JPS5952921B2 - Steel manufacturing method - Google Patents

Description

[Detailed description of the invention]

The present invention does not generate bubbles in the continuous casting process,
Rimmed steel equivalent c7) T, (AI) (totalAl
(Si content) and (Si) (Si content). Rimmed steel is a type of steel that is manufactured by intentionally performing almost no deoxidation during the smelting process, generating a large amount of CO gas during ingot formation, and washing away impurities from the solid interface through the rimming action of the bubbles. be. If this is to be manufactured by a method such as continuous casting, in which the slab is pulled out and moved as solidification progresses, if the generation of air bubbles is too intense, the movement of the molten metal in the mold will become active.
Otherwise, it is dangerous because the bumping phenomenon occurs from the deep part of unsolidified molten steel, and even if the generation of bubbles is weak, the solidification rate during continuous casting is fast, so minute bubbles are trapped near the surface and cause pinhole defects in the product. Continuous casting of rimmed steel is rarely carried out because of the problem of occurrence of . Therefore, the steel types applicable to continuous casting are limited to guild steels and semi-killed steels that have been deoxidized with AI or Si, including Si-killed steels, A1 guild steels, and Al-
(Si) in the molten steel increases due to the reduction of SiO2 in the slag or refractories of 8i killed steel by AI, and the presence of such Si causes deterioration of deep drawability, deterioration of Zn plating adhesion, and reduction of temper color. Quality problems such as cracking and bending of the collar steel sheet occurred, as well as problems in steel material processing such as deterioration of hole expandability and restrictions on cold rolling rate. In addition, especially in the case of A1 guild steel and Al-3i guild steel,
In addition to deterioration of slab quality due to the presence of Al2O3 clusters,
There are operational problems such as clogging of ladle nozzles or tundish nozzles due to AI deoxidation products, and cost problems such as the high cost of adding AI. Guild steel or semi-killed steel deoxidized by AI or Si has drawbacks due to the presence of AI or Si, so restrictions on usage cannot be avoided, and quality products that do not have the above drawbacks are required. The reality is that rimmed steel using the agglomeration and blooming method is used in these cases. However, the current trend is to expand the application of continuous casting, which has a higher yield and productivity than the ingot-blowing method, and the so-called rimmed equivalent steel, which contains almost no Si and has a low AI content Naturally, continuous casting is also desired, and several attempts have been made in response to this demand. In order to avoid the above-mentioned drawbacks such as deterioration of deep drawability, it is preferable that the (Si) content be as low as possible, but it must be at least 0.020% or less, which is acceptable as rimmed steel. In addition, in order to reduce the cost of adding A1 and to prevent an increase in [Si,l] in the molten steel caused by the reduction of SiO2 in the refractory and clogging of the ladle nozzle or tundish nozzle, it is necessary to However, in steel types such as hot-rolled steel sheets that require fixation of nitrogen in the steel, T and [AI] are preferably 0.010 to 0.
020%, and when box annealing a cold-rolled steel plate, the amount of A1 added is sufficient to prevent the generation of bubbles during continuous casting. If [AI] is 0.020% or less, it can be said that it shows a sufficient value as a product equivalent to rimmed steel. In general, for AI guild steel, T and [AI] are 0.
．． 0.030% or more. Therefore, below this value, T, (Ale.[Si,
If l is suppressed, deoxidation will naturally be insufficient, and it is thought that CO bubbles will be generated. Generally, the condition for the generation of bubbles in molten metal is that the internal pressure of the bubbles is greater than the sum of atmospheric pressure, the hydrostatic pressure of the molten metal at the location of the bubbles, and the pressure exerted on the bubbles by cohesive force. In other words, C generated by the reaction of components in molten steel
O5N2. If the sum of the pressures of gases such as H2 is smaller than the sum of the atmospheric pressure, hydrostatic pressure, and pressure due to cohesive force, bubbles will not be generated. By the way, the partial pressure of N2 gas and H2 gas is smaller than that of CO gas, so in order to prevent the generation of bubbles, it is necessary to suppress the generation of CO gas, but the aforementioned pinhole defect occurs in the thin solidified shell inside the mold. Since the small bubbles are trapped and bunched together, the hydrostatic pressure is extremely small, and the pressure exerted on the bubbles due to cohesive force is also small for bubbles of about 1 mm diameter, so in order to prevent bubble generation, the CO gas pressure must be increased. In other words, it is sufficient to reduce the atmospheric pressure to 1 atm or less. It is generated by the reaction of C in CO gas molten steel with free O as shown in equation (1), or C+0=CO(f
...(1) However, C: CO in molten steel O: Free O in molten steel CO (g): In the CO gas equilibrium state, C concentration, 1 O concentration, and CO gas pressure P
. 0, the relationship of equation (2) holds true, where K is the equilibrium constant. However, [C]: C concentration in molten steel [O]: Free C concentration in molten steel In this equilibrium state, free C concentration

