JP2007332432A - Method for refining molten steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reducing a part of Mn included in raw materials, collecting it into a workpiece of a molten steel and besides saving the resource and energy by reducing an amount of slag, though most parts of the Mn included in the raw materials are wasted together with oxide slag in a steelmaking process by using an electric furnace and employing iron scrap as a main raw material. <P>SOLUTION: Raw materials are melted and a general process is performed by normally oxidation-refining the while leaving most of oxidative slag in the furnace. The slag is moved into the ladle together with the molten steel when tapping the molten steel. Simultaneously, a reducing agent is added corresponding to the amount of a low-grade oxide in the slag. Airtight covers are attached on the top and the bottom of the ladle, decompressing the inside; and gas bubbling is performed in the molten steel to reduce, refine and collect Mn in the slag. The molten steel are cast into a slab with a continuous casing method which does not cause central segregation though many parts dephosphorized in the oxidation-refining process are rephosphorized and a P content increases. As a result, an action of impurity P as a harmful element is alleviated, and an adequate alloy can be obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method for producing steel slab by using iron scrap as a main raw material, melting and refining the raw material by arc heating in an electric furnace, and then continuously casting.

In the electric furnace steelmaking process, iron scrap is usually used as the main raw material, melted in an arc heating melting furnace, commonly known as an electric furnace, and oxidized or reduced in the electric furnace to remove or reduce various impurities, and then reduced and finished such as deoxidation desulfurization. It is refined and put into a ladle. Recently, reduction and finishing smelting are often processed in a ladle to improve electric furnace productivity. Molten steel melted at a predetermined temperature and a predetermined component is supplied to continuous casting to form a steel slab.

The oxidation refining process in the electric furnace is 1) the generation of basic slag that stabilizes the arc, improves the thermal efficiency and induces refining, 2) oxidation and removal of impurities such as P in molten steel by oxygen blowing, 3 ) Combustion of C in molten steel by oxygen blowing and degassing by CO boiling 4) Discharge of slag containing harmful impurities such as P to the outside of the furnace 5) Temperature rise.
Reduction and finishing refining in the ladle is 1) Re-smelting to regenerate slag to be non-oxidizing, 2) Removal and reduction of impurities such as O and S by adding new slag, reducing agent, deoxidizing agent, etc. 3) Consists of processes such as adjustment of ingredients and temperature.
Both oxidizing and reducing slag are treated as industrial waste after use.

P, which is an object of oxidation refining, is an impurity that causes steel to become brittle, and its content is regulated according to the steel type, steel material, and application. Iron scrap contains about 0.01 to 0.04% P depending on the original steel type. Today, several decades have passed since the refining level of converter steel has improved, and the P content in city scraps has been on the decline. The average P content in the raw material is considered to be about 0.02% depending on the composition of the scrap metal species, but it is about 0.03 to about 0.03% depending on the composition of pig iron, phosphorylated surface-treated steel sheet and other impurities. Estimated to be 05%. Accordingly, a dephosphorization process is necessary. In order to proceed with dephosphorization, the molten steel is blown with oxygen to oxidize P, and the generated oxide is absorbed by the basic slag. The slag at the time of oxidative refining contains a large amount of lower oxides such as P, Fe, and Mn. When a part of the slag is brought into reductive refining in the furnace or in the ladle, the oxide is reduced and returns to the molten steel. Therefore, it is necessary to discharge the slag out of the furnace in advance or not into the ladle in order to suppress recovery phosphorus.

The dephosphorization process simultaneously involves demanganese. The iron scrap contains about 0.2 to 2.0% of Mn, and is regarded as an average of about 0.5%. By oxidation refining, Mn is oxidized and the content in the molten steel is reduced to 0.1 to 0.2%. The rest is absorbed by the slag as oxide. Mn is a refining aid that is useful as a sulfide stabilizer, as a deoxidizing stabilizer, and as an alloying element, but most of the Mn in the raw material is discarded with no use after slag discharge.

The dephosphorization treatment is required to some extent, and the accompanying demanganese reaction has been unavoidable. Reduction refining of Mn itself is not particularly difficult. When a work method having an effect of recovering Mn is found, there is a case where a limited amount of iron scrap having a known feature is inserted into molten steel after dephosphorization in steelmaking using a blast furnace to a converter. In this case, Mn in the iron scrap is recovered, but it is not a general method for mass recycling of the iron scrap.

