JPH03254340A - Manufacture of raw material for bearing having excellent service life to rolling fatigue - Google Patents

Abstract

PURPOSE:To manufacture a raw material for bearing having excellent service life to rolling fatigue by continuously casting molten steel containing the specific wt.% of C, Si, Mn, Cr and Mo and executing the specific draft of squeezing work near crater end, which solidification in inner part of a cast billet is completed, and hot-working. CONSTITUTION:The molten steel containing wt.% of 0.60-1.50% C, 0.15-2.00% Si, 0.15-2.50% Mn, 0.05-1.00% Cr, >0.50-1.50% Mo and the balance of Fe and inevitable impurities, is continuously cast. The squeezing work is executed near the crater end, where the solidification in the inner part of cast billet is completed, at >=5% draft and successively, the hot-rolling is executed. In the molten steel, further, one or more kinds among 0.05-0.50% V, 0.05-0.50% Nb, 0.05-0.50% W, 0.10-2.00% Ni and 0.05-1.00% Cu, are incorporated. By this method, necessity for executing homogenized annealing at high temp. for long time, can be eliminated.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention provides a method for manufacturing a bearing material having excellent rolling fatigue life characteristics and suitable as a material for rolling bearings used in automobiles, other industrial machinery, etc. It is related to.

(Prior Art) Conventionally, carbon steel for machine structures, alloy steel for machine structures, high carbon chromium bearing steel, etc. have been used as steel for bearings.

Among these, high carbon chromium bearing steel is most commonly used as ball bearings and roller bearings in automobiles, industrial machinery, etc. This steel contains about 1 wt% of carbon (hereinafter simply expressed as %) and 0.
Approximately 9 to 1.6% chromium is added, and macro segregation (hereinafter referred to as center segregation) and eutectic carbides occur during continuous casting, especially in the axial center of the slab, resulting in cracking during cutting and punching. Since this increases the occurrence of eutectic carbides and deteriorates the rolling contact fatigue life characteristics, it is best to either punch out the center of the material and dispose of it as waste, or to dissipate the eutectic carbide by performing an agglomeration method or a long-term diffusion process before use. was. For this reason, a decrease in productivity and material yield could not be avoided.

In continuous casting, center segregation and eutectic carbides that cause such problems are caused by solidification shrinkage at the solidified tip and vacuum suction force of the void created by bulging of the solidified shell, which causes C, Cr, etc. to appear at the solidified tip. It is formed by the suction of concentrated molten iron content such as molten steel, etc., and due to heat treatment during product processing, large eutectic carbides or spheroidized carbides remain, an increase in the amount of retained austenite, and non-uniformity of these microstructures. etc. occur, reducing rolling fatigue life.

As a preventive measure, attempts have been made, for example, to use electromagnetic stirring in the secondary cooling zone, but this has not resulted in alleviation of semi-micro segregation and is ineffective in dissipating large eutectic carbides.

In addition, attempts have been made to apply the so-called in-line reduction method (Tetsu-to-Hagane 60th Year (1974) No. 7, pp. 875-884), in which a large reduction is applied using a pair of rolls at the final stage of solidification; If the reduction in the slab region is insufficient, cracks will occur at the solidification interface, while if the reduction is too sufficient, strong negative segregation will occur at the center of the slab in the thickness direction.

Regarding this point, Japanese Patent Application Laid-Open No. 49-121738,
Light reduction is applied with a pair of rolls near the solidified tip of the slab,
A method is proposed in which the amount of solidification shrinkage in the area is compensated by reduction, and Japanese Patent Application Laid-Open No. 52-54625 proposes a method in which a forging die is used to greatly reduce the area near the solidification completion point of the slab. .

However, in the case of light reduction by rolls, even if several pairs of rolls apply a reduction of several mm/m, it is not possible to sufficiently prevent solidification shrinkage and bulging that occur between the roll pitches, and the reduction position is If it is not appropriate, there is a problem that center segregation will worsen.

On the other hand, when a forging die is used to apply a large reduction near the solidification point of the slab, the solidification interface is less likely to crack compared to large reduction using rolls as in the in-line reduction method, and it also reduces negative segregation and even seven-dimensional macro-segregation. However, if the reduction in the area of the slab with a large unsolidified layer is insufficient, cracks will occur at the solidification interface, and conversely, if the reduction is too sufficient, the slab will The disadvantage is that strong negative segregation occurs in the center of the steel, and furthermore, the effect cannot be obtained even if the area where the unsolidified thickness is small is rolled down, so the current situation is to find the optimal rolling conditions. .

