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-

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Fig. 1.13 Melting under OsciHationThe experimental rig used for melting rate measurement under oscillation is shown In Fig. 3. 14

specific heat, J kg−1 K−1 h heat transfer coefficient, W m−1 K−1 Bhilai Steel Plant produces plates of boiler, high DH f latent heat of solidification, J kg−1 tensile and other special quality steels via the basic k eff effective thermal conductivity, W m−1 K−1 oxygen furnace-vacuum arc degassing-continuous q S local heat flux in mould/strand surface area, casting route. These plates need to be ultrasonically W m−2 sound. However, rejection of plates on account of S source term of governing heat flow equation, internal defects was high. Metallurgical W m−3 investigations of defective plates showed the T temperature variable, K presence of a martensitic zone associated with T a ambient temperature, K cracks around the centreline. The martensitic T S surface temperature of strand, K transformation is favoured by segregation of T W water temperature, K carbon and other elements. The published literature U casting speed, m min−1 confirms that, even at a normal cooling rate, W water flux, L m−2 s−1 martensite formation is possible when the x, y, z distance along x, y, z axes, respectively, m concentration of carbon at the central region is high. Bhilai Steel Plant slab casters have certain c S solid fraction of mushy zone inherent deficiencies such as a high roll pitch and e emissivity of oxidised iron surface low machine rigidity, which lead to strand bulging. r density of steel, kg m−3 High strand bulging causes centreline segregation. s Stefan-Boltzmann constant=5•67×10−8 W Segregation during solidification can be controlled m−2 K−4 by various techniques including low superheat casting, electromagnetic stirring, mechanical soft INTRODUCTION reduction, and thermal soft reduction. As a major revamping of the caster and the introduction ofThe continuous casting facility at Bhilai Steel Plant comprises additional hardware call for a large capital four single strand slab casters and one four strand bloom investment, it was planned to improve the cast caster. The continuously cast blooms are used for the proproduct quality using a less capital intensive option, duction of rails. The slabs are inspected, scarfed if necessary, thermal soft reduction. In thermal soft reduction, and rolled into plates. cooling is intensified close to the final stage of Almost 100% of plates of special quality grades are subsolidification. This creates a thermal compaction jected to an ultrasonic test (NDT or non-destructive test) for which allows the segregated liquid to move inspection of internal quality. Among various special quality upwards and dissipate. The intense cooling also plates, ASTM 537 Class I boiler quality grade exhibited high strengthens the solid shell, leading to a significant levels of internal defects leading to ultrasonic test failure. reduction of bulging and thereby reducingThe causes of the internal defects have been analysed and macrosegregation. In the present investigation, remedial measures have been taken without any capital the secondary water distribution pattern has investment. These measures have increased ...

Internal and surface quality of line pipe (API) grade steels are very important in addition to the various property requirements such as YS and UTS, YS/UTS ratio, elongation, fatigue strength at zero and sub zero temperature and weldability. Surface cracking is predominant in these steel grades because of the presence of various micro-alloying (Nb, V, etc.) elements for their precipitation in various forms like nitrides and carbo-nitrides at the prior austenite grain boundaries during continuous casting and making the solidified steel brittle. Besides this, internal soundness in terms of ultrasonic nondestructive testing is another important requirement of line pipe plates. High rejection of plates of these grades was experienced in the initial phase of production in terms of surface defects (cracks and entrapment of non-metallics) and internal defects (ultrasonic testing) including low yield at slab stage. These led to low yield of the final product. In view of this, development work was undertaken at Bhilai Steel Plant to improve processing technology to achieve higher yield of API grade plates. This paper discusses the reasons of low yield of slabs and plates in the initial stages of production and measures taken in continuous casting parameters to improve the same including the acceptability of final plates.

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