2相ステンレス鋼の低温変形挙動に及ぼす再結晶集合組織の影響

抄録

The tensile properties, the strain-induced martensite transformation and the fracture mode in the temperature range between-196°C, and room temperature were studied for a two-phase steel composed of nearly equal volumes of austenite and ferrite. Recrystallization textures were observed in both the austenite and ferrite phases. Tensile test specimens were taken in the directions parallel (θ=0°C), 45°C, and vertical (θ=90°C) to the rolling direction.
The main results obtained are as follows:
(1) Effects of the anisotropy due to the texture on tensile properties (fracture elongation, tensile strength, and 0.2% yield strength), amount of strain-induced martensite transformation, cleavage crack length, and dimple size were measured. The transformation had a marked effect on fracture elogation and tensile strength.
(2) The transformaticn-induced plasticity was evaluated with both the amount of martensite transformed per unit strain and the tensile strain at the initiation of transformation in austenite phase. As the initiation strain for martensite transformation bacame larger, the fracture elongation increased. At a constant tem perature the larger the amount of martensite per unit strain was, the larger the fracture elongation was. This phenomenon was clearly observed at -50°C. All of the strain-induced martensite in the low temperature range was observed as lath-like martensite.
(3) The behaviour of ferrite phase in the low temperature range gave characteristic aspects to the fracture mode. Cleavage cracks occurred in {001}α planes and their propagation was disturbed at spots of a martensite, therefore specimens didn't fail brittlely in the low temperature range. The cleavage crack length observed on fractured surface was in maximum at -196°C and had the length of about 35 microns at θ of 45°C. Equiaxed dimples were about 5 microns in the maximum size at -50°C in the case of 0°C or 45°C-specimen.