JP2579708B2 - Steel sheet excellent in fatigue characteristics 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 relates to a steel sheet having a structure that generates many microcracks at the tip of a fatigue crack without impairing the Z-direction characteristics in order to improve the fatigue strength of a welded structure, and a method for producing the same. is there.

[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 that is subjected to a repeated load, not only the yield strength but also the fatigue strength must be taken into consideration, and it may not be possible to meet the needs 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 weld.
In some cases, measures for improving fatigue strength such as treatment of a toe portion of a welded portion are 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 portion, and there is a strong demand for improvement in fatigue strength from the steel surface.

[0004] Since fatigue fracture of a welded joint generally occurs from 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 improve the fatigue characteristics by controlling the steel structure, it is effective to delay the propagation of the fatigue crack generated at the toe.

In order to delay the fatigue crack propagation, it is effective to branch the crack in a direction perpendicular to the fatigue crack propagation surface.
electronic conference spo
nsored by Metals Society
(21-23, October, 1981, London)
n). 79-. As a similar method, the Transactions of the Shipbuilding Society of Japan, Vol. 169, pp. 257
266 show the effect of improving the fatigue crack propagation speed by micro-separation, and reports that micro-separation is more likely to occur as the separation index increases.

However, the SI _max which is used as an ordinary separation index are those obtained from the Charpy test piece fracture, results of the examination of fatigue propagation speed relationship between SI _max, between SI _max and Fatigue Propagation May not be directly related.

[0007] Further, it is said that the effect of the separation to reduce the fatigue crack propagation speed is effective at a low ΔK value, as described in the Transactions of the Shipbuilding Society of Japan Vol. 169, pp. 257-
266, separation occurs due to plastic anisotropy between textures having different crystal orientations. In a region having a low ΔK value, the plastic region is small, so that separation is difficult to occur. Therefore, there is a demand for the development of a technique for controlling the structure required to generate a microcrack that delays the propagation of a fatigue crack even at a low K value level.

[0008]

SUMMARY OF THE INVENTION The present invention provides a steel sheet having excellent fatigue strength and a method for producing a microstructure in which a large number of microcracks are generated at the tip of a fatigue crack even in a low ΔK region in order to improve the fatigue strength. That is the task.

[0009]

The gist of the present invention is to provide a cement
Particulates that exist in a layer parallel to the steel sheet surface
The thickness of the light phase (hereinafter referred to as pearlite band) is 3μ
m and an interval of 20 μm or less and present in a striped manner, and a matrix structure between pearlite band phases is formed from a textured colony having an aspect ratio (ratio of major axis / minor axis) of 4 or more and an minor axis of 10 μm or less. It is a steel sheet with excellent fatigue characteristics.

[0010] Further, the present invention provides rolling of a slab or steel plate as a structural steel at a reduction rate of 30% or more while cooling at a cooling rate of 1 ° C / sec or more in a γ recrystallization region.
After performing the above non-recrystallization region rolling, the cumulative reduction rate at Ar ₃ to Ar ₁ at a temperature range gamma / alpha transformation proceeds alpha fraction of 80% of the temperature range 50 exerts a pressure of 20% to 50% After rolling, pearlite bands having a thickness of 3 μm or less and an interval of 20 μm or less exist in stripes, and the matrix structure between the pearlite band phases has an aspect ratio (major axis / major axis).
This is a method for producing a steel sheet having excellent fatigue properties that gives a textured colony having a ratio of the minor axis of 4 or more and a minor axis of 10 μm or less.

The structural steel to which the present invention is directed is described, for example, in Japanese Patent Publication No. 58-14849. As described below, conventional welded structural steels have been conventionally used in order to obtain required materials. Equivalent functions and effects can be obtained by using the types and amounts of the additional elements determined based on the relation between the functions and effects confirmed in the application in the industrial field. Therefore, the present invention is intended to include steels including these.

