] The method of production of the nitrided steel part of [5] wherein the gas atmosphere includes a total of 99.5 vol% of NH3, H2, and N2.
  7. [7] The method of production of the nitrided steel part of [5] or [6] wherein the steel material contains, in place of part of the Fe, one or both of Mo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
  8. [8] The method of production of the nitrided steel part of any one of [5] to [7] wherein the steel material contains, in place of part of the Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
  9. [9] The method of production of the nitrided part of any one of [5] to [8] wherein the steel material contains, in place of part of the Fe, Ti: 0.005 to less than 0.05%.

ADVANTAGEOUS EFFECTS OF INVENTION



[0025] According to the present invention, it is possible to obtain a nitrided steel part having a thin compound layer, suppressed formation of voids (porous layer), furthermore, certain surface hardness and a deep hardened layer, and an excellent bending straightening ability and bending fatigue characteristic.

BRIEF DESCRIPTION OF DRAWINGS



[0026] 

[FIGS. 1] Views showing a compound layer after nitriding, wherein FIG. 1A shows an example of formation of a porous layer containing voids in the compound layer and FIG. 1B shows an example where formation of a porous layer and voids is suppressed.

[FIG. 2] A view showing a relationship of an average value KNXave of a nitriding potential of a high KN value treatment and a surface hardness and compound layer thickness.

[FIG. 3] A view showing a relationship of an average value KNYave of a nitriding potential of a low KN value treatment and a surface hardness and compound layer thickness.

[FIG. 4] A view showing a relationship of an average value KNave of a nitriding potential and a surface hardness and compound layer thickness.

[FIG. 5] The shape of a block shaped test piece for static bending test use used for evaluating a bending straightening ability.

[FIG. 6] The shape of a columnar test piece for evaluating a bending fatigue characteristic.


DESCRIPTION OF EMBODIMENTS



[0027] Below, the requirements of the present invention will be explained in detail. First, the chemical composition of the steel material used as a material will be explained. Below, the "%" showing the contents of the component elements and concentrations of elements at the part surface mean "mass%".

C: 0.2 to 0.6%



[0028] C is an element required for securing the core hardness of a part. If the content of C is less than 0.2%, the core strength becomes too low, so the bending fatigue strength greatly falls. Further, if the content of C exceeds 0.6%, during high KN value treatment, the compound layer thickness easily becomes larger. Further, during low KN value treatment, the compound layer becomes resistant to breakdown. For this reason, it becomes difficult to reduce the compound layer thickness after nitriding and the bending straightening ability and bending fatigue strength greatly fall. The preferable range of the C content is 0.25 to 0.55%.

Si: 0.05 to 1.5%



[0029] Si raises the core hardness by solution strengthening. Further, it is a deoxidizing element. To obtain these effects, 0.05% or more is included. On the other hand, if the content of Si exceeds 1.5%, in bars and wire rods, the strength after hot forging becomes too high, so the machinability greatly falls. In addition, the bending straightening ability falls. The preferable range of the Si content is 0.08 to 1.3%.

Mn: 0.2 to 2.5%



[0030] Mn raises the core hardness by solution strengthening. Furthermore, Mn forms fine nitrides (Mn3N2) in the hardened layer at the time of nitriding and improves the bending fatigue strength by precipitation strengthening. To obtain these effects, Mn has to be 0.2% or more. On the other hand, if the content of Mn exceeds 2.5%, the effect of raising the bending fatigue strength becomes saturated. Furthermore, the effective hardened layer depth becomes shallower, so the pitting strength and the bending fatigue strength fall. Further, the bars and wire rods used as materials become too high in hardness after hot forging, so the bending straightening ability and the machinability greatly fall. The preferable range of the Mn content is 0.4 to 2.3%.

P: 0.025% or less



[0031] P is an impurity and precipitates at the grain boundaries to make a part brittle, so the content is preferably small. If the content of P is over 0.025%, sometimes the bending straightening ability and bending fatigue strength fall. The preferable upper limit of the content of P for preventing a drop in the bending straightening ability and the bending fatigue strength is 0.018%. It is difficult to make the content completely zero. The practical lower limit is 0.001%.

S: 0.003 to 0.05%



[0032] S bonds with Mn to form MnS and raise the machinability. To obtain this effect, S has to be 0.003% or more. However, if the content of S exceeds 0.05%, coarse MnS easily forms and the bending straightening ability and bending fatigue strength greatly fall. The preferable range of the S content is 0.005 to 0.03%.

Cr: 0.05 to 0.5%



[0033] Cr forms fine nitrides (CrN) in the hardened layer during nitriding and improves the bending fatigue strength by precipitation strengthening. To obtain the effects, Cr has to be 0.5% or more. On the other hand, if the content of Cr is over 0.5%, the precipitation strengthening ability becomes saturated. Furthermore, the effective hardened layer depth becomes shallower, so the pitting strength and bending fatigue strength fall. Further, the bars and wire rods used as materials become too high in hardness after hot forging, so the bending straightening ability and machinability remarkably fall. The preferable range of the Cr content is 0.07 to 0.4%.

Al: 0.01 to 0.05%



[0034] Al is a deoxidizing element. For sufficient deoxidation, 0.01% or more is necessary. On the other hand, Al easily forms hard oxide inclusions. If the content of Al exceeds 0.05%, the bending fatigue strength remarkably falls. Even if other requirements are met, the desired bending fatigue strength can no longer be obtained. The preferable range of the Al content is 0.02 to 0.04%.

N: 0.003 to 0.025%



[0035] N bonds with Al, V, and Ti to form AlN, VN, and TiN. Due to their actions of pinning austenite grains, AlN, VN, and TiN have the effect of refining the structure of the steel material before nitriding and reducing the variation in mechanical characteristics of the nitrided steel part. If the content of N is less than 0.003%, this effect is difficult to obtain. On the other hand, if the content of N exceeds 0.025%, coarse AlN easily forms, so the above effect becomes difficult to obtain. The preferable range of the content of N is 0.005 to 0.020%.

[0036] The steel used as the material for the nitrided steel part of the present invention may also contain the elements shown below in addition to the above elements.

Mo: 0.01 to less than 0.50%



[0037] Mo forms fine nitrides (Mo2N) in the hardened layer during nitriding and improves the bending fatigue strength by precipitation strengthening. Further, Mo has the action of age hardening and improves the core hardness at the time of nitriding. The content of Mo for obtaining these effects has to be 0.01% or more. On the other hand, if the content of Mo is 0.50% or more, the bars and wire rods used as materials become too high in hardness after hot forging, so the bending straightening ability and machinability remarkably fall. In addition, the alloy costs increase. The preferable upper limit of the Mo content is less than 0.40%.

