JP2009270165A - Extra-low carbon steel sheet, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for industrially mass-producing an extra-low carbon steel sheet made of a Ti-deoxidized steel with an extra-low carbon concentration and an extra-low Al concentration which has excellent surface properties while maintaining its deep drawability, and can solve the clogging of an immersion nozzle upon continuous casting and deterioration in the surface properties of the steel caused thereby by allowing the steel to have an inclusion oxygen concentration in a prescribed amount and to contain TiO<SB>x</SB>based inclusions with a predetermined composition. <P>SOLUTION: Disclosed is an extra-low carbon steel sheet having a chemical composition comprising 0.0005 to 0.025% C, 0.003 to 0.12% Si, 0.05 to 2.5% Mn, ≤0.15% P, ≤0.02% S, ≤0.006% N, 0.0002 to 0.003% sol.Al, 0.005 to 0.05% Ti and 0.005 to 0.20% Nb, having the total oxygen concentration T.O of 0.003 to 0.008% and an oxygen concentration Oinc caused by inclusions of 0.0025 to 0.007%, and the balance Fe with impurities, and in which the content of SiO<SB>2</SB>as oxide based inclusions in the steel is <1.0%, ≥90.0% is composed of a ternary system of TiOx, Al<SB>2</SB>O<SB>3</SB>and MnO, and the compositional ranges in the ternary system lie in 50.0 to 95.0% TiOx, 3.0 to 35.0% Al<SB>2</SB>O<SB>3</SB>and 2.0 to 25.0% MnO. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

The present invention relates to an ultra-low carbon steel sheet and a method for producing the same, specifically, containing a predetermined amount of total oxygen concentration in steel and oxygen concentration due to inclusions (hereinafter referred to as inclusion oxygen concentration), and having a specific composition. By including TiO _x- based inclusions, while maintaining the steel plate properties such as strength, elongation, and deep drawability, the surface properties are excellent, and the immersion material nozzle clogging during continuous casting and the steel surface properties caused thereby The present invention relates to an ultra-low carbon steel plate made of Ti deoxidized steel having an ultra-low carbon concentration and an ultra-low Al concentration, and a method for mass-producing this on an industrial scale.

  Since molten steel is in a state containing a large amount of oxygen in the steel making process, deoxidation is generally performed in order to remove excess oxygen in a process prior to continuous casting. The most popular deoxidation method is to add molten aluminum with a strong affinity for oxygen and Al that can be used in large quantities on an industrial scale to reduce the oxygen concentration in the steel. Al deoxidized steel has been widely put into practical use. This Al deoxidized steel is generally used also in steel materials widely used for automobile bodies and the like, particularly in ultra-low carbon steel sheets having a particularly low carbon concentration as a thin plate.

  Thin steel sheets, mainly for automotive applications, have a high balance of various properties such as strength, elongation, and weldability, and are in a beautiful state with no wrinkles on the surface even after hot and cold rolling. It is required to be. Since such beauty is left to sensual judgment, it is a strict and difficult problem to deal with. On the other hand, thin steel sheets, mainly for automotive applications, are positioned not only as high-grade steels that require such various properties, but also as mass-produced steel materials, so the above-mentioned various properties are highly balanced. At the same time, mass production of a thin steel sheet with a beautiful surface at a large-scale steelworks is more difficult than imagined.

One of the causes of impairing the surface properties of the rolled thin steel sheet is the presence of Al ₂ O ₃ -based inclusions generated by Al deoxidation on the surface. Since the Al ₂ O ₃ inclusions have a cluster-like shape called a cluster, they cause so-called sliver wrinkles that extend linearly on the surface of the rolled steel material. Moreover, this cluster inclusion also becomes a factor which obstruct | occludes the immersion nozzle used for continuous casting. In order to reduce this blockage, Ar gas blown from the upper part of the immersion nozzle disturbs the flow of the molten steel in the continuous casting mold, so that the surface properties of the continuous casting slab deteriorate, and as a result, a thin steel plate produced by rolling thereafter. It becomes a factor which also impairs the surface property of the. Furthermore, in the ultra-low carbon steel for thin steel plates, a state containing oxygen is also necessary for the decarburization treatment, and therefore, treatment for sufficiently removing Al ₂ O ₃ inclusions generated in the subsequent deoxidation step. Since applying time requires time and cost, it is not suitable for mass production.

As a method for suppressing these adverse effects caused by employing Al deoxidized steel, an invention using non-Al deoxidized steel for thin steel sheets is disclosed.
For example, in Patent Document 1, after adjusting free oxygen in molten steel to 50 to 150 ppm, adding Ti so as to contain 0.003 to 0.020% as a molten steel component, and continuously casting, Al: 0 An invention relating to a method for producing a low carbon steel material of 0.003% or less (in this specification, “%” means “mass%” unless otherwise specified) is disclosed. However, this invention is only intended to suppress blow holes caused by CO gas generated during continuous casting, which is a concern due to Ti deoxidation used as non-Al deoxidized steel.

In Patent Document 2, Al and Si are added to a decarburized molten steel containing C: 0.005% or less and Mn: 1.0% or less to form a semi-deoxidized molten steel. In addition, a Ti-containing material was added to further deoxidize to obtain a molten steel containing Al: 0.005% or less, Si: 0.20% or less, Ti: 0.01 to 0.10%. The main component of inclusions in molten steel is melted as a composite oxide of Ti, Al, and Si, then this molten steel is continuously cast, and then subjected to hot rolling and cold rolling, and then the obtained cold rolled steel sheet The invention which concerns on the method of manufacturing an ultra-low-carbon cold-rolled steel sheet is disclosed by carrying out continuous annealing in the temperature range of 700 degreeC-Ac _three points. This method suppresses the Al concentration, adds Si, further controls the amount of Ti added, and melts the molten steel in which the main component of inclusions is a composite oxide of Ti, Al, and Si. It is intended to suppress the generation of clusters and clogging of the immersion nozzle. However, in the present invention, it is considered difficult to control inclusions that do not allow the complex oxide of Mn that is inevitably produced while requiring the complex oxide of Si.

  Patent Document 3 contains C: 0.0001 to 0.0070%, acid-soluble Al: 0.005% or less, and Ti: 0.004 to 0.040%, and the balance Fe and unavoidable impurities. Thin steel plate with excellent defects and excellent press formability, containing titanium oxide, manganese oxide, silicon oxide and alumina composite oxide inclusions containing 30% or less of alumina. And an invention relating to the manufacturing method thereof is disclosed.

