JP2017196626A - Continuous casting method for molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method capable of stably making coarse granular crystals near the center of a casting piece (including casting pieces having a C content of 0.1 or below mass%) for high strength steel and coarse columnar crystals surrounding them into fine equiaxed crystals.SOLUTION: With use of a continuous casting apparatus having an inductive electromagnetic stirring device 13 between a meniscus 12 in a mold and 10 m below the mold, molten steel comprising 0.03 or below mass% acid soluble Al, 0.1 or below mass% acid soluble Ti and above 0 mass% of the acid soluble Al concentration and the acid soluble Ti concentration in total, 0.0003 to 0.02 mass%, in total, of at least one or more of Ce, La, Nd and Pr, and 0.0001 to 0.004 mass% Ca, is poured into the mold 4. One or both of Bi and Sn are added to the molten steel in the mold to become 0.0005 to 0.01 mass% in total, and the molten steel is cast while being revolved in a horizontal plane by the inductive electromagnetic stirring device 13.SELECTED DRAWING: Figure 1

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 method for continuous casting of molten steel that can make these coarse granular crystals and columnar crystals into fine equiaxed crystals.

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. 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 with electromagnetic stirring in the mold in order to improve the cleanliness of the slab surface layer. A technique has been proposed which simultaneously suppresses surface defects (electromagnetic stirring cleaning effect) and internal defects (solidification structure refinement effect due to MgO). Further, in Patent Document 3, an equiaxed crystallization accelerator (MgAl ₂ O ₄ , Ce oxide, sulfide, etc.) is sprayed on the surface of molten steel using a plasma heating device in a tundish, and is equiaxed in a mold. A technique for refining coarse granular crystals near the center of a slab by generating a large number of crystal nuclei is disclosed.

Japanese Patent Laid-Open No. 50-23338 JP 2000-334559 A 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 using induction electromagnetic stirring and the method of adding an equiaxed crystallization accelerator, sufficient fine equiaxed crystals are not obtained until the material of the high-strength steel can be improved. For molten steel with a C content of 0.1% by mass or less, in which crystals are difficult to form, it is difficult to sufficiently equiaxially crystallize the columnar crystals of the slab surface layer. Furthermore, the effects of equiaxed crystallization accelerators such as MgO, MgAl ₂ O ₄ and Ce oxides are not stable, and solidification nucleation ability is reduced by the influence of other oxides. It is estimated that there are no variables.

  In view of such a current situation, the present invention provides a coarse granular crystal near 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% by mass or less). An object of the present invention is to provide a continuous casting method of molten steel that can stably form fine equiaxed crystals.

  In view of such a situation, it has a continuous casting method capable of stably and finely forming a coarse equiaxed crystal near the center and a coarse columnar crystal surrounding the coarse equiaxed crystal, and a fine solidified structure cast using the same. In order to provide continuous cast slabs, elucidate the factors that fluctuate the solidification structure refinement element and refinement effect, and earnestly research on the addition method and location where the effect can be stably exhibited with small additions. The present invention has been completed by optimally combining the processes in the continuous casting process.

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: 0.1% by mass or less, Ce, La, Nd or Pr Among them, a molten steel containing at least one total: 0.0003 to 0.02% by mass, Ca: 0.0001 to 0.004% by mass, the balance being Fe and unavoidable impurities is injected into the mold. And 0.005 to 0.01 mass in total of one or more of Bi and Sn in the molten steel in the mold It allowed added to a continuous casting method of molten steel, which comprises casting while swirling the molten steel in a horizontal plane by the inductive electromagnetic stirring device.
(2) The continuous casting method for molten steel as described in (1) above, wherein 0.0001 to 0.004 mass% of Ca is contained in the tundish and then poured into the mold.
(3) A plasma arc is generated from a hollow carbon electrode installed directly above the molten steel in the tundish toward the molten steel, and a metal wire or metal particle containing Ca is contained in the plasma arc from the hollow portion of the hollow carbon electrode. The continuous casting method for molten steel as described in (2) above, wherein Ca gas is continuously blown onto the surface of the molten steel by supplying to the molten steel.
(4) Continuous casting of molten steel according to any one of (1) to (3) above, wherein a metal wire containing one or more of Bi and Sn is continuously fed into the molten steel in the mold. Method.
(5) Continuous casting of molten steel according to any one of (1) to (3) above, wherein a mold flux containing one or more of Bi and Sn is supplied onto the molten steel surface in the mold. Method.
(6) The continuous casting method for molten steel according to any one of (1) to (5) above, wherein the swirling flow velocity of the molten steel by induction electromagnetic stirring is 25 to 105 cm / s.

