JP2008240019A - Steel excellent in rolling contact fatigue life - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel which is used for machine components excellent in rolling contact fatigue life and can be obtained without prolonged ladle refining or degasification for reducing the amount and size of oxide non-metallic inclusions at melting in steelmaking process. <P>SOLUTION: The steel for machine structure shows a surface hardness of ≥58 HRC when used as machine components and includes ≤20 mass ppm O and <0.010% mass% Al. The steel includes oxide non-metallic inclusions having the largest inclusion diameter in an observation area of 3,000 mm<SP>2</SP>or all oxide non-metallic inclusions having inclusion diameter of ≥15 μm which comprise ≥30 mass% of SiO<SB>2</SB>, when inclusion diameter is defined as (length×width)<SP>1/2</SP>. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  INDUSTRIAL APPLICABILITY The present invention is used for mechanical parts and devices that are required to have a rolling fatigue life such as bearings, gears, hub units, toroidal CVT devices, constant velocity joints, piston pins, etc., and whose surface hardness is hardened to 58 HRC or more. It is related to steel.

  In recent years, with the improvement in performance of various mechanical devices, the use environment of mechanical parts and devices that require a rolling fatigue life has become very severe, and there is a strong demand for improved life and improved reliability. In response to such demands, measures from the aspect of steel materials include optimization of steel components and reduction of impurity elements to improve life and reliability.

Among impurity elements, Al ₂ O _{3 and} other oxide-based non-metallic inclusions are known to be particularly harmful because they are the starting point of damage. Furthermore, it is known that the rolling fatigue life is shortened as the diameter of these oxide-based nonmetallic inclusions increases. Therefore, various methods for producing high cleanliness steel with a small amount of oxide-based nonmetallic inclusions, that is, high cleanliness of steel and a large amount of large oxide-based nonmetallic inclusions with an inclusion diameter of 20 μm or more have been proposed. ing.

  The general manufacturing process of this high cleanliness steel is as follows: 1) Oxidative refining of molten steel by arc melting furnace or converter, 2) Reductive refining by ladle refining furnace (LF), 3) Reflux vacuum degasser (RH) ) Reflux vacuum degassing treatment (RH treatment) by 4) Casting of steel ingot by continuous casting or general ingot casting and 5) Production of steel ingot by rolling of steel ingot, processing by pressure forging and heat treatment (for example, , See Patent Document 1).

  In this case, in order to reduce oxide-based non-metallic inclusions in the steel material, deoxidation alloy is introduced in the ladle refining furnace and reduction refining is performed by deoxidation, and further ladle refining and vacuum degassing are performed. The resulting oxide is agglomerated and coalesced for floating separation. In this case, the remaining oxide that cannot be removed becomes a starting point of damage in the final product.

  As a general deoxidizing material alloy to be charged in a ladle refining furnace, Si, Mn and the like are added in combination with Al, which is a strong deoxidizing material as a main component (for example, see Patent Document 2).

  Even if the steel material manufactured by such a conventional method is used, it is not fully possible to suppress damage with a short life. Therefore, in order to reduce the oxide-based nonmetallic inclusions in the steel material and further reduce the diameter, it is necessary to treat the ladle refining time and degassing time for a long time.

JP 2001-342512 A JP 2006-200027 A

  The problem to be solved by the present invention is a rolling fatigue life that can be produced without a long ladle refining or degassing treatment in order to reduce and reduce the diameter of oxide-based nonmetallic inclusions during steelmaking melting. It is to provide steel used for machine parts that are superior to each other.

  In order to improve the rolling fatigue life of bearings and other machine parts, it is important to reduce non-metallic inclusions of a certain size or more from the steel for these machine parts. Furthermore, if there are large oxide-based non-metallic inclusions under the rolling surface of bearings and other machine parts, they will cause separation and damage. It is known that reducing the appearance of oxide-based nonmetallic inclusions is particularly important for improving the life of bearings and other mechanical parts.

