JP2003183722A - Method for smelting high cleanliness steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for smelting high cleanliness steel, which can stably remove non-metallic inclusions in molten steel in a shorter time than ever, by using RH vacuum degassing treatment. <P>SOLUTION: In the method for smelting the high cleanliness steel, which sets a ladle of holding the molted steel tapped out from a converter, in a RH vacuum degassing tank, and adds a ferroalloy and a cold charge for adjusting ingredients, into the molten steel under a reduced pressure, to reflux the molten steel between the ladle and the degassifying tank, this method is characterized by setting an adding amount of the above ferroalloy and the cold charge, so that the ferroalloy can satisfy the expression: Wa×Ca/1000≤0.5, and the cold charge satisfies the expression: Wa×Ca/1000≤0.5, [wherein Wa is a total added amount of the ferroalloy (kg/molten steel ton); Ca is concentration of [O] in the ferroalloy (ppm); Wc is the total added amount of the cold charge (kg/ molten steel ton); and Cc is the concentration of [O] in the cold charge (ppm)]. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Steel materials used for bearings and crankshafts of automobiles are repeatedly exposed to stress, so that they may be so-called "high-cleanliness steel" with very few non-metallic inclusions that are the starting points of metal fatigue fracture. Required. In order to manufacture such a high cleanliness steel, a means generally called secondary refining is adopted. For example, Japanese Patent Publication No. 6-45
Japanese Patent No. 818 discloses that when a bearing steel is melted through a sequence of converter-vacuum degassing-continuous casting, carbonaceous material and Cr alloy iron are added to the molten steel in the converter before vacuum degassing, and further the converter is used. By adding CaO-CaF ₂ -based synthetic slag to the tapped steel and continuously performing vacuum degassing for at least 30 minutes in a vacuum degassing tank (hereinafter simply referred to as degassing tank), non-metallic intervention We are trying to reduce things.

There is also a technique of centrifuging non-metallic inclusions without using vacuum degassing treatment, for example, Japanese Patent Laid-Open No. 3-1.
Japanese Laid-Open Patent Publication No. 10059 proposes a technique based on heating a molten metal in a second container after applying a horizontal rotating flow to the molten metal in a first container.

[0004]

However, in the technique described in Japanese Patent Laid-Open No. 3-110059, the molten metal is first supplied with a horizontal rotary flow, and then the molten metal is heated. There is a problem that an induction heating device is required and the equipment cost becomes excessive. Also,
In the technique described in Japanese Patent Publication No. 6-45818, the existing R
Although the H vacuum degassing tank can be used to perform the treatment relatively easily, it cannot be said that the non-metallic inclusions can be separated and removed to a sufficiently satisfactory level. In other words, the current situation is that the amount of separation and removal of non-metallic inclusions varies and is not stable. Further, the processing by this technique usually requires a long processing time of 40 minutes or more.

In view of the above circumstances, the present invention provides a method for producing high-cleanliness steel capable of stably removing non-metallic inclusions in molten steel in a shorter time than before by utilizing RH vacuum degassing treatment. It is intended to be provided.

[0006]

Means for Solving the Problems The inventor has conducted extensive studies in order to achieve the above object, and realized the results in the present invention.

That is, according to the present invention, a ladle holding molten steel discharged from a converter is set in an RH vacuum degassing tank, and alloy iron and a cold material for component adjustment are added to the molten steel under reduced pressure. ,
In a method for producing high-cleanliness steel in which the molten steel is refluxed between a ladle and a degassing tank, the addition of the alloy iron and the cold material is set to satisfy the following formula: This is a method of melting cleanliness steel.

Iron alloy: Wa × Ca / 1000 ≦ 0.5 Cold material: Wa × Ca / 1000 ≦ 0.5 However, Wa: total alloy iron addition amount (kg / per molten steel t), Ca: in alloy iron [ O] concentration (ppm), Wc; total addition amount of cold material (kg / per molten steel t), Cc; in cold material [O]
Concentration (ppm) Further, the present invention is characterized in that the alloy iron and the cold material are preliminarily selected according to the oxygen concentration contained in them, and the one suitable for the melting purpose is selected and used. This is a method of melting grade steel.

In the present invention, attention is paid to the oxygen content in the alloy iron and the cold material added in the composition adjustment in producing the high cleanliness steel. Since the one with a low concentration was preferentially selected and used, the oxygen content in the steel was reduced without spending an excessive amount of refining time after the ferroalloy was added in the RH vacuum degassing tank, Stable high cleanliness of molten steel can be achieved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below, including the background of the invention.

First, since the inventor promotes the formation of non-metallic inclusions by the oxygen existing in the molten steel, the oxygen content of the cold material represented by ferroalloy or iron scrap which has been conventionally used is contained. The amount was investigated lot by lot. As a result, it was found that the oxygen content of each lot fluctuates in a fairly wide range as shown in the frequency distributions of FIGS. This is because for ferroalloys, the oxygen content varies depending on the ferroalloy manufacturing conditions and types, and for cold materials, the oxygen content varies depending on the source (such as cut steel product scraps) and storage conditions. This is because.

In view of this, the inventor has conducted secondary refining (R) to determine how much oxygen content in molten steel can be reliably and stably separated and removed of non-metallic inclusions in the production of bearing steel.
It was decided to examine it in a test operation using the H vacuum degassing tank). An example of the examination results (marked with ○) is shown in Fig. 1 as the change over time in the oxygen concentration in the molten steel. When alloyed iron or a cold material with an oxygen concentration that is normally used is added, the oxygen concentration in the molten steel changes each time. You can see that it is increasing. Due to the increase in the oxygen concentration in the molten steel with the addition of this ferroalloy or cold material,
It has been clarified that the degassing treatment time is affected and the degassing treatment time for attaining the target cleanliness (= oxygen concentration) is extended, and eventually a treatment time of 40 minutes or more is required.

