JP3850913B2 - Manufacturing method of high strength bend pipe with excellent weld metal toughness - Google Patents

Description

【００２８】
【表２】

【００２９】
【表３】

【００３０】
【表４】

【００３１】
【表５】

【００３２】
本発明の鋼管は優れた溶接金属部の強度・低温靱性を有するのに対して、比較鋼は化学成分または鋼管製造条件が適切でなく、いずれかの特性が劣る。鋼９はＣ量が多過ぎるため、低温靱性（シャルピー吸収エネルギー、遷移温度）が悪い。鋼１０はＭｎ量が低過ぎるため、低温靱性が悪い。鋼１１はＮｂが添加されていないため、低温靱性が悪い。鋼１２はＴｉが添加されていないため、低温靱性が劣る。鋼１３はＣｒ量が多過ぎるため、低温靱性が劣る。鋼１４はＣｒ量が少な過ぎるため、Ｔ−クロス部の硬さが高く、溶接性が悪い。鋼１５はＰ値が小さ過ぎるため、低温靱性が悪い。鋼１６はＰ値が高過ぎるため、低温靱性が悪い。鋼１７は鋼管の再加熱温度が高過ぎるため、低温靱性が悪い。鋼１８は鋼管の再加熱温度が低過ぎるため、強度が低い。鋼１９は曲げ加工後空冷したため、強度が低い。
【００３３】
【発明の効果】
本発明により低温靱性に優れた極厚高強度ベンド管（ＡＰＩ規格Ｘ６５以上）が安定して製造できるようになった。その結果、パイプラインの輸送効率の向上が可能となった。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a bend pipe (bent pipe) excellent in strength of API standard X65 or higher and toughness of a welded portion.
[0002]
[Prior art]
Line pipes (straight pipes) and deformed pipes (bend pipes, elbow pipes, T-shaped pipes, etc.) used for pipelines that transport crude oil and natural gas have excellent strength, low temperature toughness, and weldability from the viewpoint of safety. Etc. are required. In particular, as the pipeline laying area becomes colder and deeper, securing low temperature toughness at, for example, −60 ° C., and increasing the thickness are demanded.
[0003]
Conventionally, since the mechanical properties (strength, low temperature toughness, etc.) of a steel pipe are deteriorated in a bend pipe or the like as compared with a straight pipe, JP-A-62-210212, JP-A-4-154913, and JP-A-7 -3330, JP-A-5-279743, JP-A-59-232225, JP-A-61-117223, etc., various methods for improving the mechanical properties of the bend pipe are proposed. Has been.
[0004]
For example, those described in JP-A-62-110212, JP-A-4-154913, JP-A-7-3330, and JP-A-5-279743 are quenched after being heated and bent. And then tempering within a specific range. However, these methods have problems from the viewpoint of productivity and manufacturing cost because tempering treatment is essential.
[0005]
Japanese Patent Laid-Open No. 61-117223 discloses a method of manufacturing a bend pipe from a steel pipe having a weld metal having a specific component by omitting the tempering treatment. However, when the thickness exceeds 20 mm, it is necessary to increase the welding heat input when welding the steel pipe, and in order to ensure the strength and good low temperature toughness of the weld metal part, It is necessary to determine the appropriate value for the weld metal component. That is, when the welding heat input is increased, the cooling rate after welding is slowed down, so that coarse grain boundary ferrite is generated from the prior austenite (γ) grain boundary and the strength and low temperature toughness are deteriorated. Therefore, development of a bend pipe having excellent productivity, high strength and excellent weld toughness at low temperatures has been strongly desired.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a manufacturing technique of a bend pipe having excellent productivity, high strength and excellent weld toughness at low temperature at a low cost.
[0007]
[Means for Solving the Problems]
That is, the gist of the present invention is as follows.
(1) By weight, C: 0.03 to 0.10%, Si: 0.3% or less, Mn: 1.67 to 2.2%, P: 0.015% or less, S: 0.030 %: Cr: 0.05-0.5%, Nb: 0.01-0.10%, Ti: 0.005-0.030%, Al: 0.05% or less, N: 0.001- Welding containing 0.010%, O: 0.04% or less, the balance being iron and inevitable impurities, and the P value defined by the following formula in the range of −0.010 to 0.010 A method for producing a high-strength bend pipe having excellent weld metal part toughness, wherein a steel pipe having a metal part is rapidly cooled while being bent at a temperature of 900 to 1000 ° C.
[0008]
P = {1.5 (O−0.89Al) + 3.4N} −Ti
(2) in wt%, C: 0.03~0.10%, Si : 0.3% or less, Mn: 1.67 ~2.2%, P : 0.015% or less, S: 0.030 %: Cr: 0.05-0.5%, Nb: 0.01-0.10%, Ti: 0.005-0.030%, Al: 0.05% or less, N: 0.001- 0.010%, O: 0.04% or less, further Ni: 0.1-1.0%, Cu: 0.1-1.0%, Mo: 0.1-1.0%, V: 0.01 to 0.10%, B: 0.0003 to 0.003%, Ca: 0.001 to 0.005%, or one or more, containing the balance of iron and inevitable A steel pipe having a weld metal part made of impurities and having a P value defined by the following formula in the range of −0.010 to 0.010 is heated to 900 to 1000 ° C. and then bent. Preparation of high strength bent pipe with excellent weld metal toughness immediately characterized by rapidly cooling while.
[0009]
P = {1.5 (O−0.89Al) + 3.4N} −Ti
Below, the manufacturing method of the high intensity | strength bend pipe | tube which has the outstanding weld metal part toughness of this invention is demonstrated in detail.
Conventionally, it is known that high strength and good low-temperature toughness can be ensured by heating and quenching a low carbon-Nb steel pipe after bending (Japanese Patent Laid-Open No. 1-444769). However, when the thickness is increased, the welding heat input is inevitably increased and the cooling rate after welding is decreased, so that coarse grain boundary ferrite or the like is generated and the low temperature toughness of the weld metal part is deteriorated. For this reason, it is necessary to further increase the amount of alloying elements. However, when the amount of alloying elements of the weld metal is simply increased, at the time of joint welding at the laying site, the portion where the steel pipe longitudinal direction welding (seam welding) and the local joint welding intersect (T-cross part) Hardness increases and problems such as cracking occur.
