JP2002060887A - Method for manufacturing high purity bearing steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high purity bearing steel in which Ti-type inclusions, particularly particles of TiN, which are supposed to greatly affect the acoustic characteristics of balls for miniature bearing can be refined. SOLUTION: Bearing steel containing, as impurities, <=10 ppm, preferably <=7 ppm, Ti and >=50 ppm, preferably <=30 ppm, N is continuously cast into a billet. The billet is heated at >=1,200 deg.C, preferably about 1,240 deg.C, for >=1 h to undergo soaking treatment. By this procedure, the billet can be made virtually free from the presence of TiN particles of >=7.5 μm length. The continuous casting is carried out at an extra low velocity of <=0.4 m/min drawing rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an ultraclean bearing steel in which Ti carbonitrides are present in a fine state. This ultra-clean bearing steel has particularly improved acoustic properties.

[0002]

BACKGROUND OF THE INVENTION Balls for miniature bearings, one of the uses of bearing steel, are often required to have good acoustic characteristics, that is, not to generate abnormal noise during use. Since it has been found that the cause of the abnormal noise is the presence of minute inclusions, production of ultra-clean bearing steel with extremely few inclusions has been attempted.

[0003] Inclusions that affect the acoustic characteristics include alumina (Al ₂ O ₃ ) inclusions and oxide composite (MgO · A) inclusions.
l ₂ O ₃ .CaO) inclusions and titanium-based (TiC, Ti
N) Inclusions are known and efforts have been made to reduce their content. For the former two, there are enhancement of degassing, control of inclusion morphology, search for non-contamination casting technology in continuous casting, etc. For the latter, in addition to examination of raw materials, Controlling the amount of TiN, etc. is such. There are also special melting methods such as VAR (vacuum arc remelting), ESR (electroslag remelting) or triple melt, which can provide almost the desired product, but inevitably suffer from disadvantages in cost. It is also desirable to produce ultra-clean bearing steel by a conventional melting-refining process.

[0004]

SUMMARY OF THE INVENTION The present invention has been focused on the acoustic characteristics of balls for miniature bearings as the amount of oxide-based inclusions is reduced.
An object of the present invention is to provide a method for producing ultra-clean bearing steel in which Ti carbonitride particles are made fine.

[0005]

According to the present invention, there is provided a method for producing an ultra-clean bearing steel, comprising the steps of: It has an alloy composition that contains the necessary alloy components for the bearing steel and that reduces the value of [% Ti] x [% N] within the limit where TiN substantially dissolves at the soaking temperature prior to slab rolling. The bearing steel is cast into a slab by continuous casting, and the slab is selected to have a temperature equal to or higher than the solid solution equilibrium temperature as the soaking temperature, and the soaking time is determined based on the diffusion rate of TiN. Is characterized by performing soaking for a period of time sufficient for reducing the particle diameter for re-solid solution of, followed by slab rolling and subsequent rolling.

[0006] More specifically, the above-mentioned manufacturing method comprises:
Ti: 10 ppm or less, preferably 7 ppm or less, N: 50 pp
m or less, preferably 30 ppm or less, a slab of bearing steel,
1200 ° C. or higher, preferably about 1240 ° C.
This is a method for producing ultra-clean bearing steel in which TiN particles having a length of 7.5 μm or more are substantially absent by performing a soaking process of heating for at least one hour.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The condition of Ti: 10 ppm or less and N: 50 ppm or less is determined by the value of [% Ti] × [% N].
This must be satisfied in order to suppress the iN generation amount. It is typical that the soaking temperature is set to 1200 ° C. or more because Ti = 4 pp.
This is because, under the conditions of m and N = 30 ppm, TiN forms a solid solution at this temperature. This was proved from the experimental results shown in FIG.

FIG. 1 shows that Ti = 4 ppm and N =
30 ppm slabs at 1180 ° C or 1240 ° C
After heating for hours, the distribution of Ti-based inclusions by particle size is
7 is a graph in comparison to an as-cast one, with a length of 7.
Although the Ti-based inclusions (mainly Ti carbonitride) particles of 5 μm or more were hardly improved by heating at 1180 ° C. (31 → 27), they were significantly reduced by heating at 1240 ° C. (2 ).

