JP5293370B2 - Steel excellent in CTOD characteristics of weld heat affected zone and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel having such unprecedented superior CTOD (Crack Tip Opening Displacement) properties as to satisfy the CTOD properties of an IC part in addition to the CTOD properties of an FL part at -60&deg;C, in multilayer welding or the like of small to intermediate heat input, and to provide a method for producing the same. <P>SOLUTION: The steel having superior CTOD properties in a weld heat-affected zone includes, by mass%, 0.015-0.045% C, 0.05-0.20% Si, 2.0-3.0% Mn, 0.008% or less P, 0.005% or less S, 0.004% or less Al, 0.005-0.015% Ti, 0.005% or less Nb, 0.0015-0.0035% O, 0.002-0.006% N and the balance iron with unavoidable impurities so that P<SB>CTOD</SB>is 0.065 or less and C<SB>eqH</SB>is 0.235 or less. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a steel excellent in CTOD characteristics of a heat affected zone (HAZ) from small heat input welding to medium heat input welding, and a method for producing the same. The present invention relates to a steel having excellent CTOD characteristics of a weld heat-affected zone exhibiting excellent toughness due to extremely good CTOD characteristics of FL and IC parts where toughness deteriorates, and a method for producing the same.

  In recent years, steel materials used in harsh usage environments have been demanded. For example, as a high-strength steel material suitable for steel structures such as marine structures and earthquake-resistant buildings used in cold regions such as the Arctic Circle, it has excellent CTOD (Cracking Tip Opening Displacement) characteristics that are indicators of fracture toughness. Steel materials are required, and excellent CTOD characteristics are required even in welds.

  The CTOD characteristics of the weld heat affected zone (HAZ) are as follows: FL portion (Fusion Line; boundary between WM (welded metal) and HAZ (weld heat affected zone)) and IC portion (Intercritical HAZ; HAZ and BM (base material)) Evaluation was made at two positions (notches) at the boundary of (1). Until now, only FL has been targeted for improvement of CTOD characteristics.

  This is because, under conditions where the test temperature is not so severe, if the CTOD characteristic of the FL part is satisfied, a sufficient value can be obtained for the CTOD characteristic of the IC part.

  However, under severe test conditions of -60 [deg.] C., it has been found that cases where a low CTOD value occurs in the IC part occur with considerable frequency, and countermeasures have been demanded.

  As a countermeasure, for example, there is a technique indicating that good CTOD characteristics can be obtained under a severe test condition of −60 ° C. with a small to medium heat input welded joint (see, for example, Patent Document 1). Here, the CTOD characteristics of the IC part are not described.

JP 2007-002271 A

  The present invention has an excellent CTOD (fracture toughness) characteristic that has never been achieved so as to satisfy the CTOD characteristic of the IC part in addition to the CTOD characteristic of the FL part at −60 ° C. in the multilayer welding with small to medium heat input. It is an object of the present invention to provide a high-strength steel and a method for producing the same.

  The present inventors have intensively studied to improve the CTOD characteristics of both the FL portion and the IC portion of the welded portion where the toughness is most deteriorated during small heat input welding to medium heat input welding. As a result, the reduction of non-metallic inclusions is the most important for improving the CTOD characteristics of both the FL part and the IC part. For this reason, reduction of O (oxygen in steel) is indispensable. Since intragranular ferrite (IGF) decreases, it is necessary to reduce the alloy elements that degrade the CTOD characteristics of the FL part, and it is difficult to improve the CTOD characteristics of the IC part by simply reducing the steel sheet oxygen. As a result, the present invention has been completed.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.015-0.045%,
Si: 0.05-0.20%,
Mn: 2.0 to 3.0%,
P: 0.008% or less,
S: 0.005% or less,
Al: 0.004% or less,
Ti: 0.005 to 0.015%,
Nb: 0.005% or less,
O: 0.0015 to 0.0035%,
N: 0.002 to 0.006%
And the balance is steel of chemical composition consisting of iron and inevitable impurities, and P _CTOD represented by the following formula (1) is 0.045 or less and C _eqH represented by the following formula (2) is 0.235 or less. A steel excellent in CTOD characteristics of the heat affected zone of welding.
here,
P _CTOD = C (1)
C _eqH = C + Si / 4.16 + Mn / 14.9 + 1.12Nb (2)

