JP6064897B2 - High-strength steel material with excellent fatigue crack propagation resistance and its determination method - Google Patents

Description

  The present invention relates to a steel material suitable for various welded structures such as ships, offshore structures, bridges, construction machines, buildings, tanks, and particularly suitable for members subjected to repeated loads, fatigue crack propagation resistance. The present invention relates to a high-strength steel material excellent in and a method for determining the same. The “steel material” here includes thick steel plates, section steels, steel pipes, cold-rolled steel plates, and hot-rolled steel plates.

  In recent years, various types of welded structures such as ships, offshore structures, bridges, construction machines, buildings, and tanks have been used to streamline design, reduce steel weight, reduce wall thickness, and save labor in welding. There are an increasing number of cases where is applied. For this reason, in addition to having excellent ductility and toughness, these high-strength steel materials are required to have excellent fatigue resistance properties in order to further ensure structural safety.

In a welded structure, a fatigue crack is often generated from the weld toe and propagates through the steel material, causing the structure to break (fatigue failure) in many cases. This is attributed to the fact that the weld toe portion tends to be a stress concentration portion due to its shape, and that a tensile residual stress is generated after welding.
For this reason, as a means to suppress crack generation from the weld toe, techniques such as additional welding to improve the shape and reduce stress concentration, techniques to introduce compressive residual stress by shot peening, etc. Is widely known.

  However, applying such a technique to a large number of weld toes on an industrial scale requires a great deal of labor and time, and is not practical from the viewpoint of productivity and cost. . Therefore, even if a fatigue crack occurs, if the subsequent crack propagation rate in the steel material can be reduced, the fatigue life of the welded structure can be extended. There is a strong demand for improved crack propagation characteristics.

  In response to such a demand, for example, Non-Patent Document 1 describes a study on the influence of the microstructure on the growth of fatigue cracks (fatigue crack propagation) in low-carbon steel. In the study described in Non-Patent Document 1, the soft phase (Vickers hardness: 149 HV) is made into a structure surrounded by a hard phase (Vickers hardness: 546 HV, fraction: 39.2%) in the form of a mesh. It is said that the fatigue crack propagation rate is greatly reduced compared to a structure in which a hard phase (Vickers hardness: 565 HV, fraction: 36.4%, average size: 149 μm) is uniformly dispersed in (Vickers hardness: 148). However, the steel sheet structure described in Non-Patent Document 1 is obtained by performing five stages of complicated heat treatment, and the complex heat treatment described in Non-Patent Document 1 is applied to normal steel sheet production. Has a problem that it is very difficult from the viewpoint of productivity. Further, the steel sheet having the structure described in Non-Patent Document 1 has reduced ductility, and there remains a problem with applying such a steel sheet to a structure.

  Patent Document 1 describes a steel sheet having a fatigue crack progress suppressing effect. The technique described in Patent Document 1 has a specific composition, a composite structure composed of a base of a hard part and a soft part dispersed in the base, and the hardness difference between the hard part and the soft part is determined by Vickers. Hardness is over 150HV. Thereby, the movement of dislocations at the crack tip is prevented at the interface between the hard part and the soft part, and the crack growth suppression characteristics of the steel sheet are improved. However, Patent Document 1 does not mention mechanical properties such as ductility and toughness, and the steel plate manufactured by the technique described in Patent Document 1 has sufficient properties as a structural steel plate. Whether it remains unknown.

Patent Document 2 describes a thick steel material having excellent fatigue crack propagation characteristics and a method for producing the same. The thick steel materials described in Patent Document 2 are, in mass%, C: 0.04 to 0.3%, Si: 0.01 to 2%, Mn: 0.1 to 3%, Al: 0.001 to 0.1%, N: 0.001 to 0.01%. It has a two-phase structure consisting of a soft phase and a hard second phase surrounding the soft phase in a network structure, and the soft phase and the hard second phase have the following conditions. (1) The soft phase is ferrite and tempered. It is composed of one or more of banite and tempered martensite, and the average Vickers hardness is 150 or less.

(2) The hard second phase is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite, and the average Vickers hardness is 250 or more.
(3) Grain boundary occupancy ratio (total sum of grain boundary lengths occupied by hard second phase / total grain boundary length) of the hard second phase is 0.5 or more.
It is a thick steel material with excellent fatigue crack propagation characteristics that satisfies all the requirements. If a thick steel material manufactured by the technique described in Patent Document 2 is used for a welded structural member, the fatigue crack growth rate in the base metal can be significantly suppressed in any crack propagation direction. However, the technique described in Patent Document 2 requires diffusion annealing at a high temperature for a long time in order to suppress the band structure, which leaves the problem that the process becomes complicated and the productivity is lowered.

  Patent Document 3 describes a thick steel plate having excellent fatigue strength. The thick steel plate described in Patent Document 3 is in mass%, C: 0.04 to 0.3%, Si: 0.01 to 2%, Mn: 0.1 to 3%, Al: 0.001 to 0.1%, N: 0.001 to 0.01%. And a composition containing at least ferrite and a hard second phase, and having a cross-sectional structure parallel to the surface, (a) a fraction of the hard second phase: 20 to 80%, (b) a hard first The two-phase average Vickers hardness: 250 to 800, (c) the average equivalent circle diameter of the hard second phase: 10 to 200 μm, (d) the maximum distance between the hard second phases: 500 μm or less, It is a thick steel plate whose two-phase structure is either bainite, martensite, or a mixed structure of both. According to the technique described in Patent Document 3, a large amount of a special or expensive alloy element, a complicated process, and a large limit on the tensile strength and the steel plate thickness are not required. It is said that the fatigue crack propagation resistance can be improved. In addition, in the technique described in patent document 3, although the characteristic which suppresses the fatigue crack progress in the plate | board thickness direction of a thick steel plate can be improved, in patent document 3, in the width direction and longitudinal direction of a thick steel plate. There is no mention of the fatigue crack growth inhibition characteristics, and there is a problem that the fatigue crack growth inhibition characteristics in the width direction and the longitudinal direction of the thick steel plate may be lowered.

  Patent Document 4 describes a steel plate having excellent fatigue crack growth resistance. The steel sheet described in Patent Document 4 has a composition including C: 0.030 to 0.30%, Si: 0.50% or less, Mn: 0.8 to 2.0%, Al: 0.01 to 0.10%, N: 0.010% or less, Number ratio of needle-shaped ferrite having an aspect ratio of 2 or more, acicular ferrite grown in the γ grain direction in an area fraction of 1 to 60%, and a major axis ranging from 5 to 100 μm Is a steel sheet having a structure of 80% or more. In the technique described in Patent Document 4, the presence of acicular ferrite having an area of 1 area% or more results in a steel sheet having excellent fatigue crack growth resistance. However, Patent Document 4 does not mention properties such as ductility and toughness, and a steel sheet manufactured by the technique described in Patent Document 4 is necessary as a structural steel sheet in addition to fatigue crack resistance. It remains unclear whether the properties are well balanced.

  Patent Document 5 describes a steel plate having excellent fatigue crack growth resistance. The steel sheet described in Patent Document 5 has a composition containing C: 0.01 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.3 to 2%, sol. Al: 0.001 to 0.1%, N: 0.0005 to 0.008%. , A bainite having an area ratio of 60 to 85%, a total of 0 to 5% martensite and pearlite, and a steel sheet having a structure in which the balance is ferrite. In the technique described in Patent Document 5, when a fatigue crack encounters bainite, the crack propagates at the boundary and is bent while avoiding bainite, so that the fatigue crack growth rate is reduced. The fatigue crack growth resistance is improved. However, in Patent Document 5, there is a description of fatigue crack growth characteristics and toughness, but there is no description of ductility, weldability, etc., which is important as a structural steel sheet, and the technique described in Patent Document 5 It remains unclear whether or not the steel plate manufactured in (1) has the properties necessary for a structural steel plate in a well-balanced manner.

  Patent Document 6 describes a thick steel plate excellent in base metal toughness and fatigue crack growth characteristics. The thick steel plate described in Patent Document 6 is a composition containing C: 0.030 to 0.300%, Si: 0.50% or less, Mn: 0.80 to 2.00%, Al: 0.01 to 0.10%, N: 0.0100% or less in mass%. And having a soft part composed of recrystallized ferrite and a multiphase structure mainly composed of a hard part composed of one or more of martensite and bainite, and the area fraction of the hard part is 15 to 85%, The average equivalent circle diameter is 10 μm or more, the average hardness is Hv 200 to 700, the difference in average hardness between the hard part and the soft part is Hv 100 or more, and the average equivalent circle diameter of the recrystallized ferrite grains is 20 μm or less. This is a thick steel plate with an average lath length of sight and bainite of 5 μm or less. In the technique described in Patent Document 6, both a fatigue crack growth characteristic and a toughness are obtained by forming a double-phase structure in which a sufficiently refined ferrite and a low-temperature transformation phase with a short lath length transformed from processed γ are combined. It is said that the characteristics can be made compatible. However, Patent Document 6 remains unclear as to whether or not it has a well-balanced property such as ductility and weldability necessary for a steel sheet for actual structures other than fatigue crack growth rate and toughness.

  Patent Document 7 describes a thick steel plate having excellent fatigue crack growth suppression characteristics. The thick steel plate described in Patent Document 7 is, by weight, C: 0.04-0.25%, Si: 0.1-0.5%, Mn: 0.4-2%, sol.Al: 0.005-0.1%, N: 0.001-0.005. %, Ti: 0 to 0.03%, B: 0 to 0.0025%, Cu: 0 to 1%, Ni: 0 to 0.5%, Cr: 0 to 1%, Mo: 0 to 0.5%, Nb: 0 to 0.06% , V: a composition containing 0 to 0.1%, a mixed structure comprising a ferrite phase and one or more hard phases, and the hardness difference between the ferrite phase and each hard phase is 150 or more in terms of Vickers hardness, one or more An aggregate of hard phases composed of the hard phases is a thick steel plate having a structure in which the average diameter is 6 to 50 μm, which is massive in the ferrite phase. According to the technique described in Patent Document 7, when the fatigue crack propagates and reaches the vicinity of the interface between the ferrite and the hard phase, the plastic deformation at the crack tip is suppressed, and the fatigue crack is retained. Even in the range, since the fatigue crack progress suppressing effect is excellent, even when a fatigue crack is generated from the welded portion, it is said that the fatigue life can be sufficiently extended as compared with the conventional case.

