JPH07242992A - Steel plate having fatigue crack arresting effect - Google Patents

Abstract

PURPOSE:To produce a steel plate hardly causing fatigue crack propagation by specifying a composition consisting of C, Si, Mn, Al, Cr, Mo, Ni, Cu, Nb, Ti, V, and Fe and also specifying a hardness difference between the hard part and the soft part in a structure. CONSTITUTION:This steel plate having a fatigue crack arresting effect can be obtained by providing a steel plate which has a composition consisting of, by weight, 0.01-0.3% C, 0.1-0.5% Si, 0.3-2.0% Mn, 0.005-0.1% sol.Al, 0-1.5% Cr, 0-0.6% Mo, 0-0.5% Ni, 0-1.0% Cu, 0-0.1% Nb, 0-0.1% Ti, 0-0.1% V, and the balance Fe with inevitable impurities and also has a structure consisting of a hard part and a soft part and having a hardness difference of >=150 Vickers hardness between these two parts. Further, it is preferable that the above structure is composed of a soft part of 50mum average grain size dispersed in a matrix of hard part or the average spacing of the hard pant is regulated to <=50mum.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet which is suitable as a material for civil engineering structures, ships, marine structures and equipment, line pipes and the like, and which has excellent fatigue crack growth characteristics in the atmosphere and in corrosive environments.

[0002]

2. Description of the Related Art In recent years, civil engineering structures and the like have been required to be larger and lighter, and therefore structural steels have been desired to have higher strength. The use of high-strength steel has also begun to be expanded for hull materials, etc., from the perspective of increasing size and cost. However, when high-strength steel is used, the fatigue stress becomes an important problem because the design stress naturally rises.

On the other hand, since thick steel plates such as structural steels are generally welded, fatigue cracks often occur at the welded portions. Therefore, if the fatigue crack generated and propagated from the weld can be retained in the steel material, it is effective in extending the fatigue life of the structure.

In order to stop the progress of fatigue cracks, there is a method of generating microcracks at the tips of fatigue cracks, as disclosed in JP-A-5-148541. However, the effect of this method is limited to a low ΔK (ΔK: difference between the maximum stress intensity factor and the minimum stress intensity factor) region, that is, when the crack is not long and the stress level is low, and it occurs to some extent in the welded portion. It is considered to be small for cracks in the middle ΔK region.

Proceedings of the Japan Society of Mechanical Engineers, Volume A, vol.45 (1979),
As shown in P.440-445, the effect of the structure on the fatigue endurance limit has been investigated in the field of mechanical structural steel, but in the case of a long crack with a length of more than about 500 μm, the progress of the structure is It has been pointed out that it is unlikely to be affected, and no knowledge has been shown for suppressing the propagation of relatively long cracks, which is necessary in order to further improve the suppression of fatigue crack growth.

Japanese Unexamined Patent Publication (Kokai) No. 4-329848 discloses a hot-rolled steel sheet for various uses such as an automobile wheel in which fatigue strength is an important characteristic, and hardness of a matrix phase and a second phase in a two-phase structure. Although it has been shown that good fatigue strength can be obtained by limiting the area ratio and the grain size of the second phase, it cannot be said that the fatigue crack growth behavior has been sufficiently investigated. The effect of suppressing the growth of fatigue cracks by focusing on the hardness difference between the structures has not been clarified.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a specific object thereof is as a material for civil engineering and building structures, hulls, marine structures and devices, line pipes and the like. It is an object of the present invention to provide a steel sheet which is a steel material used and has a property that fatigue cracks are unlikely to propagate.

[0008]

The gist of the present invention is as follows.
It is on the steel plate of (1) to (3).

(1) C: 0.01 to 0.3% and Si: 0.1% by mass.
1 to 0.5%, Mn: 0.3 to 2.0% and sol.Al: 0.005 to 0.1
%, And Cr: 1.5% or less (additional free), M
o: 0.6% or less (without addition), Ni: 0.5% or less (without addition), Cu: 1.0% or less (without addition),
A steel sheet containing Nb: 0.1% or less (or without addition), Ti: 0.1% or less (or without addition) and V: 0.1% or less (or without addition), with the balance being Fe and inevitable impurities. , Its structure consists of a hard part and a soft part.
A steel sheet having a fatigue crack growth suppressing effect, characterized in that the difference in hardness between parts is 150 or more in Vickers hardness.

