JP2009095879A - Apparatus for surface melting treatment and starting method for surface melting treatment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve reduction in size and cost of an apparatus for surface melting treatment in performing melting treatment of the surface of a cast slab using plasma. <P>SOLUTION: The apparatus 1 for surface melting treatment has a plasma torch 7 at the upper part thereof. Right below the plasma torch 7, a plasma arc electrode 11 liftable ahead of the conveyance direction D of the cast slab H is provided. After forming plasma arc P between the plasma torch 7 and the arc electrode 11 above the cast slab H, the plasma arc electrode 11 is lowered. When the plasma arc electrode 11 is lowered at the position with the same height as or lower than the upper surface of the cast slab H, the plasma arc P is shifted to the cast slab H from the plasma electrode 11. After the shift of the plasma arc P, The plasma arc electrode 11 is further lowered to the position not hindering the conveyance of the cast slab H, i. e. below the lower surface of the cast slab H. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a surface layer melting treatment apparatus and a surface layer melting treatment starting method for heating or melting a surface layer of a continuous cast slab of steel, for example, or a steel slab during rolling with plasma.

  For example, as a treatment method for modifying the surface layer of a cast steel piece such as a cast piece after continuous casting or a steel piece in the middle of rolling, for example, the surface layer of the cast steel piece to be conveyed is heated and melted by the heat of a plasma arc, and this melted A method of modifying the surface layer by adding another solute component to the surface layer portion has been proposed (Patent Document 1).

  When the surface layer of the cast steel piece is melted in this way, it is necessary to uniformly heat and melt the cast steel piece by a plasma arc, and to melt the surface layer of the cast steel piece as uniformly as possible. Therefore, a method has been proposed in which surface treatment is performed by reciprocating a plasma arc in a direction perpendicular to the direction of conveyance of the cast steel piece using electromagnetic force of an alternating magnetic field to melt the surface layer of the cast steel piece (Patent Document 2). ). In the present application, “cast steel slab” is a general term for “slab slab” and “steel slab”, “slab slab” is a steel material after casting, and “steel slab” is a slab rolled. It means later steel.

JP 2004-195512 A JP 54-142152 A

  In the surface layer melting treatment of such a cast steel piece, it is necessary to ignite the direct current plasma when starting the melting treatment with the direct current plasma for the cut cast steel piece. As an apparatus for igniting the DC plasma, for example, a slab surface melting treatment apparatus 100 shown in FIG. 6 has been conventionally used. The surface layer melting apparatus 100 has a rectangular parallelepiped chamber 101, for example. On the side surface of the chamber 101, a carry-in port 102 for carrying the slab H into the chamber 101 and a carry-out port 103 for carrying out the slab H are formed. A plasma torch 104 is formed through the upper surface of the chamber 101 so as to form a plasma arc P by DC plasma with the slab H. A plurality of plasma torches 104 are arranged in parallel in a direction perpendicular to the conveying direction D of the slab H, for example, and the plurality of plasma torches 104 can be moved up and down by an elevating device 105 provided outside the upper surface of the chamber 101. Yes. Inside the chamber 101 and above the slab H, for example, loop-shaped electromagnetic coils 106 are provided on both the front and rear sides in the conveyance direction D of the slab H with the plasma torch 104 interposed therebetween. A DC power source 107 is provided outside the chamber 101. The DC power source 107 is connected to a ground 108 that contacts the plasma torch 104 and the slab H, and a DC current can flow between the electrode (cathode) and the slab H (anode) in the plasma torch 104.

  First, as shown in FIG. 6A, the plasma torch 104 is lowered from the slab H to a distance of about 10 mm, for example. Next, a high frequency voltage of, for example, several tens of kHz is applied between an electrode (cathode) in the plasma torch 104 and a nozzle that is an outer cylinder of the plasma torch 104, and is discharged to generate DC plasma. Then, a direct current is supplied between the electrode (cathode) of the plasma torch and the plasma arc electrode (anode) by the DC power source 107, and, for example, argon gas or the like is provided between the electrode (cathode) and the nozzle in the plasma torch 104. Flow an inert gas. As a result, a plasma arc P by DC plasma is formed between the plasma torch 104 and the slab H. Then, as shown in FIG. 6B, the plasma torch 104 is raised and an alternating magnetic field is generated by the electromagnetic coil 106 to form a flat plasma arc P. And the melting process of the surface layer of slab H starts.

