Effect of heating rate on intercritical annealing of low-carbon coldrolled steel

Abstract

A study was performed on the effect of heating rate on transformations during intercritical annealing of cold-rolled low-carbon sheet steels. Two sets of experiments were developed: 1) a series of alloys (1020, 1019M, 15B25) with two different cold reductions (nominally 40 and 60 pct) were heated at different rates and transformation temperatures were determined using analysis of dilatometry and metallography of intercritically annealed samples, allowing the study of the impact of composition and cold work on transformation behavior with different heating rates. 2) A cold-rolled C-Mn-Nb steel was tested with different heating rates selected for different degrees of recrystallization during austenite formation to test the impact of ferrite recrystallization on austenite formation. Heat treated samples were analyzed with SEM, EBSD, dilatometry, and microhardness to study the changes in transformation behavior. The results of this study were extended by adding step heating tests, heat treatments with an intercritical hold, and secondary ion mass spectrometry (SIMS) measurements of Mn distribution. Austenite transformation temperatures increased logarithmically with heating rate. Greater degrees of cold work led to reduced transformation temperatures across all heating rates because the energy of cold work increased the driving force for austenite formation. The relative effects of alloying additions on transformation temperatures remained with increasing heating rate. Rapid heating minimized ferrite recrystallization and pearlite spheroidization. Austenite formation occurred preferentially in recovered ferrite regions as opposed to recrystallized ferrite boundaries. Martensite was evenly distributed in slowly heated steels because austenite formed on recrystallized, equiaxed, ferrite boundaries. With rapid heating, austenite formed in directionally-oriented recovered ferrite which increased the degree of banding. The greatest degree of banding was found with intermediate heating rates leading to partial recrystallization, because austenite formed preferentially in the remaining recovered ferrite which was located in bands along the rolling direction. Holding at the intercritical annealing temperature led to somewhat reduced martensite banding vs directly quenched specimens due to austenite growth along recrystallized ferrite boundaries. Ferrite-spheroidized carbide microstructures had somewhat reduced martensite banding when compared with ferrite-pearlite steel because austenite nucleation was not restricted to former pearlite colonies elongated in the rolling direction.