JP4641022B2 - Manufacturing method of high cleanliness steel - Google Patents

Description

  The present invention relates to a method for producing high cleanliness steel.

Conventional steel materials such as mechanical parts (for example, bearings) that require excellent fatigue life and quietness are cleanliness that reduces non-metallic inclusions as typified by Al ₂ O ₃ as much as possible. It is important to have a high steel.
Such high cleanliness steel (high cleanliness steel) is generally used after fine decarburization treatment of molten steel in a converter and then fine adjustment of chemical components in the molten steel or molten steel by secondary refining equipment. It is manufactured by reducing non-metallic inclusions contained and casting with a continuous casting apparatus.
In the production of high cleanliness steel, in order to improve the quality of steel, it is required to reduce nonmetallic inclusions as much as possible during secondary refining, and as a technology to reduce nonmetallic inclusions Is disclosed in Patent Document 1 or 2.

Patent Document 1 discloses a method of manufacturing a bearing steel that performs vacuum degassing refining in which 2/3 of the processing time is highly refluxed and 1/3 is weakly refluxed in the presence of slag having a basicity of 3 to 6. ing.
In Patent Document 2, vacuum degassing is performed using slag containing SiO ₂ : 10% or less, MgO: less than 6-15%, Al ₂ O ₃ : 25-45%, CaO: 35-60%. A method for refining clean steel is disclosed.
Japanese Patent Publication No. 5-085629 JP 2004-169147 A

The technology of Patent Document 1 discloses a processing time for high or low reflux of molten steel when vacuum degassing, that is, when refining with a reflux-type vacuum degassing apparatus, the molten steel is highly or weakly refluxed. However, strengths such as a specific reflux amount of high reflux and weak reflux are not disclosed. Moreover, although the technique of patent document 1 has disclosed the basicity of slag concretely, it is known that this basicity is too low from a viewpoint of inclusion reduction.
Therefore, even if the technique of Patent Document 1 is used, it is difficult to sufficiently reduce nonmetallic inclusions.

Moreover, although the technique of patent document 2 prescribes | regulates a slag composition and performs vacuum degassing, conditions, such as a recirculation | reflux amount, are not shown, and even if this technique is applied to an actual operation, it is nonmetallic. It was difficult to sufficiently reduce inclusions.
Then, an object of this invention is to provide the manufacturing method of the high cleanliness steel which can fully reduce a nonmetallic inclusion.

In order to achieve the above object, the present invention has taken the following measures.
That is, the present invention refining the molten steel that has been refined with the ladle refining device after the refining with the ladle refining device after refining the molten steel produced from the converter or electric furnace with the reflux vacuum degassing device. In the method for producing a high cleanliness steel, the basicity of the slag in the refining process performed in the reflux type vacuum degassing apparatus is set to 6.5 to 13.5, and in the slag In the first half treatment within a range of 1/3 to 1/2 with respect to the total treatment time of the reflux-type vacuum degassing apparatus while satisfying the formula (1), it is obtained by the formula (2). In the latter half treatment performed after the first half treatment, the molten steel reflux rate is 110 ton / min or less and 140 ton / min or less. is there.

The inventor first paid attention to FeO and MnO in slag, which affects the amount of nonmetallic inclusions in molten steel. The reason for this is that when refining is performed with slag containing a large amount of FeO or MnO, O of FeO or MnO binds to elements having strong oxygen affinity such as Al, Ti, Si in the molten steel, and non-metallic inclusions (for example, , Al ₂ O ₃ ) is increased very much.
As a result of conducting various verifications on FeO and MnO through past operational results and experiments, the inventors found that the nonmetallic inclusions in the molten steel were found if the sum of FeO and MnO satisfied Equation (1). It was found that the increase can be suppressed.

In addition to the above, the inventors focused on the fact that the increase or decrease of non-metallic inclusions is affected by the basicity of the slag, and conducted various verifications on the basicity of the slag during refining. As a result, it was found that non-metallic inclusions can be reduced when the basicity of the slag is in the range of 6.5 to 13.5.
Now, in refining in a reflux-type vacuum degassing device, there is a merit that strengthening the reflux of molten steel (high reflux) can promote the separation and floating of nonmetallic inclusions in the molten steel, There is a demerit that most of the metal that has adhered to the reflux-type vacuum degassing apparatus is melted into the molten steel, resulting in generation of non-gold metal inclusions.

