KR101355680B1 - Method for manufacturing steel - Google Patents

Abstract

(Problem) We propose a method for producing high-cleanliness steel that can reduce the occurrence rate of defects caused by oxide inclusions.
(Solving means) Calcium carbonate in a converter blowing process for blowing steel from a converter, a tapping step for tapping the molten steel blown in the converter blowing process together with slag to a ladle, and slag floating on the molten steel received in the ladle. After addition of carbon dioxide to form a state, the Al residue was dispersed on the slag and reacted with FeO in the slag, (T.Fe) in the slag: 10 mass% or less, (CaO) / (Al ₂ O ₃ ): A slag reforming step of 1 to 2 at a mass ratio, a secondary refining step of decarburizing the slag-modified molten steel in an oxygen deodorizing vacuum degassing apparatus, reducing the [C] in the molten steel to 100 mass ppm or less, and the above-mentioned. A method for producing steel, comprising a continuous casting step of continuously casting secondary refined molten steel in an oxygen-free atmosphere, and reducing [O] to 50 mass ppm or less in molten steel after completion of secondary refining.

Description

Method of manufacturing steel {METHOD FOR MANUFACTURING STEEL}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing high-clean steel, and more particularly, to a method for manufacturing high-clean steel, which can reduce the incidence of defects caused by oxide inclusions, particularly Al ₂ O ₃ inclusions in a product. .

In recent years, with the increase in demand for surface-treated steel sheets made of ultra-low carbon steel mainly used for automotive steel sheets, there is an increasing demand for improvement of surface quality and internal quality as well as improvement of workability. Extremely low carbonization and low oxygenation are strongly desired.

The conventional ultra low carbon steel solvent process includes a blowing process in which oxygen is blown into molten steel under atmospheric pressure using a deodorizing converter, and crude decarburization is carried out to 0.03 to 0.05 mass%, which has a low oxidation loss of iron. When the crude molten steel is tapped into the ladle, a slag reforming process is added to add a slag modifier to reform the slag floating on the molten steel, and vacuum decarburization degassing treatment of the molten steel having the modified slag. Usually it consists of a secondary refining process.

The secondary refining for obtaining ultra-low carbon steel having a final C of 100 mass ppm or less is carried out by vacuum decarburization using an RH vacuum degassing apparatus, or the like. It is common to decarburize to 0.03 to 0.05 mass%, to secure 300 mass ppm or more of [O] in steel, and to further reduce this to C: 100 mass ppm or less by vacuum decarburizing in an oxygen deodorizing RH vacuum degassing apparatus.

On the other hand, as a technique for reducing the amount of slag flowing out of a converter among the low oxygenation techniques of ultra low carbon steel, for example, Patent Literature 1 uses a tapping hole stopper to reduce the amount of outflow slag. Patent Documents 2 to 4 propose a method of adding a slag reduction flux onto a ladle slag.

Japanese Laid-Open Patent Publication No. 60-135511 Japanese Laid-Open Patent Publication No. 59-070710 Japanese Laid-Open Patent Publication No. 02-066111 Japanese Laid-Open Patent Publication No. 07-041824

In the said prior art, in the outflow slag amount reduction method disclosed by patent document 1 which uses the stopper hole stopper, an effect cannot be acquired stably. In particular, in the case of ultra low carbon steel having a final C of 100 mass ppm or less, it is difficult to stably reduce [O] to 20 to 50 mass ppm only by reducing the amount of outflow slag. In addition, in the method of adding the slag reduction flux on the slag disclosed in Patent Literature 2 or Patent Literature 3, P moves from slag to molten steel together with a decrease in (T.Fe) in the slag (simple P) Therefore, there exists a problem that there exists a possibility of going out of the prescribed molten steel component range. On the other hand, in the technique disclosed in Patent Document 4, this problem can be solved to some extent by regulating the content of P by molten iron pretreatment, but the effect of reducing (T.Fe) in the ladle slag after flux addition is not sufficient. There is a problem

As a result, in the prior art, surface defects and internal defects caused by oxide inclusions in the product stage frequently occur.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the prior art, and its object is to stably reduce the defect occurrence rate caused by oxide inclusions, particularly Al ₂ O ₃ inclusions, in products made of ultra low carbon steel. The present invention proposes a method of manufacturing high-cleanness steel.