The following is a description of each of these constituent elements and the reasons for their addition. C is an effective component for improving the strength of steel, but an excessive content of more than 0.20% not only increases the deformation resistance at the time of two-phase zone rolling and makes rolling difficult, but also increases the weldability. Since island martensite precipitates and significantly deteriorates the toughness of steel, the content is restricted to 0.20% or less.

Si is necessary as a deoxidizing element of molten steel,
Further, it is useful as a strength increasing element, but if it exceeds 1.0%, the workability of the steel is reduced, and the toughness of the weld is deteriorated. If the content is less than 0.01%, the deoxidizing effect is insufficient, so that the content is regulated 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, 0.3% or more is required as a component for improving the strength of the steel material. However, since Mn lowers the transformation temperature,
Since the two-phase region rolling temperature is too low and the deformation resistance is increased, the upper limit is 2.0%.

Al and N are added to refine the steel by the use of Al nitrides, and also to provide a solid solution and precipitation during the rolling process to make the steel crystal crystal orientation and recrystallization effective. However, when the addition amount is small, the effect is not obtained, and when the addition amount is excessive, the toughness of the steel is deteriorated.
0.001 to 0.20%, N: 0.020% or less.

The above are the basic components of the steel to which the present invention is applied. For the purpose of increasing the strength of the base material or improving the toughness of the joint, the alloy element is added according to the required properties. If the transformation temperature is too low, the deformation resistance in the two-phase region increases, and the rolling becomes difficult. Therefore, the addition amounts of the alloys are Ni, Cr, Mo, Cu, W, P, Co, V,
Nb, Ti, Zr, Ta, Hf, rare earth element, Y, C
One or more of a, Mg, Te, Se, and B may be added, but the total amount is regulated within 4.5%.

Next, the reasons for defining the organization in the present invention will be described. To facilitate generating microcracks in fatigue crack tip, form control of pearlite is the second phase has brittle ferrite-pearlite steel is important. It is important to disperse the pearlite band in a striped form in order to easily generate microcracks, and therefore, the thickness of the pearlite band is set to 3 μm or less. In addition, the pearlite interval was defined in order to increase the frequency of occurrence of microcracks.

Further, in order to easily generate microcracks from the pearlite band, it is necessary to use a pearlite band.
Since the development of texture in the ferrite tissue present between de is useful, therefore, binding with the same crystal orientation
The aspect ratio of a textured colony, which is an aggregate of crystal grains, was set to 4 or more.

The short axis diameter of the textured colony is 10 μm.
The reason for this is to cause local deformation due to plastic anisotropy between colonies having different crystal orientations even in a low ΔK region having a small plastic region under repeated load, thereby easily generating microcracks.

Next, regarding the production method, the recrystallization rolling amount is set to 3
The reason why the content is specified as 0% or more is to divide the micro-segregation zone interval into small pieces. The reason why the cooling rate in the recrystallization region is specified as 1 ° C./sec is to suppress the grain growth of the recrystallized ferrite phase. That's why. The reason why the unrecrystallized rolling amount is set to 30% or more is to reduce the grain size of ferrite which is a constituent unit of the ferrite textured colony existing between the pearlite bands .

The reason why the rolling temperature range is defined in the two-phase region rolling is that in the high temperature region near Ar ₃ , ferrite grains grow abnormally due to rolling strain and no textured colonies having a short axis diameter of 10 μm or less can be obtained. Α near the Ar ₁ point
This is because in the region where the fraction exceeds 80%, the formation of processed ferrite by rolling is remarkable, which is harmful to the base material toughness. Therefore,
In the temperature range where the ferrite fraction is in the range of 50 to 80 %, the cumulative rolling reduction is set to 20% or more from the viewpoint of forming a texture and to 50% or less from the viewpoint of ensuring base material toughness.

