JP2007197776A - High-strength steel material superior in delayed fracture resistance and fatigue-crack propagation resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel material for a construction, which is used in various welded structures and is superior in fatigue-crack propagation resistance and delayed fracture resistance even when having repeatedly received a load, and to provide a manufacturing method therefor. <P>SOLUTION: This steel material comprises, by mass%, 0.05-0.35% C, 0.01-0.50% Si, 0.5-2.0% Mn, 0.05% or less P, 0.02% or less S, one or more of 1.0% or less Cu, 2.0% or less Ni, 1.0% or less Cr, 1.0% or less Mo, 0.1% or less Nb, 0.1% or less V, 0.1% or less Ti and 0.005% or less B, as needed, and the balance Fe with unavoidable impurities; and has a metallographic structure that is a bainitic structure, a martensitic structure or a structure mixed thereof, which includes carbides, nitrides or carbonitrides with a maximum size of 0.5 μm or smaller in lath, and that includes ferrite with a grain size of 4 μm to 50 μm in an amount of 3% to 20% by an area fraction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a high-strength steel material having a tensile strength of 700 MPa or more that has excellent fatigue crack propagation resistance and delayed fracture resistance, and a manufacturing method thereof, such as ships, marine structures, bridges, construction machines, buildings, tanks, etc. The present invention relates to a structural steel material excellent in delayed fracture resistance and having a tensile strength of 700 MPa or more, which is suitable for various welded structures and exhibits good fatigue crack resistance when subjected to repeated loads, and a method for producing the same.

  In recent years, high strength steel materials with tensile strength have been applied to structures such as ships, offshore structures, bridges, construction machinery, buildings, tanks, etc. for the purpose of streamlining design, reducing the weight of steel materials, and reducing the thickness and labor of welding. There are an increasing number of cases.

  High-strength steel materials having a tensile strength of 700 MPa or more used for them are required to have excellent toughness and weldability and to have excellent fatigue resistance and delayed fracture resistance in order to ensure structural safety.

  In welded structures, fatigue fracture often occurs when a fatigue crack is generated from the weld toe and propagates through the steel material. This is attributed to the fact that the weld toe portion tends to be a stress concentration portion due to its shape, and in addition, tensile residual stress is generated after welding.

  For this reason, as a means to suppress the generation of cracks 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. Widely known.

  However, it is almost impossible to apply such treatment to a large number of weld toes on an industrial scale, and it is not practical in terms of cost.

  Therefore, even if a fatigue crack occurs, it is important to extend the fatigue life by reducing the propagation speed in the steel material after that. There is a strong demand.

  As a technique for improving the fatigue crack propagation characteristics of high-strength steel materials, for example, Patent Document 1 discloses that the structure is mainly composed of one or more of ferrite, pearlite, and bainite, and has an average existence interval of 20 μm or less. A steel sheet excellent in fatigue crack propagation resistance in which island-shaped martensite having an aspect ratio of 5 or more is present at a ratio of 0.5 to 5% and a method for producing the same are described.

  Patent Document 2 is mainly composed of bainite / martensite, and the lath-like structure has a minimum short side length of 1.3 μm or less, and when bainite is included, the island-like martensite having an aspect ratio of 5 or more in the structure. Steel material containing less than 5% by area ratio and a method for producing the same are disclosed.

In Patent Document 3, after hot rolling, the steel sheet is water-cooled to 500 ° C. or less, and then heated and rolled in a two-phase region of Ac ₁ point + 50 ° C. to Ac ₃ points−10 ° C. A technique for improving fatigue strength by using a mixed structure of bainite or martensite is described.
JP-A-6-271985 JP 2003-342672 A Japanese Patent Laid-Open No. 10-168542

  However, various problems are pointed out in the above-described technology. For example, in the case of Patent Document 1, since a large amount of island martensite is included, brittle fracture is likely to occur, and delayed fracture resistance is improved. There is concern about inferiority.

  In the case of Patent Document 2, since the steel material is basically only a hard layer (bainite / martensite) not containing ferrite, the fatigue crack propagation rate is inferior to that of a steel material having a soft phase / hard layer and tempering treatment is performed. If not, there is a concern that delayed destruction will occur.

  Moreover, in the case of patent document 3, since the manufacturing process reheated and rolled after cooling is complicated, a manufacturing cost rises and production efficiency falls.

