JP2000001736A - High tensile strength steel plate excellent in deformability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength steel plate less in rusting, almost free from deterioration in deformability due to inclusions and precipitates and free from surface defects caused by the cluster-like inclusions. SOLUTION: This steel plate contains, by weight, 0.03-0.20% C, <=2.0% Si, 0.05-2.5% Mn, <=0.15% P, 0.015-0.4% Ti, <=0.01% Al, <=0.02% N, and 0.0005-0.1% one kind or two kinds of Ca and REM in total and in which the total content of S and one kind or two kinds of Ca and REM satisfies the relation of expression S-5×[(32/40)Ca+(32/140)REM]<=0.0014 wt.% and the balance is composed of Fe with inevitable impurities. Further, oxide inclusions of 1-50 μm maximum size contain Ti oxide and one kind of two kinds of CaO and REM oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-tensile steel sheet, and more particularly to a high-tensile steel sheet having excellent deformability by controlling oxide-based inclusions in the steel.

[0002]

2. Description of the Related Art Conventionally, high-tensile steels have been used for various applications by utilizing their excellent strength. In particular, in recent years, high-tensile steel sheets having excellent workability have been developed, and are becoming widespread in applications requiring complicated processing such as automobile bodies. By the way, when such a high-tensile steel plate is processed into a complicated shape by a press or the like, a problem that cannot be overlooked is encountered. That is, a local crack at the time of forming processing accompanied by a large deformation locally (for example, a crack near the opening at the time of deep drawing of a material including the opening). For such processing, it is necessary to increase the deformability of the material in a state where stress is concentrated in a minute range in the material (this is referred to as local deformability herein). is there. To cope with such a lack of local deformability peculiar to a high-tensile steel sheet, conventionally, as disclosed in Japanese Patent Application Laid-Open No. 5-195144, it has been responded by exclusively controlling the components in the steel. Even with these efforts, the above problems have not been overcome.

[0003]

SUMMARY OF THE INVENTION The present invention provides an excellent high-tensile steel sheet which fundamentally solves such high tensile strength and high deformability, in particular, lack of local deformability, by means different from conventional techniques. The purpose is to propose.

[0004]

SUMMARY OF THE INVENTION The present inventors have achieved the above object.
As a result of intensive research to achieve
Controls the composition of the inclusions
Control of oxides and sulfides to improve deformability
We concluded that it was effective. That is, a huge cluster
-Size of 50μm or less
To reduce the amount of MnS in steel.
To make all oxides and sulfides in steel finer and less ductile.
Is effective in improving the deformability,
Various problems such as ball, rust and deterioration of surface properties can be solved.
I found that. The present invention based on the above knowledge,
C: 0.03 to 0.20 wt%, Si: 2.0 wt% or less, Mn: 0.05 to 2.
5 wt%, P: 0.15 wt% or less, Ti: 0.015-0.4 wt%, A
l: 0.01 wt% or less, N: 0.02 wt% or less, Ca, REM
One or two of the above in a total of 0.0005 to 0.1 wt%
The content of one or two of S and Ca, REM satisfies the following equation: S-5 × ((32/40) Ca + (32/140) REM) ≤ 0.0014 wt%, with the balance being Fe and For the composition of unavoidable impurities
Oxides with a particle size (maximum diameter; the same applies hereinafter) of 1 to 50 μm
The inclusion is one or two of Ti oxide and CaO, REM oxide
High tensile strength with excellent deformability characterized by containing
It is a steel plate. Further, in the present invention, the particle size is 1 to 50 μm.
The oxide inclusion of m is Ti oxide: 20 wt% or more and 90 wt% or less
Below, the sum of one or two of CaO and REM oxides: 10 wt% or less
Upper 40wt% or less, Al_TwoO_Three: 40% or less (Ti oxide, CaO, RE
One or two M oxides, Al_TwoO _ThreeIs less than 100%)
It is preferred that In addition, the steel sheet of the present invention
It is a high-tensile steel with a tensile stress of 440 MPa or more.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of research on which the present invention is based will be described below. The inventors simulated how the local deformability of a high-strength steel sheet was reduced by a hole expansion test of the steel sheet, and clarified the cause. As a result, it was clarified that there were elongated inclusions of MnS system and large inclusions of clustered alumina as the origins of the cracks at the cracked portions. In other words, although the composition and structure of the steel in the matrix are of course important, it was thought that a sufficient improvement in local deformability could not be achieved without appropriately controlling the inclusions generated in the steelmaking stage. Therefore, the inventors have conceived of controlling the shape of the inclusions in the steel sheet so that the inclusions are difficult to elongate and are finely dispersed.

