TWI394843B - Melt Method of Ti - containing Very Low Carbon Steel and Manufacturing Method of Ti - containing Very Low Carbon Steel Casting - Google Patents

Description

Fusion method of Ti-containing ultra-low carbon steel and manufacturing method of Ti-containing ultra-low carbon steel slab

The present invention relates to a steel casting method for Ti-containing ultralow carbon steel after deoxidation using Ti, and a Ti-containing ultra-low carbon steel slab (slab) Manufacturing method. More particularly, the present invention relates to a Ti-containing ultra-low carbon steel suitable for the manufacture of cold-rolled steel sheets excellent in surface properties and inner properties, and for the manufacture of the cast sheets thereof. method.

In recent years, in the case of melting extremely low carbon steel for cold-rolled steel sheets such as steel sheets for automobiles, the mainstream is to strongly deoxidize molten steel by using Al in a manner of leaving 0.005 mass% or more of Al in the molten steel. The steel is cleaned at low cost. In such deoxidation using Al, the molten steel is generally treated by a gas bubbling or a RH vacuum degassing device (Ruhrstahl-Hausen vacuum degasser), and the generated oxide (deoxidation product) Products) A method of aggregating and coalescing to achieve flotation and removal, but in this method, an Al oxide (Al ₂ O ₃ ) is inevitably left in the cast piece. In particular, since the residual Al ₂ O ₃ becomes a cluster-like shape, it is difficult to perform flotation separation with respect to the apparent specific gravity of the molten steel. Therefore, the size of the steel easily remains several hundred. Cluster inclusions above μm. If such cluster inclusions are entrapped to the surface layer of the cast piece during continuous casting, surface defects such as spalling and sliver may occur ( Surface defect), which impairs the surface properties of cold rolled steel sheets.

Further, there is also a solid phase Al ₂ O ₃ formed by deoxidation of Al which adheres and deposits during continuous casting, and impregnates molten steel from a tundish to a mold. On the inner surface of the immersion nozzle, the nozzle is clogged (clogging: nozzle clogging). Therefore, a method of suppressing nozzle clogging by injecting Ar gas or the like from an upper nozzle or an immersion nozzle from a feed tank is generally employed. However, in this method, the blown gas is trapped in the solidification shell together with Al ₂ O ₃ to cause surface defects such as scale, peeling, and cracking, thereby impairing the surface properties of the cold rolled steel sheet.

As described above, since Al deoxidized steel has many problems, the recent deoxidation by using Ti without adding Al has increased. The reason is as follows. In the case of Ti-deoxidized steel, compared with Al-deoxidized steel, the amount of inclusions is larger because of its higher limit oxygen concentration. However, compared with the Al deoxidized steel, it is difficult to form a cluster-like oxide, and an oxide having a size of about 5 to 20 μm exists in a state of being dispersed in steel. Therefore, surface defects caused by cluster-like oxide-based inclusions in the Ti-deoxidized steel are reduced.

However, in the case of an extremely low carbon steel having a Ti content of 0.010% by mass or more and a Ti content/Al content of ≧5, the Ti oxide is in a solid phase state in the molten steel. Therefore, in the continuous casting, the Ti oxide adheres and grows (stacks) on the inner surface of the immersion nozzle in a state in which a metal is doped, resulting in clogging of the nozzle.

In order to solve the above-mentioned problem, in the case of casting aluminum-free Ti-deoxidized steel, it is proposed to limit the oxygen content of the molten steel passing through the submerged nozzle, as disclosed in Japanese Laid-Open Patent Publication No. Hei 8-281391 (Patent Document 1). A method of suppressing adhesion of Ti ₂ O ₃ and growing on the inner surface of the impregnation nozzle. However, in the case of Ti deoxidized steel, the limiting oxygen concentration is about 30 mass ppm. Therefore, each nozzle can only cast about 800 tons. Further, as the nozzle clogging is aggravated, it becomes impossible to stably control the level of the surface of the furnace bath in the mold. For these reasons, the technique of Patent Document 1 is not a fundamental solution to the problem of Ti deoxidized steel.

Moreover, as a method of casting a very low-carbon Ti-deoxidized steel which does not cause clogging of the immersion nozzle during continuous casting, and obtaining a Ti-containing ultra-low carbon cold-rolled steel sheet which is excellent in surface properties without rust, is obtained in Japanese Patent Laid-Open In Japanese Unexamined Patent Publication No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Ca (also including alloy containing Ca), and the inclusions are formed into a low melting point composition of Ti oxide-Al ₂ O ₃ -CaO and/or rare earth metal (REM) oxide, and are continuously cast. A method in which Ar gas is not blown into the immersion nozzle to perform casting.

In addition, in the Ti-containing ultra-low carbon steel to which Ca is added, in order to suppress the coarsening of the inclusion of the oxide-based inclusions, it is proposed to add the Japanese Patent Publication No. 2001-26842 (Patent Document 5). The oxygen content in the molten steel before Al and the time from the addition of Al until the addition of Ti can establish a ₀ /t ≦ 100 (where a ₀ : oxygen content before adding Al (mass ppm), t: Ti is added in such a manner that the relationship between the addition of Al and the time (min) before the addition of Ti is added, whereby the composition of the inclusions in the cold-rolled steel sheet is adjusted to Al ₂ O ₃ : 10 to 30% by mass, Ca and/or metal REM Oxide: 5 to 30% by mass, Ti oxide: 50 to 90% by mass.

However, the present inventors have found that in the methods of Patent Documents 2 to 4, when Ca is added, the molten steel is reoxidized by the slag or the atmosphere, and the oxygen concentration and the amount of oxide-based inclusions in the molten steel are increased, and the steel after casting is added. Large oxide-based inclusions remain in the medium. Further, due to the presence of the large-sized oxide-based inclusions, there is a problem in that the cold-rolled steel sheet is subjected to press forming, and the oxide-based inclusions are used as a starting point to cause fracture.

With regard to this problem, even if the treatment time before the addition of Ti is prolonged by the method of Patent Document 5, the oxide-based inclusions are formed into a composition that is difficult to aggregate, and a fundamental solution cannot be obtained. In other words, when Ca is added, the molten steel is reoxidized by the slag or the atmosphere, and the oxygen concentration in the molten steel and the amount of the oxide-based inclusions increase, and large-sized oxide-based inclusions remain in the steel after casting. In addition, due to the presence of the large-sized oxide-based inclusions, when the cold-rolled steel sheet is press-formed, the oxide-based inclusions are used as a starting point to cause fracture.

It is an object of the present invention to provide a Ti-containing ultra-low carbon steel melting method which can solve the above problems, which is a Ti-containing ultra-low carbon steel which is deoxidized by using Ti, and the melting method is continuous casting. In this case, it is possible to prevent clogging of the immersion nozzle due to oxide-based inclusions (nozzle clogging), and to obtain a cold-rolled steel sheet having excellent surface properties and internal quality. In the present invention, in the cold-rolled steel sheet having excellent surface properties and internal quality, in particular, surface defects due to oxide-based inclusions, bubbles, and the like are small, and the fracture is caused by the oxide-based inclusions. Highly resistant cold rolled steel sheet.

Further, another object of the present invention is to provide a method for producing a cast piece which is a method for producing a cast piece by using a Ti-containing ultra-low carbon steel which is melted by the above-described melting method, and can further improve cold rolling. Surface properties and endoplasm of steel plates.

The inventors of the present invention have repeatedly conducted experiments and studies in order to solve the problems of the above-described conventional techniques, and as a result, have developed a method for melting Ti-containing ultra-low carbon steel and a method for producing a Ti-containing ultra-low carbon steel slab.

[1] A method for melting a Ti-containing ultra-low carbon steel, which melts a very low carbon Ti deoxidized steel containing C: 0.020 mass% or less, Ti: 0.010 mass% or more, and Ca: 0.0005 mass% or more The steel is subjected to a decarburization treatment, followed by adding Ti in a ladle to perform deoxidation treatment, thereby obtaining an Al content (% by mass) and a Ti content (% by mass) satisfying [%Al]≦[%Ti]/10 a deoxidized molten steel consisting of the following, wherein Ca is added to the deoxidized molten steel in the ladle, thereby adjusting the inclusion composition in the molten steel to Ti oxide: 90% by mass or less, CaO: 5 to 50 mass %, Al ₂ O ₃ : 70% by mass or less, and in the pouring slag after deoxidizing the molten steel by adding Ti as described above:

‧ The total Fe concentration and the MnO concentration are 10% by mass or less;

‧ the mass ratio of CaO concentration to SiO ₂ concentration (%CaO) / (%SiO ₂ ) is 1 or more;

‧ TiO ₂ concentration is 1% by mass or more;

‧ The concentration of Al ₂ O ₃ is 10 to 50% by mass.

