JP6953652B1 - Refining method for chromium-containing molten steel - Google Patents

Abstract

The first step of blowing a gas containing oxygen gas into the container as a pressure in the range of 400 torr or less, the second step of depressurizing the inside of the container to more than 200 torr and 400 torr or less and blowing the gas containing oxygen gas, and the inside of the container. It has a third step of reducing the pressure to 200 torr or less and blowing gas, and is characterized by adjusting the MgO concentration in the slag at the end of vacuum refining to 8 to 15% by mass and the chromium oxide concentration to 28 to 50% by mass. This is a refining method for chrome-containing molten steel. It is preferable to have the above-mentioned slag component at the start of vacuum refining and during vacuum refining.

Description

The present invention relates to a method for refining a chromium-containing molten steel, in which a gas containing oxygen gas is blown into the chromium-containing molten steel in a refining container to perform refining.

When refining chrome-containing steel, especially chrome-containing steel containing 11% or more of chromium, such as stainless steel, oxygen gas or a mixed gas of oxygen gas and inert gas is mixed in the molten steel contained in the smelting container. A method of decarburizing and refining by the blowing AOD method is widely used. In the AOD method, when the decarburization progresses and the [C] concentration in the molten steel decreases, [Cr] is easily oxidized. Therefore, as the [C] concentration decreases, Ar gas or the like in the blown gas A method has been adopted in which the ratio of the inert gas is increased to suppress the oxidation of [Cr]. However, in the low [C] concentration range, the decarburization rate decreases, so that it takes a long time to reach the desired [C] concentration, and the ratio of the inert gas in the blown gas is increased, which is expensive. Since the consumption of the non-active gas is significantly increased, it is economically disadvantageous.

As a method of promoting decarburization in such a low [C] concentration range, the use of a vacuum refining method can be mentioned. In Patent Document 1, oxygen gas or a mixed gas of oxygen gas and an inert gas is supplied as a blowing gas, and decarburization is performed under atmospheric pressure until the [C] concentration in the molten steel drops to 0.5% by mass. After the treatment and the [C] concentration drops to this value or less, a method of decarburizing the inside of the container by reducing the pressure to 200 torr or less is disclosed. As a result, the treatment is performed under reduced pressure from a relatively high [C] concentration, and the decarburization treatment is performed with a mixed gas with oxygen gas under reduced pressure. Therefore, the same oxygen supply amount is used to improve the decarbonizing efficiency. The decarburization rate can be improved, the Si basic unit for reduction and the expensive inert gas basic unit can be reduced, and the refining time can be shortened. It is said that the reason why the pressure inside the container in the depressurization treatment is 200 torr or less is that the decarboxylation efficiency decreases at a pressure higher than this.

Patent Document 2 describes, in a refining method in which a gas containing oxygen gas is blown into a chromium-containing molten steel in a refining container to perform refining, a gas containing oxygen gas is blown into the container as a pressure in the range of 400 torr to atmospheric pressure. It has a second step of depressurizing the inside of the container to 250 torr to 400 torr and blowing a gas containing oxygen gas, and a third step of depressurizing the inside of the container to 250 torr or less and blowing the gas. A method for refining molten steel is disclosed. In the medium coal region, particularly in the region of [C] 0.2 to 0.5%, high decarboxylation efficiency can be obtained even at a pressure of 250 to 400 torr by performing strong stirring of the molten steel. Further, the generation of dust can be suppressed by setting the pressure of the decompression operation to the range of 250 to 400 torr instead of the conventional pressure of 200 torr or less. Further, by setting the pressure in the range of 250 to 400 torr, the amount of bottom-blown gas blown can be increased, and as a result, the refining time can be shortened.

Patent Document 3 states that in a method for refining chromium-containing molten steel in which decarburization under atmospheric pressure is followed by decarburization under vacuum, the decarburization under vacuum is started during slag (Cr ₂ O _3). ) The concentration is 30 mass% or less, the degree of vacuum is 200 Torr or less, oxygen gas or a mixed gas of oxygen gas and inert gas is used as the blowing gas, and [C] is inactive as the blowing gas after the concentration is lowered. A vacuum decarburization treatment method for chromium-containing molten steel, which is characterized by supplying only gas, is disclosed.

