Adoptions and improvements of Continuous Steel Casting in India ( An Experience to share with Mr. S.N. Singh)

Introduction

In the last four decades, ever since continuous casting technology for solidification of crude or refined steel started millions of cusecs of water has flown down the river Ganges and so also millions of tons of steel has been produced albeit water in Ganges getting dirtier day by day while steel produced are getting cleaner and cleaner! 

Before conti casting came into commercial production line of steel mills, the liquid steel to prime finished or semi finished products used to be only in 50 to 60% range which today is as high as 95 to 97%! Yield also being a factor depending on heat size that improves considerably from 30 tons to 300 Tons or more even!

Starting with 2 to 3 million tons coverage by end of seventies today steel produced through conti caster are over 90% of capacity i.e. over 1200 MTPA in total steel production of 1350 MTPA. Definitely conti caster  technology has firmly established itself in last four decades.

This paper covering the brief history of conti-casters globally and in India, also refers to certain problems in casters and their general solutions besides reference to latest technological up-gradation done or being done or to be done.

Continuous casting is a vast field on technological and operational considerations; yet the paper tries to cover fundamentally both good and problematic aspects in very brief!

Contents of the paper are taken from the personal diary and archives of the author and some are from monthly reports of working plants in India and abroad.

Any question or point of difference will be replied only through e-mail of the author or through the organizers of the seminar.

Further, to mention here is the fact that; this paper is based entirely on practical observations and experience and does not boast of any highly technical aspects of continuous casting machines and its technology thereof! 

A Typical 2 Strand Slab Caster

salbcaster.jpg

Brief History of Conti Casters

Starting with both vertical and horizontal casting set up way back in the year 1968-69, in Europe, America and USSR, finally it became the combination of the vertical and horizontal systems as nomenclature “CURVED/STRAIGHT MOLD - apron/segment” configuration. Starting with billet, day by day sections got improving further and further to now come several types known as:

  1. Billet Casters: Curved mold of radius 4 M to 10 M producing cross sections 80x80 mm2 to 250x250 mm2. It was started even while there was no ladle purging and slide gate systems or ladle refining system until beginning of eighties. Kudos therefore to inventor and propagator of conti caster systems who despite many an odds in the beginning ventured and achieved right results, replacing conventional ingot casting by top or bottom pouring into molds! Most of these billet casters initially had metering nozzles with some kind of shrouding with loose Nitrogen gas. Later on; there came refractory tube shrouding, developed by M/S Vesuvius, Flocon, Interstop and others simultaneously in the year 1984-85.
  2. Bloom Casters: In curve mold only, could start commercial production in sizes above 200x200 mm2 as substitutes to thick cogged blooms going for wide and thicker rolled sections of common construction quality steels while for steel grades meant for crankshaft, automobile purpose gears, shafts, connecting rods, aviation, defense, line pipes etc, needing higher reduction, better inclusion ratings, lower O2 and gases etc after about 5 years of billet making machines. Also bloom casters got good acceptance when Ladle Refining furnace and Black refractory managed submerged casting methods could develop. Bloom casters usually are made in radius of curvature 8 M to 16 M as per final section of the product. Can be multi strands 2 to 6 performing casting at speed of 1 ton to 3 tons per strand per minute.
  3. Slab Casters: Starting with vertical type finally establishing in curved movement, after mold assembly. Developments in slab casters made flat steel production so easy and convenient! Slab casters highest section capability today is as much as 5000 mm x 350 mm! Radius of curvature in the cooling segments may be 6 M to 14 M or higher even!
  4. CSP: Compact slab production starting in early nineties in different names had commercial production since 1993. CSP is combination of both vertical and curved cooling systems. These plants produce slabs in 60 to 110 mm thickness range and generally width less than 3000 mm. They are found quite friendly for soft reduction and significantly reduces roles of roughing stands in hot rolling. These casters however are not found as apt technology for specialty steels like Silicone steel, API X 60 or even IF grades. However it has firmly established itself for commercial flat steel products.
  5. No. of strands starting with single 4 meters radius now the world has as many as 10 strands in one single billet caster, 6 strands in bloom caster, 5 strands in bloom blank casters and 2 strands in slab casters.  Corresponding to end products’ shaping, desired reduction ratio at high speed output, casters height (pouring in to Tundish) is as much as 16 meters. In India as of now maximum number of strand on billet caster is 7, in bloom caster 5 and in bloom blank caster 3 whereas slab caster generally are single strand.

