JPH09324258A - Nitrided parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide nitrided parts excellent in machinability and nitriding properties of the steel even if normalizing treatment after hot forging is omitted and furthermore excellent in fatigue characteristics an bending straightening properties. SOLUTION: A steel having a ferritic-pearlitic structure in which the average dimension of the crystal grains of ferrite is regulated to <=50μm and the average dimension of the crystal grains of pearlite is regulated to <=50μm as hot-forged and having a compsn. contg. 0.15 to 0.40% C, <=0.50% Si, 0.20 to 1.50% Mn and 0.05 to 0.50% Cr, and the balance Fe with inevitable impurities is prepd. The steel is subjected to gas soft-nitriding treatment to regulate the average hardening depth to >=0.3mm and the fluctuation of the hardening depth to <=0.1mm. The steel may contain <=0.50% Ni, <=0.50% Mo, 0.005 to 0.030% N, <=0.3% V, <=0.3% Nb, <=0.2% Ti, <=0.2% Zr, <=0.2% Ta, 0.01 to 0.3% S, <=0.3% Pb, <=0.05% Ca, <=0.2% Bi and <=0.05% Te.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of properties of steel parts subjected to nitriding treatment.

[0002]

2. Description of the Related Art Nitriding is applied to mechanical parts such as gears, shaft parts and crankshafts for the purpose of improving wear resistance and fatigue characteristics of steel parts. Carbon steel such as JIS S48C is used for general nitriding parts, and its manufacturing process is usually hot forging → normalizing → machining → nitriding → bending straightening. There is.

In the process of manufacturing a nitriding component, first, a rolled steel material is hot forged to produce a component raw material. However, the hot forged raw material has a metal structure because the cooling rate differs depending on the site. And hardness become non-uniform, resulting in a decrease in machinability.
In addition, when the nitriding treatment is performed with the heterogeneous metallographic structure as it is, the variation of the hardening depth depending on the site becomes large,
Fatigue properties and bend straightening properties of parts deteriorate significantly.

Therefore, after the hot forging, it is usual to perform a heat treatment such as the above-mentioned normalizing treatment in order to promote the improvement and softening of the metal structure of the steel. If this heat treatment can be omitted, a great effect can be expected in terms of improving the productivity of the nitriding component and saving energy. Further, in the nitriding treatment, it is unavoidable that a product having a complicated shape or a long product is deformed or bent. Therefore, the straightening is usually performed after the nitriding treatment. Bending straightening of a nitriding product having a hard and brittle hardened layer on the surface layer by nitriding is an extremely difficult work, and it is very important to improve the bending straightening property of the nitriding component.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object of the present invention is to cut steel even if normalizing treatment after hot forging is omitted. It is intended to provide a nitrided component which is excellent in fatigue resistance and bending straightening property as well as in fatigue resistance and nitriding property.

[0006]

