JP6064896B2 - Steel material with excellent fatigue crack propagation characteristics, method for producing the same, and method for determining steel materials with excellent fatigue crack propagation characteristics - 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 resistance, a method for producing the same, and a method for judging a steel material excellent in fatigue crack resistance. The “steel material” here includes thick steel plates, section steels, steel pipes, hot-rolled steel plates, and cold-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 low temperature 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 research described in Non-Patent Document 1, the steel sheet structure is a structure in which a soft phase (Vickers hardness: 149HV) is surrounded by a hard phase (Vickers hardness: 546HV, fraction: 39.2%) in a mesh shape. Can greatly reduce the fatigue crack propagation rate compared to a structure in which the hard phase (Vickers hardness: 565HV, fraction: 36.4%, average size: 149μm) is uniformly dispersed in the soft phase (Vickers hardness: 148HV) It is said. 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 an effect of suppressing fatigue crack growth. The technology 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 fatigue crack 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, the width direction and longitudinal direction of a thick steel plate are mentioned. There is no mention of the fatigue crack growth inhibiting property in, and there is a concern that the fatigue crack growth inhibiting property in the width direction and 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 steel sheet having excellent fatigue crack propagation resistance is obtained by the presence of 1 area% or more of acicular ferrite. 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 growth resistance. It remains unclear whether these properties are 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 including C: 0.01 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.3 to 2%, solAl: 0.001 to 0.1%, N: 0.0005 to 0.008%, and an area. It is a steel sheet having a structure of 60 to 85% bainite by rate, 0 to 5% total martensite and pearlite, and the balance being 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, fatigue crack growth characteristics and toughness are obtained by forming a multiphase structure in which sufficiently refined ferrite and a low-temperature transformation phase with a short lath length transformed from processed γ are combined. Both characteristics are said to be 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, plastic deformation at the crack tip is suppressed, and the fatigue crack is retained. Even in the range of ΔK, since the fatigue crack growth 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 compared to 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 to 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.

  Patent Document 9 describes a steel sheet having a fatigue crack growth inhibiting 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 growth-inhibiting effect can be obtained even in a moderate ΔK range, and even when a fatigue crack is generated from a welded portion, the conventional technique has been adopted. Compared to that, the fatigue life can be extended.

  Further, Patent Document 10 describes a fatigue crack propagation delay 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 rate is locally reduced when the hard phase is encountered. 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, Patent Document 7 does not mention properties other than the fatigue crack growth rate, and the thick steel plate described in Patent Document 7 is necessary as a steel sheet for welded structures together with excellent fatigue crack growth suppression properties. Whether the strength, ductility, toughness, weldability and other properties are well balanced remains unclear.
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. The problem is that the production load is large and the productivity is lowered, and flat crystals (martensite) are introduced due to high pressure at low temperature, and the anisotropy of low temperature toughness is expected to become remarkable. The problem remained as a heavy steel plate for goods.

  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 has high strength, excellent low temperature toughness, ductility, weldability, and high strength steel material with excellent fatigue crack propagation resistance and its An object is to provide a determination method. Here, “high strength” refers to the case where the yield strength is YS: 325 MPa or more. Further, herein, the term "excellent in-out fatigue crack propagation property" Fatigue Crack Growth rate da / dN is at least ^{_{ΔK I: 1.75 × 10 -8 (}} m / cycle) at 15MPa√m hereinafter, ^{_{ΔK I: 4.26 × 10 -8 (}} m / cycle) at 20MPa√m refers to the case which is below.

  The term “excellent in low temperature toughness” as used herein refers to the case where the fracture surface transition temperature vTrs in a Charpy impact test conducted in accordance with the provisions of JIS Z 2242-2005 is −20 ° C. or lower. .

In order to achieve the above-described object, the present inventors first provide a steel material having a high strength of yield strength YS: 325 MPa or more in a balanced manner with low-temperature toughness, ductility, and fatigue crack resistance. For this purpose, it has been found that the matrix structure is preferably a structure mainly composed of a bainite phase and / or a martensite phase.
In addition, we have intensively studied various factors affecting the fatigue crack propagation characteristics of steel materials having a structure mainly composed of bainite phase and / or martensite 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.
Steel materials having various compositions were subjected to treatments under various conditions to produce steel sheets (thickness: 25 mm) having a bainite single-phase structure with various changes in grain size and shape. From the obtained steel plate, a CT specimen and a three-point bending specimen were collected from the three directions 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). The test piece thickness 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 calculated, and the fatigue crack in the 500 μm section near ΔK _I = 15 MPa√m. The propagation path was observed through a cross section, and the number of fatigue crack flexions (times) was measured. From the obtained results, the relationship between the fatigue crack propagation rate da / dN of ΔK _I = 15 MPa√m and the number of bending of the crack (times) at the center of the specimen was obtained. The obtained results are shown in FIG. At the same time, the bending length and the bending angle with respect to the crack propagation direction were also determined.

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, from the observed bending behavior of fatigue cracks for steel (bainite single-phase structure) with a fatigue crack propagation rate da / dN of 5.89 × 10 ⁻⁹ m / cycle at ΔK _I = 15 MPa√m, FIG. 3 shows the relationship between the bending length r and the bending angle θ with respect to the crack propagation direction. 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.

For the steel material used (bainite single-phase structure), the structural unit closest to the crack bending length was obtained while referring to the observed bending behavior of fatigue cracks. , Found that it is a bainite packet.
In FIG. 3, the average packet size is obtained in the crack propagation direction and the direction perpendicular to the crack propagation direction (x direction and y direction in FIG. 2), and this is approximated as an ellipse with the long side and the short side as the fatigue. When placed at the crack tip, the locus indicated by the packet 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 (8) γ _p (θ) = {(K _Imax ) ² × 10 ⁶ / 4πσ _Y ² } × {(3/2) sin ² θ + (1−2ν) ² (1 + cos θ)} (8)
Defined by Is the angle (°), K _Imax is the maximum stress intensity factor (MPa√m) of the target mode I, σ _Y is the yield stress (MPa) of the steel material, and ν 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, the fatigue crack bend occurs in the range of the solid line, that is, in the structure boundary (in the bainite packet range), and tends to concentrate in the vicinity of the crack propagation direction (θ: 0 °). I understand that. 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 structure boundary are close to each other. From this, the present inventors believe that the plastic zone size and the structural unit (bainite packet) at the crack tip are closely related to the fatigue crack propagation rate through the bending of the fatigue crack. Thought.

