Method of operating electric arc furnace

Abstract

The present invention relates to a method of operating an electric arc furnace containing (a) a furnace shell having a tapping hole and/or a slag door, (b) a furnace roof having a plurality of electrodes provided so as to face downwards, and (c) a rotating apparatus that rotates the furnace shell around a vertical axis relative to the electrodes, the method contains a rotating step of rotating the furnace shell relative to the electrodes during melting of a metal material, and a holding step of stopping the rotation when any one of the plurality of electrodes reaches a holding position that is previously set close to the tapping hole or the slag door, and holding the furnace shell at the holding position.

Claims

1. A method of operating an electric arc furnace, wherein the electric arc furnace comprises: (a) a furnace shell that includes a charging opening, a cylindrical circumferential wall portion and a furnace bottom portion, and a tapping hole and/or a slag door, (b) a furnace roof that has a plurality of electrodes provided so as to face downwards, and (c) a rotating apparatus that rotates the furnace shell around a vertical axis relative to the electrodes, in which a metal material charged into the furnace shell is melted by heat of arcs formed between the electrodes and the metal material, wherein the method comprises: charging the metal material into the furnace shell via the charging opening, and thereafter, rotating the furnace shell relative to the electrodes during melting of the metal material, and stopping the rotation when any one of the plurality of electrodes reaches a holding position that is previously set relative to the tapping hole or the slag door, and holding the furnace shell at the holding position, in the case where the holding position is set relative to the tapping hole, the holding position is set in such a position that an extension line of a line connecting a rotational center of the furnace shell to any one of the plurality of electrodes is positioned in a range within 50 in a rotation direction of the furnace shell, centered at a line connecting the rotational center of the furnace shell to a center of the tapping hole, and in the case where the holding position is set relative to the slag door, the holding position is set in such a position that the extension line of the line connecting the rotational center of the furnace shell to any one of the plurality of electrodes one electrode is positioned in a range of an angle of less than 60 from an end of the slag door in the rotation direction of the furnace shell.

2. The method of operating an electric arc furnace according to claim 1, wherein the electric arc furnace is an EBT furnace having the tapping hole, in which the furnace bottom portion has a protruding portion that partially protrudes outwards from an outer surface of the circumferential wall portion in a radial direction, and the tapping hole is configured by an opening that passes through the protruding portion in a vertical direction.

3. The method of operating an electric arc furnace according to claim 1, wherein the electric arc furnace is a spout tapping electric arc furnace having the tapping hole, in which the tapping hole is configured by an opening that passes through the circumferential wall portion in an inward and outward direction, and the spout tapping electric arc furnace has a spout extending outwards from the tapping hole.

4. The method of operating an electric arc furnace according to claim 1, wherein the rotating apparatus makes the furnace shell rotate in a circumferential direction while the electrodes are fixed with respect to a rotational direction.

5. The method of operating an electric arc furnace according to claim 1, wherein, during said rotating, the furnace shell is made to rotate in a circumferential direction while the electrodes are fixed with respect to a rotational direction.

6. The method of operating an electric arc furnace according to claim 1, wherein said rotating is performed after the electrode bores the scrap.

7. The method of operating an electric arc furnace according to claim 1, wherein said charging is performed multiple times.

8. The method of operating an electric arc furnace according to claim 1, wherein the electric arc furnace further comprises a burner provided on the furnace roof so as to face downwards in a position between the electrodes which are adjacent to each other in a circumferential direction, and wherein said rotating comprises rotating the furnace shell relative to the electrodes and the burner during melting of the metal material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a view illustrating a configuration of an electric arc furnace that is used in an operation method in an embodiment of the present invention.

(2) FIG. 2 is a view illustrating another cross-section of the electric arc furnace in FIG. 1.

(3) FIG. 3 is an entire perspective view of a rotating apparatus in FIG. 1.

(4) FIG. 4 is an entire plan view of the rotating apparatus.

(5) FIG. 5 is a V-V cross-sectional view in FIG. 4.

(6) FIG. 6 is a VI-VI cross-sectional view in FIG. 4.

(7) FIG. 7A to FIG. 7F are explanation views illustrating each step of an example of the operation method.

(8) FIG. 8A and FIG. 8B are views explaining the holding positions in an EBT furnace.

(9) FIG. 9A to FIG. 9H are explanation views illustrating an example of an operation method that is different from that in FIGS. 7A to 7F.

