JP3348731B2 - Steel plate with excellent fatigue properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention improves the fatigue crack propagation resistance in the thickness direction by generating a large number of 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. It relates to a steel plate that has been made to fall.

[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 properties by controlling the steel structure, it is effective to delay the propagation of fatigue cracks generated at the toe in the thickness direction.

In order to delay fatigue crack propagation, it is effective to split a crack in a direction perpendicular to the fatigue crack propagation plane . Proceedings of an international confer
encesponsored by Metals Society (21-23, October, 198
1, London). 79-. As a similar method, the Transactions of the Shipbuilding Society of Japan, Vol. 169, p
p. 257-266 reports that micro-separation is also likely to occur.

However, SImax, which is used as a normal separation index, is obtained from the fracture surface of a Charpy test piece. As a result of investigating the relationship between SImax and fatigue propagation speed, there is a direct relationship between SImax and fatigue propagation characteristics. Was sometimes not recognized.

[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 relating to the structure control necessary for generating a microcrack that delays the propagation of a fatigue crack even at a low ΔK value level.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a microstructure control technique for generating a large number of microcracks at the tip of a fatigue crack even in a low ΔK value region in order to improve the fatigue strength. .

[0009]

The present invention has the following features to solve the above-mentioned problems . (1) By weight%, C: 0.20% or less, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.20 %, N: A steel sheet containing 0.020% or less , with the balance being Fe and inevitable impurities
Having a ferrite structure and adjacent grains of this structure
The aspect ratio of the texture roller <br/> knee comprised equal grain crystal orientation to each other by (major axis diameter / ratio of minor axis diameter) is 4 or more, and it from the minor axis diameter of 5μm or less tissue The collection
Synthetic tissue colonies should be at least
A steel sheet having excellent fatigue properties characterized by being present over a range of 5% or more . (2) The steel sheet of the above-mentioned component further contains Ni, C
r, Mo, Cu, W, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
B containing at least one kind of B in a total amount of 4.5% or less.
The steel sheet having excellent fatigue characteristics according to the above (1).

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 steels including these.

The following is a description of each of these constituent elements, the reasons for their addition, and their amounts. C is added as an effective component for improving the strength of steel. However, an excessive content exceeding 0.20% not only increases the deformation resistance during two-phase rolling, but also makes rolling difficult. In addition, since island martensite precipitates in the welded portion 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 added in excess of 1.0%, the workability of the steel is reduced, and the toughness of the weld is deteriorated. Further, if the content is less than 0.01%, the deoxidizing effect is insufficient, so 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, 0.3% or more must be added 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 means of Al nitride, and also to provide a solid solution and precipitation during the rolling process to make the crystal orientation of the steel effective and to work effectively for recrystallization. However, when the amount is small, it has no effect.
If it is excessive, the toughness of the steel is deteriorated.
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 metal or improving the toughness of the joint, the case where an alloying element is added in accordance with the required properties. If the transformation temperature is too low, deformation resistance in the two-phase region increases and rolling becomes difficult. Therefore, the amount of the alloy to be added is Ni, Cr, Mo, Cu, W, Co, V, Nb, T
i, Zr, Ta, Hf, rare earth element, Y, Ca, Mg,
One or more of Te, Se, and B may be added, but the total content is regulated within 4.5%.

The reasons for defining the organization in the present invention are as follows. In order to easily generate microcracks at the tip of the fatigue crack, the development of texture is useful. In order to obtain a texture required to generate the microcracks, the ratio (aspect ratio) between the major axis diameter and the minor axis diameter of the colonies having the same orientation between adjacent crystal grains was specified to be 4 or more.

Further, the texture was made such that the minor axis diameter of the textured colony was 5 μm or less because of local anisotropy due to plastic anisotropy between colonies having different crystal orientations even in a low ΔK region having a small plasticity region under a repeated load. And microcracks are easily generated.

[0018]

The present inventors have carried out various experiments focusing on the effect of increasing the fatigue strength due to the occurrence of separation, which has been attracting attention in the past, and as a result, see the Transactions of the Shipbuilding Society of Japan, Vol. 1
69 pp. 257-266,
S which is the separation index only in the region where the ΔK value is small
It was confirmed that even a steel material having a large Imax had no effect of delaying the fatigue propagation speed. This is because the mechanism for improving the fatigue rate due to the occurrence of separation is based on the delay effect generated when a fatigue crack propagates between the generated minute separations as shown in FIG.

