JP3860763B2 - Thick steel plate with excellent fatigue strength and its manufacturing method - Google Patents

Description

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【表２−２】

【表２−３】

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【表３】

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【発明の効果】
本発明は疲労強度が必要とされる溶接構造部材に用いられる厚鋼板において、従来、溶接部では向上が困難とされてきた、継手疲労特性の向上を特殊な合金元素や複雑な製造プロセスに頼ることなく、また、引張強度や鋼板板厚に大きな制限を受けずに製造できる点で、産業上の有用性は極めて大きい。
【図面の簡単な説明】
【図１】表面機械ノッチ３点曲げ疲労試験での破断寿命と硬質第二相の種類、分率との関係を示す図である。
【図２】上記疲労試験での破断寿命と硬質第二相のＺ面における最大間隔との関係を示す図である。
【図３】硬質第二相のＺ面における最大間隔の定義を示した図である。
【図４】母材疲労き裂伝播特性を調べるための表面機械ノッチ３点曲げ試験片と試験装置の概要図である。
【図５】疲労亀裂が母材鋼板に伝播するときの疲労寿命を測定するための４点曲げ試験片と試験装置の概要図である。
【符号の説明】
Ａ 試験片
Ｎ 機械ノッチ
Ｓ 試験片
Ｂ リブ板
Ｃ 廻し溶接
Ｆ 荷重支点[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thick steel plate used for a welded structure member that requires fatigue strength and a method for manufacturing the same. The steel sheet of the present invention can be used in general for welded steel structures such as marine structures, pressure vessels, ships, bridges, buildings, line pipes, etc., but especially marine structures, ships, bridges that require fatigue strength. It is useful as a steel plate for structures such as building structures. In addition, the present invention can also be applied to steel pipes or shaped steels made of thick steel plates.
[0002]
[Prior art]
With the increase in the size of welded structures and the demand for environmental protection, there has been a demand for increased reliability of structural members. The current structure is generally a welded structure, and the failure modes assumed for welded structures include fatigue failure, brittle failure, ductile failure, etc. Of these, the most frequent failure modes are: Brittle fracture or fatigue fracture from initial defects, and further brittle fracture following fatigue fracture. In addition, these forms of destruction are difficult to prevent 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, it is also possible to prevent it. Is the most required form of destruction.
[0003]
For brittle fracture, there are means for improving the chemical composition such as addition of Ni and optimization of the transformation structure, and the manufacturing method can be improved by refinement of the structure by controlled rolling or thermomechanical treatment. On the other hand, in the case of fatigue characteristics, it is possible to improve the smooth member by improving the strength, etc., but in the welded structure, the fatigue strength is governed by the shape of the toe of the welded part, so the strength is improved and the structure is improved. It was considered impossible to improve fatigue strength (joint fatigue strength) by metallurgical means. That is, in a structure where fatigue strength is a problem, the design strength cannot be increased even when high strength steel is used, and the advantage of using high strength steel cannot be obtained. Therefore, conventionally, in such a welded structure, joint fatigue strength has been improved by so-called toe treatment for improving the shape of the weld toe part that is a stress concentration part. For example, a method of cutting the toe by a grinder to increase the toe radius, a method of remelting the toe part by TIG welding to make the toe shape smooth (for example, Japanese Patent Publication No. 54-30386), shot peening In other words, a compressive stress is generated in the toe portion.
[0004]
However, since these toe treatments are very time-consuming, a means for improving the joint fatigue strength of the steel material itself has been awaited in order to reduce costs and improve productivity.
[0005]
Recently, several steel materials having good joint fatigue strength have been proposed in response to such demands. For example, a technique (Japanese Patent Laid-Open No. 8-73983) that can improve the fatigue strength of HAZ by setting the structure of the weld heat affected zone (HAZ) to ferrite (α) is shown. However, since the present technology requires that the HAZ structure be a ferrite structure, there is a limit to the strength level of the steel material that can be manufactured, and it is not possible to manufacture a high-strength steel material having a tensile strength exceeding 780 MPa.
[0006]
Several means for improving the joint fatigue strength of high-strength steel with a tensile strength of 590 MPa or more have been proposed. For improving the fatigue crack initiation and propagation characteristics of the HAZ bainite structure, high Si (Japanese Patent Laid-Open No. Hei 8-209295) is proposed. There is a report that high Nb (Japanese Patent Laid-Open No. 10-1743) is effective. However, when both Si and Nb are added in a large amount, it is an element that greatly deteriorates toughness, and there is also a concern that manufacturing problems such as cracking of a steel piece may occur.
[0007]
In addition, all of the above prior arts are means for improving the generation of fatigue cracks in HAZ structure and the propagation of fatigue cracks in HAZ. However, since HAZ is greatly affected by stress concentration at the toe, Depending on the shape, there may be no effect or it may be small.
[0008]
In order to improve the joint fatigue strength regardless of the shape of the toe, it is effective to delay the propagation of the fatigue crack generated from the toe at the base material. Based on this concept, a technology for improving the fatigue crack growth characteristics of a base material by forming a base material structure in which coarse ferrite is dispersed in a fine grain structure with an average ferrite grain size of 20 μm or less (special feature). (Kaihei 7-90481) is disclosed. However, in this case as well, only a steel material having a tensile strength of about 580 MPa class can be manufactured because of the necessity for a ferrite main structure.
[0009]
Furthermore, as a technique to increase fatigue strength by suppressing fatigue crack propagation in the base metal, in a structure consisting of ferrite and a hard second phase, there is a certain relationship between the hardness of the ferrite and the hardness of the hard second phase. Japanese Patent Application Laid-Open No. 11-1742 discloses a technique for defining the second phase form (aspect ratio, interval) or / and texture. This technology is one of the best methods for suppressing fatigue crack propagation among the currently shown technologies. However, the cumulative reduction in the two-phase region to the ferrite region is necessary for the formation of microstructure and texture. Since it is necessary to increase the rate, there are problems such as deterioration of productivity and deterioration of steel plate shape.
[0010]
[Problems to be solved by the invention]
The present invention provides a thick steel plate for welded structures with excellent fatigue crack propagation characteristics of the base material, regardless of whether a special or expensive alloy element is added in large amounts, productivity is inferior, or a complicated manufacturing method is used. It is another object of the present invention to provide the tensile strength and the steel plate thickness without being greatly limited.
[0011]
[Means for Solving the Problems]
The present inventors have improved the fatigue crack propagation characteristics of the base metal, thereby improving the fatigue strength of the joint without depending on the shape of the joint toe. It was found from the detailed experimental results of the relationship with the organization. That is, the fatigue crack generated from the stress concentration part of the joint toe part propagates in the thickness direction, but the type, form and characteristics of the structure on the front of the crack have a great influence on the fatigue crack growth.
[0012]
First, as the type of structure, it is preferable to use a mixed structure of a soft phase and a hard second phase rather than a uniform structure on the order of an optical microscope. This occurs when the crack progresses from the hard second phase to the soft phase, while the blunting of the crack occurs, while when the soft phase progresses to the hard second phase, the crack progress is delayed, This is because branching occurs. In order to generate such crack propagation behavior, the structure needs to contain ferrite as a soft phase and bainite or martensite as a hard second phase, or both. Based on detailed experiments shown below, the present inventors effectively suppress the propagation rate of fatigue cracks in steels having a mixed structure containing ferrite and at least bainite, martensite, or both. In order to do so, it was found that the distribution state is important in addition to the hardness and fraction of the hard second phase. In particular, regarding the distribution of the hard phase, it is important to strictly define the distribution of the hard second phase on the front surface of the fatigue crack, that is, the cross section parallel to the steel sheet surface (hereinafter referred to as the Z plane). I found it for the first time.
