PROCESS AND APPARATUS FOR SUPPLEMENTARY COOLING OF THE CENTER OF A CONTINUOUSLY CAST METAL BAR
United States Patent 3833047
An improved process and device for the continuous casting of metal bars wherein an inert gas is insufflated into the liquid metal in the mould through a metal pipe dipped into said liquid metal and having substantially the same composition of the said liquid metal.
US Patent References:
Metal casting apparatus
Hopkins - October 1944 - 2361101
Casting method
Goedecke et al. - September 1965 - 3208117
METHOD FOR IMPROVING CONTINUOUSLY CAST STRANDS
Hill - July 1973 - 3746070
Metal casting apparatus
Hopkins - October 1944 - 2361101
Casting method
Goedecke et al. - September 1965 - 3208117
METHOD FOR IMPROVING CONTINUOUSLY CAST STRANDS
Hill - July 1973 - 3746070
Application Number:
05/304893
Publication Date:
09/03/1974
Filing Date:
11/09/1972
View Patent Images:
Export Citation:
Assignee:
Dalmine S.p.A. (Milano, IT)
Primary Class:
Other Classes:
164/418, 164/415, 164/488
International Classes:
B22D11/112; B22D11/11; B22D11/10
Field of Search:
164/57,66,82,86,275,281
Primary Examiner:
Annear, Spencer R.
Attorney, Agent or Firm:
Kenyon & Kenyon Reilly Carr & Chapin
Claims:
What I claim is
1. A process for the continuous casting of metal in a vertical mold comprising the steps of
2. A continuous casting apparatus comprising
3. A continuous casting apparatus as set forth in claim 2 wherein said first means feeds said pipe axially into said mold to cool the center of the melted metal.
1. A process for the continuous casting of metal in a vertical mold comprising the steps of
2. A continuous casting apparatus comprising
3. A continuous casting apparatus as set forth in claim 2 wherein said first means feeds said pipe axially into said mold to cool the center of the melted metal.
Description:
BACKGROUND OF THE INVENTION
The present invention concerns an improved process and device for the continuous casting of bars, of the type in which a metal, such as steel, is transformed from the melted state to the solid bar state through passage through a chilled mould. More especially the invention applies to the processes for continuous casting in which the liquid metal is caused to flow into a mould of rectilinear axis which is substantially vertical.
It is known that metal bars produced by all continuous casting processes of known technique contain a porous zone in the center, which is normally no problem when the bar is used for rolled sections or structural shapes, due to the fact that the small internal holes fuse together as a result of the rolling pressure, but which instead can cause considerable inconveniences when the bar, especially in the round section, is intended for the manufacture of pipes. In that case in fact the first operation performed on the bar is that of piercing the center in a piercing rolling-mill using skew rolls which cause stresses in the center of the piece. These are not pressure stresses alone and therefore can open up the little internal holes causing subsequently defects such as scales on the internal surface of the manufactured pipe.
It is further known that in the interior of the bar which is solidifying there is a well of liquid metal, substantially in the form of an inverted cone, whose depth depends on the dimensions of the bar, on the pouring conditions of the melted metal and on the various parameters of solidification, such as the speed of extraction, intensity of the primary and secondary cooling, and so on. The greater the depth of the liquid well, or the height of its cone, the more acute is the angle to the vertex of that cone, assuming a given bar section in the melted state at the entrance of the mould. It has been found that near the vertex the crystals that grow from the walls of the bar toward the interior meet forming bridges which can hinder the flow of the liquid metal into the underlying areas when holes form in those areas as a result of shrinkage. The excessive depth of the liquid well therefore turns out to be the principal reason for the formation of the above porosity in the center of the bar produced by continuous casting.
It has already been proposed an improved process for manufacturing metal bars through continuous casting which had as its aim the avoidance of the above-mentioned inconveniences producing in the liquid metal in the act of solidifying an initial cooling in the center, which is added to the cooling performed on the exterior of the mould. According to the said known process, draught of inert gas is insufflated into the liquid metallic mass in course of solidification. The gas is introduced into a piping which ends into a pipe, made of a refractory material, which is dipped into the liquid metal. A porous plug fixed on terminal hole of the refractory pipe prevents the liquid metal come in into the pipe while letting the gas go out. The inert gas thus insufflated provides a controlled stirring of the liquid well and accelerates the cooling in the center of the melted metal.
