WO2009123272A1

WO2009123272A1 - Steel-making method for titanium-containing ultralow carbon steel and method for manufacturing titanium-containing ultralow carbon steel slab

Info

Publication number: WO2009123272A1
Application number: PCT/JP2009/056835
Authority: WO
Inventors: 鍋島誠司; 松井章敏; 高橋大輔; 三木祐司; 岸本康夫
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2008-03-31
Filing date: 2009-03-26
Publication date: 2009-10-08
Also published as: TW201002832A; JP2009242912A; TWI394843B

Abstract

After decarburizing molten steel in a vacuum degassing facility or the like, a titanium-containing alloy is added to the ladle and deoxidation is performed, thereby creating a deoxidized molten steel with a composition that satisfies the formula [% Al]≤[% Ti]/10. Next, an alloy for adjusting the inclusion composition, which includes calcium, is added to the molten steel in the ladle, thereby adjusting the inclusion composition in the molten steel to 90% or below for titanium oxide, 5 to 50% for CaO, and 70% or below for Al₂O₃. Also, by ensuring that, after performing deoxidation, the ladle slag has a T.Fe concentration + MnO concentration of 10% or below by mass, (%CaO)/(%SiO₂) of 1 or above, a TiO₂ concentration of 1% or above by mass, and an Al₂O₃ concentration of 10 to 50% by mass, the inclusion composition within the molten steel is optimized and the inclusion amount is reduced, thereby enabling the manufacturing of a titanium-containing ultralow carbon steel that makes it possible to obtain a cold-rolled steel sheet that has excellent surface properties and inner properties, and also prevents clogging of the nozzle due to inclusions.

Description

Technical field of melting Ti-containing ultra-low carbon steel and manufacturing method of Ti-containing ultra-low carbon steel

The present invention relates to a method for steel making of Ti-containing ultralow carbon steel deoxidated with Ti, and a Ti-containing ultra-low carbon steel slab. This relates to the manufacturing method. In particular, the present invention provides a Ti-containing ultra-low carbon steel and its slab that are clearly suitable for producing a cold-rolled steel sheet having excellent surface properties and inner properties. It is about how to get it.

book

Background art

In recent years, when ultra-low carbon steel for cold-rolled steel sheets such as automotive steel sheets is melted, by strong deoxidation of the molten steel with A1 so that 0.001 mass% or more of A1 remains in the molten steel, The mainstream is to clean up the steel at a low cost. In such deoxidation using A1, the molten steel is processed using a gas bubbling device or an RH vacuum degassing device (Ruhrstah 卜 Hausen vacuum degasser), and the resulting oxide (deoxidization products) ;) Agglomeration · coalescing is used to achieve floatation and removal, but in this method, the oxide of A1 (A1 ₂ 0 ₃₎ may remain. in particular, since the residual Al ₂ 0 ₃ is formed of a cluster shape (cluster-like shape), a small apparent specific gravity with respect to the molten steel, difficult to flotation Therefore. Therefore In the steel, cluster-like inclusions with a size of several hundred μ πι or more are likely to remain, and such cluster-like inclusions are entrapped at the surface of the slab during continuous casting. When you get a hege, sliver ), And the surface properties of the cold-rolled steel sheet are impaired.

Further, A1 A1 ₂ 0 ₃ of the solid phase produced in deoxidation in a continuous铸造, the inner surface of the molten steel immersion nozzle for injecting the (molten steel) from Tandi' shoe (tundish) to錶型(mold) (immersion nozzle) There is also the problem of 'deposition' and nozzle clogging. Therefore, the upper nozzle (upper nozzle) of the tundish is injected with Ar gas or the like from the immersion nozzle, and a method of suppressing the clogging of the nozzle is adopted. In this method, the injected gas is trapped in the solidification shell together with A1 ₂ 0 ₃ and causes surface defects such as scales, heges, and slivers, which impairs the surface properties of the cold-rolled steel sheet. Become. As described above, there are many problems with Al deoxidized steel, and recently, there are many cases in which A1 is not added and deoxidized with Ti. The reason is as follows. In the case of Ti deoxidized steel, the ultimate oxygen concentration is higher than that of A1 deoxidized steel, and therefore the amount of inclusions is large. However, compared to A1 deoxidized steel, it is difficult to form cluster-like oxides. Oxides with a size of about 5-20 / xm are present in a dispersed state in the steel. Therefore, in this Ti deoxidized steel, surface defects due to cluster-like oxide inclusions are reduced.

However, in the ultra-low carbon steel with Ti content greater than or equal to 0.0010 mass% and Ti content / A1 content ≥5, the Ti oxide is in a solid state in the molten steel. For this reason, during continuous fabrication, the Ti oxide adheres to the inner surface of the immersion nozzle in the form of metal (metal) and causes nozzle clogging. As a method for solving such problems, Japanese Patent Laid-Open No. 8-281391 (Patent Document 1) limits the amount of oxygen in molten steel that passes through an immersion nozzle in the fabrication of A1-less Ti deoxidized steel. the adhesion of Ti ₂ 〇 ₃ of immersion Bruno nozzle inner surface. the method of inhibiting the growth have been proposed. However, in the case of Ti deoxidized steel, the reached oxygen concentration is about 30 mass ppm. For this reason, only about 800 tons per nozzle can be produced. Moreover, the level control of the hot water surface in the mirror mold becomes unstable as the nozzle blockage proceeds. For these reasons, the technology of Patent Document 1 does not provide a fundamental solution to the problem of Ti deoxidized steel.

In addition, as a method of producing ultra-low carbon Ti deoxidized steel without causing clogging of the immersion nozzle during continuous forging, and obtaining a Ti-containing ultra-low carbon cold-rolled steel sheet with excellent surface properties without causing significant increase in the formation of heat. JP-A-10-291 053 (Patent Document 2), JP-A-11-343516 (Patent Document 3), and JP-A-2006-152444 (Patent Document 4) describe that Ca ( (Including Ca-containing alloys), the inclusions have a low melting point composition of Ti oxides A1 ₂ 0 ₃ — CaO and Z or REM (rare-earth metal) oxides. A method of forging without blowing Ar gas into the interior has been proposed.

In addition, in order to suppress the coarsening due to the aggregation of oxide inclusions in Ti-containing ultra-low carbon steel containing Ca, JP 2001-26842 A (Patent Document 5) describes that Between the oxygen content and the time from the addition of A1 to the addition of force Ti, a ₀ / t ≤ 100 (where a ₀ : oxygen content before mass addition (mass ppm), t: Al The content of inclusions in the cold-rolled steel sheet can be changed to Al ₂ O ₃ : 10 to ₃₀ mass%, Ca and Ca by adding Ti so that the relationship between the force and the time until the addition of Ti (min)) is satisfied. Metal or REM oxide: 5 to 30 mass%, Ti oxide: 50 to 90 mass% are proposed.

'Disclosure of invention

[Problems to be solved by the invention]

However, the present inventors have discovered that the methods disclosed in Patent Documents 2 to 4 do not dissolve when Ca is added. Steel is re-oxidized by slag, air, etc., and the oxygen concentration and amount of oxide inclusions in the molten steel increase, and large oxide inclusions remain in the steel after forging. The large oxide inclusions cause a problem that cracks originating from the oxide inclusions occur during press forming of the cold rolled steel sheet.

To solve this problem, even if the composition of the oxide inclusions is difficult to aggregate by extending the treatment time before adding Ti by the method of Patent Document 5, it is not a fundamental solution. That is, when Ca is added, the molten steel is re-oxidized by slag, the atmosphere, etc., and the oxygen concentration and oxide inclusions in the molten steel increase, and large oxide inclusions remain in the steel after forging. . Due to the large oxide inclusions, cracks originating from the oxide inclusions occur during the press forming of the cold-rolled steel sheet.

The object of the present invention is to solve the above-mentioned problems and to produce a Ti-containing ultra-low carbon steel that has been deoxidized with Ti. It is an object of the present invention to provide a method for melting Ti-containing ultra-low carbon steel that can prevent (nozzle clogging) and obtain a cold-rolled steel sheet having excellent surface properties and quality. In the present invention, as a cold-rolled steel sheet having excellent surface properties and internal quality, there are particularly few surface defects due to oxide inclusions and bubbles, and resistance to press cracks caused by oxide inclusions. A cold-rolled steel sheet having the following is obtained.

In addition, another object of the present invention is a method for producing a splinter from a Ti-containing ultra-low carbon steel that has been produced by such a melting method, which further improves the surface properties and quality of the cold-rolled steel sheet. An object of the present invention is to provide a method for manufacturing a piece that can be enhanced.

[Means for solving the problems]

As a result of repeated experiments and researches to solve the above-mentioned problems of the prior art, the present inventors have found that a method for melting Ti-containing ultra-low carbon steel and a Ti-containing ultra-low carbon steel piece having the following gist are described. The manufacturing method has been developed.

[1] In melting extremely low carbon Ti deoxidized steel containing C: 0.020 mass% or less, Ti: 0.0010 mass% or more, Ca: 0.005 mass% or more, the molten steel is decarburized, A1 content (ma _SS %) and Ti content (mas _S %) force [% A1] ≤ [% Ti] / lO by adding Ti and deoxidizing in ladle In the ladle, Ca is added to the deoxidized molten steel, so that the inclusion composition in the molten steel is Ti oxide: 90 mass% or less, CaO: 5 ~ 50mass%, A1 ₂ 0 ₃ : Adjusted to 70mass% or less, and,

In the ladle slag after adding the Ti and deoxidizing the molten steel,

'Total Fe concentration and MnO concentration are less than 10 mass%,

• CaO concentration Si_〇 ₂ concentration mass ratio of (% CaO) / (% Si_〇 ₂₎ one or more,

• Ti0 ₂ concentration lmass% or more, and, • A1 ₂₀ ₃ concentration 10 50mass%

A method for melting Ti-containing ultra-low carbon steel.

