738 Acier de construction: Propriétés, Applications, Guide de fabrication

738 l'acier de construction est un alliage haute performance conçu pour les applications porteuses lourdes, force de mélange, ductilité, et la maniabilité. Contrairement aux aciers au carbone standards, its tailored chemical composition (avec des éléments d'alliage comme le chrome et le molybdène) offre des propriétés mécaniques améliorées, ce qui en fait un premier choix pour la construction, automobile, et industries d'équipement lourd. Dans ce guide, nous allons décomposer ses principales caractéristiques, utilisations réelles, procédés de fabrication, and how it compares to other materials—helping you select it for projects that demand reliability under stress.

1. Key Material Properties of 738 Acier de construction

The performance of 738 structural steel starts with its balanced chemical composition, which shapes its robust propriétés mécaniques, fiable physical properties, and practical workability.

Chemical Composition

738 structural steel’s formula is optimized for strength and toughness, with key elements including:

• Carbon content: 0.20-0.25% (balances strength and weldability—higher than low-carbon steels but low enough to avoid brittleness)
• Manganese content: 1.20-1.50% (boosts tensile strength and hardenability without reducing ductility)
• Silicon content: 0.20-0.40% (aids in deoxidation during manufacturing and improves strength)
• Phosphorus content: ≤0.035% (controlled to prevent brittleness, especially in cold environments)
• Sulfur content: ≤0.035% (minimized to maintain ductility and avoid cracking during forming)
• Additional alloying elements: Chromium (0.40-0.60%, enhances corrosion resistance), nickel (0.40-0.60%, improves toughness), and molybdenum (0.15-0.25%, boosts high-temperature strength)

Physical Properties

Propriétés mécaniques

After standard heat treatment (normalizing or quenching and tempering), 738 structural steel delivers impressive load-bearing performance:

• Résistance à la traction: 650-750 MPa (higher than standard carbon structural steels like A36)
• Yield strength: 450-550 MPa (2x higher than A36, reducing material thickness for structural parts)
• Élongation: 18-22% (dans 50 mm—retains ductility to absorb impact, critical for earthquake-resistant buildings)
• Dureté: 180-220 Brinell, 80-90 Rockwell B, 190-230 Vickers (soft enough for machining, hard enough for load-bearing)
• Fatigue strength: 300-350 MPa (at 10⁷ cycles—ideal for parts under repeated stress, like bridge beams or vehicle frames)
• Impact toughness: 60-80 J (at -20°C—resists cracking in cold weather, unlike brittle high-carbon steels)

Other Critical Properties

• Weldability: Excellent—low carbon and controlled alloy content allow welding via MIG, TIG, or stick methods without preheating (critical for on-site construction).
• Usinabilité: Good—softer than high-strength alloys like titanium; uses standard high-speed steel (HSS) tools with minimal wear.
• Formabilité: Very good—can be pressed, bent, or rolled into complex shapes (par ex., curved bridge beams) sans craquer.
• Résistance à la corrosion (relative to carbon steel): Moderate—better than plain carbon steel (thanks to chromium) but requires coating (par ex., galvanisation) for marine or harsh environments.
• Ductilité: High—deforms plastically under load before failing, making it safe for structural applications where sudden failure is catastrophic.

2. Real-World Applications of 738 Acier de construction

738 structural steel’s blend of strength, ductilité, and workability makes it ideal for applications that carry heavy loads or face variable conditions. Here are its most common uses:

Construction Industry

• Poutres structurelles: Floor and roof beams in high-rise buildings use 738—its high yield strength allows thinner beams, reducing building weight and material costs.
• Colonnes: Load-bearing columns in commercial buildings (par ex., malls, bureaux) rely on 738—handles vertical loads of up to 500 kN without buckling.
• Ponts: Highway and railway bridges use 738 for girders—its fatigue strength resists stress from repeated vehicle traffic, and ductility absorbs earthquake energy.
• Buildings: Seismic-resistant buildings in earthquake-prone areas (par ex., California) use 738—its impact toughness prevents collapse during tremors.

Exemple de cas: A construction company used 738 structural steel for a 20-story office building in Tokyo. Compared to standard A36 steel, 738 beams were 20% thinner, reducing overall building weight by 15% and cutting concrete usage (for foundations) par $300,000. The building also passed seismic tests with minimal deformation.

