JP6825507B2 - Manufacturing method of low carbon steel thin wall slab and manufacturing method of low carbon steel thin wall slab and low carbon steel thin steel sheet - Google Patents

Description

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

From the viewpoint of process saving and energy saving, a technique for manufacturing a thin plate close to the final product at the casting stage, that is, near net shape continuous casting is being developed. Among these, a double-roll type continuous casting method is disclosed in Patent Document 1 as a promising thin plate type near net shape continuous casting. In the continuous casting of thin-walled slabs using a twin-roll type continuous casting device, as shown in FIG. 1, the molten steel 3 is immersed in a hot water pool 2 partitioned by a pair of cooling rolls 1 rotating in opposite directions. The thin-walled slab 6 is cast by supplying the thin-walled slab 6 from the tundish 5 via the 4 and the filter 7 provided inside the tundish 5. In order to stably cast thin-walled slabs without surface defects or internal defects in this double-roll continuous casting, it is important to rectify the flow of molten steel in the pool and prevent fluctuations in the molten metal level.

On the other hand, in Patent Document 2, a filter is built in the immersion nozzle to generate an undisturbed discharge flow over the entire width of the nozzle, and in Patent Document 3, a rectifying porous nozzle is built in the slit-shaped nozzle. Methods for rectifying the nozzle discharge flow are disclosed respectively. Further, in the double roll type continuous casting method of Al deoxidized molten steel, it is known that the turbulence of the discharge flow caused by the nozzle clogging causes the molten metal level fluctuation and destabilizes the casting. However, Ca is contained in the molten steel. Patent Document 4 proposes a method of preventing nozzle clogging by adding and modifying CaO-Al ₂ O ₃ into low melting point inclusions. Further, since it is difficult to prevent nozzle clogging and surface defects due to alumina inclusions in Al deoxidized molten steel, a method of deoxidizing with Ce and La and leaving dissolved oxygen and a method of compound deoxidizing with Ti, Ce and La. Is disclosed in Patent Document 5, and it is effective in suppressing nozzle adhesion by controlling non-alumina inclusions.

Japanese Unexamined Patent Publication No. 60-137562 Japanese Unexamined Patent Publication No. 62-282753 Japanese Unexamined Patent Publication No. 8-164454 Japanese Unexamined Patent Publication No. 10-29047 JP-A-2004-195522

The methods of Patent Documents 2 and 3 described above are effective to some extent in stainless steel (not Al deoxidized) produced by the double-roll continuous casting method, but in casting low carbon steel with Al deoxidized. Alumina inclusions, which are deoxidized products, are coarsened by agglomeration and coalescence, and also adhere to the immersion nozzle discharge hole, the filter in the immersion nozzle, and the rectifying porous nozzle, so that the discharge flow is turbulent and the molten metal level fluctuates. The re-engagement of inclusions causes problems such as frequent internal defects. Furthermore, in the double-roll continuous casting method, the molten steel injected from the tundish solidifies in an extremely short time, and it is not possible to secure the floating time of inclusions. Therefore, in Al deoxidized molten steel, most of the coarse alumina inclusions are thin-walled slabs. There is a high possibility that internal defects will occur even in a stable casting state where the molten metal is trapped inside and there is no fluctuation in the molten metal level. Further, the method for modifying alumina inclusions in Patent Document 4 is effective in preventing clogging of the immersion nozzle and clogging of the filter, but the modified CaO-Al ₂ O ₃ inclusions are in a liquid phase. It easily coalesces and coarsens in the ladle, tundish, and immersion nozzle. Since these coarse inclusions are not removed by the filter in the dipping nozzle, they are immediately trapped in the thin-walled slab as described above, which causes cracks (internal defects) during processing. Further, as described in Patent Document 5, the method of reducing the cohesiveness by using non-alumina inclusions has 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 generated in the molten steel. Therefore, if the casting time is long, the nozzle clogging prevention effect is inevitably reduced. Further, it is known that if dissolved oxygen is left in the molten steel even after deoxidation by the method of Patent Document 5, the moldability and workability of the steel sheet are deteriorated, which causes a problem that a sufficient material cannot be secured.