Japanese Unexamined Patent Publication No. 6-287629 Japanese Unexamined Patent Publication No. 2001-286694 Japanese Unexamined Patent Publication No. 6-287628

According to the invention described in Patent Document 2, in the medium coal region, particularly in the region where the [C] concentration is 0.2 to 0.5% by mass, strong stirring of the molten steel results in high decarboxylation even at a pressure of 250 to 400 torr. Carbonate efficiency is obtained. Further, the generation of dust can be suppressed by setting the pressure of the decompression operation to the range of 250 to 400 torr instead of the conventional pressure of 200 torr or less. Further, by setting the pressure in the range of 250 to 400 torr, the amount of bottom-blown gas blown can be increased, and as a result, the refining time can be shortened.

Further, if the reduction of refractory melting damage and the reduction of the amount of reducing agent used can be achieved at the same time, the refining cost can be significantly reduced.

INDUSTRIAL APPLICABILITY The present invention achieves both reduction of refractory melting damage and reduction of the amount of reducing material used in a refining method for chrome-containing molten steel in which a gas containing oxygen gas is blown into the chrome-containing molten steel by reducing the pressure inside the smelting container. The purpose is to provide a refining method that can be used.

That is, the gist of the present invention is as follows.
(1) In a refining method in which a gas containing oxygen gas is blown into a chromium-containing molten steel in a refining container to perform refining.
The first step of blowing a gas containing oxygen gas as a depressurizing pressure in the range of atmospheric pressure and / or a pressure less than 400 torr inside the container, and depressurizing the inside of the container to more than 200 torr and 400 torr or less and blowing a gas containing oxygen gas. It has a second step and a third step of depressurizing the inside of the container to 200 torr or less and blowing gas, and then the inside of the container is set to atmospheric pressure.
Switching from the first step to the second step when the [C] concentration in the molten steel is 1.5 to 0.1% by mass, and from the second step when the [C] concentration in the molten steel is 0.5 to 0.1% by mass. Switch to the third step,
The MgO concentration in the slag at the end of vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
A refining method for chromium-containing molten steel that achieves both reduction of refractory melting damage and reduction of the amount of reducing agent used.
(2) Further, the MgO concentration in the slag at the start of vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
A method for refining chromium-containing molten steel, which achieves both reduction in refractory erosion damage and reduction in the amount of reducing agent used as described in (1) above.
(3) Further, the MgO concentration in the slag during vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
A method for refining chromium-containing molten steel, which achieves both reduction in refractory erosion damage and reduction in the amount of reducing agent used as described in (2) above.

In the method for refining chromium-containing molten steel, the present invention divides the pressure in the furnace into the first step to the third step and sequentially reduces the pressure. By adjusting the slag component of the above, it is possible to realize a refining method for chrome-containing molten steel that achieves both reduction of refractory melting damage and reduction of the amount of reducing material used.

It is a figure which shows an example of the refining container of this invention, and is the figure which shows the state at the time of decompression refining. It is a figure which shows an example of the refining container of this invention, and is the figure which shows the state at the time of atmospheric pressure refining.

The present invention relates to a refining method in which a gas containing oxygen gas is blown into a chromium-containing molten steel in a refining container to perform refining. For example, the refining vessel 1 shown in FIG. 1A is used when performing vacuum refining, and the refining vessel 1 shown in FIG. 1B is used, for example, when performing atmospheric pressure refining. The molten steel 4 is housed in the refining container 1, a slag 6 is formed on the molten steel 4, and the bottom blowing gas 5 is blown into the chromium-containing molten steel in the refining container through the bottom blowing tuyere 2. Further, the smelting container 1 has a removable exhaust hood 3, and during decompression smelting, the smelting container 1 is equipped with the exhaust hood 3 as shown in FIG. 1A, and gas is sucked through the exhaust pipe 7. The pressure inside the smelting container is reduced. Since the exhaust hood 3 is not attached as shown in FIG. 1B during atmospheric pressure refining, it is possible to blow the gas by using not only the bottom blowing tuyere 2 but also the top blowing lance 12. Hereinafter,% with respect to the [C] concentration means mass%.

