JP3510989B2 - Refining method of Si alloy iron and stainless steel used for refining stainless steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for refining Si alloy iron and stainless steel used for refining stainless steel, and particularly to trace amounts of Ca, Al and Mg in additives and slag after reduction of Cr ₂ O _3. By controlling the components, the formation of harmful hard nonmetallic inclusions in molten steel is suppressed.

[0002]

2. Description of the Related Art Many techniques for making harmless stainless steel inclusions are known. For example, JP-A-4-9921
In Japanese Patent Laid-Open No. 5 (1999), regarding a trace component in Fe-Si, non-metallic inclusion Al ₂ O ₃ generated by 2 wt% Al in ferrosilicon is suppressed by using ferrosilicon having a low Al concentration. There is. However, this technique is intended for Al, and the inclusion composition control by Ca is not shown. Further, inclusion of the control target is Al ₂ O _3, is not mentioned at all the Al ₂ O ₃ · MgO is detrimental nonmetallic inclusions as well as Al ₂ O _3.

Next, in Japanese Patent Laid-Open Publication No. 7-188861, a ferrosilicon having a low Al content is used and a Ca component is blown into a ladle together with nitrogen gas to produce Al ₂ O _3-.
The MnO-based inclusions are modified to have low melting points. However, the target of inclusions to be modified by this technology is Al ₂ O ₃ -M
No mention is made of Al ₂ O ₃ .MgO, which is nO and is found in highly clean steel with low oxygen levels.

On the other hand, Al ₂ O which is a harmful non-metallic inclusion
_As a technique for preventing the formation of 3.MgO, Japanese Patent Laid-Open No. 6-3
No. 06438 discloses the slag composition after secondary refining as Mg.
Generation of Al ₂ O ₃ .MgO is suppressed by controlling O concentration ≤ 7 wt%, Al ₂ O ₃ concentration ≤ 6 wt%, and slag basicity CaO / SiO ₂ : 1.3 to 1.9. Further, in JP-A-6-306439, Al of [wt% Mg] ≦ 1.0 is used as a reducing agent for slag, and Ca or Ca alloy is added to molten steel in a ladle or a tundish after tapping. As a result, Al ₂ which is a hard non-metallic inclusion is
O ₃ · MgO and MgO are low melting point CaO-Al ₂ O
_It is controlled to ₃ .

[0005] These techniques describe only Al deoxidizing as a deoxidizing agent that most affects the composition of inclusions, and do not describe Si deoxidizing which is usually performed in stainless steel. However, even in the deoxidation of Si, harmful non-metallic inclusions such as Al ₂ O ₃ .MgO are produced, which is the cause of defects. In addition, adding a large amount of deoxidizing element at a time close to the ingot (process in a ladle or tundish) leads to an increase in the number of inclusions, deteriorating the cleanliness or increasing the ladle refining time. Cause an increase.

[0006]

SUMMARY OF THE INVENTION The present invention suppresses the formation of harmful hard non-metallic inclusions Al ₂ O ₃ --MgO in stainless steel obtained through Si deoxidation, so that the subsequent rolling or It is intended to prevent the occurrence of surface flaws and work cracks during molding.

[0007]

The present invention controls the composition of Si alloy iron and the composition of slag after reduction of Cr ₂ O ₃ used as a reducing agent of slag and a deoxidizing agent of molten steel during refining of stainless steel. This suppresses the generation of harmful hard non-metallic inclusions Al ₂ O ₃ —MgO and prevents the generation of surface flaws and work cracks during rolling and forming.

That is, the gist of the present invention is as follows. (1) Fe: 10 to 60 wt% and the balance of 40 to 90 wt% of an unavoidable impurity element containing Si, Ca, Al and Mg, wherein Ca, Al and Mg in the impurities are [wt% Ca] ≦ 2.0, [wt% Al] ≦ 3.0 and [wt% Mg] ≦ 0.1, and S used for refining stainless steel characterized by satisfying the following formula:
i Alloy iron. Note 0.15 x ([wt% Al] + 3 x [wt% Mg])-0.
02 ≦ [wt% Ca]

