So I'm reading this month's 人文历史 (National Humanity History) magazine and they have a whole bunch of articles about how Qin overcame all the other states, during the warring states period, and united China. In one paragraph they suggest that Qin's elite troops might have had steel swords instead of bronze one's for the standard troops. That made me think that if the Qin were the only state with access to steel that would have given them an obvious advantage over the other states. But I don't know. So, when was steel introduced in China and how did it spread? Did have Qin access to it, and if so did other states?
First, while it is commonly said that Qin success was due to iron swords, this isn't supported by the archaeological evidence: (a) Qin does not appear to have used more iron/steel weapons than other states of the time, and (b) Qin weapons were predominantly bronze.
The Qin iron sword debate and the lack of evidence for Qin victory through iron/steel was covered in a series of papers in the 1970s. While old, they're still worth reading:
Keightley, David N. “WHERE HAVE ALL THE SWORDS GONE? REFLECTIONS ON THE UNIFICATION OF CHINA.” Early China, vol. 2, 1976, pp. 31–34
TROUSDALE, WILLIAM. “WHERE ALL THE SWORDS HAVE GONE: REFLECTIONS ON SOME QUESTIONS RAISED BY PROFESSOR KEIGHTLEY.” Early China, vol. 3, 1977, pp. 65–66.
Barnard, Noel. “REJOINDERS: DID THE SWORDS EXIST?” Early China, vol. 4, 1978, pp. 60–65.
Second, there isn't any evidence that Qin victory was due to a single technological factor. Having better swords (assuming that the swords were steel (rather than iron) if they existed) does not necessarily win battles, especially when most soldiers are using spears (and other polearms) and bows and crossbows. Of course, this isn't always given as a qualitative argument ("steel swords are better than bronze swords, therefore Qin won") but often as a quantitative argument: "iron swords were much cheaper than bronze swords, so Qin could field much larger armies because they could afford more swords." The quantitative argument fails on the basis of the archaeological evidence: if they had very large numbers of iron swords, where have all the swords gone?
Iron and (hardened) steel weapons became standard during the Han Dynasty (mid-Han, late Western Han). This is visible archaeologically, e.g., from the numbers of steel/iron weapons from armouries in Chang'an. The archaeological visible of this suggests that the lack of Qin iron/steel weapons (in sufficient numbers to support the Qin victory through iron idea) is real rather than apparent.
So, when was steel introduced in China and how did it spread? Did have Qin access to it, and if so did other states?
Iron technology had been introduced into China possibly about the 8th century BC, probably from the north (via the Scythians/Saka/Sai). No state appears to have had a monopoly. Qin certainly had access to iron and steel, and so did its neighbours. The early spread of iron technology in China is covered in Donald B. Wagner, Iron and Steel in Ancient China, Brill (1996) and Science and Civilisation in China Vol. 5, Part 11, "Ferrous Metallurgy" (also by Donald B. Wagner) (2008). There are iron and steel swords from the Qin Dynasty and earlier. They are often richly decorated. Since iron and steel swords are less likely to survive than bronze swords, they are probably under-represented in archaeological finds. Early iron swords, and the Han transition from bronze to steel as standard for weapons is discussed in Yang Hong, Weapons in Ancient China, Science Press (1992). Steel swords appear as early as 600BC (0.5% carbon); these early steel swords are often unhardened. Qin had no monopoly, and despite the knowledge of, and production of, iron and steel swords, bronze weapons remained standard until mid-Han. The preference for bronze weapons was not due to the lack of iron (the number of iron tools and other objects, both of wrought iron and cast iron, grows over time); sufficient bronze appears to have been available to meet demand.