JP6500630B2 - Continuous casting method for molten steel and continuous cast slab - Google Patents

Description

  In the cross section of a normal continuous casting slab, coarse granular crystal parts near the center and coarse columnar crystal parts surrounding the coarse granular crystal parts are disposed so as to surround the final solidified part with porosity and segregation at the center. It is observed. If this coarse granular crystal and columnar crystal can be made fine equiaxed and central segregation and microsegregation can be significantly reduced, for example, when the slab is made into a thin plate, it becomes a thin plate with remarkably excellent formability. Also, for example, when a thick plate is used, it becomes a thick plate excellent in low temperature toughness. The present invention relates to a continuous casting method of molten steel capable of forming coarse granular crystals and columnar crystals into fine equiaxed crystals, and a continuous cast slab having a fine solidified structure cast using the same.

  Non-Patent Document 1 shows that low-temperature casting is effective for equiaxed crystallization, since equiaxed crystals increase as the degree of superheat of molten steel is low. Further, Patent Document 1 describes a technique for giving a swirling flow in one direction to molten steel in the vicinity of a solidification interface using an induction electromagnetic stirring device, and dividing columnar dendrites into equiaxed columnar crystals. There is. Furthermore, Patent Document 2 sprays an equiaxed crystallization accelerator (such as oxides and sulfides of Ce) on the surface of molten steel using a plasma heating device in a tundish to generate a large number of equiaxed nuclei in a mold. There is disclosed a technique of refining coarse granular crystals in the vicinity of the center of a cast slab by

Japanese Patent Application Laid-Open No. 50-23338 JP 2001-225153 A

Steel Manual 3rd Edition, II Steelmaking & Steelmaking, p. 653

  However, in low temperature casting, it is necessary to make the degree of superheat of the molten metal close to the liquidus and inject it into the mold from the immersion nozzle, so solidification defects such as clogging of the immersion nozzle and deckle formation in the mold May lead to For this reason, in the current continuous casting, the superheat degree of the molten metal to be injected adopts about 20 to 30 K, and under such temperature conditions, the formation of high strength thin steel plate whose production amount has been increasing due to weight reduction needs in recent years Micro equiaxed crystallization to such an extent that the processability and the low temperature toughness of a high strength thick plate can be improved has not been achieved. In addition, in the method using induction electromagnetic stirring or 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 necessarily obtained stably, For example, in the case of a molten steel having a C content of 0.1% or less, in which equiaxed crystals are not easily generated, it is difficult to sufficiently equiaxize columnar crystals in the surface layer portion of a slab.

  In view of such a present situation, the present invention provides a continuous casting method capable of making both coarse granular crystals near the center and coarse columnar crystals surrounding it into fine equiaxed crystals in a slab for high strength steel, and using the same. It is an object of the present invention to provide a continuous cast slab for high strength steel having a fine solidified structure which has been cast.

  In view of such a situation, a continuous casting method capable of making coarse granular crystals near the center and coarse columnar crystals surrounding them both into fine equiaxed crystals and a continuous cast slab having a fine solidified structure cast using the same Elucidating the solidification structure refining element, providing intensive research on the addition method and addition location that exerts the effect by small amount addition, in order to provide the process design combining the findings obtained optimally in the continuous casting process This led to the completion of the present invention.

The summary is as follows. That is,
(1) C: 0.03 to 0.20% by mass, Si: 0.08 to 1.5% by mass using a continuous casting apparatus having an induction magnetic stirring device between a meniscus in a 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% by mass or less, Mo: 0.5% by mass or less, acid-soluble Al: 0.03% by mass or less, acid-soluble Ti: less than 0.1% by mass, Ce, La, Nd or Pr In the mold, a molten steel containing a total of at least one of 0.0003 to 0.02 mass% and the balance of Fe and incidental impurities is injected into the mold, and the molten steel in the mold contains Bi and Sn. allowed added to the molten steel content is 0.0005 to 0.01 wt% of one or more in total from Continuous casting wherein the casting while swirling the molten steel in a horizontal plane by the inductive electromagnetic stirring device.
(2) The continuous casting method according to the above (1), characterized in that a metal wire containing one or more of Bi and Sn is continuously fed into molten steel in a mold.
(3) The continuous casting method according to the above (1), wherein a mold flux containing one or more of Bi and Sn is supplied onto the molten steel surface in a mold.
(4) The swirling flow velocity of the molten steel by induction electromagnetic stirring is set to 25 to 105 cm / s at the solid-liquid interface in the central portion in the width direction of the slab , any one of the above (1) to (3) The continuous casting method described.
(5) A continuous casting method characterized in that the average equiaxed grain size is 3 mm or less for each of 1⁄4 thickness and 1/4 thickness from the surface layer of the slab according to the continuous casting method described in (4) above. Slabs.

