Small, the toughness will be better

The introduction of trace amounts in the steel alloying elements, the formation of high melting point dispersed particles. These particles on the one hand by "pinning" in the form of impeding the migration of austenite grain boundaries, limiting austenite grain growth, while increasing the phase transition point in the process of nucleation,

So that the steel is more small. More current research is on the high-temperature austenitic Ti element refinement. Found, Ti in the steel to   form the dispersed TiN particles in the welding thermal cycle process to effectively prevent the austenite grain growth, and promote

Into acicular ferrite, thereby improving the HAZ toughness.
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The researchers found, Nb can enhance the refinement of Ti. Nb and N in the steel also has a strong affinity, can replace part of the Ti, and N to form (Ti, Nb) N particles, the solution temperature at 1350 ℃ or more, you can nail rolling, dragging austenite

Grain boundary migration. Further research found that, Ti-Nb microalloyed steel containing a large number of small size TixNb1-x (CyN1-x) particles, the relative content of Nb particles in the 0.25 to 0.82, nearly spherical shape. High stability of these particles

Nature of these particles in the welding process can not only effectively prevent austenite grain growth, inhibit the formation of coarse bainite, but also can promote the precipitation of acicular ferrite and MA constituent elements of the decomposition, thus significantly improving the low alloy high strength steel HAZ coarse grain zone

Toughness.

2 HAZ organizations to improve

In addition to grain refinement to improve the toughness of HAZ organizations are also a way to improve the plate. When the component to determine the steel's toughness and grain size determined by the organization. The results show that when large heat HAZ after welding with a certain amount of acicular ferrite

(AF), it will have high strength and good toughness, many studies have organizations dedicated to the HAZ get AF, and AF, the nucleation mechanism and the effects of alloying elements on the organization of doing analysis.

3 alloying elements control the microstructure of      steel

Alloy by adding trace elements, can improve the toughness of steel to improve weldability. Alloying element in steel to form fine particles of compounds, not only grain refinement, but also the shape of nuclear particles as AF, AF organizations to form more, or reduce the harmful folder

Complex levels, in order to improve toughness. Ti, Nb, V study more, in addition to Ni, Mn, Al, Si, Mo, B, Cu, and RE and other elements also have a similar effect.

Studies have shown that adding Ti in favor of steel toughness increase. TiN particles can promote the precipitation of acicular ferrite. Since TiN and ferrite particles mismatch less kept coherent between the two sides, thus contributing to the growth of ferrite nuclei. Some analysts considered

Related with the expansion coefficient for this. Because     the expansion coefficient of TiN and austenite is different around the TiN particles have a greater lattice distortion, the distortion area has a large number of dislocations, the nucleation of ferrite provides location; same time, the distortion for the C atoms diffusion