JP2023049316A - Steel material having excellent fatigue crack propagation resistance and method for manufacturing the same - Google Patents

Abstract

To provide a steel material having excellent fatigue crack propagation resistance obtained by a simple manufacturing method without complicated heat treatment.SOLUTION: There is provided a method for manufacturing a steel material in which a steel raw material having a chemical composition comprising, by mass%, 0.02 to 0.40% of C, 0.010 to 0.500% of Si, 0.05 to 2.00% of Mn, 0.050% or less of P, 0.050% or less of S, 0.100% or less of Al, 0.1000% or less of N and the balance Fe with inevitable impurities is heated to an Ac3 point or higher, reheat quenching treatment is applied and cooling is stopped at 100 to 450°C, wherein a metallic structure of the steel material is any one of structures of a bainite structure, a martensite structure and a mixed structure of a bainite structure and a martensite structure or a ferrite structure having an area ratio of 50% or less is present in these metallic structures and further one or more kinds of carbide, nitride and carbonitride precipitates are present and thus a steel material having excellent fatigue crack propagation resistance and excellent mechanical characteristics can be easily manufactured.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a steel material excellent in fatigue crack propagation resistance and a method for producing the same. More specifically, it relates to steel materials that are used for various welded structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, and that have improved fatigue crack propagation resistance even when subjected to repeated loads, and a method for manufacturing the same. .

In recent years, in structures such as ships, offshore structures, bridges, construction machinery, buildings, and tanks, high-strength steel is applied for the purpose of rationalization of design, reduction of steel weight, thinning, and labor saving of welding. Cases are increasing. In addition, these steel materials are required to have excellent fatigue resistance in order to ensure not only toughness and ductility but also weldability and structural safety.

In welded structures, there are many cases where fatigue cracks start from the weld toe, propagate through the steel material, and break. It is said that this is because the weld toe tends to become a stress concentration part due to its shape factor, and in addition, tensile residual stress is generated after welding.

For this reason, as a means to suppress the occurrence of cracks from the weld toe, techniques such as adding welding to improve the shape and reduce stress concentration, and techniques such as peening to introduce compressive residual stress are widely used. Are known.

However, it is almost impossible to apply such a treatment to many weld toes on an industrial scale, and it is difficult to say that it is realistic in terms of cost. Therefore, even if a fatigue crack occurs, it is important to extend the fatigue life by reducing the subsequent propagation speed in the steel material. There is a strong demand from industry to

Here, in Patent Document 1, the chemical composition of steel is C: 0.05 to 0.30%, Si: 0.03 to 0.35%, Cr: 0.05 to 2.0% in mass% , P: 0.03% or less, S: 0.003% or less, Al: 0.1% or less, and the balance being Fe and unavoidable impurities. After heating and quenching, reheat to a two-phase region temperature range of Ac ₁ transformation point +10°C to 790°C, quench at an average cooling rate of 5 to 60°C/s, and hold at 400 to 650°C for 10 minutes or more. A mixed structure of a ferrite phase with a Vickers hardness of 85 to 130 and a tempered martensite phase with a Vickers hardness of 15 to 85% and a tempered martensite phase with an area fraction of 15 to 85%. A certain method for producing a steel material having an absolute value of surface residual stress of 150 N/mm ² or less and having excellent fatigue crack propagation resistance is disclosed.

Further, in Patent Document 2, C: 0.04 to 0.3%, Si: 0.01 to 2%, Mn: 0.1 to 3%, Al: 0.001 to 0.1%, N: 0.001 to 0.01%, P: 0.02% or less, S: 0.01% or less, the balance being iron and unavoidable impurities, a soft phase and a hard network surrounding the soft phase It has a two-phase structure consisting of a second phase, and the soft phase and the hard second phase are: (1) the soft phase is composed of one or more of ferrite, tempered bainite, and tempered martensite, and an average (2) The hard secondary phase is composed of one or more of bainite, martensite, tempered bainite, and tempered martensite, and the average Vickers hardness is 250 or more. and (3) that the grain boundary occupancy of the hard second phase represented by the specific formula is 0.5 or less. .

In addition, in Patent Document 3, the steel composition in mass% is C: 0.05 to 0.30%, Si: 0.03 to 0.35%, Cr: 0.05 to 2.0%, P: 0 .03% or less, S: 0.003% or less, Al: 0.1% or less, the balance being Fe and unavoidable impurities, and 80% or more of the metal structure in both the plate thickness direction and the plate length direction, A mixed structure of a ferrite phase with a Vickers hardness of 130 or less and an aspect ratio of 2.5 or less and a tempered martensite phase with an area fraction of 15 to 85% and a Vickers hardness of 340 or more and an aspect ratio of 2.5 or less A steel material having small anisotropy of the material and excellent fatigue crack propagation resistance is disclosed.

Further, in Patent Document 4, in terms of % by mass, C: 0.03 to 0.30%, Si: 0.01 to 0.5%, Mn: 0.3 to 2.0%, sol. A steel sheet containing Al: 0.001 to 0.1%, the balance being Fe and unavoidable impurities, the structure of which consists of a hard portion and a soft portion, and the ratio of these two portions to the structure and a steel sheet whose average Vickers hardness satisfies a specific formula.

