JP2000345231A - Secondary refining method of molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To refine molten steel without entirely using fluorite (CaF2) in desulfu rizing slag by controlling the composition of secondary refining slag into speci fied relations when the steel containing not more than specified concns. of Al and S in the molten steel produces. SOLUTION: When the steel having <=0.005 mass % Al and <=10 ppm S in the molten steel produces, the composition of the secondary refining slag is controlled so as to satisfy the relations of (%Al2O3)<-010.(%CaO)/(% SiO2)+52.(1+0.05*(%TiO2)), (%Al2O3)+0.5.(%TiO2)>2.(%CaO)/(%SiO2), (% TiO2)<-2.(%CaO)/(%SiO2)+15, (%TiO2)>3 and (%F)=0. Activity of (Al2O3) is reduced with (TiO2) contained in the slag. The desulfurizing in the molten steel is executed by absorbing the sulfur in the molten steel into the slag. At this time, the sulfur is distributed into the slag and the molten steel, but the distributing value of the sulfur is necessary to secure >= about 250.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a steel sheet having excellent hydrogen-induced cracking resistance and excellent low-temperature toughness useful for line pipes used in petroleum and natural gas containing H ₂ S. Secondary refining slag for desulfurization of molten steel without using fluorite (CaF ₂ ) when removing sulfur concentration in molten steel to an extremely low concentration of 0.001 mass% or less under low [Al] concentration conditions The composition relates to:

[0002]

2. Description of the Related Art In the steel industry, steel plates for structural use and steel pipes for line pipes have been manufactured. At that time, in order to reduce S, desulfurization has been performed using slag at the time of secondary refining after tapping of the converter.

[0003] In the desulfurization reaction, the reaction of (CaO) + [S] = (CaS) + [O] is promoted by reacting slag mainly composed of CaO with molten steel. 2) It is necessary to reduce the oxygen activity [O] in the molten steel. In order to reduce the [O], the molten steel before desulfurization must have A
Addition of l has been done.

[0004] In this specification, [X] indicates a component X in molten steel, and (Y) indicates a component Y in slag.

In addition, in order to desulfurize, it is necessary to liquefy the slag, and a small amount of about 5% of fluorite (CaF ₂ ) has been usually added. Fluorite reacts with solid lime
This is because fluorine cuts the network of Ca and O to produce a compound having a low melting point of CaO—CaF ₂ . However, in recent years,
There has been a demand to reduce the amount of fluorine released into the environment, and a refining method that does not use fluorite is required.

[0006] On the other hand, research is also being conducted to deoxidize with titanium by lowering [Al] and control the material by using titanium oxide generated by titanium deoxidation. In this case, titanium oxide is precipitated during and after solidification for material control. Therefore, it was necessary to control [O] in the molten steel to a rather high value, and thus no active desulfurization was performed in the molten steel stage.

Further, Japanese Patent Application Laid-Open No. 7-41823 discloses a test result in which the concentration of Al ₂ O ₃ , SiO ₂ and CaO in slag is adjusted to secure the desulfurization ability and the liquid phase ratio. Also in this case, the desulfurization treatment was stabilized by adding a small amount of fluorite (CaF ₂ ) to the desulfurized slag. In other words, when desulfurization is performed without using any fluorite (CaF ₂ ), the desulfurization sometimes becomes unstable.

[0008]

In the case where desulfurization is performed by secondary refining of molten steel without using any fluorite (CaF ₂ ) in the desulfurized slag and the [S] concentration after desulfurization is set to 0.001 mass% or less. There is a need for a method for secondary refining of molten steel for performing stable desulfurization. At this time, since the material control of the steel material is performed by the titanium oxide, the Al concentration in the molten steel needs to be low, and needs to be 0.005 mass% or less.

