Cold Drawn Vs. Hot Rolled, and B500B Vs. B500C
It’s all too easy to take construction materials for granted, without giving any thought to their properties, their differences and the standards that apply to them. Yet a little knowledge can go a long way, and can help make informed purchasing decisions when it comes to buying the right product for your construction project. Take steel reinforcement products, for example – with over 3,500 grades of steel in existence worldwide, do you really know what the grades specified for steel reinforcement mean, and how they impact the mesh reinforcement products you buy? Let’s take a closer look.
Hot Rolled, Cold Rolled Or Cold Drawn Steel?
Before we look in detail at the standards that relate to steel reinforcement products and the grades that define these products, it’s worth investigating the differences between hot rolled steel, cold rolled steel and cold drawn steel. Perhaps the most obvious difference between these production processes is price – hot rolled steel is significantly cheaper than cold drawn steel. It’s important to understand the production process for each type of steel, and the physical differences between these types, before deciding whether cost should be the most significant factor in your choice of reinforcement product.
Hot Rolled Steel
Hot rolled steel is produced at temperatures of over 1,000°F, which makes it generally easier to form, compared with cold rolled steel and cold drawn steel. However, hot rolled steel is not as strong as cold rolled or cold drawn steel, and it is more difficult to form accurate shapes. Because it cools after processing, the final shape of the steel can vary slightly, making it unsuitable for precision applications. For more general construction applications, however, it is often the steel type of choice.
Cold Rolled Steel
Cold rolled steel is processed at normal temperatures, without heat, and the cold rolling process produces a stronger steel that can be formed more precisely.
Cold Drawn Steel
Cold drawn steel is produced by bringing hot rolled bars or coils down to room temperature and then drawing them through a die to shape them. Cold drawn steel has a higher tensile strength than hot rolled steel, and can be formed much more accurately.
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Understanding The Standards For Reinforcement Steel
European standard grades for steel products follow a set naming structure. Each grade starts with a single letter, and is followed by a number. The letter relates to the application and the number relates to the yield strength of the steel. Steel used for reinforcing concrete has the application letter B, and the number 500, which represents a minimum yield strength of 500MPa. The grades for reinforcement steel then have a final letter – A, B or C.
The Differences Between B500A, B500B, and B500C
The differences between the A, B and C designations are complex and relate to structural tolerances. For reinforcement mesh products, grade B500A steel is typically used, and for rebar, B500B is used. B500C steel is seldom requested and it would always be worth checking with a structural engineer whether grade B500B could be used instead of B500C, since B500C typically has a significant cost overhead.
| Welded steel type | B500A | B500B | B500C |
|---|---|---|---|
| Area | smooth, dented, ribbed | dented, ribbed | ribbed |
| Delivery form | coils, bars, spot-welded reinforcement mesh, lattice girders | coils, bars, spot-welded reinforcement mesh | |
| Nominal centerline [mm] | 4 - 16 | 6 - 50 | 6 - 50 |
| Min. flow / yield limit Re [Mpa] | 500 | 500 | 500 Re, act / Re, nom <1.25 |
| Min. Rm / Re ratio | 1.05a | 1.05 | 1.15c; <1.35 |
| Min. elongation at max load Agt [%] | 3.0a, b | 5.0b | 7.5b, c |
| Min. fatigue strength 2σad [MPa] | 100 | d ≤ 28 mm: 175 d> 28 mm: 145 | d ≤ 28 mm: 175 d> 28 mm: 145 |
| Min. shear force | 0.25 x An x Re 0.25 x A 0 / b x Re 10 / b or 0.6 x Ad x Re, d | 0.25 x An x Re 0.25 x A 0 / b x Re 10 / b or 0.6 x Ad x Re, d | 0.25 x An x Renvt |
| Tolerance nominal centerline [%] | +/- 4.5 | +/- 4.5 | +/- 4.5 |
| Chem. composition [mass%] | C <0.22, etc Ceq <0.50 | C <0.22, etc Ceq <0.50 | C <0.22, etc Ceq <0.50 |
| Min. relative opp. cross rib (dent), fr / pf | d = 4.0 - 6.0: 0.039 d = 9.0 - 10.5: 0.052 d = 6.5 - 8.5: 0.045 d = 11.0 - 50: 0.056 |
||
| a) Rm / Re1.03 and Agt 2.0 for center lines ≤ 5.5 mm | |||
| b) Agt for coils + 0.5% | |||
| c) Rm / Re min. 1.13 and Agt 7.0% for center lines ≤ 12 mm | |||
| d) Maximum voltage ripple 2σa with upper voltage 0.6Rek (300 MPa) and 1 million voltage changes. For spot welded reinforcement mesh B500B and B500C, 2σa is at least 100 MPa. For coil-oriented products, 2σa is at least 100 MPa, unless a higher value (≤ 175 MPa) is statistically demonstrated for the maximum diameter used and for the targeting machine (type) concerned. | |||
| e) For lattice girders, the lower bars must comply with B500A and / or B500B with the fR / P requirement. The upper bars and diagonals may be reinforcing steel with only the requirements of Re, d and the chemical composition. | |||
| f) For coils fR (ribbed) + 15%, fP (dented) + 5%. No requirement for weakly profiled / dented reinforcing steel (lattice girders). | |||
The specifications for these three grades of steel are covered by BS4449:2005.
More info here.
Steel grades and processing methods – is there a link?
Whilst it might be tempting to assume that certain grades of steel must be cold drawn rather than hot rolled, because of their superior ductility and tensile strength, this is not the case. Steel grades are determined by the composition of the steel and its relative performance, and not by the processed used to make it. Steels of all grades can be hot rolled, cold rolled or cold drawn.
Hopefully, this article has provided an insight into the processes involved in the production of steel used in reinforcement products, and the grades that define each different type of reinforcing steel. With that knowledge, construction professionals should be able to make better purchasing decisions for every project.
