Patent Application
20120321503
The present invention relates to an ingot steel for bearings (an ingot steel for use as a bearing steel) having excellent rolling contact fatigue life characteristics and suitable as a bearing material for use in automobiles, wind power, transport machines, electrical machines, precision machines, and other general industrial machinery. The present invention also relates to a method for manufacturing a bearing steel using the ingot steel for bearings.
High-carbon chromium steel (JIS G4805 standard SUJ2) has been widely used as a bearing steel of this type. In general, one of the important properties of bearing steels is their excellent rolling contact fatigue life characteristics. It is generally believed that the rolling contact fatigue life of steel is shortened by the presence of a non-metallic inclusion or eutectic carbide in the steel.
It has been believed in recent studies that non-metallic inclusions in steel are mainly responsible for the decrease in rolling contact fatigue life. Thus, bearing life has been improved by reducing the oxygen content of steel to control the number and size of non-metallic inclusions in the steel.
For example, PTLs 1 and 2 propose techniques for controlling the composition, shape, or distribution of an oxide-based non-metallic inclusion in steel. However, the manufacture of a bearing steel containing a decreased number of non-metallic inclusions requires expensive converter steelmaking machines or extensive modifications of conventional facilities, which entails an immense economic burden.
PTL 3 discloses a technique for improving the rolling contact fatigue life characteristics by controlling the centerline segregation rate of carbon and the oxygen and sulfur contents of steel. However, as described above, a further reduction in oxygen content to manufacture a bearing steel containing a decreased number of non-metallic inclusions requires expensive converter steelmaking machines or extensive modifications of conventional facilities, which entails an immense economic burden.
Thus, not only the decrease in the number of non-metallic inclusions in steel but also the decrease in the eutectic carbide content of steel have received attention. For example, although a high-carbon chromium steel, which contains 0.95% by mass or more C, is very hard and has high wear resistance, the central portion of a casting steel has a high degree of segregation (hereinafter referred to as centerline segregation), and furthermore enormous eutectic carbide is formed in the casting steel, which shortens the rolling contact fatigue life. Thus, the central portion of the casting steel is removed as a waste or is subjected to diffusion treatment (hereinafter referred to as soaking) for a long period of time to sufficiently eliminate the centerline segregation and the eutectic carbide.
Regarding such a segregation problem, PTL 4 discloses a method in which steel is controlled to have specific composition such as C: 0.6% to 1.2% by mass, and the total cross-sectional area of carbide having a thickness of 2 μm or more and, with respect to the center line of a longitudinal cross-section through the shaft center of a wire or rod-shaped rolled steels, existing in a central region within ⅛×D (D: the width of the longitudinal section) from the axis, including the axis of the longitudinal section, is 0.3% or less of the area of the longitudinal section. PTL 4 also quantitatively shows the influence of the amount of enormous carbide on the rolling contact fatigue life characteristics, indicating the presence of enormous eutectic carbide in steel that shortens the rolling contact fatigue life.
PTL 5 discloses a bearing steel that has a specific composition, such as C: 0.50% to 1.50% by mass and Sb: 0.0010% to 0.0150% by mass, a decreased amount of decarburized layer, and high thermal process productivity. It is an object of PTL 5 to improve thermal process productivity by the addition of Sb to decrease the formation of a decarburized layer in steel and thereby eliminate the cutting or grinding process after the thermal process. However, Sb may be highly toxic to human bodies and should therefore be treated carefully. Furthermore, the addition of Sb results in the condensation of Sb in the central segregation zone, worsening the centerline segregation. A portion containing condensed Sb can be locally hardened to have a different hardness from the base material. The different hardness may induce rolling contact fatigue fracture, shortening the rolling contact fatigue life.
In order to eliminate centerline segregation in the casting of a high-carbon chromium bearing steel and enormous eutectic carbide formed in the central segregation zone, for example, PTL 6 discloses a method for rolling a casting material into a billet and soaking the billet.