  This invention does not produce alumina in steel by restricting acid-soluble Al, but by controlling the oxygen concentration before Ti deoxidation and composite deoxidation with Ti-containing metals, titanium which tends to become coarser It suppresses the generation of oxide alone. The inclusion composition aimed here is an oxide system in which titanium oxide, manganese oxide, silicon oxide, and alumina are combined, and alumina is 30% or less, as described in paragraph 0008 of Patent Document 3. In addition, a composite inclusion that has a relatively low melting point and can suppress the formation of a hard crystallization phase with a high melting point upon cooling is obtained. The composition other than alumina is not specified, but from the description of Table 4 in paragraph 0019, the composition is easy to be crushed by rolling due to the fact that Si oxide is essential and the content of Ti oxide is low. It is thought that it is desired.

  However, according to the knowledge of the present inventors, the soft inclusions exhibit a shape extended by rolling, and deteriorate the Rankford value, which is an index of elongation and deep drawability, which are the main characteristics of a thin steel sheet. It is understood that the invention disclosed by Patent Document 3 containing an oxide is not an invention that can be implemented as it is.

Furthermore, in Patent Document 4, C: 0.01% or less, sol. Al: 0.0001 to 0.01%, excluding Ti contained as oxide-based non-metallic inclusions Ti (1): 0.003 to 0.1%, further Ti (l), sol . The contents of Al, Mn, C and N are defined by the relational expression, and 70% or more of the oxide-based nonmetallic inclusions having a width of 1 μm or more are mainly composed of TiOx, MnO and Al ₂ O ₃ and in the inclusions. A thin steel sheet containing 3% or more of TiOx, MnO, and Al ₂ O ₃ and containing the Si oxide described in Patent Document 3 when the inclusion composition ratio satisfies a predetermined relationship And a deoxidation method thereof.

As described in paragraphs 0015 to 0021 and the like of this invention, the low melting point composition of oxide inclusions in molten steel, that is, the oxide inclusion composition is reduced to a low melting point TiOx—Al ₂ O ₃ —MnO type. Alternatively, the composition of TiOx—Al ₂ O ₃ —MnO—SiO ₂ system (however, SiO ₂ content ≦ 5%) has a good surface quality that does not cause surface flaws and prevents a decrease in elongation, etc. The present invention relates to a thin steel plate excellent in workability and a melting method capable of preventing clogging of an immersion nozzle during continuous casting.

However, in the present invention, as described in paragraphs 0058 and 0059, the SiO ₂ concentration in inclusions is basically limited in order to achieve a low melting point. In addition, it was understood that the composition form of inclusions in the molten steel stage and the composition form of inclusions in the rolled steel stage were almost the same, but when the present inventors further researched, depending on the melting conditions Rather, it has been found that there is a difference between the two. That is, when the oxide inclusion having the low melting point composition is maintained as it is to the rolled steel material, it becomes a shape extended in the rolling process as described above. For this reason, even in accordance with the present invention, various properties of the thin steel sheet, particularly surface properties and properties such as elongation in the direction perpendicular to rolling, vary, and further improvement is required.
Japanese Patent Publication No. 2-9646 Japanese Patent No. 3422612 Japanese Patent No. 3436857 Japanese Patent No. 3852396

  Under such circumstances, the present inventors previously made Japanese Patent Application No. 2007-054511 contain extremely low carbon, set the Al concentration to an extremely low level, specifically 0.005% or less, and A cold-rolled steel material containing a certain amount of an alloy having an affinity for oxygen such as Ti or the like, even if the alloy components are adjusted to the same strength level, is a Rankford value (hereinafter referred to as “r value”) which is an index of deep drawability. .) Proposed an invention based on the knowledge that it would be high.

  In this study, the inventors have produced a steel containing a certain amount of an alloy having an extremely low carbon, maintaining an Al concentration at an extremely low level, and having an affinity for oxygen such as Ti. In order to achieve both the various steel properties and the beautiful surface properties of the thin steel sheet in the Ti deoxidized steel, the oxide inclusions have a composition with a high TiOx concentration, and the form is a lump-like form that is scattered in isolation. It was found that it is desirable.

  However, the oxide inclusions having such a form cause the clogging of the immersion nozzle and the deterioration of the surface properties of the thin steel plate during the continuous casting as pointed out conventionally. There is a need to prevent.

  The present invention is based on the knowledge that the above problem can be solved by specifying the O content in Ti deoxidized steel and the oxide inclusions in the steel after hot rolling.

In the present invention, C: 0.0005% to 0.025%, Si: 0.003% to 0.12%, Mn: 0.05% to 2.5%, P: 0.15% or less , S: 0.02% or less, N: 0.006% or less, sol. Al: 0.0002% to 0.003%, Ti: 0.005% to 0.05%, Nb: 0.005% to 0.20%, and the total oxygen concentration T.I. O: 0.003% to 0.008% inclusive, oxygen concentration due to inclusions Oinc: 0.0025% to 0.007% inclusive, as an optional additive element, instead of a part of Fe ( One or two selected from the group consisting of I) B: 0.0020% or less, and / or (II) Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 1.0% or less It has a chemical composition comprising more than seeds and the balance Fe and impurities, and the oxide inclusions in the steel material have a SiO ₂ content of less than 1.0%, and 90.0% or more of TiOx, Al ₂ O ₃ and The composition range of the ternary system of MnO is TiOx: 50.0% to 95.0%, Al ₂ O ₃ : 3.0% to 35.0%, MnO: 2 Extremely low carbon characterized by being in the range of 0.0% to 25.0% It is a plate.

  From another point of view, it is a manufacturing method of the ultra-low carbon steel sheet according to the present invention described above, which is performed through refining including converter refining and vacuum refining, and the oxygen concentration of molten steel before Ti adjustment by Ti addition in this refining Is added to the molten steel at a stage of 0.004% to 0.015%, the Ti concentration in the molten steel is 0.005% to 0.05% and the O concentration in the molten steel is 0.003%. It is the manufacturing method of the ultra-low carbon steel plate characterized by performing continuous casting after setting it to 0.008% or less.