(Problems to be Solved by the Invention) This invention advantageously solves the problems of the above-mentioned technology.In addition to component adjustment, continuous casting conditions are modified to require high-temperature and long-term homogenization annealing. The purpose of the present invention is to propose an advantageous manufacturing method for a bearing material that has an excellent rolling fatigue life equivalent to or better than conventional high carbon chromium bearing steel and is highly productive.

(Means for Solving the Problems) That is, this invention has the following properties: C: 0.60-1.50%, Si: 0.15-2.00%, Mn: 0.15-2.50%, Cr: 0 .05-1.00% and Mo: 0.
Molten steel containing more than 50% to 1.50%, with the remainder consisting of Fe and unavoidable impurities, is continuously cast and subjected to forging with a reduction rate of 5% or more near the crater end where the inside of the slab completes solidification. This is a method (first invention) for producing a material for a bearing having excellent rolling fatigue life, which comprises performing hot rolling.

Further, in the present invention, the composition of the molten steel is as follows: C: 0.60 to 1.50%, Si: 0.15 to 2.00%, Mn: 0.
15-2.50%, Cr: 0.05-1.00
% and Mo: more than 0.50 to 1.50%, further including V: 0.05 to 0.50%, Nb: 0.05 to 0.50%, W: 0.05 to 0.50%, Ni: 0.10-2.00% and Cu: 0.
05 to 1.00%, and the remainder consists of Fe and unavoidable impurities (Second Invention) ).

(Function) First, in this invention, the reason why the strength of the material is limited to the above range will be explained.

C: 0.60 to 1.50% C is a useful element that improves strength, wear resistance, and rolling fatigue life characteristics by forming a solid solution in the matrix.

However, if there is too much, giant carbides will grow,
This not only deteriorates the rolling fatigue life but also requires a long diffusion annealing to dissipate the fatigue, resulting in a decrease in productivity.

Therefore, taking the above points into consideration, the amount of C should be 0.60 to 1.50%.
It was decided to add within the following range.

St: 0.15-2.00% Si is a useful element that not only acts as a deoxidizing agent during steel melting, but also dissolves in the matrix and suppresses the decrease in hardness due to tempering, improving rolling fatigue life. . However, if too much Si is added, machinability and forgeability deteriorate, so Si is added in a range of 0.15 to 2.00%.

Mn: 0.15 to 2.50% Mn improves the hardenability of the steel, thereby effectively contributing to improving the base toughness and, in turn, improving the rolling fatigue life of the steel material. However, since too much Mn deteriorates machinability and forgeability, Mn is added in a range of 0.15 to 2.50%.

Cr: 0.05 to 1.00% Cr is effective in improving hardenability, increasing the strength and toughness of the matrix, and promoting the formation of carbides to improve wear resistance. This effect becomes noticeable at 0.05% or more, so this value is set as the lower limit.

However, if it exceeds 1.00%, impact resistance and machinability will deteriorate, and addition cost will increase. Furthermore, eutectic carbides are produced during casting, which not only reduces rolling fatigue life, but also requires high-temperature, long-term homogenization treatment to eliminate this negative effect.

Therefore, the upper limit was set at 1.00%.

Mo: more than 0.50 to 1.50% Mo not only improves hardenability but also has strong solid solution strengthening and precipitation hardening functions, so it effectively contributes to improving strength and rolling fatigue life. However, if the amount is too large, machinability deteriorates and addition cost increases. Therefore, the content of element 0 was determined to be in the range of more than 0.50 to 1.50%.

In this invention, in addition to the above-mentioned basic components, V
, Nb, W, Ni, and Co.
A species or two or more species can be added as strength-enhancing components within the ranges described below.

V, Nb, W: 0.05-0.50% V, Nb and W each form stable carbides at high temperatures and improve rolling fatigue life characteristics.

However, if the amount is too high, the hardness after tempering will decrease and the rolling fatigue life characteristics will deteriorate. Therefore, V.

Nb and W were each added in a range of 0.05 to 0.50%.

Ni: 0.10-2.00% Ni not only contributes to improving hardenability, but also suppresses a decrease in hardness after tempering, so it is an element useful for improving strength and rolling fatigue life.

However, if the amount is too large, a large amount of residual γ will grow and reduce the hardness of the steel material after tempering. Therefore, Ni is o,
It was supposed to be added in a range of io to 2.00%.

Cu: 0.05-1.00% Cu, like Ni, not only contributes to improving hardenability, but also suppresses the decrease in hardness after tempering, so it is useful for improving strength and rolling fatigue life. It is an element. However, when the content is too large, forgeability deteriorates.

Therefore, Cu was added in a range of 0.05 to 1.00%.

In addition, one or more selected from AI, Ca + Na, K, Mg, and Zr are used for the purpose of reducing the amount of oxygen and controlling the form of inclusions, and SI Ca, pb, and SI Ca are used for the purpose of improving machinability. One or more selected from B, Bi, and REM, one or two selected from P and N for the purpose of improving hot strength, and s for the purpose of reducing decarburization.
It is also possible to add a small amount of each of b.