(2) In the AOD refining of stainless steel, the Si alloy iron described in (1) above is used as a Cr ₂ O ₃ reducing agent for decarburized slag and a deoxidizing agent for molten steel, and C
By adding limestone as a flux and fluorite to the slag component after the reduction of r ₂ O ₃ ( wt% CaO ) / ( wt% Si
O ₂ ) : Adjusted in the range of 1.3 to 2.7, and
A method for refining stainless steel, characterized by adjusting within the range of the formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0

(3) In AOD refining of stainless steel, the Si alloy iron described in (1) above is used as a Cr ₂ O ₃ reducing agent for decarburized slag and a deoxidizing agent for molten steel, and C
After removing the slag after the reduction of r ₂ O ₃ , limestone as a flux and fluorite were added to the residue ( wt% C
aO ) / ( wt% SiO ₂ ) : Adjusted within the range of 1.3 to 2.7
In addition, the method for refining stainless steel is characterized by adjusting within the range of the following formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0

(4) In VOD or RH refining of stainless steel, the Si alloy iron described in the above (1) is used as the Cr ₂ O ₃ reducing agent of the slag after vacuum decarburization and the deoxidizing agent of molten steel, and Cr is used. By adding limestone as a flux and fluorite, the slag component after reduction of ₂ O ₃ ( wt% Ca
O ) / ( wt% SiO ₂ ) : Adjust to the range of 1.3 to 2.7
In addition, the method for refining stainless steel is characterized by adjusting within the range of the following formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0

[0012]

The Si alloy iron of the present invention is used as a deoxidizing agent for molten steel and / or a reducing agent for slag.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventors have investigated in detail the flaws of a stainless steel plate in which surface flaws and work cracks have occurred during rolling and forming, and found that the cause of surface flaws and work cracks is Al.
It was found that it is a non-metallic inclusion containing ₂ O ₃ —MgO. The inclusions containing Al ₂ O ₃ —MgO have a high melting point, tend to coarsen due to agglomeration, and are hard nonmetallic inclusions, and thus become the starting point of flaws and cracks. Based on the above findings, to prevent the formation of harmful non-metallic inclusions, low melting soft nonmetallic inclusions having a melting point of about _{1300~1400 ℃, CaO-SiO 2 -Al} 2 O 3 - A refining method in which MgO-based inclusions are preferentially generated was examined. The examination results will be described in detail below.

First, the procedures for producing a stainless steel ingot are summarized in Tables 1 to 3. That is, Table 1
The AOD refining shown in 1) is the most common refining method for stainless steel. That is, after melting in an electric furnace and adding each component element, decarburization, Cr ₂ O ₃ reduction, deoxidation, and component adjustment with AOD, and then temperature adjustment and component fine adjustment with a ladle refining device A steel ingot can be obtained by casting molten steel in a continuous casting machine or an ordinary ingot.

[0016]

[Table 1]

Another AOD refining shown in Table 2 is applied when the sulfur concentration in the molten steel is high and desulfurization is required. That is, after melting in an electric furnace and adding each component element, AOD
Decarburization, Cr ₂ O ₃ reduction, slag removal, flux addition, deoxidation, and component adjustment, then temperature adjustment and component fine adjustment with a ladle refining device, and continuous casting machine or ordinary ingot casting A steel ingot is obtained by casting.

[0018]

[Table 2]

The VOD refining shown in Table 3 is particularly applicable to the production of low carbon, low nitrogen steel. That is, after melting in an electric furnace and adding each component element, rough decarburization is performed with AOD, vacuum decarburization, Cr ₂ O ₃ reduction, deoxidation, and component adjustment are performed with VOD or RH, and a ladle is prepared. A steel ingot is obtained by adjusting the temperature and finely adjusting the components in a refining apparatus, and casting the molten steel in a continuous casting machine or an ordinary ingot.

[0020]

[Table 3]

Of the factors that influence the composition of inclusions, the most important is the composition of the deoxidizer, particularly Al, Ca and Mg in Si alloy iron, and the slag composition after reduction of Cr ₂ O ₃ , especially ( wt% CaO) / (wt% SiO ₂ ), (wt%
Al ₂ O ₃ ) and (wt% MgO), and the relationship between these factors and the composition of nonmetallic inclusions was investigated.

