JP2001279382A - Fine-diameter steel wire rod for bearing, and manufacturing method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fine-diameter steel wire rod of <=5 mm diameter for bearing, having stable quality, and its manufacture method. SOLUTION: A wire bar of steel, having a composition consisting of 0.6-1.1% C, 0.2-0.5% Si, 0.30-1.0% Mn, 1.0-1.6% Cr, <=0.0010% O and the balance Fe with inevitable impurities, is heated to 850-1,100 deg.C and hot rolled while regulating the caliber surface roughness at least at final finish rolling so that the finished arithmetic mean roughness of the resultant wire rod becomes <=2 μm Ra. Then, the wire rod is coiled under regulation of coiling temperature to 750-850 deg.C, followed by cooling at <=1 deg.C/s cooling rate. It is preferable to carry out finish rolling by using a four-high rolling mill.

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, and more particularly to a small diameter wire rod for a bearing which is preferably formed into a steel ball and a roller of a bearing.

[0002]

2. Description of the Related Art Conventionally, steel balls and rollers for bearings are pickled, as-rolled, using steel wires for bearings, pickling, spheroidizing annealing, pickling.
After being cut through a surface treatment step and a wire drawing step, it is manufactured by a cold forging-polishing-hardening / tempering-finish polishing step.

[0003] Since steel balls and rollers for bearings are required to have high dimensional accuracy, the dimensional accuracy of wires used for cold forging in the manufacturing process is strictly controlled. Therefore, prior to cold forging, wire drawing was indispensable for the purpose of accurately adjusting the wire diameter to a predetermined size.
In particular, when producing steel balls and rollers using a wire having a wire diameter of 5 mmφ or less, heat treatment (spheroidizing annealing) and wire drawing were performed twice. However, performing the heat treatment and the wire drawing twice has a problem in that the cost is increased and the cost is disadvantageous, and the energy consumption is increased and the global environment is deteriorated.

To solve such a problem, see, for example,
JP-A-279695 discloses that a four-roll rolling mill is preferably used for finish rolling to control the S content in steel and the winding temperature after hot rolling of the wire rod to keep the diameter variation after hot rolling constant. A method for producing a steel wire rod for a bearing to be adjusted to a value equal to or less than a value has been proposed. According to the technique described in Japanese Patent Application Laid-Open No. H11-279695, a wire rod with good dimensional accuracy can be stably manufactured, and even a thin wire can be omitted once.

[0005]

SUMMARY OF THE INVENTION
Even when using the technology described in JP 11-279695,
In the production of bearing steel (SUJ2) fine wire with a diameter of not more than mmφ, the rolled material is broken or a micro-martensite structure is generated in the surface layer of the wire even if it does not lead to breakage, making it stable as a bearing steel wire. There was a problem that it was difficult to obtain quality.

[0006] The smaller the diameter of the wire, the higher the cooling rate during and after rolling, so that a martensitic structure is more likely to occur in bearing steel (SUJ2) with a large amount of alloying elements and high hardenability. The present invention advantageously provides a small-diameter steel wire rod for bearings having a wire diameter of 5 mm or less, which advantageously solves the above-mentioned problems of the prior art, prevents the generation of martensite structure, has stable quality, and a method for producing the same. With the goal.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned object, the present inventors have intensively studied a method for preventing the generation of a martensite structure during the production of a bearing steel wire. As a result, it was found that the heating temperature of the steel material, the winding temperature of the wire after rolling, and the surface roughness of the wire after rolling were greatly influenced by the occurrence of the martensite structure.

The present invention has been further studied and completed based on the above findings. That is, in the first invention, in mass%, C: 0.6 to 1.1%, Si: 0.2 to 0.5%,
Mn: contains 0.30 to 1.0%, Cr: 1.0 to 1.6%, O: 0.0010% or less, has a composition consisting of the balance Fe and unavoidable impurities and a structure consisting of a pearlite single phase, and has an arithmetic average roughness of Ra: 2 μm or less, preferably a small-diameter steel wire for bearings having a wire diameter of 5 mm or less.

