JPS621444B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for refining stainless steel,
In particular, the objective is to advantageously achieve effective desulfurization during the reduction period following the oxidation period in which oxidizing gas is blown in, while also extending the life of the furnace wall refractories of the smelting furnace. Generally, when refining stainless steel, the refining furnaces used include AOD furnaces, bottom blowing converters (Q-BOP), top and bottom blowing converters (K-BOP), and LD converters. It is carried out in a process like this. That is, a Cr-containing bath obtained by melting in a melting furnace is transferred to a smelting furnace, and a mixed gas of O ₂ gas and an inert gas such as N ₂ or Ar is blown into the molten iron to remove carbon and remove carbon. Following the oxidation process (oxidation period) in which Si, etc. The process moves to a reduction process (reduction period) in which chromium is reduced and recovered, and molten iron is also desulfurized, and thus an oxidation period and a reduction period exist during one blowing period. By the way, in the refining of stainless steel, as mentioned above, inert gas is injected during the reduction period,
There is also a matching problem with the next process of RH treatment or continuous casting, and at the end of the oxidation period, it is necessary to maintain the molten iron temperature at a high temperature of 1,700 to 1,750℃, but this is due to the fact that the furnace wall refractory The conditions are harsh for the The first object of the present invention is to extend the life of the furnace by suppressing the increase in the amount of wear of the refractory as much as possible even under such adverse conditions of the refractory. Additionally, during the reduction period, auxiliary raw materials are added to desulfurize to a predetermined S concentration;
Burnt lime and lightly burnt dolomite have been added to bring the CaO/SiO ₂ ratio to about 1.5, and efforts have also been made to increase MgO in the slag to reduce wear and tear on the refractories. However, by increasing the amount of light calcined dolomite,
Simply using CaO-SiO ₂ -MgO-based slag does not stabilize slag formation during the reduction period, leaving a problem in that the S concentration in the molten iron cannot be sufficiently reduced. The second objective of this invention is to stabilize the slag state during the reduction period and achieve effective desulfurization.
The purpose of The inventors conducted numerous experiments and studies to achieve the above objectives, that is, extend the life of the furnace and stabilize desulfurization. As a result, the inventors found that by adjusting the type and amount of auxiliary materials input during the reduction period, the slag can be improved. It was newly discovered that the above object can be advantageously achieved by controlling the composition and production amount of , and based on this new knowledge, the present invention was completed. In other words, this invention provides a method for refining stainless steel by the single slug method, when charging the auxiliary raw material into the refining furnace during the reduction period following the oxidation period by blowing oxidizing gas. CaO,
Regarding MgO and _CaF2 , in the slag in the smelting furnace
Depending on the amount of _SiO2 , CaO and MgO at a ratio of CaO/ _SiO2 = 1.5-2.0 MgO/ _SiO2 = 0.3-0.5, and _CaF2 at a ratio of _CaF2 concentration in slag = 3-5% by weight, The solution to the above problem is to charge each of them, and the above refining furnace is an AOD furnace, Q-BOP or K-
BOD is particularly advantageously suited. The experimental results that led to this invention will be explained below. All experiments were conducted using K-BOP, which has a furnace capacity of 85 tons and is lined with magnesia dolomite bricks. FIG. 1 shows the relationship between the S concentration in molten iron and the actual basicity (CaO/SiO ₂ Obs) after completion of reduction refining. From the figure, in order for the S concentration to be sufficiently low [S]≦0.009%, CaO/SiO ₂ obs
It can be seen that it is sufficient to set it to ≧1.5. Figure 2 shows the results of an investigation into the relationship between the invalid CaO rate, expressed as (calculated basicity - actual basicity)/calculated basicity, that is, the proportion of CaO that is not effectively utilized, and the CaF ₂ concentration in the slag. This relationship is important for minimizing the amount of slag and for stabilizing the actual basicity. From the figure, the ratio of CaF ₂ to the slag weight is 3
% or more, it can be seen that the invalid CaO rate decreases. Figure 3 shows (calculated MgO/SiO ₂ -real MgO/
The results of an investigation of the relationship between the undissolved MgO percentage expressed as SiO ₂ )/calculated MgO and calculated MgO/SiO ₂ are shown below. If the undissolved MgO ratio is a negative value, it means that MgO is definitely eluted from the furnace wall refractories. Therefore, in order to suppress the elution of MgO as much as possible, the value of the undissolved MgO ratio needs to be positive, and in this point, if the calculated MgO/SiO ₂ is 0.3 to 0.5,
The undissolved MgO ratio can be made to a positive value without wasting any. Next, Figure 4 shows that the _CaF2 concentration in the slag is 3 to 5.
The relationship between the invalid CaO rate and the calculated basicity when expressed as % is shown. As is clear from the figure, as the calculated basicity increases, the invalid CaO rate increases. However, looking at the relationship between calculated basicity and actual basicity, when the CaF ₂ concentration in the slag is 3% or more, from Figure 4, when calculated CaO / SiO ₂ = 1.5, the invalid CaO rate ≒ 0, that is, calculated CaO / SiO 2 = 1.5. _SiO2
and real CaO/SiO ₂ are approximately equal, so the calculation
It was found that when CaO/SiO ₂ = about 1.5 to 2.0, actual CaO/SiO ₂ ≧1.5 can be achieved without increasing the invalid CaO ratio. From the above results shown in Figures 1 to 4, in order to achieve both effective desulfurization and improvement of furnace life, CaO/SiO ₂ ≧1.5, MgO/SiO ₂ = 0.3 to 0.5,
It was revealed that the auxiliary raw materials should be charged so that the CaF ₂ concentration in the slag is ≧% by weight. However, even if the above-mentioned conditions are satisfied, if the amount of auxiliary raw materials charged is too large, problems in refining will increase, such as an extension of the refining time and a corresponding drop in molten iron temperature. Therefore, in this invention, the input standard for the auxiliary raw material is set on the amount of _SiO2 in the slag at the end of the oxidation period, and this
It was determined as follows depending on the amount of _SiO2 . CaO: CaO/SiO ₂ = 1.5 to 2.0 MgO: MgO/SiO ₂ = 0.3 to 0.5 CaF ₂ : 3 to 5% by weight in slag Thus, stable desulfurization and improvement of furnace life are achieved.
This was effectively achieved without causing an unnecessary increase in the amount of slag. Next, examples of the present invention will be described in comparison with comparative examples. A Cr-containing bath housed in a K-BOP with a furnace capacity of 85 tons and lined with magnesia-dolomite bricks was subjected to oxidation refining according to a conventional method, and then auxiliary materials in the proportions shown in Table 1 were charged into the furnace. Reduction smelting was performed, and the S concentration and refractory unit index at the end of smelting were investigated, and the results are also shown in Table 1.
The refractory consumption index is based on the case where ordinary steel refining and stainless steel refining are performed in the same converter, and in all examples and comparative examples, the ratio of stainless steel refining heat to the total refining heat in one furnace charge is Approximately 45%
It is about.