  However, when the surface layer melting apparatus 100 is used, the plasma torch 104 arranged in parallel needs to be lifted and lowered mechanically by the lifting apparatus 105 together with the current cable, and the surface layer melting apparatus 100 has been enlarged. . Further, since the inside of the chamber 101 is normally maintained in an inert gas atmosphere in order to suppress oxidation, a gas sealing property is maintained between the chamber 101 and the plasma torch 104 when the plasma torch 104 moves up and down. There is a need. The gas seal in the gap between the chamber 101 and the plasma torch 104 is durable even in a harsh temperature such as a high temperature radiation atmosphere by the plasma torch 104, and is durable against raising and lowering of the plasma torch 104. It must also have sex. Since such durability is required for the gas seal, the surface layer melting apparatus 100 is an expensive apparatus.

  The present invention has been made in view of the above points, and an object of the present invention is to reduce the size and cost of a surface layer melting apparatus for performing a melting process in the surface layer melting process of a cast steel piece using plasma.

  In order to achieve the above object, a surface layer melting apparatus having a plasma torch for generating a plasma arc between a cast steel piece being conveyed and forming a plasma arc between the plasma torch and the cast steel piece The plasma arc electrode located below the plasma torch and in front of the cast steel slab, and a lifting device for raising and lowering the plasma arc electrode. The electrode can be raised to a position where a plasma arc can be formed with the plasma torch by the lifting device, and can be lowered to a position lower than the upper surface of the cast steel piece. It is characterized in that it can be retracted to a position that does not hinder the conveyance of the cast steel piece.

  According to the present invention, a plasma arc electrode that can be raised and lowered is provided below the plasma torch, and the plasma arc electrode can be raised to a position where a plasma arc can be formed with the plasma torch. At the start of processing, the plasma arc electrode can be raised to form a plasma arc between the plasma arc electrode and the plasma torch. Since the plasma arc electrode can be lowered to a position lower than the upper surface of the cast steel piece, the plasma arc electrode in a state where the plasma arc is formed is lowered, and the distance between the plasma torch and the plasma arc electrode is set to the plasma torch and the cast steel piece. Thus, the plasma arc can be smoothly transferred to the cast steel slab at a position longer than the distance between the plasma arc electrode, that is, at a position where the upper surface of the plasma arc electrode is the same height or lower than the upper surface of the cast steel slab. Accordingly, since a plasma arc can be formed between the plasma torch and the cast steel piece without raising and lowering the plasma torch as in the prior art, a large lifting device for raising and lowering the plasma torch becomes unnecessary. Therefore, the surface layer melting apparatus can be reduced in size. Further, since it is not necessary to raise and lower the plasma torch as in the prior art, an expensive gas seal provided between the plasma torch and the chamber becomes unnecessary, and the surface layer melting apparatus can be made inexpensive. Therefore, according to the present invention, it is possible to reduce the size and cost of the surface layer melting apparatus. Furthermore, since the plasma arc electrode of the present invention is located in the forward direction of the cast steel slab and can be retracted to a position that does not hinder the transport of the cast steel slab, the operating efficiency of the surface layer melting treatment of the cast steel slab is not deteriorated. .

  The plasma arc electrode may be lowered and retracted to a position lower than the lower surface of the cast steel piece.

  The plasma arc electrode may be retracted by falling forward in the conveying direction of the cast steel piece.

  You may have a control part which stops or starts the application of the voltage to the said plasma arc electrode in the position where the upper surface of the said plasma arc electrode becomes the same height as the upper surface of a cast steel piece.

  According to another aspect of the present invention, there is provided a method for generating a plasma arc between a cast steel piece to be transported and a plasma torch to melt the surface layer of the cast steel piece, and the plasma arc between the cast steel piece and the plasma torch. Is generated, a plasma arc is formed between the plasma arc electrode located above the cast steel piece and the plasma torch, and in this state, the plasma arc electrode is moved forward of the cast steel piece in the conveying direction of the cast steel piece. Lower at least to the upper surface position, transfer the plasma arc generated between the plasma arc electrode and the plasma torch to the cast steel piece side, and then retract the plasma arc electrode to a position that does not hinder the conveyance of the cast steel piece. It is characterized by.

