JP5713529B2 - Steel material with excellent rolling fatigue life - Google Patents

Description

The present invention relates to a steel material having an excellent rolling fatigue life used for machine parts such as bearings.

Conventionally, machine parts that require excellent fatigue life and quietness, especially steel materials used as rolling bearing steel (bearing), are non-metallic materials such as CaO · Al ₂ O ₃ and Al ₂ O _3. It is important that the steel is highly clean with inclusions reduced as much as possible.
Such a high cleanliness steel (high cleanliness steel), after decarburizing the molten steel in the converter, for example, the chemical component of the molten steel in the secondary refining equipment such as LF equipment and RH equipment In general, it is manufactured by fine adjustment or reduction of non-metallic inclusions contained in molten steel and casting with a continuous casting apparatus.

In manufacturing high cleanliness steel, fine adjustment of chemical components and reduction of nonmetallic inclusions are important, but the shape and number of nonmetallic inclusions in the steel are also important. Thus, as a steel material which prescribed | regulated the shape and number of the nonmetallic inclusion in steel, there exist some which are shown to patent document 1 or patent document 2, for example.
In the steel for casting of Patent Document 1, the average circularity of inclusions having a maximum chord length of 1 μm or more contained in the steel is 0.25 or more, and the number of the maximum chord length of 20 μm or more is less than 40 per 100 mm ^2. The number of inclusions having an average circularity of 0.25 or more and a maximum chord length of 1 to 10 μm is 100 or more per 100 mm ² .

In the high-strength steel sheet of Patent Document 2, 50% or more of inclusions in steel are made of either or both of spherical and spindle-shaped inclusions.
JP 2006-336092 A JP 2007-291421 A

  In Patent Document 1, the average circularity of inclusions contained in steel and the number of inclusions are defined. This regulation is set from the viewpoint of improving hydrogen cracking resistance, and is a machine part. It was not made from the viewpoint of improving the fatigue life. In addition, in Patent Document 1, the size of inclusions to be specified is about 10 μm, and there is no description of inclusions that affect the fatigue life with very small ones that do not affect the fatigue life of machine parts. In fact, even if it is this steel material, it cannot be said that the fatigue life can be sufficiently satisfied.

Further, in Patent Document 2, the number of inclusions contained in the steel is specified, but as in Patent Document 1, the size of the specified inclusion is about 10 μm, which affects the fatigue life of machine parts. Is a very small thing that does not give.
Therefore, an object of the present invention is to provide a steel material in which the fatigue life is sufficiently ensured by limiting the inclusions that affect the fatigue life.

In order to achieve the above object, the present invention has taken the following measures.
That is, the present invention is mass%, C: 0.95 to 1.10%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.50%, Cr: 1.30 to 1. .60%, Al: 0.01 to 0.03%, P: 0.025% or less, S: 0.006% or less, O: 0.0012% or less, N: 0.0028 to 0.0062%, In a steel material including an inclusion mainly including CaO.Al ₂ O ₃ or Al ₂ O ₃ having a balance Fe and unavoidable impurities and having a major axis of 30 μm or more , the roundness is 1 with respect to the inclusion. The number of inclusions having a value of 0.0 to less than 1.4 is 35 / kg or less, and the number of inclusions having a roundness of 1.4 or more is 90 / kg or less. is there.
The inventor verified the method for sufficiently ensuring the fatigue life of the steel material from various viewpoints.

  First, the inventor considered that the inclusions contained in the steel material affect the fatigue life of the steel material, and verified the relationship between the fatigue life length of the steel material and the size of the inclusion contained in the steel material through various experiments. Went. As a result, when the size of inclusions contained in the steel material (inclusion major axis) is less than 30 μm, even if the inclusions are contained in the steel material very much, the fatigue life is not so long. I found that it has no effect. On the other hand, it was found that when the size of the inclusion is 30 μm or more, the fatigue life is affected by the number of inclusions.

Therefore, the inventor further verified the inclusion of 30 μm contained in the steel material from various viewpoints. The inventor considered that the shape of inclusions affects the length of fatigue life, and conducted experiments on the shape and number of inclusions.
As a result, for inclusions having a major axis of 30 μm or more, the number of roundness, which is an index of shape, is 35 / kg or less, and the roundness is 1. It has been found that when the number of 4 or more is 90 pieces / kg or less, the fatigue life becomes very long.

  According to the present invention, for example, the fatigue life of a steel material used for mechanical parts such as a bearing can be made very long.