Chronologically Important Landmarks in Conti casters

 

Period
Event in developed countries
In India
In China
1968-69
Vertical and 4/5/6 M billet casters
No
Yes
1970-71
Slide gate system started
No
Yes
1972-73
Bloom caster & Top purging starts
1strand vertical caster starts
NO
1974-75
Shrouding + Hot tundish start
Curved  Billet Caster starts 
Bloom Casters start
1976-77
Ladle Purging, Ladle Fce, VAD start
N2/Ar Gas Shrouding
All  like developed countries
1978-79
Thicker Blooms/BB + AMLC came in
More and More casters
Do
1980-81
Ca-Si injection and Al Killing
Bloom casters start
Limited Ca-Si injection
1982-83
Use of slide gate in tundish
LRF and VAD
As in developed countries 
1984-85
Thick slab casters + EMS came
Ca-Si injection
Slab casters start
1986-87
Twin strand slab casters
Refractory tube shrouding 
Multi strand casters
1988-89
Hydraulic oscillation starts
Hot Tundish
Like developed countries     
1990-91
Digital PLC & Improved automation
AMLC and EMS came
Do
1992-93
Freedom from scarfing & grinding
More Casters + BB starts
Scarfing limited
1994-95
Limited Ca-Si injection, CSP
Slab Casters started
Process improvisation
1996-97
Speeds raised + Online width change
CSP + scarfing reduced
CSP
1998-99
Highly perfected parameters
Grinding reduces
Hot Charging 70%
2000-01
B/O prediction system
Nothing serious
Better through-put
2002-03
Non-Sinusoidal oscillation
Round Bloom produced
Quality leap
2004-05
Product perfection
Quality concerns
Big growth
2006-07
Stabilized everything
Throughput improves   
All kinds of records 
2008-09
No new plants in Europe, America 
Growth continued
Big growth continued
2010-11
Quality and Yield as main focus
Lot of new plants
All latest and biggest

Landmark events/jobs in India on Casters 

Vertical single strand with top pouring ladle started in 1972-73 by Tata Growth shop, Adityapur.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  • 4 M curved mold billet casters started in Mini steel sectors by Mukand, Poly steel, Bhoruka, Cap steel, Zenith and others in 1974-75 and then followed by others which went up to 130 mini steel plants in next 7 years!
  • First Bloom Caster started by Mukand Iron in 1982-83
  • First Bloom cum narrow slab duplex caster started by ASP Durgapur in 1983-84.
  • In Govt sector first Slab (vertical) caster started by Bhilai Steel Plant in 1987-88
  • TISCO started first slab caster (curved ) in 1992
  • In private sector first slab caster was started in 1994 at Lloyds steel and thereafter many plants came up such as ESSAR, JSL, JVSL (JSW), Tisco, Sail and others.
  • First hot tundish and full refractory shrouding started at ASP, Durgapur in 1985 followed by others.
  • First slide gate in Ladle started in India in 1981 by flo-con at ASP Durgapur. Simultaneously started in other Sail plants and TISCO. Private sector plants adopted it in 1982.
  • First Ladle Refining Furnace started at Mukand in 1980 followed by Sunflag, Ispat and all other mini and mega steel plants.
  • The first VAD in India started at ASP Durgapur in 1982-83; where as first RH degassing also started at ASP in 1968-69
  • First CSP started by Ispat India in 1997-98.
  • First Chinese AMLC started by Lloyds Steel, replacing MDH system.
  • First highly successful Chinese auto slab cutter started by Lloyds steel in 2005.
  • Use of EMS in bloom casters started in ASP and BSP in 1985/86 followed ISPAT Pune; TISCO, SAIL and SUNFLAG in 1990. Thereafter other new up coming plants including Kalyani, Indian Seamless and others followed. Rashtriya Ispat Nigam started EMS associated bloom caster in 1992.
  • Use of Mold brakes in slab casters still not started in India
  • Use of slag detection system yet to start in India. Globally it is in use in 90% plants since 2006.
  • Use of non-sinusoidal oscillation system is yet to start in India, while globally 80% plants go for it.
  • Boom in Indian Steel Sectors were in 1973-78, 1985-88, 1992-98 and 2005-onwards. Real doom periods were 1979-83 and 2000-03!
  • Demand picks up in Indian Steel market by 15% in recent times after 2005 but capacity growth is 10-12%. There is great scope to upgrade same caster machines technologically to cope up with higher liquid steel outputs in same plant.
  • Indian plants are yet to incorporate online width change in slab caster or black refractory change in bloom/slab caster.
  • Breakout prediction equipment mostly to protect machine and restart of machine faster; although of no value in good plants are there in very few plants in India.
  • Other International- Chinese bests and Indian best figures in Iron, Steel making, shaping and treating (in the following table).

