JP3697202B2 - Steel with excellent toughness of weld heat affected zone and method for producing the same - Google Patents

Description

【００４９】
【表２】

【００５０】
本発明で製造した鋼板（本発明鋼）は降伏強度（ＹＳ）が４２０Ｎ／ｍｍ²以上で、−１０℃のＣＴＯＤ値が０．６３ｍｍ以上の良好な破壊靭性を示した。
【００５１】
これに対し、比較鋼では、強度は発明鋼と同等であるが、ＣＴＯＤ値が劣り、厳しい環境下で使用される鋼板として適切でない。
【００５２】
比較鋼１６はＭｎやＡ_GB値が低いため、粒界フェライト、島状マルテンサイトが生成し、ＣＴＯＤ値が低い値であった。
【００５３】
比較鋼１７はＡ_GB値が高すぎ、Ｎｂの添加量も多すぎるため、ミクロ組織にベイナイトが多く、硬さ値も高いためＣＴＯＤ値が低い値であった。
【００５４】
比較鋼１８はＣ、Ｓｉ、Ｍｎ、Ｎｂ等は発明鋼と同じであるが、Ａ_GB値が高すぎなため、ミクロ組織にベイナイトが多く、ＣＴＯＤ値が低い値であった。
【００５６】
比較鋼１９はＡ_GBが高すぎ、窒素（Ｎ）が少ないため、ミクロ組織にベイナイトが多く観察され、低いＣＴＯＤ値であった。
【００５７】
【発明の効果】
本発明により製造した鋼は、溶接時に最も靭性が劣化する旧オーステナイト粒界とその近傍のミクロ組織を適正化して、優れた靭性を示す。
【００５８】
これにより、海洋構造物、耐震性建築物等の厳しい環境で使用される高強度の鋼材の製造を可能とした。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel excellent in toughness of a heat-affected zone (HAZ) from small heat input welding to medium heat input welding, and a manufacturing method thereof.
[0002]
[Prior art]
The HAZ toughness of low alloy steel is (1) grain size, (2) high carbon martensite (M ^* ), upper bainite (Bu) and ferrite side plate (FSP) hardened phase dispersion state, (3 It is governed by various factors such as :) precipitation hardening state, (4) presence / absence of grain boundary embrittlement, and (5) elemental microsegregation.
[0003]
These factors are known to have a great influence on toughness, and many techniques have been put to practical use in order to improve HAZ toughness.
[0004]
As a particularly excellent technology, the microstructure is refined with Ti oxide, and in addition to this, the balance of Ti, O, and N is optimized to suppress the precipitation of TiC, thereby reducing the precipitation effect and improving the toughness. Is known (Japanese Patent Laid-Open No. 5-247531).
[0005]
However, brittle cracking characteristics of the weld heat affected zone (CTOD) the effect of hardening phase of M ^* such as described above is extremely large, hardenability of the steel components for this in the previous techniques M ^* generation suppression (D _I , Ceq) must be kept low, which has been an obstacle for increasing the strength.
[0006]
[Problems to be solved by the invention]
The present invention provides a technique for inexpensively producing a high-strength steel excellent in fracture toughness (CTOD) in multilayer welding with small to medium heat input.
[0007]
The steel produced according to the present invention refines the microstructure of the multilayer weld and exhibits excellent toughness.
[0008]
[Means for Solving the Problems]
The gist of the present invention is as follows.
[0009]
(1) In mass%,
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
In the range of A _GB-1 in the range of 0.39 to 0.47, the balance is excellent in toughness of the heat affected zone, characterized in that the balance is substantially free of Al consisting of iron and inevitable impurities. steel.
Here, A _GB-1 = C + 1 / 24Si + 1 / 6Mn + 2Nb
[0010]
(2) By mass%
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
Containing
Ni: 0.3% or less,
Cu: 0.3% or less,
Mo: 0.1% or less,
V: One or more of 0.03% or less is further contained, A _GB-2 is in the range of 0.39 to 0.47, and the balance is substantially made of iron and inevitable impurities. Steel with excellent toughness of weld heat-affected zone, characterized in that it does not contain.
Here, A _GB-2 = C + 1 / 24Si + 1 / 6Mn + 1/40 (Ni + Cu) + 1 / 4Mo + 2Nb + 1 / 14V
[0011]
(3) In mass%,
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
A _GB-1 is in the range of 0.39 to 0.47, and the balance is made of iron and unavoidable impurities and substantially free of Al. A method for producing steel with excellent toughness of a heat affected zone of welding, characterized by performing a heat treatment after reheating to a temperature.
Here, A _GB-1 = C + 1 / 24Si + 1 / 6Mn + 2Nb
[0012]
(4) By mass%
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
Containing
Ni: 0.3% or less,
Cu: 0.3% or less,
Mo: 0.1% or less,
V: One or more of 0.03% or less is further contained, A _GB-2 is in the range of 0.39 to 0.47, and the balance is substantially made of iron and inevitable impurities. A method for producing a steel having excellent toughness of a weld heat affected zone, characterized in that a steel containing no slab is made into a slab by a continuous casting method, and then reheated to a temperature of 1200 ° C. or lower and then subjected to a heat treatment.
Here, A _GB-2 = C + 1 / 24Si + 1 / 6Mn + 1/40 (Ni + Cu) + 1 / 4Mo + 2Nb + 1 / 14V
[0013]
DETAILED DESCRIPTION OF THE INVENTION
According to the study by the present inventors, the brittle crack generation characteristic (CTOD characteristic at a temperature of about −10 ° C.) of small to medium heat input (for example, 1.5 to 60 kJ / mm at 50 mm) welding HAZ is extremely local. It was clarified that the toughness of this region is dominant and the microstructure control of this part is important.
[0014]
In the study by the present inventors, it has been found that the local region having the greatest influence on the CTOD characteristics is grain boundary ferrite (GBF), M ^* , and FSP in the vicinity of the prior austenite grain boundary.
[0015]
For this reason, the present inventors have examined the transformation behavior from austenite on the premise of welding heat history, and the transformation start temperature of ferrite from austenite has a great influence on the formation of microstructure, and when the transformation start temperature is high, the GBF I found out that M ^* and FSP are easy to generate.
[0016]
However, it has been found that even when the transformation start temperature is low, Bu is generated and the toughness deteriorates.
[0017]