The above invention [1] is C: 0.020 mass ^o /. In the following, in melting ultra-low carbon Ti deoxidized steel containing Ti: 0.010 maSs% or more and Ca: 0.0005 mass% or more, the molten steel produced from a converter or electric furnace is used. In a vacuum degassing facility, decarburization treatment is performed, and then the Ti-containing alloy is added to the molten steel after the decarburization treatment, followed by deoxidation treatment, whereby A1 content (mass%) and Ti content (mass% ) Is a deoxidized molten steel having a composition satisfying [% A1] ≤ [% Ti] ZlO, and then an inclusion composition adjusting alloy containing Ca is added to the deoxidized molten steel. The composition in the ladle slag after adjusting the composition to Ti oxide: 90111 ₃ % or less and CaO: 5 50 mass% A1 ₂ 0 ₃ : 70 mass% or less and adding the Ti-containing alloy to deoxidize the molten steel , 10 mass% of the total of total Fe concentration and MnO concentrations less, the weight ratio of CaO concentration and Si_〇 ₂ concentration (% CaO) Z (% Si_〇 ₂₎ one or more, Ti0 ₂ concentration Lmass% or more, A1 ₂ 0 ₃ Concentration 10 50 A method for melting Ti-containing ultra-low carbon steel characterized by mass% is preferred.

[2] In the melting method of [1] above, C: 0.020 mass% or less, Ti: 0.010 mass% or more, Ca: 0.0005 mass. /. Above, Si: 0.2mass. /. Below, Mn: 2. Omass% or less, S: 0.050 mass% or less, P: 0.00 5 0.12 mass% N: 0.0005 0.0040 mass%, ultra low carbon Ti deoxidized steel consisting of the balance Fe and inevitable impurities is melted A method for melting Ti-containing ultra-low carbon steel, characterized by:

[3] In the melting method according to [1] or [2] above, it further contains one or more of Nb: 0. 100 mass% or less, Β: 0 · 050 mass% or less, Mo: l.0 niass% or less. A method for melting Ti-containing ultra-low carbon steel, characterized by melting ultra-low carbon Ti deoxidized steel.

[4] In the melting method of any one of [1] [: 3] above, after decarburizing the molten steel, it is selected from Al Si and Mn prior to deoxidizing treatment by adding Ti A method for melting Ti-containing ultra-low carbon steel, characterized in that the dissolved oxygen concentration of molten steel is set to 200 massppm or less in advance by adding one or two or more types to perform preliminary deoxidation.

[5] Ti-containing ultra-low carbon characterized in that in the melting method according to any one of the above [1:] and [4], the deoxidation time of the molten steel performed by adding Ti is 5 minutes or more. Steel melting method.

[6] A method for producing a mirror piece by continuously forging molten steel produced by any one of the melting methods described in the above [1] to [5], wherein the tundish force is obtained through an immersion nozzle installed at the bottom of the tundish. In the mold A method for producing a Ti-containing ultra-low carbon steel piece, characterized in that when molten steel is injected, molten steel is produced without blowing gas into the molten steel flowing down the immersion nozzle.

[7] A method of continuously forging a molten steel produced by the melting method according to any one of [1:] to [5] above to produce a flake, wherein the molten steel in the mold is electromagnetically generated by a magnetic field. A method for producing a Ti-containing ultra-low carbon steel piece characterized by stirring by force.

[8] A method of continuously producing molten steel by melting the molten steel produced by any one of the above methods [1:] to [5], and applying a static magnetic field to the molten steel in the mold. A method for producing a Ti-containing ultra-low carbon steel slab characterized by applying and braking molten steel flow.

[9] A method for producing a mirror piece by continuously forging molten steel produced by the melting method of any one of [1:] to [5] above, wherein the molten steel in the mold is moved by electromagnetic force generated by a magnetic field. A method for producing a Ti-containing ultra-low carbon steel piece characterized by stirring and applying a static magnetic field to the molten steel to brake the molten steel flow.

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

[10] In the manufacturing method according to any one of [7] to [9] above, when the molten steel is poured from the tundish into the bowl through the immersion nozzle installed at the bottom of the tundish, the immersion nozzle is allowed to flow down. A method for producing a Ti-containing ultra-low carbon steel slab characterized by producing molten steel without blowing gas into the molten steel.

[11] Manufacture of Ti-containing ultra-low carbon steel slab characterized in that molten steel is continuously forged at a throughput of 4 tonZniin or less. Method. Brief Description of Drawings

Figure 1 shows the total Fe concentration and MnO concentration in ladle slag after Ti deoxidation of molten steel: (% T. Fe) + (% MnO) (horizontal axis: mass%) and cold rolled steel sheet 5 is a graph showing the relationship with the plate thickness strain rate (vertical axis:%) of a cracked portion in the bulge test.

2, the weight ratio of CaO concentration and Si_〇 ₂ concentration of ladle slag after the molten steel Ti processing deoxidation and (% 〇 & 〇) / (% Si0 ₂₎ (horizontal axis), the cold-rolled steel sheet It is a graph which shows the relationship with the plate thickness distortion rate (vertical axis:%) of the crack part in a bulge test.

3, Ti_〇 ₂ concentration of ladle slag after the molten steel Ti processing deoxidation (abscissa: ma _S s%) and, of cold-rolled steel sheet It is a graph which shows the relationship with the plate thickness distortion rate (vertical axis:%) of the crack part in a bulge test.

4, A1 ₂ 0 ₃ concentration of ladle slag after the molten steel Ti processing deoxidation (horizontal axis: mass%) and the thickness distortion of the crack portions in the bulge test cold-rolled steel plate (ordinate:% It is a graph which shows the relationship with).

Figure 5 shows the dissolved oxygen concentration (white circle and black circle) and Ti deoxidation treatment time (horizontal axis: minutes) in the molten steel before Ti deoxidation treatment, and the plate thickness distortion rate in the bulge test of cold-rolled steel sheets ( It is a graph which shows the relationship with a vertical axis | shaft:%).

Figure 6 shows the continuous forging throughput (horizontal axis: ton / black) for the test example (black circle / black square) with a magnetic field applied to molten steel in the mold and the test example without a magnetic field (white circle). 3) is a graph showing the relationship between the thickness strain rate (vertical axis:%) of the cracked portion in the bulge test of cold-rolled steel sheets. BEST MODE FOR CARRYING OUT THE INVENTION

(Main composition of steel to be melted)

In the present invention, C: 0.020 massQ /. Ti: 0. OlOmass. /. The above is a method for melting ultra-low carbon Ti deoxidized steel containing Ca: 0.0005 mass% or more. The molten steel is sequentially decarburized and deoxidized with Ti. Is added to adjust the inclusions in the molten steel (dioxide inclusions; the same shall apply hereinafter) to a predetermined composition. Molten steel to be treated after decarburization is generally discharged from a converter or electric furnace. Decarburization is preferably performed in a vacuum degassing facility. Ti deoxidation is preferably performed in the same vacuum degassing facility. Ca addition may be performed in the same vacuum degassing equipment, or may be made common with Ήdeoxidation treatment in a ladle. As the vacuum degassing equipment for performing such a series of treatments, the RH vacuum degassing equipment is particularly desirable, but the use of other vacuum degassing equipment such as VOD (Vacuum Oxygen Decarburization) equipment is not prohibited. In the ultra-low carbon steel produced by the present invention, if the C content exceeds 0.020 mass%, the deep drawability of the product cannot be ensured, so the C content is set to 0.020 mass% or less. The lower limit of the C amount is not particularly limited.

Also, if the Ti content is less than 0.001 mass%, the deoxygenation ability by Ti is weak and the total oxygen concentration becomes high, so the Ti content should be 0.001 mass% or more. On the other hand, if the amount of Ti is too large, a large amount of TiN may be generated and the immersion nozzle may be blocked, so the Ti amount is preferably 0.15 mass% or less.

In addition, when the Ca content is less than 0.0005 mass%, the CaO concentration in the inclusion does not become 5 to 50 mass%, the inclusion melting point becomes high, and nozzle clogging is likely to occur. 0005 mass% or more. On the other hand, when the Ca content exceeds 0.0050 mass%, the CaO concentration of inclusions exceeds 50 mass%, and the inclusions easily contain sulfur in the liquid phase. As a result, when the liquid phase inclusions are solidified, CaS is generated around the inclusions, which becomes the starting point of the initiation of the steel plate, and the amount of the steel plate is significantly increased. For this reason Ca The amount is preferably 0.0050 mass% or less.

The other steel compositions are not particularly limited because they do not affect the main problem to be solved of the present invention in the manufacture of molten steel pieces. A preferable composition for the steel sheet will be described later. In the present invention, in a vacuum degassing facility such as an RH vacuum degassing facility, the molten steel is first decarburized, and then Ti is added to the molten steel after the decarburizing treatment to remove the deoxidation (Ti deoxidation treatment). A1 content (mass%) [% 1] and 1 content (thigh 35%) [% Ti] deoxidized molten steel with a composition satisfying [% A1] ≤ [% Ti] ZlO And Ti is conveniently added in the form of Ti-containing alloys such as Fe-Ti alloys.