Further, the above invention [1] is preferably a method of melting a Ti-containing ultra-low carbon steel, characterized in that the melting contains C: 0.020 mass% or less, Ti: 0.010 mass% or more, and Ca: 0.0005 mass% or more. In the case of extremely low carbon Ti deoxidized steel, in a vacuum degassing apparatus, the molten steel of the tapping furnace or the electric furnace is subjected to decarburization treatment, and then, the molten steel after the decarburization treatment is performed. A Ti-containing alloy is added for deoxidation treatment, thereby obtaining a deoxidized molten steel having an Al content (% by mass) and a Ti content (% by mass) satisfying a composition of [%Al]≦[%Ti]/10, and thereafter, deoxidizing An alloy for inclusion composition adjustment containing Ca is added to the molten steel, whereby the composition of the inclusions in the molten steel is adjusted to Ti oxide: 90% by mass or less, CaO: 5 to 50% by mass, and Al ₂ O3: 70% by mass. In the following, the total Fe concentration and the MnO concentration in the ladle slag after the addition of the Ti-containing alloy to the molten steel is 10% by mass or less, and the mass ratio of the CaO concentration to the SiO ₂ concentration (%CaO) / (%SiO ₂ ) is 1 or more, the TiO ₂ concentration is 1% by mass or more, and the Al ₂ O ₃ concentration is 10 to 50% by mass.

[2] The method for melting Ti-containing ultra-low carbon steel according to the above [1], which comprises C: 0.020 mass% or less, Ti: 0.010 mass% or more, Ca: 0.0005 mass% or more, and Si: 0.2 mass. % or less, Mn: 2.0% by mass or less, S: 0.050% by mass or less, P: 0.005 to 0.12% by mass, N: 0.0005 to 0.0040% by mass, the remaining Fe and the inevitable impurities of the ultra-low carbon Ti deoxidized steel.

[3] The method for melting a Ti-containing ultra-low carbon steel according to the above [1] or [2], which further comprises Nb: 0.100% by mass or less, B: 0.050% by mass or less, and Mo: 1.0% by mass or less. One or more kinds of extremely low carbon Ti deoxidized steel.

[4] The method for melting a Ti-containing ultra-low carbon steel according to any one of the above [1] to [3] wherein, after the decarburization treatment is performed on the molten steel, Ti is added and deoxidation treatment is added, One or two or more selected from the group consisting of Al, Si, and Mn are pre-deoxidized, whereby the dissolved oxygen concentration in the molten steel is set to 200 mass ppm or less in advance.

[5] The method for melting a Ti-containing ultra-low carbon steel according to any one of the above [1] to [4] wherein the deoxidation treatment time of the molten steel obtained by adding Ti is 5 minutes or longer.

[6] A method for producing a Ti-containing ultra-low carbon steel slab, which is continuously cast by using a molten steel melted by the melting method according to any one of the above [1] to [5], thereby producing a slab Further, when the molten steel is injected into the mold from the feed tank by the dipping nozzle provided at the bottom of the feed tank, the molten steel is not cast into the molten steel flowing down the dipping nozzle to cast the molten steel.

[7] A method for producing a Ti-containing ultra-low carbon steel slab, which is continuously cast by using a molten steel melted by the melting method according to any one of the above [1] to [5], thereby producing a slab And, it agitates the molten steel in the mold by the electromagnetic force brought about by the magnetic field.

[8] A method for producing a Ti-containing ultra-low carbon steel slab, which is continuously cast by using a molten steel melted by the melting method according to any one of [1] to [5] above, thereby producing a cast piece And, it applies a static magnetic field to the molten steel in the mold to brake the flow of the molten steel.

[9] A method for producing a Ti-containing ultra-low carbon steel slab, which is continuously cast by using a molten steel melted by the melting method according to any one of the above [1] to [5], thereby producing a slab And, it agitates the molten steel in the mold by the electromagnetic force by the magnetic field, and applies a static magnetic field to the molten steel to brake the flow of the molten steel.

In the above [7] and [9], the magnetic field for stirring is preferably a moving magnetic field and/or an oscillating magnetic field, and more preferably a moving magnetic field.

[10] The method for producing a Ti-containing ultra-low carbon steel slab according to any one of the above [7] to [9] wherein, when the immersion nozzle provided at the bottom of the feed tank is sprayed into the mold from the feed tank In the case of steel, the molten steel is not cast into the molten steel flowing down the impregnation nozzle to cast the molten steel.

[11] The method for producing a Ti-containing ultra-low carbon steel slab according to any one of the above [6] to [10], wherein the molten steel is continuously cast at a throughput of 4 ton/min or less.

(The main components of the molten steel)

The present invention relates to a method for melting a very low carbon Ti deoxidized steel, which comprises a very low carbon Ti deoxidized steel containing C: 0.020 mass% or less, Ti: 0.010 mass% or more, and Ca: 0.0005 mass% or more, and The molten steel is subjected to decarburization treatment and deoxidation treatment of Ti, and further, by adding Ca to the deoxidized molten steel, the inclusions (= oxide-based inclusions, the same below) in the molten steel are adjusted to a predetermined composition. . The molten steel to be treated after decarburization is usually tapped from a converter or an electric furnace. The decarburization treatment is preferably carried out in a vacuum degassing apparatus, and the Ti deoxidation treatment is also preferably carried out in a vacuum degassing apparatus. The addition of Ca can also be carried out in a vacuum degassing apparatus or in a ladle with a deoxidation treatment of Ti. As a vacuum degassing apparatus for performing the above-described series of treatments, an RH vacuum degassing apparatus is particularly preferable, but other vacuum degassing apparatuses such as a vacuum oxygen decarburization (VOD) apparatus are not allowed.

When the amount of C of the ultra-low carbon steel melted in the present invention exceeds 0.020% by mass, the deep drawability of the product cannot be ensured, so the amount of C is made 0.020% by mass or less. The lower limit of the amount of C is not particularly limited.

Further, when the amount of Ti is less than 0.010% by mass, the deoxidizing ability of Ti is weak, and the total oxygen concentration is increased, so that the amount of Ti is 0.010% by mass or more. On the other hand, when the amount of Ti is too large, a large amount of TiN is formed to block the immersion nozzle, and therefore the amount of Ti is preferably 0.15% by mass or less.

In addition, when the amount of Ca is less than 0.0005 mass%, the CaO concentration in the inclusions is less than 5 to 50% by mass, the melting point of the inclusions is increased, and the nozzle is easily clogged, so that the amount of Ca is 0.0005 mass% or more. On the other hand, when the amount of Ca exceeds 0.0050% by mass, the CaO concentration of the inclusions exceeds 50% by mass, and the inclusions easily contain sulfur in a liquid phase state. Further, as a result, when the liquid phase inclusions are solidified, CaS is generated around the inclusions, and the CaS becomes the starting point of the rust of the steel sheet, and the amount of rust of the steel sheet remarkably increases. Therefore, the amount of Ca is preferably 0.0050% by mass or less.

The other steel composition is not particularly limited as long as it does not greatly affect the problem to be solved by the present invention in the production of steel or the production of slabs. The preferred composition of the steel sheet will be described later.

In the present invention, in a vacuum degassing apparatus such as an RH vacuum degassing apparatus, first, the molten steel is subjected to a decarburization treatment, and then Ti is added to the molten steel after the decarburization treatment to perform a deoxidation treatment (Ti deoxidation treatment) to obtain Al content (% by mass) [%Al] and Ti content (% by mass) [%Ti] Deoxidized molten steel satisfying the composition of [%Al]≦[%Ti]/10. Ti is conveniently added in the form of a Ti-containing alloy such as an Fe-Ti alloy.

If the deoxidized molten steel is out of the composition range, it becomes Al deoxidation instead of Ti deoxidation, and a large amount of Al ₂ O ₃ clusters are formed. Even if a Ti-containing alloy is added later to increase the Ti concentration, Al ₂ O ₃ cannot be sufficiently reduced, and remains as a cluster-like inclusion in the steel. Thereafter, although the composition of the inclusions is controlled by the addition of Ca, the inclusions formed become CaO‧Al ₂ O ₃ , which tends to be the starting point of rust, and the inclusions after the reaction of the Al ₂ O ₃ cluster are enormous. CaO‧Al ₂ O ₃ inclusions. Therefore, the deoxidized molten steel must satisfy [%Al]≦[%Ti]/10.

(Slag composition after deoxidation treatment of Ti)

Then, Ca is added to the Ti deoxidized molten steel, and the composition of the inclusions in the molten steel is adjusted to be Ti oxide: 90% by mass or less, CaO: 5 to 50% by mass, and Al ₂ O ₃ : 70% by mass or less. Melting point composition. Thereby, it is possible to effectively suppress adhesion of oxide-based inclusions to the inner surface of the immersion nozzle during continuous casting, and it is possible to prevent clogging of the immersion nozzle (nozzle clogging). In addition, Ca is conveniently added in the form of an alloy for adjusting inclusion composition containing Ca (hereinafter referred to as "Ca-containing flux").

Hereinafter, the purpose and effect of the Ti deoxidation treatment as described above and the subsequent Ca addition will be described. First, the molten steel is deoxidized by Ti (for example, a Ti-containing alloy such as an Fe-Ti alloy), whereby inclusions mainly composed of Ti oxide are formed. The inclusions obtained in this manner are not formed into a cluster shape when deoxidized by Al, but are formed into a granular shape having a size of about 5 to 20 μm, and are present in a state of being dispersed in steel.

If the Al concentration in the steel is at a certain level and the result becomes the same as that of Al deoxidation, a large Al ₂ O ₃ cluster is formed. In this case, even if a Ti-containing alloy is added and the Ti concentration is increased, the Al ₂ O ₃ cluster which has been formed does not disappear, but remains in the steel as an Al ₂ O ₃ cluster-like inclusion.

For the above reasons, in the present invention, it is necessary to first deoxidize the molten steel with Ti to form a Ti oxide.

