Mild Steel vs Carbon Steel: What is the Difference?

Steel is the most used material in construction, manufacturing and industry. Two of the most used types of steel are mild steel and carbon steel.

While both are used for similar purposes, there are several key differences between the two that make them better suited for different applications.

In this article, we will take a closer look at mild steel and carbon steel and examine the differences between them, including their carbon content, mechanical properties, and manufacturing and finishing processes.

Whether you are a metal fabricator, engineer or just looking to better understand mild steel and carbon steel, we will provide you with all the information you need to make an informed decision.

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What is mild steel?

Mild steel is a type of carbon steel with a low amount of carbon (typically 0.05% to 0.25%); these are also known as “low carbon steels.” Low carbon steel is considered a relatively inexpensive and versatile material that is commonly used in various construction and manufacturing applications.

The low carbon content makes mild steel more ductile and easier to shape, form and weld than other types of steel. Mild steel has good machinability and can be easily drilled, cut, and fabricated into various shapes and sizes.

In addition, low carbon steel has a relatively high tensile strength, making it suitable for use in high-stress applications such as beams, columns and machinery components. It’s versatility and affordability make it a popular choice for a wide range of applications.

Read more about mild steel here.

What is carbon steel?

Carbon steel is a type of steel that contains carbon as the main alloying element, with other elements present in smaller amounts. This metal is commonly used in the manufacturing of many products and structures due to its high strength and low cost.

Carbon steel can be further classified into various grades based on its chemical composition and mechanical properties, such as low carbon steel (mild steel), medium carbon steel, high carbon steel and ultra high carbon steel. Each grade has its own specific uses and applications, depending on the desired properties of the final product.

Medium to high carbon steel is commonly used to make machinery components, such as gears, crankshafts and shafts. Its high strength, and particularly high hardness make it an ideal choice for a wide range of tooling applications.

Carbon steel types

There are several types of carbon steel, each with unique properties and applications. These types include:

Low carbon steel

Also known as “mild steel,” this type of steel is more ductile and easier to shape, form and weld compared to other carbon steel types. This makes mild steel a popular choice over higher-carbon steels when it comes to construction and manufacturing applications.

Medium carbon steel

Contains 0.3% to 0.6% carbon content, making it stronger and harder than low-carbon steel but also more brittle. It is often used in applications that require both strength and ductility, such as machinery components, automotive parts and building frames.

High carbon steel

High carbon steel contains 0.6% to 1.5% carbon content and is known for its high strength and hardness, but high carbon steel is even more brittle than medium-carbon steel. High carbon steel is used in applications that require high strength such as knife blades, hand tools and springs.

 

Mild Steel vs Carbon Steel: What Is the Difference?

 

ComparisonMild SteelCarbon Steel
Carbon ContentLowMedium to Ultra-High
Mechanical StrengthModerateHigh
DuctilityHighModerate – Low
Corrosion resistancePoorPoor
WeldabilityGoodGenerally not suitable
CostInexpensiveSlightly higher per weight

 

Manufacturing process of mild steel and carbon steel

The manufacturing process for mild steel and carbon steel varies depending on the type of steel and the intended qualities for the final product. The manufacturing process is often divided into three stages:

  1. Primary
  2. Secondary
  3. Casting

Primary processes

Steel can be created entirely from recycled material or from a mix of recycled and virgin steel using the BOF process.

Basic Oxygen Furnace (BOF)

Mild and carbon steel are commonly produced using the basic oxygen furnace (BOF) method, which involves the transformation of raw materials such as iron ore and coke into liquid steel.

The liquid steel is poured into molds to produce slabs or ingots. Pure oxygen is pushed through the liquid steel to oxidize the extra carbon, resulting in a finished product with a carbon content of up to 0.5%.

Secondary processes

Market needs for higher-quality steel products with more consistent characteristics fueled the development of secondary steelmaking processes. This allows manufacturers to alter the carbon content to produce the resulting low carbon steel, medium carbon steel, high carbon steel or ultra-high carbon steel.

Electric Arc Furnace (EAF)

In an electric arc furnace, steel composition is changed by adding or removing specific components or by manipulating the temperature. EAF processes involve:

  • Stirring – Separating non-metallic impurities guarantees a homogenous combination and composition of the steel.
  • Ladle furnace – Enables precise temperature control and the measured injection of alloy components.
  • Ladle injection – Inert gas is injected into the bottom of the steel bath to achieve a stirring effect.
  • Degassing – Removes hydrogen, oxygen and nitrogen while also lowering the product’s sulfur content.
  • Composition adjustment – Crucial to achieve stirring (by sealed argon bubbling with oxygen blowing — CAS-OB).

Deoxidizing steel

The elimination of oxygen is a vital step in secondary steelmaking. As molten steel begins to solidify, the presence of oxygen causes a reaction with carbon, producing carbon monoxide gas.

