JP2905241B2 - Manufacturing method of bearing material with excellent rolling fatigue life - Google Patents

Description

The present invention relates to a method for producing a bearing material having excellent rolling fatigue life characteristics, which is suitable as a material for rolling bearings used in automobiles, other industrial machines, and the like. It is about.

(Prior Art) Rolling bearings are composed of rolling wheels and rolling elements, and all of them are required to have a uniform rolling contact surface. In particular, macro-segregation (hereinafter simply referred to as center segregation) and eutectic carbide generated in the center of the slab deteriorate rolling contact fatigue life. They have been used after sufficiently dissipating them by performing a time diffusion process. The eutectic carbide also increases cracking during punching and cutting. Therefore, reductions in productivity and material yield cannot be avoided.

In the case of continuous casting, in addition to solidification shrinkage of the solidification tip, vacuum segregation of voids caused by bulging of the solidified shell is applied in the case of continuous casting. As a result of the inhalation of the molten steel component, it remained as positive segregation in the center of the cross section of the slab, and due to heat treatment during product processing, excessive spheroidized carbide remained, the amount of retained austenite increased, and the microstructure of these Non-uniformity occurs and reduces rolling fatigue life.

As a preventive measure, for example, electromagnetic stirring in a secondary cooling zone has been attempted, but the effect has not been reduced to the extent of semi-micro segregation, and its effect is not sufficient.

In addition, the so-called in-line reduction method of applying a large pressure reduction using a pair of rolls at the end of solidification (iron and steel, No. 60 (1974) No. 7, pp. 875 to 884) was also attempted, but the unsolidified layer was large. If the reduction in the slab area is insufficient, cracks occur at the solidification interface, and if the reduction is too high, there is a problem that strong negative segregation occurs in the center of the slab in the thickness direction.

In this regard, Japanese Patent Application Laid-Open No. 49-121738 discloses a method in which a light reduction is performed by a pair of rolls near the solidification front end of a slab and the amount of solidification shrinkage of the portion is reduced by reduction.
Japanese Patent Application Laid-Open No. 52-54625 proposes a method in which a forging die is used to greatly reduce the pressure near the solidification completion point of a slab.

However, in the case of light rolling with rolls, solidification shrinkage and bulging that occur between roll pitches cannot be sufficiently prevented even when rolling down several mm / m with multiple pairs of rolls, and the rolling position is reduced. If not, there is a problem that the center segregation worsens.

On the other hand, when a forging die is used to greatly reduce the vicinity of the solidification completion point of a slab, the solidification interface is less likely to crack than in the case of a large reduction by a roll such as the in-line reduction method, and it also causes negative segregation and semi-macro segregation. Although it has been clarified that it can be improved, the cracking still occurs at the solidification interface if the reduction in the slab area where the unsolidified layer is large is insufficient, and conversely if the reduction is too large, the center of the slab There is a disadvantage that strong negative segregation occurs in the part, and furthermore, even if the unsolidified thickness is reduced, the effect is not obtained,
At present, we are searching for optimal rolling conditions.

(Problems to be Solved by the Invention) The present invention advantageously overcomes the problems of the above technology, and has an object to propose an advantageous method for producing a bearing material excellent in forgeability and rolling fatigue life characteristics. And

(Means for Solving the Problems) That is, the present invention provides: C: 0.60 to 1.50 wt% (hereinafter simply referred to as%), Si: 0.15 to 2.00%, Mn: 0.15 to 2.50%, and Cr: more than 1.00 to 3.00% The remainder is forged with molten steel consisting of Fe and unavoidable impurities, and subjected to forging with a draft of 5% or more near the crater end where the inside of the slab completes solidification.
This is a method for producing a bearing material excellent in rolling fatigue life by performing hot rolling (first invention).