In order to improve the deformability, it is important to 1) not coarsen oxides in steel and 2) not to coarsen sulfides in steel. Regarding the oxide of the above 1), when the content of Al is 0.01 wt% or less, the content of Ti is 0.015 wt% or more, and the content of Ca or REM satisfies 0.0005 wt% or more, the oxide is changed from the main component of Al ₂ O _3. Changes to Ti-based oxides.
Since Ti-based oxides have good wettability with molten steel, it is difficult to form clusters. For this reason, it does not become coarse unlike the inclusion mainly composed of Al ₂ O ₃ . Furthermore, Ca and / or REM is 0.0005-0.1
wt%, Ti oxide-Ca and / or RE
The oxide becomes a composite oxide of M oxide, and a sulfide to be described later is absorbed therein.

[0007] Regarding the sulfide of the above 2), it is important to suppress MnS precipitated during solidification, and if MnS is present, it extends during rolling and promotes cracking during processing. To this end, S in the steel is fixed by Ca and / or REM, which forms more stable sulfides. For this purpose, the amount of S and Ca
S-5 × ((32/40) Ca + (32/140) REM)) ≤ 0.0014 wt% (where S is the S content (wt%) and Ca is the Ca content (wt %), RE
M indicates the amount of REM (wt%). ) It was decided that it was necessary to satisfy the following relationship. That is, CaS, RE
To fix S by the formation of M sulfide, Ca, REM
The larger the amount of addition, the better, and the lower limit is given by the above inequality. That is, an experimental result was obtained that the amount of unfixed S needs to be 0.0014% or less.

However, when S in steel is fixed with Ca and / or REM, there is another concern that rust is likely to occur. Therefore, in the present invention, the content of steel in Al
Is 0.01 wt% or less, Ti is 0.015 wt% or more, and Ca and / or REM satisfies the condition of 0.0005 wt% or more. At this time, the inclusion is Ti oxide-Ca
O and / or REM oxide -Al ₂ O ₃ -SiO ₂ based oxides (Al
When they do not contain Ti, they are Ti oxides-CaO and / or REM oxides-SiO ₂ -based oxides), and rust generation starting from inclusions is suppressed. Furthermore, the Ca concentration in the inclusions
When it is 40 wt% or less, it does not become a starting point of rust, and the surface properties are good. If the amount of Al exceeds 0.01 wt%, the inclusions become Al ₂ O ₃ —CaO-based,
O concentration becomes about 50%, which becomes the starting point of rust and deteriorates corrosion resistance.

Furthermore, since the oxide-based inclusions described above have a low melting point, they hardly grow by adhering to an immersion nozzle or the like at the time of casting. Ar gas or N ₂ inside the nozzle
It was confirmed that there was almost no need to inject gas.

As a result of various studies based on the above experimental results, the present inventors have limited the present invention as follows. Less than,
The reasons for limitation for each component are shown.

(C: 0.03 to 0.20 wt%) C is a very powerful component for strengthening steel. In the steel of the present invention, it is necessary to contain 0.03 wt% or more in order to obtain a tensile strength (TS) of 440 MPa or more. However, if the amount is too large, a problem such as deterioration of weldability or remarkable decrease in ductility occurs, so the upper limit is made 0.20 wt%. The preferred lower limit is 0.05 wt%, and the preferred upper limit is 0.15 wt%.
%.

