JPH08302417A - Method for melting chromium steel - Google Patents

Abstract

PURPOSE: To realize the efficient melting by reducing the oxidation of [Cr] in the molten steel in the process to melt Cr steel in an electric furnace. CONSTITUTION: The mixed gas containing >=10% oxygen gas is blown into the molten steel by >=0.1Nm<3> /min per unit ton of the molten steel after >=20% of the material charged in an electric furnace is melted to become the molten steel. As the temperature of the molten steel rises, the ratio of the oxygen gas in the mixed gas is increased up to 90%. The decarburizing reaction by the oxygen in the mixed gas procedes under the condition where the oxidation of [Cr] in the molten steel is small, and the power consumption unit for melting is reduced by the decarburizing reaction heat, and the melting time is reduced.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In the process of melting chromium-containing steel in an electric furnace, the oxidation of [Cr] in the molten steel is reduced, the melting time is shortened and the charging material is efficiently melted. The present invention relates to a method for melting chromium steel.

[0002]

2. Description of the Related Art Chromium-containing steel such as stainless steel containing 11 mass% or more of Cr is manufactured by melting a raw material in an electric furnace and then refining it in a top-bottom blowing converter, AOD and VOD. ing. For melting chromium-containing steel in an electric furnace, alloys such as ferrochromium and ferro-nickel are also used as melting materials in addition to scrap, and are charged into the electric furnace to energize the material to be melted by electric heat.

During melting of materials in an electric furnace, oxygen gas (hereinafter,
The method of blowing an oxidizing gas such as oxygen or air into molten steel is widely used in the field of ordinary steel that does not contain Cr. This method reduces the melting time and the power consumption rate. Efficient lysis has been achieved. Further, the method of blowing oxygen into molten steel by melting chromium-containing steel in an electric furnace has been performed before the introduction of AOD or VOD. This method completely melts the material charged into the electric furnace,
When the molten steel temperature rises above about 1600 ° C, the [C] concentration in the molten steel is lowered to the concentration required by the product, and at the same time the [C] concentration decreases (decarburization) due to the blowing of oxygen. , Because oxidation of [Cr] in molten steel also progresses, AOD
It has not been implemented since the introduction of VOD and VOD.

On the other hand, Japanese Patent Laid-Open No. 58-167714 discloses a method of blowing air consisting of oxygen and nitrogen gas when melting chromium-containing steel in an electric furnace. This method is a method in which arc heating is temporarily interrupted during the reduction period and air is blown into the steel bath from the refining lance in the electric furnace melting process that is composed of the melting period, the oxidation period, and the reduction period. Although this method can promote the efficient reduction reaction in the reduction period,
There is no injection during the melting period and oxidation period, and because arc heating is interrupted even during the injection during the reduction period, there is no sufficient effect in shortening the melting time and preventing [Cr] oxidation in molten steel. Not obtained.

Further, as a method of blowing an oxidizing gas when melting chromium-containing steel in an electric furnace, Japanese Patent Laid-Open No. 52-9551 is known.
No. 8 publication is disclosed. This method is a method of injecting steam together with pure oxygen into molten steel in an electric furnace to reduce the [C] concentration to 0.2 to 0.4 mass%.
This method is also a method in which gas is blown after the material is completely melted, and the load of the refining process is reduced by lowering the concentration of [C], but the melting time in the electric furnace is shortened efficiently. It does not lead to good dissolution. As described above, in the conventional technique, the blowing time of the oxidizing gas does not bring about a sufficient effect for shortening the melting time and preventing the oxidation of [Cr] in the molten steel, and the situation is not sufficiently utilized. there were.

[0006]

In the process of melting chromium-containing steel in an electric furnace, a mixed gas containing oxygen is blown into the molten steel during melting so that the oxidation of [Cr] in the molten steel is minimized. The present invention provides an efficient dissolution method that suppresses and shortens the dissolution time.

[0007]

The present invention advantageously solves the above-mentioned problems, and the gist of the present invention is to charge a chromium-containing steel in an electric furnace in the step of melting the chromium-containing steel. After melting 20% or more of the formed material into molten steel, a mixed gas containing 10% or more oxygen is blown into the molten steel at a rate of 0.1 Nm ³ / min or more per ton of molten steel to dissolve chromium-containing steel. Is the way. Further, the gas blown into the molten steel is a mixed gas of oxygen and an inert gas, and the ratio of oxygen in the mixed gas is increased up to 90% as the molten steel temperature rises. It is a method of melting chromium-containing steel. The blowing flow rate per ton of molten steel is the blowing amount based on the molten steel amount at the end of melting in the electric furnace.

