JP2000119802A - Ultra-thin steel sheet excellent in surface characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultra-thin steel sheet of <=0.2 mm thick, excellent in surface characteristic, hardly causing problems such as rusting, and having >=540 MPa tensile strength in a rolling direction, a plated steel sheet, and a method of their manufacture. SOLUTION: This steel sheet has a composition consisting of, by weight, 0.01-0.10% C, <=0.2% Si, 0.05-1.0% Mn, <=0.02% P, 0.015-0.10% Ti, 0.001-0.01% Al, <=0.02% N, 0.0005-0.01%, in total, of either or both of Ca and REM, further S in an amount satisfying the relation of inequality S-5×((32/40)Ca+(32/140) REM)<=0.0014% in correlation with the contents of either or both of Ca and REM, and the balance Fe with inevitable impurities. Further, in this steel sheet, oxide inclusions of 1-50 μm grain size include Ti oxide and either or both of CaO and REM oxide. Such a steel sheet can be suitably used for DRD cans.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an ultra-thin steel sheet (plated steel sheet) having a good surface property, a low rust resistance, a tensile strength in the rolling direction of 540 MPa or more and a sheet thickness of 0.2 mm or less. Other surface-treated steel sheets). These steel plates are used, for example, in DRD cans (Drawn &
Used as a steel plate for Re-Drawn).

[0002]

2. Description of the Related Art Among ultra-thin steel sheets for cans, unlike ordinary ordinary cold-rolled steel sheets, a method of simultaneously performing an increase in strength (hardness) and a reduction in sheet thickness by performing secondary cold rolling after annealing. Some are used. DR for such steel plate for cans
(Double Reduced) material, DR8, DR in the standard
It is called according to the hardness (Rockwell hardness), such as 9, DR10. Such a steel plate is required to have a more beautiful surface finish than conventional steel plates for cans. In addition, the thickness of such a steel sheet is usually 0.2 mm or less, and the strength is often extremely high, such as 540 MPa or more.Therefore, if coarse non-metallic inclusions are present in the steel sheet, cracking of the steel sheet immediately occurs. There is a specific problem that it becomes a starting point and leads to a plate breakage in a manufacturing process.

[0003] For this reason, in a steel sheet used for such an application, the surface is scarfed at the stage of slab casting (which removes a contaminated layer on the surface during warm or cold).
Is carried out more strictly, and the continuous cast slab itself has not used a so-called unsteady slab in which the presence of inclusions is concerned. This is because, as described above, coarse non-metallic inclusions (including coarse alumina clusters) present in or on the steel become the starting point of cracking during high-strength cold rolling, resulting in breakage of the steel sheet. Due to the high risk. In addition, inclusions such as MnS, which extend in the longitudinal direction of the steel sheet, significantly reduce the mechanical properties in the direction perpendicular to the rolling direction, particularly the elongation value, so that such inclusions are included in a large amount. In the case of a steel plate, there is a problem that the application field of the product is limited to a very narrow range.

[0004] Improving the ductility in the extremely hard and thinned state is important for the properties of ultra-thin and high-strength steel sheets with higher versatility. In order to meet such demands, it is considered that the cleanliness of the surface and the portion immediately below the surface is particularly required.

[0005] Techniques related to these include, for example, ductility.
In order to improve the performance, see JP-A-63-192849.
Low melting point of inclusions by controlling the composition of inclusions
A method for achieving this is disclosed in JP-A-2-220735.
The precipitation of TiN and MnS is controlled by adjusting the dissolved oxygen in such steel.
A control method has been proposed. However, rolling
Due to the presence of MnS and oxides in steel,
Since the deformability of the part deteriorates, sufficient deformability is still obtained.
It was difficult to do. Also, Japanese Unexamined Patent Publication No.
In the method disclosed in the report, the inclusion is CaO-Al _TwoO_ThreeSystem
And become the starting point of rust, which is a strong requirement for steel plates for cans.
There was a fatal problem that food habits deteriorated.

[0006]

SUMMARY OF THE INVENTION The present invention has been developed as a result of experiments, investigations and studies to solve the above-mentioned problems of the prior art. That is, the present invention relates to an ultra-thin steel sheet and a plated steel sheet having good surface properties, hardly causing problems such as rust, and having a tensile strength in the rolling direction of 540 MPa or more and a sheet thickness of 0.2 mm or less. In particular, the present invention proposes a steel sheet having a feature that the surface properties are good even after performing a strong secondary cold rolling after annealing. Further, the present invention proposes a steel sheet having a good surface property so that the surface care of the slab can be reduced or omitted in the production thereof.

[0007]

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 oxide-based inclusions remaining in the steel were found to contain oxides and sulfides present in the steel. It has been concluded that controlling the composition of is advantageous for the production of ultra-thin steel sheets and plated steel sheets having excellent corrosion resistance and good surface properties and a plate thickness of 0.2 mm or less.

