JP2019018238A - Method for producing low carbon steel thin slab, low carbon steel thin slab, and method for producing low carbon thin steel sheet - Google Patents

Abstract

[PROBLEMS] In twin roll type continuous casting of a low carbon steel sheet, the inclusion composition in which molten metal inclusions are reduced as much as possible and nozzle clogging and inclusion coarsening hardly occur and anisotropy are developed. A difficult solidification structure is stably realized over the entire length of the casting. A decarburization process under a reduced pressure is performed subsequent to a decarburization process under an atmospheric pressure, and a molten steel having a dissolved oxygen concentration of 0.005 to 0.035 mass% is added to at least one of Al and Ti. Alternatively, two types are added for deoxidation, the acid-soluble Al concentration is 0.05% by mass or less, the acid-soluble Ti concentration is 0.1% by mass or less, and the acid-soluble Al concentration and the acid-soluble Ti concentration After adjusting the total to more than 0%, a twin roll of molten steel containing 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of at least one of Se or Te is added. Cast by the continuous casting method. [Selection] Figure 1

Description

  The present invention relates to a low-carbon steel thin-walled slab excellent in workability, formability and cleanliness produced by a twin-roll continuous casting method, a method for producing the same, and a method for producing a low-carbon steel thin steel plate.

  From the viewpoint of process saving and energy saving, a technology for producing a thin plate close to the final product at the casting stage, that is, a near net shape continuous casting has been developed. Among these, a twin-roll type continuous casting method is disclosed in Patent Document 1 as being effective in thin-plate type near net shape continuous casting. In continuous casting of a thin slab using a twin roll type continuous casting apparatus, as shown in FIG. 1, a molten steel 3 is immersed in a hot water pool portion 2 defined by a pair of cooling rolls 1 rotating in opposite directions. The thin cast slab 6 is casted by feeding from the tundish 5 through 4 and a filter 7 provided therein. In this twin-roll continuous casting, in order to stably cast a thin cast slab having no surface defects or internal defects, it is important to rectify the flow of molten steel in the hot water pool and prevent fluctuations in the molten metal surface.

In contrast, Patent Document 2 discloses a method in which a filter is built in an immersion nozzle to generate an undisturbed discharge flow over the entire width of the nozzle. In Patent Document 3, a slit nozzle is provided with a rectifying porous nozzle, Methods for rectifying the nozzle discharge flow are disclosed. In addition, in the twin roll continuous casting method of Al deoxidized molten steel, it is known that the turbulence of the discharge flow caused by nozzle clogging causes fluctuations in the molten metal surface and destabilizes the casting. Patent Document 4 proposes a method for preventing nozzle clogging by adding and reforming to a low melting point inclusion of CaO—Al ₂ O ₃ . Furthermore, since it is difficult to prevent clogging of nozzles and surface defects due to alumina inclusions in Al deoxidized molten steel, a method of deoxidizing with Ce and La and leaving a dissolved oxygen and a method of complex deoxidizing with Ti, Ce and La Is disclosed in Patent Document 5 and is effective in suppressing nozzle adhesion by controlling to non-alumina inclusions.

JP 60-137562 A JP-A-62-282853 JP-A-8-164454 JP-A-10-29047 JP 2004-195522 A

The methods of Patent Documents 2 to 3 described above show some effects in stainless steel (not Al deoxidation) produced by a twin-roll type continuous casting method. As the alumina inclusions, which are deoxidation products, become coarse due to agglomeration, they also adhere to the submerged nozzle discharge holes, the filter in the submerged nozzle, and the rectifying porous nozzle. There is a problem that internal defects frequently occur due to re-entrainment of inclusions. Furthermore, in the twin roll type continuous casting method, the molten steel injected from the tundish solidifies in a very short time, and the floating time of inclusions cannot be secured, so in Al deoxidized molten steel, most coarse alumina inclusions are thin-walled slabs. There is a high possibility that internal defects will occur even in a stable casting state that is trapped inside and does not fluctuate. In addition, the modification method of alumina inclusions of Patent Document 4 works effectively for preventing clogging of the immersion nozzle and clogging of the filter, but the modified CaO—Al ₂ O ₃ inclusion is a liquid phase. Easily coalesces and coarsens in ladle, tundish, immersion nozzle. Since this coarse inclusion is not removed by the filter in the submerged nozzle, it is immediately captured in the thin-walled slab as described above, and causes cracks (internal defects) during processing. Further, as described in Patent Document 5, the method of reducing the cohesiveness by using non-alumina inclusions exhibits a certain effect on the clogging of the immersion nozzle, but the molten steel decarburized to an extremely low carbon concentration range. It contains a very high concentration of dissolved oxygen (about 0.1% by mass), and even if it is deoxidized with a deoxidizing material other than Al, a large amount of non-alumina inclusions are produced in the molten steel. Therefore, if the casting time is lengthened, the effect of preventing nozzle clogging is unavoidable. Furthermore, it is known that, if the dissolved oxygen is left in the molten steel even after deoxidation by the method of Patent Document 5, it is known that the formability / workability of the steel sheet is lowered, and there is a problem that a sufficient material cannot be secured.

