JPH0772290B2 - Dechromization method of molten steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for quickly and effectively removing Cr content unintentionally mixed in molten steel melted in a converter or an electric furnace. Is.

(Prior Art) Generally, it is known that when the Cr content in steel increases, the workability such as cold workability and deep drawability deteriorates. When melting carbon steel with good workability, the Cr content in the molten steel is
It is strictly required to regulate it to 0.05% or less.

However, in recent years, it has become increasingly difficult to completely prevent a sudden increase in the Cr content in a carbon steel melting operation using a converter. Because, in the melting operation of carbon steel in a converter, scrap is usually compounded into the hot metal. This is because the chances of mixing steel scrap are increasing. Of course, although the management of scrap has been further strengthened, it is difficult to eliminate the out-of-specification of the Cr content in the molten steel after converter blowing in actual work.

As a measure against the occurrence of the Cr content out-of-specification as described above, a measure is taken such that carbon is blown down in the converter to further extend the blowing to remove Cr. However, in this case, the Cr dissolved in the hot metal
Since a part of the iron is transferred to the slag as Cr oxides that have already been oxidized during decarburization refining, there is a limit to the reduction of Cr, and during this treatment, melting loss of the converter refractory is also There is also the problem of becoming fierce.

Therefore, in the present situation, molten steel having a large degree of Cr content deviating from the standard is forced to be changed to a product other than the initially planned steel type.

In recent years, as a large amount of scrap is generated, the amount of scrap used tends to increase, and it is clear that in the future, when the carbon steel is melted, the hot metal ratio is lowered and the scrap ratio is increased to increase the number of operations. In such a situation, it is considered that the probability that the Cr content of the steel material will be out of the standard due to the inclusion of stainless steel scrap will become higher, and the width of the outlier will further increase.
It cannot be dealt with only by "changing steel type".

The above-mentioned problem of the Cr content in the converter molten steel not meeting the standard also occurs in the electric furnace molten steel containing a large amount of scrap.

In order to solve the problem of the Cr content out of specification of the molten steel as described above, the applicant of the present invention stated that "CaO, SiO ₂ , F
We have developed a "method of dechromizing molten steel" characterized by "contacting with an oxidative, preferably low basicity flux mainly composed of e ₂ O ₃ ," and filed the following patent application.

Japanese Patent Application No. 63-52099 (Japanese Patent Application Laid-Open No. 1-225717) Japanese Patent Application No. 63-83598 (Japanese Patent Application Laid-Open No. 1-255616) Japanese Patent Application No. 63-319877 (Japanese Patent Application Laid-Open No. 2-163310) Japanese Patent Application Japanese Patent Laid-Open No. 63-319878 (Japanese Unexamined Patent Publication No. 2-163311) In the above and the above, the total content of iron (T.Fe) in the slag, which is an index of the Cr removal ratio, the basicity of the flux, and the oxidizing power of the slag, Based on the relationship, a method to determine the composition of Cr-free flux was proposed. For example, in order to achieve a Cr removal rate of 20% or higher, the basicity of the flux should be 0 to 1, 1
~ 2, 2-3, and 3 or more, (T.Fe) is 7.
Fe oxide may be added to the flux so as to be 5% or more, 5% or more, 10% or more, and 15% or more. Also,
To achieve a Cr removal rate of 50% or more, (T.Fe) is 11 each.
%, 15% or more, 22% or more, and 22% or more may be mixed with Fe oxide in the flux.

In the present specification, unless otherwise specified,% means% by weight, components shown in () are components in slag or flux, and those shown in [] are components in molten steel.

In addition, in the case where it is necessary to prevent oxidative loss of Mn, which is a beneficial component, it is proposed to premix Mn oxide in the flux, and in order to further improve the yield of Mn, It was proposed to add carbonaceous material into the slag after the Cr removal treatment. Therefore, it is proposed to add MgO and / or ZrO ₂ in a total amount of 10% or more to improve the Cr removal rate,
It has the effect of preventing melting of refractory materials.

The impure Cr component mixed in the molten steel can be removed by applying the dechromization method for molten steel proposed by the applicant to the actual operation. However, if it is attempted to further reduce chromium, there is a problem that the treatment time becomes longer, leading to a decrease in productivity, and in order to compensate for the decrease in molten steel temperature during treatment, it is necessary to raise the tapping temperature. There are difficulties.

