Recrystallisation textures in cold rolled low carbon steel containing ferritic and bainitic microstructures

Abstract

Low carbon steel strip was heat treated to generate four different starting microstructures (fine and coarse polygonal ferrite, acicular ferrite and bainite) for investigating their influence on texture development during cold rolling and annealing. The starting materials were cold rolled to 50–90% reduction and annealed for various times in the temperature range 853–953 K. The resultant microstructures and textures were examined mainly by electron backscatter diffraction and X-ray diffraction. The initial microstructure strongly influenced the crystallographic rotation paths during cold rolling, whereby high strain deformation generated strong {223}〈110〉 texture components in the polygonal ferritic microstructures, whereas a strong {001}〈110〉 texture was produced in the acicular/bainitic microstructures. Subsequent annealing generated, to varying degrees, the classic {111}〈uvw〉 (γ-fibre) recrystallisation texture in all materials. Unexpectedly, coarse polygonal ferrite produced the strongest γ-fibre recrystallisation texture after 70–90% cold rolling reduction. Based on arguments involving the effect of carbon in solution, initial grain size and deformation textures on recrystallisation texture development, it was shown that a strong γ-fibre texture can indeed be generated in coarse polygonal ferrite.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Direct observation of microscopic plastic deformation of stainless steel using lattice drawn by focused ion beam of Ga+

Abstract

Visualisation of the microscopic deformation of a stainless steel was attempted. A mirror polished, flat surface specimen was subjected to a simple tension test, and the deformation of a fine lattice drawn by a focused ion beam (FIB) of Ga⁺ was observed. The depth of the lattice was of the order of a few tens of nanometres. The penetration depth of Ga⁺ was estimated using SRIM software, and the result indicated that the use of a FIB might not cause serious detriment to the mechanical properties of the lattice surface. After testing, lattices were examined by field emission scanning electron microscopy (FESEM). The results showed that displacement was continuous in the grain as well as across the grain boundary, and the microscopic deformation was categorised into three patterns: (a) a clear thin layer of shear deformation which was discontinuous across the grain boundary, (b) an area of uniform deformation inside this thin layer and (c) microscopic shear bands appearing sporadically in the grains.     

Dynamic compressive flow behaviour of S15C low carbon steel over wide temperature range

Abstract

The present study utilises the compressive split Hopkinson pressure bar to investigate the dynamic flow behaviour of S15C low carbon steel at temperatures ranging from 25 to 800°C. The effects of strain rate and temperature on the mechanical response and microstructure of the metal are evaluated. The flow stress of S15C low carbon steel is found to increase with increasing strain rate and to decrease with increasing temperature. Furthermore, the material temperature sensitivity is enhanced at higher strain rates. The study determines the strain rate sensitivity parameter and the activation volume under various strain rates and temperatures. It is found that the activation energy ΔG^* varies as a function of strain rate and temperature and attains a maximum value of 62 kJ mol^?1 under the current test conditions. A Zerilli–Armstrong bcc constitutive model is applied to describe the high strain rate plastic behaviour of S15C and is shown to produce acceptable results. Microstructural examination by TEM reveals that the dislocation density and degree of dislocation tangling both increase with increasing strain rate. Additionally, TEM observations indicate that a higher strain rate reduces the size of dislocation cells. Furthermore, it is shown that the annihilation of dislocations occurs more readily at elevated temperatures. The current results provide a valuable reference for the application of S15C low carbon steel in high speed plastic forming processes.