JP2000001748A - Steel sheet for deep drawing, excellent in surface characteristic and stretch-flange formability, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet excellent in surface characteristic and stretch- flange formability, and its manufacture. SOLUTION: The steel sheet, contains, by weight, >=0.010% C, <=1.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, <=0.01% N, 0.010-0.50% Ti, 0.001-0.1% Nb, 0.0001-0.05% B, and <=0.0005% Ca and/or metallic REM and also contains 0.005-0.1 wt.% of non-oxide Ti(Ti*) and contains Al in an amount satisfying (1) wt.% Ti/wt.% Al>=5 or (2) Al<=0.010 wt.% and wt.% Ti/wt.% Al<5, and further, the average size of oxide inclusions is regulated to <=3 μm and >=90% of the oxide inclusions contains the oxide inclusions with a size of <=10 μm. At the time of its manufacture, a slab is heated and soaked at 900 to 1,300 deg.C, finish rolling is finished at 650 to 960 deg.C, coiling is done at 400 to 750 deg.C, cold rolling is carried out at 50 to 95% draft, and the resultant sheet is subjected to recrystallization annealing at 700 to 920 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet for deep drawing having good surface properties and a method for producing the same, and more particularly to a steel sheet excellent in formability and stretch flangeability as well as surface properties, such as a cold-rolled steel sheet and a hot-dip galvanized steel sheet. , Electro-galvanized steel sheet, tin-plated steel sheet, enamel coated steel sheet,
The present invention relates to a coated steel sheet and other steel sheets for surface treatment and a method for producing the same. In particular, the present invention, by suitable Ti deoxidation, oxide-based inclusions in the steel, i.e., to improve the surface properties of the steel sheet by suppressing the formation of giant cluster-like inclusions, and to finely disperse the inclusions Accordingly, it is an object of the present invention to provide an ultra-low carbon steel sheet in which the origin of cracks during hole expansion forming is reduced and stretch flangeability is improved.

[0002]

2. Description of the Related Art Steel is initially deoxidized with ferrotitanium as disclosed in Japanese Patent Publication No. 44-18066. However, in recent years, in order to produce steel with stable oxygen concentration at low cost, Al deoxidized steel that is deoxidized with Al has become mainstream.

[0003] Al deoxidation of steel is a method of aggregating generated oxides and separating them by flotation using a gas agitator or an RH degassing apparatus. In this case, Al ₂ O ₃ oxide is contained in a slab. Will inevitably remain. In addition, since Al ₂ O ₃ is in a cluster form, it is difficult to separate, and sometimes cluster-like inclusions of several hundred μm or more remain. If such cluster-like inclusions are trapped in the surface layer of the slab,
It will lead to surface defects like sleevers,
It is a fatal defect in automotive steel plates that require beauty. In addition, Al deoxidation has a problem in that Al ₂ O ₃ adheres to an inner wall of an immersion nozzle used for injecting the material from a tundish into a mold, causing nozzle clogging.

[0004] In order to solve the above-mentioned problems associated with Al deoxidation, a method has been proposed in which Ca is added to aluminum-killed molten steel to produce a CaO, Al ₂ O ₃ composite oxide. (For example, Japanese Patent Application Laid-Open No. 61-276756,
58-1554447 and JP-A-6-49523). The purpose of the addition of Ca in this method is to solve the above-described problem by reacting Al ₂ O ₃ with Ca to form a low melting point composite oxide such as CaOAl ₂ O ₃ , 12CaOAl ₂ O ₃ , 3CaOAl ₂ O _3. We are trying to overcome the point.

[0005] However, when Ca is added to molten steel,
The Ca reacts with S in the steel to form CaS, and the CaS causes rust. In this regard, in Japanese Patent Application Laid-Open No. 6-559, in order to prevent rusting, the amount of Ca remaining in steel is reduced to 5 pp.
It proposes a method to reduce the concentration to m or more and less than 10 ppm. But,
Even if the Ca content is less than 10 ppm, CaO remaining in the steel
When the composition of -al ₂ O ₃ based oxide is not proper, especially in the case of CaO concentrations oxide 30% or more, its solubility S in the oxides increases, inclusions within the perimeter at the time and the solidification temperature drop To CaS
Is inevitably generated. As a result, rust is generated from the CaS as a starting point, leading to deterioration of the surface properties of the product plate. Also, if surface treatment such as plating or painting is performed while such rusting points remain, surface unevenness will inevitably occur after the treatment. On the other hand, if the high and the concentration of Al ₂ O ₃ CaO concentration is as low as 20% or less in inclusions, in particular Al ₂ O
_{3 When the} concentration is 70% or more, the melting point of inclusions increases,
Since inclusions are easily sintered, not only nozzle clogging is liable to occur during continuous casting, but also barges and slivers are generated on the surface of the steel sheet, which causes a problem that the surface properties are remarkably deteriorated.

On the other hand, in recent years, a method of deoxidizing with Ti without adding Al has been developed as JP-A-8-239731. The method of such Al-less Ti deoxidation is as follows:
Compared to the Al deoxidation method, the reached oxygen concentration is higher and the amount of inclusions is larger, but no cluster oxide is generated. In particular, the form of the inclusions generated is Ti oxide-Al ₂ O ₃ system, ₂ ~ 50μ
It exhibits a state in which about m granular oxides are dispersed. Therefore, the above-mentioned surface defects due to inclusions being clustered are reduced. However, in the case of this Ti deoxidation, in the molten steel with Al ≤ 0.005 wt%, when the Ti concentration becomes 0.010 wt% or more, the solid state Ti oxide adheres to the inner surface of the tundish nozzle by taking in the metal. There was a new problem of growing and instead causing nozzle blockage.