[0022]

[Action] inventors used various steel materials of different SI _max, and investigated the relationship SI _max and fatigue strength. As a result, a direct relationship was sometimes not found between SI _max and fatigue propagation characteristics. Therefore, as a result of investigating the fatigue fracture surface in detail, the separation in the region where the ΔK value is large was remarkable as the steel plate having a larger SI _max was found, but the appearance of the separation in the low ΔK region where the fatigue strength was determined was SI.
_It was found that it did not depend on _max .

Therefore, as described in Japanese Patent Application Laid-Open No. 3-44444, attention was paid to the fact that microcracks were generated from the cementite phase prior to the brittle crack during propagation and the stress state at the tip of the crack was relaxed. Various investigations were made on the morphology of the brittle second-phase structure that causes microcracks at the tip of the fatigue crack in the medium.

As a result, it is necessary to generate microcracks at a K value lower than that of a brittle crack tip at the fatigue crack tip, and it is necessary to increase the frequency of microcracks at the fatigue crack propagation speed delay. I learned.

FIG. 1 shows the results of investigation of the critical fracture stress in the thickness direction, which indicates the easiness of microcracking, by changing the thickness of the pearlite band phase. As shown in the figure,
When texture is developed (Long diameter of texture colony /
Although the tendency is different between the case where the aspect ratio which is the ratio of the minor axis is 4.1) and the case where no remarkable texture is present (when the aspect ratio is 1.2), the pearlite is obtained even when the texture is developed. When the thickness of the band phase is 4 μm or more, it is considered that the aggregation state of the cementite is insufficient and microcracks are unlikely to occur, but when the thickness is 3 μm or less, the peel strength of the ferrite and the pearlite band phase is reduced and microcracks are easily generated, Accordingly, the fatigue life of the T-shaped joint has been improved.

Further, a fatigue test was carried out by changing the distance between pearlite bands having a thickness of 3 μm or less.
As shown in the figure, a relationship was observed between the average interval of microcracks cracked parallel to the thickness direction on the fracture surface and the interval of the pearlite band phase. In order to greatly improve the fatigue strength, the pearlite band interval was set to 10 μm. We have found that it is necessary to:

FIGS. 3 (a), 3 (b) and 3 (c) show the aspect ratio of the textured colonies revealed by the temper color method, the K value at which microseparation occurs and the critical fracture stress in the thickness direction in the fatigue test. Shows the relationship. It can be seen that when the aspect ratio is 4 or more, the critical fracture stress in the thickness direction decreases, and microcracks are easily generated, and microcracks are generated even in a low ΔK region having a small plastic region size.

Next, FIG. 4 shows the relationship between the minor axis diameter of the textured colony and the microcrack occurrence limit K value in the textured tissue colony having an aspect ratio of 4 or more. The improvement of fatigue strength by separation is as follows: ΔK is 80 kg / mm ^1.5
The followings are the Transactions of the Shipbuilding Society of Japan Vol. 169,
pp. 257-266, and found that the texture colony size must be 10 μm or less in order to expect the effect of improving the fatigue strength by microcracking.

Therefore, in order to improve the fatigue strength by utilizing microcracks from the pearlite band phase, a pearlite band having a phase thickness of 3 μm or less must be reduced to 20 μm.
It was found that a tissue having a textured colony having a major axis / minor axis ratio of 4 or more and a minor axis of 10 μm or less was required to be present at intervals of not more than m.

In order to realize this structure, examination of rolling conditions was carried out. In order to reduce the pearlite band interval, it is necessary to reduce the γ particle size. However, since the γ recrystallization region is at a high temperature, grain growth generally occurs easily after γ is recrystallized, and it is not easy to obtain fine γ grains. Therefore, in order to suppress grain growth after recrystallization in the γ recrystallization region, cooling was performed during rolling in the temperature range, and the relationship between the cooling rate and the γ grain size was obtained.

From these results, if the cooling rate is 1 ° C./sec or more, grain growth after recrystallization caused by rolling at a rolling reduction of 30% in the region is suppressed, and the γ grain size is reduced, and as a result, It was found that the interval between the pearlite band phases was 10 μm or less.

