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Density =/= Strength.

You could technically epoxy and lead to the same density as steel; but you'd likely need WAY more of the material to have the same strength as even 4130.

While that example is ridiculous the same basic principle applies to steel alloys. Steel is basically just a crystaline form of iron. But if you try to just use iron only then the crystals don't align properly to get the most strength, plus it's often easy for Oxygen to work its way in and start eating apart the iron. To counter this over the course of centuries—but mostly in the era since the industrial revolution—people found that by adding some other elements to the mix of iron and using different heating and cooling cycles that you could get the iron atoms to better align into a stronger crystalline structure. It's really the balance of the very small amount of other elements that alloy iron turning it into steel. A small—but often very precise—change in materials or heating/cooling process can radically change the properties of the steel. Throw in some chromium and instead of forming oxidizing as rust, it oxidizes as sapphire; and you get stainless steel. Put in some more carbon and the steel is flexy. Put in too much carbon and it shatters. And all sorts of other cool effects just by tinkering with the mixture.

So what about 4130 vs High-Tensile Steel.
First, while bike manufacturers like to bash it, 4130 on its own is actually a great material. Aircraft quality. Easy to melt and work with, and has a great flexibility/strength to weight ratio. But some of the higher-tensile strength alloys can allow for even more flexibility/strength, but are often harder to manufacture. The higher flexibility/strength alloys mean you can make the walls of the tubing thinner and still have the same overall strength as 4130 of a thicker gauge tubing. And this is why we think high-tensile is lighter than 4130...because you can have the same dynamic strength with less material, therefore the tubes are thinner walled, use less material and are therefore lighter.

You can actually find a great overview of iron, steel, and steel alloys in this book: Stuff Matters - Exploring Marvelous Materials. Which I found easy enough to pick up an e-copy from my local library.


tl;dr: Stronger alloys allow you to use less material to make the same strength tubing.