On the other hand, weakening the reflux of molten steel (weak reflux) has the advantage of reducing the melting of the ingots attached to the reflux-type vacuum degassing device. There is a demerit that the separation and floating of inclusions is not promoted.
Therefore, the inventor decided to selectively use high reflux and weak reflux in the refining treatment of the reflux type vacuum degassing apparatus, and the degree of separation and floating of nonmetallic inclusions becomes higher than the degree of melting of the metal into the molten steel by stirring. As described above, various verifications were made regarding the timing and intensity of high reflux and weak reflux.

  As a result, at a time when a lot of nonmetallic inclusions are contained in the molten steel (first half treatment in a range of 1/3 to 1/2 with respect to the total treatment time of the reflux type vacuum degassing apparatus), the equation ( The amount of molten steel reflux determined in 2) is set to a high reflux state of 180 ton / min or more and 210 ton / min or less, and the time when nonmetallic inclusions are separated and floated and become less in the molten steel (all of the reflux type vacuum degassing apparatus) In the latter half of the treatment time in the range of 2/3 to 1/2), it has been found that the molten steel recirculation amount should be a weak recirculation state of 110 ton / min or more and 140 ton / min or less.

  As a result of various past operations and experiments, it has been found that the basicity of the slag is more preferably 8 or more and 11 or less.

  According to the method for producing high cleanliness steel in the present invention, it is possible to sufficiently reduce nonmetallic inclusions.

The manufacturing method of the high cleanliness steel of this invention is demonstrated.
Hereinafter, the manufacturing method of the high cleanliness steel of the present invention, as shown in FIG. 1, takes out the molten steel for high cleanliness steel from the converter 1 to the ladle 2, and this ladle 2 is subjected to secondary refining equipment. The steel is transported to 3 and refined by the secondary refining device 3 to produce high cleanliness steel. The molten steel for high cleanliness steel processed by the secondary refining device 3 is cast by a continuous casting device. In addition, the molten steel for high cleanliness steel 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). The molten steel for high cleanliness steel is removed. It is refined by the pot refining device 5 and then 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.

In the ladle refining device 5 described above, the molten steel is raised to a predetermined temperature by the electrode-type heating device 8, the gas is blown from the blowing device 7, and the molten steel is agitated, thereby finely adjusting the chemical composition and in the molten steel 2. The non-metallic inclusions contained in can be reduced.
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 the above 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. The gas component such as hydrogen present in the molten steel can be removed by circulating the molten steel between the degassing tank 10 and the ladle 2 under a substantially vacuum state. In the RH device 6, an alloy may be added for fine adjustment of the components.

Hereinafter, the manufacturing method of the high cleanliness steel of this invention is demonstrated in detail. FIG. 1 shows the process of the manufacturing method of the high cleanliness steel of the present invention.
As shown in FIG. 1, in the manufacturing method of high cleanliness steel, the basicity of the slag in the refining process performed in the RH apparatus (hereinafter, refining process in the RH apparatus may be referred to as RH refining) is 6. 5 or more and 13.5 or less. 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 is within the range of 6.5 to 13.5 when the refining treatment of molten steel in the LF device (hereinafter, refining treatment with the LF device is sometimes referred to as LF refining) is completed. I am doing so.

  In RH refining, the refining process is performed in a state where the composition in the slag satisfies the formula (1). Specifically, before the LF refining, the slag floating on the molten steel 2 is removed so that the amount of FeO and MnO of the slag floating on the molten steel during the LF refining is equal to or less than the formula (1). Additives are added to adjust the slag composition.

In RH refining, in the first half of the treatment, the Ar gas blown from the blowing port is strengthened (Ar gas flow rate is increased) to highly reflux the molten steel, and in the latter half of the treatment, the Ar gas blown from the blowing port is weakened (Ar gas). (The flow rate is reduced) and the molten steel is weakly refluxed. The Ar gas flow rate (recirculation gas flow rate) is calculated using Equation (2).
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.
Generally, in RH refining, the total refining time (overall processing time) is 30 minutes to 60 minutes. If the reflux rate of molten steel is 180 ton / min to 210 ton / min in the first 10 minutes (1/3 of 30) to 30 minutes (1/2 of 60) of RH refining, During the latter half of 20 minutes (2/3 of 30) to 30 minutes (1/2 of 60), it will recirculate in the range of 110 ton / min to 140 ton / min.

  Table 1 shows an example in which the manufacturing method of the high cleanliness steel of the present invention was performed and a comparative example in which the manufacturing method of the high cleanliness steel of the present invention was not performed.

