JP3405277B2 - Steel wire rod, steel bar and steel pipe for bearing element parts with excellent machinability - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel wire rod, a steel bar and a steel pipe for bearing element parts having excellent machinability. More specifically, the present invention relates to a steel wire rod, a steel bar and a steel pipe having excellent machinability suitable for use in bearing element parts such as balls, rollers, needles, shafts and races.

[0002]

2. Description of the Related Art Generally, JIS is used as a material steel for bearing element parts such as balls, rollers, needles, shafts and races.
High carbon chrome bearing steel such as SUJ2 steel standardized by G 4805 is often used.

The so-called "bearing steel" is processed by means such as hot rolling, then subjected to spheroidizing annealing for the purpose of softening, and then subjected to processing such as cold forging, cold drawing and cutting. And further subjected to heat treatment by quenching and tempering at low temperature to impart the desired mechanical properties.

In the cutting process, and most of the turning processes, among the above processes, the surface layer has a thickness of at least 200 μm in order to improve the surface texture and dimensional accuracy.
m, and in many cases, about 500 μm is cut, and the cost of this cutting process increases, so there is a need for a bearing steel with excellent machinability that can improve the cutting speed and extend the tool life. It has become extremely large.

It is well known that the machinability is improved by adding a free-cutting element (machinability improving element) such as Pb or S to the steel alone or in combination. However, high surface pressure is repeatedly applied to bearings used in various industrial machines and automobiles. Therefore, if the free cutting element is added to the bearing steel, the rolling contact fatigue life of the bearing (element component) will be significantly reduced. Furthermore, the free-cutting elements generally reduce hot workability. Therefore, there is also a problem that surface cracks and flaws are likely to occur during hot working such as hot rolling.

For this reason, Japanese Patent Laid-Open No. 3-56641 discloses that a BN compound is produced in steel to improve the machinability without lowering the rolling fatigue life. Is disclosed. However, since B has a low solubility in steel, the yield in steel is unstable and segregation easily occurs. Further, B significantly lowers the solidification start temperature of the high carbon steel, and therefore, in combination with the segregation of B,
Solidification segregation will be promoted. In addition, the lowering of the solidification start temperature leads to the lowering of hot workability, and surface cracks and flaws are likely to be generated during hot working. Therefore, even if the B content of the bearing steel is set to the value specified in the above-mentioned publication, that is, 0.004 to 0.020% by weight, the bearing element parts can be manufactured stably on an industrial scale. It wasn't something I could do.

Japanese Unexamined Patent Publication (Kokai) No. 9-227991 discloses "a bearing steel having excellent machinability and cold workability and a method for producing the same" in which heat treatment is performed under specific conditions to adjust the number of carbides in the structure and hardness. It is disclosed. However, under the annealing conditions proposed in this publication, it is necessary to perform gradual heating or isothermal holding during the heating process. Therefore, the annealing time becomes long and the productivity is lowered. Furthermore, since it is necessary to change the heat treatment conditions such as slow heating, rapid heating, and slow cooling, for example, a winding coil that is a general shape of a steel wire rod (hereinafter, "steel wire rod" is simply referred to as "wire rod") is used. When it is a target, it is difficult to uniformly heat-treat (anneal) the entire coil. Even if a uniform heat treatment can be performed, a continuous heat treatment furnace used on an industrial scale generally has a fixed temperature in each zone, and the number of zones is limited. Therefore, annealing is performed under the conditions specified in the above publication. Is difficult to carry out, and the continuous heat treatment furnace needs to be modified or renewed in order to anneal under the specified conditions, resulting in an increase in cost.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and an object thereof is to add 10 times the same as conventional ones without adding and containing a free-cutting element. To provide wire rods, steel bars and steel pipes having excellent machinability suitable for use in bearing element parts such as balls, rollers, needles, shafts, races, etc. without lowering productivity because it takes about 20 hours. Is.

As already mentioned, since high bearing pressure is repeatedly applied to the bearing, the goal is to have a rolling fatigue life of 1 × 10 ⁷ or more in the rolling fatigue test in the examples described later. To do.

[0010]

The gist of the present invention is to provide wire rods for bearing element parts having excellent machinability as shown in (1) and (2) below, and machinability as shown in (3) and (4). And a steel pipe for bearing element parts, which is excellent in machinability as shown in (5) and (6) .

(1) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
0.05% , the balance consists of Fe and unavoidable impurities, P in the impurities is 0.02% or less, S is 0.015
% Or less, N is 0.007% or less, and O (oxygen) is 0.0
015% or less, Ti 0.002% or less, Cu 0.0
Less than 5%, Ni less than 0.2%, Mo less than 0.05%
Full, V is less than 0.05%, Nb is less than 0.01%, B is
Less than 0.0003%, total rare earth elements 0.001%
Less, Ca less than 0.0001%, Mg 0.0001
% , And the average value of C content in the region from the outer periphery to the position of 200 μm in depth in the cross section of the steel wire rod is 0.4 × C to 0.9 × C% (where C is the value of the steel wire rod). A steel wire rod for bearing element parts having excellent machinability, characterized in that it has a C content).

