JP2022160777A - Smelting method of low phosphorus steel - Google Patents

Abstract

To provide a smelting method of low phosphorus steel with more improved dephosphorization efficiency during decarburization blowing.SOLUTION: In a smelting method of low phosphorus steel, in a converter type smelter equipped with a top blowing lance, dephosphorization blowing is carried out to separate the dephosphorization slag produced by dephosphorization blowing, and then decarburization blowing is carried out by feeding a lime source containing CaO or CaCO3 in the range of 30-70 kg/t-steel in total CaO equivalent mass, and the C concentration in the molten iron is reduced to less than 0.4 mass%. In the decarburization blowing, at the time of starting the blowing, 60-80% of the total CaO equivalent mass is put into the lime source, oxygen is sprayed from the top blowing lance at an acid feeding rate in the range of 150-250 Nm3/(h t-steel) to start the blowing, and at the time when 80-90% oxygen is supplied at a ratio of flow rate in the total oxygen amount from the time of starting the blowing, the rest of the lime source is put in and the acid feeding rate is reduced to 40-85% before the change.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for smelting low-phosphorus steel with improved dephosphorization efficiency during decarburization blowing.

Conventionally, dephosphorization blowing is performed to remove P from hot metal. However, in the case of smelting low-phosphorus steel, it is possible to efficiently remove P in the subsequent decarburization blowing. is important. In addition, in order to prevent phosphorus reversion from the dephosphorization slag, it is necessary to remove the dephosphorization slag as much as possible before decarburization blowing. However, since some of the dephosphorization slag remains as carryover slag, if a new lime source is added to adjust the basicity of the slag before decarburization blowing is performed to prevent replenishment of phosphorus, excess lime will be generated. and the liquid phase ratio of the slag decreases. As a result, the dephosphorization rate becomes excessively slow, so the dephosphorization efficiency is low during decarburization blowing, and sufficient dephosphorization cannot be achieved.

Therefore, various proposals have been made for efficient dephosphorization. Patent Literature 1 discloses a technique of changing the oxygen supply rate or charging a CaO source according to the blowing time. Patent Document 2 discloses a method in which a first slag agent is charged before and after the start of oxygen blowing, and a second slag agent is blown in using a carrier gas during decarburization blowing.

Japanese Patent No. 6053570 JP 2020-105562 A

However, in the technique described in Patent Document 1, the melting of slag is prioritized, and the slag basicity at the start of blowing is compared with the slag basicity in decarburization blowing of a normal low-phosphorus converter process. The dephosphorization capacity of the slag at the initial stage of blowing is greatly reduced. As a result, the CaO source additionally charged in the middle stage of blowing becomes excessive, and is not fully melted during blowing, failing to obtain a stable phosphorus reduction effect. The technique described in Patent Literature 2 requires modification of equipment for carrying out top blowing of the second slag agent in decarburization blowing. In addition, since the N concentration in the molten iron increases due to the nitrogen gas that acts as a carrier during top blowing of the second slag agent, there are restrictions on the blowing timing when applying to low nitrogen steel.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-phosphorus steel smelting method in which dephosphorization efficiency during decarburization blowing is improved by a simple operation.

If the dephosphorization slag is separated before decarburization blowing and a new lime source is added, the lime becomes excessive and the slag liquid fraction decreases. As a result, the dephosphorization rate becomes excessively slow until the middle stage of decarburization blowing, and phosphorus reversion from the carried-over slag occurs. On the other hand, in the final stage of decarburization blowing, the C concentration in the molten iron is difficult to decrease, and a large amount of FeO is produced. When a large amount of FeO is produced, the liquid phase ratio of the slag increases and the dephosphorization reaction tends to occur.

Therefore, the present inventors believe that the dephosphorization efficiency will increase if the liquid phase ratio of the slag can be secured until the end of the decarburization blowing, and a longer time can be secured at the end of the blowing while suppressing the decrease in yield. We have found that the amount of lime source input is suppressed at the start of blowing, and the remaining lime source is added at the end of decarburization blowing, and the oxygen transfer rate is reduced, leading to the present invention.

