How do I read a grade like 50W470 or 35W300?

The first number is thickness in hundredths of a millimetre (50 = 0.50 mm), W means non-oriented electrical steel, and the last number is the guaranteed maximum core loss at 1.5 T, 50 Hz, ×100 (470 = 4.70 W/kg). The equivalent IEC 60404-8-4 name is M470-50A.

When should I use thinner steel?

Eddy-current loss rises with thickness squared and frequency squared. For high-speed or high-pole-count motors where the electrical frequency is several hundred hertz, 0.35 mm or 0.27/0.20 mm grades cut losses noticeably; at 50–100 Hz, 0.5 mm is usually the economic choice.

What should come with each coil?

The mill certificate with heat number, measured core loss and polarisation (Epstein or single-sheet test), thickness and coating type. Check coating type (C3, C5, C6) — it affects interlocking, welding and insulation between laminations.

Raw material

Non-oriented Electrical Steel

Steelmaking & vacuum degassing → Hot rolling → Pickling & cold rolling → Continuous annealing → Insulation coating → Testing & slitting

✓ Expert reviewed All content on this page has been reviewed by a senior motor engineer.

Process flow

6 main process steps, with the equipment and instruments used at each step, their main suppliers, and the technical control points.

1
Steelmaking & vacuum degassing
Hot metal is refined in a converter, then RH-degassed to ultra-low carbon; silicon and aluminium are added to the target grade.
⚙ Converter & RH Vacuum DegasserAutomated
International
SMS group, Primetals Technologies, Danieli
Mainland China
CISDI
Technical control points:
- ▪Carbon content — ≤ 0.003% (avoids magnetic ageing)Check: C/S combustion analyser; every heat
- ▪Sulphur and nitrogen — S ≤ 0.003%, N ≤ 0.003% (fewer fine MnS / AlN precipitates that pin grain growth)Check: C/S and O/N analysers; every heat
- ▪Si + Al content — Within the grade window (higher Si/Al → higher resistivity, lower loss)Check: Optical emission spectrometer; every heat
- ▪Inclusion cleanliness — Per mill standardCheck: Metallographic / automated inclusion analysis; sampling per heat
2
Hot rolling
Slabs are reheated and rolled into hot-rolled coils of about 2 mm.
⚙ Hot Strip MillAutomated
International
SMS group, Primetals Technologies, Danieli
Mainland China
CISDI
Technical control points:
- ▪Slab reheating temperature — Low, about 1,050–1,150 °C, so precipitates stay coarseCheck: Furnace thermocouples / pyrometers; continuous
- ▪Finishing and coiling temperature — Per grade; high coiling temperature (≥ 650 °C) for coarser hot-band grainsCheck: Pyrometers; continuous
- ▪Thickness and crown — Gauge per schedule; crown ≤ 50 µmCheck: X-ray thickness and profile gauges; continuous
3
Pickling & cold rolling
Scale is removed and the strip is cold-rolled to final gauge (0.50 / 0.35 mm).
⚙ Pickling Line & Cold Rolling MillAutomated
International
SMS group, Primetals Technologies, Danieli
Mainland China
CISDI
Technical control points:
- ▪Thickness and transverse deviation — Within IEC 60404-8-4 / GB/T 2521 tolerance (e.g. 0.50 ± 0.03 mm); transverse deviation ≤ 0.02 mm (stack-height consistency)Check: X-ray gauge continuous; micrometer at coil ends
- ▪Flatness — Wave factor within standard / customer limitCheck: Shapemeter; continuous
- ▪Surface defects — No scale residue, scratches or roll marksCheck: Automatic surface inspection; continuous
4
Continuous annealing
Recrystallization annealing in a protective atmosphere grows grains to the size that minimises core loss.
⚙ Continuous Annealing & Coating LineAutomated
International
SMS group, Primetals Technologies
Mainland China
CISDI
Technical control points:
- ▪Soaking temperature and time — About 850–1,050 °C depending on grade (higher for low-loss grades)Check: Furnace control; continuous
- ▪Atmosphere dew point — Dry H₂/N₂, dew point about −20 to −40 °C to prevent internal oxidationCheck: Dew-point analyser; continuous
- ▪Grain size — Mean about 80–150 µm (balance of hysteresis and eddy-current loss)Check: Metallography; per coil sample
5
Insulation coating
A thin inorganic / organic insulation coating is applied and cured to limit interlaminar eddy currents and aid punchability.
⚙ Continuous Annealing & Coating LineAutomated
International
SMS group, Primetals Technologies
Mainland China
CISDI
Technical control points:
- ▪Coating thickness — About 0.5–1.5 µm per side (type per ASTM A976 / IEC 60404-1-1)Check: Gravimetric or XRF; every coil
- ▪Interlaminar resistance — ≥ specified value for the coating type (Franklin test)Check: Franklin tester (ASTM A717); every coil
- ▪Adhesion, punchability and weldability — No flaking on bend test; meets customer punching / welding trialsCheck: Bend test every coil; customer trials at qualification
6
Testing & slitting
Magnetic properties are tested per coil, then coils are slit to the stamping width.
⚙ Epstein / Single-sheet Magnetic TesterTest instrumentSemi-automatic
International
Brockhaus Measurements
Mainland China
To be added
⚙ Slitting LineAutomated
International
To be added
Mainland China
To be added
Technical control points:
- ▪Core loss and flux density — Per grade, e.g. 50W470: P1.5/50 ≤ 4.70 W/kg, B50 ≥ 1.64 T (GB/T 2521.1)Check: Epstein frame (IEC 60404-2); every coil
- ▪Slit width and burr — Width ±0.1 mm; burr ≤ 0.02 mmCheck: Width gauge and burr gauge; every slit coil
- ▪Camber — ≤ 1 mm per 2 mCheck: Straightedge; every slit coil

