Steel Making: Ingot casting
1. MY: 301 Steel Making Processes Introduction to Ingot Casting: Molten steel from BOF/EAF is tapped into a teeming ladle. Deoxidizers, decarburizes and alloying elements if required, are added for the final finishing with respect to oxygen content and other elements in steel. The steel may be degassed either before or during casting. Ingot casting is done in cast iron moulds having square, round or polygon cross section. Ingots with square cross section are used for rolling into billets, rails and other structural sections. Whereas, ingots with rectangular cross section (also known as slab), are used for rolling into flat products. Round ingots are used for tube making. Polygon ingots are used to produce tyres, wheels, etc. Typically an ingot weighing 5-20 tons for rolling, whereas few hundred to 300 tons for forging.
2. MY: 301 Steel Making Processes Teeming Ladle Uphill teeming or Indirect Teeming & Downhill or Direct Teeming:
3. MY: 301 Steel Making Processes
4. MY: 301 Steel Making Processes Ingot Mold Type: (Cast Iron) • Cast iron has good thermal shock resistance. • Cast iron has a different coefficient of expansion from that of steel and hence the mold shrinks away from the ingot face on solidification, and thereby separation the two readily. • Inner walls of the mould are coated by tar or fine carbon. The coated material decomposes during solidification which prevents sticking of solidified ingots with the inner walls of the mold.
5. MY: 301 Steel Making Processes Molds are essentially of two types: 1. Wide end up or narrow end down as shown in figure 32.1 a) 2. Narrow end up or big end down as shown in figure 32.1 (b) Figure (a) wide end up moulds (b) Narrow end up moulds Wide end up moulds are used to produce forging ingots of killed
6. MY: 301 Steel Making Processes plain carbon or alloy steels. Wide end up molds may have a solid bottom. Narrow end up molds are commonly used to produce rimming and semi-killed steel ingots. A narrow-end-up mold facilitates easy escape of rimming reaction product, CO.
7. MY: 301 Steel Making Processes Mechanism of Solidification: (Macrostructure) The grain macrostructure in ingots and most castings have three distinct regions or zones: 1. Chill zone, 2. Columnar zone, 3. Equiaxed zone. The image below depicts these zones of Killed steel. • The metal next to the mould walls and bottom is chilled by the cold mould surfaces. This is a thin layer and is known as chill, shell or skin of an ingot and has a fine equiaxed grains. Here is the nucleation phase of the solidification process takes place.
8. MY: 301 Steel Making Processes • As more heat is removed the grains grow towards the center of the casting. These are thin, long columns that are perpendicular to the casting surface, which are undesirable because they have anisotropic properties. OR • After the formation of initial chill layer further solidification results in the formation of dendrities which grow along their principal axis perpendicular to the mould walls. • Their lateral growth (on the sides) is restricted due to the growth of adjoining dendrities giving rise to elongated crystal. If the length of these is appreciable is known as columnar structure • In general columnar structure does not extend to the centre of the ingot. The central portion solidifies as equiaxed grains of bigger sizes than those in the chill due to slow cooling. These are desirable because they have isotropic properties. • One zone blends into the next gradually. The extent of each zone varies with composition and temperature of liquid steel, mould design and its temperature at the time of teeming.
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