Process for continuously casting sheet metal and apparatus for continuously producing sheet metal

The carbon content in molten steel is adjusted to be a value not lower than 0.001%, and a thin steel strip is made from this molten steel using a twin drum type continuous casting apparatus by means of direct casting. The thus obtained slab is given a reduction of not lower than 10%. The coagulated steel strip is cooled to a temperature not higher than the temperature determined by a function of the carbon content, cooling speed and ratio of reduction of in-line. After that, the steel strip is reheated and then cooled again to a temperature not higher than the temperature determined by the function of the carbon content. Then the cooled steel strip is coiled. In the above process, a metallic sheet, the surface of which is smooth and the metallic structure of which is fine, can be produced.

TECHNICAL FIELD 
The present invention relates to a method for producing metallic sheets of 
fine structure, the surfaces of which are smooth, using a twin drum type 
continuous casting apparatus. Also, the present invention relates to an 
apparatus for continuously producing metallic sheets. 
BACKGROUND ART 
Concerning a method for producing cold-rolled steel sheets, there is 
provided a method in which thin slabs, the thickness of which is 2 to 10 
mm, are made by a twin drum type continuous casting apparatus and used as 
hot-rolled sheets as they are. Also, there is provided a method in which 
the above thin slabs are subjected to acid cleaning to remove scale from 
the surfaces of the slabs, and then the thin slabs are cold-rolled to a 
predetermined thickness and annealed. 
The most important point of the above technique is the physical property of 
the thin slab made by the twin drum type continuous casting apparatus. 
According to the above conventional production process, the metallic 
structure of the thin slabs is coarse before cold rolling (as cast). 
Therefore, the thus obtained products are applied only to low grade uses. 
In order to improve the quality of the products, it is necessary to 
increase a ratio of reduction of cold rolling. 
In order to obtain a fine metallic structure, the following methods are 
disclosed. Japanese Unexamined Patent Publication No. 61-99630 describes a 
method for producing cold rolled steel sheets in which: a carbon content 
in molten steel is adjusted to an amount of not lower than 0.015%; a thin 
steel strip used for cold rolling is directly cast from the above molten 
steel; after coagulation, the steel strip is cooled to a temperature not 
higher than 800.degree. C.; the steel strip is reheated to a temperature 
not lower than 900.degree. C.; the steel strip is cooled again to a 
temperature not higher than 800.degree. C.; the cooled steel strip is 
coiled; and the steel strip is subjected to acid cleaning, cold rolling 
and annealing. Japanese Unexamined Patent Publication No. 60-30545 
describes a method for producing cold-rolled steel sheets in which: a 
continuous casting apparatus is used which has two water-cooled rollers 
arranged horizontally in parallel with each other while a clearance 
corresponding to the thickness of a metallic sheet is formed between them, 
rotated in the different direction to each other; a metallic sheet cast by 
the above apparatus is naturally cooled to a temperature not higher than 
the transformation point A.sub.1 ; the metallic sheet is heated to and 
kept at a temperature not lower than the transformation point A.sub.3 on 
the line; and the metallic sheet is cooled by gas or a mixture of gas and 
water. 
However, length of the apparatus to which the above methods are applied is 
long because a long period of time is required for the heat treatment in 
the above apparatus. For example, in the example described in Japanese 
Patent Application No. 59-226515, operation is conducted as follows. A 
slab that has been cast by the apparatus is coagulated to the thickness of 
3.2 mm; the coagulated slab is cooled by water to 700 to 950.degree. C.; 
the slab is reheated by direct heating burners for 100 seconds; the slab 
is kept at 950.degree. C. for 5 seconds; and the slab is coiled while it 
is cooled to the minimum temperature of 550.degree. C. In this case, the 
operating conditions are set as follows. The casting speed, by the twin 
drum method, is approximately 30 m/min; the water-cooling speed to cool 
the slab to the temperature of 700.degree. C. is 50.degree. C./sec; the 
reheating time at 950.degree. C. is 100 seconds; and the water-cooling 
speed to cool the slab to 550.degree. C. is 50.degree. C./sec. Then, the 
length of the apparatus of cooling--heating--cooling can be expressed by 
the following equation. 