  Furthermore, in a thin steel plate manufactured by a twin roll type continuous casting method, anisotropy appears during processing, for example, when deep drawing is performed during can making, so-called crests and troughs continue alternately in the circumferential direction of the can. Earrings occur. If these earrings are large, the yield of cans will decrease, and the earrings will come into contact with the mold and lead to troubles in can making. Therefore, thin steel sheets obtained by the twin-roll continuous casting method are required to have high formability and workability. The current situation is that it cannot be applied to other applications. Furthermore, the present inventors have newly found a method for controlling a solidified structure that reduces anisotropy during processing as described later, but its controllability is not sufficient, and the earrings are stable over the entire length. It has not yet been possible to prevent the occurrence, and it is estimated that there are fluctuation factors that have not been known in the past.

  In view of the present situation, the present invention reduces inclusions in molten steel as much as possible, and is stable in a solidified structure in which nozzle clogging and inclusion coarsening are unlikely to occur and anisotropy is difficult to develop. It is an object of the present invention to provide a twin-roll continuous casting method that can be controlled to a high level, a low-carbon steel thin-walled slab excellent in workability and formability cast using the same, and a method for producing a low-carbon steel thin steel plate.

  In view of such circumstances, the inclusions in the molten steel are reduced as much as possible, and the composition of inclusions in which nozzle clogging and inclusion coarsening are unlikely to occur and the solidified structure in which anisotropy is difficult to develop can be stably controlled. In order to provide a twin-roll continuous casting method and a low-carbon steel thin-walled slab excellent in workability and formability cast using the same, a method for reducing inclusions in low-carbon steel, nozzle clogging and inclusion coarsening Elucidation of additive elements effective for prevention, modification method of inclusions, elucidation of mechanism of anisotropy during processing, prevention measures and stable display of effects, and continuous research based on the obtained knowledge The present invention has been completed by optimally combining the processes in the process design.

The summary is as follows. That is,
(1) Decarburization treatment under reduced pressure following decarburization treatment under atmospheric pressure, and molten steel having a dissolved oxygen concentration of 0.005 to 0.035 mass%, at least one of Al and Ti 2 types are added and deacidified, acid-soluble Al concentration is 0.05 mass% or less, acid-soluble Ti concentration is 0.1 mass% or less, and the sum of acid-soluble Al concentration and acid-soluble Ti concentration After adjusting the component to more than 0%, Mg is further added to 0.0003 to 0.01 mass%, and at least one kind of Se or Te is added in a total amount of 0.0002 to 0.005 mass%. A method for producing a low-carbon steel thin-walled slab characterized by casting by a continuous casting method.
(2) The C concentration in the molten steel after decarburization treatment under atmospheric pressure is 0.05 mass% or more and 0.1 mass% or less, and the C concentration after decarburization treatment under reduced pressure is 0.01 mass%. More than 0.05 mass%, The manufacturing method of the low carbon steel thin-walled slab as described in (1) characterized by the above-mentioned.
(3) Low carbon steel thin-walled slab according to (1) or (2), wherein decarburization treatment under atmospheric pressure is performed in a converter and decarburization treatment under reduced pressure is performed with a vacuum degassing apparatus. Manufacturing method.
(4) At least one or two of Al and Ti are added for deoxidation, and the components are adjusted so that the acid-soluble Al concentration is 0.05% by mass or less and the acid-soluble Ti concentration is 0.1% by mass or less. In addition, after stirring for 3 minutes or more, molten steel added with 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass in total of at least one of Se or Te is twin-roll type. The method for producing a low-carbon steel thin-walled slab according to any one of (1) to (3), wherein the casting is performed by a continuous casting method.

(5) By mass%, C: 0.01 mass% or more and less than 0.05 mass%, Si: 0.005 to 0.03 mass%, Mn: 0.6 mass% or less, P: 0.02 mass% Hereinafter, S: 0.01% by mass or less, acid-soluble Al: 0.05% by mass or less, acid-soluble Ti: 0.1% by mass or less, and the sum of the acid-soluble Al concentration and the acid-soluble Ti concentration is More than 0% by mass, N: 0.0005 to 0.01% by mass, Mg: 0.0003 to 0.01% by mass, a total of at least one of Se or Te: 0.0002 to 0.005% by mass, The total oxygen concentration is 0.002% by mass or less, the balance is Fe and inevitable impurities, the number of oxides having a diameter of more than 30 μm is less than 5 / cm ² , and the equiaxed crystal ratio is 10% or more. A low-carbon steel thin-walled slab having a thickness of 5 mm or less.
(6) Further, Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: 0.03% by mass or less, Ni: 0.05% by mass or less The low carbon steel thin-walled slab according to (5), characterized in that

(7) A low-carbon steel thin slab characterized by subjecting the low-carbon steel thin-walled slab described in (5) or (6) to cold rolling, continuous annealing at a recrystallization temperature or higher, and subsequent temper rolling A method of manufacturing a steel sheet.

  According to the present invention, the cleanliness of molten steel is enhanced as much as possible, nozzle clogging and inclusion coarsening can be suppressed, and the anisotropy of the solidified structure can be reduced. A thin steel plate can be stably manufactured using a twin roll type continuous casting method.