V: 0.01 to less than 0.50%



[0038] V forms fine nitrides (VN) at the time of nitriding and improves the bending fatigue strength by precipitation strengthening. Further, V has the action of age hardening to improve the core hardness at the time of nitriding. Furthermore, due to the action of pinning austenite grains, it also has the effect of refining the structure of the steel material before nitriding. To obtain these actions, V has to be 0.01% or more. On the other hand, if the content of V is 0.50% or more, the bars and wire rods used for materials become too high in hardness after hot forging, so the bending straightening ability and machinability remarkably fall. In addition, the alloy costs increase. The preferable range of content of V is less than 0.40%.

Cu: 0.01 to 0.50%



[0039] Cu improves the core hardness of the part and the hardness of the nitrogen diffused layer as a solution strengthening element. To obtain the action of solution strengthening of Cu, inclusion of 0.01% or more is necessary. On the other hand, if the content of Cu exceeds 0.50%, the bars and wire rods used as materials become too high in hardness after hot forging, so the bending straightening ability and machinability remarkably fall. In addition, the hot ductility falls. Therefore, this becomes a cause of surface scratches at the time of hot rolling and at the time of hot forging. The preferable range of the content of Cu is less than 0.40%.

Ni: 0.01 to 0.50%



[0040] Ni improves the core hardness and surface layer hardness by solution strengthening. To obtain the action of solution strengthening of Ni, inclusion of 0.01% or more is necessary. On the other hand, if the content of Ni exceeds 0.50%, the bars and wire rods used as materials become too high in hardness after hot forging, so the bending straightening ability and machinability remarkably fall. In addition, the alloy costs increase. The preferable range of the Ni content is less than 0.40%.

Ti: 0.005 to 0.05%



[0041] Ti bonds with N to form TiN and improve the core hardness and surface layer hardness. To obtain this action, Ti has to be 0.005% or more. On the other hand, if the content of Ti is 0.05% or more, the effect of improving the core hardness and surface layer hardness becomes saturated. In addition, the alloy costs increase. The preferable range of content of Ti is 0.007 to less than 0.04%.

[0042] The balance of the steel is Fe and impurities. "Impurities" mean components which are contained in the starting materials or mixed in during the process of production and not components which are intentionally included in the steel. The above optional added elements of Mo, V, Cu, Ni, and Ti are sometimes included in amounts of less than the above lower limits, but in this case, just the effects of the elements explained above are not sufficiently obtained. The effect of improvement of the pitting resistance and bending fatigue characteristic of the present invention is obtained, so this is not a problem.

[0043] Below, the method of production of the nitrided steel part of the present invention will be explained. The method of production explained below is just one example. The nitrided steel part of the present invention need only have a thickness of the compound layer of 3 µm or less and an effective hardened layer depth of 160 to 410 µm. It is not limited to the following method of production.

[0044] In the method of production of the nitrided steel part of the present invention, steel having the above-mentioned components is gas nitrided. The treatment temperature of the gas nitriding is 550 to 620°C, while the treatment time A of the gas nitriding as a whole is 1.5 to 10 hours.

Treatment Temperature: 550 to 620°C



[0045] The temperature of the gas nitriding (nitriding temperature) is mainly correlated with the rate of diffusion of nitrogen and affects the surface hardness and depth of the hardened layer. If the nitriding temperature is too low, the rate of diffusion of nitrogen is slow, the surface hardness becomes low, and the depth of the hardened layer becomes shallower. On the other hand, if the nitriding temperature is over the AC1 point, austenite phases (γ phases) with a smaller rate of diffusion of nitrogen than ferrite phases (α phases) are formed in the steel, the surface hardness becomes lower, and the depth of the hardened layer becomes shallower. Therefore, in the present embodiment, the nitriding temperature is 550 to 620°C around the ferrite temperature region. In this case, the surface hardness can be kept from becoming lower and the depth of the hardened layer can be kept from becoming shallower.

Treatment Time A of Gas Nitriding as a Whole: 1.5 to 10 Hours



[0046] The gas nitriding is performed in an atmosphere including NH3, H2, and N2. The time of the nitriding as a whole, that is, the time from the start to end of the nitriding (treatment time A), is correlated with the formation and breakdown of the compound layer and the diffusion of nitrogen and affects the surface hardness and depth of the hardened layer. If the treatment time A is too short, the surface hardness becomes lower and the depth of the hardened layer becomes shallower. On the other hand, if the treatment time A is too long, the nitrogen is removed and the surface hardness of the steel falls. If the treatment time A is too long, further, the manufacturing costs rise. Therefore, the treatment time A of the nitriding as a whole is 1.5 to 10 hours.

[0047] Note that, the atmosphere of the gas nitriding of the present embodiment includes not only NH3, H2, and N2 but also unavoidable impurities such as oxygen and carbon dioxide. The preferable atmosphere is NH3, H2, and N2 in a total of 99.5% (vol%) or more. The later explained KN value is calculated from the ratio of the NH3 and H2 partial pressures in the atmosphere, so is not affected by the magnitude of the N2 partial pressure. However, to raise the stability of KN control, the N2 partial pressure is preferably 0.2 to 0.5 atm.

High KN Value Treatment and Low KN Value Treatment



[0048] The above-mentioned gas nitriding includes a step of performing high KN value treatment and a step of performing low KN value treatment. In high KN value treatment, gas nitriding is performed by a nitriding potential KNX higher than the low KN value treatment. Furthermore, after high KN value treatment, low KN value treatment is performed. In the low KN value treatment, gas nitriding is performed by a nitriding potential KNY lower than the high KN value treatment.

[0049] In this way, in the present nitriding method, two-stage gas nitriding (high KN value treatment and low KN value treatment) is performed. By raising the nitriding potential KN value in the first half of the gas nitriding (high KN value treatment), a compound layer is formed at the surface of the steel. After that, by lowering the nitriding potential KN value in the second half of the gas nitriding (low KN value treatment), the compound layer formed at the surface of the steel is broken down into Fe and N and the nitrogen (N) is made to penetrate and diffuse in the steel. By the two-stage gas nitriding, the thickness of the compound layer formed by the high KN value treatment is reduced while the nitrogen obtained by breakdown of the compound layer is used to obtain a sufficient depth of the hardened layer.

[0050] The nitriding potential of the high KN value treatment is denoted as KNX, while the nitriding potential of the low KN value treatment is denoted as KNY. At this time, the nitriding potentials KNX and KNY are defined by the following formula:





[0051] The partial pressures of the NH3 and H2 in the atmosphere of the gas nitriding can be controlled by adjusting the flow rates of the gases.