[0010]
Therefore, the present inventors have intensively studied to improve the low-temperature toughness of the ultra-thick high-strength bend pipe welded portion and reduce the hardness of the T-cross portion. As a result, the present invention has been completed. That is, the present invention provides (1) a weld metal component that includes low C-high Mn-Cr-Nb-trace amount Ti and that optimizes each addition amount in consideration of the balance of Al, N, oxygen, and Ti amount. (2) The steel pipe is heated and then bent, and then immediately hardened. As a result, high strength and excellent low temperature toughness, and further reduction in hardness of the T-cross portion can be achieved at the same time.
[0011]
The low temperature toughness of the weld metal part in the longitudinal direction of the steel pipe is governed by various metallurgical factors such as (1) the size of crystal grains and (2) the dispersion state of the hardened phase such as island martensite. In particular, since the heat input increases as the thickness increases, the cooling rate after welding inevitably decreases, and coarse ferrite or the like is generated, resulting in deterioration of low-temperature toughness. In this case, it is essential to suppress the formation of coarse grain boundary ferrite by increasing the additive amount of the alloy element. However, since an increase in the amount of alloy element added causes an increase in hardness at the T-cross portion, it is important to optimize the type and amount of the alloy element to be added. In particular, according to the present invention, the simultaneous addition of Mn and Cr is extremely effective in improving low-temperature toughness and reducing the hardness of the T-cross part by suppressing the coarse grain boundary ferrite in the longitudinal weld metal part. They found out. It was also found that the low temperature toughness can be drastically improved by optimizing the balance of the amounts of Al, N, oxygen and Ti. That is, in the formula expressed by P = {1.5 (O−0.89Al) + 3.4N} −Ti, each component is optimized so that the P value becomes −0.010 to 0.010%. As a result, the low temperature toughness is improved.
[0012]
In order to exhibit the above effects sufficiently, the Mn amount needs to be 1.67 to 2.2%, and the Cr amount needs to be 0.05 to 0.5%. When the amount of Mn and Cr is small, the welding heat input is increased in the thick material, the cooling rate after welding is decreased, coarse grain boundary ferrite is generated, and the low temperature toughness is deteriorated. In order to suppress coarse grain boundary ferrite, the Mn content is required to be 1.67% or more , and the Cr content is required to be 0.05% or more. On the other hand, when the amount of Mn and Cr increases, the hardenability increases, and island-like martensite is generated to deteriorate the low-temperature toughness. Therefore, the upper limit values are 2.2% for Mn and 0.2% for Cr. 5%.
[0013]
Furthermore, it is necessary to optimize the P value in order to improve the low temperature toughness. The P value indicates an excess or deficiency of the Ti amount. When the P value is low (minus), Ti is excessively added, and low temperature toughness deteriorates due to precipitation hardening of TiC or the like. On the other hand, when the P value is high (plus), the Ti amount is insufficient (or the oxygen amount is excessive), so that the low temperature toughness deteriorates. In order to obtain good low temperature toughness, the P value needs to be -0.010 to 0.010%.
[0014]
Al is an element usually contained in steel as a deoxidizer, and has an effect on refinement of the structure. However, if the Al content exceeds 0.05%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.
Addition of Ti forms fine Ti oxides and TiN, suppresses the coarsening of austenite grains during reheating, refines the microstructure as the nucleus of intragranular transformation ferrite during γ-α transformation, and reduces low temperature toughness. Improve. In order to exhibit such an effect, at least 0.005% of Ti should be added. However, if the amount of Ti is too large, precipitation hardening due to TiC occurs and low temperature toughness deteriorates, so the upper limit must be limited to 0.030%.
[0015]
N forms TiN and suppresses coarsening of austenite grains during reheating and improves low temperature toughness. The minimum amount required for this is 0.001%. However, if the amount of N is too large, it causes toughness deterioration due to an increase in solute N, so the upper limit must be limited to 0.010%.
Reduction of the amount of O is effective in improving low temperature toughness by reducing oxides in steel. Therefore, the lower the amount of O, the better. If the amount of O is too large, the cleanliness of steel deteriorates and the low temperature toughness deteriorates, so the upper limit value is made 0.04%.
[0016]
Next, the reasons for limiting other component elements of the present invention will be described.
The lower limit of 0.03% of C is the minimum amount for ensuring the strength of the welded portion, low temperature toughness, precipitation hardening by adding Nb and V, and the effect of refining crystal grains. However, if the amount of C is too large, the low temperature toughness and on-site weldability (T-cross part hardness) are significantly deteriorated, so the upper limit was made 0.10%.
