Papers by Keyword: Hot Shortness

Abstract: Increasing scrap usage in steelmaking is vital for resource efficiency and CO₂ reduction, but elevated residual copper limits adoption due to hot shortness during hot forming. Conventional continuous casting promotes Cu segregation in interdendritic regions, and subsequent slab reheating accelerates oxidation-driven Cu enrichment at the steel–scale interface, where liquid Cu penetrates grain boundaries and weakens cohesion. Twin-roll casting (TRC) offers a promising alternative, as its high solidification rates suppress Cu segregation and its near-net strip production eliminates slab reheating and minimizes oxidation. In this work, the hot-shortness resistance of a 0.75 wt.% Cu construction steel processed by TRC is evaluated and directly compared with a conventionally cast and reheated counterpart. The comparison reveals that TRC effectively mitigates copper-related damage mechanisms. Cu remains primarily in the thin scale without penetrating the substrate, enabling hot rolling and downstream processing without cracking. In contrast, the conventional route forms a thick, brittle, Cu-rich scale that promotes grain-boundary penetration and severe hot shortness. Overall, TRC expands the allowable copper content in flat steel production and broadens alloy design opportunities for scrap-based steelmaking.

Abstract: A novel concept of hot shortness control on copper containing weathering steels is proposed. The weathering effect of copper addition in steels and its improvement in the mechanical and impact properties by age hardening are well known. Again, it is also very well know the problem of cracks on hot processing of those steels, caused by liquid copper segregation, the “hot shortness”. Hot shortness is the main problem of commercial producing of copper containing steels because of the increase of cost production caused by cracks the Ni addition to prevent them. Recent research found that copper segregates surrounding MnS inclusions in the steel, worsening the copper segregation caused by the preferential oxidation of iron on the hot processing of copper containing steels, which is the traditional cause of the “hot shortness”. The same results also show that copper additionally precipitates as CuS in the bulk of the steel. This work points that the “hot shortness” can then be prevented by reducing the Mn in the steel, rather than by the expensive Ni addition. Reducing Mn means reduction of inclusions of MnS sites for copper segregation in the steel. Residual sulphur can therefore be trapped by the copper as finer CuS precipitates, so copper is then dissolved in the bulk of the steel rather than segregated. This paper introduces an alternative solution for the problem of the “hot shortness” by partially or fully replacing the Mn by Cu in those steels. It is then proposed a novel concept of low Mn, or Mn free, Cu containing weathering steels in which Cu replaces Mn in trapping the S. That new steel opens the possibility to a cheaper and easier controls of the “hot shortness” so the copper contamination from scraps become a technological benefit because of its age hardening and weathering effect on steels. The recycling of highly copper contaminated scraps is another important environmental advantage. The conclusion of this work is that the current results introduces a new technology of cheaper copper containing low Mn, or Mn free, high strength, environmentally-friendly weathering steels with high potential market for different welded on shore and offshore specifications.

Abstract: Since copper content in steel causes hot shortness, it is important to understand copper behaviour during high-temperature oxidation, in order to control the precipitated copper. This study examines copper distribution during the oxidation of steel. From the oxidation tests, it is shown that precipitated copper existing in the scale/steel interface is absorbed into the magnetite layer or evaporates into the atmosphere. Then, a proposed method to suppress hot shortness is tested by oxidation-tensile tests at high temperature and is proven to be effective.

Abstract: The surface cracking behaviors of Cu-bearing steels were studied under similar conditions of the direct hot charging processes. Several specimens with various Cu contents from 0.11 to 0.38% were prepared, the oxidation tests were performed at the temperature of 1100, 1150 and 1200°C, and the hot compression tests were conducted at 1050°C using the Gleeble 3500 in order to examine the Cu induced surface cracks. The surface cracks increased gradually as the Cu-rich phase above the critical level increased. The critical level of Cu-rich phase for the specimens of high Cu contents (0.38%) reaches with a thin scale layer. However, the specimens with lower Cu content (0.13%) the critical level of Cu-rich phase occurred with a thick scale layer. The oxidation potentials of steel were affected by temperature, time, atmosphere and other elements. The oxidation rates of Cu bearing steels decreased with increasing Cu contents. It is suggested that the oxidized scale thickness is the one of important factors inducing the surface cracks by Cu contained steels in direct hot charging process.

Abstract: Steel produced in Electric Arc Furnaces (EAF) contain a high amount of copper that causes a detrimental surface cracking phenomenon called hot shortness. Studies have found that nickel can alleviate hot shortness by increasing copper solubility in the Fe phase, decreasing oxidation rate and promoting occlusion [1-3]. Occlusion is a phenomenon whereby the copper-rich phase becomes incorporated into iron oxides. Nickel promotes occlusion by causing an uneven interface and increasing the number of internal oxides. The uneven interface is likely a result of the two concentration fields resulting from ternary diffusion of nickel, copper and iron in the Fe phase. This work is aimed at explaining why nickel causes wavy oxide/liquid-Cu and liquid-Cu/Fe interfaces. Constitutional super-saturation criterion [4] was applied to explain uneven interfaces caused by nickel. A model simulating diffusion behaviors of copper and nickel in Fe was developed by coupling Comsol Multiphysics® and Matlab®. Interface concentrations of copper and nickel and perturbation criterion values were calculated as a function of time. Modeling results show that (i) the nickel interface concentration first increases to a peak value then decreases slowly during oxidation process as a result of the change in oxidation rates, and (ii) the alloys with higher nickel contents have more potential for interface breakdown and this occurs within the initial linear oxidation regime.

