Flow Stress Behaviour and Static Recrystallization Characteristics of Hot Deformed Austenite in Microalloyed Medium-Carbon Bainitic Steels

Abstract

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations.

Microstructure evolution and static recrystallization kinetics in hot-deformed austenite of coarse-grained Mo-free and Mo containing low-carbon CrNiMnB ultrahigh-strength steels

“…The strain exponent of − 2.5 was also used by Hodgson and Gibbs [46] for C-Mn, Ti-, and V-steels. Moreover, the values of p in the current study are comparable with those reported by Kaikkonen et al [2] for microalloyed medium carbon bainitic steels which range from − 1.7 to − 2.7.…”

“…5a show work hardening and dynamic recovery prior to stress relaxation at all deformation temperature. Thus, the static restoration (recovery and recrystallization) process can be characterized using the relaxation after hot compression [2]. The flow stress of the examined steels is, as predicted, highly influenced by the deformation temperature.…”

“…For developing the regression model for the Q rex of hot deformed austenite, Somani et al [21,40,41] assumed the Q def of 340 kJ/mol for C-Mn steels. So by assuming the Q def is 340 kJ/mol and q is − 0.21 [2], Q rex of 277 and 283 are obtained for 0Mo and 0.25Mo steels respectively. Q rex (in J/mol) can be calculated for C-Mn and microalloyed steels using Eq.…”

“…Control of hot rolling in the recrystallization regime i. e., above the no-recrystallization temperature (T nr ) is beneficial for the refinement of austenite grain size prior to phase transformation. Therefore, static recrystallization (SRX) is the primary mechanism in most commercial hot deformation techniques [1,2] which can alter the grain characteristics i.e., size and distributions in metals and alloys [3]. In hot deformation process, SRX normally happens during the interval time, and it involves nucleation and growth processes of new grains, which require enough time to occur that could be less than one second or hundreds of seconds depending on the chemical composition, strain, and temperature.…”

The Flow Stress–Strain and Dynamic Recrystallization Kinetics Behavior of High-Grade Pipeline Steels

“…Due to the combination of high strength and toughness, high-grade pipeline steels, such as, X70, X80, and X100, has been extensively studied [ 1 , 2 , 3 , 4 , 5 ]. Thermo-mechanical control process (TMCP) is an indispensable and important technology in the manufacture of high-grade pipeline steels, which mainly involves the thermal deformation process of steel plates.…”