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H eavy section low alloy steel plates are used Jar nuclear reactor and direct oil desulj"u ri z ing reactor p ressure vessels, etc. H igh integrity is, therifore, required oJ these steel plates. The J actors which aifect the internal soundness and notch toughness oJ heavy section low alloy steel plates are discussed in relation to heat treatment after rolling. Precipitation behavior oJ aluminum nitride in A 533B steel pla tes during hea t treatment cifter rolling is described w ith respect to the austenite grain size and notch toughness if the plate aJter .final rolling. Cooling down to 400°C beJore normali z ing is recommended to obtain good notch toughness. Selection oJ tem/)erature and time Jor dehy drogenation at the J errite region is discussed and it is shown that the op timum temperature Ja r dehy drogenation must be determined by takillg microsegregation in the plate into consideration.

Low alloy steel plates with heavy sections for pressure vessels of direct desulfurization unit and nuclear reactor were produced by basic oxygen process instead of the conventional electric furnace process. There was a decrease of impurity and residual elements which increase susceptibility to neutron irradiation embrittlement and temper embrittlement. Productivity also increased by this process. Various properties of the plates thus manufactured were evaluated in comparison with those of electric furnace plates. It was found as a result that the basic oxygen process produces an improved notch toughness for use in nuclear reactor pressure vessels, and approximately the same levels of properties for application to desulfurization-purpose pressure vessels, compared with the electric furnace process in which BOP return scrap was also used especially.

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