KUČEROVÁ Ludmila UWB in Pilsen

Improvement of Cold Formability of Spring Steel by Low-Temperature Thermo-Mechanical Processing

Co-authors JIRKOVÁ Hana, ŠTÁDLER Ctibor, MAŠEK Bohuslav

Cold and warm formability of pearlite-ferrite steels can be improved by conventional soft annealing. This processing changes lamellar morphology of pearlite to a globular one. The advantage of this microstructure is a significant reduction of potential defects which would be initiated at sharp edges of carbides as a result of localized stress concentration. Annealed structures possess low yield strength and excellent ductility. This ensures their good cold formability. The problems of these technologies lie not only in long processing times, but also in high energy consumption which make the final product quite expensive. The time necessary for cementite spheroidization can be shortened by unconventional heat treatment around Ac1 temperature combined with deformation applied at various processing stages. This processing method was utilized for spring steel 54SiCr with ferrite-pearlite original microstructure and lamellar pearlite morphology. The hardness of this structure reached 290 HV10. Once Ac1 and Ac3 temperatures of phase transformations were established, different processing parameters were tested. First of all, the most suitable heating temperature was determined. Subsequently, the influence of the character and intensity of deformation on cementite spheroidization and ferrite grain refinement were investigated. Metallographic analysis was carried out by the means of light and scanning electron microscope and mechanical properties were determined by hardness measurement and mini-tensile test.