JP2006045586A - Steel for welded structure having excellent low temperature toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the low temperature toughness of HAZ including a Bond at a large heat input welded joint of ≥300 kJ/cm amount of heat input. <P>SOLUTION: The steel has a component composition composed of, by mass, 0.04 to 0.12% C, 0.01 to 0.5% Si, 0.5 to 2% Mn, 0.005 to 0.03% Ti, 0.001 to 0.003% B, 0.004 to 0.007% N and the balance essentially Fe, containing, when necessary, one or more elements among Cu, Ni, Cr, Mo, V and Nb, having an N-Ti-B balance satisfying inequality 0.9×IN≤N≤1.2×IN (wherein, IN=Ti/3.4+1.3B is satisfied; and N, Ti and B are numerical values representing their respective contents in ppm) and, further, satisfying a limiting equation consisting of the component composition and a cooling rate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel for welded structures used for offshore structures, pressure vessels, ships, bridges, buildings, line pipes, etc., and particularly to a steel excellent in low temperature toughness of a high heat input weld HAZ part including a Bond part. Is.

  In recent years, as steel structures such as marine structures, pressure vessels, ships and the like increase in size, steel materials used tend to be thickened. In the case of thick materials, high heat input welding such as electrogas welding (EGW) and submerged arc welding (SAW) is required to prevent low temperature cracking during small heat input welding such as tack welding and to improve welding work efficiency. Ensuring the low temperature toughness of the welded part when the is applied is a problem.

  Therefore, in the thick material, in order to prevent low-temperature cracking at the time of small heat input welding, component design is performed to reduce the Ceq and Pcm and prevent toughness deterioration of the high heat input HAZ part. However, in recent years, depending on the structure, welding is performed with a much larger heat input (300 kJ / cm or more) than before, and it is required to secure toughness at a low temperature of, for example, −40 ° C. for the welded portion. The required characteristics tend to become more severe, and the difficulty of component design is increasing.

  Various techniques have been proposed for improving the toughness of the high heat input welded HAZ part. However, these techniques mainly use the microstructure of the HAZ part as a fine structure, and the technical idea is (1) precipitates and inclusions. The pinning action prevents coarsening of austenite grains. (2) It is broadly divided into the formation of intragranular ferrite with inclusions as precipitation sites to make the grains substantially fine.

  Patent Document 1 and Patent Document 2 relate to (1), and Patent Document 1 describes that fine TiN is precipitated to suppress coarsening of austenite crystal grains, but reaches a temperature exceeding 1400 ° C. In the HAZ part (particularly the Bond part), most of the TiN dissolves, so that it becomes a coarse structure and the toughness is not improved.

Patent Document 2 describes that a large amount of fine Al ₂ O ₃ precipitates are precipitated and austenite grains are refined, but a large heat input that is maintained at a high temperature for a long time as in electrogas welding. The effect of welding is insufficient.

  Patent Document 3 and Patent Document 4 relate to (2), and Patent Document 3 describes that an intragranular ferrite is generated using Ti oxide particles as nucleation sites to form a fine HAZ structure. Since the oxide is finely dispersed in the steel, it has a special component composition in which Al, which is a strong deoxidizing element, is 0.007% or less, and the steel type organization becomes complicated in actual production.

  Patent Document 4 describes that intragranular ferrite is generated by using Ca oxide or Ca oxysulfide as a nucleation site to refine the HAZ structure. However, in order to stably secure Ca oxide, O is used. Because it is 0.0040% or less and Al of the strong deoxidizing element is 0.007% or less, precise control is required for the deoxidation method and addition of component elements, and depending on the welding material used in combination, the toughness of the weld metal There is also concern about the decline.

  Patent documents 5 and 6 are examples of further improving these techniques. Patent Document 5 describes that by adding N amount according to the addition amount of Ti and B, TiN and BN are finely dispersed in a large amount to improve the toughness of the welded portion. In order to make the best use of the effect, it is necessary to increase the N content, and there is a concern that embrittlement may occur in the HAZ part at 1000 to 1200 ° C. due to excess N that is not fixed.

In Patent Document 6, in ultra-high heat input welding with a heat input of 500 to 1000 kJ / cm, TiN having a size that does not dissolve and disappear even under a long high-temperature residence time is secured in the HAZ part including the Bond part. A method for producing a steel sheet for welded structure having excellent Bond part toughness and HAZ part toughness at a low temperature of −20 ° C. by precipitating a large amount of BN is described.
Japanese Patent Publication No.55-26164 Japanese Patent No. 2950076 JP-A-61-79745 JP-A-5-287374 Japanese Patent Laid-Open No. 9-20955 Japanese Patent No. 2931065

  As described above, when the high heat input weld HAZ part is made finer using precipitates, the precipitates dissolve to obtain a sufficient pinning effect or to form precipitates that are difficult to dissolve. The base material manufacturing conditions were limited, such as the need for precise component composition and precise deoxidation conditions.

