METHOD OF HEATING MOLTEN STEEL IN TUNDISH FOR CONTINUOUS CASTING APPARATUS

Abstract: A method of heating molten steel in a tundish for a continous casting apparatus introduces and recirculates the molten steel within a by-pass of a horizontal groove type of induction heater mounted on a side wall of the tundish. The power supplied to the induction heater is controlled so that, for example, it rises gradually at the beginning of pouring in proportion to the quantity of molten steel within the tun­ dish, to efficiently eliminate any disturbance in the current supply by pinching, and compensate for any drop in tem­ perature of the molten steel when it is poured into the tundish during the continuous casting.

Description: The present invention relates to a method of heating a molten steel in a vessel such as a tundish for a continuous casting apparatus. In general, the temperature of the molten steel first received in a tundish is considerably lowered through heat absorption by the refractory material of the inner lining, heat dissipation from the bath surface or the like. Consequently, a part of the cast sheet becomes poor in quality. Therefore, such a temperature drop must be compensated. For this purpose, the present invention is designed to meet the above requirement in such a manner that the above tundish is provided with a horizontal channel type induction heater. The molten steel is circuitously introduced into the interior of the induction heater to be heated and then returned into the tundish under circulation. In the continuous casting, the molten steel is poured into the tundish through a ladle, and undergoes a considerable temperature drop due to the heat dissipation from the poured flow, the heat absorption by the inner lining refractory material, and heat radiation from the surface of the bath. As the ordinary technique which compensates such a temperature drop, there has been a technique disclosed in Japanese Patent Laid-Open No. 163,730/79 in which a vertical type induction heater adapted to vertically circulate the molten metal is attached for heating to the bottom wall of the molten metal storing container. However, since in the technique disclosed therein, the vertical type induction heater is attached to the bottom wall, it is difficult to use in a tundish for a continuous casting apparatus. On the other hand, there has been proposed a technique by which a horizontal channel type induction heater is fitted to the side wall of the tundish, as disclosed in Japanese Patent Laid-Open No. 56,144/82. A skeleton view of the heater used in this technique is shown in Figs. 1 and 2. The illustrated horizontal channel type induction heater 2 is fitted to the side wall of the tundish 1. The body of the induction heater 2 is constituted by disposing a refractory material 7 inside a shell 6 defining the outer shell. A roundabout channel 8 is formed in a loop shape from the inlet port 8a to an outlet port 8b which parts communicate with the interior of the tundish 1. A through hole 9 is provided penetrating the central portion surrounded by the roundabout channel 8 in a direction orthogonal to the direction of flow of the molten steel. In Figs. 1 and 2, reference numeral 3 denotes the location of a nozzle from which the molten steel is received, reference numeral 4 an outflow port, and reference numeral 5 a partition wall for guiding the molten steel flow, which is provided if necessary. A primary induction coil 10 to generate an induction current i in the molten steel flow within the roundabout channel 8 is assembled through insertion in the inside of the above through hole 9 via a core 10a. A magnetic field Φ is produced in the core 10a when the primary induction coil 10 is energized, and a secondary induction current i is accordingly caused to flowed in the molten steel within the roundabout channel 8. Joule's heat of i²·R is thus produced to heat the molten metal. In other words, the heater is so constituted that the molten steel is heated on flowing through the roundabout channel. However, when such an induction heater 2 is used, it has been often experienced that the intended heating of the molten steel may not be appropriately and smoothly performed depending upon the schedule of the power supplied to the heater. When a normal rated electric power is constantly supplied to the coil 10 of the induction heater 2 in the heating of the molten steel, air is often entrapped in the roundabout channel 18, particularly when the stored amount of molten steel in the tundish is small, that is, in the initial stage of pouring the molten steel from the ladle to the tundish 1, at which stage heating is most necessary. Thus the sectional area of the molten steel flow is smaller and the secondary induction current density is larger in the roundabout channel 8 in which the air is entrapped, so that the pinching phenomenon