(Problems to be Solved by the Invention) The present invention has been made on the basis of the above-mentioned inventions to solve the problems in the actual operation. An object of the present invention is to provide a method for rapidly and effectively removing impurities Cr of steel melted in a converter or an electric furnace while suppressing a decrease in molten steel temperature as much as possible without lowering productivity.

(Means for Solving the Problems) The present inventors have found that the above-mentioned CaO, SiO ₂ , and Fe ₂ O ₃ as main components are oxidizable and desirably low basicity fluxes (hereinafter referred to as “deoxidized”).
The operation of removing Cr from molten steel by means of "Cr flux") was investigated, and a solution to the above problems of the treatment method was examined. As a result, the following facts were confirmed.

(1) When O ₂ gas is supplied to molten steel,
Even if the flux containing a smaller amount of the Fe oxide compounded is added by subtracting the Fe oxide equivalent amount of the O ₂ gas supply from the appropriate amount of the Fe oxide compounded, the same or higher deCr removal effect can be obtained. Therefore,
It is possible to shorten the processing time for removing Cr to the same level of [Cr].

(2) It is possible to suppress the decrease of the molten steel temperature during the treatment due to the heat of oxidation reaction of the molten steel due to the supply of O ₂ .

The present invention was made based on the above findings,
The summary is "Cr in undeoxidized molten steel produced by converter or electric furnace.
When the content exceeds the target value, a flux containing Fe oxide is added to the molten steel and oxygen gas is supplied according to the following equation (1) to determine the basicity (X) of the flux and the total Fe in the slag. (T.Fe) content is adjusted as follows, and the method for dechromizing molten steel is described in ".

When 0 ≦ X <1,… (T.Fe) ≧ 7.5% When 1 ≦ X ≦ 2 ……… (T.Fe) ≧ 5% When 2 <X ≦ 3 ……… (T.Fe ) ≧ 10% 3 <X ……… (T.Fe) ≧ 15% However, the basicity (X) of the flux is It is represented by, and the content in () is the compounding ingredient in the flux, and% is% by weight.

However, Qo ₂ : Amount of oxygen transfer (Nm ³ / T ・ molten steel) W: Dechromation when not supplying O ₂ Appropriate amount of flux (kg / T ・ molten steel) A: Dechromation when not supplying O ₂ Fe oxides in the flux proper composition of the (Fe ₂ O ₃ basis) (wt%) B: proper formulation composition (wt% of Fe oxides dechromized in flux used in the present invention (Fe ₂ O ₃ basis) (Operation) Next, details of each processing operation of the Cr removal method of the present invention will be described.

In the following description, molten steel after converter blowing will be taken as an example to explain the Cr removal operation.

In the method of the present invention, the mixed flux or the synthetic flux having the above-described composition is added after the converter blowing for the purpose of removing Cr, but the addition timing is after the converter blowing and before the deoxidation. It is preferable to add the converter slag as soon as possible after removing the converter slag after the completion of the converter blowing. Therefore, it is generally processed in the ladle after tapping, but if necessary, the converter slag may be removed in the converter before the Cr removal treatment.

In addition, when tapping in a ladle and performing Cr removal treatment in it,
It is desirable to remove the converter slag that is unavoidably mixed in the ladle, but if reconstitution P is not a problem, the slag component composition equal to the above-mentioned flux composition with a small amount of converter slag left. The composition of the added flux may be adjusted so that

The reason why the Cr removal treatment is performed before deoxidation is that it is necessary to contain oxygen in the molten steel as well as in the slag in order to oxidize and remove [Cr]. There are no particular restrictions on the converter blowing prior to the Cr removal treatment, and normal operating conditions are sufficient. Only when the Cr content finally exceeds the target value, the treatment according to the present invention may be performed.

Although the purpose of the flux addition method can be achieved even by introducing the flux from above the molten steel, the injection into the molten steel is more effective.

