Effect of Aluminum and Nitrogen on Microstructure of Ultra-Low-Carbon Nb-IF Steels
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ltra-low-carbon Nb-IF (interstitial free) steel has been extensively used for automotive panels because of superior formability and nonaging properties. Highstrength cold-rolled steel sheets with good formability have been developed, in which the major strengthening method was solid solution hardening with phosphorus, manganese, and silicon. When the IF steel is strengthened with phosphorus in the range of 0.05 to 0.l0 wt pct, it becomes susceptible to the secondary work embrittlement because of the lack of grain boundary strength, which is the essential drawback of the P-containing IF steel.[1,2] Recently, a new type of IF steel, which contains phosphorus less than 0.03 pct, has been reported to show excellent antisecondary work embrittlement and formability.[3–5] The shortage of the strength caused by reducing the phosphorus is compensated by a combination of the precipitation hardening of fine NbC precipitates and the grain refinement hardening.[3–5] The steel contains higher niobium content than the conventional Nb-IF steels, which is for the precipitation hardening and grain refinement hardening by the NbC precipitates. An investigation[6] was conducted to use AlN precipitates instead of NbC to strengthen the steel. It has been well known that aluminum is precipitated as AlN in hot rolling. The precipitate size depends on the coiling
W.C. JEONG, Associate Professor, is with the School of Mechanical and Automotive Engineering, Catholic University of Daegu, Gyeongbuk 712-702, Republic of Korea. Contact e-mail: wcjeong@ cu.ac.kr Manuscript submitted August 20, 2008. Article published online April 15, 2009 1280—VOLUME 40A, JUNE 2009
temperature in the hot rolling. Conventional Al-killed steels contain soluble aluminum content ranging from 0.03 to 0.06 pct. The Al content has not been considered to have the precipitation hardening effect. The interesting result[6] was found that, in the hot-rolled ultra-lowcarbon Nb-IF steels with Al content ranging from 0.055 to 0.16 pct, the increase in the aluminum content decreased yield and tensile strengths and increased elongation. The softening effect of aluminum is related to the decrease in the fine NbC precipitates and the increase in the coarse AlN precipitates.[6] In this study, aluminum and nitrogen contents in the ultra-low-carbon Nb-IF steels were varied to investigate the effect of aluminum and nitrogen contents on the microstructure of hot-rolled steels and continuously annealed steels. The chemical compositions of three steels are given in Table I, together with atomic ratios of Nb/C and Al/N. The vacuum-melted and cast ingots were hot rolled to 3.4-mm thickness with finish hot-rolling temperature at 900 °C. The hot-rolled strips were air cooled to 660 °C and then soaked at the temperature for an hour, followed by furnace cooling to simulate the coiling process in hot rolling. The hot-rolled sheets were pickled and cold reduced 76 pct to the nominal thickness of 0.8 mm. To simulate the continuous annealing cycle, the cold-rolled sheets were heated to 830 °C at a rate of
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