  For example, in the technique described in Patent Document 6, in order to prevent TiN from dissolving even at a long high-temperature residence time characteristic of an ultra-high heat input weld having a heat input of 500 to 1000 kJ / cm, the cooling rate of the casting solidification process is set to 5 ° C. / It is necessary to make it less than a minute. Such a restriction is not preferable from the viewpoint of productivity, for example, when a large amount of steel plate is manufactured for a container ship that is remarkably large.

  Further, in any of the patent documents, it is difficult to say that the study on the fluctuation range of the Charpy impact value is sufficient, and there remains a problem in terms of safety such as local embrittlement of the structure.

  The present invention is excellent in productivity and has a Bond portion of a high heat input weld portion having a heat input amount of 300 kJ / cm or more such that a cooling rate of 800 to 500 ° C. in a cooling process immediately after welding is 2 ° C./s or less In the Charpy impact test of the HAZ part, an object is to provide a steel having a Charpy impact value (vE-40: test temperature −40 ° C.) of at least 100 J or more.

  The present inventors have conducted a detailed study on the technique described in Patent Document 6. As a result, when the Ti content was kept low considering the surface properties and cleanliness of the steel sheet, the effect of improving toughness due to the addition of B (BN precipitation) was not stably obtained. That is, in the Charpy impact test at a low temperature such as −40 ° C., a change in toughness value was observed depending on the notch position. The reason why the toughness value is not stabilized in this way is presumed to be due to the fact that the effect of improving the hardenability by addition of B acts more strongly.

  Therefore, the present inventors have studied CCT simulating the cooling process immediately after welding for steels having various compositional compositions in order to grasp the relationship between the hardenability (transformation phenomenon) in the weld zone and the toughness improving effect by the formation of intragranular ferrite. We observed the transformation phenomenon and the formation of intragranular ferrite using curves, and obtained the following knowledge.

  1. A large amount of intragranular ferrite is expected to have a fine structure and a good low temperature toughness of the weld.

  2. When the amount of intragranular ferrite produced is large, the production start temperature of intragranular ferrite is higher than when the amount is small.

  3. Even if the initiation temperature of intragranular ferrite is the same, there may be a difference in the amount of intragranular ferrite produced observed at room temperature depending on the component composition.

  4). Therefore, in order to produce intragranular ferrite in a high heat input weld zone and obtain a fine structure, the selection of the component composition and the relationship between the component elements should be made so that the production start temperature of the intragranular ferrite is high and the production amount is increased. It is effective to specify.

  In addition, when the present inventors optimize the amount of Al and O in steel and further define the amount of N in relation to the amounts of Ti and B, the HAZ portion (including the Bond portion) of the high heat input welded portion. It was also found that the low temperature toughness of -40 ° C was excellent.

The present invention was made by further studying these findings, that is, the present invention
1. In mass%, C: 0.04 to 0.12%, Si: 0.01 to 0.5%, Mn: 0.5 to 2%, S: 0.001 to 0.01%, sol. Al: 0.04-0.08%, Ti: 0.005-0.03%, B: 0.001-0.003%, O: 0.001-0.005%, N: 0.004- A steel for welded structures including 0.007%, the balance being substantially made of Fe, and excellent in low temperature toughness of welds satisfying the following formulas (1) and (2).
0.9 × IN ≦ N ≦ 1.2 × IN (1)
Here, IN = Ti / 3.4 + 1.3B
N, Ti, and B are numerical values expressed in ppm.
T = α + β × [1 + exp {−γ (ln (1/2) −δ)}] ⁻¹ ≧ 670 (2)
Here, α, β, γ, and δ are values obtained by the following equations.

α = 714-451 × [C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5]
β = 102 + 1.8 × √ [X] √ [Y]
γ = 2.5 + 0.5 × √ [X] √ [Y]
δ = −0.6−0.025 × √ [X] √ [Y]
However, [X] is a value obtained by N-Ti / 3.4. When N-Ti / 3.4 <10, [X] = 0, and when N-Ti / 3.4> 50, X] = 50, and when N-Ti / 3.4 is 10-50, the calculated value.
[Y] is a value defined by the content of boron (B). When the content of boron (B) exceeds 18, [Y] = 18 and the content of boron (B) is less than 10 Is [Y] = 0, and when the boron (B) content is 10 to 18, the content is taken. Here, N, Ti, and B are numerical values expressed in ppm. Further, C, Mn, Cu, Ni, Cr, Mo, and V are values expressed by mass%, and 0 is not added.

