JP2003089844A - Thick steel plate for welded structure having excellent fatigue strength of welded joint, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thick steel plate for welded structures which has excel lent fatigue strength of a welded joint, and to provide a production method therefor. SOLUTION: The thick steel plate for welded structures having excellent fatigue strength of a welded joint has a composition containing, by mass, 0.015 to 0.15% C, 0.01 to 2% Si, 0.2 to 1.5% Mn, <=0.03% P, <=0.01% S and >0.1 to 1% Al, and the balance Fe with inevitable impurities, and satisfying Ceq<=0.24+0.03×√Al (Si: <1%) or Ceq<=0.275+0.03×√Al (Si; >=1%), wherein, Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5+Nb/3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fatigue of welded parts used for welded structural members such as construction, shipbuilding, bridges, construction machinery, and marine structures which require both toughness and fatigue strength of the welded parts. Welded structural mild steel with excellent characteristics and tensile strength of 5
The present invention relates to 90 MPa class high-strength steel, and more particularly to a thick steel plate for welded structures excellent in fatigue strength of welded joints and a manufacturing method thereof.

[0002]

2. Description of the Related Art With the increase in size of welded structures and the increasing demand for environmental protection, structural members are required to have higher reliability than ever before. Fatigue fracture, brittle fracture, ductile fracture, etc. are assumed as the fracture modes assumed in the welded structure, but among these, the most frequent fracture mode is the fatigue fracture or brittle fracture from the initial defect, and further the fatigue fracture. Is a brittle fracture that follows. There are many cases in which fatigue failure has become a problem, such as the recent occurrence of fatigue cracks in bridges and large tankers, and collapse due to fatigue cracks in offshore structures.

It is difficult to prevent these forms of destruction only by considering the design of the structure, and often cause sudden collapse of the structure. From the viewpoint of ensuring the safety of the structure, the prevention is the most important. This is the required form of destruction. With the increase in size of structures, the demands on the steel materials used have also increased,
In particular, it becomes more difficult to secure toughness and fatigue strength in a welded structure.

Up to now, many techniques for improving fatigue strength have been proposed, most of which are thin steel sheet base materials,
Alternatively, it relates to improvement of fatigue strength of spot welds. For example, Japanese Patent Laid-Open No. 61-96057 describes that the fatigue strength can be improved by setting the area ratio of bainite to 5 to 60%. According to a study on fatigue fracture of thick steel plate welded joints, fatigue cracks occur at stress concentration parts of welds. Since residual stress also acts on this part, it has been clarified that fatigue cracks are easily generated by the superposed action of stress concentration and residual stress.

Further, a great deal of research has been conducted on factors that govern the fatigue strength of welded members and improvements in fatigue strength. To improve the fatigue strength of the welded portion, it is necessary to grind it, and to heat and remelt the final layer of the weld bead. Most of the improvements were due to mechanical factors such as stress concentration reduction due to improvement of the weld toe shape such as shaping the shape (for example, JP-A-59-110490 and JP-A-1-30).
1823 publication). Further, the effect of reducing the residual stress by the heat treatment after welding is well known in the past.

As a result of conducting a systematic experiment on the microstructure dependence of fatigue crack initiation / propagation of welded joints, the inventors of the present invention have found that fatigue crack initiation / propagation is most likely to occur in JP-A-8-73983. It has been revealed that the HAZ structure that is effectively suppressed is ferrite. That is, it is disclosed that the fatigue strength of the weld is improved by limiting the carbon equivalent value (hereinafter, Ceq) and increasing the HAZ ferrite structure fraction.

However, in the invention disclosed in JP-A-61-96057, the fatigue strength is improved by limiting the bainite area ratio of the base metal to a specific range, which is the fatigue of the thin steel plate base metal. The present invention relates to the improvement of strength, and is not effective for improving the fatigue strength of a welded joint in butt welding of thick steel plates or fillet welding, which is the object of the present invention. Further, in the inventions described in JP-A-59-110490 and JP-A-1-301823, it is necessary to perform special work after welding, and it is impossible to improve fatigue strength as it is as welded. Furthermore, JP-A-8-7398
In the invention described in Japanese Patent Publication No. 3, the fatigue strength of the welded portion is improved by limiting the Ceq value and increasing the HAZ ferrite fraction by adding Si, but it is necessary to further improve the fatigue strength. Is 500
Up to MPa-class high-strength steel sheets, and higher-strength steel sheets beyond that are not considered.

