Metallurgical Effects of Niobium in Dual Phase Steel

Abstract

Dual phase (DP) steels are widely applied in today’s automotive body design. The favorable combination of strength and ductility in such steels is in first place related to the share of ferrite and martensite. The pronounced work hardening behavior prevents localized thinning and allows excellent stretch forming. Niobium microalloying was originally introduced to dual phase steel for improving bendability by refining the microstructure. More recently developed “high ductility” (HD) DP steel variants provide increased drawability aided by a small share of austenite retained in the microstructure. In this variant niobium microalloying produces grain refinement and produces a dispersion of nanometer-sized carbide precipitates in the steel matrix which additionally contributes to strength. This study investigates the microstructural evolution and progress of niobium precipitation during industrial processing of high-ductility DP 980. The observations are interpreted considering the solubility and precipitation kinetics of niobium. The influences of niobium on microstructural characteristics and its contributions to strength and formability are discussed.

Advanced Multiphase Steels

“…The manuscripts gathered in this Special Issue are committed to a variety of steel grades, embracing DP (Dual Phase) steels [1], medium-Mn steels [2,3], CFB (Carbide Free Bainitic steels) [4], nano-bainitic cast steel [5], δ-TRIP (TRansformation Induced Plasticity) [6] and precipitation hardened ferritic stainless steel [7].…”

“…A wide range of topics are researched that comprise the alloy design [2,6,7], optimization of both the hot working process [2] and the final heat treatment [5], and the final mechanical properties [1,3,4,6].…”

Molybdenum alloying in cast iron and steel

“…Systematic alloy variations in laboratory trials have indicated that the tensile strength of grey cast iron can be most efficiently increased by simultaneously refining the eutectic cell size and the pearlite interlamellar spacing (see Fig. 4) [21]. In that respect, the combined alloying of molybdenum and niobium has a particularly high strengthening potential.…”

“…Hypo-and hyper-eutectic alloy concepts of grey cast iron grades and specified mechanical properties according to European automotive standards Effect of niobium addition on microstructural refinement in grey cast iron[18] Effect of microstructural refinement on tensile strength in grey cast iron based on laboratory results provided by Shanghai University[21] …”

Effect of Quenching Temperature on the Strengthening Mechanism of Low‐Carbon Microalloyed Quenching and Partitioning Steel

“…More and more researchers add microalloying elements such as Nb, Ti, and V to develop new materials. [ 15–17 ] Cai et al obtained ultra‐low carbon Ti–Mo–Nb steel with high yield strength (YS) by introducing nano‐precipitates, with a precipitation strengthening of 210 MPa and a YS of 634 MPa. [ 18 ] Chen et al improved the YS of hot‐formed steel by 145.6–263.4 MPa by introducing the precipitates (Nb, V, Ti)C. [ 19 ] However, the strengthening mechanism of multiphase steels treated by the Q–P process for low‐carbon microalloyed high‐strength steels is rarely studied.…”