[0008]

The present invention will be described in detail below. In the step of melting the chromium-containing steel in the electric furnace of the present invention, the method of blowing the mixed gas containing oxygen is performed by the method illustrated in FIG. FIG. 1 (a) shows a method of blowing a mixed gas containing oxygen from a bottom blowing nozzle or a bottom blowing tuyere,
(B) shows a method of inserting an upper blowing lance from the furnace opening and blowing a mixed gas containing oxygen from the lance. In the figure, 1 is an electric furnace body, 2 is an electrode, 3 is molten steel, 4 is unmelted material, 5
Is an arc, 6 is a bottom blowing nozzle or bottom blowing tuyere, and 7 is a top blowing lance. The present invention, as illustrated in FIGS. 1 (a) and 1 (b), shows that the material charged in the electric furnace is
This is a method in which a mixed gas containing oxygen is blown into the molten steel from the time when the melting is not completely completed.

FIG. 2 shows that when SUS304 stainless steel is melted in an electric furnace, the ratio of the charged material to the melted steel at the time when the injection of the mixed gas containing oxygen is started, The relationship of the oxidation index of [Cr] is shown. In addition,
The oxidation index of [Cr] is a value roughly proportional to the average value of the oxidation amount of [Cr] in the molten steel as 100 when the mixed gas is blown from the time when the charged material is completely melted. Is. Further, blowing flow rate of the mixed gas is 0.2 Nm ³ / min from 0.01 Nm ³ / min per ton of the molten steel
And the mixed gas with argon or nitrogen gas containing 10 to 30% of oxygen was used as the mixed gas.
From FIG. 2, when the dissolved ratio of the charged materials is less than 20%, the amount of [Cr] oxidized rapidly increases. From this, [C
In order to prevent the oxidation of [r], it is necessary to start blowing the mixed gas from the time when the charged material is dissolved by 20% or more.

FIG. 3 shows that when SUS304 stainless steel is melted in an electric furnace and a mixed gas containing oxygen is blown, the flow rate of the mixed gas per ton of molten steel and the [Cr] content in the molten steel. The relationship of the oxidation index is shown. The oxidation index of [Cr] is 0.
It is a value roughly proportionally calculated with the average value of the oxidation amount of [Cr] in the molten steel being 100 in the case of 01 Nm ³ / min. In addition, the mixed gas is blown at 20 to 40% of the charged material.
Was started from the time when was dissolved, and a mixed gas with argon or nitrogen gas containing 10 to 50% oxygen was used as the mixed gas. From FIG. 3, when the flow rate of the mixed gas blown per ton of molten steel is less than 0.01 Nm ³ / min, the [C
The amount of oxidation of r] is increasing, and the flow rate of the mixed gas is 0.0 per ton of molten steel in order to prevent the oxidation of [Cr].
It should be 1 Nm ³ / min or more.

FIG. 4 shows the relationship between the proportion of oxygen in the mixed gas and the shortening rate of the melting time when the mixed gas containing oxygen is blown in melting SUS304 stainless steel in an electric furnace. The rate of shortening the melting time is the rate of shortening the melting time in the absence of gas injection. The flow rate of the mixed gas blown is 0.01 to 0.05 Nm ³ per ton of molten steel.
/ Min, and the blowing of the mixed gas was started from the time when 20 to 40% of the charged material was melted. From FIG. 4, when the ratio of oxygen gas in the blown mixed gas is less than 10%, the shortening rate of the dissolution time is very small. Therefore, in order to shorten the dissolution time and efficiently perform the dissolution, the ratio of oxygen in the mixed gas is Must be 10% or more.

FIG. 5 shows melting of SUS304 stainless steel in an electric furnace, and a mixed gas containing oxygen is supplied at a rate of 0.
When blowing is performed at a flow rate of 01 to 0.20 Nm ³ / min, the relationship between the ratio of oxygen in the mixed gas and the oxidation index of [Cr] in the molten steel is shown. In addition, in the figure, the time when the injection of the mixed gas is started is shown at the time when 20 to 40% of the charged material is dissolved and the time when 80 to 100% is dissolved. In addition, the oxidation index of [Cr] is 20% to 40% of the charged material is melted, and when the mixed gas is blown, the ratio of oxygen in the mixed gas is 10%. It is a value obtained by proportionally converting the oxidation amount of Cr] as 100.

