KR20080104940A - Steel sheet excellent in control of fatigue crack growth and brittle destruction property - Google Patents

Abstract

A steel sheet bainite to the subject is provided the excellent steel sheet with the fatigue crack tremor repression characteristic and brittle fracture repression characteristic. A steel sheet containing chemical components including C etc.. Remnant is the iron and incidental impurities, the bainite phase made of the organization deciding to the subject in the location of the depth t / 4(T: board thickness), the average circle equivalent diameter(diagonal grain boundary diameter) of the domain surrounded by the diagonal grain boundary more than 15‹ is 15mum or less. Moreover, the average rate in which the azimuth difference of the adjacent grain is 55 ~ 60 degree is 0.30 or greater.

Description

STEEL SHEET EXCELLENT IN CONTROL OF FATIGUE CRACK GROWTH AND BRITTLE DESTRUCTION PROPERTY}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to steel sheets used as structural members of ships and bridges. In particular, the present invention relates to steel sheets having excellent characteristics of suppressing the occurrence of brittle fracture while ensuring good fatigue life by suppressing the rate of growth of cracks.

In a variety of structural materials including shipbuilding and bridges, cyclic stress is hardly applied. Therefore, in order to secure the safety of the structural material, it is extremely important in design that the fatigue properties are good for the steel used as the material.

The fatigue process of steel materials can be roughly classified into two processes, namely, the occurrence of cracking at the stress concentration portion and the progression of cracking once occurred. In the normal mechanical parts, macroscopic cracking is considered to be a limit of use, and it is not considered much about the progress of cracking in design. However, in a welded structure, even if a fatigue crack occurs, the fracture does not immediately occur. If the crack propagation rate can be slowed down, the service life can be extended to failure, and cracks can be found by periodic inspection. However, it is possible to continue using the product even if it is not replaced early.

By the way, in a welded structure, there are many toe portions of welding or defect parts as stress concentration parts, and it is impossible to completely prevent the occurrence of fatigue cracking as a real problem, and such a design is economically advantageous. It cannot be said. 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 to prevent the occurrence of cracks themselves. Designs such as slowing down the pace of progress are important.

Various techniques have been proposed so far as a technique for improving the property of suppressing the rate of fatigue crack propagation (hereinafter also referred to as "fatigue crack propagation suppression characteristic"), for example, Japanese Patent Laid-Open No. 2000-17379 discloses a steel sheet. Assuming that the normal direction of the surface is ND, the crystal grains having an orientation {(100) // ND} parallel to ND when the (100) plane of α iron is aligned, and an orientation where the (111) plane of α iron is parallel to ND { The boundary between the grains having (111) // ND} crosses at least one place at least 30 μm along the crack propagation direction, but the? (111) plane strength ratio inside the steel sheet in the measurement plane parallel to the steel sheet surface. It is proposed to make a steel sheet excellent in fatigue crack propagation suppression characteristics by setting the ratio of the α (100) plane strength ratio to 1.25 to 2.0.

The higher the steel sheet used under high stress, the more attention is paid to the fatigue properties. Since the above technique mainly uses ferrite (for example, 70 area% or more), it can only cope with strength grades of about 390 to 490 MPa. There is a problem that it is not applicable to the part that becomes.

In addition, in the above technique, in order to control the crystal orientation as described above, it is disclosed to perform the steel working in the low temperature temperature region or the α temperature region of the γ-α two-phase region in which ferrite is deposited at 70 area% or more. With respect to such a ferrite structure, it is known that a structure mainly composed of bainite (sometimes referred to simply as a "bainite structure") is produced in a fixed orientation with austenite. It is not possible to control the crystal orientation.

