Using hot torsion tests, the dynamic (DRX), static (SRX) and metadynamic (MDRX) recrystallization characteristics of a Nb-microalloyed steel were studied. The torsion tests were carried out at temperatures in the range 850 to 1 050°C with strain rates ranging from 0.5 to 5/sec. At the higher temperatures, the Nb remained in solution, while precipitation was underway in the lower temperature range. The results indicate that Nb precipitation has little influence on the value of the critical strain (e c ) for dynamic recrystallization. The peak strain/Zener-Hollomon parameter equation is derived and the effect of austenite grain size on the peak strain is considered. The times for 50 % recrystallization for static and metadynamic recrystallization were established by means of interrupted torsion tests and are compared. The rate of metadynamic recrystallization increases with strain rate and temperature and is observed to be independent of the pass strain; this contrasts sharply with the observations for static recrystallization. Finally, an example is given of an industrial rolling process in which DRX/MDRX can play important roles. Here, the occurrence of dynamic/ metadynamic recrystallization causes the load to drop or else to increase less rapidly than in the case of pure strain accumulation in the absence of SRX.

In the present study, effects of notch shape and specimen thickness on drop-weight tear test (DWTT) properties of API X70 and X80 line-pipe steels fabricated by varying hot-rolling conditions were investigated. The DWTT was conducted on the rolled steels, and the results were discussed in comparison with the Charpy V-notch (CVN) impact test and crack-tip opening angle (CTOA) test data. The DWTT results indicated that the steels rolled in the singlephase region had the higher upper shelf energy (USE) than the steel rolled in the two-phase region, because their microstructures were composed of acicular ferrite (AF). The DWTT energy transition temperature (ETT) of the steel rolled in the two-phase region was the lowest, because it had a finer effective grain size and the lowest volume fraction of hard secondary phases. Chevron notch (CN) DWTT energy density did not increase much with increasing specimen thickness, whereas pressed notch (PN) DWTT energy density increased by 1 to 3 J/ mm 2 , because the total energy of the CN DWTT increased less than that of the PN DWTT due to the larger stress concentration at the CN. The ETT increased with increasing specimen thickness due to the increase in constraint state, and the ETT of the CN DWTT was slightly lower than that of PN DWTT. The measured CTOA showed better correlation with the DWTT propagation energy than with the CVN or DWTT total energy because it was related to the crack propagation speed. The value of sin (2CTOA) reliably showed a linearly proportional relation with the DWTT propagation energy density.

Using hot torsion tests, the in¯uence of Mn on the recrystallisation behaviour of a Nb microalloyed steel was studied. Continuous and interrupted torsion tests were performed in the temperature range 950 ± 1050°C at strain rates of 0 . 05 ± 5 s 1 . The kinetics of static and metadynamic recrystallisation (MRX) were characterised and appropriate expressions were formulated for the effect of Mn and of the other deformation parameters on the kinetics. The rate of MRX increases with strain rate and temperature and is observed to be independent of strain, in contrast to the observations for static recrystallisation (SRX). The strain rate dependence of MRX is about twice that of SRX, whereas the activation energy Q MRX is about Q SRX /2. Thus, results obtained under laboratory conditions must be corrected before application to industrial strain rates. When extrapolated to the much higher mill strain rates (4100 s 1 ), the present results indicate that SRX is much slower than MRX and is also unlikely to produce much softening in the 1 s interpass times available in such mills.MST/5033

Dissimilar joints between STS441, a ferritic stainless steel, and SS400, a carbon steel, were welded by GMAW (Gas Metal Arc Welding) using STS430LNb as a welding wire. The fracture behavior of the dissimilar weld was analyzed by a microstructural observation and thermo-mechanical tests. Martensite was formed at the region between SS400 and the weld metal because the Cr and Nb content in this region decreased due to the dilution of SS400 carbon steel during welding. According to results from a high temperature tensile test with a specimen aged at 900°C, it was found that the tensile strength of the dissimilar weld at high temperature was equal to that of STS441 base metal and the formation of martensite had little influence on tensile strength of the dissimilar weld at high temperature. However, in the case of thermal fatigue resistance, the dissimilar weld had an inferior thermal fatigue life to STS441 because of the presence of martensite and the softened region around the interface between the dissimilar weld metal and SS400.

Fig. 11. SEM micrographs of the cross-sectional area beneath the cleavage fracture surface of the Charpy impact specimens fractured at Ϫ20°C for the (a) S0-30, (b) S1-30, (c) S1-50, and (d) S1-100 steel HAZs, showing the crack propagation path. Fractured surfaces were coated by Ni.enhancing the HAZ toughness. By using the oxide metallurgy in which AFs are nucleated inhomogeneously inside austenite grains by forming fine complex oxides in linepipe steels as in the present study, linepipe steels with excellent Charpy impact properties in the HAZ can be fabricated. This oxide metallurgy technology can be applied to grain refinement by inhomogeneous nucleation when a high level of deformation cannot be imposed due to the limited rolling reduction ratio as in the case of thick high-strength steel plates. It can also be applied to the development of high-strength steel plates with excellent welding properties by controlling the HAZ microstructures. ConclusionsIn the present study, complex oxides were formed inside API X70 linepipe steels, and effects of these oxides on HAZ microstructures and Charpy impact properties were investigated.(1) Oxides present in API X70 linepipe steels were sized about 2 mm, and had characteristics of complex oxides composed of various elements. The S1 steel containing more Al, Ti, and Mg showed higher volume fraction of oxides than the S0 steel.(2) The HAZ microstructures of the S0 steel simulated with the heat input of 30 kJ/cm were composed of bainite, but the S1 steel HAZ contained 94 vol% of AF, together with a small amount of bainite. Thus, the absorbed energy of the S1 steel HAZ at Ϫ20°C and 0°C was five to eight times higher than that of the S0 steel HAZ.(3) When the weld simulation test was conducted with increased heat input of 50 kJ/cm and 100 kJ/cm, the absorbed energies at Ϫ20°C and 0°C of the S1 steel HAZs decreased down because of the increased amount of bainite as the cooling rate became slower.(4) In the S1 steel HAZs in which the AGS was relatively large (91 mm) and a number of complex oxides were contained, complex oxides worked as AF nucleation sites, thereby promoting the active formation of AF in the HAZ and improving Charpy impact properties. AcknowledgementThis work was supported by the National Research Lab-