Bainite Type Application Performance, Cold Work To Strengthen The Non-quenched And Tempered Steel

Fatener is a widely used general structural component. The application of cold hardening microalloyed steel instead of quenched and tempered steel to manufacture fasteners could omit quenching & tempering treatment and spheroidizing annealing treatment and thus has significant effects of energy saving and environment protection. In china, energy and resources shortage is getting worse, therefore, the application of cold hardening microalloyed steel has profound social meaning and obvious economic benefits.In this paper, microstructures and mechanical properties of a newly developed low carbon Mn-B-Ti type cold hardening microalloyed bainitic steel were studied using metallographic microscope, SEM and TEM, and hydrogen absorption and delayed fracture behaviors were studied using cathodic charging, slow strain rate test (SSRT) and hydrogen thermal desorption analysis test.Experimental results of effect of deformation on the mechanical properties show that with the increase of cold drawing reductionγ, strength increases and ductility decreases, and the increase of yield strength is more significant than that of tensile strength. The work hardening exponent n increases withγand then decreases whenγexceeds about 30%. The compression stress of 1/3 cold heading after cold drawing keeps almost constant with the increase ofγ, while the critical compression ratio after cold-drawn decreases with the increase ofγ. Compared to notch tensile strength increases with the increase ofγwith no hydrogen charging, both notch tensile strength and delayed fracture strength ratio significantly decrease with the increase ofγafter hydrogen charging. It is found that delayed fracture resistance could be significantly improved after annealing at 400℃. Therefore, with the precondition of required strength,γshould be chosen carefully to maintain a relatively high processing and application properties such as cold heading capability and delayed fracture resistance.Experimental of effect of aging temperature on the mechanical properties results show that strength increases at first and reaches a peak at about 250℃, and then decreases with the increase of aging temperature. The decrease of strength is more significant when aging temperature is higher than 400℃. Both ductility and strength ratio increase with increasing aging temperature, while the latter is more significant. The charging of hydrogen significantly impairs the delayed fracture property of the steel subject to 30% cold drawing. Hydrogen content decreases with increasing aging temperature and thus both notch tensile strength and delayed fracture strength ratio increase. The increase of notch tensile strength and delayed fracture strength ratio is more noticeable when aging temperature is higher than 200℃.The hydrogen absorption behavior was studied. It is found that there are two hydrogen desorption peaks, that is, peakⅠaround 160℃～190℃and peakⅡaround 300℃, for the charged as-rolled and as-cold drawn specimens, while there is only peakⅡfor non-charged specimens. Aging treatment has a significant effect on the hydrogen desorption curve of its desorption peak's height even disappear. The charged hydrogen content varies with charging current density, cold drawing reduction and aging temperature. It is suggests that hydrogen of peakⅠis mainly trapped by low binding energy trapping sites such as dislocation introduced by cold deformation. The hydrogen content of peakⅡCHI significantly increases after 30% cold drawing. CHⅠdecreases with increasing aging temperature up to 200℃, while CHⅠkeeps constant when aging temperature is higher than 200℃. It is suggested that hydrogen of peak II is mainly trapped by high binding energy trapping sites such as grain boundary, phase boundary and inclusions, which are already exist in the as-rolled condition. Therefore, the hydrogen content of peakⅡCHⅡremains almost unchanged with deformation, charging current density and aging under 200℃, however, ChHⅡdecreases with increasing aging temperature higher than 200℃, which is higher than it's desorption temperature.