Low carbon ultra-fine grain steel was fabricated by multi-pass rolling at room temperature

total reduction is 50% and 70% respectively and annealing at 500 〜650 ℃ for 2 min

martensite microstructure of low carbon steel Q235B obtained quenching after austenitizing. The effect of rolling reduction on microstructure and properties of ultra-fine grain steel was studied. Microstructure evolution was examined by optical microscopy and scanning electron microscope. Chemical composition of precipitate particles have been analyzed by energy dispersive spectrometer and the mechanical property was measured by an Instron-5969 tensile tester. The results indicate that cold-rolling and annealing with martensite initial structure can fabricate ultra-fine grained steel

the strength is nearly the double of that of the original steel. In addition

with the increase of the reduction amount

the recrystallization temperature decreases

and cementite particles can grow up easily

which is not effective to the improvement of the mechanical properties of ultra-fine grained steel. Under the same annealing process

the grain sizes of ferrite and cementite increase

and the comprehensive mechanical properties of specimens decrease with the reduction amount increases.