NK/K A51|NK A51 STEEL PLATE

NK A51 steel plate, also written as KA51, K A51, ClassNK A51, NK Grade A51 or market names such as AH50 / AQ51, is the extra-high-strength hull structural steel of ClassNK (Nippon Kaiji Kyokai) Rules for the Survey and Construction of Steel Ships (IACS UR W16 extra-high-strength family) with 500 MPa minimum yield, 610-770 MPa tensile strength and Charpy V-notch toughness at 0°C [32°F]. In the grade name, "A" is the toughness class, and "51" is the site strength label for about 500 MPa yield (ABS AQ51 family). Gangsteel, a steel plate exporter, supplies NK A51 / KA51 plates cut to size for shipbuilding, offshore platforms and marine structures.

Data source: Per IACS UR W16 (extra-high-strength hull structural steel) as adopted in ClassNK hull structural steel rules, cross-checked with LR AH50 / ABS AQ51 sibling pages on gangsteel.net. Official mill/certificate grade is typically KA51 (K A51); NK A51 names the same class on this site. Verify the current ClassNK edition before ordering.

NK A51 / KA51 Chemical Composition (Heat Analysis)

Per IACS UR W16 / ClassNK extra-high-strength rules. Exact maxima for Ni, Cu, Cr and Mo depend on delivery condition (N/NR, TM or QT). Carbon equivalent / CET / Pcm limits apply by thickness and condition; confirm on the mill certificate and current rule edition.

Product Analysis (Check Analysis)

When product analysis is required, apply ASTM A6/A6M Table A permitted variations to the heat limits unless the element is min-only or not listed in Table A.

Product analysis applies only when specified; for a range, results may not vary both above and below it.

NK A51 / KA51 Mechanical Properties

Per IACS UR W16 / ClassNK rules (common t≤50 mm values). Yield may step down for thicker plates per UR W16; confirm thickness-dependent values on the order.

Charpy V-Notch Impact Requirements (0°C [32°F])

Per IACS UR W16 / ClassNK extra-high-strength practice. Average of three full-size specimens; transverse values apply when required. Thicker plates by agreement.

Delivery Condition and Deoxidation

Deoxidation: Killed, fine-grain practice. Delivery condition: typically TMCP or QT for extra-high-strength plates; N/NR may apply at lower extra-high levels per rules. A-class (K A51 / AQ51) grades may be as-rolled, controlled rolled, normalized or TMCP depending on thickness; thicker plates usually N or TMCP. Optional extras: UT, Z15/Z25/Z35, ClassNK survey.

Thickness Coverage

Extra-high-strength NK A/D/E/F 43/47/51/56/63/70 plates (about 420-690 MPa family) are commonly supplied from about 6 mm to 100 mm thick (thicker by agreement under UR W16). Width 1500-4000 mm and length 6000-18000 mm are typical.

Equivalent Steel Grades of NK A51

IACS societies align extra-high-strength hull grades under UR W16; naming differs (NK A51, LR AH50, ABS AQ51, DNV A500). Dual certification is common when ordered that way.

FAQ about NK A51 / KA51 Steel Plate

What is NK A51 (KA51) steel plate?

It is an extra-high-strength hull structural steel under ClassNK Rules (IACS UR W16 extra-high-strength family), with 500 MPa minimum yield, 610-770 MPa tensile strength and Charpy toughness at 0°C [32°F].

What does NK A51 / KA51 stand for?

NK (ClassNK) is Nippon Kaiji Kyokai. The K prefix (KA51) is the ClassNK mill designation. The letter A is the toughness class (Charpy at 0°C [32°F]), and 51 is the yield-strength level in MPa. Market/LR naming often uses AH50; ABS often uses AQ51. This site uses ABS-like numbers (A43…A70), corresponding to about 500 MPa (ABS AQ51 / LR AH50 / DNV A500 family); there are no A420 paths under /nk/.

What is the yield strength of NK A51?

Minimum 500 MPa (73 ksi), tensile 610-770 MPa (88-112 ksi), elongation about 16% min (for the common thickness range).

What is the impact test temperature for NK A51 / KA51?

Charpy V-notch at 0°C [32°F]. Typical longitudinal average minimum is 50 J (transverse 33 J) for the tabulated thickness range on this grade page.

What is the difference between NK A51, D51, E51 and F51?

Same 500 MPa strength class. Toughness class differs: A at 0°C, D at -20°C, E at -40°C, F at -60°C. Chemistry (C, P, S, Mn) is tightened for colder classes.

How does NK A51 differ from higher-strength NK A40?

NK A40 is UR W11 higher-strength (390 MPa). NK A51 is UR W16 extra-high-strength (500 MPa) with different chemistry and delivery practice (often TM/QT).

Is NK A51 the same as LR AH50 or ABS AQ51?

They are IACS counterparts in the extra-high-strength family; exact chemistry/delivery options differ by society edition. Dual certification is available when surveys are arranged.

Is NK A51 weldable?

Weldable with approved procedures for extra-high-strength ship steel. Preheat, heat input and consumable matching are critical; follow the mill WPS and ClassNK welding requirements.

What heat treatment is required for NK A51?

Extra-high-strength grades are typically delivered TMCP or quenched and tempered (QT), sometimes normalized/normalized-rolled depending on strength level and thickness. A-class (K A51 / AQ51) grades may be as-rolled, controlled rolled, normalized or TMCP depending on thickness; thicker plates usually N or TMCP. Optional extras: UT, Z15/Z25/Z35, ClassNK survey.

What thickness is available for NK A51?

Common supply is about 6-100 mm (thicker by agreement under UR W16 / ClassNK rules), 1500-4000 mm wide and 6000-18000 mm long.

Other NK Steel Plate Grades

NK Grade A steel plate | NK Grade B steel plate | NK Grade D steel plate | NK Grade E steel plate | NK A32 steel plate | NK A36 steel plate | NK A40 steel plate | NK D32 steel plate | NK D36 steel plate | NK D40 steel plate | NK E32 steel plate | NK E36 steel plate | NK E40 steel plate | NK F32 steel plate | NK F36 steel plate | NK F40 steel plate | NK A43 steel plate | NK A47 steel plate | NK A56 steel plate | NK A63 steel plate | NK A70 steel plate | NK D43 steel plate | NK D47 steel plate | NK D51 steel plate | NK D56 steel plate | NK D63 steel plate | NK D70 steel plate | NK E43 steel plate | NK E47 steel plate | NK E51 steel plate | NK E56 steel plate | NK E63 steel plate | NK E70 steel plate | NK F43 steel plate | NK F47 steel plate | NK F51 steel plate | NK F56 steel plate | NK F63 steel plate | NK F70 steel plate

Equivalent Class Society Grades

NK A51 / KA51 corresponds to extra-high-strength counterparts under other IACS societies (LR AH50, ABS AQ51, etc.): LR AH50 steel plate, ABS AQ51 steel plate, CCS A500 steel plate, GL A500 steel plate, DNV A500 steel plate, BV A500 steel plate, KR A51 steel plate, RINA A500 steel plate.