JP4291941B2 - Soft nitriding steel with excellent bending fatigue strength - Google Patents

Description

【００３４】
【表２】

【００３５】
【発明の効果】
以上のように本発明によれば、疲労特性に優れた軟窒化用鋼であって、高負荷のかかるクランクシャフトや歯車等の部品に適用した場合であっても、充分な疲労強度を発揮し、高強度の軟窒化用鋼が提供できる。なお、本鋼は熱間圧延や熱間鍛造後、または更に焼きならし処理などのを行った状態でも素材の硬さが低いため、冷間鍛造部材にも好適に適用できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel for soft nitriding that can exhibit a bending fatigue strength equal to or higher than that of a carburized material after soft nitriding. The steel for soft nitriding is suitably used for high-strength structural components that dislike the occurrence of heat treatment distortion such as gears, crankshafts, and connecting rods.
[0002]
[Prior art]
In recent years, with increasing output and weight of automobiles, there has been a demand for increasing the strength of structural parts. Therefore, for parts that require fatigue strength and wear resistance such as gears and shafts, surface hardening treatment such as carburizing has been performed conventionally, but when carburizing treatment is performed, There is a problem that large distortion occurs during quenching. There is soft nitriding as a surface hardening treatment with small heat treatment strain, but this method has a problem that sufficient bending fatigue characteristics cannot be obtained, and the soft nitriding treatment is performed for several hours in a temperature range of 500 to 600 ° C. Therefore, since the internal hardness is reduced, steel having a relatively high carbon content such as SACM645 standardized by JIS is used, and there is a problem in workability such as cutting.
[0003]
Further, for the purpose of precipitation hardening during soft nitriding, there are JP-A-5-59488 , JP-A-7-138701 , and JP-A-10-306343 , which are caused by precipitation of intermetallic compounds and Cu during soft nitriding. It is intended to improve fatigue strength.
[0004]
However, conventionally, it has not always been sufficient to achieve both improvement in bending fatigue strength and workability due to the Cu precipitation effect.
[0005]
[Problems to be solved by the invention]
The present invention has been made paying attention to such circumstances, and an object of the present invention is to provide a steel for nitrocarburizing that can exhibit excellent fatigue strength characteristics over conventional nitrocarburized steel.
[0006]
[Means for Solving the Problems]
In order to obtain excellent bending fatigue characteristics after the soft nitriding treatment, the present inventors obtain excellent precipitation hardening by precipitating Cu in ferrite, and high fatigue strength is obtained. It has been found that a steel material structure for obtaining the effect is more effective in a structure mainly composed of a ferrite structure than in a martensite structure, a bainite structure, or a pearlite structure. In addition, the pearlite structure and cementite when the matrix is a ferrite structure are in a form that exists at the ferrite grain boundary, and in particular, cementite is generated in the form of a film at the grain boundary, and thus it is found that it causes a decrease in fatigue strength. As a means to solve this, it is desirable to reduce the amount of C. However, since the reduction of the amount of C lowers the hardness of the material, the carbide of Ti, V, and Nb is used to make the carbide as fine as possible, and by precipitation hardening. It has been found that the internal hardness can be increased and the fatigue strength can be improved.
[0007]
The steel for soft nitriding of the present invention includes C: 0.01% to 0.15%, Si: 0.01% to 1.5%, Mn: 0.15% to 2%, Cu: 0.5% to 2% , Al: 0.018 to 0.5%, N: limited to less than 0.005%, Ti: 0.01% to 0.5%, V: 0.05% to 0.5% %, Nb: at least one of 0.005% to 0.5% and C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3, with the balance being Fe and inevitable impurities It consists of elements, the area ratio of ferrite is 90% or more, the remainder is made of carbide, or carbide and pearlite structure, and the average size of pearlite is 20 μm or less. Although it is based on the above, the hot ductility of steel can be improved by containing Ni: 0.5%-2% as needed. Furthermore , the soft nitriding characteristic can be enhanced by containing one or more of Cr: 0.1% to 2%, and S: 0.03% to 0.1%, Pb: 0.005. It is also effective to contain one or more elements from% to 0.3%, and the addition of these elements is effective in improving machinability.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Details of the present invention will be described below.
[0009]
C: 0.01% to 0.15%
Usually, C is necessary to obtain the strength of steel. Therefore, the lower limit was made 0.01%. However, the upper limit was made 0.15% in order to suppress the pearlite structure as much as possible and keep it in a granular form of 20 μm or less.
[0010]
Si: 0.01% to 1.5%
Si is an element useful as a deoxidizer at the time of melting, and is effective in improving the strength of steel. However, if the amount is too large, the workability deteriorates, so the content was made 1.5% or less.
[0011]
Mn: 0.15% to 2%
Mn is an element useful as a deoxidizer at the time of melting, and is effective in improving the strength of steel, and is set to 0.15% or more. However, if the amount is too large, the hardenability of the steel becomes high and martensite and bainite structures tend to be generated, so the upper limit was made 2%.
[0012]
Cu: 0.5% to 2%
Cu is an element that contributes to age hardening of the core hardness during soft nitriding, and is essential in this steel in order to obtain high fatigue strength. In order to obtain the effect, 0.5% or more is necessary. However, if the amount is too large, hot brittleness occurs and cracks occur in the manufacturing process, so the content was made 2% or less.
[0013]
N: Less than 0.005% N combines with Ti, Nb, and V to form a nitride. When the N concentration increases, a large amount of Ti, Nb, and V necessary for forming a nitride is required, and TiN and the like become coarse when the amount of nitrogen is high, which causes a decrease in fatigue strength. Although the concentration is desirably as low as possible, it is allowed to be less than 0.005%, so the upper limit was made less than 0.005%.
[0014]
Ti: 0.01% to 0.5%
Ti produces carbides and is effective in suppressing film-like cementite which causes deterioration of fatigue strength.
[0015]
TiC carbide is effective for precipitation hardening in hot rolling or hot forging, and can improve the strength of the steel material. Further, the amount of addition is closely related to the amount of C in steel, and is necessary for generating carbide for suppressing film-like cementite. Since these effects can be achieved by combined addition with V or Nb, the lower limit was made 0.01% or more. Further, Ti combines with N that penetrates during soft nitriding to produce nitride, and has the effect of improving the surface hardness. Even if added over 0.5% or more, the effect is saturated, so the content was made 0.5% or less.