As shown in FIG. 5, the oxygen content of the blowing gas is increased on the side where the ratio of the melted materials at the time of starting the injection of the mixed gas is high, that is, the side where the molten steel temperature is high. Also suppresses the oxidation of [Cr]. In addition, even if the dissolved ratio of the charged material is high, the oxygen ratio of the blown gas is 90%.
When it exceeds%, the oxidation of [Cr] rapidly increases. Therefore, it is understood that by increasing the ratio of oxygen in the mixed gas up to 90% as the molten steel temperature rises, the oxidation of [Cr] can be suppressed and efficient decarburization can be performed.

From the above, in the step of melting chromium-containing steel in an electric furnace, 20% or more of the material charged in the electric furnace is melted into molten steel, and then a mixed gas containing 10% or more oxygen is melted. By blowing 0.01 Nm ³ / min or more per ton, the oxidation of [Cr] in the molten steel is small, the melting time is shortened, and efficient melting becomes possible. Further, it is effective to increase the ratio of oxygen gas in the mixed gas up to 90% as the molten steel temperature rises.
In operation, it is possible to understand the ratio of the charged materials that have melted to become molten steel from the total power input.Adjust the flow rate of the mixed gas and the oxygen ratio according to the change in the total power input. do it.

By blowing the mixed gas, the oxidation of [C] and [Si] in the molten steel proceeds, so that the heat of oxidation reaction promotes the melting, which shortens the melting time and reduces the unit consumption of the melting power. Leads to. If the [C] concentration in the molten steel becomes 0.5 mass% or less, the oxidation of [Cr] is likely to proceed. Therefore, in order to maintain the [C] concentration above this value, the total oxygen flow rate Is preferably adjusted. Also, the upper limit of the flow rate of the mixed gas blown is
The value depends on the state of stirring in the furnace and is determined by the shape of the furnace, but is generally preferably 0.5 Nm ³ / min or less per ton of molten steel.

In the process of melting chromium-containing steel in an electric furnace,
In addition to scrap, alloys such as ferrochrome and ferronickel are generally used as the melting material. These materials contain C, and when the gas containing oxygen is not blown in, the [C] concentration becomes 1 to 3 mass% at the end of melting.

When a gas containing oxygen is blown into the chromium-containing molten steel, the decarburization reaction shown in the following formula (1) proceeds and at the same time, the oxidation reaction of [Cr] in the molten steel shown in the following formula (3). May progress. The equilibrium constant K of the reaction of the equation (1) is expressed by the equation (2).

[0018]

[Equation 1]

Here, P _CO is the partial pressure (atm) of CO gas in the atmosphere, and a _C and a _O are [C] and [O] in the molten steel.
, T indicates the molten steel temperature (K), respectively. (3) suppressing the progress of oxidation reaction of [Cr] of formula (1) of the advancing the decarburization reaction efficiently than (2), lowered and the molten steel temperature of the CO gas partial pressure P _CO T Is effective.

Conventionally, in refining furnaces such as AOD and VOD, 1
It was possible to reduce _PCO at a high temperature of 650 ° C. or higher, and efficient decarburization was possible. On the other hand, in a melting furnace such as an electric furnace, since melting starts at room temperature or at a level of several hundreds of degrees Celsius, a rapid oxidation reaction of [Cr] progresses by blowing oxygen alone, resulting in efficient decarburization and melting. Could not be achieved. Further, since the electric furnace generally has no bottom-blown gas agitation and the agitation of molten steel is weak, the oxidation of [Cr] progresses due to the oxygen existing in the atmosphere during the melting. Reduction treatments have been performed to recover the oxidized [Cr].

[0021] Under such circumstances, the present inventors have found that, oxygen and argon, if put blown into the molten steel a mixed gas of an inert gas such as nitrogen, that argon, nitrogen acts to decrease in P _CO I found it. In addition, 20% of the material charged in the electric furnace
In the state where the above is melted into molten steel, the conditions in which the molten steel temperature promotes the decarburization reaction of the above formula (1) rather than the oxidation reaction of [Cr] of the above formula (3) is achieved, and oxygen 10%
It has been found that it is possible to inject the mixed gas containing the above. Furthermore, it was found that a mixed gas of 0.01 Nm ³ / min or more per ton of molten steel should be blown in as a condition for preventing the oxidation of [Cr] from the atmosphere.

Under these conditions, the oxidation of [Cr] is minimized and the decarburization reaction proceeds, but in order to increase the decarburization reaction amount, the melting ratio is increased, that is, the molten steel temperature is increased. Accordingly, it has been found that increasing the proportion of oxygen in the mixed gas up to 90% is particularly effective. By injecting the mixed gas under such conditions, the reaction of the above formula (1) proceeds, and a part or all of the generated CO gas reacts with oxygen present in the atmosphere to generate the following formula (4). Secondary combustion reaction proceeds.