[0017]
Si is an element added for deoxidation and strength improvement, but if added in a large amount, the low temperature toughness deteriorates, so the upper limit was made 0.3%. Only Ti or Al is sufficient for deoxidizing the weld metal.
Nb contributes to refinement of crystal grains and precipitation hardening, and has an effect of strengthening steel. As a minimum amount for exhibiting this effect, the lower limit was set to 0.01%. However, Nb addition exceeding 0.10% adversely affects on-site weldability and low temperature toughness, so the upper limit was made 0.10%.
[0018]
Furthermore, in the present invention, the amounts of impurity elements P and S are set to P: 0.015% or less and S: 0.030% or less, respectively. The main reason is to further improve the low temperature toughness. Reduction of the P content reduces segregation when the weld metal solidifies, prevents grain boundary fracture and improves low temperature toughness. Moreover, the reduction of the amount of S has the effect of improving ductility. Therefore, the lower the amount of P and S, the better.
[0019]
Next, the reason for adding Ni, Cu, Mo, V, B, and Ca will be described.
The main purpose of adding these elements to the basic components is to increase the plate thickness that can be produced and to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is a property that should be limited by itself.
[0020]
The purpose of adding Ni is to improve the strength of the low carbon steel of the present invention without degrading the low temperature toughness. In order to exert this effect, addition of 0.1% or more is necessary. However, if the addition amount is too large, not only the economic efficiency but also the local weldability and low temperature toughness are deteriorated, so the upper limit was made 1.0%.
Cu has substantially the same effect as Ni, and is also effective in improving the corrosion resistance and the resistance to hydrogen-induced cracking. In addition, the strength is greatly increased by Cu precipitation hardening. In order to exert this effect, addition of 0.1% or more is necessary. However, if added in excess, low temperature toughness is reduced by precipitation hardening, so the upper limit was made 1.0%.
[0021]
Mo has the effect of increasing the strength of the weld. In order to obtain such an effect, Mo needs to be at least 0.1%. However, excessive addition of Mo deteriorates low temperature toughness and on-site weldability, so the upper limit was made 1.0%.
V has substantially the same effect as Nb. In order to exert this effect, it is necessary to add 0.01% or more of V. The upper limit of V is acceptable up to 0.10% from the viewpoint of on-site weldability and low temperature toughness.
[0022]
B is extremely small and dramatically increases the hardenability of the steel. In order to obtain such an effect, B must be at least 0.0003%. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the effect of improving the hardenability of B may be lost, so the upper limit was made 0.003%.
Ca controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test). However, when the Ca content is less than 0.001%, there is no practical effect. When the Ca content exceeds 0.005%, a large amount of CaO-CaS is formed, resulting in clusters and large inclusions. Not only harms, but also adversely affects on-site weldability. For this reason, the amount of Ca added is limited to 0.001 to 0.005%.
[0023]
Next, the reasons for limiting the manufacturing conditions of the present invention will be described.
In the present invention, the steel pipe needs to be reheated to a temperature range of 900 to 1000 ° C., bent, and then quenched.
The reason why the heating temperature of the steel pipe is set to 900 ° C. or more is to sufficiently dissolve the alloy elements in the austenite region and improve the strength. However, when the heating temperature exceeds 1000 ° C., austenite grains at the time of heating grow and the crystal grains become large, resulting in deterioration of low-temperature toughness, or a predetermined dimension of the bend tube cannot be obtained. For this reason, the upper limit of heating temperature was 1000 degreeC.
[0024]
After heating, the steel pipe needs to be bent and immediately quenched. This is to obtain high strength and excellent low temperature toughness by quenching immediately after bending. If the steel pipe is not quenched immediately after bending, the temperature of the steel pipe will decrease and high strength cannot be achieved due to the formation of ferrite and the like. The cooling rate during the quenching process is preferably 15 ° C./second or more.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described.
Steel plates having the components shown in Table 1 were welded to produce steel pipes. The forming method is UOE and BR (bending roll). Thereafter, bend pipes were manufactured from steel pipes having various weld metal components, and various properties were investigated. The mechanical properties were investigated in the direction perpendicular to rolling.
[0026]
Examples are shown in Table 2, Table 3 (Continuation 1 in Table 2), Table 4 (Continuation 2 in Table 2), and Table 5 (Continuation 3 in Table 2).
[0027]
[Table 1]

