Study On Microstructure And Boron Element Distribution Of Continuous Casting Slab And Quenching Slab Of Boron-containing Steel

Addition of trace amount of boron in steels can significantly improve the hardenability,surface hardness,as well as the strength and toughness.The beneficial effect of boron addition is contolled by its amount as well as its distribution in steels.High concentration of boron addition will not continue to increase the hardenability of the steel.On the contrary,it may lead to the formation of boron-containing precipitates and thus deterioation of hardenability,or even racking.Therefore,understanding the distribution of boron in steels is of guiding significance for industrial production as well as designing steel with strong comprehensive properties.In this paper,the microstructures of a continuous casting slab and a quenched plate of the 27 Mn B5 boron steel were investigated using optical metallography,scanning electron microscope and transmission electron microscope.The distribution of boron was studied by nanoscale secondary ion mass spectrometry and atomic probe tomography.The major results are as follows:The microstructure of continuous casting slab is mainly composed of proeutectoid ferrite and pearlite.From the center to the edge of the slab,the primary austenite grains and pearlite lamellae structure are gradually refined with the increase of cooling rate.Boron is mainly distributed in following microstructures: Ti(CN)precipitates,primary austenite grain boundaries(PAGBs)and cementite particles close to the PAGBs.At the position with slow cooling rates(center and 1/4 width position),high-density Ti(CN)particles are formed on the PAGBs and boron is enriched in these Ti(CN)particles,which consumes total boron segregation on PAGBs.At the position with fast cooling rate(surface),the precipitation of Ti(CN)is significantly reduced,and obvious boron enrichment exists on the PAGBs.It indicates that boron distribution on PAGBs depends on the formation of boron-containing Ti(CN),which is controlled by local cooling condition.The precipitation of Ti(CN)mainly occurs in the austenite temperature range.With the decrease of temperature,the proeutectoid ferrite is formed,and the grain boundary structure of PAGBs disappears.Then some boron atoms diffuse into the nearby ferrite grains.Due to the extremely low solubility of boron in ferrite,these boron atoms are finally distributed in the cementite particles formed near the PAGBs.Martensite is the main microstructure of the quenched steel plate.Through the fixedposition sampling by focused ion beam on PAGBs and martensite lath interface and atom probe tomography analysis,it is found that boron is only distributed on the PAGBs.After adding a small amount of rare earth element Ce into the steel,the quenched structure is still martensite, and a large number of TiC precipitates with Ce enrichment forms.At the same time,the distribution of boron significantly changes,e.g.its segregation on PAGBs significantly decreases.Boron exists in these TiC particles containing Ce as well as in some regions of martensitic matrix.It indicates that adding trace amount of Ce has an important effect on the distribution of boron.Ce addition may hinder the diffusion of boron to the PAGBs and make boron stay in the austenite grains,and finally Ce and B will diffuse together into the TiC particles formed in the austenite grains.