The inventors earnestly examined in order to solve the said subject. As a result, when tapping the molten steel into the ladle in the converter, quicklime and / or light dolomite is added to melt the slag, and after the end of the tapping, CaCO ₃ is added to the slag to cause CO ₂ gas agitation. Thereafter, Al residues were dispersed and reacted with FeO in the slag, and further, CaO for adjustment was added to adjust (T.Fe) in the slag to 10 mass% or less, and CaO / Al ₂ O ₃ in the range of 1-2. After slag reforming, it was found out that it is preferable to carry out secondary refining and continuous casting in an oxygen uptake vacuum degassing apparatus, and came to develop this invention.

That is, the present invention is a converter blow process for blowing the steel in the converter, a tapping process for tapping the molten steel blown in the converter blow process to the ladle with slag, and floating on the molten steel received in the ladle After adding calcium carbonate to the slag to make carbon dioxide, the Al residue was dispersed on the slag and reacted with FeO in the slag, (T.Fe) in the slag: 10 mass% or less, (CaO) / (Al ₂ O ₃ ): a slag reforming step of 1 to 2 by mass ratio, and degassing the slag-modified molten steel in an oxygen squeezing vacuum degassing apparatus, and deoxidizing after reducing [C] in molten steel to 100 mass ppm or less. A refining process and a continuous casting process of continuously casting the secondary refined molten steel in an anoxic atmosphere, and a method for producing steel in which [O] in the molten steel after the completion of the secondary refining is reduced to 50 mass ppm or less.

In the method for producing a steel of the present invention, in the slag reforming step, the product of (T.Fe) and ladle slag amount in slag after slag reforming is represented by the following formula (1);

(T.Fe) (mass%) x amount of ladle slag (kg / molten steel T) ≤30 (1)

The amount of Al residue is adjusted in accordance with the analysis value of [O] or slag (T.Fe) in molten steel at the time of turning the converter.

Moreover, the manufacturing method of the steel of this invention is characterized by adding quicklime and / or light dolomite while it is outgoing in the said tapping process.

Moreover, the manufacturing method of the steel of this invention is characterized in that, in the slag reforming step, after the Al residue is dispersed on the slag, quicklime is further added.

According to the present invention, after the tapping of the molten steel blown from the converter is finished, CaCO ₃ is added to the slag to bring about CO ₂ gas agitation, and then Al residues are dispersed to (T.Fe) in the slag 10 mass. % Or less, (CaO / Al ₂ O ₃ ) is subjected to slag reforming in the range of 1 to 2, whereby [C] ≦ 100 massppm and [O] ≦ 50 massppm in steel after secondary refining stably. Since this can be achieved, it is possible to provide a steel with a very low incidence of defects resulting from inclusions in the product.

1 is a graph showing the relationship between (T.Fe) x slag amount in ladle slag and [O] in steel after secondary refining.
FIG. 2 is a graph showing the relationship between the amount of steel after the secondary refining and the defective occurrence rate resulting from the oxide inclusions in the product.
3 is a schematic cross-sectional view of an oxygen-rich RH vacuum degassing apparatus used in the present invention.

The manufacturing method of the high cleanliness steel of this invention is demonstrated concretely.

<Electric converter blow process>

In the blowdown of the steel, the necessity of obtaining a predetermined steel component in a short time is preferably using an upper bottom blowing converter. In order to prevent deviation of the component specification by the double P in the slag reforming process mentioned later, it is preferable to use the pretreatment molten iron which dephosphorized P content to 0.10 mass% or less by molten iron pretreatment. . In addition, it is preferable that C at the time of converter is 0.10 mass% or less. In addition, since the amount of slag at the time of converter conversion affects the addition amount of the reducing agent at the time of slag modification, it is preferable to make it as small as possible. As a method for this, it is preferable to use a tap for tapping holes or to perform an extrapolated slag stopper for performing slag slag refining. As another method, a method of raising and solidifying CaO / SiO ₂ may be used.