Therefore, the present inventors set the plastic zone size at the crack tip in the crack propagation direction (θ = 0 ° in FIG. 2, x direction) in the fatigue crack propagation test conducted on the obtained steel sheet by (8 ) And calculated as γ _p *. That is, γ _p * is expressed by the following equation (1) γ _p * = {(K _Imax ) ² × 10 ⁶ / 2πσ _Y ² } × {(1−2ν) ² } (1)
Defined by

On the other hand, for each of the obtained steel plates, a structural unit (bainite packet size) in the crack propagation direction (θ = 0 °, x direction in FIG. 2) was measured and defined as (D _P ) _B.
The ratio of the obtained γ _p * to the obtained (D _P ) _B , γ _p * / (D _P ) _B was calculated, and the effect of γ _p * / (D _P ) _B 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 ) _B , especially in the region where γ _p ^* / (D _P ) _B 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 ) _B becomes smaller, the fatigue crack propagation rate clearly decreases. However, gamma _p ^* / In (D _{P) B} exceeds 10 ^{_{area, γ p * / (D P}} ) less the increase in fatigue crack propagation speed increases _B, the slope of the curve is small, rather substantially It becomes horizontal, and the variation of data becomes large. That, gamma _p ^* / In (D _{P) B} exceeds 10 area, gamma _p ^* / impact to (D _P) Fatigue Crack Growth rate of _B is said to be small. In the region where γ _p ^* / (D _P ) _B exceeds 10, it is considered that the crystal grain unit is smaller than the plastic region, and the propagation of the crack is governed by the stress field at the crack tip.

The above findings were obtained for a steel sheet having a bainite single-phase structure, but as a result of a similar investigation of a martensite single-phase structure, the more the number of fatigue cracks flexed, the greater the fatigue crack propagation. The speed decreases, the bending length and the martensite organization unit (D _P ) _M have a correlation with the packet size, and the fatigue crack propagation rate can be organized by γ _p ^* / (D _P ) _M. It was confirmed that the crack propagation rate was greatly reduced in the region where γ _p ^* / (D _P ) _M was 10 or less.

In steel materials applied to actual structures, the bainite phase and the martensite phase coexist, depending on the purpose of use, depending on the component constraints and strength, and the manufacturing process used, etc., and a structure in which pearlite, ferrite phase, etc. are mixed Often occurs.
Therefore, first, a steel material having a composite structure including a bainite phase or a martensite phase or a mixed phase of a bainite phase and a martensite phase as a main phase having an area ratio of 50% or more, and containing pearlite and ferrite as a second phase. The fatigue crack propagation behavior was investigated. As a result, when a fatigue crack propagates from a soft phase to a harder phase, such as ferrite to pearlite and bainite to martensite, crack bending and branching occur, and the fatigue crack propagation rate is locally increased. Has been found to be reduced. Furthermore, as a result of more detailed observations, it was found that crack bending occurred at the packet boundary in bainite and martensite, at the ferrite grain boundary in ferrite, and at the massive or layered boundary in pearlite.

From these findings, the present inventors have also found that each phase has cracks in the composite structure mainly composed of the bainite phase and the martensite phase, as in the bainite single phase structure and the martensite single phase structure. We considered that there is an effective tissue unit that causes bending. Then, the present inventors have found that the organizational unit, the bainite defined respectively (D _{P) B,} a martensite (D _{P) M,} is a ferrite (D _{P) alpha,} in pearlite (D _{P) P,} 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 structure unit of each phase and the area ratio (AR) _B , (AR) _M, (AR) _α , (AR) _P 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” means a phase having the largest area ratio among the bainite phase and / or martensite phase exceeding 50% in total area ratio.

That is, the effective structural unit MU _eff in the composite structure having a bainite phase or martensite phase as the main phase is
_{MU eff = (AR) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase Organizational units in the direction of development (μm),
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : bainite (B), martensite (M), ferrite (α), pearlite (P), defined by average Vickers hardness of each phase it can. In the case of a steel material in which the above-described equation (2a) is a bainite single-phase structure and the equation (2b) is a martensite single-phase structure, only the first term is obtained, which is consistent with the results shown in FIG. In addition, when the bainite phase and the martensite phase each can not occupy an area ratio of 50% or more, but the mixture of the bainite phase and the martensite phase can occupy an area ratio of 50% or more. The above formula is applied by regarding a phase having a large area ratio as a main phase.

Therefore, a fatigue crack propagation test was performed on the steel material having a composite structure in which the total area ratio of the bainite phase and the martensite phase exceeds 50%, as in the case of the bainite single phase structure, and the fatigue crack propagation rate was obtained. . FIG. 5 shows the relationship between the obtained fatigue crack propagation rate and γ _p ^* / MU _eff using MU _eff defined by the above equation (2a) or (2b). In FIG. 5, the case of a bainite single-phase structure (marks ◯, Δ, □) is also shown. From FIG. 5, the fatigue crack propagation rate can be organized by γ _p ^* / MU _eff even in a composite structure having a bainite phase and a martensite phase as main phases, as in the bainite single phase. In addition, in the region of γ _p ^* / MU _eff and 10 or less, it was possible to organize within a relatively narrow band, and it was found that the fatigue crack propagation rate clearly decreases as γ _p ^* / MU _eff decreases.

In other words, even in a steel material having a composite structure in which the sum of the bainite phase and the martensite phase exceeds 50% in area ratio and pearlite or ferrite phase is mixed, γ _p ^* / MU _eff is 10 or less, It was found that the smaller the γ _p ^* / MU _eff is, the lower the fatigue crack propagation rate and the better the fatigue crack propagation resistance. Steel such as γ _p ^* / MU _eff under conditions of use is 10 or less, the fatigue crack propagation rate will be said to be superior steel materials Crack Propagation characteristics-out fatigue was reduced, gamma _p ^* / It was thought that an index with an MU _eff of 10 or less can be used as a criterion for steel materials having excellent fatigue crack propagation characteristics.

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 consists of a composition comprising the balance Fe and inevitable impurities, a bainite phase and / or martensite phase exceeding 50% in the total area ratio at the 1/4 position of the plate thickness, and the balance other than that (including 0%). It has a structure and yield strength: high strength of 325 MPa or more, high surface toughness with a fracture surface transition temperature vTrs of Charpy impact test of -20 ° C or less, and 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 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. Zone size γ _p ^* (μm)
_{MU eff = (AR) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _P ) _{B is} the average value of the packet size in the crack growth direction, martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack growth direction, ferrite structure unit (D _P ) _{α is the} crack growth The average value of the ferrite grain size in the direction, the pearlite structural unit (D _P ) _{P is} the average value of the bulk pearlite in the crack propagation direction or the thickness of the layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), average Vickers hardness of each phase or (2b) Formula
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _P ) _{B is} the average value of the packet size in the crack growth direction, martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack growth direction, ferrite structure unit (D _P ) _{α is the} crack growth The average value of the ferrite grain size in the direction, the pearlite structural unit (D _P ) _{P is} the average value of the bulk pearlite in the crack propagation direction or the thickness of the layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : bainite (B), martensite (M), ferrite (α), pearlite (P), defined by 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 * ≦ 200 (4)
MU _eff ≦ 100 (5)
High-strength steel with excellent fatigue crack propagation characteristics characterized by satisfying

(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 The high-strength steel material according to claim 1 or 2.

