What are the effects of increasing or decreasing the carbon, sulfur, silicon, manganese, and phosphorus content in steel?

What are the effects of increasing or decreasing the carbon, sulfur, silicon, manganese, and phosphorus content in steel?
1. Carbon is a key element in steel. As the carbon content increases, the steel's strength and hardness increase, but its plasticity and ductility decrease, making it brittle and more difficult to work with.
2. Sulfur is generally considered a harmful impurity. It can cause hot brittleness in steel, reduce its ductility and toughness, and potentially lead to crack formation during forging and rolling.
3. Silicon is added in steel production as a reducing agent and deoxidizer. Therefore, killed steel typically contains 0.15%-0.30% silicon. When the silicon content in steel exceeds 0.50%-0.60%, it is considered an alloying element. Silicon significantly increases the elastic modulus, yield point, and tensile strength of steel, and is therefore widely used in the manufacture of spring steel. Adding 1.0%-1.2% silicon to quenched and tempered structural steel can increase strength by 15%-20%. Silicon, combined with elements such as molybdenum, tungsten, and chromium, improves steel's corrosion and oxidation resistance, making it suitable for manufacturing heat-resistant steel. Low-carbon steel containing 1%-4% silicon exhibits extremely high magnetic permeability, making it suitable for use in silicon steel sheets in the electrical appliance industry. However, increasing silicon content can reduce the steel's weldability.
4. Manganese has multiple functions in steel: it dissolves into the metal matrix, providing solid solution strengthening; it forms carbides, providing dispersion strengthening; it improves the steel's hardenability and temper resistance, and it may induce secondary hardening, preventing type II temper embrittlement. Consequently, manganese steel exhibits excellent machinability and wear and corrosion resistance.

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