Weng Yuqing

Third generation high strength low alloy steels with improved toughness

On-line thermo mechanical controlled processing (TMCP) was conducted to develop the third generation high strength low alloy (HSLA) steel with high toughness economically. The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling. The reduction of martensite block size decreased ductile-to-brittle transition temperature (DBTT) and compensated the strength loss due to carbon reduction. Consequently, the excellent balance of strength and toughness values was obtained as 950–1060 MPa for yield strength, 180 J for Charpy V-notch impact absorbed energy at −30°C, which is much superior to that of traditional martensitic steel. Two mechanisms for the refinement of lath martensite block were proposed: One is the austenite grain refinement in the direction of thickness, and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation, possibly due to austenite hardening.

Microstructures and mechanical properties of the third generation automobile steels fabricated by ART-annealing

The mechanical properties dependence on the microstructure was reviewed and analyzed, and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility. The medium-Mn steels with different carbon contents processed by austenite reverted transformation (ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite. The lamellar typed ultrafine structure, the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper. It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility, i.e., Rm×A about 30–50 GPa%, which is about two times of that of the conventional automobile steels and close to that of the TWIP steels. It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.

Ausforming effects on anisotropy of mechanical properties in HSLA martensitic steel

In order to clarify effects of prior pancaked austenitic structure on microstructure and mechanical properties of transformed martensite in ausformed steel, a super-thin pancaked austenite was processed by multi-pass rolling in a 0.03–2.6Mn-0.06Nb-0.01Ti (wt%) low alloy steel. The evolution of prior pancaked austenite grain during multi-pass rolling was studied using Ni-30Fe model alloy. Related with the structure and texture in the prior super-thin pancaked austenite in Ni-30Fe alloy, the texture and anisotropy of mechanical properties of transformed martensite in the studied ausformed steel were focused on. There were mainly three kinds of rolling texture components in the super-thin pancaked austenite: Goss {110}〈001〉, copper {112}〈111〉 and brass {110}〈112〉. They were further transformed into the weak {001}〈110〉 and strong {112}〈110〉, {111}〈112〉 texture components in the martensitic structure. The orientation relationship (OR) of lath martensite transformation from pancaked austenite in the ausformed steel deviated larger from the exact Kurdjumov-Sachs (K-S) OR than in the case of equiaxed austenite without deformation. The tensile and yield strengths of the ausformed martensitic steel first decreased and then increased as the angle between tension direction and rolling direction increased. The main reason for the anisotropy of strength was considered as the texture component {112}〈110〉 in martensite. However, the anisotropy of impact toughness was more complex and the main reasons for it are unknown.

EFFECT OF FINISH COOLING TEMPERATURE ON MICROSTRUCTURE AND LOW TEMPERATURE TOUGHNESS OF Mn-SERIES ULTRA-LOW CARBON HIGH STRENGTH LOW ALLOYED STEEL

Recently, the steel plates used in the ship, pipeline and bridge generally required not only high strength but also excellent low temperature toughness. As a competitive candidate, the ultra- low carbon high strength low alloyed(HSLA) steel has been developed widely. The low temperature toughness depends on the microstructure of the steels. Therefore, the relationship of low temperature toughness and microstructure should be studied in detail. In the present work, the steel plates with 25 mm thickness after hot rolling were immediately water quenched to 550, 450 and 350 ℃(finish cooling temperature), respectively, and subsequently air cooled to room temperature. The effect of finish cooling temperature on the microstructure and low temperature toughness ofMn-series ultra-low carbon HSLA steel was investigated by SEM, TEM and crystallographic analysis. The results show that the granular bainite, lath bainite and martensite were obtained with finish cooling temperatures decreasing. There are three blocks with different orientations in a single packet for lath bainite microstructure in the sample with finish cooling temperature of 450 ℃, leading to the refinement of effective grain size and large amount of highangle grain boundaries. Electron backscattered diffraction analyses of the cleavage crack path show that the bainite block boundaries can strongly hinder fracture propagation, and thus the refinement of bainite blocks can improve the low temperature toughness of Mn- series ultra- low carbon HSLA steel. Finally, the yield strength of 775 MPa and ductile-brittle transition temperature of-55 ℃can be achieved when the finish cooling temperature is 450 ℃.