JP2007314819A - Steel sheet having excellent fatigue crack propagation resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet having excellent fatigue crack propagation resistance. <P>SOLUTION: The steel sheet has a composition containing 0.03 to 0.30% C, ≤0.5% Si, 0.8 to 2% Mn, 0.01 to 0.10% Al, ≤0.010% N, ≤0.03% P and ≤0.01% S, and the balance Fe with inevitable impurities, and in which the structure at the position of a depth t/4(t=sheet thickness) is a mixed structure substantially consisting of ferrite and a hard phase, the ferrite grain size is ≤30 μm, the fraction of the hard phase is 15 to 80%, the hardness (HV) of the hard phase is 200 to 800, and the grain size of the hard phase is ≤200 μm, and H value expressed by formula: H=2+8.0×10<SP>-2</SP>×(the ferrite grain size)+1.5×10<SP>-2</SP>×(the fraction of the hard phase)-2.0×10<SP>-2</SP>×(the grain size of the hard phase)-1.85×10<SP>-3</SP>×(the hardness of the hard phase), is ≤3.6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel plate excellent in fatigue crack growth resistance suitable for use in ships, offshore structures, bridges, buildings, and the like.

  Ships, offshore structures, bridges, buildings, etc. are usually assembled by welding thick steel plates with a thickness of 6 mm or more by welding. It is difficult to avoid the occurrence of fatigue cracks because stress concentration tends to occur at the welded portion of the steel plate. However, even if fatigue cracks occur in the steel sheet, if the crack growth rate can be suppressed, it becomes possible to find and repair the cracks before causing the destruction of the structure by periodic inspection or the like.

  As a steel material having excellent fatigue properties, particularly fatigue crack resistance, Patent Document 1 discloses that a ferrite structure as a soft phase is appropriate in a bainite structure as a hard phase, a martensite structure, or a mixed structure of both structures. An amount of steel is disclosed (claims and paragraph 0008).

  In Patent Document 2, in order to obtain a steel material excellent in fatigue crack growth resistance, (a) the structure of the steel material should be a composite structure having a large difference in hardness between constituent structures, and (b) the average of the soft part The particle size is not more than a specific value, or the average interval between the hard portions is not more than a certain value, and (c) the relationship between the ratio of the hard portion and the soft portion to the tissue and the hardness satisfies the specific condition (Claims and paragraphs 0021-0023).

In Patent Document 3, in order to generate microcracks that delay the propagation of fatigue cracks, pearlite bands having a thickness of 3 μm or less and an interval of 20 μm or less exist in stripes, and the aspect of the matrix structure between pearlite band phases Ferrite / pearlite steel comprising a textured colony having a ratio (ratio of major axis / minor axis) of 4 or more and a minor axis of 10 μm or less is disclosed (claims and paragraphs 0017 to 0019).
JP 2000-12392A (Claims and paragraph 0008) JP 2002-121640 A (Claims and paragraphs 0021 to 0023) JP-A-5-148541 (Claims and paragraphs 0017 to 0019)

  Various techniques as described above have been disclosed so far in order to improve fatigue crack growth resistance. However, sufficient fatigue crack growth resistance has not been obtained, and further improvement is required. Yes. This invention is made paying attention to such a situation, and the objective is to provide the steel plate which has much more excellent fatigue crack progressability.

The steel sheet of the present invention that has achieved the above-mentioned object is
C: 0.03 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 0.8-2%,
Al: 0.01 to 0.10%,
N: 0.010% or less (excluding 0%),
P: 0.03% or less (not including 0%), and S: 0.01% or less (not including 0%)
And the balance has a composition consisting of Fe and inevitable impurities,
The structure at the position of depth t / 4 (t = sheet thickness) as viewed from the Z direction (sheet thickness direction) is substantially composed of a mixed structure of ferrite and hard phase,
The ferrite particle size is 30 μm or less, the hard phase fraction is 15 to 80%, the hard phase hardness (HV) is 200 to 800, the hard phase particle size is 200 μm or less,
It is a steel plate excellent in fatigue crack growth resistance, characterized in that the H value represented by the following formula (1) is 3.6 or less.
H = 2 + 8.0 × 10 ⁻² × ferrite particle size (μm) + 1.5 × 10 ⁻² × hard phase fraction (%)
−2.0 × 10 ⁻² × hard phase particle size (μm)
−1.85 × 10 ⁻³ × hard phase hardness (HV) (1)