That is, the oxide-based inclusions in the steel are controlled, the formation of giant cluster-like inclusions is suppressed, and fine dispersion into inclusions having a size of 50 μm or less is achieved.
Good surface quality by reducing the amount of MnS and making all oxides and sulfides in steel finer and non-ductile
It has been found that ultra-thin steel sheets and plated steel sheets of 0.2 mm or less can be obtained. The present invention is based on the above findings.

That is, the present invention provides a method for producing C: 0.01 to 0.10 wt.
%, Si: 0.2 wt% or less, Mn: 0.05-1.0 wt%, P: 0.02
wt% or less, Ti: 0.015 to 0.04 wt%, Al: 0.001 to 0.01 wt
%, N: 0.02 wt% or less and one or two of Ca and REM in total of 0.0005 to 0.01 wt%, and furthermore, S and Ca, REM
The content of one or two types satisfies the following formula: S-5 × ((3/40) Ca + (32/140) REM) ≤ 0.0014 wt%, and the balance is Fe and unavoidable impurities. Oxide inclusions with a particle size of 1 to 50 μm
An ultra-thin steel sheet containing one or two of CaO and REM oxides, having a tensile strength in the rolling direction of 540 MPa or more and a sheet thickness of 0.2 mm or less and having good surface properties.

Further, the present invention relates to a method for producing C: 0.01 to 0.10 wt%,
Si: 0.2 wt% or less, Mn: 0.05-1.0 wt%, P: 0.02 wt%
Hereinafter, Ti: 0.015 to 0.04 wt%, Al: 0.001 to 0.01 wt%,
N: 0.02wt% or less and one or two of Ca and REM in total
0.0005 to 0.01 wt%, and Ni: 0.005 to 1.0 wt%
%, Cr: 0.005 to 1.0 wt% and Nb: 0.002 to 0.04 wt%
Contains one or more species, and further contains one of S and Ca, REM
The content of the species or two is represented by the following formula: S−5 × ((3/40) Ca + (32 /
140) REM) ≤ 0.0014 wt%, the remainder is composed of Fe and unavoidable impurities, and oxide inclusions having a particle size of 1 to 50 µm are composed of Ti oxide and one of CaO, REM oxide or Two
Containing seeds and having a tensile strength in the rolling direction of 540 MPa or more
Ultra-thin steel sheet with good surface properties of 0.2 mm or less.

In the present invention, the oxide-based inclusion having a particle size of 1 to 50 μm is composed of Ti oxide: 20 wt% to 90 wt%,
, REM oxide total of one or two: 10 wt% or more and 40 wt%
% Or less, and Al ₂ O ₃ : preferably 40% or less (one or two of Ti oxide, CaO and REM oxide, and the total of Al ₂ O ₃ is 100% or less). More preferably, it is made of uniform and fine crystal grains having a diameter of 15 μm or less. Further, in the steel sheet of the present invention, the grain size of the crystal grains in a cross section perpendicular to the rolling direction is 15 μm.
m or less is more preferable. The steel sheet of the present invention also includes the above-mentioned ultra-thin steel sheet coated with plating, resin, or the like.

The steel sheet of the present invention has a C: 0.01 to 0.10
wt%, Si: 0.2 wt% or less, Mn: 0.05-1.0 wt%, P: 0.
02 wt% or less, Ti: 0.015 to 0.04 wt%, Al: 0.001 to 0.01
wt%, N: 0.02 wt% or less, and one or two of Ca and REM containing 0.0005 to 0.01 wt% in total, and S and Ca, REM
After heating a slab in which the content of one or two of the following satisfies the relationship of S-5 × ((3/40) Ca + (32/140) REM) ≦ 0.0014 wt%, to a temperature of 1200 ° C. or less, Perform hot rolling with a finish rolling temperature of 850 ° C or higher, and 700 ° C
After winding at the following temperature, it is preferable to manufacture by performing pickling and cold rolling, then annealing at a recrystallization temperature or higher, and then performing secondary cold rolling at a rolling reduction of 10% or more. .

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described more specifically. In the present invention, Al is 0.001 wt% or more and 0.01 wt% or less, Ti is 0.015 wt% or more, and Ca and / or REM is
By satisfying the condition of 0.0005 wt% or more and 0.01 wt% or less, a hard steel sheet with less rust and excellent surface properties can be obtained. At this time, the inclusions are Ti ₂ O ₃ -CaO and / or REM oxide-Al
₂ O ₃ --SiO ₂ oxide and Ca in inclusions
If the total concentration of REM and REM is 40% by weight or less, it does not act as a starting point for rust, and the decrease in ductility in the direction perpendicular to the rolling direction is small. If the amount of Al exceeds 0.01 wt%, the inclusions will be Al
_{Since it is a 2} O ₃ —CaO system, the Ca concentration in the inclusions becomes about 50%, which serves as a starting point of rust, deteriorating corrosion resistance and deteriorating the beauty of the surface. In order to improve the surface properties of hard ultra-thin steel sheets and improve their mechanical properties, it is important to 1) not coarsen oxides in steel and 2) not coarsen sulfides in steel. .

