| 4Cr5MoSiV1(AISI H13)hot work die steel has been widely used in the mold industry.Due to the emergent demands of large,complex,precise and long life molds,the limitations in strength and toughness of mold steel greatly restrict its servie life and service safety.In this paper,based on the key issues of H13 steel including ingot quality optimization,strength and tougheness adjustment and fracture mechanism,we studied the rare earth(Re)microalloying,electroslag remelting and heat treatment optimization.Furthermore,we studied the strengthening and toughening mechanisms of 2000 MPa grade Re H13 steel,as well as revelaed its plastic deformation behavior and fracture mechanism.First,by the self-developed electroslag remeting equipment,the reasonable rare earth slag and electroslag remelting parameters were proposed for the effective additon of rare earth(0.01 wt.%)in H13 steel ingots.The free energys of C,Si,Al and Ca elements in the high temperature molten pool at 1600? to reduce rare earth oxides were calculated by thermodynamics.The results indicated that,Si can hardly redce the rare earth oxides,while C,Al and Ca can reduce the rare earth oxides.The RE202S can suppress the size of secondary dendrites,reduce liquid-deposited caribides and refine inclusions at grain boundaries,thereby improve the uniformity of ingots.Compared with H13 steel without rare earth addition,0.03 wt.%La addition raised the elongation after farcture of H13 steel by 15.8%,impact toughness by 67.8%and strength slightly.The pre-tempering and subsequent tempering treatment can effectively improve the comprehensive mechanicla properties of H13 steel.For the 1030? quenched non-Re H13 samples,after treated by 640?(10 min)pre-tempeing and subsequent 600?(30 min)tempering,H13 steel obtained ultimate tensile strength of ca.1921 MPa,yield strength of ca.1533 MPa,imact toughness of ca.13.8 J.cm-2 and elongation after fracture of ca.11.8%.Furthermore,for the 1030? quenched Re H13 steel by electroslag remelting,after pre-tempering and subsequent tempering,the Re H13 steel obtained superior ultimate tensile strength of ca.2029 MPa,yield strength 1654 MPa and elongtaion after fracture of ca.9.3%.Compared with the conventional tempering process,the dislocation density(5.72×1014 m-2)of the electroslag remelted Re H13 steel treated by pre-tempeirng and subsequent tempering increased by 73.3%,which notablely enhanced the dislocation strengthening.Furthermore,its number of carbides per unite area(10.1?m-2)increased by 21.7%,which notablely increased the precipitation strengthening.Besieds,we found that the rare earth addition raised the Ms points of H13 steel by 18?,which contributed to the increase of martensite transformaiton driving force and V1/V2 variant pairs content,and raised the density of HAGBs(>45°)by 55.4%.Compared with the non-Re H13 steel,the high density of HAGBs(>45°)of Re H13 steel increased it elongation after fracture(10.7%)by 55.1%and impact toughness by double.By combining in-situ TEM tensile and post-mortem EBSD analysis,we studied the plastic deformation behavior and fracture mechanism of 2000 MPa grade Re H13 steel.It is found that the microcracks nucleated by crack tip passivation mechanis and propagated by zigzag "Z" shape.During tensile deformation,the retained austenite at grain boundaries showed stress-induced transformation effect.Besides,the retaiend austenite and high angle grain boundaries can hinder the crack propagation effctively. |
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