JP2016083703A - Continuous casting method of molten steel and continuous casting cast piece - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method which can turn both of crude equiaxed crystal in the neighborhood of center and crude columnar crystal surrounding the crude equiaxed crystal to fine equiaxed crystal in a casting cast piece for high-strength steel, and to provide a continuous casting cast piece for high-strength steel which has a fine solidification structure cast by using the continuous casting method.SOLUTION: A continuous casting method is characterized in that molten steel which contains C of 0.03 to 0.20%, Si of 0.08 to 1.5%, Mn of 0.5 to 3.0%, Nb of 0.2% or less, V of 0.2% or less, Mo of 0.5% or less, acid-soluble Al of 0.03% or less, acid-soluble Ti less than 0.1% and the sum of at least one kind of Ce, La, Nd and Pr of 0.0003 to 0.02% is poured into a casting mold and at least one kind of Bi and Sn is added to the molten steel in the casting mold such that the sum gets to 0.0005 to 0.01% by using a continuous casting device having an induction electromagnetic agitation device between a position of a meniscus in the casting mold and a position of 10 m-lower than the casting mold, and the casting is performed while turning the molten steel in a horizontal plane by the induction electromagnetic agitation device.SELECTED DRAWING: Figure 2

Description

  In the cross section of a normal continuous cast slab, there are a coarse granular crystal part near the center arranged to surround the final solidified part with porosity and segregation in the center, and a coarse columnar crystal part surrounding the coarse granular crystal part. Observed. If this coarse granular crystal and columnar crystal can be made into fine equiaxed crystals and central segregation and microsegregation can be greatly reduced, for example, when a slab is made into a thin plate, it becomes a thin plate with remarkably excellent moldability. For example, when a thick plate is used, the plate has excellent low-temperature toughness. The present invention relates to a molten steel continuous casting method capable of turning coarse grain crystals and columnar crystals into fine equiaxed crystals, and a continuous cast slab having a fine solidified structure cast using the molten steel.

  Non-Patent Document 1 shows that low temperature casting is effective for equiaxed crystallization because equiaxed crystals increase when the degree of superheated molten steel is low. Patent Document 1 describes a technique for making columnar crystals equiaxed by applying a unidirectional swirling flow to the molten steel near the solidification interface using an induction electromagnetic stirrer and dividing the columnar dendrite. Yes. Furthermore, in Patent Document 2, an equiaxed crystallization accelerator (Ce oxide, sulfide, etc.) is sprayed on the surface of molten steel using a plasma heating device in a tundish, and a large number of equiaxed nuclei are generated in the mold. Thus, a technique for refining coarse granular crystals in the vicinity of the center of the slab is disclosed.

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

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

  However, in low-temperature casting, it is necessary to set the superheat degree of the molten metal to a temperature close to the liquidus and to inject it into the mold from the immersion nozzle, so solidification abnormalities such as clogging of the immersion nozzle and deckle formation in the mold May be invited. For this reason, the current continuous casting employs a superheat degree of the molten metal to be injected of about 20 to 30 K. Under such temperature conditions, the production of high-strength thin steel sheets whose production volume has been increasing due to the need for weight reduction in recent years. Fine equiaxed crystallization that can improve workability and low-temperature toughness of high-strength thick plates 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 are not stably obtained until the material of the high-strength steel can be improved, For example, it is difficult to sufficiently equiaxially crystallize columnar crystals in the slab surface layer portion for molten steel having a C content of 0.1% or less in which equiaxed crystals are not easily formed.

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

  In view of such a situation, a continuous casting method in which a coarse granular crystal in the vicinity of the center and a coarse columnar crystal surrounding the same are formed into a fine equiaxed crystal, and a continuous cast slab having a fine solidified structure cast using the same. In order to provide a solid-state structure, we will elucidate the elements that refine the solidified structure, conduct intensive research on the addition method and location where the effect can be achieved with a small amount of addition, and design the process by combining the obtained knowledge optimally in the continuous casting process. This led to the completion of the present invention.