Furthermore, in thin steel sheets manufactured by the double-roll continuous casting method, anisotropy appears during processing. For example, when deep drawing is performed during can manufacturing, peaks and valleys alternate in the circumferential direction of the can, so-called Earrings occur. If these earrings are large, the yield of can making will decrease, and the earrings will come into contact with the mold, leading to can making troubles. Therefore, the thin steel sheet obtained by the double-roll continuous casting method is required to have high formability and workability. At present, it cannot be applied to various applications. Furthermore, the present inventors have newly found a method for controlling the coagulation structure that reduces anisotropy during processing as described later, but the controllability is not sufficient and the earrings are stable over the entire length. It has not been possible to prevent the occurrence, and it is presumed that there are variable factors that have not been known so far.

In view of these current conditions, the present invention is stable in the composition of inclusions in which nozzle clogging and coarsening of inclusions are unlikely to occur and the solidified structure in which anisotropy is difficult to develop, while reducing inclusions in molten steel as much as possible. It is an object of the present invention to provide a double-roll type continuous casting method that can be controlled, a low-carbon steel thin-walled slab having excellent workability and formability cast using the same, and a method for manufacturing a low-carbon steel thin steel sheet.

In view of this situation, after reducing the inclusions in the molten steel as much as possible, it is possible to stably control the inclusion composition in which nozzle clogging and inclusions are less likely to be coarsened and the solidified structure in which anisotropy is less likely to occur. In order to provide a double-roll continuous casting method and a low-carbon steel thin-walled slab with excellent workability and formability cast using the method, a method for reducing inclusions in low-carbon steel, nozzle clogging and coarsening of inclusions. Elucidation of additive elements effective for prevention, method of modifying inclusions, elucidation of anisotropy expression mechanism during processing, preventive measures and stable exertion of effect, repeated diligent research, and obtained findings are double-roll continuous casting The present invention was completed by designing the process by optimally combining them in the process.

The summary is as follows. That is,
(1) Decarburization under atmospheric pressure is followed by decarburization under reduced pressure to make molten steel with a dissolved oxygen concentration of 0.005 to 0.035% by mass, at least one of Al and Ti, or Two kinds are added and deoxidized, 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 total of the acid-soluble Al concentration and the acid-soluble Ti concentration. The composition of molten steel is adjusted to more than 0%, and then 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of the total of at least one of Se or Te are added to the molten steel in a double roll type. A method for producing a low-carbon steel thin-walled slab, which is characterized by casting by a continuous casting method.
(2) The C concentration in the molten steel after decarburization under atmospheric pressure is 0.05% by mass or more and 0.1% by mass or less, and the C concentration after decarburization under reduced pressure is 0.01% by mass. The method for producing a low carbon steel thin-walled slab according to (1), wherein the content is less than 0.05% by mass.
(3) The low-carbon steel thin-walled slab according to (1) or (2), wherein the decarburization treatment under atmospheric pressure is performed in a converter and the decarburization treatment under reduced pressure is performed by a vacuum degassing device. Manufacturing method.
(4) At least one or two kinds of Al and Ti are added to deoxidize the components, and the acid-soluble Al concentration is adjusted to 0.05% by mass or less and the acid-soluble Ti concentration is adjusted to 0.1% by mass or less. After stirring for 3 minutes or more, molten steel containing 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of the total of at least one type of Se or Te was added in a double roll type. The method for producing a low-carbon steel thin-walled slab according to any one of (1) to (3), which comprises casting by a continuous casting method.

(5) In 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 total 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, total of at least one of Se or Te: 0.0002 to 0.005% by mass, All the oxygen concentration is not more than 0.002 mass% it is the balance Fe and unavoidable impurities, is less than 5 / cm ² oxide having a diameter of 30μm greater, is and isometric Akiraritsu 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, one or more. The low carbon steel thin-walled slab according to (5).

(7) The low-carbon steel thin-walled slab according to (5) or (6) is cold-rolled, continuously annealed at a recrystallization temperature or higher, and then temper-rolled. Steel plate manufacturing method.

According to the present invention, it is possible to suppress nozzle clogging and coarsening of inclusions while improving the cleanliness of the molten steel as much as possible, and further reduce the anisotropy of the solidified structure. Therefore, a low carbon steel having excellent workability and formability. It becomes possible to stably manufacture a thin steel sheet by using a double-roll continuous casting method.

The figure which shows the outline of the double-roll type continuous casting apparatus.