By deoxidizing Ti as described above, Ti oxide inclusions of 80% by mass of Ti ₂ O ₃生成 are formed, and the inclusions are dispersed in steel in a size of about 5 to 20 μm and are formed into a granular shape. Surface defects in the production of cold rolled steel sheets from the steel are reduced. However, in the case of extremely low carbon steel, since the solidification temperature of steel is high, the Ti oxide is in a solid phase state in the molten steel, and thus continuous casting is carried out in the form in which the oxide is doped with bare metal. Therefore, the oxide and the bare metal adhere to and grow on the inner surface of the immersion nozzle, causing the nozzle to clog.

Therefore, in the present invention, after deoxidation using a Ti-containing alloy, Ca (for example, a Ca-containing flux) is further added to the deoxidized molten steel. By adding Ca, the composition of the oxide-based inclusions in the molten steel can be changed to a low melting point containing Ti oxide: 90% by mass or less, CaO: 10 to 50% by mass, and Al ₂ O ₃ : 70% by mass or less. Low melting point inclusions of Ti oxide. That is, by changing to such a low-melting-point inclusion, it is possible to effectively prevent the Ti oxide doped with the bare metal from adhering and growing on the inner surface of the immersion nozzle.

Ca may be added to the Ti deoxidized molten steel in a ladle after Ti deoxidation treatment, or Ca may be added from the upper portion to the Ti deoxidized molten steel in a vacuum tank in a vacuum degassing treatment (after deoxidation treatment), usually The former method is added.

As the Ca-containing flux, for example, one or more of CaSi, CaNi, CaAl, CaFe, and the like are preferably used, and by appropriately adjusting the amounts of addition, the inclusions having the above composition can be obtained.

The reason for limiting the inclusion composition adjusted by adding Ca is as follows.

When the concentration of the inclusions Ti oxide exceeds 90% by mass, the melting point of the inclusions cannot be sufficiently lowered, and the inclusions do not form a cluster, but the inclusions adhere to and accumulate on the inner surface of the immersion nozzle, causing nozzle clogging. Therefore, the Ti oxide concentration is 90% by mass or less, preferably 80% by mass or less. On the other hand, when the Ti oxide concentration is low, the Al ₂ O ₃ concentration is increased, so the Ti oxide concentration of the inclusion is preferably 20% by mass or more, and more preferably 30% by mass or more. The concentration of Ti oxide in the inclusions is determined by the amount of Ti contained in the inclusions by an electron probe microanalyzer (EPMA) or an energy dispersive X-ray spectrometer (EDX). Calculated by conversion to Ti ₂ O ₃ .

When the concentration of the inclusion CaO exceeds 50% by mass, the inclusions easily contain sulfur in a liquid phase state. As a result, when the liquid phase inclusions are solidified, CaS is generated around the inclusions, and this CaS becomes the starting point of the rust of the steel sheet, and the amount of rust of the steel sheet is remarkably increased. On the other hand, when the CaO concentration is less than 5% by mass, the melting point of the inclusions is not sufficiently lowered, and the inclusions adhere to and accumulate on the inner surface of the immersion nozzle, causing the nozzle to clog. Therefore, the CaO concentration is 5 to 50% by mass, preferably 7 to 50% by mass, and more preferably 15 to 50% by mass. The CaO concentration in the inclusions was calculated by measuring the amount of Ca contained in the inclusions by EPMA or EDX, and converting it into CaO.

When the concentration of the inclusions Al ₂ O ₃ exceeds 70% by mass, the inclusions become a high melting point composition, so that the nozzle of the submerged nozzle is easily clogged, and since the inclusions become clustered, the defects of the non-metallic inclusion property in the steel sheet increase. . Even if the concentration of Al ₂ O ₃ is low, there is no problem, but in terms of cost, it is advantageous that a part of deoxidation is carried out using Al. The Al ₂ O ₃ concentration in the inclusions was calculated by measuring the amount of Al contained in the inclusions by EPMA or EDX, and converting it into Al ₂ O ₃ .

In addition to the above-mentioned Ti oxide, CaO, and Al ₂ O ₃ , the inclusions may contain an oxide which is inevitably mixed, and may contain, for example, about 5% by mass or less of MgO and about 20% by mass or less of SiO ₂ . The MgO concentration or the SiO ₂ concentration in the inclusions was determined by measuring the amount of Mg or the amount of Si contained in the inclusions by EPMA or EDX, and converting them into MgO or SiO ₂ , respectively.

(Slag composition after deoxidation treatment of Ti) (a. Total Fe concentration and MnO concentration)

The Ca-containing flux is usually added to the molten steel in the ladle using a coated iron wire or injection lance. The iron-clad metal wire is a metal wire obtained by coating an alloy powder with a thin steel plate, and the metal wire is supplied to the molten steel. Further, in the method of using a spray gun, the alloy powder is blown into the molten steel through a spray gun.

When adding Ca to the molten steel, the molten steel is stirred vigorously, because the slag present above the molten steel is taken up in the molten steel, or due to oxides such as FeO, MnO, SiO ₂ in the molten steel and the slag. The reaction causes the molten steel to reoxidize, and the amount of oxide-based inclusions in the molten steel is remarkably increased. Therefore, in the present invention, the total Fe (T.Fe) concentration and the MnO concentration in the ladle slag after the Ti deoxidation treatment of the molten steel (that is, before the addition of Ca) is 10% by mass or less. Thereby, the reoxidation of the molten steel from the addition of Ca to the continuous casting can be suppressed, and the amount of the oxide-based inclusions in the cast piece can be reduced, so that the internal quality of the cold-rolled steel sheet can be sufficiently improved. The internal quality of the cold rolled steel sheet can be evaluated by, for example, the plate thickness deformation rate of the fracture portion in the bulging test.

Figure 1 shows the total Fe concentration (% by mass) and MnO concentration (% by mass) in the poured slag after the Ti deoxidation treatment of the molten steel in a manner to achieve the composition specified in the present invention (%T.Fe) +(%MnO) (horizontal axis: mass%), and the relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion of the cold-rolled steel sheet obtained from the molten steel in the bulging test.

In this test, cold rolling was carried out by melting a Ti-containing ultra-low carbon steel and subjecting the cast piece obtained by continuous casting to hot rolling (cold rolling) and cold rolling (cold rolling). Steel plate. For the molten steel (300 tons) which is charged into the ladle after tapping in the converter, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed. Further, the Al slag is a by-product produced on the surface when the aluminum is melted, and is often used as an additive in the refining step.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to a decarburization treatment to adjust the composition of the molten steel to C: 0.0007 to 0.0150% by mass, and the oxygen concentration: 120 to 700 mass ppm. Then, Al is added to the molten steel in an amount of 0.1 to 1.2 kg/melting steel ton (relative to the amount of molten steel per ton, the same applies hereinafter), and the dissolved oxygen concentration in the molten steel is lowered to 30 to 400 mass ppm. At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Further, 0.8 to 2.0 kg of molten steel ton Fe-70% by mass Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the Fe-Ti alloy is added, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. RH vacuum degassing treatment (deoxidation treatment), the composition of the slag in the pouring tank is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, Al ₂ O ₃ concentration: 10 to 50% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration: 2 to 15% by mass, total Fe concentration: 1 to 10% by mass, MnO concentration: 0.5 to 5% by mass, all satisfying the mass ratio (%CaO) / (%SiO ₂ )≧1. Further, the composition of the slag was measured by fluorescent X-ray analysis.

After RH vacuum degassing treatment, 20-35 mass% Ca-60-75 mass% Si alloy is supplied to the molten steel in the ladle by the iron-coated metal wire, and 0.1-0.4 kg/melting steel ton is added The Ca-Si alloy gauge) adjusts the composition of the inclusions in the molten steel to Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast piece which is melted in the manner described above is continuously cast by a twin-strand continuous casting apparatus to produce a cast piece. The continuous casting system is not performed by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the molten steel output (the amount of molten steel per unit time) during casting is 2 to 6 ton/min. . The cast slab is hot-rolled until the thickness becomes 2 to 4 mm, and further cold-rolled until the thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in Fig. 1, by setting (%T.Fe)+(%MnO) in the ladle slag to 10% by mass or less, the plate thickness deformation rate of the fracture portion in the bulging test can be 50% or more. . Further, (%T.Fe)+(%MnO) is more preferably 5% by mass or less. Further, the lower limit of (%T.Fe)+(%MnO) is not particularly limited.

Further, in the present invention, the plate thickness deformation rate of the fracture portion of the cold-rolled steel sheet in the bulging test is obtained as follows. Ten pieces of 200 mm square samples were taken from a cold-rolled steel sheet having a thickness of 0.6 to 1.0 mm, and the samples were expanded by oil pressure until they were broken. The thickness of the broken (fractured) portion was measured, and the deformation rate in the thickness direction was calculated by dividing the initial thickness, and the minimum deformation rate of 10 points was defined as the "plate thickness deformation rate". The higher the plate thickness deformation rate, the less the internal defects (in this case, the large oxide-based inclusions) and the better the internal quality, the plate thickness deformation rate is preferably 50% or more.

In order to make the (%T.Fe)+(%MnO) in the ladle slag 10% by mass or less (or a more suitable value), for example, Al slag may be added before the treatment according to the amount of slag flowing out of the converter. .

