JPH0873983A - Thick steel plate for welded structure, excellent in fatigue strength in weld joint, and its production - Google Patents

Abstract

PURPOSE: To improve fatigue strength in the weld joint of a structure by control ling the structure of a weld heat-affected zone (HAZ). CONSTITUTION: This steel plate has a chemical composition which contains, by weight, 0.015-0.15% C, 0.01-2.0% Si, 0.2-1.5% Mn, <=0.03% P, and <=0.01% S as essential components and contains, if necessary, one or more elements among Al, Cu, Ni, Cr, Mo, Nb, V, Ti, N, Ca, and REM and in which the value of carbon equivalent is regulated to <=0.24% when Si is <1.0% and to <=0.275% when Si is >=1.0%.

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Application] The present invention is used in the fields of shipbuilding, offshore structures, bridges, etc., and has a welded joint mild steel excellent in fatigue strength of a welded joint and a high tensile strength of 50 kgf / mm ² class. The present invention relates to thick steel plates and their manufacturing methods.

[0002]

2. Description of the Related Art With the increasing size of welded structures and the increasing demands for environmental protection, structural members are required to have higher reliability than ever before. Fatigue fracture, brittle fracture, ductile fracture, etc. are possible fracture modes assumed in a welded structure. Of these, fatigue fracture is the most frequently occurring fracture mode in actual use environments. This is the most important issue for improving the reliability of products. There are many cases in which fatigue failure has become a problem, such as the recent occurrence of fatigue cracks in large tankers and collapse starting from fatigue cracks in offshore structures.

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 discloses 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.

To date, a great deal of research has been conducted on factors that govern the fatigue strength of welded members and improvements in fatigue strength.
Weld fatigue strength is improved by reducing the stress concentration by improving the weld toe shape, such as grinding and grinding and shaping the toe shape by heating and re-melting the final layer of the weld bead, shot peening, etc. Most of the improvements were due to mechanical factors such as the generation of compressive residual stress at the weld toe portion (JP-A-59-110490, JP-A-1-301823, etc.). Further, the effect of reducing the residual stress by the heat treatment after welding is well known in the past.

On the other hand, there has been proposed a method of improving the fatigue strength of a thick steel plate welded joint by limiting the components of the steel material without using the above-mentioned special construction and post-weld heat treatment. In Japanese Patent Application No. 4-294544, Cu is added to 0.
Ultra-low C steel containing 5 to 2.0% has low welding residual stress,
At the same time, it is described that the fatigue strength of the welded joint is improved because the strength of the weld heat affected zone (hereinafter referred to as HAZ) is secured.

Although the relationship between the microstructure and the fatigue strength of the heat-affected zone of the weld has not been clarified so far, in JP-A-5-345928, the fatigue strength of the HAZ structure is due to the formation of island martensite. It has been shown to improve. That is, it is described that when hard island martensite is present in the HAZ structure, microfatigue cracks once generated are prevented or delayed from propagating and the fatigue strength is substantially improved.

[0007]

Of these, the invention described in JP-A-61-96057 improves the fatigue strength by limiting the bainite area ratio to a specific range, but this is thin. The present invention relates to the improvement of the fatigue strength of a steel plate base material, and cannot be applied to the improvement of the fatigue strength of butt welding or fillet welding of thick steel plates, which is the object of the present invention.

In the inventions described in JP-A-59-110490 and JP-A-1-301823, it is necessary to perform special work after welding, and the fatigue strength cannot be improved as it is. The invention described in Japanese Patent Application No. 4-294544 is intended to improve the fatigue strength of a welded portion, but relates to an ultra-low carbon steel having a C content of 0.010% or less, and a general thick steel plate for welded structure. Cannot be applied to.

The invention described in Japanese Unexamined Patent Publication No. 5-345928 discloses a special post-weld heat treatment in which the welded portion is heated to an intermediate temperature range of Ac ₁ to Ac ₃ and then cooled to generate island martensite. Since it is applied, it is impossible to improve the fatigue strength as it is. The present invention does not improve the fatigue strength by the additional welding work for realizing the reduction of the stress concentration degree and the reduction of the welding residual stress, and the
Welded structural mild steel sheet with improved fatigue strength of butt welded joints or fillet welded joints by controlling the microstructure of AZ, high-strength steel sheet having a tensile strength of 50 kgf / mm ² grade, and methods for producing the same The purpose is to provide.