Incoming inspection & approval tests

Core loss and flux density (P1.5/50, B50)Epstein frame, IEC 60404-2 / GB/T 3655
Surface coating insulation resistanceIEC 60404-11
Stacking factor, lamination burr and dimensionsIncoming / in-process inspection
Rust protection (salt spray or damp heat)Per customer requirement

Buyer FAQ

How do I read a grade like 50W470 or 35W300?: The first number is thickness in hundredths of a millimetre (50 = 0.50 mm), W means non-oriented electrical steel, and the last number is the guaranteed maximum core loss at 1.5 T, 50 Hz, ×100 (470 = 4.70 W/kg). The equivalent IEC 60404-8-4 name is M470-50A.
When should I use thinner steel?: Eddy-current loss rises with thickness squared and frequency squared. For high-speed or high-pole-count motors where the electrical frequency is several hundred hertz, 0.35 mm or 0.27/0.20 mm grades cut losses noticeably; at 50–100 Hz, 0.5 mm is usually the economic choice.
What should come with each coil?: The mill certificate with heat number, measured core loss and polarisation (Epstein or single-sheet test), thickness and coating type. Check coating type (C3, C5, C6) — it affects interlocking, welding and insulation between laminations.

Supply chain / regions

Mills are outside the Pearl River Delta (Baosteel, Shougang, Ansteel); Pearl River Delta buyers get slit coil from mill service centres and steel distributors in Foshan (Lecong) and Dongguan, or buy finished cores from stamping houses. Ask for the mill certificate with every coil.

Representative suppliers

Used in

57 mm Inner-rotor BLDC MotorStator / rotor core · Specification: 50W470 or 50W800, 0.5 mm
42 mm Two-phase Hybrid Stepper Motor (NEMA 17)Stator / rotor laminations · Specification: 50W470 or 50W600, 0.5 mm (0.35 mm for high-speed variants)
57 mm Two-phase Hybrid Stepper Motor (NEMA 23)Stator / rotor laminations · Specification: 50W470 or 50W600, 0.5 mm
37 mm Brushed DC Gear Motor (spur gearbox)Armature laminations · Specification: 0.5 mm 50W800 or cold-rolled low-carbon steel for low-cost versions
86 mm Two-phase Hybrid Stepper Motor (NEMA 34)Stator / rotor laminations · Specification: 50W470 or 50W600, 0.5 mm
57 mm Integrated BLDC Motor (built-in driver)Stator / rotor core · Specification: 50W470 or 50W800, 0.5 mm
70 mm Outer-rotor BLDC MotorStator core · Specification: 35W300–50W470, 0.35–0.5 mm
AC Servo Motor (60–130 mm flange)Stator / rotor core · Specification: 35W300 / 35W360 or 50W470, low-loss
IEC Three-phase Induction Motor (frames 80–160, IE3)Stator core · Specification: 50W470 / 50W600 (IE3), 0.5 mm, C-5 / C-6 coating
IEC Three-phase Induction Motor (frames 80–160, IE3)Rotor core · Specification: Same coil as the stator, punched from the stator bore
Linear Motor & Voice-coil MotorForcer core laminations · Specification: 35W300 / 50W470, 0.35–0.5 mm
Direct-drive Torque Motor & Robot Joint ModuleStator segments · Specification: Thin-gauge non-oriented steel, 0.2–0.35 mm (e.g. 35W300 or 20/25-gauge high-frequency grades)