SUMMARY OF THE INVENTION
The object of the process according to this invention is to further improve the cooling conditions and the solidification of the billet during its formation in its passage through the chilled mould and through the subsequent secondary cooling zone. In particular this improved process allows to substantially reduce the porosity in the center of the bar by reducing the depth of the liquid well formed in its interior through a lowering of the temperature in the center area itself.
The above object is achieved through an improved casting device wherein the inert gas is insufflated into the melted mass through a piping which ends into a pipe made of metal whose composition is identical to that of the billet in course of solidification. This metallic pipe which wears out because it melts into the liquid mass in course of solidification, is introduced continuously from the top along the axis of the mould. The dimensions of this metal pipe as well as the speed with which it is fed are calculated in such a manner that the quantity of metal fed is completely melted before solidifying again. For melting the metal pipe is made use of the enthalpy of overheating of the surrounding metal mass which is in such manner further cooled. This supplementary cooling due to the melting of the metal pipe is, as well known to the skilled in the art, more intense than the caused by the insufflated gas stream.
The use of the metal pipe according to the present invention, instead of the refractory pipe of the prior art, offers another advantage because the metal pipe does not require a porous plug or diaphragm at its end. It is known in fact that such porous plugs or diaphragms are subject to occlusions due to momentary decreases of the pressure of the insufflated gas with subsequent penetration and solidification of the liquid metal into the pores of the plug or diaphragm.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages, objects and characteristics of the process and device of the present invention will become evident to the skilled in the art from the following detailed description of one embodiment referring to the attached drawings in which:
FIG. 1 is a schematic sectional view of an apparatus for continuous casting according to the process of the present invention; and
FIG. 2 shows in correspondence of two different transverse sections of the bar in the act of solidifying, and the temperature trends in the continuous casting process of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the liquid metal, particularly steel, contained in the crucible 1 is discharged into hopper 2 from which pours out a jet 3 which falls into mould 4 forming a liquid bath or well 5. Rolls 6 guide the bar partially solidified 7 through a curtain of water sprays produced by a series of nozzles 8 mounted on a manifold 9. The liquid well 5, of substantially inverted-cone form whose base is the top free surface of the melted metal, has its lower vertex 10 at the point where the solidification is completed. Rolls 11 draw from the mould 4 the bar 7 which is subjected to secondary cooling.
In FIG. 1, which shows a primary realizable method of the process covered by the present invention, hopper 2 is displaced in such a manner that jet 3 falls into the mould in off-center position, but could also provide for two or more jets symmetrical in respect to the axis of the mould. The solid metal pipe 13, having the same composition as that of the melted metal and therefore of bar 7, is fed continuously through feed-rolls 12 into the center of liquid well 5. As stated previously, bar 13 also melts assorbing heat from the surrounding melted metal in the liquid well which for that reason becomes cooled. This further reduction of the temperature at the center of the solidifying bar results in a shortening of the liquid well 5 whose vertex 10 will be therefore at a higher point and with a greater angular opening with respect to an analogous representation of the solidification in a continuous casting process of the prior art. Inside the pipe 13 flows in direction 14 an inert gas which escapes toward the bottom of the area where the pipe is melting and then re-ascends toward the surface facilitating still more the complete melting of the tubular element 13 and directly cooling the center of the melted metal.
Any gaseous substance at room temperature, which neither reacts with the liquid metal nor modifies the metal properties of the melted mass, can be used as an inert gas according to the present invention. For example nitrogen or argon can be used to this purpose. The quantity of inert gas to be insufflated into the inner part of the melted mass is approprietaly chosen according to the volume of the melted mass contained in the mould, according to the extraction speed of the solidified bar as well as according to other factors known to those skilled in the art. A stream of nitrogen between 0.3 and 1.6 liter/minute has given satisfactory results in many cases.
