JPH11229077A - Steel plate excellent in ctod characteristic in multi layer weld zone and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate excellent in CTOD characteristic in a multi- layer weld zone and suitable, e.g. for a marine structure in an extremely cold frozen sea area and its production. SOLUTION: The steel plate has a composition essentially containing, by weight, 0.02-0.15% C, 0.01-0.11% Si, 0.5-2.0% Mn, <=0.020% P, 0.002-0.010% S, 0.003-0.013% Nb, 0.005-0.025% Ti, 0.01-0.08% Al, and 0.002-0.008% N and having the balance iron with inevitable impurity elements and satisfying Si +10Nb=0.04 to 0.20, 81Nb<1/2> +42V<1/2> =4.4 to 11.0, and Ceq=0.35 to 0.45. This steel plate can be produced by heating a steel slab of the above chemical composition to 950 to 1300 deg.C, subjecting this steel slab to roughing at 10 to 90% draft in the recrystallization temperature region and successively to finish rolling at 10 to 90% draft in the unrecrystallization temperature region not lower than the Ar3 point, and subjecting the resultant steel plate, without delay, to controlled cooling at (1 to 50) deg.C/s cooling rate and then to air cooling down to room temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION
The present invention relates to a rolled steel material for a welded structure used for an offshore structure in an extremely cold and icy sea area, and particularly relates to a CTOD of a multi-layer welded portion having a welding heat input of up to about 100 kJ / cm.
It is related to steel sheets with excellent properties.

[0002]

2. Description of the Related Art In recent years, the development of oil and natural gas resources on the seabed has been actively promoted, and the developed sea area has evolved into an extremely cold ice area of -40 to -50 ° C. in search of abundant seabed resources. I am doing it. In a steel plate for an offshore structure used in such an environment, a yield stress is ensured to be 360 MPa or more, and not only a base material of the steel plate but also a welding heat affected zone (hereinafter, referred to as HA).
Z: Heat Affected Zone) requires severe fracture toughness.

In an offshore structure, since the thickness of a steel plate is large, multi-pass welding with a welding heat input of up to about 100 kJ / cm is used, but the coarse-grained area generated in the first welding heat cycle is subjected to the next welding heat cycle. It is known that the fracture toughness is significantly reduced in the coarse-grained two-phase region reheated to the two-phase region and in the coarse-grained temper in which the coarse-grained region is tempered by the next welding heat cycle.

[0004] Therefore, the evaluation of the fracture toughness of the HAZ in a multi-pass weld is not based on the commonly used Charpy impact test, but on local embrittlement regions such as coarse-grained two-phase regions and coarse-grained temper regions. Sensitive CTOD (Crack Tip
It needs to be evaluated by an Opening Displacement test.

When evaluating the fracture toughness by the CTOD test, the notch position and the groove shape of the test piece are important.
American Petroleum Institute Standard AP for Steel Plates for Offshore Structures
According to I-RP2Z (1992), the coarse area ratio was 15%.
% Is required, and FIG. 1 (Fair
lchild, ASTM STP1058, p117-1
41, 1990).
If it is less than 5%, the evaluation is looser than the original fracture toughness. Here, the coarse grain area ratio is defined by the method shown in FIG.
Also, the groove shape of the welded joint is specified to be a groove or a K groove, and the CTOD value is higher because the ratio of the coarse grain area is smaller in the X groove than in the K groove. (The Japan Welding Association Iron and Steel Subcommittee Technical Committee FTW Committee Report, p. 28, 1990).

Hitherto, many techniques have been disclosed for improving the toughness and CTOD characteristics of a welded portion, and the main ones among them are described below. A technique for improving low-temperature toughness in large heat input welding is disclosed in
No. 250917. The technology disclosed herein is to control the cooling rate after solidification, thereby increasing the number of composite precipitates of TiN and MnS, which are transformation nuclei of ferrite generated in old austenite grains during the cooling process of welding. , Welding heat input is 200kJ /
The present invention relates to a method for producing a large heat input welding steel having excellent low-temperature toughness, characterized in that the Charpy impact value of a heat affected zone at -60 ° C is 3.5 kgf · m or more at -60 ° C. is there.

A method for improving the CTOD characteristic of a weld is described in Japanese Patent Application Laid-Open No. 57-143470. The technology disclosed here is low P, trace N
The addition of b, the reduction of S, the reduction of O, and the spheroidization of inclusions are combined to reduce the non-uniformity of the material, so that the COD at -10 ° C when the heat input of welding is 50 kJ / cm or less. A high tensile steel having a high COD value, characterized by improving the average value of

A technique for improving CTOD characteristics in large heat input welding is described in JP-A-62-214126. The technique disclosed herein is to generate fine ferrite by finely dispersing oxide-based particles and to reduce the volume fraction of the island-like martensite structure generated in the HAZ to 3% or less. The present invention relates to a method for producing a high-strength steel excellent in the COD characteristic of a weld portion, characterized in that the limit CTOD value at −45 ° C. is 0.25 mm or more when the welding heat input is 25 to 130 kJ / cm. .

