304J1 is a low-nickel, high-copper variant (Ni 6.5–7%, Cu 1.5%), with an elongation rate 8% lower than 304. It exhibits noticeable “orange peel” after deep drawing.
Quick identification: Apply a drop of 8% CuSO₄ solution. After 2 minutes, J1 shows pale red copper precipitation while 304 remains unchanged.
Price: 304J1 is cheaper. Some unscrupulous factories sell it as genuine 304. Be sure to verify carefully.

National Standard GB/T 1499.2 permits “length deviation ±25mm.” 11.88m falls within the 12m±25mm range, so the theoretical weight calculated based on 12m is valid.

Rolling above the recrystallization temperature is “hot rolling,” while rolling below it is “cold rolling.”
Hot-rolled sheets feature an iron oxide scale on their surface, with a thickness ≥1.5 mm and good plasticity.
Cold-rolled sheets offer high dimensional accuracy and a bright surface, typically with a thickness ≤3 mm, and can undergo subsequent galvanizing or color coating.

Earthquake, indicating earthquake-resistant reinforcement. It requires a strength-to-yield ratio ≥1.25, a yield-to-tensile ratio ≤1.30, and a total elongation at ultimate load ≥9%, enabling ductile failure rather than brittle fracture during earthquakes.

Inspect the ends: Seamless pipes exhibit no weld marks on their inner or outer walls, with wall thickness variation ≤12.5%. Welded pipes must have welds on both sides. For high-frequency straight seam welded pipes (HFW), the outer weld is ground smooth, but the inner weld remains visible.

The Zn-6%Al-3%Mg coating achieves ≥3000 hours before red rust appears in salt spray tests—three times longer than standard Z275. This enables maintenance-free photovoltaic mounting systems for 30 years.

There is no direct correlation. Smaller zinc flowers (zero flowers) offer a more uniform appearance and result in minimal color variation after painting. Larger zinc flowers are cost-effective. Smaller zinc flowers are commonly used for home appliance casings due to aesthetic considerations.

Formula kg/m = 0.00617 × d² (where d is in mm). For φ20 round steel: 0.00617 × 400 = 2.47 kg/m. Error < 1%.

GB/T 2970 Class I, II, and III. Pressure vessel shells require Class I (no defects equivalent to ≥∅3 mm), while ordinary structures may use Class II.