[0013]
The experiment was conducted by a three-point bending test of a test piece with a surface mechanical notch shown in FIG. 4 in order to evaluate only the crack propagation characteristics of the base material. The fatigue conditions were a stress amplitude of 378 MPa and a stress ratio of 0.1. The test steel has a chemical composition of C: 0.05 to 0.2%, Si: 0.15 to 0.3%, Mn: 0.5 to 2%, P ≦ 0.01%, S: About 0.005%, Al: 0.01 to 0.05%, Nb: 0 to 0.05%, Ti: 0 to 0.02%, Ni: 0 to 3%, and Compact vacuum steelmaking with various hot rolling conditions and heat treatment conditions (including diffusion heat treatment before hot rolling) to change the type, fraction and distribution of hard second phase in the microstructure. (Steel plate thickness: 25 mm) was used. The fatigue test specimen was collected so that the longitudinal direction of the specimen was parallel to the rolling direction. Microscopic investigation and hardness measurement were performed on the Z plane of a quarter of the plate thickness. The quantification of the structure was performed using an image analysis apparatus using a structure photograph of 5 to 10 visual fields in an optical microscope structure of a cross section (Z plane) parallel to the steel sheet surface at ¼ of the plate thickness. The hardness of the hard second phase was also measured by measuring 10 or more micro Vickers hardnesses having a load of 5 to 10 g in the same cross section and evaluating the average value.
[0014]
In the test apparatus shown in FIG. 4, a mechanical notch N is provided on the surface of the convex test piece A so that fatigue cracks are easily generated, and rolls are positioned on both sides and the center. The fatigue life when alternating stress is applied is measured by three-point bending in which a force is applied in the direction of the arrow, and the fatigue crack propagation characteristics can be evaluated.
[0015]
FIG. 1 shows the structure of pearlite (perlite main phase) in which the hard second phase (hereinafter sometimes simply referred to as the second phase) is adjusted to a Vickers hardness in the range of 200 to 250, and the Vickers hardness. The bainite or martensite adjusted to the range of 550 to 600, or the mixed phase of both layers stratified for each main structure (bainite to martensite main phase); Shows the relationship. In the pearlite main phase, the second phase other than pearlite is less than 5%. On the other hand, in the bainite to martensite main phase, the fraction of the pearlite phase other than bainite and martensite is less than 5%. When at least the second phase contains bainite, martensite, or both, and the hardness is high, the fatigue properties are clearly better than when the second phase is mainly pearlite and the hardness is low.
[0016]
In addition, when the second phase is a pearlite main phase, the fatigue characteristics do not greatly depend on the fraction, whereas when the second phase is a bainite-martensite main phase, in order to ensure high fatigue characteristics It is necessary to limit the fraction. In particular, when the fraction of the second phase is as small as less than 20%, no significant improvement in fatigue properties can be expected even if the second phase is a bainite-martensite main phase. Further, even if the hard phase exceeds 80%, the fatigue characteristics tend to deteriorate. This is not preferable because the hard phase is excessive and a fatigue crack propagates while micro brittle fracture occurs.
[0017]
FIG. 1 shows that in order to improve the fatigue characteristics, it is necessary to make the second phase have a high hardness bainite to martensite main phase in the structure of 20 to 80%, however, It is suggested that there are other factors that strongly control the fatigue characteristics. Based on the fatigue crack propagation mechanism, the present inventors have conducted a more detailed study on the assumption that the variation in fatigue characteristics is due to the difference in the morphology and distribution of the second phase. Although there is some influence of elongation, for example, the ratio (aspect ratio) between the length in the rolling direction of the second phase and the length in the thickness direction, which is observed in the thickness cross-sectional structure, the second phase in the Z plane is more than that. It came to know that distribution of is important. That is, it is effective for suppressing fatigue crack growth that the second phase existing on the front surface of the growing fatigue crack is dense and uniform, and the distribution is not uniform even in the same second phase fraction, If there is a place where the second phase does not exist depending on the place, the fatigue crack propagates preferentially there, so that the fatigue crack growth suppressing effect by the second phase is not sufficiently exhibited.
[0018]
FIG. 2 is a result of clarifying the above points, and in FIG. 1, the second phase is a bainite-martensite main phase having a high hardness and the fraction is in the range of 20-80%. The distance between the second phases observed on the Z-plane based on the definition shown in Fig. 2 is measured, and the relationship between the maximum value and the fracture life of the fatigue test is shown. The second phase interval is measured in a direction perpendicular to the rolling direction on the Z plane at a 1/4 position of the plate thickness from the viewpoint of corresponding to the second phase interval existing on the front surface of the fatigue crack.
[0019]
From FIG. 2, it is clear that the fatigue characteristics are improved as the Z-plane maximum second phase interval is smaller while the type, hardness, and fraction of the second phase are limited to a certain range. In particular, when the Z-plane maximum second phase interval exceeds 500 μm, the deterioration of fatigue characteristics becomes remarkable. Below 500 μm, the adverse effect of non-uniformity of the second phase distribution is negligible.
[0020]
The present invention is based on a detailed experiment including the above-mentioned knowledge, finds a preferred structure form for fatigue crack propagation characteristics of the base material, and further industrially most preferable means for achieving the structure form Invented together, the gist of the invention is as follows.
[0021]
  (1) In mass%,
C: 0.04-0.3%
Si: 0.01-2%
Mn: 0.1 to 3%
Al: 0.001 to 0.1%,
N: 0.001 to 0.01%
As impurities,
P: 0.02% or less,
S: 0.01% or less
The balance is composed of iron and inevitable impurities, and has a structure including at least ferrite and a hard second phase, and the hard second phase in the cross-sectional structure parallel to the surface is(A)-(d)In the thick steel plate which satisfy | fills all the conditions, the structure of the said hard 2nd phase consists of either a bainite, a martensite, or a mixed structure of both, The thick steel plate excellent in fatigue strength characterized by the above-mentioned.
(A)Hard second phase fraction: 20-80%
(B)Average Vickers hardness of hard second phase: 250-800
(C)Average equivalent circle diameter of hard second phase: 10 to 200 μm
(D)Maximum distance between hard second phases: 500 μm or less
[0022]
(2) Furthermore, in mass%,
Ni: 0.01-6%,
Cu: 0.01 to 1.5%,
Cr: 0.01-2%
Mo: 0.01-2%
W: 0.01-2%
Ti: 0.003 to 0.1%,
V: 0.005-0.5%
Nb: 0.003 to 0.2%,
Zr: 0.003 to 0.1%,
Ta: 0.005 to 0.2%,
B: 0.0002 to 0.005%
The thick steel plate having excellent fatigue strength as described in (1) above, comprising one or more of the above.
[0023]
(3) Furthermore, in mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%,
REM: 0.005-0.1%
The thick steel plate excellent in fatigue strength according to any one of (1) and (2) above, comprising one or more of them.
[0024]
  (4) A steel piece having the component according to any one of (1) to (3) and having a casting thickness of 100 mm or less,_ThreeReheated to the transformation point to 1250 ° C., hot-rolled with a reduction ratio of 2 or more, and after hot rolling, at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more. After cooling, it is further quenched at 5-100 ° C./s to 500 ° C. or less.And having a structure containing at least ferrite and a hard second phase, and the hard second phase satisfies all the following conditions (a) to (d) in a cross-sectional structure parallel to the surface, and the hard The structure of the second phase is either bainite, martensite or a mixed structure of bothA method for producing a thick steel plate having excellent fatigue strength.
