Causes of transverse corner cracks in microalloyed steel in vertical bending continuous slab casters

Abstract

Transverse corner cracks have been frequently observed and are extremely difficult to prevent in some microalloyed steels produced on some vertical bending type continuous slab casters. The cracks are usually found on the fixed (outer) side corner of the slabs through inspection of the acid pickled surface and macroscopic examination. In the present study, the slab surface microstructure was investigated, and the results show that the causes of the crack formation are chain-like precipitations and film-like proeutectoid ferrite in the austenite grain boundaries when the slabs are bent. Initially, when the temperature of the slabs dropped after solidification, the Nb, V or Ti carbides and/or nitrides precipitated in chain-like way, then pinned onto the austenite grain boundaries. This process hindered grain boundary slippage and lowered the force on the grain boundaries. Second, due to the stress mismatch between the matrix and the fine precipitates during bending operations, the chain-like precipitated carbides and/or nitrides in the grain boundaries increased the cracking susceptibility of the slabs. Meanwhile, the film-like proeutectoid ferrite precipitated along the austenite grain boundaries during the austenite-ferrite transformation disrupted the continuity of the austenite matrix. In view of the fact that the strength of proeutectoid ferrite film is lower than that of the austenite grains, when the slabs were subjected to bending stress, the cracks were generated along the film-like proeutectoid ferrite.

Effect of Different Thermal Schedules on Ductility of Microalloyed Steel Slabs during Continuous Casting

“…In the SSC temperature scheme, there is a reheating period mainly due to heat conduction (physical and latent heat) from the core to surface or corner of the slab. Combining previous research [13][14][15][16][17][18], the reheating rate of 3 • C/s is assumed in this paper. 4# sample was heated to 1350 • C, held for 5 min, cooled to 600 • C with a cooling rate of 10 • C/s, held for 2 min, reheated to 1100 • C with a reheating rate of 3 • C/s, and then quenched.…”

“…Suzuki et al [16] believed that the reconstructed austenite grains only shifted, while the grain size did not change. Owing to a low temperature stage in the thermal schedule of the SSC process, Walmag et al [13], Lee et al [17], and Ma et al [18] believed that the growth time of original austenite grains was reduced, and the new austenite grains were nucleated around the ferrite grain boundary during the process of re-austenite transformation, and the newly formed austenite grains became smaller. Du et al [19] also pointed that the austenite grain grows a little bit rather than being refined after a γ-α-γ transformation.…”

Effect of Cooling Rate on the Precipitation Behavior of Carbonitride in Microalloyed Steel Slab

“…In the formula, t c is the cooling rate, KAEs À1 ; R p is the radius of precipitation particle, nm; and A, B, C, and n are the coefficients, related to the composition of steel, cooling rate, and other thermodynamic conditions during the process of cooling. Equations [3] and [4] are expressed as follows:…”

“…The precipitation of carbonitrides accelerates the nucleation of proeutectoid ferrite and forms a wide, net-like film of proeutectoid ferrite along the grain boundary. [3,4] The slab hot ductility would be significantly reduced by the formation of this kind of microstructure, and hence, the crack would be more likely to be introduced. [5,6] It also was studied that the hot ductility of microalloyed steel was influenced by the precipitation of carbonitride's volume fraction, size, shape, and distribution, and the larger volume fraction, bigger size as well as the netor chain-like shape with nonuniform distribution are more harmful to the hot ductility.…”

A Method to Control the Transverse Corner Cracks on a Continuous Casting Slab by Combining Microstructure Analysis with Numerical Simulation of the Slab Temperature Field

“…Papaefthymiou et al through the use of SEM with EDS analysis, found that the transverse cracking is attributed to copper or casting powder entrapment, thus determining that the crack occurred in the mold. Ma et al could not find any slag inclusion in cracks, but found both the chain‐like carbonitrides and film‐like proeutectoid ferrite distribution along the grain boundaries, indicating that the generation position of the transverse corner cracks was in the bending segment of the continuous caster. Takeuchi et al found that transverse cracks exhibited an interdendritic appearance located in the trough of oscillation marks, which implied that the cracks were initiated in the mold of the continuous caster.…”