JP2017024037A - Continuous casting method and continuous casting piece of molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method and a continuous casting piece capable of stably forming both a coarse granular crystal of the central vicinity of a casting piece and a coarse columnar crystal of surrounding it in a fine equiaxial crystal.SOLUTION: Molten steel of including acid soluble Al:0.03 mass% or less, acid soluble Ti:0.014-0.1 mass% and Mg:0.0003-0.006 mass%, is injected into a casting mold while blowing in inert gas of including Ngas or Nby 2 volume % or more from a gas blowing-in type immersion nozzle 3, by using a continuous casting device having an induction electromagnetic agitator 7 between a casting mold inner meniscus 12-casting mold under 10 m, and one kind or more is added to casting mold inner molten steel 11 from among Bi and Sn so as to become 0.0005-0.01 mass% in total, and is cast while turning the molten steel in a horizontal plane by the induction electromagnetic agitator 7. Thus, a continuous casting piece can be manufactured by stably, finely and equiaxially crystallizing both solidification structures of a casting piece surface layer part and the casting piece inside.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 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. 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 and MgAl ₂ O ₄ are not stable, and there is a variable factor that has not been known so far, such as the ability of other oxides to reduce the solidification nucleation ability. Presumed to exist.

  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). It is an object of the present invention to provide a continuous casting method capable of stably forming fine equiaxed crystals together and a continuous cast slab having a fine solidified structure cast using the same.

  In view of such circumstances, a continuous casting method capable of stably and finely equiaxing a coarse granular crystal near the center and a coarse columnar crystal surrounding it, and a continuous having a fine solidification structure cast using the same. In order to provide cast slabs, elucidate the factors that fluctuate the solidification structure refinement element and the refinement effect, and conduct earnest research on the addition method and location where the effect can be stably exhibited with a small amount of addition, and the knowledge obtained The present invention has been completed by optimally combining the process design 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: 0.014 to 0.1% by mass, Mg: 0. A molten steel containing 0003 to 0.006% by mass with the balance being iron and inevitable impurities is injected into the mold while injecting an inert gas containing N ₂ gas or 2% by volume of N ₂ from a gas blowing type immersion nozzle. 1 or more of Bi and Sn are added to the molten steel in the mold in a total amount of 0.0005 to 0.00. A continuous casting method characterized in that the casting is performed so that the molten steel is added in an amount of 01% by mass and the molten steel is swirled in a horizontal plane by the induction electromagnetic stirring device.
(2) The continuous casting method according to (1), wherein Mg is contained in the tundish in an amount of 0.0003 to 0.006% by mass and then injected into the mold.
(3) The continuous casting method according to (1) or (2), wherein a metal wire containing one or more of Bi and Sn is continuously fed into the molten steel in the mold.
(4) The continuous casting method according to (1) or (2), wherein a mold flux containing one or more of Bi and Sn is supplied onto the molten steel surface in the mold.
(5) The continuous casting method according to any one of (1) to (4), wherein a swirling flow velocity of the molten steel by induction electromagnetic stirring is 25 to 105 cm / s.
(6) The continuous casting casting characterized in that the average equiaxed grain size is set to 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 (5). Fragment.

  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.

Diagram for explaining a method of blowing N ₂ gas into the molten steel by using the gas blowing type immersion nozzle. The influence of electromagnetic stirring velocity on the molten steel which contains 0.0005% by mass of Mg to the average equiaxed Akiratsubu diameter of the slab inside and slab surface portion continuous casting while blowing N ₂ gas from the gas blowing type immersion nozzle FIG. The molten steel containing 0.0005% by mass of Mg is injected into the mold while N ₂ gas is being blown from a gas blown immersion nozzle, and 0.003% by weight of Bi is added in the mold and continuously cast. The figure which shows the influence of the electromagnetic stirring flow rate on the average equiaxed crystal grain diameter of a slab surface layer part.