  Accordingly, the present invention solves such problems of the prior art, and provides a tough steel material having excellent fatigue crack propagation characteristics and delayed fracture resistance, and a technique for manufacturing with high efficiency. Objective.

  The present inventors have repeated experiments and studies to solve the above problems. As a result, excellent fatigue crack propagation is achieved by dispersing ferrite in bainite or martensite with fine carbides in the lath or mixed structure of them, and keeping the ferrite grain size and area ratio within a specified range. It was found that a steel material exhibiting properties and excellent delayed fracture resistance and toughness can be obtained.

It has also been found that a steel material having such a structure can be produced with high efficiency by controlling each process of rolling, controlled cooling, and heat treatment with high precision and by forming each process continuously. The present invention was made by further study based on the obtained knowledge,
1. In mass%, C: 0.05 to 0.35%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02 % Or less, the balance Fe and inevitable impurities steel, the metallographic structure is bainite or martensite or a mixed structure in which lath carbide or nitride or carbonitride having a maximum size of 0.5 μm or less exists, Tensile strength with excellent delayed fracture resistance and fatigue crack resistance, characterized in that ferrite having a particle size of 50 μm or less and 4 μm or more is present in the metal structure at an area fraction of 3% or more and 20% or less. High-strength steel material of 700 MPa or more.
2. Furthermore, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1 in mass% %, Ti: 0.1% or less, B: 0.005% or less, containing 1 type or two or more types, with excellent delayed fracture resistance and fatigue crack propagation characteristics according to 1. High strength steel.
The steel having the components described in 3.1 or 2 is heated to 1000 ° C. or higher and 1300 ° C. or lower, rolled at an Ar ₃ point or higher and a cumulative reduction ratio of 50% or higher, and from Ar ₃ point to Ar ₃ -200 ° C. After cooling the temperature range to 3 s or more and 200 s or less at a cooling rate of less than 4 ° C./s, directly quench at a cooling rate of 5 ° C./s or less to the Ms point or less, and then increase the temperature at a rate of 1 ° C./s or more. _A method for producing a high-strength steel material having a tensile strength of 700 MPa or more and excellent in delayed fracture resistance and fatigue crack propagation characteristics, characterized by reheating to less than _one point.

  According to the present invention, it is possible to produce a steel material having improved fatigue crack propagation characteristics and having excellent delayed fracture resistance and toughness without requiring a special process or addition of a large amount of alloying elements. It is possible to provide a steel material that can increase the safety margin of fatigue failure as well as delayed fracture and brittle fracture for main members of welded structures such as ships, bridges, and buildings.

  Further, since it can be manufactured by a series of thermomechanical processing combined with rolling, cooling, and reheating, it is possible to provide a steel material having excellent characteristics as described above at a short delivery time and at a low cost, which is extremely useful industrially.

The steel material according to the present invention defines the chemical composition and the metal structure.
[Chemical component] In the following description, all percentages are by mass.
C: 0.05 to 0.35%
C needs to be added in an amount of 0.05% or more to ensure strength. However, addition exceeding 0.35% inhibits weldability. Therefore, it is limited to 0.05% or more and 0.35% or less. Preferably, the content is 0.05% or more and 0.30% or less.

Si: 0.01 to 0.50%
Si is effective as a deoxidizer and needs to be 0.01% or more for increasing the strength, but if added over 0.50%, weldability and toughness are deteriorated. Therefore, it is limited to 0.01% or more and 0.50% or less. Preferably, the content is 0.05% or more and 0.40% or less.

Mn: 0.5 to 2.0%
Mn is required not only to increase the strength at a low cost by increasing hardenability but also from the viewpoint of improving toughness, but 0.5% or more is necessary, but if it exceeds 2.0%, the weldability deteriorates. Therefore, it is limited to 0.5% or more and 2.0% or less. Preferably, the content is 0.5% or more and 1.8% or less.

P: 0.05% or less Since P deteriorates the toughness of steel, its content is desirably as low as possible. For this reason, the upper limit was made 0.05%. Preferably it is 0.03% or less.

S: 0.02% or less It is desirable to reduce S as much as possible because S decreases the toughness of steel when added in a large amount. For this reason, the upper limit was made 0.02%. Preferably, the content is 0.01% or less.