  The application of voltage to the plasma arc electrode may be stopped when the plasma arc electrode is lowered to at least the upper surface position of the cast steel piece in front of the cast steel piece in the conveying direction.

  Application of a voltage to the plasma arc electrode may be started when the plasma arc electrode is raised to at least the upper surface position of the cast steel piece in front of the cast steel piece in the conveying direction.

  According to the present invention, a conventionally provided mechanism for raising and lowering a plasma torch and a gas seal are not required, and the surface layer melting apparatus can be reduced in size and cost.

  Hereinafter, preferred embodiments of the present invention will be described. FIG.1 and FIG.2 is a longitudinal cross-sectional view which shows the outline of a structure of the surface layer fusion processing apparatus 1 of the slab H concerning this Embodiment. The slab H is, for example, a continuous cast slab having a thickness of 250 mm and a width of 1200 mm, and is carbon steel containing 0.2% by mass of carbon.

  The surface layer melting apparatus 1 is provided on a conveying line for the slab H that is conveyed in the horizontal direction. As shown in FIGS. 1 and 2, the surface layer melting apparatus 1 has a rectangular parallelepiped chamber 2, for example. A carry-in port 3 for carrying in the slab H is formed on the side surface of the chamber 2, and an open / close shutter 4 is provided in the carry-in port 3. On the side facing the carry-in port 3 of the chamber 2, a carry-out port 5 for carrying out the slab H is formed, and an open / close shutter 6 is provided at the carry-out port 5. During the melting process of the surface layer of the steel slab H, the atmosphere in the chamber 2 is maintained, for example, in an atmosphere of argon gas or a mixed gas of hydrogen and argon gas.

  A plasma torch 7 that passes through the upper surface of the chamber 2 and is fixed to the chamber 2 is provided on the upper surface of the chamber 2. The plasma torch 7 has a nozzle 7a which is an outer cylinder, and an electrode 7b is provided inside the nozzle 7a. A plurality of, for example, twelve plasma torches 7 are arranged in parallel in the direction perpendicular to the transport direction D, for example, at intervals of 100 mm. The plasma torch 7 can generate DC plasma by applying a voltage. By applying a voltage from a DC power supply 8 and supplying a DC current, the plasma torch 7 can generate a DC current between a plasma arc electrode 11 or a slab H to be described later. A plasma arc P by plasma can be formed. In order to generate the plasma arc P, for example, argon gas or a mixed gas of hydrogen and argon gas flows as plasma gas in the plasma torch 7.

  Below the plasma torch 7, a processing start mechanism 10 for forming a plasma arc P between the plasma torch 7 and the slab H is provided. The processing start mechanism 10 includes a plasma arc electrode 11 that extends in a direction perpendicular to the conveyance direction D of the slab H and that can freely move up and down in the chamber 2. The plasma arc electrode 11 is located directly below the plasma torch 7 and in front of the slab H in the transport direction D. Both ends of the plasma arc electrode 11 are supported by support members 12. An elevating device 13 that elevates and lowers the plasma arc electrode 11 is provided at the lower end of the support member 12. The lifting device 13 is provided outside the lower surface of the chamber 2. A casing 14 penetrating the lower surface of the chamber 2 is provided at the upper part of the lifting device 13, and the support member 12 can move up and down in the casing 14. A sealing material is provided in the gap between the casing 14 and the chamber 2. By the processing start mechanism 10 configured in this manner, the plasma arc electrode 11 can be raised to a position where the plasma arc P can be formed with the plasma torch 7 and does not hinder the conveyance of the slab H, that is, It can be lowered to a position lower than the lower surface of the slab H. Between the plasma arc electrode 11 and the DC power source 8, a switch 15 that controls supply of DC current to the plasma arc electrode 11 is provided. The plasma arc electrode 11 becomes an anode of DC plasma by supplying a DC current by applying a voltage from the DC power source 8 with the switch 15 turned on at a position above the slab H, and the plasma torch 7 A plasma arc P can be formed between the two. The plasma arc electrode 11 may be a tungsten rod having a length of 1200 mm and a diameter of 6 mm, for example. The tungsten rod preferably contains tria (thorium dioxide).

