JP4676871B2 - Steel sheet with excellent fatigue crack growth control - Google Patents

Description

  The present invention relates to a steel plate used mainly as a structural material for ships and bridges, and more particularly to a steel plate capable of ensuring a good fatigue life by suppressing the crack growth rate.

  Many structural materials, such as shipbuilding and bridges, are subject to repeated stresses. Therefore, in order to ensure the safety of structural materials, the steel materials used have good fatigue characteristics. It is very important to design.

  The fatigue process of steel materials can be broadly divided into two processes, namely, the generation of cracks in stress-concentrated portions and the progress of cracks once generated. In normal machine parts, the occurrence of macroscopic cracks is considered as a use limit, and the development of cracks is not considered much in terms of design. However, in welded structures, even if fatigue cracks occur, they do not immediately break, and if the crack growth rate can be slowed, the life until failure can be shortened. Cracks can be found and can be used continuously without early replacement.

  By the way, in welded structures, there are many weld toes and defects as stress concentrated parts, and it is impossible as a practical matter to completely prevent the occurrence of fatigue cracks. Such a design is economical. However, it is not a good idea. That is, in order to improve the fatigue life of the welded structure, it is effective to significantly extend the crack propagation life from the state in which the crack already exists, rather than preventing the occurrence of the crack itself. For that purpose, the design which makes the progress rate of the crack of steel materials as slow as possible becomes an important matter.

  Various techniques have been proposed so far for suppressing the rate of fatigue crack growth. For example, in Patent Document 1, when the normal direction of the steel sheet surface is ND, the (100) plane of α iron is ND. Is a boundary between a crystal grain having a parallel orientation {(100) // ND} and a crystal grain having an orientation {(111) // ND} in which the (111) plane of α iron is parallel to ND The ratio of the α (111) plane strength ratio to the α (100) plane strength ratio in the steel sheet is 1.25 to 2 at least one cross section at 30 μm along the progress direction of the steel sheet. It has been proposed that the steel sheet has excellent fatigue crack propagation (propagation) characteristics by being set to 0.0.

  Although the steel sheet used under higher stress is more interested in fatigue properties, the strength class of about 390 to 490 MPa is used because the above technique is mainly composed of ferrite (for example, 70 area% or more). There is a problem that it cannot be applied only to a portion where fatigue cracks are a problem.

  In the above technique, in order to control the crystal orientation as described above, it is shown that strong processing is performed in the low temperature temperature region or the α temperature region of the γ-α two-phase region in which 70% by area or more of ferrite is precipitated. ing. In contrast to such a ferrite structure, it is known that a structure mainly composed of a bainite phase (sometimes referred to simply as “bainite structure”) has a certain orientation relationship with austenite. The crystal orientation cannot be controlled by the same means.

On the other hand, in Patent Document 2, when a bainite structure or martensite structure is applied, a maximum tensile / compressive strain of ± 0.012, a repetition rate of 0.5 Hz, and a wave number 12 gradually increasing / decreasing repeated load up to the maximum strain of 15 times The repetitive softening parameter indicated by the ratio σ ₁ / σ ₁₅ of the stress σ ₁ at the time of _one maximum strain and the stress σ ₁₅ at the time of the maximum strain of ₁₅ is 0.65 or more and 0.95 or less. Steel materials excellent in fatigue crack growth characteristics have been proposed. In this technique, it is shown that the strain is relaxed by softening due to the movement and disappearance of the dislocation at the crack tip, and the crack progress is suppressed. In this technology, it is said that a steel plate with excellent crack growth characteristics can be obtained by producing a general manufacturing method in a component system similar to that of general-purpose steel, but the distinction from general materials is not necessarily clearly defined. However, it cannot be said that desired characteristics are exhibited only by defining the softening parameters as described above. Furthermore, there is an example in which the fracture surface transition temperature vTrs exceeds 0 ° C., and the characteristics as a structure may not be sufficiently satisfied.
JP, 2000-17379, A Claims, etc. JP, 2004-27355, A Claims etc.

  The present invention has been made paying attention to the above-mentioned circumstances, and the purpose thereof is excellent in suppressing fatigue crack growth by appropriately defining each crystal orientation relationship in a steel sheet mainly composed of bainite. It is to provide an intended steel sheet.