  Patent Document 8 describes a thick steel plate having excellent fatigue crack propagation characteristics. The thick steel plate described in Patent Document 8 is, in mass%, C: 0.03-0.2%, Si: 0.01-1.6%, Mn: 0.5-2%, Al: 0.001-0.1%, N: 0.001-0.008%. A layered composition comprising a ferrite containing a Vickers hardness of 150 or more as a parent phase, a flat martensite having a Vickers hardness of 400 to 900, an area ratio of 5 to 30%, and an aspect ratio of 3 or more as a second phase. It is a steel plate with excellent fatigue crack propagation characteristics with an average layer spacing of 3-50 μm in the thickness direction of ferrite and martensite in the structure. According to the technique described in Patent Document 8, if a welded joint is formed using a thick steel plate having such a structure, the life of the welded joint can be improved more than twice that of the conventional structure. It is said that reliability against fatigue failure can be improved.

  Further, Patent Document 9 describes a steel plate having a fatigue crack progress suppressing effect. The steel sheet described in Patent Document 9 has a composition containing C: 0.03 to 0.30%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, sol. Al: 0.001 to 0.1% in mass%. It consists of hard part A and soft part B. Hard part occupies the whole structure (%) fA, soft part occupies the whole structure (%) fB, and hard part average hardness HA in Vickers hardness HA A steel sheet having a structure in which the average hardness HB in terms of the Vickers hardness of the soft part satisfies fA · HA−fB · HB ≧ −3500. According to the technique described in Patent Document 9, a steel plate having a good fatigue crack progress suppressing effect can be obtained even in a moderate ΔK range, and even when fatigue cracks are generated from the welded portion, compared to the conventional case. The fatigue life can be extended.

  Further, Patent Document 10 describes a fatigue crack propagation delayed steel material, and Patent Document 11 describes a steel material excellent in suppressing fatigue crack propagation. The steel materials described in Patent Documents 10 and 11 contain, in mass%, C: 0.02 to 0.20%, Si: 0.01 to 0.45%, Mn: 0.5 to 2.0%, and Cu: 0.01 to 3.0%, Ni: 0.01. ~ 1.0%, Cr: 0.01 ~ 3.0%, Mo: 0.01 ~ 1.0% of the composition containing one or more and a structure composed of a hard phase and a soft phase. This is a steel material in which the product of the related parameter Vp and the Vickers hardness difference ΔHv between the hard phase and the soft phase is 50 or more. In the techniques described in Patent Documents 10 and 11, when a hard phase is present in front of the crack, the fatigue crack propagates while avoiding the hard phase via restraint of the plastic region. Suppose that crack bending and branching occur, and the decrease in fatigue crack growth rate is synergistically related to the frequency of encountering the hard phase and the extent to which the propagation velocity is locally reduced when encountering the hard phase. It is supposed to be considered.

Japanese Patent No. 2962134 Japanese Patent No. 3753992 Japanese Patent No. 3860763 Japanese Patent No. 4976749 Japanese Patent No. 4466196 Japanese Patent No.4721956 Japanese Patent Laid-Open No. 10-60575 JP 2005-320619 JP 2002-121640 A JP 2008-255468 A JP 2008-255469 A

H.SUZUKI and A.J.McEVILY: Met. Trans. A, vol.10A (1979), p.475-481

However, in Patent Document 7, there is no mention of characteristics other than the fatigue crack growth rate, and the thick steel sheet described in Patent Document 7 is necessary as a steel sheet for actual structures, together with excellent fatigue crack growth suppression characteristics. It remains unclear as to whether the properties such as strength, ductility, toughness and weldability are well balanced.
Further, according to the technique described in Patent Document 8, it is possible to suppress the fatigue crack growth in the thickness direction of the thick steel plate, but it is also possible to suppress the fatigue crack growth in the rolling direction or the width direction of the thick steel plate. Whether it remains unknown. Moreover, in the technique described in Patent Document 8, in order to obtain flat martensite having a hardness of 400 HV or more, the finish rolling temperature is set to a low temperature, the cumulative rolling reduction is limited to a high level, and rapid accelerated cooling is performed. Therefore, there is a problem that the manufacturing load is large and the productivity is lowered.

  Moreover, in the technique described in patent document 9, in order to make it the steel plate which has a favorable fatigue crack progress inhibitory effect, it aims at enlarging the difference of the hardness of a soft phase and a hard phase. Further, in the techniques described in Patent Document 10 and Patent Document 11, Vp is increased as a structure in which the hard phase (or soft phase) is close to 50% in area ratio so that the fatigue crack growth rate is reduced, Furthermore, the difference ΔHv between the Vickers hardness of the soft phase and the Vickers hardness of the hard phase is increased to increase Vp × ΔHv, thereby reducing the fatigue crack growth rate of the steel material.

However, the steel material having the above-described structure manufactured by the techniques described in Patent Documents 9 to 11 may have extremely low yield strength and yield ratio, and basically, like a bridge. There is a problem that application to a member of a structure that performs elastic design is unsuitable.
The present invention advantageously solves such problems of the prior art, and provides a high-strength steel material having high strength, excellent ductility, toughness, weldability, and excellent fatigue crack propagation characteristics, and a method for determining the same. For the purpose. Here, “high strength” refers to a case where the yield point YS (or 0.2% yield strength) has a high yield point of 315 MPa or more. In addition, “excellent fatigue crack propagation characteristics” mentioned here specifically means that the fatigue crack propagation rate da / dN is at least 1.75 × 10 ⁻⁸ (m / min) when ΔK _I = 15 MPa√m. cycle) and below, ΔK _I = 20 MPa√m and 4.26 × 10 ⁻⁸ (m / cycle) or less, preferably ΔK _I = 25 MPa√m and 8.50 × 10 ⁻⁸ (m / cycle) or less.

The term “high toughness” as used herein conforms to the provisions of “KA 32 of the Steel Ship Rules (NK class)” required by representatives of steels for welded structures that are subject to fatigue during the service period. The test temperature in the Charpy impact test conducted in this way: the case where the absorbed energy vE ₀ at 0 ° C. is 31 J or more.

  In order to achieve the above-described object, the present inventors first examined a matrix structure for improving the strength, ductility, toughness, weldability, and fatigue crack propagation characteristics in a well-balanced manner. As a result, in steel materials having a yield point YS: 315 MPa or more, the matrix structure should be made mainly of ferrite phase in order to have a good balance between ductility, toughness, and fatigue crack propagation characteristics. It was found that is preferable.

In addition, we have intensively studied various factors affecting the fatigue crack propagation characteristics of steel materials with a structure mainly composed of ferrite phase. As a result, paying attention to the plastic zone size at the tip of a fatigue crack generated by repeated stress loading, the relationship between the plastic zone size at the tip of the fatigue crack and the effective structural unit of the structure greatly affects the fatigue crack propagation rate. Obtained knowledge.
First, the experimental results on which the present invention is based will be described.

  A steel material having a specific composition was subjected to treatments under various conditions to produce a steel sheet (plate thickness: 25 mm) having a structure with a single ferrite phase and various grain sizes and shapes. From the obtained steel plate, a CT test piece and a three-point bending test piece were collected in a combination of three directions and positions shown in FIG. The CT test piece (TL) is a test piece taken so that the load direction is the steel plate width direction (T direction) and the crack propagation direction is the steel plate rolling direction (L direction). T) is a test piece sampled so that the load direction is the steel plate rolling direction (L direction) and the crack propagation direction is the steel plate width direction (T direction). The three-point bending test piece (L-Z) is a test piece taken so that the load direction is the plate thickness direction (Z direction) and the crack propagation direction is the plate thickness direction (Z direction). Note that the thickness of the test piece was the total thickness.

  A fatigue crack propagation test was performed using the collected specimens. Note that the specimen size, stress intensity factor calculation method, fatigue crack propagation test method, etc. conform to ASTM E647 when using CT specimens, and when using three-point bending specimens. Were determined with reference to the provisions of BS 7448 Part 1. The fatigue crack propagation test was performed in the atmosphere (room temperature) at a stress ratio R = 0.1 and a frequency of 20 Hz.

From the obtained results, the fatigue crack propagation rate da / dN in the mode I stress intensity factor range ΔK _I = 15 MPa√m was obtained, and at the center of the specimen thickness, ΔK _I = 15 MPa√m The fatigue crack propagation path in the 500 μm section was observed in a cross section (xy plane in FIG. 2), and the number of bending of the fatigue crack was measured. From the obtained results, the relationship between the fatigue crack propagation rate da / dN and the number of bending of the crack at ΔK _I = 15 MPa√m was obtained. At the same time, the bending length and the bending angle with respect to the crack propagation direction were also determined. The obtained results are shown in FIG.

From FIG. 1, it can be seen that the fatigue crack propagation rate decreases as the number of fatigue crack flexures increases. From this, the present inventors have come to realize that in order to improve the fatigue crack propagation resistance, it is necessary to have a structure in which the number of bending of the fatigue crack is increased.
Therefore, the observed fatigue of a steel material with a fatigue crack propagation rate da / dN at ΔK _I = 15 MPa√m of 9.57 × 10 ⁻⁹ m / cycle, which is the target value of 1.75 × 10 ⁻⁸ m / cycle or less. From the bending behavior of the crack, the relationship between the bending length r at the time of crack bending and the bending angle θ with respect to the crack propagation direction is shown by a circle in FIG. The bending length r and the bending angle θ are obtained by a cylindrical coordinate system of r−θ−z with the crack tip in the mode I (opening type) deformation mode shown in FIG. 2 as the origin, and r in FIG. It is shown in the -θ coordinate system. FIG. 2 shows an xyz orthogonal coordinate system having a crack tip as an origin and an r-θ-z cylindrical coordinate system used for the mode I (opening type) deformation mode.

In FIG. 3, the average ferrite grain size of the steel material used is determined in the crack propagation direction and the direction perpendicular to it (x direction, y direction in FIG. 2), and this is approximated as an ellipse with the long side and the short side, When arranged at the tip of a fatigue crack, the locus indicated by the grain boundary in the r-θ coordinate system is indicated by a solid line.
Next, paying attention to the relationship between the plastic zone size at the fatigue crack tip and the microstructure on the fatigue crack propagation rate, first, the plastic zone size at the crack tip based on the von Mises yield condition under plane strain. γ _p (θ) was determined. γ _p (θ) represents the distance from the crack tip to the elastoplastic boundary in the cylindrical coordinate system of FIG. 2, and the following equation (6) γ _p (θ) = {(K _Imax ) ² × 10 ⁶ / 4πσ _Y ² } × {(3/2) sin ² θ + (1−2ν) ² (1 + cos θ)} (6)
Defined by Incidentally, theta angle (°), K _Imax is the maximum stress intensity factor of mode I of interest, sigma _Y is steel yield stress (MPa), [nu is the Poisson's ratio. Here, K _Imax is the relationship between the stress ratio R and the stress intensity factor range ΔK _I ,
K _Imax = ΔK _I / (1-R)
Satisfied. In the present invention, K _Imax is set to a value in the range of 5 to 35 (MPa√m).