(Si: 2.0 wt% or less) Si is a component useful for strengthening steel and increasing tensile strength. In particular, it is a desirable component in the production of high-strength steel sheets because it increases the tensile strength without greatly reducing ductility. However, if the content exceeds 2.0 wt%, the hot deformation resistance of the steel increases significantly, which hinders the production of thin steel sheets. In addition, the paintability and corrosion resistance of the surface are reduced. Therefore, Mn is set to 2.0 wt% or less. In addition, a more preferable upper limit is 1.5 wt% from the viewpoint of paintability. The lower limit is 0.2 wt% from the viewpoint of improving the balance between strength and ductility.
% Is preferable.

(Mn: 0.05 to 2.5 wt%) Mn is a component effective for deoxidation, is a useful reinforcing component, and is a preferable additive component because it contributes to finer structure. To achieve these effects, it is necessary to add 0.05 wt% or more. However, when added in excess of 2.5 wt%, the deformation resistance during hot rolling increases and the rollability decreases. Note that a preferable lower limit is 0.30 wt% and a preferable upper limit is 2.0 wt% for securing the strength.

(P: 0.15 wt% or less) P has the effect of strengthening steel and is added in a necessary amount depending on the desired strength. However, if the added amount exceeds 0.15 wt%, the weldability is inferior. wt
% Or less. In addition, in applications requiring particularly severe weldability, 0.04 wt% or less is desirable.

(Ti: 0.015 to 0.4 wt%) Ti is an important component in the present invention. By deoxidizing Ti, fine oxide inclusions having a size of 50 μm or less are formed to control the form of sulfide. It also has an effect, and is particularly effective for improving ductility in a direction perpendicular to the rolling direction. Further, Ti plays a role of obtaining high tension by precipitation strengthening. Furthermore, the grain growth during cold rolling and annealing is controlled to improve the strength-elongation balance. Furthermore, the fine oxide is also effective for miniaturization of a hot-rolled sheet, so that stretch flange characteristics represented by hole expandability are improved. If the addition amount is less than 0.015 wt%, the effect of addition, that is, the amount of the fine oxide is too small, and the desired effect cannot be obtained. Therefore, the addition amount is limited to 0.015 wt% or more. on the other hand,
If the Ti content exceeds 0.4 wt%, the desired effect is saturated, but the hot rollability is reduced and the properties of the slab surface are deteriorated. Therefore, the Ti content is limited to 0.4 wt% or less.

Al: 0.01% by weight or less Al is a component whose content has a significant effect on the properties in the present invention. If the Al content exceeds 0.01% by weight, Al deoxidation occurs preferentially, so that giant Al _A large amount of ₂ O ₃ clusters are generated, deteriorating the surface properties, and the amount of fine oxide of 50 μm or less that can control the grain growth of the hot-rolled sheet is reduced, so that the strength-elongation balance is reduced. Therefore, 0.01wt
% Or less. More importantly, inclusion composition and Al amount is larger than this becomes the Al ₂ O ₃ -CaO or Al ₂ O _3-REM oxide, and the starting point of rust, degrade the corrosion resistance. From this viewpoint, the upper limit of Al is set to 0.01 wt%. It is not always necessary to add Al, and Al is not essential even as a deoxidizing agent by performing Ti deoxidation or the like.

(N: 0.02 wt% or less) N is one of useful components because it contributes as a solid solution strengthening component. However, the addition of more than 0.02 wt% is not only difficult to operate, but also has the effect of reducing a desirable effect of Ti because a part of the added Ti is fixed as TiN. is there. In addition, a more preferable upper limit is 0.01 wt%.

(Ca and / or metal REM: 0.0005 to 0.1 wt.
%) Ca and metal REM (refers to rare earth elements such as La and Ce)
Is an important component in the present invention, and it is necessary to add at least 0.0005% by weight of one or more of Ca and REM. That is, after the deoxidation of Ti, one or two of Ca and REM are further added so as to be 0.0005 wt% or more, and the oxide composition in the molten steel is adjusted to Ti oxide: 20 wt% or more and 90 wt% or more.
wt% or less, preferably 85 wt% or less, CaO and / or REM
Oxide: A low-melting-point oxide inclusion containing 10 wt% or more and 40 wt% or less and Al ₂ O ₃ of 40 wt% or less. Then, at the time of continuous casting, it is possible to effectively prevent the Ti oxide containing the metal from adhering to the nozzle, and to prevent the nozzle from being blocked. In addition, Ca
O 2 and / or REM oxide can contribute to grain growth after cold rolling and annealing and grain refinement of a hot rolled sheet. From these, Ca,
One or two types of REM are contained in a total of 0.0005 wt% or more.However, if the total amount exceeds 0.1 wt%, in addition to difficulty in melting and deterioration of corrosion resistance, the upper limit is set to 0.
Limited to 1 wt%.