Furthermore, in Patent Document 5, C: 0.02 to 0.16%, Si: 0.05 to 0.5%, Al: 0.005 to 0.060% by mass%, and further, Mn: 0.1 to 2.5%, Cu: 0.1 to 2.0%, Ni: 0.1 to 6.0% One or more selected from 2. The value of Mn + Cu + Ni is 3. A steel containing 5 to 6.0%, the balance being Fe and unavoidable impurities, characterized by containing 5 to 20% by volume of retained austenite with an average diameter of 0.1 to 0.5 μm. A steel plate having excellent fatigue crack propagation properties and toughness is disclosed.

Japanese Patent No. 4924047 Japanese Patent No. 3785392 Japanese Patent No. 4998708 Japanese Patent Application Laid-Open No. 2002-121640 Japanese Patent No. 4497009

However, the manufacturing techniques according to Patent Documents 1 to 5 have the following problems.

In the manufacturing method of Patent Document 1, after hot rolling, reheating and quenching treatment, reheating to a two-phase region temperature range of Ac ₁ transformation point + 10 ° C. to 790 ° C. and an average cooling rate of 5 to 60 ° C./s After quenching at 400 to 650° C. for 10 minutes or more, the steel is tempered, but the manufacturing method is complicated.

In Patent Document 2, a steel slab before hot rolling is subjected to diffusion heat treatment at a heating temperature of 1200 to 1350 ° C. for a holding time in the temperature range of 2 to 100 h, and then the heating temperature is from the Ac ₃ transformation point to 1250 ° C. Then, after rolling, hot rolling is performed with accelerated cooling at 5 to 100 ° C./s from the Ar ₃ transformation point or higher to 400 ° C. or lower, and the heating temperature is (Ac ₁ transformation point + 30 ° C.) to (Ac ₃ transformation point - 10 ° C.) and accelerated cooling at 5 to 100° C./s to 400° C. or less.

In Patent Document 3, the steel is subjected to solution heat treatment at 1200 to 1300 ° C. for 25 to 60 hours, then air-cooled, reheated to 1000 to 1200 ° C., and the hot rolling end temperature is the Ar transformation _point or higher. After rolling and air cooling, after reheating to the Ac ₃ transformation point or higher, air cooling is performed, and further reheated to a two-phase region temperature of Ac ₁ transformation point + 10 ° C. to Ac ₃ transformation point - 10 ° C., then 5 ° C./ It is characterized by quenching at an average cooling rate of s or more and tempering at 400°C to 650°C.

Patent Document 4 proposes a relational expression of fA·HA−fB·HB≧−3500. Here, fA and fB indicate the ratio of the hard portion and the soft portion in the structure in units of %, and HA and HB indicate the average values of the Vickers hardness of the hard portion and the soft portion, respectively. However, fA.HA-fB.HB and the fatigue crack propagation speed are not in a linear relationship, and the index is unclear.

Furthermore, Patent Document 5 discloses that after hot rolling the steel, it is directly quenched or reheated and quenched, followed by two-phase region heat treatment in which it is heated to a temperature of 650 ° C. or higher and less than Ac ₃ point and cooled. It is characterized by a complicated production method such as two-phase region heat treatment.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-described problems of the prior art, and to provide a steel material excellent in fatigue crack propagation resistance that can be manufactured by a simple manufacturing method without complicated heat treatment. Here, the term "steel material" includes steel plates, steel bars, bar steels, etc., but the "steel materials" will be mainly described below.

The present inventors conducted the following experiments and repeated studies in order to solve the above problems.

[Fatigue crack propagation rate]
Three types of test steel (A, B, C) were used for the experiment. Its chemical composition is shown in Table 1.

Test steel A has a steel composition of 0.12% C-0.31% Si-1.36% Mn-0.011% P-0.005% S-0.01% Ni-0.05% Cr -0.015% Ti-0.051% Al-0.0045% N steel plate (plate thickness 25 mm) is used, the steel plate is reheated to 900 ° C., quenched, and cooled at 200 ° C. stopped. After that, the relationship between the stress intensity factor range [ΔK] and the fatigue crack propagation speed [da/dN] was investigated using a CT test piece with the thickness reduced to 12 mm on both sides. Test steel B is a soft phase/hard phase dispersed steel (0.15% C-0.25% Si-1.51% Mn-0.009% P-0.005% S-0.07% Cr- A steel ingot containing 0.038% Al-0.0025% N is heated to 950° C. or higher and 1250° C. or lower, and then rolled at an Ar ₃ point or higher with a cumulative rolling reduction of 50% or higher, and the Ar ₃ point is -60° C. to 730° C. After starting accelerated cooling from the above temperature range at a cooling rate of 10°C/s or more and less than 30°C/s, the steel material was cooled to 650°C or less and 500°C or more), and test steel C was a conventional steel. (0.13%C-0.33%Si-1.45%Mn-0.009%P-0.008%S-0.05%Cr-0.012Ti-0.048%Al-0.0039 %N after heating to 950°C or higher and 1250°C or lower, rolling at 900°C or higher with a cumulative rolling reduction of 50% or higher, and starting accelerated cooling from the Ar ₃ point to the Ar ₃ point - 100°C. A steel material made by cooling to 700° C. or lower and 500° C. or higher) was used.