The present invention is as follows.
(1)
After separating the dephosphorization slag produced by dephosphorization blowing in a converter type refining apparatus equipped with a top blowing lance, a lime source containing CaO or CaCO ₃ is added at a total CaO equivalent mass of 30 to 70 kg / t. - A low-phosphorus steel smelting method in which decarburization blowing is performed by charging in the steel range and the C concentration in the molten iron is reduced to less than 0.4% by mass,
In the decarburization blowing, 60 to 80% of the total CaO-equivalent mass of the lime source is introduced at the start of blowing, and the oxygen feed rate is 150 to 250 Nm ³ /(ht- Steel) is blown to start blowing, and when oxygen is supplied at a rate of 80 to 90% of the total oxygen amount from the start of blowing, the rest of the lime source is added and fed. A method for melting low-phosphorus steel, characterized by changing the acid rate to the range of formula (1).
0.40≦F _{02_after} /F _{02_before} ≦0.85 (1)
Here, F _{O2_before} : the acid supply rate (Nm ³ /(h·t-steel)) before the rest of the lime source is added and the acid supply rate is changed, F _{O2_after} : the rest of the lime source is added and the acid feed rate (Nm ³ /(h·t-steel)) after changing the acid feed rate.

ADVANTAGE OF THE INVENTION According to this invention, the smelting method of the low phosphorus steel which further improved the dephosphorization efficiency during decarburization blowing can be provided.

It is a figure for demonstrating transition of P density|concentration in molten iron during decarburization blowing. FIG. 4 is a diagram for explaining the transition of the liquid phase ratio of decarburization slag during decarburization blowing; FIG. 4 is a diagram for explaining the extension of time in the final stage of decarburization blowing by changing the oxygen supply rate; FIG. 4 is a diagram for explaining the amount of FeO produced in the final stage of decarburization blowing; FIG. 3 is a diagram showing the relationship between the dephosphorization rate and the ratio _{FO2_after} / _{FO2_before} before and after decarburization blowing. FIG. 4 is a diagram showing the relationship between the oxygen transfer rate and the basic unit of the oxygen transfer amount;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, the low-phosphorus steel melted in the present invention is steel having a P concentration of 0.01% by mass or less.

First, before performing decarburization blowing, hot metal is subjected to dephosphorization blowing to reduce the P concentration in the hot metal in advance. In dephosphorization blowing before decarburization blowing, for example, it is preferable to set the P concentration in the hot metal to 0.04% by mass or less. After the dephosphorization blowing, the dephosphorization slag generated by the dephosphorization blowing is separated from the hot metal. The method of separating the dephosphorization slag from the hot metal after dephosphorization blowing is not particularly limited, but in the converter blowing process for low phosphorus steel, it is necessary to separate the dephosphorization slag and the hot metal as much as possible. Therefore, as a method for keeping the slag as little as possible, for example, after tapping the hot metal, the dephosphorization slag is discharged, and the tapped hot metal is charged into the converter again. It is preferable to separate 90% or more of the dephosphorization slag by these methods.

After separating the dephosphorization slag, a lime source containing CaO or CaCO ₃ is put into the converter to start decarburization blowing. In this embodiment, the details will be described later, but until the middle of the decarburization blowing, the liquid phase ratio of the decarburization slag is secured to promote the dephosphorization reaction, and furthermore, at the end of the decarburization blowing where a large amount of FeO is generated , to secure the time for the dephosphorization reaction as much as possible while suppressing the decrease in the yield of iron.

FIG. 1 is a diagram for explaining transition of P concentration in molten iron during decarburization blowing. Part of the dephosphorization slag produced by dephosphorization blowing inevitably remains as carryover slag. In order to separate the dephosphorization slag and the hot metal as much as possible as described above, the decarburization slag contains carryover slag, but the amount of _SiO2 decreases, and decarburization progresses due to excessive addition of the lime source. up to, the liquid phase ratio of the decarburized slag decreases, and the dephosphorization rate becomes excessively slow. Therefore, in the conventional example indicated by the dotted line in FIG. tend to rise.

Therefore, in this embodiment, the total amount of lime source input during decarburization blowing is set in the range of 30 to 70 kg / t-steel in terms of total CaO mass, and in order to ensure the liquid phase ratio of decarburization slag, decarburization At the time of starting blowing, the lime source is charged in an amount of 60 to 80% of the total CaO equivalent mass.

