This pa/Jer describes a part of results of continued developmental activity to obtain optimum chemical composition and processing conditions in the production of high strmgth steel with high toughness by controlled rolling .I t was noted in laboratory experiment and mill test that niobium most e.ffectively improved the frac ture ap/Jearance transition temperature (v T s) when a strong controlled rolling was applied, while vanadium was less effective in this res/Ject. Steels containing both of niobium and vanadium also /Jossessed a v T s similar to th at of Nb-bearing steels . Addition of a small amount of nickel, chromium, and cop/Jer to Nbor V -bearing steels enhanced precipitation hardening presumably by lowering the r-+a transformation temperature withou t deteriorating the v T s. The improvement of v T , by controlled rolling was consistently described by the total rolling reduction given at below 900"C, and was less de/Jendent on rolling reduction of individual pass .L owering carbon content to below a level of 0 .07 0 0 in the Nb-bearing steel considerably improved impact shelf energy without arry loss in LYS. T ransverse shelf energy was also improved by decreasing sulfur content and application of cross rolling . H owever, an intensive controlled rolling slightly decreased shelf energy.A combination of the above knowledge has made the production q! tough high strength steels possible, and a few such examples qf line/Ji/Jf plate are shown.
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