In Examples and Comparative Examples, the number of alumina inclusions was measured in order to evaluate the degree of reduction of nonmetallic inclusions in the molten steel after the reduction treatment.
The number of alumina inclusions was measured with EPMA (Electron Probe Microanalyzer). The EPMA used was “JXA-8000” series manufactured by JEOL Ltd., the measurement conditions were an acceleration voltage of 20 kv, the X-ray type was K-ray, the beam diameter was 2 μm, and an EDS detector was used.
Inclusions with a minor axis of 5 μm or more observed by EPMA and containing 50% or more of Al ₂ O ₃ and 5% or less of CaO in terms of CaO—Al ₂ O ₃ —SiO ₂ —MgO quaternary system. Those to be used were alumina inclusions, and the number of the inclusions was counted. In the measurement, 3000 mm ² or more was observed in order to ensure reliability.

Hereinafter, examples and comparative examples will be described in detail.
As shown in Table 1, in Comparative Examples 1 and 2, in the RH refining, the molten steel reflux amount in the first half treatment is in the range of 180 ton / min to 210 ton / min, and the molten steel is highly refluxed. Hereinafter, it may be referred to as a high reflux time) and is as short as less than 1/3 (high reflux ratio is less than 33%) with respect to the total processing time. As a result, since the high reflux time is short in the first half treatment, the alumina inclusions in the molten steel cannot be sufficiently separated and floated, and the number of alumina inclusions after the treatment is 4.0 pieces / cm ² . (Evaluation “×”).

In Comparative Examples 3 and 4, although the molten steel reflux amount is 110 ton / min to 140 ton / min in the latter half treatment and the molten steel is weakly refluxed, the time for weak reflux (hereinafter sometimes referred to as weak reflux time) is the total treatment. It is as short as less than 1/2 (less than 50% weak reflux ratio) with respect to time. As a result, in the latter half treatment, the degree of contamination of the molten steel due to melting of the bullion and the like attached to the dip tube 11 and the degassing tank 10 is higher than the degree of separation and floating of the alumina inclusions in the molten steel by weak reflux. As a result, the alumina inclusions in the molten steel are increased. The number of alumina inclusions after the treatment was greater than 4.0 / cm ² (evaluation “×”).

In Comparative Examples 5 and 6, although the recirculation time of the first half treatment is sufficient to be 1/3 or more of the total treatment time, the molten steel recirculation amount is 180 ton / min or less, and the molten steel is in a state of high recirculation. Absent. As a result, since the reflux state of the molten steel is weak in the first half treatment, the alumina inclusions in the molten steel cannot be sufficiently separated and floated, and the number of alumina inclusions after the treatment is 4.0 / cm ^2. (Evaluation “×”).
In Comparative Examples 7 and 8, the reflux time of the first half treatment is 1/3 or more with respect to the total treatment time, but the molten steel reflux amount is 210 ton / min or more and the reflux of the molten steel is too strong.

As a result, in the first half treatment, the molten steel reflux state is strong, so that more alumina inclusions can be separated and floated. On the other hand, the molten steel reflux state is too strong and adheres to the dip tube 11 and the degassing tank 10. The speed at which the bullion is melted will increase. That is, the speed at which the metal melts out is higher than the speed at which the alumina inclusions separate and float by reflux, and the number of alumina inclusions after the treatment is greater than 4.0 / cm ² (evaluation “×” ").
In Comparative Examples 9 and 10, the reflux time of the second half treatment is sufficient to be ½ or more of the total treatment time, but the molten steel reflux amount is 110 ton / min or less and the reflux of the molten steel is too weak.

As a result, since the reflux state of the molten steel is too weak in the latter half treatment, the alumina inclusions in the molten steel cannot be sufficiently separated and floated, and the number of alumina inclusions after the treatment is 4.0 / cm 2. More than ² (evaluation “×”)
In Comparative Examples 11 and 12, although the reflux time of the second half treatment is sufficient to be 1/2 or more of the total treatment time, the molten steel reflux amount is 140 ton / min or more, and the reflux of the molten steel is used for the second half treatment. Is too strong. Since many alumina inclusions separate and float in the first half treatment, the alumina inclusions in the molten steel are absolutely reduced at the time of the second half treatment. If the molten steel is strongly refluxed in such a state, the speed at which the metal melts out will be faster than the speed at which the alumina inclusions separate and float by the reflux. As a result, the number of alumina inclusions after the treatment was larger than 4.0 pieces / cm ² (evaluation “×”).