Regarding the above-mentioned 1) for preventing the oxide from becoming coarse, the Al content is 0.001 wt% or more and 0.01 wt% or less, and the Ti content is 0.015 wt% or less.
It can be achieved by satisfying the condition that the content of Ca and / or REM is 0.0005 wt% or more and 0.01 wt% or less.
In order to prevent the sulfide from coarsening in 2), it is important to suppress MnS precipitated during solidification. This is because MnS extends during rolling and promotes cracking during processing. For this solution, it is necessary to fix (detoxify) S in the steel with Ca and / or REM which forms more stable sulfide. Specifically, one of S and Ca, REM or
For the two types of contents, the following equation: 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%). ). That is, in order to fix S by forming CaS or REM sulfide, the larger the amount of Ca or REM added, the better. However, the lower limit is 0.0014 wt% or less as the amount of unfixed S as shown by the above inequality. It was obtained from the experimental result that it was necessary to be.
The surface properties described above may be affected by surface scratches, holes, etc., when secondary cold rolling of about 5 to 40% is applied to increase the hardness of the steel sheet after annealing and reduce the sheet thickness. It shows the property that defects do not or hardly occur. As a result of various studies based on the above experimental results, the inventors have arrived at the present invention.

Next, the reason why the composition range of the steel sheet for cans of the present invention is limited will be described. C: 0.01 to 0.10 wt%; In the steel sheet of the present invention, it is necessary to achieve a predetermined strength or more (540 MPa or more) after the secondary cold rolling step after annealing, and at the same time, to achieve a sheet thickness of 0.2 mm or less. . In producing an ultra-thin steel sheet satisfying these characteristics, the C content is an important component that governs workability.
If the C content is less than 0.01% by weight, it is difficult to stably obtain a target high strength after secondary cold rolling. On the other hand, if the C content exceeds 0.10 wt%, the amount of pearlite in the ferrite / pearlite structure is significantly increased, so that secondary cold rolling becomes extremely difficult, and at the same time, stretch flangeability is significantly reduced. In addition, the surface properties after the secondary cold rolling tend to be remarkably reduced, and it is difficult to use the steel sheet as a steel sheet for cans. From the above, C is 0.01wt% or more,
0.10 wt% or less. The preferred lower limit is 0.02wt%
And a preferable upper limit is 0.08 wt%.

Si: 0.2 wt% or less (excluding 0); Si
Is a component necessary for deoxidation during melting. However, since the effect of increasing the strength of the steel is great and the cold rollability is greatly reduced, the surface properties are also deteriorated.
Was set as the upper limit. In addition, from the viewpoint of cold rolling properties, a more preferable upper limit is 0.05 wt%.

Mn: 0.05 to 1.0 wt%; Mn is effective for deoxidation during melting, like Si. It is also useful as a component for preventing hot brittleness of steel. These desirable effects are:
Approximately 0.05% by weight or more is effective. But 1.
When added in excess of 0 wt%, the corrosion resistance, which is important as a steel sheet for cans, is reduced, and the secondary cold rolling properties after annealing and the surface properties after secondary cold rolling are reduced. Therefore, the upper limit was 1.0 wt%. The preferred lower limit is 0.2 wt%,
A preferred upper limit is 0.7 wt%.

P: 0.02% by weight or less; P is a component that lowers the corrosion resistance of a steel sheet for cans, so it is desirable to reduce P. When the content is approximately 0.02% by weight or less, it is possible to obtain corrosion resistance enough to be used as a steel plate for cans. The lower limit is determined by the cost increase factor for dephosphorization and the degree of quality improvement, but is generally about 0.005 wt%.

Ti: 0.015 to 0.10 wt%; Ti is an important component in the present invention, and fine oxide inclusions having a size of 50 μm or less are formed by deoxidation of Ti, and grain growth during cold rolling and annealing. Control the properties to improve the strength-elongation balance. Furthermore, the fine oxide of Ti is also effective for refining the structure of the hot-rolled sheet, and remarkably improves the secondary cold rolling after annealing. Further, the Ti carbide partially generated has an effect of uniformly and finely adjusting the structure of the steel sheet, and this contributes to miniaturization of the steel sheet surface after the secondary cold rolling. If the addition amount is less than 0.015 wt%, the desired effect cannot be obtained because the addition effect, that is, the amount of fine oxides and carbides is too small, so that 0.015 wt%
Limited to the above. On the other hand, if the added amount exceeds 0.10 wt%,
Hot rolling property, cold rolling property, and secondary cold rolling property are significantly reduced, and furthermore, corrosion resistance, which is an important property as a steel sheet for cans, is significantly deteriorated. Therefore, the range is 0.015 to 0.10 wt%.

