JPH05140632A - Production of stainless steel having ultrahigh cleanliness - Google Patents

Abstract

PURPOSE:To provide a producing method of stainless steel having ultrahigh cleanliness by softening and reducing mom-metallic inclusions in the stainless steel. CONSTITUTION:In refining of the molten stainless steel, the molten steel is refined to <=30ppm aluminum and 60-120ppm oxygen and tapped into a ladle, and by injecting a flux containing <=0.3% calcium oxide and calcium fluoride based on the wt. of molten steel, as the main components into the molten steel in this ladle together with an inert gas, the secondary refining is executed. Desirably, after the above treatments, by making pure calcium content of calcium alloy in either or both of the ladle and a tundish >=0.01% based on the weight of the molten steel, the non-metallic inclusion is softened and reduced and the stainless steel having ultrahigh cleanliness can be produced. In the production of an extremely fine wire, foil, etc., required inclusion cleanliness, drawability, fatigue strength and surface quality are drastically improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for softening and reducing nonmetallic inclusions by refining a molten stainless steel in a ladle with a flux containing CaO and CaF ₂ as main components together with an inert gas. It is a thing.

[0002]

Nonmetallic inclusions in stainless steel cause surface defects and work cracks, and particularly hard nonmetallic inclusions represented by Al ₂ O ₃ and Cr ₂ O ₃ are harmful.

Generally, cleaning such as removal of non-metallic inclusions of stainless steel is carried out in a secondary refining furnace such as AOD or VOD. That is, a reducing agent such as Al or Si is added, and the molten steel is vigorously stirred by blowing an inert gas in the presence of basic slag to promote deoxidation and floating removal of nonmetallic inclusions. It is recognized that such a method has the effect of reducing non-metallic inclusions. However, Al ₂ O ₃
It does not suppress the formation of hard non-metallic inclusions such as Cr ₂ O ₃ and Cr, and unless such non-metallic inclusions are softened or reduced, the occurrence of the defects described above cannot be prevented.

As a countermeasure against this, a process has been developed in which a flux containing CaO and CaF ₂ as a main component is blown into molten stainless steel in a ladle subjected to Si deoxidation together with an inert gas. J. Met. , 1 (197
2), P.I. As described in No. 103, it is known to be effective as a method for reducing nonmetallic inclusions in stainless steel and controlling the composition of nonmetallic inclusions. However, this method also does not consider that Al ₂ O ₃ -based non-metallic inclusions are generated by [Al] which is unavoidably present in molten steel, and non-metallic inclusions are controlled by controlling the components before and after the treatment. Has not reached the point where it is actively promoting the softening of.

Further, as described in Japanese Patent Application Laid-Open No. 59-6315, nonmetallic inclusions are reduced by adding Ca and Ca alloy to Si and Mn deoxidized stainless molten steel. The method is known. However, Si,
In the present situation, Mn-deoxidized stainless steel has a problem in that a stable effect cannot be obtained due to a high oxygen level, and that it is economically disadvantageous because Ca is expensive.

Although a method of desulfurization by blowing a flux of CaO-CaF ₂ or the like together with an inert gas is also known, this method also adds Al, so that a non-based Al ₂ O ₃ system is used. It is not a technology that suppresses the formation of metal inclusions.

[0007]

SUMMARY OF THE INVENTION In order to render nonmetallic inclusions in stainless steel harmless by softening and reducing them, the present invention uses Ca in stainless molten steel in a ladle.
In refining in which a flux containing O and CaF ₂ as main components is blown together with an inert gas, Al ₂ O ₃ -based and Cr ₂ O ₃ -based hard nonmetallic inclusions are softened by adjusting the components before and after the treatment, Further, it is an object to reduce the amount.

[0008]

According to the present invention, in refining molten stainless steel, [Al] in the molten steel is changed to 3%.
0 ppm or less, [O] was refined to 60 to 120 ppm and tapped in a ladle, and the molten steel in the ladle contained 0.3% of the weight of the molten steel.
This is a method for producing ultra-high cleanliness stainless steel, characterized in that the above-described amount of flux containing CaO and CaF ₂ as main components is blown together with an inert gas to carry out secondary refining.

After the secondary refining, one or both of Ca and Ca alloy is added to the stainless molten steel in the ladle or the tundish to obtain 0.01% of the weight of the molten steel.
It is characterized in that it is added as described above.

The inventor of the present invention has found that Al ₂ O ₃ and Cr _{2 which} are harmful to the occurrence of surface defects and hot work cracking of stainless steel.
For the purpose of softening and further reducing the O ₃ -based non-metallic inclusions, the composition of the molten steel before the blowing in the method of blowing the flux with the inert gas into the stainless molten steel was examined.

In order to investigate the influence of the (Al ₂ O ₃ ) and (Cr ₂ O ₃ ) concentrations in the non-metallic inclusions which adversely affect the ductility of the non-metallic inclusions, slabs of various non-metal inclusion compositions are investigated. Was subjected to hot forging (area reduction rate: 95%), and the number of nonmetallic inclusions having a width of 5 μm or more, which is considered harmful, was measured.