According to the present invention, the inclusion oxygen concentration is contained in a predetermined amount, and by including a TiO _x- based inclusion of a specific composition, the surface properties are excellent while maintaining the steel sheet characteristics such as strength and elongation, and deep drawability, Also, an ultra-low carbon steel plate made of Ti deoxidized steel having an ultra-low carbon concentration and an ultra-low Al concentration, which can solve the obstruction of the immersion nozzle during continuous casting and the deterioration of the steel surface properties caused thereby, It is possible to provide a method for mass production on a target scale.

Hereinafter, the best mode for carrying out the present invention will be described. First, the technical idea of the present invention will be described.
In order to investigate the form of oxide inclusions that do not impair the basic characteristics of thin steel sheets with an ultra-low carbon steel composition, the relationship between the strength and elongation, which are the basic characteristics of thin steel sheets, and the form of inclusions A non-Al deoxidized Ti deoxidized steel containing only a very small amount of Al was compared with an Al deoxidized steel having almost the same composition.

  As a result, in the Al deoxidized steel, alumina clusters are generated as oxide inclusions, and the alumina clusters remain on the surface of the thin steel plate, which is the cause of the so-called sliver soot, and the surface properties are significantly impaired. On the other hand, if this alumina cluster exists as a state in which inclusions are crushed by hot rolling and cold rolling and are contained in a sparse dot array, the basic properties of the steel are not limited unless the cleanliness of the steel is significantly worse. Was found to have no effect.

  On the other hand, in Ti deoxidized steel, no community inclusions are generated, so even if the inclusions remain on the surface of the thin steel plate, it does not cause deterioration of the surface properties. However, in the Ti deoxidized steel, the oxide inclusions affect the r value among the properties of the thin steel sheet, that is, the oxide inclusions are extended in the hot rolling process. It was found that the r value also tends to decrease. On the other hand, it has been found that the r-value is rather improved when the oxide inclusions have an isolated lump shape that is difficult to extend even in the hot rolling process. The reason for this is that Ti deoxidized steel is weaker deoxidized than Al deoxidized steel, so that the amount of oxide inclusions itself is large, and oxide inclusions cause heterogeneous nucleation of precipitates. It is estimated that this may be the starting point.

The isolated massive oxide in the Ti deoxidized steel which is difficult to extend has a composition mainly composed of TiO _x . Here, “TiO _x ” is a general term for Ti oxides in steel. Since Ti is capable of forming a tetravalent or trivalent as _valence, considered the presence forms such as _{TiO 2, Ti 3 O 5,} Ti 2 O 3, since there is also a further non-stoichiometric composition, and TiO _x Indicated. This oxide is composed mainly of a Ti oxide such as TiO ₂ and Ti ₂ O ₃ and a component containing Al ₂ O ₃ or MnO. On the other hand, the extended oxide in Ti deoxidized steel is composed of TiO ₂ , Ti ₂ O ₃ , Al ₂ O ₃ , MnO, SiO _2, etc., which has a low liquidus temperature and is soft even at the temperature of hot rolling. Inclusions.

  By the way, it is known that Ti deoxidized steel is easily clogged with an immersion nozzle due to oxide inclusions in a mass production process employing continuous casting depending on conditions. Especially in ultra-low carbon steel, since the oxygen concentration in the molten steel is high due to decarburization until the middle of the refining stage, the time for removing oxide inclusions generated by the subsequent deoxidation is limited. The possibility of nozzle clogging is greatly increased. In order to prevent this, there is a method in which the oxide inclusions in the molten steel are controlled to inclusions exhibiting a liquid phase, but the desired form is inconsistent with the inclusion form for the properties of the thin steel sheet.

Therefore, when the oxide inclusions of Ti deoxidized steel based on the composition of ultra-low carbon steel were investigated, the composition form of inclusions at the stage of molten steel during refining, and the slab or rolling after continuous casting It was found that the composition form of inclusions in the stage may be different. That is, in the molten steel stage, the composition is mainly composed of TiO ₂ and Ti ₂ O ₃ containing a liquid phase, whereas in the stage after the slab, the TiO ₂ and Ti ₂ O ₃ concentrations are further increased. There is a state in which the composition is in the form of a lump. In such a state, when Ti is used as a deoxidizing agent, the temperature dependence of Ti deoxidation affects, and in the process of lowering the temperature, it proceeds in the direction of further deoxidation, that is, Ti oxidation in inclusions. It is understood that the substance is concentrated to change from a state containing a liquid phase to a state of forming a lump. Furthermore, such a state can be realized by satisfying the following four conditions in the state of the rolled steel material after continuous casting.

That is, the technical ideas of the present invention are listed below.
(A) Ti deoxidized steel is used in order to prevent the alumina cluster from deteriorating the surface properties of the thin steel plate, causing the clogging of the immersion nozzle during continuous casting, and further deteriorating the surface properties of the thin steel plate. .

(B) In Ti deoxidized steel, the composition form of inclusions can adversely affect the deep drawability of the thin steel sheet, so the composition form of inclusions is controlled to a massive form that does not deteriorate the basic characteristics of the thin steel sheet.

(C) When the composition form of inclusions is controlled in such a massive form at the molten steel stage, the immersion nozzle is blocked, and mass production of the slab and deterioration of surface properties are induced. Therefore, the molten steel stage includes a liquid phase. In addition, the composition is controlled so as not to cause the clogging of the immersion nozzle, and the inclusions are controlled in a lump form in the casting process and the rolling process by utilizing the temperature dependence of the deoxidation equilibrium of Ti deoxidation.

(D) the condition of the thin steel plate that can control the composition form in inclusions containing a liquid phase in the molten steel stage, and can control the composition form into massive inclusions in the state of rolled steel after continuous casting, and such Identify the steelmaking conditions necessary to obtain steel.

Next, the features of the ultra-low carbon steel plate of the present embodiment will be described.
(Total oxygen concentration T.O: 0.003% to 0.008%)
Total oxygen concentration O is 0.003% or more and 0.008% or less. In weakly deoxidized steel, even after preliminary deoxidation, the steel material still contains oxygen. Here, T.W. If O exceeds 0.008%, even if Ti is added excessively, the oxide inclusions will be in a state where MnO is 25.0% or more and / or SiO ₂ is 1.0% or more. It is difficult to control, and inclusions are brought in with a low melting point composition. On the other hand, the total oxygen concentration T.I. If O is less than 0.003%, the form of inclusions in the molten steel stage cannot realize a liquid phase state, and there may be inclusions with high Al ₂ O ₃ concentration. It becomes impossible to maintain the superiority of the surface texture.