  According to the present invention, it is possible to manufacture a continuous cast slab in which the solidified structure of the slab surface layer and the slab are stably and finely equiaxed together. With a high-strength thick plate, a material excellent in low-temperature toughness can be produced.

The figure for demonstrating the effective Ca addition method to a tundish inner molten steel. 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 a slab surface layer part in the continuous casting experiment of the molten steel containing 0.004 mass% of Ce and 0.001 mass% of Ca. is there. Average equiaxed grains in the cast slab and in the slab surface layer, continuously cast by adding 0.003% by mass of Bi in molten steel containing 0.004% by mass of Ce and 0.001% by mass of Ca It is a figure which shows the influence of the electromagnetic stirring flow velocity which acts on a diameter.

  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 stably and maximizing the solidification structure refinement effect over the entire surface of the slab. It has been completed by optimally combining the means in the continuous casting process and designing the 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 form the equiaxed crystal nuclei in [1] are as follows: Ti deoxidized molten steel contains titania inclusions, Al deoxidized molten steel contains alumina inclusions. Although many exist, these inclusions are unlikely to be the site of formation of equiaxed crystal nuclei, and further aggregate and coalesce to form a coarse oxide, so that they do not act effectively as nuclei for the formation of 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, Pr oxysulfide, or a composite oxide thereof The present inventors have found that relatively fine 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, this is Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or these This is probably because the complex oxide is easily wetted with molten steel. 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 mass%, the amount of equiaxed nucleation sites decreases, and on the contrary, 0.02 mass% This is because, if it exceeds 1, the resulting oxide or oxysulfide is likely to be coarsened, and in any case, the effect of making the solidified structure in the slab a fine equiaxed crystal is lost.

  In actual continuous casting, molten steel is reoxidized by air, slag, etc., and alumina inclusions and titania inclusions are newly generated in the molten steel. When the amount of these inclusions increases, Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr, which become equiaxed crystal nucleation sites Since alumina inclusions and titania inclusions with small equiaxed crystal nucleation ability adhere to the surfaces of oxysulfides or these composite oxides, add the total amount of predetermined Ce, La, Nd or Pr. However, the present inventors have found that the effect of refining the solidification structure is difficult to obtain, and in the worst case, no equiaxed crystallization occurs. As a result of observing the solidification structure of the steel ingot obtained by injecting 10 kg molten steel prepared 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 is finely equiaxed and the cast structure in the air atmosphere is coarsely crystallized. Furthermore, in order to stably enjoy the effect of refining the solidified structure due to the addition of Ce, La, Nd or Pr even in the actual process in which re-oxidation of the molten steel occurs, the present inventors have added a small amount of Ca into the molten steel. Addition and lowering the melting point of alumina inclusions and titania inclusions attached to the surface of oxide, oxysulfide of Ce, La, Nd, or Pr improves the wettability with molten steel and improves Ce. Alumina inclusions and titania inclusions are cleaned from the surface of oxides, oxysulfides of La, Nd, or Pr, and equiaxed nucleation ability of oxides, oxysulfides of Ce, La, Nd, or Pr Found to be played. Here, the Ca concentration was regulated to 0.0001 to 0.004 mass%. This is because when the Ca concentration is less than 0.0001% by mass and the reoxidation of molten steel is severe, alumina inclusions and titania inclusions attached to the surface of the oxide, oxysulfide of Ce, La, Nd or Pr. On the contrary, when the Ca concentration exceeds 0.004 mass%, even the oxides and oxysulfides of Ce, La, Nd, or Pr that are the equiaxed crystal nucleation sites are melted. This is because the function of fine equiaxed crystallization is deteriorated in any case by absorbing the alumina inclusions and titania inclusions adhering thereto and changing the composition greatly.

