Research On Microstructure And Properties Of High Hole-Expansion Steel Manufactured By Hot Rolling

High Hole-Expansion steel is an important kind of Advanced High Strength Steel.It not only has high strength,but also excellent stretch flangeable performance,which can meet the high formability requirements of complex shape automotive parts,such as automobile chassis rear axle suspension,frame swing arms and rims,spokes and other components.In this paper,the composition design of high-strength hot-rolled high hole-expansion steel is based on C,Si,Mn,adding Microalloying Elements Nb,V,Ti,etc.Then,the transformation rule of microstructure,the influence of controlled rolling and cooling process on microstructure and properties,the mechanism of crack growth and the influencing factors of expanding rate had been studied by means of dynamic and static thermal simulation experiment,hot rolling experiment and expanding experiment of High Hole-Expansion steel.The specific research results are as follows:(1)With the increase of cooling rate(1～30℃/s),the transformation from Ferrite(F)→pearlite(P)→ Bainite(B)→ Martensite(M)takes place.In addition,CCT curve can be moved to the right under deformation,promoting F transformation,and increasing the starting temperature of F(Ar3 line),while Nb,V,Ti and other microalloying elements can inhibit F transformation and reduce Ar3 line.(2)The effects of hot rolling process parameters on the microstructure and properties are as follows:the finishing temperature mainly affects the grain size of F,the lower the finishing temperature is,the finer the grain is;The opening cooling temperature mainly affects the volume fraction of F in the microstructure,the greater the difference between the finishing temperature and the opening cooling temperature is,the higher the F content is;The cooling speed mainly affects the grain size,the higher the cooling speed is,the smaller the average grain size is;The coiling temperature mainly affects B or M.When the coiling temperature is 390～500℃,granular B structure is formed,and when the coiling temperature is less than 330℃,lath M structure is formed.(3)After two-stage controlled rolling+two-stage cooling+coiling heat preservation process,quasi polygonal F+granular B structure can be obtained,and it can reach the standards of 580 MPa,780 MPa and 980 MPa respectively.The specific process parameters are as:heating and heat preservation at 1200℃,rolling at 1100℃ or so,finishing rolling temperature at 850～870℃,starting cooling temperature at 810～830 ℃,laminar cooling speed at 30～36℃/s,coiling temperature at 390～500℃.(4)The crack growth mechanism of high hole expansion steel belongs to micropore aggregation system.Under the action of stress,micro holes develop into micro cracks,which will continuously pass through the soft phase F and expand along the joint of F-B.By introducing quasi polygonal F and dispersed granular B into the structure,increasing the volume fraction of F,refining its grains and controlling the content of non-metallic inclusions,the properties of the extended flange can be effectively improved and the bearizing can be increased.