  Between the plasma torch 7 and the slab H, electromagnetic coils 21 and 22 for generating an alternating magnetic field with respect to the plasma arc P are provided in parallel and facing each other across the plasma arc P (plasma arc electrode 11). It has been. The electromagnetic coils 21 and 22 have, for example, a loop shape and extend in a direction perpendicular to the conveyance direction D of the slab H. The horizontal distance between the electromagnetic coils 21 and 22 is a distance that allows the plasma arc electrode 11 to pass through, for example, 80 mm. And the electromagnetic coils 21 and 22 generate | occur | produce an alternating current magnetic field between the electromagnetic coils 21 and 22 by supply of the alternating current from alternating current power supply (not shown), and make electromagnetic force act on the plasma arc P periodically. . Then, the plasma arc P can be reciprocated in a direction perpendicular to the conveying direction D of the slab H according to the supplied AC frequency.

  The bottom surface of the slab H is in contact with the ground 23 connected to the DC power source 8. By applying a voltage from the DC power supply 8 to the ground 23 and supplying a DC current, the slab H can be made an anode of DC plasma.

  The direct current supply from the direct current power source 8 to the plasma torch 7, the direct current supply to the plasma arc electrode 11 (control of the switch 15), and the direct current supply to the ground 23 are provided outside the chamber 2. Controlled by the control unit 30.

  The surface layer melting apparatus 1 according to the present embodiment is configured as described above. Next, the melting process of the slab H performed in the surface layer melting apparatus 1 will be described. In the present embodiment, the melting process is performed on the surface layer whose depth from the upper surface of the slab H is, for example, up to 5 mm.

  First, the opening / closing shutter 4 of the carry-in port 3 is opened, and the slab H is carried into the chamber 2. The carried slab H is stopped behind the plasma arc electrode 11 in the conveying direction D of the slab H. The stop position of the slab H is a position where the horizontal distance in the transport direction D of the gap between the plasma arc electrode 11 and the slab H is equal to or less than the diameter of the plasma arc P, for example, 20 mm. At this time, the plasma arc electrode 11 is disposed at a position 10 mm below the lower end of the plasma torch 7 by the lifting device 13.

  Next, a high frequency voltage of, for example, 20 kHz and 300 V is applied between the nozzle 7a of the plasma torch 7 and the electrode 7b (cathode). Due to this high frequency voltage, DC plasma is generated in the plasma torch 7. Then, with the switch 15 turned on, for example, a DC power source of 100 A is passed between the electrode 7 b (cathode) of the plasma torch 7 and the plasma arc electrode 11 (anode) by the DC power source 8, and the nozzle 7 a of the plasma torch 7 Argon gas, for example, is caused to flow between the electrode 7b (cathode). Then, as shown in FIGS. 1 and 2, a plasma arc P by DC plasma is formed between the plasma torch 7 and the plasma arc electrode 11.

  When the plasma arc P by DC plasma is formed between the plasma torch 7 and the plasma arc electrode 11, as shown in FIG. 3, the plasma arc electrode 11 is lowered by the lifting device 13. At this time, the DC power source 8 is connected to the mold H through the ground 23, and the slab H is made an anode of DC plasma. A position where the distance between the plasma torch 7 and the plasma arc electrode 11 is longer than the distance between the plasma torch 7 and the slab H, that is, a position where the upper surface of the plasma arc electrode 11 is the same height or lower than the upper surface of the slab H. When the plasma arc electrode 11 is lowered, the horizontal distance between the plasma arc electrode 11 and the slab H is equal to or less than the diameter of the plasma arc P, so that the plasma arc P smoothly transitions from the plasma arc electrode 11 to the slab H. When the plasma arc P shifts to the slab H, the control unit 30 turns off the switch 15 at a predetermined timing and stops the supply of the direct current to the plasma arc electrode 11.

  Thereafter, the plasma arc electrode 11 is further lowered and lowered to a position lower than the slab H as shown in FIG. Then, for example, the slab H is transported at a transport speed of 5 mm / second, and the melting treatment of the surface layer of the slab H is started. At this time, the controller 30 raises the direct current between the plasma torch 7 and the slab H to, for example, 500 A, and supplies alternating current to the electromagnetic coils 21 and 22 from an alternating current power source (not shown). The plasma arc P is reciprocated in a direction perpendicular to the conveying direction D of the slab H by the electromagnetic force generated by the current. The surface layer of the slab H is melted by the heat of the plasma arc P, and the surface layer of the slab H is modified by adding other solute components to the melted portion.