Further, according to the present invention, the composition of molten steel includes C: 0.06 to 1.50%, Si: 0.15 to 2.00%, Mn: 0.15 to 2.50%, and Cr: more than 1.00 to 3.00%, and Mo: 0.05 to 1.50%, V: 0.05 to 0.50%, Nb: 0.05 to 0.50%, W: 0.05 to 0.50%, Ni: 0.10 to 2.00%, and Cu: 0.05 to 1.00%. Is
This is a method for producing a bearing material having a composition of Fe and unavoidable impurities (second invention).

Further, in the present invention, in the first and second inventions, the treatment conditions in the soaking heat treatment are set at 1100 to 1250 ° C.,
This is a time-consuming method for producing a bearing material (third invention).

(Operation) First, the reason for limiting the component composition of the material to the above range in the present invention will be described.

C: 0.60-1.50% C is a useful element that forms a solid solution in the matrix to improve the strength, wear resistance, and rolling contact fatigue life. . However, if it is too much, a huge carbide is generated, which not only deteriorates the rolling fatigue life, but also
Further, long-term diffusion annealing is required for the dissipation, which causes a decrease in productivity. Therefore, considering the above points, the amount of C is 0.
It was added in the range of 60 to 1.50%.

Si: 0.15% to 2.00% Si is a useful element that acts as a deoxidizing agent when smelting steel and also forms a solid solution in the matrix to suppress a decrease in hardness due to tempering and improve the rolling fatigue life. However, if the content is too large, the machinability and the forgeability deteriorate, so the content of Si is set to 0.
It was added in the range of 15 to 2.00%.

Mn: 0.15 to 2.50% Mn effectively contributes to the improvement of the base toughness and the improvement of the rolling fatigue life of the steel material by improving the hardenability of the steel. However, if the content is too large, the machinability and the forgeability deteriorate, so Mn was added in the range of 0.15 to 2.50%.

Cr: more than 1.00 to 3.00% Cr has a function of improving hardenability, increasing strength and toughness of the matrix, and promoting spheroidization of carbide. In order to sufficiently exhibit such an effect, an amount of Cr exceeding 1.00% is required. Therefore, in the present invention, at least 1.00% of Cr is contained. However, if the addition is too large, the carbides become coarse and the machinability and the rolling fatigue life deteriorate. Therefore, the upper limit of the Cr content is set to 3.00%.

In the present invention, in addition to the above basic components, if necessary,
One or more selected from Mo, V, Nb, W and Cu can be added as a strength improving component in the range described below.

Mo: 0.05-1.50%, W: 0.05-0.50% Mo not only enhances hardenability, but also strengthens solid solution,
Since it has a precipitation hardening function, it effectively contributes to improvement in strength and rolling fatigue life. However, if the content is too large, the machinability is deteriorated and the addition cost is increased.
Therefore, Mo was added in the range of 0.05 to 1.50%.

V, Nb, W: 0.05 to 0.50%, V, Nb, and W each form a stable carbide at a high temperature and improve rolling fatigue life characteristics. However, if the amount is too large, the hardness after tempering decreases, and on the contrary, the rolling fatigue life characteristics deteriorate. Therefore, V, Nb and W are each 0.05
It was added in the range of 0.50.50%.

Ni: 0.10 to 2.00% Ni not only contributes to the improvement of hardenability but also suppresses the decrease in strength after tempering, and is therefore an element useful for improving strength and rolling fatigue life. However, if it is too large, a large amount of retained austenite is generated, and the hardness of the steel after tempering is reduced. Therefore, Ni is 0.10-2.00%
Was added in the range described above.

Cu: 0.05 to 1.00% Cu, like Ni, not only contributes to the improvement of hardenability, but also suppresses the decrease in hardness after tempering, and is therefore an element useful for improving strength and rolling fatigue life. However, when the content is too large, forgeability is deteriorated. Therefore, Cu is added in the range of 0.05 to 1.00%.

In addition, one or two or more selected from Al, Ca, Na, K, Mg and Zr for the purpose of oxygen content reduction and inclusion morphology control, and S, Ca for the purpose of improving machinability , Pb,
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 Sb for the purpose of further reducing decarburization Can be added in small amounts.