The lower end of the pipe 13 is dipped into the inner part of the melted mass down to the desired depth, which is usually in the range between about 20 and about 80 centimeters. The metal pipe 13 is used in rolls of a considerable length. During the casting process the pipe 13 is first unrolled and then straightened by means of a known device which also provides for the continuous feeding of the pipe as its dipped end melts in the liquid well. The opposite end of metal pipe 13 is connected to the gas feeding piping through a known gas-tight rotative joint. The source of the fed inert gas can be a pressure tank or a plurality of metal bottles.
In FIG. 2, where below and in line with a schematic view in longitudinal section of a bar in the act solidifying according to the process of the present invention are the diagrams of the temperatures respectively in transverse sections AA and BB, with T s there has been indicated the temperature corresponding to the change from liquid state to solid state.
From the diagram it is evident that at the central point M of section BB the temperature is lower than that at point N thus causing a more rapid solidification in the axial area of the bar.
Although the invention has been disclosed in detail with reference to an embodiment thereof, it is to be understood that possible additions and/or modifications can be made by those skilled in the art remaining in the scope of the present invention.
The present invention concerns an improved process and device for the continuous casting of bars, of the type in which a metal, such as steel, is transformed from the melted state to the solid bar state through passage through a chilled mould. More especially the invention applies to the processes for continuous casting in which the liquid metal is caused to flow into a mould of rectilinear axis which is substantially vertical.
It is known that metal bars produced by all continuous casting processes of known technique contain a porous zone in the center, which is normally no problem when the bar is used for rolled sections or structural shapes, due to the fact that the small internal holes fuse together as a result of the rolling pressure, but which instead can cause considerable inconveniences when the bar, especially in the round section, is intended for the manufacture of pipes. In that case in fact the first operation performed on the bar is that of piercing the center in a piercing rolling-mill using skew rolls which cause stresses in the center of the piece. These are not pressure stresses alone and therefore can open up the little internal holes causing subsequently defects such as scales on the internal surface of the manufactured pipe.
It is further known that in the interior of the bar which is solidifying there is a well of liquid metal, substantially in the form of an inverted cone, whose depth depends on the dimensions of the bar, on the pouring conditions of the melted metal and on the various parameters of solidification, such as the speed of extraction, intensity of the primary and secondary cooling, and so on. The greater the depth of the liquid well, or the height of its cone, the more acute is the angle to the vertex of that cone, assuming a given bar section in the melted state at the entrance of the mould. It has been found that near the vertex the crystals that grow from the walls of the bar toward the interior meet forming bridges which can hinder the flow of the liquid metal into the underlying areas when holes form in those areas as a result of shrinkage. The excessive depth of the liquid well therefore turns out to be the principal reason for the formation of the above porosity in the center of the bar produced by continuous casting.
It has already been proposed an improved process for manufacturing metal bars through continuous casting which had as its aim the avoidance of the above-mentioned inconveniences producing in the liquid metal in the act of solidifying an initial cooling in the center, which is added to the cooling performed on the exterior of the mould. According to the said known process, draught of inert gas is insufflated into the liquid metallic mass in course of solidification. The gas is introduced into a piping which ends into a pipe, made of a refractory material, which is dipped into the liquid metal. A porous plug fixed on terminal hole of the refractory pipe prevents the liquid metal come in into the pipe while letting the gas go out. The inert gas thus insufflated provides a controlled stirring of the liquid well and accelerates the cooling in the center of the melted metal.
SUMMARY OF THE INVENTION
The object of the process according to this invention is to further improve the cooling conditions and the solidification of the billet during its formation in its passage through the chilled mould and through the subsequent secondary cooling zone. In particular this improved process allows to substantially reduce the porosity in the center of the bar by reducing the depth of the liquid well formed in its interior through a lowering of the temperature in the center area itself.
The above object is achieved through an improved casting device wherein the inert gas is insufflated into the melted mass through a piping which ends into a pipe made of metal whose composition is identical to that of the billet in course of solidification. This metallic pipe which wears out because it melts into the liquid mass in course of solidification, is introduced continuously from the top along the axis of the mould. The dimensions of this metal pipe as well as the speed with which it is fed are calculated in such a manner that the quantity of metal fed is completely melted before solidifying again. For melting the metal pipe is made use of the enthalpy of overheating of the surrounding metal mass which is in such manner further cooled. This supplementary cooling due to the melting of the metal pipe is, as well known to the skilled in the art, more intense than the caused by the insufflated gas stream.