  The reoxidation of molten steel due to air or slag occurs mainly in the ladle or the molten steel injection part in the tundish, the boiling point of Ca is 1487 ° C, lower than the melting point of molten steel (pure iron 1538 ° C), and suddenly increases when molten steel is added In order to effectively lower the melting point of alumina inclusions and titania inclusions produced by reoxidation, the tundish where the molten steel temperature is relatively low and reoxidation inclusions are produced is required. It is desirable to add Ca on the downstream side of the molten steel injection part. In addition, since the bath depth is shallow and the molten steel static pressure is low in the tundish, even if Ca is added to the tundish bottom with a wire or the like, much of the Ca gas is easily blown out by explosive gasification, and there is a concern that the yield may be reduced. . Therefore, as an effective Ca addition method in the tundish, the present inventors continuously put the molten steel 2 in the ladle 1 into the mold 4 through the tundish 3, as shown in FIG. In the continuous casting equipment 5 to be supplied, electric power is applied between the hollow carbon electrode 6 installed immediately above the molten steel 2 in the tundish and the fixed electrode 7 installed on the tundish 3 side using a DC power supply device 8, While generating the plasma arc 9 between the tip of the hollow carbon electrode 6 and the molten steel 2 in the tundish 3, the Ca wire 10 (FIG. 1) and Ca particles are supplied into the plasma arc 9 to be gasified, and the Ca gas A method was devised for spraying with a working gas on the surface of molten steel. In order to stably generate a plasma arc and promote the gasification of Ca, it is preferable that the hollow carbon electrode 6 side is a cathode, the fixed electrode 7 side is an anode, and electrons are blown to the molten steel side. be able to. In the hollow part of the carbon electrode 6, together with the Ca wire 10 and Ca particles, an inert gas such as argon or nitrogen for generating plasma is circulated as a working gas. Thereby, Ca gas can be stably absorbed by the molten steel in the tundish.

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 the formation of equiaxed crystal nuclei of the present invention, the effect of reducing the superheated degree of molten steel by this magnetic stirring is utilized, and the 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, An experiment was conducted to continuously cast molten steel of acid-soluble Al: 0.025 mass%, acid-soluble Ti: 0.04 mass%, Ce: 0.004 mass%, Ca: 0.001 mass%, and slab The influence of the swirl velocity of electromagnetic stirring on the equiaxed grain size of the inner and slab surface layer was investigated. In addition, the equiaxed crystal grain size was defined as 2 (a · b) ^0.5 so that the grain size of the branched columnar crystals (not divided) and the divided branched columnar crystals can be evaluated simultaneously ( a is the major axis of the crystal grains, and b is the minor axis of the crystal grains.For the branched columnar crystals that are not divided, one branch is taken as one crystal grain.) 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. 2 shows the influence of the magnetic stirring velocity on the average equiaxed grain size in the slab and in the slab surface layer. As can be seen from FIG. 2, the average equiaxed grain size inside the slab is as small as 3 mm or less when the swirling flow velocity of 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.

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, Acid-soluble Al: 0.025% by mass, acid-soluble Ti: 0.04% by mass, Ce: 0.004% by mass, Ca: 0.001% by mass of molten steel was poured into the mold. In a continuous casting experiment in which 0.003% by mass of Bi was added through continuous casting powder, the influence of the swirling flow rate of electromagnetic stirring on the average equiaxed crystal grain size inside the slab and the surface part of the slab was investigated. The results are 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. 3, the average equiaxed 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.

  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.

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

  In the present invention, the dissolved (acid-soluble) Al concentration in the molten steel is 0.03% by mass or less, and if the acid-soluble Al concentration exceeds this, a large amount of alumina inclusions remain, and these are Ce, La, Since it cannot be modified to an oxide or oxysulfide of Nd or Pr, the solidified structure in the slab cannot be made into fine equiaxed crystals. Alumina inclusions modified to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or complex oxides of these In order to enhance the nucleation ability of equiaxed crystals, the acid-soluble Al concentration should be low, and the lower limit value includes 0% by mass.

  If the acid-soluble Ti concentration becomes too high, the titania inclusions are changed to Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr. Since it becomes impossible to modify the oxysulfide or these complex oxides, the acid-soluble Ti concentration is 0.1% by mass or less, and the lower limit includes 0% by mass. In addition, from the viewpoint of preventing variation in molten steel components and material deterioration, it is preferable to deoxidize dissolved oxygen to some extent with Al or Ti. From the requirements, it is possible to oxidize after adding one or two of Al or Ti. The total of the dissolved (dissolved) Al concentration and the acid-soluble (dissolved) Ti concentration is preferably at least more than 0% by mass. The total of the acid-soluble Al concentration and the acid-soluble Ti concentration is more preferably 0.0005% by mass or more. Furthermore, the acid-soluble Al concentration and the acid-soluble Ti concentration are obtained by measuring the amount of Al and Ti dissolved in an acid. Dissolved Al and dissolved Ti dissolve in an acid, and alumina and titania do not dissolve in an acid. This is an analysis method that uses this. Here, the acid is a mixed acid mixed at a ratio of hydrochloric acid 1, nitric acid 1, and water 2, for example.