(A) Hard second phase fraction: 20-80%
(B) Average Vickers hardness of hard second phase: 250 to 800
(C) Average equivalent circular diameter of hard second phase: 10 to 200 μm
(D) Maximum distance between hard second phases: 500 μm or less
[0025]
(5) Before the hot rolling, the steel piece is subjected to diffusion heat treatment at a heating temperature of 1150 to 1300 ° C. and a holding time of 1 to 100 h, and has excellent fatigue strength according to (4) Manufacturing method of thick steel plate.
[0026]
  (6) The heating temperature is 1150 to 1300 ° C. and the holding time before hot rolling on a steel piece having the component according to any one of (1) to (3) and having a casting thickness of more than 100 mm. After performing diffusion heat treatment for 1 to 100 hours, AC_ThreeReheated to the transformation point to 1250 ° C., hot-rolled with a reduction ratio of 2 or more, and after hot rolling, at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more. After cooling, it is further quenched at 5-100 ° C./s to 500 ° C. or less.And having a structure containing at least ferrite and a hard second phase, and the hard second phase satisfies all the following conditions (a) to (d) in a cross-sectional structure parallel to the surface, and the hard The structure of the second phase is either bainite, martensite or a mixed structure of bothA method for producing a thick steel plate having excellent fatigue strength.
(A) Hard second phase fraction: 20-80%
(B) Average Vickers hardness of hard second phase: 250 to 800
(C) Average equivalent circular diameter of hard second phase: 10 to 200 μm
(D) Maximum distance between hard second phases: 500 μm or less
[0027]
(7) In the hot rolling, at least the start temperature is 850 ° C. or less and the end temperature is Ar_ThreeThe method for producing a thick steel plate having excellent fatigue strength according to any one of the above (4) to (6), wherein hot rolling including rolling at a transformation point or higher and a cumulative rolling reduction of 30% or higher is performed. .
[0028]
(8) In the hot rolling, at least the starting temperature is Ar_ThreeFatigue strength according to any one of the above (4) to (7), wherein hot rolling is performed including rolling at a transformation point or lower, an end temperature of 600 ° C. or higher, and a cumulative rolling reduction of 10 to 80%. A method for producing thick steel plates with excellent resistance.
[0029]
(9) After quenching to 500 ° C. or lower, after completion of hot rolling, further (AC₁Transformation point + 30 ° C) to (AC_ThreeThe fatigue as described in any one of (4) to (8) above, which is reheated to a transformation point of −50 ° C. and subjected to a two-phase heat treatment that is cooled to 5 ° C./s to 500 ° C. or less. A method for producing thick steel plates with excellent strength.
[0030]
(10) The fatigue according to any one of (4) to (9), wherein the fatigue is tempered at 250 to 500 ° C. after quenching to 500 ° C. or less or after performing a two-phase region heat treatment. A method for producing thick steel plates with excellent strength.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, optimization of the chemical composition and optimization of the organization requirements based on the above-described new knowledge are essential. First, the reason for limiting the organization requirement will be explained, and then the reason for limitation of the chemical composition will be described. Next, an embodiment of the manufacturing method proposed in the present invention will be described with respect to the method for manufacturing the thick steel plate of the present invention.
[0032]
  The structural requirement for increasing the fatigue strength is “having a structure containing at least ferrite and a hard second phase composed of a mixed structure of either bainite, martensite or both, and in the cross-sectional structure parallel to the steel sheet surface, Hard second phase,(A)Hard second phase fraction: 20-80%,(B)Average Vickers hardness of the hard second phase: 250 to 800,(C)Average equivalent circular diameter of hard second phase: 10 to 200 μm,(D)The maximum spacing between the hard second phases: satisfying all conditions of 500 μm or less ”, which was determined based on various new findings centered on the detailed experimental results described above.
[0033]
First, as a structure, it is necessary to use a structure containing ferrite and a hard second phase composed of a mixed structure of either bainite or martensite, or a mixture structure of a soft phase and a hard phase. As a result, cracking occurs when the crack propagates from the hard phase to the soft phase, whereas when the crack propagates from the soft phase to the hard phase, the crack growth is delayed, the crack is bypassed, and branching occurs. This is because the fatigue crack growth rate is significantly suppressed. In order to generate such crack propagation behavior, the structure needs to contain ferrite as a soft phase and bainite or martensite as a hard phase, or both. The reason why ferrite is preferable as the soft phase is that ferrite is the only sufficiently soft structure in the low alloy steel for welded structures. If it is a high alloy steel, it is possible to make the austenite phase a soft phase. It is very difficult and difficult to adopt. If it is a ferrite phase, a special problem will not arise in a fatigue characteristic, if this phase does not harden | cure by extreme processing or solid solution strengthening. As a guide, the ferrite phase preferably has a Vickers hardness of 220 or less.
[0034]
The reason why the hard second phase is preferably bainite, martensite, or a mixed structure of both is that the structure is more uniform than pearlite and the toughness is good for the hardness. Pearlite is not preferred because it is not easy to increase the Vickers hardness to 250 or more, and even if the hardness is increased, the pearlite itself is a layered structure of ferrite and cementite, so fatigue cracks are not generated. This is because the soft ferrite in the pearlite can be selectively propagated and the fatigue crack growth inhibiting effect is small. Further, as the hard second phase, precipitates and inclusions may be considered, but it is not easy to contain 20 to 80%, which is effective for suppressing fatigue crack growth, and the precipitates and inclusions are bainite. Since it is very brittle compared to martensite, if it is contained in such a large amount, the toughness is markedly deteriorated and cannot be practically used as a structural steel.
[0035]
For the above reasons, the hard second phase needs to be either bainite, martensite or a mixed structure of both, but the fraction, hardness, size, and distribution of the hard second phase are strictly limited. It is necessary to specify.
[0036]
The lower limit of the hard second phase fraction is 20% from FIG. This is because if the second phase fraction is less than 20%, it is not possible to clearly improve the fatigue characteristics even if other structural requirements are optimized. In the present invention, the upper limit of the hard second phase is 80%. This is because it is not easy in terms of chemical composition to make the hardness of the hard second phase over 250% and the hardness of the hard second phase to be 250 or more, and the fraction of the hard second phase is This is because if it exceeds 80%, the toughness of the steel material may be deteriorated because the toughness of the hard second phase is inferior to that of ferrite. In addition, restraining deformation of the ferrite existing between the hard second phases is disadvantageous for securing toughness, and brittle fracture may occur during fatigue crack growth, which may deteriorate fatigue characteristics. The fraction of the hard second phase in the present invention means the area fraction in the cross-sectional structure on the Z plane.
[0037]
In addition, if the fraction of the hard second phase consisting of either bainite, martensite or a mixed structure of both satisfies the present invention, even if the hard second phase other than these contains less than 10%, fatigue characteristics. Therefore, the present invention also includes the case where the second phase other than bainite and martensite is contained in an amount of less than 10%. In the present invention, cementite, carbonitride, and non-metallic inclusions do not show a clear effect on fatigue characteristics, and are therefore not included in the hard second phase.