[0009]

However, in order to satisfy the strict requirements for the CTOD characteristics of the multi-pass weld, there are the following problems. A method for improving low temperature toughness in large heat input welding is disclosed in
No. 17 discloses that the welding heat input is 200
The test temperature was kJ / cm and the test temperature was −60 ° C. The evaluation was based on small Charpy impact characteristics, but the CTOD characteristics were unknown. Further, the present invention differs from the present invention in that the cooling rate of the slab must be controlled.

Regarding a technique for improving the CTOD characteristic of a welded part, the technique disclosed in Japanese Patent Application Laid-Open No. 57-143470 is applied to the case where the welding heat input exceeds 50 kJ / cm or the CTOD at a temperature lower than -10 ° C. The problem was that the properties were unknown.

Regarding the technique for improving the CTOD characteristic in large heat input welding, the technique disclosed in Japanese Patent Application Laid-Open No. 62-214126 discloses that the Ti content is 0.003 to 0.0.
At 40%, this is partially consistent with the present invention, but the content of Al is 0.1%.
It is less than 005%, and the range of the chemical component is different from the present invention which is 0.01 to 0.08%. The technical idea is different from that of the present invention in that oxide-based particles are used for preventing coarsening of γ-particles and forming intragranular ferrite. Further, as described above, the CTOD value greatly varies depending on the coarse grain area ratio and becomes high when the coarse grain area rate is low. However, since the groove used here is the X groove, the coarse grain area is large. Low rate, API-RP2
It was unclear whether a sufficient CTOD value could be obtained when the area ratio of coarse particles based on Z was 15% or more.

In view of the problems of these techniques, the present invention provides
(1) welding heat input: 100 kJ / cm at maximum, (2) coarse grain area ratio: 15% or more, (3) test temperature: -40 to -50 ° C,
The present invention provides a steel sheet excellent in CTOD characteristics of a multi-layer welded portion and a method for producing the same, which enables a critical CTOD value of 0.1 mm or more to be satisfied.

[0013]

SUMMARY OF THE INVENTION The present invention provides (a) refinement of the effective crystal grain size, (b) reduction of island martensite and improvement of grain boundary hardenability by a small amount of Nb, and (c) precipitation hardening. And (d) reducing the HAZ hardness at the same time, it is possible to significantly improve the CTOD characteristics of the multilayer welded portion.

That is, the gist of the present invention is as follows. (1) C: 0.02 to 0.15% by weight%, Si:
0.01-0.11%, Mn: 0.5-2.0%,
P: 0.020% or less, S: 0.002-0.
010%, Nb: 0.003 to 0.013%, Ti:
0.005-0.025%, Al: 0.01-0.
08%, N: 0.002 to 0.008%,
The balance consists of iron and unavoidable impurity elements, and Si + 1
0Nb: 0.04 to 0.20, 81Nb ^1/2 + 42V
^1/2 : 4.4 to 11.0, Ceq: 0.35 to 0.45
Of high heat input multi-pass weld, characterized by satisfying
Steel plate with excellent TOD characteristics. However,

## EQU3 ## Ceq = C + Mn / 6 + (Cr + V) / 5 + (C
u + Ni) / 15.

(2) C: 0.02 to 0.1% by weight
5%, Si: 0.01 to 0.11%, Mn: 0.5
~ 2.0%, P: 0.020% or less, S: 0.
002-0.010%, Nb: 0.003-0.01
3%, Ti: 0.005 to 0.025%, Al:
0.01-0.08%, N: 0.002-0.00
8%, Cu: 0.10-1.0%, N
i: 0.10 to 2.0%, Cr: 0.05 to 0.5
0%, V: 0.005 to 0.020%, containing one or more kinds, the balance being iron and unavoidable impurity elements, Si + 10Nb: 0.04 to 0.2
0, 81 Nb ^1/2 +42 V ^1/2 : 4.4 to 11.0, Ce
q: A steel sheet which satisfies 0.35 to 0.45 and has excellent CTOD characteristics in a large heat input multi-pass weld.
However,

## EQU4 ## Ceq = C + Mn / 6 + (Cr + V) / 5 + (C
u + Ni) / 15.

(3) A steel slab having the chemical composition as described in any one of (1) and (2) above,
C., and rough rolling at a reduction rate of 10 to 90% is performed in a recrystallization temperature range, followed by finish rolling at a reduction rate of 10 to 90% in a non-recrystallization temperature range of 3 points or more of Ar, and immediately cooled. Controlled cooling at a rate of 1 to 50 ° C / s to 650 to 500 ° C,
Air-cooled to room temperature, C for multi-pass welds
A method for producing a steel sheet having excellent TOD characteristics.

