THICK STEEL PLATE HAVING GOOD MULTIPASS WELD JOINT CTOD CHARACTERISTICS AND METHOD FOR MANUFACTURING THE SAME

Abstract

A steel plate comprising, by mass %: 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%, 0: 0.0010% to 0.0050%, Ca: 0.0005% to 0.0060%, and optionally one or two or more of Cu and the like. Ti/N, Ceq, Pcm, and ACR are in particular ranges, a base metal of the plate has an effective grain size of 20 μm or less at half the thickness of the plate, and the plate contains a particular number of complex inclusions at ¼ and ½ of the thickness of the plate. The complex inclusions comprise a sulfide containing Ca and Mn and an oxide containing Al and having an equivalent circular diameter of 0.1 μm or more.

Claims

1. A thick steel plate having multipass weld joint CTOD characteristics, the steel plate having a chemical composition comprising, by mass %: 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%; and the remainder being Fe and incidental impurities, wherein a base metal of the plate has an effective grain size of 20 μm or less at half a thickness of the plate, and the plate contains 25 to 250/mm.sup.2 of complex inclusions at ¼ and ½ of the thickness (t: mm) of the plate, the complex inclusions (i) comprising a sulfide containing Ca and Mn and an oxide containing Al and (ii) having an equivalent circular diameter of 0.1 μm or more, and formulae (1) to (4) are satisfied:
1.5≦Ti/N≦5.0   (1),
0.43≦Ceq (=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.54   (2),
0.18≦Pcm (=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B])≦0.24   (3), and
0.2≦(Ca−(0.18+130*Ca)*O)/(1.25*S)≦1.4   (4).

2. The thick steel plate having multipass weld joint CTOD characteristics according to claim 1, further comprising, by mass %, at least one 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 thick steel plate having multipass weld joint CTOD characteristics, the steel plate having a chemical composition comprising, by mass %: 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%; Ca: 0.0005% to 0.0060%; and the remainder being Fe and incidental impurities, wherein a base metal of the plate has an effective grain size of 20 μm or less at half a thickness of the plate, and the plate contains 25 to 250/mm.sup.2 of complex inclusions at ¼ and ½ of the thickness (t: mm) of the plate, the complex inclusions being (i) comprising a sulfide containing Ca and Mn and an oxide containing Al and (ii) having an equivalent circular diameter of 0.1 μm or more, and formulae (1) to (4) are satisfied:
1.5≦Ti/N≦5.0   (1),
0.50<Ceq (=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5)≦0.54   (2),
0.18≦Pcm (=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B])≦0.24   (3), and
0.2≦(Ca−(0.18+130*Ca)*O)/(1.25*S)≦1.4   (4).

4. The thick steel plate having multipass weld joint CTOD characteristics according to claim 3, further comprising, by mass %, at least one 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%.

5. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 1 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

6. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 1 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

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

8. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 2 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

9. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 3 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

10. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 4 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 30% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

11. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 2 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

12. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 3 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

13. A method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics, the method comprising: heating a slab having the composition according to claim 4 to a temperature in a range of 950° C. or more and 1200° C. or less; hot rolling the slab at a cumulative rolling reduction of 33% or more with a rolling reduction of 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 a range of 3° C./s to 50° C./s.

14. The method for manufacturing a thick steel plate having 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.

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

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

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

18. The method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics according to claim 11, further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

19. The method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics according to claim 12, further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

20. The method for manufacturing a thick steel plate having multipass weld joint CTOD characteristics according to claim 13, further comprising performing tempering treatment at a temperature of 700° C. or less after the cooling.

Description

DETAILED DESCRIPTION

[0040] The reasons for defining the constituent features of the disclosed embodiments will be described below.

1. Chemical Components

[0041] First, the reason for defining the chemical components of steel according to the disclosed embodiments will be described below. The percentages are on a mass basis.

C: 0.030 to 0.12%

[0042] C is an element that can improve the strength of steel. The C content should be 0.03% or more. However, an excessively high C content of more than 0.12% results in poor joint CTOD characteristics. Thus, the C content ranges from 0.03% to 0.12%, preferably 0.03% to 0.09%, more preferably 0.04% to 0.08%.