  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 solidified structure fine equiaxed, the formation of fine equiaxed crystals is promoted by dispersing a large number of equiaxed nucleation sites 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 as follows: [1] Finely disperse oxides that act effectively as nucleation sites for equiaxed crystals in molten steel, and add electromagnetic stirring to this to take away the degree of superheat of the molten steel. [2] A metal element that lowers the solid-liquid interface energy is preferentially added to the surface of the slab, and the columnar crystals grow from the mold side toward the inside of the slab. It is intended to produce fine equiaxed crystals in the surface layer of the slab by reducing the size and dividing the fine and brittle columnar crystals with a swirling flow of electromagnetic stirring. 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, regarding the condition of the oxide that becomes the nucleation site of the equiaxed crystal of [1], there are many titania-based inclusions in the Ti deoxidized molten steel and many alumina-based inclusions in the Al deoxidized molten steel. These inclusions are unlikely to become equiaxed crystal nucleation sites, and further aggregate and coalesce to form coarse oxides, so that they do not act effectively as nuclei for forming equiaxed crystals. On the other hand, the present inventors add Mg, which is a stronger deoxidizing element than Ti and Al, to the molten steel to modify the titania inclusions and alumina inclusions to MgO or MgAl ₂ O ₄ . As a result, it has been found that relatively fine oxides can be uniformly dispersed in molten steel, and that these oxides are likely to become nuclei for the formation of fine equiaxed crystals. This is considered to be because MgO or MgAl ₂ O ₄ is easily wetted with molten steel as compared with titania and alumina. Here, the addition amount of Mg was defined to be 0.0003 to 0.006 mass%. This is because when the amount of Mg added is less than 0.0003 mass%, the amount of equiaxed crystal nucleation sites decreases, and conversely, when it exceeds 0.006 mass%, the generated oxide tends to be coarsened. This is because 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, alumina inclusions and titania inclusions with small equiaxed crystal nucleation ability adhere to the surfaces of MgO and MgAl ₂ O ₄ that are equiaxed crystal nucleation sites. The present inventors have found that even if the amount of Mg is added, 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 Mg 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 solidification structure due to the addition of Mg even in the actual process in which re-oxidation of molten steel occurs, the present inventors have intervened alumina-based media attached to the surfaces of MgO and MgAl ₂ O _4. TiN with high equiaxed crystal nucleation ability is deposited as a nucleation site on the inclusions and titania inclusions, and the nucleation ability of the composite inclusions as a whole is enhanced, so that equiaxed nuclei of MgO and MgAl ₂ O ₄ It has been found that it is effective to compensate for the decrease in productivity. Molten steel re-oxidation by air or slag mainly occurs in ladle and molten steel injection part in tundish, and TiN is relatively unstable in high temperature molten steel. In order to deposit TiN stably on titania inclusions, the molten steel temperature is as low as possible downstream of the molten steel injection part in the tundish where reoxidation inclusions are generated, and the reoxidation inclusions contact TiN. It is effective to add in a strong stirring immersion nozzle that is frequently used. However, since it is difficult to blow powder TiN into the immersion nozzle and disperse it in the molten steel, the present inventors have introduced the molten steel 2 in the tundish 1 as shown in FIG. When injecting into the mold 6 through the nozzle 3, an inert gas containing 2% by volume or more of N ₂ gas or N ₂ is blown from the gas introduction tube 5 of the gas blowing type immersion nozzle 3 through the porous inner hole body 4. A new method has been devised in which TiN is precipitated on the reoxidation inclusions by reacting with Ti in the molten steel in the immersion nozzle. The N ₂ gas blown into the molten steel is once dissolved in the molten steel near the inner wall of the nozzle by the equation (1), and then reacts with Ti in the molten steel by the equation (2) to form the MgO and MgAl ₂ O ₄ surfaces. TiN is generated on the adhered alumina inclusions and titania inclusions.
N ₂ (gas) = 2 N (dissolved) (1)
Ti (dissolved) + N (dissolved) = TiN (solid) (2)
When 100% N ₂ gas is blown, the Ti (acid-soluble Ti) concentration required for TiN generation is 0.014% by mass when calculated from thermodynamic data. Therefore, in order to generate TiN in this method, It is necessary to make the Ti concentration of 0.014% by mass or more. In addition, as will be described later, if the Ti concentration is too high, a large amount of titania inclusions are generated and the ability to produce equiaxed nuclei of MgO and MgAl ₂ O ₄ is reduced. In this case, in order to generate TiN by blowing N ₂ gas, the N ₂ gas concentration needs to be ₂ % by volume or more. Therefore, in order to compensate for the decrease in the equiaxed crystal nucleation ability of MgO and MgAl ₂ O ₄ by TiN generation, and to obtain an equiaxed crystal refinement effect stably even during reoxidation of molten steel, N ₂ gas in the blown gas The concentration needs to be 2 to 100% by volume. When the required lower limit of N ₂ gas concentration in the blowing gas is expressed as a function of Ti concentration, N ₂ gas concentration (volume%) = 0.196 / [mass% Ti] ² is obtained. The mixed gas for changing the N ₂ gas concentration needs to be an inert gas that does not react with molten steel. The inert gas refers to a rare gas in the present invention, and Ar gas is desirable from the viewpoint of cost. Further, if even a small amount of fine TiN is deposited on the alumina inclusions and titania inclusions attached to the surfaces of MgO and MgAl ₂ O ₄ , the portion becomes the nucleus of equiaxed crystals and becomes MgO and MgAl. _Since the lowering of equiaxed crystal nucleation ability of ₂ O ₄ can be compensated for, the lower limit blown N ₂ gas flow rate is not specified. On the other hand, if N ₂ gas is blown in excessively, the N content that does not react with Ti in the molten steel to become TiN reacts with Al in the molten steel to generate coarse precipitates and deteriorate workability. Although there is a concern, when N ₂ gas is blown at 10 Nl / min with the flow rate of molten steel passing through the gas blowing type immersion nozzle 3 being about 2 t / min, the N concentration in the molten steel is 0.0006% by mass at the maximum. However, the problem of material deterioration does not occur even when considering an appropriate N concentration of 0.0005 to 0.01% by mass described later. For this reason, the upper limit N ₂ gas blowing flow rate is not generally defined, but if the N concentration in the molten steel is within a range not exceeding 60% of the upper limit of 0.01% by mass under the same casting conditions, 100 Nl / min. Is a desirable upper limit. Further, in the case of blowing with a mixed gas of N ₂ gas and inert gas, the inert gas is not absorbed into the molten steel, so it is desirable that the flow rate of only N ₂ gas does not exceed 100 Nl / min.

