JP3037855B2 - Steel sheet with good fatigue crack propagation resistance and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention suppresses the expansion of a plastic zone prior to a fatigue crack by arranging a hardened layer in a steel sheet structure in order to improve the fatigue strength of a welded structure. The present invention relates to a high fatigue strength steel sheet that delays crack growth and a method for manufacturing the same.

[0002]

2. Description of the Related Art In response to demands for lighter structures and larger capacities of structures, the strength of structural steel sheets is rapidly increasing. However, in a structure subjected to repeated loads, not only the yield strength but also the fatigue strength must be taken into consideration, and it may not be possible to meet the need for higher strength. In particular, in a welded structure, fatigue cracks often occur from the weld toe, and even if the strength of the steel material is improved, the fatigue strength hardly improves. It is known that the fatigue strength of a welded structure is mainly governed by the shape of the toe of the welded portion, and measures to improve the fatigue strength such as treatment of the toe of the welded portion are sometimes applied. However, the toe treatment not only increases the number of man-hours for building the structure, but also often cannot be carried out depending on the welded part, and there is a strong demand for improvement in fatigue strength from the steel surface.

[0003] Since fatigue fracture of a welded joint generally occurs at a weld toe where stress concentration is large, it is known that the occurrence characteristics are greatly affected by the shape of the weld toe and have little effect on the steel material composition and structure. ing. Therefore, in order to control the steel structure and improve the fatigue characteristics, it is effective to delay the propagation of the fatigue crack generated in the toe in the thickness direction. Proceedings of an international conferencespons is effective in delaying fatigue crack propagation by splitting cracks perpendicular to the fatigue crack propagation plane.
ored by Metals Society (21-23, October, 1981, Londo
n). 79-. In addition, as a similar method, Proceedings of the Society of Shipbuilding Engineers of Japan, Vol. 257-26
No. 6 shows an effect of improving the fatigue crack propagation speed by minute separation, and it has been reported that the larger the separation index, the more likely the minute separation is to occur. However, according to the Western Society of Shipbuilding Engineers, even with a steel plate having an extremely large separation index (SImax: 0.8), the improvement in the bending fatigue strength by turning is not remarkable, and a new technique is required.

Japanese Unexamined Patent Publication (Kokai) No. 3-291355 discloses a technique in which a difference in strength is monotonously and continuously applied in the thickness direction of a steel sheet to reduce the propagation speed of a through-thickness crack. However, in this method, since a monotonous and asymmetrical intensity distribution is provided in the thickness direction, it is difficult to avoid deformation during the manufacturing process, and a subsequent correction step is required. Also, stress intensification
The larger the coefficient range ΔK is, the more pronounced the delay effect is,
In the present invention material in which the strength changes at a constant gradient, a delay effect in the early stage of crack propagation cannot be expected, and further new technology is eagerly desired.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet having a structure controlled to prevent propagation of a crack that propagates in the thickness direction in order to improve fatigue strength, and a technique for manufacturing the steel sheet. It is an issue.

[0006]

The gist of the present invention is as follows. (1) In wt%, C: 0.02 to 0.2%, Si: 0.01 to 1.0%, P: 0.01% or less, Al: 0.01 to 0.2%, N: 0.02% or less, Mn: An impurity element containing 0.3 to 2.0% , with the balance being Fe and inevitably contained
A steel plate comprising: an island-like martensite extending in a steel sheet rolling direction of the steel plate as a second phase ;
Resistance that site is Microstructure scattered at an area ratio of 5-50% in the matrix phase, and the hardness Hv of the island martensite is equal to or higher than 30% than the hardness Hv of the matrix Steel plate with good fatigue crack growth characteristics. (2) Aspect ratio (width width ) of the island martensite
/ Length) is 4 or more and the width is 20 μm or more.
The steel sheet excellent in fatigue crack propagation resistance according to the above (1), which is characterized in that: (3) The composition of the steel sheet is wt%, and one of Ti: 0.007 to 0.020%, Nb: 0.003 to 0.020%, and B: 0.0003 to 0.0010%. The above is characterized by containing the above
(1) The steel sheet excellent in fatigue crack growth resistance described in (2) . (4) The steel sheet according to the above-mentioned, wherein the component of the steel sheet further contains at least one of the following: Cu: 0.1-1.0%, Ni: 0.1-3.7%.
(1) The steel sheet excellent in fatigue crack propagation resistance according to any one of (1) to (3) .