In Japanese Patent Laid-Open No. 2004-27355, when bainite or martensitic tissue is subjected to 15 incremental and gradual repeated loads of ± 0.012, maximum repetition rate of 0.5 Hz, and wavenumber up to maximum strain at a maximum tensile and compressive strain, 15 times Fatigue crack propagation characteristics such as the cyclic softening parameter represented by the ratio σ ₁ / σ ₁₅ of the stress at a maximum maximum strain (σ ₁ ) and the stress at a maximum maximum strain 15 times (σ ₁₅ ) are 0.65 to 0.95. Excellent steels have been proposed. In this technique, it is disclosed that deformation is alleviated by softening by movement and dissipation of dislocations at the crack tip, and crack growth is suppressed. According to this technique, the steel sheet having excellent fatigue crack propagation suppression characteristics can be obtained by manufacturing by a general manufacturing method in a component system similar to general-purpose steel. The softening parameters as described above are not necessarily distinguished from general materials. It is not necessarily that the desired characteristic is exhibited just by prescribing. In addition, there are embodiments in which the wavefront transition degree vTrs exceeds 0 ° C, and there is a possibility that the characteristics as a structure cannot be sufficiently satisfied.

On the other hand, in order to secure safety as a structure, it is desirable to suppress the occurrence of cracking due to brittle fracture in the steel (hereinafter referred to as "brittle fracture suppression characteristics" or CTOD (Crack-Tip Opening Displacement) characteristics). do. If brittle cracks occur, the structure itself is destroyed. When such fatigue cracking occurs, brittle fracture may occur in the low temperature region even though the development of the crack (fatigue cracking) can be suppressed. Therefore, for the safety of the structure, it is also necessary to sufficiently suppress the occurrence of brittle fracture. However, the technique regarding the steel material which is excellent in both a fatigue crack propagation suppression characteristic and a brittle fracture suppression characteristic has not been proposed until now.

The present invention has been made in view of the above circumstances, and an object thereof is to suppress fatigue crack growth by appropriately defining each crystal orientation relationship at a predetermined position in the sheet thickness direction in a steel plate mainly composed of bainite. An object of the present invention is to provide a steel sheet excellent in both characteristics and brittle fracture inhibiting characteristics.

Steel sheet of the present invention that can achieve the above object is C: 0.06 to 0.12% (meaning of mass%, the same below), Si: 0.5% or less, Mn: 1.4 to 2%, Al: 0.01 to 0.06%, P: 0.025% or less, S: 0.01% or less, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002 to 0.009% and B: 0.0005 to 0.003%, respectively, the balance being iron and unavoidable impurities,

It consists of an organization mainly composed of bainite phase,

The average circle equivalent diameter of the said crystal grain is 15 when the area | region enclosed by the diagonal grain boundary of 15 degrees or more is made into a crystal grain at the position of depth t / 4 (t represents plate | board thickness, and is the same below) from the surface. The average ratio of the micrometer or less and the orientation difference between adjoining crystal grains is 55-60 degrees is 0.30 or more,

When the orientation difference of two crystal | crystallization was made into the area | region enclosed by the diagonal grain boundary more than 15 degrees at the position of depth t / 2 from a surface, the average circle equivalent diameter of the said crystal grain is 20 micrometers or less. In addition, in this invention, "the bainite is called a main body" means the state which a bainite phase occupies 90 area% or more in a structure. In addition, the average circle equivalent diameter of the said crystal grain when the area | region enclosed by diagonal grain boundary of two crystal | crystallization is 15 degrees or more may be abbreviated as "diagonal grain diameter" below.

The steel sheet of this invention is (1) Cu from 0.05 to 0.5%, Cr: 0.05 to 0.4%, Ni: 0.05 to 1%, Mo: 0.03 to 0.3%, and V: 0.005 to 0.1% as needed. At least one selected from (1) or more of (2) Ca: 0.0001 to 0.005% and rare earth elements: 0.0005 to 0.003% may be further contained. Further, the steel sheet of the present invention preferably has a sheet thickness of 40 to 100 mm and a tensile strength of 570 MPa or more.