[0023]

[Equation 2]

Both the reactions of the equations (1) and (4) are exothermic reactions, and the temperature of the molten steel rises, which leads to a reduction in the melting time and a reduction in the basic unit of melting power. Note that the [C] concentration in the molten steel decreases as the reaction of formula (1) progresses. When the concentration of [C] is about 0.5 mass% or less, the [C
The [C] concentration at the end of melting in the electric furnace is preferably at or above this level because the oxidation reaction of [r] easily proceeds.

[0025]

[Example] SUS304 stainless steel (8 mass% N
A process of blowing a mixed gas of oxygen and nitrogen gas at the time of melting (i-18 mass% Cr) in an electric furnace was carried out in the embodiment shown in FIG. Scrap, ferrochromium, or ferronickel is used as the melting material, and when the oxidizing gas containing oxygen is not blown, the [C] concentration after melting is 1.
Molten steel 60t under the condition of 8 to 2.0 mass%
Was manufactured. After melting, Fe-Si was added to reduce [Cr] that was oxidized during melting, reduction treatment was performed, steel was tapped, and then decarburization and desulfurization treatment with AOD was performed. Casting was performed by casting with a continuous casting machine.

Table 1 shows examples of blowing conditions of a mixed gas containing oxygen during melting in an electric furnace. Examples of the present invention include
It was carried out so as to satisfy the conditions shown above, and the proportion of oxygen in the mixed gas was increased as the dissolved proportion of the charged materials increased. No. of the comparative example. No. 6 is an example in which no gas is blown, No. 6 7 is an example of blowing oxygen alone,
No. 8 is an example in which the blowing start time is outside the conditions of the present invention, N
o. 9 is an example in which the amount of the blown mixed gas is outside the conditions of the present invention.

[0027]

[Table 1]

[0028]

[Table 2]

The execution results are shown in Table 2. The values in the table are the numbers of the comparative example. The result of 6 is set to 100, and the value is comparatively converted. In the example of the present invention, the oxidation of [Cr] in the molten steel is at almost the same level or less as compared with the case where no gas is blown, so the addition amount of reducing Si is also at the same level or less, and the power consumption rate is reduced. In addition, the melting time was shortened and the refining time was also shortened, resulting in a significant reduction in manufacturing cost. On the other hand, in the comparative example, although there is a slight reduction in the electric power consumption rate and a reduction in the melting time as compared with the case where no gas is blown in, a sufficient effect can be obtained because the oxidation of [Cr] in the molten steel is large. There wasn't.

[0030]

According to the method of the present invention, in the melting of chromium-containing steel in an electric furnace, decarburization of the molten steel proceeds under the condition that the oxidation of [Cr] in the molten steel is suppressed. Shortening and reduction of power consumption are achieved, and at the same time, shortening of the refining time which is the subsequent process is also achieved, resulting in reduction of manufacturing cost and significant improvement of productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing a method for injecting a mixed gas containing oxygen according to an embodiment of the present invention.

FIG. 2 is a diagram showing the reason for limiting the conditions for starting the injection of a mixed gas containing oxygen in the method of the present invention.

FIG. 3 is a diagram showing the reason for limiting the flow rate of the mixed gas containing oxygen in the method of the present invention.

FIG. 4 is a diagram showing the reason for limiting the ratio of oxygen in the blown mixed gas in the method of the present invention.

FIG. 5 is a diagram for explaining changes in the ratio of oxygen in the mixed gas to be blown in the method of the present invention.

[Explanation of symbols]

 1 Electric Furnace Body 2 Electrode 3 Molten Steel 4 Unmelted Material 5 Arc 6 Bottom Blowing Nozzle 7 Top Blowing Lance

Claims (3)

[Claims] 1. In a step of melting chromium-containing steel in an electric furnace, 20% or more of a material charged in the electric furnace is melted to form molten steel, and then a mixed gas containing 10% or more of oxygen is added. A method for melting chromium-containing steel, which comprises injecting 0.1 Nm ³ / min or more per ton of molten steel into the molten steel. 2. The method for melting chromium-containing steel according to claim 1, wherein the gas blown into the molten steel is a mixed gas of oxygen and an inert gas. 3. The method for melting chromium-containing steel according to claim 1, wherein the ratio of oxygen in the mixed gas is increased to a maximum of 90% as the molten steel temperature rises.