Table showing performance land marks

TITLE/TOPIC
International Figs
Chinese Figures
Indian Figs
Gross energy in Gig Cal (In Iron & Steel Making)
4.0/4.5
4.80/5.50
6.50/7.50
BF productivity Tons/M3
1.8/3.2
2.4/4.0
1.6/3.0
No of heats/day/Fce in SMS
35/40
45/50
30/35
Throughput in slab caster T/hr max
200
250
60/150
Throughput in Billet caster T/Str/Hr
20
25
15
Throughput in Bloom caster T/Str/Hr
30
40
25
Average Utilization Factor of Con Cast
90%
95%
60%
Breakouts at Casters
1/150000
1/250000
1/50000
Breakouts at Slab Caster: Nos
1/5 years
1/7 years
1/2months
Nozzle Choking No/No of Casts
1/100000
1/120000
1/100
Maximum output/tundish: Tons
4000
5000
2000
Grinding/scarfing of blooms/billets/Slabs (After submerged nozzle/closed casting)
5%
2%
30%
Central shrinkages in Blooms/Billets(In mm)
0-0.5
0-1.0
0-3.0
Liquid Steel to prime yield
98%
98.5%
96%
Average “S” finish
<0.02%
<0.01%
>0.025%
Ca Si/Ca Fe addition % of special heats
<5%
<5%
80%
Ultrasonically perfect
>90%
>80%
<60%

Why Indian Caster units do not compare to international performance level?

Breakouts are high due to:

  • Bad water
  • Bad mold tubes by plating/profile
  • Rising/Semi killed metal at times
  • Poor quality lubricants
  • Bad oscillation
  • Bad hardware like hoses, nozzles.
  • Big superheat operating range.
  • Human interference
  • Oscillation not smooth.
  • Slag teeming in the Tundish

All above reasons are controllable but may be that alacrity is missing sometime in Indian scenario.

Caster Utilization factor is also not comparable to China or the world due to:

  • More breakouts
  • Lower through put or casting speed.
  • Short Tundish life.
  • ONLINE WIDTH CHANGE still not there in most/None of slab casters in India!
  • Online maintenance, old styled.
  • Failures like hose leakage, choking, refractory below par all leading to shorter continuity!
  • Bad Tundish practices including slag inflow into Tundish etc.

Odd, Old and Ugly practices still going on in many Indian plants:

  • Very slow/low casting speed
  • Computerization and interfacing features not adequate.
  • Steel quality and temperature management lacking narrow range.
  • Pointless/reasonless scarfing or grinding of blooms and slabs.
  • Electro mechanical old oscillation system still going on.
  • Generally high water consumed than theoretically or really desirable as per super heat or secondary segments configuration.
  • SCARFING and Grinding still going on in big way in many plants. Such plants will always shudder to do hot charging of blooms/billets/slabs & unless they are free of surface defects.

