Hot-rolled round bar

Round bar is a hot-rolled bar of circular cross-section, the basic stock for shafts, axles, pins, fasteners and members of steel structures. Dimensions and tolerances are set by GOST 2590-2006, at mills in China by GB/T 702-2017. GOST 2590 covers diameters of 5–270 mm; round bar above 270 mm is rolled by agreement with the mill. Rolling accuracy classes under GOST 2590 are extra-high AO1, high A1–A3, raised B1 and ordinary V1–V5; for subsequent machining ordinary accuracy V1 is usually sufficient, for cold finishing and precision fits the raised class is agreed.

The grade is chosen by the purpose of the part. St3sp and St3ps to GOST 535 are for structural and non-critical parts, equivalent Q235B. Steels 20 and 45 to GOST 1050 are quality carbon steels: steel 20 goes for carburizing and welded assemblies, steel 45 for hardening and quench-and-temper treatment; equivalents 20# and 45#. 40Kh to GOST 4543 is a chromium alloy steel for shafts and gears after heat treatment, equivalent 40Cr. The mill rolls round bar from continuously cast billet; the delivery condition is hot-rolled, or normalised or annealed by specification.

During inspection at the mill the bureau measures the diameter with a micrometer in two perpendicular directions on bars from different bundles and compares the ovality with the tolerance of the chosen accuracy class, checks straightness and surface — free of cracks, laps and scabs. The heat number on the tag is checked against the mill test certificate (MTC) and the chemical composition in the specification; for steels 45 and 40Kh the as-delivered hardness is also read. The paint grade marking on the bar ends and the strapping of bundles are checked; the result is an inspection report with photographs before shipment.