Baking sheets - not trays

Does anybody know what sort of steel baking sheets are made from?
SWMBO has a couple of approx 1mm thick rectangular baking sheets with
one edge folded up by about 1cm. These sheets have had years of being heated up to 220C and cooled, sometimes quite rapidly if they get dumped in a sink full of water. They have never distorted. It felt like they are quite a soft steel when I did a test with a file.
There is now a requirement for a circular baking sheet approx 35cm dia., not non-stick and not aluminium. This specification appears unobtainable so I intend to make one. As I cant press a strengthening rim on a circle, I intend to braze a piece of welding rod round the edge.
I dont want to do all this work if the first time it is used it distorts, which is why I need to know what grade of steel to look for.
Thanks in anticipation.
John H
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In article

Make it from a steel baking sheet.
CP
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wrote:

Good plan.
Regards Tony
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There are two problems with making it from a steel baking sheet; I can't use one of the original's and I can't get a big enough diameter from from new baking sheet.
David thanks for the offer but 305mm is a bit small.
John H
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John H wrote:

How about a pizza pan either as is or as a material source http://www.acepans.co.uk/index.php?id=3 . I tossed an old one recently when clearing out my mother house and that was about 350mm diameter and had a rim about 10mm high. I think that one was bought in the US and brought back when she returned to the UK.
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On Aug 19, 11:19am, David Billington

Baking trays and tins were made from tinplate in the past. This can in fact be obtained in about 5 different thicknesses called X,XX etc. It is of course steel sheet coated with tin. Most decent trays had a wired edge, while cheap ones simply had a rolled edge. To wire the edge of a circular disc, you would use a hide mallet and rest the disc against a halfmoon stake and beat up a suitable flange. This is then beaten over again at right angles. The wire is fitted in the angle and the flange beaten in to trap the wire with a "tucking hammer" - a double ended hammer with sharpish edges at both ends. This may sound a bit complex but is really not too difficult. The equipment could be improvised. I used to obtain my sheet from a firm in Birmingham called, if I remember H.J.Edwards. I do not know if they still are trading.
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Thanks David for the suggestion. Hadn't thought of a pizza pan - it looks to be the easiest route to what I need. I've found a couple of suppliers who I will talk to tomorrow to find out what gauge they are. They need to be quite substantial to work properly as a baking sheet.
Tomol, thanks for the info about tinplate, I hadn't realised exactly what was involved in manufacturing using tinplate. After reading your info I found this site http://autonopedia.org/crafts_and_technology/Metal_work/Metalworking-The_Basics/8_Sheet_Metalwork.html which I'll retain for future reference.
John H
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John H wrote:

I you were near Bath and could accept about 305mm diameter I could spin a strengthening edge or fold it completely over around the circumference. I already have a 305mm former but no bigger at the moment.
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