##EQU1## 
The meaning of Equation (4) is described as follows. 
(1) The first term on the left side of Equation 4 expresses the length of 
the apparatus required for cooling, that is, the length of the apparatus 
required for cooling is calculated when the period of time (min) required 
for cooling the slab from 1100.degree. C. to 700.degree. C. is multiplied 
by the casting speed (30 m/min). 
(2) The second term on the left side of Equation 4 expresses the length of 
the apparatus required for reheating, that is, the length of the apparatus 
required for reheating is calculated when the period of time (min) 
required for reheating the slab from 700.degree. C. to 950.degree. C. is 
multiplied by the casting speed (30 m/min). 
(3) The third term on the left side of Equation 4 expresses the length of 
the apparatus required for cooling, that is, the length of the apparatus 
required for cooling is calculated when the period of time (min) required 
for cooling the slab from 950.degree. C. to 550.degree. C. is multiplied 
by the casting speed (30 m/min). 
In the example described in Japanese Patent Application No. 60-30545, when 
the thickness of the slab is 3 t, the casting speed is 28 m/min, and the 
heating time to heat the slab from a range of 650 to 700.degree. C., to a 
range of 900 to 950.degree. C. is 1 to 2 min. The cooling speed is 
5.degree. C./sec when the slab is coiled at the coiling temperature of 
700.degree. C. Then, the length of the apparatus of 
cooling--heating--cooling can be expressed by the following equation. 
##EQU2## 
The meaning of Equation (5) is described as follows. 
(1) The first term on the left side of Equation 5 expresses the length of 
the apparatus required for cooling, that is, the length of the apparatus 
required for cooling is calculated when the period of time (min) required 
for cooling the slab from 1100.degree. C. to 700.degree. C. is multiplied 
by the casting speed (28 m/min). 
(2) The second term on the left side of Equation 5 expresses the length of 
the apparatus required for reheating, that is, the length of the apparatus 
required for reheating is calculated when the period of time (2 minutes) 
required for reheating the slab is multiplied by the casting speed (28 
m/min). 
(3) The third term on the left side of Equation 5 expresses the length of 
the apparatus required for cooling, that is, the length of the apparatus 
required for cooling is calculated when the period of time (min) required 
for cooling the slab from 950.degree. C. to 700.degree. C. is multiplied 
by the casting speed (28 m/min). 
On the surfaces of the slabs produced by the above apparatus, there are 
irregularities, that is, the surface conditions of the slabs produced by 
the above apparatus are different from those of the hot-rolled sheets 
produced by a conventional hot rolling mill. Therefore, the use of the 
slabs produced by the above apparatus is restricted. It is an object of 
the present invention to shorten the length of the apparatus for producing 
thin slabs, so that energy can be saved in the process of production. It 
is another object of the present invention to improve the surface 
roughness of the slab and make the crystal grain size of the slab to be 
fine. 
SUMMARY OF THE INVENTION 
The present inventors have discovered the following facts. When a thin 
steel strip, which has been directly cast from molten steel, is lightly 
reduced before it is subjected to heat treatment, the temperature, at 
which the metallic structure is transformed from .gamma.-structure to 
.alpha.-structure in the process of cooling conducted after casting, is 
raised higher than that of the case in which no reduction is given to the 
slab. 
Characteristics of the method of producing steel sheets of the present 
invention will be described below. 
1. The present invention is to provide a method for continuously casting 
steel sheets comprising the steps of: adjusting a carbon content of molten 
steel to be not lower than 0.001%; directly casting a thin steel strip 
used for cold rolling from this molten steel; giving a light reduction of 
not lower than 10% to the thin steel strip; cooling the reduced thin steel 
strip; reheating the cooled thin steel strip; cooling the reheated thin 
steel strip; and coiling the cooled thin steel strip. 