In the method for refining a chromium-containing molten steel of the present invention, the first step of blowing a gas containing oxygen gas into the container as a depressurizing pressure in the range of atmospheric pressure and / or less than 400 torr, and the inside of the container of more than 200 torr and 400 torr or less. It has a second step of reducing the pressure and blowing a gas containing oxygen gas, and a third step of reducing the pressure inside the container to 200 torr or less and blowing the gas. Switching from the first step to the second step when the [C] concentration in the molten steel is 1.5 to 0.1%, and from the second step to the third step when the [C] concentration in the molten steel is 0.5 to 0.1%. Switch to step. After the completion of the third step, the inside of the container is returned to atmospheric pressure and a reduction treatment is performed.

By arranging the second step in the medium coal region and at the same time strongly stirring the molten steel, the decarboxylation efficiency in this medium coal region can be maintained at a high value, and the generation of dust can be further suppressed. Become. By setting the pressure in the range of more than 200 torr and 400 torr or less in the second step, the amount of bottom-blown gas blown can be increased, and as a result, the refining time can be shortened. The bottom blowing gas blowing speed is ^{preferably 0.4 Nm 3} / min or more per ton of molten steel. As a result, strong stirring for obtaining high decarboxylation efficiency at a pressure of more than 200 tor can be realized, and the refining time can be shortened. At a pressure of more than 200 tor, the blowing speed of the bottom blowing gas is molten steel. ^{Even if it is 0.4 Nm 3} / min or more per ton, the amount of dust generated can be suppressed to a low level. More preferable results can be obtained when the bottom blowing gas blowing speed is ^{more than 0.5 Nm 3 / min per ton of molten steel.}

The time to shift from the first step in which the pressure in the smelting vessel is more than 400 torr to the second step in which the pressure is more than 200 torr and less than 400 torr is when the [C] concentration in the molten steel is 1.5 to 0.1%. In the [C] region where the [C] concentration is higher than 1.5%, even if decompression refining is performed, refining can be performed more efficiently by setting the pressure to a pressure higher than 400 torr and increasing the oxygen gas blowing rate. This is because it is possible to secure a high oxygen gas blowing rate and perform refining efficiently by performing atmospheric pressure refining and also using top-blown oxygen gas blowing. On the other hand, if refining is continued at a pressure exceeding 400 torr to the [C] region where the [C] concentration is lower than 0.1%, the decarboxylation efficiency is lowered and the refining time is extended, which is not preferable.

The time to shift from the second step where the pressure in the smelting vessel is more than 200 torr and 400 torr or less to the third step where the pressure is 200 torr or less is when the [C] concentration in the molten steel is 0.5 to 0.1%. do. By setting the [C] region where the [C] concentration is higher than 0.5% to a pressure of more than 200 torr and 400 torr or less in the second step, the effects of the present invention such as improvement of refining efficiency and reduction of dust generation can be sufficiently achieved. This is because it can be demonstrated. On the other hand, if refining is continued at a pressure exceeding 200 torr to the [C] region where the [C] concentration is lower than 0.1%, the decarboxylation efficiency is lowered and the refining time is extended, which is not preferable.

In the first step, when decompression refining below atmospheric pressure is performed from the beginning of the first step, the start time of the first step is the start time of decompression refining. Atmospheric pressure at the beginning of the first step, and when decompression refining below atmospheric pressure is performed from the middle of the first step, the transition to decompression refining is the start of decompression refining. When the entire first step is atmospheric pressure, the start time of the second step is the start time of decompression refining.
The end of the third step is the end of decompression refining.

The method for refining a chromium-containing molten steel of the present invention is characterized in that the MgO concentration in the slag at the end of vacuum refining is adjusted to 8 to 15% by mass and the chromium oxide concentration is adjusted to 28 to 50% by mass. As a result, it is possible to obtain a refining method for chromium-containing molten steel that achieves both reduction in refractory melting damage and reduction in the amount of reducing agent used. It is preferable to have the above-mentioned slag component even at the start of vacuum refining. It is more preferable to have the above slag component even during vacuum refining.