(10) FIG. 10A to FIG. 10G are explanation views illustrating an example of an operation method that is different from those in FIGS. 7A to 7F and FIGS. 9A to 9H.

(11) FIG. 11A and FIG. 11B are views explaining holding positions in a spout tapping electric arc furnace.

(12) FIG. 12A and FIG. 12B are views explaining unmelted residue of a metal material in an EBT furnace.

(13) FIG. 13A and FIG. 13B are views illustrating a state in which burners are attached to an electric arc furnace that is used in another embodiment of the present invention.

(14) FIG. 14A to FIG. 14F are explanation views illustrating the content of an operation method step by step in another embodiment of the present invention.

(15) FIG. 15A to FIG. 15F are views illustrating an example of an operation method that is different from that in FIGS. 14A to 14F.

(16) FIG. 16 is a view illustrating an example of an operation method that is different from those in FIGS. 14A to 14F and FIGS. 15A to 15F.

(17) FIG. 17A to FIG. 17C are views explaining a problem of a conventional electric arc furnace.

DETAILED DESCRIPTION OF THE INVENTION

(18) Hereinafter, embodiments in which the present invention is applied to an electric arc furnace melting a metal material (herein, steel material) will be described in detail with reference to the drawings.

(19) FIG. 1 and FIG. 2 are views illustrating a configuration of the electric arc furnace that is used in an operation method in the present embodiment.

(20) In FIG. 1, a reference numeral 10 represents an electric arc furnace, which includes a furnace shell 16 that includes a cylindrical (herein, circular cylindrical) circumferential wall portion 12, a furnace bottom portion 14 and a charging opening 18; a furnace roof 20 that openably and closably shuts the charging opening 18 at an upper end of the furnace shell 16; and three electrodes 22 which are inserted downwards into the furnace shell 16 while passing through the furnace roof 20.

(21) Each of the electrodes 22 is disposed close to the center of the furnace roof 20 with a substantially circular shape in a plan view, while being equally spaced (at an interval of 120) around a vertical axis, which is a center axis here, of the furnace shell 16. The electrodes 22 are supported by a lifting apparatus, which is not illustrated, in such a way that the height thereof is independently adjustable. That is, the electrodes 22 are constituted in such a way that the vertical separation distance between a lower end of the electrode 22 and a metal material charged into the furnace shell 16 is adjustable.

(22) The furnace roof 20 is constituted as being movable in a vertical direction relative to the furnace shell 16 and being turnable in a horizontal direction by a lifting apparatus and a turning apparatus, which are not illustrated. The furnace roof 20 is capable of being moved and turned to open the charging opening 18 of the furnace shell 16 to charge a metal material into the furnace shell 16.

(23) The electric arc furnace 10 in the present embodiment is an EBT furnace, in which a portion of the furnace bottom portion 14 protrudes outwards in a radial direction further than the circumferential wall portion 12 of the furnace shell 16 and on the protruding shelf-like protruding portion 26 is provided a tapping hole (steel tapping hole) 27 passing therethrough in the vertical direction, as illustrated in FIG. 2.

(24) A molten metal material (molten steel) in the furnace shell 16 is tapped toward a ladle 23 (see FIG. 1) via the tapping hole 27 by tilting the entire furnace shell 16.

(25) A slag door 29 is provided at a position (symmetrical position) that is opposite to the tapping hole 27 in the radial direction. The slag door 29 is provided so as to pass through the circumferential wall portion 12 of the furnace shell 16 in an inward and outward direction.

(26) In the present embodiment, slag produced during a melting operation can be discharged to the outside via the slag door 29 by tilting the entire furnace shell 16 in the opposite direction from that in the case of tapping.

(27) In the electric arc furnace 10 in the present embodiment, the furnace shell 16 is tiltable, and is rotatable relative to the furnace roof 20 and electrodes 22.

(28) In FIG. 1, a reference numeral 34 is a tilting body that tilts while supporting the furnace shell 16 to make the furnace shell 16 tilt. The tilting body is tiltably supported by a furnace stand 36 that is provided on the ground.

(29) Engaging teeth are provided on an upper surface of the furnace stand 36 that supports the tilting body 34, and on a lower surface of a leg portion 37, which has a convex shape toward downwards, of the tilting body 34 being in contact therewith. Due to engagement between the engaging teeth with each other, the tilting body 34 is prevented from deviating relative to the furnace stand 36 during a tilting operation.

