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The Benefits of Zinc Jumbo Shapes and CGG Alloys for Continuous Galvanizing G.R. Adams and P.S. Kolisnyk Presentedat the South East Asia Iron and Steel Institute (SEAISI) Conference, Seoul, Korea, May 11-14, 1997

SouthEastAsia Iron.andSteelInstitute(SEAlS!) Conference 01.1,Session Paper V 2, 3/1-3/7,Seoul,Korea, May 11-14,1997 BENEFITS OF ZINC JUMBO SHAPES AND CONTINUOUS GALVANIZING GRADE ALLOYS FOR CONTINUOUS GALVANIZING OPERATIONS

P.S. Kolisnyk, B.A. Sc., Manager, Technical & Marketing Services,Cominco Ltd., Toronto Canada; G.R. Adams, M.A. Sc., Manager, Metallurgical Technology, Cominco Product TechnologyCentre,Mississauga,Canada.

ABSTRACT Continuous Galvanizing is the largest and the fastestgrowing market for zinc in the WesternWorld. This paperreviews recent growth in this sectorand describestrends in the use of large "Jumbo" ingots which have mostly replaced smaller zinc slabs as the preferred form for zinc bath additions in North America. This trend is expectedto continue in Asia as new Continuous Galvanizing Lines adopt modem material handling systems to reduce manual handling of zinc into the galvanizing bath. Improved plant practices,combined with a better understanding of the ternary Fe-AI-Zn phase diagram have resulted in major economic benefits for continuousgalvanizers. Much of this improvementhas resulted from more effective use of continuous galvanizing grades (CGG) of zinc, which are pre-alloyed with aluminum. The impact of using CGG strip jumbo zinc ingots on melting losses, product quality and processeconomicsare addressed herein.

1. INTRODUCTION

Continuous galvanizing is a proven process that adds durability to flat rolled steel products. Coated steel products are experiencing tremendousgrowth in all markets. Steel mills can increasethe value of their products by a protective and functional coating of zinc or zinc-basedalloys. Galvanized sheet can be painted, roll-formed, fastened, and stamped. Zinc is the "great protector" for steel. Galvanizing capacity in North America has increasedby about 100% over the last 10 years.. At the same time, there has been a trend away from High Grade (HG) zinc slabs (weighing 25 kg) towards Continuous Galvanizing Grade (CGG) zinc strip "jumbo" ingots, weighing about 1100 kg. While it is difficult to get exactnumbers,Table 1, which is based on a Cominco internal study2,showsthe trend for zinc usagefrom 1974to 1994.

Authors:

ADVANTAGES OF JUMBOS

Melting losses during remelting of the zinc result in skimmings which reduces the yield of zinc. The melting loss is usually proportional to the surface area of the ingot being melted. Cominco test results3 show that melting losses from a bundle of 25 kg,"?inc ingots (total weight of about 1 mt of zinc) are about 2% at 520 0 C. This compares to a meiting loss of 1% for a properly cast strip jumbo ingot (1.1 mt). Meltdown losses can represent a significant cost. At a zinc price of $1200/mt for a CGL using 7500 tpy of zinc, the ann\lal value of the extra skimmings is $27,000 including credit for the sale value of skimmings. In addition, the extra skmmings represent added work for the pot operator and creates the potential for dross pick-up onto the strip -a potential quality problem, especially for pre-painted coils.

Jumbos are also safer to handle at the galvanizingbath. A typical strip jumbo handling systemis describedbelow. Most systemsinvolve some form of automatedhandling that remove the pot operator from handling the ingots directly. In contrast, slab zinc ingots usually are hand-loaded into the kettle. Th~snot only represents repetitive lifting risk, but it a also exposes pot operatorto possiblemolten zinc splashingwhenchargingthe slabs. the Security is always a concernfor raw materialslike zinc, because their relatively of high value and easyresalepotential. Stripjumbo ingots needto be moved by forklift and are difficult to be hidden or taken from the storagearea. Conversely,25 kg slabs of ingot are readily portableand could possiblyrepresent theft threat. a Strip jumbos were designedfor more automated use by the CGLs. There are two cast-inholes at eachend of the ingot. Theseare a minimum diameter(76 mm or 3 inches)so as to allow a lifting chain and clevis to passthrough. Stainlesssteelproof chains (grade 316 or 316L) are normally used for loading the jumbos into the galvanizing pot. These austenitic gradesare more resistantto liquid metal embrittlementby the zinc. There are four comer legs and a centrebasepedestalfor a stripjumbo -designed to allow for easyforklift handling. The strip jumbo is usedas a standardzinc ingot shapein North America since it represents vast the majority of jumbos zinc ingot capacitycastby the zinc industry. ASTM is presentlypreparinga standardfor stripjumbo dimensions.4This is of greatadvantage the continuousgalvanizing to industry. It allows the design of standardized materials handling equipment that can accommodate severalzinc producersstripjumbos.

2.

