JP2000001749A - High tensile strength cold rolled steel sheet excellent in surface characteristic and balance between strength and elongation, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet excellent in a balance between strength and elongation as well as in surface characteristic and having >=340 MPa tensile strength, and its manufacture. SOLUTION: The steel sheet contains, by weight, <=0.20% C, <=2.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, <=0.01% N, 0.010-0.50% Ti, and >=0.0005% Ca and/or metallic REM and also 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 in which C, Si, Mn, and P are contained satisfying the relation of 9(wt.%C/12)+2(wt.%Si/28)+3(wt.%Mn/55)+30(wt.%P/31)>=0.05, and further, oxide inclusions with a size of <=50 μm are contained by 0.002-0.015 wt.%. At the time of its manufacture, a slab is heated and soaked at >=900 deg.C, finish rolling is finished at >=650 deg.C, coiling is performed at 400 to 750 deg.C, cold rolling is done, 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 high-tensile cold-rolled steel sheet having good surface properties and an excellent balance of strength and elongation, and a method for producing the same, and more particularly to controlling oxide inclusions in steel. In other words, by suppressing the formation of large cluster-like inclusions and finely dispersing them into inclusions having a size of 50 μm or less, while improving the surface properties of the high-tensile cold-rolled steel sheet, The aim is to improve the strength-elongation balance by controlling the grain growth properties of the grains.

[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 a stable oxygen concentration at low cost, an Al deoxidation method of deoxidizing 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] For such Al aforementioned problems accompanying the deoxidation by adding Ca in the molten steel was an aluminum killed, a method of generating CaO -Al ₂ O ₃ composite oxide is proposed. (For example, JP-A-61-276756, JP-A-58-1554447 and JP-A-6-49523).
The purpose of the addition of Ca in the method, to form an Al ₂ O ₃ and the Ca are reacted _{CaO · Al 2 O 3, 12CaO} · Al 2 O 3, 3CaO · Al 2 O 3 low melting point composite oxides such as Accordingly, the above-mentioned problems are to be overcome.

[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. Such an Al-less Ti deoxidation method has a higher reached oxygen concentration and a larger amount of inclusions than the Al deoxidation method, but does not generate a cluster-like oxide. In addition, the form of the generated inclusions is Ti oxide-Al ₂ O ₃ system, and 1 to 50
It exhibits a state in which a particulate oxide of about μm is dispersed. Therefore, the above-mentioned surface defects due to inclusions being clustered are reduced. However, in the case of this Ti deoxidation, in a molten steel of Al ≦ 0.005 wt%, the Ti concentration is 0.010 wt%.
As described above, there is a new problem that the Ti oxide in a solid state adheres and grows on the inner surface of the tundish nozzle while taking in the metal, and rather induces 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 0.04
In the case of 0 wt%, in order to obtain a SiO ₂ —Ti oxide, the Si concentration needs to be 2.0 wt% or more. However, such a large amount of Si causes excessive hardening of the material and also causes deterioration of the plating property. Therefore, the above proposal 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 0.030 wt%,
To obtain the required MnO-Ti oxide, the Mn concentration must be 3.0 wt.
% Or more is required. However, if the Mn content exceeds 3.0 wt%, the material is excessively hardened. Therefore, 17-31 wt% MnO
It was practically difficult to form inclusions composed of -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 to. However, there remains a problem that the blown gas is trapped by the solidified shell of the slab and becomes a cellular defect.

Incidentally, high-tensile cold-rolled steel sheets are widely used as materials for outer and inner plates of automobiles. In particular, a steel sheet having a high r-value (Rankford value) and an excellent strength-elongation balance is required for a part having a high forming difficulty. Among these, it is known that the r value increases by performing recrystallization annealing at a high temperature. However, when annealing is performed at a high temperature, the crystal grains are coarsened, so that there is a problem that the balance between strength and elongation required for press forming is reduced.

The technology for producing a high r-value cold-rolled steel sheet for deep drawing is disclosed in Japanese Patent Publication No. 7-47764 and Japanese Unexamined Patent Publication No.
No. 2,239,731 discloses a method using Ti deoxidized steel, and it is shown that Ti deoxidized steel can obtain an r value 0.1 to 0.2 higher than Al deoxidized steel. However, these prior arts have not studied the balance of strength and elongation 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 high-tensile cold-rolled steel sheet having good surface properties and excellent strength-elongation balance, and a method for producing the same. A second object of the present invention is to propose a high-tensile cold-rolled steel sheet having excellent balance of surface properties and strength-elongation and having a tensile strength of 340 MPa or more, and a method for producing the same.
A third object of the present invention is to propose a technique for producing a high-tensile cold-rolled steel sheet that is effective for preventing generation of cluster-like inclusions harmful to the surface properties and effective for preventing nozzle clogging during continuous casting. It is in.