[0052] When shifting from the high KN value treatment to the low KN value treatment, if adjusting the flow rates of the gases to lower the KN value, a certain extent of time is required until the partial pressures of NH3 and H2 in the furnace stabilize. The gas flow rates can be adjusted for changing the KN value one time or if necessary several times. To increase the amount of drop of the KN value more, the method of lowering the NH3 flow rate and raising the H2 flow rate is effective. The point of time when the KNi value after high KN value treatment finally becomes 0.25 or less is defined as the start timing of the low KN value treatment.

[0053] The treatment time of the high KN value treatment is denoted as "X"(hours), while the treatment time of the low KN value treatment is denoted as "Y" (hours). The total of the treatment time X and the treatment time Y is within the treatment time A of the nitriding overall, preferably is the treatment time A.

Various Conditions at High KN Value Treatment and Low KN Value Treatment



[0054] As explained above, the nitriding potential during the high KN value treatment is denoted as KNX, while the nitriding potential during the low KN value treatment is denoted by KNY. Furthermore, the average value of the nitriding potential during high KN value treatment is denoted by "KNXave", while the average value of the nitriding potential during low KN value treatment is denoted by "KNYave". KNXave and KNYave are defined by the following formulas:





[0055] Here, the subscript "i" is a number expressing the number of times of measurement every certain time interval. X0 indicates the measurement interval of the nitriding potential KNX (hours), Y0 indicates the measurement interval of the nitriding potential KNY (hours), KNXi indicates the nitriding potential at the i-th measurement during the high KN value treatment, and KNYi indicates the nitriding potential at the i-th measurement during the low KN value treatment.

[0056] For example, X0 is made 15 minutes. 15 minutes after the start of treatment, measurement is conducted the first time (i=1). Each 15 minutes after that, measurement is conducted the second time (i=2) and the third time (i=3). KNXave is calculated by measurement of the "n" number of times measurable up to the treatment time. KNYave is calculated in the same way.

[0057] Furthermore, the average value of the nitriding potential of the nitriding as a whole is denoted as "KNave". The average value KNave is defined by the following formula:



[0058] In the nitriding method of the present invention, the nitriding potential KNX, average value KNXave, and treatment time X of the high KN value treatment and the nitriding potential KNX, average value KNYave, treatment time Y, and average value KNave of the low KN value treatment satisfy the following conditions (I) to (IV):
  1. (I) Average value KNXave: 0.30 to 0.80
  2. (II) Average value KNYave: 0.03 to 0.20
  3. (III) KNX: 0.15 to 1.50, and KNY: 0.02 to 0.25
  4. (IV) Average value KNave: 0.07 to 0.30


[0059] Below, the Conditions (I) to (IV) will be explained.

(I) Average Value KNXave of Nitriding Potential in High KN Treatment



[0060] In the high KN value treatment, the average value KNXave of the nitriding potential has to be 0.30 to 0.80 to form a compound layer of a sufficient thickness.

[0061] FIG. 2 is a view showing the relationship of the average value KNXave and the surface hardness and compound layer thickness. FIG. 2 is obtained from the following experiments.

[0062] The steel "a" having the chemical composition prescribed in the present invention (see Table 1, below, called the "test material") was gas nitrided in a gas atmosphere containing NH3, H2, and N2. In the gas nitriding, the test material was inserted into a heat treatment furnace heated to a predetermined temperature and able to be controlled in atmosphere then NH3, N2, and H2 gases were introduced. At this time, the partial pressures of the NH3 and H2 in the atmosphere of the gas nitriding were measured while adjusting the flow rates of the gases to control the nitriding potential KN value. The KN value was found by the NH3 partial pressure and H2 partial pressure.

[0063] The H2 partial pressure during gas nitriding was measured by using a heat conduction type H2 sensor directly attached to the gas nitriding furnace body and converting the difference in standard gas and measured gas to the gas concentration. The H2 partial pressure was measured continuously during the gas nitriding. The NH3 partial pressure during the gas nitriding was measured by attachment of a manual glass tube type NH3 analysis meter outside of the furnace. The partial pressure of the residual NH3 was calculated and found every 15 minutes. Every 15 minutes of measurement of the NH3 partial pressure, the nitriding potential KN value was calculated. The NH3 flow rate and N2 flow rate were adjusted to converge to the target values.

[0064] The gas nitriding was performed with a temperature of the atmosphere of 590°C, a treatment time X of 1.0 hour, a treatment time Y of 2.0 hours, a KNYave of a constant 0.05, and a KNXave changed from 0.10 to 1.00. The overall treatment time A was made 3.0 hours.

[0065] Test materials gas nitrided by various average values KNXave were measured and tested as follows.

Measurement of Thickness of Compound Layer



[0066] After gas nitriding, the cross-section of the test material was polished, etched, and examined under an optical microscope. The etching was performed by a 3% Nital solution for 20 to 30 seconds. A compound layer was present at the surface layer of the steel and was observed as a white uncorroded layer. From five fields of the photographed structure taken by an optical microscope at 500X (field area: 2.2×104 µm2), the thicknesses of the compound layer at four points were respectively measured every 30 µm. The average value of the values of the 20 points measured was defined as the compound thickness (µm). When the compound layer thickness was 3 µm or less, peeling and cracking were largely suppressed. Accordingly, in the present invention, the compound layer thickness has to be made 3 µm or less. The compound layer thickness may also be 0.

Phase Structure of Compound Layer



[0067] The phase structure of the compound layer is preferably one where, by area ratio, γ' (Fe4N) becomes 50% or more. The balance is ε (Fe2-3N). With general soft nitriding, the compound layer becomes mainly ε (Fe2-3N), but with the nitriding of the present invention, the ratio of γ' (Fe4N) become larger. The phase structure of the compound layer can be investigated by the SEM-EBSD method.

Measurement of Void Area Ratio



[0068] Furthermore, the area ratio of the voids in the surface layer structure at a cross-section of the test material was measured by observation under an optical microscope. The ratio of voids in an area of 25 µm2 in a range of 5 µm depth from the outermost surface (below, referred to as the "void area ratio") was calculated for each field in measurement of five fields at a power of 1000X (field area: 5.6×103 µm2). If the void area ratio is 10% or more, the surface roughness of the nitrided part after gas nitriding becomes coarser. Furthermore, the compound layer becomes brittle, so the nitrided part falls in fatigue strength. Therefore, in the present invention, the void area ratio has to be less than 10%. The void area ratio is preferably less than 8%, more preferably less than 6%.

Measurement of Surface Hardness



[0069] Furthermore, the surface hardness and effective hardened layer depth of the test material after gas nitriding were found by the following method. The Vickers hardness in the depth direction from the sample surface was measured based on JIS Z 2244 by a test force of 1.96N. Further, the average value of three points of the Vickers hardness at a position of 50 µm depth from the surface was defined as the surface hardness (HV). In the present invention, 350HV to 500HV is targeted as a surface hardness equal to the case of general gas nitriding where over 3 µm of a compound layer remains.

