Study on Surface Longitudinal Crack Formation of Typical Hypoeutectoid Steel Produced on a Caster with Billet and Slab

Herein, the precipitation characteristic of sulfide is investigated by confocal laser scanning microscopy for a typical medium‐carbon sulfur‐containing 42CrMo steel under different cooling rates (100, 200, 600 °C min−1) and sulfur contents (0.0018 and 0.0249 wt%). Thermodynamic calculations show that increasing sulfur content can enlarge the liquid–solid‐phase zone and enhance the initial precipitation temperature of MnS in 42CrMo steel. Phase diagram of Mn–Mo–S demonstrates that the sulfides contain Mn, Mo, and S elements can be formed. The experimental results indicate that with an increase of the sulfur content from 0.0018 to 0.0249 wt%, more (Mn, Mo)S sulfides are formed in the steel due to the significant enhancement of supersaturation. Moreover, it is revealed that the average size of (Mn, Mo)S inclusions decreases from 16.11 to 3.55 μm when the cooling rate increases from 100 to 600 °C min−1. The typical of large‐size rodlike (Mn, Mo)S inclusions is observed in a low cooling rates of 100–200 °C min−1. As the cooling rate increases to 600 °C min−1, smaller globular (Mn, Mo)S inclusions with an average size of 3.55 μm are appeared which can act as a core to promote the ferrite formation.

Section: Introductionmentioning

confidence: 99%

Precipitation Characteristic of Sulfide in Medium‐Carbon Sulfur‐Containing Steel

Wang

et al. 2023

“…[ 41,42 ] In addition, it has been reported that proeutectoid ferrite is harmful to the toughness of steel, cracks can be initiated by proeutectoid ferrite. [ 42–45 ] Thus, it is of significant importance to control the precipitation of chain ferrite and thus inhibit the crack propagation during hot‐rolling process.…”

Section: Resultsmentioning

confidence: 99%

Mechanism of Crack Propagation during Hot‐Rolling Process of a Medium‐Carbon 40Cr Alloy Steel

Wang

et al. 2023

In this work, the mechanism of crack propagation during hot‐rolling process of a typical medium‐carbon 40Cr alloy steel is investigated by cracks characterization, thermodynamic calculation, and confocal laser scanning microscopy (CLSM). The depth of cracks is about 2.5 mm and its length along with rolling direction can even reach 2000–3000 mm. Thermodynamic calculations show that the oxide phases including MnSiO3 and MnCr2O4 can generate when the oxygen content is 0.3–1.0 wt%, suggesting that low oxygen is beneficial to the selective oxidation of Si, Mn, and Cr elements from the medium‐carbon low alloy. Furthermore, in situ experiment by CLSM indicates that the submicron Cr–Mn–Si–O particles can refine austenite grains. In addition, the contents of chain proeutectoid ferrite in the steel containing Cr–Mn–Si oxides increase by 6.3% and 12.0% at the lower cooling rates of 5 and 10 °C min−1, respectively, comparing with that of no‐oxide particles steel. The submicron Cr–Mn–Si–O particles can refine austenite grains, which induces the precipitation of chain proeutectoid ferrite. Thus, the serious surface cracks propagate along the chain proeutectoid ferrite with the submicron Cr–Mn–Si–O particles during the hot‐rolling process.

“…The temperature control scheme of the thermal expansion experiment ( Figure 7 a) was based on the results of the simulated bloom surface temperature during continuous casting. [ 32,35 ] The sample was maintained at 1100 °C for 10 min to homogenize the temperature and cool to room temperature at three different cooling rates. The linear shrinkage of Steel I during phase transformation is 0.05% greater than that of Steel II, resulting in drastic volume changes of Steel I during phase transformation and easy formation of new cracks or propagation of existing cracks (Figure 7b).…”

Section: Resultsmentioning

confidence: 99%

“…[ 28 ] Nonetheless, in actual production, a large number of surface cracks are generated in the bloom of sulfur‐containing steel when a specific Mn/S ratio is met. [ 31,32 ] Therefore, the formation mechanism of surface cracks in the bloom of medium carbon sulfur‐containing steel requires further studies.…”

Section: Introductionmentioning

confidence: 99%

Effects of Sulfur on the Surface Crack in Medium Carbon Bloom

Lü

Zhang

Zhao

et al. 2022

Self Cite

The surface cracking problem in medium carbon sulfur‐containing steel is gradually emerging in continuous casting production. Therefore, this article investigates the effect of sulfur content on the surface cracks of medium carbon steel. In addition to macroscopic cracks, network cracks are visible on the surface of medium carbon sulfur‐containing bloom. 3D confocal laser scanning microscopy and industrial computed tomography are used to characterize the 3D realistic morphology of the network and macroscopic cracks. Thermodynamic calculations and hot ductility tests’ results exhibit the higher precipitation of MnS in the sulfur‐containing steel, resulting in the second brittle zone of steel. The microstructure near the cracks revealed a high correlation between cracks and a high proportion of proeutectoid ferrite near the surface of the medium carbon sulfur‐containing bloom. In addition, the sulfur‐containing steel has 0.05% more linear shrinkage than the low‐sulfur steel during the transformation from austenite to ferrite, which increases its crack sensitivity. As a result, this article clarifies the effect of sulfur on the surface crack of medium carbon bloom.