COMMON CCM PRODUCTS’ DEFECTS IN INDIAN PLANTS

Bloom/Billets and Slabs and their Origins / Solutions:

  • Higher central shrinkage/piping/laminations are high in Indian Blooms/Billets due to: 
a. High temperature, d. High Sulphur,
b. Slower casting speed, e. Too much secondary cooling,
c. Level fluctuation, f. Improper software etc;

While in slabs big/small/tiny, centrally located longitudinal piping is found especially in medium carbon grades due to mostly low speed casting, excess secondary cooling and improper guide rolls.

  • Cracks surface / corner cracks: Can happen due to too much secondary cooling, bad mold surface, thin chilled zone and inadequate negative strip setting! Too low casting speed and poor quality mold lubricant ( casting powder or vegetable oil) can also add to cracks on surface and/or corners.
  • Cracks at centre: Happens mostly due to high inclusions in the steel, high superheat, high Sulphur, inverse cooling in the secondary and inadequate metallurgical head.
  • Pin holes/blow holes: Consequence of inadequate de-oxidation and killing or moisture containing Oil/solid powder lubricants, too much Argon in stopper of the Tundish and some time if steel is having Nitrogen beyond 120 ppm in Silicon Killed grades.
  • Double Skin: Although unusual but can happen if level fluctuation is high, metal has sticking tendency on mold wall, meniscus disturbed under manual interference and hold followed by normalization in casting.
  • Rhombodity: Mostly in billets and lesser in blooms and slabs can happen if taper in mold or mold assembly is not enough (more than 0.7%); billet mold chrome plating vanishing, water cooling not symmetrical or uniformly configured on mold walls, high superheat and chilled wall very thin! The problem gets more aggravated if it is peritectic steel. Generally highly rhombus billet/bloom do have diagonal crack due to differential solidification in X and Y axes and solidification stress. Mechanically fragile apron or segments or support rolls can also promote Rhombodity!
  • Bulging: Be it slab or bloom or billet, it happens either for secondary cooling water hose detachment or choking of nozzles or by default very high casting speed and corresponding secondary cooling water starvation!
  • Other defects: Could be there but not mentioned here as they are not universal. All sundry defects are solvable under technical due diligence and case study. Defects like Coarse grains, lot of dendrites, core not equi-axed and severe oscillation marks etc are all curable in regular course.

CONCLUSIONS

    • Buy machines mechanically sturdy!
    • Do not buy low head machines if inner soundness of products has to be maintained.
    • Do not hesitate in right/high speed of casting if all other parameters are rightly taken care of.
    • Make cleaner steel which will take care of 80% of CCM issues.
    • Origin of inner defects are 90% steel quality and superheat and balance 10% due to casters when secondary water may get high or low, AMLC is non functional and EMS is name sake! Right oscillation helps minimize both inner and outer defects in billet/bloom and slabs.
    • Run plants at over 70% capacity utilization and 100% of designed speeds.
    • As per new technologies coming in, do not delay adopting them as we have global competition to face.
    • predictive maintenance of machines will always help get best of dividends.
    • Insist more on automation rather personal skills if we want to use casters for >80% of capacity.
    • All quality problems are solvable quite easily if root cause analysis is done without pre-conceived notions.
    • Go for longer and longer sequence.
    • All caster oriented problems are over in the world and so in India too we will be attaining.
    • Breakouts do not happen like any road accident but they are done due to lack of operating disciplines and unhealthy equipments! Like in developed countries including China, if things are taken care of rightly here also it may be like one in 100000 heats.
    • With right conviction, technological up-gradation, adopting and adapting to latest technological features, Indian plants ca also be world class!

 

Acknowledgement: The author is obliged to many professionals in Steel Plants of LSIL, Ispat, Essar and Jindals who could provide valuable suggestions.