STRUCTURAL STEELS CLASSIFIED BY TENSILE STRENGTH

Structural steels classified by tensile strength are primarily selected based on their tensile stress and yield strength values. They are commonly used in steel construction, bridge building, pressure vessels and equipment, vehicle manufacturing, and machinery construction.

These steels are generally defined as non-alloy steels, with mechanical properties primarily dependent on carbon content. However, elements such as nitrogen and phosphorus, as well as manganese, silicon, copper, and sulfur—originating from raw materials and production methods—also significantly affect their properties.

Structural steels are classified into quality groups. With the latest updates in standards:

First-quality group includes only St 33 steel. No chemical analysis values are specified for St 33 steel, and the production method and casting type are left unrestricted. It can be said that first-quality steels are not meticulously refined.

Second-quality group steels contain a maximum of 0.050% phosphorus (P) and 0.050% sulfur (S) and are mainly preferred for welded constructions. Silicon and manganese contents are also specified. Fe 37 steel can be *rimmed, **killed, or ***semi-killed cast. Fe 44, Fe 50, Fe 60, and Fe 70 steels must be either killed or semi-killed cast.

Third-quality group steels are ****aging-resistant and fine-grained steels, specifically cast as killed steels. Phosphorus and sulfur contents are limited to a maximum of 0.040%. They are generally deoxidized with 0.020% aluminum.

For the machining of general structural steels, normalized or cold-formed materials should be preferred. Except for normalizing and stress-relief annealing at approximately 600–650°C, structural steels are not subjected to heat treatment. This is due to the presence of undesirable elements in the composition, which can lead to strong precipitations and hardening cracks.

General structural steels are suitable for spot and roller seam welding, as well as resistance welding. Fe 60 and Fe 70 steels should be slow-cooled or immediately annealed after welding. If the carbon content of the steel is below 0.2%, other pressure welding methods may also be used.

Definitions:
Rimmed steel is a type of steel in which the iron oxide present in the molten steel reacts with a portion of the carbon during solidification in the mold, producing carbon monoxide gas bubbles that affect the steel’s structure and typically create a thin ferrite layer on the surface.
Killed steel is steel that has been fully deoxidized in the ladle or mold using an oxygen-binding (deoxidizing) agent to remove all dissolved oxygen. (This results in a more homogeneous structure compared to other types.)
Semi-killed steel is partially deoxidized steel, where only a portion of the oxygen in the molten steel is removed using a deoxidizing agent in the ladle or mold.
Aging-resistant steel is steel that maintains its impact resistance with minimal changes even after long periods of storage.
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