Table of contents
Classification
Third generation advanced high-strength steel
Development of advanced high-strength steel
About Us
The World Steel Association divides AHSS steel into three generations based on its research and development history and characteristics:
(1) The first generation AHSS steel is based on ferrite and has a strong plastic product of less than 15 GPa%. It mainly includes dual-phase (DP) steel, multi-phase (CP) steel and transformation-induced plasticity (TRIP) steel. Steel, ferritic bainitic steel (FB/SF), martensitic steel (MS/PHS), boron steel (HF);
(2) The second generation AHSS steel is based on austenite and has a strong plasticity product of more than 50 GPa%. It mainly includes austenite twin-induced plasticity (TWIP) steel (main steel type) and induced plasticity light steel. (L-IP) and shear band reinforced (SIP) steel;
(3) The third generation AHSS steel is based on martensite, tempered martensite, submicron grain/nano grain structure or precipitation-strengthened high-strength BCC structure, with a strong plastic area of 20-40GPa%, mainly including TBF steel (TRIP Aided Bainitic Ferrite steels), medium Mn-Trip, Q&P steel (Quenching-Partitioning Steel). Under the guidance of the structure control theory of the third-generation AHSS steel characterized by "multiphase, metastable, and multi-scale", the structure control idea of the ultra-fine grain matrix and metastable phase of the high-strength plastic accumulation third-generation automotive steel was proposed. Technical ideas for new medium manganese alloying and reverse transformation austenite (ART) annealing.
Third generation advanced high-strength steel
1. TBF steel (TRIP Aided Bainitic Ferrite steels)
TBF steel is phase transformation-induced plastic iron bainitic steel, also known as Carbide-free bainitic steels (carbide-free bainitic steel), TRIP with bainitic matrix (bainite-based phase transformation-induced plastic steel) or super -bainitic TRIP (super bainite-based transformation-induced plasticity steel).
Microstructure
The structural characteristics of TBF steel are fine and regular carbide-free bainite ferrite lath bundles, film-like retained austenite and massive retained austenite distributed between the lath bundles on the bainitic ferrite matrix. , and there is also a very small amount of tempered martensite.
Performance characteristics
TBF steel contains metastable retained austenite (volume fraction is about 10%-30%), which not only has good ultra-high strength and plasticity matching, but also has high fatigue strength and good impact properties. , flanging hole expansion performance and hydrogen embrittlement resistance.
Design goals: the yield strength reaches more than 1.5GPa, the tensile strength reaches 1.77~2.2GPa, and the elongation after fracture reaches 15%.
chemical composition
The C element in TBF steel is 0.2~0.4%.
The role of chemical elements in TBF steel
2.Q&P Steel (Quenching-Partitioning Steel)
Microstructure
The microstructure of Q&P steel is carbon-poor lath martensite and fluorocarbon retained austenite (5~15%). The martensite structure ensures the strength of the steel, and the retained austenite undergoes phase transformation during the deformation process to induce plasticity, thus improving the plasticity of the steel.
Performance characteristics
Q&P steel is a new steel type with higher yield ratio (YS/TS), high strength and higher elongation. The designed tensile strength is 800~1500MPa and the elongation is 15%~40%.
Quenching and distribution process
Design idea: Through the distribution of carbon, the austenite is enriched with carbon, thereby stabilizing the austenite. Then, the TIRP effect of austenite at room temperature is exploited to obtain relatively high plasticity.
The quenching distribution process first heats the steel to a certain temperature above Ac3 to completely austenitize it. This temperature is called the austenitizing temperature AT, and then quenches it to Ms and Mf at a cooling rate greater than the critical cooling rate of martensite transformation. At a certain temperature QT between them, a mixed structure of martensite and retained austenite is formed; then the temperature is raised to the partitioning temperature PT lower than Ms and kept for a certain period of time, so that the carbon element diffuses from the carbon in the supersaturated martensite to the remaining austenite. In austenite, increases austenite stability so that it remains at room temperature during subsequent quenching.
3. Medium Mn-Trip
Microstructure
The microstructure of medium manganese steel ART steel is martensite or tempered martensite matrix containing a large amount of flaky retained austenite or ultrafine ferrite.
Austenite reverted transformation (ART)
In the ART process, the steel is first quenched to obtain quenched martensite, and then annealed in the ferrite + austenite two-phase zone to obtain retrograde austenite, accompanied by the enrichment and redistribution of solute elements in the austenite. Improved stability of retained austenite remains at room temperature.
chemical composition
Since increasing the content of metastable austenite in steel is a key factor in improving the strong plastic product of steel, it is necessary to increase the content of metastable austenite.
Mn element can expand the austenite phase area and effectively promote the formation of austenite and ultra-fine structure. Therefore, the replacement diffusion and partitioning of Mn element and the reverse transformation of austenite are ultimately the body-centered cubic (BCC) ferrite structure characterized by multiphase and sub-micron scale ultra-fine matrix and the face-centered cubic (FCC) residual austenite structure. The key to the body composite structure.
The composition of the medium-manganese steel studied experimentally is designed to have a mass fraction of C of 0.15%-0.60% and a mass fraction of Mn of 4%-10%. Some researchers have added Si and Al to the medium-manganese steel. The score is basically controlled within the range of 1.5%-3.0%. In addition, Mo and the microalloying element V were added in a few studies, aiming to improve the grain boundary strength and refine the matrix grain size.
Development of advanced high-strength steel
The development of the next generation of advanced high-strength steel should meet the following conditions: low carbon (high weldability), low cost (low alloy addition), high formability, and easy to equip and repair. In the future, the design and development of materials should be considered from the perspective of the whole process. Demand will promote the progress of related technologies, and technological progress will also stimulate the increase in demand.
About Us
GNEE Steel was established in 2008 and has become one of China's leading suppliers of automotive steel products. We have two factories and four marketing centers with more than 30 production lines and an annual production capacity of 900,000 tons.
GNEE company mainly deals in automotive steel and other steel products. We can also customize products according to orders, meeting all customer requirements by providing convenient one-stop service.