  In the steel plate of the present invention, in addition to the above components, if necessary, (a) Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%), Cr: 2 % Or less (excluding 0%), Mo: 0.5% or less (not including 0%), V: 0.1% or less (not including 0%), Nb: 0.04% or less (0% B: 0.004% or less (not including 0%), W: 2.5% or less (not including 0%), and Co: 2.5% or less (not including 0%) One or more selected from the group consisting of: (b) Ti: 0.03% or less (excluding 0%), Zr: 0.1% or less (not including 0%), and Hf: 0.05% or less 1 or more selected from the group consisting of (not including 0%), Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%) ) And rare earth elements: It is also effective to contain at least one selected from the group consisting of 0.01% or less (not including 0%), and the properties of the steel sheet depend on the type of components to be contained. Further improvement.

  By controlling the ferrite grain size, hard phase fraction, hard phase grain size, and hard phase hardness of the steel sheet structure, a steel sheet that is more excellent in fatigue crack growth resistance than the conventional one can be obtained.

  Conventionally, in order to obtain a steel sheet having excellent fatigue crack growth resistance, research has been conducted mainly focusing only on the ferrite fraction and the hardness of bainite and / or martensite, which are hard phases. However, as a result of detailed examination of the relationship between the steel structure and the crack growth rate, the present inventors have determined that by controlling not only the ferrite fraction and the hard phase hardness, but also the hard phase fraction and grain size. It has been found that fatigue crack resistance superior to the above can be achieved. Therefore, the steel sheet of the present invention is characterized in that the steel structure is appropriately controlled. Below, steel structure is demonstrated first.

<Mixed structure of ferrite and hard phase>
In the steel sheet of the present invention, the structure at the position of depth t / 4 (t = sheet thickness) when viewed from the Z direction (sheet thickness direction) is substantially composed of a mixed structure of ferrite and hard phase. Features. This is because those having a mixed structure of ferrite and hard phase are superior in fatigue crack propagation resistance than those having only ferrite or only the hard phase. Here, the “hard phase” in the present invention refers to a structure composed of only bainite, only martensite, and bainite and martensite. “The structure at the position of depth t / 4 (t = plate thickness) is substantially composed of a mixed structure of ferrite and hard phase” means that a small amount of pearlite is present at the position of depth t / 4. It means that it may exist. However, in order not to impair the effects of the present invention, the amount of pearlite (area fraction) at the position of depth t / 4 is preferably less than 5%. Further, a steel plate which does not have pearlite at the position of the depth t / 4 and consists only of a mixed structure of ferrite and a hard phase is more preferable from the viewpoint of strength and the like.

  Although the exact mechanism by which fatigue crack growth resistance is improved due to the mixed structure is unknown, it can be estimated as follows. The presence of an appropriate amount of hard phase with a certain size and hardness dispersed in the steel structure requires a large amount of energy for cracks to penetrate through the hard phase, resulting in a slow crack growth rate. I think that. It is also considered that the crack growth rate is delayed by cracks passing through the hard phase. However, the present invention is not limited to such estimation.

<Ferrite particle size is 30 μm or less>
When having a mixed structure like the steel material of the present invention, the finer the ferrite grain size, the more the number of times of crossing the grain boundary when the fatigue crack progresses, and the greater the resistance to crack propagation. Conceivable. Therefore, the ferrite particle size was determined to be 30 μm or less. A preferable ferrite particle diameter is 20 μm or less, and more preferably 10 μm or less.

  In the present invention, the value of “ferrite grain size” employs an average value measured by the method described below: First, a test piece is taken from the position of the steel sheet depth t / 4 (t = plate thickness), Apply nital corrosion. Next, the image is observed with an optical microscope at a magnification of 100 to 400 times, and a photograph is taken with 10 observation fields. The obtained ten micrographs are subjected to image analysis using “Image-Pro Plus” manufactured by Media Cybernetics, etc., and the average value of the ferrite grain size is calculated.

<Hard phase fraction 15-80%>
As explained above, a steel sheet having a mixed structure in which an appropriate amount of a hard phase is present has excellent fatigue crack growth resistance. The appropriate amount of the hard phase fraction was determined to be 15 to 80% from the experimental data (see the data of steel plates No. 36 and 37 in the following examples). Here, the “hard phase fraction” represents the area fraction of the hard phase in the structure. The measuring method is the same as the measuring method of the ferrite particle diameter, and an average value obtained from ten observation fields is adopted.

