Recently, there is a trend of using stainless steel for automobile structural components as a substitute of carbon steel for many advantages of stainless steel. Stainless steel has higher strength, toughness which makes automobile lighter, and corrosion resistance than carbon steel, as well as good formability and fancy aspect. Moreover, it is environmentally-friendly material that is easy to recycle. But, stainless steel is expensive because it has high Ni content(8~12 wt. %). So, there is a need to develop a new stainless steel with low Ni content.
For such a reason, high Mn-N austenitic stainless steels of Fe-(16~19)Cr-(1~4.5)Ni-(8~11)Mn-(0.2~0.5)N, replaced expensive Ni(austenite stabilizer) with low cost Mn(austenite stabilizer), are actively being developed by many steel companies such as AK steel, Allegheny, Carpenter, 新日鐵. Mn is 7 to 8 times cheaper per ton than Ni, and it also increases N(austenite stabilizer) solubility, which improves mechanical strength and localized corrosion resistance.
The high Mn-N austenite stainless steel is still expensive due to high Ni content (miximum 4.5 wt%). Further, high Mn content (maximum 11 wt. %) of the high Mn-N austenitic stainless steel decreases corrosion resistance. These problems can be solved by reducing the Mn and Ni content by changing structure of stainless steel. The alternative is duplex stainless steel which has low Ni, Mn content. Generally, duplex stainless steel composed of austenite and ferrite has 2~3 times higher strength and greater resistance to pitting and stress corrosion cracking than 300 austenitic stainless steel.
In this study, duplex stainless steels with chemical composition of Fe-18Cr-(4~6)Mn-(0~1)Ni-1Mo-0.2N is designed to have a structured 55%(γ)+45%(α) by using Schaeffler diagram and Thermo-Calc. The alloys were melted by VIM, and followed by hot rolling. After that, solution annealing, 35 % cold rolling and solution annealing. Finally, the mechanical and corrosion properties of the designed alloys were evaluated by tensile test, several corrosion tests and analysis of secondary phases such as sigma (σ) phase and nitride.
The optimum composition for newly developed duplex stainless steel for automobile structural components is Fe-18Cr-6Mn-1Mo-0.2N. It has about 427 MPa of Y.S., 947 MPa of UTS and 35 % of elongation. The new alloy has 2.5 times higher UTS than carbon steel and enough elongation for automobile sub frames due to its fine microstructure combined with 55%(γ)+45%(α) and solid solution hardening of nitrogen. Therefore, it is expected that it will bring 40~50 % weight reduction on comparing with carbon steel. Moreover, the new alloy exhibited high corrosion resistance in natural air environment as well as in chloride environment comparable to 304 stainless steel. Therefore, the new duplex stainless steel will be sufficiently endurable on the $CaCl_2$scattered road in winter season and under coating of automobile sub frame will not be required. Thus, durability of automobile will be improved. Finally, new alloy retard to precipitate sigma phase which causes brittle fracture and reduction of localized corrosion resistance at elevated temperature because of low Cr and Mo content in spite of duplex stainless steel. Also, nitride which causes brittle fracture and reduction of localized corrosion resistance will not be produced because of proper N content.
Therefore, new alloy is expected a optimum material for front sub frame, low arm, torsion bar, fuel tank, and members in automobiles. It can be also widely used for material of trucks, bus, large vehicles, subways, express trains because of its excellent mechanical property, corrosion resistance and low cost.