However, because of nonuniform steel temperature in soaking, a soaking temperature exceeding the solidus line in a portion could initiate melting in the portion again and induce a eutectic reaction, forming further enormous eutectic carbide.
Thus, in some applications of bearings, low-carbon alloy steel may be used in place of high-carbon chromium steel. For example, case-hardened steel is most commonly used after high-carbon chromium steel. However, case-hardened steel contains 0.23% by mass or less C and moderate amounts of Mn, Cr, Mo, and Ni or the like so as to achieve necessary quench hardenability and mechanical strength. In order to improve fatigue strength, the surface of case-hardened steel is hardened by carburization or carbonitriding.
For example, PTL 7 discloses a case-hardened steel that can be carburized in a short period of time, wherein the case-hardened steel has a specific chemical composition, such as C: 0.10% to 0.35%, and the activation energy of carbon diffusion in the steel defined by Q=34140-605[% Si]+183[% Mn]+136[% Cr]+122[% Mo] is 34000 kcal or less.
Likewise, PTL 8 discloses a technique regarding a carburized material that has excellent rolling contact fatigue characteristics, wherein the carburized material has a specific chemical composition, such as C: 0.1% to 0.45%, the austenite grain size number of a carburized layer is 7 or more, the carbon content of the surface ranges from 0.9% to 1.5%, and the retained austenite content of the surface ranges from 25% to 40%.
Although the carburization or carbonitriding can improve the rolling contact fatigue life characteristics, it may increase the manufacturing costs or decrease the yield because of a large strain or dimension change, thus increasing the product cost.
Some applications of bearing steels require a large section. This requires extensive modifications of carburization or carbonitriding facilities, which entail an immense economic burden.
With a year-by-year increase in the scale of wind power, transport machines, and general industrial machinery, there has been an urgent need to further increase the section of bearing steels. Methods for manufacturing steel ingots are broadly divided into ingot casting and continuous casting. Bearing steels having a small section to a large section, which are hithero manufactured by continuous casting steel, can be provided by manufacturing ingot casting steel in order to answer the upsizing tendency. However, steels manufactured by the ingot casting (hereinafter referred to as ingot steels) have a particular problem that enormous eutectic carbide is formed in a segregation zone, such as a V-segregation zone or an inverse V-segregation zone. This is because ingot steels have a higher degree of segregation and consequently a higher frequency of enormous eutectic carbide than continuously casting steels. Thus, it is important to decrease the formation of eutectic carbide.
Accordingly, it is an object of the present invention to provide a method for decreasing the formation of eutectic carbide in the segregation zone in an ingot steel for bearings manufactured by ingot casting.
As a result of extensive studies to solve the problems described above, the present inventors have found that the amounts of C, Si, Mn, Cr, and Al added to a conventional bearing steel are limited to a specific range and that a eutectic carbide formation index is newly introduced and is limited to a specific range. Thus, the present inventors found that these limitations can avoid the formation of enormous eutectic carbide in the V-segregation zone or the inverse V-segregation zone, which are problematic in ingot steels, and thereby an ingot steel for bearings having excellent rolling contact life characteristics can be provided.
More specifically, the present inventors manufactured an ingot steel for bearings in which the amounts of C, Si, Mn, Cr, and Al are altered and the eutectic carbide formation index Ec having the formula (I) described below is altered. As a result of extensive studies on the structure and the rolling contact fatigue life characteristics of the bearing steel, the present inventors arrived at the present invention by finding that a steel, even made of an ingot steel, having a composition and Ec in specific ranges can be free of eutectic carbide in the steel and have improved rolling contact fatigue life characteristics.
