Carbon Steels: Classification and Application | Metallurgy

In this article we will discuss about:- 1. Introduction to Carbon Steels 2. Types of Carbon Steel-Based on Deoxidation Practice 3. Classification.

Introduction to Carbon Steels:

Steels represent the most important group of engineering materials as they have the widest diversity of applications of any of the engineering materials. Even in steels, plain carbon steels form the major percentage of the steels made. In plain carbon steels, the properties are mainly effected by the principal element carbon.

These steels do, normally contain other elements, whose presence is due to:

(a) the steel making practice, such as silicon and manganese which are added as deoxidisers.

(b) almost impossible to remove in commercial steels and come from the raw materials. For example sulphur and phosphorous etc.

American Iron and Steel Institute (AISI) defines the carbon steels as iron-carbon alloys having a maximum of 1.65% Mn, 0.60% Si, 0.60% Cu.

Types of Carbon Steel-Based on Deoxidation Practice:

In the ingot mould, after liquid steel has been poured in it, the solubility of the dissolved gases (O₂, etc.) in the molten steel decreases with the decrease of the temperature. The carbon of the steel reacts with the evolved oxygen to produce CO, which, when trapped at solid-liquid interface, produces ‘blow holes’.

Carbon steels can be classified in four types, depending on the deoxidation practice (that is, depending on the amount of gases, chiefly oxygen, evolved during solidification):

(i) Rimmed steel

(ii) Capped steel

(iii) Semi-killed steel

(iv) Killed steel.

Normally, no deoxidiser is added in the furnace, while producing a rimmed steel, though some ferromanganese in the ladle, and some aluminium additions in the ingot mould, are done to control the extent of rimming action. When the steel is poured in mould, it rises due to large evolution of the gases. CO causes a boiling action, commonly called ‘rimming action’.

In a typical rimmed ingot, the gas evolution is so strong that the formation of blow holes are confined to lower quarter of the ingot, because rest of the gases escaped before the sides and top could solidify . The apparent increase in volume, due to the formation of blow holes, compensates for the shrinkage that occurred during solidification. Thus, top of the ingot does not fall, or rise appreciably during solidification.

Capped steel is a variation of rimmed steel practice, i.e. after the ingot has rimmed for a while, a cast iron cap is placed on the top of the mould to freeze the top of the ingot and to stop the rimming action. Large amount of gas is evolved inside. The resulting strong upward currents, along the sides in the upper half of the ingot, sweep away the gas bubbles from there, that otherwise would have formed the blow-holes there.

Even in the lower half of the ingot, blow-holes are formed only when the gas evolution has moderated somewhat. These results, first in a thick solid skin formation, and then followed by the zone containing the blow-holes. That is why, the blow holes are not oxidised due to atmospheric oxygen. This kind of ingot has a thin zone of relatively pure iron and also less segregation in the core than a rimmed ingot. Segregation could be positive (the content of the element is greater than the average), or negative (content of the element is lesser than the average).

The degree of segregation increases with slower rates of solidification and, by any disturbance of the liquid during freezing such as by the gas evolution. The maximum segregation takes place in rimmed, or capped steel. In a rimmed steel ingot, there is high negative segregation in the first metal to freeze at the ingot wall, or the rimmed zone (low in carbon), and core-zone shows positive segregation. As the blow-holes get welded during rolling, this zone is highly pure iron.

As the shrinkage is compensated by blow-holes, no pipe formation occurs, leading to high yield of the product. That is why, single most-widely used steels are low carbon steels having less than 0.10 % carbon (welding of blow-holes becomes increasingly difficult with the increase in carbon content of the steels, particularly when the carbon content is more than 0.3 %). Capped steels find better use, when the carbon is more than 0.15%.

Semi-killed steels have carbon in the range of 0.15% to 0.30%, when the welding of blow-holes becomes difficult during the hot-rolling of the ingots. Ferro-silicon and some aluminium are added as deoxidisers to the ladle, but the amount of these is less than that required to fully kill the steel. Here, only a slight amount of CO is evolved, but the resulting blow holes are sufficient in volume to compensate fully the shrinkage encountered during solidification.

A killed-steel (dead-killed) is one which has been sufficiently deoxidised (all the evolved oxygen reacts with the deoxidisers). Most of the products so formed are non-metallic inclusions, which join the slag. However, some oxide inclusions stay in the molten steel to prevent any gas evolution during solidification. As the molten steel lies quietly in the mould (as no CO gas formed to cause boiling) as if it has been killed, thus, the name.

As the steel solidifies, the shrinkage cavity, called pipe, forms at the top of the ingot, and that is why to minimise this volume of the steel having pipe (which is later cut off and discarded), killed steels are nearly always cast in big-end-up moulds with refractory’ hot tops, so that the pipe is contained within the small hot top. All the steels having carbon more than 0.3% are killed.

Vacuum degased steels are increasingly finding use as it has the advantages of eliminating solid deoxidation products, and thus, gives cleaner steel and also reduces the hydrogen content of the steel. Deoxidation products make deep drawing and deep-pressing difficult, whereas, reduction of hydrogen avoids internal cracks that can occur in heavy sections during cooling, after rolling, or forging.

Classification of Carbon Steels:

Based on the tendency of austenite grains, to grow when heated above the upper critical temperature, the steels could be classified:

1. Inherently fine grained steels, or fine grained steels.

2. Inherently coarse grained steels, or coarse grained steels.

This classification is based primarily on how the steel was deoxidised. Inherently fine grained steels had been deoxidised with aluminium. Aluminium, that does not combine with oxygen during deoxidation, combines with nitrogen in steel and forms a dispersion of fine aluminium nitride particles, which in turn pin the austenite grain boundaries, and thereby, inhibit grain growth, on heating. Inhibition of austenite grain growth by AlN is the classic example of grain boundary pinning by small particles of the second phase.

Steels deoxidised with silicon are called coarse grained steels. These and even, semi-killed steels, do not have particle dispersion to inhibit austenite grain growth, i.e. when these steels are heated above Ac3 temperature, austenite grains grow continuously with the rise of temperature (Fig. 1.40). Al- killed steels show very little growth of austenite grains when heated up to temperature of about 950°C to 1000°C, but if this temperature is exceeded, the grain growth is very rapid and final grain size at a given temperature can be greater than in the Si-killed steels.

Al-N particles may then, coalesce and/or dissolve. All the particles need not be dissolved before the grains can grow rapidly. This temperature of abrupt coarsening is referred to as ‘grain coarsening temperature.’

Inherently fine grained steel does not mean that the given steel is always fine grained austenite. It only means that coarse grained steel acquires a coarse grain-structure at a lower temperature than the fine grained Steel. Normally, the austenitising temperature of many heat treatments does not exceed 950°C, means, fine grained steel is able to retain a fine austenite grain size, even in long carburising cycles, and, that is why, Al-killing is an almost universally used steelmaking-practice that produces inherently fine grained steels for critical heat treated parts and for alloy steels used for carburising.