TY - THES

T1 - Influence of thermal processing on microstructure, strength and ductility of steels for cold heading applications

AU - Pfleger, Bernd

N1 - embargoed until 07-12-2022

PY - 2017

Y1 - 2017

N2 - Third generation advanced high strength steels (AHSS) provide a good combination of cost reduction and favorable mechanical properties. Whilst low costs are obtained owing to the lack of alloying elements, the mechanical properties are achieved by adjusting the microstructure with thermal treatments. Particularly promising are quenching and partitioning (Q&P) processes as well as bainitic heat treatments as they optimize both, strength and ductility due to stabilization of reverted austenite. The goal of this master thesis is to apply the fundamentals of Q&P and bainitic heat treatments on two steels that differ mainly in carbon and silicon content to obtain a combination of extremely high elongations at fracture at relatively high yield and tensile strength values. Furthermore, fractions of retained austenite at room temperature of both steels obtained by thermal processing should be compared. To specify suitable parameters for Q&P or bainitic heat treatments the cooling behavior of the two steels is analyzed by simulation with JMatPro and dilatometry. Additionally, for the characterization of the cooling behavior, the microstructure is determined by optical methods. According to the obtained data, various Q&P and bainitic heat treatments are chosen to vary phase fractions of austenite, martensite, bainite and ferrite as well as strength and ductility. Thermal treatments of specimens are applied in dilatometer. Etching according to Klemm and Nital is used to distinguish microstructural phases. Furthermore, the fractions of austenite are detected by X-Ray diffraction. Mechanical properties are determined by hardness measurements and tensile testing values. It is shown that the variation of Q&P parameter enables the achievement of a combination of high strength with low ductility and nearly the same fractions of retained austenite. Bainitic heat treatments with isothermal holding or continuous cooling lead to a combination of relatively high strength and ductility comparable to that of third generation AHSS. Furthermore, it can be shown that the steel with higher silicon content results in significant higher fractions of retained austenite.

AB - Third generation advanced high strength steels (AHSS) provide a good combination of cost reduction and favorable mechanical properties. Whilst low costs are obtained owing to the lack of alloying elements, the mechanical properties are achieved by adjusting the microstructure with thermal treatments. Particularly promising are quenching and partitioning (Q&P) processes as well as bainitic heat treatments as they optimize both, strength and ductility due to stabilization of reverted austenite. The goal of this master thesis is to apply the fundamentals of Q&P and bainitic heat treatments on two steels that differ mainly in carbon and silicon content to obtain a combination of extremely high elongations at fracture at relatively high yield and tensile strength values. Furthermore, fractions of retained austenite at room temperature of both steels obtained by thermal processing should be compared. To specify suitable parameters for Q&P or bainitic heat treatments the cooling behavior of the two steels is analyzed by simulation with JMatPro and dilatometry. Additionally, for the characterization of the cooling behavior, the microstructure is determined by optical methods. According to the obtained data, various Q&P and bainitic heat treatments are chosen to vary phase fractions of austenite, martensite, bainite and ferrite as well as strength and ductility. Thermal treatments of specimens are applied in dilatometer. Etching according to Klemm and Nital is used to distinguish microstructural phases. Furthermore, the fractions of austenite are detected by X-Ray diffraction. Mechanical properties are determined by hardness measurements and tensile testing values. It is shown that the variation of Q&P parameter enables the achievement of a combination of high strength with low ductility and nearly the same fractions of retained austenite. Bainitic heat treatments with isothermal holding or continuous cooling lead to a combination of relatively high strength and ductility comparable to that of third generation AHSS. Furthermore, it can be shown that the steel with higher silicon content results in significant higher fractions of retained austenite.

KW - Q&P

KW - quenching and partitioning

KW - martensite

KW - bainite

KW - retained austenite

KW - strength

KW - ductility

KW - Q&P

KW - Quenching and Partitioning

KW - Martensit

KW - Bainit

KW - Restaustenit

KW - Festigkeit

KW - Duktilität

M3 - Master's Thesis

ER -