[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, the size, amount and composition of oxide-based inclusions remaining in steel are within specified ranges. If so, fine dispersion can be achieved without causing the above-described nozzle clogging, and without making the inclusions gigantic in a cluster form.Moreover, by controlling the grain growth during cold rolling and annealing, And found that the strength-elongation balance can be greatly improved, and arrived at the present invention.

The present invention developed based on such knowledge,
C ≦ 0.20 wt%, Si ≦ 2.0 wt%, Mn ≦ 3.0 wt%, P ≦ 0.15
wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.010wt% ≦ Ti ≦
0.50 wt%, containing Ca and / or metal REM ≥0.0005 wt%, and satisfying the following expression (1) or (2).
Containing Al, and wherein C, Si, Mn and P are the following (3)
Surface properties characterized by containing 0.002 to 0.015 wt% of oxide-based inclusions that satisfy the relationship of the formula, the balance being Fe and unavoidable impurities, and having a width of 50 μm or less. This is a high-tensile cold-rolled steel sheet that has a good balance of strength and elongation. Notation wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 9 (wt% C / 12) +2 (wt% Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05… (3)

The steel sheet of the present invention further comprises, in addition to the above components, Nb: 0.001 to 0.1 wt%, B: 0.0001 to 0.05 wt%,
It is preferable to contain any one or more of Cr: 0.01 to 2.0 wt% and Mo: 0.01 to 1.0 wt%.
In each of the steel sheets of the present invention, inclusions in the steel,
CaO and / or REM oxide: 5wt% or more in total
It is preferable that the oxide-based inclusion mainly contains Ti oxide and contains 50 wt% or less, Ti oxide: 90 wt% or less, and Al ₂ O ₃ : 70 wt% or less.

Further, the present invention relates to a method for producing a composition comprising: C ≦ 0.20 wt%, Si ≦ 2.0
wt%, Mn ≦ 3.0 wt%, P ≦ 0.15wt%, S ≦ 0.05wt%, N
≦ 0.01 wt%, 0.010 wt% ≦ Ti ≦ 0.50 wt%, Ca and / or metal REM ≧ 0.0005 wt%, and the following (1)
Contains Al in a range satisfying the formula or the formula (2), further contains C, Si, Mn and P satisfying the relationship of the following formula (3), and the balance consists of Fe and unavoidable impurities, and has a width of 50μ
0.002 to 0.015 wt%
The steel slab is heated and soaked at 900 ° C or higher, finish rolling at 650 ° C or higher, and wound at 400-750 ° C.
Next, the present invention proposes a method for producing a high-tensile cold-rolled steel sheet having good surface properties and an excellent balance of strength and elongation, which is characterized by performing cold rolling and then recrystallization annealing at 700 to 920 ° C. Notation wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 9 (wt% C / 12) +2 (wt% Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05… (3)

In the method according to the present invention, in addition to the above components, the steel slab further contains Nb: 0.001 to 0.1 wt.
%, B: 0.0001 to 0.05 wt%, Cr: 0.01 to 2.0 wt%, Mo:
It is a preferred embodiment to contain any one or more of 0.01 to 1.0 wt%. In each of the above methods of the present invention, the inclusions in the steel are CaO and / or REM oxide: 5 wt% to 50 wt% in total, Ti oxide: 90 wt% or less, Al ₂ O ₃ : 70 wt%. % Of oxide inclusions mainly containing Ti oxide.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an experimental study which will lead to the present invention will be described. In this experiment, C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.28 wt%,
P: 0.07 wt%, S: 0.006 wt%, Al: 0.004 wt%, N:
0.002 wt%, Ti: 0.02-0.06 wt%, O: 0.002-0.020
A sheet bar consisting of a composition of wt% and Ca: 0.001 wt% is heated to 1150 ° C, soaked, and then subjected to three-pass rolling so that the finishing temperature is 900 ° C. Board. Thereafter, coil winding was performed at 650 ° C. for 1 hour. Then, after further performing cold rolling of 75%, recrystallization annealing at 880 ° C. for 40 seconds was performed.