According to the method of the present invention, a part of the Fe oxide in the Cr-free flux is converted into O ₂
One of the features is that it is replaced with gas and supplied to molten steel.
Cr content (usually about 0.05 to 0.1%) and simultaneous de-C
It is determined by the amount, etc., but is the addition amount of the deoxidized Cr flux when the O ₂ gas is not supplied, which is the normal addition amount of 10 to 50 kg / T ・ molten steel, minus the Fe oxide equivalent of the O ₂ gas supply amount. Is enough.

On the other hand, the O ₂ gas may be supplied in an amount calculated by the following formula (1) by an appropriate method such as the top blowing method or the injection method.

However, Qo ₂ : Amount of acid transfer (Nm ³ / T / molten steel) W: Proper amount of Cr removal flux when O ₂ is not supplied (kg
/ T · molten steel) A: O ₂ Fe oxides in the de Cr flux when not supplying (Fe
₂ O ₃ equivalent) Proper composition (%) B: Fe oxide in the flux used in the present invention (Fe ₂ O ₃ conversion) composition (%) where Fe oxide and O ₂ in the flux The proportion of the gas may be determined according to the cooling rate of the reaction vessel, the amount of temperature drop during the Cr removal treatment, which depends on the amount of molten steel and the like.

In order to agitate molten steel and slag to accelerate the Cr removal reaction,
It is preferable to use bubbling stirring with argon or stirring with natural dropping of molten steel to which flux is added at the time of tapping.

It is desirable to remove slag after Cr removal in order to prevent re-Cr in the degassing process such as deoxidation and RH treatment in the next process. May be adopted.

Although the above description has been given taking the case of converter steel melting as an example, it goes without saying that the method of the present invention can also be applied to the case where carbon steel is melted using scrap as the main raw material using an electric furnace.

Next, the composition of the flux used for carrying out the method of the present invention will be described more specifically.

Figure 1 is, Fe ₂ O ₃ de Cr process of the present invention and Fe ₂ O ₃ amount in the flux when the implement (O ₂ top-blown gas), after treatment in the slag (T.Fe) It is a figure which shows the relationship between a conversion amount and a Cr removal rate. The compositional composition of the flux and the amount of acid transfer are as shown in Table 1 described later. As a comparative method, a case of performing Cr removal treatment using a Cr removal flux shown in Table 1 described later without supplying O ₂ gas is shown.

As shown in the figure, in the method of the present invention, the content of Fe ₂ O ₃ in the flux is 10 to 20%, which is lower than the deCr flux of the comparative method, but the slag after the treatment by the O ₂ gas top blowing is The amount of (T.Fe) converted to Fe ₂ O ₃ increased to a level not much different from the comparative method. In addition, a higher Cr removal rate than the comparative method was obtained.

When only the Cr-free flux is used and O ₂ is not supplied, [Cr] is removed as Cr oxide into the slag by the oxidation reaction represented by the following formula (2). for that reason,
The flux must contain a large amount of Fe oxide.

[Cr] + x (FeO) → (CrO _x ) + x [Fe] (2) However, if O ₂ gas is blown onto the molten steel, the following (3)
Due to the oxidation reaction shown in the formula, (T.Fe) near the flash point increases and the [O] content increases, so
The Cr reaction is accelerated.

Since the generated Cr ₂ O ₃ is a weakly basic oxide, adding a low basicity flux is lower (T.Fe), but Cr ₂ O ₃ is stabilized in the slag and efficiently Cr can be removed.

Further, since the stirring of the slag is enhanced by the supply of O ₂ gas, the Cr ₂ O ₃ produced by the equation (4) is quickly stabilized in the slag.

Further, although the above equation (3) varies depending on the Y value, since it is an oxidation reaction with a high calorific value, it contributes to the heating of molten steel and promotes slag formation and reaction.

The flux used in the method of the present invention preferably contains at least one of CaO, SiO ₂ , CaF ₂ and Al ₂ O _{3 for} the purpose of lowering the melting point of the flux and adjusting the basicity. By adding such a melting point depressant, the melting point of the flux can be lowered to the Cr removal treatment temperature (1700 ° C to 1550 ° C) or lower. Also, this flux contains MgO
And / or ZrO ₂ is preferably added in a total amount of 10% or more for the purpose of preventing melting damage of the refractory. Cr removal of the present invention
Since the slag being treated has a high (T.Fe), it has a particularly strong erosion power against the refractory material of the converter or ladle. Therefore, in order to prevent the melting of the refractory due to the slag during the Cr removal treatment, it is desirable to preliminarily contain one or more compounds that are the main components of the refractory in the flux. In the case of slag containing MgO, MgO and Cr oxide tend to selectively form a compound. Further, in the case of slag containing ZrO ₂ , since ZrO ₂ is an acidic oxide, ZrO ₂ can stabilize Cr oxide, which is a basic oxide, in the slag. Therefore, not only to prevent melting damage of refractories but also to improve the Cr removal rate, Mg
O and ZrO ₂ are very effective additives.