Also, transformation start temperature is known to determined by the steel composition and heat history, consider the steel component to obtain a good microstructure in the range of 0.39 to 0.47 in the following A _GB formula It has been found that the generation of GBF, M ^* , and FSP that degrade toughness can be suppressed by being controllable.
A _GB-1 = C + 1 / 24Si + 1 / 6Mn + 2Nb
A _GB-2 = C + 1 / 24Si + 1 / 6Mn + 1/40 (Ni + Cu) + 1 / 4Mo + 2Nb + 1 / 14V
[0018]
Nb and Mn are the most effective for optimizing the steel components, but the addition amount also has an appropriate range as follows.
[0019]
If Nb is less than 0.01%, the effect of optimizing the microstructure is small, and if it exceeds 0.03%, the formation of Bu and the precipitation effect of Nb become noticeable in multi-layer welding and the toughness is deteriorated. 0.03% is the appropriate range.
[0020]
Mn is an inexpensive element that has a great effect on optimizing the microstructure. Therefore, it is desirable to increase the amount of addition, but if it exceeds 2.5%, the upper limit is set to 2.5% in order to easily generate Bu harmful to toughness. .
[0021]
It was also less 2.0% lower limit the effect is small (excluding 2.0%).
[0022]
Only by limiting these, good toughness cannot be obtained.
[0023]
That is, the limitation of the appropriate range of the _AGB type is effective in controlling the microstructure near the grain boundary, but the effect cannot be expected in the grains.
[0024]
In order to control the microstructure in the grains, fine Ti oxides and Ti nitrides are required, and therefore Ti, O, and N must be limited to the following ranges.
[0025]
Ti produces Ti oxide and Ti nitride to refine the microstructure inside the grains and improves toughness, but less than 0.005% is less effective, and more than 0.02% produces Ti carbide. It becomes easy and deteriorates toughness, so 0.005 to 0.02% is an appropriate range.
[0026]
O is necessary for the production of Ti oxide, but if it exceeds 0.0035%, coarse Ti oxide is produced and the toughness is extremely deteriorated, so the upper limit was made 0.0035%.
[0027]
N is necessary for Ti nitride formation, but if it is less than 0.003%, the effect is small, and if it exceeds 0.006%, surface flaws occur during the manufacture of steel slabs, so the upper limit was made 0.006%.
[0028]
The reasons for limiting the other steel components will be described below.
[0029]
C needs to be 0.03% or more in order to obtain high strength, but if it exceeds 0.09%, the toughness of the base material and HAZ is impaired, so 0.09% is made the upper limit.
[0030]
Si is preferable to be small in order to obtain good HAZ toughness, but 0.05% or more is necessary in terms of deoxidation because Al is not added in the invention steel. However, if over 0.2%, the HAZ toughness is impaired, so 0.2% is made the upper limit.
[0031]
P and S should be less in terms of base metal toughness and HAZ toughness, but there are also restrictions on industrial production, so 0.008% and 0.005% were made the upper limits, respectively.
[0032]
Al is preferably contained in a small amount to form a Ti oxide, so that Al is not substantially contained. However, there are restrictions on industrial production, and 0.004% is an allowable upper limit. Therefore, “substantially not containing Al” means that the Al content is 0.004% or less (including 0%).
[0033]
Furthermore, the purpose of adding Ni, Cu, Mo, V to the basic components is because it has little effect on the HAZ toughness and is effective in improving the strength of the base material.
[0034]
However, if Ni: 0.3%, Cu: 0.3%, Mo: 0.1%, V: over 0.03%, not only will the HAZ toughness be impaired, but also the cost of the steel material will be affected. It was.
[0035]
Even if the components of the steel are limited as described above, the intended effect cannot be exhibited unless the manufacturing method is appropriate. For this reason, it is necessary to limit the manufacturing conditions.
[0036]
The steel of the present invention is industrially required to be produced by a continuous casting method.
[0037]
The reason for this is that the solidification cooling rate of the molten steel is fast, and a large amount of fine Ti oxide and Ti nitride can be generated in the slab.
[0038]
In rolling the slab, the reheating temperature needs to be 1200 ° C. or less.
[0039]
This is because when the reheating temperature exceeds 1200 ° C., the Ti nitride becomes coarse and an effect of improving the HAZ toughness cannot be expected.
[0040]
Next, the heat treatment is essential for the manufacturing method after reheating.
[0041]
The reason is that even if excellent HAZ toughness is obtained, it is insufficient as a steel material having poor base material toughness.
[0042]
Examples of the heat treatment method include 1) controlled rolling, 2) controlled rolling-accelerated cooling, 3) direct quenching-tempering after rolling, and any method may be used, but 2) controlled rolling-accelerated cooling is preferable. Is the law.
[0043]
In addition, even if this steel is manufactured and reheated to a temperature not higher than the Ar ₃ transformation point for the purpose of dehydrogenation, the characteristics of the present invention are not impaired.
[0044]
【Example】
Thick steel plates of various steel components in the converter-continuous casting-thick plate process were manufactured, and the base material strength and the CTOD test of the welded joint were performed.
[0045]
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 / mm with a K groove so that the weld penetration line (FL) is vertical. It carried out in.
[0046]
The CTOD test was performed at the FL position with a size of t (plate thickness) × 2 t and a notch of 50% fatigue crack.
[0047]
Tables 1 and 2 show examples.
[0048]
[Table 1]