  According to the above embodiment, when starting the melting process of the surface layer of the slab H, the DC plasma is generated by the plasma torch 7 and then the plasma arc electrode 11 is lifted by the DC power source 8. Since a direct current flows between the electrode (cathode) and the plasma arc electrode (anode) of the plasma torch 7, the plasma arc P can be formed between the plasma torch 7 and the plasma arc electrode 11. The plasma arc electrode 11 is lowered to a position where the upper surface of the plasma arc electrode 11 is the same height or lower than the upper surface of the slab H, so that the plasma arc P can be smoothly transferred to the slab H. . Accordingly, since a plasma arc can be formed between the plasma torch and the cast steel piece without raising and lowering the plasma torch as in the prior art, a large lifting device for raising and lowering the plasma torch becomes unnecessary. Therefore, the surface layer melting apparatus 1 can be reduced in size. In addition, since the plasma torch 7 can be fixed to the chamber 2 without raising or lowering the plasma torch as in the prior art, an expensive gas seal provided between the conventional plasma torch and the chamber becomes unnecessary. The surface layer melting apparatus 1 can be made inexpensive while ensuring high sealing performance. Furthermore, since it is not necessary to replace the conventional gas seal, the maintenance cost of the surface layer melting apparatus 1 can be reduced. Therefore, according to the present invention, it is possible to reduce the size and cost of the surface layer melting apparatus 1.

  Further, since it is not necessary to raise and lower the plasma torch as in the conventional case, it is not necessary to secure a distance corresponding to the diameter of the plasma torch between the electromagnetic coils. This shortens the horizontal distance between the electromagnetic coils 21 and 22 (about 80 mm in this embodiment), so that the controllability of the alternating magnetic field generated between the electromagnetic coils 21 and 22 can be improved, and the electromagnetic The AC current supplied to the coils 21 and 22 can be saved, and the AC power supply can be miniaturized. Therefore, the surface layer melting apparatus 1 can be downsized.

  Furthermore, since the plasma arc electrode 11 of the present invention is located in front of the slab H in the conveying direction D and descends to a position lower than the lower surface of the slab H, the casting of the surface layer of the slab H is started even after the start of the melting process. The conveyance of the piece H is not hindered, and the operation efficiency of the surface layer melting process of the slab H is not deteriorated.

  In the above embodiment, the lifting device 13 of the processing start mechanism 10 is provided outside the chamber 2, and the plasma arc electrode 11 can be lowered to the lower side of the slab H. However, as shown in FIG. The elevating device 13 may be provided inside the chamber 2, and the processing start mechanism 10 may be configured to fall forward in the conveyance direction D of the slab H around the lower portion of the elevating device 13. In such a case, first, as in the above-described embodiment, after the plasma arc P is formed between the plasma torch 7 and the plasma arc electrode 11, the plasma arc electrode 11 is as high or low as the upper surface of the slab H. The plasma arc P is moved from the plasma arc electrode 11 to the slab H. Then, the processing start mechanism 10 is tilted around the lower end of the lifting device 13 until it becomes horizontal, for example. Thereby, even after the melting process of the surface layer of the slab H is started, the conveyance of the slab H is not hindered, and the operation efficiency of the surface layer melting process of the slab H is not deteriorated.

  In the control unit 30 in the above embodiment, the supply of direct current from the direct current power source 8 to the plasma arc electrode 11 may be automatically controlled. For example, the vertical position of the plasma arc electrode 11 is measured by the lifting device 13, and the measurement result is output from the lifting device 13 to the control unit 30. Then, the control unit 30 stops the supply of direct current to the plasma arc electrode 11 when the plasma arc electrode 11 is lowered to a position where the upper surface of the plasma arc electrode 11 is the same height as the upper surface of the slab H. Switch off the switch 15. When the supply of the direct current to the plasma arc electrode 11 is thus stopped, the plasma arc electrode 11 is no longer the anode of the direct current plasma. Then, the plasma arc P smoothly transitions from the plasma arc electrode 11 to the slab H that is an anode of DC plasma. Further, when starting the melting treatment of the surface layer of the next slab H, the plasma arc electrode 11 rises again, and the control is performed at a position where the upper surface of the plasma arc electrode 11 and the upper surface of the slab H become the same height. The switch 15 is turned on by the unit 30 to start supplying a direct current from the direct current power source 8 to the plasma arc electrode 11. A plasma arc P is formed between the plasma torch 7 and the plasma arc electrode 11. In this way, the direct current supply from the direct current power source 8 is automatically controlled by the control unit 30 based on the position of the plasma arc electrode 11, so that the waste of the direct current supply is eliminated and energy resources are saved.

