Abstract
This research is concerned with study of the effect heat treatment and the bake hardening process on dual phase steels which can be obtain by intercritical annealing process. Hardness, microstructure and tensile specimens were prepared according to standards. Used specimens was with different carbon content (0.12, 0.096 and 0.064). The normalizing heat treatment was conducted on low carbon steel, to remove the effect of the previous manufacturing processes. Intercritical annealing was used as a heat treatment to obtain the dual phase steel, the carbon steel was quenched from760 ºC this temperature lies between the critical points. It has been found that after intercritical annealing process has transformation accrue from pearlite structure resulting from the normalization process to a new structure contain ferrite and martensite. The hardness of dual-phase steel obtained by intercritical annealing is higher than of the steel that was normalized as well as the carbon content of the steel affects the hardness. Tensile test results showed that the ultimate tensile strength and yield strength increased after intercritical annealing process compared to other specimens were normalized ,content of carbon also effect on the ultimate tensile strength and yield point. The dual phase steel is characterized with no visible yield point due to the dislocations movement are restricted. Dual phase steel yield point re-appear after pre-strained at 3, 5, 8, 10%., and baked to 170 ° C. The ultimate tensile strength and the yield point for the pre-strained and baked specimens were higher than the other tensile specimens which conducted with normalizing or intercritical annealing only due to the diffused carbon atoms that pin dislocations and prevent it is movement. Baking process is affected by several factors such as carbon ratio and the amount of prestrain. It is found that when carbon
 content increase, the number of diffused carbon atoms will increase. Amount of pre-strain effect on the mechanical properties after baking process, where it was found that when the mount of pre-strain increase the ultimate tensile strength and yield point stress will increase
Highlights
The demand of environment-friendly vehicles that have better fuel economy and lower CO2 emissions compels automakers to apply advanced technology and new materials in vehicle manufacturing [1]
Dual Phase (DP) steels are a class of advanced high strength steels (AHSS) characterized by a microstructure consisting of a mixture of hard martensite and softer ferrite which combines high strength with good ductility
Additional dislocations generated during prestraining are pinned by free solute atoms that diffuse to the dislocation core during baking and as a result yield strength (YS) increases[12].The bake hardening response of BH grade steel is a function of both time and temperature and prestrain level
Summary
The demand of environment-friendly vehicles that have better fuel economy and lower CO2 emissions compels automakers to apply advanced technology and new materials in vehicle manufacturing [1]. Dual Phase (DP) steels are a class of advanced high strength steels (AHSS) characterized by a microstructure consisting of a mixture of hard martensite and softer ferrite which combines high strength with good ductility Their excellent properties have made them the most widely used of all AHSS in the automotive sector; certain aspects of DP steel behavior, notably the formability, limit the range of accessible parts[4].DP steels, the volume fractions of both phases are the most critical parameter in determining the final mechanical properties[5]. These favorable properties are definitely related to typical microstructure of dual phase steels in which, soft ferritic network provides good ductility; while, hard particles and martensite phase play the load bearing role This microstructure, shows some kind of metallic composite. Emil EVIN 2013presented the basic concepts of advanced high strength dual phase steels for automotive applications, including the design of chemical composition, microstructure and mechanical properties[19]
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