[0010]

The inventors of the present invention conducted systematic experiments on fatigue crack initiation / propagation in welds and its microstructure dependence, and found that fatigue crack initiation / propagation was most likely to occur. It has been found that the HAZ microstructure that is effectively suppressed is ferrite. Based on this, it was found that by limiting the carbon equivalent value, the ferrite structure fraction of HAZ can be increased and the fatigue strength of the welded joint can be improved.

That is, the gist of the present invention is as follows. (1) In weight%, 0.015 ≦ C ≦ 0.15, 0.01 ≦ Si <1.0, 0.2 ≦ Mn ≦ 1.5, P ≦ 0.03, S ≦ 0.01, Ceq ≦ 0.24, balance steel and unavoidable impurities, and a thick steel plate for welded structure excellent in fatigue strength of a welded joint.

However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3 (2) wt%, 0.015 ≦ C ≦ 0.15, 1.0 ≦ Si ≦ 2.0, 0.2 ≦ Mn ≦ 1.5, P ≦ 0.03, S ≦ 0.01, Ceq ≦ 0.275, balance Fe and unavoidable impurities, and a thick steel plate for welded structure excellent in fatigue strength of a welded joint.

However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3 (3) wt%, 0.005 ≦ Al ≦ 0.10 is contained, and the welded structure excellent in fatigue strength of the welded joint according to the above (1) or (2). Thick steel plate. (4) The welded structure having excellent fatigue strength of the welded joint according to any one of (1) to (3) above, characterized by containing 0.1 ≦ Cu ≦ 2.0 in weight%. Thick steel plate.

(5) The welded joint according to any one of the above items (1) to (4) is excellent in fatigue strength, characterized in that it contains 0.1 ≦ Ni ≦ 2.0 in a weight percentage. Steel plate for welded structure.

(6) One or two of 0.05≤Cr≤1.0 and 0.02≤Mo≤1.0 are contained in a weight percentage of the above (1) to (5). ) A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 1).

(7) One or two of 0.005≤Nb≤0.08 and 0.005≤V≤0.10. ) A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 1).

(8) 0.001≤Ti≤0.025, 0.001≤N≤0.010, Ti / N≤5.0 at a weight percentage of the above (1) to (1) A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of (7).

(9) One or two of 0.0005≤Ca≤0.005 and 0.0005≤REM≤0.005 are contained in a weight percentage of the above (1) to (8). ) A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 1).

(10) Any one of (1) to (9) above
A steel ingot having the same composition as the steel described in the item ₃ or more Ac,
A method for producing a thick steel plate for welded structure excellent in fatigue strength of a welded joint, which comprises heating to 1250 ° C. or lower, hot rolling in a recrystallization temperature range, and natural cooling. (11) A steel ingot having the same composition as the steel according to any one of (1) to (9) above is heated to an Ac ₃ point or higher and 1250 ° C. or lower, and then hot rolled in a recrystallization temperature range, A method for producing a thick steel plate for welded structure excellent in fatigue strength of a welded joint, which is characterized by performing hot rolling at a cumulative reduction of 40 to 90% in a non-recrystallization temperature range and then performing natural cooling.

(12) Any one of (1) to (9) above
A steel ingot having the same composition as the steel described in the item ₃ or more Ac,
After heating below 1250 ° C, hot rolling in the recrystallization temperature range,
Then, in the non-recrystallization temperature range, the cumulative rolling reduction was 40 to 9
A method for producing a thick steel plate for welded structure excellent in fatigue strength of a welded joint, which comprises performing 0% hot rolling and then cooling to 0 to 600 ° C at a cooling rate of 1 to 60 ° C / sec.

(13) Any one of (1) to (9) above
The steel ingot having the same composition as the steel described in the item₃More than points,
After heating below 1250 ° C, hot rolling in the recrystallization temperature range,
Then, in the non-recrystallization temperature range, the cumulative rolling reduction was 40 to 9
After 0% hot rolling, cooling rate of 1-60 ° C / sec
Cooling to 0-600 ℃, then 300 ℃ -Ac ₁
Welded joint characterized by heating to a point and tempering heat treatment
A method for manufacturing a thick steel plate for welded structures, which has excellent hand fatigue strength.