  According to the present invention, since it is possible to produce a continuous cast slab in which the solidified structure of the slab surface layer portion and the inside of the slab are finely equiaxed together, the high strength thin steel plate has high formability and high strength. With a thick plate, it becomes possible to manufacture a material excellent in low temperature toughness.

It is a figure which shows the influence of the electromagnetic stirring flow rate on the average equiaxed grain size inside the slab and the slab surface part in the continuous casting experiment of 0.004 mass% of Ce containing Ce. Influence of the electromagnetic stirring flow rate on the average equiaxed grain size inside and at the surface layer of continuously cast slab containing 0.003 mass% of Bi added to 0.004 mass% of Ce in the mold FIG.

  The form of the solidified structure is determined by the temperature gradient and the solidification rate of the solid-liquid interface at the time of solidification, and the smaller the temperature gradient and the larger the solidification rate, the easier the equiaxed crystals are formed. However, in continuous continuous casting, columnar crystals grow from the surface layer of the slab to the inside relatively, and it is difficult to change the cooling condition so as to change the solidified structure form to equiaxed crystal as a main component. Under such conditions, in order to micro-scale the solidified structure, nucleation sites for equiaxed crystal formation are dispersed in the molten steel and nucleation frequency is increased to promote formation of micro-scaled equiaxed crystal. There are two conceivable methods of reducing the solid-liquid interface energy by using a metal element having a high surface activity effect, and micro-equiaxially crystallizing the columnar crystals themselves. The present invention effectively combines these two solidification structure control principles, reveals a control means for maximizing the solidification structure refinement effect over the entire surface of the cast slab, and continuously controls the control means. It is completed by optimally combining process design in the casting process. The basic idea of the present invention is to finely disperse in the molten steel the oxides and oxysulfides that effectively act as nucleation sites for equiaxed crystals [1] and add electromagnetic stirring to this to deprive the superheat of the molten steel. A metal element that lowers solid-liquid interface energy is preferentially added to the surface layer portion of the [2] slab while stably micro-eaxially crystallizing the inside of the slab stably, and the growth toward the inside of the slab from the mold side In order to form fine equiaxed crystals in the surface layer portion of the slab by dividing the fine and fragile columnar crystals by the swirling flow of electromagnetic stirring. As a result, it is 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, the conditions for oxides and acid sulfides, which become nucleation sites for equiaxed crystals in [1], are titanium-based inclusions in Ti-deoxidized molten steel, and alumina-based inclusions in Al-deoxidized molten steel. Although many inclusions exist, these inclusions are extremely easy to aggregate and combine to form coarse oxides, and thus do not function effectively as nuclei for equiaxial crystal formation. On the other hand, the present inventors added at least one or more of Ce, La, Nd or Pr, which is a stronger deoxidizing element than Ti and Al, to molten steel, and included titania inclusions and alumina inclusions. Relatively fine oxidation by reforming the oxide into Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide or Pr oxysulfide It has been found that it is possible to uniformly disperse metal oxides and acid sulfides in molten steel and that these oxides and acid sulfides are likely to become nuclei of fine equiaxed crystal formation. This means that Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide are molten steel compared to titania or alumina. It is thought that it is because it is easy to get wet. Here, the total addition amount of at least one or more kinds of Ce, La, Nd or Pr is defined to 0.0003 to 0.02 mass%. This is because, if the total addition amount of at least one or more of Ce, La, Nd or Pr is less than 0.0003%, the amount of equiaxial nucleation sites is decreased, and conversely 0.02 mass% is contained. If it exceeds, the formed oxide or oxysulfide tends to be coarsened, and in any case, the effect of making the solidified structure in the slab into fine equiaxed crystals is lost.

