JP5070744B2 - Manufacturing method of steel material with excellent fatigue crack propagation resistance - Google Patents

Description

    The present invention relates to a steel material excellent in fatigue crack propagation resistance and a method for producing the same. More specifically, the present invention relates to a steel material that is used in various welded structures such as ships, offshore structures, bridges, construction machines, buildings, and tanks, and delays the fatigue crack propagation rate, and a method for manufacturing the same.

  In recent years, high-strength steel materials have been applied to structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks for the purpose of streamlining design, reducing the weight of steel materials, and reducing the thickness and labor of welding. There are many more. In addition, high-strength steel materials are required to have excellent toughness and weldability and to have excellent fatigue resistance in order to ensure structural safety.

  In a welded structure, fatigue failure often occurs when a fatigue crack occurs from the weld toe and propagates through the steel material and breaks. 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 occurrence of cracks at the weld toe, there are wide-ranging technologies such as applying additional welding to improve the shape and reducing stress concentration, and introducing compressive residual stress by shot peening. Are 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 fatigue cracks occur from the weld toe, etc., it is important to extend the fatigue life by delaying the propagation speed in the subsequent steel material, and improve the fatigue crack propagation characteristics of the steel material itself There is a strong demand from industry.

  As a method for improving fatigue crack propagation characteristics of high-strength steel materials, it has been conventionally known that the structure is a composite structure of a soft phase (mainly ferrite) and a hard phase (mainly pearlite, bainite, martensite). These forms of existence have been defined in detail.

  For example, Patent Document 1 has 0.5 to 5 island-shaped martensite whose structure is mainly composed of one or more of ferrite, pearlite, and bainite, and that has an average existence interval of 20 μm or less and an average aspect ratio of 5 or more. %, A steel plate excellent in fatigue crack propagation characteristics and a method for producing the same are described.

  In Patent Document 2, the soft phase is dispersed in the base material of the hard phase, the hardness difference between the hard phase and the soft phase is 150 Hv or more, and the average particle size of the soft portion (average interval between the hard portions) is 50 μm or less. A steel sheet that suppresses fatigue crack growth is described.

Patent Document 3 discloses a steel material excellent in fatigue crack growth resistance and a manufacturing method thereof in which a ferrite structure having a Vickers hardness of 100 or less is present in an area ratio of 10 to 50% in a hard layer structure having a Vickers hardness of 200 or more and 500 or less. Are listed.
JP-A-6-271985 Japanese Patent No. 2962134 JP 2000-129392 A

  However, in the case of the steel sheet described in Patent Document 1, since a large amount of island martensite is contained, there is a concern that the possibility of brittle fracture starting from these becomes high.

  In addition, since the average aspect ratio of the island martensite is 5 or more, it is considered that when the crack propagates in the plate thickness direction, it exhibits good fatigue crack propagation characteristics. When it progresses in the length direction, there is a concern that the fatigue crack propagation characteristics deteriorate.

  In addition, as a problem common to Patent Documents 1 to 3, the microstructure is made to yield strength in spite of the influence of the yield strength of the steel material on the size of the plastic zone at the crack tip that governs fatigue crack propagation characteristics. No consideration is given to the control in accordance with this, and the same applies to other prior arts related to other fatigue crack propagation steels.

  The present invention solves such problems of the prior art, reduces the anisotropy of the crack propagation characteristics, and appropriately builds the structure according to the yield strength level, thereby stably providing the fatigue crack resistance characteristics. An object is to provide an excellent steel material and to provide a method for producing such a steel material.

  The present inventors have repeated experiments and studies to solve the above problems. As a result, the microstructure is mainly composed of two phases, a soft phase composed of ferrite and a hard phase composed of bainite or martensite or a mixed structure thereof, and the form of the hard phase is within the range of appropriate dimensions and shapes. Furthermore, it has been found that the fatigue crack propagation characteristics are improved in a wide yield strength range by changing the average interval between the hard phases according to the yield strength. It has also been found that such a steel material has a small anisotropy in fatigue crack propagation characteristics.