Here, the total input amount of lime source mentioned above is the amount of lime source generally input in decarburization blowing for low-phosphorus steel. If the total amount of lime source input is less than 30 kg/t-steel in terms of total CaO mass, the desired low-phosphorus steel cannot be melted. Further, if the total amount of lime source input exceeds 70 kg/t-steel in terms of total CaO mass, the dephosphorization effect becomes saturated, the amount of slag increases, and the cost of lime source increases. In addition, if the lime source to be added at the time of starting decarburization blowing is less than 60% of the total CaO equivalent mass, the CaO content of the decarburized slag will be small, and the lime source that has been added will not be dissolved in time and the final The amount of dephosphorization decreases. In addition, when the lime source to be introduced at the time of starting decarburization blowing exceeds 80% of the total CaO equivalent mass, the liquid phase ratio of decarburization slag in the first half of decarburization blowing decreases, especially in the middle period of decarburization blowing. The amount of dephosphorization of will decrease.

FIG. 2 is a diagram for explaining the transition of the liquid fraction of decarburized slag during decarburization blowing. In this embodiment, at the time of starting decarburization blowing, the lime source is added by 60 to 80% of the total CaO equivalent mass to ensure the liquid phase ratio of decarburization slag, especially in the middle of decarburization blowing He is trying to raise the dephosphorization reaction efficiency up to. As a result, as shown in FIG. 1, the concentration of P in the molten iron can be kept lower than in the conventional case, particularly in the middle stage of decarburization blowing.

As described above, when decarburization blowing is started, part of the lime source is left and decarburization blowing is started at an oxygen feeding rate in the range of 150 to 250 Nm ³ /(h·t-steel). Then, when 80 to 90% of the total oxygen flow rate is supplied from the start of blowing, the remaining lime source is added, and the oxygen supply rate is changed to the range of the following formula (1).
0.40≦F _{02_after} /F _{02_before} ≦0.85 (1)

Here, _{FO2_before} represents the acid feed rate (Nm ³ /(h·t-steel)) before changing the acid feed rate, and _{FO2_after} is the acid feed rate at the stage when the remaining lime source is added. This is the acid feed rate (Nm ³ /(h·t-steel)) after the change.

Here, the range of the oxygen supply rate of 150 to 250 Nm ³ /(h·t-steel) at the start of blowing is the oxygen supply rate of oxygen generally blown in decarburization blowing. If the oxygen supply rate at this time is less than 150 Nm ³ /(h·t-steel), decarburization will not be sufficiently carried out, and a lot of time will be wasted. Also, increasing the oxygen supply rate to over 250 Nm ³ /(h·t-steel) may be difficult in terms of facility specifications, and may cause operational problems such as molten iron scattering and refractory wear.

Further, when decarburization blowing is performed under the above conditions, if 80 to 90% of the total oxygen flow rate is supplied, C in the molten iron is generally removed, and FeO is easily generated. When FeO is generated, the liquid phase ratio of the decarburized slag increases as shown in FIG. 2, making the dephosphorization reaction more likely to occur. When the remaining lime source is added at a stage where the flow rate ratio in the total oxygen amount is less than 80% from the start of blowing and the oxygen supply rate is changed to the range of formula (1), the additionally added lime source decarburizes The liquid phase ratio of the slag is excessively lowered, and the dephosphorization efficiency is lowered. Furthermore, since the oxygen supply is suppressed at the stage where the decarburization reaction is still occurring significantly (the stage where FeO is not yet generated much), excessive oxidation of Fe occurs and the yield of iron decreases. In addition, when the remaining lime source is added at the stage where the flow rate ratio in the total oxygen amount exceeds 90% from the start of blowing and the oxygen supply rate is changed to the range of formula (1), FeO is added to the decarburized slag. Sufficient time cannot be secured in the final stage of decarburization blowing, which has a high ratio, and the slag of the additionally supplied lime source becomes insufficient, so that a sufficient dephosphorization effect cannot be obtained.

FIG. 3 is a diagram for explaining the extension of time in the final stage of decarburization blowing by changing the oxygen supply rate. In FIG. 3, period II represents the middle stage of decarburization blowing in which the decarburization reaction is progressing remarkably, and period III represents the final stage of decarburization blowing in which the decarburization reaction decreases and the production of FeO increases. represents. For example, if the oxygen supply rate is set to 60000 Nm ³ /h until the middle stage of decarburization blowing and the oxygen supply rate is lowered when an additional lime source is added, the time at the end of decarburization blowing is reduced when the total amount of oxygen is the same. can be extended.