When [0] is suppressed to 70 pIlrn or less, even if [C] is at a high concentration of about 0.3%, P. 0 becomes 1 atm or less. Therefore, in a state where solidification is progressing, if the amount of free carbon dioxide is suppressed to 70 ppm or less, the generation of CO gas can naturally be suppressed. Then [Si], T. When casting molten steel with a low [AI] using the continuous casting method, deoxidation can be carried out using another method without relying on deoxidation using Si or AI.
] From this point of view, methods for producing slabs that carry out deoxidation are disclosed in JP-A-53-73422 and JP-A-54-93618.
is publicly known. These are subjected to vacuum degassing treatment after converter refining, and are converted into [C] and free

After reducing [0], deoxidation is performed by adding AI (the former) or AI and Ti (the latter),
This allows continuous casting of molten steel with low [Si] and T [AI]. However, in these methods, splash is likely to occur because the vacuum is drawn in an undeoxidized state and in a state where the free [O] in the molten steel is high, and furthermore, the vacuum equipment is required to have a large exhaust capacity. Since the base metal attached to the furnace wall due to this splash falls off into the molten steel during the vacuum treatment, it also has drawbacks such as difficulty in controlling free [O]. In addition, because the temperature drop during vacuum treatment is large, the tapping temperature of the converter has to be raised, which leads to insufficient dephosphorization in the converter process and large melting loss of the converter refractories. There is. Further, since the vacuum treatment requires a long time, there are significant restrictions on the process, and there are also disadvantages in that the cost of the vacuum treatment equipment and the running cost of the vacuum treatment are high. Now, as mentioned above, CO generated in molten steel is generated by the reaction of formula (1). When the free [O] in the molten steel is high, P is in equilibrium with the C1 free O in the molten steel obtained from equation (2). Since 0 is higher than 1 atm, the pressure of the generated CO becomes higher than the atmospheric pressure, and CO bubbles are generated in the molten steel against the atmospheric pressure. Therefore, P is in equilibrium with C1 free O in molten steel. If deoxidation is performed so that 0 becomes 1 atm or less, no CO bubbles will be generated. In the above-mentioned known example, vacuum treatment is performed for this purpose, but
CO gas partial pressure P as in vacuum treatment. As a method of lowering 0, there is a method of introducing an inert gas such as Ar into molten steel to perform C deoxidation. In this C deoxidation, the CO gas partial pressure in the bubbles in the steel is diluted with inert gas and lowered, so that the C in the molten steel and the free O
This is caused by a reaction according to formula (1) with. That is, the CO bubbles generated according to equation (1) are diluted with an inert gas to lower the CO gas pressure and the generation of CO according to equation (1) is promoted, or a large amount of Ar bubbles are distributed in the molten steel (1). The CO generated by the reaction of formula (1) is diluted and mixed into Ar bubbles and discharged from the molten steel to promote the reaction of formula (1) and reduce the free [O] content in the molten steel. However, even if such stirring refining is performed after normal converter blowing, the freedom (0) at the end point of the converter is extremely high.
Furthermore, since the converter slag has a high T and Fe content such as FeO, a large amount of oxygen is supplied from the slag to the molten steel during stirring, and it is not easy to lower the free [O] content in the molten steel. Figure 1 shows the end point on the horizontal axis.