The figure which shows the outline | summary of a twin roll type continuous casting apparatus.

The present invention is described in detail below.
In general, low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as a converter to produce molten steel having a final C concentration. The molten steel after the decarburization treatment contains a large amount of dissolved oxygen according to the C concentration, and if it is large, it may exceed 0.1% by mass. Since this dissolved oxygen is usually almost deoxidized by the addition of Al, a large amount of alumina inclusions corresponding to the amount of dissolved oxygen is generated in the molten steel, and the cleanliness of the molten steel is greatly reduced. Moreover, since the concentration of lower oxides such as FeO and MnO in the ladle slag increases at the same time as the dissolved oxygen concentration in the molten steel increases, the molten steel is reoxidized by the slag after deoxidation, and the amount of alumina inclusions further increases. This alumina inclusion floats and separates while agglomerated and coalesced in the molten steel, but the amount of inclusions in the molten steel is about 0.004% by mass in the tundish, and is produced by agglomeration in the molten steel. The large-sized alumina inclusion (alumina cluster) which reaches about several hundred μm is also included. When this molten steel is cast by the twin-roll type continuous casting method, the solidification time is very short. Therefore, unlike the conventional continuous casting apparatus for slab, inclusion floating in the mold is hardly expected. In addition, the twin roll type continuous casting immersion nozzle has a complicated structure such as a flow straightening nozzle and a filter for the purpose of rectifying the discharge flow, so it has a larger amount than a normal continuous casting immersion nozzle. Inclusions adhere to the nozzle inner wall, discharge holes and filter. When the nozzle clogging occurs, the discharge flow from the immersion nozzle becomes unstable, and inclusions are re-entrained due to the fluctuation of the molten metal surface in the hot water pool between the rolls. In this way, when casting low-carbon steel by the twin roll continuous casting method, a large amount of alumina clusters that cause cracks during processing are trapped in the thin-walled slab. It was very difficult to produce a steel plate by a twin roll continuous casting method.

  On the other hand, the solidification structure of the low-carbon steel thin-walled slab cast by the twin-roll type continuous casting method consists of columnar crystals that grow straight up to the inside of the slab. The morphology of the solidified structure is strongly influenced by the C concentration in the molten steel and the temperature gradient at the solid-liquid interface during solidification. The C concentration is 0.1% by mass or less as in low carbon steel, and the temperature gradient as in twin roll casting. When becomes large, the columnar crystals are extremely easy to grow. Thin cast slabs with a thickness of several millimeters manufactured by the twin-roll type continuous casting method are cold-rolled to the final plate thickness, but unlike the conventional continuous cast slabs cast at a thickness of about 250 mm, the rolling reduction is increased. It cannot be secured. As a result, the direction of solidification structure growth also remains in the final thin steel sheet and appears as anisotropy during processing.For example, when deep drawing is performed during canning, peaks and troughs alternate in the circumferential direction of the can. The present inventors have found that so-called earrings are generated.

  Based on the above problems, the present invention provides [1] a method for reducing inclusions in low-carbon molten steel, [2] a method for elucidating additive elements effective for preventing nozzle clogging and inclusion coarsening, and a method for modifying inclusions, 3] Research on solidification structure control based on the anisotropy mechanism at the time of processing and the stability of the control are repeated, and the obtained knowledge is used in the process from low-carbon steel melting to twin-roll continuous casting. It was completed by optimally combining and designing the process.

First, the method for reducing inclusions in molten low carbon steel of [1] will be described below. The technical idea of producing this low carbon steel is that it is refined under atmospheric pressure to blow the C concentration higher than the final component value, leaving excess carbon in the molten steel, and further decarburizing the molten steel under reduced pressure. This is to reduce the dissolved oxygen concentration to the limit and to produce a highly clean steel. Since low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as in a converter, the molten steel after decarburization contains dissolved oxygen corresponding to the C concentration. 04% by mass of low carbon steel (average component) contains about 0.06% by mass of dissolved oxygen. Since this dissolved oxygen is usually almost deoxidized by the addition of Al (reaction of the following formula (1)), a large amount of alumina inclusions corresponding to a total oxygen concentration of 0.06% by mass is formed in the molten steel, and the molten steel The cleanliness is greatly reduced.
2Al + 3O = Al ₂ O ₃ (1)

On the other hand, in the present invention, the C concentration by decarburization treatment under atmospheric pressure is higher than the product value, that is, 0.05 to 0.1% by mass, and the decarburization treatment is terminated. It becomes about 0.049-0.024 mass%, and the dissolved oxygen concentration is 0.06 mass% when decarburized to an average final C concentration (0.04 mass%) only by decarburization treatment under atmospheric pressure. Low. Since the decarburized molten steel under atmospheric pressure of the present invention is subsequently degassed under reduced pressure, the decarburization reaction of the following formula (2) further proceeds, and the C concentration is the final component value (0.04). In addition, the dissolved oxygen can be further reduced accordingly.
C + O = CO (2)

