Study On Microstructure Of 20-steel Subjected To Pipe-Inner Surface Grinding And Cold Rolling

Gradient nanostructures,owing to the unique deformation behaviors and superior properties,have become the research frontier of materials science and technology.By means of surface plastic deformation,gradient nanostructures can be fabricated on the outer surface of metal plates or rods,and inner surface of the metal pipes.Surface plastic deformation offers unique advantages in studying the evolution principles of deformation microstructure,as on the one hand,the depth-dependent gradient microstructures correspond directly to the extent of deformation,on the other hand,the large strain,high strain rate and high strain gradient allow deformation-induced grain refinement and structural evolution far below the size limit for traditional plastic deformation.Gradient nanostructure fabricated by surface plastic deformation on the inner surface of low carbon steel pipes with wide applications is expected to optimize the surface properties and benefit for their global service behaviors.In addition,it can also help to understand the structural evolution principles and grain refinement behaviors of steels with typical dual phases of proeutectoid ferrite and pearlite.Consequently,this research chose low-carbon steel pipe with a carbon content of 0.2 wt.%(20-steel)to fabricate gradient nanostructures by our independently developed pipe inner-surface grinding(PISG)technique.The deformation microstructures and hardening effect along depth were characterized and compared with traditional cold rolling(CR)to reveal the evolution principles of deformation microstructures.The following results were obtained:1)1-pass PISG with a penetration depth of 120 ?m fabricated a gradient nanostructure of about 70 ?m thick and deformation layer of 1.5-2 mm thick on the inner surface of 20 steel pipe.The structural scale of proeutectiod ferrite was reduced down to20nm,accompanied with a hardness increase from the starting 150HV to above 450HV.The ferrite and cementite lamellae in pearlite were refined to 21nm and 11nm in thickness,respectively,leading to an increase of hardness from 290HV to above 600HV.2)The structural evolution process of 20-steel during PISG was revealed.For proeutectoid ferrite,the structural evolution began with the movement and rearrangement to form dislocation tangles,dislocation cells and extended dislocation structure,which evolve with deformation to ultrafine lamellar structure and finally the nanolamellar structure;For pearlite,the coarse ferrite and cementite of pearlite were firstly subjected to deformation,refinement,the splitting of cementite lamellae,then ultrafine ferrite lamellae and finally nano-pearlite lamellae were formed.3)Water cooling during rolling was found promoting the structural refinement and strengthening effect of 20-steel.A thickness reduction of 80% could refine structural scale of proeutectiod ferrite lamellae to 127nm,leading to a hardness of 350HV,while the ferrite and cementite lamellae in pearlite were refined to 63nm and 24nm,respectively,inducing a hardness of 410HV.4)During cold rolling,proeutectoid ferrite in 20-steel formed ultrafine lamellar structure,and the lamellar thickness decreased with the increase of the thickness reduction.Pearlite form coarse lamellae,bended lamellae,fine lamellae and heterogeneous lamellae.With an increase of thickness reduction,the proportion of coarse lamellae decreased and the fraction of fine lamellae increased.