(2) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
In addition to containing 0.05% , Cu: 0.05 to 2.
0 %, Ni: 0.2 to 4.0%, Mo: 0.05 to 0.
5 %, V: 0.05 to 0.4 %, Nb: 0.01 to 0.
1%, B: 0.0003~ 0.003% , the rare earth elements:
0.001 to 0.01 % in total , Ca: 0.0001 to
0.003% and Mg: 0.0001 to 0.003%
1 or more of the above , the balance consisting of Fe and unavoidable impurities, Ti in the impurities is 0.002% or less, P is 0.02% or less, S is 0.015% or less, and N is 0. 00
7% or less, O (oxygen) is 0.0015% or less, and
In the cross section of the steel wire rod , the average value of C content in the region from the outer periphery to the position of 200 μm in depth is 0.4 × C to
Steel wire for bearing element parts with excellent machinability, characterized in that it is 0.9 x C% (where C is the C content of the steel wire material ).
Material .

(3) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
0.05% , the balance consists of Fe and unavoidable impurities, P in the impurities is 0.02% or less, S is 0.015
% Or less, N is 0.007% or less, and O (oxygen) is 0.0
015% or less, Ti 0.002% or less, Cu 0.0
Less than 5%, Ni less than 0.2%, Mo less than 0.05%
Full, V is less than 0.05%, Nb is less than 0.01%, B is
Less than 0.0003%, total rare earth elements 0.001%
Less, Ca less than 0.0001%, Mg 0.0001
Less than%, further, the outer circumference or RaFukashi of Te cross section smell bars 2
The average value of C content in the area up to the position of 00 μm is
0 . 4 × C to 0.9 × C% (however, C is the C content of the steel bar ), which is a steel bar for bearing element parts having excellent machinability.

(4) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
0.05% and Cu: 0.05-2.
0%, Ni: 0.2-4.0%, Mo: 0.05-0.
5%, V: 0.05 to 0.4%, Nb: 0.01 to 0.
1%, B: 0.0003 to 0.003% or less, rare earth element
Elementary: 0.001-0.01% in total, Ca: 0.000
1 to 0.003% and Mg: 0.0001 to 0.00
Contains at least one of 3%, the balance Fe and unavoidable
It consists of impurities, and Ti in the impurities is 0.002% or less,
P is 0.02% or less, S is 0.015% or less, and N is 0.
007% or less, O (oxygen) is 0.0015% or less,
The depth of 200 μm from the outer circumference in the cross section of the steel bar.
The average value of C content in the region up to
0.9 x C% (however, C is the C content of the steel bar)
Steel bar for bearing element parts with excellent machinability.

(5) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
Contains 0.05%, the balance from Fe and unavoidable impurities
Therefore, P in the impurities is 0.02% or less, and S is 0.015.
% Or less, N is 0.007% or less, and O (oxygen) is 0.0
0 15% or less, Ti 0.002% is less, Cu 0.0
Less than 5%, Ni less than 0.2%, Mo less than 0.05%
Full, V is less than 0.05%, Nb is less than 0.01%, B is
Less than 0.0003%, total rare earth elements 0.001%
Less, Ca less than 0.0001%, Mg 0.0001
% From the inner and outer circumferences in the cross section of the steel pipe
C content in the area up to the depth of 200 μm
Average value of 0.4 × C to 0.9 × C% (however,
C is the content of C in the steel pipe)
Steel pipe for excellent bearing element parts.

(6) C: 0.75 to 1.2 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.2 to 1.5
%, Cr: 0.2 to 2.0% and Al: 0.003 to
0.05% and Cu: 0.05-2.
0%, Ni: 0.2-4.0%, Mo: 0.05-0.
5%, V: 0.05 to 0.4%, Nb: 0.01 to 0.
1%, B: 0.0003 to 0.003% or less, rare earth element
Elementary: 0.001-0.01% in total, Ca: 0.000
1 to 0.003% and Mg: 0.0001 to 0.00
Contains at least one of 3%, the balance Fe and unavoidable
It consists of impurities, and Ti in the impurities is 0.002% or less,
P is 0.02% or less, S is 0.015% or less, and N is 0.
007% or less, O (oxygen) is 0.0015% or less,
In the transverse section of the steel pipe, a depth of 20
Average value of C content in the area up to 0 μm
The deviation is 0.4 × C to 0.9 × C% (however, C is the C of the steel pipe)
The content of the bearing is superior to the machinability.
Steel pipe for raw parts.

The "cross section" of the wire rod, the steel bar and the steel pipe in the present invention means a surface of the wire rod, the steel bar and the steel pipe cut perpendicularly to the rolling direction. Further, "the average value of the C content in the region from the outer periphery (of the wire rod, steel bar or steel pipe) to the position of depth 200 μm" means, for example, that of the wire rod, steel bar or steel pipe using a wavelength dispersion type electron beam microanalyzer. This indicates the average value of the C content at a position at a depth of 20 μm from the outer circumference when a line analysis of the C content in the outer peripheral portion (near the surface layer portion) of the cross section is performed. Similarly, “from the inner circumference (of the steel pipe) to a depth of 200μ
The average value of C content in the area up to the position of m "
For example, when line analysis of the C content in the inner peripheral portion (near the inner surface layer portion) of the steel pipe is performed using a wavelength-dispersive electron beam microanalyzer, the C content at the position of every 20 μm from the inner periphery is analyzed. It means the average value of the content.

