Papers by Keyword: Martensitic Steel

Abstract: Laser weldability was investigated for advanced high-strength steel sheets for automotive applications. Dual-phase steel (DP780) and martensitic steel (MS1300) sheets were employed as base materials; laser-butt and overlap welding experiments were conducted on combinations of steels with similar and dissimilar strength. The tensile strength and metallurgical morphology were analysed for the butt-welded specimens; tensile-shear strength and bead shapes were analysed for the overlap-welded specimens. Even with laser welding, martensite in the heat-affected zone disintegrated and resulted in a softened, heat-affected zone as compared with the base materials. The tensile strength of a butt weldment was determined by the strength of the heat-affected zone. The tensile-shear strength of an overlap weldment was determined by not only the strength of the heat-affected zone but also bead shapes such as blow holes, underfill, and the bead width at the faying surface.

Abstract: MS1470 steel is mainly used in the automotive industry, which has high strength, light weight and low cost. However, there always appear grains coarsening, brittle increasing, lower intensity and other issues. To solve this problem, we have analyzed the welding process, the fracture morphology and microstructure etc. The results shown that the microstructure were mainly ferrites and lath martensites. However, grains coarsening in the HAZ were serious, and fracture failure also occurred in the heat affected zone. Meanwhile, MS1470 steel had formed a small amount of pores during rolling process, which also is an important reason for cracks formation.

Abstract: Processing of a large number of novel steel types, such as DP, TRIP, CP and TWIP, and high-strength low-carbon bainitic and martensitic DQ-T steels, have been developed based on physical simulation and modelling studies. Among stainless steels, guidelines for processing of ultra-fine grained austenitic stainless steels have been created. Physical simulation has been used by employing a Gleeble thermo-mechanical simulator to reveal the phenomena occurring in the hot rolling stage (the flow resistance, recrystallization kinetics and microstructure evolution), and in the cooling stage (CCT diagrams) for carbon steels and in short-term annealing of cold rolled metastable austenitic steels. Connecting these data with microstructures examined in optical and electron microscopes and resultant mechanical properties have improved the understanding on complex phenomena occurring in the processing of these steels and the role of numerous process variables in the optimization of enhanced mechanical properties.

Abstract: In this study Quenching and Partitioning (Q&P) as proposed by Speer was applied to improve the ductility of C-Mn high strength Low Alloy steel (HSLAs). Microstructural observations revealed a multiphase microstructure including first martensite, fresh martensite and retained austenite in the Q&P processed steel. During tensile process, the austenite volume fraction gradually decreased with strain increasing, suggesting the phase transformation induced plasticity for the Q&P processed steel. Ultrahigh strength about 1300-1800MPa and tensile elongation about 20% were obtained after Q&P processing at specific conditions, which is significant higher than that of ~10% of conventional martensitic steel. The the product of tensile strength to total elongation increased from 25 to 35GPa% with increasing carbon content in studied steel. This improved mechanical properties were related to the ductility contribution from TRIP effects of the retained austenite and strength contribution from the hard martensitic matrix. At last it was turned out that the Q&P process is a promising way to produce ultrahigh strength steel with relative high ductility under tailored heat treatment conditions for different micro-alloyed carbon steel.

Abstract: Press hardening steel has become a much used material in car body manufacturing due to its excellent safety and lightweight potential. In some recent car models press hardening steel has reached already a weight share of more than 20% in the body structure while it is estimated that it could reach even around 40% in the future. However conventional press hardening steel based on the alloying concept 22MnB5 was designed originally not for automotive application. In spite of the high strength level, press hardening steel has generally low toughness due to the relatively high carbon content and its martensitic microstructure. Particularly important is the ductile-to-brittle transition temperature at low temperature, which might lead to unexpected failure in cold climate regions. Furthermore, hydrogen embrittlement is a major concern in press hardening steel as previous results indicated that only a few ppm of hydrogen could induce delayed cracking. Generally it is important that impact energy should be absorbed by the material to avoid unexpected disintegration of the structure. This material capability is usually characterized by the toughness. All these characteristics have not yet been systematically investigated in press hardening steel. Consequently, no attempts have been made to optimize conventional press hardening steel for obtaining better toughness, lower ductile-to-brittle transition temperature and reduced sensitivity to hydrogen induced cracking.A generally proven approach of improving the resistance against brittle failure in high strength steel is the refinement of microstructure, which can be efficiently achieved by Nb microalloying. This paper will introduce modified alloy designs for press hardening steel and explain fundamentally the metallurgical effects of Nb microalloying on the improvement of crack propagation resistance, bendability and delayed cracking behavior induced by hydrogen penetration in press hardening steel. The results reveal better toughness, improved bending behavior and increased resistance against hydrogen embrittlement.