(2) The steel plate having the composition of (1) above, the structure of which is composed of a base of a hard part and a soft part dispersed in this base, and the hardness difference between the base of the hard part and the soft part is Vickers hardness. Is 150 or more and the average grain size of the soft part is 50 μm or less.

(3) The steel sheet having the composition of (1) above, the structure of which includes a hard portion and a soft portion, the difference in hardness between these two portions is 150 or more in Vickers hardness, and the average distance between the hard portions is 50 μ.
A steel sheet having an effect of suppressing fatigue crack growth, which is characterized by being m or less.

The hard portion referred to here is martensite,
The soft part means a structure composed of at least one of bainite, pearlite, pseudo-pearlite and tempered martensite, and the soft part is composed of ferrite.

The present invention is based on the following findings obtained by examining the material factors that affect fatigue behavior.

According to the results of investigation of the fatigue behavior of steel materials, it is easy for fatigue cracks to stay when the composite structure has a large hardness difference between the constituent structures. And, this effect, by the abundance ratio (volume ratio) of the hard portion and the soft portion,
Being unaffected.

When the hardness difference between the respective structures in the composite structure is set to a certain value or more, and in addition to this, the average particle size of the soft part is suppressed to a certain value or less, or the average interval (dispersion of the hard part) When the (distance) is suppressed to a certain value or less, when the propagating crack reaches the vicinity of the boundary between the hard part and the soft part, the plastic deformation at the tip is suppressed, so that the above-mentioned fatigue crack retention occurs.

It is expected that by adding a small amount of any one of Cr, Mo, Ni and Cu, the effect of improving the corrosion resistance of the steel material and improving the fatigue crack growth characteristics even in a corrosive environment can be expected.

Further, if any of Nb, Ti and V is added, the effect of strengthening the soft part (ferrite) by producing carbide is obtained, and the fatigue crack growth rate of propagating in the soft part decreases. .

[0018]

The chemical composition of the steel sheet of the present invention, its structure, and the reasons for limiting the difference in hardness between the soft portion and the hard portion will be described together with the action and effect. % Means mass%.

1) Chemical composition of steel sheet C: 0.01 to 0.3% C is a component that enhances the strength of steel. In order to maintain the strength level required for the use of the steel sheet of the present invention, the C content is 0.01%.
That's it. Below this, it is difficult to secure the strength and structure necessary for the use of the steel sheet of the present invention. On the other hand, the main use of the steel sheet of the present invention is always subjected to welding work. Therefore, in order to prevent the occurrence of weld cracks, the upper limit of the C content is 0.3.
%. The desirable C content range is 0.03 to 0.18%.

Si: 0.1-0.5% Si is a necessary component for deoxidizing steel. Si content is 0.
If it is less than 1%, this effect cannot be expected. On the other hand, if it exceeds 0.5%, the toughness of steel is impaired. The desirable Si content range is 0.25 to 0.4%.

Mn: 0.3-2.0% Mn is a component that improves the strength of steel. Mn content is 0.3
If it is less than%, it is difficult to secure the strength required for the use of the steel sheet of the present invention. However, Mn, like C, is a component that hardens the weld heat affected zone and causes weld cracking, so its content has an upper limit. That is, if it exceeds 2.0%, weld cracking tends to occur. Desirable Mn content is
It is 0.7 to 1.4%.

Sol.Al: 0.005 to 0.1% Al has a Al content of 0.005 as sol.Al for deoxidation of steel.
It is necessary to set it to be at least%. However, if the sol.Al content exceeds 0.1%, the cleanliness and toughness of the steel are impaired.

One of the steel sheets of the present invention is, in addition to the above components,
The balance consists of Fe and inevitable impurities. It is desirable to control P and S in the impurities to 0.025% or less and 0.020% or less, respectively.

The steel sheet of the present invention may further contain, in addition to the above-mentioned components, one or two of the following seven components.
It may include more than one species.

Cr: 1.5% or less Cr is an effective component for improving fatigue crack growth characteristics in a corrosive environment, controlling the dislocation structure in the soft part, and suppressing microscopic composition deformation. Therefore, when positively added, the Cr content is preferably 0.01% or more. However, if the Cr content exceeds 1.5%, these effects are saturated, and the strength of the steel excessively increases, resulting in impaired toughness. A more desirable range is 0.3 to 1.0%.

