The purpose of the present contribution is to review the current knowledge about the relationship between the micro-structure of cold rolled intercritically annealed low alloy TRIP-aided sheet steels and their mechanical properties from a materials engineering point of view. The focus is on their production in existing industrial lines and on their application in the manufacture of passenger cars with a body-in-white which offers an improved passive safety. The review aims to make clear that although low alloy TRIP-aided sheet steel is by now starting to be an established structural material in BIW manufacturing, there is still room for the further optimization of the composition and the processing. In addition, there are still a number of problems related to their physical metallurgy that require a better fundamental understanding.

Structure–properties relationship in TRIP steels containing carbide-free bainite

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Advanced high strength steels for automotive industry

The cold rolling texture of low and extra low carbon steels is primarily made up of a nearly perfect normal direction (ND) fibre and of two other components, centred at 001 and 112 , which...

Cold rolling and annealing textures in low carbon and extra low carbon steels

A study of microstructure, transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

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Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

An excellent combination of elongation over 30% and high strength about 100kgf/mm2 is achieved in processing of a 0.4C-1.5Si-0.8Mn steel by intercritical annealing, rapid cooling into bainite transformation temperature to soak several minutes. This combination is caused by transformation induced plasticity of retained austenite. Sufficient amount of stable austenite is a requisite for the good ductility. For the rapid cooling after annealing, the soaking temperature for the best combination of strength and ductility is immediately above Ac1. On the other hand, a delay before rapid cooling provides good properties if the soaking temperature is near Ac3 and the subsequent cooling is performed at a lower rate before pearlite transformation; in this case the critical cooling rate is reduced. These phenomena are discussed in terms of the growth of ferrite and the diffusion of alloying elements inclusive of Mn during slow cooling.

Enhancement of Elongation by Retained Austenite in Intercritical Annealed 0.4C-1.5Si-O.8Mn Steel

Processing peculiarities and functions of alloying elements, such as Si and Mn, were studied for improving formability of steel sheets with mixed microstructures. Annealing a sheet steel with 0.2 pct C in the intercritical range produced very fine particles of retained austenite which were moderately stabilized due to C enrichment by subsequent holding in the bainite transformation range. Its strength-ductility balance is greatly superior to that of other dual-phase steels due to transformation-induced plasticity (TRIP). The holding time in the bainite transformation range varies with temperature, depending on the activation energy of C diffusion in austenite, and shifts to longer times with an increase of Si or Mn additions. The optimum cooling rate from the intercritical region is reduced with an increase of Mn content but is not influenced by Si content. Additional Mn makes the retained austenite content larger, although uniform elongation remains the same. In this case, the product of tensile strength and total elongation is increased due to an increase in the tensile strength. Contrary to Mn, Si does not affect retained austenite content but improves the uniform elongation by increasing its stability.

Mechanical Properties and Retained Austenite in Intercritically Heat-Treated Bainite-Transformed Steel and Their Variation with Si and Mn Additions

Trip and its kinetic aspects in austempered 0.4C-1.5Si-0.8Mn steel

The formation process of the retained austenite (γR) in cold-rolled 0.4C-1.2 to 2.0Si-1.2Mn steel sheets intercritically heated and subsequently austempered is investigated.It was concluded that granular γR grains with less than 2-3 μm dia. in the above steels and the interlath γR films in the higher C(≥0.6C)-Si-Mn steel heated at single austenite range and austempered, which have been reported by others, were the same in chemical stability of austenite by C enrichment, in spite of differences in formation site and morphology. Formation of stabilized granular γR may be attributed to the smaller granular austenite grain formed during intercritical heating. Addition of Si may cause the suppression of carbide precipitation and the increase in γR fraction in steels held isothermally to give the maximum C content in γR (Cγmax), whereas the Si addition does not change Cγmax.

Retained Austenite in 0.4C-Si-1.2Mn Steel Sheet Intercritically Heated and Austempered

An impeder for use in an apparatus for resistance tube welding by which a strip-shaped skelp continuously formed into a tubular or spirally tubular shape having spaced opposed edge portions defining a gap therebetween has the edge portions heated and pressed together for being welded. The impeder has a generally tubular casing and a core within the casing consisting of a plurality of laminated bodies each consisting of a plurality of substantially flat planar foils of metallic magnetic material, the core, when viewed in cross-section traverse to the central longitudinal axis of the casing, having the transverse cross-sections of the laminated bodies on parts of at least one circle concentric with the longitudinal axis of the casing and having the planes of the foils extending substantially radially outwardly from the longitudinal axis.

Hiroshi Takechi

Papers

Enhancement of Elongation by Retained Austenite in Intercritical Annealed 0.4C-1.5Si-O.8Mn Steel

Mechanical Properties and Retained Austenite in Intercritically Heat-Treated Bainite-Transformed Steel and Their Variation with Si and Mn Additions

Trip and its kinetic aspects in austempered 0.4C-1.5Si-0.8Mn steel

Retained Austenite in 0.4C-Si-1.2Mn Steel Sheet Intercritically Heated and Austempered

Impeder for electric resistance tube welding