Stopper for continuous casting and continuous casting method

Abstract

The precision of grasping or controlling backpressure around a gas discharge portion in a stopper for continuous casting can be improved with a stopper for continuous casting which includes a cavity for conveying gas in a vertical direction center of the stopper, one or a plurality of gas discharge holes passing through from the cavity to the outside in a distal center or a side surface of a reduced-diameter region including a fitted portion to a lower nozzle, and a pressure control component in a part of an area above the gas discharge hole within the cavity.

Claims

1. A stopper for continuous casting comprising: a cavity for conveying gas in a vertical direction center of the stopper; one or a plurality of gas discharge holes passing through from the cavity to the outside in a distal center or a side surface of a reduced-diameter region including a fitted portion to a lower nozzle; and a pressure control component in a part of the reduced-diameter region, the part being above the gas discharge hole within the cavity, wherein the pressure control component is made of a dense refractory having no gas permeability under a condition of pressurizing a sample of the refractory having a length of 20 mm at 8?10.sup.?2 MPa, wherein the pressure control component includes one or a plurality of through holes disposed within the pressure control component or between an outer periphery of the pressure control component and a body of the stopper so as to pass through from an upper end to a lower end between the pressure control component or the outer periphery of the pressure control component and the body of the stopper, wherein the through hole has a diameter having a size between ?0.2 mm and ?2 mm both inclusive, the size being obtained by assuming a cross section of the hole as a circular shape and converting the cross section into a circle, and wherein the number of through holes satisfies Equations 1 and 2:
(?0.44?Hd.sup.2+1.88Hd?0.08)?Ha?{1.67?ln(Hd)+3.66}Equation 1
Hn=Ha?(Hd.sup.2???4)Equation 2, where Ha is a numerical value of a total cross-sectional area of the through hole(s), Hn is the number of through holes, Hd is a numerical value of a diameter of the through hole, and ? is a circular constant, with the total cross-sectional area measured in mm.sup.2 and the diameter of the through hole measured in mm.

2. The stopper for continuous casting as claimed in claim 1, wherein the pressure control component is placed in an area below a diameter reduction starting position of a stopper distal end.

3. The stopper for continuous casting as claimed in claim 1, wherein the through hole has a slit shape (hereinafter referred to as slit), where a total cross-sectional area of the slit(s) is regarded as said Ha (mm.sup.2) and a thickness of the slit is regarded as said Hd (mm), a value obtained by dividing the total cross-sectional area of the slit(s) by the thickness of the slit is a total length of the slit(s).

4. A continuous casting method using the stopper for continuous casting as claimed in claim 1, the method comprising A discharging gas into molten steel from the gas discharge hole of the stopper of claim 1 by setting gas pressure in the cavity on an upstream side of the pressure control component to a value between 2?10.sup.?2 (MPa) and 8?10.sup.?2 (MPa) both inclusive.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an example of a stopper including a pressure control component and a gas discharge hole of the present invention, the gas discharge hole existing in a distal center of a reduced-diameter region.

(2) FIG. 2 is another example of the stopper including the pressure control component and gas discharge holes of the present invention, the gas discharge holes existing in a side surface of the reduced-diameter region.

(3) FIGS. 3A-3J are images of an upper end surface of the pressure control component of the present invention as viewed from above.

(4) FIG. 4 is a graph obtained by simulating a relation between a diameter and a total cross-sectional area of a through hole at a pressure of 2?10.sup.?2 (MPa) and 8?10.sup.?2 (MPa) when the total cross-sectional area of the through hole(s) is measured in mm.sup.2 and the diameter of the through hole is measured in mm.

(5) FIG. 5 is a graph illustrating an example obtained by simulating a difference in gas pressure when through holes with the shape of circle and two types of elongated circles have the same total cross-sectional area (adjusted by the number of through holes).

(6) FIG. 6 is a graph illustrating an example of gas backpressure during casting in the present invention including the pressure control component and in a conventional technique including no pressure control component.

(7) FIG. 7 is a graph illustrating an example of variation in gas backpressure and flow rate during casting in the present invention including the pressure control component and in the conventional technique including no pressure control component.

(8) FIG. 8 is a graph illustrating an example of a deposit thickness (the conventional technique is 1 as an index) of alumina-based inclusions on a nozzle inner wall in the present invention including the pressure control component and in the conventional technique including no pressure control component.

(9) FIG. 9 is a graph illustrating an example of the average number of occurrences (time/ch) of a sudden molten metal surface fluctuation of 10 mm or more in a mold in the present invention including the pressure control component and in the conventional technique including no pressure control component.

(10) FIG. 10 is an example of experiment on a water model illustrating gas flow rate/backpressure characteristics using gas discharge holes having different forms and diameters.

(11) FIG. 11 is an example of experiment on a water model illustrating a bubble diameter and an existence ratio assuming the inside of a mold using gas discharge holes having different forms and diameters.

