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Warm stretch-formability of 0.2%C-1.5%Si-(1.5-5.0)%Mn TRIP-aided steels

Sugimoto K., Tanino H., Kobayashi J.

Archives of Materials Science and Engineering

2016

Vol. 80, nr 1

5--15

Purpose: Warm stretch-formability of 0.2%C-1.5%Si-(1.5-5.0)%Mn transformation-induced plasticity (TRIP)-aided sheet steels with annealed martensite matrix was investigated for automotive applications. Additionally, the warm stretch-formability was related with the retained austenite characteristics. Design/methodology/approach: This study was aimed to enhance the stretchformability by warm forming which stabilizes mechanically a large amount of metastable retained austenite in the steels. Findings: The warm stretch-formability increased with an increase in Mn content. The stretch-formability of 5% Mn steel was improved by warm forming at peak temperatures of 150-300°C, which was the same level as that of 0.2%C-1.5%Si-1.5%Mn0.05%Nb TRIP-aided martensitic steel. The superior warm stretch-formability was caused by a large amount of mechanically stabilized retained austenite which suppresses considerably void initiation and growth at interface between matrix and transformed martensite. Higher peak temperatures for the stretch-formability than that for the total elongation was associated with high mean normal stress on stretch-forming. Research limitations/implications: The effect of warm forming on the stretchformability is smaller than that on the ductility. Practical implications: Investigation results can be easily applied to industrial technology. Originality/value: This paper presents an important result which the stretch-formability of 5% Mn TRIP-aided steel is mainly improved by stabilizing of retained austenite with low stacking fault energy. On the forming, only strain-induced α’-martensite transformation takes place and suppresses the void growth. The strain-induced bainite transformation never occurs during forming in 5% Mn steel, differing from conventional 1.5% Mn TRIP-aided steel.

Drying Behaviour of ZrOCl2 Solution Droplet in ZrO2 Fine Particles Production by Spray Pyrolysis

Kobayashi J., Itaya Y., Matsuda H., Hasatani M.

Bulletin of the Polish Academy of Sciences. Technical Sciences

2000

Vol. 48, nr 3

383-397

The difference of the mechanism to yield either dense or hollow particles may be caused rather at the stage prior to decomposition in the sequential steps because any structural change is not possible except for stress-induced strain and crack once a solid is formed. The stage of solidification and drying of the droplets plays an important role to determine the structure of the final particles in SPM. In this paper, the drying behaviour of atomized droplets of zirconium oxychloride solution is discussed in comparison between the experimental result and theortical analysis. The drying behaviour of the droplets with contraction is simulated assuming uniform temperature and concentration in droplets and surface evaporation. The hollow particles more than [formula] are experimentally observed in the obtained particles for any conditions. Drying of the atomized droplets is predicted to complete in 20-50 msec from atomizing into the reactor by theoretical analysis and the drying rate is very large. Because of the large drying rate, the concentration distribution in the droplet is formed so that the shell on the droplet surface is formed. The shell on the droplet surface is often broken by the vapour pressure and/or thermal stress with a rise in temperature. Hence the hollow spheres and the small fragments are yielded.