POT FEEDING WITH JUMBOS

A summary of ideas used to handle strip jumbos is described here. This deals with general principles only, as the design and control of specific handling systems is 'beyond the scope of this overview. The most widely used method of adding jumbos to either the coating pot or pre-melt furnace is to suspend them from a mono-rail mounted hoist and to lower them into the pot as needed. The jumbo can be supported by tongs, C-hooks or slings. Feed rate can be controlled to some extent by electronically coupling the hoist drive to the molten zinc level. The jumbo is lowered in the near-vertical orientation, usually into the back of the coating bath, near the .strip inlet. When space is limited, the jumbo ingot can also be charged into the front of the bath, near the strip exit.

In either case, it is becoming standard practice to provide a guard to protect the submergedroll hardware. This helps to guide the jumbo into a safe location in the bath. In some cases,baskets are used to contain the melting ingot. An added benefit from using a basket is to introduce the ingot more slowly into the bath. Only half of the jumbo is immersedinto the zinc. This reducesthe amountof chill that the bath experiences due to the mass of the "cold" zinc ingot. It is noted that excessivechilling of the galvanizing bath is suspected causelocalized and dross(Fe2Ais) formation.s to ;

4. SITUATIONS WHERE SLAB ZINC SHOULD BE USED

There are two situations where slab zinc must be used on a COL. In the case of the lead-zinc lines, the pre-heatfor the steel strip is gained by immersion into a lead pot below the zinc box. This box typically contains about 15 -20 tons of molten zinc with controlledaluminum content. Slabs must be used in this situation, becauseof their lighter weight. This keepsthe zinc level nearly constantand avoids the localized cooling action that occurs when a large (1.1 mt) zinc jumbo block is immersedinto the relatively small pot. The chilling effect of the jumbo would causeextra dross formation that results in coating defects. For both of thesereasons,strip galvanizersusing the lead-zinc processmust use slabzinc.

Reducing variation in zinc temperature is one critical step in reducing the formation of intermetallic compounds (Fe2 AIs) or top dross.6 A recent study by Cockerill Sambre estimated zinc strip jumbo melting times of 12 -15 minutes in an inductor heated bath of 185 mt of zinc: A temperature drop of about 4 0 C was noted during the charging of the ingot. In most cases, inductors are now being set to switch to high power sooner, increasing molten metal circulation as the jumbo is being loaded, and thus help minimize temperature variations.

Some North American lines use a premelt furnace for making galvanizing alloy that is subsequently pumped over to the coating pot. Slabs or jumbos can be used in this situation.

3.

The higher melting losses of slabs still apply. but the premelt offers greater flexibility of zinc source -either slabs or jumbos. Disadvantages of a pre-melt is the higher energy losses and greater capital cost and floor space requirements. When a premelt system is used, aluminum toning alloy is commonly added, usually at a fixed ratio to the weight of zinc added, to II1akecontinuous galvanizing alloy for the coating pot. Several Zn -Al toning alloys are commercially used, ranging from 4 to 12% aluminum. The pre-melt pot is normally free of iron, so the formation of top dross (Fe2 AIs) is.minimized in the alloying pot. The iron-free pre-melt metal allows for more efficient recovery of the aluminum from the toning alloy. However, these methods generally require more experienced operators since there is more procedures needed to maintain proper coating bath management.

BENEFITS OF CGG ALLOYS Aluminum Additions

The aluminum content in the molten zinc has been well documented as the most critical control parameter in managing a galvanizing baths. Proper control of effective aluminum -the soluble amount of aluminum in the bath -ensures proper adhesion and formability, as well as reducing the amountof top dross as well as other surface defects. A revised ternary phase diagram for the zinc-rich comer of the Fe-AI-Zn system has been developed.9 This is now being widely used by North American and European continuous galvanizersas the standardsystemfor determiningeffective aluminum. By using CGG alloys, more efficient recoveryof aluminum into the bath is achieved, as comparedto when aluminum toning alloys are used. With toning alloys, the recovery of aluminum can be as low as 10%. This is mainly due to the localized concentration of aluminum at the surfaceof the bath when using toning alloys. Thesetoning alloys tend to float on the bath surface,because their lower density from the higher aluminum content. of Thus, an aluminum -rich phasewill often segregate the top of the bath during melting, to where it is readily convertedto top dross,sincethe bath is saturatedin iron. In comparison,when CGG alloys areusedto feed the bath, the ingots tend to sink into the bath, due to the much lower aluminum content. This allows for faster melting, less segregationof the aluminum and top drossproduction.tO 5.2 Cost of CGG vs. Self-Alloying

Under normal operatingsituations,the useofCGG alloy results in the lowest cost for alloying aluminum into the zinc bath. This statement basedon the normally lower dross is production when using CGG vs. self-alloying with toning alloy. Typical top dross is 90 95% zinc (with the balancebeing Fe and Al as Fe2AIs), thus drosslossesrepresenta waste of useable zinc. The modest premium for CGG (to offset purchased aluminum and extra analytical work at the zinc refinery) is generallymore than off-set by the higher zinc yield. 5.1 5.