  In the present invention, the concentration of dissolved (acid-soluble) Al in molten steel is 0.03% by mass or less, and if the concentration of acid-soluble Al is more than this, alumina inclusions can be completely removed by Ce oxide, La oxide, Nd It can not be reformed into oxides, Pr oxides, Ce oxysulfides, La oxy sulfides, Nd oxy sulfides, or Pr oxy sulfides, and becomes complex inclusions with alumina, while aggregation and coalescence progress, Lose their ability as crystal nucleation sites. Suppresses the aggregation and combination of Ce oxide, La oxide, Nd oxide, Pr oxide, Ce acid sulfide, La acid sulfide, Nd acid sulfide, or Pr acid sulfide, and produces equiaxial nucleation In order to maintain the performance, the acid-soluble Al concentration should be low, and the lower limit includes 0% by mass. The acid-soluble Al concentration is a measurement of the amount of Al dissolved in an acid, which is an analysis method utilizing the fact that dissolved Al dissolves in an acid and alumina does not dissolve in an acid. Here, the acid is, for example, a mixed acid mixed in the proportion of hydrochloric acid 1, nitric acid 1 and water 2.

  In addition, when the acid-soluble Ti concentration is too high, the oxide is likely to aggregate and become coarse, and it is easy to form coarse TiN inclusions by being combined with N in the steel, so acid-soluble The Ti concentration is less than 0.1% by mass, and the lower limit includes 0% by mass. The acid-soluble Ti concentration, like the acid-soluble Al concentration, is obtained by measuring the amount of Ti dissolved in the acid, and corresponds to the dissolved Ti concentration.

Next, the conditions of the electromagnetic stirring of [1] will be described. Generally, in electromagnetic stirring, it is believed that this swirling flow breaks up the columnar dendrite to promote equiaxed crystallization because it imparts a swirling flow to the molten steel at the solidification interface. However, according to the findings of the present inventors, the effect of columnar crystal division at the solidification interface of the surface layer portion of the slab conventionally known is weak, and rather, heat transfer between the solidified shell and the molten steel is promoted by electromagnetic stirring, It was found that the effect of reducing the internal superheat of molten steel was high. In the promotion of nucleation of equiaxed crystals of the present invention, re-dissolution of equiaxed nuclei formed from fine oxides and oxysulfides as starting points by electromagnetic stirring utilizing the effect of lowering the degree of superheat of molten steel by the electromagnetic stirring. To prevent. However, in order to increase the reduction effect of the molten steel superheat degree by electromagnetic stirring, it is necessary to increase the swirling flow velocity, in which case fine oxides and oxysulfides become coarse due to aggregation and coalescence, resulting in equiaxed crystals. It does 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%, The continuous casting experiment of molten steel of acid soluble Al: 0.025% by mass, acid soluble Ti: 0.04% by mass, Ce: 0.004% by mass was carried out, and the equiaxed inside of the slab and the surface layer portion of the slab The influence of the swirling flow velocity of the electromagnetic stirring on the grain size was investigated. The equiaxed grain size is 2 (a · b) so that the grain sizes of branched columnar crystals (not divided), divided columnar crystals, and divided branched columnar crystals can be simultaneously evaluated. It defined as ^0.5 (a is a major axis of a crystal grain, b is a minor axis of a crystal grain. One branch is made into one crystal grain about a branched columnar crystal which is not divided). The average equiaxed grain size inside the slab, the average value of the equiaxed grain size in the cross section from the 1/4 thickness of the slab inside, and the average equiaxed grain size in the surface layer of the slab from the surface layer to the slab It is the average value of the equiaxed grain size of the cross section in 1/4 thickness. The effect of the electromagnetic stirring flow rate on the average equiaxed grain size in the inside of the slab and in the surface layer of the slab is shown in FIG. As can be seen from FIG. 1, the average equiaxed grain size inside the cast slab decreases to about 2 mm at 30 cm / s or more when the swirling velocity of the molten steel is 25 cm / s or more and 3 mm or less, but exceeds 100 cm / s On the contrary, the average equiaxed grain size starts to increase, and when it exceeds 105 cm / s, it coarsens over 3 mm. The reason for this is that if the swirling flow rate of the electromagnetic stirring is 25 cm / s or more, more specifically 30 cm / s or more, re-dissolution of equiaxed crystal nuclei generated from the fine oxide inside the slab is suppressed. On the other hand, when the swirling flow velocity exceeds 100 cm / s, coarsening by aggregation and coalescence progresses even in Ce oxide or Ce oxysulfide inside the slab, and it becomes difficult to function as an equiaxed crystal nucleus, and 105 cm / s If it exceeds, it is considered that it does not function as an equiaxed crystal nucleus. In the surface layer portion of the slab, relatively long branched columnar crystals that are aligned almost in a fixed direction from the mold side toward the inside of the slab are grown, and are not divided branch columnar crystals, divided branches The average equiaxed grain size including columnar crystals and columnar crystals was coarse. This is because the effect of columnar crystal division at the solidification interface of the surface layer portion of the slab by electromagnetic stirring is relatively weak. Therefore, in order to make the solidified structure inside the cast slab into fine equiaxed crystals, it is desirable to control the swirling flow rate of the electromagnetic stirring to 30 to 100 cm / s. In addition, since solidification of the surface layer of the slab is almost complete below 10m below the mold, induction electromagnetic stirring is placed between the position of the meniscus in the mold where solidification starts and the position of 10m below the mold Is effective. It is more desirable to set in the mold or just under the mold.