<Hard phase hardness (HV) is 200 to 800>
In order to suppress crack growth, it is considered that the higher the hard phase hardness, the better. However, from experimental data, it has been found that the crack growth rate increases conversely even if the hardness of the hard phase is too high. Therefore, the hard phase hardness (HV) was determined to be 200 to 800 from the experimental data (see data of steel plates No. 38 and 39 in the following examples). In the present invention, the value of “hard phase hardness” is obtained by taking a test piece from a position at a depth t / 4 (t = plate thickness) and randomly measuring 10 points of the hard phase at this position. The average value of Vickers hardness is adopted.

<Hard phase particle size is 200 μm or less>
In order to suppress crack growth, it is considered that the hard phase particle size is preferably as large as possible. However, even if the hard phase particle size is too large, the crack growth rate increases. Therefore, the upper limit of the hard phase particle size was set to 200 μm (refer to data of steel plate No. 40 in the following examples). The “hard phase particle size” in the present invention is not the particle size of each hard phase, but when the hard phases are adjacent to each other, the lump of the hard phase that is in contact is considered as one phase. It means the particle size of the whole mass. As one of the mechanisms by which the crack growth is suppressed, it is assumed that the crack bypasses the hard phase, so the crack is not the particle size of each adjacent hard phase, but the particle size of the entire mass. It is because it thinks that it will be influenced. In the present invention, as the value of “hard phase particle size”, an average value from 10 observation fields obtained by the same method as in the case of ferrite particle size is adopted.

<H value is 3.6 or less>
In order to improve the fatigue crack growth resistance of the steel sheet as compared with the conventional one, the present inventors have examined the relationship between the steel structure and the crack growth rate in detail, and as a result, the hard phase fraction and the hard phase as described above. It has been found that within the range of the particle size and the hard phase hardness, the crack growth rate is suppressed as the hard phase fraction is decreased, the hard phase particle size is increased, and the hard phase hardness is increased. Furthermore, it has also been found that the smaller the ferrite grain size, the more the crack growth rate is suppressed. Therefore, by regression analysis of experimental data (data of steel plates Nos. 1 to 29 in the following examples) of ferrite grain size, hard phase fraction, hard phase grain size and hard phase hardness, and crack growth rate, The H value represented by the formula (1) was calculated.
H = 2 + 8.0 × 10 ⁻² × ferrite particle size (μm) + 1.5 × 10 ⁻² × hard phase fraction (%)
−2.0 × 10 ⁻² × hard phase particle size (μm)
−1.85 × 10 ⁻³ × hard phase hardness (HV) (1)

  A graph plotting the relationship between the H value and the crack growth rate is shown in FIG. As can be seen from the graph of FIG. 2, the H value and the crack growth rate have a good correlation, and the crack growth rate is suppressed as the H value decreases. Therefore, the H value is 3.6 or less, preferably 3.0 or less, more preferably 2.5 or less, and still more preferably 2.0 or less.

  The steel sheet of the present invention is characterized in that the chemical composition is adjusted to an appropriate range in addition to being controlled to an appropriate structure in order to improve fatigue crack growth resistance. Therefore, below, the chemical component composition of a steel plate is demonstrated.

<C: 0.03-0.30%>
C is an element necessary for ensuring the strength of the base material. If the C content is less than 0.03%, the strength of the base material cannot be secured. Therefore, the lower limit of the C amount is set to 0.03%. A preferred lower limit is 0.05%. On the other hand, if the amount of C exceeds 0.30%, the entire steel becomes brittle, and the base metal toughness and fatigue characteristics deteriorate. Therefore, the upper limit of the C amount is set to 0.30%. A preferable upper limit is 0.2%.

<Si: 0.5% or less (excluding 0%)>
Si is an element necessary for deoxidation of steel. In order to fully exhibit the action, it is preferable to contain 0.1% or more, preferably 0.15% or more. However, when the amount of Si is excessive, the base metal toughness and fatigue characteristics are lowered. Therefore, the upper limit of Si content is set to 0.5%. A preferable upper limit is 0.4%.

<Mn: 0.8-2%>
Mn is an element useful for improving the hardenability and ensuring the strength of the base material. In order to ensure this effect sufficiently, the lower limit of the amount of Mn was set to 0.8%. The amount of Mn is preferably 1% or more, more preferably 1.5% or more. On the other hand, if the amount of Mn exceeds 2%, the base metal toughness and fatigue characteristics are lowered. Therefore, the upper limit of the amount of Mn is set to 2%. The amount of Mn is preferably 1.8% or less.

