The strength of any given piece of steel depends on several things:
1. physical configuration of the part - it's size and configuration, including any design flaws or stress concentrations. For example - make a leaf spring with square sharp corners, it will tend to crack at the corners due to those stress concentrations.
2. Chemistry of the steel - what hardness & toughness can it actually obtain.
3. Heat treatment - two pieces of steel can have the same hardness, but markedly different properties, depending on the heat treatment. The notorious low-numbered 1903 Springfields are an example of good steel that was incorrectly heat treated, and proved brittle.
4. Cleanliness & metallurgical quality of the steel. Sulpher is good for ease of machining, but can make steel brittle - the notorious hull rivets on the Titanic were known to be made with very high sulpher content, which made them brittle. Any impurities can cause failure.
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Good designs can overcome metallurgy - for example the Winchester High Walls were famously strong, used for proof testing by Winchester well into the smokeless era. I think it was John Campbell who had analyzed some High Walls, and found them to be of relatively low carbon steel, but well made.
