Effect of strengthening mechanisms on limit stress for slip bands formation under cyclic deformation of hot-rolled sheet steel

The purpose of this study is to clarify the effect of strengthening mechanisms on fatigue limit for slip bands formation (σws). Various kinds of hot rolled steels were prepared by adding various alloying elements (C, Si and Ti) separately to ferrite plus pearlite steel and ferrite plus martensite steel. Fatigue tests were performed under load controlled condition. Test specimens were fatigue-tested until they failed or reached 107th cycle. After the tests, slip bands were observed by SEM. The ratio of ferrite grains forming slip bands to all the ferrite grains (SBFR) was calculated by using test specimens subjected to the stresses just above σws. As a result, the followings have been clarified: (1) Strengthening of ferrite by addition of solid solution or precipitation elements markedly increased the ratio of σws to tensile strength. This result is quite the same as for fatigue limit for fracture, which was already found in the previous studies. (2) The plastic strain amplitude for slip bands formation tends to decrease as σws increases. On the other hand, SBFR increased linearly with plastic strain amplitude. Therefore increasing in σws is concluded to promote localization of slip bands. (3) And also the calculation of microscopic plastic strain limit for slip bands formation strongly suggested that the strain limit is a kind of material constant of ferrite phase.