Synopsis:For the purpose to improve the weld bond toughness through ferrite grain refinement in 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.

B asic and applied investigations are conducted Jor develo/)ing high strength ship hull steels that ensure a good notch toughness at large heatinput weld bond. Th e results are as Jollows. (I) Steels containing /)ro/)er amounts oj rare earth metals (REN!) and boron (B) show an excellent Ilotch toughness at the weld bond ill electroslag Weldi71g joint welded with a heat-input oj 506' kJ jcl1l, the weld bond in this case exhibiting a fine Jerrite and /)earlite structure. (2) W ith increasing boroll nitride (BN) content ill R EM-B steels. the number oj fine ferrite grains at the weld bond increases. (3) The amount oj BN reprecipitated during weld thermal cycle is larger ill REM-B steels than in B steels. (4) At the weld bond ill an R EM-B steel, globular particles are Jrequently observed in the center oj ferrite grains or adjacent 10 them, and mm!)' oj them give rise to electron diffraction pat/ems both oj Ce,O ,S and oJ BN. (5) According to the observatioll by 1M MA , the location; oj B+ images are in good coincidence with those oj both Ce+ and N + images. I t is concluded that Jormation oj fine Jerrite grains at the weld bond in all REM-B steel is attributed to the ~ffect oj BN accelerating the lIucleatioll oj fine ferrite grains inside the /)rior austenite grains and that, REN! , ill the Jorm oj ultra-fine RElI! ox)'Sulfide, also contributes ~Y providing the nucleation siteJor liN.

Syn op sisThe effects of austenite grain siz e and C COlltellt on the substructure and toughness of tempered martensite and isothermal bainite have been investigated to correlate the micro-structural factors to the toughness using 5%Ni-0.5% Mo steels with two levels ofC content, 0.15 and 0.3% .The increase of austenite grain size causes the coarsening of substructural units such as lath and packet, in tempered martensite, whereas it does not in the bainite structure. The increase in C content generally

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