  Nitrogen gas blowing is particularly effective when it is performed at the initial and final stages where molten steel reoxidation is likely to occur. In the middle of casting, nitrogen gas blowing may not be performed when reoxidation hardly occurs. When nitrogen gas is not blown in the middle of casting, it is preferable to blow argon gas.

  The boiling point of Mg, which is the equiaxed crystal refining agent of the present invention, is 1097 ° C., which is considerably lower than the melting point of molten steel (pure iron 1538 ° C.), and therefore explosive gasification occurs when molten steel is added. Therefore, it is difficult to leave Mg in a high yield in the mold in which equiaxed crystals are generated. In the present invention, in order to suppress as much as possible Mg loss during the molten steel transfer after adjusting the components, it is inserted as deep as possible in the bottom of the ladle at a position as close as possible to the tundish or into the ladle molten steel injection flow in the tundish using a wire or the like It is effective.

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, the alumina inclusions can be modified to MgO or MgAl ₂ O _4. However, it becomes coarse due to agglomeration and coalescence with a large amount of remaining alumina inclusions and loses its ability as a nucleation site for equiaxed crystals. In order to suppress aggregation and coalescence of MgO or MgAl ₂ O ₄ with alumina inclusions and maintain equiaxed crystal nucleation ability, the acid-soluble Al concentration should be low, and the lower limit is 0 mass. %including. 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.

