Patent Publication Number: US-9427796-B2

Title: Method for continuously casting ingot made of titanium or titanium alloy

Description:
TECHNICAL FIELD 
     The invention relates to a method for continuously casting an ingot made of titanium or a titanium alloy, in which an ingot made of titanium or a titanium alloy is continuously cast. 
     BACKGROUND ART 
     Continuous casting of an ingot has been conventionally performed by pouring metal melted by vacuum arc melting and electron beam melting into a bottomless mold and drawing the molten metal downward while being solidified. 
     Patent Document 1 discloses an automatic control method for plasma melting casting, in which titanium or a titanium alloy is melted by plasma arc melting in an inert gas atmosphere and poured into a mold for solidification. Performing the plasma arc melting in an inert gas atmosphere, unlike the electron beam melting in vacuum, allows casting of not only pure titanium, but also a titanium alloy. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Patent No. 3077387 
     SUMMARY OF THE INVENTION 
     Technical Problems 
     However, if an ingot has irregularities and flaws on casting surface after casting, it is necessary to perform a pretreatment, such as cutting the surface, before rolling, thus causing a reduction in material utilization and an increase in number of operation processes. Therefore, it is demanded to cast an ingot without irregularities or flaws on casting surface. 
     In this method, when an ingot having a large size is continuously cast by the plasma arc melting, a plasma torch is configured to horizontally move on a predetermined course to heat the entire surface of molten metal. Further, by adjusting an output and a moving location, velocity, and ingot heat extraction of the plasma torch on the surface of the molten metal, it is intended to improve the quality of casting surface over the whole ingot. 
     However, it sometimes occurs that, by abrupt changes of operational conditions, such as a change in temperature fluctuation of the molten metal poured into a mold and a change in a contacting state between the molten metal and the mold, the balance of heat input and output is locally altered, thus the quality of casting surface is deteriorated. 
     Further, when temperature conditions are largely changed, the delay of the detection of such changes would cause operation troubles. For example, when the temperature is too low, it becomes difficult to draw an ingot because of its solidification, and when the temperature is too high, a solidified shell is broken, thereby causing the leakage of the molten metal. 
     The problem has been conventionally dealt by operators, who monitor the inner state of the mold and perform operations, such as manually changing a moving pattern of the plasma torch. However, there may be cases where detecting and measuring are delayed or overlooking occurs. 
     An object of the present invention is to provide a method for continuously casting an ingot made of titanium or a titanium alloy, capable of casing an ingot having an excellent casting-surface state. 
     Solution to Problems 
     The method for continuously casting an ingot made of titanium or a titanium alloy of the present invention is a method for continuously casting an ingot made of titanium or a titanium alloy by pouring molten metal prepared by melting titanium or a titanium alloy into a bottomless mold and drawing the molten metal downward while being solidified, the method being characterized in comprising: a heating step, where, while a plasma torch is horizontally moved on the surface of the molten metal in the mold, the surface of the molten metal is heated by plasma arcs generated by the plasma torch; a temperature-measuring step for measuring the temperature of the mold by each of temperature sensors provided in a plurality of positions of the mold along the circumferential direction of the mold; and a heat input quantity control step for controlling heat input quantity per unit area applied from the plasma torch to the surface of the molten metal based on the temperature of the mold measured by the temperature sensors and a target temperature preset in each of the temperature sensors. 
     In the above configuration, based on the temperature of the mold measured by the temperature sensors and the target temperature preset in each of the temperature sensors, the heat input quantity per unit area applied from the plasma torch to the surface of the molten metal is controlled. For example, the heat input quantity per unit area applied from the plasma torch to the surface of the molten metal is increased or decreased in such a manner that the temperature measured by the temperature sensors becomes the target temperature. By changing in real time the heat input quantity per unit area applied from the plasma torch to the surface of the molten metal based on the temperature measured by the temperature sensors and the target temperature, heat input/output conditions near the molten metal surface region can be appropriately controlled. Thus, it becomes possible to cast an ingot having an excellent casting-surface state. 
