JP6728933B2 - Continuous casting method for molten steel - Google Patents

Description

In the cross section of a normal continuous cast slab, there are coarse granular crystal parts near the center arranged so as to surround the final solidified part with porosity and segregation in the center, and coarse columnar crystal parts surrounding the coarse granular crystal part. To be observed. If this coarse granular crystal and columnar crystal are made into fine equiaxed crystals and center segregation and micro segregation can be greatly reduced, for example, when a slab is made into a thin plate, it becomes a thin plate with outstandingly excellent formability. Further, for example, when made into a thick plate, it becomes a thick plate excellent in low temperature toughness. The present invention relates to a continuous casting method for molten steel capable of converting these coarse granular crystals and columnar crystals into fine equiaxed crystals.

Non-Patent Document 1 indicates that low temperature casting is effective for equiaxed crystallization because equiaxed crystals increase when the degree of superheat of molten steel is low. Further, Patent Document 1 describes a technique in which an induction electromagnetic stirrer is used to impart a unidirectional swirl flow to molten steel in the vicinity of a solidification interface to divide columnar dendrites to form columnar crystals in equiaxed crystals. There is. Patent Document 2 discloses a method of pouring molten steel containing MgO as an equiaxed crystallization accelerator into a mold and casting the molten steel while electromagnetically stirring the molten steel in the mold to improve the cleanliness of the surface layer of the slab. A technique that has been proposed to suppress surface defects (cleaning effect of electromagnetic stirring) and internal defects (effect of refining solidified structure by MgO) at the same time is disclosed. Further, in Patent Document 3, an equiaxed crystallization accelerator (MgAl ₂ O ₄ , Ce oxide, sulfide, etc.) is sprayed on the surface of the molten steel using a plasma heating device in a tundish to equiaxially move in a mold. A technique is disclosed in which coarse grain crystals near the center of the slab are refined by generating a large number of crystal nuclei.

Japanese Patent Laid-Open No. 50-23338 JP, 2000-334559, A JP, 2001-225153, A

Iron and Steel Handbook, 3rd Edition, II Ironmaking and Steelmaking, p. 653

However, in low-temperature casting, it is necessary to bring the degree of superheat of the molten metal to a temperature close to the liquidus and to inject it into the mold from the immersion nozzle.Therefore, solidification abnormalities such as blockage of the immersion nozzle and generation of deckle in the mold can occur. May be invited. For this reason, in the current continuous casting, the superheat degree of the molten metal to be injected is about 20 to 30K, and under such temperature conditions, the production of high-strength thin steel sheets has been increasing in production due to the need for weight reduction in recent years. Fine equiaxed crystallization to the extent that workability and low temperature toughness of high strength thick plates can be improved has not been achieved. Also, with respect to the method of using induction electromagnetic stirring and the method of adding an equiaxed crystallization accelerator, sufficient fine equiaxed crystals until the material of the high-strength steel can be improved are not obtained. For molten steel having a C content of 0.1% by mass or less, which is hard to form crystals, it is difficult to sufficiently finely equiaxed the columnar crystals in the surface layer of the cast slab. Moreover, the effect of the equiaxed crystallization accelerator of MgO, MgAl ₂ O ₄ and Ce oxide is not stable, and the solidification nucleation ability is lowered by the influence of other oxides. It is estimated that there are no variable factors.

In view of such a situation, the present invention provides a coarse granular crystal in the vicinity of the center and a coarse columnar crystal surrounding it in a high-strength steel slab (including a slab having a C content of 0.1 mass% or less). An object of the present invention is to provide a continuous casting method for molten steel capable of stably forming fine equiaxed crystals together.

In view of such a situation, a coarse equiaxed crystal in the vicinity of the center and a coarse columnar crystal surrounding it are continuously cast into a fine equiaxed crystal, and a fine solidification structure cast by using the continuous casting method. In order to provide continuously cast slabs, we have clarified solidification structure refining elements and the factors that affect the refining effect, and have conducted extensive research into addition methods and locations where stable addition of small amounts is effective and the findings obtained. The present invention was completed by optimally combining and designing the processes in a continuous casting process.

