CONTINUOUSLY CAST THIN PIECE AND METHOD OF CASTING THEREOF

Abstract: Formation of roughly hexagonal patterns on the continuously cast thin piece, each pattern being encircled with a groove in depth ranging from 5 to 30 µm and dimensionally corresponding to a circle having a diameter ranging from 5 to 200 µm, prevents the continuously cast thin piece from cracking at surface thereof. Such roughly hexagonal patterns are formed through the process of casting in which superheat temperature ΔT of the molten metal in the pouring basin part of the mobile mold type continuous casting machine is controlled to 15 °C or under.

Description: The present invention relates to a thin continuous cast metal plate manufactured through the use of a casting apparatus, such as a movable casting mold, for example, a twin drum system wherein use is made of a pair of cooling drums equipped with an internal cooling mechanism, a single drum system wherein use is made of a single cooling drum, or a drum-belt system wherein a pouring basin is formed between a cooling drum and a belt. In recent years, in the field of continuous casting of a metal, various proposals have been made describing a technique for casting a thin cast metal plate having a thickness (2 to 10 mm) close to that of a final article by means of a continuous casting apparatus wherein use is made of a cooling drum provided with an internal cooling mechanism for the purpose of reducing the production cost and creating a new material. In the above-described casting techniques, it is important to stably maintain the surface appearance of a cast metal plate on a high level. For this reason, proposals have been made on a casting technique wherein casting is conducted in the presence of an inert gas atmosphere for the purpose of preventing the formation of scum in a pouring basin (see Japanese Unexamined Patent Publication (Kokai) No. 62-130749), a roll brush technique wherein an oxide or the like deposited on the surface of a cooling drum is removed for the purpose of uniformly forming a solidified shell by means of a cooling drum (see Japanese Unexamined Patent Publication (Kokai) No. 62-176650), a technique as another means for achieving uniform formation of a solidified shell wherein a number of dimples are provided on the peripheral surface of a cooling drum so as to form an air gap serving as a heat insulating layer between the cooling drum and a solidifying shell (see Japanese Unexamined Patent Publication (Kokai) No. 60-184449), and other techniques. Even in the above-described conventional casting techniques, it was difficult to stably prepare a cast metal plate having good surface appearance, and longitudinal and transversal cracks often occurred. Under the above-described circumstances, an object of the present invention is to prevent the occurrence of cracking on the surface of a cast metal plate through the positive provision of a predetermined pattern on the surface of a cast metal plate as opposed to the prior art wherein the surface of the cast metal plate is made as even as possible. More specifically, an object of the present invention is to provide a cast metal plate having a tortoise shell pattern surrounded by a groove on the surface of a thin continuous cast metal plate. Another object of the present invention is to provide a process for producing said cast metal plate by means of a movable casting mold. The above objects are achieved by a continuous cast metal plate and a process according to the claims. The present inventors have made various studies and, as a result, have found that the formation of a tortoise shell pattern having a circle equivalent diameter of 5 to 200 mm surrounded by a groove having a depth of 5 to 30 µm on the surface of a cast plate is very effective for preventing the occurrence of surface cracking of the cast plate. Further, the present inventors have proved that the above-described object can be attained by a casting process wherein the overheating temperature, ΔT, of a molten metal poured into a pouring basin of a casting mold of a movable mold type continuous casting machine is regulated to 15°C or below as a means for forming the above-described pattern. The term "circle equivalent" used herein is intended to mean a value obtained by converting the area A surrounded by a groove of a closed curve to the circle area πd 2 /4 ( d = 4A/π ). Further, the term "tortoise shell pattern" is intended to mean an irregular pattern substantially surrounded by a groove. The present invention will now be described in more detail in the case of a twin drum system in connection with the drawings. Fig. 1 is a diagram showing the relationship between the overheating temperature, ΔT (°C), of a molten metal within a pouring basin and the groove depth (µm) of the tortoise shell pattern; Fig. 2 is a diagram showing the relationship between the circle equivalent diameter (mm) of a tortoise shell pattern for each rippled surface depth (µm) of the tortoise shell pattern and the overheating temperature, ΔT (°C), of a molten metal within a pouring basin; Fig. 3 is a rubbed copy of the surface state of the cast plate according to the present invention; Fig. 4 is a schematic perspective view of a twin drum continuous casting machine; Fig. 5 is