[0038]
The hardness of the hard second phase is also an essential requirement for ensuring fatigue characteristics. The hardness necessary for making the fatigue characteristics good after setting the type of the second phase to be bainite, martensite or a mixed structure of both and setting the second phase fraction to 20 to 80%, The average value of Vickers hardness is in the range of 250 to 800. If the average Vickers hardness is less than 250, the hardness of the soft phase ferrite is small, so fatigue crack propagation delay, detour, and branching occur near the soft phase / hard phase interface with sufficient frequency. Therefore, the fatigue characteristics cannot be improved. On the other hand, if the average Vickers hardness of the hard second phase is more than 800, the hard second phase becomes brittle, and the hard second phase becomes brittle fracture even during the fatigue test. Crack growth is accelerated and fatigue properties are degraded.
[0039]
For the above reasons, the fraction and hardness of the hard second phase consisting of either bainite, martensite, or a mixed structure of both are limited to an appropriate range. There is a need to further limit the size and distribution of the two phases.
[0040]
The limitation of size is that the harder the second phase, the harder it becomes, the more brittle it will lead to the deterioration of the toughness and fatigue properties of the steel. Necessary for achieving both toughness deterioration suppression of the second phase. The toughness of the hard second phase is governed by the average equivalent circle diameter, and if it exceeds 200 μm, the toughness deterioration cannot be ignored. Therefore, in the present invention, the average equivalent circle diameter of the hard second phase is limited to 200 μm or less. To do. From the viewpoint of securing toughness, the finer the size of the hard second phase, the better. However, if the size of the hard second phase is too small, the effect of suppressing fatigue crack growth becomes insufficient. The lower limit is set to 10 μm as the size of the hard second phase capable of reliably exhibiting the effect of suppressing crack propagation.
[0041]
Further, as the distribution of the hard second phase, as shown in the result shown in FIG. 3 described above, it is necessary to optimize the interval between the hard second phases observed on the Z plane. In the present invention, based on the result of FIG. 3, the maximum second phase interval on the Z plane, 500 μm, in which the fatigue characteristics are hardly deteriorated due to the expansion of the hard second phase interval, is defined as the upper limit. In addition, the space | interval between the 2nd phases in a Z surface is a space | interval in the direction corresponding to the 2nd phase space | interval which exists in a fatigue crack front surface, for example, when a crack surface progresses at right angles to a rolling direction. Is the value in the direction perpendicular to the rolling direction of the Z-plane second phase interval.
[0042]
The above-mentioned structure fraction, hardness, and distribution are all about the Z plane, but fatigue cracks are generated from the weld and propagate in the thickness direction from the surface. It is sufficient that the average tissue state up to the center satisfies the present invention. If the structure change in the plate thickness direction is small compared to the structure variation in the Z plane, evaluation may be made with the measured value on the Z plane at 1/4 of the plate thickness. When the structural change in the plate thickness direction is large, evaluation may be made at several locations in the plate thickness direction, for example, the steel plate surface of 1 to 2 mm, 1/4 of the plate thickness, and the average value of the plate thickness center.
[0043]
The above is the reason for limiting the organizational requirements in the present invention. In order to ensure the fatigue characteristics and the strength and toughness required for structural steel, it is necessary to optimize the chemical composition as shown below.
[0044]
That is, C is an effective component for increasing the hardness of the hard second phase. If it is less than 0.04%, it is not easy for 20% or more of the hard second phase having a Vickers hardness of 250 or more to be present stably, so the lower limit of C is set to 0.04% in the present invention. However, excessive content exceeding 0.3% lowers the toughness and weld crack resistance of the base metal and the welded portion, so the upper limit was made 0.3%.
[0045]
Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if it is less than 0.01%, deoxidation is insufficient and it is disadvantageous for securing the strength. On the other hand, an excessive content exceeding 2% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.01 to 2%.
[0046]
Mn is an element necessary for ensuring the strength and toughness of the base material, and it is necessary to contain at least 0.1% or more. However, if it is excessively contained, the toughness of the base material is generated due to formation of a hard phase or embrittlement at grain boundaries. In order to deteriorate the toughness of welds and weld cracks, the upper limit was made 3% within the allowable range of the material.
[0047]
Al is an element effective for deoxidation, refinement of the heated austenite grain size, etc., but in order to exert the effect, it is necessary to contain 0.001% or more. On the other hand, if it exceeds 0.1% and excessively contained, a coarse oxide is formed and the ductility is extremely deteriorated. Therefore, it is necessary to limit it to the range of 0.001% to 0.1%.
[0048]
N is effective in refining austenite grains in combination with Al and Ti, so that it is effective for improving mechanical properties if it is in a very small amount. Further, it is impossible to remove N in steel completely industrially, and reducing it more than necessary is not preferable because it places an excessive load on the manufacturing process. Therefore, the lower limit is set to 0.001% as a range that can be industrially controlled and the load on the manufacturing process is allowable. If excessively contained, solid solution N increases, which may adversely affect ductility and toughness, so the upper limit is made 0.01% as an acceptable range.
[0049]
P is an impurity element and is harmful to various properties of steel, so it is preferable to reduce it as much as possible. However, in the present invention, the upper limit is set to 0.02% as an amount that can be practically adversely affected.
[0050]
S is basically an impurity element, and is particularly preferably reduced because it has a significant adverse effect on the ductility, toughness and fatigue properties of steel. In practice, the upper limit is limited to 0.01% as an amount that can tolerate adverse effects. However, in the trace amount range, S forms fine sulfides and contributes to improvement of the weld heat affected zone (HAZ) toughness. Therefore, when considering HAZ toughness, S is added in a range of 0.0005 to 0.005%. It is preferable.
[0051]
The above is the reason for limiting the basic components of the thick steel plate of the present invention. In the present invention, for the adjustment of strength and toughness, Ni, Cu, Cr, Mo, W, Ti, V, One or more of Nb, Zr, Ta and B can be contained.
[0052]
Ni is a very effective element that can simultaneously improve the strength and toughness of the base material, but it needs to be added in an amount of 0.01% or more in order to exert its effect. As the amount of Ni increases, the strength and toughness of the base material are improved. However, when the addition exceeds 6%, the effect is saturated, but there is a concern that the HAZ toughness and weldability may be deteriorated. Since it is an expensive element, the upper limit of Ni is set to 6% in the present invention in consideration of economy.
[0053]
Cu is an element having almost the same effect as Ni. However, in order to exert the effect, addition of 0.01% or more is necessary. Addition of more than 1.5% improves hot workability and HAZ toughness. In order to raise a problem, in this invention, it limits to 0.01 to 1.5% of range.
[0054]
Cr is an element effective for improving the strength by solid solution strengthening and precipitation strengthening, and 0.01% or more is necessary to produce the effect. However, when Cr is added excessively, the quenching hardness increases and coarse precipitates. Since the base material and the toughness of the HAZ are adversely affected through the formation of the metal, etc., the upper limit is limited to 2% as an acceptable range.
[0055]
Like Cr, Mo and W are effective elements for increasing the strength by solid solution strengthening and precipitation strengthening, and are also effective elements for securing the hardness of the hard second phase. As a range that does not adversely affect other characteristics, both Mo and W are limited to 0.01 to 2%.
[0056]
Since Ti forms stable TiN in austenite and contributes not only to the base material but also to the refinement of the heated austenite grain size of HAZ, it is an element effective for improving toughness in addition to improving strength. However, in order to exert its effect, it is necessary to contain 0.003% or more, but when it is contained excessively exceeding 0.1%, coarse TiN is formed and the toughness is deteriorated conversely. In the present invention, the content is limited to a range of 0.003 to 0.1%.
