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Fe-25mass%Cr-1.1mass%N alloys with different impurity content were produced by solution nitriding and then subjected to tensile tests at ambient temperature. Yield strength and tensile strength of the alloys are much higher than those of conventional austenitic stainless steels, but the brittle intergranular fracture is caused during uniform deformation without local elongation. It was confirmed that the intergranular fracture takes place at high angle random boundary and that the grain boundary segregation of impurities significantly promotes intergranular fracture. Deformation-induced martensite did not concern the intergranular fracture. In addition, it was suggested that marked stress concentration is caused at grain boundary by the piled-up dislocations in planar array and it would be one of the important factors inducing intergranular fracture.KEY WORDS: high nitrogen steel; austenite; intergranular fracture; grain boundary segregation; impurity; deformation-induced martensite; planar dislocation array; dislocation pile-up; stress concentration. 861© 2008 ISIJ † Throughout this article, the chemical composition is expressed in mass percent, unless otherwise stated.it was reported that a Fe-Cr-Mn-N system austenitic stainless steels produced by the high-pressurized melting exhibit a sufficient ductility with local elongation.11) Although the difference between both cases has not been clarified yet, it is possible that the fracture mode might depend on the susceptibility of deformation-induced martensitic transformation, marked grain coarsening and grain boundary segregation of impurities in the case of solution-nitrided austenitic steels.In this study, two kinds of nickel-free high nitrogen austenitic stainless steel (Fe-25Cr-1N alloy) with different impurity content were produced by solution nitriding, and then the mechanical properties were investigated by means of tensile testing. The contribution of deformation-induced martensite, stress concentration at grain boundary caused by the piled-up dislocations in planar array and grain boundary segregation of impurity was then discussed for clarifying the mechanism of the occurrence of intergranular fracture. Experimental ProcedureAs the base materials for solution nitriding, two kinds of ferritic stainless steel (Fe-25Cr alloy) with different impurity content were prepared in this study. The chemical compositions of these steels are listed in Table 1. The low purity steel, Fe-25Cr-LP, contains phosphorus of 270 ppm and sulfur of 32 ppm, while the high purity steel, Fe-25Cr-HP, contains less phosphorus and sulfur contents, which are below 50 ppm and 4 ppm, respectively. The ingots (50 kg) were produced by induction melting in a vacuum and then casting into metallic molds. The ingots were homogenized by annealing at 1 473 K and then followed by hot forging to make bars of 50 mm in diameter. The specimens cut from the hot-forged bars (40 mmϫ15 mmϫ1 mm) were wet-polished and then electrically polished with a solution of phosphoric acid and chromic oxide (H 3 PO ...

In order to achieve excellent toughness of heat affected zone (HAZ) of low carbon steel plates by welding, several microstructure control techniques using fine particles have been developed. In particular, Ca-oxysulfide inclusions have been used in a lot of steel plate products, because they are thermally stable even in near the fusion line of weld joint and they have grain refining effect in HAZ. In cases where heat input is large (>100kJ/cm) and microstructure of HAZ mainly consists of polygonal ferrite, it has been clarified that Ca-oxysulfide inclusions act as nucleation sites of polygonal ferrite in HAZ during the cooling process after welding. However, in cases where heat input is medium (≈50kJ/cm) and microstructure of HAZ mainly consists of bainite, nucleation effect of Ca-oxysulfide inclusions and that of mechanism have not been clarified. This study investigated the nucleation effect of Ca-oxysulfide inclusions in medium heat input welding by in-situ laser microscope observation and the lattice misfit between Fe-matrix and Ca-oxysulfide inclusions.

The strength and the toughness required for steel plates used for offshore structures became higher as the installation areas move into arctic areas. The main property of offshore structure steel is the crack tip opening displacement (CTOD) property of weld joint, and CTOD testing is performed at the minimum design temperature of the structure. Thus, the demand for satisfying −40°C of CTOD test temperature specification has increased. For the improvement of HAZ toughness, coarse austenite grain is suppressed by TiN, and low-C, Ceq, Si, P, Nb design is adopted to decrease the formation of M-A constituents. Furthermore, by using Ca inclusion, which works as a pinning particle and a bainite nucleation site, very fine bainite microstructure are formed in HAZ and excellent low temperature toughness are achieved. The YP420 class plate with excellent low temperature toughness has been developed using these technology.