[0] and the end point free [O] on the vertical axis.
], the end point of a normal pure oxygen top-blowing converter (LD) [C
] and the end point free [O] are shown in the shaded area, and the solid line shows the relationship between [C] and the free end point in the equilibrium state when F'cc is 1 atm and the temperature is 1600°C.

This shows the relationship with [0]. In reality, CO2 is also generated, so this solid line is P. [C] in equilibrium when 0+PCO2 is 1 atm
Regarding the relationship between free [O] and free [O], if [C] is 0.1%, CO2 is less than 2% of CO+CO2, which is an extremely small amount and can be ignored. As is clear from the figure, the normal converter end point is free.

[0] is much higher than this equilibrium state, so the end point is free

In order to obtain molten steel with [0] of 300 pIITI or less, [C] must be 0.
．． It is necessary to stop blowing at 20% or more. The reason why the end point free [O] was set to be 300 ppm or less is that if it is higher than that, the following steps of stirring refining and mild AI deoxidation will result in a peak.

This is because [0] cannot be reduced to 70 ppm or less. After that, it is necessary to reduce [C] to the C concentration range required for rimmed steel by stirring and refining, but since stirring and refining takes a long time, the temperature of the molten steel decreases significantly, so it is not necessary to remove the steel from the converter. I have no choice but to raise the temperature. Therefore, as mentioned above, if you stop blowing at a high [C], dephosphorization will be insufficient and the converter refractories will be severely eroded, etc.
Various inconveniences occur, and the above-mentioned [C], free

It is extremely difficult and almost impossible to decarburize and deoxidize to a level of [0]. The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a method for producing a product equivalent to rimmed steel that can be continuously cast without generating bubbles even though it is not subjected to vacuum treatment. With the goal. The steelmaking method according to the present invention includes (1) a converter (for example, a so-called combined blowing furnace) in which gas can be introduced from below the bath surface, and oxygen blowing,
(2) Stirring and refining is carried out by introducing gas from below the surface of the steel bath during the appropriate period (all or part of the period) during oxygen blowing and after oxygen blowing, and (3) After stirring and refining, the steel is removed from the converter. (4) When casting slabs or blooms using a continuous casting machine, during tapping from the converter to the ladle after stirring and refining, and / Or, after tapping, deoxidize by adding AI to free the steel.

[0] is 7Q ppm or less, T,
(AI) Molten steel with an unavoidable content of 0.020% or less and [Si] is produced, and the molten steel is cast using a continuous casting machine, and a billet is cast using a continuous casting machine. In that case, after performing the treatments in (1) to (3), (4)' If necessary, deoxidation by adding AI during and/or after tapping from the converter to the ladle after stirring and refining. (5)' When casting the molten steel with a continuous casting machine, the continuous casting machine is A1 is added to the molten steel in the mold to perform deoxidation treatment, freeing the molten steel in the mold.

[0] below 70p, T. [Al] is made to be 0.020% or less, and [Si] is made to be an unavoidable content. Hereinafter, the steel manufacturing method according to the present invention will be explained in detail, following the processing steps in the order of the numbers mentioned above. (1) Free at the end point so that free [O] can be sufficiently reduced by the following stirring and refining steps.

