<div class="application article clearfix" id="description">
<p class="printTableText" lang="en">4 <br><br>
Priority Date(s) <br><br>
Complete Specification Fifed: <br><br>
Class: (6) <br><br>
Publication Date: ....2..S..!iQ.¥...193§.. <br><br>
P.O. Journal No: lAh!S <br><br>
NEW ZEALAND PATENTS ACT, 1953 <br><br>
No.: Date: <br><br>
COMPLETE SPECIFICATION <br><br>
//* ;15 DEC 1994 ;-O ;"CASTING MILD STEEL STRIP" ;We, ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES COMPANY LIMITED, a ;Japanese company of 2-1, Ohtemachi 2-Chome, Chiyoda-Ku, Tokyo, Japan and BHP STEEL (JLA) PTY LTD, an Australian company of 1 Castlereagh Street, Sydney, New South Wales 2000, Australia hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- ;- 1 - ;(followed by page la) ;»35 ;cur / ;- la - ;TECHNICAL FIELD ;This invention relates to the casting of steel strip and has particular application to the production of 5 mild steel strip. ;It is known to cast metal strip by continuous casting in a twin roll caster. In such a process, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal 10 shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product which is delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest 15 together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the 20 rolls immediately above the nip and extending along the length of the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls so as to dam the two ends of the casting pool against outflow. ;25 Twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, for example aluminium. However, there have been problems in applying the technique to the casting of ferrous metals. One particular problem has been the 30 propensity for ferrous metals, particularly mild steel, to produce solid inclusions which clog the very small metal flow passages required in a twin roll caster. ;The use of silicon-manganese in ladle deoxidation of steel was practised in ingot production in the early 35 days of Bessemer steelmaking and as such the equilibrium relations between the reaction product molten manganese silicates and the residual manganese, silicon and oxygen in ;- 2 - ;solution in steel are well known. However in the development of technology for the production of steel strip by slab casting and subse<iuent cold rolling, silicon/manganese deoxidation has generally been avoided, 5 except as discussed below, and it has been considered necessary to employ aluminium killed steels . In the production of steel strip by slab casting and subsequent hot rolling followed often by cold rolling, silicon/manganese killed steels produce an unacceptably 10 high incidence of stringers and other defects resulting from a concentration of inclusions in a central layer of the strip product. ;The tendency to produce inclusions in a band or layer by the slab casting process is reduced by using 15 aluminium killed steels and so silicon/manganese killed steels have generally been regarded as unsatisfactory for production of steel strip. Accordingly it has previously been thought that in order to continuously cast steel strip in a twin roll caster it would be necessary to use 20 aluminium killed steel. However in the continuous casting of steel strip it is critically important to generate a finely controlled flow of steel at constant velocity alon^r the length of the casting rolls to achieve sufficiently rapid and even cooling of steel over the casting surfaces 25 of the rolls. This reqpiires that the molten steel be constrained to flow through very small flow passages in refractory materials in the metal delivery system under conditions in which there is a tendency for solid inclusions to separate out and clog those small flow 3 0 passages. This is a particularly serious problem in the casting of mild steels which have significantly higher melting points and higher viscosity than high alloy stainless steels and are therefore sigrnif icantly harder to cast. ;3 5 After an extensive program of strip casting various grades of steel in a continuous strip roll caster we have determined that aluminium killed mild steels or ;- 3 - ;partially killed steel, with an aluminium residual content of 0.01% or greater generally cannot be cast satisfactorily because the solid inclusions agglomerate and clog the fine flow passages in the metal delivery system to form defects 5 and discontinuities in the resulting strip product. ;Surprisingly, we have found that these problems can be overcome by keeping the aluminium content below 0.01% by weight and by using a silicon/manganese killed mild steel with a carefully selected range of silicon and manganese 10 contents tending to produce liquid deoxidation products at the casting temperature. Moreover it has been found that it is possible to cast mild steel strip product without stringers and other defects normally associated with silicon/manganese killed steels. This is because the rapid 15 solidification achieved in a twin roll caster avoids the generation of large inclusions and the twin roll casting process results in the inclusions being evenly distributed throughout the strip rather than being concentrated in a central layer. ;20 In United States Patent 3,412,781 by Richards et al a process for continuous casting steel slab is disclosed in which the composition of the steel melt is adjusted to contain 