JP4645462B2 - A high-strength steel material excellent in fatigue crack propagation characteristics with low strength dependence and a method for producing the same. - Google Patents

Description

  The present invention relates to a high-strength steel material having excellent fatigue crack propagation resistance and a tensile strength of 400 MPa or more, and a method for producing the same. In particular, even when the strength (yield ratio) fluctuates in the steel material, there is little influence on the fatigue crack propagation characteristics. The present invention relates to a steel material exhibiting good fatigue crack propagation characteristics when subjected to repeated loads, which is suitable for various welded structures such as ships, marine structures, bridges, construction machines, buildings, and tanks, and a method for producing 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.

  High-strength steel materials having a tensile strength of 400 MPa or more used for them are required to have excellent toughness and weldability and to have excellent fatigue resistance properties 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, it is important to extend the fatigue life by reducing the propagation speed in subsequent steel materials, and there is a strong demand from the industry to improve the fatigue crack propagation characteristics of the steel materials themselves. Has been.

  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 and an average flatness. A steel sheet excellent in fatigue crack propagation resistance in which island-like martensite having a ratio of 5 or more is present in a proportion 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.

Patent Documents 4 and 5 describe a technique in which fatigue crack propagation characteristics are improved as the hardness difference between the phases is increased by using a composite structure composed of a soft phase: ferrite and a hard phase: bainite and martensite. .
JP-A-6-271985 JP 2003-342672 A JP 10-168542 A Japanese Patent No. 2962134 JP 2000-129392 A

  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-like martensite is contained, there is a concern that brittle fracture is likely to occur.

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

  In the case of Patent Document 3, since the manufacturing process of reheating and rolling after cooling is complicated, the manufacturing cost increases and the production efficiency decreases.

  A composite structure in which the hardness difference between phases is large as in Patent Documents 4 and 5 or the like generally has a low yield ratio (yield strength / tensile strength) (for example: Welding Structure Symposium '99 Lecture Proceedings P79). When such a steel material is used for a structure, since the yield point is low at the same tensile strength, there is a concern that plastic collapse or buckling easily occurs due to a static load.

  Therefore, the present invention aims to solve such problems of the prior art, and to provide a high-toughness steel material having excellent fatigue crack propagation characteristics and low strength dependence, and a technique for manufacturing with high efficiency. .

  The present inventors have repeated experiments and studies to solve the above problems. As a result, by dispersing ferrite in bainite or martensite with fine carbides in the lath or mixed structure thereof, and making ferrite grain size and area ratio within a specific range, strength (yield ratio) It was found that a steel material exhibiting excellent fatigue crack propagation characteristics and excellent toughness can be obtained without depending largely on.

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: 0.02 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 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, In the metal structure, ferrite having a particle size of 50 μm or less and 4 μm or more is present in an area fraction of 20% or more and 60% or less. More high strength steel.
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, containing 1 type or 2 or more types, high tensile cracking strength of 400 MPa or more with excellent fatigue crack propagation characteristics with small strength dependency according to 1 Strength steel material.
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 10 s or more and 500 s or less at a cooling rate of less than 4 ° C./s, directly quenching to the Ms point or less at a cooling rate of 5 ° C./s or more, and then increasing 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 400 MPa or more excellent in fatigue crack propagation characteristics with small strength dependency, characterized by reheating to less than _one point.

  According to the present invention, it is possible to produce a steel material that does not require a special process or addition of a large amount of alloy elements, enhances fatigue crack propagation characteristics without greatly depending on strength, and has excellent toughness, It is possible to provide a steel material that can increase the safety margin of fatigue failure as well as static failure and brittle failure for main members of welded structures such as ships, bridges, and buildings.

  In addition, 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 properties 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: 0.02-0.25%
C needs to be added in an amount of 0.02% or more to ensure strength. However, addition exceeding 0.25% 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 not only required to increase the strength through an increase in hardenability at low cost, but also 0.5% or more is necessary from the viewpoint of improving toughness. However, 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 are used for the purpose of adjusting strength, toughness, weldability, etc., and imparting weather resistance. 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 with small strength dependency according to the present invention has a metal structure of bainite, martensite, or a mixed structure thereof, and a particle size of 50 μm or less and 4 μm or more in the metal structure. Ferrite is present in an area fraction of 20% or more and 60% or less, and carbides, nitrides, or carbonitrides in the lath having a maximum size of 0.5 μm or less exist.

In-lath carbide or nitride or carbonitride having a maximum size of 0.5 μm or less Fine carbide or nitride or carbonitride having a maximum size of 0.5 μm or less in lath in bainite or martensite or a mixed structure thereof Toughness improves by generating a thing. If no carbides, nitrides or carbonitrides are present in the lath or if they exceed the maximum size of 0.5 μm, the toughness will be reduced. The carbide or nitride or carbonitride size refers to the total length in the major axis direction assuming an elliptical shape.

  Note that the maximum size of carbide, nitride or carbonitride in the lath should be 0.5 μm or less, and at the same time, carbide, nitride or carbonitride exceeding 0.5 μm at the lath interface or prior austenite grain boundary. Even if it exists, there is no significant influence on the fatigue crack propagation characteristics, strength, and toughness.