FIG. 1 shows the relationship between Ca, Al and Mg in Si alloy iron and the composition of inclusions. Al ₂ O ₃ -MgO
Is produced by the presence of Al and Mg in Si alloy iron, and by the presence of Ca, Al ₂ O ₃ -Mg
It can be seen that the generation of O is suppressed. However, Ca
Is 2.0 wt% or more, (wt% CaO) in the inclusions becomes high, which causes defects in black dots during welding. Therefore, Ca must be 2.0 wt% or less. Also,
Even if [wt% Al]> 3.0 and [wt% Mg]> 0.1, Si
Although it is possible to suppress Al ₂ O ₃ —MgO by Ca in the alloy, since it causes an increase in the number of inclusions due to over-deoxidation and an increase in the manufacturing cost of the Si alloy, unless necessary as a component,
It is desirable that [wt% Al] ≦ 3.0 and [wt% Mg] ≦ 0.1. The black dots in the figure are the values obtained by adding Ca alloy by ladle refining (LF) and converting it to [wt% Ca] in Si alloy. That is, the Ca deficiency of the Si alloy can be supplemented by the Ca alloy.

From the above results, Ca and A in Si alloy iron
The contents of 1 and Mg should be restricted within the range of the dotted line shown in FIG. 1, that is, [wt% Ca] ≦ 2.0, [wt% A
l] ≦ 3.0 and [wt% Mg] ≦ 0.1, and by limiting the ratio to satisfy the following expression (1), the generation of Al ₂ O ₃ —MgO can be suppressed. I understand. In addition, the shortage of Ca in the Si alloy can be supplemented by adding the Ca alloy. 0.15 x ([wt% Al] + 3 x [wt% Mg]) -0.02 ≤ [wt% Ca] --- (1)

The next most important factor is the basicity of the slag.
It Slag is mainly CaO-SiO.₂・ CaF₂
A mixed melt of MgO and Al in some cases
₂O₃May be included. This slag is
Then, add limestone and fluorite and deoxidize with Si alloy iron
Product SiO₂By forming a melt with
To generate. In addition, MgO should be used as a refractory protection
Al added as₂O ₃Is a trace element in Si alloy iron
It is generated by the oxidation of certain Al.

In FIG. 2, Si adjusted to the above component range is used.
The relationship between the basicity (wt% CaO) / (wt% SiO ₂ ) of Cr ₂ O ₃ reduced slag and the composition of inclusions when iron alloy is used is shown. Here, (wt% CaO) means limestone ( wt %
% Ca O), as well as Ca in fluorspar ( 56/78 × Ca
It contains CaO derived from F ₂ ). From the figure, (wt% Ca
It can be seen that when O) / (wt% SiO ₂ ) is in the range of 1.3 to 2.7, the production of Al ₂ O ₃ —MgO is suppressed.

FIG. 3 shows the relationship between the composition of slag containing Al ₂ O ₃ and MgO and the composition of inclusions. From the figure, it is clear that Al ₂ O ₃ —MgO inclusions are formed outside the range surrounded by the dotted line. Therefore, the following equation (2)
It is necessary to adjust the slag after the reduction of Cr ₂ O ₃ within the range of and to perform refining. {(Wt% Al ₂ O ₃ ) + (wt% MgO)} + (wt% CaO) / (wt% SiO ₂ ) × 10.0-38.0 ≦ 0 ---- (2) However, 1. 3 ≦ (wt% CaO) / (wt% SiO ₂ ) ≦
2.7

In addition, it is inevitably contained in Si alloy iron,
The influences of Al, Mg and Ca on the composition of inclusions are considered as follows. First, Al in Si alloy iron reacts with O in molten steel and lower oxides (SiO ₂ , MnO, Cr ₂ O ₃ ) in nonmetallic inclusions to form Al ₂
O ₃ -MgO constitutes Al ₂ O ₃ , and
MgO in the slag is reduced, and the reduced Mg reacts with the inclusions to produce MgO that constitutes Al ₂ O ₃ —MgO. Similarly, Mg in Si alloy iron also reacts with O in molten steel and lower oxides in non-metallic inclusions to form Al ₂ O ₃ —MgO.
Becomes MgO which constitutes Therefore, the higher Al and Mg in the Si alloy, the easier it is to produce Al ₂ O ₃ —MgO.

