JP2001026842A - Cold rolled steel sheet excellent in surface property and internal property and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress surface defects caused by clusterlike inclusions or the like and press cracking caused by large-granular inclusions by allowing a steel sheet to have a specified compsn. contg. C, Si, Mn, P, S, Al, N, Ta, Ca and metallic rare earth metals and composing granular or rupturelike oxide inclusions with specified size of specified amounts of Al2O3, the oxide of Ca and/or metallic rare earth metals and Ti oxide. SOLUTION: This steel sheet has a componential compsn. contg., by weight, <=0.010% C, <=1.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, <0.01% A1, <=0.01% N, 0.015 to 0.1% Ti and >=0.0005% Ca and/or rare earth metals, and the balance Fe with inevitable impurities. Then, the compsn. of granular or rupturelike oxide inclusions having the size of <=50 μm is composed of 10 to 30% Al2O3, 5 to 30% oxide of Ca and/or metallic rare earth metals and 50 to 90% Ti oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet having excellent surface properties and internal quality, in particular, by controlling the composition of oxide-based inclusions in steel to improve the resistance to press cracking due to inclusions. The present invention relates to a cold-rolled steel sheet and a method for producing the same.

[0002]

2. Description of the Related Art Steel is initially deoxidized with Fe-Ti instead of Al, 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 containing Al in an amount of 0.005 wt% or more has become mainstream.

[0003] Al deoxidation of steel is a method of aggregating and coalescing oxides produced by gas agitation or an RH degassing apparatus to separate them by flotation. In this case, Al ₂ O ₃ is contained in a slab. Oxides will inevitably remain. Moreover, since this Al ₂ O ₃ is clustered, it is difficult to separate, and sometimes several hundred μm.
The above cluster-like inclusions remain. If such cluster-like inclusions are trapped in the surface layer of the slab, they will lead to surface defects such as scabs and slivers. It becomes a defect. 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 Ca addition in this method was described above by reacting Al ₂ O ₃ and Ca to form low melting point composite oxides such as CaOAl ₂ O ₃ , 12CaOAl ₂ O ₃ , 3CaOAl ₂ O ₃ . It tries to overcome the problem.

[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. In addition, if surface treatment such as plating or painting is performed with such rusting points remaining, 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.

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:
By deoxidizing so as to be 0.03 to 1.5 wt% and Ti: 0.02 to 1.5 wt%, non-aging cold containing low melting point inclusions made of MnO-Ti oxide containing 17 to 31 wt% of MnO 2. We have proposed rolled steel sheets. In the case of this proposal, the MnO-Ti oxide has a low melting point and is in a liquid phase state in molten steel, so that even when the molten steel passes through the tundish nozzle, it is injected into the mold without adhering to the 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. 40, in order to obtain a MnO-Ti oxide containing MnO: 17-31%, Mn, Ti From the difference in affinity between oxygen and oxygen, the concentration ratio of Mn and Ti in the molten steel was calculated as follows: (wt% Mn) / (wt% Ti)> 10
Must be 0. Therefore, the Ti concentration in steel is 0.01
In the case of 0 wt%, in order to obtain the required MnO-Ti oxide,
Mn concentration must be 1.0 wt% or more. However, if the Mn content exceeds 1.0 wt%, the material hardens. Therefore, MnO
It was practically difficult to form inclusions of MnO-Ti oxide containing 17-31 wt%.

[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 nozzle clogging.On the other hand, when the oxygen concentration is low, there is a problem that the nozzle is melted, and it is not a sufficient measure .

[0011] Further, in the above-mentioned prior arts relating to nozzle clogging prevention, 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. What is more important is that if the conventional form control method is used to make granular inclusions,
Press cracking, which is difficult to occur with conventional cluster-like inclusions, is likely to occur, which may be a serious problem.

[0012]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and it is difficult to generate surface defects due to cluster-like inclusions and the like. It is an object of the present invention to propose a cold-rolled steel sheet having good surface properties and good internal quality, and a manufacturing technique therefor, which hardly causes press cracking.

[0013]

Means for Solving the Problems The inventors have conducted intensive studies to achieve the above object, and as a result, have reached the following conclusions. 1) In order to improve the surface properties of the steel sheet, it is necessary to reduce the melting point of inclusions and suppress nozzle clogging in continuous casting, thereby suppressing powder entrapment and air bubble adhesion. 2) If the granular inclusions inside the steel sheet have a specific composition, it is possible to suppress the aggregation of the inclusions. When the content is out of the specific composition range, the inclusions are rapidly aggregated, and are likely to be coarse. 3) The composition of the inclusions is not uniquely determined by the composition of the molten steel, but is determined kinetically, and is affected by the timing of addition of the deoxidizing agent in addition to the stirring during the secondary refining treatment such as RH degassing. You. This addition is performed at the time of Al ₂ O
₃ becomes a nucleus, and greatly affects the composition and particle size of the Ti-Ca-Al-O-based inclusions formed thereafter.