(4) The method for producing a high-strength steel material according to any one of (1) to (3) , wherein the steel material is subjected to hot rolling to obtain a steel material having a thickness of 100 mm or less. , 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, and the balance Fe And a steel material having a composition consisting of unavoidable impurities, and after the hot rolling is heated to a heating temperature of 950 to 1300 ° C., a cumulative finish in a temperature range of 900 ° C. or higher is 50% or higher, and the finish is rolled. Temperature: Hot rolling with an Ar3 transformation point or higher. After the hot rolling, from the temperature range above the Ar3 transformation point, the following formula (7) φ = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + Nb / 10 + 5B ...... (7)
The relationship between φ (%) and thickness t (mm) defined by
RS = (− 0.53φ ² −0.28φ + 0.67) × e ^9.10 / t ^1.63 (6)
(Where t: steel plate thickness (mm), C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: content of each element (mass%))
In defined in the cooling rate RS (℃ / s) or more cooling rate, have rows accelerated cooling to a cooling stop temperature of 600 ° C. or less,
The steel material has a structure composed of a bainite phase and / or a martensite phase with a total area ratio exceeding 50% at a 1/4 position of the plate thickness, and the balance other than that (including 0%), and Yield strength: High strength of 325MPa or more, and excellent low temperature toughness with fracture surface transition temperature vTrs of Charpy impact test of -20 ℃ or less,
Fatigue crack propagation rate da / dN is at least 1.75 × 10 ⁻⁸ (m / cycle) at ΔK _I : 15 MPa√m, and ΔK _I : 4.26 × 10 ⁻⁸ (m / cycle) at 20 MPa√m. A method for producing a high-strength steel material having excellent low-temperature toughness and fatigue crack propagation characteristics, characterized by using a steel material.

(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, by mass%, one or two selected from Ca: 0.010% or less and REM: 0.010% or less A method for producing a high strength steel material.

(7) In any one of (4) to (6), instead of the accelerated cooling, after the hot rolling is completed, the steel is cooled by any one of furnace cooling, air cooling, and water cooling, and more than the Ac3 transformation point. A high-strength steel material is produced by reheating to a temperature and then performing a quenching treatment to cool to 600 ° C. or less at a cooling rate RS (° C./s) or more defined by the above formula (6). Method.
(8) The method for producing a high-strength steel material according to any one of (4) to (7), further comprising tempering at a temperature not higher than the Ac1 transformation point after the accelerated cooling or the quenching treatment. .

(9) In 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, Structure composed of the balance Fe and inevitable impurities, a bainite phase and / or martensite phase exceeding 50% in total area ratio at the 1/4 position of the plate thickness, and the balance other phases (including 0%) And with a yield strength of 325 MPa or more, and the target high-strength steel material is subjected to structural observation and Vickers hardness measurement to determine the structure in the assumed fatigue crack propagation direction. The area ratio (AR) of each phase constituting, the structural unit (DP) of each phase, and the average Vickers hardness (HV) of each phase are obtained, and the following equation (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 (2a)
_{MU eff = (AR) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _P ) _{B is} the average value of the packet size in the crack growth direction, martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack growth direction, ferrite structure unit (D _P ) _{α is the} crack growth The average value of the ferrite grain size in the direction, the pearlite structural unit (D _P ) _{P is} the average value of the bulk pearlite in the crack propagation direction or the thickness of the layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), average Vickers hardness of each phase or (2b) Formula
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _P ) _{B is} the average value of the packet size in the crack propagation direction, martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack propagation direction, and ferrite (D _P ) _{α is} the ferrite in the crack propagation direction Average value of particle size, pearlite structural unit (D _P ) _{P is} the size of bulk pearlite in the crack propagation direction or the average value of layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : bainite (B), martensite (M), ferrite (α), pearlite (P), defined by 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 * ≦ 200 (4)
MU _eff ≦ 100 (5)
When the fatigue crack propagation rate da / dN is at least 1.75 × 10 ⁻⁸ (m / cycle) at ΔK _I : 15 MPa√m , ΔK _I : 4.26 × 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 which is equal to or less than:

(10 ) In ( 9 ), 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 : 0.1% or less, Ti: 0.1% or less, B: One or two or more types selected from 0.005% or less are contained.

( 11 ) In ( 9 ) or ( 10 ), in addition to the above-mentioned composition, it further contains, by 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, without containing a large amount of alloy elements, without performing a special process,
Yield point: High strength of 325 MPa or more, excellent low-temperature toughness, and high strength steel with excellent fatigue crack propagation characteristics, providing a remarkable industrial effect. 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 and the bending frequency of a fatigue crack in a bainite single phase structure steel plate. 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, and the bending crack length r in a bainite single phase structure steel plate. Fatigue bainite single-phase structure steel sheet Crack Propagation speed and gamma _P * / is a graph showing the relationship between (D _{P) B.} 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 a graph which shows the relationship between the calculated cooling rate CS of steel materials, and board thickness t. It is an example of the processing CCT diagram which shows the transformation characteristic after processing of steel materials. 3 is a graph showing the relationship between the lower limit cooling rate BS for obtaining a structure having an area ratio of bainite exceeding 50%, the ratio of the calculated cooling rate of a steel material having a thickness of 100 mm, and φ. 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 basic composition consisting of 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, and is a steel material having a structure mainly composed of a bainite phase or a martensite phase, and needs to contain 0.02% or more in order to ensure a desired high strength. 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 contributes to the 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% or less. In addition, Preferably it is 0.08% or less.

  The above components are basic components. In the present invention, in addition to the basic composition described above, Cu: 3.0 is added for the purpose of adjusting strength, low-temperature toughness, weldability, further weather resistance, heat resistance, and the like. %, 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 type or 2 types or more and / or 1 type or 2 types selected from Ca: 0.010% or less and REM: 0.010% or less can be selected and contained as needed.

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 dissolves and improves low-temperature 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, if the content exceeds 3.0%, weldability and toughness deteriorate. 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 increasing strength and improving 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 suppresses austenite grain recrystallization during hot rolling and contributes to an increase in strength through refinement of the structure and contributes to an increase in strength 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. 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 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 0.01% 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 a bainite phase and / or a martensite phase exceeding the total area ratio by 50% in the plate thickness 1/4 position, which is an average microstructure, and the remaining phase ( (Including 0%).

  In the steel material of the present invention, in order to have desired high strength and low temperature toughness and to have fatigue crack propagation characteristics, the structure is mainly composed of bainite phase and / or martensite phase (main phase), that is, total area The organization will exceed 50%. When the total area fraction of the bainite phase and / or martensite phase is less than 50%, the strength is lowered. The upper limit of the total area ratio of the bainite phase and / or martensite phase need not be specified. In the present invention, when either of the bainite phase and the martensite phase exceeds 50% in area ratio, they may not necessarily coexist and may be a bainite single phase or a martensite single phase. In the present invention, as long as the total area ratio of the bainite phase and the martensite phase exceeds 50%, each phase may be less than 50% in area ratio. The term “main phase” as used herein means a phase having the maximum area ratio on the condition that the total area ratio of the bainite phase and the martensite phase exceeds 50%. 2b) The phase positioned in the first term in the equation. The balance other than the main phase (including 0%) is a bainite phase, and the second phase other than the martensite phase is a pearlite or ferrite phase.