2. The present invention is to provide a method for continuously casting 
steel sheets comprising the steps of: adjusting a carbon content of molten 
steel to be not lower than 0.001%; directly casting a thin steel strip 
used for cold rolling from this molten steel; giving a light reduction of 
not lower than 10% to the thin steel strip; for controlling the 
.gamma.-grain size of the thin steel strip before recrystallization to be 
not more than 100 .mu.m, and controlling the surface roughness (R.sub.max) 
of the thin steel strip to be not more than 15 .mu.m; cooling the reduced 
thin steel strip; reheating the cooled thin steel strip; cooling the 
reheated thin steel strip; and coiling the cooled thin steel strip. 
3. The present invention is to provide a method for continuously casting 
steel sheets comprising the steps of: adjusting a carbon content of molten 
steel to be not lower than 0.001%; directly casting a thin steel strip 
from this molten steel; giving a light reduction of not lower than 10% to 
the thin steel strip; cooling the coagulated steel strip to a temperature 
not higher than T1.degree. C.; reheating the cooled thin steel strip to a 
temperature not lower than T2.degree. C.; cooling the reheated thin steel 
strip to a temperature not higher than T3.degree. C.; and coiling the 
cooled thin steel strip, wherein T1 is a function of the carbon content, 
ratio of reduction (RR) and cooling speed (CR), and T2 and T3 are 
functions of the carbon content. 
EQU T1=A(-295.45[C]-32.72)+B(363.63[C]-151.51)+(-1477.27[C]+1171.36)(Equation 1 
) 
where A: common logarithm of the cooling speed (.degree.C./s) 
[C]: carbon concentration (%) 
B: function of the ratio of in-line reduction (=750/(90.times.ILRR+1) 
ILRR: ratio of in-line reduction 
EQU T2=-2000.times.[C]+980 (.degree.C.) (Equation 2) 
EQU T3=-9000.times.[C]+920 ([C]&lt;0.02%) (.degree.C.) (Equation 3-1) 
EQU T4=740.degree. C. ([C].gtoreq.0.02%) (.degree.C.) (Equation 3-2) 
In this case, the accuracy of temperature is .+-.10.degree. C. 
4. The present invention is to provide a method for continuously casting 
steel sheets according to item 1, 2 or 3, wherein the final cold-rolled 
thin steel strip is produced by common steel, the carbon content of which 
is 0.001 to 0.25%, and the tensile strength of which is 30 to 40 
kg/mm.sup.2. 
5. The present invention is to provide an apparatus for continuously 
producing steel sheets comprising: a rolling device for giving a light 
reduction; a cooling device; a heating device; a cooling device; and a 
coiler, wherein these devices are continuously arranged in order on the 
downstream side of a twin drum type continuous casting apparatus used for 
casting steel sheets continuously.

THE MOST PREFERRED EMBODIMENT 
The present invention will be specifically explained as follows. 
(1) Ratio of Reduction 
In order to improve the surface roughness, it is necessary to conduct 
rolling at the ratio of reduction of not lower than 5% as shown in FIG. 1. 
When the slab is rolled, it is possible to raise the temperature T1. The 
reason why the temperature T1 is raised is that the .gamma.-grain size 
before recrystallization is decreased by rolling, so that the 
crystallization interface can be increased and the transformation into the 
.alpha.-region can be easily performed. According to the result of the 
experiment made by the inventors, it was found that in order to make the 
.gamma.-grain size to be not more than 100 .mu.m before recrystallization, 
it is necessary to conduct rolling at the ratio of reduction of not lower 
than 10%, and it is preferable to conduct rolling at the ratio of 
reduction of not lower than 10% and not higher than 30% as shown in FIG. 
2. 