  The steel sheet of the present invention that was able to achieve the above-mentioned object consists of a structure mainly composed of bainite, and when the crystal grain is a region surrounded by a large-angle grain boundary where the orientation difference between two crystals is 15 ° or more, It has a gist in that the average equivalent circle diameter of the crystal grains is 15 μm or less and the ratio of the orientation difference between adjacent crystal grains is 55 to 60 ° is 0.3 or more. In the present invention, “mainly composed of bainite” means a state in which the bainite phase occupies 90 area% or more in the structure.

  In the high-tensile steel plate of the present invention, the chemical component composition is effective as long as it is a component composition as a normal steel plate, but as a preferable chemical composition, C: 0.01 to 0.10% (meaning of mass%, the same applies hereinafter), Si: 0.01 to 0.6%, Mn: 0.6 to 2.0%, Al: 0.005 to 0.10%, respectively. Others: Cu: 0.03-3.0%, Cr: 0.03-3.0%, Ni: 0.03-3.0%, Mo: 0.03-1.0%, V: 0.00. One or two selected from the group consisting of 003 to 0.1%, Nb: 0.003 to 0.1%, Ti: 0.003 to 0.1% and B: 0.0003 to 0.003% Including those containing more than seeds, the balance being iron and inevitable impurities. In addition, P and S in the inevitable impurities are preferably suppressed to P: 0.025% or less and S: 0.02% or less, respectively.

In the steel sheet of the present invention, if necessary, the carbon equivalent Ceq defined by the following formula (1) is 0.40 to 0.70, and further Ca is 0.0005 to 0.005% and / or rare earth. Element (REM): It is preferable to contain 0.0005 to 0.030%, and the characteristics of the steel sheet are further improved according to the requirements to be added.
Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents (mass of C, Si, Mn, Ni, Cr, Mo and V, respectively). %).

  In the steel sheet of the present invention, in a steel sheet having a structure mainly composed of bainite, by appropriately defining each crystal orientation relationship, a steel sheet excellent in fatigue crack growth suppression can be realized, and such a steel sheet can be used in shipbuilding and bridge fields. It is useful as a material for various structural materials such as

  In order to solve the above-mentioned problems, the present inventors have focused on a steel sheet having a bainite structure, and studied means for suppressing the fatigue crack growth rate in the steel sheet from various angles. As a result, the following knowledge was obtained. That is, in the bainite structure as described above, it is generated with some orientation relationship with respect to austenite, but it is selected depending on the chemical composition of the steel sheet, the formation temperature of the structure, other conditions, etc. It has been found that the fatigue crack growth is suppressed particularly at the grain boundaries having a certain crystal orientation difference. And when the crystal orientation distribution was appropriately defined, it was found that a steel sheet capable of satisfactorily suppressing the progress of fatigue cracks could be realized, and the present invention was completed. Hereinafter, the effects of the present invention will be described along the background of the completion of the present invention.

  In a single-phase structure mainly composed of bainite phase, it is considered that the grain boundary acts as a resistance to crack growth, but if the frequency of the collision between the grain boundary and the crack is increased during crack growth, the crack growth is suppressed. It was considered possible. That is, it was found that the frequency of collision with cracks should be increased by making the grain boundaries finer. However, at a small-angle grain boundary (small tilt boundary) where the orientation difference between both ends forming a grain boundary is small (for example, a small tilt boundary), the grain boundary energy is small and the effect is small, so the orientation difference is 15 ° or more. It is necessary to target a large-angle grain boundary (large tilt boundary). It has also been found that the larger the ratio of the orientation difference between adjacent crystal grains in the large-angle grain boundary is 55 to 60 °, the more effective the crack growth is (see FIG. 1 described later).

  That is, a crystal grain surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more, and having an average diameter (equivalent circle diameter) of 15 μm or less when converted into a circle of the same area, In the distribution of misorientation of adjacent crystal grains, the ratio of misorientation of 55 to 60 ° is 0.3 or more (30% or more), thereby realizing a steel plate having an excellent fatigue crack progress suppressing effect. It is. The “orientation difference” is also referred to as “shift angle” or “inclination angle”, and may be hereinafter referred to as “crystal orientation difference”. In order to measure such a crystal orientation difference, an EBSP method (Electoron Backscattering Pattern method) may be employed.