[0008]

SUMMARY OF THE INVENTION The present invention controls not only the fatigue strength by the additional welding method for realizing the reduction of stress concentration and the reduction of welding residual stress, but the control of steel material components and manufacturing conditions. Accordingly, it is an object of the present invention to provide a thick steel plate for a welded structure and a method for producing the same, which is excellent in fatigue strength of a welded joint in a mild steel for welded structure and a high-strength steel having a tensile strength of 590 MPa class.

[0009]

[Means for Solving the Problems] As a result of detailed investigations by the present inventors to improve the fatigue strength of welds from mild steel plates for welded structures to high-strength steel sheets of 590 MPa class, the results show that Si content and Al It has been found that it is possible if the HAZ ferrite fraction is increased by limiting the amount and further limiting the upper limit of Ceq according to the amount of Al.

The present invention has been completed based on such findings, and the gist thereof is as follows. (1)% by mass, C: 0.015 to 0.15%, Si:
0.01% or more and less than 1%, Mn: 0.2 to 1.5%,
P: 0.03% or less, S: 0.01% or less, Al: 0.
A thick steel plate for welded structure excellent in fatigue strength of a welded joint, containing more than 1% and 1% or less, consisting of balance Fe and inevitable impurities, and satisfying Ceq ≦ 0.24 + 0.03 × √Al. However, Ceq = C + Mn / 6 + (Cu + N
i) / 15 + (Cr + Mo + V) / 5 + Nb / 3 (2)% by mass, C: 0.015 to 0.15%, Si:
1 to 2%, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.01% or less, Al: more than 0.1% and 1% or less, the balance Fe and unavoidable. Ceq consists of
A thick steel plate for welded structure having excellent fatigue strength of a welded joint, which satisfies ≦ 0.275 + 0.03 × √Al. However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 +
(Cr + Mo + V) / 5 + Nb / 3 (3)% by mass, further containing Cu: 0.1 to 2%, which is excellent in fatigue strength of the welded joint according to the above (1) or (2). Heavy steel plate for welded structure. (4) For welded structures excellent in fatigue strength of the welded joint according to any one of (1) to (3), characterized in that Ni: 0.1 to 2% by mass is further contained. Thick steel plate. (5) Mass%, Cr: 0.05 to 1%, Mo: 0.0
A thick steel plate for welded structure excellent in fatigue strength of the welded joint according to any one of the above (1) to (4), further containing 1 to 2% of 2-1%. (6) Mass%, Nb: 0.005 to 0.08%, V:
Weld structure thickness excellent in fatigue strength of the welded joint according to any one of the above (1) to (5), further containing 0.005 to 0.1% of 1 type or 2 types. steel sheet. (7) In mass%, Ti: 0.001 to 0.05%, N:
The thick steel plate for welded structure excellent in fatigue strength of the welded joint according to any one of the above (1) to (6), further containing 0.001 to 0.008%. (8) Mass%, Mg: 0.0001 to 0.01%, C
a: 0.0005 to 0.005%, REM: 0.000
5 to 0.005% of 1 type or 2 or more types are further contained, The thick steel plate for welded structures excellent in the fatigue strength of the welded joint in any one of said (1) thru | or (7) characterized by the above-mentioned. . (9) In the production of the steel sheet according to any one of (1) to (8), the steel ingot is heated to Ac ₃ point or higher and 1250 ° C. or lower, hot-rolled in a recrystallization temperature range, and then naturally cooled. A method for producing a thick steel plate for a welded structure, which is excellent in fatigue strength of a welded joint. (10) Following the hot rolling in the recrystallization temperature range, hot rolling at a cumulative reduction rate of 40 to 90% is performed in the non-recrystallization temperature range of the welded joint according to (9) above. A method for manufacturing a thick steel plate for welded structures having excellent fatigue strength. (11) In the production of the steel sheet according to any one of (1) to (8), the steel ingot is heated to Ac ₃ point or higher and 1250 ° C. or lower, and then hot-rolled in a recrystallization temperature range, and then unreformed. Excellent in fatigue strength of welded joints characterized by performing hot rolling at a cumulative rolling reduction of 40 to 90% in the crystallization temperature range and then cooling to a temperature of 600 ° C or less at a cooling rate of 1 to 60 ° C / sec. For manufacturing thick steel plates for welded structures. (12) The method for producing a thick steel plate for welded structure having excellent fatigue strength of the welded joint according to (11), further comprising heating to 300 ° C. to Ac ₁ point and then performing heat treatment after cooling.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail. First, improvement of fatigue strength of a welded joint, which is the essence of the present invention, will be described. The inventors performed a detailed microscopic observation of the state of crack initiation / propagation in the fatigue test piece of the welded joint. As a result, most fatigue cracks occur in weld metal and HAZ.
It has been found that it occurs at the boundary of (heat affected zone), that is, near the weld fusion line, propagates in the HAZ, and further rushes into the base metal portion, leading to total destruction of the test piece. The reason why cracks occur near the weld fusion line is that the vicinity of the weld fusion line coincides with the weld toe, and the stress concentration is highest at this portion. As described above, it has been clarified that the fatigue crack is generated near the weld fusion line and propagates in the HAZ, so that the fatigue strength greatly affects the microstructure of the HAZ.

