JPH0456083B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a method for refining (decarburizing) stainless steel using an AOD furnace, a converter, or the like. Conventional technology and its problems Stainless steel refining using AOD furnace, converter, etc.
This is done by blowing oxygen gas and inert gas such as argon into the steel bath under atmospheric pressure, but bottom blowing AOD
In the case of this method, since O ₂ and Ar gas are blown through the bottom blowing tuyere, there is a limit to the blowing flow rate and the refining time is long. In addition, in the case of the top-bottom blowing AOD method using bottom-blowing tuyeres and top-blowing lances, pure oxygen top-blowing,
With argon or oxygen gas bottom blowing, the refining time is short, but the oxidation loss of Cr is large. Also, argon,
Top blowing of a mixed gas of oxygen, bottom blowing of a mixed gas of argon and oxygen solves the problem of Cr oxidation loss, but the control range of the mixture ratio of oxygen gas and argon gas in the top blowing mixed gas is narrow, and expensive argon gas is used. need to be used in large quantities. Furthermore, in refining where oxygen gas and inert gas are injected at a certain ratio,
If a large amount of gas is strongly blown into the molten steel, the Cr in the molten steel will be oxidized and the yield will deteriorate, and if a small amount of gas is blown into the molten steel weakly to prevent oxidation, refining will take a long time. Purpose of the Invention The present invention was made in view of the above-mentioned conventional problems, and is a method for decarburizing stainless steel that prevents oxidation of Cr, increases the oxidation yield of steel, and shortens decarburization time. The purpose is to propose the following. Structure of the Invention In other words, the present invention provides a method for decarburizing stainless steel by blowing an inert gas together with oxygen gas into a steel bath under atmospheric pressure. The hole is for mixed gas of inert gas, and the side hole around the center hole is for oxygen gas, and the value α of oxygen gas amount/inert gas amount is adjusted to the range shown in the table below according to the carbon content in the molten steel. It is characterized by being blown into the air.

