Improvement of Drawability of Titanium-Stabilized Interstitial-Free Steel by Optimization of Process Parameters and Text
- PDF / 1,207,034 Bytes
- 16 Pages / 593.972 x 792 pts Page_size
- 57 Downloads / 214 Views
ON
INTERSTITIAL-FREE steels are widely used in the structural parts of automotive vehicles for their improved drawability, moderate yield strength, and nonaging properties. Among these, high value of average normal anisotropy, a measure of deep drawability, described by the engineering parameter, rm, is of great importance in optimizing the properties of IF steel sheets. The rm value for a given stress-strain condition is the ratio of broadening to thickness. Crystallographically, normal anisotropy originates from recrystallization texture. Optimum deep-drawing behavior of IF steels is obtained by a strong and homogeneous f111ghuvwi fiber texture, called the c fiber.[1–4] In industrial practice, there are numerous and complex interactions between steel chemistry and processing conditions starting from hot-rolling parameters, cold rolling, and annealing conditions, including precipitate formation during hot rolling and annealing, which critically affect the quality of annealed steel sheets. KUMKUM BANERJEE, Researcher, ANIL K. VERMA, Researcher, and T. VENUGOPALAN, Chief Technology Officer, are with the R & D Divison, Tata Steel Limited, Jamshedpur, 831007 India. Contact e-mail: [email protected] Manuscript submitted August 17, 2007. Article published online April 8, 2008 1410—VOLUME 39A, JUNE 2008
The addition of Ti plays a crucial role in controlling the desired properties of these steels by removal of interstitials from the solid solution and by forming fine precipitates. In Ti-stabilized IF steels, Ti combines with N and S prior to carbon scavenging. Thus, theoretically, the titanium level necessary to stabilize a Ti-IF steel is given by the relationship, Ti ‡ 3.42 N + 1.5S + 4C.[5,6] Under mill processing conditions, the precipitation sequence is as follows: TiN, TiS, Ti4C2S2, TiC, and FeTiP.[7] The precipitation of Ti4C2S2 in hot bands is achieved using a low slab reheating temperature (SRT) and high coiling temperature, which results in high rm values.[8] However, moving to lower SRT is difficult for some rolling mills due to roughing-mill load limitations and problems in maintaining the finish- rolling temperature (FRT) above the austenite-ferrite transformation temperature. Hence, the presence of Ti4C2S2 in hot bands depends on hot-rolling parameters. Irrespective of the advantages of good deep drawability and formability, strain aging and retardation of recrystallization sometime appear in Ti-containing IF steels due to the presence of P, which is added as solidsolution strengthening element[9] to IF high- strength steels. This is attributed to the exhaustion of Ti during the formation of ternary phosphide, FeTiP. Brun et al.[10] reported that the amount of P, present as FeTiP, varied between 10 and 50 pct of total. Shi et al.[11] found that FeTiP formed in a titanium- stabilized IF steel, even when METALLURGICAL AND MATERIALS TRANSACTIONS A
the P content was as low as £0.01 pct. Therefore, it is essential to understand the precipitation behavior of this phosphide during the annealing stage and its eff
Data Loading...
