Determination of Condition of Recrystallised Grains After Annealing of Cold Rolled IF Grade Steel Sheets Based on Ultrasonic Measurements

The Interstitial Free (IF) grade steel sheets are used for auto body applications where these sheets require good formability/drawability property. To achieve this property the cold rolled sheets are annealed to obtain desirable texture in the recrystallised grains to avoid material failure during forming operation. The paper deals with an ultrasonic technique to determine the condition of the recrystallised grains after annealing operation. 79 % cold rolled IF grade steel sheets were annealed at 650 deg. C for different times. After annealing the scale was completely eliminated from the surface by pickling to make the surface conducive for transmission of ultrasonic surface waves. Ultrasonic transit time for a fixed distance in μs was measured in rolling as well as 45 to rolling direction using 4 MHz transmitting and receiving transducers. Unrecrystallised, partially recrystallised and fully recrystallised regions in this case were determined by measuring hardness in Rockwell B scale. In fully recrystallised grains the above transit time difference was found to be more than 2.20 μs in partially recrystallised grains it was within 1.52-2.20 whereas that for unrecrystallised grains, it was less than 1.52 μs. Based on these experimental results, the condition of cold rolled and annealed sheets can be obtained which will help in optimising the annealing cycle for obtaining correct grain structure.