The aspects of the present invention are as follows:
1. An ingot steel for bearings, the ingot steel having a composition containing
C: 0.56% by mass or more and 0.70% by mass or less,
Si: 0.15% by mass or more and less than 0.50% by mass,
Mn: 0.60% by mass or more and 1.50% by mass or less,
Cr: 0.50% by mass or more and 1.10% by mass or less,
P: 0.025% by mass or less,
S: 0.025% by mass or less,
Al: 0.005% by mass or more and 0.500% by mass or less,
O: 0.0015% by mass or less,
N: 0.0030% by mass or more and 0.015% by mass or less, and
a remainder of Fe and incidental impurities,
wherein eutectic carbide formation index Ec defined by the formula (I) satisfies
0<Ec≦0.25,
wherein
Ec=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C] (1)
wherein [ ] indicates the amount of component described in parentheses (% by mass).
2. The ingot steel for bearings according to 1, wherein the composition further contains one or more selected from
Cu: 0.005% by mass or more and 0.5% by mass or less,
Ni: 0.005% by mass or more and 1.00% by mass or less, and
Mo: 0.01% by mass or more and 0.5% by mass or less.
3. The ingot steel for bearings according to 1 or 2, wherein the composition further contains one or more selected from
W: 0.001% by mass or more and 0.5% by mass or less,
Nb: 0.001% by mass or more and 0.1% by mass or less,
Ti: 0.001% by mass or more and 0.1% by mass or less,
Zr: 0.001% by mass or more and 0.1% by mass or less, and
V: 0.002% by mass or more and 0.5% by mass or less.
4. The ingot steel for bearings according to any one of 1 to 3, wherein the composition further contains
B: 0.0002% by mass or more and 0.005% by mass or less.
Summarizing these aspects 1 to 4, an ingot steel for bearings according to the present invention has a composition containing
C: 0.56% by mass or more and 0.70% by mass or less, Si: 0.15% by mass or more and less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500% by mass or less, O: 0.0015% by mass or less, and N: 0.0030% by mass or more and 0.015% by mass or less,
and optionally at least one of (A) to (C):
(A) one or more selected from Cu: 0.005% by mass or more and 0.5% by mass or less, Ni: 0.005% by mass or more and 1.00% by mass or less, and Mo: 0.01% by mass or more and 0.5% by mass or less,
(B) one or more selected from W: 0.001% by mass or more and 0.5% by mass or less, Nb: 0.001% by mass or more and 0.1% by mass or less, Ti: 0.001% by mass or more and 0.1% by mass or less, Zr: 0.001% by mass or more and 0.1% by mass or less, and V: 0.002% by mass or more and 0.5% by mass or less, and
(C) B: 0.0002% by mass or more and 0.005% by mass or less,
and a remainder of Fe and incidental impurities,
wherein eutectic carbide formation index Ec defined by the formula (I) satisfies 0<Ec≦0.25.
5. A method for manufacturing a bearing steel, including:
casting a steel ingot into an ingot steel by ingot casting; heating the ingot steel to a temperature in the range of 1000° C. to 1350° C.; and then hot forging the ingot steel at a forging ratio of 2.0% or more to manufacture a steel bar having a square section having a short side length of 400 mm or more or a circular section having a diameter of 400 mm or more,
wherein the steel ingot has a composition containing
C: 0.56% by mass or more and 0.70% by mass or less,
Si: 0.15% by mass or more and less than 0.50% by mass,
Mn: 0.60% by mass or more and 1.50% by mass or less,
Cr: 0.50% by mass or more and 1.10% by mass or less,
P: 0.025% by mass or less,
S: 0.025% by mass or less,
Al: 0.005% by mass or more and 0.500% by mass or less,
O: 0.0015% by mass or less,
N: 0.0030% by mass or more and 0.015% by mass or less, and
a remainder of Fe and incidental impurities,
wherein eutectic carbide formation index Ec defined by the formula (I) satisfies
0<Ec≦0.25,
wherein
Ec=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C] (1)
wherein [ ] indicates the amount of component described in parentheses (% by mass).
6. The method for manufacturing a bearing steel according to 5, wherein the composition further contains one or more selected from
Cu: 0.005% by mass or more and 0.5% by mass or less,
Ni: 0.005% by mass or more and 1.00% by mass or less, and
Mo: 0.01% by mass or more and 0.5% by mass or less.
