Abstract
Dissolution of δ-ferrite into austenite in the annealing temperature range of 1050 to 1250°C for 18-8 (SUS304) stainless steel continuously cast slabs has been studied experimentally together with numerical calculation of diffusion model.
(1) Secondary dendrite arm spacing (SII) increased approximately linearly with the distance from slab surface. δ content as cast did not increase linearly. At 1/4 thickness of slabs, it had minimum point which may be caused by reheating in the slab making process.
(2) At the early stage of annealing, 6 content increased. It was explained by diffusion model.
(3) The logarithm of dissolution rate decreased approximately linearly with decreasing δ content except the initial and final stages of annealing. Apparent activation energy was about 60 kcal/mol, which was nearly the same as the activation energy for diffusion in γ-phase.
(4) The relation between δ content and log (t/S2II), where t is virtual annealing time which is compensated by addition of modified reheating time in the slab making process, is nearly the same at any SII on the same temperature. The calculated result agrees with these relations except the final stage of annealing, where the dissolution rate of the experiment is slower than the calculation.
