Last night I revisited notes from a site visit to a steel plant’s continuous casting workshop two years ago. I would like to share several observations from that visit.
The caster observed was a 170 mm billet continuous casting machine with a dual tundish and 10 strands.
Start Casting with a Swing Trough
The caster was still using a swing trough to start casting.
This indicates that the plant’s operational control for metered nozzle start-casting procedures remains similar to practices widely used in the 1990s.
Today in China, very few billet casters producing rebar steel still rely on swing trough start casting. In addition, only about 6–7 plants still use stopper-rod flow control for rebar casting.
The reasons for not adopting fixed-diameter metering nozzles usually relate to:
- Operator habits
- Equipment condition
- Casting speed fluctuation tolerance
- Production rhythm
- Management awareness
Typically:
Stopper-rod controlled casting: tundish life ~15–18 hours
Metered nozzle casting: service life can exceed 60 hours
From any operational or economic perspective, the difference is significant.
Quartz Submerged Entry Nozzles Kept as Backup
Quartz submerged entry nozzles (SEN) were kept on-site as emergency spare parts.
As is well known:
- Quartz nozzles erode quickly
- Their service life is short
- They are generally used only in emergency situations
Most modern casting machines rarely use them anymore. However, this plant kept them ready for immediate use, which suggests that flow-control incidents or unexpected events occur relatively frequently.
Mold Foot-Roller Spray Nozzle Design
The continuous casting mold foot-roller spray system used:
- 3 rows of spray nozzles on the faces
- 3 rows on the corners
- Configuration: 3 × 8 = 24 nozzles
However, when checking the spray alignment, the nozzle positioning and centering condition were extremely poor and difficult to evaluate positively.
Excessive Billet Loss During SEN Replacement
Too much billet was discarded during submerged entry nozzle replacement.Loss per strand reached 4–6 meters, which is quite wasteful.
Under normal operating conditions:
- A billet caster can run up to 60 hours continuously
- Except for head and tail billets, replacing an SEN or slide gate should not require discarding billets
Even if abnormalities occur during replacement, the typical approach is:
- Inspect the billet
- Perform corrective treatment if necessary
- Release it to downstream processing
Direct scrapping or remelting is usually unnecessary.
Excessively Long Head and Tail Billet Cutting
The cutting length of head and tail billets was overly long.
During a private discussion with the site manager, it was estimated that optimizing just these two areas could reduce costs by 5–8 million RMB per year.
This represents cost reduction without additional investment, making it an obvious efficiency improvement opportunity.
Severe Billet Bending
The caster’s cooling bed was only about 7 meters long, with a bidirectional billet pushing system and stacking collection.
As a result, billet bending deformation was severe.
Frankly speaking, what surprised me most was that the rolling mill department did not raise concerns. With such excessive bending, it is difficult to imagine the billets entering rolling mill guides smoothly.
At our rolling mill, such severely bent billets would never be accepted.
Surface Slag Inclusion
Some billets showed surface slag inclusion defects, most likely formed during nozzle replacement operations.
In fact, billets with minor inclusion defects can often be repaired by grinding, rather than being directly classified as scrap.
Secondary Cooling Performance
The secondary cooling zone showed poor performance.
Billets exiting the zone appeared uneven in temperature, with red and dark patches, indicating unstable cooling.
Macrostructure examination confirmed poor internal quality.
Several months later, after technical communication with the plant, the secondary cooling water distribution ratio was optimized, which significantly improved billet macrostructure quality.The later improvements are another story.
Despite several additional abnormal conditions observed on site, some issues are not convenient to discuss openly due to various practical considerations.After all, the company is a private enterprise, and sometimes operational problems involve personnel and management sensitivities. Maintaining balance and focusing on encouragement is often the most practical approach.
What is a continuous casting mold in steelmaking?
A continuous casting mold is a water-cooled copper mold used to solidify molten steel into billets, blooms, or slabs during the continuous casting process.
What causes billet bending in continuous casting?
Billet bending can result from insufficient cooling bed length, uneven cooling, improper strand support, or poor mechanical design of the caster.
Can billets with slag inclusions be repaired?
Minor slag inclusion defects can often be removed by grinding or surface conditioning instead of scrapping the entire billet.
