Production and Quality in Hot Rolled 60kgf/mm² Class Dual Phase Steel

Abstract

Manufacturing factors and properties of hot rolled 60 kgf/mm² class dual phase (DP) steels by two producing methods, which were suitable for mass production, were discussed.
(1) As rolled extra low temperature coiling method developed using three step cooling pattern, called as "Inline controlled quenching (ICQ) method" was suitable for massproduction of DP steel with low alloying component (1.5%Mn and low Si content less than 0.15%). It was important for stabilization of mechanical properties to control the temperature and period of the second air cooling stage between water cooling stages according to variation of chemical composition.
(2) The continuous annealing (CA) method as the heat-treated type process was fit for the case that low yield ratio was especially required.
(3) As the result of investigation about formability, bake hardenability and weldability, it was proved that the hole expansion ratio of DP steel was remarkably improved with inclusion shape control by calcium-addition, bake hardenability of ICQ-DP steel was larger than the CA steel due to higher solute nitrogen content, and that weldability was excellent except flash-butt welding for auto wheel rim.
(4) DP steels were successfully appliable to auto wheel disc and most parts made of hot rolled mild steels and their applications have been useful for weight reduction.