The summary is as follows. That is,
(1) Using a continuous casting apparatus having an induction electromagnetic stirring device between the meniscus in the mold and 10 m under the mold, C: 0.03 to 0.20 mass%, Si: 0.08 to 1.5 mass%, 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 Among them, a molten steel containing at least one kind: 0.0003 to 0.02% by mass, the balance being Fe and inevitable impurities is injected into the mold, and the molten steel in the mold contains Bi and Sn. 1 or more of the above are added so that the total amount becomes 0.0005 to 0.01% by mass, Continuous casting wherein the casting while swirling the molten steel in a horizontal plane by the device.
(2) The continuous casting method as described in (1) above, wherein a metal wire containing one or more of Bi and Sn is continuously supplied into the molten steel in the mold.
(3) The continuous casting method according to (1), wherein a mold flux containing one or more of Bi and Sn is supplied onto the molten steel surface in the mold.
(4) The continuous casting method according to any one of (1) to (3) above, wherein the swirling flow velocity of the molten steel by induction electromagnetic stirring is 25 to 105 cm / s.
(5) Continuous casting characterized in that the average equiaxed crystal grain size is 3 mm or less for each of the ¼ thickness from the surface layer of the slab and the ¼ thickness to the inside by the continuous casting method described in (4) above. Slab.

  According to the present invention, it is possible to produce a continuous cast slab in which both the surface layer portion of the slab and the solidified structure inside the slab are finely equiaxed, so that the high strength thin steel sheet has high formability and high strength. With thick plates, it is possible to produce materials with excellent low-temperature toughness.

It is a figure which shows the influence of the electromagnetic stirring flow rate on the average equiaxed crystal grain diameter of the inside of a slab and the slab surface layer part in the continuous casting experiment of the molten steel containing 0.004 mass% of Ce. The effect of electromagnetic stirring flow rate on the average equiaxed grain size in the cast slab and the surface layer of the slab continuously cast by adding 0.003% by mass of Bi to molten steel containing 0.004% by mass of Ce. FIG.

  The form of the solidified structure is determined by the temperature gradient and solidification rate at the solid-liquid interface at the time of solidification. The smaller the temperature gradient and the greater the solidification rate, the easier the formation of equiaxed crystals. However, in actual continuous casting, columnar crystals grow from the slab surface layer to the inside relatively, and it is difficult to change the cooling conditions to such an extent that the solidification structure is changed to be equiaxed crystals. Under such conditions, in order to make the solidification structure fine equiaxed, the formation of fine equiaxed crystals is promoted by dispersing the nucleation sites of equiaxed crystals in the molten steel and increasing the frequency of nucleation. In addition, two methods of reducing the solid-liquid interface energy and making the columnar crystal itself into a fine equiaxed crystal using a metal element having a high surface active effect are conceivable. The present invention clarifies a control means for effectively combining these two solidification structure control principles and maximizes the effect of refining the solidification structure over the entire surface of the slab. It was completed by optimally combining the process design in the casting process. The basic idea of the present invention is that [1] oxides and oxysulfides that effectively act as nucleation sites for equiaxed crystals are finely dispersed in molten steel, and this is electromagnetically stirred to deprive the superheated degree of molten steel. While making the inside of the slab stably fine equiaxed, [2] A metal element that lowers the solid-liquid interface energy is preferentially added to the surface of the slab and grows from the mold side toward the inside of the slab. The purpose of this is to make the columnar crystals finer, and then to divide the fine and fragile columnar crystals with a swirl flow of electromagnetic stirring to produce fine equiaxed crystals on the surface of the slab. As a result, a fine equiaxed crystal structure can be obtained over the entire surface of the slab.