  INDUSTRIAL APPLICABILITY The present invention is useful, for example, for a surface layer melting apparatus and a method for starting a surface layer melting process for heating or melting a surface layer of a continuous cast slab of steel or a steel slab during rolling with plasma.

It is a longitudinal cross-sectional view which shows the outline of a structure of the surface layer fusion processing apparatus concerning this Embodiment. It is a longitudinal cross-sectional view which shows the outline of a structure of the surface layer fusion processing apparatus concerning this Embodiment. It is explanatory drawing which shows a mode that the plasma arc moved to the slab from the plasma arc electrode. It is explanatory drawing which shows a mode that the plasma arc electrode fell to the downward direction of a slab. It is a longitudinal cross-sectional view which shows the outline of a structure of the surface layer melt processing apparatus concerning another form. It is a longitudinal cross-sectional view which shows the outline of a structure of the conventional surface layer melting processing apparatus, (a) has shown the state which lowered | hung the plasma torch, (b) has shown the state which raised the plasma torch.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Surface melting processing apparatus 2 Chamber 3 Carrying-in port 4,6 Opening / closing shutter 5 Carrying-out exit 7 Plasma torch 7a Nozzle 7b Electrode 8 DC power supply 10 Process start mechanism 11 Plasma arc electrode 12 Support member 13 Lifting device 14 Casing 15 Switch 21, 22 Electromagnetic Coil 23 Ground 30 Controller D Transport direction H Cast slab P Plasma arc

Claims (7)

A surface layer melting apparatus equipped with a plasma torch that generates a plasma arc between the cast steel pieces being conveyed,
Between the plasma torch and the cast steel slab, having a processing start mechanism for forming a plasma arc,
The processing start mechanism comprises a plasma arc electrode located below the plasma torch and in front of the cast steel slab in the conveying direction, and an elevating device for raising and lowering the plasma arc electrode,
The plasma arc electrode can be raised to a position where a plasma arc can be formed with the plasma torch by the lifting device, and can be lowered to a position lower than the upper surface of the cast steel slab,
The surface layer melting apparatus according to claim 1, wherein the plasma arc electrode is retractable to a position that does not hinder the conveyance of the cast steel piece. 2. The surface layer melting apparatus according to claim 1, wherein the plasma arc electrode is lowered and retracted to a position lower than a lower surface of the cast steel piece. 2. The surface layer melting apparatus according to claim 1, wherein the plasma arc electrode is retracted by falling forward in the conveying direction of the cast steel piece. The control part which stops or starts the application of the voltage to the said plasma arc electrode in the position where the upper surface of the said plasma arc electrode becomes the same height as the upper surface of a cast steel piece, The any one of Claims 1-3 characterized by the above-mentioned. The surface layer melting apparatus according to claim 1. In the method of melting the surface layer of the cast steel piece by generating a plasma arc between the cast steel piece being conveyed and the plasma torch,
In generating a plasma arc between the cast steel piece and the plasma torch,
Forming a plasma arc between the plasma arc electrode located above the cast steel piece and the plasma torch;
After that, in this state, the plasma arc electrode is lowered to at least the upper surface position of the cast steel piece in front of the cast steel piece in the conveying direction, and the plasma arc generated between the plasma arc electrode and the plasma torch is transferred to the cast steel piece side. And
Then, the plasma arc electrode is retracted to a position that does not hinder the conveyance of the cast steel piece. The voltage application to the plasma arc electrode is stopped when the plasma arc electrode is lowered to at least the upper surface position of the cast steel piece in front of the cast steel piece in the conveying direction. Method for starting surface layer melting treatment. The voltage application to the plasma arc electrode is started when the plasma arc electrode is raised to at least the upper surface position of the cast steel piece in the transport direction front of the cast steel piece. The surface layer melting treatment start method described.

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