Next, [2] is the selection of metal elements that lower the solid-liquid interface energy, but Bi and Sn are promising as elements that can obtain the surface activity effect by adding a small amount without adversely affecting the steel sheet material, It was found by evaluating the spacing between columnar crystals in a solidification experiment of 10 kg molten steel to which these metal elements were added. It is sufficient to add 0.0005% by mass or more in total of one or more of these metal elements for the effect of refining the columnar crystals, but if it is added in excess of 0.01% by mass, the steel plate becomes brittle and is rolled. Ear cracking occurred at the end. Therefore, one or more of Bi and Sn may be added to the molten steel in a total amount of 0.0005 to 0.01% by mass. Furthermore, the locations where Bi and Sn are added are unlikely to adversely affect the entire material of the slab, and are preferably added to the molten steel in the mold so that only the effect of columnar grain refinement can be maximized at the surface layer portion of the slab. Is desirable. As a method of addition, the metal wire containing Bi and Sn is directly inserted into the upper part of the molten steel in the mold, or supplied using a mold flux containing Bi and Sn, relatively to the surface layer of the slab. It can be added efficiently. As a method of adding the refining element through the mold flux, a method of using a mold flux mixed with Bi or Sn in advance, a method of supplying Bi or Sn into the mold flux while mixing it into the mold flux just before the addition A method of supplying Bi powder or Sn powder onto a mold flux which is coated at a constant speed during casting is effective. The boiling points of Bi and Sn are 1560 ° C. and 2270 ° C., respectively, and are higher than the melting point of liquid steel (pure iron 1538 ° C.), so explosive gasification does not occur upon addition. Furthermore, the density of Bi and Sn are each 9.8 g / cm ³ and 7.3 g / cm ^3, since heavier than the density 7.0 g / cm ³ of the molten steel, is added to the molten steel surface from a wire or powder Also, it does not immediately rise and can be added relatively easily into molten steel. The content of added Bi and Sn can be evaluated by analysis of a sample collected from a slab or a rolled steel sheet.

  Furthermore, the conditions of electromagnetic stirring of [2] will be described. Here, since the columnar crystal division effect of electromagnetic stirring at the solidification interface is weak as described above, Bi and Sn are added in the mold to make the columnar crystals grown from the mold side fine and weaken, It is important to divide the columnar crystals effectively by the weak shear force of electromagnetic stirring and 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%, Molten steel with 0.025% by mass of acid-soluble Al, 0.04% by mass of acid-soluble Ti, 0.004% by mass of Ce: 0.004% by mass is poured into a mold, and Bi is continuously cast in the mold through 0.003 of continuous cast powder. An experiment was conducted to carry out continuous casting while adding mass%, and the influence of the swirling flow velocity of electromagnetic stirring on the average equiaxed grain size of the inside of the slab and the surface layer portion of the slab was investigated, and is shown in FIG. The average equiaxed grain size of the surface layer of the slab decreases to 3 mm or less when the swirling flow velocity by electromagnetic stirring is 25 cm / s or more, and to about 2 mm when the swirling flow velocity is 30 cm / s or more. The effect is maintained even over s. This is more effective when the swirling flow rate of the magnetic stirring becomes 25 cm / s or more, and the columnar crystals finely divided and embrittled by the addition of Bi in the mold begin to be divided by the magnetic stirring flow, and become 30 cm / s or more. These results show that the effect of dividing columnar crystals is obtained, and fine equiaxed crystals can be generated in the surface layer portion of the slab. On the other hand, as can be seen from FIG. 2, the average equiaxed grain size inside the slab decreases to about 2 mm when the turning flow velocity of molten steel is 30 cm / s or more, but when it exceeds 100 cm / s, the average equiaxed The particle size starts to grow. The cause of this is, as described in the previous experiment, that when the swirling velocity of the electromagnetic stirring is 30 cm / s or more, re-generation of equiaxed nuclei formed from fine Ce oxide and Ce oxysulfide inside the slab. While dissolution is effectively suppressed, when the swirling flow velocity exceeds 100 cm / s, coarsening due to aggregation and coalescence begins even with Ce oxide or Ce oxysulfide inside the slab, and it becomes an equiaxed crystal nucleus It is thought that it becomes difficult to function. Therefore, in order to micro-equiaxially crystallize the entire slab, it is effective to set the electromagnetic stirring flow rate to 30 to 100 cm / s.

