This post will explain and hopefully enlighten you about steel grades. We will show the different types of steel grades available in the United Kingdom and Europe, explain the use and classification of the different grades and why these grades are crucial to an engineers understanding.

What Are Steel Grades?

The grade of steel establishes the chemical composition and the physical properties of the steel when it is produced. .

Many different codes of practise or standards exist to classify the steel grade. A non exhausted list of standards and grades follows. European Standards (EN 10027), Japanese Grades (JIS), German Grades (DIN), Chinese Grades (GB).

Guidance can also be found from SAE (Society of Automated Engineering) which for the alloy numbing system and the International Organization for Standardization (ISO/TS 4949:2003)

Different standards ensure that quality control is followed during the creation process.

Why Are Steel Grades Important?

Structural engineers, as well as other professionals involved in any design using steel need to know both the properties of the steel they are working with in order to ensure they can design to suit the brief. 

As already stated, clues in the symbol or name give information into the physical properties of the steel. A symbol provides the meaning of the type of steel. For example, S for Structural Steel, P for Pressure Lines & Vessels. On top of this, a mechanical property is given after the symbol. The three digit number is typically the minimum yield stress of the material, especially within the European Standards.

Lets have an attempt at deciphering what S275 is and what it represents. S is for Structural Steel and 275 is the minimum yield strength. Just for reference, in countries that follow EN 10027, this is one of the most common steels available. One of which I use on a daily basis.

Not only is the mechanical strength stated, impact resistance in joules (J), personally right now my knowledge only extends as far as EN 10027, but if requested I will happily look up other codes to discuss. Three different impacts are stated in EN 10027, 27, 40 and 60.

Impact Code

Impact Force

J

27

K

40

L

60

Temperature difference also plays a role in the physical properties during impact, these range from room temperature to -60 Degrees Celsius.

Temperature

Code / Symbol

Room Temp.

R

0

0

-20

2

-30

3

-40

4

-50

5

-60

6

Types of Steel Grades

Different standards and countries have different ways to classify the various steel grades. Going forward in this post we will be covering the most common grades used in EN 10027. Sound knowledge of the principles of one code in relation to the grades is easily transferable to the others and no one expects you to remember every chemical composition, physical characteristics or numerical system.. We are engineers not encyclopedias!  

The table below shows in my opinion the most useful steel classes to be aware of, again not an exhausted list.

Symbol

Definition

S

Structural Steel

B

Reinforcement

P

Pressure Lines & Vessels

R

Rail Use

L

Pipes & Tubes

Simple right? Lets have a little test, what would S355J2 steel mean for design purposes?

Structural steel, with an impact rating of 27 joules which was tested at -20 degrees. The minimum yield strength of the steel, ignoring any effects of thickness is 355Mpa or N/mm^2.

Did you get it?

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Lets go through two examples. Calculating the bending of a steel plate with two different grades of steel, this will help demonstrate what effect the type of grade has in structural design. The examples will be simple so you don’t get lost. Whenever we design anything out of steel the grade of steel needs to be specified.

Going forward, to calculate the bending resistance of a steel plate we need knowledge of a few key areas. The centroid, the second moment of area and the physical properties of the steel (THE STEEL GRADE). Check out fundamentals to get up to speed on the first two!

Ex. 1 - Bending of a S275 Plate

Example one, lets go. The plate is shown below, 10mm (d) thick and 65mm (b) wide. We want to know the bending capacity in the minor axis.

Example 1 - Plate Bending

M / I = σ / y

Lets calculate the centroid first. 5mm. Got it? Great.

y*A = y1*A1

y = 5

And the second moment of area:

I = b*d^3 / 12

I = 5416

Note: I was lazy and rounded second moment of area. Rounded down to be conservative since we are calculating the resistance!

Rearrange the bending equation to find the bending resistance of the plate, M.

M = (σ * I) / y

We have all the parameters we need to calculate the resistance except sigma, or the yield stress of the plate. The hint is given in the type. Can you figure it out?

M = (275 * 5416) / 5

M = 297880N (297.88kN)

Ex. 2 - Bending of a S355 Plate

Now time for example two, exactly the same plate but with a steel grade of S355. Remember d = 10mm and b = 65mm. The centroid (y) and the second moment of area remain exactly the same. Geometric properties!

Example 1 - Plate Bending

Using the bending equation as before. The yield strength of S355 steel is 355Mpa or 355N/mm2.

M = (σ * I) / y

M = (355 * 5416) / 5

M = 384536N (384.536kN)

So from these two examples, a S275 plate will have a bending resistance of approximately 297kN and a S355 plate will have a bending resistance of 384kN. Quite a large difference. The next step for the designer is to find out is if the increase in the strength is required and if so the cost consideration. Another method to increase the bending resistance is to increase the geometric properties, or in simpler terms, a thicker/wider plate.

The two examples here are simple ones to just show one of the differences. As you get more into steel design you will notice areas of consideration in relation to the steel grade. Such as weld resistance and lamina tearing of which plate grade has a significant effect upon.

Anyhow, I hope you found it useful, its always good to bookmark properties such as plate grades for future reference. Using them on a daily basis, they soon become ingrained.