Al: 0.001 to 0.01 wt%; Al is an important component in the present invention. If the amount exceeds 0.01 wt%, deoxidation becomes Al deoxidation and a large amount of large Al ₂ O ₃ clusters are formed. In addition to deteriorating the surface properties, 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,
It was limited to 0.01 wt% or less. More importantly, Al amount is large, the inclusions composition Al ₂ O ₃ -CaO and / or Al ₂ O _3-REM
Because it is an oxide, such inclusions become the starting point of rust,
Deterioration of corrosion resistance. From this point, the upper limit of Al is set to 0.01 wt%. On the other hand, in the current steelmaking technology,
If the content is less than 0.001 wt%, the effect of improving the material corresponding to the cost increase cannot be obtained, so the lower limit is set to 0.001 wt%.

N: 0.02 wt% or less (excluding 0); N
Increases the strength of the steel significantly when it remains in a solid solution state, but the increase in deformation resistance due to the effect of the addition of N is relatively large in the hot rolling, cold rolling and the secondary cold rolling after annealing. small. Therefore, there is an advantage that a hard steel sheet for cans can be obtained by setting the content to about 0.02 wt% or less without difficulty in manufacturing the steel sheet. Note that N in the manufacturing process
Considering the cost increase for reduction and the effect of improving the material, the preferable lower limit is 0.0005 wt%. Here, as described above, unlike N and the like, N does not lower the hot rollability, so that the upper limit is relatively loose.

One or two types of Ca and REM are added in a total amount of 0.0005 to
0.01 wt%; Ca and metal REM (refer to rare earth elements such as La and Ce) are important components in the present invention.
One or two kinds of REM alone or in total 0.0005wt
% Must be added. That is, after Ti deoxidation,
Further, one or two of Ca and REM are added so as to be 0.0005 wt% or more, and the oxide composition in the molten steel is changed to Ti
Oxides: and more 5 wt% 40 wt% or less, oxide inclusions of low melting point Al ₂ O ₃ is less than 40 wt%: more than 20 wt% 90 wt% or less, preferably less 85 wt%, CaO and / or REM oxides I do. 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. This is extremely effective for improving the properties of the steel sheet surface. Further, CaO and / or REM oxide is cold-rolled.
It can contribute to the adjustment of the grain growth behavior after annealing and the refinement of the hot-rolled sheet. However, on the contrary, if it exceeds 0.01 wt%, the corrosion resistance is significantly reduced.

It has also been found that the proper range of the amounts of Ca and REM added has the following relationship with the total amount of S in steel. S−5 × ((32/40) Ca + (32/140) REM) ≦ 0.0014 wt% However, in the above formula, Ca and REM represent the addition amounts (wt%) of Ca and REM, respectively. By adding Ca and REM, the form and non-ductility of the sulfide are improved, and the improvement of the surface properties which is the main object of the present invention is remarkable. According to our research,
Addition of Ca and REM is thought to be harmless sulfide up to about 5 times S in atomic ratio of these elements for unknown reasons. Therefore, the amount of harmful S, that is, S-5 ×
If the value of ((32/40) Ca + (32/140) REM) is small enough,
Deterioration of surface properties due to sulfide does not occur. According to the survey,
It was found that there was no problem if the harmful S content was 0.0014 wt% or less.

0.07 wt% C, 0.01 wt% Si, 0.5 wt%
Mn, 0.010 wt% N, 0.010 wt% P, 0.025 wt% T
i, Al of 0.001 wt%, these components are kept almost constant, and the composition (especially S content and Ca, REM content) is changed over a wide range, and secondary rolling of hot rolling, cold rolling, annealing and reduction of 33% is performed. Cold rolling was performed to produce a DR9 ultrathin steel sheet having a thickness of 0.12 mm and an HR30T of 70. FIG. 1 shows the values of the above formulas and the defect rates (total of the surface defect rate and the hole defect rate) generated in the steel sheet. It can be seen that the defect rate is low and good results are obtained in the ranges of S content, Ca content and REM content according to the present invention.

S is a component that makes the steel embrittle and forms brittle sulfide in the steel. Therefore, it is desirable to reduce as much as possible in order to ensure ductility. About 0.01wt
% Or less can satisfy the characteristics required as a hard and ultra-thin steel sheet for cans. In the case where better ductility is required, the content is more preferably 0.005 wt% or less.