The steel of the present invention has the above-described composition and the above-described structure, and further has a high yield strength of 325 MPa or more, and excellent low-temperature toughness with a fracture surface transition temperature vTrs of −20 ° C. or less in the Charpy impact test. And 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 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 next (2a) or next (2b)
_{MU eff = (AR) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase Organizational units in the direction of development (μm),
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : bainite (B), martensite (M), ferrite (α), pearlite (P), defined by 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 * ≦ 200 (4)
MU _eff ≦ 100 (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, it is possible to improve the fatigue crack propagation characteristics of steel materials. become unable. 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 equation (3) is a basic equation for determining a steel material having excellent fatigue crack propagation characteristics. In the present invention, whether or not the fatigue crack propagation property of the steel material is good is determined depending on whether or not the expression (3) is satisfied.
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 corresponds to a range of 5 to 35 MPa√m. For this reason, the K _Imax is set to a value in the range of 5 to 35 MPa√m depending on the state of use of the steel material. Set. In addition, Preferably it is the range of 10-30 MPa√m. Here, σ _Y is the yield strength, and it is preferable to measure in the opening direction of the fatigue crack. However, if this is difficult, it may be a direction in which a tensile test piece 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 * ≦ 200 (4)
Is limited to a range that satisfies the above. gamma _{P *} is 200 ([mu] m) to increase beyond the, σ _Y: 325MPa and K _Imax: closer to a region beyond the scope of combinations of 5～35MPa√m, is difficult to secure the Fatigue Crack Growth characteristics Become. 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 200 (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 that is determined with reference to the fatigue crack propagation path and the constituent structure, and has the highest frequency of starting bending.
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 of each phase to the main phase {for example, (Hv) _α / (Hv) _B }, the structural unit of each phase based on the main phase can be weighted and mixed. According to the rule, the sum is _{defined as} the effective organizational unit MU _eff of the composite structure in which the main phase and each phase other than the main phase are the second phase.

The effective structural unit MU _eff of the composite structure consisting of the martensite phase, ferrite phase, and pearlite other than the bainite phase, with the main phase being the bainite phase, is expressed by the following equation (2a)
_{MU eff = (AR) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
Calculated by The effective structural unit MU _eff of the composite structure composed of a martensite phase as the main phase and a bainite phase other than the martensite phase, a ferrite phase, and pearlite is expressed by the following equation (2b):
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Calculated by

Here, (AR) _B , (AR) _M , (AR) _α , and (AR) _P are bainite (B), martensite (M), α (ferrite), pearlite (P), and area ratio of each phase. (0 to 1) and calculated by (area fraction of each phase (%)) / 100. In addition, an area ratio is performed on the xy plane shown in FIG. The area ratio can be calculated and calculated using, for example, commercially available image analysis software.

(D _P ) _B , (D _P ) _M , (D _P ) _α , and (D _P ) _P are bainite (B), martensite (M), α (ferrite), pearlite (P), and each phase. It is a structural unit (μm) in the crack propagation direction. (Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P is bainite (B), martensite (M), α (ferrite), pearlite (P), average Vickers hardness of each phase That's it.

The “structural unit in the crack propagation direction” here is a structural unit measured in the crack propagation direction related to the flexion frequency of the crack, which is closely related to the decrease in fatigue crack propagation rate. When the usage conditions are unknown, the measured values shall be used in the thickness direction.
The tissue unit closely related to crack bending is the tissue unit from the observation of the structure using optical microscope, scanning electron microscope SEM, transmission electron microscope TEM, backscattered electron diffraction EBSD, etc. 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, in bainite (B), there are packet sizes, block sizes, and prior austenite grain sizes that are closely related to crack bending, but most of fatigue crack bending is related to packets. Therefore, in the present invention, the packet size in the crack propagation direction is measured simply, and the average value is 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 is measured simply, and the average value is defined as the martensitic organization unit (D _P ) _M. In addition, in ferrite (α), most of the bending of fatigue cracks was related to the ferrite grain boundary. Therefore, simply measuring the ferrite grain size in the crack propagation direction, and calculating the average value of the ferrite (α) α) Tissue unit (D _P ) _α . 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, the thickness thereof was measured in the direction of fatigue crack propagation, and the average value was defined as the structure (D _P ) _{P of} pearlite (P). As a measurement method, for example, statistical analysis is preferably performed using a cutting method described in JIS G 0551 (2013). If necessary, it is recommended to observe the crack propagation path and statistically analyze the crack bending length, bending frequency, etc. and the structural unit of each phase to obtain the structural unit of each phase.

Note that, within the scope of the present invention, the structure has a bainite phase and / or a martensite phase as a main phase, or a structure in which a ferrite phase and pearlite are further combined. Even when intermediate structures such as ferrite, acicular ferrite, and granular bainite are generated, the structural unit contributing to fatigue crack bending is determined through microstructure observation and fatigue crack path observation, and the area ratio of each phase. The effective tissue unit MU _eff may be determined by measuring the hardness of each phase as in the present invention.

The effective organizational unit MU _eff was limited to 100 (μm) or less. When the MU _eff exceeds 100, it becomes impossible to secure a desired yield strength (325 MPa) or more. On the other hand, if MU _eff exceeds 100, the low temperature toughness is lowered and it cannot be applied to welded structures. Therefore, MU _eff is the following equation (5)
MU _eff ≦ 100 (5)
It was limited to satisfy. In addition, Preferably it is 85 (micrometer) or less.

(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P is bainite (B), martensite (M), ferrite (α), pearlite (P), average Vickers hardness of each phase That's it. The hardness measurement is carried out using a Vickers hardness meter at least 5 points in each phase on the xy plane shown in FIG. 2 at a thickness of 1/4 position, and the average value is calculated as the hardness Hv of each phase. To do. Since it is difficult to obtain a stable value at the grain boundary or phase boundary, it is preferable to perform hardness measurement by adjusting the load so that the distance between the grain boundary and the phase boundary is four times or more the indentation.

The steel of the present invention has the above composition and the above structure, the yield strength is 325 MPa or more, and the relationship between the crack tip plastic zone size γ _P * and the effective structure unit MU _eff is (3) to (5 It is a high-strength steel material with excellent fatigue crack propagation characteristics that satisfies the formula (1).
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 the Ar3 transformation point or higher. It is preferable to perform hot rolling. The temperature is the surface temperature of the steel material.
Heating temperature: 950-1300 ° C
If the heating temperature of the steel material is less than 950 ° C., the desired finish rolling temperature cannot be ensured. On the other hand, when the temperature exceeds 1300 ° C., the crystal grains become coarse, and it becomes difficult to secure desired low temperature toughness. For this reason, it is preferable to limit the heating temperature of a steel raw material to the range of 950-1300 degreeC.