The present invention will be described in detail below.
Generally, low carbon steel is decarburized by blowing oxygen under atmospheric pressure such as a converter to melt molten steel having a final C concentration. The molten steel after the decarburization treatment contains a large amount of dissolved oxygen depending on the C concentration, and in some cases, 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, which greatly reduces the cleanliness of the molten steel. Further, when the dissolved oxygen concentration in the molten steel increases, the concentration of lower oxides such as FeO and MnO in the ladle slag also increases, so that the molten steel is reoxidized by the slag after deoxidation, and the amount of alumina inclusions further increases. These alumina inclusions float and separate while agglomerating and coalescing in the molten steel, but even in the tundish, the amount of inclusions in the molten steel is about 0.004% by mass in total oxygen concentration, and they are formed by agglomeration and coalescence in the molten steel. It also contains large alumina inclusions (alumina clusters) that reach several hundred μm. When this molten steel is cast by the double-roll type continuous casting method, the solidification time is very short, so that it is very different from a normal continuous casting device for slabs, and the floating separation of inclusions in the mold can hardly be expected. In addition, the double-roll type continuous casting immersion nozzle has a complicated structure such as providing a rectifying porous nozzle and a filter for the purpose of rectifying the discharge flow, so that the amount is larger than that of a normal continuous casting immersion nozzle. Inclusions adhere to the nozzle inner wall, discharge holes and filters. When the nozzle blockage occurs, the discharge flow from the immersion nozzle becomes unstable, and inclusions due to fluctuations in the molten metal level are re-engaged in the pool portion between the rolls. In this way, when low-carbon steel is cast by the double-roll continuous casting method, a large amount of alumina clusters that cause cracks during processing are trapped in the thin-walled slab, so that high-quality low-carbon steel has been used so far. It was very difficult to manufacture a steel sheet by a double-roll continuous casting method.

On the other hand, the solidified structure of the low carbon steel thin-walled slab cast by the twin-roll 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 of the solid-liquid interface during solidification. The C concentration is 0.1% by mass or less like low carbon steel, and the temperature gradient is like double roll casting. The larger the value, the easier it is for columnar crystals to grow. A thin-walled slab with a thickness of several mm manufactured by the double-roll continuous casting method is cold-rolled to the final plate thickness, but unlike the conventional continuous-cast slab cast with a thickness of about 250 mm, the reduction ratio is large. Cannot be secured. As a result, the direction of solidification structure growth remains in the final thin steel sheet and appears as anisotropy during processing. For example, when deep drawing is performed during can manufacturing, peaks and valleys alternate in the circumferential direction of the can. The present inventors have found that the so-called earrings that follow occur.

Based on the above problems, the present invention relates to [1] a method for reducing inclusions in low carbon molten steel, [2] a method for clarifying additive elements effective for preventing nozzle clogging and inclusion coarsening, and a method for modifying inclusions. 3] We have conducted extensive research on solidification structure control based on the anisotropy expression mechanism during processing and the stability of that control, and based on the findings obtained from the melting process of low carbon steel to the double-roll continuous casting process. It was completed by optimally combining and designing the process.

First, the method for reducing inclusions in the low-carbon molten steel of [1] will be described below. The technical idea of manufacturing this low carbon steel is to refine it under atmospheric pressure to blow off the C concentration higher than the final component value, leave excess carbon in the molten steel, and decarburize this molten steel under further reduced pressure. By doing so, the dissolved oxygen concentration is reduced to the utmost limit, and high-clean steel is melted. 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 according to the C concentration. For example, the final C concentration is 0. 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 generated in the molten steel, and the molten steel is produced. Greatly reduces cleanliness.
2Al + 3O = Al ₂ O ₃ (1)

On the other hand, in the present invention, the C concentration by the decarburization treatment under atmospheric pressure is made higher than the product value, that is, 0.05 to 0.1% by mass, and the decarburization treatment is completed. Therefore, the dissolved oxygen concentration is set. It is about 0.049 to 0.024% by mass, which is higher than the dissolved oxygen concentration of 0.06% by mass when decarburized to the average final C concentration (0.04% by mass) only by decarburization under atmospheric pressure. Low. Since the molten steel decarburized 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). It can be reduced to (% by mass), and the dissolved oxygen can be further reduced accordingly.
C + O = CO (2)