  Specific methods and conditions for realizing the basic idea will be described below. First, the conditions for oxides and oxysulfides that serve as nucleation sites for equiaxed crystals in [1] are as follows. Ti deoxidized molten steel contains titania inclusions, and Al deoxidized molten steel contains alumina inclusions. Although there are many, these inclusions are very easy to agglomerate and coalesce and become coarse oxides, so they do not act effectively as nuclei for forming equiaxed crystals. On the other hand, the present inventors added at least one or more of Ce, La, Nd or Pr, which are deoxidizing elements stronger than Ti and Al, to the molten steel, and include titania inclusions and alumina inclusions. By modifying the material to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide, relatively fine oxidation The present inventors have found that oxides and oxysulfides can be uniformly dispersed in molten steel, and that these oxides and oxysulfides are likely to become nuclei for the formation of fine equiaxed crystals. Compared with titania and alumina, Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide are molten steel. It is thought that it is because it gets wet easily. Here, the total addition amount of at least one of Ce, La, Nd, or Pr was regulated to 0.0003 to 0.02 mass%. This is because, if the total addition amount of at least one of Ce, La, Nd, or Pr is less than 0.0003%, the amount of equiaxed nucleation sites is reduced, and on the contrary, 0.02% by mass. This is because if it exceeds, the resulting oxide or oxysulfide is likely to be coarsened, so that the effect of making the solidified structure in the slab a fine equiaxed crystal is lost.

  In the present invention, the dissolved (acid-soluble) Al concentration in the molten steel is 0.03% by mass or less, and when the acid-soluble Al concentration exceeds this, alumina inclusions are completely removed from the Ce oxide, La oxide, Nd. It cannot be modified to oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide, and becomes a composite inclusion with alumina, agglomeration and coalescence progress, and is equiaxed Loss the ability as a crystal nucleation site. Suppresses agglomeration and coalescence of Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide, and nucleates equiaxed crystals In order to maintain performance, the acid-soluble Al concentration should be low, and the lower limit includes 0% by mass. The acid-soluble Al concentration is an analytical method that measures the amount of Al dissolved in an acid, and utilizes that dissolved Al dissolves in an acid and alumina does not dissolve in an acid. Here, the acid is a mixed acid mixed at a ratio of hydrochloric acid 1, nitric acid 1, and water 2, for example.

  Also, if the acid-soluble Ti concentration becomes too high, the oxide tends to agglomerate and become coarse, and it is easy to produce coarse TiN inclusions by combining with N in the steel. The Ti concentration is less than 0.1% by mass, and the lower limit includes 0% by mass. The acid-soluble Ti concentration is obtained by measuring the amount of Ti dissolved in the acid, similarly to the acid-soluble Al concentration, and matches the dissolved Ti concentration.

Next, the conditions for electromagnetic stirring in [1] will be described. In general, in electromagnetic stirring, a swirl flow is imparted to the molten steel at the solidification interface, and this swirl flow is considered to break up columnar dendrites and promote equiaxed crystallization. However, according to the knowledge of the present inventors, the effect of columnar crystal fragmentation at the solidification interface of the slab surface layer portion that has been conventionally known is weak, rather, heat transfer between the solidified shell and the molten steel is promoted by electromagnetic stirring, and the slab It was found that the effect of lowering the internal superheated molten steel was high. In the promotion of nucleation of equiaxed crystals according to the present invention, the effect of reducing the superheated degree of molten steel by electromagnetic stirring is utilized, and remelting of equiaxed nuclei generated from the origin of fine oxides and oxysulfides by electromagnetic stirring. Is preventing. However, in order to increase the effect of reducing the degree of superheated molten steel by electromagnetic stirring, it is necessary to increase the swirl flow velocity. In that case, fine oxides and oxysulfides are coarsened by agglomeration and coalescence, resulting in equiaxed crystals. It will not function effectively as a nucleus. Therefore, C: 0.08% by mass, Si: 0.5% by mass, Mn: 1.0% by mass, P: 0.02% by mass, S: 0.003% by mass, N: 0.003% by mass, Conducted continuous casting experiment of molten steel of acid-soluble Al: 0.025 mass%, acid-soluble Ti: 0.04 mass%, Ce: 0.004 mass%, equiaxed inside of slab and slab surface layer The effect of swirling flow velocity of electromagnetic stirring on crystal grain size was investigated. It should be noted that the equiaxed crystal grain size is 2 (a · b) so that the grain size of the branched columnar crystals (not divided), the divided columnar crystals, and the divided branched columnar crystals can be evaluated simultaneously. ) Defined as ^0.5 (a is the major axis of the crystal grains, b is the minor axis of the crystal grains. One branch is taken as one crystal grain for the unbranched branched columnar crystals). The average equiaxed grain size inside the slab is the average value of the equiaxed grain size of the cross section inside from the slab 1/4 thickness, and the average equiaxed grain size of the slab surface layer is from the surface layer to the slab It is the average value of equiaxed grain size of the transverse section at ¼ thickness. FIG. 1 shows the influence of the electromagnetic stirring flow rate on the average equiaxed grain size in the slab and in the slab surface layer. As can be seen from FIG. 1, the average equiaxed grain size inside the slab is as small as 3 mm or less when the swirling flow velocity of the molten steel is 25 cm / s or more, and to about 2 mm when it is 30 cm / s or more. On the other hand, the average equiaxed grain size starts to increase, and when it exceeds 105 cm / s, it becomes larger than 3 mm. This is because when the swirling flow velocity of electromagnetic stirring is 25 cm / s or more, more specifically 30 cm / s or more, the remelting of equiaxed nuclei generated from fine oxides inside the slab is suppressed. On the other hand, if the swirling flow rate exceeds 100 cm / s, even Ce oxide or Ce oxysulfide inside the slab will be coarsened by agglomeration and coalescence, making it difficult to function as an equiaxed crystal nucleus. It is thought that this is because if it exceeds, it will not function as an equiaxed crystal nucleus. In 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, and undivided branched 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 slab surface layer by electromagnetic stirring is relatively weak. Therefore, in order to make the solidified structure inside the slab into a fine equiaxed crystal, it is desirable to control the swirl flow rate of electromagnetic stirring to 30 to 100 cm / s. Further, below 10 m below the mold, solidification of the slab surface has already been almost completed, and therefore induction electromagnetic stirring is installed between the position of the meniscus in the mold where solidification starts and the position of 10 m below the mold. Is effective. It is more desirable to install in the mold or directly under the mold.

