The Investigation On Strain Rate Brittleness Of Quenched-cold Rolled Low Carbon Steel

The tensile properties of metal materials are the core indicators that determine their production and processing.It is found that different tensile strain rates have different mechanical properties for the same material.Exploring the influence of tensile strain rate and determining the true properties of materials is the basic research topic of metallography.And it has a significant and far-reaching impact on the actual production and application of metallic materials.In this paper,15 steel was selected as the research object.After quenching-cold rolling process,layered nanocrystalline low carbon steel sheets were obtained.Tensile testing at different tensile strain rates was carried out in different temperatures.These experiments focused on the tensile mechanical behavior at room temperature.SEM,TEM and 3DAP methods were used to study the variation of tensile mechanical properties and its mechanism under different tensile strain rates.The results show that tensile tests at different strain rates under different conditions have different tensile mechanical properties.And there exists a strain rate,which one or several tensile properties indicators have extreme points,indicating that the uncertainty of tensile testing exists in all temperatures.Among them,at room temperature tensile testing,when the tensile strain rate is 2.1×10^-3s^-1,the corresponding elongation has a minimum value of 3.70%and the yield strength has a maximum value of 1140.0 MPa.It is shown that while the tensile testing is performed at room temperature,this strain rate is the strain rate at which the brittleness is maximum.By analyzing the tensile fracture,the tensile fracture mechanism at room temperature varies with the strain rate.At the strain rate at maximum brittleness,it is mainly stratified fracture with a certain ductile fracture.The mechanical properties are characterized by lower elongation and higher yield strength,which are macroscopically characterized by delamination fractures.Under other tensile strain rates,the tensile fracture are respectively more brittle fractures or more ductile fractures.The small amount of inclusions on the fracture is not directly related to the strain rate with the maximum brittleness and strain rate brittleness.The results of TEM and 3DAP analysis show that under the action of tensile stress,the dislocation density and configuration of the specimen change greatly under the action at room temperature,but the effect on the brittleness of the material is very limited.At the strain rate at the maximum brittleness,C atoms are non-equilibrium segregation at the martensite slab boundary,and no obvious segregation of S and P atoms was observed.This indicates that the non-equilibrium segregation of C atoms weakens the slab beam,which causes the strain rate brittle phenomenon.