(4) A steel slab having the chemical composition as described in any one of (1) and (2) above,
C., and rough rolling at a reduction rate of 10 to 90% is performed in a recrystallization temperature range, followed by finish rolling at a reduction rate of 10 to 90% in a non-recrystallization temperature range of 3 points or more of Ar, and immediately cooled. A method for producing a steel sheet having excellent CTOD characteristics of a multi-pass weld, wherein controlled cooling is performed at a rate of 1 to 50 ° C / s to 200 ° C or less, and thereafter tempering is performed at 500 ° C to 650 ° C.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The technical concept of the present invention and the reasons for limitation will be described in detail below. The present inventors conducted various studies to improve the CTOD characteristics of the multi-pass weld, and as a result, it was found that (a) the effective crystal grain size is small in large heat input welding with a welding heat input of about 50 to 100 kJ / cm. Granulation, welding heat input is 20k
In welding with a small heat input of about J / cm or a medium heat input of about 20 to 50 kJ / cm, (b) reduction of island martensite, improvement of grain boundary hardenability by a trace amount of Nb, and (c) precipitation It has been found that suppression of curing and (d) reduction of HAZ hardness are necessary. It has been found that the CTOD characteristic of the multi-pass weld can be remarkably improved in the welding heat input range up to 100 kJ / cm for the first time by carrying out the combination of these four at the same time.

(A) Refinement of effective crystal grain size In large heat input welding, since γ grains are coarsened and retained for a long time at high temperature, the unit of fracture increases, and the influence of welding heat in one pass is reduced. The CTOD characteristics of the received coarse grain area decrease. Therefore, in order to improve CTOD characteristics especially in large heat input welding,
It is necessary to make effective crystal grains, which are units of fracture, finer.

As a technique for reducing the effective crystal grain size by suppressing the growth of γ grains and reducing the grains within γ grains, the technique disclosed in Japanese Patent Laid-Open No.
-250917. This is 14
The growth of γ grains in the weld heat affected zone heated to below
Suppressed by the pinning effect of iN, to produce fine ferrite with transformation nuclei of composite precipitates of TiN and MnS in the γ grains in the very narrow welding heat affected zone near the melting line exposed to temperatures above 1400 ° C. In this technique, the effective crystal grain size of the entire heat affected zone is reduced to improve the Charpy impact characteristics. However, since the Charpy impact characteristics and CTOD characteristics do not always correspond, CTOD
The effect on properties was unknown.

Therefore, the present inventors have studied the effect of the composite precipitate of TiN and MnS on the CTOD characteristics at the time of high heat input welding, paying particular attention to the amount of Ti added. In the test, C: 0.07%, Si: 0.05%, Mn was 1.5%.
%, P: 0.005%, S: 0.003%, Cu: 0.
4%, Ni: 0.7%, Nb: 0.005%, Al:
A steel sheet was used in which 0.03% and N: 0.004% were basic components and Ti was changed from 0 to 0.04%. 1400
After 2 seconds at 800C, the cooling time to 800-500C is 1
After applying a 1-pass welding reproduction heat cycle of 30 seconds (equivalent to welding heat input of 100 kJ / cm), a 10 mm thick CTO
The D test was performed. The precipitate was observed with a transmission electron microscope. The CTOD test temperature is calculated in consideration of the thickness effect.
The temperature was lowered to −70 ° C., which is 20 ° C. lower than 50 ° C.

FIG. 3 shows the relationship between the amount of Ti and the CTOD value at -70 ° C. When the content of Ti is 0.005% to 0.025%, the CTOD value at -70 ° C is high,
If it is less than 5%, the pinning effect of TiN and ferrite transformation in the grains are not sufficient, and if it is added more than 0.025%, coarse TiN is generated, so that the CTOD characteristics are deteriorated.

FIG. 4 schematically shows an example of a precipitate observed with a transmission electron microscope. The shape of TiN was rectangular, and MnS was precipitated on a part thereof, and the size from the end of TiN to the end of MnS was almost in the range of 0.05 to 0.3 μm. From the above examination results, the technology using the composite precipitate of TiN and MnS is effective in reducing the effective crystal grain size.
It has been newly found that the CTOD characteristics in the large heat input HAZ coarse grain region are improved.

(B) Reduction of island martensite and trace N
(b) Improvement of Grain Boundary Hardenability In order to satisfy the strict requirements on the CTOD characteristics of multi-pass welding with small heat input and medium heat input, it is necessary to consider the effects of multiple welding passes. In multi-pass welding, unlike one-pass welding, in the coarse-grained region generated in the first welding heat cycle, in the coarse-grained two-phase region reheated to the two-phase region of Ac1 and Ac3 by the next welding heat cycle. It has been conventionally known that island martensite is generated and significantly reduces the CTOD characteristics of a multi-pass weld, but when Si is reduced, generation of island martensite can be suppressed and CTOD characteristics can be improved.