Next, [2] selection of a metal element that lowers the solid-liquid interfacial energy, Bi and Sn are promising 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 interval in a solidification experiment of 10 kg molten steel to which these metal elements were added. As for the effect of refining columnar crystals, it is sufficient to add one or more of these metal elements in a total amount of 0.0005% by mass or more, but if added in excess of 0.01% by mass, the steel sheet becomes brittle and during rolling. Ear cracks occurred at the edges. For this reason, what is necessary is just to add 1 or more types from Bi and Sn to molten steel so that it may become 0.0005-0.01 mass% in total. In addition, Bi and Sn are added to the molten steel in the mold so that the entire cast slab material is hardly adversely affected and only the effect of refining columnar crystals can be enjoyed to the maximum extent possible. Is desirable. As an addition method, a metal wire containing Bi and Sn is directly inserted into the molten steel upper side in the mold, or a mold flux containing Bi and Sn is used to supply relatively to the surface of the slab. Can be added efficiently. As a method of adding a fine element through a 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 a mold while mixing Bi or Sn into the mold flux immediately before the addition. For example, a method of supplying Bi powder or Sn powder onto the mold flux covering the molten metal surface at a constant speed 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 no explosive gasification occurs during addition. Further, the density of Bi and Sn is 9.8 g / cm ³ and 7.3 g / cm ³ , respectively, which is heavier than the density of molten steel 7.0 g / cm ^3. However, it does not float immediately and can be added to molten steel relatively easily. The contents of added Bi and Sn can be evaluated by analyzing samples collected from slabs or rolled steel sheets.

