Microstructures of Pearlite and Martensite Transformed from Ultrafine-Grained Austenite Fabricated through Cyclic Heat Treatment in Medium Carbon Steels

Abstract

This study investigated the microstructures of pearlite and martensite transformed from ultrafine-grained austenite in 0.45C steel and Vadded 0.45C steel. The mean prior austenite grain sizes were refined to be 4.5 µm in the 0.45C specimen and 2.4 µm in the V-added 0.45C specimen through cyclic heat treatment of austenitizing and water quenching, i.e., repetition of martensite § austenite transformations. The austenite with various grain sizes was either air-cooled or water-quenched to cause pearlitic or martensitic transformation, respectively. The fragmented lamellar structure and isolated cementite particles having particulate morphology were observed in the pearlite transformed from the ultrafine-grained austenite. Results indicated that the ferrite and cementite formed during eutectoid transformation from the ultrafine-grained austenite were not necessary to grow cooperatively maintaining lamellar shape. The packet size in lath martensite was refined significantly accompanied by decreasing of the austenite grain size, since several different variants of martensite were formed from different segments of austenite grain boundaries even when the austenite grain size was fine. In contrast, the block size did not depend on the austenite grain size.

Challenging Ultra Grain Refinement of Ferrite in Low-C Steel Only by Heat Treatment

“…Instead of the typical lamellar pearlite, the specimen showed the pearlite structures composed of fragmented cementite dispersed in ferrite, as was indicated by the arrow in Figure 8F. Shibata et al (2013) also observed such a pearlite structure in a medium-carbon steel and explained the formation of fragmented pearlite transformed from fine-grained austenite in the following way. When the austenite grain size is smaller, the distance between neighboring pearlite colonies formed on austenite grain boundaries (or triple junctions of austenite grain boundaries) becomes shorter.…”

“…On the other hand, an alternative way utilizing cyclic transformation between martensite and austenite was established by Grange to make austenite grain sizes in steels smaller than 1-2 μm without deformation (Grange, 1971). Shibata et al (2013) used the cyclic heat treatment for refining austenite grain sizes in medium carbon steels and could obtain the austenite grain size of 4.5 and 2.4 μm in a 0.44C-0.86Mn (mass %) steel and a 0.46C-0.84Mn-0.3V steel, respectively. Furuhara et al (2008) used a cyclic heat treatment followed by air-cooling and reported that fine-grained ferrite having a grain size of 2.2 μm was successfully obtained in a medium-carbon steel with a chemical composition of Fe-0.35C-1.05Cr-0.17Mo (in mass%).…”

Effect of vanadium on hydrogen embrittlement susceptibility of high-strength hot-stamped steel

“…In the present work, the purpose of adding microalloying elements into the traditional chemical composition system of hot-stamped steel (e.g., 22MnB5 and 30MnB5) is to reduce the size of austenite and enhance the strength of matrix before martensitic transformation, and a fine martensitic lath will be obtained ultimately after hot-stamping process [15][16][17][18][19]. Compared with V free hot-stamped steel, the vanadium microalloying element is introduced in 30MnBNb hot-stamped steel, the susceptibility to HE and the microstructures are analyzed, and the results are summarized in this paper.…”

Grain refinement of 2Mn-0.1C steel by repetitive heat treatment and recrystallization

“…However, it seems difficult to apply SPD processes to mass production of steels because of the limited dimensions of the SPD processed materials. Several alternative processes for refining grain size other than SPD have been also proposed, for example, repetitive heat treatment [12,13] and recrystallization of cold-rolled DP steel [14]. In the present study, grain refinement of ferrite and DP structures through repetitive heat treatment and subsequent cold-rolling and recrystallization was investigated.…”