A Study to Establish Correlation Between Intercolumnar Cracks in Slabs and Off-Center Defects in Hot-Rolled Products
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Delamination and cracking related to segregations are mostly observed at the centerline of hot-rolled products. The delamination is related to heavy concentration of manganese sulfide inclusions originating from centerline segregation in slabs. In multiple cases, similar delamination or cracking is observed at locations away from the mid thickness plane of hot-rolled products during forming operations at customer end. This leads to rejection of materials. Metallographic investigation reveals segregation line with plenty of manganese sulfide stringers at the off-center location similar to observations in cases of centerline defects. Centerline segregation is a well-understood phenomenon, but the presence of off-center segregation line in hot-rolled products has not been systematically studied before. Several literatures report intercolumnar cracks in slabs to be filled with segregations. Also, they occur away from the centerline of continuously cast slabs. A laboratory-based experimental study was conducted using a slab suspected by S-printing to contain intercolumnar cracks. The aim of this study was to evaluate the effect of intercolumnar cracks on internal quality of rolled products. This paper reports the finding of this study that established a strong correlation between intercolumnar cracks in slabs and off-center cracking in hot-rolled products.
KeywordsIntercolumnar cracks Macroetching Sulfur print Segregation
The mushy zone is the region of a continuously cast strand where the solid and liquid coexist as a mixture. In the Brittle Temperature Range (Zero Strength Temperature to Zero Ductile Temperature), this zone is characterized by extremely low-critical strain to fracture . When the combined strain arising on the solidifying front due to strand bulging, misaligned segment rolls, or uneven solidification exceeds a critical strain, there are chances of forming intercolumnar cracks [2, 3, 4, 5]. Since the surrounding of the cracks contains solute-rich interdendritic liquid, there are possibilities of the cracks to get filled with the segregated liquid. However, at some locations having heavy interdendritic bonding, the flow of fluid may be blocked and the cracks may remain open and unfilled .
The susceptibility toward forming intercolumnar cracks during solidification depends majorly on the extent of Brittle Temperature Range. Segregating elements like sulfur and phosphorus lower solidification temperature of steel below its equilibrium solidus, thereby enhancing this range . In this perspective, high-carbon steels are more prone to cracking as compared to low-carbon steels because of their larger solidification range (T solidus − T liquidus). Also segregation tendencies of sulfur and phosphorus are higher when solidification occurs in austenitic mode as in case of high-carbon steels .
Several authors have studied the origin and formation of internal defects like centerline segregation and intercolumnar cracks in continuously cast slabs. Effect of centerline segregation in cast slabs on the internal quality of the hot-rolled products is also well known [6, 7]. However, the effect of the intercolumnar cracks in slabs on the internal quality of hot-rolled product is an area which has not been significantly explored before. There is a general conception that the internal cracks get welded during hot-rolling operation. This may be true for crack portions remaining open in slab and free from segregations. However, whether the same is applicable to crack portions filled with segregated liquid is a question. Through systematic laboratory experimentations, the present work is aimed to establish a strong correlation between intercolumnar cracks in cast slab and off-center defects in hot-rolled plates or strips.
Various grades of steel produced in TATA Steel, Jamshedpur require sound internal quality for precision applications like automobile shockers, chain links, long members, etc. Such grades have carbon content ranging from 0.07 wt.% C to 0.55 wt.% C. Complaints regarding off-center cracking were reported in most of these grades, and the rejection rates were around 8%. One among such grades having 0.55 wt.% carbon was randomly taken up for the study.
Technical specification of CC#3 of LD2&SC shop of TATA Steel Jamshedpur works
Curved (bow type)
Step type tundish
15° double port (Slit SEN)
Number of segments
Continuous casting parameters of the slab under study
218 mm × 1250 mm
Primary cooling water (broad side)
Primary cooling water (narrow side)
Secondary cooling water
Microscopic Analysis of the Intercolumnar Crack Portion in the Slab
The transverse and longitudinal faces of the slab block (50 mm × 95 mm × 75 mm) were prepared using Vertical milling machine (Model: VF-1). The prepared sample was then macroetched according to ASTM E340-13 standard. The block was completely immersed for 2 h in a freshly prepared HCl solution (1:1 by volume) maintained at a temperature of 70 °C. After proper cleaning and drying, the block was examined for the presence of intercolumnar cracks.
