The ASTM A572 Gr.55 niobium vanadium steel plate is low alloy high strength steel. It is used especially for building and construction machinery parts with good weldability and toughness requirements. Besides, it is also widely used in engineering structures such as building steel structures, construction machinery, mining machinery, heavy trucks, mountain bridges and pressure vessels, etc.
In order to improve the low temperature toughness of ASTM A572 Gr.55 niobium vanadium steel plate, the purity is increased. The impurities such as phosphorus and sulphur in steel have great harm to the toughness of A572 Gr.55 steel. The adsorption of phosphorus in grain boundary reduces the surface energy required for grain boundary crack formation and reduces the toughness of A572 Gr.55 steel. Therefore, it is beneficial to the formation and propagation of cracks between particles.
In addition, nitrogen and oxygen in the molten state of ASTM A572 Gr.55 niobium vanadium steel plate will increase the brittle transition temperature of the steel. The collection of microinclusions in grain boundaries of A572 Gr.55 steel will expand the crack channel, and the inclusion of large particles will become the cradle of cracks.
Therefore, during the smelting process, BBN steel strengthened the removal of P and S and ensured the vacuum degree of the VD furnace, thus reducing the content of gas and inclusions in the A572 Gr.55 steel material.
With the decrease of sulfur content, the toughness of ASTM A572 Gr.55 niobium vanadium steel plate will be greatly improved, especially the lateral toughness will be greatly improved.