Correlating Precipitates and Texture Formation on Batch and Continuous Annealed Rolled Interstitial Free Steels

GLADYS CHARCA RAMOS; MIKE STOUT; RAUL EDUARDO BOLMARO

Rio de Janeiro

Congreso; 17th International Microscopy Congress; 2010

IMC

Automotive, beverage and appliance industries, among others, have used deep drawing processing for many years to form parts from a variety of sheet metals. The formability of a sheet can be best characterized through the R-value which, on BCC Fe alloys, strongly depends on the development of a severe {111}<uvw> recrystallization texture, what is achieved by lowering the interstitial carbon and nitrogen contents and adding stabilizing elements such as Ti. In this kind of steel, Ti scavenges C and N atoms out of the solution, forming precipitates. The times, temperatures and reductions of thermo mechanical steelmaking processes affect the size, shape and distribution of precipitates, which in turn influence the final recrystallization texture evolution.   In this investigation, we study how, after rolling, continuous and batch annealing determine the kind, size and dispersion of precipitates in interstitial free steels and the relationship between the microstructures and texture. The material used in this study was an extra low-carbon, hot-rolled steel, with 0.003% C, 0.005% Si, 0.001% Nb, 0.145% Mn, 0.018% Cu, 0.005% Sn, 0.001% V, 0.013% P, 0.009% Ni, 0.038% Al, 0.066% Ti, 0.016% Cr, 0.0001% B, 0.007% S and 0.0017% N. From its initial thickness of 4.80 mm, the steel was cold rolled to 90% reduction. The batch annealed sample (sample 1) was held at 800¡ÆC for 1 hour in vacuum (1x10-4mbar), while the continuous annealing was done under two atmospheres in the kiln: sample 2 was held at 850¡ÆC in argon atmosphere for 6 minutes and sample 3 was held at 840¨¬C in air for 8 minutes. The textures of the hot band and cold rolled and annealed samples were measured by X-ray diffraction and the grain structures of each sample were investigated with optical microscopy. The size, dispersion and composition of the precipitates were analyzed by scanning electron microscopy, employing Energy Dispersive Spectroscopy (EDS) to identify the precipitates. The continuous annealing process, sample 2, produced the strongest texture, with an intensity of about 25% larger than for the batch annealed specimen, sample 1. Widely spaced coarse carbide dispersions are known to favor the development of the {111}<uvw> texture during rapid annealing because much less C remains dissolved in the ferrite matrix [1]. Coarse, widely spaced precipitates are observed in sample 2 (Fig. 1b). Sample 1 formed FeTiP precipitates as well (Fig. 2c). Those precipitates lead to both the deterioration of drawability and the loss of strength [2]. The precipitation of copper sulphide is shown in the figure 3a-b for sample 2. The formation of S-Cu species is advantageous because more Ti is available for forming carbides. The sample 3 does not develop high intensity of gamma fiber (Fig. 1f) despite rapid annealing due to the formation of Fe-O inclusions, which act like barriers to carbon for joining to Ti (Fig. 4b-c). All samples contain TiN, Ti4C2S2 and TiS precipitates which are 5.0¥ìm, 0.5¥ìm and 2¥ìm in average diameter respectively (Figs. 2b, 3c-d and 4d).