CHAPTER 1
The Steel Material

1.1 General Points about the Steel Material

The term steel refers to a family of iron-carbon alloys characterized by well-defined percentage ratios of main individual components. Specifically, iron-carbon alloys are identified by the carbon (C) content, as follows:

• wrought iron, if the carbon content (i.e. the percentage content in terms of weight) is higher than 1.7% (some literature references have reported a value of 2%);
• steel, when the carbon content is lower than the previously mentioned limit. Furthermore, steel can be classified into extra-mild (C?<?0.15%), mild (C?=?0.15?÷?0.25%), semi-hard (C?=?0.25?÷ 0.50%), hard (C?=?0.50?÷?0.75%) and extra-hard (C?>?0.75%) materials.

Structural steel, also called constructional steel or sometimes carpentry steel, is characterized by a carbon content of between 0.1 and 0.25%. The presence of carbon increases the strength of the material, but at the same time reduces its ductility and weldability; for this reason structural steel is usually characterized by a low carbon content. Besides iron and carbon, structural steel usually contains small quantities of other elements. Some of them are already present in the iron ore and cannot be entirely eliminated during the production process, and others are purposely added to the alloy in order to obtain certain desired physical or mechanical properties.

Among the elements that cannot be completely eliminated during the production process, it is worth mentioning both sulfur (S) and phosphorous (P), which are undesirable because they decrease the material ductility and its weldability (their overall content should be limited to approximately 0.06%). Other undesirable elements that can reduce ductility are nitrogen (N), oxygen (O) and hydrogen (H). The first two also affect the strain-ageing properties of the material, increasing its fragility in regions in which permanent deformations have taken place.

The most important alloying elements that may be added to the materials are manganese (Mn) and silica (Si), which contribute significantly to the improvement of the weldability characteristics of the material, at the same time increasing its strength. In some instances, chromium (Cr) and nickel (Ni) can also be added to the alloy; the former increases the material strength and, if is present in sufficient quantity, improves the corrosion resistance (it is used for stainless steel), whereas the latter increases the strength while reduces the deformability of the material.

Steel is characterized by a symmetric constitutive stress-strain law (s-e). Usually, this law is determined experimentally by means of a tensile test performed on coupons (samples) machined from plate material obtained from the sections of interest (Section 1.7). Figure 1.1 shows a typical stress-strain response to a uniaxial tensile force for a structural steel coupon. In particular, it is possible to distinguish the following regions:

• an initial branch that is mostly linear (elastic phase), in which the material shows a linear elastic behaviour approximately up to the yielding stress (fy). The strain corresponding to fy is usually indicated with ey (yielding strain). The slope of this initial branch corresponds to the modulus of elasticity of the material (also known as longitudinal modulus of elasticity or Young's modulus), usually indicated by E, with a value between 190?000 and 210?000?N/mm2 (from 27?560 to 30?460?ksi, approximately);
• a plastic phase, which is characterized by a small or even zero slope in the s-e reference system;
• the ensuing branch is the hardening phase, in which the slope is considerably smaller when compared to the elastic phase, but still sufficient enough to cause an increase in stress when strain increases, up to the ultimate strength fu. The hardening modulus has values between 4000 and 6000?N/mm2 (from 580 to 870?ksi, approximately).

Figure 1.1 Typical constitutive law for structural steel.

Usually, the uniaxial constitutive law for steel is schematized as a multi-linear relationship, as shown in Figure 1.2 a, and for design purposes an elastic-perfectly plastic approximation is generally used; that is the hardening branch is considered to be horizontal, limiting the maximum strength to the yielding strength.

Figure 1.2 Structural steel: (a) schematization of the uniaxial constitutive law and (b) yield surface for biaxial stress states.

The yielding strength is the most influential parameter for design. Its value is obtained by means of a laboratory uniaxial tensile test, usually performed on coupons cut from the members of interest in suitable locations (see Section 1.7).

In many design situations though, the state of stress is biaxial. In this case, reference is made to the well-known Huber-Hencky-Von Mises criterion (Figure 1.2b) to relate the mono-axial yielding stress (fy) to the state of plane stress with the following expression:

where s1, s2 are the normal stresses and s12 is the shear stress.

In the case of pure shear, the previous equation is reduced to:

With reference to the principal stress directions 1´ and 2´, the yield surface is represented by an ellipse and Eq. (1.1) becomes:

1.1.1 Materials in Accordance with European Provisions

The European provisions prescribe the following values for material properties concerning structural steel design:

The mechanical properties of the steel grades most used for construction are summarized in Tables 1.1a and 1.1b, for hot-rolled and hollow profiles, respectively, in terms of yield strength (fy) and ultimate strength (fu). Similarly, Table 1.2 refers to steel used for mechanical fasteners. With respect to the European nomenclature system for steel used in high strength fasteners, the generic tag (j.k) can be immediately associated to the mechanical characteristics of the material expressed in International System of units (I.S.), considering that:

• j·k·10 represents the yielding strength expressed in N/mm2;
• j·100 represents the failure strength expressed in N/mm2.

Table 1.1a Mechanical characteristics of steels used for hot-rolled profiles.