In the steel sheet of the present invention, in a steel sheet having a structure mainly composed of bainite, both the fatigue crack propagation suppression characteristics and the brittle fracture suppression characteristics are excellent by properly defining the respective crystal orientation relationships at predetermined positions in the sheet thickness direction. Steel sheet can be realized, and such steel sheet is useful as a material for various structural materials including shipbuilding and bridge fields.

Implement the invention  Best form for

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, in particular, it focused on the steel plate which is bainite structure, and examined the means for suppressing the fatigue crack growth rate in this steel plate from various angles. As a result, the following knowledge was obtained. In other words, in the bainite structure as described above, the austenite is formed with a few azimuth relations, but the orientation of each crystal lattice selected by the chemical composition of the steel sheet, the formation temperature of the structure, and other conditions It has been found that the relationship is changed and fatigue crack growth is particularly suppressed at grain boundaries having a constant crystal orientation difference. In addition, when the crystal orientation distribution is properly defined, it has been found that a steel sheet capable of satisfactorily realizing suppression of fatigue crack growth is realized. EMBODIMENT OF THE INVENTION Hereinafter, the effect of this invention is demonstrated according to the process which this invention was completed.

In single-phase structures such as bainite phases, grain boundaries are considered to be resistance to crack growth, and if the frequency of grain boundaries and cracks is increased during crack growth, crack growth can be suppressed. It became. That is, the knowledge that what is necessary is just to increase the frequency of collision with a crack by making a grain boundary fine. However, at small angle grain boundaries (small diameter angle boundaries) having small azimuth differences (e.g., less than 15 °) forming grain boundaries, the grain boundary energy is small and its effect is small. It is necessary to target). Moreover, it turns out that it is also effective in suppressing a crack propagation, so that the ratio whose orientation difference between adjoining crystal grains is 55-60 degrees among the diagonal grain boundaries becomes high (refer FIG. 1 later).

In other words, at a depth t / 4 from the surface, crystal grains surrounded by diagonal grain boundaries having an azimuth difference of 15 ° or more, the crystal grains having an average value of 15 µm or less as an average value of diameters (circle equivalent diameters) when converted into circles of the same area. For example, in the distribution of the orientation differences of adjacent crystal grains, the steel sheet having excellent fatigue crack growth suppression can be realized by setting the average ratio of the orientation differences of 55 to 60 ° to 0.30 or more (30% or more). In improving the suppression of fatigue crack growth in the steel sheet of the present invention, it is a representative position of the entire sheet thickness that the azimuth relationship of grains is evaluated at a depth t / 4 from the surface. Because it needs to be.

In addition, the said "azimuth difference" is also called the "deviation angle" or the "diagonal angle", hereinafter also referred to as "crystal orientation difference". In addition, in order to measure such a crystal orientation difference, the EBSP method (Electron Backscattering Pattern method) may be employ | adopted as shown in the Example mentioned later.

The inventors also found that brittle fracture can be suppressed by miniaturizing the diagonal grain boundary diameter at a position t / 2 from the surface (specifically, 20 µm or less). . Since brittle fracture generally occurs at a depth t / 2 from the surface (that is, at the center of the sheet thickness), it is considered that brittle fracture can be suppressed by miniaturizing the tissue at this position. In addition, "crystal grain at the position of depth t / 2 from the surface" in this invention means "the area | region enclosed by diagonal grain boundaries of 15 degrees or more in the azimuth difference of two crystals" similarly to the position of depth t / 4 from the surface. .

One of the features of the steel sheet of the present invention is that the chemical composition is properly adjusted. Below, the reason for limiting the range of a chemical component is demonstrated.

[C: 0.06-0.12%]

C is an element necessary for securing the strength of the steel sheet. In order to obtain high strength, that is, about 570 MPa (depending on the thickness of the steel sheet to be used), it is necessary to contain 0.06% or more. However, an excessive content of more than 0.12% results in a deterioration of weldability, coarsening of block sizes, an increase in the proportion of incineration grain boundaries, and a decrease in the proportion of diagonal grain boundaries having a crystal orientation difference of 55 to 60 °. see. In this regard, the C content was 0.06 to 0.12%. Moreover, the minimum with preferable C content is 0.07%, and a preferable upper limit is 0.11%.

