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The distribution of surface macro-inclusions is an important parameter that can directly influence the surface quality of IF steel sheets. In the present work, macro-inclusions > 100 μm within a 20-mm zone from the slab surface across the whole slab width were characterized by step machining methods, and the total analyzed area was 3,300,000 mm2. Three kinds of macro-inclusions were detected: bubbles (including single and aggregated), alumina associated with bubbles and refractory-related alumina. The three-dimensional distribution of surface macro-inclusions across the whole slab width was reconstructed, which showed macro-inclusions along the thickness direction almost concentrated 8 to 20 mm from the slab surface, corresponding to the center of the upper roll zone and stagnant zone below the submerged entry nozzle bottom according to the full-scale water mold model simulation. An inclusion stability model was established that indicated that increasing the flow velocity sharply decreased the stability degree at the solidification front because of the washing effect. The calculated results by this model agree with the fact that macro-inclusions were mainly concentrated in the slab center because of the low flow velocity at this location. The present work indicates that increasing the flow velocity at the solidification front as well as eliminating the stagnant zone is a potential way to improve the surface quality of IF steel slabs.

Macro-inclusions in the surface layer of slabs are mainly responsible for the sliver defects in IF steel sheets. In the present work, macro-inclusions in six kinds of unsteady cast IF steel slabs involving four unsteady states (casting start, ladle change, nozzle change and casting end) in one casting sequence were quantitatively characterized using an automatic inclusion analyzer with a total detection area of 238,048 mm2. The cleanliness of corresponding normal slabs was also evaluated as the benchmark study, and the number density (ND) of inclusions > 50 µm was used to describe the surface slab cleanliness. Generally, alumina-based and mold powder inclusions were detected, and the former type was further subdivided into alumina clusters, alumina associated with bubbles, refractory-related alumina, Al-Ti complex oxides and CaO-Al2O3-MgO. Several large-sized alumina-based inclusions were detected in the normal slabs with an average ND of 0.31 N/cm2, although they had very low total oxygen (TO) contents (10~13 ppm). No mold powder inclusions were found in normal slabs. For all the unsteady slabs, alumina-based and mold powder inclusions were both observed, and the percentage of the former kind was ≥ 90 pct. The average density of macro-inclusions for the first slab was 1.621 N/cm2, whose cleanliness was much worse than any of the other unsteady slabs. The first slab should be downgraded. For the second slab, the ND of macro-inclusions was 0.385 N/cm2, which was still larger than the normal slabs because of the lasting effect of the re-oxidation. For the penultimate, cap-off and SEN changed slabs, the density of macro-inclusions fluctuated from 0.25~0.35 N/cm2, which was close to the average level of normal slabs. However, mold flux-originated inclusions were frequently observed in these slabs because of flow pattern variations due to the casting speed change. In terms of the slab during the ladle change process, mold powder inclusions were frequently detected even though the casting speed remained constant. A mold powder entrapment mechanism was proposed, which showed the entrapment occurred owing to the flow pattern change induced by forming/dropping of SEN clogs due to re-oxidation by FeO in the ladle slag and tundish steel temperature variations during the ladle change practice. The severity of powder entrapment and re-oxidation was also given for different kinds of unsteady slabs, which provides a direct guide to treating these slabs in the steel plant.

In this study, the austenite transformation behaviour in a medium-Mn (MMn) steel is investigated during heat treatments that replicate those occurring in low-temperature hot stamping (LTHS), with the aim of better understanding this behaviour to optimise heat-treatment design. The austenitisation behaviour and critical phase transformation temperatures during the LTHS heating process and their dependence on heating conditions are investigated using dilatometry with a Gleeble 3800 thermal-mechanical simulator, covering heating rates of 1–25 °C s−1 and soaking temperatures of 630–900 °C. Both a higher heating rate and higher soaking temperature are found to be beneficial to shorten the time required for obtaining a given austenite fraction. The martensite start temperature (Ms) shows a rapid increase with increasing soaking temperatures when austenitisation is partial, and a slower increase after full austenitisation. Excellent ultimate tensile strength values of around 1750 MPa and total elongation values of around 9.3 % are obtained for the material after the LTHS heating process. A physically based model describing the austenitic transformation under these conditions has been adopted and calibrated. The model shows good agreement with austenitic transformation diagrams constructed from experimental data, and thus can function as a guide for selecting optimum heat-treatment parameters for LTHS.

