BASICS OF STEEL

WHAT IS STEEL ?

Steel-an alloy of iron and carbon with unwanted impurities present in it.


Carbon : max. upto 2%. The most important element

Different elements (Si, Mn, P, S, Cr, Ni, Cu). In electrical steels Si is an alloying element not as in impurity.

Substitutional and interstitial atoms and its effect?

Plain carbon steel (low carbon,med carbon,high carbon)?

Alloy steel (low-alloy,medium-alloy,high-alloy)?

Plain carbon steel: Carbon is the main constituent in it.

Other constituents Si, Mn, P, S, Cr, Ni, Cu etc are present as impurities.

Types: low carbon steel: <0.4% C

Medium carbon steel: 0.4-0.6% C

High carbon steel: 0.7-1.5% C

Steels having 1.5%C-2%C are rare in use.

Alloy steel-In alloy steel alloys are added intentionally to impart specific properties in steel.

Types: low alloy steel: Alloy content <5%

Medium alloy: 5-10%

High alloy: >10%
 
 
BASICS ?
 
Steel is a combination of iron and carbon. In its annealed condition C which is in the form of cementite is present in the matrix of ferrite.


When steel is heated to prescribed temperatures, then cooled at a specific rate, it undergoes physical, internal changes which manifest themselves in the form of various micro-structures such as pearlite, bainite, and martensite.
 
These micro-structures (and others) provide a wide range of mechanical properties, making steel an extremely versatile metal.
 
 
WHY DIFFERENT TYPES OF STEELS ARE NECESSARY ? 
 
Fe-C diagram and TTT diagram?


Austenite,Bainite,ferrite,Martensite?

Pearlite?

Ferrite,Austenite and cementite/graphite are basic constituents of steel observed in Fe-C diagram.

Ferrite is solid solution of carbon in iron .

Cementite is iron carbide(Fe3C). It is ~ 10 times harder than ferrite.

Depending upon the different cooling rate we will get different distribution of carbon (diffusion) in iron matrix and correspondingly different phases appear leading to different properties.

How cooling rate is important in attaining properties of alloy steel and different constituents of that steel?

CARBON
 
Carbon is essential to the formation of cementite, carbides, pearlite, bainite, and martensite. The strength and hardness of steel is increased by the addition of more carbon, up to about 0.65 percent.



Wear resistance can be increased in amounts up to about 1.5 percent. Beyond this amount, increase of carbon reduces toughness, ductility, machinability and increases brittleness.


Carbon is the single most important alloying element in steel. Carbon has a moderate tendency to segregate within the ingot.
 
 
MANGANESE
 
Manganese increases the strength of steel (solid solution strengthening) and also increases the hardness penetration of steel in the quench by decreasing the critical quenching speed. Manganese is present in most commercially made steels.


Manganese is a deoxidizer, reacting favorably with sulfur to improve forging ability and surface quality as it converts sulfur to manganese sulfide, thereby, reducing the risk of hot shortness, or susceptibility to cracking and tearing, at rolling temperatures.

Steels with manganese can be quenched in oil rather than water, and therefore are less susceptible to cracking because of a reduction in the shock of quenching.

It has a moderate tendency to segregate. The presence of manganese increases the coefficient of thermal expansion but reduces both thermal and electrical conductivity.
 
SILICON
 
Silicon is used as a deoxidizer in the manufacture of steel. It also may be present in varying quantities up to 1% in the finished steel and has a beneficial effect on certain properties such as tensile strength of ferrite.


It is also used in spring steels in the range of 1.5% to 2.5% silicon to improve the hardenability.

In higher percentages, silicon is added as an alloy to produce certain electrical characteristics in the so-called silicon electrical steels and also finds certain applications in some tool steels where it seems to have a hardening and toughening effect.
 
Effect of Silicon on YS & UTS in Low Carbon Steel
Silicon is believed to increase the strength of steel by solid solution strengthening. The low silicon steel finds application in thin gauge rolling, drawing and extra deep drawing applications. All the above applications require steel, which is soft.


It is evident from the graph that both yield strength and ultimate tensile strength of the low carbon steel increase with increasing silicon.
 
 
NICKEL
 
Nickel increases the strength of steel. It is used in low alloy steels to increase toughness and hardenability.


Nickel also tends to help reduce distortion and cracking during the quenching phase of heat treatment. It broadens the temperature range for successful heat treatment.

It increases strength and hardness without sacrificing ductility and toughness. Certain stainless steels employ nickel up to about 20%.

It also increases resistance to corrosion and scaling at elevated temperatures when introduced in suitable quantities in high-chromium ( stainless ) steels.
 
CHROMIUM
 
Chromium has a tendency to increase hardness penetration. When 5 percent chromium or more is used in conjunction with manganese, the critical quenching speed is reduced to the point that the steel becomes air hardening.


Chromium can also increase the toughness of steel.

Chromium forms carbides that improve edge-holding capacity and wear resistance.

One of the most well known effects of chromium on steel is the tendency to resist staining and corrosion.

Steels with 12.7 percent or more chromium are referred to as stainless steels or heat resisting steels. A more accurate term would be stain resistant.
 
COPPER
 
The addition of copper in amounts of 0.2 to 0.5 percent primarily improves steel’s resistance to atmospheric corrosion.


Copper has a detrimental effect to surface quality and to hot-working behavior due to migration into the grain boundaries of the steel.
 
VANADIUM
 
Vanadium, usually in quantities from 0.15% to 0.20%, retards grain growth, even after hardening from high temperatures or after periods of extended heating. By inhibiting grain growth it helps increase the toughness and strength of the steel.


