TY - BOOK

T1 - Stability of Retained Austenite in a Bainitic Low Alloyed Steel and its Response to Mechanical and Thermal Loading

AU - Pfleger-Schopf, Bernd

N1 - embargoed until 01-07-2027

PY - 2022

Y1 - 2022

N2 - The manufacture of new products on an industrial scale is associated with constantly increasing demands on material properties such as strength and ductility of steels. In order to achieve good strength with high ductility, bainitic steels, which stabilize high fractions of retained austenite, can be used. In this case, stabilization of the retained austenite is achieved by specific selection of its chemistry, mainly carbon, and suitable heat treatment. Since the heat treatment for bainitic steels is often complex, which can lead to a variety of different bainite forms, they are usually not feasible on an industrial scale. Thus, the challenge arises of selecting a suitable bainitic appearance whose heat treatment is feasible on a large scale. The main focus is on the question of how much carbon is required as a minimum in the austenite so that, once bainite formation is complete, no further transformation of the austenite takes place during cooling to room temperature. In addition, industrial scale product manufacture requires consideration of the stability of the retained austenite against mechanical and renewed thermal stress, since forming steps and tempering processes are often part of the production process. Here, this dissertation excels in that the stability of austenite was investigated both through bainite formation and during tempering using in-situ synchrotron experiments. For the stability of austenite against mechanical loading, tensile were compared with compression tests, which were accompanied by XRD measurements. In order to show the effects of the individual process steps on the austenite, comprehensive analyses of the microstructure were carried out using high-resolution methods such as APT measurements. After comparing isothermally and continuously produced bainite, a granular bainite with <6 vol.% of equilibrium ferrite, which should be feasible on an industrial scale, with a tensile strength of 1200 MPa and an elongation at fracture of up to 30 % at a retained austenite content of 28 vol.%, could be produced in the course of this work. It has been shown that an average carbon content >1 wt.% in the austenite should be necessary to avoid transformation during cooling to room temperature, resulting in film-like retained austenite with 1.5 wt.% carbon, which is significantly more stable against mechanical loading than the blocky retained austenite with 0.8 wt.% carbon and shows high tempering resistance up to 450°C for 250 s or 400°C for 1800 s for the chosen alloy system.

AB - The manufacture of new products on an industrial scale is associated with constantly increasing demands on material properties such as strength and ductility of steels. In order to achieve good strength with high ductility, bainitic steels, which stabilize high fractions of retained austenite, can be used. In this case, stabilization of the retained austenite is achieved by specific selection of its chemistry, mainly carbon, and suitable heat treatment. Since the heat treatment for bainitic steels is often complex, which can lead to a variety of different bainite forms, they are usually not feasible on an industrial scale. Thus, the challenge arises of selecting a suitable bainitic appearance whose heat treatment is feasible on a large scale. The main focus is on the question of how much carbon is required as a minimum in the austenite so that, once bainite formation is complete, no further transformation of the austenite takes place during cooling to room temperature. In addition, industrial scale product manufacture requires consideration of the stability of the retained austenite against mechanical and renewed thermal stress, since forming steps and tempering processes are often part of the production process. Here, this dissertation excels in that the stability of austenite was investigated both through bainite formation and during tempering using in-situ synchrotron experiments. For the stability of austenite against mechanical loading, tensile were compared with compression tests, which were accompanied by XRD measurements. In order to show the effects of the individual process steps on the austenite, comprehensive analyses of the microstructure were carried out using high-resolution methods such as APT measurements. After comparing isothermally and continuously produced bainite, a granular bainite with <6 vol.% of equilibrium ferrite, which should be feasible on an industrial scale, with a tensile strength of 1200 MPa and an elongation at fracture of up to 30 % at a retained austenite content of 28 vol.%, could be produced in the course of this work. It has been shown that an average carbon content >1 wt.% in the austenite should be necessary to avoid transformation during cooling to room temperature, resulting in film-like retained austenite with 1.5 wt.% carbon, which is significantly more stable against mechanical loading than the blocky retained austenite with 0.8 wt.% carbon and shows high tempering resistance up to 450°C for 250 s or 400°C for 1800 s for the chosen alloy system.

KW - Bainit

KW - Restaustenit

KW - Austenitstabilität

KW - AHSS

KW - TRIP

KW - isothermer Bainit

KW - kontinuierlicher Bainit

KW - Anlassen

KW - Synchrotron

KW - bainite

KW - retained austenite

KW - austenite stability

KW - AHSS

KW - advanced high strength steels

KW - TRIP

KW - transformation induced plasticity

KW - tempering

KW - isothermal bainite

KW - continuous bainite

KW - synchrotron

M3 - Doctoral Thesis

ER -