KR20090016854A - Ultra high strength welding joint of 950mpa grade having excellent low temperature toughness - Google Patents

Abstract

A welding joint having more than 950MPa of tensile strength is provided to control a range of a carbon equivalent formula for a welding metal in order to secure ultra high tensile strength more than 950MPa as well as high toughness at a low temperature. A welding joint having more than 950MPa of tensile strength comprises the following components. A welding metal composed of a base metal and a filler metal satisfies a carbon equivalent value(Ceq) of the following formula in a range from 0.7% to 0.8%. Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Ni]+[Cu])/15. The welding metal comprises more than 60 area% of acicular ferrite, bainite, 10~20 area% of martensite.

Description

Ultra high strength welding joint of 950mpa grade having excellent low temperature toughness}

The present invention relates to an ultra high strength welded joint of 950 MPa or higher grade having excellent low temperature toughness of a welded structure, particularly a welded structure constructed by welding an ultra high strength steel having a tensile strength of 950 MPa or higher. More specifically, the present invention relates to a welded joint having excellent low temperature toughness and ultra high strength of 950 MPa or more by controlling the carbon equivalent and structure of the weld metal formed by the base metal and the filler metal.

In recent years, as the development of oil and natural gas resources has gradually moved to a cold district, the importance of securing excellent low-temperature toughness is gradually increasing so that marine structures, ships, and pipelines required for this operation can be stably operated below -30 ° C.

In addition, in order to improve the storage and transportation efficiency, and to reduce the construction cost by reducing the thickness and size of the structure, the high strength of the steel is urgently required.

In general, since steels tend to decrease in toughness as strength increases, many efforts have been made to derive a balance between strength and low temperature toughness.

On the other hand, when fabricating the structure, the welded joint formed by the essential welding process undergoes rapid quenching and quenching due to the welding heat source. At this time, the metal histologically, the site becomes coarse grains or a phase transformation occurs in a weak tissue. Quality characteristics inevitably deteriorate compared to the base metal.

Therefore, in securing the strength and low temperature toughness of the structure, the welded joint is always the core, and if the welded joint quality does not achieve the target level, the steel sheet can be manufactured even if the structure can not be constructed. For this reason, various efforts, such as steel design, welding materials, and welding construction technology, have been made to obtain excellent welded joint quality.

Until now, the low temperature toughness of welded joints for steel with 700MPa tensile strength has been secured, but as the demand for higher strength has increased, a structure using ultra-high strength steel of 950MPa grade has been required.

In order to obtain welded joints having the same strength and excellent low temperature toughness for such ultra-high strength steels, there is a limitation in existing technical applications. Even filler metals that satisfy their characteristics are not commercially available.

Therefore, manufacturing a weld joint of very high strength while excellent in low temperature toughness has become one of the major problems in the steel manufacturing and welding processing industry.

Conventionally, in Japanese Unexamined Patent Publication No. 2000-199036, as a means of solving such a problem, the average tensile strength of the weld metal is at least 100 MPa of tensile strength of the steel sheet, and Ni content is 1% or more higher than that of the base metal. Provided are methods for producing line pipes. However, this is limited to providing consequent data and there is a limit to introducing it into an easy and simple practical manufacturing method.

In addition, Japanese Laid-Open Patent Publication No. 2002-115032 reports a method for producing an ultra-high strength line pipe, which is characterized by a weld metal containing 1% or more of retained austenite phase, but this is to improve the low temperature toughness of the weld joint. In addition, the main purpose is to prevent cold cracking, and it is not only helpful to design the weld metal by simple calculation, but also provide consequent information.

The present invention is to improve the above-mentioned conventional problems, to provide a welded joint having excellent low-temperature toughness and ultra-high strength of 950 MPa or more in a welded structure constructed by welding an ultra high strength steel of tensile strength of 950 MPa or more, and an object thereof. There is this.

The present invention for achieving the above object,

In the welded joint of the welded structure, the weld metal formed by the base material and filler metal is in the range of 0.7-0.8% of the carbon equivalent Ceq value shown in the following formula (1), and the structure is acicular ferrite of 60 area% or more. Ultra high strength welded joint of 950MPa grade or more having excellent low temperature toughness composed of a mixed structure of and bainite and martensite of 10 to 20 area%.

Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15)... (One)

However, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] are contents of C, Mn, Cr, Mo, V, Ni and Cu, respectively (% by weight). ).

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The inventors of the present invention closely examine the balance between ultra high strength and low temperature toughness while focusing on steel plate design, filler metal design and welding conditions in order to study a method for manufacturing a super high strength welded joint having a low temperature toughness of 950 MPa or more. The optimum tissue fraction that can be secured was derived, and the proper control of the range of carbon equivalent formula of the weld metal to secure such a tissue fraction can ensure not only low-temperature toughness of the weld joint but also ultra-high strength of 950 MPa or more. The present invention was newly identified and the present invention was completed based on the results.