[ Si : 0.5% or less]

Si is an element necessary for deoxidation and ensuring strength, and it is recommended to contain it in an amount of preferably 0.01% or more, more preferably 0.05% or more. However, when it contains exceeding 0.5% excessively, weldability will deteriorate. In addition, the upper limit with more preferable Si content is 0.4%.

[ Mn : 1.4 to 2%]

Mn is an effective element for securing the strength and toughness of the steel sheet, and in order to exhibit such an effect, it is required to contain 1.4% or more. However, when it contains excessively, weldability and crack susceptibility deteriorate and it is necessary to set it as 2% or less. Moreover, the minimum with preferable Mn content is 1.5%, and a preferable upper limit is 1.8%.

[ Al : 0.01 to 0.06%]

Al is a useful element for deoxidation and has no deoxidation effect unless it reaches 0.01%. However, when it contains excessively, since the toughness of a weld part will deteriorate, it is necessary to set it as 0.06% or less.

[P: 0.025% or less]

Since P is an impurity that segregates in the crystal grains and adversely affects ductility or toughness, as few as possible is preferable, but it is preferable to suppress it to 0.025% or less in consideration of the degree of cleanliness of practical steel. In addition, P is an impurity inevitably contained in steel, and it is difficult in industrial production to make the amount 0%.

[S: 0.01% or less]

Since S is an impurity that combines with alloying elements in the steel sheet to form various inclusions and adversely affects the ductility and toughness of the steel sheet, it is preferable to use S as little as possible, but to suppress it to 0.01% or less in consideration of the degree of cleanliness of practical steel. good. In addition, S is an impurity inevitably contained in steel, and it is difficult in industrial production to make the amount 0%.

[ Nb : 0.005 to 0.025%]

Nb suppresses transformation and lowers the bainite transformation start temperature (Bs) to exert an effect of making the organization unit fine. In addition, bainite transforms with a K-S relationship (Kurdjiumov-Sachs relationship) at the time of transformation, and by transformation at low temperature, fine blocks composed of a single variant (so-called sibling) are produced. In order to exhibit such an effect, it is necessary to contain Nb in the quantity of 0.005% or more (preferably 0.012% or more). However, if excessively contained, the weldability is impaired. Therefore, the Nb content is preferably 0.025% or less (preferably 0.020% or less).

[ Ti :  0 .005 to 0.03%]

Ti, like Nb, is an element effective for forming fine blocks by suppressing transformation and lowering Bs. However, when Ti content becomes excess, weldability will be impaired. In this respect, the Ti content was set to 0.005% or more (preferably 0.007% or more) and 0.03% or less (preferably 0.025% or less).

[N: 0.002-0.009%]

N is an element which forms nitride with elements, such as Ti and Al, and improves HAZ toughness. In order to exhibit such an effect, it is necessary to contain N 0.002% or more (preferably 0.003% or more). In addition, solid solution N causes the toughness of HAZ to deteriorate. As the total amount of nitrogen increases, the above-mentioned nitride increases, but the solid solution N also becomes excessive, so the present invention is suppressed to 0.009% or less.

[B: 0.0005 to 0.003%]

B, like Nb, is effective for forming fine blocks by suppressing metamorphosis and lowering Bs. In order to exhibit such an effect, it is necessary to contain 0.0005% or more. However, since weldability will be impaired when B content becomes excess, it was made into 0.003% or less.

The basic component in the steel plate of this invention is as above-mentioned, and remainder consists of iron and an unavoidable impurity (for example, O). Moreover, it is also effective to make the steel plate of this invention contain the following component as needed other than the said component.