In order to solve such a problem (prevention of nozzle blockage), Japanese Patent Laid-Open No. 8-281391 discloses an Al-less Ti
In deoxidized steel, a method has been proposed to prevent the growth of Ti ₂ O ₃ growing on the inner surface of the nozzle by limiting the oxygen content of molten steel passing through the nozzle. However, in this method, there is another problem that the treatment amount is limited (about 800 tons) because the limitation of the oxygen amount is also limited. Also,
The fact is that the level control of the molten metal level in the mold becomes unstable with the progress of the blockage, so that it is not a fundamental solution.

The technique disclosed in Japanese Patent Application Laid-Open No. 8-281390 discloses a technique for preventing blockage of a tundish nozzle.
By the composition of inclusions by optimizing the Si concentration of the molten steel Ti ₃ O ₅ -SiO ₂ system, it has proposed a method of preventing the growth of Ti ₂ O ₃ to grow the nozzle inner surface. However, it is difficult to make the inclusions contain SiO ₂ simply by simply increasing the Si concentration, and it is necessary to satisfy at least (wt% Si) / (wt% Ti)> 50. Therefore, the Ti concentration in steel is 0.01
In the case of 0 wt%, in order to obtain a SiO ₂ —Ti oxide, the Si concentration needs to be 0.5 wt% or more. However, an increase in Si causes hardening of the material and also causes deterioration in plating property. An increase in the Si concentration has an adverse effect on the surface properties of the steel sheet, and does not provide a fundamental solution.

Next, in Japanese Patent Publication No. 7-47764, Mn:
Proposal of non-aging cold-rolled steel sheet containing low melting point inclusions consisting of 17-31 wt% MnO-Ti oxide by deoxidizing to 0.03-1.5 wt% and Ti: 0.02-1.5 wt% are doing. In the case of this proposal, since the MnO-Ti oxide has a low melting point and is in a liquid phase in molten steel, the molten steel is injected into the mold without adhering to the nozzle even when passing through the tundish nozzle, Blockage of the tundish nozzle can be effectively prevented. However, as reported by Yasuyuki Morioka, Kazuki Morita et al .: Iron and Steel, 81 (1995), p.
O: In order to obtain a MnO-Ti oxide containing 17 to 31%,
Due to the difference in affinity of Mn and Ti with oxygen, the concentration ratio of Mn and Ti in the molten steel needs to be (wt% Mn) / (wt% Ti)> 100. Therefore, when the Ti concentration in steel is 0.010 wt%,
To obtain the required MnO-Ti oxide, the Mn concentration must be 1.0 wt.
% Or more is required. However, if the Mn content exceeds 1.0 wt%, the material hardens. Therefore, it was practically difficult to form inclusions composed of 17-31 wt% MnO-Ti oxide.

[0010] Further, in Japanese Patent Application Laid-Open No. 8-281394, Al
As prevention of clogging of the tundish nozzle in less Ti-deoxidized steel, by using a material containing CaO · ZrO ₂ grains in the nozzle, if the Ti ₃ O ₅ in the molten steel was trapped in the nozzle, TiO ₂ - A method has been proposed in which a low melting point inclusion of the SiO ₂ —Al ₂ O ₃ —CaO—ZrO ₂ system is used to prevent its growth. However, when the oxygen concentration in the molten steel is high,
Since the TiO ₂ concentration is high and the melting point is not lowered, it does not prevent the nozzle from being clogged. On the other hand, when the oxygen concentration is low, there is a problem that the nozzle is melted, and this is not a sufficient measure.

Further, the above-mentioned prior arts relating to the prevention of nozzle clogging are as follows. In a continuous casting process, an immersion nozzle for injecting molten steel from a tundish nozzle into a mold is still cast by blowing Ar gas or N ₂ gas. There is a need. However, there remains a problem that the blown gas is trapped by the solidified shell of the slab and becomes a cellular defect.

Incidentally, ultra-low carbon cold rolled steel sheets are generally widely used as outer and inner plates of automobiles. In particular, high r-values (Rankford values) are required for parts where deep drawing formability is required. It is known that the r value strongly depends on the crystal orientation of the steel sheet, and can be increased by developing {111} recrystallization texture. For this reason, in order to increase the r-value, various studies have been made on steel composition, hot rolling conditions, cold rolling conditions, and annealing conditions as methods for developing {111} recrystallization texture. When recrystallization annealing is performed at high temperature,
It is known that {111} recrystallized texture develops strongly and the r-value increases. By the way, in recent years, there has been an increasing tendency to integrally mold several parts of a vehicle body for the purpose of reducing the weight of the vehicle body and improving the rigidity of the vehicle body. However, it is difficult to press-mold a large part, and particularly when a blank is punched and formed into a predetermined shape, a stretch flange crack is likely to occur around the hole. Regarding the stretch flangeability, for example, Japanese Patent Application Laid-Open No. 6-299288 discloses a technique in which phosphide having a diameter of 0.1 μm or more is reduced to 0.05% or less. However, in this technique, although the stretch flangeability is improved to some extent, the above-mentioned surface properties still remain unsolved.

The technology for producing cold-rolled steel sheets for deep drawing using Ti deoxidized steel is disclosed in Japanese Patent Publication No. 7-47764 and Japanese Patent Application Laid-Open No. Hei 8-239731, and the like.
It is disclosed that the deoxidized steel can obtain characteristics in which the r value is higher by 0.1 to 0.2 than that of the Al deoxidized steel. However, these prior arts have not studied the stretch flangeability at all, and these smelting methods originally had problems in steel making.

