EFFECT OF SLAB PULLING RATE ON FRACTURE CHARACTERISTICS AND NOTCH IMPACT TOUGHNESS OF SLAB SURFACE ZONE IN Ti-Nb MICROALLOYED STEEL

Abstract

The paper analyzes the marginal cut-outs from two slabs of Ti-Nb microalloyed steel. The first slab was transitional with initial pulling rate 0.5 m.min -1 and final pulling rate 0.8 m.min -1 . The second slab was cast at real pulling rate 1.03 m.min -1 . The V-notch Charpy impact test showed that at low pulling rate 0.5 m.min -1 , the values of impact toughness (KCV) were higher than those at high slab pulling rates. When the pulling rate was 0.5 m.min -1 , mixed fractures were obtained, occasionally ductile, but mostly brittle. On the other hand pulling at rates 0.8 m.min -1 and 1.03 m.min -1 resulted only in brittle fractures with low KCV. Comparison of the mean ferrite grain size d ST , showed that the most fine-grained structure was achieved at a low pulling rate 0.5 m.min -1 , and the coarsest structure at pulling rate 0.8 m.min -1 . Furthermore, it was shown that, at high slab-pulling rates 0.8 m.min -1 and 1.03 m.min -1 , a greater amount of tertiary cementite was found on the boundaries of ferritic grains, compared with the lowest rate of slab pulling 0.5 m.min -1 . At the highest rate of slab pulling 1.03 m.min -1 , in addition to tertiary cementite, presence particles were observed here and there on the boundaries of ferritic grains, mostly oval shaped. On the fracture surfaces of brittle samples transcrystalline cleavage facets (TCF) dominated. In small amounts smooth facet intercrystalline decohesion (FID) was also identified with max. 0.2% , and also ductile transcrystalline failure with dimples (DDTF), usually present only in the form of narrow dimple bridges between cleavage facets. Observation of failures using scanning electron microscope (SEM) revealed the presence of either clusters of inclusions or individual particles, and EDX analysis confirmed the presence of Al, S, O, Mn, i.e. most probably oxides of aluminum and manganese sulphides. Aluminum can come from poorlycleaned melt contaminated by fumes from deoxidation or from mould powders. There were also complex particles based on (C, Ti, Mn, O, Al, S, Nb, Zr, Ca) observed mainly on the TCF. Moreover, the impact of heterogeneous distribution of particles on embrittlement cannot be excluded -carbides, nitrides, and complex carbonitrides based on (Ti, Nb) (C, N).

Background of Branched Crack Formation in Surface Zone of Continuously-Cast Ti-Nb Microalloyed Steel Slab

“…Non-equilibrium microstructure observed in the marginal cut-outs arises due to increased heat dissipation. In works [9,12] it was confirmed that not only the slab cooling rate has an impact on formation of non-equilibrium microstructure, but also the slab pulling rate. The presence of coarse-grained microstructure indicative of coarse-grained original austenitic grain and larger austenite grain also promotes the formation of non-equilibrium microstructure.…”

Local Notch Toughness of Slab Surface Zone in ULC/IF and HSLA Steels

“…It has been shown that the impact toughness of cast semis depends similarly as in forgings [2] on the external conditions of the Charpy impact test, on the chemical composition of the steel and on the microstructure of the material [3]. Moreover, it also depends on the casting technology, including the pulling rate of the slab [4], cooling rate, presence of oscillation marks (OM), and inclusions. In work [5] the influence of test piece thickness on the evaluated toughness was analyzed at room temperature for different kinds of steels continuously-cast slab surface zones.…”