The scale of hot-rolled low-carbon wire rod
The scale formed on hot-rolled low-carbon wire rod should be adapted to facilitate mechanical or chemical descaling. Surface properties and cleanliness after scale removal are essential for the wire drawing performance, as residual oxide causes excessive tool wear and scratches in the finished wire. The most essential oxide scale properties in hot rolled wire rod are total thickness, structure and state of stress.
A rough surface is usually beneficial in wire drawing due to an improvement of the apprehension of the drawing lubricant to the steel wire rod surface. However, the volume share of powdery elements should be minimised in spite of the chosen descaling method. The iron oxide is usually divided into primary and secondary scale. The primary oxide is formed in the reheating furnace and is usually removed from the billet surface before rolling. However, together with wire rod rolling, secondary oxide is mainly - but not only - formed after laying at the air cooling conveyor but also during the rolling itself. Iron forms with oxygen in all the 3 different oxides: wustite, magnetite and hematite.
The named oxide phases have entirely different mechanical properties and their solubility in acids differs from each other. The interior layer, with the lowest oxygen content is wustite, the phase in the middle magnetite and the outer layer hematite. Volume changes originate in differences between thermal expansion factors in different phases in the structure and in volume amendments due to their phase transformations.
The scale thickness and composition depends in the first place on the temperature but also on the atmosphere and diffusion speeds of oxygen and iron atoms. When the oxygen share decreases, the formed layer is apparently thinner. The growth rate of wustite follows a parabolic law and it is stable at temperatures above 570°C, yet at lower temperatures it transformes to magnetite.
The growth rate of magnetite and hematite is linear during oxidation of iron at standard air pressure with the temperature of 700°C-1000°C. The thicker the wustite layer, the rougher the steel surface will be after descaling. During the oxidation of iron, iron ions diffuse through the wustite and further through magnetite to the magnetite/hematite phase boundary. Oxygen ions, however, diffuse through the hematite layer only. As a consequence, the wustite layer growth takes place at the wustite/magnetite phase boundary, but magnetite and hematite layers both grow in the same phase boundary between themselves. At temperatures below 570°C, the main part of the scale layer consists of magnetite. When the temperature exceeds 570°C, the amount of wustite also increases. Consequently, only thin layers of magnetite and hematite will cover the wustite in this case.
Between 700°C and 900°C, the scale is formed mainly by wustite and also by magnetite. Within temperatures above 900°C, the magnetite share increases at the cost of the wustite share. It is therefore necessary to ensure high cooling rates at temperatures from 570°C to 300°C to prevent wustite transformation to hematite. Through slow cooling of the wire rod, wustite transforms at 570°C to magnetite and iron.
The adherence between oxide and metal depends on inequalities in the boundary layer, the state of stress between the metal and oxide and on the plasticity in each element. The state of stress depends on the volume proportion between the growing oxide and metal, and also on differences in thermal expansion factors in the materials, It is known that the adherence between the oxide and the steel drops with rising contents of alloying elements such as aluminium, silicon, phosphorus and manganese, being nobler than iron. Furthermore, alloying elements such as chromium and Al, having a high oxygen affinity, weaken the adherence of the scale. Chromium and aluminium form chrome-iron or chrome-aluminium-iron spinels influencing the porosity of the oxide layer. As a consequence of a high porosity, the scale easily comes off during descaling.
Silicon has a great effect on the formation of the oxide layer. Particularly in silicon-killed steels, a thin silicate film is formed between the steel and its oxide layer. The silicate fayaliteis formed during oxidation as a consequence of silicon enrichment during oxidation at the oxide/iron boundary. However, silicate fayalite does not dissolve to iron oxides but enriches into that actual boundary. The melting point of fayalite is 1170°C and it forms a uniform layer between the steel surface and the iron oxides. Thus, descaling becomes difficult due to the presence of silicate fayalite, which penetrates into pores and increases the adhesion between the oxide layer and steel.