20 questions about steel

Steel is a metallic alloy made primarily from iron and carbon, with small amounts of other elements such as manganese, nickel and boron. The properties of steel can be modified through various heat treatment processes and the addition of different alloying elements. Steel is known for its strength, durability and versatility, and is used in a wide range of applications, including; construction, transportation, machinery and appliances. 

The raw material for steel is iron ore and/or recycled steel scrap. The molten iron, which is produced in a blast furnace (BF) or electric arc furnace (EAF), is purified in a converter. The required alloying elements are added in the secondary metallurgy. Finally, the steel is cast into slabs, which are then rolled and heat treated. Before delivery, the material can be leveled, blasted, galvanized, metal and color coated. 

The density of steel is approx. 7.85 g/cm³ or 490 lb/ft³. 

When deforming a piece of steel, the resistance against further deformation increases. This is strain or deformation hardening. Yield strength and hardness will increase, while some ductility is lost.

Tensile testing is one of the best-known tests for steel and can be defined by standard EN 10002. Controlled displacement is applied on a standard specimen. The relation between load and elongation is recorded and used to determine a number of material properties, e.g. yield strength and tensile strength. 

Yield strength is the maximum stress level that steel can withstand before plastic deformation occurs. Above this stress level, steel will no longer return to its initial dimensions or shape. Ultimate tensile strength is the stress level where the load is at a maximum during tensile testing and using the original cross section area.

The stress-strain curve for steel shows the relationship between applied load and elongation during tensile testing. It is used to determine mechanical properties of the material, including modulus of elasticity, yield strength, and ultimate tensile strength. The load values are transformed into stress values and the elongation is transformed into strain, resulting in a curve that provides important information on how the material behaves under different loading conditions.

The yield ratio is yield strength (R_p0.2, f_y, R_eh) divided by tensile strength (R_m, f_u). This value can give an indication of the ductility of the steel, the strain hardenability and an intuitive safety margin against plastic instability or failure. 

MPa (mega pascal) is the metric unit and ksi (kilopound per square inch) is the imperial unit for stress (force per unit area). 1 MPa ≈ 0.145 ksi or 1 ksi ≈ 6.89 MPa.

Young’s modulus, or modulus of elasticity, is the slope of the stress-strain curve in the initial elastic regime. It is defined by the initial linear part of a stress-strain curve. Young’s modulus of steel is approximately 200 GPa or 29 000 ksi at room temperature.

Stiffness is the ability of the material to resist elastic deformation, which is closely related to the modulus of elasticity. Strength is the ability to resist plastic deformation or failure. The stiffness and load carrying capacity of a real structure (e.g. a car body) are also determined by its geometry. 

An isotropic material exhibits uniform properties in all directions. For most engineering aspects, steels are isotropic but there are some exceptions. For further details, please contact SSAB Technical Support.  

Thermal Conductivity, Thermal Expansion Coefficient and Specific Heat are some examples of the thermal properties of steels. They vary with the chemical composition of the steel and with the temperature.

All steels become brittle at some low temperature, and they will lose their mechanical properties at some elevated temperature. The limiting temperatures are steel grade specific. Most SSAB steels are suitable for working at a temperature range of about -40 to +200⁰C. Contact SSAB Technical Support.

In the tempering process, a previously hardened steel is usually heated to a temperature (based on the required properties) below the lower critical temperature (A1, ~700⁰C) and cooled at a suitable rate, primarily to increase ductility and toughness, and to relieve quenching stresses. SSAB does not generally recommend tempering. For further details, please contact SSAB Technical Support.  

Annealing is a generic term for a treatment consisting of heating the steel to and maintaining it at a proper temperature (usually above A1), followed by cooling at a suitable rate (based on the type of annealing). This treatment is performed to primarily soften the steel. SSAB does not generally recommend annealing. For further details, please contact SSAB Technical Support. 

Poisson's ratio (ν) measures the deformation in the material in a direction perpendicular to the direction of the applied force. Poisson’s ratio for SSAB steels is approximately 0.3. 

Yes, all SSAB steels are ferritic and thus magnetic. Some plates might contain residual magnetism. 

Steels can be categorized in many different ways, for instance based on their microstructure, mechanical properties, application and/or their carbon or alloying content. SSAB is a supplier of some steel types such as structural, pressure vessel, wear plate and tooling steels. 

A residual element in steel is an element not wanted, but cannot be eliminate in the steel-making process. An element can be an intentional alloying element in one grade and residual element in another grade. In general, the residual elements are coming from the input raw materials as iron ore or recycled steel scrap. The content of the unwanted residual elements in SSAB plates is always lower than the critical levels.