In order to achieve the above-mentioned object, the nitriding component of the present invention is (1) a nitriding component obtained by nitriding steel, wherein the steel has a mass as an alloy component. %, C: 0.15 to 0.
40%, Si: 0.50% or less, Mn: 0.20 to 1.
50%, Cr: 0.05 to 0.50%, balance F
e and unavoidable impurities, and the hot-worked steel has a mixed structure of ferrite and pearlite, and the average crystal grain size of the ferrite is 50.
μm or less, the average size of the crystal grains of the pearlite is 50 μm or less, the average hardening depth by the nitriding treatment is 0.3 mm or more, and the fluctuation of the hardening depth is 0.1 mm or less. It is characterized by (2) In the nitriding component described in (1) above, the steel contains, in addition to the above alloy components, mass% and Ni: 0.50%.
Hereinafter, it is characterized by containing any one or two of Mo: 0.50% or less. (3) In the nitriding component according to any one of (1) and (2) above, the steel contains N: 0.005 to 0.030% and V: mass% in addition to the alloy components. 0.3
% Or less, Nb: 0.3% or less, Ti: 0.2% or less, Z
r: 0.2% or less, Ta: 0.2% or less, and one or more of them are contained. (4) In the nitriding component according to any one of (1), (2) and (3) above, the steel contains S: 0.01 to 0.3 in mass% in addition to the alloy components. %, Pb:
0.3% or less, Ca: 0.05% or less, Bi: 0.2%
Hereinafter, it is characterized by containing any one or more of Te: 0.05% or less.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The nitriding-processed component of the present invention is obtained by hot working a steel having a predetermined chemical composition into a raw material, machined if necessary, and subjected to nitriding, and if necessary, It is manufactured by straightening. The reason for defining the chemical composition of steel in the nitriding component of the present invention will be described. C: 0.15 to 0.40% C is an element necessary for maintaining the strength of parts, and for that purpose, it is necessary to contain at least 0.15% or more. However, if the C content becomes excessive, the machinability deteriorates because the hardness after hot working rises, and the crystal grain size of pearlite in the metal structure becomes large, so that it easily breaks during bending correction. Therefore, the upper limit of the C content is 0.40%. Si: 0.50% or less Si is added as a deoxidizing element of steel during melting of steel. Also, it is added to strengthen the ferrite phase. However, if added excessively, the ferrite phase becomes brittle, so the upper limit of the content is made 0.5%. Mn: 0.20 to 1.50% Like Si, Mn is added as a deoxidizing element of steel during the melting of steel. Further, it is added in order to combine with S to prevent red heat embrittlement, enhance the machinability of steel, and further refine the microstructure of steel to contribute to an increase in strength. It is necessary to add 0.2% or more in order to secure the required strength as a part.
However, if added excessively, the hardness after hot working increases and the machinability of steel deteriorates, so the upper limit of the content is set to 1.50%. Cr: 0.05 to 0.50% Cr is added in an amount of 0.05% or more in order to strengthen the ferrite phase. However, if added excessively, the machinability of the steel is deteriorated, and breakage easily occurs in the straightening of the nitriding component, so the upper limit of the content is made 0.50%. Ni: 0.50% or less, Mo: 0.50% or less Ni and Mo strengthen ferrite and increase fatigue strength. However, since it increases the hardenability of steel, if it is contained excessively, the hardness of steel is increased and the machinability is impaired.
The upper limit of the content of o is 0.5%, and one or two of them is contained. N: 0.005 to 0.030% N forms a strong compound with V, Nb, Ti, Zr, and Ta to refine ferrite crystal grains. For that purpose, it is necessary to contain at least 0.005% or more.
However, if the content is too large, a large amount of hard nitrides are formed and the machinability of steel is impaired, so the upper limit of the content is made 0.030%. V: 0.3% or less, Nb: 0.3% or less, Ti: 0.2
% Or less, Zr: 0.2% or less, Ta: 0.2% or less These elements are added in order to combine with N and refine the ferrite crystal grains. However, if added excessively, the machinability of steel is impaired, and a hard and brittle nitride phase is formed during the nitriding treatment, which makes bending straightening difficult. Therefore these elements are
V: 0.3%, Nb: 0.3%, Ti: 0.2
%, Zr: 0.2%, Ta: 0.2% as the upper limit, and 1
One kind or two or more kinds may be contained. S: 0.01 to 0.3%, Pb: 0.3% or less, Ca:
0.05% or less, Bi: 0.2% or less, Te: 0.05
% Or less These elements are added to improve the machinability of steel. Particularly, S is preferably added at least 0.01% in order to maintain the machinability of steel. However, if any of these elements is excessively added, non-metallic inclusions in the steel increase and fatigue strength decreases, so the upper limits of the contents are S: 0.3%, Pb: 0.3%, C
One or two or more may be contained as a: 0.05%, Bi: 0.2%, Te: 0.05%. The steel constituting the nitriding-treated component of the present invention has a mixed structure of ferrite and pearlite when hot-worked into a raw material. The average size of the crystal grains of ferrite and pearlite forming the mixed structure is 50 μm or less. The hot working can be performed by a usual method such as rolling or forging.
Moreover, the steel composition of the present invention can easily provide the above-mentioned metal structure.

By adjusting the metal structure as described above, it is possible to make the raw material after hot working excellent in machinability. When nitriding steel having a metallographic structure composed of ferrite and pearlite, the ferrite portion is preferentially nitrided and hardened. When the ferrite crystal grains and the pearlite crystal grains are large, the front surface of the hardened layer generated by the nitriding treatment largely undulates and changes in accordance with the crystal grain size. That is, the variation in the curing depth is large. If the variation in the hardening depth is large, the nitriding-processed component is likely to be broken when bent and the fatigue strength is also deteriorated. By setting the average size of each crystal grain of ferrite and pearlite to 50 μm or less, the variation in the hardening depth does not substantially affect the bend straightening property and the fatigue strength.

The nitriding treatment may be any of salt bath nitriding, gas soft nitriding and ion nitriding. When the hardening depth by nitriding is shallow, the fatigue strength is not sufficiently improved. Also,
If the variation of the hardening depth is large, the bend straightening property is deteriorated. In order to obtain high fatigue strength and excellent bend straightening property, the nitriding-treated part of the present invention has an average hardening depth of 0.3 in the nitriding treatment.
mm or more, and the fluctuation of the hardening depth is within 0.1 mm.

Here, as the hardening depth by the nitriding treatment, the hardness distribution is measured from the surface to the inside of the nitriding treated component according to JIS G0562, and the hardness is 50 from the core hardness.
HV The distance from the surface at the position where high hardness is obtained.

[0011]

Embodiments of the present invention will be described below. Steel having the chemical composition shown in Table 1 was melted and hot rolled to obtain a rolled steel bar having a diameter of 80 mm. The rolled steel bar is 1200
After heating and holding at 0 ° C., it was forged into a round bar having a diameter of 50 mm by hot forging to obtain a test material.

