Continuous Casting at The Connors Steel Works by J. T. Black, Jr.

The Connors Steel Works of the Connors Steel Div. of H. K. Porter Co., Inc., pioneered in the casting of billet size ingots; therefore, the change to continuous casting was a logical advancement. After several years of intensive study and research, Connors contracted Koppers Co. for a lock-ankey installation of two 2-strand continuous casting machines with the exception of stacking and palletizing equipment which Connors would design, build, and install. Connors' facility is the vertical bending type equipment consisting of two separate casting machines with horizontal run-out, cutting, and stacking arrangements, as shown in Fig. 1. Each casting unit is a double strand machine casting square billets from 3 in. to 6 in. and rectangles up to 4 x 7 in. with lengths of 3 ft. a in. to 13 ft. Continuous casting production started on March 31, 1964, on a oneturn basis. The start-up crew consisted of six men who had no previous continuous casting training. These men eventually would become leadermen of four crews that would take over production on a 168-hr basis. The second turn was added on June 29, 1964, and by October the plant was operating 24 hr per day, seven days a week (one turn on overtime) casting up to 31 heats a day. Training of the crews was provided by the Koppers Co. through August 1964. Connors presently casts 4%-in. square billets at a rate of 110 in. per minute, 5%-in. square billets at 70 in. per minute, and 4 in. x 7 in. rectangles at 85 in. per minute in a number of grades, primarily 1010 through 1090 with a few low alloy grades. The machines are fed from three electric furnaces; two 20-ton units and one 15-ton unit. The 15-ton unit has a spare shell and furnace shells can be replaced in less than 30 min. Tapping temperature is normally 2950°F when casting 1020 and less grades. Approximately 8 min are reJ. T. BLACK, JR. is general superintendent, Connors Works of the Connors Steel Div., H. K. Porter, Inc., Birmingham, Ala.

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Fig. 1-1) Hydraulic ladle shifter; 2) ladle and cradle assembly; 3) swing tundish; 4) mold assembly; 5) spray chamber; 6) mold oscillation drive; 7) pinch roll and drive assembly; 8) hydraulic bending roll; 9) straightener; 10) torch cut off; 11) stacking and palletizing equipment.

quired to move the ladles from electric furnaces to the ladle cradle over the casting machine with a temperature drop of about 25°F. The machines were designed for truly continous casting; and if both machines are operating and have not drained the ladle, the third ladle is seated behind one of the ladles that is pouring. When the ladle empties, it is removed (with cradle attached) by crane, the ladle behind is moved into pouring position by a hydraulic shifter, and casting continues without interruption of the molten metal to the molds_ When casting in this manner, the heats must be substantially the same grade. The tundishes are preheated to approximately 2200°F by natural gas burners mounted below the ladle support structure. The tun dish is positioned for casting by a motor-

driven swinging tundish holder equipped with a water cylinder for dumping the tundish at the completion of the cast. Molds are %-in. thick copper tubing with stainless steel jackets mounted in a welded steel housing. Three hundred gpm of cooling water is required for each mold. Level of the metal is controlled by a Cesium 137 gamma ray source on one side of the mold that transmits signals to two Geiger tubes on the opposite side. One tube starts the pinch rolls when the metal reaches a predetermined level, and the second tube varies the· speed of the pinch rolls in proportion to the metal level. Manual control of the machine is provided for by an operator's control console located to the side of the operator and close enough to the mold to allow full APRIL 1965, JOURNAL OF METALS-433

were experienced in the first few weeks of casting. It was determined that some were due to a heavy calcium carbonate deposit (on the coolant side of the molds) which restricted heat flow from mold to water thereby producing a very thin shell wall that would quickly reheat and run out. Steps were taken immediately to condition the water. Cooling water is pumped from a 41,500 gal recirculating sump and cooling tower of the following design: recirculation rate, 3200 gpm; temperature drop across tower, 20°F; make-up, 90 gpm; evaporation rate, 60 gpm; and bleedoff, 30 gpm.

