Abstract
The effect of the annealing cycle on microstructure, mechanical properties and draw ability of aluminium killed steel sheets at three different heating rates and holding times were investigated. Tensile test, hardness test, optical microscope and transmission electron microscope (TEM) techniques were utilized. The results of samples for different times in laboratory experiments were compared with the production samples. The results for these experiments showed that lower heating rates (12 °C/h) occurred at a lower annealing temperature, increase draw ability. This effect is attributable to the longer time available to initiate nucleation at the lowest heating rate. However, for all three heating rates (12 °C/h, 24 °C/h and 36 °C/h), the aluminium and nitrogen combine to form atmospheres or pre-precipitation clusters at polygonised sub-grains and as rolled boundaries, modifying the development of the recrystallized structure. Different amounts of AlN were precipitated prior to recrystallization for each heating rate resulting in different final recrystallized grain sizes, mechanical properties and drawabilities. The heating rate strongly influences recrystallization and drawability with a lower heating rate reducing both the temperature of the start of recrystallization and the recrystallized grain size. It is inferred that the lower rate promotes nucleation of recrystallization overgrowth of recrystallized. The effect of holding time on mechanical properties was to: (i) decrease tensile strength, yield strength and hardness and (ii) increase drawability, elongation and grain size. Tensile strength and hardness results from laboratory experiments were lower than those for production samples, but the trends were similar.
Highlights
Low carbon aluminium killed steel is one of the most important products of the steel industry today
The development of formability of low carbon aluminium killed steels depends on the alloying elements, impurities and inclusions in steel making, slab reheating furnace, finishing and coiling temperature in hot strip mill, total reduction, lubrication, heating rate, temperature and time annealing in cold rolling mill
The most important step for aluminium killed steels is coiling practice, a lower coiling temperature leads to higher r-value because ALN is retained in solution in ferrite and can precipitate out on annealing after cold working with an increase in {111} grain orientation at the expense of {100} grain [10,11,12,13,14,15]
Summary
Low carbon aluminium killed steel is one of the most important products of the steel industry today. The development of formability of low carbon aluminium killed steels depends on the alloying elements, impurities and inclusions in steel making, slab reheating furnace, finishing and coiling temperature in hot strip mill, total reduction, lubrication, heating rate, temperature and time annealing in cold rolling mill. There are five major steps in the manufacturing of cold rolled and annealed sheet steels for extra deep drawing [1,2,3,4,5,6,7,8,9], as measured by the normal anisotropy (r-value). The most important step for aluminium killed steels is coiling practice, a lower coiling temperature leads to higher r-value because ALN is retained in solution in ferrite and can precipitate out on annealing after cold working with an increase in {111} grain orientation at the expense of {100} grain [10,11,12,13,14,15]. The final step is an extension in the skin pass mill, which is important for stretching in the forming of cold rolled steel [16,17,18,19,20]
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