As a result of intensive studies by the present inventors, in addition to the above points, there is a cavity at the interface between the parent phase and the oxide-based nonmetallic inclusions or at the interface between the oxide-based nonmetallic inclusions and the oxide-based nonmetallic inclusions. It was first pioneered that the lifetime was deteriorated in the case where there was no cavity as compared to the case where there was no cavity. That is, even if the oxide-based non-metallic inclusions have the same size, if there are cavities, short-life peeling is likely to occur. If the bearing ball is a non-metallic inclusion that is in close contact with the parent phase that does not have a cavity while the ball of the bearing is rolling on the race, it is peeled only through the crack propagation process due to the shear stress under compressive stress. To. On the other hand, in the case of oxide-based nonmetallic inclusions that have cavities at the interface with the parent phase, an initial crack is first generated due to the main stress, and then crack growth due to shear stress under compressive stress occurs. Since this leads to separation, the defect at the initial stage of fatigue is more harmful than an oxide-based non-metallic inclusion in close contact with the parent phase having the same diameter. The oxide-based non-metallic inclusions contained in general steel are substantially composed of Al ₂ O ₃ , CaO, MgO, SiO ₂ , MnO, the composition of which is a molten steel composition, deoxidized alloy, or raw material. It is influenced by. Then, in the case of using the Al was subjected to deoxidation step by deoxidizer comprising mainly steel conventionally, such _{Al 2 O 3, CaO-Al} 2 O 3 and MgO-Al ₂ O ₃ in the steel Al ₂ O ₃ -based non-metallic inclusions are mainly used. These non-metallic inclusions have been found to have cavities between the steel matrix interface and the accumulated oxide-based non-metallic inclusions.

  These cavities are generated due to differences in shrinkage and deformability between the oxide-based non-metallic inclusions and the base metal itself during solidification in the steel material manufacturing process, cooling after hot working, and the like. Cavities between the accumulated oxide-based non-metallic inclusions are generated even when the steel-based non-metallic inclusions are cracked during hot working of the steel material or during rolling of the bearing.

As a result of examining the composition of oxide-based nonmetallic inclusions that do not generate cavities at the interface with the parent phase and that do not crack during hot working of this steel, it was found that SiO ₂ should be the basic composition. It was. The present invention has been made based on the above newly obtained knowledge.

That is, the means of the present invention for solving the above-mentioned problem is that, in the invention of claim 1, the steel has a surface hardness of 58 HRC or more when used for a machine part, and O is 20 ppm or less by mass ratio. Al is a structural structural steel satisfying less than 0.010%, and when the inclusion diameter is defined as (longitudinal x lateral) ^1/2 , it exists in the specular area 3,000 mm ² existing in the steel. The composition of oxide-based nonmetallic inclusions having the largest inclusion diameter or all oxide-based nonmetallic inclusions having an inclusion diameter of 15 μm or more is, by mass%, SiO ₂ : 30% or more. This steel is used for machine parts with excellent rolling fatigue life.

The invention according to claim 2 is characterized in that, among oxide-based non-metallic inclusions present in steel, the maximum inclusion diameter existing in a specular area of 3,000 mm ² is 70 μm or less. It is steel used for machine parts having excellent rolling fatigue life as described in 1.

  The steel of the present invention can be obtained without using a ladle refining or degassing treatment for a long time to reduce the oxide non-metallic inclusions and reduce the diameter when melting steel making by using the above means. It can be used with a surface hardness of 58 HRC or more, is excellent in rolling fatigue life, and has a very short steel life for machine parts.

  Prior to describing embodiments of the means of the present invention, the reasons for limiting the chemical composition of the steel of the present invention and the composition and size of oxide-based nonmetallic inclusions contained therein will be described below. In addition, ppm and% are shown by mass ratio.

O: 20 ppm or less, desirably 10 ppm or less O remains in the steel and forms oxide-based non-metallic inclusions as a starting point of separation during rolling of the bearing, so it is 20 ppm or less, but it is as low as possible Is preferable. Desirably, it is 10 ppm or less. In the present invention, it is necessary to avoid the generation of Al ₂ O ₃ -based nonmetallic inclusions having a size of 15 μm or more as much as possible. For that purpose, in the deoxidation step after oxidative refining by a converter or electric furnace, it is necessary to perform deoxidation regardless of the alloy shape such as Si and Mn without introducing Al having extremely strong deoxidizing power. . In that case, O becomes higher than deoxidation in which Al is added. However, if it is 20 ppm or less, the effect of the present invention can be obtained, and the maximum inclusion diameter existing in the speculum area of 3,000 mm ² can be stably satisfied with 70 μm or less.