The present inventors conducted extensive research and studies on the effects of the spheroidizing annealing of wire rods, steel bars and steel pipes, especially the internal and external surface layer structures on machinability, and as a result, obtained the following findings. It was

(A) In the turning of bearing steel, the surface structure (surface structure) of the work material has a great influence on the cutting speed and the tool life. Further, even in the case of the cut-off processing, the surface material of the wire rod, the steel bar and the steel pipe which first come into contact with the tool has a great influence on the tool life.

(B) If the surface layer portion is decarburized by an appropriate amount to reduce the ratio of hard cementite structure, the cutting resistance becomes small and the machinability can be remarkably enhanced. When turning bearing steel, the surface layer should have a thickness of at least 200 μm.
For cutting, the area to be decarburized from the surface layer to a depth of 200
The product characteristics are not affected at all up to the μm position.

(C) In the case where the surface layer has a structure mainly composed of a soft ferrite phase, the cutting resistance is increased and the machinability is rather deteriorated. Therefore, it is attempted to improve cold bending workability and forgeability by allowing a soft ferrite structure to exist in the surface layer of a steel material.
The technology proposed in Japanese Patent No. 5520 and Japanese Patent Application Laid-Open No. 8-109437 cannot improve the machinability even if it is applied to the cutting work.

(D) In order to reduce the proportion of hard cementite structure by decarburizing the surface layer portion and not to make the surface layer portion mainly composed of ferrite phase, in order to prevent the surface layer portion from having a structure mainly composed of ferrite phase, the wire rod for the bearing element and the steel bar It suffices if the carbon content in the vicinity of the outer circumference and in the vicinity of the inner and outer circumferences of the steel pipe is within an appropriate range.

(E) Since the cutting process is mainly performed in a state including the steel material surface layer portion, as long as the surface layer portion satisfies the above (d), machinability is irrelevant regardless of the internal region. Therefore, the cutting speed can be improved and the tool life can be extended.

The present invention has been completed based on the above findings.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. In addition, "%" of the content of a chemical component means "weight%." (A) Chemical composition C of wire rod, steel bar and steel pipe C: 0.75 to 1.2% Heat treatment by quenching and tempering at low temperature is performed to impart desired mechanical properties to the bearing steel material (bearing element part). If the C and C contents are less than 0.75%, the hardness after quenching and tempering is low, and the desired rolling fatigue life (1 × 10 ⁷ or more rolling fatigue life in the rolling fatigue test in the examples described later ) Cannot be obtained. On the other hand, when the content of C exceeds 1.2%, the solidification start temperature of the steel lowers, and during hot working, cracks and flaws frequently occur during hot pipe forming, and a huge amount of solidification occurs during steel solidification. Since carbides are easily generated, the target rolling fatigue life cannot be obtained unless the homogenizing heat treatment is performed for a long time. Therefore, the content of C is 0.75 to 1.2.
%.

Si: 0.1 to 1.5% Si is an element effective for increasing the rolling fatigue life and also an element necessary as a deoxidizing agent. However, if the content is less than 0.1%, it is difficult to obtain the above effects. In addition, S
When the content of i is 0.4% or more, the machinability improving effect also becomes large. On the other hand, when the Si content exceeds 0.8%, the descaling property begins to deteriorate, and particularly when it exceeds 1.5%,
Since it takes a long time to descale after hot rolling or spheroidizing annealing, the productivity is greatly reduced. Therefore, the Si content is set to 0.1 to 1.5%. The desirable range of Si content is 0.4 to 0.8%.

Mn: 0.2-1.5% Mn is an element necessary for improving the hardenability of steel and at the same time preventing hot brittleness due to S. In order to exert these effects, it is necessary to contain Mn in an amount of 0.2% or more. On the other hand, if the Mn content exceeds 1.0%, the machinability begins to decrease, and if it exceeds 1.5%, the machinability decreases significantly, leading to a reduction in cutting efficiency and a reduction in tool life. Therefore, the Mn content is set to 0.2 to 1.5%. The Mn content is preferably 0.2 to 1.0%.

Cr: 0.2 to 2.0% Cr is an element that easily concentrates in cementite, which increases the stability of cementite in austenite, shortens spheroidizing annealing, and improves wear resistance. It is possible. But,
If the content is less than 0.2%, it is difficult to obtain the above effects. On the other hand, if it exceeds 1.6%, the machinability will start to deteriorate and 2.0%
If it exceeds, the machinability is significantly reduced, leading to a reduction in cutting efficiency and a reduction in tool life. Therefore, the Cr content is 0.2
˜2.0%. Desirable range of Cr content is 0.2
~ 1.6%.

[0029] Al: has a 0.003 to 0.05% Al is acting deoxidation, in order to ensure this effect is 0. A content of 003% or more is required . On the other hand, however, Al forms non-metallic inclusions and reduces rolling contact fatigue life. In particular, if the content exceeds 0.05%, coarse nonmetallic inclusions are likely to be formed, so that the rolling fatigue life is significantly reduced, and the desired rolling fatigue life (rolling fatigue life in Examples described later 1 × 10 ^{7 in} fatigue test
The above rolling fatigue life) cannot be obtained. Therefore, the content of Al is set to 0.003 to 0.05 % .

Cu: 2.0% or less Cu may not be added. If added, it has the effect of enhancing corrosion resistance. To ensure this effect, Cu is 0.0
The content is preferably 5% or more. However, if the content exceeds 2.0%, segregation occurs at the grain boundaries, and cracks and flaws become prominent during hot working such as slab rolling of steel ingots and hot rolling of wire rods. Therefore, add positively
The content of Cu in the case that has to 2.0% 0.05
Good .