The present inventors have studied the effects of alloying elements on the CTOD characteristics in the coarse-grained two-phase region, not only Si but also Nb, in order to satisfy the strict requirements for CTOD characteristics. In the test, C: 0.07%, Mn: 1.
5%, P: 0.005%, S: 0.003%, Cu:
0.4%, Ni: 0.7%, Ti: 0.01%, Al:
0.03%, N: 0.004% as basic components, Si:
0.03 to 0.135%, Nb: 0.001 to 0.01
A steel sheet varied in the range of 55% was used. In consideration of the fact that the coarse-grained two-phase region is tempered by the next welding heat cycle in actual multi-pass welding,
A triple cycle was used. Specifically, after holding at 1400 ° C. for 1 second, cooling from 800 to 500 ° C. is performed in 23 seconds, then holding at 780 ° C. for 1 second, cooling to 500 ° C. in 22 seconds, and further cooling at 450 ° C. for 1 second. The heat input was equivalent to medium heat input after cooling for 2 seconds.

FIG. 5 shows the relationship between the added amounts of Si and Nb and the CTOD value at -70 ° C. Depending on the amount of Si and Nb added,
It has been found that there are conditions under which the CTOD characteristics are significantly improved. The reason is considered to be due to the following effects of Si and Nb. That is, when the amount of Si is reduced and a very small amount of Nb is added, Nb in the solid solution state becomes γ while the formation of island martensite by Si is suppressed.
In order to improve the hardenability of the grain boundaries and suppress the formation of grain boundary proeutectoid ferrite, and as a result, to promote the formation of fine ferrite from within γ grains and to reduce the effective crystal grain size, CT
Improve OD characteristics. On the other hand, when the added amount of Si is large, the effect of the trace Nb is small because the island martensite dominate the CTOD characteristics. Further, when Nb is not added even if the amount of Si added is small, grain boundary pro-eutectoid ferrite is generated, so that fine ferrite from within γ grains is not generated, and the effect of fine grains of effective crystal grains cannot be obtained. As a result, a remarkable CTOD characteristic improvement effect in the coarse-grained two-phase region cannot be expected.

The hardenability of the grain boundary is also improved by B. However, B significantly increases the HAZ hardness, so CTOD
The characteristics deteriorate. Therefore, in order to improve the CTOD characteristics, it is necessary to increase the hardenability of the grain boundaries without significantly increasing the hardness by using an extremely small amount of Nb in a solid solution state while suppressing the formation of island martensite. Is important.

Here, experimental results show that it is necessary to limit the relationship of Si + 10Nb to 0.04 to 0.20 in addition to limiting the amounts of addition of Si and Nb. If Si + 10Nb is less than 0.04 and the amount of Nb added is small, the effect of forming fine ferrite from within the γ grains cannot be sufficiently obtained, and if the amount of Si added is small, deoxidation is not sufficient. Si and N exceeding 0.20
When both the amounts of b are large, even if the respective addition amounts are within the above ranges, the ratio of the island-like martensite increases and Nb carbide tends to precipitate, so that the CTOD value decreases. Accordingly, the range of Si + 10Nb is set to 0.04 to 0.
20. Furthermore, in order to sufficiently suppress the proportion of island martensite and the precipitation of Nb carbide, 0.04
It is preferable to set it to less than 0.14.

As described above, the present invention reduces the amount of Si while simultaneously adding a very small amount of Nb,
The present invention has newly discovered a method of reducing island martensite, improving grain boundary hardenability by a trace amount of Nb, and improving CTOD characteristics in a coarse-grained two-phase region.

(C) Inhibition of precipitation hardening In order to satisfy the strict requirements on the CTOD characteristics of multi-pass welding with small heat input and medium heat input, in addition to the above-mentioned technique (b), the first welding heat cycle It is necessary to suppress the precipitation hardening in the coarse-grained temper tempered to Ac3 or less in the generated coarse-grained region by the next welding heat cycle. Precipitation hardening is
The precipitation of carbides of Nb and V during the tempering heat cycle embrittles the coarse-grained temper region and lowers the CTOD characteristics. To this end, the amounts of Nb and V added are limited. It is known that you need to.

Therefore, the present inventors investigated the amounts of Nb and V added and the effects on the increase in hardness. In the test, C: 0.07%, Si: 0.05%, Mn: 1.5
%, P: 0.005%, S: 0.003%, Cu: 0.
4%, Ni: 0.7%, Ti: 0.01%, Al: 0.
03%, N: 0.004% as basic components, Nb: 0 to
A steel sheet changed in the range of 0.013% and V: 0 to 0.02% was used. The welding heat cycle conditions were determined in consideration of the fact that the coarse-grained region was tempered by the next welding in the multi-pass welding corresponding to the actual middle heat input. 1400
C. for 1 second, and then cooled to 800-500 ° C. for 23 seconds, and a coarse-grained region material further maintained at 650 ° C. for 1 second,
About coarse grain temper material cooled to 0 ° C in 15 seconds,
Vickers hardness measurement and CTOD test (plate thickness 10 mm,-
70 ° C.).

