JP2003176507A - Long-life steel structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a girder structure prevented from brittle fracture, heightened in safety, prolonged in the service life and reduced in maintenance cost by using a steel plate formed of steel with high fatigue strength, as a steel member. <P>SOLUTION: Steel which restrains at least one of the initiation and progress of a fatigue crack is used as at least one of a web and a flange of an I-girder or a box girder jointed by welding in the steel structure, and reinforcement is performed against the fatigue of the toe of a weld part by toe grinding, pinning, using a low-temperature transformation hot charge, or the like. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel structure in, for example, a bridge, a building steel frame, a crane girder, or a crane boom. The present invention relates to a steel structure designed to be made into a material.

[0002]

2. Description of the Related Art For example, in a steel structure such as a bridge girder of a bridge, as shown in FIGS. 2 and 3, as a girder member 1 made of steel on which a steel floor plate or a concrete slab floor plate is installed, an I-shaped cross section or Members such as box-shaped cross sections are used. Then, attachments 4 such as vertical ribs, horizontal ribs, and gussets are attached to the webs 2 and flanges 3 of the girder member 1 by fillet welding 5, and act by traveling of a vehicle traveling on the floor slab. It is reinforced against live and dead loads.

In particular, a girder structure using an I-shaped cross section is
When the attachment 4 is attached to the flange 3, since the structurally highest acting stress is the flange 3 located under the girder, fatigue cracks occur around the weld end of the attachment 4, If this progresses, there is a risk that the flange 3 will break at a stretch on a day when the temperature is low. For this reason, in a girder structure having an I-shaped cross section, all attachments 4 including gussets and the like can be made into the web 2 as much as possible.
Be careful to attach to.

However, even if the attachment 4 is attached to the web 2 of the girder member 1 having an I-shaped cross section, as shown in FIG.
As shown in (a), a fatigue crack a is generated around the weld end 4a of the attachment 4, and due to the expansion of the fatigue crack a over time, the expansion b of the crack is, for example, 30
If it is equal to or larger than mm, brittle fracture may occur as shown in FIG. Therefore, when the girder member 1 is repaired, the extension b of the crack is, for example, 20 mm.
Although repair work is performed at some point, if the fatigue crack a progresses quickly, the period until transition to brittle fracture is short, and as a result, regular maintenance / inspection regarding fatigue is performed frequently. There is a need. Such brittle fracture phenomenon is also the same in a steel structure having a box-shaped cross section or another cross section.

[0005]

As described above, the steel material used for the conventional steel structure does not only gradually deteriorate the function of the steel structure due to the progress of the fatigue crack once the crack develops, but also becomes brittle. There is a case where the material moves at once to break and breaks.

In a steel structure in which fatigue is a problem due to such a live load, the fatigue strength and fatigue life of the welded portion depend only on the type of joint and the action fluctuating stress,
It has been considered that there is almost no dependence on steel grade.

By the way, the fatigue life is defined as the sum of the occurrence life and the extension life. That is, the fatigue life of a steel sheet without cracks is divided into the time until cracks occur and the time when the cracks once spread. The ratios of the lifespan to the lifespan and the lifespan to be propagated differ depending on the type of welded joint in which a crack occurs. For example, about 80% of the bending fatigue life of a T-type joint made by double-sided welding is the generated life and about 20% is the developed life. However, the axial stress fatigue life of the out-of-plane gusset type joint is about 10% of the generated life and about 90% of the developed life. The reason is that the crack growth rate of the steel sheet does not change so much unless the action fluctuating stress of the steel material changes, and therefore the rate varies depending on the easiness of crack generation. Further, it has been said that the life of crack generation in the welded part is also determined by the type of joint and does not change depending on the steel plate. Since the stress concentration factor in the T-type weld zone is small and the stress concentration factor in the out-of-plane gusset type weld zone is large, cracks are likely to occur in the out-of-plane gusset type due to stress concentration in the weld zone. The rate of occurrence life is reduced.

As described above, in the conventional steel structure, when a fatigue crack once occurs in the welded portion of the steel material, and if the extension of the crack exceeds a certain limit, it shifts to brittle fracture and bursts at once. As a result, not only does it lose its function as a structure, but it also involves the risk of developing an accident involving human life. Due to this, fatigue cracks require regular maintenance and inspection, and non-destructive inspection other than visual inspection is also required to detect cracks of 20 mm or less, which increases the scale and installation of scaffolding. Since the need for
Maintenance costs are also high.