If the acid-soluble Ti concentration becomes too high, the titania inclusions cannot be modified to MgO or MgAl ₂ O ₄ , and MgO or MgAl ₂ O ₄ aggregates with a large amount of remaining titania inclusions. Since it becomes coarse due to coalescence and loses its effectiveness as a nucleation site for equiaxed crystals, the acid-soluble Ti concentration is set to 0.1% by mass or less, and the lower limit is 100% N ₂ gas blown as described above. 0.014% by mass that enables the production of. 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 which 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 promoting the generation of equiaxed crystal nuclei of the present invention, the effect of reducing the superheated degree of molten steel by electromagnetic stirring is utilized to prevent remelting of equiaxed crystal nuclei generated from fine oxides by electromagnetic stirring. Yes. 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 are coarsened by agglomeration and coalescence, and are effective as equiaxed crystal nuclei. Stops functioning. 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, To the molten steel of acid-soluble Al: 0.025 mass%, acid-soluble Ti: 0.04 mass%, Mg wire was added in a tundish to adjust the Mg concentration to 0.0005 mass%, and the molten steel was An experiment was conducted in which continuous casting was performed while blowing a gas mixture of 15 volume% N ₂ -85 volume% Ar from the gas blowing type immersion nozzle 3 at a rate of 10 Nl / min. The effect of stirring swirl flow rate 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 velocity exceeds 100 cm / s, even with MgO or MgAl ₂ O ₄ inside the slab, coarsening due to agglomeration and coalescence progresses and it becomes difficult to function as a nucleus of equiaxed crystal, and when it exceeds 105 cm / s. This is probably because it does 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, since the solidification of the slab surface layer has already been almost completed below 10 m below the mold, the induction electromagnetic stirrer 7 is located between the position of the meniscus 12 in the mold where the solidification starts and the position of 10 m below the mold. It is effective to install.

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. Further, Bi and Sn are added to the molten steel 11 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 as much as possible at the slab surface layer. 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, Mg: 0.0005% by mass from a gas blown immersion nozzle 3 to 15% by volume N ₂ -85% by volume Ar The average equiaxed crystal grains in the slab and in the slab surface layer were obtained by continuous casting experiment in which a mixed gas was injected into the mold while blowing 10 Nl / min, and 0.003% by mass of Bi was added through the continuous casting powder. The influence of the swirling flow velocity of electromagnetic stirring on the diameter is investigated and shown in FIG. In addition, Mg added the wire in the tundish. 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, this is caused by the remelting of equiaxed nuclei generated from fine MgO and MgAl ₂ O ₄ as the starting point in the slab when the swirling flow rate of electromagnetic stirring is 30 cm / s or more. While effectively suppressed, when the swirl velocity exceeds 100 cm / s, coarsening due to agglomeration and coalescence starts even in MgO and MgAl ₂ O ₄ inside the slab, making it difficult to function as a nucleus of equiaxed crystals. This is probably because of this. 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 electromagnetic stirring flow rate, in the continuous casting conditions in which columnar crystals and branched columnar crystal structures develop, the solidified structure at the center in the width direction of the cast slab is revealed by picric acid etching. The flow rate can be evaluated from the slope. By this method, the relationship between the electromagnetic stirring thrust and the electromagnetic stirring flow rate is obtained in advance, and also in the present invention, the electromagnetic stirring is 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 Al, Ti, Mg, Bi, and Sn are as already described. Finally, the reasons for limiting the chemical components other than these 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 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. Since MnS is precipitated on hard MgO or MgAl ₂ O ₄ , deformation does not easily occur during rolling, and the inclusions are prevented from being stretched, so the upper limit of the S concentration is not particularly defined. However, if the S concentration is too high, a large amount of MgO or MgAl ₂ O ₄ that suppresses the deformation of MnS is required, and accordingly, the amount of Mg added exceeds 0.006% by mass. Therefore, MgO or MgAl ₂ O ₄ However, 0.02 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.

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

Molten steel 280t having chemical components excluding Mg, Bi, and Sn in Table 1 was melted. According to the component values of Mg, Bi and Sn in Table 1, Mg is added to the molten steel with a ladle or tundish, and N ₂ —Ar mixed gas of the gas components in Table 1 from the gas blowing type immersion nozzle 3 In the mold, Bi and Sn were added to the molten steel and cast continuously. The slab size is 250 mm thick × 1100 mm wide, and the casting speed is 1.0 m / min. The induction electromagnetic stirrer 7 is installed on the meniscus 12 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 induction electromagnetic stirrer 7. The solidification structure was observed in the early casting stage when reoxidation by air was intense, in the middle stage of casting with little reoxidation, and at the end stage of casting where intense reoxidation was caused by the entrainment of pan slag.

