High strength steel plate with ultimate tensile strength of 900 MPa or more excellent in hydrogen embrittlement resistance and method of production of same

1. A high strength steel plate with an ultimate tensile strength of 900 MPa or more, wherein in the structure of the steel plate,

(a) by volume fraction, ferrite is present in 10 to 50%, at least one of bainitic ferrite or bainite is present in 10 to 60%, and tempered martensite is present in 10 to 50%, and

(b) iron-based carbides which contain Si or Si and A1 in 0.1 mass % or more are present in 4×10 8 particles/mm 3 or more.

2. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 1 , wherein in said structure of the steel plate, by volume fraction, fresh martensite is present in 10% or less.

3. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 1 or 2 , wherein in said structure of the steel plate, by volume fraction, retained austenite is present in 2 to 25%.

4. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 1 or 2 , wherein said iron-based carbides are present in at least one of the bainite or the tempered martensite.

5. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 1 or 2 , wherein said steel plate contains, by mass %, C: 0.07% to 0.25%, Si: 0.45 to 2.50%, Mn: 1.5 to 3.20%, P: 0.001 to 0.03%, S: 0 . 0001 to 0.01%, Al: 0.005 to 2.5%, N: 0.0001 to 0.0100%, and O: 0.0001 to 0.0080% and has a balance of iron and unavoidable impurities.

6. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 5 , wherein said steel plate further contains, by mass %, one or more of Ti: 0.005 to 0.09%, Nb: 0.005 to 0.09%, B: 0.0001 to 0.01%, Cr: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 0.05%, Mo: 0.01 to 0.8%, V: 0.005 to 0.09%, and a group of one or more of Ca, Ce, Mg, and REM in a total of 0.0001 to 0.5%.

7. The high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 1 or 2 , wherein said steel plate has a galvanized layer on its surface.

8. A method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more, said method comprising:

(a) casting a slab which contains, by mass %, C: 0.07% to 0.25%, Si: 0.45 to 2.50%, Mn: 1.5 to 3.20%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 0.005 to 2.5%, N: 0.0001 to 0.0100%, and O: 0.0001 to 0.0080% and has a balance of iron and unavoidable impurities, directly, or after once cooling, heating to a 1050° C. or more temperature and hot rolling, finishing the hot rolling at a temperature of the Ara transformation point or more, coiling at a 400 to 670° C. temperature region, pickling, then cold rolling by a draft of 40 to 70%, next,

(b) using a continuous annealing line for annealing at a maximum heating temperature of 760 to 900° C., then cooling by an average cooling rate of 1 to 1000° C./sec down to the Ms point to the Ms point −100° C., next

(c) bending and unbending the steel plate by rolls of a radius of 800 mm or less, then performing heat treatment in the 150 to 400° C. temperature region for 5 seconds or more.

9. The method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 8 ,

said method further comprising galvanizing the steel plate surface after the heat treatment of (c).

10. The method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 9 , wherein said galvanization is electrogalvanization.

11. A method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more, said method comprising:

(a) casting a slab which contains, by mass %, C: 0.07% to 0.25%, Si: 0.45 to 2.50%, Mn: 1.5 to 3.20%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 0.005 to 2.5%, N: 0.0001 to 0.0100%, and O: 0.0001 to 0.0080% and has a balance of iron and unavoidable impurities, directly, or after once cooling, heating to a 1050° C. or more temperature and hot rolling, finishing the hot rolling at a temperature of the Ara transformation point or more, coiling at a 400 to 670° C. temperature region, pickling, then cold rolling by a draft of 40 to 70%, next,

(b) using a continuous hot dip galvanization line for annealing at a maximum heating temperature of 760 to 900° C., then cooling by an average cooling rate of 1 to 1000° C./sec, then dipping in a galvanization bath and cooling by an average cooling rate of 1° C./second or more down to the Ms point to the Ms point −100° C., next,

(c) bending and unbending the steel plate by rolls of a radius of 800 mm or less, then performing heat treatment in the 150 to 400° C. temperature region for 5 seconds or more.

12. The method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 11 further comprising performing alloying treatment at a 460 to 600° C. temperature after dipping in said galvanization bath, then cooling by an average cooling rate of 1° C./second or more down to the Ms point to the Ms point −100° C.

13. The method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 8 , wherein said slab further contains, by mass %, one or more of Ti: 0.005 to 0.09%, Nb: 0.005 to 0.09%, B: 0.0001 to 0.01%, Cr: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 0.05%, Mo: 0.01 to 0.8%, V: 0.005 to 0.09%, and a group of one or more of Ca, Ce, Mg, and REM in a total of 0.0001 to 0.5%.

14. The method of production for producing a high strength steel plate with an ultimate tensile strength of 900 MPa or more as set forth in claim 11 , wherein said slab further contains, by mass %, one or more of Ti: 0.005 to 0.09%, Nb: 0.005 to 0.09%, B: 0.0001 to 0.01%, Cr: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 0.05%, Mo: 0.01 to 0.8%, V: 0.005 to 0.09%, and a group of one or more of Ca, Ce, Mg, and REM in a total of 0.0001 to 0.5%.