Hot rolled high Al containing steels as a replacement for the control rolled high strength low alloy (HSLA) steels

Abstract

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~300MPa, but better impact behaviour (30-40°C lower 54J, ITT) with an impact transition temperature (ITT) of-90°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°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°C.

The effect of Al and Nb contents, cooling rate and rolling condition on the microstructure and corrosion behaviour of HSLA steel

“…The key concept of the current work emerges as a continuation of our research on the effect of Al, Nb and their combination along with variation in the cooling and rolling conditions on the changes in the microstructural features and mechanical performance of HSLA steel [8][9][10]. The inherent protection capabilities of Al and Nb are well-documented, but the impact of small alloying addition on the corrosion behaviour of steel requires further attention.…”

“…The current authors [8] reported that adding 0.16%Al to HSLA steels enhances toughness considerably without impairing strength. A further study [9] revealed that the addition of 0.018%Nb is beneficial to improve the strength through grain size refinement and precipitation hardening [10,11]. Moreover, mechanical properties are affected by the processing conditions including the rolling condition that has a marked effect on properties, in which controlled rolling is known to further increase strength and toughness in comparison with hot rolling, but the effect of cooling rate after rolling must be taken care of [12].…”

Influence of Al content on the corrosion resistance of micro-alloyed hot rolled steel as a function of grain size

“…The corrosion rate increased markedly from 0.9 to 1.2 mm/yr when the grain size grew up from 20 to 27μm. Therefore, beside its beneficial effect in enhancing mechanical properties [8], finer grain size also promoted corrosion resistance. Higher volume fraction of AlN precipitates were observed in the high Al containing steel, resulting in finer grain size.…”

“…In addition, homogeneous distribution of grain size enhanced passivation status leading to lower corrosion rate. Qaban et al [8] examined the influence of Al on grain size of HSLA and found out that AlN precipitation was beneficial in pinning grain boundaries during solidification and thus refining microstructure and encouraging homogeneous distribution of grain size.…”

Investigation of corrosion resistance of high-strength low-alloy (HSLA) steel in fresh and salt water for pipeline application

“…Recently, high-strength low-alloy (HSLA) steel has attracted much attention by various engineering applications, such as pipelines, construction, offshore structures and automobiles [1]. The main features of HSLA steel include but are not limited to high strength, good ductility and low weight which make its use possible in most of the engineering fields.…”