I am so surprised by all the responses to this thread, thank you all for your input, and there are a few things I would like to add.
Other than the fact that the Mongols and Vikings both had the ability to work iron/steel, you mean? Those fighting barbarians will take slaves, and some of those slaves will know how to work iron, and so the orcs will then have the technology.
Many "barbarian" cultures could make steel. When the Celts and Germans invaded Rome, and when Rome invaded them, both sides fought with steel. When the Huns and Mongols invaded Eastern Europe, they were shooting steel-tipped arrows at their enemies.
I could hardly consider Vikings and Mongols barbarians. Granted, they act barbaric, but they are just too advanced to be barbarians. What would you consider a barbarian? Well, lets take a look at what the game calls a barbarian: A light to medium armored battler who can induce rage in warfare. What is a viking? A battler dubbed up in steel armor, that's a barbarian, that's a warrior. Mongols fit barbarian better, and I think they mostly traded for iron and steel tools, mainly because from what I understand they were mostly nomadic, which would leave little room for building a foundry, I could be wrong though. What do I see a barbarian as? People who live in leather/dirt/maybe adobe huts who wear skins and like to crush people for fun, may eat raw meat, and believe that the reason rusty metal squeaks is because the technology spirits demand a sacrifice of oil and grease lubricant. Vikings lived in villages quite equal to that of most of Europe at that time, meaning that their homes and living were far too luxurious to be considered barbaric.
Broadly speaking, once one culture develops a technology, there are very few reasons for neighbors to not also get that technology.
Considering were talking about a barbaric species of people who have a love of war, killing for pleasure, bathing in blood, and according to Viking mythology, eating people, I think people would sooner burn down their own village then risk Orc's gaining access to a way to make better weapons easier. Plus steel making is hard, like Yora said, you can make bronze with a campfire and clay pot, steel requires much more effort. Do they have access to the masonry skills as well as carpentry needed to make a forge, foundry and device to blow air and keep the fire hot? If these people live in leather and dirt huts, no. And keeping slaves is a bit odd, it would require a bit of organization on the Orcs part. Depending on how many Orcs live in one clan, ther may not be enough Orcs to make sure the slaves don't escape, while making sure there's food and warfare to go around. I'm not saying its not possible, I'm just not sure an Orc clan can handle a bunch of slaves. Maybe one slave to make the steel, they may not be able to make the tools he needs, and he may very well be willing to die before giving up the secrets of steel so they can kill and murder more people. This is a very strange situation, as there are many factors that could keep them from achieving steel, after all, Asia was actually in the Bronze age longer than anyone else (I'm not including native Americans, they never made it into the bronze age, but merely out of choice.)
Even if orcs didn't learn how to make steel from themselves, they'd learn from others. A sympathetic half-orc blacksmith, trained by humans, could pass on the secrets.
I didn't consider this, and it seems like a good idea, but I can sum it up from a quote from the warrior cats book (I know, its a childrens book, but its well written.)
If you wish to train with us, we will have to take you into our Clan. You must either live with us and respect our ways, or return to your Twolegplace and never come back. You cannot live with a paw in each world.
That could easily go for a half-orc in the situation of human world and orc world, he cannot simply live both lives. Since Orcs would be at eternal war with the humans, raiding their villages and killing women and children, if the half-orc were ever to make friends in an Orc clan, the humans would shun him immediately, as he would have to build up their trust in the first place, and making buddies with an Orc clan would shatter that trust in an instance. He could make friends with the humans first, then go to the Orc clan and teach them the secretes of steel, but this would be a permanent decision, and he would forever be shunned, seeing that he did technically backstab them by giving the biggest danger to the village the secret to advanced, better killing tools. This could actually make a fun quest in a campaign: The secret of steel! ; Kill the traitorous Half-orc and kill all the Orcs of the clan to prevent the war hungry Orcs from evolving into a new golden-age of bloodshed and mayhem. That would actually be an interesting plot point.
To be honest, there are a lot of factors to consider, I'm not really sure whether they would be stuck to using bronze or being able to use steel, could go either way depending on many factors. It is fun to talk about.