(2) The steel is further in mass%,
Cu: less than 0.35%,
Ni: Steel having a chemical composition containing one or two of less than 0.70%, and P _CTOD represented by the following formula (3) is 0.065 or less and C _eqH represented by the following formula (4) is Steel having excellent CTOD characteristics of the weld heat affected zone according to (1) above, which is 0.235 or less.
here,
P _CTOD = C + Cu / 22 + Ni / 67 (3)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb (4)

(3) The steel is further in mass%,
V: 0.005-0.020%
The above-mentioned (3) characterized in that P _CTOD represented by the following formula (5) is 0.065 or less and C _eqH represented by the following formula (6) is 0.235 or less. Steel excellent in CTOD characteristics of the weld heat affected zone as described in 1) or (2).
here,
P _CTOD = C + V / 3 + Cu / 22 + Ni / 67 (5)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb + V / 1.82 (6)

(4) By mass%
C: 0.015-0.045%,
Si: 0.05-0.20%,
Mn: 2.0 to 3.0%,
P: 0.008% or less,
S: 0.005% or less,
Al: 0.004% or less,
Ti: 0.005 to 0.015%,
Nb: 0.005% or less,
O: 0.0015 to 0.0035%,
N: 0.002 to 0.006%
And the balance is steel of chemical composition consisting of iron and inevitable impurities, and P _CTOD represented by the following formula (1) is 0.045 or less and C _eqH represented by the following formula (2) is 0.235 or less. A method for producing a steel having excellent CTOD characteristics of the heat affected zone of welding, characterized in that the steel is a slab by a continuous casting method, and thereafter reheated to a temperature of 950 to 1100 ° C. and then subjected to a heat treatment.
here,
P _CTOD = C (1)
C _eqH = C + Si / 4.16 + Mn / 14.9 + 1.12Nb (2)

(5) The steel is further in mass%,
Cu: less than 0.35%,
Ni: Steel having a chemical composition containing one or two of less than 0.70%, and P _CTOD represented by the following formula (3) is 0.065 or less and C _eqH represented by the following formula (4) is The method for producing steel having excellent CTOD characteristics of the weld heat affected zone according to (4) above, wherein the steel is 0.235 or less.
here,
P _CTOD = C + Cu / 22 + Ni / 67 (3)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb (4)

(6) The steel is further in mass%,
V: 0.005-0.020%
_{Characterized} in that it is a steel having a chemical composition containing _PCTOD of 0.065 or less represented by the following formula (5) and C _eqH represented by the following formula (6) of 0.235 or less. A method for producing steel excellent in CTOD characteristics of the weld heat-affected zone as described in (4) or (5) above.
here,
P _CTOD = C + V / 3 + Cu / 22 + Ni / 67 (5)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb + V / 1.82 (6)

  The steel manufactured according to the present invention exhibits excellent toughness with extremely good CTOD characteristics of the FL part and the IC part where the toughness is most deteriorated during welding such as multilayer welding with small to medium heat input. This made it possible to manufacture high-strength steel materials used in harsh environments such as offshore structures and earthquake-resistant buildings.

It is a diagram showing the relationship between CTOD characteristics in P _CTOD and FL equivalent simulated thermal cycle test. It is a figure which shows the relationship between the hardness of HAZ and a CTOD characteristic in an ICHAZ equivalent reproduction | regeneration thermal cycle test. It is a figure which shows the relationship between _CeqH and HAZ hardness in an ICHAZ equivalent reproduction _| regeneration thermal cycle test.

  Details of the present invention will be described below.

  According to the study by the present inventors, in order to satisfy the CTOD characteristics of the FL part and the IC part of −60 ° C. of the welding HAZ of small to medium heat input (1.5 to 6.0 kJ / mm when the plate thickness is 50 mm). Therefore, it is most important to reduce oxide-based non-metallic inclusions for the purpose of satisfying the CTOD characteristics of the FL portion and improving the CTOD characteristics of the IC portion, and it is essential to reduce O (oxygen in steel).