In the refining of chrome-containing molten steel, two or more types of oxides containing _{CaO, MgO, SiO 2} , Al ₂ O _{3 or CaF 2} are charged into the furnace as auxiliary materials, and these are at the end of refining. It becomes a component of the slag. Further, Cr, Mn, Si, and Al in the molten steel are oxidized by oxygen refining, and the formed oxide becomes a component of slag at the end of refining. Here, the chromium oxide refers to _{Cr 2} O _3.

The inventors attempted to increase the MgO concentration in the slag for the slag component during vacuum refining. The above conditions of the present invention were applied to the transition of the pressure in the furnace during refining. For the purpose of increasing the MgO concentration in the slag, one or more of MgO alone or an oxide containing MgO was added into the refining container during refining. As a result, at least by setting the MgO concentration in the slag at the end of decompression refining to 8 to 15% by mass, the chromium oxide concentration is adjusted to 28 to 50% by mass, and the refractory erosion loss is reduced and the reducing material is reduced. It was found that the usage amount can be reduced.
If the MgO concentration in the slag is 8% by mass or more at least at the end of decompression refining, the refractory wear due to the molten slag is reduced, so that the refractory melt loss can be reduced. It is more preferable that the MgO concentration in the slag is 10% by mass or more. It is more preferable that the MgO concentration in the slag is 12% by mass or more. On the other hand, when the MgO concentration in the slag exceeds 15% by mass, the melting point of the slag rises and the fluidity of the slag significantly decreases, so that the decarburization reaction using the chromium oxide in the slag as an oxygen source is inhibited. Therefore, the upper limit of MgO concentration in the slag was set to 15% by mass at the end of vacuum refining.
Further, at least at the end of vacuum refining, the chromium oxide concentration in the slag is adjusted to 28 to 50% by mass. When the chromium oxide concentration is 28% by mass or more, the fluidity of the slag is lowered, and the infiltration of the liquid phase slag into the refractory is suppressed, so that the wear of the refractory can be suppressed. It is more preferable that the chromium oxide concentration is 30% by mass or more. On the other hand, when the chromium oxide concentration is 50% by mass or less, the amount of the reducing agent used for reducing the chromium oxide after oxygen refining can be reduced. It is more preferable that the chromium oxide concentration is 48% by mass or less. It is more preferable that the chromium oxide concentration is 45% by mass or less.
By adopting the above slag composition at the end of decompression refining, it is possible to reduce the amount of reducing agent used.
In the present invention, it is preferable that the slag component is present not only at the end of vacuum refining but also at the start of vacuum refining. It is more preferable to have the above slag component even during vacuum refining.

A preferred embodiment of vacuum refining will be described.
For the first step, either the case of refining the whole under atmospheric pressure, the case of refining the whole under reduced pressure below atmospheric pressure, or the case of initially refining under atmospheric pressure and then under reduced pressure is adopted. You may. When refining under atmospheric pressure in the first step, since the exhaust hood 3 for decompression refining is not installed on the refining container, both top blowing and bottom blowing can be used together as gas blowing. When refining under atmospheric pressure in the first step, only oxygen can be used as the gas to be blown. The first step of refining can be carried out initially under atmospheric pressure and then under reduced pressure at a pressure of over 400 torr. Decompression refining may be carried out from the beginning of the first step.

The bottom-blown gas blown gas type in the second step may be a mixed gas of oxygen and an inert gas from the beginning of the second step, but the oxygen gas is blown alone at the beginning, and the inert gas is sequentially blown in the second step. It may be a pattern for increasing the ratio. The pressure in the smelting vessel in the second step can be maintained at a constant pressure within a range of more than 200 torr and 400 torr or less, but it may be a pattern in which the pressure is gradually changed from high pressure to low pressure.

