JPH03271319A - Production of bearing steel having long service life and high strength - Google Patents

Abstract

PURPOSE:To produce a high strength bearing steel having superior rolling fatigue life characteristics by subjecting a molten high carbon steel with a specific composition to continuous casting, applying squeezing to the vicinity of the crater end of the resulting continuously cast billet, and then carrying out hot rolling. CONSTITUTION:A high carbon steel which has a composition containing, by weight, 0.60-1.50% C, 0.15-2.00% Si, 0.25-2.50% Mn, and 0.05-1.50% Mo or further containing one or >=2 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 is continuously cast at >=25 deg.C molten steel heating temp. Squeezing is applied at >=5% reduction of area to the vicinity of the crater end where the internal unsolidified molten steel of the resulting continuously cast billet is completely solidified, by which segregation in the central part is inhibited. By successively carrying out hot rolling, the high strength bearing steel stock suitable for a stock for rolling bearing and having superior rolling fatigue life characteristics can be produced.

Description

Detailed Description of the Invention (Field of Industrial Application) This invention is directed to the production of high-strength steel for bearings that has excellent rolling fatigue life characteristics and is suitable as a material for rolling bearings used in automobiles and other industrial machinery. It is about the method.

(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 after quenching, low-temperature tempering can provide the strength necessary for rolling bearings. Such a bearing is required to have a homogeneous rolling contact surface in order to extend the life of the bearing.

However, during continuous casting of steel materials, macro segregation (hereinafter referred to as center segregation) and eutectic carbides occur particularly in the axial center of the slab, increasing the occurrence of cracks during cutting and punching and deteriorating the rolling fatigue life characteristics. Therefore,
The core of the material was punched out as waste material, or the eutectic carbide was dissipated by an agglomeration method or a long diffusion process before use. As a result, productivity and material yield have been forced to decline.

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, resulting in C at the solidified tip. , Cr, and other concentrated dissolved m components are sucked in. Heat treatment during product processing results in large eutectic carbides or spheroidized carbides remaining, an increase in the amount of retained austenite, and these ξ
Non-uniformity of the black structure occurs, reducing the fatigue life due to transfer.

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 a reduction of several -11/II is applied by multiple pairs of rolls,
There was a problem in that solidification shrinkage and bulging occurring between roll pitches could not be sufficiently prevented, and center segregation would worsen if the rolling position was not appropriate.

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 than when a large reduction is performed using rolls such as in-line reduction, and negative segregation and even semi-macro segregation can be significantly reduced. 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, cracks will occur in the center of the slab. There is a disadvantage that strong negative segregation occurs in some areas, 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 optimum rolling conditions.

(Problems to be Solved by the Invention) The present invention advantageously solves the above-mentioned problems, and has high productivity and high strength with superior rolling fatigue life characteristics than conventional high carbon chromium bearing steels. The purpose of this paper is to propose an advantageous manufacturing method for copper for bearings.

(Means for Solving the Problems) That is, this invention has the following properties: C: 0.60-1.50%, Si: 0.15-2.00%, Mn: 0.25-2.50% and Mo: 0 ．．
After continuous casting of molten steel containing 05 to 1.50% Fe and unavoidable impurities at a molten steel heating degree of 25°C or higher, the reduction rate is reduced near the crater end where the molten steel inside the slab completes solidification. : A method for manufacturing a long-life, high-strength bearing steel (first invention), which comprises applying a forging process of 5% or more and then hot rolling.

Further, in the present invention, the composition of the molten steel is as follows: C: 0.60 to 1.50%, St: 0.15 to 2.00%, Mn: 0.25 to 2.50%, and Mo: 0.
Further, V: 0.05-0.50%, Nb: 0.05-0.50%, w: o, os-0.50%, Ni: 0.10-2 .00% and Cu: 0.
05 to 1.00%, and the remainder is Fe and unavoidable impurities of &11rfi (second invention). .

(Function) First, in this invention, the reason why the component composition 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 the hardness and transfer fatigue life characteristics after tempering by forming a solid solution in the matrix and strengthening martensite. However, if the amount is too large, eutectic carbides will grow, which will not only deteriorate the rolling fatigue life, but also require a long homogenization process to dissipate them, resulting in a decrease in productivity. Therefore, in consideration of the above points, the amount of C added was determined to be in the range of 0.60 to 1.50%.

Si: 0.15 to 2.00% Si is an element that not only acts as a deoxidizing agent during steel melting but also is effective in preventing surface defects in steel ingots.