In Comparative Examples 13 and 14, the slag composition does not satisfy the formula (1), and the smelting was performed using slag containing a large amount of FeO and MnO in RH refining. If the slag contains a large amount of FeO or MnO, the slag oxidizes elements with strong oxygen affinity, such as Al, Ti, Si, etc., in the molten steel, which increases non-metallic inclusions, that is, alumina. Become. As a result, the number of alumina inclusions after the treatment was larger than 4.0 pieces / cm ² (evaluation “×”).
In Comparative Examples 15 and 16, the basicity (C / S) of slag is as low as 6.5 or less in RH refining, O in the molten steel becomes large in terms of thermodynamic equilibrium, and deoxidation does not proceed. Things increase. As a result, the number of alumina inclusions after the treatment was larger than 4.0 pieces / cm ² (evaluation “×”).

In Comparative Examples 17 and 18, the basicity (C / S) of slag is as high as 13.5 or more in RH refining, and the solid phase ratio of slag is high. In other words, since the viscosity of the slag is high, in the RH refining, the slag is difficult to break when immersing the dip tube 11 in the molten steel, and not only the molten steel but also a part of the slag is easily sucked up during the reflux. The number of alumina inclusions after the treatment was larger than 4.0 pieces / cm ² (evaluation “×”). If the slag is difficult to break, not only the quality of the product is lowered, but also there is a risk of causing a great trouble in operation.
In Examples 19 to 22, the basicity of slag in RH refining is in the range of 6.5 to 13.5, and the composition in the slag satisfies the formula (1). Moreover, the reflux time of the first half treatment is in a range of 1/3 to 1/2 of the total treatment time, and the molten steel reflux is also strongly refluxed in the range of 180 ton / min to 210 ton / min. .

In Examples 19 to 22, in the first half treatment, since the reflux of the molten steel is not too strong and not too weak, the rate of separating and floating alumina inclusions in the molten steel by reflux is higher than the rate at which the metal melts by reflux. In addition to being superior, sufficient time is taken to separate and float the alumina inclusions in the molten steel. In addition to this, FeO and MnO contained in the slag also satisfy the formula (1), and since these amounts are small, the slag hardly increases alumina inclusions.
As a result, in Examples 19 to 22, the alumina inclusions contained in the molten steel could be reduced to 4.0 pieces / cm ² or less (evaluation “◯”).

In Examples 23 to 26, the conditions of Examples 19 to 22 were satisfied, but the basicity of slag in RH refining was in the range of 8.0 to 11. In Examples 23 to 26, O in the molten steel can be sufficiently reduced in terms of thermodynamic equilibrium, and more alumina inclusions are removed than in Examples 19 to 22 because the slag viscosity is not too high. We were able to. That is, in Examples 23 to 26, the alumina inclusions contained in the molten steel could be reduced to 2.0 pieces / cm ² or less (evaluation “）”).
2 to 4 summarize the relationship between the molten steel reflux amount and the number of alumina inclusions in the examples and comparative examples in Table 1. Specifically, FIG. 2 shows that in the latter half treatment, the molten steel reflux amount is in the range of 110 ton / min to 140 ton / min and the reflux time is 2/3 to 1/2, and the molten steel reflux amount in the first half treatment. Is a change. FIG. 3 shows that in the first half treatment, the molten steel recirculation amount is in the range of 180 ton / min to 210 ton / min and the recirculation time is 1/3 to 1/2, and the molten steel recirculation amount is changed in the second half treatment. It is a thing.

FIG. 4 summarizes the relationship between high reflux and weak reflux and the number of alumina inclusions in the examples and comparative examples in Table 1. Specifically, FIG. 4 shows that the molten steel reflux amount in the first half treatment is in the range of 180 ton / min to 210 ton / min, and the molten steel reflux amount in the second half treatment is in the range of 110 ton / min to 140 ton / min. (Ratio of reflux time of first half treatment and second half treatment) was changed.
As shown in FIG. 2, the number of alumina inclusions can be reduced to 4.0 pieces / cm ² or less by setting the molten steel recirculation amount in the first half treatment to a range of 180 ton / min to 210 ton / min. When the molten steel reflux amount in the first half treatment is less than 180 ton / min, or the molten steel reflux amount in the first half treatment exceeds 210 ton / min, the number of alumina inclusions is more than 4.0 pieces / cm ² .