In the third step, the inside of the container is depressurized to 200 torr or less and gas is blown. As the [C] concentration in the molten steel decreases, the optimum pressure inside the vessel for obtaining high decarboxylation efficiency decreases. Therefore, in the third step in which decarboxylation progresses, a pressure lower than that in the second step is adopted. Is preferable. At the same time, the lower the concentration of [C], the greater the influence of molten steel stirring on the decarburization reaction. At the same gas blowing speed, the lower the pressure inside the container, the larger the expansion allowance of the gas and the larger the stirring force of the molten steel. Therefore, it is preferable to set the pressure lower than that of the second step. In the third step, it is preferable to gradually reduce the pressure in the container as the concentration of [C] in the molten steel decreases. In the third step, since the concentration of [C] is sufficiently lowered, only the inert gas may be blown without using oxygen gas as the blowing gas. Further, when supplying a mixed gas of an oxygen gas and an inert gas as a blowing gas, it is preferable to gradually reduce the ratio of the oxygen gas in the mixed gas as the C concentration in the molten steel decreases.

The present invention was applied to melt SUS304 stainless steel (8% by mass Ni-18% by mass Cr) in an AOD furnace having a molten steel amount of 60 tons as shown in FIGS. 1A and 1B. In atmospheric pressure refining, bottom blowing was performed without top blowing in the manner shown in FIG. 1B, and in decompression refining, bottom blowing was performed after depressurizing the inside of the refining vessel in the manner shown in FIG. 1A. The [C] concentration in the molten steel at the start of melting is about 1.7%, and decarburization refining is performed to [C] 0.03%, and then oxidation is performed during decarburization while returning the pressure inside the container to atmospheric pressure. Fe—Si alloy iron was added as a reducing agent for reducing the resulting chromium, and the reduction treatment was carried out by blowing only Ar gas, and the steel was discharged to a pan.

For "decompression refining", the pattern shown in Table 1 was adopted for refining. The first step was atmospheric pressure refining and bottom blowing was performed. [C] A concentration of 0.8% to 0.2% was set as the second step, and the pressure inside the container was set as a two-step pressure of 400 torr and 200 torr in the second step. In the third step, the pressure inside the vessel was set to a two-step pressure of 50 torr and 40 torr, and decarburization refining was performed to a [C] concentration of 0.03%. Up to the pressure of 50 torr in the first step, the second step, and the third step, oxygen gas and argon gas were used in combination as the bottom blowing gas. At the pressure of 40 torr in the third step, Ar gas was blown alone. The start of the second step is the start of decompression smelting, and the end of the third step is the end of decompression smelting.
At the start of the second step (at the start of vacuum refining), at the start of the third step (during vacuum refining), and at the end of the third step (at the end of vacuum refining), the slag in the container was collected and the slag component was analyzed. ..
For "decompression refining not performed", refining was performed by adopting the pattern shown in Table 2. The pressure inside the container was always atmospheric pressure, and the bottom-blown oxygen gas ratio was lowered at any time according to the [C] concentration. In the final stage of refining, the bottom-blown oxygen gas ratio was set to 0, and Ar gas was blown alone. For comparison with vacuum refining, it corresponds to the start of the second step (at the start of vacuum refining), the start of the third step (during vacuum refining), and the end of the third step (at the end of vacuum refining) in vacuum refining. The slag in the container was collected at the concentration of [C], and the slag component was analyzed.
In Tables 1 and 2, as for the bottom-blown gas type, the gas type described as "A" is blown in the table, and the gas type described as "X" is not blown.

The MgO concentration in the slag was adjusted by charging either one or more of MgO alone or an oxide containing MgO into the AOD furnace at any time. When the MgO concentration at the end of vacuum refining is 8% or more and the MgO concentration at the start of vacuum refining is less than 8%, oxides containing no MgO are added into the AOD furnace at any time before the start of vacuum refining. , It was decided not to put oxides containing MgO into the AOD furnace. When the MgO concentration was 8% or more at both the end of the reduced pressure smelting and the start of the reduced pressure smelting, an oxide containing MgO was put into the AOD furnace before the start of the reduced pressure smelting.

