Bull. Mater. Sci., Vol. 12, Nos 3 & 4, September 1989, pp. 435-447. © Printed in India.

Recent trends in iron and steelmaking technology

B N S I N G H R and D Centre for Iron and Steel, Steel Authority of India Ltd., Ranchi 834002, India

Abstract. Iron and steelmaking technology, including the preparation of raw materials and utilisation of wastes, has undergone marked changes over the last five decades. The hand mining of ores/minerals has almost completely been replaced by mechanised miliing. The role of mineral beneficiation and sizing has therefore increased. The current emphasis is on maximising the use of mined materials through multiple beneficiation stages and agglomera- tion of fines.

Whilst emerging technologies for producing liquid iron are making their advent, blast furnaces continue to be the unchallenged source of hot metal production. However, the efficiency of ironmaking, both in terms of productivity and quality of hot metal has improved markedly.

Gone are the days of opefl hearth furnaces and Bessemer converters. LD steelmaking has incorporated in it a number of improvements, namely combined blowing, dynamic control along with the use of the sublance, improved refractories for lining and ladle treatment of liquid steel.

Trends emerging in the area of steel-casting include, horizontal casting, thin strip casting and hot charging of con-cast products without soaking/re-heating.

The paper deals with some of these areas including the advances in rolling mill operation.

Keywords. Ironmaking; combined blowing; continuous casting; steel processing; pre- carbonisation.

1. In troduct ion

Steelmaking technology basically involves three functions: (a) manufacture of hot metal, (b) conversion of hot metal into steel, and (c) processing of steel. Use of lump ore has almost disappeared and it is being replaced more and more by agglomerates. Thus, sinter and coke are the two primary inputs into blast furnaces as against lump ore, fluxes and other additives like quartzite, manganese ore etc. used earlier. This has led to a high degree of sophistication and improvement in the technology of both sinter-making and coke-making over the last three decades. The increased cost of liquid petroleum fuel has given a serious jolt to the iron ore pelletisation process and a number of pellet plants all over the world are now closed-down. However, improvements in pellet curing techniques and waste heat recovery systems developed during the eighties appear to help the revival of this technology again.

In the area of steelmaking, the Bessemer converters have totally disappeared, except for the ones in use at IISCO, Burnpur. O H furnaces are gradually being replaced by newer versions of basic oxygen processes, primarily using combined blowing techniques. The role of ladle metallurgy has increased both for the pre-treatment of hot metal as well as post-treatment of liquid steel.

Processing of steel, which involves three distinct functions, viz. solidification of liquid steel, shaping of steel in various forms and sizes and treatment of the finished product for achieving the desired mechanical and physical properties, has also

435

436 B N Singh

undergone marked changes. Continuous casting is gradually replacing ingot casting and it accounts for over 75~o of the steel produced in certain Japanese plants. As a result, the blooming mills and the soaking pits are gradually vanishing. The continuous-cast semi-finished products are being directly charged into finishing mills. Heat treatment now forms an integral part of the finishing lines, because of the introduction of continuous pickling and galvanising, on-line quenching and tempering etc.

Parallel developments are taking place in the area of emerging technologies with a view to produce liquid iron using non-cooking coal. Further, attempts are being made to replace finishing mills for producing thin strips directly from liquid metal.

Some of these aspects are dealt with in the following sections of this paper with relevance to the Indian context.

2. lronmaking

The blast furnace ironmaking technology has remained unchanged over decades in its broader aspects, but the refinements in the technology have lead to the installation of larger capacity units leading to economy of scale and higher labour productivity. The productivity of blast furnaces has gone beyond 2.8 in some of the blast furnaces and a 5000 M 3 blast furnace to produce 14,000 tonnes per day has become a reality. As a result, the demand on the quality of coke has increased in terms of its strength and reactivity.