Al: Less than 0.010%, desirably less than 0.008% The Al content in steel when the conventional deoxidation step with Al is added is about 0.015 to 0.025%. When this amount of Al is contained, the oxide easily contains a large amount of Al ₂ O ₃ . Therefore, it is desirable that the Al content is as small as possible, and it is desirable that it be less than 0.010%. Even if deoxidation is performed without adding Al in the deoxidation step, Al may be mixed from raw materials, slag, and furnace materials. Even if Al is mixed, if Al is not used in the deoxidation alloy, less than 0.008% can be easily achieved with Al, and as in the present invention, an oxide composed of SiO ₂ : 30% or more. The composition of the system nonmetallic inclusion can be easily obtained. On the other hand, when raw materials and furnace materials are carefully selected in order to prevent Al from being mixed, the manufacturing cost increases.

Reason why the composition of oxide-based nonmetallic inclusions having the maximum inclusion diameter or all oxide-based nonmetallic inclusions having an inclusion diameter of 15 μm or more is SiO ₂ : 30% or more Oxide-based nonmetallic inclusions If SiO ₂ is less than 30% of the composition, cavities are formed at the interface between the parent phase and the oxide-based non-metallic inclusions, and short-life peeling during rolling is likely to occur. In particular, as Al ₂ O ₃ and CaO increase, the shorter the life becomes, the more the separation becomes. Further, when containing Al in the steel, even at small content, for the standard free energy of Al ₂ O ₃ is lower than other oxides of Al ₂ O ₃ based oxide Generation is inevitable. However, if the Al content is within the range of the present invention, the produced Al ₂ O ₃ oxide is small. Moreover, if the magnitude | size is 20 micrometers or less, the harmfulness with respect to rolling fatigue is small as conventionally known.

The reason why the maximum inclusion diameter in the specular area 3,000 mm ² is 70 μm or less among the oxide-based non-metallic inclusions present in the steel is Al ₂ O ₃ , CaO—Al ₂ O _{3 in the} steel. If the steel is mainly composed of Al ₂ O ₃ -based non-metallic inclusions such as MgO-Al ₂ O _3, it is important to greatly reduce large oxide-based non-metallic inclusions of 20 μm or more, If the composition of the oxide-based nonmetallic inclusions having an inclusion diameter of 15 μm or more is SiO ₂ : 30% or more, voids are not generated at the interface between the parent phase and the oxide-based nonmetallic inclusions. Life peeling can be suppressed. However, if the maximum inclusion diameter is 70 μm or more, short-life peeling may occur.

  The implementation conditions and the obtained effects of the embodiment of the present invention will be specifically described. First, Table 1 shows component compositions of the test materials A to K according to the embodiment of the present invention. Steels to which the present invention can be applied are steels having a hardness of 58 HRC or more obtained by induction hardening, sub-quenching, and bainite quenching. In this test material, steel that satisfies the components of JIS G 4805, which is a steel frequently applied with the same quenching method, steel that has been improved (for example, high Cr, high Si), and components of JIS G 4051 are satisfied. Among the steels to be processed, the steel that satisfies C ≧ 0.42% and the steel that has been improved (for example, high Cr) were carried out. These test materials were produced by melting with a 100 kg vacuum melting furnace to produce an ingot. At the time of melting, test materials A to G and J to K do not add metal Al or Al alloy as a deoxidizer, and metal Si, Fe-Si, metal Mn, Fe-Mn (Al content is unavoidable in any case) Only less than 5%) was added as an impurity. The test materials H to I also added metal Al or Al alloy as a deoxidizing material. The obtained ingot was hot-worked to obtain a φ65 steel material. Steel materials other than the test material F were subjected to spheroidizing annealing at 800 ° C. and cut perpendicularly to the longitudinal direction of the steel material to obtain a test piece consisting of a disk having an outer diameter φ60 × inner diameter φ20 × thickness 5.8 mm. Produced. After holding this test piece at 835 ° C. for 20 minutes, it was quenched by oil cooling, and then tempered at 170 ° C. for 90 minutes to obtain a desired hardness of 58 HRC or higher. This was subjected to surface polishing and a thrust type rolling fatigue test was conducted. On the other hand, the specimen F was subjected to spheroidizing annealing at 750 ° C. and cut perpendicularly to the longitudinal direction of the steel material to obtain a test piece made of a disk having an outer diameter φ60 × inner diameter φ20 × thickness 5.8 mm. Produced. This test piece was induction-hardened and subjected to a tempering treatment at 150 ° C. for 60 minutes to obtain a desired hardness of 58 HRC or higher, and then surface polishing was performed to perform a thrust type rolling fatigue test. This thrust type rolling fatigue test was conducted at a maximum hertz stress Pmax: 5292 MPa.

