Mild steel is a steel that combines iron and carbon. Due to the iron content, it develops good magnetic properties. Carbon steel is not alloy steel. Due to the higher carbon content, its chemical composition is significantly different from other elements or other steels, as carbon steel has good surface quality and it is possible to increase its surface hardness in several ways. Let's learn more about this material.
What is carbon steel?
Steel consists mainly of iron. Depending on the carbon content, there are low carbon steels (also called carbon steel or unalloyed carbon steel), medium carbon steels and high carbon steels.
The following table shows the differences and properties of the three types of steel:
|
Carbon content (% by weight) |
Microstructure |
Characteristics |
Examples |
|
|
Low carbon steel (mild steel) |
Less than 0.25 |
Pearlite, ferrite |
Soft, cheap, very elastic, easy to machine and weld |
AISI105, AISI 316L, Q195, Q215, Q235, 08F, 15Mn, 20Mn |
|
Medium carbon steel |
0.25 to 0.60 |
Martensite |
Quite ductile, hard, strong and difficult to harden |
AISI 409, 45#, 40CR, 20CR, SCM 435 |
|
Carbon steel (tool carbon steel) |
0.60 to 1.25 |
Perlite |
Very hard, strong, inflexible, difficult to machine and weld |
T7, T7A, T8Mn, T8MnA, AISI440C |
In the table above, carbon steel is a type of steel that contains 0.05 to 0.25% carbon. Due to its malleability and ductility, the material is easy to shape and work with. Mild steel parts are generally very suitable for stamping and forging. Carbon steel is commonly used in tools, body parts, construction and infrastructure.
How is carbon steel made?
The manufacturing processes for carbon steel (low carbon steels) are similar to those for other carbon steels. These processes have changed over time and are now more efficient and cost-effective than before. In modern manufacturing processes, three main steps are performed to produce carbon steel from pure iron or iron ore.
1. Primary steel production
In this process, iron is mixed with coal and lime and heated in a blast furnace. Modern primary steelmaking uses modern furnaces such as oxygen blast furnaces or electric arc furnaces. The latter are typically used by manufacturers in developed countries, and the steel parts that come out of the arc furnace are of high quality.
2. Secondary steel production
The aim of secondary steelmaking is mainly to reduce the carbon content (less carbon) to the desired level and add other alloying elements to improve the properties of steel. This step mainly serves to control and monitor the heat treatment and cooling of the furnace.
3. Master casting and molding
Once the steel reaches the specific carbon content in the furnace and the mechanical properties are improved to a certain extent, the steel is cast into a mold. This is called cast iron, and some say it is cast iron, which is not correct. During this process, the liquid form of the steel is hardened and shaped into different geometries. These molten steel crystalline structures are then cut into smaller pieces.
Cast steel has numerous defects and imperfections. A primary forming process called hot rolling is used. After hot rolling, the steel gains more strength, ductility, weldability, etc.
4. Secondary training
There is usually a secondary forming process such as CNC machining, cold rolling, powder coating, hardening or electroplating to further improve the mechanical and chemical properties or aesthetics of mild steel parts. In these processes, a metallic coating, such as zinc coating or another type of carbon steel, is appropriately applied.
Mechanical Properties of Mild Steel
As you can see from the table above, carbon steel has a lower carbon content than other carbon steels. The carbon content of carbon steel is only 0.25%.
Carbon steel has high impact resistance, is very ductile and weldable, has good formability and can be cold formed. Due to these properties, CNC machining of carbon steel is easier than CNC machining of other types of steel.
The biggest disadvantage of carbon steel is that it has a relatively low tensile strength and therefore breaks more easily than other types of steel.
Fortunately, a heat treatment process called carburizing can be used to improve tensile strength. Carburizing is a surface hardening process in which carbon steel is heated to a certain temperature and then cooled, hardening the steel on the surface and leaving the steel core soft and ductile.
Mild Steel Applications
Mild steel is commonly used. Typical applications include common construction tools, kitchen utensils, medical instruments, machinery, construction, infrastructure, etc.
1. Construction and infrastructure
Mild steel has good tensile strength for building structures in construction projects. It meets the requirements of earthquakes and winds, cannot be affected by worms and insects, and is also erosion resistant and fire retardant, perfect for construction use.
2. Machines
Due to its high strength, ductility and hardness, carbon steel is preferred for manufacturing machine accessories. Due to the low carbon content, steel can be machined, cut, pressed, shaped and shaped without breaking.
3. Cookware
Due to its very low carbon content, carbon steel does not rust, keeping kitchen utensils clean and sharp for a long time. Carbon steel is more heat resistant than other materials, can be used as a non-stick material, and is healthier than aluminum cookware.
4. Piping
Because carbon steel is extremely ductile, carbon steel pipes are very popular in manufacturing various types of pipes and posts. Carbon steel can be used as a food-grade material, making it ideal for transporting water, beverages and natural gas in steel pipes. Due to their low carbon concentration, pipes used outdoors also do not rust and can withstand cold in harsh environments. Compared to plastic pipes, mild steel pipes are more environmentally friendly and last much longer. They can also be cut into a rectangular shape.
Application examples for carbon steel
AISI 316L, which has a low carbon content of only 0.03%, is a very commonly used typical carbon steel. Over the years, working with several companies, we have found that medical instrument manufacturers like to use this material. To produce prototypes or customize steel parts in small quantities, CNC machining is often the first choice. CNC machined 316L steel parts generally do not require surface treatment or heat treatment. Mild steel CNC parts can be used as finished products, not just prototypes. Other common products such as fishing tools also use AISI 316L a lot.
Mild steel CNC machining and small series production
Why do people often complain that steel is difficult to CNC machine? The machinability of steel depends mainly on its carbon content. High carbon steels are difficult to machine mainly due to their inflexible properties and internal stress.
However, carbon steels are also difficult to machine because they are very soft. Chips produced by the CNC machining process tend to stick and accumulate, resulting in more time spent cleaning chips and potentially damaging cutting tools. The ideal carbon content to facilitate machining is around 0.20%.
For example, some carbon steel is very soft, causing chip build-up problems. Sometimes we use one method: first harden the carbon steel. Surface hardening or surface hardening hardens the carbon steel on the outside, leaving the steel soft on the inside, eliminating chip accumulation problems. Because the steel core is still soft, the softness or ductility of carbon steel absorbs stress when external stresses are applied to the material during the machining process and ensures that the part does not break.
For small series production of mild steel parts, CNC machining is the best solution. The mild steel material used for CNC machining is the so-called “block material”, which has good quality and relatively cheap price. CNC machining is very fast compared to casting and molding. In casting and molding, the initial time and cost of manufacturing molds are very high. For low-volume or low-volume production of carbon steel parts, the overall cost is generally much lower than casting and molding because you don't need to make molds.
Long-term production of carbon steel: casting and forming
When large quantities of mild steel parts are to be produced, casting or molding (cast steel) is commonly used. Metal injection molding (MIM), which is similar to plastic injection molding, involves injecting a liquid form of metal into a mold. The liquid metal is hardened and solidified in the mold. After being ejected from the mold, the steel parts are given a rough shape. There are post-processing processes to further improve the aesthetics of molded parts.
As with plastic injection molding, a designer must consider aspects such as draft angles, draft, shrinkage, tolerances, etc. when designing the MMI molds and parts.
Conclusion
Mild steel or low carbon steel has a carbon content of less than 0.25. Due to its properties, carbon steel is used in almost all industries. The common method for producing carbon steel prototypes or quality parts in small batches is CNC machining. For larger quantities, metal injection molding is a more practical method.