In the deformed steel bar for reinforcing steel bars, the upper limit of the P content is 0.05%, so that it is acceptable to some extent. Therefore, it is likely that some Mn recovery will be possible if the oxidation slag is reduced with knowledge of the recovery phosphorus, but the reduction / deoxidation refining itself has also been simplified for the sake of productivity, and therefore the reduction function has been omitted and recovered. Absent. Thus, an economical recovery method is not practical.

Consider the regulation of P content. Standard values are set for normal processes, materials, and products. Many cases where impurities such as P and S show actual damage are caused by segregation thereof. That is, the standard is determined on the assumption that some segregation is inevitable.

Segregation is classified into three forms. The first microsegregation is concentration between dendrites in dendritic solidification, whether it is chill, columnar or equiaxed, and the concentration ratio and distribution are regular, and most of them are solid solution and show no real harm. The second is semi-macro segregation, which is unevenly distributed between equiaxed crystals as inclusions of low melting point carbides, phosphides, and sulfides. Not between columnar crystals. The size is 10 to several 100 μm. It is detrimental to steel hardening and embrittlement. The third is segregation in the center, and the second form is a group around the center, and obviously acts as a harmful impurity such as not only embrittlement of the product steel but also cracking in the hot rolling process. Therefore, if the segregation of the second and third forms can be eliminated, non-standard components can be used without any problem, and in some cases, the function as an alloy element can be exhibited.

Patent Document 1 discloses a method for eliminating the occurrence of center segregation in a continuous casting process. At the same time, a method is disclosed in which the solidification structure is made columnar throughout the entire inner surface of the chill crystal of the outer shell by controlling the casting temperature. It has been suggested that there is a possibility of a homogeneous steel ingot similar to a unidirectionally solidified steel ingot or ESR steel ingot by controlling to a chill crystal and a columnar crystal. This document suggests that impurities such as P and S are substantially harmless or harmless by eliminating segregation, but allow for non-standard amounts of impurities or excessive contamination and use them in alloying. The idea is not disclosed.

Considering a prior example of a method for reducing and recovering Mn, the contamination of impurities P is greatly allowed.
Generally referred to as the LF method, only the molten steel oxidized from the melting furnace is transferred to a ladle, and a steelmaking material and a carbonaceous material are added to the ladle, and reduced iron is formed while reheating with an arc to deoxidize and desulfurize. To promote. When part of the oxidized slag is brought in, P, Fe, and Mn in the slag are reduced and transferred into the molten steel. The yield of input Mn is close to 100%, and Mn in the slag is also recovered, but it usually takes 30 minutes or more for refining and is not very efficient.

Patent Documents 2 and 3 disclose high-speed reduction, deoxidation, and desulfurization refining methods. Both methods are basically based on the same principle. That is, 1) induction of non-oxidizing slag, 2) strong stirring between gas, slag and molten steel in non-oxidizing atmosphere by gas bubbling under reduced pressure, 3) reduction, addition of deoxidizer, 4) by electromagnetic force The reduction and deoxidation reactions of molten steel and slag are induced at high speed and high speed by strengthening stirring. Although it is disclosed that Mn in the slag is easily reduced, the idea of allowing a large amount of P in the slag is not noticed at all.

Patent No. 2998737 Patent No. 1575316 Patent No. 3654248

As described above, in the conventional steelmaking method using iron scrap as the main raw material, the molten steel is oxidized and refined to remove P as an impurity, and P is absorbed as an oxide into basic slag, and the slag is discharged out of the furnace. This prevents recovery from residual slag. At that time, most of the Mn in the iron scrap, which is originally a useful component, is taken out to the discharge slag and not recovered due to the same behavior.
The first object of the present invention is to reduce the amount of Mn alloy used as a refining aid by recovering Mn contained in iron scraps in molten steel. The second purpose is to reduce the amount of slag that will eventually become industrial waste by reducing the amount of re-fabrication.