When deoxidation molten steel outside this composition range, it becomes A1 deacidification not the Ti deoxidation, A1 ₂ 0 ₃ clusters in large quantities produced. If, A1 ₂ 0 ₃ also be增Caro the Ti concentration was added to Ti-containing alloy thereafter remained to a sufficient reduction Sarezu, clustered inclusions in the steel. Thereafter, performs the composition control of inclusions by adding Ca, inclusions produced together with easily become a starting point of Hatsu鲭becomes _{_{_{CaOAl 2 0 3, Al 2 0}}} 3 inclusions cluster one is reacted in It becomes a huge CaO 'Al ₂ O ₃ inclusion. Therefore, deoxidized molten steel satisfies [% Al] ≤ [% Ti] / lO. *

(Slag composition after Ti deoxidation treatment)

Then added Ca to the Ti deoxidation of molten steel, composition of inclusions of Ti oxides in molten steel: 90 mass% or _{less, CaO: 5~50mass%, A1 2} 0 3: a composition of 70 mass% or less of the low melting point . As a result, it is possible to effectively prevent the oxide inclusions from adhering to the inner surface of the immersion nozzle during continuous fabrication, and to prevent the immersion nozzle from being clogged (nozzle clogging). It is convenient to add Ca in the form of an inclusion composition adjusting alloy containing Ca (hereinafter referred to as “Ca-containing flux”).

Here, the purpose and effect of the above Ti deoxidation treatment and subsequent Ca addition will be explained. First, the molten steel is deoxidized with Ti (for example, a Ti-containing alloy such as an Fe-Ti alloy) to generate inclusions mainly composed of Ti oxides. The inclusions obtained in this way do not form a cluster as when deoxidized with A1, but are present in a state of being dispersed in the steel in the form of granules having a size of about 5 to 20 μm. If the A1 concentration in the steel is the same as the result of A1 deoxidation due to a certain level, a huge A1 ₂ 0 ₃ cluster is formed. In this case, even if Ti-containing alloy is added later to increase the Ti concentration, the already produced A1 ₂₀ ₃ cluster remains as a cluster-like inclusion without disappearing.

For this reason, in the present invention, it is necessary to first deoxidize molten steel with Ti to produce Ti oxide. Force generated by Ti oxide inclusions with Ti ₂ O ₃ ≥80mass% due to Ti deoxidation as described above. These inclusions should be dispersed in steel with a size of about 5 to 20 μm and be granular. Therefore, surface defects when cold-rolled steel sheets are produced from this steel are reduced. However, in the case of ultra-low carbon steel, because of the high solidification temperature of the steel, the Ti oxide is in a solid state in the molten steel, and this oxide is continuously forged in the form of taking in the metal. For this reason, oxide and metal are attached to the inner surface of the submerged nozzle and grow, which causes clogging of the nozzle.

Therefore, in the present invention, after deoxidizing with the Ti-containing alloy, Ca (for example, Ca-containing flux) is further added to the deoxidized molten steel. By adding Ca, the composition of oxide inclusions in the molten steel, Ti oxide: 90 mass% or _{less, CaO: 10~50mass%, A1 2} 0 3: containing 70 mass% or less of the low melting point Ti oxide It can be changed to a low melting point inclusion. In other words, by changing to such low melting point inclusions

8.

In this way, it is possible to effectively prevent the Ti oxide adhering to the inner surface of the immersion nozzle from growing.

Ca addition to Ti deoxidized molten steel may be performed in a ladle after Ti deoxidation treatment, or may be added to the vacuum tank during vacuum degassing treatment (after deoxidation treatment). Addition is performed by the former method.

As the Ca-containing flux, for example, it is preferable to use one or more of CaSi, CaNi, CaAl, CaFe and the like. By appropriately adjusting the amount of the applied force, inclusions having the above-described composition can be obtained. It is done.

The reasons for limiting the inclusion composition adjusted by adding Ca are as follows.

If the Ti oxide concentration of inclusions exceeds 90 mass%, the melting point of the inclusions will not sufficiently decrease, and the inclusions will adhere to and accumulate on the inner surface of the immersion nozzle. Cause. Therefore, the Ti oxide concentration is 90 mass% or less, preferably 80 mass% or less. On the other hand, Ti oxides concentration Teire, and since A1 ₂ 0 ₃ concentration will increase, it is preferred that Ti oxide concentration of the inclusions is not less than 20 mass%, further preferably at least 30 mass% . Ti oxides concentration in the inclusions, the Ti content contained in the inclusions were measured by EPMA or EDX, it shall be calculated in terms of Ti ₂ 0 _3.

When the CaO concentration of inclusions exceeds 50 mas _S %, the inclusions tend to contain sulfur in the liquid phase. As a result, when the liquid phase inclusions are solidified, CaS is generated around the inclusions, which becomes the starting point of the initiation of the steel plate, and the amount of generation of the steel plate increases significantly. On the other hand, if the CaO concentration is less than 5 mass%, the melting point of the inclusions does not decrease sufficiently, and the inclusions adhere to the inner surface of the immersion nozzle * and cause nozzle clogging. Therefore, the CaO concentration is 5 to 50 mass%, preferably 7 to 50 mass%, and more preferably 15 to 50 mass%. The CaO concentration in inclusions is calculated by measuring the amount of Ca contained in inclusions with EPMA or EDX and converting to CaO.

When the A1 ₂ 0 ₃ concentration of inclusions exceeds 70 mass%, the inclusions have a high melting point composition. As nozzle clogging easily occurs and inclusions become clustered, non-metallic inclusion physical defects in the steel sheet increase. Even if the concentration of A1 ₂ 0 ₃ is low, there is no problem, but from the viewpoint of cost, it is advantageous to use A1 as part of the deoxidation. The A1 ₂₀ ₃ concentration in inclusions is calculated by measuring the amount of A1 contained in inclusions with EPMA or EDX and converting to A1 ₂ 0 ₃ .

Inclusions, Ti oxides as described above, in addition to CaO, A1 ₂ 0 _3, may contain an oxide which inevitably mixed, for example, 5 mass% degree or less MgO, Si0 ₂ and 20 thigh ss% The following may be included. MgO concentration and Si_〇 ₂ concentration in the inclusions, the Mg content and Si content contained in the inclusions were measured by EPMA and E DX, respectively shall be calculated in terms of MgO and Si0 _2.

(Slag composition after Ti deoxidation)

(a. Total Fe concentration and MnO concentration)

Ca-containing flux is usually added to the molten steel in the ladle using an iron-clad wire or injection lance. An iron-coated wire is a wire in which alloy powder is coated with a thin steel plate, and this wire is supplied into the molten steel. In the method using an injection lance, alloy powder is blown into the molten steel through the injection lance. '

When the addition of Ca to the molten steel, the molten steel is stirred vigorously, and narrowing wind-slag present on the molten steel, FeO in the slag, MnO, molten steel is reoxidized by reaction with the oxide such as Si_〇 _2, The amount of oxide inclusions in the molten steel increases significantly. Therefore, in the present invention, the total Fe (T. Fe) concentration and MnO concentration in the ladle slag after Ti deoxidation treatment of the molten steel (that is, before Ca addition) is set to 10 mass% or less. This suppresses reoxidation of the molten steel from Ca-added calories to continuous mirroring and reduces the amount of oxide inclusions in the flakes, so that the quality of the cold-rolled steel sheet can be sufficiently increased in the end. It becomes. The internal quality of a cold-rolled steel sheet can be evaluated by, for example, the thickness distortion rate of a cracked part in a bulge test. Figure 1 shows the total (% T. Fe) of total Fe concentration (mass%) and MnO concentration (mass%) in ladle slag after Ti deoxidation treatment of the molten steel to the composition specified by the present invention. ) + (% MnO) (horizontal axis: mass%) and the thickness strain rate (vertical axis:%) of the cracked part in the bulge test of the cold-rolled steel sheet with the molten steel force . -In this test, a Ti-containing ultra-low carbon steel was melted as described below, and the hot slab (cold rolling) and hot rolling (cold rolling) were performed on the slab obtained by continuous forging. A cold-rolled steel sheet was obtained. In order to reduce FeO and MnO in the slag, A1% was added as needed to the molten steel (300 tons) that was removed from the converter and placed in the ladle. Further, in order to control the slag composition after RH vacuum degassing treatment was添Ka卩CaO, Al ₂ 〇 _3, a TiO if necessary. A1 滓 (aluminium dross) is used to dissolve aluminum. It is a by-product generated on the surface and is often used as an additive in the finishing process. Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized so that the component composition of the molten steel was C: 0.0007—0.0150 mass% and the oxygen concentration was 120 to 700 thigh sppm. Then, the molten steel to 0. The A1:! ~ 1. 2kgZ molten steel ton and (. Amount per molten steel 1 ton or less the same)添Ka卩reduced the dissolved oxygen concentration in the soluble steel to 30~400mass _P pm . The A1 concentration of the molten steel at this time was 0.001 to 0.005 mass%. Furthermore, Fe-70mass% Ti alloy was added to the molten steel at 0 · 8 to 2.0kg / toned molten steel, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was as follows: Ti concentration 0.020-0.080 mass%, A1 concentration 0. 001 to 0.006 mass%, total oxygen concentration 20 to: LOOmassppm, and [% AI] ≤ [% Ti] Zl0 was satisfied. RH vacuum degassing treatment (deacidification) slag composition during ladle later, CaO concentration: 20~60mass%, Si0 ₂ concentration: 5~20mass%, A1 ₂ 0 ₃ concentration: 10~50mass%, Ti0 ₂ Concentration: 1 ~: L0mass%, MgO concentration: 2 ~ 15mass%, Total Fe concentration: l ~ 10mass%, MnO concentration: 0.5 ~ 5mass% The force ratio (% CaO) / (% Si0 ₂ ) ≥1. The slag composition was measured by fluorescent X-ray analysis.