0.01 to 0.08% carbon, 0.20 to 0.60% manganese, 0.03 to 0.08% silicon, and not more than 0.015% aluminium. It 25 has been found that in twin roll strip casting it is not possible to cast steel of this composition to produce acceptable quality strip. ;In United States Patent 4,529,441 by Smith, a process for continuous casting electrical steel is 30 disclosed in which silicon in the range of 0.05 to 0.25% added back to melt having a composition of up to 0.06% carbon, up to 0.04% sulphur, up to 0.15% phosphorus, up to 1.0% manganese, and not more than 0.001% silicon. In the examples disclosed in this patent, it can be seen that the 35 steel is partially killed with aluminium. More specifically Example 1 employs 0.015% aluminium, Example 2 has 0.012% aluminium and Example 3 has 0.020% aluminium. ;27 ;- 4 - ;It has been found that the inclusion of aluminium in such amounts is deleterious to continuous strip casting of steel. ;DISCLOSURE OF THE INVENTION 5 According to the invention there is provided a method of continuously casting metal strip of the kind in which molten metal is introduced into the nip between a pair of parallel casting rolls via a metal delivery nozzle disposed above the nip to create a casting pool of molten 10 metal supported on casting surfaces of the rolls immediately above the nip and the casting rolls are rotated to deliver a solidified metal strip downwardly from the nip, wherein the metal is silicon/manganese killed mild steel having a manganese content of not less them. 0.20% and 15 a silicon content of not less than 0.10% by weight, wherein the ratio of manganese to silicon is in the range of 1.4:1 to 2.8:1 by weight and wherein the total aluminium content of the steel is below 0.01% by weight. ;The invention further provides a method of 20 continuously casting metal strip of the kind in which molten metal is introduced into the nip between a pair of parallel casting rolls via a metal delivery nozzle disposed above the nip to create a casting pool of molten metal supported on casting surfaces of the rolls immediately 25 above the nip and the casting rolls are rotated to deliver a solidified metal strip downwardly from the nip, wherein the metal is silicon/manganese killed steel having a carbon, manganese and silicon content in the following ranges: ;30 Carbon 0.02 - 0.15% by weight ;Manganese 0.20 - 1.0% by weight ;Silicon 0.10 - 0.5% by weight and wherein the total aluminium content of the steel is below 0.01% by weight. ;35 For optimum operation in the production of steel strip of 1 mm to 4 mm thickness, it is preferred that the carbon, manganese and silicon contents be in the following ;27 ;- 5 - ;ranges: ;Carbon 0.05 - 0.10% by weight ;Manganese 0.40 - 0.80% by weight ;Silicon 0.10 - 0.30% by weight. ;5 BRIEF DESCRIPTION OF THE DRAWINGS ;In order that the invention may be more fully explained its application to the continuous casting of steel strip will be explained with reference to the accompanying drawings in which: ;10 Figure 1 is a plan view of a continuous strip caster which may be operated in accordance with the present invention; ;Figure 2 is a side elevation of the strip caster shown in Figure 1; ;15 Figure 3 is a vertical cross-section on the line ;3-3 in Figure 1; ;Figure 4 is a vertical cross section on the line ;4-4 in Figure 1; ;Figure 5 is a vertical cross-section on the line 20 5-5 of Figure 1; and ;Figure 6 illustrates the results of trial casts using steels of varying compositions. ;BEST MODE FOR CARRYING OUT THE INVENTION ;The illustrated caster comprises a main machine 25 frame 11 which stands up from the factory floor 12. Frame 11 supports a casting roller carriage 13 which is horizontally movable between an assembly station 14 and a casting station 15. Carriage 13 carries a pair of parallel casting rollers 16 to which molten metal is supplied during 30 a casting operation from a ladle 17 via a tundish 18 and delivery nozzle 19. Casting rollers 16 are water cooled so that shells solidify on the moving roller surfaces and are brought together at the nip between them to produce a solidified strip product 20 at the roller outlet. This 35 product is fed to a standard coiler 21 and may subsequently be transferred to a second coiler 22. A receptacle 23 is mounted on the machine frame adjacent the casting station ;- 6 - ;27014 ;and molten metal can be diverted into this receptacle via an overflow spout 24 on the tundish or by withdrawal of an emergency plug 25 at one side of the tundish if there is a severe malformation of product o.r other severe malfunction 5 during a casting operation. ;Roller carriage 13 comprises a carriage frame 31 mounted by wheels 32 on rails 33 extending along part of the main machine frame 11 whereby roller carriage 13 as a whole is mounted for movement along the rails 33. Carriage 10 frame 31 carries a pair of roller cradles 34 in which the rollers 16 are rotatably mounted. Roller cradles 34 are mounted on the carriage frame 31 by interengaging complementary slide members 35, 36 to allow the cradles to be moved on the carriage under the influence of hydraulic 15 cylinder units 37, 38 to adjust the nip between the casting rollers 16 and to enable the rollers to be rapidly moved apart for a short time interval when it is required to form a transverse line of weakness across the