  When the C concentration by decarburization under atmospheric pressure is the lowest 0.05% by mass, the dissolved oxygen concentration after decarburization under reduced pressure is the highest, but still about 0.035% by mass Can be suppressed. Further, if the C concentration after the decarburization process under atmospheric pressure is higher than about 0.065% by mass, dissolved oxygen is insufficient in the subsequent decarburization process under reduced pressure, and the C concentration is reduced to the final component value ( 0.04% by mass). In that case, it is possible to supply oxygen from the outside when the decarburization reaction in the latter half of the decarburization process under reduced pressure begins to stagnate (the dissolved oxygen concentration is about 0.005% by mass). Similarly, since oxygen is consumed by the equation (2), the dissolved oxygen can be adjusted to the final C concentration while maintaining the low dissolved oxygen concentration at the time when the external oxygen starts to be supplied. For this reason, the dissolved oxygen concentration after the decarburization process under reduced pressure can be reduced to about 0.035 to 0.005 mass%. Even if Al is added and deoxidized in this state, the amount of alumina inclusions produced is the total oxygen of the low carbon steel having a C concentration of 0.04% by mass produced only by decarburization under normal atmospheric pressure. It is very low compared to the case where the concentration is 0.06% by mass. In addition, since the concentration of lower oxides such as FeO and MnO in the ladle slag is also reduced, the contamination of the molten steel by the slag after Al deoxidation is greatly reduced.

  The reason why the C concentration in the molten steel after decarburization treatment under atmospheric pressure is preferably 0.05% by mass or more and 0.1% by mass or less is that when the C concentration exceeds 0.1% by mass, the C concentration is reduced under reduced pressure. This is because the decarburization treatment becomes longer, and when the C concentration is less than 0.05% by mass, the dissolved oxygen concentration increases rapidly, and it is difficult to sufficiently reduce the dissolved oxygen concentration by decarburization treatment under reduced pressure. . Further, since the C concentration in the steel greatly affects the elongation and strength of the steel sheet, the C concentration after decarburization treatment under reduced pressure is 0.01 mass% or more, which is sufficient to obtain a material as a low carbon steel. It is desirable to be less than 05% by mass. The C concentration after decarburization corresponds to the C concentration of the slab.

  The molten steel after decarburization treatment under reduced pressure can be deoxidized by adding one or two of Al or Ti. However, if the dissolved oxygen concentration after decarburization treatment under reduced pressure exceeds 0.035% by mass, the amount of inclusions generated by adding one or two of Al or Ti increases, and Mg described later is appropriate. Even if the amount is added, alumina inclusions and titania inclusions cannot be modified, and the inclusions cannot be prevented from agglomerating and adhering to the nozzle. Conversely, reducing the dissolved oxygen concentration after decarburization under reduced pressure as much as possible is effective for improving cleanliness, but reducing the dissolved oxygen concentration to less than 0.005% by mass even under reduced pressure. Extremely difficult in terms of both cost and processing time. Therefore, it is necessary to control the dissolved oxygen concentration after decarburization treatment under reduced pressure to 0.005 to 0.035% by mass. Here, under reduced pressure means a pressure lower than atmospheric pressure.

  In the present invention, as described above, one or two of Al or Ti are added. The acid-soluble Al concentration after the addition is 0.05% by mass or less, and the acid-soluble Ti concentration is 0.1%. Less than mass. The reason for this is that when the acid-soluble Al concentration and the acid-soluble Ti concentration exceed these values, the reaction of reforming alumina inclusions and titania inclusions to magnesia or alumina magnesia spinel did not proceed as described later and remained. This is because a large amount of alumina inclusions and titania inclusions are agglomerated and coalesced, and the equiaxed crystal nucleation site is insufficient, so that a sufficient equiaxed crystal structure cannot be obtained.

  In addition, from the viewpoint of preventing variations in molten steel components and material deterioration, it is necessary to sufficiently deoxidize dissolved oxygen with Al or Ti and fix it as alumina or titania (oxide). It is important to leave dissolved Al or dissolved Ti. Therefore, the total of the acid-soluble (dissolved) Al concentration and the acid-soluble (dissolved) Ti concentration after addition of one or two of Al or Ti is at least 0 mass because of the requirement for sufficient deoxidation. %, Preferably 0.005% by mass or more, more preferably 0.01% by mass or more. By measuring the dissolved oxygen concentration after decarburization under reduced pressure and adding Al or Ti in excess of the amount of Al or Ti determined according to the stoichiometric ratio from the measured value, the above-mentioned preferred acidity can be obtained. Molten Al or acid-soluble Ti can be left in the molten steel. 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.

  In the present invention, it is preferable that the molten steel after deoxidation by adding Al or Ti is provided with a stirring time of 3 minutes or more. This is because the decarburization treatment under reduced pressure can improve the cleanliness of the molten steel, but the inclusions can be efficiently removed by further taking the stirring time, and the cleanliness can be further enhanced.

  In addition, as a decarburization treatment of molten steel under atmospheric pressure, a steelmaking furnace such as a converter or an electric furnace is usually used, and as a subsequent decarburization treatment under a reduced pressure, a vacuum degassing device, a vacuum refining device, or the like is usually used. used.