  In the test where the Mg concentration is less than 0.003 mass% (test numbers 1, 2, 5, 6, 7, 10, 16, 17, 18), the Mg wire is added in the ladle, and the Mg concentration is 0.003 mass%. In the above tests (test numbers 11, 12, 15, and 19), Mg wire was added in the tundish. In some experiments (test numbers 16, 17, and 18), the current of the induction electromagnetic stirring device 7 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 Nos. 1, 6, 11, 16, 17, and 18, which are examples of the present invention, MgO and MgAl ₂ O ₄ that are effective as nucleation sites for equiaxed crystals are finely dispersed in molten steel, and this is magnetically stirred. was added to take the degree of superheat of molten steel, further reoxidation of the molten steel is intense, in casting the initial and end-stage equiaxed crystal nucleating ability to produce MgO and MgAl ₂ O ₄ is reduced, N ₂ from gas blowing type immersion nozzle 3 By compensating for the decrease in the ability to generate equiaxed crystal nuclei by the effect of gas blowing, the inside of the slab was finely equiaxed to a particle 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 preferentially, and the columnar crystals that grow from the mold side toward the inside of the slab are made finer and more brittle. By dividing this fine and fragile columnar crystal with a swirling flow, a fine equiaxed crystal having a particle size of 3 mm or less was successfully formed in the slab surface layer throughout the casting.

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, Mg that is effective as a nucleation site for equiaxed crystals was not contained in the molten steel, so the equiaxed crystals inside the slab became coarse, and in Test Nos. 4, 9, and 14 as comparative examples, Mg was not contained. Since there was no addition of Bi and Sn in the mold, the equiaxed crystal was coarsened both in the slab and in the surface layer. Furthermore, in the test numbers 5, 10, and 15 of the comparative examples in which N ₂ gas was not blown from the gas blown immersion nozzle 3, the solidification structure was refined in the middle of casting, but in the early casting stage and the final casting stage where molten steel reoxidation was severe, Despite the addition of Mg, the equiaxed crystals inside the slab became coarse. This is because a large amount of alumina inclusions and titania inclusions generated by reoxidation of molten steel adhered to the surface of MgO and MgAl ₂ O _{4 at} the equiaxed crystal nucleation site, thereby improving the equiaxed crystal refinement ability. This is because it disappeared. Further, in Test No. 19 of the comparative example in which Mg was excessively added, MgO and MgAl ₂ O _{4 at the} generated equiaxed crystal nucleation site were coarsened, so the equiaxed crystal inside the slab was coarsened.

DESCRIPTION OF SYMBOLS 1 Tundish 2 Molten steel 3 Gas blowing type immersion nozzle 4 Porous inner hole body 5 Gas introduction pipe 6 Mold 7 Induction electromagnetic stirrer 11 Molten steel 12 in a mold Meniscus

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: 0.014-0.1% by mass, Mg: 0.0003-0 It contained .006 wt%, injecting molten steel balance of iron and unavoidable impurities into the mold while blowing an inert gas containing N ₂ gas or N ₂ 2 vol% or more from the gas blowing type immersion nozzle, 0.0005-0.01 quality in total of one or more of Bi and Sn in the molten steel in the mold A continuous casting method characterized in that it is added so as to be in an amount of%, and the molten steel is cast in a horizontal plane by the induction electromagnetic stirring device.   The continuous casting method according to claim 1, wherein 0.0003 to 0.006 mass% of Mg is contained in the tundish and then poured into the mold.   The continuous casting method according to claim 1 or 2, wherein a metal wire containing one or more of Bi and Sn is continuously fed into the molten steel in the mold.   The continuous casting method according to claim 1 or 2, 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 4, wherein a swirling flow velocity of the molten steel by induction electromagnetic stirring is set to 25 to 105 cm / s.   6. 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 5.

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