(b. ratio of CaO concentration to SiO ₂ concentration)

Further, from the same viewpoint as described above, the mass ratio (%CaO) / (%SiO ₂ ) of the CaO concentration to the SiO ₂ concentration in the ladle slag after Ti deoxidation of the molten steel is 1 or more.

Figure 2 shows the mass ratio (%CaO) / (%SiO ₂ ) (horizontal axis) in the poured slag after the Ti deoxidation treatment of the molten steel in a manner to achieve the composition specified in the present invention, and the melting The relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion of the cold-rolled steel sheet obtained by steel in the bulging test.

In this test, a Ti-containing ultra-low carbon steel was melted in the following manner, and it was continuously cast to obtain a cast piece, and the cast piece was subjected to hot rolling and cold rolling to obtain a cold rolled steel sheet. Lime was added to the molten steel (300 tons) charged into the ladle after tapping in the converter to adjust the mass ratio (%CaO) / (%SiO ₂ ). Further, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed.

Then, a series of treatments as described below were carried out in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0007 to 0.0150% by mass, and oxygen concentration: 120 to 700 mass ppm. Next, 0.1 to 1.2 kg/mel of molten steel ton is added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 30 to 400 mass ppm. At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Further, 0.8 to 2.0 kg of molten steel ton Fe-70% by mass Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the Fe-Ti alloy is added, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. The composition of the slag in the ladle after the RH vacuum degassing treatment (deoxidation treatment) is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, and Al ₂ O ₃ concentration: 10 to 50% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration: 2 to 15% by mass, total Fe concentration: 1 to 8 mass%, MnO concentration: 0.5 to 4 mass%, all satisfying (%T.Fe)+(%MnO ) ≦ 10% by mass.

After the RH vacuum degassing treatment, 20 to 35 mass% of Ca-60 to 7 mass% Si alloy is supplied to the molten steel in the ladle by the iron-coated metal wire, and 0.1 to 0.4 kg/melting steel ton is added. The composition of the inclusions in the molten steel is adjusted to be Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. This continuous casting is carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the yield of the molten steel at the time of casting is 2 to 6 ton/min. The cast slab is hot-rolled until the sheet thickness becomes 2 to 4 mm, and further cold-rolled until the sheet thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in Fig. 2, by setting (%CaO) / (%SiO ₂ ) in the ladle slag to 1 or more, the plate thickness deformation rate of the fracture portion in the bulging test can be 50% or more. Further, (%CaO) / (%SiO ₂ ) is more preferably 2 or more, still more preferably 2.5 or more. Further, the upper limit of (%CaO)/(%SiO ₂ ) is not particularly limited, but is usually at most about 6.0.

In order to make (%CaO)/(%SiO ₂ ) in the ladle slag 1 or more (or a more suitable value), for example, lime may be added to the converter tapping stream.

(c. TiO ₂ concentration)

Further, in the present invention, the concentration of TiO ₂ in the ladle slag after deoxidizing Ti in the molten steel is 1% by mass or more. Thereby, the reoxidation rate of Ti is reduced, and the increase in the amount of oxide-based inclusions can be suppressed, and the plate thickness deformation rate of the cold-rolled steel sheet in the bulging test can be 50% or more.

Figure 3 is a view showing the TiO ₂ concentration (horizontal axis: mass%) in the poured slag after the Ti deoxidation treatment of the molten steel in such a manner as to achieve the composition stipulated by the present invention, and the cold rolled steel sheet obtained from the molten steel The relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion in the bulging test.

In this test, a Ti-containing ultra-low carbon steel was melted in the following manner, and it was continuously cast to obtain a cast piece, and the cast piece was subjected to hot rolling and cold rolling to obtain a cold rolled steel sheet. After tapping in a converter, it was charged into a molten steel (300 ton) in a ladle, and a Ti-Fe alloy was added according to the oxygen concentration to adjust the TiO ₂ concentration. Further, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0007 to 0.0150% by mass, and oxygen concentration: 120 to 700 mass ppm. Next, 0.1 to 1.2 kg/mel of molten steel ton is added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 30 to 400 mass ppm. At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Further, 0.8 to 2.0 kg of molten steel ton Fe-70% by mass Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the Fe-Ti alloy is added, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. The composition of the slag in the ladle after the RH vacuum degassing treatment (deoxidation treatment) is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, and Al ₂ O ₃ concentration: 10 to 50% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration: 2 to 15% by mass, total Fe concentration: 1 to 8 mass%, MnO concentration: 0.5 to 4 mass%, all satisfying the mass ratio (%CaO) / (%SiO ₂ ) ≧1, (%T.Fe)+(%MnO)≦10% by mass.

After the RH vacuum degassing treatment, 20 to 35 mass% of Ca-60 to 7 mass% Si alloy is supplied to the molten steel in the ladle by the iron-coated metal wire, and 0.1 to 0.4 kg/melting steel ton is added. The composition of the inclusions in the molten steel is adjusted to be Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. This continuous casting is carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the yield of the molten steel at the time of casting is 2 to 6 ton/min. The cast slab is hot-rolled until the sheet thickness becomes 2 to 4 mm, and further cold-rolled until the sheet thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in Fig. 3, by setting the TiO ₂ concentration in the ladle slag to 1% by mass or more, the plate thickness deformation rate of the fracture portion in the bulging test can be 50% or more. Further, the concentration of TiO ₂ is more preferably 2% by mass or more, still more preferably 3% or more. Further, the upper limit of the TiO ₂ concentration is not particularly limited, but is usually at most about 10%.

In order to make the TiO ₂ concentration in the ladle slag 1% by mass or more, for example, Ti may be added depending on the oxygen concentration.

(d.Al ₂ O ₃ concentration)

Further, in the present invention, the concentration of Al ₂ O ₃ in the poured slag after the Ti deoxidation treatment on the molten steel is 10 to 50% by mass.

Figure 4 is a graph showing the Al ₂ O ₃ concentration (horizontal axis: mass%) in the poured slag after the Ti deoxidation treatment of the molten steel in a manner to achieve the composition specified in the present invention, and the cold obtained from the molten steel. The relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion of the rolled steel sheet in the bulging test.

In this test, a Ti-containing ultra-low carbon steel was melted in the following manner, and it was continuously cast to obtain a cast piece, and the cast piece was subjected to hot rolling and cold rolling to obtain a cold rolled steel sheet. After the tapping in the converter, the molten steel (300 ton) charged into the ladle was added to the Al slag to adjust the Al ₂ O ₃ concentration. Further, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0007 to 0.0150% by mass, and oxygen concentration: 120 to 700 mass ppm. Next, 0.1 to 1.2 kg/mel of molten steel ton is added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 30 to 400 mass ppm. At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Further, 0.8 to 2.0 kg of molten steel ton Fe-70% by mass Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the addition of the Fe-Ti alloy, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. The composition of the slag in the ladle after RH vacuum degassing treatment (deoxidation treatment) is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration : 2 to 15% by mass, total Fe concentration: 1 to 8 mass%, MnO concentration: 0.5 to 4 mass%, all satisfying the mass ratio (%CaO) / (%SiO ₂ ) ≧ 1, (% T. Fe) + (%MnO) ≦ 10% by mass.

After the RH vacuum degassing treatment, 20 to 35 mass% of Ca-60 to 7 mass% Si alloy is supplied to the molten steel in the ladle by the iron-coated metal wire, and 0.1 to 0.4 kg/melting steel ton is added. The composition of the inclusions in the molten steel is adjusted to be Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. This continuous casting is carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the yield of the molten steel at the time of casting is 2 to 6 ton/min. The cast slab is hot-rolled until the sheet thickness becomes 2 to 4 mm, and further cold-rolled until the sheet thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in FIG. 4, by making the Al ₂ O ₃ concentration in the ladle slag 10% by mass or more, the melting point of the slag is lowered, so that the absorption capacity of the slag to the oxide-based inclusion is increased, thereby reducing The amount of oxide inclusions. In addition, when the Al ₂ O ₃ concentration is 50% by mass or less, the Al ₂ O ₃ concentration in the oxide-based inclusions can be suppressed from exceeding 70% by mass, and the oxide-based inclusions can be prevented from becoming coarse. As a result of this, the plate thickness deformation rate of the fracture portion in the bulging test can be made 50% or more.

In order to make the Al ₂ O ₃ concentration in the ladle slag 10 to 50% by mass, for example, the amount of addition of the Al slag can be adjusted.

(Pre-deoxidation implementation and Ti deoxidation treatment time)

Furthermore, in the present invention, it is preferred to carry out pre-deoxidation by adding one or more selected from the group consisting of Al, Si, and Mn before the deoxidation treatment of the molten steel after the decarburization treatment. The dissolved oxygen concentration in the medium reaches 200 mass ppm or less. By this treatment, the amount of formation of oxide-based inclusions can be reduced. Therefore, the plate thickness deformation rate of the fracture portion of the cold-rolled steel sheet in the bulging test is further improved. The pre-deoxidation is preferably also carried out in a vacuum degassing process.