2.1 Coke-makino

A good blast furnace operation in large capacity units, demands coke having Ml0 index less than 8.0, coke reactivity index (CRI) less than 30 and coke strength after reaction (CSR) over 55. This is being achieved in efficient steel plants abroad by optimising blend, installation of precarbonisation technologies, automatic combustion control during coke-making and post coking treatment of coke by quenching control and coke cutting as schematically shown in figure 1 (Singh et al 1986). Indian steel plants were solely dependent, till very recently, on Indian coal which is high in ash as well as inferior in coking quality (table 1). As a result, coke produced with indigenous blends have high M10 index (generally over 10.0 and as high as 16-17), high CRI (over 35) and low CSR (less than 30). Import of low ash high grade coking coal has started only during the 80s. Pre-carbonisation technology has not been adopted by any of our steel plants so far. The partial briquette blending as well as the stamp charging of coal, which have a greater potential in Indian context, are being considered by some of the SAIL plants. Some of the relevant aspects of the different pre- carbonisation technologies are indicated in table 2 (Singh et al 1987). Since the quality of coking coal available in India is not conducive to producing high strength coke consistently, the role of stamp charging of coal becomes evident from this table. The total availability of coking coal itself is a constraint in India, and efforts have to be made to develop coke-making technology from non-coking coals using formed coke-making tec,mology. SAIL has already conducted tests with charging formed coke in the low shaft furnace at KIW, Barbil, and in one of the 500M a blast furnaces at IISCO, Burnpur. Plans are already under consideration for installing a

Iron and steel making 437

COAL 1 SELECTION

COAL PREPARATION

PRE-CARBONISATION TREATMENT

CARBONISATION

TREATMENT

--CHARACTERISTICS OF COALS -EXTENT OF WEATHERING -BLEND CONSTITUTION

MOISTURE CONTROL FINENESS CONTROL: ONCE THROUGH/SELECTIVE

CRUSHING ADDITIVES; INERTS / DENSIFIER/PLASTICISER

PBCC / PRE-HEATING I STAMP CHARGING BINDERLESS BRIQUETTING

CONTROL OF COKING PARAMETER

O, UENCHING ; CONVENTIONAL / DRY QUENCHING STABILISATION ~ CUTTING /DROPPING SCREENING

Figure 1. Improvement in coke quality - various options.

Table 1. Comparison of some important properties of good Indian prime coking coals with foreign coking coals

Parameters Indian coals Foreign coals

Approximate analysis (%) Ash 18-24 6-10 VM 22-27 22-27 FC 50-58 65-70

Cokinq properties FSI 2-5 5-8 CI (BS) 14-20 22-24 GKCT F-G2 G3-G5 PLT (ram) ! 8-22 23-26 Gieseler plasticity ddpm max 50-250 200-1000

Petrographic indices (%) Vitrimite 45-55 Over 55 RV 14)-1.10 1.10-1.20

438 B N Singh

Table 2. Comparison techniques

of some important parameters for different precarbonisation

Conventional Briquette Stamp Binderless Parameters top charging blending Preheating charging briqutting

Crushing fineness - 3-2 ram) (%) 78-80 78-80 78-80 89-91 89-91

Additive (%) 0.2 LD0 8-10 pitch - - - - - - Moisture in coal (%) 6-8 . . . . 8-11 7-8 Bulk density of coal charge (kg/m 3) 700-750 750-800 800-850 1050-1150 750-800 M 10 Index 10-11 8.5-9.5 8-9 6-8 9-95 M 40 Index 77-80 79-81 79-81 80-82 78-79 Carbonisation time Index 100 105-110 80-85 1 t0-115 105-t 10 Oven throughput Index 100 105-110 110-115 112-115 105-110 Maintenance need Index 1013 105 150-175 110-115 100-105

semi-commercial plant for producing bulk tonnage of formed coke for extended trials in the blast furnaces at Burnpur. It is important to mention at this stage that even in Japan, a 200 t/day pilot plant for producing formed coke has been installed with a view to produce bulk tonnage of such coke for trial in one of their 4000 M 3 blast furnaces.

The technology input in Indian coke ovens has been almost totally absent over the last few decades. This has now been realised and new technologies are being introduced in coking plants. Introduction of 7"2m tall ovens at Bhilai and Vizag, adoption of selective crushing and dry coke quenching technology for the Vizag Steel Plant, and installation of stamp charging in one of the new batteries at Tata Steel, are some of the examples of the adoption of such new technologies in coke-making in India.

