Steel material superior in high temperature characteristics and toughness and method of production of same

1. A steel material comprising, by mass %,

C: 0.005% to 0.03%,

Si: 0.05% to 0.40%,

Mn: 0.40% to 1.70%,

Nb: 0.02% to 0.10%,

Ti: 0.005% to 0.025%,

N: 0.0008% to 0.0045%, and

B: 0.0003% to 0.0030%,

restricting

P: 0.030% or less,

S: 0.020% or less, and Al: 0.02% or less, and

having a balance of Fe and unavoidable impurities, where the contents of C and Nb satisfy

0.008≦C—Nb/7.74≦0.02

and Ti-based oxides of a grain size of 0.05 to 10 μm are present in a density of 30 to 300/mm 2 ,

wherein the steel material is produced by a method comprising adjusting solute oxygen to within a range of from 0.003 to 0.015 mass % before the addition of Ti.

2. The steel material according to claim 1 further comprises, by mass %, one or both of

V: 0.10% or less and

Mo: 0.10% or less.

3. The steel material according to claim 1 further comprises, by mass %, one or more of

Zr: 0.03% or less and

Hf: 0.01% or less.

4. The steel material according to claim 1 further comprises, by mass %, one or more of

Cr: 1.5% or less,

Cu: 1.0% or less, and

Ni: 0.7% or less.

5. The steel material according to claim 1 further comprises, by mass %, one or more of

Mg: 0.005% or less,

REM: 0.01% or less, and

Ca: 0.005% or less.

6. The steel material according to claim 1 , wherein a mass concentration product of Nb and C is 0.0015 or more.

7. The steel material according to claim 1 , wherein the steel material is a fire resistant steel material having a heat draw rate of 30% or more.

8. The steel material according to claim 1 , wherein the steel material is an extremely thick H-section steel with a flange thickness of 40 mm or more.