[0022]

FUNCTION The present inventors first carried out a detailed microscopic observation of the state of crack initiation / propagation in a fatigue test piece of a welded joint.
As a result, most fatigue cracks are at the weld metal / HAZ boundary, or fusion line.
It was found that it is generated near the boundary between the weld metal and the HAZ), propagates in the HAZ, and further rushes into the base metal portion, resulting in the total destruction of the test piece. This is because the vicinity of the weld fusion line coincides with the weld toe and the stress concentration is highest at this portion. In this way, fatigue cracks are generated near the weld fusion line and HA
It was revealed that the fatigue strength is greatly affected by the HAZ microstructure because it propagates in the Z.

Next, a systematic experiment was carried out to clarify the effect of the HAZ structure on the fatigue strength, and the following important findings were obtained. As described above, the fatigue crack initiation portion is near the weld fusion line, and the initial stage of crack propagation is within the HAZ. 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. That is, a test piece provided with stress concentration is machined from a steel material subjected to a welding reproduction heat cycle,
A fatigue test was performed to determine the relationship between the HAZ microstructure and fatigue strength. The outer dimensions of the test piece are 10 × 10 × 55 mm, the notch depth is 2 mm, the notch tip radius is 0.75 mm, the fulcrum distance is 40 mm, and a three-point bending repeated load is applied.
Fatigue destroyed. The stress concentration factor is 2.6.

FIG. 1 shows mild steel and a tensile strength of 50 kgf /
Fatigue limit of reproduced HAZ material that was given a simulated welding cycle with maximum heating temperature of 1400 ° C and cooling time of 800 to 500 ° C for 1 to 30 seconds, using laboratory vacuum melting steel with strength of mm ² class The ratio (fatigue limit / tensile strength of the reproduced HAZ material) is plotted against the tensile strength of the reproduced HAZ material. Reproduction H observed with a 200x optical microscope
The area ratio of the microstructure was measured from the microstructure photograph of the AZ material by the point counting method, the microstructure occupying 60% or more of the area ratio was determined, and the plot data was classified according to the type of the microstructure. As is clear from the figure, the fatigue limit ratio largely depends on the HAZ microstructure. That is, it has been found that the fatigue limit ratio becomes higher in the order of martensite, lower bainite, lower bainite / upper bainite mixed structure, upper bainite, and ferrite, and the ferrite structure has the best fatigue property.

Mild steel for welded structure which is generally used (a typical composition is 0.14% C-0.2% Si-0.9).
% Mn) and tensile strength of 50 kgf / mm ² grade as-rolled high-strength steel (typical composition is 0.17% C-0.3% Si)
-1.4% Mn) has a high carbon equivalent value and high hardenability of HAZ. Therefore, in the small / medium heat input welding where the heat input is 50 kJ / cm or less, the HAZ microstructure is mainly bainite or martensite. Becomes Therefore, it can be understood from FIG. 1 that such steel has a low fatigue limit ratio and fatigue fracture easily occurs from the HAZ. From this experiment, HAZ
It was clarified for the first time that the fatigue limit ratio can be increased and the fatigue strength of the welded joint can be improved by making the microstructure of (1) a ferrite-based structure.

As shown in FIG. 1, in the fatigue test having stress concentration, the fatigue limit ratio becomes higher as the high temperature transformed structure becomes higher, and conversely, the fatigue limit ratio becomes lower as the low temperature transformed structure becomes. The reason why such fatigue strength depends on the microstructure has not been completely clarified, but (1) the low temperature transformation structure has a higher dislocation density introduced during transformation, and the dislocations are rearranged when subjected to repeated stress. Therefore, dislocation strengthening does not contribute much to fatigue strength. (2) In low temperature transformation structure, the strength of bainite or martensite lath interface or former austenite grain boundary is relatively higher than that of intragranular structure. Fatigue cracks easily occur at lath interfaces and old austenite grain boundaries. (3) Plastic deformation at the propagating crack tip is remarkable in ferrite structure, plastic absorbed energy increases, and as a result, cracks increase. Possible reasons include delaying the propagation.