No strong deoxidizing agent such as Al was added to these cast pieces, and the composition of nonmetallic inclusions was (CaO), (S
The main constituents are iO ₂ ), (Al ₂ O ₃ ), and (MnO).
As a result, the (Al ₂ O ₃ ) concentration in the non-metallic inclusion is 30
% Or less and the (Cr ₂ O ₃ ) concentration was 20% or less, it was found that the nonmetallic inclusions were divided and the number of nonmetallic inclusions having a width of 5 μm or more was small. (Al ₂ O in non-metallic inclusions
₃ ) In order to reduce the concentration to 30% or less and the (Cr ₂ O ₃ ) concentration to 20% or less, the molten steel before the flux containing CaO and CaF ₂ as main components (hereinafter, simply referred to as flux) is blown The conditions of the [Al] and [O] concentrations, the amount of flux blown, and the amounts of Ca and Ca alloy added were investigated.

Regarding the [Al] and [O] concentrations before the blowing of the flux, when the [Al] concentration is high or the [O] concentration is low, the non-metallic inclusions after the blowing treatment are (A)
l ₂ O ₃ ) as a nucleus and CaO-SiO ₂ -Al ₂ O surrounds it.
_It has a two-phase morphology in which _three types of non-metallic inclusions are collected. In this case, it is considered that Al ₂ O ₃ -based nonmetallic inclusions were already present as nuclei in the molten steel. Since such non-metallic inclusions remain without deformation even when forged (Al ₂ O ₃ ) nuclei, the number of non-metallic inclusions having a width of 5 μm or more is very large. If the [O] concentration before the flux injection is high, the non-metallic inclusions after the treatment are C
It has a two-phase form of r ₂ O ₃ —MnO system and CaO—SiO ₂ —Al ₂ O ₃ system. In this case, Cr ₂
It is considered that O ₃ -based nonmetallic inclusions were present. Such non-metallic inclusions may be Cr ₂ O ₃ -Mn even when forged.
Since the O-based portion remains without being deformed, the number of non-metallic inclusions having a width of 5 μm or more is very large.

FIG. 1 is a view of hot forging stainless steel (area reduction ratio 9
5%) and the influence of [Al] concentration on the number of non-metallic inclusions having a width of 5 μm or more is shown, and FIG. 2 shows the influence of [O] concentration on the number of non-metallic inclusions having a width of 5 μm or more. [A
l] concentration is 30 ppm or less, [O] concentration is 60 to 120
If the components are controlled to be ppm, the width will be 5μ.
The number of inclusions of m or more is small, and the non-metallic inclusions in the slab are divided or expanded. That is, it can be seen that the formation of Al ₂ O ₃ -based and Cr ₂ O ₃ -based hard nonmetallic inclusions is suppressed, and that they are divided by processing and rendered harmless.

In order to reduce the [Al] concentration to 30 ppm or less, it is important not to raise the Al concentration by selecting the melting raw material and the refractory, and the [O] concentration is 60 to 1
In order to make it 20 ppm, it is controlled by the slag composition, and especially the basicity of the slag in the refining furnace is 1.0 to 1.5.
It is desirable to set the degree. The use of such a basicity level slag is also advantageous in that it suppresses an increase in the [Al] concentration of the molten steel due to Al ₂ O ₃ in the refractory.

[O] in the slab after flux blowing treatment
When the concentration is 50 ppm or less, the composition of non-metallic inclusions is CaO.
--SiO ₂ --Al ₂ O ₃ system. It is considered that this is mainly due to the progress of diffusion deoxidation according to the following reaction formula due to the low activity flux of SiO ₂ melted at the interface between the gas and the molten steel.

Si + O → (SiO ₂ ) The composition of the non-metallic inclusions in this case is (CaO), (SiO
The ratio of ₂ ) is approximately 1: 1 and is in the low melting point region, and the non-metallic inclusions are softened. Further, in this process, since the deoxidation proceeds due to the flux having a low activity of SiO ₂ melted at the interface between the gas and the molten steel, it is advantageous in that the melting loss of the refractory is less than that in the deoxidation method using the top slag.

The composition of the flux is mainly CaO and CaF ₂ , and CaF ₂ is 20 to 20 relative to CaO + CaF ₂ .
It is desirable to contain 50% from the viewpoint of flux slagging property and prevention of melting of refractory material. From the viewpoint of controlling the composition of nonmetallic inclusions and preventing melting of refractory materials, CaO
+ 20% or less of Al ₂ O ₃ or Mn with respect to CaF ₂
It is also possible to contain one of O.

As shown in FIG. 3, the flux injection amount is
It was found that when the content was 0.3% or more based on the weight of molten steel, the number of inclusions was small, and the non-metallic inclusions in the slab were fragmented or expanded. If the amount of flux blown is large,
It is considered that this is because the Al ₂ O ₃ concentration of the nonmetallic inclusions was diluted to a low concentration of 30% or less, and soft nonmetallic inclusions with high CaO and SiO ₂ concentrations were formed.

