Structural steel: production, properties and grades

Steel is the second great structural material of civil engineering. It is strong in both tension and compression, ductile, reliable, recyclable and quick to erect. It frames buildings, spans bridges, reinforces concrete, prestresses girders and forms piles, towers and pipelines. Its properties are far more consistent than those of concrete or wood because it is made in a controlled industrial process, but they still depend on chemistry, processing and temperature, and an engineer who specifies or inspects steel needs to understand all three.

On the sports complex job, Tamarack reviews mill test certificates for the wide-flange columns and roof trusses, checks the weld procedures of the fabricator, and witnesses ultrasonic testing of the complete-joint-penetration welds in the moment frames. The structural drawings call for ASTM A992 for wide-flange shapes, ASTM A500 Grade C for the hollow structural sections, and supplementary Charpy toughness requirements for the exposed exterior truss chords.

Making steel

Steel is iron with a small amount of carbon, usually less than 0.3 percent in structural grades, plus manganese and other deliberately added elements. Two routes produce almost all modern steel. In the integrated route, iron ore, coke and limestone are smelted in a blast furnace to make molten pig iron, which is refined in a basic oxygen furnace by blowing oxygen through it to burn off excess carbon. In the electric arc furnace (EAF) route, steel scrap, sometimes with directly reduced iron, is melted by electric arcs from graphite electrodes. Most structural shapes and reinforcing bars in North America are now made by the EAF route from recycled scrap, which uses far less energy and emits far less carbon dioxide per metric ton than the blast furnace route, especially where the electricity is low-carbon.

After refining and alloying in a ladle, the liquid steel is cast continuously into slabs, blooms or beam blanks, which are then hot rolled through a series of stands into plates, wide-flange shapes, channels, angles, bars or coils. The temperature at which rolling finishes and the rate of cooling affect grain size, and therefore strength and toughness; thermomechanically controlled processing and quenching with self-tempering produce high strengths with low carbon contents. Hollow structural sections are formed from flat strip and welded along a seam, then sometimes heat treated.

Chemistry and its effects

Weldability is often judged by the carbon equivalent, which combines the hardening effect of the alloying elements into one number. A commonly used form, from the International Institute of Welding, is:

Steels with a CE below about 0.40 to 0.45 are generally easy to weld; higher values call for preheat and controlled welding procedures to avoid hard, crack-prone zones next to the weld. Welding codes base their preheat requirements on the steel's chemistry and thickness.

The stress–strain curve of structural steel

Mild and high-strength low-alloy structural steels have a characteristic stress–strain curve. It rises linearly with a modulus of about 200 GPa (29 000 ksi) to the yield point, then runs nearly flat along a yield plateau to a strain of roughly 10 to 20 times the yield strain, then rises again by strain hardening to the tensile strength, and finally falls as the specimen necks before fracture at elongations of 20 percent or more.

The yield plateau and strain hardening are what make steel structures forgiving. A locally overstressed region yields and sheds load to neighboring regions instead of breaking, and plastic hinges can form in beams, allowing the structure to redistribute moments and absorb energy in an earthquake. Design codes rely on this ductility, and that is why they limit the ratio of yield to tensile strength for seismic members and set minimum elongation requirements.

Grades of structural steel

In the United States, structural steel is specified by ASTM standards. Wide-flange shapes are almost always ASTM A992, with a minimum yield of 50 ksi, a maximum yield of 65 ksi, a minimum tensile strength of 65 ksi and a maximum yield-to-tensile ratio of 0.85. Plates and other shapes are often ASTM A572 Grade 50 or A36 (36 ksi yield); weathering steel is ASTM A588 or A709 Grade 50W for bridges; and hollow structural sections are ASTM A500 Grade C (50 ksi yield for round and rectangular sections) or A1085, which has tighter tolerances and a toughness requirement. Bridge steels are specified under ASTM A709 / AASHTO M 270 with Charpy requirements by temperature zone.

Toughness and brittle fracture

Ductility in a slow tension test does not guarantee that steel will behave in a ductile way under every condition. At low temperatures, high loading rates and where there are sharp notches, cracks or thick plates, steel can fracture in a brittle manner with little warning. The famous brittle failures of welded ships in the 1940s and of several bridges and buildings since were caused by this combination.

Toughness is measured by the Charpy V-notch test, in which a small notched bar is struck by a swinging pendulum and the energy absorbed in breaking it is recorded. Testing at several temperatures traces a transition curve: high energy on the upper (ductile) shelf, low energy on the lower (brittle) shelf, and a transition region in between. Specifications for bridges, exposed members in cold climates and fracture-critical members require a minimum Charpy energy at a test temperature related to the lowest service temperature, such as 27 J at −30 °C (20 ft·lb at −20 °F) for some categories.

