Of all the brazing alloys listed in the AWS brazing handbook, the lowest brazing temperature range listed starts at 582F, so you look to be out of luck with brazing. Soldering will be weaker than brazing. However, the strength requirement you state is relatively ambiguous for the brazing/soldering world. The strength of a brazed/soldered joint is highly dependant on the joint geometry including overlap and the gap between the parts. As the overlap gets bigger and the joint gap gets smaller, the joint strength increases, if all else remains the same. Can you change the parts to get a larger overlap and get the strength you need?
Alternative processes that do not melt the base metal that may be worth investigating depending on your requirements are:
Friction welding
Friction stir-welding (they do this on steel now too)
Ultrasonic welding
Explosion welding

Or mechanical means of fastening such as: Threads, bolts, rivets, adhesive bonding, interfereance fit, etc.

If none of the above will work for you, you could consider a welding process that has a narrow weld and heat affected zone, so that the area where properties of the part will be affected is as small as possible, such as lazer beam, or electron beam welding.