<Al: 0.01 to 0.10%>
Al is an element having a deoxidizing action. In order to fully exhibit this action, the lower limit of the Al content was set to 0.01%. The amount of Al is preferably 0.02% or more. However, if the amount of Al is excessive, the base metal toughness and fatigue characteristics are reduced. Therefore, the upper limit of the Al amount is set to 0.10%. The amount of Al is preferably 0.06% or less, more preferably 0.04% or less.

<N: 0.010% or less (excluding 0%)>
N combines with Al and Ti and has the effect of refining austenite grains. If not excessive, it works effectively on the mechanical properties of the steel sheet. Therefore, the N amount is preferably 0.003% or more, more preferably 0.004% or more. However, if the amount of N is excessive, the toughness and fatigue characteristics deteriorate, so the upper limit was set to 0.010%. The N amount is preferably 0.008% or less, more preferably 0.006% or less.

<P: 0.03% or less (excluding 0%)>
Since P adversely affects the base metal toughness and fatigue characteristics, the upper limit is set to 0.03%. The amount of P is preferably 0.01% or less. However, since P is inevitably contained, it is industrially difficult to reduce the amount to 0%.

<S: 0.01% or less (excluding 0%)>
S is an element that forms Mn and MnS and adversely affects ductility and fatigue characteristics. Therefore, the upper limit of the amount of S is set to 0.01%. The amount of S is preferably 0.005% or less. However, since S is inevitably contained, it is industrially difficult to reduce the amount to 0%.

  The basic component composition of the steel sheet of the present invention is as described above, and the balance is substantially Fe. However, as a matter of course, it is permissible for the steel sheet to contain inevitable impurities brought in depending on the situation of raw materials, materials, manufacturing equipment, and the like. Furthermore, the steel plate of this invention may contain the following arbitrary elements as needed.

<Cu: 2% or less, Ni: 2% or less, Cr: 2% or less, Mo: 0.5% or less, V: 0.1% or less, Nb: 0.04% or less, B: 0.004% or less W: 2.5% or less, and Co: one or more selected from the group consisting of 2.5% or less>
Cu, Ni, Cr, Mo, V, Nb, B, W, and Co have the effect of increasing the hardenability and improving the strength of the steel sheet, and can be contained in the steel sheet as necessary. Further, V has an effect of improving temper softening resistance in addition to hardenability, and Mo has an effect of preventing temper brittleness.

  In order to sufficiently exert these actions, the lower limit of these amounts is preferably 0.01%, more preferably 0.2% for Cu, and preferably 0.01%, more preferably for Ni. Is 0.2%, Cr is preferably 0.02%, more preferably 0.1%, Mo is preferably 0.1%, more preferably 0.2%, and V is , Preferably 0.005%, more preferably 0.010%, Nb is 0.001%, more preferably 0.01%, and B is preferably 0.0005%, more preferably 0. 0.0010%, preferably 0.1%, more preferably 0.2% for W, and preferably 0.1%, more preferably 0.2% for Co.

  However, when the amount of these elements becomes excessive, the base material toughness and fatigue characteristics are lowered. Therefore, when these elements are contained, the upper limit of these amounts is 2% for Cu, preferably 1%, 2% for Ni, preferably 1%, and 2% for Cr, preferably 1%, Mo is 0.5%, preferably 0.3%, V is 0.1%, preferably 0.05%, and Nb is 0.04%, preferably 0. 0.03%, more preferably 0.025%, for B 0.004%, preferably 0.003%, more preferably 0.002%, and for W 2.5%, preferably 1 %, And for Co, 2.5%, preferably 1%.

<One or more selected from the group consisting of Ti: 0.03% or less, Zr: 0.1% or less, and Hf: 0.05% or less>
Ti, Zr, and Hf are elements useful for improving the toughness of a steel sheet by forming a nitride with N to refine the austenite grains and fixing solid solution N. , If necessary.

  In order to sufficiently exert these actions, the lower limit of these amounts is preferably 0.002%, more preferably 0.006% for Ti, and preferably 0.0005%, more preferably for Zr. Is 0.0010%, and for Hf, it is preferably 0.0005%, more preferably 0.0010%.