In order to solve the above problems, the invention is constituted by the following element means.
1) The oxidized refining slag is transferred to the ladle along with the molten steel, and Mn in the slag is reduced and recovered. 2) Allow the P content in the molten steel, which inevitably increases with Mn recovery.
3) Take measures to eliminate center segregation in continuous casting and suppress the harmful effects of P.

A first invention is an electric furnace steelmaking method in which iron scrap is used as a main raw material and the raw material is melted and refined by an arc heating melting furnace to produce molten steel having a predetermined temperature and a predetermined component. Oxidative refining with the majority of the oxidizing slag floating on the furnace remaining in the furnace, after the refining, the molten steel and the slag are both put into a ladle and reduced and refined in the ladle to oxidize Mn in the slag. A method for refining molten steel, characterized in that the product is recovered as Mn in molten steel.

According to a second aspect of the present invention, the refining method is such that at least one of a Si-containing material, an Al-containing material, and a carbonaceous material is introduced into the ladle as a reducing agent at the time of steel output, and the amount of the reducing agent is changed to FeO in the slag, 0.8 to 1.6 times the sum of the chemical equivalents of MnO and P ₂ O ₅ , and then refining gas from the bottom of the ladle into the molten steel covered with the slag from 5 to 20 N liters / minute / ton of molten steel The molten steel refining method according to the first aspect of the present invention is characterized in that the atmospheric pressure above the molten steel is reduced and maintained at 6 to 40 kPa while gas bubbling is performed at a rate of 5%.

According to a third aspect of the present invention, a reduction refining method is performed by introducing at least one of a Si-containing material, an Al-containing material, and a carbonaceous material into a ladle as a reducing agent at the time of steel output, and the amount of the reducing agent added is FeO in the slag, 0.8 to 1.2 times the sum of the chemical equivalents of MnO and P ₂ O ₅ , and then the upper part of the ladle is covered with a cover holding an electrode for arc heating, and the molten steel covered with the slag A refining method for molten steel according to the first aspect of the present invention, characterized in that a refining gas is blown from the bottom of the ladle and stirred, and reduced while carbon carbide and slag are produced from above the molten steel by adding carbonaceous material and arc heating.

According to a fourth aspect of the present invention, there is provided a method for refining molten steel, comprising adjusting the final components of the molten steel obtained by the method of the first aspect of the invention, the second aspect of the invention, or the third aspect of the invention, and then making a billet by the following continuous casting method It is.
Note: Molten steel is cast vertically into a lower open mold to form the outer shell of the slab, and the slab continuously drawn from the lower part of the mold is arcuate and exceeds the semicircle until the center is solidified. Further, a hollow cast slab is formed by pulling upward from the casting surface over a static iron equivalent height of about atmospheric pressure (about 1.4 m), and then the slab is pressed down by a roll so that the hollow inner surfaces are pressed against each other. A continuous casting method for producing a solid slab, wherein the casting temperature is set to 20 to 50 ° C. in terms of superheat.

The first effect of the present invention is that most of Mn in the raw iron scrap is oxidized once and absorbed by the slag, but is recovered in the molten steel by the reduction treatment in the ladle, thereby reducing the amount of alloyed iron used. Moreover, no special cost is required.
The second effect is that most of the oxidized slag is brought into the reduced / finished refining slag, so that the amount of ironmaking material is greatly reduced and the amount of industrial waste is reduced. Thirdly, power energy consumed as the amount of slag decreases is also saved. Fourth, impurities are increased, but since the continuous casting is performed in a manner that does not cause segregation, the harmfulness is eliminated or reduced, and the alloying action of P (hardening, machinability, abrasion resistance, corrosion resistance) is added to improve quality. A characteristic new steel grade is easily produced.

Hereinafter, embodiments will be described with reference to the drawings.
FIG. 1 is a schematic side view of an example of equipment for carrying out the second invention.
Iron scrap 1 as a main raw material is charged into a melting furnace 2 and melted by heating the arc 3. Oxygen squeezing is performed by an oxygen lance 4 before smelting, and all or a part of impurities Mn, Si, P, S, Zn, Pb and the like in the molten steel are oxidized and removed together with decarburization treatment. An appropriate amount of lime is added as a koji making agent to form a basic slag 5 having oxidation ability. Mn, Si, and P are absorbed as oxides in the slag 5, but a part of S, Zn, Pb, and the like move into the gas phase and become dust and are collected.