Ni: 4.0% or less Ni may not be added. If added, it has a function of increasing the toughness by forming a solid solution in martensite after quenching.
To ensure this effect, the Ni content is preferably 0.2% or more. However, even if the content exceeds 4.0%, the above effect is saturated and the cost is increased. Therefore, the content of Ni to be added is aggressively preferably set to 4.0% to 0.2.

Mo: 0.5% or less Mo may not be added. If added, it forms a solid solution in the martensite after quenching, and has the effect of increasing the rolling fatigue life. To ensure this effect, Mo is 0.05%
The above content is preferable. However, if the content exceeds 0.5%, the hardenability becomes too high, martensite is likely to be formed after hot rolling, and cracking is likely to occur. Therefore, when actively adding
The content of Mo is preferably set to 0.5% 0.05.

V: 0.4% or less V may not be added. If added, it has the effect of refining the austenite crystal grains and increasing the toughness. In order to reliably obtain this effect, it is preferable that the content of V be 0.05% or more. However, even if the content exceeds 0.4%, the above effect is saturated and the cost is increased. Therefore, the content of V when positively added is
It is preferable to set it to 0.05 to 0.4%.

Nb: 0.1% or less Nb need not be added. If added, it has the effect of refining the austenite crystal grains and increasing the toughness. In order to reliably obtain this effect, the Nb content is preferably 0.01% or more. However, even if the content exceeds 0.1%, the above effect is saturated and the cost is increased. Therefore, the content of Nb to be added is aggressively preferably set to 0.1% 0.01.

B: 0.003% or less B may not be added. If added, it forms a solid solution in cementite and stabilizes cementite, enables spheroidizing annealing in a short time, and further improves wear resistance. In order to surely obtain such an effect, the content of B is preferably 0.0003% or more. However, if the content exceeds 0.003%, coarse cementite is likely to be generated, and a desired rolling fatigue life (1 × 10 ⁷ or more rolling fatigue in a rolling fatigue test in Examples described later is performed. lifespan)
Can't get Therefore, B when positively added
The content of that and 0.0003 to 0.003%
Yes .

Rare earth element: 0.01% or less in total Rare earth element may not be added. If added, it has the effect of enhancing hot workability. In order to reliably obtain this effect, the total content of rare earth elements is preferably 0.001% or more. However, the total amount of rare earth elements is 0.0
Even if the content exceeds 1%, the above effect is saturated and the cost is increased. Therefore, when adding positively
The total content of rare earth elements is 0.001 to 0. 01
It is good to set it as %.

Ca: 0.003% or less Ca may not be added. If added, it has the effect of enhancing hot workability. To ensure this effect, Ca
Is preferably 0.0001% or more.
However, even if Ca is contained in an amount of more than 0.003%, the above effect is saturated and the cost is increased. Therefore, the content of Ca when added positively is 0.000
It is preferable to set it to 1 to 0.003%.

Mg: 0.003% or less It is not necessary to add Mg. If added, it has the effect of enhancing hot workability. To ensure this effect, Mg
Is preferably 0.0001% or more.
However, even if Mg is contained in an amount of more than 0.003%, the above effect is saturated and the cost is increased. Therefore, the content of Mg when positively added is 0.000
It is preferable to set it to 1 to 0.003%.

In addition, from Cu to Mg described above
The nine optional additives are added to the examples described later, if necessary.
As shown, one or more are added positively.

In the present invention, T as an impurity element
The contents of i, P, S, N and O (oxygen) are limited as follows.

Ti: 0.002% or less Ti combines with N to form TiN and reduces the rolling fatigue life. In particular, when the content exceeds 0.002%, the rolling fatigue life is remarkably reduced, and the desired rolling fatigue life (1 × in the rolling fatigue test in Examples described later) is obtained.
A rolling fatigue life of 10 ⁷ or more) cannot be obtained. Therefore, the content of Ti is set to 0.002% or less. It is desirable that the content of Ti as an impurity element be as small as possible.

P: 0.02% or less P segregates at the grain boundaries and reduces rolling contact fatigue life.
In particular, when the content exceeds 0.02%, the rolling fatigue life is significantly reduced, and a desired rolling fatigue life (1 × 10 ⁷ or more rolling fatigue in a rolling fatigue test in Examples described later is performed. Life) will not be obtained. Therefore, the content of P is set to 0.02% or less.

S: 0.015% or less S combines with Mn to form MnS and reduces the rolling fatigue life. In particular, if the content exceeds 0.015%, coarse MnS is likely to be formed, so that the rolling fatigue life is significantly reduced, and the desired rolling fatigue life cannot be obtained. Therefore, the content of S is set to 0.015% or less.

N: 0.007% or less N easily combines with Ti or Al to form TiN or AlN, and when the N content increases and coarse TiN or AlN is formed, rolling fatigue life Will decrease. In particular, when the content exceeds 0.007%, the rolling fatigue life is significantly reduced, and a desired rolling fatigue life (1 × 10 ⁷ or more rolling fatigue in a rolling fatigue test in Examples described later is performed. Life) cannot be obtained. Therefore, the content of N is 0.00
It was set to 7% or less. On the other hand, if a large amount of so-called "free N" that does not form a compound with Ti or Al is present, the machinability will decrease. Therefore, the content of N as an impurity element should be minimized to improve the tool life. Further, it is desirable to set it to 0.005% or less.