2.2 Sintering

Sintering, which started earlier as a means of recycling some of the steel plant wastes, has now become a means of combining iron ore, fluxes and all additives in one type of feed, sinter, to the blast furnaces. Super basis sinter is considered to be the best burden material for the blast furnaces. The basicity (CaO/SiOz) of sinter in Indian steel plants has gone up to as high as 2-6-2"8 (De et al 1986), introduction of several improvements like deep bed sintering, higher suction, extended ignition hood, pre-bailing, and recycling of waste heat for preheating of ignition air as well as for soaking the sinter charge beyond the ignition hood, are a few examples of newer technologies introduced in the sinter p!ants abroad (Cappel 1985). The productivity of sinter strand has gone to as high as 1.45 t/m2/h. Thus, sinter plants having bed heights of up to 600 mm, suction levels up to 1800 mm WG, and strand areas of over 500 m z are in use in a number of steel plants around the world. The microstructural phases in the sinter are so adjusted that it does not have only a high strength and mean size but also has high reducibility, high degradation resistance and high softening temperature (table 3) (Singh 1984). This has also improved the acceptability of sinter by blast furnaces (BF) and, as a result, sinter percentage in BF feeds has gone up to 100°~£. It has also improved the wind acceptability and helped in lowering the silicon

Iron and steel making 439

Table 3. Comparison of some important properties of typical sinters produced in India with some good sinters produced abroad.

Parameters Indian sinter Foreign sinter

Al20 a content of sinter (%) 3.0-5.5 1.0-2-5 Fe content of sinter (%) 45-55 53-57 F e e content of sinter (%) 8-12 6-9 Return fines in sinter mix (% of ore mix) 45-85 25-35 Mean size of sinter (mm) 10-13 17-21 Drum index (ASTM) 63-75 67-73

Reducibility index (dR / \ d t /40 - 1.1-1"3

RDI ( - 3 ram) (%) - Less than 30 Softening temperature (°C) - Over 1150

Table 4. Effect of addition of blue dust in the iron ore mix on some sintering parameters.

Blue dust Sintering Return Productivity Shatter in ore-mix rate fines of machine index t%) (cm/min) (%) (t/m2/h) (+ 10ram%)

0 2"80 60 0"98 79 5~ 2'54 49 1"10 75

2"38 35 1'20 76

content of hot metal. The bedding and blending yards have narrowed down the fluctuations in the chemistry of sinter to almost 1/5th of the levels prevalent some two decades ago.

The use of ultrafines ( - 100 mesh fractions) in sinter mix could be increased to as high as 30% by improved pre-balling techniques. Thus, blue dust available in some of the Indian iron ore mines can be used successfully, up to 50-60% in the ore-mix, for sintering, without adversely affecting the sinter machine productivity or the quality of sinter (table 4) (De et al 1985).

Washing of iron ore is being increasingly practised to lower its alumina content. Laboratory as well as pilot plant investigations have shown that such techniques as jigging (Seetharama 1986) and Wet High Intensity Magnetic Separation (WHIMS) techniques can be used successfully to lower alumina content of Indian ores (table 5 and figure 2). Iron ore beneficiation flow charts are being developed for the future with a view to maximize the use of as-mined ore in BF iron-making, a typical conceptual ore beneficiation flow chart being shown in figure 3.

2.3 Iron-making

The modern blast furnaces are thoroughly instrumented, sometimes with as high as 500 sensing points in a furnace, including the measurement of in-wall temperatures, installation of radio-isotopes in the refractories, over and under burden probes for

440 B N Singh

Table 5. Typical results of a jigging test.

Feed

Yield Chemical analysis (%) (%) (Fe) (SiO2) (A1203)

IO3-00 62"0 1"03 4"52

Single-stage jigging Concentrate 66.3 65-35 0"65 3'03 Tailings 33-7 58-04 1.53 7.60

Two-stage rigging (roughing and cleaning) Concentrate 47.3 66"3 0-56 2.56 Tailings 52.7 59.85 1.30 6.37

Two-stage jigging (roughing and scavenging) Concentrate 90.1 64.15 0.76 3.70 Tailings 9-9 51.60 2.70 12.44

Three-stage jigging Concentrate 81.5 65.93 0.57 2.81 Tailings 18.5 52.71 2.39 11.67

1 0 0

90

80

.~- 7O

d

- 60

d z o ~ 5O

40

i - T A I U N G - I JIGGED

TWO STAGE-CONC -

,JIGGED J & ~ ~

3 0 I ~ I I I GlO i 0 ,~0 4 0 BO

PERCENTAGE DROP IN ALUMINA L E V E L

Figure 2. Effect of different j igging practices on the concentrate yield and a lumina removal for noamundi classifier fines.