The steel material of the present invention will be described. This steel is a high cleanliness steel used for machine parts such as bearings.
Specifically, the chemical composition of this steel material is, in mass%, C: 0.95 to 1.10%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.50%, Cr : 1.30 to 1.60% is contained.
Among the plurality of inclusions contained in the steel, the steel material has an inclusion whose major axis is 30 μm or more, and the number of roundness of less than 1.0 to 1.4 is 35 pieces / kg or less. ing.

In addition, among the inclusions contained in the steel, the number of steel materials having a major axis of 30 μm or more and the roundness of 1.4 or more is 90 / kg or less.
Here, the roundness of the inclusion is a value obtained using the equation (1).

The number of inclusions contained in the steel material is the number measured by the constant current electrolysis method.
Specifically, the steel material after manufacture (at least 100 g) is subjected to a solution treatment by heating at a temperature of 800 ° C. or higher, and the solution sample is determined in an aqueous solution of ferrous chloride adjusted to ph 5 to 7. Inclusions contained in the steel are extracted by current electrolysis. Then, the number of inclusions contained in the steel material is measured, and the number of inclusions per kg unit is obtained from the number and the weight of the steel material used as a sample.
The size of the inclusions included in the steel material of the present invention described above (the length of the major axis of the inclusions), the roundness of the inclusions, and the number of inclusions with respect to the roundness were determined by various experiments. . This will be described in detail below.

Table 1 is for producing a steel material (bearing steel) for a plurality of bearings and (Experiment No1～26), summarizes the major axis different in the number of inclusions in steel containing, the relationship between L ₁₀ life. Table 2 summarizes the chemical components of the steel materials in Table 1.
In Table 1, L ₁₀ life, to produce bars with bearing steel, in which to determine the rolling fatigue life by performing a thrust test. Inclusions with a major axis of less than 10 μm and a very small diameter affect the rolling fatigue life as shown in the previous literature (see Sanyo Special Steel Technical Report vol 12, No. 1, 2005, p38 to p45, fig7). In this experiment, the major axis of inclusions was set to 10 μm or more.

The steel materials in Experiment 1 to Experiment 10 were manufactured by melting electrolytic iron and an alloy in a vacuum in a vacuum high frequency induction furnace (VIF), and did not contain coarse inclusions of 30 μm or more. It is pure steel. The steel materials in Experiment 11 to Experiment 14, Experiment 16 to Experiment 18, and Experiment 20 to Experiment 26 are manufactured by dissolving electrolytic iron, an alloy, and an oxidation source (FeO) in a vacuum using VIF.
The components in steel will be described.
[C] in the steel is 0.90 to 1.10% by mass. C is an essential component for ensuring the hardness required for bearing steel. Generally, bearing steel is required to have a hardness after quenching / tempering of 60 or more in terms of HRC, and [C] is required to be 0.90% by mass or more in order to ensure this hardness. On the other hand, if the C content is excessively large, giant carbides are likely to be generated and adversely affect the rolling fatigue characteristics. Therefore, [C] is suppressed to 1.10% by mass or less at most.

[Si] in the steel is 0.15 to 0.35 mass%. In addition to effectively acting as a deoxidizing element, Si has the effect of increasing hardness by increasing the resistance to quenching and tempering, and [Si] needs to be 0.15% by mass or more. Moreover, since an increase in spheroidizing annealing hardness will be caused when content is too high, [Si] is restrained to 0.35 mass% or less.
[Mn] in the steel is 0.30 to 0.50 mass%. Mn is an element that is indispensable for improving hardenability, increasing surface layer and core hardness, preventing surface depression, and improving rolling fatigue life. Mn] needs to be 0.30% by mass or more. If the Mn content is too high, cold workability and machinability are adversely affected, so [Mn] is suppressed to 0.50% by mass or less.

[Cr] in the steel is 1.30 to 1.60 mass%. Cr is a component that effectively improves the rolling fatigue life by improving the strength and wear resistance through the improvement of hardenability and the formation of stable carbide, and [Cr] is 1.30 mass% or more is required. On the other hand, when [Cr] exceeds 1.60% by mass, the carbide is coarsened, and the machinability and the rolling fatigue life are reduced.
The steel materials in Experiment 15 and Experiment 19 were manufactured by the following steel material manufacturing process.
As shown in FIG. 3, molten steel for the steel material (for high cleanliness steel) is removed from the converter 1 to a ladle 2, and the ladle 2 is conveyed to a secondary smelting device 3 to perform the secondary smelting. Steel material was manufactured by casting the molten steel refined with the apparatus 3 and processed with the secondary refining apparatus 3 with a continuous casting apparatus. In addition, the molten steel for manufacturing a steel material may be obtained from an electric furnace.