[ Cu : 0.05 to 0.5%, Cr : 0.05 to 0.4%, Ni : 0.05 to 1%, Mo : 0.03 to 0.3% and V: at least one selected from the group consisting of 0.005 to 0.1%]

Like Nb, these elements are effective for forming fine blocks by suppressing metamorphosis and lowering Bs. Therefore, it is preferable to contain 0.05% or more of Cu, Cr and Ni, 0.03% or more of Mo, and 0.005% or more of V when containing these elements. However, if the amount is excessive, the weldability is impaired. Therefore, the upper limit in the case of containing these elements was defined as mentioned above. Moreover, even if it does not contain actively, these elements may inevitably be mixed in small amounts due to raw materials, etc., and therefore they are treated as unavoidable impurities when contained in an amount less than the above lower limit.

[ Ca : 0.0001 to 0.005% and rare earth elements ( REM ): At least one of 0.0005 to 0.003%]

Ca and REM are effective elements for the improvement of toughness by S fixation, and in order to exhibit 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 set it as 0.005% or less in Ca and 0.003% or less in REM. The minimum with more preferable at the time of containing Ca is 0.001%.

The steel sheet of the present invention is composed of a structure mainly composed of bainite. By cooling in an austenite state, the steel sheet of the present invention can be in a supercooled state and can be made of bainite structure. These bainite tissues are transformed into austenite with a certain orientation of azimuths. In particular, when transformed at low temperatures, the selectable variants are limited and a large peak in any crystal orientation difference is obtained. In order to reduce a transformation point, addition of an alloying element, an increase of a cooling rate, etc. are effective.

As specific manufacturing conditions, hot rolling is performed by heating a slab that satisfies the requirements of the above chemical composition in a temperature range of 1000 to 1200 ° C. In order to refine the metamorphic structure, it is effective to roll and recrystallize the austenite structure. The recrystallization temperature (the lowest temperature at which recrystallization is started) of austenite depends on the chemical composition of the steel, and the chemical composition of the present invention is usually about 850 to 900 ° C. In order to roll in this temperature range (above recrystallization temperature) (that is, to perform hot rolling), it is good to make heating temperature 1000 degreeC or more. However, if the heating temperature is too high, the initial austenite structure becomes too coarse, and thus the transformation structure cannot be sufficiently refined. Therefore, it is good to make heating temperature into 1200 degrees C or less.

Next, in the rolling in the temperature range where the position of depth t / 2 from the surface is 900 to 850 ° C, the cumulative reduction ratio calculated from Equation 1 below is 15% or more, and the depth 2mm and the depth t / It is good to adjust the temperature difference of 2 positions within 45 degreeC.

Cumulative reduction ratio = (t ₀ -t ₁ ) / t ₀ × 100

[In Formula 1, t ₀ represents the thickness of the steel sheet when the temperature at the t / 2 position of the steel slab is 900 ° C, and t ₁ represents the thickness of the steel sheet when the temperature at the t / 2 position of the steel slab is 850 ° C. .]

By controlling in this way, the diagonal grain boundary diameter of the crystal grain of the position of depth t / 2 from a surface can be made uniformly small. Specifically, by performing sufficient rolling in the temperature range (just above the recrystallization temperature) where the position of depth t / 2 from the surface is 900 to 850 ° C., fine austenite structures can be formed at the position of depth t / 2 from the surface. As a result, it is possible to form a fine metamorphic tissue. In contrast, if the reduction ratio is insufficient in this temperature range, coarse austenite may be formed. When the temperature range is 2 mm deep from the surface and t / 2 is deep from the surface at the time of rolling in this temperature range, coarse and fine are mixed as the resulting metamorphic structure, which is uneven in suppressing brittle fracture. This may occur. In addition, as a reference | standard which employ | adopts the temperature difference of the position with the depth t / 2 from the said surface, setting it as the position "depth of 2 mm from the surface" has a stable evaluation as a surface layer part since the surface layer part is directly influenced from the exterior, such as water. It was chosen as a possible location.

