Effect of Zr-Ti combined deoxidation on impact toughness of coarse-grained heat-affected zone with high heat input welding

In this study, the effects of Zr-Ti combined deoxidation and AI deoxidation on the impact toughness of coarse- grained heat-affected zone in high-strength low-alloy steels were investigated. More fine oxides were formed in the Zr-Ti-killed steel than in Al-killed steel. It was also found that more acicular ferrite grains were formed in the coarse-grained heat-affected zone in the Zr-Ti-killed steel than in Al-killed steel. The impact toughness of coarse-grained heat-affected zone of Zr-Ti-kiUed steel was higher than that of Al-killed steel. The good impact toughness was attributable to the pinning effect of fine oxides and the formation of acicular ferrite grains on fine oxides.

Development of oxide metallurgy technology at Baosteel

Oxide metallurgy technology has been developed at Baosteel for improving the heat affected zone toughness of steel plates for high heat input welding. After deoxidation with strong deoxidizers of Mg alloy, the complex inclusions containing parts of compounds of MgO,Ti2 O3,MnO, Al2 O3 ,MnS, CaS and TiN are formed. These kinds of inclusions are beneficial for promoting the formation of intragranular ferrite. After two-electrode vibratory electrogas arc welding with the V-type groove and a high heat input of 395 kJ/cm in a single pass,the former austenite grain is very fine in size,with an average grain size of only 85 μm. Excellent heat affected zone toughness is obtained for the developed steel plates with a thickness up to 68 mm.

High Strength Steel Plates for Large Container Ships

Heavy thickness steel plates with a good combination of strength,toughness and weldability have been demanded for building of large container ships.High strength steel plates with heavy gauge of EH36,EH40 and EH47 grades were developed by optimizing chemical compositions and TMCP process parameters.Micro alloying elements of Ti and Nb were added to the three steel grades for enhancing the strength and toughness of base plates.The strength of base plates of the EH47 grade was further enhanced with the help of the increased amount of substitutional solid solutes,such as manganese,copper,nickel,or chromium.EH36 steel plates for high heat input over 550 kJ/cm were manufactured by improving thermal stability of TiN particles for better weld heat-affected zone toughness.Thermally stabilized TiN particles effectively suppress grain growth at weld heat-affected zone,leading to better toughness.These steel plates showed excellent mechanical properties of base plates and welded joints.

Relationship between crystallographic structure of complex inclusions MgAl_(2)O_(4)/Ti2O_(3)/MnS and improved toughness of heat-affected zone in shipbuilding steel

In accordance with the minimum degree of disregistry mechanism in oxide metallurgy, the intragranular acicular ferrite (IAF) generated by microalloying elements in austenite was studied. Herein, the effect of Mg treatment on the microstructure and toughness of the heat-affected zone (HAZ) in shipbuilding steel was investigated. Mg treatment produced inclusions that influenced the formation of acicular ferrite in the microstructure. This refined the HAZ microstructure and improved its toughness. Electron backscatter diffraction was used to determine the oxides of titanium and the MgO Al_(2)O_(3) or MgAl_(2)O_(4) complex inclusions that induced the formation of IAF. MnS precipitated on MgAl_(2)O_(4) on a specific habit plane and in a specific direction. MnS had a specific orientation relationship with MgAl_(2)O_(4), i.e., f100gMgAl_(2)O_(4) //{100}MnS. The 35-mm-thick plate obtained in the industrial test after welding at a welding heat input of 120 kJ/cm had an average impact absorbed energy of 282.7 J at - 40 ℃ and 2 mm from the weld joint in the HAZ. The twodimensional disregistry index between inclusions can be used as the basis for controlling their distribution and adsorption force. Microalloy addition in the order of Al-Mg-Ti is key to obtaining abundant dispersion and fine nucleation in austenite.