JP3324272B2 - Manufacturing method of bearing steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing bearing steel by continuous casting.

[0002]

2. Description of the Related Art Continuous casting of steel has the advantage that the yield is high and the productivity is high, but C, P, and C are located at the center of the slab.
There is a drawback that components such as S tend to segregate, so that it is difficult to employ a continuous casting method depending on the type of steel. Representatives of which continuous casting is difficult are bearing steels, especially high carbon and high chromium bearing steels such as SUJ2, which have a high C content of 0.95 to 1.10% and a high demand for manufacturing quality.
Conventionally, low-speed continuous castings can only be used as materials for producing races and rollers among bearing components, and it has been impossible to continuously produce bearing steel for ball applications.

As a means for suppressing the center segregation in continuous casting, a so-called light reduction in which a roll pressure is applied at a low reduction ratio prior to solidification of a central portion is known. Light pressure reduction is often performed for the purpose of improving the close contact of the central part.

[0004] The applicant has conducted research on the suppression of center segregation due to this light reduction, casting a slab with a circular cross section, and having a center solid fraction of 0.2 to 0.8, preferably 0.4.
It has been found that it is preferable to apply a slight reduction with a flat roll while the pressure is in the range of 0.5 to 0.5, and has already proposed (Japanese Patent Application No. Hei 6 (1999)).
-15772). The degree of light reduction is 1
It has been found that about 3% is appropriate.

[0005] The term "central solid phase ratio" means the weight ratio of the solid phase in the center of the slab. In the process of forming a slab by continuous casting, there is a mixed region of solid phase and liquid phase between the outer completely solidified shell and the center liquid phase only part, and in the region where the mixed phase exists It is this term that indicates what percentage the solid phase occupies. The center solid fraction is the specific heat of the slab, the thermal conductivity,
The slab cross-section temperature distribution is obtained by heat transfer calculation based on data such as hot water temperature, and is estimated from that.

[0006] The slab of the bearing steel is placed in a heating furnace, soaked, and then slab-rolled into a billet or a product. However, a scarf treatment, that is, a flame removal is required prior to the rolling of the billet wire. Conventional soaking has been performed for a relatively long time of about 6 hours, with the intention of dissolving and eliminating the giant carbide precipitated in the slab in the matrix. On the other hand, since decarburization proceeds on the surface of the bloom during soaking, it is necessary to remove the decarburized layer together with the scale generated by prolonged soaking, and for this reason also, the scarf treatment is indispensable.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to produce a high carbon bearing steel slab by continuous casting using the above-described light reduction technology developed by the applicant, and in particular, to produce a ball material. It is an object of the present invention to provide a method of producing bearing steel that enables continuous casting, and that allows materials for races and rollers to be cast at a higher speed than before, and that eliminates the need for a scarf treatment.

[0008]

SUMMARY OF THE INVENTION The present invention relates to a method for producing bearing steel. In the production of bearing steel by continuous casting, a slab having a circular cross section is cast by a vertical continuous casting method and is drawn from a water-cooled mold. The central coagulation rate is 0.3 ~
While it is 0.6, the rolling reduction is 1 to 1 using a flat roll.
3% light reduction is applied to eliminate the center segregation, and the obtained slab is soaked at a temperature of 1150 to 1260 ° C. for 2 to 5 hours and slab-rolled, so that the bearing is not subjected to a scarf treatment. It is characterized by obtaining billets or products that can be processed into parts.

[0009]

Hereinafter, the meaning of the above-described configuration of the present invention will be described with reference to an actual example. First, as disclosed in the above-mentioned patent application, the cross-sectional shape of the slab of continuous casting is, as disclosed in the above-mentioned patent application, taking into account the volume shrinkage accompanying solidification of molten steel and the area reduction rate and the internal cracking under light pressure. Comparing the relationship with the possibility between a circular section and a square section, the latter is based on the fact that the tensile stress at the corners is large and the risk of internal cracking is high. Materials with strong crack sensitivity, such as high carbon bearing steel,
The method should be a vertical continuous casting method in which a slab having a circular cross section is drawn out without bending.

Regarding the range where the center solid phase ratio should be reduced by light, the diameter of the lightly reduced 350 at various positions in continuous casting of SUJ2 steel was examined.
The C center segregation of a slab having a circular cross section of 0.3 mm (casting speed 0.36 m / min.) was determined based on the results of FIG. "Center segregation" is the ratio of the C content at the center of the slab to the C content of the molten steel in the ladle. As shown in FIG. 1, the value of C center segregation becomes low at a low center solid fraction f _s , that is, when light reduction is performed before solidification progresses inside the slab. However, slab f _s is soft reduction at too low is because the internal cracking occurs, it can be seen that should carry out the soft reduction in the range of 0.3 to 0.6.

The optimum light reduction position is, in this case, f _s = 0.
43 points. This value varies somewhat depending on factors such as the composition of the bearing steel, the size of the slab, and the casting speed, but can be easily found in each case.