Moreover, it discovered that the steel materials which have such a microstructure could be manufactured by combining heating, rolling, accelerated cooling, and heat treatment. The present invention was made by further study based on the obtained knowledge,
1. In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02 % or less, the steel and the balance Fe and unavoidable impurities ing, heated, after hot rolling, and accelerated cooling or heating the steel, after hot rolling over cooled by reheating after quenching dual phase region , the microstructure was composed of two phases of the hard phase made of a soft phase and a bainite or martensite or their mixed structure consisting of ferrite, the hard phase average aspect ratio of the hard phase: 3 or less, a minor axis direction of the hard phase When the σ _YS after the accelerated cooling or after quenching after the two-phase region reheating satisfies the formula (1), tempering is not performed, and after the accelerated cooling or the two Σ _YS after quenching after reheating the phase region satisfies the formula (1) When there is not, the manufacturing method of the steel material excellent in the fatigue crack propagation characteristics characterized by performing tempering at the temperature below Ac1 point so that Formula (1) may be satisfied .
1000000 / σ _YS ² <L <10000000 / σ _YS ² (1)
However, L [μm]: average interval between hard phases, σ _YS : (accelerated cooling or reheating in two phases)-yield stress of steel after tempering (including accelerated cooling and reheating in two phases) [MPa ]
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% % Or less, Ti: 0.1% or less, B: 0.005% or less, one or two or more kinds , The manufacturing method of the steel material excellent in the fatigue crack propagation characteristics of 1 characterized by the above-mentioned.
3. The steel is heated at 900 ° C. to 1300 ° C. in the accelerated cooling , the hot rolling is rolled at an Ar ₃ point or higher and the cumulative rolling reduction is 50% or more, and the accelerated cooling is performed from the Ar ₃ point to Ar ₃ -200 ° C. In the temperature range, after cooling at a cooling rate of less than 4 ° C / s for 5s or more to produce ferrite, accelerated cooling to a cooling rate of 5 ° C / s or more to 500 ° C or less,
The heating of the steel when quenching after reheating after the two-phase region is 900 ° C. or more and 1300 ° C. or less, the end of hot rolling is Ar ₃ points or more, and the quenching after reheating the two-phase region is Ac ₁ point or more to Ac _{3. The} method for producing a steel material having excellent fatigue crack propagation characteristics according to 1 or 2, wherein the steel material is quenched at a cooling rate of 5 ° C./s to 500 ° C. after reheating to less than ₃ points .

  According to the present invention, it is not necessary to add a special process or a large amount of alloying elements, and it is possible to enhance fatigue crack propagation characteristics at a wide yield strength level, as represented by ships, bridges, and buildings. It is possible to provide a steel material that can increase the safety margin of fatigue failure for the main members of the welded structure.

  In addition, because it can be manufactured with high efficiency by a series of processes combining rolling, accelerated cooling, and heat treatment, it is possible to provide steel materials with excellent properties as described above at a short delivery time and at a low cost, which is extremely useful industrially. is there.

In the present invention, the chemical composition and microstructure of the steel material are defined.
[Chemical component] In the following description, all units shown in% are% by mass.
C: 0.02-0.25%
C needs to be added in an amount of 0.02% or more to ensure strength. However, addition of 0.25% or more inhibits weldability. Therefore, it is limited to 0.02% or more and 0.25% or less. Preferably, the content is 0.02% or more and 0.20% 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 the hardenability but also 0.5% or more from the viewpoint of improving toughness, but if it exceeds 2.0%, it leads to deterioration of weldability. 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, if added in a large amount, S lowers the toughness of the steel. 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 it is 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 hindered and the cost increases. Therefore, when added, the upper limit is made 2.0%. Preferably it is 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 it is 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 it is 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 it is 0.05% or less.