It is known that in a smooth test piece having a small stress concentration, the fatigue strength has little microstructure dependence, and rather has a high correlation with static tensile strength. As shown above, the fatigue strength of the reproduced HAZ material is affected by the microstructure, and the fact that the fatigue limit ratio rises 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.

As described above, it is most desirable to make the HAZ microstructure a ferrite structure in order to improve the fatigue strength, but it is particularly preferable to make the HAZ a 100% ferrite structure in the continuous cooling transformation which is received during welding. Is large
Medium heat input welding is difficult, and inevitably a structure such as bainite, which has a lower transformation temperature than ferrite, is mixed. However, since the upper bainite has the next highest fatigue limit ratio after ferrite, even if some upper bainite is mixed in, HA
It can be expected that the fatigue strength of Z will not be significantly reduced.

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 (1) the fatigue limit ratio increases as the ferrite area ratio increases. Further, if the ferrite area ratio is 60% 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. (2) Comparing with the same ferrite area ratio, the steel with Si added by 1.0% or more is Si
The fatigue limit ratio is further increased as compared with steel in which the addition amount is less than 1.0%. From these results, it was revealed that the fatigue limit ratio can be improved by setting the ferrite area ratio of HAZ to 60% or more, and the effect of improving the fatigue limit ratio becomes remarkable when Si is added to 1.0% or more. .

As described above, mild steel for welded structure and tensile strength of 50 kgf / mm ² which are generally used.
High-strength as-rolled high-strength steel has a high carbon equivalent value and high hardenability of HAZ.
The AZ microstructure becomes a bainite martensite structure. Therefore, the HAZ fatigue strength cannot be improved. HA
In order to reduce the susceptibility of Z to fatigue fracture, prevent the occurrence 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 structure. 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 investigating a carbon equivalent formula that most accurately expresses 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 + (Cr
It became clear that + Mo + V) / 5 + Nb / 3 should be used.

FIG. 3 is a plot of the ferrite area ratio of the laboratory vacuum-melted steel reproduced HAZ 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,
When compared with the same carbon equivalent value, it became clear that the steel having Si added in an amount of 1.0% or more has a further increased ferrite area ratio. From the results of FIG. 2, it is necessary to set the ferrite area ratio of HAZ to 60% or more in order to improve the HAZ fatigue strength, but in order to realize this, in the steel in which the amount of Si added is less than 1.0%, Has a carbon equivalent value of 0.24% or less, S
Carbon equivalent value is 0.27 for steel with i addition of 1.0% or more
It can be seen that it should be 5% or less. It is possible to raise the carbon equivalent upper limit to 0.275% by adding Si by 1.0% or more. Therefore, not only can the fatigue strength of the HAZ be improved by adding Si by 1.0% or more, It becomes easy to secure the strength of the base material even with a thick steel plate.

The reason for improving the fatigue limit ratio by adding Si is (1) because Si is a ferrite-forming element, in addition to increasing the ferrite area ratio of the HAZ structure, (2) solid solution strengthening of Si The resistance to the movement of dislocations during fatigue cycling increases, and (3) cross-slip is less likely to occur due to the reduction of stacking fault energy, and the reversibility of cyclic plastic deformation increases, so that it accumulates due to irreversible plastic deformation. It is considered that this is because it is difficult for the distortion to increase. Such an effect of Si exerts an effect not only on the fatigue strength of the welded portion but also on 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 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 in accordance with the range of the Si addition amount.

The reason why the range of each alloying element is limited based on the above basic idea will be described below. C is an element that increases the hardenability of HAZ, and if it is added in a large amount, bainite and martensite structure are easily generated. In order to increase the ferrite area ratio of HAZ and enhance the fatigue strength, it is desirable that the C content be low. 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%, the strength of the base material cannot be secured, 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. Furthermore, the toughness of the base material and HAZ is significantly reduced. Therefore, the upper limit of the amount of C is set to 0.15%. Considering the balance between base material strength and fatigue strength improvement, 0.02-
A C content of 0.07% is most desirable.