When the C concentration by the decarburization treatment under atmospheric pressure is set to the lowest 0.05% by mass, the dissolved oxygen concentration after the decarburization treatment under reduced pressure becomes the highest, but it is still about 0.035% by mass. Can be suppressed to. Further, when the C concentration after the completion of the decarburization treatment under atmospheric pressure is higher than about 0.065% by mass, the dissolved oxygen is insufficient in the subsequent decarburization treatment under reduced pressure, and the C concentration is set to the final component value ( It cannot be reduced to 0.04% by mass). In that case, oxygen can be supplied from the outside when the decarburization reaction in the latter half of the decarburization treatment under reduced pressure begins to stagnate (dissolved oxygen concentration is about 0.005% by mass), and the oxygen is supplied. Since oxygen is similarly 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 is started to be supplied. Therefore, the dissolved oxygen concentration after the decarburization treatment under reduced pressure can be reduced to about 0.035 to 0.005% by mass. Even if Al is added and deoxidized in this state, the amount of alumina inclusions produced is the total oxygen of low carbon steel with a C concentration of 0.04% by mass, which is melted only by decarburization under normal atmospheric pressure. It is very low compared to the case where the concentration is 0.06% by mass. Further, since the concentration of lower oxides such as FeO and MnO in the ladle slag is also reduced, the molten steel contamination by the slag after Al deoxidation is also greatly reduced.

The reason why the C concentration in the molten steel after decarburization under atmospheric pressure is preferably 0.05% by mass or more and 0.1% by mass or less is that when the C concentration is more than 0.1% by mass, the pressure is reduced. This is because the decarburization treatment is lengthened, and when the C concentration is less than 0.05% by mass, the dissolved oxygen concentration rapidly increases, and it is difficult to sufficiently reduce the dissolved oxygen concentration by the 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 the decarburization treatment under reduced pressure is 0.01% by mass or more, which is enough to obtain the material as low carbon steel. It is desirable that it is less than 05% by mass. The C concentration after the decarburization treatment corresponds to the C concentration of the slab.

The molten steel after the decarburization treatment under reduced pressure can be deoxidized by adding one or two types of Al or Ti. However, if the dissolved oxygen concentration after the decarburization treatment under reduced pressure exceeds 0.035% by mass, the amount of inclusions generated by adding one or two types of Al or Ti increases, and Mg described later is appropriate. Even if the amount is added, the alumina inclusions and titania inclusions cannot be modified, and it is not possible to prevent the agglomeration and the inclusions from adhering to the nozzle. On the contrary, 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 is possible. Extremely difficult in terms of both cost and processing time. Therefore, it is necessary to control the dissolved oxygen concentration after the decarburization treatment under reduced pressure to 0.005 to 0.035% by mass. Here, under reduced pressure means a pressure below atmospheric pressure.

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

In addition, from the viewpoint of preventing variation in molten steel components and deterioration of materials, 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, from the requirement that deoxidation is sufficiently carried out, the total of the acid-soluble (dissolved) Al concentration and the acid-soluble (dissolved) Ti concentration after the addition of one or two types of Al or Ti is at least 0 mass. It is more than%, preferably 0.005% by mass or more, and more preferably 0.01% by mass or more. After the decarburization treatment under reduced pressure, the dissolved oxygen concentration is measured, and by adding Al or Ti in excess of the Al amount or Ti amount obtained from the measured value according to the chemical quantity theory ratio, the above-mentioned suitable acid is possible. 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 the acid. Dissolved Al and dissolved Ti are dissolved in the acid, and alumina and titania are not dissolved in the acid. This is an analysis method that utilizes this. Here, the acid is, for example, a mixed acid in which 1 hydrochloric acid, 1 nitric acid, and 2 water are mixed.

In the present invention, it is preferable to provide a stirring time of 3 minutes or more for the molten steel after deoxidizing by adding Al or Ti. This is because the cleanliness of the molten steel can be improved by the decarburization treatment under reduced pressure, but the inclusions can be efficiently removed by further taking a stirring time, and the cleanliness can be further improved.

Further, as the 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 reduced pressure, a vacuum degassing device or a vacuum refining device is usually used. used.

