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Summary: China's steel structure industry is moving from plate to rolled sections and from Q235 to Q420 and above. Both shifts make Sa 2.5 harder to hold. The variable most shops overlook is grit grade, not wheel power.

Ask a blasting operator which job gives trouble and the answer is rarely flat plate. Plate blasts clean at almost any setting. Rolled sections do not. The web-to-flange fillet, the internal corner of a box column, the shadow behind a stiffener: the wheel cannot see them, and no amount of amp draw fixes a geometry problem.

That geometry is about to become the norm. The China Steel Construction Society presented its 15th Five-Year Plan outline at its annual congress in Taiyuan on 21-22 October, and two of its targets point the same way: rolled sections must exceed 30 percent of steel used in the industry, against less than 20 percent today, and strength grades shift to Q355, Q390 and Q420 and above, where Q460 and higher currently sit at around 2 percent.

The blind spot is a media problem, not a machine problem

A single-grade charge of S330 works because every pellet strikes at the same angle with the same energy. Change the workpiece to an H-section and the energy distribution becomes uneven: the flange face cleans long before the fillet does. Operators respond by raising wheel speed or dwell time, which over-blasts the exposed face and drives roughness past the coating window without touching the corner.

Angular grit solves it differently. Grit in the G18 to G25 range does not rely on a perpendicular strike. Its edges cut on contact at shallow angles, and it reaches into fillets, weld toes and stiffener shadows. Run it as a second pass, or blended with shot in the same charge, and roughness comes back into the Rz 40 to 75 micron band across the whole section rather than only on the flat.

Harder substrate, harder media

Q420 and Q460 do not just resist indentation. They return more energy to the pellet on impact. Media that was adequate on Q235 fractures early here, and the symptom appears as rising consumption rather than as a visible defect: more recharges, more fines in the dust collector, more time lost at the machine.

Our Q/HX 001-2026 internal standard sets the numbers we hold to: non-spherical particles no more than 5 percent, sulphur and phosphorus no more than 0.03 percent each, density at or above 7.4 g/cm3, cyclic life at least 2,200 cycles. For Q460 work we recommend shot hardness at HRC 45 or above; for Q355, HRC 42 is normally enough.

Roughness is a cost line, not a quality footnote

A tender from Qinghai Xikuang Hangxiao Steel Structure, open for comment from 22 to 26 October, priced outsourced blast cleaning and painting in four dry-film bands: 210 RMB per tonne below 80 microns, 350 RMB for 81 to 160, 440 RMB for 161 to 240 and 510 RMB for 241 to 320. Adjacent bands differ by 130 to 140 RMB per tonne.

Read that as a statement about surface preparation. Where roughness is stable and Sa 2.5 is genuinely achieved, a single coat at 80 microns can be specified and the job stays in the low band. Where roughness swings, the coating has to be built up to compensate, and the price crosses into the next band. Grit grading, in other words, shows up in the coating quote as well as in the abrasive invoice.

Practical settings that hold up

Three settings do most of the work. Keep the shot and grit blend matched to section thickness rather than to the average of the batch. Check media hardness against the steel grade, not against last year's order. And measure roughness on the fillet as well as on the flange face, because that is where the coating will fail first.

For a shop moving from plate to sections, a trial run on one representative member is worth more than any specification sheet. We send free samples for exactly that purpose.

Questions from the shop floor

Q: Can grit alone replace shot?

A: No. Grit cleans and cuts, shot peens and consolidates. Grit alone raises roughness and consumption; shot alone leaves fillets and weld toes short of Sa 2.5. The blend is what holds the specification.

Q: What roughness should we target before priming?

A: Rz 40 to 75 microns suits most shop primers and epoxy systems at the film thicknesses quoted in current tenders. Above Rz 100 microns, film build has to increase and the coating cost moves up a band.

Q: How quickly does media quality show up in cost?

A: Within one production month. Track recharges per week and fines removed per shift. A drop in cyclic life from 2,200 to 1,200 cycles roughly doubles recharges at constant output, which is visible long before the invoice arrives.

Mali Steel Shot (Wuhan Hongxin Lixin Metal Products) produces alloy steel shot from S230 to S780, stainless cut wire shot from 0.3 to 2.5 mm, high-carbon and high-manganese cut wire shot, and angular grit from G12 to G120. Website http://www.sisball.com/ , hotline +86 400-038-8928.