The obtained distance γ _p (θ) from the crack tip to the elastoplastic boundary, that is, the elastoplastic boundary is shown by a broken line in FIG.
From FIG. 3, it can be seen that the bending of the fatigue crack occurs in the range of the solid line, that is, in the crystal grains, and tends to concentrate in the vicinity of the crack propagation direction (θ: 0 °). In the vicinity of the crack propagation direction (θ: 0 °), the plastic zone size (broken line) at the crack tip and the locus (solid line) of the crystal grain boundary are close to each other. From this, the present inventors found that the plastic zone size at the crack tip and the crystal grain size (also referred to as a structural unit) are closely related to the fatigue crack propagation rate through bending of the fatigue crack. I thought it was related. It should be noted that in steel materials where the fatigue crack propagation rate at ΔK _I = 15 MPa√m exceeds the target value of 1.75 × 10 ⁻⁸ (m / cycle), the bending frequency of cracks is low, and the bending is other than for crystal grains. There were many occurrences in the position. In addition, the plastic zone size at the crack tip and the locus of the grain boundary were greatly different. Moreover, the locus of the plastic zone and the grain boundary at the crack tip was not close but far apart.

Therefore, the present inventors used the equation (6) to determine the plastic zone size at the crack tip in the crack propagation direction (θ = 0 °) in the fatigue crack propagation test conducted on the obtained steel sheet. Calculated and set as γ _p *. That is, γ _p * is expressed by the following formula (1) γ _p * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Defined by

On the other hand, for each steel plate obtained, the average value (structure unit) of crystal grains (ferrite grains) in the three crack propagation directions (θ = 0 °) of the fatigue crack propagation test carried out was measured, and (D _P ) defined as _α .
The ratio of the obtained γ _p * to the obtained (D _P ) _α , γ _p * / (D _P ) _α was calculated, and the effect of γ _p * / (D _P ) _α on the fatigue crack propagation rate Is shown in FIG. In FIG. 4, in addition _{ΔK I = 15MPa√m, ΔK I =} 20MPa√m, also shown for the case of ΔK _I = 25MPa√m. Of course, γ _p ^* varies according to the level of KI _max .

From Fig. 4, the fatigue crack propagation rate is unambiguously organized as γ _p ^* / (D _P ) _α , especially in the region where γ _p ^* / (D _P ) _α is 10 or less, regardless of the level of the stress intensity factor. It was possible, and it was found that as γ _p ^* / (D _P ) _α becomes smaller, the fatigue crack propagation rate clearly decreases. ^{_{However, γ p * / (D P}} ) in the region _{where alpha} is greater than ^{_{10, γ p * / (D P}} ) α is small increase in fatigue crack propagation speed increases, the slope of the curve is small, rather substantially It becomes horizontal, and the variation of data becomes large. That, gamma _p ^* / In region (D _{P) alpha} exceeds ^{_{10, γ p * / (D}} P) effects on the Fatigue Crack Growth rate of _alpha can be said to be small. This means that the crystal grain unit becomes smaller than the plastic region at the crack tip, and the crystal grain unit is unlikely to cause the crack to be bent. In such a region, the crack growth is considered to be governed only by the stress field at the crack tip.

The above findings have been obtained for steel sheets having a ferrite single phase structure. The steel material applied to the actual structure balances the desired high strength, high ductility, high toughness, and excellent weldability as a structure with a hard phase such as pearlite, bainite, and martensite, depending on the purpose of use. The steel material is well held.
Therefore, fatigue crack propagation behavior is first observed for steel materials with a composite structure that includes a ferrite phase with an area ratio of 50% or more as the main phase and the second phase as a hard phase such as pearlite, bainite, and martensite. investigated. As a result, when a fatigue crack propagates to a harder structure such as ferrite to pearlite, bainite, martensite, or bainite to martensite, the crack bends and branches, causing fatigue crack propagation. It has been found that the speed is reduced. Furthermore, as a result of more detailed observations, it was confirmed that cracks occurred at the boundaries of massive or layered layers in pearlite, and that cracks occurred at packet boundaries, block boundaries, etc. at bainite and martensite. I found out.

From such knowledge, the present inventors considered that there is an effective structural unit that causes bending of a crack in each phase, as in the ferrite crystal grains in the ferrite single phase structure, even in the composite structure. Then, the present inventors have found that the organizational unit in the ferrite (D _{P) alpha,} in pearlite (D _{P) P,} a bainite (D _{P) B,} the martensite defined respectively (D _{P) M,} and . Then, in the entire composite structure, if the contribution to the crack bending of each phase can be weighted, the mixing rule is established, and the effective structural unit MU _eff in the composite structure can be newly defined.

The contribution of each phase to crack bending is determined according to the structural unit of each phase and the area ratio (AR) _α , (AR) _P , (AR) _B , (AR) _M of each phase. Therefore, the product of the structural unit of each phase and the area ratio of each phase was used as the index. Furthermore, it has been found that by multiplying the product by the hardness ratio of each phase with respect to the main phase, the contribution of each phase to crack bending can be weighted on the basis of the main phase. Here, the “main phase” refers to a phase with an area ratio exceeding 50%.

That is, the effective structural unit MU _eff in a composite structure having a ferrite phase as a main phase and a pearlite phase, a martensite phase, and a bainite phase that are harder than the ferrite phase as
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase Organizational units in the direction of development (μm),
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), expressed in average Vickers hardness of each phase It will be. In the case of a steel material having a ferrite single phase structure, the above formula (2) is only the first term and is consistent with the result shown in FIG.

Further, a fatigue crack propagation test was performed on the composite structure steel material having the ferrite phase as the main phase and the remaining second phase consisting of the hard phase in the same manner as in the ferrite single-phase structure, and the fatigue crack propagation rate was obtained. The relationship between the obtained fatigue crack propagation rate and γ _p ^* / MU _eff was calculated and shown in FIG. In FIG. 5, the case of a ferrite single-phase structure (marks ◯, Δ, □) is also shown. From Figure 5, also in the composite structure of ferrite phase as a main phase, similarly a ferrite single phase, Fatigue Crack Growth rate can organize with γ _p ^* / MU _eff, and, γ _p ^* / MU _eff is 10 or less In this region, it was found that the fatigue crack propagation rate was clearly reduced as γ _p ^* / MU _eff became smaller and could be arranged in a relatively narrow band. That is, even in a steel material having a composite structure with a ferrite phase as the main phase, the fatigue crack propagation rate decreases as γ _p ^* / MU _eff decreases in the region where γ _p ^* / MU _eff is 10 or less. The knowledge that fatigue crack propagation resistance is improved was obtained. From this, it can be said that steel materials with γ _p ^* / MU _eff of 10 or less under the use conditions are steel materials with excellent fatigue crack propagation characteristics with reduced fatigue crack propagation rates. It has also been thought that the criterion that γ _p ^* / MU _eff is 10 or less can be used as a criterion for steel materials having excellent fatigue crack propagation resistance.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.02-0.4%, Si: 0.01-1.0%, Mn: 0.5-3.0%, P: 0.05% or less, S: 0.05% or less, Sol.Al: 0.10% or less, It has a composition composed of the remaining Fe and inevitable impurities, a ferrite having an area ratio of more than 50% as a main phase at a thickness of 1/4, and a structure composed of the main phase and the remaining hard phase. : High strength of 315 MPa or more and test temperature in Charpy impact test: Absorbed energy vE ₀ at 0 ° C and high toughness of 31 J or more.
γ _P * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Where K _{Imax is} the maximum stress intensity factor in mode I and is in the range of 5 to 35 (MPa√m), σ _{Y is the} yield stress (MPa), and ν is the crack tip plasticity defined by the Poisson's ratio. Dimension γ _p ^* (μm) and the following equation (2)
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase The structural unit (μm) in the propagation direction, the “structural unit in the crack propagation direction” here, is a structural unit closely related to the bending of the crack measured in the crack propagation direction. organizational unit (D _P) of the (alpha) _alpha feeling crack propagation direction of the average of the ferrite grain size, organizational unit (D _{P) P} wear of crack propagation direction of the bulk perlite size or layered pearlite pearlite (P) Mean thickness, bainite (B) structural unit (D _P ) _{B is} the average packet size in the crack propagation direction, martensite (M) structural unit (D _P ) _{M is the} crack propagation direction The average packet size,
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : Defined as ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase The effective structural unit MU _eff (μm) in the crack propagation direction is _expressed by the following equations (3) to (5)
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5)
Satisfies the fatigue crack propagation rate da / dN is at ^{_{least, ΔK I = 1.75 × 10 -8}} (m / cycle) or less 15MPa√m, ΔK _I = 20MPa√m at 4.26 × 10 ^-8 (m / cycle) following der Rukoto-out fatigue and wherein the crack propagation strength steel with excellent properties.
(2) In (1), in addition to the above composition, in terms of mass%, Cu: 3.0% or less, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V : High strength steel material characterized by containing one or more selected from 0.1% or less, Ti: 0.1% or less, B: 0.005% or less.
(3) In (1) or (2), in addition to the above-mentioned composition, it further contains, by mass%, one or two selected from Ca: 0.010% or less, REM: 0.010% or less Characteristic high strength steel .
( 4 ) In mass%, C: 0.02-0.4%, Si: 0.01-1.0%, Mn: 0.5-3.0%, P: 0.05% or less, S: 0.05% or less, Sol.Al: 0.10% or less, The composition consisting of the balance Fe and inevitable impurities, ferrite is more than 50% in area ratio, the balance has a structure consisting of a hard phase, the yield point: high strength steel material having a high strength of 315 MPa or more, For high-strength steel materials of interest, the microstructure observation, Vickers hardness measurement are performed, the area ratio (AR) of each phase constituting the structure in the assumed fatigue crack growth direction, the structural unit (DP) of each phase, each Obtain the average Vickers hardness (HV) of the phase and use the following formula (1)
γ _P * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Where K _{Imax is} the maximum stress intensity factor in mode I and is a value in the range of 5 to 35 (MPa√m), σ _{Y is the} yield stress (MPa), and ν is Poisson's ratio (= 0.3).
Crack tip plastic zone dimension γ _p ^* (μm) defined by the following equation (2)
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase The structural unit (μm) in the propagation direction, the “structural unit in the crack propagation direction” here, is a structural unit closely related to the bending of the crack measured in the crack propagation direction. organizational unit (D _P) of the (alpha) _alpha feeling crack propagation direction of the average of the ferrite grain size, organizational unit (D _{P) P} wear of crack propagation direction of the bulk perlite size or layered pearlite pearlite (P) Mean thickness, bainite (B) structural unit (D _P ) _{B is} the average packet size in the crack propagation direction, martensite (M) structural unit (D _P ) _{M is the} crack propagation direction The average packet size,
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : Defined by ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase The effective structural unit MU _eff (μm) in the crack propagation direction is calculated, and the following equations (3) to (5)
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5)
A case that satisfies the fatigue crack propagation rate da / dN is at ^{_{least, ΔK I = 1.75 × 10 -8}} (m / cycle) or less 15MPa√m, ΔK _I = 4.26 in 20MPa√m × 10 ^-8 ( m / cycle) A method for determining a high strength steel material having excellent fatigue crack propagation characteristics, characterized in that it is determined as a steel material having excellent fatigue crack propagation characteristics .
( 5 ) In ( 4 ), in addition to the above composition, in addition to mass, Cu: 3.0% or less, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V : 0.1% or less, Ti: 0.1% or less, B: One or two or more types selected from 0.005% or less are contained.
( 6 ) In ( 4 ) or ( 5 ), in addition to the above-mentioned composition, it further contains, in mass%, one or two selected from Ca: 0.010% or less and REM: 0.010% or less A method for judging a high strength steel material.