Industrie automobile

• Vehicle frames: Heavy-duty trucks and SUVs use 738 for chassis frames—its strength supports heavy payloads (jusqu'à 10 tonnes) and resists bending from rough roads.
• Suspension components: Truck suspension arms use 738—fatigue strength handles repeated bumps, and ductility prevents breakage during off-road use.
• Axles: Commercial vehicle axles (par ex., delivery trucks) use 738—tensile strength withstands torque from engines, and impact toughness resists pothole damage.

Génie mécanique & Heavy Equipment

• Mechanical engineering: Machine frames for industrial presses and manufacturing lines use 738—stiffness minimizes vibration during operation, and machinability allows precise mounting of components.
• Gears and shafts: Large industrial gears (par ex., in factory conveyors) use 738—hardness resists wear, and toughness prevents tooth breakage.
• Heavy equipment:
• Excavators: Bucket arms and boom structures use 738—handles digging loads of up to 20 tons and resists impact from rocks.
• Cranes: Crane booms use 738—high strength-to-weight ratio allows lifting heavy loads (jusqu'à 100 tonnes) without bending.
• Mining equipment: Mine haul truck frames use 738—resists corrosion from mine water (with galvanizing) and handles payloads of 50 tonnes.

Industrie maritime

• Ship structures: Small cargo ship hulls and deck beams use 738—with galvanizing or painting, it resists saltwater corrosion better than plain carbon steel.
• Offshore platforms: Small offshore oil rig support structures use 738—its strength handles wave loads, and weldability allows on-site assembly.

3. Manufacturing Techniques for 738 Acier de construction

Produire 738 structural steel requires precision to maintain its chemical balance and mechanical properties. Here’s the process:

1. Metallurgical Processes (Alloy Control)

• Electric Arc Furnace (EAF): The primary method—scrap steel, iron ore, and alloying elements (chrome, nickel, molybdène) are melted at 1,600-1,700°C. Sensors monitor chemical composition to ensure carbon, manganèse, and alloys meet target ranges.
• Basic Oxygen Furnace (BOF): For large-scale production—molten iron from a blast furnace is mixed with scrap steel, then oxygen is blown to adjust carbon content. Alloys are added post-blowing to fine-tune properties.

2. Rolling Processes

• Hot rolling: The molten alloy is cast into slabs (200-300 mm d'épaisseur), heated to 1,100-1,200°C, and rolled through a series of mills to form shapes like beams (I-beams, H-beams), assiettes, or bars. Hot rolling improves grain structure and strength.
• Cold rolling: Used for thin sheets (par ex., for automotive frames)—cold-rolled at room temperature to improve surface finish and dimensional accuracy. Cold rolling increases hardness slightly, so annealing may follow to restore ductility.

3. Traitement thermique (Tailoring Strength)

• Normalizing: Heated to 850-900°C and held for 30-60 minutes, then cooled in air. This refines grain size, balances strength and ductility, and is the most common heat treatment for structural applications.
• Quenching and tempering: For high-strength needs—heated to 820-860°C (austenitizing), quenched in water to harden, then tempered at 550-600°C to reduce brittleness. This boosts tensile strength to 750 MPa (used for heavy equipment axles).
• Recuit: Heated to 700-750°C and cooled slowly—softens the steel for complex forming (par ex., curved bridge beams) ou usinage.

4. Forming and Surface Treatment

• Forming methods:
• Press forming: Uses hydraulic presses (1,000-5,000 tonnes) to shape beams or columns into custom profiles (par ex., tapered columns for modern buildings).
• Pliage: Uses roll benders to create curved shapes (par ex., bridge arches)—738’s ductility allows bending to radii as small as 5x the material thickness.
• Soudage: On-site welding of structural parts (par ex., beam-to-column connections) uses MIG welding with low-carbon filler metal—no preheating needed, saving construction time.
• Usinage: CNC mills and lathes shape precision parts (par ex., arbres de transmission)—uses HSS or carbide tools with cutting fluid to prevent overheating.
• Traitement de surface:
• Peinture: Industrial paint (par ex., époxy) is applied to construction steel to prevent rust—common in buildings or inland bridges.
• Galvanisation: Hot-dip galvanizing (revêtement de zinc) is used for marine or outdoor steel (par ex., coques de navires, plateformes offshore)—provides corrosion resistance for 20+ années.
• Shot blasting: Blasts steel with tiny metal beads to remove scale and rust—improves paint/galvanizing adhesion.

