Satyendra Kumar Sarna, former ED, RINL & former MD, NINL, explains about the role and importance of technologies used in steel making process.
Technologies used in the steel industry can be broadly divided into the following four categories.
- Technologies related to the process of Iron making and supporting processes
- Technologies related to Steel making and casting
- Technologies related to Steel processing and shaping
- Technologies related to the processes which support the main processes of Iron making, Steel making and Steel processing
Iron Making and Supporting Technologies
Iron is produced either in blast furnace or through direct reduction. As per World Steel Association statistics, 1,112.4 MnT of iron was produced throughout the world during 2012, mainly by the blast furnace process out of which a small amount was produced through smelting process. The contribution of India in the production of iron was 48 MnT in 2012.
Another 74 MnT of Iron was produced in 2012 by the direct reduction processes which have set a new record for the industry, according to the data compiled by Midrex Technologies Inc and audited by World Steel Dynamics. As per this data, DRI production in India declined from 21.97 MnT in 2011 to 20.05 MnT in 2012.
In a blast furnace, Iron is produced through reduction of iron oxide charged in the furnace in the form of Sinter, Pellets and calibrated Iron ore with metallurgical coke which is produced in coke oven batteries by the carbonization of coking coals. Sinter is produced by agglomerating Iron ore fines in a sinter plant while, Pellets are normally produced from low grade Iron ores which are first beneficiated in an iron ore beneficiation plant and then pelletized in a Pellet plant. Blast furnace coke is produced either in coke ovens by indirect heating of coking coals in absence of air or in non recovery ovens where coking coal is heated with the combustion of generated gases within the ovens with controlled air.
In direct reduction process, Iron is produced either in vertical shaft kiln which uses reformed natural gas as reductant or in rotary kilns where non coking coal is used as reductant.
The Iron smelting processes are in various stages of development. Out of many Iron smelting processes, presently COREX, FINEX and HIsmelt have reached commercial stage. There are many other processes out of which HIsarna process is showing a lot of promise.
Steel Making and Casting Technologies
Steel making technologies follow three process routes which are (i) Oxygen steel making, (ii) Electric steel making and (iii) Open hearth steel making. As per World Steel Association, share of these three technologies in the steel production during 2012 was in the ratio of 69.6%: 29.3%: 1.1%. The corresponding ratio of these technologies in Indian steel production was 31.2%: 67.5%:1.3%.
Oxygen steel making process includes processes like top blown converter (LD converter), bottom blown converter, combined blowing process and energy optimizing furnace (EOF). Under these processes, impurities of molten iron (hot metal) are oxidized with pure oxygen gas (99.9%) which is blown in the converter.
In Electric steel making process, electricity is used as a medium of heat. Under this route, Steel is made either in electric arc furnace (both AC and DC arc furnaces) or in induction furnace. Originally, these furnaces were used for melting to utilize the Steel Scrap which is generated during processing of Steel or from the Steel products which are discarded when their useful life is over. But now the use of these technologies has been extended and Steel is being made with input materials like Sponge iron and even hot metal along with Scrap. Use of these input materials has helped in control of tramp elements in Steel made by these processes.
Steel made by these processes is cast into shapes either by ingot casting or by continuous casting. Casting of Steel in Ingots is inefficient way of casting and has been discarded in most of the plants around the world. As per World Steel Association the percentage of continuous casting in total steel production was 97% in the world during 2012. The corresponding figure for Indian steel industry was 79.9%.
Continuous casting process is used for casting slabs (width > 500 mm and thickness > 150 mm), Blooms (cross section > 150 mm x 150 mm) and Billets (cross section 150 mm x 150 mm or less). Continuous casting process is also available for casting of Steel directly into thin slabs (thickness 80 mm or less) or even strips in case of production of flat products.
Steel Processing Technologies
These technologies convert Cast Steel into final shaped products which are sold to the consumers. This activity is carried out mostly by rolling processes. Rolling is the process of plastically; deforming metal by passing it between rolls. It is the most widely used forming process, which provides high production and close control of final product. Rolling mill can be semi finishing mill (Blooming mill, Billet mill) or finishing mill to roll final saleable products. It can be hot rolling mill or cold rolling mill. It can be flat product mill or long product mill. The rolling mills are usually designated by the product it rolls such as rail mill, wire rod mill, plate mill and structural mill etc. Rolling mills are now-a-days set up with inbuilt heat treatment processes to roll out high quality products. Further, low temperature rolling is adopted to save on fuel. Universal rolling of structures has several advantages not only in rolling but it saves Steel weight when the rolled product is put to use.
Technologies supporting the above Processes
There are some other technologies which exist in the steel industry. These technologies are not connected directly with the making and shaping of Steels but they support main processes. Some of these technologies are calcination of limestone and dolomite, production of oxygen, nitrogen and argon gases by air separation process and production of steam and power in power plants using the by-product gases and thermal coal and waste energy. Also, in use are environment protection technologies, water and effluent treatment technologies, solid waste management technologies and many others.
Further, incorporation of computerized controls and higher level of process automation to various processes makes a big difference in productivity, reliability in operation and techno economics. Also, in each area of process technologies, several small, medium or big changes are taking place without making principle changes in the main technology. These changes are enhancing productivity, reducing specific consumptions, improving the process reliability, product quality and working environment besides making the processes safer.