(5) In wt%, C: 0.02 to 0.2%, Si: 0.01 to 1.0%, P: 0.01% or less, Al: 0.01 to 0.2% , N: 0.02% or less, Mn: 0.3 to 2.0%, Ti: 0.007 to 0.020%, Nb: 0.003 to 0.020%, B: 0. 0003 to 0.0010%, Cu: 0.1 to 1.0%, Ni: 0.1 to 3.7%, and the balance substantially inevitably contains Fe. After rolling a steel consisting of an impurity element and having a value of A03 greater than zero calculated by the following equation at a rolling reduction of 60% or more in a non-recrystallized region at an Ar ₃ point or more, from the rolling end temperature to Ar ₃ -50 ° C. Is cooled at a cooling rate of 3 ° C./sec or less, and further cooled at a cooling rate of 5 ° C./sec or more. To extend to,
The island-like martensite is a microstructure scattered in the matrix at an area ratio of 5 to 50%, and the hardness Hv of the island-like martensite is 30% or more higher than the hardness Hv of the matrix,
A method for producing a steel sheet having good fatigue crack growth resistance, wherein the aspect ratio (width / length) of the island-like martensite is 4 or more and the width is 20 μm or more. A03 = 0.9% Cr-10.5% Mo + 7.4% Ni + 7.5% Mn-12.7% Si (6) After cooling at the cooling rate of 5 ° C./sec or more, heat treatment is performed.
(5) The method for producing a steel sheet having good fatigue crack growth resistance characteristics according to the above (5) . Here, the parent phase is a microstructure that is a main body that occupies 50% or more in a predetermined region.

[0008] In the present invention, the structural steel of interest is
For example, as described in JP-B-58-14849, as described below, in order to obtain a required material for a normal welded structural steel, an operation and an operation which have been conventionally confirmed in the field of use in order to obtain a required material. Similar functions and effects can be obtained by using the types and amounts of the additional elements determined based on the relation of the effects in the same manner. Therefore, the present invention is intended to include steel containing these elements, but at least the following elements as main components include C:
0.02-0.2%, Si: 0.01-1.0%, P:
0.01% or less, Al: 0.01 to 0.2%, Mn:
The above-mentioned first to fourth means in a steel sheet having a chemical component in the range of 0.3 to 2.0% are also included in the present invention.

Here, the gist of the present invention is a large welded structural steel excellent in fatigue crack propagation resistance, characterized in that microstructural phases having different yield strengths are present in layers. The large-sized welded structural steel is an offshore structure, a bridge, a ship, or the like, and a steel plate used for them is generally a so-called thick plate having a plate thickness of about 6 to 200 mm. With the structure of the present invention, there is no particular limitation on the components, but the following component regulations fall within the scope of the present invention as a production method that easily produces the present structure from the aspect of chemical components.

[0010] Each component element, the reason for its addition and the amount are shown below. C is added as an effective component for improving the strength of the steel. However, if the content is excessively more than 0.20%, a large amount of island-like martensite is precipitated in the welded portion, and the toughness of the steel is remarkably increased. Since it deteriorates, it is restricted to 0.20% or less. Si is necessary as a deoxidizing element of molten steel and is useful as a strength increasing element, and is made 0.01 % or more in order to easily form martensite in a micro-segregated portion. On the other hand, if it is added in excess of 1.0%, the workability of the steel is reduced and the toughness of the weld is deteriorated. Therefore, the addition amount is restricted to 0.01 to 1.0%.

Mn is also required as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is reduced and the workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, since Mn lowers the transformation temperature, the excessive addition lowers the two-phase rolling temperature too much,
Since deformation resistance increases, the upper limit is 2.0%.
Al and N are added to refine the steel by Al nitrides, and also to provide a solid solution and precipitation during the rolling process to effectively work for matching the crystal orientation and recrystallization of the steel. However, no effect thereof when the amount is small added, since when excess degrades the toughness of the steel, Al: 0.01 ~0.
20%, N: 0.020% or less.

The above are the basic components of the steel targeted by the present invention. However, Ni, Cu, Mo,
Alloying elements such as Nb and V may be added as necessary.
Alternatively, for the purpose of improving the joint toughness, Ti, B, Z
r, a rare earth element, Mg, etc. may be added as necessary, but from the viewpoint of strength and toughness, Cu: 1% or less, Ni: 3.
7% or less, Nb: 0.02% or less is desirable.