  Although the above is a basic component of the present invention, one or more of Cu, Ni, Cr, Mo, Nb, V, Ti, B for the purpose of adjusting strength, toughness, weldability, etc., and imparting weather resistance, etc. May be added.

Cu: 1.0% or less Cu brings about an effect of increasing strength by solid solution and improves weather resistance. However, if the content exceeds 1.0%, weldability is impaired and flaws are likely to occur during the manufacture of the steel material. Therefore, when added, the upper limit is made 1.0%. Preferably they are 0.05% or more and 0.5% or less.

Ni: 2.0% or less Ni is effective in improving low temperature toughness and improving weather resistance and hot brittleness that occurs when Cu is added. However, if the added amount exceeds 2.0%, weldability is impaired and the cost increases. Therefore, when added, the upper limit is made 2.0%. Preferably they are 0.05% or more and 1.0% or less.

Cr: 1.0% or less Cr improves weather resistance and strength. However, if its content exceeds 1.0%, weldability and toughness are impaired. Therefore, when added, the upper limit is made 1.0%. Preferably they are 0.05% or more and 0.5% or less.

Mo: 1.0% or less Mo increases the strength. However, if its content exceeds 1.0%, weldability and toughness deteriorate. Therefore, when added, the upper limit is made 1.0%. Preferably they are 0.05% or more and 0.5% or less.

Nb: 0.1% or less Nb has the function of suppressing austenite recrystallization during rolling to achieve finer grains, and at the same time, increasing the strength through precipitation. However, when 0.1% or more is added, toughness deteriorates. Therefore, when adding, it is limited to 0.1% or less. Preferably, the content is 0.01% or more and 0.05% or less.

V: 0.1% or less V, like Nb, has a function of increasing strength by precipitation. However, addition exceeding 0.1% causes a decrease in weldability and toughness. Therefore, when adding, it is limited to 0.1% or less. Preferably, the content is 0.01% or more and 0.07% or less.

Ti: 0.1% or less Ti improves strength and weld zone toughness. However, when the content exceeds 0.1%, the cost tends to increase. Therefore, when added, the upper limit is made 0.1%. Preferably, the content is 0.01% or more and 0.05% or less.

B: 0.005% or less B contributes to an increase in hardenability and strength. However, if added over 0.005%, the weldability is impaired. Therefore, the upper limit is made 0.005%. Preferably, the content is 0.0005% or more and 0.003% or less.

[Metal structure]
The high-strength steel material excellent in fatigue crack propagation characteristics and delayed fracture resistance according to the present invention has a metal structure of bainite, martensite, or a mixed structure thereof, and the particle size in the metal structure is 50 μm or less and 4 μm or more. The ferrite is present in an area fraction of 3% or more and 20% or less, and in the lath carbide or nitride or carbonitride having a maximum size of 0.5 μm or less.

Higher strength is achieved by using ferrite bainite or martensite or a mixed structure of ferrite or bainite or martensite in an area fraction of 3% or more and 20% or less: 50 μm or less and 4 μm or more in bainite or martensite or a mixed structure thereof. In addition, by generating ferrite in these structures, the fatigue crack propagation rate shows a local decrease at the interface with the ferrite, and the fatigue crack propagation resistance is improved.

  And a fatigue crack propagation characteristic improves remarkably by making a ferrite grain size and an area ratio into 50 micrometers or less 4 micrometers or more and 3% or more and 20% or less, respectively. The ferrite area ratio is more preferably 3% or more and 10% or less.

In-lath carbide or nitride or carbonitride with a maximum size of 0.5 μm or less Produces fine carbide or nitride or carbonitride with a maximum size of 0.5 μm or less in bainite or martensite or their mixed structure As a result, delayed fracture resistance and toughness are improved while maintaining a high fatigue crack propagation rate. When no carbides, nitrides, or carbonitrides are present in the lath, or when they exceed the maximum size of 0.5 μm, delayed fracture resistance and toughness deteriorate. The carbide, nitride, or carbonitride size refers to the total length in the major axis direction when an elliptical shape is assumed.