The use of the metal pipe according to the present invention, instead of the refractory pipe of the prior art, offers another advantage because the metal pipe does not require a porous plug or diaphragm at its end. It is known in fact that such porous plugs or diaphragms are subject to occlusions due to momentary decreases of the pressure of the insufflated gas with subsequent penetration and solidification of the liquid metal into the pores of the plug or diaphragm.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages, objects and characteristics of the process and device of the present invention will become evident to the skilled in the art from the following detailed description of one embodiment referring to the attached drawings in which:
FIG. 1 is a schematic sectional view of an apparatus for continuous casting according to the process of the present invention; and
FIG. 2 shows in correspondence of two different transverse sections of the bar in the act of solidifying, and the temperature trends in the continuous casting process of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the liquid metal, particularly steel, contained in the crucible 1 is discharged into hopper 2 from which pours out a jet 3 which falls into mould 4 forming a liquid bath or well 5. Rolls 6 guide the bar partially solidified 7 through a curtain of water sprays produced by a series of nozzles 8 mounted on a manifold 9. The liquid well 5, of substantially inverted-cone form whose base is the top free surface of the melted metal, has its lower vertex 10 at the point where the solidification is completed. Rolls 11 draw from the mould 4 the bar 7 which is subjected to secondary cooling.
In FIG. 1, which shows a primary realizable method of the process covered by the present invention, hopper 2 is displaced in such a manner that jet 3 falls into the mould in off-center position, but could also provide for two or more jets symmetrical in respect to the axis of the mould. The solid metal pipe 13, having the same composition as that of the melted metal and therefore of bar 7, is fed continuously through feed-rolls 12 into the center of liquid well 5. As stated previously, bar 13 also melts assorbing heat from the surrounding melted metal in the liquid well which for that reason becomes cooled. This further reduction of the temperature at the center of the solidifying bar results in a shortening of the liquid well 5 whose vertex 10 will be therefore at a higher point and with a greater angular opening with respect to an analogous representation of the solidification in a continuous casting process of the prior art. Inside the pipe 13 flows in direction 14 an inert gas which escapes toward the bottom of the area where the pipe is melting and then re-ascends toward the surface facilitating still more the complete melting of the tubular element 13 and directly cooling the center of the melted metal.
Any gaseous substance at room temperature, which neither reacts with the liquid metal nor modifies the metal properties of the melted mass, can be used as an inert gas according to the present invention. For example nitrogen or argon can be used to this purpose. The quantity of inert gas to be insufflated into the inner part of the melted mass is approprietaly chosen according to the volume of the melted mass contained in the mould, according to the extraction speed of the solidified bar as well as according to other factors known to those skilled in the art. A stream of nitrogen between 0.3 and 1.6 liter/minute has given satisfactory results in many cases.
The lower end of the pipe 13 is dipped into the inner part of the melted mass down to the desired depth, which is usually in the range between about 20 and about 80 centimeters. The metal pipe 13 is used in rolls of a considerable length. During the casting process the pipe 13 is first unrolled and then straightened by means of a known device which also provides for the continuous feeding of the pipe as its dipped end melts in the liquid well. The opposite end of metal pipe 13 is connected to the gas feeding piping through a known gas-tight rotative joint. The source of the fed inert gas can be a pressure tank or a plurality of metal bottles.
In FIG. 2, where below and in line with a schematic view in longitudinal section of a bar in the act solidifying according to the process of the present invention are the diagrams of the temperatures respectively in transverse sections AA and BB, with T s there has been indicated the temperature corresponding to the change from liquid state to solid state.
From the diagram it is evident that at the central point M of section BB the temperature is lower than that at point N thus causing a more rapid solidification in the axial area of the bar.
Although the invention has been disclosed in detail with reference to an embodiment thereof, it is to be understood that possible additions and/or modifications can be made by those skilled in the art remaining in the scope of the present invention.