Forging steel

1. A forging steel consisting of, in mass %:

C: 0.001 to less than 0.03%,

Si: 3.0% or less,

Mn: 0.01 to 4.0%,

Cr: 5.0% or less,

P: 0.2% or less,

S: 0.35% or less,

Al: greater than 0.06 to 2.0%,

N: 0.03% or less,

B: not less than BL given by Equation (7) below and not greater than 0.008%,

Ti: 0.005 to 0.15%, and

Mg: 0.0002 to 0.003%,

one or both of Mo: 1.5% or less (including 0%) and Ni: 4.5% or less (including 0%), and

a balance of iron and unavoidable impurities; and

optionally, one or more of Nb, V, Te, Ca, Zr, and REM;

wherein Di given by the following Equation (3) is 60 or greater:

Di= 5.41× Di (Si)× Di (Mn)× Di (Cr)× Di (Mo)× Di (Ni)× Di (Al)×1.976  (3),

where

Di(Si)=0.7×[% Si]+1,

Di(Mn)=3.335×[% Mn]+1 when Mn≦1.2%,

Di(Mn)=5.1×[% Mn]−1.12 when 1.2%<Mn,

Di(Ni)=0.3633×[% Ni]+1 when Ni≦1.5%,

Di(Ni)=0.442×[% Ni]+0.8884 when 1.5%<Ni≦1.7%,

Di(Ni)=0.4×[% Ni]+0.96 when 1.7%<Ni≦1.8%,

Di(Ni)=0.7×[% Ni]+0.42 when 1.8%<Ni≦1.9%,

Di(Ni)=0.2867×[% Ni]+1.2055 when 1.9%<Ni,

Di(Cr)=2.16×[% Cr]+1,

Di(Mo)=3×[% Mo]+1,

Di(Al)=1 when Al≦0.05%, and

Di(Al)=4×[% Al]+1 when 0.05%<Al,

wherein BL= 0.0004+10.8/14×([% N]−14/47.9×[% Ti])  (7)

where

([% N]−14/47.9×[% Ti]) of less than 0 is treated as 0,

a symbol in brackets [ ] indicating content (mass %) of the element concerned, and wherein a deformation resistance at room temperature is not more than 563 MPa.

2. The forging steel according to claim 1 , wherein one or more of Nb, V, Te, Ca, Zr, and REM is present in the following amounts:

Nb: 0.005 to 0.1%,

V: 0.01 to 0.5%,

Te: 0.0002 to 0.003%,

Ca: 0.0003 to 0.003%,

Zr: 0.0003 to 0.005%, and

REM: 0.0003 to 0.005%.