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.
Highlights
According to Hall-Petch model the decrease in the ferrite grain size implies improved yield strength of low-carbon structural steels
Sub regions inside each lath were separated by dense dislocation arrangements that could be normally distinguished in transmission electron microscopy (TEM) micrographs
Such electron diffraction behavior was associated to the “fragmentation” of the TEM image contrast, especially in dark field (DF) mode due to very small located differences in diffraction orientation
Summary
According to Hall-Petch model the decrease in the ferrite grain size implies improved yield strength of low-carbon structural steels. This microstructure-properties behavior has led to the development of new microalloying compositions and thermomechanical routes aiming at the industrial manufacture of steels with ultrafine grains. A distribution of fine cementite or carbonitride particles could inhibit the ferrite grain growth at lower temperatures, where recrystallization is quite restricted. The role of this fine dispersion particles on the formation of ultrafine grains has been few explored by transmission electron microscopy (TEM). In the present paper both dispersions of cementite and carbonitride, formed during different stages of thermomechanical processing, were analyzed and its effect to obtain an ultrafine ferrite microstructure based on commercial low-carbon steel is discussed
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