Influence of ultra-fast cooling on the microstructure of X70 pipeline steel

The effects of cooling rate and finish rolling temperature on the microstructure refinement and marten-site/austenite (M/A) islands in X70 pipeline steel were studied with a Gleeble thermal-mechanical simulation machine. When the cooling rate increasing, the grain size of ferrite decreases, the volume fraction of M/A islands increases first and then decreases, but the dimensional change of M/A islands is opposite to the volume fraction. With the finish rolling temperature rising, ferrite in the steel slightly grows up and the volume fraction of M/A islands increases; but when the finish roiling temperature increases from 900 to 940℃, the volume fraction of M/A islands slightly decreases and the size of M/A islands increases.