Si is an element that is useful as a deoxidizer 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 ductility and toughness of the base material decrease. Therefore, the lower limit of the amount of Si is set to 0.01%. Si
The higher the amount added, the more remarkable the strengthening of ferrite and the increase in the ferrite area ratio of HAZ. For the purpose of improving fatigue strength, the higher the amount of Si added, the more desirable. However, the higher the amount of Si added, the lower the toughness of the base metal and HAZ. The decrease in toughness becomes remarkable when the Si content exceeds 2.0%. For this reason,
The upper limit of the amount of Si was set to 2.0%.

Mn is an element effective for securing the strength of the base material.
If the amount of Mn is less than 0.2%, the strength of the base material cannot be secured, so the lower limit was made 0.2%. On the other hand, if over 1.5% is added, the HAZ hardenability is increased, and the HAZ microstructure cannot be composed mainly of ferrite. Therefore, the upper limit of the amount of Mn is set to 1.5%. The lower P is, the more preferable.
If the content exceeds 03%, the toughness of the base material and HAZ is significantly reduced. Therefore, the upper limit of the amount of P is set to 0.03%.

The lower the S content, the more preferable it is. If it exceeds 0.01%, MnS precipitation becomes remarkable, the toughness of the base metal and HAZ is impaired, and the ductility in the sheet 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 amount of S is set to 0.01%. Al is an element added as necessary, and is an element effective in reducing inclusions in steel by deoxidation. If Al is less than 0.005%, deoxidation is insufficient and inclusions in steel cannot be reduced. Therefore, the lower limit is 0.0
It was set to 05%. On the other hand, if the content exceeds 0.10%, alumina-based inclusions increase and ductility decreases, and fatigue cracks easily occur. Therefore, the upper limit is set to 0.10%. When a strong deoxidizing element such as Ti, Ca or REM is contained, Al can be 0.005% or less.

Cu is an element effective in increasing the strength of the base material. If the amount of Cu is less than 0.1%, the effect of increasing strength cannot be expected, so the lower limit was made 0.1%. Cu not only contributes to an increase in the strength of the base material by improving the hardenability and solid solution strengthening, but also the strength of the base material can be significantly increased by precipitating fine Cu by tempering heat treatment after rolling and cooling. This effect is particularly effective in the steel of the present invention having a low carbon equivalent value. Cu is 0.4 in order to exert precipitation hardening.
% Or more must be added. However, if over 2.0% is added, the HAZ hardenability becomes high, a ferrite-based structure cannot be obtained, and casting cracks easily occur, so the upper limit of the Cu content was set to 2.0%.

Ni is an element that improves the toughness of the base metal and HAZ, and it is necessary to add 0.1% or more to improve the toughness. However, if added over 2.0%, HA
Z hardenability becomes high, and the HAZ structure cannot be mainly composed of ferrite. Therefore, the upper limit of the Ni content is 2.0
%. Cr is an element effective not only for improving the hardenability but also for improving the strength of the base material. If it is less than 0.05%, the effect of increasing the strength of the base material is not significant, so the lower limit of the Cr content was made 0.05%. Conversely, if over 1.0% is added, H
AZ hardenability becomes too high and ferrite area ratio is 60
% Or more, and the base metal and HAZ
Toughness is markedly reduced. Therefore, the upper limit of the amount of Cr is set to 1.0%.

Mo is an element effective not only for improving the hardenability but also for improving the strength of the base material. When tempering heat treatment is carried out after rolling and cooling, fine Mo carbide is precipitated to further improve strength. Mo amount is 0.
If it is less than 02%, the effect of improving the strength of the base material is not significant, so
The lower limit was 0.02%. On the contrary, if the content exceeds 1.0%, the HAZ hardenability becomes too high, and the ferrite area ratio cannot be made 60% or more, and the toughness of the base material and HAZ is significantly lowered. Therefore, the upper limit of the amount of Mo is set to 1.0%.

Nb forms a carbonitride and is effective in improving the strength and grain refining of the base material. When tempering heat treatment is carried out after rolling and cooling, fine Nb carbonitrides can be precipitated to further improve the 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 0.080%, the HAZ hardenability becomes too high, and the ferrite area ratio cannot be made 60% or more. Therefore, the upper limit of the Nb amount is 0.0
It was set to 80%.