[0016]
V: 0.05-0.5%
V produces carbides like Ti, and is effective in suppressing film-like cementite. V is an element that can combine with nitrogen entering during soft nitriding to form nitrides and increase the surface hardness. In order to obtain these effects, 0.05% or more is necessary, but even if added over 0.5%, the effect is saturated, so 0.5% or less was set.
[0017]
Nb: 0.005% to 0.5%
Nb also produces carbides like Ti and V, and is effective in suppressing film-like cementite. Nb is an element that can combine with nitrogen that enters during soft nitriding to form nitrides and increase the surface hardness. In order to obtain the effect, 0.005% or more is necessary, but even if added over 0.5%, the effect is saturated, so 0.5% or less.
[0018]
The area ratio of ferrite before nitrocarburizing is 90% or more, the remainder is carbide, or carbide and pearlite structure, and the average size of pearlite is 20 μm or less:
For the present invention, it plays an important role for the structure and fatigue strength of steel. As the structure before soft nitriding, a ferrite structure is desirable, and the lower limit is 90%. The carbide as the remaining structure has as little cementite as possible, and is desirably fine carbide such as Ti, Nb, and V. Further, even if a pearlite structure is generated, it is desirable that the size is small. When the average size of pearlite was 20 μm or less, it was found that the decrease in fatigue strength was small, and the average size was set to 20 μm or less.
[0019]
For the ferrite area ratio, for example, the structure was observed using an optical microscope, and the occupied area ratio of the ferrite portion in the field of view was determined by image analysis. In the examples described later, the structure of 10 visual fields was arbitrarily observed with an optical microscope at 400 times, and the average value was defined as the area occupied by ferrite.
[0020]
The size of the pearlite of the present invention is, for example, in the examples described later, by using an optical microscope, ten visual fields are randomly observed at 400 times, the major axis of all pearlite existing in the visual field is measured, and the average value thereof Was the average size.
[0021]
C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3:
As described above, the steel structure must be 90% or more of ferrite and the average size of pearlite must be 20 μ or less. For this purpose, it is necessary to use fine carbides of Ti, Nb, and V, and the addition amount of each element must satisfy the relationship of the above formula.
[0022]
Ni: 0.5% to 2%
Ni has an effect of suppressing hot brittleness caused by Cu. In order to obtain the effect, 0.5% or more is necessary, and if it exceeds 2%, the effect is saturated, so the upper limit is set.
[0023]
Cr: 0.1% to 2%
Cr is an element necessary for obtaining surface hardness by soft nitriding. In order to obtain the effect, 0.1% or more is necessary. However, even if it is 2% or more, the effect is small, the hardenability of the steel is increased, and martensite and bainite structure are easily generated, so 2% was made the upper limit.
[0024]
Al: 0.018 % to 0.5%
Al is an element that can combine with nitrogen that enters during soft nitriding to form nitrides and increase the surface hardness. In order to obtain the effect, 0.018% or more is necessary, but even if added over 0.5%, the effect is saturated, so the upper limit was set. Preferably, it is 0.05 to 0.5%.
[0025]
S: 0.03% to 0.1%
S is an element that improves machinability, and is added as necessary. However, if it is less than 0.03%, the effect is small, and if it exceeds 0.1%, the strength deteriorates.
[0026]
Pb: 0.005% to 0.3%
Pb is an element that improves the machinability of steel and is added as necessary. However, if it is less than 0.005%, the effect is small, and if it exceeds 0.3%, the strength deteriorates, so the upper limit is 0. .3%.
[0027]
【Example】
Hereinafter, the present invention will be described with reference to examples. After the composition of the chemical components shown in Table 1 was melted in a 150 kg vacuum melting furnace, a round bar having a diameter of 40φ was manufactured by hot rolling. This round bar was heated to 1200 ° C., forged to 20φ by hot forging, and then air-cooled to use this material as a raw material. Further, the cooling rate at this time is 1.0 ° C./second, and it is necessary to dissolve Cu in this cooling process. For that purpose, cooling should be performed at a cooling rate of 0.2 ° C./second or more. .
[0028]
Thereafter, an Ono type rotary bending test piece having a parallel portion of 10 mmφ was prepared. This test piece was subjected to gas soft nitriding at 580 ° C. for 120 minutes. Soft nitriding was performed in a gas atmosphere of RX gas: NH ₃ = 1: 1.
[0029]
The JIS-SCM415 steel used in Comparative Example 22 was carburized at 930 ° C. for 4 hours, then oil-quenched and then tempered at 180 degrees for 2 hours. In addition to measuring the internal and internal hardness of 50 μm from the surface of the test piece thus treated, an Ono type rotary bending test piece was carried out.
[0030]
Table 2 shows the hardness of the steel material of the present invention and the comparative example, the surface hardness after soft nitriding, the ferrite area ratio of the structure, the average size of carbide and pearlite, and the fatigue limit strength in the Ono type rotating bending fatigue test. Indicated.
[0031]
In Table 2, Examples 1-4 and 6-15 of the present invention have steel compositions and structures corresponding to claims 1 to 4 of the present invention, and have high fatigue strength.
[0032]
On the other hand, in Comparative Example 16, the amount of C was large, the ferrite area ratio of the structure was less than 90%, and the fatigue strength was reduced. In Comparative Example 17, the relationship of C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3 was not satisfied, the pearlite size was increased, and the fatigue strength was reduced. In Comparative Example 18, the amount of Cu added was insufficient, and the effect of improving fatigue strength was not obtained. In Comparative Example 19, the amount of Mn added was high, the ferrite area ratio of the structure was insufficient, and the fatigue strength was reduced. In Comparative Example 20, the amount of Cr added was high, the ferrite area ratio of the structure was insufficient, and the fatigue strength was reduced. Comparative Example 21 had a high N content, so that coarse TiN was generated and the fatigue strength was reduced. As described above, it can be seen that excellent fatigue strength can be obtained according to the examples of the present invention.
[0033]
[Table 1]

