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

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

Relation between microstructure and mechanical properties of a low-alloyed TRIP steel

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M
 s temperature induced by carbon enrichment.

On the Influence of Interactions between Phases on the Mechanical Stability of Retained Austenite in Transformation-Induced Plasticity Multiphase Steels

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

Steels containing 0.15 pct C and 1.2 pct Si-1.5 pct Mn or 4 pct Ni were intercritically annealed and isothermally transformed between 300 °C and 500 °C for 1 to 60 minutes. The specimens were subjected to tensile testing at room temperature, and the microstructures were evaluated by light microscopy, scanning and transmission electron microscopy (SEM and TEM, respectively), and X-ray diffraction (XRD). The microstructures consist of dispersed regions of bainite, martensite, and austenite in a matrix of ferrite, and a maximum of 11.6 pct austenite is retained after isothermal holding at 450 °C in the Si-Mn steel. In specimens where austenite transforms to martensite during quenching after isothermal holding, the stress-strain curves show continuous yielding, high ultimate tensile strength (UTS), and relatively low ductility. In specimens where higher volume fractions of austenite transform to bainite during isothermal holding, the stress-strain curves show discontinuous yielding, low UTS, and high ductility.

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation, microstructure, and room-temperature mechanical properties

Stress-strain behavior and deformation-induced transformation of retained austenite were studied for intercritically annealed and isothermally transformed Si-Mn and Ni steels as a function of testing temperature between −80 °C and 120 °C. Rapid decrease of retained austenite at small strains dominates at low-temperature testing and in microstructures containing martensite. The austenite transformation in microstructures without martensite shifts to larger strains with increasing testing temperature. The accompanying increase of strain-hardening rates at larger strains deters the onset of necking and improves ductility. The benefits of the austenite transformation lead to a peak in ductility between 20 °C and 70 °C in the Si-Mn steel and at 70 °C in the Ni steel. The peaks are dependent on the nature of the dispersed microconstituents produced in the ferrite during isothermal transformation. Higher testing temperatures enhance the mechanical stability of the austenite and result in lower ductility.

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part II. effect of testing temperature on stress-strain behavior and deformation-induced austenite transformation

A low C steel containing 1.5% Mn, 1.2% Si, and 0.1% Mo was inter-critically annealed at 760 and 820° C, and either quenched in oil to room temperature, or isothermally held at 400° C and then oil quenched to room temperature. The specimens were subjected to tensile testing at temperatures between − 80° C and 120° C, and the microstructures were evaluated by light and scanning electron microscopy and x-ray diffraction. The austenite formed during intercritical annealing transformed to martensite with negligible retained austenite as a result of the direct oil quenching, while it transformed to bainite with significant amounts of retained austenite, up to 7.5%, during isothermal transformation. The retained austenite improved ductility at strength levels between 700 and 800 MPa by strain induced transformation of austenite to martensite. The benefit of the retained austenite peaked at testing temperatures between −30 and 70° C.

Yasuharu Sakuma

Papers

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

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part I. transformation, microstructure, and room-temperature mechanical properties

Intercritically annealed and isothermally transformed 0.15 Pct C steels containing 1.2 Pct Si-1.5 Pct Mn and 4 Pct Ni: Part II. effect of testing temperature on stress-strain behavior and deformation-induced austenite transformation

Mechanical Behavior of an Intercritically Annealed and Isothermally Transformed Low C Alloy Steel with Ferrite-Bainite- Austenite Microstructures