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 are precipitated to further improve the strength. If the amount of V 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 0.10%, the HAZ hardenability becomes too high and the ferrite area ratio cannot be made 60% or more. Therefore, the upper limit of the amount of V is set to 0.10%.

Ti can be obtained by combining Ti with an appropriate amount of N.
N is generated to suppress coarsening of the austenite grains of the HAZ and to reduce the solute N so that the HAZ toughness is improved. Further, by using TiN as a nucleus, ferrite is also generated from inside the austenite grains, and the HAZ toughness is improved. If the Ti content is less than 0.005%, these effects are not remarkable, so the lower limit was made 0.005%. But,
If added in excess of 0.025% or Ti / N ratio exceeds 5.0, a large amount of Ti carbide is generated and the base metal and HAZ
Toughness is reduced. Therefore, the upper limit of the Ti addition amount is set to 0.025% and the upper limit of Ti / N is set to 5.0.

N combines with Ti to form TiN, and HA
The toughness is improved by suppressing the austenite grain growth of Z and the intragranular transformation ferrite. This effect cannot be expected if the amount of N is less than 0.001%, so the lower limit was made 0.001%. However, if added in excess of 0.010%, the amount of solute N increases and the toughness of the base material and HAZ decreases. Therefore, the upper limit of the amount of N is set to 0.010%. Although TiN can be produced by adding an appropriate amount of Ti and N to exert the above effect, in order to reduce the adverse effects of excess N or excess Ti, the Ti / N ratio is set to a range of 2.0 to 3.4. It is desirable to do.

Ca fixes S as CaS and reduces the amount of MnS produced. Coarse MnS may be a starting point of fatigue fracture, so that the addition of Ca can reduce the variation in fatigue strength. If the amount of Ca added is less than 0.0005%, the above effect is not remarkable. Therefore, the lower limit of the amount of Ca added is set to 0.0005%. On the other hand, if Ca is added in an amount of more than 0.005%, coarse Ca oxides and sulfides are generated, and this easily becomes the starting point of fatigue fracture. Therefore, the upper limit of the amount of Ca added is set to 0.005%.

REM has the same effect as Ca described above. R
As EM, both lanthanoid series and actinide series have similar effects, but typical ones are lanthanide series La and Ce. If the amount of REM added is less than 0.0005%, the effect of lowering the amount of MnS produced is not significant, so the lower limit was made 0.0005%. Conversely, REM is 0.0
If added in excess of 05%, coarse REM oxides / sulfides will be generated, and this will easily become the starting point of fatigue fracture. Therefore, R
The upper limit of the amount of EM added was 0.005%.

Next, the reasons for limiting the steel sheet manufacturing conditions will be described. The present invention provides a thick steel plate for welded structure having a tensile strength of 50 kgf / mm ² grade from mild steel having excellent weld fatigue strength, and as the strength of the steel plate, the yield stress is 24 kgf / mm ² or more in the mild steel class. , Tensile strength is 41kgf
/ Mm ² or more, 50 kgf / mm ² grade high-strength steel, yield stress is 36 kgf / mm ² or more, tensile strength is 48 kgf
/ Mm ² or more is mainly targeted. However, the improvement of the weld fatigue strength according to the present invention can be realized even for the steel having a strength level lower than that of the above mild steel.

When a mild steel having the above-mentioned yield stress and tensile strength and a 50 kgf / mm ² class high-strength steel are to be produced, it is possible to adopt a conventional hot rolling method, but the above definition is used. When the carbon equivalent value is particularly low in the range of 0.24% or less, or 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, it is necessary to convert the steel ingot to 100% austenite prior to rolling. For this purpose, the steel ingot needs to be heated to a temperature of Ac ₃ point or higher. However, if the heating temperature exceeds 1250 ° C., the austenite grains become coarse and fine particles 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. When the strength increase and the toughness improvement are measured by 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 the ferrite formation nuclei.
If the cumulative rolling reduction in the non-recrystallization region 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 region 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%.

