JP2000001739A - Welded joint excellent in corrosion resistance and fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded joint combining excellent corrosion resistance with high fatigue strength in a weld zone to a steel structures including bridges and steel towers in a seaside district and a district where there is the fear of salt damage caused by scattering of snow-melting salt or the like. SOLUTION: This welded joint uses a steel material as a base material containing, by weight, 0.02-0.20% C, 0.1-under 1.6% Si, 0.3-2.0% Mn, 0.3-1.5% Cu 1.0-5.5% Ni, over 0.05-0.30% Nb, 0.005-0.070% Al and the balance Fe with inevitable impurities and in which 0.35<=Ceq(%)<=0.65[Ceq(%)=C+Mn/6+(Cu+ Ni)/15+(Mo+V)/5+Nb/3] and also the contents of P, S, and Cr among the inevitable impurities are <=0.030%, <=0.01% and <=0.1%, respectively. Further, the fraction of bainite in the heat-affected structure of the welded joint part is >=60%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to steel structures such as bridges, steel towers, etc., which are exposed to an atmospheric environment in which salt damage is likely to occur in a high flying sea salt particle environment, such as a beach area or an area where snow melting salt is sprayed. It relates to a welded joint that has both excellent corrosion resistance and fatigue strength.

[0002]

2. Description of the Related Art In recent years, in welding structures such as bridges,
The importance of not only reducing construction costs but also controlling costs including long-term maintenance and inspection costs is becoming an issue. Along with this, attention has been paid to technologies for long-term reliability of welded structures against corrosion and fatigue failure.

[0003] In other words, corrosion prevention of steel structural members used in places where salt damage occurs, such as coastal areas, includes painting of ordinary steel materials, use of plated steel sheets, use of surface coating such as thermal spraying and mortar lining, and use of stainless steel and titanium. Use of high alloy and high corrosion resistant materials such as However, in recent years, it has been studied to use a weather-resistant steel material (JIS G3141 weather-resistant steel for welded structures) without painting for the technical purpose of reducing the maintenance cost of a steel structure.

[0004] In the case of a weather-resistant steel material, when it is used without painting, a fine stable rust having excellent corrosion resistance is formed on the surface of the steel material within several years to ten years after use, and this stable rust causes the progress of corrosion of the steel material thereafter. It is a steel material that prevents it. For steel structures, weather-resistant welded structural steel in consideration of weldability has been widely used mainly for bridges and buildings. However, as shown in the “Design and Construction Guidelines for Unpainted Weatherproof Bridges (Revised): Ministry of Construction, Public Works Research Institute, Steel Club, Japan Bridge Construction Association, March 1993,” In an area where the amount of incoming sea salt particles is large, such as an area to be sprayed, there is a problem that the formation of a stable rust with excellent protection is hindered by the salt attached to the surface of the steel material, so that it is not suitable for unpainted use.

[0005] With respect to the improvement of the weather resistance of the weather-resistant steel in the seaside area, for example, in the invention of Japanese Patent Publication No. 56-9356,
A steel material containing P (0.03 to 0.20%) and having excellent weldability and having excellent weather resistance in a corrosion phenomenon involving seawater and a general atmospheric environment is disclosed. Also, JP-A-2-125
No. 839, the low Si-P-Cu-N
It is stated that the addition of a composite oxide of Ca and Al is effective for the addition of i. In the invention of JP-A-3-223852, it is effective to finely disperse an oxide in a steel material to suppress a decrease in the pH of the steel material surface. Further, in the invention described in JP-A-6-59342,
It is stated that the composite addition of i-Cr-Al is effective.

As described above, steel materials having excellent weather resistance in the Umehama area, which is a disadvantage of conventional weather resistant steel, have been developed.
It is necessary to improve reliability against fatigue fracture, which is another problem. In particular, the weld joint often becomes a starting point of fatigue fracture due to stress concentration and the presence of welding defects. A number of techniques for improving the fatigue fracture strength have been disclosed so far, but most of them relate to improving the fatigue strength of a base material of a thin steel sheet or a spot weld. For example, Japanese Unexamined Patent Publication No. 61-96057 discloses that the fatigue strength can be improved by setting the area ratio of bainite of welder to 5 to 60%.