Mo: 0.6% or less Mo is also an effective component for improving the fatigue crack growth characteristics in a corrosive environment, controlling the dislocation structure in the soft part, and suppressing microscopic composition deformation. Therefore, when positively added, the lower limit of the Mo content is preferably 0.05%. However, when the Mo content exceeds 0.6%, these effects saturate and the strength of the steel excessively increases, resulting in impaired toughness. A more desirable range is 0.1 to 0.4%.

Ni: 0.5% or less Ni is also an effective component for improving the fatigue crack growth characteristics in a corrosive environment, controlling the dislocation structure in the soft part, and suppressing microscopic composition deformation. Therefore, the lower limit of the Ni content is preferably 0.1% when positively added. However, when the Ni content exceeds 0.5%, these effects saturate and the strength of the steel excessively increases, resulting in impaired toughness. A more desirable range is 0.2 to 0.4%.

Cu: 1.0% or less Cu is also an effective component for improving fatigue crack growth characteristics in a corrosive environment, controlling the dislocation structure in the soft part, and suppressing microscopic composition deformation. However, if the Cu content is less than 0.1%, its effect is small.
It is desirable to set it to be at least%. However, the Cu content is 1.0%
Above these values, these effects saturate and the strength of the steel excessively increases, impairing the toughness. A more desirable range is 0.3 to 0.5%.

The above Cr, Mo, Ni and Cu all have the effect of improving the corrosion resistance, and in combination with this effect, the fatigue crack growth characteristics in a corrosive environment are improved.

Nb, Ti, V: 0.1% or less for all Nb, Ti, and V all form carbides to make the soft part finer and strengthen, so fatigue crack growth characteristics in a corrosive environment It is an effective ingredient for improving.

Therefore, 1 out of these 3 kinds of ingredients
When more than one kind is selected and added positively, the content is preferably 0.01% or more. However, if the content exceeds 0.1%, the above effect is saturated and the strength of the steel excessively increases, resulting in impaired toughness. More desirable is 0.01 to 0.03% for both Ti and Nb, and 0.02 to 0.05 for V.
% Range.

2) Structure and hardness difference of steel sheet The steel sheet of the present invention having the above chemical composition is subjected to hot forging or hot rolling after ordinary melting and casting (continuous casting or ingot casting) and heat treatment. It can be obtained in the manufacturing process.

The steel sheet of the present invention has a composite structure composed of a hard portion and a soft portion. The hard part has a structure composed of one or more of martensite, bainite, pearlite, pseudo-pearlite and tempered martensite, and the soft part has a ferrite structure.

This is to form a composite structure of two kinds of structures, a hard part and a soft part, and to obtain the effect of retaining crack growth near the interface. This effect is not so affected by the existence ratio (volume ratio) of the hard part and the soft part. Therefore, in the steel sheet of the present invention, the existence ratio is not particularly limited.

One of the steel sheets of the present invention has the above-mentioned structure, and the difference in hardness between the hard portion and the soft portion is set to 150 or more in Vickers (hereinafter referred to as Hv).

The reason why the hardness difference between the soft part and the hard part is 150 or more in Hv is as follows.

When the hardness difference is 150 or more, the movement of dislocations at the crack tip is blocked at the interface between the soft portion and the hard portion, and the dislocations in which the Burgers vector is orthogonal to the interface are
Since they are arranged in the soft part in the vicinity of the interface between both parts, tilted grain boundaries are formed. Since this tilt grain boundary is composed only of grain boundary primary dislocations, it has a high grain boundary cohesive force and is liable to resist fracture. Further, since dislocations are less likely to enter the formed tilt boundaries, new tilt boundaries are formed on the soft part side adjacent to the grain boundaries when stress is repeatedly applied. By repeating such steps, a cluster of tilt grain boundaries having a large volume is formed. This gathering portion becomes a resistance against crack growth and improves the crack growth characteristics of the steel material.

Another one of the steel sheets of the present invention has a structure composed of a base material of a hard part and a soft part dispersed in the base material, and a hardness difference between the base material of the hard part and the soft part dispersed is 150 or more in Hv. Further, the average particle diameter of the soft portion is 50 μm or less.

The dispersion form of the soft part is random,
There are a plurality of parallel point sequences and a point sequence network or a tangled point sequence network, and a plurality of parallel continuous and continuous networks or a tangled continuous network. These soft parts are ferrite, and the average particle size is 50 μm or less.

Even with such a composite structure and hardness difference,
In the vicinity of the interface between the hard part and the soft part, it is possible to obtain the effect of retaining crack growth.