The secondary refining apparatus 3 has a ladle refining apparatus (LF apparatus) 5 and a reflux-type vacuum degassing apparatus 6 (hereinafter sometimes referred to as RH apparatus). Molten steel is refined by the ladle refining apparatus 5. After that, it is refined by the RH device 6.
The ladle refining device 5 is an electrode heating type refining device, and a ladle 2 in which molten steel is charged, a blowing device 7 for blowing gas into the molten steel in the ladle 2, and electrode heating for heating the molten steel. It has the apparatus 8 and the supply apparatus 9 for throwing in a flux.
The blowing device 7 includes a porous blowing port 15 that is provided at the bottom of the ladle 2 and blows gas from the bottom thereof, and a lance 16 that blows gas from the top of the ladle 2. A nozzle that blows gas into the molten steel is provided at the tip of the lance 16. The blowing device 7 may have only the porous blowing port 15 or only the lance 16.

The RH device 6 is for degassing molten steel, and has a ladle 2 in which molten steel is charged and a degassing tank 10 for degassing the molten steel in a vacuum state. The ladle 2 is the same as the ladle 2 used in the ladle refining device 5, and is arranged directly under the degassing tank 10.
Two dip pipes 11 to be immersed in the molten steel in the ladle 2 are provided at the lower part of the degassing tank 10, and one of the dip pipes 11 is blown through which an inert gas such as Ar gas is blown (illustrated). (Omitted) is provided. An exhaust port 13 for exhausting the gas in the degassing tank 10 is provided in the upper part of the degassing tank 10.

In manufacturing the steel material, the ladle refining device 5 raises the molten steel to a predetermined temperature with the electrode-type heating device 8 and finely adjusts the chemical components by blowing gas from the blowing device 7 and stirring the molten steel. The reduction of non-metallic inclusions contained in the molten steel is performed.
Further, in the RH device 6, the dip tube 11 is dipped in the molten steel in the pan 2, and an inert gas is blown from the blow port, and the gas in the degas tank 10 is exhausted from the exhaust port 13. Is evacuated and the molten steel is circulated between the degassing tank 10 and the ladle 2 to remove gas components such as hydrogen present in the molten steel. In the RH device 6, an alloy may be added for fine adjustment of the components.

The basicity of the slag in the refining process performed by the RH apparatus 6 (hereinafter, refining process performed by the RH apparatus 6 may be referred to as RH refining) is made 6.5 to 13.5. Specifically, the basicity of the slag at the initial stage of RH refining is in the range of 6.5 to 13.5. In other words, the basicity of the slag when the refining treatment of the molten steel in the LF device 5 (hereinafter, refining treatment in the LF device 5 is sometimes referred to as LF refining) is in the range of 6.5 to 13.5. I try to enter.
In RH refining, the refining process is performed in a state where the composition in the slag satisfies the formula (2). Specifically, prior to LF refining, the slag floating on the molten steel is removed so that the amount of FeO and MnO floating on the molten steel during LF refining is equal to or less than Equation (2), and flux is applied to the molten steel. The composition of the slag is adjusted by adding [the unit of the formula (2) is mass%].

  In RH refining, in the first half of the treatment, the Ar gas blown from the blow inlet is strengthened (Ar gas flow rate is increased) to highly reflux the molten steel, and in the second half of the treatment, the Ar gas blown from the blow inlet is weakened (Ar gas). (The flow rate is reduced) and the molten steel is weakly refluxed. The flow rate of Ar gas (recirculation gas flow rate) is calculated using Equation (3). Formula (3) is disclosed in the secondary refining method (Ladle refining method), the stainless steel manufacturing method, and the special steel steel manufacturing method formula (13.3).

In particular,
(I) With respect to the total treatment time of the RH apparatus, in the first half treatment within a range of 1/3 to 1/2, the molten steel reflux amount obtained by the formula (2) is 180 ton / min or more and 210 ton / min or less. As described above, the flow rate of the reflux gas, that is, the amount of Ar gas blown is adjusted.
(Ii) In the latter half treatment, the Ar gas blowing rate is adjusted so that the molten steel reflux rate is 110 ton / min or more and 140 ton / min or less.
For example, in RH refining, if the total refining time (overall processing time) is 30 minutes, the first half of RH refining is between 10 minutes (1/3 of 30) and 20 minutes (1/2 of 30) Is refluxed in a range where the molten steel reflux amount is 180 ton / min to 210 ton / min.