     Further, in the method for continuously casting an ingot made of titanium or a titanium alloy of the present invention, in the heat input quantity control step, if the temperature of the mold measured by any of the temperature sensors is lower than the target temperature, then output of the plasma torch may be increased when the plasma torch comes close to a location where such temperature sensor is installed, and if the temperature of the mold measured by any of the temperature sensors is higher than the target temperature, then the output of the plasma torch may be decreased when the plasma torch comes close to a location where such temperature sensor is installed. In the above configuration, by changing the output of the plasma torch in real time based on the temperature measured by the temperature sensors and the target temperature, the heat input/output conditions near the molten metal surface region can be appropriately controlled. 
     Further, in the method for continuously casting an ingot made of titanium or a titanium alloy of the present invention, the method may further comprise a calculation step for calculating a plasma torch output correction quantity based on the difference between the mold temperature measured by the temperature sensors and the target temperature, and then in the heat input quantity control step, correct the output of the plasma torch by adding the plasma torch output correction quantity to a standard plasma torch output pattern, which is a standard output pattern for the plasma torch. In the above configuration, the output of the plasma torch can be changed in real time based on the temperature measured by the temperature sensors and the target temperature. 
     Effect of the Invention 
     In the method for continuously casting an ingot made of titanium or a titanium alloy of the present invention, by changing in real time the heat input quantity per unit area applied from the plasma torch to the surface of the molten metal based on the temperature measured by the temperature sensors and the target temperature, the heat input/output conditions near the molten metal surface region can be appropriately controlled. Thus, it becomes possible to cast an ingot having an excellent casting-surface state. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a continuous casting apparatus. 
         FIG. 2  is a cross-section view of the continuous casting apparatus. 
         FIG. 3A  is a drawing describing a causing mechanism of surface defects. 
         FIG. 3B  is a drawing describing the causing mechanism of the surface defects. 
         FIG. 4  is a model diagram of a mold, seen from side. 
         FIG. 5  is a model diagram of the mold, seen from above. 
         FIG. 6A  is a graph showing measured temperatures and target temperatures to explain a calculation method for a plasma torch output after correction. 
         FIG. 6B  is a graph showing a standard plasma torch output pattern to explain the calculation method for the plasma torch output after correction. 
         FIG. 6C  is a graph showing a plasma torch output correction quantity to explain the calculation method for the plasma torch output after correction. 
         FIG. 6D  is a graph showing a plasma torch output to explain the calculation method for the plasma torch output after correction. 
         FIG. 7A  is a graph showing a plasma torch output correction value to explain a calculation method for a plasma torch output correction quantity. 
         FIG. 7B  is a graph showing a correction coefficient to explain the calculation method for the plasma torch output correction quantity. 
         FIG. 7C  is a graph showing a plasma torch output correction quantity to explain the calculation method for the plasma torch output correction quantity. 
         FIG. 8  is a perspective view of a continuous casting apparatus different from the one shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
     Configuration of Continuous Casting Apparatus 
     In the method for continuously casting an ingot made of titanium or a titanium alloy of the present embodiments, by pouring molten metal of titanium or a titanium alloy melted by plasma arc melting into a bottomless mold and drawing the molten metal downward while being solidified, an ingot made of the titanium or the titanium alloy is continuously cast. A continuous casting apparatus  1  carrying out the method for continuously casting an ingot made of titanium or a titanium alloy, as shown in  FIG. 1  depicting a perspective view thereof and  FIG. 2  depicting a cross-section view thereof, includes a mold  2 , a cold hearth  3 , a raw material charging apparatus  4 , plasma torches  5 , a starting block  6 , and a plasma torch  7 . The continuous casting apparatus  1  is surrounded by an inert gas atmosphere comprising argon gas, helium gas, and the like. 
     The raw material charging device  4  supplies raw materials of titanium or a titanium alloy, such as sponge titanium, scrap and the like, into the cold hearth  3 . The plasma torches  5  are disposed above the cold hearth  3  and used to melt the raw materials within the cold hearth  3  by generating plasma arcs. The cold hearth  3  pours molten metal  12  having the raw materials melted into the mold  2  through a pouring portion  3   a . The mold  2  is made of copper and formed in a bottomless shape having a rectangular cross section. At least a part of a square cylindrical wall portion of the mold  2  is configured so as to circulate water through the wall portion, thereby cooling the mold  2 . The starting block  6  is movable in the up and down direction by a drive portion not illustrated, and able to close a lower side opening of the mold  2 . The plasma torch  7  is disposed above the molten metal  12  within the mold  2  and configured to horizontally move above the surface of the molten metal  12  by a moving means not illustrated, thereby heating the surface of the molten metal  12  poured into the mold  2  by the plasma arcs. 
