Studies On Effect Of Thermomechanical Treatment On Microstructure And Mechanical Properties Of Containing Cu Crack Arrest Steels

Crack Arrest steel is a kind of high strength low alloy steel originated from carbon steel.From the engineering point of view, most of structures are constructued by welding. Welding can induce the formation of crack. These cracks can propagate quickly to the matrix, leading to the fracture of structures. Thus, it is essential to develop and apply crack arrest steels to improve the crack arrest properties. It has been reported that the crack arresting properties can be ehnaced by improving the impact energy through thermomechanical treatment. Rolling within austenite region following colling control can refine the ferrite grain size, leading to the improvement of toughness. Thus, by optimization of thermomechanical control processing,the crack arrest properties can be improved. Cu is a kind of element which makes the austenite stable and provides the precipitation strengthening. Precipitate strengthening can be applied to strengthen steels, substituting carbon strengthening to lower the carbon content in the steels. This thesis studies the crack arrest steels containing Cu. The effects of thermomechani treatments on the microsturce and properties are investigated systematically.The two-step rolling is applied to investigate the effects of rolling temperatures on the microstructure and mechanical properties.Research results show that grain refinement increases both the strength and toughness of the steel. Thermo-mechanical treatments further improve the strength and toughness of the steel by formation of bainite structure and initiation of dislocations. In order to study the thermo-mechanical treatment process on the microstructure and properties of Cu-containing steel, rolling conditions should be optimized. These optimal conditions include austenite recrystallization, region rolling distribution, and low temperature application.Homogeneous supersaturated solid solution obtained from solution treatment and aging time facilitates precipitation of fine particles. Evenly distributed carbides and lattice distortions are formed thereby increasing the strength. At this stage two different solutions at temperature 925? and 875? with two different rolling processes were studied. Steel solid solution had improved in the microstructure and mechanical properties. Two different rolling processes with different dissolution times of 15min, 30min, lhr, 2hrs and 5hrs after rolling steel solution improved the microstructure and mechanical properties.Solution treatment and aging of the Cu-containing steel crack improves microstructure and mechanical properties. Tempering process is used to enhance the performance by eliminating and reducing the quenching stress on the steel. This contributes to the rapid precipitation of Cu-rich phase. A study on rolled steel at tempering temperatures of 875? and 925? for 30 minutes improves the microstructure and mechanical properties. This is confirmed by the increase in hardness after the three tempering stages. When the tempering temperature is increased,continuous precipitation of Cu-rich phase takes place, leading to coarsening hence weakening the steel substrate.