Effect Of Cyclic Heat Treatment On Microstructure And Properties Of Ultra High Strength Steel

Ultra high strength steel is widely used in mold manufacturing,automobile transportation,navigation and shipbuilding,aerospace,military industry.With the rapid development of industry and the requirements of energy saving,resources,environment and other aspects,the steel industry puts forward higher requirements for the development and application of ultra-high strength steel with low-cost and high-performance.It is accepted that microstructure refinement is one of the effective ways to improve both the strength and toughness of materials.Many works have been shown that the microstructures of the steel can be effectively refined by cyclic heat treatment,and consequently improving the strength and toughness of the steel.It is of great significance for engineering application to study the influence of cyclic heat treatment on ultra-high strength steels.In this work,based on three type of microstructured steels: the prior austenite with coarse grain(CG)of ultrahigh-strength martensitic steel,ultrafine-structured steels produced by heavily warm rolling(HWR)of metastable austenite and ferrite-martensite dual-phase steel,we systematically investigate the effect of cyclic heat treatment(temperature,holding time and number of cycles)on the microstructure refinement and mechanical properties of three kinds of ultra-high strength steels.The conclusions we abtained are as follows:The effect of cyclic heat treatment on the microstructure and properties of ultra-high strength martensitic steel with prior coarse austenite(grain size ? 100 ? m)was studied.The results show that the cyclic heat treatment can effectively refine the prior austenite grains of CG martensitic steel,consequently improve the ductility of martensitic steel.The prior austenite grains of the studied medium-carbon low-alloy martensitic high-strength steel with coarse austenite are effectively refined and the strength and ductility of the steel are significantly improved after one cycle heat treatment at 860? for 1?20 minutes.The tensile strength and elongation of the steel are increased respectively from 1727 mpa to2322mpa and from 4.3% to 9.3% after one cycle heat treatment for 20 minutes.These results also showed that the prior austenite grain size of martensitic steel can decrease from 100?m to about 5?m and can be remain stable by three cycles treating.An ultra-high strength single-phase martensitic steel with lamellar ultrafine structures(lamellar thickness < 1 ? m)was produced by heavily warm rolling.The effect of cyclic heat treatment on the microstructure and properties of the ultra-fine martensitic steel was studied.The results show that the cyclic heat treatment can significantly improve the microstructure uniformity of the HWR ultrafine-grained martensitic steel,and effectively improve the ductility of the steel.With the increase of holding time and times of cyclic heat treatment,the proir austenite grain size of warm rolled ultra-high strength steel slightly increases,but not be significantly increased by properly controlling the temperature and holding time of cyclic heat treatment.The results of this study show that the proir austenite grain size of HWR ultra-high strength martensitic steel can be stabilized below 5 ?m under the experimental conditions of 1?6 cycles at 860?holding 1?20 minutes,without significant increase.After one cycle heat treatment at 860? for 2min,the lamellar structure of ultrafine structures of steel is changed into equiaxed fine grain.The ultimate tensile strength of the steel still remains in 2216 MPa and the elongation is increased from5.4% to 8.0%.These reulsts conclude that ultra-high strength steel with excellent comprehensive mechanical properties can be obtained by the combination of HWR and one cycle heat treatment.The ferrite / martensite dual phase steels with different grain size and phase fractions were prepared by rolling in(?+?)phase region and annealing at 930 ?,870 ?,810 ?,respectively.The effect of cyclic heat treatment on the microstructure and properties of ferrite martensite fine grained dual-phase steel was studied.The results show that the ultra-fine grained dual phase steel can be produced by the combination of dual-phase rolling and one short-time cycle heat treatment.Cyclic heat treatment can significantly improve the microstructure uniformity of as rolled dual-phase steel.it is shown that the cyclic treating can not obviously refine the prior austenite grain refinement of martensite phase,but can significantly refine the ferrite structure and stabilize to about 2?m.Short-time cyclic heat treatment can improve the strength of dual phase steel without significantly reducing the elongation.An attempted research on the preparation of ultra-high strength dual-phase steel by the combination of isothermal cyclic rolling and cyclic heat treatment was carried out.Compared with the common continuous rolling process,the microstructure of dual-phase steel by isothermal rolling and short-time heat cyclic treatment was obviously refined,and the uniformity of the microstructures of the steel was significantly improved.It is an effective method to obtain ultra-high strength and high toughness dual-phase steel by the combination of cyclic rolling and cyclic heat treatment.