The summary is as follows. That is,
(1) C: 0.03 to 0.20 mass%, Si: 0.08 to 1.5 mass%, using a continuous casting device having an induction electromagnetic stirring device between the meniscus in the mold and 10 m below the mold. Mn: 0.5 to 3.0 mass%, P: 0.05 mass% or less, S: 0.002 mass% or more, N: 0.0005 to 0.01 mass%, Nb: 0.2 mass% or less , V: 0.2 mass% or less, Mo: 0.5 mass% or less, acid-soluble Al: 0.03 mass% or less, acid-soluble Ti: 0.014 to 0.1 mass%, Ce, La, A molten steel containing at least one of Nd or Pr in a total amount of 0.0003 to 0.02% by mass and the balance of Fe and inevitable impurities is supplied from a gas injection type immersion nozzle into N ₂ gas or N ₂ gas. Is injected into the mold while blowing an inert gas containing 2% or more thereof, and one or more kinds of Bi and Sn are added to the molten steel in the mold so that the total amount becomes 0.0005 to 0.01% by mass. A continuous casting method for molten steel, characterized in that the molten steel is cast while being swirled at a swirling flow rate of the molten steel of 25 to 105 cm/s in a horizontal plane by the induction electromagnetic stirring device.
(2) The continuous casting method for molten steel according to the above (1), characterized in that a metal wire containing at least one of Bi and Sn is continuously supplied into the molten steel in the mold.
(3) The continuous casting method for molten steel according to the above (1), characterized in that a mold flux containing one or more of Bi and Sn is supplied onto the surface of the molten steel in the mold .

According to the present invention, the solidified structure of the slab surface layer and the inside of the slab, it is possible to stably produce a continuously cast slab that is finely equiaxed crystallized, therefore, in the formability in high strength thin steel sheet With high strength thick plates, it is possible to manufacture materials with excellent low temperature toughness.

Diagram for explaining a method of blowing N ₂ gas into the molten steel by using the gas blowing type immersion nozzle. The influence of the electromagnetic stirring flow rate on the average equiaxed grain size inside the cast and the surface layer of the cast, in which molten steel containing 0.004% by mass of Ce was continuously cast while blowing N ₂ gas from the gas-blowing immersion nozzle, was shown. It is a figure. Molten steel containing 0.004% by mass of Ce was injected into the mold while blowing N ₂ gas from the gas injection type immersion nozzle, and 0.003% by mass of Bi was added in the mold to continuously cast a slab. It is a figure which shows the influence of the electromagnetic stirring flow velocity on the average equiaxed grain diameter of the surface layer part of a cast piece.

The morphology of the solidification structure is determined by the temperature gradient at the solid-liquid interface and the solidification rate during solidification. The smaller the temperature gradient and the larger the solidification rate, the easier the formation of equiaxed crystals. However, in the actual continuous casting, columnar crystals grow from the surface layer of the slab to the relatively inside, and it is difficult to change the cooling conditions to change the solidification structure morphology to mainly equiaxed crystals. Under such conditions, in order to make the solidification structure into fine equiaxed crystals, the nucleation sites for equiaxed crystal formation are dispersed in the molten steel and the frequency of nucleation is increased to promote the formation of fine equiaxed crystals. Therefore, two methods are conceivable in which the solid-liquid interfacial energy is reduced by using a metal element having a high surface active effect to make the columnar crystals themselves into fine equiaxed crystals. The present invention effectively combines these two solidification structure control principles, and clarifies a control means for stably maximizing the solidification structure refining effect over the entire surface of the slab and controlling the same. It was completed by optimally combining the means in the continuous casting process and designing the process. The basic idea of the present invention is to [1] finely disperse an oxide that effectively acts as a nucleation site for equiaxed crystals in molten steel, and add electromagnetic stirring to this to deprive the superheat of molten steel to remove the inside of the slab. Stable fine equiaxed crystallization and [2] Preferential addition of a metal element that lowers the solid-liquid interface energy to the surface layer of the slab to form columnar crystals that grow from the mold side toward the inside of the slab. The aim is to make the particles finer and then to divide the fine and fragile columnar crystals by a swirling flow of electromagnetic stirring to generate fine equiaxed crystals in the surface layer of the slab. As a result, it becomes possible to obtain a fine equiaxed crystal structure over the entire surface of the cast slab.

Specific methods and conditions for realizing the above basic idea will be described below. First, regarding the condition of the oxide that becomes the formation site of equiaxed crystal nuclei in [1], there are many titania-based inclusions in Ti deoxidized molten steel and many alumina-based inclusions in Al deoxidized molten steel. However, since these inclusions do not easily serve as sites for producing equiaxed crystal nuclei and further aggregate and coalesce into coarse oxides, they do not act effectively as nuclei for equiaxed crystal formation. On the other hand, the present inventors have added at least one or more of Ce, La, Nd, and Pr, which are stronger deoxidizing elements than Ti and Al, into molten steel, and added titania-based inclusions and alumina-based inclusions. By modifying the compound into Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof. It has been found that relatively fine oxides/oxysulfides can be uniformly dispersed in molten steel, and that these oxides/oxysulfides easily become nuclei for the formation of fine equiaxed crystals. Compared with titania and alumina, this is Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or these It is considered that this is because the complex oxide easily wets the molten steel. Here, the total addition amount of at least one or more of Ce, La, Nd, and Pr is specified to be 0.0003 to 0.02 mass %. This is because when the total addition amount of at least one or more of Ce, La, Nd, and Pr is less than 0.0003% by mass, the amount of equiaxed nucleation sites becomes small, and conversely 0.02% by mass. This is because the oxides or oxysulfides that are formed are likely to coarsen if the value exceeds the above range, and in either case, the effect of making the solidified structure in the slab into fine equiaxed crystals is lost.