After RH vacuum degassing treatment, supply 20 ~ 35mass% Ca-60 ~ 75mass% Si alloy to molten steel in the ladle by iron-coated wire (Ca-Si alloy amount) 0.1 ~ 0.4kgZ molten steel ton added, the composition of the inclusions in the molten steel Ti oxides: 30~70mass%, CaO: 6~50mass% , A1 2 0 3: was adjusted to 10~70mass%. The Ca concentration in the molten steel was over 0.0005 mass%. The molten steel melted as described above was continuously forged using a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle. The produced slab was hot-rolled to a thickness of 2 to 4 mm, and further cold-rolled to a thickness of 0.6 to 1.0 mm to obtain a cold-rolled steel plate.

As shown in Fig. 1, by setting the (% T.Fe) + (% MnO) in the ladle slag to 10 mass% or less, the plate thickness distortion rate in the bulge test is set to 50% or more. It becomes possible to do. More preferably, (% T.Fe) + (% MnO) is 5 mass ° / ₀ or less. The lower limit of (% T.Fe) + (% MnO) is not particularly limited.

In the present invention, the plate thickness distortion rate of the cracked portion in the bulge test of the cold-rolled steel plate is obtained as follows. The 200mm square samples were extracted ten cold-rolled steel sheet having a thickness of 0. ⁶ 1.0 mm, inflated hydraulically to break these samples. Measure the thickness of the fractured portion Then, by dividing by the initial plate thickness, the strain rate in the plate thickness direction is calculated, and with the minimum strain rate out of 10 points

This is referred to as “plate thickness distortion rate”. It is desirable that the higher the plate thickness distortion rate, the less the internal defects (in this case, large oxides ^: existence), the better the quality, and the plate thickness distortion rate be 50% or more. . To reduce (% T.Fe) + (% MnO) in ladle slag to 10 mass% or less (or a more suitable value), for example, depending on the amount of slag flowing out of the converter power, A1 May be added.

(b. the ratio of the CaO concentration and Si0 ₂ concentration)

Also from the same viewpoint as above, the mass ratio of CaO concentration and Si0 ₂ concentration of ladle slag after the molten steel was Ti deoxidation (% CaO) / (% Si0 2) one or more.

2, the weight ratio of the ladle slag after treatment Ti deoxidation so the composition defined by the present invention a molten steel (% CaO) / (% Si0 ₂₎ (horizontal axis), obtained from the molten steel This shows the relationship with the thickness strain rate (vertical axis:%) of the cracked part in the pulge test of the obtained cold-rolled steel sheet. In this test, a Ti-containing ultra-low carbon steel was melted as follows, and a cold-rolled steel sheet was obtained through hot rolling and cold rolling from a piece obtained by continuous forging. . The mass ratio (% CaO) / (% Si0 ₂ ) was adjusted by adding lime to the molten steel (300 tons) that was removed from the converter and placed in the ladle. A1 A was added as needed to reduce FeO and MnO in the slag. Further, in order to control the slag composition after RH vacuum degassing treatment, the addition of _{_{CaO, A1 2 0 3, Ti0}} 2 as necessary.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized so that the component composition of the molten steel was C _: 0.0007 ^ 0.0150 mass% and the oxygen concentration was 120-700 sppm. Next, add A1 to the molten steel. 〜1.2kg / mol copper ton was added to reduce the dissolved oxygen concentration in molten steel to 30 ~ 400 massppm. The A1 concentration of the molten steel at this time was 0.001 to 0.005 mass%. In addition, Fe-70mass% Ti alloy was added to the molten steel 0.8 to 2. OkgZ molten steel ton, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was as follows: Ti concentration 0.020-0.080 mass%, A1 concentration 0.001 to 0.006 mass%, total oxygen concentration 20 to: lOOmassppm, and [% A1] ≤ [% Ti] / l0 was satisfied. RH vacuum degassing treatment (Datsusansho sense) slag composition during ladle later, CaO concentration: 20~60mass%, Si0 ₂ concentration: 5~20mass%, A1 ₂ 0 ₃ concentration: 10~ 5 Omass%, Ti0 ₂ concentration: 1 to: L Omass%, MgO concentration: 2 to 15 mass%, total Fe concentration: l to 8 mass%, MnO concentration: 0.5 to 4 mass%, all (% T. Fe) + 7 with (% MnO) ≤10 mass%.

After RH vacuum degassing treatment, 20 ~ 35mass% Ca-60 ~ 75mass% Si alloy is added to the molten steel in the ladle. Supplyed by iron-coated wire, 0.1 to 0.4 kgZ molten steel ton added, and the composition of inclusions in the molten steel is Ti oxide: 30 to 70 mass%, CaO: 6 to 50 mass%, A1 ₂ 0 ₃ : Adjusted to 10-70 mass%. The Ca concentration in the molten steel was over 0.0005 mass%. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the submerged nozzle, and the molten steel throughput during mirror fabrication was set to 2 to 6 tonZmin. The forged slab was hot-rolled to a thickness of 2 to 4 mm, and further cold-rolled to a thickness of 0.6 to 1.0 mm to obtain a cold-rolled steel plate.

As shown in FIG. 2, by the in ladle slag _{(% Caq / (% SiO 2} ) one or more, the thickness distortion of the crack portion in the bulge tested can be 50% or more Further, more preferable (% CaO) / (% Si0 ₂ ) is 2 or more, more preferably 2.5 or more, and the upper limit of (% CaO) Z (% Si0 ₂ ) is not particularly limited. Usually, the maximum is around 6.0 To make (% CaO) / (% Si0 ₂ ) in ladle slag 1 or more (a more suitable value), for example, Just add lime to the steel flow!

(c. Ti_〇 ₂ concentration)

Furthermore, in the present invention, the Ti0 ₂ concentration of ladle slag after the molten steel was Ti deoxidation or more lmass%. This reduces the rate of Ti reoxidation and suppresses the increase in the amount of oxide inclusions, making it possible to increase the thickness strain rate in cold-rolled steel bulge tests to 50% or more.

3, Ti0 ₂ concentration of ladle slag after treatment Ti deoxidation so the composition defined by the present invention a molten steel (horizontal axis: mass%) and, of the molten steel force resulting cold rolled steel sheet This shows the relationship with the thickness distortion rate (vertical axis:%) of the cracked part in the bulge test. In this test, a Ti-containing ultra-low carbon steel was melted as follows, and a cold rolled steel sheet was obtained from a piece obtained by continuous forging through hot rolling and cold rolling. Against soluble steel encased in a ladle and tapped from the converter (300 ton), and the Ti- Fe alloy in accordance with the oxygen concentration was adjusted to添Ka卩to Ti0 ₂ concentration. A1 A was added as needed to reduce FeO and MnO in the slag. Further, in order to control the slag composition after RH vacuum degassing treatment, the addition of _{_{CaO, A1 2 0 3, Ti0}} 2 as necessary.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized so that the component composition of the molten steel was C: 0.0007-0.150 mass% and the oxygen concentration was 120-700 massppm. Next, add A1 to the molten steel from 0.1 to 1; 1.2 kg / mol steel ton, and adjust the dissolved oxygen concentration in the molten steel to 30 to 400. Reduced to massppm. The A1 concentration of the molten steel at this time was 0.001-0.005 mass%. In addition, Fe-70mass% Ti alloy was added to the molten steel 0.8 to 2. OkgZ molten steel ton, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was as follows: Ti concentration 0.020-0.080 mass%, A1 concentration 0. 001-0. 006 mass%, total oxygen concentration was 20-100 massppm, and [% A1] ≤ [% Ti] ZlO was satisfied. RH vacuum degassing treatment (Datsusansho sense) slag composition during ladle later, CaO concentration: 20~60mass%, Si0 ₂ concentration: 5~20mass%, A1 ₂ 0 ₃ concentration: 10~50mass%, Ti0 ₂ concentration ::! ~ 10mass%, MgO concentration: 2-15mass%, total Fe concentration: l-8mass%, MnO concentration: 0.5-4mass% Forces are all mass ratio (o / oCaC Z / oSiC ≥ 1, (% T.Fe) + (% MnO) ≤10mass /.

After RH vacuum degassing treatment, supply 20 ~ 35mass% Ca-60 ~ 75mass% Si alloy to the molten steel in the ladle by iron coated wire, add 0.1 ~ 0.4kgZ molten steel ton, and intervene in the molten steel Ti oxides the composition of the object: 30~70mass%, CaO: 6~50mass% , A1 2 0 3: was adjusted to 10~70mass%. The Ca concentration in the molten steel is 0.0005 mass. /. That was all. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was set at 2 to 6 ton / min. The forged slab was hot-rolled to a thickness of 2 to 4 nun, and further cold-rolled to a thickness of 0.6 to 1.0 mm to obtain a cold-rolled steel plate.

As shown in FIG. 3, by setting the Ti_〇 ₂ concentration in the ladle slag Lmass% or more, the thickness distortion of the crack portion in Paruji test it is possible to more than 50%. A more preferable Ti02 concentration is ₂ mass% or more, more preferably 3% or more. The upper limit of Ti_〇 ₂ concentration is not essential limitation in particular, it is usually at most about 10%. The Ti0 ₂ concentration in the ladle slag to Lmass% or more, for example, may Re to adding Ti in response to the oxygen concentration.

(d. A1 ₂ 0 ₃ concentration)

Furthermore, in the present invention, the A1 ₂ 0 ₃ concentration of ladle slag after the molten steel is treated Ti deoxidation to 10~50mass%.

