Microstructure And Quench Cracking Mechanisms Of NP550 Boron Steel

NP550 boron steel is a high-strength low-alloy steel,which is widely used in armor bulletproof fields such as vehicles and ships.Adding trace boron(15-30 ppm wt.%)could produce a positive effect on hardenability and resulting in quenched structure with full martensite.However,the distribution of boron and even the final properties of quenched boron steels are seriously affected by quenching processes,such as austenitizing temperature and cooling rate.Therefore,it is of great significance to study the distribution status of boron in quenched boron steels.In this study,a healthy quench NP550 boron steel and a cracked quench boron steel have been researched to explore the forms and distribution of boron in quench martensite steels and find its influence on the mechanical properties and quenching cracking behavior.A comprehensive comparative analysis about healthy and cracked quench steel were carried out by metallography,scanning electro microscopy,electron backscatter diffraction,transmission electron microscopy and atom probe tomography.Scientific guidance is provided for optimizating quenching process and developing the high-strength boron steel from the perspectives of mechanical properties,microstructure,and element distribution.Analysis of the data leads to the following conclusions:(1)The mechanical properties of the healthy quench NP550 steel are even along the thickness direction,with high strength,plasticity and good impact toughness.The prior austenite grain size of the healthy quench steel is 6.4 μm and full martensite without precipitates was obtained after quenching.Boron only segregates at the prior austenite grain boundary in healthy quench steel and the solute excess is 2.22 n/nm2 resulting in good hardenability and mechanical properties with full quench martensite.(2)The morphologies of fracture surfaces of the quench cracked steel are obvious brittle.There are micro-cracks at prior austensite grain boundaries and extending along the grain boundaries.Macro-cracks expanded from the center region of cracked steel to the surface region.The tensile properties and impact properties of the quench cracked steel are much lower than the healthy quench steel.Whats more,the impact properties are unevenly along the thickness direction and the impact toughness of the center is worse.(3)Compared with the healthy quench steel,changes in microstructure of the cracked quench steel are as followed:(1)The prior austenite grains were coarsened obviously(especially in the center region);(2)Few ferrite precipitating along the PAGBs and there are more ferrite in the center region;(3)A large amount of nano Fe3 C and Fe3(C,B),Fe(C,B)2boron-contained precipitates and the lever of precipitation in the center region is more severe;(4)The solute excess of B on PAGB at the surface is 0.10 n/nm2.The segregation of boron along the austenite grain boundary decreases or disappears,and there is much cementite along PAGBs instead of B segregating in the center region of the cracked steel.Changes of the abovementioned microstructure are the main reason for the reduction of mechanical properties of the quench cracked steel.Since the cooling rate of the center region of steel sheet is lower than the surface during the quenching process,it is speculated that the insufficient cooling rate is the mainreason for the change of the structure and the reduction of mechanical properties of the center region of the cracked steel and evenly causing cracking.