[0057]
V is an element effective for improving the strength of the base material by precipitation strengthening, but it needs to be 0.005% or more in order to exert the effect. As the added amount increases, the strengthening amount also increases, and accordingly, the base material toughness and HAZ toughness deteriorate, and the precipitates become coarse and the strengthening effect tends to be saturated. The upper limit is 0.5% in a range where the toughness deterioration is small.
[0058]
Nb is an element effective for increasing the strength in a small amount by precipitation strengthening and transformation strengthening, and also has a large effect on the processing and recrystallization behavior of austenite, and is therefore effective in improving the base material toughness. Furthermore, it is effective for improving the fatigue characteristics of HAZ. In order to exert the effect, 0.003% or more is necessary. However, if it is added excessively exceeding 0.2%, the toughness is extremely deteriorated. Therefore, in the present invention, it is limited to the range of 0.003 to 0.2%.
[0059]
Zr is also an element effective for improving the strength mainly by precipitation strengthening, but 0.003% or more is necessary to exert the effect. On the other hand, if it is added excessively exceeding 0.1%, coarse precipitates are formed and the toughness is adversely affected, so the upper limit is made 0.1%.
[0060]
Ta has the same effect as Nb, and contributes to the improvement of strength and toughness by adding an appropriate amount. However, if it is less than 0.005%, the effect is not clearly produced, and if it exceeds 0.2%, excessive addition is not possible. Since toughness deterioration due to coarse precipitates becomes significant, the range is made 0.005 to 0.2%.
[0061]
B is an element that enhances hardenability in a very small amount and is effective for increasing the strength. Since B is segregated at the austenite grain boundary in a solid solution state to enhance the hardenability, it is effective even with a very small amount. However, if it is less than 0.0002%, the segregation amount at the grain boundary cannot be secured sufficiently, so quenching is effective. This is not preferable because the effect of improving the property is insufficient or the effect tends to vary. On the other hand, if added over 0.005%, coarse precipitates are often formed at the time of steel slab production or at the reheating stage, so the effect of improving hardenability becomes insufficient, There is also an increased risk of toughness degradation due to precipitates. Therefore, in the present invention, the range of B is set to 0.0002 to 0.005%.
[0062]
Furthermore, in this invention, 1 type, or 2 or more types of Mg, Ca, and REM can be contained as needed for the improvement of ductility and the improvement of joint toughness.
[0063]
Mg, Ca, and REM are all effective in improving ductility by suppressing extension during the hot rolling of sulfides. Effectively improves the toughness of the joint by refining the oxide. The lower limit content for exhibiting the effect is 0.0005% for Mg, 0.0005% for Ca, and 0.005% for REM. On the other hand, excessive content causes coarsening of sulfides and oxides, leading to deterioration of ductility, toughness, and fatigue properties. Therefore, the upper limit is 0.01% for Mg and Ca, and 0.1% for REM, respectively. And
[0064]
The above is the reason for limiting the microstructure and chemical composition, which are the basic requirements of the present invention. In addition, in the present invention, an appropriate manufacturing method for satisfying the organizational requirements of the present invention is also presented. However, for the microstructure of the present invention, the effect is exhibited regardless of the means for achieving it, and the method for producing a thick steel sheet having excellent fatigue strength according to claims 1 to 3 of the present invention is described in the claims. It is not limited to the method shown to 4-10.
[0065]
In the first production method, a steel piece having a casting thickness of 100 mm or less, which is subjected to diffusion heat treatment with a heating temperature of 1150 to 1300 ° C. and a holding time of 1 to 100 h, as necessary, before hot rolling. AC_ThreeReheated to the transformation point to 1250 ° C., hot-rolled with a reduction ratio of 2 or more, and after hot rolling, at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more. After cooling, it is further cooled rapidly at 5 to 100 ° C./s to 500 ° C. or less.
[0066]
The second manufacturing method is for steel slabs having a casting thickness of more than 100 mm. After hot rolling, the steel slabs are subjected to diffusion heat treatment at a heating temperature of 1150 to 1300 ° C. and a holding time of 1 to 100 h. , AC_ThreeReheated to the transformation point to 1250 ° C., hot-rolled with a reduction ratio of 2 or more, and after hot rolling, at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more. After cooling, it is further cooled rapidly at 5 to 100 ° C./s to 500 ° C. or less.
[0067]
In both the first and second methods, the start temperature is 850 ° C. or lower and the end temperature is Ar as necessary._ThreeIt includes rolling at a transformation point or higher and a cumulative rolling reduction of 30% or higher, and / or a starting temperature of Ar_ThreeBelow the transformation point, hot rolling including rolling with an end temperature of 600 ° C. or higher and a cumulative rolling reduction of 10 to 80% can be performed.
[0068]
Furthermore, (AC₁Transformation point + 30 ° C) to (AC_ThreeA two-phase region heat treatment that is reheated to a transformation point of −50 ° C. and cooled to 500 ° C. or less at 5 to 100 ° C./s, and / or tempering at a heating temperature of 250 to 500 ° C. can be performed.
[0069]
The size and distribution of the second phase are greatly influenced by the distribution and transformation behavior of microsegregated portions such as Mn and C that occur during solidification. It is effective to finely disperse the microsegregation part for the refinement of the hard second phase, which is a requirement of the present invention, and the reduction of the interval, but for that purpose, the cooling rate before and after solidification can be increased, It is effective to reduce the secondary dendrite arm spacing by hot rolling. A method of reducing the degree of segregation of microsegregation once generated by diffusion heat treatment is also effective.
[0070]
The reason why the casting thickness is 100 mm or less is that the secondary dendrite arm interval is refined by increasing the solidification rate, and the hard second phase size and interval of the Z plane in the final structure are within the scope of the present invention. is there. When the casting thickness exceeds 100 mm, it is difficult to ensure that the average equivalent circle diameter of the hard second phase is 200 μm or less and that the maximum distance between the hard second phases is 500 μm or less.
[0071]
A method for finely dispersing the hard second phase irrespective of the casting thickness is a diffusion heat treatment in which the heating temperature is 1150 to 1300 ° C. and the holding time is 1 to 100 h. In order to effectively diffuse an alloy element mainly composed of Mn, it is necessary to maintain at 1150 ° C. for 1 hour or longer. However, if the diffusion heat treatment temperature exceeds 1300 ° C., the heated austenite becomes extremely coarse and adversely affects the toughness of the steel sheet, and the steel sheet may be roughened. The holding time is preferably as long as 1 h or longer. However, if the diffusion heat treatment temperature is 1150 ° C. or higher, the holding time is within 100 h, and sufficient homogenization of the alloy elements is achieved. Therefore, in the present invention, the upper limit is set to 100 h. In addition, since the effect of the main diffusion heat treatment and the reduction of the casting thickness is additive, in the first method, if necessary, the main diffusion heat treatment is also applied to a steel piece having a casting thickness of 100 mm or less. It is valid.
[0072]
In the present invention, a steel slab having a casting thickness of 100 mm or less and / or a steel slab subjected to a diffusion heat treatment with a heating temperature of 1150 to 1300 ° C. and a holding time of 1 to 100 h,_ThreeReheated to the transformation point to 1250 ° C., hot-rolled with a reduction ratio of 2 or more, and after hot rolling, at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more. After cooling, it is further assumed that the steel plate manufacturing condition is to rapidly cool to 500 ° C. or lower at 5 to 100 ° C./s to obtain a thick steel plate.