As described above, the fatigue crack initiation portion is near the weld fusion line, and further, in the initial stage of crack propagation, HAZ.
It is within. These regions correspond to the stress concentration part.
By reproducing both the HAZ microstructure and stress concentration factors, the effect of the HAZ microstructure on fatigue strength can be investigated. Therefore, a test piece provided with stress concentration was processed from a steel material subjected to a simulated welding heat cycle and subjected to a fatigue test to determine the relationship between the HAZ microstructure and the fatigue strength. External dimensions of the test piece 10 x 10 x 55 mm, notch depth 2 m
m, notch tip radius is 0.75 mm, fulcrum distance is 40 m
A 3-point bending cyclic load was applied as m to cause fatigue failure.
The stress concentration factor is 2.6.

FIG. 1 shows a welding process using mild steel to laboratory vacuum melting steel having a tensile strength of up to 590 MPa, with a maximum heating temperature of 1400 ° C. and a cooling time of 800 to 500 ° C. for 1 to 30 seconds. Reproduced HA with reproducible heat cycle
Fatigue limit ratio of Z material (fatigue limit / reproduced HAZ tensile strength)
3 shows the dependence of the reproduced HAZ material on the tensile strength. As is clear from this figure, the fatigue limit ratio is HA
Depends largely on the Z microstructure, martensite, lower bainite, mixed structure of lower bainite + upper bainite, upper bainite, and ferrite become higher in this order. That is, in the fatigue test with stress concentration, the HAZ structure has a higher fatigue limit ratio as the high-temperature transformed structure becomes higher, and becomes lower as the low-temperature transformed structure becomes lower.

The reason why the fatigue strength depends on the microstructure has not been completely clarified, but the dislocation density introduced during the transformation is higher in the low temperature transformation structure, and the dislocations are rearranged when subjected to repeated stress. Therefore, dislocation strengthening does not contribute much to fatigue strength. The strength of the lath interface of bainite or martensite or the former austenite grain boundary becomes relatively lower than that of the intragranular structure in the low temperature transformation structure, and fatigue cracks easily occur at the lath interface and the former austenite grain boundary. To do. In the ferrite structure, the plastic deformation at the propagating crack tip is remarkable, the plastic absorbed energy increases, and as a result, the crack propagation is delayed.
The reason is considered. It is known that the fatigue strength of a smooth test piece with little stress concentration has little microstructure dependence, and rather has a high correlation with static tensile strength.

As described above, the fatigue strength of the reproduced HAZ material is influenced by the microstructure, and the fact that the fatigue limit ratio rises especially in the ferrite-based structure is a phenomenon that occurs specifically in the stress concentration part. The effect of improving the fatigue strength by forming the structure is particularly remarkable when stress concentration is present as in a welded joint. Therefore, it is most desirable to make the HAZ microstructure a ferrite-based structure in order to improve the fatigue strength, but to make the 100% ferrite structure in the continuous cooling transformation that the HAZ continuously receives is especially high in cooling rate. It is difficult to perform heat input welding, and a structure such as bainite whose transformation temperature is lower than that of ferrite is inevitably mixed. However, since upper bainite has the highest fatigue limit ratio next to ferrite,
It can be expected that the HAZ fatigue strength will not be significantly reduced even if some upper bainite is mixed.

FIG. 2 is a plot of the fatigue limit ratio of the reproduced HAZ material against the ferrite area ratio. It is clear from the figure that the fatigue limit ratio increases as the ferrite area ratio increases. Furthermore, the ferrite area ratio is 6
If it is 0% or more, the fatigue limit ratio increases significantly. The improvement of the fatigue limit ratio is particularly remarkable in the range where the ferrite area ratio is 60% or more. Comparing the area ratios of the same ferrite, the steel with Si ≧ 1% and Al <0.1% is Si
The fatigue limit ratio is higher than that of steel containing Al <0.1% at <1%. Further, comparing the area ratios of the same ferrite, the steel with Si ≧ 1% and 0.1% <Al ≦ 1% has a fatigue limit ratio higher than that of the steel with Si ≧ 1% and Al <0.1%. To rise. From this result, it is possible to improve the fatigue limit ratio by setting the ferrite area ratio of HAZ to 60% or more, and to add 0.1% or more of Si to 0.1% <Al ≦ 1.
%, It was revealed that the effect of improving the fatigue limit ratio becomes remarkable.