<Taping process>

When the molten steel after blown out taps into the ladle, the outflow converter slag discharged to the ladle together with the molten steel is preferably limited to the range of 3 to 15 kg / molten steel T. Moreover, it is preferable to add quicklime or light dolomite or both during tapping, to promote melting with slag by tapping flow, and to adjust CaO concentration in slag more uniformly.

<Slag reforming process>

On the surface of the molten steel received in the ladle, slag having a T (Fe) of 25 mass% or less is suspended, and calcium carbonate CaCO ₃ is added to the slag. After the slag is stirred with carbon dioxide CO ₂ generated from calcium carbonate, Al residue is dispersed on the slag to react the Al residue with FeO in the slag, and after completion of the reaction, quicklime CaO for composition adjustment is added as necessary. Add. The Al residue is usually added in raw units of 0.22 kg / molten steel T to 100 mass ppm of oxygen content at the time of converter, but this value increases and decreases according to the stirring ability of the converter. In addition, Al residue is a cost-effective reducing agent as a slag modifier, The main component composition is as Table 1 shows.

Here, the purpose of adding CaCO ₃ on the slag is that the slag flowing out of the ladle has poor fluidity and does not react well with the slag even if it scatters Al residues, so that CaCO ₃ is added and thermally decomposed by CO ₂ generated. This is to cause gas agitation to stir and accelerate the reaction. The addition time of CaCO ₃ is preferable because the reactivity with the diffusion transition slag is higher than that after Al residue dispersion and direct reaction with molten steel does not occur well.

In addition, the reason for adding CaO as needed is as follows. Previous studies of the inventors, the addition of Al residues Al deoxidation and absorb generated Al ₂ O ₃ is in the handling killed in the RH vacuum degassing treatment of Al ₂ O _3, and then the process resulting from the reformed slag slag composition has a large capacity, CaO / Al ₂ O _3: it is found to be that of 1.4 ~ 1.8 as a mass ratio. Therefore, in order to increase the absorption capacity of the Al ₂ O _3, it is necessary to adjust the CaO / Al ₂ O ₃ in an appropriate range, it is effective to the addition of CaO as a controlling agent therefor.

Further, in the present invention, the reason for adjusting the CaO / Al ₂ O ₃ in the range of 1 to 2 by mass ratio is that when CaO / Al ₂ O ₃ is less than 1.0, Al ₂ O ₃ absorbed in the slag is saturated and At the same time, the viscosity of the slag becomes high and it becomes difficult for the Al ₂ O ₃ inclusions to be collected in the slag. On the other hand, when CaO / Al ₂ O ₃ exceeds 2.0, the melting point of the slag becomes 1600 ° C. or more and the slag becomes This is because it becomes difficult to solidify and collect Al ₂ O ₃ -based inclusions in the slag. CaO / Al ₂ O ₃ is preferably controlled to a range of 1.4 ~ 1.8 to one, preferably the weight ratio should be controlled to between 1.0 to 2.0 in mass ratio of at least.

The CaO is, therefore may be added in the amount of slag outflow with the CaCO _3. This is because molten steel is agitated by the CO ₂ gas generated by the decomposition of CaCO ₃ , the reaction of the unreacted Al residue is promoted, and the absorption capacity of the Al ₂ O ₃ -based inclusions is ensured.

By modifying the slag, (T.Fe) in the slag is 10 mass% or less, and further adjusted to the range of CaO / Al ₂ O ₃ : 1.0 to 2.0, during the kill treatment in the subsequent RH treatment. Inclusion mainly based on the generated Al ₂ O ₃ can be efficiently absorbed in the slag.