  2. Furthermore, one or two of Cu ≦ 0.5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb ≦ 0.03% by mass%. The welded structural steel having excellent low temperature toughness of the welded portion according to 1, characterized by containing at least a seed.

  According to the present invention, a welded structural steel capable of stably securing excellent low temperature toughness in a high heat input welded joint having a heat input of 300 kJ / cm or more is obtained, which is extremely useful industrially.

  In the present invention, the component composition is defined.

C
C is added in order to ensure the strength of the steel material, but if added over 0.12%, high carbon island martensite is generated and the weld HAZ toughness is lowered. On the other hand, if it is less than 0.04%, in order to ensure sufficient strength, other hardenability improving elements must be added in a large amount, resulting in an increase in production cost. For this reason, it is made into 0.04 to 0.12%.

Si
Si is added in an amount of 0.01% or more as a deoxidizing material and in order to ensure the strength of the steel material. On the other hand, if added over 0.5%, the formation of high carbon island martensite becomes easy and the welded HAZ toughness is lowered, so 0.01 to 0.5%.

Mn
Mn is added in an amount of 0.5% or more to ensure the strength of the steel material. On the other hand, if it exceeds 2%, the hardenability increases and the weldability and the HAZ part toughness are deteriorated.

S
S forms 0.001% or more in order to form MnS which becomes a nucleation site of ferrite in the welded HAZ part. On the other hand, if it exceeds 0.01%, the toughness of the base metal and the welded HAZ part will decrease, so 0.001 to 0.01%.

Ti
Ti is necessary for the formation of TiN that suppresses the coarsening of austenite grains in the high heat input weld HAZ portion and becomes a nucleation site of ferrite, and is added in an amount of 0.005% or more. On the other hand, if added over 0.03%, TiC which is harmful to the base metal toughness and welded HAZ part toughness precipitates and the surface flaws of the steel sheet occur frequently, so 0.005 to 0.03%.

B
B is added in an amount of 0.001% or more in order to generate BN that becomes a nucleation site of ferrite. On the other hand, if added over 0.003%, the weld HAZ toughness decreases, so 0.001 to 0.003%.

sol. Al
sol. Al decreases to 0.04% or more in order to reduce solid solution N harmful to deoxidation and weld HAZ toughness and to generate Al ₂ O ₃ precipitates. On the other hand, if it exceeds 0.08%, coarse Al-based inclusions are produced and the toughness is lowered, so that the content is made 0.04 to 0.08%.

  In FIG. The Charpy impact test results of the high heat input welded joints having the Al amount within the range of the present invention and those outside the range of the present invention are shown in comparison. Welding was electrogas arc welding with a heat input of 420 kJ / cm, and in the Charpy impact test, the notch positions were Bond, HAZ 1 mm, HAZ 3 mm, HAZ 5 mm, and the test temperature was −40 ° C. Here, HAZ 1 mm represents the test result when the notch position is 1 mm from the Bond part to the HAZ side.

  As a result, it was confirmed that the steel of the present invention can obtain an excellent Charpy impact value throughout the HAZ. In addition, a test result shows the average value of 3 pieces.

  FIG. 2 shows individual impact values and average values obtained when the notch position is HAZ 1 mm in the Charpy impact test of FIG. In the case of the test steel having a low Al content outside the range of the present invention, a low impact value far from the average value was obtained, and the test result was concerned that the safety could be lowered due to local embrittlement.

  sol. In the case of the test steel with a low amount of Al, AlN is not sufficiently produced at the position of HAZ 1 mm where the maximum heating temperature is lower than the Bond part, and N that could not form Ti and B and nitrides was dissolved in the steel, It seems to have deteriorated the toughness of the ferrite texture.

N
N is sol. In order to suppress the coarsening of austenite grains by forming Al and AlN, and to produce BN and TiN that become nucleation sites of ferrite, 0.004% or more is added. On the other hand, if it exceeds 0.007%, even if AlN is formed, the amount of solid solution N decreases the toughness of the welded HAZ part, so 0.004 to 0.007%.

O
O is made 0.001% or more in order to secure Al ₂ O ₃ precipitates that will be the precipitation sites for BN and TiN. On the other hand, if it exceeds 0.005%, coarse inclusions are likely to be generated and the toughness is deteriorated, so 0.001 to 0.005%.

0.9 × IN ≦ N ≦ 1.2 × IN (1)
Here, IN = Ti / 3.4 + 1.3B, and N, Ti, and B are numerical values expressed in ppm.