Further, in order to facilitate the generation of microcracks from the pearlite band phase, it is necessary to perform rolling in an α / γ dual phase temperature range to develop a texture. FIG. 6 shows a rolling schedule in the two-phase region studied. Rolling is performed in two cases where the γ fraction is 100% to 50% and 50% to 0% to reduce the thickness of the pearlite band. It was found that there was almost no effect.

Further, the range of γ fraction from 50% to 0% is 5
When the pearlite band was changed at 0% to 20% and 20% to 0% and the rolling was performed at a γ fraction of 20% or less, the thickness of the pearlite band was reduced. did not change, the generation of deformed ferrite is that Do noticeable.

Accordingly, it has been found that in order to reduce the thickness of the pearlite band phase and facilitate the generation of microcracks, it is necessary to perform rolling with the residual γ fraction in the range of 50 to 20%.

Further, when the reduction amount was changed in this temperature range, a reduction amount of 20% or more was necessary to reduce the thickness of the separation band, and the reduction amount was 50%.
From the above, it was found that although there was no change in the thickness of the separation band, the formation of processed ferrite increased and was harmful to the base material toughness.

It was confirmed that when the rolling was performed in this temperature range at a rolling reduction of 20% or more, the aspect ratio of the textured colony, which is an index of the texture promoting the generation of microcracks, was 4 or more. .

[0037]

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.

[0038]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5] The cooling during the recrystallization zone rolling was performed according to the test numbers (hereinafter referred to as steel numbers) 1 to 12 of the examples of the present invention and 13 to 17, 19, and 2 of the comparative examples.
Test numbers 15, 19, 2
No. 1 could not satisfy the required cooling rate. Therefore, the pearlite intervals of Test Nos. 15, 19, and 21 are larger than those of the present invention.

In addition, Test Nos. 15, 17, and Comparative Examples
Sample No. 21 did not undergo rolling in the two-phase zone, so there was no texture development, the aspect ratio of the texture colonies was 4 or less, and no microcracks were observed on the fatigue fracture surface.

In Test Nos. 18 and 20, which are comparative examples, the pearlite band interval was large because rolling in the recrystallization zone was not sufficient, and in Test No. 23, the minor axis of the textured colony was large because rolling in the non-recrystallization zone was not performed. Has become.

Test numbers 13, 14, 16, 17, 22,
No. 24 satisfies the required conditions for both the recrystallization zone rolling and the non-recrystallization zone rolling. However, test numbers 13, 16, 2
In No. 2, since the γ fraction was rolled in the region near the Ar ₃ point where the γ fraction was as large as 100 to 50 % in the two-phase region rolling, abnormal growth of ferrite occurred, the ferrite grain size was coarsened, and the minor axis of the textured colonies was 10 μm or more. Had become. On the other hand, in test numbers 14, 18, and 24, in the two-phase rolling, the γ fraction was rolled in a structure mainly composed of ferrite of 20 to 0%, so that the formation of processed ferrite was remarkable and the thickness of the pearlite band phase was large. Material toughness is not good.

As shown in Table 2, the materials of Test Nos. 13 to 24 of these comparative examples are 2 × 10 ⁶ or less when the nominal stress range in the SN diagram of the T-joint is 12 kg / mm ² . No improvement in fatigue strength was observed.

On the other hand, Test Nos. 1 to 12
The material of satisfies the required manufacturing conditions as shown in Table 2,
In addition to satisfying the target strength and toughness, the fatigue characteristic of the T-joint, which is the object of the present invention, shows that the repeated nominal stress range is 12
kg / mm ² shows 3 × 10 ⁶ or more, and the fatigue strength after 2 × 10 ⁶ times becomes 13 kg / mm ² or more.
Significantly improved fatigue properties.