[0009]

That is, the gist of the present invention is to provide a secondary refining slag which has a composition of not more than 0.005 mass% of Al and not more than 10 ppm of S in molten steel. This is a secondary refining method for molten steel controlled as follows. (% Al ₂ O ₃ ) <− 10 · (% CaO) / (% SiO ₂ ) + 52 · (1 + 0.05 * (% TiO ₂ )) (1) (% Al ₂ O ₃ ) + 0.5 · (% (TiO ₂ )> 2 · (% CaO) / (% SiO ₂ ) (2) (% TiO ₂ ) <− 2 · (% CaO) / (% SiO2) +15 (3) (% TiO ₂ )> 3 ( 4) And (% F) = 0 (5)

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a secondary refining method for desulfurizing molten steel in the production of steel that requires an Al content in a product of not more than 0.005%.

Prior to determining the composition of the desulfurized slag,
(% Al ₂ O ₃ ) concentration in slag and (% CaO), (% SiO ₂ ) to control aluminum concentration in molten steel to 0.005mass% or less
The relationship between the concentrations was examined.

As a result, as disclosed in JP-A-7-41823, when the (% CaO) / (% SiO ₂ ) concentration is high and the (% Al ₂ O ₃ ) concentration is high, (Al ₂ O ₃ ) is reduced and [A
l] Since the concentration is high, it has been found that when (% TiO ₂ ) = 0, the (% Al ₂ O ₃ ) concentration in the slag needs to satisfy the following condition. (% Al ₂ O ₃ ) <− 10 · (% CaO) / (% SiO ₂ ) +52 (6)

However, in the slag (TiO 2_Two) Is included
High (% Al) exceeding the above formula (6)_TwoO_Three) Concentration
The aluminum concentration in molten steel must be less than 0.005 mass%.
I knew it. This is (TiO_Two) Is included in the slag
(Al_TwoO_ThreeIt is considered that the activity of ()) is reduced.
Specifically, (TiO_Two) Is included (% Al_TwoO
_ThreeThe condition which should be satisfied is expressed by the following equation. (% Al_TwoO_Three) <− 10 · (% CaO) / (% SiO_Two) +52 ・ (1 + 0.05 * (% TiO_Two)) (1)

[0014] Desulfurization in molten steel is carried out by absorbing sulfur in the molten steel into slag. At this time, the sulfur is distributed to the slag and the molten steel, respectively. For example, when the S concentration in the molten steel is set to 10 ppm or less, it is necessary to secure the distribution value of the sulfur by (S) / [S] of about 250 or more. Was sought from the mass balance between molten steel and slag.

To reach this distribution value: 1) Increase the desulfurization capacity of the slag. That is, the CaO activity in the slag is increased, and the oxygen activity in the molten steel is reduced. 2) Increase the liquid phase ratio of the slag. It is necessary.

Conventionally, the above 2) was addressed by increasing the (Al ₂ O ₃ ) concentration or adding a small amount of fluorite, but the former is restricted by the formula (1) due to the regulation of the aluminum concentration in molten steel. Yes, at most about 20% is the upper limit. Further, fluorite cannot be used from the gist of the present invention.

In the present invention, in order to increase the liquid phase ratio,
The use of titanium oxide was considered. Titanium oxide has been conventionally studied as a slag slagging agent at the time of dephosphorization of molten steel.For example, ilmenite (titanium ore whose main component is TiO ₂ -Fe ₂ O ₃ ) is experimentally added. But had no noticeable effect.

Furthermore, no phase diagram was found for the behavior of TiO ₂ when the (% CaO) concentration was high.

Therefore, the inventors studied the amount of titanium oxide, the fluidity of slag, and the desulfurization ability.

As a result, it was found that the liquid phase ratio of slag was improved and the desulfurization ability was improved by setting (% TiO ₂ ) to 3% or more. On the other hand, regarding the limit on the high concentration side of (% TiO ₂ ), when (% CaO) / (% SiO ₂ ) is about 2.5, Ti
O ₂ 20% or less, in the case of (% CaO) / (% SiO 2) of about 5-6
The liquid phase can be maintained at (% TiO ₂ ) below 15%. This is because TiO ₂ is Ca
It is thought to break the network of molecular bonds between O and SiO ₂ . However, when the concentration was higher than that, a high melting point precipitated phase of CaO and TiO ₂ was generated, and the liquid phase ratio was sharply reduced.