[0073]
The reheating temperature of the billet is AC_ThreeThe transformation point to 1250 ° C is that the reheating temperature is AC_ThreeIf it is less than the transformation point, it does not become an austenite single phase in the heating stage, and the solid solution of precipitates is not sufficient, so that it becomes difficult to obtain the strength and toughness required as a structural material. When the heating temperature is higher than 1250 ° C., the heated austenite grain size becomes extremely coarse and is not sufficiently refined even by the subsequent hot rolling, so that the toughness may be deteriorated.
[0074]
In hot rolling, the reduction ratio (casting thickness / steel plate thickness) is set to 2 or more. This is advantageous for fine dispersion of the hard second phase in the Z plane if the rolling ratio is 2 or more, and also reduces the interval of the hard second phase in the plate thickness direction, This is to contribute to improvement of fatigue characteristics. Furthermore, if the rolling reduction ratio is less than 2, it is difficult to press the porosity that occurs in the slab due to solidification shrinkage, which is why the rolling reduction ratio is 2 or more.
[0075]
After hot rolling, after cooling at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more, further quenching at 5 to 100 ° C./s to 500 ° C. or less is fatigue. This is to ensure 20 to 80% of the fraction in the hard second phase structure having an average Vickers hardness of 250 to 800 necessary for improving the characteristics. In order to produce the hard second phase in the low C steel having a C content of 0.3% or less as in the present invention, the transformation temperature range is quenched and C is concentrated in the austenite phase before transformation. For this purpose, it is necessary to cause ferrite transformation before quenching. If the ferrite fraction before quenching is less than 10%, the concentration of C into untransformed austenite may be insufficient. Therefore, in the present invention, the ferrite fraction before quenching is set to 10% or more. Since the main purpose of the ferrite formation is to enrich C in austenite, the ferrite transformation temperature is preferably high. For this reason, the cooling rate during the ferrite formation is 2 ° C./s or less. If the cooling rate is excessive, the ferrite transformation temperature decreases, and the diffusion rate of C becomes insufficient, which is not preferable for the concentration of C into austenite. From the viewpoint of concentrating C in austenite, the cooling rate in the ferrite formation process is preferably as low as possible. However, 0.1 ° C./s or more is sufficient, and the effect is saturated even if it is gradually cooled further. In the present invention, the lower limit cooling rate is 0.1 ° C./s.
[0076]
Untransformed austenite in which C is sufficiently concentrated is transformed at a low temperature by quenching at 5 to 100 ° C./s to 500 ° C. or less to form a hard second phase. When the average cooling rate before and after the transformation region is less than 5 ° C./s, it is difficult to stably form a hard second phase having an average Vickers hardness of 250 or more even within the chemical composition range of the present invention. It becomes. The larger the cooling rate is, the more advantageous for the formation of the hard second phase, but the effect is saturated even if it exceeds 100 ° C./s, and cooling at such an excessive cooling rate increases the production cost. It also leads to deterioration of the steel plate shape. For the above reasons, in the present invention, the cooling rate in the rapid cooling when the hard second phase is formed from untransformed austenite is in the range of 5 to 100 ° C./s. Rapid cooling at the cooling rate is necessary until the transformation is almost completed. In the chemical composition range of the present invention, if the quenching stop temperature is 500 ° C. or less, a desired hard second phase can be obtained.
[0077]
It should be noted that the effect of the present invention is not impaired even if a piece of steel such as an ingot or a slab is subjected to partial rolling for the purpose of shape adjustment before hot rolling for forming a steel plate. Moreover, as long as the conditions of the diffusion heat treatment of the present invention are satisfied, the diffusion heat treatment and the ingot rolling are combined, that is, the steel slab is 1 to 1150 to 1300 ° C., which is the diffusion heat treatment condition of the present invention. There is no problem with carrying out the partial rolling in the cooling stage after holding for 100 hours.
[0078]
The above is the reason for limiting the basic requirements of the production method in the present invention. However, in the production method of the present invention, it is necessary to obtain the structural requirements of the present invention and to improve the mechanical properties as necessary. Then, after satisfying the basic requirements of the production method of the present invention, the following treatments (a) to (d) can be additionally performed.
(A) Start temperature is 850 ° C. or lower, and end temperature is Ar_ThreeHot rolling is performed at a transformation point or higher and a cumulative rolling reduction of 30% or higher.
(B) Start temperature is Ar_ThreeHot rolling is performed at a transformation point or lower, an end temperature of 600 ° C. or higher, and a cumulative rolling reduction of 10 to 80%.
(C) After rapid cooling to below 500 ° C, further (AC₁Transformation point + 30 ° C) to (AC_ThreeA two-phase region heat treatment is performed by reheating to a transformation point of −50 ° C. and cooling to 500 ° C. or less at 5 to 100 ° C./s.
(D) After quenching to 500 ° C. or lower, or after performing a two-phase region heat treatment, temper at 250 to 500 ° C.
[0079]
(A) is a process for refining austenite before transformation and / or accumulating strain in unrecrystallized austenite to refine the transformation structure. As a result of refining the transformation structure, the hard second phase is finely dispersed and stably satisfies the average equivalent circular diameter of the hard second phase: 10 to 200 μm, and the maximum interval between the hard second phases: 500 μm or less. Can improve fatigue characteristics. In addition, since the ferrite grain size is also refined, this means is effective in achieving high toughness as well as fatigue characteristics.
[0080]
In order to exert the above effects, hot rolling with a cumulative rolling reduction of 30% or more is started at a temperature of 850 ° C. or lower and the end temperature is Ar._ThreeMust be done above the transformation point. If the cumulative rolling reduction is less than 30%, recrystallization austenite grains are not sufficiently refined in recrystallization zone rolling, and strain accumulation in austenite grains is not sufficient in non-recrystallization zone rolling. The transformation structure is not sufficiently refined regardless of the temperature. On the other hand, even if the cumulative rolling reduction is 30% or more, if the rolling temperature is not in an appropriate range, the rolling effect does not contribute to the refinement of the structure effectively, which is not preferable. That is, when the rolling start temperature exceeds 850 ° C., the recrystallized austenite grains are not sufficiently refined, the recovery rate of the introduced dislocations is large, and the strain is not accumulated effectively. In the present invention, the upper limit of the rolling start temperature is set to 850 ° C. from the viewpoint of all the rolling in the means (a) contributing to the refinement of the structure. As long as the rolling is finished in the austenite region, it is effective for refining the structure. Therefore, the rolling finishing temperature in the additional treatment (a) is Ar._ThreeIt only needs to be above the transformation point. The rolling temperature is 850 ° C. to Ar_ThreeSince the rolling effect is almost accumulated within the range of the transformation point, the rolling reduction may be defined by the cumulative rolling reduction, and it is not necessary to define the rolling conditions for each pass.
[0081]
(B) is an effective means for ensuring the hardness of the hard second phase by accelerating ferrite transformation by two-phase rolling and promoting C concentration into untransformed austenite. In order to concentrate C to austenite, it is more reliable that C is diffused from ferrite to austenite at a higher temperature, the concentration of C in austenite increases, the amount of C in the austenite increases, and the hard second after transformation Increases the hardness of the phase.
