With the increased production capacity, structural steel has become a widely used building material due to its excellent mechanical properties. Thanks to its high strength, good ductility, and excellent seismic performance, it occupies a core position in steel structure buildings. Compared with traditional concrete structures, structural steel is lightweight and can reduce the overall load of the building. Therefore, it has become an indispensable building material in workshops, warehouses, bridges, and high-rise buildings.

What is Structural Steel?

Structural steel is combined with other steel materials through welding or bolting to provide stability and safety for building structures. It has high strength, excellent plasticity and toughness, and is used for load-bearing and supporting structures. It is usually used in buildings, bridges, industrial workshops, and other projects.

Due to the emergence of structural steel, building design is more flexible and has changed the direction of development in the construction industry. Structural steel has been widely used to achieve large spans and open layouts for some buildings with high space requirements, such as airport terminals, gymnasiums, and exhibition centers. In addition, due to the recyclability of materials, structural steel has promoted the development of green buildings. Urban construction not only improves the safety of buildings but also improves the urban landscape.

Composition and characteristics

Excellent mechanical properties make structural steel widely used in load-bearing structures in the construction field. The main chemical composition includes iron, carbon, and appropriate alloying elements such as manganese, silicon, phosphorus, sulfur, etc., which optimize strength and toughness. The carbon content is usually controlled in the low to medium range to ensure good welding performance. In addition, alloying elements such as chromium, nickel, or molybdenum are added to structural steel for corrosion and high-temperature resistance.

Advantages of structural steel

The reason why structural steel occupies an irreplaceable position in modern construction and engineering is mainly attributed to the following key characteristics:

High strength and excellent bearing capacity:

High strength (yield and tensile strength) makes structural steel suitable for buildings with high load-bearing capacity, such as high-rise buildings, bridges, and long-span steel structures. Compared with concrete materials under the same load conditions, less material reduces the overall weight.

Good ductility and plastic deformation capacity:

Structural steel has good ductility and will undergo a certain degree of plastic deformation during an earthquake without sudden fracture, which improves its seismic performance. Therefore, structural steel is widely used in areas where earthquakes occur frequently.

Excellent welding performance

The carbon content of structural steel is between 0.05% and 0.25%. The low-carbon and medium-carbon range gives it good plasticity and toughness, which improves the welding quality. In addition, alloy elements such as manganese (Mn) and silicon (Si) reduce the generation of pores during welding. Good welding performance allows structural steel to be welded by arc welding, gas-shielded welding, submerged arc welding, resistance welding, and other welding methods.

Excellent impact resistance

The toughness of structural steel enables it to effectively absorb and disperse energy when subjected to the impact of external loads (wind loads and seismic forces), reducing the risk of damage.

Durability and environmental adaptability

Structural steel can prevent rust during use through surface anti-corrosion treatment (such as hot-dip galvanizing, painting), reduce subsequent maintenance costs, and extend service life.

Recyclability and sustainability

Structural steel is a recyclable building material. Ninety percent of the material can be re-smelted and reused after demolition, reducing resource waste and environmental pollution and conforming to green and sustainable development.

Challenges and considerations when using structural steel

Structural steel faces some challenges during use, such as corrosion, fire risk, and cost fluctuations. Reasonable protection measures can help increase its service life.

1. Corrosion and weathering: protection strategies

Structural steel exposed to the outdoors for a long time is prone to corrosion and weathering, which affects its structural performance. To reduce the impact of corrosion, standard protection strategies include:

Galvanizing (hot-dip galvanizing or electro-galvanizing) forms a protective layer on the surface of steel to prevent oxidation.

Spraying anti-rust paint (primer: epoxy paint, topcoat: polyurethane paint) increases the weather resistance of steel and is suitable for humid environments.

Perform regular maintenance, check rust, repaint, optimize roof drainage design, and reduce water retention.

