Thick steel sheet having excellent CTOD properties in multilayer welded joints, and manufacturing method for thick steel sheet

1. A thick steel plate having good multipass weld joint CTOD characteristics, comprising, on a mass percent basis: C: 0.03% to 0.12%, Si: 0.5% or less, Mn: 1.0% to 2.0%, P: 0.015% or less, S: 0.0005% to 0.0050%, Al: 0.005% to 0.060%, Ni 0.5% to 2.0%, Ti: 0.005% to 0.030%, N: 0.0015% to 0.0065%, O: 0.0010% to 0.0050%, and Ca: 0.0005% to 0.0060% so as to satisfy the formulae (1) to (4), the remainder being Fe and incidental impurities, a base material of the plate has an effective grain size of 20 μm or less at half the thickness of the plate, and the plate contains 25 to 250 /mm 2 of complex inclusions at ¼ and ½ of the thickness of the plate where thickness is in millimeters, the complex inclusions being composed of a sulfide containing Ca and Mn and an oxide containing Al and having an equivalent circular diameter of 0.1 μm or more:

1.5≤Ti/N≤5.0  Formula (1):

0.43≤Ceq(=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≤0.50  Formula (2):

0.18≤Pcm(=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B])≤0.22  Formula (3):

0.2≤(Ca−(0.18+130*Ca)*O)/(1.25*S)<1.4  Formula (4):

wherein alloying elements in the formulae (1) to (4) denote the corresponding contents (mass %).

2. The thick steel plate having good multipass weld joint CTOD characteristics according to claim 1 , further comprising, on a mass percent basis, one or two or more of Cu: 0.05% to 2.0%, Cr: 0.05% to 0.30%, Mo: 0.05% to 0.30%, Nb: 0.005% to 0.035%, V: 0.01% to 0.10%, W: 0.01% to 0.50%, B: 0.0005% to 0.0020%, REM: 0.0020% to 0.0200%, and Mg: 0.0002% to 0.0060%.

3. A method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 1 , comprising: heating a slab having the composition according to claim 1 to a temperature in a range of from 950° C. to 1200° C., hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction/pass being 8% or more at a half-thickness temperature of 950° C. or more and at a cumulative rolling reduction of 40% or more at a half-thickness temperature of less than 950° C., and cooling the hot-rolled plate to 600° C. or less with an average cooling rate between 700° C. and 500° C. at half the thickness of the plate being in the range of 3° C. to 50° C./s.

4. A method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 1 , comprising: heating a slab having the composition according to claim 1 to a temperature in a range of from 950° C. to 1200° C., hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction/pass being 5% or more at a half-thickness temperature of 950° C. or more and at a cumulative rolling reduction of 40% or more at a half-thickness temperature of less than 950° C., and cooling the hot-rolled plate to 600° C. or less with an average cooling rate between 700° C. and 500° C. at half the thickness of the plate being in the range of 3° C. to 50° C./s.

5. The method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 3 , further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

6. A method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 2 , comprising: heating a slab having the composition according to claim 2 to a temperature in a range of from 950° C. to 1200° C., hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction/pass being 8% or more at a half-thickness temperature of 950° C. or more and at a cumulative rolling reduction of 40% or more at a half-thickness temperature of less than 950° C., and cooling the hot-rolled plate to 600° C. or less with an average cooling rate between 700° C. and 500° C. at half the thickness of the plate being in the range of 3° C. to 50° C./s.

7. A method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 2 , comprising: heating a slab having the composition according to claim 2 to a temperature in a range of from 950° C. to 1200° C., hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction/pass being 5% or more at a half-thickness temperature of 950° C. or more and at a cumulative rolling reduction of 40% or more at a half-thickness temperature of less than 950° C., and cooling the hot-rolled plate to 600° C. or less with an average cooling rate between 700° C. and 500° C. at half the thickness of the plate being in the range of 3° C. to 50° C./s.

8. The method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 4 , further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

9. The method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 6 , further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

10. The method for manufacturing the thick steel plate having good multipass weld joint CTOD characteristics according to claim 7 , further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.