(S−5 × ((32/40) Ca + (32/140) REM)
≦ 0.0014 wt%) S can exist as various sulfides in the steel, but when it exists as MnS-based inclusions, it significantly expands in the rolling direction during hot rolling, and Deteriorate mechanical properties. By adding Ca and REM, the improvement of the deformability, which is the main purpose of the present invention, becomes remarkable,
The necessary requirement is that the amount corresponding to S not fixed by Ca and REM is 0.0014% or less.

(O: 0.010 wt% or less) O is an unavoidable component and is not particularly limited, but is a component necessary to some extent to generate fine oxides. However, if the content exceeds 0.010 wt%, a large amount of coarse Al ₂ O ₃ is generated, which lowers the deformability and rust resistance.
The upper limit was 0 wt%. The preferred upper limit is 0.007 wt.
%, More preferably 0.005 wt% or less.

In a steel satisfying the above-mentioned composition ranges, oxide inclusions having a particle size of 1 to 50 μm contain Ti oxide and Ca
It is particularly important in the present invention that the inclusions contain one or two of O 2 and REM oxides. Inclusions as such deoxidation products include Ti oxide and CaO, REM oxide.
One containing one or two kinds, more specifically, Ti oxide-CaO and / or REM oxide-Al ₂ O ₃ —SiO ₂ type oxide (If Al is not contained, Ti oxide-CaO And / or RE
M oxide-SiO ₂ oxide) -based inclusions cause less rust, little deterioration in deformability due to inclusions and precipitates, and no surface defects due to cluster-like inclusions. In addition, the high-strength steel sheet expected in the present invention, in which the Ti oxide containing the metal is not adhered to the nozzle, is obtained. The oxide-based inclusions defined in the present invention have a particle size of 1 to 50 μm.
The reason for limiting inclusions to m is that inclusions in such a range can be regarded as inclusions generated by deoxidation, and inclusions having a particle size of more than 50 μm are generally used as slag or mold powder. Exogenous inclusions are the main cause.

The above-mentioned oxide-based inclusions having a particle size of 1 to 50 μm
Composition: Ti oxide: 20 wt% or more and 90 wt% or less, CaO, REM
Total of one or two oxides: 10 wt% or more and 40 wt% or less,
Al_TwoO _Three: 40% or less (one of Ti oxide, CaO, REM oxide
Or 2 types, Al_TwoO_ThreeIs less than 100%)
More preferred.

If the content of the Ti oxide is less than 20 wt%, the steel is not Ti-deoxidized steel but Al-deoxidized steel, and the Al ₂ O ₃ concentration increases, so that nozzle clogging occurs. Also, CaO, R
As the EM oxide concentration increases, the rusting property becomes remarkable.
The oxide concentration is set to 20% wt% or less. On the other hand, if the Ti oxide concentration exceeds 90 wt%, the proportion of CaO and REM oxides will decrease and nozzle clogging will occur rather. Therefore, the Ti oxide concentration is set to 20 wt% or more and 90 wt% or less. More preferably
30 wt% or more and 80 wt% or less.

In addition, the CaO and REM oxides in the inclusions
If the total of one or two kinds is less than 10% by weight, the inclusions do not have a low melting point and cause the nozzle clogging described above. on the other hand,
If it exceeds 40% by weight, the inclusions subsequently absorb S and change to water-soluble, which becomes the starting point of rust. In addition, a more preferable range is
20 to 40% by weight.