FIG. 1 shows the effect of the amount of oxide on the strength-elongation balance (TS × EL) of the steel sheet manufactured as described above. As a result of microscopic observation of these cold-rolled steel sheets, the size of the oxide-based inclusions was 50 μm or less in the sheet width direction. As is evident from Figure 1,
In the steel sheet of this component composition, TS × EL depends on the amount of oxide, and when the amount of oxide is 0.002 to 0.015 wt%, high TS × EL is obtained.
It was found that EL was obtained, and particularly when the amount of oxide was 0.004 to 0.012 wt%, higher TS × EL was obtained.

(1) Steel composition The composition of the steel sheet according to the present invention is as follows: C ≦ 0.20 wt%, Si ≦ 2.0 wt%, Mn ≦ 3.0 wt%, P ≦
0.15wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.010wt% ≦
Ti ≦ 0.50wt%, Ca and / or metal REM ≧ 0.0005wt
%, And wt% Ti / wt% Al ≧ 5 or Al ≦
0.010 wt% and wt% Ti / wt% Al <5 is contained in a range satisfying the condition of <5, and C, Si, Mn and P are 9 (wt% C / 12).
+2 (wt% Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05, and if necessary, further Nb: 0.001-0.1 wt%, B :
0.0001-0.05 wt%, Cr: 0.01-2.0 wt%, Mo: 0.01-1.
0 wt%, one or more of which contain 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 ≦ 0.20 wt% C has the effect of strengthening steel and is added in a necessary amount according to the desired strength. However, if the added amount exceeds 0.20 wt%, ductility decreases, and strength-elongation balance is reduced. Adversely affect
Limited to 0.20 wt% or less. (b) Si ≦ 2.0 wt% Si has the effect of strengthening steel and is added in a necessary amount depending on the desired strength. However, if the content exceeds 2.0 wt%,
Since the ductility is reduced and the strength-elongation balance is adversely affected.
Limited to 2.0 wt% or less. (c) Mn ≦ 3.0 wt% Mn has the effect of strengthening steel and contains the required amount according to the desired strength. However, if the content exceeds 3.0 wt%, ductility is reduced and strength elongation is reduced. The balance was adversely affected, so the content was limited to 3.0 wt% or less. (d) P ≦ 0.15wt% P has the effect of strengthening steel and contains the necessary amount according to the desired strength. However, if the content exceeds 0.15wt%, the ductility decreases, and the strength elongation decreases. Since the balance is adversely affected, the content is limited to 0.15 wt% or less. The preferred P content is 0.10 wt% or less. (e) S ≦ 0.05 wt% S is preferable because the smaller the content, the better the ductility. However, if the content is 0.05 wt% or less, there is no significant adverse effect, so the content is limited to 0.05 wt% or less. The preferred S content is 0.01 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 ductility is. However, if the content is 0.01 wt% or less, there is no significant adverse effect. did.
The preferred N content is 0.005 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 forms fine oxide-based inclusions having a size of 50 μm or less by Ti deoxidation. It is a component that controls the grain growth during cold rolling and annealing to improve the balance between strength and elongation. Further, since the fine oxide effectively acts on miniaturization of the hot-rolled sheet, it also has an effect of improving the r-value after cold rolling and annealing. If the Ti content is less than 0.010 wt%, the effect of addition,
That is, the desired effect described above cannot be obtained because the amount of the fine oxide is too small, so the lower limit is limited to 0.010 wt% or more. This Ti acts more effectively when added at 0.025 wt% or more. However, if it is added in excess of 0.50 wt%, ductility is reduced and not only desired material properties are impaired, but also cost increases, so the upper limit is set to 0.50 wt%.

(H) Ca and / or metal REM ≧ 0.0005
wt% Ca and metal REM are heavy in the steel sheet according to the present invention.
A component that plays an important role and can be either Ca or REM
One or two or more must be added in a total of 0.0005 wt% or more
There is. That is, after deoxidizing molten steel with Ti, Ca and
0.0005wt% of one or two of REM and REM
By the above addition, the oxide composition in the molten steel is changed to Ti acid
Compound: 90 wt% or less, preferably 20 wt% or more and 85 wt% or less,
CaO and / or REM oxide: 5 wt% or more, preferred
In the range of 8 wt% to 50 wt%, Al _TwoO_ThreeIs less than 70wt%
It is adjusted to be a low melting point oxide-based inclusion. this
If such adjustments are made, during continuous casting, Ti
To prevent nozzles from sticking to the nozzle and eliminate nozzle blockage
be able to. The addition of excessive Ca and REM causes rusting.
0.005 wt% or less in total
It is desirable to add in the range.

