Formation Of Cottrell Atmosphere And Its Effect On BH Value In Ultra-low Carbon Bake Hardening Steel

This paper study the formation of Cottrell atmosphere and its effect on BH valueultra-low carbon baking hardening (ULC-BH) steel.The BH value was measuredunder different pretrain、bake hardening time and bake hardening temperature.internal friction curves were obtained by internal friction experiment. dislocationdensity was caculated by X-ray diffraction technique and internal friction Method.thevariation of solute carbon amount and the effect of Cottrell Atmosphere variation onBH Value was analysed. The result shows:Under baking process170℃×20min the BH value increases with the increasingprestrain ranging from2%to10%;Snoek peak gradually reduces while SKK peakgradually rises with the increasing prestrain; With2%,6%and10%pre-strain,dislocation density increases, the results in XRD are3.9×1010/cm2、8.4×1010/cm2�'�6.8×1011/cm2respectively, in internal friction are5.8×1011/m2、1.6×1012/m2�'�8.9×1012/m2,corresponding Cottrel atmosphere density decreases by degrees.Under2%prestain、170℃，the BH value increases rapidly at first, then basicallyremains unchanged and increases slowly at last with the increasing aging time dealingwith same pre-strain,the BH value does not increase to maximum with the normalbaking process170℃×20minbut increases slowly if samples are increased aging time;With the increasing aging time,yielding phenomenon of stress-strain curves rangefrom the continuous transition,the uniform yield finally into non-uniform yield, yieldelongation gradually extends;Snoek peak gradually reduces while SKK peakgradually rises with the increasing aging time.Under2%prestain、100℃，the BH value increases rapidly at first, then basicallyremains unchanged with the increasing aging time dealing with2%pre-strain; SnoekPeak, B Peak and SKK Peak are co-existence, Snoek Peak reduces, B Peak and SKKPeak rise with the increasing aging time; Carbon concentration of Cottrell atmosphereand the high temperature SKK damping peak plays a leading role on BH value. Theroom temperature ageing result show that original experimental steel apears roomtemperature ageing, the degree of room temperature aging increases afterpredeformation treatment.The B peak appears in the internal friction curve after shortbake hardening treatment.