Conditioning of Cooling Water

Fig. 2-0perator's control console is at the right, close enough to the mold to allow full view of the metal level.

view of the metal level. (See Fig. 2). Secondary cooling of the billet is provided by four 4-ft spray risers mounted to the spray chamber walls located immediately below the mold assembly. Approximately 100 gpm of spray water is required for each strand. As the billet emerges from the spray chamber, it is allowed to complete its solidification without induced cooling for approximately l4 ft before entering the pinch rolls. two upper and two lower. ' Immediately below the pinch rolls the starter bar is disengaged by rotating the complete bar 90 o. The bar is then lowered hydraulically into a pit below the first floor level. When the billet has emerged about 6 ft below the pinch rolls, a hydraulically actuated roll bends the strand to enable it to enter the guide chute which provides entrance to the straightener unit. The billet progresses from the straightener on a horizontal conveyor where it is automatically cut by MAPP gas-oxygen torches to a predetermined length, stacked, and palletized for removal to the billet storage area or to the rolling mill reheating furnaces. Approximately 35 min is required to cast a 20-ton heat and usually between 15 to 25 min is needed to prepare the machine for the next heat. With an average heat time of 2% hI' for the 20-ton furnaces and 272 hI' for the I5-ton furnace-the two casting machines are well within the melt shop requirements. As the merchant bar producer grudgingly accepts cobbles in the rolling mill-so must continuous casting producers accept breakouts of the molten metal. A breakout is draining of the molten center of the billet through breaks in the shell wall resulting from a number of conditions. Metal oxides, gas, slag, and refractories trapped in the billet 434-JOURNAL OF METALS. APRIL 1965

shell walls act as an insulation between the metal and the moldcreating hot spots that reheat and break through. Loss of mold oscillation or lubricating oil will allow the billet and mold to seize and part the shell walls. Hangers, i.e., metal poured over the top of the mold and extending down into the billet shell will also pull the billet apart. The most common of these causes during the first few months of operation were metal oxides, gas, etc. Breakouts usually occur just below the mold in the spray area with the metal enveloping the roller apron assembly. The strand must then be plugged off and the billet cut in two in order to run out the usable billet already cast. Mold and apron assembly must then be removed to be cleaned and realigned. An unusual number of breakouts

Bleed-off is used to regulate the number of times the water concentrates in the system. Recirculating water is tested for total hardness and bleed-off is regulated to control the total hardness at approximately 15 grains per gallon. Bleed-off is from a filter located in the main supply line. Concentrated sulfuric acid is pumped continuously to the recirculating cooling water to maintain a pH between 6.4 to 6.B. A zinc-chromate corrosion inhibitor is also pumped continuously to the water to protect all metals in the system, and both chemical feed lines are discharged into the hot well where the cooling water returns. At this point enough turbulence is provided for adequate mixing. Conditioning the water eliminated calcium carbonate buildup and greatly reduced the percentage of breakouts.

Mold Length Knowledge gained from increased production and experience led to questioning of the validity of our

~ig. 3.-At left. is the original 23-in. mold housing with conventional roller apron attached. At

right

IS

the 34-m. lengthened mold with new four-roll apron attached.

A

23-in. mold length. In July 1964 one casting machine was equipped to cast 53fs-in. billets. With the installation of the larger molds, no breakouts were experienced for the first week and comparatively few in the weeks that followed. It was felt that the lack of breakouts with the larger mold was due to a thicker shell wall resulting from the length of time the metal was in the mold. The 53fs-in. squares are cast at 60 to 75 in. per minute as opposed to 4%-in. squares cast at 110 in. per minute. With a thicker shell wall the breakouts that would normally occur from oxides and other materials pulled into the side of the casting would be less likely. Even though insulated by the entrapped material, enough of the shell wall would solidify to prevent remelting of the metal. A 41f4-in. mold length of 34 in. was decided on and two molds were promptly lengthened and put into operation. The results were so gratifying that the third arid fourth crews were scheduled, thereby bringing the operation to a full-time basis of IGS hr per week. Reliability of the operation was further increased by lengthening the 5 3/s-in. mold to 29 in. and plating the molds with .005in. hard chrome. The highest casting temperature from the ladle with the original mold was 2960°F. With the installation of the longer molds, heats with temperatures as high as 3000°F can be cast, thereby greatly improving the allowable temperatUre range, and thus permitting more heats to go to the tower without delay due to the high temperatures. The molds were lengthened with a minimum of delay, due to a mold design that can be lengthened with very little machining and expense. The original mold and the

Fig. 4 (left}-A) The experimental billets were extremely rhomboid. B} A new roller apron consisting of four V-type rolls supports the corners of the- billet to prevent it from becoming rhomboid.

