Abstract
In this paper, two low carbon microalloyed steels, named as steel A and steel B, were fabricated by ultra fast cooling (UFC). In both steels, the microstructures containing quasi polygonal ferrite (QF), acicular ferrite (AF) and granular bainite (GB) can be obtained by UFC process. The amount of AF in steel B is more than that in steel A. The size and distribution of precipitates (Nb/Ti carbonitrides) in steel B are finer and more dispersed than those of in steel A due to relatively low finish cooling temperature. The mechanical properties of both steels are effectively enhanced by UFC process. UFC process produces low-temperature transformation microstructures containing a significant amount of AF. The mechanical properties of steel B were more satisfactory than those of steel A due to the finer average grain size, the greater amount of the volume fractions and smaller size of secondary phases.
Highlights
High-strength low-alloy (HSLA) steels are those highstrength structural steels having good toughness and weldability
The cylindrical rod specimens with 8 mm diameter and 15 mm length were machined from the plates in order to measure the transformation temperature in a thermomechanical simulator
The Charpy absorbed energies level of steel B at -15oC is only slightly lower than those of steel A at -10oC, despite microalloying and ultra fast cooling (UFC) process result in higher strength and higher hardness for the former
Summary
High-strength low-alloy (HSLA) steels are those highstrength structural steels having good toughness and weldability. This combination of properties have led to their varied applications in the automotive industry, in manufacturing of large diameter pipes for gas and oil transportation in the areas of low temperature, and in fabrication of plates for naval ship’s construction[1]. Thermomechanical controlled processing (TMCP) and microalloying in order to obtain desired microstructure and properties are the essence of ultra-low carbon microalloyed steel[7]. TMCP has become the most powerful and effective manufacturing process to satisfy increased hardenability, improved strength, and superior low-temperature toughness[8].
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