  According to the present invention, it is possible to determine the fatigue crack propagation characteristics of a high strength steel material having a high yield strength of yield point: 315 MPa or more, and it has a good balance of strength, ductility, toughness, and weldability. High-strength steel material with excellent crack propagation characteristics can be provided. Moreover, if the high-strength steel material according to the present invention is applied to a main member of a welded structure such as a ship, a bridge, or a building, an effect that the safety margin of fatigue fracture of the welded structure can be expanded. is there.

It is a graph which shows the relationship between the fatigue crack propagation rate in a ferrite single phase structure steel plate, and the bending frequency of a fatigue crack. It is explanatory drawing which shows typically the relationship between a mode I crack opening and a coordinate system. It is a graph which shows the relationship between the bending angle (theta) of the fatigue crack in a ferrite single phase structure steel plate, and a bending crack length. Fatigue ferrite single-phase structure steel sheet Crack Propagation speed and gamma _P * / is a graph showing the relationship between (D _{P) α.} It is a graph which shows the relationship between the fatigue crack propagation rate of a structure steel plate, and (gamma) _P * / _MUeff . It is explanatory drawing which shows typically the sampling procedure of the fatigue crack propagation test piece.

The steel of the present invention contains C: 0.02 to 0.4%, Si: 0.01 to 1.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Sol.Al: 0.10% or less, and the balance Fe And a composition comprising inevitable impurities.
First, the reasons for limiting the composition of the steel of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.02-0.4%
C is an element that increases the strength, in order to ensure high strength of Nozomu Tokoro require inclusion of 0.02% or more. On the other hand, the content exceeding 0.4% impairs weldability. For this reason, C was limited to the range of 0.02 to 0.4%. In addition, Preferably it is 0.02 to 0.35%.

Si: 0.01-1.0%
Si is an element that effectively acts as a deoxidizer and contributes to increase in strength by increasing strength. In order to acquire such an effect, 0.01% or more of content is required. On the other hand, when it contains exceeding 1.0%, weldability and toughness will fall. For this reason, Si was limited to the range of 0.01 to 1.0%. In addition, Preferably it is 0.05 to 0.8%.

Mn: 0.5-3.0%
Mn is an element that contributes to an increase in strength through improvement in hardenability and also contributes to an improvement in toughness. In order to acquire such an effect, 0.5% or more of content is required. On the other hand, if it contains more than 3.0%, weldability is lowered. For this reason, Mn was limited to the range of 0.5 to 3.0%. In addition, Preferably it is 0.5 to 2.5%.

P: 0.05% or less
P is an element that degrades the toughness of steel, and it is desirable to reduce it as much as possible, but it is acceptable up to 0.05%. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.03% or less.
S: 0.05% or less
S exists as sulfide inclusions in steel, and lowers the ductility and toughness of the steel. For this reason, it is desirable to reduce S as much as possible, but 0.05% is acceptable. For these reasons, S is limited to 0.05% or less. In addition, Preferably it is 0.03% or less.

Sol.Al: 0.10% or less
Sol.Al is an element that acts as a deoxidizer and also contributes to refinement of crystal grains. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when it contains more than 0.10%, an oxide inclusion will increase and toughness and ductility will fall. For this reason, Sol.Al was limited to 0.10%. Note that. Preferably it is 0.08% or less.

  Although the above components are basic components, in the present invention, in addition to the above basic composition, for the purpose of adjusting strength, toughness, weldability, further weather resistance, heat resistance, etc., Cu: 3.0% Below, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less 1 One or two or more species selected from Ca or 0.010% or less and REM or 0.010% or less can be selected and contained as necessary.

Cu: 3.0% or less, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less One or more selected
Cu, Ni, Cr, Mo, Nb, V, Ti, and B are all elements that contribute to an increase in strength, and can be selected from one or more as required.
Cu is an element that contributes to increase in strength by solid solution and also contributes to improvement in weather resistance. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, a large content exceeding 3.0% lowers weldability and hot workability, and tends to generate flaws. For this reason, when it contains, it is preferable to limit Cu to 3.0% or less. More preferably, it is 2.5% or less.

  Ni is an element that improves toughness and contributes to an increase in strength. Ni also contributes effectively to improving weather resistance and improving hot brittleness that occurs when Cu is added. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 10%, the weldability deteriorates and the material cost increases. For these reasons, when Ni is contained, Ni is preferably limited to 10% or less.

Cr is an element that contributes to an increase in strength and also contributes to an improvement in weather resistance and heat resistance. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, when it contains more than 3.0%, weldability and toughness are lowered. For this reason, when contained, Cr is preferably limited to 3.0% or less. More preferably, it is 2.5% or less.
Mo is an element that contributes to an increase in strength and an improvement in heat resistance. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, if the content exceeds 2.0%, weldability and toughness are reduced. For this reason, when it contains, it is preferable to limit Mo to 2.0% or less. More preferably, it is 1.5% or less.

  Nb is an element that contributes to an increase in strength through refining of the structure by suppressing recrystallization of austenite grains during hot rolling, and through solid solution strengthening and precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, if it exceeds 0.1%, toughness is reduced. For this reason, when it contains, it is preferable to limit Nb to 0.1% or less. For this reason, when it contains, it is preferable to limit Nb to 0.1% or less. More preferably, it is 0.07% or less.

V, like Nb, is an element that contributes to an increase in strength by precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, if the content exceeds 0.1%, the toughness and weldability are lowered. For this reason, when it contains, it is preferable to limit V to 0.1% or less. More preferably, it is 0.07% or less.
Ti, like Nb, contributes to an increase in strength through precipitation strengthening and also contributes to an improvement in weld toughness. In order to acquire such an effect, it is desirable to contain 0.001% or more. On the other hand, if the content exceeds 0.1%, the material cost increases. For these reasons, when Ti is contained, Ti is preferably limited to 0.1% or less. More preferably, it is 0.07% or less.

  B is an element that contributes to an increase in strength through improved hardenability. In order to acquire such an effect, it is desirable to contain 0.0005% or more. On the other hand, if the content exceeds 0.005%, weldability decreases. For this reason, when contained, B is preferably limited to 0.005% or less. More preferably, it is 0.003% or less.

One or two selected from Ca: 0.010% or less, REM: 0.010% or less
Both Ca and REM are elements that contribute to the improvement of the ductility and toughness of the steel material through the control of the form of inclusions, and can be selected as necessary to contain one or two kinds.
Ca is an element that contributes to improving the ductility and toughness of steel through the control of the form of inclusions. In order to acquire such an effect, it is desirable to contain 0.001% or more, but when it contains a large amount exceeding 0.010%, the toughness is reduced. For this reason, when it contains, it is preferable to limit Ca to 0.010% or less. More preferably, it is 0.005% or less.
REM, like Ca, is an element that contributes to improving the ductility and toughness of steel materials through the control of the form of inclusions. In order to acquire such an effect, it is desirable to contain 0.001% or more, but when it contains a large amount exceeding 0.010%, the toughness is reduced. For this reason, when it contains, it is preferable to limit REM to 0.010% or less. More preferably, it is 0.005% or less.

The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include N: 0.01% or less and O: 0.01% or less.
The steel material of the present invention has the above-described composition, and has a main phase of ferrite exceeding 50% in terms of area ratio at the position of 1/4 of the plate thickness that is an average microstructure, and consists of the main phase and the remaining hard phase. Have an organization. In the present invention, ferrite is selected as the main phase in order to achieve both fatigue crack propagation resistance and strength, ductility, toughness, and weldability. The main phase here means a phase having a fraction exceeding 50% in area ratio. The upper limit of the structure fraction of the ferrite phase is not particularly limited, but is preferably 95% or less in order to ensure the desired yield strength: 315 MPa or more. The remainder other than the main phase is a hard phase. Examples of the hard phase include pearlite, bainite, and martensite. The steel material of the present invention includes at least one of them as a hard phase. With such a steel material, it is possible to secure a toughness of vJ ₀ required for a welded structure of 31 J or more.

And this invention steel material has the above-mentioned composition and the above-mentioned structure, yield strength: high strength of 315 MPa or more, and high toughness of vE ₀ : 31 J or more.
γ _P * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Where K _{Imax is} the maximum stress intensity factor in mode I and is in the range of 5 to 35 (MPa√m), σ _{Y is the} yield stress (MPa), and ν is the crack tip plasticity defined by the Poisson's ratio. Dimension γ _p ^* (μm) and the following equation (2)
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase Organizational units in the direction of development (μm),
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : Defined as ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase The effective structural unit MU _eff (μm) in the crack propagation direction is _expressed by the following equations (3) to (5)
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5)
Satisfied.

In the region where γ _P * / MU _eff is 10 or less, the fatigue crack propagation rate can be reduced, and the steel material has excellent fatigue crack propagation resistance as γ _P * / MU _eff decreases. . In the region where γ _P * / MU _eff is 10 or less, the effective structural unit MU _{eff and the} crack tip plastic zone dimension γ _P * show relatively close values, and the bending of the crack depends on the orientation of the structure and the phase boundary Therefore, the fatigue crack propagation rate decreases rapidly. For this reason, it can be said that the steel material satisfying the expression (3) is a steel material having excellent fatigue crack propagation characteristics. On the other hand, when γ _P * / MU _eff exceeds 10 and there is no correlation between γ _P * / MU _eff and fatigue crack propagation rate, even if MU _eff decreases, fatigue crack resistance of steel It becomes difficult to improve propagation characteristics. Therefore, γ _P * / MU _eff is limited to 10 or less.