monitoring temperature of the up-coming gas in the radial direction, and its chemistry, vertical as well as horizontal probes for measuring pressure drops across the burden column, top gas analysis by automatic sampling and analysis devices and complete sensing of the stock level through infrared cameras and laser-based devices, (figure 4) (Ojha etal 1987). A number of blast furnaces in Japan and Germany have been quenched and dissected to understand the state of reactions undergone by the burden and coke during their passage from the top to the bottom of the furnace. These

Iron and steel makiny 441

CRUSHING (-30om)

WASHING & SCREEN NG I

ICLASS' 'CA 'ON I ~ -8 + 0-25 mm

I 2 ND STAGE BENEF ICIA~'ION l '

" GRIND

"BEN EF IClATION

PRO0UC,

~ - ~ PRODUCTIII

Figure 3. Future trends in iron ore beneficiation. (Bene-beneficiated.)

investigations have helped in better understanding of the blast furnace operation and have helped in developing advanced mathematical models for automatic control of BF operation. The results of these efforts have increased productivity of the blast furnaces over 2"8 t/m3/d, lower coke rate to below 400 kg/m and production of hot metal with silicon content as low as 0"17~. Typical parameters for advanced blast furnaces abroad as compared with the Indian ones are given in table 6.

The present blast furnaces are expected primarily to reduce iron ore most efficiently. Other functions, like desiliconisation, desulphurisation and dephosphorisation are being attempted outside the blast furnaces through pre-treatment techniques in the cast house/ladles. Typical routes followed abroad for such treatments are shown in figure 5.

The role of tuyere injection in BF ironmaking is increasing. Lime injection through BF tuyeres has been found to be useful in optimising bosh slag fluidity in the Indian context and a commercial unit is already commissioning it at the Durgapur Steel Plant of SAIL (Athappan 1987).

442 B N Singh

VERTICAL ] [ ~ ~¢//1

/ O .EN, G I I " CO .,O. l I MELTING I ~ ~ /

IRA'c° °'"°NI / SUB HEARTH

COOL NG

Figure 4. Schematic representation of BF control. (Anal-analysis; distn-distribution; temp- temperature; refrac-refractory.)

Table 6. Comparison of some important operating indices of indian blast furnaces with efficient blast furnaces abroad.

Parameters Unit India Abroad

Productivity t/m3/d 0.6-1.2 1.8-2.8 Carbon rate kg/thm 500-720 360-450 HBT °C 600-1000 1100-1350 High top Pr kg/cm 2 nil-l.4 1-5-2.5 CO/CO 2 in top gas - 1.4-1.8 --- 1,20 Si in HM % 1.3-2.0 0.17-0.80 Alumina in slag % 21-26 12-15 Coke ash % 21-25 8-12 M 10 of coke - - 9"5-18 5-8 Sinter in burden % 0-70 80-100

3. Steel-making

LD steel-making, which emerged only in the 50's as a commercially viable process, posed,a threat to the well-established basic Bessemer and BOH processes. The bottom blowing process namely OBM, LWS and Q-BOP came to the forefront only during the mid-70's. However, before their full potential could be established on a commercial scale, the combined blowing processes came on the scene in the early 80's. Today, there are a number of variations of the combined blowing processes, namely, LBE, LD-KG, LD-OTB, LD-AB, STB, LD-OB, LD-HC, KBOP, LD-CL etc. operating

Iron and steel makin# 443

~ DESILICONISING B F ~ REAGENT

A~ RUNNER OESlLICONISATION ~___/LADLE

N Z BUBBLING ~ OESILICONISING (,6 BAR) "-'t I J REAGENT

B) LADLE ADDITION AND ISINTER "t" N2(6 BAR) NITROGEN BUBBLING ~ T L

~ ) O ( 1( l ( )(J ( . ) ( )O( '1 ( ) ( )

C) TORPEDO INJECTION

Figure 5. Schematic representation of various desiliconisation treatments of hot metal (l bar = 10 5 Pa).