  Next, a description will be given of a method for modifying inclusions in the low-carbon molten steel whose cleanliness has been improved by the above method into inclusions having a composition in which nozzle clogging and inclusion coarsening are unlikely to occur. Even in molten steel that has been highly purified by decarburization treatment under reduced pressure, alumina inclusions and titania inclusions are very easy to agglomerate and coalesce. In addition, due to the complicated structure of the submerged nozzle in the twin-roll continuous casting method, there is a possibility that inclusions adhere to the inner wall of the nozzle, the discharge holes and the filter, and the nozzle is blocked. The twin roll type continuous casting method is characterized by a rapid solidification process that completes solidification in a very short time. If molten steel can be injected into a twin-roll type continuous casting machine while preventing agglomeration in the molten steel, inclusions can be dispersed more uniformly and finely than the conventional slab continuous casting method due to its rapid cooling effect. And the most effective prevention of cracking during processing.

Therefore, the present inventors have modified the alumina inclusions and titania inclusions in molten steel with relatively high cleanliness, and studied additive elements that suppress inclusion adhesion to aggregated coalescence and nozzles. It has been found that Mg, which is a strong deoxidizing element compared to Ti, is an effective aggregation and adhesion preventing element.
When Mg, a strong deoxidizing element, is added to molten steel with relatively high cleanliness, some or all of the alumina inclusions and titania inclusions in the molten steel is reduced, and at least the inclusion surface layer contains magnesia or alumina magnesia spinel. Produces. Since this inclusion composition greatly reduces the interfacial energy with the low carbon molten steel, the inclusions of the inclusions on the nozzle and the filter refractory and the aggregate coalescence of the inclusions are suppressed at the same time. Here, the Mg addition amount appropriate for inclusion control is 0.0003 to 0.01% by mass. This is because when the amount of Mg added is less than 0.0003 mass%, the surface layer portion cannot be modified to magnesia or alumina magnesia spinel with alumina inclusions that are more stable than titania inclusions. On the other hand, if the amount of Mg exceeds 0.01% by mass, even if the inclusion surface layer is modified to magnesia or alumina magnesia spinel, Mg, which is a strong deoxidizing element, further reduces dissolved oxygen, and inclusions and molten steel This is because the interfacial energy is increased and the coarsening and nozzle adhesion are advanced. Here, if the amount of Mg added is 0.01% by mass or less, adsorbed oxygen of a little over 0.001% by mass remains at the oxide interface, but the amount of Mg added to the strong deoxidizing element is 0.01%. If it exceeds mass%, it is considered that even the adsorbed oxygen at the oxide interface is reduced and removed, and the interfacial energy is greatly increased.

  Furthermore, [3] a method for controlling a solidified structure in which anisotropy is less likely to occur during processing will be described. As described above, it has been found that the anisotropy occurs in the thin steel plate produced by the twin roll continuous casting method is due to the columnar crystal structure developed by rapidly solidifying the low carbon molten steel. Based on this anisotropy mechanism, the present inventors have found that equiaxed crystallization of the solidified structure of low-carbon steel is effective in reducing the anisotropy. By adding at least the surface layer of alumina inclusions and titania inclusions to magnesia or alumina magnesia spinel, and using these inclusions as equiaxed nucleation sites, the solidification structure is achieved by twin-roll continuous casting. A new method for equiaxed crystallization was devised. According to the present invention, inclusions can be modified to magnesia or alumina magnesia spinel, and the interfacial energy between molten steel and inclusions can be reduced, so Mg addition is [2] coarsening of inclusions and prevention of nozzle adhesion [3] It effectively acts on both the equiaxed crystallization of the solidified structure, and is a very effective control means in the production of a low carbon steel sheet using the twin roll type continuous casting method.

  In order to eliminate the anisotropy during processing to a level where there is no problem, the present inventors have conducted detailed examinations on the control conditions of the solidified structure by experiments, and determined the equiaxed crystal ratio (equal crystal thickness / plate thickness). Clarified that it is necessary to secure 10% or more of × 100 (%)). This is because when the equiaxed crystal ratio of the thin cast slab is 10% or more, deformation due to cold rolling is difficult to be transmitted, and the central portion of the plate thickness where the anisotropy of the solidified structure tends to remain can be stably equiaxed. It is. As in the case of inclusion composition control that prevents nozzle clogging and inclusion coarsening, the appropriate amount of Mg added to ensure an equiaxed crystallization rate of the solidified structure of the thin cast slab is 10% or more is 0.0003 to 0.01% by mass. If the amount of Mg added is less than 0.0003% by mass, the amount of magnesia or alumina magnesia spinel that becomes an equiaxed crystal nucleation site decreases. Conversely, if it exceeds 0.01% by mass, magnesia or alumina magnesia spinel is coarse. This is because the equiaxed crystallization rate of the solidified structure is less than 10%, and anisotropy appears.