Therefore, it can be said that the steel material satisfying the above-described expression (3) is a steel material excellent in fatigue crack resistance. Therefore, in the present invention, the above-described equation (3) is used as a basic equation for determining a steel material having excellent fatigue crack propagation characteristics, and the fatigue resistance of the steel material is determined by whether or not the equation (3) is satisfied. It was decided to determine the crack propagation characteristics.
Next, the crack tip plastic zone dimension γ _P * will be described.

γ _p * is a plastic zone size at the crack tip in the crack propagation direction, and the following equation (1) γ _p * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1− 2ν) ² (1)
Defined by Here, “K _Imax ” is the maximum stress intensity factor of mode I, and is a value in the range of 5 to 35 MPa√m. “Mode I” is a deformation mode in which a crack opens as shown in FIG. 2, and is a mode dominant to fatigue crack growth.

K _Imax is the following formula:
K _Imax = ΔK _I / (1-R)
It can be calculated with Here, ΔK _I is the mode I stress intensity factor range (MPa√m), and R is the stress ratio.
The fatigue crack propagation rate is generally divided into three regions (regions A to C) in relation to the stress intensity factor. In region A, the region reaches the lower limit where no crack growth is observed. In region B, the crack propagates stably, and the relationship between the crack propagation rate (logarithm) and the stress intensity factor (logarithm). The region C is a region where a linear relationship is recognized, and the region C is a region where the fatigue crack propagation rate rapidly increases with the increase of the stress intensity factor and leads to unstable fracture. The object of the present invention is to reduce the fatigue crack propagation rate in the region B. In this region B, the maximum stress intensity factor K _Imax usually corresponds to a range of 5 to 35 MPa√m. For this reason, the K _Imax is a value in the range of 5 to 35 MPa√m depending on the state of use of the steel material. Shall be set. If K _Imax is less than 5 MPa√m, the fatigue crack will stop, and if it exceeds 35 MPa√m, unstable fracture occurs. In addition, Preferably it is the range of 10-30 MPa√m. Here, σ _Y is the yield strength or 0.2% proof stress, and is preferably measured in the direction of fatigue crack opening. However, if this is difficult, it may be the direction in which a tensile specimen can be collected. Further, ν is the Poisson's ratio of the steel material and is usually 0.3.

In the steel of the present invention, γ _p * is expressed by the following equation (4)
γ _P * ≦ 250 (4)
Is limited to a range that satisfies the above. When gamma _{P *} increases beyond 250 (μm), (1) from equation sigma _Y: 315 MPa or more and K _Imax: closer to a region beyond the scope of combinations of 5～35MPa√m, Crack Propagation-out desired Fatigue It becomes difficult to ensure the characteristics. In addition, the maximum MU _eff that satisfies equation (3) also increases, making it difficult to achieve both strength and toughness. Therefore, the upper limit of γ _P * is limited to 250 (μm).

Next, the effective organizational unit MU _eff will be described.
The “effective structural unit MU _eff ” in the present invention refers to a structural unit that contributes to bending of a fatigue crack. Specifically, it refers to the tissue unit having the highest frequency of starting bending determined with reference to the fatigue crack propagation path and the constituent structure.
In steel materials having a composite structure, fatigue crack bending, that is, a decrease in fatigue crack propagation rate, is determined in accordance with the size of the structural unit of each phase (D _P ) and the area ratio (AR) of each phase. Therefore, the product (AR) × (D _P ) of the structural unit (D _P ) of each phase and the area ratio (AR) of each phase was used as the index. Then, by multiplying the product by the hardness ratio {(Hv) / (Hv) _α } of each phase with respect to the main phase, the weight of the structural unit of each phase based on the main phase can be obtained. Thus, according to the mixing rule, the main phase and each phase harder than the main phase were set as the second phase, and their sum was taken as the effective structural unit MU _eff of the composite structure.

The effective structural unit MU _eff of the composite structure in which the main phase is ferrite, the second phase other than the main phase is harder than ferrite, and pearlite, bainite, martensite is expressed by the following equation (2)
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Define in.

Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M are ferrite (α), pearlite (P), bainite (B), martensite (M), and the area ratio of each phase (0 to 1) and calculated by (area fraction of each phase (%)) / 100. The area ratio can be calculated by performing tissue observation on the xy plane shown in FIG. 2 at the position of the plate thickness ¼, and using, for example, commercially available image analysis software.

(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M are ferrite (α), pearlite (P), bainite (B), martensite (M), and each phase It is a structural unit (μm) in the crack propagation direction.
The “structural unit in the crack propagation direction” here is a structural unit closely related to the bending of the crack measured in the crack propagation direction.

  Tissue units closely related to crack bending include structural observation using optical microscope, scanning electron microscope SEM, transmission electron microscope TEM, backscattered electron diffraction (EBSD), etc. Organizational units can be determined. If necessary, it is preferable to determine the tissue unit by statistically analyzing the relationship between the crack bending length, the bending frequency, and the like and the tissue unit by observing the fatigue crack propagation path.

Specifically, since the ferrite (α) structural unit (D _P ) _α is related to the ferrite grain boundaries, most of the fatigue crack bending is related to the ferrite grains in the crack propagation direction. Use the average diameter. In pearlite (P), the pearlite colony size, bulk pearlite size, and layered pearlite thickness are closely related to fatigue crack flexion. In the case of layered pearlite, its size was measured in the direction of fatigue crack propagation, and the average value was defined as the pearlite (P) structural unit (D _P ) _P. In bainite (B), the packet size, block size, and prior austenite grain size are closely related to crack bending, but most fatigue crack bending is related to the packet. In the present invention, for convenience, the packet size in the crack propagation direction was measured, and the average value was defined as the bainite structural unit (D _P ) _B. In martensite (M), the packet size, block size, and prior austenite grain size are closely related to the crack bending, but most of the fatigue crack bending is related to the packet. Therefore, in the present invention, the packet size in the crack propagation direction was measured simply, and the average value was defined as the martensite structural unit (D _P ) _M.

As a method for measuring the tissue unit, for example, a statistical analysis method using a cutting method defined in JIS G 0551 (2013) is preferable.
Note that when use conditions is unknown, K _Imax: set any K _Imax within the 5～35MPa√m, in a direction forming a crack propagation direction perpendicular to Ki envisioned (y direction in FIG. 2) Perform a tensile test to determine σ _Y , calculate the crack tip plastic zone size γ _P * using equation (1), and further assume the structural unit and area ratio in the crack growth direction assumed for the main phase and the second phase It is preferable to measure average hardness and calculate MU _eff using equation (2).

In addition, within the scope of the present invention, the structure has a ferrite as a main phase, and a structure in which pearlite, bainite, and martensite are combined, but is outside the structure limited range of the present invention, bainitic ferrite, acicular ferrite, Even when an intermediate structure such as granular bainite is generated, the structural unit of each phase that contributes to bending of the fatigue crack is determined through the observation of the structure and the path of the fatigue crack, and the area ratio of each phase and each phase The effective tissue unit MU _eff may be determined by measuring the hardness of the same as in the present invention.

The effective grain unit MU _eff was limited to 150 (μm) or less. If MU _eff exceeds 150, the desired yield strength (315 MPa) or more cannot be secured. Also, if MU _eff exceeds 150, the toughness decreases and it cannot be applied for welded structures. Therefore, MU _eff is the following equation (5)
MU _eff ≦ 150 (5)
It was limited to satisfy. In addition, Preferably it is 120 (micrometer) or less.

Further, (Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M are ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase That's it.
In addition, hardness measurement is implemented in the xy plane shown in FIG. 2 at a thickness of 1/4 by using a Vickers hardness meter at least 5 points for each phase, and the average value is the hardness of each phase. (Hv). In addition, since it is difficult to obtain a stable value at the grain boundaries and phase boundaries, it is preferable to perform the hardness measurement by adjusting the test load so that the distance between the grain boundaries and the phase boundaries is at least four times the indentation.

In the present invention, the fatigue crack propagation characteristics are determined by equation (3) using γ _P * obtained by the above equation (1) and MU _eff obtained by the above equation (2).
The steel of the present invention has the above-described composition and the above-described structure, the yield point YS: 315 MPa or more, vE ₀ : 31 J or more, and the relationship between the crack tip plastic zone size γ _P * and the effective grain unit MU _eff , (3) to (5) are high strength steel materials with excellent fatigue crack propagation characteristics.

Next, the preferable manufacturing method of this invention steel material is demonstrated.
First, molten steel having the above-described composition is melted by a conventional melting method such as a converter, and is made into a steel material by using a conventional casting method such as a continuous casting method. Needless to say, the manufacturing method of the steel material is not particularly limited.
After the obtained steel material is heated to a temperature in the temperature range of 950 to 1300 ° C, the cumulative rolling reduction in the temperature range of 900 ° C or higher is 50% or higher, and the finish rolling finish temperature is set to (Ar3 transformation point-150 ° C) ) It is preferable to perform hot rolling as described above, and after completion of the hot rolling, accelerated cooling is performed to cool to 650 ° C. or less at a cooling rate of 5 ° C./s or more to obtain a steel material having a predetermined shape. Note that after accelerated cooling, a tempering treatment may be further performed at a temperature lower than the Ac1 transformation point. The temperature is the steel surface temperature, and the cooling rate is the average cooling rate in the steel thickness direction.

  Note that the hot rolling may be Ar3 transformation point or more, that is, austenite region rolling, and may be subjected to gentle cooling that continues cooling at a cooling rate of less than 5 ° C./s for 5 seconds or more before or during accelerated cooling. That is, after the obtained steel material is heated to a temperature in the temperature range of 950 to 1300 ° C, the cumulative rolling reduction in the temperature range of 900 ° C or higher is 50% or higher, and the finish rolling finish temperature is set to the Ar3 transformation point or higher. In the temperature range from the Ar3 transformation point to 600 ° C. after the end of the hot rolling, including a slow cooling in which a cooling rate of less than 5 ° C./s continues for 5 seconds or more, and a cooling of 5 ° C./s or more. Accelerated cooling that cools to 600 ° C. or less at a speed may be performed to obtain a steel material having a predetermined shape. Note that after accelerated cooling, tempering treatment may be performed at a temperature lower than the Ac1 transformation point. The temperature is the steel surface temperature, and the cooling rate is the average cooling rate in the steel thickness direction.

