Analysis on austenite continuous cooling behavior and hardenability of 1 300 MPa medium-thick plates
Ultra-high-strength medium-thick plates are widely used in the field of construction machinery and indus-trial protection due to its ultra-high strength,good plastic toughness and weldability.To study the austenitic contin-uous cooling behavior and hardenability of 1 300 MPa medium-thick plate,the microstructure evolution,hardness and geometrically necessary dislocation density of the experimental steel during continuous cooling of austenite were studied by thermal simulation experiments,and the CCT curve was drawn.The hardenability of the experimental steel was determined by end hardening test.And the trial production of experimental steel was carried out according to the above test results.The results show that with the increase of cooling rate,the microstructure of experimental steel gradually changes from ferrite+granular bainite to bainite+martensitie,and the hardness and geometric neces-sary dislocation density are increasing.When the cooling rate is greater than 5 ℃/s,the whole martensitic micro-structure can be obtained,the hardness and geometrically necessary dislocation density tend to be stable,and the cooling speed should be controlled at 5-10 ℃/s during quenching.The depth of the hardened layer of experimental steel is about 30 mm,and the hardenability is excellent,which can meet the hardenability requirements for medium-thick plates of 30 mm and below.After quenching and tempering treatment at 850 ℃+180 ℃,tempered martensitic microstructure is formed in the experimental steel of 30 mm,its yield strength is 1 328 MPa,tensile strength is 1 684 MPa,elongation is 13.4%,and-40 ℃ impact energy is 24.3 J,which has reached the level of 1 300 MPa grade high-strength steel,the higher density of dislocation and large angular grain boundaries are the main reasons for its high strength and good plastic toughness,which provides a basis for the formulation of the subsequent heat treatment process of experimental steel.