The limitations on the finish rolling temperature is not particularly necessary, it may be terminated rolling at Ar ₃ point or more, Ar ₃
Below the point, the coexistence region of ferrite and austenite,
Alternatively, it may be rolled in the ferrite region. After rolling, in the case of natural air cooling, ferrite is generated from the austenite grain boundaries and the intragranular deformation zone, 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 required to further increase the strength compared to natural air cooling. When 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. , Can not get enough strength. On the other hand, if the cooling rate exceeds 60 ° C./sec, the toughness of the base material decreases because a bainite-based 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 metal, 5
It is desirable to set it in the range of -30 ° C / sec.

In the present invention, in order to obtain the strength of the base material, it is necessary to continue the accelerated cooling until the transformation is completed. 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. Generally, accelerated cooling uses water as the cooling medium. In this case, the actual lower limit of the cooling stop temperature is 0 ° C.
Therefore, the lower limit was set to 0 ° C.

Since the tempering heat treatment carried out after rolling and cooling is intended to improve the toughness of the base metal structure by recovery, the heating temperature is in the temperature range where reverse transformation does not occur.
c ₁ or less The recovery is to reduce the lattice defect density due to the disappearance and coalescence of dislocations, and in order to realize this, it is necessary to heat to 300 ° C. or higher. Therefore, the lower limit of the heating temperature is set to 300 ° C. Further, as described above, when the precipitation hardening elements of Cu, Mo, Nb, and V are contained, the base material strength can be improved by forming fine precipitates by heat treatment. This effect is extremely effective in improving the base metal strength of the steel of the present invention having a low carbon equivalent value. The heating temperature for causing the precipitation hardening to act most effectively depends on the precipitation hardening element, but is generally in the range of 500 to 650 ° C. When the temperature after stopping the cooling after rolling is relatively high in the range of 600 ° C. or lower, self-tempering can be expected, and therefore this tempering heat treatment can be omitted.

[0055]

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. The chemical components are shown in Table 1 and Table 2 (continued from Table 2). Steels 1 to 24 are steels of the present invention, and steels 25 to 29 are comparative steels.

Tables 3 and 4 (continued from Table 3) show the steel plate manufacturing conditions and tensile properties. The invention steels 1, 2, 7, 16 and 17 were produced by the controlled rolling method according to the invention claim 11, and the invention steels 8 to 13, 18 to 24 and the comparative steel 2 were manufactured.
Nos. 7 and 29 were produced by the controlled rolling / controlled cooling method according to claim 12 or 13. The other steel sheets were manufactured by a conventional hot rolling method. The heating temperature is above the Ac ₃ transformation point in all steels. Further, the tempering temperature of the steels that have been subjected to tempering heat treatment after controlled rolling and controlled cooling is 600 ° C. or lower and is Ac ₁ transformation point or lower.

T-shaped fillet welded joints were prepared using these test steels. Table 5 shows the welding conditions. Fatigue strength of welded joints shows thickness dependence. In order to remove the plate thickness dependency, 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 6 shows the fatigue test results. Further, in the same table, the ferrite area ratio measured by the point counting method from the microstructure photograph of the HAZ in the vicinity of the welding fusion line is shown. In Steels 1 to 13, the Si addition amount is less than 1.0%, the carbon equivalent value defined above is 0.24% or less, and the ferrite area ratio of HAZ is 60% or more. Also, steel 14-2
In No. 4, the Si addition amount is 1.0% or more, the carbon equivalent value defined above is 0.275% or less, and the HAZ ferrite area ratio is 60% or more. The structures other than ferrite were upper bainite in all the steels of the present invention. The fatigue strength of welded joints was compared using the fatigue strength at 10 ⁶ times and the fatigue limit as indexes. The steel of the present invention has improved both fatigue strengths as compared with the comparative steel. The comparative steels 25 to 29 have a carbon equivalent of more than 0.275%, a HAZ ferrite area ratio of less than 60%, and partially contain lower bainite and martensite in addition to ferrite. The fatigue strength of the welded joint is also lower than that of the steel of the present invention.

From the above test, it was confirmed that the fatigue strength of the welded joint of the present invention steel is higher than that of the comparative steel.