     In the above configuration, solidification of the molten metal  12  poured into the mold  2  begins from a contact surface between the molten metal  12  and the mold  2  having a water-cooling system. Then, as the starting block  6  closing the lower side opening of the mold  2  is lowered at a predetermined speed, an ingot (slab)  11  in a square cylindrical shape formed by solidifying the molten metal  12  is continuously cast while being drawn downward from the mold  2 . 
     In this configuration, it is difficult to cast a titanium alloy using the electron beam melting in a vacuum atmosphere since trace components in the titanium alloy would evaporate. In contrast, it is possible to cast not only pure titanium, but also the titanium alloy using the plasma arc melting in an inert gas atmosphere. 
     Further, the continuous casting apparatus  1  may include a flux loading device for applying flux in a solid phase or a liquid phase onto the surface of the molten metal  12  in the mold  2 . In this configuration, it is difficult to apply the flux to the molten metal  12  in the mold  2  using the electron beam melting in a vacuum atmosphere since the flux would be scattered. In contrast, the plasma arc melting in an inert gas atmosphere has an advantage that the flux can be applied to the molten metal  12  in the mold  2 . 
     Operational Conditions 
     When an ingot  11  made of titanium or a titanium alloy is produced by continuous casting, if there are irregularities or flaws on the surface (casting surface) of the ingot  11 , they would cause surface detects in a rolling process, which is the next step. Thus such irregularities and flaws on the surface of the ingot  11  must be removed before rolling by cutting or the like. However, this step would decrease the material utilization and increase the number of operation processes, thereby increasing the cost of continuous casting. As such, it is demanded to perform the casting of the ingot  11  without irregularities or flaws on its surface. 
     As shown in  FIGS. 3A and 3B , in continuous casting of the ingot  11  made of titanium, the surface of the ingot  11  (a solidified shell  13 ) contacts with the surface of the mold  2  only near a surface region of the molten metal  12  (a region extending from the molten metal surface to an approximately 10-20 mm depth), where the molten metal  12  is heated by the plasma arcs or the electron beam. In a region deeper than this contact region, the ingot  11  undergoes thermal shrinkage, thus an air gap  14  is generated between the ingot  11  and the mold  2 . Then, as shown in  FIG. 3A , if the heat input to an initial solidified portion  15  (a portion of the molten metal  12  initially brought into contact with the mold  2  to be solidified) is excessive, since the solidified shell  13  becomes too thin, there occurs a “tearing-off defect”, in which the surface of the solidified shell  13  is torn off due to lack of strength. On the other hand, as shown in  FIG. 3B , if the heat input into the initial solidified portion  15  is not sufficient, there occurs a “molten metal-covering defect”, in which the solidified shell  13  that has been grown (thickened) is covered with the molten metal  12 . Therefore, it is speculated that heat input/output conditions applied to the initial solidified portion  15  near the surface region of the molten metal  12  would have a great impact on properties of the casting surface, and it is considered that the ingot  11  having an excellent casting surface can be obtained by appropriately controlling the heat input/output conditions applied to the molten metal  12  near the molten metal surface region. 
     Hence, as shown in  FIG. 4  depicting a model diagram of the mold  2  seen from the side and  FIG. 5  depicting a model diagram of the mold  2  seen from the above, thermocouples (temperature sensors)  21  are provided in a plurality of positions of the mold  2  along the circumferential direction of the mold  2 . Then, based on a temperature of the mold  2  measured by each of the thermocouples  21  and a target temperature preset in each of the thermocouples  21 , it is configured to control heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12 . In the present embodiments, based on the temperature of the mold  2  measured by each of the thermocouples  21  and the target temperature preset in each of the thermocouples  21 , it is configured to control output of the plasma torch  7  horizontally moving on the surface of the molten metal  12 . Alternatively, the heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12  may be controlled without changing the output of the plasma torch  7 , for example, by changing the distance between the plasma torch  7  and the surface of the molten metal  12  or by changing a flow rate of a plasma gas. Further, a means for measuring the temperature of the mold  2  is not limited to the thermocouples  21 , and optical fiber and the like may be used. 