Abstract: The behavior of work hardening by cold rolling and tensile deformation was investigated in an ultralow carbon and carbon bearing martensitic steels, and then the effect of carbon on the work hardening behavior was discussed in terms of the change in dislocation density and the microstructure development during deformation. In the ultralow carbon 18%Ni steel (20ppmC), the hardness is almost constant irrespective of the reduction ratio. On the other hand, the carbon bearing 18%Ni steel (890ppmC) exhibits marked work hardening. The dislocation density of these specimens was confirmed to be never increased by cold rolling. It was also found that 10% cold rolling gives no significant influence on the morphology of martensite packet and block structure. TEM images of the 10% cold-rolled steels revealed that the martensite laths in the ultralow carbon steel are partially vanished, while those in the carbon bearing steel are stably remained. These results indicate that the solute carbon retards the movement of dislocations, which results in the high work hardening rate through the formation of fine dislocation substructure within laths.

Abstract: Yield strength of highly dislocated metals is known to be directly proportional to the square root of dislocation density (ρ), so called Bailey-Hirsch relationship. In general, the microstructure of heavily cold worked iron is characterized by cellar tangled dislocations. On the other hand, the dislocation substructure of martensite is characterized by randomly distributed dislocations although it has almost same or higher dislocation density in comparison with heavily cold worked iron. In this paper, yielding behavior of ultra low carbon martensite (Fe-18%Ni alloy) was discussed in connection with microstructural change during cold working. Originally, the elastic proportional limit and 0.2% proof stress is low in as-quenched martensite in spite of its high dislocation density. Small amount of cold rolling results in the decrease of dislocation density from 6.8x1015/m-2 to 3.4x1015/m-2 but both the elastic proportional limit and 0.2% proof stress are markedly increased by contraries. 0.2% proof stress of cold-rolled martensite could be plotted on the extended line of the Bailey-Hirsch equation obtained in cold-rolled iron. It was also confirmed that small amount of cold rolling causes a clear microstructural change from randomly distributed dislocations to cellar tangled dislocations. Martensite contains two types of dislocations; statistically stored dislocation (SS-dislocation) and geometrically necessary dislocation (GN-dislocation). In the early deformation stage, SS-dislocations easily disappear through the dislocation interaction and movement to grain boundaries or surface. This process produces a plastic strain and lowers the elastic proportional limit and 0.2% proof stress in the ultra low carbon martensite.

Abstract: Nanoindentation technique was applied to evaluate nanohardness distribution in a submicron scale for two kinds of martensitic steels: Fe-0.4C binary steel and Fe-0.05C-0.22Ti steel with a stoichiometric composition of TiC. AFM images showed that Fe-C steel includes relatively coarse cementite particles with about 100~200 nm in diameter and a couple of hundreds nanometer in average spacing, while high-resolution TEM observation showed that the Fe-C-Ti steel has fine TiC precipitates with 5 nm in diameter and 15 nm for average spacing. Nanoindentation results revealed that the standard deviation was much higher for the Fe-C than that for the Fe-C-Ti. Since the typical indent size was a couple of hundreds nanometer, which was about two orders larger than the size of the TiC and comparable to the cementite size, the small distribution of nanohardness of the Fe-C-Ti was attributed to the homogeneous microstructure in sub-micron scale, while the inhomogeneity of cementite particles in the Fe-C steel leaded to large nanohardness.