Warm Deformation Behavior Of Quenched Carbon Steels And The Microstructure Evolution As Well As Properties

The aim of this dissertation is to investigate the warm deformation behavior of quenched carbon steels and the microstructure evolution and properties features during the warm deformation.The warm deformation behavior of quenched carbon steels with (0.17-1.26)C (wt.%), such as Q235,45, T8 and T12, at temperatures below Acl was studied by thermalmechanical simulation, and constitutive equations of the warm deformation were set up. Microstructure evolution during the warm deformation with different processes was examined by optical microscopy (OM) and transmission electron microscopy (TEM). In addition, the mechanical, physical and chemical properties of these steels subjected to quenching and warm deformation were also analyzed.Experimental results show that the resistance of the warm deformation of the quenched steels decreases with the deformation temperature elevating and the strain rate reducing. In certain rang of deformation temperature and strain rate, the peak stress of the warm deformation shows a trend from ascent to descent with the increase of carbon content, and the peak stress reaches maximum for 0.45C steel. Ultrafine microstructures with equiaxial ultrafine/submicron-grained ferrite and nano cementite particles can be fabricated in (0.17-1.26C) steels by quenching and warm deformation (QWD). With the increase of carbon content, the size of ferrite grain shows a trend from descent to ascent, and the finest grains can be obtained in 0.45C steel. With the increase of the reduction from 30% to 90%, the size of ferrite grain also shows a trend from descent to ascent, and the finest grains can be obtained at reduction of 50%. The yield strength and elongation are reached to 777 MPa and 19% for 0.45C steel by QWD with a reduction of 50% at 600℃and 0.01 s-1. Comparing to the yield strength (355 MPa) and elongation (16%) given in GB/T699-1999 for 0.45C steel treated by quenching at 840℃and tempering at 600℃, the yield strength is increased by～1.2 times and the elongation is slightly increased by QWD treatment. For the Q235 steel (0.17C) treated by quenching and warm deformation in reduction of 50% at 600℃and 0.001 s-1, the yield strength reaches 420 MPa which is higher than that of the as hot rolled sample by～80%, and the elongation is 16%. Thermal dilatometry measurement reveals that the precipitation of C from quenched Q235 steel starts at 250℃and completes at 550℃when heating at a rate of 5℃/min. However, for samples warmly deformed in reduction over 50%, the expansion coefficient is unchanged, suggesting that the pretipitation is not occurred, i.e. obtaining the completely recrystallized ferrite grains. The corrosion of this ultrafine grained Q235 steel in artificial sea water is mainly in mode of uniform corrosion, and the corrosion pits with uniform distribution ocuurs in the process of salt spray corrosion. The relationship between the surface roughness of the sample corroded in artificial sea water and corrosion time displays S-shape curve. The corrosion rate is slow for corrosion time less than 35 days, rapid for 35-55 days and slow for over 55 days.