commercially in several steel plants in the world (table 7) (Nilles 1981). The SAIL Combined Blowing Process (SCB), developed by SAIL, is already operating at BSL and its adoption in other steel plants is already planned. This has helped in controlling the FeO content of slag in the oxygen converter closer to the equilibrium and has eliminated chemical and thermal stratifications, which result in improved recovery of ferro alloys, improved lining life of LD converters and higher accuracy in predicting turndown chemistry of the liquid bath. The size of the converters is now not limited by any process constraints, but by the load transmitted to the trunions in supporting the weight of over 1000 tons on two contact lines and still allowing this entire weight to be rotated full circle around the trunion axis, as well as stoppage in any position. Further, the approach now is to minimise the reblows to achieve only the required degree of decarburisation of.the bath at the lowest oxygen potential. The other functions, namely desulphurisation, deoxidation, alloying, recarborisation, and

444 B N Sin,qh

Table 7. Some of the combined blowing processes operat- ing on commercial scale.

Process type

Inert gas injection through porous plug/permeable element

Inert gas injection through tuyeres

Oxygen/air injection through double well tuyeres

Mechanical agitation

Name of processes

LBE, SCB

LD-KG, LD-OTB LD-AB

BSC, STB, LD-OB, LD-HC, KBOP

LD-CL

temperature control, which were earlier done in the steel-making vessel are now performed in the ladles using secondary steelmaking techniques. This has resulted in further increasing the productivity and the lining life of the steel-making vessels. All these developments have led to the installation of steelmaking converters which can now spew out almost 500 tons of liquid steel every 40 rain for over 2000 heats without changing the refractory lining.

4. Steel processing

Continuous casting has now become almost universal with respect to its capacity to handle different steel grades (Nayar and Dutt 1984). The continuous casting of steel in sequence, totalling over 1000 ladles of liquid steel, running continuously over a period of over 40 days has been possible commercially in some of the Japanese steel plants as given in table 8 (Suzuki 1984).

Hot direct rolling and hot charge rolling are gradually replacing the reheating furnaces, so that the con-cast product can be fed directly into the finishing mills (Wakabayashi 1977). Such technologies are in use at the Nippon Steel Corporation for the last seven years at its Sakai Works. Thus, liquid steel is converted into finished product in less than 90 rain.

Development of rotary continuous casting for round bars, horizontal continuous casting for producing non-seggregated quality steel products, thin strip casting etc. are now becoming commerical realities. Automation in continuous casting has been perfected to the extent of no-man control of the con-cast operation at Kashima steel works of Sumitomo Metal Industries, Japan (Suzuki 1975). Use of electromagnetic stirring, automatic metal level control in the mould, use of slide-gate valve in the ladle as well as in tundish control of cooling and withdrawal speed, injection of aluminium wire in the ladle as well as mould etc. have significantly contributed to this high degree of automation and have made continuous processing of steel a possibility. Controls during rolling, and on-line cooling techniques have made it possible to achieve and adjust the properties of the finished product more accurately. As a result, several grades of reinforcing bars and wire rods, high strength plates and sheets can be produced directly from the rolling mills without any further heat treatment. Increase in productivity of rolling mills has been rendered possible by

Iron and steel making 445

Table 8. Sequences on some bloom casters in Japan.

Average number Maximum number Caster ladles ladles

KSC, Mizushima 6 169 SMI, Kokura 15 137 Yamata kogya 47 85 SMI Wakayama 95 t015 Daido Steel 18 23

Table 9. Rolling speed of some high speed mills.

Rolling speed Product line (m/min) Major developments

Seamless-pipe 60 More acute boring angle, making greater roll dia, faster

speed of revolution Electric- 160 Replacement of low frequency welded pipe resistance welders by high

frequency induction welders Lap-welded pipes 450 A combination of 14 stand

forging line and a sizer Hot strips 1600 Stiffening mills, rolls and

bearings, automatic gauge and shape control

2500 Thyrister-leonard control of mill drive, improved roll bearing and hydraulic press- down mechanism and better lubrication

6000 More rational laying of mills, fully continuous mills, block mills in the finishing line

Cold-rolled strips

Wire-rod

increasing the unit weight of semis like ingots, slabs, blooms, billets, increasing rolling speed, continuous rolling and the application of a high degree of automation and control.

