Summary: On October 29 rebar averaged 3,309 yuan/t and hot-rolled coil 3,229 yuan/t, yet trading stayed thin. With November schedules loosening, blast lines finally have room for inspection. Mali Steel Shot lists the four checks that matter before cold weather: media dryness, shot toughness, bag permeability and coating temperature.

The price signals at the end of October are easy to read for anyone in steel fabrication.

On October 29, the average price of 20mm rebar across 31 major Chinese cities was 3,309 yuan per tonne, up 7 yuan day on day. 4.75mm hot-rolled coil across 24 cities averaged 3,229 yuan per tonne, up 5 yuan. 20mm plate averaged 3,450 yuan, down 1 yuan, while 1.0mm cold-rolled coil held flat at 3,740 yuan. Billet ex-works in Qian'an, Tangshan, stayed at 2,900 yuan per tonne including tax. Three mills raised construction steel ex-works prices by 10 to 50 yuan per tonne that day, but traders knew the difference between a price rise and real demand.

Inventories tell the story more plainly: hot-rolled coil stocks across 33 major cities stood at 3.6048 million tonnes, down 80,800 tonnes week on week, and cold-rolled coil across 29 cities at 2.3718 million tonnes, down 21,000 tonnes. Stock is falling because buyers are taking only what they need. Nobody is building position.

For a blast shop this means two things. First, from November the line will have idle windows. Second, temperatures keep dropping, and the same machine running the same shot will not produce the same result it did in October.

Mali Steel Shot has supplied metallic abrasives for 25 years, and every year at this point we walk customers through the same pre-winter sheet. Four items, half a day to complete.

Check one: squeeze a handful of shot and see whether it is dry

This comes first because it shows up earliest and gets skipped most often.

Large day-night temperature swings cause condensation on the inner wall of bulk bags. Moisture on the shot surface freezes overnight into hard lumps that will not discharge the next morning. Clearing a blocked feed pipe costs half a shift. Even when the media does flow, shot carrying moisture accelerates corrosion inside the machine.

The acceptance line is a moisture content no higher than 0.1 percent.

Four actions: raise bulk bags at least 15 centimetres off the floor; keep the store ventilated and away from cement or sand; never use bags stored in the open; and squeeze a handful before loading. If it holds together in a ball and does not crumble, spread it out to dry or send it back.

Check two: judge toughness, not roundness

In winter many buyers switch suppliers looking at roundness. What actually matters is low-temperature toughness.

Measured data: shot held at minus 5 degrees Celsius for 1.5 hours loses roughly 18 percent of its toughness. Carbon steel at minus 20 degrees loses 60 percent of its impact toughness. The same S330 particle that peens in October may shatter in December. Broken media cannot be recovered, so consumption climbs.

Three responses:

Reduce wheel velocity by 8 to 12 percent. Impact energy per particle falls, but the breakage rate falls further, so net consumption drops.

Below minus 15 degrees, run an impact toughness check on Q235 and higher grades. Below minus 20 degrees, abrasive blasting should stop altogether. That is a brittle-fracture limit, not a productivity question.

Look at cycle life when specifying. Under our Q/HX 001-2026 enterprise standard, Mali steel shot must reach at least 2,200 cycles, with non-spherical particles held under 5 percent and sulphur plus phosphorus under 0.03 percent. Fewer irregular particles means fewer initiation points for cold cracking.

Check three: feel the dust collector housing to see whether bags are blinding

Summer dust problems mean volume. Winter problems mean blinding. They are different failures.

When hot and cold alternate, condensation forms on the inner walls of the collector. Dust on the bag surface cements together, and permeability can halve overnight. The symptoms are unmistakable: fan load rises, negative pressure falls, and dust escapes from the blast chamber.

One northern shipyard practice is worth copying. After shutdown they keep the fan running another 30 minutes to purge moisture from the housing. That single change extended cartridge life from 3 months in winter to 12 months.

Two more: insulate the collector housing, and replace bags once differential pressure exceeds 1,500 Pa rather than waiting for a full blockage.

Check four: coating temperature, where 5 degrees is a hard gate

The first three checks cover media and equipment. This one covers process sequence.

Epoxy coatings essentially stop reacting below 5 degrees Celsius. At 10 degrees, curing time runs 2 to 3 times longer than at 23 degrees. At 5 degrees, curing takes 3 to 4 times as long: a job cured in 7 days at 23 degrees needs 28 days at 5 degrees. Most winter adhesion failures are not the paint's fault. The film looked dry while the chemistry had not finished.

The sequence has to change with it. In mild weather, coat within 4 hours of blasting; in winter, compress that to 2 hours. A surface water film thicker than 5 microns destroys adhesion. Steel temperature must sit at least 3 degrees above dew point before work starts. If the margin is under 3 degrees and there is no heating or dehumidification, stop for the day. Forcing the work costs more in rework than the downtime.

Where possible, clear ice and water from plate before it enters the preheat furnace. Carrying moisture in is a direct route to pinholes and blistering.

These four checks come down to one idea: control the variables before they control you. Media dryness, shot toughness, bag permeability and coating temperature all fall in the same week. If any one slips, consumption and rework rise together.

Mali Steel Shot (Wuhan Hongxin Lixin Metal Products) has manufactured metallic abrasives for 25 years. The range covers alloy steel shot from S230 to S780, stainless cut wire shot from 0.3mm to 2.5mm, high-carbon and high-manganese cut wire shot, and angular steel grit from G12 to G120. Stock is held at 28 warehouses across China with 72-hour delivery, every batch ships with a test report, and free samples are available for trial runs.

Website: http://www.sisball.com/ Hotline: +86 400-038-8928 (alternate +86 27-84841595)

FAQ

Q: Why does shot consumption rise in winter?

A: Low temperature and moisture act together. Shot held at minus 5 degrees for 1.5 hours loses about 18 percent of its toughness, which raises the breakage rate, while condensation inside bulk bags makes feed uneven and causes local over-blasting. Reducing wheel velocity by 8 to 12 percent and holding moisture below 0.1 percent usually brings winter consumption back to summer levels.

Q: How much shot should be stocked before winter?

A: Plan on 1.5 times the normal monthly consumption. Logistics in northern China can stretch from 72 hours to 5 or 7 days between December and January, and higher breakage rates add 10 to 20 percent to actual usage. With 28 warehouses, deliveries can be staged rather than filling the store at once.

Q: Blasted steel flashes rust within hours. What should we do?

A: Compress the interval between blasting and coating to under 2 hours, against 4 hours in mild weather, and measure dew point before starting. Steel temperature must exceed dew point by 3 degrees. Anchor profiles exposed to cold damp air can flash rust in a few hours, and once rust appears the surface has to be re-blasted rather than painted over.

Q: Can caked steel grit still be used?

A: Lightly caked grit can be spread out, dried and screened. Hard lumps that will not break apart should be rejected. Moist media carries water into the machine, blocking feed pipes and accelerating corrosion. Judge by moisture content: below 0.1 percent is acceptable.

Q: Should we switch to harder shot in cold weather?

A: Higher hardness is not the answer. Harder particles fracture more readily at low temperature and push consumption up. Adjust wheel velocity, down 8 to 12 percent, and the replenishment rhythm instead. Where ambient temperature stays below minus 15 degrees, specify media with high cycle life and low non-spherical content: at least 2,200 cycles and under 5 percent irregular particles under Q/HX 001-2026.