Measurement of Effective Hardened Layer Depth



[0070] In the present invention, the effective hardened layer depth (µm) is defined as the depth in a range where the Vickers hardness in the distribution measured in the depth direction from the surface of the test material using the hardness distribution in the depth direction obtained by the above Vickers hardness test is 250HV or more.

[0071] At the treatment temperature of 570 to 590°C, in the case of general gas nitriding where a compound layer of 10 µm or more is formed, if the treatment time of the gas nitriding as a whole is A (hours), the effective hardened layer depth becomes the value found by the following formula (A)±20 µm.



[0072] In the nitrided steel part of the present invention, the effective hardened layer depth was made 130×{treatment time A (hours)}1/2. In the present embodiment, the treatment time A of the gas nitriding as a whole, as explained above, was 1.5 to 10 hours, so the effective hardened layer depth was targeted as 160 to 410 µm.

[0073] As a result of the above-mentioned measurement test, if the average value KNYave is 0.20 or more, the effective hardened layer depth was 160 to 410 µm (when A=3, effective hardened layer depth 225 µm). Furthermore, in the results of the measurement tests, the surface hardnesses and thicknesses of the compound layers of the test materials obtained by gas nitriding at the different average values KNXave were used to prepare FIG. 2.

[0074] The solid line in FIG. 2 is a graph showing the relationship of the average value KNXave and surface hardness (HV). The broken line in FIG. 2 is a graph showing the relationship of the average value KNXave and the thickness of the compound layer (µm).

[0075] Referring to the solid line graph of FIG. 2, if the average value KNYave at the low KN value treatment is constant, as the average value KNXave at the high KN value treatment becomes higher, the surface hardness of the nitrided part remarkably increases. Further, when the average value KNXave becomes 0.30 or more, the surface hardness becomes the targeted 350HV or more. On the other hand, if the average value KNXave is higher than 0.30, even if the average value KNXave becomes further higher, the surface hardness remains substantially constant. That is, in the graph of the average value KNXave and surface hardness (solid line in FIG. 2), there is an inflection point near KNXave=0.30.

[0076] Furthermore, referring to the broken line graph of FIG. 2, as the average value KNXave falls from 1.00, the compound thickness remarkably decreases. Further, when the average value KNXave becomes 0.80, the thickness of the compound layer becomes 3 µm or less. On the other hand, with an average value KNXave of 0.80 or less, as the average value KNXave falls, the thickness of the compound layer is decreased, but compared with when the average value KNXave is higher than 0.80, the amount of reduction of the thickness of the compound layer is small. That is, in the graph of the average value KNXave and surface hardness (solid line in FIG. 2), there is an inflection point near KNXave=0.80.

[0077] From the above results, in the present invention, the average value KNXave of the nitriding potential of the high KN value treatment is made 0.30 to 0.80. By controlling it to this range, the nitrided steel can be raised in surface hardness and the thickness of the compound layer can be suppressed. Furthermore, a sufficient effective hardened layer depth can be obtained. If the average value KNXave is less than 0.30, the compound is insufficiently formed, the surface hardness falls, and a sufficient effective hardened layer depth cannot be obtained. If the average value KNXave exceeds 0.80, sometimes the thickness of the compound layer exceeds 3 µm and, furthermore, the void area ratio becomes 10% or more. The preferable lower limit of the average value KNXave is 0.35. Further, the preferable upper limit of the average value KNXave is 0.70.

(II) Average Value KNYave of Nitriding Potential at Low KN Value Treatment



[0078] The average value KNYave of the nitriding potential of the low KN value treatment is 0.03 to 0.20.

[0079] FIG. 3 is a view showing the relationship of the average value KNYave and the surface hardness and compound layer thickness. FIG. 3 was obtained by the following test.

[0080] Steel "a" having the chemical composition prescribed in the present invention was gas nitrided by a temperature of the nitriding atmosphere of 590°C, a treatment time X of 1.0 hour, a treatment time Y of 2.0 hours, an average value KNXave of a constant 0.40, and an average value KNYave changed from 0.01 to 0.30. The overall treatment time A was 3.0 hours.

[0081] After the nitriding, the above-mentioned methods were used to measure the surface hardness (HV), effective hardened layer depth (µm), and compound layer thickness (µm) at the different average values KNYave. As a result of measurement of the effective hardened layer depth, if the average value KNYave is 0.02 or more, the effective hardened layer depth became 225 µm or more. Furthermore, the surface hardnesses and the compound thicknesses obtained by the measurement tests were plotted to prepare FIG. 3.

[0082] The solid line in FIG. 3 is a graph showing the relationship of the average value KNYave and the surface hardness, while the broken line is a graph showing the relationship of the average value KNYave and the depth of the compound layer. Referring to the solid line graph of FIG. 3, as the average value KNYave becomes higher from 0, the surface hardness remarkably increases. Further, when KNYave becomes 0.03, the surface hardness becomes 570HV or more. Furthermore, when KNYave is 0.03 or more, even if KNYave becomes higher, the surface hardness is substantially constant. Due to the above, in the graph of the average value KNYave and the surface hardness, there is an inflection point near the average value KNYave=0.03.

[0083] On the other hand, if referring to the broken line graph in FIG. 3, the thickness of the compound layer is substantially constant until the average value KNYave falls from 0.30 to 0.25. However, as the average value KNYave falls from 0.25, the thickness of the compound layer remarkably decreases. Further, when the average value KNYave becomes 0.20, the thickness of the compound layer becomes 3 µm or less. Furthermore, when the average value KNYave is 0.20 or less, as the average value KNYave falls, the thickness of the compound layer decreases, but compared with when the average value KNYave is higher than 0.20, the amount of decrease of the thickness of the compound layer is small. Due to this, in the graph of the average value KNYave and the thickness of the compound layer, there is an inflection point near the average value KNYave=0.20.

[0084] From the above results, in the present invention, the average value KNYave of the low KN value treatment is limited to 0.03 to 0.20. In this case, the gas nitrided steel becomes higher in surface hardness and the thickness of the compound layer can be suppressed. Furthermore, it is possible to obtain a sufficient effective hardened layer depth. If the average value KNYave is less than 0.03, nitrogen is removed from the surface and the surface hardness falls. On the other hand, if the average value KNYave exceeds 0.20, the compound insufficiently breaks down, the effective hardened layer depth is shallow, and the surface hardness falls. The preferable lower limit of the average value KNYave is 0.05. The preferable upper limit of the average value KNYave is 0.18.

(III) Scope of Nitriding Potentials KNX and KNY During Nitriding



[0085] In gas nitriding, a certain time is required after setting the gas flow rates until the KNi value in the atmosphere reaches the equilibrium state. For this reason, the KNi value changes with each instant until the KNi value reaches the equilibrium state. Furthermore, when shifting from the high KN value treatment to low KN value treatment, the setting of the KNi value is changed in the middle of the gas nitriding. In this case as well, the KNi value fluctuates until reaching the equilibrium state.