The second half of RH refining is refluxed in the range of 110 ton / min to 140 ton / min for 20 minutes (2/3 of 30) to 30 minutes.
When the LF refining and the RH refining are finished, the molten steel is transferred to the continuous casting apparatus through the ladle, and the steel material is cast (manufactured) by the continuous casting apparatus.
FIG. 1 summarizes the experimental results shown in Table 1, and shows the relationship between the number of inclusions (number of separate major diameters) and the L ₁₀ life (rolling fatigue life). In Table 1, 100 × 10 ⁶ times or more with a sufficiently long L ₁₀ life was determined as good (evaluation: “◯”), and less than 100 × 10 ⁶ times with a short L ₁₀ life was determined as poor (evaluation: “×”). .

As shown in Experiments 1 to 5 and FIG. 1, in the bearing steel that does not include large inclusions of 20 μm or more, even if the number of inclusions increases, the L ₁₀ life is extremely high regardless of the number of inclusions. (100 × 10 ⁶ times or more).
As shown in Experiments 6 to 10 and FIG. 1, in the bearing steel that does not include large inclusions of 30 μm or more, even if the number of diameters is increased, the L ₁₀ life is very similar to the above. It became long (100 × 10 ⁶ times or more).
That is, according to Experiments 1 to 10, it can be seen that if the major axis of inclusions is less than 30 μm, the L ₁₀ life is very long regardless of the number of inclusions.

On the other hand, as shown in Experiments 11 to 26 and FIG. 1, some bearing steels containing inclusions having a major axis of 30 μm or more had an L ₁₀ life of less than 100 × 10 ⁶ times. Looking closely at the relationship between the L ₁₀ life and the number of inclusions whose major axis is 30 μm or more, the L ₁₀ life tends to decrease gradually as the number of inclusions 30 μm or more increases, and there are few inclusions 30 μm or more. and, L ₁₀ life tends to be high. However, not always assert also L ₁₀ life decreases with increasing inclusions.
As described above, as shown in Experiments 1 to 26 and FIG. 1, it was found that the L ₁₀ life was affected at least when the major axis was 30 μm or more. Here, as affecting the life of the L ₁₀ life, not only the size and the number of major axis discussed above, other factors are also presumed to be associated with. Since the shape of the inclusion was considered as one factor, paying attention to the roundness as an index indicating the shape, further measuring the roundness of the inclusion with respect to a steel material containing inclusions of 30 μm or more Verification was performed.

Tables 3 and 4 summarize the relationship between the roundness and the L ₁₀ life in the inclusions having a major axis of 30 μm or more among the inclusions contained in the steel material. Table 5 shows chemical components in the steel materials shown in Tables 3 and 4.
Table 3 summarizes the relationship between the average composition of inclusions at each roundness and the L ₁₀ life, and Table 4 summarizes the relationship between the number of inclusions at each roundness and the L ₁₀ life. Is. In Table 3 and Table 4, 100 × 10 ⁶ times or more with a sufficiently long L ₁₀ life is considered good (evaluation: “◯”), and less than 100 × 10 ⁶ times with a short L ₁₀ life is poor (evaluation: “×”). ).

In this experiment, inclusions were extracted by the constant current electrolysis method described above, and the number of inclusions with respect to roundness was measured. The major axis of the inclusion was measured by imaging the extracted inclusion and performing image processing analysis. Formula (1) was used for calculation of roundness.
Specifically, solution treated steel 1kg prepared in advance 800 ° C. or higher, the solution samples were extracted inclusions that is part of the constant current electrolysis to steel. The particle size and roundness were measured using a Luzex AP (desktop automatic multifunctional image processor, manufactured by Nireco Co., Ltd.). The image of the inclusion was a photograph shown in FIG.

Table 3, as shown in Table 4, L ₁₀ life is very long (100 × 10 ⁶ times or more) and, classified in the L ₁₀ life is short (less than 100 × 10 ⁶ times), the roundness The relationship between the number of inclusions (number of inclusions by roundness) and the L ₁₀ life is verified.
As shown in Table 4, when the average composition of inclusions by roundness (inclusion average composition by roundness) is viewed as a whole, the roundness is 1.4 or more and less than 1.4 And tend to have different composition of average composition of roundness inclusions. Specifically, looking at the average composition of inclusions having a roundness of less than 1.4, the CaO content is 10% or more, and it is considered that the inclusions of CaO.Al ₂ O ₃ are mainly used. Further, looking at the average composition of inclusions having a roundness of 1.4 or more, CaO is 5% or less on average, Al ₂ O ₃ is very much, and Al ₂ O ₃ is the main component. .