     Specifically, the temperature of the mold  2  measured by each of the thermocouples  21  is inputted to a control device  22 . In the control device  22 , target temperature values preset in each of the thermocouples  21  and plasma torch output correction quantity are inputted. The control device  22 , then, outputs a plasma torch output control signal based on the temperature of the mold  2  measured by each of the thermocouples  21  and the target temperature to the plasma torch  7 . In this manner, if the temperature of the mold  2  measured by any of the thermocouples  21  is lower than the target temperature, the control device  22  controls the output of the plasma torch  7  so as to increase the output of the plasma torch  7  when the plasma torch  7  comes close to a location where such thermocouple  21  is installed. Further, if the temperature of the mold  2  measured by any of the thermocouples  21  is higher than the target temperature, the control device  22  controls the output of the plasma torch  7  so as to decrease the output of the plasma torch  7  when the plasma torch  7  comes close to a location where such thermocouple  21  is installed. 
     As described above, by changing in real time the heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12  based on the temperature measured by the thermocouples  21  and the target temperature, the heat input/output conditions near the surface region of the molten metal  12  can be appropriately controlled. Thus, it becomes possible to cast an ingot  11  having an excellent casting-surface state. 
     Further, by changing the output of the plasma torch  7  in real time based on the temperature measured by the thermocouples  21  and the target temperature, the heat input/output conditions near the surface region of the molten metal  12  can be appropriately controlled. 
     In performing a control of the plasma torch  7 , a standard plasma torch output pattern PA(L)[W], which is a standard output pattern of the plasma torch  7 , capable of casting an ingot  11  having an excellent casting-surface state, is first determined in advance. Here, PA(L) represents an output value of the plasma torch  7  at a position L[m] on a moving route of the plasma torch  7 . Further, a target temperature Ta(i)[° C.] of the mold  2  at each position i for measuring the temperature is determined in advance by operation results in the past, simulations, and the like. Specifically, when the casting is performed using the standard plasma torch output pattern PA(L), a measured temperature where the quality of the ingot surface is known to be excellent or a temperature where the quality of the ingot surface is predicted to be excellent is used as the target temperature Ta(i). The target temperature Ta(i) may be a measured value or a calculated value by simulations. Further, a plasma torch output correction quantity ΔP(L, ΔT(i))[W] is determined in advance based on the difference ΔT(i) between a measured temperature Tm(i)[° C.] by the thermocouples  21  and the target temperature Ta(i) of the mold  2 . Here, ΔT(i) is given by
 
 ΔT ( i )= Tm ( i )− Ta ( i ).
 
     Then it is configured to measure the measured temperature Tm(i) of the mold  2  in real time during the casting. The plasma torch output P(L)[W] is then controlled according to the following formula 1.
 
 P ( L )= PA ( L )+Δ P ( L,Tm ( i )− Ta ( i ))  (Formula 1)
 
     Output adjustment described above is performed in every preset time interval. 
     More specifically, as shown in  FIG. 5 , torch positions A to D are designated at corner parts of a moving track  23  of the plasma torch  7 . Further, the thermocouples  21  are each provided on the center parts of the long sides of the mold  2  and on the center parts of the short sides of the mold  2 . The positions of the thermocouples  21  are hereinafter referred to as positions ( 1 ) to ( 4 ). 
       FIG. 6A  shows the measured temperatures Tm(i) by the thermocouples  21  located on each of the positions ( 1 ) to ( 4 ) and the target temperatures Ta(i). Further  FIG. 6B  shows the standard plasma torch output pattern PA(L) at the torch positions A to D. 
     In  FIG. 6A , the plasma torch output correction quantity ΔP(L, ΔT(i)) can be obtained based on the difference ΔT(i) between the measured temperature Tm(i) and the target temperature Ta(i).  FIG. 6C  shows the plasma torch output correction quantity ΔP(L, ΔT(i)) at the torch positions A to D. The plasma torch output P(L) after correction is then obtained by adding the plasma torch output correction quantity ΔP(L, ΔT(i)) to the standard plasma torch output pattern PA(L).  FIG. 6D  shows the plasma torch output P(L) after correction at the torch positions A to D. 
     As shown above, the output of the plasma torch  7  is corrected by adding the plasma torch output correction quantity ΔP(L, ΔT(i)) to the standard plasma torch output pattern PA(L). By this correction, the output of the plasma torch  7  can be changed in real time based on the measured temperature by the thermocouples  21  and the target temperature. 
