Promising bending properties of a new as-rolled medium-carbon steel achieved with furnace-cooled bainitic microstructures

Abstract

The bending properties of a new thermomechanically processed medium-carbon (0.40 wt% C) low-alloy steel, intended for the slurry transportation pipeline application, have been investigated. The studied material was hot-rolled to 10 mm thick strips followed by direct quenching to two different quench-stop temperatures (QST) of 560 °C and 420 °C. The samples were subsequently cooled slowly to room temperature in a furnace, producing two different bainitic microstructures. In general, the final microstructures on the centerline consisted of different bainitic features with yield strengths of a ~700 MPa and ~1200 MPa for QST 560 °C and 420 °C, respectively. To determine the factors affecting bendability, as determined by three-point brake press bending tests, local microstructural features and texture were characterized with transmission and scanning electron-microscoy and macrohardness tests. Detailed quantitative microstructural evaluation of both subsurface and mid-thickness regions revealed that the bainitic sample produced at the higher temperature (QST 560) consisted of almost equal amounts of bainite types B1 and B2, where B1 has bainitic sheaves with a low dislocation density and intralath cementite, and B2 a very dislocation-dense morphology with mainly interlath cementite. In the QST 420 sample, the high dislocation density components B2 bainite and martensite were dominant, although martensite was only present near the strip surfaces. Different post-rolling cooling conditions did not change the general crystallographic theme but resulted in a slight increase in the texture intensity of the QST 420 sample. Neither the concave nor convex subsurface regions showed significant changes in texture after bending. The more favorable distributions of microstructural components and textural component intensities in the QST 560 sample resulted in higher elongation to fracture and work-hardening capacity, resulting a smaller minimum usable punch radius than for the stronger QST 420 sample. Fractographic examination of the cracked surfaces revealed that cracks developed by shear band formation followed by surface roughening, which promoted subsequent void and micro crack nucleation and growth.