The core principle of carbon steel plate can be developed from five dimensions: material composition, crystal structure, phase change mechanism, performance regulation and application logic. Its essence is to achieve precise control of mechanical properties through the distribution and organizational evolution of carbon elements in the iron matrix. The following is an in-depth analysis based on metallurgy and materials science:
1. Composition and crystal structure principle: synergistic effect of carbon and iron
Basic composition of iron-carbon alloy
Iron matrix: Carbon steel plate is mainly composed of iron (Fe) (accounting for >98%). At room temperature, iron atoms form body-centered cubic crystals (α-Fe), which have good plasticity but low strength (pure iron tensile strength is about 200MPa).
The role of carbon: The carbon (C) content is usually between 0.02% and 2.11%, and exists in two forms:
Solid solution: Carbon dissolves in the interstitial space of the iron lattice to form ferrite, which is slightly stronger than pure iron (for example, the tensile strength of ferrite with 0.1% carbon is about 250MPa);
Compound: When the carbon content is greater than 0.77%, cementite (Fe₃C) is formed, which has extremely high hardness (HV≈800), but is brittle and needs to be combined with ferrite to form a composite structure.
Correlation between microstructure and performance
Low carbon steel (C≤0.25%): The structure is mainly ferrite, containing a small amount of pearlite (ferrite + cementite layered structure), good plasticity (elongation>25%), suitable for stamping (such as automobile shell);
Medium carbon steel (C=0.25%~0.6%): The proportion of pearlite increases, the strength is improved (tensile strength 500~800MPa), and can be strengthened by quenching (such as 45# steel after quenching and tempering, the strength reaches 850MPa);
High carbon steel (C＞0.6%): Pearlite + network cementite, high hardness (HRC＞50), but low plasticity, mostly used for cutting tools (such as T10 steel files).
2. Principle of heat treatment phase transformation: regulating organizational properties through temperature
Austenitization (heating stage)
When the temperature rises to above 727℃ (eutectoid temperature), ferrite and cementite transform into face-centered cubic crystals (γ-Fe, austenite), and the solubility of austenite is high (2.11% carbon can be dissolved), which lays the foundation for subsequent quenching.
Example: 45# steel plate is heated to 850℃ and kept warm to form uniform austenite. When it is quickly cooled by water, carbon atoms cannot diffuse and are forced to remain in the iron lattice to form martensite (body-centered square structure). The hardness rises suddenly to HRC50+, but it is brittle and needs to be tempered to eliminate stress.
Tempering troostitization (performance optimization)
The quenched martensite is tempered at medium temperature (350~500℃), and fine carbides are precipitated to form tempered troostite, which has both strength (σb≈800MPa) and toughness (AKV≥40J), and is suitable for gear and shaft parts.
Structural adjustment of annealing and normalizing
Annealing (slow cooling): eliminate the work hardening of cold-rolled steel plates (such as the hardness of 08F steel plates after annealing drops from HB200 to HB140), and restore plasticity;
Normalizing (air cooling): refine the grains (grain size above 8 levels), and improve the cutting performance of medium carbon steel (such as 20CrMnTi cutting resistance after normalizing is reduced by 30%).
III. Mechanical properties regulation principle: quantitative relationship between carbon content and structure
Strength-carbon content curve
For every 0.1% increase in carbon content, the tensile strength increases by about 70~100MPa, but after exceeding 0.9%, the cementite network distribution leads to a decrease in strength. For example:
Q235 steel (C≈0.12%): σb=375~500MPa, used for construction steel plates;
65Mn steel (C≈0.65%): σb=900~1100MPa, used for spring steel plates.
Inverse relationship between plasticity and carbon content
Low carbon steel has an elongation (δ) of ＞25% and can be deep-drawn (such as DC01 steel plate for cans); high carbon steel has δ＜10%, and needs to be improved by spheroidizing annealing (cementite spheroidization) to improve plasticity (such as GCr15 bearing steel plate δ reaches 12% after spheroidization).
Fracture toughness and microstructure refinement
By refining the grains to less than 5μm through controlled rolling and controlled cooling (TMCP process), the fracture toughness (KIC) is increased by more than 40%. For example: Q355ND steel plate (grain size 12) for bridges, -20℃ impact energy ≥34J, meeting the seismic requirements.
4. Forming process principle: adaptability from rolling to welding
Rolling deformation and recrystallization
Hot rolling (temperature > recrystallization temperature, about 900℃): The steel plate eliminates casting defects through roller pressure, and the grains are elongated along the rolling direction to form a fibrous structure, and the strength is increased by 15%~20% (such as Q345B hot-rolled steel plate σb=470~630MPa);
Cold rolling (normal temperature rolling): The grains are broken into subgrains, dislocation entanglement is generated, and the strength is doubled (such as SPCC cold-rolled steel plate σb from 300MPa to 600MPa), but annealing is required to eliminate stress.
Welding Metallurgy Principles
When welding carbon steel plates, the temperature of the weld zone reaches above 1500℃, forming a fusion zone (semi-molten state) and a heat-affected zone (HAZ):
The HAZ softening of low-carbon steel (C≤0.2%) is not obvious and can be welded directly (such as Q235 steel plate with E43 welding rod);
Medium-high carbon steel (C＞0.3%) needs to be preheated before welding (such as 35# steel preheated to 150℃) to prevent the formation of hardened martensite in HAZ and cause cracks.
V. Principle Adaptation in Application Scenarios
Structural Bearing Scenarios (Buildings, Bridges)
Using the plastic deformation capacity of low-carbon steel, the safety factor (usually 1.5~2.0) is designed through the yield strength (such as Q355B's σs=355MPa) to ensure that it deforms first and then breaks when overloaded to avoid brittle failure.
Wear-resistant scenarios (mines, construction machinery)
High carbon steel or wear-resistant steel plates (such as NM400) improve wear resistance through martensite + carbide structure, hardness HRC40~45, and wear reduction of 50% compared with ordinary steel plates (such as excavator bucket lining).
Deep drawing scenarios (automobiles, home appliances)
Ultra-low carbon steel (C≤0.02%) is used, and "pie-shaped grains" are obtained through annealing. During stamping, the grains slide along the thickness direction to avoid cracking (such as Baosteel DC06 steel plate can be stamped with complex door parts).
VI. Defect control principle: optimization from metallurgy to processing
Inclusion regulation
Through calcium treatment (Ca content 0.001%~0.003%), sulfide (MnS) is modified into spherical calcium aluminate to reduce cracks during cold bending of steel plates (such as pipeline steel X70 inclusion rating ≤1.0 grade).
Residual stress management
Thick steel plates (>50mm) need stress relief annealing (550~650℃) after welding to eliminate welding residual stress (from 300MPa to below 100MPa) through atomic diffusion to prevent hydrogen-induced cracking.