Why Hot-Rolled Stainless Steel Needs No.4 Grinding Before 8K Mirror Polishing
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Why Hot-Rolled Stainless Steel Needs No.4 Grinding Before 8K Mirror Polishing

Metal Surface Finishing Equipment Industry News

By:Lebon

Sep 29,2026

For stainless steel plates thicker than 5 mm, a good mirror finish starts well before the mirror polishing machine. Hot-rolled plate typically arrives with oxide scale and surface irregularities. If these are left for the mirror polishing heads to remove, polishing becomes slower and No.4 lines or head marks may remain visible in the final surface.

Stage 1: Prepare the plate with wet belt grinding

A heavy-duty wet belt grinding machine removes the hot-rolled oxide layer and creates a more uniform surface. This is the No.4 preparation stage. For the project described here, the target after grinding is Ra ≤ 0.2 μm.

The plate is now ready for mirror polishing, but it is not yet a mirror surface. The No.4 stage leaves directional grinding lines that must be removed progressively in the next stage.

For 6 m long hot-rolled plates, our engineers estimate the following daily output from a two-head wet belt grinding line:

Plate thicknessEstimated output
3–8 mm50–80 plates/day
9–15 mm30–40 plates/day
16–25 mm15–20 plates/day
26–40 mm10–15 plates/day

These are project estimates. Actual output depends on oxide scale, plate flatness, the required material removal, abrasive condition and operating hours.

Stage 2: Remove No.4 lines before fine polishing

Rough mirror polishing is the bridge between No.4 grinding and the final 8K finish. Successive rough polishing groups use progressively finer abrasives to remove the directional No.4 lines and reduce surface roughness further. For this process, our engineers recommend aiming for approximately Ra 0.05 μm after rough polishing, before the fine polishing groups raise the gloss.

If there are too few rough polishing groups, operators may have to reduce the feed speed to remove the No.4 lines. Otherwise, residual lines or polishing-head marks can remain. Fine polishing improves reflectivity, but it should not be expected to compensate for incomplete rough polishing.

How many polishing groups are needed?

A four-rough + two-fine configuration can process properly prepared hot-rolled plates. For 6 m plates, the estimated output of a six-group line is approximately 14–16 plates per day, depending on the incoming surface and finish requirement.

Where higher throughput is needed, our engineers recommend five rough groups + three fine groups. The additional rough group provides more capacity to remove No.4 lines, while the additional fine group supports the final gloss at a higher feed speed. Based on our processing experience, this configuration may run more than 30% faster than a four-rough + two-fine arrangement under comparable conditions. The actual difference should be verified with representative customer plates.

The process principle

For thick hot-rolled stainless steel, the two machines perform different jobs:

Wet belt grinding removes oxide scale and establishes a uniform No.4 base. Rough mirror polishing removes the No.4 lines. Fine polishing develops the final mirror appearance and gloss.

Selecting the polishing line without considering the quality of the incoming No.4 surface can lead to slower production or visible defects. The most reliable configuration starts with the plate condition, the target finish and a trial using representative material.


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