On the other hand, the inventor paid attention to the oxygen content of the ferro-alloy and the cold material added by adjusting the composition, repeated the experiment by using ferro-alloy and the cold material with various oxygen concentrations, and It has been found that if the formulas (1) and (2) are satisfied, it does not affect the increase of the oxygen concentration in the molten steel at the time of addition.

Iron alloy: Wa × Ca / 1000 ≦ 0.5 (1) Cold material: Wa × Ca / 1000 ≦ 0.5 (2) where Wa: total iron alloy addition (kg / per molten steel t), Ca: [O] concentration (ppm) in ferroalloy, Wc: total addition amount of cold material (kg / per molten steel t), Cc: [O] in cold material
Concentration (ppm) An example of the results of this study is shown in Fig. 1 as the change over time in the oxygen concentration in the molten steel. In this experimental example (indicated by a black square), the addition amounts and timings of the ferroalloy and the cooling material are the same as those in the above-described conventional example (a white circle). As a result, if the oxygen content contained in the ferroalloy satisfies the relationship of (1) and (2) above, the oxygen concentration of the molten steel does not increase during the treatment and the target oxygen concentration (8 ppm in FIG. 1) is reached. It was revealed that the RH degassing treatment up to 30 minutes can be performed in less than 30 minutes, and it is not necessary to take the treatment time longer than necessary. Therefore, the present invention as described above has been completed with the requirement of utilizing this relationship.

Further, in the practice of the present invention, when the above-mentioned low oxygen content product is selected and used as the alloy iron to be added for adjusting the composition at the time of tapping from the converter, the molten steel before vacuum degassing is selected. It is possible to reduce the oxygen concentration and to shorten the degassing treatment time.

[0016]

[Example] Basic composition of automobile bearing steel (C: 1% by mass,
Si: 0.25 mass%, Mn: 0.4 mass%, Cr:
Molten steel having 1.5 mass% and Mo: 0.02 mass% is smelted in a converter (capacity: 180 tons) over a large number of charges, and RH vacuum degassing treatment is performed as secondary smelting. Omitted). At the time of this RH vacuum degassing treatment, the melting method of the high cleanliness steel according to the present invention was applied to reduce non-metallic inclusions. The reducing effect is to count the number of non-metallic inclusions with a diameter of 10 μm or more visually observed in the unit cross-sectional area (per 1 cm ² ) of a sample sampled by continuously casting the obtained molten steel into a steel slab. It was evaluated by that.

Table 1 shows the operating conditions such as the type of ferroalloy or cold material added, the oxygen content, the amount added, the timing of addition, the molten steel temperature, the number of non-metallic inclusions and RH corresponding to these operating conditions.
The vacuum degassing treatment time is shown. From Table 1, it is clear that according to the present invention, the standard for the number of non-metallic inclusions of 0.3 / cm ² which is the standard for cleanliness steel is cleared. It was also confirmed that the time required for the RH vacuum degassing process was stable and was significantly shortened compared to the conventional case.

[0018]

[Table 1]

In the present inventions 1 to 4, which are examples of the present invention,
According to the above relational expression, the oxygen amounts are 90, 108 and 8 respectively.
Although an amount of 1,84 ppm or less is required, an alloy iron of 80 ppm or less was selected and used. Similarly, the cold material is also the present invention 1 to
4, the oxygen amount was 142, 178, 185, 1 respectively.
47 ppm or less is required, but those below that value were selected. On the other hand, the oxygen content of the ferroalloy and the cold material used in Conventional Examples 1 and 2 has an average oxygen content that is normally used, as shown in FIGS. 2 and 3. is there. In addition, although the above-mentioned embodiment is applied to bearing steel, the present invention is resistant to HI that requires high cleaning.
It has been confirmed that it can be similarly applied to C (hydrogen-induced cracking) steel and the like.

[0020]

As described above, according to the present invention, R
By utilizing the H 2 vacuum degassing treatment, it becomes possible to stably remove non-metallic inclusions in molten steel in a shorter time than before.

[Brief description of drawings]

FIG. 1 is a diagram showing changes over time in the oxygen content of molten steel during RH treatment.

FIG. 2 is a result of examining the frequency distribution of oxygen content for each lot of ferroalloys used conventionally.

FIG. 3 is a result of examining the frequency distribution of the oxygen content for each lot of the cooling material that has been conventionally used.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroshi Nomura             1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama             Shi) Kawasaki Steel Co., Ltd. Mizushima Steel Works F-term (reference) 4K013 AA07 BA08 BA09 BA14 CE01                       CE06 FA02

Claims (2)

[Claims] 1. A ladle holding molten steel discharged from a converter is set in an RH vacuum degassing tank, and alloy iron and a cold material for component adjustment are added to the molten steel under reduced pressure to remove the molten steel. In a method for producing high-cleanliness steel that is refluxed between a ladle and a degassing tank, the addition of the alloy iron and the cold material is set to satisfy the following formula: Method of melting. Iron alloy: Wa × Ca / 1000 ≦ 0.5 Cold material: Wa × Ca / 1000 ≦ 0.5 where Wa: total alloy iron addition amount (kg / per molten steel t), Ca: [O] concentration in alloy iron (Ppm), Wc; total addition amount of cold material (kg / per molten steel t), Cc: in cold material [O]
Concentration (ppm) 2. The high-cleanliness according to claim 1, wherein the ferroalloy and the cold material are preliminarily selected according to the oxygen concentration contained in them, and those suitable for the melting purpose are selected and used. Method of melting grade steel.