Materials are typically ductile at higher temperatures and become brittle at lower temperatures. In contrast to the typical ductile-to-brittle transition behavior of body-centered cubic (bcc) steels, we observed an inverse temperature dependence of toughness in an ultrahigh-strength bcc steel with an ultrafine elongated ferrite grain structure that was processed by a thermomechanical treatment without the addition of a large amount of an alloying element. The enhanced toughness is attributed to a delamination that was a result of crack branching on the aligned {100} cleavage planes in the bundles of the ultrafine elongated ferrite grains strengthened by nanometer-sized carbides. In the temperature range from 60 degrees to -60 degrees C, the yield strength was greater, leading to the enhancement of the toughness.
Bulk ultrafine-grained (UFG) low-carbon steel bars were produced by caliber rolling, and the impact and tensile properties were investigated. Initial samples with two different microstructures, ferrite-pearlite and martensite (or bainite), were prepared and then caliber rolling was conducted at 500°C. The microstructures in the rolled bars consisted of an elongated UFG structure with a strong a-fiber texture. The rolled bar consisting of spheroidal cementite particles that distributed uniformly in the elongated ferrite matrix of transverse grain sizes 0.8 to 1.0 lm exhibited the best strength-ductility balance and impact properties. Although the yield strength in the rolled bar increased 2.4 times by grain refinement, the upper-shelf energy did not change, and its value was maintained from 100°C to À40°C. In the rolled bars, cracks during an impact test branched parallel to the longitudinal direction of the test samples as temperatures decreased. Delamination caused by such crack branching appeared, remarkably, near the ductile-to-brittle transition temperature (DBTT). The effect of delamination on the impact properties was associated with crack propagation on the basis of the microstructural features in the rolled bars. In conclusion, the strength-toughness balance is improved by refining crystal grains and controlling their shape and orientation; in addition, delamination effectively enhances the low-temperature toughness.
(0.2-0.6)%C-2%Si-1%Cr-1%Mo steels were quenched and tempered at 773 K and deformed by multipass caliber rolling (i.e. warm tempforming) with a rolling reduction of 78%, in order to obtain ultrafine elongated grain (UFEG) structures. The tensile and Charpy impact properties of the warm tempformed (TF) steels were investigated to determine the influence of the carbon content on toughening in the UFEG structures. The TF samples consisted of UFEG structures with strong <110>//rolling direction (RD) fiber textures. The transverse grain size and aspect ratio in the UFEG structure tended to reduce as the carbon content increased, whilst the carbide particle size became slightly larger. The increase in the carbon content resulted in an increase in the yield strength from 1.68 to 1.95 GPa at room temperature; however, it was accompanied by a loss of tensile ductility. In contrast to quenched and tempered samples exhibiting ductile-to-brittle transitions, the TF samples exhibited inverse temperature dependences of the impact toughness. This was due to delaminations, where cracks were observed to branch in the longitudinal direction (//RD) of the impact test bars. The upper-shelf energy of the TF sample was enhanced as the carbon content decreased, and higher absorbed energy was also achieved as delamination occurred at lower temperatures. The delamination was found to be controlled not only by the transverse grain size, the grain shape, and the <110>//RD fiber texture but also by carbide particle distribution in the UFEG structure.
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