As a result of the experiment, the Vickers hardness of the coarse-grained temper material was increased by adding a small amount of Nb and V compared with the coarse-grained region material, and this effect was saturated when the added amount was increased. It became clear to go. Rate of increase in hardness due to each ^{element, Nb: 81 (% 1/} 2), V: 42 from becoming (% ^1/2) equivalent expression of hardness due to these elements,

## EQU5 ## It can be expressed by Phv = 81Nb ^1/2 + 42V ^1/2 .

Therefore, the relationship between the value of Phv and the limit CTOD value was examined, and this is shown in FIG. The value of Phv is 4.
In the range of 4 to 11.0, CTOD characteristics were improved. The reason is considered to be the following effect of Nb and V. That is, when Phv is less than 4.4, the hardenability of the γ grain boundary is improved by Nb to suppress the generation of the grain boundary proeutectoid ferrite,
As a result, it is not possible to expect a sufficient effect of promoting the formation of fine ferrite from within the γ grains to reduce the effective crystal grain size. It can be said that the CTOD characteristic was lowered because the hardness of the CTOD increased. Therefore, the precipitation hardening is suppressed by limiting the addition amounts of Nb and V, and the CTO in the coarse-grained temper region is suppressed.
D characteristics can be improved.

(D) Reduction of HAZ Hardness In order to satisfy the strict requirements for the CTOD characteristics of multi-pass welding with small heat input and medium heat input, in addition to the above-mentioned techniques (b) and (c), HAZ It is necessary to reduce hardness. The HAZ hardness is determined by the chemical composition of the steel sheet when the cooling rate is constant,
It is known that the effect of an additional element can be evaluated by a carbon equivalent formula evaluated by a ratio to carbon. Many formulas have been proposed so far as the carbon equivalent formula, and particularly in foreign countries, the carbon equivalent formula proposed by IIW is often used. In the present invention, in consideration of not adding Mo, the carbon equivalent is defined by the following formula.

Ceq = C + Mn / 6 + (Cr + V) / 5 +
(Cu + Ni) / 15

The present inventors have investigated carbon equivalent and CTOD characteristics. P: 0.005%, S: 0.00
3%, Cu: 0.4%, Ni: 0.7%, Ti: 0.0
1. Al: 0.03%, N: 0.004% as basic components, C: 0.04 to 0.15%, Mn: 1.0 to 1.8.
%, The steel plate was changed in carbon equivalent by changing the carbon equivalent, then kept at 1400 ° C. for 1 second, cooled from 800 to 500 ° C. in 23 seconds, then kept at 650 ° C. for 1 second,
After applying a welding reproducible heat cycle equivalent to two-pass medium heat input for cooling to 0 ° C. in 15 seconds, a CTOD test was performed. As a result, the CTOD characteristics decreased when the carbon equivalent exceeded 0.45. If the carbon equivalent is less than 0.35, it is difficult to secure the strength of the base material. Therefore, the range of the carbon equivalent is set to 0.35 to 0.45.

In the present invention, the CTOD characteristics of the multi-pass weld are improved by simultaneously combining the above four (a) to (d), and at the same time, the base material Since it is necessary to ensure strength and base metal toughness, it is necessary to limit the range of chemical components.

C is an important element for securing the strength of the base material. If it is less than 0.02%, it is difficult to secure the strength of the base material, and if it exceeds 0.15%, the HAZ hardness increases, the amount of island martensite in the coarse-grained two-phase region increases, and the CTOD characteristics deteriorate. Therefore, the content is set in the range of 0.02 to 0.15%.

Si is an important element for suppressing the island-like martensite in the coarse-grained two-phase region, deoxidizing and improving the strength of the base material. If it is less than 0.01%, deoxidation is not sufficient,
If it exceeds 0.11%, the amount of island-like martensite in the coarse-grained two-phase region increases.

Mn is an essential element in the present invention not only to secure the base material strength but also to form a composite precipitate of TiN and MnS and to improve the CTOD characteristics during large heat input welding. If it is less than 0.5%, sufficient base material strength cannot be obtained, and sufficient MnS cannot be generated.
If it exceeds 0%, the HAZ hardness increases and the CTOD characteristics decrease, so the range is 0.5 to 2.0%.

P is an element that microsegregates in the steel and easily causes grain boundary embrittlement. If the content exceeds 0.020%, the effect becomes remarkable, so the content is set to 0.020% or less. Preferably it is 0.006% or less.

