Refining method of chromium-containing molten steel

Abstract

To provide a refining method of chromium-containing molten steel with which a treating condition can be optimized so as to shorten a treating time of reduction treatment of chromium oxide and problems of the increase of consumption of non-oxidizing gas and the lowering of molten steel temperature can be restrained.SOLUTION: A refining method of chromium-containing molten steel includes a step of atmospheric refining, a step of reduced pressure refining after the atmospheric refining, and a step of reduction treatment in which, after the reduced pressure refining, a reducing agent is added to the molten steel and a non-oxidizing gas is blown into the molten steel under reduced pressure to reduce chromium oxide. The reduction treatment is performed under a condition that a stirring power defined by an integral value of a stirring-energy density and a reduction treatment time defined by the following formula (1) is within a range of 0.90-1.10 in comparison with a reference reduction treatment condition, and the reduction treatment time is shorter than the reference reduction treatment condition. Stirring-energy density ε(kW / cht)=12.2Q / W{0.85+ln(1+H / 148P)}...Equation (1), where Q is a stirring gas flow rate, P is a pressure in the vessel, H is a bath depth of the molten steel, and W is a molten steel weight.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for refining molten steel containing chromium, such as stainless steel. [Background technology]

[0002] When producing chromium-containing steel (such as stainless steel), a mixture of O2 gas and non-oxidizing Ar gas is blown into the molten steel contained in a refining furnace (AOD furnace) from below the bath surface in the atmosphere to cause a reaction between O2 and the C in the molten steel, i.e., a decarburization reaction, thereby reducing the amount of C in the molten steel. This method is known as the AOD process.

[0003] In decarburization refining in air, when the carbon content in the molten steel decreases, the injected oxygen does not work effectively for decarburization and oxidizes the chromium, resulting in poor decarburization efficiency. Therefore, when the carbon content in the molten steel decreases to a certain level, the furnace is depressurized and only a non-oxidizing gas such as Ar gas is injected into the molten steel as a stirring gas to cause a reaction between the chromium oxide and the carbon in the molten steel, thereby performing reduced-pressure refining for decarburization. Thereafter, a reducing agent is added to the molten steel and a reduction treatment is performed in which the chromium oxide remaining in the slag is reduced and recovered in the molten steel. Such a series of refining methods is described in, for example, Patent Document 1 listed below. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-194125 Summary of the Invention [Problem to be solved by the invention]

[0005] In the reduction treatment carried out following vacuum refining, the treatment conditions have traditionally been set for each steel type based on the Cr reduction rate, the number of inclusions, and other past performance data. However, there has been a demand for a shorter reduction treatment time (Cr recovery time) to prevent problems such as an increase in the consumption of non-oxidizing gases and a decrease in the molten steel temperature.

[0006] In view of the above circumstances, an object of the present invention is to provide a method for refining chromium-containing molten steel, which can optimize treatment conditions so as to shorten the treatment time when reducing chromium oxide, and can suppress problems such as an increase in the consumption of non-oxidizing gas and a decrease in the temperature of molten steel. [Means for solving the problem]

[0007] As a result of extensive research into solving the above problems, the present inventors have found that the treatment time can be shortened while maintaining Cr reducibility if the stirring force, which is defined by the integral of the stirring energy density and the reduction treatment time, is the same. This invention was made based on this finding.

[0008] Thus, the method for refining chromium-containing molten steel according to a first aspect of the present invention is defined as follows: A refining method in which decarburization is carried out by injecting gas into chromium-containing molten steel in a refining vessel, An atmospheric refining step in which a mixture of non-oxidizing gas and O2 gas is blown into the molten steel to decarburize it; a step of vacuum refining in which, after the atmospheric refining step, the pressure in the refining vessel is reduced and a non-oxidizing gas is blown into the molten steel to subsequently carry out decarburization; a reduction step after the vacuum refining step, in which a reducing agent is added to the molten steel and a non-oxidizing gas is blown into the molten steel under reduced pressure to reduce chromium oxide contained in slag above the molten steel; Including, A method for refining chromium-containing molten steel, comprising carrying out the reduction treatment under conditions in which a stirring force defined by an integral of the stirring energy density defined by the following formula (1) and the reduction treatment time is within a range of 0.90 to 1.10 compared to reference reduction treatment conditions, and the reduction treatment time is shorter than the reference reduction treatment conditions: Mixing energy density ε (kW / cht) = 12.2Q / W × {0.85 + ln(1 + H / 148P)} ... Equation (1) However, Q: Stirring gas flow rate (Nm 3 / min), P: Pressure inside the vessel (atm), H: Depth of the molten steel bath (cm), W: Weight of molten steel (tons)