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 conducted on the method of generating microcracks at the tip of a fatigue crack during propagation. As a result, it was found that microcracks need to be generated at a fatigue crack tip at a lower ΔK value than a brittle crack tip, and that the occurrence frequency of microcracks also needs to be increased to reduce the fatigue crack propagation speed.

First, in order to determine the conditions under which microcracks are formed even in a low ΔK region having a small plastic region size, FIG.
As shown in Table 2, the relationship between the aspect ratio of the textured colonies revealed by the temper color method and the K value at which minute separation occurs and the critical fracture stress in the thickness direction was determined in a fatigue test. It has been found 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, in order to increase the frequency of occurrence of microcracks, an experiment was conducted by changing the crystal grain size and the texture, and the size of the texture colonies revealed by the temper color method in the fatigue crack growth direction ( The relationship between the minor axis diameter) and the microcrack occurrence density was investigated. As a result, as shown in FIG. 2, the microcrack occurrence density was greatly dependent on the colony size of the texture.

FIG. 3 shows the relationship between the minor axis diameter of the textured colonies and the number of breaks at the nominal repeated bending stress Δσ = 160 MPa in the surface crack propagation test piece. When the minor axis diameter of the textured colonies was 10 μm or more, the rupture life was not changed, and when the short axis diameter was 5 μm or less, the rupture life was greatly improved. In order to secure a rupture life twice or more as long as the short diameter of the textured colony at the level of 10 μm, it was found that the short diameter of the textured colony should be 5 μm or less. As a result of investigating the relationship between the fracture surface and the structure of these test pieces in detail, it was found that, in the test pieces having a structure in which the minor axis diameter of the texture is 5 μm or less, the density of microcracks at the crack tip is high and the crack tip is substantially at the crack tip. It was shown that the stress of was reduced. Further, the undulation of the fracture surface became more remarkable as the minor axis diameter became smaller, and it was found that the effect of reducing ΔKeff by the crack closure was also exhibited.

Therefore, by reducing the minor axis diameter of the texture, the fatigue crack propagation delay mechanism of the conventional separation-producing steel material was completely different. It has been found that the fatigue characteristics can be greatly improved by the stress drop at the crack tip due to the generation of many microcracks and the fatigue crack propagation delay mechanism by the crack closure.

In order to achieve this structure, for example, if a certain amount of processing is applied to the ferrite during the heating process and the reverse transformation to austenitization is prevented, the dislocation introduced into the processed ferrite is recovered. It has been found that the microstructure of the present invention can be achieved by causing rearrangement, ultrafine graining of the matrix structure such as ferrite, and leaving the texture developed by the processing applied to the ferrite as it is.

Further, by paying attention to the fatigue fracture generated from the stress-concentrated portion of the steel plate additive such as a turning weld, the growth process was observed. As a result, more than 80% of the fatigue life was caused by the generation of fatigue cracks and the thickness direction. It turned out that it needed to grow to a depth of about 2 mm. From this, it was found that the structure modification of the surface layer of the sheet was important, and the fatigue rupture life was investigated by changing the thickness of the surface layer modified portion.

As a result, as shown in FIG.
If the surface is modified, it shows the same life as when the entire sheet thickness is modified, and the surface layer necessary to achieve a fatigue life more than twice the industrially meaningful current material It was found that the thickness of the layer was 5% or more of the plate thickness.

In order to form the structure of the present invention on the surface layer of the steel sheet, for example, the surface of the steel sheet is water-cooled during rolling, and Ar ₁
If it is below the point, it will be transformed into ferrite once,
Using the sensible heat at the center of the sheet thickness, the temperature of which hardly decreases even by cooling, further rolling is performed while raising the temperature of the ferrite structure in the surface layer, and the short axis diameter of the textured colonies in the surface layer becomes 5 μm or less. It has been found that a modified structure can be formed.

[0028]

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.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

[Table 3]

[0032]

[Table 4]

[0033]

[Table 5]

Test Nos. 1 to 12, which are examples of the present invention, have a texture having a minor axis diameter of 5 μm or less and an aspect ratio of 4 or more of a colony of a textured tissue revealed by the temper color method over the surface layer or the entire plate thickness. The fatigue strength is improved according to the thickness of the structure.

