Mechanism of Improvement of Mechanical Properties of Cu-contained Low Alloy Steel by Intercritical Quenching

Better balance of strength and toughness is a strong demand for the ASTM A707 5 L grade steel as offshore structural material. In our previous study, therefore, inter-critical quenching from dual-phase of ferrite and austenite region, which is called lamellarizing (L) treatment, brought a clear improvement of balance between strength and toughness of Cu containing low alloy steel based on A707 5 L grade. However it is important to investigate the effects of C and Cu amount and microstructure before the quenching on mechanical properties of L treated Cu-contained low alloy steel in order to clarify the mechanism of improvement of the properties by the L treatment.From present study, the investigation revealed that C act on strength of harder phases that were inherited the transformed γ phase region in L treatment in the C rage from 0.01 to 0.05 mass%. Moreover, the steel was given better balance of strength by optimized L temperature which was (AC3-15) K. Additionally, Cu was useful to adjust balance of strength and toughness during tempering by aging. However, Cu precipitates were not observed after tempering in the steel was added 0.61 mass% Cu. Hence, the results suggest that the steel need to be contained over 1.0 mass% of Cu amount in order to make good use of Cu. Microstructure before L treatment had an effect upon optimized L temperature. The temperature was (AC3-15) K in the case of granular bainitic ferrites (αB) that was granular structure but it was the middle between AC1 and AC3 in the bainitic ferrites (α°B) and martensite (α’m). From results of in-situ EBSD (Electron Back Scatter Diffraction), the transformed γ phase did not show γ memory effect when microstructure before L treatment was granular structure. On the other hand, it had the γ memory effect in the case of acicular structure. It is inferred that the difference between the granular and the acicular structure of optimized L temperature results from generating behavior of the transformed γ phase during the lamellarizing.