B. Strength increases while ductility decreases with increasing carbon content.
Carbon acts as a hardening agent in steel. As you increase the carbon content, the steel becomes harder and stronger. However, this increase in strength comes at the cost of ductility. Ductility is the ability of a material to deform under tensile stress without fracturing. So, while the steel becomes stronger, it also becomes more brittle.
Why the other options are incorrect:
- A. Strength and ductility increase proportionally with carbon content. While it's true that strength increases with carbon content, ductility decreases. They have an inverse relationship, not a proportional one.
- C. Ductility increases while strength decreases with increasing carbon content. This is exactly opposite to the actual relationship. As explained, increasing carbon content increases strength but decreases ductility.
- D. There is no significant change in mechanical properties with increasing carbon content. This is incorrect. Carbon is a key alloying element in steel and has a substantial impact on its mechanical properties.