Control of transverse cracking in special quality slabs

Abstract

due to various types of surface defects, i.e. transverse and longitudinal cracks and surface entrapments from mould Rolled plates from continuously cast slabs at Bhilai powder origin. Of these, transverse cracking was of major Steel Plant have been frequently rejected due to the concern, since the average rejection of plates due to this was formation of irregular shaped fine surface cracks. more than 4%. These cracks used to appear as fine irregular These cracks originate from fine transverse cracks shaped surface cracks on the plates. Minor scarfing on the located below oscillation marks/surface depressions slab surface also revealed this defect. This clearly indicated of the cast slabs. The random occurrence of this that the origin of the transverse cracks was primarily from defect from heat to heat was of major concern to continuous casting. The principal features of this defect Bhilai Steel Plant where slabs are cast of special were: grades such as boiler, microalloying, and high (i) there is a wide variation in rejection level from heat tensile quality. The mechanism of formation of this to heat defect was identified through a detailed (ii) a high rejection heat is preceded or succeeded by metallographic study of defect bearing slabs and several zero rejection heats in the same sequence plates and analysis of rejection patterns with (iii) the occurrence of cracks were random in nature. respect to various important process parameters.Various investigators1-3 have outlined the reasons for trans-The crack formation mechanism was verified verse crack formation during continuous casting of slab and through a process perturbation trial. A defect control measures. The occurrence of transverse cracks could potential index was developed to explain the be both on the narrow side and/or at the corner of slabs.4,5 random occurrence of these cracks. The index Cracks may also be seen below the surface depression.6 incorporated parameters which influence crackThese cracks are perpendicular to the casting direction, and formation in the Bhilai casters. A significant can be seen below the oscillation marks. In general, transreduction in this type of cracking was observed, verse cracks form on the inner side of the strand surface in based on the recommendations made in this study a curved continuous caster having a single unbending point I&S/1559 between the curved region and straightening rolls.1 The above observations confirmed that the formation of trans-

Development of New Testing Procedure for Physical Simulation of the Continuous‐Casting Process for Steels

“…It is well known6–9 that the most critical part of the cast strand is its surface layer, since it is exposed to the highest temperature and mechanical loading variations. Therefore, it is essential to obtain input simulation parameters that are as close as possible to the conditions encountered in industrial applications.…”

“…From the quality point of view, the upper surface of the strand is the most problematic. In the case of reduced ductility, in conjunction with tensile stresses and a very low strain rate, there is a possibility of transverse surface cracks occurring 6–9. This leads to a smaller yield or even to the need to scrap the produced material.…”

Control of defects during continuous casting of line pipe (API) quality steels

“…The slabs were rejected largely due to surface cracks, which is common in steel grades containing Nb and V as micro alloying elements. Since the aimed carbon was in the range of 0.10–0.14% (peritectic region), surface cracking becomes more prominent [1, 2]. In addition to surface cracks, large quantities of slabs were also rejected due to entrapment of non‐metallic inclusion (NMI) [3].…”

“…Precipitation of these types of carbo‐nitrides in micro‐alloyed steel in addition to AlN precipitation is common in low carbon aluminium‐killed steel. The precipitation phenomenon and thus, high temperature embrittlement is further accentuated if the carbon content is close to the peritectic region (0.1–0.16% C) [2, 6–9]. Therefore, reduction of carbon less than peritectic range, low Al and N content in steel and proper secondary cooling to avoid of high degree of strand re‐heating as well as avoidance low ductility trough (700–850°C) at unbending point are the main considerations.…”

A Model Estimating the Slab Corner Transverse Cracking Susceptibility of Microalloyed Steel Based on Microstructure

“…1,2) In the temperature range, the hot ductility is found to be low and can be characterized by the reduction of area in a simple hot tensile test. 3,4) Hence, the hot tensile test is often used for assessing slab transverse corner cracking susceptibility during the continuous casting.…”