Cumulative rolling reduction at 900 ° C or higher: 50% or higher
By setting the cumulative rolling reduction at 900 ° C. or more to 50% or more, the austenite grains can be refined, and the strength, low temperature toughness and fatigue crack propagation resistance are improved. If the cumulative rolling reduction above 900 ° C is less than 50%,
The desired structure refinement cannot be achieved. For this reason, it is preferable to limit the cumulative rolling reduction at 900 ° C. or higher in hot rolling to 50% or higher.

Finishing rolling end temperature: Ar3 transformation point or higher If the finishing rolling finishing temperature of the hot rolling is less than the Ar3 transformation point, a large amount of ferrite is generated, and the desired high strength cannot be secured. In addition, the working texture develops and anisotropy occurs in the fatigue crack propagation characteristics, and the low temperature toughness decreases. For this reason, it is preferable that the finish rolling finish temperature is limited to the Ar3 transformation point or higher.

Next, after the hot rolling is completed, from the temperature range above the Ar3 transformation point, the following formula (7) φ = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + Nb / 10 + 5B (7)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: content of each element (mass%))
The relationship between φ (%) and thickness t (mm) defined by
RS = (− 0.53φ ² −0.28φ + 0.67) × e ^9.10 / t ^1.63 (6)
Preferably, the steel plate is accelerated and cooled to a cooling stop temperature of 600 ° C. or lower at a cooling rate RS (° C./s) or higher defined by The temperature is the steel surface temperature, and the cooling rate is the average cooling rate in the steel thickness direction.

Accelerated cooling start temperature: temperature range above the Ar3 transformation point In order to obtain a structure mainly composed of bainite and martensite phases, the accelerated cooling start temperature is set above the Ar3 transformation point. Below the Ar3 transformation point, a large amount of ferrite is produced, and the total area ratio of the bainite phase and martensite phase cannot exceed 50%. The Ar3 transformation point is based on the component content of the steel material,
Ar3 (° C) = 910−310C−80Mn−20Cu−15Cr−55Ni−80Mo
(However, C, Mn, Cu, Cr, Ni, Mo: Content of each element (mass%))
It shall be calculated using In the calculation, the elements that are not included among the listed elements are treated as zero.

Cooling rate of accelerated cooling: RS or more When the cooling rate of accelerated cooling is less than RS (° C./s), it is not possible to obtain a structure having a desired bainite phase and / or martensite phase as a main phase. RS is the following equation (6)
RS = (− 0.53φ ² −0.28φ + 0.67) × e ^9.10 / t ^1.63 (6)
Defined by Here, φ is the following equation (7) φ = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + Nb / 10 + 5B (7)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: content of each element (mass%))
Given in. t is the thickness (mm) of the steel material.

The cooling rate RS (° C./s) is a lower limit cooling rate for obtaining a bainite phase-based structure obtained by the inventors' research.
The present inventors measured the average cooling rate CS in the thickness direction of the steel material by performing accelerated cooling using the steel material with the plate thickness t changed in the latest actual machine and laboratory equipment. As a result, as shown in FIG. 6, the average cooling rate CS (° C./s) and the plate thickness t (mm) in the thickness direction of the steel material are expressed in the natural logarithm as follows:
Ln (CS) = − 1.63 × Ln (t) +9.10
It was found that the relationship of By expanding the above formula,
CS = e ^9.10 / t ^1.63
Is obtained.

On the other hand, the transformation behavior by accelerated cooling after hot rolling strongly depends on the chemical composition of the steel material. Therefore, a machining CCT diagram was created to investigate the transformation behavior of a general-purpose steel material having a φ defined by the equation (7) of φ = 0.20 (%). The processed CCT diagram was prepared under the conditions of 1150 ° C heating-850 ° C finish rolling-850 ° C accelerated cooling start. The obtained results are shown in FIG. In the figure, for steel materials with t = 12mm and t = 100mm, the following formula
CS = e ^9.10 / t ^1.63
The calculated cooling curve is also shown. In addition, for t = 12 mm, a cooling curve is also shown in the case of air cooling after accelerated cooling to 550 ° C.

  From FIG. 7, in order to obtain a bainite-based structure, a cooling rate that does not apply to the ferrite nose, such as when the plate thickness t is 100 mm, or the total area of bainite and martensite, even if applied to the ferrite nose. It is important to cool at a rate higher than the cooling rate at which a 50% structure can be obtained, and when the sheet thickness t is in the bainite generation region, as in the case where the thickness t is 12 mm, or It can be seen that it is necessary to stop the accelerated cooling once in the temperature range of 600 ° C. or lower, and then air-cool.

Thus, in order to obtain a bainite-based structure, it is necessary to perform accelerated cooling at a cooling rate that avoids ferrite nose as much as possible. However, the generation behavior of ferrite varies depending on the composition of the steel material.
Therefore, a processed CCT diagram was created for a steel material having a composition of φ = 0.05 to 0.5, and a lower limit cooling rate BS (° C./s) at which a structure having a bainite phase exceeding 50% in area ratio was obtained.

And as a thick steel material with a slow cooling rate, assuming the case of a plate thickness of 100 mm, the following equation is the relational expression between the plate thickness and cooling rate obtained with the actual cooling device
CS = e ^9.10 / t ^1.63
Was used to determine the average cooling rate CS (100) (° C./s) in the thickness direction of the steel with a plate thickness of 100 mm, and the relationship between BS / CS (100) and φ was investigated. The result is shown in FIG. FIG. 8 shows that BS / CS (100) decreases as φ increases. This is thought to be because the hardenability of the steel material increased due to an increase in φ, that is, an increase in the amount of alloying elements (higher component), and the ferrite nose shifted to the long time side. From Fig. 8, BS / CS (100) is
BS / CS (100) = − 0.53φ ² −0.28φ + 0.67
It can be seen that it can be roughly arranged by a quadratic function of φ shown in FIG.

From these findings, the lower limit cooling rate RS (° C./s) for obtaining a bainite-based structure is a term BS / CS that varies from the φ related to the composition to the cooling rate term CS determined by the plate thickness. The following equation (6) multiplied by (100)
RS = (− 0.53φ ² −0.28φ + 0.67) × e ^9.10 / t ^1.63 (6)
Can be expressed as

Therefore, if the cooling rate of accelerated cooling is less than RS defined by Equation (6), a large amount of ferrite is generated, and a bainite-based structure (a structure in which the sum of bainite and martensite exceeds 50% in area ratio) Can't get.
As can be seen from FIG. 7, in the accelerated cooling, when the cooling stop temperature is set to 600 ° C. or higher, ferrite and pearlite are increased in a large amount in the subsequent air cooling process. For this reason, in the present invention, accelerated cooling is performed at a cooling rate equal to or higher than the cooling rate represented by the formula (6), to a cooling stop temperature: 600 ° C. or lower, thereby obtaining a steel material having a bainite-based structure. It can be set as the steel material which has a desired characteristic.