[0086] Such fluctuations in the KNi value have an effect on the compound layer and depth of the hardened layer. Therefore, in the high KN value treatment and low KN value treatment, not only are the average value KNXave and average value KNYave made the above ranges, but also the nitriding potential KNx during the high KN value treatment and the nitriding potential KNY during the low KN value treatment are controlled to predetermined ranges.

[0087] Specifically, in the present invention, to form a sufficient compound layer, the nitriding potential KNX during the high KN value treatment is made 0.15 to 1.50. To make the compound layer thin and the depth of the hardened layer larger, the nitriding potential KNY during the low KN value treatment is made 0.02 to 0.25.

[0088] Table 1 shows the compound layer thickness (µm), void area ratio (%), effective hardened layer depth (µm), and surface hardness (HV) of the nitrided part in the case of nitriding steel containing C: 0.45%, Si: 0.70%, Mn: 1.01%, P: 0.015%, S: 0.015%, Cr: 0.25%, Al: 0.028%, and N: 0.0009% and having a balance of Fe and impurities (below, referred to as "steel 'a'") by various nitriding potentials KNX and KNY. Table 1 was obtained by the following tests.
Table 1
Test no.Temp.High Kn value treatmentLow Kn value treatmentNitridingCompound layer thicknessVoid area ratioEffective hardened layer depth (actual)Surface hardness
Time XNitriding potentialTime YNitriding potentialTime ANitriding potential
Min. valueMax. valueAver. valueMin. valueMax. valueAver. valueAver. value
(°C)(h)KnXminKnXmaxKnXave(h)KnYminKnYmaxKnYave(h)Knave(µm)(%)(µm)(Hv)
1 590 1.0 0.12 0.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 None 2 195 310
2 590 1.0 0.14 0.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 None 2 243 335
3 590 1.0 0.15 0.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 1 4 241 391
4 590 1.0 0.25 0.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 1 4 240 394
5 590 1.0 0.25 1.40 0.40 2.0 0.05 0.15 0.10 3.0 0.20 2 8 238 400
6 590 1.0 0.25 1.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 2 9 241 403
7 590 1.0 0.30 1.55 0.40 2.0 0.05 0.15 0.10 3.0 0.20 3 14 242 408
8 590 1.0 0.30 1.60 0.40 2.0 0.05 0.15 0.10 3.0 0.20 6 16 250 401
9 590 1.0 0.30 0.50 0.40 2.0 0.01 0.15 0.10 3.0 0.20 None 3 242 283
10 590 1.0 0.30 0.50 0.40 2.0 0.02 0.15 0.10 3.0 0.20 None 3 243 390
11 590 1.0 0.30 0.50 0.40 2.0 0.03 0.15 0.10 3.0 0.20 None 3 247 390
12 590 1.0 0.30 0.50 0.40 2.0 0.05 0.15 0.10 3.0 0.20 1 3 241 396
13 590 1.0 0.30 0.50 0.40 2.0 0.05 0.20 0.10 3.0 0.20 2 4 240 400
14 590 1.0 0.30 0.50 0.40 2.0 0.05 0.22 0.10 3.0 0.20 2 4 242 399
15 590 1.0 0.30 0.50 0.40 2.0 0.05 0.25 0.10 3.0 0.20 3 5 244 402
16 590 1.0 0.30 0.50 0.40 2.0 0.05 0.27 0.10 3.0 0.20 5 5 252 409


[0089] Using the steel "a" as a test material, the gas nitriding shown in Table 1 (high KN value treatment and low KN value treatment) was performed to produce a nitrided part. Specifically, the atmospheric temperature of the gas nitriding in the different tests was made 590°C, the treatment time X was made 1.0 hour, the treatment time Y was made 2.0 hours, KNXave was made a constant 0.40, and KNYave was made a constant 0.10. Further, during gas nitriding, the minimum values KNXmin and KNYmin and the maximum values KNXmax and KNYmax of KNX and KNY were changed to perform high KN value treatment and low KN value treatment. The treatment time A of the nitriding as a whole was made 3.0 hours.

[0090] In the case of general gas nitriding where a compound layer of 10 µm or more is formed at a treatment temperature of 570 to 590°C, if making the treatment time of the gas nitriding as a whole 3.0 hours, the effective hardened layer depth became 225 µm±20 µm. The nitride part after gas nitriding was measured for compound layer thickness, void area ratio, effective hardened layer depth, and surface hardness by the above measurement methods to obtain Table 1.

[0091] Referring to Table 1, in Test Nos. 3 to 6 and 10 to 15, the minimum value KNXmin and maximum value KNXmax were 0.15 to 1.50 and the minimum value KNYmin and maximum value KNYmax were 0.02 to 0.25. As a result, the compound thickness was a thin 3 µm or less and voids were kept down to less than 10%. Furthermore, the effective hardened layer depth was 225 µm or more, while the surface hardness was 350HV or more.

[0092] On the other hand, in Test Nos. 1 and 2, KNXmin was less than 0.15, so the surface hardness was less than 570HV. In Test No. 1, furthermore, KNXmin was less than 0.14, so the effective hardened layer depth was less than 225 µm.

[0093] In Test Nos. 7 and 8, KNXmax exceeded 1.5, so the voids in the compound layer became 10% or more. In Test No. 8, furthermore, KNXmax exceeded 1.55, so the thickness of the compound layer exceeded 3 µm.

[0094] In Test No. 9, KNYmin was less than 0.02, so the surface hardness was less than 350HV. This is believed because not only was the compound layer eliminated by the low KN value treatment, but also denitration occurred from the surface layer. Furthermore, in Test No. 16, KNYmax exceeded 0.25. For this reason, the thickness of the compound layer exceeded 3 µm. KNYmax exceeded 0.25, so it is believed that the compound layer did not sufficiently break down.

[0095] From the above results, the nitriding potential KNX in the high KN value treatment is made 0.15 to 1.50 and the nitriding potential KNY in the low KN value treatment is made 0.02 to 0.25. In this case, in the part after nitriding, the thickness of the compound layer can be made sufficiently thin and voids can be suppressed. Furthermore, the effective hardened layer depth can be made sufficiently deep and a high surface hardness is obtained.

[0096] If the nitriding potential KNX is less than 0.15, the effective hardened layer becomes too shallow and the surface hardness becomes too low. If the nitriding potential KNX exceeds 1.50, the compound layer becomes too thick and voids excessively remain.

