Second phase precipitation in ultrafine-grained ferrite steel

Abstract

Grain size refinement is one of the most efficient strengthening mechanisms applied to modern High-Strength Low-Alloy steels (HSLA) because yield strength and toughness are both improved. This paper discusses the distribution of carbides by using transmission electron microscopy (TEM) in a low-carbon steel with ultrafine grained (UFG) ferrite. Fine cementite particles were formed during water quenching due to the auto-tempering of highly distorted martensite. Other fine particles observed under the same condition were nucleated due to the presence of carbide formers such as niobium, titanium and vanadium. TEM analysis showed that cementite particles underwent Ostwald ripening during warm rolling but they were still able to inhibit ferrite grain growth, which was maintained 1µm size approximately.

Characterisation of precipitation and carbide coarsening in low carbon low alloy Q&T steels during the early stages of tempering

“…Auto-tempering occurs in the three steels during water quenching, where cementite and ε'carbide are both observed, in contrast with some reports for these grades [1][2][3][4]. In the Base and Base-Mo-V steels, the dominant as-quenched carbide is cementite.…”

“…Quenched and tempered (Q&T) steels, which normally have a relatively low carbon content and also contain alloying elements such as Mn, Mo and Cr, are widely used in applications such as cranes and earth movers, due to their desirable mechanical properties combining high strength and good toughness. Lath martensite normally forms during quenching with the occurrence of auto-tempering due to the low carbon and alloying contents resulting in high Ms temperatures [1][2][3][4]. Cementite is observed as the predominant as-quenched carbide in low carbon alloyed steels with relatively high Ms temperatures (>400 °C) [2,4], e.g.…”

“…Lath martensite normally forms during quenching with the occurrence of auto-tempering due to the low carbon and alloying contents resulting in high Ms temperatures [1][2][3][4]. Cementite is observed as the predominant as-quenched carbide in low carbon alloyed steels with relatively high Ms temperatures (>400 °C) [2,4], e.g. observed in agitated water quenched Fe-0.16C-1.49Mn-0.27Si-0.28Cr-0.048Nb-0.0044V-0.0046Al steel [4].…”

“…Cementite is observed as the predominant as-quenched carbide in low carbon alloyed steels with relatively high Ms temperatures (>400 °C) [2,4], e.g. observed in agitated water quenched Fe-0.16C-1.49Mn-0.27Si-0.28Cr-0.048Nb-0.0044V-0.0046Al steel [4]. Whereas, close-packed hexagonal ɛ-carbides, proposed by Jack using powder diffraction patterns [5], or ɛ'-carbides with orthorhombic symmetry, named by Taylor [6] with the carbide being partially ordered, have been identified within the laths during quenching for steels with relatively lower Ms temperatures [1,3], such as after oil quenching of AISI 4340 steel where Ms is approximately 304 °C [3].…”

The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling

“…The suppression of grain boundary migration due to microalloying is caused by either the solute dragging effect caused by segregation of alloying elements to the boundaries, or the pinning effect caused by carbo-nitride precipitates. These very fine precipitates are responsible for the fine-grained ferritic structure 23 . Noting that the size and distribution of precipitates (Nb/Ti carbonitrides) in steel B are finer and more dispersed than those of in steel A (Fig.…”