Well, molten steel adjusted to a suitable component composition as described above,
Although it is continuously cast into a slab, in the present invention, it is important to perform forging at a compression ratio of 5% or more in the vicinity of the crater end where the internal molten steel of the obtained continuous cast slab is solidified. Thus, the generation of segregation at the center of the slab is prevented.

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

In other words, in the final stage of solidification of the internal molten steel, molten steel with a high concentration of alloying elements exists near the crater end, so if it solidifies as it is, central segregation will occur, but if forging processing is performed before solidification, Since the molten steel is extruded upward, the alloy element concentration in the central portion does not increase so much, and as a result, generation of center segregation, eutectic carbide, etc., which cause deterioration of rolling contact fatigue life, is prevented.

As described above, the center segregation and the eutectic carbide are effectively suppressed, and as a result, the diffusion annealing time conventionally performed using the soaking furnace is greatly reduced.

In FIG. 1a, 1.00% C-0.45% Si-0.70% Mn-1.30%
For continuous casting of Cr steel, slabs obtained by continuous forging during continuous casting or slabs obtained by the conventional method without forging are each subjected to soaking at 1240 ° C for 2 hours. after subjected to a steel bar rolling, D / 4 parts: shows the results of examining the rolling fatigue life L ₀ of (D steel bar diameter) and the center (taken test piece so that the center of the steel bar comes to the surface).

FIG. 1b shows the result of examining the castability of the central portion.

As is apparent from the figure, the rolling fatigue life characteristics of the steel bar center member are improved by 5 times or more by performing forging with a draft of 5% or more, compared to the conventional method without such forging. Cracking is completely prevented.

Here, in order to apply the bar steel center member or wire as a material for the bearing steel ball, the rolling fatigue life of the center member may be equal to or greater than that of the D / 4 member.

Therefore, in the present invention, the lower limit of the rolling reduction by forging is set to 5%.

However, if the rolling reduction is too large, there is a problem in that the material accuracy after rolling is reduced.
It is preferable to set it to the following level.

In addition, as forging working method, the inventors first disclosed in
The continuous forging method disclosed in Japanese Patent No. 82257 was used.

Next, FIG. 2, 1.0% C-0.45% Si-0.70% Mn-1.30% C
r The relationship between the reduction ratio and the size of eutectic carbide at the center of the slab (area per piece) when forging is performed at various reduction ratios in continuous casting of steel.

As is clear from the figure, the size of the eutectic carbide tends to decrease as the rolling reduction in the forging process increases, and by setting the rolling ratio to 5% or more, the size of the eutectic carbide is reduced by 1 / in the case where the forging process is not performed. It could be reduced to 5 or less.

FIG. 3 shows that the rolling reduction in the forging process is 0%, 2%.
The relationship between the soaking temperature and the holding time required for dissipating the eutectic carbide in the case of%, 5% and 10% is shown.

If the soaking temperature is 1200 ° C or more or the soaking time is 5 hours or more, a heating furnace directly connected to the rolling line cannot be used due to productivity and material issues. Required.

However, if forging is performed at a rolling reduction of 5% or more, the eutectic carbide can be dissipated under extremely high heating and holding time conditions of a heating temperature of 1100 ° C. or more and a holding time of about 2 hours or more.

(Examples) Various molten steels having the chemical components shown in Table 1 were processed by a converter to a continuous casting method under the forging conditions shown in Table 2 to obtain cast slabs.

Then, after soaking under the conditions shown in Table 2, hot rolling was performed to obtain a 65 mmφ steel bar, and after spheroidizing annealing, 12 mm
Specimens of φ × 22 mm were taken from the D / 4 part and the center (taken so that the center of the steel bar was on the surface of the specimen), quenched, tempered, and examined for rolling fatigue life characteristics. Also shown in Table 2.