Hereinafter, the ultra-high strength welded joint of 950 MPa or more excellent in low temperature toughness of the present invention will be described in detail.

In general, in the case of manufacturing the welding metal of 700MPa or less, the component design is aimed at acicular ferrite structure that can obtain the same strength and excellent low temperature toughness. Needle-like ferrite tissues cannot match equal strength, thus requiring optimal mixing of new tissues for strength improvement.

According to the present invention, in the welded joint of the welded structure, the weld metal formed by the base material and the filler metal is a mixed structure of acicular ferrite, bainite, and martensite of 10-20 area% or more. It is done.

If the high martensite structure ratio is high, the strength of the weld metal may be easily achieved, but may be unsatisfactory in terms of low temperature toughness. On the contrary, when the ratio of the needle-like ferrite and lower bainite structure having excellent toughness is increased, the toughness of the weld metal may be improved while the strength may not reach the ultra high strength level. Therefore, it is preferable that the structure of the weld metal is made of a mixed structure of acicular ferrite and acicular ferrite and bainite, and martensite of 10 to 20 area%.

In addition, in order to obtain a mixed structure of the weld metal, the base metal and the filler metal are designed so that the weld metal formed by diluting the base metal and the filler metal under ordinary welding conditions is in the range of 0.7 to 0.8% of the carbon equivalent Ceq value shown below. Should be done.

Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15)... (One)

However, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] are contents of C, Mn, Cr, Mo, V, Ni and Cu, respectively (% by weight). ).

Here, the carbon equivalent Ceq value must be in the range of 0.7 to 0.8 to achieve low-temperature toughness and ultra-high strength balance.At this time, the weld metal structure has acicular ferrite and bainite and bainite and martensite of 10 to 20 area%. It consists of a mixed structure of. When the carbon equivalent Ceq value of the weld metal is less than 0.7, low temperature toughness is excellent but ultra high strength is not achieved, whereas when the carbon equivalent Ceq value is higher than 0.8, ultra high strength may be achieved but low temperature toughness may be reduced.

In addition, according to an embodiment of the present invention, the base material is% by weight, C: 0.03 to 0.10%, Si: 0.6% or less, Mn: 1.6 to 2.1%, Cu: 1.0% or less, Ni: 1.0% or less, Nb : 0.01 to 0.06%, V: 0.01 to 0.1%, Mo: 0.1 to 0.5%, Cr: 1.0% or less, Ti: 0.005 to 0.03%, Al: 0.001 to 0.06%, B: 0.0005 to 0.0025%, N: 0.001 It may be desirable to make up to 0.006%, P: 0.02% or less, S: 0.005% or less, Ca: 0.006% or less, remaining Fe and other unavoidable impurities.

In addition, according to an embodiment of the present invention, the filler metal is in weight percent, C: 0.05 to 0.1%, Si: 0.3 to 0.6%, Mn: 1.0 to 3.0%, Ni: 1.5 to 6.0%, Cr + Mo + V : 0.5-2.5%, N: 0.006% or less, P: 0.02% or less, S: 0.005% or less, O: 0.03% or less, and the remaining Fe and other unavoidable impurities may be preferable.

According to the present invention, there is an effect of providing a welded joint having excellent low-temperature toughness and ultra-high strength of 950 MPa or more in a welded structure constructed by welding an ultra high strength steel of 950 MPa or more of tensile strength.

(Example)

In the preparation of the base material, the slab is obtained by controlling the chemical composition through the refining of molten metal in a 300 ton converter. After that, after reheating at 1100 ° C., the rolling was controlled to 16 mm in which the cumulative reduction was 80% through a rolling process, and thereafter, water cooling was performed at a cooling rate of 20 to 30 ° C./sec to obtain a steel sheet. Table 1 shows the chemical components and mechanical properties of the steel sheet obtained above.

Grater Chemical composition (wt.%) Mechanical properties C Si Mn P S Cr + Mo + V Ni + Cu Ceq TS (MPa) vE-30 ℃ (J) A 0.05 0.15 1.9 0.006 0.001 0.64 0.5 0.52 1018 240 B 0.09 0.15 1.9 0.005 0.001 0.64 0.2 0.55 1077 186

As shown in Table 1, the steel sheet A had a high strength within the target range and at the same time high temperature toughness (absorbed energy at -30 ° C through Charpy V notch impact test). On the other hand, compared with the steel sheet A, the strength of the steel sheet B having a high C and Cr content and a low Mo and Ni content is in the target range, but the low temperature toughness value is rather low.

Submerged arc welding was performed about such a steel plate using the filler metals a-c shown in Table 2 below. Flux used a commercially available sintered high base type and a welding speed of 1.6m / min and a welding heat input of 2.0 ~ 5.0kJ / mm were applied using a three-electrode electrode. After performing experiments on the chemical composition, tissue fraction and mechanical properties of the welded joint, the results are shown in Table 3 below.