  In the twin roll type continuous casting, the ladle temperature and the tundish capacity are small compared to the normal continuous casting, so the molten steel temperature is likely to be lowered, and the ladle molten steel temperature is adjusted to be higher for stable casting. In particular, at the initial stage of ladle molten steel injection, the superheat of molten steel (difference between molten steel temperature and liquidus temperature) injected into a twin-roll continuous casting machine is very high, and magnesia, which becomes an equiaxed crystal nucleation site. Alternatively, even if there is sufficient alumina magnesia spinel, the condition is such that equiaxed crystal nuclei are not easily generated. In addition, at the beginning of casting where the molten steel in the ladle starts to be poured into the tundish or at the seam of the continuous casting, the molten steel height in the tundish is low, and the pouring flow from the ladle does not cover the long nozzle and falls as a falling flow. Therefore, molten steel reoxidation occurs due to the entrainment of air and slag, and new alumina inclusions and titania inclusions are generated in the molten steel in the tundish. If measures against reoxidation of molten steel, such as Ar sealing in the tundish, are implemented, there is no effect on relatively large inclusions that cause nozzle clogging and cracking during steel plate processing. However, it is difficult to completely prevent reoxidation of molten steel, and newly formed relatively small and low equiaxed nucleation ability of alumina inclusions and titania inclusions become equiaxed nucleation sites. Since it adheres to the surface of magnesia or alumina magnesia spinel, the equiaxed crystallization ability is greatly reduced. As described above, the present inventors have found that, at the initial casting stage and continuously in the casting seam portion, the increase in molten steel superheat and molten steel reoxidation occur simultaneously, and even if a predetermined amount of Mg is added, almost no equiaxed crystallization occurs. ing.

  The inventors have also added a small amount of Se and Te to the molten steel to reduce the solid-liquid interface energy and to make the columnar crystals themselves fine and brittle. It has been found that nuclei are newly formed and can compensate for a decrease in equiaxed crystal nucleation ability caused by an increase in the degree of superheat of molten steel and reoxidation of molten steel. The reason for the growth of equiaxed nuclei due to the addition of Se and Te is that the columnar crystals that grow from the cooling roll side to the molten steel side become fine and brittle due to the decrease in the solid-liquid interface energy, and this columnar crystals are used to rotate the cooling roll. This is because it is divided by the accompanying compressive force and the shearing force of the molten steel flow to form new equiaxed crystal nuclei.

  Therefore, in the present invention, as described above, Mg was added to the molten steel, and one or both of Se and Te were added to perform twin-roll continuous casting. As a result, it has been clarified that equiaxed crystals are generated in the cast slab not only in the casting stable part but also in the casting initial stage and the continuous casting seam part. Here, the effect of refining columnar crystals is sufficient if at least one of Se or Te is added in a total amount of 0.0002 mass% or more, but if added over 0.005 mass%, the steel sheet becomes brittle. Cracks occur at the ends during recoil after casting or during cold rolling. For this reason, what is necessary is just to add 1 type or more from Se and Te so that it may become 0.0002-0.005 mass% in total in molten steel.

  By combining the methods [1], [2] and [3] above, it is possible to ensure high cleanliness and to prevent inclusion clogging and inclusion coarsening from occurring, and inclusion composition and anisotropy from which anisotropy hardly develops. A thin-walled slab controlled to a structure can be stably cast using a twin-roll continuous casting method. In the present invention, the thin slab refers to a slab having a thickness of 5 mm or less.

When the presence of large inclusions in the thin cast slab obtained by the present invention was evaluated, there were less than 5 / cm ^{2 of} large oxides exceeding 30 μm, and the oxides were refined. Here, the dispersion state of the inclusions was evaluated by observing the polished surface (C cross section) of the slab or the steel plate with a 100-fold optical microscope and evaluating the inclusion particle size distribution within the unit area. The particle diameter of the inclusions was determined by measuring the major axis and the minor axis, and taking the equivalent diameter as ^0.5 (major axis x minor axis). Furthermore, when the cleanliness of the thin cast slab of the present invention was evaluated by the total oxygen concentration, it was 0.002% by mass or less and was very good. Here, the total oxygen concentration is the sum of oxygen contained in the slab (including all oxide oxygen and dissolved oxygen) and can be analyzed by a normal gas analyzer. Further, the slab of the present invention has an equiaxed crystal ratio of 10% or more and the solidified structure in the center of the slab is equiaxed. In this way, the cleanliness of the slab is increased, the inclusions in the slab are dispersed as fine oxides, and the solidification structure in the center of the slab is equiaxed to generate cracks in the steel sheet during processing. Therefore, it is possible to provide a thin cast slab that becomes a thin steel plate material excellent in workability and formability.

  The thin-walled slab cast according to the present invention can be produced by performing normal cold rolling, continuous annealing at a recrystallization temperature or higher, and subsequent temper rolling.

  Finally, among the chemical components of the thin cast slab of the present invention, mention will be made of the action of chemical components other than C, acid-soluble Al, acid-soluble Ti, Mg, Se, and Te already described.

  It is preferable that Si is 0.005 mass% or more and 0.03 mass% or less. This is because if the Si concentration is less than 0.005 mass%, the strength of the plate is insufficient, and if the Si concentration exceeds 0.03 mass%, the workability of the plate decreases.

  Mn is an element effective for improving the strength of the steel sheet together with C and Si. If necessary, Mn is preferably contained in an amount of 0.1% by mass or more, but if it exceeds 0.6% by mass, coarse MnS is contained. It is preferable to make it 0.6% by mass or less because there is a possibility of reducing the ductility. Even if Mn is not present, the present invention is not impaired, so a lower limit is not determined.

