Steel Making Processes; Historical Development

Steel classifications

What is steel?Steel is an alloy of basically iron & carbon ranging from 0.025-2%. Other alloying elements are occasionally added to improve different properties in the ultimate steel.The composition of steel broadly divides it into,i) Plain carbon steelii) Alloy steels

The plain carbon steels are of three major types:i) Low carbon or mild steels upto 0.25%Cii) Medium carbon steels 0.25-0.65%Ciii) High carbon steels 0.65-1.70%CAlloy steels are broadly divided into three different types:iv) Low alloy steels total alloy content < ~5%v) Medium alloy steels ‘’ 5-10%vi) High alloy steels ‘’ >10%

Steelmaking is a process of selective oxidation of impurities i.e. reverse of iron making. During steelmaking impurities like C, Si, Mn, P are oxidized to their respective oxides and eliminated either as gas or as liquid slag. Other impurity ‘S’ can be minimized in reducing condition by a slag-metal interface reaction.

Steelmaking flow chart

Bessemer Converter Process

The Bessemer Converter Process was first developed in 1856. the original process was suitable for only low ‘P’ iron as it used acidic refractory lining. The basic bessemer process, suitable for high ‘P’ hot metal was developed later by Thomas Gilchrist and is known as Thomas process.

The Bessemer converter process has now been completely replaced by basic oxygen converters.

The converter & operational practice

The bessemer converter is a pear shaped vessel with a detachable bottom. It is mounted on a trunnion and can be rotated through 360⁰. After charging the hot metal blowing is started through the bottom tuyeres. At the end of the blow, the converter is again tilted upside down and the metal and slag are tapped out.

The Bessemer converter process is completely autogeneous, i.e. no external heat supply is needed. The energy requirement for steelmaking is provided by the exothermic chemical reaction of the bath.

Silicon oxidation occurs first, then Mn and ‘C’ oxidation peaks only after ‘Si’ and ‘Mn’. Carbon oxidation is indicated by appearance of the long flame over the mouth of the converter. It drops after the completion of C-O reaction.

‘P’ oxidation reaction continues for another 3-4 mins. This period is known as after-blow period.

Composition of suitable hot metalElement% Acidic process Basic process

C 3.5-4.0 3-3.6

Si 2.0-2.5 0.6-1.0

S 0.04max. 0.08-0.10

P 0.04max. 1.8-2.5

Mn 0.75-1.0 1.0-2.5

Decline of the Bessemer Process

In spite of the basic simplicity, the Bessemer steel suffered from several limitations:

• A large part of the heat generated through exothermic reactions is lost in the form of sensible heat of the N2 gas. So, the scrap melting ability of this process is very limited.

• The ‘N’ content in the Bessemer steel is high-of the order of 0.012%. This is not desired with extremely low level (50-60ppm) required in most commercial steels.

• The need to change the bottom section frequently was a severe problem.

• The Bessemer process could not refine high ‘Si’ medium ‘P’ iron in one single stage. In Indian condition, a duplex or triplex process became necessary to treat this type of hot metal.

Open hearth furnace steelmaking

The process was originally developed by Siemens in Germany and Martin in France in late nineteenth century. The ability of OH furnaces to melt both light & heavy scraps to produce liquid steel of any ‘C’ content and any desired chemical compositions had saved this process till eighties.

Construction of OH Furnace

It is basically a reverberatory furnace in which hot metal and molten steel scraps are refined in a shallow basic lined hearth. The fuel burners heat up the bath through the intermediate slag layer. The hot exhaust gases are conducted through a set of regenerators. The sensible heat of hot gases preheat the refractory checker works. The regenerators preheat the air and fuel.

OH Furnace

Operation of the basic OH furnaceThere is usually a wide choice of raw materials in basic OH

furnace. In Indian plants , usually steel scrap + hot metal mix constitutes the basic charge materials. The ‘Si’ content of the hot metal was maintained as low as possible usually ≤ 1.0%, to ensure optimum basicity of the slag. The ‘P’ content ranged from 0.22-0.30%. The formation of basic oxidising slag required addition of iron ore + limestone.

In integrated steel plants the usual practice was to charge steel scrap, lime/limestone and iron ore first. The charge was heated upto a state of incipient fusion. Then hot metal was charged.