Although the relationship between the microstructure of the heat affected zone and the fatigue strength has not been clarified so far, Japanese Patent Application Laid-Open No. Hei 5-34592 discloses that the fatigue strength of the HAZ structure depends on the formation of island martensite. It has been shown to improve. That is, the hard island martensite is HA
It is described that when present in the Z structure, the micro fatigue crack once generated is prevented or retarded from propagating, and the fatigue strength is substantially improved.

Further, the inventors of the present invention have developed fatigue cracks in welds.
A systematic experiment on propagation and its microstructure dependence revealed that the HAZ microstructure that most effectively suppresses the initiation and propagation of fatigue cracks is ferrite, ie, the carbon equivalent value ( There has been previously proposed a method of increasing the HAZ ferrite structure fraction by limiting Ceq) and improving the fatigue strength of the welded contact portion (JP-A-8-73983).

[0009] Further, the present inventors have found that 60 to 80 kgf / m
If HAZ structure as m ² class high strength steel is bainite, suppression of fatigue crack generation and propagation, Si added and Ceq
Clarifies that the limitation of the above is effective. In other words, it is proposed that the addition of Si has the effect of strengthening the lath boundary by solid solution strengthening of the ferrite in bainite while suppressing martensitic transformation, and it is possible to improve the fatigue strength of welders by strengthening the entire bainite by limiting Ceq. (JP-A-8-209295). Furthermore, high Nb
It is clear that addition is effective.

However, in these inventions, no mention is made of the corrosion resistance, and it is not clear whether or not they can be used without painting. As described above, heretofore, a welded joint that can be used without painting even in a beach area and has high fatigue strength has not been obtained.

[0011]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a welding method in which a stable rust is formed in an environment where there is concern about salt damage and excellent weather resistance is exhibited, and at the same time, the welding HAZ has high reliability against fatigue fracture. It is intended to provide a joint.

[0012]

Means for Solving the Problems The present inventors have paid attention to the fact that it is difficult for a weather-resistant steel to form a protective rust film having excellent corrosion resistance in an air environment with a lot of flying sea salt particles. We have been conducting research based on the composition of low alloy steels. As a result, Cr, which is effective in improving the weather resistance of steel materials in inland areas where the amount of incoming sea salt particles is small, is less resistant to weathering in environments with high incoming sea salt particles such as beach areas and areas where snowmelt is sprayed. Significant adverse effects were found.

[0013] Further, as a result of examining the improvement of the weather resistance of various alloy elements, it was found that the combined addition of Cu-Ni has a remarkable effect on the formation of stable rust in the beach area. Further, it has been found that the application limit of the Cu-Ni system (the upper limit of the annual average amount of sea salt particles that can form stable rust) can be roughly controlled by the amount of Ni added. Since Ni is an expensive additive element, it has been found that this finding makes it possible to provide the most economical steel material according to the severity of the corrosive environment.

Further, regarding the improvement of the fatigue strength of the welded HAZ, the present inventors have verified through many experiments that the HAZ structure is made mainly of bainite even if Ni necessary for securing the weather resistance is added. By adding Nb,
It has been found that not only does the fatigue strength of the welded HAZ have no problem, but that the addition of Ni works in the direction of improving the fatigue strength.

The present invention is based on the above findings, and the gist thereof is as follows. (1) By weight%, C: 0.02 to 0.20%, S
i: 0.1 to less than 1.6%, Mn: 0.3 to 2.0%,
Cu: 0.3 to 1.5%, Ni: 1.0 to 5.5%, N
b: more than 0.05 to 0.30%, Al: 0.005 to 0.
070%, and 0.30 ≦ Ceq (%) ≦ 0.65 [where Ceq (%) = C + Mn / 6 + (Cu + N
i) / 15 + (Mo + V) / 5 + Nb / 3], the balance being Fe and unavoidable impurities, and among the unavoidable impurities, P, S and Cr are P: P.
030% or less, S: 0.01% or less, Cr: 0.1%
A welded joint having excellent corrosion resistance and fatigue strength, characterized in that a steel material as a base material is used as a base material and a bainite fraction of a heat-affected structure of the welded joint is 60% or more.

(2) Mo: 0.1 to 1.0%, V: 0.005 to 0.
(1) The welded joint according to (1), further comprising one or more of Ti: 0.001 to 0.03%.