The present invention has been made in view of the above circumstances, and prevents fatigue fracture and the accompanying brittle fracture by using a steel plate having a long fatigue life and less likely to cause brittle fracture as a member for transmitting stress. However, it is an object of the present invention to obtain a steel structure capable of improving safety and extending the life and reducing the maintenance cost.

[0010]

In order to solve the above-mentioned problems, the first invention is that in a steel structure, fatigue cracks are generated in at least one of the I-digit web and the flange to be joined by welding. It is characterized by using a steel material that suppresses at least one of:

A second aspect of the present invention is a steel structure,
At least one of the web and the flange of the box girder to be joined by welding is made of a steel material that suppresses at least one of the occurrence and propagation of fatigue cracks.

A third aspect of the present invention is the steel structure of the first or second aspect of the present invention, in which the toe of the welded portion is strengthened against fatigue by toe grinding, pinning, use of a low temperature transformation liquid material and the like. Is characterized by.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Fatigue fracture of a steel sheet consists of crack initiation and propagation. The sum of the crack initiation life and the crack growth life is the total life leading to a fatigue crack. Then, in the welded part of the steel plate, the stress concentration is often generated from the welded HAZ that coincides with the weld toe, and the generated cracks penetrate into the base metal part after propagating in the welded HAZ, and Continuing to progress, eventually leading to breakage of the member. To improve the fatigue life of steel plate welds, weld HAZ
It is necessary to suppress the occurrence of fatigue cracks in the interior and the propagation of fatigue cracks into the base material.

As a steel for suppressing fatigue crack growth, there is a steel sheet in which a hard phase is uniformly formed in an island shape by performing a special rolling treatment. Fatigue cracks do not linearly spread in the formation part of such a hard phase, and they develop by causing a minute separation so as to avoid the hard phase while detouring and meandering. In this way, the lengthening of the extension path can delay the fatigue cracks, so that the apparent fatigue crack growth rate can be suppressed.

As an example of a steel material capable of suppressing the propagation of such fatigue cracks, a new metallurgical principle that a fine grain can be made into a fine grain by processing a ferrite structure while heating is applied to a steel sheet. Steel sheet having a superfine grain structure in the surface layer (SUF steel sheet: Surface layer wit)
h Ultra Fine grain micros
structure) has been developed. (Proceedings of the Welding Society, Volume 15, No. 1, "Characteristics of Highly Arrested Steel Sheets by Superfine Graining of Surface Layer", p. 148-154, 1997)

Surface ultra fine grain steel sheet (hereinafter referred to as SUF steel sheet)
Using as a steel sheet, forcibly generating a brittle crack in the weld, and as a result of a crack propagation test, the crack does not propagate along the weld, and it is difficult for the brittle crack to propagate along the weld. confirmed. Further, similar test results were obtained also on the one-sided SUF steel plate in which the SUF portion on one side of the SUF steel plate was deleted.

A steel material that suppresses the development of fatigue cracks in addition to suppressing the development of fatigue cracks like the above steel sheet was developed by the applicant of the present invention and applied for a patent as Japanese Patent Application No. 2000-322415. There is something like.

% By mass, C: 0.005 to 0.15
%, Si: 0.01 to 1.6%, Mn: 0.5 to 2%,
P: 0.01% or less, S: 0.005% or less, Nb: 0.05 to 0.3%, V: 0.
05-0.3% 1 type or 2 types, and the balance is Fe
And Ceq (%) and Pcm (%) defined by the following formulas are Ceq: 0.55.
% Or less, Pcm: 0.25% or less, and the microstructure of the welded HAZ portion is 6 in area fraction of the upper bainite.
More than 0%, and the microstructure of the base material further contains 15 to 80% by area fraction of ferrite.
The (200) texture strength of the plane parallel to the rolled surface in the center part of the plate thickness measured by line diffraction is 2 to 15 in relative strength ratio to the random sample, and the rest has Vickers hardness of 2
A welded structural steel excellent in fatigue properties of a welded portion, which has a structure of one or more kinds of pearlite, bainite, and martensite of 50 to 700.

With respect to the steel material, the mechanism of suppressing fatigue crack initiation and fatigue crack growth will be considered. The welded HAZ structure was reproduced by a thermal cycler, the test piece provided with stress concentration part was subjected to a fatigue test, and the effect of the microstructure on the fatigue strength of the welded HAZ was systematically investigated. It was found that the limit ratio (fatigue limit / tensile strength of reproduced HAZ material) was high.

Further, the welded HAZ structure of steel having a tensile strength of 570 MPa or higher is inevitably changed from bainite to martensite structure, so that the tensile strength is 490 M.
It is not possible to expect improvement in weld HAZ fatigue strength by making a welded HAZ structure such as a steel material of Pa class or less into ferrite.
Therefore, it is effective to add one or two of Nb and V in order to further suppress the occurrence of fatigue cracks after the upper bainite structure having the highest fatigue limit ratio next to ferrite is formed. In the welded HAZ in the upper bainite structure, fatigue cracks are generated and propagated from the lath interface, but Nb,
If one or two kinds of V are added in an appropriate amount, segregation and fine precipitation on the lath interface strengthens the lath interface.
It is possible to significantly suppress the occurrence of fatigue cracks in the AZ portion. Further, the limitation of the carbon equivalent value (hereinafter, referred to as Ceq) contributes to the improvement of the fatigue strength of the welded portion by strengthening the entire welded HAZ bainite structure.

The texture is developed in order to deviate the crack from the main path and to appropriately distribute the slow-growing orientations based on the knowledge that the fatigue crack growth rate depends on the crystal orientation. is there. As a result of investigating the texture for increasing the crack growth resistance, the strength of the (200) texture in the plane parallel to the rolled surface at the center part of the plate thickness measured by X-ray diffraction is shown by the relative strength ratio to the random sample. It was found that the fatigue life of fatigue cracks in the base material portion was improved by setting the respective values to 2 to 15.

Further, when the hard phase exists in the crack propagation path, the second phase having a strength of a certain level or more needs to be distributed in an appropriate amount in order to provide resistance to crack propagation. From the detailed experiment, the strength of the second phase is required to be 250 or more in Vickers hardness. In addition, as the second phase, a material such as an inclusion that easily forms a gap with the base material is not effective in resistance to crack propagation and adversely affects toughness, which is not preferable.

On the other hand, fine precipitates such as carbonitrides and cement have too small Vickers hardness of the strength of the second phase and are liable to hinder the development of cracks. So, as the second phase,
It is preferable that the second phase has a certain degree of ductility and toughness. Therefore, the second phase has a structure of one or more of pearlite, bainite, and martensite.

Next, the reason why the range of each alloying element is limited based on the above basic idea will be described below. In addition, "%" in the description means "mass%".

C is contained as an effective component for improving the strength of the steel material. If it is less than 0.005%, it is difficult to secure the strength of the base material. If it exceeds 0.15%, the toughness and weld crack resistance of the base material and the welded portion are reduced, so 0.00
It was set to 5% to 0.15%.

Si is an essential element as a deoxidizing element in addition to securing strength, and solid-solution strengthens ferrite in bainite to strengthen the lath boundary and also improves fatigue strength. To obtain that effect, it is necessary to add 0.01% or more,
On the other hand, if it exceeds 1.6%, on the contrary, cracks are likely to occur from the lath boundary, and the fatigue strength decreases. Therefore, the amount is
It was set to 0.01 to 1.6%.

Mn is an essential element for increasing the strength, but if it is less than 0.5%, the strength of the base material cannot be secured. On the other hand, if it exceeds 2%, the welded HAZ structure is mainly composed of martensite, and the fatigue strength of the welded portion decreases, so the amount was made 0.5 to 2%.

P is an element that affects the toughness of the steel material, and if it exceeds 0.01%, not only the base metal but also the weld HA
Since the toughness of the Z part is significantly impaired, it is preferable that the amount is as small as possible, and the amount is set to 0.01% or less.

As with P, the lower S is, the better.
If it exceeds 005%, the precipitation of MnS becomes remarkable, the toughness of the welded HAZ of the base material is impaired, and the ductility in the plate thickness direction also decreases. Furthermore, if a large amount of MnS inclusions is present, this becomes the starting point of fatigue cracks and causes variations in fatigue strength, so the amount was made 0.005% or less.