In addition, as shown in FIG. 4, when the oxygen supply rate is lowered, the FeO concentration in the decarburization slag becomes lower than when the oxygen supply rate is not lowered. It is not supply rate-limiting, but mass transfer rate-limiting of P between molten iron and decarburized slag. Therefore, even if the oxygen supply rate is lowered, the amount of dephosphorization per unit time is almost unchanged. Therefore, if the total oxygen content is the same, the P concentration in the molten iron decreases as the oxygen supply rate is lowered and the time is extended. . Therefore, it is possible to avoid excessive oxidation of Fe (reduced yield of iron) due to an increase in the total amount of oxygen and a significant extension of time, thereby improving dephosphorization efficiency.

FIG. 5 is a diagram showing the relationship between the dephosphorization rate and the ratio _{FO2_after} / _{FO2_before} before and after decarburization blowing. As shown in FIG. 5, it can be seen that the dephosphorization rate is 75% or more by adjusting the oxygen feeding rate so as to satisfy the formula (1). If the ratio F _{02_after} /F _{02_before} is adjusted to more than 0.85, the dephosphorization effect cannot be obtained because the final stage of decarburization blowing, in which the dephosphorization reaction tends to occur, cannot be ensured sufficiently. On the other hand, when the ratio F _{02_after} /F _{02_before} was adjusted to less than 0.40, the difference between the actual value of the P concentration, which is the driving force for the dephosphorization reaction, and the equilibrium value decreased, and the time at the end of the decarburization blowing was extended. Therefore, the dephosphorization effect is saturated. In addition, the top-blown oxygen jet becomes an excessively soft blow, and the FeO reduction reaction rate in the decarburized slag due to the bottom-blown stirring relative to the FeO generation rate increases, and the increase in the FeO concentration in the decarburized slag slows down. The dephosphorization efficiency rather decreases.

Also, before or after adding the lime source, the oxygen supply rate may be changed as appropriate. FIG. 6 is a diagram showing the relationship between the oxygen transfer rate and the basic unit of the oxygen transfer amount. If the oxygen supply rate is changed before changing the oxygen supply rate at the stage of adding the lime source, the average oxygen supply rate obtained by dividing the amount of oxygen supplied during that period by the blowing time is set to _{FO2_before} . . Similarly, when the acid feeding rate is changed after changing the acid feeding rate at the stage of adding the lime source, the average acid feeding rate is set to F _{O2_after} .

Moreover, in decarburization blowing in this embodiment, C concentration in molten iron shall be reduced to less than 0.4 mass %. This is because, when melting high-carbon steel having a C concentration of 0.4% by mass or more, it is necessary to stop blowing oxygen at a stage where FeO is difficult to generate. Furthermore, the stirring conditions are not particularly limited, but the general decarburization blowing conditions are used according to the type of bottom-blown gas and the shape of the tuyeres.

As described above, at the start of decarburization blowing, 60 to 80% of the total CaO equivalent mass of the lime source is added, and from the start of blowing, at the time of supplying 80 to 90% of the total oxygen flow rate. By adding the remaining lime source and changing the oxygen supply rate to the range of formula (1), the dephosphorization reaction is promoted even in the middle stage of decarburization blowing, and the decarburization blowing reaction is likely to occur. It is possible to extend the final stage of smelting and improve the dephosphorization efficiency without lowering the iron yield.

Next, an example of the present invention will be described, but this condition is an example of conditions for confirming the feasibility and effect of the present invention, and the present invention is limited to the description of this example. is not. The present invention can be implemented in various ways to achieve the objects of the present invention without departing from the gist of the present invention.

Hot metal tapped from a blast furnace was charged into a converter, and dephosphorization blowing was performed to remove P until the P concentration in the hot metal became 0.040% by mass or less. Thereafter, hot metal was tapped and dephosphorization slag was discharged to separate the hot metal and dephosphorization slag to obtain 320 tons of pretreated hot metal. After that, the treated hot metal is returned to the converter, and the carbonaceous material, FeSi, and lime source (CaO) as heating materials are added to decarburize the hot metal, and the C concentration is reduced to 0.03% by mass. It was reduced to less than 0.4% by mass. At this time, the P concentration in the hot metal before decarburization blowing is in the range of 0.02 to 0.04% by mass, and the total amount of lime source newly added in the decarburization blowing is the total CaO equivalent mass per 1 ton of hot metal. 50 to 60 kg, and the average oxygen supply rate F _{O2_before} was set to 191 to 213 Nm ³ /(h·t-steel).