a diagram showing the relationship between the overheating temperature, ΔT (°C), of a molten metal within a pouring basin and the occurrence of a tortoise shell groove pattern and the degree of occurrence of cracking (m/m 2 ); and Figs. 6A and 6B are respectively a plan view and a cross-sectional view showing the surface state of position I in Fig. 3, Figs. 6C and 6D are a plan view and a cross-sectional view, respectively, showing the surface state of position II in Fig. 3, and Fig. 6E is a cross-sectional view showing the surface state of the cast metal plate of the present invention. In Fig. 4, a molten metal 6 fed into a pouring basin 5 defined by cooling drums 1 and 2 and side weirs 3 and 4 is rapidly cooled and solidified by means of the cooling drums 1 and 2 to form a solidified shell and extruded downward to form a cast metal plate 7. The surface of the cast metal plate 7 according to the present invention has a tortoise shell pattern as shown in Fig. 3. The tortoise shell pattern is identified, for example, by spraying a particulate carbon on the cast metal plate and making a rubbing of the tortoise shell pattern by means of a plastic pressure-sensitive tape to identify the pattern of a substantially closed curve (see Fig. 3). This pattern is defined by a groove having a depth of about 5 µm or more, and in Fig. 3, the groove is shown as a continuously linked white portion. The real surface area of the cast metal plate having a tortoise shell pattern is larger than that of a smooth cast plate. The conditions under which this pattern is formed in the step of cooling and solidification are such that the formation of a solidified shell is slow at the initial stage of solidification. This corresponds to the case where the overheating temperature of the molten metal is low. Under this condition, a solidified shell having a sufficient surface area is formed on the surface layer of a cast metal plate, and the shrinkage caused by the subsequent cooling and solidification of the inside of the cast metal plate causes a tortoise shell groove pattern to be formed on the surface of the solidified shell, so that no cracking occurs on the surface of the cast metal plate. This is because the critical strain is so large with respect to the fracture, by virtue of the thin shell of the surface layer, that the deformation according to the shrinkage stress is possible within the tolerable range. When the solidified shell thickness is too large due to excessive time for the formation of the solidified shell, it often becomes difficult to form the tortoise shell pattern by the subsequent shrinkage. In this case, there is a high possibility that the deformation is locally concentrated and consequently cracking occurs. The pattern is in a tortoise shell form having a depth, D, of 5 to 30 µm and a circle equivalent diameter of 5 to 200 mm as shown in Fig. 6E. When the depth of the groove exceeds 30 µm, this pattern is often left as uneven brightness at the time of cold rolling of the cast plate. When the circle equivalent diameter is less than 5 mm, since there is not significant difference in the real surface area between this cast metal plate and the smooth cast metal plate, the deformation falling within the critical strain range cannot absorb the shrinkage stress, so that cracking occurs. On the other hand, when the circle equivalent diameter exceeds 200 mm, the deformation caused by the solidification stress often concentrates on a very small portion of the groove constituting the tortoise shell pattern, so that cracking occurs. By contrast, the cast metal plate having a tortoise shell pattern brings about neither longitudinal cracking nor transversal cracking and can stably maintain a good surface appearance of the cast plate. Fig. 1 shows the relationship between the overheating temperature, ΔT (°C), of the molten metal 6 within the pouring basin 5 and the groove depth (µm) of the tortoise shell pattern in a continuous casting of an austenitic stainless steel thin cast plate through the use of a continuous casting apparatus of a twin drum system shown in Fig. 4. As is apparent from the drawing, there is a tendency that the higher the overheating temperature, the smaller the groove depth. Fig. 2 shows the relationship between the overheating temperature, ΔT (°C), of the molten metal within the pouring basin and the circle equivalent diameter (mm) of the tortoise shell pattern of each groove depth (µm) manufactured under the same condition as shown in Fig. 1. As is apparent from the drawing, there is a tendency that the higher the overheating temperature, the larger the circle equivalent diameter of the tortoise shell pattern and the smaller the groove depth. In order to attain conditions which do not bring about the occurrence of surface cracking of the cast plate, i.e., a tortoise shell pattern having a circle equivalent diameter of 200 mm or less and a groove depth of 5 µm or more, as can be seen from Figs. 1 and 2, it is necessary that the overheating temperature, ΔT (°C), of the molten metal within the pouring basin be 15°C or below. The present invention will now be described by way of the following Examples. EXAMPLES An austenitic stainless steel having an SUS304 composition manufactured by the conventional