[0097] Further, if the nitriding potential KNY is less than 0.02, denitration occurs and the surface hardness falls. On the other hand, if the nitriding potential KNY is over 0.20, the compound layer becomes too thick. Therefore, in the present embodiment, the nitriding potential KNX during the high KN value treatment is 0.15 to 1.50, and the nitriding potential KNY in the low KN value treatment is 0.02 to 0.25.

[0098] The preferable lower limit of the nitriding potential KNX is 0.25. The preferable upper limit of KNX is 1.40. The preferable lower limit of KNY is 0.03. The preferable upper limit of KNY is 0.22.

(IV) Average Value KNave of Nitriding Potential During Nitriding



[0099] In gas nitriding of the present embodiment, furthermore, the average value KNave of the nitriding potential defined by formula (2) is 0.07 to 0.30.



[0100] FIG. 4 is a view showing the relationship between the average value KNave, surface hardness (HV), and depth of the compound layer (µm). FIG. 4 was obtained by conducting the following tests. The steel "a" was gas nitrided as a test material. The atmospheric temperature in the gas nitriding was made 590°C. Further, the treatment time X, treatment time Y, and range and average value of the nitriding potential (KNX, KNY, KNXave, KNYave) were changed to perform gas nitriding (high KN value treatment and low KN value treatment).

[0101] The test materials after gas nitriding under the various test conditions were measured for the compound layer thicknesses and surface hardnesses by the above methods. The obtained compound layer thicknesses and surface hardnesses were measured and FIG. 4 was prepared.

[0102] The solid line in FIG. 4 is a graph showing the relationship between the average value KNave of the nitriding potential and the surface hardness (HV). The broken line in FIG. 4 is a graph showing the relationship between the average value KNave and the thickness of the compound layer (µm).

[0103] Referring to the actual line graph of FIG. 4, as the average value KNave becomes higher from 0, the surface hardness remarkably rises. When the average value KNave becomes 0.07, the hardness becomes 350HV or more. Further, if the average value KNave becomes 0.07 or more, even if the average value KNave becomes higher, the surface hardness is substantially constant. That is, in the graph of the average value KNave and surface hardness (HV), there is an inflection point near the average value KNave=0.07.

[0104] Furthermore, referring to the broken line graph of FIG. 4, as the average value KNave falls from 0.35, the compound thickness becomes remarkably thinner. When the average value KNave becomes 0.30, it becomes 3 µm or less. Further, if the average value KNave becomes less than 0.30, as the average value KNave becomes lower, the compound thickness gradually becomes thinner, but compared with the case where the average value KNave is higher than 0.30, the amount of reduction of the thickness of the compound layer is small. Due to the above, in the graph of the average value KNave and the thickness of the compound layer, there is an inflection point near the average value KNave=0.30.

[0105] From the above results, with the gas nitriding of the present embodiment, the average value KNave defined by formula (2) is made 0.07 to 0.30. In this case, in the gas nitrided part, the compound layer can be made sufficiently thin. Furthermore, a high surface hardness is obtained. If the average value KNave is less than 0.07, the surface hardness is low. On the other hand, if the average value KNave is over 0.30, the compound layer exceeds 3 µm. The preferable lower limit of the average value KNave is 0.08. The preferable upper limit of the average value KNave is 0.27.

Treatment Time of High KN Value Treatment and Low KN Value Treatment



[0106] The treatment time X of the high KN value treatment and the treatment time Y of the low KN value treatment are not particularly limited so long as the average value KNave defined by the formula (2) is 0.07 to 0.30. Preferably, the treatment time X is 0.50 hour or more and the treatment time Y is 0.50 hour or more.

[0107] Gas nitriding is performed under the above conditions. Specifically, high KN value treatment is performed under the above conditions, then low KN value treatment is performed under the above conditions. After the low KN value treatment, gas nitriding is ended without raising the nitriding potential.

[0108] The steel having the components prescribed in the present invention is gas nitrided to thereby produce a nitrided part. In the nitrided part produced, the surface hardness is sufficiently deep and the compound layer is sufficiently thin. Furthermore, the effective hardened layer depth can be made sufficiently deep and voids in the compound layer can also be suppressed. Preferably, in the nitrided part produced by nitriding in the present embodiment, the surface hardness becomes a Vickers hardness of 350HV or more and the depth of the compound layer becomes 3 µm or less. Furthermore, the void area ratio becomes less than 1 0%. Also, the nitrided part satisfies the formula (B). Furthermore, the effective hardened layer depth becomes 160 to 410 µm.

EXAMPLES



[0109] Steels "a" to "z" having the chemical components shown in Table 2 were melted in 50 kg amounts in a vacuum melting furnace to produce molten steels. The molten steels were cast to produce ingots. Note that, in Table 2, "a" to "q" are steels having the chemical components prescribed in the present invention. On the other hand, steels "r" to "z" were steels of comparative examples off from the chemical components prescribed in the present invention in at least one element.
Table 2
SteelChemical components (mass%)*1Remarks
CSiMnPSCrAlNMoCuNiVTi
a 0.30 0.26 1.26 0.011 0.010 0.20 0.026 0.015            
b 0.58 0.20 1.15 0.012 0.012 0.22 0.024 0.010 0.15          
c 0.26 1.31 0.88 0.015 0.021 0.11 0.019 0.014   0.10        
d 0.41 0.35 2.33 0.010 0.009 0.07 0.023 0.015     0.25      
e 0.36 0.53 0.95 0.019 0.031 0.18 0.021 0.018       0.18    
f 0.43 1.03 0.66 0.009 0.013 0.45 0.025 0.014   0.15     0.010  
g 0.46 0.15 1.45 0.009 0.013 0.23 0.042 0.024 0.31       0.006  
h 0.39 0.42 0.91 0.010 0.010 0.17 0.023 0.012 0.22   0.17   0.005  
i 0.37 0.24 0.42 0.009 0.026 0.16 0.026 0.017   0.20   0.41   Inv. ex.
j 0.25 0.20 1.51 0.009 0.011 0.07 0.020 0.006 0.33 0.19        
k 0.21 0.29 1.00 0.015 0.021 0.21 0.021 0.010   0.11 0.24 0.22    
l 0.54 0.06 1.01 0.016 0.006 0.24 0.022 0.008 0.19     0.05 0.008  
m 0.53 0.30 0.32 0.012 0.009 0.22 0.033 0.008   0.35     0.008  
n 0.45 0.21 1.25 0.011 0.007 0.05 0.021 0.017 0.44   0.10   0.011  
o 0.34 0.33 0.95 0.010 0.010 0.25 0.018 0.004   0.18 0.22   0.020  
p 0.50 0.25 1.01 0.008 0.010 0.10 0.022 0.009 0.15 0.16 0.05 0.08    
q 0.21 0.06 0.22 0.015 0.015 0.05 0.025 0.015 0.39 0.30 0.26 0.22 0.008  
r 0.62 0.32 1.56 0.015 0.020 0.39 0.031 0.010 0.24 0.22     0.006 Comp. ex.
s 0.18 0.35 1.02 0.010 0.013 0.20 0.021 0.012          
t 0.33 0.04 1.33 0.013 0.040 0.23 0.019 0.004     0.11    
u 0.33 0.77 0.19 0.013 0.012 0.15 0.021 0.011 0.10     0.30  
v 0.36 0.36 0.80 0.026 0.051 0.26 0.034 0.007 0.23   0.20   0.016
w 0.36 0.13 0.95 0.014 0.022 0.04 0.021 0.007 0.08 0.06     0.008
x 0.44 0.78 0.40 0.014 0.009 0.26 0.052 0.015   0.25      
y 0.40 1.28 0.18 0.011 0.010 0.55 0.025 0.011 0.05 0.06 0.41 0.48 0.006
z 0.11 0.25 0.99 0.008 0.006 0.95 0.022 0.009 0.14 0.18 0.05 0.05  
*1. Balance of chemical components is Fe and impurities.
*2. Empty fields indicate alloy element not intentionally added.