Fig. 5 (above}-Units for stacking palletizing the billets on rails.

lengthened mold are shown in Fig.

swinging the launder to stop the flow of metal into the billet cavity and allowing the breakout to heal, the cast could be continued. Breakouts usually occur several inches below the mold; therefore, the molds are seldom removed due to breakouts. Life of the original molds varied from 35 to 90 heats depending on alignment and wear of the roller apron. The redesigned mold and apron units are pulled at approximately 200 heats for check up ana mold life is now from 200 to 400 heats, depending on wear of the mold resulting from repeated starter-bar insertions. Tundish refractory life now averages about 55 heats and we hope to improve this performance because we have experienced up to 92 heats per lining. Tundishes are lined with 4lh in. of 75% alumina Multex chemically bonded standard brick. The nozzles are %-in. zircon and are replaced new after every cast. The tun dish bottom has a solid 2 x 9 x 18-in. Multex slab to withstand the errosive action of the ladle stream. This block is replaced after 20 to 25 heats. Connors chose to design the stacking and palletizing units· because equipment was not available that would palletize the billets on rails as was the practice in small ingot production (see Fig. 5). This equipment receives one to four billets, depending on the number to be stacked, from a hydraulically actuated off-bearer. The entire stack is then rotated 90° to a vertical position where it continues to be moved horizontally on rail skids over the pallet. Each succeeding

3.

This so-called shot-in-the-arm motivated a further study of the roller apron unit. Although universally accepted as the proper support of the solidifying billet, we could not accept this multi-roller cage type arrangement as perfect. Extensive study of mold wear and failure revealed that the billet shell upon solidifying would become rhomboid and wear two opposing corners of the mold. The accepted purpose of the roller cage or apron is to correct this rhomboid condition and hold the billet central in the mold thereby providing equal cooling of all billet surfaces and support to the sides of the billet shell as it solidifies. We had long felt that the roller cage was not needed to support the sides of billets which were 6 in. or less, so a number of experimental heats were cast without the roller cage. This experiment proved that support of the billet sides was not necessary but support for centering the billet in the mold was very important. The experimental billets were extremely rhomboid and the billet obtuse corners which had pulled away from the mold greatly reheated (see Fig. 4A). A new roller apron • was designed that consisted of four V type rolls equipped with tapered roller bearings, housed and attached securely to the bottom of the mold assembly. These V type rolls support the corners of the billet to prevent it from becoming rhomboid-and by using a dummy set-up bar, the rolls can be set to hold the billet within a few thousand ths of perfect (see Fig. 4B). With this apron assembly the operators soon discovered, when experiencing a breakout, that by

and

• Patent applied for.

APRIL 1965, JOURNAL OF METALS-435

(supplying cooling water) are disconnected and the mold removed by crane as shown in Fig. 6. Molds are replaced in the same manner. The only changes required other than molds are replacing a 3-ft transition section of the starter bar, by removing a dowell pin, and a change of hydraulic pressure to the pinch rolls by adjusting the pressure control valve. Connors' two rolling mills produce a full line of merchant bars, 3 and 4-in. structural channels, a number of special shapes, and reinforcing bars-No. 3-18S. Continuous cast billets make possible longer hot bed lengths for larger sections such as No. 18S reinforcing steel that can now be produced in lengths up to 100 ft as opposed to only 60 ft using ingots.

Fig. 6-After quick-connecting hoses are disconnected, the mold is removed by a crane.

stack moves the preceding stack further until the desired number of billets are in place over the rail pallet. The rails and billets are then moved by crane to the storage area or direct to the reheating furnaces.

436-JOURNAL OF METALS, APRIL 1965

Provision in the basic design for quick mold replacement makes possible a size change in about 20 to 30 min. Molds are secured with four wedges that can be removed in seconds. Four quick connecting hoses

Casting of a 4 x 7-in. slab makes possible the production of 4-in. angles, and a feasibility study is now being made of casting beam blanks for production of 5 and 6in. channels. The cold finishing department is presently producing cold drawn products with hot stock produced from continuous cast billets without condi tioning. Scrap loss through processing of continuously cast billets is approximately one-half that previously encountered on conventionally cast ingots. Rejection of material in the finishing department has been reduced from approximately 8% to less than .25%, with most of these rejects resulting from defects caused by the mill, hot bed, or shearing. Continuous casting is tailormade for companies in small ingot practice.