Microstructure and Mechanical Properties of Austempered Medium Carbon Steels

Abstract

Synopsis : Microstructures and mechanical properties for medium carbon steels including practical spring steels austempered at 400°C and 300°C were investigated. The larger strain was applied, the larger the fraction of retained austenite became martensite through strain induced transformation. 0.2% proof stress was increased by the increase of the fraction of bainitic ferrite. Uniform elongation was increased by increasing the fraction of retained austenite. Good strength-ductility balance with tensile stress over 1800 MPa for high strength spring was achieved in austempered SUP12 at 300°C composed of three phases of retained austenite, martensite and lower bainite.

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.…”

Deformation-Induced Martensitic Transformation at Tensile and Compressive Deformations of High-Carbon Bainitic Steel

“…The published data about the tensile strength of the steels with different carbon content are summarized in Fig. 1(b) [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] . This figure indicates rough tendency that the TRIP aided steel with higher carbon content is useful to obtain the higher tensile strength.…”