The slag is usually discharged out of the furnace before the steel is discharged, but in the present invention, the majority remains in the furnace and is discharged into the ladle 7 together with the molten steel 6 after the completion of the oxidation refining. The slag 5 contains about 20 to 30% of lower oxides P ₂ O ₅ , MnO and FeO. At the time of steel production, in addition to ferrosilicon corresponding to the standard Si value, the ladle is charged with ferrosilicon, silicomanganese, carbonaceous material, etc. as a reducing agent. Almidros or the like can also be used as an auxiliary. An appropriate amount is preferably 0.8 to 1.6 times the chemical equivalent of the amount of the lower oxide. An appropriate amount of lime or the like is added to the ladle for adjusting the slag component. A part of the lower oxide in the slag is reduced by C, Si, and Al only by stirring the steel, and returns to the molten steel. Take a sample for analysis and prepare for final component adjustment.

The received steel ladle 7 is transferred and left on the lower hermetic cover 8. Next, an upper hermetic cover 11 connecting the alloy addition hopper 9 and the exhaust device 10 is mounted on the upper side of the ladle 7. Since the ladle 7 has an airtight side wall, it is integrated with the upper and lower airtight covers to form an airtight structure. Next, a non-oxidizing gas such as Ar gas is blown from a plug 12 of a breathable refractory attached to the bottom of the ladle, and the space above the ladle is depressurized by the exhaust device 10, and the gas bubbling under the depressurization is performed at a gas flow rate of 5. It is made to act at a rate of ˜20 N liter / min / ton of molten steel. On the other hand, the atmospheric pressure above the molten steel is reduced and maintained at 6 to 40 kPa.

The injected gas rises as bubbles and stirs the molten steel. When the bubbles reach the upper layer of the molten steel, the external pressure rapidly decreases and expands rapidly. As a result, the three phases of molten steel, gas, and slag are vigorously mixed and become a foam rather than a boiling state, and the reaction between the three phases is strongly stimulated and attempts to shift to equilibrium. At the beginning of the pressure reduction, carbon in the carbon material and molten steel reacts with the lower oxide, and reduction proceeds while generating CO gas. Thereafter, Si in the molten steel is mainly reacted, and the reduction proceeds. After 3 to 6 minutes, the total of the oxides in the slag is easily reduced to 2% or less, and the slag becomes white. That is, most of Mn and Fe in the slag are recovered in the molten steel. P is almost fully regressed but is acceptable. Along with the reduction, deoxidation, desulfurization, and non-metallic inclusions proceed.

Final adjustment of the components of C, Si, Mn and other alloys is completed. After the refining is completed, the ladle is transferred to a continuous casting machine, and molten steel is continuously cast into steel pieces. The molten steel 21 (in FIG. 2) according to the present invention contains more harmful impurities P than before, and sometimes exceeds 0.05%. Obviously harmful if segregation occurs. A special continuous casting method is therefore essential.

The continuous casting method used by this invention is demonstrated along FIG.
The molten steel 21 is poured from a ladle into a tundish 22 as an intermediate container, and cast from above into a mold 23 opened below the tundish 22 to form an outer shell to form a slab 24. The slab 24 is formed into a circular arc downward. The steel plate is continuously drawn out and cooled by the spray cooling device 25, exceeds a semicircle until the center portion of the slab 24 is solidified, and further reaches a static iron pressure height equivalent to atmospheric pressure from the casting surface (about 1.. 4m) Pulling upward beyond the point P to form a hollow cast slab 26, the cast slab 26 is stretched by a straightening roll 27, pressed down by a pressure rolling mill 28, and the inner surfaces are pressed together to form a solid casting. Let it be a piece 29. Since there is no solidification end point, central segregation is eliminated in principle.
Since the details of the continuous casting method are disclosed in the above-mentioned Patent Document 1, they are omitted here.