  P makes the material brittle, and if contained excessively, it segregates at the grain boundaries and causes deep drawing cracks, so it is preferable to make it 0.02% by mass or less, which is clearly not problematic for practical use. Even if P is not present, the present invention is not impaired, so a lower limit is not determined.

  Since S produces coarse MnS and deteriorates ductility and formability, it is preferable to be 0.01% by mass or less. Even if it does not contain S, the present invention is not impaired, so the lower limit is not particularly defined.

  If N is added too much, even if it is a trace amount of Al, a coarse precipitate is generated and the workability is deteriorated. Therefore, the N content is preferably 0.01% by mass or less. On the other hand, since it takes cost to make it less than 0.0005 mass%, it is preferable to make it 0.0005 mass% or more.

  Although the effects of the main additive elements of the present invention have been described, other elements such as Nb, V, Mo, and Ni also include Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: If it is the range of 0.03 mass% or less and Ni: 0.05 mass% or less, since workability is not deteriorated, it can be added. By adding each element within this range, deep drawability is improved by Nb, strength is improved by V and Mo, and corrosion resistance is improved by Ni. In addition, the incorporation of trace amounts of Cu, Ni, Cr and other inevitable impurities due to the use of scrap does not impair the present invention.

  In the following Tables 1 and 2, the present invention will be described with reference to Examples and Comparative Examples. In Tables 1 and 2, numerical values and items deviating from the present invention are underlined.

表１、表２の試験番号１〜１３については、転炉での脱炭処理によりＣ濃度を０．０５５質量％まで低下させ、続いて真空脱ガス装置により表１のＣ濃度まで脱炭処理した溶鋼にＡｌまたはＴｉを添加して脱酸し、必要に応じて攪拌を実施した後、Ｍｇと、ＳｅもしくはＴｅの少なくとも１種以上を添加して表１成分の溶鋼１００ｔを溶製した。試験番号１４の実験では、転炉のみで表１のＣ濃度まで脱炭処理した溶鋼にＡｌを添加して脱酸し、続いてＭｇと、ＳｅもしくはＴｅの少なくとも１種以上を添加して最終表１の成分の溶鋼を溶製した。これらの溶鋼を、図１に示すような双ロール式連続鋳造を用いて、厚み２．５ｍｍ、幅１２００ｍｍの薄肉鋳片に鋳造した。

For test numbers 1 to 13 in Tables 1 and 2, the C concentration was reduced to 0.055% by mass by decarburization treatment in the converter, and then decarburization treatment was performed to the C concentration in Table 1 by a vacuum degasser. Al or Ti was added to the molten steel for deoxidation, and after stirring as necessary, at least one of Mg and Se or Te was added to melt 100t of molten steel 100t in Table 1. In the experiment of test No. 14, Al was added to the molten steel decarburized to the C concentration shown in Table 1 by a converter alone, followed by deoxidation, followed by the addition of at least one of Mg and Se or Te. Molten steel having the components shown in Table 1 was produced. These molten steels were cast into thin-walled slabs having a thickness of 2.5 mm and a width of 1200 mm using twin-roll continuous casting as shown in FIG.

  The dissolved oxygen concentration before deoxidation was evaluated using a solid electrolyte oxygen sensor, and the results are shown in Table 2. Test No. 14 was evaluated at the stage where the decarburization process in the converter was completed and the steel was put out in a ladle. The total oxygen concentration was evaluated in the slab.

  Inclusion adhesion to the nozzle during twin-roll casting is “None” if the nozzle opening is almost constant, “Yes” if the nozzle opening is gradually increasing, and no molten steel at the end of casting. If there is any “occlusion”, it is listed in Table 2.

The oxide number density of more than 30 μm is shown in Table 2 by observing the polished surface (C cross section) of the slab with a 100 × optical microscope. The particle size of the inclusion is the equivalent circle diameter obtained by measuring the major axis and the minor axis and (major axis x minor axis) ^0.5 .

  The equiaxed crystal ratio of the slab was defined as the ratio of the equiaxed crystal region thickness to the slab thickness by revealing the solidified structure by picric acid etching in the slab C cross section.

  The manufactured thin cast slab was cold-rolled, then continuously annealed at an annealing temperature of 700 ° C., and further subjected to temper rolling to obtain a cold-rolled steel sheet having a thickness of 0.16 mm. In addition, since the recrystallization temperature is less than 650 ° C., if the annealing temperature is 700 ° C., the continuous annealing can be reliably performed at the recrystallization temperature or higher.

  Cans were made with a can-making tester for 2-piece cans, which had difficult-to-make conditions where cracking was about 1000 times higher than actual cans. The crack occurrence rate at the time of can making was evaluated as a value obtained by further multiplying the ratio of the number of cracked cans with respect to the number of cans made by this can making tester by 1/1000 times. The earring height was evaluated as the difference (mm) between the maximum height (mountain portion) and the minimum height (valley portion) in the circumferential direction of the can, and the results are shown in Table 2.

  In Test Nos. 1-7, which are examples of the present invention, there was no inclusion adhesion to the immersion nozzle, casting was stable, and both high cleaning and refinement of inclusions were achieved over the entire length of the thin cast slab. The crack occurrence rate during can making was 1 ppm or less. In addition, the solidification structure in the center of the thickness of the thin slab also secured an equiaxed crystal ratio of 10% or more over the entire length of the slab, and the anisotropy could be eliminated. It has become possible to make it equal to or lower than 1.5 mm of ordinary continuous cast material.