Next, a method of modifying the inclusions in the low-carbon molten steel whose cleanliness has been improved by the above method into inclusions having a composition that is less likely to cause nozzle clogging and inclusion coarsening will be described. Even in molten steel that has been highly purified by decarburization under reduced pressure, alumina inclusions and titania inclusions are very easy to aggregate and coalesce, so the aggregation and coalescence of inclusions gradually occurs in the subsequent ladle and tundish. In addition, due to the complicated structure of the immersion nozzle in the double-roll continuous casting method, inclusions may adhere to the inner wall of the nozzle, the discharge hole and the filter, causing nozzle blockage. The biggest feature of the double-roll continuous casting method is that it is a quenching solidification process that completes solidification in a very short time. If molten steel can be injected into a twin-roll continuous casting machine by preventing agglutination in molten steel, it is possible to disperse inclusions more uniformly and finely than in a normal slab continuous casting method due to its characteristic quenching effect. Therefore, the occurrence of cracks during processing can be prevented most effectively.

Therefore, the present inventors have investigated additive elements that modify alumina inclusions and titania inclusions in molten steel with relatively high cleanliness to suppress agglomeration and inclusions and adhesion of inclusions to nozzles. It was found that Mg, which is a strong deoxidizing element, becomes an effective aggregation / adhesion prevention element compared to Ti.
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 are reduced, and at least the surface layer of the inclusions is magnesia or alumina magnesia spinel. Is generated. Since this inclusion composition greatly reduces the interfacial energy with the low-carbon molten steel, it simultaneously suppresses the adhesion of inclusions to the nozzle and filter refractory and the aggregation and coalescence of inclusions. Here, the amount of Mg added appropriate for inclusion control is 0.0003 to 0.01% by mass. This is because if the amount of Mg added is less than 0.0003% by mass, the surface layer portion cannot be modified to magnesia or alumina magnesia spinel, especially with alumina inclusions that are more stable than titania inclusions. On the contrary, when the amount of Mg added exceeds 0.01% by mass, even if the surface layer of inclusions is modified to magnesia or alumina magnesia spinel, Mg, which is a strong deoxidizing element, further lowers the dissolved oxygen and the inclusions and molten steel. This is because the interfacial energy with and is increased to promote coarsening and nozzle adhesion. Here, if the amount of Mg added is 0.01% by mass or less, more than 0.001% by mass of adsorbed oxygen remains at the oxide interface, but the amount of Mg added as a strongly deoxidizing element is 0.01. If it exceeds mass%, even the adsorbed oxygen at the oxide interface is reduced and removed, which is considered to greatly increase the interface energy.

Further, [3] a method for controlling a coagulated structure in which anisotropy is unlikely to appear during processing will be described. As described above, it has been found that the anisotropy of the thin steel sheet produced by the twin-roll continuous casting method is caused by the columnar crystal structure developed by quenching and solidifying the low-carbon molten steel. Based on this anisotropy expression mechanism, the present inventors have found that equiaxed crystallization of the solidified structure of low carbon steel is effective in reducing anisotropy. Therefore, Mg is added to low carbon molten steel. By adding at least the surface layer of alumina inclusions and titania inclusions to magnesia or alumina magnesia spinel and utilizing those inclusions as equiaxed crystal nucleation sites, the solidified structure is formed by a biroll continuous casting method. We devised a new method for equiaxed crystallization. According to the present invention, inclusions can be modified to magnesia or alumina magnesia spinel to reduce the interfacial energy between molten steel and inclusions. Therefore, addition of Mg [2] coarsens inclusions and prevents nozzle adhesion. [3] It works effectively on both equiaxed crystallization of the solidified structure, and is an extremely effective control means in the production of low carbon steel thin steel sheets using the twin-roll continuous casting method.

In order to eliminate the anisotropy during processing to a level that does not cause any problems, the present inventors have conducted a detailed experimental study on the control conditions of the solidified structure, and equiaxed crystal ratio (equal crystal thickness / plate thickness). It was clarified that it is necessary to secure 10% or more of × 100 (%)). This is because when the equiaxed crystal ratio of the thin-walled slab is 10% or more, the 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. Is. Similar to the inclusion composition control for preventing nozzle clogging and coarsening of inclusions, the appropriate amount of Mg added to secure the equiaxed crystallization rate of the solidified structure of the thin-walled slab is 0.0003 to 10% or more. It is 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 equiaxed nucleation sites decreases, and conversely if it exceeds 0.01% by mass, magnesia or alumina magnesia spinel becomes coarse. This is because the equiaxed crystallization rate of the coagulated structure becomes less than 10% and anisotropy develops.