The unit weight of ingots have increased to as high as 40 tons and mill designs which can roll up to 70 ton ingots are already in progress. The width of the heavy plate mills have increased to as much as 5-5 m to meet the specific requirements of ship-building, land structures and line pipes. The speed of finishing strands in the continuous hot strip mills, which was less than 400 m/min in the 50s has gone up to 1600 m/min during the 80s. Rolling speeds of up to 2500 m/min have been achieved in cold roiling mills and the same in the wire rod mills has increased to as much as 5000m/min (table 9) (Chattopadhyay 1984). These speeds are now possible because of improvements in the stiffening of rolls, improved roll bearings and mill housings, introduction of non-twist blocks for wire rod rolling and advances in automatic gauge and shape control techniques. Lubrication technology has also been developed to match the above rolling speed requirements.

446 B N Singh

Continuous rolling by welding coils together end-to-end has further helped in increasing the productivity of the finishing mills. Continuous annealing, tinning and galvanising are in practice since the beginning of the 70s. High speed rolling and heavier reductions have made it possible to roll steel without any marked drop in temperature of the rolling stock. This has made the rolling of high temperature alloys possible, which were considered unsuitable for this purpose earlier. The relationship between the chemistry of steel, draft schedule, finishing temperatures, microstructures and properties of finished products have been well-established by simulated laboratory studies followed by plant trials. This has made it possible to use process computers extensively in controlling the rolling operation as well as the quality of rolled products.

Controlled cooling of finished products on the run out table, is another aspect of steel processing for imparting desired properties to the rolled products. Water-cooling is very successfully used, particularly in hot strip mills for producing microalloyed high strength steel strips. Controlled cooling of high carbon wire rods has eliminated the need for separate patenting treatment before cold drawing. The Stelmor and the Ashlow processes, fluidised bed cooling and boiling water-cooling methods have found applications in heat treatment of finished products on the run out table. On-line quenching and self-tempering by Tempcore process has been able to produce the reinforcing bars with high yield strength, good weldability and superior ductility (Simon 1984). On-line roller quenching and tempering of plates is another example of achieving high strength through separate heat treatment after hot rolling.

5. Conclusions

Advances in the technology of steel-making and its processing are taking place at a very fast pace, both in terms of the scale of operation as well as the processes themselves. These developments coupled with adoption of secondary steelmaking processes have made it possible to precisely control the c~hemistry and the properties of the finished products. Steel-making has also become more energy-efficient.

References

Athappan R and Thakur B 1987 Trans. Indian Inst. Met. 40 55 Cappel F 1985 Proceedings of the Short Course on Sintering Technology, Jamshedpur, p.I.1 Chattopadhyay R N, Singh B N and Mukherjee T 1984 Paper presented at the seminar on Recent Advances

in the Processing of Metals, Bangalore De A, Singh B N, Chatterjee A and Mukherjee T 1985 TISCO Tech. J. 40(3) 79 De A, Singh B N, Mitra A N and Mukherjee T 1986 Proceedings oflnternational Symposium on Beneficiation

and Agglomeration, Bhuwaneshwar, p. 263 Nayar N K and Dutt N K 1984 Proceedings of the short course on Continuous Casting of Steel, Jamshedpur,

p. 76 Nilles P 1981 Proceedings of Int. Syrup. on Modern Developments in Steelmaking, Jamshedpur, p. 395 Ojha S K, Verma R K, Roy P K and Singh B N 1987 Paper presented at the Workshop on BF

Instrumentation, Jamshedpur Seetharama V N e t al 1986 Proc. Int. Conf. on Progress in Mater. Res. Fundamental and Applied Aspects,

Kanpur p. 31 Simon Pe t al 1984 Iron Steel Eng. p. 53 Singh B N, De A, Rawat Y F, Das R C and Chatterjee A 1984 Iron Steel Int. 57(4) 135

Iron and steel makin9 447

Singh B N, Rao P U T, Poddar M N and Thakur C 1986 TISCO Tech. J 31(4) 79 Singh B N, Singh B K, Venugopal T, Mitra A N and Mukherjce T 1987 Presented at the First International

Cokemaking Congress, Essen (W. Germany) Suzuki A 1975 J. Jpn. Soc. Mech. Eng. 78 683 Suzuki A 1984 Trans. ISI Jpn. 24 228 Wakabayashi S 1977 Continuous casting of steel (The Metals Society) p. 209