Abstract: To develop cold-rolled SK4 steel sheet that can replace imports, the effects of spheroidizing annealing of hot-rolled pickled coils under (680-720) ℃×(16-20) h and recrystallization annealing of cold-rolled coils under (660-700) ℃×(16-20) h on the microstructure and mechanical properties of steel coils were studied by using an industrial bell furnace. The results show that after spheroidizing annealing, the lamellar caibides in the hot-rolled pickled SK4 steel coils are significantly spheroidized, while when the annealing temperature or time reaches the maximum, clusters of acicular carbide appear in the spheroidized structure. The yield strength of the spheroidizing annealed coils ranges from 365 MPa to 515 MPa, the tensile strength ranges from 578 MPa to 647 MPa, the elongation ranges from 17.2% to 24.1%, and the hardness ranges from 170 HV5 to 197 HV5. With the increase of spheroidizing annealing temperature or holding time, the strength and hardness of the steel coils are obviously reduced, while the plasticity is obviously improved, which is beneficial to the subsequent cold rolling. The optimal spheroidizing annealing process is 680 ℃ for 20 h or 700 ℃ for 16-20 h. Recrystallization annealing also has a spheroidizing effect, and as the annealing temperature or annealing time increases, the carbides are almost completely spheroidized, with the spheroidized particles coarsening. After recrystallization annealing, the yield strength ranges from 399 MPa to 464 MPa, the tensile strength ranges from 522 MPa to 598 MPa, the elongation ranges from 17.9% to 25.3%, and the hardness ranges from 158 HV5 to 176 HV5. With the increase of recrystallization annealing temperature, the strength and hardness first decrease, then slightly increase, while the elongation is opposite. The optimal recrystallization annealing process is 680-700 ℃ for 16-20 h.

Key words: SK4 carbon tool steel, bell furnace, spheroidizing annealing, recrystallization annealing, microstructure, mechanical properties