S is an essential element in the present invention because it forms a composite precipitate of TiN and MnS and improves CTOD characteristics during large heat input welding. If the content is less than 0.002%, the effect cannot be sufficiently obtained. If the content is more than 0.010%, coarse and elongated MnS is formed in the steel, which causes anisotropy of the material and lowers the toughness. 002-0.010%.

Nb, when present in a solid solution state, improves the hardenability of γ grain boundaries, promotes the formation of fine ferrite from within γ grains, and reduces the effective crystal grain size. C
Improve TOD characteristics. Further, in order to suppress the recrystallization by hindering the movement of crystal grain boundaries during rolling, to enable rolling in the non-recrystallization temperature range, to reduce the crystal grain size and increase the base material strength, the present invention, It is an essential element. 0.00
If it is less than 3%, these effects cannot be obtained sufficiently, and
If it exceeds 3%, Nb carbide precipitates during the tempering cycle and lowers the CTOD characteristic.
The range was 3%.

Ti combines with N to form TiN to form γ
In order to suppress the coarsening of the grains and, in the large heat input welding, to form precipitates with MnS and to form nuclei for intragranular ferrite to refine the effective crystal grain size and improve the CTOD characteristics, It is an essential element in the invention. If Ti is less than 0.005%, these effects cannot be sufficiently obtained, and 0.025%
If it is excessive, coarse TiN is generated and the toughness is reduced.
The range was 0.005 to 0.025%. Note that the addition ratio of Ti and N is preferably in the range of 2.0 to 3.0.

Al is an important element in fixing solid solution N and deoxidizing. If it is less than 0.01%, these effects cannot be sufficiently obtained, and if it exceeds 0.08%, coarse inclusions and island-like martensite are easily formed to lower toughness. 0.08%.

N is an essential element in the present invention because it produces TiN as described above. If it is less than 0.002%, TiN cannot be sufficiently generated, and if it exceeds 0.008%, solid solution N
In this case, the toughness is lowered, and island martensite is easily formed.

Cu is an element effective for improving the strength of the base material. If it is less than 0.10%, the effect cannot be obtained sufficiently,
If it exceeds 1.0%, a large amount of ε-Cu is precipitated, and the CTOD characteristic is lowered.

Ni is an element effective for improving the strength of the base material by increasing the hardenability and for improving the base material toughness and the HAZ toughness. If it is less than 0.10%, a sufficient effect of improving base material strength and toughness cannot be obtained, and if it is more than 2.0%, it is not economical.

Cr is an element effective in improving the strength of the base material by increasing the hardenability. 0.05%
If it is less than 0.50%, the effect cannot be sufficiently obtained. If it exceeds 0.50%, HAZ is hardened and HAZ toughness is reduced.
-0.50%.

V is an element effective for improving the strength of the base material. If it is less than 0.005%, the effect cannot be sufficiently obtained, and if it exceeds 0.020%, it precipitates during the tempering cycle, lowering the CTOD characteristics, and lowering the HAZ toughness. %.

In the present invention, Ceq is set to 0.35 to 0.45
Since it is impossible to secure a sufficient base metal strength by a manufacturing method in which air cooling is performed after rolling, it is necessary to limit the following manufacturing conditions when manufacturing a steel sheet.

After the steel slab is melted in a converter and an electric furnace,
Ladle scouring and vacuum degassing are performed as necessary, and the product is manufactured by a continuous casting method. Alternatively, the slab may be formed by lumping with a mold or a one-way solidification mold and then lumping.

The heating is performed to reduce the deformation resistance during rolling and to form a solid solution of part or all of Nb and V contained in the steel. If the temperature is lower than 950 ° C., they cannot be dissolved. If the temperature exceeds 1300 ° C., the austenite grain size becomes coarse, and it becomes difficult to reduce the crystal grains by rolling, so that sufficient base material toughness cannot be secured. Therefore, the heating temperature is 95
The range was 0 to 1300 ° C. In addition, the holding time of heating is preferably 60 to 180 minutes.

The rough rolling is performed to gradually reduce the austenite grain size by recrystallization in each rolling pass. Since it is necessary to recrystallize at the time of rolling, the temperature is defined as a recrystallization temperature range. If the rolling reduction of the rough rolling is less than 10%, sufficient austenite grain refinement cannot be obtained,
If it exceeds 90%, the rolling reduction of finish rolling cannot be secured,
The rolling reduction was in the range of 10 to 90%. Here, the rolling reduction is ((slab thickness)-(transfer thickness)) / (slab thickness).