  That is, in the prior art, in order to ensure excellent CTOD characteristics of the FL portion, an oxide-based nonmetallic inclusion represented by Ti oxide is used as an intragranular ferrite (IGF) as a transformation nucleus. Therefore, some addition of O was necessary. On the other hand, the present inventors have found that it is necessary to reduce oxide-based nonmetallic inclusions in order to improve the CTOD characteristics of the FL portion and the IC portion at −60 ° C.

Since IGF is reduced by reducing O, it is necessary to reduce alloy elements that deteriorate the CTOD characteristics of the FL portion. Figure 1 shows the relationship between the FL equivalent simulated HAZ of CTOD characteristics and P _CTOD. Here, P _CTOD , which can be expressed by the following equation (7) as a parameter of the steel component, is the CTOD characteristic (T _{δc0.1 (FL)} ) of the HA equivalent of many laboratory molten steels and the steel component This is an empirical formula derived from the analysis.
P _CTOD = C + V / 3 + Cu / 22 + Ni / 67 (7)
In addition, the element in the said formula means content mass%, and is set to 0 when not containing.
In the FL equivalent reproduction HAZ shown in FIG. 1, the target level of T _{δc0.1 (FL)} ≦ −110 ° C. is a knowledge obtained in many experiments, and is an actual joint FL notch of a steel plate having a thickness of 50 to 100 mm. In this case, it is a value necessary for stably obtaining a CTOD value of 0.25 mm or more at −60 ° C. From FIG. 1, it can be seen that in the FL equivalent reproduction HAZ, in order to _satisfy T _{δc0.1 (FL)} ≦ −110 ° C., it is necessary to control the steel component parameter P _CTOD to 0.065% or less.

With respect to T _{δc0.1 (FL) in} FIG. 1, FL equivalent thermal cycle processing (Triple cycle), 1st: 1400 ° C. (800 to 500 ° C .: 15 sec), 2nd: 760 ° C. (760 to 500 ° C .: 22 sec), 3rd : Obtained by a CTOD test in which a test piece having a cross section of 10 mm × 20 mm subjected to 500 ° C. (500 to 300 ° C .: 60 sec) was carried out in accordance with BS7448 (British Standard). This T _{δc0.1 (FL)} means a temperature (° C.) at which the minimum value of three CTOD (δ) values carried out at each test temperature exceeds 0.1 mm. In view of the plate thickness effect in CTOD test, in order to obtain a stable CTOD value of more than 0.25mm and the -60 ° C. in a real joint FL notch steel sheet having a thickness 50~100mm is empirically _{T? C0 .1 (FL)} needs to be −110 ° C. or lower.

  Further, it has been found that it is difficult to improve the CTOD characteristics of the IC part only by reducing the oxygen in the steel, and it is effective to reduce the hardness.

FIG. 2 shows the relationship between the CTOD characteristics of specimens subjected to reproducible thermal cycles equivalent to Intercritical HAZ (ICHAZ), which will be described later, and hardness equivalent to ICHAZ, and FIG. 3 shows the steel component hardness parameter C _eqH and ICHAZ equivalent HAZ hardness. Show the relationship.

Here, the target level that T _{δc0.1 (ICHAZ)} of the reproduction HAZ (cross section 10 mm × 20 mm) equivalent to ICAZ shown in FIG. 2 is −110 ° C. or less is a knowledge obtained in many experiments, and the plate thickness is 50 to 100 mm. This is a necessary value for obtaining a CTOD value of about 0.25 mm at −60 ° C. of the IC notch of the actual joint of the steel plate.

2 and 3, in order to set T _{δc0.1 (ICHAZ)} of the reproduced HAZ to −110 ° C. or less, the hardness is Hv 176 or less, and the steel component hardness parameter C _eqH represented by the following equation (8) is _set to 0. It turns out that it is necessary to control to .235 or less. In order to further reduce the hardness, 0.225 or less is desirable.

In addition, BS7448 (British Standard) of the CTOD test method is applied as a test method, and the ICHAZ equivalent reproduction thermal cycle process (Triple cycle) used is 1st: 950 ° C. (800 to 500 ° C .: 20 sec), 2nd. : 770 ° C. (770 to 500 ° C .: 22 sec), 3rd: 450 ° C. (450 to 300 ° C .: 65 sec). here,
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb + V / 1.82 (8)
Is defined. _{Even if} the amount of P _CTOD or C _eqH is limited, steel with high strength and excellent CTOD characteristics cannot be produced unless other alloy elements are optimized.
In addition, the element in the said formula means content mass%, and is set to 0 when not containing.