[0013]

In the process of studying the mechanism of fatigue crack propagation, fatigue crack propagation is closely related to the plastic zone propagation at the crack tip, and the slip deformation that occurs at the crack tip is repeated and gradually grows as a crack. I figured out what to do. Furthermore, it was clarified that fatigue cracks are difficult to propagate when there is a region where slip deformation is difficult at the fatigue crack tip. This phenomenon is
Japanese Patent Application Laid-Open No. 3-29135 discloses a technique of “giving a monotonic (constant gradient) and continuous strength gradient in the thickness direction” to gradually reduce the plastic strain range accumulated at the crack tip. Unlike the technology that attempts to prevent the growth of fatigue cracks, a region where slip deformation that changes the macroscopic sudden yield strength is difficult to exist over the entire crack tip, and the region that receives plastic strain from the crack growth direction This is caused by dispersing.

The present inventors believe that this finding can be further applied to the control of microscopic fatigue crack propagation, and a mixed grain structure in which a phase having a higher yield strength than the parent structure is dispersed in the microstructure. A prototype was fabricated and the behavior of fatigue cracks passing through the boundary between the high yield strength phase and the parent phase was observed in detail. As a result, it was found that even in a microstructure, when the fatigue crack reaches a region having a high yield strength, slip deformation is suppressed and propagation of the fatigue crack is greatly delayed. FIG. 1 shows a comparison of the number of repetitions required in the process of propagating a crack when a second phase having a high hardness exists over a range of about 20 μm at the tip of the crack. Island-shaped circle
The hardness Hv (H) of Tensite is equal to the hardness Hv (O) of the matrix.
It has been found that a state higher than 30% is an obstacle to crack growth.

The microscopic difference in hardness can be realized by dispersing a cementite phase or a martensite phase. After austenitizing by a reproducible heat cycle test, various types of second cycles were applied to the sample that had been rapidly cooled to form a lath-like structure, and a fatigue test was performed using a sample prepared by changing the form of island martensite did. As a result, as shown in FIG. 2 (a), when the lateral length L of the island-like martensite is 20 μm or less , the crack propagates simply around the martensite, and The speed did not slow. Also shown in FIG.
As described above, when the lateral length L of the island-like martensite was 20 μm or more, crack propagation was stagnated, and a propagation delay effect was observed. Further, as shown in FIG. 3A, when the aspect ratio ( width L / length H ) of the martensite shape is about 2, even if the width L of the martensite is 20 μm or more, the martensite and along the interface between the matrix cracks propagate, the delay effect can not be obtained, FIG. 3 (b), (c)
As shown in the figure, it was found that a form having an aspect ratio (L / H) of 4 or more was required.

Means for producing the steel material according to the present invention, that is, a steel plate having a microstructure composed of phases having microscopically different yield strengths, need not be limited at all.
As a manufacturing method that can be implemented industrially easily at low cost,
Various relations between chemical components and islands were studied. as a result,
The island -structure generation index A03, which is an index indicating the degree of development of the island-like structure, is A03 = 0.9% Cr-10.5% Mo + 7.
If 4% Ni + 7.5% Mn-12.7% Si> 0, micro-segregation is easy to generate, and the Si content is reduced to 0.1% .
It has been found that the formation of martensite is facilitated if the content is secured to not less than 01 %.

In this component system, the material is rolled in a non-recrystallized region so that the material properties can be secured even if an island- like structure develops, and then slowly cooled to give time for promoting microsegregation, and the component is concentrated. After the γ region is generated, when accelerated cooling, the part is apt to undergo martensitic transformation, and the aspect ratio is 4 or more and the width is 20 μm or more.
They found that island martensite could be formed. In order to industrially exert the above-mentioned effects, at least a 50% or more area ratio was required in the cross-sectional structure in the thickness direction of the steel sheet.

[0018]

EXAMPLES The components of the test steels of the examples are shown in Table 1, and the production conditions and the obtained materials are shown in Table 2 together with comparative examples.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

In order to evaluate the fatigue characteristics, a test piece width 80
The surface fatigue crack propagation test was devised to facilitate the generation of fatigue cracks and extract the fatigue crack propagation characteristics by making a notch in the center of the test specimen with a protrusion with a height of 2 mm and a bending span of 220 mm. The crack propagation speed in the thickness direction was measured by applying the beach mark method. Further, a welding test was carried out around the center of the test plate having a test piece width of 80 mm, and a fatigue test was carried out by an axial force to obtain a fatigue strength of 2 × 10 ⁶ times. In Test Nos. 1 to 12, which are examples of the present invention, the hard second phase is present in a layer in the matrix, and the fracture life and the fatigue strength of 2 × 10 ⁶ times in the lap welding joint are also improved as compared with the comparative example. ing. On the other hand, in Comparative Example 12, since the rolling reduction in the unrecrystallized region was 50% or less, the second phase did not have a predetermined shape, and the fatigue characteristics were deteriorated as compared with the inventive examples. Comparative Example 13 was applied up to a predetermined rolling, but the subsequent cooling start temperature was too high and a predetermined cooling speed could not be obtained, so that the entire phase became bainite-like and the fatigue characteristics were at the level of the present invention. Did not reach. In Comparative Example 14, after the predetermined rolling, the cooling rate from Ar ₃ -50 ° C. was too low, the component enriched region did not undergo martensitic transformation, and a second phase harder than the parent phase could not be obtained. In Comparative Example 15, although the cooling after the rolling was performed under predetermined conditions, the rolling temperature range was high and the unrecrystallized region rolling could not be performed. For this reason, although the hard second phase was formed, it did not have a predetermined shape, and the fatigue characteristics were inferior to those of the present invention.
In Comparative Example 16, a hard second phase was not obtained because the air was cooled after the rolling.

