Abstract
An investigation has been made of the effects of the annealing conditions and its cooling rates on the Charpy impact value and the mechanical properties of Ti and Nb stabilized low C, N-19%Cr-2%Mo stainless steel sheet. The main results obtained in this paper are as follows:
(1) The precipitation of Laves phase has a great influence upon the mechanical properties of this steel sheet. Its structure is determined as MgZn2 (C14) type (Fe, Cr) 2 (Mo, Nb, Ti) Laves phase using electron and X-ray diffraction techniques and electron probe microanalysis. The lattice constants obtained are a0=4.78 Å, c0=7.84 Å and c0/a0=1.64. The orientation relationship between Laves phase and matrix is determined as (0001) Laves// (110) α, [1120] Laves// [111] α. The precipitation rate of Laves phase reaches the maximum at 700°C and its dissolution occurs over 900°C.
(2) Laves phase is formed at grainboundary and then in matrix during cooling. Large Laves phase at grainboundary shifts the ductilebrittle transition temperature to higher one. The large amount of Laves phase degrades the room temperature ductility of cold rolled and annealed sheet and enhances its strength very much. The degradation of these mechanical properties can be overcome by rapid cooling.
(3) When the effect of cold reduction on the mechanical properties of cold rolled and annealed sheet is examined between 0 and 92%, a peculiarly rapid precipitation of Laves phase is observed at 20% cold reduction and its mechnical properties are severely degraded because of the rapid precipitation.
(4) The optimum properties of cold rolled and annealed sheet are obtained by the following conditions: Hot rolled sheet is annealed at 950°C and cooled rapidly. Cold reduction is more than 80% in order to improve r value. Cold rolled sheet is annealed at 920°C and cooled rapidly.
