Effects of Rolling and Cooling Conditions on Microstructure and Mechanical Properties of Low Carbon Cold Heading Steel

Effects of rolling and cooling conditions on microstructure and mechanical properties of low carbon cold heading steel were investigated on a laboratory hot rolling mill. The results have shown that the mechanical properties of low carbon steels exceed the standard requirements of ML30, ML35, ML40, and ML45 steel, respectively due to thermomechanical controlled processing (TMCP). This is attributed to a significant amount of pearlite and the ferrite-grain refinement. Under the condition of relatively low temperature rolling, the mechanical properties exceed standard requirements of ML45 and ML30 steel after water cooling and air cooling, respectively. Fast cooling which leads to more pearlite and finer ferrite grains is more critical than finish rolling temperatures for low carbon cold heading steel. The specimen at high finish rolling temperature exhibits very good mechanical properties due to fast cooling. This result has great significance not only for energy saving and emission reduction, but also for low-carbon economy, because the goals of the replacement of medium-carbon by low-carbon are achieved with TMCP.