  In addition, after hot rolling, the air-cooled steel material may be reheated to a two-phase temperature range and then subjected to heat treatment for accelerated cooling. That is, the obtained steel material is heated and hot-rolled and air-cooled to room temperature to form a steel material having a predetermined shape, and then reheated to a temperature not lower than the Ac1 transformation point and lower than the Ac3 transformation point, and then 5 ° C / s. You may perform the heat processing cooled to 600 degrees C or less with the above cooling rate. Further, tempering treatment may be performed at a temperature lower than the Ac1 transformation point. The temperature is the steel surface temperature, and the cooling rate is the average cooling rate in the steel thickness direction.

The Ar3 transformation point is, for example, the following formula
Ar3 (° C) = 910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
(Here, C, Mn, Cu, Cr, Ni, Mo: content of each element (mass%))
Can be used to calculate.
The Ac1 transformation point is, for example,
Ac1 (° C) = 723-14Mn + 22Si-14.4Ni + 23.3Cr
(Where Mn, Si, Cr, Ni: content of each element (mass%))
Can be used to calculate.

The Ac3 transformation point is, for example,
Ac3 (℃) = 854−180C + 44Si-14Mn−17.8Ni−1.7Cr
(Here, C, Si, Mn, Ni, Cr: Content of each element (mass%))
Can be used to calculate.
Moreover, according to this invention, the high strength steel materials excellent in the fatigue crack propagation characteristics can be determined from the target high strength steel materials. Here, as a high strength steel material of interest, the above composition, the above ferrite has an area ratio of more than 50%, the remainder has a structure composed of a hard phase, and a high strength steel material having a yield strength of 315 MPa or more To do.

Yield strength σ _Y in the crack opening direction (y direction in FIG. 2), structure type and fraction, structure unit in crack growth direction of each phase, average Vickers of each phase Measure hardness. Then, using the obtained values, the crack tip plastic zone size γ _P * and the effective structural unit MU _eff are calculated using the equations (1) and (2), and the equations (3) to (5) are calculated.
γ _P * / MU _eff ≦ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5)
It is determined whether or not the above is satisfied. When the conditions (3) to (5) are satisfied, the steel material is evaluated as having excellent fatigue crack propagation characteristics. On the other hand, when the conditions (3) to (5) cannot be satisfied, the fatigue crack resistance characteristics are satisfied. The steel material is judged by evaluating it as a steel material inferior to. As described above, a steel material having γ _P * / MU _eff of 10 or less is a high-strength steel material having a reduced fatigue crack propagation rate and excellent fatigue crack propagation resistance. According to such a method, it is possible to easily determine a high-strength steel material having excellent fatigue crack propagation characteristics, and there is an advantage that the pre-life evaluation accuracy for fatigue of the welded structure is increased.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method.

Example 1
To the obtained steel material (A, B, D, E, G, H, I, K, M, N, S, T, U), as manufacturing method A, heating under the conditions shown in Table 2, hot rolling, After the rolling, accelerated cooling was performed to obtain a steel plate having a thickness of 12 to 100 mm. Some of the steel plates were subjected to heat treatment (tempering treatment).

Specimens were collected from the obtained steel plates and subjected to fatigue crack propagation tests, structure observations, tensile tests, impact tests, hardness tests, and weldability tests. The test method was as follows.
(1) Fatigue crack propagation test From the obtained steel plate, a CT test piece and a three-point bending test piece were collected from the three directions shown in FIG. The test piece TL is a CT test piece in which the load direction is the width direction T and the crack propagation direction is the rolling direction L, and the test piece LT is the load direction is the rolling direction L and the crack propagation direction is the width direction T. This is a CT specimen. For test piece TL and test piece LT, if the steel plate thickness is 25 mm or less, the test piece thickness is the full thickness of the steel plate, and if the steel plate thickness is more than 25 mm to 50 mm or less, the steel plate surface is ground. A single-sided reduced thickness test piece with a thickness of 25 mm was obtained. Further, when the steel plate thickness exceeded 50 mm, both sides of the steel plate were ground to obtain a double-sided thickness test piece having a thickness of 25 mm centered on the 1/4 position of the plate thickness.

  The test piece LZ is a three-point bending test piece in which the load direction is the plate thickness direction Z and the crack propagation direction is the plate thickness direction Z. For test piece L-Z, if the steel plate thickness is 25 mm or less, the test piece thickness is the full thickness of the steel plate, and if the steel plate thickness is more than 25 mm to 50 mm or less, the steel plate is ground to the Z direction thickness. A single-side thickness reduction test piece having a thickness of 25 mm was used. When the steel plate thickness exceeded 50 mm, both sides of the steel plate were ground to obtain a double-sided reduced thickness test piece having a Z-direction thickness of 25 mm centered at the 1/4 position of the plate thickness.

  In the test using the CT specimen, the specimen size, fatigue crack propagation test method, calculation of stress intensity factor, and the like were performed in accordance with ASTM E647 regulations. In the test using the three-point bending test piece, the test piece size and the load mode were determined with reference to the provisions of BS 7448 Part1. In the test using the three-point bending test piece, a crack gauge with a pitch of 0.1 mm was attached to the area in front of the notch on both sides of the test piece to determine the crack length during the test.

The calculation of the stress intensity factor K ₁ of the formula in Srawly
K ₁ = (3SP / 2W ² B) √ (πa) F ₁ (ζ) (7)
(Where S: span (= 4W), P: load, W: specimen thickness in the crack propagation direction, B: specimen thickness in the width direction T (= W / 2), a: notch Including crack length)
Was used. F ₁ (ζ) is a shape factor when a / W = ζ, and the following equation (8)
F ₁ (ζ) = {1.99−ζ (1−ζ) (2.15−3.93ζ + 2.7ζ ² )} / {√π (1 + 2ζ) (1−ζ) ^3/2 } (8)
Calculated using

All fatigue crack propagation tests were performed under the conditions of stress ratio R: 0.1 and frequency: 20 Hz in a room temperature atmosphere, stress expansion range ΔK _I : 10 MPa√m, and constant load ΔK _I gradually increasing condition The relationship between the fatigue crack propagation rate da / dN and ΔK _I was obtained.
In addition, the evaluation of fatigue crack propagation characteristics is based on the stress intensity factor range and fatigue crack propagation rate for the NK class KA steel described in “Metallic Material Fatigue Crack Propagation Resistance Data” Vol. The upper limit of the related data band was used as a reference value, and a steel sheet with excellent fatigue crack propagation resistance was obtained when the fatigue crack growth rate was 1/2 or less of the reference value within the same stress intensity factor range. Specifically, the fatigue crack propagation rate is 1/2 or less of this reference value. Specifically, the fatigue crack propagation rate da / dN is 1.75 × 10 ^-8 (m / cycle) or less when ΔK _I = 15 MPa√m, This is the case where ΔK _I = 20 MPa√m and 4.26 × 10 ⁻⁸ (m / cycle) or less, and ΔK _I = 25 MPa√m and 8.50 × 10 ⁻⁸ (m / cycle) or less. A steel sheet with excellent fatigue crack propagation characteristics is assumed to have a fatigue crack propagation rate that satisfies the above-mentioned levels at least at two levels of ΔK _I = 15 MPa√m and 20 MPa√m.
(2) Microstructure observation A specimen for microstructural observation was collected from the position of the steel plate with a thickness of 1/4, the observation surface was polished, corroded with 2% nital corrosive solution, the microstructure was revealed, and an optical microscope (magnification: 100) Using a scanning electron microscope (magnification: 100 to 1000 times), the tissue was observed and imaged in at least 5 fields of view. In addition, in the structure observation, a transmission electron microscope and EBSD were used as necessary. In addition, the observation surface is specifically a surface parallel to the plate surface of the steel plate at a position corresponding to the plate thickness ¼ position of the steel plate in the test piece TL and the test piece LT. In −Z, a cross section in the rolling direction was used. Note that the observation surface was an xy plane shown in FIG. 2 in relation to the crack.

Using the photographed tissue photograph, the area ratio of each phase was measured using tissue identification and commercially available image analysis software. Also, with reference to the provisions of JIS G 0551 (2013), the cutting unit determines the structural unit (D _P ) for each phase in each visual field and the average value of each phase (D _P ) It was. Separately, the structural unit was determined from the bending behavior of the cracks performed. Specifically, in the ferrite phase, the ferrite grains, in the case of pearlite, the size of the bulk, in the case of an elongated layer, the thickness of the layered pearlite, in the bainite phase, the size of the packet, in the martensite phase. The size of the packet was taken as each organizational unit (D _P ).
(3) Tensile test From the obtained steel plate, with reference to the Japan Maritime Society steel ship regulations, a tensile test piece was collected so that the tensile direction was the rolling direction L or the direction T perpendicular to the rolling direction. A tensile test was performed in accordance with the regulations, and tensile properties (up-yield point or 0.2% proof stress YS, tensile strength TS, elongation El) were determined. As the tensile test piece, a U1 test piece sampled in full thickness was used for a steel sheet having a thickness of 40 mm or less, and a U14A test specimen sampled from the plate thickness / 4 position was used for a steel sheet having a thickness of more than 40 mm.
(4) Impact test From the obtained steel plate, with reference to the Japan Maritime Society steel ship regulations, the test piece length direction is the rolling direction L and the crack propagation direction is the direction T perpendicular to the rolling direction, or the test piece length An impact test piece is taken so that the length direction is a direction T perpendicular to the rolling direction and the crack propagation direction is the rolling direction L, and a Charpy impact test is performed at a test temperature of 0 ° C. in accordance with the provisions of JIS Z 02242. Then, the absorbed energy vE ₀ (J) was obtained and the toughness was evaluated. The number of test pieces was three each, and the average absorbed energy value was obtained. For the impact test piece, a U4 Charpy 2 mm V notch test piece was sampled centered on the plate thickness 1/2 position when the plate thickness was less than 20 mm, and centered at the plate thickness 1/4 position when the plate thickness was 20 mm or more.
(5) Hardness test Using the structure observation test piece used in (1) as a hardness measurement test piece, for each phase, the load is adjusted so that the distance between grain boundaries and phase boundaries is at least four times the indentation. Then, Vickers hardness Hv was measured. For the hardness measurement, five or more points were measured for each phase, and the average value thereof was defined as the hardness (Hv) of each phase. The thickness measurement surface was the same as the tissue observation surface.
(6) Weldability test In accordance with the provisions of JIS Z 3158, a y-type weld crack test piece was collected from the obtained steel sheet, the preheating temperature was 25 ° C, the temperature was 20 ° C, and the humidity was 60%. A y-type weld cracking test for MAG welding (heat input 14 kJ / cm) was carried out in an atmosphere to investigate whether cracks occurred. The case where no crack was generated was evaluated as ◯, and the case other than that was evaluated as ×.

