What is Billet?
Billet is the solid form of liquid metal given a shape in continuous casting plant (CCM) provided as a raw material to produce different sizes of MS DEFORMED BARS, ANGLES, CHANNELS and other steel products in rolling mill.
Billets are the appropriate raw material of MS DEFORMED BARS because the way of making billets in CCM is superior to the conventional one with regard to surface finish and internal porosity. There are many kinds of billets differ in sizes, shapes and grades. We produce 100mm x 100mm square, 60 grade and 40 grade billets for our rolling mill.
We also import billets for 40 grade deformed bars and angles from overseas to fulfill the increasing demand of the market but billets for 60 grade, we produce in our own care.
Production Procedure of Billets
Solid selected scrap brought from scrap yard to furnace floor for charging. Charging means to fill up the blank chamber of crucible with melt able scrap. Charging by magnet or by hand both are effective. Heavy scrap cut in sizes charged at the bottom first and lighter scrap above. Shredded scrap charged also which helps to fill the gaps inside and make the volume compact. A small quantity of cast iron added purposely for fast melting.
As the furnace is induction type so the metal is heated by its resistance to induced currents. The scrap charging goes on periodically up to the molten metal inside crucible come to a certain level of its capacity. A quantity of sponge iron also charged by maintaining a balanced proportion with other kinds of scrap. Slag taken out at intervals.
When solid scrap become liquid, first bath sample from molten metal send to laboratory for chemical analysis. This analysis determine the presence and percentage of various elements in metal such as carbon, manganese, silicon, sulfur, phosphorous etc.
After completion of analysis and the molten metal raised in a certain level inside crucible, calculated quantity of ferro alloys like ferro manganese (FeMn), ferro silicon (Fe Si), silico manganese (SiMn) added primarily with molten metal. All addition must be completed at least half an hour before tapping the heat in order to permit them become thoroughly mixed with the metal.
During the last part of the process opportunity is given for the gases and non metallic impurities remaining in the metal to raise to the slag (A fused mass formed by the action of the flux on the gangue of the scrap and ore is known as slag produce in oxidation and desulfurization periods). Pieces of aluminium also added as deoxidizer.
Temperature measured by a pyrometer. If it indicate the desired temperature(1600°c – 1620°c) it means that the metal is ready for tapping.The molten metal is tapped from the furnace to a pre heated ladle and then carried by the overhead crane to LRF (Ladle refinery furnace). It is to be mentioned that, refining of steel by LRF is now an essential part of the production in order to produce super quality steel billets. Hence graphite electrodes come down and arc the liquid simultaneously to raise the desire temperature for casting. Meanwhile purging by nitrogen gas continue from the time of tapping helps to destroy the existing temperature stratification pattern and homogenize the ladle contents.
Second bath sample collect and send to laboratory for spectrometer analysis. As soon as the analysis done, a calculated quantity of ferro alloys finally added again with the metal to make particular grade of billets and bar size to be rolled. Addition of ferro alloys in LRF reduce manganese loss, heat loss, advantageous to enhance the quality of billet. Temperature measure by the pyrometer again. 1585 to 1610°c (depend on composition of steel grade) should be the appropriate temperature to send the ladle to CCM for continuous casting.
From ladle the steel poured through a tundish into a water cooled copper mould. The bottom of this mould is sealed in the beginning by means of a dummy billet. As soon as the mould full, the dummy billet is withdrawn and as the casting proceeds, the desired height of liquid metal in the mould is maintained by adjusting the pouring speed and rate of with drawl.
The billet coming out of the bottom of the mould, is passed through another cooling chamber, which is sprayed with water to aid in complete solidification of steel. The billet then passes through the with drawl rolls and the continuously cast billet is then cut into desired length.
An external inspection and chemical analysis taken place by our quality department soon after the billets are somewhat cooling down. if no obvious defects are apparent and the chemical analysis establish the desire grade, heat number and suitable bar size marked on billets and cut into length for rolling.