Correlation of the Solidification Path with As-Cast Microstructure and Precipitation of Ti,Nb(C,N) on a High-Temperature
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DURING solidification, macroscopic and microscopic partition of the solutes between the liquid and the crystals formed in the solid phase occurs. This phenomenon leads to a nonuniform distribution of the chemical elements in the solid, which remains in the final product[1] even after normal industrial processing. The heterogeneity formed during solidification is called ‘‘segregation’’ and usually occurs in microscopic scale. Depending on the fluid dynamics during solidification, significant movements of the segregated liquid may also cause macrosegregation,[2–4] which can achieve scales close to the dimensions of the slabs, around centimeters or meters in the case of heavy ingots. In the case of
D. PE´REZ ESCOBAR, C.S. BATISTA CASTRO, E. CAVICHIOLI BORBA, and M. SPANGLER ANDRADE are with the SENAI Innovation Institute in Metallurgy and Special Alloys, Center for Innovation and Technology SENAI FIEMG, Av. Jose´ Caaˆndido da Silveira, 2000 Horto, Belo Horizonte, MG, Brazil. Contact e-mail: descobar@fiemg.com.br A.P. OLIVEIRA and K. CAMEY are with Gerdau Ouro Branco, Ouro Branco, MG, Brazil. E. TAISS is with Companhia Brasileira de Metalurgia e Minerac¸a˜o (CBMM), Araxa´, MG, Brazil. A. COSTA E SILVA is with Escola de Engenharia Industrial Metalu´rgica de Volta Redonda EEIMVR, Universidade Federal Fluminense UFF, Volta Redonda, RJ, Brazil. Manuscript submitted January 08, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
continuous casting slabs, macrosegregation is usually more relevant in the centerline of the products.[2] While microsegregation may be reduced by long heat treatments, which are usually economically inviable and depend on the elements, their mobility and treatment temperatures, it is almost impossible to eliminate macrosegregation. Macrosegregation leads to great chemical variation through the material, causing relevant changes in the microstructure and mechanical properties.[5] High-performance steel thick slabs usually present higher levels of Mn, which cause undesired effects related to the segregation levels during solidification. Banding and uneven formation and distribution of MnS inclusions,[6] which affects the mechanical properties and hydrogen embrittlement resistance in sour environments of the final product, are some of the problems associated with this segregation. Microalloying elements, such as titanium, niobium, and vanadium, are usually added to steels aiming at improving grain refinement or causing precipitation hardening.[7] Niobium is known to significantly improve the mechanical properties in highstrength low alloy, even at small levels.[8–10] At temperatures between 1000 C and 1300 C, Nb is known to retard austenite recrystallization and inhibit grain growth. At lower temperatures, it precipitates as carbonitrides, inhibiting austenite recovery, recrystallization, and grain growth prior to the c/a transformation;
this increases the density of ferrite nucleation sites and, thus, reduces the final ferrite grain size.[11] However, segregation heterogeneities, associated with the enrichment
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