Microstructure and Mechanical Properties of a Lamellar-Structured Low-Alloy TRIP Steel

In this paper, we report a lamellar-structured low-alloy transformation-induced plasticity (TRIP) steel; the microstructure of the steel consists of alternate lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. Such a microstructure was produced by a heat treatment process similar to that for producing conventional TRIP-assisted steels, i.e. intercritical annealing followed by austempering. Nevertheless, quenched martensite rather than a mixture of ferrite and pearlite was used as the starting structure for intercritical annealing to form austenite, and the resulting austenite was then transformed to bainite by austempering treatment. This steel exhibits much enhanced strength-ductility combinations as compared with those conventional polygonal-structured low-alloy TRIP steels.

Enhancing the Robustness and Efficiency in the Production of Medium Mn Steels by Al Addition

The narrow process window during intercritical annealing and discontinuous yielding have limited the commercialization of medium Mn steels. In this study, a double-annealing process based on the commercial continuous annealing line is proposed. The cold-rolled medium Mn steels were first fully austenitized and quenched during the first annealing, followed by intercritical annealing for reverted austenite transformation. The microstructure of duplex lath-shaped austenite and ferrite is produced and steel exhibits a desirable continuous yielding during tensile deformation. Al is added into the medium Mn steel to enlarge the process window and to improve the partitioning efficiency of Mn. The produced steel is more robust with temperature fluctuation during the industrial process due to the enlarged intercritical region. Mn partitioning is more efficient owing to the elevated annealing temperature, which results in the improvement of ductility in the Al-added steel with increased austenite stability.

Age Hardening Characteristics of an Ultra-Low Carbon Cu Bearing Steel

We studied the influence of aging temperature on microstructure and mechanical properties in an ultra-low carbon Cu bearing steel in the present study. During the aging process, a continuous recovery of matrix associated with formation and growth of Cu precipitates could be observed during aging processes, exerting significant effects on the mechanical properties of the steel. At aging temperature below 600 °C, the mechanical properties were dominated by the precipitation strengthening effect, leading to excessive matrix strengthening and poor low-temperature toughness. Conversely, steel aged at temperatures above 650 °C exhibited an extraordinary improvement in toughness at the expense of strength, which can be attributed to the synergistic effects of softening matrix, coarsened Cu precipitates and formation of reverted austenite. After aging at 650 °C, reverted austenite formed at the lath boundaries. Increasing the aging temperature to 700 °C lowered the thermal stability of reverted austenite, consequently, the reverted austenite was partially transformed to fresh martensite. After aging at 650 °C for 0.5 h, the mechanical properties were optimized as follows—yield strength = 854 MPa, tensile strength = 990 MPa, elongation = 19.8% and Charpy impact energy = 132 J at −80 °C.

Aging Effect on Microstructure and Machinability of Corrax Steel

In this paper, the influence of the aging process on the microstructure and machinability of Corrax Steel was investigated for four samples: a solution heat-treated (A0) and three samples aged at 400ºC (A4), 525ºC (A5.25) and 600ºC (A6) for four hours. The effect of aging temperature on hardness was examined. Machining tests were carried out using a CNC lathe with a multi-layer coated PVD (AlTiN) cutting tool, at various cutting speeds (50, 100, 150, 200, 250, 300, 350m/min) with constant feed rate (0.1mm/rev) and 1mm constant cutting depth. The microstructure was investigated using an optical microscope and EDS attached SEM. The effect of aging on reverted austenite formation was also evaluated. In order to understand the changes in surface topology, cutting forces and vibrations were measured. With increasing aging temperature, the lath martensite was transformed to plate martensite because of the formation of precipitates and reverted austenite. Aging at different temperatures increased hardness up to 58%, cutting forces up to 117% and surface roughness up to 450%. The results describe the effect of the aging treatment on cutting forces, surface topology, tool wear and vibrations.