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%, Al <0.01wt%, N ≦ 0.01wt%,
0.015 wt% ≦ Ti ≦ 0.1 wt%, Ca and / or metal RE
M ≧ 0.0005 wt%, and the balance is a component composition of Fe and unavoidable impurities. The composition of the particulate or fractured oxide-based inclusion having a size of 50 μm or less is Al ₂ O.
₃ : A cold-rolled steel sheet having excellent surface properties and internal quality, characterized by 10 to 30 wt%, oxide of Ca and / or metal REM: 5 to 30 wt%, and Ti oxide: 50 to 90 wt%. .

Further, the steel sheet according to the invention is characterized in that the molten steel after tapping is made of Al
In the production through the step of adding Ca and / or REM after deoxidation with Ti and Ti, between the amount of oxygen before the addition of Al and the time from the addition of Al to the addition of Ti, a _{O 2} / t ≦ 100 (however, a _O : the amount of oxygen (ppm) before the addition of Al, t: the time (min) from the addition of Al to the addition of Ti) so as to satisfy the relationship. It is manufactured by

[0016]

Next, the reason why the constituent elements of the present invention are limited to the above range will be described. (a) C ≦ 0.010 wt% As the smaller the content of C, the deeper the drawability, the better the formability can be obtained by setting the content to 0.010 wt% or less.

(B) Si ≦ 1.0 wt% Si has an effect of strengthening steel, and contains a necessary amount according to a desired strength. However, if the content exceeds 1.0 wt%, deep drawability deteriorates. Therefore, the content was limited to 1.0 wt% or less.

(C) Mn ≦ 3.0 wt% Mn has the effect of strengthening the steel and contains a necessary amount according to the desired strength. However, if the content exceeds 3.0 wt%, the deep drawability decreases. Therefore, it was limited to 3.0 wt% or less. In addition, a preferable range is 0.01 to 2.0 wt%.

(D) P ≦ 0.15 wt% P has the effect of strengthening steel and contains a necessary amount according to the desired strength. If the content exceeds 0.15 wt%, the deep drawability becomes poor. The content was limited to 0.15% by weight or less because it decreased.

(E) S ≦ 0.05 wt% It is preferable that S is smaller, since the deep drawability is improved.
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.

(F) Al <0.01 wt% Al is a component that plays an important role in the present invention. If the content is 0.01 wt% or more, Al deoxidation occurs, and a large amount of giant Al ₂ O ₃ clusters are generated, deteriorating the surface properties. Further, the amount of fine oxide of 50 μm or less that can control the grain growth during cold rolling and annealing is reduced, and the strength-elongation balance is reduced. Therefore, the Al content was less than 0.01 wt%.

(G) N ≦ 0.01 wt% It is preferable to reduce the N content because the smaller the content, the better the deep drawability is. However, if the content of N is 0.01 wt% or less, there is no significant adverse effect. It was limited to wt% or less.

(H) 0.015 wt% ≦ Ti ≦ 0.1 wt% Ti is an important element in the steel sheet of the present invention. By deoxidizing Ti, fine oxide-based inclusions having a size of 50 μm or less are formed, A component that controls grain growth during elongation-annealing and improves strength-elongation balance. Further, since this fine oxide effectively acts on miniaturization of a hot-rolled sheet, {111} recrystallization texture develops after cold rolling and annealing, and the r-value is improved. If the Ti content is less than 0.015 wt%, the desired effect cannot be obtained because the effect of addition, that is, the amount of the fine oxide is too small, so the lower limit is limited to 0.015 wt%. On the other hand, if added in excess of 0.1% by weight, the material is hardened and the moldability is impaired, and the cost is increased. Therefore, the upper limit is set to 0.1% by weight.

(I) Ca and / or metal REM ≧ 0.0005
wt% (total amount) Ca and metal REM are important elements in the present invention, and any one or two of Ca and REM are used in total.
It is necessary to add 0.0005 wt% or more. In other words, molten steel
After Ti deoxidation, one or more of Ca and REM are added by a total of 0.0005 wt% or more,
It can be adjusted to Ti-Ca-Al-O-based low melting point oxide-based inclusions. By performing such adjustment, it is possible to prevent the Ti oxide containing the base metal from adhering to the nozzle during continuous casting, thereby preventing nozzle blockage. In addition, CaO and / or RE
The M oxide also contributes to grain growth after cold rolling and annealing and grain refinement of a hot rolled sheet. Since excessive addition of Ca and REM may cause rusting of the steel sheet, it is preferable to add Ca and REM in an amount of 0.005 wt% or less (total amount).