in the roundabout channel 8 becomes evident and, in the worst case, the molten steel is cut off in the channel 8 and the induction current is interrupted. When such a pinching phenomenon occurs, fluctuation in the electric current flowing through the coil 10 becomes larger so that the electric power necessary for heating the molten steel can not be steadly supplied, and in some cases, there takes place a tripping of the electric power source. In the case where the electric current continuity is interrupted due to the pinching phenomenon, it takes a long time to recover, and similar electric current continuity interruption repeatedly occurs. When the above pinching phenomenon becomes more evident, the damage to the refactory material layer is too large to be repaired, and it is possible that a leakage of molten steel may occur. Although restriction of the electric power to be supplied is effective for preventing such a phenomenon, the temperature drop of the molten steel in the initial stage can not be avoided. The presence or absence of the pinching phenomenon and the appropriateness of the electric power supply schedule is not ordinarily a problem in the case of the ordinary vessel for holding the molten metal other than the tundish for the continuous casting. For, in the case of such a holding vessel, it is not too late to apply the electric power after the bath surface level is so raised that the electric power may be stably supplied. However, when the temperature drop is large in the pouring initial stage in the case of the tundish for the continuous casting, the casting sheet quality is adversely affected. Thus, the control of the electric power supply is indispensable for meeting the requirement that the application of the electric power is appropriate at the early stage to provide heating. In this sense, it is necessary to develop a technique to effectively prevent the temperature drop by supplying the maximum electric power within a range in which no pinching is caused and at the initial stage of pouring the molten metal into the tundish. It is an object of the present invention to provide a method of heating the molten steel in the tundish by using horizontal channel type induction heater which meets such a requirement. According to the present invention there is provided a method of heating molten steel in a vessel for molten metal by means of a channel type induction heater, which induction heater comprises a roundabout channel the ends of which communicate with the interior of said vessel and through which said molten steel flows, and a coil which, when electrical power us applied thereto, generates a magnetic flux passing through said channel whereby an induction current is caused to flow through said molten steel in said channel, which induction current heats said molten steel by Joule's heating, characterised in that the electrical power supplied to the induction heater is controlled according to the amount of molten steel in the vessel. Advantageously the vessel is a tundish for a continuous casting apparatus. In an embodiment of the invention the electrical power supplied to the induction heater is controlled such that the electrical current density D (A/cm²/n) in the molten steel in the channel fulfills the rationship D ⋞ 0.01 H + 4.5 where H (mm) is the depth of molten steel in the tundish. By so supplying the electric power, the interruption of the secondary induction current flowing through the molten steel flow in the roundabout channel owing to the pinching is avoided, and the molten steel in the tundish is stably heated. For a better understanding of the invention and to show how the same may be carried into effect reference will now be made, by way of example only, to the accompanying drawings in which:- Fig.1 is a horizontally sectional view of a tundish equipped with an induction heater having a coil removed; Fig.2 is a vertically sectional view along line A-A portion in Fig.1; Fig.3 is a graph illustrating the relation between the depth of the steel in the bath and the supply of electric current in connection with the occrrence of pinching; Fig. 4 is a graph illustrating the influence of the induction electric current density (coil and end count N: 22) in different heaters in connection with a pinching occurrence; Fig. 5 is a graph illustrating the comparison between the method of the present invention and the conventional method, and the influence thereof on the temperature of the steel bath; Figs. 6a and b, Figs. 7a and b, and Figs. 8a and b are each a graph showing the relationship between electric power supply pattern to the induction heater and molten metal temperature shifting caused thereby. Reference will now be made to Fig. 3 in which is shown the relation between the stored amount of the molten steel in the tundish, that is, the depth H mm of the steel bath in the tundish (the distance from the upper edge of the roundabout channel to the bath surface) and the supply of electric power KW to the heater 2. It is understood that there exists an appropriate electric power supply which gives no pinching depending upon the depth of the steel bath. In the illustrated embodiment according to the present invention, the sectional profile of the roundabout channel 8 was a long elliptical form of about 100 x 200 mm with a sectional area of 184 cm². The composition of the molten steel was C 0.1-0.15%, Si 0.25-0.35%, Mn 0.65-1.10%, P 0.01-0.018%, S 0.005-0.010%, Al 0.02-0.03% as an ordinary plate. When similar experiments were done with respect to a heater with the roundabout channel of a substantially annular sectional profile (100 mm Φ) having a sectional area 79 cm² different from that shown in Fig. 1, a slight difference in the pinching occurred zone was observed. Accordingly, as indicated in Fig. 4, when examination was similarly done on the relation between the depth H mm of the bath which means the distance from the upper edge of the roundabout channel to the bath surface and the induction electric current density D(A/cm²/n) in the molten steel per one turn of the primary coil with end count of n, in connection with the pinching occurrence in two types of roundabout channel 8 of different section profile, it was revealed that the appropriate induction electric current density is constant with respect to the depth H of the steel bath irrespective of the different sectional area. As a consequence, it is seen from the figure that in any case, the zone which gives the appropriate induction electric current density D(A/cm²/n) with no pinching produced is the left upper portion in the graph given by the following formula: D ≦αµρ¨ 0.01 H + 4.5. From this, it is clear that the required induction heating is possible at the range at which the electric power is stably supplied, so long as the relation between the appropriate induction electric current density and the depth H is controlled to meet the following relation: D ≦αµρ¨ 0.01 H + 4.5. As to the value of D, if it is too small, the effect aimed at by the invention, that is, the minimizing of the decrease in the temperature cannot be attained. Therefore, the operation may be done in conditions meeting the above formula Fig. 5 shows an example in which the molten steel was heated by an appropriate induction electric current value i obtained, an example in which an electric current was first passed through the coil 10 such that the induction electric current density was 10.3 A/cm²/n after the depth of the steel bath in the tundish reached 700 mm, and an example in which no current was applied. The drop in the temperature during the initial pouring stage into the tundish is conspicuously small in the case where heating was done while being controlled through an appropriate secondary induction electric current according to the present invention, and a drop in the temperature of the molten steel is to a greater or lesser extent observed in the other two examples. Next, an example to which the invention is applied at the pouring initial stage will be explained below. In the following description, the stored amount of the molten steel in the tundish, that is, the depth H mm of the steel bath, is represented by the pouring time in min.- i.e. the lapse of time after pouring commences - under the conditions that the pouring flow rate of the molten steel per time was kept constant. An experiment was done at a pouring flow rate 7 tons/min by using a tundish 1 of volume 7 tons (the depth of the bath being 600 mm on full charging). When the maximum electric power was applied by the induction heater 2 of the normal rated output 1,000 KW/minute after pouring was begun, the above-mentioned pinching was produced as shown in Fig. 6a, and the resistance heat generation due to the induction current i was not observed. Subsequently 1 and 2 minutes thereafter, electric power of 1,000 KW was applied again, but pinching was produced. When electric power was applied again 2.5 minutes after the pouring, the first stable heating could be done under current application. However, such delayed initial heating, as much as 2.5 minutes after the commencement of the pouring, does not serve to prevent a drop in the temperature of the molten steel in the tundish 1 immediately after pouring, and the difference ▵T between the lowest temperature and the temperature at which the stationary casting zone is reached is considerable, as shown in Fig. 6b. Thus, as shown in Fig. 7a, when a restricted electric power of 200 Kw (H: 200 mm) and then 300 KW (H: 400 mm) was successively applied for 17 seconds 0.25 minute and 0.7 minute after the pouring at 7 tons/min. and the maximum electric power of 1,000 kw was applied 1 minute after the pouring was commenced, the pinching is still