In actual continuous casting, molten steel reoxidation occurs due to air, slag, etc., and alumina-based inclusions and titania-based inclusions are newly generated in the molten steel. When the amount of these inclusions is increased, Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr which become the equiaxed nucleation site are formed. Since alumina-based inclusions or titania-based inclusions having a low equiaxed crystal nucleation ability adhere to the surface of oxysulfides or these composite oxides, the total amount of predetermined Ce, La, Nd or Pr should be added. However, the present inventors have found that it is difficult to obtain the effect of refining the solidified structure, and in the worst case, no equiaxed crystal is formed. This finding was based on the observation that the solidified structure of the obtained steel ingot was observed by injecting 10 kg molten steel melted by adding Ce in an inert gas atmosphere into a mold in an inert atmosphere and an air atmosphere. This is confirmed by the fact that the cast structure turns into fine equiaxed crystals, while the cast structure in the air atmosphere turns into coarse-grained crystals. Furthermore, in order to stably enjoy the solidification structure refining effect of the addition of Ce, La, Nd, or Pr even in the actual process in which the reoxidation of molten steel occurs, the present inventors have found that Ce, La, Nd, or Pr should be used. On the surface of alumina-based inclusions and titania-based inclusions adhering to the oxides and oxysulfides of TiN, TiN with high equiaxed nucleation power is deposited as nucleation sites, and the nucleation power of the entire composite inclusions is increased. It has been found to be effective to raise. Molten steel reoxidation due to air, slag, etc. mainly occurs in the ladle and molten steel injection part in the tundish, and TiN is relatively unstable in molten steel at high temperature. In order to stably deposit TiN on the titania-based inclusions, the molten steel temperature should be as low as possible at the downstream side of the molten steel injection part in the tundish where the reoxidized inclusions are formed, and the reoxidized inclusions should contact TiN. It is effective to add it in the immersion nozzle of strong stirring which is frequently performed. When the molten steel 2 in the tundish 1 is injected into the mold 6 through the gas injection type immersion nozzle 3 as shown in FIG. A new method was devised in which N ₂ gas or N ₂ -Ar mixed gas was blown through the pores 4 and reacted with Ti in the molten steel in the immersion nozzle to precipitate TiN on the reoxidized inclusions. The N ₂ gas blown into the molten steel is once dissolved in the molten steel in the vicinity of the inner wall of the nozzle by the formula (1) and then reacts with Ti in the molten steel by the formula (2) so that the alumina-based inclusions and titania-based inclusions To produce TiN.
N ₂ (gas) = 2 N (dissolved) (1)
Ti (dissolved)+ N (dissolved)=TiN (solid) (2)
When 100% N ₂ gas is blown in, the Ti concentration required for TiN production is calculated from thermodynamic data to be 0.014% by mass. Therefore, in order to produce TiN by this method, the Ti concentration in the molten steel should be 0. It is necessary to make it 014 mass% or more. Further, as will be described later, if the Ti concentration is too high, the titania-based inclusions are converted into Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, which have high equiaxed nucleation ability. Since it cannot be modified into Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof, the Ti concentration must be 0.1% by mass or less. In that case, TiN is generated by blowing N ₂ gas. Therefore, it is necessary to set the N ₂ gas concentration to 2% or more. Therefore, in order to compensate for the decrease in the equiaxed crystal nucleation ability of the oxide/oxysulfide of Ce, La, Nd or Pr due to the formation of TiN, and to obtain the effect of refining the equiaxed crystal stably even during the reoxidation of molten steel, It is necessary to set the concentration of N ₂ gas in the blown gas to 2 to 100%. Further, the mixed gas for changing the N ₂ gas concentration needs to be an inert gas that does not react with the molten steel. In the present invention, the inert gas means a rare gas, and Ar gas is preferable in terms of cost. In addition, if even a small amount of fine TiN is deposited on the alumina-based inclusions or titania-based inclusions adhering to the surface of the oxide/oxysulfide of Ce, La, Nd, or Pr, that portion is equiaxed. Since it can serve as nuclei of Ni, and compensate for the decrease in the equiaxed crystal nucleation ability of oxides/oxysulfides of Ce, La, Nd, or Pr, the lower limit blown N ₂ gas flow rate is not specified. On the other hand, if N ₂ gas is blown in excessively, the N content that has not reacted with Ti in the molten steel to form TiN may react with Al in the molten steel to form coarse precipitates and deteriorate workability. There is concern, but when N ₂ gas is blown at 10 Nl/min with a standard flow rate of molten steel passing through the gas injection type immersion nozzle being about 2 t/min, the N concentration in the molten steel will increase by 0.0006 mass% at the maximum. The problem of material deterioration does not occur even when considered from the proper N concentration of 0.0005 to 0.01 mass% described later. Therefore, the upper limit N ₂ gas blowing flow rate is not specified unconditionally, but if the N concentration in the molten steel does not exceed 60% of the upper limit of 0.01 mass% under the same casting conditions, it is 100 Nl/min. Is a desirable upper limit. Further, when the mixed gas of the N ₂ gas and the inert gas is blown in, the inert gas is not absorbed in the molten steel, so it is desirable that the flow rate of the N ₂ gas alone does not exceed 100 Nl/min.