The temperature of the position of depth t / 2 from a surface can be calculated | required by the method as shown in the following Example. The temperature difference between the position of 2 mm deep and t / 2 from the surface can be controlled by adjusting the plate thickness, the water cooling time, and the air cooling time.

In addition, even if rolling is carried out in the austenite uncrystallized temperature range and strain is introduced into the austenite grains, the resulting metamorphic structure can be made fine, but the effect is not great. So US not expecting a rolling refinement of the effect of the recrystallization temperature region, it is preferable to terminate the rolling at a temperature of (Ar ₃ transformation point + 10 ℃). This is because the effects of high strength and finer structure of accelerated cooling (for example, water cooling) after rolling can be sufficiently exhibited.

About cooling after rolling, it is preferable to perform accelerated cooling (for example, water cooling) at the cooling rate of 5 degrees-C / sec or more. Moreover, about the stop temperature of accelerated cooling, since it is necessary to cool to the temperature which a structure becomes a bainite main body, it shall be 450 degrees C or less. By performing such accelerated cooling, the metamorphic structure (particularly, the metamorphic structure at the position of t / 4) can be sufficiently refined, and the strength of the steel sheet and the sufficient suppression of fatigue crack growth can be achieved.

In addition to the above manufacturing process, a tempering treatment may be performed in a temperature range of 500 to 700 ° C as necessary.

By the manufacturing method as described above, a steel sheet which satisfies the chemical composition composition requirements and structure requirements of the present invention and has a tensile strength of 570 MPa or more can be produced. It is preferable that the plate | board thickness of the steel plate of this invention is 40-100 mm.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is, of course, not limited by the following Examples, and of course, the present invention is not limited by the following examples. It is possible that they all fall within the technical scope of the present invention.

( Example )

Example  One

Steel of the chemical composition shown in Table 1 was melt-manufactured in a basic oxygen furnace gas, and steel sheets were manufactured by various cooling and rolling conditions. The manufacturing conditions at this time are shown in Table 2 below. In addition, in Table 2 below, "AcC" means "accelerated cooling (cooling by water)", "AC" means "air cooling", and "QT" means "quenching and tempering", respectively. In addition, the temperature of t / 2 part of a steel piece was computed by the process computer which used the difference method, and the temperature difference of 2 mm in depth and t / 2 part was adjusted by controlling the plate thickness, cooling time, air cooling time, etc. which are temperature-controlled. The specific temperature management procedure is as follows.

[Measurement of Temperature During Rolling]

1. Using a process computer, the predetermined temperature (position of t / 2 from the surface) of the steel slab is calculated based on the ambient temperature from the start of heating to the end of heating and the existence time in the furnace.

2. A method suitable for calculating a rolling temperature at an arbitrary position in the sheet thickness direction, such as a differential method, based on data of a rolling pass schedule during rolling or a cooling method (water cooling or air cooling) between rollings using the calculated heating temperature. Rolling is carried out while calculating using.

3. The surface temperature of the steel sheet is measured using a radial thermometer placed on a rolling line. However, the theoretical value is also calculated by the process computer.

4. The surface temperature of the steel sheet actually measured at the start of rough rolling, at the end of rough rolling, and at the start of finish rolling is compared with the calculated temperature calculated from the process computer.

5. If the difference between the calculated temperature and the measured temperature is more than ± 30 ℃, the calculated surface temperature is recalculated to match the measured temperature to be the calculated temperature on the process computer, and if less than ± 30 ℃, the calculated temperature calculated from the process computer Use as is.

6. Using the calculated temperature calculated above, the rolling temperature of the region to be controlled is managed.

For each of the obtained steel sheets, the bainite fraction, the ratio of the diagonal grain diameter and the crystal orientation difference at the position of depth t / 4 from the surface are 55 to 60 °, the diagonal grain diameter and tensile strength at the position of depth t / 2 from the surface. (TS), fatigue crack growth rate, and brittle crack suppression characteristics (δc _{-10 ° C} ) were measured by the following method. These results are collectively shown in Table 3 below.