[0009] Ti corresponding to each [% Ti] and [% N]
A typical example of the calculated value of the N solid solution temperature is as follows. Ti (ppm) N (ppm) TiN solid solution temperature (° C.) 330 1207 4 202 1224 4 30 1248 4 50 1281 4 60 1293 5 30 1262 5 40 1281 5 50 1295 6 20 1248 6 40 1293 6 50 1308 1303 7 50 1318

According to the present invention, continuous casting is performed as a means for casting a smelted bearing steel, and the casting is performed at a drawing speed of 0.4 m.
/ Minute / min.
Such ultra-low-speed continuous casting aims at rapidly solidifying the molten metal poured into the water-cooled mold, and improves segregation and reduces generation of coarse Ti-based inclusions. There is expected.

In the research leading to the present invention, the inventors have found that coarse Ti-based inclusions are present in the thickened molten steel portion, which is the final solidified portion in continuous casting, and the effect of segregation on this fact. Thought it was big. Then, the generation mechanism of the coarse Ti-based inclusions was estimated as follows. Based on this, the degree of segregation depends on the solidification rate. (Segregation of Ti ・ NC ・ C at dendrite tree / tip) → (Ti ・
Temperature drop to a temperature exceeding the solubility product of N / C) → (T
Crystallization of iC / TiN) → (Growth of crystallized TiC / TiN)

FIG. 2 is a graph showing how the solidification differs between the case where the drawing speed is 0.45 m / min and the case where the drawing speed is 0.30 m / min. In the case of extremely low-speed continuous casting, the solid-liquid temperature width is reduced, and an improvement in the solidification rate and an improvement in segregation can be expected.

[0013] In performing ultra-low speed continuous casting, all operations that reduce the solid-liquid temperature difference, improve the solidification rate, and reduce segregation are preferable. Specifically, first, there is a low-temperature casting using a tundish heater, which obviously means a reduction in the solid-liquid temperature difference. Electromagnetic stirring is also useful for preventing segregation, but when it is applied to the casting of round slabs using a circular mold, stirring can be performed effectively. Light reduction at an appropriate central solids fraction leads to improved segregation, and enhanced secondary cooling causes rapid solidification. It is desirable to carry out one or more of these operations, as much as possible, in addition to the above-mentioned extremely low speed continuous casting.

[0014] In the method of the present invention, if the step of heating the bloom obtained by the slab rolling to the soaking temperature again is added, the object of miniaturization of Ti-based inclusions mainly composed of TiN can be further achieved. . This is because the structure of the bearing steel is subdivided by the slab rolling, and the refinement by the processing is facilitated.

[0015] The bearing steel which can make the particles of TiN fine by the production method of the present invention generally has a C: 0.6 to 1.5% by weight and a Si:
2.0% or less, Mn: 2.0% or less, Ni: 0 to 2.0
%, Cr: 0.3-5.0% and Mo: 0-1.0%
And Ti: 10 ppm or less, preferably 7 ppm or less,
N: 50 ppm or less, preferably 30 ppm or less, with the balance substantially consisting of Fe.

[0016]

EXAMPLE A bearing steel having the following alloy composition (this TiN
The solid solution temperature was 1240 ° C. as described above, and continuous casting was performed at a drawing speed of 0.45 m / min or 0.30 m / min to obtain a round slab having a diameter of 350 mm. CSiMnCrMoTiNFe 1.0% 0.25% 0.45% 1.5% 0.1% 3ppm 20ppm Remainder

For the two types of round slabs, 1260 ° C. × 10
After performing the soaking process for a long time, the Ti-based inclusions in the transverse direction and the axial direction in the central portion were examined by size of 3 μm or more, 5 μm or more, and 7.5 μm or more. The results are shown in FIG. 3 (Comparative Example) and FIG. 4 (Example).