Next, the condition of electromagnetic stirring of [1] will be described. Generally, in electromagnetic stirring, since a swirl flow is imparted to the molten steel at the solidification interface, it is considered that this swirl flow divides the columnar dendrites and promotes equiaxed crystallization. However, according to the knowledge of the present inventors, the effect of columnar crystal fragmentation at the solidification interface of the surface layer of the slab, which has been conventionally said, is weak, but rather the heat transfer between the solidified shell and the molten steel is promoted by electromagnetic stirring, It was found that the effect of lowering the superheat of molten steel inside is high. In the promotion of the production of equiaxed crystal nuclei of the present invention, the effect of lowering the degree of superheating of molten steel by electromagnetic stirring is utilized, and the remelting of equiaxed crystal nuclei generated from the origin of fine oxides/oxysulfides by electromagnetic stirring Is being prevented. However, in order to enhance the effect of reducing the degree of superheat of molten steel by electromagnetic stirring, it is necessary to increase the swirling flow rate. In that case, fine oxides/oxysulfides become coarse due to agglomeration/coalescing, resulting in equiaxed crystal formation. It will not function effectively as a nucleus. Therefore, C: 0.08 mass%, Si: 0.5 mass%, Mn: 1.0 mass%, P: 0.02 mass%, S: 0.003 mass%, N: 0.003 mass%, Acid-soluble Al: 0.025% by mass, acid-soluble Ti: 0.04% by mass, Ce: 0.004% by mass, molten steel was mixed with 15% N ₂ -85% Ar mixed gas from a gas injection type immersion nozzle. An experiment of continuous casting while blowing 10 Nl/min was carried out to investigate the influence of the swirling flow velocity of electromagnetic stirring on the equiaxed grain size inside the slab and on the surface layer of the slab. For the branched columnar crystals (not divided) and the divided branched columnar crystals, the equiaxed crystal grain size was defined as 2(a·b) ^0.5 so that the grain size could be evaluated simultaneously ( a is the major axis of the crystal grain, b is the minor axis of the crystal grain, and one branch is regarded as one crystal grain for the undivided branched columnar crystals.). The average equiaxed grain size inside the slab is the average value of the equiaxed grain size of the transverse section from the slab ¼ thickness to the inside, and the average equiaxed grain size of the surface layer of the slab is from the surface layer to the slab It is the average value of the equiaxed crystal grain size in the transverse section at a thickness of 1/4.

FIG. 2 shows the effect of the electromagnetic stirring flow rate on the average equiaxed grain size inside the cast and in the surface layer of the cast. As can be seen from FIG. 2, the average equiaxed crystal grain size inside the slab decreases to 3 mm or less when the swirling flow velocity of molten steel is 25 cm/s or more, and to about 2 mm when 30 cm/s or more, but exceeds 100 cm/s. On the contrary, the average equiaxed crystal grain size starts to increase, and if it exceeds 105 cm/s, the grain size becomes larger than 3 mm and becomes coarse. The reason for this is that when the swirling flow rate of electromagnetic stirring is 25 cm/s or more, more specifically 30 cm/s or more, re-dissolution of equiaxed nuclei generated from fine oxides inside the slab is suppressed. On the other hand, when the swirling flow velocity exceeds 100 cm/s, even Ce oxide and Ce oxysulfide in the slab become coarser due to aggregation and coalescence, and it becomes difficult to function as a nucleus of equiaxed crystal, and 105 cm/s It is considered that if it exceeds the limit, it will not function as equiaxed nuclei. Incidentally, for the slab surface layer portion, relatively long branched columnar crystals that are aligned in a certain direction from the mold side toward the inside of the slab are growing, branched columnar crystals that are not divided, and branched branches that are not divided. The columnar crystals and the average equiaxed grain size including the columnar crystals were coarse. This is because the effect of columnar crystal division at the solidification interface of the surface layer of the cast slab by electromagnetic stirring is relatively weak. Therefore, in order to make the solidification structure inside the slab into fine equiaxed crystals, it is desirable to control the swirling flow rate of the electromagnetic stirring to 30 to 100 cm/s. Further, below 10 m below the mold, solidification of the surface layer of the slab is almost completed, so induction electromagnetic stirring should be installed between the position of the meniscus in the mold where solidification begins and the position 10 m below the mold. Is effective.