In order to lower [0], an inert gas such as Ar is introduced below the surface of the steel bath for an appropriate period during oxygen blowing in the converter to stir the steel bath.
So-called complex blowing is performed in which oxygen blowing and stirring refining are combined. The blow stop [C] in this composite blowing is set to 0.05% or more for the reason described later. Introducing gas from below the surface of the steel bath compensates for the drop in stirring power, especially at the final stage of oxygen blowing, efficiently decarburizes oxygen blowing, prevents an increase in free CO in the molten steel, and removes FeO from the slag. Prevent concentration increase. Furthermore, Ar lowers the CO gas partial pressure and promotes the above-mentioned C deoxidation. Stirring gas may be introduced by using a normal converter and inserting a lance for the stirring gas into the steel bath in the converter, but as shown in Figure 2, a lance for stirring gas may be inserted into the bottom of the converter. The most efficient method is to use a composite blowing furnace equipped with a tuyere 1 and introduce stirring gas through the tuyere 1 to stir the steel bath. It goes without saying that the same effect can be obtained even if the tuyere is installed at the lower part of the furnace wall. The gases introduced from below the bath surface during oxygen blowing or the gases introduced during carbon deoxidation after blowing are Ar, Xe, and Kr.
Although inert gases such as Ar are preferable, Ar
It is preferable to use Considering the cost of stirring gas, C09CO2,
It is best to stir with an inexpensive gas such as N2, but for steel types that are sensitive to N absorption in molten steel, N2 gas such as Ar may be used during the N2 stirring.
It is necessary to switch to a gas that does not contain 2. Note that it is also possible to use 02 as a part of the stirring gas during oxygen blowing, or a combination of these gases may be used. The stirring gas during C deoxidation after oxygen blowing is preferably an inert gas such as Ar. CO is not suitable as a stirring gas during C deoxidation, and CO2 is also undesirable because it decomposes into CO and O2, increasing the CO gas partial pressure. Now, the stirring gas supply rate is preferably 0.03 Nm3/min or more per molten steel. If it is less than 0.038 m3/min, the stirring effect will be small, and if only stirring refining □ is performed, the deoxidation rate will be slow, resulting in a longer treatment time and a problem of lowering the molten steel temperature during treatment. Generally, decarburization progresses by oxygen injection, but the free [O] in the molten steel increases. Figure 1 shows [C] and free flow at the end point in the case of composite blowing using Ar as a stirring gas.

The relationship with [0] is shown by the black circle, and as mentioned above, stirring prevents the decarburization efficiency from decreasing at the final stage of oxygen blowing, increases free [O] in the molten steel, and increases Fe in the slag.
[Cax[O] is P because it prevents the increase in O concentration. It is close to the equilibrium state when o = 1 atm, and due to the C deoxidizing effect 1) P. 0 may drop to 1 atm or less. In addition, the broken line in the figure is P. O=0.5 atm, dashed line is P. . = 0.1 atm, each [Cax

[0] represents an equilibrium relationship. In this way, compound blowing has a free C compared to normal blowing (LD).
O) remains low, and the -CC) is free to be deoxidized in the post-process.

It has the advantage of having less [0] or

[0] The relationship is close to equilibrium and there is little variation. Therefore P. By controlling 0 and the end point [C]

[0] is almost uniquely controlled, and it can be said that compound blowing is an extremely excellent refining method. Next, only stirring is performed to measure C deoxidation, but in order not to impair this C deoxidation effect, oxygen blowing in combined blowing is performed so that [C] is 0.05% or more. , preferably at 0.08% or more. If free [C] is less than 0.05%, free [0]
] is too high at 400 ppm or more, C cannot be sufficiently deoxidized even by stirring, and when deoxidizing using AI, which will be described later, there is a large variation in free [0) and it is difficult to control it to a predetermined value. (2) Next, as an extension of the above-mentioned composite blowing, only the gas introduction from below the surface of the steel bath is continued. The results of stirring and refining the molten steel obtained by composite blowing using Ar are shown in Figure 1 by white circles, and as is clear from this figure,
The decarburization and deoxidation effects of stirring are remarkable, and [C] in molten steel is
,freedom

P in equilibrium with [0]. 0 has been deoxidized to about 0.1 to 0.5 atm. In addition, if the oxygen blowing end point [C] is 0.05% or more, free