The finish rolling is performed in order to roll austenite in an unrecrystallized state and to introduce a deformation zone in austenite grains. At low temperatures than Ar ₃ point, since the ferrite is formed, can not rolling in the austenite single phase, also non-it is necessary to roll by recrystallization state, non-recrystallized temperature the temperature above the Ar ₃ point Area. On the other hand, if the rolling reduction of the finish rolling is less than 10%, a sufficient effect of refining the ferrite grains cannot be obtained. If the rolling reduction exceeds 90%, it takes a long time for the finish rolling, so that it is difficult to secure the temperature. And the rolling reduction was in the range of 10 to 90%. Here, the rolling reduction is ((transfer thickness)-(finish thickness)) / (transfer thickness).

The controlled cooling is carried out in order to promote the nucleation of ferrite transformation from the deformation zone in the unrecrystallized austenite grains, to make the ferrite grains finer, and to obtain a sufficient base material strength. If the cooling rate is less than 1 ° C./s, these effects cannot be obtained, and it is difficult to obtain a cooling rate of 50 ° C./s or more due to facility restrictions.
Range. When the tempering described below is not performed, the above effect cannot be sufficiently obtained when the cooling stop temperature is higher than 650 ° C, and when the cooling stop temperature is lower than 500 ° C, the tempering effect during air cooling is insufficient and the base material toughness is reduced. Therefore, the cooling stop temperature was set to 650 to 500 ° C.

When tempering is performed, a temperature of 200 ° C.
Since sufficient base material strength cannot be obtained unless cooled below, the cooling stop temperature was set to 200 ° C. or lower. Tempering is performed to improve the base material toughness that has been reduced by controlled cooling. If the tempering temperature is lower than 500 ° C, the base metal toughness is not sufficiently improved, and if the tempering temperature is higher than 650 ° C, the base material toughness is improved. The holding time of the tempering is 20 to 120.
Minutes are preferred.

Based on the technical concept described above, the present invention provides
An object of the present invention is to provide a steel sheet excellent in CTOD characteristics of a multi-pass weld and a method for producing the same. In addition, in order to prevent low-temperature cracking from occurring in the welded portion as it is, it is preferable that the low-temperature cracking susceptibility evaluated by the following equation be 0.22 or less in addition to the claims of the present invention.

Pcm = C + Si / 30 + Mn / 20 + Cu /
20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10
+ 5B

[0058]

Embodiments of the present invention will be described below. In order to clarify the relationship between the chemical composition of steel, the manufacturing method, the properties of the base metal and the properties of the welded joint, steel having various chemical compositions shown in Table 1 was melted, and a slab obtained by continuous casting was obtained. Under the conditions shown in Table 2, a steel plate having a thickness of 25 to 75 mm was manufactured. Table 3 shows the base material properties obtained by the tensile test and the Charpy test. Furthermore, a welded joint was created to evaluate the welded joint characteristics. API-RP2Z
In order to carry out a test conforming to JIS, the groove shape was K groove.
The welding method was submerged arc welding (SAW), and the welding heat input was at two levels of 30, 100 kJ / cm 2.

Table 4 shows the Charpy absorbed energy at -70 ° C, the critical CTOD values at -30 ° C and -50 ° C, the coarse grain area ratio, the island martensite ratio (M ^* ratio), the HAZ hardness,
Shows the effective crystal grain size (def). The Charpy test is
Make the notch position coincide with the vertical bond part, and
This test was performed to determine the average value of the absorbed energy. C
The TOD test was performed at -30 ° C and -50 ° C for each five test pieces using test pieces whose notch positions were aligned with the vertical bond parts.
Calculate each CTOD value and calculate the limit C from the lowest value.
The TOD value was determined. The coarse grain area ratio is an average value measured at the fatigue notch portion of the test specimen after the test, and it was confirmed that three or more test specimens had a value of 15% or more. The ratio of island martensite is the ratio of island martensite generated in the coarse-grained two-phase region, the HAZ hardness is the hardness of the coarse-grained temper region, and the effective crystal grain size is the particle size of the coarse-grained region at the bond boundary. It is.

Inventive steels 1 to 10 have a base metal property of 360
A yield stress of not less than MPa and a sufficient absorbed energy at -70 ° C were obtained. Further, as for the characteristics of the welded joint, the absorbed energy at -70 ° C and
Since a good value was obtained as to the limit CTOD value at 0 ° C. and −50 ° C., it was confirmed that the CTOD characteristic of the multilayer welded portion was excellent.

On the other hand, the comparative steel 11 uses a slab of steel B, and its components are within the scope of the present invention. However, since the manufacturing conditions are different from those of the present invention, the base material strength is good although the welded joint characteristics are good. Run out. The comparative steel 12 has a large effective crystal grain size because the amounts of Ti and S added are different from the ranges of the present invention, and particularly, the CTOD characteristic at −50 ° C. during large heat input welding is significantly reduced. Since the amount of Si added to the comparative steel 13 is different from the range of the present invention, a large amount of island-like martensite is generated, and particularly, the CTOD characteristic at −50 ° C. during medium heat input welding is significantly reduced.
In Comparative Steel 14, since the addition amounts of Nb and V are different from the range of the present invention, the hardness of the coarse-grained temper region is high, and particularly, the CTOD characteristic at −50 ° C. during medium heat input welding is significantly reduced. Comparative steel 1
5, each component is within the range of the present invention, but the carbon equivalent is different from the range of the present invention, so that the hardness of the coarse-grained temper region is high, and particularly the CTOD characteristics at −50 ° C. during medium heat input welding are deteriorated. .
From the above examples, it has been clarified that by satisfying the claims of the present invention, it is possible to obtain a steel sheet excellent in CTOD characteristics of a multi-pass weld.

