I want to solder a ground wire to the d-sub connector's metal shield( the bright color metal part of the connector), using tin-lead solder 60-40. I just wonder what kind of metal it is. I searched the web and couldn't find the answer.
Finding the datasheet or mechanical drawing of your part would be the best way of getting the answer.
Taking one connector at random on digi-key, http://www.assmann-wsw.com/fileadmin/datasheets/ASS_4884_CO.pdf you can find that the shell is SPCC. So soft steel. Not so fun to solder on.
Some (most?) shells are plated though, with copper, tin and/or nickel, which are much nicer to solder on.
The other factor is that the shell is basically a HUGE heat sink (in terms of soldering electronic components) so you might have a hard time getting it hot enough to have a good solder join.
It is commonly nickel-plated steel.
Soldering to this requires an immersion flash-gold plating process, which is probably beyond your means.
Otherwise, HCl acid with ZCl and NH4Cl and cleaners with toxic fumes when soldering is also not recommended.
The recommended solution is to use a thin ring-term. lug on outside screw connection to chassis ground and crimp wire connection to lug.
One example might be to terminate a shield at one source only to circuit 0V to prevent ground fault currents yet provide 0V coupling to reduce noise. The shield area has a 1/4 wavelength of several GHz so it practically won't see any egress in normal applications but ingress might only come from arc welders nearby or more likely ESD discharge. In which case I would prefer 10kV insulation over a low impedance current path for ESD to go around a plastic shell screws with heat shrink. But you may have different requirements. Others might terminate to pin 7 ground, but then ESD tests to shell may induce a false start bit. This may be irrelevant to you but are real issues for commercial equipment.
They tend to be steel, sometimes plated with nickel. I have also seen some that are die cast zinc or nickel, too. The actual manufacturer will generally give you this information in their datasheets.
In general, the d-sub connector housing material is SPCC, but there are also some SUS301 and SUS304 stainless steel.