The results are shown in FIG. In FIG. 1, □ (indicated by “white frame filled in black” in the figure) is the data of the test steel A, Δ is the data of the test steel B, and ◯ is the test steel C. In addition, the solid line in the figure is the level of the fatigue crack propagation rate [da/dN] with respect to the stress intensity factor range [ΔK] in general conventional steel such as test steel C, where ΔK = 15 MPa·m ¹ A straight line connecting 3.50×10 ⁻⁸ m/cycle at ^/2 and 1.70×10 ⁻⁷ m/cycle at ΔK=25 MPa·m ^1/2 . The dotted line below it is the 1/2 level of the fatigue crack propagation rate level of test steel C, 1.75 × 10 ^-8 m/cycle when ΔK = 15 MPa m ^1/2 , and ΔK = It is a straight line connecting 8.50×10 ⁻⁸ m/cycle at 25 MPa·m ^1/2 . The one-dot chain line below it is the 1/5 level of the fatigue crack propagation speed level of test steel C, and when ΔK = 15 MPa m ^1/2, 7.00 × 10 ^-9 m / cycle, It is a straight line connecting 3.40×10 ⁻⁸ m/cycle when ΔK=25 MPa·m ^1/2 . From this experimental result, the fatigue crack propagation rate level of test steel B is about half that of test steel C, whereas the fatigue crack propagation rate level of test steel A is about half that of test steel C. , and it was confirmed that the fatigue crack propagation resistance was the best.

Next, a thin film sample was prepared from a half thickness of the steel plate of test steel A, and metallographic observation was performed with a TEM (transmission electron microscope) (magnification: 10,000 times). An example of the result is shown in FIG. In FIG. 2, 28 carbide precipitates were observed in one field of view (3.8 μm ² ). As a result of performing the same observation in 10 fields with a field interval of 50 μm, precipitates composed of one or more carbides, nitrides or carbonitrides were confirmed in 6 fields (equivalent to 60% of the 10 fields).

In addition, observation of the metal structure is performed by polishing and corroding (nital etching) a test piece taken at the center of the thickness of the steel (steel plate) (1/2 of the thickness) and using a scanning electron microscope (SEM). (Magnification: 500 times), the structure was observed, an image was taken, and the metal structure was determined using an image analyzer. As a result, the metal structure observed in the above test steel A was a bainite structure. Moreover, the above test steel B had a structure in which massive pearlite and massive bainite were generated in the ferrite matrix. Further, the sample steel C had a structure in which band-like pearlite was generated in the ferrite matrix.

From the above experimental results, the steel material has a specific composition, the metal structure is a bainite structure, and the steel material contains one or more kinds of precipitates of carbides, nitrides, and carbonitrides. It has been found that the fatigue crack propagation characteristics are greatly improved by

Similarly, in the case of a martensite structure, a mixed structure of a bainite structure and a martensite structure, and even if a ferrite structure exists in those structures, the ferrite structure has an area A reduction in the fatigue crack propagation rate is similarly observed in a steel material having a structure in which the ratio is 50% or less and in which one or more kinds of precipitates of carbides, nitrides and carbonitrides are present. rice field.

The present invention was completed based on these findings and further studies, and the gist of the present invention is as follows.
[1] In mass %, C: 0.02 to 0.40%, Si: 0.010 to 0.500%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: A steel material having a chemical composition containing 0.050% or less, Al: 0.100% or less, N: 0.1000% or less, and the balance being Fe and inevitable impurities, wherein the metal structure of the steel material is a bainite structure. , a martensite structure or a mixed structure of a bainite structure and a martensite structure, and further, one or more kinds of precipitates of carbides, nitrides and carbonitrides are present. Steel material with excellent fatigue crack propagation resistance.
[2] In [1], in addition to the chemical composition, Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01 to 3, in mass % .00%, Mo: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, REM: excellent in fatigue crack propagation resistance according to claim 1, characterized by containing one or more selected from 0.001 to 0.100% steel material.
[3] In [1] or [2], a steel material having excellent fatigue crack propagation resistance, characterized in that each metal structure and a ferrite structure having an area ratio of 50% or less are present.
[4] In any one of [1] to [3], the steel material having excellent fatigue crack propagation resistance is characterized in that the thickness of the steel material is 5 to 100 mm.
[5] In any one of [1] to [4], the number of precipitates is 1 to 100/3.8 μm ² at the thickness center of the steel material. Steel material with excellent fatigue crack propagation properties.
[6] In any one of [1] to [5], when the stress intensity factor range [ΔK] in the fatigue crack propagation test of the steel material is 20 MPa m ^1/2 , the fatigue crack propagation rate A steel material having excellent fatigue crack propagation resistance, wherein [da/dN] is 4.26×10 ⁻⁸ m/cycle or less.
[7] The method for manufacturing the steel material according to any one of [1] to [6], wherein, in mass%, C: 0.02 to 0.40%, Si: 0.010 to 0.500 %, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.050% or less, N: 0.1000% or less, Al: 0.100% or less, the balance being Fe and A steel material with a chemical composition consisting of unavoidable impurities is heated to Ac ₃ point or higher, subjected to reheating and quenching, and cooling is stopped at 100 to 450 ° C. Excellent fatigue crack propagation resistance. method of manufacturing steel.
[8] In [7], in addition to the chemical composition of the steel material, further, by mass%, Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01% to 2.00%. 01-3.00%, Mo: 0.01-1.00%, Nb: 0.001-0.100%, V: 0.001-0.100%, Ti: 0.001-0.100% , B: 0.0001 to 0.0100%, and REM: 0.001 to 0.100%. Production method.
[9] In [7] or [8], the reheating and quenching treatment is performed at a cooling rate of 5 to 275° C./s.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to manufacture a steel material excellent in fatigue crack propagation resistance by a simple manufacturing method, and it has a remarkable industrial effect.

