Steel production and processing are connected with high temperatures. Due to a reaction between hot surface of the steel and oxygen contained in surrounding atmosphere, oxides are formed on the surface of the steel. Created layer of oxides is called scales and has influence on cooling and quality of steel. Thickness and structure of scale layer are influenced by chemical composition of the steel, temperature and atmosphere during oxidation. Scale layer can be considerably porous which has a significant influence on thermal conductivity of this layer, because air pores have much lower thermal conductivity compared to scales. Steel samples were prepared and porosity of scale layer was studied. Further, the average thermal conductivity of porous scale layer was determined for different regimes of oxidation by FEM modelling. It was found that the average thermal conductivity of porous scale layer is influenced not only by porosity of scale layer, but also by distribution of air pores, which can has a significant effect.
Steel production and processing are connected with the formation of an oxide layer on a hot surface of steel. The oxide layer influences cooling and the final quality of the steel. Spray cooling is mainly influenced by water impingement density and by surface temperature, but the influence of the oxide layer is not negligible. The oxide layer can be considerably porous. The porosity of the oxide layer significantly influences its thermal conductivity, because air pores have much lower thermal conductivity compared to pure oxides. In this paper, the influence of the oxide layer on water spray cooling is experimentally and numerically investigated. The heat transfer coefficient of an oxidized steel surface and a clean steel surface are compared and the effect of the oxide layer on the Leidenfrost temperature is studied. Also, the porosity of the oxide layer and the average thermal conductivity of the porous oxide layer are determined for different oxidation regimes.
Hydraulic descaling is an inherent part of the hot rolling process but can sometimes also be applied in the heat treatment process, continuous casting and other processes. The need for optimal descaling is linked with the quality of the final product. The goal is usually simplified to the complete removal of the scale layer from the hot surface. The descaled surfaces are often wide and a number of nozzles must be used. The quality problems are almost exclusively connected with the overlap of water jets. An experimental study of overlap optimization is presented in this paper. A new approach using in-line configuration of jets is introduced and discussed. This paper also describes why even the completely oxide-free surface achieved after descaling the unit can be a far from optimal solution. Thermal strips on the hot surface cause much more intensive oxidation of the hot part and much slower oxidation in the cold strips on the descaled surface. The speed of oxide formation on the steel surface is exponentially dependent on the surface temperature. Temperature nonhomogeneity after descaling in the rolling process can cause the same defects on the surface of the final product as poor descaling. Temperature aspects with links to heat loss and secondary oxidation are discussed.
The temperature of the bloom during hot rolling is a crucial factor for the quality of the final product. Heat losses during hot rolling which affect bloom temperatures were therefore experimentally investigated. Experiments were designed to simulate heat transfer on the bloom surface during rolling when: 1. the bloom does not move and is far from the work roll (heat transfer by radiation and free convection in air); 2. the bloom moves far from the work roll (heat transfer by radiation and forced convection in air); 3. the bloom moves close to the work roll so the residual water from roll cooling falls on its surface (heat transfer by radiation and forced convection by residual spray from work roll cooling). Roll bite is excluded in this paper. Experiments were performed with an austenitic steel plate embedded with thermocouples. The plate was firstly heated up to 900 °C and then repeatedly moved by a computer controlled mechanism through a section which simulated the blooming line. Three different velocities of 1, 3, and 5 m•s -1 were tested to observe the influence of velocity on the heat losses. The temperatures gathered during the experiments are evaluated numerically by an inverse heat conduction task and analytically. The final results show that the highest heat loss is caused by radiation; its value is 171 W•m -2 •K -1 for 1,100 °C. The heat loss due to residual water from work roll cooling is 80-88 W•m -2 •K -1 and the heat loss by forced air is found to be from13-29 W•m -2 •K -1 .
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