Further, the flux used in the method of the present invention preferably contains 20% or more of Mn oxide for the purpose of preventing the oxidation loss of [Mn]. Since the slag during the Cr removal treatment has a strong oxidizing power, [Mn], which is a beneficial component in molten steel, is also oxidized and removed at the same time as [Cr]. Depending on the steel type, it is necessary to contain a large amount of (MnO) in order to prevent this. For example, in order for the molten steel to have an Mn yield of more than 50%, (MnO) in the slag must be 15% or more, and the amount of Mn oxide in the flux must be about 20% or more.

The form of the flux of the present invention may be a mixed flux obtained by mixing various compounds, or a mixture of synthetic flux obtained by melting and cooling the mixture once before adding it to molten steel and then pulverizing it. It may be used in the form.
The latter synthetic flux is preferable from the viewpoint of stability of flux properties.

Hereinafter, the method of the present invention will be described more specifically with reference to Examples and Comparative Examples.

(Example 1) 10 tons of undeoxidized low carbon steel molten steel melted in an electric furnace was placed in a ladle, and the composition shown in Table 1 was added while bubbling stirring with argon gas of 0.05 Nm ³ / min · T molten steel. 42.5 kg of flux
/ T / molten steel was added, and further, O ₂ gas 1.58 Nm ³ / T / molten steel was top-blown to remove Cr for about 20 minutes. In this example, the amount of Fe ₂ O ₃ added from the flux was set to 1 of Comparative Example 1.
This is the case where the amount is reduced to / 2 and the reduced amount is replaced by O ₂ top blowing.

(Comparative Example 1) Same as Example 1 except that 50 kg / T / molten steel of the Cr removal flux shown in Table 1 having a high Fe ₂ O ₃ composition of 30% was added and no O ₂ gas top blowing was performed. The Cr removal treatment was performed under the conditions.

The results of the Cr removal treatment are shown in Table 2. As shown in the table, in Example 1, although the (Fe ₂ O ₃ )% in the flux was as low as 17.6%, the converted (Fe ₂ O ₃ )% in the slag after the treatment was 30.
It became as high as 0%, and strong oxidizing slag was generated. The amount of decrease in the molten steel temperature during the treatment was compared with 100 ° C. of Comparative Example 1 in Example 1
It was halved to 50 ℃. The Cr removal rate of Example 1 was 64 of Comparative Example 1.
It was 78%, which was higher than the%, and the [Cr] after the treatment could be lowered in the same treatment time.

(Example 2) 10 tons of undeoxidized low carbon steel molten steel melted in an electric furnace was placed in a ladle, and the composition shown in Table 1 was added while bubbling and stirring the molten steel with argon gas of 0.05 Nm ³ / min · T molten steel. 40 kg of flux
/ T / molten steel was added, and further O ₂ gas 2.1 Nm ³ / T / molten steel was sprayed on to perform Cr removal treatment for about 20 minutes. In this example, the amount of Fe ₂ O ₃ added from the flux was set to 1/3 of that of Comparative Example 2.
This is the case when the amount is reduced and the reduced amount is replaced by blowing over O ₂ . In addition, (MnO ₂ ) was mixed in the flux.

(Comparative Example 2) Same as Example 2 except that 50 kg / T / melting addition of the Cr removal flux shown in Table 1 with a high composition of Fe ₂ O ₃ of 30% was not performed and O ₂ gas top blowing was not performed. The Cr removal treatment was performed under the conditions.