Si: 0.5% or less

[0043] An excessively high Si content of more than 0.5% results in poor joint CTOD characteristics. Thus, the Si content is 0.5% or less, preferably 0.2% or less, more preferably less than 0.15%.

Mn: 1.0% to 2.0%

[0044] Mn is an element that can improve the quenching hardenability of steel and thereby improve the strength of the steel. However, an excessive addition of Mn significantly impairs joint CTOD characteristics. Thus, the Mn content ranges from 1.0% to 2.0%, preferably 1.2% to 1.8%,

P: 0.015% or less

[0045] P is an element that is inevitably contained in steel as an impurity and decreases the toughness of steel. Thus, it is desirable to minimize P. In particular, a P content of more than 0.015% results in very poor joint CTOD characteristics. Thus, the P content is limited to 0.015% or less, preferably 0.010% or less.

S: 0.0005% to 0.0050%

[0046] S is an element necessary for inclusions to improve multipass weld HAZ toughness. The S content should be 0.0005% or more. However, a S content of more than 0.0050% results in poor joint CTOD characteristics. Thus, the S content is limited to 0.0050% or less, preferably 0.0045% or less.

Al: 0.005% to 0.060%

[0047] Al is an element necessary for inclusions to improve multipass weld HAZ toughness. The Al content should be 0.005% or more. An Al content of more than 0.060% results in poor joint CTOD characteristics. Thus, the Al content is limited to 0.060% or less.

Ni: 0.5% to 2.0%

[0048] Ni is an element that can reinforce a base metal and a joint without significantly reducing the toughness of the base metal and the joint. This effect requires a Ni content of 0.5% or more. However, the reinforcement is saturated at a Ni content of 2.0%, and a Ni content of more than 2.0% incurs increased costs. Thus, the Ni content is limited to 2.0% or less, preferably 0.5% to 1.8%.

Ti 0.005% to 0.030%

[0049] Ti is an element that can be precipitated as TiN and is effective in suppressing austenite grain coarsening in HAZ, making a HAZ microstructure finer, and improving the toughness of steel. These effects require a Ti content of 0.005% or more. An excessively high Ti content of more than 0.030% results in low heat affected zone toughness due to dissolved Ti or precipitation of coarse TiC. Thus, Ti is limited to the range of 0.005% to 0.030%, preferably 0.005% to 0.025%.

N: 0.0015% to 0.0065%

[0050] N is an element that can be precipitated as TIN and is effective in suppressing austenite grain coarsening in HAZ, making a HAZ microstructure finer, and improving the toughness of steel. These effects require a N content of 0.0015% or more. An excessively high N content of more than 0.0065% results in low heat affected zone toughness. Thus, the N content is limited to the range of 0.0015% to 0.0065%, preferably 0.0015% to 0.0055%,

O: 0.0010% to 0.0050%

[0051] O is an element necessary for inclusions to improve multipass weld HAZ toughness. The O content should be 0.0010% or more. An O content of more than 0.0050% results in poor joint CTOD characteristics. Thus, the O content is limited to the range of 0.0010% to 0.0050%, preferably 0.0010% to 0.0045%.

Ca: 0.0005% to 0.0060%

[0052] Ca is an element necessary for inclusions to improve multipass weld HAZ toughness. The Ca content should be 0.0005% or more. A Ca content of more than 0.0060% results in poor joint CTOD characteristics. Thus, the Ca content is limited to the range of 0.0005% to 0.0060%, preferably 0.0007% to 0.0050%.

1.5≦Ti/N≦5.0   (1)

[0053] The amount of dissolved N in HAZ and the precipitation state of TiC depend on Ti/N. Ti/N of less than 1.5 results in low HAZ toughness due to dissolved N not fixed as TiN. Ti/N of more than 5.0 results in low HAZ toughness due to precipitation of coarse TiC. Thus, Ti/N is limited to 1.5 or more and 5.0 or less, preferably 1.8 or more and 4.5 or less, The alloying elements in the formula (1) denote the corresponding contents (mass %).

Ceq: 0.43% or more and 0.54% or less

[0054] Strength decreases with decreasing Ceq. Ceq of less than 0.43% results in unsatisfactory strength characteristics.

