The Transformation Behavior,Microstructure And Mechanical Properties Of Low Carbon Steel Austempered Below Ms Temperature

This paper takes a low-carbon Al-contained steel as research material,the effects of isothermal temperatures below-Ms on transformation behavior,microstructure,mechanical properties and strain distribution during deformation were analyzed,and the results are compared with the traditional process of isothermal quenching above-Ms.In addition,the effects of warm rolling at different temperatures before isothermal quenching above and below-Ms on microstructure and mechanical properties were studied.As a result,it was found that a small amount of prior martensite was formed when quenched below-Ms,and then bainite adhered to the prior martensite to nucleate and the bainite transformation occurred rapidly.Finally,the bainite ferrite lath and blocky martensite/austenite(M/A)constituents in the structure have been significantly refined.Especially for bainite laths,the thickness is 221±42 nm when austempering at 400℃(above-Ms),but only 123±23 nm when quenched at 365℃(below-Ms).The strength of the samples austempering above-Ms are higher,while the elongation and product of strength and elongation of the samples austempering below-Ms are higher.In addition,the impact performance of the samples austempering below-Ms is much higher than that above-Ms.Particularly,as the steel is austempering below-Ms(350℃),its impact toughness reaches190 J/cm~2 while maintaining excellent tensile properties,which is 2.6 times higher than the impact toughness(73 J/cm~2)when the steel is austempering above-Ms(440℃).As a result,it was found that the thickness of bainite lath and the size of blocky martensite/austenite(M/A)constituents can be refined by the combination of ausforming and austempering process.The combination of warm rolling and austempering above-Ms improves strength and reduces plasticity and toughness.In contrast,the combination of warm rolling deformation and isothermal quenching below-Ms can increase strength while increasing plasticity and toughness.For example,after ausforming at 600℃and austempering at 350℃(below-Ms),the strength of the sample is 1354 MPa,the elongation is 17.5%,and the impact toughness can still reach 163.9 J/cm~2.