  The reason for limitation of the present invention will be described below. First, the reasons for limiting the composition of the steel of the present invention will be described. In the following composition,% means mass%.

C: 0.015-0.045%
C needs to be 0.015% or more in order to obtain strength, but if it exceeds 0.045%, the characteristics of the welded HAZ are deteriorated, and the CTOD characteristic at −60 ° C. cannot be satisfied, so 0.045% is made the upper limit.

Si: 0.05-0.20%
In order to obtain good HAZ toughness, it is preferable that the amount of Si is small. However, in the steel according to the present invention, Al is not added. However, if over 0.20%, the HAZ toughness is impaired, so 0.20% is made the upper limit. In order to obtain better HAZ toughness, 0.15% or less is desirable.

Mn: 2.0 to 3.0%
Mn is an element that has a large effect of optimizing the microstructure and is inexpensive, and has an effect of suppressing transformation from grain boundaries that are harmful to HAZ toughness. However, if it exceeds 3.0%, the hardness of ICHAZ increases and the toughness deteriorates, so 3.0% was made the upper limit. Further, if the content is less than 2.0%, the effect of optimizing the microstructure is small, so the lower limit was made 2.0%.

P: 0.008% or less P is contained as an unavoidable impurity and segregates at the grain boundaries to deteriorate the toughness of the steel. Therefore, it is desirable to reduce it as much as possible, but there are also restrictions on industrial production. Was the upper limit. In order to obtain better HAZ toughness, 0.005% or less is desirable.

S: 0.005% or less S is contained as an inevitable impurity, and it is preferable that S is less in terms of base material toughness and HAZ toughness. However, due to industrial production restrictions, 0.005% was made the upper limit. In order to obtain better HAZ toughness, 0.003% or less is desirable.

Al: 0.004% or less Al is preferably less in order to produce a Ti oxide, but there are restrictions on industrial production, and the upper limit is set to 0.004%.

Ti: 0.005 to 0.015%
Ti produces a Ti oxide and refines the microstructure, but if it is too much, TiC is produced and the HAZ toughness is deteriorated, so 0.005 to 0.015% is an appropriate range. In order to further improve the HAZ toughness, 0.013% or less is desirable.

Nb: 0.005% or less Nb is beneficial from the viewpoint of the strength and toughness of the base material, but is harmful to the HAZ toughness. For this reason, it can add to 0.005% which is the range which does not reduce HAZ toughness remarkably. However, in order to further improve the HAZ toughness, it is more desirable to limit it to 0.002% or less.

N: 0.002 to 0.006%
N is necessary for the formation of Ti nitride, but if less than 0.002%, the effect is small, and if it exceeds 0.006%, surface flaws occur during the production of steel slabs, so the upper limit was made 0.006%. In order to obtain better HAZ toughness, 0.005% or less is desirable.

O: 0.0015 to 0.0035%
O is required to be 0.0015% or more from the productivity of oxides as IGF production nuclei in the FL part of Ti. However, if O is too much, the size and number of oxides become problems, and the CTOD characteristics of the IC portion are deteriorated. In order to obtain better HAZ toughness, 0.0030% or less, more preferably 0.0028% or less is desirable.

  Next, Cu, Ni, and V, which are selectively added components, will be described.

Cu: less than 0.35% Cu is less effective in reducing the HAZ toughness and effective in improving the strength of the base material, and is effective with little increase in hardness of ICHAZ, but is an expensive alloy. The limit range was less than 0.35%. The lower limit is not particularly limited, but is desirably 0.01% or more.

Ni: Less than 0.70% Ni is effective because it has little effect on the deterioration of HAZ toughness and improves the strength of the base metal, and is effective with little increase in hardness of ICHAZ. The limit range was less than 0.70%. The lower limit is not particularly limited, but is desirably 0.01% or more.

V: 0.005-0.020%
The purpose of further adding V to the basic component is to improve the strength of the base material, but in order to exert this effect, it is necessary to make it 0.005% or more. On the other hand, if added over 0.020%, the HAZ toughness is impaired, so the upper limit of V is set to 0.020% or less as a range that does not significantly impair the HAZ toughness.

