Effect of Phosphorus Content on Properties of Warm-Rolled Interstitial-Free Steel Sheets

Microstructures, precipitates, and textures of experimental phosphorus additions to high-strength and traditional Ti-bearing interstitial-free (IF) steels were evaluated by uniaxial tensile test, transmission electron microscope and electron backscatter diffraction. The results show that IF steel is strengthened significantly by addition of phosphorus. Yield strength increases by 60 MPa, from 105 to 165 MPa; and tensile strength increases by 80 MPa, from 245 to 325 MPa. Phosphorus impairs formability while improving strength. Elongation, n value (work hardening), and r value (transverse strain/vertical anisotropy) are decreased from 50%, 0.31, and 1.75 to 40%, 0.25, and 1.38, respectively. Research shows that phosphorus degradation in steel is due to two mechanisms: (1) high-temperature aging and formation of FeTiP particles, and (2) solid-solution strengthening and the role FeTiP particles play in blocking nucleation and growth of textures in the {111} orientation. This results in a weak, annealed {111} texture, so that the r value decreases.