(I) 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, it becomes Al deoxidized steel, a large amount of Al ₂ O ₃ clusters are generated, and the surface properties of the steel slab are deteriorated, and the grain growth during cold rolling-annealing is controlled. Therefore, the amount of fine oxide of 50 μm or less is reduced, and the strength-elongation balance is inferior. Therefore, it is necessary that the Al content satisfies the above conditions or the above conditions, and the above conditions are particularly preferable ranges for achieving the object of the present invention.

(J) 9 (wt% C / 12) + 2 (wt% Si / 28) + 3 (wt%
Mn / 55) +30 (wt% P / 31) ≧ 0.05 As described above, all four elements of C, Si, Mn and P are strengthening elements necessary for increasing the tensile strength of steel, and have a tensile strength of 340.
9 (wt% C / 12) +2 (wt%
It is necessary to satisfy Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05.

(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 saturates and leads to deterioration of ductility, so it was limited to the range of 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%, the ductility will be degraded, so the content was limited to 0.0001 to 0.05 wt%. (m) Cr: 0.01 to 2.0 wt% Cr has the effect of strengthening steel and is added according to the desired strength. However, if the added amount is less than 0.01 wt%, there is no effect. If it exceeds, the ductility decreases and the strength-elongation balance deteriorates, so the content was limited to 0.01 to 2.0 wt% or less. (n) Mo: 0.01 to 1.0 wt% Mo has the effect of strengthening steel and is added depending on the desired strength. However, if the amount of Mo added is less than 0.01 wt%, there is no effect. If it exceeds, the ductility decreases and the strength-elongation balance deteriorates, so the content was limited to 1.0 wt% or less.

(2) Inclusion of Slab and Steel Sheet For the steel sheet of the present invention, the width direction of the steel sheet (direction perpendicular to the rolling direction)
It is necessary to adjust so as to contain 0.002 to 0.015 wt% of fine oxide inclusions having a size of 50 μm or less. 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. For this reason,
Control of fine oxide-based inclusions contained in a billet is one of the important components of the present invention. In particular, the inclusions produced under the method of the invention have a width (dimension in the direction perpendicular to the rolling) of 50 μm.
It is a granular or fractured oxide-based inclusion having the following size. If the oxide inclusions have a width of 50 μm or less,
It is possible to suppress crystal grain refinement during hot rolling and grain growth during cold rolling and annealing. However, inclusions having a width larger than 50 μm do not have the above-described effect. For this reason, the oxide inclusions were limited to those having a width of 50 μm or less.
When the content of the oxide-based inclusions is less than 0.002 wt%, the effect of controlling the grain growth is not effective.
If the content is more than wt%, the strength-elongation balance will be deteriorated, so the content is limited to 0.002 to 0.015 wt%. From the viewpoint of strength-elongation balance, the content of oxide-based inclusions is preferably 0.004 to 0.012 wt%. Where the width is 50μ
Granular or fractured oxide inclusions with a size of less than m are oxide inclusions formed from steel slabs, and relatively large ones are cut in the rolling direction by hot rolling and cold rolling. A fractured oxide-based inclusion that has been broken is referred to, 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.
For the material, Ti ≧ 0.010 wt%, wt% Ti / wt% Al ≧ 5,
Alternatively, it is necessary to melt steel having a component composition satisfying any of the following conditions: Al ≦ 0.010 wt% and wt% Ti / wt% Al <5. In this case, Ti as an adjustment component is replaced by Ti
The reason for setting ≧ 0.010 wt% is that when Ti <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% Ti) / (wt% Al) ≧ 5,
It is preferable that (wt% Mn) / (wt% Ti) <100. In this case, the concentration of Ti oxide in 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, a steel sheet in which Ti oxide-based inclusions are formed by deoxidation of Ti means that the inclusions are 1 to 50%.
Since the particles are dispersed in steel with a size of about μm, surface defects due to cluster-like inclusions are eliminated. However,
Ti oxide is in a solid phase in molten steel, and in ultra-low carbon steel, the solidification temperature of the steel is high, so it grows on the inner surface of the tundish nozzle with the metal ingested, causing nozzle blockage There is a possibility that.