In the above continuous casting method, when the casting temperature is set to 20 to 50 ° C. in terms of superheat (= molten steel temperature−liquid phase temperature of the component), the solidified structure is composed of chill crystals in the outer shell and columnar crystals in the interior. It is substantially free of equiaxed crystals. The conditions and results are described in detail in Patent Document 1.
The columnar crystals themselves are essentially homogeneous, but central segregation occurs when they are solidified to the end of solidification. Therefore, generally, center segregation is dispersed by equiaxed crystallization. In this case, equiaxed crystals around the center are accompanied by semi-macro segregation. Since no equiaxed crystal exists under the above conditions, semi-macro segregation can also be avoided. Since it does not have a harmful segregation structure, normal inconvenience does not occur even if the P content is outside the conventional standard, and it can be put to practical use.

The main points of the process will be described below.
The reason for limiting the majority of the smelting slag to remain in the furnace is that an increase in the recovered amount of Mn can be expected as the residual amount increases, but an increase in P also becomes a problem. Some slag flows out of the furnace due to foaming of the slag in the process of boiling in the furnace. The above amount was taken as an effective and safe working condition.

The reason why the amount of the reducing agent is 0.8 to 1.6 times the chemical equivalent of the amount of the lower oxide in the reduction refining in the ladle of the second invention is as follows. In the reduction method of the present invention, the reducibility is large, so about 1.0 is originally necessary and sufficient. However, 1) Combustion loss of C during steel output, 2) Dispersion of slag discharge, 3) Non-uniformity of slag components, 4) Analytical values cannot be obtained immediately and must be treated statistically. A certain range was empirically established. If it is 0.8 or less, a part of Si for the product component is consumed for reduction, and if it is 1.6 or more, the product Si% and C% tend to increase and become inconvenient.
In the same invention, the grounds for working conditions of gas bubbling under reduced pressure are described in Patent Documents 2 and 3, and are omitted because they are not particularly changed.

Although the details of the reduction and refining method of the third invention are omitted since they are well-known matters, the lower limit value of the amount of the reducing agent is set to the same value for the same reason as in the second invention. This is because the upper limit is inconvenient because the Si% and C% of the product tends to increase at 1.2 or more due to the effect of generation of carbide soot by introducing a new carbon material during arc heating.

A supplementary explanation of the solidified structure and segregation will be given.
As the solidification progresses, a concentrated liquid layer of solute elements such as C, Mn, P, and S is formed on the front surface. When the layer progresses in the form of columnar crystals, it is taken up between the dendritic solidification branches and distributed in a certain ratio with the inside of the dendrites. That is, microsegregation is formed. Columnar crystals are semi-macro-homogeneous, and impure elements are often in solid solution. Since it is not a precipitated phase, the segregation is not harmful. After the transition from columnar crystals to equiaxed crystals, the equiaxed nuclei grow separately in the liquid phase, and in the region around the center, the accumulation and flow of concentrated layers surrounding the equiaxed crystals occur in parallel with the formation of the porous structure. Deadlocks get involved and form semi-macro segregation.
FIG. 3 shows the solidification structure of the high carbon steel billet. As shown in the left figure, there is no precipitation of non-metallic phase at the columnar grain boundaries, and there is precipitation of ferrite. On the other hand, a nonmetallic phase of phosphide, sulfide, carbide or a solid solution thereof is precipitated at the equiaxed grain boundary located around the center shown in the right figure. The segregation rate and size of the phase overwhelm microsegregation. The phase itself can be detrimental to certain mechanical properties or the phase population can be detrimental as central segregation.

The second reason for avoiding equiaxed crystals is that in the present invention, the P content is considerably increased from the normal level, and as a result, the segregation tendency and the degree of harm are further increased, so it is necessary to avoid semi-macro segregation occurring between equiaxed crystals as much as possible. This is because.

In the present invention, P does not appear as a non-metallic precipitation phase but is dissolved, and the content increases from the usual level, so that it acts as an alloy element. Like P-containing steel, the tendency of hardening, embrittlement, machinability improvement, corrosion resistance improvement, wear resistance improvement, temper embrittlement, etc. is strengthened.

