Heat Pattern Scale formation on grades 1035A4 and 1035S1

[Ikarise]

We're having a ton of HPS buildup on these grades coming out of our hot mill and into the pickle line in the cold mill, and we don't really know why. One idea floating around is heat retention between the coils when they're sat in groups to coil... anyone have any insight on the validity of that? Something must be keeping these coils at higher temps (scale formation range) for longer. Coiling and finishing temps varying is another idea. These, among other details, are things I could look into, but I'd love to know a good place to start. Any ideas?

Purdue Materials Science & Metallurgy - Metallurgical Engineering Intern

 

[MetalsGirl23]

We see issues of HPS with coils placed too close together after HM... It's an easy trial to at least check out, and I'd start there.

 

[Ikarise]

Thank you, that's gonna be the first thing I check. Hopefully all it will take is to spread those coils out in the first stage of cooling.

Purdue Materials Science & Metallurgy - Metallurgical Engineering Intern

 

[AndrewFinAustralia]

Former cold mill and pickle line technical assistant (process metallurgist) in a 1000tpd plant in the late 80's/early 90's when I finished my degree so knowledge is a bit dated.

Easiest option is the trial as mentioned above, but you need to find a way to measure it. While time at temperature is important in scale formation, my gut feel (and the 80's were a long time ago) is that there is minimal access to oxygen to form a scale layer in the middle of the coil that is much thicker than that which forms in rolling. The air would not enter the wraps of the coil as it heats and expands/pushes straight back out. I never worked in the strip mill and never had to worry about this - we ran a sulphuric acid immersion line, not a hydrochloric spray line.

1035 can form martensite if quenched too quickly, so I'd be guessing (and it is only a guess) that the coiling temperature would be 680-710C to avoid the knee of the TTT curve and your oxide layer problem would be hot mill runout table related. An increase of 10-20 degrees on the runout table would cause far more issues with thicker scale. As a gut feel, I'd start looking there first.

The scale, as you know, forms in three distinct layers, FeO closest to the surface of the metal, Fe304 in middle of scale layer and Fe2O3 on the outside of the scale layer. As the Fe2O3 is acid insoluble, you have to crack it to allow the acid access to the soluble FeO layer underneath.

I'm guessing that the problem is that the acid-insoluble Fe2O3 is building up in the tanks and/or transferring through the pickle line and entering the cold mill as a smudge layer on the surface of the steel, causing premature mill roll wear. There is obviously also the issue of increased acid consumption and decreased throughput due to the thicker layer.

If this is the case, I'd look to the scrubber ex pickle line to make sure strip is as clean as possible as well as trying to improve the amount of scale cracking pre-entry to the acid bath from the scale breaker to remove more scale early so the strip is cleaner.

You'll need immediate proof that the store-separately post strip-mill trial has worked rather than waiting for the sludge to build up. Do you have access to lab - if so, take 5 mid-coil samples from your current stock at the entry welder and measure scale thickness (micrograph) pre-trial, repeat post trial for another 5 coils. Head-end and tail-end crops are useless for this exercise. You may have a research lab associated with your company that has access to a SEM with glancing XRD capability or similar - if so and you need the proof, I'd look at the ratio of Fe3O4 to FeO in each sample to prove scale difference - I'm sure there's an old empirical way as well - be acid-insolubility for the woostite, haematite and magnetite.

Good luck with this - I'm expecting that the proposed trial will reduce scale buildup on the strip edges noticeably. Unsure on the effects on scale thickness across the width of the strip in the body of the coil.

 

[AndrewFinAustralia]

Just noticed you said you were an intern.

I did a quick google search for some papers to help guide you - I was a trainee metallurgist (similar to your internship) in our local steel company - degree was 7 years part time while I worked in the trade from the age of 18.

Here's two immediate links of interest that summarise effect of coiling temp on scale formation, oddly enough, sponsored by my old employer.

http://link.springer.com/article/10.1023/A:1004637127231#page-2

http://link.springer.com/article/10.1023/A:1010395419981#page-1

edit - a third paper available here is available for download. Covers the scale formation mechanism well

http://ro.uow.edu.au/cgi/viewcontent.cgi?article=2017&context=eispapers

Edit II - just checked formation of Heat Pattern Scale if you're sure what it is - out of early stands in Strip Mill

http://files.engineering.com/getfile.aspx?folder=84506990-6be2-48f4-9134-bec4016069f4&file=31.png

 

[Ikarise]

Andrew,

Thanks so much for your suggestions, insight, and information. I decided to do some reading and ask a few questions around here before I responded back to you. I think its pretty well known that the coiling temperature would affect the scale build up (and we think the heat pattern scale comes from coiling with bad shape factor, so the oxygen gets onto the edges of the wavy strip), but what's unfortunate is that we intentionally increased that coiling temperature in order to correct for sagging issues in our coils.
It seems as though the sagging might be the more pressing issue, so I don't believe I'll be able to mess with that coiling temperature *too* much, but I'll see about it.

Purdue Materials Science & Metallurgy - Metallurgical Engineering Intern