Characterization And Control Of Carbide In D2 Cold Work Die Steel

D2 cold work die steel has been widely used in cold working die fields such as cold punching,cold heading and cold extrusion due to its extraordinary hardenability,remarkable wear resistance,and high temperature oxidation resistance.As a Laitenite steel,D2 cold work die steel has severe dendrite segregation in its as-cast microstructure due to its high carbon and alloy composition,and it is easy to precipitate M₇C₃ carbides with high melting point,high hardness and large size at grain boundary.Large size strip and fishbone M₇C₃ carbides formed along dendrite boundary were difficult to eliminate in the subsequent process,resulting in carbide segregation and banded structure problems in the subsequent production process,which made D2 cold work die steel crack under the action of external forces,resulting in early service cracking of the material.Therefore,it has become an urgent problem to regulate the large size carbides in D2 cold work die steel,elucidate the growth mechanism of carbides and improve the solidification structure of D2 cold work die steel.Based on above problems,effect of solidification cooling rate on carbide characteristics of D2 cold work die steel was studied in this thesis,and effect of cooling rate on growth mechanism of rod-like M₇C₃ carbide was studied from a three-dimensional perspective.In order to further refine large size M₇C₃ carbides,transformation behavior of carbides during high-temperature treatment of D2 cold work die steel,influence of carbides morphology on high-temperature heating decomposition and,influence of hot working forging treatment on the decomposition and breakage of carbides in the matrix were studied.Phase transformation activation energy of M₇C₃ carbides during isothermal deformation was calculated to refine large size carbides gathered at grain boundaries.To optimize structure of D2 cold work die steel.The results show that:（1）During solidification process of D2 cold work die steel,with increase of cooling rate,quantity and size of M₇C₃ carbide increase,spacing between carbide layers decreases,and type of carbide is not affected by cooling rate,mainly M₇C₃,including a small amount of M₂₃C₆ and MC.（2）Three-dimensional morphology of carbides is mainly hexagonal hollow rod,and there are a few lamellar,curved cluster rod.Growth of hexagonal hollow rod-shaped M₇C₃ carbides is mainly affected by Cr concentration and temperature gradient.When solidification rate is fast,diffusion time of C and Cr elements required by the carbide decreases,and carbide begins to grow into a plane shape along nucleate particle.When it grows to a certain extent,it grows in a lateral encircling helical dislocation.（3）After high-temperature treatment,large size M₇C₃ carbides in D2cold work die steel matrix are refined,and bright black vanadium-rich MC carbides are formed around matrix,and bright white Mo₂C alloy carbides are formed on the surface of the matrix.Compared with strip M₇C₃ carbides,thin hollow rod-shaped carbides have smaller curvature radius and are more likely to decompose into small particles and nearly spherical carbides after high-temperature transformation.（4）The M₇C₃ carbides in D2 cold work die steel matrix were fractured during hot working forging process,and activation energy of phase transition was226.974KJ/mol.