  In the steel plate of the present invention, the chemical component composition is effective as long as it is a component composition as a normal steel plate, but a preferable chemical component composition is C: 0.01 to 0.10. %, Si: 0.01-0.6%, Mn: 0.6-2.0%, Al: 0.001-0.10%, Cu: 0.03-3.0% Cr: 0.03-3.0%, Ni: 0.03-3.0%, Mo: 0.03-1.0%, V: 0.003-0.1%, Nb: 0.003 -0.1%, Ti: 0.003-0.1% and B: One or more selected from the group consisting of 0.0003-0.003%, with the balance being iron and inevitable impurities Some are listed. The reasons for limiting the ranges of these components are as follows.

C: 0.01 to 0.10%
C is an element necessary for ensuring the strength of the steel sheet. In order to obtain the minimum strength as a steel sheet, that is, about 490 MPa (depending on the thickness of the steel material to be used), it is preferable to contain 0.01% or more. However, if the content exceeds 0.10% excessively, the weldability deteriorates, the block size becomes coarse, the proportion of small angle grain boundaries increases, and a large angle grain boundary having a crystal orientation difference of 55 to 60 °. This shows a tendency that the ratio of decreases. For these reasons, the C content is set to 0.01 to 0.10%. A more preferable lower limit of the C content is 0.03%, and a preferable upper limit is 0.08%.

Si: 0.01 to 0.6%
Si is a necessary element for deoxidation and securing strength, and if it is less than 0.1%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 0.6%, weldability deteriorates. In addition, the upper limit with more preferable Si content is 0.40%.

Mn: 0.6 to 2.0%
Mn is an element effective for securing the strength and toughness of the steel sheet, and in order to exert such effects, it is preferable to contain 0.6% or more. However, if over 2.0% is contained, the weldability and cracking susceptibility deteriorate, so the content is preferably made 2.0% or less. A more preferable lower limit of the Mn content is 0.8%.

Al: 0.001 to 0.10%
Al is an element useful for deoxidation, and if less than 0.001%, there is no deoxidation effect. However, if the content exceeds 0.10%, the toughness of the welded portion deteriorates, so the content is preferably 0.10% or less.

Cu: 0.03-1.0%, Cr: 0.03-1.0%, Ni: 0.03-1.0%, Mo: 0.03-1.0%, V: 0.003- One or more selected from the group consisting of 0.1%, Nb: 0.003-0.1%, Ti: 0.003-0.1% and B: 0.0003-0.003% These elements exhibit an effect of making the texture unit fine by suppressing transformation and lowering the bainite transformation start temperature Bs. In addition, bainite transforms with a KS relationship (Kurdjiumov-Sachs relationship) during transformation, but by transforming at low temperature, fine blocks consisting of a single variant (so-called siblings) are generated. It becomes like this. In order to exert such an effect, it is preferable to contain the above lower limit values or more. However, if contained in a large amount, the weldability is impaired.

  The basic components in the steel sheet of the present invention are as described above, and the balance is made of iron and inevitable impurities (for example, P, S, N, O, etc.). P and S in the inevitable impurities are as follows: In view of the above, it is preferable to suppress the content to 0.025% or less and 0.02% or less, respectively. Further, from the viewpoint of obtaining a bainite structure and maintaining good weldability, it is preferable to control Ceq represented by the above formula (1) within a range of 0.40 to 0.70 (%). .

P: 0.025% or less and S: 0.02% or less P is an impurity that segregates in the crystal grains and adversely affects the ductility and toughness, so it is preferable that the content be as small as possible (including 0%). In consideration of the degree of cleanliness of practical steel, it is preferable to suppress it to 0.025% or less. Further, S is an impurity which forms various inclusions by combining with alloy elements in the steel sheet and adversely affects the ductility and toughness of the steel sheet, so it is preferable that S be as small as possible (including 0%). In consideration of the degree of cleanliness of practical steel, it is preferable to suppress it to 0.02% or less.