[0014]

The object of the present invention is to solve the above-mentioned problems of the prior art. A first object of the present invention is to propose a steel sheet having excellent surface properties, formability and deep drawability, and a method for producing the same.
A second object of the present invention is to propose a technique for manufacturing a steel sheet for deep drawing which is effective for preventing nozzle clogging during continuous casting and effective for preventing generation of cluster-like inclusions. A third object of the present invention is to provide a steel sheet for deep drawing which has excellent stretch flangeability in addition to surface properties. A fourth object of the present invention is to provide a surface treatment for cold-rolled steel sheets, hot-dip galvanized steel sheets, electro-galvanized steel sheets, tin-plated steel sheets, painted steel sheets, etc., which has good r-value and stretch flangeability in addition to the surface properties. It is an object of the present invention to propose a deep drawing steel sheet manufacturing technique for manufacturing a steel sheet.

[0015]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, oxide inclusions remaining in steel have a size and a composition within a specified range. In this case, fine dispersion can be achieved without causing the above-described nozzle clogging and without making the inclusions large in clusters, and furthermore, the starting point of cracks during hole expansion molding can be reduced and the extension flange can be formed. The present inventors have found that the performance can be greatly improved, and arrived at the present invention.

The present invention developed based on such knowledge,
C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.0 wt%, P ≦ 0.
15wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.010wt% ≦ Ti
≦ 0.50 wt%, but 0.005 to 0.1 wt% of the above Ti is contained in the form of non-oxide Ti (Ti ^* ), and Nb: 0.001 to 0.1 wt%
%, B: 0.0001 to 0.05 wt%, Ca and / or metal RE
M ≧ 0.0005wt% and the following formula (1) or (2)
Contains Al in a range satisfying the formula, the balance is composed of Fe and unavoidable impurities, and the oxide-based inclusions have an average size of 3
A deep drawing steel sheet having good surface properties and excellent stretch flangeability, characterized in that 90 μm or less and 10 μm or less account for 90% or more thereof. Note wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2)

In the steel sheet of the present invention, the inclusions in the steel are CaO and / or REM oxide: 5% in total.
wt% or more and 50 wt% or less, Ti oxide: 90 wt% or less, Al ₂ O ₃ :
70 wt% or less, or even SiO _2: 30 wt% or less, MnO:
It is preferable to contain 15 wt% or less.

Further, the present invention provides a method for producing a liquid crystal composition comprising: C ≦ 0.010 wt%, Si ≦ 1.
0 wt%, Mn ≦ 3.0 wt%, P ≦ 0.15 wt%, S ≦ 0.05 wt%,
N ≦ 0.01wt%, 0.010wt% ≦ Ti ≦ 0.50wt%, but above
0.005 to 0.1 wt% of Ti is contained in the form of non-oxide Ti (Ti ^* ), Nb: 0.001 to 0.1 wt%, B: 0.0001 to 0.05 wt%
%, Ca and / or metal REM ≧ 0.0005 wt%, a steel slab containing Al in a range satisfying the following formula (1) or (2), and the balance being Fe and unavoidable impurities:
Heating and soaking at 00-1300 ° C, finish rolling at 650-960 ° C, winding at 400-750 ° C,
Cold-rolled at a reduction rate of 50-95%, then 700-920
The oxide inclusions having an average size of 3 μm or less and 10 μm or less occupy 90% or more of the oxide-based inclusions, which are characterized by being subjected to recrystallization annealing at 0 ° C., and have excellent surface properties and excellent stretch flangeability. A method for manufacturing a steel sheet for deep drawing is proposed. Note wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2)

In the above method according to the present invention, the inclusions in the steel are CaO and / or REM oxide: 5 wt% to 50 wt% in total, and Ti oxide: 90 wt%.
Hereinafter, Al ₂ O ₃ : 70 wt% or less, or further SiO ₂ : 30 wt%
% MnO: 15 wt% or less.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the present invention will be described.
The experimental research that triggered this was explained. This experiment
Are: C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.1 wt%,
P: 0.01 wt%, S: 0.006 wt%, Al: 0.005 wt%, N:
0.002 wt%, Nb: 0.005 wt%, B: 0.0004 wt%, Ti: 0.
02-0.05wt%, O: 0.001-0.022wt%, Ca: 0.001wt
%, (Ti ^*/ 48)-(N / 14 + S / 32) ≒ 3.0 × (C / 12) (Ti^*:
Non-oxide Ti)
After heating to ℃, soak the finish temperature to 890 ℃
Was subjected to three-pass rolling to obtain a hot-rolled sheet having a thickness of 4.0 mm. That
After that, coil winding treatment was performed at 600 ° C for 1 hour.
Was. After that, it is further cold rolled by 80%, and then 830
A recrystallization annealing of 40 ° C. was performed.

FIG. 1 shows the effect of the average size of inclusions on the formability of the steel sheet produced as described above, particularly on the hole expansion rate. Here, the hole expansion ratio was measured with a plate thickness: 0.7 mm, a punched hole: 10 mmφ, a punched clearance: 0.1 mm, a burr: die side, a bunch: cone 60 °, and a die: shoulder 8.0 mmR. As is clear from FIG. 1, in the present component system (compatible material), the hole expandability depends on the size of the inclusions, and it was found that a high stretch flangeability can be obtained when the average size of the inclusions is 3 μm or less. did. Hereinafter, the present invention will be described in detail.

(1) Steel composition The composition of the steel sheet according to the present invention is as follows: C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.0 wt%, P
≦ 0.15wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.010wt%
≦ Ti ≦ 0.50 wt%, Nb: 0.001 to 0.1 wt%, B: 0.0001 to
0.05% by weight, Ca and / or metal REM ≥0.0005% by weight, and wt% Ti / wt% Al≥5, or Al≤0.01
0 wt% and wt% Ti / wt% Al <5
And the content of the non-oxide form of Ti (Ti ^* ) is 0.005 to 0.1 wt%, with the balance being Fe and unavoidable impurities.