produced. Although the stable heating can be possible under electric current application by reapplying 1.5 minutes after the pouring was commenced, the change of the temperature in the tundish is still considerable as shown in Fig. 7b, and the ▵T reaches near -10°C. As shown in Fig. 8a, when electric power of 300 KW and 650 KW were applied at the lapses of time of 0.2 minute and 0.7 minute for 17 seconds to be in proportion to the lapse of time after pouring which is in proportion with the stored amount of the molten metal in the tundish, and the electric power of 1,000 KW was applied 1 minute after the commencement of the pouring, the flow of the molten steel in the channel was not interrupted due to the pinching, and the heating under stable electric current application was possible, so that as shown in Fig. 8b, the reduction in the temperature in the molten steel in the tundish was thus decreased to an insignificant extent. When the preliminary and stepwise electric power application was tried at 800 KW or 950 KW which was in no relation to the stored amount of the molten steel and thus deviating from the above-mentioned proportional linearity, the occurrence of the pinching could not be avoided. In the above examples, when the operation as shown in Fig. 8a is applied to the tundish 1 with a volume of 7 tons, in which the depth H=600 mm of the steel bath at a time of one minute after the commencement of the pouring, which is the time period at which the necessary maximum electric power is applied to the induction heater 2, is deemed as a standard bath surface level, and an electric power application pattern corresponding to 30% (300 KW) and 65% (650 KW) of the normal rated electric power 1,000 KW of the induction heater 2 at the points of 200 mm (33% of the standard level) and 400 mm (67%) was applied to the heater 2, casting can be realized as shown in Fig. 8b free from interference due to pinching and accompanied by substantially no drop in the temperature of the molten steel in the tundish. Although in the above explanation, a tundish of 7 tons in volume was considered, the invention may similarly be applied in the case of a tundish of a large volume of 35 tons, or 75 tons, and a so-called consecutively ascending schedule in which the electric power is gradually increased depending upon the stored amount may be adopted. However, since the heat absorption of the refractory material of the inner lining becomes larger and the heat dissipation from the bath surface becomes greater as the volume of the tundish increases, it may be necessary to increase the pouring speed in the initial stage to some extent in order to decrease the above-mentioned ▵T. In the above description, the inventions is explained only in relation to the initial pouring stage. However, the control of the supply of the electric power to the induction heater depending upon the stored amount of the molten steel in the tundish is also applicable where the depth H of the steel bath varies due to the changes in the bath surface levels which occur between charges when continuous casting is successively done. As mentioned above, the method of heating the molten steel according to the present invention can be advantageously applied to a tundish for a continuous steel casting apparatus, and it may also be applied to a metal melt hold vessel with an induction heater comprising a roundabout channel where a drop in temperature of the molten metal due to significant heat absorption, must be avoided.

Claims: A method of heating molten steel in a vessel for molten metal by means of a channel type induction heater, which induction heater comprises a roundabout channel the ends of which communicate with the interior of said vessel and through which said molten steel flows, and a coil which, when electrical power is applied thereto, generates a magnetic flux passing through said channel whereby an induction current is caused to flow through said molten steel in said channel, which induction current heats said molten steel by Joule's heating, characterised in that the electrical power supplied to the induction heater is controlled according to the amount of molten steel in the vessel. A method according to claim 1 characterised in that the vessel is a tundish for a continuous casting apparatus. A method according to claim 2 characterised in that the electrical power supplied to the induction heater is controlled such that the electrical current density D(A/cm²/n) in the molten steel in the channel fulfills the rationship D ⋞ 0.01 H + 4.5 where H (mm) is the depth of molten steel in the tundish.

Inventors: HABU, YASUHIRO KAWASAKI STEEL CORPORATION, UEDA, TSUNEHIRO KAWASAKI STEEL CORPORATION, YAMANAKA, HIROMITSU KAWASAKI STEEL CORPORATION, YOSHII, YUTAKA KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1984
Main classification: B22D11
IPC codes: B22D11, B22D41