How many tons of steel in a Boeing 747?

Answer 1

Approximately 19 tonnes, mostly high-strength steel alloys.

Sources:

http://www.Boeing.com/commercial/aeromagazine/aero_22/alloy_story.html
Steel treatment guidelines, focusing on landing gear.

http://www.pbs.org/kcet/chasingthesun/planes/747.html
"Weight: 358,000 pounds (empty)"

http://www.c3titanium.com/technology.html
"10% of the Boeing 747 airframe is made of titanium"

http://www.sndgems.com/titanium-jewelry.htm
"There are 40000 pounds of titanium on a Boeing 747"

http://forward.msci.org/articles/1106showdown.cfm
"The body of the Boeing 747 is made up of 81% aluminum, for example."
"Composite will constitute about 50% of the 787, Boeing says. The balance is 20% aluminum, 15% titanium and 10% steel [5% other]."

http://en.wikipedia.org/wiki/Boeing_747-400
"Weight empty 394,100 lb"

http://www.boeing.com/commercial/747family/pf/pf_facts.html
"A 747-400 consists of 147000 pounds (66150 kg) of high-strength aluminum."

http://enhancinginnovation.wustl.edu/Sankaran.pdf
Slide 11 has pie chart showing elemental composition of 747, 757, etc.
Looks like the 747 contains about:
1% composite
1% other
12% steel (266-224 deg, 11.6%; comes out at 41,767 pounds = 18.94 tonnes)
5% titanium
balance aluminum (including alloying titanium? otherwise numbers don't match up)
I've assumed these figures are by weight, not volume.