Hereinafter, the component composition of the steel sheet according to the present invention will be described.
The reason for limiting as described above will be described. (a) C ≦ 0.010 wt% It is preferable to reduce the content of C as the smaller the content, the better the deep drawability is improved. However, considering the refining load and the like, the upper limit is set to 0.010 wt% or less without adverse effects. (b) Si ≦ 1.0 wt% Si has the effect of strengthening steel, and contains the necessary amount according to the desired strength. However, if the content exceeds 1.0 wt%, deep drawability deteriorates. It was limited to 1.0 wt% or less. (c) Mn ≦ 3.0 wt% Mn has the effect of strengthening steel and contains the necessary amount according to the desired strength. However, if the content exceeds 3.0 wt%, deep drawability deteriorates. It was limited to 3.0 wt% or less. (d) P ≦ 0.15wt% P has the effect of strengthening steel and contains a necessary amount according to the desired strength. However, if the content exceeds 0.15wt%, deep drawability deteriorates. Limited to 0.15 wt% or less. (e) S ≦ 0.05 wt% It is preferable to reduce the content of S because the smaller the content, the better the deep drawability. However, if the content is 0.05 wt% or less,
Since there is no significant adverse effect, the content was limited to 0.05 wt% or less. (f) N ≦ 0.01 wt% It is preferable to reduce the content of N as the smaller the content, the better the deep drawability. However, if the content of N is 0.01 wt% or less,
Since there is no significant adverse effect, the content was limited to 0.01 wt% or less. (g) 0.010 wt% ≦ Ti ≦ 0.50 wt% Ti is a component that plays the most important role in the steel sheet of the present invention, and the size of oxide-based inclusions is reduced by Ti deoxidation.
Those having an average of 3 μm or less and 10 μm or less occupy 90% or more thereof (area ratio, the same applies hereinafter). Due to the formation of these fine oxide-based inclusions, the origin of cracks during hole expansion molding is reduced and the elongation is reduced. Improves flangeability. further,
Since this fine oxide effectively acts on miniaturization of a hot-rolled sheet, it develops {111} recrystallization texture after cold rolling and annealing to increase the r value. If the Ti content is less than 0.010 wt%, the effect of addition, that is, the amount of the fine oxide is too small, so that the desired effect described above cannot be obtained. Therefore, the lower limit is limited to 0.010 wt% or more. This Ti is 0.025 wt%
The above addition works more effectively. However, if it is added in excess of 0.50 wt%, the material hardens in a thin steel sheet and not only impairs the desired material properties, but also increases the cost. Therefore, the upper limit is set to 0.50 wt%.

(H) Al Al is a component that plays an important role in the present invention, and is represented by wt% Ti / wt% Al ≧ 5 or Al ≦ 0.010 wt%
In addition, it is necessary to satisfy one of the conditions of wt% Ti / wt% Al <5. When the above conditions are no longer satisfied, Al deoxidized steel is formed, a large amount of huge clusters of Al ₂ O ₃ are generated, and the surface properties of the steel slab are deteriorated, and the oxide inclusions have an average size of 3 μm or less, 90% of those with 10μm or less
The above condition is not satisfied, and the stretch flangeability is reduced. Therefore, the Al content needs to satisfy the above conditions or the following conditions.
Is a preferable range for achieving the object of the present invention.

(I) Ca and / or metal REM ≧ 0.0005
The wt% Ca and the metal REM are components that play an important role in the steel sheet according to the present invention, and it is necessary to add one or two of Ca and REM in a total of 0.0005 wt% or more. That is, after deoxidizing molten steel with Ti, one or two of Ca and REM are further added in a total amount of 0.0005 wt%.
By adding the above, the oxide composition in the molten steel is adjusted to Ti oxide: 90 wt% or less, preferably 20 wt% or more and 90 wt% or less,
More preferably 85% by weight or less, CaO and / or RE
M oxide: 5 wt% or more, preferably 8 wt% to 50 wt%, and adjusted so as to be low melting point oxide-based inclusions in which Al ₂ O ₃ is 70 wt% or less. By performing such an adjustment, it is possible to prevent the Ti oxide containing the base metal from adhering to the nozzle during continuous casting, thereby eliminating nozzle blockage. further,
CaO and / or REM oxide can contribute to grain growth after cold rolling and annealing and grain refinement of a hot rolled sheet. By controlling the oxide to have such a low melting point and good wettability with the molten steel, it is possible to prevent aggregation of the oxides in the molten steel and cluster-like growth. In addition, since excessive addition of Ca and REM causes rusting, it is desirable to add the total amount in a range of 0.005 wt% or less.

(J) Non-oxide Ti (Ti ^* ) = 0.005-0.1 wt
% Non-oxide Ti means the total amount of Ti that does not exist in the oxide state in the steel, that is, exists as carbides, nitrides, sulfides, etc., or exists in the solid solution state, out of the total Ti. , Determined by the following method. Non-oxide Ti content = total Ti content−oxide Ti Here, oxide Ti = total O content × Ti concentration of inclusions in steel by EPMA (wt%) / O concentration of inclusions in steel by EPMA (wt%) ). And the Ti concentration and the O concentration by EPMA are selected from 10 oxide inclusions of 3 to 10 μm randomly present in the steel, and the concentration is measured by EPMA.
Use the average value. The non-oxide Ti obtained in this way is a component that plays a very important role in the steel sheet according to the present invention, and converts solid solution C, solid solution N, and solid solution S in the steel as carbide, nitride, and sulfide. By reducing the precipitation and fixing, there is an effect of preventing the deterioration of the deep drawability. The amount
If the content is less than 0.005 wt%, there is no effect, and if it exceeds 0.1 wt%, no further effect is obtained, and conversely, deep drawability is deteriorated. Therefore, the content is limited to 0.005 to 0.1 wt%. In addition, this Ti ^*
The amount is desirably in the range of 0.01% by weight or more. The amount of the non-oxide Ti (Ti ^* ) is expressed as follows: (C / 12) ≦ (Ti ^* / 48) − (N / 14 + S / 32) ≦ 10 in relation to the respective contents of C, N and S. (C / 1
It is preferable to satisfy the relational expression 2). The reason is that when (C / 12)> (Ti ^* / 48)-(N / 14 + S / 32), a large amount of solid solution C remains in the hot-rolled sheet, so that the deep drawability after cold rolling and annealing Is poor, while (Ti ^* / 48)-(N / 14 + S / 32)> 1
This is because the non-oxide Ti in an amount of 0 (C / 12) conversely deteriorates the deep drawability.