7. The method for manufacturing a bearing steel according to 5 or 6, wherein the composition further contains one or more selected from
W: 0.001% by mass or more and 0.5% by mass or less,
Nb: 0.001% by mass or more and 0.1% by mass or less,
Ti: 0.001% by mass or more and 0.1% by mass or less,
Zr: 0.001% by mass or more and 0.1% by mass or less, and
V: 0.002% by mass or more and 0.5% by mass or less.
8. The method for manufacturing a bearing steel according to any one of 5 to 7, wherein the composition further contains
B: 0.0002% by mass or more and 0.005% by mass or less.
Summarizing these aspects 5 to 8, a method for manufacturing a bearing steel according to the present invention includes:
casting a steel ingot into an ingot steel by ingot casting; heating the ingot steel to a temperature in the range of 1000° C. to 1350° C.; and then hot forging the ingot steel at a forging ratio of 2.0% or more to manufacture a steel bar having a square section having a short side length of 400 mm or more or a circular section having a diameter of 400 mm or more, wherein the steel ingot has a composition containing
C: 0.56% by mass or more and 0.70% by mass or less, Si: 0015% by mass or more and
less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500% by mass or less, O: 0.0015% by mass or less, and N: 0.0030% by mass or more and 0.015% by mass or less,
and optionally at least one of (A) to (C):
(A) one or more selected from Cu: 0.005% by mass or more and 0.5% by mass or less, Ni: 0.005% by mass or more and 1.00% by mass or less, and Mo: 0.01% by mass or more and 0.5% by mass or less,
(B) one or more selected from W: 0.001% by mass or more and 0.5% by mass or less, Nb: 0.001% by mass or more and 0.1% by mass or less, Ti: 0.001% by mass or more and 0.1% by mass or less, Zr: 0.001% by mass or more and 0.1% by mass or less, and V: 0.002% by mass or more and 0.5% by mass or less, and
(C) B: 0.0002% by mass or more and 0.005% by mass or less,
and a remainder of Fe and incidental impurities,
wherein eutectic carbide formation index Ec defined by the formula (I) satisfies 0<Ec≦0.25.
In accordance with the present invention, an ingot steel for bearings having much better rolling contact fatigue life characteristics than conventional bearing steels can be stably manufactured. Thus, bearing steels having a small section to those having a large section can be manufactured. The present invention also contributes to the upsizing of wind power generators, transport machines, and general industrial machinery, providing industrially advantageous effects.
An ingot steel for bearings according to the present invention will be described in detail below.
First, reasons for limiting the percentage of each component of the composition of an ingot steel for bearings according to the present invention will be described below.
C: 0.56% by Mass or More and 0.70% by Mass or Less
C can increase the strength of steel and effectively improve the rolling contact fatigue life characteristics of steel. The C content in the present invention is 0.56% by mass or more. On the other hand, the C content of more than 0.70% by mass results in the formation of enormous eutectic carbide during the casting of the material, shortening the rolling contact fatigue life. Thus, the C content is 0.56% by mass or more and 0.70% by mass or less.
Si: 0.15% by Mass or More and Less than 0.50% by Mass
Si can act as a deoxidizing agent, increase the strength of steel owing to solid-solution hardening, and improve the rolling contact fatigue life characteristics of steel. Si is added to produce these effects. In order to produce these effects, 0.15% by mass or more Si is added in the present invention. However, the addition of 0.50% by mass or more Si results in deterioration in the machinability and the forgeability of steel. Si can be bound to oxygen in steel and remain as an oxide in the steel, causing deterioration in the rolling contact fatigue life characteristics. Furthermore, Si condensed in a segregation zone facilitates the formation of eutectic carbide. Thus, the upper limit of Si is less than 0.50% by mass.