FIG. 2 is a correlation diagram showing fatigue crack propagation resistance test results of the steel material of the present invention and a conventional steel material. It is a photograph taken in the observation of the metal structure of the steel material of the present invention.

Hereinafter, embodiments according to the present invention will be specifically described.

[Basic chemical composition of steel]
Next, the basic chemical composition of the steel material according to the present invention will be explained. In addition, hereinafter, "%" in the chemical composition means "% by mass".

[C: 0.02 to 0.40%]
C needs to be added in an amount of 0.02% or more to ensure strength. However, addition of 0.40% or more impairs weldability. Therefore, the range is limited to 0.02 to 0.40%. Preferably, it is 0.02 to 0.35%. More preferably, it is 0.02 to 0.33%. More preferably, it is 0.02 to 0.30%.

[Si: 0.010 to 0.500%]
Si is effective as a deoxidizing agent and needs to be 0.010% or more for high strength, but if added in excess of 0.500%, it deteriorates weldability and toughness. Therefore, the range is limited to 0.010 to 0.500%. Preferably, it is 0.050 to 0.450%. More preferably, it is 0.050 to 0.430%. More preferably 0.050 to 0.400%.

[Mn: 0.05 to 2.00%]
Mn is required to be 0.05% or more from the viewpoint of not only increasing strength by increasing hardenability at a low cost, but also improving toughness. Therefore, it is limited to the range of 0.05 to 2.00%. Preferably, it is 0.05 to 1.90%. More preferably, it is 0.05 to 1.85%. More preferably, it is 0.05 to 1.80%.

[P: 0.050% or less]
P is an impurity and degrades the toughness, so the smaller the content, the better, and the range is limited to 0.050% or less from the viewpoint of manufacturing costs. Preferably, it is 0.040% or less. More preferably, it is 0.030% or less. More preferably, it is 0.020% or less.

[S: 0.050% or less]
Since S is an impurity and deteriorates the toughness, the smaller the content, the better. Preferably, it is 0.040% or less. More preferably, it is 0.030% or less. More preferably, it is 0.020% or less.

[Al: 0.100% or less]
Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process for high strength steel. In addition, N in the steel is fixed as AlN, contributing to improvement of the toughness of the base material. On the other hand, addition of more than 0.100% lowers the toughness of the base material, and also mixes in the weld metal portion during welding to deteriorate the toughness. Therefore, Al is limited to 0.100% or less. Preferably, it is 0.080% or less. More preferably, it is 0.070% or less. More preferably, it is 0.060% or less.

[N: 0.1000% or less]
N is not preferable because it adversely affects ductility and toughness in a solid solution state. It is valid. However, if it is contained excessively, solid solution N may increase and adversely affect ductility and toughness, so N is limited to a range of 0.1000% or less. Preferably, it is 0.0900% or less. More preferably, it is 0.0800% or less. More preferably, it is 0.0700% or less.

[Optional chemical composition of steel]
The chemical composition described above is the basic chemical composition of the steel material of the present invention, and in the present invention, in addition to this basic chemical composition, as an optional chemical composition, strength, toughness, weldability, etc. can be adjusted as necessary. , For the purpose of imparting weather resistance, etc., Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01 to 3.00%, Mo: 0.01 to 1 .00%, Nb: 0.001 to 0.100%, V: 0.001 to 0.100%, Ti: 0.001 to 0.100%, B: 0.0001 to 0.0100%, REM: One or more selected from 0.001 to 0.100% can be selected and contained.

[Cu: 0.01 to 2.00%]
Cu brings about the effect of increasing the strength by solid solution and improves the weather resistance. Therefore, it is preferable to set the lower limit to 0.01%. However, if the content exceeds 2.00%, the weldability is impaired and flaws are likely to occur during steel production. Therefore, when it is contained, it is preferably 0.01 to 2.00%. More preferably, it is 0.01 to 1.50%. More preferably,
0.01 to 1.00%.

[Ni: 0.01 to 5.00%]
Ni improves low-temperature toughness and is effective in improving weather resistance and hot brittleness that occurs when Cu is added. Therefore, it is preferable to set the lower limit to 0.01%. However, if the content exceeds 5.00%, it impairs weldability and leads to an increase in cost. Therefore, when it is contained, it is preferably 0.01 to 5.00%. More preferably, it is 0.01 to 4.00%. More preferably, it is 0.01 to 3.00%.

[Cr: 0.01 to 3.00%]
Cr improves weather resistance and strength. Therefore, it is preferable to set the lower limit of addition to 0.01%. However, if its content exceeds 3.00%, it impairs weldability and toughness. Therefore, when it is contained, it is preferably 0.01 to 3.00%. More preferably, it is 0.01 to 2.50%. More preferably, it is 0.01 to 2.00%.