(K) Nb: 0.001 to 0.1 wt% Nb has the effect of improving the r-value after cold rolling and annealing by making the structure of the hot rolled sheet finer. The addition amount is 0.00
If it is less than 1 wt%, there is no effect of addition, while if it exceeds 0.1 wt%, the effect of the addition is saturated and conversely leads to deterioration of deep drawability, so the content was made 0.001 to 0.1 wt%. (l) B: 0.0001 to 0.05 wt% B is added for improving the secondary work brittleness resistance of steel. However, if the addition amount is less than 0.0001 wt%, there is no addition effect.
On the other hand, if added in excess of 0.05 wt%, it will lead to deterioration of the deep drawability, so it was limited to 0.0001 to 0.05 wt%.

(2) Inclusions of Slab and Steel Sheet In the steel sheet of the present invention, the oxide-based inclusions have a size in the width direction of the steel sheet (direction perpendicular to the rolling direction) and an average size of 3 μm or less.
In addition, it is necessary that those having a size of 10 μm or less contain fine inclusions occupying 90% or more (in area ratio). By the way, the size of the inclusions present in the slab (slab) is elongated in the rolling direction by rolling, but hardly changes in the plate width direction. Therefore, in order to keep the size of the inclusion in the width direction of the steel sheet within a predetermined range, it is necessary to control the size of the inclusion at the stage of the billet. Therefore, control of fine oxide-based inclusions contained in the billet is one of the important components of the present invention.
One. In particular, the inclusions produced under the method of the invention are:
It is a granular or fractured oxide-based inclusion having a width (dimension in a direction perpendicular to the rolling direction) satisfying the size. If the width of the oxide-based inclusions satisfies the above-mentioned size, it is possible to achieve refinement of crystal grains during hot rolling and improvement in stretch flangeability due to a decrease in crack starting points during hole expansion forming. Such improvement effects cannot be obtained with large inclusions that do not satisfy the above conditions.
From this, the average size of the oxide inclusions was 3 μm.
It is necessary that 90% or more (area ratio) of m or less and 10 μm or less exist. When the content of such oxide-based inclusions is less than 0.002 wt%, there is no effect on grain growth during cold rolling and annealing, while when the content is more than 0.015 wt%, deep drawability is adversely affected. 0.002 to 0.01
It is desirable to be in the range of 5 wt%. From the viewpoint of deep drawability, the content of oxide-based inclusions is 0.004 to 0.01.
2 wt% is more preferred. Here, the granular or fractured oxide-based inclusions having the above-mentioned size are oxide-based inclusions formed by steel slabs, and relatively large ones are cut in the rolling direction by hot rolling and cold rolling. A broken oxide-based inclusion that has been broken, and a relatively small one is a particulate oxide-based inclusion that maintains its shape.

(3) Steel Sheet Manufacturing Method Steelmaking Step: This step is not particularly limited in the case of the present invention, but preferred processing methods are exemplified below.
The material is ultra-low carbon steel, Ti ≧ 0.010 wt%,
wt% Ti / wt% Al ≧ 5 or Al ≦ 0.010 wt% and wt
% Ti / wt% Al <5 It is necessary to melt a steel having a component composition that satisfies either condition. In this case, the reason for making Ti ≧ 0.010 wt% as the adjusting component is Ti <
At 0.010 wt%, the deoxidizing ability is weak, the total oxygen concentration in the molten steel increases, and the material properties such as elongation and drawing are deteriorated. However, even in this case, it is conceivable that the deoxidizing power is increased by increasing the concentration of Si and Mn. However, when Ti <0.010 wt%, a large amount of inclusions containing SiO ₂ or MnO is generated, and the hardening of steel material and Deterioration of plating property is caused. To prevent this, (wt%
It is preferable that Ti) / (wt% Al) ≧ 5, (wt% Mn) / (wt% Ti) <100. In this case, the Ti oxide concentration in the inclusions is 20% or more.

Also, wt% Ti / wt% Al ≧ 5, or
The reason for setting any of the conditions of Al ≦ 0.010 wt% and wt% Ti / wt% Al <5 is that if these conditions are not satisfied, Ti
This is because Al deoxidized steel is used instead of deoxidized steel, and a large amount of Al ₂ O ₃ clusters having an Al ₂ O ₃ concentration of 70% or more are generated. The present invention is intended to achieve the intended purpose by including CaO and REM oxide in inclusions mainly composed of Ti oxide as described later. In this regard, it is preferable that the above two conditions be adjusted particularly to the condition of wt% Ti / wt% Al ≧ 5.

In manufacturing the steel sheet according to the present invention,
First, molten steel is deoxidized with a Ti-containing alloy such as Fe-Ti to generate oxide-based inclusions mainly composed of Ti oxides in the steel.
The inclusions are not in the form of huge clusters as in the case of deoxidation with Al, but mostly in the form of granules or fractures having a size of about 1 to 50 μm. However, at this time, if the above or the above conditions are not satisfied, a huge Al ₂ O ₃ cluster is generated. Such Al ₂ O ₃ clusters can be obtained by adding Ti alloy.
Even if the Ti concentration is increased, it cannot be reduced and remains as cluster-like inclusions in the steel. Therefore, in the steel sheet according to the present invention, at this stage of production, first, an appropriate Ti
Preferably, an oxide is formed.