(2) Inclusions in Steel Sheet In the present invention, the composition of inclusions in the steel is Al ₂ O ₃ : 10 to ₃₀ wt.
%, Oxide of Ca and / or metal REM: 5-30 wt%
%, Ti oxide: 50 to 90 wt% to lower the melting point of the inclusions and increase the wettability with the molten steel, thereby reducing the cohesiveness of the inclusions. As a result, large-sized particulate inclusions in the slab do not remain, and finally press cracking of the steel sheet is prevented, and the pressability is improved. Figure 1 shows a TiO ₂ -CaO -A1 ₂ O ₃ phase diagram. The low melting point composition from this phase diagram is particularly due to TiO ₂ : Al ₂ O
The weight ratio of ₃ : CaO is around 7: 1.5: 1.5. Further studies were conducted based on such a tendency. In the present invention, Al ₂ O ₃ : 10 to ₃₀ wt%, Ca and / or metal REM oxide: 5 to 30 wt% (total Amount), Ti oxide: 50 to 90 wt%. If the composition is out of this range, the inclusions are in the liquid phase in the process before the tundish, but the temperature of the molten steel drops in the mold, so that the inclusions become solid and agglomerate rapidly, and large inclusions are removed. Will be generated. Thus, in the composition range according to the present invention, the generation of large inclusions is suppressed, and it is possible to suppress press cracking.

In the present invention, the inclusion diameter defined in the above composition is 50 μm.
m or less, since inclusions having a size exceeding approximately 50 μm include those resulting from the incorporation of slag and mold flux, to determine the composition of the deoxidation product,
This is because inclusions having a size exceeding μm must be excluded from the composition. The size of the inclusions in the steel sheet in the present invention means the size in the width direction of the steel sheet (the direction perpendicular to the rolling direction). Because the size of the inclusions present in the slab is elongated in the rolling direction by rolling,
It hardly changes in the width direction. Therefore, those having the inclusion size in the width direction of the steel sheet within the above range can be considered to be fine within this size range even at the slab stage. Further, the granular or fractured oxide-based inclusions referred to in the present invention are, when the oxide-based inclusions generated by the steel slab are relatively small, granular ones that maintain their shape, A relatively large one means a fractured one which is divided in the rolling direction by hot rolling and cold rolling.

(3) Production Method The steel sheet of the present invention is produced by deoxidizing molten steel after tapping with Al and Ti and then adding Ca and / or REM. At this time, between the amount of oxygen before the addition of Al and the time from the addition of Al to the addition of Ti (that is, the deoxidation time of Al alone), a _O / t ≦ 100 (where a _O : It is necessary to add oxygen in such a way that the relationship between the amount of oxygen (ppm) before Al addition and the time (t: the time from the addition of Al to the addition of Ti (min)) is satisfied. When Al is added to molten steel (usually, molten steel after decarburization treatment by RH degassing equipment), Al ₂ O ₃ clusters are generated immediately thereafter. FIG. 2 shows a photograph of the inclusion after addition of each deoxidizing agent. The Al ₂ O ₃ cluster is likely to be large, and if it is large, it will be 1 mm or more. The size and diameter of the Al ₂ O ₃ clusters, substantially determined by dissolving oxygen a _O before Al addition, as large as a _O large amount of clusters, both diameter. This Al ₂ O ₃ cluster is composed of Ti ₂ O ₃ after Ti addition and Ti-Al-
Affects the composition and particle size of Ca (REM) -based spherical inclusions.
If the amount of Al ₂ O ₃ before adding Ti is large, the final inclusion Al ₂ O 3
₃ It was found that the concentration was increased, and the inclusions were easily aggregated and coarsened. Also, after adding Al, Ti
Time until the addition of Ti
Through the ₂ O ₃ cluster, it ultimately affects the amount of large inclusions in the product.

If the value of the parameter a _O / t derived on the basis of these facts is set to 100 or less, the composition of the inclusion becomes Al ₂ O ₃ : 10 to ₃₀ wt%, Ca and / or metal REM.
Oxide: 5 to 30% by weight, Ti oxide: 50 to 90% by weight, and a lower melting point can be achieved. As a result, it is not necessary to blow a gas such as Ar or N ₂ into the tundish or the immersion nozzle to prevent the adhesion of oxides and the like, and it is possible to prevent the bubble defect due to the blown gas. Moreover, this slab is further
By hot rolling, cold rolling and annealing, it becomes possible to produce a cold rolled steel sheet having excellent surface properties and internal quality. In the present invention, REM oxide (for example, La ₂ O ₃ , CeO ₂ )
Confirmed the same role as CaO. In the secondary refining equipment, processing other than RH degassing, for example, VOD
It is needless to say that the same effect can be obtained even when the composition is controlled within the range of the present invention.

