Research And Development Of Hot Rolled Dual Phase Steels Sheets For Automobiles

Dual phase steels are a kinds of high strength steels with main ferrite and a little martensite, which are low carbon steels or low carbon alloy steels were made in critical heat treatment region or by controlling rolling and controlling cooling processes. Dual phase steels have the advantage that they have continuous yielding and high initial work-hardening rates, high strength, good ductility, are a new high strength and good formability stamping steels having been widely being considered for use in structural parts by the automotive industry. It is very important to develop the lower cost hot rolled dual phase steels for saving energy source.This paper's purpose is to investigate the hot rolled DP600 dual phase steels by laboratory simulating, pilot trials, and full scale testing, using Thermal/Mechanical simulator, metallographic structures examination, TEM analyzing, texture analyzing, mechanical properties testing. The research results show as following:1) Having determined the CCT curve of dual phase steels and optimized the process parameters of pilot trial by hot rolling test in laboratory.2) By pilot trial, the conclusion is that the microstructures with DP600 steels properties have 88% ferrite, the others is the second phase martensite island mainly, and the hot rolled dual phase DP600 steels have formed aγ-fiber texture, it issuitable to deep drawing ability, mainly {111}<112> and {111}<110>.3) Having given the basic parameters of DP600 dual phase steels in full scale testing:Soaking Temperature: 1200±10℃;Initial Temperature: 1100±10℃;Finishing Temperature: 860±10℃;Water cooling processes were divided by 3 stages,ⅠStage water cooling : (860—760)±10℃(30℃/s) ;ⅡStage air cooling: (760-700)±10℃;ⅢStage water cooling: (700-640)±10℃(40℃/s) ;Coiling Temperature: 600-650℃.4) Having finished the full scale testing of the hot rolled dual phase DP600 steels, its sample yield strength are greater than 340MPa, the YS/TS is less than 0.6, and tensile strength are all greater than 600MPa, the minimum elongation is 26%, generally are greater than 30%, cooled bend properties are all qualified, the tensile curve of trial steels is continuous with the typical dual phase steels characteristic.