Mn: 0.60% by Mass or More and 1.50% by Mass or Less
Mn can be added to improve quench hardenability, increase the toughness of steel, and improve the rolling contact fatigue life characteristics of steel. 0.60% by mass or more Mn is added in the present invention. However, the addition of more than 1.50% by mass Mn results in deterioration in machinability. Furthermore, Mn condensed in a segregation zone facilitates the formation of eutectic carbide. Thus, the upper limit of Mn is 1.50% by mass.
Cr: 0.50% by Mass or More and 1.10% by Mass or Less
In the same manner as in Mn, Cr can be added to increase the toughness of steel and improve the rolling contact fatigue life characteristics of steel. 0.50% by mass or more Cr is added in the present invention. However, the addition of more than 1.10% by mass Cr results in deterioration in machinability. Thus, the upper limit of Cr is 1.10% by mass.
P: 0.025% by Mass or Less
P is a detrimental element that can decrease the base material toughness or the rolling contact fatigue life of steel and is preferably decreased as much as possible. In particular, the P content of more than 0.025% by mass results in a significant decrease in base material toughness and rolling contact fatigue life. Thus, the P content is 0.025% by mass or less, preferably 0.020% by mass or less. It is industrially difficult to achieve the P content of 0%. Thus, the P content is often 0.002% by mass or more.
S: 0.025% by Mass or Less
S is contained in steel as a non-metallic inclusion MnS. Since bearing steels contain a decreased amount of oxide, which can often induce rolling contact fatigue, a large amount of MnS in steel can shorten the rolling contact fatigue life. Thus, S is preferably decreased as much as possible. The S content in the present invention is 0.025% by mass or less, preferably 0.020% by mass or less. It is industrially difficult to achieve the S content of 0%. Thus, the S content is often 0.0001% by mass or more.
Al: 0.005% by Mass or More and 0.500% by Mass or Less
Al can act as a deoxidizing agent, form a nitride and decrease the size of austenite grains, and improve the toughness and the rolling contact fatigue life characteristics. Al is added to produce these effects. In order to produce these effects, 0.005% by mass or more Al is added in the present invention. However, the addition of more than 0.500% by mass Al results in the formation of a coarse oxide type inclusion in steel, causing deterioration in the rolling contact fatigue life characteristics of steel. Furthermore, Al condenced in a segregation zone facilitates the formation of eutectic carbide. Thus, the upper limit of the Al content is 0.500% by mass, preferably 0.450% by mass or less.
O: 0.0015% by Mass or Less
O can be bound to Si or Al to form a hard oxide-based non-metallic inclusion, shortening the rolling contact fatigue life. Thus, O is preferably decreased as much as possible and is 0.0015% by mass or less. It is industrially difficult to achieve the O content of 0%. Thus, the O content is often 0.0003% by mass or more.
N: 0.0030% by Mass or More and 0.015% by Mass or Less
N can be bound to Al to form a nitride-based non-metallic inclusion, decrease the size of austenite grains, and improve the toughness and the rolling contact fatigue life characteristics. Thus, 0.003% by mass or more N is added. However, the addition of more than 0.015% by mass N results in the formation of a large number of nitride-based inclusions in steel, causing deterioration in the rolling contact fatigue life characteristics. This also results in the presence of a large amount of N that does not form a nitride in steel (free N), thus decreasing the toughness of steel. Thus, the upper limit of the N content is 0.015% by mass, preferably 0.010% by mass or less.
Eutectic Carbide Formation Index Ec: 0<Ec≦0.25
The present inventors smelted steels having various compositions in a vacuum melting furnace. The resulting steel ingot was examined for the presence of eutectic carbide. Regression calculation on the result was performed with various selected sets of parameters (main influential elements). As a result, it was found that the eutectic carbide index Ec defined by the formula (1) must satisfy 0<Ec≦0.25 as a steel composition with which the formation of eutectic carbide can be reduced.
Ec=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C] (1)
wherein [ ] indicates the amount of component described in parentheses (% by mass).
