JP2855333B2 - Modification method of molten steel slag - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for reforming molten steel slag, and in particular, taps molten steel smelted in a refining furnace such as a converter into a ladle. The slag reforming method in the slag reforming process of the high cleanliness steel subjected to the secondary refining of the vacuum degassing after the slag reforming is applied. Used in

[Conventional technology]

In recent years, demands for high cleanliness steel are increasing, and high cleanliness steel is generally manufactured by secondary refining by vacuum treatment of molten steel such as RH and DH.

This vacuum treatment of RH, DH, etc. is extremely effective in degassing and promoting the floating of inclusions, but oxides such as FeO, MnO, which are present in large amounts in the slag, are subjected to vacuum treatment of RH, DH, etc. In addition, there is a problem that it reacts with Al and the like in molten steel to form minute inclusions and exists in steel. That is, the slag flowing into the ladle during tapping of the converter is an oxidizing slag having a high concentration of FeO and MnO, which reacts with Al in the steel before casting to form Al ₂ O ₃ . This Al ₂ O ₃ remains in the steel as nonmetallic inclusions and adversely affects the quality of the product.

Therefore, slag reforming that renders slag flowing into a ladle (hereinafter referred to as a ladle slag) harmless during tapping is required for smelting high-cleanliness steel.

Although many studies have been disclosed in the past as a method of ladle slag reforming, Japanese Patent Application Laid-Open No. Sho 59-70710 discloses a method of adding almost a necessary amount of a deoxidizing agent as a product at the time of tapping from a steelmaking furnace, and This is a method in which a flux is added to promote the combined floating of the acid products, or a method in which a slag reducing agent for further modifying the oxidizable slag is added. In addition,
60-152611 discloses a method in which a gas generating substance is added together with a slag reducing agent to stir the slag.

However, when ladle slag is reduced by these methods, Fe
Not only O and MnO but also P ₂ O ₅ and SiO ₂ contained in slag
To reduce the concentration of P and Si, which are harmful elements, in steel.

[Problems to be solved by the invention]

An object of the present invention is to solve the drawbacks of the prior art relating to the reforming of ladle slag, and to reduce the amount of harmful elements such as P and Si while suppressing FeO, MnO and the like. An object of the present invention is to provide a method for reforming slag.

[Means and actions for solving the problems]

The gist of the present invention is as follows. That is, in a method of reforming molten steel slag which reforms oxidizing slag floating on molten steel blown in a converter and tapped into a ladle, the molten iron having a P concentration of 0.030% by weight or less in the converter blowing. And a step of performing blowing with the basicity of the blow-stop slag to 5.5 or more, and adding a metal Al or a reducing agent containing metal Al onto the slag flowing into the ladle after tapping from the converter. And a process for modifying molten steel slag.

The details of the present invention will be described with reference to the accompanying drawings. In reforming the ladle slag, if the P concentration in the hot metal before charging the converter is reduced, the amount of P removed during blowing decreases, so the amount of P transferred into the slag also decreases. Therefore, the present inventors consider that it is possible to suppress the pick-up of P from the slag to the molten steel when slag is reduced by this, and to blow the molten iron in the converter using hot metal having various P concentrations. After that, slag reduction was performed, and the amount of return P at the time of slag reduction was investigated. The results are as shown in FIG. In the experiment shown in FIG. 1, the P concentrations in the hot metal were 0.02%, 0.04%, and 0.
06% and 0.08% of hot metal is blown and returned to the steel when the slag is reduced to the slag on the molten steel by tapping into the ladle with the same amount of metallic Al as 1kg / t as a reducing agent. It is a survey of phosphorus content.

As is clear from FIG. 1, the P concentration of the hot metal used for converter blowing is 0.030% or less, so that the amount of rephosphorized iron in the steel is a P level that does not affect the operation and the steel quality.
It turned out that it can be less than 0.002%. Therefore, in the present invention, the use of pretreated hot metal having a P concentration of 0.030% by weight or less in the converter blowing is limited.

Next, when the hot metal with low P concentration is blown in a converter,
Since the amount of dephosphorization during blowing can be small, the basicity of slag
It was thought that there would be no particular problem in operation even if CaO / SiO ₂ was increased and the slagging property deteriorated, so a test to increase the basicity of slag and reduce the amount of Si pickup during slag reduction. Went into operation. That is, the basicity CaO /
In the case of ladle slag reforming for molten steel blown with various changes of SiO ₂ from 3 to 8, as in the previous test, when using 1 kg / metal Al of molten steel t as a slag reducing agent in each case The amount of Si picked up in molten steel was investigated. The results are as shown in FIG. As is clear from FIG. 2, by setting the basicity of the blow-up slag of the converter to 5.5 or more, the amount of Si pick-up in the molten steel during the reforming reduction of the ladle slag can be made 0.01% or less. There was found. Therefore, in the present invention, the basicity Ca of the converter blow-up slag
It was limited to blowing with O / SiO ₂ of 5.5 or more.