O: 0.010 wt% or less; O is a necessary component to generate fine oxides to a certain extent, but if added in excess of 0.010 wt%, large amounts of coarse Al ₂ O ₃ will be produced. When formed, the cold rollability, particularly the secondary cold rollability, is reduced, so the upper limit was made 0.010 wt%. The preferable upper limit is 0.007 wt%, and more preferably 0.005 wt% or less.

Ni: 0.005 to 1.0 wt%: Cr: 0.005 to 1.0
wt%; Ni and Cr are one of the goals of the present invention, which is to reduce the structure of the steel sheet without solid solution strengthening or to facilitate the deformation in a low temperature and high strain rate environment. Workability (shallow drawing) and stretch flange properties can be improved. In addition, since each component has an effect of reducing the transformation point of steel, it is also effective in relaxing the regulation condition of the hot finishing temperature. Therefore, in the present invention, one or two of Ni and Cu can be added as necessary. Ni
In addition, both Cr and Cr exhibit a remarkable effect when added in an amount of 0.005 wt% or more, and this effect is not offset even when they are added in combination. However, even if added in excess of 1.0 wt%, the effect tends to saturate.
wt%. 0.01-0.5 wt from the viewpoint of material stabilization
% Is more preferred.

Nb: 0.002 to 0.04 wt%; Nb is extremely effective in refining the crystal grains of the steel sheet. Therefore, in the present invention, Nb can be added as needed. By refining the crystal grains, particularly in the steel sheet to which the present invention is applied, a remarkable effect can be exerted on the prevention of surface roughness after forming and the improvement of ductility in connection with this. Nb is about 0.00
A remarkable effect is exhibited by adding 2 wt% or more. But 0.
Even if Nb is added in excess of 04 wt%, the effect tends to saturate, and on the contrary, there is a possibility of causing a problem that the hot and cold deformation resistance of the steel is significantly increased.
-0.04 wt%. In terms of material stabilization,
0.01-0.5 wt% is more preferred.

In the steel satisfying the above 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, more specifically, Ti oxide-CaO and / or REM oxide-Al ₂ O _3- SiO ₂ oxide-based inclusions, less rust, The steel sheet expected in the present invention, which has almost no deterioration in deformability due to inclusions and precipitates, has no surface defects due to cluster-like inclusions, and has no adhesion of Ti oxides containing metal to the nozzle. Becomes

The reason why the oxide inclusions defined in the present invention are limited to those having a particle size of 1 to 50 μm is that inclusions in such a range can be regarded as inclusions formed by deoxidation. Inclusions having a particle size of more than 50 μm are generally caused mainly by exogenous inclusions such as slag or mold powder. Some Al ₂ O ₃ clusters are larger than this, but if the oxide composition of inclusions with a particle size of 50 μm or less satisfies the above requirements, a large Al ₂ O ₃ cluster can be sufficient. It can be considered that it is decreasing. The content of such inclusions is set to 80% by weight or more. The reason is that if the content is less than 80 wt%, the control of inclusions is insufficient, which may cause a surface defect of the coil or a nozzle clogging.

The above 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.

When the content of Ti oxides in the inclusions is less than 20% by weight, not Ti deoxidized steel but Al deoxidized steel is obtained, and nozzle clogging occurs due to an increase in Al ₂ O ₃ concentration. 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 decreases, and nozzle clogging rather occurs. Therefore, the Ti oxide concentration is set to 20 wt% or more and 90 wt% or less. More preferably, the content is 30 wt% or more and 80 wt% or less.

The CaO and REM oxides in the inclusions
If the total of one or two kinds is less than 10% by weight, the inclusion does not have a low melting point and causes the nozzle to be blocked as described above. On the other hand, if it exceeds 40% by weight, the inclusions subsequently absorb S and change to water-soluble, and become the starting point of rust, so that the corrosion resistance decreases. In addition, a more preferable range is 20 to 40 wt%.

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.

The oxide-based inclusions may contain oxides other than those listed above. In this case, the amount of the oxides other than those listed above is 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.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, 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 /
It is preferable to set Ti> 50, but in this case, hardening of the steel and deterioration of the surface properties are caused.

The steel sheet of the present invention has a tensile strength of 540 MPa or more. If the tensile strength (TS) is less than 540 MPa, the ultra-thin steel sheet or the plated steel sheet targeted by the present invention has insufficient strength of the can body and causes a problem in use. In addition, TS is controlled to a target value by the secondary cold rolling reduction ratio in addition to the component and the annealing temperature.

The reason why the thickness of the steel sheet of the present invention is set to 0.2 mm or less is that the merit of improving the surface properties by applying the present invention is exhibited because the thickness of the steel sheet is about 0.2 mm or less. This is because the effect of controlling the oxide cannot be sufficiently exerted with a thicker material.