Further, in the present invention, instead of the above-described accelerated cooling, after the above hot rolling, after cooling in any one of furnace cooling, air cooling, water cooling, and further reheating to a temperature above the Ac3 transformation point, The reheating and quenching treatment for cooling to 600 ° C. or lower may be performed at a cooling rate equal to or higher than the cooling rate RS (° C./s) defined by the equation (6). By such quenching treatment, a structure mainly composed of a bainite phase can be obtained. When the reheating temperature is less than the Ac3 transformation point, austenitization becomes incomplete, and a structure mainly composed of a bainite phase cannot be obtained even by subsequent cooling. The Ac3 transformation point is
Ac3 transformation point (℃) = 854−180C + 441Si-14Mn−17.8Ni−1.7Cr
(Here, C, Si, Mn, Ni, Cr: content of each element (mass%))
Can be calculated. In addition, the element which does not contain among the displayed elements shall be calculated as zero.

In order to adjust the balance between strength, ductility and low temperature toughness, tempering may be performed after accelerated cooling or after reheating and quenching. The tempering temperature is preferably less than the Ac1 transformation point. When the tempering temperature is higher than the Ac1 transformation point, it is partially austenitic, and the subsequent cooling produces island martensite, which reduces toughness.
The Ac1 transformation point is
Ac1 (° C) = 723-14Mn + 22Si-14.4Ni + 23.3Cr
(Here, the element symbol is the content of each element in the steel (mass%))
Can be used to calculate. If the specified element is not included, it shall be calculated as zero.

  Moreover, in 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, the target high-strength steel materials include the above composition, as described above, the bainite phase and / or the martensite phase exceeds 50% in area ratio, and the remainder is the other second phase (0% A high strength steel material with a yield strength of 325 MPa or more. The target high-strength steel material is a steel material that also has excellent low-temperature toughness.

With respect to the target steel material, the yield strength σ _Y in the crack opening direction, the type and fraction of the structure, the structural unit in the crack propagation direction of each phase, and the average Vickers hardness of each phase are measured. Then, using the obtained value, the effective organizational unit MU _eff is obtained by the equations (2a) and (2b). In addition, the crack tip plastic zone size γ _P * is obtained from the equation (1) according to the operating stress state. From the obtained value, the relationship between the crack tip plastic zone dimension γ _P * and the effective structural unit MU _eff (3) to (5)
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 200 (4)
MU _eff ≦ 100 (5)
It is determined whether or not the above is satisfied. The case where the expressions (3) to (5) are satisfied is evaluated as a steel material having excellent fatigue crack propagation characteristics. On the other hand, the case where the expressions (3) to (5) cannot be satisfied is evaluated as a steel material inferior in fatigue crack resistance. 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, a high-strength steel material having excellent fatigue crack propagation characteristics can be easily determined.

  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 materials (steel No. A, B, C, E, F, G, H, I, J, K, L, N, O, P, Q, R, S, T, U) As A, heating, hot rolling, and accelerated cooling after rolling were performed under the conditions shown in Table 2 to obtain a steel sheet 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)
(Where S: span (= 4W), P: load, W: specimen thickness in the crack propagation direction, B: specimen thickness in the width direction T (= 2W), a: notch included Crack length)
Was used. F ₁ (ζ) is a shape factor when a / W = ζ.
F ₁ (ζ) = {1.99−ζ (1−ζ) (2.15−3.93ζ + 2.7ζ ² )} / {√π (1 + 2ζ) (1−ζ) ^3/2 }
Calculated using

All fatigue crack propagation tests were conducted in the atmosphere at room temperature under the conditions of stress ratio R: 0.1, frequency: 20Hz, stress expansion range ΔK _I : 10MPa√m, and with constant load ΔK _I gradually increasing 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 when the fatigue crack propagation rate was ½ or less of the reference value within the same stress intensity factor range, a steel plate having excellent fatigue crack propagation resistance was obtained. Specifically, the fatigue crack propagation rate is 1/2 or less of the 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, ΔK _{When I} = 20 MPa√m, 4.26 × 10 ^-8 (m / cycle) or less, and ΔK _I = 25 MPa √m, 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 1/4 of the thickness of the steel plate, the observation surface was polished, corroded with a 2% nital corrosive solution, the microstructure was revealed, and an optical microscope (magnification: 100- 400 times) or 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. -Z is a cross section in the rolling direction. All of the observation planes were xy planes (z planes) shown in FIG. 2 in relation to cracks.

Using the photographed tissue photograph, tissue identification and tissue fraction of each phase were measured using commercially available image analysis software. In addition, with reference to JIS G 0551 (2013), by the cutting method, the structural unit (D _P ) for each phase in each visual field in the direction of crack propagation was obtained, and the average value thereof was defined as (D _P ) for each phase. . In the bainite phase, the packet size is set. In the martensite phase, the packet size is set. In the ferrite phase, the ferrite grains are set. Were each organizational unit (D _P ).
(3) Tensile test Full-thickness tensile test piece (No. 5 tensile test piece so that the tensile direction is the rolling direction L or the direction T perpendicular to the rolling direction, with reference to the Japan Maritime Association Steel Ship Rules. ) Were collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength (0.2% yield strength) YS, tensile strength TS, elongation El).
(4) Impact test From the obtained steel plate, in accordance with the provisions of JIS Z 2242, 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. Impact test specimens are taken so that the length direction is the direction T perpendicular to the rolling direction and the crack propagation direction is the rolling direction L. In accordance with the provisions of JIS Z 2242, three test specimens at each test temperature. The impact test was carried out at 5 levels or more, the fracture surface transition temperature vTrs (° C.) was determined, and the low temperature toughness was evaluated. The impact test piece was a V-notch test piece centered on the plate thickness 1/2 position when the plate thickness was less than 20 mm, and centered on the plate thickness 1/4 position when the plate thickness was 20 mm or more.
(5) Hardness test The specimen for structure observation used in (1) is a hardness measurement test piece, and for each phase, a load is applied so that the distance between grain boundaries and phase boundaries is at least four times the indentation. The Vickers hardness HV was measured after adjustment. 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 hardness measurement surface was the xy plane shown in FIG. 2 in relation to the crack.
(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 ×.

  Among the obtained results, the results of the structure observation, tensile properties, toughness, and weldability are shown in Table 3. In addition, a structure | tissue shows the observation result in the board thickness 1/4 position which is a typical structure | tissue, z plane.