4, A1 ₂ 0 ₃ concentration of ladle slag after treatment Ti deoxidation so the composition defined by the present invention a molten steel (abscissa: mas _S%) and was obtained from the molten steel cold This shows the relationship with the thickness distortion rate (vertical axis:%) of the cracked part in the bulge test of the rolled steel sheet. In this test, a Ti-containing ultra-low carbon steel was melted as follows, and a cold rolled steel sheet was obtained from a piece obtained by continuous forging through hot rolling and cold rolling. Respect converter tapped to a ladle put was dissolved steel from (300 ton), to adjust the A1 ₂ 0 ₃ concentration by the addition of A1 slag. A1 A was added as needed to reduce FeO and MnO in the slag. Further, in order to control the slag composition after RH vacuum degassing treatment, the addition of _{_{CaO, A1 2 0 3, Ti0}} 2 as necessary.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten copper was decarburized so that the component composition of the molten steel was C: 0.0007—0.0150 mass% and the oxygen concentration was 120-700 massppm. Next, 0.1 to 1.2 kgZ molten steel ton was added to the molten steel, and the dissolved oxygen concentration in the molten steel was lowered to 30 to 400 massppm. The A1 concentration of the molten steel at this time was 0.001-0.005 mass%. In addition, Fe-70mass% Ti alloy was added to the molten steel 0.8 to 2. OkgZ molten steel ton, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was as follows: Ti concentration 0.020—0.0080 mass%, A1 The concentration was 0.001 to 0.006 mass%, the total oxygen concentration was 20 to 100 thigh ssppm, and [% Al] ≤ [% Ti] Zl0 was satisfied. RH vacuum degassing treatment (Datsusansho sense) slag composition during ladle later, CaO concentration: 20~60mass%, Si_〇 ₂ concentration: ⁵ ~ 20mass%, Ti_〇 ₂ concentration: l ~ 10mass%, MgO concentration: 2 to 15 mass%, total Fe concentration: l to 8 mass%, MnO concentration: 0.5 to 4 mass%, both of which are mass ratio (%. & 0) / (% 310 ₂ ) 1, (% T. Fe) + (% MnO) ≤10 mass ^o / _o .

After RH vacuum degassing treatment, supply 20 ~ 35mass% Ca-60 ~ 75mass% Si alloy to the molten steel in the ladle by iron coated wire, add 0.1 ~ 0.4kgZ molten steel ton, and intervene in the molten steel Ti oxides the composition of the object: 30~70mass%, CaO: 6~50mass% , A1 2 0 3: was adjusted to 10~70mass%. The Ca concentration of the molten steel was 0.0005 mass% or more. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was 2 to 6 ton / min. The forged slab was hot-rolled to a thickness of 2 to 4 mm, and further cold-rolled to a thickness of 0.6 to 1.0 mm to obtain a cold-rolled steel plate.

As shown in FIG. 4, by the A1 ₂ 0 ₃ concentration in the ladle slag and 10 mass% or more, the slag is low melting point, absorption capacity of the slag oxide inclusions are increased, The amount of oxide inclusions can be reduced. Further, by the A1 ₂ 0 ₃ concentration below 50mass%, Al ₂ 0 ₃ concentration of oxide inclusions in it is possible to suppress to become 70 mass% greater, coarsening of the oxide inclusions Can be prevented. As a result, the thickness distortion rate of the cracked part in the bulge test should be 50% or more. It becomes possible. In order to set the A1 ₂₀ ₃ concentration in the ladle slag to 10 to 50 mass%, for example, the amount of A1 滓 added may be adjusted.

(Preliminary deoxidation and Ti deoxidation time)

Furthermore, in the present invention, prior to Ti deoxidation of the molten steel after decarburization treatment, one or more selected from Al, Si, and Mn are added to perform preliminary deoxidation, thereby It is preferable that the dissolved oxygen concentration be 200 mass ppm or less in advance. By this treatment, the amount of oxide inclusions can be reduced, so that the thickness distortion rate of the cracked portion in the bulge test of the cold rolled steel sheet is further improved. This preliminary deoxidation is preferably performed by vacuum degassing.

Figure 5 shows the dissolved oxygen concentration (white circles and black circles) and Ti deoxidation treatment time (horizontal axis: minutes) of the molten steel before Ti deoxidation treatment, and the cracks in the bulge test of the cold-rolled steel sheet obtained from the molten steel. This shows the relationship with the plate thickness strain rate. The dissolved oxygen concentration of molten steel before Ti deoxidation treatment: 50 to 200 massppm (white circles in the figure) is a test example in which preliminary deoxidation was performed as needed. The dissolved oxygen concentration of molten steel before Ti deoxidation treatment: More than 200 to 500 Massppm (black circle in the figure) is a powerful test example without preliminary deoxidation. In this test, a Ti-containing ultra-low carbon steel was melted as follows, and a cold rolled steel sheet was obtained from a piece obtained by continuous forging through hot rolling and cold rolling. Respect converter tapped to a ladle put was dissolved steel from (300 ton), to adjust the A1 ₂ 0 ₃ concentration by the addition of A1 slag. A1 A was added as needed to reduce FeO and MnO in the slag. Further, in order to control the slag composition after RH vacuum degassing treatment was添Ka卩the _{_{CaO, A1 2 0 3, Ti0}} 2 as necessary.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized so that the component composition of the molten steel was C: 0.0007 to 0.0150 mass% and the oxygen concentration was 120 to 700 mass _P p: m. Next, when preliminary deoxidation was performed, 0.1 to 1.2 kgZ of molten steel ton was added to the molten steel, and the dissolved oxygen concentration in the molten steel was reduced to 50 to 200 massppm. The A1 concentration of the molten steel at this time was 0 · 001–0.005 mass%. Then, Fe-70 mass% Ti alloy was added to the molten steel in an amount of 0.8 to 2.0 kg / toned molten steel, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was Ti concentration 0 · 02 0 to 0.080 mass%, A1 concentration 0.001—0.006 mass%, total oxygen concentration was 20-100 massppm, and [% A1] ≦ [% Ti] / 10 was satisfied. Slag assembly in ladle after RH vacuum degassing (deoxidation) T JP2009 / 056,835 formed is, CaO concentration: 20~60mass%, Si0 ₂ concentration: 5~20mass%, A1 ₂ 0 ₃ concentration: 10~50mass%, Ti 0 ₂ concentration: 1~: L0mass%, Mg_〇 Concentration: 2~15mass%, total Fe concentration: l~8mass%, MnO concentration force which was a 0. 5~4mass% S, both the mass ratio (% CaO) / (% Si0 2) ≥l, (% T. Fe) + (% MnO) ≤10 mass%.

After RH vacuum degassing treatment, supply 20-35 mass% Ca-60-75 mass% Si alloy to the molten steel in the ladle with iron-covered wire, add 0.1-0.4 kgZ molten steel ton, and intervene in the molten steel the composition of the objects of the Ti oxide: 30~70mass%, CaO: 6~50mass% , A1 2 0 3: was adjusted to 10~70mass%. The Ca concentration in the molten steel was over 0.0005 mass%. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was 2 to 6 ton / min. The forged slab was hot-rolled to a thickness of 2 to 4 mm, and further cold-rolled to a thickness of 0.6 to 1.0 mm to obtain a cold-rolled steel plate.

As shown in Fig. 5, the pre-deoxidation before Ti deoxidation treatment reduces the dissolved oxygen concentration of the molten steel to 200 massppm or less in advance, thereby suppressing the formation of oxide inclusions and the cracking in the bulge test. It is possible to further improve the plate thickness distortion rate. On the other hand, excessive preliminary deoxidation may increase the risk of nozzle clogging, so the presence or absence and degree of preliminary deoxidation may be appropriately selected according to the need for inclusion suppression. Further, as shown in FIG. 5, the Ti deoxidation treatment time (RH treatment time after addition of the Ti-containing alloy) is preferably 5 minutes or longer, whereby the effects of the present invention can be appropriately obtained, and cold rolling is performed. It is possible to increase the plate thickness distortion rate of the cracked portion in the bulge test of the steel plate to a desired level. The upper limit of Ti deoxidation time is not particularly limited, but it is usually about 5 minutes or less from the viewpoint of operation efficiency.

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

Next, a method for producing a Ti-containing ultra-low carbon steel piece by continuously forging the molten steel produced by the above-described method of the present invention will be described.

When the molten steel is poured into the vertical mold in continuous forging, the oxide inclusions contained in the molten steel penetrate deep into the unsolidified layer of the flakes and are trapped in the solidified shell. In addition, in order to prevent oxide inclusions from adhering to the immersion nozzle, an inert gas such as Ar gas is blown into the immersion nozzle, and the bubbles of this inert gas rise in the molten steel. In the process, it is trapped by the solidified shell due to the turbulence of the molten steel flow near the molten steel surface in the vertical mold. These oxide inclusions and gas trapped in the sepal Bubbles cause surface flaw defects in thin steel sheets. In many cases, oxide inclusions are attached to the inert gas bubbles, and the oxide inclusions are trapped in the solidified shell together with the inert gas bubbles.

For such a problem, several types of strip manufacturing methods as described below are effective. That is, in the first method for producing a Ti-containing ultra-low carbon steel piece, when the molten steel is poured into the mold from the tundish through the immersion nozzle installed at the bottom of the tundish in the continuous forging apparatus, It is preferable to forge the molten steel without blowing gas (inert gas such as Ar or non-oxidizing gas such as N ₂ ) into the molten steel flowing down the nozzle. By melting molten steel by the method of the present invention described above, it is possible to prevent nozzle clogging without blowing gas into the molten steel flowing down the immersion nozzle. In addition, by not injecting gas, the occurrence of bubble defects in the flakes due to gas injection is suppressed, and surface defects such as heges, slivers, and scales are greatly increased in the final product, cold-rolled steel sheet and plated steel sheet. It can be reduced.

