Towards Improved Usages Of Induction Furnace To Produce Micro Alloyed Steels
S.I. Singh Director (Actg.), N.I.S.S.T., Mandi Gobindgarh, Punjab
R.K. Bagchi Manager (Tech.), N.I.S.S.T., Mandi Gobindgarh, Punjab
Induction crucible furnaces differ from arc furnaces in ways, which are important metallurgically, althrough both types of furnaces are used for making steel from scrap and alloys. As compared to arc furnaces, Induction melting furnaces do possess following characteristics:
1. High, relatively narrow melting vessel (low d/h ratio),
2. Low crucible wall thickness,
3. Relatively small area of metal in contact with slag,
4. Low slag temperature,
5. No carburizing during melting down,
6. Powerful bath motion.
Therefore sections such as oxidation, decarburising and dephosphorising as well as deoxidizing and desulphurizing, which are usually requited in quality steelmaking, can only we applied to a limited extent in an induction crucible furnace used for steel production. Those reactions, which take place inside the metl, oral the metal-lining interface, are assisted by the powerful path motion. On the other hand, the metal-slag reaction suffers because of the smaller contact-area and lower slag temperature. Any attempts at enhancing this reaction, by using sings that are more reactive and less viscous ore further limited by the requirement of production of alloy steels by the remelting process where precious elements are to be recovered. Also dissolution of any alloy addition is very effective, because the powerful melt motion dissolves alloying additions quickly. For this reason, even small induction furnaces can be operated economically.
Possibility of Melting H.S.L.A. Steels
High strength low alloy steels, known as micro alloyed steels, are basically low alloy steels, having micro alloy addition (added in small proportions) of various elements e.g. NB, Ti, Al, B, V, Cb, etc. These elements are either added alone or in combination of small proportions, so that total alloy addition becomes less than 0.30% apart from C, Mn, Si, S & P. Usefulness of micro alloyed steels for engineering applications is well understood now. Its production applicabililies are to be seriously considered by the secondary steel sector. As reported in various literatures by micro alloying, tensile strength values of particular plain carbon steel can easily be enhanced by 10.15 kg/mm2 without adopting much expensive cooling cycles like TMT process. For that matter some trails have been planned to produce H.S.L.S. steel through induction furnace rout.
Selection of Chemistry of H.S.L.A. Steels
As a base chemistry, composition of rebar steels have been selected due to fact that these contain around 0.20-0. 25%C which is easily achievable by melting of mild steel scrap.
Major effect of silicon on strength is through substitutional solid solution hardening. It also increases the ductile to brittle transition temperature. So, silicon contents of base metal are selected to be in range of 0.35-50%. This can be obtained by FeSi for SiMn additions.
Manganese refines the pearlitic spacing and increases toughness of steels. It has an extremely potent effect on hardenability and improves weldability. So, its content has been limited to 1.2 - 1.4% and can be attained by FeMn & / or SiMn additions.
Vanadium increases the strength of carbon steels through a combination of precipitation hardening and refinement of the interlameller spacing of pearlite. As reported ill various literatures, the economic level of vanadium addition for effective strengthening is about 0.7%, but the strengthening factor drops above 0.1 1%. Hence, for trials. vanadium is to be limited from 0.6%-10%.
Vanadium in steels precipitates in the form of carbonitrides. In presence of free nitrogen in steels containing vanadium, VN precipitates at austenite ahead of VC. Precipitation continues and at the austenite-ferrite (Y-cc) transformation boundary, simultaneous precipitation of VN and VC in the form of mixed vanadium carbonitrides occurs.
Precipitation in austenite considerably retards grain growth. This refines the ferritic grains and thus, favourably effects strengthening. In view of the dissolved power of C, V & N in austenite?ferrite and their mutual interaction, nitrogen content is selected to be in the nitrogen of 0.009%-0.016% i.e. 90-160 ppm. Vanadium and nitrogen could well be added through Nitrovan alloy, but due to non-availability of this alloy in our country at present, addition of FeV and purging of nitrogen gas has been planned.
Additionally a set of trials has been planned with addition of columbium (Cb) in place of V+N. Cb contents shall be varied between 0.6%-10% to study the ef'f'ect.
Steelmakers of present generation are more concerned about quality steel production. This may be attributed mainly to the growing awareness of the consumers, resulting in a very tough competition. For quality steelmaking, steelmakers must have some flexibility of operations. Low alloy, especially low carbon steels, are considered a problem due to the refining limitations encountered. That is why coreless inducdon furnaces arc often nicknamed as very efficient "Dead Melter". Quality alloy steel grades, fetching handsome premium at the market-place, have drawn attention of the steel producers. To achieve the quality standards, induction furnaces may be balanced/coupled with a refining unit to draw benefits of induction furnace as an economic melting unit and remove the bottleneck, of lack of refining, hence very limited flexibility.
Maintaining a good level of 'S' in steels, i.e. upto 0.025% max has been the need of day particularly in the context of H.S.L.A. steels and other, provides sufficient challenge to the producers using
Induction furnace. A close control of charge chemistry is believed to be required.
Often level of 'S' presents problems as there are sources for 'S' pick up by the melt. Reduction of 'S' in induction furnace in difficult, as for disulphurization, a time slag with high basicity of around 2-4 and very low oxygen potential of the bath required apart from other factors.
Low sulphur levels are rapidly achieved using silicon carbide (SiC) a, primary deoxidiser. This practice is well established in EAF industry in spite of so much flexibility in slag conditioning. SiCcan easily be used in induction furnaces because of the reaction.
SiC+3(Fe0) = SiO + 3Fe + CO2
The products (at RHS) are SiO2 (acidic), Fe (metallic) and CO (gas). So, there has been no danger of reactions with lining. CO gas provides additional bubbling, thereby combination of these effects helps to rise the desulphurization of these effects helps to raise the desulphurization level. An addition of 50gm SiC granules (about 5mm size) per ton of molter, metal is recommended.
SiC also increase the fluidity of metal, helps in removal inclusions thereby providing a cleaning effect. This enhances the product quality. Thus it is recommended that for production of quality steels addition of SiC is to be adopted.