(30) The tilting body 34 includes a tilting bed 35 for supporting the furnace shell 16, and the furnace shell 16 is supported by the tilting bed 35 of the tilting body 34 via the rotating apparatus 32 described later.

(31) In the present invention, the rotating apparatus may be configured as below.

(32) That is, the rotating apparatus may be configured to include 1) a support unit that rotatably supports a furnace shell above a tilting body; 2) a guide rail that is provided between the furnace shell and the tilting body, and guides the furnace shell to rotate around a center axis thereof; 3) a driven unit that is provided on a furnace shell side, and receives rotation drive force; 4) a drive member that is provided on a tilting body side, and is engaged with the driven unit to transmit rotation drive force to the driven unit; and 5) a drive source that is provided on the tilting body side, and produces drive force. The rotating apparatus may be provided to tilt integrally with the tilting body.

(33) In the present embodiment, the rotating apparatus is specifically configured as below.

(34) First, a tilting mechanism will be described.

(35) As illustrated in FIG. 2, one end of a drive cylinder 38 is rotatably connected to a right end in the drawing of the tilting body 34, and the other end of the drive cylinder 38 is rotatably connected to the ground side. When the drive cylinder 38 extends, the tilting body 34 is tilted along with the furnace shell 16 so that a left side in the drawing is moved downwards and a right side is moved upwards. As a result, molten steel in the furnace shell 16 is tapped toward the ladle 23 via the tapping hole 27.

(36) FIG. 3 illustrates an entire perspective view of the rotating apparatus 32 in the present embodiment, and FIG. 4 illustrates a plan view thereof.

(37) The rotating apparatus 32 includes a circular ring-shaped support frame 40 with multiple upstanding walls, and the furnace shell 16 is mounted on and fixed to an upper surface of the circular ring-shaped support frame 40.

(38) A ring-shaped gear 42 is provided on an inner circumferential portion of a lower surface of the circular ring-shaped support frame 40, and teeth are provided on an inner circumference of the ring-shaped gear 42.

(39) In contrast, as illustrated in FIG. 5, an outer circumferential middle portion of the ring-shaped gear 42 protrudes outwards in a square cross-sectional shape, and forms an inner race portion 46 of a bearing member 44.

(40) An outer race portion 48 with a C-shaped cross-section is provided to surround the inner race portion 46, and roller bearings 49 are provided between a concave surface of the outer race portion 48 and a convex surface of the inner race portion 46, that is, upper and lower surfaces and an outer circumferential end surface.

(41) Owing to such a structure, the circular ring-shaped support frame 40 is supported by the bearing member 44 and is rotatable around a ring center thereof in a plane parallel with the tilting bed 35. Accordingly, the furnace shell 16 supported by the rotating apparatus 32 is rotatable relative to the electrodes 22 around a center axis of the furnace shell 16 as a rotational center.

(42) Gear boxes 50 (see FIG. 4) are provided on the tilting bed 35 at radially symmetrical positions inside the ring of the circular ring-shaped support frame 40, and thereinside are provided gears. In FIG. 5, a hydraulic motor 52 is provided as a drive source on the tilting bed 35 side, and on an output shaft thereof is mounted a gear 53. The gear 53 meshes with a gear 55 that is rotatably supported by a shaft 54 vertically arranged on the tilting bed 35, and the gear 55 meshes with the teeth of the ring-shaped gear 42.

(43) Accordingly, when the hydraulic motor 52 is made rotate in a clockwise direction or a counter-clockwise direction, the circular ring-shaped support frame 40 is also rotated in the clockwise direction or the counter-clockwise direction via the gears 53, 55, and 42.

(44) In the present embodiment, the circular ring-shaped support frame 40, the gear 42, and the bearing member 44 serve as the support unit, and the inner race portion 46 and the outer race portion 48 of the bearing member 44 among them also serve as the guide rail.

(45) The teeth of the gear 42 serves as the driven unit, the gears 55 and 53 serve as the drive unit, and the hydraulic motor 52 serves as the drive source.

(46) In the present embodiment, the circular ring-shaped support frame 40, that is, the furnace shell 16 can be rotated by the hydraulic motor 52, in a range of 60 in the counter-clockwise direction from an original position at which the tapping hole 27 of the furnace shell 16 faces a tapping yard as illustrated in FIG. 4.