  Fig. 1 shows the grain size and area ratio of ferrite in a ferrite and bainite or martensite or a mixed structure thereof in which a carbide, nitride, or carbonitride in a lath having a maximum size of 0.5 µm or less exists. The results of investigating fatigue crack propagation characteristics and delayed fracture properties of steel materials having a tensile strength of 700 MPa or more by changing the above are shown. The test material manufactured the steel of the component shown by A of Table 1 mentioned later on various conditions.

  As shown in FIG. 1, in order to obtain a steel having high strength and excellent delayed fracture resistance and fatigue crack propagation characteristics, ferrite having a grain size of 50 μm or less and 4 μm or more has an area fraction of the ferrite of 3% or more and 20% or more. % Or less.

  The reason why there is a unique ferrite grain size and area ratio range that improves delayed fracture resistance and fatigue crack propagation characteristics in high-strength steel is that the ferrite grain size and area ratio are less than 4 μm and less than 3%, respectively. In addition, since the hard phase with high delayed fracture susceptibility increases, the delayed fracture property is inferior. Furthermore, since fatigue cracks propagate preferentially in the hard phase, a propagation speed delay effect at the ferrite interface can be expected. On the other hand, if the ferrite grain size and area ratio exceed 50 μm and 20%, respectively, high strength cannot be obtained, and fatigue cracks are preferentially propagated in the ferrite, resulting in a propagation speed delay at the bainite interface. This is probably because the effect cannot be expected.

  In the present invention, a small amount of other structures are inevitably mixed in the metal structure described above.

Steel steel having the above components of the present invention, heating to 1000 ° C. or higher 1300 ° C. or less, perform rolling cumulative reduction of 50% or more Ar ₃ point or more, the temperature range of _Ar 3 -200 ° C. from Ar ₃ point After cooling at a cooling rate of less than 4 ° C./s for 3 s or more and 200 s or less, directly quenching at a cooling rate of 5 ° C./s or more to the Ms point or less, and then at a temperature rising rate of 1 ° C./s or more to less than Ac ₁ point Obtained by reheating. The temperature is the steel surface temperature, and the cooling rate is the average value in the thickness direction of the steel.

Heating temperature: 1000 ° C. or higher and 1300 ° C. or lower If the heating temperature is lower than 1000 ° C., the subsequent rolling temperature cannot be secured. On the other hand, if the temperature exceeds 1300 ° C., the crystal grains of the steel become coarse, and it becomes difficult to ensure toughness.

Rolling at an Ar ₃ point or higher and a cumulative reduction ratio of 50% or higher at an Ar ₃ point or higher to refine the prior austenite grains to improve toughness, and ferrite in the subsequent cooling process Promote generation. In rolling with less than Ar ₃ points, ferrite is excessively generated, and if the cumulative rolling reduction is less than 50%, refinement of prior austenite grains cannot be achieved.

As a means of generating a ferrite 3s than 200s in less cooling cooling below 4 ° C. / s at a cooling rate in the temperature range of _Ar 3 -200 ° C. from Ar _{3 point,} cooled at the temperature range of Ar ₃ to Ar 3 -200 ° C. A step of a rate of less than 4 ° C./s is provided for 3 seconds or more. Temperature does not produce ferrite, if below or when Ar ₃ point -200 ° C. greater than ₃ points Ar.

  Also in that temperature range, if the cooling rate is less than 4 ° C./s and the cooling time is not more than 3 s, sufficient ferrite content (area fraction) to improve fatigue crack propagation resistance and delayed fracture resistance 3% or more) is not obtained.

  On the other hand, when the ferrite generation time exceeds 200 s, fatigue characteristics and strength are reduced due to excessive ferrite generation exceeding the area fraction of 20% as the production efficiency decreases. More preferably, the cooling time is 3 seconds or more and 100 seconds or less.

After the above cooling process, it is directly quenched at a cooling rate of 5 ° C./s or higher to the Ms point or lower, and then reheated to a temperature less than ₁ point at a temperature rising rate of 1 ° C./s or higher. Before the transformation is completed, the remaining untransformed austenite is bainite, martensite, or a mixed structure thereof, and is defined to produce fine carbides, nitrides, or carbonitrides in the laths.

  When the cooling rate is less than 5 ° C./s, ferrite and pearlite are generated. Further, the strength can be increased by cooling to the Ms point or lower and using untransformed austenite as bainite, martensite, or a mixed structure thereof.