[Mo: 0.01 to 1.00%]
Mo may be contained in an amount of 0.01% or more to increase strength. However, when its content exceeds 1.00%, deterioration of weldability and toughness occurs. Therefore, when it is contained, it is preferably 0.01 to 1.00%. More preferably, it is 0.01 to 0.90%. More preferably, it is 0.01 to 0.80%.

[Nb: 0.001 to 0.100%]
Nb suppresses recrystallization of austenite during rolling to achieve grain refinement, and at the same time, has the function of increasing strength by precipitation, so it may be contained in an amount of 0.001% or more. However, if the content exceeds 0.100%, the toughness deteriorates. Therefore, when it is contained, it is preferably 0.001 to 0.100%. More preferably 0.001 to 0.090%. More preferably, it is 0.001 to 0.080%.

[V: 0.001 to 0.100%]
Like Nb, V also has a function of increasing the strength by precipitation, so it may be contained in an amount of 0.001% or more. However, when the content exceeds 0.100%, the weldability and toughness are lowered. Therefore, when it is contained, it is preferably 0.001 to 0.100%. More preferably, it is 0.001 to 0.090%. More preferably, it is 0.001 to 0.080%.

[Ti: 0.001 to 0.100%]
Ti may be contained in an amount of 0.001% or more in order to increase the strength and improve the toughness of the weld zone. However, when the content exceeds 0.100%, there is a tendency for the cost to increase. Therefore, when it is contained, it is preferably 0.001 to 0.100%. More preferably, it is 0.001 to 0.090%. More preferably, it is 0.001 to 0.080%.

[B: 0.0001 to 0.0100%]
B may be contained in an amount of 0.0001% or more in order to improve hardenability and contribute to an increase in strength. However, if the content exceeds 0.0100%, the weldability is impaired. Therefore, when it is contained, it is preferably 0.0001 to 0.0100%. More preferably, it is 0.0001 to 0.0090%. More preferably, it is 0.0001 to 0.0080%.

[REM: 0.001 to 0.100%]
REM refers to rare earth elements such as Sc, Y, La and Ce. When added in a small amount, it may be contained in an amount of 0.001% or more because it contributes to the improvement of HAZ toughness. However, if the content exceeds 0.100%, the weldability is impaired. Therefore, when REM is contained, it is preferably 0.001 to 0.100%. More preferably, it is 0.001 to 0.090%. More preferably, it is 0.001 to 0.080%.

[Residual chemical composition]
The balance chemical composition other than the chemical composition described above consists of Fe and unavoidable impurities. Examples of the unavoidable impurity elements include O (oxygen), Sn, Sb, As, Pb, Bi, Ca, Mg, etc., and a total content of 0.10% or less is acceptable. In addition, as long as the basic chemical composition and optional chemical composition described above are satisfied, it does not prevent the inclusion of unavoidable impurity elements other than these, and such embodiments are also included in the technical scope of the present invention. be

[Metal structure of steel]
In the present invention, the steel material having the chemical composition described above can be obtained by the manufacturing method described below. The metal structure in the central part of the plate thickness of the obtained steel has a different chemical composition, and by changing the cooling rate according to the plate thickness of the steel material, depending on the heat treatment conditions such as quenching treatment, as follows. It was found to have a fine metallographic structure. The structure is any one of a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure, and a ferrite structure with an area ratio of 50% or less is included in the structure of the metal structure. may exist. That is, it has the following metallographic structure.
(a) Bainite structure (b) Martensite structure (c) Mixed structure of bainite structure and martensite structure (d) Ferrite structure with an area ratio of 50% or less and bainite structure (e) Ferrite structure with an area ratio of 50% or less and marten Site structure (f) Mixed structure of ferrite structure, bainite structure and martensite structure with an area ratio of 50% or less Incidentally, as described above, these metal structures were observed with a scanning electron microscope (SEM) (magnification: 500 times). It can be determined by observing the tissue used.

Each of the above structures can be obtained by adjusting the chemical composition, cooling rate, heat treatment conditions such as quenching, etc., as described above.

Specifically, when the cooling rate is high (50 ° C./s or higher) and the cooling stop temperature is low (150 ° C. or lower), the martensitic structure is likely to occur, and the cooling rate is low (30 ° C./s or lower) and the cooling stop temperature is low. If the temperature is high (100° C. to 450° C.), it tends to form a bainite structure. If the cooling rate is even lower (5° C./s or less), the ferrite structure will be mixed in the matrix of the bainite structure, the martensite structure, or the bainite structure and the martensite structure. That is, the above-described structures (a) to (f) can be obtained by the above heat treatment.

[Precipitates in metal structure]
In the above metallographic observation by TEM, it was found that it is important that one or more of carbide, nitride and carbonitride precipitates are present in the metallographic structure. Specifically, when the number of precipitates is 1 to 100/3.8 μm ² at the central portion of the thickness of the steel material in metallographic observation, a steel material with excellent fatigue crack propagation resistance can be obtained. be done. That is, when observing the metallographic structure at the center of the sheet thickness of steel, one measurement field of view is 3.8 μm ² , and 10 fields of view are observed at intervals of 50 µm. (above) It is preferable that the number of the precipitates is 1 to 100/3.8 μm ² .