(47) A stopper mechanism 56 is provided at a middle position in a circumferential direction of the circular ring-shaped support frame 40 between both of the gear boxes 50. The details of the stopper mechanism 56 are illustrated in FIG. 6. In FIG. 6, a sheath member 57 is provided inwards in the circular ring-shaped support frame 40. The sheath member 57 is a circular cylindrical body, an inner side half of which is made in a tapered shape in which the inner circumference gradually increases inwards.

(48) In contrast, on a stand 58 of the tilting bed 35 side is provided a plug member 60, which is straightly moved forwards and backwards in an inward and outward direction by a drive cylinder 59. The plug member 60 is a circular columnar body, a tip end portion of which positioned on the outside side has a diameter gradually decreasing toward the tip end, and a rear end of the plug member 60 is connected to a rod 61 of the drive cylinder 59.

(49) When the circular ring-shaped support frame 40 is at the original position, as illustrated in FIG. 6, the sheath member 57 faces the plug member 60, and when the plug member 60 is made move forwards by the drive cylinder 59, the plug member 60 enters the sheath member 57 and the tapered tip end portion of the plug member 60 is fitted into the tapered half portion of the sheath member 57. Accordingly, rotation of the circular ring-shaped support frame 40, that is, the furnace shell 16 is reliably restricted, and in this state, the furnace shell 16 can be tilted for tapping or tilted for discharging slag.

(50) Next, a method of operating the electric arc furnace 10 to melt a metal material will be described with reference to FIG. 7A to FIG. 7F.

(51) Typically, a metal material such as scrap is charged into a furnace in multiple times. In this example, the metal material is charged into the furnace in twice.

(52) First, the charging opening 18 of the furnace shell 16 is opened by turning and withdrawing the furnace roof 20, a scrap bucket containing the metal material is moved to a position above the charging opening 18 by a crane, and the metal material in the scrap bucket is charged into the furnace.

(53) After the completion of the charging of the metal material, the furnace roof 20 and the electrodes 22 are placed above the furnace shell 16, and arcs are produced from the tip ends of the electrodes 22 downward to preferentially melt the metal material positioned around the tip end portions of the electrodes 22. The electrodes 22 are made bore into the metal material (this step is referred to as a boring step hereinafter). FIG. 7A illustrates a state in which the boring is performed.

(54) When the boring is completed and the electrodes 22 reach the vicinity of a furnace bottom portion 14, the material around the electrodes 22 is melted (FIG. 7B).

(55) When unmelted metal material around the electrodes 22 is disappeared as a result, the furnace shell 16 becomes rotatable relative to the electrodes 22. Portions in the furnace illustrated by half-tone dots in the drawing represent the unmelted metal material, and portions illustrated by white backgrounds represent the molten metal material. In this stage, extremely ununiform melting has not occurred yet.

(56) After the melting proceeds and the volume of unmelted metal material in the furnace is reduced, the remainder of the metal material is additionally charged. The furnace roof 20 is turned and withdrawn, the furnace shell 16 is rotated by 60 from the original position in the counter-clockwise direction in the drawing, and thereafter, the metal material is additionally charged (FIG. 7C). Thereafter, the furnace roof 20 and the electrodes 22 are placed above the furnace shell 16, and the melting of the metal material is performed again by arc discharge including the boring step, then, the metal material is ununiformly melted by forming three hot spots and three cold spots in the circumferential direction of the furnace shell 16 (FIG. 7D).

(57) Then the unmelted metal material is moved to the hot spot by rotating the furnace shell 16 by 60 in the clockwise direction in the drawing to the original position, in a state where the furnace roof 20 is separated upwards (FIG. 7E). In this state, the furnace roof 20 is closed and the discharge from the electrodes 22 is started, whereby the unmelted metal material can be effectively heated by the electrodes 22. As illustrated in FIG. 7F, the melting of the metal material charged into the furnace is completed.

(58) However, in an operation of merely switching hot spots and cold spots by rotating the furnace shell and the electrodes relative to each other according to the above operation method, the metal material in the vicinity of the tapping hole 27 is positioned distantly from the electrodes 22 and is difficult to be satisfactorily melted.