Similarly, Ca reacts with O in molten steel and lower oxides in non-metallic inclusions to form CaO-SiO ₂ -A ₁₂ O.
_3- CaO is formed which constitutes a part of the MgO-based non-metallic inclusion. Since Ca is easier to form an oxide than A1 or Mg, even if it is present in the molten steel in a trace amount, Ca is present in the non-metallic inclusions.
aO is present, and as a result, Al ₂ O ₃ −
This will suppress the production of MgO.

The influence of the slag composition is considered as follows. That is, as the slag basicity increases, the oxygen potential in the molten steel decreases, the activity of MgO in the slag increases, MgO in the slag decomposes, and dissolved Mg is generated in the molten steel. And dissolved Mg
Reacts with the inclusions, resulting in (wt% MgO) in the inclusions.
Rises and Al ₂ O ₃ —MgO is produced. On the other hand, when (wt% CaO) / (wt% SiO ₂ ) is less than 1.3, the solubility of MgO in slag increases, so MgO
Even if the addition amount of Al is small, (wt% MgO) in the slag rises due to melting loss from the refractory, the activity of MgO rises, and by the same mechanism, Al ₂ O ₃ -MgO inclusions are removed. It is easy to generate. Similarly, when the contents of Al ₂ O ₃ and MgO in the slag exceed 2.7, the activities of these oxides increase and Al ₂ O ₃ —MgO inclusions are easily generated.

[0030]

Example: 60 tons of stainless steel was melted in an electric furnace,
In AOD or VOD, various Si alloy irons are used as a reducing agent for Cr ₂ O ₃ of slag and a deoxidizing agent for molten steel, and the slag after Cr ₂ O ₃ reduction is basically adjusted. Refining was performed in each step shown in 3. Then, after casting with a continuous casting machine, hot rolling and then cold rolling were performed to manufacture a stainless steel plate having a thickness of 1 mm. The number of surface defects of the 1 mm-thick stainless steel sheet thus obtained and the number of internal defects that cause work cracking were investigated. The results are shown in Tables 4-6. The surface defects were visually observed, and the internal defects were measured by an ultrasonic flaw detector.

That is, according to the steps shown in Table 1, the table
The Si alloy iron shown in 4 is the slag of Cr ₂O₃Reducing agent and
Used as a deoxidizer for molten steel and Cr₂O₃Of slag after reduction
Adjustment is Cr₂O₃After reducing, analyze the composition of the slag,
CaO, CaF so that the target slag composition is obtained₂Such
It was carried out by throwing in the flux. In Table 4, No. 4
-1 to 4-6 are comparative examples, No. 4-7 to 4-12 are
Here is a clear example. No. 4-1, 4-5, 4-9, 4-12
This is an example of supplementing Ca deficiency in Si alloy by adding Ca alloy.
It is expressed in terms of Ca concentration in Si alloy iron.

In addition, according to the steps shown in Table 2,
Shows Si alloy iron as slag Cr₂O ₃Reducing agent and melt
Used as a deoxidizer for steel, Cr₂O₃Adjustment of slag after reduction
Is Cr₂O₃After reduction, analyze the composition of the slag and remove the slag.
The Ca residue so that the target slag composition is obtained
O, CaF₂It was done by adding flux such as. Table 5
No. 5-1 to 5-6 are comparative examples, and No. 5-
7 to 5-12 show invention examples. No. 5-1, 5-5, 5
-9, 5-12 are the Ca alloy shortage in the Si alloy iron
It is an example supplemented by addition, converted to Ca concentration in Si alloy iron
Is written.

Further, according to the steps shown in Table 3, Table 6
The Si ferroalloy shown in with the deoxidizer of the reducing agent and the molten steel Cr ₂ O ₃ in the slag, adjusting the slag after Cr ₂ O ₃ reduction is carried out composition analysis of the slag after Cr ₂ O ₃ reduction, A flux such as CaO or CaF ₂ was added so that the desired slag composition was obtained. In Table 6, No. 6-
Nos. 1-6 to 6-6 are comparative examples, and Nos. 6-7 to 6-12 are inventive examples. No. 6-1, 6-5, 6-9, 6-12 is S
This is an example in which the Ca deficiency in the i alloy is supplemented by the addition of the Ca alloy, and is expressed in terms of the Ca concentration in the Si alloy iron.

