Improvement in Impact Properties of 80kg/mm² Grade High Tensile Strength Steel Plate

Abstract

High tensile strength steel plate of 80kg/mm² grade which contains rather higher Al content in order to make a small amount of boron effective, shows in some cases very poor impact property in transverse direction.
Authors made systematic investigations about manufacturing processes from slab rolling to plate rolling as well as chemical compositions such as nitrogen and titanium.
It was found that poor impact energy in transverse direction was caused by AIN which precipitated concentratedly at the austenite grain boundaries during slow cooling after slab rolling and afterwards on the stage of plate rolling modified its distribution to line in row to longitudinal direction.
The ways to get the improved impact energy in transverse direction were also found as follows:
1. lowering the finishing temperature in slab rolling below 1 100°C.
2. increasing the cooling rate after slab rolling above 15°C/ min.
3. raising the heating temperature before plate rolling above 1 100°C so that AIN re-dissolves.
4. decreasing the nitrogen content as low as possible.
5. a small amount of Ti addition.
The methods 1-3 are intended to modify the distribution of AN and the ones 4-5 to decrease the amount of it.