Further, regarding Al ₂ O ₃ in the inclusions,
If it exceeds 40% by weight, the composition will have a high melting point and not only will the nozzle be clogged, but also the inclusions will be clustered and defects of non-metallic inclusions on the product plate will increase. When Al is hardly contained in steel, the concentration of Al ₂ O _{3 in} inclusions is almost negligible.

Incidentally, oxides other than those listed above may be mixed in the oxide-based inclusions. In this case, the amount of the oxides other than those listed above is not particularly limited. but not, for SiO ₂ is less than 30wt%, MnO
Is preferably controlled to 15 wt% or less. The reason for this is that if they exceed the respective amounts, they cannot be said to be the titanium-killed steels targeted in the present invention, and 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, it is preferable that the molten steel have Si and Mn concentrations of Mn / Ti> 100 and Si / Ti> 50. If hardened steel,
This leads to deterioration of surface properties.

Next, the method for producing steel according to the present invention will be described. In the present invention, Ti as an adjusting component is
The reason for setting i: 0.015 wt% or more is that if Ti is less than 0.015 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.
In this case, it is conceivable to increase the deoxidizing power by increasing the concentrations of Si and Mn. However, if Ti is less than 0.015 wt%, SiO ₂ or MnO
Inclusion inclusions are generated in large quantities, causing hardening of the steel material and deterioration of the plating property. To prevent this, (wt% Mn) / (wt% Ti) <10
It is necessary to contain Ti so as to be 0. In that case, the Ti oxide concentration in the inclusion becomes 20% or more.

In manufacturing the titanium-killed steel sheet according to the present invention, first, molten steel is deoxidized with a Ti-containing alloy such as FeTi to generate an oxide-based inclusion mainly composed of Ti oxide in the steel. The inclusions are not in the form of giant clusters as in the case of deoxidation with Al, but are mostly in the form of grains or fractures having a size of about 1 to 50 μm. However, at this time, Al
If the concentration exceeds 0.010 wt%, huge Al ₂ O ₃ clusters are formed. Such Al ₂ O ₃ clusters cannot be reduced even if the Ti concentration is increased by adding a Ti alloy, and remain as cluster-like inclusions in the steel. Therefore, regarding the steel sheet according to the present invention, during the manufacturing stage,
Preferably, a Ti oxide is formed.

Under the present invention, the yield of Ti alloy is lower than the conventional method of deoxidizing with Al, and
Alloys for adjusting the composition of inclusions are expensive because they contain Ca and REM. From this, the addition of such alloys to molten steel
It is economically preferable that the amount of inclusions be controlled so as to be as small as possible within the controllable range. In this sense, before adding a deoxidizing agent such as a Ti-containing alloy, preliminary deoxidation is performed so that the dissolved oxygen concentration becomes 200 ppm or less in order to reduce dissolved oxygen in molten steel and FeO and MnO in slag. It is desirable. This preliminary deoxidation is preferably performed by stirring the molten steel in a vacuum, deoxidizing with a small amount of Al such that Al ≦ 0.010 wt% in the molten steel after deoxidation, and adding Si, FeSi, Mn, and FeMn. . If deoxidation with Ti is performed immediately after preliminary deoxidation, a large number of inclusions with insufficient modification will remain in the molten steel, making it difficult to control the composition of the target inclusions. Therefore, by stirring for 3 to 4 minutes after the addition of the preliminary deoxidizing agent and for 8 to 9 minutes after the addition of Ti, the inclusions are 20 wt% or more and 90 wt% or less of Ti oxide, and one of CaO and REM oxide. Alternatively, the total of the two types has a composition of 10% by weight or more and 40% by weight or less, and Al ₂ O ₃ : 40% or less, and becomes an inclusion controlled by Ti deoxidation.

As described above, Ti produced by Ti deoxidation
Oxide-based inclusions are dispersed in steel with a size of about 2 to 20 μm, so that surface defects due to cluster-like inclusions are eliminated. However, Ti oxide is in a solid phase state in molten steel, and ultra-low carbon steel has a high solidification temperature. May trigger.