  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. MnS is deposited on hard Ce oxide, La oxide, Nd oxide, Pr oxide, Ce oxysulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof. The upper limit of the S concentration is not particularly specified because deformation hardly occurs during rolling and the inclusions are prevented from being stretched. However, if the S concentration is too high, a large amount of Ce, La, Nd, or Pr oxide / oxysulfide that suppresses deformation of MnS is required, and accordingly at least one of Ce, La, Nd, or Pr is required. Since the total addition amount of seeds or more exceeds 0.02% by mass, there is a disadvantage that the generated oxide or oxysulfide tends to be coarsened, and 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 needs to be considerably strengthened by secondary refining, the desulfurization treatment cost becomes excessively high, and the secondary of the present invention Therefore, 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.

  Below, an Example and a comparative example are given and this invention is demonstrated based on FIG.

  300t of molten steel containing chemical components excluding Ca, Bi and Sn in Table 1 was produced. According to the component values of Ca, Bi and Sn in Table 1, Ca is added to the molten steel with a ladle or tundish 3 (capacity 50 t), and Bi and Sn are added to the molten steel in the mold 4. While casting. For test numbers 11, 13, and 15 in Table 1, a mold flux containing at least one of Bi and Sn was supplied onto the surface of the molten steel 11 in the mold and continuously cast. The slab size is 250 mm thick × 1500 mm wide, and the casting speed is 1.3 m / min.

  The induction electromagnetic stirrer 13 was installed in the in-mold meniscus 12. During casting, the molten steel was stirred at 40 cm / s by passing a current of 500 A and a frequency of 2 Hz through the induction electromagnetic stirrer 13. In some experiments (test numbers 16, 17, and 18), the current of the induction electromagnetic stirring device 13 was changed, and the molten steel was stirred at 90 cm / s, 25 cm / s, and 105 cm / s. As for the electromagnetic stirring flow rate, as described above, the flow rate was evaluated from the inclination of the columnar crystals and the branched columnar crystals at the center in the width direction of the cast slab.

  In the test (test numbers 11, 12, and 13) with a Ca concentration of less than 0.001% by mass, a Ca wire was added in the ladle, and the test with a Ca concentration of 0.001 to 0.002% by mass (test number) 1, 2, 3, 16, 17, 18, 19), the Ca wire is added in the tundish, and the test in which the Ca concentration exceeds 0.002 mass% (test numbers 6, 7, 8) is shown in FIG. Thus, by supplying the metal Ca wire 10 to the hollow part from the upper part of the hollow carbon electrode 6 installed immediately above the molten steel 2 in the tundish, Ca gas was sprayed on the molten steel surface while generating a plasma arc. The hollow carbon electrode 6 had a diameter of 120 mm, the hollow portion had an inner diameter of 10 mm, and Ar gas was flowed as a working gas at a flow rate of 250 Nl / min. A power of 600 KW was applied using the hollow carbon electrode side as a cathode and the molten steel side as an anode.

  The solidification structure was observed in the early casting stage where reoxidation by air was intense (site until the empty tundish was filled with molten steel: about 0 to 50 tons), in the middle casting period where the reoxidation was low (the molten steel in the tundish was filled). The subsequent steady part: casting about 50 to 250 t), and at the end of casting where the reoxidation was intense due to the entrainment of pan slag (site where the molten steel in the tundish starts to decrease: casting about 250 to 300 t).

As described above, the average equiaxed grain size inside the slab was defined as the average value of the equiaxed grain size of the cross section inside from the thickness of the slab ¼. Further, the average equiaxed grain size of the slab surface layer portion was the average value of the equiaxed grain size of the transverse section from the surface layer to the slab thickness ¼. In addition, the equiaxed crystal grain size was defined as 2 (a · b) ^0.5 so that the grain size of the branched columnar crystals (not divided) and the divided branched columnar crystals can be evaluated simultaneously ( a is the major axis of the crystal grains, and b is the minor axis of the crystal grains.For the branched columnar crystals that are not divided, one branch is taken as one crystal grain.)