The above precipitates include carbides, nitrides or carbonitrides such as Fe, Si, Mn, Al, etc. in the basic chemical composition of the steel material, and optionally selected chemical compositions of Cr, Mo, Nb, V, Ti Such as carbides and nitrides or carbonitrides. Precipitates in these metal textures are scattered in the form of fine particles of various sizes, and their sizes are about 10 to 500 nm, as in the example shown in FIG.

In addition, these precipitates are obtained by cutting a 1 mm thin piece from the central part of the plate thickness of the steel material (steel plate), polishing the cut thin piece from both sides to reduce the thickness to 50 μm, punching it with a disk punch to 3 mmφ, and electrolyzing the disk. It is sliced by a polishing method, washed with methanol, dried on filter paper to obtain a sample for TEM observation, and observed by TEM observation (observation field: 3.8 μm ² , magnification: 10000 times). Furthermore, carbides, nitrides, and carbonitrides were identified by energy dispersive X-ray spectroscopy (EDX) analysis. This makes it possible to determine whether the precipitates are carbide, nitride, or carbonitride precipitates. Here, methods for identifying carbides, nitrides, and carbonitrides include energy dispersive X-ray spectroscopy (EDX) analysis, etc. According to this method, carbides have peaks of Fe and C, Fe and N peaks appear in nitrides, and Fe, C and N peaks appear in carbonitrides.

[Manufacturing method of steel]
The steel manufacturing method according to the present invention is characterized by heating the steel material adjusted to have the above-mentioned composition to Ac ₃ point or higher, performing reheating and quenching, and stopping cooling at 100 to 450 ° C. and Here, the temperature is the temperature at the center of the plate thickness of the steel material (steel plate). As a result, the cooling rate is adjusted according to the thickness of the steel material at the central portion of the plate thickness, and the aforementioned metallographic structure and precipitates are obtained in the metallographic structure. It was also confirmed that the steel material having such a metal structure has improved fatigue crack propagation resistance as described later. Such an effect has been confirmed when the plate thickness of the steel material (steel plate) is in the range of 5 to 100 mm.

Further, individual steps in the steel manufacturing method of the present invention will be described in detail.

[Heating at Ac ₃ or more]
In order to completely austenite the metal structure of the steel material, it is heated to the Ac ₃ point (austenite transformation temperature) or higher. This heating is also called reheating, and this temperature is also called reheating temperature. Heating below the Ac ₃ point causes the structure before heating to remain and the strength to decrease. This Ac ₃ point can be calculated by the following formula (1) depending on the composition % of the steel material.
Ac ₃ =937.2-436.5*C+56*Si-19.7*Mn-16.3*Cu-26.6*Ni-4.9*Cr+38.1*Mo+124.8*V+136.3*Ti -19.1 x Nb + 198.4 x Al + 3315 x B (1)
In addition, it is set to 0 when it is not contained.

[Quenching treatment]
A quenching treatment is performed in order to make the metal structure at the central portion of the plate thickness of the steel material the above-described bainite structure, martensite structure, or mixed structure of bainite structure and martensite structure. These structures can be appropriately obtained by changing the cooling rate during water cooling according to the plate thickness and by adjusting the chemical composition. Even if the ferrite structure is not precipitated, or if the precipitation is 50% or less in area ratio, the desired mechanical properties described later can be obtained, but if the ferrite structure exceeds 50% in area ratio , the property is not obtained.

Specifically, the quenching treatment is to control-cool the heated steel material with jet water at a cooling rate of 5 to 275° C./s or less, preferably 5 to 260° C./s. The reason why the cooling rate is set to 275° C./s or less is to obtain a martensite structure and a bainite structure.

As for the quenching treatment method, in the case of laboratory experiments, controlled cooling was performed by jet water cooling or the like from Ar ₃ points or higher. In the case of an actual machine scale, a desired metal (micro) structure can be obtained by cooling by jet water cooling, laminar cooling, or the like.

[Cooling stop temperature]
By stopping cooling at 100 to 450° C. in the process of performing the quenching treatment and cooling, it becomes a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure, and the carbides and nitrides described above. Alternatively, precipitates that are carbonitrides appear.

If the cooling stop temperature, which is the temperature at which the cooling is stopped, is not 100°C or higher, none of the carbides, nitrides and carbonitrides will precipitate, and if it exceeds 450°C, the strength will decrease. Therefore, the cooling stop temperature is preferably 100 to 450°C. More preferably, it is 150 to 450°C. To control the temperature within this range, TMCP technology such as controlled cooling is used. Control cooling is performed by jet cooling, laminar cooling, or the like in the case of an actual machine, and water cooling is stopped at 100°C to 450°C. In the case of laboratory cooling, it is obtained by controlling cooling by jet cooling or the like and stopping water cooling at 150 to 450°C. A bainite structure is likely to be obtained when the cooling stop temperature is 150°C to 450°C. If the cooling stop temperature is less than 150°C, a martensite structure is likely to be obtained.