(Oxygen concentration due to inclusion: Oinc: 0.0025% to 0.007%)
The oxygen concentration Oinc due to inclusions is 0.0025% or more and 0.007% or less. In Ti weakly deoxidized steel, not all of the total oxygen concentration contained in the steel becomes oxide inclusions. Even in the weakly deoxidized steel, it is desirable that the deoxidation is sufficiently performed and most of the total oxygen concentration exists as oxide inclusions, and the oxygen concentration Oinc due to the inclusions is 0.0025% or more. . However, since the cleanliness of steel deteriorates when the amount of inclusions is too large, the oxygen concentration Oinc due to inclusions is desirably 0.007% or less.

  The method for measuring the oxygen concentration by inclusions is not particularly limited. For example, the average composition of oxide inclusions is obtained with an energy dispersive X-ray microanalyzer, the particle size distribution is measured with an optical microscope, and the unit area is measured. There is a method in which the amount of inclusion per unit volume is obtained from the amount of inclusions per unit and the oxygen concentration Oinc due to the inclusions is converted from both.

(Oxide inclusions)
Oxide inclusions in the steel material after hot rolling are composed of TiOx, Al ₂ O ₃ , MnO, SiO ₂ , MgO, CaO and impurities. Here, TiOx ranges from x = 1.5 to 2.0, means an oxide configured in other words from TiO ₂ and _Ti 2 _{O 3.} In the composition range of the Ti deoxidized steel of the present invention, since the proportion of TiO ₂ is dominant, the value converted to the TiO ₂ concentration is used.

In the oxide inclusions, the total of TiOx, Al ₂ O ₃ and MnO is 90% or more, and SiO ₂ is less than 1.0%. In order to exhibit a massive form containing TiOx, the content is 90.0% or more. On the other hand, if the SiO ₂ concentration exceeds 1.0%, the inclusion contains a liquid phase, and the inclusion is formed in an extended shape in the rolled steel material. Therefore, the SiO ₂ concentration is less than 1.0%. It is. The CaO and MgO concentrations can be obtained even if a small amount is contained, so that a bulk form can be obtained.

The ratios of TiOx, Al ₂ O ₃ and MnO are as follows: TiOx: 50.0% to 95.0%, Al ₂ O ₃ : 3.0% to 35. 0% or less, MnO: 2.0% or more and 25.0% or less. If the TiOx is less than 50.0%, it becomes inclusions that have been extended in the hot rolling zone and adversely affects the deep drawability of the thin steel sheet. This is because it is not included. Also, if Al ₂ O ₃ is less than 3.0%, the TiOx concentration exceeds 95.0%, or liquid phase inclusions of MnO—TiOx are present, making it difficult to control at the molten steel stage, while 35.0% It is because it will be in the state which does not contain a liquid phase in a molten steel stage when exceeding. Furthermore, if MnO is less than 2.0%, inclusions are in a state not containing a liquid phase at the molten steel stage, and if it exceeds 25.0%, liquid phase inclusions of MnO—TiOx increase. Because.

Next, the reasons for limiting the chemical composition of the ultra-low carbon steel sheet according to the present embodiment will be described.
(C: 0.0005% or more and 0.025% or less)
C combines with a carbide-forming element such as Ti to form TiC, NbC or a composite carbonitride precipitate thereof. For this reason, by optimizing the C content, the volume fraction of these precipitates is limited to improve the formability of the steel, that is, the effect of precipitation strengthening by these precipitates, and solid solution carbon during annealing The deep drawability can be improved by reducing the solid solution nitrogen.

  However, if the C content is less than 0.0005%, sufficient tensile strength cannot be obtained. In the molten steel stage, carbon reacts with oxygen present in the molten steel and is decarburized by removing the reaction product under reduced pressure. For this reason, in order to make C content less than 0.0005%, it will be necessary to perform a long-time vacuum processing, and it is not preferable also from an economical viewpoint. On the other hand, when the carbon content exceeds 0.025%, the yield strength increases, the elongation decreases, and the moldability deteriorates.

  Therefore, the C content is 0.0005% or more and 0.025% or less. In order to secure further moldability, particularly r value, the C content is preferably 0.01% or less. A more preferable range is 0.0010% or more and 0.0030% or less.

(Si: 0.003% to 0.12%)
Si is an inexpensive solid solution strengthening element, and can increase the strength of a thin steel sheet at low cost. Therefore, Si is contained for the purpose of improving the strength. Further, in the case of Ti weakly deoxidized steel, the effects of preliminary deoxidation and complex deoxidation can be expected. However, if SiO ₂ is contained in the inclusions, the original performance of the thin steel sheet may not be exhibited. Therefore, it is desirable that the concentration be low. If the Si concentration exceeds 0.12%, it is difficult to make the SiO ₂ concentration in the inclusions less than 1%. Desirably, it is 0.08% or less, and more desirably 0.05% or less. On the other hand, Si is contained at the stage of the coarse molten steel, and in order to drastically reduce the Si content, a predetermined treatment is required, leading to a decrease in productivity. % Or more. For these reasons, the Si content is set to 0.003% or more and 0.12% or less.

(Mn: 0.05% to 2.5%)
Mn has the effect of increasing the strength of the steel sheet by solid solution strengthening, and in the case of Ti weakly deoxidized steel, it has a function of strengthening deoxidation by the effect of combined deoxidation with Mn. Therefore, the Mn content is 0.05% or more. Desirably, it is 0.10% or more, more desirably 0.15%. On the other hand, when the Mn content exceeds 2.5%, the yield strength and the deterioration of the elongation become remarkable, and wrinkles and cracks are likely to occur during processing. Moreover, the usage-amount of Mn alloy iron also increases and the manufacturing cost increases in order to remove the carbon contained in Mn alloy iron. For this reason, the upper limit of the Mn content is set to 2.5%. A desirable upper limit is less than 0.80%, and a more preferred upper limit is less than 0.31%.

(P: 0.15% or less)
P is a useful element for increasing the strength of a steel sheet by solid solution strengthening, and is contained for the purpose of improving the strength. However, when the P content is excessive, there is a concern about embrittlement due to grain boundary segregation. Moreover, in the steel type which carries out the hot dip galvanization on the surface of a cold-rolled steel plate, alloying processability falls and plating adhesiveness falls, or the streak pattern resulting from P segregation appears on the plating surface. Therefore, the P content is 0.15% or less. It is preferable to make it 0.06% or less. About a lower limit, it is preferable to set it as 0.005% or more from a viewpoint of suppressing the raise of the manufacturing cost required for P removal process.

