Preparation and Mechanical Properties Tailoring of Micro/nano Structure Carbon Steel and Austenitic Stainless Steel

Author:Li Zheng Ning

Supervisor:la pei qing

Database:Doctor

Degree Year:2019

Download:16

Pages:153

Size:23029K

Keyword:

1045 carbon steel and 304,316 L stainless steel are common engineering materials in the modern industrial.With the exhaustion of resources and energy,and the increasing pressure on the environment,a higher requirement is put forward for the comprehensive mechanical properties of the steels.The cold rolling and subsequent annealing process is an important route to obtain block high strength and high plastic steels,the tailoring mechanisms of microstructure and mechanical properties were also investigated,it provides a guidance for all high strength and high plastic alloy design efforts by this route.In this paper,the large dimensional bulk micro/nanostructured 1045 carbon steel with nanolaminate pearlite was prepared by aluminothermic reaction casting method.The steel was rolled at room temperature with different thickness reduction.With the increase of thickness reduction,the size of ferrite grain was refined,many lamella cementites were squeezed,a portion of cementite were crushed,and the dislocation density increased.Finally,1045 steel with composite structure of microcryst alline/ultrafine grain(UFG)ferrite + nano-lamella cementite + nano-cementite particles were obtained.Under various strengthening mechanisms and high work hardening ability,the 1045 carbon steel obtained a good strength-plasticity synergy after rolling with 70% thickness reduction.The rolled 1045 steel with 85% thickness reduction was annealed at lower temperature,the deformation ferrite grains recrystallized and the number of UFG ferrite increased,in addition,a large number of nano-cementite particles were precipitated out,and a heterogeneous composite structure of microcrystalline/UFG ferrite + nano-cementite particle was formed.The rolled steel was annealed at different temperature with 1 h,with the increase of annealing temperature,the average size of ferrite grain decreased slightly,the number of nano-cementite particles increased,the content of lamellar cementite decreased,meanwhile the tensile strength and elongation of rolled steel were both improved significantly.Annealing at 400℃ for different time,with the increase of annealing duration,the change of microstructure and mechanical property is similar to that at different temperature.Then,the 316 L and 304 stainless steels prepared by aluminothermic method were rolled at room temperature with different thickness reduction,respectively.With the increase of thickness red uction,the average size of austenite grain decreased,the content of strain-induced martensite increased,and the dislocation density increased obviously.Finally,the microstructure composed of high-density dislocation,deformation twins,strain-induced martensite and microcrystalline/UFG/ nanocrystalline austenite matrix was obtained.After rolling at room temperature with same thickness reduction,the strain-induced martensite content of 304 stainless steel is higher than that of 316 L stainless steel,and the martensite strengthening effect is more significant.Under the action of multiple strengthening mechanisms such as dislocation strengthening,fine grain strengthening,especially martensite strengthening,with the increase of thickness reduction,th e yield strength and tensile strength of 304 and 316 L stainless steel gradually increased.Although the strength of 304 and 316 L stainless steels w ere greatly improved by rolling at room temperature,but the plasticity of 304 and 316 L stainless steels were also deteriorated significantly.In order to improve the plasticity of cold rolled 316 L and 304 stainless steels,the rolled 316 L stainless steels with 70% thickness reduction were annealed.With the increase of annealing temperature,the deformation austenite grains occurred recovery and recrystallization,and more α′ martensite transformed to austenite.The number of UFG austenite grain increased first and then decreased,the average grain size decreased first and then increased,and the average grain size of nanocrystalline austenite decreased first and then increased.Therefore,the high strength was maintained,and elongation increased significantly.After annealing at 600 ℃ with different time,with the increase of annealing duration,the size of recrystallization ultrafine grains grew slightly,the content of strain-induced martensite decreased.The dislocation,twin and other defects in the deformed austenite are gradually eliminated.With the increase of annealing duration,the strength and plasticity changed slightly compared with annealing at different temperature.The rolled 304 stainless steels with 70% thickness reduction were annealed at 700℃ with different time,and the variation of microstructure is basically consistent with that of 316 L stainless steels.After annealing,a heterogeneous composite structure with austenite of grain size microcrystalline/UFG/nanocrystalline distribution and different contents of residual martensite was also formed.With the increase of annealing duration,the deformation austenite grains occurred recovery and recrystallization,more α′ martensite transformed to austenite,the dislocation and twin defects in the deformed austenite gradually eliminated,and the strength decreased slightly,but the plasticity increased dramatically.