The reason for controlling the average particle diameter of the soft part to be 50 μm or less and its effect are as follows.

By this control, the crystal orientation is changed within a short range to suppress the movement of dislocations within the crystal grains, and the Burgers vector also changes when the dislocations reaching the grain boundaries move into the adjacent crystal grains. Therefore, the grain boundary dislocations can remain and strengthen the grain boundaries, and a high crack retention effect is exhibited. If the average particle size of the soft part exceeds 50 μm, a high crack retention effect cannot be obtained.

Still another one of the steel sheets of the present invention is such that its structure is composed of a hard portion and a soft portion, the difference in hardness between these two portions is 150 or more in Hv, and the average interval (dispersion distance) between the hard portions. Is 50 μm or less. In this case, either the hard part or the soft part may be the base material, and the existence form of either part is random, parallel point array and point array network or entangled point network, parallel. There are continuous and continuous networks or entangled continuous networks. In such a composite structure, the hard portions are dispersed so that the average distance between them is 50 μm or less. Here, the interval refers to the center-to-center distance.

Since such a composite structure is also composed of both hard and soft parts and has an appropriate hardness difference, it is possible to obtain the effect of retaining crack growth in the vicinity of the interface.

When the average interval of the hard parts is controlled to be dispersed at 50 μm or less, this control suppresses microscopic plastic deformation in the soft part, and the crack retention effect becomes remarkable.
If the average interval of the hard parts exceeds 50 μm, the remarkable effect cannot be obtained.

In the steel sheet of the present invention, if the hardness difference from the above-mentioned composite structure is maintained, and further, the conditions that the average grain size of the soft portion is 50 μm or less and the average interval of the hard portions is 50 μm or less are satisfied,
A more preferable effect can be obtained.

[0047]

EXAMPLE Steels having the chemical compositions shown in Table 1 were continuously cast into slabs having a thickness of 160 mm, which were directly fed hot and subjected to hot rolling to obtain steel plates having a thickness of 40 mm. In order to control the structure and hardness difference of the obtained steel plates, these steel plates were subjected to thermomechanical treatment under the conditions shown in Table 2.

[0048]

[Table 1]

[0049]

[Table 2]

That is, by appropriately controlling the heating temperature, rolling conditions, cooling rate, etc. for each of the steels Nos. 1 to 24, various structures, hardness differences between the soft and hard parts, and the average of the soft parts are obtained. A steel sheet having a grain size and an average interval (dispersion distance) of hard parts and having a tensile strength in the range of 41 to 86 kgf / mm ² was used.

The test materials sampled from these steel sheets are trial No. 1
To 96, the structure, the average grain size of the soft portion, the average interval of the hard portion, the hardness difference, and the fatigue crack growth characteristics were investigated.

The structural investigation and hardness measurement of the steel sheet were carried out by embedding the sample in an epoxy resin, cutting, polishing the cross section and etching, and observing with a microscope and measuring the hardness of a minute area.

The fatigue crack growth characteristics were investigated by the fatigue test method using the CT test piece 1 shown in FIG. 1 (b) and the servo pulser device shown in FIG. 1 (a).

In the apparatus shown in FIG. 1 (a), 1 is a CT test piece, 2 is a test solution tank, 3 is a solution circulation pump, 4 is a load cell, 5 is a hydraulic cylinder, 6 is a hydraulic source, 7 is a servo valve, Reference numeral 8 is a waveform generator, and 9 is a load controller, which can repeatedly apply stress to the test piece 1 in the test solution tank 2 by the hydraulic cylinder 5. The fatigue test conditions are as follows.

F (repetition rate) = 30 Hz R (stress ratio) = 0.1 T (test temperature) = room temperature Test atmosphere: moist hydrogen sulfide environment (hydrogen sulfide concentration 1% in suspended crude oil containing 10% water) However, nitrogen gas is constantly blown into the rest during the test period) Artificial seawater specified in ASTM, D-1141-52 Atmosphere According to the fatigue test results, the medium ΔK range ( In the test, in the second region at about 50 to 300 kgf / mm ^3/2 ), the Paris rule [Trans.ASME, Ser.D.85.523 (196
3))], that is, da / dN = C (ΔK) ^m , where [ΔK]: kgf / mm ^3/2 ,
[Da / dN]: It was found that mm / cycle holds. Therefore, the fatigue crack growth characteristics are as follows: ΔK = 50 and 100 kgf / mm ^3/2 in ΔK region
The average value of the crack growth rate da / dN (mm / cycle) at (da
/ dN) _m was used for evaluation. Tables 3 to 6 show the results of the above investigations, measurements and fatigue tests.