5. Contrôle de qualité (Structural Safety Focus)

• Ultrasonic testing: Checks for internal defects (par ex., fissures, voids) in thick beams or columns—critical for load-bearing parts.
• Radiographic testing: Inspects welds for flaws (par ex., porosité, lack of fusion) in bridge or building connections—ensures welds can carry load.
• Essais de traction: Verifies tensile strength (650-750 MPa) and yield strength (450-550 MPa) to meet 738 caractéristiques.
• Microstructure analysis: Examines the alloy under a microscope to confirm uniform grain structure—no brittle phases (par ex., martensite) that could cause failure.
• Tests d'impact: Conducts Charpy V-notch tests at -20°C to ensure impact toughness (60-80 J)—critical for cold-weather or seismic applications.

4. Étude de cas: 738 Structural Steel in Heavy-Duty Crane Booms

A construction equipment manufacturer used standard A572 Grade 50 steel for crane booms. The booms weighed 800 kg and could lift 50 tons—too heavy for mobile cranes, which needed lighter booms to improve maneuverability. They switched to 738 structural steel, with the following results:

• Force & Poids: Le 738 boom was 15% thinner (depuis 20 mm à 17 mm) et pesé 680 kilos (15% plus léger) while lifting the same 50 tons—thanks to 738’s higher yield strength.
• Performance: The lighter boom reduced the crane’s total weight by 1.2 tonnes, améliorer le rendement énergétique en 8% and allowing access to narrow construction sites (where heavier cranes couldn’t fit).
• Économies de coûts: The manufacturer saved $500 per boom in material costs (less steel used) and received 20% more orders for the lighter, more agile cranes.

5. 738 Structural Steel vs. Other Materials

How does 738 structural steel compare to other common structural materials? Let’s break it down with a detailed table:

Application Suitability

• High-Rise Buildings: 738 is better than A36/A572 (thinner beams, lower weight) and cheaper than HSLA or composites.
• Mobile Cranes: 738 outperforms A572 (lighter for same strength) and is far cheaper than aluminum or composites.
• Ponts: 738 is superior to A36 (better fatigue strength for traffic) and more cost-effective than HSLA for most projects.
• Marine Structures: 738 (with galvanizing) is cheaper than aluminum and easier to weld than composites, though aluminum has better corrosion resistance.

Yigu Technology’s View on 738 Acier de construction

Chez Yigu Technologie, we see 738 structural steel as a cost-effective workhorse for heavy-duty applications. Its balanced strength, ductilité, and weldability make it ideal for our construction, automobile, and heavy equipment clients. We often recommend 738 for high-rise beams, crane booms, and seismic-resistant structures—where it cuts material costs (thinner profiles) and simplifies on-site welding. While it needs coating for harsh environments, its low cost and reliable performance deliver better value than premium HSLA or composite materials, aligning with our goal of sustainable, solutions pratiques.

FAQ

1. Can 738 structural steel be used in cold weather?

Oui. 738 has excellent impact toughness (60-80 J at -20°C), so it resists cracking in cold climates (par ex., northern Europe, Canada). It’s commonly used for bridges, buildings, and heavy equipment that operate in sub-zero temperatures.

2. Does 738 structural steel need to be coated for outdoor use?

Oui. Alors que 738 has better corrosion resistance than plain carbon steel, it still rusts in rain, snow, or saltwater. For outdoor applications (par ex., ponts, outdoor cranes), use painting (époxy) or galvanizing (revêtement de zinc) to extend its lifespan to 20+ années.

3. How does 738 compare to HSLA steel for structural projects?

738 has similar yield strength to HSLA Grade 65 but is 10% cheaper and easier to weld (no preheating needed). HSLA has slightly better corrosion resistance, mais 738 is a better choice for most construction/heavy equipment projects—offering similar strength at lower cost and simpler fabrication.