Therefore, in the steel sheet according to the present invention, after being deoxidized with a Ti alloy, the steel sheet is further adjusted to have a content of 0.0005 wt% or more.
One or two of Ca and REM are added, and oxide inclusions having a particle size of 1 to 50 μm in molten steel are converted to Ti oxide: 20 wt.
% To 90% by weight, preferably 85% by weight or less, CaO and / or
Or REM oxides: more than 10 wt% 40% or less, Al ₂ O ₃ is 40 wt%
The following low melting point oxide-based inclusions are used. Then, it is possible to effectively prevent the Ti oxide containing the metal from adhering to the nozzle. The composition of such oxide-based inclusions is measured by a quantitative analysis of each inclusion using EPMA. It is most desirable that all the inclusions in the steel analyzed in this way satisfy the above composition, but in practice, inclusions of 50% or more in the number of inclusions having a size of 1 to 50 μm are included. Is within the above composition range, the characteristics of the high-strength steel sheet aimed at by the present invention are achieved.

In the present invention, when the composition of the generated 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 or 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.

In the hot rolling step after continuous casting, hot rolling may be performed according to a conventional method. Slab heating temperature is 900
Preferably it is 11300 ° C. 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, at a high temperature exceeding 1300 ° C., the crystal grain size before rolling becomes too large, and the hot-rolled sheet does not become fine. Therefore, the slab heating temperature is 900 ~
1300 ° C. is preferred. A slab heating temperature of 1200 ° C. or less is preferable from the viewpoint of deep drawability. Further, CC-DR (continuous casting-direct rolling) or DHCR (direct hot charge rolling) is preferable from the viewpoint of energy saving. The hot rolling end temperature is preferably 650 to 960 ° C. As for the coil winding temperature after hot rolling, the higher the temperature, the more advantageous is the coarsening of precipitates, but if it is too high, problems such as the scale becoming too thick will occur.
Is preferred.

[0034]

[Example] (Invention example) After tapping from a converter, 300 ton molten steel was subjected to RH
Decarburized by degassing equipment, C = 0.05 ~ 0.09wt%, Si =
0.60 ~ 0.80wt%, Mn = 1.0 ~ 1.3wt%, P = 0.005 ~ 0.
030 wt%, S = 0.004 to 0.008 wt%, and the molten steel temperature was adjusted to 1585 to 1615 ° C. Al was added to the molten steel in an amount of 0.2 to 0.8 kg / ton, and preliminarily deoxidized for 3 to 4 minutes to lower the dissolved oxygen concentration in the molten steel to 55 to 260 ppm. The Al concentration in the molten steel at this time was 0.001 to 0.005 wt%. Then, a 70 wt% Ti-Fe alloy is added to this molten steel for 0.8-
1.8 kg / ton was added over 8 to 9 minutes to deoxidize Ti. Then, after the composition adjustment, 30wt% Ca-60wt% in molten steel
% Si alloy, an additive mixed with metallic Ca, Fe, 5 to 15 wt% REM, or 90 wt% Ca-5 wt% Ni alloy
An alloy and a REM alloy Fe-coated wire were added at 0.05 to 0.5 kg / ton for treatment. The Ti concentration after this treatment is 0.026-0.
058 wt%, Al concentration 0.001-0.005 wt%, Ca concentration 0.00
05-0.0018wt%, REM concentration was 0.0000-0.0020wt%.

Next, the steel was cast using a two-strand slab continuous casting apparatus to produce a continuously cast slab. During casting, Ar gas was not blown into the tundish and the immersion nozzle. Observation after continuous casting showed that there was almost no deposit in the tundish and in the immersion nozzle.