[Fatigue crack propagation resistance]
The fatigue crack propagation resistance of the steel material is evaluated by obtaining the stress intensity factor range [ΔK] and the fatigue crack propagation speed [da/dN] by a fatigue crack propagation test conforming to the standard of ASTM E647. This stress intensity factor range [ΔK] is ΔK=Kmax−Kmin, and represents the difference between the maximum and minimum values of the stress intensity factor. In addition, the fatigue crack propagation rate [da/dN] is the amount of fatigue crack growth [m/cycle] in one cycle of the stress waveform, especially in the middle region of ΔK (approximately ΔK = about 10 MPa m ^{1 / 2} to ΔK=approximately 70 MPa·m ^1/2 ), a linear relational expression da/dN=CΔK ^m (where C and m are constants), which is called the Paris' rule, holds.

Here, in the present invention, as a steel material excellent in fatigue crack propagation resistance, when the stress intensity factor range [ΔK] is 20 MPa m ^1/2 , the fatigue crack propagation speed is 4.26 × 10 ^-8 m/cycle or less.

[Mechanical properties of steel]
The target values for the mechanical properties of the steel material of the present invention are: yield stress [YS] ≥ 400 MPa, tensile strength [TS] ≥ 500 MPa in a tensile test, test temperature in a Charpy impact test: absorbed energy [ _V E ₀ ] at 0 ° C. ≥ 27 J, fatigue crack propagation speed [da/dN] ≤ 4.26 × 10 ^-8 [m/cycle] when the stress intensity factor range [ΔK] in the fatigue crack propagation test is 20 MPa m ^1/2 and

Twenty-six types of steel having the steel compositions A to Z shown in Table 2 were prepared, and the test pieces (plate Nos. 1 to 32) shown in Table 3 were heated under the manufacturing conditions shown in Table 3 (Ac ₃ points or more). and quenched, and the cooling was stopped during the cooling process).

A thin piece of 1 mm is cut out from the central part of the plate thickness of the steel material (steel plate) treated as described above, and the cut thin piece is polished from both sides to reduce the thickness to 50 μm. After washing with methanol, the sample was dried on filter paper to obtain a sample for TEM observation, and TEM observation (field of view: 3.8 μm ² , magnification: 10,000×) was performed for 10 fields of view at a pitch of 50 μm. Furthermore, carbides, nitrides, and carbonitrides were identified by attached energy dispersive X-ray spectrometer (EDX) analysis. Table 4 shows the observation results obtained. All of the test pieces of the present invention contained one or more kinds of precipitates in the metallographic structure. In 24, 27 to 29, 32, no precipitate was observed.

Next, in the tensile test, an ASTM-F (6φ×24GL) round bar test piece was sampled from the center of the plate thickness and perpendicular to the rolling direction and subjected to the test. In the Charpy impact test, three full-size test pieces were sampled from the center of the plate thickness in the rolling direction and subjected to the test at a test temperature of 0°C. For the fatigue crack propagation speed, a fatigue crack propagation test was performed on a CT test piece with a thickness of 12 mm on both sides, with an R ratio of 0.1, a sine wave, and 10 Hz. YS [MPa], TS [MPa], _V E ₀ [J], and fatigue crack propagation speed [da/dN] [m/cycle] at ΔK = 20 MPa·m ^1/2 at this time are shown in Table 4. show. Microscopic observation was performed by nital etching at a position of 1/2 of the plate thickness and using SEM (magnification: 500 times). For the TEM observation, a thin film was sampled from a half-thickness position as described above, and then observed and photographed (field of view: 3.8 μm ² , magnification: 10,000 times).

[Mechanical properties of steel]
A tensile test piece (parallel portion diameter 6 mmφ) and a Charpy impact test piece (V notch) were taken from the obtained test pieces in accordance with JIS Z 2241 and 2242, and tensile tests and impact tests were performed.

In the tensile test, the value obtained as the upper yield point when the yield point appears, and the value obtained as 0.2% yield strength when the yield point does not appear is the YS of the test piece, and the maximum value in the stress-strain curve is taken as the tensile stress. The obtained value was taken as TS.

Also, the Charpy impact test was performed three times each, the absorbed energy _V E ₀ at the test temperature of 0° C. was determined, and the average value was taken as the value of the test piece.

Table 5 shows the results obtained.

Plate No. of the test piece having the chemical composition of the steel specified in the present invention and prepared by the method for manufacturing the steel specified in the present invention. 1 to 22, when YS is 400 MPa or more, TS is 500 MPa or more, _V E ₀ is 27 J or more, and ΔK = 20 MPa·m ^1/2 , the fatigue crack propagation speed [da/dN] is 4.26 × 10 ^{- 8} [m/cycle] or less, and a steel sheet having an excellent fatigue crack propagation rate was obtained. Moreover, at this time, as shown in Table 4, one or more kinds of precipitates were present in metallographic observation. In addition, as shown in Table 3, the metal structure is either a bainite structure, a martensite structure, or a mixed structure of a bainite structure and a martensite structure. The structure had a ferrite structure of 50% or less.