[0055] An increase in Ceq results in low HAZ toughness due to an increased amount of low-toughness microstructure, such as island martensite or bainite, in a HAZ microstructure. Ceq of more than 0.54% results in low HAZ matrix microstructure toughness and unsatisfactory joint CTOD characteristics even using a technique for improving HAZ toughness with inclusions. Thus, Ceq ranges from 0.43% to 0.54%, preferably more than 0.45% and 0.53% or less. Ceq is preferably more than 0.45 in order to consistently achieve the desired strength of a base metal and a joint. Ceq should be more than 0.50% in order to consistently achieve YP of 550 MPa or more, Ceq is preferably 0.53 or less in order for consistent HAZ toughness. Furthermore, Ceq=[C]+[Mn]/6+([Cu]+[Ni])/15+([Cr]+[Mo]+[V])/5 (2), wherein the alloying elements denote the corresponding contents (mass %).

Pcm: 0.18 or more and 0.24% or less

[0056] Strength decreases with decreasing Pcm. Pcm of less than 0.18% results in unsatisfactory strength characteristics. An increase in Pcm results in low HAZ toughness due to an increased amount of low-toughness microstructure, such as island martensite or bainite, in a HAZ microstructure. Pcm, of more than 0.24% results in low HAZ matrix microstructure toughness and unsatisfactory joint CTOD characteristics even using a technique for improving HAZ toughness with inclusions. Thus, Pcm ranges from 0.18% to 0.24%, preferably 0.18% to 0.23%. Furthermore, Pcm=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B](3), wherein the alloying elements denote the corresponding contents (mass %).

0.2≦(Ca−(0.18+130*Ca)*O)/(1.25*S)≦1.4

[0057] The atomic concentration ratio (ACR) of Ca, O, and S in steel is represented by (Ca−(0.18+130*Ca)*O)/(1.25* S). An ACR of less than 0.2 indicates that sulfide inclusions are mainly MnS. MnS has a low melting point and melts in the vicinity of a weld line during welding. Thus, MnS does not have the effect of suppressing austenite grain coarsening in the vicinity of a weld line and the transformation nucleus effect during cooling after welding. On the other hand, (Ca−(0.18+130*Ca)*O)/(1.25*S) of more than 1.4 indicates that sulfide inclusions are mainly CaS. Because a Mn-poor layer, which is required for transformation nucleation, is not formed around CaS, no transformation nucleus effect is produced. Thus, (Ca−(0.18+130*Ca)*O)/(1.25*S) is 0.2 or more and 1.4 or less, preferably 0.2 or more and 1.2 or less. The alloying elements in the formula (4) denote the corresponding contents (mass %).

[0058] A thick steel plate according to the disclosed embodiments is composed essentially of the components described above, and the remainder is Fe and incidental impurities. In order to improve strength, toughness control, and joint toughness, a thick steel plate according to the disclosed embodiments can further contain 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%.

Cu: 0.05% to 2.0%

[0059] Cu is an element that can reinforce a base metal and a joint without significantly reducing the toughness of the base metal and the joint. This effect requires a Cu content of 0.05% or more. However, an addition of 2.0% or more may cause steel plate cracking resulting from a Cu-rich layer formed directly under scales. Thus, when Cu is added, the Cu content ranges from 0.05% to 2.0%, preferably 0.1% to 1.5%.

Cr: 0.05% to 0.30%

[0060] Cr is an element that can improve the strength of steel by improving quenching hardenability, An excessive addition of Cr results in poor joint CTOD characteristics. Thus, when Cr is added, the Cr content ranges from 0.05% to 0.30%.

Mo: 0.05% to 0.30%

[0061] Mo is an element that can improve the strength of steel by improving quenching hardenability. However, an excessive addition of Mo results in poor joint CTOD characteristics. Thus, when Mo is added, the Mo content ranges from 0.05% to 0.30%.

Nb: 0.005% to 0.035%

[0062] Nb is an element that can extend the non-recrystallization temperature range of an austenite phase and is effective for efficient rolling in a non-recrystallization region and the formation of microstructures. These effects require a Nb content of 0.005% or more. However, a Nb content of more than 0.035% results in poor joint CTOD characteristics. Thus, when Nb is added, the Nb content ranges from 0.005% to 0.035%.

