Effect of Ti Additions on Micro‐Alloyed Nb TRIP Steel

Abstract

In the present study, the influence of Ti-additions on the mechanical properties of Nb-microalloyed TRIP steel is investigated. Ti micro-alloying additions to multi-phase Nb TRIP steel result in a substantial increase of the yield strength and a reduction of strain hardening. The increase of the yield strength can be attributed mainly to grain refinement with a relatively small contribution of precipitation hardening. Based on general principles and well-known models of alloying strengthening, metallurgical reasons for the observed mechanical behavior of the steel can be formulated. The contribution of precipitation hardening is relatively small as Ti-addition result in the formation of coarse (Nb,Ti)(C,N) particles. The addition of Ti to a Nb-microalloyed TRIP steel leads to a pronounced enhancement of precipitation kinetics of (Nb,Ti)(C,N), thereby increasing their phase fraction. The precipitates coarsen and tend to form groups of aggregates of particles rather than single isolated particles with increasing intercritical annealing time. In addition, Ti-addition to Nb-microalloyed TRIP steel has a direct influence on the chemical composition of the precipitates, which become Ti-rich.

Effect of Finishing Hot‐Working Temperature on Microstructure of Thermomechanically Processed Mn–Al Multiphase Steels

“…The microstructures consist of a mixture of degenerate upper bainite and degenerate lower bainite. These bainite morphologies are beyond classification based on conventional concepts due to the incomplete austenite transformation phenomenon, often observed for steels alloyed with Si and Al . The degenerate lower bainite consists of very fine, fragmented martensite–austenite (M–A) particles instead of cementite located within bainitic ferrite laths.…”

“…The difference between Mn and Nb content is the basis for steel coding (3Mn, 3MnNb, 5Mn, and 5MnNb). Manganese was used to stabilize austenite whereas low‐Si high‐Al concept was chosen to obtain carbide‐free bainite . Mo and Nb were used to increase strength due to solid solution strengthening, grain refinement, and precipitation strengthening.…”

“…Advanced high‐strength steels (AHSS) dedicated to the automotive industry have received much interest in recent years due to their outstanding combination of strength, ductility, technological formability, and superior capability of energy absorption under dynamic deformation conditions. They include dual phase (DP), transformation induced plasticity (TRIP), and Complex Phase (CP) steels, in which final mechanical and technological properties are formed by a precise control of chemical composition of steel and processing route . Presently, there is a need to further improve a strength‐ductility relation of steel sheets to satisfy the safety requirements of the automotive industry.…”

Dilatometric study of phase transformations in advanced high-strength bainitic steel

“…This behaviour is due to a high Al content in the steel. The martensite start temperature (M s ) is relatively low, what favours austenite stabilization in TRIP steels [1][2][3]. Pearlite is delayed to long times and the bainitic field is left-shifted.…”

“…They consist of different soft and hard structural constituents in various proportions, which enable to obtain a very wide range of mechanical and technological properties. The microstructure of dual phase (DP) steel contains soft ferrite and hard martensite whereas the multiphase microstructure of TRIP (Transformation Induced Plasticity) steel consists of ferrite, bainite and retained austenite [1][2][3][4][5][6]. New demands of the automotive industry for relatively low-cost steel sheets characterized by tensile strength above 1,000 MPa require further searching of new chemical composition strategies.…”