The test numbers 1 to 6, 8 to 12 of the present invention and the test numbers 13 to 16, 21, 22, 24 of the comparative example were
Although cooling was applied after the rough rolling, in Test Nos. 14, 21 and 22 of the comparative examples, the cooling rate was slow, so that the temperature of the entire steel sheet decreased, and the rolling after cooling did not become the temperature-raising process. . Further, in Test No. 24 of the comparative example, the elapsed time after cooling was too long, and the required conditions for rolling after cooling could not be satisfied. Therefore, Test No. 14, which is a comparative example,
The structure of the surface layer portion of 21, 22, 24 was not refined, and the aspect ratio of the texture was 4 or more, but the minor axis diameter was 5
μm or less.

In addition, Test Nos. 17 to 20, 23 of Comparative Examples
No cooling was performed after rough rolling in any of the samples, and a predetermined structure was not obtained. In Comparative Example 16, although the predetermined cooling / rolling was performed, the ferrite grain size did not become 3 μm or less because air cooling was performed after the completion of the rolling.
In the heat recovery process after the rolling, the heat recovered to Ac ₃ or more, so that partial grain growth occurred, and a predetermined structure could not be obtained.

In Comparative Examples 13 and 14, predetermined cooling and rolling were performed, and although the structure of the surface layer had the desired characteristics,
The thickness is less than 5% of the sheet thickness, and although the fatigue properties have been improved, the normal rolled material at the target level (Comparative Example 1)
It did not reach twice the fatigue life of 7).

Also, as shown in Table 2, the materials of Comparative Examples 16 to 24 did not reach a breaking life of 3 × 10 ⁶ times when the repetitive nominal stress range in the surface crack propagation test was 160 MPa, so 2 × 10 ⁶ fatigue strength of 110MPa
It was below.

On the other hand, Test Nos. 1 to 12
The material of satisfies the required manufacturing conditions as shown in Table 2,
While satisfying the strength and toughness of the target, the repeating nominal stress range at the surface crack propagation test is at rupture life in the case of 160MPa is 3 × 10 ⁶ times or more, 2 × 10 in turn welded portion ⁶
The fatigue strength at one time was 110 MPa or more, and the fatigue properties were greatly improved as compared with conventional steel materials.

[0040]

According to the present invention, since the above-mentioned action is utilized by using the above-mentioned means, microcracks are generated on the fracture surface during fatigue crack propagation, and as a result, from the surface of the steel sheet such as a turning welded joint or a T joint. It can reduce and prevent fatigue damage in the form of fatigue cracks that propagate in the thickness direction and have a great effect on related fields as well as the technical field.

[Brief description of the drawings]

FIG. 1 (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.

FIG. 2 is a table showing the relationship between the size (small axis diameter) of a textured colony in the fatigue crack growth direction and the microcrack occurrence density of the textured colonies revealed by the temper color method.

FIG. 3 is a table showing the relationship between the minor axis diameter of textured colonies revealed by the temper color method and the rupture life at a nominal cyclic stress Δσ = 160 MPa in a surface crack propagation test.

FIG. 4 shows the ratio of the thickness of the surface layer texture modified portion to the plate thickness, and the nominal cyclic bending stress Δσ = 1 in the surface crack propagation test.
4 is a table showing a relationship with a breaking life at 60 MPa.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hidesato Mabuchi 1 Nishinoshima, Oita, Nippon Steel Corporation Inside Oita Works (56) References JP-A-63-20414 (JP, A) JP-A-61-235534 (JP, A) JP-A-5-148541 (JP, A) JP-A-5-148540 (JP, A) JP-A-5-148542 (JP, A) JP-A-5-202444 ( JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/00-8/10

Claims (2)

(57) [Claims] C .: 0.20% or less, Si: 0.01 to 1.0%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.20% by weight . N: a steel sheet containing 0.020% or less , with the balance being Fe and unavoidable impurities
Having a ferrite structure and adjacent grains of this structure
The aspect ratio of the texture roller <br/> knee comprised equal grain crystal orientation to each other by (major axis diameter / ratio of minor axis diameter) is 4 or more, and it from the minor axis diameter of 5μm or less tissue The collection
Synthetic tissue colonies should be at least
A steel sheet having excellent fatigue properties characterized by being present over a range of 5% or more . 2. The steel sheet of the above component further comprises N
i, Cr, Mo, Cu, W, Co, V, Nb, Ti, Z
r, Ta, Hf, rare earth element, Y, Ca, Mg, Te,
The steel sheet having excellent fatigue properties according to claim 1, wherein the steel sheet contains at least one of Se and B in a total amount of 4.5% or less.