Tool steels containing vanadium seem to resist shock better than those which do not contain this element.

Refines the primary grain; hence also the as-cast structure.

Additions of vanadium up to 0.05% increase the hardenability of medium-carbon steels.

It is a strong carbide former, increases wear resistance, retention of cutting edges and high temperature strength. Therefore, preferred as an additional alloy material in high speed steels, hot work and high temperature steels. Vanadium greatly improves red hardness and diminishes overheating sensibility.

Vanadium forms almost no austenite precipitates and is plentifully available for precipitation hardening during or after the GAMMA/ ALPHA transformation. Even though the specific efficiency of vanadium compounds is comparably low, the high volume fraction of fine precipitates compensates for this, especially in steels with a relatively high carbon content.
 
NIOBIUM
 
In low carbon alloy steels niobium lowers the transition temperature and aids in a fine grain structure.


Niobium retards tempering and can increase the hardenability of steel because it forms very stable carbides. This can mean a reduction in the amount of carbon dissolved into the austenite during heat treating.

It has an outstanding status in retarding recrystallization during austenite processing via thermo mechanical rolling, resulting in grain refinement, which cannot be obtained by any heat treatment process.

Other outstanding effects of niobium, such as lowering the GAMMA/ ALPHA transformation temperature by a solute drag effect or the effective precipitation hardening potential can be used only to a certain extent owing to its limited solution in austenite.
 
TITANIUM
 
Titanium forms nitrides, which are stable at high temperatures and these titanium nitrides provide control of the austenite grain size at the reheating temperature before hot working and also in the weld-ment, in particular in the heat affected zone close to the fusion boundary.


The elimination of free nitrogen due to the formation of TiN is positive for the toughness and indirectly makes niobium more effective.

The influence of titanium on sulphide shape control had been widely used at a time, when the production of a low sulphur content was not standard.

Titanium is added to 18-8 stainless steels to make them immune to harmful carbide precipitation. It is sometimes added to low carbon sheets to make them more suitable for porcelain enameling.

This element when used in conjunction with boron, increases the effectiveness of the boron in the hardenability of steel.
 
PHOSPHOROUS
 
In appreciable amounts, phosphorus increases the yield strength (interstitial solid solutin strengthening) and hardness of hot rolled steel to about the same degree as carbon, but at the sacrifice of ductility at low temperature and toughness, particularly in the quenched and tempered condition.


In certain low carbon free machining steels, higher phosphorus content is specified for its beneficial effect on machinability.

Phosphorus has a pronounced tendency to segregate.

Phosphorus is believed to increase resistance to atmospheric corrosion ( e.g. Corten steel )
 
BORON
 
Boron can significantly increase the hardenability of steel without loss of ductility. Its effectiveness is most noticeable at lower carbon levels. The addition of boron is usually in very small amounts ranging from 0.0005 to 0.003 percent.


Unlike many other elements baron does not affect the ferrite strength of steel. It can be used to increase the hardenability of steel without sacrificing ductility, formability or machinability of steel in the annealed condition.
 
ALUMINUM
 
Strongest and most frequently used de-oxidizer and degasifier.


It prevents reapperance of yield point by combining with Nitrogen

Added in small amounts, it helps fine grain formation.

Since it combines with nitrogen to form very hard nitride, it is a favorable alloy constituent in nitriding steels.

Aluminum - killed steels exhibit a high order of fracture toughness.
 
TUNGSTEN
 
Used in small amounts, tungsten combines with the free carbides in steel during heat treatment, to produce high wear resistance with little or no loss of toughness.


High amounts (17-20%) combined with chromium gives steel a property known as red hardness. This means that the steel will not lose its working hardness at high temperatures. An example of this would be tools designed to cut hard materials at high speeds, where the friction between the tool and the material would generate high temperatures.

Tungsten is also used in certain heat resisting steel where the retention of strength at high temperatures is important. It is usually used in combination with chrome or other alloying elements.
 
MOLYBDENUM
 
Molybdenum increases the hardness penetration of steel, slows the critical quenching speed, and increases high temperature tensile strength.


It is chiefly used in conjunction with other alloying elements. Its presence reduces the critical cooling rate and improves harden ability, hardness, and toughness, as well as creep resistance and strength at elevated temperatures.

It helps to prevent temper brittleness and promotes fine grained structure.

It increases both yield point and tensile strength.

It forms carbides readily and thus improves the cutting properties in high speed steels.

It improves machinability and resistance to corrosion and it intensifies the effects of other alloying elements.
 
HYDROGEN
 
Harmful to steel, it causes embitterment by decreasing of elongation and reduction of area without any increase of yield point and tensile strength.


Atomic hydrogen engendered by pickling penetrates into the steel and forms blowholes.

At elevated temperatures moist hydrogen acts as a de-carburizing agent.
 
 
NITROGEN


It is present in all steels, but usually in small amounts; it will combine with certain other elements to precipitate as a nitride. This increases hardness, tensile and yield strength, but it decreases toughness and ductility.

OXYGEN

Injurious to steel, its specific influence depends on the type and composition of its compounds in steel and on their shape and distribution. It weakens mechanical properties, in particular impact strength, especially in the transverse direction, whereas the tendency to aging brittleness, red shortness, woody and slanty fracture is increased.

Integrated Steel Plant Flowchart

The choice of the technology is based on the merits and demerits of various technologies, the status of technology, flexiblity of operation, availablity of raw materials, capital investment, market demand and present capacity of supply available in the market. The DRI - IF/EAF/BOF - BLOOM/SLAB - RM route is considered for the company's integrated steel plant.