JPH0143807B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for refining high S chromium-containing steel. [Prior Art] In the refining of chromium-containing steel, as in the refining of ordinary steel, the effect of removing S from the steel and the life of the refractory vary significantly depending on the basicity (CaO/SiO ₂ ) of the slag.
In general, increasing the basicity promotes S removal, and conversely, decreasing the basicity suppresses S removal. Regarding refractories, it is known that in a furnace using basic refractories, increasing the basicity reduces melting loss more than decreasing the basicity. By the way, in the refining of high S steel, it is necessary to suppress S removal, but if the basicity is lowered in an attempt to suppress S removal, a problem arises in that the refractory is eroded. As a method for solving this problem, for example, a method for refining S-based free-cutting steel described in Japanese Patent Application Laid-Open No. 54-38213 is known. In this method, hot metal [S]
When refining to 0.05%, the [S] reached is 0.04%, the (MgO) in the slag is 6 to 30%, and the slag basicity is 1.0 to 3.6. However, when the attained [S] is high, for example in the refining of chromium steel containing 0.1 to 0.5% [S] in the steel, the desulfurization effect fluctuates greatly due to fluctuations in slag basicity, so it is necessary to aim for the right amount of [S]. Therefore, basicity must be controlled to less than 1.2.
On the other hand, when refining chromium-containing steel, it is necessary to have a basicity of 1.2 or higher. In other words, in the refining of chromium-containing steel, oxidation of [Cr] in the steel increases as decarburization progresses. In order to suppress this oxidation of [Cr], it is sufficient to lower the CO partial pressure. As a method for efficiently decarburizing by lowering the partial pressure of CO, decarburization in a vacuum or dilution decarburization, in which diluent gas is simultaneously blown in with oxygen gas, is widely used. However, none of the above methods can completely eliminate the oxidation of [Cr], so a method is adopted in which after decarburization, a reducing agent (Fe-Si, etc.) is introduced to reduce and recover the oxidized [Cr]. ing. This [Cr]
The recovery ratio (hereinafter referred to as Cr yield) is strongly influenced by the basicity of the slag, and for efficient recovery, the basicity of the slag needs to be 1.2 or higher. In other words, if the slag basicity is set to less than 1.2 in the refining method of high S chromium-containing steel in order to target [S] in the steel, there will be problems such as significant erosion of the refractories and a significant decrease in the Cr yield. On the other hand, if an attempt is made to increase the slag basicity to 1.2 or more in order to improve the Cr yield and prevent the erosion of refractories, there is a problem in that the accuracy of [S] is reduced. [Problems to be Solved by the Invention] The present invention solves the above-mentioned problems in the refining method for high-sulfur-containing chromium steel. [S]
The present invention provides a refining method that can accurately achieve the target value. [Means and effects for solving the problems] The present invention is aimed at, in refining chromium-containing steel containing 0.1 to 0.5% [S], adjusting (S) in slag to 1.5 to 6.0 during refining, and It is characterized by refining so that the slag basicity (CaO/SiO ₂ ) after refining is 1.2 to 2.0. When refining chromium-containing steel in a converter or AOD furnace lined with basic refractories, the slag basicity at the end of reduction (hereinafter referred to as "after refining"), the amount of erosion of the furnace refractory, and Cr
Figure 1 shows the relationship between yields. Slag basicity is 1.2
If it is below, the melting loss of the furnace refractory will become significantly large, and the Cr yield will also decrease significantly. Furthermore, if the slag basicity exceeds 2.0, the Cr yield will decrease, which is disadvantageous in terms of cost. This is because when the basicity becomes high, the slag becomes hard and the progress of the reduction reaction is inhibited. Therefore, from the viewpoint of refractory erosion and Cr yield, the optimum slag basicity after refining is 1.2 to 2.0. Next, FIG. 2 shows the relationship between the basicity of the slag after refining and the desulfurization reaction. In FIG. 2, the distribution ratio ([S]/[S]) between [S] in the steel and (S) in the slag after the completion of refining is shown as an index of the S-removal reaction. The greater the S distribution ratio, the greater the progress of the S removal reaction. When the [S] in the steel after refining is 0.030% or less and the (S) in the slag during refining is 0.5% or less (○ mark in the figure), the S distribution ratio increases when the basicity becomes 1.2 or more. It can be seen that desistance is promoted. On the other hand, [S] in steel after refining is 0.1 to 0.5%,