Reasons for decline of OH process

• OH steelmaking is a very slow process. It cannot match the productivity of modern BOF where the tap to tap time is of the order of 40-60mins.

• The dependence of external fuel supply is a serious constraint of the process.

• Construction and maintenance of the roof and substructure of the OH furnace is more difficult than the overall maintenance of a basic oxygen converter.

Top-blown Basic Oxygen Converter Process

The easy availability of oxygen gas in the post Second World War period facilitated research on oxygen lancing through the throat of the converter. As a result the top-blown basic oxygen converter process, popularly known as LD process was developed. LD stands for Linz and Donawitz towns in Austria, where the developmental work was carried out. In course of time the process also came to be known as Basic Oxygen Furnace (BOF) process.

Because of its flexibility, it can refine hot metal of varying compositions to produce low ‘C’, high ‘C’ and low alloy steels.

A basic oxygen converter is a pear-shaped vessel with a concentrically positioned oxygen lance. The steel shell is suitably lined with basic refractories. O2 (99.9%) is blown through a water cooled lance fitted with a copper nozzle. The position of the lance w.r.to the bath and the flow rate of O2 are automatically controlled. The capacity of a modern converter ranges from 100T-400T.

BOF Steelmaking practice

1. Scrap Charging• Scrap Iron and Steel are tipped into the Furnace. The Iron and

Steel comes from old or scraped cars, bridges, buildings, etc. Also used is Iron or Steel that when manufactured into a product was not of good enough quality to be used for its intended purpose.

2. Molten Iron Charging• Molten Iron, which comes straight from the Blast Furnace is

then tipped into the Furnace. The Furnace is now ready for the blow. The hot metal to scrap ratio ranges70:30 to 100:1.

SCRAP & MOLTEN IRON CHARGING

3. The Blow• The Gas Offtake Hood is lowered onto the Furnace. The water

cooled Oxygen Lance is then lowered. This carries the hot Oxygen to the surface of the hot metal, increasing the temperature in the Furnace and melting all of the metal. The Oxygen combines with the impurities to form oxides in the form of gases and slag.

4. Sampling• During the Blow the temperature of the Furnace is monitored,

and at regular intervals samples of the molten metal are taken to be analysed. When the Steel is of the right composition, then the Steel workers can move onto the next stage.

THE BLOW SAMPLING

5. Pouring• When the Steel is of the right composition the Gas Offtake

Hood and the Oxygen Lance are removed. The molten Steel is then poured out the Top-hole by turning the Furnace to one side. The Steel is then cast into ingots, or processed by continuous casting.

6. Slagging• When all of the Steel has been poured out, the Furnace is

turned upside down, in the opposite direction to that when pouring, and the Slag is removed.

POURING SLAGGING

Oxygen jet characteristics• In BOF process, O2 is blown at a pressure of 8-10atm. through

a convergent –divergent nozzle. The O2 jet is supersonic and has a speed of 1.5-2.2 times the speed of sound. A supersonic jet is characterized by a supersonic core in which the jet velocity is higher than the speed of sound.

• As the jet travels away from the nozzle, it is retarded by the converter atmosphere so that the supersonic core shrinks radially and the axial velocity gradually decreases until at some distance away from the nozzle, the jet becomes fully subsonic. This point marks the end of supersonic core.

• The jet ultimately impinges on the liquid metal surface to form a cavity. The impingement of the jet and the dissipation of the jet momentum causes circulation of the liquid bath in the upward direction at the vessel central axis. The intensity of the jet-bath interaction is expressed in terms of ‘Jet Force No.’ (JFN) and is defined as,

JFN = Gas pressure x Nozzle dia./ Lance height

• At low JFN, dimpling with a slight surface depression is observed.

• At medium to high JFN, splashing with a shallow depression.

• At very high JFN, penetrating mode of cavity with reduction in splashing.

Only the last two types of behavior are encountered in BOF. Metal droplets are formed on the lip of the cavity and get ejected in both modes.

Mechanism of refining• During refining, controlled oxidation of the impurities takes

place once O2 is blown at supersonic speed. The interaction of the O2 jet with the bath produces crater on the surface, from the outer lips of which a large number of tiny metal droplets get splashed. These droplets reside for a short time in the slag above the bath.