V: 0.1% or less V, like Nb, has a function of increasing strength by precipitation. However, addition of 0.1% or more causes a decrease in weldability and toughness. Therefore, when adding, it is limited to 0.1% or less. Preferably it is 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 it is 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 it is 0.003% or less.
[Microstructure]
The steel material for delaying the fatigue crack propagation speed according to the present invention is mainly composed of a soft phase composed of ferrite and a hard phase composed of bainite, martensite, or a mixed structure thereof, and has the following characteristics.
(1) Average aspect ratio of hard phase: 3 or less (2) Average length of hard phase in short axis direction: 5 μm or more and 100 μm or less (3) Average interval between hard phases: L [μm]
1000000 / σ _YS ² <L <10000000 / σ _YS ²
However, σ _YS : Yield stress of steel [MPa]
Hereinafter, the reasons for limiting the microstructure will be described.

Fatigue crack propagation resistance is improved by using a composite phase composed of a soft phase composed of ferrite and a hard phase composed of bainite or martensite or a mixed structure thereof. In addition, a hardness difference can be provided by using ferrite as the soft phase and bainite or martensite as the hard phase or a mixed structure thereof, and the fatigue crack propagation characteristics are improved as compared with ferrite and pearlite steel.

Average aspect ratio of hard phase: 3 or less By setting the aspect ratio of the hard phase (length in the rolling direction / length in the plate thickness direction) to 3 or less, the fatigue crack propagation direction (plate thickness, plate width, plate length) Regardless of this, it is possible to obtain a good fatigue crack propagation rate that is stable and slow. When the aspect ratio exceeds 3, anisotropy occurs in the propagation speed. More preferably, the aspect ratio is 2 or less.

Average length in the minor axis direction of the hard phase: 5 μm or more and 100 μm or less When cracks penetrate from the soft phase to the hard phase, the fatigue crack propagation rate is locally increased by setting the average length in the minor axis direction of the hard phase to 5 μm or more. Will be delayed. Such an effect becomes conspicuous with an increase in the size of the hard phase, but when its length exceeds 100 μm, the toughness deteriorates. More preferably, it is 5 μm or more and 75 μm or less.

Average interval between hard phases: L [μm]
1000000 / σ _YS ² <L <10000000 / σ _YS ²
However, σ _YS : Yield stress of steel [MPa]
By setting the average interval between the hard phases with the yield strength as a medium, the fatigue crack propagation characteristics are stably improved in a wide range of yield strength.

With respect to steel sheets (σ _YS : about 300 MPa to 600 MPa) in which the yield strength is changed in various component ranges satisfying the provisions of the present invention, and ferrite and bainite or martensite or mixed structures thereof are changed by various methods. A fatigue crack propagation test in the width direction (ΔK = 20 MPa√m) was performed. The results are shown in FIG.

  From FIG. 1, it can be seen that the fatigue crack propagation rate is stably reduced by setting the average interval of the hard phase: L within the range of this regulation. The reason why the fatigue crack propagation rate decreases in such a wide yield strength range is that the average interval between the hard phases is within the range specified in the present invention, so that even if the plastic zone size changes due to the yield strength, it remains within that size. It is considered that the interface between the hard phase and the soft phase can be inherent in the structure, and as a result, fatigue cracks are locally delayed at the interface.

The average distance between the hard phase, if below the prescribed value (1000000 / σ _{YS ^2),} for fatigue cracks mainly propagate hard phase, fatigue crack propagation characteristics is reduced.

On the other hand, when it exceeds the specified value (10000000 / σ _YS ² ), the interface between the hard phase and the soft phase is not stably present in the plastic region, and the fatigue crack propagation resistance is deteriorated. A more preferable average interval between the hard phases is 2000000 / σ _YS ² <L <8000000 / σ _YS ² . However, σ _{YS is} the yield stress [MPa] of the steel material.
[Production conditions]
Steel material according to the present invention is a steel having the above components, 900 ° C. or higher, 1300 ° C. by heating below performs cumulative rolling reduction of 50% or more rolling at Ar ₃ point or more, from the Ar ₃ point of _Ar 3 -200 ° C. By providing a step of cooling for 5 seconds or more at a cooling rate of less than 4 ° C / s in the temperature range, and then tempering at a cooling rate of 5 ° C / s or more to 500 ° C or less, or thereafter tempering at less than Ac ₁ point. can get. The temperature is the steel surface temperature, and the cooling rate is the average value in the thickness direction of the steel.