Therefore, (% TiO ₂ ) is expressed by the following equation (3) in relation to (% CaO) / (% SiO ₂ ). (% TiO ₂ ) <-2 · (% CaO) / (% SiO ₂ ) +15 (3)

Also, as (% TiO ₂ ) increased, the desulfurization ability improved. This reduces the activity of (MnO) and (FeO), which remains in the slag and affects the oxygen activity of the molten steel, thereby lowering the oxygen concentration in the molten steel and increasing the desulfurization capacity. It is considered that this has been improved.

Further, the (% Al ₂ O ₃ ) concentration also affects the desulfurization ability,
It was effective to make the concentration equal to or more than the following equation (2). (% Al ₂ O ₃ ) + 0.5 · (% TiO ₂ )> 2 · (% CaO) / (% SiO ₂ ) (2) and (% TiO ₂ )> 3 (4)

In the present invention, since no fluorite is used, the following equation is further satisfied. (% F) = 0 (5) Conventionally, when slag formation was poor, some fluorite was added at the operator's discretion. However, in the case of the present invention using no fluorite, some fluorite was added. It is considered that the addition of MgO is also effective for slag slag formation.

[0025]

[Example] [C] = 0.01-0.07mass%, [Si] = 0.05-0.5mass%,
[Mn] = 0.8-1.5mass%, [P]: 0.015mass% or less, [Al]: 0.005m
Desulfurization treatment of ass% or less molten steel was performed. The desulfurizing agent was added to the molten steel in the ladle during tapping. The basic unit of desulfurization slag is 12
It was about kg / ton, and fluorite was not added. Agitation of the molten steel during the desulfurization of the molten steel was performed by blowing Ar gas into the molten steel. Deoxidation was performed by adding Ti during the blowing of Ar gas. Table 1 shows the composition of the produced desulfurized slag. The oxygen concentration of the molten steel was about 30 ppm.

An experiment was also conducted in which lime powder was blown together with Ar gas during the stirring of the molten steel. In this case, although the treatment time could be shortened slightly, the ultimate S concentration was determined by the slag composition, and there was no difference in desulfurization ability depending on the presence or absence of lime powder injection for the same slag composition.

[0027]

[Table 1]

In the examples shown in Table 1, the slag composition satisfies the above formulas (1) to (4), and the obtained molten steel components are as shown in Table 1 above. [S] concentration became 0.001 mass% or less.

FIG. 1 shows (% Al ₂ O ₃ ) and (%%) for each (% CaO) / (% SiO ₂ ).
It shows the range of the appropriate range ([S] <0.001%) for desulfurization of TiO ₂ ).

[0030]

According to the present invention, secondary refining without using fluorite can be performed, and one of the steel making methods that does not pollute the environment has become possible.

[Brief description of the drawings]

Fig. 1 (% Al ₂ O at each slag basicity ((% CaO) / (% SiO ₂ ))
₃ ) shows the appropriate range ([S] <0.001%) for desulfurization of (% TiO ₂ ).

Claims (1)

[Claims] 1. A secondary refining method for molten steel in which the composition of secondary refining slag is controlled as described below when producing steel with 0.005 mass% or less of Al and 10 ppm or less of S in molten steel. (% Al ₂ O ₃ ) <− 10 · (% CaO) / (% SiO ₂ ) + 52 · (1 + 0.05 * (% TiO ₂ )) (1) (% Al ₂ O ₃ ) + 0.5 · (% (TiO ₂ )> 2 · (% CaO) / (% SiO ₂ ) (2) (% TiO ₂ ) <− 2 · (% CaO) / (% SiO ₂ ) +15 (3) (% TiO ₂ )> 3 (4) And (% F) = 0 (5)