Abstract: The residual element copper in recycled steels embrittles grain boundaries, causing a surface cracking phenomenon known as hot shortness. Embrittlement is caused by a copper-rich liquid phase that forms at the oxide/metal interface during steel oxidation. Another residual element, nickel, enriches along with copper and reduces hot shortness cracking. The mechanisms by which nickel affects copper enrichment behavior have not yet been adequately studied. This work examines the effects of nickel and copper on the oxidation behavior and oxide/metal interface microstructure of iron. Iron-0.3 wt% copper alloys containing 0.1 wt% nickel and 0.05 wt% nickel were compared. Pure iron was used as a reference material. Alloy samples were oxidized in air at 1150°C. The parabolic oxidation rates for both alloys were found to decrease by a factor of two from that of pure iron. Both alloys had perturbed oxide/metal interfaces consisting of alternating solid/liquid regions. The interface development is due to stabilization of perturbations in the solid/liquid interface. The interface morphology can also explain the observed decrease in oxidation rate.

Abstract: The presence of the residual element copper in recycled steels causes a surface cracking phenomenon during thermo-mechanical processing which is known as “hot shortness”. The cracks result from a copper-rich liquid that forms at the oxide/metal interface and subsequently embrittles austenite grain boundaries. Minimizing formation of the liquid phase would reduce or eliminate cracking. Thus, the evolution of the liquid layer is an important consideration when designing an optimal thermomechanical processing cycle in scrap-based steel plants. The time evolution of the liquid phase is dependent on the competing processes of enrichment rate due to iron oxidation and the rate of copper back-diffusion into the steel. This paper presents a fixed grid finite difference model that predicts the evolution of the enriched region as a result of a given oxidation kinetics and solution of Fick’s 2nd law. The model predictions are in agreement with measured data for the case of an iron alloy containing 0.3 wt% copper oxidized in air at 1150°C. Model predictions indicate that initial copper content, oxidation kinetics, and alloy microstructure (i.e. grain boundary diffusion) have the most significant influence on the copper-rich layer whereas the solubility increase due to nickel additions was not found to have an appreciable influence.

Abstract: The recycling of scrap steels can be difficult due to the tramp elements that they can contain. During the steelmaking process, tramp elements such as Cu and Sn are difficult to be removed; and it is these elements that cause surface cracking of steels during hot rolling process (i.e. Cu and Sn liquid embrittlement).The paper consists of three different experiments into the suppression of surface cracking during the hot rolling process. For the oxidation in air, the surface cracking most severely occurred in the specimens which were oxidized around 1100°C in the tested range of 950-1200°C after a 1250°C heating. For the change in oxidation atmosphere from air to water vapor, the surface cracking occurred more severely although the mass gains were smaller in water vapor than in air oxidation. For the addition of Si and Ni in the water vapor conditions of 0%-30%H2O, the surface cracking was found to be suppressed effectively when the mass gain increased. The Cu and Sn enriched alloys at the scale/steel interface were closely observed by optical microscopy and scanning electron microscopy. The mechanism for suppression of the surface cracking was explained in terms of back diffusion of Cu and Sn into the steel and/or occlusion of Cu and Sn into the scale through the development of a rugged scale/steel interface.

Abstract: Copper and nickel are accumulated in steels when steel scrap is used as steel sources. It is well known that copper causes hot shortness problem and nickel suppresses the effect of copper. In this paper, the behaviour of copper and nickel during oxidation is investigated. Steels containing copper and nickel were oxidized and the distribution of copper and nickel in the scale was examined. It was found that copper is not only enriched at the scale/metal interface but also exists in upper magnetite layer as a state of solid solution and along grain boundaries of the wustite layer as metal phase. From these results an assumption has been proposed that the liquid copper migrates from the scale/metal interface to the magnetite layer along the grain boundaries. On the other hand, nickel enriched in steel side near the scale/metal interface with copper. The metal particles containing nickel and copper remain inside the scale. Nickel also has an effect of the uneven scale/metal interface formation.

Abstract: Recent 7xxx aluminum alloys have been designed for the finite use of thick semiproduct with contolled amount of constituent phases which mostly evolve during ingot preheat. In this study, the effects of constitutional change and preheat conditions of 7175 and 7050 type alloys on the evolution of constituent phases [M-, T-, S-phase and dispersoid] are presented. The constiuents evolve depending on the constitutional effect, primarily the change of Zn:Mg ratio, preheat condition comprising temperature and cooling rate following preheat. T- and M-phase are reprecipitated during cooling after preheat, depending on the alloy constitutions. S-phase is evolved depending on the constitution and preheat temperature, rather than preheat cooling rate. Prominent precipitation temperature interval of constituents are discussed in view of quaternary phase evolutions. In addition, evolutions of dispersoids together with M-phase are discussed. Specific alloy designs and preheat conditions could provide controlled microstructures for the thick 7xxx semiproducts.