Application of 980 MPa Grade Advanced High Strength Steel Sheet with High Formability

Paper #:
  • 2018-01-0625

Published:
  • 2018-04-03
Abstract:
For curbing global warming, vehicles are strongly required to achieve weight reduction and crash safety of body-in-white (BIW) simultaneously to improve fuel economy to reduce CO2 emissions. One of effective measures for the issue is to expand the application of advanced high strength steel (AHSS) over 780 MPa to the vehicle body. In order to replace the conventional 590 MPa grade steel, Nissan has newly developed a 980 MPa grade AHSS with high formability in collaboration with steel manufacturers. The newly developed steel has both 980 MPa tensile strength and high formability which is comparable to that of the conventional 590 MPa grade steel. To accomplish that, a complex metallographic structure including the residual austenite (γ) phase possessing transformation-induced plasticity (TRIP) effect is utilized in this new steel as same as 1.2 GPa grade AHSS which Nissan developed. However the content of its residual γ phase in the new steel exceeds that of 1.2 GPa grade AHSS. As a result, it displays high elongation over 20%. That made it possible to form such parts as the front pillar hinge brace and the rear side member, which are extremely difficult to form. There was also concern about resistance spot weldability, specifically, toughness decline of weld and the occurrence of weld cracking due to the high carbon content. To improve weld strength, the crystal size of the weld and the hardness of the heat-affected zone were optimized through temperature control by improving the welding current pattern. With regard to the impact properties of automotive parts, energy absorption is markedly superior to that of the conventional steels. As a result of developing various application technologies, it has become possible to apply 980 MPa grade AHSS extensively to the complex shape structural parts of the vehicle frame to which such application was difficult previously.
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