(3) The steel material as the base material is, by weight%, Ca: 0.0005-0.0100%, Mg
: 0.0005 to 0.0100%, REM: 0.0
The welded joint according to (1) or (2), comprising one or more of 005 to 0.0100%.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described. First, the reasons for limiting the chemical composition of the steel of the present invention and the effects thereof will be described. C is necessary to secure the strength as a structural material, and is added in an amount of 0.02% or more.
Since the structure cannot be bainite, its upper limit is 0.20
%.

Si is an essential element for deoxidation and is an element for improving fatigue strength, and is added in an amount of 0.1% or more. However, if added in excess of 1.6%, toughness is significantly reduced. The upper limit was set to 1.6%. Mn has an effect of fixing S, which is an impurity, as MnS in addition to the effects of deoxidation and strength adjustment, and preventing hot brittleness due to S.
It is necessary to add 0.3% or more, but if added over 2.0%, the weldability is impaired.
To 2.0%.

Cu elutes together with Fe in steel, suppresses crystallization and coarsening of the rust particles during formation of the rust layer, and maintains the rust density. Is an element indispensable for improving the content, and is effective when added at 0.30% or more. The effect increases as the content increases, but if the content exceeds 1.5%, the weldability is reduced and cracking during hot working becomes a problem.
5%.

When Ni is contained in the rust layer in an amount of 0.5% or more, Ni is eluted together with Fe in the steel, and is uniformly contained in the rust layer, whereby C adhering to the surface of the rust layer as flying sea salt particles.
By suppressing the penetration of l-ions into the rust layer / base iron interface and suppressing the crystallization and coarse growth of rust particles by setting the inside of the rust layer to a low Cl environment, the rust layer has an effect of maintaining the denseness of the rust layer. Further, as the amount of Ni added to the steel increases, the corrosion resistance of the steel material improves in a dry-wet repeated corrosive environment in an aqueous solution containing Cl ions. According to the study of the present inventors, it has been clarified that the addition of 1.0% or more of Ni is effective in containing 0.5% or more of Ni in the rust layer. Also,
If it exceeds 5.5%, the cost increases, so that the
5.5%.

Nb is one of the most important elements as a component of the present invention, and Nb and fine Nb (CN) segregate and precipitate at the lath boundary in bainite to strengthen the lath boundary and to form a weld contact portion. Improves fatigue strength with 0.6% Si addition
If less than this, the fatigue strength can be sufficiently improved. More than 0.05% is required to obtain the effect. On the other hand, 0.30
%, Coarse Nb (CN) precipitates, which serves as a starting point, so that production is likely to occur, and the fatigue strength decreases. Therefore, the amount of addition is set to more than 0.05 to 0.30%.

Al is required to be 0.005% or more as a deoxidizing element, but if the addition amount is large, inclusions increase, so that
The upper limit is set to 0.070%. The steel of the present invention contains the above elements as essential components, and the balance is substantially Fe and inevitable impurities. Among the unavoidable impurities, especially P, S, C
Since r is likely to be present in an amount that affects the properties of the steel material, its content is limited as follows.

P is generally an effective element for improving the weather resistance of the steel material itself. However, in a steel material as a welding base material, if the content of P exceeds 0.030%, the content of the base material and the welding HAZ is reduced. Since the toughness is reduced and the performance as a structure is impaired, the range is set to 0.030% or less. S is an unavoidable impurity that degrades the toughness and weather resistance of the steel material, and is preferably small. In particular, if the content is suitably 0.01%, inclusions increase and the toughness of the welded HAZ is remarkably reduced, so the range is set to 0.01% or less.

Since Cr is a metal that is lower than Fe, several percent
In addition to the addition of H, the weather resistance in an environment with a large amount of sea salt particles and the weldability are hindered. If it is 0.1% or less, the inhibitory effect on weather resistance and weldability can be almost ignored, so the range is set to 0.1% or less. The above is the basic component system in the present invention, and in the present invention, the addition amount of the above components and C
The important point is to satisfy 0.30 ≦ Ceq (%) ≦ 0.65 between eq and eq, thereby improving the fatigue strength of the welded portion.

That is, if Ceq is less than 0.30%, H
The strength difference between the cement truss in bainite and the lath boundary in the AZ structure increases, and fatigue cracks easily occur at the lath boundary. Therefore, although Ceq is high, it is preferable that Ceq.
If it exceeds 65%, the structure changes from bainite to martensite and the fatigue strength decreases. Therefore, the range was limited to 0.30 to 0.65%.