Next, the continuous cast slab was hot-rolled and then pickled to obtain a hot-rolled sheet. At this time, the size of the oxide-based inclusions was 50 μm or less in width. Non-metallic inclusion defects such as barbs, slivers, scales and the like were found only in 0.00 to 0.02 / 1000 m-coils in this hot rolled sheet. Table 1 shows the steel composition of the hot-rolled sheet,
Table 2 shows hot rolling conditions, mechanical properties, and rust generation area ratio index. The area ratio of rust is 10% at a temperature of 50 ° C and a humidity of 95%.
The rusting area when left for a time was indicated by an index. There was no problem with the amount of rust as in the conventional Al deoxidized steel. FIG. 1 shows the results of the hole expansion test of the obtained hot rolled sheet. Here, the hole expandability λ is defined by the following formula from a critical hole diameter D ′ (mm) in which a 10 mm hole is formed in a 2 mm hot-rolled sheet and no crack occurs when the hole is expanded by a cone. λ = (D′−10) / 10 × 100 (%) The surface quality of the steel sheet subjected to the electroplating and the hot-dip galvanizing after the cold rolling was also good.

[0037]

[Table 1]

[0038]

[Table 2]

(Comparative example) After tapping the converter, molten steel of 300 ton
Decarburized by RH vacuum degasser, C = 0.05 ~ 0.09wt
%, Si = 0.60-0.80 wt%, Mn = 1.0-1.3 wt%, P = 0.
005 to 0.030 wt%, S = 0.004 to 0.008 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, and deoxidation was performed for 15 minutes. The Al concentration in the molten steel after the deoxidation treatment was 0.035 wt%. Thereafter, FeTi was added and the components were adjusted. After this treatment, the Ti concentration was 0.040 wt%.

Next, the molten steel was cast using a two-strand slab continuous casting apparatus to produce a continuously cast slab. Incidentally, in this case, the average composition of inclusions in the tundish molten steel, a cluster-like inclusions of _{95~98wt% Al 2 O 3, 5} % or less of Ti ₂ O ₃ was mainly.

When Ar gas was not blown into the tundish and immersion nozzle during casting, Al ₂ O ₃ was remarkably adhered to the nozzle, and the opening degree of the sliding nozzle was significantly increased at the third charge. Casting was stopped. 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 and then pickled to obtain a hot-rolled sheet. The steel composition is shown in Table 1, and the inclusion composition, hot rolling conditions and mechanical properties, and the rust generation area ratio index are shown in Table 2. This annealed sheet has 0.45 defects / 100 non-metallic inclusions such as barbs, slivers and scales.
0 m-coil was observed. FIG. 1 shows the results of the hole expansion test of the obtained hot rolled sheet.

[0043]

As described above, the high-strength steel sheet according to the present invention does not cause clogging of the immersion nozzle during continuous casting in the production thereof, and the surface of the rolled steel sheet is a surface caused by nonmetallic inclusions. As a steel sheet which has very few defects and is extremely clean, and which has excellent properties of deformability and corrosion resistance, it is actually suitably used as an automobile part or the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of unfixed S and hole expandability as an index of local deformability.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Kenichi Sorimachi 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Inside the Technical Research Institute of Kawasaki Steel Corporation (72) Inventor Akio Tosaka 1-Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside Kawasaki Steel Engineering Laboratory

Claims (2)

[Claims] 1. C: 0.03 to 0.20 wt%, Si: 2.0 wt% or less, Mn: 0.05 to 2.5 wt%, P: 0.15 wt% or less, Ti: 0.01
5 to 0.4 wt%, Al: 0.01 wt% or less, N: 0.02 wt% or less, 0.005 to 0.1 wt% in total of one or two of Ca and REM
And the content of S and / or one or two of Ca and REM satisfy the relationship of S-5 x ((32/40) Ca + (32/140) REM) ≤ 0.0014 wt%. The remainder has a composition of Fe and unavoidable impurities, and oxide inclusions having a particle size of 1 to 50 μm are composed of Ti oxide and
High tensile strength steel sheet with excellent deformability characterized by containing one or two kinds of CaO and REM oxides. 2. An oxide-based inclusion having a particle size of 1 to 50 μm is made of Ti acid.
Oxide: 20 wt% or more and 90 wt% or less, one of CaO and REM oxides
Or the total of two types: 10 wt% or more and 40 wt% or less, Al _TwoO_Three： 40%
The following (one or two of Ti oxide, CaO, REM oxide, Al
_TwoO_ThreeIs less than 100%)
Item 4. A high-tensile steel sheet excellent in deformability according to item 1.