  Furthermore, the electromagnetic stirring conditions of [2] will be described. Here, as described above, since the columnar crystal fragmentation effect of electromagnetic stirring at the solidification interface is weak, Bi and Sn are added into the mold to make the columnar crystal growing from the mold side fine and brittle. It is important to effectively divide the columnar crystals by the shearing force with weak electromagnetic stirring and to build fine equiaxed crystals in the slab surface layer. Therefore, C: 0.08% by mass, Si: 0.5% by mass, Mn: 1.0% by mass, P: 0.02% by mass, S: 0.003% by mass, N: 0.003% by mass, Molten steel of acid-soluble Al: 0.025 mass%, acid-soluble Ti: 0.04 mass%, Ce: 0.004 mass% is injected into the mold, and Bi is 0.003 through the continuous casting powder in the mold. An experiment of continuous casting while carrying out mass% addition was carried out, and the influence of the swirling flow velocity of electromagnetic stirring on the average equiaxed crystal grain size inside the slab and on the surface layer of the slab was investigated and is shown in FIG. The average equiaxed grain size of the slab surface layer portion is reduced to 3 mm or less when the swirling flow rate by electromagnetic stirring is 25 cm / s or more, and is decreased to about 2 mm when the swirling flow rate is 30 cm / s or more. The effect is maintained even if it exceeds s. This is more effective when the swirling flow velocity of electromagnetic stirring is 25 cm / s or more, and columnar crystals refined and weakened by addition of Bi in the mold begin to be divided by the electromagnetic stirring flow, and further 30 cm / s or more. It is a result which shows that the parting effect of a columnar crystal is obtained, and that a fine equiaxed crystal can be generated in the slab surface layer portion. On the other hand, as can be seen from FIG. 2, the average equiaxed crystal grain size inside the slab decreases to about 2 mm when the swirling flow velocity of the molten steel is 30 cm / s or more. The particle size begins to increase. As described in the previous experiment, the cause of this is that the recrystallization of equiaxed nuclei generated from fine Ce oxides and Ce oxysulfides in the slab when the swirling flow velocity of electromagnetic stirring is 30 cm / s or more. While dissolution is effectively suppressed, when the swirling flow velocity exceeds 100 cm / s, coarsening due to agglomeration and coalescence begins even in the case of Ce oxide or Ce oxysulfide inside the slab. It is thought that it becomes difficult to function. Therefore, in order to make the entire slab fine equiaxed, it is effective to set the magnetic stirring flow rate to 30 to 100 cm / s.

  With regard to the magnetic stirring speed, the solidified structure at the center in the width direction of the cast slab is revealed by picric acid etching under normal continuous casting conditions in which columnar crystals and branched columnar crystal structures develop. The flow rate can be evaluated from the inclination of the crystal. In 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 may be performed by selecting the electromagnetic stirring thrust for obtaining the target electromagnetic stirring flow rate.

  By combining the above [1] and [2], the average equiaxed grain size is 3 mm or less for each of the ¼ thickness from the surface layer of the slab and the ¼ thickness to the inside (electromagnetic stirring flow rate 25 to 105 cm / s). Desirably, a solidified structure having a thickness of 2 mm or less (electromagnetic stirring flow rate of 30 to 100 cm / s) can be obtained. The electromagnetic stirring flow rate is more preferably 70 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 bloom or billet, a sufficient solidification structure refinement effect can be obtained.

  Finally, the reasons for limiting the chemical components of the present invention will be described.

  C is an essential element for securing the strength of the steel sheet, and at least 0.03 mass% is necessary to obtain a high-strength steel sheet. However, if excessively contained, even if C is fixed by an additive element such as Ti or the cooling conditions are fully utilized, the formation of a cementite phase that is not preferable for stretch flange characteristics is unavoidable. To do.

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

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

  P is effective as a solid solution strengthening element, but since there is a concern about deterioration of workability due to segregation, it is necessary to make it 0.05% by mass or less. If solid solution strengthening is not necessary, it is not necessary to add P, and the lower limit value of P includes 0% by mass.

  Since S forms coarsely stretched inclusions of MnS and degrades workability, conventionally, ultra-low sulfidation with an S concentration of 0.002% by mass has been essential for securing workability. Precipitates MnS on hard Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, or Pr oxysulfide and deforms during rolling. The upper limit of the S concentration is not particularly defined because it hardly occurs and prevents the inclusions from being stretched. 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 that suppresses deformation of MnS A large amount of the product is required, and accordingly, the total added amount of at least one of Ce, La, Nd or Pr exceeds 0.02% by mass, so that the generated oxide or oxysulfide is easily coarsened. 0.02% by mass or less is desirable. In addition, in order to reduce the S concentration to less than 0.002% by mass, the desulfurization treatment must be considerably strengthened by secondary refining, and the desulfurization treatment cost becomes too high. Since the significance of enjoying the effect of controlling the form of the next MnS is reduced, the lower limit value of the S concentration is set to 0.002% by mass.