[0034]
[Table 2]

[0035]
【The invention's effect】
As described above, according to the present invention, a soft nitriding steel having excellent fatigue characteristics, which exhibits sufficient fatigue strength even when applied to components such as crankshafts and gears that are subjected to high loads. High strength steel for soft nitriding can be provided. In addition, since this steel has low material hardness even after hot rolling or hot forging, or in a state of further normalizing, it can be suitably applied to cold forged members.

Claims (4)

% By mass
C: 0.01% to 0.15%,
Si: 0.01% to 1.5%,
Mn: 0.15% to 2%,
Cu: 0.5% ~2%,
Al: 0.018 to 0.5%
Containing
N: limited to less than 0.005%,
Ti: 0.01% to 0.5%,
Nb: 0.005% to 0.5%,
V: 0.05% to 0.5%
Containing one or more of
And C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3,
Bending fatigue, characterized in that the balance is Fe and inevitable impurity elements, the area ratio of ferrite is 90% or more, the balance is carbide, or carbide and pearlite structure, and the average size of pearlite is 20 μm or less Soft nitriding steel with excellent strength. % By mass
C: 0.01% to 0.15%,
Si: 0.01% to 1.5%,
Mn: 0.15% to 2%,
Cu: 0.5% ~2%,
Al: 0.018 to 0.5%
Containing
N: limited to less than 0.005%,
Ti: 0.01% to 0.5%,
Nb: 0.005% to 0.5%,
V: 0.05% to 0.5%
Containing one or more of
Ni: 0.5% to 2%
And C + N ≦ Ti / 4.0 + Nb / 7.7 + V / 4.3, the balance being made of Fe and inevitable impurity elements, the area ratio of ferrite being 90% or more, and the remainder being carbide, or A nitrocarburizing steel excellent in bending fatigue strength, characterized by comprising a carbide and a pearlite structure and having an average pearlite size of 20 μm or less. The steel component according to claim 1 or 2, further in mass%,
Cr: 0.1% to 2%
Containing, in area ratio of ferrite is 90% or more, from the rest carbides or carbides and pearlite, soft-nitriding steel for the average size of pearlite and excellent bending fatigue strength, characterized in that at 20μm or less . The steel component according to claims 1 to 3, further in mass%,
S: 0.03% to 0.1%
Pb: 0.005% to 0.3%
The bending fatigue strength is characterized in that it contains at least one of the above, the ferrite area ratio is 90% or more, the remainder consists of carbide, or carbide and pearlite structure, and the average size of pearlite is 20 μm or less. Excellent nitrocarburizing steel.