[0044]

According to the present invention, the above-mentioned action is utilized by using the above-mentioned means, so that the rolling is carried out while cooling in the recrystallization temperature range, the rolling in the non-recrystallization region is carried out, and then the rolling is performed under a predetermined condition. If phase rolling is performed, microcracks occur on the fracture surface during fatigue crack propagation, and as a result, fatigue cracks are generated from the steel sheet surface as in a T-shaped joint, and the fatigue damage propagates in the thickness direction. It is possible to reduce and prevent it, and it has a great effect on related fields as well as this field.

[Brief description of the drawings]

FIG. 1 shows the relationship between the thickness of the pearlite band phase and the critical fracture stress in the thickness direction, and the thickness and T of the pearlite band phase.
5 is a table showing the relationship between the fatigue life when the repeated nominal stress range of the U-shaped joint is 12 kg / mm ² .

FIG. 2 shows the relationship between the spacing of the pearlite band phase and the spacing of the microcracks observed on the fatigue fracture surface, and the relationship between the spacing of the pearlite band phase and the fatigue life when the cyclic nominal stress range in the T-joint is 12 kg / mm ^2. 6 is a table showing the relationship of FIG.

FIG. 3 (a) shows the relationship between the aspect ratio (major axis / minor axis) of the colony of the texture obtained by the temper color method and the critical fracture stress in the plate thickness direction, and the critical ΔK required to generate microcracks. 6 is a chart showing a relationship with a value. (B) is a schematic diagram of an aspect ratio. (C) is a schematic diagram of a pearlite band.

FIG. 4 is a chart showing a relationship between a minor axis and a limit ΔK value required for generating microcracks when the aspect ratio of a textured colony is about 4 and about 10.

FIG. 5 is a table showing a relationship between a cooling rate in a recrystallization temperature range of 1000 ° C. to 850 ° C., a γ particle size at 850 ° C., and a pearlite band interval.

FIG. 6: α fraction when rolling with a rolling reduction of 40% is performed as dual phase rolling, the minor axis of textured colonies and the thickness of pearlite band phase obtained by rolling, and the structure of processed ferrite. It is a chart which shows a relationship with an area ratio.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroshi Yoshikawa 1 Nishinosu, Oita, Oita City, Oita Prefecture Inside Nippon Steel Corporation Oita Works (72) Inventor Yoshiki Kawashima 1 Nishinosu, Oita, Oita City, Oita Prefecture New Japan (56) References JP-A-51-134306 (JP, A) JP-A-3-64413 (JP, A) JP-A-58-67823 (JP, A) JP-A-2 JP-A-129318 (JP, A) JP-A-5-202444 (JP, A) JP-A-5-9651 (JP, A) JP-A-5-148540 (JP, A)

Claims (2)

(57) [Claims] 1. A pearlite band having a thickness of 3 μm or less and an interval of 20 μm or less exists in a stripe pattern, and a matrix structure between pearlite band phases has an aspect ratio (major axis / major axis / length).
A steel sheet having excellent fatigue properties, comprising a textured colony having a ratio of minor axis of 4 or more and a minor axis of 10 μm or less. 2. A slab or steel plate as a structural steel is rolled at a reduction rate of 30% or more while cooling at a cooling rate of 1 ° C./sec or more in a γ recrystallization region, and subsequently, a non-recrystallization of 30% or more is performed. After performing the zone rolling, in the Ar _{3 to} Ar ₁ temperature range, γ
/ Α transformation proceed α fraction accumulated in a temperature range of 50 to 80%
Performs rolling product reduction ratio added pressure of 20% to 50%,
A pearlite band having a thickness of 3 μm or less and an interval of 20 μm or less exists in stripes, and the matrix structure between the pearlite band phases has an aspect ratio (major axis / minor axis ratio) of 4
A method for producing a steel sheet having excellent fatigue characteristics, wherein a textured colony having a minor axis of 10 μm or less is obtained as described above.