Next, regarding [2] selection of a metal element that lowers the solid-liquid interface energy, it is confirmed that Bi and Sn are promising elements as elements that can obtain a surface active effect by adding a small amount without adversely affecting the steel plate material. It was found by evaluating the columnar crystal spacing in a solidification experiment of 10 kg molten steel to which these metal elements were added. The effect of refinement of columnar crystals is sufficient if one or more of these metal elements are added in a total amount of 0.0005 mass% or more, but if added in excess of 0.01 mass%, the steel sheet becomes brittle and during rolling. Ear cracks occurred at the edges. Therefore, at least one of Bi and Sn may be added to the molten steel in a total amount of 0.0005 to 0.01% by mass. Further, the locations where Bi and Sn are added are unlikely to adversely affect the entire slab material, and are added to the molten steel in the mold so as to maximize the effect of columnar grain refinement in the surface layer of the slab as much as possible. Is desirable. As a method of addition, a metal wire containing Bi and Sn is directly inserted into the molten steel upper side in the mold, or is supplied by using a mold flux containing Bi and Sn, so that the surface layer of the slab can be relatively exposed. Can be added efficiently. As a method of adding the refinement element via the mold flux, a method of using a mold flux mixed with Bi or Sn in advance, or a method of supplying Bi or Sn into the mold while mixing Bi or Sn into the mold flux immediately before the addition is performed. A method of supplying Bi powder or Sn powder onto the mold flux coating the molten metal surface at a constant rate during casting is effective. The boiling points of Bi and Sn are 1560° C. and 2270° C., respectively, which are higher than the melting point of molten steel (pure iron 1538° C.), so that explosive gasification does not occur during addition. Further, the densities of Bi and Sn are 9.8 g/cm ³ and 7.3 g/cm ³ , respectively, which are heavier than the density of molten steel of 7.0 g/cm ³ , so that they should be added from the wire or powder to the surface of molten steel. However, it does not float immediately and can be added to molten steel relatively easily. The content of added Bi and Sn can be evaluated by analysis of a sample taken from a slab or a rolled steel plate.

Furthermore, the condition of electromagnetic stirring in [2] will be described. Here, since the columnar crystal dividing effect of electromagnetic stirring at the solidification interface is weak as described above, Bi and Sn are added to the mold to make the columnar crystals growing from the mold side fine and weak, It is important to effectively divide the columnar crystals by the weak shearing force of electromagnetic stirring to form fine equiaxed crystals in the surface layer of the slab. Therefore, C: 0.08 mass%, Si: 0.5 mass%, Mn: 1.0 mass%, P: 0.02 mass%, S: 0.003 mass%, N: 0.003 mass%, Acid-soluble Al: 0.025% by mass, acid-soluble Ti: 0.04% by mass, Ce: 0.004% by mass, molten steel was mixed with 15% N ₂ -85% Ar mixed gas from a gas injection type immersion nozzle. The average equiaxed grain size inside the slab and on the surface layer of the slab is determined by a continuous casting experiment in which 0.003 mass% of Bi is added through continuous casting powder in the mold while pouring 10 Nl/min. The influence of the swirling flow velocity of electromagnetic stirring was investigated and shown in FIG. The average equiaxed crystal grain size of the surface layer of the slab becomes 3 mm or less when the swirling flow rate by electromagnetic stirring becomes 25 cm/s or more, and about 2 mm when the swirling flow rate becomes 30 cm/s or more, and the swirling flow rate becomes 100 cm/s. Even over s, the effect is maintained. This is because when the swirling flow rate of the electromagnetic stirring becomes 25 cm/s or more, the columnar crystals that are finely and weakened by the addition of Bi in the mold begin to be separated by the electromagnetic stirring flow, and are more effective when it becomes 30 cm/s or more. This is a result showing that the columnar crystals can be effectively divided and fine equiaxed crystals can be generated in the surface layer of the cast slab. On the other hand, as can be seen from FIG. 3, when the swirling flow velocity of the molten steel becomes 30 cm/s or more, the average equiaxed crystal grain size inside the slab decreases to about 2 mm, but when it exceeds 100 cm/s, the average equiaxed crystal conversely. The particle size begins to grow. As described in the previous experiment, the cause of this is that when the swirling flow rate of electromagnetic stirring becomes 30 cm/s or more, re-equilibration of the equiaxed nuclei generated from fine Ce oxide or Ce oxysulfide inside the slab. While the dissolution is effectively suppressed, when the swirling flow rate exceeds 100 cm/s, even Ce oxides and Ce oxysulfides inside the slab begin to coarsen due to agglomeration and coalescence, and become cores of equiaxed crystals. It is thought that it is difficult to function. Therefore, in order to make the entire slab into fine equiaxed crystals, it is effective to set the electromagnetic stirring flow rate to 30 to 100 cm/s.

