Vanadium - Transforming Possibilities

The Effects of Accelerated Processing on the Microstructure and Mechanical Properties of V- and V+ Cb HSLA Steels

Abstract:

THE EFFECTS OF HOLD-PERIOD accelerated cooling on microstructure and mechanical properties are investigated for a series of vanadium- and vanadium- columbium microalloy plate steels. 

Water-spray cooling vanadium-columbium steels decreases the hold period by a factor of 2-3, and results in a smaller ferrite grain size and improved strength and toughness compared with the normally processed plate. However, this treatment also produces a layer of hard, pancaked grains approximately 500 um thick at the plate surfaces. The pancaked grains are deformed, unrecrystallized ferrite which occur because of enhanced Cb(C,N) precipitation and an elevated Ar3 temperature in the surface region during the hold period.

Pancaked grains can be avoided in vanadium columbium steels by cooling during the hold period with a compressed air-water mixture. By using this treatment the plate surfaces remain above Ar3, the hold-period is shortened by a factor of two and there is still some improvement in strength and toughness of the rolled plate.

For the vanadium steels, water-spray cooling can be used to shorten the hold period, no pancaked grains occur and there is no change in the mechanical properties of the rolled plate. In this case, V(C,N) precipitates that form in the surface region during the hold period cooling redissolve when the plate is thermally equilibrated prior to final rolling.

Keywords:

vanadium, niobium, microalloyed steel, hot rolling. 

PDF Preview: