Reading Chinese GB Grade Names: Q355, 42CrMo, GCr15 and NM400 Decoded

GB designations look dense but follow strict rules: yield letters for structural steels, spelled-out chemistry for alloy steels, and hardness numbers for wear plate. This guide decodes the common patterns and their prefixes and suffixes.

Start with the family signal

GB names begin by telling you the family:

Q — structural steel named by yield strength (Q from the Chinese term for yield point). Q355 guarantees 355 MPa minimum yield at ≤ 16 mm thickness.
Digits only — carbon steel named by carbon content: grades 20, 35 and 45 mean roughly 0.20%, 0.35% and 0.45% nominal carbon (GB/T 699 quality carbon steels; GB/T 700 uses the Q system instead).
ZG — cast steel, followed by a yield–tensile pair (ZG230-450).
NM — wear-resistant plate (from the Chinese for abrasion-resistant): the number in NM400 is nominal Brinell hardness, not strength.

Alloy steels: read the recipe left to right

In alloy structural steels (GB/T 3077), the leading digits give carbon in hundredths of a percent; each element symbol follows, with a number for its approximate percentage. An element written without a number is present at roughly 1% or below:

42CrMo = 0.42% C, about 1% Cr, a small Mo addition.
20CrMnTi = 0.20% C with Cr, Mn and Ti — a carburizing gear steel.
40Cr = 0.40% C with about 1% Cr.

A trailing A marks the higher-quality variant with tighter sulfur and phosphorus limits (40CrNiMoA); E denotes special quality in several GB families.

The special notations

GCr15 — G marks rolling bearing steel. After Cr, the number gives chromium in tenths of a percent: GCr15 ≈ 1.5% Cr. Carbon (about 1%) is not written at all, and GCr15SiMn adds silicon and manganese for hardenability.
Spring steels follow the alloy rule: 65Mn = 0.65% C with elevated Mn; 60Si2Mn = 0.60% C, about 2% Si, about 1% Mn.
Q355 suffix letters B/C/D/E are quality levels defined by impact test temperature: B at +20 °C, C at 0 °C, D at −20 °C, E at −40 °C. Further letters refine the use: NH = weathering resistant, GJ = for building structures.
Stainless numbers spell out the full recipe: 06Cr19Ni10 has carbon in the 0.06% class (≤ 0.08%), 19% Cr, 10% Ni. For very low carbon grades the leading figure switches to thousandths: 022Cr19Ni10 ≈ 0.02% C class (≤ 0.03%), the 304L counterpart. Older names such as 0Cr18Ni9 still circulate on legacy drawings.

Worked examples

Name	Decode
Q355D	Structural steel, 355 MPa yield, impact tested at −20 °C
42CrMo	0.42% C chromium-molybdenum alloy steel
GCr15SiMn	Bearing steel, about 1.5% Cr, with added Si and Mn
NM500	Wear plate, nominal hardness about 500 HBW
06Cr19Ni10	Stainless, 0.06% C class, 19% Cr, 10% Ni (304 counterpart)