Further, even if the amount of flux blown is 0.7% or more with respect to the weight of molten steel, the effect of reducing the number of inclusions is small, and if the amount of flux blown is too large, slag formation deteriorates and deoxidation efficiency decreases. Therefore, the effect is small. Therefore, it is desirable that the upper limit of the amount of flux blown is 0.7%.

According to the second and third aspects of the present invention, as another method for controlling the CaO concentration in the non-metallic inclusions, Ca, C is added to the molten stainless steel in the ladle or the tundish.
This is a method of adding alloy a. In the present invention, one or both of Ca and Ca alloys are added for the purpose of finely adjusting the composition of the non-metallic inclusions after the flux blowing. Si, M
Since the addition of Ca and Ca alloys at high oxygen concentration level by only deoxidation of n is not stable and costly, after sufficiently reducing the oxygen concentration by the flux injection, C
a, Ca alloy is added.

4 and 5 show that C was added to the stainless molten steel in the container after the flux was blown by 0.3% with respect to the molten steel weight.
Fig. 4 shows the relationship between the amount of Ca and Ca alloy added when a and Ca alloys are added and the number of non-metallic inclusions after forging. Fig. 4 shows the case where the container is a ladle, and Fig. 5 shows the container is a tundish. Is the case. In addition, the horizontal axis represents the ratio of the added Ca content to the molten steel weight. If the addition amount is 0.01% or more of the molten steel weight in terms of Ca amount,
The number of non-metallic inclusions tends to decrease as compared with the case of only flux blowing, and it can be seen that the softening of non-metallic inclusions was further promoted.

As described above, CaO, Ca in the ladle
In the process of blowing the flux containing F ₂ as a main component together with the inert gas, by adjusting the [Al] and [O] concentrations of the molten steel before the treatment and the amount of the flux blown, preferably the amount of Ca and Ca alloy added. , Al ₂ O ₃ system and C
It has become possible to stably suppress the formation of r ₂ O ₃ -based hard nonmetallic inclusions and soften the nonmetallic inclusions.

[0024]

[Examples] SUS304 stainless steel was melted in a 60 ton electric furnace and refined in an AOD furnace, and then secondary refining was performed by blowing flux with argon gas in a ladle. Or C in the tundish
a, Ca alloy was added. The various steels obtained are
A 150 mm square billet was obtained by continuous casting, and then a wire rod having a diameter of 5.5 mm was obtained by rolling the wire rod, and nonmetallic inclusions and surface flaws of the wire rod were investigated. 1 cm ^{2 for} non-metallic inclusions
The area of the sample was observed with an optical microscope at a magnification of 500, and the width was 5μ.
The number of non-metallic inclusions of m or more was counted. The results are shown in Table 1.

In the present invention, by adjusting the [Al] and [O] concentrations and the amount of flux blown before the treatment, preferably Ca and Ca alloy addition, the Al ₂ O ₃ system and Cr are adjusted.
_{Since the generation of 2} O ₃ -based hard non-metallic inclusions can be stably suppressed, the non-metallic inclusions are reduced and softened, and the non-metallic inclusions after wire rolling are deformed or divided. Therefore, the number of nonmetallic inclusions having a width of 5 μm or more is extremely small, and the maximum flaw depth of the wire is also extremely shallow.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As described above, according to the present invention, in the process of blowing the flux containing CaO and CaF ₂ as a main component together with the inert gas into the molten stainless steel in the ladle, [Al] before the process is performed. By adjusting the concentration and [O] concentration and adjusting the amount of flux blown or the amount of Ca or Ca alloy added, it is possible to stabilize the formation of Al ₂ O ₃ -based and Cr ₂ O ₃ -based hard nonmetallic inclusions. Since it can be suppressed, it has become possible to manufacture stainless steel products with very few surface defects.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of [Al] concentration before treatment on the number of non-metallic inclusions after forging.

FIG. 2 is a diagram showing the effect of [O] concentration before treatment on the number of non-metallic inclusions after forging.

FIG. 3 is a diagram showing the influence of the amount of flux blown on the number of non-metallic inclusions after forging.

[Fig. 4] Effect of C on a ladle on the number of non-metallic inclusions after forging
The figure which shows the influence of the addition amount.

FIG. 5 is a diagram showing the effect of the amount of Ca added in the tundish on the number of non-metallic inclusions after forging.

Claims (3)

[Claims] 1. In refining molten stainless steel, [Al] in the molten steel is 30 ppm or less, and [O] is 60 to 120.
After refining to ppm and tapping in a ladle, a flux containing CaO and CaF ₂ as a main component in an amount of 0.3% or more of the molten steel weight is blown into the molten steel in the ladle together with an inert gas to carry out the secondary operation. A method for producing ultra-high cleanliness stainless steel, characterized by refining. 2. After the secondary refining, one or both of Ca and Ca alloy is added to the molten steel in the ladle in an amount of 0.01% or more of the weight of the molten steel.
A method for producing the described ultra-high cleanliness stainless steel. 3. After secondary refining, the molten steel in the ladle is received by a tundish for continuous casting, and one or both of Ca and a Ca alloy is added to the molten steel in the tundish to obtain a Ca content. The method for producing ultra-high cleanliness stainless steel according to claim 1, wherein 0.01% or more by weight is added.