strip as will be explained in more detail below. The carriage is movable as 20 a whole along the rails 33 by actuation of a double acting hydraulic piston and cylinder unit 39, connected between a drive bracket 40 on the roller carriage and the main machine frame so as to be actuable to move the roller carriage between the assembly station 14 and casting 25 station 15 and vice versa. ;Casting rollers 16 are contra rotated through drive shafts 41 from an electric motor and transmission mounted on carriage frame 31. Rollers 16 have copper peripheral walls formed with a series of longitudinally 30 extending and circumferentially spaced water cooling passages supplied with cooling water through the roller ends from water supply ducts in the roller drive shafts 41 •which are coimected to water supply hoses 42 through rotary glands 43. The roller may typically be about 500 mm 35 diameter and up to 1300 mm long in order to produce 1300 mm wide strip product• ;Ladle 17 is of entirely conventional construction ;- 7 - ;270147 ;and is supported via a yoke 45 on an overhead crane whence it can be brought into position from a hot metal receiving station. The ladle is fitted with a stopper rod 46 actuable by a servo cylinder to allow molten metal to flow 5 from the ladle through an outlet nozzle 47 and refractory shroud 48 into tundish 18. ;Tundish 18 is also of conventional construction. It is formed as a wide dish made of a refractory material such as magnesium oxide (MgO) . One side of the tundish 10 receives molten metal from the ladle and is provided with the aforesaid overflow 24 and emergency plug 25. The other side of the tundish is provided with a series of longitudinally spaced metal outlet openings 52. The lower part of the tundish carries mounting brackets 53 for 15 mounting the tundish onto the roller carriage frame 31 and provided with apertures to receive indexing pegs 54 on the carriage frame so as to accurately locate the tundish. ;Delivery nozzle 19 is formed as an elongate body made of a refractory material such as alumina graphite. 20 Its lower part is tapered so as to converge inwardly and downwardly so that it can project into the nip between casting rollers 16. It is provided with a mounting bracket 60 whereby to support it on the roller carriage frame and its upper part is formed with outwardly projecting side 25 flanges 55 which locate on the mounting bracket. ;Nozzle 19 may have a series of horizontally spaced generally vertically extending flow passages to produce a suitably low velocity discharge of metal throughout the width of the rollers and to deliver the 3 0 molten metal into the nip between the rollers without direct impingement on the roller surfaces at which initial solidification occurs. Alternatively, the nozzle may have a single continuous slot outlet to deliver a low velocity curtain of molten metal directly into the nip between the 3 5 rollers and/or it may be immersed in the molten metal pool. ;The pool is confined at the ends of the rollers by a pair of side closure plates 56 which are held against ;?G 1 4 / ;- 8 - ;stepped ends 57 of the rollers when the roller carriage is at the casting station. Side closure plates 56 are made of a strong refractory material, for example boron nitride, and have scalloped side edges 81 to match the curvature of 5 the stepped ends 57 of the rollers. The side plates can be mounted in plate holders 82 which are movable at the casting station by actuation of a pair of hydraulic cylinder units 83 to bring the side plates into engagement with the stepped ends of the casting rollers to form end 10 closures for the molten pool of metal formed on the casting rollers during a casting operation. ;During a casting operation the ladle stopper rod 46 is actuated to allow molten metal to pour from the ladle to the tundish through the metal delivery nozzle whence it 15 flows to the casting rollers. The clean head end of the strip product 20 is guided by actuation of an apron table 96 to the jaws of the coiler 21. Apron table 96 hangs from pivot mountings 97 on the main frame and can be swung toward the coiler by actuation of an hydraulic cylinder 20 unit 98 after the clean head end has been formed. Table 96 may operate against an upper strip guide flap 99 actuated by a piston and a cylinder unit 101 and the strip product 20 may be confined between a pair of vertical side rollers 102. After the head end has been guided in to the jaws of 25 the coiler, the coiler is rotated to coil the strip product 20 and the apron table is allowed to swing back to its inoperative position where it simply hangs from the machine frame clear of the product which is taken directly onto the coiler 21- The resulting strip product 20 may be 3 0 subsequently transferred to coiler 22 to produce a final coil for transport away from the caster. ;It has been found in the operation of the above apparatus that aluminium killed steel will not produce a satisfactory strip product because of clogging problems 35 caused by aluminate inclusions when the liquid steel passes through the small orifices in the tundish 18 and delivery nozzle 19- This disturbs the even flow of steel to the ;0147 ;- 9 - ;casting pool with the result that a continuous strip product cannot be produced or a product is produced with severe defects. Silicon and