The present inventors manufactured bearing steels having the compositions and Ec's listed in Table 1 and examined their rolling contact fatigue life characteristics. The rolling contact fatigue life characteristics were performed by the test method described below in the examples.
In order to examine the presence of eutectic carbide and the influence of the composition and Ec on the rolling contact fatigue life characteristics, bearing steels were manufactured under fixed conditions. More specifically, after smelting in a converter, an ingot steel (ingot) having a 1350 mm×1250 mm section (top side) and a 1280×830 mm section (bottom side) was formed by ingot casting. The ingot steel was forged to have a 550 mm square section. The forging ratio was 5.6 at the top and 3.5 at the bottom. Test specimens for observing formed eutectic carbide illustrated in
0.55
0.58
Table 2 shows the evaluation results of the rolling contact fatigue life.
The reason for limiting Ec to produce a steel free of eutectic carbide is that, as described above, the formation of eutectic carbide in steel can cause rolling contact fatigue originating from the eutectic carbide, which causes deterioration in the rolling contact fatigue life characteristics.
In accordance with the present invention, the formation of eutectic carbide can be decreased even with an ingot steel manufactured by ingot casting. In particular, the rolling contact fatigue life is significantly improved. Thus, the present invention is limited to ingot steels. Furthermore, steels manufactured by ingot casting can be used to provide bearing products having a small section to those having a large section.
In addition to these base components, the following components can appropriately be added,
(A) One or More Selected from Cu: 0.005 to 0.5% by Mass, Ni: 0.005 to 1.00% by Mass, and Mo: 0.01 to 0.5% by Mass
Cu, Ni, and Mo can improve the quench hardenability, the strength after tempering, and the rolling contact fatigue life characteristics of steel and can be selectively added in accordance with the strength required (more specifically, any one of Cu, Ni, Mo, Cu+Ni, Cu+Mo, Ni+Mo, and Cu+Ni+Mo can be selectively added). In order to produce such effects, the amounts to be added are preferably 0.005% by mass or more for Cu and Ni and 0.01% by mass or more for Mo. However, the addition of more than 0.5% by mass Cu or Mo or more than 1.00% by mass Ni results in deterioration in the machinability of steel. Thus, Cu, Ni, and Mo are preferably added in amounts equal to or below these upper limits.
Likewise, in order to increase the strength or improve the rolling contact fatigue life characteristics of steel, in addition to the components described above, the following components may be added to a bearing steel according to the present invention.
(B) One or more of W: 0.001 to 0.5% by mass, Nb: 0.001 to 0.1% by mass, Ti: 0.001 to 0.1% by mass, Zr: 0.001 to 0.1% by mass, and V: 0.002 to 0.5% by mass
W, Nb, Ti, Zr, and V can improve the quench hardenability, the strength after tempering, and the rolling contact fatigue life characteristics of steel and can be selectively added in accordance with the strength required (more specifically, any one of W, Nb, Ti, Zr, V, W+Nb, W+Ti, W+Zr, W+V, Nb+Ti, Nb+Zr, Nb+V, Ti+Zr, Ti+V, Zr+V, W+Nb+Ti, W+Nb+Zr, W+Nb+V, W+Ti+Zr, W+Ti+V, W+Zr+V, Nb+Ti+Zr, Nb+Ti+V, Nb+Zr+V, Ti+Zr+V, W+Nb+Ti+Zr, W+Nb+Ti+V, W+Nb+Zr+V, W+Ti+Zr+V, Nb+Ti+Zr+V, and W+Nb+Ti+Zr+V can be selectively added). In order to produce such effects, the amounts to be added are preferably 0.001% by mass or more for W, Nb, Ti, and Zr and 0.002% by mass or more for V. However, the addition of more than 0.5% by mass W or V or more than 0.1% by mass of Nb, Ti, or Zr results in deterioration in the machinability of steel. Thus, these elements are preferably added in amounts equal to or below these upper limits.