Therefore, in the present invention, after deoxidation with a Ti alloy, one or two of Ca and REM are further added so that the content becomes 0.0005 wt% or more, and oxidation in molten steel and further in steel sheet is performed. The composition is as follows: Ti oxide: 90 wt% or less, preferably 20 wt% to 85 wt%, CaO and / or REM oxide: 5 wt% or more, preferably 8 wt% or more.
wt% or less, Al ₂ O ₃ is an oxide inclusions of low melting point which is below 70 wt%. Then, it becomes possible to effectively prevent the Ti oxide incorporating the metal from adhering to the nozzle.
A more desirable inclusion composition is as follows: Ti ₂ O ₃ : 30 wt% or more and 80 wt% or less, CaO, REM oxide (La ₂ O ₃ , Ce ₂ O
₃ ): 10 wt% or more and 40 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, it is desirable to control the content of SiO ₂ in the inclusions to 30 wt% or less and the content of MnO to 15 wt% 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 are set to Mn / Ti> 100 and Si / Ti> 50.
Is preferable. 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. Incidentally, the composition of the oxide-based inclusions in the steel described above, arbitrarily extract ten oxide-based inclusions,
It is determined from 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.

The size of the inclusion controlled as described above has a size of 50 μm or less. here,
The reason for limiting the size of inclusions to those of 50 μm or less is as follows:
This is because, in the deoxidation method according to the present invention, inclusions having a size of 50 μm or more are hardly generated, and the inclusions having a size of 50 μm or more are mainly caused by extrinsic inclusions such as slag or mold powder.
The amount of inclusions of 50 μm or less is 80% of the total amount of oxide-based inclusions.
It is desirable to make the presence of wt% or more in order to prevent coil surface defects and nozzle clogging.

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 in order to prevent the attachment 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: Heating of the slab prior to hot rolling is performed at a temperature of 900 ° C. or more. If the slab heating temperature is lower than 900 ° C., the load applied during rolling becomes too high, which causes operational problems. However, heating at a high temperature exceeding 1200 ° C. lowers the workability of the steel sheet, so the upper limit is desirably 1200 ° C. Therefore,
Slab heating temperature is 900 ℃ or more, preferably 900-1200 ℃
And 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 must be 650 ° C. 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 finishing rolling end temperature is limited to 650 ° C. or higher. However, if hot rolling is completed at a temperature higher than 960 ° C., scale-type surface defects may occur. Therefore, the finish rolling finish temperature is preferably set to 960 ° C. or lower. If the coil winding temperature after hot rolling exceeds 750 ° C, problems such as the scale becoming too thick will occur, and if it is lower than 400 ° C, the shape of the steel sheet will deteriorate.
Limit to the range of 750 ° C.

Cold Rolling Step: This step is necessary for obtaining a beautiful surface and high dimensional accuracy. Although the rolling reduction of the cold rolling is not particularly specified, it is 50 to 95 to achieve a high r value.
% Is desirable. This is because if the draft 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 cannot be obtained any more, and conversely, the r value decreases.

Annealing step: It is necessary to perform recrystallization annealing on the cold-rolled steel sheet after the cold rolling step. Annealing temperature is 700 ~ 920
° C. This is because if the annealing temperature is below 700 ° C,
This is because a sufficient recrystallized structure cannot be formed. Meanwhile, 92
Even if annealing is performed in a high temperature region exceeding 0 ° C., the crystal grains are coarsened, the strength-elongation balance is lowered due to α → γ transformation, and the r value is also deteriorated. Therefore, the annealing temperature is 700 ~
Limit to 920 ° C. The cooling process after annealing is not particularly limited, and depending on the structure of the steel sheet and the necessity of material adjustment,
There is no problem even if an appropriate temperature history is adopted. Further, the steel strip after annealing may be subjected to 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. Examples of the surface treatment include metal plating such as electrogalvanizing, hot-dip galvanizing, and alloyed zinc plating, as well as resin or oil film and various types of coating. At this time, the annealing may be performed in equipment in which the next step such as plating is continuous. Further, after annealing or surface treatment, the steel sheet of the present invention may be further subjected to a special treatment to improve the chemical conversion treatment property, coating property, corrosion resistance, weldability, press formability, and the like.