Further, the second present invention provides a method of the present invention wherein C: 0.
6 to 1.1%, Si: 0.2 to 0.5%, Mn: 0.30 to 1.0%, C
r: 1.0 to 1.6%, O: 0.0010% or less, the balance being Fe
And 850-
It is heated to a temperature in the range of 1100 ° C., and hot-rolled at least by adjusting the caliber surface roughness of the final finish rolling so that the finished arithmetic average roughness of the wire is Ra: 2 μm or less, preferably a wire diameter of 5 mmφ. After making the following fine wire,
The fine wire is wound at a temperature of 750 to 850 ° C.
Next, the present invention is a method for producing a small-diameter steel wire rod for a bearing, characterized by cooling at a cooling rate of 1 ° C./s or less. In the second invention, the finish rolling in the hot rolling is performed by a four-roll rolling mill. It is preferable to carry out using.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The steel wire of the present invention is
C: 0.6 to 1.1%, Si: 0.2 to 0.5%, Mn: 0.30 to 1.0
%, Cr: 1.0 to 1.6%, O: 0.0010% or less, and has a composition consisting of the balance of Fe and inevitable impurities, and is preferably a small-diameter steel wire for bearings having a wire diameter of 5 mm or less. In a steel wire rod having a bearing steel composition with a wire diameter exceeding 5 mmφ, a martensite structure does not occur under ordinary heating, rolling and cooling conditions.

First, the reasons for limiting the composition of the steel wire and steel material of the present invention will be described. Note that mass% is simply referred to as%
I will write it. C: 0.6 to 1.1% C is an element that increases the hardenability and increases the hardness. In the present invention, the content of at least 0.6% is required to secure the desired hardness. On the other hand, if the content exceeds 1.1%, the hardenability is saturated and the amount of residual γ increases, making it difficult to obtain a desired hardness. For this reason, C was limited to 1.1% or less.

Si: 0.2-0.5% Si is an element that acts as a deoxidizing agent. Such an effect is recognized when the content is 0.2% or more, but the material becomes brittle even when the content exceeds 0.5%. May be caused. For this reason, Si
Was limited to the range of 0.2-0.5%. Incidentally, preferably 0.
20 to 0.30%.

Mn: 0.30% to 1.0% Mn is an element that improves hardenability similarly to C.
If less than 30%, the effect is not recognized. On the other hand, if the content exceeds 1.0%, the effect saturates, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, Mn is 0.30
Limited to ~ 1.0%. In addition, it is preferably 0.35 to 0.50%.

Cr: 1.0 to 1.6% Cr is an element that improves hardenability, generates carbides, and stably forms a spheroidized structure. In the present invention, Cr is contained in an amount of 1.0% or more. On the other hand, if the content exceeds 1.6%, the degree of spheroidization is not affected, and the effect is saturated, and an effect corresponding to the content cannot be expected, which is economically disadvantageous. Therefore, Cr is 1.0 to
Limited to 1.6%.

O: 0.0010% or less O is limited to 0.0010% or less in the present invention in order to reduce oxide-based nonmetallic inclusions and improve fatigue life.
If a longer life is required, the content is preferably 0.0006% or less. The balance other than the above is Fe and inevitable impurities. As unavoidable impurities, P that affects rolling fatigue life is 0.015% or less and S that affects rolling fatigue life and grindability is 0.010% or less.

Arithmetic average roughness: Ra ≦ 2.0 μm The arithmetic average roughness of the surface of the steel wire rod is 2.0 μm in Ra as rolled.
If it exceeds m, a martensite structure is likely to be formed in the surface layer, and breakage and other accidents occur frequently. For this reason, in the present invention, the arithmetic mean roughness of the finish of the steel wire is limited to Ra: 2.0 μm or less. Preferably, Ra is 0.8 to 1.3 μm.
m. The arithmetic average roughness is measured with a surface roughness meter as-rolled at a plurality of locations with a measurement length of 10 mm or more.