  In addition to the above components, it is also effective for the steel sheet of the present invention to contain Ca: 0.0005 to 0.005% and / or REM: 0.0005 to 0.0030%. By including the element, the properties of the steel sheet are further improved.

Ca: 0.0005 to 0.005% and / or REM: 0.0005 to 0.030%
Ca and REM are effective elements for improving the toughness by fixing S, and in order to exert the effect, it is preferable to contain both 0.0005% or more. However, since the effect is saturated even if it contains excessively, it is preferable to make it 0.005% or less by Ca and 0.030% or less by REM. A more preferable lower limit when Ca is contained is 0.001%.

The steel sheet of the present invention is composed of a structure mainly composed of bainite. However, by cooling in the austenite state, the steel sheet becomes supercooled, lowers the Ar ₃ transformation point, and can have a bainite structure. Such a bainite phase transforms with austenite in a certain orientation relationship, but when it is transformed at a particularly low temperature, the variants that can be selected are limited, and a large peak appears at a certain crystal orientation difference. In order to lower the transformation point, it is effective to add an alloying element or increase the cooling rate.

As specific manufacturing conditions, after heating to a temperature range of 950 to 1250 ° C. and finishing rolling in a temperature range of Ar ₃ transformation point to 900 ° C., accelerated cooling (for example, water cooling) may be performed. If the heating temperature at this time is less than 950 ° C., the austenite state is not sufficiently obtained. However, when the heating temperature exceeds 1250 ° C., the austenite grains are coarsened, and the large-angle grain boundary diameter after transformation is also coarsened.

  As for cooling after rolling, accelerated cooling is preferably performed at a cooling rate of 10 ° C./second or more. The accelerated cooling stop temperature is set to 450 ° C. or lower because the structure needs to be cooled to a temperature at which the structure is mainly bainite.

  As another method, after heating to a temperature range of 950 to 1250 ° C. and rolling in a recrystallization temperature range, cooling to a temperature range of 600 to 700 ° C. at a cooling rate of 1 ° C./second or more is continued. There is a method in which rolling at a reduction rate of 30% or more is performed in the state of supercooled austenite at that temperature, and then accelerated cooling is performed again. In this method, many deformation bands can be introduced by rolling austenite at a low temperature, and nucleation sites increase due to the effect of ausforming (processing heat treatment), so the structure is refined and fatigue crack growth is suppressed. The effect can be enhanced.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Example 1
Steel materials having the chemical composition shown in Table 1 below were melted in a converter, and steel sheets were produced under various cooling and rolling conditions. The manufacturing conditions at this time are shown in Table 2 below. In Table 2, “AcC” means accelerated cooling (cooling with water), “AC” means air cooling, and “QT” means quenching / tempering.

  About each obtained steel plate, while measuring mechanical characteristics (yield point YP, tensile strength TS, elongation El), a large-angle grain boundary diameter (average circle equivalent diameter), a ratio of crystal orientation difference of 55-60 °, Fatigue crack growth rate, fracture surface transition temperature vTrs, etc. were measured by the following methods. These results are collectively shown in Table 3 below.

[Large-angle grain boundary diameter (average equivalent circle diameter)]
The cross section parallel to the rolling direction of the steel sheet was measured by FE-SEM-EBSP (electron backscatter diffraction image method using an electron emission scanning electron microscope). Specifically, an EBSP apparatus (trade name: “OIM”) manufactured by Tex SEM Laboratories is used in combination with EF-SEM, and a crystal grain is formed with a boundary having an inclination (crystal orientation difference) of 15 ° or more as a grain boundary. The diameter was measured. The measurement conditions at this time are: measurement area: 200 μm, measurement step: 0.5 μm interval, and measurement points with a confidence index (Confidence Index) indicating the reliability of the measurement direction are excluded from the analysis target. did. The average value of the crystal grain sizes thus obtained was calculated and used as the average crystal grain size in the present invention. Incidentally, those having a crystal grain size of 2.0 μm or less were judged as measurement noise and excluded from the target of calculating the average value of the crystal grain size.

[Proportion of crystal orientation difference of 55-60 °]
By measuring the orientation difference at each grain boundary with OIM automatic analysis software, the ratio of the crystal orientation difference of 55-60 ° was determined (calculated).