[0009] According to the method for refining chromium-containing molten steel of the first aspect defined as above, the reduction treatment can be carried out in a short treatment time while maintaining a stirring force substantially equivalent to that of the standard reduction treatment conditions conventionally adopted, and problems such as an increase in the consumption of non-oxidizing gas and a decrease in the temperature of the molten steel can be suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing a refining vessel used in a method for refining chromium-containing molten steel according to one embodiment of the present invention. [Figure 2] FIG. 10 is an explanatory diagram of the stirring force in the reduction treatment. [Figure 3] 10A and 10B are diagrams illustrating the effects of the refining method of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Next, an embodiment of the present invention will be described. The refining method of this embodiment involves injecting gas into chromium-containing molten steel to decarburize it, and sequentially performs the steps of air refining, reduced-pressure refining, and reduction treatment, which will be described later. The chromium-containing molten steel produced by the refining method of this embodiment has a chromium content of 5 to 50 mass %. There are no particular restrictions on the C content, but the refining method of this embodiment is particularly suitable for application to chromium-containing molten steel with a C content of 0.15 mass % or less.

[0012] FIG. 1 is a diagram showing a refining vessel used in the method for refining chromium-containing molten steel according to this embodiment. In the figure, 1 denotes an AOD furnace as a refining vessel, and a tuyere 3 is provided near the bottom of the furnace. Molten steel W and slag S are contained within the AOD furnace 1. In atmospheric refining, as shown in Figure 1(A), oxygen (O2 gas) is blown into the molten steel contained inside the furnace body from a tuyere 3 near the bottom of the furnace in the atmosphere, along with a non-oxidizing diluent gas (Ar or N2 gas), to decarburize the C in the molten steel and reduce its concentration. In atmospheric refining, the reaction between the blown O2 gas and the C and chromium in the molten steel generates heat, which increases the temperature of the molten steel W.

[0013] Following air refining, reduced-pressure refining is performed. In air refining, as the carbon concentration in the molten steel decreases, the injected oxygen (O2) becomes ineffective for decarburization and oxidizes chromium (Cr), resulting in poor decarburization efficiency. Therefore, when the carbon concentration in the molten steel (W) decreases to a certain level during air refining (for example, when the carbon concentration in the molten steel decreases to 0.50-0.05 mass%), a pressure reduction lid 5 is installed to seal the furnace, as shown in Figure 1(B), and the refining of the molten steel (W) is switched to reduced-pressure refining. Specifically, as shown in Figure 1(B), the furnace is sealed with the pressure reduction lid 5, vented through duct 6, and the furnace is depressurized. Then, only Ar gas, an inert gas, is injected through tuyere 3 as a stirring gas and a non-oxidizing gas. Note that other non-oxidizing gases may also be used as the stirring gas.

[0014] In vacuum refining, a non-oxidizing stirring gas (Ar gas in this case) is blown into the molten steel (W) under reduced pressure to stir the molten steel (W) and slag (S). The chromium oxide in the slag (S) generated during the atmospheric refining reacts with the carbon in the molten steel to continue decarburization. (During this process, the carbon in the molten steel (W) becomes CO and is discharged from the molten steel (W).) As a result, the carbon in the molten steel (W) is effectively decarburized, resulting in a low carbon content. Furthermore, the chromium oxide in the slag (S) is partially reduced by the carbon in the molten steel and recovered in the molten steel.

[0015] Next, after the vacuum refining, a reduction treatment is carried out. In the reduction treatment, a reducing agent feed device (not shown) attached to the vacuum lid 5 is operated to feed a reducing agent into the furnace body, and a non-oxidizing stirring gas (Ar gas in this case) is blown in under reduced pressure to reduce the chromium oxide remaining in the slag S and recover it in the molten steel. The reducing agent can be a material primarily composed of a metal that is more reactive with oxygen than iron and chromium, such as Fe-Si (ferrosilicon) or metallic aluminum. During the vacuum refining and reduction treatment, the temperature of the molten steel decreases over time.