Generally, in the case of SUJ2 steel, the allowable limit of C center segregation is 1.1 for balls and 1.2 for race rollers. FIG. 1 shows that it is easy to obtain products within this limit according to the invention.

The effect of casting speed on C center segregation is as shown in FIG. The SUJ2 steel was continuously cast at different casting speeds circular slab with a diameter of 350 mm, was subjected to 1.5% of soft reduction in known optimal soft reduction position in Figure 1 (f _s = 0.43), It has been found that higher casting speeds can be employed by performing the light reduction according to the present invention to achieve the same level of C center segregation. In particular,
If the casting speed is lower than 0.4 m / min, C center segregation 1.1
The following slabs are obtained, which can be used as a material for bearing balls as described above. On the other hand, if the allowable limit of C center segregation is expanded to 1.2 for race rollers,
A casting speed increased to 0.4 m / min or more can be adopted. This increase in casting speed is a big step forward, compared to the speed at which continuous casting of SUJ2 steel could hardly exceed 0.3 m / min even for race roller materials.

Regarding soaking, FIG. 3 shows the reason for adopting the above conditions. When light reduction is not performed on a slab having a circular cross section with a diameter of 350 mm, in order to eliminate the giant carbide by soaking, it is necessary to maintain the temperature within the range of the nose shown by the thin solid line in the drawing, that is, at a temperature of about 1200 to 1240 ° C. Heating was required for up to 6 hours, but when a suitable light reduction was applied according to the present invention, the nose moved to the left as shown by the thick solid line, and even with a short soaking time, the material was free of giant carbide. Become. As is clear from the figure, at a soaking temperature of less than 1150 ° C., there is no effect of shortening the soaking time, while at a high temperature exceeding 1260 ° C., the once lost macro carbides crystallize again. Preferred conditions are temperatures from 1200 to 1240
° C, time 2.5-3.0 hours.

If the time required for soaking is reduced,
As described above, slab rolling can be performed before decarburization progresses deeply on the slab surface, and generation of scale can be reduced. When the bloom obtained by the bulk rolling is further rolled, surface scratches are removed by grinder grinding, but the bloom obtained according to the present invention may be ground only on a limited portion of the surface. Thus, the same results as those obtained by the conventional scarf treatment and the whole surface grinder grinding can be obtained.

[0016]

As is apparent from the above data, if the bearing steel is manufactured according to the present invention, even if the high carbon bearing steel is continuously cast, the slab which has no or little center segregation and is directed to the ball can be obtained. It is easy to obtain, and if it is a material for race rollers, even if the casting speed is higher than before, a material with center segregation within the allowable limit can be obtained.

Since the soaking time before the slab rolling of the slab is about half the time required in the conventional method, the depth of the decarburized layer is not deep and the amount of scale generated is small. Therefore, the surface will fall off as a rolling scale in the subsequent rolling process,
Even without performing the scarf treatment, it can be sent to a processing step such as rolling.

Thus, the present invention increases the productivity and reduces the cost of producing bearing steel and bearings therefrom.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the position under light pressure and the C-center segregation of a slab in vertical continuous casting in which SUJ2 steel is cast into a slab having a circular cross section, which is implementation data of the present invention.

FIG. 2 is a graph showing the relationship between the casting speed and C center segregation, which is practical data following FIG.

FIG. 3 is a graph, following the data of FIG. 2, showing whether giant carbide is present in the slab at various times and temperatures for soaking prior to slab rolling of the slab.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-254342 (JP, A) JP-A-63-215353 (JP, A) JP-A-1-162551 (JP, A) JP-A-5-254 228598 (JP, A) JP-A-63-123516 (JP, A) JP-A-7-204812 (JP, A) JP-A-6-33190 (JP, A) JP-A-5-3177091 (JP, A) JP-A-3-254341 (JP, A) JP-A-3-254340 (JP, A) JP-A-3-254339 (JP, A) JP-A-2-92444 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/00 B22D 11/20

Claims (4)

(57) [Claims] In the production of bearing steel by continuous casting,
A slab having a circular cross section is cast by a vertical continuous casting method, and a slab drawn from a water-cooled mold is subjected to a rolling reduction using a flat roll while its central solidification rate is 0.3 to 0.6. Applying a 1 to 3% light reduction to eliminate center segregation, subjecting the obtained slab to soaking by heating at a temperature of 1150 to 1260 ° C for 2 to 5 hours and subjecting it to sparf rolling to perform a scarf treatment A method for producing bearing steel, characterized in that a billet or a product is obtained without the need. 2. The bearing steel is high carbon chromium bearing steel SUJ1.
The method according to claim 1, wherein the number is 5. 3. The method according to claim 2, wherein the casting speed is in the range of 0.3 to 0.5 m / min to obtain a slab having a C center segregation ratio of 1.2 or less. 4. The method according to claim 2, wherein the casting speed is set in the range of 0.3 to 0.4 m / sec to obtain a slab having a C center segregation ratio of 1.1 or less for use in balls.