[0082]
Ferrite transformation is promoted by applying two-phase rolling. Therefore, in the chemical composition range of the present invention, the starting temperature is Ar._ThreeIt is effective to additionally perform hot rolling at a transformation point or lower, an end temperature of 600 ° C. or higher, and a cumulative rolling reduction of 10 to 80%. Start temperature as Ar_ThreeBelow the transformation point is Ar_ThreeIf the transformation point is exceeded, the amount of machining in the two-phase region becomes insufficient even when the transformation point is raised by machining. The rolling end temperature is set to 600 ° C. or more because if it is less than 600 ° C., transformation from austenite occurs during or after processing and before the start of quenching, which is caused by the hard second phase to be generated by quenching. This is because the pearlite transformation with low hardness may occur. In order to ensure the effect of promoting the ferrite transformation by the two-phase region rolling, the cumulative reduction ratio of the two-phase region rolling needs to be 10% or more. If it is less than 10%, the ferrite transformation is not sufficiently promoted, and there is no point in additionally performing two-phase rolling. The upper limit of the cumulative rolling reduction of the two-phase zone rolling is 80%. This is because if excessive two-phase rolling exceeding 80% is applied, transformation from untransformed austenite is also promoted, and the desired hard second phase may not be formed. This is because it becomes difficult to secure 600 ° C., which is the lower limit of the end temperature. In addition, by using this means additionally, a texture develops, and it can be expected to improve fatigue characteristics and toughness as a result.
[0083]
(C) is to ensure the hardness and fraction of the hard second phase more reliably by the two-phase region heat treatment. When heated in the two-phase region, austenitization in the reverse transformation occurs from the micro-segregated portion and pearlite portion where the components are concentrated, so if the hot rolling conditions before heat treatment satisfy the requirements of the present invention, the dispersion The state is within the scope of the present invention. Depending on the heating conditions of the two-phase region heat treatment, the fraction of reverse transformed austenite and the degree of concentration of C in the austenite are determined. That is, the fraction and hardness of the hard second phase after transformation from austenite are determined. The heating temperature in the two-phase region heat treatment is (AC₁If it is less than the transformation point + 30 ° C., the reverse transformed austenite fraction, and thus the resulting hard second phase fraction, is too small compared to the present invention, and the fatigue characteristics deteriorate. On the other hand, the heating temperature in the two-phase region heat treatment is (AC_ThreeIf the transformation point exceeds -50 ° C), the austenite fraction increases, but instead, the concentration of C in the austenite is insufficient, and the hardness of the hard second phase deviates from the present invention. Therefore, the fatigue characteristics are also inferior, which is not preferable. Therefore, in the present invention, the two-phase region heat treatment temperature is (AC₁Transformation point + 30 ° C) to (AC_ThreeTransformation point-50 ° C). (AC₁Transformation point + 30 ° C) to (AC_ThreeAfter reheating to a transformation point of −50 ° C., cooling is performed at 5 to 100 ° C./s to 500 ° C. or less. This is similar to the rapid cooling after hot rolling described above, and C is sufficiently concentrated. This is because untransformed austenite is transformed at a low temperature to form a hard second phase that satisfies the structural requirements of the present invention.
[0084]
(D) is a tempering treatment performed after rapid cooling to 500 ° C. or less or after performing a two-phase region heat treatment, and is performed to adjust strength and toughness as necessary. However, it is necessary to consider not to excessively reduce the hardness of the hard second phase. That is, in the present invention, the upper limit of the tempering temperature is set to 500 ° C. If the tempering temperature exceeds 500 ° C., depending on the chemical composition, the average Vickers hardness of the hard phase was 250 or more in the hot rolling or the two-phase region heat treatment stage. This is because there is a concern that it will decrease to less than. In the present invention, the lower limit of the tempering temperature is defined as 250 ° C. This is because when the tempering temperature is less than 250 ° C., the material adjustment effect by tempering is not clear.
[0085]
Next, the effects of the present invention will be described more specifically with reference to examples.
[0086]
【Example】
Table 1 shows the chemical composition of the test steel used in the examples. Each test steel is made into a steel slab by ingot rolling, by ingot rolling, or by continuous casting. In Table 1, steel slab numbers 1 to 10 satisfy the chemical composition range of the present invention, and steel slab numbers 11 to 15 do not satisfy the chemical composition range of the present invention. Table 1 also shows the heating transformation point (AC₁, AC_ThreeThis indicates that the rate of temperature rise is 5 ° C./min. Although it is an actual measurement value at this time, it almost coincides with the transformation point shown in Table 2 under the actual temperature rise condition during billet heating or heat treatment of the steel sheet.
[0087]
  Steel slabs with the chemical composition of Table 1 are shown in Table 2.(Table 2-1 to Table 2-3)Were subjected to diffusion heat treatment, hot rolling, heat treatment, and tempering under the conditions shown in (2) above, to produce a steel plate having a thickness of 25 mm or 50 mm, and the tensile properties at room temperature, 2 mm V notch Charpy impact properties, and fatigue properties of welded joints were investigated. Tensile test pieces and Charpy impact test pieces were sampled perpendicularly to the rolling direction (C direction) from the center of the plate thickness. Tensile properties were measured at room temperature, and Charpy impact properties were evaluated at 50% fracture surface transition temperature (vTrs). The fatigue test was performed on the turn welded joint shown in FIG. 5 in order to evaluate the fatigue characteristics when a fatigue crack is generated from the weld toe of the structure and propagates through the base metal. The test piece S is a test plate having a size from steel plate to steel plate longitudinal direction length: 300 mm, width direction length: 80 mm, plate thickness: 25 mm (total thickness for 25 mm thick material, sampled from the surface for 50 mm thick material). The rib plate B having a width of 10 mm, a length of 30 mm, and a height of 30 mm was welded with carbon dioxide gas (CO₂Welding), the test plate was turned to the center and welded by welding C. In this case, carbon dioxide gas welding uses a 1.4 mm diameter welding wire having a chemical composition of C: 0.06 mass%, Si: 0.5 mass%, Mn: 1.4 Mass%, current: 270A, Voltage: 30 V, welding speed: 20 cm / min. I went there. In the fatigue test, the span of the load fulcrum F is set to the lower span: 70 mm, the upper span: 220 mm, the maximum load (Pmax): 5500 kgf, the stress ratio (R): 0.1, and the fatigue life is measured. did.
[0088]
Table 3 shows the structure of the hard second phase of the steel sheet (type, fraction, Vickers hardness, average equivalent circle diameter, maximum interval) and mechanical properties. The structure was quantified with respect to an optical microscope structure having a cross section (Z plane) parallel to the steel plate surface at ¼ of the plate thickness. Using a tissue photograph of 5 to 10 fields of view, it was quantified by an image analyzer. The hardness of the hard second phase was also measured by measuring 10 or more micro Vickers hardnesses having a load of 5 to 10 g in the same cross section and evaluating the average value.
[0089]
Steel plates Nos. A1 to A13 in Tables 2 and 3 are steel plates that satisfy all the requirements regarding the chemical composition and structure of the present invention, and all of them have the strength and toughness (2 mm V notch Charpy impact properties required as structural steels). It is clear that it also has very good joint fatigue properties.
[0090]
On the other hand, steel plate numbers B1 to B11 are comparative steel plates that do not satisfy any of the requirements of the present invention, and have joint fatigue properties and toughness as compared with the steel plates of the present invention having the same composition and strength level. It is clear that it is inferior.
[0091]
Steel plate numbers B1 to B5 have good characteristics even though they cannot satisfy the structural requirements of the present invention because the chemical composition does not satisfy the present invention, or satisfy the structural requirements of the present invention. This is an example that could not be achieved.
[0092]
That is, the steel plate number B1, because the C amount is excessive, the toughness is inferior, as well as the toughness is extremely inferior, and the hard phase is brittle fracture even in the fatigue test, compared to the present invention, Joint fatigue properties are inferior.