Under the method of the present invention, the yield of Ti alloy is lower than that of the conventional method of deoxidizing with Al,
a, Inclusion composition adjusting alloys are expensive because they contain REM. For this reason, it is economically preferable to add the Ti alloy to the molten steel so that the addition of the Ti alloy is as small as possible within a range where the composition of the inclusions can be controlled. In this sense, prior to the addition of a deoxidizing material such as a Ti-containing alloy, it is desirable to carry out preliminary deoxidation in order to reduce dissolved oxygen in the molten steel and FeO and MnO in the slab. This preliminary deoxidation is performed by deoxidation with a small amount of Al such that Al ≦ 0.010 wt% in the molten steel after deoxidation, and addition of Si, FeSi, Mn, and FeMn.

As described above, Ti produced by Ti deoxidation
A steel sheet with Ti oxide-based inclusions of ₂ O ₃ ≧ 70% means that the inclusions are dispersed in the steel with a size of about 2 to 20 μm. Disappears. However, Ti oxide is in a solid state in molten steel, and ultra-low carbon steel grows on the inner surface of a tundish nozzle with metal incorporation due to the high solidification temperature of the steel. May be induced.

Therefore, in the present invention, after deoxidation with a Ti alloy, one or more of Ca and REM are further added so as to be 0.0005 wt% or more, and the molten steel, and thus the steel Oxide composition, Ti oxide: 20 wt% or more 90
wt% or less, preferably 85 wt% or less, CaO and / or R
EM oxide: a low melting point oxide-based inclusion containing 5 wt% or more, preferably 8 wt% or more and 50 wt% or less, and Al ₂ O ₃ of 70 wt% or less. Then, it becomes possible to effectively prevent the Ti oxide incorporating the metal from adhering to the nozzle. In addition,
A more desirable inclusion composition is Ti ₂ O ₃ : 30 wt% or more and 80 wt%
% Or less, CaO, REM oxide (La ₂ O ₃ , Ce ₂ O ₃ etc.): 10
It is not less than wt% and not more than 40 wt%. 20 Ti oxides of the above inclusions
If it is less than wt%, it will not be Ti deoxidized steel but Al deoxidized steel, and Al ₂ O
Nozzle clogging because the growing ₃ concentration is generated, also, Ca
Since the rusting deteriorates as the O, REM oxide concentration increases, the Ti oxide concentration is set to 20 wt% or more. On the other hand, when the Ti oxide concentration is 90 wt% or more, CaO and REM oxides are small and nozzle clogging occurs, so that the Ti oxide concentration is 20 wt% or more and 90 wt% or more.
wt% or less.

Further, regarding Al ₂ O ₃ in the inclusions,
If the content exceeds 70 wt%, the composition will have a high melting point, and not only will the nozzle be blocked, but the inclusions will also form clusters, which will increase nonmetallic inclusion defects on the product plate.

As described above, the oxide-based inclusions in the steel according to the present invention include CaO and / or REM oxide in a total amount of 5 wt% to 50 wt%, Ti oxide in a total amount of 90 wt% or less, and Al ₂ O ₃ Ti oxide containing less 70 wt% of the need as the main. The oxide-based inclusions may further include oxides such as SiO ₂ and MnO in addition to the oxides. In this case, regarding the content of SiO _{2 in} the inclusions,
It is desirable to control the content to 30% by weight or less and MnO to 15% by weight or less. The reason is that if these contents exceed the respective amounts, it cannot be said that the titanium-killed steel is the target of the present invention.Under such a composition, there is no nozzle clogging without Ca addition, and there is no problem of rusting. It is because it disappears.
Moreover, as described above, in order to include SiO ₂ and MnO in the inclusions, the Si and Mn concentrations of the molten steel must be Mn / Ti> 100 and Si
/ Ti> 50 is preferred. In addition, the oxide of the present invention is allowed to contain ZrO ₂ , MgO and the like in a range of 5 wt% or less. The composition of the oxide-based inclusions in the steel described above is determined by arbitrarily extracting ten oxide-based inclusions and calculating the average value (analytical value).

In the steel sheet according to the present invention, the conventional Al
Compared to deoxidized steel, the yield of Ti alloy is poor, and Ca, REM
Is expensive due to the addition of For this reason, it is preferable to adjust the composition of inclusions in steel so that the amount is as small as possible.If possible, pre-deoxidize so that the dissolved oxygen concentration in the molten steel before Ti deoxidation becomes 200 ppm or less. Is desirable. This preliminary deoxidation is preferably performed by stirring the molten steel in a vacuum, deoxidizing with a small amount of Al (Al after deoxidation is 0.010 wt% or less in the molten steel), and deoxidizing with Si, FeSi, Mn, or FeMn.

In the above-described deoxidation method of the present invention, 50 μm
Almost no inclusions of m or more are generated. This is because the inclusions having a size of 50 μm or more are generally caused by foreign inclusions such as slag and mold powder. It is desirable that the amount of inclusions of 50 μm or less be present in an amount of 80% by weight or more of the total amount of oxide-based inclusions in order to prevent coil surface defects and nozzle clogging. In particular, in the present invention, the oxide-based inclusions are controlled to a finer size so that those having an average size of 3 μm or less and 10 μm or less occupy 90% or more of the oxide-based inclusions. It reduces cracks and improves stretch flangeability.

In the present invention, when the composition of the formed inclusions is controlled as described above, it is possible to completely prevent oxides and the like from adhering to the inner surfaces of the tundish nozzle and the immersion nozzle of the mold during continuous casting. . Therefore, it is not necessary to blow a gas such as Ar or N ₂ into the tundish or the immersion nozzle for preventing adhesion of oxides and the like. As a result, it is possible to obtain an effect that it is possible to prevent powdery defects of the cast slab due to powder entrainment during continuous casting and bubble-like defects due to the blown gas from being generated in the cast slab.