  This parameter formula (1) prescribes the N content in relation to Ti and B. By satisfying this parameter formula, most of N contained is TiN and BN.

  The balance being substantially made of Fe means containing inevitable impurities such as P and other trace elements as long as the effects of the present invention are not impaired.

FIG. 3 is a diagram showing the transformation phenomenon and the formation start temperature of intragranular ferrite in the CCT curve when the cooling rate is 2 ° C./s, and schematically shows the measurement result of the transformation point by a thermal dilatometer.
The composition of the sample steel 1 is within the range of the present invention, and the sample steel 2 is out of the range of the present invention.

In the figure, A ₁ and A ₂ indicate the formation start temperature of the grain boundary ferrite, and T ₁ and T ₂ indicate the generation start temperature of the intragranular ferrite at the minimum point on the thermal expansion curve. Comparing the thermal expansion curves of Steel 1 and Steel 2, the formation start temperatures A ₁ and A ₂ of the grain boundary ferrite are the same, but the temperatures T ₁ and T ₂ indicating the minimum points (transformation points) of the thermal expansion curves are Is different.

That is, the minimum point T ₁ of the steel ₁ is higher than the minimum point T ₂ of the steel 2. This is after generation start of the grain boundary ferrite due precipitate sufficient of BN in the steel 1, immediately indicates that the formation of intragranular ferrite occurs, generation starting temperature of T ₁ i.e. intragranular ferrite 670 ° C. or higher It is.

  When the relationship between the minimum point (transformation point) on the thermal expansion curve obtained in the same experiment and the toughness of the high heat input welded joint for the steel with various amounts of Ti and B was obtained, as shown in FIG. It was confirmed that an excellent Charpy impact test value was obtained with a steel having a transformation point of 670 ° C. or higher.

  Therefore, in order to improve the toughness as a microstructure by generating intragranular ferrite in the high heat input welding HAZ part (including the Bond part), the transformation point (starting temperature of intragranular ferrite generation) in the cooling process after welding is It is necessary to adjust the component composition so as to be 670 ° C. or higher.

The transformation point T in the CCT curve, which is the formation start temperature of intragranular ferrite, is determined by the composition of the base material and the cooling rate, and is determined by the following formula (3) according to the study by the present inventors. .
T (v) = α + β × [1 + exp {−γ (ln (1 / v) −δ)}] ⁻¹ (2)
In this parameter equation (2), v is the cooling rate (° C./s) of the CCT curve, and α, β, γ, and δ are values obtained by the following equations.

α = 714-451 × [C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5]
β = 102 + 1.8 × √ [X] √ [Y]
γ = 2.5 + 0.5 × √ [X] √ [Y]
δ = −0.6−0.025 × √ [X] √ [Y]
However, [X] is a value obtained by N-Ti / 3.4. When N-Ti / 3.4 <10, [X] = 0, and when N-Ti / 3.4> 50, X] = 50, and when N-Ti / 3.4 is 10 to 50, the value is obtained by calculation.

  [Y] is a value defined by the content of boron (B). When the content of boron (B) exceeds 18, [Y] = 18, and when the content of boron (B) is less than 10, When [Y] = 0 and the boron (B) content is 10 to 18, the content is determined. Here, N, Ti, and B are numerical values expressed in ppm.

  In addition, C, Mn, Cu, Ni, Cr, Mo, and V are numbers in which the content is expressed in mass%, and 0 is not added.

  In the high heat input welding in which the heat input amount targeted by the present invention exceeds 300 kJ / cm, the cooling rate of 800 to 500 ° C. in the welding cooling process is a very slow cooling state of 2 ° C./s or less. For this reason, steel in which the composition of the base material is adjusted so that the T value when satisfying the cooling rate v = 2 in equation (3) satisfies 670 or more (that is, steel satisfying equation (2)) is A microstructure in which a large amount of intragranular ferrite is generated is obtained, and good weld bond and HAZ toughness is obtained.

  The steel of the present invention can obtain excellent characteristics by satisfying the above-described component composition and parameter formula, but in order to further improve the characteristics, one or more of Cu, Ni, Cr, Mo, V, and Nb are used. In mass%, Cu ≦ 0.5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb ≦ 0.03% Can do.

  The steel according to the present invention can be cast at a conventional casting cooling rate, and thereafter can be formed into a thick steel plate by conventional rolling and heat treatment according to desired characteristics, and the production method is not limited.

