Iíd guess theyíve also increased the amount of carbon content in the steels, this adds strength to the structure, and probably allows car manufacturers to use steels that are somewhat thinner, and also somewhat cheaper.
Iím an ex-welder and my preference when welding on car bodies was always use oxyacetylene (gas welding) the problem with this method was it was slow and there was always vast amounts of heat spreading out, even in areas that were not being welded.
Other than distortion this isnít a biggi when welding bog standard mild steels, but on medium or high carbon steels it does become a problem because the carbon content is burned out of the metal during the welding process, the objective with carbon steels is keeping this heat affected area as small as possible.
To retain structural strength now you need to use a process that doesnít spread boatloads of heat outside the joint area, which blows out any further idea of using gas welding.
The accepted norm for welding now on car bodies is MIG (Metal Inert Gas) this is similar to arc welding, but is so much faster thus keeping the heat input very low other than in the joint area to be welded.
BTW Iím no big fan of MIG welding, I used to do structural nuclear work, rail and road bridges etc, using arc welding which sitting on a stool was fairly relaxing, then the gaffa found out MIG was faster, so the stool had to go.
MIG welding on this type of work is an on yer elbows and knees job, head down, a$$e up.
While I was on the tools, I used to call it sh!t