[0045]

EXAMPLES Inventive example: After tapping the converter, the molten steel was decarburized by an RH degassing device, and the components were preliminarily adjusted. In this molten steel,
Al is added at 0.2-1.0 kg / ton to reduce the dissolved oxygen concentration in the molten steel.
Reduced to 84-128 ppm. At this time, the Al concentration in the molten steel was 0.002 to 0.005 wt%. And to this molten steel, 70
0.8% to 2.2 kg / ton of a wt% Ti-Fe alloy was added to deoxidize Ti. Then, after adding iron alloys and adjusting the composition, the molten steel contained 30wt% Ca-60wt% Si alloy and Met.
Ca, Fe, additive mixed with 5 to 15 wt% REM, or Ca alloy such as 90 wt% Ca-5 wt% Ni alloy, REM alloy
The treatment was performed by adding 0.05 to 0.5 kg / ton of a Fe-coated wire. After this treatment, the Ti concentration is 0.038 to 0.071 wt%, the Al concentration is 0.002 to 0.005 wt%, and the Ca concentration is 0.0005 to 0.0016 wt%.
% And the REM concentration were 0.0000 to 0.0018 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 ~ 45wt% CaO-0 ~ 20wt% R
EM oxide-5-40 wt% 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 casting slab was heated at a slab heating temperature of 11
50 ° C, rolling end temperature 850-900 ° C, coil winding temperature 600
After hot rolling to a thickness of 4.0 mm at ~ 650 ° C, the thickness is 1.
It was cold rolled to 0 mm and further recrystallized and annealed in a continuous annealing line (CAL) or a hot dip galvanizing line (CGL). Table 1 shows the steel composition and the content of oxide-based inclusions at 1 μm.
Table 2 shows the average composition of the inclusions in the above main steel sheets.
Table 3 shows the manufacturing conditions and mechanical properties. In this case, the size of the oxide-based inclusions in the width direction was 50μ.
m or less. All the steel sheets had a tensile strength of 340 MPa or more, and TS × El was high, indicating an excellent strength-elongation balance. In addition, only 0.03 / 1000 m ² or less of surface defects of nonmetallic inclusions such as barbs and slivers were found in this annealed plate. And there was no problem in the amount of rust as in the conventional Al deoxidation. In addition, the surface quality of the steel sheet subjected to the electrogalvanizing and the hot-dip galvanizing after the cold rolling was also good.

[0048]

[Table 1]

[0049]

[Table 2]

[0050]

[Table 3]

Comparative Example: After tapping from the converter, the molten steel was decarburized by an RH degassing device, and the components were preliminarily adjusted. 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 0.034 to 0.037 wt%. After that, the composition of the components was adjusted by adding iron alloys. After this treatment, the Ti concentration was 0.041 to 0.068 wt%. Next, the molten steel was cast by a two-strand slab continuous casting apparatus to produce a continuously cast slab. Incidentally, in this case, the average composition of inclusions contained in the tundish molten steel, principal 93~96wt% Al ₂ O _3, cluster-like inclusions consisting 6 wt% or less of Ti ₂ O ₃ Met. If Ar gas was not blown into the tundish and immersion nozzle during casting, Al ₂ O ₃ was remarkably adhered to the nozzle and 2
At the 4th charge, the opening of the sliding nozzle significantly increased, and the casting was stopped due to nozzle clogging. Also,
Even when Ar gas was blown, a large amount of Al ₂ O ₃ adhered in the nozzle, and the fluctuation of the molten metal level in the mold became large at the 6th to 8th charges, and the casting was stopped. Next, the continuous cast slab was heated at a slab heating temperature of 1150 ° C and a rolling end temperature of 850 to 90.
After hot rolling to a sheet thickness of 4.0 mm at 0 ° C. and coil winding temperature of 600 to 650 ° C., cold rolling to a sheet thickness of 1.0 mm, and recrystallization annealing were performed in a continuous annealing line. Steel composition,
Table 1 shows the content of the oxide-based inclusions, Table 2 shows the average composition of the main inclusions in the steel sheet of 1 μm or more, and Table 3 shows the production conditions and mechanical properties as comparative examples. Was. Non-metallic inclusion defects such as barbs and slivers were found in the annealed sheet at 0.48 to 0.56 / 1000 m ² .

