Steel making

Use of steel Immense and varied use

Desirable properties Low cost and availability Hot and cold formable Weldable Suitable machinability Hard, tough and wear resistant Corrosion resistant Heat resistant and resistance to deformation at high temperatures.

Purpose of steel making Pig iron usually contains 3-4% of carbon, 2-4% of silicon, 1-2% of manganese and 1-1.2% of phosphorous. which makes it very brittle and not useful directly as a material except for limited applications Primary Steel making is about refining pig iron to reduce these impurity amounts Typical MS composition Carbon 0.16-0.18% Silicon 0.40% max Manganese 0.70-0.90% Sulphur 0.040% Max Phosphorus

Steel Production-Path

Refining reactions [Fe] + [O] = (FeO) [C] + [O] = CO [Si] + 2 [O] = (SiO2) [Mn] + [O] = (MnO) 2[P] + 5[O] = (P2O5) [FeS/MnS] + (CaO) = (CaS) + (FeO/MnO)

Steel making processes Bessemer process Open hearth process LD process Electric process

Bessemer and open hearth furnace

LD converter

Electric arc furnace

The LD Process of Steel Making

Construction: LD converter is a welded construction of non-ageing steel plates (8mm), the height of the vessel varies from 7-10m. L.D converter has a basic lining of magnesite bricks (permanent lining) and Dolomite bricks (working lining). Oxygen lance (8-10m & 20-25cm dia) is made of concentric steel tubes and the tip of the lance is made with Copper.

Why basic lining? The refining of the impurities specially P and S require the slag to be basic in nature. If the slag is not basic, these elements cannot be kept in the slag and revert back into the melt Thus most modern steel refining operations utilize a basic process, i.e create a basic slag, by adding fluxes appropriately If the refractory lining of the furnace is not basic as well then the slag and lining will react with each other, causing unacceptable lining wear

Steps involved in LD process: Charging Blowing Sampling Tapping Slag off

1. Charging: Charge materials: i) Scrap: Home scrap generated in the plant is charged. It acts as a coolant & utilizes the excess heat energy generated during refining. LD process can take upto 25% of the metal charge as scrap. Any more scarp, if charged, cannot be melted and heated to the desired temperature because of insufficient heat generation

i) Hot Metal (75-90%): A minimum amount of scrap has to be charged. Otherwise the temperature will get uncontrollably high. The analysis of iron required to use in LD process as follows: C 4.10 - 4.30% Si 0.50 – 0.85% Mn 0.50 – 0.80% S 0.02 – 0.03% P 0.10 – 0.25%

iii) Fluxes: Lime (95+%CaO) and dolomite (58%CaO, 39%MgO) are the two primary fluxes. iv) Coolants: Limestone, scrap, iron ore, and sponge iron are all potential coolants that can be added to a heat that has been overblown and is excessively hot.  v) Oxygen: 99.5% of pure oxygen is used as refining agent.

2.Blowing: After charging, the vessel is rotated to vertical position, lance is lowered to blowing position and O2 is turned on. Oxygen blows at a pressure of 150 psi . which increases temperature (16000C) ( at hot spot around 25000C)and burns off impurities. The blow continues for about 18 minutes. Oxygen consumption: 50-60 Nm3/t of steel.

Emulsion results from vigorous evolution of CO This accelerates the refining process greatly, as the surface area of the metal exposed is increased many folds

3.Sampling: Slag and metal samples are taken out for analysis. Temperature of the bath is measured by immersion of thermocouple.

4.Tapping: If the analysis & tapping temperature are in the required range, then the molten steel is tapped in the laddle. Deoxidizers and alloying additions are made in the laddle. Tap-to-tap time is 40 – 50 min.

De-oxidation of Converter Steel De-oxidation is the final process in which dissolved oxygen in the steel is removed. The de-oxidizers i.e. Al, Fe-Si and Fe-Mn are added to the steel, which combines with dissolved oxygen and forms their oxides.                  FeO + Al                Fe + Al2O3                  FeO + Fe-Si          Fe + SiO2           FeO + Fe-Mn      Fe + MnO

5.Slag off: After tapping steel into the ladle, and turning the vessel upside down and tapping the remaining slag into the "slag pot“.