The Si pick-up amount of 0.01% or less is within a range where no problem occurs in operation and steel quality at all, and even when the basicity CaO / SiO ₂ of the blow-up slag is 5.5 or more, the blow-off P concentration is as follows. Confirmed that no problem occurred.

Further, when the basicity of the blow stop slag is set to 5.5 or more according to the present invention, the melting point of the slag is high, and part of the slag solidifies during tapping and remains in the furnace, so that the amount of slag flowing out to the ladle is reduced. third, as shown in FIG gradually becomes less with increasing basicity CaO / SiO _2, the outflow amount of slag in the case basicity of 5.5 or more is 3 kg /
It is less than molten steel t.

The present inventors have found that the amount of slag effluent (kg /
Phosphorus recovery during slag reduction with respect to change in molten steel t) (%)
Was investigated. The results are as shown in FIG.
As is clear from FIG. 4, if the amount of effluent slag is small, the amount of rephosphorus will naturally be small.
It was also found that there was a secondary effect that was less than 002%.

〔Example〕

P: 0.025% by weight of pre-treated hot metal, with the basicity of the blow-off slag CaO / SiO ₂ = 6.5. To perform slag reforming according to the present invention, together with P: 0.060
Comparative Example 1 and Comparative Example 2 in which molten steel with a basicity of 4.5 g and 6.3 in the blow stop slag was used, respectively, using hot metal of 0.062 weight% and 0.062 weight%, and P: 0.023 weight% of pretreated hot metal Used, basicity of blow slag CaO / SiO ₂ =
Regarding Comparative Example 3 of 4.6, slag reduction was performed by the same method in which the same amount of metal Al of 1 kg / t was added to molten steel as a slag reducing agent, and a comparative test was performed to compare the results. The results are as shown in Table 1.

As is clear from Table 1, in Comparative Example 1, since the P concentration of the hot metal used in the converter blowing was high, the rephosphorization amount was as high as 0.005%, and the basicity of the blowoff slag was as low as 4.5. Pick-up amount is as high as 0.03%. Furthermore, since the amount of slag flowing out of the converter is large, T.Fe after the reforming treatment is 4.5
% And high.

In Comparative Example 2, since the basicity of the blow stop slag met the requirements of the present invention, Δ [% Si] was as low as 0.01% or less, and the slag amount was small. Fe is 3.
Although it is as low as 2, Δ [% P] is high because the P concentration of the hot metal used is as high as 0.062%.

In Comparative Example 3, Δ [% Si] was high due to the low basicity of blow slag being 4.6, and the slag outflow from the converter was large. Δ
[% P] is increasing.

On the other hand, Examples of the present invention satisfying all the requirements according to the present invention have Δ [% P] = 0.001% and Δ [% Si] = 0.01
% Or less, indicating that the pickup amounts of P and Si are kept extremely low. As is clear from each of the above comparative examples, even if one of the requirements of the present invention is satisfied, Δ [% P] Δ [% Si] unless all other requirements are satisfied.
In addition, a satisfactory value cannot be obtained in any of the T.Fe% after the modification treatment. In other words, the reforming method that satisfies all the requirements according to the present invention always has a small amount of oxides, suppresses the pickup of harmful elements such as P and Si, and enables effective reforming of ladle slag. It has been found.

〔The invention's effect〕

As is apparent from the above-described embodiment, the present invention has been made in view of the disadvantage that not only FeO and MnO are reduced but also harmful elements P and Si are reduced by the conventional ladle slag reforming method. Uses pretreated hot metal with a P concentration of 0.030% by weight or less in converter blowing and basicity of blow-off slag CaO / Si
Since the method of adding metal Al or a reducing agent containing metal Al to the ladle slag after performing blowing with O ₂ of 5.5 or more was adopted, the following effects were obtained.

(B) The amount of pick-up of harmful elements such as P and Si during ladle slag reforming can be minimized.

(B) T.Fe etc. in ladle slag can be reduced, and the amount of Al ₂ O ₃ produced by the reaction between Al in steel and FeO and MnO in slag decreases, resulting in deterioration of steel quality. The resulting non-metallic inclusions could be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the P concentration of the hot metal used for converter blowing in the experiment for obtaining the present invention on the rephosphorization amount during ladle slag reforming, and FIG. Basicity CaO / Si
Diagram showing the effect of O _{2 on} the amount of Si pick-up (%) during ladle slag reforming. FIG. 4 is a graph showing the effect of the amount of slag (kg / t of molten steel) flowing out of the converter on the amount of reconstituted phosphorus at the time of ladle slag reforming.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-169320 (JP, A) JP-A-51-12321 (JP, A) JP-A-55-76013 (JP, A) (58) Field (Int. Cl. ⁶ , DB name) C21C 7/00 F27D 15/00

Claims (1)

(57) [Claims] 1. A method of reforming molten steel slag for reforming oxidized slag floating on molten steel blown in a converter and discharged into a ladle, wherein a P concentration of 0.030% by weight is used in the converter blowing. A step of performing blowing with the basicity of the blow-stop slag of 5.5 or more using the following hot metal, and a reducing agent containing metal Al or metal Al on the slag flowing into the ladle after tapping from the converter. And a step of adding slag.