Abstract
In the present paper, the medium-C Si-rich steel with a quenched martensite microstructure was heated to intercritical annealing temperatures at 750 °C, 760 °C and 770 °C after warm rolling deformation to obtain ferrite with varying volume fractions. Subsequently, bainite/ferrite multiphase microstructures were attained via austempering near Ms temperature. The microstructures of the test steel after different heat treatments were characterized by scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction, and corresponding tensile and impact properties were tested. The results showed that, with the increase of intercritical annealing temperature, the austenite content increased, which limited the growth of ferrite grains, and the grain size decreased from ~1.6 μm to ~1.4 μm. In addition, the degree of ferrite recrystallization was almost complete. At the same intercritical annealing temperature, compared with austempering above Ms, prior athermal martensite (PAM) was obtained after austempering below Ms, which effectively refined the size of bainite ferrite lath. Moreover, with the increase of intercritical annealing temperature, the bainite content of the test steel increased after austempering, resulting in the increase of yield strength, tensile strength and impact energy. In contrast, while the decrease in ferrite content led to a significant decrease in uniform elongation. At constant intercritical annealing temperature, the tensile strength decreased slightly, and the impact property improved after austempering above Ms.
Highlights
Publisher’s Note: MDPI stays neutralAs an important branch of high-strength low-alloy (HSLA) steel, dual-phase steels have been widely used in some industrial fields because of their excellent mechanical properties [1–4]
When the intercritical annealing temperature increased from 750 ◦ C to 770 ◦ C, the retained austenite content decreased from 12.5–15.3% to 5.8–8.6%
Ultrafine-grained bainite multiphase microstructure was obtained in the medium-Cferrite/low-temperature
Summary
As an important branch of high-strength low-alloy (HSLA) steel, dual-phase steels have been widely used in some industrial fields because of their excellent mechanical properties [1–4]. Researchers have tried to use bainite with better toughness to replace martensite to form bainite/ferrite dual-phase steel. This approach has improved the toughness significantly at the expense of strength [5,6]. In the 1980s, Bhadeshia et al [7–11] first obtained and defined the low-temperature bainite under isothermal transformation at a low-temperature condition of T = 0.25Tm (Tm is the absolute melting temperature) for several days in some high-carbon high-silicon steels This low-temperature bainite microstructure was composed of ultra-fine bainite ferrite lath and thin-film retained austenite between laths, which had high strength and good toughness. Warm-rolling deformation, which was applied to medium-carbon Si-rich low-alloy steel, and different intercritical annealing temperatures were selected to obtain ferrite with different volume fractions (~30%, ~15% and ~5%). The effects of different heat treatment processes on the microstructure and mechanical properties (including tensile and notch impact toughness) were compared and analyzed
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