This article reports the morphology, size, and distribution evolution of MnS inclusions in non-quenched and tempered steel during heat treatment. The variation of single large-sized MnS inclusions at high temperature was observed in situ using a confocal scanning laser microscope (CSLM). The slender MnS inclusions first changed to pearl-like strings. These small-sized pearls subsequently coalesced and became closer together as the temperature increased. Large-sized MnS inclusions in non-quenched and tempered steel samples with different thermal histories were investigated with respect to the evolution of their morphology, size, and distribution. After 30 min of ovulation at 1573 K, the percentage of MnS inclusions larger than 3 µm decreased from 50.5% to 3.0%. After a 3 h soaking period, Ostwald ripening occurred. Most MnS inclusions moved from the grain boundaries to the interior. The present study demonstrates that heat treatment is an effective method of changing the morphology, size, and distribution of MnS inclusions, especially large-sized ones.

The flow control devices (FCDs) in the tundish play an important role in steel cleanliness and removal of macro-inclusions. To eliminate the spot-like defects during deep-drawing process, the liquid steel samples were taken from tundish in three industrial IF steel heats, and the effect of weir structure in the tundish on the steel cleanliness were evaluated and compared with the aid of ASPEX with the total detection area of over 54 000 mm2. The results showed that non-metallic inclusions over 5 µm observed in the liquid tundish samples were categorized into two types: type 1: alumina based inclusions and type 2: calcium aluminates. Compared to the tundish without weir, the number density of macro-inclusions reduces by half when furnished with weir. In addition, several large sized calcium aluminates were occasionally found without weir whereas no calcium aluminates were detected with the weir. The mechanism of the inclusion distributions with the weir was proposed based on the experimental results, which shows that the weir act as a baffle that block the large sized inclusions from moving toward the mould. After using the weir in the actual casting practice, the occurrence ratio of spot-like defects was decreased by about 25%.

Subsurface macro-inclusions and hooks are detrimental to the surface quality of deep-drawing steel sheets. However, little is known about the relationship between macro-inclusions and hooks. Thus, in this work, two ultralow carbon (ULC) steel slabs and two low carbon (LC) aluminum-killed steel slabs were sampled to study the relationship between hooks and subsurface macro-inclusions, which were detected on the cross-sections of steel samples with an area of 56058 mm2 using an automated scanning electron microscopy/energy-dispersive X-ray spectroscopy system. Results show that subsurface inclusions larger than 200 μm were almost entrapped by hook structures, whereas the location of other inclusions smaller than 200 μm had no obvious dependence on the location of solidified hooks. Furthermore, the number density (ND) of subsurface inclusions larger than 200 μm decreased from 0.02 to 0 cm−2 in ULC steel as the mean hook depth decreased from 1.57 to 1.01 mm. Similar trends were also observed in LC steel. In addition, the detected inclusions larger than 200 μm were concentrated in the region near the slab center (3/8 width-5/8 width), where hook depths were also larger than those at any other locations. Therefore, minimizing the hook depth is an effective way to reduce inclusion-induced sliver defects in deep-drawing steels.

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tensile test. The results show that Si can promote the transformation of austenite (γ) to ferrite (α), enlarge the (α+γ) region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite (M/A) constituents, as well as the decomposition of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

The “Slag-Remaining+Double-Slag” BOF steelmaking process has been developed and applied in Shougang Corporation, Ltd., by which consumption of lime and volume of slag in BOF steelmaking are remarkably decreased. In this paper, three important technologies taken in application of the new steelmaking process are introduced: 1. To solve the two most serious problems, i.e. difficult to make fast and enough amount of deslagging and decrease the metallic Fe droplets in the slag, low basicity slag is used in the dephosphorization stage. 2. Hard blow pattern is adopted to utilize the top blown O2 jet to strengthen agitation of the bath in the dephosphorization stage, through which good dephosphorization has been obtained. (3) By speeding up the steelmaking operations and particularly better matching the BOF, secondary refining and continuous casting productions, productivity has not been reduced though the BOF tap to tap time has been increased by about 4min after using the new process.

Deep-drawing steels are widely used in the automobile industry and macro-inclusions in continuous casting slabs should be strictly controlled because they cause surface defects in the final products. In this paper, industrial tests with different casting speeds from 1.0 m.min-1 to 2.5 m.min-1 and different tundish superheats were carried out, then subsurface inclusions in high-speed casting slabs were investigated with a large detection area by the ASPEX automatic inclusion analysis system, and the effect of the casting speed and tundish superheat was studied. The results show that there are six different types of macro-inclusions found beneath the slab surface, named cluster alumina, bubble plus cluster alumina, lump alumina, cluster TiOx-Al2 O3 , entrapped mold powder and CaO-Al2 O3 -MgO, respectively. The above first three types account for about 98% of total inclusions in the slabs cast with the mold fluctuation within ±3 mm. The size of more than 80% of such inclusions ranges from 20 to 50 μ m. The number density of inclusions larger than 50 μ m decreases with the increase in the casting speed and tundish superheat. The subsurface macro-inclusions in deep-drawing steel slabs are closely related to the hook structure. Increasing the casting speed and superheat is effective for reducing the hook depth and large-sized inclusions on the subsurface of slabs.