DESCRIPTION OF EMBODIMENTS

(12) Embodiments of the present invention will be described together with examples (water model experiment examples).

(13) FIG. 1 illustrates a vertical cross-sectional view of main parts of a stopper as an example of the present invention together with a lower nozzle. A stopper 10 illustrated in FIG. 1 includes a cavity 2 for conveying gas in a vertical direction center of the stopper. That is, the cavity 2 is provided so as to extend vertically in the center of a stopper body 1, and an unillustrated gas supply source is connected to an upper end of the cavity 2. The stopper 10 is typically located in a tundish so as to control a flow rate of molten steel by being fitted from above to a nozzle (lower nozzle) 20 placed in a bottom of the tundish.

(14) The stopper 10 includes a gas discharge hole 4 passing through from the cavity 2 to the outside in a distal center of a reduced-diameter region including a fitted portion 3 to the lower nozzle 20. The stopper 10 further includes a pressure control component 5 in a part of the reduced-diameter region above the gas discharge hole 4 within the cavity 2.

(15) The gas discharge hole 4 may be also provided in a side surface of the reduced-diameter region, and may be provided at a plurality of positions as illustrated in FIG. 2. Additionally, the gas discharge hole 4 may be formed in a slit shape.

(16) As described above, the stopper of the present invention includes the pressure control component in a part of an area above the gas discharge hole, preferably in an area immediately above the gas discharge hole. This is because it is preferable to grasp and control pressure at a position as close as possible to the discharge hole in order to more accurately and precisely grasp and control a state of gas discharged from around a distal end of the stopper. The position as close as possible to the discharge hole is an area roughly below a diameter reduction starting position of the stopper distal end. To be more specific, the area is roughly within 150 mm from the distal end of the stopper body.

(17) The gas discharge hole in the stopper of the present invention is a distal opening of the cavity for conveying gas. The discharge hole may be located at one position in the distal center of the reduced-diameter region or at a plurality of positions around the fitted portion (side surface). It should be noted that a total opening area of the gas discharge hole is preferably about 3.1 mm.sup.2 (equivalent to an opening area of a hole having a diameter of 2 mm) or less.

(18) While the pressure control component may have any one of a porous body (porous refractory) form or a through hole form, the pressure control component preferably controls a flow rate of gas under higher pressure. The gas permeability characteristics of the pressure control component and the gas discharge hole defined in the above Equation 1 are individually measured in a laboratory.

(19) Additionally, a decrease in gas amount, clogging or the like may occur when the pressure control component is a porous body (porous refractory). In this case, it is preferable to use a dense refractory for the pressure control component as described above and form a through hole within the pressure control component or between the outer periphery of the pressure control component and the stopper body so as to satisfy conditions in the equations or the like in the above item 3.

(20) FIGS. 3A to 3J illustrate formation and shape examples of the through hole.

(21) FIG. 3A is an example in which the pressure control component 5 having a through hole 6 is placed in the stopper body 1 via a joint filler 7.

(22) FIG. 3B is an example in which the pressure control component 5 having a plurality of through holes 6 is placed in the stopper body 1 via the joint filler 7.

(23) FIG. 3C is an example in which the through holes 6 are formed as grooves in the outer periphery of the pressure control component 5 placed in the stopper body 1 without the joint filler.

(24) FIG. 3D is an example in which the through holes 6 are formed in the joint filler 7 between the outer periphery of the pressure control component 5 and the stopper body 1.

(25) FIG. 3E is an example in which the through holes 6 are formed as grooves in the cavity 2 of the stopper body 1 between the outer periphery of the pressure control component 5 and the stopper body 1, and the pressure control component 5 is placed without using the joint filler.

(26) FIG. 3F is an example in which the pressure control component 5 having the slit-shaped through holes (slits) 6 is placed in the stopper body 1 via the joint filler 7.

(27) FIG. 3G is an example in which the slit-shaped through holes (slits) 6 are formed between the outer periphery of the pressure control component 5 and the stopper body 1.

(28) FIG. 3H is an example in which the pressure control component 5 made of a porous refractory is placed in the stopper body 1. While no joint filler is used in FIG. 3H, the joint filler may be used.

(29) FIG. 3I is a view illustrating a thickness t and a length L of an example in which the through hole 6 has a slit shape.

(30) FIG. 3J is a view illustrating a thickness t and a length L of another example in which the through hole 6 has a slit shape.

(31) In the present invention, the through hole may have various shapes as in the examples of the through hole illustrated in FIGS. 3A to 3G, 3I, 3J, and 5. While FIG. 3H is an example in which the pressure control component 5 is the porous body (porous refractory), the pressure control component 5 may have various forms. For example, the pressure control component 5 may be wholly or partially made of the porous body, or may be placed via the joint filler.

