Toshinobu Nishibata

Decarburization of 0.21C-1.3Mn-0.2Si Steel for Hot Stamping at Various Heating Temperatures

In order to examine the decarburization behavior in the hot stamping (HS) method, the dependence of the microstructure evolution on the annealing temperature was experimentally studied using a Fe-0.21 mass% C-1.3 mass% Mn-0.2 mass% Si steel. The steel was isothermally annealed in the temperature range of T = 773-1173 K for various times of t = 100-12800 s in an ambient atmosphere. Here, the steel possesses the ferrite (α) + cementite (θ) two-phase microstructure at T = 773-923 K, the α + austenite (γ) two-phase microstructure at T = 1013-1073 K, and the γ single-phase microstructure at T = 1093-1173 K. During annealing at T = 1013-1073 K for t = 1600 s, however, the α layer with a uniform thickness is formed at the surface of the steel due to decarburization and gradually grows into the inside. Such formation of the a layer was not clearly observed at T 973 K and T 1093 K. Thus, the formation of the α layer hardly occurs under the HS annealing conditions. At T = 1033 K, the thickness of the α layer is mostly proportional to the square root of the annealing time. Such a relationship is called the parabolic relationship. Furthermore, the grain size of the α layer monotonically increases with increasing annealing time. Hence, the parabolic relationship guarantees that the growth of the α layer is controlled by volume diffusion.

Effect of Ni content on weld hot cracking susceptibility of fully austenitic Fe-Cr-Ni alloy - investigation on weld hot cracking susceptibility of fully austenitic fe-cr-ni alloy (report 1)

The generally acknowledged reduction in hot cracking susceptibility using the p ferritic phase cannot be achieved for fully austenitic (hereinafter inscribed as fully y) stainless steel such as SUS310S and fully y group alloys with a high Ni content, such as high Ni heat and corrosion resisting alloys; there is a tendency for hot weld cracking susceptibility to increase. Consequently, there are many cases where the initiation of hot cracking during the welding procedure becomes a problem. Possible countermeasures involve the use of materials, so numerous investigations have been carried out, including the study of the effects of impurity elements (P, S etc.) upon hot cracking susceptibility; many investigations have been limited to within the scope of restricted major constituents (for example, under the category ofSUS310S group, high Ni group heat resisting alloys). However, where the major constituent group varies, the effects of the alloying elements upon hot cracking susceptibility are thought to be not necessarily similar. In other words, the effects of the alloying elements upon hot cracking susceptibility in different major constituent groups and the information which can generally explain the effect mechanisms are not necessarily established. This study aims to clarify the effects of alloying elements upon hot cracking susceptibility and the effect mechanisms for a wide range of Ni content of fully y Fe-Cr-Ni alloys. Consequently, the effects of Ni content upon hot cracking susceptibility were evaluated by means of a Trans-Varestráint test and the hot ductility test; hot cracking regions and fracture surfaces were observed; the solidification temperature range and the residual liquid phase content were investigated using Thermo-Calc (Version L, SSOL data base) and the weld solidification process using the liquid tin quenching system was examined.