manganese are very effective deoxidants in combination and they act together with 5 synergistic effect. We have found that a silicon/manganese killed steel used in a strip caster of the illustrated kind can produce steel strip with acceptable oxygen levels.' The caster can be operated so that the resulting strip has fine, evenly distributed inclusions in the form of 10 manganese silicates which do not have serious detrimental effect on the mechanical properties of the strip even when their density is moderately high with oxygen contents of about 100-150 ppm. This level of oxygen in aluminium killed steels would be unacceptable because aluminate 15 inclusions cluster together and can form planar defects in the strip. The silicate inclusions do not form clusters and in any event the operation of the strip casting is such that it restricts their agglomeration to form excessively large inclusions which would be detrimental to the 20 mechanical properties of the strip. Moreover, since the strip product is not rolled out extensively the inclusions are not strung out. ;,;vith silicon and manganese levels selected in accordance with the present invention, it is possible to 25 maintain the deoxidation products in the liquid state during the casting operation, thus eliminating clogging problems in the restricted orifices of the tundish and metal distribution nozzle. ;The carbon, manganese and silicon levels selected 3 0 in accordance with the present invention are such as to maintain the solidification process in the fully ferritic (§) region. Ideally the composition is selected to maximise the difference between the liquidus and solidus temperatures to maintain a sufficient mushy layer during 35 casting as this makes the quality of the resultant cast strip less susceptible to the detrimental effects that disturbances to the casting pool can have during the rapid ;- 10 - ;14 ;solidification process. However it is important not to come too close to the austenite plus ferrite region on the equilibrium phase diagram "since equilibrium is not maintained under the very rapid cooling conditions in the 5 strip caster and some care must be taken so as not to inadvertently have solidification occur in the austenite plus ferrite region. ;Increasing the silicon level and decreasing the carbon level promotes the formation of 5 ferrite during 10 solidification and so reduces the tendency to fozm uneven strip surfaces known as "crocodile skin" defects. These defects are usually associated with localised variations in cooling rate (and subsequently variations in microstructure) caused by inconsistent contact of the 15 solidifying shell with the cooling roll surfaces. ;If the carbon content is too low a defect known as "herring bone" can occur. This defect is a periodic thickness variation which essentially extends across the strip width (or at an angle to the strip width direction) 20 and is thought to be associated with the behaviour of the mushy zone as the solidifying shells are brought together at the rolls nip. If the temperature range in which the . mushy layer exists is too narrow, the disturbances at the casting pool meniscus can have a significant influence on 25 the thickness and viscosity of the mushy layer and so can influence the forces that the strip exerts on the casting rolls, which subsequently affect the strip thickness. ;We have carried out extensive operations of a strip caster of the illustrated kind with various grades of 30 steel. We have determined that for the production of steel strips of thicknesses ranges from about 1 mm to 4 mm, the optimum steel composition is as follows: ;Carbon 0.05 - 0.10% by weight ;Manganese 0.50 - 0.70% by weight ;3 5 Silicon 0.20 - 0.30% by weight ;Aluminium less than 0.008% by weight. ;The steel is preferably preheated to a ;270147 ;temperature in the. range 1500°C to 1600°C for casting. Preferably the tundish is preheated to a temperature in the range 1000°C to 1300°C and the delivery nozzle is also preheated to a temperature in the range 1000°C to 1300°C. ;Figure 6 illustrates the results of various trial casts using mild steels of various silicon and manganese contents and atmospheric melting. In this figure, the points indicate steels which have been cast successfully to produce steel strip of good quality whereas the crosses identify steels which have consistently resulted in casting problems and strip defects. The trials have indicated that steels having compositions falling within Region A in Figure 6 produce strip sheeting due to poor deoxidising. ;Steels with manganese contents greater than 1.0% by weight and therefore falling within the Region B produce casting problems due to MnO fume deposits on the casting rolls. Steels with silicon contents greater than 0.5% by weight and therefore falling within the Region C result in strip with poor forming characteristics. ;It will be seen from Figure 6 that the band of positive results conforms to a line of best fit corresponding to a manganese to silicon ratio of 2:1 by weight. More specifically, the band of positive results generally corresponds to manganese to silicon ratios in the range of 1.4:1 to 2.8:1 by weight and ratios within this range are clearly desirable in order to avoid casting problems. *<br><br></p>
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