V: 0.01% to 0.10%

[0063] V is an element that can improve the strength of a base metal. A V content of 0.01% or more is effective. However, a V content of more than 0.10% results in low HAZ toughness. Thus, when V is added, the V content ranges from 0.01% to 0.10%, preferably 0.02% to 0.05%.

W: 0.01% to 0.50%

[0064] W is an element that can improve the strength of a base metal. A W content of 0.01% or more is effective. However, a W content of more than 0.50% results in low HAZ toughness. Thus, when W is added, the W content ranges from 0.01% to 0.50%, preferably 0.05% to 0.35%.

B: 0.0005% to 0.0020%

[0065] B is an element. that is effective in improving quenching hardenability at a very low B content and thereby improving the strength of a steel plate. These effects require a B content of 0.0005% or more. However, a B content of more than 0.0020% results in low HAZ toughness. Thus, when B is added, the B content ranges from 0.0005% to 0.0020%.

REM: 0.0020% to 0.0200%

[0066] REM can form oxysulfide inclusions and thereby suppress austenite grain growth in HAZ and improve HAZ toughness. These effects require a REM content of 0.0020% or more. However, an excessively high REM content of more than 0.0200% results in low base metal and HAZ toughness. Thus, when REM is added, the REM content ranges from 0.0020% to 0.0200%.

Mg: 0.0002% to 0.0060%

[0067] Mg is an element that can form oxide inclusions and is thereby effective in suppressing austenite grain growth in a heat affected zone and improving heat affected zone toughness. These effects require a Mg content of 0.0002% or more. However, these effects are saturated at a Mg content of 0.0060%, and a Mg content of more than 0.0060% is not worth the content and is economically disadvantageous. Thus, when Mg is added, the Mg content ranges from 0.0002% to 0.0060%.

2. Microstructure of Base Metal

[0068] In order to improve the joint CTOD characteristics at an SC/ICHAZ boundary, the effective grain size of a base metal microstructure at half the thickness of a plate is 20 μm or less such that the toughness of the base metal is improved by decreasing the crystal grain size at half the thickness of the plate where center segregation is likely to occur. The base metal microstructure is not particularly limited, provided that desired strength is achieved. The term “effective grain size”, as used herein, refers to the equivalent circular diameter of a crystal grain surrounded by a high-angle grain boundary having an orientation difference of 15 degrees or more with respect to adjacent crystal grains.

3. Inclusions

[0069] Complex inclusions composed of a sulfide containing Ca and Mn and an oxide containing Al: 25 to 250/mm.sup.2 at an equivalent circular diameter of 0.1 μm or more

[0070] A Mn-poor region around inclusions formed by formation of a sulfide containing Mn is effective for transformation nucleation. The sulfide further containing Ca has an increased melting point, is resistant to a temperature rise in the vicinity of a weld line in HAZ, and has the effect of suppressing austenite grain growth and the transformation nucleus effect. In order to produce these effects, the complex inclusions have an equivalent circular diameter of 0.1 μm or more, and the number of complex inclusions ranges from 25 to 250/mm.sup.2, preferably 35 to 170/mm.sup.2, at ¼ and ½ of the thickness of the plate.

4. Manufacturing Method

[0071] The reasons for limiting the conditions of the manufacturing method will be described below. Unless otherwise specified, the temperatures are steel surface temperatures.

Slab Heating Conditions

[0072] A slab is made of continuous cast steel and is heated to a temperature of 950° C. or more and 1200° C. or less. A heating temperature of less than 950° C. results in a residual untransformed zone after heating and a residual coarse microstructure after solidification. Thus, a desired fine grain microstructure cannot be formed. On the other hand, a heating temperature of more than 1200° C. results in austenite grain coarsening, and a desired fine grain microstructure cannot be formed by controlled rolling. Thus, the heating temperature is limited to 950° C. or more and 1200° C. or less, preferably 970° C. or more and 1170° C. or less.