  Table 3 shows the results of tensile properties, toughness, and weldability among the obtained results. Table 3 also shows the results of observing the structure on the z-plane at the thickness ¼ position indicating a typical structure.

Further, the effective tissue unit MU _eff in the composite tissue was calculated by the formula (2) using the tissue observation result and the Vickers hardness measurement result. In addition, when the bainite phase and the martensite phase exceeded 0.5 by area ratio (50% by area%), the calculation was performed according to the following formula using the composite rule in the same manner as when the ferrite phase was the main phase.
_{MU eff = (AR) B ×} (D P) B + (AR) α × (D P) α × {(Hv) α / (Hv) B} + (AR) P × (D P) P × {( _{Hv) P / (Hv) B} } + (AR) M × (D P) M × {(Hv) M / (Hv) B} ‥‥ (9)
MU _eff = (AR) _M × (D _P ) _M + (AR) _α × (D _P ) _α × {(Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {( _{Hv) P / (Hv) M} } + (AR) B × (D P) B × {(Hv) B / (Hv) M} ‥‥ (10)
The obtained MU _eff results are shown in Table 4 together with the structure observation results and the Vickers hardness measurement results.

In addition, from the conditions of the fatigue crack propagation test performed, the crack tip plastic zone size γ _p ^* was calculated using equation (1). Here, K _Imax can be calculated from the stress intensity factor range ΔK _I and the stress ratio R by K _Imax = ΔK _I / (1-R). As the yield stress σ _Y , the value obtained in the tensile test was used. In the case of the CT test piece TL, the yield strength YS in the width direction (direction perpendicular to the rolling direction) T is set to be the same as that in the rolling direction L for the CT test piece LT and the three-point bending test piece LZ. Yield strength YS was used.

Table 5 shows calculation results of γ _p ^* , γ _p ^* / MU _eff and measurement results of fatigue crack propagation rate.

Each of the examples of the present invention has a structure composed of ferrite having an area ratio of more than 50% as a main phase, the main phase and the remaining hard phase, a high strength at a yield point of 315 MPa or more, and an absorbed energy vE ₀ : It has a high toughness of 31J or more, and also has excellent weldability in the y-type weld cracking test, with no weld cracking, MU _eff of 150 (μm) or less, and at least ΔK _I = 15 MPa√m, ΔK _I = 20 MPa√m, γ _P * is 250 (μm) or less, γ _P * / MU _eff is 10 or less, and the fatigue crack propagation rate da / dN is at least ΔK _I = 15 MPa√m Less than a certain 1.75 × 10 ^-8 (m / cycle), ΔK _I = 20 MPa√m and the target value of 4.26 × 10 ^-8 (m / cycle) or less, reducing fatigue crack resistance. It is a high-strength steel material.

On the other hand, in the comparative example outside the scope of the present invention, any of strength, toughness and weldability is reduced, or γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is reduced. .
Steel plate No. A11 (comparative example), C etc. deviated from the scope of the present invention, toughness and weldability decreased, the structure became bainite single phase and deviated from the scope of the present invention, and ΔK _I = 20 MPa√m In addition, γ _P * / MU _eff exceeds 10 and the fatigue crack propagation characteristics are degraded. Moreover, as for steel plate No. A12 (comparative example), P and S are outside the scope of the present invention, and ductility and toughness are reduced. Steel plate No. A13 (comparative example) has C and Mn outside the range of the present invention, and the strength (yield point YS or 0.2% proof stress YS) cannot secure the target value. Therefore, ΔK _I = 15 MPa√m When γ _P * exceeds 250 μm and ΔK _I = 20 MPa√m, γ _P * / MU _eff exceeds 10 and the fatigue crack propagation resistance is degraded. Steel plate No. A14 (comparative example) has a high heating temperature, the structure becomes coarse, and the MU _eff exceeds 150 μm. Therefore, the yield strength cannot be secured and the toughness is lowered. Steel plate No. A15 (comparative example) has a low heating temperature and a low cumulative rolling reduction of 900 ° C. or higher, so the austenite grains are coarsened and hardenability is improved, the structure becomes a bainite single phase, and MU _eff is When it exceeds 150 μm, the toughness is reduced, and when ΔK _I = 20 MPa√m, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is reduced. Steel plate No. A16 (comparative example) had a low rolling finish temperature and accelerated cooling was after the generation of ferrite and pearlite was completed, so ferrite refinement by accelerated cooling and second-phase strengthening could not be obtained, yielding Strength has not secured the target value. For this reason, when ΔK _I = 15 MPa√m, γ _P * / MU _eff exceeds 10, and the fatigue crack resistance is deteriorated. Steel plate No. A17 (comparative example) has a slow cooling rate after hot rolling, ferrite refinement by accelerated cooling, and second-phase strengthening cannot be obtained, and the target value of yield strength cannot be secured. . For this reason, when ΔK _I = 20 MPa√m, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is degraded. Steel plate No. A18 (comparative example) has a high cooling stop temperature after rolling, ferrite refinement due to accelerated cooling, and second phase strengthening cannot be obtained, and the yield strength cannot be secured. . For this reason, when ΔK _I = 20 MPa√m, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is degraded. Steel plate No. A19 (comparative example) has a high tempering temperature, and a large amount of MA is produced, so that the toughness is reduced.
(Example 2)
The obtained steel material (steel No. A, B, E, G, I, N, O, P, Q, R) is heated, hot-rolled and cooled after rolling as the manufacturing method B under the conditions shown in Table 6. And a steel plate having a thickness of 12 to 100 mm. The cooling after rolling is three-stage cooling consisting of first-stage accelerated cooling, slow cooling and second-stage accelerated cooling, two-stage cooling consisting of slow cooling and accelerated cooling, or one-stage cooling consisting of only accelerated cooling and only slow cooling. It was. Some steel plates were subjected to heat treatment (tempering treatment).

Specimens were collected from the obtained steel plates and subjected to fatigue crack propagation tests, structure observations, tensile tests, impact tests, hardness tests, and weldability tests. The test method was the same as in Example 1.
Table 7 shows the results of tensile properties, toughness, and weldability among the obtained results.

The obtained MU _eff results are shown in Table 8 together with the structure observation results and the Vickers hardness measurement results.

Table 9 shows the calculation results of γ _p ^* and γ _p ^* / MU _eff and the measurement results of fatigue crack propagation rate.

Each of the examples of the present invention has a structure composed of ferrite having an area ratio of more than 50% as a main phase, the main phase and the remaining hard phase, a high strength at a yield point of 315 MPa or more, and an absorbed energy vE ₀ : It has a high toughness of 31J or more, and also has excellent weldability in the y-type weld cracking test, with no weld cracking, MU _eff of 150 (μm) or less, and at least ΔK _I = 15 MPa√m, ΔK _I = 20 MPa√m, γ _P * is 250 (μm) or less, γ _P * / MU _eff is 10 or less, and the fatigue crack propagation rate da / dN is at least ΔK _I = 15 MPa√m Less than a certain 1.75 × 10 ^-8 (m / cycle), ΔK _I = 20 MPa√m and the target value of 4.26 × 10 ^-8 (m / cycle) or less, reducing fatigue crack resistance. It is a high-strength steel material. Even if the manufacturing method is changed, all of the examples of the present invention are high-strength steel materials having improved fatigue crack propagation characteristics.

On the other hand, in the comparative example outside the scope of the present invention, any of strength, toughness and weldability is reduced, or γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is reduced. .
Steel plate No. B30 (comparative example) has a high heating temperature of the steel material, austenite grains cannot be refined, the ferrite phase becomes coarse even if hot rolling and accelerated cooling are properly performed, and the yield point YS is 315 MPa. The desired high strength cannot be secured. Further, when ΔK _I = 20 MPa√m or more, γ _P * exceeds 250 μm, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is deteriorated. In addition, toughness is also reduced. Steel plate No. B31 (comparative example) has a low heating temperature of the steel material, a low cumulative rolling reduction of 900 ° C. or higher, and austenite grains cannot be refined, resulting in a coarse ferrite phase, The yield point YS is less than 315 MPa, and the desired high strength cannot be secured. Moreover, the toughness is also lowered. Further, when ΔK _I = 20 MPa√m or more, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is deteriorated. Steel plate No. B32 (comparative example) has a low hot rolling finishing temperature, and accelerated cooling is performed after ferrite formation and pearlite precipitation is completed, resulting in finer ferrite and accelerated second-phase strengthening by accelerated cooling. The yield strength YS is less than 315 MPa, and the desired high strength cannot be secured. Further, when ΔK _I = 20 MPa√m or more, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is deteriorated.

Steel plate No. B33 (comparative example) was not subjected to slow cooling at a cooling rate of less than 5 ° C / s for cooling after hot rolling, but only accelerated cooling, so the structure became a bainite single phase and ΔK _{When I} = 20 MPa√m or more, γ _P * / MU _eff exceeds 10, and the fatigue crack resistance is deteriorated. Steel plate No. B34 (comparative example) was not subjected to accelerated cooling after hot rolling, so ferrite refinement and second-phase strengthening were not obtained, and the desired high strength was obtained with a yield point YS of less than 315 MPa. It is not secured. Further, when ΔK _I = 20 MPa√m or more, γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is deteriorated. Steel plate No. B35 (comparative example) has a high tempering temperature, so a large amount of MA is produced and toughness is reduced.
(Example 3)
The obtained steel material (steel No. C, F, I, J, P) is heated, hot-rolled and cooled after rolling under the conditions shown in Table 10 as production method C. After that, reheating was performed under the conditions shown in Table 10, followed by heat treatment for cooling at various cooling rates. Some of the steel plates were subjected to heat treatment (tempering treatment).

Specimens were collected from the obtained steel plates and subjected to fatigue crack propagation tests, structure observations, tensile tests, impact tests, hardness tests, and weldability tests. The test method was the same as in Example 1.
Table 11 shows the results of tensile properties, toughness, and weldability among the obtained results.

The obtained MU _eff results are shown in Table 12 together with the structure observation results and the Vickers hardness measurement results.

Table 13 shows the calculation results of γ _p ^* and γ _p ^* / MU _eff and the measurement results of the fatigue crack propagation rate.

