Development And Microstructure Research Of A600MPa Grade High Al Cold Rolling Dual Phase Steels

A600MPa grade high Al cold rolling dual phase steel was developed successfully, this steel canproduced not only by continuous annealing line, but also by continuous hot dip galvanized line. With both ofthe two processes, the steels was got well mechanical properties with yield strength from360to390MPa,tensile strength from600to640MPa, elongation rate is above24%, and the BH value is above65MPa, all theproperties agreement the standard GB/T20564.2-2006and the standard GMW3399.The laboratory and pilottest steels were systematically invested, including continuous annealing test, mechanical properties test,microstructures analysis by optic microscopy(OM), transmission electron microscopy(TEM) and scanningelectron microscopy(SEM), respectively. The morphology of the surface and inhibition layer of thegalvanized steel was studied, and the different phases of ferrite and island martensite were analyzed by bothmicroscopy and EBSD, the mechanics of the microstructure transformation of the new steel was revealed.The inhibition layer was studied by both SEM and TEM, influences of the quality and the adhesion of thesurface were figured out.The composition of the new steel was mainly composed of C, Mn, Al, Mo and Si, which was based onthe600MPa grade dual phase steel’s composition of ThyssenKrupp Steel Company, and some elementscontent were optimized, Si was decreased from0.50%to0.10%, and Al was increased from0.5%to0.9%,meanwhile, the contents of Mn and Mo were fine adjusted. It is well known that Si and Al were used for solidsolution strengthening and suppress the carbide formation during cooling from annealing temperature,however, the segregation and selective oxidation of the alloy element Si, Mn, and some other elements duringannealing process deteriorate the surface quality of the hot dip galvanized steels, Al have the same effect withSi to improve the steel’s strength, and the selected oxidation effect is not so bad as Si, the new compositiondual phase steel was employed as continuous annealing sheet, what’s more, it also fit for produce hot dipgalvanized steel.Serials annealing tests were carried out on HDPS hot dip galvanizing simulator, the most suitableproduction technology was got for continuous annealing sheet and continuous hot dip galvanized steel aftermechanical test, and the influence of annealing process parameters on the mechanical properties andmicrostructure of the steel was found out.For the continuous annealing steel, with the increase of the annealing temperature, the size of islandmartensite grown, meanwhile, the tensile strength decreased; the best annealing temperature of this steel was820℃. Along with the decrease of the slow cooling temperature, the yield strength of the steel decreased, andthe best slow cooling temperature was630℃. With the aging temperature increased, the amount of temperingcarbide in the island martensite grown, the size of which increased, and the tensile strength of the steeldecreased accordingly, the best aging temperature is300℃. When aging treatment at300℃, with the agingtime expand, the change of yield and tensile strength was not obviously, however, the elongation increasedgradually. The aging time was about350s to460s when pilot produced, following this production process theelongation of the test dual phase steel can reach to above24%.There are differences between hot dip galvanizing process and continuous annealing produce process,one is the structure length of the annealing furnace, and the other is the processing parameter. Annealingtemperature made a little influence on microstructure and mechanical properties of ho dip galvanized dualphase steel, rapid cooling temperature was the critical factor which influences the microstructure andmechanical properties of the steel. With the rapid cooling temperature increased from470℃to520℃, thetensile strength increased gradually, when this temperature was below500℃, the tensile strength of the steelhardly reach600MPa, rapid cooling temperature must be controlled in the range from495℃to505℃, whichwas the main factor to get a good mechanical properties of the galvanized dual phase steel. The inhibition layer of hot dip galvanized steel was analyzed by both SEM and TEM, the result showedthat the density of the particles in the inhibition layer is direct influence the adhesion of Zn coating layer.Manganese oxides on the surface of the sheet were reduced by Al in the Zn bath according to thealuminothermic reduction reaction, which formed Al2O3particles, and then were trapped in the inhibitionlayer during the formation and growth of Fe2Al5grains. When the manganese oxides were thin film shape, itis hardly to react with Al in the Zn bath, an this film inhibit the nucleation of Fe2Al5phase at the interfaceof the iron in contact and the liquid zinc, so the adhesion of Zn coating become worth.. Several factorsinfluenced the adhesion of hot dip galvanized steel such as the selected oxidation of the steel surface duringannealing process, the atmosphere state of the annealing furnace, the content of Al and Fe in the Zn bath, andetc. For the production of dual phase steel, it is necessary that the surface of the sheet should be cleaned tothe greatest extent, the content of H2in the annealing furnace kept above5%, and the content of Al in the Znbath was controlled in0.19～0.22%.Both of the continuous annealing and the continuous hot dip galvanized sheet were tested by automobilefactories, there are made into automobile parts such as the B-column and internal plates of the doorsill, theperformance test and user’s feedback show that the products meet the requirement of600MPa grade dualphase steel.