(S: 0.02% or less)
S is present in the steel sheet as an impurity, but if the S content is excessive, scale wrinkles are likely to occur on the surface of the thin steel sheet, and the appearance quality may be impaired. Therefore, the S content is 0.02% or less. It is preferable to make it 0.01% or less.

(N: 0.006% or less)
When N is contained excessively, the yield strength is increased or stretcher strain is generated, and surface distortion of the thin steel sheet is likely to occur during processing. For this reason, N content shall be 0.006% or less. It is preferable to make it 0.003% or less.

(Sol.Al: 0.0002% or more and 0.003% or less)
Al in steel has a form that does not dissolve in acid used for analysis of oxides and the like, and a form that dissolves in acid such as solid solution or nitride, generally Al content is expressed in an amount soluble in acid, This is sol. Indicated as Al. sol. Since the Al content is related to the amount of dissolved Al in the molten steel stage, it strongly affects the deoxidation of steel and the composition form of inclusions.

In the present invention, the Al ₂ O ₃ concentration needs to be 35.0% or less at the stage of the thin steel plate, but when Al is high, it exceeds this, so that the sol. Al content shall be 0.003% or less. Desirably, it is 0.001% or less.

  On the other hand, Al itself can be used for preliminary deoxidation and temperature adjustment in the manufacturing process of molten steel, and the concentration of alumina inclusions in the thin steel plate is required to be 3.0% or more. 0002% or more. It is preferable to set it as 0.0003% or more.

(Ti: 0.005% to 0.05%)
In the ultra-low carbon steel sheet according to the present embodiment, Ti in the steel is an important element having a function of deoxidizing in the molten steel stage and generating inclusions of TiOx necessary for stabilizing various properties of the thin steel sheet. is there. Further, a part of Ti precipitates as TiN, thereby suppressing the stretcher strain and the yield strength increase due to N, and suppressing the surface distortion during processing. Therefore, the Ti content is set to 0.005% or more.

  On the other hand, if the Ti content exceeds 0.05%, the amount of TiC deposited increases and the elongation deteriorates, and surface distortion and cracking occur during processing. In addition, when hot dip galvanizing is performed on the ultra-low carbon steel sheet according to the present embodiment, there is a problem that a streak pattern is formed on the surface of the plating film. Therefore, the Ti content is 0.05% or less. In addition, since Ti is a relatively expensive element and affects the manufacturing cost, when it is allowed from the viewpoint of maintaining the characteristics of the thin steel sheet and from the viewpoint of preventing the occurrence of surface distortion during processing, the content thereof is set to 0. It is preferable to make it 025% or less.

(Nb: 0.005% to 0.20%)
Nb is in steel and combines with C to form NbC precipitates, improving the mechanical properties of the steel sheet. In particular, in Ti deoxidized steel, the amount of Ti associated with C is not always sufficient, so Nb is essential. In order to obtain such an effect, Nb is contained by 0.005% or more. If the Nb content is less than 0.005%, it may be difficult to stably obtain the tensile strength. In order to obtain a more stable effect, the content is preferably 0.01% or more. On the other hand, when the Nb content exceeds 0.20%, the Nb content becomes excessive with respect to the C content, and on the contrary, the yield strength increases and the elongation decreases. Therefore, the upper limit of the Nb content is 0.20%. A desirable upper limit is 0.05% or less.

  The element listed below as an optional additive element instead of a part of Fe is contained alone or in combination of two or more to further improve the properties of the thin steel sheet while enjoying the effects of the present invention. be able to.

(B: 0.0020% or less)
Since B has the effect | action which prevents secondary work embrittlement, it is preferable to contain. However, when the B content exceeds 0.0020%, the r value is significantly reduced. For this reason, it is desirable that the B content be 0.0020% or less. If the B content is 0.0003% or more and 0.0010% or less, the effect of B addition can be enjoyed efficiently, which is desirable.

(Cu: 1.0% or less, Ni: 1.0% or less, Cr: 1.0% or less)
Cu, Ni, and Cr may all be contained in order to ensure strength. However, even if added excessively, the effect is saturated and the manufacturing cost only increases. Therefore, the contents of Cu, Ni, and Cr are all preferably set to 1.0% or less.

Compositions other than those described above are Fe and impurities.
Next, the manufacturing method of the ultra-low carbon steel plate of this Embodiment is demonstrated in order of a process.

(Refining process including converter refining and vacuum refining, and continuous casting process)
A decarburization process is performed in a steelmaking furnace such as a converter, and the steel is discharged into a container such as a ladle without deoxidization as low carbon molten steel having a C concentration of 0.04% to 0.07%. The molten steel containing C is further transported to a vacuum degassing apparatus such as an RH apparatus and subjected to vacuum decarburization treatment, and becomes an ultra-low carbon molten steel having a C concentration of 0.025% or less. In this decarburization reaction, it is required that the molten steel contains O that reacts with C, and the O concentration is about 0.03% to 0.08% when the steel is discharged from the converter. .

In order to compensate for the molten steel temperature drop during the processing time required for this vacuum decarburization, heat treatment of the molten steel is often performed before and after the vacuum decarburization treatment. In this heat treatment, there is a method of heating the molten steel by an oxidation reaction between a metal such as Al and oxygen gas, or a method of generating an electric arc from a graphite electrode and electrically heating the molten steel by supplying Joule heat, Any may be used in the present invention. In the case of heat treatment by Al oxidation reaction, a large amount of suspension of Al ₂ O ₃ inclusions generated by Al combustion is generated, so in the stage of component adjustment before Ti adjustment described later, molten steel such as reflux It is desirable to remove to the extent that the composition control of the oxide inclusions is not hindered by the stirring operation. On the other hand, the addition of Al also serves as a means for satisfying the trace Al content of the present invention, and the oxygen gas supply serves as a means for adjusting the oxygen concentration before Ti adjustment.

  After decarburization by vacuum refining, the components of elements having a weak affinity with oxygen other than Ti are adjusted. Here, when elements such as Si, Nb, and B are adjusted by alloying iron such as ferrosilicon, ferroniobium, and ferroboron, an auxiliary role of containing a small amount of Al in the molten steel by a small amount of Al contained in these elements. Can be fulfilled.