[0016] Here, chromium recovery also proceeds through a reaction between chromium oxide in the slag and C in the molten steel due to contact, and the chromium recovery rate at this time varies depending on the stirring force of the stirring gas. In light of this point, in this embodiment, the reduction treatment conditions are set so that a stirring force approximately equivalent to that under the reference reduction treatment conditions is obtained (specifically, so that the stirring force is within a range of 0.90 to 1.10 compared to the reference reduction treatment conditions), and so that the reduction treatment time is shorter than that under the reference reduction treatment conditions.

[0017] The stirring force in the reduction treatment is defined by the integral of the stirring energy density ε and the reduction treatment time (corresponding to the area of ​​the hatched portion in FIG. 2). The stirring energy density ε is defined by the following formula (1). Mixing energy density ε (kW / cht) = 12.2Q / W × {0.85 + ln(1 + H / 148P)} ... Equation (1) In this equation (1), Q is the stirring gas flow rate (Nm 3 / min), P is the pressure inside the vessel (atm), H is the bath depth of the molten steel (cm) (see Figure 1), and W is the weight of the molten steel (tons).

[0018] In the reduction process, increasing the stirring energy density ε increases the chromium recovery rate. According to the above formula (1), in order to increase the stirring energy density ε, excluding the molten steel weight W and bath depth H, it is effective to reduce the vessel pressure (vacuum degree) P and increase the stirring gas flow rate Q. If the stirring energy density ε is increased, it becomes possible to obtain a stirring force that is approximately equivalent to the stirring force under the reduction treatment conditions (reference reduction treatment conditions) that have been conventionally performed and have proven to be effective, even with a treatment time that is shorter than conventional. The standard stirring force for the reduction treatment conditions is: stirring gas flow rate Q: 38.6 (Nm 3 / min) and the pressure inside the vessel P is 760 (torr), and the stirring energy density ε can be calculated as the integral value (stirring force) obtained when the reduction treatment is continued for 5 minutes.

[0019] Table 1 below shows an example of reduction treatment conditions for chromium-containing molten steel that are carried out following atmospheric refining and reduced-pressure refining. The conventional examples shown in Table 1 show proven reduction treatment conditions that have been used in the past under atmospheric pressure. Examples 1 and 2 are examples in which the reduction treatment conditions were set so as to obtain stirring forces under reduced pressure that are approximately equivalent to those in the conventional examples. Examples 1 and 2 enable the reduction treatment time to be shorter than in the conventional examples while maintaining the same chromium recovery capacity as in the conventional examples.

[0020] By shortening the reduction treatment time, it is possible to suppress the consumption of non-oxidizing gas (Ar gas) used as a stirring gas, which increases with the reduction treatment time, as shown in Figure 3. Furthermore, as shown in the same figure, it is possible to suppress the decrease in molten steel temperature, which progresses with the reduction treatment time. This makes it possible to reduce the energy required to heat the molten steel in response to the decrease in molten steel temperature. In other words, according to Examples 1 and 2, it is possible to reduce the production costs associated with refining in accordance with the shortening of the reduction treatment time. The cost reduction effect is greater in Example 2, which has a shorter reduction treatment time.

[0021] [Table 1]

[0022] Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be configured in various modified forms without departing from the spirit of the present invention. [Explanation of symbols]

[0023] 1 AOD furnace (refining vessel) 3 Tuyere 5. Vacuum Lid W Molten steel S slug

Claims

[Claim 1] A refining method in which decarburization is carried out by injecting gas into chromium-containing molten steel in a refining vessel, Non-oxidizing gas and O 2 an atmospheric refining step in which a mixture of slag and gas is blown in to decarburize the slag; a step of vacuum refining in which, after the atmospheric refining step, the pressure in the refining vessel is reduced and a non-oxidizing gas is blown into the molten steel to subsequently carry out decarburization; a reduction step after the vacuum refining step, in which a reducing agent is added to the molten steel and a non-oxidizing gas is blown into the molten steel under reduced pressure to reduce chromium oxide contained in slag above the molten steel; Including, A method for refining chromium-containing molten steel, comprising carrying out the reduction treatment under conditions in which a stirring force defined by an integral of the stirring energy density defined by the following formula (1) and the reduction treatment time is within a range of 0.90 to 1.10 compared to reference reduction treatment conditions, and the reduction treatment time is shorter than the reference reduction treatment conditions: Stirring energy density ε (kW / cht) = 12.2Q / W × {0.85 + ln(1 + H / 148P)} ... Equation (1) Where, Q: stirring gas flow rate (Nm 3 / min), P: pressure inside the vessel (atm), H: bath depth of molten steel (cm), W: weight of molten steel (tons)

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