Research On Mechanical Properties Of Advanced High Strength Steel Considering Cold Working Effect

Due to the changes of chemical composition,multiphase microstructure and microscopic mechanism,the strength of Advanced High Strength Steel(AHSS)can reach 3～5 times that of traditional carbon steel.AHSS,which is widely used in automotive field,has high application potential in cold formed steel structure engineering field.Therefore,the properties and cold working effects of materials must be correctly understood and quantified so that they can be used economically and rationally in structural design to give full play to their advantages.In this paper,a series of DP1180 and Q&P1180 advanced high strength steels have been tested for tensile properties,and their key mechanical properties have been captured.Combined with AHSS material database,the universality of various material modified constitutive models and their key parameter models has been explored.By introducing modified yield strength and equivalent ultimate strength,a new universal three-stage mixed material constitutive model for advanced high strength steel was established,and the prediction equations of many parameters were given.Secondly,aiming at the cold processing effect of press brake,a semi-empirical model of stress strengthening for advanced high strength steel was proposed based on section size,corner test and modified constitutive model of virgin material.Based on the superposition assumption of flexural plus springback,equivalent plastic stress and elastic-plastic modified constitutive model of virgin material,the zigzag distribution pattern of transverse and longitudinal residual stress along the thickness direction is given separately.The accuracy of the theoretical pattern is verified based on the refined finite element model of press brake forming process.Finally,aiming at the cold processing effect of cold rolling,based on the experimental data of corner area,the universality of semi-empirical model of bending strengthening was verified,and a simplified material strengthening model suitable for cold rolling was proposed.Based on the assumption of rectangular block distribution and analysis of X-ray diffractometer scanning results,the actual and simplified distribution pattern of transverse and longitudinal bending residual stress along the thickness direction were given separately.Based on the refined finite element model of cold rolling forming process,the actual pattern of residual stress "M" shape and the experimental simplified pattern were both verified.The main conclusions are as follows:(1)The existing prediction models of steel constitutive and required parameters are difficult to reliably characterize the highly nonlinear stress-strain behavior of AHSS materials.The conventional yield stress used as AHSS significantly underestimates the material strength,and the strain hardening exponential of the classical R-O model increases blindly to accommodate the gentle end at a 1% increment before the stress reaches the ultimate strength.Therefore,it is necessary to perfect the modification of constitutive model by reassigning the demarcation points.(2)In this paper,the plastic offset of yield point is analyzed iteratively with AHSS database,and it is found that there is a positive linear correlation between the optimum offset and the ratio of ultimate strength to yield strength of virgin material.The three-stage AHSS mixed constitutive model based on modified yield stress and equivalent ultimate strength has good accuracy and wide applicability.Based on the statistical analysis of AHSS database,the linear regression model is given one by one based on the modified yield strength of virgin material,which greatly simplifies the prediction difficulty of mechanical properties.(3)The modified yield strength of press braked and cold rolling AHSS members is positively correlated with the thickness ratio of the inner radius,but negatively correlated with the modified strength ratio of the virgin material.Therefore,the power-law semi-empirical stress strengthening model and its bilinear regression parameters model are proposed to simplify the prediction difficulty and realize the same standardization with the constitutive parameters of virgin material.(4)The ultimate strength and modified yield strength of press braked and cold rolling AHSS members show a positive linear correlation trend consistent with the virgin material,which makes the prediction model of ultimate strength of virgin material applicable.Due to the limited strength reserve after yielding,the ratio of ultimate strength to yield strength of the materials in the corner area is positively correlated with that of the virgin material.Compared with stainless steel,the ratio coefficient is adjusted from 0.75 to nearly 1.0.Based on this,the prediction model proposed also has high precision.(5)The residual stress along the thickness direction of AHSS in corner area is influenced by springback and tends to show the linear anti-symmetric distribution pattern of zigzag shape.In the process of moving from the surface to the neutral layer,the transverse residual stress will be transformed into tension and compression gradually,and its flexion point is 1/3 of the wall thickness away from the neutral layer,while the longitudinal stress form remains unchanged and keeps increasing,but suddenly drops markedly to zero at the neutral layer.In addition,when considering the neutral layer is shifted towards the compression side,the transverse and longitudinal residual stresses reach their peak values at 0.025 wall thickness away from the neutral layer,which are 2 and 10 times of the surface,respectively.(6)The residual stress along the thickness direction of AHSS in corner area tends to show rectangular block distribution pattern due to the influence of multi-pass forming,and the longitudinal bending residual stresse are much larger than the transverse ones.The mean value of the dimensionless residual tensile stress in the longitudinal and transverse direction of the corner area is 0.66 and 0.22 respectively,and the dimensionless residual tensile stress in the longitudinal and transverse direction of the flat plate adjacent to the corner area,whose length equals to the arc length is 0.20 and 0.22 respectively.