In the double-roll type continuous casting, the molten steel temperature tends to decrease because the capacity of the ladle and the tundish is smaller than that of the normal continuous casting, and the hot pot molten steel temperature is adjusted higher for stable casting. In particular, the superheat degree (difference between the molten steel temperature and the liquidus temperature) of the molten steel injected into the twin-roll continuous casting machine is extremely high at the initial stage of ladle molten steel injection, and magnesia becomes an equiaxed crystal nucleation site. Alternatively, even if a sufficient amount of alumina magnesia spinel is present, the condition is such that equiaxed crystal nuclei are difficult to be formed. Furthermore, the height of the molten steel in the tundish is low at the initial stage of casting when the molten steel in the ladle begins to be injected into the tundish and at the seams of continuous casting, and the injection flow from the ladle is a falling flow without being covered by the long nozzle. As a result, 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 molten steel reoxidation such as Ar sealing in the tundish are taken, there is no effect on relatively large inclusions that cause nozzle clogging and cracking during steel sheet processing. However, it is difficult to completely prevent the reoxidation of molten steel, and newly generated alumina-based inclusions and titania-based inclusions with relatively small equiaxed nucleation ability become equiaxed crystal 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 the molten steel superheat degree and the molten steel reoxidation occur at the same time at the initial stage of casting and at the continuous casting seam, and almost no equiaxed crystallization occurs even when a predetermined amount of Mg is added. ing.

The present inventors also add a small amount of Se and Te to the molten steel to reduce the solid-liquid interfacial energy and make the columnar crystals themselves fine and fragile, so that equiaxed crystals are formed in the pool portion in the twin rolls. It was found that new nuclei are generated, and it is possible to compensate for the decrease in equiaxed nucleation ability due to the increase in molten steel superheat and the reoxidation of molten steel. The reason why equiaxed crystal nuclei proliferate 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 fragile due to the decrease in solid-liquid interface energy, and these columnar crystals become the rotation of 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 a new equiaxed crystal nucleus.

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 double roll continuous casting. As a result, it was clarified that equiaxed crystals are formed in the slab not only in the casting stable part but also in the initial stage of casting and the joint part of continuous casting. Here, for the effect of columnar crystal refinement, it is sufficient to add at least one type of Se or Te in a total amount of 0.0002% by mass or more, but if it is added in excess of 0.005% by mass, the steel sheet becomes fragile. , Cracks occur at the ends during recoil after casting or cold rolling. Therefore, one or more of Se and Te may be added to the molten steel so as to have a total of 0.0002 to 0.005% by mass.

By combining the above methods [1], [2] and [3], while ensuring high cleanliness, nozzle clogging and inclusion coarsening are unlikely to occur, and inclusion composition and anisotropy are unlikely to develop. The structure-controlled thin-walled slab can be stably cast by using the double-roll continuous casting method. In the present invention, the thin-walled slab means a slab having a thickness of 5 mm or less.

When the state of existence of large inclusions in the thin-walled slab obtained by the present invention was evaluated, only 5 large oxides exceeding 30 μm / cm ² were present, and the oxides were refined. Here, as for the dispersed state of inclusions, the polished surface (C cross section) of the slab or steel plate was observed with a 100x optical microscope, and the particle size distribution of inclusions within a unit area was evaluated. The particle size of this inclusion was the equivalent diameter obtained by measuring the major axis and the minor axis and determining (major axis x minor axis) ^0.5 . Further, when the cleanliness of the thin-walled slab of the present invention was evaluated by the total oxygen concentration, it was 0.002% by mass or less, which was very good. Here, the total oxygen concentration is the total amount of oxygen contained in the slab (including all oxide oxygen and dissolved oxygen), and can be analyzed by a normal gas analyzer. Further, in the slab of the present invention, the equiaxed crystal ratio is 10% or more, and the solidified structure at the center of the slab is equiaxed. In this way, the cleanliness of the slab is improved, the inclusions in the slab are dispersed as fine oxides, and the solidified structure at the center of the slab is isotropically crystallized to generate cracks in the steel sheet during processing. Since the anisotropy can be suppressed, it is possible to provide a thin-walled slab which is a thin steel plate material having excellent workability and formability.

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

Finally, among the chemical components of the thin-walled slab of the present invention, the actions of chemical components other than C, acid-soluble Al, acid-soluble Ti, Mg, Se, and Te described above will be mentioned.