[0028]
[Table 2]

[0029]
[Table 3]

[0030]
[Table 4]

[0031]
[Table 5]

[0032]
The steel pipe of the present invention has excellent weld metal strength and low temperature toughness, whereas the comparative steel has inadequate chemical composition or steel pipe manufacturing conditions and is inferior in any of the characteristics. Since steel 9 has too much C content, low temperature toughness (Charpy absorption energy, transition temperature) is poor. Steel 10 has a low Mn content and therefore low temperature toughness. Steel 11 has poor low-temperature toughness because Nb is not added. Since the steel 12 is not added with Ti, the low temperature toughness is inferior. Since the steel 13 has too much Cr, the low temperature toughness is inferior. Since the amount of Cr in the steel 14 is too small, the hardness of the T-cross portion is high and the weldability is poor. Since the P value of steel 15 is too small, the low temperature toughness is poor. Since the steel 16 has an excessively high P value, the low temperature toughness is poor. Steel 17 has poor low-temperature toughness because the reheating temperature of the steel pipe is too high. Steel 18 has low strength because the reheating temperature of the steel pipe is too low. Since the steel 19 is air-cooled after bending, the strength is low.
[0033]
【The invention's effect】
According to the present invention, an ultra-thick high-strength bend pipe (API standard X65 or higher) having excellent low-temperature toughness can be stably produced. As a result, pipeline transportation efficiency can be improved.

Claims (2)

% By weight
C: 0.03-0.10%,
Si: 0.3% or less,
Mn: 1.67 to 2.2%,
P: 0.015% or less,
S: 0.030% or less,
Cr: 0.05 to 0.5%,
Nb: 0.01-0.10%,
Ti: 0.005 to 0.030%,
Al: 0.05% or less,
N: 0.001 to 0.010%,
O: A steel pipe having a weld metal part containing 0.04% or less, the balance being iron and inevitable impurities, and a P value defined by the following formula in the range of −0.010 to 0.010 A method for producing a high-strength bend pipe having excellent weld metal toughness, characterized in that, after being heated to 900 to 1000 ° C., it is immediately cooled while being bent.
P = {1.5 (O−0.89Al) + 3.4N} −Ti % By weight
C: 0.03-0.10%,
Si: 0.3% or less,
Mn: 1.67 to 2.2%,
P: 0.015% or less,
S: 0.030% or less,
Cr: 0.05 to 0.5%,
Nb: 0.01-0.10%,
Ti: 0.005 to 0.030%,
Al: 0.05% or less,
N: 0.001 to 0.010%,
O: 0.04% or less, further Ni: 0.1-1.0%,
Cu: 0.1 to 1.0%
Mo: 0.1 to 1.0%,
V: 0.01 to 0.10%,
B: 0.0003 to 0.003%,
Ca: 0.001 to 0.005%
1 or 2 or more of them, the balance being iron and inevitable impurities, and having a weld metal part having a P value defined by the following formula in the range of -0.010 to 0.010 A method for producing a high-strength bend pipe having excellent weld metal toughness, characterized in that the steel pipe is heated to 900 to 1000 ° C. and then immediately cooled while being bent.
P = {1.5 (O−0.89Al) + 3.4N} −Ti