(59) In the present embodiment, as illustrated in FIGS. 8A and 8B and FIG. 9A to 9H, a holding position is set to a position relative to the tapping hole 27, the rotation of the furnace shell 16 is stopped when any one of the electrodes reaches the holding position, and in a state of keeping the electrode at the holding position, the heating by arc discharge from the electrodes 22 is performed, whereby, the metal material in the vicinity of the tapping hole 27 can be melted.

(60) FIG. 8A illustrates a holding position set relative to the tapping hole 27 in the case where the electric arc furnace is an EBT furnace.

(61) The holding position is desirably set as such a position that an extension line OP of a line connecting a rotational center O of the furnace shell to any electrode (electrode that is closest to the tapping hole 27 in the drawing) of a plurality of the electrodes 22 passes through between one end and the other end of the protruding portion (tapping-hole bottom portion) 26 in a circumferential direction in the vicinity of the tapping hole 27. Typically, an angle range is approximately 100.

(62) By setting the holding position in this manner, the electrode 22 can be positioned close to the metal material in the vicinity of the tapping hole 27, and the metal material in the vicinity of the tapping hole 27 can be effectively heated.

(63) Further preferable is that the angle formed by the extension line OP and the line connecting the rotational center O of the furnace shell to a center of the tapping hole 27 is less than /4 (approximately less than 25) and more desirably less than /6 (approximately less than 17).

(64) By this manner, the metal material which positions on the outside in the radial direction and positions in a region proximate to the tapping hole 27 that is particularly separated from the electrodes can be effectively heated.

(65) FIGS. 9A to 9H illustrate an example of an operation method of setting a holding position relative to the tapping hole 27 and performing a heating by the electrodes at the holding position.

(66) In this example, steps illustrated in FIGS. 9A to 9D are the same as steps of FIGS. 7A to 7D.

(67) In this example, the furnace shell 16 is not directly rotated up to the original position by rotating by 60 from a state of FIG. 9D, and instead, as illustrated in FIG. 9E, the furnace shell 16 is rotated by 30 in the clockwise direction in the drawing from the state of FIG. 9D, and then the rotation is stopped at a position in which the furnace shell 16 rotates from the original position by 30 in the counter-clockwise direction.

(68) This state illustrated in FIG. 9E represents a state in which the electrode 22 positioned closest to the tapping hole 27 reaches the holding position that is set relative to the tapping hole 27 in this example. In this example, the holding position is set as a position that the extension line OP of the line connecting the rotational center of the furnace shell 16 to the electrode 22 positioned closest to the tapping hole 27 passes through the tapping hole 27.

(69) By performing arc discharge from the electrodes 22 in a state where the electrode 22 is held at the holding position, the metal material in the vicinity of the tapping hole 27 can be effectively heated (FIG. 9F).

(70) In this example, after the metal material in the vicinity of the tapping hole 27 is heated by arc discharge in a state where the electrode 22 is kept at the holding position, the furnace shell 16 is rotated to the original position in a state where the furnace roof 20 is separated upwards (FIG. 9G), and the melting of unmelted metal material remaining on an inner surface of the circumferential wall portion 12 is promoted. Finally, as illustrated in FIG. 9H, the melting of the metal material charged into the furnace is completed.

(71) That is, by adding the steps of FIGS. 9E and 9F to the example illustrated in FIGS. 7A to 7F, in the example of the operation illustrated in FIGS. 9A to 9H, the formation of unmelted residue in the vicinity of the tapping hole 27 can be effectively prevented.

(72) In the above-described examples of FIGS. 7A to 7F and FIGS. 9A to 9H, after rotated by 60 at the time of the additional charging, the furnace shell 16 is rotated again until the electrode 22 positioned close to the tapping hole 27 reaches the holding position set relative to the tapping hole 27. However, as in the example illustrated in FIGS. 10A to 10G, the furnace shell 16 may be rotated to reach the holding position set relative to the tapping hole 27 from the start at the time of the additional charging in FIG. 10C, the metal material may be heated at the holding position, thereby making the metal material in the vicinity of the tapping hole 27 be melted after the charging of the metal material.

(73) In this example, thereafter, the furnace shell 16 is rotated up to the original position (FIG. 10F), unmelted metal material remaining on the inner surface of the circumferential wall portion 12 is melted, and as illustrated in FIG. 10G, the melting of the metal material charged into the furnace is completed.