Each of the examples of the present invention has a structure composed of ferrite having an area ratio of more than 50% as a main phase, the main phase and the remaining hard phase, a high strength at a yield point of 315 MPa or more, and an absorbed energy vE ₀ : It has a high toughness of 31J or more, and also has excellent weldability in the y-type weld cracking test, with no weld cracking, MU _eff of 150 (μm) or less, and at least ΔK _I = 15 MPa√m, ΔK _I = 20 MPa√m, γ _P * is 250 (μm) or less, γ _P * / MU _eff is 10 or less, and the fatigue crack propagation rate da / dN is at least ΔK _I = 15 MPa√m Less than a certain 1.75 × 10 ^-8 (m / cycle), ΔK _I = 20 MPa√m and the target value of 4.26 × 10 ^-8 (m / cycle) or less, reducing fatigue crack resistance. It is a high-strength steel material. Even if the manufacturing method is changed, all of the examples of the present invention are high-strength steel materials having improved fatigue crack propagation characteristics.

On the other hand, in the comparative example outside the scope of the present invention, any of strength, toughness and weldability is reduced, or γ _P * / MU _eff exceeds 10, and the fatigue crack propagation resistance is reduced. .
Steel plate No. C41 (comparative example) was water-cooled after hot rolling, so a structure with a ferrite phase as the main phase could not be obtained, ΔK _I = 15 MPa√m or more, and γ _P * / MU _eff exceeded 10 As a result, fatigue crack propagation characteristics are degraded. Steel plate No. C42 (comparative example) had a reheat temperature exceeding the Ac3 transformation point, and thus became a martensite single-phase structure, ΔK _I = 15 MPa√m or more, and γ _P * / MU _eff was Beyond 10, fatigue crack propagation resistance is degraded. Steel plate No. C43 (comparative example) has a slow cooling after reheating, so ferrite refinement and second phase strengthening cannot be obtained, and the desired high strength cannot be obtained when the yield point YS is less than 315 MPa. ΔK _I = 15 MPa√m or more, γ _P * / MU _eff exceeds 10, and fatigue crack propagation resistance is degraded. Steel plate No. C44 (comparative example) has a high cooling stop temperature in cooling after reheating, the yield point YS is less than 315 MPa, and the desired high strength cannot be obtained. ΔK _I = 20 MPa√m or more, γ _P * / MU _eff exceeds 10 and fatigue crack resistance is degraded. Steel plate No. C45 (comparative example) has a high tempering temperature, so a large amount of MA is produced and toughness is reduced.
Example 4
Fatigue crack resistance of a steel sheet (plate thickness: 12 to 100 mm) having the composition shown in Table 14 and subjected to normal hot rolling, cooling after rolling, heat treatment, etc. and having the structure and strength shown in Table 15 Propagation characteristics were determined.

  In the same way as in Example 1, the structure was observed and the Vickers hardness was measured at the ¼ thickness position of the target steel sheet, the area ratio (AR) of each phase constituting the structure, and the fatigue crack of each phase. The crystal grain unit (DP) in the progress direction and the average Vickers hardness (HV) of each phase were determined. The obtained results are shown in Table 16. Note that the fatigue crack propagation test was performed using only the three-point bending specimen LZ shown in FIG. 6 where the crack propagates in the plate thickness direction. Therefore, the structure observation and the hardness measurement were performed only in the cross section in the rolling direction (xy plane shown in FIG. 2).

Using the obtained structure observation result and the Vickers hardness measurement result, the effective structure unit MU _eff was calculated by the equation (2). Further, the crack tip plastic zone size γ _p ^* (μm) defined by the equation (1) was calculated. Table 17 shows the obtained results. Note that using equation (1), when calculating the gamma _p ^* is, K _{Imax is, ΔK I = 15MPa√m, ΔK I} = 20MPa√m, the ΔK _I = 25MPa√m, R is using 0.1 And calculated.

If at least ΔK _I = 15 MPa√m and ΔK _I = 20 MPa√m, and γ _P * / MU _eff was 10 or less, it was determined that the steel plate had excellent fatigue crack propagation resistance. Other than that, it was determined that the fatigue crack propagation resistance was inferior.
In this determination, a fatigue crack propagation test is separately performed to confirm consistency. In the fatigue crack propagation test, a three-point bending test piece LZ was sampled from the target steel plate as shown in FIG. The test method of the fatigue crack propagation test was the same as in Example 1. If the steel plate thickness is 25 mm or less, the test piece thickness is the total thickness of the steel plate. If the steel plate thickness is more than 25 mm to 50 mm or less, the single-sided steel plate is ground to have a Z-direction thickness of 25 mm. A thinned specimen was obtained. When the steel plate thickness exceeded 50 mm, both sides of the steel plate were ground to obtain a double-sided reduced thickness test piece having a Z-direction thickness of 25 mm centered at the 1/4 position of the plate thickness. The obtained fatigue crack propagation rates are also shown in Table 17.

All steel plates judged to have excellent fatigue crack propagation resistance have a fatigue crack propagation rate da / dN of at least ΔK _I = 15 MPa√m and the target value of 1.75 × 10 ^-8 (m / cycle) or less. , ΔK _I = 20MPa√m and the target value is reduced to 4.26 × 10 ^-8 (m / cycle) or less. On the other hand, the steel plate determined to have poor fatigue crack propagation characteristics had a fatigue crack propagation rate da / dN that was larger than the above target value.

Claims (6)

% By mass
C: 0.02 to 0.4%, Si: 0.01 to 1.0%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.05% or less, Sol.Al: 0.10% or less, the balance Fe and inevitable impurities,
At a thickness of 1/4, the main phase is ferrite with an area ratio of more than 50%, and it has a structure consisting of the main phase and the remaining hard phase. Yield point: High strength of 315 MPa or more, Charpy impact test Test temperature at: High toughness with absorbed energy at 0 ° C of 31J or more,
The crack tip plastic zone dimension γ _p ^* (μm) defined by the following equation (1) and the effective structural unit MU _eff (μm) in the crack growth direction defined by the following equation (2) are 3) satisfied ~ (5), fatigue Crack Growth rate da / dN is at least, ΔK _I = 15MPa√m at ^{1.75 × 10 -8 (m / cycle} ) or less, at ΔK _I = 20MPa√m 4.26 × 10 ^-8 (m / cycle) or less der Rukoto-out fatigue and wherein the crack propagation strength steel with excellent properties.
Record
γ _P * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Where K _{Imax is} the maximum stress intensity factor in mode I and is in the range of 5 to 35 (MPa√m), σ _{Y is the} yield stress (MPa), and ν is Poisson's ratio.
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase The structural unit (μm) in the propagation direction, the “structural unit in the crack propagation direction” here, is a structural unit closely related to the bending of the crack measured in the crack propagation direction. organizational unit (D _P) of the (alpha) _alpha feeling crack propagation direction of the average of the ferrite grain size, organizational unit (D _{P) P} wear of crack propagation direction of the bulk perlite size or layered pearlite pearlite (P) Mean thickness, bainite (B) structural unit (D _P ) _{B is} the average packet size in the crack propagation direction, martensite (M) structural unit (D _P ) _{M is the} crack propagation direction The average packet size,
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5)   In addition to the above composition, Cu: 3.0% or less, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1 % Or less, B: 1 type or 2 types or more chosen from 0.005% or less are contained, The high strength steel materials of Claim 1 characterized by the above-mentioned.   The high content according to claim 1 or 2, further comprising one or two selected from Ca: 0.010% or less and REM: 0.010% or less in mass% in addition to the composition. Strength steel material. % By mass
C: 0.02 to 0.4%, Si: 0.01 to 1.0%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.05% or less, Sol.Al: 0.10% or less
A high-strength steel material having a composition comprising the balance Fe and inevitable impurities, ferrite having an area ratio exceeding 50%, the balance having a hard phase structure, and a high yield strength of 315 MPa or more For
For the high-strength steel material of interest, the structure observation, Vickers hardness measurement is performed, the area ratio (AR) of each phase constituting the structure in the assumed fatigue crack growth direction, the structural unit (DP) of each phase, Find the average Vickers hardness (HV) of each phase,
Calculate the crack tip plastic zone dimension γ _p ^* (μm) defined by the following equation (1) and the effective structural unit MU _eff (μm) in the crack propagation direction defined by the following equation (2). (3) a is satisfied - (5), the fatigue Crack Growth rate da / dN, at ^{least, 1.75 × 10 -8 (m /} cycle) at ΔK _I = 15MPa√m below, ΔK _I = 20MPa√ A method for determining a high strength steel material having excellent fatigue crack propagation characteristics, characterized in that the steel material is determined to have excellent fatigue crack propagation characteristics at m of 4.26 × 10 ^-8 (m / cycle) or less .
Record
γ _P * = {(K _Imax ) ² × 10 ⁶ / (2πσ _Y ² )} × (1−2ν) ² (1)
Where K _{Imax is} the maximum stress intensity factor in mode I and is a value in the range of 5 to 35 (MPa√m), σ _{Y is the} yield stress (MPa), and ν is Poisson's ratio (= 0.3).
MU _eff = (AR) _α × (D _P ) _α + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _α } + (AR) _B × (D _P ) _B × {( _{Hv) B / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α} ‥‥ (2)
Here, (AR) _α , (AR) _P , (AR) _B , (AR) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), area ratio of each phase ( 0-1),
(D _P ) _α , (D _P ) _P , (D _P ) _B , (D _P ) _M : Ferrite (α), pearlite (P), bainite (B), martensite (M), crack of each phase The structural unit (μm) in the propagation direction, the “structural unit in the crack propagation direction” here, is a structural unit closely related to the bending of the crack measured in the crack propagation direction. organizational unit (D _P) of the (alpha) _alpha feeling crack propagation direction of the average of the ferrite grain size, organizational unit (D _{P) P} wear of crack propagation direction of the bulk perlite size or layered pearlite pearlite (P) Mean thickness, bainite (B) structural unit (D _P ) _{B is} the average packet size in the crack propagation direction, martensite (M) structural unit (D _P ) _{M is the} crack propagation direction The average packet size,
(Hv) _α , (Hv) _P , (Hv) _B , (Hv) _M : ferrite (α), pearlite (P), bainite (B), martensite (M), average Vickers hardness of each phase
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 250 (4)
MU _eff ≦ 150 (5) In addition to the above composition, Cu: 3.0% or less, Ni: 10% or less, Cr: 3.0% or less, Mo: 2.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1 % Or less, B: 1 type or 2 types or more selected from 0.005% or less are contained, The determination method of the high strength steel materials of Claim 4 characterized by the above-mentioned. The high content according to claim 4 or 5 , further comprising one or two selected from Ca: 0.010% or less and REM: 0.010% or less in mass% in addition to the composition. Method for judging strength steel.

Images