The steel sheet of the present invention has a particularly remarkable effect of improving the surface properties when it has a uniform and fine structure of crystal grains having a crystal grain size of 15 μm or less and having a cross section perpendicular to the rolling direction.
This includes the so-called roughening after press molding. By making the steel sheet thinner than usual as the steel sheet becomes thinner, it is possible to eliminate the roughening. Therefore, it is preferable to have a structure composed of uniform and fine crystal grains having a crystal grain size of 15 μm or less, and it is more preferable that the grain size be 12 μm or less. In addition, uniform means that it is not a so-called mixed grain structure including coarse grains, and such a uniform and fine structure is obtained by adjusting the steel composition and hot rolling conditions (slab heating temperature, finishing temperature, etc. described later). Can be obtained.

The ultra-thin steel sheet of the present invention can be plated to form a plated steel sheet. The type of plating is not particularly limited, and tin, nickel, chrome plating, zinc, etc.
Any conventionally known one can be applied. Further, a steel plate or a plated steel plate may be coated with an organic resin such as polyethylene by sticking or coating.

Next, the method for producing steel of 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, prior to the addition of a deoxidizing agent 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 preferably carried out by deoxidation with a small amount of Al such that the content of Al in the molten steel after deoxidation becomes 0.010 wt% or less, and addition of Si, FeSi, Mn, or FeMn.

As described above, Ti produced by Ti deoxidation
A steel sheet in which ₂ O ₃ contains 70% or more of Ti oxide-based inclusions means that such inclusions are dispersed in the steel in a size of about 2 to 20 μm, and thus, surface defects due to cluster-like inclusions are generated. Is gone. 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, the Ti alloy
After deoxidation, so that it becomes more than 0.0005wt%
Add one or two of Ca and REM and add
Oxide inclusions with a particle size of 1 to 50 μm
% To 90% by weight, preferably 85% by weight or less, CaO and / or
Or REM oxide: 5 wt% or more and 40 wt% or less, Al_TwoO_ThreeIs 40wt
% Or less of low melting point oxide-based inclusions. Do so
Effectively prevents Ti oxides containing ingots from adhering to the nozzle.
It becomes possible to stop. More preferred inclusion composition
Is Ti oxide: 30 wt% or more and 80 wt% or less, CaO, REM oxidation
Things (La_TwoO_Three, Ce_TwoO _ThreeEtc.): 10 wt% or more and 40 wt% or less, Al
_TwoO_Three : 20 wt% or less, others (SiO_Two, MnO, etc.): 10% or less
It is.

The measurement of the composition of such oxide-based inclusions is based on EP
It is performed by performing quantitative analysis on each inclusion using MA or using a scanning electron microscope with EDX function. It is most desirable that all the inclusions in the steel analyzed in this way satisfy the above composition, but in practice, the number of inclusions of 1 to 50 μm in size is
When the content of 50% or more is within the above composition range, various properties of the hot-rolled steel sheet aimed at by the present invention are achieved. In addition,
As the particle diameter, the maximum diameter of each particle is used.

In the present invention, when the composition of the formed inclusions is controlled as described above, it is possible to completely prevent oxides and the like from adhering to the inner surfaces of the tundish nozzle and the immersion nozzle of the mold during continuous casting. . Therefore, it is not necessary to blow a gas such as Ar or N ₂ into the tundish or the immersion nozzle for preventing adhesion of oxides 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, the slab heating temperature is preferably 1300 ° C. or less. 1300 ℃
If the slab heating temperature exceeds the above range, the structure of the steel sheet becomes non-uniform, and there is a possibility that a shape defect may occur when the can body is formed. Above all, a slab heating temperature of 1050 to 1250 ° C. is preferable from the viewpoint of material and operation stability. In addition, continuous casting
Direct hot rolling (CC-DR) or DHCR (direct hot charge rolling) in which a continuously cast slab is inserted into a heating furnace with a hot piece is preferable from the viewpoint of energy saving.

The hot rolling end temperature is preferably 850 ° C. or higher. If the temperature is lower than 850 ° C., the material in the width direction of the steel sheet tends to be non-uniform, and the shape and the like of the cold-rolled steel sheet are undesirably reduced. On the other hand, if the temperature is higher than 960 ° C., the thickness of the steel sheet may be small and scale flaws are likely to occur, and the crystal grain size increases, which is not preferable. The coil winding temperature after hot rolling is preferably 700 ° C. or less in order to make the distribution of the product material in the width and length directions uniform. Winding at temperatures above 700 ° C increases the grain size.

