To investigate the role of stress induced martensitic transformations in the TRIP effect, stress strain curves and TRIP effects in two types of metastable austenite steel (JIS SUS304 and JIS SUS301L) were compared. In static tensile tests, the tensile strength (TS) of SUS301L steel was larger than that of SUS304, and the 0.2 proof stress and uniform elongation (U.El) were both similar for the two steels. The transformation rate and strength of martensite in the stress induced martensitic transformation were compared by X ray diffraction and Vickers hardness tests. At the same strain, SUS301L contained a larger volume fraction of martensite than did SUS304. Both steels showed similar values of the Vickers hardness. The difference between SUS301L and SUS304 is mainly due to the transformation rate of stress induced martensite. The effects of the transformation rate and martensite strength on the TS/U.El balance in metastable austenite steels were demonstrated by means of the Weng secant method, based on a micromechanical model.

Effect of Strain Rate on TRIP Effect in a 0.2C-1.5Si-1.2Mn Steel Noriyuki Tsuchida and Keiko OsakiSynopsis : Effect of strain rate on tensile properties and stress-induced martensitic transformation behavior in a 0.2C-1.5Si-1.2Mn (0.2C TRIP) steel was investigated at strain rates between 3.3×10 -6 s -1 and 10 3 s -1 . The 0.2% proof stress and tensile strength increased and uniform elongation decreased with an increase in strain rate in the 0.2C TRIP steel. At low strain rates below 10 -4 s -1 , the 0.2C TRIP steel was obtained good uniform elongation. In the strain-rate dependence on stress-induced martensitic transformation behavior, the volume fraction of stress-induced martensite decreased at strain rates higher than about 10 -2 s -1 due to the temperature rise caused by adiabatic deformation. The difference of stress-induced transformation behavior between the 0.2C and 0.4C TRIP steels seems to be associated with the stress partitioning to retained austenite. Furthermore, the stress partitioning is affected by the volume fraction of not only retained austenite but also ferrite and bainite.

緒言TRIP（TransformationSynopsis : Effect of strain rate on mechanical properties and TRIP effect on TRIP-aided multi-phase steel obtained by 0.4C-1.5Si-1.2Mn steel (0.4C TRIP steel) was investigated by tensile tests with strain rates between 3.3×10 −6 s −1 and 10 3 s −1 at room temperature. The 0.4C TRIP steel showed good uniform elongation at low strain rates below 10 −3 s −1 due to TRIP effect and tensile strength increased with an increase in strain rate. The strain rate dependence on stress-induced martensitic transformation was also investigated by x-ray diffraction experiments at strain rates between 3.3×10 −5 s −1 and 10 0 s −1 . These results clearly show the stress-induced transformation should be induced momentarily by straining at the latter stage of deformation in order to obtain larger uniform elongation. By comparing the experimental results between the 0.4C TRIP and the metastable austenitic steels, it is found that the volume fraction of retained austenite and the stability of ausutenite are associated with the difference of the strain rate dependence on TRIP effect.

True stress (s) true strain (e) relationships until just before fracture, i.e., the plastic deformation limit, were estimated by the stepwise tensile test and the Bridgman equation for various metals and alloys with different crystal structures. The estimated s e relationships were different from the nominal stress strain curves including the conventional tensile properties. In the relationships between the true stress (s pdl ) and true strain (e pdl ) at the plastic deformation limit, SUS304 and SUS329J4L indicated a better s pdl  e pdl balance. On the other hand, SUS329J4L, tempered martensite, and an ultrafine grained steel showed superior results in the yield strength e pdl balance. The estimated s e relationship for the ultrafine grained steel suggests that grain refinement strengthening can improve s and e up until the plastic deformation limit. The value of e pdl became larger with increasing the reduction in area and a decrease in the fracture stress. The products of s pdl and e pdl became larger with increasing work hardening rate at the plastic deformation limit.

Effects of temperature and strain rate on stress-strain curves for two types of dual-phase (DP) steel with different carbon contents were investigated from the viewpoints of tensile tests and the Kocks-Mecking (KM) model based on thermal activation theory. In the tensile tests, flow stress increased but elongation decreased with decreasing temperature or increasing strain rate. Uniform elongation for each DP steel was almost independent of deformation temperature between 123 and 373K. In the comparison of strain rate sensitivity exponent (m), the mvalue increased with decreasing the volume fraction of martensite. The calculated true stress-true strain curves by using the KM model agreed with the measured ones at various temperatures and strain rates for the DP steels. In terms of the parameters for the KM model, the athermal stress and mechanical threshold stresses were different between the two types of DP steel. This seems to be associated with the difference of volume fractions of ferrite and martensite.