It is also useful as an element that strengthens the hardened structure and suppresses the decrease in hardness due to tempering. However, if it is too much, machinability and forgeability deteriorate, so St.
shall be added in a range of 0.15 to 2.00%.

Mn: 0.25 to 2.50% Mn improves the hardenability of the steel, thereby increasing the strength and toughness of the matrix, and effectively contributing to improving the rolling fatigue life of the steel material. However, if it is too large, impact resistance and machinability will deteriorate, so Mn is 0.25 to 2.5.
It was assumed that it was added in a range of 0%.

Mo: 0.05 to 1.50%, Mo lowers the solid solution temperature of carbides and reduces C by quenching.
It is a useful element that increases the amount of solid solution. Furthermore, since it has strong solid solution strengthening properties, it effectively contributes to improving the strength and rolling fatigue life of the product (after quenching and tempering). However, if the amount is too large, machinability deteriorates and addition cost increases. Therefore, Mo was added in a range of 0.05% to 1.50%.

In this invention, in addition to the above-mentioned basic components, V
, Nb, W, Ni and Cu selected from
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, N
b and W each form stable carbides at high temperatures and improve transfer fatigue life characteristics.

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

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

Ni: 0.10 to 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 retained austenite will grow and reduce the hardness of the steel material after tempering. Therefore, Ni was added in a range of 0.10 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 an element useful for improving strength and rolling fatigue life. It is. However, when the content is too large, forgeability deteriorates.

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

In addition, AI, Ca, Na, K, Mg, and Zr were added for the purpose of reducing the amount of oxygen and controlling the morphology of inclusions.
For the purpose of improving machinability, one or more selected from S, Ca, Pb, B, Bi, and REM are used to further improve hot strength. For this purpose, one or two selected from P and N may be added, and a small amount of sb may be added for the purpose of reducing decarburization.

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 center 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, the occurrence of center segregation, eutectic carbides, etc. that cause deterioration of transfer fatigue life characteristics in the center can be effectively avoided.

In Figure 1, C: 1.00%, Si: 0.75%,
When continuously casting molten steel with a composition containing Mn: 0.45% and Mo: 1.00%, it is obtained by continuously performing forging pressure processing during continuous casting, or conventional method without forging processing. The slabs obtained by this method were each rolled into 65 mmφ steel bars, and the transfer fatigue life characteristics at the center portion (the specimens were taken so that the center of the steel bar was on the surface of the specimen) were investigated.The results are shown below.

As is clear from the figure, the rolling fatigue life characteristics of the central steel bar member are improved by more than 6 times when subjected to forging with a rolling reduction of 5% or more compared to conventional materials that are not subjected to such forging. .

Therefore, in this invention, the rolling reduction rate by forging process is 5.
% or more. However, if the rolling reduction ratio is too large, a problem arises in that the accuracy of the material after rolling decreases, so the rolling reduction ratio is preferably about 60% or less.

By the way, the inventors have conducted further research with the aim of further improving the rolling fatigue life characteristics, and have found that setting the molten steel heating degree ΔT during continuous casting to 25°C or higher is extremely effective in achieving the intended purpose. We obtained the knowledge that

Figure 2 shows the results of investigating the relationship between the heating degree ΔT of molten steel during continuous casting and the transfer fatigue life characteristics of the central member in each case where the reduction rate by forging was 0% (conventional method) and 10%. show. As the forging method, the continuous forging method previously disclosed by the inventors in JP-A-60-82257 was used.

As is clear from the figure, in the conventional method, the peak of the transfer fatigue life characteristic occurs when the molten steel heating degree ΔT is approximately 20°C;
When ΔT is lower than that, the floating and separation of non-metallic inclusions is insufficient, while when ΔT is higher than that, the transfer fatigue life tends to decrease due to the remaining of dense center segregation. .

In contrast, according to the present invention, continuous casting is performed under conditions where the molten steel heating degree ΔT is 25°C or higher, and the forging process is performed near the crater end where the molten steel inside the slab completes solidification, thereby further increasing the rolling fatigue life. extension has been achieved.

The reason for this is that center segregation of the product can be suppressed by performing forging near the crater end where the molten steel inside the slab has solidified. Can adopt casting,
As a result, it is thought that the floating and separation of inclusions is promoted and the transfer fatigue life is improved.

Since this effect is significant when the molten steel heating degree ΔT is 25° C. or higher, in the present invention, the molten steel heating degree ΔT during continuous casting is limited to a range of 25° C. or higher (preferably 85° C. or lower).

