Structural steels are the most common class of steels. In this post we’ll cover the uses, mechanical properties and chemical composition of the most used grades – S235, S275, S355 and S420.
Structural steels are the most widely used among steels because of their sufficient qualities and comparatively low price. This combination makes them useful in different sectors. Every year, about 25% of structural steels are used in the construction of buildings.
Mechanical engineers favour it when building machinery that does not have many special needs like high durability or resistance to heavy loads.
Sometimes resistance to heavy wear is important, while maintaining a low weight itself. This is the case with forestry and mining equipment, for example. It is then advisable to turn to high strength steels.
Structural steels require an extra layer of protection against the weather. They tend to corrode easily, so powder coating is commonly used, irrespective of a machine’s surroundings – inside or outside. For a lasting coating, the metal surface should be pretreated (chemical etching or phosphating) and cleaned (blasting, washing or wiping) beforehand.
Another more durable way to protect structural steels against corrosion is galvanization. The quality and applicability of a zinc coating depends on a steel’s chemical composition – the percentage of phosphorus (P) and silicium (Si) are decisive:
- Si+2,5P<0,05% = 1st class quality
- 0,05%≤Si+2,5P≤0,15% = bad result (Sandelin range)
- 0,15%<Si+2,5P<0,25% = 2nd class quality
- 0,25%<Si+2,5P = bad result
In case of harsher conditions, for example sea-harbours, stainless steels are used instead.
|Tensile strength, Rm, MPa (N/mm2)||0,2% Yield strength, Rp0, 2min, (MPa)||Brinell hardness, HB max|
|360 – 510||235||100 – 154|
|370 – 530||275||121 – 163|
|470 – 630||355||146 – 187|
|480 – 620||420||143 – 184|
The number in a structural steel’s name is its yield strength. It indicates the maximum load, in MPa, that does not end in plastic deformation. Stress above that value will deform the metal permanently. In S235’s case, this value is 235 MPa. Anything below results in elastic deformation, meaning that your detail will regain its former shape after removing the load.
Tensile strength refers to the maximum load that does not break the metal. Continuing with the S235 example, this value is somewhere between 360…510 MPa. While it is noticeably higher than yield strength, you should consider the yield strength value when choosing the right material for your conditions. This is because exceeding tensile strength means failure, while exceeding yield strength only ends in a deformation.
The properties table shows that increasing grades mean higher strength. S235 and S355 steels are the most popular among those presented here, as they cover most needs. S275 does not find much use and may be harder to find. S420 is suitable when looking for higher strength but may also not be readily available with every manufacturer.
The hardness measures of all structural steels are quite low. If needed, those values can be improved by hardening.
Prefixes and suffixes
You can find different letters before and after the numbers, which we already explained. Structural steels have an S as a prefix, standing for… structural steels. There is a variety of alternatives – for example P for pressure vessel steels – meant for different applications.
Ferritic steels change their behaviour with temperature. They become more brittle at lower and a little more ductile at higher temperatures. This has to be accounted for when the conditions need it. A construction that lies outside through winter, at -20°C, may get knocked by a forklift and result in a brittle fracture. Therefore, you should select a steel with the right impact resistance.
|Impact code||Testing strength||Temperature code||Testing temperature|
|J||27 J||R||+20 °C|
|K||40 J||0||0 °C|
|L||60 J||2||-20 °C|
S235JR and S235L2 act differently when facing blows. J indicates that it can take a hit with a maximum energy of 27 Joules, K 40 Joules and L 60 Joules. R means that the minimum temperature for that capacity is at room temperature (20°C), 0 means 0°C and 2 means -20°C. Therefore, S235JR can only take a blow of 27 Joules at room temperature, while S235L2 can take 60 Joules at -20°C. It is all well illustrated in this video about the Charpy impact test.
When you encounter a steel with a name like S235J0+N, you now it is normalized. Normalizing is a heat treatment method used on steel to refine its crystal structure and provide a more uniform grain size distribution. It eliminates internal stresses and strains. This, in turn, improves the steel’s machinability, toughness and ductility without compromising hardness and strength.
While there are some other suffixes you may see, we are limiting ourselves to the most common ones. The last of which is MC. S355MC steel is thermomechanically processed which combines its strength with suitable characteristics for cold forming like bending.
|Composition||Chemical content % by weigth|
|Manganese (Mn) max||1.6||1.6||1.6||1.6|
|Silicon (Si) max||0.05||0.05||0.05||0.5|
|Carbon (C) max||0.22||0.25||0.23||0.12|
|Phosphorus (P) max||0.05||0.04||0.05||0.025|
|Sulfur (S) max||0.05||0.05||0.05||0.015|
The exact chemical composition will change depending on the needed durability. For example, stronger structural steel that has to be resistant to weather (e.g S355W) will have a bit different chemistry than the standard S355.
Even if the chemistry is almost identical, combination of different production processes (rolling, heat treatment and cooling) determines the final strength. For example, the more steel is rolled, the stronger it becomes.
Which to choose?
It really depends on your needs. Are you just going to laser cut some details? Do you want to bend the details afterwards? What kind of coating is necessary for the conditions? What kind of load does it carry?
Simply put, a better material costs more. In case of assemblies, it is best to keep the amount of different materials at a minimum because otherwise all kinds of setup costs will get added to the final price.
The most common method for choosing is by yield strength value. You needs can be calculated, using structural analysis. An easy way for doing this is to use the simulation options available in most 3D CAD programs. The critical points should still be checked by manual calculation to make sure that at least the order of magnitude is correct.
Not everything comes in all sizes and thicknesses. Moreover, not everything is in stock. Cold-rolled sheets are up to 3 mm, over that you get hot-rolled sheets. Cold-rolled ones have the advantage of a more uniform structure. S355MC also provides a uniform structure and good bending qualities, if your construction needs metals thicker than 3 mm. If thinner is better, go with DC01 (another steel class).
You may be wondering why 2 manufacturers are offering very different prices to the “same material”, if you ask for S235. It’s because there is no steel like S235 – the suffixes are always present. One may be offering you more quality. Make sure you know what you are getting by asking what exact metal is used. If you need quality, ask for it. If you need a low price, ask for it.