The savings from using CGG alloy are increasedat higher basezinc prices. This is explained by the reducedwastageof increasinglymore valuablezinc when using CGG alloy.

A further benefit is the greater ease of operation using COO. The "guesswork" of toning alloy additions is removed from the bath management strategy and this ultimately should lead to higher yields of prime galvanized steel. Typically, analytical results for bath samples take hours to be completed and interpreted. In these periods, considerable off-prime steel can be galvanized. The need for bath analysis can be reduced when pre-alloyed COG zinc is used as the bath feed material.

5.3 CGG Alloy Compositions

Table 3 is a simplified breakdown of nominal CGG alloy grades along with the sheet product produced on the galvanizing line. This is based on North American product mix and CGG alloys actually used to control the bath aluminum.

Table 2 : General Breakdown of CGG Alloy as Compared to the Product Mix Made on the Line. Aluminum contentof

CGG

Steel Product % by weight)

Comments

CR auto -painted (galvanneal)

0.50

Lead-free improves corrosion performance for OTforming.

CR auto -unexposed

0.45

Heavier gauges than above.

CR sheet-construction

0.35

Usually with low lead additions for spangle.

light gauge sheet -pre-painted

0.55

roofing

Higher line speedand lower coating weight. Lead-free best for OTbends.

HR -construction

0.30 or 0.60

Strip entry temperature greatly affects CGG.

Aluminum usage is dictated by many factors, specifically gauge, line speed, coating weight, strip entry temperature, bath temperature, bath aluminum composition, and strip immersion time. I I A mathematical model proposed by Dr. Tang (see reference 10) is consistent with line practices in North America for COO alloys, in that these seven COL parameters can be used to approximately predict the nominal COO composition needed to maintain the galvanizing bath. Cominco's experience shows that unique COO alloys are usually preferred for each line because of the complex relationships between the operating parameters listed above. Further discussion on the applications of this model are beyond the scope of this paper.

6. CONCLUSIONS

1. Strip jumbo zinc ingots have grown to be the dominant zinc shapeused by continuous galvanizers in North America. This shape is safer to handle and reducesmelting losses as comparedto slabzinc. 2. CustomizedCGG (continuousgalvanizing grade)zinc alloys are most commonly used by galvanizing lines in North America. This is believed to be the lowest net cost method for aluminum additions for bath management: However, there are several lines successfully using HG (High Grade) zinc together with toning alloys, but bath managementpractices becomemore complexand critical when self-alloying is used. 3. The optimum CO alloy composition is dependenton coated sheetproduct specifications and line operatingparameters.

References

1 Cominco Market Research -Western World Continuous Galvanizing Survey (1995). ;see above footnote Internal Cominco Zinc Melting memo, Trail B.C., (1996). 4 ASTM Subcommittee B2.04, Zinc & Cadmium Products, Task Group, 1996. 5 Dr NY Tang, presentation to AISI Metallic Coated Sheet Practices committee, June 1995. 6 International Lead Zinc Research Organization (ILZRO) program ZCO8-2 summary report, 1995. (privateet al., Experimental Study & Mathematical Modelling of Zinc Ingot Melting 7 M. Dubois, to project supporters) Behavior in the Continuous Hot Dip Galvanizing Process, Galvatech 1995 Proceedings, Fhicago, ISS. S. Belisle, Physical Metallurgy of Zinc Coated Steel, TMS annual meeting, San Francisco

1994.N- Y Tang, Journal of Phase Equilibria 9 Dr.

1 see footnote 6 ...,. . 1 Dr. N- Y Tang, Modelling Alummum Enrichment In GalvanIzed Coatings, MetallurgIcal & Material Transactions A, Vol26A July 1995.

10

Table 1 : Zinc Alloy & Shape Trends for Continuous Galvanizing in North America, (1974 -1994), Total 'Capacity

Hot Dip'"

HG Slab

HG Jumbo

CGG Slab CGG Jumbo

Total Tons of Zinc

Galvanize Capacity (mt steel)

1974 mt % of zinc

47,000

41

28,000 24

19,000 17

21,000 18

115,000

2,750,000

1984 mt % of zinc

45,000 15

100,000

38,000

13

33

120,000 40

303,000

7,196,000

1994 mt % of nnc

40,000 8

132,000

27

60,000 12

259,000 53

491,000

12,662,000

These figures show that all the significant growth in zinc usage by continuous galvanizers beenin stripjumbos, most of which are CGG alloys. The use of HG zinc slabs has has actuallydecreased about15%,asmeasured total tons of zinc since1974. by by The percentageof continuousgalvanizing production using self-alloying (instead of purchasingpre-alloyed zinc) has reducedfrom 65% in 1974to 35 % in 1994. Virtually all new CGLs started since 1994 in North America have used CGG jumbos as their primary method of zinc addition and bath management. The main reasons thought to be: health & are safety issues,higher quality needs from the customeras well as an overall trend to reduced manpower on the lines. It is also generallyaccepted that CGG zinc alloys, tailored for each specific line conditions, represents lowestnet costalternative. the

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