[Table] The method of this invention will be explained in detail below.
Here, as equipment for carrying out the method of this invention,
This will be explained using an AOD furnace as an example. Figure 1 shows the decarburization method using an AOD furnace, and Figure 2 shows the decarburization method using an AOD furnace.
The double-flow type top-blowing lances used for refining in AOD furnaces are shown. The AOD furnace 1 is equipped with O ₂ gas and inert gas (hereinafter referred to as "Ar gas") on the side wall of the bottom of the molten steel 2 stored in the furnace.
A tuyere 3 is provided for blowing in (represented by ),
A top blowing lance 4 for refining is installed in the upper part so as to be movable up and down. During refining, the upper blowing lance 4 is lowered into the furnace and O ₂ gas 5 or a mixed gas 6 of O ₂ gas and Ar gas is blown into the furnace, and a mixed gas of O ₂ gas and Ar gas is also blown from the tuyere 3. 6 is blown into the AOD furnace 1, and refining is performed while sucking the exhaust gas with an exhaust gas suction cover 7 that covers the upper part of the AOD furnace 1. In this invention, the above-mentioned top blowing lance for refining 4
It is characterized in that the refining process is performed by replacing the lance 14 with a double flow type lance 14 shown in FIG. This double flow type top-blowing lance 14 has a center hole 14-1 and a plurality of holes 14-2 arranged around the center hole 14-1, and a mixed gas 6 of O ₂ gas and Ar gas flows through the center hole 14-1. Only the O ₂ gas 5 is allowed to flow through the outer holes 14-2. In this way, by using the center hole of the lance for a mixed gas of O ₂ gas and Ar gas, and using the surrounding porous holes for O ₂ gas, the amount of O ₂ gas and Ar gas amount can be reduced depending on the carbon content of the molten steel 2. and [O ₂ gas amount/Ar gas amount] values can be adjusted, improving the efficiency of decarburization refining. Furthermore, this double flow lance injects a mixture of O ₂ gas and Ar gas from the center hole.
By diluting the oxygen potential with Ar gas
The oxidation loss of Cr is suppressed, and the mixed gas jet blown vertically from the center hole penetrates deeply into the molten steel surrounded by the oxygen gas jet from the side holes around the center, which reduces the decarburization reaction and Cr in the molten steel. Effective in preventing oxidation. In addition, the oxygen gas jet from the side holes around the center hole is effectively diffused into the furnace, so it acts effectively on normal decarburization blowing and secondary combustion. In Figure 3, the horizontal axis shows the carbon content (wt%) in molten steel.
The vertical axis shows the amount of Cr oxidation loss (Kg/
T-st) and inject _O2 gas and Ar into the molten steel.
The figure shows the relationship between the carbon content in molten steel and the amount of Cr oxidation loss by varying the gas ratio [O ₂ gas amount/Ar gas amount = α]. Furthermore, this result is
This is the case of SUS430. In other words, in molten steel with a high carbon content, the amount of Cr oxidation loss is small even if the amount of oxygen gas blown is large, but
Since the amount of Cr oxidation loss tends to increase as the carbon content decreases, in the case of molten steel with a low carbon content, the ratio of the amount of O ₂ gas must be reduced. Therefore, as is well known _, the decarburization reaction due to O ₂ gas injection is 2C + O ₂ → 2CO. The lower the CO partial pressure (Pco) and the higher the O ₂ partial pressure, the more rapidly the decarburization reaction will proceed and the decarburization refining time will be shortened. Based on this knowledge, this invention is based on the knowledge that when decarburizing stainless steel, if the carbon content in molten steel is high, O ₂
By increasing the gas injection amount as much as possible to shorten the decarburization time and reduce the carbon content in the molten steel.
This method aims to reduce the amount of Cr oxidation loss while sufficiently stirring the molten steel by decreasing the amount of O ₂ gas blown and increasing the amount of Ar gas blown. That is, based on the results shown in Fig. 3, the present invention adjusts the [O ₂ gas amount/Ar gas amount] value within the range shown in Table 1 (hatched area in Fig. 3) according to the carbon content in molten steel. This is a method of decarburizing, and in order to easily adjust the gas amount, a double flow type top blowing lance shown in Fig. 2 is used, and O ₂ gas and Ar gas are mixed from the center hole 14-1 of the lance. It was decided to blow in the gas 6 and the _O2 gas 5 through the outer holes 14-2. Moreover, according to this double flow type lance, a wide adjustment range of the value of [O ₂ gas flow rate/Ar gas flow rate] can be obtained. In this invention, the reason why [O ₂ gas flow rate/Ar gas flow rate] is adjusted to the range shown in Table 1 according to the carbon content in molten steel is that if the upper limit of each range is exceeded,
This is because increasing the O ₂ gas flow rate not only hardly increases the decarburization rate but also oxidizes the Cr in the molten steel and increases the amount of loss.Also, below the lower limit, the oxidation of Cr in the molten steel decreases. , the decarburization reaction is slow;
This is because the decarburization treatment time is extended, resulting in deterioration of treatment efficiency. Example: Decarburization of SUS430 series stainless steel by 90 tons AOD
When processing in a furnace, 60 tons of molten steel with the initial composition shown in Table 2 (molten steel composition before high carbon ferrochrome addition) is
The mixture was stored in an AOD furnace, and while a mixed gas of O ₂ gas and Ar gas was blown in from the tuyeres at the bottom of the furnace, O ₂ gas and Ar gas were blown in using the top blowing lance shown in Figure 2. At that time, the blowing ratio of the O ₂ gas and Ar gas [O ₂ gas flow rate/Ar gas flow rate] was adjusted according to the estimated carbon content. Table 3 shows the relationship between the carbon content in the molten steel and [O ₂ gas flow rate/Ar gas flow rate] at that time. Note that high carbon ferrochrome was introduced from the hopper above the furnace at the beginning of blowing while blowing was being carried out. The results of the molten steel composition after decarburization in this example are shown in the fourth table.
The table shows the history of decarbonization in Figure 4. Table 4 also shows the results of decarburization using the conventional method shown in Table 3. From the results in Table 4, it can be seen that compared to the conventional method, the oxidation loss of Cr is reduced, so less ferrosilicon is needed for reduction, and the decarburization time is also greatly shortened.

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[Table] Effects of the invention As explained above, according to the method of this invention,
Since the oxygen delivery rate can be increased without increasing Cr oxidation loss, decarburization time can be shortened, and a wide range of inert gas ratios can be selected, minimizing Cr oxidation loss. This has a great effect on the refining of stainless steel.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the decarburization method using an AOD furnace.
Figure 2 is a vertical cross-sectional view showing the structure of a top blowing lance for carrying out the method of this invention, and Figure 3 shows decarburization using the same lance and varying the value of [ _O2 gas amount/Ar gas amount]. FIG. 4 is a chart showing the relationship between the carbon content in molten steel and the amount of Cr oxidation loss. 1...AOD furnace, 2...molten steel, 3...tuyere, 4
... Lance, 5 ... O ₂ gas, 6 ... Mixed gas,
14...Double flow type lance.

Claims (1)

[Claims] 1. In a method for decarburizing stainless steel by blowing inert gas together with oxygen gas into a steel bath under atmospheric pressure, the top blowing lance is of a double flow type,
The center hole is used for a mixed gas of oxygen gas and inert gas, and the side holes around the center hole are used for oxygen gas, and the value of oxygen gas amount / inert gas amount is determined according to the carbon content [C] in molten steel. A method for decarburizing stainless steel, which is characterized by adjusting α to the range shown in the table below and blowing into the stainless steel. 【table】