However, board no. Steel plate No. 23 had a Charpy impact test value lower than 27 J because C exceeded the specified value of the present invention. Board No. In the steel sheet No. 24, C, Si, and Mn are below the specified values of the present invention, so precipitates such as carbides are not observed in any of the 10 fields of view in metallographic observation, and YS and TS are the targets of the present invention. value, the Charpy impact test value is below the target value of the present invention, the fatigue crack propagation speed at ΔK = 20 MPa m ^1/2 exceeds 4.26 × 10 ^-8 [m/cycle], The fatigue crack propagation properties were inferior. Board No. The steel sheet No. 25 had P, S, N, and Al exceeding the specified values of the present invention, so the Charpy impact test values were below the target values of the present invention. Board No. The steel sheet No. 26 had a Charpy impact test value lower than the target value of the present invention because Si and Mn exceeded the specified values of the present invention. Board No. No. 27 steel plate had a reheating temperature lower than the Ac ₃ point of the present invention, so the metal structure had an area ratio of 80% ferrite structure + 20% pearlite structure. No precipitate was observed. As a result, YS and TS fell below the target values of the present invention. Board No. For the steel sheet No. 28, the cooling stop temperature was below the specified value of the present invention, so precipitates such as carbides were not observed in any of the 10 fields of view in metallographic observation, and the Charpy impact test value was the target value of the present invention. and the fatigue crack propagation speed at ΔK=20 MPa·m ^1/2 exceeded 4.26×10 ⁻⁸ [m/cycle], indicating poor fatigue crack propagation resistance. Board No. The steel sheet No. 29 has a heating temperature lower than the Ac ₃ point of the present invention, so the metal structure is 85% ferrite structure + 15% pearlite structure, and precipitates such as carbides are observed in all of the 10 fields of view in the metal structure observation. I couldn't. As a result, YS and TS fell below the target values of the present invention. Board No. In the steel sheet No. 30, the cooling stop temperature exceeded the specified value of the present invention, so the metal structure was 82% ferrite structure + 18% pearlite structure, and YS and TS were below the target values of the present invention. Board No. Similarly, the steel sheet No. 31 also had a cooling stop temperature exceeding the specified value of the present invention, so the metal structure was 81% ferrite structure + 19% pearlite structure, and YS and TS were below the target values of the present invention. Board No. For the steel sheet No. 32, the cooling stop temperature was below the specified value of the present invention, so precipitates such as carbides were not observed in any of the 10 fields of view in metallographic observation, and the Charpy impact test value was the target value of the present invention. and the fatigue crack propagation speed at ΔK=20 MPa·m ^1/2 exceeded 4.26×10 ⁻⁸ [m/cycle], indicating poor fatigue crack propagation resistance.

Claims (9)

% by mass, C: 0.02 to 0.40%, Si: 0.010 to 0.500%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.050 % or less, Al: 0.100% or less, N: 0.1000% or less, and the balance being Fe and inevitable impurities, the metal structure of the steel material having a bainite structure and martensite A fatigue resistant crack characterized in that it is either a structure or a mixed structure of a bainite structure and a martensite structure, and further includes one or more kinds of precipitates of carbides, nitrides and carbonitrides. Steel material with excellent crack propagation characteristics. In addition to the chemical composition described above, Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01 to 3.00%, Mo: 0.01% to 2.00%, and Mo: 0.01% to 2.00% by mass. 01-1.00%, Nb: 0.001-0.100%, V: 0.001-0.100%, Ti: 0.001-0.100%, B: 0.0001-0.0100% , and REM: 0.001 to 0.100%. 3. The steel material having excellent fatigue crack propagation resistance according to claim 1 or 2, wherein a ferrite structure having an area ratio of 50% or less is present with each metal structure. The steel material having excellent fatigue crack propagation resistance according to any one of claims 1 to 3, wherein the steel material has a plate thickness of 5 to 100 mm. The fatigue crack propagation resistance crack propagation according to any one of claims 1 to 4, wherein the number of precipitates is 1 to 100/3.8 µm ² at the center of the plate thickness of the steel material. Steel with excellent properties. When the stress intensity factor range [ΔK] in the fatigue crack propagation test of the steel material is 20 MPa m ^1/2 , the fatigue crack propagation speed [da/dN] is 4.26 × 10 ^-8 m/cycle The steel material having excellent fatigue crack propagation resistance according to any one of claims 1 to 5, characterized by: % by mass, C: 0.02 to 0.40%, Si: 0.010 to 0.500%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.050 % or less, Al: 0.100% or less, N: _0.1000 % or less, and the balance is Fe and unavoidable impurities. The method for producing a steel material excellent in fatigue crack propagation resistance according to any one of claims 1 to 6, characterized in that the cooling is stopped at 100 to 450°C. In addition to the chemical composition of the steel material, in mass%, Cu: 0.01 to 2.00%, Ni: 0.01 to 5.00%, Cr: 0.01 to 3.00%, Mo : 0.01-1.00%, Nb: 0.001-0.100%, V: 0.001-0.100%, Ti: 0.001-0.100%, B: 0.0001-0 0100%, and REM: 0.001 to 0.100%. The method for manufacturing a steel material excellent in fatigue crack propagation resistance according to claim 7, characterized by containing one or more selected from 0.001 to 0.100%. . 9. The method for manufacturing a steel material excellent in fatigue crack propagation resistance according to claim 7 or 8, wherein the reheating and quenching treatment is performed at a cooling rate of 5 to 275° C./s.

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