When the (S) content in the slag during refining is 1.5 to 6.0% (marked with an x in the figure) and the basicity is in the range of 0.7 to 2.0, the fluctuation of the S distribution ratio after the completion of refining is as described above (marked with a circle in the figure). ), the relationship with slag basicity is not significant and the variation is small. On the other hand, if the (S) in the slag during refining is less than 1.5%, the distribution ratio (S) / [S] will increase when the basicity after refining increases, so the [S] in the steel will be lower than the target. This may result in poor accuracy.
Also, if the (S) in the slag during refining is 6.0% or more and the basicity after refining becomes low, the distribution ratio [S] / [S] will decrease, so the [S] in the steel will be higher than the target. This may result in poor accuracy. Therefore, in the slag before or at the start of refining (S)
If you adjust it to 1.5-6.0% in advance and maintain this range during refining, the distribution ratio (S)/
[S] is stable without significant increase,
The variation is the same as when the basicity is low, and in steel [S], the same accuracy as when the basicity is low can be obtained. [Example] The present invention will be explained based on an example. Example 1 In the refining of high S chromium steel in a 60Ton AOD furnace using basic refractories, the slag (S) at the start of refining was set at 3.5% and decarburized, and the slag basicity after refining was 1.6. Table 1 shows the average results of the embodiments of the invention and comparative examples using the conventional method. In Table 1, even though the AOD start [S] of the examples according to the present invention is significantly increased from 0.3% to 0.6% compared to the comparative example, the value of [S] after scouring is the same as 0.25%. This is because the slag basicity after refining is
This is because the basicity has increased from 0.9 to 1.6, so the desulfurization reaction has progressed during the reduction period, and the (S) in the slag is controlled at 3.0%, resulting in a distribution ratio (S)/
This is because [S] is stable, making it possible to accurately hit [S] in steel. Example 2 Example of the present invention and comparative example of conventional method when raw material is melted in a 60Ton electric furnace and high S chromium-containing steel is refined in a 60Ton AOD furnace, when 1% [S] is mixed in the raw material Table 2 shows the average performance. As shown in Table 2, in the example according to the present invention, the content of [S] increased from 0.5% to 1.0% compared to the comparative example, and since the S removal reaction progresses even in the electric furnace, the [S] after electric furnace refining has increased from 0.3% to 0.6%, but the content after AOD refining remains at the same value as 0.3%. This is because the basicity has become high from 0.8 to 1.4 after refining, so desulfurization has progressed during the reduction period, and since the (S) in the slag is controlled to 3.0%, the distribution ratio (S) )/[S] is stable and it has become possible to accurately hit [S] in steel.

【table】

【table】

〔Effect of the invention〕

Since the method for refining high S chromium-containing steel according to the present invention is configured as described above, it has the following effects. (1) Since the slag can be refined with high basicity, there is little erosion of the furnace refractories and the Cr yield is high. (2) By adjusting the (S) content in the slag to 1.5 to 6.0% in advance, it is possible to reduce the variation in the distribution ratio (S)/[S], and even refining with high basicity is equivalent to that with low basicity. You can hit [S] with the accuracy of . (3) Since it is possible to increase the proportion [S] and use inexpensive high-sulfur raw materials such as Ni matu and sulfide ore, the raw sulfur cost can be significantly reduced.

[Brief explanation of drawings]

Figure 1 shows slag basicity, amount of refractory erosion, and
A diagram showing the relationship between Cr yield and FIG. 2 is a diagram showing the relationship between slag basicity and distribution ratio (S)/[S].

Claims (1)

[Claims] 1. In the refining of chromium-containing steel containing 0.1 to 0.5% [S], (S) in the slag during refining is
A method for refining high S chromium-containing steel, which comprises adjusting the slag basicity (CaO/SiO 2 ) to 1.5 to 6.0% and refining the slag basicity (CaO/SiO ₂ ) to 1.2 to 2.0.