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

As is clear from Tables 4 to 6, in the invention examples, generation of harmful Al ₂ O ₃ --MgO composite inclusions was prevented, and as a result, generation of surface defects and internal defects could be avoided. .

[0038]

According to the present invention, A contained in a trace amount
It is possible to provide a Si-alloy iron in which the concentrations of 1, Mg and Ca are advantageously controlled, and further to use the Si-alloy iron for refining stainless steel, and to control the slag composition after the Cr ₂ O ₃ reduction period. As a result, it is possible to suppress the generation of harmful hard non-metallic inclusions in the molten steel, so that it is possible to provide a stainless steel that is free from surface defects and work cracks during rolling and forming.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between [wt% Ca], [wt% Al] and [wt% Mg] in a Si alloy and the composition of inclusions.

FIG. 2 Slag basicity (wt% CaO) / (wt% Si
3 is a graph showing the relationship between O ₂ ) and the composition of inclusions.

FIG. 3 is a graph showing the relationship between slag composition and inclusion composition.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 33/04 C22C 33/04 C 38/00 302 38/00 302Z 38/06 38/06 (72) Inventor Hideki Tanaka Kanagawa Prefecture No. 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi Nippon Metallurgical Industry Co., Ltd. Kawasaki Plant (72) Inventor Shinichi Sasayama No. 4-2 Kojima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Nippon Metallurgy Co., Ltd. Kawasaki Plant (56) References JP 53-11112 (JP, A) JP 2000-1718 (JP, A) JP 10-158720 (JP, A) JP 9-104912 (JP, A) JP 11-50127 ( JP, A) JP-A-3-211214 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21C 7/04 C21C 7/00 C21C 7/072 C21C 7/10

Claims (5)

(57) [Claims] 1. Fe: 10 to 60 wt%, balance 40 to 90
It consists of inevitable impurity elements containing wt% of Si and Ca, Al and Mg, and Ca, Al and M in the impurities.
Stainless steel characterized in that g is in the range of [wt% Ca] ≦ 2.0, [wt% Al] ≦ 3.0 and [wt% Mg] ≦ 0.1 and satisfies the following formula: Si alloy iron used for refining. 2. The Si according to claim 1, which is used as a deoxidizing agent for molten steel and / or a reducing agent for slag.
Alloy iron. 3. In the AOD refining of stainless steel, the Si alloy iron according to claim 1 is used as a Cr ₂ O ₃ reducing agent for decarburizing slag and a deoxidizing agent for molten steel, and after reducing Cr ₂ O ₃ Of the slag component of limestone as a flux and fluorite ( wt% CaO 2 ) / ( wt% SiO ₂ ) :
A method for refining stainless steel, characterized by adjusting the range of 1.3 to 2.7 and adjusting the range of the following formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0 4. In the AOD refining of stainless steel, the Si alloy iron according to claim 1 is used as a Cr ₂ O ₃ reducing agent for decarburized slag and a deoxidizing agent for molten steel, and after Cr ₂ O ₃ reduction. Slag was removed, and then limestone as a flux and fluorite were added to the residue ( wt% CaO ) / ( wt% SiO ₂ ) : 1.3-
A method for refining stainless steel, characterized in that the range is adjusted to the range of 2.7 and the range of the following formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0 5. In the VOD or RH refining of stainless steel, the Si alloy iron according to claim 1 is used as a Cr ₂ O ₃ reducing agent for the slag after vacuum decarburization and a deoxidizing agent for the molten steel, and Cr ₂ O is used. ₃ Add the slag component after reduction to limestone as flux and fluorite ( wt% CaO ) / ( w
t% SiO ₂ ) : Adjusted within the range of 1.3 to 2.7.
In addition, a method for refining stainless steel, characterized by adjusting to the range of the following formula . Serial _{{(wt% Al 2 O 3} ) + (wt% MgO)} + (wt% Ca
O) / ( wt % SiO ₂ ) × 10.0-38.0 ≦ 0