Sequence of elimination of impurities

Advantages of LD converter  L-D process is about ten times faster than the open hearth process. Instead of air pure oxygen is used, this eliminates the harmful effects of nitrogen. It produces steel with low S & P content from raw materials of ordinary quality. It does not use an external source of heat or fuel.

Disadvantages of LD converter The major disadvantage of L-D process is that the charge must include a considerable quantity of molten pig, thus limiting the amount of scrap that can be used. Steel wastage due to splashes by oxygen lancing is more. Insufficient depth of penetration of O2, leads to thermal gradient in the bath.

EAF Ref : http://www.steel.org/making-steel/how-its- made/processes/processes-info/electric-arc-furnace-steelmaking.aspx

Furnace Construction Outer steel shell Inner lining of firebricks upto slag line, silica bricks in roof Three graphite electrodes Each can weigh upto 40 tons and be upto 1-2 stories tall Current furnaces can reach sizes of upto 400 tons

Modern operations aim for a tap-to-tap time of less than 60 minutes. Steps of operation Furnace charging Melting Refining De-slagging Tapping Modern operations aim for a tap-to-tap time of less than 60 minutes. 

Charging The roof and electrodes are raised and are swung to the side of the furnace to allow the scrap charging crane to move a full bucket of scrap into place over the furnace The scarp is charged, the roof swung back into position and electrodes are lowered. Charging may be done two or three times in total

Melting Melting is the heart of the operation Low power arc is struck at first so as to not damage the roof After some molten metal forms the electrodes can ‘bore-in’ and a high voltage maximum power arc can be struck Once enough scrap has melted 2nd charging can commence After all charges have been melted flat-bath conditions are achieved and refining can commence

Refining Oxygen lance is used to blow oxygen for refining purposes just like LD C,Si and Mn can be removed sufficiently without problem P and S on the otherhand require more control It has not been discussed previously but, the removal condition for P and S are opposite to each other

Removing P and S (applicable to any steelmaking process) P removal is favored by low slag temperature, high basicity and high FeO content (oxidizing environment) P2O5 + 3FeO  (FeO)3.P2O5 (FeO)3.P2O5 + 3CaO  (CaO)3P2O5 + 3FeO S removal is facilitated by a basic slag (which matches with P removal) and reducing atmosphere (low FeO content – opposite to P removal!) FeS + CaO  FeO + CaS So if aiming to remove P, bath oxidation level must be high which would then not make it possible to remove S! So how can both be removed effectively?

The Solution P is separated into the slag as early into the melting stage as possible where the temperature is still low The highly oxidizing slag has to be removed before de-oxidation may be carried out because otherwise P will revert back into melt. So this P-rich slag is removed first and then another reducing slag may be setup to remove S in the furnace itself or in the ladle after tapping.

De-Slagging and Tapping Similar to LD process Only difference is, as discussed just now, de-slagging may be done more than once

Advantages and disadvantages Great flexibility – cold charge or hot metal Heat available as desired, refining can be carried out as pleased Minimum impurities introduced because of clean energy Furnace atmosphere is controllable to a high degree making it ideal for alloy steel making Disadvantages High capital cost Not suitable for places where electrical energy is not cheap Can cause problems for the electric grid

Steel making route in bangladesh In Bangladesh, maximum steel mills use induction furnace to melt scrap metal Since appreciable refining is not possible in induction furnace, the scrap must be graded properly Induction furnace functions simply as a melting furnace and any further refining is done in a ladle refining furnace (LRF) Scrap grading  Induction furnace  LRF  CCM  billets

Induction furnace Some ferro alloys (Fe-Mn, Fe-Si, Si-Mn) may be added during melting for composition control

Functions of LRF Reheating of liquid steel through electric power conducted by graphite electrodes Homogenization of steel temperature and chemistry through inert gas stirring Formation of a slag layer that protects refractory from arc damage, concentrates and transfers heat to the liquid steel, trap inclusions and metal oxides, and provide the means for desulphurization. Alloy additions to provide bulk or trim chemical control Provide a means for deep desulphurization Provide a means for dephosphorization Act as a buffer for down stream steelmaking equipment

Continuous Casting Machine (CCM)