Figure 5 shows the dissolved oxygen concentration (white dots, black dots) and Ti deoxidation treatment time (horizontal axis: minutes) in the molten steel before the Ti deoxidation treatment, and the chilling test of the cold rolled steel sheet obtained from the molten steel. The relationship between the plate thickness deformation rate of the fracture part in the middle. The dissolved oxygen concentration in the molten steel before the Ti deoxidation treatment: 50 to 200 mass ppm (white dots in the figure) is a test example in which pre-deoxidation is performed as needed, and the dissolved oxygen concentration in the molten steel before the Ti deoxidation treatment: 200 to 500 mass ppm (black dot in the figure) is a test example in which pre-deoxidation is not performed.

In this test, a Ti-containing ultra-low carbon steel was melted in the following manner, and it was continuously cast to obtain a cast piece, and the cast piece was subjected to hot rolling and cold rolling to obtain a cold rolled steel sheet. After the tapping in the converter, the molten steel (300 ton) charged into the ladle was added to the Al slag to adjust the Al ₂ O ₃ concentration. Further, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0007 to 0.0150% by mass, and oxygen concentration: 120 to 700 mass ppm. Next, in the case of pre-deoxidation, 0.1 to 1.2 kg/mel of molten steel ton is added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 50 to 200 mass ppm.

At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Then, 0.8 to 2.0 kg/melt ton of Fe-70 mass% Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the addition of the Fe-Ti alloy, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. The composition of the slag in the ladle after the RH vacuum degassing treatment (deoxidation treatment) is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, and Al ₂ O ₃ concentration: 10 to 50% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration: 2 to 15% by mass, total Fe concentration: 1 to 8 mass%, MnO concentration: 0.5 to 4 mass%, all satisfying the mass ratio (%CaO) / (%SiO ₂ ) ≧1, (%T.Fe)+(%MnO)≦10% by mass.

After the RH vacuum degassing treatment, the molten iron is supplied to the molten steel in the ladle by the iron-coated metal wire by supplying 20 to 35 mass% of Ca-60 to 75 mass% Si alloy and adding 0.1 to 0.4 kg/melting steel ton. The composition of the inclusions in the middle is adjusted to be Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. This continuous casting is carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the yield of the molten steel at the time of casting is 2 to 6 ton/min. The cast slab is hot-rolled until the sheet thickness becomes 2 to 4 mm, and further cold-rolled until the sheet thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in FIG. 5, by pre-deoxidizing the Ti before the deoxidation treatment, the dissolved oxygen concentration in the molten steel is 200 mas ppm ppm or less, thereby suppressing the formation of oxide-based inclusions and further improving the fracture portion in the bulging test. Plate thickness deformation rate. Conversely, excess pre-deoxidation increases the risk of nozzle clogging, so the degree of pre-deoxidation and pre-deoxidation is preferably selected as appropriate for inclusion suppression requirements.

Further, as shown in FIG. 5, the Ti deoxidation treatment time (RH treatment time after the addition of the Ti-containing alloy) is preferably 5 minutes or longer, whereby an appropriate effect of the present invention can be obtained, and the cold-rolled steel sheet can be expanded. The plate thickness deformation rate of the fracture portion in the test was raised to the required level. Further, the upper limit of the Ti deoxidation treatment time is not particularly limited, and is usually about 5 minutes or less from the viewpoint of operational efficiency.

(Manufacturing method of Ti-containing ultra-low carbon steel slab)

Hereinafter, a method of continuously casting a molten steel melted by the method of the present invention to produce a Ti-containing ultra-low carbon steel slab will be described.

In continuous casting, when molten steel is injected into a mold, the oxide-based inclusions contained in the molten steel are infiltrated into the deep portion of the unsolidified layer of the cast piece by the downflow and are caught by the solidified shell. Further, in order to prevent oxide inclusions or the like from adhering to the immersion nozzle, an inert gas such as Ar gas is blown into the immersion nozzle, and the bubble of the inert gas is floated in the molten steel due to the molten steel furnace in the mold. The molten steel flow near the bath surface is trapped by the solidified shell. The oxide-based inclusions or bubbles trapped in the cast piece cause surface defects in the steel sheet. Further, in many cases, oxide-based inclusions adhere to the bubbles of the inert gas, and the oxide-based inclusions are caught by the solidified shell together with the bubbles of the inert gas.

In view of the above problems, the slab manufacturing method of several forms as described below is effective.

That is, in the method for producing the first Ti-containing ultra-low carbon steel slab, it is preferable that in the continuous casting apparatus, when the molten steel is injected into the casting mold from the feeding tank by the immersion nozzle provided at the bottom of the feeding tank, A molten steel (a inert gas such as Ar or a non-oxidizing gas such as N ₂ ) is blown into the molten steel flowing down the dipping nozzle to cast the molten steel. By melting the molten steel by the method of the present invention described above, nozzle clogging can be prevented without blowing a gas into the molten steel flowing down the dipping nozzle. Further, by not blowing the gas, it is possible to suppress the occurrence of bubble defects in the cast piece due to the entrapment of the gas, and it is possible to significantly reduce surface defects such as peeling, cracking, and scale of the cold-rolled steel sheet or the gold-plated steel sheet as the final product.

Further, in the method for producing a Ti-containing ultra-low carbon steel slab, it is preferred to carry out casting at a throughput of 4 ton/min or less.

Fig. 6 is a graph showing the thickness deformation rate of the fracture portion of the cold-rolled steel sheet in the bulging test of the continuous casting yield (horizontal axis: ton/min) and the cold-rolled steel sheet obtained by using the steel sheet obtained by the continuous casting ( Vertical axis: %) relationship. In this test, a Ti-containing ultra-low carbon steel was melted in the following manner, and a cast piece was obtained by continuous casting, and the cast piece was subjected to hot rolling and cold rolling to obtain a cold-rolled steel sheet. After the tapping in the converter, the Al slag was added to the molten steel (300 ton) charged into the ladle, and the Al ₂ O ₃ concentration was adjusted. Further, in order to reduce FeO and MnO in the slag, Al slag is added as needed. Further, in order to control the composition of the slag after the RH vacuum degassing treatment, CaO, Al ₂ O ₃ , and TiO ₂ are added as needed.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0007 to 0.0150% by mass, and oxygen concentration: 120 to 700 mass ppm. Next, 0.1 to 1.2 kg/mel of molten steel ton is added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 30 to 400 mass ppm. At this time, the Al concentration in the molten steel is 0.001 to 0.005 mass%. Further, 0.8 to 2.0 kg of molten steel ton Fe-70% by mass Ti alloy was added to the molten steel to carry out Ti deoxidation treatment. In the Ti deoxidation treatment, after the addition of the Fe-Ti alloy, the RH vacuum degassing treatment is completed within 2 to 15 minutes, and after completion, the composition of the molten steel is Ti concentration of 0.020 to 0.080% by mass, Al concentration of 0.001 to 0.006 mass%, and total The oxygen concentration is 20 to 100 mass ppm, which satisfies [%Al]≦[%Ti]/10. The composition of the slag in the ladle after the RH vacuum degassing treatment (deoxidation treatment) is CaO concentration: 20 to 60% by mass, SiO ₂ concentration: 5 to 20% by mass, and Al ₂ O ₃ concentration: 10 to 50% by mass, TiO ₂ concentration: 1 to 10% by mass, MgO concentration: 2 to 15% by mass, total Fe concentration: 1 to 8 mass%, MnO concentration: 0.5 to 4 mass%, all satisfying the mass ratio (%CaO) / (%SiO ₂ ) ≧1, (%T.Fe)+(%MnO)≦10% by mass.

After the RH vacuum degassing treatment, 20 to 35 mass% of Ca-60 to 7 mass% Si alloy is supplied to the molten steel in the ladle by the iron-coated metal wire, and 0.1 to 0.4 kg/melting steel ton is added, and the melting is performed. The composition of the inclusions in the steel is adjusted to be Ti oxide: 30 to 70% by mass, CaO: 6 to 50% by mass, and Al ₂ O ₃ : 10 to 70% by mass. The Ca concentration in the molten steel to be melted is 0.0005 mass% or more.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. This continuous casting is carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the yield of the molten steel at the time of casting is 2 to 6 ton/min. The cast slab is hot-rolled until the sheet thickness becomes 2 to 4 mm, and further cold-rolled until the sheet thickness becomes 0.6 to 1.0 mm, thereby obtaining a cold-rolled steel sheet.

As shown in Fig. 6, by casting at a throughput of 4 ton/min or less, the amount of inclusion of oxide-based inclusions is reduced, and as a result, the thickness of the fracture portion of the cold-rolled steel sheet in the bulging test is deformed. The rate is increased.

Further, as a method of manufacturing the second Ti-containing ultra-low carbon steel slab, it is preferable to carry out (i) agitating the molten steel in the mold by electromagnetic force by a moving magnetic field and/or an oscillating magnetic field, (ii) Either or both of the operations in which the molten steel in the mold applies a static magnetic field to brake the flow of the molten steel. According to such a manufacturing method, the amount of oxide-based inclusions captured by the solidified shell can be reduced without performing flotation separation in the mold, and as a result, the plate thickness deformation rate of the fracture portion of the cold-rolled steel sheet in the bulging test is further increased. improve. Further, by performing both of the above (i) and (ii), a particularly excellent effect can be obtained.