Regarding the electromagnetic stirring flow rate, under normal continuous casting conditions where columnar crystals and branched columnar crystal structures develop, the solidified structure in the widthwise center of the cast slab was revealed by picric acid etching, and The flow velocity can be evaluated from the slope of the crystal. By this method, the relationship between the electromagnetic stirring thrust and the electromagnetic stirring flow rate is obtained in advance, and in the method of the present invention, the electromagnetic stirring thrust for obtaining the target electromagnetic stirring flow rate may be selected to carry out the electromagnetic stirring.

By the combination of the above [1] and [2], the average equiaxed crystal grain size is 3 mm or less (electromagnetic stirring flow rate 25 to 105 cm/s) from the surface layer of the cast slab to the quarter thickness and from the quarter thickness to the inside. Desirably, it is possible to obtain a solidified structure of 2 mm or less (electromagnetic stirring flow rate 30 to 100 cm/s).

As can be seen from the above description, the present invention is not limited to application to slabs, and even when applied to blooms and billets, a sufficient solidification structure refining effect can be obtained.

Of the chemical components in the molten steel of the present invention, the reasons for limiting Ce, La, Nd, Pr, Bi, and Sn are as described above. Finally, the reasons for limiting the chemical components other than these are described.

In the present invention, the dissolved (acid-soluble) Al concentration in the molten steel is 0.03 mass% or less, and a large amount of alumina-based inclusions remain at the acid-soluble Al concentration exceeding this, and these are contained in Ce, La, Since it cannot be modified into Nd or Pr oxide/oxysulfide, the solidified structure in the cast cannot be made into a fine equiaxed crystal. Alumina-based inclusions are modified to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or composite oxide of these. However, in order to enhance the nucleation ability of equiaxed crystals, it is better that the acid-soluble Al concentration is lower, and the lower limit value includes 0 mass %.

Further, if the acid-soluble Ti concentration is too high, the titania-based inclusions are converted to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr. The acid-soluble Ti concentration should be 0.1% by mass or less because it cannot be modified into oxysulfides or their composite oxides, and the lower limit value is 100% N ₂ gas blowing as described above, and TiN can be generated. And 0.014% by mass. The acid-soluble Al concentration and the acid-soluble Ti concentration are measured by measuring the amount of Al dissolved in the acid and the amount of Ti. Dissolved Al and dissolved Ti are dissolved in the acid, and alumina and titania are not dissolved in the acid. It is an analysis method that utilizes this. Here, the acid is, for example, a mixed acid obtained by mixing hydrochloric acid 1, nitric acid 1, and water 2 in a ratio.

C is an essential element for ensuring the strength of the steel sheet, and at least 0.03 mass% is necessary to obtain a high-strength steel sheet. However, if it is contained excessively, even if C is fixed by an additive element such as Ti or the cooling conditions are fully used, the generation of a cementite phase which is unfavorable to the stretch flange characteristic cannot be avoided, so the content is 0.20 mass% or less. To do.

Si is an element that relatively suppresses the deterioration of bendability and contributes to the improvement of strength, and in order to exert its effect, 0.08 mass% or more must be added. If added excessively, the weldability and ductility are adversely affected, so the upper limit is 1.5% by mass.

Mn, together with C and Si, is an element effective in increasing the strength of the steel sheet, and it is necessary to contain 0.5 mass% or more, but if it exceeds 3.0 mass%, the ductility deteriorates, so the upper limit is set. The amount is 3.0% by mass.

P is effective as a solid solution strengthening element, but since it is feared that the workability will deteriorate due to segregation, it is necessary to set it to 0.05% by mass or less. If solid solution strengthening is not necessary, it is not necessary to add P, and the lower limit of P includes 0 mass %.