[0060]

[Table 1]

[0061]

[Table 2]

[0062]

[Table 3]

[0063]

[Table 4]

[0064]

[Table 5]

[0065]

[Table 6]

[0066]

As described above, the steel of the present invention has the HAZ
By controlling the microstructure to have a ferrite-based structure, it is possible to improve the fatigue strength of the welded joint without reducing the stress concentration by additional welding.
By using the steel of the present invention, it is possible to improve the reliability of the welded structure for fatigue fracture.

[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.

Claims (13)

[Claims] 1. By weight%, 0.015 ≦ C ≦ 0.15, 0.01 ≦ Si <1.0, 0.2 ≦ Mn ≦ 1.5, P ≦ 0.03, S ≦ 0.01 , Ceq ≦ 0.24, balance Fe and unavoidable impurities, and a thick steel plate for welded structure excellent in fatigue strength of a welded joint. However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr
+ Mo + V) / 5 + Nb / 3 2. In weight%, 0.015 ≦ C ≦ 0.15, 1.0 ≦ Si ≦ 2.0, 0.2 ≦ Mn ≦ 1.5, P ≦ 0.03, S ≦ 0.01 , Ceq ≦ 0.275, balance Fe and unavoidable impurities, and a thick steel plate for welded structure excellent in fatigue strength of a welded joint. However, Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (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, which contains 0.005 ≦ Al ≦ 0.10 in weight%. 4. The composition according to any one of claims 1 to 3, characterized in that, by weight%, 0.1 ≦ Cu ≦ 2.0 is contained.
A thick steel plate for a welded structure having excellent fatigue strength of the welded joint according to the item. 5. The composition according to any one of claims 1 to 4, characterized in that it contains 0.1 ≦ Ni ≦ 2.0 in weight%.
A thick steel plate for a welded structure having excellent fatigue strength of the welded joint according to the item. 6. The composition according to claim 1, which contains, in wt%, one or two of 0.05 ≦ Cr ≦ 1.0 and 0.02 ≦ Mo ≦ 1.0.
A thick steel plate for a welded structure having excellent fatigue strength of the welded joint according to any one of 1 to 5. 7. The composition according to claim 1, which contains, by weight, one or two of 0.005 ≦ Nb ≦ 0.08 and 0.005 ≦ V ≦ 0.10.
7. A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 1 to 6. 8. The composition according to claim 1, wherein the content of 0.005 ≦ Ti ≦ 0.025, 0.001 ≦ N ≦ 0.010, and Ti / N ≦ 5.0 is included by weight. Which one 1
A thick steel plate for a welded structure having excellent fatigue strength of the welded joint according to the item. 9. The composition according to claim 1, which contains, by weight, one or two of 0.0005 ≦ Ca ≦ 0.005 and 0.0005 ≦ REM ≦ 0.005.
A thick steel plate for welded structure having excellent fatigue strength of the welded joint according to any one of 1 to 8. 10. A steel ingot having the same composition as that of the steel according to any one of claims 1 to 9 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. 11. A steel ingot having the same composition as that of the steel according to any one of claims 1 to 9 has an Ac ₃ point or more at 1250 ° C.
After heating below, hot rolling in a recrystallization temperature range, followed by hot rolling at a cumulative rolling reduction of 40 to 90% in a non-recrystallization temperature range, and then natural cooling fatigue of a welded joint A method of manufacturing a thick steel plate for a welded structure having excellent strength. 12. A steel ingot having the same composition as that of the steel according to any one of claims 1 to 9 has an Ac ₃ point or more at 1250 ° C.
After heating below, hot rolling is performed in a recrystallization temperature range, and subsequently hot rolling is performed at a cumulative reduction of 40 to 90% in a non-recrystallization temperature range, and then 0 to 0 at a cooling rate of 1 to 60 ° C./sec. 6
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 00 ° C. 13. A steel ingot having the same composition as that of the steel according to any one of claims 1 to 9 has an Ac ₃ point or more at 1250 ° C.
After heating below, hot rolling is performed in a recrystallization temperature range, and subsequently hot rolling is performed at a cumulative reduction of 40 to 90% in a non-recrystallization temperature range, and then 0 to 0 at a cooling rate of 1 to 60 ° C./sec. 6
A method for producing a thick steel plate for welded structure excellent in fatigue strength of a welded joint, which comprises cooling to 00 ° C., further heating to 300 ° C. to Ac ₁ point and tempering heat treatment.