A powerful deoxidizer and nitride former. In small amounts, can serve as a powerful, inexpensive grain refiner ( i.e., restricts Austenitic grain growth ). Can improve toughness, especially at low temperatures.

Also Boron reduces the brittleness of hardened steel, by having a positive influence on the transformation from Ferrite to Austenite to Martensite Structure. Especially on steels with lower carbon content (e.g. C=0.27%) that tend to transform inert, Boron strongly improves the transformation process.

The principal element responsible for hardness in steel, due to formation of Fe 3C upon cooling through the Transformation Temperature, when Gamma Iron ( Austenite ) decomposes into Alpha Iron ( Ferrite ) + Fe3C ( Iron Carbide ). Increases tensile strength in steels. Ductility generally decreases as C increases, but in most cases this effect can be offset with proper heat treatment.^ Weldability decreases as C increases. Excess oxygen usage may be required to remove excess C ( takes furnace time ).

Cr is a strong carbide former, and can improve wear resistance and somewhat increase resistance to softening during tempering. 

Improves hardenability depth. Promotes the response of steel containing Cr the effects of carburizing heat treatment. In combination with even very low P, Sn, As or Sb contents, Chromium and Ni-Cr alloy steels are particularly susceptible to "temper embrittlement" (loss of ductility when tempering or slow cooling in the range 700-1100 F). ^ When Cr > 4%, corrosion resistance greatly improves ( Responsible for corrosion resistance in Stainless Steels ). ^ Not readily oxidized from bath; requires high temperatures, increased heat time and slag volume.

Cr makes steel oil and air hardenable.

Mo is a strong Carbide former and has a high effect on hardenability.  Improves control of heat treatment by inhibiting formation of certain microstructures ( e.g., Pearlite ). Can improves high temperature corrosion resistance.  Can improve toughness & fatique properties. Expensive.

Vanadium V and Molybdenum Mo Both have influence on better toughness and have the same negative influence as Cr they are mostly used as fragility compensatory in High carbon steels like D2 or in extreme cause in 440V steel.

Improves harden ability. Reduces distortion in heat treating. Permits use of milder quenching media. Improves weldability, plasticity & fatique properties. ^ Improves toughness, especially at low temperatures. ^Improves corrosion resistance.

Si is an element (like Mn) that is contained in every kind of steel since already iron ore has a certain amount. In content up to 0,5% it has positive influence on mechanical properties and helps to perform hot forming of steel.  Si deoxidizes and increases durability and strongly increases elasticity. Only steel with more than 0.40% is called silicon steel.

V is an effective grain refiner ( i.e., restricts Austenitic grain growth ). Strong carbide and nitride former ( improves abrasion resistance ).  Improves yield strength, toughness and hot hardness. ^ Strongly increases resistance to softening during tempering. Expensive.