As described above, since the slag composition affects the absorption ability of Al ₂ O ₃ , the slag composition also greatly affects the failure rate resulting from the oxide inclusions in the final product. According to the previous studies of the inventors, as shown in Figs. 1 and 2, the product of (T.Fe) in slag after slag modification and the amount of slag per molten steel T in ladle is expressed by the following formula (1);

(T.Fe) [mass%] x amount of ladle slag (kg / molten steel T) ≤30 (1)

When the relationship is satisfied, it becomes clear that a product free of defects resulting from inclusions can be produced. Therefore, in order to reduce [O] in steel, it is preferable to reduce either (T.Fe) in a slag, or both or both amounts of slag, and to reduce the product of (T.Fe) and slag amount. Therefore, in the present invention, the addition amount of the Al residue is adjusted from the analysis of the [O] concentration at the time of turning the converter and the (T.Fe) concentration analysis value in the slag so that the slag in the ladle after slag reforming satisfies the above expression (1). It is preferable.

<2nd refining process>

In the secondary refining of the ultra low carbon steel in the present invention, it is preferable to use an RH vacuum degassing apparatus having a fresh oxygen lance, such as an oxygen-rich RH vacuum degassing apparatus used in the embodiment of the present invention. (T.Fe) in slag after slag reforming process is 10 mass% or less, but generally [O] in molten steel has an amount of 300 massppm or more, so in vacuum decarburization treatment,

[C] + [O] → CO ↑

Decarburization reaction is promoted.

However, when [O] in molten steel is insufficient, vacuum decarburization is carried out while blowing oxygen from the upper lance. As shown in FIG. 3, the oxygen deodorizing vacuum degassing apparatus raises the immersion pipe 6 by the reflux gas 5 blown into the molten steel 1 in the ladle 2 from the reflux gas inlet 4. Degassing in the degassing tank 7 and decarburizing and degassing, but simultaneously deoxidizing the molten steel 1 by deodorizing oxygen from the lance 8 drowned downward in the degassing tank. The degassed molten steel 1 is returned to the ladle 2 through the descending immersion tube 9. By repeating this reflux, it is decarburized and degassed and the ultralow carbon steel of [C]: 100 mass ppm or less is dissolved. The molten steel decarburized and degassed in this way is then deoxidized by adding Al from the addition chute 10 or the like, and then subjected to a kill process which continues refluxing for about 5 to 15 minutes, so that the concentration of [O] in molten steel is 30. Reduced to massppm or less.

Here, there is a relationship shown in FIG. 2 between [O] in the steel and the failure occurrence rate attributable to the oxide inclusions in the product, and when [O] in the steel is 50 mass ppm or less, the failure occurrence rate attributable to the inclusion is It can be reduced up to the level (defect rate: 1% or less) which is practically no problem. When [O] is 20 mass ppm or less, the defect occurrence rate is almost zero (zero). However, since the manufacturing cost is relatively high, the preferable level seen from the economic point of view is about 30 mass ppm.

<Continuous Casting Process>

The molten steel after the completion of the secondary refining is then injected into the mold via tundish from the ladle for continuous casting. In the continuous casting, a seal other than the injection hole in the upper portion of the tundish is sealed, and an inert gas such as Ar gas sufficient to fill the tundish is flowed to prevent reoxidation of the molten steel. It is preferable to prevent the slag from mixing in the tundish at the end of the injection of. In this case, the sealing method of the tundish may be, for example, a means of covering with a lid made of cast iron, and a method of preventing slag from mixing at the end of injection into the mold. Although there is a method of installing a slag automatic detector for detecting a difference between molten steel and slag permeability near the nozzle, the present invention is not limited to these means, and other molten steel may be reclaimed if it can be continuously cast in an oxygen-free atmosphere. You may use the prevention method and the prevention method of slag mixing.

[Example]

After the steel was blown in the upper blast furnace, the steel was subjected to slag reforming by tapping the steel on the ladle under the two conditions of the invention examples and the comparative examples shown below, followed by secondary refining by using an oxygen odor vacuum degassing apparatus, Process casting was carried out by continuously casting the steel slab to various steel products, and comparing the defective occurrence rate resulting from the oxide inclusions in the respective products. In addition, both the invention examples and the comparative examples use a tapping hole stopper and an extra slag stopper at the time of tapping to reduce the flow rate of the slag into the ladle, and in the continuous casting, all of them are completely injected into the mold. The gas was shut off and injected into the mold under an Ar gas atmosphere, and a slag automatic detector was mounted near the lower ladle nozzle to prevent the slag from being mixed.

