1. Carbon (C) Content and Its Impact
Carbon levels are optimized to balance strength and weldability.
Grade-Specific Limits:
LR Grade A: C ≤ 0.23% (higher than ABS Grade A's 0.21%).
LR EH36: C ≤ 0.18% (reduced for better weldability).
Trade-offs:
Higher carbon increases strength but raises CEV (carbon equivalent), complicating welding.
TMCP-processed steels achieve strength with lower carbon (≤0.16%).
LR Rule Reference: Pt.3, Ch.2 specifies max carbon based on thickness and grade.
Example: LR FH40 restricts carbon to ≤0.16% to minimize HAZ cracking.
2. Manganese (Mn) and Strength Enhancement
Manganese boosts strength and toughness while mitigating sulfur's harmful effects.
Typical Ranges:
Grade A: 0.60–1.50% Mn.
EH36/FH40: 1.20–1.60% Mn (supports TMCP strengthening).
Functions:
Forms pearlite for tensile strength.
Binds sulfur into MnS inclusions, reducing hot cracking.
LR Flexibility:
Allows higher Mn in TMCP steels (up to 1.70%) vs. normalized grades.
Note: Excessive Mn (>1.80%) may require Ni additions for Arctic grades.
3. Sulfur (S) and Phosphorus (P) Control
Strict sulfur/phosphorus limits ensure ductility and corrosion resistance.
Standard Limits:
S ≤ 0.035%, P ≤ 0.035% for Grade A (vs. ABS's 0.025%).
EH36: S ≤ 0.025%, P ≤ 0.025% (tighter for HIC resistance).
Harmful Effects:
High S causes hot shortness; high P increases cold brittleness.
LR Exceptions:
Seismic-resistant steels may allow P ≤ 0.040% for cost savings.
Case Study: LR rejected a Grade A batch for S = 0.038% (exceeding mill cert).
4. Microalloying Elements (Nb, V, Ti)
Microalloys refine grain structure and enhance strength without carbon penalties.
Typical Additions:
Niobium (Nb): 0.02–0.05% for grain refinement in TMCP.
Vanadium (V): 0.05–0.10% for precipitation strengthening.
LR Requirements:
Nb+V+Ti ≤ 0.15% to avoid excessive hardness.
Mandatory for FH40+ grades in thick plates.
Welding Impact:
Microalloys reduce preheat needs by lowering CEV.
Example: POSCO's LR-certified EH36 uses 0.03% Nb + 0.08% V.
5. Residual Elements and Impurity Restrictions
LR controls trace elements (Cu, Ni, Cr) to prevent unintended effects.
Copper (Cu):
≤0.35% (avoids hot cracking during welding).
≥0.20% required for weathering steels (LR-approved variants).
Nickel (Ni):
≤0.50% in standard grades; up to 3.5% in Arctic steels.
Harmful Residuals:
Tin (Sn) ≤ 0.03%, Arsenic (As) ≤ 0.02% (prevents temper embrittlement).
