A: Steel is an alloy with iron as the main element and contains small amounts of carbon (usually less than 2%) and other elements. What we commonly refer to as "iron" (such as cast iron) has a higher carbon content and is brittle and hard. By adjusting its chemical composition and heat treatment, steel can achieve superior comprehensive properties such as strength, toughness, and plasticity, making its applications far wider than cast iron.

A: Yield strength: The stress value at which a material begins to undergo permanent deformation (yield). In structural design, this is the most critical parameter because it determines the maximum load a structure can withstand under normal operating conditions without permanent damage.

Tensile strength: The maximum stress a material can withstand before breaking. It represents the material's ultimate load-bearing capacity.

Simply put, yield strength relates to safety and service life, while tensile strength relates to the ultimate limit of failure.

A: The grade is like an "identity card" for steel, representing its chemical composition and mechanical properties.

Q235B: "Q" represents yield strength, "235" means the yield strength is not less than 235 MPa, and "B" is the quality grade, indicating specific impact toughness requirements at 0°C.

SS400: Japanese standard, "SS" represents ordinary structural steel, and "400" means the tensile strength is not less than 400 MPa.

304: A common stainless steel grade, representing that it contains approximately 18% chromium and 8% nickel, possessing good corrosion resistance and machinability.

A: Characteristics: Hot-rolled steel vs. Cold-rolled steel

Process: Rolled at high temperature vs. further rolled at room temperature

Surface: Oxide scale, dark color, slightly rough feel vs. smooth, bright surface, good feel

Precision: Larger dimensional tolerances vs. precise dimensions, uniform thickness

Performance: Slightly lower strength, better plasticity vs. higher strength and hardness

Cost: Lower vs. Higher

Applications: Building structures, bridges, truck beams, etc. vs. automotive bodies, appliance panels, precision instruments

Selection Recommendation: Choose cold-rolled steel for products requiring high strength, attractive appearance, and high precision; for structural components with less stringent surface and dimensional requirements, hot-rolled steel offers better cost-effectiveness.