[0] is deoxidized to 2009 Iln or less, and the amount of AI added in the AI deoxidation treatment described later can be extremely small. (3) By the way, if converter slag is present on the molten steel surface during AI deoxidation in the ladle, the slag component SiO
2 is partially reduced by formula (3), 3S102 + 4
Al→3Si + 2AI,,O"...
(3) A so-called Si pickup occurs in which [Si] in the molten steel increases. Therefore, in order to suppress the mixing of converter slag into the ladle during tapping, so-called slag cut tapping is performed to prevent slag from flowing out at the start of tapping and from flowing out at the end of tapping. One way to prevent slag from flowing out at the start of tapping is to stuff the tap hole, which is usually installed on the upper part of the furnace wall of a converter (combined blowing furnace), with rags, scales, etc. In addition, methods for preventing slag outflow at the final stage of tapping include the slag ball method, in which a refractory spherical object (slag ball) is dropped onto the tap to block the tap, and the slag stopper method, in which the tap is closed with a stopper. There is a known quicklime powder method in which quicklime powder is added to slag floating on the molten steel surface near the tap to solidify the slag and prevent the slag from flowing out from the tap. Furthermore, as a method for preventing slag outflow both at the start of tapping and at the end of tapping, there is also known a tapping loss sliding gate method in which a sliding valve is installed at the tapping port and tapping is controlled by opening and closing the sliding valve. There is. Furthermore, the amount of slag on the surface of the molten steel in the ladle can be reduced by adding quicklime to solidify the slag on the surface of the molten steel in the ladle, or by removing the slag in the ladle using a slag tracker, etc. This is possible, and in short, such a method may be appropriately adopted when implementing the method of the present invention. Although this method cannot completely prevent slag from flowing out during tapping or remove ladle slag, it is sufficient to reduce the thickness of slag on the molten steel surface in the ladle to 50 mm or less. . (4), (4)', (5)' Next, when the molten steel is cast in a continuous casting machine, it is deoxidized by adding Al to remove the free material in the unsolidified molten steel in the mold that constitutes the continuous casting machine. 0
] to below 70p1mJ. In this process, the nozzle diameter of the ladle or tundish is small (15 mmφ or less) like in a continuous billet casting machine.
In this case, nozzle clogging is likely to occur due to the AI deoxidation product Al2O3. Specifically, the free content in the molten steel in the ladle and tumbler is obtained by deoxidizing with the addition of Al.

When [0] is less than IQOppm, nozzle clogging occurs frequently. Therefore, when continuously casting billets, the free [O] should be adjusted to 100 to 180 p[ to avoid nozzle clogging in the stage up to the tundish, and after that, the free [O] in the molten steel in the mold [0 negative ffOOI] Deoxidation is performed by adding AI to the molten steel in the mold so that the concentration is below pm. In addition, it is free at the tanditshu stage.

The reason [0] was set to 180 pIm or less is that if this value is exceeded, Al will enter the molten steel in the mold.
Too many additions, freedom

This is because it becomes difficult to control [0] and the amount of Al added. Furthermore, if the free [O] exceeds 1 go, ppm, the melting loss of refractories such as tundish nozzles becomes severe, and the melted refractory mixes into the molten steel in the mold, causing quality defects. Al can be added by adding Al grains, but as described later, it is easier to add when the gap between the tundish and the mold is sealed with an inert gas, and it is easier to freely adjust [O] by supplying an AI wire. law is preferred. Meanwhile, ladle nozzle, like slab and bloom continuous casting machine
If the nozzle diameter of the tundish nozzle is 15mmφ or more, there is no need to divide the AI deoxidation treatment into two stages as described above, and the AI deoxidation treatment can be carried out at any time before casting.

[0]
The deoxidizing treatment may be performed by adding the minimum amount of AI necessary to reduce the temperature to 70 pI] T1 or less. The addition location may be during tapping from the converter (combined blowing furnace) to the ladle as described in (2) above, in the ladle after tapping, in the tantest, or even in the mold. You can go anywhere. A1 is added to free [0) 10 in molten steel when producing billets, as well as when producing slabs or blooms.
Even when adjusting to 0 to 1 soppm, it is preferable to do it in a ladle. It is advantageous in the pouring process, and more uniform deoxidation is possible than when At is added in the tundish, and it is more effective if Ar is introduced into the solid solution steel in the ladle and stirred. This is because deoxidation is possible. Note that, as described above, in the present invention, slag cut steel tapping is performed, so that the pickup of Si from the slag due to the addition of AI can be suppressed. Figure 3 shows the relationship between the amount of AI added and [Si] in molten steel.
The broken line is for normal tapping (slag thickness 200 mm on the surface of the molten steel in the ladle), the solid line is for slag-cut tapping (slag thickness 50 mm), and 1 is for slagless tapping (slag thickness Omm).
Each is indicated by a dotted chain line. As is clear from the figure, when the slag thickness is 50 mm, [Si] after adding AI can be suppressed to 172 or less compared to the case where slag cutting and tapping are not performed. And, by combined blowing and stirring refining, it is already free in molten steel.

