Hot stamping of ultra-high strength steel parts

The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 µm, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needle-shaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650°C to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.

Precipitation behavior and its effect on strengthening of an HSLA-Nb/ti steel

Microstructure based prediction of strain hardening behavior of dual phase steels

Microstructure-based model for the static mechanical behaviour of multiphase steels

Kinetics of the simultaneous decomposition of austenite into several transformation products

To make quantitative prediction of microstructure in HSLA steels produced by isothermal or continuous cooling transformation, transformation kinetics of various phases were modeled thermodynamically. Five kinds of phases, i.e. polygonal ferrite, Widmanstatten ferrite, pearlite, bainite and martensite, and 10 alloying elements were taken into account. A program was developed on the basis of this model. Out line of the involved transformation kinetics are presented in this paper. The effects of work-hardening of austenite on the nucleation and growth rates of various phases are discussed and a mathematical model of transformation kinetics from work-hardened austenite is also presented.

https://www.jstage.jst.go.jp/article/isijinternational1989/32/3/32_3_306/_pdf

Computer Modelling of Phase Transformation from Work-hardened Austenite

An approach to predict mechanical properties of hot-rolled multi-phase steels referring to the stress-strain curves is proposed. Different from a conventional approach of regression analysis about the relationships between properties and chemical compositions and processing factors, a proposed one is based on the analysis and application of stress-strain curve: several commonly used mechanical properties such as yield strength, tensile strength, uniform elongation, total elongation, work-hardening exponent (n) and Vickers hardness, are derived systematically from the stress-strain curve of a multi-phase steel, which is calculated by using concentration factor, i.e., strain partition ratio and stress-strain curves of constituent phases. Stress-strain curves of individual component structures such as ferrite, pearlite, bainite, and martensite are expressed by Swift's equation. Physical background of the concentration factor is discussed by examining theoretical models of deformation for two-phase materials. Evaluation of plastic relaxation related to microstructural topology might be the most difficult point of this approach and some trials are presented.

Yuichi Higo

Papers

Computer Modelling of Phase Transformation from Work-hardened Austenite

Prediction of mechanical properties of multi-phase steels based on stress-strain curves