[0049]
[Table 2]

[0050]
The steel sheet produced in accordance with the present invention (present invention steel) exhibited a good fracture toughness with a yield strength (YS) of 420 N / mm ² or more and a CTOD value of −10 ° C. of 0.63 mm or more.
[0051]
On the other hand, the comparative steel has the same strength as that of the invention steel, but the CTOD value is inferior, and it is not suitable as a steel sheet used in a severe environment.
[0052]
Comparative Steel 16 has a low Mn and A _GB value, generated by the grain boundary ferrite, island martensite, CTOD value was low.
[0053]
Comparative Steel 17 too high A _GB value, since the addition amount too much of Nb, bainite microstructure lot was value CTOD value is lower for higher hardness values.
[0054]
Comparative Steel 18 C, Si, Mn, although Nb and the like are the same as the invention steels, since such A _GB value is too high, many bainite microstructure, CTOD value was low.
[0056]
In Comparative Steel 19 , A _GB was too high and nitrogen (N) was small, so that a lot of bainite was observed in the microstructure, and the CTOD value was low.
[0057]
【The invention's effect】
The steel produced according to the present invention exhibits excellent toughness by optimizing the prior austenite grain boundaries where the toughness is most deteriorated during welding and the microstructure in the vicinity thereof.
[0058]
This made it possible to manufacture high-strength steel materials used in harsh environments such as offshore structures and earthquake-resistant buildings.

Claims (4)

% By mass
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
In the range of A _GB-1 in the range of 0.39 to 0.47, the balance is excellent in toughness of the heat affected zone, characterized in that the balance is substantially free of Al consisting of iron and inevitable impurities. steel.
Here, A _GB-1 = C + 1 / 24Si + 1 / 6Mn + 2Nb % By mass
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
Containing
Ni: 0.3% or less,
Cu: 0.3% or less,
Mo: 0.1% or less,
V: One or more of 0.03% or less is further contained, A _GB-2 is in the range of 0.39 to 0.47, and the balance is substantially made of iron and inevitable impurities. Steel with excellent toughness of weld heat-affected zone, characterized in that it does not contain.
Here, A _GB-2 = C + 1 / 24Si + 1 / 6Mn + 1/40 (Ni + Cu) + 1 / 4Mo + 2Nb + 1 / 14V % By mass
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
A _GB-1 is in the range of 0.39 to 0.47, and the balance is made of iron and unavoidable impurities and substantially free of Al. A method for producing steel with excellent toughness of a heat affected zone of welding, characterized by performing a heat treatment after reheating to a temperature.
Here, A _GB-1 = C + 1 / 24Si + 1 / 6Mn + 2Nb % By mass
C: 0.03 to 0.09%,
Si: 0.05-0.2%
Mn: 2.0 to 2.5% (excluding 2.0%)
P: 0.008% or less,
S: 0.005% or less,
Ti: 0.005 to 0.02%,
Nb: 0.01-0.03%,
O: 0.0035% or less,
N: 0.003 to 0.006%
Containing
Ni: 0.3% or less,
Cu: 0.3% or less,
Mo: 0.1% or less,
V: One or more of 0.03% or less is further contained, A _GB-2 is in the range of 0.39 to 0.47, and the balance is substantially made of iron and inevitable impurities. A method for producing a steel having excellent toughness of a weld heat affected zone, characterized in that a steel containing no slab is made into a slab by a continuous casting method, and then reheated to a temperature of 1200 ° C. or lower and then subjected to a heat treatment.
Here, A _GB-2 = C + 1 / 24Si + 1 / 6Mn + 1/40 (Ni + Cu) + 1 / 4Mo + 2Nb + 1 / 14V