Heating temperature: 900 ° C. or more and 1300 ° C. or less If the heating temperature is less than 900 ° C., the subsequent rolling temperature cannot be secured. Further, when the temperature exceeds 1300 ° C., the crystal grains of the steel become coarse, ferrite is not generated during the subsequent cooling process, fatigue crack propagation resistance deteriorates, 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 less than Ar ₃ points, the hard phase is stretched and anisotropy occurs in the fatigue crack propagation resistance. On the other hand, if the cumulative rolling reduction is less than 50%, refinement of prior austenite grains cannot be achieved.

From Ar ₃ point in the temperature range of _Ar 3 -200 ° C. The 5s or cooling ferrite is less than the cooling rate of 4 ° C. / s as a means for uniformly generated in the steel material, the cooling in the temperature range of Ar ₃ point _{to Ar} 3 -200 ° C. A cooling or slow cooling step at a rate of less than 4 ° C./s is provided for 5 s or more. Temperature does not produce ferrite, if below or when Ar ₃ point -200 ° C. greater than ₃ points Ar.

  Even in this temperature range, if the cooling rate is less than 4 ° C./s and the cooling time is not longer than 5 s, ferrite for improving fatigue crack propagation characteristics cannot be uniformly distributed in the steel.

  The above-mentioned cooling or gradual cooling step, which is the ferrite formation time, may be 5 s or longer, and there is no particular upper limit, but it is preferably 500 s or shorter for the purpose of preventing the formation of pearlite and reducing efficiency.

In addition, the above-described cooling or gradual cooling step is not defined if it is within the temperature range from Ar ₃ point to Ar ₃ -200 ° C., and is performed immediately after reaching Ar ₃ point, followed by water cooling. (Slow cooling (relative cooling) → water cooling), after water cooling, the water cooling may be stopped once and then performed, and then water cooling may be performed again (water cooling → slow cooling (cooling) → water cooling). Moreover, you may divide | segment into multiple times.

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 to make the balance bainite, martensite, or a mixed structure thereof.

After cooling or slow cooling, accelerated cooling to 500 ° C. or less at a cooling rate of 5 ° C./s or more After the above cooling or slow cooling, the remaining untransformed austenite is made bainite, martensite, or a mixed structure thereof. When the cooling rate is less than 5 ° C./s or the stop temperature is 500 ° C. or more, ferrite and pearlite are generated.

In addition, water cooling is started from Ar ₃ point, the water cooling is stopped once, and the water cooling in the case of performing the above-described cooling or gradual cooling is also set to a cooling rate of 5 ° C./s or more.

In the present invention, the hard phase interval is defined by the yield strength. Therefore, when the yield strength of the obtained microstructure does not satisfy the above-mentioned definition while being accelerated and cooled, the yield strength is adjusted by tempering so as to satisfy the above-mentioned specification. However, when the tempering temperature exceeds ₁ Ac, island-shaped martensite is generated and toughness deteriorates.

In addition, the steel material according to the present invention is heated, rolled and cooled under arbitrary conditions, and after reheating the steel material having a desired thickness to Ac ₁ point or more to less than Ac ₃ point, at a cooling rate of 5 ° C./s or more. It can also be obtained by quenching to 500 ° C. or lower, or afterwards tempering at less than ₁ Ac.

In this case, the type of the structure before quenching is not particularly limited, but since the aspect ratio of the hard phase, the length in the minor axis direction, and the average interval between the hard phases are affected by the previous structure, the heating temperature is 900 ° C. or higher and 1300 ° C. or lower. The rolling is preferably completed at Ar ₃ points or more.

When the reheating quenching temperature exceeds Ac ₃ points, ferrite cannot be obtained by subsequent quenching. When the reheating quenching temperature is less than Ac ₁ point, recrystallized ferrite cannot be obtained when the previous structure does not contain ferrite, and when pearlite is contained, pearlite remains after quenching without reverse transformation.

