Mechanical Behavior and Microstructure of High Carbon Si–Mn–Cr Steel with Trip Effect

Abstract

The influence of the thermal cycle and austempering treatment on the mechanical behavior of a 0.56C-1.43Si-0.58Mn-0.47Cr (wt%) steel has been investigated. The thermal cycle consisted of heating the steel to 800°C or 900°C, in the intercritical and austenitic region respectively, fast cooling down to 600°C or 400°C, followed by 300 s hold. After austempering the materials were cooled at different rates and then submitted to tensile testing. The total elongation of 15-20 % and tensile strength of 1 300-1 400 MPa were reached after heating to 900°C and transformation at 400°C. The strain-induced austenite transformation to martensite during the plastic deformation (Transformation Induced Plasticity Effect) is responsible for this combination of high strength and ductility. Silicon acts to stabilize the austenite during austempering. However, the stabilized austenite can be transformed and the microstructure modified, resulting in the formation of others constituents such as bainitic ferrite, upper bainite and martensite.

Bainite Transformation and Resultant Tensile Properties of 0.6%C Low Alloyed Steels with Different Prior Austenite Grain Sizes

“…12,13) One of the early works on the Si-bearing steels was reported by Sinoda et al 14) and this report exhibited that the austempering of a medium carbon steel (0.59wt%C-1.53%Si-0.89%Mn-0.02%Ni-0.14%Cr-Fe) at a temperature ranging from 300°C to 500°C brought the retained austenite in bainite with superior tensile properties. Similar experi-ments of the austempering were reported by Suzuki et al 15) (0.57%C-1.4%Si-0.75Mn-0.70%Cr-0.08%Ni; JIS-SUP12 and other spring steels) and Santos et al 16) [Santos 2009] (0.56%C-0.58%Mn-1.43%Si-0.47%Cr-Fe) at various cooling rate for the cooling after the austempering. In other works, [17][18][19] the austempering was applied after the reheating at inter critical temperature to obtain the mixture of both the ferrite, bainite and the retained austenite.…”

“…However, the studies on the kinetics of bainitic transformation, microstructure and the tensile strength -ductility balance of the medium carbon Si-bearing steel by the austempering at higher temperature are limited, except for some papers. [14][15][16][17]22) The aim of this work is to clarify the effect of prioraustenite grain size on the bainite transformation, microstructure and mechanical properties of upper bainite with the retained austenite in the medium-carbon Si-bearing steel, so that the present results will indicate that the utilization of upper bainite of medium carbon steel is one of the possible choice for steel to improve the strength -ductility balance.…”

“…The data of the Si-bearing steels reported by Shinoda et al 14) (sheet steel, ~0.04/ sec) and Matsumura et al 17) (sheet steel, ~0.003/sec) were also plotted. Additionally, data from more recent reports by Suzuki et al 15) (round bar, 0.0028/sec), Santos et al 16) (round bar, ~0.001/sec) and Gartia-Mateo et al, 22) (round bar, 0.004/sec) are added. The dotted lines indicate the contour lines designating the product of tensile strength and elongation for the guide of the strength -ductility balance.…”

Ultrahigh strength-ductility steel treated by a novel quenching–partitioning–tempering process

“…It can be seen from Fig. 7b that the work-hardening behavior of Fe-0.4C Q-P-T sample can be divided into three stages: (1) when εo0.015, the work-hardening exponent decreases sharply in a normal way (stage I); (2) when 0.015rεrε u (0.108), it has an obvious increase corresponding to the uniform elongation with wide plateaus, which agrees to that of high carbon Si-Mn-Cr steel with TRIP effect [20] (stage II); (3) when ε4ε u , the exponent slowly decreases during necking stage (stage III). For Fe-0.2C Q-P-T sample, there are also three stages, but the second stage appears in small strain range from 0.012 to 0.054, indicating that a weak TRIP effect occurs.…”

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

“…[13][14][15][16] The second stage with a plateau is considered to be associated with the mechanical stability of retained austenite and the TRIP effect produced by the transformation of retained austenite to martensite, which enhance the elongation of TRIP steel. The third stage shows a weak effect of retained austenite and a tendency of tensile specimens to fracture.…”