Si is preferably 0.005% by mass or more and 0.03% by mass or less. This is because if the Si concentration is less than 0.005% by mass, the strength of the plate is insufficient, and if the Si concentration is more than 0.03% by mass, the workability of the plate is lowered.

Mn is an element effective for improving the strength of the steel sheet together with C and Si, and if necessary, it is preferably contained in an amount of 0.1% by mass or more, but if it is contained in excess of 0.6% by mass, coarse MnS Is generated and may reduce ductility, so it is preferably 0.6% by mass or less. Since the present invention is not impaired even if there is no Mn, the lower limit is not set.

P makes the material brittle, and if it is contained excessively, it segregates at the grain boundaries and causes deep drawing cracks. Therefore, it is preferable that P is 0.02% by mass or less, which is clear that there is no problem in practical use. Since the present invention is not impaired without P, the lower limit is not set.

Since S produces coarse MnS and deteriorates ductility and moldability, it is preferably 0.01% by mass or less. Since the present invention is not impaired even if S is not contained, the lower limit value is not particularly set.

If N is added too much, even a small amount of Al will generate a coarse precipitate and deteriorate the processability. Therefore, the amount of N is preferably 0.01% by mass or less. On the other hand, it is costly to make it less than 0.0005% by mass, so it is preferably 0.0005% by 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 have Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: If it is in the range of 0.03% by mass or less and Ni: 0.05% by mass or less, it can be added because it does not deteriorate the workability. By adding each element within this range, Nb improves the deep drawing property, V and Mo improve the strength, and Ni improves the corrosion resistance. Further, mixing of trace amounts of Cu, Ni, Cr and the like as unavoidable impurities by using scrap does not impair the present invention.

The present invention will be described in Tables 1 and 2 below with reference to Examples and Comparative Examples. In Tables 1 and 2, numerical values and items that deviate 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 in a converter, and then decarburized to the C concentration in Table 1 by a vacuum degassing device. Al or Ti was added to the molten steel to deoxidize it, and stirring was carried out as necessary, and then Mg and at least one of Se or Te were added to melt 100 tons of the molten steel as the component of Table 1. In the experiment of test number 14, Al was added to deoxidize the molten steel that had been decarburized to the C concentration in Table 1 only in the converter, and then Mg and at least one of Se or Te were added to make the final result. The molten steel of the components shown in Table 1 was melted. These molten steels were cast into thin-walled slabs having a thickness of 2.5 mm and a width of 1200 mm by using a double-roll type 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 number 14 was evaluated at the stage when the decarburization treatment in the converter was completed and the steel was put out in the ladle. In addition, the total oxygen concentration was evaluated on the slab.

The state of inclusions adhering to the nozzle during double-roll casting is "none" if the nozzle opening is almost constant, "yes" if the nozzle opening is gradually increasing, and there is no molten steel at the end of casting. If there is, it is listed in Table 2 as "occlusion".

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 times optical microscope. The particle size of the inclusions is the equivalent diameter of a circle obtained by measuring the major axis and the minor axis and calculating (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 on the slab C cross section.

The produced thin-walled slab was cold-rolled, then subjected to continuous annealing at an annealing temperature of 700 ° C., and further temper-rolled to obtain a cold-rolled steel sheet having a plate thickness of 0.16 mm. Since the recrystallization temperature is less than 650 ° C., continuous annealing can be surely performed at an annealing temperature of 700 ° C. or higher.

Cans were made with a can-making test machine for two-piece cans, which had difficult can-making conditions in which cracking was about 1000 times higher than that of an actual can-making machine. The crack generation rate during can production was evaluated as a value obtained by further multiplying the ratio of the number of cracked cans to the number of cans produced by this can manufacturing tester by 1/1000. The earring height was evaluated as the difference (mm) between the maximum height (mountain part) and the minimum height (valley part) in the circumferential direction of the can, and the results are shown in Table 2.