  Even if the components of the steel are limited as described above, the intended effect cannot be exhibited unless the production method is appropriate. For this reason, it is necessary to limit the manufacturing conditions. Hereinafter, the reason for limiting the manufacturing conditions will be described.

  The steel of the present invention is industrially required to be produced by a continuous casting method. The reason for this is that the solidified cooling rate of the molten steel is high, and a large amount of fine Ti oxide and Ti nitride can be generated in the slab.

  In rolling the slab, the reheating temperature needs to be 950 to 1100 ° C. This is because if the reheating temperature exceeds 1100 ° C., the Ti nitride becomes coarse, and the toughness deterioration of the base metal and the effect of improving the HAZ toughness cannot be expected. Further, at a reheating temperature of less than 950 ° C., the rolling load is large and the productivity is remarkably impaired, so 950 ° C. is the lower limit reheating temperature. Even if the reheating temperature is 950 to 1100 ° C., a sufficiently excellent base material toughness can be secured. However, when extremely excellent base material toughness is required, the reheating temperature should be 950 to 1050 ° C. .

  The heat treatment is essential for the production method after reheating. The thermomechanical treatment is a process in which the rolling temperature is controlled within a range suitable for the steel components, and then water cooling or the like is performed as necessary. By this process, the austenite grains can be refined and the microstructure can be refined. it can. Thereby, it is the manufacturing method which can improve the intensity | strength improvement and toughness of steel materials. Even in the steel according to the present invention, even if excellent HAZ toughness is obtained, a work heat treatment method is indispensable because the steel material is insufficient if the toughness of the base material is inferior. As a condition of the thermomechanical processing method, it is desirable that the cumulative rolling reduction at the non-recrystallization temperature is 30% or more.

  Examples of the heat treatment method include (1) controlled rolling, (2) controlled rolling-accelerated cooling, and (3) direct quenching-tempering after rolling. A preferred method is (2) controlled rolling-accelerated cooling. In addition, even if this steel is reheated to a temperature below the Ar3 transformation point for the purpose of dehydrogenation after production, the characteristics of the present invention are not impaired.

  Hereinafter, the present invention will be described based on examples and comparative examples.

  Thick steel plates of various steel components were manufactured in the converter-continuous casting-thick plate process, and the base material strength and the CTOD test of the welded joint were performed. Welding is a submerged arc welding (SAW) method that is generally used as test welding. The welding heat input is 4.5 to 5.0 kJ / K with a K groove so that the weld penetration line (FL) is vertical. mm.

  The CTOD test was performed with a size of t (plate thickness) x 2t and a notch with a 50% fatigue crack, and the notch position was 2 (FL (boundary between WM and HAZ)) and IC (boundary between HAZ and BM (base metal)). In each place, five tests were carried out at -60 ° C.

Table 1 shows the chemical composition of steel, and Table 2 shows the manufacturing conditions, the base metal, and the CTOD characteristics of the welded joint. The symbols of the heat treatment method in Table 2 mean the following heat treatment methods.
CR: Controlled rolling (rolling at a temperature range optimal for strength and toughness)
ACC: Accelerated cooling (controlled rolling and cooling to 400 to 600 ° C after water cooling)
DQ: quenching immediately after rolling-tempering treatment (water cooling to room temperature immediately after rolling, then tempering treatment)
Moreover, in the CTOD test result of the welded joint in Table 2, δ _CAve represents the average value of the five test results, and δ _Cmin represents the lowest value of the five tests.

The steel sheet manufactured in the present invention (the steel of the present invention) has a yield strength (YS) of 424 N / mm ² or more, a tensile strength (TS) of 501 N / mm ² or more, and a CTOD value of −60 ° C. is δ _Cmin of FL notch. Good fracture toughness of 0.39 mm or more and IC notch δ _Cmin of 0.62 mm or more was exhibited.

  On the other hand, the strength of the comparative steel is almost the same as that of the invention steel, but the CTOD value is inferior, and it is not suitable as a steel plate used in a severe environment.