[0062]

[Table 1]

[0063]

[Table 2]

[0064]

[Table 3]

[0065]

[Table 4]

[0066]

As described in detail above, according to the present invention,
Regarding a steel plate for a welding structure used for an offshore structure used in an extremely cold ice sea area of about -40 to -50 ° C, the C of a multi-pass welded part having a welding heat input of up to about 100 kJ / cm.
It is possible to provide steel sheets with excellent TOD characteristics,
By using the steel sheet of the present invention, it has become possible to significantly improve the reliability of the above welded structure against brittle fracture. The significance of the steel sheet of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a view showing a relationship between a coarse grain area ratio and a CTOD value of a multi-pass weld.

FIG. 2 is a diagram showing a method of measuring a coarse grain area ratio of a multi-pass weld.

FIG. 3 is a diagram showing the relationship between the amount of Ti added and the limit CTOD value at −70 ° C.

FIG. 4 is a view showing a composite precipitate of TiN and MnS.

FIG. 5 is a graph showing the influence of the amounts of Si and Nb on the limit CTOD value at −70 ° C.

FIG. 6 is a limit CT when the value of the relational expression between Nb and V is −70 ° C.
It is a figure which shows the influence which affects OD value.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshi Nagao 5-3 Tokai-cho, Tokai City, Aichi Prefecture Inside Nagoya Works, Nippon Steel Corporation (72) Inventor Naoki Saito 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] 1. C: 0.02 to 0.15% by weight, Si: 0.01 to 0.11%, Mn: 0.5 to 2.0%, P: 0.020% or less by weight, S : 0.002 to 0.010%, Nb: 0.003 to 0.013%, Ti: 0.005 to 0.025%, Al: 0.01 to 0.08%, N: 0.002 to 0 0.008%, with the balance being iron and unavoidable impurity elements, Si + 10Nb: 0.04 to 0.20, 81Nb1 ^{/ 2} + 42V1 ^{/ 2} : 4.4 to 11.0, Ceq: 0. CTOD of a multi-pass weld, characterized by satisfying 35-0.45
Steel plate with excellent properties. Here, Ceq = C + Mn / 6 + (Cr + V) / 5 + (Cu
+ Ni) / 15. 2. In% by weight, C: 0.02 to 0.15%, Si: 0.01 to 0.11%, Mn: 0.5 to 2.0%, P: 0.020% or less, S : 0.002 to 0.010%, Nb: 0.003 to 0.013%, Ti: 0.005 to 0.025%, Al: 0.01 to 0.08%, N: 0.002 to 0 0.008%, Cu: 0.10 to 1.0%, Ni: 0.10 to 2.0%, Cr: 0.05 to 0.50%, V: 0.005 to 0.020 %, One or more of them are contained, and the balance consists of iron and inevitable impurity elements. Si + 10Nb: 0.04 to 0.20, 81Nb ^1/2 + 42V ^1/2 : 4.4 to 11 0.0, Ceq: 0.35 to 0.45
Steel plate with excellent properties. Where Ceq = C + Mn / 6 + (Cr + V) / 5 + (Cu
+ Ni) / 15. 3. A steel slab having the chemical composition according to claim 1 is heated to 950 to 1300 ° C. and subjected to rough rolling at a recrystallization temperature range with a rolling reduction of 10 to 90%. Subsequently, finish rolling is performed at a rolling reduction of 10 to 90% in a non-recrystallization temperature range of Ar ₃ or more, and the cooling rate is immediately increased to 1 to 90%.
Controlled cooling to 650-500 ° C at 50 ° C / s and air cooling to room temperature, CTOD for multi-pass welds
Manufacturing method of steel sheet with excellent characteristics. 4. A steel slab having the chemical composition according to claim 1 is heated to 950 to 1300 ° C., and is subjected to rough rolling in a recrystallization temperature range at a rolling reduction of 10 to 90%. Subsequently, finish rolling is performed at a rolling reduction of 10 to 90% in a non-recrystallization temperature range of Ar ₃ or more, and the cooling rate is immediately increased to 1 to 90%.
Controlled cooling to 200 ° C or less at 50 ° C / s,
A method for producing a steel sheet having excellent CTOD characteristics of a multi-pass weld, wherein tempering is performed at 0 ° C to 650 ° C.