[0093]
Steel plate number B2 is significantly inferior to the present invention in both toughness and fatigue properties for the same reason as in the case where the amount of C is excessive because the amount of Mn is excessive.
[0094]
Steel plate numbers B3 and B4 are excessively inferior to the present invention in both toughness and fatigue properties because the P and N contents are excessive and cause the steel to become brittle.
[0095]
Steel plate number B5 is inferior in fatigue characteristics as compared with the present invention because the amount of S is excessive and the fatigue characteristics are greatly degraded through ductility degradation.
[0096]
Steel plate numbers B6 to B11 are examples in which the joint fatigue characteristics are inferior because the chemical composition satisfies the present invention but the structural requirements do not satisfy the present invention.
[0097]
That is, steel plate numbers B6 and B9 are examples in which the fatigue characteristics are inferior compared to the steel of the present invention having the same composition because the interval between the hard second phases is excessive.
[0098]
Steel plate numbers B7, B8 and B10, B11 are pearlite in which the second phase is not preferred for fatigue properties, so the fatigue properties of the second phase compared to the present invention although the dispersed state satisfies the present invention. Is greatly inferior.
[0099]
From the above examples, it is apparent that according to the present invention, it is possible to obtain excellent joint fatigue characteristics while ensuring sufficiently high toughness as structural steel.
[0100]
[Table 1]

[0101]
[Table 2-1]

[Table 2-2]

[Table 2-3]

[0102]
[Table 3]

[0103]
【The invention's effect】
The present invention relies on special alloying elements and complex manufacturing processes to improve joint fatigue characteristics, which have been difficult to improve in welded parts, in thick steel plates used for welded structural members that require fatigue strength. In addition, the industrial utility is extremely large in that it can be manufactured without being greatly restricted in the tensile strength and the steel plate thickness.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the fracture life in a surface mechanical notch three-point bending fatigue test, the type and fraction of a hard second phase.
FIG. 2 is a diagram showing the relationship between the fracture life in the fatigue test and the maximum spacing on the Z plane of the hard second phase.
FIG. 3 is a diagram showing a definition of a maximum interval on the Z plane of a hard second phase.
FIG. 4 is a schematic diagram of a surface machine notch three-point bending test piece and a test apparatus for examining base metal fatigue crack propagation characteristics.
FIG. 5 is a schematic diagram of a four-point bending test piece and a test apparatus for measuring a fatigue life when a fatigue crack propagates to a base steel plate.
[Explanation of symbols]
A Test piece
N Mechanical notch
S specimen
B Rib plate
C Turning welding
F Load fulcrum

Claims (10)

% By mass
C: 0.04-0.3%
Si: 0.01-2%
Mn: 0.1 to 3%
Al: 0.001 to 0.1%,
N: 0.001 to 0.01%
As impurities,
P: 0.02% or less,
S: 0.01% or less, the balance being iron and inevitable impurities, having a structure containing at least ferrite and a hard second phase, and in the cross-sectional structure parallel to the surface, the hard second phase is In the thick steel plate that satisfies all the following conditions (a) to (d) , the hard second phase structure is composed of either bainite, martensite, or a mixed structure of both, and is excellent in fatigue strength. Thick steel plate.
(A) Hard second phase fraction: 20-80%
(B) Average Vickers hardness of hard second phase: 250 to 800
(C) Average equivalent circular diameter of hard second phase: 10 to 200 μm
(D) Maximum distance between hard second phases: 500 μm or less Furthermore, in mass%,
Ni: 0.01-6%,
Cu: 0.01 to 1.5%,
Cr: 0.01~2%,
Mo: 0.01-2%
W: 0.01-2%
Ti: 0.003 to 0.1%,
V: 0.005-0.5%
Nb: 0.003 to 0.2%,
Zr: 0.003 to 0.1%,
Ta: 0.005 to 0.2%,
B: 0.0002 to 0.005%
The thick steel plate excellent in fatigue strength according to claim 1, comprising one or more of the following. Furthermore, in mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%,
REM: 0.005-0.1%
The steel plate having excellent fatigue strength according to any one of claims 1 and 2, wherein one or more of them are contained. A steel slab having the component according to any one of claims 1 to 3 and having a casting thickness of 100 mm or less is reheated to an AC ₃ transformation point to 1250 ° C, and hot rolling with a reduction ratio of 2 or more is performed. After hot rolling, after cooling at a cooling rate of 0.1 to 2 ° C./s to a temperature at which the ferrite fraction becomes 10% or more, further quenching at 5 to 100 ° C./s to 500 ° C. or less , at least ferrite And the hard second phase satisfies all the following conditions (a) to (d) in a cross-sectional structure parallel to the surface, and the hard second phase The manufacturing method of the thick steel plate excellent in the fatigue strength characterized by making a structure | tissue into either a bainite, a martensite, or a mixed structure of both .
(A) Hard second phase fraction: 20-80%
(B) Average Vickers hardness of hard second phase: 250 to 800
(C) Average equivalent circular diameter of hard second phase: 10 to 200 μm
(D) Maximum distance between hard second phases: 500 μm or less   The steel plate having excellent fatigue strength according to claim 4, wherein the steel piece is subjected to diffusion heat treatment at a heating temperature of 1150 to 1300 ° C and a holding time of 1 to 100 hours before the hot rolling. Method. Diffusion with a heating temperature of 1150 to 1300 ° C. and a holding time of 1 to 100 h before hot rolling on a steel slab having the component according to claim 1 and having a casting thickness of more than 100 mm After heat treatment, reheat to AC ₃ transformation point to 1250 ° C., perform hot rolling with a reduction ratio of 2 or more, and after hot rolling, 0.1 to a temperature at which the ferrite fraction becomes 10% or more. After cooling at a cooling rate of 2 ° C./s, it is further quenched at 5-100 ° C./s to 500 ° C. or less , and has a structure containing at least ferrite and a hard second phase, and is a cross-sectional structure parallel to the surface The hard second phase satisfies all the following conditions (a) to (d), and the hard second phase structure is either bainite, martensite, or a mixed structure of both. A method for producing thick steel plates with excellent fatigue strength.
(A) Hard second phase fraction: 20-80%
(B) Average Vickers hardness of hard second phase: 250 to 800
(C) Average equivalent circular diameter of hard second phase: 10 to 200 μm
(D) Maximum distance between hard second phases: 500 μm or less In the hot rolling, hot rolling including rolling with at least a start temperature of 850 ° C. or less, an end temperature of Ar ₃ transformation point or more, and a cumulative reduction ratio of 30% or more is performed. The manufacturing method of the thick steel plate excellent in the fatigue strength in any one of. In the hot rolling, hot rolling including rolling with at least a start temperature of Ar ₃ transformation point or less, an end temperature of 600 ° C or higher, and a cumulative reduction ratio of 10 to 80% is performed. 8. A method for producing a thick steel plate having excellent fatigue strength according to any one of 7 above. After rapidly cooling to 500 ° C. or lower, after completion of hot rolling, it is further reheated to (AC ₁ transformation point + 30 ° C.) to (AC ₃ transformation point−50 ° C.) to 500 ° C. or lower at 5-100 ° C./s. The method for producing a thick steel plate having excellent fatigue strength according to any one of claims 4 to 8, wherein a two-phase heat treatment for cooling is performed.   The steel plate excellent in fatigue strength according to any one of claims 4 to 9, wherein the steel plate is tempered at 250 to 500 ° C after being rapidly cooled to 500 ° C or less or subjected to a two-phase region heat treatment. Manufacturing method.

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