As apparent from Table 1, Steels 23 to 42 show comparative examples deviating from the present invention in terms of chemical components. These steels are respectively C amount (steel 23, steel 41), Si amount (steel 24), Mn amount (steel 25, steel 26), P amount (steel 27), S amount (steel 28), Nb amount ( Steel 29, Steel 34), V amount (Steel 30), Al amount (Steel 31, Steel 42), Ti amount (Steel 32, Steel 33), O amount (Steel 35, Steel 36), N amount (Steel 37, The conditions of Steel 38), P _CTOD (Steel 39), and C _eqH amount (Steel 40) are different from those of the invention. The strength of the comparative steel is almost the same as that of the invention steel, but the CTOD value is inferior, and it is not suitable as a steel plate used in a severe environment.

  Further, as shown in the example of steel 22, it can be seen that the invention steel is inferior in CTOD value if the heating conditions are not satisfied.

Claims (6)

% By mass
C: 0.015-0.045%,
Si: 0.05-0.20%,
Mn: 2.0 to 3.0%,
P: 0.008% or less,
S: 0.005% or less,
Al: 0.004% or less,
Ti: 0.005 to 0.015%,
Nb: 0.005% or less,
O: 0.0015 to 0.0035%,
N: 0.002 to 0.006%
And the balance is steel of chemical composition consisting of iron and inevitable impurities, and P _CTOD represented by the following formula (1) is 0.045 or less and C _eqH represented by the following formula (2) is 0.235 or less. A steel excellent in CTOD characteristics of the heat affected zone of welding.
here,
P _CTOD = C (1)
C _eqH = C + Si / 4.16 + Mn / 14.9 + 1.12Nb (2) The steel is further mass%,
Cu: less than 0.35%,
Ni: Steel having a chemical composition containing one or two of less than 0.70%, and P _CTOD represented by the following formula (3) is 0.065 or less and C _eqH represented by the following formula (4) is The steel excellent in CTOD characteristics of the weld heat affected zone according to claim 1, wherein the steel is 0.235 or less.
here,
P _CTOD = C + Cu / 22 + Ni / 67 (3)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb (4) The steel is further mass%,
V: 0.005-0.020%
And P _CTOD represented by the following formula (5) is 0.065 or less, and C _eqH represented by the following formula (6) is 0.235 or less. A steel excellent in CTOD characteristics of the weld heat-affected zone as described in 1 or 2.
here,
P _CTOD = C + V / 3 + Cu / 22 + Ni / 67 (5)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb + V / 1.82 (6) % By mass
C: 0.015-0.045%,
Si: 0.05-0.20%,
Mn: 2.0 to 3.0%,
P: 0.008% or less,
S: 0.005% or less,
Al: 0.004% or less,
Ti: 0.005 to 0.015%,
Nb: 0.005% or less,
O: 0.0015 to 0.0035%,
N: 0.002 to 0.006%
And the balance is steel of chemical composition consisting of iron and inevitable impurities, and P _CTOD represented by the following formula (1) is 0.045 or less and C _eqH represented by the following formula (2) is 0.235 or less. A method for producing a steel having excellent CTOD characteristics of the heat affected zone of welding, characterized in that the steel is a slab by a continuous casting method, and thereafter reheated to a temperature of 950 to 1100 ° C. and then subjected to a heat treatment.
here,
P _CTOD = C (1)
C _eqH = C + Si / 4.16 + Mn / 14.9 + 1.12Nb (2) The steel is further mass%,
Cu: less than 0.35%,
Ni: Steel having a chemical composition containing one or two of less than 0.70%, and P _CTOD represented by the following formula (3) is 0.065 or less and C _eqH represented by the following formula (4) is The method for producing a steel excellent in CTOD characteristics of the weld heat affected zone according to claim 4, wherein the steel is 0.235 or less.
here,
P _CTOD = C + Cu / 22 + Ni / 67 (3)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb (4) The steel is further mass%,
V: 0.005-0.020%
_{Characterized} in that it is a steel having a chemical composition containing _PCTOD of 0.065 or less represented by the following formula (5) and C _eqH represented by the following formula (6) of 0.235 or less. A method for producing a steel excellent in CTOD characteristics of the weld heat-affected zone according to claim 4 or 5.
here,
P _CTOD = C + V / 3 + Cu / 22 + Ni / 67 (5)
C _eqH = C + Si / 4.16 + Mn / 14.9 + Cu / 12.9
+ 1.12Nb + V / 1.82 (6)

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