As described above, mild steel for welding structures and high-strength as-rolled steel having a tensile strength of 590 MPa, which are generally used, have a high carbon equivalent value and a high HAZ hardenability. The medium heat input welding HAZ microstructure becomes the bainite-martensite structure. Therefore HA
No improvement in fatigue strength of Z can be expected. In order to reduce the susceptibility of HAZ to fatigue fracture, suppress the initiation of fatigue cracks even under stress concentration, or delay the propagation of cracks that have occurred, it is effective to make the HAZ microstructure a ferrite-based microstructure. Target. In order to make the HAZ microstructure mainly ferrite, it is necessary to reduce the HAZ hardenability. For this reason, it is necessary to limit the value of carbon equivalent, which is an indicator of the HAZ hardenability, to a limit value or less. Here, as a result of examining a carbon equivalent formula that most accurately represents the ferrite area ratio of HAZ, the following formula that considers the hardenability increasing effect of Nb in the commonly used carbon equivalent formula of IIW, Ceq (%) = C + Mn / 6 + (Cu + Ni) / 15 +
It was clarified that (Cr + Mo + V) / 5 + Nb / 3 should be used.

FIG. 3 is a plot of the ferrite area ratio of the laboratory HAZ reproduced vacuum melting steel against the above carbon equivalent. It is clear from the figure that the HAZ ferrite area ratio shows a good correlation with the carbon equivalent, and the lower the carbon equivalent value, the higher the HAZ ferrite area ratio. But,
Comparing with the same carbon equivalent value, it became clear that the steel having 1.0% or more Si increases the ferrite area ratio, and the steel having 0.5% Al added further increases the ferrite area ratio. It was From the results of FIG. 2, it is necessary to set the HAZ ferrite area ratio to 60% or more in order to improve the HAZ fatigue strength.
The upper limit of carbon equivalent is 0.2 for steel with an addition amount of less than 1.0%.
It is understood that the upper limit of carbon equivalent value should be 0.275% or less for the steel having 4% or less and the Si addition amount of 1.0% or more. It is also understood that the upper limit of the carbon equivalent value of steel containing 0.5% Al should be 0.295% or less.

As a result of examining the upper limit of carbon equivalent value by adding Al based on the result of FIG. 3, 0.1% ≦ Al ≦ 1
%, Depending on the amount of Al added, 0.025 × √
It was found that the pulling rate was Al (%). Thereby,
The upper limit carbon equivalent value is 0.24% + 0.03 × √Al (%) or less for steel with Si addition amount less than 1.0%, and the upper limit carbon equivalent value for steel with Si addition amount of 1.0% or more. 0.275
It has been clarified that it may be set to be not more than% + 0.03 × √Al (%).

The reason for improving the fatigue limit ratio by adding Al and Si is that, since both elements are ferrite-forming elements, in addition to increasing the ferrite area ratio of the HAZ structure, dislocation during fatigue repetition due to solid solution strengthening. Since the resistance to movement increases, further, the cross-slip is less likely to occur due to the decrease in stacking fault energy, and the reversibility of repeated plastic deformation is increased, so that the strain accumulated by irreversible plastic deformation is less likely to increase. Is considered to be. Such effects of Al and Si are effective not only for improving the fatigue strength of the welded portion, but also for improving the fatigue strength of the base material having a ferrite-based structure.

The area where the stress concentration is high in the HAZ of the actual welded joint is within 1.0 mm from the weld fusion line, and it is within this area that fatigue cracks occur. Therefore, it is important to set the ferrite area ratio to 60% or more in the HAZ within 1.0 mm from the weld fusion line. As is clear from the above-mentioned examination results, the skeleton of the present invention is to reduce the fatigue fracture susceptibility of HAZ and improve the fatigue strength of the welded joint by making the HAZ microstructure mainly ferrite, and to realize this. The carbon equivalent value defined above is limited according to the range of the added amounts of Al and Si.