Further, the effective structural unit MU _eff in the composite structure is obtained by using the structural observation result and the Vickers hardness measurement result. In each case, the calculation was made using equation (2b). If the ferrite phase exceeds 0.5 in area ratio (50% in area%), the following equation using the composite rule is used as in the case where the bainite phase or martensite phase is the main phase.
_{MU eff = (AR) α ×} (D P) α + (AR) B × (D P) B × {(Hv) B / (Hv) α} + (AR) P × (D P) P × {( _{Hv) P / (Hv) α} } + (AR) M × (D P) M × {(Hv) M / (Hv) α}
Calculated by

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 inventive examples has a structure in which the total area ratio of the bainite phase and the martensite phase exceeds 50%, yield strength: high strength of 325 MPa or more, and excellent fracture surface transition temperature vTrs: −20 ° C. or less. In addition, it has excellent low temperature toughness and has excellent weldability in the y-type weld cracking test, with no weld cracking, and at least ΔK _I = 15 MPa√m, 20 MPa√m, γ _p ^* is 200 or less, MU _eff is 100 or less, and γ _p ^* / MU _eff is 10 or less, and the fatigue crack propagation rate da / dN is at least ΔK _I = 15 MPa√m, which is the target value of 1.75 × 10 ^-8 (m / cycle) and below, ΔK _I = 20 MPa√m and the target value is reduced to 4.26 × 10 ^-8 (m / cycle) or less, resulting in a high strength steel material with improved fatigue crack propagation resistance.

On the other hand, in the comparative example that is out of the scope of the present invention, any of strength, toughness, and weldability is reduced, or γ _P * / MU _eff exceeds 10, and fatigue crack propagation resistance is reduced. .
The No. 17 steel sheet in which C, Si, and Mn exceed the scope of the present invention has low ductility and low temperature toughness, and is inferior in weldability. The No. 18 steel sheet in which P and S exceed the range of the present invention has low ductility and low temperature toughness. In addition, No. 19 in which C and Mn are below the range of the present invention does not generate bainite and martensite, and has a single phase structure of ferrite, has low yield strength YS, and low temperature toughness. Also, since yield strength YS is low, ΔK _I = 15 MPa√m or more, γ _p ^* exceeds 200, γ _p ^* / MU _eff also exceeds 10, and ΔK _I = 15 MPa√m or more, fatigue crack The propagation speed da / dN exceeds the target value, and the fatigue crack propagation resistance is degraded. In addition, the No. 20 steel sheet with a high heating temperature of 900 ° C or higher and a cumulative rolling reduction of less than the preferred range of the present invention, the prior austenite grain size is coarsened, the MU _eff exceeds 100, the yield strength YS is low, and the low temperature Toughness is also reduced. In addition, the No. 21 steel plate with a low heating temperature and a cumulative rolling reduction of 900 ° C. or more below the preferred range of the present invention has a coarse austenite grain size, MU _eff exceeds 100, and low temperature toughness decreases. Yes. In addition, the No. 22 steel plate whose cooling start temperature is lower than the Ar3 transformation point, the No. 23 steel plate whose cooling rate of accelerated cooling is lower than the RS and slow cooling, and the No. 24 steel plate whose cooling stop temperature exceeds 600 ° C are In any case, the total area ratio of the bainite phase and the martensite phase is 50% or less, and the yield strength YS is low. Therefore, ΔK _I = 15 MPa√m or more for No. 22 steel plate and No. 24 steel plate. In the steel plate of 23, ΔK _I = 20 MPa√m or more, and γ _p ^* / MU _eff exceeds 10, respectively, and the fatigue crack propagation resistance is deteriorated. In addition, the No. 25 steel sheet having a tempering temperature exceeding the Ac1 transformation point has low temperature toughness due to the formation of a large amount of island martensite.
(Example 2)
The obtained steel material (steel No. C, D, E, M, R) was heated, hot-rolled and cooled after rolling under the conditions shown in Table 6 as production method B, and a steel plate having a thickness of 25 to 100 mm was obtained. After that, reheating was performed under the conditions shown in Table 6, followed by heat treatment for cooling at various cooling rates and cooling stop temperatures. 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 7 shows the results of the structure observation, 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 ^* , γ _p ^* / MU _eff and the measurement results of the fatigue crack propagation rate.

Each of the inventive examples has a structure in which the total area ratio of the bainite phase and the martensite phase exceeds 50%, yield strength: high strength of 325 MPa or more, and excellent fracture surface transition temperature vTrs: −20 ° C. or less. In addition, it has excellent low temperature toughness and has excellent weldability in the y-type weld cracking test, with no weld cracking, and at least ΔK _I = 15 MPa√m, 20 MPa√m, γ _p ^* is 200 or less, MU _eff is 100 or less, and γ _p ^* / MU _eff is 10 or less, and the fatigue crack propagation rate da / dN is at least ΔK _I = 15 MPa√m, which is the target value of 1.75 × 10 ^-8 (m / cycle) and below, ΔK _I = 20 MPa√m and the target value is reduced to 4.26 × 10 ^-8 (m / cycle) or less, resulting in a high strength steel material with improved fatigue crack propagation resistance. 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. .
No.B31 steel sheet with reheating temperature less than Ac3 transformation point, No.B32 steel sheet with cooling rate after reheating below RS lower limit, No.B33 steel sheet with cooling stop temperature exceeding 600 ° C Since the total area ratio of the bainite phase and the martensite phase is 50% or less and the structure is mainly composed of a ferrite phase, the yield strength YS is low. Therefore, No. B32 steel plate and No. B33 steel plate have ΔK _I = 15 MPa√m or more, and No. B31 steel plate has ΔK _I = 20 MPa√m and γ _p ^* / MU _eff exceeds 10, Fatigue crack propagation characteristics are degraded. Further, the No. B34 steel sheet having a tempering temperature exceeding the Ac1 transformation point has a low temperature toughness because a large amount of island-like martensite is generated.
(Example 3)
A high strength steel plate (thickness: 12 to 100 mm) having the composition shown in Table 10 and subjected to normal hot rolling, cooling after rolling, heat treatment, etc. and having the structure, strength, and toughness shown in Table 11 The fatigue crack propagation characteristics were judged.

  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 structural unit (DP) in the development direction and the average Vickers hardness (HV) of each phase were determined. The results obtained are shown in Table 12. The fatigue crack propagation direction was only the width direction (LT test piece). The structure observation was performed on the xy plane shown in FIG. 2 in correspondence with the crack propagation direction of the test piece.

Using the obtained structure observation result and the Vickers hardness measurement result, the effective structure unit MU _eff was calculated by the expression (2a) or (2b). Further, the crack tip plastic zone size γ _p ^* (μm) defined by the equation (1) was calculated. The obtained results are shown in Table 13. In calculating γ _p ^* using equation (1), K _Imax was calculated using ΔK _I = 15 MPa√m and ΔK _I = 20 MPa√m, and R was 0.1.