2. Fire resistance and the role of intumescent coatings

Steel loses strength at temperatures exceeding 500°C, causing structural instability or collapse. Therefore, steel structures need to enhance their fire resistance. The specific measures are as follows:

Expandable fire retardant coating: expands at high temperatures to form a foam insulation layer, delaying the temperature rise of steel, extending the load-bearing time of the structure, and buying time for personnel evacuation and fire fighting.

Thick fire retardant coating: Steel is coated with refractory materials such as gypsum or cement-based coatings, suitable for high-risk environments such as tunnels and chemical plants.

Fireproof coating: steel beams and columns are coated with fireproof boards, refractory bricks or concrete to improve the fire resistance limit.

Structural steel manufacturing process

The manufacturing design of structural steel has multiple links, from raw material extraction to the final product, and each link affects the quality of the steel.

1. Raw material extraction and refining

The production of structural steel begins with the mining and smelting of iron ore. The screened iron ore is fed into a blast furnace together with coke and limestone for smelting to produce pig iron, which is then refined in a converter to remove impurities and adjust the alloy composition to obtain molten steel, which is then solidified into billets through a continuous casting process to provide the base material for subsequent rolling.

2. The role of rolling, cutting, and manufacturing in production

The billets are fed into the rolling mill for hot or cold rolling to form structural steels of different shapes, such as I-beams, H-beams, angles, channels, and pipes. The rolling process can improve the mechanical properties of the steel and increase its strength and toughness.

Classification of structural steel:

1. General purpose structural steel: carbon steel

Low-carbon steel contains a maximum of about 0.30 wt% C and about 1.65 wt% Mn. Most rolled steels used for welding are low-carbon steels, and their weldability varies greatly.

For example, most welding processes can weld mild steels with carbon contents less than 0.15% and mild steels with 0.15 – 0.30% C (sometimes called low-carbon steels) up to 25 mm (1 in) thick. However, thicker sections of these steels may require additional measures to weld successfully.
This type of steel is generally called non-alloy or general-purpose steel and is included in the following standards.

Standard

International Standard “ISO630-2Structural steels — Part 2: Technical delivery conditions for structural steels for general purposes.”

European Standard “EN10025-2 Hot rolled products of Structural steels — Part 2: Technical delivery conditions for non-alloy structural steels.”

Chinese Standard “GB/T34560.2-2017_Structural Steel Part 2: Technical Delivery Conditions for General-Purpose Structural Steels.”

Classification of Carbon Steel:

Low Carbon Steel
Low carbon steel is less than 0.25%C, especially 08F, 08Al, etc. with a carbon content of less than 0.10%

Medium Carbon Steel
0.25~0.60 % C is medium carbon steel, primarily used in the quenching and tempering state to make parts for the machinery manufacturing industry. Quenching and tempering of 22~34HRC can obtain comprehensive mechanical properties, and it is easy to cut.

High carbon steel
High-carbon steel with more than 0.6 %C is mainly used to make springs, gears, rollers, etc.

Low alloy high strength steel

High-strength low-alloy steel is generally carbon manganese, with tiny amounts of niobium and vanadium added to ensure grain refinement and precipitation hardening.

The design aims to provide better mechanical properties than traditional carbon steels. The yield strength of these steels is generally 290 to 550 MPa (42 to 80 ksi).

Standard

International standard “ISO 630-3:2012 Structural steels — Part 3: Technical delivery conditions for fine-grain
structural steels “N type.

European standard “EN10025-3 Hot rolled products of Structural steels — Part 3: Technical delivery conditions for normalized/normalized rolled weldable fine grain structural steels.”

Chinese standard “GB T 34560.3 Structural steel Part 3 Technical delivery conditions for fine grain structural steels”.

Thermomechanical rolled steels

Thermomechanical rolling was developed to meet the demand for high-strength steels for large-diameter pipelines with good low-temperature toughness and weldability.

The yield strength of these steels reaches 700 MPa (100 ksi) or more. They can generally be welded without preheating. However, preheating may be required at high strength levels to prevent weld metal cracking.

