Effect of FDT on microstructure and crystallographic texture of 600 MPa grade high-titanium high-formability ferrite-pearlite pickling steel

Obtaining a near-γ texture that parallels that of a rolling plane enhances the r value of steel, thereby improving its formability. A high-angle grain boundary with a misorientation greater than 45 degrees is another crucial factor contributing to the formability of steel. Steel's crack arrest capability is dramatically improved by increasing the density of the high-angle grain boundary. The primary factors associated with texture evolution include chemical composition, finishing delivery temperature (FDT), rolling speed, and cooling rate after final rolling, of which the FDT is the most critical. Previous studies, which emphasized only pipelines and interstitial-free steels, have suggested that there is a discrepancy in the relationship between FDT and texture, and this relationship remains unclear with respect to high-titanium high-formability ferrite-pearlite steel. In this study, the microstructure and crystallographic texture of a 600 MPa grade high-titanium high-formability ferrite-pearlite steel with differential FDT were investigated by scanning electron microscopy and electron backscatter diffraction techniques. The results reveal that its microstructure comprises ferrite and pearlite irrespective of FDT, but increases in the FTD cause an increase in the high-angle grain-boundary density. The primary microstructure is ferrite in both these samples, with a small amount of pearlite dispersed between the ferrite grains. The texture dramatically changes with elevated FDT. The intensity of all the textures significantly increases as the FDT increases from 850℃ to 875℃, with the transformation of a large amount of apparent near-γ textures, which is beneficial to formability. The intensities of near-α texture and γ texture are low in the sample with an FDT of 850℃, wherein the primary textures include 001110, 113471, 114110, and 223110. The intensity of the textures disadvantageous for formability is stronger than that of the advantageous textures in the sample with a lower FDT, which constrains formability and should be avoided. A positive change was observed in the textures as the FDT increased to 875℃. A strong near-γ texture was transformed in the steel that was finally rolled at 875℃, and its fraction increased to 41% from 19.9% at 850℃. A strong 001110 rotated cubic texture also occurred in the 875℃ finally rolled steel, which is bad for formability. However, superior formability can be guaranteed in general as the transformation of more advantageous textures than disadvantageous textures was observed.