Research On Microstructure Of 355MPa Ultra-fine Ferrite Carbon Steel

The yield strength of an ultra-fine ferrite steel made by Q235 can reach to 355MPa by lowing final rolling temperature, increasing rolling percentage in the hot continuous rolling finish rolling segment and declining the coiling temperature. Several research works were conducted to the new steel, including mechanical performance testing, ferrite grain size measuring, fine microstructure observation under transmission electron microscope (TEM), the electron back scatter diffraction (EBSD) analysis under scanning electron microscope (SEM). All above was aimed to analysis the microstructure and performance of the new steel. Yield strength of the ultra-fine Q235 steel can be raised to 390-455MPa and its ferrite grain size can be thinned to 3-6μm by using new techniques. A Hall-Patch equation of the new steel is σ_S =232.7 +12.429d^-1/2. It can be used for the new steel in grain size of 3～10μm and the error between mechanical test and account is below 5%. Some 0.2～1.0μm fine etc. axis deformation induced transformation ferrite in higher strength samples were observed under TEM. The second microstructure in each specimen also has some difference. A lamella form cementite membrane around fine grain in the higher strength samples can be observed. The next in order for follow is the chain form cementite flake of the grain boundary distribution. The second microstructure in lower strength samples is agglomerate pearlite. Partly coherent relationship is exist for the agglomerate pearlite and the ferrite close by, but for the fine cementite and ferrite. The sample surface performed by right press stress and shearing stress parallel to the plate coming from roll appears some difference with the center performed only by right press stress. Such as the grain size on the sample surface is smaller than that in center, little small angle grain boundary and integrity grain exist on the surface comparing to the center unit. Texture of surface regard (110) as principle, but texture of center unit appears difference. Sample 8168#, whose strength is the highest, dominates the texture of (100). Sample 335# and 345#, whose strength appear lower, dominate the texture of (112) and (111) respectively.