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Effect of Normalizing on Properties of High Strength Shipbuilding Steel Plate

Effect of Normalizing on Properties of High Strength Shipbuilding Steel Plate

shipbuilding steel plate

With the development of the ship industry, the demand for high-strength shipbuilding steel plate has also increased rapidly. Therefore, research on high-strength steel for ships has become one of the hot spots. Under normal circumstances, the low carbon micro-alloy shipbuilding plate, like CCSA shipbuilding steel plates structure is ferrite plus a small amount of pearlite. The structural control of steel is the key to obtain high strength and toughness, and the main strengthening methods are fine grain strengthening, precipitation strengthening and so on. Rolling and cooling control technology are generally used to control grain refinement and carbonitride precipitation strengthening for  shipbuilding steel plate to obtain the best mechanical properties such as high strength, high toughness, high weldability and good forming properties under hot rolling conditions. However, for shipbuilding plates with large thickness specifications, the product structure obtained by the controlled rolling and controlled cooling process is usually uneven, which affects the use of materials. Therefore, it is necessary to do post-heat treatment on thicker ship plates to obtain the best overall performance.
After normalizing, the mechanical properties of the shipbuilding steel plates showed a slight decrease in strength and a slight improvement in elongation. The impact toughness values at -40 °C were all above 200 J, and the comprehensive mechanical properties were high and the stability was good. After the combination of structure and performance, the strength and toughness of 850 °C and 880 °C after normalizing are the best match. After normalizing at 850 °C and 880 °C, the sample structure is finer and evener, and the normalized structure of 910 °C begins to be larger. The best normalizing temperature should be between 850 and 880 °C.
The low-carbon microalloyed high-strength shipbuilding steel plates subjected to normalizing at 850 ° C and 880 ° C obtain fine uniform ferrite structure and block-shaped pearlite. The normalized steel has slightly lower strength, more improved elongation, better low temperature impact toughness, higher comprehensive mechanical properties and better stability than that of original hot rolled steel plates with rolling and cooling control.
Through the observation surface of the microstructure of the sample observed by H-800 transmission electron microscope, carbonitrides of niobium and  vanadium were precipitated on the grain boundaries and dislocations, and were dispersed and distributed in a size of about ten nanometers, which could strengthen steel. In addition, some nitrides of Ti precipitate in the structure, and such precipitates have good stability, can inhibit austenite grains and refine grains.

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