(74) The above is the operation method of preventing the formation of unmelted residue in the vicinity of the tapping hole 27. For effectively heating the metal material in the vicinity of the slag door 29, after heating is performed at the holding position set relative to the tapping hole 27, the furnace shell 16 may be rotated to reach a holding position that is set relative to the slag door 29, stopped rotating, and held at the second holding position, thereby effectively heat the metal material positioned in the vicinity of the slag door 29.

(75) FIG. 8B illustrates a holding position set relative to the slag door 29 in the case where the electric arc furnace is an EBT furnace.

(76) In many cases, unmelted residue in the vicinity of the slag door 29 is formed over a range of approximately 60 from an end of the slag door 29 on a side of a dust collection hole 94 toward the dust collection hole 94. Therefore the holding position relative to the slag door 29 is desirably set in such a way that an extension line OQ of a line connecting a rotational center O of a furnace shell to any electrode of the plurality of electrodes 22 is positioned in a range of an angle of less than 60 from the end of the slag door 29 on a side of the dust collection hole 94 toward the dust collection hole 94.

(77) Since the largest amount of unmelted residue is present in many cases in a range of less than approximately 45, and particularly, less than approximately 30 from the end of the slag door 29 on a side of the dust collection hole 94 toward the dust collection hole 94, the holding position is more desirably set in such a way that an angle illustrated in the drawing is positioned in a range of less than 45, and more desirably in a range of less than 30.

(78) By setting the holding position in this manner, the metal material which has remained unmelted in an end portion of the slag door 29 can be efficiently melted.

(79) FIGS. 11A and 11B are views illustrating the configuration of a spout tapping electric arc furnace.

(80) In this type of electric arc furnace, a tapping hole 65 is configured by an opening that passes through a circumferential wall portion 12 of a furnace shell 16 in a radial direction, and a spout 66 extends from the tapping hole 65.

(81) In the spout tapping electric arc furnace, the tapping hole 65 is kept in an open state during a melting operation, and thus external air infiltrates into the furnace therethrough, flow of cool air is generated in the furnace from the tapping hole 65 toward the dust collection hole 94 attached to the furnace roof, the metal material inside the furnace is cooled in the vicinity of the end portion of the tapping hole 65 on a side of the dust collection hole 94, and thus unmelted residue is formed. That is, the vicinity of the tapping hole 65 is in substantially the same state as that of the slag door of the EBT furnace.

(82) For this reason, in the case of the spout tapping electric arc furnace, the holding position set relative to the tapping hole 65 is desirably set in such a way that the extension line OR of the line connecting the rotational center O of the furnace shell 16 to an electrode positioned closest to the tapping hole 65 of the plurality of electrodes 22 is positioned in the range of an angle of less than 60 from an end of the tapping hole 65 on the side of the dust collection hole 94 toward the dust collection hole 94.

(83) Further preferred is to set an angle in the drawing in a range of less than 45, and more desirably, in a range of less than 30.

(84) By setting the holding position in this manner, the metal material positioned in the vicinity of the end portion of the tapping hole 65 can be efficiently melted.

(85) In the spout tapping electric arc furnace, the slag door has the same structure as that in the EBT furnace, and the holding position set relative to the slag door is desirably set so as to be in the same range as in the case of the EBT furnace.

(86) The embodiments of the present invention have been described above in details but they are merely examples. In the present invention, for example, a holding step may be performed independent from the method of switching hot spots and cold spots.

(87) Further, the electrodes may be rotated instead of the furnace shell, and the number of electrodes may two, four, or other than those numerals.

(88) Furthermore, it is also applicable to a method of operating an electric arc furnace that melts metal other than steel. As such, the present invention can be realized in various modified forms insofar as they do not depart from the gist thereof.

(89) Hereinafter, modified embodiments of the present invention, other than the above-described embodiment, will be described in detail with reference to FIGS. 13A to 17B. The same or similar component as that in the above-described embodiment will be noted by the same reference numeral and the detailed description thereof will be omitted.

(90) In the electric arc furnace 10 in the modified embodiment, as illustrated in FIG. 13A, three burners 19 are attached to the furnace roof 20 for accelerating the melting of the metal material.

(91) Each burner 19 is disposed between the electrode 22 and the circumferential wall portion 12 of the furnace shell 16 when the arrangement is seen in the radial direction.