Reinforcing and prestressing steel

Reinforcing bars are hot-rolled with deformations (ribs) that grip the concrete. ASTM A615 covers carbon-steel bars, most often Grade 60 (60 ksi yield), and ASTM A706 covers low-alloy bars with controlled chemistry for weldability and ductility, required in many seismic applications. Bar sizes run from No. 3 to No. 18, where the number is the nominal diameter in eighths of an inch. Corrosion-resistant options include epoxy-coated, galvanized, stainless, low-carbon chromium (ASTM A1035) and glass-fiber-reinforced polymer (GFRP) bars.

Prestressing strand is made of high-carbon cold-drawn wires twisted into seven-wire strand, with a tensile strength of about 1860 MPa (270 ksi), roughly four times that of mild steel. Its stress–strain curve has no yield plateau, so its yield is defined by an offset or extension method. Low-relaxation strand is stress-relieved under tension so that it loses less force over time.

Mill test certificates and inspection

Every heat of steel is tested at the mill, and a mill test certificate (or material test report) records the heat number, specification and grade, chemical analysis, yield and tensile strength, elongation and, where required, Charpy results. The heat number is marked on the product so the material can be traced. Reviewing certificates is a routine job for inspectors, who check that each property meets the specified grade, that the heat numbers on the steel match the documents, and that supplementary requirements such as toughness were tested.

def carbon_equivalent(c):
    return c["C"] + c["Mn"]/6 + (c["Cr"] + c["Mo"] + c["V"])/5 + (c["Ni"] + c["Cu"])/15

def review(cert, fy_min, fy_max, fu_min, ratio_max):
    notes = []
    if not fy_min <= cert["fy"] <= fy_max: notes.append("yield out of range")
    if cert["fu"] < fu_min: notes.append("tensile below minimum")
    if cert["fy"] / cert["fu"] > ratio_max: notes.append("Fy/Fu too high")
    return notes

certs = [
    {"heat": "H41872", "fy": 392, "fu": 488, "chem": dict(C=0.08, Mn=1.10, Cr=0.08, Mo=0.02, V=0.03, Ni=0.10, Cu=0.25)},
    {"heat": "H41903", "fy": 455, "fu": 505, "chem": dict(C=0.11, Mn=1.35, Cr=0.12, Mo=0.04, V=0.05, Ni=0.15, Cu=0.35)},
]
for c in certs:
    ce = carbon_equivalent(c["chem"])
    notes = review(c, 345, 450, 450, 0.85)
    print(f"{c['heat']}: CE = {ce:.2f}, Fy/Fu = {c['fy']/c['fu']:.2f}, {'OK' if not notes else '; '.join(notes)}")

H41872: CE = 0.31, Fy/Fu = 0.80, OK
H41903: CE = 0.41, Fy/Fu = 0.90, yield out of range; Fy/Fu too high

The second heat has a yield strength above the 450 MPa (65 ksi) maximum, a yield-to-tensile ratio of 0.90, above the limit, and a higher carbon equivalent of 0.41, so Tamarack would ask the supplier for replacement material or an engineering review before it is used in the moment frames, where ductility matters most.

Residual stresses, cold work and fatigue

Hot-rolled shapes cool unevenly: flange tips and web centers cool first, and the slower-cooling junctions are left in residual tension while the tips are in residual compression. These locked-in stresses, often 30 percent or more of the yield strength, do not change the strength of a member in tension, but they make columns yield earlier at the flange tips and reduce buckling resistance, which is why column design curves differ for hot-rolled, welded and stress-relieved sections. Welding also leaves residual stresses near the weld that can reach the yield strength.

Cold working, such as bending a bar or cold-forming a hollow section, raises the yield strength locally but reduces ductility, and if the steel later ages, especially after welding near the cold-worked zone, it can become brittle. Reinforcing bar bend radii are therefore limited, and bars should not be re-bent cold in the field without approval. Under repeated loading, cracks can start at weld toes, bolt holes and abrupt changes of section and grow slowly with each cycle; fatigue design of bridges and crane girders limits the stress range at each detail according to its category, and inspectors look for fatigue cracks at those details throughout a bridge's life.

Steel at high and low temperatures

Steel loses strength and stiffness as it heats: by about 550 to 600 °C (1000 to 1100 °F) its yield strength has fallen to roughly half the room-temperature value, which is why building codes require fire protection by sprayed or board insulation, intumescent coatings or concrete encasement, or a performance-based fire design. Thermal expansion of steel, about 12 millionths per degree Celsius, is almost the same as that of concrete, which is one reason reinforced concrete works.