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Failure Mode Analysis and a Mechanism for Hot-Ductility

Failure Mode Analysis and a Mechanism for Hot-Ductility -...

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Failure Mode Analysis and a Mechanism for Hot-Ductility Improvement in the Nb-Microalloyed Steel FARAMARZ ZARANDI and STEVE YUE Loss of hot ductility at the straightening stage of the continuous casting of high-strength low-alloy steel is attributed to different microalloying elements, in particular, Nb. However, such elements are essential for the desired mechanical characteristics of the final product. Since the chemistry cannot be altered to alleviate the problem, thermomechanical processing was studied in order to improve the hot ductility. Two Nb-microalloyed steels, one also containing B, were examined. The thermal history occurring in the continuous casting process was taken into account as well. First, it was noticed that the steel with B has a higher hot ductility than the other after being subjected to in-situ melting followed by the thermal schedule. Grain boundary sliding was recognized as the failure mechanism. Then, the effect of deformation applied in the vicinity of the . transformation, while the ther- mal schedule was being executed, was investigated. Such deformation appeared to improve the hot ductility remarkably. Finally, the mechanism of such improvement in the hot ductility was elaborated. d g I. INTRODUCTION C ONTINUOUSLY cast steel, whether cast in a curved or straight and vertical mold, eventually has to be straight- ened horizontally when it has solidified throughout its cross section. During this straightening operation, a tensile strain of about 1 to 2 pct at a strain rate between 10 3 and 10 4 s 1 can be generated on the top surface of a slab/billet. [1] Loss of hot ductility during the straightening operation, where the surface temperature can vary from 700 °C to 1200 °C, has been a serious problem in carbon and low alloy steels since it is associated with transverse surface cracking. Even though extensive work has been done to resolve this problem, hot cracking still persists. The process of crack removal (scarf- ing) interferes with productivity, and cracks can even lead to scrapping of a coil. In addition, a current trend in steel processing technology is to integrate the rolling process with the continuous casting process through “direct rolling” (rolling of hot slabs without reheating) or “hot charging” (charging of hot slabs into the reheating furnace). This does not allow for any tolerance of surface cracks, since there is no interruption between casting and subsequent hot rolling processes for inspection and scarfing. Generally, the mechanisms of the hot ductility loss in steel have been attributed to the grain boundary, or the region adja- cent to the grain boundary, which can be weaker than the grain interior. [1–7] This leads to strain concentrations at or near the grain boundary and, consequently, grain boundary decohesion.
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