Influence of quenching temperature on the character of steel K60 strain hardening

The article analyses the strain-stress diagrams of steel K60, previously normalized and quenched in water from two-phase (-) interval; the degree of hardening, even elongation and hardening coefficient were calculated as the ratio of the tensile strength and yield strength difference to the relative elongation of tensile test specimens on the diagram tension sector, that corresponds to the even elongation. Hardening temperature increase from 700 to 830С results in opposite change of в and т values: в slowly increases from 516,8 to 527,0 MPa while т rather sharply decreases from 498,7 to 478,6 MPa. Thus, obtaining of diphasic structure of low-carbon steel after hardening from the diphasic area, consisting of pure ferrite and martensite in separate joints of grains, predetermines the low values of yield limit and the high values of ultimate resistance. The nature of strain hardening coefficient * change has been determined. Its value becomes  1,7 times higher with the increase of quenching temperature from the intercritical interval. Herewith the compound ferritic- martensitic structure is formed, its dispersion increases with the increase of the quenching temperature from the mentioned interval. It is coefficient * that determines plasticity of low-carbon sheet steel. At the temperatures in the upper part top of the intercritical interval its growth is related to forming of plenty of dispersed austenite grains, to size reduction of martensite packages, and to active atoms of carbon in martensite that block the sources of dislocations. The results are recommended to be used in the development of thermal hardening modes for low-carbon and low-alloy steels, as well as in the evaluating the parameters of their mechanical and operating characteristics