One of the most important objectives in the development of automotive steel sheet is the combination of strength and formability. Formability is required when the sheet is shaped into an automobile body panel, and high strength is required after assembly. Bake-hardenable steel sheet was developed by exploiting the fact that these two properties are not needed simultaneously. The lowest possible strength in an automotive steel sheet is desirable for good formability when the sheet is subjected to press forming; therefore, the carbon atoms in the steel sheet are kept in the solid-solution condition. This can be accomplished by rapid cooling of the steel sheet from high temperature during the sheet production process, and this rapid cooling has become industrially possible by the development of the continuous annealing technique for strips. Before the development of the continuous annealing technique, rapid cooling was impossible because annealing of heavy coil with big heat capacity was conducted as a batch process. Dislocations are introduced by press forming a steel sheet, and strength is increased by the action of work hardening in which accumulated dislocations prevent the movement of other dislocations. When an automobile body is being manufactured, painting and baking are carried out after assembly. These processes involve heating the steel body panels to about 443K (170). At this temperature, the carbon atoms dissolved in the steel diffuse by jumping between lattice points, which occurs 103 to 105 times a second, segregating in the regions around dislocations where the stresses are compressive. This results in locking of the dislocations which is called strain aging. This mechanism makes the steel panels harder after baking than after press forming, and is referred to as bake hardening. The utilization of this bake hardening phenomenon has made it possible to utilize steel sheet that has good formability during press forming and that can withstand severe working, while being hard and less prone to denting when assembled in the automobile body.