Hot Rolling Conditions

[0073] In hot rolling, the pass conditions in a recrystallization temperature range and the pass conditions in a non-recrystallization temperature range are defined. In the recrystallization temperature range, the cumulative rolling reduction is 30% or more for rolling reduction with a rolling reduction/pass of 8% or more at a half-thickness temperature of 950° C. or more. Alternatively, in the recrystallization temperature range, the cumulative rolling reduction is 33% or more for rolling reduction with a rolling reduction/pass of 5% or more at a half-thickness temperature of 950° C. or more.

[0074] Rolling at less than 950° C. rarely causes recrystallization, and the austenite grain size is insufficiently decreased. Thus, the temperature is limited to 950° C. or more.

[0075] In rolling reduction with a rolling reduction/pass of less than 8%, a decrease in grain size due to recrystallization does not occur. Even for rolling reduction with a rolling reduction/pass of 8% or more, a decrease in crystal grain size due to recrystallization is insufficient at a cumulative rolling reduction of 30% or less. Thus, for rolling reduction with a rolling reduction/pass of 8% or more, the cumulative rolling reduction is 30% or more. As a result of further studies, the present inventors found that even for rolling reduction with a rolling reduction/pass of 5% or more, a cumulative rolling reduction of 33% or more results in a sufficient decrease in crystal grain size due to recrystallization. Thus, for rolling reduction with a rolling reduction/pass of 5% or more, the cumulative rolling reduction is 33% or more.

Cumulative rolling reduction of 40% or more at half-thickness temperature of less than 950° C. in non-recrystallization temperature range

[0076] In the rolling of steel according to the disclosed embodiments at less than 950° C., recrystallization rarely occurs, and strain in the steel is not relieved by recrystallization and is accumulated, acts as transformation nuclei in subsequent cooling, and thereby makes a final microstructure finer. A cumulative rolling reduction of less than 40% results in an insufficient effect of decreasing the crystal grain size. Thus, the cumulative rolling reduction is 40% or more at a half-thickness temperature of less than 950° C.

Cooling Conditions

[0077] Cooling after hot rolling is performed such that the average cooling rate between 700° C. and 500° C. at half the thickness of the plate ranges from 3° C. to 50° C./s. The cooling stop temperature is 600° C. or less.

[0078] An average cooling rate of less than 3° C./s at half the thickness of the plate results in the formation of a coarse ferrite phase in a base metal microstructure and poor CTOD characteristics a t SC/ICHAZ. An average cooling rate of more than 50° C./s results in poor CTOD characteristics at SC/ICHAZ due to increased base metal strength. Thus, the average cooling rate between 700° C. and 500° C. at half the thickness of the plate is limited to the range of 3° C. to 50° C./s When the cooling stop temperature is more than 600° C., transformation strengthening due to cooling is insufficient, and the base metal strength is insufficient. Thus, the cooling stop temperature is 600° C. or less.

[0079] In order to decrease base metal strength and improve toughness, tempering can be performed at 700° C. or less after cooling. A tempering temperature of more than 700° C. results in the formation of a coarse ferrite phase and low toughness of SCHAZ. Thus the tempering temperature is limited to 700° C. or less, preferably 650° C. or less.

EXAMPLES

[0080] Table 1 lists the composition of steel specimens. A slab was continuously casted with a continuous casting machine having a vertical length of 17 m at a casting speed in the range of 0.2 to 0.4 m/min and at a water flow rate in the range of 1000 to 2000 l/min/m2 in a cooling zone. Steel specimens A to K according to examples have compositions within the scope of the disclosed embodiments. Steel specimens L to T according to comparative examples have compositions outside the scope of the disclosed embodiments. These steel specimens were used to manufacture thick steel plates under conditions listed in Table 2. A multipass weld joint was formed from each thick steel plate. The half-thickness temperature was measured during hot rolling with a thermocouple disposed at the center of the plate in the longitudinal, width, and thickness directions,

[0081] The base metal strength and the distribution of inclusions in the thickness direction were examined in each thick steel plate. The average effective grain size was measured by taking a sample from the center of a plate in the longitudinal, width, and thickness directions, subjecting the sample to mirror polish finishing, performing an EBSP analysis under the following conditions, and from the resulting crystal orientation map determining, as the effective grain size, the equivalent circular diameter of a microstructure surrounded by a high-angle grain boundary having an orientation difference of 15 degrees or more with respect to adjacent crystal grains.

