JP2009521601A5 - - Google Patents

Description

In order to achieve the above object, the extra-thick steel plate of the present invention is C: 0.05-0.10 % , Si: 0.10-0.5 % , Mn: 1.3-1. 7 % , B: 0.0005-0.0025 % , Ti: 0.005-0.03 % , N: 0.010 % or less, Nb: 0.005-0.03 % , Sol. Al: 0.005 to 0.055 % , balance Fe and inevitable impurities, the Ti / N content ratio Ti / N is 2.0 or more, the component index (CP) represented by the following relational expression 1 ) Is between 27.6 and 38.1 .

At this time, in addition to the above composition, it was selected from the group consisting of Cu: 0.5 % or less, Ni: 0.5 % or less, Cr: 0.15 % or less, and Mo: 0.15 % or less in terms of % by weight. It is preferable that 1 type or 2 types or more are further included.

The extra-thick steel sheet according to the present invention has an area fraction of polygonal ferrite of 10% or less in the thickness center portion (in the range of t / 4 to 3t / 4, where t is the total thickness) in addition to the above characteristics. Thus, the area fraction of the martensite is preferably 10% or less (in the range from 1 mm to t / 4 below the surface layer, the same on the opposite side).

The method for producing the steel sheet of the present invention with advantageous effects described above, in weight%, C: 0.05~0.10%, Si : 0.10~0.5%, Mn: 1.3 ~1.7%, B: 0.0005~0.0025%, Ti: 0.005~0.03%, N: 0.010% or less, Nb: 0.005~0.03%, Sol. Al: 0.005 to 0.055 % , balance Fe and inevitable impurities, the Ti / N content ratio Ti / N is 2.0 or more, the component index (CP) represented by the following relational expression 1 ) is reheated steel slab is between 27.6 to 38.1 within a temperature range of 1000 to 1250 ° C., subjected to reduction of 30% or more of the finish rolling in the temperature range of the recrystallization temperature of the Ar ₃ ~ austenite After that, cooling is started at a cooling rate of 1.5 ° C./sec or more at a temperature of Ar ₃ or higher with reference to the center of the steel sheet, and cooling is stopped at 350 to 550 ° C.

[Relational expression 1 ]
CP = 165 ×% C + 6.8 ×% Si + 10.2 ×% Mn + 80.6 ×% Nb + 9.5 ×% Cu + 3.5 ×% Ni + 12.5 ×% Cr + 14.4 ×% Mo

At this time, in addition to the above advantageous composition, the slab targeted by the present production method is Cu: 0.5 % or less, Ni: 0.5 % or less, Cr: 0.15 % or less, and Mo: It is preferable to further include one or more selected from the group consisting of 0.15 % or less.

(Steel composition)
The composition of the steel sheet used in the present invention is as follows: C: 0.05 to 0.10%, Si: 0.10 to 0.5%, Mn: 1.3 to 1.7%, P: 0 by weight percentage. 0.012% or less, S: 0.005% or less, B: 0.0005 to 0.0025%, Ti: 0.005 to 0.03%, Nb: 0.005 to 0.03%, Sol. Al: 0.005 to 0.055%, N: 0.01% or less, with the balance being Fe and other inevitable impurities. Further, the Ti / N ratio is 2.0 or more, and the index (CP) represented by the following relational expression 1 must be in the range of 27.6 to 38.1 .

In addition to the above composition, the steel according to the present invention must have a component index (CP) represented by the following relational expression 1 in the range of 27.6 to 38.1 . The above component index can determine how much polygonal ferrite can be suppressed at the center of the thickness when water cooling a thick steel plate, and how much martensite can be suppressed at the surface layer. It is a scale that can be determined at the same time.

[Relational expression 1 ]
CP = 165 ×% C + 6.8 ×% Si + 10.2 ×% Mn + 80.6 ×% Nb + 9.5 ×% Cu + 3.5 ×% Ni + 12.5 ×% Cr + 14.4 ×% Mo

Seen in index represented by the equation 1 (CP) is the area fraction of polygonal ferrite is 27.6 or higher than 10% in FIG. 1, if this CP is 27.6 or more 1. This means that the area fraction of polygonal ferrite can be maintained at 10% or less even at 5 ° C./sec (a cooling rate slower than 3 ° C./sec, which is a normal cooling rate of the central portion of 50 to 100 mm thickness). . On the other hand, when the CP is 38.1 or less, the martensite area fraction is 10% or less, and when the CP is 38.1 or less, the normal cooling rate of 40 ° C. at 1 mm below the surface layer of 50 to 100 mm thickness. / Sec means that the area fraction of martensite can be maintained at 10% or less.

