METHOD OF CONTROLLING DRAWING IN HORIZONTAL CONTINUOUS CASTING

Abstract: A method of controlling drawing of a cast piece in horizontal continuous casting, in which drawing is controlled in accordance with an inwardly curved passage to keep the acceleration low at the start of the drawing and to increase it gradually in the drawing process for the purpose of reducing the number of bubbles appearing on the surface layer of the cast piece by sucking the ambient air into the mold when drawing the cast piece.

Description: TECHNICAL FIELD The present invention relates to a withdrawal process of horizontal continuous casting, and particularly relates to a withdrawal control process for controlling acceleration at the velocity-up stage in a pull step of pulling a cast matter. BACKGROUND OF THE INVENTION In horizontal continuous casting, a casting process in which a cast matter pulling cycle is composed of a pull step, a pause step and a push back step is known (Japanese Unexamined Patent Publication No. Sho-58-44950). Fig. 2 typically shows a pattern of the pulling velocity in the above steps. In a horizontal continuous casting process for casting a cast matter having a cross sectional size of 80 - 350 mm at a high speed (not lower than 1.6 m/min), the pulling cycle is set to about 120 cycle/min, and time t o of one pulling cycle is set to about 0.5 sec. The respective times t 1 , t 2 and t 3 of the pull, pause and push back steps are set to 0.2 sec, 0.1 sec and 0.2 sec respectively. In this case, the pulling velocity V c in the pull step is controlled by increasing the velocity straight initially in about 0.04 sec. That is, a cast matter is pulled suddenly with a constant velocity gradient k (=tan  ) from a start point A to a point B. Then, the cast matter is pulled at a uniform speed from the point B to a point C, and the velocity is decreased suddenly from the point C to a point D. Then the pulling is paused from the point D to a point E, and next the cast matter is pushed in the reverse direction, that is, back to the mold side slightly from the point E. Then, returning to the start point A through points E, G and H, one pulling cycle is finished. In such a conventional withdrawal control process, there is a particular problem in that outside air enters into a mold in the velocity-up stage in the step of pulling a cast matter. This phenomenon causes residual bubbles 2 in a surface layer portion of a cast matter 1 as shown in Fig. 3, and if the number of residual bubbles increases, the bubbles appear as linear flaws in the surface of products at the time of rolling so that the quality deteriorates. The cause of such residual bubbles is that the pull velocity in the velocity-up stage is so high that negative pressure is produced in a portion called a triple point 5 between a mold 3 and a brake ring 4 to lead the outside air therein, the air being brought as bubbles into a surface layer portion of a shell 6 which is still in a melted state. In order to solve the problem of residual bubbles in a surface layer portion of a cast matter, a device of preventing the outside air from entering into the above-mentioned triple point has been made (Japanese Unexamined Utility Model Publication No. Hei-1-30687). A seal mechanism disclosed in this application is constituted by three members, that is, a mold 3, a brake ring 4 and a feed tube 7 which are joined with each other with a flexible thin plate 8 (carbon sheet or the like) inserted into a joint portion of the three members. According to such a seal mechanism, even if accurate fitting in the joint portion of the three members, that is, the mold, the brake ring and the feed tube is insufficient more or less, a thin plate flexes itself to prevent a very small space in the joint portion, so that there is an effect to prevent the air from entering. To employ such a mechanical seal mechanism, however, it is necessary not only to make machining on each part with high accuracy, but also to performing careful working at the time of attaching a thin plate. The same work must be done every time when a brake ring or any other member is replaced. Particularly in a multi-strand equipment mounted with two or more molds, troublesome increases more in working. The present invention is intended to prevent the outside air from entering into a mold only through pulling acceleration control in the consideration of the foregoing disadvantage and inconvenience caused by employing such a mechanical seal mechanism, and it is an object of the present invention to provide a withdrawal control process of horizontal continuous casting in which the number of bubbles in a surface layer portion of a cast matter can be reduced extremely. A discontinuous cast strand withdrawing apparatus and process is known from DE-A-3503865. In the method of this disclosure, a cast strand is withdrawn by a drive providing a linear motion with superimposed periodic oscillations. Typically, the motion of the strand is arranged such that it is resting or moving backwards for less than 10% of the time of the oscillation cycle. DISCLOSURE OF THE INVENTION In order to attain the aforesaid object of reducing the number of bubbles in the surface layer portion, the withdrawal control process of horizontal continuous casting according to the present invention comprises a cast matter pulling cycle composed of a pull step, a pause step and a push back step, and is characterized in that said pull step is controlled along a pulling velocity trace downwardly curved with respect to a linear velocity trace having a constant velocity gradient from a starting speed to a maximum speed in a pull acceleration stage at the beginning of the pull step and that the value of acceleration of a cast matter pulling velocity at the beginning part of the pull acceleration stage in the pull step is not larger than 0.6m/sec 2 . That is, the pulling velocity pattern is made to be an inward curved shape (from a point A to a point B as depicted in Figure 1). Specially, the initial acceleration is reduced to 0.4 ∼ 0.6 m/sec 2 which is about a quarter of conventional acceleration, in a period of withdrawal of 2 mm. According to the present invention, since withdrawal of a cast matter is begun with the acceleration is made small at the beginning in the velocity-up stage in the pull step, no phenomenon of negative pressure is produced at the triple point. Therefore, the outside air hardly enters into a mold even if no conventional mechanical seal mechanism is provided, so that the number of bubbles in a surface layer portion of a cast matter can be reduced extremely. Succeeding the initial control under the small acceleration, though the acceleration is made larger than that in the conventional case, an air space has been produced at this time between the triple point and a shell, and no phenomenon of negative pressure is not caused. As has been described above, according to the present invention, the acceleration in the pull step is controlled along a curved trace so that the value of acceleration is made small at an acceleration beginning stage and made large succeedingly. Accordingly, no phenomenon of negative pressure is produced, so that it is possible to prevent the output air from entering into a mold to make it possible to reduce the number of produced bubbles in a surface layer portion of a cast matter extremely. It is therefore unnecessary to provide any mechanical seal mechanism to make it possible to eliminate the disadvantage and inconvenience due to the provision of the seal mechanism. Accordingly, the above-mentioned effect can be obtained only by controlling the acceleration in the velocity-up stage. In addition, according to the present invention, it was possible to obtain an improved cast matter of Ca-S free cutting steel stably. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a typical diagram illustrating the configuration of a pulling velocity pattern according to the present invention; Fig. 2 is a typical diagram illustrating a conventional pulling velocity pattern; Fig. 3 is a configuration diagram illustrating a conventional mold device, or an explanatory diagram showing a state of residue of bubbles in a surface layer portion of a cast matter; and Fig. 4 is an explanatory diagram of a mold showing the quantity of projection of a triple point. BEST MODE FOR REALIZING THE INVENTION Fig. 1 is a typical diagram illustrating a pulling velocity pattern according to the present invention. That is, although the acceleration in the pulling velocity-up stage in the pull step is always kept constant as shown by a dotted line in Fig. 1 according to the conventional method, the acceleration according to the present invention is divided into two stages so that control is made so as to make the acceleration small in the beginning and then make it large succeedingly along a curve. The pulling velocity pattern is the same as in the conventional case, except that it is different from the conventional case in its region of from the point A to the point B. In practice, the acceleration from the point A to the point A 1 is made 0.4 ∼ 0.6 m/sec 2 . Since the acceleration was made 1.6 m/sec 2 in the conventional case, the acceleration is reduced into about a quarter thereof. In the period of the initial pulled quantity 2 mm (since the pulling stroke varies according to the size of a cast cast matter, it is easier to perform control based on the pulling quantity than based on time), no phenomenon of negative pressure shown in Fig. 3 is produced at the triple point 5 because the withdrawal is made with a low velocity. If this pulling velocity is made too low, the solidification of a shell is so progressed that a cast matter cannot be pulled. The pulling quantity is detected by using a measure roll (not shown) or the like provided on the output side of a mold. After reaching the point A 1 , though the pulling velocity is made up suddenly, no phenomenon of negative pressure is produced because an air space has been produced at the triple point 5 at that time. As has been described above, since no phenomenon of negative pressure is produced at the triple point, it is possible to prevent the outside air from entering into a mold even if no conventional seal mechanism is provided, and it is possible to reduce bubbles in a surface layer portion of a cast matter. If the process according to the present invention is used in addition to the provision of a seal mechanism, needless to say, a more advantageous effect can be obtained. The results of carrying-out of the process according to the present invention and the conventional process with respect to Ca-S free cutting steels was obtained as follows. Cast Matter Size:  120 mm Pulling Cycle: 120 cpm Pulling Velocity: 1.6 m/min Melted Steel Overheat Degree (in crucible): 20 °C Melted Steel Component (%): C Si Mn P S Al Cr Ca 0.33 0.24 0.76 0.008 0.058 0.007 0.03 0.0095 Used Mold: quantity of projection of triple point h=4.0 mm, l = 8.0 mm (see Fig. 4) Pulling Velocity: process of present invention 0.4 m/sec 2 conventional process 1.6 m/sec 2 Seal Mechanism: not provided in the both the processes After horizontal casting was performed under the above conditions, the number of produced bubbles in 3mm-depth portion under a surface layer of a cast matter at each of a bottom portion thereof (portion at the beginning of withdrawal), a middle portion thereof (intermediate portion), and a top portion thereof (portion at the termination of withdrawal) was examined. The results are shown in Table 1. Bottom portion Middle portion Top portion Conventional Process 392 722 397 Process of the Invention 32 9 0 As understood from the results, the number of produced bubbles was reduced extremely in the process according to the present invention. In addition, the distribution of Ca in cross section was uniform.

