The Effect Of Soaking Conditions On The Properties Of The Super-heavy Steel Plate

The deformation and heat transfer of quenching and tempering super-heavy steel plate are influenced by the thickness during production, rolling and heat treatment processes. Microstructures and mechanical properties in the normal direction are prone to inhomogeneous, which seriously affect the overall properties of the plate. With the requirement of thickness specification increasing, the non-uniform properties in the cross-section occurred. But the strength and toughness and good strength and toughness balance of steel plate must also be guaranteed. which means the technical issue is challenging. China’s the rapid development in heavy industry including shipbuilding, marine engineering, oil and gas transportation, machinery industry require super- heavy plate in an increased quantity, and high end quality as well.Super-heavy steel plate could gain higher strength, finer low-temperature toughness, and the better weldability and corrosion resistance after heat treatment process. In the shipbuilding, marine engineering industries, the application of quench and tempered steel with moderate Ni, Cr, Mo addition are widely used in recent years. In this work, based on the characteristics of the production process, simulation on different parts of the plate was carry out and the heat treatment simulation experiments to attentively investigation the prior austenite grain growth at different heat temperatures was conducted as well. According to GB/T6394-2003 "metal average grain size measurement method", the prior austenite grain size after being quenched was measured, which determines the temperature of the austenite grains grow rapidly. The results indicated that when the heat temperature is higher than 1220℃, the austenite grain grows rapidly. On this basis, a laboratorial rolling simulation experiment was conducted. Experimental steel subjected to high temperature and low temperature, rolled and air cooling, respectively, using the same heat treatment: 900℃quenching and 640 ℃ tempering. Using optical microscope(OM), scanning electron microscopy(SEM), transmission electron microscopy(TEM) and electron backscattered diffraction(EBSD) to investigate the microstructure at different temperatures, and quantitatively analyze on the substructure as well, conventional mechanical properties are tested by uniaxial tensile and impacted test, which shows that the strength changes slightly at different temperatures, but during low temperature test, the result is the plate could obtain higher toughness, therefore ductile-brittle transition temperature is lower. With the decrease of the experimental temperature, the impact toughness value of high temperature processed plate drops to 110 J. When the experimented temperature is-80℃, the impact toughness value of low temperature processed plate reaches 190 J. Comprehensive analysis revealed that the experimental steel after evenly temperature does not completely eliminate the adverse effects of high temperature through subsequent heat treatment, which causes the substructure growingly increase, especially the block size, and the length of the grain boundaries per unit area reduces, resulting in insufficient low-temperature toughness.