Bainite or martensite or a mixed structure thereof, area fraction: 20% to 60%, particle size: 50 μm or less, 4 μm or more of ferrite By setting ferrite having a particle size of 50 μm or less and 4 μm or more in martensite or a mixed structure thereof to an area fraction of 20% or more and 60% or less, the fatigue crack growth rate is locally reduced at the interface with ferrite. Thus, fatigue crack propagation resistance is improved. The ferrite area fraction is more preferably 30% or more and 60% or less.

  In the steel material according to the present invention, the fatigue crack propagation rate greatly depends on the strength by combining ferrite with a specified area fraction in the bainite or martensite having a lath structure as described above or a mixed structure thereof as described above. Stable improvement without

  FIG. 1 shows that ferrite, bainite, martensite, or a mixed structure thereof has a grain size and area ratio of ferrite within the range of the present invention, and carbide, nitride, or carbonitride in the lath has a maximum size of 0.5 μm. The result of investigating the influence of the yield ratio on the fatigue crack propagation rate for steel materials having the following metal structures and metal structures exceeding 0.5 μm is shown. The test material manufactured the steel of the component shown by A of Table 1 mentioned later on various conditions.

  From FIG. 1, it is recognized that the fatigue crack propagation rate is stable and low without greatly depending on the yield ratio (yield strength) when the form of the carbide, nitride or carbonitride in the lath is within the scope of the present invention. . In particular, in a region with a high yield ratio, it is possible to obtain a steel material that is strong in static strength against plastic deformation and excellent in fatigue crack propagation characteristics.

  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 10 s or more and 500 s or less, it is directly quenched at a cooling rate of 5 ° C./s or less to the Ms point or less, and then at a heating 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 10s or 500s 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 speed of less than 4 ° C./s is provided for 10 seconds or longer. 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 longer than 10 s, there is a sufficient amount of ferrite (area fraction of 20% or higher) for improving the fatigue crack propagation resistance. I can't get it.

  On the other hand, when the ferrite generation time exceeds 500 s, fatigue characteristics and strength are reduced due to excessive ferrite generation exceeding the area fraction of 60% as the production efficiency decreases. More preferably, the cooling time is 10 s or more and 200 s or less.

After the above cooling process, it is directly quenched at a cooling rate of 5 ° C./s or higher up to the Ms point or lower, and then reheated to a temperature of 1 ° C./s or higher to below Ac ₁ point 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, so the temperature is set to 5 ° C./s or more. 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, Toughness is improved.

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

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 + 13.3Cr (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 mmt is cooled from 800 ° C. by the above-mentioned method 2 and provided with a cooling period at 620-610 ° C., then cooled again to 130 ° C. and immediately reheated to 500 ° C. The relationship between the average temperature within the plate thickness and time when cooled again is shown in (a), and the schematic diagram of the resulting structure 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 100 s 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.

  Based on the conditions shown in Table 2, 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. With respect to these steel sheets, the structure observation, tensile strength, toughness, and fatigue crack propagation rate were investigated 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).

  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 on one side is reduced to 25 mm when the thickness exceeds 25 mm), stress ratio 0.1, frequency 20 Hz, This was performed in accordance with ASTM E647 in the room temperature atmosphere.

In the present invention, 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 characteristics are fatigue cracks when ΔK = 25 MPa√m in the atmosphere. The case where the propagation speed was 1.0 × 10 ⁻⁷ 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 all the steel plates No. 8 have high fatigue crack propagation characteristics regardless of the yield ratio (YR), and also have 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 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 sheet No. 10 has a ferrite area ratio and a ferrite grain size not lower than the lower limit values of the present invention, and exhibits a high fatigue crack propagation rate.

No. in which a cooling or slow cooling step of less than 4 ° C./s was not provided for 10 s or more in the temperature range of Ar _{3 to} Ar ₃ -200 ° C. 11-No. No. 12 steel plate is inferior in fatigue crack propagation characteristics because ferrite cannot be introduced into the structure.

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

No. in which reheating temperature exceeds Ac ₁ point. No. 15 steel plate has no carbides in the lath (partially 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 toughness and poor fatigue crack propagation characteristics.

The figure which shows the relationship between a yield ratio and fatigue crack propagation characteristics when changing the carbide | carbonized_material form in a lath as ferrite and bainite, a martensite, or those mixed structure. The schematic diagram of the microstructure obtained by each process of the relationship between the plate thickness average temperature at the time of performing water cooling, natural cooling, water cooling, and reheating after rolling.

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 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, In the metal structure, ferrite having a particle size of 50 μm or less and 4 μm or more is present in an area fraction of 20% or more and 60% or less. More high strength steel.   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, tensile strength of 400 MPa or more excellent in fatigue crack propagation characteristics with small strength dependence according to claim 1 High strength steel material. 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 10 s or more and 500 s or less at a cooling rate of less than 4 ° C./s, directly quenching to the Ms point or less at a cooling rate of 5 ° C./s or more, and then increasing 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 400 MPa or more excellent in fatigue crack propagation characteristics with small strength dependency, characterized by reheating to less than _one point.

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