[0012]

[Table 1]

The metallographic structure in the cross section of the test material was examined by light irradiation.
Analysis with a microscope and image analyzer
The average size of the grit and pearlite grains was determined.
That is, for each crystal grain appearing in the cross section,
Measure the maximum diameter in two directions and calculate the average value.
Is the diameter of each crystal grain. Analysis is 0.5 mm per field of view ^Two
As an example, it is included in this for any 30 fields of view.
Find the diameters of all the crystal grains and calculate the ferrite crystal grains.
And the average diameter of pearlite grains for each phase
Therefore, the average size of each is used.

A round bar having a diameter of 10 mm and a length of 200 mm was cut out from the D / 4 position of the test material by machining,
This was subjected to gas soft nitriding treatment which was held at 555 ° C. for 3 hours. The hardness distribution from the surface to the center was measured using a Vickers hardness meter for 10 arbitrary cross sections of the nitriding material. The cure depth was defined as the distance from the surface exhibiting a hardness 50 HV higher than the hardness at the center, and the average value of the cure depths at the 10 locations was determined and defined as the average cure depth. Also,
The maximum value and the minimum value of the 10 curing depths were taken, the difference between them was determined, and this was used as the variation of the curing depth.

Table 2 shows the average size of the ferrite crystal grains, the average size of the pearlite crystal grains, the average hardening depth, and the measurement results of variations in the hardening depth.

[0016]

[Table 2]

The machinability was judged by the tool life during outer diameter turning with a cutting tool, using the test material. Cutting conditions: cutting speed: 150 mm / min, feed: 0.2 mm / r
ev, depth of cut: 2 mm, tool boundary wear is 0.2
The time until reaching mm was defined as the tool life time. According to JIS Z2274, a rotary bending fatigue test piece having a notch with a test portion diameter of 8 mm and a shape factor of 1.63 was carved out from the test material, and a gas soft material kept at 555 ° C. for 3 hours was cut out from the test material. Nitriding was performed. Fatigue characteristics were evaluated by obtaining 10 ⁷ times strength.

A round bar having a diameter of 10 mm and a length of 200 mm was cut out from the D / 4 position of the test material by machining,
This was subjected to a gas soft nitriding treatment that was held at 555 ° C. for 3 hours to obtain a bending test piece. Bending test: Distance between fulcrums is 150m
m, and a three-point bending test method in which a central concentrated load is applied. The deflection at the load point was measured with a dial gauge, and the bending straightness was evaluated by the maximum deflection until breakage.

Table 3 shows the measurement results of tool life time, 10 ⁷ times strength, and maximum deflection amount.

[0020]

[Table 3]

As is clear from Table 3, in the examples of the present invention, the tool life in the as-forged state is longer and the machinability is superior to the comparative examples. Further, it can be seen that the fatigue characteristics after the gas nitrocarburizing treatment are also high, and particularly the bending straightening property is excellent.

[0022]

As described above, according to the present invention, the chemical composition, metallographic structure and nitriding hardening depth of steel can be adjusted without heat treatment such as normalizing after hot working of steel. , Excellent in machinability, easy to straighten after nitriding,
In addition, it is possible to economically provide a nitriding component having high fatigue strength.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takaki Mizuno 1-4-1 Chuo, Wako-shi, Saitama Inside the Honda R & D Co., Ltd. (72) Inventor Masaki Amano 1-4-1 Wako, Saitama Prefecture Stock Company Honda Technical Research Institute

Claims (4)

[Claims] 1. A nitriding component obtained by nitriding steel, wherein the steel is C: 0.1 in mass% as an alloy component.
5 to 0.40%, Si: 0.50% or less, Mn: 0.2
0 to 1.50%, Cr: 0.05 to 0.50%, the balance Fe and unavoidable impurities, and the steel is a hot-worked as-mixed structure composed of ferrite and pearlite. The average size of the crystal grains of the ferrite is 50 μm or less, the average size of the crystal grains of the pearlite is 50 μm or less, the average hardening depth by the nitriding treatment is 0.3 mm or more, and A nitriding component, wherein the variation of the hardening depth is within 0.1 mm. 2. The nitriding component according to claim 1,
The steel contains Ni: 0.
A nitriding component containing 50% or less and Mo: 0.50% or less any one kind or two kinds. 3. The nitriding component according to claim 1, wherein the steel contains N: 0.005 to 0.030% and V in mass% in addition to the alloy components. : 0.3% or less, Nb: 0.3% or less, Ti: 0.2
% Or less, Zr: 0.2% or less, Ta: 0.2% or less, and one or more kinds of nitriding-treated parts. 4. The nitriding component according to any one of claims 1, 2 and 3, wherein the steel contains, in mass% in addition to the alloy components, S: 0.01 to 0. Three
%, Pb: 0.3% or less, Ca: 0.05% or less, B
i: 0.2% or less, Te: 0.05% or less 1
A nitriding component, which contains one or more species.