[Table] In Table 1, Comparative Example 1 is a case in which CaO and MgO are slightly higher than the appropriate range and the amount of _CaF2 is small, and desulfurization is unstable despite the large amounts of CaO and MgO. be. In Comparative Example 2, the amount of MgO was lower than the appropriate range, and although the desulfurization ability was good, the refractory was severely worn out. Furthermore, Comparative Example 3 is a case where MgO and CaF ₂ are lower than the appropriate range, and both the desulfurization ability and the refractory unit consumption are unsatisfactory. On the other hand, in Example 1 according to the present invention, both the desulfurization ability and the refractory unit consumption after the completion of refining were good, and stable desulfurization was achieved without increasing wear on the refractories.

[Brief explanation of the drawing]

Figure 1 shows the S concentration and CaO/ after reduction refining.
Graph showing the relationship with SiO ₂ obs, 2nd
The figure is a graph showing the relationship between the invalid CaO rate and the CaF ₂ concentration in slag. Figure 3 is a graph showing the relationship between the undissolved MgO rate and calculated MgO/SiO ₂ (MgO/SiO ₂ cal). The figure shows invalid CaO rate and calculation
It is a graph showing the relationship with CaO/SiO ₂ (CaO/SiO ₂ cal).

Claims (1)

[Claims] 1. In refining stainless steel by the single slug method, when charging the auxiliary raw material into the refining furnace during the reduction period that follows the oxidation period by blowing oxidizing gas, the auxiliary raw material Regarding CaO, MgO and CaF ₂ in the slag, CaO, MgO are added at a ratio of CaO/SiO ₂ = 1.5 to 2.0 MgO/SiO ₂ = 0.3 to 0.5, depending on the amount of SiO ₂ in the slag in the slag. A method for refining stainless steel, characterized in that CaF ₂ is charged at a medium CaF ₂ concentration of 3 to 5% by weight. 2 The refining furnace is an AOD furnace, a bottom-blowing converter, or an upper
Claim 1, which is any bottom blowing converter
The method described in section.