LR EH40 steel plates have rather good low temperature impact toughness and rather low brittle transition temperature. In order to ensure -40 DEG C impact toughness, reducing carbon content is a good way. But that will reduce LR EH40 steel plates strength to a certain extent, and the following normalizing will inevitably lead to a further strength decline.
Therefore, we need to add some fine crystallization micro alloy elements such as Nb, Ti. Nb and Ti elements can expand the temperature range of deformation austenite non recrystallization region. It is advantageous in the hot rolling process by increasing rolling in austenite non recrystallization region deformation. Relying on the generated small dispersion of nitride, carbide and carbon nitride compound grain boundary pinning, it prevents the austenite grain grow, and make grain refinement and grain dispersion effect. Meanwhile, the precipitated grain also plays the role of precipitation strengthening. Thus, the strength loss can be made up by the strengthening action of fine grain and precipitation strengthening.
In addition, the addition of microalloying elements combine with controlled rolling and controlled cooling process, can improve LR EH40 steel plates ductility. The harmful elements in steel, such as S, P, H, O and inclusion, have obvious influence on toughness and ductile brittle transition temperature. N element is the fundamental factor that results in strain aging effect just now. So LR EH40 steel plates put forward higher requirements for steel purity. Therefore, the composition design principle of normalizing LR EH40 steel plates is low C, high Mn, Nb, Ti microalloying.
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