[0056]

[Table 3]

[0057]

[Table 4]

[0058]

[Table 5]

[0059]

[Table 6]

As can be seen from (da / dN) _{m of} each test piece,
In Test Nos. 1 to 72, which are examples of the present invention, (da / dN) _m is small and has high fatigue crack growth resistance. On the other hand, in the case of Test Nos. 73 to 90 in which the hardness difference is outside the range defined by the present invention, (da / dN) _m is large and the fatigue crack growth characteristics are not improved.

[0061]

EFFECTS OF THE INVENTION The steel sheet of the present invention is excellent in fatigue crack growth characteristics even in the middle ΔK region, and even if fatigue cracks occur in the welded portion, the fatigue life can be expected to be extended compared to conventional steels. Therefore, it is suitable as a steel material used for civil engineering and building structures, hulls, marine structures, marine equipment, line pipes, and the like.

[Brief description of drawings]

1A is a diagram showing an outline of a test apparatus, and FIG. 1B is a diagram showing a shape of a fatigue test piece.

[Explanation of symbols]

1: CT test piece, 2: test solution tank, 3: solution circulation pump, 4: load cell, 5: hydraulic cylinder, 6:
Hydraulic power source, 7: servo valve, 8: waveform generator, 9:
Load controller

 ─────────────────────────────────────────────────── ─── Continued front page (72) Hideaki Sachi, Inventor Hideaki Sachi, 4-53-3 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (72) Hideharu Okaguchi 4-chome, Kitahama, Chuo-ku, Osaka City, Osaka Prefecture No. 5-33 Sumitomo Metal Industries Ltd. (72) Inventor Takeshi Ichinose 4-5-33 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries Ltd. (72) Inventor Takahiro Kushida Central Osaka City, Osaka Prefecture 4-533 Kitahama, Sumitomo Metal Industries, Ltd. (72) Inventor Noboru Honda, 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd.

Claims (3)

[Claims] 1. In mass%, C: 0.01 to 0.3%, Si: 0.1 to
0.5%, Mn: 0.3-2.0% and sol.Al: 0.005-0.1
%, Further, Cr: 1.5% or less (may be added free), Mo:
0.6% or less (without addition), Ni: 0.5% or less (without addition), Cu: 1.0% or less (without addition), N
b: 0.1% or less (without addition), Ti: 0.1% or less (without addition) and V: 0.1% or less (without addition), with the balance being Fe and inevitable impurities. , Its structure consists of a hard part and a soft part.
A steel sheet having a fatigue crack growth suppressing effect, characterized in that the difference in hardness between parts is 150 or more in Vickers hardness. 2. In mass%, C: 0.01-0.3%, Si: 0.1-
0.5%, Mn: 0.3-2.0% and sol.Al: 0.005-0.1
%, Further, Cr: 1.5% or less (may be added free), Mo:
0.6% or less (without addition), Ni: 0.5% or less (without addition), Cu: 1.0% or less (without addition), N
b: 0.1% or less (without addition), Ti: 0.1% or less (without addition) and V: 0.1% or less (without addition), with the balance being Fe and inevitable impurities. , Its structure is composed of the base of the hard part and the soft part dispersed in this base, the hardness difference between the base of the hard part and the soft part is 150 or more in Vickers hardness, and the average particle diameter of the soft part is 50 μm or less. A steel plate having a characteristic fatigue crack growth suppressing effect. 3. In mass%, C: 0.01-0.3%, Si: 0.1-
0.5%, Mn: 0.3-2.0% and sol.Al: 0.005-0.1
%, Further, Cr: 1.5% or less (may be added free), Mo:
0.6% or less (without addition), Ni: 0.5% or less (without addition), Cu: 1.0% or less (without addition), N
b: 0.1% or less (without addition), Ti: 0.1% or less (without addition) and V: 0.1% or less (without addition), with the balance being Fe and inevitable impurities. , Its structure consists of a hard part and a soft part.
A steel sheet having an effect of suppressing fatigue crack growth, characterized in that the difference in hardness between parts is 150 or more in Vickers hardness and the average interval of hard parts is 50 μm or less.