  After melting steel of the component composition shown in Table 1 and making it a slab by a continuous casting method, it is heated to 1100 to 1250 ° C. and is subjected to TMCP (controlled rolling / controlled cooling) or DQ-T (direct quenching / tempering). A steel plate having a thickness of 50 to 70 mm was used.

  These steel plates were subjected to electrogas arc welding with a heat input of 340 to 530 kJ / cm depending on the plate thickness. The Charpy impact test piece obtained was collected, and the Charpy impact absorption energy value at a test temperature of −40 ° C. was determined.

Table 2 shows manufacturing conditions, base metal strength, toughness, and welded joint test results. No. 1 to 14 are steels of the present invention, which have a base metal tensile strength of 510 N / mm ² or more and a Charpy impact value of 200 J or more at -40 ° C. An impact value of 100 J or more is obtained.

  On the other hand, no. 15 to 30 are comparative steels, and even if the component composition is outside the range of the present invention, or even within the range of the present invention, the balance relationship between the component elements is poor, so the formula (1) or (2) is not satisfied. However, the weld zone toughness is inferior to the steel of the present invention.

  No. Nos. 15, 16, and 27 have a poor balance of Ti, B, and N, and do not satisfy the formula (1).

  No. 20 and 25 satisfy the formula (1), but B or Ti is low, so the formula (2) is not satisfied, and the toughness at the Bond part and HAZ of 1 mm is inferior.

  No. 17, 18, 21, 22, 24 are sol. Since the amount of Al is low, the toughness at HAZ 3 mm is inferior. 28 is sol. Since the amount of Al is high, the Bond part and HAZ 1 mm are inferior.

  No. In Nos. 23 and 30, the amount of O is outside the range of the present invention, so the toughness of the Bond part is inferior to the steel of the present invention.

  No. Nos. 19 and 26 have component compositions within the scope of the present invention. 19 is the formula (1), No. 19; No. 26 does not satisfy the formula (2), and the Bond part toughness is inferior.

The effect on the Charpy impact test results of the large heat input welded joint (notch position: Bond, HAZ 1 mm, 3 mm, 5 mm). The figure which shows the influence of Al amount. The average value in the Charpy impact test result (notch position: HAZ 1 mm) of the high heat input welded joint HAZ and the sol. The figure which shows the influence of Al amount. The figure explaining the relationship between the transformation point in a thermal expansion curve, and the intragranular ferrite generation start temperature. The figure which shows the relationship between the minimum point (transformation point) on a thermal expansion curve, and large heat-input welding Bond part toughness (Charpy impact absorption energy value in -40 degreeC).

Claims (2)

In mass%, C: 0.04 to 0.12%, Si: 0.01 to 0.5%, Mn: 0.5 to 2%, S: 0.001 to 0.01%, sol. Al: 0.04-0.08%, Ti: 0.005-0.03%, B: 0.001-0.003%, O: 0.001-0.005%, N: 0.004- A steel for welded structures including 0.007%, the balance being substantially made of Fe, and excellent in low temperature toughness of welds satisfying the following formulas (1) and (2).
0.9 × IN ≦ N ≦ 1.2 × IN (1)
Here, IN = Ti / 3.4 + 1.3B
N, Ti, and B are numerical values expressed in ppm.
T = α + β × [1 + exp {−γ (ln (1/2) −δ)}] ⁻¹ ≧ 670 (2)
Here, α, β, γ, and δ are values obtained by the following equations.
α = 714-451 × [C + Mn / 6 + (Cu + Ni) / 15
+ (Cr + Mo + V) / 5]
β = 102 + 1.8 × √ [X] √ [Y]
γ = 2.5 + 0.5 × √ [X] √ [Y]
δ = −0.6−0.025 × √ [X] √ [Y]
However, [X] is a value obtained by N-Ti / 3.4. When N-Ti / 3.4 <10, [X] = 0, and when N-Ti / 3.4> 50, X] = 50, and when N-Ti / 3.4 is 10-50, the calculated value.
[Y] is a value defined by the content of boron (B). When the content of boron (B) exceeds 18, [Y] = 18 and the content of boron (B) is less than 10 Is [Y] = 0, and when the boron (B) content is 10 to 18, the content is taken. Here, N, Ti, and B are numerical values expressed in ppm. Further, C, Mn, Cu, Ni, Cr, Mo, and V are values in which the content is expressed by mass%, and 0 is not added. Furthermore, one or two of Cu ≦ 0.5%, Ni ≦ 1.0%, Cr ≦ 0.5%, Mo ≦ 0.5%, V ≦ 0.1%, Nb ≦ 0.03% by mass%. The welded structural steel having excellent low-temperature toughness of the welded portion according to claim 1, comprising at least a seed.

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