Consider the economics of Mn reduction recovery.
Si, Al and carbon materials are used as the reducing agent. During the reduction of Mn, iron oxide and phosphorus oxide are also reduced and the reducing agent is consumed. Although the concentration of the lower oxide has already been described as about 20 to 30%, MnO in the oxide is considered to be 30 to 40%. As a rough estimate, 3 equivalents of reducing agent are required for 1 equivalent of Mn.
As shown in the formula (1), Si may be 28/110 × 1 = 0.25 kg for the reduction of 1 kg of Mn. The price of alloy iron is slightly lower than that of Mn. Therefore, even if 3 equivalents are consumed in the operation, it is sufficiently attracted. As a supplement, since the following reaction is an exothermic reaction, there is no problem in power consumption.
_{2MnO + Si = 2Mn + SiO 2} --- (1)
Atomic weight 28 110
Complementing the lower oxide concentration, the amount of FeO generated due to the raw materials and blowing is considerably large, but a carbonaceous material is also blown to foam slag in order to improve the thermal efficiency during blowing. This unexpectedly reduces FeO, and is stable in the above concentration range.

When the reaction amount of the carbonaceous material that is cost-effective as a reducing agent is increased, the consumption of Si is reduced as shown in equations (2) and (3). The combination of alumidoros is further cost-effective.
MnO + C = Mn + CO --- (2)
MnO + CO = Mn + CO _2- (3)
When a carbon material is added to a slag containing a large amount of a lower oxide and bubbling under reduced pressure, the CO foaming reaction is promoted and acts to reduce the oxide.
Another effective method is to react a basic slag having a reducing ability with a carbonaceous material by arc heating to induce a carbide-containing slag. Reduction refining in the third invention follows this method. This method is commonly referred to as the LF method, and the refining is performed by adding a basic slag carbon material while covering the upper part of the ladle and heating the arc. Even in this method, most of Mn can be reduced and recovered easily, and the reaction amount of the carbonaceous material is preferably increased, but there is a problem that the reaction rate is slow.

Example 1: The verification test of the second invention was conducted. Using a 30-ton electric furnace, the entire amount of iron scrap was melted as a raw material, and 0.8% C high carbon steel was melted. After oxidative refining, most of the slag in the furnace was discharged and part of it remained. The remaining amount was set at two levels, one was the conventional working condition and the other was the amount of the standard, which was doubled. As furnace reductive refining, about 300 kg of lime was newly added with arc heating to dilute the lower oxides, and after the reduction carbon material was added, the steel was put into a ladle. At the time of steel production, the ferrosilicon was increased from 5 to 10 kg from the usual amount and charged into the ladle. The slag composition immediately after the steel was found to have an average lower oxide concentration of 3% in the conventional normal operation and 7 to 11% in the test. Next, the upper and lower airtight covers are attached to the ladle, the pressure is reduced and maintained at about 0.1 atm by a water seal pump, Ar gas is blown in for about 6 minutes at a flow rate of 160 N liters / minute, and gas reduction is performed. We proceeded with desulfurization and non-metal inclusion treatment. In the slag composition after refining, the lower oxide concentration was all reduced to 1.0% or less in the 10 charge test. It can be seen that the ladle refining method is extremely effective in reducing lower oxides.
The P concentration in the molten steel usually increased from 0.010 to 0.017% compared to 0.010 to 0.013%. The Mn concentration in the molten steel increased by 0.04%.

Example 2: The reduction test of Cr oxide was similarly conducted for Si-Cr steel for springs. In the same operation as before, a powdery Cr ore (33% Cr content) was charged into the furnace in an amount of 450 kg corresponding to 0.5% Cr steel before the steel was released (the estimated lower oxide concentration was about 50). At the time of steel production, lime for neutralization and Si equivalent to the chemical equivalent of Cr in the ore were charged into the ladle in the form of ferrosilicon. After the reduction refining by reduced pressure gas bubbling, the concentration of the lower oxide (FeO + MnO + Cr ₂ O ₃ ) in the slag was reduced to 1.5%. The ladle refining method was found to be sufficiently reduced even when the initial concentration of the lower oxide was large.

Example 3: Similar to the experiment of Example 1, about half of the slag after oxidative refining is discharged outside the furnace, half is left and about 300 kg of lime is added to dilute the lower oxide, reducing carbon material Was put on the ladle. The same reduction, deoxidation, and desulfurization processes were performed. In some cases, P before reduction exceeds 0.010% and after reduction may exceed 0.025%, which causes a problem in the standard of JIS hard steel wire rod. The recovery of Mn has an effect of 0.10 to 0.15%. It was expected that recovery of 0.2-0.3% Mn would be possible if rebound was ignored.

