The local corrosion of high-strength steel of 440N/mm2 class manufactured by the thermo-mechanical control process was examined.
The local corrosion of welded joints of icebreakers resulting from damage of paint and cathodic protection by ice collision is a problem to be discussed  [l] . The steels for icebreakers are required to have (i) high resistance to local corrosion in seawater, (ii) high toughness at low temperature and (iii) good weldability. In this paper, the local corrosion of high strength steel of Y.P. 440 N/mm2 class manufactured by a thermo-mechanical control process (TMCP) is studied. And the properties of the steel, and welding materials developed on the basis of the results of the local corrosion resistance are also discussed.
Table I shows the chemical compositions of the steels tested. PI and CT are high-strength steels having yield strength of 440 N/mrn2 class produced by the TMCP. Both belong to the Cu-Ni series. A small amount of Nb was added to improve the notch toughness of the base metal, and a similar amount of Ti was added to improve the notch toughness of the weld heat affected zone (HAZ) . CF and TB are the reference steels. Both belong to the Si-Mn series; CF is low Mn steel and TB is low C steel. Photo. 1 shows the basemetal microstructures. PI and CT have fine proeutectoid ferrite (F) and bainite (Bu) microstructures. CF and TB have ferrite and pearlite (F+P) microstructures.
Table 2 lists the test welding materials. N100, N100Cu, N10, N13, ASl5 and NUSN were used for shielded metal are welding (SHAW) and YE-NB55E, IN-NB55E and YDN-NB55L for submerged are welding (SAW).