O (oxygen): 0.0015% or less O forms oxide inclusions and reduces rolling fatigue life. In particular, when the content exceeds 0.0015%, the rolling fatigue life is remarkably reduced, and the desired rolling fatigue life (1 × 10 ⁷ in the rolling fatigue test in Examples described later) is achieved.
The above rolling fatigue life) cannot be obtained. Therefore, O
Content was 0.0015% or less. It should be noted that it is desirable to reduce the O content as an impurity element as much as possible. (B) Carbon content distribution in cross section of wire rod, steel bar and steel pipe In the present invention, in the cross section of wire rod, steel bar, average value of C content in the region from the outer periphery to the position of 200 μm in depth, and the steel pipe In the cross section of, the average value of the C content in each of the regions from the inner and outer circumferences to the position of 200 μm in depth is 0.4 × C to 0.9 × C% (C
Is specified in the target wire rod, steel bar and steel pipe C content). This is for the following reason.

When a wire rod, a steel bar or a steel pipe is cut, especially by turning or parting, the wire rod, the steel bar or the steel pipe is rotated while the longitudinal direction of the wire rod, the steel bar or the steel pipe is rotated. Therefore, in the case of turning, cutting is always performed in a state including the surface layer, that is, the surface layer portion. Further, in the case of the cut-off processing, the surface layer portion is always cut in the initial stage of the processing.
Therefore, in both cases of turning and parting off, if the surface layer is easy to cut,
Good machinability can be secured.

On the other hand, the part to be removed by cutting, especially turning is 200 μm in the surface layer,
That is, since the depth from the outer or inner circumference is 200 μm,
In order to ensure the desired characteristics of the bearing element component after cutting, the chemical composition (C content) of the deeper portion must be the same as the "C content of the wire rod, steel bar or steel pipe". Therefore, when the target is a wire rod or steel bar, the degree of decarburization in the region from the outer circumference to a position of 200 μm in depth, and when the target is a steel pipe, from the inner and outer circumferences to a position of 200 μm in depth, respectively. The degree of decarburization in the area of 1 may be specified. In the following description, the "region from the outer periphery to the position of 200 μm in depth" in the cross section of the wire rod and the steel bar and the "region from the inner and outer periphery to the position of 200 μm in depth" in the cross section of the steel pipe will be combined. It may also be referred to as "surface layer area".

In the present invention, the above-mentioned "surface layer region", that is, the region up to the position of 200 μm in depth from the outer or inner periphery is decarburized by an appropriate amount to reduce the hard cementite structure ratio in that region. In addition, the main feature is that machinability is enhanced by not forming a structure mainly composed of a ferrite phase. When the average value of the C content in the region is 0.9 × C% or less, the hard cementite structure ratio decreases and the cutting resistance decreases, so that the machinability can be greatly improved. On the other hand, when the average value of the C content in the region is less than 0.4 × C%, the “surface layer region” has a structure mainly composed of the ferrite phase and the cutting resistance increases, so that the machinability is rather deteriorated. However, it is difficult to prevent the reduction (decarburization) of the C content in the portion deeper than this region. Therefore, the average value of the C content in the “surface layer region” is set to 0.4 × C to 0.9 × C%. In addition, in order to extend the tool life and further improve the machinability, it is preferable that the average value of the C content in the region is 0.4 × C to 0.7 × C%.

Here, "C" is "C of wire rod, steel bar, and steel pipe."
The term "content" is as described above, and means, for example, a ladle analysis value. In addition, “the average value of C content in the region from the outer circumference (of the wire rod, steel bar or steel pipe) to the position of 200 μm in depth” is, for example, using a wavelength-dispersive electron beam microanalyzer When the line content of the C content in the outer peripheral part (near the surface layer part) is analyzed, it means the average value of the C content at the position of each depth of 20 μm from the outer periphery, and it means “from the inner periphery (of the steel pipe) to the depth. 200 μm
Similarly, the "average value of C content in the region up to the position of" is similarly analyzed by, for example, a line analysis of the C content of the inner peripheral portion (in the vicinity of the inner surface layer portion) of the cross section of the steel pipe using a wavelength dispersive electron beam microanalyzer. As described above, the average value of the C content at the depths of 20 μm from the inner circumference in the case of performing is described above. The C content of the surface layer portion can also be determined by secondary ion mass spectrometry.

In order to make the average value of C content in the "surface layer region" of a wire rod, steel bar or steel pipe 0.4 x C to 0.9 x C%, there is almost no decarburized layer. When spheroidizing annealing a hot-rolled steel bar or a hot-rolled steel tube using a bright furnace, for example, RX gas (gas containing N ₂ , H ₂ , and CO as the main components) and NX gas (containing N ₂ as the main component, Trace C
The atmosphere in the bright furnace is adjusted by (a gas containing O and CO ₂ ), and the (CO partial pressure) ² / (CO ₂ partial pressure) is adjusted to 8
0 to 120 and steel material (wire rod, steel bar, steel pipe) 770
After heating to ~ 790 ° C and holding for 2-6 hours, then 5-15
It may be cooled to 680 to 650 ° C at a cooling rate of ° C / hour. When the hot-rolled wire rod, hot-rolled steel bar or hot-rolled steel pipe has a decarburized layer, the furnace atmosphere, that is, (CO partial pressure) ² / (CO ₂ The pressure may be changed to be larger than 120.