EBSP Conditions

[0082] Analysis area: 1 mm * 1 mm area at half the thickness of the plate

[0083] Step size: 0.4 μm

[0084] The density of inclusions was measured by taking samples from a plate at ¼ and ½ of the thickness of the plate in the longitudinal, width, and thickness directions, subjecting the samples to mirror polish finishing with a diamond buff and an alcohol, identifying inclusions in a 1 mm * 1 mm evaluation area by an EDX analysis with a field-emission scanning electron microscope (FE-SEM), and measuring the density of the inclusions. In the evaluation of the type of inclusions, an element was considered to be an inclusion when the atomic percentage of the element was 3% or more of the chemical composition of inclusions quantified by a ZAF method.

[0085] In a tensile test, a round bar tensile test piece having a diameter 14 mm and a length of 70 mm was taken from a plate in the plate width direction at ¼ of the thickness (t) of the plate, and the tensile test was performed according to EN10002-1. The yield strength in Table 2 refers to upper yield stress in the presence of an upper yield point and refers to 0.2% proof stress in the absence of an upper yield point.

[0086] A weld joint used in a joint CTOD test was formed by submerged arc welding (multipass welding) with a K groove shape and a heat input of 5.0 kJ/mm. The test method conformed to BS standard EN10225 (2009). A test specimen had a t (thickness) * t (thickness) cross-section. The CTOD value (δ) was determined at a test temperature of −10° C. For each type of steel, three test pieces for each notch position were tested. Test pieces having an average CTOD value of 0.35 mm or more in CGHAZ and/or an SC/ICHAZ boundary were judged to be a steel plate having good joint CTOD characteristics.

[0087] The notch positions were CGHAZ on a straight line shape side of the K groove (a straight line shape and a bent shape) and the SC/ICHAZ boundary. After the test, a tip of a fatigue precrack on a test specimen fracture surface was observed in CGHAZ and the SC/ICHAZ boundary defined by EN10225 (2009). In a multipass weld joint CTOD test, a notch position in CGHAZ includes a certain area of ICCGHAZ, and the test results reflect both CGHAZ toughness and ICCGHAZ toughness.

[0088] Table 2 shows the test results. Nos. 1 to 11, 17, 18, 29, 30, and 32 according to examples, which have chemical components, an effective grain size of a base metal, an inclusion density, and manufacturing conditions within the scope of the disclosed embodiments, have high base metal tensile strength and good joint CTOD characteristics

[0089] Nos. 12 to 16, 19 to 28, and 31 according to comparative examples have poor joint CTOD characteristics.

