Hot Rolled High Al Containing Steels as a Replacement for the Control Rolled High Strength Low Alloy (HSLA) Steels

The extent to which Al and Nb can be used to improve the properties of hot rolled steels has been investigated with the aim of obtaining mechanical properties similar to those given by the more expensive, control rolled or normalised route, eg. API X52 line pipe. Three steels with 0.02%Al, 0.16%Al and 0.16%Al, 0.018%Nb have been examined and their strength and impact behaviour obtained. The 0.16%Al steel had a similar strength to the 0.02%Al containing steel similar to 300MPa, but better impact behaviour (30-40 degrees C lower 54J, ITT) with an impact transition temperature (ITT) of -90 degrees C which from previous work will be due to a refinement of the grain boundary carbides. The present work shows that the addition of Nb to this high Al containing steel, although beneficial to strength, giving a lower yield strength (LYS) of 385 MPa, close to that given by some of the control rolled steels gives very poor impact behaviour with a 54J ITT of only -20 degrees C. The improvement of strength is mainly a result of precipitation hardening by NbCN with some benefit from grain refinement while the deterioration of impact behaviour might be due to the presence of lower transformation products or coarser carbides. Further work is required to positively clarify the cause of this deterioration and to explore further options in achieving the aim of obtaining a hot rolled steel with strength in the range 350-400MPa and 54J ITT of -50 degrees C.