The reason why the range of each alloying element is limited based on the above basic idea will be described below. In addition, the following% shall mean the mass%. C is an element that increases the hardenability of HAZ, and if added in a large amount, bainite and martensite structure are likely to be formed. It is desirable that the C content is low in order to increase the ferrite area ratio of HAZ and enhance the fatigue strength. However, C is an element that increases the strength of the base material, and it is desirable to add a large amount of C to increase the strength of the base material. If the C content is less than 0.015%, it is difficult to secure the strength of the base material, so the lower limit was made 0.015%. On the other hand, if it exceeds 0.15%, the HAZ hardenability becomes too high, the ferrite area ratio decreases, and the fatigue strength cannot be improved. Further, since the toughness and weld crack resistance of the base material and HAZ are reduced, the upper limit of the C content is set to 0.15%. Considering the balance between base material strength and fatigue strength, 0.02 to 0.
A C content of 09% is most desirable.

Si is an element that is essential as a deoxidizing element in addition to ensuring strength, and is an additive element that exerts an effect of improving fatigue strength as described above. If the Si content is less than 0.01%, deoxidation becomes insufficient, inclusions increase, and the toughness and ductility of the base material decrease. Therefore, the lower limit of the amount of Si is 0.01%
And The higher the Si content, the stronger the ferrite and HA.
The Z ferrite area ratio increases remarkably, and for the purpose of improving fatigue strength, it is desirable to add Si by 1% or more. However, the higher the amount of Si added, the lower the toughness of the HAZ. The decrease in toughness becomes remarkable when the Si amount exceeds 2%, so the upper limit of the Si amount was set to 2%.

Mn is an essential element for increasing the strength, but if it is less than 0.2%, the strength of the base material cannot be secured.
The lower limit value was 0.2%. On the other hand, if added in excess of 1.5%, the HAZ hardenability is increased and the HAZ microstructure cannot be made mainly of ferrite. Therefore, the upper limit of the amount of Mn is set to 1.5%. P is an element that affects the toughness of steel, and if it exceeds 0.03%, it significantly impairs the toughness of not only the base metal but also the HAZ, so it is better to minimize it as much as possible, and the upper limit of its amount is 0.03%. And S is preferably as low as P, and if it exceeds 0.01%, MnS
Precipitation becomes remarkable, the HAZ toughness of the base material is hindered, and the ductility in the plate thickness direction is also reduced. Furthermore, when a large amount of MnS inclusions are present, this becomes the starting point of fatigue cracks and causes variations in fatigue strength. Therefore, the upper limit of the S content is 0.01
%.

Al is an element which is effective for deoxidation and grain refinement of the austenite grain size, and is an additive element which exerts an effect of improving fatigue strength as described above. For the purpose of improving fatigue strength, the amount of Al added needs to be more than 0.1%, and the higher the amount, the more preferable. However, if it exceeds 1%, the fatigue strength improving effect is saturated and the toughness of the HAZ decreases, so the upper limit of the Al amount was made 1%.

Cu is an element effective for increasing the strength without lowering the toughness, but if it is less than 0.1%, it has no effect. Two
%, The HAZ hardenability becomes high, a ferrite-based structure cannot be obtained, and cracks are likely to occur during heating of the slab and welding, so the upper limit of the Cu content was set to 2%.

Ni is an element effective in improving toughness and strength, and it is necessary to add 0.1% or more to obtain the effect. If it exceeds 2.0%, the HAZ hardenability will be high, and it will not be possible to form a ferrite-based structure, which will reduce the fatigue strength, so the upper limit of the Ni content is 2.0.
%.

Cr is required to be 0.05% or more in order to enhance hardenability and ensure strength. On the other hand, if it exceeds 1%, it is not preferable for the same reason as Ni. Therefore, the upper limit of the amount of Cr is set to 1%. Mo is an element effective for improving hardenability, strength, temper embrittlement resistance, and suppressing recrystallization, and it is necessary to add 0.02% or more to obtain the effect. 1%
Since it is not possible to obtain a ferrite-based structure exceeding 1.0 and fatigue strength is lowered, and further, the base material toughness and weldability are deteriorated, the upper limit of the amount of Mo is set to 1%.

Nb forms a carbonitride and is effective in improving the strength and grain refining of the base material. When tempering heat treatment is performed after rolling and cooling, fine Nb carbonitrides can be precipitated to further improve strength. If the amount of Nb is less than 0.005%, this effect is not remarkable, so the lower limit was made 0.005%. On the contrary, if the content exceeds 0.08%, the HAZ hardenability becomes too high and the ferrite area ratio cannot be set to 60% or more. Therefore, the upper limit of the Nb amount is set to 0.
It was set to 08%. V forms a carbonitride and is effective in improving the strength of the base material and making it finer. When tempering heat treatment is carried out after rolling and cooling, fine V carbonitrides can be precipitated to further improve the strength. If the amount of V is less than 0.005%, this effect is not significant, so the lower limit was made 0.005%. On the contrary, if the content exceeds 0.1%, the HAZ hardenability becomes too high and the ferrite area ratio cannot be set to 60% or more. Therefore, the upper limit of the V content was set to 0.1%.

