Ambient Aging Behavior of Hot Dip Coated Bake Hardenable Steel Sheets Relevant to Automotive Industry

The ambient temperature aging behavior of hot dip coated cold-rolled bake hardenable steel sheets manufactured by an in-line continuous annealing technique has been investigated. These materials exhibit good and uniform mechanical and bake hardening properties as-produced and after aging. For the same chemistry, carbon and nitrogen solute levels can be higher in galvanized (GI) products than in galvannealed (GA) products, with the result, of course, of higher bake hardening and aging indexes. Based on the classical static strain aging theory, the interactions between interstitial solutes and dislocations during the ambient aging of these steel sheets can be classified into four stages. Optimal press forming or stamping operations are best performed in the first or second stages of aging but no significant forming difficulties should be expected even if these operations are conducted during the later aging stages.