  If N is added too much, even if it is a trace amount of Al, coarse precipitates are generated and the workability is deteriorated, so 0.01 mass% is made the upper limit. On the other hand, since it costs to make it less than 0.0005 mass%, 0.0005 mass% is made the lower limit.

  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. However, 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 strength improvement effect, 0.005% by mass or more is preferably added.

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

  From the viewpoint of securing the material, the main additive elements are as described above. However, the incorporation of trace amounts of Cu, Ni, Cr, etc. as unavoidable impurities by using scrap does not impair the present invention.

  Hereinafter, the present invention will be described with reference to examples and comparative examples.

  Molten steels having chemical components other than Bi and Sn in Table 1 were melted and injected into the tundish. According to the component values of Bi and Sn in Table 1, casting was performed while adding wires of Bi and Sn into the molten steel in the mold. The slab size is 250 mm thick × 1500 mm wide, and the casting speed is 1.3 m / min. Induction electromagnetic stirring was installed at the meniscus in the mold. During casting, the molten steel was stirred at 40 cm / s by flowing a current of 500 A and a frequency of 2 Hz through the electromagnetic stirring device. In some experiments (Examples 18, 19, and 20), the current of the electromagnetic stirrer was changed and the molten steel was stirred at 90 cm / s, 25 cm / s, and 105 cm / s. Table 1 shows the results of investigating the solidification structure of the slab obtained in this experiment. In test numbers 1, 5, 9-14, and 18-21, which are examples of the present invention, by containing at least one of Ce, La, Nd, and Pr in the molten steel, equiaxed nuclei Ce, La, Nd, Pr oxides and oxysulfides effective as production sites are finely dispersed in the molten steel, and electromagnetic stirring is added to this to take away the degree of superheat of the molten steel. In the following, fine equiaxed crystallization and Bi and Sn that preferentially reduce the solid-liquid interface energy in the mold are added preferentially, so that the columnar crystals that grow from the mold side toward the inside of the slab are refined and weakened. Then, by dividing the fine and fragile columnar crystals with a swirl flow of electromagnetic stirring, the inventors succeeded in generating fine equiaxed crystals with a particle size of 3 mm or less in the slab surface layer. On the other hand, in the test numbers 2, 6 and 15 which are comparative examples, Bi and Sn addition in the mold was not performed, so that the branched columnar crystals in the slab surface layer portion were coarsened, and test numbers 3 and 7 which were comparative examples. In No. 16, Ce, La, Nd, and Pr that are effective as nucleation sites for equiaxed crystals were not contained in the molten steel, so the equiaxed crystals inside the slab became coarse, and test numbers 4, 8, which are comparative examples. In No. 17, Ce, La, Nd, and Pr were not contained, and Bi and Sn were not added in the mold, so that the equiaxed crystal was coarsened both in the slab and in the surface layer portion.

Claims (5)

  Using a continuous casting apparatus having an induction electromagnetic stirring device 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 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, among Ce, La, Nd or Pr, Total of at least one or more types: 0.0003 to 0.02% by mass, molten steel consisting of iron and unavoidable impurities is poured into the mold, and one kind of Bi and Sn is injected into the molten steel in the mold. The above was added to a total of 0.0005 to 0.01% by mass, and the induction electromagnetic stirring device Continuous casting wherein the casting while swirling the molten steel in Ri horizontal plane.   The continuous casting method according to claim 1, wherein a metal wire containing one or more of Bi and Sn is continuously fed into the molten steel in the mold.   2. The continuous casting method according to claim 1, wherein a mold flux containing at least one 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 a swirling flow velocity of the molten steel by induction electromagnetic stirring is set to 25 to 105 cm / s.   5. A continuous cast slab characterized in that the average equiaxed crystal grain size is 3 mm or less for each of the ¼ thickness from the surface layer of the slab and the ¼ thickness to the inside by the continuous casting method according to claim 4.

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