(32) The through hole(s) may be located so as to fall under a range of an approximate curve representing a relation between a diameter and a total cross-sectional area of a circular through hole at a pressure of 2?10.sup.?2 (MPa) and 8?10.sup.?2 (MPa) (pressure of the cavity on an upstream side of the pressure control component) as illustrated in FIG. 4. In other words, the number of through holes equal to a value obtained by dividing a value (Ha) of the total cross-sectional area of the through hole(s) represented on the vertical axis of the graph in FIG. 4 by a cross-sectional area (Hd.sup.2???4) of the through hole having a value (Hd) of the diameter of the through hole on the horizontal axis thereof may be located in the pressure control component with the total cross-sectional area of the through hole(s) measured in mm.sup.2 and the diameter of the through hole measured in mm.

(33) The through hole may have a single hole shape such as the above circular shape, an elliptical or another shape having a curved surface (non-perfect circle), and a polygonal shape, or may have a slit shape.

(34) FIG. 5 illustrates an example in which the shape of the through hole is compared between the circular shape and the slit shapes. The slit in this example is shaped such that its opposite ends have partially circular shapes, which are elongated outward from the opposite ends. In this example, pressure values (pressure values of the cavity on the upstream side of the pressure control component) obtained when the through holes have the same total cross-sectional area were observed. Here, the same total cross-sectional area was obtained by changing the numbers of the respective through holes.

(35) The result shows that the circular shape and the slit shapes have little pressure difference. That is, for the slit-shaped through hole, the shape and number thereof may be determined using the conversion method described in the above item 4.

(36) FIG. 6 illustrates an example of gas (Ar) backpressure during casting in the present invention including the pressure control component (FIGS. 1 and 3A, the same applies hereinafter) and in a conventional technique including no pressure control component. It is shown that the backpressure is extremely low in the conventional technique including no pressure control component, whereas the backpressure can be controlled to be high in the present invention including the pressure control component.

(37) FIG. 7 illustrates an example of variation in gas (Ar) backpressure and flow rate during casting in the present invention including the pressure control component and in the conventional technique including no pressure control component. It is shown that not only the backpressure but the gas flow rate (discharge amount) is also more stable in the present invention including the pressure control component than in the conventional technique including no pressure control component.

(38) FIG. 8 illustrates an example of a deposit thickness (the conventional technique is 1 as an index) of alumina-based inclusions on a nozzle inner wall in the present invention including the pressure control component and in the conventional technique including no pressure control component. It is shown that the deposit thickness of alumina-based inclusions on a nozzle inner wall is smaller in the present invention including the pressure control component than in the conventional technique including no pressure control component.

(39) FIG. 9 illustrates an example of the average number of occurrences (time/ch) of a sudden molten metal surface fluctuation of 10 mm or more in a mold in the present invention including the pressure control component and in the conventional technique including no pressure control component. It is shown that the average number of occurrences of a sudden molten metal surface fluctuation of 10 mm or more in a mold is also smaller in the present invention including the pressure control component than in the conventional technique including no pressure control component.

(40) When the gas discharge hole is located at one position in the distal center of the reduced-diameter region of the stopper, the gas discharge hole is preferably disposed at a position within +10 mm in a radial direction of the stopper from the vertical center axis of the stopper. This is because disposing the gas discharge hole at the above position makes it hard for the discharged gas flow to receive the effect of a molten steel flow flowing along the outer periphery of the stopper distal end (so-called head portion), and bubbles to hardly join together, thereby preventing generation of coarse bubbles. As a result, nozzle clogging can be efficiently prevented, and inclusion floatation in the mold can be efficiently promoted.

(41) When the gas discharge hole is located at a plurality of positions around the distal end of the reduced-diameter region of the stopper, the gas discharge hole is preferably disposed at positions away from the vertical center axis of the stopper by 10 mm or more in the radial direction of the stopper up to the fitted portion (contact point with the lower nozzle). This is because disposing the gas discharge hole at the above positions allows the discharged gas flow to be dispersed, and makes it difficult for bubbles to join together, thereby preventing generation of coarse bubbles. As a result, nozzle clogging can be efficiently prevented, and inclusion floatation in the mold can be efficiently promoted. Discharging gas below the fitted portion (contact point with the lower nozzle) makes it possible to certainly blow the gas into an inner hole of the lower nozzle.

(42) When the gas discharge hole is located at one position of the distal center or at a plurality of positions of the side surface of the reduced-diameter region of the stopper, experiment shows that the distal opening (discharge port) of the gas discharge hole preferably has a diameter of 2 mm or less. This is because the flow rate can be controlled more precisely, and there is a higher ratio of bubbles having a small diameter (roughly less than 3 mm), which allow inclusions in molten steel to easily float up and make it difficult to produce steel defects. FIGS. 10 and 11 illustrate these water model experiment results.

REFERENCE SIGNS LIST

(43) 10 STOPPER 1 STOPPER BODY 2 CAVITY 3 FITTED PORTION 4 GAS DISCHARGE HOLE 5 PRESSURE CONTROL COMPONENT 6 THROUGH HOLE 7 JOINT FILLER 20 LOWER NOZZLE