As shown in FIG. 3, the number of alumina inclusions can be reduced to 4.0 pieces / cm ² or less by setting the molten steel recirculation amount in the latter half treatment to a range of 110 ton / min to 140 ton / min. When the molten steel reflux amount in the latter half treatment is less than 110 ton / min, or the molten steel reflux amount in the latter half treatment exceeds 140 ton / min, the number of alumina inclusions is more than 4.0 pieces / cm ² .
As shown in FIG. 4, the number of alumina inclusions is reduced to 4.0 pieces / cm ² or less by setting the reflux time of the second half treatment to a range of 1/2 to 2/3 of the total treatment time. be able to. If the reflux time of the second half treatment is less than 1/2 or the reflux time of the second half treatment exceeds 2/3, the number of alumina inclusions exceeds 4.0 pieces / cm ² .

FIG. 5 summarizes the relationship between the number of alumina inclusions contained in the molten steel and the rolling life of a bearing produced from the molten steel. In FIG. 5, “◎”, “◯”, and “×” plot symbols correspond to Table 1.
As shown in FIG. 5, as the number of alumina inclusions decreases, the number of rolling times until the bearing breaks increases and the rolling life is longer. On the other hand, it can be seen that as the number of alumina inclusions increases, the number of rolling of the bearing, which is the rolling life, decreases and the rolling life is short. A trend line a indicating a tendency between the rolling life and the number of alumina inclusions is downwardly sloping. Here, paying attention to the trend line a, in the range where the number of alumina inclusions exceeds 4.0 / cm ² , there is not much change in rolling life as the number of alumina inclusions increases. The lifetime is 50 × 10 ⁶ times or less.

However, looking at the trend line a, as the number of alumina inclusions becomes less than 4.0 / cm ² , the rolling life increases remarkably, and the number of alumina inclusions becomes 4.0 / The position where cm ² is the critical line. That is, when attention is paid to the trend line a, the rolling life is rapidly increased from 50 × 10 ⁶ times at the critical point.
When the number of alumina inclusions is 2.0 pieces / cm ² or less, the rolling life is 100 × 10 ⁶ times or more, which is very excellent as the fatigue life of the bearing. Therefore, the number of alumina inclusions is preferably less than 2.0 / cm ² .

FIG. 6 summarizes the relationship between the basicity of slag and the number of alumina inclusions in the examples.
As shown in FIG. 6, in the RH refining, when the basicity of slag is refined in the range of 6.5 to 13.5, the number of alumina inclusions is 4.0 pieces / cm ² or less. It is possible to manufacture a long-life bearing having a rolling life of 50 × 10 ⁶ times or more. Furthermore, when the basicity of the slag is refined in the range of 8 to 11, the number of alumina inclusions is 2.0 pieces / cm ² or less, so that the rolling life is 100 × 10 ⁶ times or more. A strong bearing with a long life can be manufactured.

  Therefore, when the high cleanliness steel manufacturing method of the present invention is adopted in manufacturing the high cleanliness steel, the fatigue life (rolling life) of the machine parts (for example, bearings) manufactured with the high cleanliness steel is reduced. It becomes possible to make it excellent.

It is the figure which showed the process of manufacture of high cleanliness steel. It is a related figure of the amount of molten steel reflux when changing the amount of molten steel reflux of the first half process, and the number of alumina inclusions. FIG. 6 is a relationship diagram between the molten steel reflux amount and the number of alumina inclusions when the molten steel reflux amount in the latter half treatment is changed. It is a related figure of the ratio (the second half weak reflux ratio) of the reflux amount, and the number of alumina inclusions. FIG. 4 is a relationship diagram between the number of alumina inclusions and the rolling life (rolling fatigue life) of a bearing. It is a related figure of the basicity (C / S) of slag and the number of alumina inclusions.

Explanation of symbols

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

Claims (2)

After refining the molten steel from the converter or electric furnace with a ladle refining device, refining the molten steel refined with the ladle refining device with a reflux-type vacuum degassing device to produce high cleanliness steel. In the manufacturing method of high cleanliness steel to be manufactured,
The basicity of the slag in the refining process performed in the reflux-type vacuum degassing apparatus is 6.5 or more and 13.5 or less, and the composition in the slag satisfies the formula (1),
In the first half treatment within a range of 1/3 to 1/2 with respect to the total treatment time of the reflux type vacuum degassing apparatus, the molten steel reflux amount obtained by the formula (2) is 180 ton / min or more and 210 ton / min or less. A high reflux state,
In the second half treatment performed after the first half treatment, a method for producing a high cleanliness steel, characterized in that a molten steel reflux amount obtained by the formula (2) is in a weak reflux state of 110 ton / min or more and 140 ton / min or less.

  The method for producing a high cleanliness steel according to claim 1, wherein the basicity of the slag is 8 or more and 11 or less.

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