Standard

International Standard: Class M in the “ISO 630-3:2012 Structural steels — Part 3: Technical delivery conditions for fine-grain structural steels.”

European standard “Hot rolled products of structural steels – Part 4: Technical delivery conditions for thermomechanical rolled weldable fine grain structural steels.”

European standard “EN10149 Hot rolled flat products made of high yield strength steels for cold forming.”

Chinese standard: Class M in the “GB T 34560.3 Structural steel Part 3 Technical delivery conditions for fine grain structural steels.”

Quenched and tempered steel

Quenched and tempered steel is heat treated to obtain a yield strength YS of 350 to 1030 MPa.

Welded parts of these materials generally do not require further heat treatment unless post-weld heat treatment (stress relief) is performed in some exceptional cases.

Standard

International standard “ISO 630-4 Structural steels — Part 3: Technical delivery conditions for fine-grain structural steels.”
European standard “EN10025 Hot rolled products of Structural
steels — Part 6: Technical delivery conditions for flat products of high yield strength structural steels in the quenched and tempered condition.”
The Chinese standard is “GB T 34560.4 Structural steel—Part 4: Technical delivery conditions for quenched and tempered high-yield strength structural steels.”

Common types of structural steel:

1. I-beam

An I-beam is a rolled iron beam, steel beam, or cast steel beam with an I-shaped cross-section, and it also refers to a composite beam with an I-shaped cross-section.

Application field
Building structure: I-beam is often used as a material for building structures such as beams, columns, stairs, and platforms

Bridges: I-beams are often used as bridges and parts on bridges, such as railings, beams, columns, and abutments. Its high strength and good bending resistance make bridges more stable and safe.

2. H-beam

The H-beam is an efficient profile with an economical structural cross-section. It requires more optimization of effective cross-sectional area and distribution problems and a more scientific and reasonable strength-to-weight ratio. It is named because its cross-section resembles the English letter “H.”

Application field:
H-beam is mainly used for beam and column components in industrial and civil structures, such as steel structure load-bearing brackets of industrial structures, steel piles and support structures of underground projects, industrial equipment structures such as petrochemicals and power, large-span steel bridge components, ship, and machinery manufacturing frame structures, etc.

3. Channel steel

Channel steel is a long steel strip with a grooved cross-section. It belongs to carbon structural steel for construction and machinery. It is a steel section with a complex cross-section and a grooved cross-section. Channel steel is mainly used in building structures, curtain wall engineering, mechanical equipment, vehicle manufacturing, etc.

Application areas:

Channel steel is mainly used in steel structure frames, supporting structures, and other construction applications. Its high strength and good plasticity make the building structure more stable and reliable.

4. Angle steel

Angle steel is a long steel strip with two sides perpendicular to each other. There are equal angle steel and unequal angle steel. The two sides of the equal-angled steel are equal in width.

Application:
It is widely used in various building and engineering structures, such as beams, bridges, transmission towers, lifting and transportation machinery, ships, industrial furnaces, reaction towers, container racks, and warehouse shelves.

5. T-shaped steel

T-shaped steel has a specific shape and purpose, and its cross-section is T-shaped. Due to its unique shape and performance, this steel is widely used in construction, bridges, vehicles, ships, and machinery manufacturing.

Application fields:

T-shaped steel is widely used in construction to manufacture structural components such as beams and columns, which can improve a building’s bearing capacity and stability.

6. Circular tube

A circular tube is steel, with two ends open and hollow and a concentric circle cross-section. Its length is relatively large compared to its perimeter.

Application:
It can be used for pipelines, thermal equipment, machinery, petroleum geological drilling, containers, the chemical industry, and special purposes.

7. Rectangular tube

A rectangular tube is a hollow, long strip of steel. The weight is light when the bending and torsional strengths are the same, so it is widely used in manufacturing mechanical parts and engineering structures.

Application:
It can be used in construction, machinery manufacturing, steel construction projects, shipbuilding, solar power generation brackets, steel structure engineering, power engineering, power plants, agricultural and chemical machinery, glass curtain walls, automobile chassis, airports, etc.