[0] has decreased considerably, so the amount of AI added for deoxidation only needs to be small, and if A1 is added to the extent that T, (AI) of molten steel to be continuously cast is 0.020% or less, ( The increase in Si) is extremely small and remains at an unavoidable content. The amount of AI added is determined by measuring the free [O] in molten steel before addition using an oxygen probe that utilizes an electrochemical phenomenon such as a solid-state battery, but the molten steel has already been desorbed by combined blowing and stirring refining. Because it has been acid-treated, a very small amount of AI can be added to free the molten steel.

[0] can be deoxidized to below 70 ppm, and T, (AI) in molten steel after adding A1 is 0.020%, which is required for quality as steel equivalent to rimmed steel.
The following can suffice. In addition, when adding AI into the ladle, slag-cut tapping is performed, so the amount of slag present on the surface of the molten steel is small, and the loss of AI due to oxygen in the slag is small. freedom

[0] can be controlled with high precision. Further, alloying elements such as Mn, which are required depending on the purpose of the steel material, are adjusted by adding the alloy to the ladle from the converter during or after tapping into the ladle. Furthermore, in order to prevent the molten steel from being oxidized by air during tapping from the converter to the ladle, it is desirable to seal the area around the molten steel with an inert gas such as Ar during tapping. Furthermore, since the surface of the molten steel tapped into the ladle is in contact with the air, it is preferable to add flux, rice husks, etc. to prevent heat radiation and air oxidation. In the above-mentioned series of steps, the processing time is longer due to the time required for stirring and refining than the steps required in the pre-continuous casting stage, but it is significantly shorter than in the case of vacuum processing as described above, and There is also no need to make the temperature particularly higher than in the case of normal continuous casting. Next, the molten steel that has been subjected to such deoxidation treatment is cast in a continuous casting machine, but the free [O] in the molten steel is 70 μm as mentioned above.
Since the following conditions are satisfied, CO is not generated and slabs without defects such as pinholes can be stably produced. During continuous casting, measures are taken to prevent re-oxidation of molten steel, that is, a long nozzle is used as the ladle nozzle, a submerged nozzle is used as the tumbler nozzle, and Ar is applied between the ladle and tundifushi 1 and between the tandice and the mold. It is desirable to take known measures such as sealing with an inert gas between the parts. Next, an example of the present invention was prepared using a 250 mm thick slab and a 116 mm thick slab.
The case of producing billets of mm diameter will be described in detail. First, when manufacturing a 250 mm thick slab, Table 1 A
Hot metal 270 with the component concentration and temperature listed in the column (before treatment)
t is charged into the 270 t converter 2 shown in Fig. 2, and the hot metal 1
40 kg of quicklime and 18 kg of lightly burnt dolomite were added to conduct composite blowing. Note that iron ore was appropriately added during blowing in order to adjust the molten steel temperature. Tuyere 1 has an inner diameter of 12.7 mm, and a gap of 1.1 m between the inner and outer tubes.
It has a double tube structure with an inner tube made of Cu and an outer tube made of stainless steel. The oxygen blowing rate during oxygen blowing was 40,000 Nm3/hour, and the stirring Ar blowing rate was 9 Nm3/min. As shown in column B of Table 1 (after composite blowing), [C] is 0.09% and free [O] is 2781)n
As shown in Figure 1, this is close to the equilibrium state when the PCO is 21 atmospheres. In addition, [C] is blown at a value of 0.05% or more, and [P] is also 0.1% due to the dephosphorizing effect of quicklime.
It has been reduced from 18% to 0.013%. Next, after composite blowing, a total of 25N is applied from each inner pipe of the two tuyeres 1.
Ar was blown into the molten steel for 8 minutes at a rate of m3/min for stirring and refining. As a result, as shown in column C of Table 1 (after stirring and refining), free [O] was deoxidized to 180 ppm. Next, the steel was tapped from the converter into a ladle, and during tapping, the flow of converter slag into the ladle was suppressed as much as possible using the tapping loss writing gate method. As a result, the slag thickness on the self-melting steel surface of the ladle could be set to 50 mm. At this time of tapping, 100 kg of rod-shaped AI was added to the molten steel.
(approximately 0.37 kg per molten steel), and at the same time Mn
An appropriate amount of HCFe-Mn (Fe-Mn alloy) was added for component adjustment. . In addition, in order to promote the floating of Al2O3-based inclusions produced by AI deoxidation, after the ladle was covered, an immersion lance was inserted into the molten steel, and the steel was stirred with Ar at a pressure of 3 kg/cm2 for 5 minutes. The concentrations of each component after this AI deoxidation treatment are shown in the first table column (after AI deoxidation treatment); (Si) is 0.010%, T[A
I] was 0.005%, which is a sufficient value for molten steel for products equivalent to rimmed steel, and free [O] was 53 pIlrn, with no risk of bubbles being generated during continuous casting. Then, in a continuous slab casting machine with a thickness of 250 mm, the inside of the ladle and the tundish and between the tundish and the mold were sealed with Ar, and a long nozzle was used for the ladle nozzle, and a submerged nozzle was used for the tundish nozzle, and the drawing speed was set at l. , 2 m/min, no air bubbles or pinholes were generated, and a slab with good surface quality was produced. Next, the case of producing a 116 mm billet will be described. 77 tons of molten steel having the component concentration and temperature listed in column A (before treatment) of Table 2 was charged into a composite blowing furnace (70 having the same structure as shown in Fig. 2), and the molten steel (31 kg of quicklime and light burnt dolomite) was added to the furnace.
Composite blowing was performed by adding 8 kg of fluorite and 1.3 kg of fluorite. Note that iron ore was appropriately added during blowing in order to adjust the molten steel temperature. The oxygen supply rate was 11゜00ONm3/hour, and the stirring Ar
The blowing speed is 4 Nm3/min. The material of the lining is Cu for the inner tube and stainless steel for the outer tube, which is the same as in the case of FIG. 2, but the inner diameter of the inner tube is 7.75 mmφ, and the gap between the inner and outer tubes is 0.8 mm, which is slightly smaller. As shown in column B of Table 2 (after composite blowing), [C amount is 0.09%, free