TABLE-US-00001 TABLE 1 (Mass % of each component) Steel type C Si Mn P S Al Ni Ti N O Ca Cu Cr A 0.05 0.30 2.0 0.004 0.0013 0.022 1.7 0.010 0.0043 0.0024 0.0016 B 0.11 0.20 1.6 0.005 0.0048 0.029 1.5 0.008 0.0033 0.0039 0.0039 C 0.12 0.10 1.2 0.008 0.0007 0.018 2.0 0.016 0.0034 0.0038 0.0037 D 0.08 0.20 1.6 0.005 0.0019 0.037 0.9 0.021 0.0053 0.0026 0.0026 E 0.09 0.50 2.0 0.007 0.0035 0.015 0.6 0.014 0.0041 0.0015 0.0046 F 0.07 0.20 1.7 0.005 0.0026 0.031 1.4 0.007 0.0037 0.0019 0.0027 G 0.03 0.30 1.5 0.006 0.0024 0.036 0.6 0.005 0.0018 0.0042 0.0047 1.60 H 0.08 0.40 1.3 0.003 0.0009 0.016 1.7 0.026 0.0063 0.0015 0.0007 0.25 J 0.08 0.30 1.8 0.007 0.0016 0.047 1.1 0.017 0.0051 0.0019 0.0035 K 0.09 0.30 1.7 0.006 0.0012 0.023 1.3 0.015 0.0041 0.0028 0.0029 L 0.14 0.10 1.0 0.004 0.0016 0.028 1.8 0.017 0.0055 0.0018 0.0013 M 0.09 0.20 1.4 0.005 0.0008 0.036 1.5 0.006 0.0048 0.0017 0.0019 0.15 N 0.08 0.20 1.5 0.006 0.0015 0.036 0.7 0.014 0.0051 0.0025 0.0048 0.30 0.26 O 0.08 0.20 1.3 0.006 0.0024 0.018 0.8 0.025 0.0036 0.0036 0.0041 0.71 P 0.06 0.20 1.7 0.006 0.0006 0.024 1.2 0.007 0.0031 0.0008 0.0003 0.26 Q 0.10 0.20 1.8 0.008 0.0021 0.038 1.1 0.013 0.0028 0.0032 0.0024 0.16 R 0.09 0.40 1.6 0.005 0.0018 0.031 0.9 0.022 0.0045 0.0026 0.0028 S 0.07 0.30 1.6 0.006 0.0013 0.041 1.0 0.003 0.0020 0.0035 0.0032 0.45 T 0.09 0.30 1.6 0.003 0.0014 0.025 1.4 0.009 0.0043 0.0045 0.0022 U 0.08 0.30 1.5 0.005 0.0047 0.024 0.9 0.022 0.0051 0.0042 0.0075 0.35 W 0.10 0.18 1.8 0.007 0.0004 0.032 0.6 0.013 0.0032 0.0059 0.0034 0.16 X 0.07 0.03 1.8 0.004 0.0012 0.028 1.3 0.010 0.0038 0.0022 0.0019 0.50 Y 0.05 0.05 1.5 0.005 0.0009 0.019 1.6 0.011 0.0041 0.0027 0.0017 0.45 0.28 Z 0.08 0.27 1.9 0.012 0.0031 0.029 0.7 0.009 0.0029 0.0021 0.0029 0.55 0.16 AA 0.08 0.12 1.9 0.004 0.0009 0.023 1.8 0.011 0.0033 0.0022 0.0018 (Mass % of each component) Steel type Mo Nb V W B REM Mg Ti/N Ceq(%) Pcm(%) ACR Examples A 2.3 0.50 0.19  0.4 Example B 2.4 0.48 0.22  0.2 Example C 0.022 4.7 0.45 0.22  1.4 Example D 0.22 4.0 0.45 0.20  0.5 Example E 0.03 3.4 0.47 0.22  0.8 Example F 0.002 1.9 0.45 0.19  0.5 Example G 2.8 0.43 0.21  0.5 Example H 4.1 0.46 0.20  0.3 Example J 0.002 3.3 0.45 0.21  1.1 Example K 0.008 3.7 0.46 0.21  0.9 Example L 3.1 0.43 0.22  0.3 Comparative   example M 0.007 1.3 0.45 0.20  1.2 Comparative   example N 2.7 0.45 0.20  1.5 Comparative   example O 0.24 6.9 0.45 0.22  0.5 Comparative   example P 0.08 2.3 0.44 0.18  0.2 Comparative   example Q 4.6 0.51 0.22  0.3 Example R 0.23 0.04 0.002 4.9 0.47 0.23  0.6 Example S 1.5 0.43 0.20  0.7 Comparative   example T 0.001 2.1 0.45 0.20  0.1 Comparative   example U 0.09 0.011 0.02 4.3 0.44 0.21  0.5 Comparative example W 0.04 4.1 0.48 0.22 −0.5 Comparative example X 0.24 0.011 2.6 0.53 0.22  0.6 Example Y 0.15 0.024 2.7 0.52 0.20  0.5 Example Z 0.23 0.04 3.1 0.57 0.25  0.4 Comparative   example AA 3.3 0.52 0.21  0.8 Example Note 1: Underlined data are outside the scope of the disclosed embodiments. Note 2: Ceq = [C] + [Mn]/6 + ([Cu] + [Ni])/15 + ([Cr] + [Mo] + [V])/5, Pcm = [C] + [Si]/30 + ([Mn] + [Cu]) + ([Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B] ACR = (Ca-(0.18 + 130 × Ca) × O)/(1.25 × S) The alloying elements in the formulate denote the corresponding contents (mass %).