In addition, as shown in FIG. 2, even when the state of the inclusion is seen, in the inclusion having a roundness of less than 1.4, the shape of the inclusion is close to a sphere (a part of the sphere is recessed) When the roundness is 1.4 or more, the shape is closer to a polyhedron than the spherical shape.
Here, it is fully conceivable that the type of inclusion (difference in composition) affects the L ₁₀ life, and the shape of the inclusion also sufficiently affects the L ₁₀ life. as a boundary 1.4 roundness and organize the relationship between the number of inclusions and L ₁₀ life.

As shown in Table 4, when the roundness distribution is viewed, as a whole, the number of inclusions changes when the roundness is around 1.4. For example, when the roundness is 1.3 to less than 1.4 (the former) and the roundness is 1.4 to less than 1.5 (the latter), The number is greatly increased or decreased compared to the former or the latter. Also from this point, it is very effective to organize the relationship with the fatigue life with the roundness as 1.4 boundary.
When the comparative example of experiment 30 to experiment 33 and the example of experiment 36 to experiment 37 are compared, the comparative example is equivalent to inclusions having a roundness of 1.4 or more compared to the examples, or 10 pieces / kg or less. There are very few. Here, in the comparative example of Experiment 30 to Experiment 33, when the total of inclusions having a roundness of 1.0 or more and less than 1.4 is seen, the total is more than 35 / kg, and the L ₁₀ life is 70 × 10 ⁶ times or less.

On the other hand, in the examples of Experiment 36 to Experiment 37, when the total of inclusions having a roundness of 1.0 or more and less than 1.4 is viewed, the total is 35 pieces / kg or less, and L ₁₀ The lifetime is 100 × 10 ⁶ times or more.
According to the comparative example of experiment 30 to experiment 33 and the example of experiment 36 to experiment 37, in the inclusion whose major axis contained in the steel is 30 μm or more, the roundness is less than 1.0 to less than 1.4. If the number is 35 pieces / kg or less, the fatigue life can be made very long.
When the comparative example of experiment 34 and experiment 35 and the examples of experiment 38 to experiment 44 are compared, both have 35 or less inclusions with roundness of 1.0 or more and less than 1.4. And in the comparative example of Experiment 35, the number of inclusions having a roundness of 1.4 or more is more than 90 / kg, and the L ₁₀ life is less than 80 × 10 ⁶ times.

In contrast, in the embodiment of experiments 38 through experiment 44, comprising at circularity 1.4 or more inclusions than 90 pieces / _{kg, L 10} life has a 100 × 10 ⁶ times or more. In particular, in the example of Example 44, the number of inclusions having a roundness of 1.0 or more and less than 1.4 is 1 / kg, and the number of inclusions having a roundness of 1.4 or more is 5 / kg. The L ₁₀ life was 141 × 10 ⁶ times, which was the steel material with the longest rolling fatigue life.
On the other hand, in the comparative examples of Experiments 27 and 28, the number of inclusions having a roundness of 1.0 or more and less than 1.4 is much more than 35 / kg, and the number of inclusions having a roundness of 1.4 or more is 90. The steel material was much shorter than the number of pieces / kg and the L ₁₀ life was 21 × 10 ⁶ times or less, and the rolling fatigue life was short.

  As described above, according to the steel material of the present invention, in mass%, C: 0.95 to 1.10%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.50%, Cr: 1 In steel containing 30 to 1.60%, the number of rounds having a roundness of 1.0 to less than 1.4 is 35 with respect to inclusions having a major axis of 30 μm or more contained in the steel. When the number of roundness is 1.4 or more is 90 pieces / kg or less, the steel material can be a steel material having an extremely excellent rolling fatigue life.

It is a diagram illustrating a relationship between the number and the L ₁₀ life of inclusions (rolling fatigue life). It is an image of inclusions. It is the figure which showed the manufacturing process of steel materials.

Explanation of symbols

1 Converter 2 Ladle 3 Secondary refining equipment 5 Ladle refining equipment 6 RH equipment

Claims (1)

In mass%, C: 0.95 to 1.10%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.50%, Cr: 1.30 to 1.60%, Al: 0.01 to 0.03%, P: 0.025% or less, S: 0.006% or less, O: 0.0012% or less, N: 0.0028 to 0.0062%, remaining Fe and inevitable impurities In which the major axis is 30 μm or more and includes inclusions mainly composed of CaO.Al ₂ O ₃ or Al ₂ O ₃ ,
Relates to the aforementioned inclusions, it is the number is less than 35 / kg of inclusions roundness is less than 1.0 to 1.4, and the number of inclusions is 1.4 or more roundness 90 A steel material with excellent rolling fatigue life, characterized by being no more than pieces / kg.