Research On Flatness Evolution Of Cold-rolled High Strength Steel Strip During Heat Treatment

High strength steel are widely used in areas of construction, traffic, bridges, ships, engineering machinery, oil and gas pipeline, large tower structure and defense equipment due to its excellent mechanical properties. The flatness defect of cold-rolled high strength steel strip during heat treatment process is a practical problem urgently to solve. Based on National Science Foundation project "Basic Research on Residual Stress Control of Leveling and Straightening Process for Ultra-high-strength Steel (NO.51075031)", shape evolution mechanism of cold-rolled high strength steel strip with initial flatness defects during heat treatment process is studied. The change law of strip flatness is revealed. And then, flatness control strategy of cold-rolled high strength steel strip during heat treatment process is put forward to provide theoretical guidance for actual production. In this paper, main contents and research results are as follows:(1)A modified Johnson-Cook model is proposed based on isothermal tensile experiments of22MnB5steel and DP780steel. And the coupling effects of strain hardening and temperature softening are considered in the model. By user subroutine UMAT, numerical implementation of the model is conducted on the finite element software ABAQUS. At last, the FEM model of typical tensile experiment is established and validated with experimental results.(2)Based on physical simulation of22MnB5steel and DP780steel, elastoplastic constitutive model considering the effects of phase transformation and temperature is built. The influence of stress on transformation kinetics and transformation plasticity is studied. And then, multi-field coupling numerical simulation with temperature field, structure field and stress-strain field is realized by secondary development of ABAQUS finite element software. Through thermal simulation experiments of quenching and annealing process, the multi-field coupling FEM models of22MnB5steel and DP780steel are validated.(3)For buckled22MnB5steel strips, evolution mechanism of buckling in heat treatment process is studied by simulation. The research indicates that the mainly reason of flatness change is longitudinal extension difference in transverse direction caused by temperature gradient and sequential phase transformation in width direction. And then, the effects of tension and initial transverse temperature difference on flatness change are obtained. At last, the flatness control strategy of 22MnB5steel with initial buckling defects is proposed.(4)For warped22MnB5steel strips, evolution mechanism of warping in heat treatment process is studied by simulation. The research indicates that the mainly reason of flatness change is longitudinal extension difference in thickness direction caused by temperature gradient and sequential phase transformation in thickness direction. And then, the effects of tension and initial transverse temperature difference on flatness change are obtained. At last, the flatness control strategy of22MnB5steel with initial warping defects is proposed.(5)The quenching experiment system is designed and set up under laboratory conditions. Quenching experiments of22MnB5steel plates with initial buckling and warping defects are carried out. The influence of quenching process on buckling and warping defects is studied. And the experimental results validate the simulation model.