Next, a method for producing a small-diameter steel wire rod will be described. The steel material having the above composition is heated and hot-rolled to obtain a thin steel wire having a diameter of preferably 5 mmφ or less. Heating temperature: 850 to 1100 ° C One method for preventing martensite generation in steel wires is to reduce the hardenability of the steel wires. In the present invention, coarsening of the austenite (γ) particle size of the steel material is suppressed, and the hardenability of the steel wire is reduced. In order to suppress the coarsening of the γ particle size, the heating of the steel material is preferably performed at a low temperature, but in the present invention, the heating temperature of the steel material is 850.
The range was limited to ~ 1100 ° C. If the heating temperature is lower than 850 ° C., it is difficult to perform rolling with high dimensional accuracy. On the other hand, 1100
When the temperature exceeds ℃, coarsening of the γ particle size becomes remarkable.

After the steel material is heated to the above-mentioned temperature, it is hot-rolled to obtain a fine wire having a diameter of preferably 5 mmφ or less. In the present invention, the hot rolling also comprises rough rolling and finish rolling. In the present invention, at least the caliper surface roughness of the final rolling pass of the finish rolling is calculated by calculating the final arithmetic average roughness of the wire rod as R.
a: Adjust so as to be 2 μm or less. If the finished arithmetic mean roughness of the wire exceeds Ra: 2 μm, the surface of the wire is rapidly cooled upon cooling after hot rolling, a martensite structure is generated on the surface layer, and breakage and uneven wire quality are likely to occur.

The rough rolling in the hot rolling may be performed by a generally known method. On the other hand, in finish rolling, rolling may be performed while shifting the rolling direction in a rolling mill row in which a plurality of two-roll rolling mills are arranged, but a plurality of four-roll rolling mills described in JP-A-5-38502 are used. It is preferable to perform at least two-pass rolling using the arranged rolling mill row while shifting the rolling direction. Winding temperature: 750 to 850 ° C. The winding temperature also affects the γ grain size of the steel wire after hot rolling, and the lower the temperature, the smaller the γ grain size. Winding temperature after hot rolling,
Preferably, the wire is cooled by a water jet or the like and adjusted to a desired temperature. Cooling for adjusting the winding temperature is preferably water cooling at a cooling rate of 50 ° C./s or more. If the cooling rate is too high, the surface layer may undergo martensitic transformation. Therefore, the cooling rate is preferably 100 ° C./s or less. If the winding temperature is lower than 750 ° C, temperature control becomes difficult. On the other hand, when the temperature exceeds 850 ° C., the growth of the γ particle size becomes remarkable. For this reason, the winding temperature was limited to 750 to 850 ° C.

Cooling rate after winding: 1.0 ° C./s or less The wound wire is gradually cooled at a cooling rate: 1.0 ° C./s or less. When the cooling rate exceeds 1.0 ° C./s, the generation of a martensite structure becomes remarkable. Therefore, the cooling rate after winding is limited to 1.0 ° C./s or less. Preferably,
0.5 ° C./s or less, more preferably 0.3 to 0.5 ° C./s.

[0021]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and cast into a slab of 400 × 560 mm by continuous casting. These slabs were made into 170 mmφ round billets (steel material) by slab rolling. These steel materials were formed into coiled steel wires having a wire diameter of 4.3 mm by hot rolling under the heating rolling conditions shown in Table 2. The finish rolling (last two stands) in the hot rolling was performed by a four-roll rolling mill as shown in FIG. In the final rolling pass of finish rolling and in two passes before the final rolling pass, rolls in which the surface roughness of the caliber was adjusted so that the finished arithmetic average roughness of the wire rod was Ra: 2 μm or less were used.