[0] is 295pII
I'n, which is close to the equilibrium state when Pco=1 atm as shown in FIG. In addition, CC)] is blown at a value of 0.05% or more, and [P] is also zero due to the dephosphorizing effect of quicklime.
, has been reduced from 120% to 0.015%. Next, after compound blowing, a total of 8Nm is extracted from each inner pipe of the two tuyeres.
Ar gas was blown into the molten steel for 10 minutes at a rate of 3/min to perform stirring and refining. As a result, as shown in column C of Table 2 (after stirring and refining), the free [O] decreased to 200 pIITI. Steel was then tapped from the converter into a ladle, and at the start of tapping, the tapping opening was stuffed with rags to prevent slag from flowing out. In addition, at the final stage of tapping, the tapping port was closed using the slag ball method as shown in Figure 4 to prevent slag from flowing out. Figure 4 schematically shows the state in which the converter 3 is overturned and molten steel is being tapped from the tap port 4 installed in the converter wall soil, and the molten steel is being tapped in the horizontal direction from the converter head opening. A refractory slag ball 6 stored at the tip of a ball dropping device 5 inserted into the tap is dropped from above the tap hole toward the tap hole at an appropriate point in the final stage of tapping to block the tap hole. This shows how converter slag is prevented from flowing out. By performing slag cutting and tapping in this manner, the slag thickness on the surface of the molten steel in the ladle was able to be 30 mm. At this time of tapping, 7.7 kg of rod-shaped AI was added to the molten steel.
(approximately 0.1 kg per molten steel) was added to perform a slight deoxidation treatment, and at the same time, an appropriate amount of HCFe-Mn was added to adjust the Mn component. The free [O] in the molten steel after this AI deoxidation treatment is 150 ppm as shown in the second table (after AI deoxidation treatment), which is the standard 1 up to the tundish stage when billets are continuously cast.
00-180p is fully compatible. In this example, deoxidation was performed by adding AI, but when the end point CC,l is high, the end point free [O] is low, and the free [O] in the molten steel is approximately 180 pm or less after stirring and refining. In this case, it goes without saying that there is no need to perform deoxidation by adding AI up to the tanditsu stage. When casting the molten steel produced in this way using a continuous billet casting machine with a diameter of 116 mm, each molten steel weighs 200 g.
The AI wire was supplied to the molten steel in the mold at a rate of . The drawing speed was 2.2 m/min, and Ar sealing was performed to prevent re-oxidation of the molten steel. As a result, there was no generation of air bubbles or pinholes, and a slab with good surface quality could be produced. The concentration of each component of the molten steel in the mold is shown in Table 2 column E (molten steel in the mold), but since no Si deoxidation was performed, [Si
] is naturally as low as 0.012%, but T,
(:A1) is also 0.010%, which is 0 for conventional Al-killed steel.
This is extremely low compared to 0.30% or more, and is sufficient for molten steel equivalent to rimmed steel. more freedom

