For low-carbon steel used in construction and manufacturing, the yield strength and tensile strength are commonly improved by adding which alloying element in practical compositions? Pick one best microalloying choice.

Introduction / Context:
Most structural and sheet steels are low in carbon and rely on modest alloy additions to tune strength, ductility, and hardenability. This question focuses on the most ubiquitous element used to raise strength in low-carbon steels without dramatically increasing cost or compromising weldability.

Given Data / Assumptions:

• Base steel is low-carbon (typically up to ~0.25% C).
• We seek the most common addition that improves both yield and tensile strength in ordinary grades.
• Cost-effectiveness and typical mill practice matter.

Concept / Approach:

Manganese (generally ~0.4–1.6%) is the standard strengthener in low-carbon steels. It increases hardenability, promotes solution strengthening, ties up sulphur as MnS (improving hot workability), and raises yield/tensile with balanced ductility. Chromium and nickel also strengthen, but they are more associated with alloy/stainless grades and are added in larger, more expensive proportions for corrosion resistance or deep hardenability. Vanadium is a microalloy that forms carbides/nitrides and strongly raises yield in microalloyed steels, yet it is not as universally used across everyday low-carbon grades as manganese. Tungsten is typical of tool steels (hot-work, high-speed) rather than common construction steels.

Step-by-Step Solution:

Verification / Alternative check:

Standard specifications (e.g., mild steel plates, rebars, low-carbon structural steels) list Mn as a routine addition aligned with target yield/tensile levels.

Why Other Options Are Wrong:

Chromium/Nickel: effective but not the default, cost-efficient choice in plain low-carbon steels. Tungsten: tool-steel domain. Vanadium: potent microalloy but not the most commonly used across general low-carbon steels.

Common Pitfalls:

Assuming any alloying element that raises strength is equally common; overlooking cost and typical composition windows.

Final Answer:

manganese