Hot Working of the Slab Sample to a Plate of 9-mm Thickness
The slab block containing the intercolumnar cracks was reheated to a temperature of 1220 °C in electric trolley furnace and soaked for 1 h. The sample was then hot worked using laboratory facility to a plate of 9-mm thickness, through multiple passes.
Macroetching of the Transverse Section of the Hot Worked Plate
Microscopic Analysis of the Hot Worked Plate
A microsample was separated from the defect portion of the macroetched hot worked plate sample. In order to identify the exact nature of the defect, optical microscopic investigation was carried out on the polished microsample in unetched and etched (2 wt.% nital) condition. The sample was then subjected to SEM-EDX analysis using FE-SEM (Model: Ziess, Vorpommern, Germany) with EDAX and X-ray mapping attachments.
Results and Discussions
Investigation of Intercolumnar Crack in Slab
Investigation of Defect in the Hot Worked Plate
The manganese sulfide stringers in the rolled plate originate from the globular sulfide inclusions observed in association with the intercolumnar crack in the slab. The presence of stringers at the defect location in plate suggests that the segregations inside and around the intercolumnar cracks do not disappear on rolling. Rather, they accumulate along some plane in the rolled product, forming a segregation band. The location of this segregation band with respect to the thickness direction of the rolled plate is dependent on the location of the intercolumnar cracks with respect to the transverse direction of the slab. During cold-forming operations, delamination or cracking may occur along this plane containing heavy concentration of segregates and inclusions.
Recommended Actions to Reduce Off-Center Cracking
From this experimental study, a strong correlation is seen to exist between the intercolumnar crack in slabs and off-center defect in hot-rolled strips. Thus, any countermeasure taken to reduce occurrence of intercolumnar cracks is likely to reduce generation of off-center defects in the rolled products.
Sulfur content of steel plays a crucial role in the generation of intercolumnar cracks. It is recommended to lower sulfur levels in steel to avoid intercolumnar cracks in slabs and thereby off-center defects in rolled products. In TATA Steel, the sulfur content is restricted to 0.005% for all grades facing off-center cracking at customer end during forming applications. Secondly, casting parameters like superheat and casting speed are similarly optimized to reduce crack occurrences in slabs. Casting speed is restricted to 1 m/min and superheat is maintained in the range of 15–25 °C. Thirdly, misaligned segment rolls in caster increase propensity for developing intercolumnar cracks. Therefore, it is recommended to cast the critical grades only after roll gaps and alignments in the caster segments are properly checked. Also, for these grades, no coils are sent to customers if its corresponding slab revealed intercolumnar cracks during S-printing.
Intercolumnar cracks are not specific to any particular grade. Therefore, chances of observing off-center defects span all grades. Similarly, recommendations for preventing off-center cracking apply to all grades. High-carbon grades are more prone to cracking and therefore forming off-center defects because of reasons already discussed. Implementing the aforesaid actions has reduced rejection rates at customer end from 8% in FY’13 to 0.3% in FY’15.
Intercolumnar cracks lead to off-center segregation bands in rolled products which may result in delamination or cracking during forming operations.
Reduction of the occurrences of intercolumnar cracks reduces chances of off-center cracking.
The occurrence of off-center cracking can be reduced by (a) reducing the sulfur content of the steel (b) properly checking the roller gaps and alignment of caster segments prior casting complaint facing grades, and (c) ensuring that coils rolled from slabs containing intercolumnar cracks do not reach customers.
The authors wish to thank colleagues from Tata Steel Ltd for their support and encouragement in completion of the work. Help received from National Metallurgical Laboratory, Jamshedpur for carrying out experiments using their laboratory facilities is also appreciated.
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