Controlling deoxidation can be used to change the material properties of the final product and hence the steel’s suitability for various desired applications. Deoxidizing steel processes involve:

  • Rimming steels – Non-deoxidized or partially deoxidized steels.
  • Capped steels – Initially comparable to rimming, but the mold is capped to prevent the creation of carbon monoxide.
  • Semi-killed steels – Partially deoxidized and have a carbon content in the range of 0.15–0.3%.
  • Killed steels – Totally deoxidized to the point where no carbon monoxide is produced during solidification.

Casting

Traditional casting methods entail pouring molten steel into individual molds positioned on rail cars. Continuous casting of molten steel into shapes more appropriate for downstream processing is possible with casting machines.

Ingots are moved to soaking pits to be reheated for hot rolling. In a continuous casting machine, steel is produced into slabs, blooms or billets.

Finishing processes of mild steel and carbon steel

The finishing procedure for mild steel and carbon steel can have a significant impact on the end product’s appearance and performance. Carbon steel is finished using:

  1. Rolling
  2. Heat treatment
  3. Surface treatment
  4. Downstream secondary processing

Rolling

Product rolling

Solid cast ingots must be rolled into more usable shapes and sizes, similar to continuous casting ingots. The rolls rotate faster than the steel as it enters the machine, propelling it forward and compressing it.

Hot forming

To break up the as-cast microstructure, steel is heated above the recrystallization temperature. This results in a more uniform grain size and an even carbon distribution throughout the steel.

Cold forming

Cold forming is done at temperatures lower than the recrystallization temperature. This procedure improves the finish while increasing the strength by up to 20% through strain hardening. In a rolling mill, semi-finished materials are further processed into intermediate products. They are then ready for downstream industries to manufacture and process them.

Heat treatment

The goal of heat-treating steel is to change the distribution of carbon in the product and the interior microstructure which modifies its mechanical properties. When the mechanical qualities of steel are changed by heat treatments, an increase in ductility leads to a decrease in hardness and strength (and vice versa).

Normalizing

Steel is heated to approximately 55 °C (130 °F) over its top critical temperature. The upper critical temperature is maintained until the entire product has been uniformly heated, at which point it is air-cooled. This is the most frequent type of heat treatment, and it imparts exceptional strength and hardness to steel.

Annealing

Steel is heated to a solid solution temperature for one hour before cooling at a rate of 21 °C (70 °F) per hour. Internal tensions are eliminated, resulting in soft and ductile steel.

Quenching

This is similar to normalizing heat treatment, except that cooling is expedited by quenching the steel in water, brine or oil. The resulting material is extremely hard but extremely brittle, leaving it prone to breaking and cracking.

As a result, for exact control of the steel’s properties, it is usually followed by a controlled cooling rate down to room temperature in a process known as tempering or stress relief.

Surface treatment

Approximately one-third of all steel manufactured is surface coated to prevent corrosion and increase weldability and paintability.

Hot dip galvanizing

Galvanizing is the application of a zinc surface layer to steel. The steel is heated before entering a zinc bath, where liquid zinc coats the product’s surface. Gas-knives are used to adjust the coating thickness. A small amount of aluminum is added to the zinc solution to prevent the zinc coating from breaking.

Electrolytic galvanizing

Electrolytic galvanizing is another method for putting a zinc layer on steel goods. By regulating the current in an electrolyte solution, zinc is deposited onto the surface of the steel. This approach allows for more precise control of coating thickness.

For more on metal finishing options, check out our video blog.

Downstream secondary processing

Steel raw materials are further processed by downstream companies into the desired finished products. Various processing procedures, such as machining and joining, which include uniformly removing surface metal with machine tools and welding, are common.

Does carbon steel rust easily?

Carbon steel is predominantly made of iron, which makes it more susceptible to rust. When exposed to moist or humid conditions, carbon steel can corrode and form rust, which is a reddish-brown oxide of iron. This is because the steel reacts with oxygen in the air to produce iron oxide (rust). The same applies for mild steel.

However, the rate of corrosion for carbon steel can be influenced by several factors, including the environment in which it is used, the presence of other metals or substances that can accelerate corrosion as well as the specific type of carbon steel.

Some types of carbon steel may be more resistant to rust than others, and the use of coatings, such as paint or electroplating, can help to reduce the risk of rust formation.

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Is carbon steel better than mild steel?

Both types of carbon steel have their own unique properties and advantages that make them more suitable for some applications than others. Which one is better typically comes down to your specific requirements.

Mild steel is better used for low-stress applications due to its ease of fabrication and low cost, while carbon steel (from medium carbon steel to ultra high carbon steel) is better used for high-strength applications due to its high carbon content and strength.

Carbon steel has a significant advantage over mild steel in terms of strength. Carbon steel can be up to 20% stronger than mild steel, making it an excellent choice for high-strength applications or where high hardness is required.

One of the most significant disadvantages of carbon steel is its high cost. Because of its increased carbon content, carbon steel is often more expensive than mild steel.

Furthermore, carbon steel is more difficult to weld than mild steel, making it less appropriate for welding applications.



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