  Further, when the cooling rate during reheating and quenching is less than 5 ° C./s, or when the stop temperature is over 500 ° C., pearlite is generated.

When the yield strength of the obtained microstructure is as-quenched and does not satisfy this rule, the yield strength is adjusted by tempering so that it becomes the specified value. However, when the tempering temperature exceeds ₁ Ac, island-shaped martensite is generated and toughness deteriorates.

Ar ₃ point, Ac ₃ point, and Ac ₁ point are, for example, Ar ₃ (° C.) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo, Ac ₃ = 854-180C + 44Si-14Mn-17.8Ni-1. .7Cr, 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) Can be guided by.

  In addition, when applying the present invention to actual production, the sample structure is used to obtain the microstructure and yield strength after accelerated cooling, determine whether to continue accelerated cooling or temper, and further temper. When performing, tempering conditions are calculated | required so that desired yield strength may be obtained, and product manufacture will be performed on the obtained conditions. The same applies to the two-phase reheating treatment. Tempering is not performed when the desired yield strength is obtained with accelerated cooling and with reheating in the two-phase region.

  Based on the conditions shown in Tables 2 and 3, steel plates having a thickness of 12 to 100 mm were produced from steel pieces obtained by melting steel having the composition shown in Table 1. The obtained steel sheet was examined for structure observation, tensile strength, toughness, and fatigue crack propagation rate by the following procedure. Table 3 shows manufacturing conditions by reheating heat treatment.

    The structure observation was carried out with a sample of full thickness, the surface parallel to the rolling direction was etched with 2% nital etchant, and the plate thickness / 4 position was observed in 10 fields with an optical microscope (magnification × 400). From these structure observations, the structural structure of the steel material, the hard phase average aspect ratio, the hard phase minor axis average length, and the hard phase average interval were determined. The average interval between the hard phases was determined by obtaining the interval between the closest hard phase ends and the hard phase ends and averaging them.

  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).

  Fatigue crack propagation rate is taken by CT test of full thickness (thickness of one side is reduced to 25mmt when the thickness exceeds 25mm) where the crack propagates in the direction perpendicular to the rolling direction and the rolling direction, stress ratio 0.1, frequency 20Hz, room temperature atmosphere In accordance with ASTM E647.

  In addition, the rate of progress in the plate thickness direction was measured in a three-point bending test piece of full thickness (thickness on one side was reduced to 25 mm when the plate thickness exceeded 25 mm), stress ratio 0.1, frequency 10 Hz, and room temperature atmosphere. .

  The purpose of the present invention is to reduce the propagation speed when a crack generated from a weld toe in a welded structure propagates through a steel material. The stress intensity factor range (ΔK) was investigated in the range of 10 to 30 MPa√m.

In the examples, the strength is 400 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, and the fatigue crack propagation rate is ΔK = 20 MPa√m regardless of the crack propagation direction. The case where it was 5.0 × 10 ⁻⁸ or less was regarded as acceptable (within the scope of the present invention).

  Table 4 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. 8, no. It is recognized that the 18-20 steel plates have a low fatigue crack propagation rate regardless of the propagation direction, and have both high strength and toughness.

  On the other hand, P and S are No. exceeding the scope of the present invention. Steel plate No. 9 shows low toughness even when the production method and structure specified in the present invention are obtained. In addition, no. Since the steel plate No. 10 produces a large amount of ferrite as a manufacturing method specified in the present invention, the average length in the minor axis direction of the hard phase is less than the specified value, and the average interval between the hard phases exceeds the specified value. For this reason, it is inferior to fatigue crack propagation performance.

The heating temperature is higher than the upper limit of the present invention, and the cumulative rolling reduction at ₃ or more points of Ar is less than the lower limit of the present invention. No. 11 steel plate does not produce ferrite, and the average length in the minor axis direction of the hard phase exceeds this specified value. For this reason, toughness is low and fatigue crack propagation characteristics are inferior.