  Table 2 shows the results of investigating the solidification structure of the slab obtained in this experiment. In Tables 1 and 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. Sulfide, La oxysulfide, Nd oxysulfide, Pr oxysulfide, or complex oxides of these are finely dispersed in molten steel, and electromagnetic stirring is added to this to take away the superheat of the molten steel. Oxidation is severe, and the ability to produce equiaxed nuclei is reduced by the effect of Ca addition at the beginning and end of casting, when the ability to produce equiaxed nuclei of Ce, La, Nd, or Pr oxides and oxysulfides decreases. By making up, the inside of the slab was finely equiaxed with a grain size of 3 mm or less over the entire 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 that grow from the mold side toward the inside of the slab, and then the vortex flow of electromagnetic stirring Thus, by dividing the fine and brittle columnar crystals, it was possible to produce fine equiaxed crystals with a particle size of 3 mm or less in the slab surface layer throughout the entire casting area.

  On the other hand, in the test numbers 2, 7, and 12 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 8 which were comparative examples. In No. 13, Ce, La, Nd, or Pr effective as an equiaxed crystal nucleation site was not contained in the molten steel, so that the equiaxed crystal inside the slab was coarsened, and test numbers 4, 9, which were comparative examples, In No. 14, Ce, La, Nd, or Pr was not contained, Bi and Sn were not added in the mold, and Ca was not added. Therefore, the equiaxed crystal was coarsened both in the slab and in the surface layer portion. .

  Furthermore, in the test numbers 5, 10, and 15 of the comparative examples in which Ca was not added, the solidification structure was refined in the middle of the casting, but Ce, La, Nd, or Pr in the early casting stage and the late casting stage where the molten steel was reoxidized severely. However, the equiaxed crystal inside the slab was coarsened. This is because a large amount of alumina inclusions and titania inclusions generated by reoxidation of molten steel are Ce oxides, La oxides, Nd oxides, Pr oxides, Ce oxysulfides at equiaxed nucleation sites, This is because the ability to refine equiaxed crystals has disappeared due to adhesion to the surface of La oxysulfide, Nd oxysulfide, Pr oxysulfide, or a composite oxide thereof.

  In addition, in the test number 19 of the comparative example in which Ce was excessively added, since the Ce oxide, Ce oxysulfide, or complex oxide of these formed equiaxed nucleation sites was coarsened, the equiaxed inside of the slab Crystals became coarse.

DESCRIPTION OF SYMBOLS 1 Ladle 2 Molten steel 3 Tundish 4 Mold 5 Continuous casting equipment 6 Hollow carbon electrode 7 Fixed electrode 8 DC power supply device 9 Plasma arc 10 Ca wire 11 Molten steel 12 Meniscus 13 Induction electromagnetic stirring device

Claims (6)

  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: 0.1% by mass or less, Ce, La, Nd or Pr, At least one or more total: 0.0003 to 0.02% by mass, Ca: 0.0001 to 0.004% by mass, with the balance being Fe and unavoidable impurities injected into the mold, One or more of Bi and Sn are added to the molten steel in the mold in a total amount of 0.0005 to 0.01% by mass. Addition allowed the continuous casting method of molten steel, which comprises casting while swirling the molten steel in a horizontal plane by the induction stirrer as.   The molten steel continuous casting method according to claim 1, wherein 0.0001 to 0.004 mass% of Ca is contained in the tundish and then poured into the mold.   A plasma arc is generated from the hollow carbon electrode placed directly above the molten steel in the tundish toward the molten steel, and a metal wire or metal particles containing Ca is supplied into the plasma arc from the hollow portion of the hollow carbon electrode. The continuous casting method of molten steel according to claim 2, wherein Ca gas is continuously sprayed onto the surface of the molten steel.   4. The molten steel continuous casting method according to claim 1, wherein a metal wire containing at least one of Bi and Sn is continuously fed into the molten steel in the mold. 5. .   The molten steel continuous casting method according to any one of claims 1 to 3, 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 of molten steel according to any one of claims 1 to 5, wherein a swirling flow velocity of the molten steel by induction electromagnetic stirring is set to 25 to 105 cm / s.

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