| The production and demand of medium and thick plates are increasing with the development of shipbuilding, construction industry, bridges manufacture, offshore platform, pressure vessel, petroleum transportation, etc. And the superior performances of above plates are required, such as strength, toughness, etc. Especially, excellent welding property is necessary. The coarsened structure in heat affected zone (HAZ) deteriorates the toughness and causes the micro-crack, which is the key factor affecting steel wielding performance. Many ways are put out to improve toughness in HAZ. The most effective way is to refine the structure in HAZ by "oxide metallurgy".In the present work, the nucleation and growth of many kinds of inclusions were studied based on the metallurgical thermodynamics and kinetics and experiments. Especially, the components, distribution of Ti-Mg/Zr deoxidization and the control conditions were studied systematically. The effects of the inclusions by Ti-Mg/Zr deoxidization on microstructure and performances were studied by isothermal heat treatment, welding simulation and observation of CLSM. Finally, industrial test was carried to testing the process of Ti-Mg/Zr deoxidization.The required number and size of inclusions and relevant oxygen content for refining structure were calculated based on Zener equation. The nucleation and growth of inclusions were discussed during deoxidization by Si, Mn, Ti, Al, Mg, Ca and Zr based on thermodynamics and Ostwald ripening theory. It is indicated that the sequence of inclusions nucleation rate is CaOMgO Al2O3>ZrO2> TiO2> Ti2O3>SiO2> MnO, and the order of the particles size changes over time is MgO<CaO<ZrO2<Al2O3<TiO2<Ti2O3<MnO<SiO2. The experiments of deoxidization by Ti-Al, Ti-Zr, Ti-Mg, Mg and Ca were carried on the theoretical basis, whichshows that the particles by Ti-Mg/Zr deoxidization are compounded inclusions, whose number, size and stability are suitable for "oxidizing metallurgy".The calculation about Ti deoxidization in high heat input steel shows thatthe soluble oxygen should be controlled below 0.0063% to insure that Ti2O3 particles precipitate prior during melting; Only Ti2O3 particles precipitate in the front of solid-liquid phase region with the initial w[Ti]=0.005%～0.015%, w[O]=0.001%～0.005% and w[N]=0.002%～0.006% during E36 steel solidification, and TiN only precipitates in solid-state phase change; The particles by Ti-Mg deoxidization is Ti-Mg-O compound, including Ti2O3, Mg2TiO4 and MgO during steel making; The inclusions change from Mg-rich compound to Ti-rich compound with soluble oxygen changing from 0.001% to 0.005%; the deoxidized inclusions are Mg2TiO4 and MgO at initial soluble oxygen of 0.005%; the soluble Al is easy to react with Mg and O to produce MgAl2O4, which is harmful to "oxide metallurgy" as well as liquid inclusions.The experiment of Ti-Mg/Zr deoxidization shows that the optimum Ti content exists at a certain Zr content; the optimum Ti content is 0.012% at Zr content is 0.005%, and the Ti-Zr-O deoxidized inclusions are dispersive and fine, which promote acicular ferrite (AF) nucleation and growth to improve the toughness in HAZ at low temperature; the sample with 0.012%Ti-0.005%Zr achieves 273 J impact energy at 253 K in HAZ with 150 kJ/cm heat input simulation welding; the optimum Mg content exists at a certain Ti content; the optimum Mg content is 0.0018% at Ti content is 0.012%, and the Ti-Mg-O deoxidized inclusions are dispersive and fine, which promote AF nucleation and growth to improve the toughness in HAZ at low temperature. The sample with 0.012%Ti-0.0018%Zr achieves 283 J impact energy at 253 K in HAZ with 150 kJ/cm heat input simulation welding.Isothermal heat treatment and high-temperature metallograpic observation show that:the overall trend of austenite grains is same in the samples deoxidized by Ti, Ti-Zr and Ti-Mg at a certain cooling rate, that is, the high the temperature and the longer the holding time, the more coarse the austenite grains are. Due to the greater activation energy of grain growth in samples deoxidized by Ti-Mg and Ti-Zr, the austenite incubation period in samples deoxidized by Ti-Mg and Ti-Zr is longer than that in sample deoxidized by Ti at same isothermal temperature, and the sharply-coarsening temperature is increased from 1373 K to 1573 K at the same keeping time. The finer and denser particles are, the stronger pinning effect is, and the more the nucleation sites are for acicular ferrite forming, so the higher acicular ferrite ratio is in microstructure.Industrial test shows that both the mechanical property of the steel deoxidized by Ti-Mg/Zr and the toughness in its HAZ meet the national standard. The absorbed energy at 253 K is 57 J with 150 kJ/cm simulation welding in HAZ of the steel deoxidized by Ti-Zr, while the absorbed energy for the steel deoxidized by Ti-Mg is 170 J with 410 kJ/cm welding, which is far more than the standard value of 41 J. |
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