  However, when the amounts of Ti, Zr and Hf are excessive, the base material toughness, the HAZ toughness and the fatigue characteristics are lowered. Therefore, when these elements are contained, the upper limit of these amounts is 0.03% for Ti, preferably 0.02%, 0.1% for Zr, preferably 0.05%, And Hf is 0.05%, preferably 0.02%.

<One or more selected from the group consisting of Ca: 0.005% or less, Mg: 0.005% or less, and rare earth elements: 0.01% or less>
Ca, Mg, and rare earth elements (hereinafter abbreviated as “REM”) are all elements that have an effect of improving toughness and fatigue characteristics, and can be contained as necessary. Specifically, Ca and REM have the effect of reducing anisotropy by controlling the form of inclusions to spheroidize MnS, thereby improving the base material toughness and fatigue characteristics. On the other hand, Mg forms MgO and suppresses the coarsening of HAZ austenite grains.
Improves toughness and further improves fatigue properties after welding.

  In order to fully exert these actions, the lower limit of these amounts is preferably 0.0005%, more preferably 0.0010% for Ca, and preferably 0.0005%, more preferably for Mg. Is 0.0010%, and for REM it is preferably 0.0005%, more preferably 0.0010%.

  However, when the amount of Ca, Mg and REM becomes excessive, the base material toughness, the HAZ toughness and the fatigue characteristics are lowered. Therefore, when these elements are contained, the upper limit of these amounts is 0.005%, preferably 0.003% for Ca, 0.005%, preferably 0.003% for Mg, And REM is 0.01%, preferably 0.005%.

  As described above, the steel sheet of the present invention is characterized by its structure and chemical composition. Therefore, the manufacturing method of the steel plate of this invention, especially the adjustment method of structure | tissue are demonstrated. First, in order to make the ferrite finer, for example, the rolling finishing temperature may be lowered or the rolling reduction rate at a low temperature may be increased. Further, the holding temperature for ferrite transformation should be lowered and the holding time should be shortened. Next, in order to decrease the hard phase fraction and increase the hard phase particle size, the holding time for ferrite transformation may be shortened. Further, in order to increase the hardness of the hard phase, it is only necessary to increase the amount of ferrite generated by increasing the holding time for ferrite transformation and increase the amount of C to the hard phase. Alternatively, rapid cooling may be performed after ferrite transformation. However, when manufacturing conditions are changed to adjust one tissue factor, other tissue factors also change. Therefore, it is important to balance production conditions under specific chemical components. Specific production conditions are shown in the following examples.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  The steel having the composition shown in Table 1 was melted by a normal melting method to form a slab, and then heated to the temperature shown in Table 2 (described as “heating temperature” in Table 2). Finish rolling was performed. Table 2 shows the finish rolling temperature (described as “finishing temperature” in Table 2) and the rolling reduction up to + 50 ° C. (described as “rolling rate” in Table 2). And after finish rolling, it is cooled to the temperature for ferrite transformation at the cooling rate shown in Table 2 (described as “first cooling rate” in Table 2) (described as “holding temperature” in Table 2), It was held for a predetermined time (described as “holding time” in Table 2). After ferrite transformation, cooling is performed at the cooling rate shown in Table 2 (described as “second cooling rate” in Table 2), and in some cases, cooling is stopped at a predetermined temperature (described as “Cooling stop temperature” in Table 2). ), A steel plate was produced.

  The structure, ferrite grain size, hard phase fraction, hard phase hardness and hard phase grain size, and crack growth rate of the steel sheet produced as described above were measured as follows. The results are shown in Table 3. Table 3 also shows the H value calculated from the measurement results of the ferrite grain size and the like.

  In order to observe and measure the structure, ferrite grain size, hard phase fraction and hard phase grain size of the steel plate, first, a test piece of 15 mm × 15 mm × 10 mm from the position of the depth t / 4 (t = plate thickness) of the steel plate Was collected and subjected to nital corrosion. Subsequently, the photomicrograph was observed with an optical microscope at a magnification of 400, and photographs were taken with 10 observation fields. Furthermore, the average value of the structure, ferrite particle size, hard phase fraction, and hard phase particle size was calculated by image analysis using “Image-Pro Plus” manufactured by Media Cybernetics for the 10 micrographs obtained. .

