Refinement of Ferrite Grain Size near the Ultrafine Range by Multipass, Thermomechanical Compression

Abstract

Plane-strain compression testing was carried out on a Nb-Ti-V microalloyed steel, in a GLEEBLE3500 simulator using a different amount of roughing, intermediate, and finishing deformation over the temperature range of 1373 K to 1073 K (1100°C to 800°C). A decrease in soaking temperature from 1473 K to 1273 K (1200°C to 1000°C) offered marginal refinement in the ferrite (a) grain size from 7.8 to 6.6 lm. Heavy deformation using multiple passes between A e3 and A r3 with true strain of 0.8 to 1.2 effectively refined the a grain size (4.1 to 3.2 lm) close to the ultrafine size by dynamic-strain-induced austenite (c) fi ferrite (a) transformation (DSIT). The intensities of microstructural banding, pearlite fraction in the microstructure (13 pct), and fraction of the harmful ''cube'' texture component (5 pct) were reduced with the increase in finishing deformation. Simultaneously, the fractions of high-angle (>15 deg misorientation) boundaries (75 to 80 pct), beneficial gamma-fiber (ND//h111i) texture components, along with {332}h133i and {554}h225i components were increased. Grain refinement and the formation of small Fe 3 C particles (50-to 600-nm size) increased the hardness of the deformed samples (184 to 192 HV). For the same deformation temperature [1103 K (830°C)], the difference in a-grain sizes obtained after single-pass (2.7 lm) and multipass compression (3.2 lm) can be explained in view of the static-and dynamic-strain-induced c fi a transformation, strain partitioning between c and a, dynamic recovery and dynamic recrystallization of the deformed a, and a-grain growth during interpass intervals.

Niobium in Microalloyed Rail Steels

“…Some articles [34,138,142] have reported equations for austenite recrystallisation kinetics in non-microalloyed pearlitic rail compositions. Laboratory methods like double-hit deformation testing, multipass hot torsion or, multipass plane-strain compression testing, stress relaxation testing, tension-compression testing etc [90,134,[143][144][145][146][147][148][149][150] can be utilised to determine T nR experimentally. The advantage of multistep hot torsion [90] or plane-strain compression [150] testing is that single experiments can approximate the T nR for a given deformation schedule as shown in Figure 8…”

“…-1.42Mn-0.05Nb-0.02Ti-0.05V / wt% (b) Multipass plane-strain compression T nR T nR determination by (a) multipass hot torsion testing[90], (b) multipass plane-strain compression testing[150].8.2 Determination of no-recrystallisation temperatureIn order to approximate the no-recrystallisation temperature (T nR ), multipass axisymmetric compression tests were carried out on the water-quenched cylindrical samples according to the schedule illustrated inFigure 8.2. The deformation temperature ranges from 1100 • C to 800 • C, soaking temperature and time are 1200 • C for 2 min.…”