  After substantially adjusting the components of elements other than Ti, the O concentration is adjusted. Adjustment of O concentration can be carried out by stirring operations such as Si, Mn concentration, O concentration, composition control of ladle slag, addition of slag modifier, molten steel reflux, etc. during component adjustment. For the control of the O concentration, a semi-empirical control method that has been conventionally used, feedback control using an analysis value by a rapid analyzer of O concentration that has been put into practical use in recent years, and the like can be applied.

The O concentration before Ti adjustment is 0.004% or more and 0.015% or less. The reason is that if the O concentration before Ti adjustment is less than 0.004%, the TiOx concentration and / or Al ₂ O ₃ concentration in the oxide inclusions after Ti adjustment and / or other oxides such as MgO In such a state, the concentration cannot be obtained stably in the molten steel stage. On the other hand, if the oxygen concentration exceeds 0.015%, the MnO concentration and / or the SiO ₂ concentration in the inclusions become high, and it becomes impossible to obtain massive inclusions in the rolling stage after solidification.

Further, when the Ti concentration is adjusted to 0.005% or more and 0.05% or less by Ti adjustment, the total oxygen concentration is set to 0.003% or more and 0.008% or less. The reason is as described above.
Thus, by adjusting the total oxygen concentration before and after the Ti adjustment to a predetermined range, the composition form of inclusions at the molten steel stage can be a TiOx-based inclusion containing a liquid phase, which is a continuous casting process. In this way, it is possible to suppress the clogging of the immersion nozzle, and the steel according to the present embodiment can be mass-produced stably. Furthermore, the deterioration of the surface property of the slab induced by the blockage of the immersion nozzle and the injection of Ar gas inside the immersion nozzle performed to prevent this, and the deterioration of the surface property of the rolled steel material based on this slab. Any of these can be suppressed.

  Thus, in the refining process and the continuous casting process of the present embodiment, the molten steel that has been refined has the above-described chemical composition, and the oxygen concentration of the molten steel before Ti adjustment by adding Ti in the refining process is 0.004. Ti is added to the molten steel at a level of not less than 0.01% and not more than 0.015%, so that the Ti concentration in the molten steel is 0.005% to 0.05% and the O concentration in the molten steel is 0.003% to 0.008. % Or less, then continuous casting.

(After solidification in continuous casting)
By performing the above operation, in the rolling stage after solidification in continuous casting, the composition of oxide inclusions is less than 1.0% of SiO ₂ , 90% or more of which is TiOx, Al ₂ O ₃ and MnO. The composition range of a ternary system composed of TiOx, Al ₂ O ₃ and MnO is TiOx: 50.0% to 95.0%, Al ₂ O ₃ : 3.0% to 35.0% , MnO: 2.0% or more and 25.0% or less can be satisfied. Thereby, there is no deterioration of various properties as Ti deoxidized steel, and a thin steel plate having a beautiful surface quality can be produced.

Although the manufacturing method of the steel plate in the subsequent process of continuous casting is not specified, it is preferable to follow the following method.
The steel ingot obtained by continuous casting is reheated, or the hot steel ingot after continuous casting is subjected to hot rolling as it is or after auxiliary heating. In order to keep the surface properties of the steel ingot in good condition, it is preferable that the surface of the steel ingot be kept cold or warm before heating. If the heating temperature is low, the rolling load increases and rolling becomes difficult, so the heating temperature is preferably over 1150 ° C.

The hot rolling conditions are not particularly specified, but the Ar ₃ transformation is used to refine the hot rolled steel sheet grains by performing finish rolling in a low temperature range of austenite and to develop a recrystallized texture preferable for deep drawability during annealing. It is desirable to perform the final reduction in a temperature range of not less than the point (Ar ₃ transformation point + 100 ° C.) and less, and more desirably if the final reduction is performed in the range of 890 ° C. to less than 920 ° C. Moreover, in order to suppress the surface defect of a scale property, it is preferable to ensure the difference of 100 degreeC or more between the start temperature of finish rolling, and the finish temperature of finish rolling.

  In addition, in order to perform finish rolling in these temperature ranges, you may heat a rough rolling material with a bar heater between rough rolling and finish rolling. At this time, it is desirable to suppress the temperature fluctuation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or less by heating so that the rear end of the rough rolled material is higher than the front end. Thereby, the uniformity of the product characteristic in a coil can be improved.

  For the heating of the rough rolled material, for example, a solenoid induction heating device is provided between the rough rolling mill and the finish rolling mill, and the heating temperature is increased based on the longitudinal temperature distribution on the upstream side of the induction heating device. A method of controlling the amount is illustrated.

  After the hot rolling is finished, the steel plate is cooled and wound into a coil. In order to reduce the yield due to the generation of scale, it is desirable to wind up at less than 700 ° C. On the other hand, in order to sufficiently precipitate Ti and Nb carbonitrides after winding and develop a recrystallized texture preferable for deep drawability, the winding temperature is preferably over 610 ° C.

  Cold rolling is performed according to a conventional method after descaling by pickling or the like. In order to develop a recrystallized texture preferable for deep drawability by recrystallization annealing performed after cold rolling, the rolling reduction is preferably set to 70% or more. If the reduction ratio is excessively high, the load on the rolling equipment increases, leading to a decrease in productivity. Therefore, it is preferable that the rolling reduction is less than 90% and the final plate thickness is 0.40 mm or more.

The cold-rolled steel sheet is subjected to a treatment such as degreasing according to a known method, if necessary, and is recrystallized and annealed. If the heating rate at the time of recrystallization annealing is too fast, ferrite becomes finer and ductility is deteriorated. For this reason, it is preferable that the heating rate to soaking temperature shall be less than 60 degreeC / s. In addition, when the annealing temperature is equal to or higher than the Ac ₃ transformation point, the recrystallization texture preferable for deep drawability is reduced by transformation. Therefore, it is preferable that the upper limit of the annealing temperature is less than the Ac ₃ transformation point. In addition, although recrystallization annealing may be performed by either continuous annealing or box annealing, it is preferable to perform continuous annealing from the viewpoint of productivity. Temper rolling may be performed after annealing.

  After annealing, hot dip galvanizing treatment may be performed according to a conventional method. From the viewpoint of productivity and corrosion resistance, it is preferable to perform recrystallization annealing and plating with a continuous hot dip galvanizing apparatus, and to perform alloying treatment. Further, temper rolling may be performed before plating or after plating.

