Dong-Chul Chae

Ridging Control in Transformable Ferritic Stainless Steels

An alloy design concept leading to an improved ridging resistance in the transformable ferritic stainless steels is introduced. It is based on achieving a small γ-phase fraction at the ingot soaking temperature. The γ-phase fraction is then increased to a maximum value during the early stages of hot rolling. The nucleation of γ-phase islands in the ferritic matrix increases the fraction of transformed ferrite. The multiplicity of crystallographic orientations resulting from the α→γ and γ→α transformations leads to a pronounced weakening of the as-cast texture and an increased ridging resistance.

Influence of Texture on Ridging and Formability of 16%Cr Ferritic Stainless Steel

The influence of the texture on ridging and forming properties of transformable 16mass-%Cr steel was studied for two different specific processing routes. In the first route (HACA), hot strips were annealed prior to cold rolling, whereas in the second route (HCA), un-annealed hot strips were directly subjected to cold rolling. Results indicate that compared to HACA, HCA results in an improved surface smoothness, i.e. reduced roping, but a lower mean r-value, as a result of strengthening of the α-fiber texture components. Differences in the roping and forming properties could also be achieved by compositional differences resulting in differences in the fraction of austenite at hot rolling temperatures.

Cold formability prediction by the modified maximum force criterion with a non-associated Hill48 model accounting for anisotropic hardening

Experimental and numerical investigations on the characterisation and prediction of cold formability of a ferritic steel sheet are performed in this study. Tensile tests and Nakajima tests were performed for the plasticity characterisation and the forming limit diagram determination. In the numerical prediction, the modified maximum force criterion is selected as the localisation criterion. For the plasticity model, a non-associated formulation of the Hill48 model is employed. With the non-associated flow rule, the model can result in a similar predictive capability of stress and r-value directionality to the advanced non-quadratic associated models. To accurately characterise the anisotropy evolution during hardening, the anisotropic hardening is also calibrated and implemented into the model for the prediction of the formability.

Influence of the Cold Rolling Reduction on the Texture Development and r-Value Anisotropy in Ti-stabilized 18% Cr Ferritic Stainless Steel

Ti stabilized AISI 439 type 18% Cr ferritic stainless steel offers an attractive alternative for replacing Ni-containing austenitic steels. The influence of the cold rolling reduction on texture development and the planar anisotropy of Ti-stabilized 18% Cr ferritic stainless steel have been studied in order to find an optimized processing route to obtain improved formability properties. In the present study, both annealed and un-annealed hot strips were given different cold rolling reductions up to 86%, and recrystallization annealed. The recrystallization texture was analyzed by means of XRD. Tensile tests were carried out on cold rolled and annealed samples oriented in seven different directions to the rolling direction to evaluate the planar anisotropy in detail. The mean normal anisotropy rm increased with increasing cold reduction for both annealed hot band and un-annealed hot band. The planar anisotropy revealed a behavior related to the development of the recrystallization texture component in the annealed hot band and the texture component in the un-annealed hot band, with increasing cold reduction.