What is steel?

Steel is an alloy of iron. It is formed by heating iron and combining carbon (0.1-1.7%) with it and minor amounts of other elements such sulfur,  chromium, manganese, nickel, etc. Thousands of steel grades have been published, registered, or regulated around the world, each with its chemical composition, and special numbering systems have been devised in various countries to classify the vast number of alloys.

Chemical composition of steel

According to the chemical composition, steels are classified into three groups

• carbon steels
• low alloy steels
• high alloy steels

A tiny quantity of incidental components leftover from steelmaking can be found in all steels. Scrap used in the steelmaking furnace introduces copper and other metals, known as residuals. These elements aren't considered alloys because they typically make up less than 1% of the steel.

Carbon steels

They are the most common and widely utilized, accounting for over 90% of global steel production. 

• High-carbon steels have a carbon content of more than 0.6%.
• Medium-carbon steels have a carbon content of 0.3 to 0.6%.
• Low-carbon steels have a carbon content of less than 0.3%.
• Extra-low-carbon steels have a carbon content of 0.015 to 0.05%.
• Ultra Low-carbon steels have a carbon content of less than 0.015%. 

Carbon steels are also classified as having a manganese content of less than 1.65%, the silicon content of 0.6%, and copper content of 0.6%, with a total of these additional components of less than 2%.

Low-alloy steels

It includes up to 8% alloying elements, while high-alloy steels contain more than 8% alloying elements. Aside from carbon, there are roughly 20 alloying elements. Aluminum, molybdenum, manganese, nickel, chromium, vanadium, tungsten, lead, titanium, copper, and many more are used as examples of these elements.

High-alloy steels

The high-strength low-alloy (HSLA) steels meet the demand for high strength, improved weldability, and higher resistance to atmospheric corrosion. These grades contain minor concentrations of one or more elements, such as nickel, chromium, titanium, molybdenum, vanadium, and niobium, and have low carbon levels (nearly 0.05%). Oil and gas pipelines, ships, offshore constructions, and storage tanks are all HSLA steel. Advanced high strength steels (AHSS) are used for safety components.

Other types of steels

Machining steels

This group, which is used to make nuts-bolts, and screws, which have (nearly 0.35%) sulfur and lead (up to 0.35%), including selenium and tellurium. Numerous inclusions are generated by the components that are undesired but required in this case since these breakdowns down the long, hazardous metal thread which are normally created while milling into little bits.

Wear-resistant steels

These are made of austenitic steels with a carbon content of 1.2% and a manganese content of 12%. Hadfield steels are mainly manganese steels named after their inventor, Robert Hadfield. These types are mainly used to make wear plates for crushers, gear, rock-processing, and power shovels.

Moreover, wear resistance is achieved by pounding (or deforming) the surface, which causes a significant number of disruptions and passage of dislocations. The more you pound the steel, the stronger it becomes. Cold forming is also used to make high-strength, cold-drawn wire for prestressed concrete and vehicle tires. The tensile strength of piano wire drawn from 0.8% carbon steel can reach 275 kilograms-force per square millimeter in a unique scenario.

Bearing steels

Steels used in roller and ball bearings are one major group that exemplifies the various advances in manufacturing potentials. After heat treatment, these steels commonly comprise 1% carbon, 1.2% chromium, 0.25% nickel, and 0.25% molybdenum and are extremely hard. Most importantly, they are exceedingly clean, vacuum treated to remove virtually all impurities from the liquid steel. An invisible, self-healing chrome oxide coating formed when chromium is introduced at concentrations greater than 10.5%, giving this remarkable group its stainless features. The austenitic, ferritic, and martensitic are the three primary groups.

All alloying elements are retained in solid solution in their microstructures, which are exceptionally pure fcc crystals. These steels have 16 to 26% chromium and up to 35% nickel, a powerful austenizer like manganese.

The bcc microstructure is seen in both the ferritic and martensitic groups. The latter has a more significant carbon content (up to 1.2%), is hardenable, and is used to make blades and tools. Carbon content in ferritic stainless steels is as low as 0.12%. Both varieties have low corrosion resistance and are ferromagnetic.

At high temperatures, such as 800°C (1,450°F), a particular group of stainless steel is used. To keep the steels robust at such high temperatures, used in solution hardening. They contain up to 25% chromium and 20% nickel and tiny amounts of solid carbide formers like niobium or titanium to bind the carbon and prevent chromium depletion at grain boundaries. Superalloys are utilized in aircraft jet engines and gas turbines for even more demanding use.