Further, for the oxide-based non-metallic inclusion diameter and composition evaluation in each test material, hot working was performed in the same manner as described above to obtain a φ65 material, and spheroidizing annealing similar to that of the thrust type rolling fatigue test piece was performed. Later, 30 micro samples having a size of 10 × 10 mm were cut out from a surface parallel to the rolling direction from a surface portion of ¼ diameter D of φ65 material of each test material. These micro samples were subjected to the same quenching and tempering treatment as described above, and then the sample measurement surface was polished, and among the oxide-based non-metallic inclusions present in each 100 mm ² (vertical x horizontal) ¹ The size of oxide-based nonmetallic inclusions having a thickness of 15 μm or more at ^1/2 was measured, and the maximum inclusion diameter existing in 3000 mm ² was evaluated. Furthermore, the inclusion composition was quantified by an energy dispersive X-ray fluorescence analyzer.

  In Table 1, A to G of the test materials satisfy the constituent requirements of the present invention. In terms of mass ratio, the Al content is at most 0.009% of the test material A, and the O content is at most 20 ppm of the test material C. Among the test materials H to K, the test material H and the test material I have an Al content outside the scope of the present invention, and as shown in the remarks in Table 1, the test material J and the test material K are O content is outside the scope of the present invention due to insufficient deoxidation.

For each test piece made of each sample shown in these tables 1, the maximum inclusion diameter present in 3000 mm ^2, SiO ₂ is the main product of the oxide-based nonmetallic inclusions having the above 15 [mu] m, Al ₂ The average composition of O ₃ , CaO + MnO, and MgO, and the SiO ₂ content of the oxide-based non-metallic inclusions of the test piece having the smallest amount of SiO ₂ among the oxide-based non-metallic inclusions having 15 μm or more. Are shown in Table 2.

なお、ＭｇＯ欄の−は１％未満を示す。

In the MgO column,-indicates less than 1%.

In Table 2, the specimens A to G satisfy Claims 1 and 2 of the present invention. On the other hand, among the oxide-based nonmetallic inclusions having the above 15 [mu] m, for the most amount of SiO ₂ structure of SiO ₂ is less oxide-based nonmetallic inclusions, test materials A~G is in Table 1 The component range of the present invention is satisfied. Furthermore, the SiO ₂ content of the oxide-based non-metallic inclusions with the smallest amount of SiO ₂ is 20% of the specimen A at the lowest.

Further, among the 10 test pieces of each specimen subjected to the surface hardness after quenching and tempering and the thrust type rolling fatigue test in the above, the number of 10 pieces separated in less than 1 × 10 ⁷ cycles and L Table 3 shows the ₁₀ lifespan.

In Table 3, A to G of the test materials satisfying the configuration of the present invention had a surface hardness of 61.5 HRC or more, and the number of test pieces separated in less than 1 × 10 ⁷ cycles was 0 out of 10 pieces. and it is L ₁₀ life be of minimum test material 10.7 × 10 ⁶ cycles over G. On the other hand, the specimens H to K which are other than the present invention are the specimens separated in less than 1 × 10 ⁷ cycles, and 10 specimens are specimens H, specimen I and specimen K. 2 pieces, and in the specimen J, it is 1 piece out of 10 pieces. Further, the L ₁₀ life is the maximum, less than 9.5 × 10 ⁶ cycles, which is inferior to that of the present invention.

Claims (2)

A steel for machine structural use that has a surface hardness of 58 HRC or more when used in a machine part, and satisfies a mass ratio of O of 20 ppm or less and Al of less than 0.010%. (Length x width) When defined as ^1/2, it has an oxide-based nonmetallic inclusion having a maximum inclusion diameter existing in the speculum area of 3,000 mm ² in the steel or an inclusion diameter of 15 μm or more. Steel used for machine parts having excellent rolling fatigue life, characterized in that the composition of all oxide-based nonmetallic inclusions is SiO ₂ : 30% or more by mass. ^2. The rolling fatigue life according to claim 1, wherein, among the oxide-based nonmetallic inclusions present in the steel, the maximum inclusion diameter present in the specular area of 3,000 mm ² is 70 μm or less. Steel used for excellent machine parts.