Nb or V is one of the main elements as a component of the present invention, and Nb or V is present at the lath boundary in bainite.
Fine carbides or nitrides of V or fine carbides or nitrides of V are precipitated to strengthen the lath boundary and improve the fatigue strength of the welded joint, and the addition of Si is less than 0.6%. Even in the case, the fatigue strength can be sufficiently improved. To obtain that effect, 0.05
% Is required. On the other hand, if it exceeds 0.3%, the precipitates become coarse, which becomes a starting point, and cracks easily occur, and the fatigue strength and the toughness of the welded HAZ decrease. Therefore, the addition amount is set to 0.05 to 0.3%. Note that N
Both b and V may be added up to 0.3% at the same time, but if the total amount of Nb and V exceeds 0.5%, coarse precipitates tend to precipitate, so 0.5%. It is preferable to add it as follows.

The above is the basic component system of the steel sheet used for the steel structure of the present invention. Furthermore, the addition amount of the above components and the hardenability Ceq which is the hardenability of the weld heat affected zone and the weldability are considered. Ceq ≦
It is an important essence to satisfy 0.55% and Pcm ≦ 0.25%, and it improves the weldability, the toughness of the weld and the fatigue strength.

That is, the higher the Ceq is, the better it is for improving the strength, but if it exceeds 0.55%, the structure mainly of martensite is formed from bainite, and the fatigue strength of the welded portion is lowered. Therefore, the upper limit value is set to 0.55%. Further, if Pcm exceeds 0.25%, there is a possibility that low temperature cracking may occur, and it becomes difficult to improve the fatigue strength with welding. Therefore, the upper limit is set to 0.
It was set to 25%.

The production conditions for producing the above steel sheet are as follows: 100% of the steel ingot prior to hot rolling.
It is necessary to convert to austenite, and for this purpose, it is necessary to heat the temperature of the steel ingot to the Ac3 transformation point or higher. However, when heated above 1350 ° C., the austenite grains are remarkably coarsened and fine-grained phenite cannot be obtained after rolling. Therefore, the upper limit of the heating temperature is 135
Set to 0 ° C.

Further, the subsequent hot rolling of the steel ingot is performed with Ar3.
It is necessary to limit the temperature range from the transformation point to 1200 ° C.
The reason is that by performing rolling in the austenite single phase region, the transformation temperature is raised and the transformation structure is refined, and fine grain ferrite is obtained in the two-phase region rolling. If the cumulative rolling reduction is 10%, such an effect is small, and if it exceeds 80%, it is not possible to secure the rolling reduction in the subsequent two-phase region rolling. Therefore, the upper limit is set to 80%. In this case, the controlled rolling in the austenite region,
It is preferable to further refine the austenite grains before the two-phase region rolling.

Further, in the above process, it is necessary to increase the (200) diffraction strengthening ratio in the plate thickness direction of the steel plate. For this reason, finish rolling below the Ar3 transformation point plays a very important role. That is, from only the viewpoint of increasing the (200) diffraction strengthening ratio in the plate thickness direction,
It is desirable to reduce the rolling temperature, but the lower the temperature, the higher the deformation resistance, so the rolling load increases and it becomes difficult to perform rolling. Further, when the temperature is 600 ° C. or lower, it becomes impossible to form one or more structures of pearlite, bainite, and martensite having a Vickers hardness of 250 to 700, which is a structural requirement. Therefore, the rolling end temperature is 600
℃ or above.

As a method for cooling the steel sheet after the completion of the two-phase rolling, depending on the desired strength and toughness level, the steel sheet may be cooled in the air as it is, or at a cooling rate of 5 to 80 ° C./sec. You may accelerate-cool to 20-600 degreeC. The reason why the cooling rate in the case of accelerated cooling is set to 5 to 80 ° C./sec is that changes in the structure due to accelerated cooling are not clear and reliable improvement in strength and toughness cannot be expected.
This is because if it exceeds sec, there is a large difference in structure or characteristics between the surface layer and the inside, which is not preferable. In addition, the accelerated cooling is performed in accordance with the strength at a desired temperature of the steel material and the toughness level.
Stop at ~ 600 ° C. Setting the stop temperature of the accelerated cooling below 20 ° C. has no effect in controlling the material,
It simply makes the manufacturing cost higher and is meaningless. On the other hand, if the accelerated cooling is stopped above 600 ° C., the effects such as improvement in strength and toughness due to accelerated cooling do not clearly occur, and there is no point in performing accelerated cooling.

