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

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

Recent advances in the development of high performance steels presenting improved properties of strength and ductility rely on the TRIP effect, i.e. on the mechanically-induced martensitic transformation of the retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilisation and retention of austenite at room temperature have become of primary importance, leading to specifically designed steel grades and thermal or thermomechanical treatments. Particularly, carbon enrichment of the austenite during intercritical annealing and bainite transformation was found to be very effective in retaining austenite. This metastable austenite then progressively transforms during straining, bringing about a large increase of the work hardening rate. This increase results from the stress and strain partitioning continuously evolving with the appearance of the hard martensite. (c) 2004 Elsevier Ltd. All rights reserved.

Transformation-induced plasticity for high strength formable steels

The influence of forming temperature and strain rate on the ductility and strain-induced transformation behavior of retained austenite in a ferritic 0.4C-1.5Si-1.5Mn (wt pct) dual-phase steel containing fine retained austenite islands of about 15 vol pct has been investigated. Ex- cellent combinations of total elongations (TELs), about 48 pct, and tensile strength (TS), about 1000 MPa, were obtained at temperatures between 100 °C and 200 °C and at a strain rate of 2.8 X 10-4/s. Under these optimum forming conditions, the flow curves were characterized by intensive serrations and increased strain-hardening rate over a large strain range. The retained austenite islands were mechanically the most stable at temperatures between 100 °C and 200 °C, and the retained austenite stability appeared to be mainly controlled by strain-induced martensite and bainite transformations (SIMT and SIBT, respectively), with deformation twinning occur- ring in the retained austenite. The enhanced TEL and forming temperature dependence of TEL were primarily connected with both the strain-induced transformation behavior and retained aus- tenite stability.

Ductility and strain-induced transformation in a high-strength transformation-induced plasticity-aided dual-phase steel

The effect of phosphorus and silicon on the formation of retained austenite has been investigated in a low-carbon steel cold rolled, intercritically annealed, and isothermally held in a temperature range of bainitic transformation followed by air cooling. The steel sheet containing phosphorus after final heat-treatment consisted of ferrite, retained austenite, and bainite or martensite. Phosphorus, especially in the presence of silicon, in steel was useful to assist the formation of retained austenite. Mechanical properties, such as tensile strength, uniform elongation, and the combination of tensile strength/ductility, were improved when phosphorus was increased up to 0.07 pct in 0.5 pct Si steel. This could be attributed to the strain-induced transformation of retained austenite during tensile deformation. Furthermore, two types of retained austenite were observed in P-containing steel. One is larger than about 1 μm in size and usually exists adjacent to bainite; the other one is of submicron size and usually exists in a ferrite matrix. High phosphorus content promotes the formation of stable (small size) austenites which are considered to be stabilized mainly by their small size effect and have a different formation mechanism from the coarser retained austenite in the lower P steels. The retained austenites of submicron size showed mechanical stability even after 10 pct deformation, suggesting that these small austenites have little effect on ductility. The 0.07 pct P-0.5 pct Si-1.5 pct Mn-0.12 pct C steel showed a high strength of 730 MPa and a total elongation of 36 pct.

Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet

The effect of manganese on the development of 111 recrystallization textures of P-containing low-carbon Al-killed steel sheet has been investigated. A proper combination of phosphorus and manganese content in steel gave a strong 111 texture through a simulated box annealing cycle. The experimental data suggest that 111 recrystallization textures could be developed providing that the steels contained both high manganese and phosphorus. Phosphorus in steel was useful in enhancing the uniform dispersion of precipitates such as (FeMn)3C and A1N. This might be attributed to softening the possible interaction between manganese and carbon or nitrogen atoms by the addition of phosphorus which is regarded as a repulsive element for carbon or nitrogen. The 0.2 pct P-1.2 pct Mn Al-killed steel sheet tested had a highr- value of 2.0 through the precipitates effect and a high strength of 430 MPa through the effect of a solid solution hardening due to phosphorus.

Effect of manganese on the recrystallization texture of P-containing low-carbon steel sheet

The development of {111} recrystallization textures in 0.5 pct and 1 pct Cu-containing steels through a simulated continuous annealing cycle has been investigated. A low-carbon content of 0.05 pct compared with extra low-carbon content of 0.005 pct accelerated the precipitation of e-Cu by preaging prior to cold rolling, and a nickel content up to 0.95 pct gave the adverse effect. Pole intensity ratio, I222/I200, of the cold rolled and continuous-annealed sheet depended more strongly upon the distribution density of e-Cu particles than upon the amount of carbon in solid solution. The optimum dispersion of e-Cu precipitates for producing a strong {111} texture was about 300 particles per μm3 in distribution density, and 20 to 30 nm in particle size. Also, it was suggested that through continuous annealing by utilizing the effect of prior precipitates such as e-Cu, a high strength, dual-phase steel with a highr-value could be produced.

M. Shimizu

Papers

Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet

Effect of manganese on the recrystallization texture of P-containing low-carbon steel sheet

Effect of copper precipitation on the development of recrystallization textures through continuous annealing of low-carbon sheet steel