[0110] The ingots were hot forged to rods of a diameter of 35 mm. Next, rods were annealed, then machined to prepare plate-shaped test pieces for evaluation of the thickness of the compound layer, volume ratio of the voids, effective hardened layer depth, and surface hardness. The plate shaped test pieces were made vertical 20 mm, horizontal 20 mm, and thickness 2 mm. Further, a block shaped test pieces for four-point bending tests for evaluating the bending straightening ability were prepared (FIG. 5). Furthermore, columnar test pieces were prepared for evaluating the bending fatigue characteristic (FIG. 6).

[0111] The obtained test pieces were gas nitrided under the next conditions. The test pieces were loaded into a gas nitriding furnace then NH3, H2, and N2 gases were introduced into the furnace. After that, the high KN value treatment was performed, then the low KN value treatment was performed under the conditions of Tables 3 and 4. The test pieces after gas nitriding were oil cooled using 80°C oil.




Test for Measurement of Thickness of Compound Layer and Void Area Ratio



[0112] The cross-sections of test pieces after gas nitriding in a direction vertical to the length direction were polished to mirror surfaces and etched. An optical microscope was used to examine the etched cross-sections, measure the compound layer thicknesses, and check for the presence of any voids in the surface layer parts. The etching was performed by a 3% Nital solution for 20 to 30 seconds.

[0113] The compound layers can be confirmed as white uncorroded layers present at the surface layers. The compound layers were examined from five fields of photographed structures taken at 500X (field area: 2.2×104 µm2). The thicknesses of the compound layers at four points were measured every 30 µm. Further, the average values of the 20 points measured were defined as the compound thicknesses (µm).

[0114] Furthermore, the etched cross-sections were examined at 1000X in five fields and the ratios of the total areas of the voids in areas of 25 µm2 in the ranges of 5 µm depth from the outermost surface (void area ratio, unit: %) were found.

Test for Measurement of Surface Hardness and Effective Hardened Layer



[0115] The steel rods of the different tests after gas nitriding were measured for Vickers hardnesses based on JIS Z 2244 by test forces of 1.96N at 50 µm, 100 µm, and every subsequent 50 µm increments from the surfaces until depths of 1000 µm. The Vickers hardnesses (HV) were measured at five points each and the average values were found. The surface hardnesses were made the average values of five points at positions of 50 µm from the surfaces.

[0116] The depths of ranges becoming 250HV or more in the distribution of Vickers hardnesses measured in the depth direction from the surfaces were defined as the effective hardened layer depths (µm).

[0117] If the thicknesses of the compound layers are 3 µm or less, the ratios of voids are less than 10%, and the surface hardnesses are 350HV to 500HV, the test pieces are judged as good. Furthermore, if the effective hardened layer depths are 160 to 410 µm, the test pieces are judged as good.

[0118] Below, good and poor test pieces were used to evaluate the bending straightening ability and rotating bending fatigue characteristic.

Test for Evaluation of Bending Straightening Ability



[0119] The block shaped test pieces used for gas nitriding were subjected to static bending tests. The shapes of the block shaped test pieces are shown in FIG. 5. Note that in FIG. 5, the units of the dimensions are "mm". The static bending tests were performed by four-point bending with inside support point distances of 30 mm and outside support point distances of 80 mm. The strain rate was 2 mm/min. A strain gauge was attached to the rounded parts of the block shaped test pieces in the longitudinal direction. The maximum amount of strain (%) at the time when cracks formed at the rounded parts and measurement by the strain gauges was no longer possible was found as the bending straightening ability. In the parts of the present invention, a bending straightening ability of 1.3% or more was targeted.

Test for Evaluation of Bending Fatigue Characteristic



[0120] Columnar test pieces used for gas nitriding were tested by an Ono-type rotating bending fatigue test. The speed was 3000 rpm, the cutoff of the test was made 107 cycles showing the fatigue limit of general steel, and the maximum stress amplitude in a rotating bending fatigue test piece when reaching 107 cycles without fracture was made the fatigue limit of the rotating bending fatigue test piece. The shapes of the test pieces are shown in FIG. 6. In a part of the present invention, the target is a maximum stress at the fatigue limit of 500 MPa or more.

Test Results



[0121] The results are shown in Tables 3 and 4. In Table 3, the "Effective hardened layer depth (target)" column describes the values calculated by the formula (A) (target value), while the "Effective hardened layer depth (actual)" describes the measured values of the effective hardened layer (µm).

[0122] Referring to Tables 3 and 4, in Test Nos. 17 to 41, the treatment temperatures in gas nitriding were 550 to 620°C and the treatment times A were 1.5 to 10 hours. Furthermore, the KNX's at the high KN value treatment were 0.15 to 1.50, while the average values KNXave's were 0.30 to 0.80. Furthermore, the KNY's at the low KN value treatment were 0.02 to 0.25, while the average values KNYave's were 0.03 to 0.20. Furthermore, the average values KNave's found by formula (2) were 0.07 to 0.30. For this reason, in each test, the thicknesses of the compound layers after nitriding were 3 µm or less, while the void area ratios were less than 10%.

[0123] Furthermore, the effective hardened layers satisfied 160 to 410 µm and the surface hardnesses were 350 to 500HV. Both the bending straightening ability and bending fatigue strengths satisfied their targets of 1.3% and 500 MPa or more. Note that the cross-sections of the surface layers of the test pieces with the compound layers were investigated for phase structures of the compound layers by the SEM-EBSD method, whereupon by area ratio, the γ"s (Fe4N) were 50% or more and the balances were ε (Fe2-3N).