(92) When the arrangement is seen in the circumferential direction, the burners 19 are respectively provided at every position between the electrodes 22 (specifically, at every intermediate position in the circumferential direction between the adjacent electrodes 22). These three burners 19 are disposed with equally intervals (at intervals of 120) in the circumferential direction.

(93) As illustrated in FIG. 13B, in order to effectively heat a metal material M that is remained unmelted on a lower side of the circumferential wall portion 12 of the furnace shell 16, the burner 19 is attached in such a way that a tip end thereof slightly faces toward the circumferential wall portion 12 rather than directly below. However, insofar as not departing from the gist of the present invention, the arrangement of the burner 19 can be properly designed. For example, the burner 19 may be disposed in such a way that the tip end thereof faces directly below.

(94) Next, a method of operating the electric arc furnace 10 to melt a metal material will be described with reference to FIGS. 14A to 14F.

(95) Typically, a metal material such as scrap is charged into a furnace in multiple times. In this example, the metal material is charged into the furnace in twice.

(96) First, the charging opening 18 of the furnace shell 16 is opened by turning and withdrawing the furnace roof 20, a scrap bucket containing the metal material is moved to a position above the charging opening 18 by a crane, and the metal material in the scrap bucket is charged into the furnace.

(97) After the completion of the charging of the metal material, the furnace roof 20 and the electrodes 22 are placed above the furnace shell 16, and arcs are produced from the tip ends of the electrodes 22 downward to preferentially melt the metal material positioned around the tip end portions of the electrodes 22. The electrodes 22 are made bore into the metal material (this step is referred to as a boring step hereinafter). FIG. 14A illustrates a state in which the boring is performed.

(98) When the boring is completed and the electrodes 22 reach the vicinity of a furnace bottom portion 14, the material around the electrodes 22 is melted (FIG. 14B).

(99) When unmelted metal material around the electrodes 22 is disappeared as a result, the furnace shell 16 becomes rotatable relative to the electrodes 22. Portions in the furnace illustrated by half-tone dots in the drawing represent the unmelted metal material, and portions illustrated by white backgrounds represent the molten metal material. In this stage, extremely ununiform melting has not occurred yet.

(100) After the melting proceeds and the volume of unmelted metal material in the furnace is reduced, the remainder of the metal material is additionally charged. The furnace roof 20 is turned and withdrawn, the furnace shell 16 is rotated by 60 from the original position in the counter-clockwise direction in the drawing, and thereafter, the metal material is additionally charged (FIG. 14C). Thereafter, the furnace roof 20 and the electrodes 22 are placed above the furnace shell 16, and the melting of the metal material is performed again by arc discharge including the boring step, then, the metal material is ununiformly melted by forming three hot spots and three cold spots in the circumferential direction of the furnace shell 16 (FIG. 14D).

(101) In the modified embodiment, the metal material between the electrodes 22 (between in the circumferential direction) is heated and melted by flames from the three burners 19 provided in the furnace roof 20 at the same time of heated and melted by the electrodes 22.

(102) For example, in a state illustrated in FIG. 14D, the metal material at the cold spot between the electrodes 22 is heated and melted.

(103) After the heating of FIG. 14D, the furnace shell 16 is rotated to the original position by 60 in the clockwise direction in the drawing in a state where the furnace roof 20 is separated upwards, to thereby relatively move the metal material at a hot spot to a cold spot and the metal material at a cold spot to a hot spot. The furnace shell 16 is held at the original position, the furnace roof 20 is closed and heating by the electrode 22 and heating by the burner 19 are performed. Specifically, the metal material, the position of which is changed from a cold spot to a hot spot, is heated and melted by the electrode 22 and the metal material, the position of which is changed from a hot spot to a cold spot, is heated and melted by the burner 19.

(104) In this case, since the metal material, the position of which is changed from a cold spot to a hot spot, has been applied a heating and melting action by the burner 19 at the position of the original cold spot, the amount of unmelted material is small as compared to the case of a melting operation by a conventional electric arc furnace with a rotating apparatus 32. Therefore, the heating and melting by the electrodes 22 can be rapidly completed.

(105) In contrast, as for the metal material, which has been newly reached to a cold spot from a hot spot, even if an unmelted metal material remains there, this can be rapidly heated and melted by the burner 19.