Ti is an element that contributes to the improvement of the strength of the base material by precipitation strengthening, and is effective for refining the heated austenite grain size by forming TiN that is stable even at high temperatures. Further, as will be described later, MgO necessary for improving the HAZ toughness,
Contributes to fine dispersion of the Mg-containing oxide. In order to exert the effect, it is necessary to contain 0.001% or more. on the other hand,
If it exceeds 0.05%, coarse oxides are formed, the ductility is extremely deteriorated, and the origin of fatigue cracks is caused. Therefore, the upper limit of the Ti content is set to 0.05%. N is Al
When combined with or Ti, it works effectively for refining austenite grains, so a small amount contributes to the improvement of mechanical properties. Also,
Industrially, it is impossible to completely remove N in steel, and it is not preferable to reduce N more than necessary because it puts an excessive load on the manufacturing process. Therefore, the lower limit is set to 0.001% as a range in which industrial control is possible and the load on the manufacturing process is allowable. If it is contained in excess, the amount of solid solution N increases, which may adversely affect the ductility and toughness. Therefore, the upper limit of the amount of N is set to 0.008% as an allowable range.

Next, in order to improve ductility and HAZ toughness, one or more of Mg, Ca and REM may be contained, if necessary. Mg is 0.000
If less than 1% is added, it is not possible to fully expect the generation of oxides necessary for pinning particles for intragranular transformation and austenite grain refinement, so the lower limit is 0.0001%.
And If it exceeds 0.01%, a coarse oxide is likely to be formed, resulting in deterioration of the base material and HAZ toughness. Therefore, the upper limit of the amount of Mg is set to 0.01%. Ca, REM
All of these are effective in improving the ductility characteristics by suppressing the expansion of the sulfide during hot rolling. It also works effectively to improve the HAZ toughness by refining the oxide. 0.000 for both Ca and REM
If it is less than 5%, this effect cannot be obtained, so the lower limit is set to 0.
It was 0005%. On the contrary, if it exceeds 0.005%, C
a, the number of oxides in REM increases, the number of ultrafine Mg-containing oxides decreases, or coarsening of sulfides and oxides occurs, leading to deterioration of ductility and toughness, so its upper limit value is 0. It was set to 0.005%.

Next, the reason for limiting the manufacturing conditions will be described. The present invention provides a thick steel plate for welded structure having a tensile strength of 590 MPa grade from mild steel which is excellent in fatigue strength of the welded portion while ensuring the toughness of the welded portion. When manufacturing mild steel having the above-mentioned tensile strength and thick steel plate for welded structure of 590 MPa class, it is possible to adopt the conventional hot rolling method, but when the carbon equivalent value defined above is low. Alternatively, when the plate thickness is large, the required strength may not be obtained by the conventional hot rolling method. In such a case, the base material strength can be increased by the controlled rolling method or the controlled rolling / accelerated cooling method.

In both conventional hot rolling and controlled rolling, the steel ingot needs to be austenitized to 100% before rolling, and therefore the steel ingot needs to be heated to Ac ₃ point or higher. However, if heating is performed above 1250 ° C., austenite grains become coarse and fine grains cannot be obtained after rolling. Therefore, it is necessary to set the heating temperature to 1250 ° C. or lower.

Since the austenite grains are coarsened by heating the steel ingot, it is necessary to reduce the austenite grain size by rolling in the recrystallization temperature range in both the conventional hot rolling and controlled rolling methods. When the strength is increased and the toughness is improved by using the controlled rolling method, it is effective to introduce a deformation zone into the austenite grains by further rolling in the non-recrystallization temperature range to increase ferrite formation nuclei.
If the cumulative rolling reduction in the non-recrystallization temperature range is less than 40%, the deformation zone is not sufficiently formed, so the lower limit of the cumulative rolling reduction in the non-recrystallization temperature range was set to 40%. However, the cumulative reduction rate is 90%
When it exceeds, the upper shelf impact value in the base material Charpy test remarkably decreases and the low cycle fatigue property deteriorates. Therefore, the upper limit of the cumulative rolling reduction in the non-recrystallization temperature range was set to 90%.