Furthermore, fine carbide, nitride, or carbonitride can be generated in the lath by rapidly reheating to less than Ac ₁ point at a temperature rising rate of 1 ° C./s or more, Delayed fracture resistance and toughness are improved while having high fatigue crack propagation characteristics.

When the rate of temperature rise is less than 1 ° C / s, carbides, nitrides, or carbonitrides become coarse, and when reheated to _one or more points of Ac, island-like martensite is generated, thereby toughness and delay resistance respectively. Destructibility is reduced.

The temperature rising rate is more preferably 3 ° C./s or more. Ar ₃ point, Ms point, and Ac ₁ point are, for example, Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, Ms (° C.) = 517-300C-33Mn-22Cr-17Ni— 11Mo-11Si, Ac ₁ (° C.) = 723-14Mn + 22Si-14.4Ni + 23.3Cr (where the element symbol represents the content in mass% of each element in the steel) It can be derived based on the composition.

The series of cooling as described above is performed, for example, after the rolling is completed. 1. Water cooling → Slow cooling → Water cooling 2. water cooling → cooling → water cooling; 3. Slow cooling → water cooling It may be performed by a process such as cooling to water cooling, and ferrite is introduced by providing a slow cooling and cooling period between Ar ₃ points to Ar ₃ -200 ° C., and then water cooling is performed, and the remainder is bainite and martensite. It may be a site or a mixed organization of them.

  In FIG. 2, a steel material having a thickness of 25 mm is cooled from 800 ° C. by the above method 2 and provided with a cooling period near 670 ° C., then cooled again to 40 ° C., immediately reheated to 450 ° C., and again The average temperature and time within the plate thickness when allowed to cool are shown in (a), and the schematic diagram of the structure obtained at that time is shown in (b) (the test material was a steel plate of Example No. 1 described later). . The desired structure is obtained in a series of steps and in a very short time (less than 50 seconds from the start of cooling to the end of reheating).

  In addition, when accelerated cooling and reheating are most efficiently performed as shown in FIG. 2 in the present manufacturing method, the accelerated cooling device and the heating device are preferably laid out on the same line.

  In addition, when the distance between the heating device and the acceleration cooling device is long, the normal off-line Temper may be applied in terms of characteristics. Any heating method such as induction heating or atmosphere heating may be used as long as the rate of temperature increase is achieved.

  In addition, reheating may or may not be performed at the target temperature within the range where desired strength and toughness can be obtained, and after reheating, any of furnace cooling / cooling / quenching is selected. But it is not particularly specified.

  Steel plates having a thickness of 12 to 100 mm were manufactured from the steel pieces having the compositions shown in Table 1 under the conditions shown in Table 2. The obtained steel sheet was examined for structure observation, tensile strength, toughness, delayed fracture resistance, and fatigue crack propagation rate by the following procedure.

  For the tissue observation, a sample taken from the position of plate thickness / 4 was polished. Regarding the microstructure observation, the structure, the area fraction of ferrite, and the particle size of ferrite were measured by optical microscope observation on the surface etched with 2% nital etchant.

  Regarding the ferrite area fraction and the ferrite particle size, five visual fields were measured for one sample, and the average values were obtained. In addition, a thin film was sampled by electrolytic polishing from the same sampling position, and the largest carbide, nitride, or carbonitride size in the lath was measured by TEM observation.

  The strength was evaluated by a full thickness test piece of JIS Z2201 1A collected in a direction perpendicular to the rolling direction (JIS Z2201 No. 4 round bar test piece at a thickness of 4 mm for a plate thickness of 50 mm or more).

  Toughness was evaluated by a V-notch Charpy impact test piece of JIS Z 2202 taken in a direction parallel to the rolling direction at a plate thickness / 4 position (plate thickness / 2 position is less than 25 mm).

  Delayed fracture resistance is evaluated by a cantilever type constant load delayed fracture test in which a steel sheet is immersed in a 3.5% NaCl aqueous solution and a constant load is applied. Stress intensity factor). In addition, the measurement of the rupture time at this time was 1000 hours as the longest.

  Fatigue crack propagation rate was obtained by taking a CT test of the full thickness in the LT direction where cracks propagate in the direction perpendicular to the rolling direction (thickness exceeding 25 mm is reduced on both sides to 25 mmt), stress ratio 0.1, frequency The test was carried out in accordance with ASTM E647 in the atmosphere of 20 Hz and room temperature.

