Carbide Investigation Of A Novel Steel For Cold Work Roll And Roll-forging Simulation

Carbide is an important hard phase existed in the steel, which determines the servicelife of cold work roll. Considering the carbides with different types, shapes, sizes,quantities and distributions, it is challenging to control the carbide generation for bettermechanical properties of the cold work roll in the roll manufacturing field. In thisdissertation, a novel steel for cold work roll was developed. The phase transformationduring solidification process and the precipitation law, as well as the micro-mechanicalbehavior of the carbide were investigated in details. The cracking behavior around thecarbide and the influence of the carbide size, tetragonal degree(b/a value), orientation andload on the crack propagation were also discussed. Finally, the forging process of thenovel cold work roll was simulated, and the forging critical value of the roll was predicted.During solidification process, eutectic carbides, which consist of MC and M2C, formin the ternary eutectic reaction at1240oC, grow in the form of dendrite and mix togetheralong the crystal grain boundary. Eutectic MC is leaf-like shape and grows dispersedly intwo dimension, while eutectic M2C is fibrous shape and arranges tightly. Eutectic MCcontains VC and WC, while eutectic M2C contains V2C and Mo2C. With temperaturedecreasing, the eutectic carbides at the grain boundary are gradually replaced by thesecondary carbides. Secondary MC with strip-like and bulk-like features precipitates at1200oC and grows accompanied by the dissolving of eutectic MC. Secondary M2C withstrip-like and honeycomb-like features is the product after eutectic M2C grows up. M7C3precipitates at1150oC and is relatively large with chrysanthemum-like feature andirregular bulk-like feature. M7C3exists in the form of hexagonal (Fe,Cr)7C3. M23C6precipitates from the matrix at900oC and is distributed dispersively inside the grain.M23C6is indexed to be cubic Cr23C6.Among the carbides in the steel for cold work roll, the hardness and elastic modulusof MC are the highest, while the fracture toughness of M7C3is the best. During the wearprocess, the main abrasion mechanisms of eutectic carbides are cutting, brittle fracture andthe carbide detaching from matrix. The brittle fracture is the main cause of transgranular cracks inside the carbide, while the carbide detaching from matrix leads to theintergranular cracks. The aggregate distribution of eutectic carbide dendrites can improvethe surface resistance and prevent the crack propagation effectively. The transgranularcracks of the secondary carbides in the middle of the scratch are perpendicular to thescratch, while the internal cracks in the secondary carbides in the edge of scratch areusually parallel to scratch direction. The cracks are easily formed and propagated in thesecondary MC, which results in carbide broken and peeling. With better toughness, thebroken and peeling phenomena in the secondary M2C and M7C3are not obvious, whichcan resist the abrasion effectively. The carbides can protect the matrix, while the matrixcan relieve the high stress around the carbides by means of plastic deformation, play asupporting and protection role and also can prevent the crack propagation.In the deformation process, the crack initiation location is affected by the size,tetragonal degree(b/a value) and orientation of the carbide, as well as the load and itsdirection. The transgranular cracks are usually formed inside the carbide with largercarbide size and smaller tetragonal degree. Otherwise, the intergranular cracks are formedon the edge of the carbide conversely. With increasing load, the intergranular crackstransform into the transgranular ones, while the crack positions of transgranular cracksexpand to two poles of the carbide. When the crack extend around the carbide, the stressconcentration phenomenon appears inside the carbide. Carbide plays a blocking effect oncrack propagation, among which the carbide size plays the most significant role.Seven passes are carried out during the forging process. The temperature of the billetdecreases fast at the early stage, while it decreases unobviously late during the forging.The temperature field and stress field of the forging zone distribute un-uniformly. Parts ofparameters have exceeded the measured limit under1000oC, which indicates that it is notsuitable to go on forging. Moreover, the critical state is investigated, and the forging limitdiagram of the roll is drawn. If the forging temperature is above1050oC, the reduction isthe largest at the billet end. At1050-1000oC, the reduction at1/4position is the largest,while that at the end is the smallest.