The effects of V, Ti and B on the synthetic HAZ structures and toughness of 80 kgf/mm grade HSLA steels (80HSLA) have been investigated. With addition of V, Ti and B at the content described in this paper, the low temperature toughness of 80 HSLA HAZ at high heat input has been improved remarkably. If heat input is 5 kJ/mm and 4 kJ/mm, the V-notched impact toughness of HAZ at -40 C is increased to 70.6 J/cm and 95.1 J/cm in the V-Ti and B-Ti bearing steels respectively, which are 1.5 and 6 times of that without micro-alloy elements. The experiments reveal that the formation of much trapezoidal ferrite because of the addition of V-Ti is one of the major reasons for the good HAZ toughness. This kind of ferrite can prevent the propagation of cracks because of its special boundary structures, arrangement and small size. It has been proved that the granular bainite with trapezoidal ferrite matrix has a good toughness. With different welding parameters, it depends upon the degree of lattice distortion caused by B atoms and the stress relaxation during phase transformation whether the addition of B can improve the HAZ toughness. When the heat input varies from 2.5 to 6.0 kJ/mm, the B-alloyed steel has a good low temperature toughness. Both of V and Ti can mutually improve the HAZ toughness strongly. Therefore, the combination of V-Ti is called "Promoting Type Combination". Ti can compensate for effects of the harmful aspect of B. The combination of B-Ti is called "Compensating Type Combination". The effects of micro-alloy elements on HAZ toughness of HSLA steels is explained by the kinetics of phase transformation for the first time. The formation mechanism of bainite trapezoidal ferrite has been also discussed. V, Ti can increase the number of C-poor areas, prevent the growth ledges of ferrite from moving. They therefore can prevent ferrite laths from growing longitudinally. So, the trapezoidal ferrite can be formed.
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