     The plasma torch output correction quantity ΔP(L, ΔT(i)) can be obtained by the following formula 2.
 
 ΔP ( L,ΔT ( i ))=Σ( i= 1, N )(Δ Pu ( L,i )× fd ( Tm ( i )− Ta ( i )))  (Formula 2)
 
     In this formula, N represents a measurement number of the temperature, ΔPu(L, i)[W/° C.] represents a plasma torch output correction value when the measured temperature by the thermocouple  21  at the i-th position is deviated from its target temperature by unit temperature, and fd(ΔT)[° C./° C.] represents a correction coefficient based on a deviated amount from the measured temperature value. 
       FIG. 7A  shows the plasma torch output correction value ΔPu(L, i), and  FIG. 7B  shows the correction coefficient fd(ΔT). When the difference between the target temperature and the measured temperature becomes extremely large, operational troubles may occur due to abnormal solidification. Thus, when the difference between the target temperature and the measured temperature exceeds a predetermined threshold value, it may be configured to take actions such as outputting an alarm to an operator, reducing a drawing speed, and stopping the casting.  FIG. 7C  shows the plasma torch output correction quantity ΔP(L, ΔT(i)) calculated from the plasma torch output correction value ΔPu(L, i) and the correction coefficient fd(Tm(i)−Ta(i)). 
     Effects 
     As described hereinabove, in the method for continuously casting an ingot  11  made of titanium or a titanium alloy according to the present embodiments, based on the temperature of the mold  2  measured by the thermocouples  21  and the target temperature preset in each of the thermocouples  21 , the heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12  is controlled. For example, the heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12  is increased or decreased in such a manner that the temperature measured by the thermocouples  21  becomes the target temperature. By changing in real time the heat input quantity per unit area applied from the plasma torch  7  to the surface of the molten metal  12  based on the temperature measured by the thermocouples  21  and the target temperature, the heat input/output conditions near the surface region of the molten metal  12  can be appropriately controlled. Thus, it becomes possible to cast an ingot  11  having an excellent casting-surface state. 
     Further, if the temperature of the mold  2  measured by any of the thermocouples  21  is lower than the target temperature, then the output of the plasma torch  7  is increased when the plasma torch  7  comes close to a location where such thermocouple  21  is installed. Further, if the temperature of the mold  2  measured by any of the thermocouples  21  is higher than the target temperature, then the output of the plasma torch  7  is decreased when the plasma torch  7  comes close to a location where such thermocouple  21  is installed. In this manner, by changing the output of the plasma torch  7  in real time based on the temperature measured by the thermocouples  21 , the heat input/output conditions near the surface region of the molten metal  12  can be appropriately controlled. 
     Further, by adding the plasma torch output correction quantity to the standard plasma torch output pattern, the output of the plasma torch  7  is corrected. In this manner, the output of the plasma torch  7  can be changed in real time based on the temperature measured by the thermocouples  21 . 
     Modifications 
     It is noted that a continuous casting apparatus  201  carrying out the continuous casting method of the present embodiments, as shown in  FIG. 8 , may be configured so as to continuously cast an ingot  211  having a cylindrical shape using a mold  202  having a circular cross section. 
     Modifications of the Present Embodiments 
     The embodiments of the present invention are described hereinabove, however, it is obvious that the above embodiments solely serve as specific examples and are not to limit the present invention. The specific structures and the like of the present invention may be modified and designed according to the needs. Further, the actions and effects of the present invention described in the above embodiments are no more than most preferable actions and effects achieved by the present invention, thus the actions and effects of the present invention are not limited to those described in the above embodiments of the present invention. 
     The present application is based on Japanese Patent Application (Japanese Patent Application No. 2013-012034) filed on Jan. 25, 2013, the contents of which are incorporated herein by reference. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           1 ,  201  Continuous casting apparatus 
           2 ,  202  Mold 
           3  Cold hearth 
           3   a  Pouring portion 
           4  Raw material charging apparatus 
           5  Plasma torch 
           6  Starting block 
           7  Plasma torch 
           11 ,  211  Ingot 
           12  Molten metal 
           13  Solidified shell 
           14  Air gap 
           15  Initial solidified portion 
           21  Thermocouples 
           22  Control device 
           23  Moving track