In the above method (i) of applying a moving magnetic field (AC magnetic field), an AC moving magnetic field applying device is provided, and the molten steel in the mold is rotated in the horizontal direction by the electromagnetic force of the magnetic field applying device, and the casting is performed while casting. sheet. Thereby, it is possible to suppress the oxide-based inclusions from being caught by the solidified shell, and it is possible to obtain a clean cast piece having less oxide-based inclusions. It is also effective to apply a non-moving oscillating magnetic field as an alternating magnetic field to agitate the molten steel in the mold. Further, a method of imparting an oscillating magnetic field (moving an oscillating magnetic field) accompanied by a movement in the horizontal direction is also effective.

In the method of (ii) applying a static magnetic field, a static magnetic field applying means is provided at a position surrounding the discharge flow of the molten steel from the discharge hole of the submerged nozzle, and the static magnetic field is applied by the static magnetic field applying means to cause the discharge flow The flow rate is reduced. Thereby, it is possible to promote the floating of the oxide-based inclusions, thereby suppressing the solidified shell from capturing the oxide-based inclusions, and obtaining a clean cast piece having less oxide-based inclusions.

In Fig. 6, the "applying moving magnetic field" (black dot) is the test example of the above (i), and the "static magnetic field applied" (black square) is the test example of the above (ii). As shown in FIG. 6, in the case where the molten steel in the mold is subjected to the above-described (i) or (ii) stirring by applying a magnetic field or braking the molten steel flow, compared with the case where no magnetic field is applied. The plate thickness deformation rate of the fracture portion in the bulging test is further improved.

In the case of performing both (i) and (ii) above, the moving magnetic field and/or the oscillating magnetic field is preferably applied to the upper portion of the relatively static magnetic field, and is preferably applied to the vicinity of the surface of the molten bath.

In the method for producing the second Ti-containing ultra-low carbon steel slab, the casting method at a throughput of 4 ton/min or less is also effective.

(other components of the molten steel)

Next, in the composition of the Ti-containing ultra-low carbon steel melted by the present invention, preferred conditions of the contents of the main components other than C, Ti, and Ca as described above will be described. Among these, it is not so much the viewpoint of further promoting the above-mentioned surface quality and the effect of improving the internal quality, but rather a preferred steel composition from the viewpoint of enjoying the advantages of the improvement effect.

The amount of Si is preferably 0.5% by mass or less. When the amount of Si exceeds 0.5% by mass, the material properties of the product steel sheet are deteriorated, and when used as a plated steel sheet, surface properties are deteriorated due to deterioration of plating properties. When the characteristics are emphasized, it is more preferable to make the amount of Si 0.2% by mass or less.

Further, if the mass ratio of Si to Ti (%Si)/(%Ti) ≧50, SiO ₂ is formed in the inclusions, and the characteristics as the bismuth deoxidized steel are also enhanced, so it is preferably (%Si)/(% Ti) <50.

The lower limit of the amount of Si is not particularly limited.

The amount of Mn is preferably 2.0% by mass or less. When the amount of Mn exceeds 2.0% by weight, the material is easily cured. The amount of Mn is preferably 1.5% by mass or less, more preferably 1.0% by mass or less, still more preferably 0.5% by mass or less.

Further, when the mass ratio (%Mn)/(%Ti) ≧100 between Mn and Ti, MnO is formed in the inclusions, and the characteristics as manganese deoxidized steel are also enhanced, so (%Mn)/(%Ti is preferable. ) <100.

The lower limit of the amount of Mn is not particularly limited.

The amount of S is preferably 0.050% by mass or less. When the amount of S exceeds 0.050% by mass, the CaS or REM sulfide in the molten steel increases, and the steel sheet as a product tends to rust. The amount of S is preferably 0.030% by mass or less. The lower limit of the amount of S is not particularly limited.

The amount of P is preferably from 0.005 to 0.12% by mass. When a large amount of P is contained, the amount of grain boundary segregation increases to cause grain boundary embrittlement, and in particular, the secondary steel sheet as a product is deteriorated in secondary work embrittlement resistance, so it is desirable to reduce P as much as possible. The amount of P is preferably 0.050% by mass or less. However, even if the amount of P is less than 0.005% by mass, the material cannot be expected to be further improved, and the melting cost is increased. On the other hand, it is acceptable as long as the amount of P is 0.12% by mass or less.

The amount of N is preferably 0.0005 to 0.0040% by mass. N is the same as C. In order to improve the deep drawability of the thin steel sheet as a product, it is desirable to reduce N as much as possible. However, even if the content is less than 0.0005 mass%, the material cannot be expected to be further improved, but the melting cost is increased. . On the other hand, when the amount of N exceeds 0.0040% by mass, the deterioration of the material of the steel sheet starts to increase. Among them, when the strength of the steel sheet is emphasized, N may be added to the upper limit of about 0.0200% by mass, and there is no problem in using such a steel.

Further, depending on the purpose, one or more of the elements listed below may be appropriately selected and added.

‧Nb: 0.100% by mass or less...Improve the deep drawability of thin steel sheets

‧B: 0.050% by mass or less...Improving the secondary processing brittleness of the steel sheet

‧Mo: 1.0% by mass or less... Increase the tensile strength of the steel sheet

‧Sb: 0.0200% by mass or less... to prevent nitriding when the billet is heated

‧Ce: 0.0050% by mass or less...the melting point of the inclusions is lowered, thereby suppressing nozzle clogging more stably

‧La: 0.0050% by mass or less... Reduces the melting point of inclusions, thereby suppressing nozzle clogging more stably

Furthermore, one or more selected from the group consisting of Ni, Cu, and Cr may be added in a range of 0.01% by mass or less, respectively. Adding these elements can improve the corrosion resistance of the steel sheet.

It is also possible to appropriately add an alloying element other than the above, which is about 1% in total.

The remainder is Fe and unavoidable impurities.

[Examples] [Inventive Example 1]

For the molten steel (300 tons) charged into the ladle after tapping in the converter, 400 kg of Al slag is added to reduce FeO and MnO in the slag, and the composition of the slag after controlling the vacuum degassing treatment is Add CaO.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0010% by mass, Si: 0.01% by mass, Mn: 0.15% by mass, P: 0.015% by mass, S: 0.005% by mass, and oxygen concentration. : 500 mass ppm (the remainder is Fe and unavoidable impurities, the same in the following composition after decarburization), and the temperature of the molten steel is adjusted to 1600 °C. Next, 0.5 kg of molten steel ton Al was added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 120 mass ppm. At this time, the Al concentration in the molten steel was 0.002% by mass. Further, 1.0 kg/melt ton of Fe-70% by mass Ti alloy was added to the molten steel, and Ti deoxidation treatment was performed for 7 minutes. In the Ti deoxidation treatment, the vacuum degassing treatment is completed within 7 minutes after the addition of the Fe-Ti alloy, and the Ti concentration in the molten steel in the ladle is 0.040% by mass, the Al concentration is 0.002% by mass, and the total oxygen concentration is completed. For 30mass ppm. Further, the composition of the slag in the ladle after the vacuum degassing treatment (deoxidation treatment) was CaO concentration: 35 mass%, SiO ₂ concentration: 15 mass%, Al ₂ O ₃ concentration: 35 mass%, and TiO ₂ concentration: 3 Mass%, MgO concentration: 7 mass%, total Fe concentration: 2 mass%, and MnO concentration: 2 mass% (other unavoidable oxide: 1 mass%).

After the vacuum degassing treatment is completed, 0.3kg/melt steel 30% by mass Ca-70 mass% Si alloy is added to the molten steel in the ladle by the iron-coated metal wire, and the inclusions in the molten steel are added. The composition is controlled. The Ca concentration in the molten steel to be melted was 0.0010% by mass.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. The morphology and composition of the inclusions in the feed tank at the time of casting were examined, and as a result, spherical inclusions of 70% by mass of Ti ₂ O ₃ -15 mass% CaO-15 mass% Al ₂ O ₃ were obtained. The continuous casting was carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the molten steel output at the time of casting was 3.8 ton/min. Furthermore, there is almost no deposit on the inner surface of the impregnated nozzle after casting.

The cast slab was hot rolled until the sheet thickness became 3.5 mm, and further cold rolling was performed until the sheet thickness became 0.8 mm, and then continuous annealing was performed under annealing conditions of 780 ° C × 45 seconds. On the annealed sheets obtained in such a manner, only defects of 0.2/1000 m of non-metallic inclusion properties and bubble properties were confirmed. Further, the plate thickness deformation ratio of the fracture portion of the cold-rolled steel sheet in the bulging test was 50%, which was good.

[Inventive Example 2]

For the molten steel (300 tons) which is charged into the ladle after tapping in the converter, 500 kg of Al slag is added to reduce FeO and MnO in the slag, and the slag composition after the vacuum degassing treatment is controlled. Add CaO, TiO ₂ .

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0015 mass%, Si: 0.01 mass%, Mn: 0.10 mass%, P: 0.012 mass%, S: 0.006 mass%, and oxygen concentration. : 450 mass ppm and the molten steel temperature was adjusted to 1600 °C. Then, 0.4 kg/melt ton of Al was added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 150 mass ppm. At this time, the Al concentration in the molten steel was 0.002% by mass. Further, 1.2 kg/melt ton of Fe-70% by mass Ti alloy was added to the molten steel, and Ti deoxidation treatment was performed for 6 minutes. In the Ti deoxidation treatment, after the Fe-Ti alloy was added, the vacuum degassing treatment was completed in 6 minutes. At the end, the Ti concentration of the molten steel was 0.045% by mass, the Al concentration was 0.002% by mass, and the total oxygen concentration was 30 mass ppm. Further, the composition of the slag in the ladle after the vacuum degassing treatment (deoxidation treatment) was CaO concentration: 30% by mass, SiO ₂ concentration: 17% by mass, Al ₂ O ₃ concentration: 40% by mass, and TiO ₂ concentration: 2 Mass%, MgO concentration: 8 mass%, total Fe concentration: 1 mass%, and MnO concentration: 2 mass%.