With respect to the obtained steel wire, the presence or absence of breakage and the occurrence of martensite in each part of the wire were examined. The investigation position was, first, after cutting off the dimensionally inferior portions at the front and rear ends of each steel wire (coil), and then forming a continuous five-turn ring at each of the front, rear, and center portions. From each of these rings, four test pieces {(4 pieces / 1 ring) × 5 = 20 pieces (total 20 × 3 positions = 60 pieces)} were sampled, and the C section of the wire rod was corroded (Niter Then, the presence or absence of martensite structure was observed with an optical microscope.

Further, these steel wire rod coils were divided into two parts and then tied. After binding, the bundle was unwrapped and carefully examined for breakage. When a martensite structure is generated in the wire rod of bearing steel, the external force due to binding causes
There is a possibility of breakage. The hardness of the obtained steel wire rod after spheroidizing annealing was examined. In addition, the rolling fatigue life was investigated.

The results are shown in Table 2.

[0025]

[Table 1]

[0026]

[Table 2]

In each of the examples of the present invention, the generation of martensite structure was not recognized, the whole coil had a stable pearlite structure, there was no breakage after unbundling, further less variation in characteristics, and uniform quality characteristics. This is a small-diameter steel wire rod having In a comparative example outside the scope of the present invention, a martensite structure was observed in a part of the coil. In addition, breakage after unbundling has been observed, and the unevenness in quality has also increased.

Further, regarding the steel wires No. 1, No. 2 and No. 5,
As-rolled spheroidized as-rolled material (cooling rate after holding at 790 ° C × 8 h)
(Cooling at 10 ° C./h), followed by pickling, surface treatment (bonderite treatment), and cold forging into steel balls. The steel wire rods No. 1 and No. 2 of the present invention could be processed without wire drawing, and did not have steel ball forging cracks, dimensional defects, and the like. Further, a quenching / tempering treatment was performed through a polishing step, and Rockwell hardness was measured. As a result, the steel wire rod No. 1, N
o.2 is an average hardness HRB85.
9-86.4 were obtained. No.1, No.2, No.5, N
Rolling fatigue life tests were performed on steel balls manufactured from o.7 and No.8. The results are shown in Table 2 in terms of the life ratio with respect to steel wire No. 5 (comparative example), with steel wire No. 5 (comparative example) as 1. The example of the present invention has a longer life than the comparative example out of the scope of the present invention.

[0029]

According to the present invention, it is possible to easily provide a small-diameter steel wire rod for bearings having a wire diameter of 5 mm or less, which prevents generation of martensite structure and has a stable quality. And / or omission of wire drawing can be omitted, and a remarkable industrial effect can be expected.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing one example of a rolling method using a four-roll rolling mill suitable for carrying out the present invention.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takao Ogawa 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama (No address) Inside the Mizushima Works of Kawasaki Steel Corporation F-term (reference) 4K032 AA06 AA07 AA12 AA16 AA26 AA31 BA02 CA01 CA02 CD01 CE02

Claims (3)

[Claims] (1) In mass%, C: 0.6 to 1.1%, Si: 0.2 to 0.5%, Mn: 0.30 to 1.0%, Cr: 1.0 to 1.6%, O: 0.0010% or less, the balance being Fe and inevitable A fine steel wire for bearings, characterized in that it has a composition consisting of chemical impurities and a structure consisting of a pearlite single phase, and has an arithmetic mean roughness of Ra: 2 μm or less. 2. In% by mass, C: 0.6 to 1.1%, Si: 0.2 to 0.5%, Mn: 0.30 to 1.0%, Cr: 1.0 to 1.6%, O: 0.0010% or less, the balance being Fe and inevitable Is heated to a temperature in the range of 850 to 1100 ° C., and at least the caliber surface roughness of the final finish rolling is adjusted so that the finished arithmetic average roughness of the wire rod is Ra: 2 μm or less. Hot-rolled to obtain a small-diameter wire,
Wind up after adjusting to a temperature of 0 to 850 ° C, then 1 ° C / s
A method for producing a small-diameter steel wire rod for a bearing, characterized by cooling at the following cooling rate. 3. The method according to claim 2, wherein the finish rolling in the hot rolling is performed using a four-roll rolling mill.