The Research On Strengthening And Toughening Mechanism By Adding Calcium And Magnesium Induced Intragranular Acicular Ferrite In High Stength Low Alloy Steel

High strength low alloy steel was widely used in the production of pressure vessels,bridges,locomotives,vehicles and other engineering components.However,with the continuous improvement of welding technology,in order to improve the production efficiency,the ability of high strength low alloy steel to adapt to the high heat input welding has been paid more and more attention.Using Ca and Mg oxides particles with high melting point and high stability to pin the grain boundary and to become the nuclear of the toughness organization in welding heat affected zone(HAZ),the high strength low alloy steel for high heat input welding with favorable strength and toughness,also with excellent weldability,can be obtained.In this paper,the comprehensive application of metallurgical thermodynamics and metallurgical physical and chemical methods,combined with metallographic microscope,scanning electron microscope(SEM)and energy spectrum(EDX),welding thermal simulation technology,mechanical properties and other modern analytical testing methods,the effect of Ca and Mg elements on microstructure,inclusion morphology,composition and distribution and mechanical properties of the high strength low alloy steel welding HAZ was studied.The strengthening and toughening mechanism by Ca and Mg elements induced intragranular acicular ferrite(IAF)in the welding HAZ of high strength low alloy steel was revealed.In this paper,by vacuum induction melting experiment and controlled rolling and controlled cooling process,different groups of Ca and Mg elements content experimental steel were obtained,then the welding thermal simulation was tested.Under the experimental conditions,the study concluded that the microstructure of the experimental steel welding HAZ is mainly composed of polygonal ferrite,acicular ferrite and granular bainite after the addition of Ca and Mg elements.The inclusions of effective induced IAF nucleation are obviously increased and the distribution is moreuniform.The low temperature impact toughness is significantly improved.Compared with other experimental steels,the welded HAZ comprehensive performance of adding5wt% Mg element steel is optimal.The number of effective inclusions in the unit area is the largest,up to 579.10/mm2,and most of the inclusions exist in the form of small size MgO or complex inclusions containing Mg.The low temperature impact absorbing energy is the highest,reaching 163 J.The impact fracture is mainly the dimple fracture.After adding Ca and Mg elements,the complex inclusions containing Ca and Mg elements are formed in the welded HAZ.The nucleation mechanism of IAF induced by Ca and Mg elements in HAZ is mainly the local component change mechanism and the minimum mismatch mechanism,respectively.At the same time,Ca and Mg elements and their composite inclusions in the welded HAZ of the experimental steel mainly play the role of fine grain strengthening and dispersion strengthening.