procedure was cast into a thin cast plate having a plate width of 800 mm and a plate thickness of 2 mm at a casting speed of 80 m/min through the use of a continuous casting machine of a twin drum system shown in Fig. 4. In this case, the temperature of the molten metal 6 at the pouring basin 5 was varied by varying the overheating temperature, ΔT, and use was made of cooling drums 1, 2 having depressions in a circular or elliptical form having a diameter of 0.1 to 1.2 mm and a depth of 5 to 100 µm ununiformly provided on the periphery thereof. The surface appearance and degree of cracking (m/m 2 ) of the resultant cast plate are shown in Table 1 and Fig. 5. The groove depth of the tortoise shell pattern was measured by the following method. Specifically, a portion including a closed curve was detected by a rubbed copy in the case of a groove depth of 5 µm or more and by optical means in the case of a groove depth of less than 5 µm. The roughness of the portion was measured by means of a roughness meter, and the maximum value was regarded as the above-described groove depth. The circle equivalent diameter of the tortoise shell pattern was regarded as the circle equivalent diameter of the detected portion. As given in Nos. 1 to 4 of Table 1, it has been confirmed that when the overheating temperature, ΔT, of the molten metal 6 is 15°C or below, the tortoise shell pattern as shown in Fig. 3 according to the present invention is formed and the degree of cracking is substantially zero. Thus, the casting through the use of a molten metal having an overheating temperature, ΔT, of 15°C or below contributes to alleviation in the occurrence of cracking derived from the heat shrinkage of the cast metal plate and, at the same time, enables a tortoise shell groove to be formed on the surface of the cast metal plate, and the relaxation of the cooling of the cast metal plate and the prevention of rapid lowering of the surface temperature of the cast metal plate by means of the cooling drums having depressions ensures the formation of the tortoise shell pattern and can suppress the variation in the dimension of the pattern. In this case, the width, W (see Fig. 6), of the groove of the tortoise shell pattern shown in Fig. 3 was about 2 mm. The cold-rolling of this cast metal plate brought about no surface defect. There is a tendency that the lower the overheating temperature, ΔT, of the molten metal, the larger the groove depth. As is apparent from Nos. 6 to 12 as Comparative Examples of Table 1, when the casting was conducted under condition of an overheating temperature, ΔT, higher than 15°C, even in the case of use of the same cooling drums as those of the present invention, no tortoise shell pattern was formed and the degree of cracking increased. In particular, when the casting was conducted at a high temperature of a ΔT value of 40°C or more, the degree of cracking was rapidly increased and reached 0.1 m/m 2 . The degree of cracking was quantified by pickling the cast metal plate having a length of 4 m after casting to measure the flaw present in the cast metal plate and converting the measured value to the unit area. No. Overheating temp. (°C) Tortoise shell pattern Groove depth (µm) Circle equivalent diameter of tortoise shell pattern (mm) Degree of cracking (m/m 2 ) Uneven brightness of tortoise shell pattern Example of present invention SUS304 1 5 Yes 27 6 - 10 0 No 2 6 Yes 20 7 - 15 0 No 3 10 Yes 11 50 - 110 0 No 4 12 Yes 5 100 - 190 0 No Comparative Example SUS304 5 0 Yes 32 6 - 10 0 Yes 6 16 No 4 200 - 250 0.01 No 7 17 No 4 210 - 250 0.05 No 8 23 No 4 230 - 320 0.04 No 9 36 No 2 1 - 4 0.07 No 10 40 No 2 1 - 4 0.10 No 11 42 No 2 1 - 4 0.09 No 12 45 No 2 1 - 4 0.13 No As is apparent also from the foregoing Examples, in the present invention, the occurrence of the cracking and uneven brightness is suppressed by positively forming a desired pattern on the surface of a thin continuous cast metal plate, which enables reliable results unattainable by the prior art to be obtained, so that it becomes possible to provide a product having better surface quality and material quality.

Claims: A process for continuously casting a thin cast metal plate by means of a movable casting mold of a continuous casting machine, said movable casting mold comprising cooling drums equipped with depressions and side weirs, characterized in that the casting is conducted while regulating the overheating temperature, ΔT, of a molten metal in a pouring basin formed by said movable casting mold at 15°C or below. A thin continuous cast metal plate having on its surface a tortoise shell pattern comprising a plurality of areas having a circle equivalent diameter of 5 to 200 mm surrounded by a groove having a depth in the range of from 5 to 30 µm, wherein said pattern is formed on the surface of said cast metal plate by the process of claim 1. A thin cast metal plate according to claim 2, wherein said thin cast metal plate is an austenitic stainless steel plate.

Inventors: SUICHI ISAO, TANAKA SHIGENORI, SUICHI, ISAO, TANAKA, SHIGENORI, SUICHI, ISAO NIPPON STEEL CORPORATION HIKARI WORKS, TANAKA, SHIGENORI NIPPON STEEL CORPORATION HIKARI
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1991
Main classification: B22D11
IPC codes: B22D11