Of the selectively added Mo, V, and Ti,
Mo and V are elements that enhance hardenability and increase Ceq.
It is effective to contain one or more of the basic components. Ti is an element effective in refining the crystal grains, and improves the toughness of the welding HAZ. Hereinafter, the reasons for limiting the components for the strength and other effects of each element will be described.
Mo not only increases the strength but also prevents temper embrittlement. Further, the non-recrystallization temperature range is expanded to promote the effect of grain refinement by low-temperature rolling. These effects are not sufficiently exhibited at less than 0.1%. On the other hand, if the content exceeds 1.5%, coarse carbides are formed to lower toughness and the HAZ is hardened remarkably. Therefore, the addition amount is set to 0.1 to 1.5%.

V forms carbon and nitride during tempering and increases the strength by precipitation hardening. Therefore 0.005%
The above addition is necessary, but if it exceeds 0.07%, HA
Reduces Z toughness. Therefore, the addition amount is 0.005
To 0.07%. When Ti is added to steel,
N, particularly effective for refining the weld HAZ,
In order to further improve the toughness, it may be added.
001% or more is added, but if it exceeds 0.03%, carbides are formed, conversely increasing hardness and adversely affecting toughness, so the range is made 0.001 to 0.03%.

[0029] Ca and Mg are added as needed, and when they are added, they are present as oxides or sulfides in the steel and eluted from the base iron to form C and Mg.
Since a (OH) ₂ and Mg (OH) ₂ are formed to suppress the initial growth of rust colloid particles, fine precipitation and aggregation of rust particles are promoted. Since the effect is obtained by adding 0.05% or more and saturates at 0.0100%, the content of these elements is set to 0.0005% to 0.0100%.

REM is also added as needed, and it is effective in controlling the form of inclusions to improve the tensile properties in the thickness direction, reducing lamellar tears and improving low-temperature toughness. For this reason, the content is 0.0005% or more, but if the addition amount is too large, inclusions increase, so the upper limit is set to 0.010
0%. The welded joint of the present invention uses a steel sheet having the above chemical composition as a base material, and has a bainite fraction of 60% or more in the microstructure of the heat affected zone (HAZ). By setting the bainite fraction to 60% or more, high fatigue strength is ensured. Here, the bainite fraction specified in the present invention is a value obtained by the following measuring method.

[Bainite fraction measuring method] The bainite fraction was determined as a ratio of bainite structure to 200 or more measurement points by a point counting method using a mesh of 5 mm intervals in an optical microscope photograph at a magnification of 200 times. The welded joint of the present invention can be formed by various welding methods using a steel material having the above chemical composition as a base material. That is, for example, manual welding (SMAW), carbon dioxide welding, various kinds of automatic welding (SAW, GTAW, CES), and the like, and other methods that can be used in place of them, there is no problem in the present invention. The inventors' research has revealed that the bainite fraction of the welded HAZ structure becomes approximately 60% or more when a normal welding method is used using the steel material of the present invention as a base material. However, since the HAZ structure is affected by welding heat input, in order to increase the fatigue strength of the welding HAZ aimed at by the present invention, the welding conditions are set so that the bainite fraction of the heat-affected structure is 60% or more. It is desirable to adjust.

Further, it has been found that the steel material according to the present invention has a far superior coating durability even in a state of being subjected to anticorrosion coating such as painting or thermal spraying, as compared with ordinary steel or conventional weathering steel. This means that even if corrosion of the base steel sheet progresses from a local defect in the film, the generated rust is dense and has excellent protection, so that further peeling of the anticorrosion film and the progress of under-film corrosion are suppressed. Inferred. In this case, there is no effect on the fatigue strength of the welded HAZ, and both weather resistance and high fatigue strength can be achieved.

[0033]

EXAMPLE Steel having the chemical components shown in Table 1 was melted and heated.
The sheet thickness 2
A 0 mm steel plate was manufactured. Using these test steels,
Corrosion test and fatigue test of welded joints
did. That is, as an index indicating corrosion resistance, five years Chiba
Coastal area of Futtsu City, Prefecture (0.5-0.8mg / 100cm ^Two/
rust appearance of unpainted surface exposed in day)
The evaluation was made according to points 1 to 4. Here, the appearance score is 4
Is the best and forms a stable rust, and 1 is a layered peel rust
Can be expected to stabilize rust and prevent corrosion progress
This is an indicator that the status is not available.
You.

