Pre-Deformation Induced Carbide Precipitation At Low Temperature And Its Effect On Residual Stress

The residual stress inside the high-strength steel strip can cause problems such as flatness defects,reduced service performance,and stress corrosion.Tempering is an important means of residual stress control,but for a long time tempering is based on experience,by increasing the tempering temperature,prolong the tempering time to reduce the residual stress in the material.Not only the production cost is high,but it is also easy to cause the change of material performance,and the residual stress control method is lacking.The study found that the "precipitation plasticity" produced by the precipitation of carbides during tempering has an important influence on the residual stress,which may become a new way to control the tempering residual stress.Carbide precipitation is essentially the diffusion of solute atoms in the material.The driving force for diffusion includes both the chemical potential gradient and the influence of the external field.Among various external fields,the stress field is easy to obtain and has the most engineering application prospects.Based on this,this paper adopts the method of applying tensile pre-deformation and rolling compression pre-deformation to 700L high-strength steel,using thermal expansion phase change instrument,SEM,TEM,XRD and other methods to study the precipitation behavior of carbides during tempering after pre-deformation.The residual stress in the thickness direction of the sample after pre-deformation and tempering was measured by the crack compliance method,and the effect of the precipitation plasticity caused by carbide precipitation on the residual stress was analyzed.It is found that pre-deformation can induce carbide precipitation at low temperature in the tempering process of 700L high strength steel and reduce the controlling temperature of residual stress in the tempering process.The specific conclusions are as follows:(1)The effect of pre-deformation on the precipitation of cementite is restricted by the amount of deformation;when the amount of deformation is small,low-temperature precipitation of cementite can be induced.After 5%pre-deformation,the precipitation temperature of cementite can be reduced from 247℃ to 189℃ at most;When the deformation is large,the precipitation of cementite is suppressed.After 10%pre-deformation,the precipitation temperature of cementite can be increased from 247℃ to 309℃ at most.The reason for the change in the precipitation temperature of cementite is that the change of dislocations affects the instantaneous solubility of solute Catoms.(2)Pre-deformation can induce low-temperature precipitation of alloy carbides,but compression pre-deformation is more conducive to reducing the precipitation temperature of alloy carbides.After 5%compression pre-deformation,the precipitation temperature of alloy carbides is reduced from 584℃ to 461℃,and after 5%tensile pre-deformation,the precipitation temperature of alloy carbides is reduced from 584℃to 489℃,which is better than compression pre-deformation high 28℃.(3)During the tempering process of the compressed pre-deformed sample,the grain boundary carbides are broken and form a composite with the vacancies,moving from the grain boundary to the grain,which is conducive to the dispersion and distribution of the carbide;the tempering process of the tensile pre-deformed sample Among them,the carbide and vacancy complexes move from the inner grain to the grain boundary,and the grain boundary carbide increases.(4)Tempering after compression and pre-deformation is more conducive to the improvement of residual stress.The residual stress of the 5%compression pre-deformed sample is reduced to 33.33%after the cementite precipitation temperature is tempered;after tempering at carbide precipitation temperature,the residual stress of 5%and 10%pre-deformed specimens decreases to 14.91%and 43.3%,respectively.This paper studies the influence of rolling compression pre-deformation and tensile pre-deformation on the low-temperature precipitation of carbides during tempering,and the regulation mechanism of carbide precipitation on residual stress during pre-deformation and tempering.It provides some theoretical guidance for solving the residual stress problems encountered in the on-site production process,which is conducive to improving and perfecting the current residual stress reduction methods,improving industrial production efficiency and reducing production costs.