Further, in this method for producing a Ti-containing ultra-low carbon steel piece, it is preferable to produce with a throughput of 4 tonZmin or less. Figure 6 shows the throughput of continuous forging (horizontal axis: tonZmin) and the plate thickness strain rate in the bulge test of cold-rolled steel sheets using the steel slab obtained by continuous forging (vertical axis:% ). In this test, a Ti-containing ultra-low carbon steel was melted as follows, and a cold rolled steel sheet was obtained through hot rolling and cold rolling from a piece obtained by continuous forging. Relative to tapped from the converter molten steel encased in a ladle (300 ton), to adjust the A1 ₂ 0 ₃ concentration by添Ka卩the A1 slag. A1 A was added as needed to reduce FeO and MnO in the slag. Further, in order to control the slag composition after RH vacuum degassing treatment was添Ka卩the _{_{CaO, A1 2 0 3, Ti0}} 2 as necessary.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized so that the component composition of the molten steel was C: 0.0007 to 0.0150 mass% and the oxygen concentration was 120 to 700 massppm. Next, AI was added to the molten steel to 1 to 1.2 kgZ molten steel ton, and the dissolved oxygen concentration in the molten steel was reduced to 30 to 400 massppm. The A1 concentration of the molten steel at this time was 0.001 to 0.005 mass%. In addition, Fe-70mass% Ti alloy was added to the molten steel 0.8 to 2. OkgZ molten steel ton, and Ti deoxidation treatment was performed. In this Ti deoxidation treatment, the RH vacuum degassing treatment was completed 2 to 15 minutes after the addition of the Fe-Ti alloy, and the composition of the molten steel after completion was as follows: Ti concentration 0.020-0.080 mass%, A1 concentration It was 0.001 to 0.006 mass% and the total oxygen concentration was 20 to 100 massppm, and [% A1] ≤ [% Ti] / l0 was satisfied. RH vacuum degassing treatment (Datsusansho sense) slag composition during ladle later, CaO concentration: 20~60mass%, Si_〇 ₂ concentration: 5~20mass%, A1 ₂ 0 ₃ Concentration: 10-50 mass%, Ti0 ₂ concentration: 1-: 10 mass%, MgO concentration: 2-15 mass%, Total Fe concentration: l-8 mass%, MnO concentration: 0.5-4 mass% the ratio (% CaO) Z (% Si_〇 ₂₎ ≥ 1, was (% T. Fe) + ( % MnO) ≤10mass%.

After RH vacuum degassing treatment, supply 20 ~ 35mass% Ca-60 ~ 75mass% Si alloy to the molten steel in the ladle with iron-covered wire. 0:! ~ 0.4kg Molten steel the composition of the inclusions Ti oxides in: 30~70mass%, CaO: 6~50mass% , A1 2 0 3: was adjusted to 10~70mass%. The Ca concentration in the molten steel was over 0.0005 mass%. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. This continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was set at 2 to 6 ton / min. The forged slab was hot-rolled to a thickness of 2 to 4 mm, and further cold-rolled to a thickness of 0.6 to 1. Onrni to obtain a cold-rolled steel plate.

As shown in Fig. 6, forging with a throughput of 4 tonZmin or less, the amount of oxide inclusions is reduced, and as a result, the thickness distortion rate of cracks in the bulge test of cold-rolled steel sheets is improved. The second method for producing Ti-containing ultra-low carbon steel pieces is as follows: (i) stirring the molten steel in the mirror mold by electromagnetic force generated by a moving magnetic field and / or an oscillating magnetic field; (ii) It is preferable to apply one or both of applying a static magnetic field to the molten steel and damping the molten steel flow. According to such a manufacturing method, the amount of oxide inclusions trapped in the solidified shell without floating and separating in the vertical mold is reduced, and as a result, the thickness distortion of the crack portion in the bulge test of the cold rolled steel sheet is reduced. The rate is further improved. In addition, a particularly excellent effect can be obtained by performing both G) and (ii).

In the method of applying the moving magnetic field (alternating magnetic field) of (i) above, an alternating moving magnetic field applying device is installed, and the molten steel in the mold is horizontally swirled and stirred by the electromagnetic force of this magnetic field applying device. Forging. As a result, trapping of oxide inclusions in the solidified shell is suppressed, and a clean piece with less oxide inclusions can be obtained. It is also effective to apply a non-moving oscillating magnetic field as an alternating magnetic field and stir the molten steel in the mold. It is also effective to apply an oscillating magnetic field that accompanies horizontal movement (moving oscillating magnetic field).

In the method (ii) of applying a static magnetic field, a static magnetic field application device is installed at a position surrounding the discharge flow of molten steel from the discharge hole of the immersion nozzle, and the static magnetic field application device applies a static magnetic field for discharge. Decrease the flow velocity. As a result, the floating of the oxide inclusions is promoted, the trapping in the solidified shell is suppressed, and a clean flake with few oxide inclusions is obtained. In FIG. 6, “application of moving magnetic field” (black circle) is the test example of (i) above, and “application of static magnetic field” (black square) is the test example of (ii) above. As shown in Fig. 6, when the stirring or melting of the molten steel flow by applying the magnetic field (i) or (ii) above is performed on the molten steel in the bowl, compared to the case where no magnetic field is applied. In addition, the thickness distortion rate of the cracked part in the bulge test is further improved.

When performing both (i) and (ii) above, it is preferable to apply the moving magnetic field and the Z or oscillating magnetic field above the static magnetic field, and it is also preferable to apply it near the molten metal surface.

To錄造throughput below _4ton Bruno _min is effective in producing how the second containing Ti ultra-low carbon steel铸片.

(Other composition of steel to be melted)

Next, preferable conditions for the contents of the main components other than C, Ti and Ca described above in the composition of the Ti-containing ultra-low carbon steel produced by the present invention will be described. However, these exemplify preferred steel compositions from the viewpoint that the benefits of the improvement effect can be enjoyed more than the above-described surface quality / interior quality improvement effect.

The amount of Si is preferably 0.5 mass% or less. When the Si content exceeds 0.5 mass%, the material properties of the thin steel sheet, which is a product, deteriorate, and when used as a matte steel sheet, the surface properties are likely to deteriorate due to deterioration of the tackiness. When emphasizing these characteristics, it is more preferable to reduce the Si content to 0.2 mass% or less.

Incidentally, at the 31 and the mass ratio of 7 (% 31) (% Ding 1) 50, inclusions Si_〇 ₂ is produced in, since stronger character as Shirikonki field steel, (% Si) Z (% Ti) is preferably 50.

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

The Mn content is preferably 2.0 mass% or less. When the amount of Mn exceeds 2.0 wt%, the material is easily cured. Preferably it is 1.5 mass% or less, more preferably 1.0 mass% or less, and even more preferably 0.5 mmass% or less.

When the mass ratio of Mn to Ti (% Mn) / (% Ti) ≥100, ΜηΟ is generated in the inclusions and the properties of Mangan killed steel become stronger, so (% Mn) / (% Ti 100 is preferred.

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

The amount of S is preferably not more than 0.050 mass%. When the S content exceeds 0.050 mass%, CaS and REM sulfides increase in the molten steel, and defects are likely to occur in the thin steel sheet that is the product. Preferably it is 0.030 mass% or less. The lower limit of the amount of S is not particularly limited. The amount of P is preferably 0.005-0.12 mass%. If P is contained in a large amount, the amount of segregation at the grain boundary increases and grain boundary embrittlement occurs, and it is desirable to reduce it as much as possible, especially in the thin steel sheet as a product, which causes deterioration of secondary work brittleness resistance. Preferably it is 0.050 mass% or less. However, even if the amount of P is made lower than 0.005 mass%, further improvement of the material cannot be expected, and the melting cost increases. On the other hand, it is acceptable if it is 0.12 mass% or less.

The amount of N is preferably 0.0005-0.0040 mass% mass%. N is a force that should be reduced as much as possible to improve deep drawability in the thin steel plate product, as in C. Even if the content is lower than 0.0005 mass%, further improvement of the material cannot be expected. Conversely, the cost of melting increases. On the other hand, when it exceeds 0.0040 mass%, the material deterioration in the thin steel sheet begins to increase. However, in steel plates where strength is important, N may be added up to an upper limit of about 0.0200 mass%, but there is no particular problem even if such steel is used. Depending on the purpose, one or more of the following elements may be appropriately selected and added.

• Nb: 0.1 OOmass% or less ··· Improves deep drawability of thin steel sheet

• B: 0.050 mass% or less · · · Improves secondary work brittleness of thin steel sheet

• Mo: l.0mass% or less ····· Increase the tensile strength of thin steel sheet

• Sb: 0.0200 mass% or less · · · Prevents nitriding during slab heating

• Ce: 0.0050mass% or less · ■ 'Slower nozzle clogging due to lower melting point of inclusions

• La: 0.0050 mass% or less · · 'Nozzle clogging is more stably suppressed by lowering the melting point of inclusions.

Furthermore, if necessary, one or more selected from Ni, Cu, and Cr may be added within a range of 01 mass% or less. Addition of these elements can improve the corrosion resistance of the steel sheet.

Alloy elements other than the above may be added as appropriate as long as the total is up to about 1%.