After the vacuum degassing treatment is completed, 0.25 kg / molten steel ton of 30% by mass of Ca-70 mass% Si alloy is added to the molten steel in the ladle by the iron-coated metal wire, and the composition of the inclusions in the molten steel is added. Control it. The Ca concentration in the molten steel to be melted was 0.0005 mass%.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. The morphology and composition of the inclusions in the feed tank at the time of casting were examined, and as a result, spherical inclusions of 70% by mass of Ti ₂ O ₃ -12% by mass of CaO-18% by mass of Al ₂ O ₃ were obtained. The continuous casting was carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the molten steel output at the time of casting was 4.0 ton/min. Further, the molten steel is stirred in the mold by an electromagnetic stirring device having a moving magnetic field. Furthermore, there is almost no deposit on the inner surface of the impregnated nozzle after casting.

The cast slab was hot rolled until the sheet thickness became 3.5 mm, and further cold rolling was performed until the sheet thickness became 0.8 mm, and then continuous annealing was performed under annealing conditions of 780 ° C × 45 seconds. On the annealed sheets obtained in such a manner, only defects of 0.2/1000 m of non-metallic inclusion properties and bubble properties were confirmed. Further, the plate thickness deformation ratio of the fracture portion of the cold-rolled steel sheet in the bulging test was 55%, which was good.

[Inventive Example 3]

For the molten steel (300 tons) which is charged into the ladle after tapping in the converter, 300 kg of Al slag is added to reduce FeO and MnO in the slag, and the slag composition after vacuum degassing treatment is controlled. Add CaO.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0015 mass%, Si: 0.01 mass%, Mn: 0.12 mass%, P: 0.015 mass%, S: 0.006 mass%, and oxygen concentration. : 400 mass ppm and adjust the temperature of the molten steel to 1600 °C. Next, 0.4 kg/mel of molten steel ton was added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 100 mass ppm. At this time, the Al concentration in the molten steel was 0.002% by mass. Further, 1.1 kg/melt ton of Fe-70% by mass Ti alloy was added to the molten steel, and Ti deoxidation treatment was performed for 5 minutes. In the Ti deoxidation treatment, after the Fe-Ti alloy was added, the vacuum degassing treatment was completed in 5 minutes. At the end, the Ti concentration of the molten steel was 0.042% by mass, the Al concentration was 0.002% by mass, and the total oxygen concentration was 30 mass ppm. Further, the composition of the slag in the ladle after the vacuum degassing treatment (deoxidation treatment) was CaO concentration: 42% by mass, SiO ₂ concentration: 13% by mass, Al ₂ O ₃ concentration: 30% by mass, and TiO ₂ concentration: 4 Mass%, MgO concentration: 6% by mass, total Fe concentration: 1% by mass, MnO concentration: 2% by mass (other unavoidable oxide: 2% by mass). After the vacuum degassing treatment is completed, 0.27 kg/melting steel 30% by mass Ca-70 mass% Si alloy is added to the molten steel in the ladle by the iron-coated metal wire, and the composition of the inclusions in the molten steel is added. control. The Ca concentration in the molten steel to be melted was 0.0006 mass%.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. The morphology and composition of the inclusions in the feed tank at the time of casting were examined, and as a result, spherical inclusions of 72% by mass of Ti ₂ O ₃ -12% by mass of CaO-16% by mass of Al ₂ O ₃ were obtained. The continuous casting was carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the molten steel output at the time of casting was 4.0 ton/min. Further, a static magnetic field of a DC magnetic field is applied to the molten steel in the mold to brake the flow of the molten steel. Furthermore, there is almost no deposit on the inner surface of the impregnated nozzle after casting.

The cast slab was hot rolled until the sheet thickness became 3.5 mm, and further cold rolling was performed until the sheet thickness became 0.8 mm, followed by continuous annealing under annealing conditions of 780 ° C × 45 seconds. On the annealed sheets obtained in such a manner, only defects of 0.2/1000 m of non-metallic inclusion properties and bubble properties were confirmed. Further, the plate thickness deformation ratio of the fracture portion of the cold-rolled steel sheet in the bulging test was 55%, which was good.

[Inventive Example 4]

The cast steel melted under the same conditions as in Inventive Example 1 was continuously cast by a double-strand continuous casting apparatus to produce a cast piece (the form and composition of the inclusions in the feed tank during casting were the same as in Invention Example 1). ). In the continuous casting apparatus, a static magnetic field applying means is provided at a position 500 mm below the lower end of the discharge hole of the submerged nozzle. Further, the shape of the discharge hole of the immersion nozzle was a square of 80 mm in length and width.

When continuously casting the molten steel, the flow rate of the Ar gas in the submerged nozzle is 0 to 10 NL/min, and the magnetic field strength (DC static magnetic field) applied by the static magnetic field applying device is 0.1 to 0.3 tes. Within the range of variation, a billet having a width of 1200 to 1500 mm and a thickness of 250 mm was cast at a casting speed of 4.5 to 6.0 ton/min.

After the cast slab is hot rolled and cold rolled to form a steel sheet, the steel sheet is subjected to hot-dip galvanizing. In the hot-dip galvanized steel sheet obtained in this manner, surface defects of inclusion properties and bubble properties were extremely small, and it was confirmed that a billet which was cleaned on both the surface and the inside by casting a static magnetic field was confirmed.

[Inventive Example 5]

The cast steel melted under the same conditions as in Inventive Example 1 was continuously cast by a double-strand continuous casting apparatus to produce a cast piece (the form and composition of the inclusions in the feed tank during casting were the same as in Invention Example 1). ). In the continuous casting apparatus, an AC moving magnetic field applying device is disposed at a position 2 m from the molten steel bath surface in the mold. Further, the shape of the discharge hole of the immersion nozzle was a square of 80 mm in length and width.

When the molten steel is continuously cast, the flow rate of the Ar gas in the immersion nozzle is 0 to 10 NL/min, and the magnetic field strength (AC moving magnetic field) applied by the AC moving magnetic field applying device is pulled at 0.05 to 0.2. Within the range of variation, a billet having a width of 1200 to 1500 mm and a thickness of 250 mm was cast at a casting speed of 4.5 to 6.0 ton/min.

After the cast slab is hot rolled and cold rolled to form a steel sheet, the steel sheet is subjected to hot-dip galvanizing. In the hot-dip galvanized steel sheet obtained in such a manner, surface defects of inclusion properties and bubble properties were extremely small, and it was confirmed that a billet which was cleaned on both the surface and the inside by casting an alternating magnetic field during casting was confirmed.

[Inventive Example 6]

Prepared to use the steel composition of Invention Example 1 as a basis,

A: further containing molten steel having Nb=0.010% by mass and B=0.0010% by mass, and

B: Further, a molten steel of Mo = 0.0100% by mass was contained, and other conditions were adjusted to the same conditions as in Inventive Example 1 to produce an annealed sheet.

In the annealed sheets of A and B, only defects of 0.2/1000 m of non-metallic inclusion properties and bubble properties were confirmed. Further, the plate thickness deformation ratio of the fracture portion of the cold-rolled steel sheet in the bulging test was 50%, which was good.

[Comparative Example 1]

For the molten steel (300 ton) charged into the ladle after tapping in the converter, 100 kg of Al slag was added to reduce FeO and MnO in the slag.

Then, a series of treatments as shown below were performed in the RH vacuum degassing apparatus. First, the molten steel is subjected to decarburization treatment, and the composition of the molten steel is adjusted to C: 0.0010% by mass, Si: 0.01% by mass, Mn: 0.15% by mass, P: 0.015% by mass, S: 0.005% by mass, and oxygen concentration. : 500 mass ppm, and the temperature of the molten steel was adjusted to 1600 °C. Then, 0.3 kg/mel of molten steel ton was added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 220 mass ppm. At this time, the Al concentration in the molten steel was 0.002% by mass. Further, 1.2 kg/melt ton of Fe-70% by mass Ti alloy was added to the molten steel, and Ti deoxidation treatment was performed for 7 minutes. In the Ti deoxidation treatment, after the Fe-Ti alloy was added, the vacuum degassing treatment was completed in 7 minutes, and at the end, the Ti concentration of the molten steel was 0.035 mass%, the Al concentration was 0.001 mass%, and the total oxygen concentration was 40 mass ppm. Further, the composition of the slag in the ladle after the vacuum degassing treatment (deoxidation treatment) was CaO concentration: 23% by mass, SiO ₂ concentration: 27% by mass, Al ₂ O ₃ concentration: 20% by mass, and TiO ₂ concentration: 0.8. Mass%, MgO concentration: 9% by mass, total Fe concentration: 8% by mass, MnO concentration: 6% by mass (other unavoidable oxide: 6.2% by mass).

After the vacuum degassing treatment is completed, 0.2kg/melt steel 30% by mass Ca-70 mass% Si alloy is added to the molten steel in the ladle by the iron-coated metal wire, and the composition of the inclusions in the molten steel is added. control.