  Hard phase hardness (HV) is obtained by collecting 15 mm × 15 mm × 10 mm test pieces from the position of the steel sheet depth t / 4 (t = plate thickness), and randomly selecting 10 hard phases at this position. The average value of Vickers hardness obtained by measurement was adopted.

  In order to measure the crack growth rate, the compact described in ASTM standard E 647 is set so that the crack propagation direction is the C direction (plate width direction) from the position of the depth t / 4 (t = plate thickness) of the steel plate. A tension test piece (CT test piece) was cut out (see FIG. 1). This CT test piece was subjected to a fatigue test with a servo pulser device at room temperature, a repetition rate of 30 Hz, and a stress ratio (the ratio of the minimum stress to the maximum stress) of 0.1, and the stress intensity factor range (ΔK) = 20 MPa√ The fatigue crack growth rate (mm / cycle) at m was measured.

  As shown in Tables 1 to 3, the steel plate No. 1 satisfying both the structure and composition requirements of the present invention. 1 to 19 show that the crack growth rate is suppressed and the fatigue crack growth resistance is excellent. On the other hand, steel plate No. which does not satisfy the requirement of H value. In 20 to 29, the crack growth rate is increased. Steel plate No. A graph showing the relationship between the H value of 1 to 29 and the fatigue crack growth rate is shown in FIG. As can be seen from FIG. 2, there is a good correlation between the H value and the fatigue crack growth rate, and the fatigue crack growth rate decreases as the H value decreases.

  Steel plate No. Nos. 30 to 34 do not satisfy the composition requirements of the present invention, and all of them have increased fatigue crack growth rates. Specifically, steel plate No. No. 30 has a high C content. No. 31 has a high Si content. No. 32 has a high Mn content. No. 33 has a high Al content. Since 34 has a high N content, it is brittle and the crack growth rate is increased.

  Steel plate No. 35 to 40 do not satisfy the structural requirements of the present invention, and the fatigue crack growth rate is increased in all cases. Specifically, steel plate No. No. 35 has a large ferrite particle size. No. 36 has a small hard phase fraction, so No. 37 has a high hard phase fraction. No. 38 has a high hard phase hardness. No. 39 has a low hard phase hardness. Since No. 40 has a large hard phase particle size, fatigue crack growth is increased.

It is the schematic of the fatigue test piece extract | collected from the steel plate in order to measure the fatigue crack growth rate of the steel plate manufactured in the Example. Steel plate No. manufactured in the examples. It is a graph which shows the relationship between H value of 1-29, and a crack growth rate.

Claims (4)

C: 0.03 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.5% or less (excluding 0%),
Mn: 0.8-2%,
Al: 0.01 to 0.10%,
N: 0.010% or less (excluding 0%),
P: 0.03% or less (not including 0%), and S: 0.01% or less (not including 0%)
And the balance has a composition consisting of Fe and inevitable impurities,
The structure at the position of depth t / 4 (t = sheet thickness) as viewed from the Z direction (sheet thickness direction) is substantially composed of a mixed structure of ferrite and hard phase,
The ferrite particle size is 30 μm or less, the hard phase fraction is 15 to 80%, the hard phase hardness (HV) is 200 to 800, the hard phase particle size is 200 μm or less,
A steel sheet excellent in fatigue crack growth resistance, wherein the H value represented by the following formula (1) is 3.6 or less.
H = 2 + 8.0 × 10 ⁻² × ferrite particle size (μm) + 1.5 × 10 ⁻² × hard phase fraction (%)
−2.0 × 10 ⁻² × hard phase particle size (μm)
−1.85 × 10 ⁻³ × hard phase hardness (HV) (1)   Furthermore, Cu: 2% or less (not including 0%), Ni: 2% or less (not including 0%), Cr: 2% or less (not including 0%), Mo: 0.5% or less (0%) V: 0.1% or less (not including 0%), Nb: 0.04% or less (not including 0%), B: 0.004% or less (not including 0%), The steel plate according to claim 1, comprising at least one selected from the group consisting of W: 2.5% or less (not including 0%) and Co: 2.5% or less (not including 0%).   Further, selected from the group consisting of Ti: 0.03% or less (not including 0%), Zr: 0.1% or less (not including 0%), and Hf: 0.05% or less (not including 0%) The steel plate according to claim 1 or 2, which contains one or more kinds.   Further, Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (not including 0%), and rare earth elements: 0.01% or less (not including 0%) The steel plate according to any one of claims 1 to 3, comprising one or more selected.

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