[0018]

When the bearing steel is manufactured by the method of the present invention, the particles of Ti-based inclusions centering on TiN therein can be refined. Using this bearing steel as a material, a ball for a miniature bearing having excellent acoustic characteristics, which is in great demand for HDD parts and the like, can be manufactured.

[Brief description of the drawings]

Fig. 1 Slab of bearing steel (Ti = 4ppm, N = 30ppm)
Is a graph comparing the distribution of Ti-based inclusions by particle size after being heated to 1180 ° C. or 1240 ° C. for 6 hours, respectively, as compared with the as-cast.

FIG. 2 is a conceptual longitudinal sectional view showing a vicinity of a water-cooled mold for explaining an extremely low-speed continuous casting employed in the present invention, wherein a drawing speed is 0.45 m / min and a drawing speed is 0.30 m / min. FIG.

FIG. 3 shows data of a comparative example of the present invention, wherein the diameter is 350;
The graph which shows the size and the number of Ti type | system | group inclusions of the transverse direction and the axial direction center part at the time of continuously casting a round cast piece of mm at a drawing speed of 0.45 m / min.

FIG. 4 shows data of an embodiment of the present invention, wherein the diameter is 350;
The graph which shows the size and the number of Ti type | system | group inclusion of the center part of a transverse direction and an axial direction at the time of continuously casting the round cast piece of mm at a drawing speed of 0.30 m / min.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) B22D 11/12 B22D 11/12 C 11/128 350 11/128 350A 11/20 11/20 A 11/22 11/22 B // C22C 38/00 301 C22C 38/00 301Z 38/58 38/58

Claims (7)

[Claims] 1. A method for producing an ultra-clean bearing steel in which Ti carbonitrides are present in a fine state, comprising an alloy component necessary for the bearing steel, and a soaking temperature prior to slab rolling. [% Ti] × within the limit where TiN substantially dissolves
A bearing steel having an alloy composition with a reduced [% N] value is cast into a slab by continuous casting, and the slab or ingot is selected to have a temperature equal to or higher than a solid solution equilibrium temperature as a soaking temperature. The thermal time is determined based on the diffusion rate of Ti.
A method for producing an ultra-clean bearing steel, characterized in that a soaking is carried out for a time sufficient for reducing the diameter of the particles due to re-dissolution of N, followed by slab rolling and subsequent rolling. 2. A soaking slab of bearing steel having a titanium content of 10 ppm or less and a nitrogen content of 50 ppm or less is subjected to a soaking process of heating to a temperature of 1200 ° C. or more for 1 hour or more, and a Ti having a length of 7.5 μm or more.
2. The method for producing ultra-clean bearing steel according to claim 1, wherein a bearing steel substantially free of N particles is obtained. 3. The method for producing ultra-clean bearing steel according to claim 1, wherein the casting of the bearing steel is carried out by continuous casting at a very low speed at a drawing speed of 0.4 m / min or less to produce cast slabs. 4. In carrying out extremely low-speed continuous casting, one of low-temperature casting by using a tundish heater, use of a circular mold and electromagnetic stirring thereto, low pressure at an appropriate central solid phase ratio, and enhancement of secondary cooling. 4. The method for producing ultra-clean bearing steel according to claim 3, wherein the method is performed by adding one or more kinds. 5. The method for producing an ultraclean bearing steel according to claim 1, further comprising a step of heating the bloom or billet after the bulk rolling to the soaking temperature again. 6. The bearing steel has a C: 0.6 to 1.5 by weight.
%, Si: 2.0% or less, Mn: 2.0% or less, Ni:
0 to 2.0%, Cr: 0.3 to 5.0%, and Mo: 0
-1.0%, Ti: 10 ppm or less, N: 50 ppm
The method for producing an ultra-clean bearing steel according to any one of claims 1 to 5, wherein the remainder has an alloy composition substantially consisting of Fe. 7. Ti: 7 ppm or less, N: 3 in bearing steel
The method for producing ultra-clean bearing steel according to claim 6, wherein the content is 0 ppm or less.