As another condition for producing the steel sheet of the present invention, the slab is desirably heated at a temperature of 900 to 1300 ° C. in view of the rolling load and the crystal grain size. For the same reason, the temperature is preferably in the range of 650 to 960 ° C. It is desirable that the rolling reduction in cold rolling in cold rolling be 50 to 95% from the viewpoint of ensuring workability, and the temperature of recrystallization annealing after cold rolling be 700 to 920 ° C.

[0030]

EXAMPLE Al was added to molten steel that had been rimmed after tapping in a RH treatment facility, and then Ti was added. Then, Ca
(REM for some) with a wire,
A slab was formed by continuous casting. This slab is hot rolled (finish temperature: 820 ° C), cold rolled, and continuously annealed (annealing temperature: 800 ° C)
Thus, a cold-rolled sheet and a galvanized sheet having a sheet thickness of 0.75 mm were obtained. Tables 1 and 2 collectively show the above production conditions and the results of the investigation on the properties of the obtained cold-rolled steel sheets. In the above manufacturing process, Al added before lysis oxygen a _o was measured with an oxygen probe utilizing concentration cell. The composition of the inclusions in the steel sheet was determined by analyzing the composition of each inclusion having a diameter of 50 μm or less by an analysis method such as EPMA or EDX, and determining the average value of 10 values. In addition, the surface properties of cold-rolled steel sheets (cold-rolled sheets and galvanized sheets) were checked by visual inspection of the steel sheets. The number was converted to an index. From Tables 1 and 2, _ao /
It can be seen that the invention example in which the composition of the inclusion was adjusted by setting t to 100 (ppm / min) or less significantly improved both the surface properties and the press crack resistance. In the present invention, the clogging of the immersion nozzle in the continuous casting process did not occur, but when the Al concentration in the steel was 0.01% or more (Al-killed steel), Ar blowing from the immersion nozzle was not performed. Nozzle clogging occurred and casting became impossible.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, according to the present invention,
Without infused with Ar and N ₂ or the like of gas during continuous casting without causing clogging of the immersion nozzle, moreover the surface of the cold-rolled steel sheet has no surface defects caused by nonmetallic inclusions,
Since there are no coarse inclusions inside, press cracking can be avoided and excellent pressability can be obtained.

[Brief description of the drawings]

FIG. 1 is a phase diagram of a Ti—Ca—Al—O system.

FIG. 2 is a view showing a shape change of an oxide-based inclusion.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Kenichi Sorimachi 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel Co., Ltd. (72) Inventor Seiji Nabeshima 1-chome Mizushima Kawasaki-dori, Kurashiki City, Okayama Prefecture None) Kawasaki Steel Corporation Mizushima Works F term (reference) 4K020 AA22 AC07 AC09 BB22

Claims (2)

[Claims] 1. C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.
0 wt%, P ≦ 0.15 wt%, S ≦ 0.05 wt%, Al <0.01 wt%,
N ≦ 0.01 wt%, 0.015 wt% ≦ Ti ≦ 0.1 wt%, Ca and / or
Or, the composition of the oxide-based inclusions containing metal REM ≧ 0.0005 wt%, the balance being Fe and unavoidable impurities, and having a size of 50 μm or less, and having a granular or fractured form of Al ₂ O ₃ : 10-30 wt%, Ca and / or metal R
A cold rolled steel sheet having excellent surface properties and internal quality, characterized in that EM oxide: 5 to 30 wt% and Ti oxide: 50 to 90 wt%. 2. A process in which molten steel after tapping is deoxidized with Al and Ti and then Ca and / or REM is added.
C ≦ 0.010 wt%, Si ≦ 1.0 wt%, Mn ≦ 3.0 wt%, P ≦ 0.
15wt%, S ≦ 0.05wt%, Al <0.01wt%, N ≦ 0.01wt%,
0.015 wt% ≦ Ti ≦ 0.1 wt%, Ca and / or metal RE
In producing a cold-rolled steel sheet containing M ≧ 0.0005 wt%, with the balance being Fe and unavoidable impurities, between the amount of oxygen before adding Al and the time from adding Al to adding Ti. to, a _O / t ≦ 100, however, a _O: the amount of Al added before oxygen (ppm), t: Al
A method for producing a cold-rolled steel sheet having excellent surface properties and internal quality, characterized in that a time (min) from the addition of Ti to the addition of Ti is established.