As shown in Table 2, in Example 2, (Fe ₂ O
₃ )% was as low as 12.5%, but the converted (Fe ₂ O ₃ )% in the slag after treatment became high at 30.3% by the O ₂ top blowing, and strong oxidizing slag was generated. The amount of decrease in molten steel temperature during the treatment was halved to 40 ° C. in Example 2 compared to 80 ° C. in Comparative Example 2. The Cr removal rate of Example 2 was 67%, which was higher than the 50% of Comparative Example 2.
In each of Example 2 and Comparative Example 2, a high value of 87.5% [M
n] The yield was obtained.

(Example 3) 90 tons of undeoxidized low carbon steel molten steel melted in a converter was placed in a ladle, and the composition shown in Table 1 was obtained by bubbling and stirring the molten steel with argon gas of 0.05 Nm ³ / min · T molten steel. Flux of 40kg / T
・ By adding molten steel and further blowing O ₂ gas 2.1 Nm ³ / T ・ molten steel, the Cr removal treatment was performed for about 20 minutes. In this example, the amount of Fe ₂ O ₃ added from the flux was reduced to 1/3 of that of Comparative Example 3, and the reduced amount was replaced by O ₂ top blowing. In addition, 25% (MnO ₂ ) in the flux, (MgO)
And (ZrO ₂ ) were added in a total amount of 25%.

(Comparative Example 3) Same as Example 3 except that 50 kg / T of molten steel was added as the Cr removal flux shown in Table 1 in which the composition of Fe ₂ O ₃ was as high as 30%, and no O ₂ gas top blowing was performed. The Cr removal treatment was performed under the conditions.

As shown in Table 2, in Example 3, (Fe ₂ O
₃ )% is as low as 12.5%, but the converted (Fe ₂ O ₃ )% in the slag after treatment by O ₂ top blowing is as high as 34.9%.
It produced a strong oxidizing slag similar to. The amount of decrease in molten steel temperature during the treatment was significantly reduced to 20 ° C. in Example 3 compared to 45 ° C. in Comparative Example 3.

The Cr removal rate of Example 3 was 75%, which was higher than 60% of Comparative Example 3, and the [Cr] after the treatment could be lowered in the same treatment time. The [Mn] yield was as high as 85% in both Example 3 and Comparative Example 3.

In addition, since there is no difference in the (ZrO ₂ )% in the slag before and after the treatment, the zircon brick (60% ZrO ₂ ,
It is considered that there is almost no melting loss of (% SiO ₂ ).

(Effect of the Invention) According to the method of the present invention, a strong oxidizing slag required for Cr removal during treatment can be generated even if the iron oxide content in the flux is reduced. Further, since the stirring of the slag is strengthened and the decrease of the molten steel temperature during the treatment is suppressed, the Cr removal reaction is promoted and [Cr] exceeding the target value can be removed quickly and efficiently. In addition, since the time required for the Cr removal treatment is short and the tapping temperature does not need to be particularly high, the Cr removal treatment can be performed without reducing the production efficiency.

[Brief description of drawings]

Figure 1 is, Fe ₂ of the Fe ₂ O ₃ amount in the flux when carrying out the de-Cr process of the present invention, the processed slag (T.Fe)
O ₃ is a diagram showing the relationship between the equivalent amount and de Cr ratio.

Claims (1)

[Claims] 1. When the Cr content in undeoxidized molten steel produced by a converter or an electric furnace exceeds a target value, a flux containing Fe oxide is added to the molten steel and the following ( Oxygen gas is supplied according to the formula 1), and the relationship between the basicity (X) of the flux and the content of total Fe (T.Fe) in the slag is adjusted as follows: Dechromization of molten steel Method. When 1 ≦ X ≦ 2 ………… (T.Fe) ≧ 5% When 2 <X ≦ 3 ………… (T.Fe) ≧ 10% When 3 <X ………… (T.Fe) ≧ 15% However, In (), the compounding ingredients in the flux are shown, and% is% by weight. However, Qo ₂ : Amount of oxygen transfer (Nm ³ / T ・ molten steel) W: Dechromation when not supplying O ₂ Appropriate amount of flux (kg / T ・ molten steel) A: Dechromation when not supplying O ₂ Fe oxides in the flux proper composition of the (Fe ₂ O ₃ basis) (wt%) B: proper formulation composition (wt% of Fe oxides dechromized in flux used in the present invention (Fe ₂ O ₃ basis) )