(C) B: 0.0002 to 0.005% by Mass
B can improve quench hardenability and thereby increase the strength of steel after tempering and improve the rolling contact fatigue life characteristics of steel. B can be added to steel on an as-needed basis. In order to produce these effects, 0.0002% by mass or more of B is preferably added. However, the addition of more than 0.005% by mass B results in deterioration in workability. Thus, 0.0002% to 0.005% by mass B is preferably added.
When an element other than the base components is added, any combination of elements of (A), (B), and (C) groups is available. More specifically, an element selected from any one element group may be added, elements selected from each of two element groups may be added, or elements selected from each of all the element groups may be added.
In an ingot steel for bearings according to the present invention, the components other than the components described above are Fe and incidental impurities. Examples of the incidental impurities include, but are not limited to, Sn, Sb, As, and Ca.
An ingot steel for bearings having the composition described above is smelted in a vacuum melting furnace or a converter and further by a known refining method, such as a degassing process, and is then formed into a cast billet by ingot casting. In accordance with the present invention, when a cast billet is formed by ingot casting by which eutectic carbide is particularly easily precipitated, ingot steels can be manufactured without the formation of eutectic carbide. Thus, large materials that are difficult to manufacture particularly by continuous casting can be manufactured.
A method for manufacturing a bearing steel according to the present invention will be described below.
An ingot steel for bearings is subjected to solid forging (a forging process of elongating a material by radially pressing the material) by hot forging to form a steel bar having a square section (hereinafter referred to as a square steel bar) or a steel bar having a circular section (hereinafter referred to as a round steel bar). The steel bar is then forged by upsetting to form a component of a bearing, such as an outer ring or an inner ring. The reason for the temporary solid forging of an ingot steel for bearings into a steel bar having a square section or a circular section is that a cast billet manufactured by ingot casting tends to have an internal defect, such as a cavity defect (a group of coarse porosities), which can be decreased by hot forging.
The forging for forming a round steel bar or a square steel bar is performed while an ingot steel is heated at a temperature in the range of 1000° C. to 1350° C. A heating temperature of less than 1000° C. results in an increase in load during the hot forging of a cast billet manufactured by ingot casting, making the forging difficult. This necessitates additional heating and hot forging, thus decreasing productivity and increasing the manufacturing costs. On the other hand, a heating temperature of more than 1350° C. results in partial melting within the cast billet, thus causing a crack in the round steel bar or the square steel bar. Thus, the heating temperature is 1000° C. or more and 1350° C. or less.
The forging must be accompanied with drawing at a forging ratio of 2.0 or more. As described above, a cast billet manufactured by ingot casting tends to have an internal defect, such as a cavity defect (a group of coarse porosities), which can be decreased by hot forging. In order to prevent the internal defect, the forging ratio is preferably 2.0 or more. A forging ratio of less than 2.0 results in the presence of a defect, such as a coarse cavity defect, causing an internal crack during the processing for forming the bearing steel. Thus, the solid forging is preferably performed at a forging ratio of 2.0 or more, more preferably 2.5 or more. The upper limit is not particularly specified but is realistically 8.0 or less.
The forging ratio refers to the forging ratio in solid forging described in JIS G 0701. More specifically, the forging ratio refers to the ratio A/a of the cross-sectional area A before forging to the cross-sectional area a after the forging.
A large scale wind power, transport machines, other general industrial machinery require large steels. For example, some bearing outer rings of wind power generators have an outer diameter of more than 3 m. Such a large bearing necessitates the processing of a large steel material. With respect to the dimensions of a steel bar having a circular section or a square section in the present invention, the square steel bar preferably has a short side length of 400 mm or more, and the round steel bar preferably has a diameter of 400 mm or more.