In the present invention, when at least ΔK _I = 15 MPa√m and ΔK _I = 20 MPa√m, if γ _P * / MU _eff is 10 or less, it is determined that the steel plate has excellent fatigue crack propagation resistance. Other than that, it was determined that the fatigue crack propagation resistance was inferior.
This determination is confirmed by separately conducting a fatigue crack propagation test. In the fatigue crack propagation test, CT specimens (LT) were collected 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. When the steel plate thickness is 25 mm or less, the test piece thickness is the total thickness of the steel plate. did. 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 obtained fatigue crack propagation rates are also shown in Table 13.

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 higher than the above target value.

Claims (11)

% 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,
It has a structure composed of a bainite phase and / or a martensite phase exceeding 50% in total area ratio at the 1/4 position of the plate thickness, and the remainder (including 0%), and yield strength: It has a high strength of 325 MPa or more, an excellent low temperature toughness with a fracture surface transition temperature vTrs of −20 ° C. or less in the Charpy impact test, and the crack tip plastic zone dimension γ _p ^* defined by the following formula (1) The effective structural unit MU _eff (μm) in the crack growth direction defined by (μm) and the following equation (2a) or the following equation (2b) satisfies the following equations (3) to (5) , and fatigue cracks velocity da / dN is at least [Delta] K _I: characterized in that ^{4.26 × 10 -8 (m / cycle} ) at 20MPa√m or ^{_{less: 1.75 × 10 -8 (m /}} cycle) at 15MPa√m below, [Delta] K _I High-strength steel with excellent fatigue crack propagation characteristics.
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) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _P ) _{B is} the average value of the packet size in the crack growth direction, martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack growth direction, ferrite structure unit (D _P ) _{α is the} crack growth The average value of the ferrite grain size in the direction, the pearlite structural unit (D _P ) _{P is} the average value of the bulk pearlite in the crack propagation direction or the thickness of the layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), average Vickers hardness of each phase
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 200 (4)
MU _eff ≦ 100 (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. It is a manufacturing method of the high-strength steel material in any one of Claims 1 thru | or 3, Comprising: On steel material, when hot-rolling and making it steel material with a plate thickness of 100 mm or less,
The steel material is 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 steel material having a composition consisting of the remainder Fe and inevitable impurities,
The hot rolling is performed at a heating temperature of 950 to 1300 ° C., then at a cumulative rolling reduction in a temperature range of 900 ° C. or higher: 50% or higher, and a rolling finish temperature: Ar 3 transformation point or higher,
After the hot rolling, from the temperature range above the Ar3 transformation point, the relationship between φ (%) defined by the following formula (7) and the sheet thickness t (mm) is defined by the following formula (6). at a cooling rate RS (℃ / s) or more cooling rate, have rows accelerated cooling to a cooling stop temperature of 600 ° C. or less,
The steel material has a structure composed of a bainite phase and / or a martensite phase with a total area ratio exceeding 50% at a 1/4 position of the plate thickness, and the balance other than that (including 0%), and Yield strength: High strength of 325 MPa or more, excellent low temperature toughness with fracture surface transition temperature vTrs of Charpy impact test of −20 ° C. or less, and fatigue crack propagation rate da / dN is at least ΔK _I : Low temperature toughness characterized by a steel material of 1.75 × 10 ⁻⁸ (m / cycle) or less at 15 MPa√m and ΔK _I : 4.26 × 10 ⁻⁸ (m / cycle) or less at 20 MPa√m And high-strength steel manufacturing method with excellent fatigue crack resistance.
Record
RS = (− 0.53φ ² −0.28φ + 0.67) × e ^9.10 / t ^1.63 (6)
Where φ = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + Nb / 10 + 5B (7),
t: steel plate thickness (mm),
C, Si, Mn, Cu, Ni, Cr, Mo, V, Nb, B: Content of each element (mass%)   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 manufacturing 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. A manufacturing method for high strength steel.   Instead of the accelerated cooling, after the hot rolling is finished, it is cooled by any one of furnace cooling, air cooling, and water cooling, and further reheated to a temperature higher than the Ac3 transformation point, and then defined by the above equation (6). The method for producing a high-strength steel material according to any one of claims 4 to 6, wherein a quenching treatment is performed at a cooling rate of not less than RS (° C / s) and cooling to 600 ° C or less. The method for producing a high-strength steel material according to any one of claims 4 to 7, wherein after the accelerated cooling or after the quenching treatment, a tempering treatment is further performed at a temperature below the Ac1 transformation point . % 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
Including the balance Fe and inevitable impurities,
It has a structure consisting of a bainite phase and / or martensite phase exceeding 50% in total area ratio at the 1/4 position of the plate thickness, and the remainder (including 0%), and yield strength: 325 MPa For the above high-strength steel materials,
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, The average Vickers hardness (HV) of each phase is obtained, and the crack tip plastic zone dimension γ _p ^* (μm) defined by the following formula (1), the following (2a) formula, or the following (2b) formula is defined. When the effective structural unit MU _eff (μm) in the crack growth direction is calculated and the following equations (3) to (5) are satisfied, the fatigue crack propagation rate da / dN is at least ΔK _I : 15 MPa√m 1.75 × 10 ⁻⁸ (m / cycle) or less, ΔK _I : 20 MPa√m or less, and 4.26 × 10 ⁻⁸ (m / cycle) or less. Of high strength steel with excellent fatigue crack propagation characteristics.
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) B ×} (D P) B + (AR) M × (D P) M × {(Hv) M / (Hv) B} + (AR) α × (D P) α × {( Hv) _α / (Hv) _B } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _B } (2a)
MU _eff = (AR) _M × (D _P ) _M + (AR) _B × (D _P ) _B × {(Hv) _B / (Hv) _M } + (AR) _α × (D _P ) _α × {( Hv) _α / (Hv) _M } + (AR) _P × (D _P ) _P × {(Hv) _P / (Hv) _M } (2b)
Here, (AR) _B , (AR) _M , (AR) _α , (AR) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), area ratio of each phase ( 0-1),
(D _P ) _B , (D _P ) _M , (D _P ) _α , (D _P ) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), crack of each phase The structural unit (μm) in the propagation direction , where “the structural unit in the crack propagation direction of each phase” is a structural unit closely related to the bending of the crack. For convenience, the bainite structural unit (D _{In P} ) _B , the average value of the packet size in the crack propagation direction, the martensite structure unit (D _P ) _{M is} the average value of the packet size in the crack growth direction, and the ferrite structure unit (D _P ) _{α is the} crack growth The average value of the ferrite grain size in the direction, the pearlite structural unit (D _P ) _{P is} the average value of the bulk pearlite in the crack propagation direction or the thickness of the layered pearlite in the crack growth direction,
(Hv) _B , (Hv) _M , (Hv) _α , (Hv) _P : Bainite (B), martensite (M), ferrite (α), pearlite (P), average Vickers hardness of each phase
γ _P * / MU _eff ≤ 10 (3)
γ _P * ≦ 200 (4)
MU _eff ≦ 100 (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 9 characterized by the above-mentioned. The high content according to claim 9 or 10 , 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