There is no particular limitation on the finish rolling temperature, and the rolling may be completed at the Ar ₃ point or higher.
Rolling in the ferrite and austenite coexistence region or in the ferrite region at ₃ points or less is acceptable. After rolling, in the case of natural air cooling, ferrite is generated from austenite grain boundaries and intragranular deformation zones, and fine-grained ferrite can be obtained compared to conventional rolling without rolling in the non-recrystallization temperature range. Increased strength and toughness can be achieved.

Accelerated cooling is necessary to further increase the strength compared to natural air cooling. If the cooling rate is less than 1 ° C / sec, the degree of supercooling is small and the ferrite after transformation is not sufficiently refined. At the same time, the diffusion of C during transformation is easy and the C concentration in ferrite decreases, It is not possible to obtain sufficient strength. On the other hand, if the cooling rate exceeds 60 ° C./sec, the toughness of the base material decreases because a bainite structure is generated. Therefore, the cooling rate is limited to 1 to 60 ° C./sec. Considering the balance between strength and toughness of the base material, the range of 5 to 30 ° C./sec is desirable.

In the present invention, accelerated cooling must be continued until the transformation is completed in order to obtain the strength of the base material. Therefore, the upper limit of the cooling stop temperature is set to 600 ° C. 600
At a stopping temperature above 0 ° C, the transformation does not end, so sufficient strength cannot be obtained. Normally, accelerated cooling uses water as a coolant. In this case, the lower limit of the actual cooling stop temperature is 0 ° C, so the lower limit was set to 0 ° C.

Tempering heat treatment performed after rolling and cooling
The purpose is to improve the toughness of the base metal structure by recovery.
Therefore, the heating temperature is a temperature range where reverse transformation does not occur.
Ac ₁Must be below the point. Recovery is the disappearance of dislocations
Coalescence reduces the lattice defect density.
In order to realize
is there. Therefore, the lower limit of the heating temperature is set to 300 ° C. Well
Also, as described above, the precipitation elements of Cu, Mo, Nb, and V
When it contains, fine precipitates are generated by heat treatment.
By doing so, the strength of the base material can be improved. This
Is extremely effective in improving the base metal strength of the steel of the present invention having a low carbon equivalent value.
It is also very effective. Most effective precipitation effect
The heating temperature for volatilization depends on the precipitation effect element, but
It is in the range of 500 to 650 ° C. Cooling stop temperature after rolling
If the temperature is 600 ° C or less and the temperature is relatively high, self-baking
Since tempering can be expected, this tempering heat treatment can be omitted.
Both are possible.

[0039]

EXAMPLES Examples of the present invention will be described below. A steel plate having a plate thickness of 20 to 40 mm was manufactured from the slab manufactured by continuous casting. Table 1 shows the chemical components. Steels 1 to 22 are steels of the present invention, and steels 23 to 32 are comparative steels. Table 2 shows the manufacturing conditions and tensile properties of the steel sheet. Invention Steels 1-3, Comparative Steel 2
Nos. 3 and 24 were produced by the controlled rolling method shown in claim 10 of the present invention, and the present invention steels 8 to 11, 15 to 22, and comparative steel 29,
No. 32 was manufactured by the controlled rolling / controlled cooling method according to claim 11 or 12. The other steel plates were manufactured by a conventional hot rolling method. The heating temperature is above the Ac ₃ transformation point in all steels. Further, the tempering temperatures of the steels subjected to tempering heat treatment after controlled rolling / controlled cooling are all 600 ° C. or lower and not higher than the Ac ₁ transformation point. From this, in the steel of the present invention, mild steel to 590
A strength of MPa was confirmed.

T-shaped fillet welded joints were prepared using these test steels. Table 4 shows the welding conditions. Fatigue strength of welded joints shows thickness dependence. In order to remove the plate thickness dependence, the back surface of a steel plate having a plate thickness of more than 20 mm was cut to a thickness of 20 mm and then welded. FIG. 4 shows the shape of a 3-point bending fatigue test piece prepared from a T-shaped fillet welded joint. A fatigue test was carried out under the condition that the ratio of the maximum load and the minimum load was 0.1.

Table 3 shows the fatigue test results. The fatigue strength of welded joints was compared using the fatigue strength of 10 ⁶ times and the fatigue limit as indexes. From this, it was confirmed that in the steel sheet for high-strength welded structural steel having a tensile strength of 390 to 590 MPa class in which the HAZ structure is more than 60% of ferrite, the steel of the present invention has a higher fatigue strength than that of the comparative steel. Comparative steels 23-2
The fatigue strength of No. 6 is relatively high because Ceq is within the range of the present invention.
Was confirmed to be significantly improved over Comparative Steels 23 to 26.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

[0045]

[Table 4]

[0046]

As described above, according to the present invention, the HAZ microstructure is controlled so as to have a ferrite-based structure, so that the fatigue strength of the welded joint is improved without reducing stress concentration due to additional welding. The present invention makes it possible to improve the reliability of the welded structure against fatigue fracture. Therefore, it can be said that the industrial value of the present invention is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing tensile strength and microstructure dependence of a fatigue limit ratio in a fatigue test of a notched reproduced HAZ material.