Invention Example

C: 0.04 mass% in the bottom-lowering converter, and the steel is blown, and in the first half when the steel is pushed to the ladle, 0.9 kg / melt T lime is added to the ladle, and after finishing , 0.45 kg / molten steel T was added to generate CO ₂ gas, and the slag agitation was caused to occur. Then, 1.4 kg / mold T of Al residue was added as a slag modifier, followed by 0.87 kg / molten steel T. Slag was modified by addition of quicklime. Next, vacuum decarburization (limd treatment) was carried out in an RH vacuum degassing apparatus for 15 minutes to reduce C in the steel to 20 mass ppm, and then Al was added to deoxidize the remaining [O] in the molten steel. The killing time after Al addition was secured for 10 minutes to float the deoxidation product. In addition, the tapping conditions, slag modification conditions, and the details of the secondary refining are shown in Table 2.

Thereafter, the molten steel refined secondary was continuously cast into slab while preventing molten steel reoxidation and mixing of slag in tundish.

The slab obtained as described above was subjected to hot rolling, cold rolling, and finish annealing after that to obtain a product plate. In addition, in the final inspection line made of the said product board | plate, the incidence rate of defects resulting from an interference | inclusion was measured with the online automatic surface inspection apparatus, mailing a coil.

<Comparative Example>

The steel was blown out and pulled out under the same conditions as the invention example, and after the end of the tapping, 1.6 kg / molten steel T of Al residue was added as a modifier of slag, followed by 0.45 kg / molten steel and quicklime T, 0.87 kg / mold steel T, respectively. The slag was added to modify the slag, after which the RH degassing treatment was carried out under the same conditions as in the invention example, followed by continuous casting to obtain a slab. In addition, the tapping conditions, slag modification conditions, and the details of the secondary refining are shown in Table 2.

From Table 2, in the present invention, despite the addition amount of Al residue less than the comparative example, the product of (T.Fe) x slag amount is reduced and the slag is efficiently modified, as a result, the molten steel after secondary refining It can be seen that the [O] in the present invention can be reduced to 30 mass ppm or less, which is lower than that of the comparative example, and the defect occurrence rate caused by inclusions in the product is also reduced to 1/2 or less.

1: molten steel
2: ladle
3: slag
4: reflux gas inlet
5: reflux gas
6: rising immersion tube
7: vacuum degassing tank
8: Lance
9: descent immersion pipe
10: addition chute

Claims (5)

The converter blow process which blows a river from a converter,
A tapping step of tapping the molten steel blown in the converter blowing step together with the slag to the ladle,
After adding calcium carbonate to the slag suspended on the molten steel received on the ladle to generate carbon dioxide, Al residues were dispersed on the slag and reacted with FeO in the slag, and (T.Fe): 10 mass in the slag. % Or less, (CaO) / (Al ₂ O ₃ ): slag reforming step of 1 to 2 by mass ratio,
A secondary refining step of decarburizing the slag-modified molten steel in an oxygen odorous vacuum degassing apparatus, and deoxidizing the molten steel by reducing it to 100 mass ppm or less, and
It has a continuous casting process of continuously casting the secondary refined molten steel in an anoxic atmosphere,
A method for producing steel for reducing [O] to 50 mass ppm or less in molten steel after completion of secondary refining. The method of claim 1,
In the slag reforming step, in the molten steel at the time of the converter tapping or in the slag (T.Fe) so that the product of the slag after slag reforming (T.Fe) and the ladle slag amount satisfies the relation of the following formula (1). A method for producing steel, characterized by adjusting the amount of Al residue added in accordance with the analytical value of Fe).
group
(T.Fe) (mass%) x amount of ladle slag (kg / molten steel T) ≤30 (1) 3. The method according to claim 1 or 2,
In the tapping step, quicklime, light borosilicate, and produced ash and dolomite are added during the tapping step. 3. The method according to claim 1 or 2,
In the slag reforming step, the Al residue is dispersed on the slag, and then quicklime is further added. The method of claim 3, wherein
In the slag reforming step, the Al residue is dispersed on the slag, and then quicklime is further added.

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