| As a cheap microalloy element,boron has been widely studied because of its excellent role in steel.For example,by using boron to improve hardenability,cold heading steel containing boron(hereinafter referred to as boron steel)has developed rapidly in the field of fasteners.In addition,as the application of cold heading,high quality boron steel needs to have good microstructure,strength and plasticity,excellent surface quality and fatigue properties.The composition and process have a great influence on the transformation,microstructure and properties of the boron steel.In this paper,the hardenability,microstructure,strength and plasticity are studied and analyzed,the crack source,crack evolution rule,inclusion and fatigue properties are also studied,which will provide guidance for improving the quality of boron steel.The hardenability of boron steel was studied,it is found that adding both B and Ti can improve hardenability significantly,but adding B only can not obviously improve hardenability,which is mainly because Ti can increase the effective B.The increase of Cr or Mn in boron steel is beneficial to hardenability,while the increase of S is unfavorable to hardenability.The effect of Ti/N on hardenability was studied,and the results show that the quench hardness of 10B21 increase with the increase of Ti/N,and when Ti/N is greater than 6,the sample can be fully quenched.The effect of quenching temperature on the hardness of boron steel was also studied.The results show that with the increase of quenching temperature,the hardness increased first and then decreased slowly,when quenched at 870℃,the hardness was the highest.By using JMatPro simulation method,ideal critical diameter method and nonlinear equation method,the Jominy curves of boron steel is calculated and compared with Jominy test method.For boron steel,there are some deviations between different calculation methods and test methods,and the Jominy curves of boron steel can not be calculated well.Based on the experimental data,the relationship equation between the critical quenching diameter when H is 0.35 and the main chemical elements was obtained by regression method:DH=0.35=-23.9+19.3×C+17.9×Si+ 28.1×Mn+23.8×Cr+6403×B+24.3×Ti.The influence of chemical composition and process on the microstructure and mechanical properties of different boron steel were studied.For the medium carbon boron steel containing 0.0021%B+0.035%Ti,the tensile strength of the wire rod can be reduced to below 595MPa by adopting high temperature rolling and slow cooling process,which meets the requirements of annealing free process.For low carbon boron steel containing 0.0050%B+0.066%Ti,the tensile strength under conventional rolling process is as high as 469MPa,but the plasticity decrease is not obvious,which is mainly due to the microstructure of quasi-polygonal ferrite.The tensile strength of the wire rod can be reduced to 373MPa by optimizing the controlled cooling process.For ultra-low carbon boron steel,when adding 0.0055%B,the grain size Grade is reduced from 7.5 to 6,and the tensile strength is reduced from 295MPa to 275MPa.But When adding 0.0020%B,the grain size and mechanical properties have no obvious change.This is related to B/N,the larger the B/N is,the more obvious the grain coarsening is.It is found that more than 80%of the surface defects of the coil bar are caused by the billet defects,and the billet cracks mainly exist in the vibration mark of the corner of the billet,and the cracks are generated and propagated along the grain boundary.The high temperature thermoplasticity of boron steel with Ti is better than that without Ti.When Ti/N≥4,the crack sensitivity of boron steel can be reduced.The evolution rule of cracks in rolling process was studied by means of artificial preset cracks on the billet surface.The crack depth decreases with the increase of deformation.It is assumed that the crack depth of wire rod is≤0.05mm,the relationship between the critical crack depth d0 of the billet and the diameter D of the rolled wire rod is deduced:do=8.28/D.After several passes of rolling,the transverse crack at the corner of billet surface changes into short longitudinal crack,and the cross section of the crack is folded in a small angle under the metallographic microscope.The influence of non calcium treatment process on the size and type of inclusions was studied.The results show that compared with calcium treatment process,the non calcium treatment process can change the type of oxide inclusions from CaS-CaO-Al2O3-MgO to MgO·Al2O3,and the size of inclusions is obviously reduced.The fatigue of 8.8 grade bolt made by boron steel was studied.When the stress amplitude is 10%of the average load,the fatigue cycles can reach 5 million when the average load is≤65%of the guarantee load;the conditional fatigue limit is 438.96MPa;when the average load is 50%of the guarantee load,the fatigue S-N curve can be expressed as lgΔσ=3.317-0.252×lgN.After conversion to effective stress,the fatigue S-N curve can be expressed as lgσ=3.24-0.152×lgN.When the 8.8 grade bolt bears the service life of million times,the relation curve of normalized pretightening stress and pretightening torque to stress ratio is obtained through conversion.And through this relation curve,the fatigue performance of 8.8 grade bolt under different stress ratio can be predicted,and the reasonable control of pretightening stress and pretightening torque of bolt can be realized. |
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