If the short side length of a cross section of an ingot steel perpendicular to the drawing direction is smaller than the long side length of a cross section of a square steel bar perpendicular to the drawing direction in the solid forging of the square steel bar or smaller than the diameter of a round steel bar in the drawing of the round steel bar, a cast billet can be buckled during the forging, thus decreasing the forging efficiency and increasing the manufacturing costs. Thus, the short side length of a cross section of an ingot steel perpendicular to the drawing direction is preferably larger than the short side length (in the case of a square steel bar) or the diameter (in the case of a round steel bar) of the steel after drawing.
A steel having a composition listed in Table 3 was smelted by converter refining and a degassing process and was then formed into an ingot having a size listed in Table 4 by ingot casting. The ingot was heated to a temperature in the range of 1000° C. to 1350° C. in a furnace and was then forged to have a section size listed in Table 4. The presence of eutectic carbide and the rolling contact fatigue life characteristics of the forged product were examined as described below.
[Presence of Eutectic Carbide]
The presence of eutectic carbide was examined by taking a sample for microstructure observation from a (T1/2, T2/2) portion (central portion) and a (T1/2, T2/4) portion (T1=T2 denote a side length of a square-forged steel billet:
[Rolling Contact Fatigue Life Characteristics]
The rolling contact fatigue life characteristics are preferably determined in actual use after forging, cutting, quenching, and tempering. However, this requires long term. Thus, the rolling contact fatigue life characteristics were determined with a thrust type rolling contact fatigue machine as described below. A 60 mmφ×5.3 mm disk was cut from a (T1/2, T2/4) portion (T1=T2 denote a side length of a square-forged steel billet:
The rolling contact fatigue life characteristics were determined as described below. The stress loading frequency when the test specimen underwent detachment was measured for 10 to 15 test specimens. The relationship between the cumulative probability and the stress loading frequency was organized using Weibull probability paper. After that, the cumulative probability 10% (hereinafter referred to as B10 life) was determined. The rolling contact fatigue life characteristics were judged to be improved when the B10 life was improved by 10% or more with respect to a reference steel (A-1: SUJ2 equivalent steel).
[Machinability]
The machinability is preferably determined in actual processing after forging, cutting, quenching, and tempering. However, this requires long term. Thus, the machinability was determined in a turning test (lathe turning test of the outer surface) as described below. A 60 minφ×270 mm round bar was cut from a (T1/2, T2/4) portion (T1=T2 denote a side length of a square-forged steel billet:
0.47
0.92
0.69
0.530
1.20
0.40
0.80
0.0020
0.0160
1.8
1.8
1.9
1.2
Table 5 shows the presence or absence of eutectic carbide, the rolling contact fatigue life characteristics, and the results of machinability test. It was shown that steels B-1 and B-2, B-4 and B-5, B-7, B-12 to B-18, and B-20 to B-28, which satisfy the composition and Ec according to the present invention, contained no eutectic carbide in the steel and had excellent rolling contact fatigue life characteristics. In contrast, steels B-3, B-6, and B-11, which have a composition within the scope of the present invention but Ec outside the scope of the present invention, contained eutectic carbide in the steel and had a shortened rolling contact fatigue life. Steels B-8 to B-10, B-19, and B-30 to B-33, which have a composition outside the scope of the present invention, had a shortened rolling contact fatigue life. B-23 to B-25 have a composition within the scope of the present invention and a forging ratio of less than 2.0. B-26 to B-28 have a composition within the scope of the present invention and a forging ratio of 2.0 or more. Comparison of B-23 to B-25 with B-26 to B-28 shows that a forging ratio of 2.0 or more further improves the rolling contact fatigue life characteristics. Steel B-29, which has Ec within the scope of the present invention but a Cr content outside the scope of the present invention, had insufficient machinability.
In accordance with the present invention, ingot steels for bearings having excellent rolling contact fatigue life characteristics can be manufactured at low cost, and industrially very valuable bearing steels can be provided.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-272906 | Nov 2009 | JP | national |
| 2010-242692 | Oct 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/071779 | 11/30/2010 | WO | 00 | 9/5/2012 |