8. Square tube

A square tube is a tubular material with a square cross-section. Different production processes and uses allow square tubes to be divided into seamless and welded tubes.

Application:
Square tubes are widely used in the construction field, such as steel structure frames, bridges, highway guardrails, etc. Due to their high strength, lightweight, and beautiful appearance, square tubes have become an important supporting material for modern buildings.

9. Flat steel

Flat steel refers to a long steel strip with a flat cross-section. It is characterized by a width much more significant than its thickness and is usually used to manufacture various structural parts, components, etc.

Application:
Flat steel is a common material in construction engineering and is widely used in culverts, columns, beams, walls, etc. It can be processed into different shapes to construct structures and improve building safety and stability.

Property of structural steel:

1. Strength
   Strength is one of the most essential properties of structural steel, mainly including yield strength and tensile strength. Yield strength refers to the stress value at which steel begins to undergo plastic deformation after being subjected to force. Tensile strength: refers to the maximum stress value that steel can withstand during the stretching process.
   Example: The yield strength of Q235 steel is 235 MPa, and the tensile strength is 375-460 MPa.
2. Plasticity
   Plasticity refers to the ability of steel to deform without breaking after being subjected to force. Steel with good plasticity can absorb more energy after being stressed, which improves the safety of the structure.
   Example: The elongation of Q345 steel can reach 21%, showing good plasticity.
3. Toughness
   Toughness refers to the ability of steel to resist brittle fracture at low temperatures. High-toughness steel can still maintain good mechanical properties in low temperature environments.
   Example: The impact toughness of Q345E steel at -40°C can reach 34 J.
4. Fatigue property
   Fatigue performance refers to the ability of a material to resist fracture under cyclic stress. Fatigue performance is particularly important for steel structures, especially in structures such as bridges and cranes that are subjected to repeated loads.
   Example: For Q235 steel, the fatigue limit is about 200 MPa.
5. Welding property
   Welding performance refers to the performance of a material in forming a high-quality welded joint during welding. The quality of welding performance directly affects the reliability and safety of the structure.
   Welding crack sensitivity refers to the tendency of a material to crack during welding.
   It is usually evaluated by carbon equivalent (CE).
   Example: The carbon equivalent CE of Q235 steel is about 0.3%, and its welding performance is good.
6. Weld joint strength
   The strength of the weld joint should not be lower than the strength of the parent material to ensure the overall performance of the structure. Example: For Q345 steel, the yield strength of the weld joint should not be lower than 345 MPa.
7. Corrosion resistance
   Corrosion resistance refers to the ability of the material to resist environmental corrosion. Structural steel is often corroded by the atmosphere, water, chemical media, etc. during use.
   Example: Coating protection: Apply anti-rust paint on the surface of the steel structure to form a protective layer.

FAQS about Structural Steel

1. What is the most common structural steel?

Low-alloy high-strength steel is the most commonly used structural steel. Its carbon content is low, generally between 0.1% and 0.2%, and it contains alloying elements such as manganese, silicon, chromium, and nickel.

2. What is the difference between rebar and structural steel?

Rebar is a slender, rod-shaped steel used in concrete to enhance strength.
Structural steel is a type of steel used in construction or engineering, such as I-beams, H-beams, angle steels, channel steels, and steel pipes.

3. What are the characteristics of structural steel?

Structural steel has high strength, good plasticity, good low temperature toughness and aging hardening properties. It is mainly used in steel structures of various industrial buildings and infrastructure, such as bridges, ships, vehicles, buildings, etc.

4. What are the grades of structural steel?

Structural steel is usually classified according to yield strength, tensile strength, and impact properties. Common structural steel grades include Q235B, Q355B (Chinese standard), S235, S355 (European standard,) and ASTM A36, A572 (American standard). Reasonable selection should be made in steel structure building design to ensure the safety and stability of the structure.