FIG. 2 shows the difference between the maximum value and the minimum value of the Vickers hardness measured at an interval of 2 mm in the thickness direction with respect to a 100 mm-thick steel plate of the above-described alloy with varying CP. As described above, it can be seen that the hardness difference can be controlled to 50 Hv or less when the CP range of the present invention is between 27.6 to 38.1. As described above, this is a result of martensite being suppressed to 10% or less in the surface layer portion and polygonal ferrite being suppressed to 10% or less in the thickness center portion.

Cooling start temperature: Ar ₃ or higher If water cooling is not started at a temperature of Ar ₃ or higher even when finish rolling is completed when the steel is Ar ₃ or higher, coarse polygonal ferrite is formed during air cooling. In such a case, the target structure of the present invention cannot be obtained , and the strength and toughness are also lowered. Therefore, unless the cooling is started before the temperature of the steel material enters the ferrite generation region, that is, before it reaches Ar ₃ , the intended purpose provided by the present invention cannot be achieved.

Claims (6)

% By weight, C: 0.05-0.10 % , Si: 0.10-0.5 % , Mn: 1.3-1.7 % , B: 0.0005-0.0025 % , Ti: 0.005 to 0.03 % , N: 0.010 % or less, Nb: 0.005 to 0.03 % , Sol. Al: 0.005 to 0.055 % , P: 0.012% or less, S: 0.005% or less, balance Fe and inevitable impurities, Ti / N content ratio Ti / N is 2.0 or more state, and it is between the component index (CP) is 27.6 to 38.1, which is represented by the equation 1 below,
The area fraction of polygonal ferrite in the central portion of the thickness (the range of t / 4 to 3t / 4, where t is the total thickness) is 10% or less, and the surface layer portion (from 1 mm below the surface layer to t / 4) The area fraction of martensite in the same range and the opposite side) is 10% or less,
Hardness deviation in the thickness direction is within Hv50,
The thickness is 50-100mm,
An ultra-thick steel plate for welded structures having excellent ductility-brittle transition temperature of -50 ° C. or less and excellent strength and toughness at the center of thickness with little material deviation.
[Relational expression 1]
CP = 165 ×% C + 6.8 ×% Si + 10.2 ×% Mn + 80.6 ×% Nb + 9.5 ×% Cu + 3.5 ×% Ni + 12.5 ×% Cr + 14.4 ×% Mo One or more selected from the group consisting of Cu: 0.5 % or less, Ni: 0.5 % or less, Cr: 0.15 % or less, and Mo: 0.15 % or less by weight% The super-thick steel plate for welded structures having excellent strength and toughness at the center of thickness and having a small material deviation. P in P and S are each wt% present as the impurity: not more than 0.010% and S: according to claim 1 or claim 2, characterized in that controlled 0.003% or less Extremely thick steel plate for welded structures with excellent strength and toughness at the center of thickness and with little material deviation. % By weight, C: 0.05-0.10 % , Si: 0.10-0.5 % , Mn: 1.3-1.7 % , B: 0.0005-0.0025 % , Ti: 0.005 to 0.03 % , N: 0.010 % or less, Nb: 0.005 to 0.03 % , Sol. Al: 0.005 to 0.055 % , P: 0.012% or less, S: 0.005% or less, balance Fe and inevitable impurities, Ti / N content ratio Ti / N is 2.0 With the above, a steel slab having a component index (CP) represented by the following relational expression 1 between 27.6 to 38.1 is reheated in a temperature range of 1000 to 1250 ° C., and the reduction amount is 30% or more. After finishing rolling in the temperature range of the recrystallization temperature of Ar ₃ to austenite, cooling is started at a cooling rate of 1.5 ° C./sec or more at a temperature of Ar ₃ or higher with reference to the center of the steel sheet, and 350 to 550 A method for producing a very thick steel plate for welded structures having excellent strength and toughness at the thickness center and having a small material deviation , wherein cooling is stopped at 0 ° C. and a steel plate having a thickness of 50 to 100 mm is produced.
[Relational expression 1 ]
CP = 165 ×% C + 6.8 ×% Si + 10.2 ×% Mn + 80.6 ×% Nb + 9.5 ×% Cu + 3.5 ×% Ni + 12.5 ×% Cr + 14.4 ×% Mo One or more selected from the group consisting of Cu: 0.5 % or less, Ni: 0.5 % or less, Cr: 0.15 % or less, and Mo: 0.15 % or less by weight% the thickness center intensity, and a manufacturing method of the toughness less for welded structures extra heavy product steel sheet material deviation superior in claim 4, characterized in that it comprises. 6. The thickness center portion according to claim 4, wherein P and S present as the impurities are controlled by weight percent to P: 0.010% or less and S: 0.003% or less, respectively. For producing super-thick steel plate for welded structure with excellent material strength and toughness and less material deviation.

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