Hot rolling step: The slab is heated at a temperature of 900 to 1300 ° C. prior to the hot rolling. The reason for this is that at a slab heating temperature of 900 ° C. or less, the load applied during rolling becomes too high, which causes operational problems. On the other hand, 1300
At a high temperature exceeding ℃, the crystal grain size before rolling becomes too large, so that the hot-rolled sheet does not become fine. Therefore, the slab heating temperature is limited to 900 to 1300 ° C. The slab heating temperature is preferably 1200 ° C. or less from the viewpoint of deep drawing. In the process from continuous casting to rolling, CC-DR (continuous casting-direct rolling) or
Adopting HCR (hot charge rolling) is a preferable method from the viewpoint of energy saving.

The end temperature of the hot rolling is 650 to 960 ° C.
And The reason is that when hot rolling is completed at a temperature higher than 960 ° C., the crystal grains of the hot-rolled sheet are coarsened, and the deep drawability after cold rolling and annealing is deteriorated. On the other hand, hot rolling may be terminated in the α region below the Ar ₃ transformation point, but if the temperature is lower than 650 ° C., the rolling load increases, so the finish rolling end temperature is set to 650 to 960 ° C. limit. As for the coil winding temperature after hot rolling, higher temperatures are more advantageous for coarsening of precipitates.However, if the temperature exceeds 750 ° C, the scale becomes too thick. Is not coarsened, so it is limited to the range of 400 to 750 ° C.

Cold rolling step: This step is a process performed to obtain a high r value, and in order to achieve this purpose, it is necessary to set the cold rolling reduction to 50 to 95%. This is because if the rolling reduction is less than 50%, excellent deep drawability cannot be obtained. On the other hand, even if cold rolling is performed at a rolling reduction of 95% or more, a higher r value is obtained. Is not obtained, and conversely, the r-value is reduced, so that it is limited to 50 to 95%.

Annealing step: The cold-rolled steel sheet that has undergone the cold rolling step must be subjected to recrystallization annealing. Annealing temperature is 700 ~ 92
Set to 0 ° C. This is because if the annealing temperature is below 700 ° C,
The preferred {111} recrystallization texture for deep drawability does not develop.On the other hand, even if annealing is performed in a high temperature region exceeding 920 ° C, further deep drawability cannot be obtained. This is because the texture is randomized and the r value is degraded. Therefore, the annealing temperature is limited to 700-920 ° C. The steel strip after annealing may be subjected to a temper rolling of 10% or less for shape correction, adjustment of surface roughness, and the like.

The cold-rolled steel sheet thus obtained can be used not only as a cold-rolled steel sheet for processing but also as an original sheet of a surface-treated steel sheet for processing. The surface treatment includes zinc plating (including alloys), tin plating, enamel resin coating, and the like. Further, the steel sheet of the present invention may be subjected to a special treatment after annealing or galvanization to improve the chemical conversion treatment property, the weldability, the press formability, the corrosion resistance and the like.

[0045]

[Example] Inventive example: After tapping from the converter, 300 ton of molten steel was decarburized by an RH degassing apparatus, and C: 0.0008 to 0.0022 wt%, S
i: 0.004 to 0.120 wt%, Mn: 0.06 to 0.45 wt%, P: 0.0
09 to 0.060 wt%, S: 0.003 to 0.009 wt%, Nb: 0.002
0.009 wt%, B: 0.0002 to 0.0006 wt%, and the molten steel temperature was adjusted to 1585 to 1615 ° C. Al was added to the molten steel at 0.2 to 0.8 kg / ton to reduce the dissolved oxygen concentration in the molten steel to 55 to 250 ppm. Al in molten steel at this time
The concentration was between 0.001 and 0.003 wt%. Then, Ti was deoxidized by adding 0.8 to 1.8 kg / ton of a 70 wt% Ti-Fe alloy to the molten steel. Then, after adding FeNb, FeB, etc. to adjust the components, the molten steel contains 30 wt% Ca-60 wt% Si alloy and an additive obtained by mixing Met.Ca, Fe, and 5-15 wt% REM,
Alternatively, a Ca alloy such as 90 wt% Ca-5 wt% Ni alloy or a Fe-coated wire of a REM alloy was added at 0.05 to 0.5 kg / ton to perform the treatment. The Ti concentration after this treatment is 0.026-0.058 wt%,
Al concentration 0.001 to 0.003 wt%, Ca concentration 0.0005 to 0.0018
wt%, and the REM concentration was 0.0000 to 0.0020 wt%.

Next, this molten steel was continuously used for two strand slabs.
The cast slab was manufactured by casting using a casting apparatus. Note that this
Average structure of molten steel inclusions in a tundish
Is 25-85wt% Ti_TwoO_Three− 5 to 45 wt% CaO − 0 to 18 wt% R
EM oxide-6 ~ 41wt% Al_TwoO _ThreeFine spherical inclusions
Was. During this casting, tundish and immersion
Ar gas was not blown into the fuel cell. Observe after continuous casting
In a tundish and immersion nozzle
Had almost no deposits.

