For the purpose to improve the weld bond toughness through ferrite grain refinement in 40-60kg/mmmm2 tensile strength steels, the effects of various alloying elements on the grain structure and notch toughness in the weld bond were investigated. Of the alloying elements investigated, small additions of both rare earth metal (REM) and boron (B) were found to accelerate the formation of fine ferrite grains inside prior austenite grains, resulting in a remarkable improvement weld bond thoughnss.The main results obtained are summarized as follows:(1) The optimum concentration range of REM and B are 0.020-0.040% and 0.0020-0.0035%, respectively.(2) With an increase of BN content, the number of fine ferrite grains increases remarkably.(3) The amount of BN re-precipitated during welding thermal cycle is larger in REM-B steel than in B steel.(4) Spherical inclusions are frequently observed in the center of ferrite grains or adjacent to them and many of them identified as Ce2O2S and BN.(5) According to the observation by IMMA, the location of B+-images is in good accordance with that of both Ce+-and N+-images.By the use of commercial heats of REM-B steel, brittle fracture tests were conducted and good results were obtained.
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Funakoshi et al. (1977) studied this question.