  With regard to the electromagnetic stirring flow rate, under normal continuous casting conditions in which columnar crystals and branched columnar crystal structures develop, the solidified structure in the central portion in the width direction of the cast slab is revealed by picric acid etching. The flow velocity can be estimated from the inclination of the crystal. The relationship between the electromagnetic stirring thrust and the electromagnetic stirring flow velocity is previously obtained by this method, and in the method of the present invention, the electromagnetic stirring thrust for obtaining the target electromagnetic stirring flow velocity may be selected to carry out the electromagnetic stirring.

  By the combination of the above [1] and [2], the average equiaxed grain size is 3 mm or less (electromagnetic stirring flow rate 25 to 105 cm / s) for each of 1/4 thickness and 1/4 thickness from the surface layer of the slab Preferably, a coagulated structure of 2 mm or less (magnetic stirring flow rate of 30 to 100 cm / s) can be obtained. The magnetic stirring flow rate is more preferably 70 to 100 cm / s.

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

  Finally, the reasons for limitation of the chemical composition of the present invention will be described.

  C is an essential element to secure the strength of the steel plate, and at least 0.03 mass% is required to obtain a high strength steel plate. However, if it is contained excessively, even if C is fixed by an additive element such as Ti or the cooling condition is used, the formation of a cementite phase which is not preferable for the stretch flange characteristics can not be avoided, so 0.20 mass% or less Do.

  Si is an element which contributes to strength improvement by relatively suppressing deterioration of bendability, and addition of 0.08 mass% or more is necessary to exhibit the effect. Excessive addition adversely affects weldability and ductility, so the upper limit is 1.5% by mass.

  Mn is an element effective for strengthening the steel sheet together with C and Si, and 0.5% by mass or more needs to be contained, but if it is contained in excess of 3.0% by mass, the ductility deteriorates, so the upper limit is made It is 3.0 mass%.

  P is effective as a solid solution strengthening element, but since there is a concern about deterioration of processability due to segregation, it is necessary to make it 0.05 mass% or less. If there is no need for solid solution strengthening, it is not necessary to add P, and the lower limit value of P includes 0% by mass.

  Since S forms coarse drawn inclusions of MnS and degrades processability, conventionally, extremely low sulfurization with an S concentration of 0.002 mass% has been essential for securing processability, but in the present invention, it is fine MnS is deposited on hard Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide or Pr oxysulfide, and deformation occurs even during rolling The upper limit of the S concentration is not particularly defined because it is unlikely to occur and stretching of inclusions is prevented. However, if the S concentration is too high, Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide which suppress deformation of MnS In addition, the total amount of at least one of Ce, La, Nd, or Pr is more than 0.02% by mass, so that the resulting oxide or oxysulfide tends to coarsen. And is preferably at most 0.02 mass%. In addition, in order to reduce the S concentration to less than the conventional 0.002 mass%, it is necessary to considerably strengthen the desulfurization treatment in the secondary refining, and the desulfurization treatment cost becomes too high, that is, The lower limit value of the S concentration is set to 0.002% by mass because the significance of receiving the effect of the next form control of MnS decreases.

  When N is added too much, coarse precipitates are formed even with a slight amount of Al and the processability is degraded, so the upper limit is 0.01 mass%. On the other hand, the cost is less than 0.0005% by mass, so the lower limit is 0.0005% by mass.

  Nb and V are elements added to obtain higher strength, and use precipitation strengthening by carbonitrides formed by bonding with these elements. Precipitation strengthening is obtained by adding these elements singly or in combination, but since excessive addition deteriorates the processability, one or two of these elements have an upper limit of 0.2 mass% each. In order to obtain the strength improvement effect, it is preferable to add each in 0.005% by mass or more.

