Influence of rolling and cooling conditions on the mechanical properties of low-carbon cold forging steel

Abstract

The thermomechanical control process (TMCP) was performed using a laboratory hot rolling mill. A satisfactory high strength class can be obtained by the TMCP for lowcarbon cold forging steel. This is attributed to the increase in the amount of pearlite and the ferrite-grain refinement. The TMCP conditions ensured that the slabs with fast cooling to 600 °C, 650 °C and 550 °C at the rate of 37 °C/s, 28 °C/s and 15 °C/s, respectively, after high-temperature (950 °C) rolling were suited for industrial production of ML40, ML30 and ML25 steel. Fast cooling after high-temperature rolling is the optimum processing for manufacture of cold forging steel. The reason is that it is relatively easy for a rolling mill to produce cold forging steel bars under higher temperature rolling because of the low load during rolling. When the rolling mill is large enough to allow the production of cold forging steel bars, air cooling after low-temperature (800 °C) rolling can also meet the standard requirements of ML30 steel due to the grain-refining effect, the increased amount of pearlite and the improvement of the pearlite morphology. Bar products which have not only outstanding formability due to their low carbon content, but also desired mechanical properties, are manufactured by the TMCP. The substitution of medium-carbon with low-carbon cold forging steel is achieved by the TMCP in a laboratory hot rolling mill.