The effect of the present invention was evaluated by the dephosphorization rate of hot metal and the FeO concentration in slag. The dephosphorization rate was calculated by the following formula (2) from the P concentration obtained by chemical analysis of the metal samples collected before and after decarburization blowing, and the FeO concentration was analyzed by slag chemical analysis after decarburization blowing. In this example, it was determined that the effects of the invention were obtained under the conditions that both the dephosphorization rate was 75% or more and the FeO concentration was less than 30% by mass.
Dephosphorization rate = 100 * ([P] _before deC - [P] _after deC) / [P] _before deC (2)
In addition, [P] _before decarburization represents the P concentration in molten iron before decarburization blowing, and [P] _after decarburization represents the P concentration in molten steel after decarburization blowing.

As shown in Table 1, Example No. 1 to No. In No. 4, the dephosphorization rate was 75% or more, and the FeO concentration in the decarburized slag was less than 30% by mass.

On the other hand, Comparative Example No. In 5, since the lime source was not divided and the oxygen supply rate was not changed, the liquid phase ratio of the decarburization slag was low until the middle stage of the decarburization blowing, and the dephosphorization reaction easily progressed at the end of the decarburization blowing. could not be extended. As a result, the dephosphorization rate was low. Comparative example no. In 6, the amount of lime source input at the start of decarburization blowing was small, so the lime source input at the end of decarburization blowing increased, and as a result, the additionally input lime source was not completely dissolved and degassed. Phosphorus rate was low. Comparative example no. In No. 7, the amount of lime source input at the start of decarburization blowing was too large, so the liquid phase ratio of decarburization slag was low until the middle stage of decarburization blowing. As a result, the dephosphorization rate was low.

Comparative example no. In No. 8, since the remaining lime source was added and the oxygen supply rate was changed too quickly, the decarburization slag liquid phase ratio was low in the middle stage of decarburization blowing, and the dephosphorization rate was low. Furthermore, the FeO concentration in the decarburized slag was also high due to excessive oxidation of Fe. Comparative example no. In 9, since the remaining lime source was added and the oxygen transfer rate was changed too slowly, sufficient time could not be secured at the end of decarburization blowing, and the additionally added lime source was insufficiently turned into slag. Phosphorus rate was low.

Comparative example no. In No. 10, since the oxygen supply rate was too low when the remaining lime source was added, the increase in the FeO concentration in the decarburization slag slowed down, the increase in the liquid fraction of the decarburization slag became too slow, and the dephosphorization rate decreased. was low. Comparative example no. In No. 11, the decrease in the oxygen supply rate at the time of charging the remaining lime source was too small, so the effect of extending the time at the end of decarburization blowing was not so much obtained, and the dephosphorization rate was low.

Claims (1)

After separating the dephosphorization slag produced by dephosphorization blowing in a converter type refining apparatus equipped with a top blowing lance, a lime source containing CaO or CaCO ₃ is added at a total CaO equivalent mass of 30 to 70 kg / t. - A low-phosphorus steel smelting method in which decarburization blowing is performed by charging in the steel range and the C concentration in the molten iron is reduced to less than 0.4% by mass,
In the decarburization blowing, 60 to 80% of the total CaO-equivalent mass of the lime source is introduced at the start of blowing, and the oxygen feed rate is 150 to 250 Nm ³ /(ht- Steel) is blown to start blowing, and when oxygen is supplied at a rate of 80 to 90% of the total oxygen amount from the start of blowing, the rest of the lime source is added and fed. A method for melting low-phosphorus steel, characterized by changing the acid rate to the range of formula (1).
0.40≦F _{02_after} /F _{02_before} ≦0.85 (1)
Here, F _{O2_before} : the acid supply rate (Nm ³ /(h·t-steel)) before the rest of the lime source is added and the acid supply rate is changed, F _{O2_after} : the rest of the lime source is added and the acid feed rate (Nm ³ /(h·t-steel)) after changing the acid feed rate.

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