FIG. 2 is a diagram showing a ferrite area ratio dependency of a fatigue limit ratio in a fatigue test of a notched reproduced HAZ material.

FIG. 3 is a diagram showing a carbon equivalent dependency of a ferrite area ratio of a reproduced HAZ material.

FIG. 4 is a view showing the shape of a T-shaped fillet welded joint fatigue test piece.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shuji Aiwahara             20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel shares             Company Technology Development Division F-term (reference) 4K032 AA01 AA04 AA05 AA08 AA11                       AA14 AA15 AA16 AA19 AA21                       AA22 AA23 AA24 AA27 AA29                       AA31 AA32 AA35 AA36 AA40                       BA01 CA01 CA02 CA03 CB02                       CD01 CD02 CD03 CF01

Claims (12)

[Claims] 1. In mass%, C: 0.015 to 0.15%, Si: 0.01% to less than 1%, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.01% or less, Al: more than 0.1% and 1% or less, the balance Fe and unavoidable impurities, and Ceq ≦ 0.24 + 0.03 × √Al. Steel plate for welded structure with excellent fatigue strength.
However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 +
(Cr + Mo + V) / 5 + Nb / 3 2. In mass%, C: 0.015 to 0.15%, Si: 1 to 2%, Mn: 0.2 to 1.5%, P: 0.03% or less, S: 0.0. 01% or less, Al: more than 0.1% and 1% or less, consisting of the balance Fe and unavoidable impurities, and satisfying Ceq ≦ 0.275 + 0.03 × √Al. Excellent thick steel plate for welded structures. However, Ceq = C + Mn / 6 + (Cu + Ni) / 1
5+ (Cr + Mo + V) / 5 + Nb / 3 3. The thick steel plate for welded structure having excellent fatigue strength of the welded joint according to claim 1 or 2, further containing Cu: 0.1 to 2% by mass%. 4. The welded structure with excellent fatigue strength of the welded joint according to claim 1, further comprising Ni: 0.1 to 2% by mass. Thick steel plate. 5. The composition according to claim 1, further comprising one or two of Cr: 0.05 to 1% and Mo: 0.02 to 1% by mass. A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to item 1. 6. The composition according to claim 1, further comprising, in mass%, one or two of Nb: 0.005 to 0.08% and V: 0.005 to 0.1%. 5. A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 5 above. 7. The composition according to claim 1, further comprising: Ti: 0.001 to 0.05% and N: 0.001 to 0.008% in mass%. A thick steel plate for welded structures, which has excellent fatigue strength of the welded joint described in. 8. In mass%, Mg: 0.0001 to 0.01%, Ca: 0.0005 to 0.005%, and REM: 0.0005 to 0.005% of one or more kinds, A thick steel plate for a welded structure having excellent fatigue strength of the welded joint according to any one of claims 1 to 7, further comprising: 9. In the production of the steel sheet according to any one of claims 1 to 8, a steel ingot has an Ac ₃ point or more at 1250 ° C.
A method for producing a thick steel plate for welded structure excellent in fatigue strength of a welded joint, characterized by comprising heating to the following, hot rolling in a recrystallization temperature range, and natural cooling. 10. The hot rolling in the recrystallization temperature range is followed by a cumulative reduction of 40 to 90% in the non-recrystallization temperature range.
10. The method for producing a thick steel plate for welded structure having excellent fatigue strength of the welded joint according to claim 9, wherein the hot rolling is performed. 11. In the production of the steel sheet according to claim 1, the steel ingot has Ac ₃ points or more and 1250 or more.
After heating to ℃ or less, hot rolling in a recrystallization temperature range, followed by hot rolling at a cumulative reduction of 40 to 90% in a non-recrystallization temperature range, and then 600 at a cooling rate of 1 to 60 ° C / sec. ℃
A method for producing a thick steel plate for welded structure, which is excellent in fatigue strength of a welded joint, characterized by cooling to the following temperature. 12. The method according to claim 11, further comprising, after cooling, heating at 300 ° C. to an Ac ₁ point for tempering heat treatment.
The method for producing a thick steel plate for welded structures, which has excellent fatigue strength of the welded joint according to.