• So, the existance of metal-slag- gas emulsion within the vessel during the entire blowing period is an integral part of the BOF steelmaking. This is the reason why slag-metal (dephosphorization) and gas-metal (decarburization) reaction proceed so rapidly in BOF steelmaking.

Bath conditions at various stages of blow

0 min. 5 min. 7 min. 20 min.

GAS-SLAG-METAL EMULSION

Reactions in BOF

0 5 10 15 20 250

0.51

1.52

2.53

3.54

4.55

C%Si%Mn%P%

Blow time, min

C,Si

,Mn,

wt%

• When the metal bath reacts with O2 jet oxidation of iron occurs,

2[Fe] + {O2} = 2(FeO) 4(FeO) +{O2} = 2(Fe2O3)An iron oxide rich slag forms in the early stage of

blowing due to oxidation of iron and addition of iron ore/mill scale in the charge.

• The (FeO) reacts with the impurity elements in the metal & slag.

(FeO) + [Si] = [Fe] + (SiO2) (SiO2) + 2(FeO) = (2FeO.SiO2) [Mn] + (FeO) = (MnO) + [Fe] 2(MnO) + (SiO2) = (2MnO.SiO2)A (FeO) rich slag quickly dissolves lime, and the following

reactions proceed, (2MnO.SiO2) + 2(CaO) = 2(CaO.SiO2)

Carbon removal

• The most important reaction in BOF is oxidation of carbon. The rate decarburization is initially low, it increases to a peak value in the middle of the blow and then decreases again.

• In the initial period the (FeO) in slag may raise to 14-16%. During peak period it decreases to 7-9%, because during this period more O2 is consumed than supplied. When the ‘C’ content of the bath drops to <0.2% the decarburization kinetics drops and (FeO) rises again.

Variation of rate of decarburization during the blow

0 20 40 60 80 100 1200

0.05

0.1

0.15

0.2

0.25

0.3

0.35

C oxidation rate

% of Blow time

Carb

on o

xida

tion

rate

(%/m

in)

Mn removal

• [Mn] from the bath is lost in the slag as (2MnO.SiO2), so there is reduction in Mn% in the bath in the initial period. As the slag basicity increases due to lime dissolution, (MnO) is gradually released & is reduced by carbon,

(MnO) + [C] = [Mn] + {CO}So, the [Mn] content in the bath increases again. As the

intensity of C-O reaction decreases towards the end of the blow, Mn is reoxidised from the bath. This accounts the characteristic “Mn hump” in the reaction diagram.

Phosphorus removal

• The early formation of a basic slag enables dephosphorization to proceed simultaneously with decarburization. The reactions are,

2[P] + 5 (FeO) = (P2O5) + 5 [Fe] (P2O5) + 4 (CaO) = (4CaO.P2O5)

Sulphur removal• Although the oxidizing slag in BOF is not suitable for desulphurization,

some ‘S’ removal may occur due to highly basic character of the slag and high temperature (1680-1700⁰C).

The slag-metal desulphurization reactions are, (FeS) + (MnO) = (MnS) + (FeO) (FeS) + (MgO) = (MgS) + (FeO) (FeS) + (CaO) = (CaS) + (FeO)Part of the ‘S’ may be removed in the initial stage of the blow through the

reaction with Mn, [Mn] + [S] = (MnS)

Production of High Carbon Steels in BOF

• High carbon steels like rail steels(0.65- 0.74%C, 0.6-1.0%Mn, 0.27-0.3%Si), Ball bearing steels (1.0%C, 1.2%Cr) etc. are manufactured in BOF converter by “catch carbon technique”.

• In this technique, dephosphorization is accelerated and completed before decarburization. Extra lime and fluorspar are charged and the lance is raised to a higher position for maintaining a soft blow condition till ‘P’ removal is completed.

• Thereafter, decarburization is continued by a harder blow till the bath carbon content drops to the desired level.

• Alternatively, blowing may be continued to complete both dephosphorization and decarburization. Required amount of carburizer (petroleum coke /graphite) is then added to the low carbon steel bath to raise the ‘C’ content to the desired level. However, this method involves risk of increasing the inclusion & nitrogen % in steel, picked up from the carburizer.