Since S forms coarse stretched inclusions of MnS and deteriorates the workability, conventionally, extremely low sulfurization with an S concentration of 0.002 mass% was essential to secure the workability, but in the present invention, it is fine. Precipitating MnS on hard Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or their composite oxides. The upper limit of the S concentration is not particularly specified because deformation is unlikely to occur during rolling and the stretching of inclusions is prevented. However, if the S concentration is too high, a large amount of Ce, La, Nd, or Pr oxide/oxysulfide that suppresses the deformation of MnS is required, and at least one of Ce, La, Nd, or Pr is required. Since the total addition amount of at least one species exceeds 0.02 mass %, there is a disadvantage that the produced oxide or oxysulfide is likely to coarsen, and 0.02 mass% or less is desirable. Further, in order to reduce the S concentration to less than 0.002% by mass, which is the same level as the conventional one, it is necessary to considerably strengthen the desulfurization treatment in the secondary refining, and the desulfurization treatment cost becomes too high. The lower limit of the S concentration is set to 0.002% by mass because it becomes difficult to enjoy the effect of controlling the morphology of MnS.

If N is added too much, a coarse precipitate is generated even with a trace amount of Al and the workability is deteriorated. Therefore, the upper limit is 0.01% by mass. On the other hand, since it takes a cost to make it less than 0.0005 mass %, the lower limit is made 0.0005 mass %.

Nb and V are elements added to obtain higher strength, and utilize precipitation strengthening by carbonitride formed by combining with these elements. Precipitation strengthening can be obtained by adding these elements alone or in combination, but excessive addition deteriorates workability, so the upper limit is 0.2% by mass for one or two of these elements. In order to obtain the effect of improving strength, it is preferable to add 0.005 mass% or more of each.

Mo is also an element used to improve strength, but is mainly added to improve hardenability. If added excessively, ductility will be deteriorated, so 0.5% by mass is made the upper limit. When ensuring hardenability, it is preferable to add 0.05 mass% or more. Nb, V, and Mo may not be contained.

The main additive elements are as described above from the viewpoint of ensuring the material quality, but the inclusion of trace amounts of unavoidable impurities such as Cu, Ni and Cr due to the use of scrap does not impair the present invention.

Hereinafter, the present invention will be described based on FIG. 1 with reference to Examples and Comparative Examples.

300 t of molten steel containing chemical components other than Bi and Sn in Table 1 was melted and poured into the tundish 1 (volume: 50 t). While injecting the N ₂ —Ar mixed gas shown in Table 2 from the gas injection type immersion nozzle 3, the molten steel 2 in the tundish is injected into the mold 6, and in the mold 6 according to the component values of Bi and Sn in Table 1. Then, Bi and Sn were added to the molten steel 11 by wire and continuously cast. Regarding test numbers 11, 13, and 15 in Table 1, mold flux containing at least one of Bi and Sn was supplied onto the surface of the molten steel 11 in the mold for continuous casting. The slab size is 250 mm in thickness×1100 mm in width, and the casting speed is 1.0 m/min. The induction electromagnetic stirring device 7 is installed on the meniscus 12 in the mold, and during casting, a current of 500 A and a frequency of 2 Hz was passed through the induction electromagnetic stirring device 7 to stir the molten steel at 40 cm/s. In some experiments (Examples 16, 17, and 18), the electric current of the induction electromagnetic stirrer 7 was changed to stir the molten steel at 90 cm/s, 25 cm/s, and 105 cm/s. Regarding the electromagnetic stirring flow rate, as described above, the flow rate was evaluated from the inclination of the columnar crystals or the branched columnar crystals in the widthwise center of the cast slab.

The solidification structure was observed in the early stage of casting where reoxidation was severe due to air (a part until the empty tundish 1 was filled with molten steel 2: about 0 to 50t), in the middle stage of casting with little reoxidation (molten steel in tundish 1). It was carried out at a steady site after 2 was filled: about 50 to 250 t casting) and at the end of casting where reoxidation due to the inclusion of ladle slag was severe (site where molten steel in the tundish begins to decrease: about 250 to 300 t casting).

As described above, the average equiaxed crystal grain size inside the cast piece was the average value of the equiaxed crystal grain diameter of the transverse section inside the cast piece 1/4 thickness. The average equiaxed crystal grain size of the surface layer of the cast slab was the average value of the equiaxed crystal grain size of the transverse section of the cast slab ¼ thickness. For the branched columnar crystals (not divided) and the divided branched columnar crystals, the equiaxed crystal grain size was defined as 2(a·b) ^0.5 so that the grain size could be evaluated simultaneously ( a is the major axis of the crystal grain, b is the minor axis of the crystal grain, and one branch is regarded as one crystal grain for the undivided branched columnar crystals.).

