LD PROCESS HEAT CYCLE

Heat Cycle


In the steel making shop one heat cycle is subjected to charging of solid scrap and liquid iron to tapping of refined steel within which total process is going on.

● After the previous heat tapped and slag is drained the vessel is brought to vertical (0°) position for bottom batch then inclined for 50-55° to received scrap. The scrap is virtually poured in the vessel from the charging chute.

● The vessel is rocked to gently drop the scrap on the bottom and is brought back to inclined for 38-45° position to receive hot metal. The metal is kept waiting in the ladle for being charged.

● The vessel is rotated to vertical position, Lance is lowered to its first blowing position and the oxygen is turned on.

● As soon as ignition takes place part of the lime is charged along with other fluxes to form early slag. The rest of the fluxes are added as per predetermined intervals but in the early part of the blow. Some prefer to add all fluxes in one lot after good ignition.

● The blow continues for nearly 15 to 25 min during which period the Lance height may be altered as per bath level. The oxygen is put off and the Lance is raised above.

● The vessel is rocked back to horizontal position +90° and slag and metal sample are taken for analysis. Temperature of the bath is measured by immersion thermocouple. And excess slag is dumped in slag pot.

● If the analysis and tapping temperature are correct it is rocked in tapping position and steel is tapped in the ladle at an angle 80-103°. Deoxidizer and alloying additions are made in the ladle in the same way as described for open hearth practices, and through the additional tapping chute for other process.

1. Here tap to tap time of 30-50 mins. Delays due to tap hole repair, cleaning of Lance tip etc may increase heat time by a few minutes. The most efficient plants have an average tap to tap time 30-35min where as 40-50min is considered a good average heat time.

2. If the analysis is not correct according to the desire plan, the vessel is re-blown for a short duration which may be about 1 to 2 min. On retaining of the previous slag for the next heat to help from early slag. This also a bad impact such time which leads to sloping during blowing in the initial stages.

3. The Lance is located at the highest distance from the bath level and slowly lowered to required position during the course of blow.The Lance height depends on the shape, size and raw materials used and blowing condition.
LD process control
Fig. LD process control

Process Economics :

● Ejection from the vessel that is slopping should be kept to a minimum. It is not leads to loss of metal but affect smooth operation. It caused by lower Lance position during blowing for long. Build up of FeO in the slag when carbon is high causes violent and intermittent reaction resulting in sloping.

● Lowering the Lance too close to the bath cause metal particles to be ejected from the mouth in the from of sparks. The ejection can damage the Lance. Early formation of the slag is beneficial in minimising sparking.

● Being a basic process, more silicon content in the metal leads to increase in the slag bulk which affects the lining life adversely. A very high silicon and phosphorus content needs double slag practice.

● Spar or alumina dross should be used sparingly to improve lining life. Its use adds to the cost of steel making.

● The Lance should be absolutely vertical in blowing position. Inclination on any one side causes the gases to escape in some preferential direction resulting in excessive erosion of lining.

● If the scrap is wet or greasy it should be held in the vessel for a while, before charging the hot metal, to drive off the moisture. Blast furnace slag should not flow in along with the metal since it will alter the basicity of refining slag.

● The cleaning of skull at the mouth of the vessel may protect the lining but excessive skull formation reduces the mouth area. It is generally cut in to two rings by a flame so that the lower part falls in due to its own weight and outer one is removed through machine.

● Extra superheat at turndown unnecessary affects lining life adversely. Surplus oxygen supply may burn CO to CO2 at the mouth and cause lining wear.

● Delay in tapping and extra holding of a heat due to other delays reduces lining life.





References :
1. Modern Steel Making :  Dr R.H. Tupkary and V.R. Tupkary.
2. Ironmaking and Steelmaking Theory an d Practice : A. Ghosh and A. Chatterjee
3. Steel Making : A.K.Chakravorty



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