In the present invention, the strength is 700 MPa or more in TS, the toughness is a ductile / brittle fracture surface transition temperature in a Charpy impact test of −30 ° C. or less, the delayed fracture resistance is a delayed fracture occurrence stress intensity factor of 980 N / mm ^3/2 or more. As for the fatigue crack propagation characteristics, the case where the fatigue crack propagation rate when ΔK = 15 MPa√m in the atmosphere was 2.0 × 10 ⁻⁸ m / cycle or less was regarded as acceptable.

  Table 3 shows the test results. The components and production methods defined in the present invention were adopted, and No. 1 having the microstructure defined in the present invention. 1-No. It can be seen that the steel plate No. 8 has excellent fatigue crack propagation characteristics, and also has high strength, toughness, and delayed fracture resistance.

On the other hand, P and S are No. exceeding the scope of the present invention. Steel plate No. 9 is inferior in toughness as a manufacturing method and structure prescribed in the present invention. Moreover, No. _{3 in which} the cumulative rolling reduction of Ar ₃ points or more does not reach the lower limit of the present invention. Steel 10 _were not provided Ar ₃ to Ar 3 -200 at a temperature in the range of ° C. 4 ° C. / s of less than cool and the slow cooling step or 3s No. 11, no. No. 12 steel has no ferrite introduced into the hard phase, is inferior in fatigue crack resistance, and No. 10 steel sheet has toughness and delayed fracture resistance, 11 steel plate is also inferior in delayed fracture resistance compared to the steel of the present invention.

  No. No water cooling. Steel plate No. 13 has a ferrite-pearlite structure, and the area ratio and grain size of ferrite exceed the upper limit of the present invention. For this reason, it is low strength and inferior toughness and fatigue crack propagation characteristics. No reheating was performed after cooling to the Ms point or lower. No. 14 steel plate has no carbides in the lath, has low toughness, and is inferior in delayed fracture resistance.

No. in which reheating temperature exceeds Ac ₁ point. No. 15 steel plate has no carbide in the lath (island martensite is generated) and has low toughness. No. in which the temperature rising rate during reheating is lower than the lower limit of the present invention. Since the size of carbide in the lath exceeds the upper limit of the present invention, the steel plate No. 16 has low strength and low toughness and inferior fatigue crack propagation characteristics.

The figure which shows the relationship between the delayed fracture resistance and fatigue crack propagation characteristics at the time of introduce | transducing a ferrite by changing a particle size and an area ratio in a bainite, a martensite, or those mixed structures. (A) is a figure which shows the relationship between sheet thickness average temperature at the time of performing water cooling, natural cooling, water cooling, and reheating after rolling, and (b) is a schematic diagram of the microstructure obtained by each process.

Claims (3)

  In mass%, C: 0.05 to 0.35%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02 % Or less, the balance Fe and inevitable impurities steel, the metallographic structure is bainite or martensite or a mixed structure in which lath carbide or nitride or carbonitride having a maximum size of 0.5 μm or less exists, Tensile strength with excellent delayed fracture resistance and fatigue crack resistance, characterized in that ferrite having a particle size of 50 μm or less and 4 μm or more is present in the metal structure at an area fraction of 3% or more and 20% or less. High-strength steel material of 700 MPa or more.   Furthermore, Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1 in mass% %, Ti: 0.1% or less, B: 0.005% or less, or a tensile strength of 700 MPa excellent in delayed fracture resistance and fatigue crack propagation resistance according to claim 1 More high strength steel. The steel of the component according to claim 1 or 2 is heated to 1000 ° C. or more and 1300 ° C. or less, rolled at an Ar ₃ point or more and a cumulative reduction ratio of 50% or more, and from Ar ₃ point to Ar ₃ -200 ° C. After cooling the temperature range to 3 s or more and 200 s or less at a cooling rate of less than 4 ° C./s, directly quench at a cooling rate of 5 ° C./s or less to the Ms point or less, and then increase the temperature at a rate of 1 ° C./s or more. _A method for producing a high-strength steel material having a tensile strength of 700 MPa or more and excellent in delayed fracture resistance and fatigue crack propagation characteristics, characterized by reheating to less than _one point.

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