According to the present invention, Mn contained in iron scrap in the electric furnace steelmaking method using iron scrap as a main raw material is once oxidized and removed, but part of it is recovered in the product molten steel by reduction refining in the ladle, and the alloy iron Usage is saved. On the other hand, the impurity P may also be reduced and returned to exceed the normal regulation value.
The above can be easily implemented by partially modifying the current processes, equipment, and operations. For molten steel with increased P, steel slabs are manufactured by adopting a continuous casting method (solidification by pressure welding of hollow cast slabs) that does not generate center segregation and does not generate semi-macro segregation between equiaxed crystals. By doing so, the harmfulness of P can be eliminated or suppressed. As a result, 1) the amount of Mn alloy iron used is reduced, 2) the amount of slag is reduced in terms of both consumption and disposal, 3) energy is saved by reducing the amount of slag, and 4) the alloying action of P is induced.・ It can be used.

It is a general | schematic cross-sectional view of the example of an installation which implements 2nd invention. It is a general | schematic cross-sectional view of the principal part of the example of an installation which implements 4th invention. It is a structure | tissue photograph which shows a solidification structure | tissue and an impurity precipitation phase.

Explanation of symbols

1: Iron scrap 2: Melting furnace 3: Arc 4: Oxygen lance 5: Slag 6: Molten steel 7: Ladle 8: Lower airtight cover 9: Hopper 10: Exhaust device 11: Upper airtight cover 12: Plug 21: Molten steel 22: Tundish 23: Mold 24: Cast slab 25: Spray cooling device 26: Hollow cast slab 27: Straight roll 28: Press-welding mill 29: Solid slab

Claims (4)

In an electric furnace steelmaking method in which iron scrap is used as the main raw material and the raw material is melted and refined by an arc heating melting furnace to produce molten steel with a predetermined temperature and predetermined components, it is generated during and after melting and floats on the molten steel Oxidative smelting with the majority of the oxidizing slag remaining in the furnace, after the smelting, the molten steel and the slag are both put into a ladle and reduced and refined in the ladle to convert the Mn oxide in the slag into Mn. A method for refining molten steel, characterized by being recovered. In the refining method, at least one of a Si-containing material, an Al-containing material, and a carbonaceous material is introduced into the ladle as a reducing agent at the time of steel output, and the amount of the reducing agent added is FeO, MnO, P ₂ O _{5 in the} slag. The refining gas was blown into the molten steel covered with the slag at a rate of 5 to 20 N liters / minute / ton of molten steel into the molten steel covered with the slag. The method for refining molten steel according to claim 1, wherein the atmospheric pressure above the molten steel is reduced and maintained at 6 to 40 kPa while bubbling. In the refining method, at least one of a Si-containing material, an Al-containing material, and a carbonaceous material is introduced into the ladle as a reducing agent at the time of steel output, and the amount of the reducing agent added is FeO, MnO, P ₂ O _{5 in the} slag. 0.8 to 1.2 times the sum of the chemical equivalents, and then the upper part of the ladle is covered with a cover holding an electrode for arc heating, and the gas for refining from the bottom of the ladle into the molten steel covered with the slag 2. The method for refining molten steel according to claim 1, wherein carbon steel slag is reduced while carbon carbide is added and arc heating is performed from above the molten steel. A method for refining molten steel, comprising adjusting the final components of the molten steel obtained by the method according to claim 1, claim 2, or claim 3, and then making a billet by the following continuous casting method.
Note: Molten steel is cast vertically into a lower open mold to form the outer shell of the slab, and the slab continuously drawn from the lower part of the mold is arcuate and exceeds the semicircle until the center is solidified. Further, a hollow cast slab is formed by pulling upward from the casting surface over a static iron equivalent height of about atmospheric pressure (about 1.4 m), and then the slab is pressed down by a roll so that the hollow inner surfaces are pressed against each other. A continuous casting method for producing a solid slab, wherein the casting temperature is set to 20 to 50 ° C. in terms of superheat.