Hot ductility and fracture mechanisms of a C-Mn-Nb-Al steel
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ing of steel is usually performed in a curved mold, and the strand is straightened when the steel has solidified throughout its cross section. This straightening or unbending operation brings about tension stresses on the top surface of a slab. The unbending temperature lies in the range of 750 ⬚C to 1000 ⬚C, over which a ductility trough is commonly found by isothermal tension tests.[1,2] For the current continuous casting machines, the maximum bending and strain rates are 2 pct and 3 ⫻ 10⫺4 s⫺1, respectively. According to some researchers,[3] cracking of slabs begins at the roots of oscillation marks formed in the mold. These cracks propagate through the microsegregation area, which penetrates deeply into the austenitic grain boundaries. Depending on the cooling rate at the secondary cooling system, these cracks may grow and their internal surfaces may be oxidized, making it impossible for them to be healed during further hot-rolling operations. Secondary cooling and AIN precipitation are directly linked with the formation of transverse corner cracks, according to other authors.[4] Transverse corner cracks are easily formed in steels containing 0.020 mass pct or more aluminum and 50 to 60 ppm nitrogen. Precipitation of AIN is enhanced by the repetition of cooling and heating cycles, especially by a reheating treatment which includes a cool-down below 700 ⬚C. According to those authors, a secondary cooling pattern involving a temperature plateau is the most appropriate procedure in order to avoid transversal corner cracks. To assess the hot ductility of steels, a hot tensile test has been employed as the main research tool and has proven to be very useful in designing secondary cooling patterns to avoid the ductility trough at the unbending point. In this test, an initial high-temperature solution treatment is given to the tensile samples to dissolve any precipitate present (usually at around 1200 ⬚C) and to produce a coarse grain structure corresponding to that observed in cast steel before E. HURTADO-DELGADO, Postdoctoral Student, and R.D. MORALES, Professor, are with the Department of Metallurgy and Materials Engineering, Instituto Politecnico Nacional - ESIQIE, Apdo. 75-875, Mexico City, CP 07300. Manuscript submitted June 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
the unbending operation. Cooling conditions to a given testing temperature are carried out at rates similar to those usually found in continuous casting machines. Strain rates applied during the test are on the order of those mentioned previously. Ductility is measured by the reduction of area (pct RA) of the sample cross section, after the tensile test, as a function of temperature. The hot-ductility curve can be divided into a central trough of poor hot ductility and a temperature range on either side, where ductility is good. Specifically, the continuous casting of C-Mn-Nb-Al steels is very difficult due to the high propensity of these steels to form transversal cracks. Very poor ductilities within the trough are reported for these steels
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