(Example) Slabs were manufactured from various molten steels having the chemical acid content shown in Table 1 by a converter->continuous casting method. At this time, steel material No.
No. 1 was subjected to forging processing, and all other steel materials were subjected to forging processing. Further, the target value of the molten steel heating degree ΔT during continuous casting was set to 30°C. Furthermore, the target values for the reduction ratio in the forging process were 2% and 20% for the prepared material Nα4.5, and 20% for other steel materials.

Next, steel material NO,i was heated to 1240℃ in a soaking furnace.
l2h and 20h, and 1 for other steels.
After soaking at 240″Cl2h, it was hot rolled into a 651φ steel bar.

After that, a spheroidizing annealing treatment was performed, and transfer fatigue life test pieces were taken from the 074 part and the center part (taken so that the center of the steel bar was on the surface of the test piece), and after quenching and tempering, a transfer fatigue life test was performed. did.

The transfer fatigue life test was conducted using a cylindrical transfer fatigue life testing machine under the conditions of Hertzian maximum contact stress of 600 kgf/mm and repeated stress number of 46,240 cpm, and the test results were summarized on probability paper assuming that they followed the Weibull distribution. , D/4 part LlO (number of stress loads until peeling when cumulative failure probability is 10%) of 20h diffusion annealed steel material-11 was set as 1, and relative evaluation was made.

The obtained results are also listed in Table 2.

The transfer fatigue life characteristics at the center of the 2-hour diffusion annealed material of steel material No. 1 (comparative example) are the same as those of the same 20-hour-treated material (conventional example).
It is extremely inferior at 173 times compared to .

Further, in steel materials No. 4, 5, if the rolling reduction during forging is less than 5%, the rolling fatigue life characteristics in the center part will not be superior to that in the D/4 part.

Further, steel material No. 2, in which C deviates from the proper composition range of this invention, and steel material No. 2, in which ■ deviates,
11, and steel material No. 15, in which Ni deviates, even when subjected to forging with a reduction rate of 5% or more, both the center and D/4 portions were 20h treated material of steel material No. 1 (conventional). The transfer fatigue life characteristics were inferior to Example).

On the other hand, although the composition range, the degree of heating of molten steel, and the reduction rate during forging satisfy the appropriate ranges of this invention, the transfer fatigue life characteristics of steel material No.

Compared to the 20h diffusion annealed material of No. 1 (conventional material), it is 2.1 to 3.9 times better in the D/4 part and 2.4 to 4.1 times better in the center, and both steel materials The transfer fatigue life characteristics of the D/4 section were superior to those of the D/4 section.

(Effects of the Invention) Thus, according to the present invention, it is possible to reduce not only the non-metallic inclusions in the axial center of the cross section, which were a problem in conventional continuously cast slabs, but also the center segregation. As a result, it is possible to obtain steel for bearings with excellent transfer fatigue life characteristics.

In addition, in this invention, the same rolling fatigue life characteristics as the D/4 part can be obtained for the center part, so the part corresponding to the center part of the slab can be used as a rolling wheel, etc., without having to be discarded as waste material, as in the past. Material yield is significantly improved compared to conventional methods.

[Brief explanation of drawings]

Figure 1 shows the rolling reduction rate during forging and the rolling fatigue life of the center of the slab. The graph shown in Figure 2 shows the relationship between
It is a graph showing the relationship between the degree of heating ΔT of molten steel during continuous casting and the rolling fatigue life characteristics of the central member in the cases where the reduction rate by forging is 0% (conventional method) and 10%.

Claims (1)

[Claims] 1. C: 0.60 to 1.50 wt%, Si: 0.15 to 2.00 wt%, Mn: 0.25 to 2.50 wt%, and Mo: 0.05 to 1.50 wt%. %, with the remainder consisting of Fe and unavoidable impurities. After continuous casting at a heating temperature of 25°C or higher, the molten steel is cast at a rolling reduction rate of 5% or higher near the crater end where the molten steel inside the slab completes solidification. A method for producing long-life, high-strength bearing steel, which is characterized by forging and then 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: 0.25 to 2.50 wt%, and Mo: 0.05 to 1.50 wt%. %, and further includes 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. 2. The method for producing a long-life, high-strength bearing steel according to claim 1, wherein the steel contains one or more selected from: 0.05 to 1.00 wt%, and the remainder is Fe and unavoidable impurities. .