Mechanism of crack initiation and propagation in high-alloy steel slabs during the cooling and scarfing processes after the continuous casting process

Although remarkable progress has been made in manufacturing high-strength and lightweight steel materials for different applications, the steel manufacturing process is still susceptible to various issues and limitations. Typically, the initial step involved in manufacturing most alloys is the continuous casting process, which involves forming slabs and solidifying molten alloys. Most studies focus on microstructural changes and the formation of cracks during continuous casting processes. Nevertheless, there are limited studies on the formation of cracks during cooling process following continuous casting.

The formation of cracks and fractures in slabs during cooling is prevalent. Most cracks reported in steel slabs are mid-face, transverse and corner cracks. After continuous casting, steel slabs can be cooled using either slow or fast cooling methods. Slow cooling entails stacking of slabs, while fast cooling consists of lateral placement of the slabs individually on the ground. The cooling process is characterized by phase transformation that differs depending on the cooling rate. The slow cooling rate is mostly dominated by austenite to pearlite transformation and less dominated by bainite and pearlite phases.

Currently, different alloying elements like chromium, boron and titanium are added to high-carbon steels to achieve high-strength and lightweight alloy. Unfortunately, cracks occur in most of these steel alloys during cooling after preceding the subsequent manufacturing process after continuous casting. Although high manganese and carbon contents have been speculated to initiate transverse cracks, the underlying mechanism responsible for crack initiation during cooling processes is still poorly understood.

On this account, Assistant Professor Yong-Seok Lee from The University of Suwon and Professor Soon-Bok Lee from Korea Advanced Institute of Science and Technology (KAIST) in collaboration with Dr. Seongwoo Kim from POSCO Technical Research Laboratories and Assistant Professor Dong-Won Jang from Myongji University investigated the underlying mechanism of thermal crack initiation and propagation in high-alloy steel slabs during cooling process using finite element analysis. The work is currently published in the journal, Mechanics of Materials.

In their approach, the fracture shape of the slab was confirmed using 2000 MPa hot press forming steel. The research team evaluated the phase transformation, impact toughness and fracture toughness of six different steel alloys. Each toughness was measured by altering the chemical composition of the slab. In addition, the scarfing slabs and subsequent scarfing process were simulated to determine the alloying element facilitating crack formation and growth.

The authors reported that cracks were mostly formed at the corner of the slab during cooling process, allowing the prediction of fracture occurrence probability. During the cooling process, the steel slabs experienced larger thermal stresses, although this is not the case for all high-alloy steels. Consequently, fractured occurred following the formation of transverse cracks on the steel slab surface during scarfing process.

The addition of boron and chromium to the steel increased the likelihood of fracture occurrence, while the magnitude of the negative coefficient of thermal expansion for the samples added to boron and chromium increased than those of other alloys subjected to similar austenizing conditions. In natural cooling, however, the cracking probability was higher after scarfing process when the steel contained 0.3 wt.% carbon content without other additives like boron, niobium and titanium. These are crucial factors responsible for the initiation and growth of cracks in steel slabs. Furthermore, crack formation mechanism was proposed. Adjusting ambient temperature or processing the slab before completing cooling were identified as possible ways of preventing cracking during cooling.

In summary, the causes of random formation and growth of cracks in high-alloy steel slabs during cooling and scarfing processes after continuous casting process were evaluated, identified and determined. There were high possibility of cracks occurring differently during cooling and scarfing processes. After scarfing, cracks were predominately caused by martensization and austenitization processes. In a statement to Advances in Engineering, first author Assistant Professor Yong-Seok Lee who is currently affiliated with the Department of Mechanical Engineering, School of Industrial and Mechanical Engineering at The University of Suwon explained their findings will advance developing effective methods for preventing crack formation in steels.