The balance is Fe and inevitable impurities. 〔Example〕

[Invention Example 1]

In order to reduce FeO and MnO in the slag, 400 kg of A1ton was added to the molten steel (300 tons) that was removed from the converter and placed in the ladle, and the slag composition after vacuum degassing was changed. CaO was added for control.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel is decarburized, and the composition of the molten steel is changed to C: 0.0010% s%, Si: 0.01 mass%, Mn: 0.15 mass%, P: 0.015 mass%, S: 0.005 mass. %, Oxygen concentration: 500 massppm (the balance is Fe and inevitable impurities. The same applies to the molten steel composition after decarburization), and the molten steel temperature was adjusted to 1600 ° C. Next, 0.5 kg of molten steel ton was added to the molten steel, and the dissolved oxygen concentration in the molten steel was reduced to 120 massppm. The AI concentration of the molten steel at this time was 0.002 mass%. Furthermore, Fe-70 mass ° / ₀ Ti alloy was added to the molten steel by 1. OkgZ molten steel ton, and Ti deoxidation treatment was performed for 7 minutes. In this Ti deoxidation treatment, the vacuum degassing treatment was completed 7 minutes after the addition of the Fe-Ti alloy, and the Ti concentration of the molten steel in the ladle at the end was 0.040 mass%, and the A1 concentration was 0.002 mass%. The oxygen concentration was 30 massppm. Further, the slag composition in the ladle after the vacuum degassing treatment (deacidification) is, CaO concentration: 35 mass%, Si0 ₂ concentration: 15mass%, A1 ₂ 0 ₃ concentration: 35 mass%, Ti_〇 ₂ concentration: 3 mass% MgO concentration: 7 mass%, total Fe concentration: 2 mass%, MnO concentration: 2 mass% (other unavoidable acids: lmass%).

After the vacuum degassing treatment, 30 mass% Ca-70mass% Si alloy was added to the molten steel in the ladle by adding 0.3 kgZ of molten steel ton with an iron-coated wire to control the composition of inclusions in the molten steel. The Ca concentration of the molten steel is 0.OOlOmass. /. Met. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. As a result of investigating the form and composition of inclusions in the tundish at the time of fabrication, it was a spherical inclusion of 70 mass% Ti ₂ 0 ₃ —15 mass% CaO—15 mass% Al ₂ 0 ₃ . Continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was set at 3.8 ton Zmin. There was almost no deposit on the inner surface of the immersion nozzle after fabrication.

The fabricated slab is hot-rolled to a thickness of 3.5 mm, then cold-rolled to a thickness of 0.8 mm, and then subjected to continuous annealing under annealing conditions of 780 ° CX for 45 seconds. It was. In the annealed plate thus obtained, only 0.2 / 1000 m non-metallic inclusion physical properties and bubble defects were observed. Furthermore, the thickness distortion rate of the cracked part in the bulge test of the cold-rolled steel sheet was 50%, which was good. [Invention Example 2]

In order to reduce FeO and MnO in the slag, 500 kg of A1ton was added to the molten steel (300 tons) that was removed from the converter and placed in the ladle, and the slag composition after vacuum degassing was changed. and添Ka卩CaO, the Ti_〇 ₂ to control.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel is decarburized, and the composition of the molten steel is changed to C: 0.0015 mass%, Si: 0.01 mass%, Mn: 0.1 mass%, P: 0.012 mass%, S: 0.006 mass%, The oxygen concentration was 450 massppm, and the molten steel temperature was adjusted to 1600 ° C. Then, the A1 to the molten steel by adding 0. 4KgZ molten steel ton, were made as low a dissolved oxygen concentration in the molten steel to 150masspp _m. The A1 concentration of the molten steel at this time was 0.002 mass%. Furthermore, Fe-70 mass% Ti alloy was added to the molten steel by 1.2 kg / molten steel ton, and Ti deoxidation treatment was performed for 6 minutes. In this Ti deoxidation treatment, the vacuum degassing treatment was completed 6 minutes after the addition of the Fe-Ti alloy, and the Ti concentration of the molten steel at the end was 0.045% ss%, the A1 concentration was 0.002 mass%, total oxygen The concentration was 30 massppm. Moreover, the slag composition in the ladle after the vacuum degassing treatment (deacidification) is, CaO concentration: 30 mass%, Si0 ₂ concentration: 17mass%, A1 ₂ 0 ₃ concentration: 40 mass%, Ti0 ₂ concentration: 2mass% MgO concentration: 8 mass%, total Fe concentration: lmass%, MnO concentration: 2 mass%.

After the vacuum degassing treatment, 0.25kg / mol ton of 30mass% Ca-70mass% Si alloy was added to the molten steel in the ladle with an iron-coated wire to control the composition of inclusions in the molten steel. The Ca concentration in the molten steel was 0.0005 mass%. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. As a result of investigating the morphology and composition of inclusions in the tundish during the fabrication, it was found to be spherical inclusions of 70 mass% Ti ₂ 0 ₃ —12 mass% CaO—18 mass% Al ₂ 0 ₃ . Continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was 4. Oton Zmin. In the vertical type, the molten steel was stirred by an electromagnetic stirring device having a moving magnetic field. There was almost no deposit on the inner surface of the immersion nozzle after fabrication.

The fabricated slab was hot-rolled to a thickness of 3.5 mm, further cold-rolled to a thickness of 0.8 mm, and then subjected to continuous annealing at 780 ° C x 45 seconds. The annealed sheet obtained in this way had a force with only 0.2 and 1000 m non-metallic inclusions and bubble defects. Furthermore, the thickness distortion rate of the cracked portion in the bulge test of the cold-rolled steel sheet was 55%, which was good.

[Invention Example 3]

FeO and MnO in slag are reduced to molten steel (300 tons) put out from the converter and placed in the ladle For this purpose, 300 kg of A1 was added, and CaO was added to control the slag composition after vacuum degassing.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized, and the composition of the molten steel was changed to C: 0.0015 mass%, Si: 0.01 mass%, Mn: 0.12 mass%, P: 0.015 mass%, S: 0.006 mass%, The oxygen concentration was 400 massppm, and the molten steel temperature was adjusted to 1600 ° C. Next, A1 was added to the molten steel at 0.4 kg / mol steel ton, and the dissolved oxygen concentration in the molten steel was lowered to lOOmassppm. The A1 concentration of the molten steel at this time was 0.002 mass%. Furthermore, Fe-70mass% Ti alloy was added to the molten steel by 1. lkgZ molten steel ton, and Ti deoxidation treatment was performed for 5 minutes. In this Ti deoxidation treatment, the vacuum degassing treatment was completed 5 minutes after the addition of the Fe-Ti alloy, and the Ti concentration of the molten steel at the end was 0.042 mass%, the A1 concentration was 0.002 mass%, and the total oxygen concentration was It was 30 massppm. Moreover, the slag composition in the ladle after the vacuum degassing treatment (deacidification) is, CaO concentration: 42mass%, Si_〇 ₂ concentration: 13mass%, A1 ₂ 0 ₃ concentration: 30 mass%, Ti0 ₂ concentration: 4 mass% MgO concentration: 6 mass%, total Fe concentration: lmass%, MnO concentration: 2 mass% (other inevitable oxides: 2 mass%). After the vacuum degassing treatment, 0.2 mass / ton of 30mass% Ca-70mass% Si alloy was added to the molten steel in the ladle with iron-coated wire, and the composition of inclusions in the molten steel was controlled. The Ca concentration of the molten steel was 0.0006 thigh ss%. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. As a result of investigating the form and composition of inclusions in the tundish at the time of fabrication, it was a spherical inclusion of 7 ² mass% Ti ₂ 0 ₃ —12 mass% CaO—16 mass% Al ₂ O ₃ . Continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing through the immersion nozzle, and the molten steel throughput during forging was 4. Oton Zmin. In addition, a static magnetic field by a direct current magnetic field was applied to the molten steel in the vertical mold to brake the molten steel flow. There was almost no deposit on the inner surface of the immersion nozzle after fabrication.

The fabricated slab was hot-rolled to a thickness of 3.5 mm, further cold-rolled to a thickness of 0.8 mm, and then subjected to continuous annealing at 780 ° C x 45 seconds. In the annealed sheet thus obtained, only 0.2 / 1000 m non-metallic inclusion physical properties and bubble defects were observed. Furthermore, the thickness distortion rate of the cracked portion in the bulge test of the cold-rolled steel sheet was 55%, which was good.

[Invention Example 4]

The molten steel melted under the same conditions as in Invention Example 1 was continuously forged using a two-strand slab continuous forging device to produce a flaw (the form and composition of inclusions in the tundish during forging were the same as in Inventive Example 1) . In the continuous mirror device, a static magnetic field application device was installed at a position 500 nm below the lower end of the discharge hole of the immersion nozzle. In addition, the shape of the discharge hole of the immersion nozzle was a square of 80mm length and width. In continuous forging of molten steel, the Ar gas blowing flow rate in the immersion nozzle is set to 0 to LONLZmin, and the magnetic field strength (DC static magnetic field) applied by the static magnetic field application device is in the range of 0.1 to 0.3 Tesla. A slab having a width of 12,000 to 1,500 mm and a thickness of 250 mm was produced at a pouring speed of 4.5 to 6.0 tons Ζπώι.

The forged slab was hot-rolled and cold-rolled to form a thin steel plate, and this thin steel plate was hot dip galvanized. The hot dip galvanized steel sheet obtained in this way is capable of producing a clean slab both on the surface and inside by applying a static magnetic field during the fabrication with very few inclusions and bubble surface defects. Was confirmed.

[Invention Example 5]

The molten steel melted under the same conditions as in Invention Example 1 was continuously forged using a two-strand slab continuous forging machine to produce a flake (the form and composition of inclusions in the tundish at the time of forging were as in Inventive Example 1. The same). In the continuous forging machine, an AC moving magnetic field applying device was installed at a position 2m from the molten steel surface of the vertical mold. In addition, the shape of the discharge hole of the immersion nozzle was a square of 80 mm in length and width.

In continuous casting of molten steel, the Ar gas blowing flow rate in the immersion nozzle is set to 0 to 10 NL / min, and the magnetic field strength (AC moving magnetic field) applied by the AC moving magnetic field application device is in the range of 0.05 to 0.2 stellar. Slabs with a width of 1200 to 150011 ^ 1 and a thickness of 250 mm were fabricated at a pouring rate of 4.5 to 6.0 tons Zmin.