The rolling fatigue life test was performed using a cylindrical rolling fatigue life tester under the conditions of a Hertz maximum contact stress of 600 kgf / mm ² and a repetitive stress number of 46,240 cpm. 1240 of steel material No.1
° C., D / 4 parts L ₁₀ of 2h diffusion annealing treatment material (cumulative damage established 10%
In this case, the number of times of stress loading until peeling) was set to 1 and the relative evaluation was performed.

In addition, after soaking under the conditions shown in Table 2, hot rolling was performed to obtain a steel bar of 65 mmφ, and a casting test was taken from the center (taken so that the center of the steel bar came to the surface of the test piece). The results are shown in Table 2.

In the forging test, the compression ratio was 50% with the end face completely restrained.
The cracking rate of the 20-hour diffusion annealing material of steel material No. 1 was also relatively evaluated assuming that it was 1.

As is evident from Table 2, when the component composition satisfies the appropriate range and the rolling reduction during forging is 5% or more, the rolling fatigue life characteristics at the center are D / 4 parts. Equal to or slightly better, but the rolling fatigue life characteristics of both the D / 4D part and the center part are improved.

In addition, the forging property was significantly improved by applying a forging process with a rolling reduction of 5% or more with a shorter soaking time than the conventional example, and sufficient performance was obtained.

(Effects of the Invention) Thus, according to the present invention, it is possible to achieve the miniaturization of nonmetallic inclusions and the reduction of eutectic carbide at the axial center of the cross section, which are problems in the conventional continuous cast slab, and Heat soaking and holding time can be greatly simplified, and a bearing material excellent in not only forgeability but also rolling fatigue life characteristics can be obtained.

In addition, there is no need to perform high-temperature and long-time diffusion annealing, which has been conventionally unavoidable due to center segregation and dissipation of eutectic carbide, and a dedicated soaking furnace is not required.

Further, since the entire cross section of the continuous cast slab can be applied as a bearing material, it is advantageous in terms of productivity and material yield.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 a is a graph showing the relationship between the rolling contact fatigue life L ₀ of D / 4 parts of rolling reduction and billet and the center of the forging process,
FIG. 2B is a graph showing the relationship between the reduction ratio in forging and the forging property and L ₀ at the center of the slab, and FIG. 2 is a graph showing the relationship between the reduction ratio in forging and the eutectic carbide at the center of the slab. FIG. 3 is a graph showing the relationship between the soaking temperature and the holding time required for dissipating the eutectic carbide as a rolling reduction parameter in forging.

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁶ Identification symbol FI C22C 38/18 C22C 38/18 (56) References JP-A-3-226337 (JP, A) JP-A-2-92444 (JP) , A) JP-A-48-71318 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/00 B21J 5/00 C22C 38/00 301 C22C 38/18

Claims (3)

(57) [Claims] 1. A molten steel containing C: 0.06 to 1.50% by weight, Si: 0.15 to 2.00% by weight, Mn: 0.15 to 2.50% by weight and Cr: more than 1.00 to 3.00% by weight, with the balance being Fe and inevitable impurities. Is continuously cast, subjected to forging at a rolling reduction of 5% or more near the crater end where the inside of the slab completes solidification, and then subjected to hot rolling after soaking heat treatment. Manufacturing method of excellent bearing material. 2. The composition of molten steel comprises: C: 0.60-1.50 wt%, Si: 0.15-2.00 wt%, Mn: 0.15-2.50 wt%, and Cr: more than 1.00-3.00 wt%, and Mo: 0.05 1 selected from the following: 1.50 wt%, V: 0.05-0.50 wt%, Nb: 0.05-0.50 wt%, W: 0.05-0.50 wt%, Ni: 0.10-2.00 wt%, and Cu: 0.05-1.00 wt%. Contains two or more species, with the balance being
2. The method for producing a bearing material according to claim 1, wherein the composition of Fe and unavoidable impurities is obtained. 3. The treatment condition in the soaking treatment is 1100-1250.
The method for producing a bearing material according to claim 1, wherein the temperature is 2 ° C. for 2 to 10 hours.