The concentration of chromium oxide in the slag was adjusted by changing the mixing ratio of the bottom-blown oxygen gas and the inert gas, and controlling the amount of Cr in the molten steel oxidized by oxygen refining. When the chromium oxide concentration at the end of vacuum refining is 28% or more and the chromium oxide concentration at the start of vacuum refining is less than 28%, the oxygen gas ratio of the bottom blowing gas before the start of vacuum refining is lowered. After suppressing the formation of chromium oxide, it was decided to increase the oxygen gas ratio at the initial stage of vacuum refining to promote the formation of chromium oxide. When the chromium oxide concentration is 28% or more at both the end of vacuum refining and the start of vacuum refining, the oxygen gas ratio of the bottom blowing gas should be increased before the start of vacuum refining to promote the production of chromium oxide. And said.

Regarding the evaluation of the slag component during decompression smelting, from the inside of the scouring container at the start of decompression smelting (at the start of the second step), during decompression smelting (at the start of the third step), and at the end of decompression smelting (at the end of the third step). Slag was collected, component analysis was performed, and it is shown in Table 3. At each level, the amount of Fe—Si alloy iron used as a reducing agent was evaluated and shown in Table 3. No. which is a general example of the conventional manufacturing method. Based on the amount of reducing agent used in 5, A was used when the amount was equal to or less than that, and X was used in other cases.
The refractory erosion status of the refining furnace was evaluated by inserting a measuring rod with a scale from the outside of the bottom blowing tuyere after refining and recording the remaining dimensions as needed. Table 3 shows the case where the amount of refractory erosion is small and has an economic effect, and the case where the refractory is significantly eroded or locally unevenly eroded and interferes with the operation is designated as X.
Regarding the operation evaluation, it was evaluated whether the reduction of the amount of reducing agent used and the reduction of the state of wear of refractories were compatible. A is given when both the amount of reducing agent used and the state of wear of the refractory are good, and X is given when the result of any one or more of the amount of reducing material used or the state of wear of the refractory is not good. Shown in 3. In Table 3, items outside the scope of the present invention are underlined.

As is clear from Table 3, the present invention No. Nos. 1 to 5 satisfy the conditions of the present invention in which the MgO concentration in the slag at the end of vacuum refining is 8 to 15% by mass and the chromium oxide concentration is 28 to 50% by mass, and as a result, refractory erosion reduction and reduction are achieved. We were able to reduce the amount of material used at the same time. On the other hand, Comparative Example No. In No. 6, the MgO concentration in the slag at the end of vacuum refining was too low, and Comparative Example No. In No. 7, the chromium oxide concentration in the slag at the end of decompression refining was too low, and the refractory wear condition was poor in each case. Comparative Example No. In No. 8, decompression refining had not been carried out, and the chromium oxide concentration in the slag at the end of refining was too high, so that the amount of reducing agent used was not reached.

1 Refining container 2 Bottom blown tuyere 3 Exhaust hood 4 Molten steel 5 Bottom blown gas 6 Slag 7 Exhaust pipe 12 Top blown lance

Claims (3)

In a refining method in which a gas containing oxygen gas is blown into a chromium-containing molten steel in a refining container to perform refining.
The first step of blowing a gas containing oxygen gas as a depressurizing pressure in the range of atmospheric pressure and / or a pressure less than 400 torr inside the container, and depressurizing the inside of the container to more than 200 torr and 400 torr or less and blowing a gas containing oxygen gas. It has a second step and a third step of depressurizing the inside of the container to 200 torr or less and blowing gas, and then the inside of the container is set to atmospheric pressure.
Switching from the first step to the second step when the [C] concentration in the molten steel is 1.5 to 0.1% by mass, and from the second step when the [C] concentration in the molten steel is 0.5 to 0.1% by mass. Switch to the third step,
The MgO concentration in the slag at the end of vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
A refining method for chromium-containing molten steel that achieves both reduction of refractory melting damage and reduction of the amount of reducing agent used. Further, the MgO concentration in the slag at the start of vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
The method for refining a chromium-containing molten steel according to claim 1, wherein both reduction of refractory melting damage and reduction of the amount of reducing agent used are achieved. Further, the MgO concentration in the slag during vacuum refining is adjusted to 8 to 15% by mass, and the chromium oxide concentration is adjusted to 28 to 50% by mass.
The method for refining a chromium-containing molten steel according to claim 2, wherein both reduction of refractory melting damage and reduction of the amount of reducing agent used are achieved.

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