[L1] 1 Carbon Steel [L2] 1) Definition of Carbon Steel [L4] - Carbon steel is a steel material containing carbon, silicon, and manganese, with trace amounts of phosphorus and sulfur as impurities, and no other alloying elements such as chromium or nickel added. [L5] * Based on carbon content of 0.8%, less than this is classified as steel (0.02~0.8) and more than this as cast iron (0.8~2.0). [L4] - Generally, carbon steel is classified by carbon content into low carbon steel (0.3% or less), medium carbon steel (0.3~0.6%), and high carbon steel (above 0.6%). [L2] 2) Characteristics of Carbon Steel [L4] - Carbon steel encompasses various steels from very mild steel with about 0.05%C to high carbon steel, and their properties differ significantly, leading to varying uses based on carbon content. [L4] - Mechanical properties can be easily altered through heat treatment and machining. [L4] - As the carbon content of carbon steel increases, its hardenability and hardness increase. [L4] - Although manufactured by hot rolling, it can also be produced through hot working processes such as extrusion and forging, and is supplied in various forms such as shaped steel (e.g., bar, square), wire rod, and plate. [L1] 2 Types of Carbon Steel [L2] 1) Low Carbon Steel with 0.15%C or less [L4] - Used after increasing strength and hardness through cold working processes such as pressing and cold drawing. [L4] - Used where cold workability, weldability, and corrosion resistance are required. [L4] - Small containers, automotive parts, bolts and nuts, steel plates/pipes for boilers. [L2] 2) 0.15~0.25%C Carbon Steel [L4] - A carbon content range where machinability is important. [L4] - Materials around 0.15%C are for cold working. [L4] - Materials around 0.25%C are widely used for construction, bridges, boilers, bolts, nuts, pins, steel pipes, etc. [L4] - SM490 = 0.18% C 이하 [L2] 3) 0.25~0.35%C Carbon Steel [L4] - When quenched and tempered, hardness increases, making it suitable for general machine parts, casting, forging, machining, and welding. [L2] 4) 0.35~0.45%C Medium Carbon Steel [L4] - Used for products requiring high strength in relatively large forgings or for parts demanding toughness. [L4] - Used in shipbuilding, bridges, boilers, axles, crankshafts, etc. [L4] - Low weldability. [L2] 5) 0.5~1.7%C High Carbon Steel [L4] - Quenching is possible, but due to high hardenability, it is widely used primarily for tools, etc. [L1] 3 Phase Transformation of Iron [L2] 1) Iron Phase Diagram [L2] 2) Transformation [L4] - Magnetic Transformation: A change in the magnetic properties of iron. [L4] - Allotropic Transformation: When the same substance changes its crystal structure at a certain temperature boundary. [L4] - A0 Transformation Point: The magnetic transformation point of cementite, where its magnetic properties change around 210℃. Below 210℃, it is ferromagnetic, but above 210℃, it exhibits paramagnetic properties. [L4] - A1 Transformation Point [L5] a. This is the eutectoid point, where the eutectoid reaction of steel begins. It occurs around 723℃. [L5] b. The A1 transformation point exists only in steel and cast iron, showing a maximum transformation amount at 0.85%C. The transformation when heated is called Ac1, and when cooled, Ar1. At this Ar1 transformation point, if steel is observed in a dark place, it suddenly emits light, which is also called the recalescence point. [L5] c. At the Ac1 point, steel contracts and electrical resistance increases (the opposite phenomenon occurs at Ar1). [L4] - A2 Transformation Point [L5] a. This is the magnetic transformation of iron, occurring around 768℃, also known as the Curie point. [L5] b. Below the A2 transformation point, iron is ferromagnetic, but above it, it becomes paramagnetic. [L4] - A3 Transformation Point [L5] a. This is an allotropic transformation point where BCC changes to FCC, occurring around 910℃. [L5] b. At the eutectoid point, the temperatures of A1 and A3 are the same. [L5] c. The A3 transformation point is crucial for quenching, annealing, and tempering of steel. These heat treatments are performed after heating the steel to a temperature approximately 50℃ higher than the Acm transformation point. [L4] - A4 Transformation Point: One of the allotropic transformations where the crystal lattice changes at 1390℃.