Thus, in this embodiment, SiO ₂ of oxide inclusions in the steel after solidification in continuous casting is less than 1.0%, and 90.0% or more is composed of TiOx, Al ₂ O ₃ and The composition range of the ternary system of MnO is TiOx: 50.0% to 95.0%, Al ₂ O ₃ : 3.0% to 35.0%, MnO: 2 It should be in the range of 0% to 25.0%.

  In this way, the ultra-low carbon steel plate of the present embodiment is manufactured. The ultra-low carbon steel sheet of the present embodiment is made of a Ti deoxidized steel having an ultra-low carbon concentration and an ultra-low Al concentration, contains a predetermined amount of inclusion oxygen concentration, and includes a TiOx-based inclusion having a specific composition. It has excellent surface properties while maintaining properties such as strength, elongation, and deep drawability, and the clogging of the immersion nozzle during continuous casting of the material slab and the deterioration of the surface properties caused thereby. Etc. can also be solved.

Furthermore, the present invention will be described more specifically with reference to examples.
In order to clarify the relationship between the steel composition according to the present invention, the inclusion composition form, and the steel sheet characteristics, the following steel sheets were made as prototypes.

  Steel ingots having the compositions shown in Table 1 were produced by a 17 kg induction heating furnace capable of melting in an inert gas atmosphere. Immediately before casting, a bomb sample having a diameter of 17 mm was taken from the molten steel, and the form of inclusions in the molten steel was investigated. The number of inclusions observed on the cross section is observed with an optical microscope to count the number of inclusions, and the ratio of spherical inclusions that are considered to have exhibited a liquid phase at the molten steel stage with respect to the total number of inclusions is 20% or more. In the case, the inclusion was determined to have a liquid phase.

  The oxygen concentration Oinc due to inclusions is obtained by calculating the average composition of oxide inclusions with an energy dispersive X-ray microanalyzer and measuring the particle size distribution with an optical microscope. The quantity was calculated and calculated from both.

  The cast steel ingot is forged to produce a material having a width of 210 mm, a length of 130 mm, and a thickness of 20 mm, and further maintained at a heating temperature of 1250 ° C., and then hot-rolled at a finishing temperature of 910 ° C. to a thickness of 4.0 mm A hot rolled steel sheet was obtained. Immediately after hot rolling, it is cooled to 650 ° C. by water spray cooling to make it a winding temperature, charged in an electric heating furnace maintained at the same temperature, held for 30 minutes, and then cooled at 20 ° C./h. The furnace was cooled at a speed and the annealing was performed after winding.

  A cross section of the obtained hot-rolled steel sheet, a micro sample was taken in the longitudinal direction at a position of about 100 mm in width, and inclusions observed in the cross section were evaluated. The evaluation method uses a scanning electron microscope and an energy dispersive X-ray analyzer to conduct composition analysis by randomly selecting 10 to 20 oxide inclusions having a length of 4 μm or more and a width of 2 μm or more. Assuming a stoichiometric composition, it was converted to an oxide, and the average value was obtained.

Furthermore, after pickling the hot-rolled steel sheet, it was cold-rolled to a thickness of 0.7 mm at a rolling reduction of 82.5%, and annealed at a soaking temperature of 850 ° C. to obtain a cold-rolled steel sheet. After subjecting the obtained cold-rolled steel sheet to temper rolling with an elongation of 1.0%, a JIS No. 5 tensile test piece was sampled and subjected to a tensile test, yield stress (YS) in the rolling direction, and tensile strength (TS). The total elongation (El) was determined. The r value is obtained by conducting a tensile test on a JIS No. 5 tensile specimen taken from the rolling direction (0 ° direction), the direction forming 45 ° with the rolling direction (45 ° direction), and the direction orthogonal to the rolling direction (90 ° direction). Using the r value in the 0 ° direction (r _{0 °} value), the r value in the 45 ° direction (r _{45 °} value), and the r value in the 90 ° direction (r _{90 °} value), the average r value = (r _{0 (} Average value + 2 × r _{45 °} value + r _{90 °} value) / 4) to obtain an average r value.

  The evaluation of the cold rolled material was carried out as follows. When a cold rolled material has a surface width of about 200 mm and a length of 900 mm, sliver-like wrinkles are observed, and when oxide marks having the same composition as the oxide inclusions are observed in the sliver-like flaws, It was determined that there was an object.

  The results are summarized in Tables 1 and 2.

  Sample Nos. 1-1 to 1-7 are examples that satisfy all the conditions defined in the present invention. 1-8 to 1-15 are comparative examples that do not satisfy at least one of these.

  Sample No. 1-1 to 1-7 maintain strength, elongation, and deep drawability. Particularly, the average r value is 1.90 or more and the deep drawability is good, and inclusions in the molten steel stage are liquid. There was no example in which inclusions were observed in the buttocks while being in a state containing phases.

In contrast, sample no. In 1-8, 1-9, 1-11, 1-13, and 1-15, SiO _{2 in} inclusions is contained in excess of 1%, or the MnO concentration is 25 in terms of ternary inclusions. By exceeding%, the average r value was unsatisfactory.

Sample No. No. 1-10 has a TiO ₂ concentration of more than 95.0%. In No. 1-12, the Al ₂ O ₃ concentration exceeds 35.0%. In 1-14, since the MnO concentration was less than 2.0%, in all cases, no liquid phase was observed in the molten steel inclusions, and inclusion traces were observed in the collar portion of the cold-rolled steel sheet.

290 tons of molten steel was decarburized and refined in a converter, the ladle containing the undeoxidized molten steel was transferred to an RH apparatus, and vacuum decarburization processing was performed using the RH apparatus. After the vacuum decarburization was completed in the RH apparatus, metal Al was added for both the preliminary deoxidation of the undeoxidized molten steel and the temperature raising operation of the molten steel. After addition of Al, oxygen was supplied to the molten steel in the vacuum chamber at 38 Nm ³ / min, and heat was applied to the molten steel by an appropriate oxidation reaction. Then adjust the addition of various alloys other than Ti in consideration of the concentration already contained in the molten steel in the state of oxygen concentration, and finally adjust the addition of Ti to the chemical composition shown in Table 3 did.