After hot rolling, pickling and cold rolling are performed, followed by annealing. In the pickling, the scale layer on the surface is removed with ordinary hydrochloric acid or sulfuric acid. Particularly in the case of a steel sheet having a thin scale phase, the pickling step can be omitted. In the cold rolling, the rolling reduction of (primary) cold rolling is usually 80% or more in the ultra-thin steel sheet targeted for the steel sheet of the present invention.

The annealing can be applied to either continuous annealing or batch annealing. However, continuous annealing is recommended from the viewpoint of the efficiency of the annealing work and the uniformity of the material. Annealing must be performed at a temperature higher than the recrystallization temperature. At a temperature lower than the recrystallization temperature, partial recrystallization occurs, and it is extremely difficult to obtain a steel sheet shape that satisfies the specifications after secondary cold rolling after annealing. On the other hand, if the annealing temperature is too high, it will be difficult to keep the crystal grain size fine. Therefore, the annealing temperature is preferably 800 ° C or lower, more preferably 720 ° C or lower.

In the secondary cold rolling after annealing, the rolling reduction is adjusted from the viewpoint of obtaining a target hardness and a target plate thickness. There are no particular regulatory requirements, but generally 5-30
% Is recommended from the viewpoint of material stability.

[0052]

[Example] (Example 1) After tapping the converter, molten steel of 300 ton was used.
Decarburized by RH degassing equipment, C = 0.07 ~ 0.08wt%, Si
= 0.65 to 0.75 wt%, Mn = 1.1 to 1.2 wt%, P = 0.010 to
0.020 wt%, S = 0.005 to 0.009 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 to reduce the dissolved oxygen concentration in the molten steel to 55 to 260 ppm. At this time,
The Al concentration was 0.001 to 0.005 wt%. Then, Ti was deoxidized by adding 0.8 to 1.8 kg / ton of a 70 wt% Ti-Fe alloy to the molten steel. Then, after adjusting the composition, 30
wt% Ca-60wt% Si alloy and metal Ca, Fe, 5 ～ 15wt
% REM or 90wt% Ca-5wt% Ni
Alloys such as Ca alloy and REM alloy Fe-coated wire 0.05 ~
0.5 kg / ton was added for treatment. After this treatment, the Ti concentration is 0.026 to 0.058 wt%, the Al concentration is 0.001 to 0.005 wt%,
Ca concentration is 0.0010-0.0035wt%, REM concentration is 0.0000-0.00
20 wt%, and the sum of the concentrations of Ca and REM was 0.0010 to 0.0043 wt%.

Next, this steel was cast by a two-strand slab continuous casting apparatus to produce a continuously cast slab. The average composition of the inclusions of the molten steel in the tundish at this time was
_{25~85wt% Ti 2 O 3, 5} ~45wt% CaO, 0 ~18wt% REM oxides was fine spherical inclusions 6 ~41wt% Al ₂ O _3.
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. The hot rolling conditions are as follows: slab heating temperature: 1180 ° C, finish rolling temperature:
The temperature was 880 ° C. and the hot-rolling winding temperature was 620 ° C. The hot-rolled steel sheet was pickled and cold-rolled by about 90% to obtain a cold-rolled sheet having a thickness of 0.22 mm. After performing recrystallization annealing at 700 ° C. in a continuous annealing line (CAL), a secondary cold rolling of 20% was performed.

Table 1 shows the steel composition and the inclusion composition. Most of the oxide inclusions had a width of 50 μm or less. In addition, this cold-rolled sheet contains 0.00-0.02 / 1000m non-metallic inclusion defects such as barbs, slivers, and scales.
-Only less than the coil was found. In addition, rust generation had no problem similarly to conventional Al deoxidation. Also, change this to 0.18
No troubles such as cracks and breaks caused by non-metallic inclusions in steel when secondary cold-rolled to mm. After cold rolling, the surface quality of the electroplated or chromium-plated steel sheet was also very good. The results of the corrosion resistance test of the obtained rolled sheet are also shown in Table 1.

[0056]

[Table 1]

On the other hand, for comparison, 300 to
n is decarburized by RH vacuum degasser, and C = 0.07
~ 0.08wt%, Si = 0.65 ~ 0.75wt%, Mn = 1.1 ~ 1.2wt
%, P = 0.010-0.020 wt%, S = 0.001-0.008 wt%
, And the temperature of molten steel was adjusted to 1590 ° C. Al was added to the molten steel in an amount of 1.2 to 1.6 kg / ton for deoxidation. The Al concentration in the molten steel after the deoxidation treatment was 0.035 wt% (Al killed steel). Then, while adding FeTi,
The components were adjusted. The Ti concentration after this treatment is 0.040 wt%
Met.