[0023]

The steel sheet according to the present invention is used for large structures in which fatigue cracks are generated and propagated from the surface, such as longitudinal ribs of a hull and marine structures.
It has become possible to impart high fatigue crack propagation prevention performance to the large-sized structure without requiring special consideration in design and construction. Therefore, it is possible to sufficiently secure the large-sized structure without increasing the cost, which has a great effect not only in this technical field but also in related fields.

[Brief description of the drawings]

FIG. 1 is a chart of the effect of hardness differences on fatigue crack propagation delay phenomena.

FIGS. 2A and 2B are schematic diagrams of the influence of the length of a second phase (a phase having a higher hardness than a parent phase) on the fatigue crack propagation behavior.

3 (a), (b), and (c) are diagrams showing the influence of the shape (aspect ratio) of the second phase on the fatigue crack propagation delay phenomenon.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/02

Claims (6)

(57) [Claims] 1. wt%, C: 0.02-0.2%, Si: 0.01-1.0 %, P: 0.01% or less, Al: 0.01-0.2%, N: 0.02% or less, Mn: 0.3 to 2.0% , the balance being Fe and unavoidable impurity elements
A steel plate comprising: an island-like martensite extending in a steel sheet rolling direction of the steel plate as a second phase ;
Resistance that site is Microstructure scattered at an area ratio of 5-50% in the matrix phase, and the hardness Hv of the island martensite is equal to or higher than 30% than the hardness Hv of the matrix Steel plate with good fatigue crack growth characteristics. 2. The aspect ratio of the island martensite.
(Width / length) is 4 or more and width is 20 μm or more
The steel sheet having good fatigue crack growth resistance characteristics according to claim 1 , characterized in that: 3. The composition of a steel sheet in wt%, wherein Ti: 0.007 to 0.020%, Nb: 0.003 to 0.020%, and B: 0.0003 to 0.0010%. good steel fatigue crack growth resistance characteristics according to claim 1 or 2, characterized in that it contains one or more. 4. The steel sheet is characterized in that the steel sheet further contains at least one of the following: wt.% : Cu: 0.1-1.0%, Ni: 0.1-3.7%. The steel sheet excellent in fatigue crack propagation resistance according to any one of claims 1 to 3 . 5. In% by weight, C: 0.02 to 0.2%, Si: 0.01 to 1.0%, P: 0.01% or less, Al: 0.01 to 0.2%, N: 0.02% or less, Mn: 0.3 to 2.0%, Ti: 0.007 to 0.020%, Nb: 0.003 to 0.020%, B: 0.0003 -0.0010%, Cu: 0.1-1.0%, Ni: 0.1-3.7%, and the balance is substantially Fe and inevitable impurities. After rolling a steel consisting of elements and having A03 greater than zero calculated by the following equation at a rolling reduction of 60% or more in a non-recrystallized region at an Ar ₃ point or more, the temperature from the rolling end temperature to Ar ₃ -50 ° C. Cool at a cooling rate of 3 ° C / sec or less, and further cool at a cooling rate of 5 ° C / sec or more, and roll the island martensite as a second phase of the steel sheet in the rolling direction. Zaisa was a microstructure that island martensite is dispersed at an area ratio of 5-50% in the matrix phase, and the hardness H of the hardness Hv mother phase of the island martensite
v is 30% or more, and the aspect ratio (width / length) of the island martensite is 4 or more, and the width is 20 μm.
A method for producing a steel sheet having good fatigue crack propagation resistance, characterized by the above. A03 = 0.9% Cr-10.5% Mo + 7.4% Ni + 7.5% Mn-12.7% Si 6. Cooling at a cooling rate of 5 ° C./sec or more.
The method for producing a steel sheet having good fatigue crack growth resistance characteristics according to claim 5, wherein heat treatment is performed thereafter .