Internal Friction Study Of The Bake-hardening Properties On Low Carbon Steel Influenced By The Degree Of Pre-deformation And Baking Time

This dissertation selected cold-rolled low carbon steel as the object of study.The annealed sample was baked in different degree of pre-deformation and differenttime span to measure its baking hardening property. The sample was analyzedmicro-structurally after annealed, pre-deformed and baked. And the internal frictionwas measured in terms of different process conditions. The main results are asfollows:(1)The sample was set in different pre-deformation. When the bakingtemperature was170℃and the time span was20min the results released that whenthe degree of pre-deformation was between2%and5%, BH value increased with theincreasing degree of pre-deformation; When the degree of pre-deformation wasbetween5%and10%, BH value decreased with the increasing degree ofpre-deformation; when the degree of pre-deformation is5%, BH value reachedmaximum. BH value increased along with the rising of SKK peak, while decreasedwith the falling of SKK peak, which means that the increase of BH value was mainlyowing to the intensify of Cottrell atmosphere. As the pre-deformation increased, thesum of hSnoek+hSKKdecreased, while under the same circumstances, the sum ofhSnoek+hSKKincreased after baking.(2)When the degree of pre-deformation was5%and the baking temperature was170℃， BH value increased then unchanged as the time went by. When baking timespan was1000min, BH value came to the maximum, while remain the sameafterwards. When the degree of pre-deformation was5%, baking temperature was170℃and baking time was20min, the SKK peak came to the maximum. Whenbaking time was1000min and constantly extended, the height of SKK peak rarelychanged.(3) The samples with the degree of pre-deformation of2%,5%and10%werebaked under the condition of170℃×20min separately, and observed the dislocationmorphology with TEM, this reveals that when degree of pre-deformation was5%,dislocation morphology was the largest compared to2%and5%; Synthesizing theresults of dislocation morphology and internal friction experiment, the dislocationdensity was calculated under different degree of pre-deformation. When bakingtemperature was170℃and baking time was20min, the dislocation density of5%ofpre-deformation was ρ=1.06×1016m-2; Also the Solute carbon concentration of thesample was calculated and analyzed. Solute carbon concentration was high under the Annealed sample which provides the free carbon atom for baking hardening steel.