No. in which a cooling and gradual cooling step of less than 4 ° C./s in the temperature range of Ar _{3 to} Ar ₃ −200 ° C. was not provided for 5 s or more. No. 12 steel plate does not generate enough ferrite, and as a result, the average interval between the hard phases is less than the specified value. For this reason, it is inferior to the fatigue crack propagation characteristics.

No. ₃ rolled at a temperature below ₃ points. In the steel plate No. 13, the hard phase is extended, the aspect ratio exceeds the specified value, and the average length in the minor axis direction is lower than the specified value. For this reason, the fatigue crack propagation characteristics in the rolling direction and the direction perpendicular to the rolling direction are inferior to those in the plate thickness direction.

No. which was not provided with a cooling or slow cooling step in a temperature range of Ar _{3 to} Ar ₃ -200 ° C. No. 14 steel plate is not introduced with ferrite and has poor fatigue crack propagation characteristics.

  No. No accelerated cooling at 5 ° C./s or higher. No. 15 steel plate, the accelerated cooling rate is less than 5 ° C / s, and the stop temperature is over 500 ° C Steel No. 16 has a structure of ferrite / pearlite, has low strength and toughness, and is inferior in fatigue crack propagation resistance.

No. with tempering temperature exceeding Ac1 point. Steel sheet No. 17 has low toughness because island martensite (MA) was generated in the structure. The reheating temperature is higher than Ac ₃ point. 21 steel plate, less than the Ac ₁ point No. No. 22 steel is inferior in fatigue crack propagation characteristics because no ferrite is introduced into the structure.

  No. in which the cooling rate and stop temperature during reheating and quenching exceed this regulation In the steel plate No. 23, pearlite is mixed in the structure, and as a result, the average interval between the hard phases (bainite) exceeds the specified value. For this reason, it is low strength and inferior in fatigue crack propagation characteristics.

The figure which shows the relationship of a hard phase average space | interval, yield strength, and fatigue crack propagation velocity.

Claims (3)

In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.02 % or less, the steel and the balance Fe and unavoidable impurities ing, heating, after hot rolling, and accelerated cooling, or after hot rolling over cooled by reheating after hardening two-phase region, the microstructure of ferrite comprising a soft phase and bainite or martensite or is composed of two phases of the hard phase consisting of mixed structure, the hard phase average aspect ratio of the hard phase: 3 or less, the hard phase short axial average length: 5 [mu] m When the σ _YS after the accelerated cooling or after quenching after reheating in the two-phase region satisfies the formula (1), tempering is not performed, and quenching is performed after the accelerated cooling or after reheating in the two-phase region. If the later σ _YS does not satisfy the formula (1), (1 A method for producing a steel material having excellent fatigue crack propagation resistance, characterized by performing tempering at a temperature of Ac1 point or less so as to satisfy the formula (1) .
1000000 / σ _YS ² <L <10000000 / σ _YS ² (1)
However, L [μm]: average interval between hard phases, σ _YS : (accelerated cooling or reheating in two phases)-yield stress of steel after tempering (including accelerated cooling and reheating in two phases) [MPa ] 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% % Or less, Ti: 0.1% or less, B: 0.005% or less, or a combination of two or more kinds, The method for producing a steel material having excellent fatigue crack propagation resistance according to claim 1 . The steel is heated at 900 ° C. to 1300 ° C. in the accelerated cooling , the hot rolling is rolled at an Ar ₃ point or higher and the cumulative rolling reduction is 50% or more, and the accelerated cooling is performed from the Ar ₃ point to Ar ₃ -200 ° C. In the temperature range, after cooling at a cooling rate of less than 4 ° C / s for 5s or more to produce ferrite, accelerated cooling to a cooling rate of 5 ° C / s or more to 500 ° C or less,
The heating of the steel when quenching after reheating after the two-phase region is 900 ° C. or more and 1300 ° C. or less, the end of hot rolling is Ar ₃ points or more, and the quenching after reheating the two-phase region is Ac ₁ point or more to Ac _The method for producing a steel material having excellent fatigue crack propagation characteristics according to claim 1 or 2, wherein the steel material is quenched at a cooling rate of 5 ° C / s to 500 ° C after reheating to less than ₃ points.

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