[0124] On the other hand, in Test No. 42, the minimum value of KNX at the high KN value treatment was less than 0.15. For this reason, a compound layer was not stably and periodically formed during the high KN value treatment, so the effective hardened layer depth became less than 160 µm, and the bending fatigue strength was less than 500 MPa.

[0125] In Test No. 43, the maximum value of KNX at the high KN value treatment exceeded 1.50. For this reason, the void area ratio became 10% or more, the bending straightening ability was less than 1.3%, and the bending fatigue strength was less than 500 MPa.

[0126] In Test No. 44, the average value KNXave in the high KN value treatment was less than 0.30. For this reason, a compound layer of a sufficient thickness was not formed during the high KN value treatment and the compound layer ended up breaking down at the early stage of the low KN value treatment, so the effective hardened layer depth became less than 160 µm and the surface hardness also was less than 350HV, so the bending fatigue strength was less than 500 MPa.

[0127] In Test No. 45, the average value KNXave at the high KN value treatment exceeded 0.80. For this reason, the compound layer thickness exceeded 3 µm, the void area ratio became 10% or more, the bending straightening ability was less than 1.3 %, and the bending fatigue strength was less than 500 MPa.

[0128] In Test No. 46, the minimum value of KNY at the low KN value treatment was less than 0.02. For this reason, at the early stage of the low KN value treatment, the compound layer ended up breaking down, so the effective hardened layer depth became less than 160 µm and the surface hardness also was less than 350HV, so the bending fatigue strength was less than 500 MPa.

[0129] In Test No. 47, the minimum value of KNY at the low KN value treatment was less than 0.02, and the average value KYave at the low KN value treatment was less than 0.03. For this reason, the effective hardened layer depth became less than 160 µm and the surface hardness was also less than 350HV, so the bending fatigue strength was less than 500 MPa.

[0130] In Test No. 48, the average value KNave was less than 0.07. For this reason, the surface hardness was less than 350HV, so the bending fatigue strength was less than 500 MPa.

[0131] In Test No. 49, the average value KYave at the low KN value treatment exceeded 0.20. For this reason, the compound layer thickness exceeded 3 µm, so the bending straightening ability was less than 1.3% and the bending fatigue strength was less than 500 MPa.

[0132] In Test No. 50, the average value KNave exceeded 0.30. For this reason, the compound layer thickness exceeded 3 µm, so the bending straightening ability was less than 1.3% and the bending fatigue strength was less than 500 MPa.

[0133] In Test No. 51, no high KNlow KN value treatment was performed and the average value KNave was controlled to 0.07 to 0.30. As a result, the compound layer thickness exceeded 3 µm, so the bending straightening ability became less than 1.3% and the bending fatigue strength became less than 500 MPa.

[0134] In Test Nos. 52 to 60, steels "r" to "z" having components outside the scope prescribed in the present invention were used and nitrided as prescribed in the present invention. As a result, at least one of the bending straightening ability and bending fatigue strength failed to meet the target value.

[0135] Above, embodiments of the present invention were explained. However, the above-mentioned embodiments are only illustrations for working the present invention. Therefore, the present invention is not limited to the above-mentioned embodiments. The above-mentioned embodiments can be suitably changed within a scope not departing from the gist of the invention.

[0136] 
  1. 1. porous layer
  2. 2. compound layer
  3. 3. nitrogen diffused layer



Claims

1. A nitrided steel part comprising a steel material as a material, the steel material consisting of, by mass%,
C: 0.2 to 0.6%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 2.5%,
P: 0.025% or less,
S: 0.003 to 0.05%,
Cr: 0.05 to 0.5%,
Al: 0.01 to 0.05%,
N: 0.003 to 0.025% and
a balance of Fe and impurities,
the nitrided steel part comprising a compound layer of a thickness of 3 µm or less comprising iron, nitrogen, and carbon formed on the steel surface and a hardened layer formed under the compound layer,
an effective hardened layer depth of the nitrided steel part being 160 to 410 µm.
 
2. The nitrided steel part of claim 1 wherein the steel material contains, in place of part of Fe, one or both of Mo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
 
3. The nitrided steel part of claim 1 or 2 wherein the steel material contains, in place of part of Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
 
4. The nitrided part of any one of claims 1 to 3 wherein the steel material contains, in place of part of Fe, Ti: 0.005 to less than 0.05%.
 
5. A method of nitriding comprising
using as a material a steel material consisting of, by mass%,
C: 0.2 to 0.6%,
Si: 0.05 to 1.5%,
Mn: 0.2 to 2.5%,
P: 0.025% or less,
S: 0.003 to 0.05%,
Cr: 0.05 to 0.5%,
Al: 0.01 to 0.05%,
N: 0.003 to 0.025% and
a balance of Fe and impurities and
gas nitriding by heating the steel material in a gas atmosphere containing NH3, H2, and N2 to 550 to 620°C, and making the overall treatment time A 1.5 to 10 hours,
the gas nitriding comprised of high KN value treatment having a treatment time of X hours and a low KN value treatment after the high KN value treatment having a treatment time of Y hours,
the high KN value treatment having a nitriding potential KNX determined by formula (1) of 0.15 to 1.50 and having an average value KNXave of the nitriding potential KNX determined by formula (2) of 0.30 to 0.80,
the low KN value treatment having a nitriding potential KNY determined by formula (3) of 0.02 to 0.25, having an average value KNYave of the nitriding potential KNY determined by formula (4) of 0.03 to 0.20 and having an average value KNave of the nitriding potential determined by formula (5) of 0.07 to 0.30:









wherein, in formula (2) and formula (4), the subscript "i" is a number indicating the number of measurements for each constant time interval, X0 indicates the measurement interval (hours) of the nitriding potential KNX, Y0 indicates the measurement interval (hours) of the nitriding potential KNY, KNXi indicates the nitriding potential at the i-th measurement during the high KN value treatment, and KNYi indicates the nitriding potential at the i-th measurement during the low KN value treatment.
 
6. The method of production of the nitrided steel part of claim 5 wherein the gas atmosphere includes a total of 99.5 vol% of NH3, H2, and N2.
 
7. The method of production of the nitrided steel part of claim 5 or 6 wherein the steel material contains, in place of part of the Fe, one or both of Mo: 0.01 to less than 0.50% and V: 0.01 to less than 0.50%.
 
8. The method of production of the nitrided steel part of any one of claims 5 to 7 wherein the steel material contains, in place of part of the Fe, one or both of Cu: 0.01 to less than 0.50% and Ni: 0.01 to less than 0.50%.
 
9. The method of production of the nitrided part of any one of claims 5 to 8 wherein the steel material contains, in place of part of the Fe, Ti: 0.005 to less than 0.05%.
 




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Cited references

REFERENCES CITED IN THE DESCRIPTION



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Patent documents cited in the description