THE EFFECT OF RATE OF COOLING FROM THE AUSTENITE RANGE UPON THE MICROSTRUCTURE OF LOW-CARBON RIMMED STEEL

On the formation of pearlite-free structure

Abstract

The microstructure of low-carbon rimmed steel specimens cooled continuously from their austenite range to the room temperature at the various cooling rates was investigated in relation to their cooling rates. All specimens were first given a homogenizing annealing at 950°C for 30mn. They were then rapidly coold at the rates of 300°C/mn, 150°C/mn, or 20°C/mn to the different temperatures, 800°C, 750°C, 700°C and 650°C and subsequently cooled slowly to the room temperature at the rate within the range from 3°C/mn to 96°C/mn.
The results obtained were as follows.
1) Cooling slowly at the rate 3°C/mn below 700°C a pearlite-free structure was formed with the 0.08% C specimens cooled at the rate above 150°C/mn to 700°C and with the 0.05% C specimens cooled at the rate above 20°C/mn to the same temperature. In other words, the formation of the pearlite-free structure was affected by the temperature at which the slow cooling began and also by the slow cooling rate.
2) The specimens which showed the pearlite-free structure gave the exceedingly high Charpy impact values in comparison with the annealed specimens of the same chemical composition.