(2) Cooling Temperature (T1) 
Temperature T1 at which the .gamma.-grain is transformed into the 
.alpha.-grain is affected by the .gamma.-grain size before rolling, the 
cooling speed and the carbon concentration. The .gamma.-grain size before 
rolling is a function of the ratio of reduction of in-line. The 
.gamma.-grain size is 500 to 1000 .mu.m after the slab has been cast. When 
the slab is rolled at the ratio of reduction of 10%, the .gamma.-grain 
size is decreased to a value not more than 100 .mu.m. In FIG. 3, there is 
shown a relation between the cooling speed and the temperature T1 when the 
carbon concentration is 0.05%. When the slab is rolled at the ratio of 
reduction of 10%, temperature T1 is raised. This temperature is changed by 
the carbon concentration. That is, when the carbon concentration is 
increased, this temperature is decreased as shown by Equation (1). The 
relation between the cooling speed and the temperature T1 is shown in FIG. 
4 when the carbon concentration is 0.16%. 
EQU T1=A(-295.45[C]-32.72)+B(363.63[C]-151.51)+(-1477.27[C]+1171.36)(Equation 1 
) 
where A: common logarithm of the cooling speed (.degree.C./s) 
[C]: carbon concentration (%) 
B: function of the ratio of in-line reduction (=750/(90.times.ILRR+1) 
ILRR: ratio of in-line reduction 
(3) Reheating Temperature (T2) 
The reheating temperature is determined by the carbon concentration. This 
relation is shown by Equation 2. That is, the reheating temperature is a 
temperature at which the .gamma.-crystal is generated again on the 
interface of the .alpha.-grain. When the temperature is lower than T2, the 
.gamma.-crystals are not sufficiently generated. 
EQU T2=-2000.times.[C]+980 (.degree.C.) (Equation 2) 
(4) Coiling Temperature (T3) 
Coiling temperature (T3) is determined to be not higher than the 
temperature of recrystallization. This temperature is affected by the 
carbon concentration and expressed by Equation 3. 
EQU T3=-9000.times.[C]+920 ([C]&lt;0.02%) (.degree.C.) (Equation 3-1) 
EQU T3=740.degree. C. ([C].gtoreq.0.02%) (.degree.C.) (Equation 3-2) 
In this connection, the cold-rolled steel strip, which is the final product 
according to the present invention, is produced by common steel, the 
carbon content of which is 0.001 to 0.25% and the tensile strength of 
which is 30 to 40 kg/mm.sup.2. This cold-rolled steel strip of the final 
product can be produced in such a manner that after the slab according to 
the present invention has been made, it is subjected to the arbitrary 
processes of acid cleaning, cold rolling, annealing and so forth. 
In order to realize the method of the present invention, it is preferable 
to use a continuous sheet producing apparatus as illustrated in FIG. 5, 
including: a rolling device to give a light reduction arranged on the 
downstream side of a twin drum type continuous casting apparatus, a 
cooling device, a heating device, a cooling device and a coiling device. 
In this connection, the cooling system of each cooling device described 
above may be a water cooling system or a mist cooling system. The heating 
system of each heating device described above may be a gas heating system 
or an induction heating system by which slabs can be quickly heated. 
EXAMPLES 
Example 1 
The following is an example in which a slab of 3 mm thickness, the carbon 
content of which was 0.05%, was made by means of casting. The casting 
conditions are described as follows. The casting speed was 30 m/min, the 
ratio of reduction was 10%, the water cooling speed was 50.degree. C./sec, 
the heating speed was 2.5.degree. C./sec, and the cooling speed after 
heating was 5.degree. C./sec. The temperature T1 was 767.degree. C., the 
reheating temperature T2 was 880.degree. C., and the coiling temperature 
was 740.degree. C. 
Then, the length of the apparatus of heating--cooling--heating can be 
expressed by the following equation. 
##EQU3## 
The meaning of Equation 6 is described as follows. 
(1) The first term on the left side of Equation 6 expresses the length of 
the apparatus required for cooling after rolling has been conducted at the 
ratio of reduction of 10%, that is, the length of the apparatus required 
for cooling is calculated when a period of time (minute) necessary for 
cooling from 1100.degree. C. to 767.degree. C. is multiplied by a casting 
speed (30 m/min). 
(2) The second term on the left side of Equation 6 expresses the length of 
the apparatus required for reheating, that is, the length of the apparatus 
required for reheating is calculated when a period of time necessary for 
reheating from 767.degree. C. to 880.degree. C. at 2.5.degree. C./sec is 
multiplied by a casting speed (30 m/min). 