FIG. 1 shows the apparent fatigue crack growth rates of the steel sheet of Japanese Patent Application No. 2000-322415 used as one of the steel materials of the present invention and the conventional standard steel sheet. As is clear from the figure, in the conventional standard steel materials of A steel, B steel and C steel, the apparent fatigue crack growth rate hardly changes in the fatigue crack propagation region (ΔK = 20 MPa√m). It is apparent that the steel sheets used in the present invention have an apparent fatigue crack growth rate of about 1/3, which is extremely small, as compared with these conventional standard steel sheets.

As described above, in the steel sheet used in the present invention, the true rate of fatigue cracking has not changed. However,
In the steel sheet, the hard phase is uniformly formed in an island shape, and cracks do not spread linearly at the hard phase forming part, and by causing a minute separation so as to avoid the hard layer. , Make a detour and meander. Therefore, the apparent fatigue crack growth rate decreases. Therefore, by subjecting the steel sheet to the special rolling treatment as described above, the fatigue crack can be delayed, so that it is possible to suppress the apparent fatigue crack growth.

Further, in the steel sheet used in the steel structure of the present invention, it is possible to suppress the occurrence of fatigue cracks by adding an appropriate amount of one or two of Nb and V as described above.

In each of the steel sheets used in the present invention as described above, the hard phase is uniformly formed in an island shape,
Cracks do not spread linearly in the formation part of such hard phase,
By avoiding the hard phase, the micro separation causes a detour and meandering. This effect was remarkably effective also in suppressing the conversion of fatigue cracks into brittle fracture, and brittle fracture did not occur even when the CTOD test was actually carried out.

The first and second inventions are the above-mentioned Japanese Patent Application No. 2000.
Long-life steel structure by using a steel material such as a steel plate of No. 322415 that suppresses at least one of the occurrence and development of fatigue cracks in at least one of the I-girder web and flange, and at least one of the box girder web and flange. It is a thing.

The effects of the steel sheets used in the steel structures of the first and second inventions have been proposed in recent years by welding toe grinding and pinning by a grinder, which are conventional fatigue durability improving methods, and in recent years. It can be further improved by using it at the same time as a post-treatment technique such as a low temperature transformation material.

[0043]

As described above, according to the steel structure of the present invention, a steel plate made of a steel material for suppressing at least one of the occurrence and propagation of fatigue cracks is used as at least one of the flange and the web of the I girder and box girder. Can be used to improve the fatigue life of the steel member. Also,
Since the steel sheet used in the present invention does not easily undergo brittle fracture even if a fatigue crack occurs, as in the past, safety can be improved. Moreover, since maintenance / inspection intervals regarding fatigue can be taken very long, maintenance costs can be reduced. Further, since it is only necessary to change the steel material, it is not necessary to change the structure of the steel structure. But conversely,
Since the fatigue life can be made longer than before, it is usually taken care to weld the web. Attachments such as ribs and gussets can also be welded to the flange, increasing design flexibility.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an apparent fatigue crack growth rate of a steel material used for a steel structure of the present invention in comparison with a conventional standard steel material.

FIG. 2 is an explanatory view of a main part showing a state in which an attachment is attached to an I-digit.

FIG. 3 is an explanatory view of a main part showing an attachment state of an attachment to a box girder.

FIG. 4A is an explanatory view showing a fatigue crack generation state in an I-digit web, and FIG. 4B is an explanatory view showing a fatigue fracture state in the web.

[Explanation of symbols]

1 digit member 2 web 3 flange 4 attachments 4a Weld end 5 welds

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kiyotaka Nakajima             20-1 Shintomi, Futtsu City Nippon Steel Co., Ltd.             Inside the surgical development headquarters (72) Inventor Tadashi Kasuya             20-1 Shintomi, Futtsu City Nippon Steel Co., Ltd.             Inside the surgical development headquarters F-term (reference) 2D059 AA07 AA08 GG01                 2E164 EA01

Claims (3)

[Claims] 1. A long-life steel structure, characterized in that at least one of a web and a flange of an I-digit to be joined by welding is made of a steel material which suppresses at least one of fatigue crack initiation and propagation. . 2. A long-life steel structure characterized in that at least one of a web and a flange of a box girder to be joined by welding is made of a steel material for suppressing at least one of the occurrence and propagation of fatigue cracks. 3. The long-life steel structure according to claim 1 or 2, wherein the weld toe is strengthened against fatigue by toe grinding, pinning, use of a low temperature transformation liquid material and the like.