Although the structure of the wire rod for the bearing element parts, the steel bar and the steel pipe is not particularly specified, the shape and grain size of the carbide mainly composed of cementite affect the machinability, and the shape of the carbide is spherical, and The larger the particle size, the better the machinability. However, as shown in Tables 2, 3, and 4 of Examples described below, the machinability improving effect is almost saturated at a spheroidization rate of 90% or more, so that the spheroidization rate of the carbide in the wire rod, steel bar or steel pipe is It is preferably 90% or more. To increase the spheroidization rate of carbides in wire rods, steel bars and steel pipes to 90% or more,
For example, hot-rolled wire rods, steel bars and steel pipes are 770-79.
After heating to 0 ° C. and holding for 2 to 6 hours, it may be cooled to 680 to 650 ° C. at a cooling rate of 5 to 15 ° C./hour.
The spheroidization rate of the above-mentioned carbides means the “ratio (%) of the carbides whose“ major axis / minor axis ”is less than 2 with respect to the carbides (cementite) in the field of view” when observed under a microscope. . If the average particle size of the spheroidized carbide is 0.4 μm or more, the tool life becomes longer.

The chemical composition and the book (B) described in the item (A).
The wire rods, steel bars and steel pipes having the distribution of carbon content described in the item are subjected to processing such as cold forging, cold drawing and cutting in a usual method, and further subjected to heat treatment by quenching and tempering at low temperature. And then finished into bearing element parts having desired mechanical properties, and then assembled into bearings as final products which are precision machine parts.

Hereinafter, the present invention will be described in more detail with reference to examples.

[0054]

EXAMPLE 30 steels A to R having the chemical compositions shown in Table 1 were used.
It was melted using a 0 kg vacuum furnace. Steel B in Table 1
F, H, and OR have chemical compositions satisfying the above-mentioned item (A), while steels A, G, and I-N have the content of any of the components specified in the present invention of the above-mentioned item (A). It is a comparative example out of the range. In addition, among those whose chemical composition satisfies the above (A), Steel C is SUJ standardized by JIS G 4805.
Steel equivalent to 2.

[0055]

[Table 1]

Then, these steels were hot forged by a conventional method into a round bar having a diameter of 40 mm, and then a diameter of 35 mm.
Mechanically ground to completely remove the decarburized layer. After that, the round bar having a diameter of 35 mm was subjected to spheroidizing annealing using an electric furnace whose atmosphere was adjusted. The atmosphere in the furnace is composed of CO, H ₂ , N ₂ and CO ₂ , and (CO
The partial pressure of ² ) / (the partial pressure of CO ₂ ) was changed to adjust the C content in the outer peripheral portion.

The heat treatment conditions (heat pattern) for spheroidizing annealing are the following three conditions. Among these, the one corresponding to the heat pattern of the conventional annealing treatment is “condition X”,
The time required to cool to 660 ° C after heating to 0 ° C is 1
2 hours. In the case of "condition Y", the time is 24 hours, and in the case of "condition Z", the time is 3 hours.

Condition X: Heating to 780 ° C. and holding for 2 hours, then cooling to 660 ° C. at a cooling rate of 10 ° C./hour. Condition Y: After heating to 780 ° C. and holding for 5 hours, 5 ° C. /
Cool to 660 ° C at a cooling rate of time. Condition Z: After heating to 780 ° C. and holding for 20 minutes, 40 ° C.
Cooling to 660 ° C at a cooling rate of / hour.

The atmospheric conditions of the electric furnace are the following four conditions. Condition 1: (partial pressure of CO) ² / (partial pressure of CO ₂ ) = 40
0. Condition 2: (CO partial pressure) ² / (CO ₂ partial pressure) = 20
0. Condition 3: (CO partial pressure) ² / (CO ₂ partial pressure) = 10
0. Condition 4: (CO partial pressure) ² / (CO ₂ partial pressure) = 1
0.

A transverse sample was taken from the round bar after the spheroidizing annealing, and the C content in the outer peripheral portion (in the vicinity of the surface layer portion) was measured. That is, using a wavelength-dispersive electron probe microanalyzer, perform a linear analysis of the C content, read the C content from the outer periphery to a position of a depth of 200 μm every 20 μm from the measurement chart, and calculate the average value of these points. By determining, the average value of C content in the region from the outer periphery to the position of 200 μm in depth was obtained.

Further, the spheroidization rate and average particle size of the carbide after spheroidization annealing were measured. That is, a sample is cut out in the cross-sectional direction of the round bar, and after polishing and corrosion are performed by a usual method,
From the center of each sample using a scanning electron microscope, "R /
Position 2 "(where R is the radius of the round bar) at a magnification of 5000
The 0 field of view was photographed, and the spheroidization rate was investigated by image analysis of this photograph by a usual method. Note that, as described above, the spheroidization rate refers to "a ratio (%) of carbides in which" major axis / minor axis "is less than 2 with respect to carbides (cementite) in the visual field.

Further, the average cross-sectional area of each carbide is obtained from the image analysis using the above photograph, the diameter is obtained by assuming that the shape of the carbide is a sphere (hence, a circle on the photograph), and this is the average grain size of the carbide. The diameter.