The forged slab was hot-rolled and cold-rolled to form a thin steel plate, and this thin steel plate was hot dip galvanized. The hot-dip galvanized steel sheet obtained in this way is capable of forging a clean slab both on the surface and inside by applying an AC moving magnetic field during fabrication with very few inclusions and bubble surface defects. Was confirmed.

[Invention Example 6]

Based on the steel composition of Invention Example 1,

A: Molten steel further containing Nb = 0.010 mass%, B = 0.0010 mass%, and

B: Molten steel further containing Mo = 0.0100 mass%

The other conditions were the same as in Invention Example 1, and an annealed sheet was manufactured.

In both A and B, only 0.2 / 1000m of non-metallic inclusion physical properties and bubble defects were observed on the annealed plate. Furthermore, the thickness distortion rate of the cracked part in the bulge test of cold-rolled steel sheet is 50%, which is good. [Comparative Example 1]

In order to reduce FeO and MnO in the slag, 100 kg of A1 滓 was added to the molten steel (300 tons) that was removed from the converter and placed in the ladle.

Next, the following series of treatments were performed in the RH vacuum degassing facility. First, the molten steel was decarburized, and the composition of the molten steel was changed to C: 0.0010 mass%, Si: 0.01 mass%, Mn: 0.15 mass% P: 0.15 mass%, S: 0.005 mass%, oxygen. Concentration: 500 mass _P pm, and the molten steel temperature was adjusted to 1600 ° C. Then 0. 3KgZ molten steel ton添Ka卩the A1 to molten steel was made as low a dissolved oxygen concentration in the molten steel to 220mass _P pm. The molten steel concentration at this time was 0.002 mass%. Furthermore, Fe-70mass% Ti alloy was added to the molten steel by 1.2kg / molten steel ton, and Ti deoxidation treatment was performed for 7 minutes. In this Ti deoxidation treatment, the vacuum degassing treatment was completed 7 minutes after the addition of the Fe-Ti alloy. At the end, the Ti concentration of the molten copper was 0.035 mass%, the Al concentration was 0.001 mass%, and the total oxygen concentration Was 40 massppni. Moreover, the slag composition in the ladle after the vacuum degassing treatment (deacidification) is, CaO concentration: 23mass%, Si0 ₂ concentration: 27mass%, A1 ₂ 0 ₃ concentration: 20 mass%, Ti0 ₂ concentration: 0. 8 mass%, MgO concentration: 9 mass%, total Fe concentration: 8 mass%, MnO concentration: 6 mass% (other inevitable oxides: 6.2 mass%).

After the vacuum degassing treatment, 30mass% Ca-70mass% Si alloy was added to the molten steel in the ladle by 0.2kgZ molten steel with iron-coated wire, and the composition of inclusions in the molten steel was controlled. The molten steel melted as described above was continuously forged with a two-strand slab continuous forging device to produce a piece. As a result of investigating the morphology and composition of inclusions in the tundish during the fabrication, it was a spherical inclusion of 70 mass% Ti ₂ 0 ₃ — 15 mass% CaO — 15 mass% Al ₂ 0 ₃ . Continuous forging was performed without blowing gas such as Ar or N ₂ into the molten steel flowing down the immersion nozzle, and the molten steel throughput during forging was set to 4.8 ton / min. There was almost no deposit on the inner surface of the immersion nozzle after fabrication.

The fabricated slab was hot-rolled to a thickness of 3.5 nun, further cold-rolled to a thickness of 0.8 mm, and then subjected to continuous annealing under annealing conditions of 780 ° C x 45 seconds. In the annealed sheet thus obtained, 0.5 non-metallic inclusions and 0.5 bubble defects were observed. Furthermore, the plate thickness distortion rate of the cracked part in the bulge test of the cold-rolled steel sheet was 25%, which was poor.

[Comparative Example 2]

An annealed plate was manufactured under almost the same conditions as in Comparative Example 2, except that the A1 deoxidation (500 massppm → 220 massppm) time before Ti deoxidation treatment was set to 120 seconds (a ₀ Zt = 4.2 ). The thickness distortion rate of the cracked portion in the bulge test of the annealed plate thus obtained was 30%, which was an improved force compared to Comparative Example 1, but did not reach the target level of the present application. Non-metallic inclusions and cellular Defects were found to be 0.4 / 1000m. Industrial use possible 'I' raw

According to the method for melting Ti-containing ultra-low carbon steel of the present invention, the composition of oxide inclusions in the molten steel is optimized and the amount of inclusions is reduced. For this reason, it is possible to prevent clogging of the smashing nozzle (nozzle clogging) due to oxide inclusions during continuous forging, as well as cold-rolled steel sheets with excellent surface properties and quality, especially oxide inclusions and bubbles. Ti-containing ultra-low carbon steel that can produce cold-rolled steel sheets with few surface defects due to the above and high resistance to press cracking caused by oxide inclusions can be produced.

Further, according to the method for producing a Ti-containing ultra-low carbon steel piece of the present invention, by optimizing the continuous forging conditions, from the Ti-containing ultra-low carbon steel produced by the above-described melting method, A piece that can further enhance the surface properties and quality of the steel sheet can be produced.

Claims

The scope of the claims

1. In melting ultra-low carbon Ti deoxidized steel containing C: 0.020 mass% or less, Ti: 0.010 mass% or more, Ca: 0.0005 mass% or more,

By decarburizing the molten steel, then adding Ti to the ladle and deoxidizing it, the A1 content (mass%) and Ti content (mass%) are [% A1] ≤ [ % De] deoxidized molten steel with a composition satisfying ZlO,

After that, by adding Ca to the deoxidized molten steel in the ladle, the inclusion composition in the molten steel is Ti oxide: 90 mass% or less, CaO: 5 to 50 mass%, A1 ₂ 0 ₃ : 70 mass% or less And adjust

In the ladle slag after adding the Ti and deoxidizing the molten steel,

'Total Fe concentration and MnO concentration are less than 10 mass%,

• CaO concentration Si_〇 ₂ concentration mass ratio of (% CaO) Z (% Si0 2) one or more,

.Ti0 ₂ concentration over lmass%, and

• A1 ₂₀ ₃ concentration 10-50mass%

A method for melting Ti-containing ultra-low carbon steel. ·

2. A method for melting Ti-containing ultra-low carbon steel according to claim 1,

C: 0.020 mass% or less, Ti: 0.010 mass% or more, Ca: 0 · 0005 mass% or more, Si: 0.2 mass% or less, Mn: 2. Omass. /. Hereafter, S: 0.050 mass% or less, P: 0.005 to 0.12 mass%, N: 0.0005-0.0040 mass%, the method of melting ultra-low carbon Ti deoxidized steel composed of the balance Fe and inevitable impurities.

3. A method for melting Ti-containing ultra-low carbon steel according to claim 2,

In addition to the composition according to claim 2, an ultra-low carbon Ti deoxidized steel containing at least one of Nb: 0.100 mass% or less, B: 0.050II ^ SS% or less, Mo: 1.0 mass% or less is melted. how to.

4. A method for melting Ti-containing ultra-low carbon steel according to any one of claims 1 to 3,

After decarburizing the molten steel, prior to deoxidizing by adding Ti, by adding one or more selected from Al, Si and Mn and pre-deoxidizing, A method in which the dissolved oxygen concentration is set to 200 massppm or less in advance.

5. A method for melting Ti-containing ultra-low carbon steel according to any one of claims 1 to 3,

A method in which the deoxidation time of molten steel is 5 minutes or longer by adding Ti.

6. A method of continuously forging molten steel melted by the melting method according to any one of claims 1 to 5 to produce a flake piece,

Ti-containing ultra-low carbon steel steel forging molten steel without injecting gas into the molten steel flowing down the immersion nozzle when the molten steel is poured into the mold from the tundish through the immersion nozzle installed at the bottom of the tundish A manufacturing method of a piece.

7. A method of continuously forging molten steel produced by the melting method according to any one of claims 1 to 5 to produce a flake piece,

A method for producing Ti-containing ultra-low carbon steel pieces, in which molten steel in the mold is agitated by electromagnetic force generated by a magnetic field.

8. A method of continuously forging molten steel produced by the melting method according to any one of claims 1 to 5 to produce a flake piece,

A method for producing Ti-containing ultra-low carbon steel pieces that applies a static magnetic field to the molten steel in the mold and brakes the molten steel flow.

9. A method for continuously producing molten steel produced by the melting method according to any one of claims 1 to 5 to produce a flake piece,

A method for producing Ti-containing ultra-low carbon steel pieces that stirs molten steel in a mold by electromagnetic force generated by a magnetic field and applies a static magnetic field to the molten steel to brake the molten steel flow.

10. A method for producing a Ti-containing ultra-low carbon steel piece according to claim 7,

A method of forging molten steel without blowing gas into the molten steel flowing down the immersion nozzle when the molten steel is poured into the mold from the tundish through the immersion nozzle installed at the bottom of the tundish.

11. A method for producing a Ti-containing ultra-low carbon steel piece according to claim 8,

A method of forging molten steel without injecting gas into the molten steel flowing down the immersion nozzle when the molten steel is poured into the mold from the tundish through the immersion nozzle installed at the bottom of the tundish.

12. A method for producing a Ti-containing ultra-low carbon steel flake according to claim 9,

A method of forging molten steel that does not blow gas into the molten steel flowing down the immersion nozzle when the molten steel is poured into the mirror mold from the tundish through the immersion nozzle installed at the bottom of the tundish.

13. A method for producing a Ti-containing ultra-low carbon steel flake according to claim 6,

A method of continuously forging molten steel at a throughput of 4ΐοηΖπΰη or less.