[ Bainite  Fraction]

From the region of the steel plate depth t / 4, the sample is cut out so as to expose a surface parallel to the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet, which is ground using wet emery polishing papers of # 150 to # 1000. After that, mirror finishing was performed using a diamond slurry as an abrasive. After this mirror polished piece was etched with a 2% nitric acid-ethanol solution (nital solution), a 150 μm × 200 μm field of view was observed at an observation magnification of 400 times, and the bainite fraction was measured by image analysis. In addition, all lath-like structures other than ferrite were regarded as bainite. The bainite fraction of 5 o'clock was calculated | required in total, and the average value was employ | adopted.

[Diagonal at position t / 4 and t / 2 Boundary diameter ]

In the cross section parallel to the rolling direction of the steel sheet at the positions t / 4 and t / 2 from the surface, the diagonal grain boundary diameter is determined by FE-SEM-EBSP (electron backscattering diffraction imaging method using an electron emission scanning electron microscope). Measured. Specifically, Tex SEM Laboratories Inc.'s EBSP apparatus (trade name: "OIM") was used in combination with EF-SEM, and the diagonal grain diameter was measured using a boundary with a grain angle (crystal orientation difference) of 15 DEG or more. The measurement conditions at this time were measured at intervals of 200 µm and measurement steps: 0.5 µm, and measurement points having a confidence index (Confidence Index) indicating reliability of the measurement orientation were less than 0.1 were excluded from the analysis. The average value of the diagonal grain boundary diameter obtained in this way was computed, and it was set as "diagonal grain boundary diameter (average circle equivalent diameter)" in this invention. In addition, the thing with a diagonal grain diameter of 2.0 micrometers or less was judged as measurement noise, and was excluded from the target of average value calculation.

[At the position of depth t / 4 from the surface Decision defense difference  Ratio of 55 to 60 °]

By measuring the orientation difference at each grain boundary by OIM automatic analysis software, the ratio whose crystal orientation difference was 55-60 degrees at the position of depth t / 4 from a surface was calculated | required (calculated). When measuring this ratio, suppose that the area | region enclosed by the diagonal grain boundary of 15 degrees or more is made into crystal grains (large grain), for example, and there are six crystal grains adjacent to this diagonal grain among them, When only one satisfies the azimuth difference 55 to 60 °, the "ratio" is 1/6. These calculations are performed for all the large grains and averaged to find an average ratio of 55 to 60 degrees of azimuth difference between adjacent crystal grains. In other words, even if the ratio of the crystal orientation difference is 55 to 60 degrees among the large grains is less than 0.30, the average of the present invention satisfies when the average is 0.30 or more (at an average ratio).

[ The tensile strength ]

Tensile strength TS was measured by extracting the NK U 14A test piece from the site | part of the depth t / 4 of each steel plate, and performing a tensile test according to JISZ2241. The tensile strength was set at 570 MPa or more.

[Fatigue Crack Growth Rate]

Based on ASTM E647, the fatigue crack growth rate was calculated | required by performing a fatigue crack growth test using a compact test piece. At this time, it evaluated by making into a representative value the value in the stable growth area | region ((DELTA) K = 20 (MPa * rm)) which the Paris law prescribed | regulated by following formula (2) holds. In addition, about the evaluation criteria of the fatigue crack growth rate, since the normal steel is a growth rate of about 4 to 6 x 10 ^-5 mm / cycle (When △ K = 20), less than 3.0 × 10 ^-5 mm / cycle As a guide.

da / dn = C (△ K) ^m

[In Formula 2, the integers determined on conditions, such as a: crack length, n: number of repetitions, C, m: material, and load, are shown, respectively.]