A cutting test was also performed on a round bar that had been spheroidized and annealed.
That is, the spheroidized and annealed round bar is pickled by a usual method to remove the scale, and then the SK of JIS standard is applied to the tool.
Using a H4 triangular tip, no lubrication, peripheral speed 50 m / min, depth of cut 0.5 mm, feed 0.25 mm / rev. The tool life was investigated by turning under the above conditions and used as an index of machinability. The tool life was defined as the processing time until the tool became worn or chipped and could not be cut when the surface layer of the round bar was turned under the above conditions.

The target of machinability was to satisfy both the following conditions (a) and (b).

(A) With respect to each steel, the tool life of the round bar annealed under the heat treatment condition X and the atmosphere condition 1 is used as a reference,
20% or more longer tool life than this. From the condition (a), the round bar for each steel was subjected to the heat treatment condition X,
When the spheroidizing annealing is performed under the atmospheric condition 1, it is evaluated that the machinability target is not satisfied.

(B) SUJ2 standardized by JIS G 4805
The tool life is 20% or more longer than the tool life when a round bar of steel C corresponding to steel is spheroidized and annealed under heat treatment condition X and atmosphere condition 1. That is, Table 2 described later
The tool life is 20% or more longer than the tool life of 4.1 minutes of the test number 13 (specifically, the tool life is 4.92 minutes or more).

Further, a test piece having a diameter of 12 mm and a length of 22 mm was cut out from the cut round bar of each test number by machining, and the test piece was quenched and tempered (82
It was held at 0 ° C. for 30 minutes, oil-quenched, and then tempered at 160 ° C. for 1 hour) and subjected to a rolling fatigue test. That is, using a cylindrical rolling fatigue tester, using # 68 turbine oil as the lubricating oil, the Hertz maximum contact stress was 600 kgf / m.
A rolling fatigue test was conducted under the conditions of m ² and a test piece load frequency of 46000 cycles / minute. For each steel, 10 test pieces were provided, and the number of rotations when the surface was first peeled from the 10 test pieces was defined as "rolling fatigue life". When the rolling fatigue life was 1.0 × 10 ⁷ or more, the rolling fatigue property was evaluated as excellent.

Tables 2 to 5 show the average value of C content in the region from the outer periphery to the position of 200 μm in depth after spheroidizing annealing (in each table, “average value of C content in outer peripheral portion”). ), The spheroidization rate and average grain size of carbide, tool life in turning, and rolling fatigue life.

[0069]

[Table 2]

[0070]

[Table 3]

[0071]

[Table 4]

[0072]

[Table 5]

As is clear from Tables 2 to 5, in the case of the test numbers using the steels A and J to N of the comparative examples, that is, the test numbers using the steel A having a C content of less than 0.75%. 1 to
6, test numbers 55 to 60 using steel J having an Al content exceeding 0.05%, test numbers 61 to 66 using steel K having a Ti content exceeding 0.002%, P content and S content Test Nos. 67 to 72 using steel L whose amounts exceed 0.02% and 0.015%, test Nos. 73 to 78 using steel M whose N content exceeds 0.007%, and O-containing Test No. 79-8 with steel N whose amount exceeds 0.0015%
No. 4 did not reach the rolling fatigue life of 1.0 × 10 ⁷ times. Of the above, test numbers 1, 55, 61, 67, 73
In Nos. 79 and 79, the average value of the C content in the region from the outer periphery to the position at a depth of 200 μm exceeds 0.9 × C%, and therefore the tool life has not reached the target value. Also, test numbers 4, 58, 64, 70, 76 and 82 are 20 from the outer circumference.
Since the average C content in the region up to the depth of 0 μm is less than 0.4 × C%, the tool life has not reached the target value.

In the case of the test numbers using the steels G and I of the comparative example, that is, the test numbers 37 to 42 using the steel G having a Mn content exceeding 1.5%, and the Cr content of 2. 0%
Test Nos. 49 to 54 using Steel I having a hardness of more than 1 do not reach the target value of the tool life.

Even if the steel has a chemical composition within the content range specified in the present invention, test numbers 7, 13, 19, 2
For 5, 31, 43, 85, 91, 95 and 99, the average C content in the region from the outer periphery to the position at a depth of 200 μm exceeds 0.9 × C%, so the tool life becomes the target value. Has not reached. Also, test numbers 10, 16, 22, 2
Nos. 8, 34, 46, 88, 94, 98 and 102 have an average C content of less than 0.4 × C% in the region from the outer periphery to the position of 200 μm depth, and therefore the tool life is still the target value. Has not reached.

In contrast to the above comparative example, in the case of the present invention satisfying the conditions specified in the present invention, the tool life was longer than that in the case where the round bar of each steel was spheroidized and annealed under the heat treatment condition X and the atmosphere condition 1. Is longer than 20%, and the tool life is 4.9.
The machinability target is satisfied in 2 minutes or more, and the rolling fatigue life exceeds 1.0 × 10 ⁷ times.

[0077]

The wire rod, steel bar and steel pipe of the present invention are excellent in machinability and have a long rolling contact fatigue life.
It can be used as a material for bearing element parts such as needles, shafts and races.