Claims: A withdrawal control process of horizontal continuous casting, comprising a cast matter pulling cycle composed of a pull step, a pause step and a push back step, characterized in that said pull step is controlled along a pulling velocity trace downwardly curved with respect to a linear velocity trace having a constant velocity gradient from a starting speed to a maximum speed in a pull acceleration stage at the beginning of the pull step and that the value of acceleration of a cast matter pulling velocity at the beginning part of the pull acceleration stage in the pull step is not larger than 0.6m/sec 2 . A withdrawal control process of horizontal continuous casting according to Claim 1, characterized in that the mean value of acceleration of said cast matter pulling velocity along said downwardly curved pulling velocity trace in the acceleration stage is not larger than 0.6m/sec 2 . A withdrawal control process of horizontal continuous casting according to Claim 1, characterized in that said pull acceleration stage from a starting speed to a maximum speed of the pulling velocity includes: a first acceleration stage with a small value of acceleration not larger than 0.4m/sec 2 , a second acceleration stage in which the acceleration is gradually increased to a value not larger than 0.6m/sec 2 succeeding to said first acceleration stage; and a third acceleration stage in which the acceleration is gradually increased to a value not smaller than 0.6m/sec 2 succeeding to said second acceleration stage.

Inventors: NAKAJIMA AKIHIRO, SAIKI TATUO, TSURU MASAHIRO, NAKAJIMA, AKIHIRO, SAIKI, TATUO, TSURU, MASAHIRO, NAKAJIMA, AKIHIRO NKK CORPORATION, SAIKI, TATUO NKK CORPORATION, TSURU, MASAHIRO NKK CORPORATION
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1991
Main classification: B22D11
IPC codes: B22D11