[0] was also extremely low at 53 px, so naturally no bubbles were generated. In both of the above two examples, the amount of C in the product was 0.0.
This is about a product equivalent to low carbon rimmed steel with a carbon content of 0.11 to 0.25%.
It goes without saying that the present invention is also applicable to products equivalent to high carbon rimmed steel. The higher the level [C], the freer the oxygen blowing end point.

[0] is low;
This is because the subsequent deoxidation treatment is mild. As detailed above, 1. The series of steel manufacturing methods centered on composite blowing and stirring refining according to the present invention enables continuous casting of molten steel with no Si deoxidation at all and only slight AI deoxidation. It is possible to obtain slabs equivalent to rimmed steel without defects such as break-in and pinholes at a high yield and with high efficiency, and since vacuum treatment is not performed, the equipment is easy to use. Moreover, since the time required for the entire process is shorter than when deoxidizing is performed in a vacuum, the drop in molten steel temperature is small, which is extremely advantageous in terms of operation. In this way, the steel manufacturing method according to the present invention achieves low A
This made it possible to continuously cast Si, L steel, that is, a product equivalent to rimmed steel, and it can be said to be an epoch-making contribution to this type of technology.

[Brief explanation of drawings]

Figure 1 is a graph showing the effects of composite blowing and stirring refining, Figure 2 is a schematic vertical cross-sectional view of a composite blowing furnace, Figure 3 is a graph showing the relationship between the amount of AI added and (Si), Figure 4 is a conceptual diagram of slag cut steel tapping equipment using the slag ball method. 1...tuyere, 2,3...converter, 4...
Curved steel opening, 5...Ball dropping device, 6...
...Slug ball.

Claims (1)

[Scope of Claims] 1. In a converter in which gas can be introduced from below the bath surface, oxygen blowing is carried out to achieve a blow stop [C] of 0.05% or more, and an appropriate period and time during oxygen blowing are carried out. After oxygen blowing, stirring and refining is performed by introducing gas from below the surface of the steel bath, and after the stirring and refining, steel is tapped while suppressing the mixing of converter slag from the converter to the ladle. Or after tapping, deoxidize by adding AI,
Freedom [0] is 70 ppm or less, T, (AI'l is 0.
A steel manufacturing method characterized by producing molten steel having an unavoidable content of [Si] of 0.020% or less, and casting the molten steel using a continuous casting machine. 2 In a converter where gas can be introduced from below the bath surface, oxygen blowing is carried out to achieve a blow stop [C] of 0.05% or more, and during the appropriate period during oxygen blowing and after oxygen blowing, steel Stirring and refining is performed by introducing gas from below the bath surface, and after the stirring and refining, steel is tapped while suppressing the contamination of converter slag from the converter to the ladle. After that, deoxidation was performed by adding AI, and free (0) was 100 to 180 pI1m, [Si
), and when casting the molten steel using a continuous casting machine, AI is added to the molten steel in the casting constituting the continuous casting machine to deoxidize it and When the freedom [0] of molten steel is 70 pμs or less, 'T, (Al, l
A steel manufacturing method characterized by making (Si) an unavoidable content of 0.020% or less.