Table 2 shows the results of investigating the solidification structure of the cast pieces obtained in this experiment. In Table 1 and Table 2, numerical values outside the scope of the present invention are underlined.

In test numbers 1, 6, 11, 16, 17, and 18, which are examples of the present invention, Ce oxide, La oxide, Nd oxide, Pr oxide, and Ce acid effective as nucleation sites for equiaxed crystals are used. Finely disperse sulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or their complex oxides into molten steel, and add electromagnetic stirring to remove the superheat of molten steel. Due to the effect of N ₂ gas injection from the gas injection type immersion nozzle at the initial and final stages of casting when the oxidation is severe and the equiaxed nucleation ability of Ce, La, Nd, or Pr oxides/oxysulfides decreases. By compensating for the decrease in the ability to generate axial crystal nuclei, the inside of the slab was finely equiaxed with a grain size of 3 mm or less throughout the casting period. At the same time, Bi and Sn, which lower the solid-liquid interface energy in the mold, are added to make the columnar crystals growing from the mold side toward the inside of the slab fine and weak, and then the swirling flow of electromagnetic stirring. By dividing the fine and fragile columnar crystals, we succeeded in forming fine equiaxed crystals with a grain size of 3 mm or less in the surface layer of the slab throughout the casting.

On the other hand, in the test numbers 2, 7 and 12 which are comparative examples, since Bi and Sn were not added in the mold, the branched columnar crystals in the surface layer of the slab become coarse, and the test numbers 3 and 8 which are comparative examples, In No. 13, since Ce, La, Nd, or Pr which is effective as a nucleation site for equiaxed crystal was not contained in the molten steel, the equiaxed crystal inside the slab coarsened, and further, test numbers 4, 9 as comparative examples, No. 14 contained neither Ce, La, Nd, or Pr, did not add Bi and Sn in the mold, and did not blow N ₂ gas from the gas blowing type immersion nozzle. Also in, the equiaxed crystals became coarse.

Further, test numbers 5, 10 and 15 of Comparative Examples in which N ₂ gas was not blown from the gas blowing type immersion nozzle, and N ₂ gas was blown from the gas blowing type immersion nozzle, but the acid-soluble Ti concentration was too low. In the test No. 20, the solidification structure was refined in the middle stage of casting, but in the early stage of casting and the final stage of casting where molten steel reoxidation was intense, despite the addition of Ce, La, Nd, or Pr, The axial crystals became coarse. This is because a large amount of alumina-based inclusions or titania-based inclusions generated by molten steel reoxidation are Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide of equiaxed nucleation site, This is because the equiaxed crystal refining ability disappears due to the deposition on the surface of La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof.

Moreover, in the test number 19 of the comparative example in which Ce was excessively added, the Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide of the produced equiaxed nucleation site, Since Nd oxysulfide, Pr oxysulfide, or a complex oxide thereof was coarsened, the equiaxed crystal inside the slab was coarsened.

1 Tundish 2 Molten Steel 3 Gas Blowing Type Immersion Nozzle 4 Porous Inner Porous Body 5 Gas Introducing Pipe 6 Mold 7 Induction Electromagnetic Stirrer 11 Molten Steel 12 Meniscus

Claims (3)

Using a continuous casting device having an induction electromagnetic stirrer between the meniscus in the mold and 10 m below the mold, C: 0.03 to 0.20 mass%, Si: 0.08 to 1.5 mass%, Mn: 0 .5-3.0 mass%, P:0.05 mass% or less, S:0.002 mass% or more, N:0.0005-0.01 mass%, Nb:0.2 mass% or less, V: 0.2 mass% or less, Mo: 0.5 mass% or less, acid-soluble Al: 0.03 mass% or less, acid-soluble Ti: 0.014 to 0.1 mass%, Ce, La, Nd or Pr. Among them, a total of at least one kind: 0.0003 to 0.02% by mass, the balance of molten steel consisting of Fe and unavoidable impurities, N ₂ gas or 2% of N ₂ from a gas injection type immersion nozzle. The above-mentioned inactive gas is blown into the mold while being blown, and one or more kinds of Bi and Sn are added to the molten steel in the mold so that the total amount becomes 0.0005 to 0.01% by mass. A continuous casting method for molten steel, characterized in that the molten steel is cast while being swirled at a swirling flow rate of 25 to 105 cm/s in a horizontal plane by an electromagnetic stirrer. The continuous casting method for molten steel according to claim 1, wherein a metal wire containing at least one of Bi and Sn is continuously supplied into the molten steel in the mold. The continuous casting method for molten steel according to claim 1, wherein a mold flux containing at least one of Bi and Sn is supplied onto the surface of the molten steel in the mold.

Images