In Test No. 1-7, which is an example of the present invention, there was no inclusions adhering to the immersion nozzle, casting was stable, and both high purification and miniaturization of inclusions were achieved over the entire length of the thin-walled slab. The cracking rate during can manufacturing was 1 ppm or less. In addition, the solidified structure at the center of the plate thickness of the thin-walled 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 reduce the thickness to the same level as or even lower than the 1.5 mm of a normal 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, the Al and Ti concentrations are not appropriate. Therefore, nozzle adhesion and coarsening of inclusions occur in each of the test numbers 11 and 12, and cracks occur during processing. did. In addition, 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 the adhesion of inclusions to the immersion nozzle and the coarsening of inclusions were suppressed and cracks occurred during can making. The ratio was also 1 ppm or less, but the earrings became a problem because a sufficient equiaxed crystal ratio could not be obtained at the initial stage of casting due to the high casting temperature and the influence of molten steel reoxidation. In Test No. 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 is blown in the vacuum degassing device, and the dissolved oxygen concentration exceeds 0.035% by mass. As the amount of inclusions increased, the aggregation of inclusions and nozzle adhesion could not be suppressed even if Mg was added, the immersion nozzle was completely closed at the end of casting, and cracks frequently occurred during deep drawing. did. Of course, the anisotropy of the coagulated tissue could not be eliminated, and large earrings were generated.

Further, in the experiment of Test No. 14 of the comparative example, Al was added to deoxidize the molten steel that had been decarburized to the C concentration in Table 1 only in the converter, and then Mg was added to obtain the components of the final Table 1. Since the molten steel was melted, the dissolved oxygen concentration became excessive at more than 0.035% by mass, the amount of inclusions increased, and even if Mg was added, aggregation of inclusions and nozzle adhesion could not be suppressed, and the immersion nozzle was in the final stage of casting. It was completely closed and cracks occurred frequently during deep drawing. Large earrings were generated because the solidified structure in the center of the plate thickness could not be crystallized equiaxed.

1. 1. Cooling roll 2. Hot water pool 3. Molten steel 4. Nozzle 5. Tandish 6. Thin-walled slab 7. Rectifying perforated nozzle or filter

Claims (7)

Following the decarburization treatment under atmospheric pressure, the decarburization treatment under reduced pressure was performed to add at least one or two types of Al and Ti to the molten steel having a dissolved oxygen concentration of 0.005 to 0.035% by mass. Add and deoxidize, 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 total of the acid-soluble Al concentration and the acid-soluble Ti concentration is 0%. A double-roll continuous casting method in which molten steel is further added with 0.0003 to 0.01% by mass of Mg and 0.0002 to 0.005% by mass of a total of at least one type of Se or Te after adjusting the composition to a high degree. A method for producing a low-carbon steel thin-walled slab, which is characterized by casting in. The C concentration in the molten steel after decarburization under atmospheric pressure is 0.05% by mass or more and 0.1% by mass or less, and the C concentration after decarburization under reduced pressure is 0.01% by mass or more and 0. 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 production of low carbon steel thin-walled slabs according to claim 1 or 2, wherein the decarburization treatment under atmospheric pressure is performed in a converter and the decarburization treatment under reduced pressure is performed by a vacuum degassing device. Method. At least one or two of Al and Ti are added to deoxidize the components to adjust the acid-soluble Al concentration to 0.05% by mass or less and the acid-soluble Ti concentration to 0.1% by mass or less, and 3 A double-roll continuous casting method in which molten steel is 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 after stirring for at least a minute. The method for producing a low-carbon steel thin-walled slab according to any one of claims 1 to 3, which is characterized by casting in. In mass%, 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% by mass or less, acid-soluble Al: 0.05% by mass or less, acid-soluble Ti: 0.1% by mass or less, and the total of acid-soluble Al concentration and acid-soluble Ti concentration is 0% by mass. Super, N: 0.0005 to 0.01% by mass, Mg: 0.0003 to 0.01% by mass, total of at least one of Se or Te: 0.0002 to 0.005% by mass, total oxygen concentration Is 0.002% by mass or less, the balance is Fe and unavoidable impurities, the number of oxides having a diameter of more than 30 μm is less than 5 pieces / cm ² , and the equiaxed crystal ratio is 10% or more. A low-carbon steel thin-walled slab with a thickness of 5 mm or less. Further, it is characterized by containing one or more of Nb: 0.05% by mass or less, V: 0.03% by mass or less, Mo: 0.03% by mass or less, and Ni: 0.05% by mass or less. The low carbon steel thin-walled slab according to claim 5. A low carbon steel thin steel sheet according to claim 5 or 6, characterized in that the low carbon steel thin-walled slab is cold-rolled, continuously annealed at a recrystallization temperature or higher, and then temper-rolled. Production method.

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