(3) The third term on the left side of Equation 6 expresses the length of 
the apparatus required for cooling, that is, the length of the apparatus 
required for cooling is calculated when a period of time (minutes) 
necessary for cooling from 880.degree. C. to 740.degree. C., at which the 
cooled strip is coiled, is multiplied by a casting speed (30 m/min). 
In the case where no reduction is given to the slab, the above result can 
be directly compared with Equation 5 described in Japanese Patent 
Application No. 60-30545, because the heating time from 650.degree. C. to 
950.degree. C. in Equation 5 has the same meaning as the heating speed of 
2.5.degree. C./sec. Therefore, when a reduction is given to the slab, the 
length 83 m of the heat treatment device can be shortened to 40 m. The 
surface roughness R.sub.max of the thus obtained slab was 10 .mu.m, which 
was equivalent to the surface roughness of a hot-rolled steel sheet. The 
crystal grain size of the thus obtained slab was 20 .mu.m, which was 
equivalent to the crystal grain size of a hot-rolled steel sheet used at 
present. Concerning the mechanical property, surface roughness and 
brittleness, excellent results were provided by the thus obtained product. 
Example 2 
Table 1 shows the results of experiments in which steel sheets were 
produced while the length of the heating furnace zone was variously 
changed. 
In Table 1, Example Nos. 1 to 6 are the examples of the present invention. 
In Nos. 1 to 3, the carbon concentration was changed in a range from 0.05 
to 0.16. Comparative Examples are shown in No. 1-ref to No. 3-ref. In all 
cases, the length of the heat treatment apparatus was shortened by about 
10 m. 
In Example Nos. 4 to 6, the periods of time T1, T2 and T3 were changed by 
10%. 
According to the above examples, it is clear that the heating furnace zone 
could be shortened by conducting rolling on the slab. The crystal grain 
size of the thus obtained slab was approximately 20 .mu.m, and quality of 
the slab was high with respect to surface roughness and brittleness. 
TABLE 1 
__________________________________________________________________________ 
Ratio of 
Cooling 
[C] 
reduction 
speed 
T1 T2 T3 Vc length 
NO (%) 
(%) (.degree. C./s) 
(.degree. C.) 
(.degree. C.) 
(.degree. C.) 
(m/min) 
(m) 
__________________________________________________________________________ 
Example of 
1 0.05 
10 10 800 
880 
740 
30 26 
the present 
2 0.02 
10 10 833 
940 
740 
30 29 
invention 
3 0.16 
10 10 680 
660 
740 
30 16 
4 0.05 
10 5 814 
880 
740 
30 49 
5 0.05 
10 10 720 
968 
814 
30 39 
6 0.05 
10 10 720 
792 
592 
30 33 
Comparative 
1-ref 
0.05 
0 10 667 
880 
740 
30 39 
example 
2-ref 
0.02 
0 10 688 
940 
740 
30 43 
(no reduction) 
3-ref 
0.16 
0 10 587 
660 
740 
30 25 
4-ref 
0.05 
0 5 681 
880 
740 
30 76 
__________________________________________________________________________ 
INDUSTRIAL AVAILABILITY 
As described above, according to the present invention, after a reduction 
has been given to a cast metallic slab, it is cooled from the 
.gamma.-transformation point to a temperature not higher than the 
.alpha.-transformation point. After that, the slab is heated from the 
.alpha.-transformation point to a temperature not lower than 
.gamma.-transformation point. Then the slab is cooled. Due to the 
foregoing heat treatment process, as compared with a simple heat treatment 
process in which the slab is cooled and heated to make the crystal grains 
fine, it is possible to obtain a thin slab, the metallic structure of 
which is fine, by a production apparatus, the length of which is 
shortened. Accordingly, while energy is saved and the production apparatus 
is made compact, it is possible to obtain a slab, the quality of which is 
equivalent to that of a good hot-rolled steel sheet.