Recently, automotive industries are subjected to strong pressure to reduce exhaust emission of their automobiles, which is an important cause of air pollution. To address this imposition, automotive industries have focused on the development of a lightweight body-in-white using high-strength sheet s...
Recently, automotive industries are subjected to strong pressure to reduce exhaust emission of their automobiles, which is an important cause of air pollution. To address this imposition, automotive industries have focused on the development of a lightweight body-in-white using high-strength sheet steels. The development of lightweight automobile bodies has a variety of other goals as well, such as increasing fuel efficiency, increasing strength and safety, and improving durability. Advanced high-strength steels (AHSS) with excellent tensile properties such as dual-phase (DP), transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), quenching and partitioning (Q & P), and light-weight steels have been developed to meet these goals. However, compared to conventional low-strength steels, AHSS have superior tensile properties, but inferior formability, such as bendability, drawability, and stretchability. Especially, AHSS have extremely poor stretch-flangeability, which is the ability to resist edge fracture during complex forming. This inferior characteristic impedes use of AHSS in automobile parts. To overcome inferior stretch-flangeability of AHSS, it is necessary to investigate what is the governing factors of its stretch-flangeability and how to design the microstructural features to improve its stretch-flangeability.
The aims of this thesis consist of two specific goals. The first goal of this thesis is to figure out the universal governing factors of stretch-flangeability of sheet metallic materials. To achieve this purpose, various sheet materials including AHSS, low-strength grade steels, and non-ferrous materials were evaluated to analyze the correlation between mechanical properties and stretch-flangeability. The mechanical properties and stretch-flangeability of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion test. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability.
The second goal of this thesis is to demonstrate the possibility of improvement of the AHSS’ stretch-flangeability through the improvement of microstructure affecting their fracture toughness, which is the universal factor governing stretch-flangeability. To achieve this purpose, 1) the effect of grain size on stretch-flangeability of single-phase steels and 2) the effect of secondary phase on stretch-flangeability of dual-phase steels were investigated.
Firstly, the effect of grain size on stretch-flangeability was investigated to determine its influence on stretch-flangeability of AHSS. To avoid other effects of microstructure, single-phase TWIP steels were selected for the investigation. To control the grain size of the two types of TWIP steels, 1) the initial specimen was annealed at 1100 ℃ to increase its grain size, or 2) subjected to high-pressure torsion then annealed at 650 ℃ to reduce the grain size. The microstructural features were analyzed using the electron backscatter diffraction. The stretch-flangeability of TWIP steels with various grain sizes was evaluated using a hole-expansion test. It was found that the hole-expansion ratio follows the Hall-Petch correlation as does fracture toughness.
Secondly, the effect of secondary phase on stretch-flangeability of DP steels were investigated. Microstructure of relatively simple ferrite-martensite DP steel was modified by intercritical annealing, then the effects of this modification on stretch-flangeability, tensile properties, and fracture resistance of the steels were systematically quantified. The hole-expansion ratio increased linearly with an increase in the apparent fracture initiation energy, but was not correlated significantly with the fraction of martensite, the carbon content of martensite, or the hardness difference between ferrite and martensite. In this thesis, it is discovered that the universal factor governing stretch-flangeability is fracture toughness, and suggested that microstructural features should be designed to increase their fracture toughness in order to improve the stretch-flangeability of AHSS.
Recently, automotive industries are subjected to strong pressure to reduce exhaust emission of their automobiles, which is an important cause of air pollution. To address this imposition, automotive industries have focused on the development of a lightweight body-in-white using high-strength sheet steels. The development of lightweight automobile bodies has a variety of other goals as well, such as increasing fuel efficiency, increasing strength and safety, and improving durability. Advanced high-strength steels (AHSS) with excellent tensile properties such as dual-phase (DP), transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), quenching and partitioning (Q & P), and light-weight steels have been developed to meet these goals. However, compared to conventional low-strength steels, AHSS have superior tensile properties, but inferior formability, such as bendability, drawability, and stretchability. Especially, AHSS have extremely poor stretch-flangeability, which is the ability to resist edge fracture during complex forming. This inferior characteristic impedes use of AHSS in automobile parts. To overcome inferior stretch-flangeability of AHSS, it is necessary to investigate what is the governing factors of its stretch-flangeability and how to design the microstructural features to improve its stretch-flangeability.
The aims of this thesis consist of two specific goals. The first goal of this thesis is to figure out the universal governing factors of stretch-flangeability of sheet metallic materials. To achieve this purpose, various sheet materials including AHSS, low-strength grade steels, and non-ferrous materials were evaluated to analyze the correlation between mechanical properties and stretch-flangeability. The mechanical properties and stretch-flangeability of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion test. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability.
The second goal of this thesis is to demonstrate the possibility of improvement of the AHSS’ stretch-flangeability through the improvement of microstructure affecting their fracture toughness, which is the universal factor governing stretch-flangeability. To achieve this purpose, 1) the effect of grain size on stretch-flangeability of single-phase steels and 2) the effect of secondary phase on stretch-flangeability of dual-phase steels were investigated.
Firstly, the effect of grain size on stretch-flangeability was investigated to determine its influence on stretch-flangeability of AHSS. To avoid other effects of microstructure, single-phase TWIP steels were selected for the investigation. To control the grain size of the two types of TWIP steels, 1) the initial specimen was annealed at 1100 ℃ to increase its grain size, or 2) subjected to high-pressure torsion then annealed at 650 ℃ to reduce the grain size. The microstructural features were analyzed using the electron backscatter diffraction. The stretch-flangeability of TWIP steels with various grain sizes was evaluated using a hole-expansion test. It was found that the hole-expansion ratio follows the Hall-Petch correlation as does fracture toughness.
Secondly, the effect of secondary phase on stretch-flangeability of DP steels were investigated. Microstructure of relatively simple ferrite-martensite DP steel was modified by intercritical annealing, then the effects of this modification on stretch-flangeability, tensile properties, and fracture resistance of the steels were systematically quantified. The hole-expansion ratio increased linearly with an increase in the apparent fracture initiation energy, but was not correlated significantly with the fraction of martensite, the carbon content of martensite, or the hardness difference between ferrite and martensite. In this thesis, it is discovered that the universal factor governing stretch-flangeability is fracture toughness, and suggested that microstructural features should be designed to increase their fracture toughness in order to improve the stretch-flangeability of AHSS.
주제어
#Stretch-flangeability Hole-expansion ratio Formability Fracture toughness
※ AI-Helper는 부적절한 답변을 할 수 있습니다.