  On the other hand, in the test numbers 8 and 9 of the comparative example, the Mg concentration is not appropriate, and in the test numbers 11 and 12, since the Al and Ti concentrations are not appropriate, both nozzle adhesion and inclusion coarsening occur, and cracking occurs during processing. did. Further, since the solidified structure could not be equiaxed, earrings were generated. In Test No. 10 of the comparative example, Mg was added properly, but neither Se nor Te was added, so that inclusion adhesion to the immersion nozzle and inclusion coarsening were suppressed, and cracking occurred during canning. Although the rate was also 1 ppm or less, sufficient equiaxed crystal ratio could not be obtained at the early stage of casting, which was affected by the re-oxidation of molten steel due to the high casting temperature, and earrings became a problem. In the test number 13 of the comparative example, it is necessary to supplement oxygen in order to reduce the C concentration to 0.01% by mass or less, oxygen was blown in a vacuum degassing apparatus, and the dissolved oxygen concentration exceeded 0.035% by mass. In addition to increasing the amount of inclusions, even if Mg is added, inclusion aggregation and nozzle adhesion cannot be suppressed, the immersion nozzle is completely blocked at the end of casting, and frequent cracking occurs during deep drawing. did. Of course, the anisotropy of the solidified structure could not be eliminated and a large earring was generated.

  Furthermore, in the experiment of test number 14 of the comparative example, Al was added to the molten steel decarburized to the C concentration shown in Table 1 only by a converter, followed by deoxidation, and then Mg was added to complete the components of Table 1 Since the molten steel was melted, the dissolved oxygen concentration exceeded 0.035% by mass, resulting in an increase in the amount of inclusions. Even when Mg was added, aggregation of inclusions and nozzle adhesion could not be suppressed. It closed completely and cracking occurred frequently during deep drawing. Since the equiaxed crystallization of the solidified structure at the center of the plate thickness could not be performed, large earrings were generated.

1. 1. Cooling roll 2. Hot water pool Molten steel 4. Nozzle 5. Tundish Thin wall slabs 7. Rectified porous nozzle or filter

Claims (7)

  Following decarburization treatment under atmospheric pressure, decarburization treatment under reduced pressure is performed, and at least one or two of Al and Ti are added to the molten steel having a dissolved oxygen concentration of 0.005 to 0.035 mass%. Add and deoxidize, acid soluble Al concentration is 0.05 mass% or less, acid soluble Ti concentration is 0.1 mass% or less, and the total of acid soluble Al concentration and acid soluble Ti concentration is 0% After adjusting the components to ultra, molten steel with 0.0003 to 0.01 mass% Mg added and 0.0002 to 0.005 mass% total of at least one of Se or Te is added to the twin roll continuous casting method. A method of producing a low-carbon steel thin-walled slab characterized by casting with   The C concentration in the molten steel after decarburization treatment under atmospheric pressure is set to 0.05% by mass or more and 0.1% by mass or less, and the C concentration after decarburization treatment under reduced pressure is 0.01% by mass or more to 0.00%. The method for producing a low-carbon steel thin-walled slab according to claim 1, wherein the content is less than 05% by mass.   The decarburization process under atmospheric pressure is performed in a converter, and the decarburization process under reduced pressure is performed with a vacuum degassing apparatus. Method.   At least one or two of Al and Ti are added for deoxidation, and the components are adjusted so that the acid-soluble Al concentration is 0.05% by mass or less and the acid-soluble Ti concentration is 0.1% by mass or less. After stirring for min. Or more, molten steel added with 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of at least one of Se or Te is added to the twin roll continuous casting method. The method for producing a low-carbon steel thin-walled slab according to any one of claims 1 to 3, wherein the casting is carried out by casting. C: 0.01% by mass or more and less than 0.05% by mass, Si: 0.005 to 0.03% by mass, Mn: 0.6% by mass or less, P: 0.02% by mass or less, S : 0.01 mass% or less, acid-soluble Al: 0.05 mass% or less, acid-soluble Ti: 0.1 mass% or less, and the total of acid-soluble Al concentration and acid-soluble Ti concentration is 0 mass% Ultra, N: 0.0005 to 0.01 mass%, Mg: 0.0003 to 0.01 mass%, total of at least one of Se or Te: 0.0002 to 0.005 mass%, total oxygen concentration 0.002% by mass or less, the balance Fe and inevitable impurities, the number of oxides having a diameter of more than 30 μm is less than 5 / cm ² , and the equiaxed crystal ratio is 10% or more. Low carbon steel thin-walled slab having a thickness of 5 mm or less.   Further, Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: 0.03% by mass or less, Ni: 0.05% by mass or less, containing one or more types The low carbon steel thin-walled slab according to claim 5.   A low carbon steel thin steel sheet according to claim 5 or 6, wherein the low carbon steel thin cast slab is subjected to cold rolling, continuous annealing at a recrystallization temperature or higher, and subsequently subjected to temper rolling. Production method.

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