Electrical steels

The high-silicon electrical steels are an essential group of steels used to generate and transmit electrical power. To decrease the eddy currents and less current losses and heat production, alternating current electromagnets are always produced by laminating multiple thin sheets that are insulated. The condition is improved even further by adding up to 4.5 percent silicon, which imparts strong electrical resistance. Grain-oriented sheets containing 3.5 percent silicon are commonly in use in electric transformers.

Properties of steel materials

The essential component of steel is iron, a metal with a hardness similar to copper in its pure form. Except for the significant conditions, iron, like all other metals, is polycrystalline in its solid state, meaning it is made up of several crystals that are linked at their edges. A crystal is a well-ordered atom arrangement that resembles spheres touching one another. The easiest method to see the lattice arrangement for iron is to look at a unit cube with eight iron atoms at its turnings. Each of the six sides of the unit cube has an extra iron atom in its center in the face-centered cubic (fcc) arrangement. The sides of the face-centered cube, or the lengths between adjoining lattices in the fcc arrangement, are approximately 25% bigger than the bcc arrangement, implying that the fcc structure has more area to hold foreign (i.e., alloying) atoms in solid solution.

The melting point of iron is 1,538°C (2,800°F). Its bcc allotropy is below 912°C (1,674°F) and between 1,394°C (2,541°F) and 912°C (1,674°F). Iron in its bcc structure is also known as alpha iron in the lower temperature range and delta iron in the higher temperature range and is referred to as ferrite. Iron is in its fcc order, called austenite or gamma iron, between 912° and 1,394°C. Steel retains the allotropic behavior of iron with few exceptions, even when the alloy contains significant amounts of other elements.

Hooke's Law

There are roughly 3,500 different grades of steel, according to the World Steel Association, each has its own set of physical, chemical, and cultural characteristics. From the figure of stress-strain curve containing the elastic and plastic region, where the elastic region is following Hooke's law.

From the figure of stress-strain curve containing the elastic and plastic region, where the elastic region follows Hooke's law.

σ α εσ =E ε

Also,

$\sigma$=$\frac{F}{A}$

$\epsilon$=$\frac{∆L}{L}$

Whereas

Stress =σ (unit=Pascal; Pa or N/$m^2$)

strain =ε(unit=m/m)

elasticity constant=E (unit =Pascal)

The term beta iron also alludes to iron's high magnetic properties rather than its mechanical capabilities. Iron is ferromagnetic below 770°C (1,420°F); the temperature above it loses this property is known as the Curie point.

Context and Applications

This topic is useful for the students who are undertaking the following courses:

• Bachelors in Technology (Civil and Mechanical)
• Masters in Technology (Civil and Mechanical)
• Bachelors in Science in Chemistry
• Masters in Science in Chemistry

Practice Problems

1. Which of the following materials is used to make the surgical knives?

1. stainless steel
2. low carbon steel alloys
3. high carbon steel alloys
4. medium carbon steel

Answer- c

Explanation: Surgical knives are made of high carbon steel alloys material.

2.  Which of the following materials is used to make the vehicle body components?

1. stainless steel
2. low carbon steel alloys
3. high carbon steel alloys
4. medium carbon steel

Answer- b

Explanation: Most of the vehicle body components are made of low carbon steel alloys material.

3. Which of the following materials is used to make the railway tracks and wheels?

1. stainless steel
2. low carbon steel alloys
3. high carbon steel alloys
4. medium carbon steel

Answer- d

Explanation: Most of the railway tracks and wheels are made of medium carbon steel material.

4. What is the percentage of carbon content in the medium-carbon steels?

1. more than 0.6%
2.  0.3% to 0.6%
3. Less than 0.6%
4. None

Answer- b

Explanation:  The medium-carbon steels have a carbon content between 0.3% to 0.6%.

5. What is the carbon content in the high-carbon steels?

1. more than 0.6%;
2. 3 to 0.6%;
3. Less than 0.6%;
4. None

Answer- a

Explanation: The high-carbon steels have a carbon content of more than 0.6%.

Formulae

• Hooke’s law concept

σ α εσ =E ε

• Stress formulae

σ =$\frac{F}{A}$

• Strain formulae

 ε=$\frac{∆L}{L}$

Related concepts

• Different classification based on its phase, properties, and types of steel based its application
• Different chemical composition based application
• Stress-strain concept (Hooke’s law)
• Iron-carbon concept