[ Brittle cracks  Evaluation of Inhibition Characteristics]

The brittle crack suppression characteristics are based on the crack tip opening displacement test (CTOD test) specified by WES1108 (established on February 1, 1995) issued by the Japan Welding Association (WES), which is a crack tip opening displacement test. The opening displacement (δc) at the start of unstable fracture was measured and evaluated based on this result. In addition, when conducting the crack tip opening displacement test, the "Guidelines on the CTOD test method of the weld heat influence part" prescribed in WES1109 (established on April 1, 1995) was also taken into consideration.

The test piece used the "standard three-point bending test piece" shown in FIG. 6 of P.6 of WES1108 (established on February 1, 1995). The test temperature was -10 ° C, and δc _{-10 ° C} (mm) was measured. In this invention, the case where (delta) c- ₁₀ degreeC is 0.20 mm or more is set as the pass.

(-): Not measured due to low TS

From the result of Table 3, it can consider as follows. First, Test Nos. 3, 7, 8, 11, 13, 16, 18, 21, and 23 satisfy the organizational requirements of the present invention, and fatigue crack growth and brittle fracture are sufficiently suppressed. On the contrary, any of the structure requirements of the present invention is not satisfactory _{at the} crack propagation rate or δc _{-10 ° C.}

Based on the results in Table 3, the relationship between the ratio of the crystal orientation difference at the depth t / 4 position of 55 to 60 ° and the fatigue crack growth rate is shown in FIG. 1, wherein the diagonal grain diameter at the depth t / 4 position is coarsened. Except for the test pieces (test Nos. 15, 19, and 26), it can be seen that the fatigue crack propagation is suppressed by setting the ratio to 0.30 or more (crack propagation rate <3.0 × 10 ⁻⁵ mm / cycle). Moreover, although the relationship between the diagonal grain boundary diameter at the depth t / 2 position and delta c- ₁₀ degreeC is shown in FIG. 2, it turns out that brittle fracture generation is suppressed by making the diagonal grain boundary diameter in this position into 20 micrometers or less ( delta c- _10> 0.20 mm).

BRIEF DESCRIPTION OF THE DRAWINGS The graph which shows the relationship between the average ratio whose crystal orientation difference is 55-60 degrees at the depth t / 4 position in a Example, and a fatigue crack growth rate.

2 is a graph showing a relationship between a diagonal grain boundary diameter at a depth t / 2 position and δc _{-10 ° C} in an example.

Claims (4)

As a steel plate, C: 0.06 to 0.12% (meaning of mass%, the same below), Si: 0.5% or less, Mn: 1.4-2%, Al: 0.01-0.06%, P: 0.025% or less, S: 0.01% or less, Nb: 0.005 to 0.025%, Ti: 0.005 to 0.03%, N: 0.002-0.009% and B: contains 0.0005 to 0.003%, respectively The balance is iron and inevitable impurities, It consists of an organization mainly composed of bainite phase, The average circle equivalent diameter of the said crystal grain is 15 when the area | region enclosed by the diagonal grain boundary of 15 degrees or more is made into a crystal grain at the position of depth t / 4 (t represents plate | board thickness, and is the same below) from the surface. The average ratio of the micrometer or less and the orientation difference between adjoining crystal grains is 55-60 degrees is 0.30 or more, The steel plate whose average circle equivalent diameter of the said crystal grain is 20 micrometers or less when the area where the orientation difference of two crystal | crystallization is 15 degree or more diagonally surrounded by the grain boundary from the surface is made into crystal grains. The method of claim 1, Cu: 0.05-0.5%, Cr: 0.05 to 0.4%, Ni: 0.05-1%, Mo: 0.03 to 0.3% and V: 0.005 to 0.1% A steel sheet further containing at least one selected from the group consisting of: The method of claim 1, A steel sheet further containing at least one of Ca: 0.0001 to 0.005% and rare earth element: 0.0005 to 0.003%. The method according to any one of claims 1 to 3, A steel sheet having a sheet thickness of 40 to 100 mm and a tensile strength of 570 MPa or more.

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