  Mo is also an element used to improve strength, but is mainly added to improve hardenability. Excessive addition causes deterioration of ductility, and the upper limit is 0.5% by mass. When securing hardenability, it is preferable to add 0.05 mass% or more. Nb, V and Mo may not be contained.

  The main additive elements are the above from the viewpoint of securing the material, but the inclusion as unavoidable impurities such as trace amounts of Cu, Ni and Cr by utilization of scrap does not impair the present invention.

  The present invention will be described below with reference to examples and comparative examples.

  The molten steel of the chemical component except Bi and Sn of Table 1 was melted and poured in a tundish. According to the component value of Bi and Sn of Table 1, it casted, adding Bi and Sn in the molten steel in a casting_mold | die in a wire. The slab size is 250 mm thick × 1500 mm wide, and the casting speed is 1.3 m / min. The induction magnetic stirring was placed on the meniscus in the mold, and during casting, a current of 500 A and a frequency of 2 Hz was supplied to the magnetic stirring device to stir the molten steel at 40 cm / s. In some experiments (Examples 18, 19, 20), the current of the magnetic stirrer was varied and the molten steel was stirred at 90 cm / s, 25 cm / s and 105 cm / s. Table 1 shows the results of investigation of the solidified structure of the slab obtained in the present experiment. In Test Nos. 1, 5, 9 to 14 and 18 to 21 which are examples of the present invention, the core of equiaxed crystal is obtained by containing at least one or more of Ce, La, Nd or Pr in molten steel. Finely disperse Ce, La, Nd, Pr oxides and oxysulfides that are effective as formation sites in molten steel, adding electromagnetic stirring to this and removing the superheat of the molten steel to stabilize the inside diameter of the slab by 3 mm In addition to the following, micro-equiaxial crystallization and preferential addition of Bi and Sn, which lower the solid-liquid interface energy in the mold, make fine and weakening of columnar crystals grown from the mold side toward the inside of the slab. We succeeded in generating fine equiaxed crystals with a grain size of 3 mm or less even in the surface layer of the slab by dividing the fine and fragile columnar crystals with a swirling flow of electromagnetic stirring on top of that. On the other hand, in Test Nos. 2, 6 and 15 as Comparative Examples, Bi and Sn were not added in the mold, so branched columnar crystals in the surface layer portion of the slab became coarse, and Test Nos. 3, 7 and Comparative Examples. In No. 16 because Ce, La, Nd and Pr effective as nucleation sites for equiaxed crystals were not contained in the molten steel, the equiaxed crystals in the inside of the slab became coarser, and test numbers 4, 8 which are comparative examples, Since No. 17 contained neither Ce, La, Nd nor Pr and did not add Bi and Sn in the mold, the equiaxed crystals were coarsened in both the inside of the slab and the surface layer.

Claims (5)

C: 0.03 to 0.20% by mass, Si: 0.08 to 1.5% by mass, Mn: 0 using a continuous casting apparatus having an induction electromagnetic stirrer between the in-mold meniscus and the under-mold 10 m. .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% by mass or less, Mo: 0.5% by mass or less, acid-soluble Al: 0.03% by mass or less, acid-soluble Ti: less than 0.1% by mass, among Ce, La, Nd or Pr, A molten steel containing at least one of a total of 0.0003 to 0.02 mass% and the balance consisting of iron and unavoidable impurities is injected into the mold, and the molten steel in the mold is one of Bi and Sn. allowed added to the molten steel content is 0.0005 to 0.01 mass% in total of the above, the induction Continuous casting wherein the casting while swirling the molten steel in a horizontal plane by 磁攪 拌 device.   The continuous casting method according to claim 1, characterized in that a metal wire containing one or more of Bi and Sn is continuously fed into molten steel in a mold.   The continuous casting method according to claim 1, wherein a mold flux containing one or more of Bi and Sn is supplied onto the molten steel surface in the mold. The continuous casting method according to any one of claims 1 to 3, wherein the swirling flow velocity of the molten steel by induction electromagnetic stirring is set to 25 to 105 cm / s at the solid-liquid interface in the widthwise central portion of the slab .   A continuous cast slab characterized in that the average equiaxed grain size is 3 mm or less for each of 1⁄4 thickness and 1/4 thickness from the surface layer of the slab by the continuous casting method according to claim 4.

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