Dragonfly Chemical composition (Wt.%) C Si Mn Ni Cr + Mo + V a 0.06 0.51 1.6 2.0 0.53 b 0.08 0.40 2.6 5.7 2.34 c 0.07 0.44 2.3 4.5 1.74

Lost Number Grater Dragonfly Welding heat input (kJ / mm) Weld metal properties Chemical composition Tissue fraction Mechanical properties C Mn Cr + Mo + V Ni + Cu Ceq Martensiite (%) AF + B (%) TS (MPa) vE- 30 ℃ (J) One A b 5.0 0.07 1.8 1.33 2.5 0.8 19 81 1096 126 2 A c 2.8 0.07 1.9 1.0 2.3 0.74 12 88 988 130 3 A c 3.2 0.07 1.9 1.1 2.2 0.75 17 83 1004 144 4 B a 2.0 0.09 2.2 0.68 1.55 0.70 11 89 963 98 5 B c 3.2 0.08 1.9 1.1 1.9 0.74 12 88 971 106 6 A a 3.2 0.06 1.7 0.55 1.2 0.52 0 100 898 94 7 A b 3.2 0.08 2.2 1.45 3.3 0.94 76 24 1114 65 8 B a 3.2 0.08 1.7 0.6 1.1 0.55 0 100 904 43 9 B b 3.2 0.1 2.2 1.41 2.9 0.95 84 16 1102 50

As shown in Table 3, in the case of the invention examples (1 to 5) satisfying the carbon equivalent range of the present invention, the mixed structure of the target martensite with a target area of 10 to 20 area%, and at least 80% acicular ferrite and bainite Could secure. Accordingly, it can be seen that the inventive examples (1 to 5) exhibited excellent absorption energy of 90J or more even at -30 ° C while satisfying ultrahigh strength of 950 MPa or more.

On the other hand, Comparative Examples (6 and 8) showed a value of carbon equivalent (Ceq) of the weld metal is smaller than the limited range (0.7 ~ 0.8) of the present invention. Therefore, due to this component system, the hardenability during cooling after welding is low, and thus no martensite transformation, which plays a major role in strength increase, is achieved (martensite fraction of 0%). As a result, the target tensile strength cannot be secured. It was.

In Comparative Examples (7 and 9), the carbon equivalent (Ceq) of the weld metal was larger than the limited range of the present invention. This excessive component system increases the hardenability during cooling after welding, resulting in excessive martensite formation, which plays a major role in the strength increase (martensite fraction 76% (Comparative Example 7), 84% (Comparative Example 9)). On the other hand, the fraction of acicular ferrite, which has a favorable role in improving low temperature toughness, was much smaller than the target 60% (24% (Comparative Example 7), 16% (Comparative Example 9)). As a result, the tensile strength of the weld metal was 1100 MPa or more, which is higher than the 950 MPa grade, but the low temperature toughness of Comparative Example 7 65J and Comparative Example 9 50J at −30 ° C. showed unsatisfactory characteristics.

Claims (3)

In the welded joint of the welded structure, the weld metal formed by the base material and filler metal is in the range of 0.7-0.8% of the carbon equivalent Ceq value shown in the following formula (1), and the structure is acicular ferrite of 60 area% or more. Ultra high strength welded joint of 950MPa grade or more with excellent low temperature toughness composed of mixed structure of bainite and martensite of 10 ~ 20 area%. Ceq = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 15)... (One) However, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] are contents of C, Mn, Cr, Mo, V, Ni and Cu, respectively (% by weight). ). According to claim 1, wherein the base material is by weight, C: 0.03 ~ 0.10%, Si: 0.6% or less, Mn: 1.6 ~ 2.1%, Cu: 1.0% or less, Ni: 1.0% or less, Nb: 0.01 ~ 0.06 %, V: 0.01-0.1%, Mo: 0.1-0.5%, Cr: 1.0% or less, Ti: 0.005-0.03%, Al: 0.001-0.06%, B: 0.0005-0.0025%, N: 0.001-0.006%, P: 0.02% or less, S: 0.005% or less, Ca: 0.006% or less, ultra-high strength welded joint of 950 MPa or more having excellent low temperature toughness composed of remaining Fe and other unavoidable impurities. The filler metal according to claim 1 or 2, wherein the filler metal is in weight percent, C: 0.05 to 0.1%, Si: 0.3 to 0.6%, Mn: 1.0 to 3.0%, Ni: 1.5 to 6.0%, Cr + Mo + V. : 0.5 ~ 2.5%, N: 0.006% or less, P: 0.02% or less, S: 0.005% or less, O: 0.03% or less, ultra high strength welded joint of 950 MPa or more with excellent low temperature toughness made of remaining Fe and other unavoidable impurities .