Question 1
Conventional automotive torsion bars are manufactured using steel with a carbon content of 0.3% to 0.5%, which is
Not quite. Low carbon steel stops at 0.30%. The range starts on that edge, but everything from just above 0.3% up to 0.5% is past it. Reading only the first number lands here. In 2021, 9.1% of Queensland students chose (A).
Not quite. High carbon steel starts at 0.60%, so the whole range is below it. A torsion bar twists like a spring, and springs are on the high carbon list, which may be what pulled you here. In 2021, 4.4% of Queensland students chose (B).
Not quite. Mild sounds like the middle of the scale, but under the 2019 syllabus mild carbon steel was the band just above low, 0.15% to 0.30%. This range only touches it at 0.3%. The 2025 syllabus has dropped the name, and mild steel now means low carbon steel. In 2021, 23.0% of Queensland students chose (C).
That is it. Step through the hints below to see why. In 2021, 63.2% of Queensland students chose (D).
The 2025 syllabus gives three plain-carbon bands as numbers you have to know. The formula book has none of them.
The stem gives you a range, so place both ends of it.
The bottom end sits on the edge where medium begins, and the top end is well under the edge where high begins.
$$0.30\%\le0.3\%\qquad0.5\%<0.60\%$$
The whole range is inside the medium band. Move the slider from 0.30% to 0.50% and the band never changes.
Mild is the trap. This paper was set under the 2019 syllabus, where mild carbon steel was 0.15% to 0.30%, the band just above low. Turn on the 2019 bands and you will see it ends exactly where this range starts.
For 2026 there is no mild band at all. Mild steel is low carbon steel.
The answer makes engineering sense. A torsion bar is a suspension spring that twists every time the wheel moves, so it needs strength and toughness together. That is the medium carbon mix of ferrite and pearlite.
$$\frac{0.3}{0.83}\approx36\%\qquad\frac{0.5}{0.83}\approx60\%$$
Across the range, the pearlite rises from about 36% to about 60%.
$$0.30\%\le0.3\%\text{ to }0.5\%<0.60\%$$
Both ends of the range sit inside the medium carbon band, 0.30% to 0.60%. Its ferrite and pearlite give the strength and toughness a torsion bar needs.
The most common wrong answer was (C), from reading mild as the middle of the scale. Under the 2019 syllabus mild was the band just above low, and for 2026 it has gone. A 2026 question that says mild steel means low carbon steel, 0.07% to 0.30%.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2021 QCAA Engineering external assessment, © State of Queensland (QCAA) 2021, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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