(106) In the present embodiment, depending on the circumstances, during the furnace shell 16 is rotated by 60 in the clockwise direction in the drawing to the original position from the state of FIG. 14D, that is, during the electrodes 22 and the burners 19 are rotated relative to the metal material in the furnace shell 16, the metal material can be heated and melted by the electrodes 22 and the burners 19 while the furnace shell 16 is continuously rotated.

(107) In particular in this case, the heating position by the electrodes 22 and the burners 19 to the metal material can be continuously changed, and the heating action can be applied over a wide range of the metal material. In this case, the holding step in the above-described embodiment can be omitted. Further, this operation method of the modified embodiment is applicable to an electric arc furnace without tapping hole and slag door.

(108) The example of the operation illustrated in FIGS. 14A to 14F is an example in which the burners are used after the metal material is additionally charged; however, in the modified embodiment, as illustrated in FIGS. 15A to 15F, the metal material can be heated and melted by the electrodes 22 and the burners 19 from an initial charging stage (refer to FIG. 15A) at which melting is started.

(109) As described above, according to the modified embodiment, by rotating the furnace shell 16 relative to the electrodes 22 provided in the furnace roof 20 by the rotating apparatus 32, a portion at a cold spot can be moved to a hot spot, and a portion at a hot spot can be moved to a cold spot, whereby the metal material in the furnace shell can be more uniformly heated.

(110) Furthermore, in the modified embodiment, since the burners 19 is provided on the furnace roof 20 so as to face downwards in a position between the electrodes 22 in a circumferential direction, the metal material in the furnace shell can be heated also at a position between the electrodes 22 (between the electrodes 22 in the circumferential direction) by the burners.

(111) Further, in the modified embodiment, since the heating position from the burners to the metal material can be changed by the rotation by the rotating apparatus 32, the metal material can be heated by the burners 19 over a wide range in the circumferential direction and the heating from the burners 19 can be made more uniform.

(112) In the modified embodiment, the metal material can be uniformly heated, and in addition, the speed of heating can be increased as compared to that in the conventional art, time required to melt the metal material can be further reduced, and high-speed melting can be realized.

(113) In the modified embodiment, the burners 19 are disposed in a position between the electrodes 22 and the circumferential wall portion 12 of the furnace shell in the radial direction of the furnace roof 20. Therefore, the unmelted metal material, which is positioned distant from the electrode 22 and positioned close to the circumferential wall portion 12, can be heated by the burner 19 from a position closer than the electrode 22. Therefore, heating efficiency can be further improved and time required for a melting operation can be further reduced.

(114) For example, in the holding step of the above-described embodiment, the furnace shell 16 may be rotated until the any one of the burners 19, instead of the electrodes 22, reaches the holding position that is set close to tapping hole or slag door and held at the state.

(115) In the operation method in the modified embodiment, flames emanate from the three burners 19 at the same time; however, when metal remains unmelted at one location inside the furnace as illustrated in FIG. 16, the furnace shell 16 may be rotated up to a position at which flame can emanate from any one of the burners 19 directly to the unmelted metal material, and the unmelted metal may be melted by the flame from the burner 19. Further, the electric arc furnace used in the above-described modified embodiment has the burners provided at every position between the electrodes. However, the present invention is not limited to such a configuration. For achieving more excellent uniform melting, the burners are preferably provided at every position between the electrodes.

(116) The modified embodiments of the present invention have been described above in details but they are merely an example. The above-mentioned modified embodiment is an example in which three burners are provided in the furnace roof; however, the number of burners may be two, four, or more, and it may be a case where instead of the furnace shell being rotated, the electrodes and the burners are rotated.

(117) Furthermore, the present invention is applicable to an electric arc furnace that melts metal other than steel and an operation method thereof. As such, the present invention can be realized in various modified forms insofar as they do not depart from the gist thereof.

(118) The present application is based on the Japanese patent applications No. 2014-225630 filed on Nov. 5, 2014 and No. 2014-225634 filed on Nov. 5, 2014, which contents are incorporated herein by reference.

DESCRIPTION OF REFERENCE NUMERALS

(119) 10: electric arc furnace 12: circumferential wall portion 14: furnace bottom portion 16: furnace shell 18: charging opening 19: burner 20: furnace roof 22: electrode 26, 86: protruding portion 27, 65, 88: tapping hole 29, 91: slag door 30, 92: slag-door bottom portion 32: rotating apparatus 66: spout