[Appearance Rating] 1: Red rust on the entire surface 2: Red rust on black rust exceeding 30% area ratio 3: Red rust on black rust 30% or less area ratio 4: Fully dense black rust (Note) “Black” includes dark brown and chocolate.

Further, the fatigue strength of the welded portion was determined by using a three-point bending fatigue test piece shown in FIG. 1 to produce a T-shaped fillet joint by semi-automatic welding of CO ₂ [heat input: 18 kJ / cm]. the ratio of the load and the minimum load (stress ratio) were tested at stress conditions such that 0.1 was evaluated by 10 ⁶ times fatigue strength. Table 2 summarizes the results of the above tests. The table also shows the base metal tensile strength and the HAZ structure of the steel sheet.

From the results shown in Table 2, the steels A to G of the present invention showed that
All of the ^six- time fatigue strengths were as high as 290 MPa or more, and the rust scores were all 4, indicating good corrosion resistance. On the other hand, the comparative steels H to N whose chemical compositions deviated from the scope of the present invention had inferior properties as described below.

That is, Comparative Steel H is an example in which the C content is excessively added to 0.22% beyond the range of the present invention.
The hardenability of the HAZ reduced the proportion of the bainite structure in the HAZ to 35%, and the fatigue strength was 210%.
Low with MPa. Comparative steels I, J, and K are examples in which chemical components related to corrosion resistance deviated from the scope of the present invention. That is, Comparative Steel I had a Cu content of 0.05%, Comparative Steel J had a Ni content of 0.65%, all of which were lower than the range of the present invention, and Steel K had a Cr content of 0.19%. This is an example of excessive addition beyond the range. The welded joints using these steels have good fatigue strength, but all have low corrosion resistance.

Further, Comparative Steel L is an example in which the Nb content is 0.042%, which is lower than the range of the present invention, and the corrosion resistance is good, but the fatigue strength is as low as 200 MPa. Comparative steel M, N
In each case, the range of Ceq exceeds the range of the present invention, and Comparative Steel M is as low as 0.25% and Steel N is as high as 0.67%. As a result, the corrosion resistance was good in both cases, but the fatigue strength was 200 and 220 MPa, respectively, lower than the steel of the present invention.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

As described above, according to the present invention,
A welded joint having both excellent corrosion resistance and high weld fatigue strength is provided for a steel structure such as a bridge or a steel tower in a beach area or an area where salt damage such as spraying of snow-melting salt is concerned.

[Brief description of the drawings]

FIG. 1 is a front view showing a three-point bending fatigue test method for a T-shaped fillet joint.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuji Tanabe 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Tadashi Ozeki 20-1 Shintomi, Futtsu-shi, Chiba New Japan (72) Inventor Yukio Tomita 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division of Nippon Steel Corporation (reference) 4E001 AA03 BB09 CA07 4E081 YB10

Claims (3)

[Claims] C .: 0.02 to 0.20%, Si: 0.1 to less than 1.6%, Mn: 0.3 to 2.0%, Cu: 0.3 to 100% by weight. 1.5%, Ni: 1.0 to 5.5%, Nb: more than 0.05 to 0.30%, Al: 0.005 to 0.070%, 0.30 ≦ Ceq (% ) ≦ 0.65 [However, Ceq (%) = C + Mn / 6 + (Cu + N)
i) / 15 + (Mo + V) / 5 + Nb / 3], the balance being Fe and unavoidable impurities, and among the unavoidable impurities, P, S and Cr are: P: 0.030% or less; S: 0.01% or less, Cr: 0.1% or less The base material is a steel material, and the bainite fraction of the heat-affected structure of the welded joint is 60% or more, and the corrosion resistance and fatigue strength are characterized in that: Excellent welded joint. 2. The steel material as the base material is, by weight%, Mo: 0.1 to 1.0%, V: 0.005 to 0.07%, Ti: 0.001 to 0.03%. The welded joint according to claim 1, further comprising one or more of the following. 3. The steel material as a base material is, by weight%, Ca: 0.0005 to 0.0100%, Mg: 0.0005 to 0.0100%, REM: 0.0005 to 0.0100%. 3. The composition according to claim 1, wherein the composition contains one or more of the following.
The described welded joint.