TABLE-US-00002 TABLE 2 Cumu- Cumu- lative lative rolling rolling reduc- reduc- tion tion with with rolling rolling reduc- reduc- Cumu- Aver- Den- Den- tion/ tion/ lative age sity sity pass pass rolling cooling of Ca of Ca δ at being being reduc- rate com- com- YS SC/ 8% 5% tion be- Tem- Ef- plex plex of ICHAZ or more or more at tween pering fec- inclu- inclu- base Num- bound- Heating at at less 700° C. tem- tive sions sions metal ber δ in ary Thick- temper- 950° C. 950° C. than and per- grain at at at of CGHAZ at Steel ness ature or more or more 950° C. 500° C. ature size 1/4t 1/2t 1/4t weld −10° C. −10° C. No. type (mm) (° C.) (%) (%) (%) (° C./s) (° C.) (μ m) (/mm.sup.2) (/mm.sup.2) (MPa) passes (mm) (mm) Examples 1 A 35 1100 48 53 53 18 — 13 58 50 551 18 0.94 1.56 Example 2 B 76 1020 36 36 45 7 — 17 73 65 553 45 0.74 0.97 Example 3 C 30 1190 53 53 67 31 610 9 66 61 570 16 0.97 1.37 Example 4 D 51 1050 38 43 55 10 560 11 61 66 563 23 1.37 1.57 Example 5 E 25 970 31 31 67 48 650 8 89 80 585 15 0.97 1.21 Example 6 F 80 1070 48 60 50 5 — 19 42 48 503 48 1.88 1.97 Example 7 G 80 1100 42 42 42 6 550 17 112 104 526 49 1.56 1.86 Example 8 H 34 1120 43 49 49 22 — 13 31 28 574 17 0.67 0.89 Example 9 I 102 1090 38 44 51 3 520 18 48 40 549 58 1.55 2.07 Example 10 J 51 1030 33 33 56 13 580 12 67 60 542 24 1.23 1.34 Example 11 K 63 1150 46 46 50 9 — 12 49 45 567 36 1.54 1.64 Example 12 L 63 1040 41 41 48 8 — 17 39 35 553 37 0.26 0.39 Comparative example 13 M 50 1090 38 38 45 13 600 20 53 46 546 22 0.29 0.88 Comparative example 14 N 38 1070 37 37 58 18 — 14 18 13 524 20 0.30 1.07 Comparative example 15 O 60 1160 38 52 53 10 650 13 78 65 598 35 0.33 0.49 Comparative example 16 P 34 1080 43 43 63 20 — 16 6 2 511 18 0.17 0.87 Comparative example 17 Q 76 1150 56 56 39 6 — 18 69 68 592 45 0.52 0.68 Example 18 R 50 1050 36 36 58 13 580 13 56 59 594 23 0.38 0.69 Example 19 S 40 1100 45 50 53 16 550 13 69 60 536 20 0.11 0.78 Comparative example 20 T 34 1070 40 40 60 19 — 12 13 11 550 17 0.23 0.97 Comparative example 21 U 76 1130 47 52 46 7 560 15 266 250 578 46 0.15 0.32 Comparative example 23 J 76 1140 40 40 45 6 760 23 65 61 518 44 1.24 0.31 Comparative example 24 C 35 920 39 39 55 18 — 30 52 47 539 19 0.64 0.27 Comparative example 25 H 76 1030 35 35 35 8 — 33 36 33 509 46 0.58 0.26 Comparative example 26 B 40 1230 43 49 66 15 — 28 70 72 585 20 0.67 0.33 Comparative example 27 E 41 1070 25 25 50 14 610 31 70 73 550 21 0.88 0.24 Comparative example 28 W 102 1120 38 44 50 3 — 20 51 74 507 60 0.22 0.29 Comparative example 29 X 76 1070 32 45 45 7 580 18 108 78 568 43 0.88 1.57 Example 30 Y 50 1030 35 50 55 10 — 12 99 89 579 22 0.97 1.26 Example 31 Z 70 1120 35 47 50 7 — 17 38 49 588 40 0.11 0.14 Comparative example 32 AA 50 1090 35 50 50 10 — 13 107 97 607 22 0.74 0.89 Example Note 1: Underlined data are outside the scope of the disclosed embodiments. Note 2: t denotes thickness (mm)