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 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. The hot rolling conditions were as follows: slab heating temperature: 1170 ° C, finish rolling temperature: 880
℃, hot rolling winding temperature: 630 ℃. After pickling.
After cold rolling to 22 mm, recrystallization annealing was performed at 700 ° C. in a continuous annealing line. Table 1 shows the steel composition and the inclusion composition.
Non-metallic inclusion defects such as barbs, slivers and scales were found in the cold rolled steel sheet at 0.45 / 1000 m-coil. For this reason, when this was secondarily cold rolled to 0.18 mm,
1.2% vacancy defects occurred. Table 1 shows the evaluation results of the corrosion resistance of the obtained product plate. The rust generation area ratio as a method of evaluating corrosion resistance is an index showing the rust generation area when left at 50 ° C. for 10 hours in a humidity of 95%.

(Example 2) Including the component composition shown in Table 2,
The remainder is made of steel substantially made of iron in a converter, and the steel slab is subjected to hot rolling, cold rolling, continuous annealing, and secondary cold rolling under the conditions shown in Table 3, and its mechanical properties And surface properties. Table 4 shows the results.
Shown in In Table 4, the surface properties were evaluated as defective when a defect having a coil length ratio of 0.5% or more on the surface was judged to be defective, and when 0.1% to 0.5% was judged to be slightly defective, 0.1% to 0.5% was judged as poor. % Was determined to be good. The corrosion resistance was evaluated by the same test as in Example 1 when the rust generation area ratio (index) was 0.5% or less, and was judged to be good.
% Was judged to be slightly defective, and 1.0% was judged to be defective.

As is evident from Table 4, the steel of the present invention shows sufficient mechanical properties and excellent surface properties. The plasticity of the oxide-based inclusions in Tables 1 and 4 was determined by examining inclusions having a particle size of 1 to 50 μm, and taking the average value (without weighting according to the inclusion size). Samples that fall within the component plasticity range of the present invention have 50% or more of the number of inclusions.
Ti oxide: 20 wt% or more and 90 wt% or less, one of CaO and REM oxide
Species or the sum of two: 10 wt% or more and 40 wt% or less, Al ₂ O ₃ : 40
%.

[0063]

[Table 2]

[0064]

[Table 3]

[0065]

[Table 4]

[0066]

The steel sheet of the present invention is manufactured by
Extremely stable continuous casting is possible without causing the clogging of the immersion nozzle during continuous casting, and there is no trouble such as cracking or breakage in the rolling process, etc., excellent corrosion resistance, due to non-metallic inclusions This is a steel sheet that has almost no surface defects and is excellent in surface properties, and is extremely excellent when used as a hard steel sheet for extremely thin cans.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the amounts of S, Ca, and REM in steel and the incidence of surface defects.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuji Miki 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Inside the Technical Research Institute of Kawasaki Steel (72) Inventor Makoto Aratani 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki (72) Inventor Hideo Kukuminato 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Chiba Works

Claims (4)

[Claims] 1. C: 0.01 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.05 to 1.0 wt%, P: 0.02 wt% or less, Ti: 0.015 to 0.04 wt%, Al: 0.001 to 0.01 wt% , N: 0.02 wt% or less and one or two of Ca and REM in total
0.0005 to 0.01 wt%, and the content of one or two of S, Ca, and REM is represented by the following formula: S-5 × ((3/40) Ca + (32/140) REM) ≦ 0.0014 wt% And the remainder has the composition of Fe and unavoidable impurities, and the oxide-based inclusions having a particle size of 1 to 50 μm contain Ti oxide and
Ultra-thin steel sheet containing one or two kinds of CaO and REM oxides, having a tensile strength in the rolling direction of 540 MPa or more and a sheet thickness of 0.2 mm or less and excellent surface properties. 2. C: 0.01 to 0.10 wt%, Si: 0.2 wt% or less, Mn: 0.05 to 1.0 wt%, P: 0.02 wt% or less, Ti: 0.015 to 0.04 wt%, Al: 0.001 to 0.01 wt% , N: 0.02 wt% or less and one or two of Ca and REM in total
0.0005 to 0.01 wt%, Ni: 0.005 to 1.0 wt%, Cr: 0.005 to 1.0 wt%, and Nb: 0.002 to 0.04 wt%. REM
The content of one or two types satisfies the following formula: S-5 × ((3/40) Ca + (32/140) REM) ≤ 0.0014 wt%, and the balance is Fe and unavoidable impurities. Oxide inclusions with a particle size of 1 to 50 μm
Ultra-thin steel sheet containing one or two kinds of CaO and REM oxides, having a tensile strength in the rolling direction of 540 MPa or more and a sheet thickness of 0.2 mm or less and excellent surface properties. 3. 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 3. An ultra-thin steel sheet having excellent surface properties according to item 1 or 2. 4. A crystal grain having a cross section perpendicular to the rolling direction having a grain size of 15 μm.
The ultra-thin steel sheet having a good surface texture according to any one of claims 1 to 3, which is not more than m.