Now, the molten steel adjusted to the preferred composition as described above is continuously cast into slabs. In this invention, the reduction rate is set near the crater end where the internal molten steel of the obtained continuously cast slabs completes solidification. It is important to perform a forging process of 5% or more, thus preventing the formation of segregation in the center of the slab.

Here, the reason why the forging process as described above improves segregation at a position corresponding to the center of the slab is considered to be as follows.

In other words, at the final stage of solidification of internal molten steel, molten steel containing large nonmetallic inclusions and a high concentration of alloying elements exists near the crater end, so if it solidifies as it is, nonmetallic inclusions will remain and central segregation will occur. However, when forging is performed before solidification, the concentrated molten steel containing such non-metallic inclusions is pushed upwards, so the amount of non-metallic inclusions and alloying elements in the center do not increase significantly; As a result, rolling fatigue life characteristics in the center area are improved.

As described above, as center segregation and eutectic carbides are effectively suppressed, the time required for diffusion annealing, which was conventionally performed using a soaking furnace, is significantly shortened.

In Figure 1, C: 1.01%, Si: 0.80%,
Mn: 0.45%, Cr: 0.25% and M
o: Slabs obtained by continuous forging during continuous casting of M-molten steel containing 0.80%, or slabs obtained by conventional methods without forging. 65 mm diameter steel bars were obtained by steel bar rolling, and the rolling contact fatigue life characteristics at the center portion (the test pieces were taken so that the center of the steel bars was on the surface of the test piece) were investigated. The results are shown below.

As is clear from the figure, the rolling contact fatigue life characteristics of the central steel bar member were improved by more than five times the conventional method without such forging by applying forging with a rolling reduction of 5% or more.

Therefore, in this invention, the rolling reduction rate by forging process is 5.
% or more. However, the reduction rate is 6
If it exceeds 0%, there will be a problem that the precision of the material after rolling will decrease, so the rolling reduction ratio is preferably 60% or less.

(Example) Various molten steels having the chemical compositions shown in Table 1 were processed into slabs by a converter->continuous casting method under the conditions shown in Table 2.

Then, after homogenizing in a soaking furnace at 1240°C for 2 or 20 hours, the 65IIIlφ steel bar was hot rolled and then subjected to spheroidizing annealing. A rolling contact fatigue life test piece was taken from the surface of the piece), and after quenching and tempering, a rolling contact fatigue life test was conducted.

The rolling contact fatigue life test was conducted using a cylindrical rolling contact fatigue life testing machine under the conditions of a maximum Hertzian contact stress of 600 kgf/mn+" and a repeated stress number of 46,240 cpm, and the test results were calculated on probability paper assuming that they follow the Weibull distribution. summarized in
A relative evaluation was made by setting D/4 part L1° (the number of stress loads until peeling when the cumulative probability of failure is 10%) of a 20-h diffusion annealed material of rope material Sou 1 as 1.

The obtained results are also listed in Table 2.

As is clear from Table 2, all steels whose rolling force range and rolling reduction during forging meet the appropriate ranges of this invention have rolling contact fatigue life characteristics of l! 20h of Ll
This is a significant improvement over diffusion annealed materials (conventional materials).

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain a bearing material having a rolling contact fatigue life superior to that of conventional steel materials without the need for homogenizing annealing at high temperatures and for a long period of time.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between rolling reduction rate and rolling fatigue life characteristics in forging. Pressure reduction (%)

Claims (1)

[Claims] 1. C: 0.60 to 1.50 wt%, Si: 0.15 to 2.00 wt%, Mn: 0.15 to 2.50 wt%, Cr: 0.05 to 1.00 wt%. % and Mo: more than 0.50 to 1.50 wt%, with the remainder consisting of Fe and unavoidable impurities. Molten steel is continuously cast, and the reduction rate is 5% near the crater end where the inside of the slab completes solidification. A method for manufacturing a bearing material with excellent rolling fatigue life, characterized by subjecting the above forging process to hot rolling. 2. The composition of the molten steel is: C: 0.60 to 1.50 wt%, Si: 0.15 to 2.00 wt%, Mn: 0.15 to 2.50 wt%, Cr: 0.05 to 1.00 wt% % and Mo: more than 0.50 to 1.50 wt%, further V: 0.05 to 0.50 wt%, Nb: 0.05 to 0.50 wt%, W: 0.05 to 0.50 wt%, Claim 1: Contains one or more selected from Ni: 0.10 to 2.00 wt% and Cu: 0.05 to 1.00 wt%, and the remainder is Fe and inevitable impurities. A method of manufacturing the bearing material described above.