After these refining, the ladle containing the molten steel was conveyed to a continuous casting machine to obtain a slab-shaped slab having a width of 960 mm to 1200 mm and a thickness of 250 mm.
After the surface of the obtained slab was cleaned, it was heated and hot rolled under the conditions shown in Table 4, pickled and cold rolled. Subsequently, the cold-rolled sheet was annealed by continuous galvanizing equipment, galvanized, and alloyed. Thereafter, temper rolling was performed at an elongation rate of 1.0% to obtain an galvannealed steel sheet. In some steel plates, the alloying treatment after hot dip galvanization was omitted, and hot dip galvanized steel plates were obtained. Moreover, in some steel plates, after cold rolling, they were annealed with a continuous annealing facility and subjected to temper rolling at an elongation rate of 1.0% to obtain cold rolled steel plates.

  Table 4 shows the total oxygen concentration before and after the Ti adjustment. Table 5 shows the composition form of oxide inclusions in the molten steel stage and the composition form of oxide inclusions in the hot-rolled steel. The investigation method of the inclusion composition, etc. in Table 5 is in accordance with the method shown in Example 1. Inclusions at the molten steel stage are bomb samples taken from molten steel contained in the tundish of a continuous casting machine, and spherical inclusions are observed among oxide inclusions having a diameter of 5 μm or more observed in the mirror-finished section. The percentage is shown. Moreover, the rolled steel material produced the micro sample of 3.2 mm x 20 mm in the width center of a steel strip and a plate cross section in the rolling longitudinal direction, and performed the same evaluation as Example 1.

  Table 6 shows Δ% / heat as an index of the clogging state of the immersion nozzle in the continuous casting process, and the amount of increase per one heat of the sliding gate opening for controlling the flow rate of the molten steel installed on the upper part of the nozzle. . Moreover, about the sliver rod of a cold-rolled steel plate, the surface defect generation | occurrence | production number per 1000 m of steel strips was shown. And the defect incidence rate of the steel strip was considered to be good when it was less than 0.5 pieces / 1000 m, and 1.0 piece / 1000 m or more as bad.

  JIS No. 5 tensile specimens were collected from hot-dip galvanized steel sheets, galvannealed steel sheets, and cold-rolled steel sheets, subjected to tensile tests, and yield stress (YS), tensile strength (TS) and total elongation (El) in the rolling direction. ) The r value was determined in the same manner as in Example 1.

  The results are shown in Tables 4 to 6, respectively.

  Sample Nos. Examples 2-1 to 2-4 are examples that satisfy all the conditions defined in the present invention. 2-5 to 2-10 are comparative examples that do not satisfy at least one of these.

  Sample No. In 2-1 to 2-4, the inclusion composition ratio in the molten steel is as high as 30% or more, and therefore, the increase in the nozzle opening, which is an index of nozzle clogging, is also as small as 2% or less per heat, and is stable in multiples. Casting is possible. Furthermore, since the defect occurrence rate of the steel sheet is as small as 0.5 / 1000 m, it can be seen that a thin steel sheet having a beautiful surface property, which is a feature of the steel, can be obtained. Furthermore, the average r value is 1.8 or more and the deep drawability is excellent.

In contrast, sample no. 2-5 has a high total oxygen concentration before Ti adjustment, the SiO ₂ concentration in the rolled steel exceed 1%, the average r value is decreased.
Sample No. In 2-6, since the Ti concentration was small, the MnO concentration and the SiO ₂ concentration in the inclusions were increased, and the average r value was decreased.

Sample No. In 2-7, since the oxygen concentration after Ti adjustment is low and the Al concentration is 0.003% or more, the Al ₂ O ₃ concentration in the rolled steel material exceeds 35%, and as a result, the liquid phase ratio in the molten steel stage is high. The nozzle opening was as low as 7%, and the nozzle opening was 5% or more per heat, making it difficult to achieve stable multiple casting.

Sample No. In No. 2-8, the total oxygen concentration before Ti adjustment was high, and as a result, the TiO ₂ concentration of inclusions in the rolled steel material was less than 50%, resulting in a low average r value.
Sample No. 2-9, since the total oxygen concentration before Ti adjustment is low, inclusion composition control after Ti adjustment becomes difficult, and as a result, the concentration of other oxides of inclusions in the rolled steel exceeded 10% in this example. The average r value was low.

Furthermore, sample no. 2-10, since the Ti concentration exceeds 0.05%, the TiO ₂ concentration in inclusions in the rolled steel material exceeds 95%, and the liquid phase inclusion ratio of inclusions in the molten steel is as low as 7%. The change in the nozzle opening was also large, making it difficult to apply to multiple casting, and the defect rate, which is a feature of Ti deoxidized steel, could not be reduced.

Claims (4)

C: 0.0005% to 0.025%, Si: 0.003% to 0.12%, Mn: 0.05% to 2.5%, P: 0.15% or less , S: 0.02% or less, N: 0.006% or less, sol. Al: 0.0002% to 0.003%, Ti: 0.005% to 0.05%, Nb: 0.005% to 0.20%, and the total oxygen concentration T.I. O: 0.003% or more and 0.008% or less, oxygen concentration due to inclusions Oinc: 0.0025% or more and 0.007% or less, and having a chemical composition comprising the balance Fe and impurities, and steel material The oxide inclusions in the SiO ₂ is less than 1.0%, 90.0% or more is composed of ternary system of TiOx, Al ₂ O ₃ and MnO, the composition range in the ternary system is TiOx: 50.0% or more and 95.0% or less, Al ₂ O ₃ : 3.0% or more and 35.0% or less, MnO: 2.0% or more and 25.0% or less Carbon steel plate.   The ultra-low carbon steel sheet according to claim 1, wherein the chemical composition has B: 0.0020% or less in mass% instead of part of Fe.   The chemical composition may be one selected from the group consisting of Cu: 1.0% or less, Ni: 1.0% or less, and Cr: 1.0% or less in mass% instead of a part of Fe. The ultra-low carbon steel sheet according to claim 1 or 2 having two or more kinds. It is the manufacturing method of the ultra-low-carbon steel sheet according to any one of claims 1 to 3, which is performed through refining including converter refining and vacuum refining,
When the oxygen concentration of the molten steel before Ti adjustment by Ti addition in the refining is 0.004% or more and 0.015% or less, Ti is added to the molten steel, and the Ti concentration in the molten steel is 0.005% or more. A manufacturing method of an ultra-low carbon steel sheet characterized by continuously casting after setting the O concentration in the molten steel to be 0.003% or more and 0.008% or less while being 0.05% or less.