ABSTRACT: The local corrosion of high-strength steel of 440N/mm 2 class manufactured by the thermo-mechanical control process was examined. INTRODUCTION 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 [2] [l] [5]. 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/mm 2 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. 2. LOCAL CORROSION OF THE TMCP STEELS 2-1. Test Specimens 2-1-1. Tested Steels 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) [6]. 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. 2-1-2. Test Welding Materials 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).

ABSTRACT: Influence of the local brittle zone on HAZ CTOD was discussed. From weld HAZ CTOD tests and simulated HAZ CTOD tests, it was clarified that the local brittle zone of a multi-pass weld HAZ is intercritically reheated coarse-grained zone, where high-carbon martensitic island was formed. Factors influencing the formation and decomposition of high-carbon martensitic island (M*) in the intercritically reheated coarsegrained HAZ was investigated. Based on a simple estimation model of volume fraction of the M*, it became possible to estimate quantitatively the influencing factors on the formation and decomposition of the M*. It was clarified that combination of steel with low carbon, low microa110y chemical composition and suitable welding condition, such as application of preheating and thin weld bead placement, is important for reducing or eliminating the H*. 1. INTRODUCTION Since low CTOD (Crack-Tip Opening Displacement) value was obtained in a HAZ (Heat-Affected Zone) CTOD test of offshore structural steel in the early 1980s, extensive studies have been made to clarify the cause of the low CTOD value. It has been made clear that the low value was caused by the LBZ (Local Brittle Zone) in the HAZ. Factors influencing the microscopic toughness of the LBZ and measures to prevent low CTOD value have been studied (OMAE, 1988),(OMAE, 1989) and new steels which have high HAZ CTOD value have been developed (e.g. Chijiiwa et aI, 1988). For improving the LBZ toughness, selection of chemical composition which has low susceptibility to cleavage fracture is primarily important. In addition, selection of a suitable welding condition (heat input, preheating and bead placement etc.) is also important. For this purpose, it is of urgent necessity to elucidate these factors quantitatively. From this standpoint, the present paper will discuss the factors controlling the LBZ toughness.

ABSTRACT: The demand of high strength steel for ship and offshore structure application has been changed from conventionally heat treated steel to TMCP steel, because the latter provides the improved strength - toughness balance and weldability. Enhanced property of TMCP steel comes from the effective conditioning of austenite with controlled rolling and pertinent selection of microstructure by accelerated cooling process. This new thermo-mechanical control process, named PILAC(POSCO In-Line Accelerated Cooling) has been developed to produce the high qual1ty structural steel. The production conditions of thermo-mechanical control process were studied to obtain the appropriate combination of strength and toughness of base metal. The effects of carbon to manganese ratio and Ti addition on the heat affected zone toughness were evaluated to optimize the chemical composition of steel. Based on these studies, the 50kg/mm2 tensile strength grade steels for ship and offshore structure were produced with PILAC process. These steels showed good heat affected zone toughness, even in heat input of 18kJ/mm, and good weld :rack resistance. Introduction Recently, the demand of structural steel has moved from mild steel to high strength steel and the consumed quantity of high strength steel has continuously increased. During last five years, the amount of high strength steel used in Korean industry have continuously increased as shown in Fig. I. From this figure, it could be found that there is a big jump in consumption rate of high strength steel from 36% in 1986 to 50-55% in 1989. Recent increase in consumption rate of high strength steel is largely dependent on a change in the demand of steel user and a quality renovation of high strength steel through TMCP technology. In ship building industries, as a measure of energy saving, the use of high strength steel is increased to reduce the total weight of ship.