4TH International Congress on Technology - Engineering & SCIENCE - Kuala Lumpur - Malaysia (2017-08-05)

Effect Of Annealing On The Microstructure And Mechanical Properties Of A Low Carbon Steel With Nanostructured Microstructure
The low carbon steel (0.165C-0.211Si-0.448Mn-0.014P-0.013S-0.002Al, wt. %) was chosen to investigate the effect of annealing on the microstructure and mechanical properties of a low carbon steel with nanostructured microstructure. The samples with martensite starting structure were cold-rolled by 50% in four passes at room temperature. The cold-rolled samples were annealed at various temperatures from 500 ºC to 650 ºC for 2 minutes and 30 minutes, respectively. The microstructures of the specimens annealed at 500 ºC and 550 ºC for 2 minutes are still elongated and no equiaxial grains are observed, indicating that no recrystallization occurs. The microstructure of the specimen annealed at 600 ºC for 2 minutes shows some small equiaxial nanostructure grains and it is obviously different from those annealed at 500 ºC and 550 ºC. For the specimen annealed at 650 ºC for 2 minutes, the equiaxed ultrafine ferrite grains with a mean diameter of 310 nm appear in most areas with more nanostructured carbide precipitates. The microstructure annealed at 500 ºC for 30 minutes is still similar to the as-deformed one, except some small carbide in the microstructure. However, in the specimen annealed at 550 ºC for 30 minutes, the equiaxed and nanostructured ferrite grains with an average size of 330 nm in most areas are observed. When the specimens were annealed at 600 ºC and 650 ºC for 30 minutes, the recrystallized grains grow and carbides coarsened obviously. Moreover, the tensile strength and total elongation are determined to be 867 MPa and 16.7% for the sample annealed at 550 ºC for 30 minutes, and 844 MPa and 15.7% for the sample annealed at 650 ºC for 2 minutes. The sample annealed at 550 ºC for 30 minutes has a better combination of strength and elongation. It may be attributed to the difference of carbides sizes in two samples. The uniform and nano-scale carbides cause a pinning effect on dislocations, which enhances the strength and elongation as the sample annealed at 550 ºC for 30 minutes. It indicates that the sample annealed at 550 ºC for 30 minutes shows a mixed microstructure composed of nanostructured and equiaxed ferrite grains (the mean grain size of 330 nm) and uniformly distributed nano-carbides, and has a good combination of high tensile strength (867 MPa) and elongation (16.7%). In addition, the recrystallization diagram indicating recrystallization conditions for tested steel is obtained. The whole area is divided into three parts, i.e., recovery, recrystallization and grain growth areas. The recrystallization diagram presents the annealing temperature and time for the recrystallization of the deformed steels. It also provides the reference for the selection of the annealing temperature and time.
Junyu Tian, Guang Xu, Haijiang Hu, Zishan Yao