The cast steel melted in the manner described above is continuously cast by a double-strand continuous casting apparatus to produce a cast piece. The morphology and composition of the inclusions in the feed tank at the time of casting were examined, and as a result, spherical inclusions of 70% by mass of Ti ₂ O ₃ -15 mass% CaO-15 mass% Al ₂ O ₃ were obtained. The continuous casting was carried out by blowing a gas such as Ar or N ₂ into the molten steel flowing down the dipping nozzle, and the molten steel output at the time of casting was 4.8 ton/min. Furthermore, there is almost no deposit on the inner surface of the impregnated nozzle after casting.

The cast slab was hot rolled until the sheet thickness became 3.5 mm, and further cold rolling was performed until the sheet thickness became 0.8 mm, and then continuous annealing was performed under annealing conditions of 780 ° C × 45 seconds. On the annealed sheets obtained in such a manner, only defects of 0.5/1000 m of non-metallic inclusion properties and bubble properties were confirmed. Further, the plate thickness deformation rate of the fracture portion of the cold-rolled steel sheet in the bulging test was 25%, which was not preferable.

[Comparative Example 2]

An annealed sheet was produced under substantially the same conditions as in Comparative Example 2, wherein the Al deoxidation (500 mass ppm → 220 mass ppm) time before the Ti deoxidation treatment was set to 120 seconds (a _{0 /} t ₌ 4.2) with reference to the technique of Patent Document 5. . The plate thickness deformation rate of the fracture portion in the bulging test of the annealed sheet obtained in this manner was 30%, which was improved as compared with Comparative Example 1, but still did not reach the level of the object of the present application. Furthermore, the defects of non-metallic inclusion properties and bubble properties of 0.4/1000 m were confirmed.

(industrial availability)

According to the melting method of the Ti-containing ultra-low carbon steel according to the present invention, the composition of the oxide-based inclusions in the molten steel can be optimized, and the amount of inclusions can be reduced. Therefore, it is possible to prevent clogging of the immersion nozzle due to oxide-based inclusions during continuous casting (nozzle clogging), and it is possible to produce a cold-rolled steel sheet having excellent surface properties and internal quality, particularly caused by oxide-based inclusions and bubbles. There are few surface defects and cold-rolled steel sheets having high resistance to press fracture due to oxide-based inclusions.

Further, according to the method for producing a Ti-containing ultra-low carbon steel slab according to the present invention, by optimizing the continuous casting conditions, the Ti-containing ultra-low carbon steel melted by the above-described melting method can be used to further improve cold rolling. The surface properties of the steel sheet and the cast of the endogenous material.

Figure 1 is a graph showing the total Fe concentration and MhO concentration in the ladle slag after Ti deoxidation treatment of molten steel: (%T.Fe) + (%MnO) (horizontal axis: mass%), and cold rolling A graph showing the relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion of the steel sheet in the bulging test.

Figure 2 is a graph showing the mass ratio (%CaO) / (%SiO ₂ ) (horizontal axis) of CaO concentration to SiO ₂ concentration in the ladle slag after Ti deoxidation treatment of molten steel, and bulging with cold rolled steel sheet A graph showing the relationship between the plate thickness deformation rate (vertical axis: %) of the fracture portion in the test.

Fig. 3 is a graph showing the TiO ₂ concentration (horizontal axis: mass%) in the ladle slag after the Ti deoxidation treatment of the molten steel, and the plate thickness deformation ratio of the fracture portion of the cold rolled steel sheet in the bulging test (vertical axis) :%) The chart of the relationship.

Fig. 4 is a graph showing the Al ₂ O ₃ concentration (horizontal axis: mass%) in the ladle slag after the Ti deoxidation treatment of the molten steel, and the plate thickness deformation ratio of the fracture portion of the cold rolled steel sheet in the bulging test ( A graph of the relationship of the vertical axis: %).

Figure 5 is a graph showing the dissolved oxygen concentration (white dots, black dots) and Ti deoxidation treatment time (horizontal axis: minutes) in the molten steel before the Ti deoxidation treatment, and the fracture portion of the cold rolled steel sheet in the bulging test. A graph showing the relationship between the plate thickness deformation rate (vertical axis: %).

Fig. 6 is a graph showing the yield of continuous casting in a test example (black dot, black square) for applying a magnetic field to a molten steel in a mold, and a test example (white dot) in which no magnetic field is applied (horizontal axis: ton/ Min) A graph showing the relationship between the thickness deformation rate (vertical axis: %) of the fracture portion of the cold rolled steel sheet in the bulging test.

Claims (13)

A method for melting a Ti-containing ultra-low carbon steel, which is obtained by melting a very low carbon Ti deoxidized steel containing C: 0.020 mass% or less, Ti: 0.010 mass% or more, and Ca: 0.0005 mass% or more Decarburization treatment is carried out, followed by deoxidation treatment by adding Ti to the ladle, thereby obtaining deoxidation melting in which the Al content (% by mass) and the Ti content (% by mass) satisfy the composition of [%Al]≦[%Ti]/10. Steel, after which Ca is added to the deoxidized molten steel in the ladle, thereby adjusting the inclusion composition in the molten steel to Ti oxide: 90% by mass or less, CaO: 5 to 50% by mass, and Al ₂ O ₃ : 70% by mass or less, and the above-described state of the poured slag after the deoxidation treatment of the molten steel by adding Ti is as follows: ‧ the total Fe concentration and the MnO concentration are 10% by mass or less; ‧ CaO concentration and The mass ratio of SiO ₂ concentration (%CaO) / (%SiO ₂ ) is 1 or more; ‧ TiO ₂ concentration is 1% by mass or more; and ‧ Al ₂ O ₃ concentration is 10 to 50% by mass. A method for melting a Ti-containing ultra-low carbon steel according to the first aspect of the patent application, which melts a very low carbon Ti deoxidized steel having the following composition: C: 0.020% by mass or less, Ti: 0.010% by mass or more, Ca: 0.0005 mass% or more, Si: 0.2 mass% or less, Mn: 2.0 mass% or less, S: 0.050 mass% or less, P: 0.005 to 0.12 mass%, N: 0.0005 to 0.0040 mass%, the remainder of Fe, and inevitable Impurities. The method for melting a Ti-containing ultra-low carbon steel according to the second aspect of the patent application, wherein, in addition to the composition of the second item of the patent application, further comprising Nb: 0.100% by mass or less, B: 0.050% by mass or less, Mo: One or more of 1.0% by mass or less. The method for melting Ti-containing ultra-low carbon steel according to any one of claims 1 to 3, wherein, after decarburization treatment of the molten steel, addition of Ti and deoxidation treatment, addition of Al, Si The pre-deoxidation is carried out by selecting one or more of Mn and two or more kinds of Mn, whereby the dissolved oxygen concentration in the molten steel is set to 200 ppm by mass or less in advance. The method for melting Ti-containing ultra-low carbon steel according to any one of claims 1 to 3, wherein the deoxidation treatment time of the molten steel obtained by adding Ti is set to 5 minutes or longer. A method for producing a Ti-containing ultra-low carbon steel slab, which is a continuous casting of a molten steel melted by the melting method according to any one of claims 1 to 5, thereby producing a cast piece, wherein When the molten steel is injected into the mold from the feed tank by the dipping nozzle provided at the bottom of the feed tank, the molten steel is not cast into the molten steel flowing down the dipping nozzle. A method for producing a Ti-containing ultra-low carbon steel slab, which is a continuous casting of a molten steel melted by the melting method according to any one of claims 1 to 5, thereby producing a cast piece, wherein The molten steel in the mold is stirred by the electromagnetic force generated by the magnetic field. A method for producing a Ti-containing ultra-low carbon steel slab, which is a continuous casting of a molten steel melted by the melting method according to any one of claims 1 to 5, thereby producing a cast piece, wherein A static magnetic field is applied to the molten steel in the mold to brake the flow of the molten steel. A method for producing a Ti-containing ultra-low carbon steel slab, which is a continuous casting of a molten steel melted by the melting method according to any one of claims 1 to 5, thereby producing a cast piece, wherein The molten steel in the mold is stirred by the electromagnetic force generated by the magnetic field, and a static magnetic field is applied to the molten steel to brake the flow of the molten steel. The method for manufacturing a Ti-containing ultra-low carbon steel slab according to Item 7 of the patent application, wherein when the molten steel is injected into the mold from the feeding tank through the immersion nozzle provided at the bottom of the feeding tank, the impregnation is not performed The molten steel flowing down the nozzle is blown into the gas to cast the molten steel. The method for manufacturing a Ti-containing ultra-low carbon steel slab according to Item 8 of the patent application, wherein when the molten steel is injected into the mold from the feeding tank through the immersion nozzle provided at the bottom of the feeding tank, the impregnation is not performed The molten steel flowing down the nozzle is blown into the gas to cast the molten steel. The method for manufacturing a Ti-containing ultra-low carbon steel slab according to claim 9 wherein the molten steel is injected into the mold from the feeding tank through the immersion nozzle provided at the bottom of the feeding tank, and the impregnation is not performed. The molten steel flowing down the nozzle is blown into the gas to cast the molten steel. A method for producing a Ti-containing ultra-low carbon steel slab according to Item 6 of the patent application, wherein the molten steel is continuously cast at a throughput of 4 ton/min or less.