Claims (6)

(57) [Claims] 1. C: 0.75 to 1.2% by weight, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2-2.0%And Al: 0.003 to 0.0
5%And the balance consists of Fe and unavoidable impurities,
In impuritiesP is 0.02% or less, S is 0.015% or less
, N is 0.007% or less, O (oxygen) is 0.0015
% Or less, Ti 0.002% or less, Cu 0.05% or less
Full, Ni less than 0.2%, Mo less than 0.05%, V
Less than 0.05%, Nb less than 0.01%, B 0.00
Less than 03%, total rare earth elements less than 0.001%, C
a is less than 0.0001%, Mg is less than 0.0001%
In addition, a depth of 200 from the outer periphery in the cross section of the steel wire rod.
The average value of C content in the region up to the position of μm is 0.
4 x C to 0.9 x C% (where C is the C content of the steel wire)
For bearing element parts with excellent machinability
Steel wire rod. 2. C: 0.75 to 1.2% by weight, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2 to 2.0% and Al: 0.003 to 0.0
5% and Cu: 0.05-2.0 %,
Ni: 0.2 to 4.0%, Mo: 0.05 to 0.5 %,
V: 0.05 to 0.4 %, Nb: 0.01 to 0.1 %,
B: 0.0003~ 0.003%, the rare earth elements: a total of
0.001-0.01 %, Ca: 0.0001-0.0
0.3% and Mg: out of 0.0001 to 0.003%
1 or more of , and the balance is Fe and unavoidable impurities, Ti in the impurities is 0.002% or less, P is 0.0
2% or less, S is 0.015% or less, N is 0.007% or less, O (oxygen) is 0.0015% or less, and further, at a depth of 200 μm from the outer periphery in the cross section of the steel wire . The average value of C content in the area up to the position is 0.4 × C to 0.9 ×
C% (where, C is the C content of the steel wire rod) steel wire rod for bearing element parts having excellent machinability, which is a. 3. By weight%, C: 0.75 to 1.2%, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2-2.0%And Al: 0.003 to 0.0
5%And the balance consists of Fe and unavoidable impurities,
In impuritiesP is 0.02% or less, S is 0.015% or less
, N is 0.007% or less, O (oxygen) is 0.0015
% Or less, Ti 0.002% or less, Cu 0.05% or less
Full, Ni less than 0.2%, Mo less than 0.05%, V
Less than 0.05%, Nb less than 0.01%, B 0.00
Less than 03%, total rare earth elements less than 0.001%, C
a is less than 0.0001%, Mg is less than 0.0001%
And then,Steel barCross section odorOutsideLapDeep200μ
Average C content in the area up to mIs 0． Four
XC-0.9xC% (however, C isSteel barC content of)
For bearing element parts with excellent machinability characterized byrod
steel. 4. % By weight, C: 0.75 to 1.2%, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2 to 2.0% and Al: 0.003 to 0.0
5% and Cu: 0.05-2.0%,
Ni: 0.2-4.0%, Mo: 0.05-0.5%,
V: 0.05 to 0.4%, Nb: 0.01 to 0.1%,
B: 0.0003 to 0.003% or less, rare earth element: compound
0.001-0.01% in total, Ca: 0.0001-
0.003% and Mg: 0.0001 to 0.003%
Containing one or more of the above, with the balance being Fe and unavoidable impurities.
Made of a material, Ti in the impurities is 0.002% or less, and P is
0.02% or less, S 0.015% or less, N 0.00
7% or less, O (oxygen) is 0.0015% or less, and
In the cross section of the steel bar, from the outer periphery to a position at a depth of 200 μm.
The average value of C content in the region of 0.4 × C to 0.
9 × C% (however, C is the C content of steel bar)
A steel bar for bearing element parts with excellent machinability. 5. % By weight, C: 0.75 to 1.2%, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2 to 2.0% and Al: 0.003 to 0.0
5%, the balance consists of Fe and unavoidable impurities,
Impurity P is 0.02% or less, S is 0.015% or less
, N is 0.007% or less, O (oxygen) is 0.0015
% Below, Ti is 0.002% or less, Cu is 0.05% or less.
Full, Ni less than 0.2%, Mo less than 0.05%, V
Less than 0.05%, Nb less than 0.01%, B 0.00
Less than 03%, total rare earth elements less than 0.001%, C
a is less than 0.0001%, Mg is less than 0.0001%
In addition, in the cross section of the steel pipe,
Average of C content in the area up to the position of 200 μm
All values are 0.4 x C to 0.9 x C% (where C is steel
Excellent machinability characterized by the C content of the pipe)
Steel pipe for bearing element parts. 6. % By weight, C: 0.75 to 1.2%, S
i: 0.1-1.5%, Mn: 0.2-1.5%, C
r: 0.2 to 2.0% and Al: 0.003 to 0.0
5% and Cu: 0.05-2.0%,
Ni: 0.2-4.0%, Mo: 0.05-0.5%,
V: 0.05 to 0.4%, Nb: 0.01 to 0.1%,
B: 0.0003 to 0.003% or less, rare earth element: compound
0.001-0.01% in total, Ca: 0.0001-
0.003% and Mg: 0.0001 to 0.003%
Containing one or more of the above, with the balance being Fe and unavoidable impurities.
Made of a material, Ti in the impurities is 0.002% or less, and P is
0.02% or less, S 0.015% or less, N 0.00
7% or less, O (oxygen) is 0.0015% or less, and
In the cross section of the steel pipe, the depth is 200μ from the inner and outer circumferences.
What is the average C content in the area up to the position of m?
0.4 × C to 0.9 × C% (however, C is the C content of the steel pipe)
Amount)), which has excellent machinability
Steel pipe for products.