Effect of Niobium or Vanadium on Mechanical Properties of Hot Rolled High Strength Steel Sheets for Automotive Use

Nb-bearing and V-bearing steels have been well known as typical high-strength low-alloy (HSLA) steels. However, little comparison has been made so far between the formability of Nb-bearing steels and that of V-bearing steels for automotive use. The hot direct rolling (HDR) process has been noticed because it is a very energy saving process. This paper aims to clarify the differences between the formability of Nb-bearing hot-rolled steel sheets and that of V-bearing hot-rolled steel sheets produced by both HDR process and cold charge rolling (CCR) process. A low temperature (450°C) simulated coiling characterized by good formability of steels was applied in a laboratory to 0.025% Nb (25 Nb), 0.049% Nb (50 Nb) and 0.078% V (80 V) steels with the ultimate tensile strengths (TS) ranging from 490 to 590 MPa. Hole expansion test was carried out in addition to the tensile test because stretch flangeability is very important for automobile structural uses. As a result, TS-flangeability balance of Nb-bearing steels was found to be superior to that of V-bearing steel. This is because their microstructures were distinctly different: Nb-bearing steels had a ferrite+bainite microstructure and V-bearing steel had a ferrite+fine pearlite microstructure. For the linearly precipitated carbides, V-bearing steel had a low TS-flangeability balance. It was concluded that Nb-bearing steels is strengthened by ferrite grain refining and bainite transformation hardening rather than precipitation hardening, which is the main strengthening mechanism for V-bearing steels. Change from CCR to HDR increased the strengths of all the steels studied, resulting in nearly the same TS-elongation and TS-uniform elongation balances, but the TS-flangeability balance was still maintained through this change.