[Fatigue crack growth rate]
In accordance with ASTM E647, a fatigue crack growth rate was determined by carrying out a fatigue crack growth test using a compact test piece. At this time, the value in the stable growth region ΔK = 20 (MPa · √m) where the Paris law defined by the following equation (2) is satisfied was evaluated as a representative value. Incidentally, the fatigue crack growth rate evaluation, for reference, since the conventional steel is an advance speed of about 4~6 × ¹⁰ -5 mm / cycle (when ^{ΔK = 20), 3.0 × 10} -5 The standard was mm / cycle or less.
da / dn = C (ΔK) ^m (2)
Here, a: crack length, n: number of repetitions, C, m: constants determined by conditions such as material and load, respectively.

[Fracture surface transition temperature vTrs]
Using a JIS No. 4 impact test piece produced by machining, an impact test is conducted by a test method in accordance with JIS Z2242, and a brittle fracture surface ratio (or “ductile fracture surface ratio”) is obtained by a method in accordance with JIS. The transition temperature vTrs at which the brittle fracture surface ratio becomes 50% was determined from the curve of (temperature vs. brittle fracture surface ratio).

From the results in Table 3, it can be considered as follows. Test No. Nos. 1 to 6 satisfy the requirements specified in the present invention, and exhibit a sufficient fatigue crack progress suppressing effect (3.0 × 10 ⁻⁵ mm / cycle or less in progress rate). I understand.

In contrast, test no. 7 to 15 lack any of the requirements defined in the present invention, and none of them exerts the effect of suppressing fatigue progress. That is, test no. In the case of Nos. 7 to 13 , since accelerated cooling is not performed, the rate of the crystal orientation difference being 55 to 60 ° is low, so the fatigue crack growth rate is high. In addition, Test No. In the case of No. 14 , the ratio of the crystal orientation difference is 55 to 60 ° is high, but the large-angle grain boundary diameter is large and the fatigue crack growth rate is high. Furthermore, test no. No. 15 has a structure of (ferrite + pearlite), and since the ratio of the crystal orientation difference of 55 to 60 ° is low, the fatigue crack growth rate is high.

  Based on the results in Table 3, the relationship between the crystal orientation difference of 55-60 ° and the fatigue crack growth rate is shown in FIG. 1 except for the above-mentioned ratio except for the coarsened grain size (Test No. 14). It can be seen that the fatigue crack growth rate is sufficiently low when the value is 0.3 or more.

It is a graph which shows the relationship between the ratio of the crystal orientation difference in an Example, and 55-60 degrees, and a fatigue crack growth rate.

Claims (2)

When a region composed of a structure mainly composed of a bainite phase and surrounded by a large-angle grain boundary where the orientation difference between two crystals is 15 ° or more is used as a crystal grain, the average equivalent circle diameter of the crystal grain is 15 μm or less. And the ratio that the orientation difference between adjacent crystal grains is 55 to 60 ° is 0.311 or more,
C: 0.01-0.10% (meaning of mass%, the same applies hereinafter), Si: 0.01-0.6%, Mn: 0.6-2.0%, Al: 0.005-0. 10%, C r: 0.03~3.0% , M o: 0.03~1.0%, and B: containing from 0.0003 to 0.003%, further
Selected from the group consisting of Ni: 0.03-3.0%, V: 0.003-0.1%, Nb: 0.003-0.1%, and Ti: 0.003-0.1% 1 type or 2 types or more
Selected from the group consisting of Ni: 0.03-3.0%, V: 0.003-0.1%, Nb: 0.003-0.1%, and Ti: 0.003-0.1% And containing Ca: 0.0005-0.005%,
The balance is iron and inevitable impurities,
A steel sheet excellent in fatigue crack growth suppression, characterized in that a carbon equivalent Ceq defined by the following formula (1) is 0.52 to 0.70.
Ceq = [C] + [Si] / 24 + [Mn] / 6 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1)
However, [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents (mass of C, Si, Mn, Ni, Cr, Mo and V, respectively). %). The steel plate according to claim 1 , wherein P in the inevitable impurities is suppressed to 0.025% or less and S is suppressed to 0.02% or less.

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