Next, the continuous cast slab is hot-rolled, cold-rolled to 0.8 mm, and further subjected to a continuous annealing line (C
AL) or recrystallization annealing was performed in a hot-dip galvanizing line (CGL). Table 1 shows the steel composition, and Table 2 shows the composition of the inclusions and the surface properties of the coil. Table 3 shows the conditions of hot rolling, cold rolling and annealing. The values of non-oxide Ti, mechanical properties, hole expansion ratio, etc. were determined in the same manner as described above. Non-metallic inclusion defects such as barbs, slivers, scales and the like were found in the annealed plate in a range of 0.00 to 0.04 / 1000 m-coil or less. In addition, there was no problem with the amount of rust as in conventional Al deoxidation. The surface quality of the steel sheet subjected to the electroplating and the hot-dip galvanizing treatment after the cold rolling was also good. Table 3 summarizes the mechanical properties of the obtained steel sheet, the size of the oxide-based inclusions, the hole expansion ratio, and the like.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

Comparative example: After tapping from the converter, 300 ton of molten steel was RH
Decarburized by degassing equipment, C: 0.0015wt%, Si: 0.00
6 wt%, Mn: 0.15 wt%, P: 0.020 wt%, S: 0.005 wt
%, And the molten steel temperature was adjusted to 1590 ° C.
Al was added to the molten steel in an amount of 1.2 to 1.6 kg / ton to perform a deoxidation treatment. The Al concentration in the molten steel after the deoxidation treatment was 0.035 wt%. Then, while adding Fe-Ti, Fe-Nb, Fe
-B was added to adjust the component composition. The Ti concentration after this treatment was 0.040 wt%. Next, the molten steel was cast by a two-strand slab continuous casting apparatus to produce a continuously cast slab. At this time, the average composition of inclusions contained in the molten steel in the tundish is 95 to 98 wt.
% Al ₂ O ₃ , and mainly cluster-like inclusions of 5 wt% or less of Ti ₂ O ₃ . If Ar gas was not blown into the tundish and immersion nozzle during casting,
Al ₂ O ₃ was remarkably adhered to the nozzle, the opening of the sliding nozzle was significantly increased at the third charge, and the casting was stopped due to nozzle clogging. In addition, even when Ar gas was blown, a large amount of Al ₂ O ₃ adhered to the nozzle, and at the eighth charge, the level of the molten metal in the mold became large, and the casting was stopped. Next, the continuous cast slab was hot-rolled to 4.0 mm, cold-rolled to 0.8 mm, and further recrystallized and annealed in a continuous annealing line. Table 1 shows the steel composition and Table 2 shows the inclusion composition. Table 3 shows the respective conditions (manufacturing conditions) of hot rolling, cold rolling and annealing. Non-metallic inclusion defects such as barbs, slivers and scales were found in the annealed sheet at 0.30 / 1000 m-coil. The mechanical properties, the size of the oxide-based inclusions, and the hole expansion ratio are shown in Table 3 as comparative examples.

[0052]

As described above, the steel sheet according to the present invention does not cause clogging of the immersion nozzle during continuous casting in producing the steel sheet, and the surface of the rolled thin steel sheet is caused by nonmetallic inclusions. This is an extremely clean deep drawing steel sheet with almost no surface defects. Since it has an excellent hole expansion ratio, it is actually suitably used as a thin steel sheet for automobiles and the like.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the size of oxide-based inclusions on the hole expansion ratio.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Seiji Nabeshima 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Chome (without address) Kawasaki Steel Corporation Mizushima Works (72) Inventor Hirohide Uehara 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture (without address) Kawasaki Steel Corporation Mizushima Works F-term (reference) 4K037 EA02 EA04 EA09 EA15 EA16 EA18 EA19 EA23 EA25 EA27 EA31 EA36 EB02 EB03 EB06 EB08 EB09 FA01 FA02 FA03 FC02 FC03 FC04 FE01 FE02 FE03 FJ05 FJ06 HA05

Claims (4)

[Claims] 1. C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.
0 wt%, P ≦ 0.15 wt%, S ≦ 0.05 wt%, N ≦ 0.01 wt%,
0.010 wt% ≦ Ti ≦ 0.50 wt%, but 0.005% of the above Ti
~ 0.1 wt% is contained in the form of non-oxide Ti (Ti ^* ), and Nb:
0.001 to 0.1 wt%, B: 0.0001 to 0.05 wt%, Ca and / or
Or containing metal REM ≥0.0005wt% and
Contains Al in the range satisfying the formula (1) or (2), and the balance Fe
Deep drawing having good surface properties and excellent stretch flangeability, characterized in that oxide inclusions having an average size of 3 μm or less and 10 μm or less occupy 90% or more thereof. For steel plate. Note wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 2. The steel sheet according to claim 1, wherein the inclusions in the steel are CaO and / or REM oxide: 5 wt% or more in total.
50 wt% or less, Ti oxides: 90 wt% or less, Al ₂ O _3: 70wt% or less, characterized in that it mainly contains oxide inclusions of, for deep drawing of surface properties excellent good stretch flangeability steel sheet. 3. C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.
0 wt%, P ≦ 0.15 wt%, S ≦ 0.05 wt%, N ≦ 0.01 wt%,
0.010 wt% ≦ Ti ≦ 0.50 wt%, but 0.005% of the above Ti
~ 0.1 wt% is contained in the form of non-oxide Ti (Ti ^* ), and Nb:
0.001 to 0.1 wt%, B: 0.0001 to 0.05 wt%, Ca and / or
Or containing metal REM ≥0.0005wt% and
Contains Al in the range satisfying the formula (1) or (2), and the balance Fe
The steel slab consisting of unavoidable impurities is heated and soaked at 900-1300 ° C, finish rolling at a temperature of 650-960 ° C, wound up at a temperature of 400-750 ° C, and then 50-95% Cold-rolled at a reduction ratio and then recrystallization annealing at 700 to 920 ° C. The average inclusion size of oxide-based inclusions is 3 μm or less and 10 μm or less is 90% or more. A method for producing steel sheets for deep drawing that occupies a good surface property and has excellent stretch flangeability. Note wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 4. The method according to claim 3, wherein the inclusions in the steel are CaO and / or REM oxide: at least 5 wt% in total.
50 wt% or less, Ti oxides: 90 wt% or less, Al ₂ O _3: 70wt% or less, characterized in that it mainly comprises an oxide of the production of deep-drawing steel sheet for the surface properties excellent in stretch flange formability was good Method.