[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. It is a very high-strength steel sheet with almost no surface defects and extremely high tensile strength of 340 MPa and high TS × El. Further, since it has a superior strength-elongation balance as compared with a comparative steel having the same strength level, it is actually suitably used as a thin steel sheet for automobiles.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the amount of fine oxide-based inclusions on TS × EL.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Osamu Furukun, Inventor: 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Pref. Kawasaki Steel Engineering Co., Ltd. No address) Inside Mizushima Works of Kawasaki Steel Corporation (72) Inventor Taro Yahiro 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama Prefecture 1-chome Kawasaki-dori, Mizushima-shi (without address) F-term (reference) 4K037 EA02 EA04 EA05 EA06 EA09 EA11 EA15 EA16 EA17 EA18 EA19 EA23 EA25 EA27 EA28 EA31 FC02 FA03 FC03 FA03 FC05 FE01 FE02 FE03 FJ05 FJ06 HA05

Claims (6)

[Claims] 1. C ≦ 0.20 wt%, Si ≦ 2.0 wt%, Mn ≦ 3.0
wt%, P ≦ 0.15wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.
010 wt% ≦ Ti ≦ 0.50 wt%, Ca and / or metal REM
≧ 0.0005 wt%, and Al in a range satisfying the following formula (1) or (2), and the C, Si, Mn and P satisfy the relationship of the following formula (3), and The balance consists of Fe and unavoidable impurities, and contains 0.002 to 0.015 wt% of oxide-based inclusions having a width of 50 μm or less, and is characterized by good surface properties and excellent strength-elongation balance. Tension cold rolled steel sheet. Notation wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 9 (wt% C / 12) +2 (wt% Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05… (3) 2. The steel sheet according to claim 1, further comprising Nb: 0.001 to 0.1 wt%, B: 0.0001 to
It is characterized by containing one or more of 0.05 wt%, Cr: 0.01 to 2.0 wt%, and Mo: 0.01 to 1.0 wt%, and has high surface properties and excellent strength-elongation balance. Tension cold rolled steel sheet. 3. 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.
It is an oxide-based inclusion mainly composed of Ti oxide, containing 50 wt% or less, Ti oxide: 90 wt% or less, Al ₂ O ₃ : 70 wt% or less, good surface properties and strength High tension cold rolled steel sheet with excellent elongation balance. 4. C ≦ 0.20 wt%, Si ≦ 2.0 wt%, Mn ≦ 3.0
wt%, P ≦ 0.15wt%, S ≦ 0.05wt%, N ≦ 0.01wt%, 0.
010 wt% ≦ Ti ≦ 0.50 wt%, Ca and / or metal REM
≧ 0.0005 wt%, and Al in a range satisfying the following formula (1) or (2), and the C, Si, Mn and P satisfy the relationship of the following formula (3), and The remainder consists of Fe and unavoidable impurities. Heating and equalizing a steel slab containing 0.002 to 0.015 wt% of oxide-based inclusions with a width of 50 μm or less at 900 ° C or higher and finish rolling at 650 ° C or higher. Finishing and winding at a temperature of 400 to 750 ° C, then cold rolling, and then recrystallization annealing at 700 to 920 ° C. High tensile strength with good surface properties and excellent balance of strength and elongation Manufacturing method of cold rolled steel sheet. Notation wt% Ti / wt% Al ≧ 5 (1) Al ≦ 0.010 wt%, and wt% Ti / wt% Al <5 ... (2) 9 (wt% C / 12) +2 (wt% Si / 28) +3 (wt% Mn / 55) +30 (wt% P / 31) ≧ 0.05… (3) 5. The method according to claim 4, wherein the billet further comprises Nb: 0.001 to 0.1 wt%, B:
0.0001-0.05 wt%, Cr: 0.01-2.0 wt%, Mo: 0.01-1.
A method for producing a high-tensile cold-rolled steel sheet having good surface properties and excellent strength-elongation balance, characterized by containing one or more of 0 wt%. 6. The method according to claim 4, wherein the inclusions in the steel are CaO and / or REM oxide: at least 5 wt% in total.
It is an oxide-based inclusion mainly composed of Ti oxide, containing 50 wt% or less, Ti oxide: 90 wt% or less, Al ₂ O ₃ : 70 wt% or less, good surface properties and strength A method for manufacturing high-tensile cold-rolled steel sheets with excellent elongation balance.